- Conference main webpage with info on the venue, accommodation, payment of the fee, etc:
www.qcd-n2021.com
- This edition will be held in a hybrid format, with invited and contributed talks.
- There is a small fee for remote participation
- Abstract submission deadline: September 19, 2021
- Notification for contributed talks: September 22, 2021
- Connection details will be given only to registered participants.
QCD-N2021 is the 5th edition of the series of workshops on the QCD Structure of the Nucleon, previously held in Ferrara (2002), Frascati (2006), Bilbao (2012) and Getxo (2016). The main focus of this series of workshops is the investigation of the multi-dimensional nucleon structure and related topics in quantum chromodynamics.
This edition will emphasize the recent progress in the field from theory, lattice-QCD and phenomenology, as well as new developments coming from synergies with Quantum Information Science. The workshop will be complemented by presentations of projects planned all over the world at current and future high-energy facilities for a deeper understanding of the nucleon structure.
Organizing Committee:
José Manuel Alarcón (U. Alcalá)
Miguel G. Echevarría (chair, U. Alcalá)
Rafael Fernández (U. Complutense Madrid)
Gunar Schnell (U. Basque Country)
Ignazio Scimemi (U. Complutense Madrid)
Andrea Signori (U. Pavia & JLab)
International Program Committee:
Harut Avakian (JLab, US)
Alessandro Bacchetta (co-chair, U. Pavia & INFN, Italy)
Miguel G. Echevarría (U. Basque Country, Spain)
Michael Engelhardt (U. New Mexico State, US)
Salvatore Fazio (BNL, US)
Wolf-Dieter Nowak (U. Johannes Gutenberg, Germany)
Barbara Pasquini (U. Pavia & INFN, Italy)
Catarina Quintans (LIP, Portugal)
Gunar Schnell (co-chair, U. Basque Country, Spain)
Marco Stratmann (U. Tübingen, Germany)
Nature has two sources of mass. One is the Higgs boson, which is responsible for every mass scale that appears explicitly in the Standard Model Lagrangian. The Higgs was discovered at CERN in 2012; and with that discovery, the Standard Model became complete.
However, in connection with everyday matter, the material which constitutes our computers and ourselves, the Higgs produces little more than 1% of the mass. Some agent, far more powerful than the Higgs mechanism, is responsible for the remaining 99%, i.e. for the vast bulk of visible mass in the Universe. This is the phenomenon of emergent hadron mass (EHM) and it is characterized by the size of the proton’s mass, mp ≈ 1 GeV, which is roughly 2000-times the size of the electron mass. Uncovering the source and revealing the impacts of EHM stand as some of the greatest challenges in modern science.
Within the Standard Model, the proton is supposed to be described by quantum chromodynamics (QCD), a Poincaré-invariant local gauge theory of gluons (gauge fields) and quarks (matter fields); but no one can really be certain because, even fifty years after its formulation, there is no solution to QCD. Of course, there is a perturbative definition of the theory, valid at high energies. However, a perturbative approach cannot be used to compute any bound-state mass or radius, etc. And there is another challenge; namely, the pion. Discovered more than 70 years ago, the Standard Model describes the pion as a bound-state of a quark+antiquark pair. It is a hadron, like the proton; yet, the pion is atypically light, with a mass on the same scale as the μ lepton. It is a rare thing for a single theory in Nature to produce such a large disparity in masses from “nothing”: in the perturbative formulation of QCD, the gluons are massless and the quarks/antiquarks in the pion and proton possess masses that are less-than 1% of mp.
Solving the riddle posed by EHM is today the primary focus of experiments in nuclear and particle physics at the world’s leading accelerator facilities and the impetus behind construction of an electron ion collider in the USA and planning for such a facility in China. This presentation will describe a paradigm for understanding EHM within the Standard Model; and explain how a synergistic effort in experiment, phenomenology, and theory is capable of validating the picture. The sketch begins with the emergence of a mass for gluons, QCD’s gauge bosons – something that many imagined to be impossible; and culminates with an explanation of how deep ploughing structural studies of the pion and kaon, Nature’s lightest hadrons, may deliver answers to the profound questions that probe for the origin of 99% of the visible mass in the Universe.
With $e^-$ constituents in a classical potential atoms are at the borderline between hard processes and soft classical physics. Data and phenomenology indicate that hadrons are similar. The importance of this cannot be overemphasized: It suggests a first-principles, analytic QCD approach to strong dynamics analogous to that long developed for QED atoms.
Bound state perturbation theory is sometimes characterized as an ''art''. Atoms may be considered ''non-perturbative'', yet their binding energies can be expanded in powers of $\alpha$ and $\log\alpha$. The essential difference compared to standard perturbation theory is that the ''lowest order'' wave function (given, e.g., by the Schrödinger or Bethe-Salpeter equation) already has all powers of $\alpha$. Hence the ordering of the expansion is not unique, and may be chosen for optimal convergence.
Gauge theories have instantaneous interactions in non-local gauges. In temporal $(A^0=0)$ gauge the longitudinal electric field $\boldsymbol{E}_L$ provides a potential also for relativistic constituents. Gauss' law determines $\boldsymbol{\nabla}\cdot\boldsymbol{E}_L$ for each state from the instantaneous positions of its (color) charged constituents. Poincaré invariance allows including a homogeneous solution of Gauss' law for color singlet states in QCD. This gives rise to an ${\cal O}(\alpha_s^0)$ confining potential that is fully determined for any quark and gluon state, up to a universal scale $\Lambda$.
In a ''Bound Fock expansion'' the constituents of each state propagate in their instantaneous potential. Requiring the valence state ($e^+e^-$ for Positronium, $q\bar q$ for mesons, $qqq$ for baryons) to be an eigenstate of the Hamiltonian $\mathcal{H}$ determines its wave function. The bound state equation reduces to the Schrödinger equation in the non-relativistic limit. For quarkonia one obtains the ''Cornell'' potential with linear confinement. Higher Fock states are determined perturbatively by the transverse gauge boson interactions of $\mathcal{H}$. The QCD coupling $\alpha_s(Q) \simeq 0.5$ is frozen at hadronic scales $Q < \Lambda$, where the classical $\boldsymbol{E}_L^a$ field dominates. Considering all Fock states one obtains a formally exact expression for a hadron in any frame.
Mesons have no transverse gluon constituents at $\ {\cal O}(\alpha_s^0)$, but $\ q\bar q$ pairs arise due to time ordered ''$Z$''-diagrams. These have the properties of sea quarks and contribute to Deep Inelastic Scattering at low $x_{Bj}$. For small quark masses the meson states lie on linear Regge trajectories with parallel daughter trajectories. Features of the parton model and duality appear in the relevant limits. The approach seems promising, although many aspects remain to be investigated [1,2].
[1] Paul Hoyer, ``Journey to the Bound States'', SpringerBriefs in Physics (Springer, 2021) arXiv:2101.06721 [hep-ph].
[2] Paul Hoyer, “Hadrons as QCD Bound States,” in 14th Conference on Quark Confinement and the Hadron Spectrum (2021) arXiv:2109.06257 [hep-ph].
I review recent progress in the determination of the collinearly-integrated parton distribution functions of the proton. I will focus on the recent NNPDF4.0 determination, a state-of-the-art analysis based on almost independent 100 datasets. NNPDF4.0 is constructed by means of a novel methodology through hyperparameter optimisation, leading to an efficient fitting algorithm built upon stochastic gradient descent. Theoretical improvements in the PDF description include a systematic implementation of positivity constraints and integrability of sum rules. We validate our methodology by means of closure tests and "future tests" (i.e. tests of backward and forward data compatibility), and assess its stability, specifically upon changes of PDF parametrization basis. I will present the implications for NNPDF4.0 for our understanding of nucleon structure, from the proton antimatter asymmetry to the strange and charm quark content. The software framework used to produce NNPDF4.0 is made available as an open-source package together with documentation and examples.
Originally conceived for precise luminosity measurements, the gas injection system SMOG currently allows the unique LHCb detector capabilities to be exploited for fixed-target studies in proton-gas collisions at $\sqrt{s}$ ∼ 100 GeV. The first results obtained with SMOG data are reported: antiproton production with a He target and $J/\psi$, $D^0$ productions in p-He and p-Ar collisions. The upgraded system SMOG2, which will be used during Run 3 of LHC, will extend the target species available and increase the areal gas density, offering a unique opportunity for measurements related to hadron production, cosmic rays physic and nucleon structure at the LHC. An overview of the SMOG2 system and its prospects is reported along with a selection of interesting physical measurements.
For a long time, lattice QCD was unable to address the x-dependence of partonic distributions, direct access to which is impossible in Euclidean spacetime. Recent years have brought a breakthrough for such calculations when it was realized that partonic light-cone correlations can be accessed through spatial correlations computable on the lattice. Appropriately devised observables can be factorized into physical PDFs via a perturbative procedure called matching, analogous to the standard factorization of experimental cross sections. In this talk, I will discuss the tremendous progress of this research program in the last few years, concentrating on the two most popular approaches of quasi- and pseudo-PDFs. I will present the cases of the most studied twist-2 PDFs, as well as the exploratory direction of twist-3 distributions. Possibilities of synergy with phenomenology will be outlined.
We develop a procedure to renormalize the quasi parton distribution in the recently proposed hybrid scheme and match it to the $\overline{\rm MS}$ scheme light-cone parton distribution at next-to-next-to-leading-order in perturbation theory. Under this procedure we calculate the pion valence quark
distribution using two fine lattices with spacing
$a=0.04$ fm and $0.06$ fm and valence pion mass $m_\pi=300$ MeV, at boost momentum as large as $2.42$ GeV. We demonstrate that perturbative matching in the Bjorken-$x$ space yields
reliable determination of the valence quark
distribution for a moderate region of $x$.
The structure of nucleons as well as nuclei at large longitudinal momenta, x, is crucial for understanding QCD confinement, the origin of non-perturbative charm and beauty in the proton, or for explaining the origins of the nuclear EMC effect. Parton distribution functions (PDFs), which describe the longitudinal structure of hadrons, are typically determined in global analysis of experimental data. Unfortunately, the high-x region is hard to determine due to different theoretical effects that enter in this challenging region. In this talk I review such theoretical effects and present a recent analysis where we have accounted for them and determined nuclear PDFs using high-x data from JLAB experiments. I also give a brief overview of other possibilities to constrain the high-x structure of nucleons as well as nuclei.
We present the first lattice calculation of pion valence parton distribution using matching formula at NNLO level. We use the Wilson-Clover fermion on three 2+1 flavor HISQ ensembles of lattice spacings a = 0.04, 0.06 and 0.076 fm, with two pion mass including the physical one. Two unitary Domain-Wall calculations at physical point are also presented. This allows us to control the continuum limit, quark mass effects as well as the chiral symmetry. Our analysis use ratio-based schemes to renormalize the equal-time bilocal quark-bilinear matrix elements. We extract first few moments model independently and reconstruct the x-dependent PDF.
We developed a non-perturbative model for valence parton distribution functions (PDFs) based on the mean field interactions of valence quarks in the nucleonic interior. The model is based on the separation of the valence three-quark cluster and residual system in the nucleon. Then the nucleon structure function is calculated within the effective light-front diagrammatic approach introducing nonperturbative light-front valence quark and residual system wave functions. Based on the model one obtained a new relation between the position of the peak of $xq_V(x)$ distribution of the valence quark and the effective mass of the residual system, $m_R$: $x_{peak} \approx {1\over 4} (1-{m_R\over m_N})$ at starting $Q^2$ and explain the difference in the peak positions for d- and u- quarks to be due to an expected larger residual mass in the case of valence d- quark distribution. The parameters of the model are fixed by fitting the calculated valence quark distributions to the phenomenological PDFs. This allowed us to estimate the overall mean field contribution in baryonic and momentum sum rules. The evaluated parameters of the non-perturbative wave functions of valence 3q- and residual system can be used in calculation of other observables such as nucleon form factors, generalized partonic and transverse momentum distributions.
We review the theory, interpretation, and applications of the nucleon elastic form factors (vector, scalar, axial, other QCD operators) at low/moderate momentum tranfers (Q2 ~< 1 GeV2). We emphasize the importance of analyticity and dispersion theory for both empirical analysis and dynamical calculations. Topics include: (a) Dispersive analysis of the nucleon electromagnetic form factors, including modern approaches incorporating dynamical input from chiral EFT and other sources. (b) Extraction of the proton electric and magnetic radii from electron scattering data using theory-guided dispersive analysis. (c) First-principles calculations of the nucleon transverse charge and magnetization densities based on dispersion theory and chiral EFT, with new insights into peripheral nucleon structure and quark-hadron duality. (d) Dispersion theory for nucleon scalar, axial, and energy-momentum tensor form factors.
In 2010, precise determination of the proton radius from muonic lamb shift data, a result that systematically disagreed with previous atomic lamb shift and electron scattering data, sparked what was to become known as the proton radius puzzle. Since then, there have many new measurements and new theory calculations done to understand the source of the discrepancy. A preponderance of the new evidences points to the muonic lamb shift data being correct and now both CODATA and the Particle Data Group recommend using a value of ~0.84 fm for the proton's charge radius. I will review the new experimental results, the re-analyses, and new theoretical work that resulted in this paradigm shift as well as discuss planned future measurements as we go from puzzle to precision.
Effective field theories are exceptionally suited to define and determine the proton radius and its relatives from low energy observables. They not only provide a unified and unambiguous definition of the low energy constants that naturally includes electromagnetic corrections, but they also yield a robust determination of the theoretical uncertainty. In this talk, I will present the effective field theory that allows us to extract the proton charge radius and its relatives from spectroscopic measurements. As an application, I will review the determination of the proton radius and the Zemach radius from measurements of the Lamb shift and the hyperfine splitting in regular and muonic hydrogen, emphasizing on the advantages of using effective field theories.
The Generalized Polarizabilities (GPs) are fundamental properties of the
nucleon. They characterize the nucleon's response to an applied
electromagnetic field, offering access to the polarization densities inside
the nucleon, and as such they represent an essential part for a complete
understanding of the nucleon structure and dynamics. The GPs can be
explored through the measurement of the Virtual Compton Scattering
reaction. Previous measurements of the proton electric GP at
intermediate four-momentum transfer squared have challenged the
predictions of theoretical calculations, raising questions in regard to the
underlying reasons responsible for a local enhancement of the electric
GP. The measurement of the magnetic GP, on the other hand, allows to
quantify the interplay of the paramagnetism and diamagnetism inside
the proton. The VCS experiment (E12-15-001) at JLab has accessed the
proton GPs with high precision in the intermediate four-momentum
transfer squared region, namely from Q2=0.3 (GeV2) to Q2=0.7
(GeV2). Preliminary results from the Hall C VCS experiment will be
presented and future prospects will be discussed in this talk.
This work has been supported by the US Department of Energy Office
of Science, office of Nuclear Physics under contract no. DESC0016577.
In this talk, I will present the details of a new factorized approach to inclusive lepton-hadron scattering, in particular to semi-inclusive deep-inelastic scattering, which treats QED and QCD radiation on equal footing, and provides a systematically improvable approximation to the extraction of transverse momentum dependent parton distributions. We demonstrate how the QED contributions can be well approximated by collinear factorization, and systematically included to all orders. For semi-inclusive processes, we show how radiation effects prevent a well-defined ``photon-nucleon'' frame, forcing one to use a two-step process to account for the radiation. We illustrate the utility of the new method by explicit application to the spin-dependent Sivers and Collins asymmetries at the future EIC.
It has been more than 30 years since the EMC published a surprising result on the spin structure of the proton: the spins of its three quark components account for only a small part of the spin of the proton. In this talk, we discuss what has been learned so far, what is still missing and what could be learned from the upcoming experiments, including the Jefferson Lab 12 GeV upgrade and the proposed Electron-Ion Collider.
The structure of hadrons depends strongly on the confinement mechanism. This talk illustrates the dynamics of confinement at the border between the interior and exterior of hadrons. The extended chiral structure is illustrated in chromostatics and amplified by a discussion of vortices and the orbital angular momentum resulting in the pion tornado.
I will report on recent developments in the extraction of the transversity distribution from various methodologies, including transverse momentum dependent (TMD) and collinear twist-3 observables, collinear dihadron fragmentation processes, and lattice QCD. Connected to this is the calculation of the tensor charges of the nucleon, which are important quantities that sit at the intersection of TMD/collinear QCD phenomenology, beyond the Standard Model physics, and lattice QCD. In addition, I will discuss the impact of the Electron-Ion Collider on our understanding of the transversity distribution and associated tensor charges and the importance of achieving a coherent picture of extractions across multiple approaches.
The Standard Model (SM) has besides the basic chiral left-right (L-R) symmetry and the charge conjugation symmetry, an intriguing tri-partite structure and corresponding $Z_3$ symmetry. This is most manifest for quarks with both electroweak and color charges. We conjecture a basic underlying 1D structure for the SM as the origin of many of its features. The special status of QCD in 3D emerges because of basic entanglement properties of a tri-partite chiral qubit space. I discuss some consequences for parton structure including transverse momentum dependence and fragmentation.
We determine approximate next-to-next-to-leading order (NNLO) corrections to unpolarized and polarized semi-inclusive DIS. They are derived using the threshold resummation formalism, which we fully develop to next-to-next-to-leading logarithmic (NNLL) accuracy, including the two-loop hard factor. In terms of the customary SIDIS variables x and z they include all double distributions in the partonic variables. Our numerical estimates suggest much significance of the approximate NNLO terms, along with a reduction in scale dependence.
We review the recent progress on the extraction of TMD PDFs and TMD FFs from global data of Semi-Inclusive Deep-Inelastic Scattering, Drell-Yan and Z boson production. In particular, we address the tension between the low-energy SIDIS data and the theory predictions.
I review the latest extractions of TMD distributions made with artemide. The main emphasis is made on the first determination of realistic uncertainty band for unpolarized TMDPDF, and on the interplay between collinear and TMD distributions.
I will give a brief overview of our current understanding of fragmentation from phenomenological analyses of (polarized) collinear and TMD fragmentation functions, including some recent extractions, formulations of new factorization theorems, and Monte Carlo studies.
COMPASS is a fixed-target experiment located at the CERN SPS, which has been collecting data since 2002. One of the key aspects of the broad COMPASS physics program is the investigation of the transverse-momentum and transverse-spin structure of the nucleon, which has been pursued via measurements of semi-inclusive deep inelastic scattering using a 160 GeV/c muon beam and transversely polarized and unpolarized proton and deuteron targets, and of the Drell-Yan process using a 190 GeV/c pion beam and a transversely polarized proton target. An overview of the main COMPASS results on TMD observables obtained so far is presented.
After fifty years of investigations, the nucleon structure is still far from being understood and continues to represent a unique test bench for QCD. Despite the enormous progresses achieved in five decades of deep-inelastic scattering (DIS) experiments, a number of crucial open questions are still on the carpet and subject of intense theoretical and experimental studies. In the last two decades, semi-inclusive DIS was established as a unique tool for the study of the non-collinear structure of nucleons, involving the parton transverse momentum pT as an additional degree of freedom. Requiring the detection of at least one final state-hadron in coincidence with the scattered lepton, it opened the way not only to measure of the chiral-odd transversity distribution, the last missing leading-twist collinear parton distribution function, but also to a variety of new pT-dependent PDFs, known as TMDs. Describing correlations between the quark transverse momentum and the quark or the nucleon spin (spin-orbit correlations), TMDs account for a number of intriguing effects observed in polarized and unpolarized reactions, and allow for a 3-dimensional description of the nucleon in momentum space. Furthermore, they could provide insights into the yet unmeasured quark orbital angular momentum. At leading-twist, eight TMDs enter the SIDIS cross section in conjunction with a fragmentation function. In addition, going to the twist-3 level allows us to probe novel quark-gluon correlations.
The HERMES experiment collected a wealth of data using the 27.6 GeV polarized HERA lepton beam and various polarized and unpolarized gaseous targets. This allows for a series of unique measurements of observables sensitive to this multidimensional (spin) structure of the nucleon, probed through specific azimuthal modulations in the distribution of hadrons produced in semi-inclusive DIS. Amplitudes of some of these modulations sensitive to the beam and/or target polarization, recently extracted for the first time also in a three-dimensional kinematic space, will be presented in more detail.
A small-$x$ model for gluon GTMDs will be discussed. With this model a reasonable description of the H1 data on diffractive dijet production in electron-proton collisions can be obtained. Predictions for the EIC will be given for both electroproduction and photoproduction which may allow to further test the underlying GTMD description.
The SpinQuest experiment (E1039) at Fermilab aims to extract Sivers functions for the light sea-quarks in the range of 0.1 < xB < 0.5 through Transverse Single Spin Asymmetry (TSSA) measurements from the Drell-Yan (DY) process, using an unpolarized 120 GeV proton beam interacting with polarized fixed target of either NH3 or ND3. In addition to the DY TSSA measurements, J/ψ TSSAs will also be measured to extract the gluon Sivers function. The SpinQuest experiment will be put into a global context, considering its unique kinematic coverage, which will serve as a bridge between probing valence quarks and kinematics at future EIC experiments. The goals, as well as the importance of the measurements, will be discussed in detail.
I give an overview of generalized parton distributions (GPDs), a vital theoretical tool for understanding the 3D partonic structure of hadrons. GPDs provide a generalization of collinear parton distributions that additionally encode the elastic form factors associated with electromagnetic, axial, and gravitational currents. They can be experimentally studied through hard inclusive reactions such as deeply virtual Compton scattering that are a major focus of both Jefferson Lab and the future Electron Ion Collider. I will discuss their properties and experimental status, and briefly touch on the deconvolution problem and whether GPDs can be fully experimentally measured.
We compute the matching of TMD distributions to collinear distributions of twist-2 and twist-3 to all powers of small-$b$ (large-$p_T$).
The computation is based on the operator product expansion of the non-local TMD operators.
For the first time, a non-trivial expression for the pretzelosity distribution is derived.
We explain possible transverse-momentum-dependent parton distribution functions (TMDs) for spin-1 hadrons up to twist 4 by decomposing a quark correlation function with the conditions of the Hermiticity and parity invariance [1]. In the TMDs, there exist time-reversal-odd functions in addition to the time-reversal-even ones. We showed that 40 TMDs exist in the tensor-polarized spin-1 hadron in the twist 2, 3, and 4. In particular, we found 30 new structure functions in the twist 3 and 4 in our work. Since time-reversal-odd terms of the collinear correlation function should vanish after integrals over the partonic transverse momentum, we obtain new sum rules for the time-reversal-odd structure functions, $\int d^2 k_T g_{LT} = \int d^2 k_T h_{LL} = \int d^2 k_T h_{3LL} =0$. In addition, we indicated that new transverse-momentum-dependent fragmentation functions exist in tensor-polarized spin-1 hadrons.
Sum rules for structure functions and their twist-2 relations have important roles in constraining their magnitudes and $x$ dependencies and in studying higher-twist effects. The Wandzura-Wilczek (WW) relation and the Burkhardt-Cottingham (BC) sum rule are such examples for the polarized structure functions $g_1$ and $g_2$. Recently, new twist-3 and twist-4 parton distribution functions were proposed for spin-1 hadrons [1], so that it became possible to investigate spin-1 structure functions including higher-twist ones. We show in this work that an analogous twist-2 relation and a sum rule exist for the tensor-polarized parton distribution functions $f_{1LL}$ and $f_{LT}$ [2], where $f_{1LL}$ is a twist-2 function and $f_{LT}$ is a twist-3 one. Namely, the twist-2 part of $f_{LT}$ is expressed by an integral of $f_{1LL}$ (or $b_1$) and the integral of the function $f_{2LT} = (2/3) f_{LT} -f_{1LL}$ over $x$ vanishes. If the parton-model sum rule for $f_{1LL}$ ($b_1$) is applied by assuming vanishing tensor-polarized antiquark distributions, another sum rule also exists for $f_{LT}$ itself. These relations should be valuable for studying tensor-polarized distribution functions of spin-1 hadrons and for separating twist-2 components from higher-twist terms, as the WW relation and BC sum rule have been used for investigating $x$ dependence and higher-twist effects in $g_2$. In deriving these relations, we indicate that four twist-3 multiparton distribution functions $F_{LT}$, $G_{LT}$, $H_{LL}^¥perp$, and $H_{TT}$ exist for tensor-polarized spin-1 hadrons. These multiparton distribution functions are also interesting to probe multiparton correlations in spin-1 hadrons.
In the near future, we expect that physics of spin-1 hadrons will become a popular topic, since there are experimental projects to investigate spin structure of the spin-1 deuteron at the Jefferson Laboratory, the Fermilab, the nuclotron-based ion collider facility [3], the LHCspin, the electron-ion colliders in US and China in 2020's and 2030's.
[1] S. Kumano and Qin-Tao Song, Phys. Rev. D 103 (2021) 014025.
[2] S. Kumano and Qin-Tao Song, arXiv:2106.15849 (J. High Energy Phys. in press).
[3] A. Arbuzov et al. (S. Kumano, 16th author; Q.-T. Song, 27th author), Prog. Nucl. Part. Phys. 119 (2021) 103858.
We present a recent calculation of the gravitational
form factors and the distributions of pressure and shear force inside the proton in a light-front quark-diquark model constructed by the soft-wall AdS/QCD.
We compare the results with the lattice as well as those extracted from JLab data. We present a comparative study of different model calculations of these quantities in the literature.
Effective chiral Lagrangian of nucleons and pions in external gravitational field and the corresponding energy-momentum tensor will be considered. Gravitational form factors of the nucleon and their relation to internal forces will be discussed.
In this contribution I will revisit GPD evolution in momentum space. I will highlight some fundamental properties of the evolution kernels and present a leading-order numerical implementation that quantitatively fulfils all these properties.
Deeply virtual Compton scattering (DVCS) is a particularly promising channel to extract generalized parton distributions (GPDs) allowing the study of the three-dimensional structure and the energy-momentum tensor of the nucleon. However, the access to GPDs from DVCS experimental data is complicated by the so-called deconvolution problem and the lack of precise data in some kinematic regions. The recent highlight on "shadow GPDs", that is GPDs with arbitrarily small imprint on DVCS observables in a next-to-leading order study even with evolution effects, gives interesting possibilities to quantify the extent of the deconvolution problem and suggest modelling procedures to propagate the associated theoretical uncertainty. We discuss their impact on the extraction of energy-momentum tensor form factors and some statistical procedures to assess the impact of future experimental setups.
Generalized Parton Distributions (GPDs) describe the correlations between the longitudinal momentum and the transverse position of the partons inside the nucleon. They are nowadays the subject of an intense effort of research, in the perspective of understanding nucleon spin and mechanical properties.
In this talk, we present the first measurement of the Timelike Compton Scattering (TCS) process, $\gamma p \rightarrow \gamma^*p \rightarrow e^+ e^- p' $, measured using the CLAS12 detector at Jefferson Lab, with a 10.6 GeV electron beam impinging on a liquid-hydrogen target. The initial photon polarization and the decay lepton angular asymmetries are reported in the range of timelike photon virtualities $2.25< Q'^2<9~{\rm GeV}^2$ and the squared momentum transferred $0.1<-t<0.8~{\rm GeV}^2$ at the average total center mass energy squared of $\bar{s}=14.5~{GeV^2}$. The polarization asymmetry, similar to the beam spin asymmetry in Deeply Virtual Compton Scattering (DVCS), projects out the imaginary part of the Compton Form Factors (CFFs, which are complex quantities linked to the GPDs) and provides a way to test the universality of Generalized Parton Distributions. The angular asymmetry of the decay leptons, on the other hand, accesses the real part of the CFF $\mathcal{H}$ which contains the D-term, a quantity directly linked to the mechanical properties of the nucleon.
Following simple large Nc arguments and perturbative QCD constraints complemented with uncertainty estimates based on the idea of meson dominance and the half-width rule, we describe the pseudoscalar form factors of the nucleon. We analyze their implications in the space-like region at intermediate and low energies and compare to recent lattice QCD determinations. Our analysis allows for a simple determination of the pion-nucleon coupling constant at a precision level that matches the most accurate determination to date based on the analysis of the Granada nucleon-nucleon database (8000 experimental NN scattering data).
A lattice QCD determination of the nucleon sigma terms, which are closely related to the scalar charges of the nucleons, will be presented. These sigma terms determine the strength of the spin-independent interactions of nucleons with WIMPs searched for in direct dark matter detection experiments. They are also closely related to the masses of the nucleons. The calculation presented here is based on the Feynman-Hellmann theorem and features Wilson and staggered fermion formulations at different stages of the calculation to exploit their respective advantages.
Generalized parton distributions (GPDs) are important quantities that characterize the structure of hadrons. They provide information about the partons’ momentum distribution and also on their distribution in position space. Most of the information from lattice QCD is on the Mellin moments of GPDs, namely form factors and their generalizations. Recent developments in calculations of matrix elements of boosted hadrons coupled with non-local operators opened a new direction of extracting the x dependence of GPDs. In this talk, we will discuss selected results on the two approaches to get information on GPDs and highlight their advantages and disadvantages. I also discuss novel calculations to extract the x-dependence on twist-3 GPDs using the quasi-distribution functions approach and LaMET. Information on twist-3 GPDs is basically non-existing, but they are essential for several reasons.
In the last few years, several new techniques for quantum many-body physics based on quantum information methods and concepts have been developed and applied. In one such approach, quantum simulation, an inaccessible physical model is mapped to another quantum device which can be manipulated and measured in the lab, hence serving as its simulator. In another one, by considering the special entanglement properties of physically relevant states, one can significantly reduce the computational complexity of many-body models and study them on a classical computer.
Starting in the field of condensed matter physics, these tools have now entered the particle physics toolbox as well. I will introduce the ideas behind them and how they could help, and discuss quantum simulation and tensor network techniques for lattice gauge theories.
We present a first attempt to design a quantum circuit for the determination of the parton content of the proton through the estimation of parton distribution functions (PDFs), in the context of high energy physics (HEP). The growing interest in quantum computing and the recent developments of new algorithms and quantum hardware devices motivates the study of methodologies applied to HEP. In this work we identify architectures of variational quantum circuits suitable for PDFs representation (qPDFs). We show experiments about the deployment of qPDFs on real quantum devices, taking into consideration current experimental limitations. Finally, we perform a global qPDF determination from collider data using quantum computer simulation on classical hardware and we compare the obtained partons and related phenomenological predictions involving hadronic processes to modern PDFs.
Quantum computers can efficiently simulate quantum field theory observables. Basis light front quantized systems have been modeled for meson states using confining effective field theory. Some form factors and parton distributions were obtained from an available quantum computer not yet able to implement error correction – a NISQ device. The procedure will be outlined and some early results shown.
Double parton scattering (DPS) is the phenomenon in which, during a nucleon-nucleon scattering, two partons from each nucleon undergo two distinct hard interactions.
The cross section resulting from DPS interactions is normally power-suppressed with respect to the traditional single parton scattering (SPS) cross section. However, the two contributions can be similar in size in some circumstances, for example in processes that are highly suppressed in SPS like same-sign W-boson production, or when studying jet emissions in particular phase-space sectors. Moreover, the study of DPS is closely related to the three-dimensional structure of nucleons.
In this talk we focus on the derivation of the DPS cross section formula and the relevant factorization theorems, both in collinear and TMD factorization schemes. We will also introduce the double parton distributions (DPDs), which are the DPS analog of PDFs, and the subtraction formalism that is necessary to combine SPS and DPS calculations.
In factorization theorems for double parton scattering (DPS) the non-perturbative parts of the cross section are encoded in universal functions, double parton distributions (DPDs) in the case of collinear factorization and double transverse momentum dependent parton distributions (dTMDs) for transverse momentum dependent (TMD) factorization.
These distributions contain a wealth of information about the structure of the proton, in particular about correlations between two partons inside a proton with respect to their spin, colour, and spatial separation.
In this talk we will give an introduction to these distributions, their spin and colour structure, and their renormalisation and rapidity scale dependence.
Since presently DPDs and dTMDs are largely unconstrained experimentally, DPS cross section calculations are at present performed using model DPDs. We will briefly review one of the most used approximations, highlighting its shortcomings, and show how improved models can be constructed using DPD number and momentum sum rules.
In this contribution, I discuss the possibilities offered by double parton scattering (DPS) processes initiated by photon-proton interactions. In fact, the DPS cross section depends on the double parton distribution functions (dPDFs) of hadrons. These new quantities encode new informations on the 3D partonic structure of the proton, complementary to TMDs and GPDs. In fact, dPDFs are sensitive to unknown double parton correlations in hadrons which cannot be accessed through, e.g., GPDs. However, dPDFs are almost unknown and, in particular, their dependence on the transverse distance of partons. In our analyses [1, 2, 3, 4] we discussed the impact of both perturbative and non perturbative double parton correlations in dPDFs. In addition, our collaboration studied the impact
of these effects on the experimental observable called effective cross section, $\sigma_{eff}$ [5, 6]. However, as proved in Refs. [7, 8] in proton-proton collisions, e.g. at the LHC, only limited information on the partonic proton structure can be extracted from data due to the lack of information on dPDFs. Therefore, in Ref. [12], we proposse to consider DPS initiated by quasireal photons. In this case, the offshellness of the photons is controlled by measuring leptons, proton or ions from the impinging beam scattered at low angle. At such low virtualities, the photon can fluctuate in a $q − \bar q$ pair which then initiates the double parton scattering with the proton. In this scenario, the photon transverse size could be almost controlled by measuring the virtuality ($Q^2$) and, in turn, the interaction rate in the DPS mechanism. Such a condition leads to the extraction of information on the transverse proton structure from the dependence on $Q^2$ on $\sigma_{eff}^{\gamma p}(Q^2)$. The latter quantity has been calculated, for the first time by using different models for the proton and the photon splitting mechanism. These results have been then used to estimate the DPS cross section for the four jets production via DPS for the HERA kinematics. In fact, the ZEUS collaboration reported significant MPI effects in this channel for the four jets cross sections, and exposed in their analyses possible contamination of the DPS processes. By estimating the expected number of events at given integrated luminosity, we conclude that DPS process in photoproduction gives a significant fraction of the four jet production cross section if cuts on transverse momenta of the jets are low enough.
References
[1] M. Rinaldi, S. Scopetta and V. Vento, Phys. Rev. D 87, 114021 (2013)
[2] M. Rinaldi, S. Scopetta, M. Traini and V. Vento, JHEP 1412, 028 (2014)
[3] M. Rinaldi, S. Scopetta, M. C. Traini and V. Vento, JHEP 1610, 063 (2016)
[4] M. Rinaldi and F. A. Ceccopieri, Phys. Rev. D 95, no. 3, 034040 (2017)
[5] F. A. Ceccopieri, M. Rinaldi and S. Scopetta, Phys. Rev. D 95, no. 11, 114030 (2017)
[6] M. Rinaldi, S. Scopetta, M. Traini and V. Vento, Phys. Lett. B 752, 40 (2016)
[7] M. Rinaldi and F. A. Ceccopieri, JHEP 1909, 097 (2019)
[8] M. Rinaldi and F. A. Ceccopieri, Phys. Rev. D 97, no. 7, 071501 (2018)
[9] G. S. Bali et al., JHEP 1812, 061 (2018)
[10] M. Rinaldi, S. Scopetta, M. Traini and V. Vento, Eur. Phys. J. C 78, no. 9, 781
(2018)
[11] M. Rinaldi, Eur. Phys. J. C 80, no. 7, 678 (2020)
[12] M. Rinaldi and F. A. Ceccopieri, arXiv:2103.13480.
Understanding the structure and dynamics of the proton constitute one of the most important challenges in hadron physics. From the theoretical point of view, one of the challenges is to extract from Lattice QCD calculations, performed in Euclidean space, Minkowskian quantities such as the proton parton distribution function. Due to the inherent difficulties associated with the mapping of Euclidean quantities to the corresponding Minkowskian ones, it is advantageous to have a solution defined directly in Minkowski space for calculations of dynamical observables such as momentum distributions.
In this contribution we present results for the proton calculated using a simple but dynamical model defined in Minkowski space [1]. Our starting point is the Bethe-Salpeter-Faddeev equation for a system of three spin-less bosons interacting through a contact interaction. Recently, the solution to this equation was studied in great detail by us in the papers [2, 3, 4]. In this work, the equation is solved in the valence approximation and the parameters of the model are set by comparing the calculated Dirac form factor with experimental data. The single- and double parton distributions of the proton are then computed. The proton image on the null plane in the space given by the transverse coordinates and the Ioffe times $\tilde{x}_{1,2}$ is also studied, by performing numerically the Fourier transformation of the distribution amplitude.
[1] E. Ydrefors and T. Frederico, arXiv:2108.02146 [hep-ph].
[2] E. Ydrefors, J.H. Alvarenga Nogueira, V. Gigante, T. Frederico and V.A. Karmanov, Phys. Lett. B 770 (2017) 131.
[3] E. Ydrefors, J.H. Alvarenga Nogueira, V.A. Karmanov and T. Frederico, Phys. Lett. B 791 (2019) 276.
[4] E. Ydrefors, J.H. Alvarenga Nogueira, V.A. Karmanov and T. Frederico, Phys. Rev. D 101 (2020) 096018.
We present exploratory studies of the 3D gluon content of the proton, as a result of analyses on leading-twist transverse-momentum-dependent (TMD) gluon distribution functions, calculated in a spectator model for the parent proton. Our formalism embodies a fit-based parameterization for the spectator-mass density, suited to describe both the small- and the moderate-x regime. Particular attention is paid to the T-odd gluon TMDs, which represent a key ingredient in the description of relevant spin-asymmetries emerging when the nucleon is polarized, as the gluon Sivers effect. All these analyses are helpful to shed light on the gluon dynamics inside nucleons and nuclei, which is one of the primary goals of new-generation colliders, as the Electron-Ion Collider, the High-Luminosity LHC and NICA-SPD.
In the context of a theory for only one heavy flavor in QCD, we use the renormalization group procedure for effective particles (RGPEP) to derive the effective potential for $Q\bar Q$ and $QQQ$ that arises at the energy scale at which bound states are formed. The RGPEP provides the connection between low- and high-energy interactions in QCD through the construction of effective particles.
The approach reproduces the correct behavior of the coupling constant at high energies (asymptotic freedom) and, at the current level of approximation, the second-order solution of the renormalization group equations yields a Coulomb potential with Breit-Fermi spin couplings, which is corrected by a harmonic oscillator term. These results are obtained assuming that, beyond perturbation theory, gluons get an effective mass.
We study the polar and azimuthal decay angular distributions of $J/\psi$ mesons produced in semi-inclusive, deep-inelastic electron-proton scattering. For the description of the quarkonium formation mechanism, we adopt the framework of Non-Relativistic Quantum Chromodynamics, with the inclusion of the intermediate color octet-channels that are suppressed at most by a factor $v^4$ in the velocity parameter $v$, relative to the leading color-singlet channel. We put forward factorized expressions for the helicity structure functions in terms of transverse momentum dependent gluon distributions and shape functions, which are valid when the $J/\psi$ transverse momentum is small and correctly match with the collinear factorization results at high transverse momentum. It turns out that the $\cos 2\phi$ azimuthal decay asymmetry originates from the distribution of linearly polarized gluons inside an unpolarized proton.
We therefore suggest a novel experiment for the extraction of this so-far unknown parton density that could be performed, in principle, at the future Electron-Ion Collider.
We revisit inclusive $J/\psi$ and $\Upsilon$ photoproduction at lepton-hadron colliders, namely in the limit when the exchange photon is quasi real. Our computation includes the leading-$v$ next-to-leading order (NLO) $\alpha_s$ corrections. Similarly to the case of NLO charmonium-hadroproduction processes, the resulting cross sections obtained in the $\overline{\text{MS}}$ factorisation scheme are sometimes found to be negative. We show that the scale fixing criteria which we derived in a previous study of $\eta_c$ production successfully solves this problem. In turn, we argue that both $J/\psi$ and $\Upsilon$ photoproduction can be used to set stringent constraints on the poorly constrained gluon densities at scales as low as a couple of GeV.
I will show the strategy to access the transverse-momentum dependent parton distribution function (TMD-PDF) through the large momentum effective theory (LaMET), and also summarize the present Lattice QCD progresses on the related topics, likes the renormalization, soft function, TMD wave function, Collins-Soper kernel and so on.
The COMPASS experiment continues the investigation of the transverse spin and transverse momentum structure of the nucleon.
Very recently, a new set of measurements has been performed in SIDIS of high energy muons off unpolarised protons. This talk will review the results on the transverse momentum distributions of the final state hadrons. A fairly complete study of the kinematic dependence has been performed, and will be summarised. Also, ideas for a leading order extraction of the mean value of the squared intrinsic transverse momentum of the quarks will be presented.
The transverse single-spin asymmetry in inclusive electron-nucleon scattering, e + N(S_T) -> e’ + X, represents a pure two-photon exchange observable and is of fundamental interest for exploring higher-order QED effects in electron scattering. We compute this observable in the resonance region, where excitation of Delta isobars occurs in both intermediate and final states. We employ a novel theoretical method based on the large-Nc limit of QCD, which allows us to consistently combine nucleon and Delta states and predict the elastic, inelastic, and inclusive spin-dependent cross section. Our results aim at disentangling the different contributions of nucleon and Delta states organizing them according to their 1/Nc scaling. The case of the target single spin asymmetry will be discussed in detail. Our predictions could be tested in future measurements of electron nucleon scattering with polarized targets in the few-GeV energy range. Such experiments would complement earlier measurements of the inclusive single-spin asymmetry in the DIS regime (JLab, HERMES) and allow one to study the unknown dependence of two-photon exchange dynamics on the energy/momentum of the probe.
Primary authors: GOITY, Jose (Hampton University, Jefferson Lab); WEISS, Christian (Jefferson Lab); WILLEMYNS, Cintia (University of Mons)
The Relativistic Heavy Ion Collider (RHIC) is the world's only polarized proton+proton collider, capable of reaching center of mass energies up to 510 GeV. The STAR experiment at RHIC has been carrying out a cold QCD program in order to gain deeper insight into the proton's spin structure and dynamics.
Data from longitudinally polarized $p$+$p$ collisions allow one to study the gluon helicity distribution function ($\Delta g(x)$), by measuring the longitudinal double-spin asymmetries ($A_{LL}$) of inclusive jets and dijets. On the other hand, the transversely polarized proton collisions at RHIC enable the studies of the transverse spin structure, such as the transversity and Sivers distributions, as well as polarized fragmentation functions. These studies can be used to test universality of transverse-momentum dependent distributions (TMDs) with respect to $e$+$p$ processes, and constrain their evolution effects. Furthermore, unpolarized measurements of differential cross sections of weak bosons at RHIC provide important constraints on the scale dependence of unpolarized TMDs in an $x$ range ($0.1 < x < 0.3$) that naturally complements the phase space accessed at the LHC.
In this talk, we present the recent measurements for longitudinal and transverse polarization, besides selected unpolarized results. STAR is currently installing a suite of new sub-detectors in the forward pseudorapidity region ($2.5 < \eta < 4$). How those upgrades will supplement previous spin measurements at RHIC will also be briefly discussed.
I will survey a number of new developments in hadron physics
which can be derived from the application of super-conformal quantum
mechanics and lightฉ\front holography -- its embedding in higher
dimensional gravity theory. This includes new insights into the
physics of color confinement, chiral symmetry, the spectroscopy and
dynamics of hadrons, as well as surprising supersymmetric relations
between the masses of mesons, baryons, and tetraquarks, I also will
briefly discuss some novel features of QCD-- such as color transparency,
hidden color, and intrinsic heavy-quark phenomena.
Within a recently developed formalism for $e^+e^-$ one-hadron production, we present preliminary results on the extraction of unpolarized transverse momentum dependent fragmentation functions (TMDFFs). We address possible constrains at large values of impact parameter $b_T$, for the TMDFF and the Collins-Soper kernel. We outline a work plan for global extractions of fragmentation functions.
By adopting the helicity formalism within a TMD scheme, we present the complete structure of the azimuthal dependences and polarization observables for two-hadron production in $e^+e^-$ annihilation processes. The leading-twist TMD fragmentation functions (FF) for spin-1/2 hadrons, with their properties and their probabilistic interpretation, are fully accounted for.
The role of the polarizing FF is discussed in detail and its extraction from Belle data for the transverse polarization of $\Lambda$'s is shown. Estimates for SIDIS processes at the EIC are presented.
In this talk I will make an overview on recent progress in the field of massive event shapes. In particular, the status of computations carried out in fixed-order QCD as well as in effective field theories (SCET and bHQET) will be reviewed. I will show how to efficiently carry out large-log resummation in the presence of heavy-quark masses and present some phenomenological studies. If time permits I will also show an analysis based on the large-$\beta_0$ limit of QCD.
We consider four and five parton scattering amplitudes in QCD and analyse their high-energy limit and colour structure. On the one hand, our main tools are the Reggeization of $2\rightarrow n$ scattering amplitudes in QCD, known as the effective action that governs their behaviour in the Regge limit or multi-Regge (MR) limit in the case of five or more partons. On the other hand, we use the dipole formula, which governs Infrared factorisation and is a compact ansatz for all-order infrared-singular terms of scattering amplitudes of massless partons. Building upon previous results, we use the compatibility between Regge factorisation and Infrared factorisation to obtain new results like the one-loop central-emission vertex for a gluon with positive helicity.
The anomalous magnetic moment of the muon, $(g-2)_{\mu}$, is an interesting quantity to search for new physics. In particular, it has attracted quite attention due to the persisting discrepancy among theory and experiment, that has been confirmed recently this year at Fermilab.
In this talk, I review the current status of its theoretical determination, with special attention to the hadronic contributions. Notoriously, the latter play an important role in determining $(g-2)_{\mu}$ and represent an extremely active field in the hadronic physics community.
Recently the first part of the proposed measurements of the AMBER collaboration
were approved as NA66 at CERN. AMBER will use the M2 beamline of the CERN SPS
with muon as well as hadron beams to perform a variety of hadron structure and
spectroscopy studies. These studies will allow to adress fundamental questions
of QCD. The talk will cover the proposed measurements and the preparation of the
first measurements.
Prospects for quarkonium-production studies accessible during the upcoming high-luminosity phases of the CERN Large Hadron Collider operation after 2021 are reviewed. Current experimental and theoretical open issues in the field are assessed together with the potential for future studies in quarkonium-related physics. This will be possible through the exploitation of the huge data samples to be collected in proton–proton, proton–nucleus and nucleus–nucleus collisions, both in the collider and fixed-target modes.
A polarized gaseous target, operated in combination with the high-energy, high-intensity LHC beams and a highly performing LHC particle detector, has the potential to open new physics frontiers and to deepen our understanding of the intricacies of the strong interaction in the non-perturbative regime of QCD. Specifically, the LHCspin project aims to develop, in the next few years, innovative solutions and cutting-edge technologies to access spin physics in high-energy polarized fixed-target collisions using the LHCb detector. Given its forward geometry (2<𝜂<5), the LHCb spectrometer is, in fact, perfectly suitable to cope with the forward kinematics of these collisions. Furthermore, being designed and optimized for the detection of heavy hadrons, it will allow to inspect the nucleon’s structure by exploiting new probes, such as inclusive production of c- an b-hadrons, and ideal tool to access, e.g., the essentially unexplored spin-dependent gluon TMDs. This configuration will allow to explore the nucleon’s internal dynamics at unique kinematic conditions, by covering a wide backward rapidity region, including the poorly explored high x-Bjorken and high x-Feynman regimes. With the installation of the proposed polarized target system, LHCb will become the first experiment delivering simultaneously unpolarized beam-beam collisions at 14 TeV and both polarized and unpolarized beam-target collisions at center-of-mass energies of the order of 100 GeV. The status of the LHCspin project is presented along with a selection of physics opportunities.
I will present my thoughts about how heavy-ion physics
can profit from eA measurements, paying a particular attention to cold nuclear matter effects that can be studied at
the future Electron-Ion Collider and the implications of these measurements on the interpretation of RHIC and LHC data.