We argue that the Drell–Yan process in the <math><msub><mrow><mi>x</mi></mrow><mrow><mi>A</mi></mrow></msub><mo>≥</mo><mn>0.15</mn></math> kinematics recently studied at FNAL by the E906/SeaQuest experiment may allow to observe an analogous of the EMC effect for antiquarks. The effects of Fermi motion and energy loss are considered. The preliminary E906/SeaQuest data are inconsistent with the growth of the <math><msub><mrow><mi>σ</mi></mrow><mrow><mi>A</mi></mrow></msub><mo>/</mo><mi>A</mi><msub><mrow><mi>σ</mi></mrow><mrow><mi>N</mi></mrow></msub></math> ratio expected in the Fermi motion scenario at <math><msub><mrow><mi>x</mi></mrow><mrow><mi>A</mi></mrow></msub><mo>≥</mo><mn>0.25</mn></math>. The pattern of the <math><msub><mrow><mi>x</mi></mrow><mrow><mi>A</mi></mrow></msub></math> dependence of the ratio seems also inconsistent with a scenario in which the dominant nuclear effect is a suppression of the cross section due to the energy loss experienced by a quark of the projectile proton involved in the Drell–Yan process. All together the data suggest the possibility of a modification of the antiquark parton distributions in nuclei, with a pattern similar to the one observed in the EMC effect. We argue that optimal kinematics to look for an antiquark EMC–like effect would be to measure the <math><msubsup><mrow><mi>σ</mi></mrow><mrow><mi>A</mi></mrow><mrow><mi>D</mi><mi>Y</mi></mrow></msubsup><mo>/</mo><mi>A</mi><msubsup><mrow><mi>σ</mi></mrow><mrow><mi>N</mi></mrow><mrow><mi>D</mi><mi>Y</mi></mrow></msubsup></math> ratios for <math><msub><mrow><mi>x</mi></mrow><mrow><mi>A</mi></mrow></msub></math> = 0.2 – 0.4 and <math><msub><mrow><mi>x</mi></mrow><mrow><mi>p</mi></mrow></msub></math> ≈ constant.

Italy

Massimiliano Alvioli

United States

Mark Strikman

Hunting for an EMC–like effect for antiquarks

The goal of this white paper is to give a comprehensive overview of the rich field of forward physics. We discuss the occurrences of BFKL re-summation effects in special final states, such as Mueller–Navelet jets, jet–gap–jets, and heavy quarkonium production. It further addresses TMD factorization at low $x$ and the manifestation of a semi-hard saturation scale in (generalized) TMD PDFs. More theoretical aspects of low-$x$ physics, probes of the quark–gluon plasma, as well as the possibility to use photon–hadron collisions at the LHC to constrain hadronic structure at low $x$, and the resulting complementarity between LHC and the EIC are also presented. We also briefly discuss diffraction at colliders as well as the possibility to explore further the electroweak theory in central exclusive events using the LHC as a photon–photon collider.

Mexico

Martin Hentschinski

Christophe Royon

Marco Alcazar Peredo

France

Cristian Baldenegro

Andrea Bellora

Renaud Boussarie

Francesco Giovanni Celiberto

Türkiye

Salim Cerci

Portugal

Grigorios Chachamis

Czechia

J.G. Contreras

Brazil

Sylvain Fichet

Michael Fucilla

Gero von Gersdorff

Germany

Pablo González

Poland

Andreas van Hameren

Jamal Jalilian-Marian

Mats Kampshoff

United Kingdom

Valery Khoze

Michael Klasen

Spencer Robert Klein

Georgios Krintiras

Piotr Kotko

Krzysztof Kutak

Jean-Philippe Lansberg

Emilie Li

Constanin Loizides

Mohammed M.A. Mohammed

Maxim Nefedov

Melih A. Ozcelik

Alessandro Papa

CERN

Michael Pitt

Spain

Agustin Sabio Vera

Jens Salomon

Sebastian Sapeta

Gustavo Gil da Silveira

Victor Paulo Gonçalves

Deniz Sunar Cerci

Lech Szymanowski

Daniel Tapia Takaki

Marek Taševský

Samuel Wallon

White Paper on Forward Physics, BFKL, Saturation Physics and Diffraction

While the phenomenon of gluon nuclear shadowing at small <math><mi>x</mi></math> has been getting confirmation in QCD analyses of various LHC measurements involving heavy nuclei, it has not been possible so far to establish experimentally the number of target nucleons responsible for nuclear shadowing in a given process. To address this issue, we study coherent <math><mi>J</mi><mo>/</mo><mi>ψ</mi></math> electroproduction on <math><mrow><mmultiscripts><mrow><mi>He</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>4</mn></mrow></mmultiscripts></mrow></math> and <math><mrow><mmultiscripts><mrow><mi>He</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>3</mn></mrow></mmultiscripts></mrow></math> in the kinematics of a future electron-ion collider and show that this process has the power to disentangle the contributions of the interaction with a specific number of nucleons <math><mi>k</mi></math>, in particular, with two nucleons at the momentum transfer <math><mi>t</mi><mo>≠</mo><mn>0</mn></math>. We predict a dramatic shift of the <math><mi>t</mi></math> dependence of the differential cross section toward smaller values of <math><mo>|</mo><mi>t</mi><mo>|</mo></math> due to a nontrivial correlation between <math><mi>x</mi></math> and <math><mi>k</mi></math>. This calculation, which makes use for the first time of realistic wave functions, provides a stringent test of models of nuclear shadowing and a novel probe of the 3D imaging of gluons in light nuclei. In addition, thanks to this analysis, unique information on the real part of the corresponding scattering amplitude could be accessed.

Russia

Vadim Guzey

Matteo Rinaldi

Sergio Scopetta

Michele Viviani

Coherent <math><mi>J</mi><mo>/</mo><mi>ψ</mi></math> Electroproduction on <math><mrow><mmultiscripts><mrow><mi>He</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>4</mn></mrow></mmultiscripts></mrow></math> and <math><mrow><mmultiscripts><mrow><mi>He</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>3</mn></mrow></mmultiscripts></mrow></math> at the Electron-Ion Collider: Probing Nuclear Shadowing One Nucleon at a Time

We present a model of exclusive ϕ-meson lepto-production <math><mi>e</mi><mi>p</mi><mo>→</mo><msup><mrow><mi>e</mi></mrow><mrow><mo>′</mo></mrow></msup><msup><mrow><mi>p</mi></mrow><mrow><mo>′</mo></mrow></msup><mi>ϕ</mi></math> near threshold which features the strangeness gravitational form factors of the proton. We argue that the shape of the differential cross section <math><mi>d</mi><mi>σ</mi><mo>/</mo><mi>d</mi><mi>t</mi></math> is a sensitive probe of the strangeness D-term of the proton.

Yoshitaka Hatta

-meson Lepto-production Near Threshold and the Strangeness -term

We investigate the prospects of the diffractive production of <math><mi>J</mi><mo>/</mo><mi>ψ</mi></math> mesons at a large momentum transfer <math><mo>|</mo><mi>t</mi><mo>|</mo></math> at the future Electron Ion Collider (EIC) in electron-proton collisions. In particular, we focus on the measurements of the rapidity gap size. The model used for the calculations is based on the diffractive exchange of the Balitsky-Fadin-Kuraev-Lipatov perturbative Pomeron. Calculations for the cross section and the estimates for the rates assuming integrated luminosity of <math><mn>10</mn><mtext> </mtext><mtext> </mtext><msup><mi>fb</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></math> are provided. Two experimental strategies were considered. First, measuring the rapidity gap size directly, by observing the activity in the forward part of the central detector, and second by putting a lower limit on the rapidity gap size in the case when the detector cannot measure forward activity. We find that it is possible to measure at the EIC the dependence of the cross section on rapidity gap interval up to four units in rapidity. This should allow us to measure the change of the cross section by a factor of 1.6 expected due to the BFKL exchange. This is possible with the present setup of the detector which projects the coverage up to 3.5 units of rapidity. We conclude however, that the extension of the detector up to higher rapidity, for example to 4.5 would be desirable and provide even better lever arm for testing rapidity gap physics at the EIC.

Michal Deák

Anna M. Staśto

High <math><mo>|</mo><mi>t</mi><mo>|</mo></math> diffractive vector meson production at the EIC

It is well understood that the leading logarithmic approximation for the amplitudes of high energy processes is insufficient and that the next-to-leading logarithmic effects are very large and lead to instability of the solution. The resummation at low x, which includes kinematical constraints and other corrections leads to stable result. Using previously established resummation procedure we study in detail the preasymptotic effects which occur in the solution to the resummed BFKL equation when the energy is not very large. We find that in addition to the well known reduction of the intercept, which governs the energy dependence of the gluon Green’s function, resummation leads to the delay of the onset of its small x growth. Moreover the gluon Green’s function develops a dip or a plateau in wide range of rapidities, which increases for large scales. The preasymptotic region in the gluon Green’s function extends to about 8 units in rapidity for the transverse scales of the order of 30–100 GeV. To visualize the expected behavior of physical processes with two equal hard scales we calculate the cross section of the process $$\gamma ^{*}+\gamma ^{*}\rightarrow X$$ <math><mrow><msup><mi>γ</mi><mrow><mrow></mrow><mo>∗</mo></mrow></msup><mo>+</mo><msup><mi>γ</mi><mrow><mrow></mrow><mo>∗</mo></mrow></msup><mo>→</mo><mi>X</mi></mrow></math>  to be probed at future very high-energy electron-positron colliders. We find that at $$\gamma ^*\gamma ^*$$ <math><mrow><msup><mi>γ</mi><mo>∗</mo></msup><msup><mi>γ</mi><mo>∗</mo></msup></mrow></math>  energies below $$100 \; \mathrm{{GeV}}$$ <math><mrow><mn>100</mn><mspace width="0.277778em"></mspace><mi>GeV</mi></mrow></math>  the BFKL Pomeron leads to smaller value of the cross section than the Born approximation, and only starts to dominate at energies about $$100 \; \mathrm{{GeV}}$$ <math><mrow><mn>100</mn><mspace width="0.277778em"></mspace><mi>GeV</mi></mrow></math> . This pattern is significantly different from the one which we find using LLx approximation. We also analyze the transverse momentum contributions to the cross section for different virtualities of the photons and find that the dominant contributions to the integral over the transverse momenta comes from lower values than the the external scales in the process under consideration.

Michal Deak

Israel

Leonid Frankfurt

Anna Staśto

Taming of preasymptotic small x evolution within resummation framework

We study sub-threshold heavy quarkonium (<math><mi>J</mi><mo>/</mo><mi>ψ</mi></math> and ϒ) photo-productions in γA collisions as an independent test of the universality of the nucleon-nucleon short range correlation (SRC) in nuclear scattering processes. Just below the γp threshold, the cross section is dominated by the mean field contribution of nucleons inside the nucleus. The SRC contributions start to dominate at lower photon energies, depending on the fraction of the SRC pairs in the target nucleus. We give an estimate of the cross sections in the sub-threshold region both for <math><mi>J</mi><mo>/</mo><mi>ψ</mi></math> and ϒ. This may be helpful for future measurements at JLab as well as at the Electron-Ion Collider in the U.S., and especially in China.

China

Ji Xu

Feng Yuan

Sub-threshold / And ϒ Production in Collisions

One of the main challenges hampering an accurate measurement of the double parton scattering (DPS) cross sections is the difficulty in separating the DPS from the leading twist (LT) contributions. We argue that such a separation can be achieved, and cross section of DPS measured, in proton–nucleus scattering by exploiting the different centrality dependence of DPS and LT processes. We developed a Monte Carlo implementation of the DPS processes which includes realistic nucleon–nucleon (NN) correlations in nuclei, an accurate description of transverse geometry of both hard and soft NN collisions as well as fluctuations of the strength of interaction of nucleon with nucleus (color fluctuation effects). Our method allows the calculation of probability distributions of single and double dijet events as a function of centrality, also distinguishing double hard scatterings originating from a single target nucleon and from two different nucleons. We present numerical results for the rate of DPS as a function of centrality, following the model developed by the ATLAS collaboration which relates the distribution over the number of wounded nucleons to the distribution over the sum of transverse energies of hadrons produced at large negative (along the nucleus direction) rapidities, which is experimentally measurable. We suggest a new quantity which allows to test the geometry of DPS and we argue that it is a universal function of centrality for different DPS processes. This quantity can be tested by analyzing existing LHC data. The method developed in this work can be extended to the search for triple parton interactions.

Maxim Azarkin

Boris Blok

Revealing minijet dynamics via centrality dependence of double parton interactions in proton–nucleus collisions

In hadronic collisions, interference between different production channels affects momentum distributions of multi-particle final states. As this QCD interference does not depend on the strong coupling constant α s , it is part of the no-interaction baseline that needs to be controlled prior to searching for other manifestations of collective dynamics, e.g., in the analysis of azimuthal anisostropy coefficients v n at the LHC. Here, we introduce a model that is based on the QCD theory of multi-parton interactions and that allows one to study interference effects in the production of m particles in hadronic collisions with N parton-parton interactions (“sources”). In an expansion in powers of 1/( N c 2  − 1) and to leading order in the number of sources N , we calculate interference effects in the m -particle spectra and we determine from them the second and fourth order cumulant momentum anisotropies v n {2} and v n {4}. Without invoking any azimuthal asymmetry and any density dependent non-linear dynamics in the incoming state, and without invoking any interaction in the final state, we find that QCD interference alone can give rise to values for v n {2} and v n {4}, n even, that persist unattenuated for increasing number of sources, that may increase with increasing multiplicity and that agree with measurements in proton-proton (pp) collisions in terms of the order of magnitude of the signal and the approximate shape of the transverse momentum dependence. We further find that the non-abelian features of QCD interference can give rise to odd harmonic anisotropies. These findings indicate that the no-interaction baseline including QCD interference effects can make a sizeable if not dominant contribution to the measured v n coefficients in pp collisions. Prospects for analyzing QCD interference contributions further and their possible relevance for proton-nucleus and nucleus-nucleus collisions are discussed shortly.

Christian Jäkel

Urs Wiedemann

Collectivity from interference

With an explicit example, we show that Jordan frame and the conformally transformed Einstein frames clearly lead to different physics for a non-minimally coupled theory of gravity, namely Brans–Dicke theory, at least at the quantum level. The example taken up is the spatially flat Friedmann cosmology in Brans–Dicke theory.

India

Narayan Banerjee

Barun Majumder

A question mark on the equivalence of Einstein and Jordan frames