### Conveners

#### WG2: Small-x, Diffraction and Vector Mesons 1

- Feng Yuan
- Sylvester Joosten

#### WG2: Small-x, Diffraction and Vector Mesons 2

- Feng Yuan
- Sylvester Joosten

#### WG2: Small-x, Diffraction and Vector Mesons 3

- Feng Yuan
- Sylvester Joosten

#### WG2: Small-x, Diffraction and Vector Mesons 4

- Feng Yuan
- Sylvester Joosten

#### WG2: Small-x, Diffraction and Vector Mesons 5

- Sylvester Joosten
- Feng Yuan

### Description

Small-x, Diffraction and Vector Mesons

Recently, a novel factorization scheme has been put forward in the context of DIS . This new approach allows to connect the moderate x regime where the partonic picture is manifest to the small x regime best described by strong classical fields. In this work, we explore quantum evolution of the associated 3D gluon distribution that encodes saturation effects. In this framework, we obtain a...

We revisit the problem of small Bjorken-$x$ evolution of the gluon and flavor-singlet quark helicity distributions in the shock wave (s-channel) formalism. Earlier works on the subject in the same framework resulted in an evolution equation for the gluon field-strength $F^{12}$ and quark “axial current” $\bar{\psi} \gamma^+ \gamma^5 \psi$ operators in the double-logarithmic approximation...

We revisit the problem of the small Bjorken-$x$ asymptotics of the quark and gluon orbital angular momentum (OAM) distributions in the proton utilizing the revised formalism for small-$x$ helicity evolution derived recently in \footnote{F. Cougoulic, Y. V. Kovchegov, A. Tarasov, and Y. Tawabutr, Journal of High Energy Physics 2022,

10.1007/jhep07(2022)095 (2022).}. We relate the quark and...

When parton momentum faction $x$ of hadron becomes small, an enhancement from small-$x$ logarithms shows up, and eventually, we enter into a partonic saturation region. A consistent treatment of the small-$x$ logarithms requires an all-order resummation which can be achieved with the BFKL formalism. However, a boundary to delineate the small-$x$ resummation region from saturation one is...

A crucial ingredient in all calculations in the Color Glass Condensate framework is the non-perturbative input to the perturbative small-$x$ evolution equation such as the Balitsky-Kovchegov (BK) equation. Due to the non-perturbative nature, it has to be determined from experimental data, most naturally from total DIS cross section measurements.

So far it has not been possible in leading...

We study the single-inclusive particle production from proton-nucleus collisions in the dilute-dense framework of the color glass condensate (CGC) at next-to-leading order (NLO) accuracy. In this regime, the cross section factorizes into hard impact factors and dipole-target scattering amplitude describing the eikonal interaction of the partons in the target color field. We combine, for the...

Using the CGC effective theory together with the hybrid factorisation, we study forward photon+jet production in proton-nucleus collisions beyond leading order. We first compute the "real" next-to-leading order (NLO) corrections, i.e. the radiative corrections associated with a three-parton final state, out of which only two are being measured. Then we move to the "virtual" NLO corrections to...

One very promising observable to study gluon saturation effects at high energy is photon+jet production at forward rapidity in proton-nucleus collisions. Since the produced photon does not rescatter on the target, this observable provides a clean environment to study the interaction of the quark probe with the dense target.

In this talk, we will present the results for the photon-quark...

During the last ten years, a key problem in our understanding of particle production at small $x$ has been the fact that single inclusive particle spectra computed at NLO in pA collisions at forward rapidities using the hybrid model become negative at large transverse momenta. Different solutions have been proposed in the literature in the last years, including Sudakov and threshold...

The LHCb detector's forward geometry provides unprecedented kinematic

coverage at low Bjorken-x. LHCb's excellent momentum resolution,

vertex reconstruction, and particle identification enable precision

measurements at low transverse momentum and high rapidity in

proton-lead collisions, probing x as small as 10^-6. In this

contribution, we present recent studies of low-x physics using...

The elastic scattering of protons at 13 TeV is measured in the range of the protons' transverse momenta allowing the access to the Coulomb-Nuclear-Interference region. The data were collected thanks to dedicated special LHC beta* = 2.5km optics. The total cross section as well as rho-parameter, the ratio of the real to imaginary part of the forward elastic scattering amplitude, are measured...

This talk presents ATLAS recent measurements of distributions sensitive to Underlying event, the hadronic activity observed in relationship with the hard scattering in the event using the full ATLAS dataset at center-of-mass energy of 13 TeV. Measurement of charged-particle distributions as a function of Upsilon momentum and different Upsilon states will be discussed. The measurement benefits...

Gluons are found to become increasingly dominant constituents of nuclear matter when being probed at higher energies or smaller Bjorken-$x$ values. This has led to the question of the ultimate fate of nuclear gluonic structure and its interaction with external probes at extreme density regimes when approaching the limit allowed by unitarity. In ultraperipheral collisions (UPCs) of relativistic...

Since 2011 a wide variety of measurements suggest the existence of strong collectivity in collisions of small systems such as proton-proton (pp) and proton-nucleus (pPb) with hydrodynamic models and gluon saturation in the initial state as two theory alternatives showing consistency with the observations. These results raise the question as to whether such phenomena may be present in even...

We present a phenomenological analysis of events with two high transverse momentum jets separated by a large rapidity interval void of particle activity, also known as jet-gap-jet events. In the limit where the collision energy is much larger than any other momentum scale, the jet-gap-jet process is described in terms of perturbative pomeron exchange between partons within the...

In order to solve the proton spin problem, the small-$x$ asymptotics of the helicity parton distribution functions (hPDFs) need to be understood. New theory has been developed for the small-$x$ evolution of these hPDFs, able to extrapolate the small-$x$ behaviour of the quark and gluon hPDFs. At large $N_c \& N_f$, these evolution equations close and are amenable to numerical computation. In...

We consider the novel small-$x$ helicity evolution equations previously derived using the light-cone operator treatment (LCOT) [1,2]. In the double logarithmic approximation (summing powers of $\alpha_s\ln^2(1/x)$) and in the large-$N_c$ limit, the evolution yields a closed system of equations for which we construct an analytic solution. This solution can then provide small-$x$, large-$N_c$...

Double spin asymmetries for particle and jet productions in longitudinally polarized proton-proton collisions are among the key measurements at RHIC to extract the spin fraction of gluons inside the proton. Although next-to-leading order perturbative QCD predictions have been quite successful in fitting experimental data within the RHIC kinematics, to constrain gluons at even smaller x, one...

We study the small-$x$ asymptotics of the flavor non-singlet T-odd leading-twist quark transverse momentum dependent parton distributions (TMDs). While the leading eikonal small-$x$ asymptotics of the quark Sivers function is given by the spin-dependent odderon, we are interested in revisiting the sub-eikonal correction considered by us earlier. We first simplify the expression for the TMD at...

It is well known that the small x calculations for the variety of observables are characterized by large corrections at the next-to-leading order (NLO). Resummation procedure for the gluon Green's function was constructed some time ago, which takes into account correct collinear limits, through appropriate subtractions of higher order poles and shifts of the leading poles. In the present work...

The potential discovery of gluon saturation is one of the chief goals of the future Electron-Ion Collider (EIC) program. The Color Glass Condensate (CGC) is an effective field theory (EFT) to characterize this novel regime of nuclear matter. In recent years, tremendous efforts have been carried out to promote the theory and phenomenology of the CGC EFT to higher precision.

We contribute to...

In this talk, we will discuss the back-to-back limit of the DIS dijet production at next-to-eikonal accuracy computed in a highly boosted gluon background field within the Color Glass Condensate (CGC) framework. We will show that the various types of next-to-eikonal corrections can be written as field strength insertions on the CGC Wilson lines which provide direct relation with the gluon TMDs...

We examine the process of producing a quark-gluon dijet in DIS in the CGC at leading order. We have to go beyond the eikonal approximation by including quark exchanges with the target that we take to be unpolarised. We provide final expressions for the DIS cross sections including quark masses. This might be used as a leverage for experimentally tagging one heavy quark jet and one light jet....

We propose semi-inclusive diffractive deep inelastic scattering (SIDDIS) to investigate the gluon tomography in the nucleon and nuclei at small-x. The relevant diffractive quark and gluon parton distribution functions (DPDF) can be computed in terms of the color dipole S-matrices in the fundamental and adjoint representations, respectively.

We calculate [1] the contribution from the $q \bar q g$ state production to the diffractive cross sections in deep inelastic scattering [2,3] at high energy. The obtained cross section is finite by itself and a part of the full next-to-leading order result for the diffractive structure functions. We perform the calculation in exact kinematics in the eikonal limit, and show that the previously...

The gluon density has been observed to increase rapidly with energy, which would eventually violate unitarity. At high energies, however, nonlinear effects start to become important, slowing down the evolution of the gluon density and hence giving rise to gluon saturation. To study this saturation region of QCD one possibility is to look at diffractive processes, as being approximately...

In this talk, we present numerical results on diffractive dissociation of virtual photon in the scattering off hadron. The calculation employs the dipole picture of diffractive deep inelastic scattering and solutions to nonlinear Kovchegov-Levin equation, taking into account the running coupling correction and a simple treatment of impact parameter dependence. The (generalized)...

Measurement of near threshold quarkonia photoproduction cross section provides a unique tool to probe gluonic structure inside the nucleon, hence allowing extraction of gluonic form factors and mass radii. $J/\Psi$-007 experiment (E12-16-007) was conducted at Hall-C of the Thomas Jefferson National Accelerator Facility to measure near threshold 2-D differential $J/\Psi$ photoproduction cross...

Exclusive charmonium photoproduction near threshold opens the door for studying the gluonic properties of the proton: gluonic GPDs, anomalous contribution to the mass of the proton, gravitational form factors, the mass radius of the proton. However, such an ambitious program requires precise measurements to validate the theoretical assumptions that relate the experimental results to the above...

When beginning to describe the structure of $^4$He, one often begins by invoking a description based on nucleon degrees of freedom -- a bound system with two protons and two neutrons -- leaving the partonic description at the level of each nucleon aside. However, the ultimate goal to understand $^4$He within Quantum Chromodynamics (QCD)k is to connect its intrisic properties with the...

for SoLID Collaboration

The proposed Solenoidal Large Intensity Device (SoLID) in Hall A at Jefferson Lab, will fully utilize the great physics potential of the 12-GeV energy upgrade by combining high luminosities and large acceptance to explore physics needs large statistics. We plan a common

setup to measure JPsi production near threshold, timeline compton scattering and double deeply...

One of the golden measurements at the Electron-Ion Collider (EIC) is to measure the coherent diffractive Vector Meson (VM) production on a heavy-nucleus target. The measurement is expected to be sensitive to the non-linear gluon dynamics - saturation, and most importantly, it also provides the gluon density distribution of the nucleus. While the measurement was proposed in the EIC White Paper...

The Good-Walker paradigm is widely used for studying ultra-peripheral collisions (UPCs) and underlies much of the exclusive photoproduction/electroproduction program at the future electron-ion collider (EIC). It relates the cross-section for coherent photoproduction, $d\sigma/dt$ to the transverse distribution of scattering centers in a target, and ties the incoherent cross-section to...

Photon-induced reactions in ultra-peripheral collisions (UPCs) of heavy nuclei at the LHC have been studied using the ALICE detector for several years. The ALICE detector can measure the photoproduction cross section for vector mesons at various rapidities, centre-of-mass energies and

collision systems. Beyond the recent ALICE studies of the rapidity and momentum transfer dependence of...

High energy photons originating from the electromagnetic field of ultrarelativistic lead nuclei can interact with the other lead beam. These reactions are studied in the ultra-peripheral heavy ion collisions to probe the physics of strong interactions. The analysis of dipion and dikaon photoproduction was carried out using the ALICE 2015 Pb-Pb data at center-of-mass energy $\sqrt{s_{NN}}=5.02$...

Baryon number is a strictly conserved quantity in QCD. Conventionally it is assumed to be split between the three valence quarks, but the baryon junction has also been proposed as a way of constructing a gauge-invariant baryon wave function. The baryon junction is a nonperturbative Y-shaped configuration of gluons that is connected to all three valence quarks and carries the baryon number. ...

Exclusive coherent and incoherent \jpsi photoproduction in Au$+$Au ultra-peripheral collisions (UPC) are measured at

$\sqrt{s_{_\mathrm{NN}}}=200$ GeV using the STAR detector.

Measurements of the differential cross sections as functions of rapidity $y$ and \ptSquare $\approx -t$ of the $J/\psi$ are reported.

Results for different combinations of neutron emission, detected in zero degree...

In this contribution I would like to present our recent computation of exclusive eta_c production in e+p and e+A collisions [1] in the Color Glass Condensate framework. While the original suggestion for this channel as a potential probe of the QCD odderon [2] was made some time ago [3], obtaining a realistic estimate for the cross section remains a challenge. A recent attempt at moderate-x was...

We provide the first calculation of two-gluon production at mid-rapidity in ultra-peripheral collisions in the Color Glass Condensate framework. To estimate systematic uncertainty associated with poor understanding of the wave function of the nearly real photon, we consider two diametrically different models: the dilute quark-antiquark dipole approximation and a

vector meson, in which...

Photonuclear interactions are studied in ultra-peripheral p-Pb collisions with the ALICE experiment, where the photon radiated by a Pb nucleus probes the gluon density of the proton at low Bjorken-x. The exclusive J/𝜓 photoproduction cross section 𝜎(𝛾+𝑝→𝐽/𝜓+𝑝) is expected to follow a power law trend as x decreases, but it should deviate from this trend at low x due to gluon saturation. In...

We describe the status of the ATLAS Forward Proton Detectors (AFP) in Run 2 and the outlook for Run 3 analyses.

The performance as well as Run 3 improvements of detector subsystems are discussed in detail. This includes the Tracking and Time-of-Flight Detectors, the electronics, the trigger, the readout, detector control and data quality monitoring.

Additionally, key physics results from the...

The purpose of the ALFA detector is to measure protons scattered at very small angles. ALFA aims to study elastic and diffractive events, exclusive production and photon-induced interactions. In LHC Run 2, ALFA participated in the ATLAS high-luminosity data taking, as well as, special runs with reduced luminosity and dedicated optics.

This report presents the status of the ALFA detector...

The hotspot model has proven to be an efficient tool to study coherent and incoherent diffraction HERA data by modeling the initial state fluctuations of the gluon density of the proton. The hotspot model in its original form is a non-perturbative model applicable for low momentum transfer and underestimates the incoherent cross section in orders of magnitude when extended for large momentum...