abstract

We analyze the recently discovered phenomena in elastic proton–proton scattering at the LHC, challenging the standard Regge-pole theory: the low-\(|t|\) “break” (departure from the exponential behavior of the diffraction cone), the accelerating rise with energy of the forward slope \(B(s,t=0)\), absence of secondary dips and bumps on the cone, and the role of the odderon in the forward phase of the amplitude, \(\rho (13\) TeV\()=0.1\pm 0.01\) and, especially, its contribution at the dip region, measured recently by TOTEM. Relative contributions from different components to the scattering amplitude are evaluated from the fitted model.

abstract

We show that the impact of the maximal odderon amplitude at \(t=0\) and \(\sqrt {s} = 13 \) TeV is small. We obtain a value of \(\rho (t=0)\) at \(\sqrt {s}= 13 \) TeV of the order of \(0.12\). The real part of the odderon amplitude grows like \(\log (s/s_0)\) at high energies, and is calculated from the analytic properties of the amplitude. In the framework of the HEGS model, taking the same intercept for the odderon and the Pomeron leads to a good fit of the new LHC data at \( \sqrt {s}= 13 \) TeV. We also show that the main effect of the odderon can be seen in the region of the diffraction minimum of the differential elastic cross section. The form and energy dependence of the odderon amplitude determined in the HEGS model reproduce the characteristics of the diffraction minimum at \(\sqrt {s}= 7, 8\) and \( 13 \) TeV.

abstract

We investigate the single-spin asymmetry, \(A_{\mathrm {N}}(t)\), arising from Coulomb-nuclear interference (CNI) in the small-angle elastic scattering of polarized protons. Previous theoretical predictions failed to explain the non-trivial \(t\) dependence of \(A_{\mathrm {N}}\) in elastic proton–gold scattering, measured recently at RHIC. We found that the absorptive corrections make the Coulomb amplitude of \(pA\) elastic scattering significantly different from \(eA\) scattering, leading to dramatic changes in the \(t\) dependence. Moreover, we also found the absorptive corrections being significant for the analysis of data for polarized \(pp\) elastic scattering, which previously revealed a zero spin-flip part of the Pomeron. We concluded with an essentially non-zero hadronic spin-flip.

abstract

A measurement of the inelastic proton–proton (\(pp\)) cross section is presented. The analysis is based on the data collected in 2015 at a center-of-mass energy of 13 TeV with the CMS detector at the LHC. The inelastic \(pp\) cross section is measured in two fiducial ranges and the results are compared to the predictions of various models.

abstract

This contribution gives an overview of the recent measurements of the differential cross sections for final states involving photons and/or jets at the centre-of-mass energies of 8 and 13 TeV, published by the ATLAS Collaboration. The results are compared with the state-of-art theoretical predictions. Two measurements of dijet correlations allowing the strong coupling constant to be extracted are also presented.

abstract

We describe an attempt to use the renormalization group equations for interpolating between the BFKL Pomeron in perturbative QCD and the soft Pomeron in \(pp\) scattering.

abstract

We suggest the inclusive detection at the LHC of a light charged hadron and of a jet widely separated in rapidity as a new probe channel for the study of the BFKL resummation. Predictions for cross section and azimuthal correlations, shaped on the CMS and CASTOR acceptances, are presented.

abstract

A novel model-independent Lévy imaging method is employed for reconstruction of the elastic \(pp\) and \(p\bar p\) scattering amplitudes at low and high energies. The four-momentum transfer \(t\)-dependent elastic slope \(B(t)\), the nuclear phase \(\phi (t)\) as well as the excitation function of the shadow profile \(P(b)\) have been extracted from data at ISR, Tevatron and the LHC energies. We found qualitative differences in properties of \(B(t)\) and \(\phi (t)\) between \(pp\) and for \(p\bar p\) collisions that indicate an odderon effect. A proton substructure has also been identified and found to have two different sizes, comparable to that of a dressed quark at the ISR and a dressed diquark at the LHC energies, respectively.

abstract

A model is presented in which the electro-production of light and heavy vector mesons is treated using the color dipole approach including the quantum fluctuations of the target structure. These fluctuations are generated by hot spots randomly placed in the transverse plane. The number of hot spots grows with decreasing Bjorken-\(x\), which introduces an energy dependence of the target structure. This model successfully reproduces the exclusive and dissociative vector meson photo-production data from HERA and the LHC. Moreover, it predicts that, once the proton structure starts to resemble the gluon saturation picture, the dissociative cross section reaches a maximum and then decreases steeply with energy. This signal is present also in electro-production cross section and it has well-defined mass and scale dependence measurable at EIC/LHeC energies.

abstract

A study is presented to extend the measurements of photon–photon scattering in ultra-peripheral Pb–Pb collisions at the LHC into the mass region of the pseudoscalar resonances \(\eta \) and \(\eta '\). The elementary photon–photon scattering cross section is presented. The cross section for photon–photon scattering in Pb–Pb is derived by convoluting the elementary photon–photon cross section with the Pb–Pb photon luminosity. The main background to two-photon final states, arising from double \(\pi ^{0}\) production with two of the four decay photons escaping detection, is examined, and possible kinematical conditions are discussed to optimize the signal-to-background ratio for such measurements at mid-rapidity.

abstract

The existence of an axion-like particle (ALP) would induce anomalous scattering of light-by-light. This process can be probed at the LHC in central exclusive production of photon pairs in \(p\)–\(p\) collisions by tagging the surviving protons using forward proton detectors. We show that the proposed search in central exclusive production of photon pairs is competitive and complementary to other collider bounds for masses above 600 GeV, especially for resonant ALP production between 600 GeV and 2 TeV.

abstract

We discuss the sensitivities to photon quartic anomalous couplings and to axion-like particles using tagged intact protons at the LHC.

abstract

We present preliminary differential fiducial cross sections on Central Exclusive Production (CEP, \(pp\to pXp\)) of two opposite-charge mesons (\(X=\pi ^{+}\pi ^{-}\), \(K^{+}K^{-}\)) in midrapidity region with small squared four-momentum transferred from forward protons (\(t_{1}\), \(t_{2}\)), 0.03 GeV\(^{2}\lt |t_{1,2}|\lt 0.2\) GeV\(^{2}\). The process was measured in the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) in proton–proton collisions at the center-of-mass energy \(\sqrt {s}=200\) GeV.

abstract

The exclusive photoproduction of the \(J/\psi \) state is investigated in peripheral \(AA\) collisions for the energies available at the LHC, \(\sqrt {s}=2.76\) TeV and \(\sqrt {s}=5.02\) TeV, in different centrality classes. The rapidity distribution and the nuclear modification factor (\(R_{AA}\)) were calculated from the light-cone color dipole formalism. Three scenarios were considered: (1) a similar formalism adopted in the UPC regime is used, (2) one considers that only the spectators in the target are the ones that interact coherently with the photon, and (3) the photonuclear cross section is modified using the same geometrical constraints applied in scenario 2. The results were compared with the ALICE measurements (only \(J/\psi \) at the moment) and show a better agreement in the more complete approach (scenario 3), mainly in the more central regions (30%–50% and 50%–70%) where the dependence on \(b\) is deeper.

abstract

The recently discovered correspondence between the distribution of rapidity gaps in electron–nucleus diffractive processes and the statistics of the height of genealogical trees in branching random walks is reviewed. In addition, a new comparison of numerical solutions of exact equations for diffraction on the one hand, and for ancestry on the other hand, both established in the framework of the color dipole model, is presented.

abstract

So far, the analyses of the polarized structure functions of the proton and neutron have been limited to the evaluation of their integrals and comparing them to the prediction of the static-quark model of the nucleon. We extended our analysis to the \(x\) dependence of the polarized structure functions and observe: the measured structure function excellently agrees with the prediction of the static-quark model for Bjorken \(x\gt 0.2\) and drops rapidly for \(x\lt 0.2\). It is suggested that for Bjorken \(x\gt 0.2\), electrons get scattered on the undamaged constituent quarks (alias valence quarks) denoted as quasi-elastic scattering on the constituent quarks and for \(x\lt 0.2\), the constituent quarks fragment. In the fragmentation, strong interaction is involved which does not preserve the polarization.

abstract

Very forward neutral particle production was measured in the RHICf experiment for polarized \(p\)+\(p\) collisions at \(\sqrt {s}=510\) GeV. An electromagnetic calorimeter was installed in the zero-degree area of the STAR experiment and took neutron, photon and neutral pion data at pseudorapidity \(\eta \gt 6\). The cross-section measurement will give us new inputs to develop high-energy collision models which is essential to understand air-shower from ultra-high energy cosmic rays. The asymmetry measurement will enable us to understand the hadron collision mechanism based on QCD. The data were taken in 2017 RHIC run with three detector positions in order to cover wide kinematic regions. STAR detector data were also recorded for combined data analysis. We obtained preliminary results of the transverse single-spin asymmetry, \(A_{\mathrm {N}}\), of very forward \(\pi ^0\) which show large asymmetries even at low \(p_{\mathrm {T}}\) and larger at high \(p_{\mathrm {T}}\).

abstract

We consider the production of \(\chi _{cJ}\) mesons at the LHC conditions and show predictions for polarization observables for the original \(\chi _{cJ}\) mesons and their decay products. We find that the polarization of \(\chi _{cJ}\) and \(J/\psi \) mesons is large and shows nontrivial behavior as a function of \(\chi _{cJ}\) transverse momentum. A comparison between collinear and \(k_{\mathrm {T}}\)-factorization predictions is presented.

abstract

The factorization theorem for Drell–Yan process and semi-inclusive deep inelastic scattering holds for all leading-twist transverse momentum distributions. In this context, a QCD perturbative calculation shows several important characteristics of spin-dependent distributions. We consider all the different spin-dependent distributions which can be matched onto integrated twist-2 functions, focusing on the matching of the transversity and pretzelosity distributions up to next-to-next-to-leading-order. The pretzelosity case is specially relevant because, using a direct perturbative calculation, we obtain a null result up to two loops which agrees with the experimental measurements.

abstract

We study in the framework of collinear QCD factorization the photoproduction of a \(\gamma \,\pi \) pair with a large invariant mass and a small transverse momentum, as a new way to access generalized parton distributions (GPDs). In the kinematics of JLab 12 GeV, we demonstrate the feasibility of this measurement and show the extreme sensitivity of the unpolarized cross section to the axial quark GPDs.

abstract

We evaluate the nucleon spin decomposition to quarks and gluons contributions using lattice QCD. One ensemble of maximally twisted mass fermions with two degenerate light quarks tuned to reproduce the physical value of the pion mass has been analyzed. State-of-the-art techniques have been employed to increase the statistical accuracy of sea quarks contributions. Both spin and momentum sum rules are found to be satisfied within the statistical and systematic uncertainty.

abstract

Measurements of open charm and beauty production cross sections in \(ep\) deep inelastic scattering (DIS) at HERA from the H1 and ZEUS collaborations are combined. Reduced cross sections are obtained in a restricted kinematic range. Perturbative QCD calculations are compared to the combined data. Next-to-leading order QCD analysis is performed using these data together with combined inclusive HERA DIS cross sections. The running charm and beauty quark masses are determined.

abstract

There has been a revived interest in small-\(x\) resummation in recent times. The main motivation was its success in describing small-\(x\) HERA data without the inclusion of non-perturbative corrections. In this contribution, I will review the recent developments in the field.

abstract

We present latest results of soft and small-\(x\) QCD measurements with the CMS experiment, such as minimum bias, underlying event physics, and studies on forward jet production.

abstract

The PROSA parton distribution function fit was the first one appeared in the literature incorporating data on open charm and open bottom production at the LHCb, in order to reduce the uncertainties on gluons and sea quarks at low \(x\)s (\(x \lt 10^{-4}\)). We discuss aspects of the PROSA PDFs of particular relevance for their usage in the field of neutrino astronomy, and their application in the computation of prompt neutrino fluxes.

abstract

The gluon content of the proton, in the high-energy regime, is embodied by the unintegrated gluon distribution (UGD), which describes the gluon emission probability, with a given longitudinal momentum fraction and transverse momentum. The UGD, formulated within the \(\kappa \)-factorization approach, has universal validity, and several models for it have been proposed so far. We will show that the polarized \(\rho \)-meson leptoproduction at HERA is a non-trivial testfield for discriminating among existing models of UGD.

abstract

The forward Drell–Yan (DY) lepton-pair production together with a backward jet is proposed as a new way to study the BFKL effects due to a large rapidity gap between the two systems. The predictions for quantities to be measured are computed using the leading order DY impact factors and the BFKL kernel with a consistency condition which takes into account an important part of the next-to-leading order corrections.

abstract

Using the formalism of the light-cone wave function in perturbative QCD together with the hybrid factorization, we compute the cross section for three particle production at forward rapidities in proton–nucleus collisions. In this picture, the three produced partons — a quark accompanied by a gluon pair, or two quarks plus one antiquark — are all generated via one or two successive splittings of a quark from the incoming proton, that was originally collinear with the latter. The three partons are put on-shell by their scattering off the nuclear target, described as the Lorentz-contracted shockwave. We explicitly compute the three-parton Fock space components of the light-cone wave function of the incoming quark and its outgoing state, which encodes the information both on the evolution in time as well as the scattering process. This outgoing state is also an ingredient for other interesting calculations, like the next-to-leading order correction to the cross section for the production of a pair of jets.

abstract

Due to its unique pseudorapidity coverage (2 \(\lt \eta \lt \) 5) and excellent performance, the LHCb detector allows the study of various aspects of particle correlations at large rapidities and low transverse momenta. Selected results are summarized, such as the first measurement of the Bose–Einstein correlations of the same-sign pions and kinematic correlations for pairs of beauty hadrons performed using large samples of proton–proton collision data accumulated with the LHCb detector at \(\sqrt {s}=7\) and 8 TeV, together with the long-range correlations on the near side measured in proton–lead and lead–proton collisions at a nucleon–nucleon centre-of-mass energy of \(\sqrt {s} = 5\) TeV. The results provide a unique insight into particle production in the forward region at the LHC.

abstract

The recent results that demonstrate that the vector meson photoproduction in hadronic collisions can be used to constrain the QCD dynamics at high energies and improve our understanding of the gluon Sivers function are reviewed.

abstract

In this work, we provide a scaling function that describes the cross section for all available data from DESY-HERA for the exclusive production of vector mesons \(\rho \), \(\phi \), \(J/\psi \) and Deeply Virtual Compton Scattering (DVCS). Our description is based on the geometric scaling phenomenon, in which cross sections are functions of a single variable \(\tau = Q^2/Q_{\mathrm {sat}}^2\), where \(Q^2\) is the the photon virtuality and \(Q_{\mathrm {sat}}^2\) represents the saturation scale and drives the energy dependence and nuclear effects. We briefly extend our results to nuclear targets to be tested in future EICs.

abstract

In this contribution, we present a status report on the recent progress towards an analysis of nuclear parton distribution functions (nPDFs) using the NNPDF methodology. We discuss how the NNPDF fitting approach can be extended to account for the dependence on the atomic mass number \(A\), and introduce novel algorithms to improve the training of the neural network parameters within the NNPDF framework. Finally, we present preliminary results of an nPDF fit to neutral current deep-inelastic lepton–nucleus scattering data, and demonstrate how one can validate the new fitting methodology by means of closure tests.

abstract

Exclusive \(\rho ^{0}(770)\) photoproduction is studied for the first time in ultra-peripheral \(p\)Pb collisions at \(\sqrt {s_{NN}} = 5.02\) TeV. The cross section, \(\sigma (\gamma p \rightarrow \rho ^{0}(770)p)\), is measured for photon–proton centre-of-mass energies, \(W_{\gamma p}\), between \(29\) and \(213\) GeV with the CMS experiment. The results are compared to previous measurements and theoretical predictions. The measured cross section \(\sigma (\gamma p \rightarrow \rho ^{0}(770)p)\) has a power-law energy dependence, consistent with the electron–proton collision measurements performed at HERA.

abstract

Central exclusive production studies at the LHCb are reported. A search is performed for the central exclusive production of pairs of charmonia produced in proton–proton collisions using data corresponding to an integrated luminosity of 3 fb\(^{-1}\) collected at centre-of-mass energies of 7 and 8 TeV. Pairs of \(J/\psi J/\psi \) and \(J/\psi \psi (2S)\) are observed which is a unique process at hadron machines in which particles are produced via colourless propagators. The central exclusive production of \(J/\psi \) and \(\psi (2S)\) mesons is measured in \(pp\) collisions at a centre-of-mass energy of 13 TeV, and backgrounds are significantly reduced compared to previous measurements through the use of new forward shower counters.

abstract

We discuss novel ways to probe high-energy diffraction, first, inclusive diffraction and then, central exclusive processes at the LHC. Our new Monte Carlo synthesis and analysis framework, GRANIITTI, includes differential screening, an expendable set of scattering amplitudes with adaptive Monte Carlo sampling, spin systematics and modern computational technology.

abstract

In the framework of the Instanton Liquid Model, we evaluate the heavy-quark \(\bar {Q}Q\) potential at nonzero temperature \(T\). The potential has two components: a contribution due to direct interaction with instantons, and a modification of the one-gluon exchange contribution via instanton-generated dynamical “electric” gluon mass \(M_{\mathrm {el}}(q,T)\). We conclude that the nonperturbative ILM contributions to the \(Q\bar {Q}\) potential have pronounced temperature dependence, which might be tested in phenomenological analyses of charmonia production data.

abstract

A novel way of computing high-order amplitudes in the multi-Regge limit of the planar maximally supersymmetric Yang–Mills theory is presented. In this framework, we are able to obtain high-loop and high-leg results by an easy operation on known amplitudes with fewer loops and lower multiplicity. This mechanism will be reviewed, along with an ensuing factorisation which allows us to determine leading logarithmic MHV results for any number of legs at a fixed loop order.

abstract

We review the main features of event-by-event fluctuations of the content of the Fock states of onia (as models for dilute hadrons, or as bare hadronic components of virtual photons), as well as some of their observable consequences. We briefly address the total scattering cross section of a small onium off a nucleus, then of two small onia. Finally, we explain that the multiplicity in the final state of collisions of large onia with nuclei may directly be related to the gluon density in the former. We provide first predictions for the event-by-event fluctuations of the gluon density.

abstract

We extend the numerical analysis of the energy and centrality dependence of particle multiplicities at midrapidity in high-energy \(p+A\) and \(A+A\) collisions from a running coupling \(k_{\mathrm {T}}\)-factorization formula made in A. Dumitru, A.V. Giannini, M. Luzum, Y. Nara, Phys. Lett. B 784, 417 (2018) by considering two unintegrated gluon distributions that were left out. While a good agreement with the experimental data in \(A+A\) collisions is achieved, improving the description of those observables in \(p+A\) collisions calls for a better understanding of the proton unintegrated gluon distribution at larger values of \(x\) and also the use of a realistic impact parameter dependence.

abstract

Pomeron is a term introduced in the 1960s in the framework of the phenomenological Regge theory. It describes the behavior of the total cross sections of any hadronic reaction at extremely large values of the invariant energy \(s\). In the QCD context, the best-known contributions to the Pomeron come from the BFKL equation. The BFKL equation accounts for total resummation of Leading Logarithmic (LL) contributions, i.e. the terms where single-logarithmic contributions are multiplied by the overall factor \(s\). The high-energy asymptotics of such resummation is known as the BFKL Pomeron. It predicts the total cross section to be \(\sim s^{\mit \Delta }\), where the exponent \({\mit \Delta }\) is called the intercept of the BFKL Pomeron. In contrast, the Double-Logarithmic (DL) contributions are not accompanied by the overall factor \(s\), so resummation of them leads to the asymptotic form \(\sim s^{({\mit \Delta }_{\mathrm {DL}} - 1)}\) which looks as negligibly small compared to the BFKL result. However, the intercept \({\mit \Delta }_{\mathrm {DL}}\) proves to be so large that its value compensates for the lack of the extra factor of \(s\) and makes the DL Pomeron of importance comparable to the BFKL Pomeron. By this reason, contributions of the DL Pomeron should be taken into account whenever the BFKL Pomeron applies.

abstract

Coherent production of \(J/\psi \) mesons is studied in lead–lead collision data at a nucleon–nucleon centre-of-mass energy of 5 TeV collected by the LHCb experiment. The data set corresponds to an integrated luminosity of about 10 \(\mu \)b\(^{-1}\). The \(J/\psi \) mesons are reconstructed in the dimuon final state, where the muons are detected within the pseudorapidity region \(2.0 \lt \eta \lt 4.5\). The \(J/\psi \) mesons are required to have transverse momentum \(p_{\mathrm {T}} \lt 1\) GeV and rapidity \(2.0 \lt y \lt 4.5\). The cross section within this fiducial region is measured to be \(\sigma =5.3\pm 0.2\,\mathrm {(stat.)} \pm 0.5\,\mathrm {(syst.)} \pm 0.7\,\mathrm {(lumi.)}\) mb. The differential cross section is also measured in five bins of \(J/\psi \) rapidity. The results are compared to predictions from phenomenological models.

abstract

Data from the ALICE experiment demonstrate universality of suppression of different hadrons produced with high-\(p_{\mathrm {T}}\) in heavy-ion collisions (HICs) at the Large Hadron Collider (LHC). Moreover, the recent data from ATLAS on production of prompt charmonia at large \(p_{\mathrm {T}}\) demonstrate a similar attenuation as observed for light hadrons. Such a universality could hardly be explained by the energy-loss mechanism, since the rate of medium induced energy loss strongly correlates with the parton mass. However, the hadronization length has been evaluated to be very short, especially at high \(p_{\mathrm {T}}\), therefore, attenuation of the high-\(p_{\mathrm {T}}\) hadrons originates mainly from the possibility to be broken-up by inelastic collisions during propagation through the dense medium. The survival probability is controlled by the effect of color transparency, which makes the attenuation dependent on the dipole size, related mainly to \(p_{\mathrm {T}}\). This naturally explains the observed universality of hadron suppression. With the same value of the transport coefficient, we predict similar suppression factors \(R_{AA}\) for inclusive production of pions, kaons, protons, charmonia and even bottomonia as a function of \(p_{\mathrm {T}}\) and centrality in good agreement with data.

abstract

We present the inclusive and identified particle (pion, proton and their antiparticle) production in Single Diffraction Dissociation and Central Diffraction processes with the STAR detector at RHIC. The forward-scattered proton(s) are tagged in the STAR Roman Pot system, while the charged particle tracks are reconstructed in the STAR Time Projection Chamber. Ionization energy loss and time of flight of charged particles are used for particle identification. In addition, the proton–antiproton production asymmetry as a function of rapidity is measured and used to study the baryon number transfer over a large rapidity interval in the Single Diffraction Dissociation.