V. Guzey

Proton-nucleus collisions at the LHC: Scientific opportunities and requirements

Proton–nucleus (p+A) collisions have long been recognized as a crucial component of the physics program with nuclear beams at high energies, in particular for their reference role to interpret and understand nucleus–nucleus data as well as for their potential to elucidate the partonic structure of matter at low parton fractional momenta (small-x). Here, we summarize the main motivations that make a proton–nucleus run a decisive ingredient for a successful heavy-ion program at the Large Hadron Collider (LHC) and we present unique scientific opportunities arising from these collisions. We also review the status of ongoing discussions about operation plans for the p+A mode at the LHC.

Leading Twist Nuclear Shadowing Phenomena in Hard Processes with Nuclei

We present and discuss the theory and phenomenology of the leading twist theory of nuclear shadowing which is based on the combination of the generalization of the Gribov-Glauber theory, QCD factorization theorems, and the HERA QCD analysis of diffraction in lepton-proton deep inelastic scattering (DIS). We apply this technique for the analysis of a wide range of hard processes with nuclei---inclusive DIS on deuterons, medium-range and heavy nuclei, coherent and incoherent diffractive DIS with nuclei, and hard diffraction in proton-nucleus scattering---and make predictions for the effect of nuclear shadowing in the corresponding sea quark and gluon parton distributions. We also analyze the role of the leading twist nuclear shadowing in generalized parton distributions in nuclei and in certain characteristics of final states in nuclear DIS. We discuss the limits of applicability of the leading twist approximation for small x scattering off nuclei and the onset of the black disk regime and methods of detecting it. It will be possible to check many of our predictions in the near future in the studies of the ultraperipheral collisions at the Large Hadron Collider (LHC). Further checks will be possible in pA collisions at the LHC and forward hadron production at the Relativistic Heavy Ion Collider (RHIC). Detailed tests will be possible at an Electron-Ion Collider (EIC) in the USA and at the Large Hadron-Electron Collider (LHeC) at CERN.

Complete analysis of spin structure function g₁ of ³He

We present a comprehensive analysis of the nuclear effects important in DIS on polarized He-3 over a wide range of Bjorken x, 10^{-4} \leq x \leq 0.8. Effects relevant for the extraction of the neutron spin structure function, g1^n, from the He-3 data are emphasized.

Color fluctuation approximation for multiple interactions in leading twist theory of nuclear shadowing

Abstract The leading twist theory of nuclear shadowing predicts the shadowing correction to nuclear parton distributions at small x by connecting it to the leading twist hard diffraction in electron–nucleon scattering. The uncertainties of the predictions are related to the shadowing effects resulting from the interaction of the hard probe with N ⩾ 3 nucleons. We argue that the pattern of hard diffraction observed at HERA allows one to reduce these uncertainties. We develop a new approach to the treatment of these multiple interactions, which is based on the concept of the color fluctuations and accounts for the presence of both point-like and hadron-like configurations in the virtual photon wave function. Using the developed framework, we update our predictions for the leading twist nuclear shadowing in nuclear parton distributions of heavy nuclei at small x .

Revealing “flickering” of the interaction strength in p A collisions at the CERN LHC

We analyze pA interactions at ultra-high energies within the semiclassical approximation for high energy processes accounting for the diffractive processes and a rapid increase with the incident energy of the coherence length. The fluctuations of the strength of interaction expected in QCD and momentum conservation are taken into account also. We evaluate the number of wounded nucleons in soft and hard processes, the multiplicity of jets in the proton fragmentation region as a function of the variance of the distribution over the interaction strengths directly measured in forward diffractive pN scattering for RHIC and LHC energies. We argue that these results could be used to test whether parton configurations containing a parton carrying the x ≥ 0.5 fraction of the projectile momentum interact significantly weaker than on average. We also study leading twist shadowing and the EMC effect for superdense nuclear matter configurations probed in the events with larger than average number of wounded nucleons. 1 ar X iv :1 40 2. 28 68 v2 [ he pph ] 1 7 M ar 2 01 4

Neutron spin structure with polarized deuterons and spectator proton tagging at EIC

The neutrons deep-inelastic structure functions provide essential information for the flavor separation of the nucleon parton densities, the nucleon spin decomposition, and precision studies of QCD phenomena in the flavor-singlet and nonsinglet sectors. Thus, traditional inclusive measurements on nuclear targets are limited by dilution from scattering on protons, Fermi motion and binding effects, final-state interactions, and nuclear shadowing at x << 0.1. An Electron-Ion Collider (EIC) would enable next-generation measurements of neutron structure with polarized deuteron beams and detection of forward-moving spectator protons over a wide range of recoil momenta (0 < pR << several 100 MeV in the nucleus rest frame). The free neutron structure functions could be obtained by extrapolating the measured recoil momentum distributions to the on-shell point. The method eliminates nuclear modifications and can be applied to polarized scattering, as well as to semi-inclusive and exclusive final states. We review the prospects for neutron structure measurements with spectator tagging at EIC, the status of R&D efforts, and the accelerator and detector requirements.

Neutron contribution to nuclear deeply virtual Compton scattering asymmetries

Abstract Using a simple model for nuclear GPDs, we study the role of the neutron contribution to nuclear DVCS observables. As an example, we use the beam-spin asymmetry ALU measured in coherent and incoherent DVCS on a wide range of nuclear targets in the HERMES and JLab kinematics. We find that at small values of the momentum transfer t, ALU is dominated by the coherent-enriched contribution, which enhances ALU compared to the free proton asymmetry A p LU , A A LU (φ)/A p LU (φ) = 1.8− 2.2. At large values of t, the nuclear asymmetry is dominated by the incoherent contribution and ALU/(φ)A p LU (φ) = 0.66 − 0.74. The deviation of ALU (φ)/A p LU (φ) from unity at large t is a result of the neutron contribution, which gives a possibility to constain neutron GPDs in incoherent nuclear DVCS. A similar trend is expected for other DVCS asymmetries.

Impact of nuclear dependence of

We study the impact of the nuclear dependence of R=\sigma_L/\sigma_T on the extraction of the F_2^A/F_2^D and F_1^A/F_1^D structure function ratios from the data on the \sigma^A/\sigma^D cross section ratios. Guided by indications of the nuclear dependence of R from the world data, we examine selected sets of EMC, BCDMS, NMC and SLAC data and find that F_1^A/F_1^D < \sigma^A/\sigma^D \leq F_2^A/F_2^D. In particular, we observe that the nuclear enhancement (antishadowing) for F_1^A/F_1^D in the interval 0.1 < x < 0.3 becomes significantly reduced or even disappears, which indicates that antishadowing is dominated by the longitudinal structure function F_L. We also argue that precise measurements of nuclear modifications of R and F_L^A have the potential to constrain the poorly known gluon distribution in nuclei over a wide range of x.

R=\sigma_L/\sigma_T

We explore the theoretical observation that within the leading twist approximation, the nuclear effects of shadowing and antishadowing in non-perturbative nuclear parton distribution functions (nPDFs) at the input QCD evolution scale involve diffraction on nucleons of a nuclear target and originate from merging of two parton ladders belonging to two different nucleons, which are close in the rapidity space. It allows us to propose that for a given momentum fraction

on antishadowing in nuclear structure functions

x_P