High Parton Density Research Articles

Back-to-back inclusive dijets in DIS at small x: Sudakov suppression and gluon saturation at NLO

Back-to-back dijet cross-sections in deeply inelastic scattering (DIS) at small xBj are suppressed by many-body multiple scattering and screening effects arising from gluon saturation at high parton densities. They are similarly sensitive in these kinematics to large Sudakov logarithms from soft gluon radiation. Uncovering novel physics in this DIS channel therefore requires understanding the interplay of the two phenomena. In this work, we compute the small xBj inclusive dijet DIS cross-section in back-to-back kinematics at next-to-leading order (NLO) in the Color Glass Condensate effective field theory (CGC EFT). Our result includes, for the first time, all real and virtual NLO contributions to the impact factor. These include all Sudakov double and single logarithm contributions, as well as all other finite mathcal{O} (αs) terms that contribute at this order. We demonstrate explicitly that resummations of small x and Sudakov logarithms can be performed simultaneously in the CGC EFT. This requires that the JIMWLK kernel for small x evolution of the Weizsäcker-Williams (WW) gluon distribution satisfies a kinematic constraint imposed by lifetime ordering of successive gluon emissions; the corresponding modifications to the kernel, corresponding to resummations of large double transverse logarithms, are precisely of the type required to stabilize JIMWLK evolution beyond leading logarithmicaccuracy. We compute the azimuthal harmonics of the NLO back-to-back distributions and show their sensitivity to both the unpolarized and linearly polarized WW gluon distributions. Finally, we discuss how TMD factorization is broken by an emergent saturation scale at small x.

Evidence for Nonlinear Gluon Effects in QCD and Their Mass Number Dependence at STAR.

The STAR Collaboration reports measurements of back-to-back azimuthal correlations of di-π^{0}s produced at forward pseudorapidities (2.6<η<4.0) in p+p, p+Al, and p+Au collisions at a center-of-mass energy of 200GeV. We observe a clear suppression of the correlated yields of back-to-back π^{0} pairs in p+Al and p+Au collisions compared to the p+p data. The observed suppression of back-to-back pairs as a function of transverse momentum suggests nonlinear gluon dynamics arising at high parton densities. The larger suppression found in p+Au relative to p+Al collisions exhibits a dependence of the saturation scale Q_{s}^{2} on the mass number A. A linear scaling of the suppression with A^{1/3} is observed with a slope of -0.09±0.01.

Di-$\pi^0$ correlations in p+p, p+Al and p+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV at STAR

The STAR Collaboration reports measurements of back-to-back azimuthal correlations of di-\pi^0π0 produced at forward pseudorapidity (2.6&lt;\eta&lt;4.02.6&lt;η&lt;4.0) in p+p+pp, p+p+\mathrm{Al}Al and p+p+\mathrm{Au}Au collisions at a center-of-mass energy per nucleon-nucleon pair of 200 GeV. A clear suppression of the correlated away-side yields is observed in p+p+\mathrm{Au}Au and for the first time in p+p+\mathrm{Al}Al collisions, compared with the pp++pp data. The enhanced suppression found in p+p+\mathrm{Au}Au with respect to p+p+\mathrm{Al}Al collisions exhibits the saturation scale (Q_s^2Qs2) dependence on the mass number AA. The observed suppression of back-to-back pairs as a function of event activity and transverse momentum points to non-linear gluon dynamics arising at high parton densities.

Strangeness enhancement and flow-like effects in $$e^+e^-$$ annihilation at high parton density

Strangeness enhancement and collective flow are considered signatures of the quark gluon plasma formation. These phenomena have been detected not only in relativistic heavy ion collisions but also in high energy, high multiplicity events of proton-proton and proton-nucleus ("small systems") scatterings. Indeed, a universal behavior emerges by considering the parton density in the transverse plane as the dynamical quantity to specify the initial condition of the collisions. On the other hand, $e^+ e^-$ annihilation data at LEP and lower energies indicate that there is no strangeness enhancement and no flow-like effect. We show that the parton density in the transverse plane generated in $e^+ e^-$ annihilation at the available energy is too low to expect such effects. The event-by-event multiplicity where strangeness suppression and flow-like phenomenon could show up in $e^+ e^-$ is evaluated.

Collectivity in pp from resummed interference effects?

Azimuthal asymmetries vn in the soft transverse momentum spectra of hadronic collisions can result as a consequence of quantum interference and color flow which translates spatial anisotropies into momentum anisotropies via multipole radiation patterns. Here, we analyze to what extent these effects result in signal strengths vn{2s} that can persist in higher order (2s) cumulants. In a simple model of soft multi-particle production with quantum interference effects in which m particles are emitted from N sources and in which interference contributions appear naturally ordered in inverse powers of the adjoint color trace, 1/(Nc2−1), we provide the first resummed calculation of all powers of m2/(Nc2−1). This allows one to determine all higher order flow cumulants vn{2s} with the same parametric accuracy. For a phenomenologically relevant range of N sources emitting m particles, we find that the even flow coefficients vn{2s} decrease very mildly with increasing cumulants. This provides a proof of principle that non-vanishing higher order cumulants vn{2s} can persist in systems that exhibit neither final state interactions nor phenomena related to high (saturated) initial parton densities.

Classical gluon fields and collective dynamics of color-charge systems

An investigation of color fields that arise in collisions of relativistic heavy ions reveals that, in the non-Abelian case, a change in the color charge leads to the appearance of an extra term that generates a sizable contribution of color-charge glow in chromoelectric and chromomagnetic fields. The possibility of the appearance of a color echo in the scattering of composite color particles belonging to the dipole type is discussed. Arguments are adduced in support of the statement that such effects are of importance in simulating the first stage of ultrarelativistic heavy-ion collisions,where the initial parton state is determined by a high nonequilibrium parton density and by strong local color fluctuations.

High density effects in ultrahigh neutrino interactions

The high parton density present at high energies and large nuclei is expected to modify the lepton-hadron cross section and the associated observables. In this paper we analyze the impact of the high density effects in the average inelasticity and the neutrino-nucleus cross section at ultrahigh energies. We compare the predictions associated to the linear Dokshitzer-Gribov-Lipatov-Altarelli-Parisi dynamics with those from the color glass condensate formalism, which includes nonlinear effects. Our results demonstrated that the nonlinear effects reduce the average inelasticity and that the predictions of the distinct approaches for the neutrino-nucleus cross section at ultrahigh energies are similar.

Open questions in QCD at high parton density

I present the state of our understanding of the QCD dynamics at play in the parton saturation regime of nuclear wave functions. I explain what are the biggest open questions in the field, their intrinsic interest, but also why is it important to answer them from the quark-gluon-plasma physicists' perspective. Focusing on those aspects that proton-nucleus collisions cannot investigate to a satisfactory degree, I show that future high-energy electron-ion colliders have the potential to address these questions, providing thorough answers in most cases, and exploratory measurements otherwise.

Inclusive hadron and photon production at the LHC in dipole momentum space

Using a momentum space model for the dipole scattering amplitude we present an analysis of the saturation effects at LHC energies, describing the data on proton-proton and proton-lead collisions. The model is based on the asymptotic solutions of the Balitsky-Kovchegov equation, being ideal in the saturation domain where the target wave function has a high occupation number. We also make predictions for the nuclear modification ratios on charged hadron and prompt photon production in the forward region, where the high parton density effects are important.

Baryon anomaly and strong color fields in Pb + Pb collisions at 2.76ATeV at the CERN Large Hadron Collider

With the HIJING/B$\ifmmode\bar\else\textasciimacron\fi{}B$ v2.0 heavy ion event generator, we explore the phenomenological consequences of several high parton density dynamical effects predicted in central Pb+Pb collisions at the Large Hadron Collider (LHC) energies. These include (1) jet quenching due to parton energy loss ($dE/dx$), (2) strangeness and hyperon enhancement due to strong longitudinal color field (SCF), and (3) enhancement of baryon-to-meson ratios due to baryon-antibaryon junction (J$\ifmmode\bar\else\textasciimacron\fi{}J$) loops and SCF effects. The saturation/minijet cutoff scale ${p}_{0}(s,A)$ and effective string tension $\ensuremath{\kappa}(s,A)$ are constrained by our previous analysis of LHC $p+p$ data and recent data on the charged multiplicity for Pb+Pb collisions reported by the ALICE collaboration. We predict the hadron flavor dependence (mesons and baryons) of the nuclear modification factor ${R}_{AA}({p}_{T})$ and emphasize the possibility that the baryon anomaly could persist at the LHC up to ${p}_{T}\ensuremath{\sim}10$ GeV, well beyond the range observed in central Au+Au collisions at RHIC energies.

The initial spectrum of fluctuations in the little bang

High parton densities in ultra-relativistic nuclear collisions suggest a description of these collisions wherein the high energy nuclear wavefunctions and the initial stages of the nuclear collision are dominated by classical fields. This underlying paradigm can be significantly improved by including quantum fluctuations around the classical background fields. One class of these contributes to the energy evolution of multi-parton correlators in the nuclear wavefunctions. Another dominant class of unstable quantum fluctuations grow rapidly with proper time $\tau$ after the collision. These secular terms appear at each loop order; the leading contributions can be resummed to all loop orders to obtain expressions for final state observables. The all-order result can be expressed in terms of the spectrum of fluctuations on the initial proper time surface. We compute, in $A^\tau=0$ gauge, the essential elements in this fluctuation spectrum--the small quantum fluctuation modes in the classical background field. With our derivation in QCD, we have all the ingredients to compute inclusive quantities in heavy ion collisions at early times including i) all--order leading logs in Bjorken $x_{1,2}$ of the two nuclei, ii) all strong multiple scattering contributions, and iii) all-order leading secular terms. In the simpler analogous formalism for a scalar $\phi^4$ theory, numerical analysis of the behavior of the energy-momentum tensor is strongly suggestive of early hydrodynamic flow in the system. In QCD, in addition to studying the possible early onset of hydrodynamic behavior, additional important applications of our results include a) the computation of sphaleron transitions off-equilibrium, and b) "jet quenching", or medium modification of parton spectra, in strong color fields at early times.

Gribov-Regge theory, partons, remnants, strings – and the EPOS model for hadronic interactions

We discuss the key ingredients of realistic hadronic interaction models for air shower developments, like parton evolutions, high parton density effects, remnants, multiple scattering, and string fragmentation. We will focus in particular on the EPOS approach, which has been extensively tested against RHIC, SPS, and TEVATRON data. We will also discuss recent developments concerning collective effects like the hydrodynamic evolution of a quark gluon plasma in hadronic interactions.

Probing High Parton Densities at Low- x in d+Au Collisions at PHENIX Using the New Forward and Backward Muon Piston Calorimeters

The new forward Muon Piston Calorimeters allow PHENIX to explore low- x parton distributions in d+Au collisions with hopes of observing gluon saturation. We present a two-particle azimuthal Δ ϕ correlation measurement made between a mid-rapidity particle (| η 1 | < 0.35 ) and a forward π 0 ( 3.1 < η 2 < 3.9 ) wherein we compare correlation widths in d+Au to p+p and compute I d A .

Implications of cosmic ray results for UHE neutrinos

Recent measurements of the spectrum and composition of ultrahigh energy cosmic rays suggest that their extragalactic sources may be accelerating heavy nuclei in addition to protons. This can suppress the cosmogenic neutrino flux relative to the usual expectation for an all-proton composition. Cosmic neutrino detectors may therefore need to be even larger than currently planned but conversely they will also be able to provide valuable information concerning astrophysical accelerators. Moreover measurement of ultrahigh energy cosmic neutrino interactions can provide an unique probe of QCD dynamics at high parton density.

Baryon enhancement in high-density QCD and relativistic heavy ion collisions

We argue that the collinear factorization of the fragmentation functions in high-energy nuclear collisions breaks down at transverse momenta pT ≲ Qs/g due to high parton densities in the colliding hadrons and/or nuclei. We find that gluon recombination dominates in that pT region. We calculate the inclusive cross section for π meson and nucleon production using the low-energy theorems for the scale anomaly in QCD, and compare our quantitative baryon-to-meson ratio to the RHIC data.

The hadronic interaction model EPOS

EPOS is a sophisticated multiple scattering approach based on partons and Pomerons (parton ladders), with special emphasis on high parton densities. The latter aspect, particularly important in proton-nucleus or nucleus-nucleus collisions, is taken care of via an effective treatment of Pomeron-Pomeron interactions, referred to as parton ladder splitting. In addition, collective effects are introduced after separating the high density central core from the peripheral corona. EPOS is the successor of the NEXUS model.

J/ψ RAPIDITY DISTRIBUTIONS IN THE SATURATION REGIME

In this work we calculate the J/ψ production in the initial stage of hadronic and nuclear collisions at RHIC and LHC energies, taking into account the high parton density regime of Quantum Chromodynamics (QCD), where the physics of parton saturation is expected to be dominant. We perform a quantitative analysis of the rapidity distributions in these collisions with the Color Glass Condensate (CGC) approach. The ratio between distributions with or without saturated gluons shows that this mechanism produces a suppression of the J/ψ yield in the forward region only at very high energies.

Gluon recombination in high parton density QCD: Inclusive pion production

We argue that the collinear factorization of the fragmentation functions in high energy hadron and nuclei collisions breaks down at transverse momenta ${k}_{T}\ensuremath{\lesssim}{Q}_{s}/g$ due to high parton densities in the colliding hadrons and/or nuclei. We calculate, at next-to-leading order in projectile parton density and to all orders in target parton density, the double-inclusive cross-section for production of a pair of gluons in the scalar ${J}^{PC}={0}^{++}$ channel. Using the low energy theorems of QCD we find the inclusive cross-section for $\ensuremath{\pi}$-meson production.

J/psi production in the saturation regime

In this work we calculate the J/psi production in the initial stage of proton-proton, proton-nucleus and nucleus-nucleus collisions at RHIC and LHC energies, taking into account the high parton density regime of QCD, where the physics of parton saturation is expected to be dominant. We perform a quantitative analysis of the xF distributions in these collisions with the Color Glass Condensate (CGC) approach. The ratio between distributions with or without saturated gluons shows that this mechanism produces a suppression on the J/psi yield in the forward region and presents a dip in intermediate values of xF, which is visible only at LHC energies.

Heavy quark production in colour glass condensate

We discuss heavy quark production in high parton density QCD in quasi-classical approximation and including low-x quantum evolution. We also consider an alternative approach based on the effect of pair production in external fields.