Showing papers on "Color-glass condensate published in 2016"

Revealing proton shape fluctuations with incoherent diffraction at high energy

Abstract: The differential cross section of exclusive diffractive vector meson production in electron proton collisions carries important information on the geometric structure of the proton. More specifically, the coherent cross section as a function of the transferred transverse momentum is sensitive to the size of the proton, while the incoherent or proton dissociative cross section is sensitive to fluctuations of the gluon distribution in coordinate space. We show that at high energies the experimentally measured coherent and incoherent cross sections for the production of $J/\mathrm{\ensuremath{\Psi}}$ mesons are very well reproduced within the color glass condensate framework when strong geometric fluctuations of the gluon distribution in the proton are included. For $\ensuremath{\rho}$ meson production, we also find reasonable agreement. We study in detail the dependence of our results on various model parameters, including the average proton shape, analyze the effect of saturation scale and color charge fluctuations and constrain the degree of geometric fluctuations.

Heavy-ion collisions at the Large Hadron Collider: a review of the results from Run 1

Abstract: We present an overview of the results obtained in pPb and PbPb collisions at the Large Hadron Collider during Run 1. We first discuss the results for global characteristics: cross sections, hadron multiplicities, azimuthal asymmetries, correlations at low transverse momentum, hadrochemistry, and femtoscopy. We then review hard and electroweak probes: particles with high transverse momentum, jets, heavy quarks, quarkonia, electroweak bosons and high transverse momentum photons, low transverse momentum photons and dileptons, and ultraperipheral collisions. We mainly focus on the experimental results, and present very briefly the main current theoretical explanations.

Tracing the origin of azimuthal gluon correlations in the color glass condensate

Abstract: We examine the origins of azimuthal correlations observed in high energy proton-nucleus collisions by considering the simple example of the scattering of uncorrelated partons off color fields in a large nucleus. We demonstrate how the physics of fluctuating color fields in the color glass condensate (CGC) effective theory generates these azimuthal multiparticle correlations and compute the corresponding Fourier coefficients v n within different CGC approximation schemes. We discuss in detail the qualitative and quantitative differences between the different schemes. We will show how a recently introduced color field domain model that captures key features of the observed azimuthal correlations can be understood in the CGC effective theory as a model of non-Gaussian correlations in the target nucleus.

Single inclusive forward hadron production at next-to-leading order

Abstract: We discuss single inclusive hadron production from a high energy quark scattering off a strong target color field in the Color Glass Condensate formalism. Recent calculations of this process at the next-to-leading order accuracy have led to negative cross sections at large transverse momenta. We identify the origin of this problem as an oversubtraction of the rapidity divergence into the Balitsky-Kovchegov evolution equation for the target. We propose a new way to implement the kinematical restriction on the emitted gluons to overcome this difficulty.

Single spin asymmetries from a single Wilson loop

Abstract: We study the leading-power gluon transverse-momentum-dependent distributions (TMDs) of relevance to the study of asymmetries in the scattering off transversely polarized hadrons. Next-to-leading-order perturbative calculations of these TMDs show that at large transverse momentum they have common dynamical origins but that in the limit of a small longitudinal momentum fraction x, only one origin remains. We find that in this limit, only the dipole-type gluon TMDs survive and become identical to each other. At small x, they are all given by the expectation value of a single Wilson loop inside the transversely polarized hadron, the so-called spin-dependent odderon. This universal origin of transverse spin asymmetries at small x is of importance to current and future experimental studies, paving the way to a better understanding of the role of gluons in the three-dimensional structure of spin-polarized protons.

Wigner, Husimi, and generalized transverse momentum dependent distributions in the color glass condensate

Abstract: We study the phase-space distributions of gluons inside a nucleon/nucleus in the small-$x$ regime including the gluon saturation effect. This can be done by using the relation between the gluon Wigner distribution and the dipole S matrix at small $x$, which latter satisfies the Balitsky-Kovchegov (BK) equation. By efficiently solving the BK equation with impact parameter dependence, we compute the Wigner, Husimi and generalized transverse momentum dependent distributions in the saturation regime. We also investigate the elliptic angular dependence of these distributions which has been recently shown to be measurable in DIS experiments.

Energy dependence of the ridge in high multiplicity proton-proton collisions

Abstract: We demonstrate that the recent measurement of azimuthally collimated, long-range rapidity (``ridge'') correlations in $\sqrt{s}=13\text{ }\text{ }\mathrm{TeV}$ proton-proton ($p+p$) collisions by the ATLAS Collaboration at the LHC are in agreement with expectations from the color glass condensate effective theory of high-energy QCD. The observation that the integrated near-side yield as a function of multiplicity is independent of collision energy is a natural consequence of the fact that multiparticle production is driven by a single semihard saturation scale in the color glass condensate framework. We argue further that the azimuthal structure of these recent ATLAS ridge measurements strongly constrains hydrodynamic interpretations of such correlations in high-multiplicity $p+p$ collisions.

Semi-holography for heavy ion collisions: self-consistency and first numerical tests

Abstract: We present an extended version of a recently proposed semi-holographic model for heavy-ion collisions, which includes self-consistent couplings between the Yang-Mills fields of the Color Glass Condensate framework and an infrared AdS/CFT sector, such as to guarantee the existence of a conserved energy-momentum tensor for the combined system that is local in space and time, which we also construct explicitly. Moreover, we include a coupling of the topological charge density in the glasma to the same of the holographic infrared CFT. The semi-holographic approach makes it possible to combine CGC initial conditions and weak-coupling glasma field equations with a simultaneous evolution of a strongly coupled infrared sector describing the soft gluons radiated by hard partons. As a first numerical test of the semi-holographic model we study the dynamics of fluctuating homogeneous color-spin-locked Yang-Mills fields when coupled to a homogeneous and isotropic energy-momentum tensor of the holographic IR-CFT, and we find rapid convergence of the iterative numerical procedure suggested earlier.

Simulating collisions of thick nuclei in the color glass condensate framework

Abstract: We present our work on the simulation of the early stages of heavy-ion collisions with finite longitudinal thickness in the laboratory frame in $3+1$ dimensions. In particular we study the effects of nuclear thickness on the production of a glasma state in the McLerran-Venugopalan model within the color glass condensate framework. A finite thickness enables us to describe nuclei at lower energies, but forces us to abandon boost invariance. As a consequence, random classical color sources within the nuclei have to be included in the simulation, which is achieved by using the colored particle-in-cell method. We show that the description in the laboratory frame agrees with boost-invariant approaches as a limiting case. Furthermore we investigate collisions beyond boost invariance, in particular the pressure anisotropy in the glasma.

Photon-jet ridge at RHIC and the LHC

Abstract: We investigate long range rapidity correlations of pairs of prompt photon and jet in the Color Glass Condensate (CGC) framework in proton-proton and proton-nucleus collisions at RHIC and the LHC. We show that photon-jet correlations exhibit long-range azimuthal collimation at near-side for low transverse momenta of the produced photon and jet in high-multiplicity events. These ridge-like features are strikingly similar to the observed ridge effect for di-hadron correlations at RHIC and the LHC. We show that correlations in the relative rapidity and the relative azimuthal angle between pairs of prompt photon and jet strongly depend on the gluon saturation dynamics at small-x kinematics and such measurements can help to understand the true origin of the observed di-hadron ridge in p+A collisions, and address whether the ridge is a universal phenomenon for all two particle correlations at high energy and high multiplicity events. We also investigate if there is a ridge-like structure for photon-hadron pair correlations at RHIC and the LHC. We found that the hadronization of jet has non-trivial effects on the photon-jet correlations.

Quark production in heavy ion collisions: formalism and boost invariant fermionic light-cone mode functions

Abstract: We revisit the problem of quark production in high energy heavy ion collisions, at leading order in α s in the color glass condensate framework. In this first paper, we setup the formalism and express the quark spectrum in terms of a basis of solutions of the Dirac equation (the mode functions). We determine analytically their initial value in the Fock-Schwinger gauge on a proper time surface Q s τ 0 ≪ 1, in a basis that makes manifest the boost invariance properties of this problem. We also describe a statistical algorithm to perform the sampling of the mode functions.

Gluon TMDs in quarkonium production

Abstract: Quarkonium production offers good possibilities to study gluon TMDs. In this proceedings contribution this topic is explored for the linearly polarized gluons inside unpolarized hadrons and unpolarized gluons inside transversely polarized hadrons. It is argued that $\chi_{b0/2}$ and $\eta_b$ production at LHC are best to study the effects of linearly polarized gluons in hadronic collisions, by means of angular independent ratios of ratios of cross sections. This can be directly compared to $\cos 2\phi$ asymmetries in heavy quark pair and dijet production in DIS at a future high-energy Electron-Ion Collider (EIC), which probe the same TMDs. In the small-$x$ limit this corresponds to the Weizsacker-Williams (WW) gluon distributions, which should show a change in behavior for transverse momenta around the saturation scale. Together with investigations of the dipole (DP) gluon distributions, this can provide valuable information about the polarization of the Color Glass Condensate if sufficiently small $x$ are reached. Quarkonia can also be useful in the study of single transverse spin asymmetries. For transversely polarized hadrons the gluon distribution can be asymmetric, which is referred to as the Sivers effect. It leads to single spin asymmetries in for instance $J/\psi$ (pair) production at AFTER@LHC, which probe the WW or $f$-type gluon Sivers TMD. It allows for a test of a sign-change relation w.r.t. the gluon Sivers TMD probed at an EIC in open heavy quark pair production. Single spin asymmetries in backward inclusive $C$-odd quarkonium production, such as $J/\psi$ production, may offer probes of the DP or $d$-type gluon Sivers TMD at small $x$-values in the polarized proton, which in that limit corresponds to a correlator of a single Wilson loop, describing the spin-dependent odderon.

Probing gluon saturation with next-to-leading order photon production at central rapidities in proton-nucleus collisions

Abstract: We compute the cross section for photons emitted from sea quarks in proton-nucleus collisions at collider energies. The computation is performed within the dilute-dense kinematics of the Color Glass Condensate (CGC) effective field theory. Albeit the result obtained is formally at next-to-leading order in the CGC power counting, it provides the dominant contribution for central rapidities. We observe that the inclusive photon cross section is proportional to all-twist Wilson line correlators in the nucleus. These correlators also appear in quark-pair production; unlike the latter, photon production is insensitive to hadronization uncertainties and therefore more sensitive to multi-parton correlations in the gluon saturation regime of QCD. We demonstrate that $k_\perp$ and collinear factorized expressions for inclusive photon production are obtained as leading twist approximations to our result. In particular, the collinearly factorized expression is directly sensitive to the nuclear gluon distribution at small $x$. Other results of interest include the realization of the Low-Burnett-Kroll soft photon theorem in the CGC framework and a comparative study of how the photon amplitude is obtained in Lorenz and light-cone gauges.

CGC/saturation approach for high energy soft interactions: ‘soft’ Pomeron structure and $$v_{n}$$ in hadron and nucleus collisions from Bose–Einstein correlations

Abstract: In the framework of our model of soft interactions at high energy based on the CGC/saturation approach, we show that Bose–Einstein correlations of identical gluons lead to large values of $$v_n$$ . We demonstrate how three dimensional scales of high energy interactions, hadron radius, typical size of the wave function in diffractive production of small masses (size of the constituent quark), and the saturation momentum, influence the values of BE correlations, and in particular, the values of $$v_n$$ . Our calculation shows that the structure of the ‘dressed’ Pomeron leads to values of $$v_n$$ which are close to experimental values for proton–proton scattering, 20 % smaller than the observed values for proton–lead collisions and close to lead–lead collisions for 0–10 % centrality. Bearing this result in mind, we conclude that it is premature to consider that the appearance of long range rapidity azimuthal correlations are due only to the hydrodynamical behaviour of the quark–gluon plasma.

Evolution to the Quark-Gluon Plasma

Abstract: Theoretical studies on the early-time dynamics in the ultra-relativistic heavy-ion collisions are reviewed including pedagogical introductions on the initial condition with small-x gluons treated as a color glass condensate, the bottom-up thermalization scenario, plasma/glasma instabilities, basics of some formulations such as the kinetic equations and the classical statistical simulation. More detailed discussions follow to make an overview of recent developments on the fast isotropization, the onset of hydrodynamics, and the transient behavior of momentum spectral cascades.

Analytic calculation of the energy-momentum tensor in heavy ion collisions from color glass condensate

Abstract: We generalize calculations of the energy-momentum tensor for classical gluon fields in the boost-invariant McLerran-Venugopalan model using the small-$\ensuremath{\tau}$ power series expansion method. Results to all orders for the energy density and pressures are given in the leading ${Q}^{2}$ approximation and with the inclusion of estimated running coupling effects. The energy density and transverse pressure decrease monotonically with time while the longitudinal pressure starts from a negative value and increases towards zero.

Testing the running coupling kT -factorization formula for the inclusive gluon production

Abstract: The inclusive gluon production at midrapidities is described in the color glass condensate formalism using the kT-factorization formula, which was derived at fixed coupling constant considering the scattering of a dilute system of partons with a dense one. Recent analysis demonstrated that this approach provides a satisfactory description of the experimental data for the inclusive hadron production in pp/pA/AA collisions. However, these studies are based on the fixed coupling kT-factorization formula. This formula does not take into account the running coupling corrections, which are important to set the scales present in the cross section. In this paper we consider the running coupling corrected kT-factorization formula conjectured some years ago and investigate the impact of the running coupling corrections on the observables. In particular, the pseudorapidity distributions and charged hadrons' multiplicity are calculated considering pp, dAu/pPb, and AuAu/PbPb collisions at RHIC and LHC energies. We compare the corrected running coupling predictions with those obtained using the original kT-factorization assuming a fixed coupling or a prescription for the inclusion of the running of the coupling. Considering the Kharzeev-Levin-Nardi unintegrated gluon distribution and a simplified model for the nuclear geometry, we demonstrate that the distinct predictions are similar for the pseudorapidity distributions in pp/pA/AA collisions and for the charged hadrons' multiplicity in pp/pA collisions. On the other hand, the running coupling corrected kT-factorization formula predicts a smoother energy dependence for dN/dη in AA collisions. (Less)

Ultraforward particle production from color glass condensate and Lund fragmentation

Abstract: We present an analysis of data on single inclusive pion production measured by the LHCf Collaboration in high-energy proton-proton and proton-nucleus at ultraforward rapidities, $8.8\ensuremath{\le}y\ensuremath{\le}10.8$. We also analyze forward RHIC data for calibration purposes. Our analysis relies on the use of a Monte Carlo event generator that combines a perturbative description of the elementary scattering process at partonic level based on the hybrid formalism of the color glass condensate with an implementation of hadronization in the framework of the Lund string fragmentation model. This procedure allows us to reach values of the momenta of the produced particles as low as detected experimentally, ${p}_{t}\ensuremath{\sim}0.1\text{ }\text{ }\mathrm{GeV}$. We achieve a good description of single inclusive spectra of charged particles and neutral pions at the RHIC and the LHC, respectively, and nuclear modification factors for proton-lead collisions at the LHC. Our results add evidence to the idea that particle production in the domain of a very small Bjorken $x$ is dominated by the saturation effects encoded in the unintegrated gluon distribution of the target. With forward particle production being of key importance in the development of air showers, we stress that this approach allows for a theoretically controlled extrapolation of our results to the scale of ultra-high-energy cosmic rays, thus serving as the starting point for future works on this topic.

An Experimental Review on Elliptic Flow of Strange and Multistrange Hadrons in Relativistic Heavy Ion Collisions

Abstract: Strange hadrons, especially multistrange hadrons, are good probes for the early partonic stage of heavy ion collisions due to their small hadronic cross sections. In this paper, I give a brief review on the elliptic flow measurements of strange and multistrange hadrons in relativistic heavy ion collisions at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC).

Two-Particle Correlations in Heavy-Light Ion Collisions

Abstract: We study the initial, high-energy scatterings in heavy ion collisions using the saturation/Color Glass Condensate framework. We focus on two-particle long-range rapidity correlations which are modeled as two-gluon correlations. We calculate the two-gluon production cross section using the saturation framework in the heavy-light ion regime, including all-order saturation effects in the heavy nucleus while considering only two-orders in the light ion. The two-gluon production cross section generates four types of long-range in rapidity correlations: (i) geometric correlations, (ii) Hanbury Brown and Twiss (HBT) like correlations accompanied by a back-to-back maximum, (iii) near-side correlations, and (iv) away-side azimuthal correlations. The geometric correlations (i) are due to the fact that nucleons are correlated by simply being confined within the same nucleus. Correlations (iii) and (iv) have exactly the same amplitudes along with azimuthal and rapidity shapes: one centered around $\Delta \phi =0$ and the other one centered around $\Delta \phi =\pi$ (here $\Delta \phi$ is the azimuthal angle between the two produced gluons). The geometry dependence of the correlation function leads to stronger azimuthal near- and away-side correlations in the tip-on-tip U+U collisions than in the side-on-side U+U collisions, an exactly opposite behavior from the correlations generated by the elliptic flow of the quark-gluon plasma: a study of azimuthal correlations in the U+U collisions may help to disentangle the two sources of correlations. Finally we rewrite our result for the two-gluon production cross-section in a $k_T$-factorized form resulting in an expression involving a convolution of one- and two-gluon Wigner distributions over the transverse momenta and impact parameters. This differs from the $k_T$-factorized forms used in the literature.

Initial State and Thermalization in the Color Glass Condensate Framework

Abstract: In this review, I present the description of the early stages of heavy ion collisions at high energy in the Color Glass Condensate framework, from the pre-collision high energy nuclear wave function to the point where hydrodynamics may start becoming applicable.

Transverse momentum distributions of primary charged particles in pp, p–Pb and Pb–Pb collisions measured with ALICE at the LHC

Abstract: According to the standard model of Big Bang cosmology the earliest universe contained an extremely hot and dense medium that subsequently expanded and cooled. The evolution of the early universe happened through a phase with of deconfined quarks and gluons: the quark-gluon plasma (QGP). This phase ended about ten microseconds after the Big Bang when the temperature dropped below the critical temperature Tc and quarks and gluons became confined into hadrons. The existence of a QGP phase at high temperature is also predicted by Quantum Chromodynamics (QCD), the fundamental field theory describing the strong interaction of quarks and gluons. In high-energy collisions of heavy nuclei a QGP can be created and studied experimentally. The energy loss of high energy partons in the hot QCD medium results in a suppression of particle production at large transverse momenta. Measurements of the parton energy loss can be used to characterize the QGP properties. The Large Hadron Collider (LHC) at CERN provides hadron-hadron (pp), nucleus-nucleus (Pb–Pb) and proton-nucleus (p–Pb) collisions at the highest energies reached in an accelerator so far. The ALICE experiment at the LHC is dedicated to the investigation of the QGP in heavy-ion collisions and complemented by the study of pp and p–Pb collisions. In p–Pb collisions the QGP is not expected to be formed and the nuclear initial state and cold nuclear matter effects can be studied. Measurements in pp collisions serve as a reference for p–Pb and Pb–Pb collisions. They also allow to tune phenomenological models and are a test of theoretical predictions from perturbative QCD. The data analyzed for this thesis were collected in pp, p–Pb and Pb–Pb collisions by ALICE in 2010-2013 with different minimum bias triggers using the two VZERO scintillators and the Silicon Pixel Detector (SPD). Charged tracks are reconstructed using combined information from the Inner Tracking System (ITS) and the Time Projection Chamber (TPC), the main tracking detectors of ALICE. Events used for the analysis are required to have a reconstructed primary vertex. The selection of tracks is optimized to provide high purity of primary particles and optimal pT resolution. Measured pT distributions are corrected for tracking efficiency and acceptance effects as well as contamination from secondary particles originating from particle decays or particle production in the detector material. Both corrections are evaluated from full detector simulations using GEANT3 for particle transport through the detector with events generated from the Monte Carlo event generators PYTHIA (pp), DPMJET (p–Pb) and HIJING (Pb–Pb). Corrections for the finite momentum resolution of the detector are derived from the measured pT resolution and the pT spectra in an unfolding procedure. Spectra are normalized to inelastic yields and cross sections (pp), respectively non-single-diffractive yields (p–Pb), taking into account the efficiencies of the trigger and the vertex reconstruction. Possible sources of systematic uncertainties are identified and their contribution is estimated. Transverse momentum (pT) distributions of primary charged particles have been measured at mid rapidity |eta| as a function of the true multiplicity is constructed using a reweighting procedure with effective corrections obtained from Monte Carlo simulations which account for limited acceptance, tracking efficiency and contamination. The multiplicity dependence of the average transverse momentum is compared to predictions from Monte Carlo event generators and (in p–Pb and Pb–Pb collisions) to expectations from binary collision scaling. In pp collisions the expected power-law behavior at large transverse momentum is observed, but the measured cross section does not agree with next-to-leading order (NLO) perturbative QCD calculations. At small transverse momenta the shape of the spectrum is approximately exponential. The data are compared to the MC event generators PHOJET and PYTHIA; none of them agrees with the data over the full pT range. In p–Pb collisions pT spectra are softer at forward pseudorapidity (in the Pb fragmentation region). The nuclear modification factor RpPb reveals that at low pT approximate participant scaling is in place. In the intermediate pT region, a hint of Cronin enhancement is visible, but at the edge of the experimental uncertainties. At high pT no deviation from binary collision scaling is observed and RpPb is consistent with unity. Theoretical predictions from a Color Glass Condensate initial state model are in agreement with the measured RpPb, as well as as calculations based on NLO pQCD with EPS09s nuclear parton distribution functions. In Pb–Pb collisions particle production is suppressed compared to the expectation from binary collision scaling. The suppression is largest for central collisions but remains substantial also for peripheral collisions. For central (0-5%) collisions the nuclear modification factor is about 0.4 at the largest measured momenta. The observations are in quantitative agreement only with part of theoretical models that incorporate medium-induced parton energy loss. In the future, improvements in the simulations and analysis procedure are expected to reduce the systematic uncertainties to about half the current values. With the inclusion of Pb–Pb data recorded in 2011 and the use of other triggers than minimum bias the pT reach could be extended up to 100 GeV/c.