Conveners
Parallel 1: Initial State: Low-x QCD
- Jamal Jalilian-Marian (Baruch College)
Parallel 1: Initial State
- Xin-Nian Wang
Parallel 1: Jets & high-pT: Photon-jet studies
- David Morrison (BNL)
Parallel 1: Jets & high-pT: Jet-medium interactions
- Abhijit Majumder (Wayne state university)
Parallel 1: Initial State: Nuclear PDFs
- Carlos Albert Salgado Lopez (Universidade de Santiago de Compostela (ES))
Parallel 1: Initial State: Nuclear PDFs
- Yen-Jie Lee (Massachusetts Inst. of Technology (US))
Parallel 1: Initial State
- Eero Aleksi Kurkela (CERN)
Parallel 1: Jets & high-pT
- William Horowitz (University of Cape Town)
Deep Inelastic Scattering (DIS) is the cleanest tool available to probe the content of a fast proton or nucleus. In the regime of low Bjorken x, one enters in the nonlinear regime of gluon saturation, where the gluons are better described as a strong coherent semi-classical field (a.k.a. Color Glass Condensate) than as a collection of quasi on-shell partons. Hence, that regime lies outside the...
We study quantitatively [1] the importance of the recently derived NLO corrections [2,3,4] to the DIS structure functions at small x in the dipole formalism. We show that these corrections can be significant and depend on the factorization scheme used to resum large logarithms of energy into renormalization group evolution with the BK equation. This feature is similar to what has recently been...
The small-x evolution of protons is determined from numerical solutions of the JIMWLK equations, starting from an initial condition at moderate $x$ for a finite size proton. The resulting dipole amplitude is used to calculate the total reduced cross section $\sigma_r$ and charm contribution to it, as well as diffractive vector meson production.
We compare results to experimental data from...
The Large Hadron-electron Collider LHeC is a proposed upgrade of the LHC. An energy recovery linac would provide 60 GeV electrons to collide with the proton and nuclear beams from the HL-LHC and, eventually, with those from the HE-LHC and the FCC-hh. Working concurrently with the pp, pPb or PbPb LHC modes, it will provide ep (ePb) collisions in the TeV regime with luminosities $\sim 10^{34}...
We perform a first computation of inclusive photon production in DIS to NLO accuracy in the impact factor and to NLLx in QCD evolution at small x. We also discuss the corresponding formalism in p+A collisions, and numerical results for the same. We outline an efficient framework to perform computations in the Regge limit of QCD to higher orders. This will be important for quantitative...
The low-x gluon density in the proton and, in particular, in nuclei is only very poorly constrained, while a better understanding of the low-x structure is crucial for measurements at the LHC and also for the planning of experiments at future hadron colliders. In addition, deviations from linear QCD evolution are expected to appear at low x, potentially leading to gluon saturation and a...
Measurements of correlations between two particles separated in pseudorapidity and azimuthal angles have shown striking similarities between results obtained in $pp$, $p$+A and A+A collision systems. In $pp$ collisions, unlike in $p$+A and A+A, the strength of the correlations, quantified by the anisotropy parameter $v_2$, shows little dependence on the observed charged-particle multiplicity....
The produced matter in the high energy nuclear collisions reinteracts and form a plasma which ultimately equilibrates and exhibits collective hydrodynamic flow. The connection between the early gluon production in classical field simulations and hydrodynamic expansion at later times is given by the QCD kinetic theory. In this work we construct a set of non-equilibrium Green functions...
The quark-gluon plasma, which is produced at an early stage of ultrarelativistic heavy-ion collisions, is expected to be initially strongly populated with chromodynamic fields. We address the question how heavy quarks interact with such a turbulent plasma in comparison with an equilibrated one of the same energy density. For this purpose we derive a Fokker-Planck transport equation of heavy...
Direct photon-hadron correlations are an excellent probe for QCD effects, including parton energy loss in the Quark-Gluon Plasma. At leading order, direct photons balance the pT of the away-side jet. In addition, as a colorless probe, direct photons do not interact strongly with the colored medium providing a less biased trigger than a single high-pT hadron. PHENIX has measured direct...
Events containing a high-transverse momentum ($p_T$) prompt photon offer a useful tool to study the dynamics of the hot, dense medium produced in heavy ion collisions. Because photons do not carry color charge, they are unaffected by the medium, and thus provide information about the momentum, direction, and flavor (quark or gluon) of the associated hard-scattered parton before it begins to...
In the recent two papers [1,2], we develop the first systematic theoretical approach to dijet and photon-jet $p_T$ imbalance in hadron-hadron collisions based on the perturbative QCD (pQCD) expansion and the Sudakov resummation formalism. We find that the pQCD calculation at next-to-leading order is indispensable to describe the experimental data, while the Sudakov resummation formalism is...
We present a measurement of semi-inclusive recoil jets for both direct-photon and $\pi^{0}$ triggers, within various trigger transverse momentum ($\rm{p_{T}}$) ranges between 9 GeV/c and 30 GeV/c, in central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, using 13 $\rm{nb}^{-1}$ of data recorded in the year 2014 by the STAR collaboration. Details of the analysis will be discussed, including...
**We use the state of art CoLBT-hydro to investigate the importance of jet-induced medium excitation in calculating γ-jet fragmentation function. CoLBT-hydro model is Linear Boltzmann Transport model cocurrently coupled with 3+1D hydrodynamic model in real time. It is designed for simultaneous event-by-event simulations of jet propagation and hydrodynamics evolution of the bulk medium...
One can impose constraints on theories for parton-medium interactions by measuring jet substructure observables, such as fragmentation functions or jet momentum density profiles. Tagging jets with an associated photon helps to reduce the ambiguities in such measurements, since the photon, not interacting with the QCD matter, gives precise information about the parton's momentum before...
The photon-tagged correlation of jets and jet fragments is a promising channel for the study of parton energy loss in nuclear collisions. While existing measurements in pp and nuclear collisions have used high energy photons and jets, we focus on an unexplored kinematic range given by $10 < p_{\rm T} < 20$ GeV/$c$ photons and the corresponding low jet $p_{\rm T}$. We present results obtained...
We present new results on the energy loss of light partons traversing a highly dynamical strongly coupled quark-gluon plasma. As QGP has large gradients in both temperature and the fluid velocity, it is crucial to study energy loss without assuming a homogeneous plasma, especially as it is known that energy loss depends on the virtuality of the partons, which evolves in the plasma. In a...
We study the phenomenon of transverse momentum broadening for a high-$p_T$ parton propagating through a weakly-coupled quark-gluon plasma undergoing boost-invariant longitudinal expansion. We propose a boost-invariant description for this phenomenon, in which the broadening refers to the angular variables $\eta$ (the pseudo-rapidity) and $\phi$ (the azimuthal angle) --- the same variables as...
In this work, we extend the re-summation of multiple medium-induced emissions to apply to dynamically expanding media. This is done by recasting the quenching weight as the solution of a rate equation with medium-induced partonic splitting functions that are sensitive to the expansion. We perform the calculations in the framework of Baier-Dokshitzer-Mueller-Peigne-Schiff-Zakharov (BDMPSZ)...
The momentum exchanged by hard partons with the quark-gluon plasma ranges from frequent soft interactions to rare large angle scatterings. The larger number of soft interactions makes possible an effective stochastic description of parton-plasma interactions in terms of a small number of transport coefficients, drag and diffusion~[1]. In this work, we study jet energy loss in heavy ion...
The QCD factorization theorems allows one to write any hard-scattering cross section involving hadrons in the initial state as a convolution between perturbative matrix elements and non-perturbative parton distributions functions (PDFs), which need to be extracted from experimental data by means of a global QCD analysis. Thanks to recent progress from both experiment and theory, the PDFs of...
Yields of W and Z bosons can be used to probe the nuclear parton distribution functions of quarks and antiquarks. Final results on W boson and Drell-Yan production in pPb collisions at a nucleon-nucleon center-of-mass energy of 8.16 TeV using the CMS detector will be presented. The muon decay channel is used to study both positive and negative W bosons as a function of muon pseudorapidity....
W and Z bosons are electroweak probes and, therefore, insensitive to the strong interaction, they are clean observables of the initial state of heavy-ion collisions. Despite their low production rates, their relatively clean signatures in the leptonic decay channels allow their study in heavy-ion collisions at the LHC. W and Z bosons measurements in p--Pb and Pb--Pb collisions provide...
Electroweak bosons produced in Pb+Pb collisions are an excellent tool to constrain initial state effects which affect hard scattering process rates in nucleon-nucleon interactions. The production yields of massive electroweak bosons, observed via their leptonic decay channels, offer a high-precision test of the binary collision scaling expected in Pb+Pb and a way to quantify nuclear...
Precision of the current nuclear PDF (nPDF) analyses is limited due to the lack of data constraints especially at small-x region. Ultimately the best way to pin down the nPDF uncertainties would be the clean photon-induced processes in a high-energy lepton-ion collider. To some extend this can also be accomplished with ultra-peripheral heavy-ion collisions at the LHC where a quasi-real photon...
Dijet measurements in pPb collisions have been shown to be one of the most important tools for constraining the gluon nuclear parton distribution functions (nPDFs) at large Bjorken-x. The dijet pseudorapidity distributions are measured as a function of dijet average transverse momentum in order to study the nuclear modifications of PDFs at various factorization scales. The final results from...
The CMS measurement of dijet pseudorapidity distributions in pPb versus pp collisions at 5.02 TeV [1] provides a direct constraint on nuclear gluon PDFs. We show that while the pPb pseudorapidity distributions have sizable free-proton PDF uncertainties, the ratios of the pPb and pp distributions at high average transverse momentum are practically insensitive to scale and free-proton PDF...
Using the LHCb and ALICE detectors in the fixed-target mode at the LHC offers unprecedented possibilities to study the quark, gluon and heavy-quark content of the nuclei in the poorly known region of the high-momentum fractions. We will review our projections for studies of Drell-Yan, W, charm, beauty and quarkonium production with both detector set-ups used with various nuclear targets and...
Measurement of identified charged particle production as a function of multiplicity in pp and p-Pb collisions are important tools for understanding the similarities and differences between small and large interacting systems. Due to their short lifetime, resonances are useful tools to understand the mechanism of particle production and properties of the hadronic phase. The yield of...
We describe a new unified formalism which contains both large and small x gluons in a target proton or nucleus wave function. It generalizes the Color Glass Condensate formalism (appropriate for low pt only) and therefore can be applied to particle production in high energy hadronic/nuclear collisions in both small and large transverse momentum regions and all rapidities.
We compute multi gluon production at mid rapidity in proton-nucleus collisions within the Color Glass Condensate framework. We show that, in the dilute-dense limit valid for such collisions, the terms responsible for the multi gluon correlation have two origins. On the one hand, the Hanbury-Brown-Twiss interference in the final state. On the other hand, the Bose enhancement of gluons in the...
We report on recent progress in understanding multi-particle correlations in small systems from the initial state using the Color Glass Condensate Effective Field Theory (CGC EFT). Working in the dilute-dense limit, applicable for asymmetric collision systems, we first study the Fourier harmonics $v_2$ and $v_3$ for the the systems p/d/$^3$He+Au. This semi-analytic framework, when paired with...
The Large Hadron-electron Collider LHeC is a proposed upgrade of the LHC. An energy recovery linac would provide 60 GeV electrons to collide with the proton and nuclear beams from the HL-LHC and, eventually, with those from the HE-LHC and the FCC-hh. Working concurrently with the pPb or PbPb LHC modes, it will provide ePb collisions in the TeV regime with luminosities $\sim 10^{33}$...
By looking for rare (but not exponentially rare) large-angle deflections of a jet or of partons within a jet, experimentalists can find the weakly coupled short-distance quark and gluon quasiparticles (scatterers) within the strongly coupled liquid quark-gluon plasma produced in heavy ion collisions, as was proposed in Ref.~1. In this previous work, the probability for picking up a given...
We study the modification of reconstructed jets and high transverse momentum (high-$p_T$) hadrons, produced in the fragmentation of these jets, as they propagate through a dense medium. Vacuum and medium modified fragmentation functions are used to define the properties of hadrons produced in hard interactions, in $p$-$p$ and $A$-$A$ collisions respectively. Jets and jet modification are...
We present a novel method to obtain spectral properties of a non-Abelian gauge theory in the region where occupation numbers are high. The method to measure the (single-particle) spectral function is based on linear response theory and classical-statistical lattice simulations. Although we use a system close to its nonthermal fixed point, a situation that typically occurs in the weak-coupling...
The jet transport coefficient $\hat{q}$ is the leading parameter that controls the modification of hard jets in a dense extended medium. In this talk, we outline an ab-initio framework to compute $\hat{q}$ on a quenched SU(3) lattice. We consider a leading order diagram where a hard parton probes the thermal medium by exchanging a Glauber gluon. The non-perturbative part of this process is...
By means of a global fit to all available central hadron and jet RAA measurements in heavy ion collisions at the LHC, we constrain the hybrid model free parameter while obtaining an execellent description of those data. With the narrow range of parameters obtained by this procedure, we perform a detailed investigation of the mechanism of jet suppression in heavy ions as a function of the jet...
