arXiv subjects
Publications and source records attributed to Enke Wang.
We evaluate the octupole in the large-$N_c$ limit in the McLerran-Venugopalan model, and derive a general expression of the 2n-point correlator, which can be applied in analytical studies of the multi-particle production in the scatterings between hard probes and dense targets.
The transverse momentum ($p_T$) spectra from heavy-ion collisions at intermediate momenta are described by non-extensive statistical models. Assuming a fixed relative variance of the temperature fluctuating event by event or alternatively a fixed mean multiplicity in a negative binomial distribution (NBD), two different linear relations emerge between the temperature, $T$, and the Tsallis parameter $q-1$. Our results qualitatively agree with that of G.~Wilk. Furthermore we revisit the "Soft+Hard" model, proposed recently by G.~G.~Barnaföldi \textit{et.al.}, by a $T$-independent average $p_T^2$ assumption. Finally we compare results with those predicted by another deformed distribution, using Kaniadakis' $κ$ parametrization.
Within the framework of higher-twist collinear factorization, transverse momentum broadening for the final hadrons in semi-inclusive deeply inelastic $e+A$ collisions is studied at the next-to-leading order (NLO) in perturbative QCD. Through explicit calculations of real and virtual corrections at twist-4, the transverse-momentum-weighted differential cross section due to double scattering is shown to factorize at NLO and can be expressed as a convolution of twist-4 nuclear parton correlation functions, the usual twist-2 fragmentation functions and hard parts which are finite and free of any divergences. A QCD evolution equation is also derived for the renormalized twist-4 quark-gluon correlation function which can be applied to future phenomenological studies of transverse momentum broadening and jet quenching at NLO.
Nuclear modification factor $R_{AA}$ for large $p_T$ single hadron is studied in a next-to-leading order (NLO) perturbative QCD (pQCD) parton model with medium-modified fragmentation functions (mFFs) due to jet quenching in high-energy heavy-ion collisions. The energy loss of the hard partons in the QGP is incorporated in the mFFs which utilize two most important parameters to characterize the transport properties of the hard parton jets: the jet transport parameter $\hat q_{0}$ and the mean free path $λ_{0}$, both at the initial time $τ_0$. A phenomenological study of the experimental data for $R_{AA}(p_{T})$ is performed to constrain the two parameters with simultaneous $χ^2/{\rm d.o.f}$ fits to RHIC as well as LHC data. We obtain for energetic quarks $\hat q_{0}\approx 1.1 \pm 0.2$ GeV$^2$/fm and $λ_{0}\approx 0.4 \pm 0.03$ fm in central $Au+Au$ collisions at $\sqrt{s_{NN}}=200$ GeV, while $\hat q_{0}\approx 1.7 \pm 0.3$ GeV$^2$/fm, and $λ_{0}\approx 0.5 \pm 0.05$ fm in central $Pb+Pb$ collisions at $\sqrt{s_{NN}}=2.76$ TeV. Numerical analysis shows that the best fit favors a multiple scattering picture for the energetic jets propagating through the bulk medium, with a moderate averaged number of gluon emissions. Based on the best constraints for $λ_{0}$ and $τ_0$, the estimated value for the mean-squared transverse momentum broadening is moderate which implies that the hard jets go through the medium with small reflection.
The production of the massive bosons $Z^0$ and $W^{\pm}$ could provide an excellent tool to study cold nuclear matter effects and the modifications of nuclear parton distribution functions (nPDFs) relative to parton distribution functions (PDFs) of a free proton in high energy nuclear reactions at the LHC as well as in heavy-ion collisions (HIC) with much higher center-of mass energies available in the future colliders. In this paper we calculate the rapidity and transverse momentum distributions of the vector boson and their nuclear modification factors in p+Pb collisions at $\sqrt{s_{NN}}=63$TeV and in Pb+Pb collisions at $\sqrt{s_{NN}}=39$TeV in the framework of perturbative QCD by utilizing three parametrization sets of nPDFs: EPS09, DSSZ and nCTEQ. It is found that in heavy-ion collisions at such high colliding energies, both the rapidity distribution and the transverse momentum spectrum of vector bosons are considerably suppressed in wide kinematic regions with respect to p+p reactions due to large nuclear shadowing effect. We demonstrate that in the massive vector boson productions processes with sea quarks in the initial-state may give more contributions than those with valence quarks in the initial-state, therefore in future heavy-ion collisions the isospin effect is less pronounced and the charge asymmetry of W boson will be reduced significantly as compared to that at the LHC. Large difference between results with nCTEQ and results with EPS09 and DSSZ is observed in nuclear modifications of both rapidity and $p_T$ distributions of $Z^0$ and $W$ in the future HIC.
The transverse momentum spectrum of $η$ meson in relativistic heavy-ion collisions is studied at the next-to-leading-order (NLO) within the perturbative QCD, where the jet quenching effect in the QGP is incorporated with the effectively medium-modified $η$ fragmentation functions using the higher-twist approach. We show that the theoretical simulations could give nice descriptions of PHENIX data on $η$ meson in both $\rm p+p$ and central $\rm Au+Au$ collisions at the RHIC, and also provide numerical predictions of $η$ spectra in central $\rm Pb+Pb$ collisions with $\sqrt{s_{NN}}=2.76$~TeV at the LHC. The ratios of $η/π^0$ in $\rm p+p$ and in central $\rm Au+Au$ collisions at $200$~GeV are found to overlap in a wide $p_T$ region, which matches well the measured ratio $η/ π^0$ by PHENIX. We demonstrate that, at the asymptotic region when $p_{T} \rightarrow \infty$ the ratios of $η/π^{0}$ in both $\rm Au+Au$ and $\rm p+p$ are almost determined only by quark jets fragmentation and thus approach to the one in $e^{+} e^{-}$ scattering; in addition, the almost identical gluon (quark) contribution fractions to $η$ and to $π$ result in a rather moderate variation of $η/π^{0}$ distribution at intermediate and high $p_T$ region in $\rm A+A$ relative to that in $\rm p+p$; while a slightly higher $η/π^{0}$ at small $p_T$ in $\rm Au+Au$ can be observed due to larger suppression of gluon contribution fraction to $π^{0}$ as compared to the one to $η$. The theoretical prediction for $η/ π^0$ at the LHC has also been presented.
The productions of massive gauge bosons, $Z^0$ and $W^+/W^-$, in heavy-ion reactions at the LHC, provide an excellent tool to study the cold nuclear matter effects in high-energy nuclear collisions. In this paper we investigate $Z^0$ and $W^+/W^-$ productions in p+Pb and Pb+Pb at the LHC, at NLO and NNLO with DYNNLO incorporating the nuclear PDFs (nPDFs) parametrization sets EPS09 and DSSZ, within the framework of perturbative QCD. The numerical simulations of the transverse momentum spectra, rapidity dependence, and related nuclear modification factors for $Z^0$ and $W$ particles, as well as the charge asymmetry for W boson, are provided and tested against the latest experimental data. It is found that the theoretical results with EPS09 and DSSZ nPDFs can give good descriptions of the recent data on $Z^0$ and $W^{\pm}$ particles in p+Pb and Pb+Pb within the experimental error bars, though some differences between results with EPS09 and DSSZ can be observed, especially in the rapidity dependence of the $Z^0$ yield. Theoretical predictions for future measurements on $Z^0$ and $W$ in p+Pb and Pb+Pb collisions at the LHC are also provided.
We study the incoherent multiple scattering effects on heavy meson production in the backward rapidity region of p+A collisions within the generalized high-twist factorization formalism. We calculate explicitly the double scattering contributions to the heavy meson differential cross sections by taking into account both initial-state and final-state interactions, and find that these corrections are positive. We further evaluate the nuclear modification factor for muons that come form the semi-leptonic decays of heavy flavor mesons. Phenomenological applications in d+Au collisions at a center-of-mass energy $\sqrt{s}=200$ GeV at RHIC and in p+Pb collisions at $\sqrt{s}=5.02$ TeV at the LHC are presented. We find that incoherent multiple scattering can describe rather well the observed nuclear enhancement in the intermediate $p_T$ region for such reactions.
Within the framework of a high-twist approach, we show the first complete next-to-leading order calculation of transverse momentum broadening in semi-inclusive deeply inelastic $e+A$ scattering and Drell-Yan dilepton production in $p+A$ collisions. We demonstrate at one-loop level how QCD factorization holds for multiple scattering in nuclear medium, and the university of the associated quark-gluon correlation function. Our calculation also identifies QCD evolution equation for the quark-gluon correlation function, which determines the QCD factorization scale and energy dependence of jet transport parameter $\hat q$.
In the presence of collective flow a new model potential describing the interaction of the hard jet with scattering centers is derived based on the static color-screened Yukawa potential. The flow effect on jet quenching with detailed balance is investigated in pQCD. It turns out, considering the collective flow with velocity $v_z$ along the jet direction, the collective flow decreases the LPM destructive interference comparing to that in the static medium. The gluon absorption plays a more important role in the moving medium. The collective flow increases the energy gain from gluon absorption, however, decreases the energy loss from gluon radiation, which is $(1 - v_z )$ times as that in the static medium to the first order of opacity. In the presence of collective flow, the second order in opacity correction is relatively small compared to the first order. So that the total effective energy loss is decreased. The flow dependence of the energy loss will affect the suppression of high $p_T$ hadron spectrum and anisotropy parameter $v_2$ in high-energy heavy-ion collisions.
Within the framework of a high-twist approach, we calculate the next-to-leading order (NLO) perturbative QCD corrections to the transverse momentum broadening in semi-inclusive hadron production in deeply inelastic $e+A$ collisions, as well as lepton pair production in $p+A$ collisions. With explicit calculations of both real and virtual contributions, we verify, for the first time, the factorization theorem at twist 4 in NLO for the nuclear-enhanced transverse momentum weighted differential cross section and demonstrate the universality of the associated twist-4 quark-gluon correlation function. We also identify the QCD evolution equation for the twist-4 quark-gluon correlation function in a large nucleus, which can be solved to determine the scale dependence of the jet transport parameter in the study of jet quenching.
We study the nuclear enhancement of the transverse momentum imbalance for back-to-back particle production in both p+A and e+A collisions. Specifically, we present results for photon+jet and photon+hadron production in p+A collisions, di-jet and di-hadron production in e+A collisions, and heavy-quark and heavy-meson pair production in both p+A and e+A collisions. We evaluate the effect of both initial-state and final-state multiple scattering, which determine the strength of the nuclear-induced transverse momentum imbalance in these processes. We give theoretical predictions for the experimentally relevant kinematic regions in d+Au collisions at RHIC, p+Pb collisions at LHC and e+A collisions at the future EIC and LHeC.
Modifications of jets in the existence of a hot and dense QCD medium have recently attracted a lot of attentions. In this talk, we demonstrate how jet-medium interactions change the behavior of jets by offering examples of inclusive jet and dijet productions at $ {\cal O}(α_s^3)$ in heavy ion collisions including initial-state cold nuclear effects and especially the final-state parton energy loss effect. The suppression of inclusive jet spectrum varying with jet radii and a flatter dijet momentum imbalance as compared to those in hadron-hadron collisions are observed in high-energy nuclear collisions.
Within the generalized high-twist factorization formalism, we express the contribution from multiple parton scattering and induced gluon radiation to the DY dilepton spectra in terms of nuclear modified effective beam quark distribution function. We show that beam quark energy loss is characterized by jet transport parameter $\hat{q}$, which is related to the local gluon density of the medium. Using the value of $\hat{q}$ determined from the deeply inelastic scattering (DIS) data, we evaluate the nuclear modification factor in Drell-Yan process in p+A collisions. Effects of parton energy loss in the DY spectra are found negligible in the Fermilab experimental data at $E_{\rm lab}=800$ GeV relative to parton shadowing while the predicted suppression of DY spectra are significant at $E_{\rm lab}=120$ GeV.
Jet physics in relativistic heavy ion collisions, which combines perturbative QCD jet production with quark and gluon energy loss and in-medium parton shower modification, has emerged as a powerful tool to probe the properties of strongly-interacting matter formed in high-energy nuclear reactions. We present selected results for the modification of jet cross sections and related observables in the ambiance of hot and/or dense nuclear medium. We focus on the inclusive jet spectrum and dijets [${\cal O}(α_s^3) $], and $Z^0/γ^*$ tagged jets [${\cal O}(G_Fα_s^2) $] in the framework of perturbative QCD.
We investigate the cold nuclear matter(CNM) effects on dijet productions in high-energy nuclear collisions at LHC with the next-to-leading order perturbative QCD. The nuclear modifications for dijet angular distributions, dijet invariant mass spectra, dijet transverse momentum spectra and dijet momentum imbalance due to CNM effects are calculated by incorporating EPS, EKS, HKN and DS param-etrization sets of parton distributions in nucleus . It is found that dijet angular distributions and dijet momentum imbalance are insensitive to the initial-state CNM effects and thus provide optimal tools to study the final-state hot QGP effects such as jet quenching. On the other hand, the invariant mass spectra and the transverse momentum spectra of dijet are generally enhanced in a wide region of the invariant mass or transverse momentum due to CNM effects with a feature opposite to the expected suppression because of the final-state parton energy loss effect in the QGP. The difference of EPS, EKS, HKN and DS parametrization sets of nuclear parton distribution functions is appreciable for dijet invariant mass spectra and transverse momentum spectra at p+Pb collisions, and becomes more pronounced for those at Pb+Pb reactions.
Within the framework of generalized collinear factorization in perturbative QCD (pQCD), we study the effect of initial multiple parton scattering and induced parton energy loss in Drell-Yan (DY) process in proton-nucleus collisions. We express the contribution from multiple parton scattering and induced gluon radiation to the DY dilepton spectra in terms of nuclear modified effective beam quark distribution functions. The modification depends on the quark transport parameter in nuclear medium. This is similar to the final-state multiple parton scattering in deeply inelastic scattering (DIS) off large nuclei and leads to the suppression of the Drell-Yan cross section in $p+A$ relative to $p+p$ collisions. With the value of quark transport parameter determined from the nuclear modification of single-inclusive DIS hadron spectra as measured by the HERMES experiment, we calculate DY spectra in $p+A$ collisions and find the nuclear suppression due to beam parton energy loss negligible at the Fermilab energy $E_{\rm lab}$=800 GeV in the kinematic region as covered by the E866 experiment. Most of the observed nuclear suppression of DY spectra in E866 experiment can be described well by parton shadowing in target nuclei as given by the EPS08 parameterization. The effect of beam parton energy loss, however, becomes significant for DY lepton pairs with large beam parton momentum fraction $x^{\prime}$ or small target parton momentum fraction $x$. We also predict the DY cross section in $p+A$ collisions at lower beam proton energy $E_{\rm lab}$=120 GeV and show significant suppression due to initial state parton energy loss at moderately large $x^{\prime}$ where the effect of parton shadowing is very small.
Nuclear modification factor $R_{AA}(p_{T})$ for large transverse momentum pion spectra in $Pb+Pb$ collisions at $\sqrt{s}=2.76$ TeV is predicted within the NLO perturbative QCD parton model. Effect of jet quenching is incorporated through medium modified fragmentation functions within the higher-twist approach. The jet transport parameter that controls medium modification is proportional to the initial parton density and the coefficient is fixed by the RHIC data on suppression of large $p_{T}$ hadron spectra. Data on charged hadron multiplicity $dN_{ch}/dη=1584 \pm 80$ in central $Pb+Pb$ collisions from the ALICE Experiment at the LHC are used to constrain the initial parton density both for determining the jet transport parameter and the 3+1D ideal hydrodynamic evolution of the bulk matter that is employed for the calculation of $R_{PbPb}(p_{T})$ for neutral pions.