arXiv subjects
Publications and source records attributed to Xin-Nian Wang.
Recent experimental measurements of high $p_T$ hadron spectra and jet correlation at RHIC are analyzed within a parton model which incoporates initial jet production and final propagation in heavy-ion collisions. The suppresion of single hadron spectra, back-to-back correlation, their centrality dependence and azimuthal anisotropy point to a dense matter with an initial parton density about 30 times of that in a cold heavy nucleus.
Quark and gluon parton dihadron fragmentation functions and their evolution are studied in the process of e+e- annihilation. We provide definitions of such dihadron fragmentation functions in terms of parton matrix elements and derive the momentum sum rules and their connection to single hadron fragmentation functions. We parameterize results from the Lund Monte Carlo model JETSET as the initial conditions for the parton dihadron fragmentation functions at the scale Q_0^2=2 GeV^2. The evolution equations for the quark and gluon fragmentation functions are solved numerically and the results at different higher scales Q^2 agree well with JETSET results. The importance of the input from the single fragmentation functions is pointed out.
We propose the formulation of a dihadron fragmentation function in terms of parton matrix elements. Under the collinear factorization approximation and facilitated by the cut-vertex technique, the two hadron inclusive cross section at leading order (LO) in e+ e- annihilation is shown to factorize into a short distance parton cross section and the long distance dihadron fragmentation function. We also derive the DGLAP evolution equation of this function at leading log. The evolution equation for the non-singlet and singlet quark fragmentation function and the gluon fragmentation function are solved numerically with the initial condition taken from event generators. Modifications to the dihadron fragmentation function from higher twist corrections in DIS off nuclei are computed. Results are presented for cases of physical interest.
We review the physics of nuclear matter at high energy density and the experimental search for the Quark-Gluon Plasma at the Relativistic Heavy Ion Collider (RHIC). The data obtained in the first three years of the RHIC physics program provide several lines of evidence that a novel state of matter has been created in the most violent, head-on collisions of $Au$ nuclei at $\sqrt{s}=200$ GeV. Jet quenching and global measurements show that the initial energy density of the strongly interacting medium generated in the collision is about two orders of magnitude larger than that of cold nuclear matter, well above the critical density for the deconfinement phase transition predicted by lattice QCD. The observed collective flow patterns imply that the system thermalizes early in its evolution, with the dynamics of its expansion consistent with ideal hydrodynamic flow based on a Quark-Gluon Plasma equation of state.
Recent observation of high-$p_T$ hadron spectra suppression and mono-jet production in central $Au+Au$ collisions and their absence in $d+Au$ collisions at RHIC have confirmed the long predicted phenomenon of jet quenching in high-energy heavy-ion collisions. Detailed analyses of the experimental data also show parton energy loss as the mechanism for the discovered jet quenching. Using a pQCD parton model that incorporates medium modified parton fragmentation functions and comparing to experimental data from deeply inelastic scattering off nuclei, one can conclude that the initial gluon (energy) density of the hot matter produced in central $Au+Au$ collisions that causes jet quenching at RHIC is about 30 (100) times higher than in a cold $Au$ nucleus. Combined with data on bulk and collective properties of the hot matter, the observed jet quenching provides strong evidence for the formation of a strongly interacting quark gluon plasma in central $Au+Au$ collisions at RHIC.
Suppression of high $p_T$ hadron spectra in high-energy heavy-ion collisions at different energies is studied within a pQCD parton model incorporating medium induced parton energy loss. The $p_T$ dependence of the nuclear modification factor $R_{AA}(p_T)$ is found to depend on both the energy dependence of the parton energy loss and the power-law behavior of the initial jet spectra. The high $p_T$ hadron suppression at $\sqrt{s}=62.4$ GeV and its centrality dependence are studied in detail. The overall values of the modification factor are found to provide strong constraints on the lifetime of the dense matter.
Dihadron fragmentation functions and their evolution are studied in the process of $e^+e^-$ annihilation. Under the collinear factorization approximation and facilitated by the cut-vertex technique, the two hadron inclusive cross section at leading order (LO) is shown to factorize into a short distance parton cross section and a long distance dihadron fragmentation function. We provide the definition of such a dihadron fragmentation function in terms of parton matrix elements and derive its DGLAP evolution equation at leading log. The evolution equation for the non-singlet quark fragmentation function is solved numerically with a simple ansatz for the initial condition and results are presented for cases of physical interest.
The observed suppression of high-$p_T$ hadron spectra, finite azimuthal anisotropy, disappearance of jet-like back-to-back correlations, and their centrality dependence in $Au+Au$ collisions at RHIC are shown to be quantitatively described by jet quenching within a pQCD parton model. The difference between $h^{\pm}$ and $π^0$ suppression in intermediate $p_T$ is consistent with the observed $(K+p)/π$ enhancement which should disappear at $p_T>6$ GeV/$c$. The suppression of back-to-back correlations is shown to be directly related to the medium modification of jet fragmentation functions(FF).
Multiple scattering, modified fragmentation functions and radiative energy loss of a heavy quark propagating in a nuclear medium are investigated in perturbative QCD. Because of the quark mass dependence of the gluon formation time, the medium size dependence of heavy quark energy loss is found to change from a linear to a quadratic form when the initial energy and momentum scale are increased relative to the quark mass. The radiative energy loss is also significantly suppressed relative to a light quark due to the suppression of collinear gluon emission by a heavy quark.
Significant jet quenching in central $Au+Au$ collisions has been discovered at RHIC. This paper provides theoretical arguments and lists experimental evidence that the observed jet quenching at RHIC is due to parton energy loss instead of hadron rescattering or absorption in a hadronic medium. These include: (1) hadron formation time based on the uncertainty principle, (2) $p_T$ dependence and (3) centrality dependence of the observed jet quenching, (4) jet-like leading hadron correlations (5) high-$p_T$ azimuthal anisotropy and (6) experimental data from $Pb+Pb$ collisions at SPS and $e+A$ collisions. Direct measurements of the parton energy loss in the direction of a triggered high-$p_T$ hadron and the medium modified fragmentation function on the back-side are proposed to further verify the partonic nature of the observed jet quenching. The importance of jet quenching studies at lower energies at RHIC is also discussed.
High-$p_T$ particle spectra in $p+p$ ($\bar p + p$), $p+A$ and $A+B$ collisions are calculated within a QCD parton model in which intrinsic transverse momentum, its broadening due to initial multiple parton scattering, and jet quenching due to parton energy loss inside a dense medium are included phenomenologically. The intrinsic $k_T$ and its broadening in $p+A$ and $A+B$ collisions due to initial multiple parton scattering are found to be very important at low energies ($\sqrt{s}<50$ GeV). Comparisons with $S+S$, $S+Au$ and $Pb+Pb$ data with different centrality cuts show that the differential cross sections of large transverse momentum pion production ($p_T>1$ GeV/$c$) in $A+B$ collisions scale very well with the number of binary nucleon-nucleon collisions (modulo effects of multiple initial scattering). This indicates that semi-hard parton scattering is the dominant particle production mechanism underlying the hadron spectra at moderate $p_T \stackrel{>}{\sim} 1$ GeV/$c$. However, there is no evidence of jet quenching or parton energy loss. Within the parton model, one can exclude an effective parton energy loss $dE_q/dx>0.01$ GeV/fm and a mean free path $λ_q< 7$ fm from the experimental data of $A+B$ collisions at the SPS energies. Predictions for high $p_T$ particle spectra in $p+A$ and $A+A$ collisions with and without jet quenching at the RHIC energy are also given. Uncertainties due to initial multiple scattering and nuclear shadowing of parton distributions are also discussed.
We review recent finite opacity approaches (GLV, WW, WOGZ) to the computation of the induced gluon radiative energy loss and their application to the tomographic studies of the density evolution in ultra-relativistic nuclear collisions.
In contrast to the recent prediction of high $p_T$ hadron suppression within the parton saturation model, it is shown that multiple parton scattering suffered by the projectile will enhance high $p_T$ hadron spectra in $d+A$ collisions relative to a superposition of binary $p+p$ collisions at RHIC. A stronger enhancement in the forward rapidity region of the projectile is also predicted, resulting in a unique rapidity asymmetry of the hadron spectra at high $p_T$. The shape of the rapidity asymmetry should be reversed for low $p_T$ hadrons that are dominated by soft and coherent interactions which suppress hadron spectra in the projectile rapidity region. The phenomenon at the LHC energies is shown to be qualitatively different because of parton shadowing.
Medium modification of jet fragmentation function due to multiple scattering and induced gluon radiation leads directly to jet quenching or suppression of leading particle distribution from jet fragmentation. One can extract an effective total parton energy loss which can be related to the total transverse momentum broadening. For an expanding medium, both are shown to be sensitive to the initial parton density and formation time. Therefore, one can extract the initial parton density and formation time from simultaneous measurements of parton energy loss and transverse momentum broadening. Implication of the recent experimental data on effects of detailed balance in parton energy loss is also discussed.
Multiple parton scattering and induced parton energy loss in deeply inelastic scattering (DIS) off heavy nuclei is studied within the framework of generalized factorization in perturbative QCD with a complete calculation beyond the helicity amplitude (or soft bremsstrahlung) approximation. Such a calculation gives rise to new corrections to the modified quark fragmentation functions. The effective parton energy loss is found to be reduced by a factor of 5/6 from the result of helicity amplitude approximation.
The first order correction to the parton fragmentation functions in a thermal medium is derived in the leading logarithmic approximation in the framework of thermal field theory. The medium-modified evolution equations of the parton fragmentation functions are also derived. It is shown that all infrared divergences, both linear and logarithmic, in the real processes are canceled among themselves and by corresponding virtual corrections. The evolution of the quark number and the energy loss (or gain) induced by the thermal medium are investigated.
Medium modification of parton fragmentation functions induced by multiple scattering and gluon bremsstrahlung is shown to describe the recent HERMES data in deeply inelastic scattering (DIS) very well, providing the first evidence of $A^{2/3}$-dependence of the modification. The energy loss is found to be $ \approx 0.5$ GeV/fm for a 10-GeV quark in a $Au$ nucleus. Including the effect of expansion, analysis of the $π^0$ spectra in central $Au+Au$ collisions at $\sqrt{s}=130$ GeV yields an averaged energy loss equivalent to $ \approx 7.3$ GeV/fm in a static medium. Predictions for central $Au+Au$ collisions at $\sqrt{s}=200$ GeV are also given.
The effect of detailed balance in induced gluon radiation for an energetic parton propagating inside a quark-gluon plasma is discussed. The stimulated thermal gluon emission reduces while absorption increases the parton's energy. The net contribution results in a reduction of the effective parton energy loss. The relative reduction of the parton energy loss is found to be important for intermediate parton energies and negligible for very high energies. This will increase the energy dependence of the parton energy loss and will affect the shape of suppression of moderately high p_T hadron spectra.