arXiv subjects
Publications and source records attributed to G. Fai.
Calculations of the nuclear modification factor, R_dAu, for pi^0 production in dAu collisions at s_NN^1/2 = 200 GeV are presented. The applied pQCD-improved parton model incorporates intrinsic k_T. Nuclear multiscattering and nuclear shadowing are considered in the Au nucleus. Theoretical results are displayed for midrapidity and high pseudorapidity (eta), and compared to preliminary PHENIX and BRAHMS data.
Recent RHIC results on pion production in AuAu collision at sqrt(s)=130 and 200 AGeV display a strong suppression effect at high p_T. This suppression can be connected to final state effects, namely jet energy loss induced by the produced dense colored matter. Applying our pQCD-based parton model we perform a quantitative analysis of the measured suppression pattern and determine the opacity of the produced deconfined matter.
This manuscript is the outcome of the subgroup ``PDFs, shadowing and $pA$ collisions'' from the CERN workshop ``Hard Probes in Heavy Ion Collisions at the LHC''. In addition to the experimental parameters for $pA$ collisions at the LHC, the issues discussed are factorization in nuclear collisions, nuclear parton distributions (nPDFs), hard probes as the benchmark tests of factorization in $pA$ collisions at the LHC, and semi-hard probes as observables with potentially large nuclear effects. Also, novel QCD phenomena in $pA$ collisions at the LHC are considered. The importance of the $pA$ program at the LHC is emphasized.
We study pion production in proton-proton collisions within a pQCD-improved parton model in next-to-leading order augmented by intrinsic transverse momentum ($k_\perp$) of the partons. We find the introduction of $k_\perp$ necessary to reproduce the experimental data in the CERN SPS to RHIC energy range. We study the influence of intrinsic $k_\perp$ on the so-called $K$ factor, the ratio of the NLO cross section to the Born term. A strong $p_T$ dependence is seen, especially in the 3-6 GeV transverse momentum region of the outgoing pion, where nuclear effects (e.g. the Cronin effect) play an important role.
The influence of nuclear multiscattering and shadowing on pion spectra is investigated in $pA$ collisions from CERN SPS to RHIC energies. The calculations are performed in a next-to-leading order (NLO) pQCD-improved parton model, including intrinsic partonic transverse momentum distributions. The nuclear modification of the pion spectra (Cronin effect) is considered at different targets in a wide energy range. Theoretical predictions are displayed for planned $pA$ experiments at CERN SPS and recent $dAu$ experimental results are analysed at RHIC.
Perturbative QCD results on pion production are presented in proton-proton, proton-nucleus and nucleus-nucleus collisions from CERN SPS up to RHIC energy. A K_{jet}(s, p_T, Q) factor obtained from jet production is applied to perform next-to-leading order calculations. Using the intrinsic transverse momentum (k_T) we determined transverse momentum spectra for pions in wide energy region. We have investigated nuclear multiscattering and the Cronin effect at RHIC energies.
We summarize new pQCD results on pion production in proton-proton (pp), proton-nucleus (pA) and nucleus-nucleus (AA) collisions. Our calculation introduces intrinsic parton transverse momentum (k_T) and is performed effectively at next-to-leading order (NLO), applying a K factor extracted for jet events. Two different factorization scales, Q=p_{T,jet}/2 and p_{T,jet} are used. Experimental data in pA collisions imply a preference for the latter choice at NLO level. We display our results at CERN SPS for AA collisions.
Non-abelian energy loss in quark gluon plasma is shown to lead to novel hadron ratio suppression patterns in ultrarelativistic nuclear collisions. We apply recent (GLV) estimates for the gluon radiative energy loss, which increases linearly with the jet energy up to E<20 GeV and depends quadratically on the nuclear radius, R. The K+/π+ ratio is found to be most sensitive to the initial density of the plasma.
We study, in a pQCD calculation augmented by nuclear effects, the jet energy loss needed to reproduce the pi^0 spectra in Au+Au collisions at large p_T, measured by PHENIX at RHIC. The transverse width of the parton momentum distributions (intrinsic-k_T) is used phenomenologically to obtain a reliable baseline pp result. Jet quenching is applied to the nuclear spectra (including shadowing and multiscattering) to fit the data.
High-p_T pion and kaon production is studied in relativistic proton-proton, proton-nucleus, and nucleus-nucleus collisions in a wide energy range. Cross sections are calculated based on perturbative QCD, augmented by a phenomenological transverse momentum distribution of partons (``intrinsic k_T''). An energy dependent width of the transverse momentum distribution is extracted from pion and charged hadron production data in proton-proton/proton-antiproton collisions. Effects of multiscattering and shadowing in the strongly interacting medium are taken into account. Enhancement of the transverse momentum width is introduced and parameterized to explain the Cronin effect. In collisions between heavy nuclei, the model over-predicts central pion production cross sections (more significantly at higher energies), hinting at the presence of jet quenching. Predictions are made for proton-nucleus and nucleus-nucleus collisions at RHIC energies.
In ultrarelativistic heavy-ion collisions, in the 2 GeV$<p_\perp<$ 6 GeV transverse momentum region, the soft and semi-hard multiple scattering of the incoming nucleons in the nuclear medium results in the broadening of the expected hadronic (e.g. pion) $p_\perp$ spectra relative to proton-proton ($pp$) collisions. Thus, higher transverse-momentum regions are populated than in $pp$ collisions. In a perturbative QCD based calculation we include the intrinsic transverse momentum ($k_\perp$) of the partons in the nucleon (determined from $pp$ collisions), augmented by the extra broadening obtained via a systematic analysis of proton-nucleus ($pA$) collisions in the energy range 17$<\sqrt{s}<$ 39 AGeV. The original polynomial spectra are modified, and a nearly exponential spectrum appears in the region 2$\lesssim p_\perp\lesssim 3.5$ GeV. At present RHIC energies ($\sqrt{s}=$130 AGeV), the slope of the calculated spectra is reminiscent of that of fluid-dynamical descriptions, but lacks any thermal origin. We determine and discuss the size of the modifications originating in multiple scattering, which lead to this state of affairs.
The predicted quenching of jets in A+A at RHIC energies has been discovered by STAR and PHENIX in preliminary data reported at this conference. We apply the GLV theory of QCD radiative energy loss to estimate the opacity, L/λ_g, of the gluon plasma produced in Au+Au collisions at 130 AGeV. We show that (in contrast to the factor of two Cronin enhancement of π^0 found at the SPS by WA98) the factor of 5-8 suppression of p_T \sim 2-4 GeV π^0 reported by PHENIX can be accounted for with an effective static plasma opacity L/λ_g \approx 3-4.
Higher-order pQCD corrections play an important role in the reproduction of data at high transverse momenta in the energy range 20 GeV $ \leq \sqrt{s} \leq 200$ GeV. Recent calculations of photon and pion production in $pp$ collisions yield detailed information on the next-to-leading order contributions. However, the application of these results in proton-nucleus and nucleus-nucleus collisions is not straightforward. The study of nuclear effects requires a simplified understanding of the output of these computations. Here we summarize our analysis of recent calculations, aimed at handling the NLO results by introducing process and energy-dependent $K$ factors.