Suppression Of High pT Hadrons Research Articles

Recent PHENIX results on high-pT light hadron production

Production of Quark Gluon Plasma (QGP) has been established in central heavy ion collisions (Au+Au, Cu+Cu) at RHIC energies. Observation of strong suppression of hadron yields at high transverse momentum served as one of the most important evidences in favor of production of a new state of matter in such collisions. Recent RHIC run with asymmetric collision system (Cu+Au) provides the means to systematically study suppression pattern of hadrons in different nuclear overlap geometry needed to improve theoretical description of parton energy loss in QGP. Non-zero elliptic flow and a hint of suppression of high pT hadrons suggests that mini-QGP can be formed in collisions of light and heavy nuclei characterized by high charged particle multiplicities. To address the question of collective behavior in small systems RHIC provided series of geometry controlled experiments with highly asymmetric systems (p+Al, p+Au, 3He+Au). The recent results from the PHENIX experiment at RHIC on π0 and η mesons production in asymmetric systems will be presented and discussed.

Transverse momentum spectra and nuclear modification factors of identified charged hadrons in p-Pb and Pb-Pb collisions at √sNN = 5.02 TeV with ALICE

The ALICE experiment has measured the production of identified light-flavour hadrons in p-Pb and Pb-Pb collisions at 5.02 TeV in a wide range of transverse momentum. The newest ALICE results on pion, kaon and proton transverse momentum spectra, yield ratios and nuclear modification factors will be presented and discussed in comparison to lower energy results. In particular, the production of identified hadrons in most central Pb-Pb collisions relative to pp collisions is found to be strongly suppressed at high transverse momenta (pT > 8 GeV/c) whereas in p-Pb collisions the nuclear modification factors are consistent with unity. This indicates that the strong suppression of high-pT hadrons measured in central Pb-Pb collisions is not due to an initial state effect but instead to the energy loss of partons traversing a hot and dense QCD medium.

Energy conservation in high-pT nuclear reactions

The Cronin effect, which is nuclear enhancement of high-pT hadron production in pA collisions was successfully predicted prior the measurements at RHIC and LHC. The restrictions imposed by energy conservation lead to spectacular effects. Energy deficit becomes an issue for hadron production in pA collisions at large xL and/or large xT toward the kinematic bounds xL, T = 1. It leads to a suppression, which has been indeed observed for hadrons produced at forward rapidities and large pT. Intensive energy dissipation via gluon radiation by a highly virtual parton produced with large pT, makes this process impossible to continue long. Color neutralization and creation of a colorless dipole must occur promptly. When this happens inside a hot medium created in AA collisions, attenuation of dipoles, rather than induced energy loss, becomes a dominant mechanism for suppression of high-pT hadrons.

Results on identified particle production in pp, p-Pb and Pb-Pb collisions measured with ALICE at the LHC

Using the unique capabilities of the ALICE detectors for particle identification, different measurements have been performed to study the properties of the hot and dense matter created in the Pb-Pb collisions at sqrt(sNN)= 2.76 TeV. The analysis of the p-Pb data at sqrt(sNN) = 5.02 TeV reveals that the suppression of high pt hadrons observed in heavy nuclei collisions can not be explained as due to initial state effects. The systems created in the p-Pb collisions do not show evidence of jet quenching but, surprisingly, exhibit characteristics of flow. In this paper a review of the main results on identified particle production measured in different colliding systems is presented, data are also compared to models.

Suppression of high pT hadrons in Pb + Pb collisions at 2.76 TeV

The nuclear modification factor RAA(pT) for large pT hadrons in central Pb + Pb collisions at TeV/n is calculated within the next-to-leading order perturbative QCD parton model with medium-modified fragmentation functions and agree well with the new data. The jet transport parameter that controls medium modification is assumed to be proportional to the initial parton density and the coefficient is fixed by the RHIC data. The charged hadron multiplicity dNch/dη = 1584 ± 80 in central Pb + Pb collisions from the ALICE experiment at the LHC is used to determine both the jet transport parameter and the initial condition for (3+1)D ideal hydrodynamic evolution of the bulk matter that is employed for the calculation of RAA(pT).

High-p t and jet physics from RHIC to LHC

The observation of the strong suppression of high pT hadrons in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) at BNL has motivated a large experimental program using hard probes to characterize the deconfined medium created. However what can be denoted as ``leading particle physics'' accessible at RHIC presents some limitations which motivate at higher energy the study of much more penetrating objects: jets. The gain in center of mass energy expected at the Large Hadron Collider (LHC) at CERN will definitively improve our understanding on how the energy is lost in the system opening a new major window of study: the physics of jets on an event-by-event basis. We will concentrate on the expected performance for jet reconstruction in ALICE using the EMCal calorimeter.

Heavy flavor in STAR

The strong suppression of high pT hadrons in central Au+Au collisions observed at RHIC is generally interpreted as being a consequence of the energy loss of energetic partons in the hot and dense matter before fragmenting. The study of heavy quark production is testing our understanding of this scenario.The recent results on heavy flavor measurements from the STAR experiment at RHIC are presented. The total charm production cross section, extracted from d+Au, Au+Au and Cu+Cu collisions at s√S N N = 200 GeV, scales with the number of binary collisions. The high pT non-photonic electron spectra show a strong suppression in central Au+Au collisions. Current theoretical models do not explain this observation satisfactorily. The relative contribution of electrons from bottom decays in the non-photonic electron spectrum predicted in FONLL calculations is consistent with the value determined from azimuthal electron-hadron correlations.The measurement of the T cross section in p+p collision is also presented and is found to be consistent with NLO calculations.

Collisional energy loss and the suppression of high pT hadrons

We calculate nuclear suppression factor ( R AA ) for light hadrons by taking only the elastic processes and argue that in the measured p T domain of RHIC, collisional rather than the radiative processes is the dominant mechanism for partonic energy loss.

Direct photons measured by the PHENIX experiment at RHIC

Results from the PHENIX experiment at RHIC on direct photon production in p+p, d+Au, and Au+Au collisions at $\sqrt{s_{NN}}$ =200 GeV are presented. In p+p collisions, direct photon production at high pT behaves as expected from perturbative QCD calculations. The p+p measurement serves as a baseline for direct photon production in Au+Au collisions. In d+Au collisions, no effects of cold nuclear matter are found within the large uncertainty of the measurement. In Au+Au collisions, the production of high pT direct photons scales as expected for particle production in hard scatterings. This supports jet quenching models, which attribute the suppression of high pT hadrons to the energy loss of fast partons in the medium produced in the collision. Low pT direct photons, measured via e+e- pairs with small invariant mass, are possibly related to the production of thermal direct photons.

Suppression of high-pT non-photonic electrons in Au+Au collisions at $\sqrt{s_{NN}}$ =200 GeV

The strong suppression of high pT hadrons observed at RHIC has led to the interpretation that energetic partons lose their energy via induced gluon radiation in the hot and dense matter before fragmenting into hadrons. The study of heavy quark production can extend our understanding of this scenario. Due to the dead cone effect, the suppression of heavy quark mesons at high pT is expected to be smaller than that observed for charged hadrons at the same energy. The measurement of non-photonic single electrons up to high pT provides information on charm and beauty production. The semi-leptonic decays of D and B mesons are the dominant contribution to the non-photonic electron spectra. The preliminary spectra from p+p, d+Au and Au+Au collisions at $\sqrt{s_{NN}}$ =200 GeV have been extracted for mid-rapidity non-photonic electrons in the range 1.5<pT (GeV/c)<10. The corresponding nuclear modification factors (RAA) are presented and show a large suppression in central Au+Au collisions, indicating an unexpectedly large energy loss for heavy quarks in the hot and dense matter created at RHIC. This observed suppression is compared to recent theoretical models.

Relativistic heavy ion physics: An experimental review

A review of relativistic heavy ion physics is given, with emphasis on selected interesting results. The charged particle multiplicity is compared from measurements at the AGS, SPS, and RHIC. Collective motion due to a build-up of pressure is discussed, with implications on estimates of the energy density of the produced medium. The suppression of J/ψ and the enhancement of low-mass dileptons observed in experiments at the SPS are highlighted. The high p T phenomena at RHIC are discussed in some detail. The suppression of high p T hadrons in central Au+Au collisions is shown, as well as the absence of such a suppression in d+Au collisions. PACS: 25.75.-q Relativistic heavy-ion collisions

Parton saturation and Npart scaling of semi-hard processes in QCD

We argue that the suppression of high pt hadrons discovered recently in heavy ion collisions at RHIC may be a consequence of saturation in the color glass condensate. We qualitatively and semi-quantitatively describe the data, in particular, the dependence upon the number of nucleon participants. We show that if parton saturation sets in at sufficiently small energy, then in nucleus–nucleus collisions at RHIC and LHC energies the cross sections of semi-hard processes should scale approximately with the number of participants, Npart. Our results provide a possible explanation of both the absence of apparent jet quenching at SPS energies and its presence at RHIC. Under the same assumption we predict that in semi-central and central pA (dA) collisions at collider energies the dependence of semi-hard processes on the number of participating nucleons of the nucleus will change to ∼(NpartA)1/2. The forthcoming data on dA collisions will provide a crucial test of this description.