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Nuclear matter, that takes the form of protons and neutrons under normal conditions, is subject to a phase transition at high temperatures and densities, liberating the quarks and gluons that are usually confined in nucleons and creating a medium of free partons: the Quark-Gluon-Plasma. It is generally believed that this state of matter can be created in relativistic collisions of heavy nuclei. The study of the medium created in these collisions is the subject of heavy-ion physics. One topic within this field are particles with high transverse momentum, that are created in initial hard collisions between partons of the incoming nuclei. The energetic partons lose energy due to interactions with the medium before they fragment into a jet of hadrons. Due to momentum conservation, these jets are usually created as back-to-back pairs, or less commonly as three-jet or photon-jet events, where a single jet is balanced by a hard photon. The energy loss can be measured using correlations between particles with high transverse momenta. A trigger particle is selected with very high transversemomentum and the distribution of the azimuthal angle of associated particles in the same event is studied, relative to the azimuth of the trigger particle.These azimuthal correlations show a peak for opening angles around 0 from particles selected from the same jet, and a second peak at opening angles around 180 degrees from back-to-back di-jets. Random combinations with the underlying event generate a flat background, extending over the full range of opening angles. The STAR experiment observed a modification of these correlations in central Au+Au collisions, where trigger particles with 4GeV < pT(trigger) < 6GeV and associated particles with 2GeV < pT(trigger) < 4GeV were selected. A strong suppression has been observed for away-side correlations in central Au+Au collisions, relative to p+p, d+Au and peripheral Au+Au data. This can be explained by assuming two partons going in opposite directions, where at least one has to travel a large distance through the medium, causing energy loss and effectively removing the event from the analysis. For near-side correlations, no significant modification has been observed, which can be explained by surface emission, assuming that the observed jets have travelled only a short distance in themedium, not leaving enough time for interactions with the medium. Both trigger- and associated particles in a correlation analysis with charged hadrons are subject to modifications due to the medium. This can be avoided by using photon-jet events instead of di-jets, because the photon does not interact with the medium and therefore provides the best available measure of the properties of the opposite jet in the presence of the underlying event. This thesis studies azimuthal correlations between regions of high energy deposition in the electro-magnetic calorimeter as trigger- and charged tracks as associated particles. The data sample had been enriched by online event selection, allowing for the selection of trigger particles with a transverse energy of more than 10GeV and associated particles with more than 2,3 or 4 GeV. The away-side yield per trigger particle is strongly suppressed like in correlations between charged particles. The near-side yield is also reduced by about a factor two, clearly different from charged correlations. The trigger particles are a mixture of photon pairs from the decays of neutral pions and single photons, mainly from photon-jet events, with small contributions from other hadron decays and fragmentation photons. Pythia simulations predict a ratio of neutral pions to prompt photons of 3.5:1 in p+p collisions with the same cuts as in the presented analysis. Single particle suppression further reduces this ratio in central Au_Au collisions, down to about 0.8:1, indicating that the majority of trigger particles in central Au+Au collisions are prompt photons. The increasing fraction of prompt photon triggers without an accompanying jet and therefore zero associated yield reduces the average yield per trigger particle. The magnitude of the observed effect agrees well with the expectation from Pythia simulations and the assumption of a single particle suppression by a factor 4-5. An analysis of away-side correlations is more difficult, because both photon-jet and di-jet events contribute. The aim is the separation of these two contributions. As a clear separation is not possible with the available dataset, a comparison with two different scenarios is given, where a surprisingly small suppression by only a factor of about 5 is favoured for both dijet- and photon-jet-correlations. A separate measurement of both contributions will be possible by a shower-shape analysis with the EM calorimeter or a comparison with charged correlations in the same kinematic region.
Hard physics in STAR
(2005)
The hot and dense matter created in high-energy nuclear collisions is believed to undergo a transition into a deconfined phase where partonic degrees of freedom determine the dynamics of the medium. High-p⊥ partons, that are produced in the initial collisions between nucleons of the incoming nuclei, lose energy as they propagate through the medium. This effect, called jetquenching, is observed in high-p⊥ particle spectra, in azimuthal correlations with the reaction plane (elliptic flow) and jet-like two-particle correlations.
STAR consists of tracking detectors and electromagnetic calorimetry with large and azimuthally symmetric acceptance and is exceptionally well suited for single particle detection and correlation studies at high p⊥. In the last five years, it has collected a large dataset including Au+Au and Cu+Cu collisions at different energies and reference data from p+p and d+Au collisions.
We present particle spectra and two-particle correlations at high-p⊥, and relate these measurements to the properties of the medium.
The ALICE Zero Degree Calorimeter system (ZDC) is composed of two identical sets of calorimeters, placed at opposite sides with respect to the interaction point, 114 meters away from it, complemented by two small forward electromagnetic calorimeters (ZEM). Each set of detectors consists of a neutron (ZN) and a proton (ZP) ZDC. They are placed at zero degrees with respect to the LHC axis and allow to detect particles emitted close to beam direction, in particular neutrons and protons emerging from hadronic heavy-ion collisions (spectator nucleons) and those emitted from electromagnetic processes. For neutrons emitted by these two processes, the ZN calorimeters have nearly 100% acceptance.
During the √sNN = 2.76 TeV Pb-Pb data-taking, the ALICE Collaboration studied forward neutron emission with a dedicated trigger, requiring a minimum energy deposition in at least one of the two ZN. By exploiting also the information of the two ZEM calorimeters it has been possible to separate the contributions of electromagnetic and hadronic processes and to study single neutron vs. multiple neutron emission.
The measured cross sections of single and mutual electromagnetic dissociation of Pb nuclei at √sNN = 2.76 TeV, with neutron emission, are σsingle EMD = 187:4 ± 0.2 (stat.)−11.2+13.2 (syst.) b and σmutual EMD = 5.7 ± 0.1 (stat.) ±0.4 (syst.) b, respectively [1]. This is the first measurement of electromagnetic dissociation of 208Pb nuclei at the LHC energies, allowing a test of electromagnetic dissociation theory in a new energy regime. The experimental results are compared to the predictions from a relativistic electromagnetic dissociation model.
The STAR Collaboration reports the first observation of exclusive rho 0 photoproduction, AuAu-->AuAu rho 0, and rho 0 production accompanied by mutual nuclear Coulomb excitation, AuAu-->Au [star] Au [star] rho 0, in ultraperipheral heavy-ion collisions. The rho 0 have low transverse momenta, consistent with coherent coupling to both nuclei. The cross sections at sqrt[sNN]=130 GeV agree with theoretical predictions treating rho 0 production and Coulomb excitation as independent processes.
We report the first observation of K*(892)0--> pi K in relativistic heavy ion collisions. The transverse momentum spectrum of (K*0+K*0)/2 from central Au+Au collisions at sqrt[sNN]=130 GeV is presented. The ratios of the K*0 yield derived from these data to the yields of negative hadrons, charged kaons, and phi mesons have been measured in central and minimum bias collisions and compared with model predictions and comparable e+e-, pp, and p-barp results. The data indicate no dramatic reduction of K*0 production in relativistic heavy ion collisions despite expected losses due to rescattering effects.
Elliptic flow holds much promise for studying the early-time thermalization attained in ultrarelativistic nuclear collisions. Flow measurements also provide a means of distinguishing between hydrodynamic models and calculations which approach the low density (dilute gas) limit. Among the effects that can complicate the interpretation of elliptic flow measurements are azimuthal correlations that are unrelated to the reaction plane (nonflow correlations). Using data for Au + Au collisions at sqrt[sNN]=130 GeV from the STAR time projection chamber, it is found that four-particle correlation analyses can reliably separate flow and nonflow correlation signals. The latter account for on average about 15% of the observed second-harmonic azimuthal correlation, with the largest relative contribution for the most peripheral and the most central collisions. The results are also corrected for the effect of flow variations within centrality bins. This effect is negligible for all but the most central bin, where the correction to the elliptic flow is about a factor of 2. A simple new method for two-particle flow analysis based on scalar products is described. An analysis based on the distribution of the magnitude of the flow vector is also described.
We report STAR results on the azimuthal anisotropy parameter v2 for strange particles K0S, Lambda , and Lambda -bar at midrapidity in Au+Au collisions at sqrt[sNN]=130 GeV at the Relativistic Heavy Ion Collider. The value of v2 as a function of transverse momentum, pt, of the produced particle and collision centrality is presented for both particles up to pt~3.0 GeV/c. A strong pt dependence in v2 is observed up to 2.0 GeV/c. The v2 measurement is compared with hydrodynamic model calculations. The physics implications of the pt integrated v2 magnitude as a function of particle mass are also discussed.
Azimuthal anisotropy (v2) and two-particle angular correlations of high pT charged hadrons have been measured in Au+Au collisions at sqrt[sNN]=130 GeV for transverse momenta up to 6 GeV/c, where hard processes are expected to contribute significantly. The two-particle angular correlations exhibit elliptic flow and a structure suggestive of fragmentation of high pT partons. The monotonic rise of v2(pT) for pT<2 GeV/c is consistent with collective hydrodynamical flow calculations. At pT>3 GeV/c, a saturation of v2 is observed which persists up to pT=6 GeV/c.
Inclusive transverse momentum distributions of charged hadrons within 0.2<pT<6.0 GeV/c have been measured over a broad range of centrality for Au+Au collisions at sqrt[sNN]=130 GeV. Hadron yields are suppressed at high pT in central collisions relative to peripheral collisions and to a nucleon-nucleon reference scaled for collision geometry. Peripheral collisions are not suppressed relative to the nucleon-nucleon reference. The suppression varies continuously at intermediate centralities. The results indicate significant nuclear medium effects on high-pT hadron production in heavy-ion collisions at high energy.
We report the first measurement of strange ( Lambda ) and antistrange ( Lambda -bar) baryon production from sqrt[sNN]=130 GeV Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC). Rapidity density and transverse mass distributions at midrapidity are presented as a function of centrality. The yield of Lambda and Lambda -bar hyperons is found to be approximately proportional to the number of negative hadrons. The production of Lambda -bar hyperons relative to negative hadrons increases very rapidly with transverse momentum. The magnitude of the increase cannot be described by existing hadronic string fragmentation models alone.