2,582 research outputs found

    Hard physics in STAR

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    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

    Jets and Photons in ALICE

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    ALICE measured transverse momentum spectra of pi0 and eta mesons via the two photon decay in pp collisions at sqrt(s)=0.9, 2.76 and 7 TeV and Pb-Pb collisions at sqrt(sNN)=2.76 TeV. NLO pQCD calculations agree with p-p measurements at 0.9 TeV, but overestimate the data at 2.76 and 7 TeV. The nuclear modification factor for neutral pions shows a strong suppression of high-pt particle production in central Pb-Pb collisions. Raw spectra of charged particle jets have been measured in Pb-Pb collisions. Detailed studies of background fluctuations have been performed and will allow us to unfold the spectra even for low momentum cut offs, giving access to soft fragmentation products in quenched jets.Comment: EPIC@LHC - International Workshop on Early Physics with heavy-Ion Collisions at LH

    Azimuthale Korrelation von Photonen und Hadronen mit hohem Transversalimpuls in Au+Au-Kollisionen bei STAR

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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.Diese Arbeit untersucht azimuthale Korrelationen von Photonen und geladenen Hadronen mit hohem Transversalimpuls in Kollisionen von zwei Goldkernen bei einer Schwerpunktsenergie von sgrt(sNN) = 200GeV. Die Daten wurden mit dem STAR-Experiment am Relativistic Heavy Ion Collider (RHIC) aufgezeichnet. Kernmaterie, die unter normalen Bedingungen in Form von Protonen und Neutronen vorkommt, durchlĂ€uft bei hohen Temperaturen und Dichten einen PhasenĂŒbergang, bei dem die in den Nukleonen enthaltenen Quarks und Gluonen freigesetzt werden und ein Medium aus freien Partonen entsteht, das Quark-Gluon-Plasma. Man geht davon aus, dass man diese Form von Materie im Labor in relativistischen Kollisionen schwerer Kerne erzeugen kann. Das in Schwerionenkollisionen erzeugte Medium ist Gegenstand der Schwerionenphysik. Ein Teilbereich beschĂ€ftigt sich mit der Teilchen mit hohem Transversalimpuls, die nur in harten Kollisionen von Partonen der einfliegenden Kerne erzeugt werden. Aufgrund der Impulserhaltung werden diese Partonen meist paarweise produziert, seltener zu dritt oder als Parton-Photon-Paar. Durch Wechselwirkungen mit dem Medium verlieren die so erzeugten Partonen Energie, bevor sie das Medium verlassen und in einen Jet fragmentieren. Dieser Effekt kann anhand von Korrelationen zwischen Teilchen mit hohem Transversalimpuls untersucht werden. Man wĂ€hlt ein Triggerteilchen mit hohem Transversalimpuls aus und betrachtet dann die Verteilung des azimuthalen Öffnungswinkels anderer, assoziierter Teilchen aus dem selben Ereignis. Diese azimuthalenKorrelationen zeigen eine HĂ€ufung bei kleinen Winkeln aufgrund von Teilchen aus dem selben Jet und eine zweite HĂ€ufung bei großen Winkeln um 180 Grad, aufgrund von Teilchen aus gegenĂŒberliegenden Dijets. ZufĂ€llige Kombinationen erzeugen einen flachen Untergrund ĂŒber den gesamten Winkelbereich. Diese Analyse wurde zuerst von STAR mit Triggerteilchen mit Transversalimpuls von 4-6 GeV/c und assoziierten Teilchen mit 2-4 GeV/c durchgefĂŒhrt. WĂ€hrend diesseitige Korrelationen in p+p, peripheren und zentralen Au+Au-Kollisionen nahezu unverĂ€ndert bleiben, sind die jenseitigen Korrelationen in zentralen Kollisionen stark unterdrĂŒckt. Dies wird damit erklĂ€rt, dass im Fall zweier, in entgegengesetzte Richtungen laufender Partonen mindestens eines eine lange Wegstrecke im Medium zurĂŒckzulegen hat, dadurch Energie verliert und letztlich nicht mehr in die Korrelationsanalyse eingeht. Die Abwesenheit von VerĂ€nderungen diesseitiger Korrelationen wird so erklĂ€rt, dass durch die Wahl des Triggerteilchens nur Jets selektiert werden, die nahe der OberflĂ€che des Mediums entstanden sind und deshalb kaum modifiziert wurden. Bei dieser Analyse wechselwirken Trigger- und assoziierte Teilchen mit dem Medium, so dass die Modifikation eines einzelnen Jets nicht direkt messbar ist. Dies kann durch die Analyse von Photon-Jet-Ereignissen umgangen werden, bei denen dem Parton ein hartes Photon gegenĂŒbersteht. Das Photon reagiert nicht mit dem Medium und liefert deshalb die beste Messung der Energie des gegenĂŒberliegenden Jets, die in einer Au+Au-Kollision möglich ist. Diese Arbeit untersucht azimuthale Korrelationen zwischen neutralen Triggerteilchen, die mit dem elektromagnetische Kalorimeter gemessen wurden, und geladenen assoziierten Teilchen aus der Spurendriftkammer. Durch Online-Ereignisselektion konnten Triggerteilchen mit Transversalimpulsen von mehr als 10 GeV/c und assoziierte Teilchen mit mehr als 2,3 oder 4 GeV/c untersucht werden. Die Korrelationen zeigen diesseitige Korrelationen von Teilchen aus dem selben Jet und jenseitige Korrelationen aus gegenĂŒberliegenden Jets oder Photon-Jet-Ereignissen. Wie bei geladenen Teilchen sind jenseitige Korrelationen in zentralen Au+Au-Kollisionen stark unterdrĂŒckt, aufgrund des geringeren Untergrundes allerdings noch klar sichtbar. Neu ist eine UnterdrĂŒckung von Korrelationen mit kleinem Öffnungswinkel um etwa die HĂ€lfte. Die Triggerteilchen sind Photonpaare mit kleinem Öffnungswinkel aus dem Zerfall neutraler Pionen oder direkte Photonen aus Photon-Jet-Ereignissen, mit kleineren BeitrĂ€gen aus anderen Prozessen. Pythia-Simulationen ergeben ein VerhĂ€ltnis von Pionen zu direkten Photonen von 3,5:1 in p+p-Kollisionen. Durch die HadronunterdrĂŒckung bei hohem Transversalimpuls verschiebt sich das VerhĂ€ltnis in zentralen Au+Au-Kollisionen so, dass etwa 60% der Triggerteilchen aus Photon-Jet-Ereignissen stammen. Diese Reduktion des Anteils neutraler Pionen von 80% auf 40% geht geht einher mit einer Verringerung der mittleren Zahl assoziierter Teilchen bei kleinem Öffnungswinkel um 50%, wie dies beobachtet wurde. Bei jenseitigen Korrelation ĂŒberlagern sich Dijet- und Photon-Jet-Anteile. Ziel ist eine separate Bestimmung der UnterdrĂŒckung dieser Anteile. Da eine Unterscheidung mit den analysierten Daten nicht möglich ist, konnten die Daten nur mit zwei möglichen Szenarien verglichen werden, wobei eine ĂŒberraschend geringe UnterdrĂŒckung von sowohl Dijet- als auch Photon-Jet-Korrelation um nur einen Faktor von etwa 5 favorisiert wird. Eine separate Bestimmung der beiden Anteile wird durch Vergleich mit Korrelationen geladener Teilchen in diesem kinematischen Bereich, sowie durch die Messung des Profils der Schauer im Kalorimeter möglich sein

    Elliptic flow from two- and four-particle correlations in Au+Au collisions at sqrt[sNN]=130 GeV

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    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.alle Autoren: C. Adler11, Z. Ahammed23, C. Allgower12, J. Amonett14, B. D. Anderson14, M. Anderson5, G. S. Averichev9, J. Balewski12, O. Barannikova9,23, L. S. Barnby14, J. Baudot13, S. Bekele20, V. V. Belaga9, R. Bellwied31, J. Berger11, H. Bichsel30, A. Billmeier31, L. C. Bland2, C. O. Blyth3, B. E. Bonner24, A. Boucham26, A. Brandin18, A. Bravar2, R. V. Cadman1, H. Caines33, M. CalderĂłn de la Barca SĂĄnchez2, A. Cardenas23, J. Carroll15, J. Castillo26, M. Castro31, D. Cebra5, P. Chaloupka20, S. Chattopadhyay31, Y. Chen6, S. P. Chernenko9, M. Cherney8, A. Chikanian33, B. Choi28, W. Christie2, J. P. Coffin13, T. M. Cormier31, J. G. Cramer30, H. J. Crawford4, W. S. Deng2, A. A. Derevschikov22, L. Didenko2, T. Dietel11, J. E. Draper5, V. B. Dunin9, J. C. Dunlop33, V. Eckardt16, L. G. Efimov9, V. Emelianov18, J. Engelage4, G. Eppley24, B. Erazmus26, P. Fachini2, V. Faine2, K. Filimonov15, E. Finch33, Y. Fisyak2, D. Flierl11, K. J. Foley2, J. Fu15,32, C. A. Gagliardi27, N. Gagunashvili9, J. Gans33, L. Gaudichet26, M. Germain13, F. Geurts24, V. Ghazikhanian6, O. Grachov31, V. Grigoriev18, M. Guedon13, E. Gushin18, T. J. Hallman2, D. Hardtke15, J. W. Harris33, T. W. Henry27, S. Heppelmann21, T. Herston23, B. Hippolyte13, A. Hirsch23, E. Hjort15, G. W. Hoffmann28, M. Horsley33, H. Z. Huang6, T. J. Humanic20, G. Igo6, A. Ishihara28, Yu. I. Ivanshin10, P. Jacobs15, W. W. Jacobs12, M. Janik29, I. Johnson15, P. G. Jones3, E. G. Judd4, M. Kaneta15, M. Kaplan7, D. Keane14, J. Kiryluk6, A. Kisiel29, J. Klay15, S. R. Klein15, A. Klyachko12, A. S. Konstantinov22, M. Kopytine14, L. Kotchenda18, A. D. Kovalenko9, M. Kramer19, P. Kravtsov18, K. Krueger1, C. Kuhn13, A. I. Kulikov9, G. J. Kunde33, C. L. Kunz7, R. Kh. Kutuev10, A. A. Kuznetsov9, L. Lakehal-Ayat26, M. A. C. Lamont3, J. M. Landgraf2, S. Lange11, C. P. Lansdell28, B. Lasiuk33, F. Laue2, A. Lebedev2, R. LednickĂœ9, V. M. Leontiev22, M. J. LeVine2, Q. Li31, S. J. Lindenbaum19, M. A. Lisa20, F. Liu32, L. Liu32, Z. Liu32, Q. J. Liu30, T. Ljubicic2, W. J. Llope24, G. LoCurto16, H. Long6, R. S. Longacre2, M. Lopez-Noriega20, W. A. Love2, T. Ludlam2, D. Lynn2, J. Ma6, R. Majka33, S. Margetis14, C. Markert33, L. Martin26, J. Marx15, H. S. Matis15, Yu. A. Matulenko22, T. S. McShane8, F. Meissner15, Yu. Melnick22, A. Meschanin22, M. Messer2, M. L. Miller33, Z. Milosevich7, N. G. Minaev22, J. Mitchell24, V. A. Moiseenko10, C. F. Moore28, V. Morozov15, M. M. de Moura31, M. G. Munhoz25, J. M. Nelson3, P. Nevski2, V. A. Nikitin10, L. V. Nogach22, B. Norman14, S. B. Nurushev22, G. Odyniec15, A. Ogawa21, V. Okorokov18, M. Oldenburg16, D. Olson15, G. Paic20, S. U. Pandey31, Y. Panebratsev9, S. Y. Panitkin2, A. I. Pavlinov31, T. Pawlak29, V. Perevoztchikov2, W. Peryt29, V. A. Petrov10, M. Planinic12, J. Pluta29, N. Porile23, J. Porter2, A. M. Poskanzer15, E. Potrebenikova9, D. Prindle30, C. Pruneau31, J. Putschke16, G. Rai15, G. Rakness12, O. Ravel26, R. L. Ray28, S. V. Razin9,12, D. Reichhold8, J. G. Reid30, G. Renault26, F. Retiere15, A. Ridiger18, H. G. Ritter15, J. B. Roberts24, O. V. Rogachevski9, J. L. Romero5, A. Rose31, C. Roy26, V. Rykov31, I. Sakrejda15, S. Salur33, J. Sandweiss33, A. C. Saulys2, I. Savin10, J. Schambach28, R. P. Scharenberg23, N. Schmitz16, L. S. Schroeder15, A. SchĂŒttauf16, K. Schweda15, J. Seger8, D. Seliverstov18, P. Seyboth16, E. Shahaliev9, K. E. Shestermanov22, S. S. Shimanskii9, V. S. Shvetcov10, G. Skoro9, N. Smirnov33, R. Snellings15, P. Sorensen6, J. Sowinski12, H. M. Spinka1, B. Srivastava23, E. J. Stephenson12, R. Stock11, A. Stolpovsky31, M. Strikhanov18, B. Stringfellow23, C. Struck11, A. A. P. Suaide31, E. Sugarbaker20, C. Suire2, M. Sumbera20, B. Surrow2, T. J. M. Symons15, A. Szanto de Toledo25, P. Szarwas29, A. Tai6, J. Takahashi25, A. H. Tang14, J. H. Thomas15, M. Thompson3, V. Tikhomirov18, M. Tokarev9, M. B. Tonjes17, T. A. Trainor30, S. Trentalange6, R. E. Tribble27, V. Trofimov18, O. Tsai6, T. Ullrich2, D. G. Underwood1, G. Van Buren2, A. M. VanderMolen17, I. M. Vasilevski10, A. N. Vasiliev22, S. E. Vigdor12, S. A. Voloshin31, F. Wang23, H. Ward28, J. W. Watson14, R. Wells20, G. D. Westfall17, C. Whitten, Jr.6, H. Wieman15, R. Willson20, S. W. Wissink12, R. Witt33, J. Wood6, N. Xu15, Z. Xu2, A. E. Yakutin22, E. Yamamoto15, J. Yang6, P. Yepes24, V. I. Yurevich9, Y. V. Zanevski9, I. ZborovskĂœ9, H. Zhang33, W. M. Zhang14, R. Zoulkarneev10, and A. N. Zubarev

    Open charm yields in d+Au collisions at sqrt[sNN]=200 GeV

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    Midrapidity open charm spectra from direct reconstruction of D0(D0-bar)-->K± pi ± in d+Au collisions and indirect electron-positron measurements via charm semileptonic decays in p+p and d+Au collisions at sqrt[sNN]=200 GeV are reported. The D0(D0-bar) spectrum covers a transverse momentum (pT) range of 0.1<pT<3 GeV/c, whereas the electron spectra cover a range of 1<pT<4 GeV/c. The electron spectra show approximate binary collision scaling between p+p and d+Au collisions. From these two independent analyses, the differential cross section per nucleon-nucleon binary interaction at midrapidity for open charm production from d+Au collisions at BNL RHIC is d sigma NNcc-bar/dy=0.30±0.04(stat)±0.09(syst) mb. The results are compared to theoretical calculations. Implications for charmonium results in A+A collisions are discussed

    Pseudorapidity asymmetry and centrality dependence of charged hadron spectra in d+Au collisions at sqrt[sNN ]=200 GeV

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    The pseudorapidity asymmetry and centrality dependence of charged hadron spectra in d+Au collisions at sqrt[sNN ]=200 GeV are presented. The charged particle density at midrapidity, its pseudorapidity asymmetry, and centrality dependence are reasonably reproduced by a multiphase transport model, by HIJING, and by the latest calculations in a saturation model. Ratios of transverse momentum spectra between backward and forward pseudorapidity are above unity for pT below 5 GeV/c . The ratio of central to peripheral spectra in d+Au collisions shows enhancement at 2< pT <6 GeV/c , with a larger effect at backward rapidity than forward rapidity. Our measurements are in qualitative agreement with gluon saturation and in contrast to calculations based on incoherent multiple partonic scatterings

    Measurements of transverse energy distributions in Au+Au collisions at sqrt [sNN ]=200 GeV

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    Transverse energy ( ET ) distributions have been measured for Au+Au collisions at sqrt[sNN ]=200 GeV by the STAR Collaboration at RHIC. ET is constructed from its hadronic and electromagnetic components, which have been measured separately. ET production for the most central collisions is well described by several theoretical models whose common feature is large energy density achieved early in the fireball evolution. The magnitude and centrality dependence of ET per charged particle agrees well with measurements at lower collision energy, indicating that the growth in ET for larger collision energy results from the growth in particle production. The electromagnetic fraction of the total ET is consistent with a final state dominated by mesons and independent of centrality

    K(892)* resonance production in Au+Au and p+p collisions at sqrt[sNN]=200GeV

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    The short-lived K(892)* resonance provides an efficient tool to probe properties of the hot and dense medium produced in relativistic heavy-ion collisions. We report measurements of K* in sqrt[sNN]=200GeV Au+Au and p+p collisions reconstructed via its hadronic decay channels K(892)*0-->K pi and K(892)*±-->K0S pi ± using the STAR detector at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The K*0 mass has been studied as a function of pT in minimum bias p+p and central Au+Au collisions. The K*pT spectra for minimum bias p+p interactions and for Au+Au collisions in different centralities are presented. The K*/K yield ratios for all centralities in Au+Au collisions are found to be significantly lower than the ratio in minimum bias p+p collisions, indicating the importance of hadronic interactions between chemical and kinetic freeze-outs. A significant nonzero K*0 elliptic flow (v2) is observed in Au+Au collisions and is compared to the K0S and Lambda v2. The nuclear modification factor of K* at intermediate pT is similar to that of K0S but different from Lambda . This establishes a baryon-meson effect over a mass effect in the particle production at intermediate pT (2<pT <= 4GeV/c)