Heavy-ion collisions - hot QCD in a lab

High-energy heavy-ion collisions provide a unique opportunity to study the properties of the hot and dense strongly-interacting system composed of deconfined quarks and gluons -- the quark-gluon plasma (QGP) -- in laboratory conditions. The formation of a QGP is predicted by lattice QCD calculations as a crossover transition from hadronic matter (at zero baryochemical potential) and is expected to take place once the system temperature reaches values above 155 MeV and/or the energy density above $0.5~\mathrm{GeV}/\mathrm{fm}^{3}$. The nature of such a strongly coupled QGP has been linked to the early Universe at some microseconds after the Big Bang. To characterize the physical properties of the short-lived matter (lifetime of about $10~\mathrm{fm}/c$) experimental studies at Relativistic Heavy-Ion Collider and the Large Hadron collider use auto-generated probes, such as high-energy partons created early in the hadronic collisions, thermally emitted photons, and a set of particle correlations that are sensitive to the collective expansion and the dynamics of the system. The lectures briefly introduced some of the experimental techniques and provided a glimpse at some of the results.Comment: Proceedings of the XIV International Workshop on Hadron Physics, Florianopolis, Brazil, March 201

Two-particle azimuthal correlations at high transverse momentum in Pb - Au at 158 AGeV/c

The study of two-particle azimuthal correlations at high transverse momentum has become an important tool to investigate the interaction of hard partons with the medium formed in high-energy nucleus-nucleus collisions. At SPS energies, pioneering studies by the CERES collaboration indicated a significant modification of the away-side structure in central collisions. Here we present new results emerging from the analysis of the year 2000 data set recorded with the CERES Time-Projection Chamber, which provides excellent tracking efficiency and significantly improved momentum determination

Two particle azimuthal correlations at high transverse momentum in Pb - Au at 158 AGeV/c

The analysis of two-particle azimuthal angular correlations at high transverse momentum from Pb+Au collisions at 158 AGeV/c at SPS reveals substantial modifications of the away-side peak as compared to the distributions from p+p reactions. The data recorded with the CERES Time-Projection Chamber, which provides excellent tracking efficiency, suggest that the observed modification of the back-to-back structure implies significant re-interactions of the scattered partons within the medium. These findings at top SPS energy show features qualitatively similar to the results obtained at RHIC from Au+Au collisions at $\sqrt{s} = 200$ GeV/c and Cu+Cu collisions at $\sqrt{s} = 62$ GeV/c. We present the centrality and charge dependent conditional di-jet yields for similar transverse momentum windows as investigated by the RHIC experimentsThe analysis of two-particle azimuthal angular correlations at high transverse momentum from Pb+Au collisions at 158 AGeV/c at SPS reveals substantial modifications of the away-side peak as compared to the distributions from p+p reactions. The data recorded with the CERES Time-Projection Chamber, which provides excellent tracking efficiency, suggest that the observed modification of the back-to-back structure implies significant re-interactions of the scattered partons within the medium. These findings at top SPS energy show features qualitatively similar to the results obtained at RHIC from Au+Au collisions at s = 200  GeV / c and Cu+Cu collisions at s = 62  GeV / c [S.S. Adler et al., PHENIX Collaboration, arXiv:nucl-ex/0507004 , M. McCumber and J. Frantz, arXiv:nucl-ex/0511048 ]. We present the centrality and charge dependent conditional di-jet yields for similar transverse momentum windows as investigated by the RHIC experiments [S.S. Adler et al., PHENIX Collaboration, arXiv:nucl-ex/0507004 , M. McCumber and J. Frantz, arXiv:nucl-ex/0511048 , J. Adams et al., STAR Collaboration, Phys. Rev. Lett. 95 (2005) 152301, arXiv:nucl-ex/0501016 ]

Identifying groomed jet splittings in heavy-ion collisions

Measurements of jet substructure in heavy-ion collisions may provide key insight to the nature of jet quenching in the quark-gluon plasma. Jet grooming techniques from high-energy physics have been applied to heavy-ion collisions in order to isolate theoretically controlled jet observables and explore possible modification to the hard substructure of jets. However, the grooming algorithms used have not been tailored to the unique considerations of heavy-ion collisions, in particular to the experimental challenge of reconstructing jets in the presence of a large underlying event. We report a set of simple studies illustrating the impact of the underlying event on identifying groomed jet splittings in heavy-ion collisions, and on associated groomed jet observables. We illustrate the importance of the selection of grooming algorithm, as certain groomers are more robust to these effects, while others, including those commonly used in heavy-ion collisions, are susceptible to large background effects -- which, when uncontrolled, can mimic a jet quenching signal. These experimental considerations, along with appropriate theoretical motivation, provide input to the choice of grooming algorithms employed in heavy-ion collisions.Comment: Final version after journal revie

Novel subjet observables for jet quenching in heavy-ion collisions

Using a novel observable that relies on the momentum difference of the two most energetic subjets within a jet $\Delta S_{12}$ we study the internal structure of high-energy jets simulated by several Monte Carlo event generators that implement the partonic energy-loss in a dense partonic medium. Based on inclusive jet and di-jet production we demonstrate that $\Delta S_{12}$ is an effective tool to discriminate between different models of jet modifications over a broad kinematic range. The new quantity, while preserving the colinear and infrared safety of modern jet algorithms, it is experimentally attractive because of its inherent resiliance against backgrounds of heavy-ion collisions.Comment: v1: 10 pages. v2: Includes (i) additional discussion about best discriminant by calculating the RSD (ii) new section about hadronization effects on the reconstructed subjets; version to be published in European Physical Journal

Two- and three-particle azimuthal correlations of high-pt charged hadrons in Pb-Au collisions at 158A GeV/c

Azimuthal correlations of hadrons with high transverse momenta serve as a measure to study the energy loss and the fragmentation pattern of jets emerging from hard parton-parton interactions in heavy ion collisions. Preliminary results from the CERES experiment on two- and three-particle correlations in central Pb-Au collisions are presented. A strongly non-Gaussian shape on the away-side of the two-particle correlation function is observed, indicating significant interactions of the emerging partons with the medium. Mechanisms like deflection of the initial partons or the evolution of a mach cone in the medium can lead to similar modifications of the jet structure on the away-side. An analysis based on three-particle correlations is presented which helps to shed light on the origin of the observed away-side pattern.Comment: 4 pages, 2 figures, contribution to the Quark Matter conference 200

Azimuthale Winkelverteilung von K+- und K-- Mesonen in Au+Au-Kollisionen bei einer Kinetische-Strahl-Energie von 1.5 AGeV

The Kaon-Spectrometer (KaoS) at the heavy-ion synchrotron (SIS) at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt has been used to study production and propagation of K+ and K- mesons from Au+Au collisions at a kinetic beam energy of 1.5 AGeV. This energy for K+ mesons is close to the corresponding production threshold in binary nucleon-nucleon collisions and far below for K- mesons. The azimuthal angular distributions of particles as a function of the collision centrality and particle transverse momenta have been measured. The properties of strange mesons are expected to be modified by the in-medium meson-baryon potential. Theoretical calculations show that the superposition of the scalar and vector potentials leads to a small repulsive K+N and a strong attractive K-N potential. Additionally, the interaction of kaons and antikaons with nuclear matter is different. The strangeness conservation law inhibits the absorption probability of K+ mesons as they contain an s-quark. K- mesons, however, interact with nucleons via strangenessexchange (K- + N ->Y + pion, where Y = lambda, sigma). Moreover, the reverse process (pion + Y -> K- + N) is the dominant production mechanism of K- mesons at SIS energies. The azimuthal angular emission patterns of kaons are expected to be sensitive to the in-medium potentials. An enhanced out-of-plane emission of K+ mesons was observed in Au+Au reactions at 1.0 AGeV and 1.5 AGeV, and also in Ni+Ni at 1.93 AGeV. The out-of-plane emission of K+ mesons in Au+Au reactions at 1.0 AGeV was interpreted as a consequence of a repulsive K+N potential in the nuclear medium, however, recent transport calculations show that the emission patterns obtained in Au+Au at 1.5 AGeV and Ni+Ni at 1.93 AGeV are additionally influenced by the re-scattering of kaons. For K- mesons the calculations predict an almost isotropic emission pattern due to the attractive K-N potential which counteracts the absorption of K- mesons in the spectator fragments. In Ni+Ni collisions at 1.93 AGeV the azimuthal distribution of K- mesons has been found to be isotropic. In this case, however, the spectators are rather small and have large relative velocities. In addition, the delay of antikaon emission due to strangenessexchange reaction minimizes the interaction with the spectators. As a consequence the sensitivity of the K- meson emission pattern to the K-N in-medium potential is reduced. In Au+Au collisions we found a dependence of the K- meson azimuthal emission pattern on the transverse momentum. The antikaons registered with pt 0.5 GeV/c show strong out-of-plane enhancement. The emission patterns of K- can be explained in terms of two competing phenomena: one of them is indeed the influence of the attractive K-N potential, however, the second one originates from the strangeness-exchange process.Mit dem Kaonen-Spectrometer (KaoS) am Schwerionen-Synchrotron (SIS) der Gesellschaft für Schwerionenforschung (GSI) in Darmstadt wurde die Produktion und Propagation von K+ und K- Mesonen in Au+Au Kollisionen bei einer kinetischen Strahlenergie von 1,5 AGeV untersucht. Diese Energie liegt nahe der Produktionsschwelle für K+ Mesonen in binären Nukleon-Nukleon Kollisionen und weit unter derjenigen für K- Mesonen. Die azimutalen Winkelverteilungen der Teilchen als Funktion der Stoßzentralität sowie ihre Transversalimpulsverteilungen wurden gemessen. Für Mesonen mit Strangeness wird eine Modifikation ihrer Eigenschaften durch Meson-Baryon-Potentiale in Materie erwartet. Theoretische Rechnungen zeigen, daß die Überlagerung des skalaren und des Vektorpotentials zu einem leicht repulsiven K+N und einem stark attraktiven K-N Potential führen. Desweiteren ist die Wechselwirkung von Kaonen und Antikaonen mit Kernmaterie unterschiedlich. Die Erhaltung der Strangeness vermindert die Absorptionswahrscheinlichkeit für K+ Mesonen, da diese ein s-quark enthalten. K- Mesonen dagegen wechselwirken mit Nukleonen durch den strangenessexchange Kanal (K- + N ->Y + pion, mit Y = lambda, sigma). Gleichzeitig ist der umgekehrte Prozeß (pion + Y -> K- + N) der dominante Produktionsmechanismus für K- Mesonen bei SIS Energien. Es wird erwartet, daß die azimutale Verteilung der Kaonenemission sensitiv auf In-Medium Potentiale ist. Eine bevorzugte Emission senkrecht zur Reaktionsebene wurde für K+ Mesonen in Au+Au Reaktionen bei 1,0 AGeV und 1,5 AGeV sowie in Ni+Ni-Reaktionen bei 1,93 AGeV beobachtet. In Au+Au Reaktionen bei 1,0 AGeV wurde dies als Konsequenz eines repulsiven K+N Potentials in Kernmaterie interpretiert. Neuere Transportmodellrechnungen zeigen allerdings, daß die azimutalen Verteilungen in Au+Au bei 1,5 AGeV und in Ni+Ni St¨oßen bei 1,93 AGeV zusätzlich durch die elastische Streuung der Kaonen beeinflußt werden. Für K- Mesonen sagen die Rechnungen eine im wesentlichen isotrope Verteilung voraus, bedingt durch das attraktive K-N Potential, welches der Absorption der K- Mesonen in den Spektator Fragmenten entgegenwirkt. In Ni+Ni Kollisionen bei 1,93 AGeV wurde eine isotrope azimutale Verteilung der K- Mesonen gefunden. In diesem Fall sind die Spektatoren allerdings relativ klein und haben große Relativgeschwindigkeiten. Zusätzlich wird die Wechselwirkung der Antikaonen mit den Spektatoren durch ihre späte Emission verringert. Als Kosequenz hiervon ist die Sensitivität der azimutalen Verteilung von K- Mesonen auf K-N in-Medium Potentiale reduziert. In Au+Au Kollisionen wurde eine Abhängigkeit der azimutalen Verteilung der K-Emission vom Transversalimpuls beobachtet. Antikaonen mit pt 0, 5 GeV/c zeigen dagegen eine stark bevorzugte Emission senkrecht zur Reaktionsebene. Dieses Emissionsverhalten der K- kann durch zwei konkurrierende Phänomene erklärt werden: Eines davon ist tatsächlich der Einfluß des attraktiven K-N Potentials, das andere dagegen basiert auf dem strangeness-exchange Prozeß

Data-driven analysis methods for the measurement of reconstructed jets in heavy ion collisions at RHIC and LHC

We present data-driven methods for the full reconstruction of jets in heavy ion collisions, for inclusive and co-incidence jet measurements at both RHIC and LHC. The complex structure of heavy ion events generates a large background of combinatorial jets, and smears the measured energy of the true hard jet signal. Techniques to correct for these background effects can induce biases in the reported jet distributions, which must be well controlled for accurate measurement of jet quenching. Using model studies, we evaluate the proposed methods for measuring jet distributions accurately while minimizing the fragmentation bias of the measured population.Comment: 5 pages, 14 figures, Hard Probe 2012 Conference Proceedin

The Hot QCD White Paper: Exploring the Phases of QCD at RHIC and the LHC

The past decade has seen huge advances in experimental measurements made in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) and more recently at the Large Hadron Collider (LHC). These new data, in combination with theoretical advances from calculations made in a variety of frameworks, have led to a broad and deep knowledge of the properties of thermal QCD matter. Increasingly quantitative descriptions of the quark-gluon plasma (QGP) created in these collisions have established that the QGP is a strongly coupled liquid with the lowest value of specific viscosity ever measured. However, much remains to be learned about the precise nature of the initial state from which this liquid forms, how its properties vary across its phase diagram and how, at a microscopic level, the collective properties of this liquid emerge from the interactions among the individual quarks and gluons that must be visible if the liquid is probed with sufficiently high resolution. This white paper, prepared by the Hot QCD Writing Group as part of the U.S. Long Range Plan for Nuclear Physics, reviews the recent progress in the field of hot QCD and outlines the scientific opportunities in the next decade for resolving the outstanding issues in the field.Comment: 110 pages, 33 figures, 429 references. Prepared as part of the U.S. Long-Range Plan for Nuclear Physic