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

Azimuthal anisotropy of charged jet production in root s(NN)=2.76 TeV Pb-Pb collisions

We present measurements of the azimuthal dependence of charged jet production in central and semi-central root s(NN) = 2.76 TeV Pb-Pb collisions with respect to the second harmonic event plane, quantified as nu(ch)(2) (jet). Jet finding is performed employing the anti-k(T) algorithm with a resolution parameter R = 0.2 using charged tracks from the ALICE tracking system. The contribution of the azimuthal anisotropy of the underlying event is taken into account event-by-event. The remaining (statistical) region-to-region fluctuations are removed on an ensemble basis by unfolding the jet spectra for different event plane orientations independently. Significant non-zero nu(ch)(2) (jet) is observed in semi-central collisions (30-50% centrality) for 20 <p(T)(ch) (jet) <90 GeV/c. The azimuthal dependence of the charged jet production is similar to the dependence observed for jets comprising both charged and neutral fragments, and compatible with measurements of the nu(2) of single charged particles at high p(T). Good agreement between the data and predictions from JEWEL, an event generator simulating parton shower evolution in the presence of a dense QCD medium, is found in semi-central collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Production of He-4 and (4) in Pb-Pb collisions at root(NN)-N-S=2.76 TeV at the LHC

Results on the production of He-4 and (4) nuclei in Pb-Pb collisions at root(NN)-N-S = 2.76 TeV in the rapidity range vertical bar y vertical bar <1, using the ALICE detector, are presented in this paper. The rapidity densities corresponding to 0-10% central events are found to be dN/dy4(He) = (0.8 +/- 0.4 (stat) +/- 0.3 (syst)) x 10(-6) and dN/dy4 = (1.1 +/- 0.4 (stat) +/- 0.2 (syst)) x 10(-6), respectively. This is in agreement with the statistical thermal model expectation assuming the same chemical freeze-out temperature (T-chem = 156 MeV) as for light hadrons. The measured ratio of (4)/He-4 is 1.4 +/- 0.8 (stat) +/- 0.5 (syst). (C) 2018 Published by Elsevier B.V.Peer reviewe

Forward-central two-particle correlations in p-Pb collisions at root s(NN)=5.02 TeV

Two-particle angular correlations between trigger particles in the forward pseudorapidity range (2.5 2GeV/c. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B. V.Peer reviewe

Event-shape engineering for inclusive spectra and elliptic flow in Pb-Pb collisions at root(NN)-N-S=2.76 TeV

Peer reviewe

Pseudorapidity and transverse-momentum distributions of charged particles in proton-proton collisions at root s=13 TeV

The pseudorapidity (eta) and transverse-momentum (p(T)) distributions of charged particles produced in proton-proton collisions are measured at the centre-of-mass energy root s = 13 TeV. The pseudorapidity distribution in vertical bar eta vertical bar <1.8 is reported for inelastic events and for events with at least one charged particle in vertical bar eta vertical bar <1. The pseudorapidity density of charged particles produced in the pseudorapidity region vertical bar eta vertical bar <0.5 is 5.31 +/- 0.18 and 6.46 +/- 0.19 for the two event classes, respectively. The transverse-momentum distribution of charged particles is measured in the range 0.15 <p(T) <20 GeV/c and vertical bar eta vertical bar <0.8 for events with at least one charged particle in vertical bar eta vertical bar <1. The evolution of the transverse momentum spectra of charged particles is also investigated as a function of event multiplicity. The results are compared with calculations from PYTHIA and EPOS Monte Carlo generators. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Elliptic flow of muons from heavy-flavour hadron decays at forward rapidity in Pb-Pb collisions at root s(NN)=2.76TeV

The elliptic flow, v(2), of muons from heavy-flavour hadron decays at forward rapidity (2.5 <y <4) is measured in Pb-Pb collisions at root s(NN)= 2.76TeVwith the ALICE detector at the LHC. The scalar product, two- and four-particle Q cumulants and Lee-Yang zeros methods are used. The dependence of the v(2) of muons from heavy-flavour hadron decays on the collision centrality, in the range 0-40%, and on transverse momentum, p(T), is studied in the interval 3 <p(T)<10 GeV/c. A positive v(2) is observed with the scalar product and two-particle Q cumulants in semi-central collisions (10-20% and 20-40% centrality classes) for the p(T) interval from 3 to about 5GeV/c with a significance larger than 3 sigma, based on the combination of statistical and systematic uncertainties. The v(2) magnitude tends to decrease towards more central collisions and with increasing pT. It becomes compatible with zero in the interval 6 <p(T)<10 GeV/c. The results are compared to models describing the interaction of heavy quarks and open heavy-flavour hadrons with the high-density medium formed in high-energy heavy-ion collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.Peer reviewe

Where Brain, Body and World Collide

The production cross section of electrons from semileptonic decays of beauty hadrons was measured at mid-rapidity (|y| &lt; 0.8) in the transverse momentum range 1 &lt; pt &lt; 8 Gev/c with the ALICE experiment at the CERN LHC in pp collisions at a center of mass energy sqrt{s} = 7 TeV using an integrated luminosity of 2.2 nb^{-1}. Electrons from beauty hadron decays were selected based on the displacement of the decay vertex from the collision vertex. A perturbative QCD calculation agrees with the measurement within uncertainties. The data were extrapolated to the full phase space to determine the total cross section for the production of beauty quark-antiquark pairs