29 research outputs found
Measurement of Prompt D0, Λc+, and Σc 0, ++ (2455) Production in Proton-Proton Collisions at √s = 13 TeV
No full textThe pT-differential production cross sections of prompt D0, Λc+, and Σc0,++(2455) charmed hadrons are measured at midrapidity (|y|<0.5) in pp collisions at s=13 TeV. This is the first measurement of Σc0,++ production in hadronic collisions. Assuming the same production yield for the three Σc0,+,++ isospin states, the baryon-to-meson cross section ratios Σc0,+,++/D0 and Λc+/D0 are calculated in the transverse momentum (pT) intervals 2<pT<12 and 1<pT<24 GeV/c. Values significantly larger than in e+e- collisions are observed, indicating for the first time that baryon enhancement in hadronic collisions also extends to the Σc. The feed-down contribution to Λc+ production from Σc0,+,++ is also reported and is found to be larger than in e+e- collisions. The data are compared with predictions from event generators and other phenomenological models, providing a sensitive test of the different charm-hadronization mechanisms implemented in the models
Charm-quark fragmentation fractions and production cross section at midrapidity in pp collisions at the LHC
No full textRecent pT-integrated cross-section measurements of the ground-state charm mesons and baryons, D0, D+, Ds+, Λc+, and Ξc0, are used to evaluate the charm fragmentation fractions and production cross section per unit of rapidity at midrapidity (|y|<0.5), in pp collisions at s=5.02 TeV at the LHC. The latter is dσcc¯/dy||y|<0.5=1165±44(stat)-101+134(syst) μb. These measurements were obtained for the first time in hadronic collisions at the LHC, including the charm baryon states, recently measured by ALICE at midrapidity. The charm fragmentation fractions differ significantly from the values measured in e+e- and ep collisions, providing evidence of the dependence of the parton-to-hadron fragmentation fractions on the collision system, indicating that the assumption of their universality is not supported by the measured cross sections. An increase of a factor of about 3.3 for the fragmentation fraction for the Λc+ with a significance of 5σ between the values obtained in pp collisions and those obtained in e+e- (ep) collisions is reported. The fragmentation fraction for the Ξc0 was obtained for the first time in any collision system. The measured fragmentation fractions were used to update the cc¯ cross sections per unit of rapidity at |y|<0.5 at s=2.76 and 7 TeV, which are about 40% higher than the previously published results. The data were compared with perturbative-QCD calculations and lie at the upper edge of the theoretical bands
Charakterisierung und Signalanalyse von TRD-Prototypen für das CBM Experiment
No full textAls es in den 1920er und 1930er Jahren zur Entdeckung des expandierenden Raums, gemessen über die kosmologische Rotverschiebung, kommt, wird daraus erstmals die Idee eines kleinsten Ursprungspunkts zu Beginn der Zeit postuliert. Es dauerte jedoch weitere 30 Jahre, bis das Konzept des „Big Bang“ als Ursprungsmodell veröffentlicht wurde [Lum07]. Seitdem untersuchen Wissenschaftler fortschreitend die Theorie des Big Bang. Der Zustand der Materie zum Zeitpunkt Null ist nach wie vor ungeklärt, ab ca. 10 exp (-44) Sekunden nach dem Urknall wird in der Theorie des Big Bang davon ausgegangen, dass sich die Materie im Zustand des Quark-Gluon-Plasmas befand. Zunächst galt es eine experimentelle Bestätigung für die generelle Existenz eines solchen Zustands zu finden. Die experimentelle Suche nach dem Quark-Gluon-Plasma im Labor begann vor fast 30 Jahren am Bevalac in Berkley [Gus84], dort konnte bei Niobium-Kollisionen kollektiver Fluss beobachtet werden. Zehn Jahre später begannen die Messungen des Schwerionen-Forschungsprogramms am CERN1. Im Jahr 1994 wurden dort die ersten Schwerionenkollisionen durchgeführt, von denen man sich erhoffte, dass sie zu einer kurzzeitigen Erzeugung des Quark-Gluon-Plasmas führen. Im Jahr 2000 gab es dann eine zusammenfassende Pressemitteilung des CERN, in der die Messungen eines neuartigen Materiezustands beschrieben wurden [CER00]. Weitere fünf Jahre später wurde ein Bericht der bis dahin erreichten Ergebnisse der vier Quark-Gluon-Plasma Experimente am Relativistic Heavy Ion Collider (RHIC) des Brookhaven National Labratory veröffentlicht [BNL05]. Dabei konnten einige Ergebnisse aus den Messungen am CERN bestätigt werden, andere hingegen nicht. Die Annahme, das Quark-Gluon-Plasma verhalte sich wie ein Gas, musste beispielsweise nach den Messungen am RHIC verworfen werden. Diese zeigen, dass das Verhalten des Quark-Gluon-Plasma eher dem von Flüssigkeiten ähnelt [BNL05].
Seit den ersten Schritten zur Untersuchung des Quark-Gluon-Plasma am CERN, in denen vor allem die Messung der Existenz des Quark-Gluon-Plasmas an sich im Vordergrund stand, soll der Phasenübergang nun quantitativ untersucht werden. Dazu werden Dichte und Temperatur der betrachteten Materie variiert und die vorhandene Zustandsphase gemessen. Eines dieser Experimente soll das Compressed Baryonic Matter (CBM) Experiment werden. Das Ziel des Experiments ist die Untersuchung von Materie bei sehr hohen Dichten, aber im Vergleich zu anderen aktuellen Experimenten relativ niedrigen Temperaturen. Im Jahr 2009 wurde der erste Spatenstich auf dem Gelände der Gesellschaft für Schwerionenforschung in Darmstadt für den Bau der Facility for Antiproton and Proton Research (FAIR) getätigt. Mit Hilfe der FAIR-Beschleuniger soll dann das CBM Experiment das Quark-Gluon-Plasma bei hohen Materiedichten mit bisher nicht erreichter Statistik untersuchen können. Jedoch gerade das Erreichen solch hoher Ereignisraten stellt nicht nur eine zentrale Herausforderung an die Beschleuniger dar, sondern auch an die messenden Detektoren. Diese Arbeit beschäftigt sich mit der Entwicklung eines Transition Radiation Detektors für das CBM Experiment.
Nach einer kurzen Einführung in die generelle Untersuchung des Quark-Gluon-Plasmas folgt die Beschreibung des geplanten Aufbaus des CBM Experiments mit dessen Subsystemen. Danach wird die theoretische und praktische Funktionsweise eines Transition-Radiation Detektors (TRD) erklärt, um darauf aufbauend die Charakterisierung und Signalanalyse der entwickelten TRD-Prototypen darzustellen. Abschließend werden die Ergebnisse in Bezug auf den für das CBM Experiment zu entwickelnden Detektor diskutiert
Inclusive J / ψ production at midrapidity in pp collisions at √s=13 TeV
No full textWe report on the inclusive J / ψ production cross section measured at the CERN Large Hadron Collider in proton–proton collisions at a center-of-mass energy s=13 TeV. The J / ψ mesons are reconstructed in the e +e - decay channel and the measurements are performed at midrapidity (| y| TT-integrated J / ψ production cross section at midrapidity, computed using the minimum-bias data sample, is dσ/dy|y=0=8.97±0.24(stat)±0.48(syst)±0.15(lumi)μb. An approximate logarithmic dependence with the collision energy is suggested by these results and available world data, in agreement with model predictions. The integrated and pT-differential measurements are compared with measurements in pp collisions at lower energies and with several recent phenomenological calculations based on the non-relativistic QCD and Color Evaporation models
Event-shape and multiplicity dependence of freeze-out radii in pp collisions at sqrt(s) = 7 TeV s = 7 TeV
No full textTwo-particle correlations in high-energy collision experiments enable the extraction of particle source radii by using the Bose-Einstein enhancement of pion production at low relative momentum q ∝ 1/R. It was previously observed that in pp collisions at s√ = 7TeV the average pair transverse momentum kT range of such analyses is limited due to large background correlations which were attributed to mini-jet phenomena. To investigate this further, an event-shape dependent analysis of Bose-Einstein correlations for pion pairs is performed in this work. By categorizing the events by their transverse sphericity ST into spherical (ST > 0:7) and jet-like (ST < 0:3) events a method was developed that allows for the determination of source radii for much larger values of kT for the first time. Spherical events demonstrate little or no background correlations while jet-like events are dominated by them. This observation agrees with the hypothesis of a mini-jet origin of the non-femtoscopic background correlations and gives new insight into the physics interpretation of the kT dependence of the radii. The emission source size in spherical events shows a substantially diminished kT dependence, while jet-like events show indications of a negative trend with respect to kT in the highest multiplicity events. Regarding the emission source shape, the correlation functions for both event sphericity classes show good agreement with an exponential shape, rather than a Gaussian one
Investigating Λ baryon production in p -Pb collisions in jets and the underlying event using angular correlations
No full textFirst measurements of hadron-Λ (h-Λ) azimuthal angular correlations in p-Pb collisions at sNN =5.02 TeV using the ALICE detector at the Large Hadron Collider are presented. These correlations are used to separate the production of associated Λ baryons into three different kinematic regions, namely those produced in the direction of the trigger particle (near side), those produced in the opposite direction (away side), and those whose production is uncorrelated with the jet axis (underlying event). The per-trigger associated Λ yields in these regions are extracted, along with the near- and away-side azimuthal peak widths, and the results are studied as a function of associated particle pT and event multiplicity. Comparisons with the dpmjet event generator and previous measurements of the φ(1020) meson are also made. The final results indicate that strangeness production in the highest multiplicity p-Pb collisions is enhanced relative to low multiplicity collisions in both the jetlike regions and the underlying event. The production of Λ relative to charged hadrons is also enhanced in the underlying event when compared to the jetlike regions. Additionally, the results hint that strange quark production in the away-side of the jet is modified by soft interactions with the underlying event
Proton emission in ultraperipheral Pb-Pb collisions at √sNN = 5.02 TeV
No full textThe first measurements of proton emission accompanied by neutron emission in the electromagnetic dissociation (EMD) of Pb208 nuclei in the ALICE experiment at the Large Hadron Collider are presented. The EMD protons and neutrons emitted at very forward rapidities are detected by the proton and neutron zero degree calorimeters of the ALICE experiment. The emission cross sections of zero, one, two, and three protons accompanied by at least one neutron were measured in ultraperipheral Pb208-Pb208 collisions at a center-of-mass energy per nucleon pair sNN=5.02TeV. The 0p and 3p cross sections are described by the RELDIS model within their measurement uncertainties, while the 1p and 2p cross sections are underestimated by the model by 17-25%. According to this model, these 0p, 1p, 2p, and 3p cross sections are associated, respectively, with the production of various isotopes of Pb, Tl, Hg, and Au in the EMD of Pb208. The cross sections of the emission of a single proton accompanied by the emission of one, two, or three neutrons in EMD were also measured. The data are significantly overestimated by the RELDIS model, which predicts that the (1p,1n), (1p,2n), and (1p,3n) cross sections are very similar to the cross sections for the production of the thallium isotopes Tl206,205,204
J/psi elliptic and triangular flow in Pb-Pb collisions at root s(NN)=5.02 TeV
No full textThe inclusive J/psi (v(2)) and triangular (v(3)) flow coefficients measured at forward rapidity (2.5 < y < 4) and the v2 measured at midrapidity (vertical bar y vertical bar < 0.9) in Pb-Pb collisions at root s(NN) = 5.02TeV using the ALICE detector at the LHC are reported. The entire Pb-Pb data sample collected during Run 2 is employed, amounting to an integrated luminosity of 750 mu b(-1) at forward rapidity and 93 mu b(-1) at midrapidity. The results are obtained using the scalar product method and are reported as a function of transverse momentum p(T) and collision centrality. At midrapidity, the J psi v(2) is in agreement with the forward rapidity measurement. The centrality averaged results indicate a positive J/psi v(3) with a significance of more than 5 sigma at forward rapidity in the p(T) range 2 < p(T) < 5 GeV/c. The forward rapidity v(2), v(3), and v(3)/v(2) results at low and intermediate p(T) (p(T) < 8 GeV/c) exhibit a mass hierarchy when compared to pions and D mesons, while converging into a species-independent curve at higher p(T). At low and intermediate p(T), the results could be interpreted in terms of a later thermalization of charm quarks compared to light quarks, while at high p(T), path-length dependent effects seem to dominate. The J/psi v(2) measurements are further compared to a microscopic transport model calculation. Using a simplified extension of the quark scaling approach involving both light and charm quark flow components, it is shown that the D-meson vn measurements can be described based on those for charged pions and J/flow
Multiplicity dependence of pi, K, and p production in pp collisions at root s=13 TeV
No full textThis paper presents the measurements of π±, K±, p and p¯¯¯ transverse momentum (pT) spectra as a function of charged-particle multiplicity density in proton–proton (pp) collisions at s√ = 13 TeV with the ALICE detector at the LHC. Such study allows us to isolate the center-of-mass energy dependence of light-flavour particle production. The measurements reported here cover a pT range from 0.1 to 20 GeV/c and are done in the rapidity interval |y|<0.5. The pT-differential particle ratios exhibit an evolution with multiplicity, similar to that observed in pp collisions at s√ = 7 TeV, which is qualitatively described by some of the hydrodynamical and pQCD-inspired models discussed in this paper. Furthermore, the pT-integrated hadron-to-pion yield ratios measured in pp collisions at two different center-of-mass energies are consistent when compared at similar multiplicities. This also extends to strange and multi-strange hadrons, suggesting that, at LHC energies, particle hadrochemistry scales with particle multiplicity the same way under different collision energies and colliding systems
Charged-particle production as a function of the relative transverse activity classifier in pp, p–Pb, and Pb–Pb collisions at the LHC
No full textAbstract Measurements of charged-particle production in pp, p–Pb, and Pb–Pb collisions in the toward, away, and transverse regions with the ALICE detector are discussed. These regions are defined event-by-event relative to the azimuthal direction of the charged trigger particle, which is the reconstructed particle with the largest transverse momentum p T trig in the range 8 < p T trig < 15 GeV/c. The toward and away regions contain the primary and recoil jets, respectively; both regions are accompanied by the underlying event (UE). In contrast, the transverse region perpendicular to the direction of the trigger particle is dominated by the so-called UE dynamics, and includes also contributions from initial- and final-state radiation. The relative transverse activity classifier, R T = N ch T / N ch T , is used to group events according to their UE activity, where N ch T is the charged-particle multiplicity per event in the transverse region and N ch T is the mean value over the whole analysed sample. The energy dependence of the R T distributions in pp collisions at s = 2.76, 5.02, 7, and 13 TeV is reported, exploring the Koba-Nielsen-Olesen (KNO) scaling properties of the multiplicity distributions. The first measurements of charged-particle p T spectra as a function of R T in the three azimuthal regions in pp, p–Pb, and Pb–Pb collisions at s NN = 5.02 TeV are also reported. Data are compared with predictions obtained from the event generators PYTHIA 8 and EPOS LHC. This set of measurements is expected to contribute to the understanding of the origin of collective-like effects in small collision systems (pp and p–Pb)
