Nucleon-nucleon Center Of Mass Energy Research Articles

Studies of Beauty Suppression via Nonprompt D^{0} Mesons in Pb-Pb Collisions at sqrt[s_{NN}]=5.02 TeV.

The transverse momentum spectra of D^{0} mesons from b hadron decays are measured at midrapidity (|y|<1) in pp and Pb-Pb collisions at a nucleon-nucleon center of mass energy of 5.02TeV with the CMS detector at the LHC. The D^{0} mesons from b hadron decays are distinguished from prompt D^{0} mesons by their decay topologies. In Pb-Pb collisions, the B→D^{0} yield is found to be suppressed in the measured p_{T} range from 2 to 100 GeV/c as compared to pp collisions. The suppression is weaker than that of prompt D^{0} mesons and charged hadrons for p_{T} around 10 GeV/c. While theoretical calculations incorporating partonic energy loss in the quark-gluon plasma can successfully describe the measured B→D^{0} suppression at higher p_{T}, the data show an indication of larger suppression than the model predictions in the range of 2<p_{T}<5 GeV/c.

Study of Z+jet correlations in PbPb and pp collisions at [formula omitted

The production of Z+jet pairs is measured for the first time in pp and central PbPb collisions at a nucleon-nucleon center of mass energy of 5.02 TeV per nucleon pair, using data samples collected by the CMS experiment at the LHC. The Z+jet azimuthal angle difference and pT imbalance are analysed for events containing a Z boson with transverse momentum pTZ>60 GeV/c and an associated jet with pTJet>30 GeV/c. A moderate shift in the pTJet/pTZ ratio is seen in central PbPb collisions with respect to the ratio found using pp data.

Centrality dependence of ψ(2S) suppression in p-Pb collisions at s N N = 5.02 $$ {\sqrt{s}}_{\mathrm{NN}}=5.02 $$ TeV

The inclusive production of the $\psi$(2S) charmonium state was studied as a function of centrality in p-Pb collisions at the nucleon-nucleon center of mass energy $\sqrt{s_{\rm NN}}$ = 5.02 TeV at the CERN LHC. The measurement was performed with the ALICE detector in the center of mass rapidity ranges $-4.46<y_{\rm cms}<-2.96$ and $2.03<y_{\rm cms}<3.53$, down to zero transverse momentum, by reconstructing the $\psi$(2S) decay to a muon pair. The $\psi$(2S) production cross section $\sigma_{\psi(\rm 2S)}$ is presented as a function of the collision centrality, which is estimated through the energy deposited in forward rapidity calorimeters. The relative strength of nuclear effects on the $\psi$(2S) and on the corresponding 1S charmonium state J/$\psi$ is then studied by means of the double ratio of cross sections $[\sigma_{\psi(\rm 2S)}/\sigma_{\rm J/\psi}]_{\rm pPb}/[\sigma_{\psi(\rm 2S)}/\sigma_{\rm J/\psi}]_{\rm pp}$ between p-Pb and pp collisions, and by the values of the nuclear modification factors for the two charmonium states. The results show a large suppression of $\psi$(2S) production relative to the J/$\psi$ at backward (negative) rapidity, corresponding to the flight direction of the Pb-nucleus, while at forward (positive) rapidity the suppressions of the two states are comparable. Finally, comparisons to results from lower energy experiments and to available theoretical models are presented.

Suppression of ψ(2S) production in p-Pb collisions at s N N $$ \sqrt{s_{\mathrm{NN}}} $$ = 5.02 TeV

The ALICE Collaboration has studied the inclusive production of the charmonium state $\psi(2S)$ in proton-lead (p-Pb) collisions at the nucleon-nucleon centre of mass energy $\sqrt{s_{NN}}$ = 5.02 TeV at the CERN LHC. The measurement was performed at forward ($2.03<y_{cms}<3.53$) and backward ($-4.46<y_{cms}<-2.96$) centre of mass rapidities, studying the decays into muon pairs. In this paper, we present the inclusive production cross sections $\sigma_{\psi(2S)}$, both integrated and as a function of the transverse momentum $p_{T}$, for the two $y_{cms}$ domains. The results are compared to those obtained for the 1S vector state (J/$\psi$), by showing the ratios between the production cross sections, as well as the double ratios $[\sigma_{\psi(2S)}/\sigma_{J/\psi}]_{pPb}/[\sigma_{\psi(2S)}/\sigma_{J/\psi}]_{pp}$ between p-Pb and proton-proton collisions. Finally, the nuclear modification factor for inclusive $\psi(2S)$ is evaluated and compared to the measurement of the same quantity for J/$\psi$ and to theoretical models including parton shadowing and coherent energy loss mechanisms. The results show a significantly larger suppression of the $\psi(2S)$ compared to that measured for J/$\psi$ and to models. These observations represent a clear indication for sizeable final state effects on $\psi(2S)$ production.

J/ψ production and nuclear effects in p-Pb collisions at $ \sqrt{{{{\mathrm{s}}_{\mathrm{NN}}}}} $ = 5.02 TeV

Inclusive J/$\psi$ production has been studied with the ALICE detector in p-Pb collisions at the nucleon-nucleon center of mass energy $\sqrt{s_{\rm NN}}$ = 5.02 TeV at the CERN LHC. The measurement is performed in the center of mass rapidity domains $2.03<y_{\rm cms}<3.53$ and $-4.46<y_{\rm cms}<-2.96$, down to zero transverse momentum, studying the $\mu^+\mu^-$ decay mode. In this paper, the J/$\psi$ production cross section and the nuclear modification factor $R_{\rm pPb}$ for the rapidities under study are presented. While at forward rapidity, corresponding to the proton direction, a suppression of the J/$\psi$ yield with respect to binary-scaled pp collisions is observed, in the backward region no suppression is present. The ratio of the forward and backward yields is also measured differentially in rapidity and transverse momentum. Theoretical predictions based on nuclear shadowing, as well as on models including, in addition, a contribution from partonic energy loss, are in fair agreement with the experimental results.

Electroweak bosons in heavy-ion collisions with the CMS detector at =2.76 TeV

Electroweak gauge bosons W and Z, do not interact strongly, and thus constitute clean probes of the initial state of nucleus-nucleus collisions. The comparison of their production cross-sections in pp and in nuclear collisions provides an estimate of the nuclear parton distribution functions. Despite the low production cross section of weak bosons compared to other nuclear processes, the relatively clean signal of their leptonic decay channel allows their detection. This paper reports measurements of Z and W bosons, produced in PbPb and pp collisions both at nucleon-nucleon center of mass energy =2.76 TeV with the CMS detector. The Z boson yield and the nuclear modification factor (RAA) corresponding to the integrated luminosity of 150 μb−1 for PbPb collisions are presented. The search for W bosons has been performed in the muon plus neutrino channel, using the data sample with integrated luminosity of 7.2 μb−1 for PbPb collisions. Event centrality an muon pseudorapidity dependencies are studied for the complete W candidate sample as well as samples separated by charge (W+ and W−).

The ATLAS heavy ion program: status prior to the first LHC Pb+Pb run

The LHC will begin colliding lead nuclei at a nucleon-nucleon center of mass energy of 2.76 TeV in November, 2010. The luminosity during this first LHC heavy ion run is expected to be roughly one hundredth of the Pb+Pb “design” luminosity. The ATLAS experiment is prepared to carry out measurements during this first Pb+Pb run of bulk observables such as charged particle multiplicity, charged particle spectra and elliptic flow and to make first measurements of jet spectra and jet fragment spectra. The prospects for these measurements are briefly described.

QCD at ATLAS: The story so far

Since the start of Large Hadron Collider (LHC) operation at √ s = 7 TeV in Spring 2010, the number of collisions produced has grown roughly exponentially with running time. By the end of the 2010 run, a sample of integrated luminosity around 45 pb had been collected, at a peak luminosity of around 10 cms. In addition, the first LHC run with lead ion collisions in November yielded approximately 9 μb at a nucleon-nucleon centre of mass energy of √ s = 2.76 TeV. The ATLAS detector [1] performed well throughout the 2010 run and its response was quickly understood. The results contained in this talk [2] are based on varying sample sizes taken from the 2010 data. LHC physics is an extremely fast-moving field. By the time of writing (May 2011), the 2011 data sample has already reached an integrated luminosity of over 250 pb, such that many of the results here will soon be superceded. For a complete current list of ATLAS publications and preliminary results, see [3].

Model for hypernucleus production in heavy ion collisions

We estimate the production cross sections of hypernuclei in projectile like fragment (PLF) in heavy ion collisions. The discussed scenario for the formation cross section of hypernucleus is: (a) Lambda particles are produced in the participant region but have a considerable rapidity spread and (b) Lambda with rapidity close to that of the PLF and total momentum (in the rest system of PLF) up to Fermi motion can then be trapped and produce hypernuclei. The process (a) is considered here within Heavy Ion Jet Interacting Generator HIJING-BBbar model and the process (b) in the canonical thermodynamic model (CTM). We estimate the production cross-sections for light hypernuclei for C + C at 3.7 GeV total nucleon-nucleon center of mass energy and for Ne+Ne and Ar+Ar collisions at 5.0 GeV. By taking into account explicitly the impact parameter dependence of the colliding systems, it is found that the cross section is different from that predicted by the coalescence model and large discrepancy is obtained for 6_He and 9_Be hypernuclei.