Simulation of Jet Quenching Observables in Heavy Ion Collisions at the LHC

Large transverse momentum jets provide unique tools to study dense QCD matter in high-energy heavy-ion collisions. Results from RHIC on suppression of high transverse momentum particles in Au+Au collisions indicate a significant energy loss of leading partons in the dense and strongly interacting matter formed in these collisions. The LHC will collide Pb ions at sqrt(s_NN)=5500 GeV, where the cross section of hard scattering will increase dramatically. Large production rates, the large acceptance of the CMS calorimeters and tracking system, combined with the capability of triggering on jets, will extend the transverse momentum reach of charged particle spectra and nuclear modification factors up to p_T>200 GeV/c.Comment: 6 pages, 5 figures. Presented at the 19th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions (QM2006), Shanghai, China, November 14-20, 2006. [Grant numbers added in the acknowledgment.

Measurement of charged particle R_AA at high pT in PbPb collisions at sqrt(sNN)=2.76 TeV with CMS

Charged particle transverse momentum (pT) spectra have been measured by CMS for pp and PbPb collisions at the same sqrt(sNN)=2.76 TeV collision energy per nucleon pairs. Calorimeter-based jet triggers are employed to enhance the statistical reach of the high-pT measurements. The nuclear modification factor (R_AA) is obtained in bins of collision centrality for the PbPb data sample dividing by the measured pp reference spectrum. In the range pT = 5-10 GeV/c, the charged particle yield in the most central PbPb collisions is suppressed by up to a factor of 7. At higher pT, this suppression is significantly reduced, approaching a factor of 2 for particles with pT = 40-100 GeV/c.Comment: Proceedings of the talk was given at Hard Probes 2012, 27 May - 1 June, Cagliari, Ital

Hadronfizika a CMS detektorral = Hadronic physics at the CMS experiment

A CERN CMS kísérletében kifejlesztettük a töltött részecskék nyomkövetésének kis tévesztésű, nagyon kis impulzus esetén is jó hatásfokú módszerét. A szilíciumdetektorokban leadott energia, valamint a bomlási topológia figyelembe vételével kidolgoztuk a töltött és semleges részecskék azonosításának technikáját. Megmutattuk, hogy új matematikai eszközökkel a semleges pionok spektruma az egyfoton eloszlásból visszafejthető. A fenti eszközök segítségével elkészítettük a 0.9-7-10 TeV-es proton-proton ütközésekben keletkezett hadronok számával és eloszlásával foglalkozó példa-analíziseket, melyek a 2009-es év végén a CMS kísérlet első publikációi lesznek. A Pb-Pb ütközésben keletkezett anyagon áthaladó partonok energiaveszteségének vizsgálatához kidolgoztuk a nukleáris elnyomási faktorok mérésének módszerét, nagy transzverzális impulzusokra, amely csak jet triggerek alkalmazásával érhető el. A módszerrel várhatóan többszörösére nő majd az az impulzustartomány, ahol a mérés elvégezhető. Létrehoztunk egy megbízható, szcintillátorokon alapuló triggert, amely az első nehézion mérések alapvető triggere lehet 2010-ben. | Working in the CERN CMS experiment we have developed a new method of charged particle tracking that has good efficiency even at very low momentum. With help of the measured energy deposited in the silicon detectors, as well as by using the decay topology, we have worked out a technique of particle identification of charged and neutral hadrons. We have shown that with new mathematical methods the spectrum of neutral pions can be extracted from that of the photons. The above mentioned tools enabled us to prepare and accomplish the example analyses dealing with the measurement of the number distribution and momentum spectrum of hadrons produced in proton-proton collisions at 0.9-7-10 TeV. They will be the first publications of the CMS Collaboration at the end of 2009. For the study of energy loss of partons produced in Pb-Pb collisions, we have developed a method to measure the nuclear modification factors, for high transverse momenta. It was possible with help of jet triggers which multipled the reach of the measurement. We have created a reliable, scintillator based trigger, that can be a basic trigger for the first heavy ion collisions in 2010

Top rediscovery at ATLAS and CMS

A measurement is presented for the energy flow of minimum bias events in the forward region (3.15 <|\eta |< 4.9, where \eta is the pseudorapidity) of the CMS detector at the LHC for center-of-mass energies sqrt(s) = 0.9 TeV, 2.36 TeV and 7 TeV. The measurement is compared to Monte Carlo simulations, which use a model of multiparton interactions for the underlying event. In addition, production of forward jets was studied for the very first pp collisions at sqrt(s) = 0.9 TeV at LHC.L

Measurement of the B0 s → μ+μ− branching fraction and search for B0 → μ+μ− with the CMS experiment

Results are presented from a search for the rare decays B0 s → μ+μ− and B0 → μ+μ− in pp collisions at √s = 7 and 8 TeV, with data samples corresponding to integrated luminosities of 5 and 20 fb−1, respectively, collected by the CMS experiment at the LHC. An unbinned maximum-likelihood fit to the dimuon invariant mass distribu- tion gives a branching fraction B(B0 s → μ+μ−) = (3.0+1.0 −0.9) × 10−9, where the uncer- tainty includes both statistical and systematic contributions. An excess of B0 s → μ+μ− events with respect to background is observed with a significance of 4.3 standard de- viations. For the decay B0 → μ+μ− an upper limit of B(B0 → μ+μ−) < 1.1 × 10−9 at the 95% confidence level is determined. Both results are in agreement with the expectations from the standard model

Search for long-lived particles in events with photons and missing energy in proton-proton collisions at √ s = 7 TeV

Results are presented from a search for long-lived neutralinos decaying into a photon and an invisible particle, a signature associated with gauge-mediated supersymmetry breaking in supersymmetric models. The analysis is based on a 4.9 fb−1 sample of proton-proton collisions at √ s = 7 TeV, collected with the CMS detector at the LHC. The missing transverse energy and the time of arrival of the photon at the electromagnetic calorimeter are used to search for an excess of events over the expected background. No significant excess is observed, and lower limits at the 95% confidence level are obtained on the mass of the lightest neutralino, m > 220 GeV (for cτ 6000 mm (for m < 150 GeV)

Search for anomalous production of highly boosted Z bosons decaying to μ+μ− in proton-proton collisions at√s = 7 TeV

Results are reported from a search for the anomalous production of highly boosted Z bosons with large transverse momentum and decaying to μ+μ−. Such Z bosons may be produced in the decays of new heavy particles. The search uses pp collision data at √s = 7 TeV, corresponding to an integrated luminosity of 5.0 fb−1 recorded with the CMS detector. The shape of the observed transverse momentum distribution of Z bosons is consistent with standard model expectations. Constraints are obtained on models predicting the production of excited quarks decaying via electroweak pro- cesses. Assuming a compositeness scale that is equal to the excited quark mass as well as transition coupling strengths between Z bosons and excited quarks that are equal to standard model couplings to quarks, masses of excited quarks below 1.94 TeV are excluded at the 95% confidence level. For excited quark production via a novel con- tact interaction, masses below 2.22 TeV are excluded, even if the excited quarks do not couple to gluons