Observation of Z decays to four leptons with the CMS detector at the LHC

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Measurement of the relative prompt production rate of chi(c2) and chi(c1) in pp collisions at root s=7 TeV

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Measurement of the differential cross section d sigma/dt in elastic p(p)over-bar scattering at root s=1.96 TeV

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Search for long-lived particles decaying into electron or photon pairs with the D0 detector

In this Letter we report on a search for long-lived particles that decay into final states with two electrons or photons. Such long-lived particles arise in a variety of theoretical models, such as hidden valleys and supersymmetry with gauge-mediated breaking. By precisely reconstructing the direction of the electromagnetic shower we are able to probe much longer lifetimes than previously explored. We see no evidence of the existence of such long-lived particles and interpret this search as a quasi model-independent limit on their production cross section, as well as a limit on a long-lived fourth generation quark. © 2008 The American Physical Society.

Measurement of the angular and lifetime parameters of the decays Bd0→J/ψK*0 and Bs0→J/ψ

We present measurements of the linear polarization amplitudes and the strong relative phases that describe the flavor-untagged decays Bd0→J/ψK*0 and Bs0→J/ψ in the transversity basis. We also measure the mean lifetime τ̄s of the Bs0 mass eigenstates and the lifetime ratio τ̄s/τd. The analyses are based on approximately 2.8fb-1 of data recorded with the D0 detector. From our measurements of the angular parameters we conclude that there is no evidence for a deviation from flavor SU(3) symmetry for these decays and that the factorization assumption is not valid for the Bd0→J/ψK*0 decay. © 2009 The American Physical Society.

Search for resonant pair production of neutral long-lived particles decaying to bb̄ in pp̄ collisions at s=1.96TeV

We report on a first search for resonant pair production of neutral long-lived particles (NLLP) which each decay to a bb̄ pair, using 3.6fb-1 of data recorded with the D0 detector at the Fermilab Tevatron collider. We search for pairs of displaced vertices in the tracking detector at radii in the range 1.6-20cm from the beam axis. No significant excess is observed above background, and upper limits are set on the production rate in a hidden-valley benchmark model for a range of Higgs boson masses and NLLP masses and lifetimes. © 2009 The American Physical Society.

Direct measurement of the mass difference between top and antitop quarks

We present a measurement of the mass difference between t and t̄ quarks in lepton+jets final states of tt̄ events in 1fb-1 of data collected with the D0 detector from Fermilab Tevatron Collider pp̄ collisions at s=1.96TeV. The measured mass difference of 3.8±3.7GeV is consistent with the equality of t and t̄ masses. This is the first direct measurement of a mass difference between a quark and its antiquark partner. © 2009 The American Physical Society.

Search for a resonance decaying into WZ boson Pairs in pp̄ collisions

We present the first search for an electrically charged resonance W′ decaying to a WZ boson pair using 4.1fb-1 of integrated luminosity collected with the D0 detector at the Fermilab Tevatron pp̄ collider. The WZ pairs are reconstructed through their decays into three charged leptons (≤=e, μ). A total of 9 data events is observed in good agreement with the background prediction. We set 95% C.L. limits on the W′WZ coupling and on the W′ production cross section multiplied by the branching fractions. We also exclude W′ masses between 188 and 520 GeV within a simple extension of the standard model and set the most restrictive limits to date on low-scale technicolor models. © 2010 The American Physical Society.

Measurement of the charge ratio of atmospheric muons with the CMS detector

We present a measurement of the ratio of positive to negative muon fluxes from cosmic ray interactions in the atmosphere, using data collected by the CMS detector both at ground level and in the underground experimental cavern at the CERN LHC. Muons were detected in the momentum range from 5 GeV/. c to 1 TeV/. c. The surface flux ratio is measured to be 1.2766±0.0032(stat.)±0.0032(syst.), independent of the muon momentum, below 100 GeV/. c. This is the most precise measurement to date. At higher momenta the data are consistent with an increase of the charge ratio, in agreement with cosmic ray shower models and compatible with previous measurements by deep-underground experiments. © 2010.

Search for events with Leptonic jets and missing transverse energy in pp̄ collisions at √s=1.96TeV

We present the first search for pair production of isolated jets of charged leptons in association with a large imbalance in transverse energy in pp̄ collisions using 5.8fb⊃-1 of integrated luminosity collected by the D0 detector at the Fermilab Tevatron Collider. No excess is observed above the standard model background, and the result is used to set upper limits on the production cross section of pairs of supersymmetric chargino and neutralino particles as a function of dark-photon mass, where the dark photon is produced in the decay of the lightest supersymmetric particle. © 2010 The American Physical Society.

Measurement of the muon stopping power in lead tungstate

A large sample of cosmic ray events collected by the CMS detector is exploited to measure the specific energy loss of muons in the lead tungstate (PbWO4) of the electromagnetic calorimeter. The measurement spans a momentum range from 5 GeV/c to 1 TeV/c. The results are consistent with the expectations over the entire range. The calorimeter energy scale, set with 120 GeV/c electrons, is validated down to the sub-GeV region using energy deposits, of order 100 MeV, associated with low-momentum muons. The muon critical energy in PbWO4 is measured to be 160+5 -68 GeV, in agreement with expectations. This is the first experimental determination of muon critical energy. © 2010 IOP Publishing Ltd and SISSA.

Commissioning of the CMS experiment and the cosmic run at four tesla

The CMS Collaboration conducted a month-long data-taking exercise known as the Cosmic Run At Four Tesla in late 2008 in order to complete the commissioning of the experiment for extended operation. The operational lessons resulting from this exercise were addressed in the subsequent shutdown to better prepare CMS for LHC beams in 2009. The cosmic data collected have been invaluable to study the performance of the detectors, to commission the alignment and calibration techniques, and to make several cosmic ray measurements. The experimental setup, conditions, and principal achievements from this data-taking exercise are described along with a review of the preceding integration activities. © 2010 IOP Publishing Ltd and SISSA.

Performance of the CMS Level-1 trigger during commissioning with cosmic ray muons and LHC beams

The CMS Level-1 trigger was used to select cosmic ray muons and LHC beam events during data-taking runs in 2008, and to estimate the level of detector noise. This paper describes the trigger components used, the algorithms that were executed, and the trigger synchronisation. Using data from extended cosmic ray runs, the muon, electron/photon, and jet triggers have been validated, and their performance evaluated. Efficiencies were found to be high, resolutions were found to be good, and rates as expected. © 2010 IOP Publishing Ltd and SISSA.

Performance of the CMS drift-tube chamber local trigger with cosmic rays

The performance of the Local Trigger based on the drift-tube system of the CMS experiment has been studied using muons from cosmic ray events collected during the commissioning of the detector in 2008. The properties of the system are extensively tested and compared with the simulation. The effect of the random arrival time of the cosmic rays on the trigger performance is reported, and the results are compared with the design expectations for proton-proton collisions and with previous measurements obtained with muon beams. © 2010 IOP Publishing Ltd and SISSA.