A measurement is presented of W-boson production in PbPb collisions carried out at a nucleon-nucleon (NN) centre-of-mass energy s NN of 2.76 TeV at the LHC using the CMS detector. In data ...corresponding to an integrated luminosity of 7.3 mu b - 1 , the number of W ? mu ? mu decays is extracted in the region of muon pseudorapidity | ? mu | 25 GeV / c . Yields of muons found per unit of pseudorapidity correspond to (159 +/- 10 (stat.) +/- 12 (syst.)) X 10 - 8 W + and (154 +/- 10 (stat.) +/- 12 (syst.)) X 10 - 8 W - bosons per minimum-bias PbPb collision. The dependence of W production on the centrality of PbPb collisions is consistent with a scaling of the yield by the number of incoherent NN collisions. The yield of W bosons is also studied in a sample of pp interactions at s = 2.76 TeV corresponding to an integrated luminosity of 231 nb - 1 . The individual W + and W - yields in PbPb and pp collisions are found to agree, once the neutron and proton content in Pb nuclei is taken into account. Likewise, the difference observed in the dependence of the positive and negative muon production on pseudorapidity is consistent with next-to-leading-order perturbative QCD calculations.
A search for narrow, high-mass resonances decaying to electron or muon pairs has been performed using pp collision data collected at s = 7 TeV by the CMS experiment in 2011. The data sample ...corresponds to an integrated luminosity of approximately 5 fba1. The event yields observed in the signal regions are consistent with predictions of the standard model backgrounds, and upper limits on the cross section times branching fraction for a resonance decaying to dileptons are extracted from a shape analysis of the dilepton invariant mass distribution. The resulting mass limits at 95% confidence level are 2330 GeV for the Z a2 in the Sequential Standard Model, 2000 GeV for the superstring-inspired Z I a2 resonance, 890 (540) GeV for the Stueckelberg extension Z St a2 with the mass parameter Im = 0.06 (0.04), and 2140 (1810) GeV for Kaluza-Klein gravitons with the coupling parameter k / M A= Pl of 0.10 (0.05). These limits are the most stringent to date.
The I b differential production cross section and the cross section ratio I ( I A= b ) / I ( I b ) are measured as functions of transverse momentum p T I b and rapidity | y I b | in p p collisions at ...s = 7 TeV using data collected by the CMS experiment at the LHC. The measurements are based on I b decays reconstructed in the exclusive final state J / I I , with the subsequent decays J / I a mu + mu a and I a p I , using a data sample corresponding to an integrated luminosity of 1.9 fb a 1 . The product I ( I b ) X B ( I b a J / I I ) versus p T I b falls faster than that of b mesons. The measured value of I ( I b ) X B ( I b a J / I I ) for p T I b > 10 GeV and | y I b | < 2.0 is 1.16 +/- 0.06 +/- 0.12 nb , and the integrated I ( I A= b ) / I ( I b ) ratio is 1.02 +/- 0.07 +/- 0.09 , where the uncertainties are statistical and systematic, respectively.
The NA48 experiment at the CERN SPS accelerator aims to measure Re(ε′/ε), the direct CP violation parameter in
K
0 → 2π decays, to the precision of 2 × 10
−4. In the year 1999, the NA48 collaboration ...has published its first measurement based on the 1997 data. The presented result is a preliminary measurement from data collected in the 1998 run which contain 2.5 times more
K
L
→ 2π
0 decays than the 1997 data. A combined result of Re(ε′/ε)=(14.0±4.3)×10
−4 has been obtained.
A search for neutral Higgs bosons decaying to tau pairs at a center-of-mass energy of 7 TeV is performed using a dataset corresponding to an integrated luminosity of 4.6 fba1 recorded by the CMS ...experiment at the LHC. The search is sensitive to both the standard model Higgs boson and to the neutral Higgs bosons predicted by the minimal supersymmetric extension of the standard model (MSSM). No excess of events is observed in the tau-pair invariant-mass spectrum. For a standard model Higgs boson in the mass range of 110a145 GeV upper limits at 95% confidence level (CL) on the production cross section are determined. We exclude a Higgs boson with m H = 115 GeV with a production cross section 3.2 times of that predicted by the standard model. In the MSSM, upper limits on the neutral Higgs boson production cross section times branching fraction to tau pairs, as a function of the pseudoscalar Higgs boson mass, m A , sets stringent new bounds in the parameter space, excluding at 95% CL values of tan I2 as low as 7.1 at m A = 160 GeV in the m h max benchmark scenario.
Dijet production in PbPb collisions at a nucleonanucleon center-of-mass energy of 2.76 TeV is studied with the CMS detector at the LHC. A data sample corresponding to an integrated luminosity of 150 ...I14ba1 is analyzed. Jets are reconstructed using combined information from tracking and calorimetry, using the anti- k T algorithm with R = 0.3 . The dijet momentum balance and angular correlations are studied as a function of collision centrality and leading jet transverse momentum. For the most peripheral PbPb collisions, good agreement of the dijet momentum balance distributions with pp data and reference calculations at the same collision energy is found, while more central collisions show a strong imbalance of leading and subleading jet transverse momenta attributed to the jet-quenching effect. The dijets in central collisions are found to be more unbalanced than the reference, for leading jet transverse momenta up to the highest values studied.
Results are presented from a search for large, extra spatial dimensions in events with either two isolated muons or two isolated electrons. The data are from proton-proton interactions at s = 7 TeV ...collected with the CMS detector at the LHC. The size of the data sample corresponds to an integrated luminosity of approximately 2 fb - 1 . The observed dimuon and dielectron mass spectra are found to be consistent with standard-model expectations. Depending on the number of extra dimensions, the 95% confidence level limits from the combined mu mu and ee channels range from M s > 2.4 TeV to M s > 3.8 TeV , where M s characterizes the scale for the onset of quantum gravity.
We present a measurement of the ratio of the decay rates Γ(KL→π+π−)/Γ(KL→π±e∓ν), denoted as ΓK2π/ΓKe3. The analysis is based on data taken during a dedicated run in 1999 by the NA48 experiment at the ...CERN SPS. Using a sample of 47 000 K2π and five million Ke3 decays, we find ΓK2π/ΓKe3=(4.835±0.022stat.±0.016syst.)×10−3. From this we derive the branching ratio of the CP violating decay KL→π+π− and the CP violation parameter |η+−|. Excluding the CP conserving direct photon emission component KL→π+π−γ, we obtain the results BR(KL→π+π−)=(1.941±0.019)×10−3 and |η+−|=(2.223±0.012)×10−3.
The direct CP violation parameter Re(epsilon'/epsilon) has been measured from the decay rates of neutral kaons into two pions using the NA48 detector at the CERN SPS. With 3.3 million KL->pi0pi0 ...events collected during the 1998 and 1999 running periods, a result of Re(epsilon'/epsilon) = (15.0 +- 2.7)x10-4 has been obtained. The result combined with the published 1997 sample is Re(epsilon'/epsilon) = (15.3 +- 2.6)x10-4.
The KS→π+π−e+e− decay mode was investigated using the data collected in 2002 by the NA48/1 Collaboration. With about 23 k KS→π+π−e+e− events and 59 k KL→π+π−πD0 normalization decays, the KS→π+π−e+e− ...branching ratio relative to the KL→π+π−πD0 one was determined to be BR(KS→π+π−e+e−)/BR(KL→π+π−πD0)=(3.28±0.06stat±0.04syst)×10−2. This result was used to set the upper limit |gE1/gBR|<3.0 at 90% CL on the presence, in the decay amplitude, of an E1 direct emission (gE1) term relative to the dominant inner bremsstrahlung (gBR) term. The CP-violating asymmetry Aϕ in the sinϕcosϕ distribution of KS→π+π−e+e− events, where ϕ is the angle between the π+π− and the e+e− decay planes in the kaon centre of mass, was found to be Aϕ=(−0.4±0.8)%, consistent with zero. These results are in good agreement with a description of the KS→π+π−e+e− decay amplitude dominated by the CP-even inner bremsstrahlung process.
PDF
