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MIT Open Access Articles

Mostrando recursos 41 - 60 de 13,979

41. Search for new physics in trilepton events and limits on the associated chargino-neutralino production at CDF - Gomez-Ceballos, Guillelmo; Goncharov, Maxim; Paus, Christoph M. E.
We perform a search for new physics using final states consisting of three leptons and a large imbalance in transverse momentum resulting from proton-antiproton collisions at 1.96 TeV center-of-mass energy. We use data corresponding to 5.8  fb[superscript −1] of integrated luminosity recorded by the CDF II detector at the Tevatron collider. Our main objective is to investigate possible new low-momentum (down to 5  GeV/c) multileptonic final states not investigated by LHC experiments. Relative to previous CDF analyses, we expand the geometric and kinematic coverage of electrons and muons and utilize tau leptons that decay hadronically. Inclusion of tau leptons is particularly important...

42. Measurement of the Inclusive Leptonic Asymmetry in Top-Quark Pairs that Decay to Two Charged Leptons at CDF - Gomez-Ceballos, Guillelmo; Goncharov, Maxim; Paus, Christoph M. E.
We measure the inclusive forward–backward asymmetry of the charged-lepton pseudorapidities from top-quark pairs produced in proton–antiproton collisions and decaying to final states that contain two charged leptons (electrons or muons). The data are collected with the Collider Detector at Fermilab and correspond to an integrated luminosity of 9.1  fb[superscript −1]. We measure the leptonic forward–backward asymmetry, A[ℓ over FB], to be 0.072 ± 0.060 and the leptonic pair forward–backward asymmetry, A[ℓℓ over FB], to be 0.076 ± 0.082. The measured values can be compared with the standard model predictions of A[ℓ over FB] = 0.038 ± 0.003 and A[ℓℓ over FB]...

43. Search for s-Channel Single-Top-Quark Production in Events with Missing Energy Plus Jets in p[bar over p] Collisions at √s = 1.96 TeV - Gomez-Ceballos, Guillelmo; Goncharov, Maxim; Paus, Christoph M. E.
The first search for single-top-quark production from the exchange of an s-channel virtual W boson using events with an imbalance in the total transverse energy, b-tagged jets, and no identified leptons is presented. Assuming the electroweak production of top quarks of mass 172.5  GeV/c[superscript 2] in the s channel, a cross section of 1.12[+0.61 over −0.57] (stat + syst) pb with a significance of 1.9 standard deviations is measured. This measurement is combined with the result obtained from events with an imbalance in total transverse momentum, b-tagged jets, and exactly one identified lepton, yielding a cross section of 1.36[+0.37 over −0.32]...

44. Evidence for the Baryonic Decay [bar over B][superscript 0] → D[superscript 0]Λ[bar over Λ] - Cowan, Ray Franklin; Dujmic, Denis; Sciolla, Gabriella
Evidence is presented for the baryonic B meson decay [bar over B][superscript 0] → D[superscript 0]Λ[bar over Λ] based on a data sample of 471 × 10[superscript 6] B[bar over B] pairs collected with the BABAR detector at the PEP-II asymmetric e[superscript +]e[superscript −] collider located at the SLAC National Accelerator Laboratory. The branching fraction is determined to be B([bar over B][superscript 0] → D[superscript 0]Λ[bar over Λ]) = (9.8[+2.9 over −2.6] ± 1.9) × 10[superscript −6], corresponding to a significance of 3.4 standard deviations including additive systematic uncertainties. A search for the related baryonic B meson decay [bar over...

45. Study of Beauty Hadron Decays into Pairs of Charm Hadrons - Counts, Ian Thomas Hunt; Cukierman, A. R.; Dorothy, Connor D.; Ilten, Philip James; Williams, Michael
First observations of the decays Λ[0 over b] → Λ[+ over c]D[− over (s)] are reported using data corresponding to an integrated luminosity of 3  fb[superscript −1] collected at 7 and 8 TeV center-of-mass energies in proton-proton collisions with the LHCb detector. In addition, the most precise measurement of the branching fraction B(B[0 over s] → D[superscript +]D[− over s]) is made and a search is performed for the decays B[0 over (s)] → Λ[+ over c]Λ[− over c]. The results obtained are B(Λ[0 over b] → Λ[+ over c]D[superscript −])/B(Λ[0 over b] → Λ[+ over c]D[− over s]) = 0.042 ±...

46. Search for massive resonances in dijet systems containing jets tagged as W or Z boson decays in pp collisions at √s = 8 TeV - Apyan, Aram; Barbieri, R.; Bauer, Gerry P.; Busza, Wit; Cali, Ivan Amos; Chan, M.; Di Matteo, Leonardo; Dutta, V.; Gomez-Ceballos, Guillelmo; Goncharov, Maxim; Gulhan, Doga Can; Klute, Markus; Lai, Yue Shi; Lee, Y.-J.; Levin, A.; Luckey, P. David, Jr.; Ma, Teng; Paus, Christoph M. E.; Ralph, Duncan Kelley; Roland, Christof E.; Roland, Gunther M.; Stephans, George S. F.; Stockli, Fabian; Sumorok, Konstanty C.; Velicanu, Dragos Alexandru; Veverka, Jan; Wyslouch, Boleslaw; Yang, M.; Zanetti, Marco; Zhukova, Victoria
A search is reported for massive resonances decaying into a quark and a vector boson (W or Z), or two vector bosons (WW, WZ, or ZZ). The analysis is performed on an inclusive sample of multijet events corresponding to an integrated luminosity of 19.7 fb[superscript −1], collected in proton-proton collisions at a centre-of-mass energy of 8 TeV with the CMS detector at the LHC. The search uses novel jet-substructure identification techniques that provide sensitivity to the presence of highly boosted vector bosons decaying into a pair of quarks. Exclusion limits are set at a confidence level of 95% on the...

47. Studies of dijet transverse momentum balance and pseudorapidity distributions in pPb collisions at √s[subscript NN] = 5.02 TeV - Apyan, Aram; Barbieri, R.; Bauer, Gerry P.; Busza, Wit; Cali, Ivan Amos; Chan, M.; Di Matteo, Leonardo; Dutta, V.; Gomez-Ceballos, Guillelmo; Goncharov, Maxim; Gulhan, Doga Can; Klute, Markus; Lai, Yue Shi; Lee, Y.-J.; Levin, A.; Luckey, P. David, Jr.; Ma, Teng; Paus, Christoph M. E.; Ralph, Duncan Kelley; Roland, Christof E.; Roland, Gunther M.; Stephans, George S. F.; Stockli, Fabian; Sumorok, Konstanty C.; Velicanu, Dragos Alexandru; Veverka, Jan; Wyslouch, Boleslaw; Yang, M.; Yoon, A. S.; Zanetti, Marco; Zhukova, Victoria
Dijet production has been measured in pPb collisions at a nucleon–nucleon centre-of-mass energy of 5.02 TeV . A data sample corresponding to an integrated luminosity of 35 nb[superscript −1] was collected using the Compact Muon Solenoid detector at the Large Hadron Collider. The dijet transverse momentum balance, azimuthal angle correlations, and pseudorapidity distributions are studied as a function of the transverse energy in the forward calorimeters ( E[4<|η|<5.2 over T] ). For pPb collisions, the dijet transverse momentum ratio and the width of the distribution of dijet azimuthal angle difference are comparable to the same quantities obtained from a simulated...

48. Electron reconstruction and identification efficiency measurements with the ATLAS detector using the 2011 LHC proton–proton collision data - Taylor, Frank E.
Many of the interesting physics processes to be measured at the LHC have a signature involving one or more isolated electrons. The electron reconstruction and identification efficiencies of the ATLAS detector at the LHC have been evaluated using proton–proton collision data collected in 2011 at √s = 7 TeV and corresponding to an integrated luminosity of 4.7 fb[superscript −1]. Tag-and-probe methods using events with leptonic decays of W and Z bosons and J/ψ mesons are employed to benchmark these performance parameters. The combination of all measurements results in identification efficiencies determined with an accuracy at the few per mil level...

49. Measurement of the underlying event in jet events from 7 TeV proton–proton collisions with the ATLAS detector - Taylor, Frank E.
Distributions sensitive to the underlying event in QCD jet events have been measured with the ATLAS detector at the LHC, based on 37 pb[superscript −1] of proton–proton collision data collected at a centre-of-mass energy of 7 TeV . Charged-particle mean p[subscript T] and densities of all-particle E[subscript T] and charged-particle multiplicity and p[subscript T] have been measured in regions azimuthally transverse to the hardest jet in each event. These are presented both as one-dimensional distributions and with their mean values as functions of the leading-jet transverse momentum from 20 to 800 GeV . The correlation of charged-particle mean p[subscript T]...

50. Search for WWγ and WZγ Production and Constraints on Anomalous Quartic Gauge Couplings in pp Collisions at √s = 8 TeV - Apyan, Aram; Barbieri, R.; Bauer, Gerry P.; Busza, Wit; Cali, Ivan Amos; Chan, M.; Di Matteo, Leonardo; Dutta, V.; Gomez-Ceballos, Guillelmo; Goncharov, Maxim; Gulhan, Doga Can; Klute, Markus; Lai, Yue Shi; Lee, Y.-J.; Levin, A.; Luckey, P. David, Jr.; Ma, Teng; Paus, Christoph M. E.; Ralph, Duncan Kelley; Roland, Christof E.; Roland, Gunther M.; Stephans, George S. F.; Stockli, Fabian; Sumorok, Konstanty C.; Velicanu, Dragos Alexandru; Veverka, Jan; Wyslouch, Boleslaw; Yang, M.; Yoon, A. S.; Zanetti, Marco; Zhukova, Victoria
A search for WVγ triple vector boson production is presented based on events containing a W boson decaying to a muon or an electron and a neutrino, a second V (W or Z) boson, and a photon. The data correspond to an integrated luminosity of 19.3  fb[superscript −1] collected in 2012 with the CMS detector at the LHC in pp collisions at √s = 8  TeV. An upper limit of 311 fb on the cross section for the WVγ production process is obtained at 95% confidence level for photons with a transverse energy above 30 GeV and with an absolute value of...

51. Evidence of b-Jet Quenching in PbPb Collisions at √s[subscript NN] = 2.76 TeV - Apyan, Aram; Barbieri, R.; Bauer, Gerry P.; Busza, Wit; Cali, Ivan Amos; Chan, M.; Di Matteo, Leonardo; Dutta, V.; Gomez-Ceballos, Guillelmo; Goncharov, Maxim; Gulhan, Doga Can; Klute, Markus; Lai, Yue Shi; Lee, Y.-J.; Levin, A.; Luckey, P. David, Jr.; Ma, Teng; Paus, Christoph M. E.; Ralph, Duncan Kelley; Roland, Christof E.; Roland, Gunther M.; Stephans, George S. F.; Stockli, Fabian; Sumorok, Konstanty C.; Velicanu, Dragos Alexandru; Veverka, Jan; Wyslouch, Boleslaw; Yang, M.; Yoon, A. S.; Zanetti, Marco; Zhukova, Victoria
The production of jets associated to bottom quarks is measured for the first time in PbPb collisions at a center-of-mass energy of 2.76 TeV per nucleon pair. Jet spectra are reported in the transverse momentum (p[subscript T]) range of 80–250  GeV/c, and within pseudorapidity |η| < 2. The nuclear modification factor (R[subscript AA]) calculated from these spectra shows a strong suppression in the b-jet yield in PbPb collisions relative to the yield observed in pp collisions at the same energy. The suppression persists to the largest values of p[subscript T] studied, and is centrality dependent. The R[subscript AA] is about 0.4...

52. Search for high-mass dilepton resonances in pp collisions at √s = 8 TeV with the ATLAS detector - Taylor, Frank E.
The ATLAS detector at the Large Hadron Collider is used to search for high-mass resonances decaying to dielectron or dimuon final states. Results are presented from an analysis of proton-proton (pp) collisions at a center-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3  fb[superscript −1] in the dimuon channel. A narrow resonance with Standard Model Z couplings to fermions is excluded at 95% confidence level for masses less than 2.79 TeV in the dielectron channel, 2.53 TeV in the dimuon channel, and 2.90 TeV in the two channels combined. Limits on other model interpretations are also presented, including...

53. Search for Dark Matter in Events with a Z Boson and Missing Transverse Momentum in pp Collisions at √s = 8 TeV with the ATLAS Detector - Taylor, Frank E.
A search is presented for production of dark-matter particles recoiling against a leptonically decaying Z boson in 20.3  fb[superscript −1] of pp collisions at √s = 8  TeV with the ATLAS detector at the Large Hadron Collider. Events with large missing transverse momentum and two oppositely charged electrons or muons consistent with the decay of a Z boson are analyzed. No excess above the Standard Model prediction is observed. Limits are set on the mass scale of the contact interaction as a function of the dark-matter particle mass using an effective field theory description of the interaction of dark matter with quarks...

54. Study of the Reaction e[superscript +]e[superscript −] → ψ(2S)π[superscript +]π[superscript − via Initial-State Radiation at BaBar - Cowan, Ray Franklin; Dujmic, Denis; Sciolla, Gabriella
We study the process e[superscript +]e[superscript −] → ψ(2S)π[superscript +]π[superscript −] with initial-state-radiation events produced at the PEP-II asymmetric-energy collider. The data were recorded with the BABAR detector at center-of-mass energies at and near the Υ(nS) (n = 2,3,4) resonances and correspond to an integrated luminosity of 520  fb[superscript −1]. We investigate the ψ(2S)π[superscript +]π[superscript −] mass distribution from 3.95 to 5.95  GeV/c[superscript 2], and measure the center-of-mass energy dependence of the associated e[superscript +]e[superscript −] → ψ(2S)π[superscript +]π[superscript −] cross section. The mass distribution exhibits evidence of two resonant structures. A fit to the ψ(2S)π[superscript +]π[superscript −] mass distribution corresponding to...

55. Search for Gravitational Waves Associated with γ-Ray Bursts Detected by the Interplanetary Network - Aggarwal, Nancy; Barsotti, Lisa; Biscans, Sebastien; Bodiya, Timothy Paul; Buikema, Aaron; Donovan, Frederick J.; Essick, Reed Clasey; Evans, Matthew J.; Fritschel, Peter K.; Gras, Slawek; Isogai, Tomoki; Katsavounidis, Erotokritos; Lee, P. J.; Libson, Adam; MacInnis, Myron E.; Mason, Kenneth R.; Matichard, Fabrice; Mavalvala, Nergis; Miller, John; Mittleman, Richard K.; Oelker, Eric Glenn; Sankar, S.; Shoemaker, David H.; Vaulin, Ruslan; Vitale, Salvatore; Weiss, Rainer; Wipf, C. C.; Yam, William; Zhang, Fan; Zucker, Michael E.
We present the results of a search for gravitational waves associated with 223 γ-ray bursts (GRBs) detected by the InterPlanetary Network (IPN) in 2005–2010 during LIGO’s fifth and sixth science runs and Virgo’s first, second, and third science runs. The IPN satellites provide accurate times of the bursts and sky localizations that vary significantly from degree scale to hundreds of square degrees. We search for both a well-modeled binary coalescence signal, the favored progenitor model for short GRBs, and for generic, unmodeled gravitational wave bursts. Both searches use the event time and sky localization to improve the gravitational wave search...

56. Dielectron Mass Spectra from Au + Au Collisions at √s[subscript NN] = 200 GeV - Stevens, Justin; van Nieuwenhuizen, Gerrit Jan
We report the STAR measurements of dielectron (e[superscript +]e[superscript −]) production at midrapidity (|y[subscript ee]| < 1) in Au + Au collisions at √s[subscript NN] = 200  GeV. The measurements are evaluated in different invariant mass regions with a focus on 0.30–0.76 (ρ-like), 0.76–0.80 (ω-like), and 0.98–1.05 (ϕ-like) GeV/c[superscript 2]. The spectrum in the ω-like and ϕ-like regions can be well described by the hadronic cocktail simulation. In the ρ-like region, however, the vacuum ρ spectral function cannot describe the shape of the dielectron excess. In this range, an enhancement of 1.77 ± 0.11(stat) ± 0.24(syst) ± 0.33(cocktail) is determined with...

57. Measurement of the resonant and CP components in B¯[superscript 0]-->J/ψπ+π− decays - Counts, Ian Thomas Hunt; Ilten, Philip James; Williams, M.
The resonant structure of the reaction B¯0→J/ψπ+π− is studied using data from 3  fb−1 of integrated luminosity collected by the LHCb experiment, one third at 7 TeV center-of-mass energy and the remainder at 8 TeV. The invariant mass of the π+π− pair and three decay angular distributions are used to determine the fractions of the resonant and nonresonant components. Six interfering π+π− states, ρ(770), f0(500), f2(1270), ρ(1450), ω(782) and ρ(1700), are required to give a good description of invariant mass spectra and decay angular distributions. The positive and negative charge parity fractions of each of the resonant final states are determined....

58. Precision Measurement of the Mass and Lifetime of the Ξ[0 over b] Baryon - Counts, Ian Thomas Hunt; Ilten, Philip James; Williams, M.; LHCb collaboration
Using a proton-proton collision data sample corresponding to an integrated luminosity of 3  fb[superscript −1] collected by LHCb at center-of-mass energies of 7 and 8 TeV, about 3800 Ξ0b→Ξ+cπ−, Ξ+c→pK−π+ signal decays are reconstructed. From this sample, the first measurement of the Ξ0b baryon lifetime is made, relative to that of the Λ0b baryon. The mass differences M(Ξ0b)−M(Λ0b) and M(Ξ+c)−M(Λ+c) are also measured with precision more than 4 times better than the current world averages. The resulting values are τΞ0bτΛ0b=1.006±0.018±0.010,M(Ξ0b)−M(Λ0b)=172.44±0.39±0.17  MeV/c2,M(Ξ+c)−M(Λ+c)=181.51±0.14±0.10  MeV/c2, where the first uncertainty is statistical and the second is systematic. The relative rate of Ξ0b to Λ0b baryon production is measured to...

59. Search for Invisible Decays of a Higgs Boson Produced in Association with a Z Boson in ATLAS - Taylor, Frank E.
A search for evidence of invisible-particle decay modes of a Higgs boson produced in association with a Z boson at the Large Hadron Collider is presented. No deviation from the standard model expectation is observed in 4.5  fb−1 (20.3  fb−1) of 7 (8) TeV pp collision data collected by the ATLAS experiment. Assuming the standard model rate for ZH production, an upper limit of 75%, at the 95% confidence level is set on the branching ratio to invisible-particle decay modes of the Higgs boson at a mass of 125.5 GeV. The limit on the branching ratio is also interpreted in terms of...

60. Search for massive resonances decaying into pairs of boosted bosons in semi-leptonic final states at s $$\sqrt{s}$$ = 8 TeV - Apyan, Aram; Barbieri, Richard Alexander; Bauer, Gerry P.; Busza, Wit; Cali, Ivan Amos; Chan, M.; Di Matteo, Leonardo; Dutta, Valentina; Gomez-Ceballos, Guillelmo; Goncharov, Maxim; Gulhan, Doga Can; Klute, Markus; Lai, Yue Shi; Lee, Yen-Jie; Levin, Andrew Michael; Luckey, P. David; Ma, T.; Paus, Christoph M. E.; Ralph, Duncan Kelley; Roland, Christof E.; Roland, Gunther M.; Stephans, George S. F.; Stoeckli, Fabian; Sumorok, Konstanty C.; Velicanu, Dragos Alexandru; Veverka, Jan; Wyslouch, Boleslaw; Yang, M.; Zanetti, Marco; Zhukova, Victoria
A search for new resonances decaying to WW, ZZ, or WZ is presented. Final states are considered in which one of the vector bosons decays leptonically and the other hadronically. Results are based on data corresponding to an integrated luminosity of 19.7 fb−1 recorded in proton-proton collisions at s√ = 8 TeV with the CMS detector at the CERN LHC. Techniques aiming at identifying jet substructures are used to analyze signal events in which the hadronization products from the decay of highly boosted W or Z bosons are contained within a single reconstructed jet. Upper limits on the production of...