Kenneth Bloom Publications

Search for Supersymmetry at the LHC in Events with Jets and Missing Transverse Energy

CMS Collaboration et al. — Fri, 19 Oct 2012 09:03:01 PDT

A search for events with jets and missing transverse energy is performed in a data sample of pp collisions collected at √s = 7 TeV by the CMS experiment at the LHC. The analyzed data sample corresponds to an integrated luminosity of 1.14 fb^-1. In this search, a kinematic variable α_T is used as the main discriminator between events with genuine and misreconstructed missing transverse energy. No excess of events over the standard model expectation is found. Exclusion limits in the parameter space of the constrained minimal supersymmetric extension of the standard model are set. In this model, squark masses below 1.1 TeV are excluded at 95% C.L. Gluino masses below 1.1 TeV are also ruled out at 95% C.L. for values of the universal scalar mass parameter below 500 GeV.

Study of W boson production in PbPb and pp collisions at √sNN = 2.76 TeV

E. Avdeeva et al. — Fri, 19 Oct 2012 08:57:12 PDT

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 μb⁻¹, the number of W → μν_μ decays is extracted in the region of muon pseudorapidity |η^μ| < 2.1 and transverse momentum p^μ_T > 25 GeV/c. Yields of muons found per unit of pseudorapidity correspond to (159 ± 10(stat.) ± 12(syst.)) × 10⁻⁸ W⁺ and (154 ± 10(stat.) ± 12(syst.)) × 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.

Search for the standard model Higgs boson decaying to bottom quarks in pp collisions at √s = 7 TeV

CMS Collaboration et al. — Fri, 19 Oct 2012 08:50:21 PDT

A search for the standard model Higgs boson (H) decaying to bb̅ when produced in association with weak vector bosons (V) is reported for the following modes: W(μν)H, W(eν)H, Z(μμ)H, Z(ee)H and Z(νν)H. The search is performed in a data sample corresponding to an integrated luminosity of 4.7 fb⁻¹, recorded by the CMS detector in proton–proton collisions at the LHC with a center-of-mass energy of 7 TeV. No significant excess of events above the expectation from background is observed. Upper limits on the V H production cross section times the H→bb̅ branching ratio, with respect to the expectations for a standard model Higgs boson, are derived for a Higgs boson in the mass range 110–135 GeV. In this range, the observed 95% confidence level upper limits vary from 3.4 to 7.5 times the standard model prediction; the corresponding expected limits vary from 2.7 to 6.7 times the standard model prediction.

Search for the standard model Higgs boson decaying into two photons in pp collisions at √s = 7 TeV

CMS Collaboration et al. — Fri, 19 Oct 2012 08:42:33 PDT

A search for a Higgs boson decaying into two photons is described. The analysis is performed using a dataset recorded by the CMS experiment at the LHC from pp collisions at a center-of-mass energy of 7 TeV, which corresponds to an integrated luminosity of 4.8 fb⁻¹. Limits are set on the cross section of the standard model Higgs boson decaying to two photons. The expected exclusion limit at 95% confidence level is between 1.4 and 2.4 times the standard model cross section in the mass range between 110 and 150 GeV. The analysis of the data excludes, at 95% confidence level, the standard model Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The largest excess of events above the expected standard model background is observed for a Higgs boson mass hypothesis of 124 GeV with a local significance of 3.1σ. The global significance of observing an excess with a local significance ≥ 3.1σ anywhere in the search range 110–150 GeV is estimated to be 1.8σ. More data are required to ascertain the origin of this excess.

Search for physics beyond the standard model in events with a Z boson, jets, and missing transverse energy in pp collisions at √s = 7 TeV

CMS Collaboration et al. — Fri, 19 Oct 2012 08:34:25 PDT

A search is presented for physics beyond the standard model (BSM) in events with a Z boson, jets, and missing transverse energy (E^miss_T ). This signature is motivated by BSM physics scenarios, including supersymmetry. The study is performed using a sample of proton–proton collision data collected at √s = 7 TeV with the CMS experiment at the LHC, corresponding to an integrated luminosity of 4.98 fb⁻¹. The contributions from the dominant standard model backgrounds are estimated from data using two complementary strategies, the jet-Z balance technique and a method based on modeling E^miss_T with data control samples. In the absence of evidence for BSM physics, we set limits on the non-standard-model contributions to event yields in the signal regions and interpret the results in the context of simplified model spectra. Additional information is provided to facilitate tests of other BSM physics models.

Search for narrow resonances in dilepton mass spectra in pp collisions at √s = 7 TeV

CMS Collaboration et al. — Fri, 19 Oct 2012 08:29:38 PDT

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 fb⁻¹. 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' in the Sequential Standard Model, 2000 GeV for the superstring-inspired Z'_ψ resonance, 890 (540) GeV for the Stueckelberg extension Z'_St with the mass parameter є = 0.06 (0.04), and 2140 (1810) GeV for Kaluza–Klein gravitons with the coupling parameter k/M̅_Pl of 0.10 (0.05). These limits are the most stringent to date.

Search for large extra dimensions in dimuon and dielectron events in pp collisions at √s = 7 TeV

CMS Collaboration et al. — Fri, 19 Oct 2012 08:19:40 PDT

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 μμ 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.

Search for high-mass resonances decaying into τ -lepton pairs in pp collisions at √s = 7 TeV

CMS Collaboration et al. — Fri, 19 Oct 2012 08:15:57 PDT

A search for high-mass resonances decaying into τ⁺τ⁻ is performed using a data sample of pp collisions at √s = 7 TeV. The data were collected with the CMS detector at the LHC and correspond to an integrated luminosity of 4.9 fb⁻¹. The number of observed events is in agreement with the standard model prediction. An upper limit on the product of the resonance cross section and branching fraction into τ -lepton pairs is calculated as a function of the resonance mass. Using the sequential standard model resonance Z'_SSM and the superstring-inspired E₆ model with resonance Z'_ψ as benchmarks, resonances with standard model couplings with masses below 1.4 and 1.1 TeV, respectively, are excluded at 95% confidence level.

Search for heavy, top-like quark pair production in the dilepton final state in pp collisions at √s = 7 TeV

CMS Collaboration et al. — Fri, 19 Oct 2012 08:10:42 PDT

The results of a search for pair production of a heavy, top-like quark, t', in the decay mode t't̅' → bW⁺ b̅W⁻ → bℓ⁺νb̅ℓ⁻ν̅ are presented. The search is performed with a data sample corresponding to an integrated luminosity of 5.0 fb⁻¹ in pp collisions at a center-of-mass energy of 7 TeV, collected by the CMS experiment at the LHC. The observed number of events agrees with the expectation from standard model processes, and no evidence of t't̅' production is found. Upper limits on the production cross section as a function of t' mass are presented, and t' masses below 557 GeV/c² are excluded at the 95% confidence level.

Search for heavy Majorana neutrinos in μ±μ± +jets and e±e± + jets events in pp collisions at √s = 7 TeV

CMS Collaboration et al. — Fri, 19 Oct 2012 08:03:52 PDT

A search is performed for heavy Majorana neutrinos (N) using an event signature defined by two samesign charged leptons of the same flavour and two jets. The data correspond to an integrated luminosity of 4.98 fb⁻¹ of pp collisions at a centre-of-mass energy of 7 TeV collected with the CMS detector at the Large Hadron Collider. No excess of events is observed beyond the expected standard model background and therefore upper limits are set on the square of the mixing parameter, |V_ℓN|², for ℓ = e,μ, as a function of heavy Majorana-neutrino mass. These are the first direct upper limits on the heavy Majorana-neutrino mixing for m_N > 90 GeV.

Search for heavy long-lived charged particles in pp collisions at √s = 7 TeV

CMS Collaboration et al. — Fri, 19 Oct 2012 07:49:33 PDT

The result of a search for heavy long-lived charged particles produced in pp collisions at √s = 7 TeV at the LHC is described. The data sample has been collected using the CMS detector and corresponds to an integrated luminosity of 5.0 fb⁻¹. The inner tracking detectors are used to define a sample of events containing tracks with high momentum and high ionization energy loss. A second sample of events, which have high-momentum tracks satisfying muon identification requirements in addition to meeting high-ionization and long time-of-flight requirements, is analyzed independently. In both cases, the results are consistent with the expected background estimated from data. The results are used to establish cross section limits as a function of mass within the context of models with long-lived gluinos, scalar top quarks and scalar taus. Cross section limits on hyper-meson particles, containing new elementary longlived hyper-quarks predicted by a vector-like confinement model, are also presented. Lower limits at 95% confidence level on the mass of gluinos (scalar top quarks) are found to be 1098 (737) GeV/c². A limit of 928 (626) GeV/c² is set for a gluino (scalar top quark) that hadronizes into a neutral bound state before reaching the muon detectors. The lower mass limit for a pair produced scalar tau is found to be 223 GeV/c2. Mass limits for a hyper-kaon are placed at 484, 602, and 747 GeV/c² for hyper-ρ masses of 800, 1200, and 1600 GeV/c², respectively.

Search for charge-asymmetric production of W' bosons in tt + jet events from pp collisions at √s = 7 TeV

CMS Collaboration et al. — Fri, 19 Oct 2012 07:45:50 PDT

A search is presented for charge-asymmetric production of a W' boson that has been proposed to accommodate the forward–backward asymmetry observed in the production of top–antitop quark pairs at the Tevatron. The new heavy W' boson would be produced in association with a top quark and would decay into top and down quarks. The data correspond to an integrated luminosity of 5.0 fb⁻¹ in pp collisions at a center-of-mass energy of 7 TeV, recorded by the CMS detector at the LHC. No significant excess above the standard model expectations is observed, and, from a combination of the electron-plusjets and muon-plus-jets channels, a 95% confidence level lower limit of 840 GeV/c² is set on the W' boson mass for a W' boson model with values for coupling constants to top and down quarks g_L = 0 and g_R = 2. In addition, a kinematic reconstruction of the W[1] resonance mass using the inherent charge asymmetry of this model finds no indication of the presence of W' events in the data.

Search for neutral Higgs bosons decaying to tau pairs in pp collisions at √s = 7 TeV

CMS Collaboration et al. — Fri, 19 Oct 2012 07:40:08 PDT

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 fb⁻¹ 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 super symmetric 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 110–145 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, mA, sets stringent new bounds in the parameter space, excluding at 95% CL values of tan β as low as 7.1 at m_A = 160 GeV in the m^max_h benchmark scenario.

Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC

CMS Collaboration et al. — Fri, 19 Oct 2012 07:34:01 PDT

Results are presented from searches for the standard model Higgs boson in proton–proton collisions at √s = 7 and 8 TeV in the Compact Muon Solenoid experiment at the LHC, using data samples corresponding to integrated luminosities of up to 5.1 fb⁻¹ at 7 TeV and 5.3 fb⁻¹ at 8 TeV. The search is performed in five decay modes: γγ, ZZ, W⁺W⁻, τ⁺τ⁻, and bb̅. An excess of events is observed above the expected background, with a local significance of 5.0 standard deviations, at a mass near 125 GeV, signalling the production of a new particle. The expected significance for a standard model Higgs boson of that mass is 5.8 standard deviations. The excess is most significant in the two decay modes with the best mass resolution, γγ and ZZ; a fit to these signals gives a mass of 125.3±0.4(stat.)±0.5(syst.) GeV. The decay to two photons indicates that the new particle is a boson with spin different from one.

Measurement of the Λb cross section and the Λb to Λb ratio with J/ψΛ decays in pp collisions at √s = 7 TeV

CMS Collaboration et al. — Fri, 19 Oct 2012 07:28:16 PDT

The Λ_b differential production cross section and the cross section ratio σ(Λ̅_b )/σ(Λ_b) are measured as functions of transverse momentum p^Λ^b_T and rapidity |y^Λb | in pp collisions at √ s = 7 TeV using data collected by the CMS experiment at the LHC. The measurements are based on Λ_b decays reconstructed in the exclusive final state J/ψΛ, with the subsequent decays J/ψ →μ⁺μ⁻ and Λ→pπ, using a data sample corresponding to an integrated luminosity of 1.9 fb⁻¹. The product σ(Λ_b) × B(Λ_b →J/ψΛ) versus p^Λ^b_T falls faster than that of b mesons. The measured value of σ(Λ_b) × B(Λ_b →J/ψΛ) for p^Λ^b_T > 10 GeV and |y^Λ^b | < 2.0 is 1.16±0.06±0.12 nb, and the integrated σ(Λ̅_b)/σ(Λ_b) ratio is 1.02±0.07±0.09, where the uncertainties are statistical and systematic, respectively.

Measurement of isolated photon production in pp and PbPb collisions at √sNN = 2.76 TeV

CMS Collaboration et al. — Fri, 19 Oct 2012 07:15:48 PDT

Isolated photon production is measured in proton–proton and lead–lead collisions at nucleon–nucleon centre-of-mass energies of 2.76 TeV in the pseudorapidity range |η| < 1.44 and transverse energies E_T between 20 and 80 GeV with the CMS detector at the LHC. The measured E_T spectra are found to be in good agreement with next-to-leading-order perturbative QCD predictions. The ratio of PbPb to pp isolated photon E_T -differential yields, scaled by the number of incoherent nucleon–nucleon collisions, is consistent with unity for all PbPb reaction centralities.

Jet momentum dependence of jet quenching in PbPb collisions at √sNN = 2.76 TeV

CMS Collaboration et al. — Fri, 19 Oct 2012 07:02:54 PDT

Dijet production in PbPb collisions at a nucleon–nucleon 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 μb⁻¹ 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.

Inclusive and differential measurements of the tt̅ charge asymmetry in proton–proton collisions at √s= 7 TeV

CMS Collaboration et al. — Fri, 19 Oct 2012 06:49:46 PDT

The tt̅ charge asymmetry is measured in events containing a charged lepton (electron or muon) and at least four jets, one of which is identified as originating from b-quark hadronization. The analyzed data set corresponds to an integrated luminosity of 5.0 fb⁻¹ collected with the CMS detector at the LHC. An inclusive and three differential measurements of the tt̅ charge asymmetry as a function of rapidity, transverse momentum, and invariant mass of the tt̅ system are presented. The measured inclusive tt̅ charge asymmetry is A_C = 0.004 ± 0.010 (stat.) ± 0.011 (syst.). This result and the three differential measurements are consistent with zero asymmetry as well as with the predictions of the standard model.

Search for Universal Extra Dimensions in pp Collisions

V. M. Abazov et al. — Thu, 10 May 2012 13:25:36 PDT

We present a search for Kaluza-Klein (KK) particles predicted by models with universal extra dimensions (UED) using a data set corresponding to an integrated luminosity of 7.3 fb^-1, collected by the D0 detector at a pp̅ center-of-mass energy of 1.96 TeV. The decay chain of KK particles can lead to a final state with two muons of the same charge. This signature is used to set a lower limit on the compactification scale of R^-1 > 260 GeV in a minimal UED model.

Search for Charged Massive Long-Lived Particles

V. M. Abazov et al. — Thu, 10 May 2012 13:22:00 PDT

We report on a search for charged massive long-lived particles (CMLLPs), based on 5.2 fb^-1 of integrated luminosity collected with the D0 detector at the Fermilab Tevatron pp̅ collider. We search for events in which one or more particles are reconstructed as muons but have speed and ionization energy loss (dE/dx) inconsistent with muons produced in beam collisions. CMLLPs are predicted in several theories of physics beyond the standard model. We exclude pair-produced long-lived gauginolike charginos below 267 GeV and Higgsino-like charginos below 217 GeV at 95% C.L., as well as longlived scalar top quarks with mass below 285 GeV.