Results from the Commissioning of the ATLAS Pixel Detector with Cosmic data

The ATLAS pixel detector is the innermost detector of the ATLAS experiment at the Large Hadron Collider at CERN. With approximately 80 million readout channels, the ATLAS silicon pixel detector is a high-acceptance, high-resolution, low-noise tracking device. Providing the desired refinement in charged track pattern recognition capability in order to meet the stringent track reconstruction requirements, the pixel detector largely defines the ability of ATLAS to effectively resolve primary and secondary vertices and perform efficient flavor tagging essential for discovery of new physics. Being the last sub-system installed in ATLAS by July 2007, the pixel detector was successfully connected, commissioned, and tested in situ while meeting an extremely tight schedule, and was ready to take data upon the projected turn-on of the LHC. Since fall 2008, the pixel detector has been included in the combined ATLAS detector operation, collecting cosmic muon data. Details from the pixel detector installation and commissioning, as well as details on calibration procedures and the results obtained with collected cosmic data, are presented along with a summary of the detector status.Comment: To be published in the proceedings of DPF-2009, Detroit, MI, July 2009, eConf C090726. Contents: 9 pages, 13 figures, 9 reference

Application of bent crystals at IHEP 70-GeV accelerator to enhance the efficiency of its usage

Bent crystal was extracting 70-GeV protons with average intensity 4*10^11 (as measured in external beamline) per spill of 1.6 s duration, in parallel to the simultaneous work of two internal targets in the accelerator ring. An additional crystal, placed in the external beamline, was deflecting a small part of the extracted beam with intensity 10^7 protons toward another physics experiment. Crystal-extracted beam had a typical size of 4 mm by 4 mm fwhm at the end of the external beamline. Measurements for the extraction efficiency and other characteristics at the simultaneous work of four experimental set-ups are presented. With crystal working in the above-said regime during one month, no degradation of channeling was observed. The studies of extraction efficiency have been continued with new crystals.Comment: 6pp. Presented at EPAC 200

Direct Measurement of the Top Quark Mass at D0

We determine the top quark mass m_t using t-tbar pairs produced in the D0 detector by \sqrt{s} = 1.8 TeV p-pbar collisions in a 125 pb^-1 exposure at the Fermilab Tevatron. We make a two constraint fit to m_t in t-tbar -> b W^+bbar W^- final states with one W boson decaying to q-qbar and the other to e-nu or mu-nu. Likelihood fits to the data yield m_t(l+jets) = 173.3 +- 5.6 (stat) +- 5.5 (syst) GeV/c^2. When this result is combined with an analysis of events in which both W bosons decay into leptons, we obtain m_t = 172.1 +- 5.2 (stat) +- 4.9 (syst) GeV/c^2. An alternate analysis, using three constraint fits to fixed top quark masses, gives m_t(l+jets) = 176.0 +- 7.9 (stat) +- 4.8 (syst) GeV/C^2, consistent with the above result. Studies of kinematic distributions of the top quark candidates are also presented.Comment: 43 pages, 53 figures, 33 tables. RevTeX. Submitted to Phys. Rev.

Search for Charged Higgs Bosons in Decays of Top Quark Pairs

We present a search for charged Higgs bosons in decays of pair-produced top quarks using 109.2 +- 5.8 pb^-1 of data recorded from ppbar collisions at sqrt{s} = 1.8 TeV by the D0 detector during 1992-96 at the Fermilab Tevatron. No evidence is found for charged Higgs production, and most parts of the [m(H+),tan(beta)] parameter space where the decay t -> bH+ has a branching fraction close to or larger than that for t -> bW+ are excluded at 95% confidence level. Assuming m(t) = 175 GeV and sigma(ppbar -> ttbar) = 5.5 pb, for m(H+) = 60 GeV, we exclude tan(beta) 40.9.Comment: 11 pages, 3 figures, submitted to PR

Search for Electroweak Production of Single Top Quarks in ppbar Collisions

We present a search for electroweak production of single top quarks in the electron+jets and muon+jets decay channels. The measurements use ~90 pb^-1 of data from Run 1 of the Fermilab Tevatron collider, collected at 1.8 TeV with the DZero detector between 1992 and 1995. We use events that include a tagging muon, implying the presence of a b jet, to set an upper limit at the 95% confidence level on the cross section for the s-channel process ppbar->tb+X of 39 pb. The upper limit for the t-channel process ppbar->tqb+X is 58 pb.Comment: 11 pages, 2 figures. This is the published versio

Probing BFKL Dynamics in the Dijet Cross Section at Large Rapidity Intervals in ppbar Collisions at sqrt{s}=1800 and 630 GeV

Inclusive dijet production at large pseudorapidity intervals (delta_eta) between the two jets has been suggested as a regime for observing BFKL dynamics. We have measured the dijet cross section for large delta_eta in ppbar collisions at sqrt{s}=1800 and 630 GeV using the DO detector. The partonic cross section increases strongly with the size of delta_eta. The observed growth is even stronger than expected on the basis of BFKL resummation in the leading logarithmic approximation. The growth of the partonic cross section can be accommodated with an effective BFKL intercept of a_{BFKL}(20GeV)=1.65+/-0.07.Comment: Published in Physical Review Letter

Search for Squarks and Gluinos in Events Containing Jets and a Large Imbalance in Transverse Energy

Using data corresponding to an integrated luminosity of 79 pb-1, D0 has searched for events containing multiple jets and large missing transverse energy in pbar-p collisions at sqrt(s)=1.8 TeV at the Fermilab Tevatron collider. Observing no significant excess beyond what is expected from the standard model, we set limits on the masses of squarks and gluinos and on the model parameters m_0 and m_1/2, in the framework of the minimal low-energy supergravity models of supersymmetry. For tan(beta) = 2 and A_0 = 0, with mu < 0, we exclude all models with m_squark < 250 GeV/c^2. For models with equal squark and gluino masses, we exclude m < 260 GeV/c^2.Comment: 10 pages, 3 figures, Submitted to PRL, Fixed typo on page bottom of p. 6 (QCD multijet background is 35.4 events

Direct Search for Charged Higgs Bosons in Decays of Top Quarks

We present a search for charged Higgs bosons in decays of pair-produced top quarks in pbar p collisions at sqrt(s) = 1.8 TeV using 62.2 pb^-1 of data recorded by the D0 detector at the Fermilab Tevatron collider. No evidence is found for signal, and we exclude at 95% confidence most regions of the (M higgs, tan beta) parameter space where the decay t->H b has a branching fraction greater than 0.36 and B(H -> tau nu) is large.Comment: 11 pages, 4 figures, submitted to Phys. Rev. Let