73 research outputs found
ATLAS monitored drift tube chambers for super-LHC
After the high-luminosity upgrade of the Large Hadron Collider (LHC) at CERN,
the ATLAS muon spectrometer is expected to work at 10 times increased
background rates of gammas and neutrons. This is challenging as the momentum
resolution of the spectrometer is expected to be 10 %. This requires a single
tube resolution of the muon drift tubes of 80 mum. At background rates around
1000 Hz/cm2 space charge effects will lead in the slow and non-linear AR:CO2 =
93:7 gas mixture to a degradation of the drift-tube spatial resolution. This
was studied before experimentally for gammas and low energetic neutrons. Almost
no information exists for fast neutrons. Therefore, we organized our studies
under the following aspects: - We investigated the influence of 11 MeV neutrons
on the position resolution of ATLAS MDT chambers. At flux densities between 4
and 16 kHz/cm2, almost no influence on the position resolution was found, it
degrades by only 10 mum at a detection efficiency of only 4*10-4. - We
investigated inert gas mixtures on fastness and linearity of their
position-drifttime (r-t) relation. At a reduction of the maximum drift time by
a factor of 2, the use of the present hardware and electronics might be
possible. For our experimental studies we used our Munich cosmic ray facility.
Two gas mixtures show almost identical position resolution as the standard gas.
- For spectrometer regions of highest background rates we contributed to the
investigation of newly developed 15 mm drift tubes. Position resolutions have
been measured as a function of gamma background rates between 0 and 1400
Hz/cm2. - Garfield simulations have been performed to simulate space charge
effects due to gamma irradiation. Results will be presented for the standard
geometry as well as for the new 15 mm drift tubes.Comment: 3 pages, 7 figures, conferenc
Measurement of the charge asymmetry in top-quark pair production in the lepton-plus-jets final state in pp collision data at root s=8 TeV with the ATLAS detector
This paper reports inclusive and differential measurements of the t (t) over bar charge asymmetry A(C) in 20.3 fb(-1) of root s = 8 TeV pp collisions recorded by the ATLAS experiment at the Large Hadron Collider at CERN. Three differential measurements are performed as a function of the invariant mass, transverse momentum and longitudinal boost of the t (t) over bar system. The t (t) over bar pairs are selected in the single-lepton channels (e or mu) with at least four jets, and a likelihood fit is used to reconstruct the t (t) over bar event kinematics. A Bayesian unfolding procedure is performed to infer the asymmetry at parton level from the observed data distribution. The inclusive t (t) over bar charge asymmetry is measured to be A(C) = 0.009 +/- 0.005 (stat. + syst.). The inclusive and differential measurements are compatible with the values predicted by the Standard Model
Measurement of the b(b)over-bar dijet cross section in pp collisions at root s=7TeV with the ATLAS detector
The dijet production cross section for jets containing a b-hadron (b-jets) has been measured in protonproton collisions with a centre-of-mass energy of root s = 7TeV, using the ATLAS detector at the LHC. The data used correspond to an integrated luminosity of 4.2 fb(-1). The cross section is measured for events with two identified b-jets with a transverse momentum pT > 20GeV and a minimum separation in the eta-phi plane of Delta R = 0.4. At least one of the jets in the event is required to have p(T) > 270GeV. The cross section is measured differentially as a function of dijet invariant mass, dijet transverse momentum, boost of the dijet system, and the rapidity difference, azimuthal angle and angular distance between the b-jets. The results are compared to different predictions of leading order and next-to-leading order perturbative quantum chromodynamics matrix elements supplemented with models for parton-showers and hadronization
A search for prompt lepton-jets in pp collisions at root s=8 TeV with the ATLAS detector
A search is presented for a new, light boson with a mass of about 1 GeV and decaying promptly to jets of collimated electrons and/or muons (lepton-jets). The analysis is performed with 20.3 fb(-1) of data collected by the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a centre-of-mass energy of 8 TeV. Events are required to contain at least two lepton-jets. This study finds no statistically significant deviation from predictions of the Standard Model and places 95% confidence-level upper limits on the contribution of new phenomena beyond the SM, incuding SUSY-portal and Higgs-portal models, on the number of events with lepton-jets
Centrality and rapidity dependence of inclusive jet production in root(NN)-N-S=5.02 TeV proton-lead collisions with the ATLAS detector
Measurements of the centrality and rapidity dependence of inclusive jet production in root(NN)-N-S = 5.02 TeV proton-lead (p + Pb) collisions and the jet cross-section in root s = 2.76 TeV proton-proton collisions are presented. These quantities are measured in datasets corresponding to an integrated luminosity of 27.8 nb(-1) and 4.0 pb(-1),respectively, recorded with the ATLAS detector at the Large Hadron Collider in 2013. The p + Pb collision centrality was characterised using the total transverse energy measured in the pseudorapidity interval -4.9 < eta < -3.2 in the direction of the lead beam. Results are presented for the double-differential per-collision yields as a function of jet rapidity and transverse momentum (p(T)) for minimum-bias and centrality-selected p + Pb collisions, and are compared to the jet rate from the geometric expectation. The total jet yield in minimum-bias events is slightly enhanced above the expectation in a p(T)-dependent manner but is consistent with the expectation within uncertainties. The ratios of jet spectra from different centrality selections show a strong modification of jet production at all p(T) at forward rapidities and for large pT at mid-rapidity, which manifests as a suppression of the jet yield in central events and an enhancement in peripheral events. These effects imply that the factorisation between hard and soft processes is violated at an unexpected level in proton-nucleus collisions. Furthermore, the modifications at forward rapidities are found to be a function of the total jet energy only, implying that the violations may have a simple dependence on the hard parton-parton kinematics. (C) 2015 CERN for the benefit of the ATLAS Collaboration. Published by Elsevier B.V
Fiducial and differential cross sections of Higgs boson production measured in the four-lepton decay channel in pp collisions at root s=8 TeV with the ATLAS detector
Measurements of fiducial and differential cross sections of Higgs boson production in the H -> ZZ* -> 4l decay channel are presented. The cross sections are determined within a fiducial phase space and corrected for detection efficiency and resolution effects. They are based on 20.3 fb(-1) of pp collision data, produced at root s= 8 TeV centre-of-mass energy at the LHC and recorded by the ATLAS detector. The differential measurements are performed in bins of transverse momentum and rapidity of the four-lepton system, the invariant mass of the subleading lepton pair and the decay angle of the leading lepton pair with respect to the beam line in the four-lepton rest frame, as well as the number of jets and the transverse momentum of the leading jet. The measured cross sections are compared to selected theoretical calculations of the Standard Model expectations. No significant deviation from any of the tested predictions is found. Published by Elsevier B.V
High Rate Proton Irradiation of 15mm Muon Drifttubes
Future LHC luminosity upgrades will significantly increase the amount of
background hits from photons, neutrons and protons in the detectors of the
ATLAS muon spectrometer. At the proposed LHC peak luminosity of 5*10^34
1/cm^2s, background hit rates of more than 10 kHz/cm^2 are expected in the
innermost forward region, leading to a loss of performance of the current
tracking chambers. Based on the ATLAS Monitored Drift Tube chambers, a new high
rate capable drift tube detecor using tubes with a reduced diameter of 15mm was
developed. To test the response to highly ionizing particles, a prototype
chamber of 46 15mm drift tubes was irradiated with a 20 MeV proton beam at the
tandem accelerator at the Maier-Leibnitz Laboratory, Munich. Three tubes in a
planar layer were irradiated while all other tubes were used for reconstruction
of cosmic muon tracks through irradiated and non-irradiated parts of the
chamber. To determine the rate capability of the 15mm drift-tubes we
investigated the effect of the proton hit rate on pulse height, efficiency and
spatial resolution of the cosmic muon signals
High-Rate Capable Floating Strip Micromegas
We report on the optimization of discharge insensitive floating strip
Micromegas (MICRO-MEsh GASeous) detectors, fit for use in high-energy muon
spectrometers. The suitability of these detectors for particle tracking is
shown in high-background environments and at very high particle fluxes up to
60MHz/cm. Measurement and simulation of the microscopic discharge behavior
have demonstrated the excellent discharge tolerance. A floating strip
Micromegas with an active area of 48cm50cm with 1920 copper anode
strips exhibits in 120GeV pion beams a spatial resolution of 50m at
detection efficiencies above 95%. Pulse height, spatial resolution and
detection efficiency are homogeneous over the detector. Reconstruction of
particle track inclination in a single detector plane is discussed, optimum
angular resolutions below are observed. Systematic deviations of this
TPC-method are fully understood. The reconstruction capabilities for
minimum ionizing muons are investigated in a 6.4cm6.4cm floating strip
Micromegas under intense background irradiation of the whole active area with
20MeV protons at a rate of 550kHz. The spatial resolution for muons is not
distorted by space charge effects. A 6.4cm6.4cm floating strip
Micromegas doublet with low material budget is investigated in highly ionizing
proton and carbon ion beams at particle rates between 2MHz and 2GHz. Stable
operation up to the highest rates is observed, spatial resolution, detection
efficiencies, the multi-hit and high-rate capability are discussed.Comment: Presented at ICHEP 2014, accepted for publication in Nuclear Physics
B Proceedings Supplement
Performance of Drift-Tube Detectors at High Counting Rates for High-Luminosity LHC Upgrades
The performance of pressurized drift-tube detectors at very high background
rates has been studied at the Gamma Irradiation Facility (GIF) at CERN and in
an intense 20 MeV proton beam at the Munich Van-der-Graaf tandem accelerator
for applications in large-area precision muon tracking at high-luminosity
upgrades of the Large Hadron Collider (LHC). The ATLAS muon drifttube (MDT)
chambers with 30 mm tube diameter have been designed to cope with and neutron
background hit rates of up to 500 Hz/square cm. Background rates of up to 14
kHz/square cm are expected at LHC upgrades. The test results with standard MDT
readout electronics show that the reduction of the drift-tube diameter to 15
mm, while leaving the operating parameters unchanged, vastly increases the rate
capability well beyond the requirements. The development of new small-diameter
muon drift-tube (sMDT) chambers for LHC upgrades is completed. Further
improvements of tracking efficiency and spatial resolution at high counting
rates will be achieved with upgraded readout electronics employing improved
signal shaping for high counting rates
Search for direct pair production of the top squark in all-hadronic final states in proton-proton collisions at s√ = 8 TeV with the ATLAS detector
The results of a search for direct pair production of the scalar partner to the top quark using an integrated luminosity of 20.1 fb−1 of proton-proton collision data at s√ = 8 TeV recorded with the ATLAS detector at the LHC are reported. The top squark is assumed to decay via t¯ →tχ¯01 or t¯ →bχ¯±1 →bW(∗)χ¯01 , where χ¯01 (χ¯±1) denotes the lightest neutralino (chargino) in supersymmetric models. The search targets a fully-hadronic final state in events with four or more jets and large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits are reported in terms of the top squark and neutralino masses and as a function of the branching fraction of t¯ →tχ¯01 . For a branching fraction of 100%, top squark masses in the range 270–645 GeV are excluded for χ¯01 masses below 30 GeV. For a branching fraction of 50% to either t¯ →tχ¯01 or t¯ →bχ¯±1 , and assuming the χ¯±1 mass to be twice the χ¯01 mass, top squark masses in the range 250–550 GeV are excluded for χ¯01 masses below 60 GeV
- …