Results of three searches are presented for the production of supersymmetric particles decaying into final states with missing transverse momentum and exactly two isolated leptons, e or μ. The ...analysis uses a data sample collected during the first half of 2011 that corresponds to a total integrated luminosity of 1 fb−1 of s=7 TeV proton–proton collisions recorded with the ATLAS detector at the Large Hadron Collider. Opposite-sign and same-sign dilepton events are separately studied, with no deviations from the Standard Model expectation observed. Additionally, in opposite-sign events, a search is made for an excess of same-flavour over different-flavour lepton pairs. Effective production cross sections in excess of 9.9 fb for opposite-sign events containing supersymmetric particles with missing transverse momentum greater than 250 GeV are excluded at 95% CL. For same-sign events containing supersymmetric particles with missing transverse momentum greater than 100 GeV, effective production cross sections in excess of 14.8 fb are excluded at 95% CL. The latter limit is interpreted in a simplified electroweak gaugino production model excluding chargino masses up to 200 GeV, under the assumption that slepton decay is dominant.
Using data recorded in 2011 with the ATLAS detector at the Large Hadron Collider, a search for evidence of extra spatial dimensions has been performed through an analysis of the diphoton final state. ...The analysis uses data corresponding to an integrated luminosity of 2.12 fb−1 of s=7 TeV proton–proton collisions. The diphoton invariant mass (mγγ) spectrum is observed to be in good agreement with the expected Standard Model background. In the large extra dimension scenario of Arkani-Hamed, Dimopoulos and Dvali, the results provide 95% CL lower limits on the fundamental Planck scale between 2.27 and 3.53 TeV, depending on the number of extra dimensions and the theoretical formalism used. The results also set 95% CL lower limits on the lightest Randall–Sundrum graviton mass of between 0.79 and 1.85 TeV, for values of the dimensionless coupling k/M¯Pl varying from 0.01 to 0.1. Combining with previously published ATLAS results from the dielectron and dimuon final states, the 95% CL lower limit on the Randall–Sundrum graviton mass for k/M¯Pl=0.01 (0.1) is 0.80 (1.95) TeV.
The LEP spectrometer performed calibrations of the beam energy in the 2000 LEP run, in order to provide a kinematical constraint for the W boson mass measurement. The beam was deflected in the ...spectrometer by a steel core dipole, and the bending angle was measured by Beam-Position Monitors on either side of the magnet. The energy determination relies on measuring the change in bending angle when ramping the beam from a reference point at 50
GeV to an energy within the LEP W physics regime, typically 93
GeV. The ratio of integrated bending fields at these settings (approximately 1.18
Tm/0.64
Tm) must be known with a precision of a few
10
-
5
.
The paper reports on the field mapping measurements which were conducted to determine the bending integral under a range of excitation currents and coil temperatures. These were made in the laboratory before and after spectrometer operation, using a test-bench equipped with a moving arm, carrying an NMR probe and Hall probes, and in the LEP tunnel itself, with a mapping trolley inside the vacuum chamber. The mapping data are related to local readings supplied by fixed NMR probes in the dipole, and a predictive model developed which shows good consistency for all datasets within the estimated uncertainty, which is
14
×
10
-
5
for the moving arm, and
3
×
10
-
5
for the mapping trolley. Measurements are also presented of the field gradient inside the dipole, and of the environmental magnetic fields in the LEP tunnel. When applied to the spectrometer energy calibrations, the bending field model calculates the ratio of integrated fields with an estimated uncertainty of
1.5
×
10
-
5
.
This Letter presents a search at the LHC for s-channel single top-quark production in proton–proton collisions at a centre-of-mass energy of 8 TeV. The analyzed data set was recorded by the ATLAS ...detector and corresponds to an integrated luminosity of 20.3 fb−1. Selected events contain one charged lepton, large missing transverse momentum and exactly two b-tagged jets. A multivariate event classifier based on boosted decision trees is developed to discriminate s-channel single top-quark events from the main background contributions. The signal extraction is based on a binned maximum-likelihood fit of the output classifier distribution. The analysis leads to an upper limit on the s-channel single top-quark production cross-section of 14.6 pb at the 95% confidence level. The fit gives a cross-section of σs=5.0±4.3 pb, consistent with the Standard Model expectation.
The b-hadron production cross section is measured with the ATLAS detector in pp collisions at s=7 TeV, using 3.3 pb−1 of integrated luminosity, collected during the 2010 LHC run. The b-hadrons are ...selected by partially reconstructing D⁎+μ−X final states. Differential cross sections are measured as functions of the transverse momentum and pseudorapidity. The measured production cross section for a b-hadron with pT>9 GeV and |η|<2.5 is 32.7±0.8(stat.)−6.8+4.5(syst.)μb, higher than the next-to-leading-order QCD predictions but consistent within the experimental and theoretical uncertainties.
This paper presents a measurement of the polarisation of
τ
leptons produced in
Z
/
γ
∗
→
τ
τ
decays which is performed with a dataset of proton—proton collisions at
s
=
8
TeV, corresponding to an ...integrated luminosity of 20.2 fb
-
1
recorded with the ATLAS detector at the LHC in 2012. The
Z
/
γ
∗
→
τ
τ
decays are reconstructed from a hadronically decaying
τ
lepton with a single charged particle in the final state, accompanied by a
τ
lepton that decays leptonically. The
τ
polarisation is inferred from the relative fraction of energy carried by charged and neutral hadrons in the hadronic
τ
decays. The polarisation is measured in a fiducial region that corresponds to the kinematic region accessible to this analysis. The
τ
polarisation extracted over the full phase space within the
Z
/
γ
∗
mass range of 66
<
m
Z
/
γ
∗
<
116 GeV is found to be
P
τ
=
-
0.14
±
0.02
(
stat
)
±
0.04
(
syst
)
. It is in agreement with the Standard Model prediction of
P
τ
=
-
0.1517
±
0.0019
, which is obtained from the ALPGEN event generator interfaced with the PYTHIA 6 parton shower modelling and the TAUOLA
τ
decay library.
Dijet angular distributions from the first LHC pp collisions at center-of-mass energy s=7 TeV have been measured with the ATLAS detector. The dataset used for this analysis represents an integrated ...luminosity of 3.1 pb−1. Dijet χ distributions and centrality ratios have been measured up to dijet masses of 2.8 TeV, and found to be in good agreement with Standard Model predictions. Analysis of the χ distributions excludes quark contact interactions with a compositeness scale Λ below 3.4 TeV, at 95% confidence level, significantly exceeding previous limits.
PDF
