A search for the Standard Model Higgs boson in proton–proton collisions with the ATLAS detector at the LHC is presented. The datasets used correspond to integrated luminosities of approximately 4.8 ...fb−1 collected at s=7 TeV in 2011 and 5.8 fb−1 at s=8 TeV in 2012. Individual searches in the channels H→ZZ(⁎)→4ℓ, H→γγ and H→WW(⁎)→eνμν in the 8 TeV data are combined with previously published results of searches for H→ZZ(⁎), WW(⁎), bb¯ and τ+τ− in the 7 TeV data and results from improved analyses of the H→ZZ(⁎)→4ℓ and H→γγ channels in the 7 TeV data. Clear evidence for the production of a neutral boson with a measured mass of 126.0±0.4(stat)±0.4(sys) GeV is presented. This observation, which has a significance of 5.9 standard deviations, corresponding to a background fluctuation probability of 1.7×10−9, is compatible with the production and decay of the Standard Model Higgs boson.
The reconstruction of the signal from hadrons and jets emerging from the proton–proton collisions at the Large Hadron Collider (LHC) and entering the ATLAS calorimeters is based on a ...three-dimensional topological clustering of individual calorimeter cell signals. The cluster formation follows cell signal-significance patterns generated by electromagnetic and hadronic showers. In this, the clustering algorithm implicitly performs a topological noise suppression by removing cells with insignificant signals which are not in close proximity to cells with significant signals. The resulting
topological cell clusters
have shape and location information, which is exploited to apply a local energy calibration and corrections depending on the nature of the cluster. Topological cell clustering is established as a well-performing calorimeter signal definition for jet and missing transverse momentum reconstruction in ATLAS.
A
bstract
Results of a search for
H
→
ττ
decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data ...correspond to integrated luminosities of 4.5 fb
−1
and 20.3 fb
−1
at centre-of-mass energies of
s
=
7
TeV and
s
=
8
TeV respectively. All combinations of leptonic (
τ
→
ℓ
ν
ν
¯
with
ℓ
=
e, μ
) and hadronic (
τ
→ hadrons
ν
) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalised to the Standard Model expectation, of
μ
= 1. 43
− 0.37
+ 0.43
is consistent with the predicted Yukawa coupling strength in the Standard Model.
A
bstract
A summary of the constraints from the ATLAS experiment on
R
-parity-conserving supersymmetry is presented. Results from 22 separate ATLAS searches are considered, each based on analysis of ...up to 20.3 fb
−1
of proton-proton collision data at centre-of-mass energies of
s
=
7
and 8 TeV at the Large Hadron Collider. The results are interpreted in the context of the 19-parameter phenomenological minimal supersymmetric standard model, in which the lightest supersymmetric particle is a neutralino, taking into account constraints from previous precision electroweak and flavour measurements as well as from dark matter related measurements. The results are presented in terms of constraints on supersymmetric particle masses and are compared to limits from simplified models. The impact of ATLAS searches on parameters such as the dark matter relic density, the couplings of the observed Higgs boson, and the degree of electroweak fine-tuning is also shown. Spectra for surviving supersymmetry model points with low fine-tunings are presented.
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
-
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 for electron transverse energy greater than 30 GeV.
A search for the associated production of the Higgs boson with a top quark pair is performed in multilepton final states using 20.3 fb−1 of proton–proton collision data recorded by the ATLAS ...experiment at s=8 TeV at the Large Hadron Collider. Five final states, targeting the decays H→WW⁎, ττ, and ZZ⁎, are examined for the presence of the Standard Model (SM) Higgs boson: two same-charge light leptons (e or μ) without a hadronically decaying τ lepton; three light leptons; two same-charge light leptons with a hadronically decaying τ lepton; four light leptons; and one light lepton and two hadronically decaying τ leptons. No significant excess of events is observed above the background expectation. The best fit for the tt¯H production cross section, assuming a Higgs boson mass of 125 GeV, is 2.1−1.2+1.4 times the SM expectation, and the observed (expected) upper limit at the 95% confidence level is 4.7 (2.4) times the SM rate. The p-value for compatibility with the background-only hypothesis is 1.8σ; the expectation in the presence of a Standard Model signal is 0.9σ.
This paper presents a new method of reconstructing the individual charged and neutral hadrons in tau decays with the ATLAS detector. The reconstructed hadrons are used to classify the decay mode and ...to calculate the visible four-momentum of reconstructed tau candidates, significantly improving the resolution with respect to the calibration in the existing tau reconstruction. The performance of the reconstruction algorithm is optimised and evaluated using simulation and validated using samples of
Z
→
τ
τ
and
Z
(
→
μ
μ
)
+jets events selected from proton–proton collisions at a centre-of-mass energy
s
=
8
TeV
, corresponding to an integrated luminosity of 5
fb
-
1
.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted; see image) Abstract The results of a search for pair production of supersymmetric partners of the Standard Model third-generation ...quarks are reported. This search uses 20.1 fb^sup -1^ of pp collisions at ... TeV collected by the ATLAS experiment at the Large Hadron Collider. The lightest bottom and top squarks (... and ... respectively) are searched for in a final state with large missing transverse momentum and two jets identified as originating from b-quarks. No excess of events above the expected level of Standard Model background is found. The results are used to set upper limits on the visible cross section for processes beyond the Standard Model. Exclusion limits at the 95 % confidence level on the masses of the third-generation squarks are derived in phenomenological supersymmetric R-parity-conserving models in which either the bottom or the top squark is the lightest squark. The ... is assumed to decay via ... and the ... via ..., with undetectable products of the subsequent decay of the ... due to the small mass splitting between the ... and the ... Figure not available: see fulltext.
A
bstract
The differential cross section for the process
Z
/
γ
∗
→
ℓℓ
(
ℓ
=
e, μ
) as a function of dilepton invariant mass is measured in
pp
collisions at
s
= 7 TeV at the LHC using the ATLAS ...detector. The measurement is performed in the
e
and
μ
channels for invariant masses between 26 GeV and 66 GeV using an integrated luminosity of 1
.
6 fb
−1
collected in 2011 and these measurements are combined. The analysis is extended to invariant masses as low as 12 GeV in the muon channel using 35 pb
−1
of data collected in 2010. The cross sections are determined within fiducial acceptance regions and corrections to extrapolate the measurements to the full kinematic range are provided. Next-to-next-to-leading-order QCD predictions provide a significantly better description of the results than next-to-leading-order QCD calculations, unless the latter are matched to a parton shower calculation.
This Letter presents measurements of the polarization of the top quark in top-antitop quark pair events, using 4.7 fb(-1) of proton-proton collision data recorded with the ATLAS detector at the ...Large Hadron Collider at √s=7 TeV. Final states containing one or two isolated leptons (electrons or muons) and jets are considered. Two measurements of α(ℓ)P, the product of the leptonic spin-analyzing power and the top quark polarization, are performed assuming that the polarization is introduced by either a CP conserving or a maximally CP violating production process. The measurements obtained, α(ℓ)P(CPC)=-0.035±0.014(stat)±0.037(syst) and α(ℓ)P(CPV)=0.020±0.016(stat)(-0.017)(+0.013)(syst), are in good agreement with the standard model prediction of negligible top quark polarization.
PDF
