分类： 物理学 >> 核物理学 提交时间： 2016-09-10
摘要： With an integrated luminosity of 2.47 fb−1 recorded by the ATLAS experiment at the LHC, the exclusive decays B0s→J/ψϕ and B0d→J/ψK∗0 of B mesons produced in pp collisions at s√=7 TeV are used to determine the ratio of fragmentation fractions fs/fd. From the observed B0s→J/ψϕ and B0d→J/ψK∗0yields, the quantity fsfdB(B0s→J/ψϕ)B(B0d→J/ψK∗0) is measured to be 0.199 ± 0.004(stat) ± 0.008(sys). Using a recent theory prediction for B(B0s→J/ψϕ)B(B0d→J/ψK∗0) yields fsfd= 0.240 ± 0.004(stat) ± 0.010(sys) ± 0.017(th). This result is based on a new approach that provides a significant improvement of the world average.
分类： 物理学 >> 核物理学 提交时间： 2016-09-10
摘要： An observation of the Λ0b→ψ(2S)Λ0 decay and a comparison of its branching fraction with that of the Λ0b→J/ψΛ0 decay has been made with the ATLAS detector in proton--proton collisions at s√=8TeV at the LHC using an integrated luminosity of 20.6fb−1. The J/ψ and ψ(2S) mesons are reconstructed in their decays to a muon pair, while the Λ0→pπ− decay is exploited for the Λ0 baryon reconstruction. The Λ0bbaryons are reconstructed with transverse momentum pT>10GeV and pseudorapidity |η|<2.1. The measured branching ratio of the Λ0b→ψ(2S)Λ0 and Λ0b→J/ψΛ0 decays is Γ(Λ0b→ψ(2S)Λ0)/Γ(Λ0b→J/ψΛ0)=0.501±0.033(stat)±0.019(syst), lower than the expectation from the covariant quark model.
分类： 物理学 >> 核物理学 提交时间： 2016-09-10
摘要： The decays B+c→J/ψD+s and B+c→J/ψD∗+s are studied with the ATLAS detector at the LHC using a dataset corresponding to integrated luminosities of 4.9 fb−1 and 20.6 fb−1 of pp collisions collected at centre-of-mass energies s√=7 TeV and 8 TeV, respectively. Signal candidates are identified through J/ψ→μ+μ−and D(∗)+s→ϕπ+(γ/π0) decays. With a two-dimensional likelihood fit involving the B+c reconstructed invariant mass and an angle between the μ+ and D+s candidate momenta in the muon pair rest frame, the yields of B+c→J/ψD+s and B+c→J/ψD∗+s, and the transverse polarisation fraction in B+c→J/ψD∗+sdecay are measured. The transverse polarisation fraction is determined to be Γ±±(B+c→J/ψD∗+s)/Γ(B+c→J/ψD∗+s)=0.38±0.23±0.07, and the derived ratio of the branching fractions of the two modes is BB+c→J/ψD∗+s/BB+c→J/ψD+s=2.8+1.2−0.8±0.3, where the first error is statistical and the second is systematic. Finally, a sample of B+c→J/ψπ+ decays is used to derive the ratios of branching fractions BB+c→J/ψD+s/BB+c→J/ψπ+=3.8±1.1±0.4±0.2 and BB+c→J/ψD∗+s/BB+c→J/ψπ+=10.4±3.1±1.5±0.6, where the third error corresponds to the uncertainty of the branching fraction of D+s→ϕ(K+K−)π+ decay. The available theoretical predictions are generally consistent with the measurement.
分类： 物理学 >> 核物理学 提交时间： 2016-09-09
摘要： We search for evidence of physics beyond the Standard Model in the production of final states with multiple high transverse momentum jets, using 20.3 fb−1 of proton-proton collision data recorded by the ATLAS detector at s√=8 TeV. No excess of events beyond Standard Model expectations is observed, and upper limits on the visible cross-section for non-Standard Model production of multi-jet final states are set. Using a wide variety of models for black hole and string ball production and decay, the limit on the cross-section times acceptance is as low as 0.16 fb at the 95% CL for a minimum scalar sum of jet transverse momentum in the event of about 4.3 TeV. Using models for black hole and string ball production and decay, exclusion contours are determined as a function of the production mass threshold and the gravity scale. These limits can be interpreted in terms of lower-mass limits on black hole and string ball production that range from 4.6 to 6.2 TeV.
分类： 物理学 >> 核物理学 提交时间： 2016-08-29
摘要： Searches for new resonances decaying into two photons in the ATLAS experiment at the CERN Large Hadron Collider are described. The analysis is based on proton--proton collision data corresponding to an integrated luminosity of 3.2 fb−1 at s√=13 TeV recorded in 2015. Two searches are performed, one targeted at a spin-2 particle of mass larger than 500 GeV, using Randall--Sundrum graviton states as a benchmark model, and one optimized for a spin-0 particle of mass larger than 200 GeV. Varying both the mass and the decay width, the most significant deviation from the background-only hypothesis is observed at a diphoton invariant mass around 750 GeV with local significances of 3.8 and 3.9 standard deviations in the searches optimized for a spin-2 and spin-0 particle, respectively. The global significances are estimated to be 2.1 standard deviations for both analyses. The consistency between the data collected at 13 TeV and 8 TeV is also evaluated. Limits on the production cross section times branching ratio to two photons for the two resonance types are reported.
分类： 物理学 >> 核物理学 提交时间： 2016-08-26
摘要： This paper presents the measurement of the relative width difference ΔΓd/Γd of the B0-B¯0 system using the data collected by the ATLAS experiment at the LHC in pp collisions at s√=7 TeV and s√=8TeV and corresponding to an integrated luminosity of 25.2 fb−1. The value of ΔΓd/Γd is obtained by comparing the decay-time distributions of B0→J/ψKS and B0→J/ψK∗0(892) decays. The result is ΔΓd/Γd=(−0.1±1.1 (stat.)±0.9 (syst.))×10−2. Currently, this is the most precise single measurement of ΔΓd/Γd. It agrees with the Standard Model prediction and the measurements by other experiments.
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： The production of D∗±, D± and D±s charmed mesons has been measured with the ATLAS detector in ppcollisions at s√=7 TeV at the LHC, using data corresponding to an integrated luminosity of 280nb−1. The charmed mesons have been reconstructed in the range of transverse momentum 3.5
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： This Letter describes a model-agnostic search for pairs of jets (dijets) produced by resonant and non-resonant phenomena beyond the Standard Model in 3.6 fb−1 of proton-proton collisions with a centre-of-mass energy of s√=13 TeV recorded by the ATLAS detector at the Large Hadron Collider. The distribution of the invariant mass of the two leading jets is examined for local excesses above a data-derived estimate of the smoothly falling prediction of the Standard Model. The data are also compared to a Monte Carlo simulation of Standard Model angular distributions derived from the rapidity of the two jets. No evidence of anomalous phenomena is observed in the data, which are used to exclude, at 95% CL, quantum black holes with threshold masses below 8.3 TeV, 8.1 TeV, or 5.3 TeV in three different benchmark scenarios; resonance masses below 5.2 TeV for excited quarks, 2.6 TeV in a W′ model, a range of masses starting from mZ′=1.5 TeV and couplings from gq=0.2 in a Z′model; and contact interactions with a compositeness scale below 12.0 TeV and 17.5 TeV respectively for destructive and constructive interference between the new interaction and QCD processes. These results significantly extend the ATLAS limits obtained from 8 TeV data. Gaussian-shaped contributions to the mass distribution are also excluded if the effective cross-section exceeds values ranging from approximately 50-300 fb for masses below 2 TeV to 2-20 fb for masses above 4 TeV.
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： The distribution of particles inside hadronic jets produced in the decay of boosted W and Z bosons can be used to discriminate such jets from the continuum background. Given that a jet has been identified as likely resulting from the hadronic decay of a boosted W or Z boson, this paper presents a technique for further differentiating Zbosons from W bosons. The variables used are jet mass, jet charge, and a b-tagging discriminant. A likelihood tagger is constructed from these variables and tested in the simulation of W′→WZ for bosons in the transverse momentum range 200 GeV
分类： 物理学 >> 核物理学 提交时间： 2016-09-09
摘要： Results of a search for decays of massive particles to fully hadronic final states are presented. This search uses 20.3 fb−1 of data collected by the ATLAS detector in s√=8 TeV proton--proton collisions at the LHC. Signatures based on high jet multiplicities without requirements on the missing transverse momentum are used to search for R-parity-violating supersymmetric gluino pair production with subsequent decays to quarks. The analysis is performed using a requirement on the number of jets, in combination with separate requirements on the number of b-tagged jets, as well as a topological observable formed from the scalar sum of the mass values of large-radius jets in the event. Results are interpreted in the context of all possible branching ratios of direct gluino decays to various quark flavors. No significant deviation is observed from the expected Standard Model backgrounds estimated using jet-counting as well as data-driven templates of the total-jet-mass spectra. Gluino pair decays to ten or more quarks via intermediate neutralinos are excluded for a gluino with mass mg~<1 TeV for a neutralino mass mχ~01=500 GeV. Direct gluino decays to six quarks are excluded for mg~<917 GeV for light-flavor final states, and results for various flavor hypotheses are presented.
分类： 物理学 >> 核物理学 提交时间： 2016-08-25
摘要： A search for squarks and gluinos in final states containing hadronic jets, missing transverse momentum but no electrons or muons is presented. The data were recorded in 2015 by the ATLAS experiment in s√= 13 TeV proton--proton collisions at the Large Hadron Collider. No excess above the Standard Model background expectation was observed in 3.2 fb−1 of analyzed data. Results are interpreted within simplified models that assume R-parity is conserved and the neutralino is the lightest supersymmetric particle. An exclusion limit at the 95% confidence level on the mass of the gluino is set at 1.51 TeV for a simplified model incorporating only a gluino octet and the lightest neutralino, assuming the lightest neutralino is massless. For a simplified model involving the strong production of mass-degenerate first- and second-generation squarks, squark masses below 1.03 TeV are excluded for a massless lightest neutralino. These limits substantially extend the region of supersymmetric parameter space excluded by previous measurements with the ATLAS detector.
分类： 物理学 >> 核物理学 提交时间： 2016-08-25
摘要： The production of Z bosons with one or two isolated high-energy photons is studied using pp collisions at s√ = 8 TeV. The analyses use a data sample with an integrated luminosity of 20.3 fb−1 collected by the ATLAS detector during the 2012 LHC data taking. The Zγ and Zγγ production cross sections are measured with leptonic (e+e−, μ+μ−, νν¯) decays of the Z boson, in extended fiducial regions defined in terms of the lepton and photon acceptance. They are then compared to cross-section predictions from the Standard Model, where the sources of the photons are radiation off initial-state quarks and radiative Z-boson decay to charged leptons, and from fragmentation of final-state quarks and gluons into photons. The yields of events with photon transverse energy ET> 250 GeV from ℓ+ℓ−γ events and with ET> 400 GeV from νν¯γ events are used to search for anomalous triple gauge-boson couplings ZZγ and Zγγ. The yields of events with diphoton invariant mass mγγ> 200 GeV from ℓ+ℓ−γγ events and with mγγ> 300 GeV from νν¯γγ events are used to search for anomalous quartic gauge-boson couplings ZZγγ and Zγγγ. No deviations from Standard Model predictions are observed and limits are placed on parameters used to describe anomalous triple and quartic gauge-boson couplings.
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： The ZZ production cross section in proton-proton collisions at 13 TeV center-of-mass energy is measured using 3.2 fb−1 of data recorded with the ATLAS detector at the Large Hadron Collider. The considered Z boson candidates decay to an electron or muon pair of mass 66-116 GeV. The cross section is measured in a fiducial phase space reflecting the detector acceptance. It is also extrapolated to a total phase space for Z bosons in the same mass range and of all decay modes, giving 16.7+2.2−2.0(stat.)+0.9−0.7(syst.)+1.0−0.7(lumi.) pb. The results agree with standard model predictions.
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： The ATLAS experiment at the CERN Large Hadron Collider has performed searches for new, heavy bosons decaying to WW, WZ and ZZ final states in multiple decay channels using 20.3 fb−1 of pp collision data at s√=8 TeV. In the current study, the results of these searches are combined to provide a more stringent test of models predicting heavy resonances with couplings to vector bosons. Direct searches for a charged diboson resonance decaying to WZ in the ℓνℓ′ℓ′ (ℓ=μ,e) , ℓℓqq¯, ℓνqq¯ and fully hadronic final states are combined and upper limits on the rate of production times branching ratio to the WZ bosons are compared with predictions of an extended gauge model with a heavy W′ boson. In addition, direct searches for a neutral diboson resonance decaying to WW and ZZ in the ℓℓqq¯, ℓνqq¯, and fully hadronic final states are combined and upper limits on the rate of production times branching ratio to the WW and ZZ bosons are compared with predictions for a heavy, spin-2 graviton in an extended Randall--Sundrum model where the Standard Model fields are allowed to propagate in the bulk of the extra dimension.
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： The large rate of multiple simultaneous proton--proton interactions, or pile-up, generated by the Large Hadron Collider in Run 1 required the development of many new techniques to mitigate the adverse effects of these conditions. This paper describes the methods employed in the ATLAS experiment to correct for the impact of pile-up on jet energy and jet shapes, and for the presence of spurious additional jets, with a primary focus on the large 20.3 fb−1 data sample collected at a centre-of-mass energy of s√=8 TeV. The energy correction techniques that incorporate sophisticated estimates of the average pile-up energy density and tracking information are presented. Jet-to-vertex association techniques are discussed and projections of performance for the future are considered. Lastly, the extension of these techniques to mitigate the effect of pile-up on jet shapes using subtraction and grooming procedures is presented.
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： The electroweak production and subsequent decay of single top quarks is determined by the properties of the Wtb vertex. This vertex can be described by the complex parameters of an effective Lagrangian. An analysis of angular distributions of the decay products of single top quarks produced in the t-channel constrains these parameters simultaneously. The analysis described in this paper uses 4.6 fb−1 of proton-proton collision data at s√ = 7 TeV collected with the ATLAS detector at the LHC.Two parameters are measured simultaneously in this analysis. The fraction f1 of decays containing transversely polarised W bosons is measured to be 0.37±0.07(stat.⊕syst.). The phase δ− between amplitudes for transversely and longitudinally polarised W bosons recoiling against left-handed b-quarks is measured to be −0.14π±0.036π (stat.⊕syst.).The correlation in the measurement of these parameters is 0.15. These values result in two-dimensional limits at the 95% confidence level on the ratio of the complex coupling parameters gR and VL, yielding Re [gR/VL] ∈[−0.36,0.10] and Im [gR/VL] ∈[−0.17,0.23] with a correlation of 0.11. The results are in good agreement with the predictions of the Standard Model.
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： A search for vector-like quarks and excited quarks in events containing a top quark and a W boson in the final state is reported here. The search is based on 20.3fb−1 of proton--proton collision data taken at the LHC at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector. Events with one or two leptons, and one, two or three jets are selected with the additional requirement that at least one jet contains a b-quark. Single-lepton events are also required to contain at least one large-radius jet from the hadronic decay of a high-pT W boson or a top quark. No significant excess over the expected background is observed and upper limits on the cross-section times branching ratio for different vector-like quark and excited-quark model masses are derived. For the excited-quark production and decay to Wt with unit couplings, quarks with masses below 1500 GeV are excluded and coupling-dependent limits are set.
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： The ATLAS experiment has performed extensive searches for the electroweak production of charginos, neutralinos and staus. This article summarizes and extends the search for electroweak supersymmetry with new analyses targeting scenarios not covered by previously published searches. New searches use vector-boson fusion production, initial-state radiation jets, and low-momentum lepton final states, as well as multivariate analysis techniques to improve the sensitivity to scenarios with small mass splittings and low-production cross-sections. Results are based on 20 fb−1 of proton-proton collision data at s√=8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. The new and existing searches are combined and interpreted in terms of 95% confidence-level exclusion limits in simplified models, where a single production process and decay mode is assumed, as well as within phenomenological supersymmetric models.
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： The production cross sections of top-quark pairs in association with massive vector bosons have been measured using data from pp collisions at s√=8 TeV. The dataset corresponds to an integrated luminosity of 20.3 fb−1collected by the ATLAS detector in 2012 at the LHC. Final states with two, three or four leptons are considered. A fit to the data considering the tt¯W and tt¯Z processes simultaneously yields a significance of 5.0σ (4.2σ) over the background-only hypothesis for tt¯W (tt¯Z) production. The measured cross sections are σtt¯W=369+100−91 fb and σtt¯Z=176+58−52 fb. The background-only hypothesis with neither tt¯W nor tt¯Z production is excluded at 7.1σ. All measurements are consistent with next-to-leading-order calculations for the tt¯W and tt¯Z processes.
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： The momentum-weighted sum of the charges of tracks associated to a jet is sensitive to the charge of the initiating quark or gluon. This paper presents a measurement of the distribution of momentum-weighted sums, called jet charge, in dijet events using 20.3 fb−1 of data recorded with the ATLAS detector at s√=8 TeV in pp collisions at the LHC. The jet charge distribution is unfolded to remove distortions from detector effects and the resulting particle-level distribution is compared with several models. The pT-dependence of the jet charge distribution average and standard deviation are compared to predictions obtained with several LO and NLO parton distribution functions. The data are also compared to different Monte Carlo simulations of QCD dijet production using various settings of the free parameters within these models. The chosen value of the strong coupling constant used to calculate gluon radiation is found to have a significant impact on the predicted jet charge. There is evidence for a pT-dependence of the jet charge distribution for a given jet flavor. In agreement with perturbative QCD predictions, the data show that the average jet charge of quark-initiated jets decreases in magnitude as the energy of the jet increases.
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