Test of CP invariance in vector-boson fusion production of the Higgs boson using the Optimal Observable method in the ditau decay channel with the ATLAS detector

November 29, 2016, 12:00 am

≫ Next: Measurement of the photon identification efficiencies with the ATLAS detector using LHC Run-1 data

≪ Previous: Luminosity determination in pp collisions at $$\sqrt{s}$$ s = 8 TeV using the ATLAS detector at the LHC

Abstract

A test of CP invariance in Higgs boson production via vector-boson fusion using the method of the Optimal Observable is presented. The analysis exploits the decay mode of the Higgs boson into a pair of $\tau $ leptons and is based on 20.3 $\mathrm{fb}^{-1}$ of proton–proton collision data at $\sqrt{s}$ = 8 $\,\mathrm{TeV}$ collected by the ATLAS experiment at the LHC. Contributions from CP-violating interactions between the Higgs boson and electroweak gauge bosons are described in an effective field theory framework, in which the strength of CP violation is governed by a single parameter $\tilde{d}$. The mean values and distributions of CP-odd observables agree with the expectation in the Standard Model and show no sign of CP violation. The CP-mixing parameter $\tilde{d}$ is constrained to the interval $(-0.11,0.05)$ at 68% confidence level, consistent with the Standard Model expectation of $\tilde{d}=0$.

↧

Measurement of the photon identification efficiencies with the ATLAS detector using LHC Run-1 data

December 3, 2016, 12:00 am

≫ Next: Measurement of the $$b\overline{b}$$ b b ¯ dijet cross section in pp collisions at $$\sqrt{s} = 7$$ s = 7 TeV with the ATLAS detector

≪ Previous: Test of CP invariance in vector-boson fusion production of the Higgs boson using the Optimal Observable method in the ditau decay channel with the ATLAS detector

Abstract

The algorithms used by the ATLAS Collaboration to reconstruct and identify prompt photons are described. Measurements of the photon identification efficiencies are reported, using 4.9 fb$^{-1}$ of pp collision data collected at the LHC at $\sqrt{s} = 7$ $\text {TeV}$ and 20.3 fb$^{-1}$ at $\sqrt{s} = 8$ $\text {TeV}$. The efficiencies are measured separately for converted and unconverted photons, in four different pseudorapidity regions, for transverse momenta between 10 $\text {GeV}$ and 1.5 $\text {TeV}$. The results from the combination of three data-driven techniques are compared to the predictions from a simulation of the detector response, after correcting the electromagnetic shower momenta in the simulation for the average differences observed with respect to data. Data-to-simulation efficiency ratios used as correction factors in physics measurements are determined to account for the small residual efficiency differences. These factors are measured with uncertainties between 0.5% and 10% in 7 $\text {TeV}$ data and between 0.5% and 5.6% in 8 $\text {TeV}$ data, depending on the photon transverse momentum and pseudorapidity.

↧

Measurement of the $$b\overline{b}$$ b b ¯ dijet cross section in pp collisions at $$\sqrt{s} = 7$$ s = 7 TeV with the ATLAS detector

December 5, 2016, 12:00 am

≫ Next: Search for squarks and gluinos in events with hadronically decaying tau leptons, jets and missing transverse momentum in proton–proton collisions at $$\sqrt{s}=13$$ s = 13 TeV recorded with the ATLAS detector

≪ Previous: Measurement of the photon identification efficiencies with the ATLAS detector using LHC Run-1 data

Abstract

The dijet production cross section for jets containing a b-hadron (b-jets) has been measured in proton–proton collisions with a centre-of-mass energy of $\sqrt{s} = 7$ TeV, using the ATLAS detector at the LHC. The data used correspond to an integrated luminosity of $\mathrm 4.2\,\text {fb}^{-1}$. The cross section is measured for events with two identified b-jets with a transverse momentum $p_{\text {T}} > 20$ GeV 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 $p_{\text {T}} > 270$ GeV. 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.

↧

Search for squarks and gluinos in events with hadronically decaying tau leptons, jets and missing transverse momentum in proton–proton collisions at $$\sqrt{s}=13$$ s = 13 TeV recorded with the ATLAS detector

December 10, 2016, 12:00 am

≫ Next: The domain interface method in non-conforming domain decomposition multifield problems

≪ Previous: Measurement of the $$b\overline{b}$$ b b ¯ dijet cross section in pp collisions at $$\sqrt{s} = 7$$ s = 7 TeV with the ATLAS detector

Abstract

A search for supersymmetry in events with large missing transverse momentum, jets, and at least one hadronically decaying tau lepton has been performed using 3.2 fb$^{-1}$ of proton–proton collision data at $\sqrt{s}=13{\mathrm {\ TeV}}$ recorded by the ATLAS detector at the Large Hadron Collider in 2015. Two exclusive final states are considered, with either exactly one or at least two tau leptons. No excess over the Standard Model prediction is observed in the data. Results are interpreted in the context of gauge-mediated supersymmetry breaking and a simplified model of gluino pair production with tau-rich cascade decays, substantially improving on previous limits. In the GMSB model considered, supersymmetry-breaking scale ($\Lambda $) values below $92 {\mathrm {\ TeV}}$ are excluded at the 95% confidence level, corresponding to gluino masses below $2000 {\mathrm {\ GeV}}$. For large values of $\tan \beta $, values of $\Lambda $ up to $107 {\mathrm {\ TeV}}$ and gluino masses up to $2300 {\mathrm {\ GeV}}$ are excluded. In the simplified model, gluino masses are excluded up to $1570 {\mathrm {\ GeV}}$ for neutralino masses around $100 {\mathrm {\ GeV}}$. Neutralino masses below $700 {\mathrm {\ GeV}}$ are excluded for all gluino masses between 800 and $1500 {\mathrm {\ GeV}}$, while the strongest exclusion of $750 {\mathrm {\ GeV}}$ is achieved for gluino masses around $1450 {\mathrm {\ GeV}}$.

↧

The domain interface method in non-conforming domain decomposition multifield problems

December 26, 2016, 12:00 am

≫ Next: A measurement of the calorimeter response to single hadrons and determination of the jet energy scale uncertainty using LHC Run-1 pp -collision data with the ATLAS detector

≪ Previous: Search for squarks and gluinos in events with hadronically decaying tau leptons, jets and missing transverse momentum in proton–proton collisions at $$\sqrt{s}=13$$ s = 13 TeV recorded with the ATLAS detector

Abstract

The Domain Interface Method (DIM) is extended in this contribution for the case of mixed fields as encountered in multiphysics problems. The essence of the non-conforming domain decomposition technique consists in a discretization of a fictitious zero-thickness interface as in the original methodology and continuity of the solution fields across the domains is satisfied by incorporating the corresponding Lagrange Multipliers. The multifield DIM inherits the advantages of its irreducible version in the sense that the connections between non-matching meshes, with possible geometrically non-conforming interfaces, is accounted by the automatic Delaunay interface discretization without considering master and slave surfaces or intermediate surface projections as done in many established techniques, e.g. mortar methods. The multifield enhancement identifies the Lagrange multiplier field and incorporates its contribution in the weak variational form accounting for the corresponding consistent stabilization term based on a Nitsche method. This type of constraint enforcement circumvents the appearance of instabilities when the Ladyzhenskaya–Babuška–Brezzi (LBB) condition is not fulfilled by the chosen discretization. The domain decomposition framework is assessed in a large deformation setting for mixed displacement/pressure formulations and coupled thermomechanical problems. The continuity of the mixed field is studied in well selected benchmark problems for both mixed formulations and the objectivity of the response is compared to reference monolithic solutions. Results suggest that the presented strategy shows sufficient potential to be a valuable tool in situations where the evolving physics at particular domains require the use of different spatial discretizations or field interpolations.

↧

A measurement of the calorimeter response to single hadrons and determination of the jet energy scale uncertainty using LHC Run-1 pp -collision data with the ATLAS detector

January 13, 2017, 12:00 am

≫ Next: Measurement of the $${\varvec{t\bar{t}Z}}$$ t t ¯ Z and $${\varvec{t\bar{t}W}}$$ t t ¯ W production cross sections in multilepton final states using 3.2 fb $$^{-1}$$ - 1 of $$\varvec{pp}$$ p p collisions at $$\sqrt{s}$$ s = 13 TeV with the ATLAS detector

≪ Previous: The domain interface method in non-conforming domain decomposition multifield problems

Abstract

A measurement of the calorimeter response to isolated charged hadrons in the ATLAS detector at the LHC is presented. This measurement is performed with 3.2 nb$^{-1}$ of proton–proton collision data at $\sqrt{s}=7$ $\,\mathrm{TeV}$ from 2010 and 0.1 nb$^{-1}$ of data at $\sqrt{s}=8$ $\,\mathrm{TeV}$ from 2012. A number of aspects of the calorimeter response to isolated hadrons are explored. After accounting for energy deposited by neutral particles, there is a 5% discrepancy in the modelling, using various sets of Geant4 hadronic physics models, of the calorimeter response to isolated charged hadrons in the central calorimeter region. The description of the response to anti-protons at low momenta is found to be improved with respect to previous analyses. The electromagnetic and hadronic calorimeters are also examined separately, and the detector simulation is found to describe the response in the hadronic calorimeter well. The jet energy scale uncertainty and correlations in scale between jets of different momenta and pseudorapidity are derived based on these studies. The uncertainty is 2–5% for jets with transverse momenta above 2 $\,\mathrm{TeV}$, where this method provides the jet energy scale uncertainty for ATLAS.

↧

Measurement of the $${\varvec{t\bar{t}Z}}$$ t t ¯ Z and $${\varvec{t\bar{t}W}}$$ t t ¯ W production cross sections in multilepton final states using 3.2 fb $$^{-1}$$ - 1 of $$\varvec{pp}$$ p p collisions at $$\sqrt{s}$$ s = 13 TeV with the ATLAS detector

January 20, 2017, 12:00 am

≫ Next: Measurement of the ZZ production cross section in proton-proton collisions at s = 8 $$ \sqrt{s}=8 $$ TeV using the ZZ → ℓ − ℓ + ℓ ′− ℓ ′+ and Z Z → ℓ − ℓ + ν ν ¯ $$ ZZ\to {\ell}^{-}{\ell}^{+}\nu \overline{\nu} $$ decay channels with the ATLAS detector

≪ Previous: A measurement of the calorimeter response to single hadrons and determination of the jet energy scale uncertainty using LHC Run-1 pp -collision data with the ATLAS detector

Abstract

A measurement of the $t\bar{t}Z$ and $t\bar{t}W$ production cross sections in final states with either two same-charge muons, or three or four leptons (electrons or muons) is presented. The analysis uses a data sample of proton–proton collisions at $\sqrt{s} = 13$ TeV recorded with the ATLAS detector at the Large Hadron Collider in 2015, corresponding to a total integrated luminosity of 3.2 fb$^{-1}$. The inclusive cross sections are extracted using likelihood fits to signal and control regions, resulting in $\sigma _{t\bar{t}Z} = 0.9 \pm 0.3$ pb and $\sigma _{t\bar{t}W} = 1.5 \pm 0.8$ pb, in agreement with the Standard Model predictions.

↧

Measurement of the ZZ production cross section in proton-proton collisions at s = 8 $$ \sqrt{s}=8 $$ TeV using the ZZ → ℓ − ℓ + ℓ ′− ℓ ′+ and Z Z → ℓ − ℓ + ν ν ¯ $$ ZZ\to {\ell}^{-}{\ell}^{+}\nu \overline{\nu} $$ decay channels with the ATLAS detector

January 24, 2017, 12:00 am

≫ Next: Measurements of ψ (2 S ) and X (3872) → J/ψπ + π − production in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

≪ Previous: Measurement of the $${\varvec{t\bar{t}Z}}$$ t t ¯ Z and $${\varvec{t\bar{t}W}}$$ t t ¯ W production cross sections in multilepton final states using 3.2 fb $$^{-1}$$ - 1 of $$\varvec{pp}$$ p p collisions at $$\sqrt{s}$$ s = 13 TeV with the ATLAS detector

Abstract

A measurement of the ZZ production cross section in the ℓ⁻ℓ⁺ℓ^{′ −}ℓ^{′ +} and $ {\ell}^{-}{\ell}^{+}\nu \overline{\nu} $ channels (ℓ = e, μ) in proton-proton collisions at $ \sqrt{s}=8 $TeV at the Large Hadron Collider at CERN, using data corresponding to an integrated luminosity of 20.3 fb⁻¹ collected by the ATLAS experiment in 2012 is presented. The fiducial cross sections for ZZ→ ℓ⁻ℓ⁺ℓ^{′ −}ℓ^{′ +} and $ ZZ\to {\ell}^{-}{\ell}^{+}\nu \overline{\nu} $ are measured in selected phase-space regions. The total cross section for ZZ events produced with both Z bosons in the mass range 66 to 116 GeV is measured from the combination of the two channels to be 7.3 ± 0.4(stat) ± 0.3(syst)_− 0.1^− 0.2(lumi) pb, which is consistent with the Standard Model prediction of 6. 6_− 0.6^+ 0.7 pb. The differential cross sections in bins of various kinematic variables are presented. The differential event yield as a function of the transverse momentum of the leading Z boson is used to set limits on anomalous neutral triple gauge boson couplings in ZZ production.

↧

Measurements of ψ (2 S ) and X (3872) → J/ψπ + π − production in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

January 26, 2017, 12:00 am

≫ Next: Search for lepton-flavour-violating decays of the Higgs and Z bosons with the ATLAS detector

≪ Previous: Measurement of the ZZ production cross section in proton-proton collisions at s = 8 $$ \sqrt{s}=8 $$ TeV using the ZZ → ℓ − ℓ + ℓ ′− ℓ ′+ and Z Z → ℓ − ℓ + ν ν ¯ $$ ZZ\to {\ell}^{-}{\ell}^{+}\nu \overline{\nu} $$ decay channels with the ATLAS detector

Abstract

Differential cross sections are presented for the prompt and non-prompt production of the hidden-charm states X(3872) and ψ(2S), in the decay mode J/ψπ⁺π⁻, measured using 11.4 fb⁻¹ of pp collisions at $ \sqrt{s}=8 $ TeV by the ATLAS detector at the LHC. The ratio of cross-sections X(3872)/ψ(2S) is also given, separately for prompt and non-prompt components, as well as the non-prompt fractions of X(3872) and ψ(2S). Assuming independent single effective lifetimes for non-prompt X(3872) and ψ(2S) production gives $ {R}_B=\frac{\mathrm{\mathcal{B}}\left(B\to X(3872)+\mathrm{any}\right)\mathrm{\mathcal{B}}\left(X(3872)\to J/\psi {\pi}^{+}{\pi}^{-}\right)}{\mathrm{\mathcal{B}}\left(B\to \psi (2S)+\mathrm{any}\right)\mathrm{\mathcal{B}}\left(\psi (2S)\to J/\psi {\pi}^{+}{\pi}^{-}\right)}=\left(3.95\pm 0.32\left(\mathrm{s}\mathrm{tat}\right)\pm 0.08\left(\mathrm{s}\mathrm{y}\mathrm{s}\right)\right)\times {10}^{-2} $ separating short- and long-lived contributions, assuming that the short-lived component is due to B_c decays, gives R_B = (3.57 ± 0.33(stat) ± 0.11(sys)) × 10⁻², with the fraction of non-prompt X(3872) produced via B_c decays for p_T(X(3872)) > 10 GeV being (25 ± 13(stat) ± 2(sys) ± 5(spin))%. The distributions of the dipion invariant mass in the X(3872) and ψ(2S) decays are also measured and compared to theoretical predictions.

↧

Search for lepton-flavour-violating decays of the Higgs and Z bosons with the ATLAS detector

February 4, 2017, 12:00 am

≫ Next: Measurement of the prompt J / $$\psi $$ ψ pair production cross-section in pp collisions at $$\sqrt{s} = 8$$ s = 8 TeV with the ATLAS detector

≪ Previous: Measurements of ψ (2 S ) and X (3872) → J/ψπ + π − production in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

Abstract

Direct searches for lepton flavour violation in decays of the Higgs and Z bosons with the ATLAS detector at the LHC are presented. The following three decays are considered: $H\rightarrow e\tau $, $H\rightarrow \mu \tau $, and $Z\rightarrow \mu \tau $. The searches are based on the data sample of proton–proton collisions collected by the ATLAS detector corresponding to an integrated luminosity of 20.3 $\mathrm{fb}^{-1}$ at a centre-of-mass energy of $\sqrt{s}=8$ TeV. No significant excess is observed, and upper limits on the lepton-flavour-violating branching ratios are set at the 95$\%$ confidence level: Br$(H\rightarrow e\tau )<1.04\%$, Br$(H\rightarrow \mu \tau )<1.43\%$, and Br$(Z\rightarrow \mu \tau )<1.69\times 10^{-5}$.

↧

Measurement of the prompt J / $$\psi $$ ψ pair production cross-section in pp collisions at $$\sqrt{s} = 8$$ s = 8 TeV with the ATLAS detector

February 7, 2017, 12:00 am

≫ Next: Measurements of charge and CP asymmetries in b -hadron decays using top-quark events collected by the ATLAS detector in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV

≪ Previous: Search for lepton-flavour-violating decays of the Higgs and Z bosons with the ATLAS detector

Abstract

The production of two prompt $J/\psi $ mesons, each with transverse momenta $p_{\mathrm {T}}>8.5$ GeV and rapidity $|y| < 2.1$, is studied using a sample of proton-proton collisions at $\sqrt{s} = 8$ TeV, corresponding to an integrated luminosity of 11.4 fb$^{-1}$ collected in 2012 with the ATLAS detector at the LHC. The differential cross-section, assuming unpolarised $J/\psi $ production, is measured as a function of the transverse momentum of the lower-$p_{\mathrm {T}}$$J/\psi $ meson, di-$J/\psi $$p_{\mathrm {T}}$ and mass, the difference in rapidity between the two $J/\psi $ mesons, and the azimuthal angle between the two $J/\psi $ mesons. The fraction of prompt pair events due to double parton scattering is determined by studying kinematic correlations between the two $J/\psi $ mesons. The total and double parton scattering cross-sections are compared with predictions. The effective cross-section of double parton scattering is measured to be $\sigma _{\mathrm {eff}} = 6.3 \pm 1.6 \mathrm {(stat)} \pm 1.0 \mathrm {(syst)}$ mb.

↧

Measurements of charge and CP asymmetries in b -hadron decays using top-quark events collected by the ATLAS detector in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV

February 7, 2017, 12:00 am

≫ Next: Measurements of top-quark pair to Z -boson cross-section ratios at s = 13 $$ \sqrt{s}=13 $$ , 8 , 7 TeV with the ATLAS detector

≪ Previous: Measurement of the prompt J / $$\psi $$ ψ pair production cross-section in pp collisions at $$\sqrt{s} = 8$$ s = 8 TeV with the ATLAS detector

Abstract

Same- and opposite-sign charge asymmetries are measured in lepton+jets $ t\overline{t} $ events in which a b-hadron decays semileptonically to a soft muon, using data corresponding to an integrated luminosity of 20.3 fb⁻¹ from proton-proton collisions at a centre-of-mass energy of $ \sqrt{s}=8 $ TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. The charge asymmetries are based on the charge of the lepton from the top-quark decay and the charge of the soft muon from the semileptonic decay of a b-hadron and are measured in a fiducial region corresponding to the experimental acceptance. Four CP asymmetries (one mixing and three direct) are measured and are found to be compatible with zero and consistent with the Standard Model.

↧

Measurements of top-quark pair to Z -boson cross-section ratios at s = 13 $$ \sqrt{s}=13 $$ , 8 , 7 TeV with the ATLAS detector

February 23, 2017, 12:00 am

≫ Next: Search for triboson $$W^{\pm }W^{\pm }W^{\mp }$$ W ± W ± W ∓ production in pp collisions at $$\sqrt{s}=8$$ s = 8 $$\text {TeV}$$ TeV with the ATLAS detector

≪ Previous: Measurements of charge and CP asymmetries in b -hadron decays using top-quark events collected by the ATLAS detector in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV

Abstract

Ratios of top-quark pair to Z-boson cross sections measured from proton-proton collisions at the LHC centre-of-mass energies of $ \sqrt{s}=13 $ TeV, 8 TeV, and 7 TeV are presented by the ATLAS Collaboration. Single ratios, at a given $ \sqrt{s} $ for the two processes and at different $ \sqrt{s} $ for each process, as well as double ratios of the two processes at different $ \sqrt{s} $, are evaluated. The ratios are constructed using previously published ATLAS measurements of the $ t\overline{t} $ and Z-boson production cross sections, corrected to a common phase space where required, and a new analysis of Z→ ℓ⁺ℓ⁻ where ℓ = e, μ at $ \sqrt{s}=13 $ TeV performed with data collected in 2015 with an integrated luminosity of 3.2 fb⁻¹. Correlations of systematic uncertainties are taken into account when evaluating the uncertainties in the ratios. The correlation model is also used to evaluate the combined cross section of the Z→ e⁺e⁻ and the Z→ μ⁺μ⁻ channels for each $ \sqrt{s} $ value. The results are compared to calculations performed at next-to-next-to-leading-order accuracy using recent sets of parton distribution functions. The data demonstrate significant power to constrain the gluon distribution function for the Bjorken-x values near 0.1 and the light-quark sea for x < 0.02.

↧

Search for triboson $$W^{\pm }W^{\pm }W^{\mp }$$ W ± W ± W ∓ production in pp collisions at $$\sqrt{s}=8$$ s = 8 $$\text {TeV}$$ TeV with the ATLAS detector

March 2, 2017, 12:00 am

≫ Next: Search for new phenomena in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum in $$\varvec{\sqrt{s}=13}$$ s = 13 $$\text {TeV}$$ TeV $$\varvec{pp}$$ p p collisions with the ATLAS detector

≪ Previous: Measurements of top-quark pair to Z -boson cross-section ratios at s = 13 $$ \sqrt{s}=13 $$ , 8 , 7 TeV with the ATLAS detector

Abstract

This paper reports a search for triboson $W^{\pm }W^{\pm }W^{\mp }$ production in two decay channels (${W^{\pm }W^{\pm }W^{\mp } \rightarrow \ell ^\pm \nu \ell ^\pm \nu \ell ^\mp \nu }$ and ${W^{\pm }W^{\pm }W^{\mp } \rightarrow \ell ^\pm \nu \ell ^\pm \nu jj}$ with $\ell =e, \mu $) in proton-proton collision data corresponding to an integrated luminosity of 20.3 $\mathrm{fb}^\mathrm{-1}$ at a centre-of-mass energy of 8 $\text {TeV}$ with the ATLAS detector at the Large Hadron Collider. Events with exactly three charged leptons, or two leptons with the same electric charge in association with two jets, are selected. The total number of events observed in data is consistent with the Standard Model (SM) predictions. The observed 95% confidence level upper limit on the SM $W^{\pm }W^{\pm }W^{\mp }$ production cross section is found to be 730 fb with an expected limit of 560 fb in the absence of SM $W^{\pm }W^{\pm }W^{\mp }$ production. Limits are also set on WWWW anomalous quartic gauge couplings.

↧

Search for new phenomena in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum in $$\varvec{\sqrt{s}=13}$$ s = 13 $$\text {TeV}$$ TeV $$\varvec{pp}$$ p p collisions with the ATLAS detector

March 4, 2017, 12:00 am

≫ Next: Measurements of top quark spin observables in t t ¯ $$ t\overline{t} $$ events using dilepton final states in s = 8 $$ \sqrt{s}=8 $$ TeV pp collisions with the ATLAS detector

≪ Previous: Search for triboson $$W^{\pm }W^{\pm }W^{\mp }$$ W ± W ± W ∓ production in pp collisions at $$\sqrt{s}=8$$ s = 8 $$\text {TeV}$$ TeV with the ATLAS detector

Abstract

Two searches for new phenomena in final states containing a same-flavour opposite-sign lepton (electron or muon) pair, jets, and large missing transverse momentum are presented. These searches make use of proton–proton collision data, collected during 2015 and 2016 at a centre-of-mass energy $\sqrt{s}=13$ $\text {TeV}$ by the ATLAS detector at the large hadron collider, which correspond to an integrated luminosity of $14.7~\mathrm {fb}^{-1}$. Both searches target the pair production of supersymmetric particles, squarks or gluinos, which decay to final states containing a same-flavour opposite-sign lepton pair via one of two mechanisms: a leptonically decaying Z boson in the final state, leading to a peak in the dilepton invariant-mass distribution around the Z boson mass; and decays of neutralinos (e.g. $\tilde{\chi }_2^0 \rightarrow \ell ^+\ell ^- \tilde{\chi }_1^0$), yielding a kinematic endpoint in the dilepton invariant-mass spectrum. The data are found to be consistent with the Standard Model expectation. Results are interpreted in simplified models of gluino-pair (squark-pair) production, and provide sensitivity to gluinos (squarks) with masses as large as 1.70 $\text {TeV}$ (980 $\text {GeV}$).

↧

Measurements of top quark spin observables in t t ¯ $$ t\overline{t} $$ events using dilepton final states in s = 8 $$ \sqrt{s}=8 $$ TeV pp collisions with the ATLAS detector

March 22, 2017, 12:00 am

≫ Next: Electron efficiency measurements with the ATLAS detector using 2012 LHC proton–proton collision data

≪ Previous: Search for new phenomena in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum in $$\varvec{\sqrt{s}=13}$$ s = 13 $$\text {TeV}$$ TeV $$\varvec{pp}$$ p p collisions with the ATLAS detector

Abstract

Measurements of top quark spin observables in $ t\overline{t} $ events are presented based on 20.2 fb⁻¹ of $ \sqrt{s}=8 $ TeV proton-proton collisions recorded with the ATLAS detector at the LHC. The analysis is performed in the dilepton final state, characterised by the presence of two isolated leptons (electrons or muons). There are 15 observables, each sensitive to a different coefficient of the spin density matrix of $ t\overline{t} $ production, which are measured independently. Ten of these observables are measured for the first time. All of them are corrected for detector resolution and acceptance effects back to the parton and stable-particle levels. The measured values of the observables at parton level are compared to Standard Model predictions at next-to-leading order in QCD. The corrected distributions at stable-particle level are presented and the means of the distributions are compared to Monte Carlo predictions. No significant deviation from the Standard Model is observed for any observable.

↧

Electron efficiency measurements with the ATLAS detector using 2012 LHC proton–proton collision data

March 27, 2017, 12:00 am

≫ Next: Measurement of charged-particle distributions sensitive to the underlying event in s = 13 $$ \sqrt{s}=13 $$ TeV proton-proton collisions with the ATLAS detector at the LHC

≪ Previous: Measurements of top quark spin observables in t t ¯ $$ t\overline{t} $$ events using dilepton final states in s = 8 $$ \sqrt{s}=8 $$ TeV pp collisions with the ATLAS detector

Abstract

This paper describes the algorithms for the reconstruction and identification of electrons in the central region of the ATLAS detector at the Large Hadron Collider (LHC). These algorithms were used for all ATLAS results with electrons in the final state that are based on the 2012 pp collision data produced by the LHC at $\sqrt{\mathrm {s}}$ = 8 $\text {TeV}$. The efficiency of these algorithms, together with the charge misidentification rate, is measured in data and evaluated in simulated samples using electrons from $Z\rightarrow ee$, $Z\rightarrow ee\gamma $ and $J/\psi \rightarrow ee$ decays. For these efficiency measurements, the full recorded data set, corresponding to an integrated luminosity of 20.3 fb$^{-1}$, is used. Based on a new reconstruction algorithm used in 2012, the electron reconstruction efficiency is 97% for electrons with $E_{\mathrm {T}}=15$ $\text {GeV}$ and 99% at $E_{\mathrm {T}}= 50$ $\text {GeV}$. Combining this with the efficiency of additional selection criteria to reject electrons from background processes or misidentified hadrons, the efficiency to reconstruct and identify electrons at the ATLAS experiment varies from 65 to 95%, depending on the transverse momentum of the electron and background rejection.

↧

Measurement of charged-particle distributions sensitive to the underlying event in s = 13 $$ \sqrt{s}=13 $$ TeV proton-proton collisions with the ATLAS detector at the LHC

March 29, 2017, 12:00 am

≫ Next: Measurement of jet activity produced in top-quark events with an electron, a muon and two b -tagged jets in the final state in pp collisions at $$\sqrt{s}=13$$ s = 13 TeV with the ATLAS detector

≪ Previous: Electron efficiency measurements with the ATLAS detector using 2012 LHC proton–proton collision data

Abstract

We present charged-particle distributions sensitive to the underlying event, measured by the ATLAS detector in proton-proton collisions at a centre-of-mass energy of 13 TeV, in low-luminosity Large Hadron Collider fills corresponding to an integrated luminosity of 1.6 nb⁻¹. The distributions were constructed using charged particles with absolute pseudorapidity less than 2.5 and with transverse momentum greater than 500 MeV, in events with at least one such charged particle with transverse momentum above 1 GeV. These distributions characterise the angular distribution of energy and particle flows with respect to the charged particle with highest transverse momentum, as a function of both that momentum and of charged-particle multiplicity. The results have been corrected for detector effects and are compared to the predictions of various Monte Carlo event generators, experimentally establishing the level of underlying-event activity at LHC Run 2 energies and providing inputs for the development of event generator modelling. The current models in use for UE modelling typically describe this data to 5% accuracy, compared with data uncertainties of less than 1%.

↧

Measurement of jet activity produced in top-quark events with an electron, a muon and two b -tagged jets in the final state in pp collisions at $$\sqrt{s}=13$$ s = 13 TeV with the ATLAS detector

April 7, 2017, 12:00 am

≫ Next: Performance of algorithms that reconstruct missing transverse momentum in $$\sqrt{s}$$ s = 8 TeV proton–proton collisions in the ATLAS detector

≪ Previous: Measurement of charged-particle distributions sensitive to the underlying event in s = 13 $$ \sqrt{s}=13 $$ TeV proton-proton collisions with the ATLAS detector at the LHC

Abstract

Measurements of jet activity in top-quark pair events produced in proton–proton collisions are presented, using 3.2 fb$^{-1}$ of pp collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS experiment at the Large Hadron Collider. Events are chosen by requiring an opposite-charge $e\mu $ pair and two b-tagged jets in the final state. The normalised differential cross-sections of top-quark pair production are presented as functions of additional-jet multiplicity and transverse momentum, $p_{\text {T}}$. The fraction of signal events that do not contain additional jet activity in a given rapidity region, the gap fraction, is measured as a function of the $p_{\text {T}}$ threshold for additional jets, and is also presented for different invariant mass regions of the $e\mu b\bar{b}$ system. All measurements are corrected for detector effects and presented as particle-level distributions compared to predictions with different theoretical approaches for QCD radiation. While the kinematics of the jets from top-quark decays are described well, the generators show differing levels of agreement with the measurements of observables that depend on the production of additional jets.

↧

Performance of algorithms that reconstruct missing transverse momentum in $$\sqrt{s}$$ s = 8 TeV proton–proton collisions in the ATLAS detector

April 13, 2017, 12:00 am

≫ Next: Measurement of the inclusive cross-sections of single top-quark and top-antiquark t -channel production in pp collisions at s = 13 $$ \sqrt{s}=13 $$ TeV with the ATLAS detector

≪ Previous: Measurement of jet activity produced in top-quark events with an electron, a muon and two b -tagged jets in the final state in pp collisions at $$\sqrt{s}=13$$ s = 13 TeV with the ATLAS detector

Abstract

The reconstruction and calibration algorithms used to calculate missing transverse momentum ($E_{\text {T}}^{\text {miss}}$ ) with the ATLAS detector exploit energy deposits in the calorimeter and tracks reconstructed in the inner detector as well as the muon spectrometer. Various strategies are used to suppress effects arising from additional proton–proton interactions, called pileup, concurrent with the hard-scatter processes. Tracking information is used to distinguish contributions from the pileup interactions using their vertex separation along the beam axis. The performance of the $E_{\text {T}}^{\text {miss}}$ reconstruction algorithms, especially with respect to the amount of pileup, is evaluated using data collected in proton–proton collisions at a centre-of-mass energy of 8 $\text {TeV}$ during 2012, and results are shown for a data sample corresponding to an integrated luminosity of $20.3\, \mathrm{fb}^{-1}$. The simulation and modelling of $E_{\text {T}}^{\text {miss}}$ in events containing a Z boson decaying to two charged leptons (electrons or muons) or a W boson decaying to a charged lepton and a neutrino are compared to data. The acceptance for different event topologies, with and without high transverse momentum neutrinos, is shown for a range of threshold criteria for $E_{\text {T}}^{\text {miss}}$ , and estimates of the systematic uncertainties in the $E_{\text {T}}^{\text {miss}}$ measurements are presented.

↧

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Latest Images