Search for resonances in diphoton events at s = 13 $$ \sqrt{s}=13 $$ TeV with the ATLAS detector

September 1, 2016, 12:00 am

≫ Next: Measurement of total and differential W + W − production cross sections in proton-proton collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector and limits on anomalous triple-gauge-boson couplings

≪ Previous: Measurement of the angular coefficients in Z -boson events using electron and muon pairs from data taken at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

Abstract

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 $ \sqrt{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.

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Measurement of total and differential W + W − production cross sections in proton-proton collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector and limits on anomalous triple-gauge-boson couplings

September 6, 2016, 12:00 am

≫ Next: Measurement of jet activity in top quark events using the eμ final state with two b -tagged jets in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

≪ Previous: Search for resonances in diphoton events at s = 13 $$ \sqrt{s}=13 $$ TeV with the ATLAS detector

Abstract

The production of W boson pairs in proton-proton collisions at $ \sqrt{s}=8 $ TeV is studied using data corresponding to 20.3 fb⁻¹ of integrated luminosity collected by the ATLAS detector during 2012 at the CERN Large Hadron Collider. The W bosons are reconstructed using their leptonic decays into electrons or muons and neutrinos. Events with reconstructed jets are not included in the candidate event sample. A total of 6636 WW candidate events are observed. Measurements are performed in fiducial regions closely approximating the detector acceptance. The integrated measurement is corrected for all acceptance effects and for the W branching fractions to leptons in order to obtain the total WW production cross section, which is found to be 71.1 ± 1.1(stat) _− 5.0^+ 5.7(syst) ± 1.4(lumi) pb. This agrees with the next-to-next-to-leading-order Standard Model prediction of 63. 2_− 1.4^+ 1.6(scale) ± 1.2(PDF) pb. Fiducial differential cross sections are measured as a function of each of six kinematic variables. The distribution of the transverse momentum of the leading lepton is used to set limits on anomalous triple-gauge-boson couplings.

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Measurement of jet activity in top quark events using the eμ final state with two b -tagged jets in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

September 13, 2016, 12:00 am

≫ Next: Charged-particle distributions at low transverse momentum in $$\sqrt{s} = 13$$ s = 13 TeV pp interactions measured with the ATLAS detector at the LHC

≪ Previous: Measurement of total and differential W + W − production cross sections in proton-proton collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector and limits on anomalous triple-gauge-boson couplings

Abstract

Measurements of the jet activity in $ t\overline{t} $ events produced in proton-proton collisions at $ \sqrt{s}=8 $ TeV are presented, using 20.3 fb⁻¹ of data collected by the ATLAS experiment at the Large Hadron Collider. The events were selected in the dilepton eμ decay channel with two identified b-jets. The numbers of additional jets for various jet transverse momentum (p_T) thresholds, and the normalised differential cross-sections as a function of p_T for the five highest-p_T additional jets, were measured in the jet pseudo-rapidity range |η| < 4.5. The gap fraction, the fraction of events which do not contain an additional jet in a central rapidity region, was measured for several rapidity intervals as a function of the minimum p_T of a single jet or the scalar sum of p_T of all additional jets. These fractions were also measured in different intervals of the invariant mass of the $ e\mu b\overline{b} $ system. All measurements were corrected for detector effects, and found to be mostly well-described by predictions from next-to-leading-order and leading-order $ t\overline{t} $ event generators with appropriate parameter choices. The results can be used to further optimise the parameters used in such generators.

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Charged-particle distributions at low transverse momentum in $$\sqrt{s} = 13$$ s = 13 TeV pp interactions measured with the ATLAS detector at the LHC

September 15, 2016, 12:00 am

≫ Next: Study of the rare decays of $$B^0_s$$ B s 0 and $$B^0$$ B 0 into muon pairs from data collected during the LHC Run 1 with the ATLAS detector

≪ Previous: Measurement of jet activity in top quark events using the eμ final state with two b -tagged jets in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

Abstract

Measurements of distributions of charged particles produced in proton–proton collisions with a centre-of-mass energy of 13 TeV are presented. The data were recorded by the ATLAS detector at the LHC and correspond to an integrated luminosity of 151 $\upmu\text{b}^{-1}$. The particles are required to have a transverse momentum greater than 100 MeV and an absolute pseudorapidity less than 2.5. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity and the dependence of the mean transverse momentum on multiplicity are measured in events containing at least two charged particles satisfying the above kinematic criteria. The results are corrected for detector effects and compared to the predictions from several Monte Carlo event generators.

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Study of the rare decays of $$B^0_s$$ B s 0 and $$B^0$$ B 0 into muon pairs from data collected during the LHC Run 1 with the ATLAS detector

September 21, 2016, 12:00 am

≫ Next: Search for supersymmetry in a final state containing two photons and missing transverse momentum in $$\varvec{\sqrt{s}}$$ s = 13 TeV $$\varvec{pp}$$ p p collisions at the LHC using the ATLAS detector

≪ Previous: Charged-particle distributions at low transverse momentum in $$\sqrt{s} = 13$$ s = 13 TeV pp interactions measured with the ATLAS detector at the LHC

Abstract

A study of the decays $B^0_s \rightarrow \mu ^+\mu ^-$ and $B^0 \rightarrow \mu ^+\mu ^-$ has been performed using data corresponding to an integrated luminosity of 25 fb$^{-1}$ of 7 and 8 TeV proton–proton collisions collected with the ATLAS detector during the LHC Run 1. For the $B^0$ dimuon decay, an upper limit on the branching fraction is set at $\mathcal{B}(B^0 \rightarrow \mu ^+\mu ^-) < 4.2 \times 10^{-10}$ at 95 % confidence level. For $B^0_s$, the branching fraction $\mathcal{B}(B^0_s \rightarrow \mu ^+\mu ^-) = \left( 0.9^{+1.1}_{-0.8} \right) \times 10^{-9}$ is measured. The results are consistent with the Standard Model expectation with a p value of 4.8 %, corresponding to 2.0 standard deviations.

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Search for supersymmetry in a final state containing two photons and missing transverse momentum in $$\varvec{\sqrt{s}}$$ s = 13 TeV $$\varvec{pp}$$ p p collisions at the LHC using the ATLAS detector

September 24, 2016, 12:00 am

≫ Next: The performance of the jet trigger for the ATLAS detector during 2011 data taking

≪ Previous: Study of the rare decays of $$B^0_s$$ B s 0 and $$B^0$$ B 0 into muon pairs from data collected during the LHC Run 1 with the ATLAS detector

Abstract

A search has been made for supersymmetry in a final state containing two photons and missing transverse momentum using the ATLAS detector at the Large Hadron Collider. The search makes use of $3.2{~\mathrm{fb}^{-1}}$ of proton-proton collision data collected at a centre-of-mass energy of 13 TeV in 2015. Using a combination of data-driven and Monte-Carlo-based approaches, the Standard Model background is estimated to be $0.27^{+0.22}_{-0.10}$ events. No events are observed in the signal region; considering the expected background and its uncertainty, this observation implies a model-independent 95 % CL upper limit of 0.93 fb (3.0 events) on the visible cross section due to physics beyond the Standard Model. In the context of a generalized model of gauge-mediated supersymmetry breaking with a bino-like next-to-lightest supersymmetric particle, this leads to a lower limit of 1650 GeV on the mass of a degenerate octet of gluino states, independent of the mass of the lighter bino-like neutralino.

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The performance of the jet trigger for the ATLAS detector during 2011 data taking

September 27, 2016, 12:00 am

≫ Next: Searches for heavy diboson resonances in pp collisions at s = 13 $$ \sqrt{s}=13 $$ TeV with the ATLAS detector

≪ Previous: Search for supersymmetry in a final state containing two photons and missing transverse momentum in $$\varvec{\sqrt{s}}$$ s = 13 TeV $$\varvec{pp}$$ p p collisions at the LHC using the ATLAS detector

Abstract

The performance of the jet trigger for the ATLAS detector at the LHC during the 2011 data taking period is described. During 2011 the LHC provided proton–proton collisions with a centre-of-mass energy of 7 TeV and heavy ion collisions with a 2.76 TeV per nucleon–nucleon collision energy. The ATLAS trigger is a three level system designed to reduce the rate of events from the 40 MHz nominal maximum bunch crossing rate to the approximate 400 Hz which can be written to offline storage. The ATLAS jet trigger is the primary means for the online selection of events containing jets. Events are accepted by the trigger if they contain one or more jets above some transverse energy threshold. During 2011 data taking the jet trigger was fully efficient for jets with transverse energy above 25 GeV for triggers seeded randomly at Level 1. For triggers which require a jet to be identified at each of the three trigger levels, full efficiency is reached for offline jets with transverse energy above 60 GeV. Jets reconstructed in the final trigger level and corresponding to offline jets with transverse energy greater than 60 GeV, are reconstructed with a resolution in transverse energy with respect to offline jets, of better than 4 % in the central region and better than 2.5 % in the forward direction.

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Searches for heavy diboson resonances in pp collisions at s = 13 $$ \sqrt{s}=13 $$ TeV with the ATLAS detector

September 29, 2016, 12:00 am

≫ Next: Dark matter interpretations of ATLAS searches for the electroweak production of supersymmetric particles in s = 8 $$ \sqrt{s}=8 $$ TeV proton-proton collisions

≪ Previous: The performance of the jet trigger for the ATLAS detector during 2011 data taking

Abstract

Searches for new heavy resonances decaying to WW, WZ, and ZZ bosons are presented, using a data sample corresponding to 3.2 fb⁻¹ of pp collisions at $ \sqrt{s}=13 $ TeV collected with the ATLAS detector at the CERN Large Hadron Collider. Analyses selecting ννqq, ℓνqq, ℓℓqq and qqqq final states are combined, searching for an arrow-width resonance with mass between 500 and 3000 GeV. The discriminating variable is either an invariant mass or a transverse mass. No significant deviations from the Standard Model predictions are observed. Three benchmark models are tested: a model predicting the existence of a new heavy scalar singlet, a simplified model predicting a heavy vector-boson triplet, and a bulk Randall-Sundrum model with a heavy spin-2 graviton. Cross-section limits are set at the 95% confidence level and are compared to theoretical cross-section predictions for a variety of models. The data exclude a scalar singlet with mass below 2650 GeV, a heavy vector-boson triplet with mass below 2600 GeV, and a graviton with mass below 1100 GeV. These results significantly extend the previous limits set using pp collisions at $ \sqrt{s}=8 $ TeV.

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Dark matter interpretations of ATLAS searches for the electroweak production of supersymmetric particles in s = 8 $$ \sqrt{s}=8 $$ TeV proton-proton collisions

September 30, 2016, 12:00 am

≫ Next: Measurements of top-quark pair differential cross-sections in the lepton+jets channel in pp collisions at $$\sqrt{s}=8\,~{\mathrm {TeV}}$$ s = 8 TeV using the ATLAS detector

≪ Previous: Searches for heavy diboson resonances in pp collisions at s = 13 $$ \sqrt{s}=13 $$ TeV with the ATLAS detector

Abstract

A selection of searches by the ATLAS experiment at the LHC for the electroweak production of SUSY particles are used to study their impact on the constraints on dark matter candidates. The searches use 20 fb⁻¹ of proton-proton collision data at $ \sqrt{s}=8 $ TeV. A likelihood-driven scan of a five-dimensional effective model focusing on the gaugino-higgsino and Higgs sector of the phenomenological minimal supersymmetric Standard Model is performed. This scan uses data from direct dark matter detection experiments, the relic dark matter density and precision flavour physics results. Further constraints from the ATLAS Higgs mass measurement and SUSY searches at LEP are also applied. A subset of models selected from this scan are used to assess the impact of the selected ATLAS searches in this five-dimensional parameter space. These ATLAS searches substantially impact those models for which the mass $ m\left({\tilde{\chi}}_1^0\right) $ of the lightest neutralino is less than 65 GeV, excluding 86% of such models. The searches have limited impact on models with larger $ m\left({\tilde{\chi}}_1^0\right) $ due to either heavy electroweakinos or compressed mass spectra where the mass splittings between the produced particles and the lightest supersymmetric particle is small.

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Measurements of top-quark pair differential cross-sections in the lepton+jets channel in pp collisions at $$\sqrt{s}=8\,~{\mathrm {TeV}}$$ s = 8 TeV using the ATLAS detector

October 3, 2016, 12:00 am

≫ Next: Search for new phenomena in different-flavour high-mass dilepton final states in pp collisions at $$\sqrt{s}=13$$ s = 13 Tev with the ATLAS detector

≪ Previous: Dark matter interpretations of ATLAS searches for the electroweak production of supersymmetric particles in s = 8 $$ \sqrt{s}=8 $$ TeV proton-proton collisions

Abstract

Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, $t\bar{t}$ system and event-level kinematic observables in proton–proton collisions at a centre-of-mass energy of $\sqrt{s}=8\,\mathrm{TeV}$. The observables have been chosen to emphasize the $t\bar{t}$ production process and to be sensitive to effects of initial- and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fb$^{-1}$, recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a b-quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used.

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Search for new phenomena in different-flavour high-mass dilepton final states in pp collisions at $$\sqrt{s}=13$$ s = 13 Tev with the ATLAS detector

October 4, 2016, 12:00 am

≫ Next: Search for bottom squark pair production in proton–proton collisions at $$\sqrt{s}=13$$ s = 13 TeV with the ATLAS detector

≪ Previous: Measurements of top-quark pair differential cross-sections in the lepton+jets channel in pp collisions at $$\sqrt{s}=8\,~{\mathrm {TeV}}$$ s = 8 TeV using the ATLAS detector

Abstract

A search is performed for a heavy particle decaying into different flavour dilepton pairs ($e\mu $, $e\tau $ or $\mu \tau $), using 3.2 fb$^{-1}$ of proton–proton collision data at $\sqrt{s}=13$ TeV collected in 2015 by the ATLAS detector at the Large Hadron Collider. No excess over the Standard Model prediction is observed. Limits at the 95 % credibility level are set on the mass of a $Z^\prime $ boson with lepton-flavour-violating couplings at 3.0, 2.7 and 2.6 TeV, and on the mass of a supersymmetric $\tau $ sneutrino with R-parity-violating couplings at 2.3, 2.2 and 1.9 TeV, for $e\mu $, $e\tau $ and $\mu \tau $ final states, respectively. The results are also interpreted as limits on the threshold mass for quantum black hole production.

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Search for bottom squark pair production in proton–proton collisions at $$\sqrt{s}=13$$ s = 13 TeV with the ATLAS detector

October 6, 2016, 12:00 am

≫ Next: Search for gluinos in events with an isolated lepton, jets and missing transverse momentum at $$\sqrt{s}$$ s = 13 Te V with the ATLAS detector

≪ Previous: Search for new phenomena in different-flavour high-mass dilepton final states in pp collisions at $$\sqrt{s}=13$$ s = 13 Tev with the ATLAS detector

Abstract

The result of a search for pair production of the supersymmetric partner of the Standard Model bottom quark ($\tilde{b}^{}_{1} $) is reported. The search uses 3.2 fb$^{-1}$ of pp collisions at $\sqrt{s}=13$ TeV collected by the ATLAS experiment at the Large Hadron Collider in 2015. Bottom squarks are searched for in events containing large missing transverse momentum and exactly two jets identified as originating from b-quarks. No excess above the expected Standard Model background yield is observed. Exclusion limits at 95 % confidence level on the mass of the bottom squark are derived in phenomenological supersymmetric R-parity-conserving models in which the $\tilde{b}^{}_{1} $ is the lightest squark and is assumed to decay exclusively via $\tilde{b}^{}_{1} \rightarrow b \tilde{\chi }^{0}_{1}$, where $\tilde{\chi }^{0}_{1}$ is the lightest neutralino. The limits significantly extend previous results; bottom squark masses up to 800 (840) GeV are excluded for the $\tilde{\chi }^{0}_{1}$ mass below 360 (100) GeV whilst differences in mass above 100 GeV between the $\tilde{b}^{}_{1}$ and the $\tilde{\chi }^{0}_{1}$ are excluded up to a $\tilde{b}^{}_{1} $ mass of 500 GeV.

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Search for gluinos in events with an isolated lepton, jets and missing transverse momentum at $$\sqrt{s}$$ s = 13 Te V with the ATLAS detector

October 21, 2016, 12:00 am

≫ Next: Performance of pile-up mitigation techniques for jets in $$pp$$ p p collisions at $$\sqrt{s}=8$$ s = 8 TeV using the ATLAS detector

≪ Previous: Search for bottom squark pair production in proton–proton collisions at $$\sqrt{s}=13$$ s = 13 TeV with the ATLAS detector

Abstract

The results of a search for gluinos in final states with an isolated electron or muon, multiple jets and large missing transverse momentum using proton–proton collision data at a centre-of-mass energy of $\sqrt{s} = 13 \mathrm{{\ Te V}}$ are presented. The dataset used was recorded in 2015 by the ATLAS experiment at the Large Hadron Collider and corresponds to an integrated luminosity of 3.2 fb$^{-1}$. Six signal selections are defined that best exploit the signal characteristics. The data agree with the Standard Model background expectation in all six signal selections, and the largest deviation is a 2.1 standard deviation excess. The results are interpreted in a simplified model where pair-produced gluinos decay via the lightest chargino to the lightest neutralino. In this model, gluinos are excluded up to masses of approximately 1.6 Te V depending on the mass spectrum of the simplified model, thus surpassing the limits of previous searches.

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Performance of pile-up mitigation techniques for jets in $$pp$$ p p collisions at $$\sqrt{s}=8$$ s = 8 TeV using the ATLAS detector

October 27, 2016, 12:00 am

≫ Next: Search for minimal supersymmetric standard model Higgs Bosons H / A and for a $$Z^{\prime }$$ Z ′ boson in the $$\tau \tau $$ τ τ final state produced in pp collisions at $$\sqrt{s}= 13$$ s = 13 TeV with the ATLAS detector

≪ Previous: Search for gluinos in events with an isolated lepton, jets and missing transverse momentum at $$\sqrt{s}$$ s = 13 Te V with the ATLAS detector

Abstract

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 $\mathrm{fb}^{-1}$ data sample collected at a centre-of-mass energy of $\sqrt{s} = 8~\mathrm {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.

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Search for minimal supersymmetric standard model Higgs Bosons H / A and for a $$Z^{\prime }$$ Z ′ boson in the $$\tau \tau $$ τ τ final state produced in pp collisions at $$\sqrt{s}= 13$$ s = 13 TeV with the ATLAS detector

October 27, 2016, 12:00 am

≫ Next: Search for the Higgs boson produced in association with a W boson and decaying to four b -quarks via two spin-zero particles in pp collisions at 13 TeV with the ATLAS detector

≪ Previous: Performance of pile-up mitigation techniques for jets in $$pp$$ p p collisions at $$\sqrt{s}=8$$ s = 8 TeV using the ATLAS detector

Abstract

A search for neutral Higgs bosons of the minimal supersymmetric standard model (MSSM) and for a heavneutral $Z^{\prime }$ boson is performed using a data sample corresponding to an integrated luminosity of 3.2 fb$^{-1}$ from proton–proton collisions at $\sqrt{s} = 13$ ${\mathrm {TeV}}$ recorded by the ATLAS detector at the LHC. The heavy resonance is assumed to decay to a $\tau ^+ \tau ^-$ pair with at least one $\tau $ lepton decaying to final states with hadrons and a neutrino. The search is performed in the mass range of 0.2–1.2 ${\mathrm {TeV}}$ for the MSSM neutral Higgs bosons and 0.5–2.5 ${\mathrm {TeV}}$ for the heavy neutral $Z^{\prime }$ boson. The data are in good agreement with the background predicted by the Standard Model. The results are interpreted in MSSM and $Z^{\prime }$ benchmark scenarios. The most stringent constraints on the MSSM $m_A$–$\tan \beta $ space exclude at 95 % confidence level (CL) $\tan \beta > 7.6$ for $m_A = 200$ $\text {GeV}$ in the $m_{h}^{\text {mod+}}$ MSSM scenario. For the Sequential Standard Model, a $Z^{\prime }_\mathrm {SSM}$ mass up to 1.90 ${\mathrm {TeV}}$ is excluded at 95 % CL and masses up to 1.82–2.17 ${\mathrm {TeV}}$ are excluded for a $Z^{\prime }_{\mathrm {SFM}}$ of the strong flavour model.

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Search for the Higgs boson produced in association with a W boson and decaying to four b -quarks via two spin-zero particles in pp collisions at 13 TeV with the ATLAS detector

November 5, 2016, 12:00 am

≫ Next: Search for the Standard Model Higgs boson produced by vector-boson fusion and decaying to bottom quarks in s = 8 $$ \sqrt{s}=8 $$ TeV pp collisions with the ATLAS detector

≪ Previous: Search for minimal supersymmetric standard model Higgs Bosons H / A and for a $$Z^{\prime }$$ Z ′ boson in the $$\tau \tau $$ τ τ final state produced in pp collisions at $$\sqrt{s}= 13$$ s = 13 TeV with the ATLAS detector

Abstract

This paper presents a dedicated search for exotic decays of the Higgs boson to a pair of new spin-zero particles, $H \rightarrow aa$, where the particle a decays to b-quarks and has a mass in the range of 20–60 GeV. The search is performed in events where the Higgs boson is produced in association with a $W$ boson, giving rise to a signature of a lepton (electron or muon), missing transverse momentum, and multiple jets from b-quark decays. The analysis is based on the full dataset of pp collisions at $\sqrt{s} = 13\,\mathrm{{TeV}}$ recorded in 2015 by the ATLAS detector at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 3.2 $\text{ fb }^{-1}$. No significant excess of events above the Standard Model prediction is observed, and a $95\,\%$ confidence-level upper limit is derived for the product of the production cross section for $pp \rightarrow WH$ times the branching ratio for the decay $H \rightarrow aa \rightarrow 4b$. The upper limit ranges from 6.2 pb for an a-boson mass $m_a = 20\,\mathrm{{GeV}}$ to 1.5 pb for $m_a = 60\,\mathrm{{GeV}}$.

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Search for the Standard Model Higgs boson produced by vector-boson fusion and decaying to bottom quarks in s = 8 $$ \sqrt{s}=8 $$ TeV pp collisions with the ATLAS detector

November 21, 2016, 12:00 am

≫ Next: Study of hard double-parton scattering in four-jet events in pp collisions at s = 7 $$ \sqrt{s}=7 $$ TeV with the ATLAS experiment

≪ Previous: Search for the Higgs boson produced in association with a W boson and decaying to four b -quarks via two spin-zero particles in pp collisions at 13 TeV with the ATLAS detector

Abstract

A search with the ATLAS detector is presented for the Standard Model Higgs boson produced by vector-boson fusion and decaying to a pair of bottom quarks, using 20.2 fb⁻¹ of LHC proton-proton collision data at $ \sqrt{s}=8 $ TeV. The signal is searched for as a resonance in the invariant mass distribution of a pair of jets containing b-hadrons in vector-boson-fusion candidate events. The yield is measured to be −0.8 ± 2.3 times the Standard Model cross-section for a Higgs boson mass of 125 GeV. The upper limit on the cross-section times the branching ratio is found to be 4.4 times the Standard Model cross-section at the 95% confidence level, consistent with the expected limit value of 5.4 (5.7) in the background-only (Standard Model production) hypothesis.

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Study of hard double-parton scattering in four-jet events in pp collisions at s = 7 $$ \sqrt{s}=7 $$ TeV with the ATLAS experiment

November 21, 2016, 12:00 am

≫ Next: Addendum to ‘Measurement of the $$t\bar{t}$$ t t ¯ production cross-section using $$e\mu $$ e μ events with b -tagged jets in pp collisions at $$\sqrt{s}$$ s = 7 and 8 $$\,\mathrm{TeV}$$ TeV with the ATLAS detector’

≪ Previous: Search for the Standard Model Higgs boson produced by vector-boson fusion and decaying to bottom quarks in s = 8 $$ \sqrt{s}=8 $$ TeV pp collisions with the ATLAS detector

Abstract

Inclusive four-jet events produced in proton-proton collisions at a centre-of-mass energy of $ \sqrt{s}=7 $ TeV are analysed for the presence of hard double-parton scattering using data corresponding to an integrated luminosity of 37.3 pb⁻¹, collected with the ATLAS detector at the LHC. The contribution of hard double-parton scattering to the production of four-jet events is extracted using an artificial neural network, assuming that hard double-parton scattering can be approximated by an uncorrelated overlaying of dijet events. For events containing at least four jets with transverse momentum p_T≥ 20 GeV and pseudorapidity |η| ≤ 4.4, and at least one having p_T≥ 42.5 GeV, the contribution of hard double-parton scattering is estimated to be f_DPS = 0.092_− 0.011^+ 0.005(stat.)_− 0.037^+ 0.033(syst.). After combining this measurement with those of the inclusive dijet and four-jet cross-sections in the appropriate phase space regions, the effective cross-section, σ_eff , was determined to be σ_eff = 14. 9_− 1.0^+ 1.2(stat.)_− 3.8^+ 5.1(syst.) mb. This result is consistent within the quoted uncertainties with previous measurements of σ_eff , performed at centre-of-mass energies between 63 GeV and 8 TeV using various final states, and it corresponds to 21_− 6^+ 7 % of the total inelastic cross-section measured at $ \sqrt{s}=7 $ TeV. The distributions of the observables sensitive to the contribution of hard double-parton scattering, corrected for detector effects, are also provided.

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Addendum to ‘Measurement of the $$t\bar{t}$$ t t ¯ production cross-section using $$e\mu $$ e μ events with b -tagged jets in pp collisions at $$\sqrt{s}$$ s = 7 and 8 $$\,\mathrm{TeV}$$ TeV with the ATLAS detector’

November 23, 2016, 12:00 am

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

≪ Previous: Study of hard double-parton scattering in four-jet events in pp collisions at s = 7 $$ \sqrt{s}=7 $$ TeV with the ATLAS experiment

Abstract

The ATLAS measurement of the inclusive top quark pair ($t\bar{t}$) cross-section $\sigma _{t\bar{t}}$ in proton–proton collisions at $\sqrt{s}=8$ $\,\mathrm{TeV}$ has been updated using the final 2012 luminosity calibration. The updated cross-section result is: $$\begin{aligned} \sigma _{t\bar{t}}= 242.9\pm 1.7\pm 5.5\pm 5.1\pm 4.2\,\mathrm pb, \end{aligned}$$where the four uncertainties arise from data statistics, experimental and theoretical systematic effects, knowledge of the integrated luminosity and of the LHC beam energy. The result is consistent with theoretical QCD calculations at next-to-next-to-leading order. The measurement of the ratio of $t\bar{t}$ cross-sections at $\sqrt{s}=8$ $\,\mathrm{TeV}$ and $\sqrt{s}=7$ $\,\mathrm{TeV}$, and the $\sqrt{s}=8$ $\,\mathrm{TeV}$ fiducial measurement corresponding to the experimental acceptance of the leptons, have also been updated.

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

November 28, 2016, 12:00 am

≫ Next: 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

≪ Previous: Addendum to ‘Measurement of the $$t\bar{t}$$ t t ¯ production cross-section using $$e\mu $$ e μ events with b -tagged jets in pp collisions at $$\sqrt{s}$$ s = 7 and 8 $$\,\mathrm{TeV}$$ TeV with the ATLAS detector’

Abstract

The luminosity determination for the ATLAS detector at the LHC during pp collisions at $\sqrt{s} =$ 8 TeV in 2012 is presented. The evaluation of the luminosity scale is performed using several luminometers, and comparisons between these luminosity detectors are made to assess the accuracy, consistency and long-term stability of the results. A luminosity uncertainty of $\delta \mathcal{L}/ \mathcal{L} = \pm 1.9\%$ is obtained for the $22.7\,\mathrm {fb}^{-1}$ of pp collision data delivered to ATLAS at $\sqrt{s} =$ 8 TeV in 2012.

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