A search for cascade decays of charged sleptons and sneutrinos using final states characterized by three leptons (electrons or muons) and missing transverse momentum is presented. The analysis is based on a dataset with 140 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of $\sqrt{s}$=13 TeV recorded by the ATLAS detector at the Large Hadron Collider. This paper focuses on a supersymmetric scenario that is motivated by the muon anomalous magnetic moment observation, dark mattter relic density abundance, and electroweak naturalness. A mass spectrum involving light higgsinos and heavier sleptons with a bino at intermediate mass is targeted. No significant deviation from the Standard Model expectation is observed. This search enables to place stringent constraints on this model, excluding at the 95% confidence level charged slepton and sneutrino masses up to 450 GeV when assuming a lightest neutralino mass of 100 GeV and mass-degenerate selectrons, smuons and sneutrinos.
Distribution of $m_{3\ell}$ in SROS-on-$eee$. The SR selections are applied for each distribution, except for the variable shown, for which the selection is indicated by a black arrow. The last bin includes the overflow. The `Others' category contains the production of Higgs boson, 3-top, 4-top, and single-top processes. Distributions for SBH signals are overlaid. The bottom panels show the ratio of the observed data to the predicted total background yields. The hatched band includes all statistical and systematic uncertainties.
Distribution of $m_{3\ell}$ in SROS-on-$e\mu\mu$. The SR selections are applied for each distribution, except for the variable shown, for which the selection is indicated by a black arrow. The last bin includes the overflow. The `Others' category contains the production of Higgs boson, 3-top, 4-top, and single-top processes. Distributions for SBH signals are overlaid. The bottom panels show the ratio of the observed data to the predicted total background yields. The hatched band includes all statistical and systematic uncertainties.
Distribution of $E_{\text{T}}^{\text{miss}}$ in SROS-on-b-$eee$. The SR selections are applied for each distribution, except for the variable shown, for which the selection is indicated by a black arrow. The last bin includes the overflow. The `Others' category contains the production of Higgs boson, 3-top, 4-top, and single-top processes. Distributions for SBH signals are overlaid. The bottom panels show the ratio of the observed data to the predicted total background yields. The hatched band includes all statistical and systematic uncertainties.
A search for decays of the Higgs boson into a $Z$ boson and a light resonance, with a mass of 0.5-3.5 GeV, is performed using the full 140 fb$^{-1}$ dataset of 13 TeV proton-proton collisions recorded by the ATLAS detector during Run~2 of the LHC. Leptonic decays of the $Z$ boson and hadronic decays of the light resonance are considered. The resonance can be interpreted as a $J/ψ$ or $η_c$ meson, an axion-like particle, or a light pseudoscalar in two-Higgs-doublet models. Due to its low mass, it would be produced with high boost and reconstructed as a single small-radius jet of hadrons. A neural network is used to correct the Monte Carlo simulation of the background in a data-driven way. Two additional neural networks are used to distinguish signal from background. A binned profile-likelihood fit is performed on the final-state invariant mass distribution. No significant excess of events relative to the expected background is observed, and upper limits at 95% confidence level are set on the Higgs boson's branching fraction to a $Z$ boson and a light resonance. The exclusion limit is 10% for the lower masses, and increases for higher masses. Upper limits on the effective coupling $C^\text{eff}_{ZH}/Λ$ of an axion-like particle to a Higgs boson and $Z$ boson are also set at 95% confidence level, and range from 0.9 to 2 TeV$^{-1}$.
The angularity, for data, background (pre- and post-reweighting) and three $H\rightarrow Za$ signal hypotheses (for $a\rightarrow q\bar{q}/gg$ inclusively). Events are required to pass the complete event selection but not the classification NN requirement. The background normalization is set equal to that of the data for events passing the preselection and being in the $m_{\ell\ell j}$ 100-180 GeV region. The signal normalization assumes the SM Higgs boson inclusive production cross-section, $\mathcal{B}(H\to Za)=100\%$, and it is scaled up by a factor of 100. The error bars (hatched regions) represent the data (MC) sample's statistical uncertainty in the histograms and the ratio plots. Vertical arrows indicate data points that fall outside the displayed $y$-axis range.
The modified energy correlation function, for data, background (pre- and post-reweighting) and three $H\rightarrow Za$ signal hypotheses (for $a\rightarrow q\bar{q}/gg$ inclusively). Events are required to pass the complete event selection but not the classification NN requirement. The background normalization is set equal to that of the data for events passing the preselection and being in the $m_{\ell\ell j}$ 100-180 GeV region. The signal normalization assumes the SM Higgs boson inclusive production cross-section, $\mathcal{B}(H\to Za)=100\%$, and it is scaled up by a factor of 100. The error bars (hatched regions) represent the data (MC) sample's statistical uncertainty in the histograms and the ratio plots. Vertical arrows indicate data points that fall outside the displayed $y$-axis range.
$Z$ boson transverse momentum, for data, background (pre- and post-reweighting) and three $H\rightarrow Za$ signal hypotheses (for $a\rightarrow q\bar{q}/gg$ inclusively). Events are required to pass the complete event selection but not the classification NN requirement. The background normalization is set equal to that of the data for events passing the preselection and being in the $m_{\ell\ell j}$ 100-180 GeV region. The signal normalization assumes the SM Higgs boson inclusive production cross-section, $\mathcal{B}(H\to Za)=100\%$, and it is scaled up by a factor of 100. The error bars (hatched regions) represent the data (MC) sample's statistical uncertainty in the histograms and the ratio plots. Vertical arrows indicate data points that fall outside the displayed $y$-axis range.
Charged Higgs bosons produced either in top-quark decays or in association with a top-quark, subsequently decaying via $H^{\pm} \to \tau^{\pm}\nu_{\tau}$, are searched for in 140 $\text{fb}^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV recorded with the ATLAS detector. Depending on whether the top-quark produced together with the $H^{\pm}$ decays hadronically or semi-leptonically, the search targets $\tau$+jets or $\tau$+lepton final states, in both cases with a $\tau$-lepton decaying into a neutrino and hadrons. No significant excess over the Standard Model background expectation is observed. For the mass range of $80 \leq m_{H^{\pm}} \leq 3000$ GeV, upper limits at 95% confidence level are set on the production cross-section of the charged Higgs boson times the branching fraction $\mathrm{\cal{B}}(H^{\pm} \to \tau^{\pm}\nu_{\tau})$ in the range 4.5 pb-0.4 fb. In the mass range 80-160 GeV, assuming the Standard Model cross-section for $t\bar{t}$ production, this corresponds to upper limits between 0.27% and 0.02% on $\mathrm{\cal{B}}(t\to bH^{\pm}) \times \mathrm{\cal{B}}(H^{\pm} \to \tau^{\pm}\nu_{\tau})$.
This paper presents a search for massive charged long-lived particles produced in pp collisions at $\sqrt{s}=$ 13 TeV at the LHC using the ATLAS experiment. The dataset used corresponds to an integrated luminosity of 3.2 fb$^{-1}$. Many extensions of the Standard Model predict the existence of massive charged long-lived particles, such as $R$-hadrons. These massive particles are expected to be produced with a velocity significantly below the speed of light, and therefore to have a specific ionization higher than any Standard Model particle of unit charge at high momenta. The Pixel subsystem of the ATLAS detector is used to measure the ionization energy loss of reconstructed charged particles and to search for such highly ionizing particles. The search presented here has much greater sensitivity than a similar search performed using the ATLAS detector in the $\sqrt{s}=$ 8 TeV dataset, thanks to the increase in expected signal cross-section due to the higher center-of-mass energy of collisions, to an upgraded detector with a new silicon layer close to the interaction point, and to analysis improvements. No significant deviation from Standard Model background expectations is observed, and lifetime-dependent upper limits on $R$-hadron production cross-sections and masses are set. Gluino $R$-hadrons with lifetimes above 0.4 ns and decaying to $q\bar{q}$ plus a 100 GeV neutralino are excluded at the 95% confidence level, with lower mass limit ranging between 740 GeV and 1590 GeV. In the case of stable $R$-hadrons the lower mass limit at the 95% confidence level is 1570 GeV.
The centrality dependence of the mean charged-particle multiplicity as a function of pseudorapidity is measured in approximately 1 $\mu$b$^{-1}$ of proton--lead collisions at a nucleon--nucleon centre-of-mass energy of $\sqrt{s_{_{\rm{NN}}}} = 5.02$ TeV using the ATLAS detector at the Large Hadron Collider. Charged particles with absolute pseudorapidity less than 2.7 are reconstructed using the ATLAS pixel detector. The $p$+Pb collision centrality is characterised by the total transverse energy measured in the Pb-going direction of the forward calorimeter. The charged-particle pseudorapidity distributions are found to vary strongly with centrality, with an increasing asymmetry between the proton-going and Pb-going directions as the collisions become more central. Three different estimations of the number of nucleons participating in the $p$+Pb collision have been carried out using the Glauber model as well as two Glauber--Gribov inspired extensions to the Glauber model. Charged-particle multiplicities per participant pair are found to vary differently for these three models, highlighting the importance of including colour fluctuations in nucleon--nucleon collisions in the modelling of the initial state of $p$+Pb collisions.
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 $\sqrt{s}=8$ 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 $p_T$-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 $p_{T}$-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.
This paper presents measurements of distributions of charged particles which are produced in proton--proton collisions at a centre-of-mass energy of $\sqrt{s} = 8$ TeV and recorded by the ATLAS detector at the LHC. A special dataset recorded in 2012 with a small number of interactions per beam crossing (below 0.004) and corresponding to an integrated luminosity of $160 \mathrm{\mu b^{-1}}$ was used. A minimum-bias trigger was utilised to select a data sample of more than 9 million collision events. The multiplicity, pseudorapidity, and transverse momentum distributions of charged particles are shown in different regions of kinematics and charged-particle multiplicity, including measurements of final states at high multiplicity. The results are corrected for detector effects and are compared to the predictions of various Monte Carlo event generator models which simulate the full hadronic final state.
This paper reports a search for a light CP-odd scalar resonance with a mass of 20 GeV to 90 GeV in 13 TeV proton-proton collision data with an integrated luminosity of 140 fb$^{-1}$ collected with the ATLAS detector at the Large Hadron Collider. The analysis assumes the resonance is produced via gluon-gluon fusion and decays into a $\tau^{+}\tau^{-}$ pair which subsequently decays into a fully leptonic $\mu^{+}\nu_{\mu} \bar{\nu}_{\tau} e^{-} \bar{\nu}_{e} \nu_{\tau}$ or $e^{+}\nu_{e}\bar{\nu}_{\tau} \mu^-\bar{\nu}_{\mu}\nu_{\tau}$ final state. No significant excess of events above the predicted Standard Model background is observed. The results are interpreted within a flavour-aligned two-Higgs-doublet model, and a model-independent cross-section interpretation is also given. Upper limits at 95$%$ confidence level between 3.0 pb and 68 pb are set on the cross-section for producing a CP-odd Higgs boson that decays into a $\tau^+\tau^-$ pair.
Post-fit $m_\mathrm{MMC}$ distribution in the low-mass SR for the $m_A = 20\,\mathrm{GeV}$ signal mass hypothesis. $m_\mathrm{MMC}$ is the mass reconstructed by the Missing Mass Calculator. Processes contributing to the background Others are $Z/\gamma^* \rightarrow ee/\mu\mu$ and SM Higgs. The subscript on the $A\to\tau\tau$ process indicates the mass of the $A$ boson. Total includes all backgrounds and the signal process. The low-mass Signal Region is defined as: - 1 electron and 1 muon with opposite charge - $p_\mathrm{T}$ requirements of the leptons are a combination of the following: - $p_\mathrm{T}^e > 18\,\mathrm{GeV}$ and $p_\mathrm{T}^\mu > 15\,\mathrm{GeV}$ - $p_\mathrm{T}^e > 10\,\mathrm{GeV}$ and $p_\mathrm{T}^\mu > 25\,\mathrm{GeV}$ - $p_\mathrm{T}^e > 27\,\mathrm{GeV}$ and $p_\mathrm{T}^\mu > 10\,\mathrm{GeV}$ - $\vert \eta_e \vert < 2.47$, excluding $1.37 < \vert \eta_e \vert < 1.52$ - $\vert \eta_\mu \vert < 2.7$ - no jets with $b$-quarks - $\Delta R_{\ell\ell} < 0.7$ - $E_\mathrm{T}^\mathrm{miss} > 50\,\mathrm{GeV}$ - $m_\mathrm{T}^\mathrm{tot} = \sqrt{\left(p_\mathrm{T}^e+p_\mathrm{T}^\mu+E_\mathrm{T}^\mathrm{miss}\right)^2-\left(\vec{p}_\mathrm{T}^{\,e}+\vec{p}_\mathrm{T}^{\,\mu}+\vec{E}_\mathrm{T}^{\,\mathrm{miss}}\right)^2} < 45\,\mathrm{GeV}$ - $m_\mathrm{MMC} > 0\,\mathrm{GeV}$
Post-fit $m_\mathrm{MMC}$ distribution in the low-mass SR for the $m_A = 20\,\mathrm{GeV}$ signal mass hypothesis. $m_\mathrm{MMC}$ is the mass reconstructed by the Missing Mass Calculator. Processes contributing to the background Others are $Z/\gamma^* \rightarrow ee/\mu\mu$ and SM Higgs. The subscript on the $A\to\tau\tau$ process indicates the mass of the $A$ boson. Total includes all backgrounds and the signal process. The low-mass Signal Region is defined as: - 1 electron and 1 muon with opposite charge - $p_\mathrm{T}$ requirements of the leptons are a combination of the following: - $p_\mathrm{T}^e > 18\,\mathrm{GeV}$ and $p_\mathrm{T}^\mu > 15\,\mathrm{GeV}$ - $p_\mathrm{T}^e > 10\,\mathrm{GeV}$ and $p_\mathrm{T}^\mu > 25\,\mathrm{GeV}$ - $p_\mathrm{T}^e > 27\,\mathrm{GeV}$ and $p_\mathrm{T}^\mu > 10\,\mathrm{GeV}$ - $\vert \eta_e \vert < 2.47$, excluding $1.37 < \vert \eta_e \vert < 1.52$ - $\vert \eta_\mu \vert < 2.7$ - no jets with $b$-quarks - $\Delta R_{\ell\ell} < 0.7$ - $E_\mathrm{T}^\mathrm{miss} > 50\,\mathrm{GeV}$ - $m_\mathrm{T}^\mathrm{tot} = \sqrt{\left(p_\mathrm{T}^e+p_\mathrm{T}^\mu+E_\mathrm{T}^\mathrm{miss}\right)^2-\left(\vec{p}_\mathrm{T}^{\,e}+\vec{p}_\mathrm{T}^{\,\mu}+\vec{E}_\mathrm{T}^{\,\mathrm{miss}}\right)^2} < 45\,\mathrm{GeV}$ - $m_\mathrm{MMC} > 0\,\mathrm{GeV}$
Post-fit $m_\mathrm{MMC}$ distribution in the high-mass SR for the $m_A = 90\,\mathrm{GeV}$ signal mass hypothesis. $m_\mathrm{MMC}$ is the mass reconstructed by the Missing Mass Calculator. Processes contributing to the background Others are $Z/\gamma^* \rightarrow ee/\mu\mu$ and SM Higgs. The subscript on the $A\to\tau\tau$ process indicates the mass of the $A$ boson. otal includes all backgrounds and the signal process. The high-mass Signal Region is defined as: - 1 electron and 1 muon with opposite charge - $p_\mathrm{T}$ requirements of the leptons are a combination of the following: - $p_\mathrm{T}^e > 18\,\mathrm{GeV}$ and $p_\mathrm{T}^\mu > 15\,\mathrm{GeV}$ or - $p_\mathrm{T}^e > 10\,\mathrm{GeV}$ and $p_\mathrm{T}^\mu > 25\,\mathrm{GeV}$ or - $p_\mathrm{T}^e > 27\,\mathrm{GeV}$ and $p_\mathrm{T}^\mu > 10\,\mathrm{GeV}$ - $\vert \eta_e \vert < 2.47$, excluding $1.37 < \vert \eta_e \vert < 1.52$ - $\vert \eta_\mu \vert < 2.7$ - no jets with $b$-quarks - $\Delta R_{\ell\ell} < 1.0$ - $E_\mathrm{T}^\mathrm{miss} > 30\,\mathrm{GeV}$ - $m_\mathrm{T}^\mathrm{tot} = \sqrt{\left(p_\mathrm{T}^e+p_\mathrm{T}^\mu+E_\mathrm{T}^\mathrm{miss}\right)^2-\left(\vec{p}_\mathrm{T}^{\,e}+\vec{p}_\mathrm{T}^{\,\mu}+\vec{E}_\mathrm{T}^{\,\mathrm{miss}}\right)^2} < 65\,\mathrm{GeV}$ - $35\,\mathrm{GeV} < m_\mathrm{MMC} < 130\,\mathrm{GeV}$
This article reports on a search for dark matter pair production in association with bottom or top quarks in 20.3 fb$^{-1}$ of $pp$ collisions collected at $\sqrt{s} = 8$ TeV by the ATLAS detector at the LHC. Events with large missing transverse momentum are selected when produced in association with high-momentum jets of which one or more are identified as jets containing $b$-quarks. Final states with top quarks are selected by requiring a high jet multiplicity and in some cases a single lepton. The data are found to be consistent with the Standard Model expectations and limits are set on the mass scale of effective field theories that describe scalar and tensor interactions between dark matter and Standard Model particles. Limits on the dark-matter--nucleon cross-section for spin-independent and spin-dependent interactions are also provided. These limits are particularly strong for low-mass dark matter. Using a simplified model, constraints are set on the mass of dark matter and of a coloured mediator suitable to explain a possible signal of annihilating dark matter.
The differential cross-section for pair production of top quarks with high transverse momentum is measured in 20.3 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 8 TeV. The measurement is performed for $t\bar{t}$ events in the lepton+jets channel. The cross-section is reported as a function of the hadronically decaying top quark transverse momentum for values above 300 GeV. The hadronically decaying top quark is reconstructed as an anti-$k_t$ jet with radius parameter $R=1.0$ and identified with jet substructure techniques. The observed yield is corrected for detector effects to obtain a cross-section at particle level in a fiducial region close to the event selection. A parton-level cross-section extrapolated to the full phase space is also reported for top quarks with transverse momentum above 300 GeV. The predictions of a majority of next-to-leading-order and leading-order matrix-element Monte Carlo generators are found to agree with the measured cross-sections.
Forum