A search for dark matter produced in association with a Higgs boson in final states with two hadronically decaying $\tau$-leptons and missing transverse momentum is presented. The analysis uses $139$ fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. No evidence for physics beyond the Standard Model is found. The results are interpreted in terms of a 2HDM+$a$ model. Exclusion limits at 95% confidence level are derived. Model-independent limits are also set on the visible cross section for processes beyond the Standard Model producing missing transverse momentum in association with a Higgs boson decaying to $\tau$-leptons.
Expected CLs values in the Low mA SR, mA vs tanB signal grid.
Observed CLs values in the Low mA SR, mA vs tanB signal grid.
Expected CLs values in the High mA SR, mA vs ma signal grid.
A search for Majorana neutrinos in same-sign $WW$ scattering events is presented. The analysis uses $\sqrt{s}= 13$ TeV proton-proton collision data with an integrated luminosity of 140 fb$^{-1}$ recorded during 2015-2018 by the ATLAS detector at the Large Hadron Collider. The analysis targets final states including exactly two same-sign muons and at least two hadronic jets well separated in rapidity. The modelling of the main backgrounds, from Standard Model same-sign $WW$ scattering and $WZ$ production, is constrained with data in dedicated signal-depleted control regions. The distribution of the transverse momentum of the second-hardest muon is used to search for signals originating from a heavy Majorana neutrino with a mass between 50 GeV and 20 TeV. No significant excess is observed over the background expectation. The results are interpreted in a benchmark scenario of the Phenomenological Type-I Seesaw model. In addition, the sensitivity to the Weinberg operator is investigated. Upper limits at the 95% confidence level are placed on the squared muon-neutrino-heavy-neutrino mass-mixing matrix element $\vert V_{\mu N} \vert^{2}$ as a function of the heavy Majorana neutrino's mass $m_N$, and on the effective $\mu\mu$ Majorana neutrino mass $|m_{\mu\mu}|$.
Observed and expected 95% CL upper limits on the heavy Majorana neutrino mixing element $\vert V_{\mu N} \vert^{2}$ as a function of $m_N$ in the Phenomenological Type-I Seesaw model.
Cutflow for a selection of signal samples used in this analysis. The flavour-aligned scenario (in which $\vert V_{\mu N} \vert^{2}=1$) is considered for heavy Majorana neutrino samples. The event yields include all correction factors applied to simulation, and is normalised to 140 fb$^{-1}$. The `Skim' selection requires 2 baseline muons and 2 jets satisfying the object definitions described in Section 3 and $m_{jj} > 150$ GeV. Uncertainties are statistical only.
A search is reported for excited $\tau$-leptons and leptoquarks in events with two hadronically decaying $\tau$-leptons and two or more jets. The search uses proton-proton (pp) collision data at $\sqrt{s} = 13$ TeV recorded by the ATLAS experiment during the Run 2 of the Large Hadron Collider in 2015-2018. The total integrated luminosity is 139 fb$^{-1}$. The excited $\tau$-lepton is assumed to be produced and to decay via a four-fermion contact interaction into an ordinary $\tau$-lepton and a quark-antiquark pair. The leptoquarks are assumed to be produced in pairs via the strong interaction, and each leptoquark is assumed to couple to a charm or lighter quark and a $\tau$-lepton. No excess over the background prediction is observed. Excited $\tau$-leptons with masses below 2.8 TeV are excluded at 95% CL in scenarios with the contact interaction scale $\Lambda$ set to 10 TeV. At the extreme limit of model validity where $\Lambda$ is set equal to the excited $\tau$-lepton mass, excited $\tau$-leptons with masses below 4.6 TeV are excluded. Leptoquarks with masses below 1.3 TeV are excluded at 95% CL if their branching ratio to a charm quark and a $\tau$-lepton equals 1. The analysis does not exploit flavour-tagging in the signal region.
Observed and expected upper 95% CL limit on the $\tau^\ast$ production cross-section as a function of $m_{\tau^\ast}$ for a fixed value of the contact interaction scale, $\Lambda = 10$ TeV.
Observed and expected lower 95% CL limit on the contact interaction scale $\Lambda$ as a function of $m_{\tau^\ast}$.
Observed and expected upper 95% CL limit on the LQ production cross-section as a function of $m_\mathrm{LQ}$. The LQ couples to a tau lepton and a c-quark. The limits are also valid for scenarios in which the LQ couples to lighter quarks.
We present a search for magnetic monopoles and high-electric-charge objects using LHC Run 2 $\sqrt{s} =$13 TeV proton$-$proton collisions recorded by the ATLAS detector. A total integrated luminosity of 138 fb$^{-1}$ was collected by a specialized trigger. No highly ionizing particle candidate was observed. Considering the Drell-Yan and photon-fusion pair production mechanisms as benchmark models, cross-section upper limits are presented for spin-0 and spin-$\frac{1}{2}$ magnetic monopoles of magnetic charge $1g_\textrm{D}$ and $2g_\textrm{D}$ and for high-electric-charge objects of electric charge $20 \leq |z| \leq 100$, for masses between 200 GeV and 4000 GeV. The search improves by approximately a factor of three the previous cross-section limits on the Drell-Yan production of magnetic monopoles and high-electric charge objects. Also, the first ATLAS limits on the photon-fusion pair production mechanism of magnetic monopoles and high-electric-charge objects have been obtained.
Observed 95% CL upper limits on the cross section for all masses and charges of Drell-Yan spin-0 monopoles production as a function of mass for magnetic charges $|g|=1g_D$ and $|g|=2g_D$.
Observed 95% CL upper limits on the cross section for all masses and charges of Drell-Yan spin-1/2 monopoles production as a function of mass for magnetic charges $|g|=1g_D$ and $|g|=2g_D$.
Observed 95% CL upper limits on the cross section for all masses and charges of photon-fusion pair-produced spin-0 monopoles as a function of mass for magnetic charges $|g|=1g_D$ and $|g|=2g_D$.
This paper reports cross-section measurements of $ZZ$ production in $pp$ collisions at $\sqrt{s}=13.6$ TeV at the Large Hadron Collider. The data were collected by the ATLAS detector in 2022, and correspond to an integrated luminosity of 29 fb$^-1$. Events in the $ZZ\rightarrow4\ell$ ($\ell = e$, $\mu$) final states are selected and used to measure the inclusive and differential cross-sections in a fiducial region defined close to the analysis selections. The inclusive cross-section is further extrapolated to the total phase space with a requirement of 66 $< m_Z <$ 116 GeV for both $Z$ bosons, yielding $16.8 \pm 1.1$ pb. The results are well described by the Standard Model predictions.
The measured differential cross-sections compared to the predictions in the $m_{4\ell}$ bins
The measured differential cross-sections compared to the predictions in the $p_T^{4\ell}$ bins
This Letter presents a differential cross-section measurement of Lund subjet multiplicities, suitable for testing current and future parton shower Monte Carlo algorithms. This measurement is made in dijet events in 140 fb$^{-1}$ of $\sqrt{s}=13$ TeV proton-proton collision data collected with the ATLAS detector at CERN's Large Hadron Collider. The data are unfolded to account for acceptance and detector-related effects, and are then compared with several Monte Carlo models and to recent resummed analytical calculations. The experimental precision achieved in the measurement allows tests of higher-order effects in QCD predictions. Most predictions fail to accurately describe the measured data, particularly at large values of jet transverse momentum accessible at the Large Hadron Collider, indicating the measurement's utility as an input to future parton shower developments and other studies probing fundamental properties of QCD and the production of hadronic final states up to the TeV-scale.
$N_{Lund}, k_t \geq 0.5~\text{GeV}$, All $p_T$ bins, Central $\eta$
$N_{Lund}, k_t \geq 0.5~\text{GeV}$, All $p_T$ bins, Forward $\eta$
$N_{Lund}, k_t \geq 0.5~\text{GeV}$, $300~\text{GeV} \leq p_T < 500~\text{GeV}$, Inclusive $\eta$
A combination of searches for a new resonance decaying into a Higgs boson pair is presented, using up to 139 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}=13$ TeV recorded with the ATLAS detector at the LHC. The combination includes searches performed in three decay channels: $b\bar{b}b\bar{b}$, $b\bar{b}\tau^+\tau^-$ and $b\bar{b}\gamma\gamma$. No excess above the expected Standard Model background is observed and upper limits are set at the 95% confidence level on the production cross section of Higgs boson pairs originating from the decay of a narrow scalar resonance with mass in the range 251 GeV-5 TeV. The observed (expected) limits are in the range 0.96-600 fb (1.2-390 fb). The limits are interpreted in the Type-I Two-Higgs-Doublet Model and the Minimimal Supersymmetric Standard Model, and constrain parameter space not previously excluded by other searches.
Local p-value as a function of the resonance mass $m_{X}$. Some table entries are empty because resonance masses from 251 GeV to 5 TeV are considered, whereas some channels only use masses in a certain, more restricted, range.
Observed significance as a function of the resonance $m_{X}$. Some table entries are empty because resonance masses from 251 GeV to 5 TeV are considered, whereas some channels only use masses in a certain, more restricted, range.
Observed and expected upper limits at the 95% CL on the resonant Higgs boson pair production cross section as a function of the resonance mass $m_{X}$. Some table entries are empty because resonance masses from 251 GeV to 5 TeV are considered, whereas some channels only use masses in a certain, more restricted, range.
This paper presents a search for pair production of higgsinos, the supersymmetric partners of the Higgs bosons, in scenarios with gauge-mediated supersymmetry breaking. Each higgsino is assumed to decay into a Higgs boson and a nearly massless gravitino. The search targets events where each Higgs boson decays into $b\bar{b}$, leading to a reconstructed final state with at least three energetic $b$-jets and This paper presents a search for pair production of higgsinos, the supersymmetric partners of the Higgs bosons, in scenarios with gauge-mediated supersymmetry breaking. Each higgsino is assumed to decay into a Higgs boson and a nearly massless gravitino. The search targets events where each Higgs boson decays into $b\bar{b}$, leading to a reconstructed final state with at least three energetic $b$-jets and missing transverse momentum. Two complementary analysis channels are used, with each channel specifically targeting either low or high values of the higgsino mass. The low-mass (high-mass) channel exploits 126 (139) fb$^{-1}$ of $\sqrt{s}=13$ TeV data collected by the ATLAS detector during Run 2 of the Large Hadron Collider. No significant excess above the Standard Model prediction is found. At 95% confidence level, masses between 130 GeV and 940 GeV are excluded for higgsinos decaying exclusively into Higgs bosons and gravitinos. Exclusion limits as a function of the higgsino decay branching ratio to a Higgs boson are also reported.
Post-fit SR yields of the high-mass channel. The upper panel shows the observed number of events, as well the post-fit background predictions in each region. The bottom panel shows the ratio of the observed data and the total background prediction. The shaded areas correspond to the total statistical and systematic uncertainties obtained after the fit and described in Section 6.
Post-fit SR yields of the high-mass channel. The upper panel shows the observed number of events, as well the post-fit background predictions in each region. The bottom panel shows the ratio of the observed data and the total background prediction. The shaded areas correspond to the total statistical and systematic uncertainties obtained after the fit and described in Section 6.
Post-fit SR yields of the high-mass channel. The upper panel shows the observed number of events, as well the post-fit background predictions in each region. The bottom panel shows the ratio of the observed data and the total background prediction. The shaded areas correspond to the total statistical and systematic uncertainties obtained after the fit and described in Section 6.
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