A search for a new heavy boson produced via gluon-fusion in the four-lepton channel with missing transverse momentum or jets is performed. The search uses proton-proton collision data equivalent to an integrated luminosity of 139 fb$^{-1}$ at a centre-of-mass energy of 13 TeV collected by the ATLAS detector between 2015 and 2018 at the Large Hadron Collider. This study explores the decays of heavy bosons: $R\rightarrow SH$ and $A\rightarrow ZH$, where $R$ is a CP-even boson, $A$ is a CP-odd boson, $H$ is a CP-even boson, and $S$ is considered to decay into invisible particles that are candidates for dark matter. In these processes, $S\rightarrow \textrm{invisible}$ and $H\rightarrow ZZ$. The $Z$ boson associated with the heavy scalar boson $H$ decays into all decay channels of the $Z$ boson. The mass range under consideration is 390-1300 (320-1300) GeV for the $R$ ($A$) boson and 220-1000 GeV for the $H$ boson. No significant deviation from the Standard Model backgrounds is observed. The results are interpreted as upper limits at a 95% confidence level on the cross-section times the branching ratio of the heavy resonances.
Observed and expected distributions of the invariant mass of the four-lepton system in the $R\to SH\to 4\ell+E^{\textrm{miss}}_{\textrm{T}}$ search for SR1 under a background-only fit to data. The total background includes the $q\overline{q}\to ZZ$, $gg\to ZZ$, $q\overline{q}\to ZZ$ (EW), $VVV$, $t\overline{t}V$, $t\overline{t}$, $Z$+jets and $WZ$ processes. The distribution of the $(m_{R}, m_{H}) = (500, 300)$ GeV signal is normalised to the observed upper limit on the cross-section (25.0 fb).
Observed and expected distributions of the invariant mass of the four-lepton system in the $R\to SH\to 4\ell+E^{\textrm{miss}}_{\textrm{T}}$ search for SR2 under a background-only fit to data. The total background includes the $q\overline{q}\to ZZ$, $gg\to ZZ$, $q\overline{q}\to ZZ$ (EW), $VVV$, $t\overline{t}V$, $t\overline{t}$, $Z$+jets and $WZ$ processes. The distribution of the $(m_{R}, m_{H}) = (500, 300)$ GeV signal is normalised to the observed upper limit on the cross-section (25.0 fb).
Observed and expected distributions of the invariant mass of the four-lepton system in the $R\to SH\to 4\ell+E^{\textrm{miss}}_{\textrm{T}}$ search for SR3 under a background-only fit to data. The total background includes the $q\overline{q}\to ZZ$, $gg\to ZZ$, $q\overline{q}\to ZZ$ (EW), $VVV$, $t\overline{t}V$, $t\overline{t}$, $Z$+jets and $WZ$ processes. The distribution of the $(m_{R}, m_{H}) = (500, 300)$ GeV signal is normalised to the observed upper limit on the cross-section (25.0 fb).
A search for heavy right-handed Majorana or Dirac neutrinos $N_{\mathrm{R}}$ and heavy right-handed gauge bosons $W_{\mathrm{R}}$ is performed in events with energetic electrons or muons, with the same or opposite electric charge, and energetic jets. The search is carried out separately for topologies of clearly separated final-state products (``resolved'' channel) and topologies with boosted final states with hadronic and/or leptonic products partially overlapping and reconstructed as a large-radius jet (``boosted'' channel). The events are selected from $pp$ collision data at the LHC with an integrated luminosity of 139 fb$^{-1}$ collected by the ATLAS detector at $\sqrt{s}$ = 13 TeV. No significant deviations from the Standard Model predictions are observed. The results are interpreted within the theoretical framework of a left-right symmetric model, and lower limits are set on masses in the heavy right-handed $W_{\mathrm{R}}$ boson and $N_{\mathrm{R}}$ plane. The excluded region extends to about $m(W_{\mathrm{R}}) = 6.4$ TeV for both Majorana and Dirac $N_{\mathrm{R}}$ neutrinos at $m(N_{\mathrm{R}})<1$ TeV. $N_{\mathrm{R}}$ with masses of less than 3.5 (3.6) TeV are excluded in the electron (muon) channel at $m(W_{\mathrm{R}})=4.8$ TeV for the Majorana neutrinos, and limits of $m(N_{\mathrm{R}})$ up to 3.6 TeV for $m(W_{\mathrm{R}}) = 5.2$ (5.0) TeV in the electron (muon) channel are set for the Dirac neutrinos. These constitute the most stringent exclusion limits to date for the model considered.
Observed 95% CL exclusion contours in the $(m(W_{R}), m(N_{R}))$ plane in the electron channel for boosted.
Expected 95% CL exclusion contours in the $(m(W_{R}), m(N_{R}))$ plane in the electron channel for boosted.
Observed 95% CL exclusion contours in the $(m(W_{R}), m(N_{R}))$ plane in the muon channel for boosted.
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
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 $|g|=2g_D$.
Searches for new resonances are performed using an unsupervised anomaly-detection technique. Events with at least one electron or muon are selected from 140 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV recorded by ATLAS at the Large Hadron Collider. The approach involves training an autoencoder on data, and subsequently defining anomalous regions based on the reconstruction loss of the decoder. Studies focus on nine invariant mass spectra that contain pairs of objects consisting of one light jet or $b$-jet and either one lepton ($e$, $\mu$), photon, or second light jet or $b$-jet in the anomalous regions. No significant deviations from the background hypotheses are observed.
Distributions of the anomaly score from the AE for data and five benchmark BSM models. Their legends, from top to bottom, are; (1) charged Higgs boson production in association with a top quark, $tbH^{+}$ with $H^{+} \rightarrow t\bar{b}$; (2) a Kaluza-Klein gauge boson, $W_{KK}$, with the SM $W$ boson and a radion $\phi$; (3) a $Z'$ boson decaying to a composite lepton $E$ and $\ell$, with $E \rightarrow Z\ell$ with a mass of 0.5 TeV; (4) the SSM $W$'$\rightarrow W Z' \rightarrow \ell\nu q\bar{q}$; (5) a simplified dark-matter model with an $Z$ axial-vector mediator $Z' \rightarrow q\bar{q}$, where one of the quarks radiates a $W$ boson decaying to $\ell\nu$. The BSM predictions represent the expected number of events from 140 $fb^{-1}$ of data for heavy particle ($H^{+}$ ,$W_{KK}$ , $Z'$ , $W'$ and $Z'$, respectively) masses around 2 TeV. The distributions for the BSM models are smoothed to remove fluctuations due to low MC event counts. The vertical lines indicate the start of the three anomaly regions (ARs). The labels of the three ARs indicate the visible cross section for hypothetical processes yielding the same number of events as observed in the 140 $fb^{-1}$ dataset. The AE is applied to preselected events without any requirements on invariant mass distributions.
Invariant mass distributions of jet+Y for $M_{jY}$ > 0.3 TeV in the 10 pb AR along with the fit of Eq. (1). The fits are represented by the lines, while the associated statistical uncertainties are indicated by the shaded bands. The lower panels show the bin-by-bin significances of deviations from the fit, calculated as $(d_{\textit{i}} - f_{i})/\delta_{\textit{i}}$, where $d_{i}$ is the data yield, $f_{\textit{i}}$ is the fit value, and $\delta_{i}$ is the data uncertainty in the $\textit{i}$-th bin.
Values of $\Delta Z$ for the discovery sensitivity, as defined in the text, as a function of the invariant mass $\textit{m}$. The j+j invariant mass distribution is calculated in the 10 pb AR. Positive percentages indicate improvements in sensitivity. Horizontal dashed lines are drawn at 100% and 200% to guide the eye. The five benchmark BSM models are (1) charged Higgs boson production in association with a top quark, $tbH^{+}$ with $H^{+} \rightarrow t\bar{b}$; (2) a Kaluza-Klein gauge boson, $W_{KK}$, with the SM $W$ boson and a radion $\phi$; (3) a $Z'$ boson decaying to a composite lepton $E$ and $\ell$, with $E \rightarrow Z\ell$; (4) the sequential standard model $W' \rightarrow W Z' \rightarrow \ell\nu q\bar{q}$; (5) a simplified dark-matter model with an axial-vector mediator $Z' \rightarrow q\bar{q}$, where one of the quarks radiates a $W$ boson decaying to $\ell\nu$. The multiple markers shown for the composite-lepton model at the same invariant mass values correspond to different composite lepton ($E$) masses between 0.25 and 3.5 TeV. The center positions of the markers are set to the masses of the corresponding heavy particles.
Semi-visible jets, with a significant contribution to the event's missing transverse momentum, can arise in strongly interacting dark sectors. This results in an event topology where one of the jets can be aligned with the direction of the missing transverse momentum. The first search for semi-visible jets produced via a $t$-channel mediator exchange is presented. The analysis uses proton-proton collisions with an integrated luminosity of 139 fb$^{-1}$ and a centre-of-mass energy of 13 TeV, collected with the ATLAS detector during the Run 2 of the LHC. No excess over Standard Model predictions is observed. Assuming a coupling strength of unity between the mediator, a Standard Model quark and a dark quark, mediator masses up to 2.7 TeV are excluded at the 95% confidence level. Upper limits on the coupling strength are also derived.
The post-fit yields in the nine bins of the $(p_\textrm{T}^{\textrm{bal}}, |\phi_{\textrm{max}} - \phi_{\textrm{min}}|)$ grid. Error band includes all the systematic uncertainties.
The post-fit distributions of HT for the SR. Data are compared with background predictions, and six signal predictions covering a representative mediator mass and invisible fraction range are overlaid. The uncertainties include all systematic and statistical components. The last bin contains the overflow.
The post-fit distributions of $E_{\text{T}}^{\text{miss}}$ for the SR. Data are compared with background predictions, and six signal predictions covering a representative mediator mass and invisible fraction range are overlaid. The uncertainties include all systematic and statistical components. The last bin contains the overflow.
A novel search for exotic decays of the Higgs boson into pairs of long-lived neutral particles, each decaying into a bottom quark pair, is performed using 139 fb$^{-1}$ of $\sqrt{s} = 13$ TeV proton-proton collision data collected with the ATLAS detector at the LHC. Events consistent with the production of a Higgs boson in association with a leptonically decaying $Z$ boson are analysed. Long-lived particle (LLP) decays are reconstructed from inner-detector tracks as displaced vertices with high mass and track multiplicity relative to Standard Model processes. The analysis selection requires the presence of at least two displaced vertices, effectively suppressing Standard Model backgrounds. The residual background contribution is estimated using a data-driven technique. No excess over Standard Model predictions is observed, and upper limits are set on the branching ratio of the Higgs boson to LLPs. Branching ratios above 10% are excluded at 95% confidence level for LLP mean proper lifetimes $c\tau$ as small as 4 mm and as large as 100 mm. For LLP masses below 40 GeV, these results represent the most stringent constraint in this lifetime regime.
95% CL exclusion limits on $\mathcal{B}(H\rightarrow aa \rightarrow b\bar{b}b\bar{b})$ for $m_a = 16$ GeV.
95% CL exclusion limits on $\mathcal{B}(H\rightarrow aa \rightarrow b\bar{b}b\bar{b})$ for $m_a = 25$ GeV.
95% CL exclusion limits on $\mathcal{B}(H\rightarrow aa \rightarrow b\bar{b}b\bar{b})$ for $m_a = 35$ GeV.
Measurements of the production cross-sections of the Standard Model (SM) Higgs boson ($H$) decaying into a pair of $\tau$-leptons are presented. The measurements use data collected with the ATLAS detector from $pp$ collisions produced at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=13\,\text{TeV}$, corresponding to an integrated luminosity of $139\,\text{fb}^{-1}$. Leptonic ($\tau\to\ell\nu_{\ell}\nu_{\tau}$) and hadronic ($\tau\to\text{hadrons}~\nu_{\tau}$) decays of the $\tau$-lepton are considered. All measurements account for the branching ratio of $H\to\tau\tau$ and are performed with a requirement $|y_H|<2.5$, where $y_H$ is the true Higgs boson rapidity. The cross-section of the $pp\to H\to\tau\tau$ process is measured to be $2.94 \pm 0.21 \text{(stat)} ^{+\,0.37}_{-\,0.32} \text{(syst)}$ pb, in agreement with the SM prediction of $3.17\pm0.09~ \mbox{pb}$. Inclusive cross-sections are determined separately for the four dominant production modes: $2.65 \pm 0.41 \text{(stat)} ^{+\,0.91}_{-\,0.67} \text{(syst)}$ pb for gluon$-$gluon fusion, $0.197 \pm 0.028 \text{(stat)} ^{+\,0.032}_{-\,0.026} \text{(syst)}$ pb for vector-boson fusion, $0.115 \pm 0.058 \text{(stat)} ^{+\,0.042}_{-\,0.040} \text{(syst)}$ pb for vector-boson associated production, and $0.033 \pm 0.031 \text{(stat)} ^{+\,0.022}_{-\,0.017} \text{(syst)}$ pb for top-quark pair associated production. Measurements in exclusive regions of the phase space, using the simplified template cross-section framework, are also performed. All results are in agreement with the SM predictions.
Observed yields in the boost_0_1J signal region category of the hh channel.
Observed yields in the boost_0_1J signal region category of the hh channel.
Observed yields in the boost_0_ge2J signal region category of the hh channel.
A search for forward proton scattering in association with light-by-light scattering mediated by an axion-like particle is presented, using the ATLAS Forward Proton spectrometer to detect scattered protons and the central ATLAS detector to detect pairs of outgoing photons. Proton-proton collision data recorded in 2017 at a centre-of-mass energy of $\sqrt{s} = 13$ TeV were analysed, corresponding to an integrated luminosity of 14.6 fb$^{-1}$. A total of 441 candidate signal events were selected. A search was made for a narrow resonance in the diphoton mass distribution, corresponding to an axion-like particle (ALP) with mass in the range 150-1600 GeV. No excess is observed above a smooth background. Upper limits on the production cross section of a narrow resonance are set as a function of the mass, and are interpreted as upper limits on the ALP production coupling constant, assuming 100% decay branching ratio into a photon pair. The inferred upper limit on the coupling constant is in the range 0.04-0.09 TeV$^{-1}$ at 95%confidence level.
Signal selection efficiency as a function of ALP mass $m_{\textrm{X}}$ for the exclusive (EL), single-dissociative (SD), and double-dissociative (DD) processes. The ratio of the number of selected events to the number of generated MC events is given (black points) and is parameterised by an analytic function (red solid line). The linear (black dashed line) and cubic (blue chain line) interpolations of the black points are used to derive the envelopes (cyan filled region) which are regarded as systematic uncertainties.
The diphoton mass distribution of the mixed-data sample (black points).
The $(\xi_{\gamma\gamma}^{+},\xi_{\gamma\gamma}^{-})$ distribution of the selected data candidates after the full event selection in $m_{\gamma\gamma}$ in [150,1600] GeV with $m_{\gamma\gamma}$ contours (blue) and $y_{\gamma\gamma}$ contours (black). The range of $\xi_{\gamma\gamma}$ in which forward-proton matching is possible, $[0.035-\xi_{\textrm{th}}, 0.08+\xi_{\textrm{th}} ]$, for events that pass the matching requirement to the A or C side as indicated. No event passed the matching requirement for both the A-side and C-side.
A search for light long-lived neutral particles with masses in the $O$(MeV-GeV) range is presented. The analysis targets the production of long-lived dark photons in the decay of a Higgs boson produced via gluon-gluon fusion or in association with a $W$ boson. Events that contain displaced collimated Standard Model fermions reconstructed in the calorimeter or muon spectrometer are selected in 139 fb$^{-1}$ of $\sqrt{s} = 13$ TeV $pp$ collision data collected by the ATLAS detector at the LHC. Background estimates for contributions from Standard Model processes and instrumental effects are extracted from data. The observed event yields are consistent with the expected background. Exclusion limits are reported on the production cross-section times branching fraction as a function of the mean proper decay length $c\tau$ of the dark photon, or as a function of the dark-photon mass and kinetic mixing parameter that quantifies the coupling between the Standard Model and potential hidden (dark) sectors. A Higgs boson branching fraction above 1% is excluded at 95% CL for a Higgs boson decaying into two dark photons for dark-photon mean proper decay lengths between 10 mm and 250 mm and dark photons with masses between 0.4 GeV and 2 GeV.
The reconstruction efficiency for μDPJ objects satisfying the cosmic-ray tagger selection produced in the decay of a γ<sub>d</sub> into a muon pair. The reconstruction efficiency is shown for γ<sub>d</sub> with 0<|η|<1 as a function of the transverse decay length L<sub>xy</sub>.
The reconstruction efficiency for μDPJ objects satisfying the cosmic-ray tagger selection produced in the decay of a γ<sub>d</sub> into a muon pair. The reconstruction efficiency is shown for γ<sub>d</sub> with 0<|η|<1 as a function of the γ<sub>d</sub> transverse momentum in events where the γ<sub>d</sub> L<sub>xy</sub> is below 6 m.
The reconstruction efficiency for caloDPJs produced by the decay of γ<sub>d</sub> into e<sup>+</sup>e<sup>-</sup> or qq̄. The reconstruction efficiency is shown for γ<sub>d</sub> with 0<|η|<1.1 as a function of the transverse decay length L<sub>xy</sub>. The efficiency drop at 2.5 m corresponds to the end of the first layer of the HCAL.
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