A High-Granularity Timing Detector for the ATLAS Phase-II upgrade

The increase of the particle flux (pile-up) at the HL-LHC with instantaneous luminosities up to L ~ 7.5 × $10^{34} cm^{−2}s^{−1}$ will have a severe impact on the ATLAS detector reconstruction and trigger performance. The end-cap and forward region where the liquid Argon calorimeter has coarser granularity and the inner tracker has poorer momentum resolution will be particularly affected. A High Granularity Timing Detector (HGTD) will be installed in front of the LAr end-cap calorimeters for pile-up mitigation and luminosity measurement. The HGTD is a novel detector introduced to augment the new all-silicon Inner Tracker in the pseudo-rapidity range from 2.4 to 4.0, adding the capability to measure charged-particle trajectories in time as well as space. Two silicon-sensor double-sided layers will provide precision timing information for minimum-ionising particles with a resolution as good as 30 ps per track in order to assign each particle to the correct vertex. Readout cells have a size of 1.3 mm × 1.3 mm, leading to a highly granular detector with 3.7 million channels. Low Gain Avalanche Detectors (LGAD) technology has been chosen as it provides enough gain to reach the large signal over noise ratio needed. The requirements and overall specifications of the HGTD will be presented as well as the technical design and the project status. The on-going R&D effort carried out to study the sensors, the readout ASIC, and the other components, supported by laboratory and test beam results, will also be presented

A High-Granularity Timing Detector for the ATLAS Phase-II upgrade

The increase of the particle flux (pile-up) at the HL-LHC with instantaneous luminosities up to $7.5 \times 10^{34}$$cm{^−2}$$s^{−1}$ will have a severe impact on the ATLAS detector reconstruction and trigger performance. The end-cap and forward region where the liquid Argon calorimeter has coarser granularity and the inner tracker has poorer momentum resolution will be particularly affected. A High Granularity Timing Detector (HGTD) will be installed in front of the LAr end-cap calorimeters for pile-up mitigation and luminosity measurement. The HGTD is a novel detector introduced to augment the new all-silicon Inner Tracker in the pseudo-rapidity range from 2.4 to 4.0, adding the capability to measure charged-particle trajectories in time as well as space. Two silicon-sensor double-sided layers will provide precision timing information for minimum-ionising particles with a resolution as good as 30 ps per track in order to assign each particle to the correct vertex. Readout cells have a size of 1.3 mm × 1.3 mm, leading to a highly granular detector with 3.7 million channels. Low Gain Avalanche Detectors (LGAD) technology has been chosen as it provides enough gain to reach the large signal over noise ratio needed. The requirements and overall specifications of the HGTD will be presented as well as the technical design and the project status. The on-going R&D effort carried out to study the sensors, the readout ASIC, and the other components, supported by laboratory and test beam results, will also be presented.The High Luminosity Large Hadron Collider is scheduled to begin in 2027 where faster and more radiation hard detectors are needed to cope with higher track multiplicity and higher radiation levels. With a luminosity of 7.5×1034cm−2 s −1 the average pile-up will be about 200 in the High Luminosity Large Hadron Collider, resulting in 1.8 vertices/mm on average. A powerful new way to address this challenge is to exploit the time spread of the interactions to distinguish between collisions occurring very close in space but well separated in time. In this context, timing information can be used to resolve the ambiguities. In a high pile-up event, multiple vertices will occur in the same but, by adding timing information, vertices could be distinguished. A HighGranularity Timing Detector, based on low-gain avalanche detector technology, is proposed for the ATLAS Phase-II upgrade to mitigate pile-up effects and improve the overall ATLAS performance in the forward region by combining the High-Granularity Timing Detector high-precision time measurement with the Inner Tracker position information. The High-Granularity Timing Detector layout, including sensors and readout electronics are presented. The test beam results are discussed as well

Jet Energy Scale and Resolution in the High-Granularity Timing Detector in ATLAS Upgrades at HL-LHC

The large increase of pileup is one of the main experimental challenges for the High Luminosity-Large Hadron Collider (HL-LHC) physics program. HL-LHC is expected to start in 2027 and to provide an in- tegrated luminosity of 3000fb1 in ten years, a factor 10 more than what will be collected by 2023. A pow- erful new way to address this challenge is to exploit the time spread of the interactions to distinguish between collisions occurring very close in space but well separated in time. A High-Granularity Timing Detector (HGTD), based on low gain avalanche detector technology, is proposed for the ATLAS Phase-II upgrade. Covering the pseudo rapidity region between 2.4 and 4.0, with a timing resolution of 30 ps for minimum-ionizing particles, this device will significantly improve the performance in the forward region. The improvement of the jet energy scale and resolution in the forward region by reducing the pileup track contamination in hard scatter jets from nearby pileup interactions is presented. The impact of HGTD in reducing pileup track contamination in the jets reconstruction in the forward region is investigated. The performance is evaluated in terms of jet energy response and resolution as a function of pseudo rapidity η, transverse momentum pT

Search for events with a pair of displaced vertices from long-lived neutral particles decaying into hadronic jets in the ATLAS muon spectrometer in pp collisions at √s=13 TeV

A search for events with two displaced vertices from long-lived particle (LLP) pairs using data collected by the ATLAS detector at the LHC is presented. This analysis uses 139  fb−1 of proton-proton collision data at s=13  TeV recorded in 2015–2018. The search employs techniques for reconstructing vertices of LLPs decaying to jets in the muon spectrometer displaced between 3 and 14 m with respect to the primary interaction vertex. The observed numbers of events are consistent with the expected background and limits for several benchmark signals are determined. For the Higgs boson with a mass of 125 GeV, the paper reports the first exclusion limits for branching fractions into neutral long-lived particles below 0.1%, while branching fractions above 10% are excluded at 95% confidence level for LLP proper lifetimes ranging from 4 cm to 72.4 m. In addition, the paper present the first results for the decay of LLPs into tt¯ in the ATLAS muon spectrometer.publishedVersio

Search for Higgs boson pair production in the two bottom quarks plus two photons final state in pp collisions at √s=13 TeV with the ATLAS detector

Searches are performed for nonresonant and resonant di-Higgs boson production in the b¯bγγ final state. The dataset used corresponds to an integrated luminosity of 139  fb−1 of proton–proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. No excess above the expected background is found and upper limits on the di-Higgs boson production cross sections are set. A 95% confidence-level upper limit of 4.2 times the cross section predicted by the Standard Model is set on pp→HH nonresonant production, where the expected limit is 5.7 times the Standard Model predicted value. The expected constraints are obtained for a background hypothesis excluding pp→HH production. The observed (expected) constraints on the Higgs boson trilinear coupling modifier κλ are determined to be [−1.5,6.7] ([−2.4,7.7]) at 95% confidence level, where the expected constraints on κλ are obtained excluding pp→HH production from the background hypothesis. For resonant production of a new hypothetical scalar particle X (X→HH→b¯bγγ), limits on the cross section for pp→X→HH are presented in the narrow-width approximation as a function of mX in the range 251  GeV≤mX≤1000  GeV. The observed (expected) limits on the cross section for pp→X→HH range from 640 fb to 44 fb (391 fb to 46 fb) over the considered mass range.publishedVersio

Study of B+c→J/ψD+s and B+c→J/ψD∗+s decays in pp collisions at √s = 13 TeV with the ATLAS detector

A study of B+c→J/ψD+sBc+→J/ψDs+ and B+c→J/ψD∗+sBc+→J/ψDs∗+ decays using 139 fb−1 of integrated luminosity collected with the ATLAS detector from s√s = 13 TeV pp collisions at the LHC is presented. The ratios of the branching fractions of the two decays to the branching fraction of the B+cBc+ → J/ψπ+ decay are measured: B(B+c→J/ψD+s)/B(B+c→J/ψπ+)B(Bc+→J/ψDs+)/B(Bc+→J/ψπ+) = 2.76 ± 0.47 and B(B+c→J/ψD∗+s)/B(B+c→J/ψπ+)B(Bc+→J/ψDs∗+)/B(Bc+→J/ψπ+) = 5.33 ± 0.96. The ratio of the branching fractions of the two decays is found to be B(B+c→J/ψD∗+s)/B(B+c→J/ψD∗+s)B(Bc+→J/ψDs∗+)/B(Bc+→J/ψDs∗+) = 1.93 ± 0.26. For the B+c→J/ψD∗+sBc+→J/ψDs∗+ decay, the transverse polarization fraction, Γ±±/Γ, is measured to be 0.70 ± 0.11. The reported uncertainties include both the statistical and systematic components added in quadrature. The precision of the measurements exceeds that in all previous studies of these decays. These results supersede those obtained in the earlier ATLAS study of the same decays with s√s = 7 and 8 TeV pp collision data. A comparison with available theoretical predictions for the measured quantities is presented.publishedVersio

Search for invisible Higgs-boson decays in events with vector-boson fusion signatures using 139 fb−1 of proton-proton data recorded by the ATLAS experiment

A direct search for Higgs bosons produced via vector-boson fusion and subsequently decaying into invisible particles is reported. The analysis uses 139 fb−1 of pp collision data at a centre-of-mass energy of s√ = 13 TeV recorded by the ATLAS detector at the LHC. The observed numbers of events are found to be in agreement with the background expectation from Standard Model processes. For a scalar Higgs boson with a mass of 125 GeV and a Standard Model production cross section, an observed upper limit of 0.145 is placed on the branching fraction of its decay into invisible particles at 95% confidence level, with an expected limit of 0.103. These results are interpreted in the context of models where the Higgs boson acts as a portal to dark matter, and limits are set on the scattering cross section of weakly interacting massive particles and nucleons. Invisible decays of additional scalar bosons with masses from 50 GeV to 2 TeV are also studied, and the derived upper limits on the cross section times branching fraction decrease with increasing mass from 1.0 pb for a scalar boson mass of 50 GeV to 0.1 pb at a mass of 2 TeV.publishedVersio

Measurement of Higgs boson decay into b-quarks in associated production with a top-quark pair in pp collisions at √s = 13 TeV with the ATLAS detector

The associated production of a Higgs boson and a top-quark pair is measured in events characterised by the presence of one or two electrons or muons. The Higgs boson decay into a b-quark pair is used. The analysed data, corresponding to an integrated luminosity of 139 fb−1, were collected in proton-proton collisions at the Large Hadron Collider between 2015 and 2018 at a centre-of-mass energy of s√s = 13 TeV. The measured signal strength, defined as the ratio of the measured signal yield to that predicted by the Standard Model, is 0.35+0.36−0.340.35−0.34+0.36. This result is compatible with the Standard Model prediction and corresponds to an observed (expected) significance of 1.0 (2.7) standard deviations. The signal strength is also measured differentially in bins of the Higgs boson transverse momentum in the simplified template cross-section framework, including a bin for specially selected boosted Higgs bosons with transverse momentum above 300 GeV.publishedVersio

Search for flavour-changing neutral-current interactions of a top quark and a gluon in pp collisions at √s=13 TeV with the ATLAS detector

A search is presented for the production of a single top quark via left-handed flavour-changing neutral-current (FCNC) interactions of a top quark, a gluon and an up or charm quark. Two production processes are considered: u+g→t and c+g→t. The analysis is based on proton–proton collision data taken at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. The data set corresponds to an integrated luminosity of 139 fb−1. Events with exactly one electron or muon, exactly one b-tagged jet and missing transverse momentum are selected, resembling the decay products of a singly produced top quark. Neural networks based on kinematic variables differentiate between events from the two signal processes and events from background processes. The measured data are consistent with the background-only hypothesis, and limits are set on the production cross-sections of the signal processes: σ(u+g→t)×B(t→Wb)×B(W→ℓν)<3.0pb and σ(c+g→t)×B(t→Wb)×B(W→ℓν)<4.7pb at the 95% confidence level, with B(W→ℓν)=0.325 being the sum of branching ratios of all three leptonic decay modes of the W boson. Based on the framework of an effective field theory, the cross-section limits are translated into limits on the strengths of the tug and tcg couplings occurring in the theory: |CutuG|/Λ2<0.057TeV−2 and |CctuG|/Λ2<0.14TeV−2. These bounds correspond to limits on the branching ratios of FCNC-induced top-quark decays: B(t→u+g)<0.61×10−4 and B(t→c+g)<3.7×10−4

Measurements of the Higgs boson inclusive and differential fiducial cross-sections in the diphoton decay channel with pp collisions at √s = 13 TeV with the ATLAS detector

A measurement of inclusive and differential fiducial cross-sections for the production of the Higgs boson decaying into two photons is performed using 139 fb−1 of proton-proton collision data recorded at s√ = 13 TeV by the ATLAS experiment at the Large Hadron Collider. The inclusive cross-section times branching ratio, in a fiducial region closely matching the experimental selection, is measured to be 67 ± 6 fb, which is in agreement with the state-of-the-art Standard Model prediction of 64 ± 4 fb. Extrapolating this result to the full phase space and correcting for the branching ratio, the total cross-section for Higgs boson production is estimated to be 58 ± 6 pb. In addition, the cross-sections in four fiducial regions sensitive to various Higgs boson production modes and differential cross-sections as a function of either one or two of several observables are measured. All the measurements are found to be in agreement with the Standard Model predictions. The measured transverse momentum distribution of the Higgs boson is used as an indirect probe of the Yukawa coupling of the Higgs boson to the bottom and charm quarks. In addition, five differential cross-section measurements are used to constrain anomalous Higgs boson couplings to vector bosons in the Standard Model effective field theory framework.publishedVersio