Boosted Higgs →bbˉ\rightarrow b\bar{b} in vector-boson associated production at 14 TeV

The production of the Standard Model Higgs boson in association with a vector boson, followed by the dominant decay to $H \rightarrow b\bar{b}$, is a strong prospect for confirming and measuring the coupling to $b$-quarks in $pp$ collisions at $\sqrt{s}=14$ TeV. We present an updated study of the prospects for this analysis, focussing on the most sensitive highly Lorentz-boosted region. The evolution of the efficiency and composition of the signal and main background processes as a function of the transverse momentum of the vector boson are studied covering the region $200-1000$ GeV, comparing both a conventional dijet and jet substructure selection. The lower transverse momentum region ($200-400$ GeV) is identified as the most sensitive region for the Standard Model search, with higher transverse momentum regions not improving the statistical sensitivity. For much of the studied region ($200-600$ GeV), a conventional dijet selection performs as well as the substructure approach, while for the highest transverse momentum regions ($> 600$ GeV), which are particularly interesting for Beyond the Standard Model and high luminosity measurements, the jet substructure techniques are essential.Comment: 13 pages.(Fixed figure layout error

Search for Heavy Neutral Leptons in Decays of W Bosons Using a Dilepton Displaced Vertex in √s = 13 TeV pp Collisions with the ATLAS Detector

A search for a long-lived, heavy neutral lepton (N) in 139 fb^{−1} of √s = 13 TeV p p collision data collected by the ATLAS detector at the Large Hadron Collider is reported. The N is produced via W → N_{μ} or W → N e and decays into two charged leptons and a neutrino, forming a displaced vertex. The N mass is used to discriminate between signal and background. No signal is observed, and limits are set on the squared mixing parameters of the N with the left-handed neutrino states for the N mass range 3 GeV < m_{N} < 15 GeV . For the first time, limits are given for both single-flavor and multiflavor mixing scenarios motivated by neutrino flavor oscillation results for both the normal and inverted neutrino-mass hierarchies

Soft interactions in Herwig++

We describe the recent developments to extend the multi-parton interaction model of underlying events in Herwig++ into the soft, non-perturbative, regime. This allows the program to describe also minimum bias collisions in which there is no hard interaction, for the first time. It is publicly available from versions 2.3 onwards and describes the Tevatron underlying event and minimum bias data. The extrapolations to the LHC nevertheless suffer considerable ambiguity, as we discuss.Comment: 10 pages, talk given by Manuel Bahr at First International Workshop on Multiple Partonic Interactions at the LHC, "MPI@LHC'08", Perugia, Italy, October 27-31 200

Prospects for (non-SUSY) new physics with first LHC data

The ATLAS and CMS experiments will take first data soon. I consider here the prospects for new physics (excluding SUSY) with a few inverse fb of data. This means processes with signal cross sections of a few 100 fb or less, with clear and fairly simple signatures - precision comparison of data to Standard Model tails will take longer, needing more luminosity and very good understanding of detector calibrations, resolutions and trigger efficiencies. The approach I take here is signature rather than model based, but examples of models will be given.Comment: 4 pages, 1 figure. Contribution to 13th Annual Symposium of Particles, Strings and Cosmology (PASCOS 07), July 200

Discovering baryon-number violating neutralino decays at the LHC.

Recently there has been much interest in the use of single-jet mass and jet substructure to identify boosted particles decaying hadronically at the LHC. We develop these ideas to address the challenging case of a neutralino decaying to three quarks in models with baryonic violation of R parity. These decays have previously been found to be swamped by QCD backgrounds. We demonstrate for the first time that such a decay might be observed directly at the LHC with high significance, by exploiting characteristics of the scales at which its composite jet breaks up into subjets

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

Measurements of the suppression and correlations of dijets in Pb+Pb collisions at √sNN = 5.02 TeV

Studies of the correlations of the two highest transverse momentum (leading) jets in individual Pb+Pb collision events can provide information about the mechanism of jet quenching by the hot and dense matter created in such collisions. In Pb+Pb and p p collisions at √ s N N = 5.02 TeV , measurements of the leading dijet transverse momentum ( p T ) correlations are presented. Additionally, measurements in Pb+Pb collisions of the dijet pair nuclear modification factors projected along leading and subleading jet p T are made. The measurements are performed using the ATLAS detector at the LHC with 260 pb − 1 of p p data collected in 2017 and 2.2 nb − 1 of Pb+Pb data collected in 2015 and 2018. An unfolding procedure is applied to the two-dimensional leading and subleading jet p T distributions to account for experimental effects in the measurement of both jets. Results are provided for dijets with leading jet p T greater than 100 GeV . Measurements of the dijet-yield-normalized x J distributions in Pb+Pb collisions show an increased fraction of imbalanced jets compared to p p collisions; these measurements are in agreement with previous measurements of the same quantity at 2.76 TeV in the overlapping kinematic range. Measurements of the absolutely normalized dijet rate in Pb+Pb and p p collisions are also presented, and show that balanced dijets are significantly more suppressed than imbalanced dijets in Pb+Pb collisions. It is observed in the measurements of the pair nuclear modification factors that the subleading jets are significantly suppressed relative to leading jets with p T between 100 and 316 GeV for all centralities in Pb+Pb collisions

Search for pair-production of vector-like quarks in pp collision events at √s = 13 TeV with at least one leptonically decaying Z boson and a third-generation quark with the ATLAS detector

A search for the pair-production of vector-like quarks optimized for decays into a Z boson and a third-generation Standard Model quark is presented, using the full Run 2 dataset corresponding to 139 fb−1 of pp collisions at √s = 13 TeV, collected in 2015–2018 with the ATLAS detector at the Large Hadron Collider. The targeted final state is characterized by the presence of a Z boson with high transverse momentum, reconstructed from a pair of same-flavour leptons with opposite-sign charges, as well as by the presence of b-tagged jets and high-transverse-momentum large-radius jets reconstructed from calibrated smaller-radius jets. Events with exactly two or at least three leptons are used, which are further categorized by the presence of boosted W , Z, and Higgs bosons and top quarks. The categorization is performed using a neural-network-based boosted object tagger to enhance the sensitivity to signal relative to the background. No significant excess above the background expectation is observed and exclusion limits at 95% confidence level are set on the masses of the vector-like partners T and B of the top and bottom quarks, respectively. The limits depend on the branching ratio configurations and, in the case of 100% branching ratio for T → Z t and 100% branching ratio for B → Zb, this search sets the most stringent limits to date, allowing mT > 1.60 TeV and mB > 1.42 TeV, respectively

Search for a new pseudoscalar decaying into a pair of muons in events with a top-quark pair at √ s = 13 TeV with the ATLAS detector

A search for a new pseudoscalar a -boson produced in events with a top-quark pair, where the a -boson decays into a pair of muons, is performed using √ s = 13  TeV pp collision data collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 139    fb − 1. The search targets the final state where only one top quark decays to an electron or muon, resulting in a signature with three leptons e μμ and μμμ . No significant excess of events above the Standard Model expectation is observed and upper limits are set on two signal models: p p → tt̄ a and p p → tt̄ with t → H ± b , H ± → W ± a , where a → μμ, in the mass ranges 15 GeV < m a < 72 GeV and 120 GeV ≤ m H ± ≤ 160  GeV

The ATLAS inner detector trigger performance in pp collisions at 13 TeV during LHC Run 2

The design and performance of the inner detector trigger for the high level trigger of the ATLAS experiment at the Large Hadron Collider during the 2016–2018 data taking period is discussed. In 2016, 2017, and 2018 the ATLAS detector recorded 35.6 fb−1, 46.9 fb−1, and 60.6 fb−1respectively of proton–proton collision data at a centre-of-mass energy of 13 TeV. In order to deal with the very high interaction multiplicities per bunch crossing expected with the 13 TeV collisions the inner detector trigger was redesigned during the long shutdown of the Large Hadron Collider from 2013 until 2015. An overview of these developments is provided and the performance of the tracking in the trigger for the muon, electron, tau and b-jet signatures is discussed. The high performance of the inner detector trigger with these extreme interaction multiplicities demonstrates how the inner detector tracking continues to lie at the heart of the trigger performance and is essential in enabling the ATLAS physics programme