Full LHC Run Research Articles

Combination of searches for pair-produced leptoquarks at [formula omitted] TeV with the ATLAS detector

A statistical combination of various searches for pair-produced leptoquarks is presented, using the full LHC Run 2 (2015–2018) data set of 139 fb−1 collected with the ATLAS detector from proton–proton collisions at a centre-of-mass energy of s=13 TeV. All possible decays of the leptoquarks into quarks of the third generation and charged or neutral leptons of any generation are investigated. Since no significant deviations from the Standard Model expectation are observed in any of the individual analyses, combined exclusion limits are set on the production cross-sections for scalar and vector leptoquarks. The resulting lower bounds on leptoquark masses exceed those from the individual analyses by up to 100GeV, depending on the signal hypothesis.

Search for a CP-odd Higgs boson decaying into a heavy CP-even Higgs boson and a Z boson in the ℓ+ℓ−tt¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\ell}^{+}{\\ell}^{-}t\\overline{t} $$\\end{document} and νν¯bb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \ u \\overline{\ u}b\\overline{b} $$\\end{document} final states using 140 fb−1 of data collected with the ATLAS detector

A search for a heavy CP-odd Higgs boson, A, decaying into a Z boson and a heavy CP-even Higgs boson, H, is presented. It uses the full LHC Run 2 dataset of pp collisions at s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{s} $$\\end{document} = 13 TeV collected with the ATLAS detector, corresponding to an integrated luminosity of 140 fb−1. The search for A → ZH is performed in the ℓ+ℓ−tt¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\ell}^{+}{\\ell}^{-}t\\overline{t} $$\\end{document} and νν¯bb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \ u \\overline{\ u}b\\overline{b} $$\\end{document} final states and surpasses the reach of previous searches in different final states in the region with mH > 350 GeV and mA > 800 GeV. No significant deviation from the Standard Model expectation is found. Upper limits are placed on the production cross-section times the decay branching ratios. Limits with less model dependence are also presented as functions of the reconstructed m(tt¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ t\\overline{t} $$\\end{document}) and m(bb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ b\\overline{b} $$\\end{document}) distributions in the ℓ+ℓ−tt¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\ell}^{+}{\\ell}^{-}t\\overline{t} $$\\end{document} and νν¯bb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \ u \\overline{\ u}b\\overline{b} $$\\end{document} channels, respectively. In addition, the results are interpreted in the context of two-Higgs-doublet models.

Search for the decay of the Higgs boson to a Z boson and a light pseudoscalar particle decaying to two photons

A search for the decay of the Higgs boson to a Z boson and a light, pseudoscalar particle, a, decaying respectively to two leptons and to two photons is reported. The search uses the full LHC Run 2 proton–proton collision data at s=13 TeV, corresponding to 139 fb−1 collected by the ATLAS detector. This is one of the first searches for this specific decay mode of the Higgs boson, and it probes unexplored parameter space in models with axion-like particles (ALPs) and extended scalar sectors. The mass of the a particle is assumed to be in the range 0.1–33 GeV. The data are analysed in two categories: a merged category where the photons from the a decay are reconstructed in the ATLAS calorimeter as a single cluster, and a resolved category in which two separate photons are detected. The main background processes are from Standard Model Z boson production in association with photons or jets. The data are in agreement with the background predictions, and upper limits on the branching ratio of the Higgs boson decay to Za times the branching ratio a→γγ are derived at the 95% confidence level and they range from 0.08% to 2% depending on the mass of the a particle. The results are also interpreted in the context of ALP models.

LHC tau-pair production constraints on $a_\tau$ and $d_\tau$

We point out that relevant constraints on the anomalous magnetic (a_\tauaτ) and electric (d_\taudτ) moment of the tau lepton can be derived from tau-pair production measurements performed at the LHC. Our conclusion is based on the observation that the leading relative deviations from the Standard Model prediction for pp \to \tau^+ \tau^-pp→τ+τ− due to a_\tauaτ and d_\taudτ are enhanced at high energies. Less precise measurements at hadron colliders can therefore offer the same or better sensitivity to new physics with respect to high-precision low-energy measurements performed at lepton machines. We derive bounds on a_\tauaτ and d_\taudτ using the full LHC Run II data set on tau-pair production and compare our findings with the current best limits on the tau anomalous moments.

Search for heavy long-lived multi-charged particles in the full LHC Run 2 pp collision data at [formula omitted] TeV using the ATLAS detector

A search for heavy long-lived multi-charged particles is performed using the ATLAS detector at the LHC. Data collected in 2015–2018 at s=13 TeV from pp collisions corresponding to an integrated luminosity of 139 fb−1 are examined. Particles producing anomalously high ionization, consistent with long-lived spin-½ massive particles with electric charges from |q|=2e to |q|=7e are searched for. No statistically significant evidence of such particles is observed, and 95% confidence level cross-section upper limits are calculated and interpreted as the lower mass limits for a Drell–Yan plus photon-fusion production mode. The least stringent limit, 1060 GeV, is obtained for |q|=2e particles, and the most stringent one, 1600 GeV, is for |q|=6e particles.

Inclusive and differential cross-sections for dilepton toverline{t} production measured in sqrt{s} = 13 TeV pp collisions with the ATLAS detector

Differential and double-differential distributions of kinematic variables of leptons from decays of top-quark pairs ( toverline{t} ) are measured using the full LHC Run 2 data sample collected with the ATLAS detector. The data were collected at a pp collision energy of sqrt{s} = 13 TeV and correspond to an integrated luminosity of 140 fb−1. The measurements use events containing an oppositely charged eμ pair and b-tagged jets. The results are compared with predictions from several Monte Carlo generators. While no prediction is found to be consistent with all distributions, a better agreement with measurements of the lepton pT distributions is obtained by reweighting the toverline{t} sample so as to reproduce the top-quark pT distribution from an NNLO calculation. The inclusive top-quark pair production cross-section is measured as well, both in a fiducial region and in the full phase-space. The total inclusive cross-section is found to beσtt¯=829±1stat±13syst±8lumi±2beampb,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\sigma}_{t\\overline{t}}=829\\pm 1\\ \\left(\ extrm{stat}\\right)\\pm 13\\ \\left(\ extrm{syst}\\right)\\pm 8\\ \\left(\ extrm{lumi}\\right)\\pm 2\\ \\left(\ extrm{beam}\\right)\\ \ extrm{pb}, $$\\end{document}where the uncertainties are due to statistics, systematic effects, the integrated luminosity and the beam energy. This is in excellent agreement with the theoretical expectation.

Search for pair-produced scalar and vector leptoquarks decaying into third-generation quarks and first- or second-generation leptons in pp collisions with the ATLAS detector

A search for pair-produced scalar and vector leptoquarks decaying into quarks and leptons of different generations is presented. It uses the full LHC Run 2 (2015–2018) data set of 139 fb−1 collected with the ATLAS detector in proton–proton collisions at a centre-of-mass energy of sqrt{s} = 13 TeV. Scalar leptoquarks with charge −(1/3)e as well as scalar and vector leptoquarks with charge +(2/3)e are considered. All possible decays of the pair-produced leptoquarks into quarks of the third generation (t, b) and charged or neutral leptons of the first or second generation (e, μ, ν) with exactly one electron or muon in the final state are investigated. No significant deviations from the Standard Model expectation are observed. Upper limits on the production cross-section are provided for eight models as a function of the leptoquark mass and the branching ratio of the leptoquark into the charged or neutral lepton. In addition, lower limits on the leptoquark masses are derived for all models across a range of branching ratios. Two of these models have the goal of providing an explanation for the recent B-anomalies. In both models, a vector leptoquark decays into charged and neutral leptons of the second generation with a similar branching fraction. Lower limits of 1980 GeV and 1710 GeV are set on the leptoquark mass for these two models.

Collider prospects for the muon g−2 in a general two-Higgs-doublet model

Recent progress on muon $g-2$ measurement prompts one to take it even more seriously. In the general two Higgs doublet model that allows extra Yukawa couplings, we take a simplified approach of single enhanced coupling. We fix the charged lepton flavor violating coupling, $\rho_{\tau\mu} = \rho_{\mu\tau}$, via the one-loop mechanism, for illustrative masses of the heavy scalar $H$ and pseudoscalar $A$, where we assume $m_A = m_{H^+}$. Since extra top Yukawa couplings are plausibly the largest, we turn on $\rho_{tt}$ and find that LHC search for $gg \to H,\,A \to \tau\mu$ gives more stringent bound than from $\tau\to \mu\gamma$ with two-loop mechanism. Turning on a second extra top Yukawa coupling, $\rho_{tc}$, can loosen the bound on $\rho_{tt}$, but LHC constraints can again be more stringent than from $B \to D\mu\nu$ vs $De\nu$ universality. This means that evidence for $H,\,A \to \tau\mu$ may yet emerge with full LHC Run 2 data, while direct search for $\tau^\pm\mu^\mp bW^+$ or $t\bar cbW^+$ (plus conjugate) may also bear fruit.

Closing in on t-channel simplified dark matter models

A comprehensive analysis of cosmological and collider constraints is presented for three simplified models characterised by a dark matter candidate (real scalar, Majorana fermion and real vector) and a coloured mediator (fermion, scalar and fermion respectively) interacting with the right-handed up quark of the Standard Model. Constraints from dark matter direct and indirect detection and relic density are combined with bounds originating from the re-interpretation of a full LHC run 2 ATLAS search targeting final states with multiple jets and missing transverse energy. Projections for the high-luminosity phase of the LHC are also provided to assess future exclusion and discovery reaches, which show that analogous future search strategies will not allow for a significant improvement compared with the present status. From the cosmological point of view, we demonstrate that thermal dark matter is largely probed (and disfavoured) by constraints from current direct and indirect detection experiments. These bounds and their future projections have moreover the potential of probing the whole parameter space when combined with the expectation of the high-luminosity phase of the LHC.

High-precision limits on W\u2013W' and Z\u2013Z' mixing from diboson production using the full LHC Run 2 ATLAS data set

The full ATLAS Run 2 data set with time-integrated luminosity of 139 , {text {fb}}^{-1} in the diboson channels in hadronic final states is used to probe a simple model with an extended gauge sector (EGM), proposed by Altarelli et al., and often taken as a convenient benchmark by experimentalists. This model accommodates new charged W' and neutral Z' vector bosons with modified trilinear Standard Model gauge couplings, decaying into electroweak gauge boson pairs WZ or WW, where W/Z decay hadronically. Exclusion limits at the 95% CL on the Z' and W' resonance production cross section times branching ratio to electroweak gauge boson pairs in the mass range of sim 1–5 TeV are here converted to constraints on W–W' and Z–Z' mixing parameters and masses for the EGM. We present exclusion regions on the parameter space of the W' and Z' by using the full Run 2 data set comprised of pp collisions at sqrt{s}=13 , hbox {TeV} and recorded by the ATLAS detector at the CERN LHC. The obtained exclusion regions are significantly extended compared to those obtained from the previous analysis performed with Tevatron data as well as with LHC data collected at 7 and 8 TeV in Run 1 and are the most stringent bounds to date.

Topic model for four-top at the LHC

We study the implementation of a Topic Model algorithm in four-top searches at the LHC as a test-probe of a not ideal system for applying this technique. We study this Topic Model behavior as its different hypotheses such as mutual reducibility and equal distribution in all samples shift from true. The four-top final state at the LHC is not only relevant because it does not fulfill these conditions, but also because it is a difficult and inefficient system to reconstruct and current Monte Carlo modeling of signal and backgrounds suffers from non-negligible uncertainties. We implement this Topic Model algorithm in the Same-Sign lepton channel where S/B is of order one and all backgrounds cannot have more than two b-jets at parton level. We define different mixtures according to the number of b- jets and we use the total number of jets to demix. Since only the background has an anchor bin, we find that we can reconstruct the background in the signal region independently of Monte Carlo. We propose to use this information to tune the Monte Carlo in the signal region and then compare signal prediction with data. We also explore Machine Learning techniques applied to this Topic Model algorithm and find slight improvements as well as potential roads to investigate. Although our findings indicate that still with the full LHC run 3 data the implementation would be challenging, we pursue through this work to find ways to reduce the impact of Monte Carlo simulations in four-top searches at the LHC.

Search for supersymmetry in proton-proton collisions at 13 TeV in final states with jets and missing transverse momentum

Results are reported from a search for supersymmetric particles in the final state with multiple jets and large missing transverse momentum. The search uses a sample of proton-proton collisions at sqrt{s} = 13 TeV collected with the CMS detector in 2016–2018, corresponding to an integrated luminosity of 137 fb−1, representing essentially the full LHC Run 2 data sample. The analysis is performed in a four-dimensional search region defined in terms of the number of jets, the number of tagged bottom quark jets, the scalar sum of jet transverse momenta, and the magnitude of the vector sum of jet transverse momenta. No significant excess in the event yield is observed relative to the expected background contributions from standard model processes. Limits on the pair production of gluinos and squarks are obtained in the framework of simplified models for supersymmetric particle production and decay processes. Assuming the lightest supersymmetric particle to be a neutralino, lower limits on the gluino mass as large as 2000 to 2310 GeV are obtained at 95% confidence level, while lower limits on the squark mass as large as 1190 to 1630 GeV are obtained, depending on the production scenario.

SUSY fits with full LHC Run I data

We present the latest results from the MasterCode collaboration on supersymmetric models, in particular on the CMSSM, the NUHM1, the NUHM2 and the pMSSM. We combine the data from LHC Run I with astrophysical observables, flavor and electroweak precision observables. We determine the best fit regions of these models and analyze the discovery potential of squarks and gluinos at LHC Run II and direct detection experiments.

Search for Higgs bosons decaying into photons at CMS

We present a new result in the search for Higgs bosons decaying into two photons at the CMS detector using the full LHC Run 1 dataset. The data samples correspond to integrated luminosities of 5.1 fb-1 and 19.7 fb-1 at centre-of-mass energies of 7 TeV and 8 TeV respectively. A clear signal is observed in the diphoton channel at a mass close to 125 GeV with a local significance of 5.7 σ, where 5.2 σ is expected from the standard model Higgs boson. The mass is measured to be 124.70 ± 0.34 GeV = 124.70 ± 0.31(stat) ± 0.15(syst) GeV and the signal strength relative to the standard model prediction is found to be 1.14−0.23+0.26=1.14±0.21(stat)−0.05+0.09(syst)−0.09+0.13(theo). Additional results include measurements of the signal strength modifiers with respect to different production mechanisms.

The NUHM2 after LHC Run 1.

We make a frequentist analysis of the parameter space of the NUHM2, in which the soft supersymmetry (SUSY)-breaking contributions to the masses of the two Higgs multiplets, m^2_{H_{u,d}}, vary independently from the universal soft SUSY-breaking contributions m^2_0 to the masses of squarks and sleptons. Our analysis uses the MultiNest sampling algorithm with over 4 times 10^8 points to sample the NUHM2 parameter space. It includes the ATLAS and CMS Higgs mass measurements as well as the ATLAS search for supersymmetric jets + {/!!E}_T signals using the full LHC Run 1 data, the measurements of mathrm{BR}(B_s rightarrow mu ^+mu ^-) by LHCb and CMS together with other B-physics observables, electroweak precision observables and the XENON100 and LUX searches for spin-independent dark-matter scattering. We find that the preferred regions of the NUHM2 parameter space have negative SUSY-breaking scalar masses squared at the GUT scale for squarks and sleptons, m_0^2 < 0, as well as m^2_{H_u} < m^2_{H_d} < 0. The tension present in the CMSSM and NUHM1 between the supersymmetric interpretation of (g-2)_mu and the absence to date of SUSY at the LHC is not significantly alleviated in the NUHM2. We find that the minimum chi ^2 = 32.5 with 21 degrees of freedom (dof) in the NUHM2, to be compared with chi ^2/mathrm{dof} = 35.0/23 in the CMSSM, and chi ^2/mathrm{dof} = 32.7/22 in the NUHM1. We find that the one-dimensional likelihood functions for sparticle masses and other observables are similar to those found previously in the CMSSM and NUHM1.