Leptons In Pp Collisions Research Articles

Search for third-generation vector-like leptons in pp collisions at sqrt{s} = 13 TeV with the ATLAS detector

A search for vector-like leptons in multilepton (two, three, or four-or-more electrons plus muons) final states with zero or more hadronic τ-lepton decays is presented. The search is performed using a dataset corresponding to an integrated luminosity of 139 fb−1 of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the LHC. To maximize the separation of signal and background, a machine-learning classifier is used. No excess of events is observed beyond the Standard Model expectation. Using a doublet vector-like lepton model, vector-like leptons coupling to third-generation Standard Model leptons are excluded in the mass range from 130 GeV to 900 GeV at the 95% confidence level, while the highest excluded mass is expected to be 970 GeV.

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.

Measurement of the top quark pair production cross section in dilepton final states containing one \u03c4 lepton in pp collisions at sqrt{s} = 13 TeV

The cross section of top quark pair production is measured in the mathrm{t}overline{mathrm{t}}to left(ell {nu}_{ell}right)left({uptau}_{mathrm{h}}{nu}_{uptau}right)mathrm{b}overline{mathrm{b}} final state, where τh refers to the hadronic decays of the τ lepton, and ℓ is either an electron or a muon. The data sample corresponds to an integrated luminosity of 35.9 fb−1 collected in proton-proton collisions at sqrt{s} = 13 TeV with the CMS detector. The measured cross section is {sigma}_{mathrm{t}overline{mathrm{t}}} = 781 ± 7 (stat) ± 62 (syst) ± 20 (lumi) pb, and the ratio of the partial width Γ(t → τντb) to the total decay width of the top quark is measured to be 0.1050 ± 0.0009 (stat) ± 0.0071 (syst). This is the first measurement of the mathrm{t}overline{mathrm{t}} production cross section in proton-proton collisions at sqrt{s} = 13 TeV that explicitly includes τ leptons. The ratio of the cross sections in the ℓτh and ℓℓ final states yields a value {R}_{ell {uptau}_{mathrm{h}}/mathcal{ll}} = 0.973 ± 0.009 (stat) ± 0.066 (syst), consistent with lepton universality.

Search for a right-handed gauge boson decaying into a high-momentum heavy neutrino and a charged lepton in pp collisions with the ATLAS detector at [formula omitted] TeV

A search for a right-handed gauge boson WR, decaying into a boosted right-handed heavy neutrino NR, in the framework of Left-Right Symmetric Models is presented. It is based on data from proton–proton collisions with a centre-of-mass energy of 13 TeV collected by the ATLAS detector at the Large Hadron Collider during the years 2015, 2016 and 2017, corresponding to an integrated luminosity of 80 fb−1. The search is performed separately for electrons and muons in the final state. A distinguishing feature of the search is the use of large-radius jets containing electrons. Selections based on the signal topology result in smaller background compared to the expected signal. No significant deviation from the Standard Model prediction is observed and lower limits are set in the WR and NR mass plane. Mass values of the WR smaller than 3.8–5 TeV are excluded for NR in the mass range 0.1–1.8 TeV.

Study of the hard double-parton scattering contribution to inclusive four-lepton production in pp collisions at [formula omitted] TeV with the ATLAS detector

The inclusive production of four isolated charged leptons in pp collisions is analysed for the presence of hard double-parton scattering, using 20.2 fb−1 of data recorded in the ATLAS detector at the LHC at centre-of-mass energy s=8 TeV. In the four-lepton invariant-mass range of 80<m4ℓ<1000 GeV, an artificial neural network is used to enhance the separation between single- and double-parton scattering based on the kinematics of the four leptons in the final state. An upper limit on the fraction of events originating from double-parton scattering is determined at 95% confidence level to be fDPS=0.042, which results in an estimated lower limit on the effective cross section at 95% confidence level of 1.0 mb.

Search for electroweak production of charginos in final states with two \u03c4 leptons in pp collisions at sqrt{s}=8 TeV

Results are presented from a search for the electroweak production of supersymmetric particles in pp collisions in final states with two τ leptons. The data sample corresponds to an integrated luminosity between 18.1 fb−1 and 19.6 fb−1 depending on the final state of τ lepton decays, at sqrt{s}=8 TeV, collected by the CMS experiment at the LHC. The observed event yields in the signal regions are consistent with the expected standard model backgrounds. The results are interpreted using simplified models describing the pair production and decays of charginos or τ sleptons. For models describing the pair production of the lightest chargino, exclusion regions are obtained in the plane of chargino mass vs. neutralino mass under the following assumptions: the chargino decays into third-generation sleptons, which are taken to be the lightest sleptons, and the sleptons masses lie midway between those of the chargino and the neutralino. Chargino masses below 420 GeV are excluded at a 95% confidence level in the limit of a massless neutralino, and for neutralino masses up to 100 GeV, chargino masses up to 325 GeV are excluded at 95% confidence level. Constraints are also placed on the cross section for pair production of τ sleptons as a function of mass, assuming a massless neutralino.

Search for direct top squark pair production in final states with two tau leptons in pp collisions at [Formula: see text]TeV with the ATLAS detector.

A search for direct pair production of the supersymmetric partner of the top quark, decaying via a scalar tau to a nearly massless gravitino, has been performed using 20 fb^{-1} of proton–proton collision data at sqrt{s}=8~mathrm{TeV}. The data were collected by the ATLAS experiment at the LHC in 2012. Top squark candidates are searched for in events with either two hadronically decaying tau leptons, one hadronically decaying tau and one light lepton, or two light leptons. No significant excess over the Standard Model expectation is found. Exclusion limits at 95~% confidence level are set as a function of the top squark and scalar tau masses. Depending on the scalar tau mass, ranging from the 87~mathrm{GeV} LEP limit to the top squark mass, lower limits between 490 and 650~mathrm{GeV} are placed on the top squark mass within the model considered.

Search for a very light NMSSM Higgs boson produced in decays of the 125 GeV scalar boson and decaying into τ leptons in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV

A search for a very light Higgs boson decaying into a pair of tau leptons is presented within the framework of the next-to-minimal supersymmetric standard model. This search is based on a data set corresponding to an integrated luminosity of 19.7 inverse femtobarns of proton-proton collisions collected by the CMS experiment at a centre-of-mass energy of 8 TeV. The signal is defined by the production of either of the two lightest scalars, h[1] or h[2], via gluon-gluon fusion and subsequent decay into a pair of the lightest Higgs bosons, a[1] or h[1]. The h[1] or h[2] boson is identified with the observed state at a mass of 125 GeV. The analysis searches for decays of the a[1] (h[1]) states into pairs of tau leptons and covers a mass range for the a[1] (h[1]) boson of 4 to 8 GeV. The search reveals no significant excess in data above standard model background expectations, and an upper limit is set on the signal production cross section times branching fraction as a function of the a[1] (h[1]) boson mass. The 95% confidence level limit ranges from 4.5 pb at m(a[1]) (m(h[1])) = 8 GeV to 10.3 pb at m(a[1]) (m(h[1])) = 5 GeV.

Search for heavy lepton resonances decaying to a Z boson and a lepton in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

A search for heavy leptons decaying to a $Z$ boson and an electron or a muon is presented. The search is based on $pp$ collision data taken at $\sqrt{s}=8$ TeV by the ATLAS experiment at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb$^{-1}$. Three high-transverse-momentum electrons or muons are selected, with two of them required to be consistent with originating from a $Z$ boson decay. No significant excess above Standard Model background predictions is observed, and 95% confidence level limits on the production cross section of high-mass trilepton resonances are derived. The results are interpreted in the context of vector-like lepton and type-III seesaw models. For the vector-like lepton model, most heavy lepton mass values in the range 114-176 GeV are excluded. For the type-III seesaw model, most mass values in the range 100-468 GeV are excluded.

Search for new phenomena in events with three or more charged leptons in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

A generic search for anomalous production of events with at least three charged leptons is presented. The data sample consists of $pp$ collisions at $\sqrt{s}=8$ TeV collected in 2012 by the ATLAS experiment at the CERN Large Hadron Collider, and corresponds to an integrated luminosity of 20.3 fb$^{-1}$. Events are required to have at least three selected lepton candidates, at least two of which must be electrons or muons, while the third may be a hadronically decaying tau. Selected events are categorized based on their lepton flavour content and signal regions are constructed using several kinematic variables of interest. No significant deviations from Standard Model predictions are observed. Model-independent upper limits on contributions from beyond the Standard Model phenomena are provided for each signal region, along with prescription to re-interpret the limits for any model. Constraints are also placed on models predicting doubly charged Higgs bosons and excited leptons. For doubly charged Higgs bosons decaying to $e\tau$ or $\mu\tau$, lower limits on the mass are set at 400 GeV at 95% confidence level. For excited leptons, constraints are provided as functions of both the mass of the excited state and the compositeness scale $\Lambda$, with the strongest mass constraints arising in regions where the mass equals $\Lambda$. In such scenarios, lower mass limits are set at 3.0 TeV for excited electrons and muons, 2.5 TeV for excited taus, and 1.6 TeV for every excited-neutrino flavour.

Search for type-III seesaw heavy leptons in pp collisions at √s = 8 TeV with the ATLAS detector

Search for type-III seesaw heavy leptons in pp collisions at √s = 8 TeV with the ATLAS detector

Search for neutral MSSM Higgs bosons decaying to a pair of tau leptons in pp collisions

A search for neutral Higgs bosons in the minimal supersymmetric extension of the standard model (MSSM) decaying to tau-lepton pairs in pp collisions is performed, using events recorded by the CMS experiment at the LHC. The dataset corresponds to an integrated luminosity of 24.6 inverse femtobarns, with 4.9 inverse femtobarns at 7 TeV and 19.7 inverse femtobarns at 8 TeV. To enhance the sensitivity to neutral MSSM Higgs bosons, the search includes the case where the Higgs boson is produced in association with a b-quark jet. No excess is observed in the tau-lepton-pair invariant mass spectrum. Exclusion limits are presented in the MSSM parameter space for different benchmark scenarios, m[h,max}, m[h,mod+], m[h,mod-], light-stop, light-stau, tau-phobic, and low-m[H]. Upper limits on the cross section times branching fraction for gluon fusion and b-quark associated Higgs boson production are also given.

Search for direct top-squark pair production in final states with two leptons in pp collisions at $ \sqrt{s} $ = 8 TeV with the ATLAS detector

A search is presented for direct top-squark pair production in final states with two leptons (electrons or muons) of opposite charge using 20.3fb-1 of pp collision data at sqrt(s)=8TeV, collected by the ATLAS experiment at the Large Hadron Collider in 2012. No excess over the Standard Model expectation is found. The results are interpreted under the separate assumptions (i) that the top squark decays to a b-quark in addition to an on-shell chargino whose decay occurs via a real or virtual W boson, or (ii) that the top squark decays to a t-quark and the lightest neutralino. A top squark with a mass between 150 GeV and 445 GeV decaying to a b-quark and an on-shell chargino is excluded at 95% confidence level for a top squark mass equal to the chargino mass plus 10 GeV, in the case of a 1 GeV lightest neutralino. Top squarks with masses between 215 (90) GeV and 530 (170) GeV decaying to an on-shell (off-shell) t-quark and a neutralino are excluded at 95% confidence level for a 1 GeV neutralino.

Search for excited leptons in pp collisions at [formula omitted

Results are presented of a search for compositeness in electrons and muons using a data sample of pp collisions at a center-of-mass energy s=7 TeV collected with the CMS detector at the LHC and corresponding to an integrated luminosity of 5.0 fb−1. Excited leptons (ℓ⁎) are assumed to be produced via contact interactions in conjunction with a standard model lepton and to decay via ℓ⁎→ℓγ, yielding a final state with two energetic leptons and a photon. The number of events observed in data is consistent with that expected from the standard model. The 95% confidence upper limits for the cross section for the production and decay of excited electrons (muons), with masses ranging from 0.6 to 2 TeV, are 1.48 to 1.24 fb (1.31 to 1.11 fb). Excited leptons with masses below 1.9 TeV are excluded for the case where the contact interaction scale equals the excited lepton mass. The limits on the cross sections are the most stringent ones published to date.

Search for excited leptons in pp collisions at (square root) s (equivalent to) 7 TeV

Search for excited leptons in pp collisions at (square root) s (equivalent to) 7 TeV

Search for anomalous production of multilepton events in pp collisions at $ \sqrt {s} = 7\,TeV $

Abstract A search for anomalous production of events with three or more isolated leptons in pp collisions at $ \sqrt {s} = 7\,TeV $ is presented. The data, corresponding to an integrated luminosity of 4.98 fb−1, were collected by the CMS experiment at the LHC during the 2011 run. The search is applicable to any model of new physics that enhances multiple lepton production. The observed multilepton events are categorized into exclusive search channels based on the identity and kinematics of the objects in the events. An estimate of the standard-model background rates from data is emphasized, but simulation is also used to estimate some of the background rates. The search results are interpreted in the context of supersymmetry, including both R-parity-conserving and R-parity-violating models. We derive exclusion limits as a function of squark, gluino, and chargino masses.

Search for a Low-Mass Standard Model Higgs Boson in theττDecay Channel inpp¯Collisions ats=1.96 TeV

We report on a search for the standard model Higgs boson decaying into pairs of τ leptons in pp collisions produced by the Tevatron at sqrt[s]=1.96 TeV. The analyzed data sample was recorded by the CDFII detector and corresponds to an integrated luminosity of 6.0 fb(-1). The search is performed in the final state with one τ decaying leptonically and the second one identified through its semihadronic decay. Since no significant excess is observed, a 95% credibility level upper limit on the production cross section times branching ratio to the ττ final state is set for hypothetical Higgs boson masses between 100 and 150 GeV/c2. For a Higgs boson of 120 GeV/c2 the observed (expected) limit is 14.6 (15.3) the predicted value.

A search for excited leptons in pp collisions at [formula omitted

A search for excited leptons is carried out with the CMS detector at the LHC, using 36 pb−1 of pp collision data recorded at s=7 TeV. The search is performed for associated production of a lepton and an oppositely charged excited lepton pp→ℓℓ⁎, followed by the decay ℓ⁎→ℓγ, resulting in the ℓℓγ final state, where ℓ=e,μ. No excess of events above the standard model expectation is observed. Interpreting the findings in the context of ℓ⁎ production through four-fermion contact interactions and subsequent decay via electroweak processes, first upper limits are reported for ℓ⁎ production at this collision energy. The exclusion region in the compositeness scale Λ and excited lepton mass Mℓ⁎ parameter space is extended beyond previously established limits. For Λ=Mℓ⁎, excited lepton masses are excluded below 1070 GeV/c2 for e⁎ and 1090 GeV/c2 for μ⁎ at the 95% confidence level.

Measurement ofσ(pp¯→Z)·B(Z→ττ)inpp¯collisions ats=1.96 TeV

We present a measurement of the inclusive production cross section for Z bosons decaying to tau leptons in pp collisions at {radical}(s)=1.96 TeV. We use a channel with one hadronically-decaying and one electronically-decaying tau. This measurement is based on 350 pb{sup -1} of CDF Run II data. Using a sample of 504 opposite sign e{tau} events with a total expected background of 190 events, we obtain {sigma}(pp{yields}4Z){center_dot}B(Z{yields}{tau}{tau})=264{+-}23(stat){+-}14(syst){+-}15(lumi) pb, in agreement with the next-to-next-to-leading order QCD prediction. This is the first CDF cross section measurement using hadronically-decaying taus in Run II.

Do direct leptons with large transverse momentum originate from ψ production?

We show that the energy as well as the transverse momentum dependence of the yield of direct leptons in pp collisions can be obtained from the production and subsequent decay of vector mesons (ϱ, ω, φ and ψ). This firstly requires that the total ψ yield σ(pp→ ψ) be 0.02 μb at 70 GeV, ≈ 2 μb at 300 GeV, 3–10 μb at 1500 GeV and secondly that the transverse momentum distribution be broader than that of other hadrons, with 〈p T 2〉 ≳ 0.5 (GeV/c) 2.