Leptonic Decay Modes Research Articles

Observation of γγ→ττ in proton–proton collisions and limits on the anomalous electromagnetic moments of the τ lepton

The production of a pair of τ leptons via photon-photon fusion,γγ→ττ, is observed for the first time in proton-proton collisions, with a significance of 5.3 standard deviations. This observation is based on a data set recorded with the CMS detector at the LHC at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 138 fb-1. Events with a pair of τ leptons produced via photon-photon fusion are selected by requiring them to be back-to-back in the azimuthal direction and to have a minimum number of charged hadrons associated with their production vertex. The τ leptons are reconstructed in their leptonic and hadronic decay modes. The measured fiducial cross section ofγγ→ττisσobsfid=12.4-3.1+3.8fb. Constraints are set on the contributions to the anomalous magnetic moment (aτ) and electric dipole moments (dτ) of the τ lepton originating from potential effects of new physics on theγττvertex:aτ=0.0009-0.0031+0.0032and|dτ|<2.9×10-17ecm(95% confidence level), consistent with the standard model.

Measurements of electroweak W±Z boson pair production in association with two jets in 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 with the ATLAS detector

Measurements of integrated and differential cross-sections for electroweak W±Z production in association with two jets (W±Zjj) in proton-proton collisions are presented. The data collected by the ATLAS detector at the Large Hadron Collider from 2015 to 2018 at a centre-of-mass energy of 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 are used, corresponding to an integrated luminosity of 140 fb−1. The W±Zjj candidate events are reconstructed using leptonic decay modes of the gauge bosons. Events containing three identified leptons, either electrons or muons, and two jets are selected. Processes involving pure electroweak W±Zjj production at Born level are separated from W±Zjj production involving a strong coupling. The measured integrated fiducial cross-section of electroweak W±Zjj production per lepton flavour is σWZjj−EW→ℓ′νlljj\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\sigma}_{WZjj- EW\ o {\\ell}^{\\prime}\ u \\mathcal{ll} jj} $$\\end{document} = 0.368 ± 0.037 (stat.) ± 0.059 (syst.) ± 0.003 (lumi.) fb, where ℓ and ℓ′ are either an electron or a muon. Respective cross-sections of electroweak and strong W±Zjj production are measured separately for events with exactly two jets or with more than two jets, and in three bins of the invariant mass of the two jets. The inclusive W±Zjj production cross-section, without separating electroweak and strong production, is also measured to be σWZjj→ℓ′νlljj\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\sigma}_{WZjj\ o {\\ell}^{\\prime}\ u \\mathcal{ll} jj} $$\\end{document} = 1.462 ± 0.063 (stat.) ± 0.118 (syst.) ± 0.012 (lumi.) fb, per lepton flavour. The inclusive W±Zjj production cross-section is measured differentially for several kinematic observables. Finally, the measurements are used to constrain anomalous quartic gauge couplings by extracting 95% confidence level intervals on dimension-8 operators.

Precise estimate of charged Higgsino/Wino decay rate

Higgsinos and Winos in the supersymmetric Standard Model are prime candidates for dark matter due to their weakly interacting nature. The mass differences between their charged components (charginos) and neutral components (neutralinos) become degenerate when other superparticles are heavy, resulting in long-lived charginos. In the case of the Winos, the mass difference is approximately 160 MeV across a wide range of the parameter space. Consequently, the chargino decays into the lightest neutralino, emitting a single charged pion. For Higgsinos, however, mass differences ranging from O(0.1) GeV to O(1) GeV are possible, leading to a variety of decay channels. In this paper, we extend our previous analysis of Wino decay to the chargino with a larger mass difference. We emphasize characterizing its decay signatures through leptonic and hadronic modes. By utilizing the latest experimental data, we perform a comprehensive study of the decay rate calculations incorporating these hadronic modes to determine the impact on the predicted chargino lifetime. Additionally, we conduct next-to-leading order (NLO) calculations for the leptonic decay modes. Our NLO results can be applied to the case of more general fermionic electroweak multiplets, e.g., quintuplet dark matter.

Observation of gauge boson joint-polarisation states in W±Z production from pp collisions at [formula omitted] TeV with the ATLAS detector

Measurements of joint-polarisation states of W and Z gauge bosons in W±Z production are presented. The data set used corresponds 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 CERN Large Hadron Collider. The W±Z candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons and muons. The simultaneous pair-production of longitudinally polarised vector bosons is measured for the first time with a significance of 7.1 standard deviations. The measured joint helicity fractions integrated over the fiducial region are f00=0.067±0.010, f0T=0.110±0.029, fT0=0.179±0.023 and fTT=0.644±0.032, in agreement with the next-to-leading-order Standard Model predictions. Individual helicity fractions of the W and Z bosons are also measured and found to be consistent with joint helicity fractions within the expected amounts of correlation. Both the joint and individual helicity fractions are also measured separately in W+Z and W−Z events. Inclusive and differential cross sections for several kinematic observables sensitive to polarisation are presented.

Disentangling longitudinal and transverse modes of the ϕ meson through dilepton and kaon decays

Angular distributions of $\phi$ meson decay amplitudes of $e^+e^-$ and $K^+K^-$ channels are computed using both specific interaction Lagrangians and simple arguments relying on angular momentum conservation. Based on the obtained results, we assess methods to experimentally disentangle the longitudinal and transverse polarization modes of the $\phi$ meson and discuss advantages and disadvantages of employing either the leptonic or hadronic decay modes for this task.

Revealing the origin of neutrino masses through the Type II Seesaw mechanism at high-energy muon colliders

The future muon collider can play as an ideal machine to search for new physics at high energies. In this work, we study the search potential of the heavy Higgs triplet in the Type II Seesaw mechanism at muon colliders with high collision energy and high luminosity. The latest neutrino oscillation data are taken into account for realizing the leptonic decay modes of the charged Higgs bosons (H±±, H±) in the Type II Seesaw. We show the impact of neutrino mass and mixing parameters on the purely leptonic decays. The pair production of doubly charged Higgs H++H−− is through direct μ+μ− annihilation and vector boson fusion (VBF) processes at muon collider. The associated production H±±H∓ can only be induced by VBF processes. We simulate both the purely leptonic and bosonic signal channels of charged Higgs bosons in Type II Seesaw, together with the Standard Model backgrounds. We show the required luminosity for the discovery of the charged Higgses and the reachable limits on the leptonic decay branching fractions.

Studying Same-Sign Top Pair Production through Top-Higgs FCNC Interactions at the HL-LHC

We investigate the potential of the HL-LHC for discovering new physics effects via the same-sign top pair signatures. We focus on the semileptonic (electron and muon) decay of the top quarks and study the reach for a simplified model approach where top quark flavor changing could occur through a neutral scalar exchange. A relatively smaller background contribution and clean signature are the advantages of the leptonic decay mode of the same-sign W bosons in the same-sign production processes of top quark pairs. Assuming the FCNC between top quark, up-type quark, and scalar boson from the new physics interactions, the branchings could be excluded of the order O 1 0 − 4 . We use angular observables of the same-sign lepton pairs and the top quark kinematics in the process which provide the possibility of separation of new physics signal from the SM backgrounds using machine-learning techniques. We find that the same-sign top quark pair production is quite capable of testing the top-Higgs FCNCs at the HL-LHC.

Future projections on the anomalous WWγγ couplings in hadron–hadron interactions at the FCC-hh

We carry out a study in the approach of a dimension-eight effective field theory on the anomalous quartic WWγγ couplings that arise from the process pp → Wγγ at the Future Circular Collider-hadron hadron (FCC-hh). The process is sensitive to the WWγγ vertex, and future projections on the anomalous f M,i /Λ4 and f T,j /Λ4 couplings can be put by measurements of Wγγ production. We illustrate the physics opportunities and reach of the FCC-hh operating for a high integrated luminosity scenario of of proton–proton collisions at a center-of-mass energy of TeV and systematic uncertainties of δ sys = 0%, 3%, 5%, 10%. Production cross-sections of the signal and the sensitivity constraints on the anomalous f M,i /Λ4 and f T,j /Λ4 couplings at 95% C.L. are identified through the leptonic and hadronic decay modes of the W-boson. The FCC-hh sensitivity on these anomalous couplings is expected might reach up to the order of magnitude . These sensitivity perspectives indicate that the pp → Wγγ channel at the FCC-hh has a greater potential to search for the anomalous quartic WWγγ couplings and to search for new physics than present colliders.

Laser-assisted charged Higgs boson decay in Two Higgs Doublet Model - type II

In this paper, we investigate the charged Higgs boson decay in the context of the type-II two-Higgs-doublet model in the presence of a circularly polarized electromagnetic field of laser radiation. The calculations are performed by adopting the Furry picture approach of non-perturbative interactions with the external electromagnetic field. Using the method of exact solutions for charged particles states in the presence of a circularly polarized electromagnetic wave field and evaluating the S-matrix elements, an exact analytic expression is derived for the decay width of leptonic, hadronic and bosonic decay modes. The branching ratios of different decay modes with multiple photon emission and absorption from the laser beam are analyzed and found to be dramatically modified in the region of superstrong fields. The dependencies of the decay width on the laser field strength and frequency are also examined. The results obtained may be interesting for future experimental and theoretical investigations.

Lepton collider probes for Majorana neutrino effective interactions

The extension of the standard model with new high-scale weakly coupled physics involving right-handed neutrinos in an effective field theory framework (SMNEFT) allows for a systematic study of heavy neutrinos phenomenology in current and future experiments. We exploit the outstanding angular resolution in future lepton colliders to study the sensitivity of forward–backward asymmetries to discover the possible single production of heavy Majorana neutrinos via e^{+}e^{-} rightarrow N nu , followed by a purely leptonic decay N rightarrow mu ^{-} mu ^{+} nu or a semi-leptonic decay N rightarrow mu ^{-} text {j} text {j} , for masses m_N > 50 GeV. In this regime, we consider the N production and decays to be dominated by scalar and vectorial four-fermion d=6 single N_R operators. This is an alternative analysis to searches using displaced vertices and fat jets, in a higher mass regime, where the N is short-lived but can be found by the angular distribution of its decay products. We find that a forward–backward asymmetry between the final muons in the pure leptonic decay mode provides a sensitivity up to 12sigma for m_N=100 GeV, for effective couplings alpha =0.2 and new physics scale Lambda =1 TeV. In the case of the semi-leptonic decay, we can compare the final muon and higher p_T jet flight directions, again finding up to 12sigma sensitivity to the effective signals.

Search for flavour-changing neutral-current interactions of a top quark and a gluon in pp collisions at sqrt{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+grightarrow t and c+grightarrow 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: sigma (u+grightarrow t)times mathcal {B}(trightarrow Wb)times mathcal {B}(Wrightarrow ell nu )<3.0,pb and sigma (c+grightarrow t)times mathcal {B}(trightarrow Wb)times mathcal {B}(Wrightarrow ell nu )<4.7,pb at the 95% confidence level, with mathcal {B}(Wrightarrow ell nu )=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: |C^{,ut}_{uG}|/Lambda ^2 < 0.057,TeV^{-2} and |C^{,ct}_{uG}|/Lambda ^2 < 0.14,TeV^{-2}. These bounds correspond to limits on the branching ratios of FCNC-induced top-quark decays: mathcal {B}(trightarrow u+g)< 0.61times 10^{-4} and mathcal {B}(trightarrow c+g)< 3.7times 10^{-4}.

Four-lepton production in gluon fusion at NLO matched to parton showers

We present a calculation of the next-to-leading order (NLO) QCD corrections to gluon-induced electroweak gauge boson pair production, gg rightarrow ZZ and gg rightarrow W^+W^-, matched to the PYTHIA 8 parton shower in the POWHEG approach. The calculation consistently incorporates the continuum background, the Higgs-mediated ggrightarrow H^* rightarrow VV process, and their interference. We consider leptonic decay modes of the vector bosons and retain offshell and non-resonant contributions. The processes considered are loop-induced at leading order and thus contain two-loop virtual contributions as well as loop-squared real contributions. Parton-shower effects are found to be marginal in inclusive observables and quite sizeable in observables that are exclusive in additional jet radiation. The Monte Carlo generator presented here allows for realistic experimental effects to be incorporated in state-of-the-art precision analyses of diboson production and of the Higgs boson in the offshell regime.

Standard model EFT effects in vector-boson scattering at the LHC

The assumption that the Standard Model is an Effective Field Theory (SM EFT) of a more fundamental theory at a higher, than electroweak, energy scale, implies a growth of cross-sections for electroweak Vector Boson Scattering (VBS) processes signalling the appearance of a resonance (or resonances) nearby that scale. In this article, we investigate in detail SM EFT effects from dimension-6 operators in VBS with like-sign-$W$ production in fully leptonic decay modes at the High Luminosity LHC (HL-LHC). We find that these effects are important for a handful of operators, most notably for the operator composed of three $SU(2)$ field strength tensors responsible for strong transversely polarized vector boson interactions. Current global fits on Wilson-coefficients allow for a signal immediately permissive at the HL-LHC if not accessible at the current LHC-dataset.

Same-Sign Dilepton Signature in the Inert Doublet Model * *Supported by the National Natural Science Foundation of China (11805081), Natural Science Foundation of Shandong Province (ZR2019QA021, ZR2018MA047)

In this study, we perform a detailed analysis on the same-sign dilepton signature in the inert doublet model. Focusing on the low dark matter mass region, we randomly scan the corresponding parameter space. Viable samples allowed by various constraints are obtained, and among them are twenty benchmark points that are selected for further study on collider signature. At hadron colliders, the same-sign dilepton signature is produced via with the leptonic decay mode , where H represents the dark matter candidate. We investigate the testability of this signal at the high-luminosity LHC (HL-LHC) and the proposed 27 TeV high-energy LHC (HE-LHC). According to our simulation, the HL-LHC with can barely probe this signal. Meanwhile, for the HE-LHC with , it is promising to obtain a significance when GeV with dark matter mass or 71 GeV.

Chasing a power supply in Siberia

Arthur Liberman’s Letter to the Editor on Cold War particle-physics collaborations (Physics Today, October 2020, page 12) reminded me of a visit I made in 1969 to Akademgorodok, a small research town near Novosibirsk, Siberia. At that time our Northeastern University research group had theoretically postulated, and done an initial experiment on, ρ–ω interference in the leptonic decay mode.11. R. G. Parsons, R. Weinstein, Phys. Rev. Lett. 20, 1314 (1968). https://doi.org/10.1103/PhysRevLett.20.1314 There were several interesting features to be experimentally or theoretically studied, and a flurry of work followed. We sought to do definitive experiments at the well-suited colliding-beams accelerator in Gersh Budker’s research facilities in Akademgorodok.During discussions with the facility’s director, Veniamin Siderov, concerning the proposed experiments, we compared the structure of the Novosibirsk research group with that of the Northeastern group. The numbers that emerged gave significant insight to the hurdles faced by physics researchers in the Soviet Union.Northeastern’s high-energy research group consisted of five PhDs and seven support technicians. When I requested that Siderov provide comparable information about his facility, he replied that he had 2000 technicians. I assumed that he had misunderstood my question, but he actually did have that many technicians working for him. To help me understand the staggering difference, he described the procedure he would follow when he needed, for example, a power supply.In the US one would simply go out and buy a power supply, but such devices were not available on the Soviet market, and Siderov had no access to hard currencies. He would have to set up a production line in a subgroup of his 2000 technicians, and they would produce perhaps 100 power supplies. He would then act as a vendor of the devices—for several years if need be—to support his technicians and make purchases of his own from other similar manufacturing centers. The amount of organization, energy, and manpower required to obtain a power supply was staggering.I had come prepared to discuss the details of the proposed experiment, but Budker was already expert in the details of ρ–ω interference. Instead he wanted to discuss the proposed experimental setup—in particular, the number of photomultiplier tubes and power supplies. He agreed to make the accelerator available for the proposed experiment on one condition—that after completion of the measurements, the equipment would be left at his lab.I carried that proposition back to my supporting agency, which had previously supplied support for Soviet experiments in the US. However, the arrangement was not approved; it amounted to the US paying for Soviet beam time while Soviet researchers got US beam time for free.ReferenceSection:ChooseTop of pageReference <<1. R. G. Parsons, R. Weinstein, Phys. Rev. Lett. 20, 1314 (1968). https://doi.org/10.1103/PhysRevLett.20.1314, Google ScholarCrossref© 2021 American Institute of Physics.

Measurement of the inclusive and differential Higgs boson production cross sections in the leptonic WW decay mode at sqrt{s} = 13 TeV

Measurements of the fiducial inclusive and differential production cross sections of the Higgs boson in proton-proton collisions at sqrt{s} = 13 TeV are performed using events where the Higgs boson decays into a pair of W bosons that subsequently decay into a final state with an electron, a muon, and a pair of neutrinos. The analysis is based on data collected with the CMS detector at the LHC during 2016–2018, corresponding to an integrated luminosity of 137 fb−1. Production cross sections are measured as a function of the transverse momentum of the Higgs boson and the associated jet multiplicity. The Higgs boson signal is extracted and simultaneously unfolded to correct for selection efficiency and resolution effects using maximum-likelihood fits to the observed distributions in data. The integrated fiducial cross section is measured to be 86.5 ± 9.5 fb, consistent with the Standard Model expectation of 82.5 ± 4.2 fb. No significant deviation from the Standard Model expectations is observed in the differential measurements.

Sensitivity measuring expected on the electromagnetic anomalous couplings in the tt̄γ vertex at the FCC-he

Starting from an effective Lagrangian for the [Formula: see text] interaction of the top-quark, we consider the electroweak production cross-section via charged current and through the [Formula: see text] signal. Further, we derive the sensitivity expected for the magnetic dipole moment [Formula: see text] and the electric dipole moment [Formula: see text] of the top-quark at the Future Circular Collider-hadron electron (FCC-he). We present our study for [Formula: see text], 10 TeV, [Formula: see text], 100, 300, 500, 1000 fb[Formula: see text], [Formula: see text] and [Formula: see text], 80%, [Formula: see text], respectively. We find that the sensitivity estimated on dipole moments of the top-quark is of the order of magnitude [Formula: see text] for both hadronic and leptonic decay modes of [Formula: see text]: [Formula: see text] and [Formula: see text] at 95% C.L. in the hadronic channel with unpolarized electron beam [Formula: see text]. Our results regarding the polarized electron beam for [Formula: see text] and [Formula: see text] are [Formula: see text], [Formula: see text] and [Formula: see text], [Formula: see text] at 95% C.L. in the hadronic channel. The corresponding results for the leptonic channel with [Formula: see text], 80% [Formula: see text] are [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text], [Formula: see text], respectively. The results for [Formula: see text] and [Formula: see text] in the leptonic channel are weaker than those corresponding to the hadronic channel by a factor of 0.75. Given these prospective sensitivities, the FCC-he is an alternative to study the anomalous couplings of the quark-top that would complement the studies of the LHC with cleaner environments.

Measurements of production cross sections of WZ and same-sign WW boson pairs in association with two jets in proton-proton collisions at [formula omitted

Measurements of production cross sections of WZ and same-sign WW boson pairs in association with two jets in proton-proton collisions at s=13TeV at the LHC are reported. The data sample corresponds to an integrated luminosity of 137fb−1, collected with the CMS detector during 2016–2018. The measurements are performed in the leptonic decay modes ▪ and ▪, where ℓ,ℓ′=e, ▪. Differential fiducial cross sections as functions of the invariant masses of the jet and charged lepton pairs, as well as of the leading-lepton transverse momentum, are measured for W±W± production and are consistent with the standard model predictions. The dependence of differential cross sections on the invariant mass of the jet pair is also measured for WZ production. An observation of electroweak production of WZ boson pairs is reported with an observed (expected) significance of 6.8 (5.3) standard deviations. Constraints are obtained on the structure of quartic vector boson interactions in the framework of effective field theory.

Search for lepton flavour violating decays of a neutral heavy Higgs boson to \u03bc\u03c4 and e\u03c4 in proton-proton collisions at sqrt{s} = 13 TeV

A search for lepton flavour violating decays of a neutral non-standard-model Higgs boson in the μτ and eτ decay modes is presented. The search is based on proton-proton collisions at a center of mass energy sqrt{s} = 13 TeV collected with the CMS detector in 2016, corresponding to an integrated luminosity of 35.9 fb−1. The τ leptons are reconstructed in the leptonic and hadronic decay modes. No signal is observed in the mass range 200–900 GeV. At 95% confidence level, the observed (expected) upper limits on the production cross section multiplied by the branching fraction vary from 51.9 (57.4) fb to 1.6 (2.1) fb for the μτ and from 94.1 (91.6) fb to 2.3 (2.3) fb for the eτ decay modes.

Angular and CP-Violation Analyses of B-→D⁎+l-ν-l Decays at Hadron Collider Experiments

The B-→D⁎+l-ν-l branching fractions ratio between muon and tau lepton decay modes R(D⁎) has shown intriguing discrepancies between the Standard Model prediction and measurements performed at BaBar, Belle, and LHCb experiments, a possible sign of beyond the Standard Model physics. Theoretical studies show how observables related to the B-→D⁎+l-ν-l differential decay distribution can be used to further constrain New Physics contributions, but their experimental measurements are lacking to date. This article proposes the measurement of B-→D⁎+l-ν-l angular and CP-violating observables at hadron collider experiments, by exploiting approximate reconstruction algorithms using information from detectable final-state particles only. The resolution on the phase space variables is studied using B-→D⁎+l-ν-l decays simulated in a forward detector geometry like LHCb. A method to correct the observable values for the reconstruction inaccuracies based on detector simulation is successfully tested on simulated data and the decrease in precision with respect to a perfect reconstruction is evaluated. The D⁎+ longitudinal polarization fraction and the AT(2)CP-violating observable can be measured losing a factor of 2 and 5 in precision, respectively. The extraction of angular distributions from the template fit selecting B-→D⁎+l-ν-l decays and associated systematic uncertainties are also discussed.