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DOE OSTI2013

Lepton mixing under the lepton charge nonconservation, neutrino masses and oscillations and the 'forbidden' decay Micro-Sign {sup -} {yields} e{sup -} + {gamma}

The lepton-charge (L{sub e}, L{sub {mu}}, L{sub {tau}}) nonconserving interaction leads to the mixing of the electron, muon, and tau neutrinos, which manifests itself in spatial oscillations of a neutrino beam, and also to the mixing of the electron, negative muon, and tau lepton, which, in particular, may be the cause of the 'forbidden' radiative decay of the negative muon into the electron and {gamma} quantum. Under the assumption that the nondiagonal elements of the mass matrices for neutrinos and ordinary leptons, connected with the lepton charge nonconservation, are the same, and by performing the joint analysis of the experimental data on neutrino oscillations and experimental restriction for the probability of the decay Micro-Sign {sup -} {yields} e{sup -} + {gamma} per unit time, the following estimate for the lower bound of neutrino mass has been obtained: m{sup ({nu})} > 1.5 eV/c{sup 2}.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Measurement of the top quark mass in the $t\bar{t}\rightarrow $ lepton+jets channel from $\sqrt{s}=8$ TeV ATLAS data and combination with previous results

The top quark mass is measured using a template method in the $t\bar{t}$→ lepton+jets channel (lepton is e or μ) using ATLAS data recorded in 2012 at the LHC. The data were taken at a proton–proton centre-of-mass energy of √s = 8 TeV and correspond to an integrated luminosity of 20.2 fb –1 . The $t\bar{t}$ → lepton+jets channel is characterized by the presence of a charged lepton, a neutrino and four jets, two of which originate from bottom quarks (b). Exploiting a three-dimensional template technique, the top quark mass is determined together with a global jet energy scale factor and a relative b-to-light-jet energy scale factor. The mass of the top quark is measured to be m top = 172.08 ± 0.39(stat) ± 0.82(syst) GeV. A combination with previous ATLAS m top measurements gives m top = 172.69 ± 0.25(stat) ± 0.41(syst) GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

FCC-ee: The Lepton Collider

Particle physics has arrived at an important moment of its history. The discovery of the Higgs boson, with a mass of 125 GeV, completes the matrix of particles and interactions that has constituted the “Standard Model” for several decades. This model is a consistent and predictive theory, which has so far proven successful at describing all phenomena accessible to collider experiments. On the other hand, several experimental facts do require the extension of the Standard Model and explanations are needed for observations such as the domination of matter over antimatter, the evidence for dark matter and the non-zero neutrino masses. Theoretical issues that need to be addressed include the hierarchy problem, the neutrality of the Universe, the stability of the Higgs boson mass upon quantum corrections and the strong CP problem. This report contains the description of a novel research infrastructure based on a highest-luminosity energy frontier electron-positron collider (FCC-ee) to address the open questions of modern physics. It will be a general precision instrument for the continued in-depth exploration of nature at the smallest scales, optimised to measure precisely the properties of the Higgs boson at the per-cent level, the Z and W bosons, the top quark and the Higgs coupling to the Z at the per-mil level. FCC-ee will provide unprecedented sensitivity to signs of new physics appearing either in the form of small deviations from the Standard Model or as rare decay processes. This collider will be implemented in stages, successively spanning the entire energy range from the Z pole over the WW threshold and H production peak to the t"t" ̅ threshold. Most of the infrastructure (e.g. underground structures, surface sites, electrical distribution, cooling & ventilation, RF systems) can be directly re-used for a subsequent highest-energy hadron collider (described in the FCC conceptual design report volume 3), serving the world-wide particle-physics community in a highly synergetic and cost-effective manner throughout the 21st century. The European Strategy for Particle Physics (ESPP) update 2013 stated “To stay at the forefront of particle physics, Europe needs to be in a position to propose an ambitious post-LHC accelerator project at CERN by the time of the next Strategy update”. The FCC study has implemented the ESPP recommendation by developing a long-term vision for an “accelerator project in a global context”. This document describes the detailed design and preparation of a construction project for a post-LHC circular lepton collider “in collaboration with national institutes, laboratories and universities worldwide”, and enhanced by a strong participation of industrial partners. Now, a coordinated preparation effort can be based on a core of an ever-growing consortium of already more than 135 institutes worldwide. The technology for constructing a high-energy, highest-luminosity circular lepton collider exists today. The FCC-ee concept comprises a power-saving twin-aperture magnet system, a continuous top-up injection scheme for stable operation and maximum integrated luminosity. Combined with an energy staging scheme, the FCC-ee represents the most efficient and most sustainable route for executing the research required to discover signs of new physics beyond the Standard Model. The step-wise energy increase of the FCC-ee does not require any additional civil engineering activities. Strategic R&D for FCC-ee aims at minimising construction cost and energy consumption, while maximising the socio-economic impact. It will mitigate residual technology-related risks and ensure that industry can benefit from an acceptable economic utility. Concerning the implementation, a preparatory phase of about eight years is both necessary and adequate to establish the project governance and organisation structures, building the international machine and experiment consortia, developing a territorial implementation plan in agreement with the host-states’ requirements, optimising the disposal of land and underground volumes and preparing the civil engineering project. Such a large-scale, international fundamental research infrastructure, tightly involving industrial partners and providing training at all education levels, will be a strong motor of economic and societal development in all participating nations. The FCC study has implemented a set of actions towards a coherent vision for the world-wide high-energy and particle physics community, providing a collaborative framework for topically complementary and geographically well-balanced contributions. This conceptual design report lays the foundation for a subsequent infrastructure preparatory and technical design phase.

43 PARTICLE ACCELERATORS↗
DOE OSTI2016

A search for prompt lepton-jets in pp collisions at √s = 8 TeV with the ATLAS detector

We present a search for a new, light boson with a mass of about 1 GeV and decaying promptly to jets of collimated electrons and/or muons (lepton-jets). The analysis is performed with 20.3 fb -1 of data collected by the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a centre-of-mass energy of 8 TeV. Events are required to contain at least two lepton-jets. This study finds no statistically significant deviation from predictions of the Standard Model and places 95% confidence-level upper limits on the contribution of new phenomena beyond the SM, incuding SUSY-portal and Higgs-portal models, on the number of events with lepton-jets.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Search for long-lived heavy neutral leptons in proton-proton collision events with a lepton-jet pair associated with a secondary vertex at $\sqrt{s}$ = 13 TeV

A search for long-lived heavy neutral leptons (HNLs) using proton-proton collision data corresponding to an integrated luminosity of 138 fb −1 collected at $\sqrt{s}$ = 13 TeV with the CMS detector at the CERN LHC is presented. Events are selected with a charged lepton originating from the primary vertex associated with the proton-proton interaction, as well as a second charged lepton and a hadronic jet associated with a secondary vertex that corresponds to the semileptonic decay of a long-lived HNL. No excess of events above the standard model expectation is observed. Exclusion limits at 95% confidence level are evaluated for HNLs that mix with electron and/or muon neutrinos. Limits are presented in the mass range of 1–16.5 GeV, with excluded square mixing parameter values reaching as low as 2 × 10 −7 . For masses above 11 GeV, the presented limits exceed all previous results in the semileptonic decay channel, and for some of the considered scenarios are the strongest to date.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Search for lepton-flavor-violating τ decays into a lepton and a vector meson using the full Belle data sample

Charged-lepton-flavor-violation is predicted in several new physics scenarios. We update the analysis of τ lepton decays into a light charged lepton (ℓ = e ± or μ ± ) and a vector meson (V 0 = ρ 0 , Φ, ω, K *0 , or $\overline{K}$ *0 ) using 980 fb -1 of data collected with the Belle detector at the KEKB collider. No significant excess of such signal events is observed, and thus 90% credibility level upper limits are set on the τ → ℓV 0 branching fractions in the range of (1.7–4.3) × 10 -8 . These limits are improved by 30% on average from the previous results.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Search for leptonic charge asymmetry in $t\overline{t}W$ production in final states with three leptons at $\sqrt{s}$ = 13 TeV

A search for the leptonic charge asymmetry (${A}_{c}^{\ell}$) of top-quark-antiquark pair production in association with a $W$ boson ($t\overline{t}W$) is presented. The search is performed using final states with exactly three charged light leptons (electrons or muons) and is based on $\sqrt{s}$= 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider at CERN during the years 2015–2018, corresponding to an integrated luminosity of 139 fb –1 . A profile-likelihood fit to the event yields in multiple regions corresponding to positive and negative differences between the pseudorapidities of the charged leptons from top-quark and top-antiquark decays is used to extract the charge asymmetry. At reconstruction level, the asymmetry is found to be –0.12 ± 0.14 (stat.) ± 0.05 (syst.). An unfolding procedure is applied to convert the result at reconstruction level into a charge-asymmetry value in a fiducial volume at particle level with the result of –0.11 ± 0.17 (stat.) ± 0.05 (syst.). The Standard Model expectations for these two observables are calculated using Monte Carlo simulations with next-to-leading-order plus parton shower precision in quantum chromodynamics and including next-to-leading-order electroweak corrections. They are ${–0.084}_{–0.003}^{+0.005}$ (scale) ± 0.006 (MC stat.) and ${–0.063}_{–0.004}^{+0.007}$ (scale) ± 0.004 (MC stat.) respectively, and in agreement with the measurements.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2013

Measurements of branching fractions of leptonic and hadronic $ D_s^{+} $ meson decays and extraction of the $ D_s^{+} $ meson decay constant

We present measurements of absolute branching fractions of hadronic and lep-tonic \( D_s^{+} \) decays to K - K + π + , \( {{\overline{K}}^0} \) K + , ηπ + , μ + ν μ and τ + ν τ and report a search for the leptonic \( D_s^{+} \) → e + ν e decays. The results are obtained from a data sample of 913 fb -1 collected at or near the \( \varUpsilon \) (4 S ) and \( \varUpsilon \) (5 S ) resonances with the Belle detector at the KEKB asymmetric-energy e + e - collider. The branching fractions of hadronic decays are measured to be $$ \begin{array}{*{20}{c}} {\mathcal{B}\left( {D_s^{+}\ \to\ {K^{-}}{K^{+}}{\pi^{+}}} \right) = \left( {5.06\pm 0.15\pm 0.21} \right)\%,} \\ {\mathcal{B}\left( {D_s^{+}\ \to\ {{\overline{K}}^0}{K^{+}}} \right) = \left( {2.95\pm 0.11\pm 0.09} \right)\%,} \\ {\mathcal{B}\left( {D_s^{+}\ \to\ \eta {\pi^{+}}} \right) = \left( {1.82\pm 0.14\pm 0.07} \right)\%,} \\ \end{array} $$ where the first and second uncertainties are statistical and systematic, respectively. The branching fractions of leptonic decays are measured to be $$ \begin{array}{*{20}{c}} {\mathcal{B}\left( {D_s^{+}\ \to\ {\mu^{+}}{\nu_{\mu }}} \right)=\left( {0.531\pm 0.028\pm 0.020} \right)\%,} \\ {\mathcal{B}\left( {D_s^{+}\to {\tau^{+}}{\nu_{\tau }}} \right)=\left( {5.70\pm 0.21_{-0.30}^{-0.31 }} \right)\%,} \\ \end{array} $$ which are combined to determine the \( D_s^{+} \) meson decay constant $$ {f_{{{D_s}}}}=\left( {255.5\pm 4.2\pm 5.1} \right)\ \mathrm{MeV}. $$ We find no significant signal for \( D_s^{+} \) → e + ν e decays and set an upper limit of \( \mathcal{B}\left( {D_s^{+}\ \to\ {e^{+}}{\nu_e}} \right) \) < 1 . 0(0 . 83) × 10 -4 at 95% (90%) confidence level.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Search for charged Higgs bosons decaying via H ± → τ ± ν τ in the τ+jets and τ+lepton final states with 36 fb -1 of $pp$ collision data recorded at $ \sqrt{s}=13 $ TeV with the ATLAS experiment

Charged Higgs bosons produced either in top-quark decays or in association with a top-quark, subsequently decaying via H ± → τ ± ν τ , are searched for in 36.1 fb -1 of proton-proton collision data at $ \sqrt{s}=13 $ TeV recorded with the ATLAS detector. Depending on whether the top-quark produced together with H ± decays hadronically or leptonically, the search targets τ+jets and τ+lepton final states, in both cases with a hadronically decaying τ-lepton. No evidence of a charged Higgs boson is found. For the mass range of mH ± = 90–2000 GeV, upper limits at the 95% confidence level are set on the production cross-section of the charged Higgs boson times the branching fraction &beta;(H ± → τ ± ν τ ) in the range 4.2–0.0025 pb. In the mass range 90–160 GeV, assuming the Standard Model cross-section for $ t\bar{t} $ production, this corresponds to upper limits between 0.25% and 0.031% for the branching fraction &beta;(t→bH ± )&times;&beta;(H ± →τ ± ν τ ).

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Lepton universality violation with lepton flavor conservation in B-meson decays

Anomalies in semileptonic B-meson decays present interesting patterns that might be revealing the shape of the new physics to come. Under the assumption that neutrino and charged lepton mass terms are the only sources of flavor violation and given the hierarchy between the two, we find that charged lepton universality violation without charged lepton flavor violation naturally arises. This can account for a deficit of B + → K + μμ over B + → K + ee decays with new physics coupled predominantly to muons and a new physics scale of a few TeV. A generic prediction of this scenario is a large enhacement of tauonic B decay rates that, in particular, could accommodate an excess in B → D (*) τ ν. For the most part, the study is carried out in an effective field theory framework with an underlying SU(2) L × U(1) Y symmetry that emphasizes the model-independent correlations between low- and high-energy observables. As an example, a connection between B-decays and top physics is pointed out. To complement the discussion, all possible (spin 0 and 1) leptoquark models are matched to the low-energy field theory so that the effective analysis can be used to survey these candidates for new physics.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

First evaluation of meson and τ lepton spectra and search for heavy neutral leptons at ILC beam dump

A beam dump experiment can be seamlessly added to the proposed Inter- national Linear Collider (ILC) program because the high energy electron beam should be dumped after the collision point. The ILC beam dump experiment will provide an excellent opportunity to search for new long-lived particles. Since many of them can be produced by a rare decay of standard model particles, we evaluate spectra of the mesons and τ lepton at the decay based on the PHITS and PYTHIA8 simulations. As a motivated physics case, we study the projected sensitivity of heavy neutral leptons at the ILC beam dump experiment. The heavy neutral leptons can also be produced via deep inelastic scattering and Z boson decay at the ILC main detector, which we include in the projection. With the multi-track signal, the reach would be greatly extended in mass and coupling, even compared with the other proposed searches.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Search for charged-lepton flavour violation in top quark interactions with an up-type quark, a muon, and a $τ$ lepton in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A search for charged-lepton flavour violation (CLFV) in top quark (t) production and decay is presented. The search uses proton-proton collision data corresponding to 138 fb$^{-1}$ collected with the CMS experiment at $\sqrt{s}$ = 13 TeV. The signal consists of the production of a single top quark via a CLFV interaction or top quark pair production followed by a CLFV decay. The analysis selects events containing a hadronically decaying $τ$ lepton and a muon of opposite electric charge, as well as at least three jets, one of which is identified as originating from the fragmentation of a bottom quark. Machine learning classification techniques are used to distinguish signal from standard model background events. The results of this search are consistent with the standard model expectations. The upper limits at 95% confidence level on the branching fraction $\mathcal{B}$ for CLFV top quark decays to a muon, a $τ$ lepton, and an up or a charm quark are set at $\mathcal{B}$(t $\to$ $μτ$u) $\lt$ (0.04, 0.08, and 0.12) $\times$ 10$^{-6}$, and $\mathcal{B}$(t $\to$ $μτ$c) $\lt$ (0.81, 1.71, and 2.05) $\times$ 10$^{-6}$ for scalar, vector, and tensor-like operators, respectively.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Review of searches for vector-like quarks, vector-like leptons, and heavy neutral leptons in proton–proton collisions at $\sqrt{s} = 13$ TeV at the CMS experiment

The LHC has provided an unprecedented amount of proton–proton collision data, bringing forth exciting opportunities to address fundamental open questions in particle physics. These questions can potentially be answered by performing searches for very rare processes predicted by models that attempt to extend the standard model of particle physics. The data collected by the CMS experiment in 2015–2018 at a center-of-mass energy of 13 TeV can be used to test the standard model with high precision and potentially uncover evidence for new particles or interactions. An interesting possibility is the existence of new fermions with masses ranging from the MeV to the TeV scale. Such new particles appear in many possible extensions of the standard model and are well motivated theoretically. New fermions may explain the appearance of three generations of leptons and quarks, the mass hierarchy across these generations, and the nonzero neutrino masses. In this report, the results of searches targeting vectorlike quarks, vector-like leptons, and heavy neutral leptons at the CMS experiment are summarized. The complementarity of current searches for each type of new fermion is discussed, and combinations of several searches for vector-like quarks are presented. The discovery potential for some of these searches at the High-Luminosity LHC is also discussed.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Combination of searches for heavy resonances decaying into bosonic and leptonic final states using 36 fb - 1 of proton-proton collision data at s = 13 TeV with the ATLAS detector

Searches for new heavy resonances decaying into different pairings of W, Z, or Higgs bosons, as well as directly into leptons, are presented using a data sample corresponding to 36.1 fb -1 of pp collisions at $ \sqrt{s}=13 $ TeV collected during 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Analyses selecting bosonic decay modes in the qqqq, ννqq, ℓνqq, ℓℓqq, ℓνℓν, ℓℓνν, ℓνℓℓ, ℓℓℓℓ, qqbb, ννbb, ℓνbb, and ℓℓbb final states are combined, searching for a narrow-width resonance. Likewise, analyses selecting the leptonic ℓν and ℓℓ final states are also combined. These two sets of analyses are then further combined. No significant deviation from the Standard Model predictions is observed. Three benchmark models are tested: a model predicting the existence of a new heavy scalar singlet, a simplified model predicting a heavy vector-boson triplet, and a bulk Randall-Sundrum model with a heavy spin-2 Kaluza-Klein excitation of the graviton. Cross section limits are set at the 95% confidence level using an asymptotic approximation and are compared with predictions for the benchmark models. These limits are also expressed in terms of constraints on couplings of the heavy vector-boson triplet to quarks, leptons, and the Higgs boson. The data exclude a heavy vector-boson triplet with mass below 5.5 TeV in a weakly coupled scenario and 4.5 TeV in a strongly coupled scenario, as well as a Kaluza-Klein graviton with mass below 2.3 TeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for Quantum Black Hole Production in High-Invariant-Mass Lepton + Jet Final States Using p p Collisions at s = 8 TeV and the ATLAS Detector

This Letter presents a search for quantum black-hole production using 20.3 fb -1 of data collected with the ATLAS detector in $pp$ collisions at the LHC at $ \sqrt{s}$ = 8 TeV. The quantum black holes are assumed to decay into a final state characterized by a lepton (electron or muon) and a jet. In either channel, no event with a lepton-jet invariant mass of 3.5 TeV or more is observed, consistent with the expected background. Limits are set on the product of cross sections and branching fractions for the lepton + jet final states of quantum black holes produced in a search region for invariant masses above 1 TeV. The combined 95% confidence level upper limit on this product for quantum black holes with threshold mass above 3.5 TeV is 0.18 fb. This limit constrains the threshold quantum black-hole mass to be above 5.3 TeV in the model considered.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗