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At least 307 records · Page 17

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DOE OSTI2024

Search for the lepton flavor violating τ → 3μ decay in proton-proton collisions at s = 13 TeV

A search for the lepton flavor violating τ → 3 μ decay is performed using proton-proton collision events at a center-of-mass energy of 13 TeV collected by the CMS experiment at the LHC in 2017–2018, corresponding to an integrated luminosity of 97.7 fb − 1 . Tau leptons produced in both heavy-flavor hadron and W boson decays are exploited in the analysis. No evidence for the decay is observed. The results of this search are combined with an earlier null result based on data collected in 2016 to obtain a total integrated luminosity of 131 fb − 1 . The observed (expected) upper limits on the branching fraction B ( τ → 3 μ ) at confidence levels of 90 and 95% are 2.9 × 10 − 8 ( 2.4 × 10 − 8 ) and 3.6 × 10 − 8 ( 3.0 × 10 − 8 ), respectively.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Measurement of the production cross section of a Higgs boson with large transverse momentum in its decays to a pair of τ leptons in proton-proton collisions at s = 13 TeV

A measurement of the production cross section of a Higgs boson with transverse momentum greater than 250GeV is presented where the Higgs boson decays to a pair of τ leptons. It is based on proton-proton collision data collected by the CMS experiment at the CERN LHC at a center-of-mass energy of 13TeV. The data sample corresponds to an integrated luminosity of 138 fb − 1 . Because of the large transverse momentum of the Higgs boson the τ leptons from its decays are boosted and produced spatially close, with their decay products overlapping. Therefore, a dedicated algorithm was developed to reconstruct and identify them. The observed (expected) significance of the measured signal with respect to the standard model background-only hypothesis is 3.5 (2.2) standard deviations. The product of the production cross section and branching fraction is measured to be 1.64 − 0.54 + 0.68 times the standard model expectation. The fiducial differential production cross section is also measured as functions of the Higgs boson and leading jet transverse momenta. This measurement extends the probed large-transverse-momentum region in the ττ final state beyond 600GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Test of lepton flavor universality in B ± → K ± μ + μ – and B ± → K ± e + e – decays in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A test of lepton flavor universality in B ± → K ± μ + μ – and B ± → K ± e + e – decays, as well as a measurement of differential and integrated branching fractions of a nonresonant B ± → K ± μ + μ – decay are presented. The analysis is made possible by a dedicated data set of proton-proton collisions at $\sqrt{s}$ = 13 TeV recorded in 2018, by the CMS experiment at the LHC, using a special high-rate data stream designed for collecting about 10 billion unbiased b hadron decays. The ratio of the branching fractions B(B ± → K ± μ + μ – ) to B(B ± → K ± e + e – ) is determined from the measured double ratio R(K) of these decays to the respective branching fractions of the B ± → J/ψK ± with J/ψ → μ + μ – and e + e – decays, which allow for significant cancellation of systematic uncertainties. The ratio R(K) is measured in the range 1.1 < q 2 < 6.0 GeV 2 , where q is the invariant mass of the lepton pair, and is found to be R(K) = 0.78$^{+0.47}_{-0.23}$, in agreement with the standard model expectation R(K) ≈ 1. This measurement is limited by the statistical precision of the electron channel. The integrated branching fraction in the same q 2 range, B(B ± → K ± μ + μ – ) = (12.42 ± 0.68) x 10 -8 , is consistent with the present world-average value and has a comparable precision.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2019

Long live the Higgs factory: Higgs decays to long-lived particles at future lepton colliders

We initiate the study of exotic Higgs decays to long-lived particles (LLPs) at proposed future lepton colliders, focusing on scenarios with displaced hadronic final states. Our analysis entails a realistic tracker-based search strategy involving the reconstruction of displaced secondary vertices and the imposition of selection cuts appropriate for eliminating the largest irreducible backgrounds. The projected sensitivity is broadly competitive with that of the LHC and potentially superior at lower LLP masses. In addition to forecasting branching ratio limits, which may be freely interpreted in a variety of model frameworks, we interpret our results in the parameter space of a Higgs portal Hidden Valley and various incarnations of neutral naturalness, illustrating the complementarity between direct searches for LLPs and precision Higgs coupling measurements at future lepton colliders.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

Heterogeneous Graph Neural Network for identifying hadronically decayed tau leptons at the High Luminosity LHC

Here, we present a new algorithm that identifies reconstructed jets originating from hadronic decays of tau leptons against those from quarks or gluons. No tau lepton reconstruction algorithm is used. Instead, the algorithm represents jets as heterogeneous graphs with tracks and energy clusters as nodes and trains a Graph Neural Network to identify tau jets from other jets. Different attributed graph representations and different GNN architectures are explored. We propose to use differential track and energy cluster information as node features and a heterogeneous sequentially-biased encoding for the inputs to final graph-level classification.

47 OTHER INSTRUMENTATION↗

DOE OSTI2018

Measurement of the $τ$ Michel parameters $\bar{\eta}$ and $\xi\kappa$ in the radiative leptonic decay $\tau^- \rightarrow \ell^- \nu_{\tau} \bar{\nu}_{\ell}\gamma$

We present a measurement of the Michel parameters of the τ lepton, η ¯ and ξκ, in the radiative leptonic decay τ - →ℓ - ν τ ν ¯ ℓγ using 711~fb -1 of collision data collected with the Belle detector at the KEKB e + e - collider. The Michel parameters are measured in an unbinned maximum likelihood fit to the kinematic distribution of e + e - →τ + τ - →(π + π 0 ν ¯ τ)(ℓ - ν τ ν ¯ ℓγ) (ℓ=e or μ). The measured values of the Michel parameters are η ¯ =-1.3±1.5±0.8 and ξκ=0.5±0.4±0.2, where the first error is statistical and the second is systematic. This is the first measurement of these parameters. These results are consistent with the Standard Model predictions within their uncertainties and constrain the coupling constants of the generalized weak interaction.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Test of lepton flavor universality with measurements of 𝑅⁡(𝐷 + ) and 𝑅⁡(𝐷* + ) using semileptonic 𝐵 tagging at the Belle II experiment

We report measurements of the ratios of branching fractions ℛ⁡(𝐷 (*)+ ) = ℬ⁡($\bar{𝐵}$ 0 → 𝐷 (*)+ ⁢𝜏 −⁢ $\bar{𝜈}$ 𝜏 )/ℬ⁡($\bar{𝐵}$ 0 → 𝐷 (*)+ ⁢ℓ − $\bar{𝜈}$ ℓ ), where ℓ denotes either an electron or a muon. These ratios test the universality of the charged-current weak interaction. The results are based on a 365 fb −1 data sample collected with the Belle II detector at the SuperKEKB 𝑒 + ⁢𝑒 − collider, which operates at a center-of-mass energy corresponding to the ϒ⁡(4⁢𝑆) resonance, just above the threshold for $𝐵\bar{𝐵}$ production. Signal candidates are reconstructed by selecting events in which the companion 𝐵 meson from the ϒ⁡(4⁢𝑆) → $𝐵\bar{𝐵}$ decay is identified in semileptonic modes. The 𝜏 lepton is reconstructed via its leptonic decays. We obtain ℛ⁡(𝐷 + ) = 0.418$^{+0.075}_{−0.073}$⁢(stat)$^{+0.049}_{−0.056}$⁢(syst) and ℛ⁡(𝐷 *+ ) = 0.306$^{+0.035}_{−0.033}$⁢(stat)$^{+0.016}_{−0.018⁢}$(syst), which are consistent with world average values. Accounting for the correlation between them, these values differ from the Standard Model expectation by a collective significance of 1.7 standard deviations.

bottom quark↗

DOE OSTI2025

One- and two-body current contributions to lepton-nucleus scattering

Modeling lepton-nucleus scattering with the accuracy required to extract neutrino-oscillation parameters from long- and short-baseline experiments necessitates retaining most quantum-mechanical effects. One such effect is the interference between one- and two-body current operators in the transition currents, which has been known to enhance the cross sections, especially in transverse kinematics. Here, we incorporate such interference in the spectral function formalism, which combines relativistic currents and kinematics with an accurate description of the initial target state. Our analysis of lepton-scattering off 12 C demonstrates that interference effects appreciably enhance the transverse electromagnetic response functions and the flux-folded neutrino-nucleus cross section, in both cases, improving the agreement with experimental data. We discuss the impact on the neutrino-oscillation program and the determination of nucleon axial form factors.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Electron-positron, parton-parton, and photon-photon production of τ -lepton pairs: Anomalous magnetic and electric dipole moments spin effects

Anomalous contributions to the electric and magnetic dipole moments of the τ lepton from new physics scenarios have brought renewed interest in the development of new charge-parity violating signatures in τ -pair production at Belle II energies, and also at higher energies of the Large Hadron Collider and the Future Circular Collider. In this paper, we discuss the effects of spin correlations, including transverse degrees of freedom, in the τ -pair production and decay. These studies include calculating analytical formulas, obtaining numerical results, and building semirealistic observables sensitive to the transverse spin correlations induced by the dipole moments of the τ lepton. The effects of such anomalous contributions to the dipole moments are introduced on top of precision simulations of e − e + → τ − τ + , q q ¯ → τ − τ + and γ γ → τ − τ + processes, involving multibody final states. The τ decays are simulated along with radiative corrections, in particular electroweak box contributions of W W and Z Z exchanges are taken into account. Respective extensions of the Standard Model amplitudes and the reweighting algorithms are implemented into the Monte Carlo, which is used to simulate τ -pair production in e − e + collisions, and the aupinner program, which is used to reweight events with τ pair produced in p p collisions. Published by the American Physical Society 2024

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Search for charged-lepton flavor violation in the production and decay of top quarks using trilepton final states in proton-proton collisions at $\sqrt{s}$ =13 TeV

A search is performed for charged-lepton flavor violating processes in top quark (𝑡) production and decay. The data were collected by the CMS experiment from proton-proton collisions at a center-of-mass energy of 13 TeV and correspond to an integrated luminosity of 138 fb −1 . The selected events are required to contain one opposite-sign electron-muon pair, a third charged lepton (electron or muon), and at least one jet of which no more than one is associated with a bottom quark. Boosted decision trees are used to distinguish signal from background, exploiting differences in the kinematics of the final states particles. The data are consistent with the standard model expectation. Upper limits at 95% confidence level are placed in the context of effective field theory on the Wilson coefficients, which range between 0.024–0.424 TeV −2 depending on the flavor of the associated light quark and the Lorentz structure of the interaction. These limits are converted to upper limits on branching fractions involving up (charm) quarks, 𝑡 → 𝑒⁢𝜇⁢𝑢 (𝑡 → 𝑒⁢𝜇⁢𝑐), of 0.032⁢(0.498) × 10 −6 , 0.022⁢(0.369) × 10 −6 , and 0.012⁢(0.216) × 10 −6 for tensorlike, vectorlike, and scalarlike interactions, respectively.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Search for heavy neutral resonances decaying to tau lepton pairs in proton-proton collisions at $\sqrt{s}$=13 TeV

A search for heavy neutral gauge bosons (𝑍′) decaying into a pair of tau leptons is performed in proton-proton collisions at $\sqrt{s}$ =13 TeV at the CERN LHC. The data were collected with the CMS detector and correspond to an integrated luminosity of 138 fb −1 . The observations are found to be in agreement with the expectation from standard model processes. Limits at 95% confidence level are set on the product of the 𝑍′ production cross section and its branching fraction to tau lepton pairs for a range of 𝑍′ boson masses. For a narrow resonance in the sequential standard model scenario, a 𝑍′ boson with a mass below 3.5 TeV is excluded. This is the most stringent limit to date from this type of search.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Test of lepton flavor universality in semileptonic 𝐵$^+_𝑐$ meson decays in proton-proton collisions at $\sqrt{𝑠}$ = 13 TeV

A measurement of the ratio of branching fractions 𝑅⁢(𝐽/𝜓) =ℬ⁡(𝐵$^+_𝑐$ →𝐽/𝜓⁢𝜏 + ⁢𝜈 𝜏 )/ℬ⁡(𝐵$^+_𝑐$ →𝐽/𝜓⁢𝜇 + ⁢𝜈 𝜇 ) in the 𝐽/𝜓 →𝜇 + ⁢𝜇 − , 𝜏 + →𝜇 + ⁢𝜈 𝜇 ⁢$\bar{𝜈}$ 𝜏 decay channel is presented. This measurement uses a sample of proton-proton collision data collected at a center-of-mass energy of 13 TeV by the CMS experiment in 2018, corresponding to an integrated luminosity of 59.7 fb −1 . The measured ratio, 𝑅⁢(𝐽/𝜓) = 0.1⁢7$^{+0.18}_{−0.17}⁢$(stat)$^{+0.21}_{−0.22⁢}$(syst)$^{+0.19}_{−0.18}$⁢(theo) =0.17 ± 0.33, agrees with the value of 0.2582 ± 0.0038 predicted by the standard model, which assumes lepton flavor universality. By testing lepton flavor universality, this measurement is a probe of new physics using 𝐵$^+_𝑐$ mesons, which are currently only produced at the LHC.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

Observation of $\tau$ lepton pair production in ultraperipheral lead-lead collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV

We present an observation of photon-photon production of $\tau$ lepton pairs in ultraperipheral lead-lead collisions. The measurement is based on a data sample with an integrated luminosity of 404 $\mu$b$^{-1}$ collected by the CMS experiment at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The $\gamma\gamma$$\to$$\tau^+\tau^-$ process is observed for $\tau\tau$ events with a muon and three charged hadrons in the final state. The measured fiducial cross section is $\sigma(\gamma\gamma$$\to$$\tau^+\tau^-)$ = 4.8 $\pm$ 0.6 (stat) $\pm$ 0.5 (syst) $\mu$b, in agreement with leading-order QED predictions. Using $\sigma(\gamma\gamma$$\to$$\tau^+\tau^-)$, we estimate a model-dependent value of the anomalous magnetic moment of the $\tau$ lepton of $a_\tau$ = 0.001 $^{+0.055}_{-0.089}$.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2026

Lattice calculation of light meson radiative leptonic decays

In this work, we perform a lattice QCD calculation of the branching ratios and the form factors\r\nof radiative leptonic decays P →ℓνℓγ (P= π,K) using Nf = 2+1 domain wall fermion ensembles\r\ngenerated by the RBC and UKQCD collaborations at the physical pion mass. We adopt the\r\ninfinite-volume reconstruction (IVR) method, which extends lattice data to infinite volume and\r\neffectively controls the finite-volume effects. This study represents a first step toward a complete\r\ncalculation of radiative corrections to leptonic decays using the IVR method, including both real\r\nphoton emissions and virtual photon loops. For decays involving a final-state electron, collinear\r\nradiative corrections, enhanced by the large logarithmic factors such as ln(m2\r\nπ/m2e) and ln(m2K/m2e), can reach the level of O(10%) and are essential at the current level of theoretical and experimental precision. After including these corrections, our result for π →eνeγ agrees with the PIBETA measurement; for K →eνeγ, our results are consistent with the KLOE data and exhibit a 1.7σtension with E36; and for K →µνµγ, where radiative corrections are negligible, our results confirm the previously observed discrepancies between lattice results and the ISTRA/OKA measurements at large photon energies, and with the E787 results at large muon–photon angles.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2017

Dark matter and baryogenesis from non-Abelian gauged lepton number

A simple model is constructed based on the gauge symmetry SU(3) $c$ × SU(2) $L$ × U(1) $Y$ × SU(2) ℓ , with only the leptons transforming nontrivially under SU(2) ℓ . The extended symmetry is broken down to the Standard Model gauge group at TeV-scale energies. We show that this model provides a mechanism for baryogenesis via leptogenesis in which the lepton number asymmetry is generated by SU(2) ℓ instantons. In conclusion, the theory also contains a dark matter candidate — the SU(2) ℓ partner of the right-handed neutrino.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2018

Leptonic dark matter with scalar dilepton mediator

Here, a simple and elegant mechanism is proposed to resolve the problem of having a light scalar mediator for self-interacting dark matter and the resulting disruption to the cosmic microwave background (CMB) at late times by the former’s enhanced Sommerfeld production and decay. The crucial idea is to have Dirac neutrinos with the conservation of U(1) or $Z_N$ lepton number extended to the dark sector. The simplest scenario consists of scalar or fermion dark matter with unit lepton number accompanied by a light scalar dilepton mediator, which decays to two neutrinos

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2022

Testing Lepton Flavor Universality with Pion, Kaon, Tau, and Beta Decays

We present an overview of searches for violation of lepton flavor universality with a focus on low energy precision probes using π, K, τ, and nuclear beta decays. We review the current experimental results, summarize the theoretical status within the context of the Standard Model, and discuss future prospects (both experimental and theoretical). We review the implications of these measurements for physics beyond the Standard Model by performing a global model-independent fit to modified W couplings to leptons and four-fermion operators. We also discuss new physics in the context of simplified models and review Standard Model extensions with a focus on those that can explain a possible deviation from unitarity of the Cabibbo–Kobayashi–Maskawa quark mixing matrix.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2021

Using Tau Leptons as a Tool for Studying the Higgs in the SM and Beyond

In the Standard Model, the mass generation of fermions is achieved through the Higgs Yukawa coupling. There exists several more fundamental theories, such as super- symmetry, that could imply deviations of the couplings of the observed Higgs boson to down-type fermions, such as the τ lepton. Therefore, it is necessary to demonstrate the direct coupling of the Higgs boson to the fundamental fermions and to demonstrate the proportionality of the Higgs boson coupling strength to their mass. Furthermore, any deviations from Standard Model expectation could hint at beyond the Standard Model physics. We summarize a measurement of Higgs to Tau leptons at CMS using a simplified template cross section scheme as well as a differential measurement as a function of Higgs transverse momentum, leading jet transverse momentum and the number of jets present in the event. We also summarize recent calorimeter trigger related activities which will help to improve Higgs to fermion measurements in the future runs at the LHC.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗