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At least 613 records · Page 34

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DOE OSTI2018

Measurement of the Ratio of the B 0 → D * – τ + ν τ and B 0 → D * – μ + ν μ Branching Fractions Using Three-Prong τ -Lepton Decays

The ratio of branching fractions R( D * – ) ≡ B( B 0 → D * – τ + ν τ )/B( B 0 → D * – μ + ν μ ) is measured using a data sample of proton-proton collisions collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of 3 fb –1 . For the first time, R( D * – ) is determined using the τ-lepton decays with three charged pions in the final state. The B 0 → D * – τ + ν τ yield is normalized to that of the B 0 → D * – π + π – π + mode, providing a measurement of B( B 0 → D * – τ + ν τ )/B( B 0 → D * – π + π – π + ) = 1.97 ± 0.13 ± 0.18, where the first uncertainty is statistical and the second systematic. The value of B( B 0 → D * – τ + ν τ )=(1.42 ± 0.094 ± 0.129 ± 0.054)% is obtained, where the third uncertainty is due to the limited knowledge of the branching fraction of the normalization mode. Using the well-measured branching fraction of the B 0 → D * – μ + ν μ decay, a value of R( D * – ) = 0.291 ± 0.019 ± 0.026 ± 0.013 is established, where the third uncertainty is due to the limited knowledge of the branching fractions of the normalization and B 0 → D * – μ + ν μ modes. This measurement is in agreement with the standard model prediction and with previous results.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Search for Heavy Neutral Leptons in Electron-Positron and Neutral-Pion Final States with the MicroBooNE Detector

We present the first search for heavy neutral leptons (HNLs) decaying into ν e + e − or ν π 0 final states in a liquid-argon time projection chamber using data collected with the MicroBooNE detector. The data were recorded synchronously with the NuMI neutrino beam from Fermilab’s main injector corresponding to a total exposure of 7.01 × 10 20 protons on target. We set upper limits at the 90% confidence level on the mixing parameter | U μ 4 | 2 in the mass ranges 10 ≤ m HNL ≤ 150 MeV for the ν e + e − channel and 150 ≤ m HNL ≤ 245 MeV for the ν π 0 channel, assuming | U e 4 | 2 = | U τ 4 | 2 = 0 . These limits represent the most stringent constraints in the mass range 35 < m HNL < 175 MeV and the first constraints from a direct search for ν π 0 decays. Published by the American Physical Society 2024

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2024

Observation of D s + → η ′ μ + ν μ , Precision Test of Lepton Flavor Universality with D s + → η ( ′ ) l + ν l , and First Measurements of D s + → η ( ′ ) μ + ν μ Decay Dynamics

By analyzing 7.33 fb − 1 of e + e − annihilation data collected at center-of-mass energies between 4.128 and 4.226 GeV with the BESIII detector, we report the observation of the semileptonic decay D s + → η ′ μ + ν μ , with a statistical significance larger than 10 σ , and the measurements of the D s + → η μ + ν μ and D s + → η ′ μ + ν μ decay dynamics for the first time. The branching fractions of D s + → η μ + ν μ and D s + → η ′ μ + ν μ are determined to be ( 2.235 ± 0.05 1 stat ± 0.05 2 syst ) % and ( 0.801 ± 0.05 5 stat ± 0.02 8 syst ) % , respectively, with precision improved by factors of 6.0 and 6.6 compared to the previous best measurements. Combined with the results for the decays D s + → η e + ν e and D s + → η ′ e + ν e , the ratios of the decay widths are examined both inclusively and in several ℓ + ν ℓ four-momentum transfer ranges. No evidence for lepton flavor universality violation is found within the current statistics. The products of the hadronic form factors f + , 0 η ( ′ ) ( 0 ) and the c → s Cabibbo-Kobayashi-Maskawa matrix element | V c s | are determined. The results based on the two-parameter series expansion are f + , 0 η ( 0 ) | V c s | = 0.452 ± 0.01 0 stat ± 0.00 7 syst and f + , 0 η ′ ( 0 ) | V c s | = 0.504 ± 0.03 7 stat ± 0.01 2 syst , which help to constrain present models on f + , 0 η ( ′ ) ( 0 ) . The forward-backward asymmetries are determined to be ⟨ A FB η ⟩ = − 0.059 ± 0.03 1 stat ± 0.00 5 syst and ⟨ A FB η ′ ⟩ = − 0.064 ± 0.07 9 stat ± 0.00 6 syst for the first time, which are consistent with the theoretical calculation. Published by the American Physical Society 2024

Physics↗

DOE OSTI2024

Measurement of Angular Coefficients of B ¯ → D * ℓ ν ¯ ℓ : Implications for | V c b | and Tests of Lepton Flavor Universality

We measure the complete set of angular coefficients J i for exclusive B ¯ → D * ℓ ν ¯ ℓ decays ( ℓ = e , μ ). Our analysis uses the full 711 fb − 1 Belle dataset with hadronic tag-side reconstruction. The results allow us to extract the form factors describing the B ¯ → D * transition and the Cabibbo-Kobayashi-Maskawa matrix element | V cb | . Using recent lattice QCD calculations for the hadronic form factors, we find | V cb | = ( 40.7 ± 0.7 ) × 10 − 3 using the Boyd-Grinstein-Lebed parametrization, compatible with determinations from inclusive semileptonic decays. We search for lepton flavor universality violation as a function of the hadronic recoil parameter w and investigate the differences of the electron and muon angular distributions. We find no deviation from standard model expectations. Published by the American Physical Society 2024

Physics↗

DOE OSTI2024

Heavy Neutral Leptons via Axionlike Particles at Neutrino Facilities

Heavy neutral leptons (HNLs) are often among the hypothetical ingredients behind nonzero neutrino masses. If sufficiently light, they can be produced and detected in fixed-target-like experiments. We show that if the HNLs belong to a richer—but rather generic—dark sector, their production mechanism can deviate dramatically from expectations associated with the standard-model weak interactions. In more detail, we postulate that the dark sector contains an axionlike particle (ALP) that naturally decays into HNLs. Since ALPs mix with the pseudoscalar hadrons, the HNL flux might be predominantly associated with the production of neutral mesons (e.g., π 0 , η ) as opposed to charge hadrons (e.g., π ± , K ± ). In this case, the physics responsible for HNL production and decay are not directly related and experiments like DUNE might be sensitive to HNLs that are too weakly coupled to the standard model to be produced via weak interactions, as is generically the case of HNLs that play a direct role in the type-I seesaw mechanism. Published by the American Physical Society 2024

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Test of Lepton Flavor Universality with B + → K + π + π − ℓ + ℓ − Decays

The first test of lepton flavor universality between muons and electrons using B + → K + π + π − ℓ + ℓ − ( ℓ = e , μ ) decays is presented. The measurement is performed with data from proton-proton collisions collected by the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb − 1 . The ratio of branching fractions between B + → K + π + π − e + e − and B + → K + π + π − μ + μ − decays is measured in the dilepton invariant-mass-squared range 1.1 < q 2 < 7.0 GeV 2 / c 4 and is found to be R K π π − 1 = 1.3 1 − 0.17 + 0.18 ( stat ) − 0.09 + 0.12 ( syst ) , in agreement with the standard model prediction. The first observation of the B + → K + π + π − e + e − decay is also reported. © 2025 CERN, for the LHCb Collaboration 2025 CERN

Aaij, R. (ORCID:0000000305331952)↗

DOE OSTI2025

Search for displaced leptons in $\sqrt{𝑠}$ =13 TeV and 13.6 TeV 𝑝⁢𝑝 collisions with the ATLAS detector

A search for leptons displaced from the primary vertex is performed with the ATLAS detector at the Large Hadron Collider. The search includes the full proton-proton collision dataset collected during Run 2 at $\sqrt{𝑠}$ = 13 TeV and a partial dataset collected during Run 3 in 2022–2023 at $\sqrt{𝑠}$ = 13.6 TeV, corresponding to integrated luminosities of 140 fb −1 and 56.3 fb −1 , respectively. Final states with displaced electrons or muons are considered, and novel triggers introduced in Run 3 are employed that use large impact parameter tracking to reconstruct displaced tracks with low momentum. In addition, photon reconstruction and multivariate techniques are employed to broaden the sensitivity to channels with large background rates or highly displaced electrons, respectively. The results are consistent with the Standard Model background expectations and are used to set model-independent limits on the production of displaced electrons and muons. The analysis is also interpreted in the context of a gauge-mediated supersymmetry breaking model with pair-produced long-lived sleptons and a dark sector model with pair-produced chargino-like states. The results include 95% confidence level exclusions of selectrons with lifetimes from 4 ps to 60 ns and a mass of 150 GeV, and exclusions of selectrons, smuons, and staus with a lifetime of 0.3 ns for masses up to 740, 830, and 440 GeV, respectively. Dark charginos with masses up to 380 GeV are excluded for a mass difference with the neutral state of 40 GeV, and mass differences down to 17 GeV are excluded for dark charginos with a 100 GeV mass.

supersymmetric models↗

DOE OSTI2024

HTS Accelerator Magnets Conceptual Design for Future Lepton Colliders

There is an interest in designing superconducting magnet systems for future lepton circular colliders. This application requires many low-field iron-dominated dipole and quadrupole magnets. Conventional room-temperature magnets are often used because of their low field, low total current, and low power losses. However, high electricity bills for large accelerators drive magnet design to superconductivity. High-temperature superconducting (HTS) magnets can substantially reduce energy losses in magnet systems. The present study investigated the conceptual design of HTS dipole and quadrupole magnets operating in persistent current mode. Energy is transferred into the magnet from an external detachable power source. A continuously circulating current generates a stable magnetic field. The iron-dominated magnet system concept was investigated using OPERA3D code, and the results confirmed the proposed approach’s validity.

43 PARTICLE ACCELERATORS↗

DOE OSTI2015

Gravitating lepton bag model

The Kerr–Newman (KN) black hole (BH) solution exhibits the external gravitational and electromagnetic field corresponding to that of the Dirac electron. For the large spin/mass ratio, a ≫ m, the BH loses horizons and acquires a naked singular ring creating two-sheeted topology. This space is regularized by the Higgs mechanism of symmetry breaking, leading to an extended particle that has a regular spinning core compatible with the external KN solution. We show that this core has much in common with the known MIT and SLAC bag models, but has the important advantage of being in accordance with the external gravitational and electromagnetic fields of the KN solution. A peculiar two-sheeted structure of Kerr’s gravity provides a framework for the implementation of the Higgs mechanism of symmetry breaking in configuration space in accordance with the concept of the electroweak sector of the Standard Model. Similar to other bag models, the KN bag is flexible and pliant to deformations. For parameters of a spinning electron, the bag takes the shape of a thin rotating disk of the Compton radius, with a ring–string structure and a quark-like singular pole formed at the sharp edge of this disk, indicating that the considered lepton bag forms a single bag–string–quark system.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2019

Neutrino Oscillation Processes with a Change of Lepton Flavor in Quantum Field-Theoretical Approach

The oscillating probabilities of lepton flavor changing neutrino oscillation processes, where neutrinos are detected by weak charged-current and neutral-current interactions, are calculated in a quantum field-theoretical approach to neutrino oscillations based on a modification of the Feynman propagator in the momentum representation. The approach is most similar to the standard Feynman diagram technique in the momentum representation. It is found that the oscillating distance-dependent probabilities of detecting an electron in experiments with neutrino production in the muonic decay of π{sup +}-meson and the detection of the produced neutrino by charged-current and neutral-current interactions exactly coincide with the corresponding probabilities calculated in the standard approach.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2019

Opportunities for Lattice QCD in quark and lepton flavor physics

This document is one of a series of white papers from the USQCD Collaboration. Here, we discuss opportunities for Lattice QCD in quark and lepton flavor physics. New data generated at Belle II, LHCb, BES III, NA62, KOTO, and Fermilab E989, combined with precise calculations of the relevant hadronic physics, may reveal what lies beyond the Standard Model. We outline a path toward improvements of the precision of existing Lattice QCD calculations and discuss groundbreaking new methods that allow Lattice QCD to access new observables.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2021

Probing charged lepton flavor violation with a positron beam at CEBAF (JLAB)

The addition of a high intensity 11 GeV polarized positron beam at the Continuous Electron Beam Accelerator Facility (CEBAF) at JLAB would allow for a search of Charged Lepton Flavor Violation (CLFV) via the process $e^+N \rightarrow \mu^+ X$. The proposed Solenoidal Large Intensity Detector (SoLID) spectrometer, in the configuration with muon chambers, would be ideal for such CLFV searches. Various new physics scenarios, including the phenomenologically convenient Leptoquark (LQ) framework, predict CLFV rates that are within reach of current or planned experiments. A positron beam with instantaneous luminosity, ${\cal L}\sim 10^{38}$ cm$^{-2}$s$^{-1}$, could improve on existing HERA limits by two or three orders of magnitude. The availability of positron beam polarization would also allow for distentangling CLFV effects mediated by left-handed vs. right-handed LQs.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2014

Searches for electroweak production of charginos, neutralinos, and sleptons decaying to leptons and W, Z, and Higgs bosons in pp collisions at 8 TeV

Our searches for the direct electroweak production of supersymmetric charginos, neutralinos, and sleptons in a variety of signatures with leptons and W, Z, and Higgs bosons are presented. Results are based on a sample of proton-proton collision data collected at center-of-mass energy √s=8TeV with the CMS detector in 2012, corresponding to an integrated luminosity of 19.5 fb -1 . These observed event rates are in agreement with expectations from the standard model. Finally, these results probe charginos and neutralinos with masses up to 720 GeV, and sleptons up to 260 GeV, depending on the model details.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2016

Search for lepton flavour violating decays of heavy resonances and quantum black holes to an eμ pair in proton-proton collisions at √s = 8 TeV

A search for narrow resonances decaying to an electron and a muon is presented. The $\mathrm {e}$ ${\mu }$ mass spectrum is also investigated for non-resonant contributions from the production of quantum black holes (QBHs). The analysis is performed using data corresponding to an integrated luminosity of 19.7 $~\text {fb}^\text {-1}$ collected in proton-proton collisions at a centre-of-mass energy of 8 $~\text {TeV}$ with the CMS detector at the LHC. With no evidence for physics beyond the standard model in the invariant mass spectrum of selected $\mathrm {e}\mu $ pairs, upper limits are set at 95 $\%$ confidence level on the product of cross section and branching fraction for signals arising in theories with charged lepton flavour violation. In the search for narrow resonances, the resonant production of a $\mathrm {\tau }$ sneutrino in R-parity violating supersymmetry is considered. The $\mathrm {\tau }$ sneutrino is excluded for masses below 1.28 $~\text {TeV}$ for couplings $\lambda _{132}=\lambda _{231}=\lambda '_{311}=0.01$ , and below 2.30 $~\text {TeV}$ for $\lambda _{132}=\lambda _{231}=0.07$ and $\lambda '_{311}=0.11$ . These are the most stringent limits to date from direct searches at high-energy colliders. In addition, the resonance searches are interpreted in terms of a model with heavy partners of the ${\mathrm {Z}} $ boson and the photon. In a framework of TeV-scale quantum gravity based on a renormalization of Newton’s constant, the search for non-resonant contributions to the $\mathrm {e}$ ${\mu }$ mass spectrum excludes QBH production below a threshold mass $M_{\mathrm {th}}$ of 1.99 $~\text {TeV}$ . In models that invoke extra dimensions, the bounds range from 2.36 $~\text {TeV}$ for one extra dimension to 3.63 $~\text {TeV}$ for six extra dimensions. Furthermore, this is the first search for QBHs decaying into the $\mathrm {e}$ ${\mu }$ final state.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2016

Forward–backward asymmetry of Drell–Yan lepton pairs in pp collisions at $\sqrt{s}$ = 8 TeV

A measurement of the forward-backward asymmetry $A_{\mathrm{FB}}$ of oppositely charged lepton pairs ($\mu\mu$ and $\mathrm{ ee } $) produced via $\mathrm{ Z }/\gamma^*$ boson exchange in pp collisions at $\sqrt{s} =$ 8 TeV is presented. The data sample corresponds to an integrated luminosity of 19.7 fb$^{-1}$ collected with the CMS detector at the LHC. The measurement of $A_{\mathrm{FB}}$ is performed for dilepton masses between 40 GeV and 2 TeV and for dilepton rapidity up to 5. As a result, the $A_{\mathrm{FB}}$ measurements as a function of dilepton mass and rapidity are compared with the standard model predictions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2016

Search for the lepton flavour violating decay $\mu ^+ \rightarrow \mathrm {e}^+ \gamma $ with the full dataset of the MEG experiment: MEG Collaboration

Our final results of the search for the lepton flavour violating decay μ+→e+γ based on the full dataset collected by the MEG experiment at the Paul Scherrer Institut in the period 2009–2013 and totalling 7.5×1014 stopped muons on target are presented. Furthermore, there was not a significant excess of events observed in the dataset with respect to the expected background and a new upper limit on the branching ratio of this decay of B(μ+→e+γ)<4.2×10-13 (90 % confidence level) is established, which represents the most stringent limit on the existence of this decay to date.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2017

Averages of b -hadron, c -hadron, and τ-lepton properties as of summer 2016

Here, this article reports world averages of measurements of b-hadron, c-hadron, and τ-lepton properties obtained by the Heavy Flavor Averaging Group using results available through summer 2016. For the averaging, common input parameters used in the various analyses are adjusted (rescaled) to common values, and known correlations are taken into account. The averages include branching fractions, lifetimes, neutral meson mixing parameters, $C\!P$ violation parameters, parameters of semileptonic decays, and Cabbibo–Kobayashi–Maskawa matrix elements.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2017

Search for direct top squark pair production in final states with two leptons in $\sqrt{s} = 13$ TeV pp collisions with the ATLAS detector

Here, the results of a search for direct pair production of top squarks in events with two opposite-charge leptons (electrons or muons) are reported, using 36.1 fb –1 of integrated luminosity from proton–proton collisions at √s=13 TeV collected by the ATLAS detector at the Large Hadron Collider. To cover a range of mass differences between the top squark t ~ and lighter supersymmetric particles, four possible decay modes of the top squark are targeted with dedicated selections: the decay t ~ →bχ ~ ± 1 into a b-quark and the lightest chargino with χ ~ ± 1 →Wχ ~0 1 , the decay t ~ →tχ ~0 1 into an on-shell top quark and the lightest neutralino, the three-body decay t ~ →bWχ ~0 1 and the four-body decay t ~ →bℓνχ ~0 1 . No significant excess of events is observed above the Standard Model background for any selection, and limits on top squarks are set as a function of the t ~ and χ ~0 1 masses. The results exclude at 95% confidence level t ~ masses up to about 720 GeV, extending the exclusion region of supersymmetric parameter space covered by previous searches.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗