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All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Searches for lepton-flavour-violating decays of the Higgs boson into $eτ$ and $μτ$ in $ \sqrt{s}$ = 13 TeV pp collisions with the ATLAS detector

This paper presents direct searches for lepton flavour violation in Higgs boson decays, H → eτ and H → μτ, performed using data collected with the ATLAS detector at the LHC. The searches are based on a data sample of proton-proton collisions at a centre-of-mass energy $\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of 138 fb –1 . Leptonic (τ → ℓν ℓ ν τ ) and hadronic (τ → hadrons ν τ ) decays of the τ-lepton are considered. Two background estimation techniques are employed: the MC-template method, based on data-corrected simulation samples, and the Symmetry method, based on exploiting the symmetry between electrons and muons in the Standard Model backgrounds. No significant excess of events is observed and the results are interpreted as upper limits on lepton-flavour-violating branching ratios of the Higgs boson. The observed (expected) upper limits set on the branching ratios at 95% confidence level, $\mathcal{B}$(H → eτ) < 0.20% (0.12%) and $\mathcal{B}$(H → μτ ) < 0.18% (0.09%), are obtained with the MC-template method from a simultaneous measurement of potential H → eτ and H → μτ signals. The best-fit branching ratio difference, $\mathcal{B}$(H → μτ) → $\mathcal{B}$(H → eτ), measured with the Symmetry method in the channel where the τ-lepton decays to leptons, is (0.25 ± 0.10)%, compatible with a value of zero within 2.5σ.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Quantum entanglement and Bell inequality violation in semi-leptonic top decays

Abstract Quantum entanglement is a fundamental property of quantum mechanics. Recently, studies have explored entanglement in the$$ t\overline{t} $$ t t ¯ system at the Large Hadron Collider (LHC) when both the top quark and anti-top quark decay leptonically. Entanglement is detected via correlations between the polarizations of the top and anti-top and these polarizations are measured through the angles of the decay products of the top and anti-top. In this work, we propose searching for evidence of quantum entanglement in the semi-leptonic decay channel where the final state includes one lepton, one neutrino, twob-flavor tagged jets, and two light jets from theWdecay. We find that this channel is both easier to reconstruct and has a larger effective quantity of data than the fully leptonic channel. As a result, the semi-leptonic channel is 60% more sensitive to quantum entanglement and a factor of 3 more sensitive to Bell inequality violation, compared to the leptonic channel. In 139 fb −1 (3 ab −1 ) of data at the LHC (HL-LHC), it should be feasible to measure entanglement at a precision of ≲ 3% (0.7%). Detecting Bell inequality violation, on the other hand, is more challenging. With 300 fb −1 (3 ab −1 ) of integrated luminosity at the LHC Run-3 (HL-LHC), we expect a sensitivity of 1.3σ(4.1σ). In our study, we utilize a realistic parametric fitting procedure to optimally recover the true angular distributions from detector effects. Compared to unfolding this procedure yields more stable results.

Physics↗
DOE OSTI2024

Indirect constraints on third generation baryon number violation

Abstract The non-observation of baryon number violation suggests that the scale of baryon-number violating interactions at zero temperature is comparable to the GUT scale. However, the pertinent measurements involve hadrons made of the first-generation quarks, such as protons and neutrons. One may therefore entertain the idea that new flavour physics breaks baryon number at a much lower scale, but only in the coupling to a third generation quark, leading to observable baryon-number violatingb-hadron decay rates. In this paper we show that indirect constraints on the new physics scale Λ BNV from the existing bounds on the proton lifetime do not allow for this possibility. For this purpose we consider the three dominant proton decay channelsp→$$ {\ell}^{+}{\nu}_{\ell}\overline{\nu} $$ ℓ + ν ℓ ν ¯ ,p→$$ {\pi}^{+}\overline{\nu} $$ π + ν ¯ andp→π 0 ℓ + mediated by a virtual bottom quark.

Physics↗
DOE OSTI2021

Unitarity violation and the geometry of Higgs EFTs

We derive the scale of unitarity violation from the geometry of Effective Field Theory (EFT) extensions of the Standard Model Higgs sector. The high-energy behavior of amplitudes with more than four scalar legs depends on derivatives of geometric invariants with respect to the physical Higgs field h, such that higher-point amplitudes begin to reconstruct the scalar manifold away from our vacuum. In theories whose low-energy limit can be described by the Higgs EFT (HEFT) but not the Standard Model EFT (SMEFT), non-analyticities in the vicinity of our vacuum limit the radius of convergence of geometric invariants, leading to unitarity violation at energies below 4πv. Our results unify approaches to the HEFT/SMEFT dichotomy based on unitarity, analyticity, and geometry, and more broadly illustrate the sense in which observables probe the geometry of an EFT. Along the way, we provide novel basis-independent results for Goldstone/Higgs boson scattering amplitudes expressed in terms of geometric covariant quantities.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Search for heavy Higgs bosons with flavour-violating couplings in multi-lepton plus b-jets final states in pp collisions at 13 TeV with the ATLAS detector

A search for new heavy scalars with flavour-violating decays in final states with multiple leptons and b-tagged jets is presented. The results are interpreted in terms of a general two-Higgs-doublet model involving an additional scalar with couplings to the top-quark and the three up-type quarks (ρ tt , ρ tc , and ρ tu ). The targeted signals lead to final states with either a same-sign top-quark pair, three top-quarks, or four top-quarks. The search is based on a data sample of proton-proton collisions at $\sqrt{s}$ = 13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb –1 . Events are categorised depending on the multiplicity of light charged leptons (electrons or muons), total lepton charge, and a deep-neural-network output to enhance the purity of each of the signals. Masses of an additional scalar boson m H between 200 – 630 GeV with couplings ρ tt = 0.4, ρ tc = 0.2, and ρ tu = 0.2 are excluded at 95% confidence level. Additional interpretations are provided in models of R-parity violating supersymmetry, motivated by the recent flavour and (g – 2) μ anomalies.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Effects of the CPT-even and Lorentz violation on the Bhabha scattering at finite temperature

In this paper a Lorentz-violating CPT-even non-minimal coupling term is considered. A new interaction term between fermions and photons emerges. In this context, the differential cross-section for Bhabha scattering at finite temperature is calculated. The temperature effects are introduced using the Thermo-Field Dynamics (TFD) formalism. It is shown that the differential cross-section is changed due to both effects, Lorentz violation and finite temperature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

The Mu2e experiment — Searching for charged lepton flavor violation

The Mu2e experiment will search for a Standard Model violating rate of neutrinoless conversion of a muon into an electron in the presence of an aluminum nucleus. Observation of this charged lepton flavor violating process would be an unambiguous sign of new physics. Mu2e will improve upon previous searches for this process by four orders of magnitude. This requires the world’s highest-intensity muon beam, a detector system capable of efficiently reconstructing the 105 MeV/c conversion electron signal, and minimizing sensitivity to background events. A pulsed 8 GeV proton beam strikes a target, producing pions that decay into muons. Beam outside the pulse must be suppressed to < 1 0 – 10 to reduce beam-related backgrounds. The muon beam is guided from the production target along the transport system and onto the aluminum stopping target. Conversion electrons leave the stopping target and propagate inside a solenoidal magnetic field to the tracker and electromagnetic calorimeter. The tracker is a system of straw tube panels filled with Ar/CO 2 at 1 atm that tracks particles inside of a solenoidal B-field and measures their momenta with ~ 100 keV/c resolution to resolve signal events from decay-in-orbit backgrounds. The CsI calorimeter provides E / p and is used to seed the track reconstruction algorithm with σ E / E ~ 10 % and σ t < 500 ps . Additionally, a novel cosmic ray veto with greater than 99.99% efficiency brings the expected number of background events to fewer than one over three years of running. Finally, to normalize the experiment, the stopping target monitor measures the rate of capture photons from muons incident on the stopping target by using a system of high-purity germanium and lanthanum bromide scintillators.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2022

QCD analysis of Δ S = 0 hadronic parity violation

We present a QCD analysis of the effective weak Hamiltonian at hadronic energy scales for strangeness-nonchanging (ΔS = 0) hadronic processes. Performing a leading-order renormalization group analysis in QCD from the W to the O(2 GeV) energy scale, we derive the pertinent effective Hamiltonian for hadronic parity violation, including the effects of both neutral and charged weak currents. We compute the complete renormalization group evolution of all isosectors and the evolution through heavy-flavor thresholds for the first time. We show that the additional four-quark operators that enter below the W mass scale from QCD operator mixing effects form a closed set, and they result in a 12 x 12 anomalous dimension matrix. Computing the resulting effective Hamiltonian and comparing to earlier results, we affirm the importance of operator mixing effects and find, as an example, that the parity-violating pion-nucleon coupling constant, using the factorization Ansatz and an assessment of the pertinent quark charge of the nucleon in lattice QCD at the 2 GeV scale, is in better agreement with recent experiments.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Searches for violation of Lorentz invariance in top quark pair production using dilepton events in 13 TeV proton-proton collisions

A search for violation of Lorentz invariance in the production of top quark pairs ($t\bar{t}$) is presented. The measured normalized differential $t\bar{t}$production cross section, as a function of the sidereal time, is examined for potential modulations induced by Lorentz-invariance breaking operators in an effective field theory extension of the standard model (SM). The cross section is measured from collision events collected by the CMS detector at a center-of-mass-energy of 13 TeV, corresponding to an integrated luminosity of 77.8 fb −1 , and containing one electron and one muon. The results are found to be compatible with zero, in agreement with the SM, and are used to place upper limits at 68% confidence level on the magnitude of the Lorentz-violating couplings ranging from 1–8 × 10 −3 . This is the first precision test of the isotropy in special relativity with top quarks at the LHC, restricting further the bounds on such couplings by up to two orders of magnitude with respect to previous searches conducted at the Tevatron.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Flavor-changing Lorentz and CPT violation in muonic atoms

Flavor-changing signatures of Lorentz and CPT violation involving muon-electron conversions in muonic atoms are studied using effective field theory. Constraints on coefficients for Lorentz violation at parts in 10 -12 GeV −1 for flavor-changing electromagnetic muon decays and parts in 10 -13 GeV −2 for flavor-changing 4-point quark-lepton interactions are extracted using existing data from the SINDRUM II experiment at the Paul Scherrer Institute. Estimates are provided for sensitivities attainable in the forthcoming experiments Mu2e at Fermilab and COMET at the Japan Proton Accelerator Complex.

Alan Kostelecký, V. [Indiana Univ., Bloomington, I↗
DOE OSTI2026

Violation of detailed balance in non-equilibrium magnons observed by inelastic neutron scattering

Traditional inelastic neutron scattering (INS) characterizes excitations—such as phonons and magnons—in condensed matter systems at thermodynamic equilibrium. However, the most intriguing and puzzling many-body effects in open quantum systems often emerge from dissipative dynamics that are inherently out of equilibrium. Here, we use a combination of laser pumping and INS to experimentally observe long-lived nonequilibrium magnons in a two-dimensional (2D) square-lattice Heisenberg antiferromagnet. These nonequilibrium magnons manifest themselves as a violation of detailed balance in the dynamic structure factor and reach steady states under periodic driving, analogous to nonequilibrium steady states in driven dissipative systems. Furthermore, we show that the violation of detailed balance reflects the quantum-mechanical nature of the underlying dynamical system, where out-of-time-ordered correlations of creation and annihilation operators do not satisfy commutation relations. The in operando INS technique developed here provides a new approach to studying nonequilibrium magnons in prototypical 2D quantum magnets and can be extended to other systems, including one-dimensional spin chains and topological many-body spin systems, where nonequilibrium effects are widespread and rich in discovery potential.

Hua, Chengyun [Oak Ridge National Laboratory (ORNL↗
DOE OSTI2023

N -representability violations in truncated equation-of-motion coupled-cluster methods

One-electron reduced density matrices (1RDMs) from equation-of-motion (EOM) coupled-cluster with single and double excitations (CCSD) calculations are analyzed to assess their N-representability (i.e., whether they are derivable from a physical N-electron state). We identify EOM-CCSD stationary states whose 1RDMs violate either ensemble-state N-representability conditions or pure-state conditions known as generalized Pauli constraints. As such, these 1RDMs do not correspond to any physical N-electron state. Unphysical states are also encountered in the course of time-dependent EOM-CC simulations; when an external field drives transitions between a pair of stationary states with pure-state N-representable 1RDMs, the 1RDM of the time-dependent state can violate ensemble-state conditions. Furthermore, these observations point to potential challenges in interpreting the results of time-dependent EOM-CCSD simulations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Measurement of the parity-violating asymmetry in the 𝑁 → Δ transition at low 𝑄 2

Here, we report the measurement of the parity-violating asymmetry in the 𝑁 → Δ transition via the 𝑒 − + 𝑝 → 𝑒 − + Δ + reaction at two different kinematic points with low four-momentum transfer 𝑄 2 . Measurements were made with incident electron beam energies of 0.877 and 1.16 GeV, corresponding to 𝑄 2 values of 0.0111 and 0.0208 (GeV/c) 2 , respectively. These measurements put constraints on a low-energy constant in the weak Lagrangian, 𝑑 Δ , corresponding to a parity-violating electric-dipole transition matrix element. This matrix element has been shown to be large in the strangeness-changing channel, via weak hyperon decays such as Σ + → 𝑝⁢𝛾. The measurements reported here constrain 𝑑 Δ in the strangeness-conserving channel. The final asymmetries were −0.65±1.00⁢(stat.)±1.02⁢(syst.) ppm (parts per million) for 0.877 GeV and −3.59±0.82⁢(stat.)±1.33⁢(syst.) ppm for 1.16 GeV. With these results we deduce a small value for 𝑑 Δ , consistent with zero, in the strangeness-conserving channel, in contrast to the large value for 𝑑 Δ previously reported in the strangeness-changing channel.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

Search for charged lepton flavor violating 𝑍 and 𝑍′ boson decays in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A search for flavor violating decays of the 𝑍 boson to charged leptons is performed using data from proton-proton collisions at $\sqrt{s}$ =13 TeV collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 138 fb −1 . Each of the decays 𝑍 → 𝑒⁢𝜇, 𝑍 → 𝑒⁢𝜏, and 𝑍 → 𝜇⁢𝜏 is considered. The data are consistent with the backgrounds expected from standard model processes. For the 𝑍 → 𝑒⁢𝜇 channel the observed (expected) 95% confidence level upper limit on the branching fraction is 1.9⁢(2.0) × 10 −7 , which is the most stringent direct limit to date on this process; the corresponding limits for the 𝑍 → 𝑒⁢𝜏 and 𝑍 → 𝜇⁢𝜏 channels are 13.8⁢(11.4) × 10 −6 and 12.0⁢(5.3) × 10 −6 , respectively. Additionally, the 𝑒⁢𝜇 final state is used to search for lepton flavor violating decays of 𝑍′ resonances in the mass range from 110 to 500 GeV. No significant excess is observed above the predicted background levels.

Hayrapetyan, A. [Yerevan Physics Institute]↗
DOE OSTI2022

Causality violations in realistic simulations of heavy-ion collisions

Causality is violated in the early stages of state-of-the-art heavy-ion hydrodynamic simulations. Such violations are present in up to 75% of the fluid cells in the initial time and only after 2–3 fm/c of evolution do we find that 50% of the fluid cells are definitely causal. Superluminal propagation reaches up to 15% the speed of light in some of the fluid cells. The inclusion of pre-equilibrium evolution significantly reduces the number of acausal cells. Here our findings suggests that relativistic causality may place constraints on the available parameter space of heavy-ion collision simulations when factored into more thorough statistical analyses.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Search for Lorentz-invariance violation with the first KATRIN data

Some extensions of the Standard Model of particle physics allow for Lorentz invariance and charge-parity-time invariance violations. In the neutrino sector strong constraints have been set by neutrino-oscillation and time-of-flight experiments. However, some Lorentz-invariance-violating parameters are not accessible via these probes. In this work, we focus on the parameters (a$^{(3)}_{of}$)00, (a$^{(3)}_{of}$)10, and (a$^{(3)}_{of}$)11 which would manifest themselves in a nonisotropic β-decaying source as a sidereal oscillation and an overall shift of the spectral endpoint. Based on the data of the first scientific run of the KATRIN experiment, we set the first 90% confidence-level limit on (a$^{(3)}_{of}$)11| of < 0.9 × 10 –6 GeV to 3.7 × 10 –6 GeV, depending on the phase. Furthermore, we derive new constraints on (a$^{(3)}_{of}$)00 and (a$^{(3)}_{of}$)10.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Flavor-violating ALPs, electron g − 2 , and the Electron-Ion Collider

We revisit the possibility that light axionlike particles (ALPs) with lepton-flavor-violating couplings could give significant contributions to the electron’s anomalous magnetic moment g e − 2 . Unlike flavor diagonal lepton-ALP couplings, which are exclusively axial, lepton-flavor-violating couplings can have arbitrary chirality. Focusing on the e − τ ALP coupling, we find that the size of the contribution to g e − 2 depends strongly on the chirality of the coupling. A significant part of the parameter space for which such a coupling can explain experimental anomalies in g e − 2 can be probed at the Electron-Ion Collider, which is uniquely sensitive to the chirality of the coupling using the polarization of the electron beam. Published by the American Physical Society 2024

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Flavor-violating Higgs and Z boson decays at a future circular lepton collider

Recent advances in b , c , and s quark tagging coupled with novel statistical analysis techniques will allow future high energy and high statistics electron-positron colliders, such as the FCC-ee, to place phenomenologically relevant bounds on flavor violating Higgs and Z decays to quarks. We assess the FCC-ee reach for Z / h → b s , c u decays as a function of jet tagging performance. We also update the standard model (SM) predictions for the corresponding branching ratios, as well as the indirect constraints on the flavor violating Higgs and Z couplings to quarks. Using the type III two Higgs doublet model as an example of beyond the standard model physics, we show that the searches for h → b s , c u decays at FCC-ee can probe new parameter space not excluded by indirect searches. We also reinterpret the FCC-ee reach for Z → b s , c u in terms of the constraints on models with vectorlike quarks. Published by the American Physical Society 2024

Kamenik, Jernej F. (ORCID:0000000151838637)↗