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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

Evidence for Higgs boson decay to a pair of muons

Evidence for Higgs boson decay to a pair of muons is presented. This result combines searches in four exclusive categories targeting the production of the Higgs boson via gluon fusion, via vector boson fusion, in association with a vector boson, and in association with a top quark-antiquark pair. The analysis is performed using proton-proton collision data at $ \sqrt{s} $ = 13 TeV, corresponding to an integrated luminosity of 137 fb$^{−1}$, recorded by the CMS experiment at the CERN LHC. An excess of events over the back- ground expectation is observed in data with a significance of 3.0 standard deviations, where the expectation for the standard model (SM) Higgs boson with mass of 125.38 GeV is 2.5. The combination of this result with that from data recorded at $ \sqrt{s} $ = 7 and 8 TeV, corresponding to integrated luminosities of 5.1 and 19.7 fb$^{−1}$, respectively, increases both the expected and observed significances by 1%. The measured signal strength, relative to the SM prediction, is $ {1.19}_{-0.39}^{+0.40}{\left(\mathrm{stat}\right)}_{-0.14}^{+0.15}\left(\mathrm{syst}\right) $. This result constitutes the first evidence for the decay of the Higgs boson to second generation fermions and is the most precise measurement of the Higgs boson coupling to muons reported to date.[graphic not available: see fulltext]

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2021

Search for the lepton flavor violating decay $\tau$ $\to$ 3$\mu$ in proton-proton collisions at $\sqrt{s} =$ 13 TeV

Results are reported from a search for the lepton flavor violating decay τ → 3μ in proton-proton collisions at $ \sqrt{\mathrm{s}} $ = 13 TeV. The data sample corresponds to an integrated luminosity of 33.2 fb$^{−1}$ recorded by the CMS experiment at the LHC in 2016. The search exploits τ leptons produced in both W boson and heavy-flavor hadron decays. No significant excess above the expected background is observed. An upper limit on the branching fraction ℬ(τ → 3μ) of 8.0 × 10$^{−8}$ at 90% confidence level is obtained, with an expected upper limit of 6.9 × 10$^{−8}$.[graphic not available: see fulltext]

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2022

Amplitude analysis and branching fraction measurement of the decay $D_s^{+}$ → $π^+π^0π^0$

Using a data set corresponding to an integrated luminosity of 6.32 fb –1 recorded by the BESIII detector at center-of-mass energies between 4.178 and 4.226 GeV, an amplitude analysis of the decay $D_s^{+}$ → $π^+π^0π^0$ is performed, and the relative fractions and phases of different intermediate processes are determined. The absolute branching fraction of the decay $D_s^{+}$ → $π^+π^0π^0$ is measured to be (0.50 ± 0.04 stat ± 0.02 syst )%. The absolute branching fraction of the intermediate process $D_s^{+}$ → $f_o$(980)$π^+$, $f_o$(980) → $π^oπ^o$ is determined to be (0.28 ± 0.04 stat ± 0.04 syst )%.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2022

Search for exotic decays of the Higgs boson into $b\bar{b}$ and missing transverse momentum in $pp$ collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

A search for the exotic decay of the Higgs boson (H) into a $b\bar{b}$ resonance plus missing transverse momentum is described. The search is performed with the ATLAS detector at the Large Hadron Collider using 139 fb -1 of pp collisions at $\sqrt{s}$ = 13 TeV. The search targets events from $ZH$ production in an NMSSM scenario where H → $\tilde{χ}^0_2\tilde{χ}^0_1$, with $\tilde{χ}^0_2$ → a$\tilde{χ}^0_1$, where a is a light pseudoscalar Higgs boson and $\tilde{χ}^0_{1,2}$ are the two lightest neutralinos. The decay of the a boson into a pair of b-quarks results in a peak in the dijet invariant mass distribution. The final-state signature consists of two leptons, two or more jets, at least one of which is identified as originating from a b-quark, and missing transverse momentum. Observations are consistent with Standard Model expectations and upper limits are set on the product of cross section times branching ratio for a three-dimensional scan of the masses of the $\tilde{χ}^0_2\tilde{χ}^0_1$ and a boson.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

Measurement of two-photon decay width of χ c2 (1 P ) in γγ → χ c2 (1 P ) → J/ψγ at Belle

We report the measurement of the two-photon decay width of χ c2 (1P) in two-photon processes at the Belle experiment. We analyze the process γγ → χ c2 (1P) → J/ψγ, J/ψ → ℓ + ℓ - (ℓ = e or μ) using a data sample of 971 fb -1 collected with the Belle detector at the KEKB e + e - collider. In this analysis, the product of the two-photon decay width of χ c2 (1P) and the branching fraction is determined to be ${\Gamma}_{\gamma \gamma}({\chi}_{c2}(1P))\mathcal{B}({\chi}_{c2}(1P)\to J/\psi \gamma) \mathcal{B} (J/\psi \to {\ell}^{+}{\ell}^{-})=$ 14.8 ± 0.3(stat.) ± 0.7(syst.) eV, which corresponds to Γ γγ (χ c2 (1P)) = 653 ± 13(stat.) ± 31(syst.) ± 17(B.R.) eV, where the third uncertainty is from $\mathcal{B}$ (χ c2 (1P) → J/ψγ) and $\mathcal{B}$ (J/ψ → ℓ + ℓ - ).

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2026

Scalar-induced neutrinoless double beta decay in SU(5)

We discuss the role of heavy scalar fields in mediating neutrinoless double beta decay (0νββ) within the SU(5) Grand Unified Theory framework, extended suitably to include neutrino mass. In such a minimal realistic SU(5) setup for fermion masses, the scalar contributions to 0νββ are extremely suppressed as a consequence of the proton decay bound. We circumvent this problem by imposing a discrete Z 3 symmetry. However, the scalar contributions to 0νββ remain suppressed in this SU(5) x Z 3 model due to the neutrino mass constraint. We find that the 0νββ contribution can be enhanced by extending the scalar sector with an additional 15-dimensional scalar representation with suitable Z 3 charge. Such an extension not only yields realistic fermion mass spectra but also leads to experimentally testable predictions in upcoming ton-scale 0νββ searches, which can be used as a sensitive probe of the new scalars across a broad range, from LHC-accessible scales up to ∼ 10 10 GeV.

Baryon/Lepton Number Violation↗

DOE OSTI2021

Study of ${\mathrm{B}}_{\mathrm{s}}^0$ → J/ψπ + π – K + K – decays

The decays ${\mathrm{B}}_{\mathrm{s}}^0$ → J/ψπ + π – K + K – are studied using a data set corresponding to an integrated luminosity of 9 fb –1 , collected with the LHCb detector in proton-proton collisions at centre-of-mass energies of 7, 8 and 13 TeV. The decays ${\mathrm{B}}_{\mathrm{s}}^0$ → $\mathrm{J}/{ψ \mathrm{K}}^{\ast 0}{\overline{\mathrm{K}}}^{\ast 0}$ and ${\mathrm{B}}_{\mathrm{s}}^0$ → χ c1 (3872)K + K – , where the K + K – pair does not originate from a Φ meson, are observed for the first time. Precise measurements of the ratios of branching fractions between intermediate χ c1 (3872)Φ, $\mathrm{J}/{ψ \mathrm{K}}^{\ast 0}{\overline{\mathrm{K}}}^{\ast 0}$, ψ(2S)Φ and χ c1 (3872)K + K – states are reported. A structure, denoted as X(4740), is observed in the J/ψΦ mass spectrum and, assuming a Breit-Wigner parameterisation, its mass and width are determined to be ${\displaystyle \begin{array}{c}{m}_{\mathrm{X}(4740)}=4741\pm 6\pm 6\kern0.5em \mathrm{MeV}/{c}^2,\\ {}{\Gamma}_{\mathrm{X}(4740)}=53\pm 15\pm 11\kern0.5em \mathrm{MeV},\end{array}}$ where the first uncertainty is statistical and the second is systematic. In addition, the most precise single measurement of the mass of the ${\mathrm{B}}_{\mathrm{s}}^0$ meson is performed and gives a value of ${m}_{{\mathrm{B}}_{\mathrm{s}}^0}=5366.98\pm 0.07\pm 0.13\kern0.5em \mathrm{MeV}/{c}^2.$

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Search for a CP-odd Higgs boson decaying into a heavy CP-even Higgs boson and a Z boson in the ${\ell}^{+}{\ell}^{-}t\overline{t}$ and $\nu\overline{\nu}b\overline{b}$ final states using 140 fb –1 of data collected with the ATLAS detector

A search for a heavy CP-odd Higgs boson, A, decaying into a Z boson and a heavy CP-even Higgs boson, H, is presented. It uses the full LHC Run 2 dataset of pp collisions at $\sqrt{s}$ = 13 TeV collected with the ATLAS detector, corresponding to an integrated luminosity of 140 fb –1 . The search for A → ZH is performed in the $ℓ^+ℓ^–t\bar{t}$ and $ν\bar{ν}b\bar{b}$ final states and surpasses the reach of previous searches in different final states in the region with mH > 350 GeV and mA > 800 GeV. No significant deviation from the Standard Model expectation is found. Upper limits are placed on the production cross-section times the decay branching ratios. Limits with less model dependence are also presented as functions of the reconstructed $m(t\bar{t}$) and $m(b\bar{b}$) distributions in the $ℓ^+ℓ^–t\bar{t}$ and $ν\bar{ν}b\bar{b}$ channels, respectively. In addition, the results are interpreted in the context of two-Higgs-doublet models

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Search for single-production of vector-like quarks decaying into Wb in the fully hadronic final state in pp collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

A search for T and Y vector-like quarks produced in proton-proton collisions at a centre-of-mass energy of 13 TeV and decaying into Wb in the fully hadronic final state is presented. The search uses 139 fb −1 of data collected by the ATLAS detector at the LHC from 2015 to 2018. The final state is characterised by a hadronically decaying W boson with large Lorentz boost and a b-tagged jet, which are used to reconstruct the invariant mass of the vector-like quark candidate. The main background is QCD multijet production, which is estimated using a data-driven method. Upon finding no significant excess in data, mass limits at 95% confidence level are obtained as a function of the global coupling parameter, κ. The observed lower limits on the masses of Y quarks with κ = 0.5 and κ = 0.7 are 2.0 TeV and 2.4 TeV, respectively. For T quarks, the observed mass limits are 1.4 TeV for κ = 0.5 and 1.9 TeV for κ = 0.7.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Search for a heavy charged Higgs boson decaying into a W boson and a Higgs boson in final states with leptons and b-jets in s = 13 TeV pp collisions with the ATLAS detector

This article presents a search for a heavy charged Higgs boson produced in association with a top quark and a bottom quark, and decaying into a W boson and a 125 GeV Higgs boson h. The search is performed in final states with one charged lepton, missing transverse momentum, and jets using proton-proton collision data at s$$ \sqrt{s} $$ = 13 TeV recorded with the ATLAS detector during Run 2 of the LHC at CERN. This data set corresponds to a total integrated luminosity of 140 fb−1. The search is conducted by examining the reconstructed invariant mass distribution of the Wh candidates for evidence of a localised excess in the charged Higgs boson mass range from 250 GeV to 3 TeV. No significant excess of data over the expected background is observed and 95% confidence-level upper limits between 2.8 pb and 1.2 fb are placed on the production cross-section times branching ratio for charged Higgs bosons decaying into Wh.

Aad, G↗

DOE OSTI2021

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.[graphic not available: see fulltext]

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

Study of η(1405)/η(1475) in $J/\psi \to \gamma {K}_S^0{K}_S^0{\pi}^0$ decay

Using a sample of (10 . 09 ± 0 . 04) × 10 9 J/ψ decays collected with the BESIII detector, partial wave analyses of the decay $ J/\psi \to \gamma {K}_S^0{K}_S^0{\pi}^0$ are performed within the ${K}_S^0{K}_S^0{\pi}^0$ invariant mass region below 1.6 GeV/ c 2 . The covariant tensor amplitude method is used in both mass independent and mass dependent approaches. Both analysis approaches exhibit dominant pseudoscalar and axial vector components, and show good consistency for the other individual components. Furthermore, the mass dependent analysis reveals that the ${K}_S^0{K}_S^0{\pi}^0$ invariant mass spectrum for the pseudoscalar component can be well described with two isoscalar resonant states using relativistic Breit-Wigner model, i.e., the η (1405) with a mass of $1391.7\pm {0.7}_{-0.3}^{+11.3}$ MeV/ c 2 and a width of $60.8\pm {1.2}_{-12.0}^{+5.5}$ MeV, and the η (1475) with a mass of $1507.6\pm {1.6}_{-32.2}^{+15.5}$ MeV/ c 2 and a width of $115.8\pm {2.4}_{-10.9}^{+14.8}$ MeV. The first and second uncertainties are statistical and systematic, respectively. Alternate models for the pseudoscalar component are also tested, but the description of the ${K}_S^0{K}_S^0{\pi}^0$ invariant mass spectrum deteriorates significantly.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Search for Long-Lived Heavy Neutral Leptons with Lepton Flavour Conserving or Violating Decays to a Jet and a Charged Lepton

A search for long-lived heavy neutral leptons (HNLs) is presented, which considers the hadronic final state and coupling scenarios involving all three lepton generations in the 2–20 GeV HNL mass range for the first time. Events comprising two leptons (electrons or muons) and jets are analyzed in a data sample of proton-proton collisions, recorded with the CMS experiment at the CERN LHC at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$^{−1}$. A novel jet tagger, based on a deep neural network, has been developed to identify jets from an HNL decay using various features of the jet and its constituent particles. The network output can be used as a powerful discriminating tool to probe a broad range of HNL lifetimes and masses. Contributions from background processes are determined from data. No excess of events in data over the expected background is observed. Upper limits on the HNL production cross section are derived as functions of the HNL mass and the three coupling strengths V$_{ℓN}$ to each lepton generation ℓ and presented as exclusion limits in the coupling-mass plane, as lower limits on the HNL lifetime, and on the HNL mass. In this search, the most stringent limit on the coupling strength is obtained for pure muon coupling scenarios; values of |$ {V}_{\mu \textrm{N}}^2 $| > 5 (4) × 10$^{−7}$ are excluded for Dirac (Majorana) HNLs with a mass of 10 GeV at a confidence level of 95% that correspond to proper decay lengths of 17 (10) mm.[graphic not available: see fulltext]

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Probing light quark Yukawa couplings through angularity distributions in Higgs boson decay

We propose to utilize angularity distributions in Higgs boson decay to probe light quark Yukawa couplings at e + e - colliders. Angularities $τ$ a are a class of 2-jet event shapes with variable and tunable sensitivity to the distribution of radiation in hadronic jets in the final state. Using soft-collinear effective theory (SCET), we present a prediction of angularity distributions from Higgs decaying to quark and gluon states at e + e - colliders to NNLL + $\mathcal{O}$(α s ) accuracy. Due to the different color structures in quark and gluon jets, the angularity distributions from H → $q\overline{q}$ and H → gg show different behaviors and can be used to constrain the light quark Yukawa couplings. We show that the upper limit of light quark Yukawa couplings could be probed to the level of ~ 15% of the bottom quark Yukawa coupling in the Standard Model in a conservative analysis window far away from nonperturbative effects and other uncertainties; the limit can be pushed to ≲ 7 – 9% with better control of the nonperturbative effects especially on gluon angularity distributions and/or with multiple angularities.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Constraints on standard model effective field theory for a Higgs boson produced in association with W or Z bosons in the H → $\textrm{b}\overline{\textrm{b}}$ decay channel in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A standard model effective field theory (SMEFT) analysis with dimension-six operators probing nonresonant new physics effects is performed in the Higgs-strahlung process, where the Higgs boson is produced in association with a W or Z boson, in proton-proton collisions at a center-of-mass energy of 13 TeV. The final states in which the W or Z boson decays leptonically and the Higgs boson decays to a pair of bottom quarks are considered. The analyzed data were collected by the CMS experiment between 2016 and 2018 and correspond to an integrated luminosity of 138 fb −1 . An approach designed to simultaneously optimize the sensitivity to Wilson coefficients of multiple SMEFT operators is employed. Likelihood scans as functions of the Wilson coefficients that carry SMEFT sensitivity in this final state are performed for different expansions in SMEFT. The results are consistent with the predictions of the standard model.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2026

Model-independent measurement of the Higgs boson associated production with two jets and decaying to a pair of W bosons in proton-proton collisions at $\sqrt{s}=13$ TeV

A model-independent measurement of the differential production cross section of the Higgs boson decaying into a pair of W bosons, with a final state including two jets produced in association, is presented. In the analysis, events are selected in which the decay products of the two W bosons consist of an electron, a muon, and missing transverse momentum. The model independence of the measurement is maximized by employing a discriminating variable, developed through machine learning, that is agnostic to the signal hypothesis. The analysis is based on proton-proton collision data at $\sqrt{s}=13$ TeV collected with the CMS detector from 2016–2018, corresponding to an integrated luminosity of 138 fb −1 . The production cross section is measured as a function of the difference in azimuthal angle between the two jets. The differential cross section measurements are used to constrain Higgs boson couplings within the standard model effective field theory framework.

Hadron-Hadron Scattering↗

DOE OSTI2020

Search for heavy Higgs bosons decaying to a top quark pair in proton-proton collisions at $ \sqrt{s} $ = 13 TeV

A search is presented for additional scalar (H) or pseudoscalar (A) Higgs bosons decaying to a top quark pair in proton-proton collisions at a center-of-mass energy of 13 TeV. The data set analyzed corresponds to an integrated luminosity of 35.9 fb$^{-1}$ collected by the CMS experiment at the LHC. Final states with one or two charged leptons are considered. The invariant mass of the reconstructed top quark pair system and variables that are sensitive to the spin of the particles decaying into the top quark pair are used to search for signatures of the H or A bosons. The interference with the standard model top quark pair background is taken into account. A moderate signal-like deviation compatible with an A boson with a mass of 400 GeV is observed with a global significance of 1.9 standard deviations. New stringent constraints are reported on the strength of the coupling of the hypothetical bosons to the top quark, with the mass of the bosons ranging from 400 to 750 GeV and their total relative width from 0.5 to 25%. The results of the search are also interpreted in a minimal supersymmetric standard model scenario. Values of $m_\mathrm{A}$ from 400 to 700 GeV are probed, and a region with values of $\tan\beta$ below 1.0 to 1.5, depending on $m_\mathrm{A}$, is excluded at 95% confidence level.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗