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DOE OSTI2020

Properties of 187 Ta Revealed through Isomeric Decay

Mass-separated 187 Ta114 in a high-spin isomeric state has been produced for the first time by multi-nucleon transfer reactions, employing an argon gas stopping cell and laser ionisation. Here, internal $γ$ rays revealed a $T$ 1/2 = 7.3±0.9 s isomer at 1778±1 keV, which decays through a rotational band with perturbations associated with the approach to a prolate-oblate shape transition. Model calculations show less influence from triaxiality compared to heavier elements in the same mass region. The isomer decay reduced $E$2 hindrance factor, $f$ $ν$ = 27±1, supports the interpretation that axial symmetry is approximately conserved.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2020

Many-Body Signatures of Collective Decay in Atomic Chains

Fully inverted atoms placed at exactly the same location synchronize as they deexcite, and light is emitted in a burst (known as “Dicke’s superradiance”). We investigate the role of finite interatomic separation on correlated decay in mesoscopic chains and provide an understanding in terms of collective jump operators. We show that the superradiant burst survives at small distances, despite Hamiltonian dipole-dipole interactions. However, for larger separations, competition between different jump operators leads to dephasing, suppressing superradiance. Collective effects are still significant for arrays with lattice constants of the order of a wavelength, and lead to a photon emission rate that decays nonexponentially in time. We calculate the two-photon correlation function and demonstrate that emission is correlated and directional, as well as sensitive to small changes in the interatomic distance. Furthermore, these features can be measured in current experimental setups, and are robust to realistic imperfections.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2021

New Fe 59 Stellar Decay Rate with Implications for the Fe 60 Radioactivity in Massive Stars

The discrepancy between observations from γ-ray astronomy of the 60 Fe/ 26 Al γ-ray flux ratio and recent calculations is an unresolved puzzle in nuclear astrophysics. The stellar β-decay rate of 59 Fe is one of the major nuclear uncertainties impeding us from a precise prediction. The important Gamow-Teller strengths from the low-lying states in 59 Fe to the 59 Co ground state are measured for the first time using the exclusive measurement of the 59 Co(t,3He+γ) 59 Fe charge-exchange reaction. The new stellar decay rate of 59 Fe is a factor of 3.5±1.1 larger than the currently adopted rate at T=1.2 GK. Stellar evolution calculations show that the 60 Fe production yield of an 18 solar mass star is decreased significantly by 40% when using the new rate. Our result eliminates one of the major nuclear uncertainties in the predicted yield of 60 Fe and alleviates the existing discrepancy of the 60 Fe/ 26 Al ratio.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2022

Disentangling Pauli Blocking of Atomic Decay from Cooperative Radiation and Atomic Motion in a 2D Fermi Gas

The observation of Pauli blocking of atomic spontaneous decay via direct measurements of the atomic population requires the use of long-lived atomic gases where quantum statistics, atom recoil, and cooperative radiative processes are all relevant. In this work, we develop a theoretical framework capable of simultaneously accounting for all these effects in the many-body quantum degenerate regime. We apply it to atoms in a single 2D pancake or arrays of pancakes featuring an effective Λ level structure (one excited and two degenerate ground states). We identify a parameter window in which a factor of 2 extension in the atomic lifetime clearly attributable to Pauli blocking should be experimentally observable in deeply degenerate gases with ~10 3 atoms. We experimentally observe a suppressed excited-state decay rate, fully consistent with the theory prediction of an enhanced excited-state lifetime, on the 1 S 0 – 3 P 1 transition in 87 Sr atoms.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2023

Limits to Gauge Coupling in the Dark Sector Set by the Nonobservation of Instanton-Induced Decay of Super-Heavy Dark Matter in the Pierre Auger Observatory Data

Instantons, which are nonperturbative solutions to Yang-Mills equations, provide a signal for the occurrence of quantum tunneling between distinct classes of vacua. They can give rise to decays of particles otherwise forbidden. Using data collected at the Pierre Auger Observatory, we search for signatures of such instanton-induced processes that would be suggestive of super-heavy particles decaying in the Galactic halo. These particles could have been produced during the post-inflationary epoch and match the relic abundance of dark matter inferred today. The nonobservation of the signatures searched for allows us to derive a bound on the reduced coupling constant of gauge interactions in the dark sector: α X ≲ 0.09, for 10 9 ≲ M X /GeV < 10 19 . Conversely, we obtain that, for instance, a reduced coupling constant α X = 0.09 excludes masses M X ≳ 3 x 10 13 GeV. In the context of dark matter production from gravitational interactions alone, we illustrate how these bounds are complementary to those obtained on the Hubble rate at the end of inflation from the nonobservation of tensor modes in the cosmological microwave background.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

First Measurement of the Michel Parameter ξ ' in the τ - → μ - ν ¯ μ ν τ Decay at Belle

We report the first measurement of the Michel parameter ξ' in the $\tau$ - →μ - ⁢ $\overline{v}$ μ ⁢v $\tau$ decay with a new method proposed just recently. The measurement is based on the reconstruction of the $\tau$ - →μ -⁢ $\overline{v}$ μ v $\tau$ events with subsequent muon decay in flight in the Belle central drift chamber. The analyzed data sample of 988 fb -1 collected by the Belle detector corresponds to approximately 912×10 6 $\tau$ + ⁢$\tau$ - pairs. We measure ξ'=0.22±0.94⁢(stat)±0.42⁢(syst), which is in agreement with the standard model prediction of ξ' =1. Statistical uncertainty dominates in this study, being a limiting factor, while systematic uncertainty is well under control. Our analysis proved the practicability of this promising method and its prospects for further precise measurement in future experiments.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

Search for Higgs Boson Decay to a Charm Quark-Antiquark Pair in Proton-Proton Collisions at s = 13 TeV

A search for the standard model Higgs boson decaying to a charm quark-antiquark pair, $H → c\bar{c}$, produced in association with a leptonically decaying $V (W \text{or} Z)$ boson is presented. The search is performed with proton-proton collisions at $\sqrt{s}=13\,\text {TeV}$ collected by the CMS experiment, corresponding to an integrated luminosity of 138 fb -1 . Novel charm jet identification and analysis methods using machine learning techniques are employed. The analysis is validated by searching for $Z → c\bar{c}$ in $VZ$ events, leading to its first observation at a hadron collider with a significance of 5.7 standard deviations. The observed (expected) upper limit on $σ(VH)\mathscr{B}(H → c\bar{c}$) is 0.94 (0.50$^{+0.22}_{– 0.15}$)pb at 95% confidence level (C.L.), corresponding to 14 (7.6$^{+3.4}_{–2.3}$) times the standard model prediction. For the Higgs-charm Yukawa coupling modifier, $κ_c$, the observed (expected) 95% C.L. interval is 1.1 < |$κ_c$| < 5.5 (|$κ_c$| < 3.4), the most stringent constraint to date.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

Measurement of C P Violation in B 0 → K S 0 π 0 Decays at Belle II

We report a measurement of the C P -violating parameters C and S in B 0 → K S 0 π 0 decays at Belle II using a sample of 387 × 10 6 B B ¯ events recorded in e + e − collisions at a center-of-mass energy corresponding to the ϒ ( 4 S ) resonance. These parameters are determined by fitting the proper decay-time distribution of a sample of 415 signal events. We obtain C = − 0.0 4 − 0.15 + 0.14 ± 0.05 and S = 0.7 5 − 0.23 + 0.20 ± 0.04 , where the first uncertainties are statistical and the second are systematic. Published by the American Physical Society 2023

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

Search for a Nonrelativistic Boson in Two-Body Antimuon Decay

We demonstrate the feasibility of probing the charged lepton-flavor-violating decay μ + → e + X 0 for the presence of a slow-moving neutral boson X 0 capable of undergoing gravitational binding to large structures and, as such, able to participate in some cosmological scenarios. A short exposure to surface antimuons from beam line M20 at TRIUMF generates a branching ratio limit of ≲ 10 –5 . This is comparable to or better than previous searches for this channel, although in a thus-far-unexplored region of X 0 phase space very close to the kinematic limit of the decay, where m X 0 approaches m μ+ . The future improved sensitivity of the method using a customized p -type point-contact germanium detector is described.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Observation of Cabibbo-Suppressed Two-Body Hadronic Decays and Precision Mass Measurement of the Ω c 0 Baryon

The first observation of the singly Cabibbo-suppressed Ω c 0 → Ω − K + and Ω c 0 → Ξ − π + decays is reported, using proton-proton collision data at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb − 1 , collected with the LHCb detector between 2016 and 2018. The branching fraction ratios are measured to be B ( Ω c 0 → Ω − K + ) B ( Ω c 0 → Ω − π + ) = [ 6.08 ± 0.51 ( stat ) ± 0.40 ( syst ) ] % , B ( Ω c 0 → Ξ − π + ) B ( Ω c 0 → Ω − π + ) = [ 15.81 ± 0.87 ( stat ) ± 0.44 ( syst ) ± 0.16 ( ext ) ] % . In addition, using the Ω c 0 → Ω − π + decay channel, the Ω c 0 baryon mass is measured to be M ( Ω c 0 ) = 2695.28 ± 0.07 ( stat ) ± 0.27 ( syst ) ± 0.30 ( ext ) MeV , improving the precision of the previous world average by a factor of 4. © 2024 CERN, for the LHCb Collaboration 2024 CERN

Aaij, R. (ORCID:0000000305331952)↗

DOE OSTI2024

Amplitude Analysis of the B 0 → K * 0 μ + μ − Decay

An amplitude analysis of the B 0 → K * 0 μ + μ − decay is presented using a dataset corresponding to an integrated luminosity of 4.7 fb − 1 of p p collision data collected with the LHCb experiment. For the first time, the coefficients associated to short-distance physics effects, sensitive to processes beyond the standard model, are extracted directly from the data through a q 2 -unbinned amplitude analysis, where q 2 is the μ + μ − invariant mass squared. Long-distance contributions, which originate from nonfactorizable QCD processes, are systematically investigated, and the most accurate assessment to date of their impact on the physical observables is obtained. The pattern of measured corrections to the short-distance couplings is found to be consistent with previous analyses of b - to s -quark transitions, with the largest discrepancy from the standard model predictions found to be at the level of 1.8 standard deviations. The global significance of the observed differences in the decay is 1.4 standard deviations. © 2024 CERN, for the LHCb Collaboration 2024 CERN

Aaij, R. (ORCID:0000000305331952)↗

DOE OSTI2024

Search for Baryon Number Violation in Top Quark Production and Decay Using Proton-Proton Collisions at $\sqrt{𝑠}$ = 13 TeV

A search is presented for baryon number violating interactions in top quark production and decay. The analysis uses data from proton-proton collisions at a center-of-mass energy of 13 TeV, collected with the CMS detector at the LHC with an integrated luminosity of 138 fb −1 . Candidate events are selected by requiring two oppositely charged leptons (electrons or muons) and exactly one jet identified as originating from a bottom quark. Multivariate discriminants are used to separate the signal from the background. No significant deviation from the standard model prediction is observed. Upper limits are placed on the strength of baryon number violating couplings. For the first time the production of single top quarks via baryon number violating interactions is studied. This allows the search to set the most stringent constraints to date on the branching fraction of the top quark decay to a lepton, an up-type quark (𝑢 or 𝑐), and a down-type quark (𝑑, 𝑠, or 𝑏). The results improve the previous bounds by 3 to 6 orders of magnitude based on the fermion flavor combination of the baryon number violating interactions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Gamma Decay of the 154 Sm Isovector Giant Dipole Resonance: Smekal-Raman Scattering as a Novel Probe of Nuclear Ground-State Deformation

𝛾 decays of the isovector giant dipole resonance (IVGDR) of the deformed nucleus 154 Sm were measured using 2$^{+}_{1}$-Smekal-Raman and elastic scattering of linearly polarized, quasimonochromatic photon beams. The two scattering processes were disentangled through their distinct angular distributions. Their branching ratio and cross sections were determined at six excitation energies covering the 154 Sm IVGDR. Both agree with the predictions of the geometrical model for the IVGDR and confirm 𝛾 decay as an observable sensitive to the structure of the resonance. Consequently, the data place strong constraints on the nuclear shape, including the degree of triaxiality. The derived 154 Sm shape parameters 𝛽 = 0.2925⁢(25) and 𝛾 = 5.0⁢(15)° agree well with other measurements and recent Monte Carlo shell-model calculations.

150 ≤ A ≤ 189↗

DOE OSTI2025

Measurement of the Branching Fraction Ratios R ( D + ) and R ( D * + ) Using Muonic τ Decays

The branching fraction ratios of B ¯ 0 → D + τ − ν ¯ τ and B ¯ 0 → D * + τ − ν ¯ τ decays are measured with respect to their muonic counterparts, using a data sample corresponding to an integrated luminosity of 2.0 fb − 1 collected by the LHCb experiment in proton-proton collisions at s = 13 TeV . The reconstructed final states are formed by combining D + mesons with τ − → μ − ν ¯ μ ν τ candidates, where the D + is reconstructed via the D + → K − π + π + decay. The results are R ( D + ) = 0.249 ± 0.043 ± 0.047 , R ( D * + ) = 0.402 ± 0.081 ± 0.085 , where the first uncertainties are statistical and the second systematic. The two measurements have a correlation coefficient of − 0.39 and are compatible with the standard model. © 2025 CERN, for the LHCb Collaboration 2025 CERN

Aaij, R. (ORCID:0000000305331952)↗

DOE OSTI2025

Sensitivity of JWST to eV-Scale Decaying Axion Dark Matter

The recently launched James Webb Space Telescope can resolve eV-scale emission lines arising from dark matter decay. We forecast the end-of-mission sensitivity to the decay of axions, a leading dark matter candidate, in the Milky Way using the blank-sky observations expected during standard operations. Searching for unassociated emission lines will constrain axions in the mass range 0.18 to 2.6 eV with axion-photon couplings g a γ γ ≳ 5.5 × 10 − 12 GeV − 1 . In particular, these results will constrain nucleophobic QCD axions to masses ≲ 0.2 eV . Published by the American Physical Society 2025

Roy, Sandip (ORCID:0000000276387454)↗

DOE OSTI2025

Observation of D + → η ′ μ + ν μ and First Experimental Study of D + → η ′ ℓ + ν ℓ Decay Dynamics

Using 20.3 fb − 1 of e + e − collision data collected at the center-of-mass energy 3.773 GeV with the BESIII detector, we report the first observation of the semileptonic decay D + → η ′ μ + ν μ with significance of 8.6 σ including systematic uncertainties, and an improved measurement of D + → η ′ e + ν e . The branching fractions of D + → η ′ μ + ν μ and D + → η ′ e + ν e are determined to be ( 1.92 ± 0.2 8 stat ± 0.0 8 syst ) × 10 − 4 and ( 1.79 ± 0.1 9 stat ± 0.0 7 syst ) × 10 − 4 , respectively. The ratio of the two branching fractions is determined to be R μ / e = 1.07 ± 0.1 9 stat ± 0.0 3 syst , which agrees with the theoretical expectation of lepton flavor universality within the standard model. From an analysis of the D + → η ′ ℓ + ν ℓ decay dynamics, the product of the hadronic form factor f + η ′ ( 0 ) and the Cabibbo-Kobayashi-Maskawa matrix element | V c d | is measured for the first time, giving f + η ′ ( 0 ) | V c d | = ( 5.92 ± 0.5 6 stat ± 0.1 3 syst ) × 10 − 2 . The η − η ′ mixing angle in the quark flavor basis is determined to be ϕ P = ( 39.8 ± 0.8 stat ± 0.3 syst ) ° . Published by the American Physical Society 2025

Ablikim, M.↗

DOE OSTI2025

Test of Lepton Flavor Universality with B s 0 → ϕ ℓ + ℓ − Decays

Lepton flavor universality in rare b → s transitions is tested for the first time using B s 0 meson decays. The measurements are performed using p p collision data collected by the LHCb experiment between 2011 and 2018, corresponding to a total integrated luminosity of 9 fb − 1 . Branching fraction ratios between the B s 0 → ϕ e + e − and B s 0 → ϕ μ + μ − decays are measured in three regions of dilepton mass squared, q 2 , with 0.1 < q 2 < 1.1 , 1.1 < q 2 < 6.0 , and 15 < q 2 < 19 GeV 2 / c 4 . The results agree with the standard model expectation of lepton flavor universality. © 2025 CERN, for the LHCb Collaboration 2025 CERN

Aaij, R. (ORCID:0000000305331952)↗

DOE OSTI2025

Search for a Hidden Sector Scalar from Kaon Decay in the Dimuon Final State at ICARUS

We present a search for long-lived particles (LLPs), produced in kaon decays, that decay to two muons inside the ICARUS neutrino detector. This channel would be a signal of hidden sector models that can address outstanding issues in particle physics such as the strong CP problem and the microphysical origin of dark matter. The search is performed with data collected in the Neutrinos at the Main Injector (NuMI) beam at Fermilab corresponding to 2.41 × 10 20 protons-on-target. No new physics signal is observed, and we set world leading limits on heavy QCD axions, as well as for the Higgs portal scalar among dedicated searches. Limits are also presented in a model-independent way applicable to any new physics model predicting the process 𝐾 → 𝜋 + 𝑆⁡(→ 𝜇⁢𝜇), for a LLP 𝑆. This result is the first search for new physics performed with the ICARUS detector at Fermilab. It paves the way for the future program of LLP searches at ICARUS.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗