Constraining axionlike particles from rare pion decays
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72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
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Simulating rare kaon decays K + → π + ℓ + ℓ − using domain wall lattice QCD with physical light quark masses
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72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Universality of weak interactions and rare pion decays
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Atomic, Nuclear and Particle Physics↗
Analysis of Neutral B -Meson Decays into Two Muons
Branching fraction and effective lifetime measurements of the rare decay B s 0 → μ + μ - and searches for the decays B 0 → μ + μ - and B s 0 → μ + μ - γ are reported using proton-proton collision data collected with the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV, corresponding to a luminosity of 9 fb - 1 . The branching fraction B ( B s 0 → μ + μ - ) = ( 3.0 9 - 0.43 - 0.11 + 0.46 + 0.15 ) × 10 - 9 and the effective lifetime τ ( B s 0 → μ + μ - ) = 2.07 ± 0.29 ± 0.03 ps are measured, where the first uncertainty is statistical and the second systematic. No significant signal for B 0 → μ + μ - and B s 0 → μ + μ - γ decays is found and upper limits B ( B 0 → μ + μ - ) < 2.6 × 10 - 10 and B ( B s 0 → μ + μ - γ ) < 2.0 × 10 - 9 at the 95% C.L. are determined, where the latter is limited to the range m μ μ > 4.9 GeV / c 2 . The results are in agreement with the standard model expectations.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for the Higgs boson decays to a ρ 0 , ϕ, or K ⁎0 meson and a photon in proton-proton collisions at $\sqrt{s} = 13$ TeV
Three rare decay processes of the Higgs boson to a ρ(770) 0 , Φ(1020), or K ⁎ (892) 0 meson and a photon are searched for using $\sqrt{s} = 13$ TeV proton-proton collision data collected by the CMS experiment at the LHC. Events are selected assuming the mesons decay into a pair of charged pions, a pair of charged kaons, or a charged kaon and pion, respectively. Depending on the Higgs boson production mode, different triggering and reconstruction techniques are adopted. The analyzed data sets correspond to integrated luminosities up to 138 fb -1 , depending on the reconstructed final state. After combining various data sets and categories, no significant excess above the background expectations is observed. Upper limits at 95% confidence level on the Higgs boson branching fractions into ρ(770) 0 $γ$, Φ(1020)$γ$, and K ⁎ (892) 0 are determined to be 3.7 x 10 -4 , 3.0 x 10 -4 , and 3.0 x 10 -4 , respectively. In case of the ρ(770) 0 $γ$ and Φ(1020)$γ$ channels, these are the most stringent experimental limits to date.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
New strategies for new physics search with Λ b → Λ ν ν ¯ decays
We examine the new physics sensitivity of the rare decay Λ b → Λ ν ν ¯ , which can be accessible at future Z -pole machines like Future Circular Collider and Circular Electron Positron Collider. We find that the longitudinal polarization of Λ b baryons produced in Z decays introduces a novel observable, the forward-backward asymmetry A FB ↑ in the angle between the outgoing Λ momentum and the Λ b spin. We provide Standard Model predictions for the Λ b → Λ ν ν ¯ branching ratio and A FB ↑ , and show that future precision measurements of these observables are complementary and probe new physics scales comparable to other b → s ν ν ¯ and b → s ℓ + ℓ − processes. We also show that the zero crossing of the forward-backward asymmetry offers a robust test of form factor calculations independent of new physics. Published by the American Physical Society 2025
Altmannshofer, Wolfgang (ORCID:0000000316212561)↗
Search for Dark Photons in Rare $Z$ Boson Decays with the ATLAS Detector
A search for events with a dark photon produced in association with a dark Higgs boson via rare decays of the standard model Z boson is presented, using 139 fb –1 of $\sqrt{s}$ = 13 TeV proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider. The dark boson decays into a pair of dark photons, and at least two of the three dark photons must each decay into a pair of electrons or muons, resulting in at least two same-flavor opposite-charge lepton pairs in the final state. The data are found to be consistent with the background prediction, and upper limits are set on the dark photon’s coupling to the dark Higgs boson times the kinetic mixing between the standard model photon and the dark photon, α D ϵ 2 , in the dark photon mass range of [5, 40] GeV except for the $Υ$ mass window [8.8, 11.1] GeV. This search explores new parameter space not previously excluded by other experiments.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Fluorescence decay of rare-earth ions in crystals.
Decay of fluorescent transitions excited by monochromatic radiations for most rare earths in hexagonal lanthanum chloride and other lattices
TRANSITION PROBABILITY↗
Addressing R D(*) , R K(*) , muon g -2 and ANITA anomalies in a minimal R-parity violating supersymmetric framework
We analyze the recent hints of lepton flavor universality violation in both charged-current and neutral-current rare decays of B mesons in an R -parity-violating supersymmetric scenario. Motivated by simplicity and minimality, we had earlier postulated the third-generation superpartners to be the lightest (calling the scenario “RPV3”) and explicitly showed that it preserves gauge coupling unification and of course has the usual attribute of naturally addressing the Higgs radiative stability. Here we show that both R D ( * ) and R K ( * ) flavor anomalies can be addressed in this RPV3 framework. Interestingly, this scenario may also be able to accommodate two other seemingly disparate anomalies, namely, the longstanding discrepancy in the muon ( g - 2 ), as well as the recent anomalous upgoing ultrahigh-energy Antarctic Impulsive Transient Antenna events. Based on symmetry arguments, we consider three different benchmark points for the relevant RPV3 couplings and carve out the regions of parameter space where all (or some) of these anomalies can be simultaneously explained. We find it remarkable that such overlap regions exist, given the plethora of precision low-energy and high-energy experimental constraints on the minimal model parameter space. The third-generation superpartners needed in this theoretical construction are all in the 1–10 TeV range, accessible at the LHC and/or next-generation hadron colliders. We also discuss some testable predictions for the lepton-flavor-violating decays of the tau lepton and B mesons for the current and future B -physics experiments, such as LHCb and Belle II. Complementary tests of the flavor anomalies in the high- p T regime in collider experiments such as the LHC are also discussed.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Light quark loops in ${K}^{\pm}\to {\pi}^{\pm}\nu \overline{\nu}$ from vector meson dominance and update on the Kaon Unitarity Triangle
We use vector meson dominance to calculate non-perturbative contributions to the branching ratio of the rare decay $K$ ± → π ± $v\overline{v}$ stemming from matrix elements involving up-quark loops. The importance of this observable as well as of K 0 → π 0 l + l - and of the direct CP violation parameter ϵ' K is then discussed in the context of a Unitarity Triangle sqtudy based on Kaon sector observables only.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Collisional-radiative electron-ion recombination rate in rare-gas plasmas.
Collisional radiative electron-ion recombination rates measured in decaying rare gas plasmas produced by transient discharge
Chen, C. J.↗
Direct observation of the exotic β – γ – α decay mode in the T z = – 1 nucleus Na 20
The exotic β – γ – α decay mode of 20 Na has been directly observed for the first time in the Day-one experiment at the Beijing Radioactive Ion-beam Facility. The 20 Na source was produced by using a 100-MeV proton beam bombarding a stack of microporous MgO thick target and delivered as an intense mass separated beam after online ionization. A high-efficiency simultaneous measurement of β, γ, and α transitions enables the β-delayed γ – γ and α – γ coincidence spectroscopy. Here three β – γ – α exotic decay sequences in 20 Na are discovered, which expands the rare decay modes observed in β decay. Moreover, a β – α –decay sequence to the 6130-keV 3 – state of 16 O is observed, which is likely through the 12367-keV 1 + state in 20 Ne. The experimentally deduced B(F) and B(GT) are compared to the shell-model calculation, the allowed β transition strengths can be well accounted for by using sd shell-model space interactions.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Search for the decay B 0 → γ γ using Belle and Belle II data
We report the result of a search for the rare decay B 0 → γ γ using a combined dataset of 753 × 10 6 B B ¯ pairs collected by the Belle experiment and 387 × 10 6 B B ¯ pairs collected by the Belle II experiment from decays of the ϒ ( 4 S ) resonance produced in e + e − collisions. A simultaneous fit to the Belle and Belle II data sets yields 11.0 − 5.5 + 6.5 signal events, corresponding to a 2.5 σ significance. We determine the branching fraction B ( B 0 → γ γ ) = ( 3.7 − 1.8 + 2.2 ( stat ) ± 0.5 ( syst ) ) × 10 − 8 and set a 90% credibility level upper limit of B ( B 0 → γ γ ) < 6.4 × 10 − 8 . Published by the American Physical Society 2024
Adachi, I. (ORCID:0000000322870173)↗
Searches for 25 rare and forbidden decays of D + and ${D}_s^{+}$ mesons
A search is performed for rare and forbidden charm decays of the form ${D}_{(s)}^{+}\to {h}^{\pm }{\mathrm{\ell}}^{+}{\mathrm{\ell}}^{\left(\prime \right)\mp } $, where h ± is a pion or kaon and ℓ (')± is an electron or muon. The measurements are performed using proton-proton collision data, corresponding to an integrated luminosity of 1.6 fb –1 , collected by the LHCb experiment in 2016. No evidence is observed for the 25 decay modes that are investigated and 90 % confidence level limits on the branching fractions are set between 1.4 × 10 –8 and 6.4 × 10 –6 . In most cases, these results represent an improvement on existing limits by one to two orders of magnitude.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Rare K 40 Decay with Implications for Fundamental Physics and Geochronology
Potassium-40 is a widespread, naturally occurring isotope whose radioactivity impacts subatomic rare-event searches, nuclear structure theory, and estimated geological ages. A predicted electron-capture decay directly to the ground state of argon-40 has never been observed. Here, the KDK (potassium decay) collaboration reports strong evidence of this rare decay mode. A blinded analysis reveals a nonzero ratio of intensities of ground-state electron-captures (I EC 0 ) over excited-state ones (I EC $\star$ ) of I EC 0 /I EC $\star$ = 0.0095 $^{stat}_{±}$ 0.0022 $^{sys}_{±}$ 0.0010 (68% C.L.), with the null hypothesis rejected at 4σ. In terms of branching ratio, this signal yields I EC 0 = 0.098 % $^{stat}_{±}$ 0.023 % $^{sys}_{±}$ 0.010 % , roughly half of the commonly used prediction, with consequences for various fields [L. Hariasz et al., companion paper, Phys. Rev. C 108, 014327 (2023)].
39 ≤ A ≤ 58↗
Bridging nuclear physics across energy scales: from neutrinoless double-beta decay to high-energy heavy-ion collisions
This paper exemplifies how connecting methods at disparate energy scales can illuminate fundamental questions. By demonstrating that nuclear wave function properties governing rare decay processes also influence collective behavior at extreme temperatures and densities, the authors have opened a new pathway for constraining physics beyond the Standard Model. With multiple ton-scale 0νββ experiments under construction, any method reducing NME uncertainties will directly impact our ability to interpret discoveries or constrain neutrino properties. The general principle—that collective phenomena in high-energy collisions can illuminate subtle features of many-body correlations in the colliding nuclei—may find applications across nuclear and particle physics. Furthermore, this intersection of nuclear structure theory, heavy-ion physics, and fundamental symmetry tests represents fertile ground for future discoveries in modern physics.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
β -delayed charged-particle decay of Si 22 , 23
The β decay of 22,23 Si by emission of delayed charged particles was investigated in an experiment at the Cyclotron Institute of Texas A&M University by means of an optical-readout time-projection chamber. The previously-known decay of the two isotopes by β-delayed one- and two-proton emission was confirmed. Here for the first time, a new, rare, decay mode for 23 Si, β-delayed three-proton emission, was observed and β-delayed proton-α emission tentatively identified. Moreover, the β-decay pattern for the ground state of 23 Si was studied by means of multireference density-functional-rooted calculations.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DAMSA Experiment Conceptual Design White Paper
DAMSA (DArk Messenger Searches at an Accelerator) is a novel short-baseline accelerator experiment aimed at probing short-lived physics processes, including searches for evidence of a dark sector of particle physics and well-motivated Standard Model signals. Motivated by open questions in neutrino physics and the absence of conclusive evidence for conventional weakly interacting massive particles, DAMSA targets MeV-to-sub-GeV dark-sector messengers with feeble couplings that can be produced in abundance at the PIP-II LINAC. By employing an ultra-short baseline of order one meter, DAMSA is uniquely positioned to overcome the beam-dump "ceiling" that limits sensitivity to promptly decaying particles in longer-baseline experiments. The conceptual design emphasizes a beam-dump production scheme combined with a compact detector optimized for rare decays while mitigating intense neutron-induced backgrounds inherent to high-power proton beams. To validate the experimental strategy and detector technologies, the Little DAMSA Path-Finder (LDPF) proof-of-concept experiment is proposed, focusing on axion-like particles decaying to two photons and operating with 300 MeV electron beams at FAST. Successful realization of LDPF will establish the feasibility of the DAMSA approach, enabling a broad and powerful program to explore short-lived new physics and precision Standard Model processes in a previously inaccessible regime. This conceptual design document outlines the technical details of DAMSA's physics goals, the beam facility proposals, key experimental challenges and how to overcome them, and the proposed experimental staging campaigns.
Bhattarai, Prithak [Texas U., Arlington]↗