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Tests of Lepton Universality Using B 0 → K S 0 ℓ + ℓ − and B + → K * + ℓ + ℓ − Decays
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Test of Lepton Universality in b → $s$$\ell$ + $\ell$ – Decays
The first simultaneous test of muon-electron universality using $B^+$ → $K^+$$\ell^+$$\ell^–$ and $B^0$ → $K^{*0}$$\ell^+$$\ell^–$ decays is performed, in two ranges of the dilepton invariant-mass squared, q 2 . The analysis uses beauty mesons produced in proton-proton collisions collected with the LHCb detector between 2011 and 2018, corresponding to an integrated luminosity of 9 fb –1 . Each of the four lepton universality measurements reported is either the first in the given q 2 interval or supersedes previous LHCb measurements. The results are compatible with the predictions of the Standard Model.
Current Status and Future Prospects for the Light Dark Matter eXperiment
The constituents of dark matter are still unknown, and the viable possibilities span a vast range of masses. The physics community has established searching for sub-GeV dark matter as a high priority and identified accelerator-based experiments as an essential facet of this search strategy. A key goal of the accelerator-based dark matter program is testing the broad idea of thermally produced sub-GeV dark matter through experiments designed to directly produce dark matter particles. The most sensitive way to search for the production of light dark matter is to use a primary electron beam to produce it in fixed-target collisions. The Light Dark Matter eXperiment (LDMX) is an electron-beam fixed-target missing-momentum experiment that realizes this approach and provides unique sensitivity to light dark matter in the sub-GeV range. This contribution provides an overview of the theoretical motivation, the main experimental challenges, how LDMX addresses these challenges, and projected sensitivities. We further describe the capabilities of LDMX to explore other interesting new and standard physics, such as visibly-decaying axion and vector mediators or rare meson decays, and to provide timely electronuclear scattering measurements that will inform the modeling of neutrino-nucleus scattering for DUNE.
Nucleon Decays into Light New Particles in Neutrino Detectors
Proton and neutron decays into light new particles 𝑋 can drastically change the experimental signatures and benefit from the complementarity of large water-Cherenkov neutrino detectors such as Super- and Hyper-Kamiokande and tracking detectors such as JUNO and DUNE. The proton decays 𝑝 → ℓ + 𝑋 and 𝑝 → 𝜋 + 𝑋 with 𝑚 𝑋 near phase-space closure lead to charged particles below the Cherenkov threshold, rendering them practically invisible in Super- and Hyper-Kamiokande but not in JUNO and DUNE, which are therefore uniquely positioned for these baryon-number-violating signatures despite their smaller size. As an additional signature, such nucleon decays in the Earth can produce a sizable flux of 𝑋 particles in underground detectors. We present a simple model in which nucleons decay into sub-GeV sterile neutrinos that subsequently decay through active-sterile neutrino mixing, with a promisingly large number of events in Super-Kamiokande even in the seesaw-motivated parameter space.
Neutrinoless double beta decay sensitivity of the XLZD rare event observatory
The XLZD collaboration is developing a two-phase xenon time projection chamber with an active mass of 60–80 t capable of probing the remaining weakly interacting massive particle-nucleon interaction parameter space down to the so-called neutrino fog. In this work we show that, based on the performance of currently operating detectors using the same technology and a realistic reduction of radioactivity in detector materials, such an experiment will also be able to competitively search for neutrinoless double beta decay in 136 Xe using a natural-abundance xenon target. XLZD can reach a 3σ discovery potential half-life of 5.7 × 10 27 years (and a 90% CL exclusion of 1.3 × 10 28 years) with 10 years of data taking, corresponding to a Majorana mass range of 7.3–31.3 meV (4.8–20.5 meV). XLZD will thus exclude the inverted neutrino mass ordering parameter space and will start to probe the normal ordering region for most of the nuclear matrix elements commonly considered by the community.
PASSAT at future neutrino experiments: Hybrid beam-dump-helioscope facilities to probe light axionlike particles
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Discovering heavy neutrino oscillations in rare B c ± meson decays at HL-LHCb
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Measurement of the 𝐷 0 →𝐾 − 𝜋 + 𝑒 + 𝑒 − branching fraction and search for 𝐷 0 → 𝜋 + 𝜋 − 𝑒 + 𝑒 − and 𝐷 0 →𝐾 + 𝐾 − 𝑒 + 𝑒 − decays at Belle
We present a study of the rare charm meson decays 𝐷 0 →𝐾 + 𝐾 − 𝑒 + 𝑒 − , 𝜋 + 𝜋 − 𝑒 + 𝑒 − , and 𝐾 − 𝜋 + 𝑒 + 𝑒 − using a 942 fb −1 dataset collected by the Belle detector at the KEKB asymmetric-energy 𝑒 + 𝑒 − collider. We identify 𝐷 0 candidates via the charge of the pion from 𝐷* + →𝐷 0 𝜋 + decays and normalize the branching fractions to 𝐷 0 → 𝐾 − 𝜋 + 𝜋 − 𝜋 + decays. The branching fraction for decay 𝐷 0 → 𝐾 − 𝜋 + 𝑒 + 𝑒 − is measured to be (39.6 ± 4.5 (stat) ± 2.9 (syst)) × 10 −7 , with the dielectron mass in the 𝜌/𝜔 mass region 675 < 𝑚 𝑒𝑒 < 875 MeV/𝑐 2 . We also search for 𝐷 0 → ℎ − ℎ (′)+ 𝑒 + 𝑒 − (ℎ (′) = 𝐾, 𝜋) decays with the dielectron mass near the 𝜂 and 𝜙 resonances, and away from these resonances for the 𝐾 + 𝐾 − 𝑒 + 𝑒 − and 𝜋 + 𝜋 − 𝑒 + 𝑒 − modes. For these modes, we find no significant signals and set 90% confidence level upper limits on their branching fractions at the 𝒪(10 −7 ) level.
Unified explanation of flavor anomalies, radiative neutrino masses, and ANITA anomalous events in a vector leptoquark model
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Measurement of branching fraction and search for C P violation in B → ϕ ϕ K
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Search for K + decays into the π + e + e - e + e - final state
The first search for ultra-rare K + decays into the π + e + e - e + e - final state is reported, using a dataset collected by the NA62 experiment at CERN in 2017–2018. An upper limit of 1.4 x 10 -8 at 90% CL is obtained for the branching ratio of the K + → π + e + e - e + e - decay, predicted in the Standard Model to be (7.2 ± 0.7) x 10 -11 . Upper limits at 90% CL are obtained at the level of 10 -9 for the branching ratios of two prompt decay chains involving pair-production of hidden-sector mediators: K + → π + aa,a → e + e - and K + → π + S,S → A'A',A' → e + e - .
First Results on the Search for Lepton Number Violating Neutrinoless Double-𝛽 Decay with the LEGEND-200 Experiment
The LEGEND Collaboration is searching for neutrinoless double-beta (0𝜈𝛽𝛽) decay by operating high-purity germanium detectors enriched in 76 Ge in a low-background liquid argon environment. Building on key technological innovations from the GERmanium Detector Array (GERDA) experiment and the MAJORANA DEMONSTRATOR experiment, LEGEND-200 has performed a first 0𝜈𝛽𝛽 decay search based on 61.0 kg yr of data. Over half of this exposure comes from our highest performing detectors, including newly developed inverted-coaxial detectors, and is characterized by an estimated background level of 0.5$^{+0.3}_{−0.2}$ cts/(keV ton yr) in the 0𝜈𝛽𝛽 decay signal region. A combined analysis of data from GERDA, the MAJORANA DEMONSTRATOR, and LEGEND-200, characterized by a 90% confidence level exclusion sensitivity of 2.8 ×10 26 yr on the half-life of 0𝜈𝛽𝛽 decay, reveals no evidence for a signal and sets a new observed lower limit at 𝑇$^{0𝜈}_{1/2}$ > 1.9 × 10 26 yr (90% confidence level). Assuming the decay is mediated by Majorana neutrinos, this corresponds to an upper limit on the effective Majorana mass in the range 𝑚 𝛽𝛽 < 75–200 meV, depending on the adopted nuclear matrix element.
The Experimental Programs on Light Pseudoscalar Meson Decays
The decays of the low-lying pseudoscalar mesons (?0, ?, and ??) allow access to a plethora of physics probes, from measurements of fundamental properties of matter such as the up-down quark mass ratio, to studies of channels that access higher-order terms in Chiral Perturbation theory, to tests of fundamental symmetries and searches for signatures of physics beyond the Standard Model. Several experiments have studied pseudoscalar decays: these experiments include PrimEx in Hall B at Jefferson Lab, A2 at the MAMI electron accelerator facility, the KLOE-II experiment at the DA?NE ?-factory, WASA-at-COSY at the cooler synchrotron COSY storage ring, and BESIII at the Beijing charm factory. In Hall D at Jefferson Lab, PrimEx-eta uses the Primakoff process to measure the ? radiative width using the GlueX spectrometer. In the near future the Jefferson Lab Eta Factory (JEF) experiment will use the GlueX spectrometer with an upgraded forward calorimeter to study ?(?) decay channels with a focus on all-neutral final states. Finally, a proposal for a new experiment, ``Rare Eta Decays with a TPC for Optical Photons'' (REDTOP) utilizing a proton beam on fixed targets, is under development. Highlights from previous and ongoing experiments and plans for future experiments will be presented.
The FRIB Decay Station: New Horizons with Rare Isotopes
In May 2022, the Facility for Rare Isotope Beams (FRIB), located on the campus of Michigan State University (MSU), began delivering exotic isotopes to an international community of scientists. New discoveries are now being reported from radioactive decay of neutron-rich nuclei near N = 20 and N = 28.FRIB is expected to produce roughly 80% of the unstable or radioactive isotopes predicted to exist up to uranium (Z = 92). The new user facility is supported by the U.S. Department of Energy, and it is operated by MSU. A high-power superconducting linear accelerator, shaped like a paper-clip, drives the production of these rare isotopes by colliding stable nuclei moving at half the speed of light with a rotating, water-cooled graphite tar-get. These collisions cause the primary stable beam to fragment into a wide variety of unstable nuclei, which can be subsequently filtered through a multistage magnetic separator, the Advanced Rare Isotope Separator, and transported to one of several experimental stations. The FRIB Decay Station initiator (FDSi) (see Figure 1) was developed to enable comprehensive radio-active decay studies of the exotic nuclei produced by FRIB and it was used in the first two experiments in 2022. Further, the FDSi is a highly reconfigurable multidetector system with two focal planes (FP1 for discrete spectroscopy and FP2 for total absorption spectroscopy) that can be optimized for the specific science goals of each experiment. It is designed, built, and operated by a community of users with the sup-port of U.S. funding agencies, including the Department of Energy and National Science Foundation.
Hidden-sectors search and probe of discrete symmetries at the REDTOP experiment
The $η$ and $η^{\prime}$ mesons are nearly unique in the particle universe since they are nearly Goldstone bosons, and their decay dynamics are strongly constrained. While earlier experiments collected samples of order $\sim 10^{9}η$, the proposed REDTOP (Rare Eta Decays To Observe Physics Beyond the Standard Model) facility targets $\mathcal{O}(10^{14})η$ and $\mathcal{O}(10^{12})η^\prime$, enabling broad searches for physics beyond the Standard Model. In this work, we present studies evaluating REDTOP sensitivity to processes that couple the Standard Model to New Physics through four portals: the Vector (dark photon), the Scalar (Higgs-mixing), the Axion-like, and the Heavy Lepton. In parallel, the proposed statistics allow precise tests of $CP$ and $T$ invariance and lepton universality and improve determinations of the $η/η'$ transition form factors, which are crucial inputs to the hadronic light-by-light contribution to the muon anomalous magnetic moment $(g-2)_μ$.