Rare Decays Research Articles

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.

First search for resonant enhanced neutrinoless double electron capture in 152\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{152}$$\\end{document}Gd and other rare decays in natural Gd isotopes

A first search for rare decays of gadolinium isotopes was performed with an ultra-low background high-purity germanium detector at Gran Sasso Underground Laboratory (Italy). A 198 g Gd2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Gd}_2$$\\end{document}O3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {O}_3$$\\end{document} powder sample was measured for 63.8 d with a total Gd exposure of 12.6 kg ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes $$\\end{document} d. 152\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{152}$$\\end{document}Gd is the most promising isotope for resonant enhanced neutrinoless double electron capture which could significantly increase the decay rate over other neutrinoless double beta decay processes. The half-life for this decay was constrained to >4.2×1012\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$>4.2\ imes 10^{12}$$\\end{document} year (90% credibility). This limit is still orders of magnitude away from theoretical predictions but it is the first established limit on the transition paving the way for future experiments. In addition, other rare alpha and double beta decay modes were investigated in 152\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{152}$$\\end{document}Gd, 154\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{154}$$\\end{document}Gd, and 160\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{160}$$\\end{document}Gd with half-life limits in the range of 1017-20\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^{17-20}$$\\end{document} year.

Flavor-changing top quark rare decays in the Bestest Little Higgs Model

This paper investigates rare flavor-changing decays of the top quark in the Bestest Little Higgs Model (BLHM). As a result, flavor-changing phenomena are introduced in the BLHM for the first time. In this study, we incorporate new flavor mixing terms between the light quarks of the Standard Model (SM) and the fermions and bosons of the BLHM. We compute the one-loop contributions from the heavy quark (B) and the heavy bosons (W′±,ϕ±,η±,H±)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(W^{\\prime \\pm }, \\phi ^{\\pm }, \\eta ^{\\pm },H^{\\pm })$$\\end{document}. Our findings demonstrate that the branching ratios of decays t→qV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$t\\rightarrow qV$$\\end{document} and t→qh0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$t\\rightarrow qh^0$$\\end{document}, where q=u,c\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$q=u,c$$\\end{document} and V=Z,γ,g\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$V=Z, \\gamma , g $$\\end{document}, can reach larger branching ratios compared to their counterparts in the SM, except for the gluon case. Moreover, we observe that the processes with the highest sensitivity are Br(t→cZ)∼10-5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Br(t\\rightarrow cZ)\\sim 10^{-5}$$\\end{document}, Br(t→cγ)∼10-6\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Br(t\\rightarrow c\\gamma )\\sim 10^{-6}$$\\end{document} and Br(t→ch0)∼10-8\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Br(t \\rightarrow ch^0) \\sim 10^{-8}$$\\end{document} within the appropriate parameter space.

New dark matter candidate in the B-L\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$B-L$$\\end{document} symmetric SSM

The B-L symmetric SUSY model (B-LSSM) introduces a new Z′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Z'$$\\end{document} gauge boson and two singlet scalars, which indicates there are more neutralinos in the B-LSSM comparing with the minimal SUSY extension of standard model (MSSM). These new neutralinos provide more dark matter (DM) candidates in the B-LSSM, we find that the super partner of the new scalar singlet can serve as the lightest supersymmetric particle (LSP) and be considered as a DM candidate. The properties of this new DM candidate are analyzed in this work. Taking into account the experimental constraints on Higgs boson mass, B meson rare decays and muon anomalous magnetic dipole moment, the numerical results are obtained and presented. The properties of new DM candidate in the B-LSSM satisfy the observed relic density Ωh2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Omega h^2$$\\end{document} and comply with the latest upper bounds on σSIχ10-n\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _\ extrm{SI}^{\\chi _1^0-n}$$\\end{document}, and its mass is expected to greater than 80GeV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$80\\;\ extrm{GeV}$$\\end{document} approximately.

New constraint on dark photon at T2K off-axis near detector

The T2K experiment is one of the most powerful long-baseline experiments to investigate neutrino oscillations. The off-axis near detector called ND280 is installed 280 m downstream from the neutrino production target to measure the neutrino energy spectrum. In this paper, we study the capability of the ND280 detector to search for the dark photon produced through the meson rare decay and proton bremsstrahlung processes at the proton beam dump. We find that the ten-year operation of T2K with the ND280 detector excludes the unexplored parameter region for the dark photon mass and kinetic mixing. We also show that a broader parameter region can be searched by the ND280 in the future T2K operation for dark photon as well as U(1)B−L gauge boson.

A model with vectorlike fermions and U(1)X symmetry: CKM unitarity, b → s transitions, and prospect at Belle II

The updated analysis of the LHCb Collaboration on the lepton flavor violation suggests that the new physics should couple to muons and electrons with comparable magnitudes, resulting in the anomalies in both rare decay channels, b→sμ+μ− and b→se+e−. Meanwhile, the recent result of the Muon g−2 experiment with higher precision has increased the existing tension with the standard model prediction. In this paper, we consider an extension of the standard model with a new sector consisting of vectorlike fermions and two scalar charged under an extra U(1)X gauge symmetry. The exotic Yukawa interactions in this model lead to the quark mixing responsible for the additional contributions to the flavor changing neutral currents in B-meson decays, and solve the muon g−2 discrepancy. We derive the analytic expression of the new physics contributions to the Wilson coefficient C7 in the effective Hamiltonian, and point out that the CKM unitarity violation can be explained within this context. By calculating the branching ratio of the inclusive radiative B decay, the impact of current experimental data of the b→sγ transition on the model and the future prospect at the Belle II experiment are investigated. Taking into account the current data on the muon anomalous magnetic moment, the CKM unitarity violation, the constraints on the flavor observables relevant to the b→s transitions, the LHC searches for vectorlike quarks, and the perturbation limits of the couplings, the viable parameter regions of the model are identified.

Spectroscopic performance evaluation and modeling of a low background HPGe detector using GEANT4

Low background gamma spectrometry employing HPGe detectors is a sensitive technique for measuring low-level radioactivity in environmental applications, material screening, and for rare decay searches. This work presents spectroscopic performance evaluation and modeling of a low background measurement setup developed at IIT Ropar in Punjab, India, to measure trace natural radioactive elements, with a particular interest in studying low-level radioactivity in soil and/or rock samples to generate specific inputs for low background experiments. The performance test and characterization of a low background cryocooled HPGe detector with relative efficiency of ∼33% have been carried out. An effective detector model has been developed using GEANT4 Monte Carlo simulation to determine the response of the detector over an energy range of 80.9–1408 keV and compared with the experimental performance of the detector. The response of the detector obtained using Monte Carlo simulations agrees reasonably well within 93% level of confidence, indicating only 7% deviation in the comparison. The present setup offers improved detection limits of primordial radionuclides (U/Th and K) to measure radioactive contamination in environmental matrices, which has been used elsewhere (Thakur, 2023).

NA62 liquid krypton purity monitor

A system for determining the purity of liquid krypton employed in the NA62 rare kaon decay experiment at CERN was developed based on the use of a time projection chamber. The attenuation of drifting ionization electrons from absorption of 511 keV gamma rays in liquid krypton was measured to estimate the purity. The setup was tested with krypton purified from commercial sources.

Measurement of the Λ_{b}^{0}→Λ(1520)μ^{+}μ^{-} Differential Branching Fraction.

The branching fraction of the rare decay Λ_{b}^{0}→Λ(1520)μ^{+}μ^{-} is measured for the first time, in the squared dimuon mass intervals q^{2}, excluding the J/ψ and ψ(2S) regions. The data sample analyzed was collected by the LHCb experiment at center-of-mass energies of 7, 8, and 13TeV, corresponding to a total integrated luminosity of 9 fb^{-1}. The result in the highest q^{2} interval, q^{2}>15.0 GeV^{2}/c^{4}, where theoretical predictions have the smallest model dependence, agrees with the predictions.

Kaon theory: 50 years later

Abstract We summarize the status of the kaon theory 50 years after the seminal paper of Kobayashi and Maskawa [Prog. Theor. Phys. 49, 652 (1973)], who pointed out that six quarks are necessary to have CP violation in the Standard Model (SM) and presented a parametrization of a 3 × 3 unitary matrix that, after the discovery of the charm quark in 1974 and the b quark in 1977, dominated the field of flavor-changing processes. One of the main goals of flavor physics since then has been the determination of the four parameters of this matrix, which we will choose here to be |Vus|, |Vcb|, and the two angles of the unitarity triangle, β and γ, with |Vus| introduced by Cabibbo in 1963. I will summarize the recent strategy for determination of these parameters without new physics (NP) infection. It is based on the conjecture of the absence of relevant NP contributions to ΔF = 2 processes that indeed can be demonstrated by a negative rapid test: the |Vcb|–γ plot. This in turn allows one to obtain SM predictions for rare K and B decays that are the most precise to date. We present strategies for the explanation of the anticipated anomaly in the ratio ε′/ε and the observed anomalies in b → sμ+μ− transitions that are consistent with our ΔF = 2 conjecture. In particular, the absence of NP in the parameter εK still allows for significant NP effects in ε′/ε and in rare kaon decays, moreover, in a correlated manner. Similarly, the absence of NP in ΔMs combined with anomalies in b → sμ+μ− transitions hints at the presence of right-handed quark currents. We also discuss how the nature of neutrinos, Dirac vs. Majorana ones, can be probed in $K\rightarrow \pi \nu \bar{\nu }$ and $B\rightarrow K(K^{*})\nu \bar{\nu }$ decays. The present status of the ΔI = 1/2 rule and ε′/ε is summarized.

Physics of the Top Quark at the LHC: An Appraisal and Outlook of the Road Ahead

Since its start, the Large Hadron Collider (LHC) has helped advance both theory and experiment on the production and properties of the heaviest fundamental particle, the top quark. This review focuses on a selected set of measurements and associated searches for new physics, which have opened the door for unprecedented precision in this area of high-energy physics. Fundamental parameters of the theory such as mt, αS, Vtb, and ytare measured from top quark events with relative uncertainties that are smaller than 0.5%, 1.8%, 2%, and 10%, respectively, and that are expected to improve with more data, better experimental methods, and more accurate theory predictions. Several results, even if statistically limited, already significantly constrain the phase space of new physics: measurements of associated production with bosons, processes with four top quarks, and searches for rare decays, among others. It is expected that until the completion of the LHC program, top quark physics will keep providing unique insights regarding the consistency of the Standard Model and the energy scale of new physics.

Particle identification with the NA62 RICH detector

NA62 is a new generation kaon experiment at the CERN SPS aiming at studying rare and forbidden decays. One of main challenges of the experiment is the suppression of background decay channels with branching ratios up to 10 orders of magnitude higher than the signal and with similar experimental signatures. To provide such suppression, a powerful particle identification (PID) is needed.A key element of PID in NA62 is the Ring Imaging Cherenkov (RICH) detector. The RICH has successfully operated during the 2016–2018 data taking periods, being essential in the measurement of the branching ratio of the K+→π+νν̄ decay. The detector was also used for searches for lepton number violation in 3-track kaon decays. The results on the π/μ and π/e separation directly measured with the data for the aforementioned decays are presented.

Electroweak baryogenesis via top-charm mixing

We investigate a scenario of electroweak baryogenesis in the two Higgs doublet model with quark flavor mixing. In general, off-diagonal components of quark Yukawa interactions with additional Higgs bosons are strongly constrained by the data for flavor changing neutral currents. However, top-charm quark mixing is not the case, so that a large off-diagonal element can be taken, which can contribute to generating baryon asymmetry of the universe. It turns out that CP violating phases of the off-diagonal element in the source term in the Boltzmann equation are eliminated by the rephasing. This result is somewhat different from previous works on electroweak baryogenesis by flavor off-diagonal Yukawa couplings. Instead, we find that the absolute value of the top-charm off-diagonal element enhances CP violating phases in the Higgs potential, by which sufficient amount of the baryon number can be generated to explain the observed baryon asymmetry of the universe. We find that such a scenario is viable under the current experimental data. The model can be tested by the current and future measurements of various flavor experiments like Kaon rare decays, in addition to high energy collider experiments as well as gravitational wave observations. Characteristic predictions of our model would be deviations in Kaon rare decays. Branching ratios of K+→π+νν¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {K}^{+}\ o {\\pi}^{+}\ u \\overline{\ u} $$\\end{document} and KL→π0νν¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {K}_L\ o {\\pi}^0\ u \\overline{\ u} $$\\end{document} can deviate by the order of 10% and 1%, respectively, which may be tested at future Kaon experiments such as NA62 and KOTO step-2.

Non universality of leptons as a source of new physics from non standard interactions of pseudoscalar mesons

The rare decays, K+→π+νν¯, B+→π+νν¯ and D+→π+νν¯, are investigated in the frame work of nonstandard neutrino interactions (NSI). It is claimed that NSI are arising due to non-universality of the leptons and bounds on NSI parameters ϵll′uL are determined. For this purpose, the branching ratios (Br) of these processes with in the Standard Model (SM) and beyond the SM are calculated by applying the non-universality of the leptons. The NSI in K+→π+νν¯ and B+→π+νν¯ proceed with u quark in the loop while other two with d quark in the loop. Very stringent 1 σ level constrains on non-standard parameters (ϵττuL and ϵτl′uL) are pointed out.

Complementarity of Brightarrow K^{(*)} mu bar{mu } and Brightarrow K^{(*)} + textrm{inv} for searches of GeV-scale Higgs-like scalars

The rare decays B^+rightarrow K^+ mu bar{mu } and B^0rightarrow K^{*0} mu bar{mu } provide the strongest constraints on the mixing of a light scalar with the Higgs boson for GeV-scale masses. The constraints sensitively depend on the branching ratio to muons. Additional decay channels like an invisible partial width may substantially weaken the constraints. This scenario will be probed at Belle II in Brightarrow K^{(*)} + textrm{inv}. We illustrate the complementarity of scalar decays to muons and invisible decays using the currently available results of LHCb and BaBar. We provide two simple model realisations providing a sizeable invisible scalar width, one based on a real scalar and one based on a U(1)_{B-L} gauge symmetry. In both examples the scalar decays into heavy neutral leptons which can be motivated by the seesaw mechanism for neutrino masses.

Observation of the Rare Decay of the η Meson to Four Muons.

A search for the rare η→μ^{+}μ^{-}μ^{+}μ^{-} double-Dalitz decay is performed using a sample of proton-proton collisions, collected by the CMS experiment at the CERN LHC with high-rate muon triggers during 2017 and 2018 and corresponding to an integrated luminosity of 101 fb^{-1}. A signal having a statistical significance well in excess of 5 standard deviations is observed. Using the η→μ^{+}μ^{-} decay as normalization, the branching fraction B(η→μ^{+}μ^{-}μ^{+}μ^{-})=[5.0±0.8(stat)±0.7(syst)±0.7(B_{2μ})]×10^{-9} is measured, where the last term is the uncertainty in the normalization channel branching fraction. This work achieves an improved precision of over 5 orders of magnitude compared to previous results, leading to the first measurement of this branching fraction, which is found to agree with theoretical predictions.

Measurement of the Branching Fractions B(B^{0}→pp[over ¯]pp[over ¯]) and B(B_{s}^{0}→pp[over ¯]pp[over ¯]).

Searches for the rare hadronic decays B^{0}→pp[over ¯]pp[over ¯] and B_{s}^{0}→pp[over ¯]pp[over ¯] are performed using proton-proton collision data recorded by the LHCb experiment and corresponding to an integrated luminosity of 9 fb^{-1}. Significances of 9.3σ and 4.0σ, including statistical and systematic uncertainties, are obtained for the B^{0}→pp[over ¯]pp[over ¯] and B_{s}^{0}→pp[over ¯]pp[over ¯] signals, respectively. The branching fractions are measured relative to the topologically similar normalization decays B^{0}→J/ψ(→pp[over ¯])K^{*0}(→K^{+}π^{-}) and B_{s}^{0}→J/ψ(→pp[over ¯])⁢ϕ(→K^{+}K^{-}). The branching fractions are measured to be B(B^{0}→pp[over ¯]pp[over ¯])=(2.2±0.4±0.1±0.1)×10^{-8} and B(B_{s}^{0}→pp[over ¯]pp[over ¯])=(2.3±1.0±0.2±0.1)×10^{-8}. In these measurements, the first uncertainty is statistical, the second is systematic, and the third one is due to the external branching fraction of the normalization channel.

Search for the rare decays and at LHCb

A search for the rare decays and is performed using proton-proton collision data collected by the LHCb experiment at a centre-of-mass energy of 13, corresponding to an integrated luminosity of 2.0. No significant signal is observed for either decay mode and upper limits on their branching fractions are set using and decays as normalization channels. The upper limits are and at 95% confidence level for the and decay modes, respectively. This is the first reported search for the decay, while the upper limit on the branching fraction improves upon the previous best limit.

Performance study of the aerogel RICH at the Belle II experiment

The Belle II experiment at the SuperKEKB asymmetric e+e− collider in High Energy Accelerator Research Organization (KEK, Tsukuba, Japan) is searching for CP asymmetries in different rare decays in the Standard Model. We will collect a large data sample to improve precision of Standard Model measurements (of which CP asymmetries are one example) and search for New Physics. We study the Aerogel Ring Imaging Cherenkov (ARICH) detector response in terms of loglikelihood functions using decay samples of D∗+→D0π+ and D∗−→D̄0π− resonances. The particle identification efficiencies and mis-identification probabilities are presented in 2D momentum and polar angle bins.

Rare ^{40}K 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. 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^{*}}) of I_{EC^{0}}/I_{EC^{*}}=0.0095±[over stat]0.0022±[over 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%±[over stat]0.023%±[over sys]0.010%, roughly half of the commonly used prediction, with consequences for various fields [27L. Hariasz et al., companion paper, Phys. Rev. C 108, 014327 (2023)PRVCAN2469-998510.1103/PhysRevC.108.014327].