Forbidden Decays Research Articles

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.

Scattering versus forbidden decay in dark matter freeze-in

Scattering versus forbidden decay in dark matter freeze-in

Dark matter freeze-in via a light fermion mediator: forbidden decay and scattering

The connection between a hidden nonthermal sector and a thermal plasma can be established by a light thermal fermion mediator. When the fermion mediator is much lighter than the hidden species, kinematically forbidden decay of the mediator can be opened at finite temperatures to produce the hidden species. Unlike bosons having quartic couplings, renormalizable forbidden fermion decay generically shares the same order of couplings with the scattering. We present a dedicated investigation into the freeze-in dark matter production via a thermal fermion mediator. We demonstrate that the plasma-induced decay rate differs from that calculated via the tree-level amplitude, but the former can be obtained from the latter via constant rescaling. Furthermore, we find that the relative effect of the forbidden decay and the scattering on the dark matter relic density can be simply estimated via the thermal coupling between the plasma and the mediator. Applying to different thermal interactions, we show that the forbidden decay contribution can reach the level of 4%- 45% for a thermal coupling at 0.1- 1.

Improvements in Antineutrino Spectrum by Including Fission Product Corrections and Calculation of Scatter-Based Pulse Height Distributions

Modeling a reactor’s antineutrino flux spectrum is a critical step in studying its detector response. The first objective of this paper is to study the importance of fission product libraries in the construction of the antineutrino spectrum using the summation method and with various other corrections, including excited states in fission products, finite size, weak magnetism, and radiative corrections. We have used the ENDF/B-VIII.0 and JEFF-3.3 nuclear libraries as our base data to model the antineutrino spectrum. We have also included the total absorption gamma spectroscopy (TAGS) data, which is free from the pandemonium effect, when such data are available. Our analysis includes the newest TAGS data sets from Gombas et al. [Phys. Rev. C, Vol. 103, p. 35803 (2021)] with additions made after the Estienne et al. [EPJ Web Conf., Vol. 211, p. 01001 (2019)] reactor antineutrino spectra study involving TAGS data. The excited state correction has the highest impact on the antineutrino energy spectra, increasing the values 29% to 37% on average in the energy range of 0.5 to 2 MeV. This antineutrino spectra correction also shows an increase of 4.71% to 7.13% in the range of 0 to 2 MeV, with improving excited states using the TAGS data from published literature. Next, antineutrino spectra including the excited state correction using the Gross Theory causes reduction by 11.56% to 69.46% for all four fissionable isotopes in the range of 6 to 8 MeV. The finite size correction, radiative correction, and weak magnetism corrections cause no more than a 3.27% difference between the corrected and uncorrected spectra. We studied the impact of various corrections to the antineutrino spectra and quantified the improvements made in the antineutrino spectrum calculation due to these changes. However, we have not included forbidden decays to simplify the calculations. The second objective of this work is to determine the impact of spectrum improvements on the coherent-elastic-neutrino-nucleus-scatter (CEνNS)-based detector response because this detection mechanism is more sensitive to lower energy antineutrinos, as expected from a nuclear reactor. We calculate pulse height distributions of Ge- and Si-based CEνNS sensors assuming a 20-eV nuclear recoil threshold. Toward this objective, we formulate pulse height distribution probabilities for different incident antineutrino energies in Ge and Si for a 100-kg detector placed 10 m away from the 1-MW TRIGA reactor with a 20-eV nuclear recoil energy threshold. Our results show that the reaction rate with corrected spectra for a CEνNS-based natural Ge detector is 20.6 events/day and a natural Si detector is 7.18 events/day. The biggest impact on the reaction rates between 38% and 41% is observed due to the excited state corrections. To benchmark our results, we show excellent agreement with the previous antineutrino spectrum calculated by Huber [Phys. Rev. C, Vo. 84, p. 24617 (2011)] and Hayes et al. [Phys. Rev. Lett., Vol. 112, p. 202501 (2014)].

Search for new physics with rare charm decays at LHCb

The LHCb experiment is playing a crucial role in the study of rare and forbidden decays of charm hadrons, which are unique probes for hints of physics beyond the Standard Model. These proceedings present results of the search for semileptonic three-body decays of the form $D_{(s)}^+$ \rightarrow $h^\pm$ \ellp $\ell^{(')\mp}$, where $h^\pm$ is a charged pion or kaon and $\ell^{(')\mp}$ is an electron or a muon. Furthermore, studies of the four-body neutral meson charm decays $D^0 \rightarrow \pi^+ \pi^- \mu^+ \mu^-$ and $D^0 \rightarrow K^+ K^- \mu^+ \mu^-$ are discussed. Prospects for leptonic and semileptonic rare charm decays at LHCb in the future are presented in the end.

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.

Light particles with baryon and lepton numbers

We consider light new particles χ and ϕ that carry baryon and lepton numbers. If these particles are lighter than nucleons they lead to exotic decays such as p→π+χ and p→e+ϕ, not yet fully constrained by dedicated searches. For χ and ϕ masses in the GeV range, proton decays are kinematically forbidden but other decays of the forms baryon → meson + χ, meson → baryon + χ¯, and baryon → anti-lepton + ϕ involving heavy initial hadrons are allowed. This opens up the possibility to search for apparent baryon number violation not just in underground experiments such as Super-Kamiokande and DUNE but also in decays of heavy hadrons in charm and B factories.

Searches for lepton flavor and lepton number violation in K + decays with NA62

The NA62 experiment at the CERN SPS provides unique opportunity to study rare and forbidden decays of K + mesons. Using the data collected in 2017 a search for the lepton number violating decays K + → π − µ + µ + and K + → π − e + e + has been performed. New upper limits on the branching ratio have been obtained: 4.2×10−11 for the muon and 2.2×10−10 for the electron mode (90% confidence level). These results improve the existing limits by factors of 2 and 3 respectively.

Quenching of gA deduced from the β-spectrum shape of 113Cd measured with the COBRA experiment

A dedicated study of the quenching of the weak axial-vector coupling strength gA in nuclear processes has been performed by the COBRA collaboration. This investigation is driven by nuclear model calculations which show that the β-spectrum shape of the fourfold forbidden non-unique decay of 113Cd strongly depends on the effective value of gA. Using an array of CdZnTe semiconductor detectors, 45 independent 113Cd spectra were obtained and interpreted in the context of three nuclear models. The resulting effective mean values are g‾A(ISM)=0.915±0.007, g‾A(MQPM)=0.911±0.013 and g‾A(IBFM-2)=0.955±0.022. These values agree well within the determined uncertainties and deviate significantly from the free value of gA. This can be seen as a first step towards answering the long-standing question regarding quenching effects related to gA in low-energy nuclear processes.

The fun (?) of rare event searches

Various kinds of rare events can be studied but only a small fraction of these are mentioned in these proceedings. The search for rare events is quite some fun but also very hard work as typically very high backgrounds must be tackled for a very small signal. The first process discussed is double beta decay, which is an extremely rare process and requires half-live measurements around 1020 years for the neutrino accompanied mode, while for the neutrinoless mode much longer half-lives are already explored. Nuclear physics input is needed for the matrix elements and a quenching measurement of gA will be presented. In this case the focus is on the highly forbidden Cd-113 beta decay. New half-lives for the forbidden EC/beta decay measurement of V-50 will be presented. In addition new precision half-lives of long-living alpha decays will be presented as well.

New investigation of half-lives for the decay modes of V50

A new search for the decay modes of the 4-fold forbidden non-unique decay of $^{50}$V has been performed at the Gran Sasso Underground Laboratory (LNGS). In total an exposure of 197 kg $\times$ d has been accumulated. The half-life for the electron capture into the first excited state of $^{50}$Ti has been measured with the highest precision to date as $2.67_{-0.18}^{+0.16} \times 10^{17}$ yr (68% C.I.) in which systematics uncertainties dominate. The search for the $\beta$-decay into the first excited state of $^{50}$Cr resulted in a lower limit of ${1.9} \times 10^{19}$ yr (90% C.I.), which is an improvement of almost one order of magnitude compared to existing results. The sensitivity of the new measurement is now in the region of theoretical predictions.

Search for exotics at NA62

Fixed target experiments are a particularly useful tool in the search of very weakly coupled particles in the MeV-GeV range, which are of interest as potential dark matter mediators. Owing to the high beam energy and a hermetic detector coverage, NA62 also has the opportunity to directly search for a variety of long-lived beyond the Standard Model particles, such as axion-like particles and dark photons. Furthermore, the large sample of K+ decays available at NA62 allows a rich program for rare and forbidden decay studies. In this paper the status of these searches is reviewed, giving prospects for future data taking at NA62.

η meson physics with WASA-at-COSY

In recent years, the η meson has been a focal point of research for the WASA experiment at the Cooler Synchrotron COSY of the Research Center Jülich. Production experiments using nucleon-nucleon and nucleon-nucleus collisions have been performed, studying the η − N interaction in various configurations. A better understanding of this interaction is a key aspect in the ongoing search for η-nuclear bound states. In addition, the η meson itself represents an ideal laboratory for precision studies of the strong and electromagnetic interactions as well as for searches for beyond Standard Model physics. Large datasets were assembled using the WASA experiment to enable studies on rare and forbidden decay modes. An overview over recent highlights of the WASA η meson physics programme was given.

Search for Exotic Particles at the NA62 Experiment

The NA62 experiment at the CERN Super Proton Synchrotron (SPS) is currently taking data to measure the ultra-rare decay K + → π + ν ν ¯ . The high-intensity setup, trigger flexibility, detector performance and high-efficiency vetoes make NA62 also suitable for direct searches of long-lived, beyond-the-Standard-Model particles, such as Heavy Neutral Leptons (HNLs), Axion-Like Particles (ALPs) and Dark Photons (DPs); moreover, many rare and forbidden decays are studied at NA62. The status of all these searches is reviewed, together with prospects for future data taking at NA62 after the CERN Long Shutdown 2 (LS2).

Improved measurements of two-photon widths of the χcJ states and helicity analysis for χc2→γγ

Based on $448.1 \times 10^6$ $\psi(3686)$ events collected with the BESIII detector, the decays $\psi(3686)\to\gamma\chi_{cJ}, \chi_{cJ} \to \gamma\gamma~(J=0, 1, 2)$ are studied. The decay branching fractions of $\chi_{c0,c2} \to \gamma\gamma$ are measured to be $\mathcal{B}(\chi_{c0}\to\gamma\gamma) = (1.93 \pm 0.08 \pm 0.05 \pm 0.05)\times 10^{-4}$ and $\mathcal{B}(\chi_{c2}\to\gamma\gamma) = (3.10 \pm 0.09 \pm 0.07 \pm 0.11)\times 10^{-4} $, which correspond to two-photon decay widths of $\Gamma_{\gamma\gamma}(\chi_{c0}) = 2.03 \pm 0.08 \pm 0.06 \pm 0.13 ~\rm{keV}$ and $\Gamma_{\gamma\gamma}(\chi_{c2}) = 0.60 \pm 0.02 \pm 0.01 \pm 0.04 ~\rm{keV}$ with a ratio of $\mathcal{R}=\Gamma_{\gamma\gamma}(\chi_{c2})/\Gamma_{\gamma\gamma}(\chi_{c0})= 0.295 \pm 0.014 \pm 0.007 \pm 0.027$, where the uncertainties are statistical, systematic and associated with the uncertainties of $\mathcal{B}(\psi(3686)\to\gamma\chi_{c0,c2})$ and the total widths $\Gamma(\chi_{c0,c2})$, respectively. For the forbidden decay of $\chi_{c1}\to\gamma\gamma$, no signal is observed, and an upper limit on the two-photon width is obtained to be $\Gamma_{\gamma\gamma}(\chi_{c1})<5.3 ~\rm{eV}$ at the 90\% confidence level. The ratio of the two-photon widths between helicity-zero and helicity-two components in the decay $\chi_{c2}\to\gamma\gamma$ is also measured to be $f_{0/2} = \Gamma^{\lambda=0}_{\gamma\gamma}(\chi_{c2})/\Gamma^{\lambda=2}_{\gamma\gamma}(\chi_{c2}) = (0.0 \pm 0.6 \pm 1.2)\times 10^{-2}$, where the uncertainties are statistical and systematic, respectively.

Radiation Hardness tests with neutron flux on different Silicon photomultiplier devices

Radiation hardness is an important requirement for solid state readout devices operating in high radiation environments common in particle physics experiments. The MEG II experiment, at PSI, Switzerland, investigates the forbidden decay μ+ → e+ γ. Exploiting the most intense muon beam of the world. A significant flux of non-thermal neutrons (kinetic energy Ek⩾ 0.5 MeV) is present in the experimental hall produced along the beam-line and in the hall itself. We present the effects of neutron fluxes comparable to the MEG II expected doses on several Silicon Photomultiplier (SiPMs). The tested models are: AdvanSiD ASD-NUV3S-P50 (used in MEG II experiment), AdvanSiD ASD-NUV3S-P40, AdvanSiD ASD-RGB3S-P40, Hamamatsu and Excelitas C30742-33-050-X. The neutron source is the thermal Sub-critical Multiplication complex (SM1) moderated with water, located at the University of Pavia (Italy). We report the change of SiPMs most important electric parameters: dark current, dark pulse frequency, gain, direct bias resistance, as a function of the integrated neutron fluency.

The MEGII detector

We present a report of the MEG II experiment, the upgrade of MEG, whose goal is to search for the forbidden decay μ+ → e+γ with increased precision. After having briefly reviewed the motivation for such a search and the current limit due to MEG, we present the conceptual design of the detector detailing for each subdetector the motivations and the extent of the upgrade and the expected resolution improvements. Novel subdetectors and calibration hardware are introduced. We conclude with the expected sensitivity of the MEGII experiment.

A quark model study of strong decays of X(3915)

Strong decays of are analyzed from two quark model descriptions of , a conventional one in terms of the Cornell potential and an unconventional one from a generalized screened potential. We conclude that the experimental suppression of the OZI allowed decay might be explained in both cases due to the momentum dependence of the decay amplitude. However, the experimental significance of the OZI forbidden decay could favor an unconventional description.

Spectrum-shape method and the next-to-leading-order terms of theβ-decay shape factor

Effective values of the axial-vector coupling constant ${g}_{\mathrm{A}}$ have lately attracted much attention due to the prominent role of ${g}_{\mathrm{A}}$ in determining the half-lives of double $\ensuremath{\beta}$ decays, in particular their neutrinoless mode. The half-life method, i.e., comparing the calculated half-lives to the corresponding experimental ones, is the most widely used method to access the effective values of ${g}_{\mathrm{A}}$. The present paper investigates the possibilities offered by a complementary method: the spectrum-shape method (SSM). In the SSM, comparison of the shapes of the calculated and measured $\ensuremath{\beta}$ electron spectra of forbidden nonunique $\ensuremath{\beta}$ decays yields information on the magnitude of ${g}_{\mathrm{A}}$. In parallel, we investigate the impact of the next-to-leading-order terms of the $\ensuremath{\beta}$-decay shape function and the radiative corrections on the half-life method and the SSM by analyzing the fourfold forbidden decays of $^{113}\mathrm{Cd}$ and $^{115}\mathrm{In}$ by using three nuclear-structure theory frameworks; namely, the nuclear shell model, the microscopic interacting boson-fermion model, and the microscopic quasiparticle-phonon model. The three models yield a consistent result, ${g}_{\mathrm{A}}\ensuremath{\approx}0.92$, when the SSM is applied to the decay of $^{113}\mathrm{Cd}$ for which $\ensuremath{\beta}$-spectrum data are available. At the same time the half-life method yields results which are in tension with each other and the SSM result.

Search for theC-violating meson decayη→π0e+e−with WASA-at-COSY

TheWASA-at-COSY experiment is, besides meson production processes, dedicated to studies on rare and forbidden decays of light mesons. In extensive η -production beam times, a high-statistics data set have been collected by means of proton-proton scattering which opens the door to studies on forbidden decays like the C -parity violating process η → π 0 e + e − . In this article, an optimized detector calibration leading to a significantly improved missing mass resolution and a preliminary decay analysis are presented.