Two-body Decays Research Articles

Decay of the mediator particle at threshold

A light mediator particle is often predicted in the dark sector scenario, which weakly interacts with the standard model (SM) particles. The weakness of the interaction is usually described by a small coupling; however, the small coupling does not always guarantee the weakness of the interaction. When the mass of the mediator particle lies in a threshold region, the so-called threshold singularity may emerge, and then the perturbative calculation fails. This singularity causes several effects, e.g., the mixing between the mediator particle and bound states, the Sommerfeld effect on the final state of the mediator particle decay, etc. Taking the minimal model of the vector mediator particle decaying mainly into the SM particles as an example, we develop a method to describe the singularity quantitatively. We also calculate some physical quantities using this method, such as the lifetime of the mediator particle and find that those could be significantly altered compared with the result of the perturbative calculation.

Effective portals to heavy neutral leptons

The existence of right-handed neutrinos, or heavy neutral leptons (HNLs), is strongly motivated by the observation of neutrino masses and mixing. The mass of these new particles could lie below the electroweak scale, making them accessible to low-energy laboratory experiments. Additional new physics at high energies can mediate new interactions between the Standard Model particles and HNLs, and is most conveniently parametrized by the neutrino Standard Model Effective Field Theory, or νSMEFT for short. In this work, we consider the dimension six νSMEFT operators involving one HNL field in the mass range of O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(1) MeV < MN<O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(100) GeV. By recasting existing experimental limits on the production and decay of new light particles, we constrain the Wilson coefficients and new physics scale of each operator as a function of the HNL mass.1

Results on light (anti)hypernuclei production with ALICE at the LHC

The high collision energies reached at the LHC lead to significant production yields of light (anti)hypernuclei in proton–proton (pp), proton–lead (p–Pb) and, in particular, Pb–Pb collisions. The lightest known hypernucleus is the hypertriton, which is a bound state of a proton, a neutron, and a Λ hyperon. It decays weakly with a decay length of a few centimeters. The excellent tracking and particle identification capabilities of the ALICE detector, exploiting the energy loss measurement of the Time Projection Chamber (TPC) and using the Inner Tracking System (ITS) to distinguish between primary and secondary (decay) vertices, allow for the determination of the hypertriton yield across different collision systems, its lifetime, and its binding energy. The latest hypertriton lifetime measurement in Pb–Pb collisions performed in the 2-body decay channel will be presented. This measurement contributes to the solution of the hypertriton lifetime puzzle. In addition, the hypertriton production in different collision systems and at different energies will be compared to model predictions. Due to its low binding energy, and hence to its large size, the hypertriton is the ideal candidate to distinguish between statistical hadronization and coalescence models. With the precision of the presented yield measurements some variants of the aforementioned models can be excluded.

Optimizing Injection Locations Relaxes Altitude‐Lifetime Trade‐Off for Stratospheric Aerosol Injection

AbstractStratospheric Aerosol Injection (SAI) aims to offset some climate hazards by injecting aerosols into the stratosphere to reflect solar radiation. The lifetime of injected particles influences SAI's radiative efficacy—the ratio of radiative forcing to particle mass flux. We employ a Lagrangian trajectory model with particle sedimentation to simulate how background circulations influence the transport of passive particles (without microphysical growth) in the stratosphere and quantify sensitivities of particle lifetime to injection locations. At 20 km, optimizing injection locations can increase particle lifetime by &gt;40%. Injection strategies can be constrained to maintain an interhemispheric balance of particle lifetime without significantly decreasing total lifetime. Generally, increasing injection altitude increases particle lifetime while also increasing costs and environmental impacts of deployment aircraft. Optimizing injection latitude and longitude can relax this altitude‐lifetime trade‐off by increasing lifetime without needing to increase altitude, which warrants further testing in global climate models with aerosol microphysics.

Can the new states around 2.2 GeV be identified as the ω(3D)?

Recently, the BESIII Collaboration reported that there exist two explicit enhancement structures near 2.25[Formula: see text]GeV, one has the mass of [Formula: see text][Formula: see text]MeV and the width of [Formula: see text][Formula: see text]MeV and another structure has the mass of [Formula: see text][Formula: see text]MeV and the width of [Formula: see text][Formula: see text]MeV when analyzing the cross section of subprocesses. Earlier, the BESIII Collaboration reported the existence of the [Formula: see text], with a mass of [Formula: see text][Formula: see text]MeV and width of [Formula: see text][Formula: see text]MeV. Based on these experimental results, we carried out a study. We present the comparison of the mass spectrum between theoretical and experimental masses and examine the two-body strong decays of these states within two different approaches to the [Formula: see text] model. We find that the newly discovered states are most likely to be an [Formula: see text] state. This discovery could be useful in establishing entire [Formula: see text] mesons.

Robust and provably monotonic networks

The Lipschitz constant of the map between the input and output space represented by a neural network is a natural metric for assessing the robustness of the model. We present a new method to constrain the Lipschitz constant of dense deep learning models that can also be generalized to other architectures. The method relies on a simple weight normalization scheme during training that ensures the Lipschitz constant of every layer is below an upper limit specified by the analyst. A simple monotonic residual connection can then be used to make the model monotonic in any subset of its inputs, which is useful in scenarios where domain knowledge dictates such dependence. Examples can be found in algorithmic fairness requirements or, as presented here, in the classification of the decays of subatomic particles produced at the CERN Large Hadron Collider. Our normalization is minimally constraining and allows the underlying architecture to maintain higher expressiveness compared to other techniques which aim to either control the Lipschitz constant of the model or ensure its monotonicity. We show how the algorithm was used to train a powerful, robust, and interpretable discriminator for heavy-flavor-quark decays, which has been adopted for use as the primary data-selection algorithm in the LHCb real-time data-processing system in the current LHC data-taking period known as Run 3. In addition, our algorithm has also achieved state-of-the-art performance on benchmarks in medicine, finance, and other applications.

Keeping it simple: simplified frameworks for long-lived particles at neutrino facilities

Modern-day accelerator neutrino facilities are excellent venues for searches for new-physics particles. Many distinct new-physics models predict overlapping signatures and phenomenology in these experiments. In this work, we advocate for the adoption of simplified frameworks when studying these types of new-physics signatures, which are characterized by a small number of primary variables, including particle masses, lifetimes, and production and decay modes/rates that most directly control signal event rates and kinematics. In particular, taking the example of long-lived particles that decay inside a neutrino detector as a test case, we study formulate and study simplified frameworks in the context of light scalars/fermions produced in kaon decays which then decay into final states containing an electron-positron pair. We show that using these simplified frameworks can allow for individual experimental analyses to be applicable to a wide variety of specific model scenarios. As a side benefit, we demonstrate that using this approach can allow for the T2K collaboration, by reinterpreting its search for Heavy Neutral Leptons, to be capable of setting world-leading limits on the Higgs-Portal Scalar model. Furthermore, we argue the simplified framework interpretation can serve as a bridge to model identification in the hopeful detection of a new-physics signal. As an illustration, we perform a first determination of the likelihood that, in the presence of a new-physics signal in a detector like the DUNE ND-GAr, multiple different new-physics hypotheses (such as the Higgs-Portal Scalar and Heavy Neutral Lepton ones) can be disentangled. We demonstrate that this model discrimination is favorable for some portions of detectable new-physics parameter space but for others, it is more challenging.

Looking for an axion in a haystack of muons

The search for axion-like particles X in muon decays is an excellent opportunity for the MEG II and Mu3e experiments to extend their horizons beyond μ + → e + γ and μ + → e + e - e +. A suitable process for both experiments is the two-body decay μ + → e + X, whose only signature is a monochromatic peak close to the kinematic endpoint of the positron energy spectrum of the μ + → e + νeν̅μ background. The hunt for such an elusive signal in a vast amount of irreducible background requires extremely accurate theoretical predictions to be implemented in a Monte Carlo event generator. This work presents a new state-of-the-art computation of μ + → e + νeν̅μ for polarised muons, accomplished with the McMule framework. The calculation includes next-to-next-leading order QED corrections and logarithmically enhanced terms at even higher orders. The results are also used to estimate the sensitivity of both experiments on the branching ratio of μ + → e + X, in order to evaluate the impact of the theoretical error.

Multi-species thermalization cascade of energetic particles in the early universe

Heavy long-lived particles are abundant in BSM physics and will, under generic circumstances, get to dominate the energy density of the universe. The resulting matter dominated era has to end through the decay of the heavy matter component particles of mass M, and before the universe cools down to a temperature of a few MeV, in order not to upset the successful predictions of Big Bang Nucleosynthesis. The process of thermalization of their decay products, of initial energy ∼ M/2, into a thermal bath of temperature T primarily involves near-collinear splittings of energetic particles into two particles with lower energy. The correct treatment of these processes requires the inclusion of coherence effects which suppress the splitting rate. We write down and numerically solve the resulting coupled Boltzmann equations including all gauge bosons and fermions of the unbroken phase of the Standard Model (SM). We then comment on the dependence of the nonthermal spectra on the ratio M/T, as well as on the matter decay rate and branching ratios into various SM particles.

Study of tagged neutrino beam characteristics at the U-70 accelerator

The method of tagged neutrino beams is considered, based on the reconstruction of the kinematics of π+ → µ+νμ decay of the parent particle one can accurately recovers the momentum and type of the produced neutrino. We described the technique of the π+ parent particle beam formation on the U-70 accelerator (Protvino) channel. Also, the calculated characteristics of the corresponding beam are given.

Search for long-lived particles using out-of-time trackless jets in proton-proton collisions at sqrt{s} = 13 TeV

A search for long-lived particles decaying in the outer regions of the CMS silicon tracker or in the calorimeters is presented. The search is based on a data sample of proton-proton collisions at sqrt{s} = 13 TeV recorded with the CMS detector at the LHC in 2016–2018, corresponding to an integrated luminosity of 138 fb−1. A novel technique, using nearly trackless and out-of-time jet information combined in a deep neural network discriminator, is employed to identify decays of long-lived particles. The results are interpreted in a simplified model of chargino-neutralino production, where the neutralino is the next-to-lightest supersymmetric particle, is long-lived, and decays to a gravitino and either a Higgs or Z boson. This search is most sensitive to neutralino proper decay lengths of approximately 0.5 m, for which masses up to 1.18 TeV are excluded at 95% confidence level. The current search is the best result to date in the mass range from the kinematic limit imposed by the Higgs boson mass up to 1.8 TeV.

Search for new phenomena in multi-body invariant masses in events with at least one isolated lepton and two jets using sqrt{s} = 13 TeV proton–proton collision data collected by the ATLAS detector

A search for resonances in events with at least one isolated lepton (e or μ) and two jets is performed using 139 fb−1 of sqrt{s} = 13 TeV proton–proton collision data recorded by the ATLAS detector at the LHC. Deviations from a smoothly falling background hypothesis are tested in three- and four-body invariant mass distributions constructed from leptons and jets, including jets identified as originating from bottom quarks. Model-independent limits on generic resonances characterised by cascade decays of particles leading to multiple jets and leptons in the final state are presented. The limits are calculated using Gaussian shapes with different widths for the invariant masses. The multi-body invariant masses are also used to set 95% confidence level upper limits on the cross-section times branching ratios for the production and subsequent decay of resonances predicted by several new physics scenarios.

JUNO sensitivity on proton decay p → ν K + searches* *We are grateful for the ongoing cooperation from the China General Nuclear Power Group. This study was supported by the Chinese Academy of Sciences, the National Key R&D Program of China, the CAS Center for Excellence in Particle Physics, Wuyi University, and the Tsung-Dao Lee Institute of Shanghai Jiao

The Jiangmen Underground Neutrino Observatory (JUNO) is a large liquid scintillator detector designed to explore many topics in fundamental physics. In this study, the potential of searching for proton decay in the mode with JUNO is investigated. The kaon and its decay particles feature a clear three-fold coincidence signature that results in a high efficiency for identification. Moreover, the excellent energy resolution of JUNO permits suppression of the sizable background caused by other delayed signals. Based on these advantages, the detection efficiency for the proton decay via is 36.9% ± 4.9% with a background level of events after 10 years of data collection. The estimated sensitivity based on 200 kton-years of exposure is years, which is competitive with the current best limits on the proton lifetime in this channel and complements the use of different detection technologies.

Asymmetric dark matter abundance including non-thermal production

We investigate the relic abundance of asymmetric dark matter where the asymmetric dark matter is non–thermally produced from the decay of heavier particles in addition to the usual thermal production. We discuss the relic density of asymmetric dark matter including the decay of heavy particles in low-temperature scenarios. Here, we still assume that the Universe is radiation-dominated and there is asymmetry before the decay of heavy particles. We obtain an increased abundance of asymmetric dark matter when there is an additional contribution from the decay of heavier particles. Finally, we find the constraints on the asymmetry factor and annihilation cross-section using Planck data.

UFO 2.0: the ‘Universal Feynman Output’ format

We present an update of the Universal FeynRules Output model format, commonly known as the UFO format, that is used by several automated matrix-element generators and high-energy physics software. We detail different features that have been proposed as extensions of the initial format during the last ten years, and collect them in the current second version of the model format that we coin the Universal Feynman Output format. Following the initial philosophy of the UFO, they consist of flexible and modular additions to address particle decays, custom propagators, form factors, the renormalisation group running of parameters and masses, and higher-order quantum corrections.

Dynamics of laser-induced formation of color centers in a lithium fluoride crystal

The dynamics of the laser-induced formation of aggregate color centers under the action of the first (950 nm) and second (475 nm) harmonics of a femtosecond Ti:sapphire laser has been studied. During the irradiation of the samples with laser radiation, the photoluminescence spectra of the induced centers were repeatedly measured. It was found that intensity of the spectral luminescence band of F3+ centers exceeds intensity of the band of F2 centers in samples with color centers induced by the first harmonic. At the same time, the opposite ratio of the intensities of these bands was observed in the luminescence spectra of color centers produced by second harmonic radiation for the same energy of laser pulses. It has been established that the observed effect is caused by different degree of heating of the crystalline medium in the regions of passage of laser filaments. Heating accelerates the migration of kinetic particles that carry out the aggregation process. The ratio of the concentrations of F2 and F3+ centers depends on the ratio of the duration of irradiation and the lifetime of kinetic particles, which significantly depends on temperature. A multiphoton-tunneling mode of interband ionization of a crystalline substance was implemented in the experiment. It is shown that the identity of the absorption spectra of color centers induced by the first and second harmonics of femtosecond laser radiation is a new distinguishing characteristic of experimental implementation of the tunneling ionization mechanism in experiment.

Recent developments in data reconstruction for aerogel RICH at Belle II

In the forward end-cap of the Belle II spectrometer, particle identification is provided by a proximity focusing RICH detector with an aerogel radiator (ARICH). The ARICH’s primary function is to effectively distinguish between pions and kaons in the momentum range of 0.5GeV/c to about 4GeV/c, as well as to contribute to identification of low-momentum leptons. Since its operation began, Belle II has collected over 420fb−1 of data. Based on this large data sample, studies of several effects that impact the performance of the ARICH detector were carried out. In this paper, we present a comparison of the observed Cherenkov ring image and detector particle identification performance in the measured data and detector simulation. Furthermore, we highlight recent efforts aimed at enhancing the ARICH’s performance by taking into account the effects of particle decay in flight and scattering in materials before the detector, as well as by refining the probability density function used for particle identification likelihood evaluation.

Investigating the gamma-ray burst from decaying MeV-scale axion-like particles produced in supernova explosions

We investigate the characteristics of the gamma-ray signal following the decay of MeV-scale Axion-Like Particles (ALPs) coupled to photons which are produced in a Supernova (SN) explosion. This analysis is the first to include the production of heavier ALPs through the photon coalescence process, enlarging the mass range of ALPs that could be observed in this way and giving a stronger bound from the observation of SN 1987A. Furthermore, we present a new analytical method for calculating the predicted gamma-ray signal from ALP decays. With this method we can rigorously prove the validity of an approximation that has been used in some of the previous literature, which we show here to be valid only if all gamma rays arrive under extremely small observation angles (i.e. very close to the line of sight to the SN). However, it also shows where the approximation is not valid, and offers an efficient alternative to calculate the ALP-induced gamma-ray flux in a general setting when the observation angles are not guaranteed to be small. We also estimate the sensitivity of the Fermi Large Area Telescope (Fermi-LAT) to this gamma-ray signal from a future nearby SN and show that in the case of a non-observation the current bounds on the ALP-photon coupling gaγ are strengthened by about an order of magnitude. In the case of an observation, we show that it may be possible to reconstruct the product gaγ 2 ma , with ma the mass of the ALP.

Placing of the recently observed bottom strange states BsJ(6063) and BsJ(6114) in bottom spectra

Abstract We employ the heavy quark effective theory (HQET) to give spin-parity quantum numbers for the bottom strange states BsJ(6063) and BsJ(6114) recently observed by the LHCb Collaboration [9]. By exploring flavor-independent parameters $\Delta _{F}^{(c)} =\Delta _{F}^{(b)}$ and $\lambda _{F}^{(c)} = \lambda _{F}^{(b)}$ appearing in the HQET Lagrangian, we calculate the masses of the experimentally missing bottom strange meson states 2S, 1P, 1D. The parameter ΔF appears in the HQET Lagrangian and gives the spin-averaged mass splitting between excited-state doublets (F) and ground-state doublets (H). Another parameter λF comes from first-order corrections in the HQET Lagrangian and gives hyperfine splittings. We also analyze these bottom strange masses by taking 1/mQ corrections, which lead to modifications of parameter terms of $\Delta _{F}^{(b)} =\Delta _{F}^{(c)} + \delta \Delta _F$ and $\lambda _{F}^{(b)} = \lambda _{F}^{(c)}\delta \lambda _F$. Further, we analyze their two-body decays, couplings, and branching ratios via the emission of light pseudoscalar mesons. Based on the predicted masses and decay widths, we tentatively identify the states BsJ(6063) as 23S1 and BsJ(6114) as 13D1. Our predictions provide crucial information for future experimental studies.

On the Global Temperature of the Schwarzschild–de Sitter Spacetime

It is shown that the Schwarzschild–de Sitter spacetime has the universal temperature. This temperature describes the thermal processes of decay of the composite particles and the other processes, which are energetically forbidden in the Minkowski spacetime, but are allowed in the de Sitter and in Schwarzschild–de Sitter backgrounds. In particular, this temperature describes the probability of ionization of the atom in the Schwarzschild–de Sitter, which is observed by the stationary observer at the point where the shift function (velocity) in the Arnowitt–Deser–Misner formalism changes sign. This activation temperature does not depend on the black hole mass and is fully determined by the Hubble parameter, {{T}_{{{text{SdS}}}}} = sqrt 3 H{text{/}}pi . This temperature is twice the Bousso–Hawking temperature {{T}_{{{text{BH}}}}}, which characterizes the limit of degenerate Lorentzian Schwarzschild–de Sitter universe, when the cosmological and black hole horizons are close to each other, {{T}_{{{text{SdS}}}}} = 2{{T}_{{{text{BH}}}}}. The similar doubling of the temperature of Hawking radiation is known in the pure de Sitter spacetime, where the corresponding local temperature describing the ionization of atoms is twice the Gibbons–Hawking temperature, {{T}_{{{text{dS}}}}} = 2{{T}_{{{text{GH}}}}} = H{text{/}}pi . We suggest that the activation temperature {{T}_{{{text{dS}}}}} can be considered as the thermodynamic temperature of the de Sitter state, which determines the local entropy in this state, s = 3H{text{/}}4G.