Decay In Flight Research Articles

First Measurement of the Michel Parameter ξ^{'} in the τ^{-}→μ^{-}ν[over ¯]_{μ}ν_{τ} Decay at Belle.

We report the first measurement of the Michel parameter ξ^{'} in the τ^{-}→μ^{-}ν[over ¯]_{μ}ν_{τ} decay with a new method proposed just recently. The measurement is based on the reconstruction of the τ^{-}→μ^{-}ν[over ¯]_{μ}ν_{τ} events with subsequent muon decay in flight in the Belle central drift chamber. The analyzed data sample of 988 fb^{-1} collected by the Belle detector corresponds to approximately 912×10^{6} τ^{+}τ^{-} pairs. We measure ξ^{'}=0.22±0.94(stat)±0.42(syst), which is in agreement with the standard model prediction of ξ^{'}=1. Statistical uncertainty dominates in this study, being a limiting factor, while systematic uncertainty is well under control. Our analysis proved the practicability of this promising method and its prospects for further precise measurement in future experiments.

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.

First results for searches of exotic decays with NA62 in beam dump mode

We report the results of a search for dark photon in-flight decays to μ + μ − pairs at NA62. Dark photons could be produced by dumping protons onto a copper-iron absorber, reach the NA62 decay volume more than 80 m downstream of the dump and decay therein. The measurement is based on data collected in 2021 which correspond to 1.4 × 1017 dumped protons.

Modeling the Albedo Neutron Decay Source of Radiation Belt Electrons and Protons

Modeling the Albedo Neutron Decay Source of Radiation Belt Electrons and Protons

Heavy neutral leptons below the kaon mass at hodoscopic neutrino detectors

Heavy neutral leptons ($N$) below the kaon mass are severely constrained by cosmology and lab-based searches for their decays in flight. If $N$ interacts via an additional force, $N\to\nu e^+e^-$ decays are enhanced and cosmological limits can be avoided. We show that the T2K and MicroBooNE neutrino experiments provide the best limits on the mixing of $N$ with muon-neutrinos, outperforming past-generation experiments, previously thought to dominate. We constrain models with electromagnetically-decaying and long-lived $N$, such as in a transition-magnetic-moment portal and in a leptophilic axion-like particle portal, invoked to explain the MiniBooNE excess. By considering these models as representative examples, our results show that explanations of the MiniBooNE excess that involve $e^+e^-$ pairs from long-lived particles are in tension with T2K, PS191, and MicroBooNE data. Similarly, these searches also constrain MiniBooNE explanations based on single photons due to the associated $e^+e^-$ decay mode via a virtual photon.

An LGAD-Based Full Active Target for the PIONEER Experiment

PIONEER is a next-generation experiment to measure the charged pion branching ratios to electrons vs. muons Re/μ=Γπ+→e+ν(γ)Γπ+→μ+ν(γ) and pion beta decay (Pib) π+→π0eν. The pion to muon decay (π→μ→e) has four orders of magnitude higher probability than the pion to electron decay (π→eν). To achieve the necessary branching-ratio precision it is crucial to suppress the π→μ→e energy spectrum that overlaps with the low energy tail of π→eν. A high granularity active target (ATAR) is being designed to suppress the muon decay background sufficiently so that this tail can be directly measured. In addition, ATAR will provide detailed 4D tracking information to separate the energy deposits of the pion decay products in both position and time. This will suppress other significant systematic uncertainties (pulse pile-up, decay in flight of slow pions) to <0.01%, allowing the overall uncertainty in to be reduced to O (0.01%). The chosen technology for the ATAR is Low Gain Avalanche Detector (LGAD). These are thin silicon detectors (down to 50 μm in thickness or less) with moderate internal signal amplification and great time resolution. To achieve a 100% active region several emerging technologies are being evaluated, such as AC-LGADs and TI-LGADs. A dynamic range from MiP (positron) to several MeV (pion/muon) of deposited charge is expected, the detection and separation of close-by hits in such a wide dynamic range will be a main challenge. Furthermore, the compactness and the requirement of low inactive material of the ATAR present challenges for the readout system, forcing the amplifier chip and digitizer to be positioned away from the active region.

Hunting wino and higgsino dark matter at the muon collider with disappearing tracks

We study the capabilities of a muon collider experiment to detect disappearing tracks originating when a heavy and electrically charged long-lived particle decays via X+→ Y+Z0, where X+ and Z0 are two almost mass degenerate new states and Y+ is a charged Standard Model particle. The backgrounds induced by the in-flight decays of the muon beams (BIB) can create detector hit combinations that mimic long-lived particle signatures, making the search a daunting task. We design a simple strategy to tame the BIB, based on a detector-hit-level selection exploiting timing information and hit-to-hit correlations, followed by simple requirements on the quality of reconstructed tracks. Our strategy allows us to reduce the number of tracks from BIB to an average of 0.08 per event, hence being able to design a cut-and-count analysis that shows that it is possible to cover weak doublets and triplets with masses close to sqrt{s}/2 in the 0.1–10 ns range. In particular, this implies that a 10 TeV muon collider is able to probe thermal MSSM higgsinos and thermal MSSM winos, thus rivaling the FCC-hh in that respect, and further enlarging the physics program of the muon collider into the territory of WIMP dark matter and long-lived signatures. We also provide parton-to-reconstructed level efficiency maps, allowing an estimation of the coverage of disappearing tracks at muon colliders for arbitrary models.

Progress of experimental studies on two-proton emission

<p indent=0mm>Radioactivity is one of the most significant characteristics of unstable nuclei, the familiar modes including α, β, γ decay and fission. Two-proton (2p) emission is an exotic radioactivity mode of nuclei in the proton drip-line area, which is very significant for studying the structure, mass and correlation between proton pair of extremely proton-rich nuclei and one of the frontiers in radioactive ion beam physics. In this article, progress of experimental studies on two-proton emission was briefly reviewed. Several experimental measurement methods were introduced. The identification method for the mechanism of two-proton emission was discussed. Two-proton emission was predicted to be existed in extremely proton-rich nuclei in 1960. For 2p emission from the ground state of a nucleus with long half-life, it was observed for the first time in 2002 due to the difficulty in the production of a candidate. Only few decay events were observed for the four proton drip-line nuclei with a long half-life ground state. For the detection of 2p emission process, the in-flight decay, implantation decay and optical time projection chamber (OTPC) plus CCD method are the three main experimental methods. The in-flight decay method is suitable for the candidate with very short half-life and almost inapplicable for the candidate with a half-life longer than 1 μs. The implantation decay method is suitable for the candidate with long half-life, especially for β-delayed decay nuclei. In this case, the measured decay signal will be very clear with low background due to the separation recording of the implantation and decay events. The OTPC plus CCD method could record a picture of 2p emission process directly for the candidate. Generally, there are three mechanisms for proton-rich nuclei to emit two protons: (1) ²He cluster emission; (2) three-body emission; (3) sequential emission. The ²He cluster emission is basically two protons constrained by the pair correlation into a quasi-bound singlet, i.e., ¹S₀ configuration. Such a quasi-bound state can only exist for very short time and then becomes separated after penetrating through the Coulomb barrier. Usually, the two-proton relative momentum and the opening angle distributions are used to identify the mechanism of 2p emission in experimental investigations. However, this method can only distinguish it being ²He cluster emission or not. For the three-body and sequential emissions, the key difference is the emission time between the two protons. The proton-proton momentum correlation function (also called HBT) method is a widely used analysis technique in nuclear physics, which can extract the emission time of a particle. It has been demonstrated that the three-body and sequential emissions could be distinguished by using the HBT method. The difficulty in the study of exotic radioactivity is the low production rate of the unstable nuclei in the drip-line area. The high-intensity heavy-ion accelerator facility (HIAF) under construction in China has an advantage of high beam intensity, the most frontier work in the field of exotic radioactivity for nuclei around the drip-line could be performed on it by adopting the superior detection technique and investigation method.

Experimental studies of clustering in light nuclei: 11,12,13,16C

Different, complementary, techniques are used to experimentally probe clustering aspects in several carbon isotopes: $$^{11,12,13,16}$$C. Our approaches involve breakup reactions with radioactive cocktail beams ($$^{16}$$C), compound nucleus reactions and resonant scattering at low energies ($$^{11,13}$$C), and direct reactions with the detection of in-flight resonance decay fragments ($$^{12}$$C). In this paper, we discuss results of our experimental campaign: in $$^{11}$$C, we unveil the existence of a new excited state, characterized by clustering nature, at an excitation energy of 9.36 MeV ($$5/2^-$$); the decay path of the Hoyle state in $$^{12}$$C (7.654 MeV, $$0^+$$) is investigated with unprecedented precision; we refine the spectroscopy of $$^{13}$$C above the $$\alpha $$-threshold supporting the possible appearance of the $$K^\pi ={3/2}^\pm $$ molecular bands, based on the $$\alpha + ^{9}$$Be structure, previously discussed in the literature; in $$^{16}$$C, we find evidence for $$^{6}$$He–$$^{10}$$Be decays. Our findings have an impact on the understanding of clustering phenomena in light nuclear systems.

Single-muon rate reduction for beam dump operation of the K12 beam line at CERN

The Physics Beyond Colliders study investigates the feasibility of future projects and experiments using CERN facilities. In the scope of this study, a future operation of the NA62 experiment using the existing K12 beam line operated in beam dump mode is discussed. Such a setup allows for a search for dark sector particles, e.g. heavy neutral leptons, dark photons and axions. Production of these hypothetical particles requires the 400GeV/c proton beam extracted from the Super Proton Synchrotron (SPS) to be dumped on a massive dump collimator located in the first part of the K12 beam line. The decay of these dark sector particles, for example, into muons could be detected by the NA62 experimental setup. However, muons created in particle interactions with material and from in-flight decays of secondary hadrons pose a critical background for this kind of experiment. Magnetic sweeping serves as an efficient tool for reducing this background. This paper discusses the simulation studies conducted with the software framework G4beamline to reduce the expected single-muon rate for this experiment. The required beam line modifications, in particular the removal of the beryllium target and repositioning of the dump, have been implemented into this model. A comparison of simulated muon distributions and spectra available experimental data, already taken in test runs, has been made in order to validate the beam line simulation. Finally, additional beam line modifications to further suppress the muon background are elaborated upon. These studies show a potential further reduction of a factor four of the total single-muon rate relative to the nominal setup over the full momentum range, and up to a factor twenty considering only muons with momenta [Formula: see text].

Intruder dominance in the 02+ state of Mg32 studied with a novel technique for in-flight decays

The development of advanced γ-ray tracking arrays allows for a sensitive new technique to investigate elusive states of exotic nuclei with fast rare-isotope beams. By taking advantage of the excellent energy and position resolution of the Gamma-Ray Energy Tracking In-beam Nuclear Array, we developed a novel technique to identify in-flight isomeric decays of the 0⁺₂ state in 32Mg populated in a two-proton removal reaction. We confirm the 0⁺₂→2+1γ-ray transition of ³²Mg and constrain the 0⁺₂ decay lifetime, suggesting a large collectivity. The small partial cross section populating the 0⁺₂ state in this reaction provides experimental evidence for the reduced occupancy of the normal configuration of the 0⁺₂ state, indicating the intruder dominance of this state.

An annular parallel plate avalanche counter for heavy-ion [formula omitted]-ray coincidence measurements

The characteristics and performance of an annular parallel plate avalanche counter (APPAC) developed for performing heavy ion nuclear physics experiments are presented. The main motive of developing this detector system is to perform Coulomb excitation (Coulex) experiments using particle–γ coincidence technique. The Coulex measurements are performed with APPAC and clover germanium detectors. The APPAC has a two electrode geometry and provides information about the scattering angles of the reaction products, which along with their velocity and/or energy information is used for correcting Doppler energy shift of γ-ray events coming from the in-flight decay of reaction products. The detector has an active area of ∼ 410 cm2 and provides scattering angle information in two coordinates namely polar angle θ and azimuthal angle ϕ. The ϕ resolution is fixed at 22.5°, and the θ resolution for the present experimental configuration is about 0.6°. Average rise times of the signals after amplification was observed to be 5 ns. During the experiments, the detector could handle heavy ion count rates of about 25 x 104 pps without any deterioration in the quality of signals.

Searching for dark sector with missing mass technique in fixed target experiments

Currently, the existence of a dark sector almost completely decoupled from the Standard Model is a viable solution for numerous long-standing problems in physics, including the nature of dark matter and the muon anomalous magnetic moment. A new gauge mediator, the dark photon, could be the portal to this hidden sector. The most general probe to its existence is the missing mass technique which requires a precise knowledge of the initial state of the process but does not put constraints on the dark photon final states. The experimental approaches to the search for dark photons in positron-on-target annihilation and in mesons decay in flight are presented and the physics reach is discussed.

Signals of HE atmospheric μ decay in flight around the Sun’s albedo versus astrophysical νμ and ντ traces in the Moon shadow

The Sun albedo of Cosmic Rays (CRs) at GeVs energy has been discovered recently by the FERMI satellite. They are traces of atmospheric CRs hitting solar atmosphere and reflecting skimming gamma photons. Even if relevant for astrophysics, as being a trace of atmospheric solar CR noises they cannot offer any signal of neutrino astronomy. On the contrary, the Moon with no atmosphere, may become soon a novel filtering calorimeter and an amplifier of energetic muon astronomical neutrinos (at TeV up to hundred TeVs energy); these lepton tracks leave an imprint in their beta decay while in flight to Earth. Their TeV electron air-shower are among the main signals. Also, a more energetic, but more rare, PeV up to EeV tau lunar neutrino events may be escaping as a tau lepton from the Moon: [Formula: see text] PeV secondaries, then, may be shining on Earth’s atmosphere in lunar shadows in a surprising way. One or a few gamma air-shower events inside the Moon shadows may occur each year in near future Cherenkov telescope array (CTA) or large high altitude air shower observatory (LHAASO) TeV gamma array detector, assuming a nonnegligible astrophysical TeV up to hundred TeV neutrino component (with respect to our terrestrial ruling atmospheric ones); these signals will open a new wonderful passe-partout keyhole for neutrino, been seen along the Moon. The lunar solid angle is small and the muon or tau expected rate is rare, but with the future largest tau radio array as the giant radio array for neutrino detection (GRAND), one might well discover such neutrino imprint.

Spectroscopy of excited states of unbound nuclei Ar30 and Cl29

Several states of proton-unbound isotopes $^{30}$Ar and $^{29}$Cl were investigated by measuring their in-flight decay products, $^{28}$S+proton+proton and $^{28}$S+proton, respectively. A refined analysis of $^{28}$S-proton angular correlations indicates that the ground state of $^{30}$Ar is located at $2.45^{+0.05}_{-0.10}$ MeV above the two-proton emission threshold. The theoretical investigation of the $^{30}$Ar ground state decay demonstrates that its mechanism has the transition dynamics with a surprisingly strong sensitivity of the correlation patterns of the decay products to the two-proton decay energy of the $^{30}$Ar ground state and the one-proton decay energy as well as the one-proton decay width of the $^{29}$Cl ground state. The comparison of the experimental $^{28}$S-proton angular correlations with those resulting from Monte Carlo simulations of the detector response illustrates that other observed $^{30}$Ar excited states decay by sequential emission of protons via intermediate resonances in $^{29}$Cl. Based on the findings, the decay schemes of the observed states in $^{30}$Ar and $^{29}$Cl were constructed. For calibration purposes and for checking the performance of the experimental setup, decays of the previously-known states of a two-proton emitter $^{19}$Mg were remeasured. Evidences for one new excited state in $^{19}$Mg and two unknown states in $^{18}$Na were found.

Recent results from NA62 and NA48

This paper reviews recent results form the NA62 and NA48 experiments at CERN. NA62 has searched for the K → πνν¯ with an in-flight decay technique. The analysis of 1.2 · 1011 decays has found one event compatible with the Standard Model prediction, giving an upper limit for the branching ratio of 1.4 · 10-9 (95% CL) in the background assumption. Prospects for improvements in the measurement are given. Results on the Kl3 decay form factors from NA48/2 are presented as well as the first observation of the decay K± → π ± π0e+e- and the measurement of its branching ratio from the same experiment.

VALOR joint oscillation analysis using multiple LAr-TPCs in the Booster Neutrino Beam at Fermilab

Anomalies observed by different experiments, the most significant ones being the ∼3.8 sigma νe appearance in a ∼50 MeV νµ beam from muon decay at rest observed by the LSND experiment and the ∼3.8 sigma νe and appearance in a ∼1 GeV neutrino beam from pion decay in flight observed by MiniBooNE, suggest the existence of sterile neutrinos. The Short Baseline Neutrino (SBN) program at Fermilab aims to perform a sensitive search for sterile neutrinos by performing analyses of νe appearance and νµ disappearance employing three Liquid Argon Time Projection Chambers (LAr-TPCs) at different baselines.The VALOR neutrino fitting group was established within the T2K experiment and has led numerous flagship T2K oscillation analyses, and provided sensitivity and detector optimisation studies for DUNE and Hyper-K. The neutrino oscillation framework developed by this group is able to perform fits of several samples and systematic parameters within different neutrino models and experiments. Thus, VALOR is an ideal environment for the neutrino oscillation fits using multiple LAr-TPC detectors with proper treatment of correlated systematic uncertainties necessary for the SBN analyses.

Towards T2K neutrino flux predictions using the replica target measurements by NA61/SHINE

The precise knowledge of the neutrino flux composition and kinematics is one of the biggest challenges of long-baseline experiments such as T2K. Neutrinos are made by the in-flight decay of unstable hadrons produced by the interactions of 31GeV/c protons in a long graphite target. Mostly π+ (π−) are created, leading to the enhanced flux. As kaons and muons are also produced, an irreducible background of electron (anti-)neutrino is also present. The main source of uncertainty in the flux prediction is driven by the lack of data on the proton-carbon interaction in this energy range. The measurements performed by the NA61/SHINE large-acceptance experiment at CERN are used by the T2K collaboration to improve the flux predictions. Two datasets have been taken: using a thin target to study the primary interaction, and a replica of the T2K target to account for the re-interactions. The recently released differential multiplicity distributions of π± along the replica target measured in NA61/SHINE will be presented. This dataset is now in the process of being used by T2K to further tune the flux prediction as 90% of the neutrinos will be directly constrained.

Gamma-ray triangles: A possible signature of asymmetric dark matter in indirect searches

We introduce a new type of gamma-ray spectral feature, which we denominate gamma-ray triangle. This spectral feature arises in scenarios where dark matter self-annihilates via a chiral interaction into two Dirac fermions, which subsequently decay in flight into another fermion and a photon. The resulting photon spectrum resembles a sharp triangle and can be readily searched for in the gamma-ray sky. Using data from the Fermi-LAT and H.E.S.S. instruments, we find no evidence for such spectral feature and therefore set strong upper bounds on the corresponding annihilation cross section. A concrete realization of a scenario yielding gamma-ray triangles consists of an asymmetric dark matter model where the dark matter particle carries lepton number. We show explicitly that this class of models can lead to intense gamma-ray spectral features, potentially at the reach of upcoming gamma-ray telescopes, opening a new window to explore asymmetric dark matter through indirect searches.

Tau Now

Ultra High Energy Cosmic Rays and UHE neutrinos may lead to a new deep astronomy. However the most recent results on their correlations and clustering seem to most authors inconclusive. We briefly remind some UHECR models and past and recent results. Our reading and overlapping of IR-gamma-UHECR maps and their correlations seem to answer to several key puzzles, offering a first hope of the UHECR astronomy, mostly ruled by lightest nuclei from nearby Universe. Regarding the UHE neutrino we recently noted that the flavor ratio and the absence of double bang in IceCube within highest energetic ten events may suggest still a dominant noisy prompt component. However a first correlated UHE crossing muon with expected location (through going upward muon neutrino or horizontally) in IceCube is in our view a milestone in neutrino astronomy road map, possibly partially related, to galactic UHECR narrow clustering. The disturbing and persistent atmospheric neutrino noises, both conventional and prompt, call for a better filtered neutrino astronomy: the tau neutrino ones. There are no yet (at present, detectable) TeV-PeVs or more energetic tau neutrino of atmospheric, conventional or prompt nature; only astrophysical ones might soon shine. Double bangs in IceCube and in particular the tau air-showers in large array are the unique definitive expected signatures of astrophysical signals. In particular tau air-shower amplify in a huge way the otherwise single lepton track, once in decay in flight, into a richest three of secondaries (up to a million of billion Cherenkov photons for PeV tau energy) whose wide areas may extend up to nearly kilometer size. Such airshowers are very directional. PeVs energetic tau lepton penetrate hundreds meters inside the rock before its decay. Therefore horizontal tau air-shower in front of deep, wide valleys or mountain cliff [D. Fargion, A. Aiello, R. Conversano; 26th ICRC, He 6.1.09, 6 p. 396–398. (1999). Ed. D. Kieda, et al. arXiv:astro-ph/9906450], as well as up-going tau air showers escaping our Earth, observable in air by their fluorescence lights as in AUGER and TA, might be a signal at EeVs energies. At lower energies blazing few PeVs tau airshower flashes are better observable from the top of the mountains, by an array located in a crown edge (as water Cherenkov, telescope Cherenkov or radio array), as it has been done in ASHRA or it might be done on ideal modified GRAND experiments constructed within aeolian towers by radio array possibly in mountains, facing the wide peculiar τ neutrino sky: our own Earth.