Lepton Number Research Articles

Resonant Majorana neutrino effects in ΔL=2 four-body hyperon decays

We computed the $\Sigma^{-}\to n\pi^+e^-\ell^-$ ($\ell=e,\mu$), $\Xi^-\to \Lambda\pi^+e^-e^-$, and $\Lambda\to p\pi^+e^-e^-$ lepton number violating (LNV) hyperon decays mediated by a resonant Majorana neutrino. The expected hyperon production rate of experiments like BES-III of around $10^6-10^8$ may allow searching for these rare hyperon decays at enough sensitivities. We illustrate the limits on the new heavy mixing parameters derived from these hyperon channels and compare them with other LNV meson decays in similar mass regions of the resonant neutrino state.

Neutrino flavor conversion, advection, and collisions: Toward the full solution

At high densities in compact astrophysical sources, the coherent forward scattering of neutrinos onto each other is responsible for making the flavor evolution non-linear. Under the assumption of spherical symmetry, we present the first simulations tracking flavor transformation in the presence of neutrino-neutrino forward scattering, neutral and charged current collisions with the matter background, as well as neutrino advection. We find that, although flavor equipartition could be one of the solutions, it is not a generic outcome, as often postulated in the literature. Intriguingly, the strong interplay between flavor conversion, collisions, and advection leads to a spread of flavor conversion across the neutrino angular distributions and neighboring spatial regions. Our simulations show that slow and fast flavor transformation can occur simultaneously. In the light of this, looking for crossings in the electron neutrino lepton number as a diagnostic tool of the occurrence of flavor transformation in the high-density regime is a limiting method.

Simulating lepton number violation induced by heavy neutrino-antineutrino oscillations at colliders

We study pseudo-Dirac pairs of two almost mass-degenerate sterile Majorana neutrinos which generate light neutrino masses via a low-scale seesaw mechanism. These pseudo-Dirac heavy neutral leptons can oscillate between interaction eigenstates that couple to leptons and antileptons and thus generate oscillations between lepton number conserving and lepton number violating processes. With the phenomenological symmetry protected seesaw scenario (pSPSS), we introduce a minimal framework capable of describing the dominant features of low-scale seesaws at colliders and present a FeynRules implementation usable in Monte Carlo generators. Additionally, we extend MadGraph to simulate heavy neutrino-antineutrino oscillations and present results from such simulations.

Probing the non-standard neutrino interactions using quantum statistics

Using the well established principles of Lorentz invariance, CP and CPT symmetry, and quantum statistics we do a model-independent study of effects of possible non-standard couplings of (Dirac and Majorana) neutrinos. The study is sensitive to the different quantum statistical properties of the Dirac and Majorana neutrinos which, contrary to neutrino-mediated processes of lepton number violation, could lead to observable effects not suppressed by the small ratios of neutrino and heavier particle masses. For processes with a neutrino-antineutrino pair of the same flavor in the final state, we formulate the “Dirac Majorana confusion theorem (DMCT)” showing why it is normally very difficult to observe the different behaviour of both kinds of neutrinos in experiments if they have only the standard model (SM)-like left-handed vector couplings to gauge bosons. We discuss deviations from the confusion theorem in the presence of non-standard neutrino interactions, allowing to discover or constrain such novel couplings. We illustrate the general results with two chosen examples of neutral current processes, Z rightarrow nu , {overline{nu }} and {mathcal {P}}_i rightarrow {mathcal {P}}_f , nu , {overline{nu }} (with {mathcal {P}}_{i,f} denoting pseudoscalar mesons, such as B,K,pi ). Our analysis shows that using 3-body decays the presence of non-standard interactions can not only be constrained but one can also distinguish between Dirac and Majorana neutrino possibilities.

HNL mass degeneracy: implications for low-scale seesaws, LNV at colliders and leptogenesis

Low-scale seesaw variants protected by lepton number symmetry provide a natural explanation of the smallness of neutrino masses but, unlike their higher-scale counterparts, with potentially testable phenomenology. The approximate lepton number symmetry arranges the heavy neutrinos in pseudo-Dirac pairs, which might be accessible at collider or even beam dump experiments if their mass is low enough and their mixing with the active neutrinos sufficiently large. Despite their pseudo-Dirac nature, their small mass splittings may lead to oscillations that prevent the cancellation of their potential lepton-number-violating signals. Interestingly, these small splittings may also resonantly enhance the production of a lepton number asymmetry for low-scale leptogenesis scenarios or, for extremely degenerate states, lead to an asymmetry large enough to resonantly produce a keV sterile neutrino dark matter candidate with the correct relic abundance via the Shi-Fuller mechanism. In this work we explore the parameter space of the different low-scale seesaw mechanisms and study the size of these splittings, given their important and interesting phenomenological consequences. While all low-scale seesaw variants share the same dimension 5 and 6 operators when integrating out the heavy states, we point out that the mass splitting of the pseudo-Dirac pairs are very different in different realizations such as the inverse or linear seesaw. This different phenomenology could offer a way to discriminate between low-scale seesaw realizations.

RGESolver: a C++ library to perform renormalization group evolution in the Standard Model Effective Theory

Renormalization group evolution above the electroweak scale is a crucial ingredient in the phenomenology of the Standard Model Effective Theory. The RGESolver open-source C++ library performs the evolution at leading order for dimension-six operators in the most general flavour scenario (assuming lepton and baryon number conservation). Given its efficiency, RGESolver can be used to include the effects of renormalization group evolution in extensive phenomenological analyses in the framework of the Standard Model Effective Theory.

Heavy neutral leptons at the Electron-Ion Collider

The future Electron-Ion Collider (EIC) at Brookhaven National Laboratory, along with its primary capacity to elucidate the nuclear structure, will offer new opportunities to probe physics beyond the Standard Model coupled to the electroweak sector. Among the best motivated examples of such new physics are new heavy neutral leptons (HNLs), which are likely to play a key role in neutrino mass generation and lepton number violation. We study the capability of the EIC to search for HNLs, which can be produced in electron- proton collisions through charged current interactions as a consequence of their mixing with light neutrinos. We find that, with the EIC design energy and integrated luminosity, one is able to probe HNLs in the mass range of 1 – 100 GeV with mixing angles down to the order of 10−4 − 10−3 through the prompt decay signatures, and in the mass range of 1 10 GeV with |Ue|2 ~ 10−6 – 10−4 via the displaced decay signatures. We also consider the invisible mode where an HNL is undetected or decaying to dark sector particles. One could potentially probe heavy HNLs for mixing angles in the window 10−3 – 10−2, provided SM background systematics can be brought under control. These searches are complementary to other probes of HNLs, such as neutrino-less double-β decay, meson decay, fixed-target, and high-energy collider experiments.

Lepton- and baryon-number violation in nuclear effective field theory

Lepton and baryon numbers are accidental symmetries in the effective field theory (EFT) of the Standard Model. Their violation would lead to spectacular nuclear decays which can be described with nuclear EFTs. I discuss ways in which two such decays — the neutrinoless double-beta decay of heavy nuclei and the disappearance of the deuteron — provide complementary information to processes that take place outside the nuclear environment — neutrino masses, nucleon decay, and neutron-antineutron oscillations.

Rare weak decays and neutrino mass

The question whether neutrinos are Majorana fermions (i.e., their own anti-particles) remains among the most fundamental open questions of subatomic physics. If neutrinos are Majorana particles it would revolutionize our understanding of physics. Although neutrinoless double beta decay, 0νββ, was proposed more than 80 years ago to establish the nature of neutrinos, it remains the most sensitive probe into the non-conservation of lepton number. 0νββ-decay is a postulated extremely slow and yet unobserved radioactive process in which two neutrons (or protons) inside a nucleus transform into two protons (or neutrons) emitting two electrons (or positrons), respectively, but no neutrinos. Its observation would be a breakthrough in the description of elementary particles and would provide fundamental information on the neutrino masses, their nature, and origin. In this paper double beta decay, its connection to neutrino mass, and mechanisms beyond the standard mass mechanism are discussed from a theoretical point of view. The current situation is then addressed by combining theoretical results with recent experimental limits.

Dineutron decay into sterile antineutrinos in neutron stars and its observable consequences

In some extensions of the Standard Model (SM), two neutrons are allowed to decay into two sterile antineutrinos ($nn\ensuremath{\rightarrow}\overline{\ensuremath{\chi}}\overline{\ensuremath{\chi}}$) via new scalar bosons. This process violates both the baryon number ($\mathcal{B}$) and the lepton number ($\mathcal{L}$) by two units but conserves their difference $(\mathcal{B}\ensuremath{-}\mathcal{L})$. Neutron stars contain a large number of neutrons, and thus the $nn\ensuremath{\rightarrow}\overline{\ensuremath{\chi}}\overline{\ensuremath{\chi}}$ process can be greatly enhanced inside a neutron star. This process could result in nontrivial effects that are different from the SM predictions and can be explored through astrophysical and laboratory observations. Furthermore, a large number of sterile antineutrinos, which may be dark matter candidates, can be emitted from the interior of the neutron star. The properties of the emitted particles show a particular pattern that can be uniquely determined by the mass and radius of the neutron star. In addition, the dineutron decay may contribute to the orbital-period change of the binary systems containing neutron stars. We analyze the possibility of constraining the mass of the new scalar bosons using the observations of the binary's orbital-period changes. It is found that the mass of the new scalar bosons is roughly restricted in the range from 1 TeV to several TeV, which is possibly within the reach of direct searches at the LHC or future high-energy experiments. The joint analysis which combines the astrophysics and particle phenomenology could provide an excellent opportunity for the study of the new physical effects beyond the SM.

Search for Majoron-like particles with CUPID-0

We present the first search for the Majoron-emitting modes of the neutrinoless double β decay (0νββχ0) using scintillating cryogenic calorimeters. We analyzed the CUPID-0 Phase I data using a Bayesian approach to reconstruct the background sources activities, and evaluate the potential contribution of the Se82 0νββχ0. We considered several possible theoretical models which predict the existence of a Majoron-like boson coupling to the neutrino. The energy spectra arising from the emission of such bosons in the neutrinoless double β decay have spectral indices n=1, 2, 3, or 7. We found no evidence of any of these decay modes, setting a lower limit (90% of credibility interval) on the half-life of 1.2×1023 yr in the case of n=1, 3.8×1022 yr for n=2, 1.4×1022 yr for n=3 and 2.2×1021 yr for n=7. These are the best limits on the 0νββχ0 half-life of the Se82, and demonstrate the potentiality of the CUPID-0 technology in this field.Received 27 September 2022Accepted 19 January 2023DOI:https://doi.org/10.1103/PhysRevD.107.032006Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Physical SystemsMajorana neutrinosPropertiesBaryon & lepton number symmetriesTechniquesBayesian methodsParticles & FieldsNuclear Physics

Recent results from the NA62 and NA48/2 experiments at CERN

A measurement of the K+→π0e+νγ radiative decay and a search for the K+→π−(π0)e+e+ lepton number violating decays by the NA62 experiment are presented, together with the first observation and branching fraction measurement of the rare decay K+→π0π0μ+ν by the NA48/2 experiment.

Lepton PDFs and multipurpose single-lepton searches at the LHC

A final state consisting of one charged lepton, at least one jet, and little missing transverse energy can be a very promising signature of new physics at the LHC across a wide range of models. However, it has received only limited attention so far. In this work we discuss the potential sensitivity of this channel to various new physics scenarios. To demonstrate our point, we consider its application to lepton parton distribution functions (PDFs) at the LHC in the context of supersymmetry. These lepton PDFs can lead to resonant squark production (similar to leptoquarks) via lepton number violating couplings present in $R$-parity violating supersymmetry (RPV-SUSY). Unlike leptoquarks, in RPV-SUSY there are many possible decay modes leading to a wide range of signatures. We propose two generic search regions: (a) a single first or second generation charged lepton, exactly 1 jet and low missing transverse energy, and (b) a single first or second generation charged lepton, at least 3 jets, and low missing transverse energy. We demonstrate that together these cover a large range of RPV-SUSY signatures, and have the potential to perform better than existing low-energy bounds, while being general enough to extend to a wide range of possible models hitherto not explored at the LHC.

Search for heavy Majorana neutrinos at future lepton colliders* *Supported in part by the National Natural Science Foundation of China (12235008, 11875179), and the Natural Science Foundation of Shandong Province, China (ZR2021QA040)

A nonzero neutrino mass may be a sign of new physics beyond the standard model (SM). To explain the small neutrino mass, we can extend the SM using right-handed Majorana neutrinos in a low-scale seesaw mechanism, and the CP violation effect can be induced due to the CP phase in the interference of heavy Majorana neutrinos. The existence of heavy Majorana neutrinos may lead to lepton number violation processes, which can be used to search for the signals of heavy Majorana neutrinos. In this paper, we focus on the CP violation effect related to two generations of heavy Majorana neutrinos at GeV GeV in the pair production of W bosons and rare decays. It is valuable to investigate Majorana neutrino production signals and the related CP violation effects in rare W boson decays at future lepton colliders.

Searches for lepton flavour and lepton number violating K + decays at the NA62 experiment

The NA62 experiment at CERN collected the world’s largest dataset of charged kaon decays to di-lepton final states in 2016-2018, using dedicated trigger lines. Upper limits on the rates of several K + decays violating lepton flavour and lepton number conservation, obtained by analysing this dataset, are reported.

Foreword

The International Conference on Kaon Physics 2022 – KAON2022 – was held on September 13-16, 2022 at the Nambu Yoichiro Hall in the Toyonaka Campus of Osaka University, Japan.At the previous KAON2019 which was held in September 2019 at Perugia, it was decided that the next conference will be held in Osaka in September 2022. At that time, nobody knew what the entire world had to go through between the two KAON Conferences. Many other conferences were canceled or postponed, and many have switched to a pure online style. In November 2021, as we saw some hope for having a face-to-face meeting, the International Advisory Committee (IAC) for KAON2022 decided to hold the Conference as scheduled in hybrid style. Still, due to unpredictable variants of the virus and border restrictions, the expected number of attendants had a large uncertainty. The conference had to be prepared for a wide range of possibilities.At the end, however, the Conference had 53 participants attending in person, of which 29 were from 13 countries abroad. We also had 53 people registered for online participation. There were 47 oral presentations of which 25 were given in person, and 19 poster presentations.Oral presentations covered experimental and theoretical aspects of kaon rare decays, semileptonic decays, radiative decays, the CKM matrix, violations in CP, T, CPT, and lepton number, hadron physics, quantum mechanics, exotic particle searches, and future perspectives. We also had review talks on flavor physics in neighboring fields, including theoretical overview, and experiments on B mesons, neutrinos, and muons.The poster session was also held in hybrid style. All the posters were posted physically in the Nambu Hall, and their PDF files were also posted in a virtual poster session room on a platform called Gather. For each poster in the Hall, a rented iPad was hung next to it. Each iPad was connected to Gather, and showed the same poster. Effectively, the iPad served as a window between the real and virtual worlds. Through the window, the presenter and audience could see each other and have discussions just as if they were standing in front of the poster.List of Conference Home Page, Participants, Posters are available in this Pdf.

HIKE: High Intensity Kaon Experiments at the CERN SPS

The availability of high intensity kaon beams at the CERN SPS North Area gives rise to unique possibilities for sensitive tests of the Standard Model in the quark flavor sector. Precise measurements of the branching ratios for the flavor-changing neutral current decays can provide unique constraints on CKM unitarity and, potentially, evidence for new physics. Building on the success of the NA62 experiment, plans are taking shape at CERN for a comprehensive program that will include experimental phases to measure the branching ratio for to ∼5% and to to ∼20% precision. These planned experiments would also carry out lepton flavor universality tests, lepton number and flavor conservation tests, and perform other precision measurements in the kaon sector, as well as searches for exotic particles in kaon decays. We overview the physics goals, detector requirements, and project status for the next generation of kaon physics experiments at CERN.

A search for the K+ → μ−νe+e+ decay

A search for the K+→μ−νe+e+ decay, forbidden within the Standard Model by either lepton number or lepton flavour conservation depending on the flavour of the emitted neutrino, has been performed using the dataset collected by the NA62 experiment at CERN in 2016–2018. An upper limit of 8.1×10−11 is obtained for the decay branching fraction at 90% CL, improving by a factor of 250 over the previous search.

Axionic Dirac seesaw mechanism and electroweak vacuum stability

We explore the connection between tree-level Dirac neutrino masses and axion physics in a scenario where the Peccei-Quinn (PQ) symmetry enforces lepton number conservation perturbatively. Requiring that the PQ scale ${f}_{a}$ is the only heavy scale to play a role in neutrino mass generation, we are led to the construction of a Kim--Shifman--Vainshtein--Zakharov-type model where Dirac neutrino masses are inversely proportional to ${f}_{a}$, provided a real scalar triplet (zero hypercharge) is added to the standard model scalar sector. We analyze this extended scalar sector, focusing on the stabilization of the electroweak vacuum. The contribution of the triplet vacuum expectation value to the $W$ mass may also be responsible for the recent hint of beyond-the-SM physics by the CDF collaboration.

Search for baryon and lepton number violating decay D±→n(n¯)e±

Using a data set of electron-positron collisions corresponding to an integrated luminosity of ${\rm 2.93~fb^{-1}}$ taken with the BESIII detector at a center-of-mass energy of 3.773 GeV, a search for the baryon ($B$) and lepton ($L$) number violating decays $D^{\pm}\to n(\bar{n})e^{\pm}$ is performed. No signal is observed and the upper limits on the branching fractions at the $90\%$ confidence level are set to be $1.43\times10^{-5}$ for the decays $D^{+(-)}\to \bar{n}(n)e^{+(-)}$ with $\Delta|B-L|=0$, and $2.91\times10^{-5}$ for the decays $D^{+(-)}\to n(\bar{n})e^{+(-)}$ with $\Delta|B-L|=2$ , where $\Delta|B-L|$ denotes the change in the difference between baryon and lepton numbers.