Search

We study the potential to probe the origin of neutrino masses, by searching for long-lived right-handed neutrinos~(RHNs) $N$ in the $B-L$ gauge model. Despite the small active-sterile mixing $|V_{\ell N}|^2$, RHNs are produced abundantly via SM and exotic Higgs production, as long as the Higgs mixing is sufficiently large. We reinterpret a search for displaced showers in the CMS muon system and we find that it is sensitive to parameter space at and below the canonical seesaw floor, $|V_{\ell N}|^2 \approx 10^{-12}$ ($\ell = e$, $\tau$) for $m_N \approx 40$ GeV. With existing data constraining such a well-motivated scenario of neutrino mass generation, we determine the projected sensitivity at the HL-LHC, motivating dedicated searches for long-lived RHNs with decay lengths $\approx 10$ m., Comment: 12 pages, 5 figures + appendix

A large experimental program is being mounted to search for neutrinoless double-beta decay over the next decade. Multiple experiments using different target isotopes are being prepared to explore the whole parameter space allowed for inverted-ordered light neutrinos, and have the potential to make discoveries in several other scenarios, including normal-ordered light neutrinos and other exotic mechanisms. We investigate to what extent long-range and exotic short-range contributions may be distinguished by combining measurements of the decay half-life across isotopes. We demonstrate how measurements in two isotopes may facilitate a joint measurement up to a two-fold degeneracy, and how a further measurement in a third isotope may remove this degeneracy. We also highlight the precision of the nuclear matrix elements needed to convert half-lives into information on the underlying decay mechanism. Considering their correlations and uncertainties in the framework of a global Bayesian analysis, we assess the consequences of a possible future 10-fold reduction in uncertainties. Our work motivates an experimental program measuring neutrinoless double-beta decay in more than one isotope, as this would break parameter degeneracies and advance our understanding of particle physics beyond the Standard Model., Comment: 30 pages, 9 figures

This is the Snowmass2021 Energy Frontier (EF) Beyond the Standard Model (BSM) report. It combines the EF topical group reports of EF08 (Model-specific explorations), EF09 (More general explorations), and EF10 (Dark Matter at Colliders). The report includes a general introduction to BSM motivations and the comparative prospects for proposed future experiments for a broad range of potential BSM models and signatures, including compositeness, SUSY, leptoquarks, more general new bosons and fermions, long-lived particles, dark matter, charged-lepton flavor violation, and anomaly detection., Comment: 108 pages + 38 pages references and appendix, 37 figures, Report of the Topical Group on Beyond the Standard Model Physics at Energy Frontier for Snowmass 2021. The first nine authors are the Conveners, with Contributions from the other authors

We examine a macroscopic system in a quantum superposition of two spatially separated localized states as a detector for a stream of weakly interacting relativistic particles. We do this using the explicit example of neutrinos with MeV-scale energy scattering from a solid object via neutral-current neutrino-nucleus scattering. Presuming the (anti-)neutrino source to be a nuclear fission reactor, we utilize the estimated flux and coherent elastic neutrino-nucleus cross section to constrain the spatial separation ${\Delta}$x and describe the temporal evolution of the sensing system. Particularly, we find that a potentially measurable relative phase between quantum superposed components is obtained for a single gram scale mass placed in a superposition of spatial components separated by $10^{-14}$m under sufficient cooling and background suppression., Comment: 12 pages, 3 figures, 1 table

The existence of non-zero neutrino masses points to the likely existence of multiple SM neutral fermions. When such states are heavy enough that they cannot be produced in oscillations, they are referred to as Heavy Neutral Leptons (HNLs). In this white paper we discuss the present experimental status of HNLs including colliders, beta decay, accelerators, as well as astrophysical and cosmological impacts. We discuss the importance of continuing to search for HNLs, and its potential impact on our understanding on key fundamental questions, and additionally we outline the future prospects for next-generation future experiments or upcoming accelerator run scenarios., Comment: 82 pages, 34 figures. Contribution to Snowmass 2021

High energy collisions at the High-Luminosity Large Hadron Collider (LHC) produce a large number of particles along the beam collision axis, outside of the acceptance of existing LHC experiments. The proposed Forward Physics Facility (FPF), to be located several hundred meters from the ATLAS interaction point and shielded by concrete and rock, will host a suite of experiments to probe Standard Model (SM) processes and search for physics beyond the Standard Model (BSM). In this report, we review the status of the civil engineering plans and the experiments to explore the diverse physics signals that can be uniquely probed in the forward region. FPF experiments will be sensitive to a broad range of BSM physics through searches for new particle scattering or decay signatures and deviations from SM expectations in high statistics analyses with TeV neutrinos in this low-background environment. High statistics neutrino detection will also provide valuable data for fundamental topics in perturbative and non-perturbative QCD and in weak interactions. Experiments at the FPF will enable synergies between forward particle production at the LHC and astroparticle physics to be exploited. We report here on these physics topics, on infrastructure, detector, and simulation studies, and on future directions to realize the FPF's physics potential., Comment: 429 pages, contribution to Snowmass 2021

Usually considered a background for experimental searches for the hypothetical neutrinoless double beta decay process, two-neutrino double beta decay nevertheless provides a complementary probe of physics beyond the Standard Model. In this paper we investigate how the presence of a sterile neutrino, coupled to the Standard Model either via a left-handed or right-handed current, affects the energy distribution and angular correlation of the outgoing electrons in two-neutrino double beta decay. We pay particular attention on the behaviour of the energy distribution at the kinematic endpoint and we estimate the current limits on the active-sterile mixing and effective right-handed coupling using current experimental data as a function of the sterile neutrino mass. We also investigate the sensitivities of future experiments. Our results complement the corresponding constraints on sterile neutrinos from single beta decay measurements in the 0.1 - 10 MeV mass range., Comment: 28 pages, 7 figures, extended discussion of end point behaviour, matches published version

Neutrino Self-Interactions ($\nu$SI) beyond the Standard Model are an attractive possibility to soften cosmological constraints on neutrino properties and also to explain the tension in late and early time measurements of the Hubble expansion rate. The required strength of $\nu$SI to explain the $4\sigma$ Hubble tension is in terms of a point-like effective four-fermion coupling that can be as high as $10^9\, G_F$, where $G_F$ is the Fermi constant. In this work, we show that such strong $\nu$SI can cause significant effects in two-neutrino double beta decay, leading to an observable enhancement of decay rates and to spectrum distortions. We analyze self-interactions via an effective operator as well as when mediated by a light scalar. Data from observed two-neutrino double beta decay is used to constrain $\nu$SI, which rules out the regime around $10^9\, G_F$., Comment: 10 pages, 5 figures

We investigate the potential of LHC resonance searches in leptonic final states to probe the $Z'$ in the minimal $U(1)_{B-L}$ model. Considering the current constraints on the $Z'$ in terms of its mass $m_{Z'}$ and the associated gauge coupling $g_{B-L}$ as well as constraints in the Higgs sector, we analyse the potential of dilepton and four lepton final states for $Z'$ production. This includes Drell-Yan production, Higgs mediated decays and final state radiation processes concentrating only on the ATLAS and CMS detectors at the LHC. We show that the four-lepton final state is sensitive to $m_{Z'}$ as low as 0.25 GeV. Furthermore, setting the Higgs mixing to $\sin\alpha = 0.3$, this final state has a strong sensitivity and it probes regions of parameter space where the $Z'$ is long-lived. We demonstrate the sensitivity at the High Luminosity LHC and comment on the potential of probing displaced vertices due to long-lived $Z'$. Finally, we also comment on the strength of $Z'$ and Higgs mediated heavy neutrino processes by taking into account the constraints derived., Comment: 26 pages, 9 figures, 9 references added, matches published version

Particles beyond the Standard Model (SM) can generically have lifetimes that are long compared to SM particles at the weak scale. When produced at experiments such as the Large Hadron Collider (LHC) at CERN, these long-lived particles (LLPs) can decay far from the interaction vertex of the primary proton-proton collision. Such LLP signatures are distinct from those of promptly decaying particles that are targeted by the majority of searches for new physics at the LHC, often requiring customized techniques to identify, for example, significantly displaced decay vertices, tracks with atypical properties, and short track segments. Given their non-standard nature, a comprehensive overview of LLP signatures at the LHC is beneficial to ensure that possible avenues of the discovery of new physics are not overlooked. Here we report on the joint work of a community of theorists and experimentalists with the ATLAS, CMS, and LHCb experiments --- as well as those working on dedicated experiments such as MoEDAL, milliQan, MATHUSLA, CODEX-b, and FASER --- to survey the current state of LLP searches at the LHC, and to chart a path for the development of LLP searches into the future, both in the upcoming Run 3 and at the High-Luminosity LHC. The work is organized around the current and future potential capabilities of LHC experiments to generally discover new LLPs, and takes a signature-based approach to surveying classes of models that give rise to LLPs rather than emphasizing any particular theory motivation. We develop a set of simplified models; assess the coverage of current searches; document known, often unexpected backgrounds; explore the capabilities of proposed detector upgrades; provide recommendations for the presentation of search results; and look towards the newest frontiers, namely high-multiplicity "dark showers", highlighting opportunities for expanding the LHC reach for these signals.

We investigate the pair-production of right-handed neutrinos via the Standard Model (SM) Higgs boson in a gauged $B-L$ model. The right-handed neutrinos with a mass of few tens of GeV generating viable light neutrino masses via the seesaw mechanism naturally exhibit displaced vertices and distinctive signatures at the LHC and proposed lepton colliders. The production rate of the right-handed neutrinos depends on the mixing between the SM Higgs and the exotic Higgs associated with the $B-L$ breaking, whereas their decay length depends on the active-sterile neutrino mixing. We focus on the displaced leptonic final states arising from such a process, and analyze the sensitivity reach of the LHC and proposed lepton colliders in probing the active-sterile neutrino mixing. We show that mixing to muons as small as $V_{\mu N} \approx 10^{-7}$ can be probed at the LHC with 100 fb$^{-1}$ and at proposed lepton colliders with 5000 fb$^{-1}$. The future high luminosity run at LHC and the proposed MATHUSLA detector may further improve this reach by an order of magnitude., Comment: 23 pages, 9 figures, matches published version

We perform a Markov Chain Monte Carlo (MCMC) statistical analysis of a number of measured ground-state-to-ground-state single $\beta^+$/electron-capture and $\beta^-$ decays in the nuclear mass range A = 62 - 142. The corresponding experimental comparative half-lives (log ft values) are compared with the theoretical ones obtained by the use of the proton-neutron quasiparticle random-phase approximation (pnQRPA) with G-matrix based effective interactions. The MCMC analysis is performed separately for 47 isobaric triplets and 28 more extended isobaric chains of nuclei to extract values and uncertainties for the effective axial-vector coupling constant g_A in nuclear-structure calculations performed in the pnQRPA framework. As far as available, measured half-lives for two-neutrino double beta-minus decays occurring in the studied isobaric chains are analyzed as well., Comment: 20 pages, 8 figures, 4 tables, matches version published in PRC

The recent diphoton excess signal at an invariant mass of 750 GeV can be interpreted in the framework of left-right symmetric models with additional scalar singlets and vector-like fermions. We propose a minimal scenario for such a purpose. Extending the LRSM framework to include these new vector-like fermionic fields, on the other hand, results in interesting phenomenological implications for the LRSM fermion masses and mixing. Furthermore, existence of such vector-like fermions can also have interesting implications for baryogenesis and the dark matter sector. The introduction of a real bi-triplet scalar which contains a potential DM candidate will allow the gauge couplings to unify at $\approx 10^{17.7}$ GeV., Comment: 11 pages, 1 figure, matches published version

We interpret the 2 TeV excesses at the LHC in a left-right symmetric model with Higgs doublets and spontaneous $D$-parity violation. The light neutrino masses are understood via a linear seesaw, suppressed by a high $D$-parity breaking scale, and the heavy neutrinos have a pseudo-Dirac character. In addition, with a suppressed right-handed gauge coupling $g_R / g_L \approx 0.6$ in an $SO(10)$ embedding, we can thereby interpret the observed $eejj$ excess at CMS. We show that it can be reconciled with the diboson and dijet excesses within a simplified scenario based on our model. Moreover, we find that the mixing between the light and heavy neutrinos can be potentially large which would induce dominant non-standard contributions to neutrinoless double beta decay via long-range $\lambda$ and $\eta$ neutrino exchange., Comment: References added, typos fixed, matches published version, 12 pages, 4 figures

Interactions that manifest themselves as lepton number violating processes at low energies in combination with sphaleron transitions typically erase any preexisting baryon asymmetry of the Universe. In this article, we discuss the constraints obtained from an observation of neutrinoless double beta decay in this context. If a new physics mechanism of neutrinoless double beta decay other than the standard light neutrino exchange is observed, typical scenarios of high-scale baryogenesis will be excluded unless the baryon asymmetry is stabilized via some new mechanism. We also sketch how this conclusion can be extended beyond the first lepton generation by incorporating lepton flavor violating processes., Comment: 5 pages, 2 figures; minor additions, matches published version

We review the collider phenomenology of neutrino physics and the synergetic aspects at energy, intensity and cosmic frontiers to test the new physics behind the neutrino mass mechanism. In particular, we focus on seesaw models within the minimal setup as well as with extended gauge and/or Higgs sectors, and on supersymmetric neutrino mass models with seesaw mechanism and with $R$-parity violation. In the simplest Type-I seesaw scenario with sterile neutrinos, we summarize and update the current experimental constraints on the sterile neutrino mass and its mixing with the active neutrinos. We also discuss the future experimental prospects of testing the seesaw mechanism at colliders and in related low-energy searches for rare processes, such as lepton flavor violation and neutrinoless double beta decay. The implications of the discovery of lepton number violation at the LHC for leptogenesis are also studied., Comment: 38 pages, 12 figures; invited review prepared for the New Journal of Physics; journal version

We propose a class of left-right symmetric models (LRSMs) with spontaneous D parity breaking, where SU(2)_R breaks at the TeV scale while discrete left-right symmetry breaks around 10^9 GeV. By embedding this framework in a non-supersymmetric SO(10) Grand Unified Theory (GUT) with Pati-Salam symmetry as the highest intermediate breaking step, we obtain g_R / g_L ~ 0.6 between the right- and left-handed gauge couplings at the TeV scale. This leads to a suppression of beyond the Standard Model phenomena induced by the right-handed gauge coupling. Here we focus specifically on the consequences for neutrinoless double beta decay, low energy lepton flavour violation and LHC signatures due to the suppressed right handed currents. Interestingly, the reduced g_R allows us to interpret an excess of events observed recently in the range of 1.9 TeV to 2.4 TeV by the CMS group at the LHC as the signature of a right handed gauge boson in LRSMs with spontaneous D parity breaking. Moreover, the reduced right-handed gauge coupling also strongly suppresses the non-standard contribution of heavy states to the neutrinoless double beta decay rate as well as the amplitude of low energy lepton flavour violating processes. In a dominant type-II Seesaw mechanism of neutrino mass generation, we find that both sets of observables provide stringent and complimentary bounds which make it challenging to observe the scenario at the LHC., Comment: 20 pages, 11 figures

The discovery of lepton number violation (LNV) at the LHC would have profound consequences for the viability of high-scale leptogenesis models. As an example, we discuss the case of observing a signal with two same-sign leptons, two jets and no missing energy. This would imply a large washout factor for the lepton number density in the early Universe, which leads to a significant constraint on any high-scale model for the generation of the observed baryon asymmetry. In a standard leptogenesis scenario, the corresponding washout factor would strongly decrease a pre-existing lepton asymmetry and thus would render leptogenesis models that generate a (B-L) asymmetry far above the LHC scale ineffective. Therefore, LHC searches focused on LNV processes without missing energy are powerful probes for high-scale leptogenesis models and correspondingly shed light on the nature of baryogenesis and neutrino masses., Comment: 6 pages, 3 figures, contribution to the proceedings of the second annual Large Hadron Collider Physics conference (LHCP, 2014)

The non-observation of supersymmetric signatures in searches at the Large Hadron Collider strongly constrains minimal supersymmetric models like the CMSSM. We explore the consequences on the SUSY particle spectrum in a minimal SO(10) with large D-terms and non-universal gaugino masses at the GUT scale. This changes the sparticle spectrum in a testable way and for example can sufficiently split the coloured and non-coloured sectors. The splitting provided by use of the SO(10) D-terms can be exploited to obtain light first generation sleptons or third generation squarks, the latter corresponding to a compressed spectrum scenario., Comment: 35 pages, 10 figures, published version

Measuring a non-zero value for the cross section of any lepton number violating (LNV) process would put a strong lower limit on the washout factor for the effective lepton number density in the early universe at times close to the electroweak phase transition and thus would lead to important constraints on any high-scale model for the generation of the observed baryon asymmetry based on LNV. In particular, for leptogenesis models with masses of the right-handed neutrinos heavier than the mass scale observed at the LHC, the implied large washout factors would lead to a violation of the out-of-equilibrium condition and exponentially suppress the net lepton number produced in such leptogenesis models. We thus demonstrate that the observation of LNV processes at the LHC results in the falsification of high-scale leptogenesis models. However, no conclusions about the viability of leptogenesis models can be drawn from the non-observation of LNV processes., Comment: 5 pages, 5 figures, matches published version

Searches for rare processes such as mu --> e gamma put stringent limits on lepton flavour violation expected in many Beyond the Standard Model physics scenarios. This usually precludes the observation of flavour violation at high energy colliders such as the LHC. We here discuss a scenario where right-handed neutrinos are produced via a Z' portal but which can only decay via small flavour violating couplings. Consequently, the process rate is unsuppressed by the small couplings and can be visible despite unobservably small mu --> e gamma rates., Comment: 5 pages, 4 figures, matches published version