Neutrino mixing parameters are subject to quantum corrections and hence are scale dependent. This means that the mixing parameters associated to the production and detection of neutrinos need not coincide since these processes are characterized by di
fferent energy scales. We show that, in the presence of relatively light new physics, the scale dependence of the mixing parameters can lead to observable consequences in long-baseline neutrino oscillation experiments, such as T2K and NOvA, and in neutrino telescopes like IceCube. We discuss some of the experimental signatures of this scenario, including zero-baseline flavor transitions, new sources of CP-invariance violation, and apparent inconsistencies among measurements of mixing angles at different experiments or oscillation channels. Finally, we present simple, ultraviolet-complete models of neutrino masses which lead to observable running of the neutrino mixing matrix below the weak scale.
Nonzero neutrino masses imply the existence of degrees of freedom and interactions beyond those in the Standard Model. A powerful indicator of what these might be is the nature of the massive neutrinos: Dirac fermions versus Majorana fermions. While
addressing the nature of neutrinos is often associated with searches for lepton-number violation, there are several other features that distinguish Majorana from Dirac fermions. Here, we compute in great detail the kinematics of the daughters of the decays into charged-leptons and neutrinos of hypothetical heavy neutral leptons at rest. We allow for the decay to be mediated by the most general four-fermion interaction Lagrangian. We demonstrate, for example, that when the daughter charged-leptons have the same flavor or the detector is insensitive to their charges, polarized Majorana-fermion decays have zero forward/backward asymmetry in the direction of the outgoing neutrino (relative to the parent spin), whereas Dirac-fermion decays can have large asymmetries. Going beyond studying forward/backward asymmetries, we also explore the fully-differential width of the three-body decays. It contains a wealth of information not only about the nature of the new fermions but also the nature of the interactions behind their decays.
One proposed component of the upcoming Deep Underground Neutrino Experiment (DUNE) near detector complex is a multi-purpose, magnetized, gaseous argon time projection chamber: the Multi-Purpose Detector (MPD). We explore the new-physics potential of
the MPD, focusing on scenarios in which the MPD is significantly more sensitive to new physics than a liquid argon detector, specifically searches for semi-long-lived particles that are produced in/near the beam target and decay in the MPD. The specific physics possibilities studied are searches for dark vector bosons mixing kinetically with the Standard Model hypercharge group, leptophilic vector bosons, dark scalars mixing with the Standard Model Higgs boson, and heavy neutral leptons that mix with the Standard Model neutrinos. We demonstrate that the MPD can extend existing bounds in most of these scenarios. We illustrate how the ability of the MPD to measure the momentum and charge of the final state particles leads to these bounds.
The planned DUNE experiment will have excellent sensitivity to the vector and axial couplings of the electron to the $Z$-boson via precision measurements of neutrino--electron scattering. We investigate the sensitivity of DUNE-PRISM, a movable near d
etector in the direction perpendicular to the beam line, and find that it will qualitatively impact our ability to constrain the weak couplings of the electron. We translate these neutrino--electron scattering measurements into a determination of the weak mixing angle at low scales and estimate that, with seven years of data taking, the DUNE near-detector can be used to measure $sin^2theta_W$ with about 2% precision. We also discuss the impact of combining neutrino--electron scattering data with neutrino trident production at DUNE-PRISM.
We explore the hypothesis that the unexplained data from LSND and MiniBooNE are evidence for a new, heavy neutrino mass-eigenstate that mixes with the muon-type neutrino and decays into an electron-type neutrino and a new, very light scalar particle.
We consider two different decay scenarios, one with Majorana neutrinos, one with Dirac neutrinos; both fit the data equally well. We find a reasonable, albeit not excellent, fit to the data of MiniBooNE and LSND. The decaying-sterile-neutrino hypothesis, however, cleanly evades constraints from disappearance searches and precision measurements of leptonic meson decays, as long as $1~{rm MeV}gtrsim m_4gtrsim 10$~keV. The SBN program at Fermilab should be able to definitively test the decaying-sterile-neutrino hypothesis.
Lepton-number violation (LNV), in general, implies nonzero Majorana masses for the Standard Model neutrinos. Since neutrino masses are very small, for generic candidate models of the physics responsible for LNV, the rates for almost all experimentall
y accessible LNV observables -- except for neutrinoless double-beta decay -- are expected to be exceedingly small. Guided by effective-operator considerations of LNV phenomena, we identify a complete family of models where lepton number is violated but the generated Majorana neutrino masses are tiny, even if the new-physics scale is below 1 TeV. We explore the phenomenology of these models, including charged-lepton flavor-violating phenomena and baryon-number-violating phenomena, identifying scenarios where the allowed rates for $mu^-to e^+$-conversion in nuclei are potentially accessible to next-generation experiments.
We explore the capabilities of the upcoming Deep Underground Neutrino Experiment (DUNE) to measure $ u_tau$ charged-current interactions and the associated oscillation probability $P( u_mu to u_tau)$ at its far detector, concentrating on how such re
sults can be used to probe neutrino properties and interactions. DUNE has the potential to identify significantly more $ u_tau$ events than all existing experiments and can use this data sample to nontrivially test the three-massive-neutrinos paradigm by providing complementary measurements to those from the $ u_e$ appearance and $ u_mu$ disappearance channels. We further discuss the sensitivity of the $ u_tau$ appearance channel to several hypotheses for the physics that may lurk beyond the three-massive-neutrinos paradigm: a non-unitary lepton mixing matrix, the $3+1$ light neutrinos hypothesis, and the existence of non-standard neutral-current neutrino interactions. Throughout, we also consider the relative benefits of the proposed high-energy tune of the Long-Baseline Neutrino Facility (LBNF) beam-line.
The discovery of neutrino oscillations invites many fundamental physics questions that have yet to be answered. Two of these questions are simple, easy to state, and essential: What are the values of the neutrino masses? Are neutrinos Majorana fermio
ns? The reason we dont know the answer to those questions is that it is difficult to measure neutrino properties outside of the ultrarelativistic regime. We discuss the physics of $egammato e ubar{ u}$ near threshold, where one has access to nonrelativistic neutrinos and only nonrelativistic neutrinos. Near threshold, $egammato e ubar{ u}$ is a rich phenomenon and its cross section is sensitive to the individual values of the neutrino masses and the nature of the neutrinos. We show that if one could scan the threshold region, it would be simple to identify the mass of the lightest neutrino, the neutrino mass ordering, and whether the neutrinos are Majorana fermions. In practice, however, event rates are tiny and backgrounds are huge; the observation of $egammato e ubar{ u}$ in the sub-eV regime appears to be utterly inaccessible in the laboratory. Our results, nonetheless, effectively illustrate the discriminatory power of nonrelativistic neutrino observables.
Experimentally, baryon number minus lepton number, $B-L$, appears to be a good global symmetry of nature. We explore the consequences of the existence of gauge-singlet scalar fields charged under $B-L$ -- dubbed lepton-number-charged scalars, LeNCS -
- and postulate that these couple to the standard model degrees of freedom in such a way that $B-L$ is conserved even at the non-renormalizable level. In this framework, neutrinos are Dirac fermions. Including only the lowest mass-dimension effective operators, some of the LeNCS couple predominantly to neutrinos and may be produced in terrestrial neutrino experiments. We examine several existing constraints from particle physics, astrophysics, and cosmology to the existence of a LeNCS carrying $B-L$ charge equal to two, and discuss the emission of LeNCSs via neutrino beamstrahlung, which occurs every once in a while when neutrinos scatter off of ordinary matter. We identify regions of the parameter space where existing and future neutrino experiments, including the Deep Underground Neutrino Experiment, are at the frontier of searches for such new phenomena.
There is no guarantee that the violation of lepton number, assuming it exists, will primarily manifest itself in neutrinoless double beta decay ($0 ubetabeta$). Lepton-number violation and lepton-flavor violation may be related, and much-needed infor
mation regarding the physics that violates lepton number can be learned by exploring observables that violate lepton flavors other than electron flavor. One of the most promising observables is $mu^- rightarrow e^+$ conversion, which can be explored by experiments that are specifically designed to search for $mu^- rightarrow e^-$ conversion. We survey lepton-number-violating dimension-five, -seven, and -nine effective operators in the standard model and discuss the relationships between Majorana neutrino masses and the rates for $0 ubetabeta$ and $mu^- rightarrow e^+$ conversion. While $0 ubetabeta$ has the greatest sensitivity to new ultraviolet energy scales, its rate might be suppressed by the new physics relationship to lepton flavor, and $mu^- rightarrow e^+$ conversion offers a complementary probe of lepton-number-violating physics.
