arXiv subjects
Publications and source records attributed to Andre de Gouvea.
In this talk I review what we know and don't know about neutrinos, neutrino masses and lepton mixing. I also discuss the importance of the discovery that neutrinos have nonzero masses, and illustrate how little is currently known about the physics behind them.
If the dark matter (DM) consists of a weakly interacting massive particle (WIMP), it can be produced and studied at future collider experiments like those at the LHC. The production of collider-stable WIMPs is characterized by hard scattering events with large missing transverse energy. Here we point out that the discovery of this well-characterized DM signal may turn out to be a red herring. We explore an alternative explanation -- fake dark matter -- where the only sources of missing transverse energy are standard model neutrinos. We present examples of such models, focusing on supersymmetric models with R-parity violation. We also briefly discuss means of differentiating fake dark matter from the production of new collider-stable particles.
We study the impact of very light sterile neutrinos (Delta m^2_new around 1 to 10 times 10^(-2) eV^2, sin^2 2theta_new<10^(-1)) on upcoming theta_13-driven reactor antineutrino experiments like Double-CHOOZ and Daya Bay. Oscillations driven by these vales of Delta m^2_new affect data in the near and far detectors differently and hence potentially modify the capability of these experimental setups to constrain and measure sin^2 2theta_13. We find that the hypothesis theta_new different from zero negatively impacts one's ability to either place an upper bound on sin^2 2theta_13 in the advent of no oscillation signal or measure sin^2 2theta_13 if a theta_13-driven signal is observed. The impact of sterile neutrino effects, however, depends significantly on one's ability to measure the recoil positron energy spectrum. If sin^2 2theta_new is larger than 10^(-2), upcoming theta_13-driven reactor antineutrino experiments should be able to measure sin^2 2theta_new and Delta m^2_new, along with sin^2 2theta_13, as long as one is sensitive to distortions in the recoil positron energy spectrum in the near (and far) detectors.
We survey 129 lepton number violating effective operators, consistent with the minimal Standard Model gauge group and particle content, of mass dimension up to and including eleven. Upon requiring that each one radiatively generates the observed neutrino masses, we extract an associated characteristic cutoff energy scale which we use to calculate other observable manifestations of these operators for a number of current and future experimental probes, concentrating on lepton number violating phenomena. These include searches for neutrinoless double-beta decay and rare meson, lepton, and gauge boson decays. We also consider searches at hadron/lepton collider facilities in anticipation of the LHC and the future ILC. We find that some operators are already disfavored by current data, while more are ripe to be probed by next-generation experiments. We also find that our current understanding of lepton mixing disfavors a subset of higher dimensional operators. While neutrinoless double-beta decay is the most promising signature of lepton number violation for the majority of operators, a handful is best probed by other means. We argue that a combination of constraints from various independent experimental sources will help to pinpoint the ``correct'' model of neutrino mass, or at least aid in narrowing down the set of possibilities.
If the Standard Model particle content is extended by gauge singlet fermions (right-handed neutrinos), active neutrinos generically acquire (Majorana) masses, in agreement with our current understanding of the lepton sector. If the right-handed neutrino masses are of the same order as the electroweak symmetry breaking scale, it is usually expected that these will not mediate any experimentally observable effects. Here, I explore the fact that this is not necessarily the case. If neutrino masses are "accidentally small", active-sterile mixing angles can, according to current experimental bounds, be as large as several percent. If this is the case, I argue that the dominant decay mode of light (M_H less than 130 GeV) Higgs bosons could be into an active and a sterile neutrino. The sterile neutrino decays promptly into a charge lepton and an on- or off-shell W-boson, so that the dominant Higgs boson decay mode is H to leptons + jets + missing transverse energy.
We show that under a new U(1) gauge symmetry, which is non-anomalous in the presence of one ``right-handed neutrino'' per generation and consistent with the standard model Yukawa couplings, the most general fermion charges are determined in terms of four rational parameters. This generalization of the B-L symmetry with generation-dependent lepton charges leads to neutrino masses induced by operators of high dimensionality. Neutrino masses are thus naturally small without invoking physics at energies above the TeV scale, whether neutrinos are Majorana or Dirac fermions. This ``Leptocratic'' Model predicts the existence of light quasi-sterile neutrinos with consequences for cosmology, and implies that collider experiments may reveal the origin of neutrino masses.
Right-handed neutrinos offer us the possibility of accommodating neutrino masses. In a supersymmetric model, this implies the existence of right-handed sneutrinos. Right-handed sneutrinos are expected to be as light as other supersymmetric particles if the neutrinos are Dirac fermions, or if the lepton-number breaking scale is at (or below) the supersymmetry (SUSY) breaking scale, assumed to be around the electroweak scale. Depending on the mechanism of SUSY breaking, the lightest right-handed sneutrino may be the lightest supersymmetric particle (LSP). We consider the unique hadron collider signatures of a weak scale right-handed sneutrino LSP, assuming R-parity conservation. For concreteness, we concentrate on stop pair-production and decay at the Tevatron and the Large Hadron Collider, and briefly comment on the production and decay of other supersymmetric particles.
The Standard Model augmented by the presence of gauge-singlet right-handed neutrinos proves to be an ideal scenario for accommodating nonzero neutrino masses. Among the new parameters of this ``New Standard Model'' are right-handed neutrino Majorana masses M. Theoretical prejudice points to M much larger than the electroweak symmetry breaking scale, but it has recently been emphasized that all M values are technically natural and should be explored. Indeed, M around 1-10 eV can accommodate an elegant oscillation solution to the LSND anomaly, while other M values lead to several observable consequences. We consider the phenomenology of low energy seesaw scenarios with M less than and equal to approximately 1 keV. By exploring such a framework with three right-handed neutrinos, we can consistently fit all oscillation data -- including those from LSND -- while partially addressing several astrophysical puzzles, including anomalous pulsar kicks, heavy element nucleosynthesis in supernovae, and the existence of warm dark matter. Furthermore, low-energy seesaws -- regardless of their relation to the LSND anomaly -- can also be tested by future tritium beta-decay experiments, neutrinoless double-beta decay searches, and other observables. We estimate the sensitivity of such probes to M.
We investigate whether postulating the existence of Lorentz-violating, CPT-conserving interactions allows three-neutrino solutions to the LSND anomaly that are also consistent with all other neutrino data. We show that Lorentz-violating interactions that couple only to one of the active neutrinos have the right properties to explain all the data. The details of the data make this solution unattractive. We find, for example, that a highly non-trivial energy dependence of the Lorentz-violating interactions is required.
Precision tests of the standard model are essential for constraining models of new physics. Neutrino-electron elastic scattering offers a clean probe into many electroweak effects that are complimentary to the more canonical measurements done at collider facilities. Such reactions are rare, even as compared with the already tiny cross-sections for neutrino-nucleon scattering, and competitive precision measurements have historically been challenging to obtain. Due to new existing and proposed high-flux neutrino sources, this is about to change. We present a topical survey of precision measurements that can be done with neutrino-electron scattering in light of these new developments. Specifically, we consider four distinct neutrino sources: nuclear reactors, neutrino factories, beta-beams, and conventional beams. For each source we estimate the expected future precision of several representative observables, including the weak mixing angle, neutrino magnetic moments, and potential leptonic Z' couplings. We find that future neutrino-electron scattering experiments should add non-trivially to our understanding of fundamental physics.
When the minimal supersymmetric standard model is augmented by three right-handed neutrino superfields, one generically predicts that the neutrinos acquire Majorana masses. We postulate that all supersymmetry (SUSY) breaking masses as well as the Majorana masses of the right-handed neutrinos are around the electroweak scale and, motivated by the smallness of neutrino masses, assume that the lightest supersymmetric particle (LSP) is an almost-pure right-handed sneutrino. We discuss the conditions under which this LSP is a successful dark matter candidate. In general, such an LSP has to be nonthermal in order not to overclose the universe, and we find the conditions under which this is indeed the case by comparing the Hubble expansion rate with the rates of the relevant thermalizing processes, including self-annihilation and co-annihilation with other SUSY and standard model particles.
We discuss the experimental requirements for a mass hierarchy measurement for $θ_{13}=0$ using muon neutrino disappearance. We find that a specially optimized neutrino factory at $L \simeq 6 \,000 \, \mathrm{km}$ could do this measurement using extreme luminosities. In particular, we do not require charge identification for this purpose. In order to measure the mass hierarchy for more adequate luminosities, we explore the capabilities of low energy narrow band off-axis beams, which have relatively more events at low energies. We find that, in this case, the energy resolution of the detector quickly becomes the limiting factor of the measurement, and significantly affects the baseline optimization for determining the mass hierarchy.
If the neutrinos are Majorana fermions, there are at least three new, potentially observable CP-odd phases that parameterize CP-invariance violating phenomena. We currently have no information regarding any of them and know that two out of the three, the so-called Majorana phases, are very hard to access experimentally. Here, we discuss the effect of Majorana phases on charged-lepton electric dipole moments (EDM), and explicitly show that neutrino Majorana phases induce EDMs even in the absence of other sources of CP-invariance violation. We also argue that while the Majorana neutrino contribution to EDMs is tiny, there is one generic ultraviolet completion to the standard model plus massive Majorana neutrinos -- the standard model plus a triplet Higgs boson -- that leads to significantly enhanced contributions which are still proportional to the low-energy neutrino Majorana phases. If this particular scenario is realized in nature, it seems possible, at least in principle, to measure the Majorana phases by precisely measuring charged-lepton EDMs.
One of the outstanding issues in neutrino physics is the experimental determination of the neutrino mass hierarchy: Is the order of the neutrino masses ``normal'' - m_1^2<m_2^2<m_3^2 - or is it inverted - m_3^2<m_1^2<m_2^2, with m_2^2-m_1^2 << m_2^2,m_1^2? While this issue can be resolved in next-generation long-baseline nu_{mu} to nu_e neutrino oscillation studies if |U_{e3}|^2 is large enough, a clear strategy on how to resolve it if |U_{e3}|^2 is sufficiently small is still lacking. We study the capability of non-oscillation probes of neutrino masses to determine the neutrino mass ordering. We concentrate on studies of m_{nu_e}, the kinematical neutrino mass to which precise studies of tritium beta-decay are sensitive, m_{ee}, the effective mass to which the rate for neutrinoless double-beta decay is sensitive if the neutrinos are Majorana fermions, and Sigma, the sum of the neutrino masses, to which cosmological probes of the energy budget of the Universe are sensitive. We find that combined measurements of m_{ee}, Sigma, and m_{nu_e} are capable of establishing the neutrino mass hierarchy if these measurements are precise enough and if one ``gets lucky.'' We quantify the previous sentence in detail by performing a numerical analysis of a large number of theoretical data sets, for different measured values of m_{nu_e}, m_{ee}, and Sigma, keeping in mind the ultimate sensitivity that can be reached by next (and next-to-next) generation experiments.
Is the relatively isolated member of the neutrino mass spectrum heavier or lighter than the two closely-spaced members? This question - the character of the neutrino mass hierarchy - is of great theoretical interest. All previously identified experiments for addressing it via neutrino oscillations require that the currently unknown size of the U_e3 element of the leptonic mixing matrix (parameterized by the unknown theta_13 mixing angle) be sufficiently large, and will utterly fail in the limit theta_13=0. For this reason, we explore alternative oscillation approaches that would still succeed even if theta_13 vanishes. We identify several alternatives that require neither a nonzero |U_e3| nor the presence of significant matter effects. All include multiple percent-level neutrino oscillation measurements, usually involving muon-neutrino (or antineutrino) disappearance and very long baselines. We comment on the degree of promise that these alternative approaches show.
In this brief review, I discuss the new physics unveiled by neutrino oscillation experiments over the past several years, and discuss several attempts at understanding the mechanism behind neutrino masses and lepton mixing. It is fair to say that, while significant theoretical progress has been made, we are yet to construct a coherent picture that naturally explains non-zero, yet tiny, neutrino masses and the newly revealed, puzzling patterns of lepton mixing. I discuss what the challenges are, and point to the fact that more experimental input (from both neutrino and non-neutrino experiments) is dearly required - and that new data is expected to reveal, in the next several years, new information. Finally, I draw attention to the fact that neutrinos may have only just begun to reshape fundamental physics, given the fact that we are still to explain the LSND anomaly and because the neutrino oscillation phenomenon is ultimately sensitive to very small new-physics effects.
The most general, renormalizable Lagrangian that includes massive neutrinos contains ``right-handed neutrino'' Majorana masses of order M. While there are prejudices in favor of M much larger than the weak scale, virtually nothing is known about the magnitude of M. I argue that the LSND anomaly provides, currently, the only experimental hint: M around 1 eV. If this is the case, the LSND mixing angles are functions of the active neutrino masses and mixing and, remarkably, adequate fits to all data can be naturally obtained. I also discuss consequences of this ``eV-seesaw'' for supernova neutrino oscillations, tritium beta-decay, neutrinoless double-beta decay, and cosmology.
I discuss, in a semi-pedagogical way, our current understanding of neutrino physics. I present a brief history of how the neutrino came to be ``invented'' and observed, and discuss the evidence that led to the recent discovery that neutrinos change flavor. I then spend some time presenting mass-induced neutrino flavor change (neutrino oscillation), and how it pieces all the neutrino puzzles except for the LSND anomaly, which is also briefly discussed. I conclude by highlighting the importance of determining the nature of the neutrinos, i.e., are they Dirac or Majorana fermions.