The Majorana nature of neutrinos can be experimentally verified only via lepton-number violating processes involving charged leptons. We study 36 lepton-number violating (LV ) processes from the decays of tau leptons and pseudoscalar mesons. These decays are absent in the Standard Model but, in presence of Majorana neutrinos in the mass range ∼ 100 MeV to 5 GeV, the rates for these processes would be enhanced due to their resonant contribution. We calculate the transition rates and branching fractions and compare them to the current bounds from direct experimental searches forL = 2 tau and rare meson decays. The experimental non-observation of such LV processes places stringent bounds on the Majorana neutrino mass and mixing and we summarize the existing limits. We also extend the search to hadron collider experiments. We find that, at the Tevatron with 8 fb −1 integrated luminosity, there could be 2σ (5σ) sensitivity for resonant production of a Majorana neutrino in the � ± � ± modes in the mass range of ∼ 10 − 180 GeV (10 − 120 GeV). This reach can be extended to ∼ 10 − 375 GeV (10 − 250 GeV) at the LHC of 14TeV with 100 fb −1 . The production cross section at the LHC of 10TeV is also presented for comparison. We study the � ± e ± modes as well and find that the signal could be large enough even taking into account the current bound from neutrinoless double-beta decay. The signal from the gauge boson fusion channel W + W + → l + l + at the LHC is found to be very weak given the rather small mixing parameters. We comment on the search strategy when a τ lepton is involved in the final state.

https://iopscience.iop.org/article/10.1088/1126-6708/2009/05/030/pdf

The Search for Heavy Majorana Neutrinos

http://home.thep.lu.se/~torbjorn/preprints/lutp1028.pdf

General-purpose event generators for LHC physics

We review the present status of global analyses of neutrino oscillations, taking into account the most recent neutrino data including the latest KamLAND and K2K updates presented at Neutrino2004, as well as state-of-the-art solar and atmospheric neutrino flux calculations. We give the two-neutrino solar + KamLAND results, as well as two-neutrino atmospheric + K2K oscillation regions, discussing in each case the robustness of the oscillation interpretation against departures from the Standard Solar Model and the possible existence of non-standard neutrino physics. Furthermore, we give the best fit values and allowed ranges of the three-flavour oscillation parameters from the current worlds' global neutrino data sample and discuss in detail the status of the small parameters $\alpha \equiv \Dms/\Dma$ as well as $\sin^2\theta_{13}$, which characterize the strength of CP violating effects in neutrino oscillations. We also update the degree of rejection of four-neutrino interpretations of the LSND anomaly in view of the most recent developments.

Status of global fits to neutrino oscillations

We review the particle physics aspects of neutrino-less double beta decay. This process can be mediated by light massive Majorana neutrinos (standard interpretation) or by something else (non-standard interpretations). The physics potential of both interpretations is summarized and the consequences of future measurements or improved limits on the half-life of neutrino-less double beta decay are discussed. We try to cover all proposed alternative realizations of the decay, including light sterile neutrinos, supersymmetric or left-right symmetric theories, Majorons, and other exotic possibilities. Ways to distinguish the mechanisms from one another are discussed. Experimental and nuclear physics aspects are also briefly touched, alternative processes to double beta decay are discussed, and an extensive list of references is provided.

/pdf/neutrino-less-double-beta-decay-and-particle-physics-4fssdtwcty.pdf

Neutrino-less double beta decay and particle physics

A Call for New Physics : The Muon Anomalous Magnetic Moment and Lepton Flavor Violation

We demonstrate how to systematically test a well-motivated mechanism for neutrino mass generation (type II seesaw) at the LHC, in which a Higgs triplet is introduced In the optimistic scenarios with a small Higgs triplet vacuum expectation value ${v}_{\ensuremath{\Delta}}l{10}^{\ensuremath{-}4}\text{ }\text{ }\mathrm{GeV}$, one can look for clean signals of lepton-number violation in the decays of doubly charged (${H}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}$) and singly charged (${H}^{\ifmmode\pm\else\textpm\fi{}}$) Higgs bosons to distinguish the normal hierarchy (NH), the inverted hierarchy (IH), and the quasidegenerate (QD) spectrum for the light neutrino masses The observation of either ${H}^{+}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{+}\overline{\ensuremath{\nu}}$ or ${H}^{+}\ensuremath{\rightarrow}{e}^{+}\overline{\ensuremath{\nu}}$ will be particularly robust for the spectrum test since they are independent of the unknown Majorana phases The ${H}^{++}$ decays moderately depend on a Majorana phase ${\ensuremath{\Phi}}_{2}$ in the NH, but sensitively depend on ${\ensuremath{\Phi}}_{1}$ in the IH In a less favorable scenario ${v}_{\ensuremath{\Delta}}g2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\text{ }\text{ }\mathrm{GeV}$, when the leptonic channels are suppressed, one needs to observe the decays ${H}^{+}\ensuremath{\rightarrow}{W}^{+}{H}_{1}$ and ${H}^{+}\ensuremath{\rightarrow}t\overline{b}$ to confirm the triplet-doublet mixing which in turn implies the existence of the same gauge-invariant interaction between the lepton doublet and the Higgs triplet responsible for the neutrino mass generation In the most optimistic situation, ${v}_{\ensuremath{\Delta}}\ensuremath{\sim}{10}^{\ensuremath{-}4}\text{ }\text{ }\mathrm{GeV}$, both channels of the lepton pairs and gauge boson pairs may be available simultaneously The determination of their relative branching fractions would give a measurement for the value of ${v}_{\ensuremath{\Delta}}$

Neutrino masses and the CERN LHC: Testing the type II seesaw mechanism

We study the pair production of doubly charged Higgs bosons at the CERN Large Hadron Collider (LHC), assuming the doubly charged Higgs to be part of an SU(2){sub L} triplet which generates Majorana masses for left-handed neutrinos. Such pair production has the advantage that it is not constrained by the triplet vacuum expectation value, which tends to make the single production rate rather small. We point out that, in addition to the Drell-Yan production mechanism, two-photon processes also contribute to H{sup ++}H{sup --} production at a level comparable to the QCD corrections to the Drell-Yan channel. Decays of the doubly charged Higgs into both the l{sup +}l{sup +} and W{sup +}W{sup +} modes are studied in detail to optimize the signal observation over the backgrounds. Doubly charged scalars should be observable at the LHC with 300 fb{sup -1} integrated luminosity in the l{sup {+-}}l{sup {+-}} channel up to the mass range of 1 TeV even with a branching fraction of about 60%, and in the W{sup {+-}}W{sup {+-}} channel up to a mass of 700 GeV. Such a doubly charged Higgs, if it is a member of a triplet that generates neutrino masses, cannot be long-lived on the scale ofmore » collider detectors although it might lead to a displaced secondary vertex during its decay if it is lighter than about 250 GeV.« less

Pair production of doubly charged scalars: Neutrino mass constraints and signals at the CERN LHC

We investigate the predictions of a simple extension of the standard model where the Higgs sector is composed of one SU(2)L doublet and one real triplet. We discuss the general features of the model, including its vacuum structure, theoretical and phenomenological constraints, and expectations for Higgs collider studies. The model predicts the existence of a pair of light charged scalars and, for vanishing triplet vacuum expectation value, contains a cold dark matter candidate. When the latter possibility occurs, the charged scalars are long-lived, leading to a prediction of distinctive single charged track with missing transverse energy or double charged track events at the large hadron collider. The model predicts a significant excess of two-photon events compared to SM expectations due to the presence of a light charged scalar.

/pdf/triplet-scalars-and-dark-matter-at-the-lhc-24lhw1uh33.pdf

Triplet Scalars and Dark Matter at the LHC

We consider the recent measurement of the top quark forward-backward asymmetry at the Fermilab Tevatron, which shows a discrepancy of slightly more than $2\ensuremath{\sigma}$ compared to the standard model prediction. We find that $t$-channel exchange of a color sextet or triplet scalar particle can explain the measurement, while leaving the cross section for $t\overline{t}$ production within measured uncertainties. Such particles have good discovery prospects by study of the kinematic structure of $t\overline{t}+\mathrm{\text{jets}}$ at the LHC.

Kai Wang

Papers

Neutrino masses and the CERN LHC: Testing the type II seesaw mechanism

Pair production of doubly charged scalars: Neutrino mass constraints and signals at the CERN LHC

Triplet Scalars and Dark Matter at the LHC

Explorations of the top quark forward-backward asymmetry at the Tevatron

Axigluon as possible explanation for pp¯→tt¯ forward–backward asymmetry