Neutrino physics beyond neutrino masses

We briefly summarise the current status of neutrino masses and mixing, paying special attention to the prospects for observing new leptonic interactions.Comment: Latex 6 pages. To appear in the proceedings of 9th Hellenic School and Workshops on Elementary Particle Physics and Gravity (CORFU2009), Corfu, Greece, August 30-September 20, 2009

Evidence for right-handed neutrinos at a neutrino factory

We emphasize that a muon based neutrino factory could show the existence of light right-handed neutrinos, if a deficit in the number of detected events is observed at a near detector. This could be as large as ~10% if the size of the new interactions saturates the present limits from electroweak precision data, what is not excluded by the oscillation experiments performed up to now. A simple model realizing such a scenario can be obtained adding right-handed neutrinos to the minimal Standard Model, together with an extra scalar doublet and a triplet of hypercharge 1. In this case, however, the possible deficit is reduced by a factor of ~3, and the Yukawa couplings must be adequately chosen. This is also generically required if lepton flavour violation must be below present bounds.Comment: Latex 18 pages, 2 eps figure

Heavy lepton pair production at LHC: model discrimination with multi-lepton signals

If new leptons exist close to the electroweak scale, they can be produced in pairs at LHC through standard or new interactions. We study the production of heavy lepton pairs in SM extensions with: (i) a Majorana or Dirac lepton triplet, as those appearing in type-III seesaw; (ii) a lepton isodoublet (N,E)_{L,R}; (iii) a charged isosinglet E_{L,R}; (iv) a Majorana or Dirac neutrino singlet N and an additional Z' gauge boson. It is shown that the trilepton final state l+-l+-l-+, which has a small SM background, constitutes the golden channel for heavy neutrino searches, being very sensitive to Majorana or Dirac neutrinos in triplet, doublet or singlet SU(2)_L representations. For higher luminosities, signals in this final state can also distinguish lepton triplets from doublets and singlets. The Majorana or Dirac nature of the heavy neutrinos is revealed by the presence or not of like-sign dilepton l+-l+- signals without significant missing energy. Notably, l+-l+- signals but with large missing energy are characteristic of Dirac triplets, distinguishing them from the other two models with a heavy Dirac neutrino. Further discrimination is achieved with the analysis of the clean l+l+l-l- final state.Comment: LaTeX 38 pages, 28 PS figures. Added cross section plots, comments and references. Final version to appear in NP

Heavy Majorana Neutrinos in the Effective Lagrangian Description: Application to Hadron Colliders

We consider the effects of heavy Majorana neutrinos N with sub-TeV masses. We argue that the mere presence of these particles would be a signal of physics beyond the minimal seesaw mechanism and their interactions are, therefore, best described using an effective Lagrangian. We then consider the complete set of leading effective operators (up to dimension 6) involving the N and Standard Model fields and show that these interactions can be relatively easy to track at high-energy colliders. For example, we find that an exchange of a TeV-scale heavy vector field can yield thousands of characteristic same-sign lepton number violating l^+ l^+ j j events (j=light jet) at the LHC if m_N < 600 GeV, which can also have a distinctive forward-backward asymmetry signal; even the Tevatron has good prospects for this signature if m_N < 300 GeV.Comment: 4 pages, 1 figur

Seesaw Options for Three Neutrinos

The seesaw mechanism for three neutrinos is discussed, clarifying the situation where the seesaw texture results in three approximately zero mass eigenvalues. The true underlying mechanism is shown to be just the $inverse$ (or $linear$) seesaw, which explains why there could be large mixing. However, these zeroes cannot occur naturally, unless there is a conserved symmetry, i.e. lepton number $L$, either global or gauged, which is softly or spontaneously broken at the TeV scale. We discuss in particular the case where the three heavy singlet neutrinos have $L=3,-2,-1$.Comment: Latex, 11 Pages with 1 figure. Some minor modifications and a reference adde

Distinguishing seesaw models at LHC with multi-lepton signals

We investigate the LHC discovery potential for electroweak scale heavy neutrino singlets (seesaw I), scalar triplets (seesaw II) and fermion triplets (seesaw III). For seesaw I we consider a heavy Majorana neutrino coupling to the electron or muon. For seesaw II we concentrate on the likely scenario where the new scalars decay to two leptons. For seesaw III we restrict ourselves to heavy Majorana fermion triplets decaying to light leptons plus gauge or Higgs bosons, which are dominant except for unnaturally small mixings. The possible signals are classified in terms of the charged lepton multiplicity, studying nine different final states ranging from one to six charged leptons. Using a fast detector simulation of signals and backgrounds, it is found that the trilepton channel ℓ±ℓ±ℓ∓ is by far the best one for scalar triplet discovery, and for fermion triplets it is as good as the like-sign dilepton channel ℓ±ℓ±. For heavy neutrinos with a mass O(100) GeV, this trilepton channel is also better than the usually studied like-sign dilepton mode. In addition to evaluating the discovery potential, we make special emphasis on the discrimination among seesaw models if a positive signal is observed. This could be accomplished not only by searching for signals in different final states, but also by reconstructing the mass and determining the charge of the new resonances, which is possible in several cases. For high luminosities, further evidence is provided by the analysis of the production angular distributions in the cleanest channels with three or four leptons.This work has been supported by MEC project FPA2006-05294 and Junta de Andalucía projects FQM 101, FQM 437 and FQM03048. The work of J.A.A.S. has been supported by a MEC Ramón y Cajal contract

A Little Higgs model of neutrino masses

Little Higgs models are formulated as effective theories with a cut-off of up to 100 times the electroweak scale. Neutrino masses are then a puzzle, since the usual see-saw mechanism involves a much higher scale that would introduce quadratic corrections to the Higgs mass parameter. We propose a model that can naturally accommodate the observed neutrino masses and mixings in Little Higgs scenarios. Our framework does not involve any large scale or suppressed Yukawa couplings, and it implies the presence of three extra (Dirac) neutrinos at the TeV scale. The masses of the light neutrinos are induced radiatively, they are proportional to small (\approx keV) mass parameters that break lepton number and are suppressed by the Little Higgs cut-off.Comment: 10 pages, 3 figure

Leptophobic U(1)'s and the R_b - R_c Crisis

In this paper, we investigate the possibility of explaining both the R_b excess and the R_c deficit reported by the LEP experiments through Z-Z' mixing effects. We have constructed a set of models consistent with a restrictive set of principles: unification of the Standard Model (SM) gauge couplings, vector- like additional matter, and couplings which are both generation-independent and leptophobic. These models are anomaly-free, perturbative up to the GUT scale, and contain realistic mass spectra. Out of this class of models, we find three explicit realizations which fit the LEP data to a far better extent than the unmodified SM or MSSM and satisfy all other phenomenological constraints which we have investigated. One realization, the \eta-model coming from E_6, is particularly attractive, arising naturally from geometrical compactifications of heterotic string theory. This conclusion depends crucially on the inclusion of a U(1) kinetic mixing term, whose value is correctly predicted by renormalization group running in the E_6 model given one discrete choice of spectra.Comment: LaTeX, 26 pages, 5 embedded EPSF figures. Version to be published in Phys. Rev.

Gauging U(1) symmetries and the number of right-handed neutrinos

In this letter we consider that assuming: a) that the only left-handed neutral fermions are the active neutrinos, b) that $B-L$ is a gauge symmetry, and c) that the $L$ assignment is restricted to the integer numbers, the anomaly cancellation imply that at least three right-handed neutrinos must be added to the minimal representation content of the electroweak standard model. However, two types of models arise: i) the usual one where each of the three identical right-handed neutrinos has total lepton number L=1; ii) and the other one in which two of them carry L=4 while the third one carries $L=-5$.Comment: Published version in PLB forma

Semileptonic decays of polarised top quarks: V + A admixture and QCD corrections

The semileptonic decays of polarised top quarks are analysed for a general chirality-conserving tbW vertex. We calculate double differential distributions for the charged lepton and the neutrino to order \alpha_s in the QCD coupling. We present these QCD corrections in terms of compact parameterisations that should be useful for the future investigation of the structure of the top decay vertex on the basis of large data samples.Comment: 7 pages, 2 figures, Late