69 research outputs found
TeV-scale Seesaw with Quintuplet Fermions
We propose a new seesaw model based on fermionic hypercharge zero weak
quintuplet in conjunction with additional scalar quadruplet which attains an
induced vev. The model provides both tree-level seesaw ~ v^6/M^5 and a
loop-suppressed radiative ~ (1 / 16 \pi^2) v^2/M contributions to active
neutrino masses. The empirical masses m_\nu ~ 10^{-1} eV can be achieved with M
~ TeV new states, accessible at the LHC. For 5 fb^{-1} of accumulated
integrated luminosity at the LHC, there could be ~ 500 doubly-charged
\Sigma^{++} or \bar{\Sigma^{++}} fermions with mass M_\Sigma = 400 GeV, leading
to interesting multi-lepton signatures. The neutral component of the fermion
quintuplet, previously identified as minimal dark matter candidate, becomes
unstable in the proposed seesaw setup. The stability can be restored by
introducing a Z_2 symmetry, in which case neutrinos get mass only from
radiative contributions.Comment: 19 pages, 9 figures, corresponds to published versio
Novel TeV-scale seesaw mechanism with Dirac mediators
We propose novel tree level seesaw mechanism with TeV-scale vectorlike Dirac
mediators that produce Majorana masses of the known neutrinos. The gauge
quantum number assignment to the Dirac mediators allows them to belong to a
weak triplet and a five-plet of nonzero hypercharge. The latter leads to new
seesaw formula m_\nu ~ v^6/M^5, so that the empirical masses m_\nu ~ 10^{-1} eV
can be achieved by M ~ TeV new states. There is a limited range of the
parameter space with M < a few 100 GeV where the tree level contribution
dominates over the respective loop contributions and the proposed mechanism is
testable at the LHC. We discuss specific signatures for Dirac type heavy
leptons produced by Drell-Yan fusion at the LHC.Comment: 10 pages, 1 figure, version corresponding to PL
Exotic Seesaw-Motivated Heavy Leptons at the LHC
We study the LHC potential for discovering TeV-scale SU(2)_L 5-plet fermions
introduced recently to explain small neutrino masses. We show that the
Drell-Yan production and the decays of new exotic Sigma leptons are testable at
the LHC. Their production is abundant due to nontrivial electroweak gauge
charges. For 1 fb^-1 of integrated luminosity at the present LHC sqrt{s}=7 TeV,
there can be 270 Sigma-Sigmabar pairs produced for M_Sigma = 400 GeV. Besides
producing same-sign dilepton events, they could lead, due to a chosen small
mixing between heavy and light leptons, to ~10 golden decays
Sigma^{+++}(Sigma^{+++}-bar) --> W^\pm W^\pm l^\pm with a specific decay
signature.Comment: 31 pages, 10 figures, corresponds to the PRD versio
Critique of Fermionic R\nuMDM and its Scalar Variants
We examine the stability of minimal dark matter (MDM) particle-candidates in
the setup in which they participate in radiative neutrino (R\nu) masses. We
first point out the existence of an additional renormalizable term in recently
proposed R\nuMDM Lagrangian, which violates the claimed accidental Z_2 symmetry
and spoils the stability of the fermionic MDM quintuplet component. We then
explore the viability of RMDM variants based on scalar MDM multiplets.
There are ubiquitous super-renormalizable terms in the scalar potential which
make these scalar multiplets unstable.Comment: 9 pages, 4 figures, corresponds to published versio
Bounds and Decays of New Heavy Vector-like Top Partners
We study the phenomenology of new heavy vector-like fermions that couple to
the third generation quarks via Yukawa interactions, covering all the allowed
representations under the standard model gauge groups. We first review tree and
loop level bounds on these states. We then discuss tree level decays and
loop-induced decays to photon or gluon plus top. The main decays at tree level
are to W b and/or Z and Higgs plus top via the new Yukawa couplings. The
radiative loop decays turn out to be quite close to the naive estimate: in all
cases, in the allowed perturbative parameter space, the branching ratios are
mildly sensitive on the new Yukawa couplings and small. We therefore conclude
that the new states can be observed at the LHC and that the tree level decays
can allow to distinguish the different representations. Moreover, the
observation of the radiative decays at the LHC would suggest a large Yukawa
coupling in the non-perturbative regime.Comment: 32 pages, 2 tables, 10 figure
A clean signal for a top-like isosinglet fermion at the Large Hadron Collider
We predict a clean signal at the Large Hadron Collider (=14 TeV for
a scenario where there is a top-like, charge +2/3 vectorlike isosinglet
fermion. Such a quark, via mixing with the standard model top, can undergo
decays via both flavour-changing Z-boson coupling and flavour-changing Yukawa
interactions. We concentrate on the latter channel, and study the situation
where, following its pair-production, the heavy quark pair gives rise to two
tops and two Higgs boson. We show that the case where each Higgs decays in the
channel, there can be a rather distinct and background-free signal
that can unveil the existence of the vectorlike isosinglet quark of this kind.Comment: 14 pages, 5 figures, 4 table
Hyperon weak radiative decays in chiral perturbation theory
We investigate the leading-order amplitudes for weak radiative decays of
hyperons in chiral perturbation theory. We consistently include contributions
from the next-to-leading order weak-interaction Lagrangian. It is shown that
due to these terms Hara's theorem is violated. The data for the decays of
charged hyperons can be easily accounted for. However, at this order in the
chiral expansion, the four amplitudes for the decays of neutral hyperons
satisfy relations which are in disagreement with the data. The asymmetry
parameters for all the decays can not be accounted for without higher-order
terms. We shortly comment on the effect of the 27-plet part of the weak
interaction.Comment: 8 pages of REVTeX and using macro-package "feynman.tex" (available at
http://xxx.lanl.gov/ftp/hep-ph/papers/macros) for the 2 figure
Systematic study of the d=5 Weinberg operator at one-loop order
We perform a systematic study of the Weinberg operator at the one-loop
level. We identify three different categories of neutrino mass generation: (1)
finite irreducible diagrams; (2) finite extensions of the usual seesaw
mechanisms at one-loop and (3) divergent loop realizations of the seesaws. All
radiative one-loop neutrino mass models must fall into one of these classes.
Case (1) gives the leading contribution to neutrino mass naturally and a
classic example of this class is the Zee model. We demonstrate that in order to
prevent that a tree level contribution dominates in case (2), Majorana fermions
running in the loop and an additional symmetry are needed for a
genuinely leading one-loop contribution. In the type-II loop extensions, the
lepton number violating coupling will be generated at one loop, whereas the
type-I/III extensions can be interpreted as loop-induced inverse or linear
seesaw mechanisms. For the divergent diagrams in category (3), the tree level
contribution cannot be avoided and is in fact needed as counter term to absorb
the divergence.Comment: 18 pages, 5 Figures, 4 Tables - Version to appear in JHE
The Hunt for New Physics in the Flavour Sector with up vector-like quarks
We analyse the possible presence of New Physics (NP) in the Flavour Sector
and evaluate its potential for solving the tension between the experimental
values of \AJPKs and \BTNu with respect to the Standard Model (SM)
expectations. Updated model independent analyses, where NP contributions are
allowed in Bd - anti-Bd and Bs - anti-Bs transitions, suggest the need of New
Physics in the sector. A detailed analysis of recent Flavour data is then
presented in the framework of a simple extension of the SM, where a
vector-like isosinglet quark is added to the spectrum of the SM. Special
emphasis is given to the implications of this model for correlations among
various measurable quantities. We include constraints from all the relevant
quark flavour sectors and give precise predictions for selected rare processes.
We find important deviations from the SM in observables in the sector like
the semileptonic asymmetry \asld, and
. Other potential places where NP can show up include
, , , and
among others. The experimental data favours in this
model the existence of an up vector-like quark with a mass below
GeV at .Comment: 36 pages, 41 figures; minor changes and references added, matches
version accepted for publicatio
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