192 research outputs found
A sterile neutrino at MiniBooNE and IceCube
We discuss the possibility that a sterile neutrino of mass around 50 MeV
slightly mixed with the muon flavor may be the origin of the MiniBooNE anomaly.
We show that its production in the atmosphere in a fraction of kaon decays
would imply an excess of contained showers at IceCube from down-going and
near-horizontal directions.Comment: 12 pages, talk presented at II Russian-Spanish Congress: Particle and
Nuclear Physics at all Scales, Saint-Petersburg, October 1-4, 201
A minimal Little Higgs model
We discuss a Little Higgs scenario that introduces below the TeV scale just
the two minimal ingredients of these models, a vectorlike T quark and a singlet
component (implying anomalous couplings) in the Higgs field, together with a
pseudoscalar singlet \eta. In the model, which is a variation of Schmaltz's
simplest Little Higgs model, all the extra vector bosons are much heavier than
the T quark. In the Yukawa sector the global symmetry is approximate, implying
a single large coupling per flavour, whereas in the scalar sector it is only
broken at the loop level. We obtain the one-loop effective potential and show
that it provides acceptable masses for the Higgs h and for the singlet \eta
with no need for an extra \mu term. We find that m_\eta can be larger than
m_h/2, which would forbid the (otherwise dominant) decay mode h -> \eta\eta.Comment: 16 pages. References added, fine tuning analysis included. Version to
appear in PR
Extra Higgs bosons in ttbar production at the LHC
The top quark has a large Yukawa coupling with the Higgs boson. In the usual
extensions of the standard model the Higgs sector includes extra scalars, which
also tend to couple strongly with the top quark. Unlike the Higgs, these fields
have a natural mass above 2m_t, so they could introduce anomalies in ttbar
production at the LHC. We study their effect on the ttbar invariant mass
distribution at sqrt{s}=7 TeV. We focus on the bosons (H,A) of the minimal SUSY
model and on the scalar field (r) associated to the new scale f in Little Higgs
(LH) models. We show that in all cases the interference with the standard
amplitude dominates over the narrow-width contribution. As a consequence, the
mass difference between H and A or the contribution of an extra T-quark loop in
LH models become important effects in order to determine if these fields are
observable there. We find that a 1 fb^{-1} luminosity could probe the region
tan beta \le 3 of SUSY and v/(sqrt{2}f) \ge 0.3 in LH models.Comment: 18 pages, version to appear in PR
Heavy neutrino decays at MiniBooNE
It has been proposed that a sterile neutrino \nu_h with m_h \approx 50 MeV
and a dominant decay mode (\nu_h -> \nu\gamma) may be the origin of the
experimental anomaly observed at LSND. We define a particular model that could
also explain the MiniBooNE excess consistently with the data at other neutrino
experiments (radiative muon capture at TRIUMF, T2K, or single photon at NOMAD).
The key ingredients are (i) its long lifetime (\tau_h\approx 3-7x10^{-9} s),
which introduces a 1/E dependence with the event energy, and (ii) its Dirac
nature, which implies a photon preferably emitted opposite to the beam
direction and further reduces the event energy at MiniBooNE. We show that these
neutrinos are mostly produced through electromagnetic interactions with nuclei,
and that T2K observations force BR(\nu_h -> \nu_\tau\gamma) \le 0.01 \approx
BR(\nu_h -> \nu_\mu\gamma). The scenario implies then the presence of a second
sterile neutrino \nu_{h'} which is lighter, longer lived and less mixed with
the standard flavors than \nu_h. Since such particle would be copiously
produced in air showers through (\nu_h -> \nu_{h'}\gamma) decays, we comment on
the possible contamination that its photon-mediated elastic interactions with
matter could introduce in dark matter experiments.Comment: 18 pages, typo in Eq.(6) correcte
Heavy-neutrino decays at neutrino telescopes
It has been recently proposed that a sterile neutrino \nu_h of mass
m_h=40--80 MeV, mixing |U_{\mu h}|^2=10^{-3}--10^{-2}, lifetime \tau_h \lsim
10^{-9} s, and a dominant decay mode (\nu_h \to \nu_\mu \gamma) could be the
origin of the experimental anomalies observed at LSND, KARMEN and MiniBooNE.
Such a particle would be abundant inside air showers, as it can be produced in
kaon decays (K -> \nu_h \mu, K_L -> \nu_h \pi \mu). We use the Z-moment method
to evaluate its atmospheric flux and the frequency of its decays inside
neutrino telescopes. We show that the \nu_h would imply around 10^4 contained
showers per year inside a 0.03 km^3 telescope like ANTARES or the DeepCore in
IceCube. These events would have a characteristic energy and zenith-angle
distribution (E_\nu = 0.1--10 TeV and \theta < 90^o), which results from a
balance between the reach of the heavy neutrino (that disfavors low energies)
and a sizeable production rate and decay probability. The standard background
from contained neutrino events (\nu_e N \to e X and neutral-current
interactions of high inelasticity) is 100 times smaller. Therefore, although it
may be challenging from an experimental point of view, a search at ANTARES and
IceCube could confirm this heavy-neutrino possibility.Comment: 10 pages. Comments on constraints from muon capture and cosmology
added, minor corrections, references added. Version to appear as a Rapid
Communication in PR
A new physics interpretation of the IceCube data
IceCube has recently observed 37 events of TeV-PeV energies. The angular
distribution, with a strong preference for downgoing directions, the spectrum,
and the small muon to shower ratio in the data can not be accommodated assuming
standard interactions of atmospheric neutrinos. We obtain an excellent fit,
however, if a diffuse flux of ultrahigh energy (cosmogenic) neutrinos
experiences collisions where only a small fraction of the energy is transferred
to the target nucleon. We show that consistent models of TeV gravity or other
non-Wilsonian completions of the standard model provide cross sections with
these precise features. An increased statistics could clearly distinguish our
scenario from the one assumed by IceCube (a diffuse flux of astrophysical
neutrinos with a E^{-2} spectrum) and establish the need for new physics in the
interpretation of the data.Comment: 13 pages, version to appear in Astroparticle Physic
Black hole gas in TeV-gravity models
In a plasma at temperature close to the fundamental scale a small fraction of
particles will experience transplanckian collisions that may result in
microscopic black holes (BHs). We study the dynamics of a system (a black hole
gas) defined by radiation at a given temperature coupled to a distribution of
BHs of different mass. Our analysis includes the production of BHs in
photon-photon collisions, BH evaporation, the absorption of radiation,
collisions of two BHs to give a larger one, and the effects of the expansion.
We find that the system may follow two different generic paths depending on the
initial temperature of the plasma.Comment: 4 pages. Talk given by M.B. in ERE-200
Gluon excitations in t tbar production at hadron colliders
We argue that a relatively light massive gluon with mass <= 1 TeV, small
purely axial couplings to light quarks and sizable vector and axial couplings
to the top quark can reproduce the large forward-backward asymmetry observed at
the Tevatron without conflicting with the t tbar and the dijet invariant mass
distributions measured at the Tevatron and the LHC. We show that realistic
Higgsless models with warped extra dimensions naturally fulfil all the
necessary ingredients to realize this scenario. While current data is unable to
discover or exclude these heavy gluons with masses 850 GeV, they should be
observed at the (7 TeV) LHC with a luminosity of the order of 300 pb^{-1}.Comment: 16 pages, 6 figure
Black hole gas in the early universe
We consider the early universe at temperatures close to the fundamental scale
of gravity (M_D << M_Planck) in models with extra dimensions. At such
temperatures a small fraction of particles will experience transplanckian
collisions that may result in microscopic black holes (BHs). BHs colder than
the environment will gain mass, and as they grow their temperature drops
further. We study the dynamics of a system (a black hole gas) defined by
radiation at a given temperature coupled to a distribution of BHs of different
mass. Our analysis includes the production of BHs in photon-photon collisions,
BH evaporation, the absorption of radiation, collisions of two BHs to give a
larger one, and the effects of the expansion. We show that the system may
follow two different generic paths depending on the initial temperature of the
plasma.Comment: 17 pages, version to appear in JCA
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