191 research outputs found
Predicting the locations of possible long-lived low-mass first stars: Importance of satellite dwarf galaxies
The search for metal-free stars has so far been unsuccessful, proving that if
there are surviving stars from the first generation, they are rare, they have
been polluted, or we have been looking in the wrong place. To predict the
likely location of Population~III (Pop~III) survivors, we semi-analytically
model early star formation in progenitors of Milky Way-like galaxies and their
environments. We base our model on merger trees from the high-resolution dark
matter only simulation suite \textit{Caterpillar}. Radiative and chemical
feedback are taken into account self-consistently, based on the spatial
distribution of the haloes. Our results are consistent with the non-detection
of Pop III survivors in the Milky Way today. We find that possible surviving
Population III stars are more common in Milky Way satellites than in the main
Galaxy. In particular, low mass Milky Way satellites contain a much larger
fraction of Pop~III stars than the Milky Way. Such nearby, low mass Milky Way
satellites are promising targets for future attempts to find Pop~III survivors,
especially for high-resolution, high signal-to-noise spectroscopic
observations. We provide the probabilities for finding a Pop~III survivor in
the red giant branch phase for all known Milky Way satellites to guide future
observations.Comment: 17 pages, 12 figures, 1 table, submitted to MNRA
A Higgs Quadruplet for Type III Seesaw and Implications for and Conversion
In Type III seesaw model the heavy neutrinos are contained in leptonic
triplet representations. The Yukawa couplings of the triplet fermion and the
left-handed neutrinos with the doublet Higgs field produce the Dirac mass
terms. Together with the Majorana masses for the leptonic triplets, the light
neutrinos obtain non-zero seesaw masses. We point out that it is also possible
to have a quadruplet Higgs field to produce the Dirac mass terms to facilitate
the seesaw mechanism. The vacuum expectation value of the quadruplet Higgs is
constrained to be small by electroweak precision data. Therefore the Yukawa
couplings of a quadruplet can be much larger than those for a doublet. We also
find that unlike the usual Type III seesaw model where at least two copies of
leptonic triplets are needed, with both doublet and quadruplet Higgs
representations, just one leptonic triplet is possible to have a
phenomenologically acceptable model because light neutrino masses can receive
sizable contributions at both tree and one loop levels. Large Yukawa couplings
of the quadruplet can induce observable effects for lepton flavor violating
processes and conversion. Implications of the
recent limit from MEG and also limit on conversion
on Au are also given. Some interesting collider signatures for the doubly
charged Higgs boson in the quadruplet are discussed.Comment: Latex 11 pages, 1 figure. A few references adde
Effective Lagrangian approach to neutrinoless double beta decay and neutrino masses
Neutrinoless double beta () decay can in general produce
electrons of either chirality, in contrast with the minimal Standard Model (SM)
extension with only the addition of the Weinberg operator, which predicts two
left-handed electrons in the final state. We classify the lepton number
violating (LNV) effective operators with two leptons of either chirality but no
quarks, ordered according to the magnitude of their contribution to \znbb
decay. We point out that, for each of the three chirality assignments, and , there is only one LNV operator of the corresponding type
to lowest order, and these have dimensions 5, 7 and 9, respectively. Neutrino
masses are always induced by these extra operators but can be delayed to one or
two loops, depending on the number of RH leptons entering in the operator.
Then, the comparison of the decay rate and neutrino masses
should indicate the effective scenario at work, which confronted with the LHC
searches should also eventually decide on the specific model elected by nature.
We also list the SM additions generating these operators upon integration of
the heavy modes, and discuss simple realistic examples of renormalizable
theories for each case.Comment: Accepted for publication. Few misprints corrected and new references
adde
Low energy consequences from supersymmetric models with left-right symmetry
We consider several low energy consequences arising from a class of
supersymmetric models based on the gauge groups and in which the gauge
hierarchy and problems have been resolved. There are important
constraints on the MSSM parameters , and
, and we discuss how they are reconciled with radiative electroweak
breaking. We also consider the ensuing sparticle and Higgs spectroscopy, as
well as the decays and . The latter process
may be amenable to experimental tests through an order of magnitude increase in
sensitivity.Comment: 17 pages, latex2
TeV-scale bileptons, see-saw type II and lepton flavor violation in core-collapse supernova
Electrons and electron neutrinos in the inner core of the core-collapse
supernova are highly degenerate and therefore numerous during a few seconds of
explosion. In contrast, leptons of other flavors are non-degenerate and
therefore relatively scarce. This is due to lepton flavor conservation. If this
conservation law is broken by some non-standard interactions, electron
neutrinos are converted to muon and tau-neutrinos, and electrons - to muons.
This affects the supernova dynamics and the supernova neutrino signal. We
consider lepton flavor violating interactions mediated by scalar bileptons,
i.e. heavy scalars with lepton number 2. It is shown that in case of TeV-mass
bileptons the electron fermi gas is equilibrated with non-electron species
inside the inner supernova core at a time-scale of order of (1-100) ms. In
particular, a scalar triplet which generates neutrino masses through the
see-saw type II mechanism is considered. It is found that supernova core is
sensitive to yet unprobed values of masses and couplings of the triplet.Comment: accepted to Eur.Phys.J.
Neutrinoless double beta decay in seesaw models
We study the general phenomenology of neutrinoless double beta decay in
seesaw models. In particular, we focus on the dependence of the neutrinoless
double beta decay rate on the mass of the extra states introduced to account
for the Majorana masses of light neutrinos. For this purpose, we compute the
nuclear matrix elements as functions of the mass of the mediating fermions and
estimate the associated uncertainties. We then discuss what can be inferred on
the seesaw model parameters in the different mass regimes and clarify how the
contribution of the light neutrinos should always be taken into account when
deriving bounds on the extra parameters. Conversely, the extra states can also
have a significant impact, cancelling the Standard Model neutrino contribution
for masses lighter than the nuclear scale and leading to vanishing neutrinoless
double beta decay amplitudes even if neutrinos are Majorana particles. We also
discuss how seesaw models could reconcile large rates of neutrinoless double
beta decay with more stringent cosmological bounds on neutrino masses.Comment: 34 pages, 5 eps figures and 1 axodraw figure. Final version published
in JHEP. NME results available in Appendi
Spontaneous R-Parity Violation, Flavor Symmetry and Tribimaximal Mixing
We explore the possibility of spontaneous R parity violation in the context
of flavor symmetry. Our model contains singlet matter chiral superfields which are arranged as triplet of
and as well as few additional Higgs chiral superfields which are singlet
under MSSM gauge group and belong to triplet and singlet representation under
the flavor symmetry. R parity is broken spontaneously by the vacuum
expectation values of the different sneutrino fields and hence we have
neutrino-neutralino as well as neutrino-MSSM gauge singlet higgsino mixings in
our model, in addition to the standard model neutrino- gauge singlet neutrino,
gaugino-higgsino and higgsino-higgsino mixings. Because all of these mixings we
have an extended neutral fermion mass matrix. We explore the low energy
neutrino mass matrix for our model and point out that with some specific
constraints between the sneutrino vacuum expectation values as well as the MSSM
gauge singlet Higgs vacuum expectation values, the low energy neutrino mass
matrix will lead to a tribimaximal mixing matrix. We also analyze the potential
minimization for our model and show that one can realize a higher vacuum
expectation value of the singlet
sneutrino fields even when the other sneutrino vacuum expectation values are
extremely small or even zero.Comment: 18 page
Do experiments suggest a hierarchy problem?
The hierarchy problem of the scalar sector of the standard model is
reformulated, emphasizing the role of experimental facts that may suggest the
existence of a new physics large mass scale, for instance indications of the
instability of the matter, or indications in favor of massive neutrinos. In the
see-saw model for the neutrino masses a hierarchy problem arises if the mass of
the right-handed neutrinos is larger than approximatively GeV: this
problem, and its possible solutions, are discussed.Comment: revtex, 4 pages, 1 figur
Examining leptogenesis with lepton flavor violation and the dark matter abundance
Within a supersymmetric (SUSY) type-I seesaw framework with flavor-blind
universal boundary conditions, we study the consequences of requiring that the
observed baryon asymmetry of the Universe be explained by either thermal or
non-thermal leptogenesis. In the former case, we find that the parameter space
is very constrained. In the bulk and stop-coannihilation regions of mSUGRA
parameter space (that are consistent with the measured dark matter abundance),
lepton flavor-violating (LFV) processes are accessible at MEG and future
experiments. However, the very high reheat temperature of the Universe needed
after inflation (of about 10^{12} GeV) leads to a severe gravitino problem,
which disfavors either thermal leptogenesis or neutralino dark matter.
Non-thermal leptogenesis in the preheating phase from SUSY flat directions
relaxes the gravitino problem by lowering the required reheat temperature. The
baryon asymmetry can then be explained while preserving neutralino dark matter,
and for the bulk or stop-coannihilation regions LFV processes should be
observed in current or future experiments.Comment: 20 pages, 5 figures, 1 tabl
Magnetic Properties of Monomer and Dimer Tetrahedral VOx Entities Dispersed on Amorphous Silica-based Materials: Prediction of EPR Parameters from Relativistic DFT Calculations and Broken Symmetry Approach to Exchange Couplings
Molecular structures of the isolated tetrahedral oxovanadium(IV) and bridged μ-oxo-divanadium(IV) complexes hosted by the clusters mimicking surfaces of amorphous silica-based materials were investigated using density functional theory (DFT) calculations. Principal values of the g and A tensors for the monomer vanadyl species were obtained using the coupled-perturbed DFT level of theory and the spin–orbit mean-field approximation (SOMF). Magnetic exchange interaction for the μ-oxo bridged vanadium(IV) dimer was investigated within the broken symmetry approach. An antiferromagnetic coupling of the individual magnetic moments of the vanadium(IV) centers in the [VO–O–VO]2+ bridges was revealed and discussed in detail. The coupling explains pronounced decrease of the electron paramagnetic resonance signal (EPR) intensity, observed for the reduced VOx/SiO2 samples with the increasing coverage of vanadia, in terms of transformation of the paramagnetic monomer species into the dimers with S = 0 ground state
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