75 research outputs found
Neutrino Masses and Leptogenesis with Heavy Higgs Triplets
A simple and economical extension of the minimal standard electroweak gauge
model (without right-handed neutrinos) by the addition of two heavy Higgs
scalar triplets would have two significant advantages. \underline {Naturally}
small Majorana neutrino masses would become possible, as well as leptogenesis
in the early universe which gets converted at the electroweak phase transition
into the present observed baryon asymmetry.Comment: 12 pages including one figur
Leptogenesis with Heavy Majorana Neutrinos Reexamined
The mass term for Majorana neutrinos explicitly violates lepton number.
Several authors have used this fact to create a lepton asymmetry in the
universe by considering CP violating effects in the one loop self-energy
correction for the decaying heavy Majorana neutrino. We compare and comment on
the different approaches used to calculate the lepton asymmetry including those
using an effective Hamiltonian and resummed propagators. We also recalculate
the asymmetry in the small mass difference limit.Comment: 16 pages, LaTex, 1 figure included. 2 footnotes and 1 reference adde
Unstable Heavy Majorana Neutrinos and Leptogenesis
We propose a new mechanism producing a non-vanishing lepton number asymmetry,
based on decays of heavy Majorana neutrinos. If they are produced out of
equilibrium, as occurs in preheating scenario, and are superpositions of mass
eigenstates rapidly decaying, their decay rates contains interference terms
provided the mass differences are small compared to widths .
The resulting lepton asymmetry, which is the analogue of the time-integrated CP
asymmetry in system, is found to be proportional to .Comment: 18 pages, latex, revised version to be published in Phys. Rev.
Soft breaking of symmetry: Light neutrino spectrum and Leptogenesis
Continuous symmetry can generate quasi degenerate mass
spectrum for both left handed light and right handed heavy Majorana neutrinos
assuming that the symmetry preserving non zero parameters are nearly same.
There is an accidental exchange symmetry in the light and heavy
neutrino Majorana mass terms. This implies and
. In addition it generates another zero mixing angle
and one zero mass difference. We restrict ourselves to type-I See-Saw mechanism
for generation of light neutrino mass. We have found that under
symmetry cosmological lepton asymmetry vanishes. We break
such a way that the exchange symmetry preserves
in the neutrino sector. We have seen that light neutrino phenomenology can be
explained under soft breaking of this symmetry. We have observed that softness
of this symmetry breaking depends on the degeneracy of the light neutrino mass
spectrum. Quasi-degeneracy of right handed neutrino mass spectrum opens an
option for resonant leptogenesis. The degeneracy of the right handed neutrino
mass spectrum is restricted through light neutrino data. We observed that for
generation of right sized baryon asymmetry common neutrino mass scale
have to be of the order of and corresponding
right handed neutrino mass scale have to be nearly GeV. We also have
discussed the effect of RG evolution on light neutrino spectrum and also on
baryon asymmetry.Comment: 21 pages, no figure, Revised with the comments on RG effec
Electromagnetic Leptogenesis
We present a new leptogenesis scenario, where the lepton asymmetry is
generated by CP violating decays of heavy electroweak singlet neutrinos via
electromagnetic dipole moment couplings to the ordinary light neutrinos. Akin
to the usual scenario where the decays are mediated through Yukawa
interactions, we have shown, by explicit calculations, that the desired
asymmetry can be produced through the interference of the corresponding
tree-level and one-loop decay amplitudes involving the effective dipole moment
operators. We also find that the relationship of the leptogenesis scale to the
light neutrino masses is similar to that for the standard Yukawa-mediated
mechanism.Comment: 6 pages, 6 figures; v2: some references added, minor change to
discussion, accepted by PR
Monte Carlo study of the potential reduction in out-of-field dose using a patient-specific aperture in pencil beam scanning proton therapy
This study is aimed at identifying the potential benefits of using a patientspecific aperture in proton beam scanning. For this purpose, an accurate Monte Carlo model of the pencil beam scanning (PBS) proton therapy (PT) treatment head at Massachusetts General Hospital (MGH) was developed based on an existing model of the passive double-scattering (DS) system. The Monte Carlo code specifies the treatment head at MGH with sub-millimeter accuracy. The code was configured based on the results of experimental measurements performed at MGH. This model was then used to compare out-of-field doses in simulated DS treatments and PBS treatments. For the conditions explored, the penumbra in PBS is wider than in DS, leading to higher absorbed doses and equivalent doses adjacent to the primary field edge. For lateral distances greater than 10 cm from the field edge, the doses in PBS appear to be lower than those observed for DS. We found that placing a patient-specific aperture at nozzle exit during PBS treatments can potentially reduce doses lateral to the primary radiation field by over an order of magnitude. In conclusion, using a patient-specific aperture has the potential to further improve the normal tissue sparing capabilities of PBS
A new silicon detector for microdosimetry applications in proton therapy
A silicon-on-insulator diode array with a sensitive depth of 10 microns has been developed for microdosimetry in proton therapy. The detector was coupled to a radiation-hard charge sensitive amplifier with the probe assembly capable of measuring an LET down to 1.2 keV/μm. The device has been successfully tested at two proton therapy centers. The 230 MeV Northeastern Proton Therapy Center, Boston and the 250 MeV Proton Medical Research Center at Tsukuba, Japan. The device offers much improved spatial resolution compared with a proportional gas counter particularly in the critical high dose region around the proton Bragg peak. Due to its small cross-sectional area (0.04 cm2) measurements may also be made in facilities with short high intensity beams
Signatures of heavy Majorana neutrinos and HERA's isolated lepton events
The graph of neutrinoless double beta decay is applied to HERA and
generalized to final states with any two charged leptons. Considered is the
case in which one of the two escapes typical identification criteria and the
case when a produced tau decays hadronically. Both possibilities give one
isolated lepton with high transverse momentum, hadronic activity and an
imbalance in transverse momentum. We examine the kinematical properties of
these events and compare them with the high p_T isolated leptons reported by
the H1 collaboration. Their positive charged muon events can be explained by
the ``double beta'' process and we discuss possibilities for the precise
determination which original final state produced the single isolated lepton.
To confirm our hypothesis one should search in the data for high pseudorapidity
and/or low p_T leptons or for additional separated jets.Comment: 19 pages with 14 figures, minor change
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
Resonant leptogenesis in a predictive SO(10) grand unified model
An SO(10) grand unified model considered previously by the authors featuring
lopsided down quark and charged lepton mass matrices is successfully predictive
and requires that the lightest two right-handed Majorana neutrinons be nearly
degenerate in order to obtain the LMA solar neutrino solution. Here we use this
model to test its predictions for baryogenesis through resonant-enhanced
leptogenesis. With the conventional type I seesaw mechanism, the best
predictions for baryogenesis appear to fall a factor of three short of the
observed value. However, with a proposed type III seesaw mechanism leading to
three pairs of massive pseudo-Dirac neutrinos, resonant leptogenesis is
decoupled from the neutrino mass and mixing issues with successful baryogenesis
easily obtained.Comment: 22 pages including 1 figure; published version with reference adde
- …