12,600 research outputs found
Long-lived Heavy Neutrinos from Higgs Decays
We investigate the pair-production of right-handed neutrinos via the Standard
Model (SM) Higgs boson in a gauged model. The right-handed neutrinos with
a mass of few tens of GeV generating viable light neutrino masses via the
seesaw mechanism naturally exhibit displaced vertices and distinctive
signatures at the LHC and proposed lepton colliders. The production rate of the
right-handed neutrinos depends on the mixing between the SM Higgs and the
exotic Higgs associated with the breaking, whereas their decay length
depends on the active-sterile neutrino mixing. We focus on the displaced
leptonic final states arising from such a process, and analyze the sensitivity
reach of the LHC and proposed lepton colliders in probing the active-sterile
neutrino mixing. We show that mixing to muons as small as can be probed at the LHC with 100 fb and at proposed lepton
colliders with 5000 fb. The future high luminosity run at LHC and the
proposed MATHUSLA detector may further improve this reach by an order of
magnitude.Comment: 23 pages, 9 figures, matches published versio
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Zero-one IP problems: Polyhedral descriptions & cutting plane procedures
A systematic way for tightening an IP formulation is by employing classes of linear inequalities that define facets of the convex hull of the feasible integer points of the respective problems. Describing as well as identifying these inequalities will help in the efficiency of the LP-based cutting plane methods. In this report, we review classes of inequalities that partially described zero-one poly topes such as the 0-1 knapsack polytope, the set packing polytope and the travelling salesman polytope. Facets or valid inequalities derived from the 0-1 knapsack and the set packing polytopes are algorithmically identifie
Comments on the paper ``Bare Quark Surfacees of Strange Stars and Electron-Positron Pair Emission''
In a recent paper (Ushov, PRL, 80, 230, 1998), it has been claimed that the
bare surface of a strange star can emit electron-positron pairs of luminosity
\~10^{51} ergs/s for about 10s. If true, obviously, this mechanism may explain
the origin of cosmic Gamma Ray Bursts. However, we point out that such a
mechanism is does not work because (i) if pair production really occurs the
supposed pre-existing supercritical electric field will be quenched and this
discharge process may at best release ~10^{24} ergs of electromagnetic energy,
and (ii) there is no way by which the trapped core thermal energy of few
10^{52} ergs can be transmitted electromagnetically on a time scale of ~10s or
even on a much larger time scale. The only way the hot core can cool on a time
scale of ~10 s or much shorter is by the well known process of emission of
nu-antinu pairs.Comment: Final version accepted in Phy. Rev. Lett. Main conclusion that the
mechanism by Usov does not work remains unchanged,
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Tunneling conduction in graphene/(poly)vinyl alcohol composites
Graphene/(Poly)vinyl alcohol (PVA) composite film with thickness
were synthesized by solidification of a PVA solution comprising of dispersed
graphene nanosheets. The close proximity of the graphene sheets enables the
fluctuation induced tunneling of electrons to occur from one sheet to another.
The dielectric data show that the present system can be simulated to a parallel
resistance-capacitor network. The high frequency exponent of the frequency
variation of the ac conductivity indicates that the charge carriers move in a
two-dimensional space. The sample preparation technique will be helpful for
synthesizing flexible conductors.Comment: 10 pages, 8 figure
Sensitivity of the Hazard Ratio to Non-Ignorable Treatment Assignment in an Observational Study
In non-randomized studies, estimation of treatment effects generally requires adjustment for imbalances in observed covariates. One such method, based on the propensity score, is useful in many applications but may be biased when the assumption of strongly ignorable treatment assignment is violated. Because it is not possible to evaluate this assumption from the data, it is advisable to assess the sensitivity of conclusions to violations of strong ignorability. Lin et al [1] have implemented this idea by investigating how an unmeasured covariate may affect the conclusions of an observational study. We extend their method to assess sensitivity of the treatment hazard ratio to hidden bias under a range of covariate distributions. We derive simple formulas for approximating the true from the apparent treatment hazard ratio estimated under a specific survival model, and assess the validity of these formulas in simulation studies. We demonstrate the method in an analysis of SEER-Medicare data on the effects of chemotherapy in elderly colon cancer patients
Brane-worlds and theta-vacua
Reductions from odd to even dimensionalities ( or ), for
which the effective low-energy theory contains chiral fermions, present us with
a mismatch between ultraviolet and infrared anomalies. This applies to both
local (gauge) and global currents; here we consider the latter case. We show
that the mismatch can be explained by taking into account a change in the
spectral asymmetry of the massive modes--an odd-dimensional analog of the
phenomenon described by the Atiyah-Patodi-Singer theorem in even
dimensionalities. The result has phenomenological implications: we present a
scenario in which a QCD-like -angle relaxes to zero on a certain
(possibly, cosmological) timescale, despite the absence of any light axion-like
particle.Comment: 44 pages, 4 figure
Turbulent dynamo with advective magnetic helicity flux
Many astrophysical bodies harbor magnetic fields that are thought to be
sustained by a dynamo process. However, it has been argued that the production
of large-scale magnetic fields by mean-field dynamo action is strongly
suppressed at large magnetic Reynolds numbers owing to the conservation of
magnetic helicity. This phenomenon is known as {\it catastrophic quenching}.
Advection of magnetic fields by stellar and galactic winds toward the outer
boundaries and away from the dynamo is expected to alleviate such quenching.
Here we explore the relative roles played by advective and turbulent--diffusive
fluxes of magnetic helicity in the dynamo. In particular, we study how the
dynamo is affected by advection. We do this by performing direct numerical
simulations of a turbulent dynamo of type driven by forced
turbulence in a Cartesian domain in the presence of a flow away from the
equator where helicity changes sign. Our results indicate that in the presence
of advection, the dynamo, otherwise stationary, becomes oscillatory. We confirm
an earlier result for turbulent--diffusive magnetic helicity fluxes that for
small magnetic Reynolds numbers (\Rm\lesssim 100...200, based on the
wavenumber of the energy-carrying eddies) the magnetic helicity flux scales
less strongly with magnetic Reynolds number (\Rm^{-1/2}) than the term
describing magnetic helicity destruction by resistivity (\Rm^{-1}). Our new
results now suggest that for larger \Rm the former becomes approximately
independent of \Rm, while the latter falls off more slowly. We show for the
first time that both for weak and stronger winds, the magnetic helicity flux
term becomes comparable to the resistive term for \Rm\gtrsim 1000, which is
necessary for alleviating catastrophic quenching.Comment: 9 pages, 9 figures, accepted for publication in MNRA
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