1,568 research outputs found
Toward Full LHC Coverage of Natural Supersymmetry
We argue that combining just a handful of searches for new physics at Run I
of the LHC is sufficient to exclude most supersymmetric extensions of the
Standard Model in which the gluino is kinematically accessible and the spectrum
is natural. Such models typically give rise to significant MET, top quarks
and/or high object multiplicity, and we show that having even one of these
signatures generally results in stringent limits. We also identify, among
models that lack these signatures, the few gaps in coverage remaining, and
propose search strategies to close these gaps. Our results are general and
independent of the details of the spectrum, assumptions about minimality,
R-parity, etc. Our analysis strategy should remain applicable when the LHC
moves to higher energy. Central to our argument are ATLAS and CMS searches for
many jets and low MET, a proposed lepton + many jets search, an ATLAS search
for 6-7 high-pT jets, and a reexamination of the control and signal regions of
the CMS black hole search.Comment: 53 pages, 16 figures, journal versio
Size-independence of statistics for boundary collisions of random walks and its implications for spin-polarized gases
A bounded random walk exhibits strong correlations between collisions with a
boundary. For an one-dimensional walk, we obtain the full statistical
distribution of the number of such collisions in a time t. In the large t
limit, the fluctuations in the number of collisions are found to be
size-independent (independent of the distance between boundaries). This occurs
for any inter-boundary distance, including less and greater than the
mean-free-path, and means that this boundary effect does not decay with
increasing system-size. As an application, we consider spin-polarized gases,
such as 3-Helium, in the three-dimensional diffusive regime. The above results
mean that the depolarizing effect of rare magnetic-impurities in the container
walls is orders of magnitude larger than a Smoluchowski assumption (to neglect
correlations) would imply. This could explain why depolarization is so
sensitive to the container's treatment with magnetic fields prior to its use.Comment: 5 page manuscript with extra details in appendices (additional 3
pages
Hadronic Calorimeter Shower Size: Challenges and Opportunities for Jet Substructure in the Superboosted Regime
Hadrons have finite interaction size with dense material, a basic feature
common to known forms of hadronic calorimeters (HCAL). We argue that
substructure variables cannot use HCAL information to access the microscopic
nature of jets much narrower than the hadronic shower size, which we call
superboosted massive jets. It implies that roughly 15% of their transverse
energy profile remains inaccessible due to the presence of long-lived neutral
hadrons. This part of the jet substructure is also subject to order-one
fluctuations. We demonstrate that the effects of the fluctuations are not
reduced when a global correction to jet variables is applied. The above leads
to fundamental limitations in the ability to extract intrinsic information from
jets in the superboosted regime. The neutral fraction of a jet is correlated
with its flavor. This leads to an interesting and possibly useful difference
between superboosted W/Z/h/t jets and their corresponding backgrounds. The QCD
jets that form the background to the signal superboosted jets might also be
qualitatively different in their substructure as their mass might lie at or
below the Sudakov mass peak. Finally, we introduce a set of zero-cone
longitudinal jet substructure variables and show that while they carry
information that might be useful in certain situations, they are not in general
sensitive to the jet substructure.Comment: 6 pages, 4 figures; v2: minor improvements of presentation; published
versio
Efficiency and spectrum of internal gamma-ray burst shocks
We present an analysis of the Internal Shock Model of GRBs, where gamma-rays
are produced by internal shocks within a relativistic wind. We show that
observed GRB characteristics impose stringent constraints on wind and source
parameters. We find that a significant fraction, of order 20 %, of the wind
kinetic energy can be converted to radiation, provided the distribution of
Lorentz factors within the wind has a large variance and provided the minimum
Lorentz factor is higher than 10^(2.5)L_(52)^(2/9), where L=10^(52)L_(52)erg/s
is the wind luminosity. For a high, >10 %, efficiency wind, spectral energy
breaks in the 0.1 to 1 MeV range are obtained for sources with dynamical time
R/c < 1 ms, suggesting a possible explanation for the observed clustering of
spectral break energies in this range. The lower limit to wind Lorenz factor
and the upper limit, around (R/10^7 cm)^(-5/6) MeV to observed break energies
are set by Thomson optical depth due to electron positron pairs produced by
synchrotron photons. Natural consequences of the model are absence of bursts
with peak emission energy significantly exceeding 1 MeV, and existence of low
luminosity bursts with low, 1 keV to 10 keV, break energies.Comment: 10 pages, 5 ps-figures. Expanded discussion of magnetic field and
electron energy fraction. Accepted for publication in Astrophysical Journa
Paramagnetic anisotropic magnetoresistance in thin films of SrRuO3
SrRuO3 is an itinerant ferromagnet and in its thin film form when grown on
miscut SrTiO3 it has Tc of ~ 150 K and strong uniaxial anisotropy. We measured
both the Hall effect and the magnetoresistance (MR) of the films as a function
of the angle between the applied field and the normal to the films at
temperatures above Tc. We extracted the extraordinary Hall effect that is
proportional to the perpendicular component of the magnetization and thus the
MR for each angle of the applied field could be correlated with the magnitude
and orientation of the induced magnetization. We successfully fit the MR data
with a second order magnetization expansion, which indicates large anisotropic
MR in the paramagnetic state. The extremum values of resistivity are not
obtained for currents parallel or perpendicular to the magnetization, probably
due to the crystal symmetry.Comment: 3 pages, 3 figure
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