5,717 research outputs found
Preconditioning of low-frequency repetitive transcranial magnetic stimulation with transcranial direct current stimulation: evidence for homeostatic plasticity in the human motor cortex
Recent experimental work in animals has emphasized the importance of homeostatic plasticity as a means of stabilizing the properties of neuronal circuits. Here, we report a phenomenon that indicates a homeostatic pattern of cortical plasticity in healthy human subjects. The experiments combined two techniques that can produce long-term effects on the excitability of corticospinal output neurons: transcranial direct current stimulation (TDCS) and repetitive transcranial magnetic stimulation (rTMS) of the left primary motor cortex. "Facilitatory preconditioning" with anodal TDCS caused a subsequent period of 1 Hz rTMS to reduce corticospinal excitability to below baseline levels for >20 min. Conversely, "inhibitory preconditioning" with cathodal TDCS resulted in 1 Hz rTMS increasing corticospinal excitability for at least 20 min. No changes in excitability occurred when 1 Hz rTMS was preceded by sham TDCS. Thus, changing the initial state of the motor cortex by a period of DC polarization reversed the conditioning effects of 1 Hz rTMS. These preconditioning effects of TDCS suggest the existence of a homeostatic mechanism in the human motor cortex that stabilizes corticospinal excitability within a physiologically useful range
LHC discovery potential for models with continuously distributed mass
We study the Large Hadron Collider (LHC) discovery potential for models
with continuously distributed mass for and 14 TeV
centre-of-mass energies. One of possible LHC signatures for such models is the
existence of broad resonance in Drell-Yan reaction .Comment: 14 pages, some references and formula adde
Discriminating graviton exchange effects from other new physics scenarios in e^+e^- collisions
We study the possibility of uniquely identifying the effects of graviton
exchange from other new physics in high energy e^+e^- annihilation into
fermion-pairs. For this purpose, we use as basic observable a specific
asymmetry among integrated differential distributions, that seems particularly
suitable to directly test for such gravitational effects in the data analysis.Comment: 18 pages, including figures; v2: additional references and
acknowledgements. To appear in PR
Quantum Gravity Effects in Black Holes at the LHC
We study possible back-reaction and quantum gravity effects in the
evaporation of black holes which could be produced at the LHC through a
modification of the Hawking emission. The corrections are phenomenologically
taken into account by employing a modified relation between the black hole mass
and temperature. The usual assumption that black holes explode around TeV
is also released, and the evaporation process is extended to (possibly much)
smaller final masses. We show that these effects could be observable for black
holes produced with a relatively large mass and should therefore be taken into
account when simulating micro-black hole events for the experiments planned at
the LHC.Comment: 14 pages, 8 figures, extended version of hep-ph/0601243 with new
analysis of final products, final version accepted for publication in J.
Phys.
Constraints From on the Left-Right Symmetric Model
Recent results from the CLEO Collaboration on both inclusive and exclusive
radiative decays are used to constrain the parameter space of two versions
of the Left-Right Symmetric Model. In the first scenario, when the left- and
right-handed Cabibbo-Kobayashi-Maskawa mixing matrices are equal, ,
the radiative decay data is shown to lead to strong bounds on the
mixing angle that are quite insensitive to either the top quark or mass.
The second scenario examined is that of Gronau and Wakaizumi wherein -quark
decays proceed only via right-handed currents and and are quite
distinct. For this model, the combined constraints from Tevatron
searches, the lifetime, and radiative decays lead to a very highly
restricted allowed range for the mixing angle.Comment: 16 pages, 9 figures(not included), LaTex, SLAC-PUB-642
Cartography with Accelerators: Locating Fermions in Extra Dimensions at Future Lepton Colliders
In the model of Arkani-Hamed and Schmaltz the various chiral fermions of the
Standard Model(SM) are localized at different points on a thick wall which
forms an extra dimension. Such a scenario provides a way of understanding the
absence of proton decay and the fermion mass hierarchy in models with extra
dimensions. In this paper we explore the capability of future lepton colliders
to determine the location of these fermions in the extra dimension through
precision measurements of conventional scattering processes both below and on
top of the lowest lying Kaluza-Klein gauge boson resonance. We show that for
some classes of models the locations of these fermions can be very precisely
determined while in others only their relative positions can be well measured.Comment: 32 pages, 10 figs, LaTe
Linear Collider Test of a Neutrinoless Double Beta Decay Mechanism in left-right Symmetric Theories
There are various diagrams leading to neutrinoless double beta decay in
left-right symmetric theories based on the gauge group SU(2)_L x SU(2)_R. All
can in principle be tested at a linear collider running in electron-electron
mode. We argue that the so-called lambda-diagram is the most promising one.
Taking the current limit on this diagram from double beta decay experiments, we
evaluate the relevant cross section e e to W_L W_R, where W_L is the Standard
Model W-boson and W_R the one from SU(2)_R. It is observable if the life-time
of double beta decay and the mass of the W_R are close to current limits. Beam
polarization effects and the high-energy behaviour of the cross section are
also analyzed.Comment: 17 pages, 6 figures. v2: minor changes, references added, to be
published in EPJ
Higgs-Boson Decay to Four Fermions Including a Single Top Quark Below Threshold
The rare decay modes Higgs four light fermions, and Higgs
single top-quark + three light fermions for , are
presented, and phenomenologically interpreted. The angular correlation between
fermion planes is presented as a test of the spin and intrinsic parity of the
Higgs particle. In Higgs decay to single top, two tree-level graphs contribute
in the standard model (SM); one couples the Higgs to , and
one to t\bar t(\sim g_{top\;yukawa}=m_t/246\GeV). The large Yukawa coupling
for m_t>100\GeV makes the second amplitude competitive or dominant for most
values. Thus the Higgs decay rate to single top directly probes the
SM universal mechanism generating both gauge boson and fermion masses, and
offers a means to infer the Higgs- Yukawa coupling when is kinematically disallowed. We find that the modes at the SSC, and at future high energy,
high luminosity colliders, may be measureable if is not too far above
. We classify non-standard Higgses as gaugeo-phobic, fermio-phobic or
fermio-philic, and discuss the Higgs single top rates for these
classes.Comment: 30 pages, 6 figures (figures available upon request); VAND-TH-93/
TeV-Scale Black Hole Lifetimes in Extra-Dimensional Lovelock Gravity
We examine the mass loss rates and lifetimes of TeV-scale extra dimensional
black holes (BH) in ADD-like models with Lovelock higher-curvature terms
present in the action. In particular we focus on the predicted differences
between the canonical and microcanonical ensemble statistical mechanics
descriptions of the Hawking radiation that results in the decay of these BH. In
even numbers of extra dimensions the employment of the microcanonical approach
is shown to generally lead to a significant increase in the BH lifetime as in
case of the Einstein-Hilbert action. For odd numbers of extra dimensions,
stable BH remnants occur when employing either description provided the highest
order allowed Lovelock invariant is present. However, in this case, the time
dependence of the mass loss rates obtained employing the two approaches will be
different. These effects are in principle measurable at future colliders.Comment: 27 pages, 9 figs; Refs. and discussion adde
Realistic constraints on the doubly charged bilepton couplings from Bhabha scattering with LEP data
Upper limits on doubly charged bilepton couplings and masses are extracted
from LEP data for Bhabha scattering at energy range GeV
using standard model program ZFITTER which calculates radiative corrections. We
find that at 95% C.L. for scalar and
vector bileptons.Comment: 5 pages, 1 EPS figur
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