184,357 research outputs found
Exclusive Radiative Higgs Decays as Probes of Light-Quark Yukawa Couplings
We present a detailed analysis of the rare exclusive Higgs-boson decays into
a single vector meson and a photon and investigate the possibility of using
these processes to probe the light-quark Yukawa couplings. We work with an
effective Lagrangian with modified Higgs couplings to account for possible
new-physics effects in a model-independent way. The h->V\gamma{} decay rate is
governed by the destructive interference of two amplitudes, one of which
involves the Higgs coupling to the quark anti-quark pair inside the vector
meson. We derive this amplitude at next-to-leading order in \alpha_s using QCD
factorization, including the resummation of large logarithmic corrections and
accounting for the effects of flavor mixing. The high factorization scale
\mu~m_h ensures that our results are rather insensitive to the hadronic
parameters characterizing the light-cone distribution amplitude of the vector
meson. The second amplitude arises from the loop-induced effective
h\gamma\gamma* and h\gamma Z* couplings, where the off-shell gauge boson
converts into the vector meson. We devise a strategy to eliminate theoretical
uncertainties related to this amplitude to almost arbitrary precision. This
opens up the possibility to probe for O(1) modifications of the c- and b-quark
Yukawa couplings and O(30) modifications of the s-quark Yukawa coupling in the
high-luminosity LHC run. In particular, we show that measurements of the ratios
Br(h->\Upsilon(nS)\gamma)/Br(h->\gamma\gamma) and Br(h->bb)/Br(h->\gamma\gamma)
can provide complementary information on the real and imaginary parts of the
b-quark Yukawa coupling. More accurate measurements would be possible at a
future 100 TeV proton-proton collider.Comment: 25 pages plus appendices, 9 figures, 5 tables; v2: NLO evolution of
Gegenbauer moments implemented in Appendix B, some references added; v3: typo
in the value of C_{\gamma Z}(0) after eq. (35) fixe
Non-Equilibrium relation between mobility and diffusivity of interacting Brownian particles under shear
We investigate the relation between mobility and diffusivity for Brownian
particles under steady shear near the glass transition, using mode coupling
approximations. For the two directions perpendicular to the shear direction,
the particle motion is diffusive at long times and the mobility reaches a
finite constant. Nevertheless, the Einstein relation holds only for the
short-time in-cage motion and is violated for long times. In order to get the
relation between diffusivity and mobility, we perform the limit of small
wavevector for the relations derived previously [Phys. Rev. Lett. 102 (2009),
135701], without further approximation. We find good agreement to simulation
results. Furthermore, we split the extra term in the mobility in an exact way
into three terms. Two of them are expressed in terms of mean squared
displacements. The third is given in terms of the (less handy) force-force
correlation function.Comment: 14 pages, 4 figures, accepted for Prog. Theor. Phys. Suppl., issue
for the workshop "Frontiers in Nonequilibrium Physics", Kyoto, 200
On the spin parameter of dark-matter haloes
The study by White (1984) on the growth of angular momentum in dark haloes is
extended towards a more detailed investigation of the spin parameter
. Starting from the Zel'dovich
approximation to structure formation, a dark halo is approximated by a
homogeneous ellipsoid with the inertial tensor of the (highly irregular)
Lagrangian region from which the dark halo forms. Within this
approximation, an expression for the spin parameter can be derived, which
depends on the geometry of , the cosmological density parameter
, the overdensity of the dark halo, and the tidal torque exerted on
it. For Gaussian random fields, this expression can be evaluated statistically.
As a result, we derive a probability distribution of the spin parameter which
gives , consistent with numerical
investigations. This probability distribution steeply rises with increasing
spin parameter, reaching its maximum at . The 10 (50,90)
percentile values are (0.05,0.11, respectively). There is a weak
anticorrelation of the spin parameter with the peak height of the density
fluctuation field . The dependence on
and the variance of the density-contrast field is very weak; there is
only a marginal tendency for the spin parameter to be slightly larger for
late-forming objects in an open universe. Due to the weak dependence on
, our results should be quite generally applicable and independent onComment: 16 pages, preprint MPA 79
A Comparison of X-ray and Strong Lensing Properties of Simulated X-ray Clusters
We use gas-dynamical simulations of galaxy clusters to compare their X-ray
and strong lensing properties. Special emphasis is laid on mass estimates. The
cluster masses range between 6 x 10^14 solar masses and 4 x 10^15 solar masses,
and they are examined at redshifts between 1 and 0. We compute the X-ray
emission of the intracluster gas by thermal bremsstrahlung, add background
contamination, and mimic imaging and spectral observations with current X-ray
telescopes. Although the beta model routinely provides excellent fits to the
X-ray emission profiles, the derived masses are typically biased low because of
the restricted range of radii within which the fit can be done. For beta values
of ~ 2/3, which is the average in our numerically simulated sample, the mass is
typically underestimated by ~ 40 per cent. The masses of clusters which exhibit
pronounced substructure are often substantially underestimated. We suggest that
the ratio between peak temperature and emission-weighted average cluster
temperature may provide a good indicator for ongoing merging and, therefore,
for unreliable mass estimates. X-ray mass estimates are substantially improved
if we fit a King density profile rather than the beta model to the X-ray
emission, thereby dropping the degree of freedom associated with beta. Clusters
selected for their strong lensing properties are typically dynamically more
active than typical clusters. Bulk flows in the intracluster gas contain a
larger than average fraction of the internal energy of the gas in such objects,
hence the measured gas temperatures are biased low. The bulk of the optical
depth for arc formation is contributed by clusters with intermediate rather
than high X-ray luminosity. Arcs occur predominantly in clusters which exhibit
substructure and are not in an equilibrium state. Finally we explain why theComment: 22 pages including figures, submitted to MNRA
Limit theory for the Gilbert graph
For a given homogeneous Poisson point process in two points
are connected by an edge if their distance is bounded by a prescribed distance
parameter. The behaviour of the resulting random graph, the Gilbert graph or
random geometric graph, is investigated as the intensity of the Poisson point
process is increased and the distance parameter goes to zero. The asymptotic
expectation and covariance structure of a class of length-power functionals are
computed. Distributional limit theorems are derived that have a Gaussian, a
stable or a compound Poisson limiting distribution. Finally, concentration
inequalities are provided using a concentration inequality for the convex
distance
Velocity Distribution of a Homogeneously Cooling Granular Gas
In contrast to molecular gases, granular gases are characterized by inelastic
collisions and require therefore permanent driving to maintain a constant
kinetic energy. The kinetic theory of granular gases describes how the average
velocity of the particles decreases after the driving is shut off. Moreover it
predicts that the rescaled particle velocity distribution will approach a
stationary state with overpopulated high-velocity tails as compared to the
Maxwell-Boltzmann distribution. While this fundamental theoretical result was
reproduced by numerical simulations, an experimental confirmation is still
missing. Using a microgravity experiment which allows the spatially homogeneous
excitation of spheres via magnetic fields, we confirm the theoretically
predicted exponential decay of the tails of the velocity distribution.Comment: 11 pages, 14 figure
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