187 research outputs found
Gluon-induced QCD corrections to pp --> ZZ --> l anti-l l' anti-l'
A calculation of the loop-induced gluon-fusion process gg --> Z(photon)Z(photon) --> l anti-l l' anti-l' is presented, which provides an important background for Higgs boson searches in the H --> ZZ channel at the LHC. We find that the photon contribution is important for Higgs masses below the Z-pair threshold and that the gg-induced process yields a correction of about 15% relative to the NLO QCD prediction for the q anti-q-induced process when only a M(l anti-l), M(l' anti-l') > 5 GeV cut is applied
Neutrino clustering in the Milky Way and beyond
The standard cosmological model predicts the existence of a Cosmic Neutrino Background, which has not yet been observed directly. Some experiments aiming at its detection are currently under development, despite the tiny kinetic energy of the cosmological relic neutrinos, which makes this task incredibly challenging. Since massive neutrinos are attracted by the gravitational potential of our Galaxy, they can cluster locally. Neutrinos should be more abundant at the Earth position than at an average point in the Universe. This fact may enhance the expected event rate in any future experiment. Past calculations of the local neutrino clustering factor only considered a spherical distribution of matter in the Milky Way and neglected the influence of other nearby objects like the Virgo cluster, although recent N-body simulations suggest that the latter may actually be important. In this paper, we adopt a back-tracking technique, well established in the calculation of cosmic rays fluxes, to perform the first three-dimensional calculation of the number density of relic neutrinos at the Solar System, taking into account not only the matter composition of the Milky Way, but also the contribution of the Andromeda galaxy and the Virgo cluster. The effect of Virgo is indeed found to be relevant and to depend non-trivially on the value of the neutrino mass. Our results show that the local neutrino density is enhanced by 0.53% for a neutrino mass of 10 meV, 12% for 50 meV, 50% for 100 meV or 500% for 300 meV
Cosmic ray electrons and positrons from discrete stochastic sources
The distances that galactic cosmic ray electrons and positrons can travel are
severely limited by energy losses to at most a few kiloparsec, thereby
rendering the local spectrum very sensitive to the exact distribution of
sources in our galactic neighbourhood. However, due to our ignorance of the
exact source distribution, we can only predict the spectrum stochastically. We
argue that even in the case of a large number of sources the central limit
theorem is not applicable, but that the standard deviation for the flux from a
random source is divergent due to a long power law tail of the probability
density. Instead, we compute the expectation value and characterise the scatter
around it by quantiles of the probability density using a generalised central
limit theorem in a fully analytical way. The uncertainty band is asymmetric
about the expectation value and can become quite large for TeV energies. In
particular, the predicted local spectrum is marginally consistent with the
measurements by Fermi-LAT and HESS even without imposing spectral breaks or
cut-offs at source. We conclude that this uncertainty has to be properly
accounted for when predicting electron fluxes above a few hundred GeV from
astrophysical sources.Comment: 16 pages, 8 figures; references and clarifying comment added; to
appear in JCA
Diffusive propagation of cosmic rays from supernova remnants in the Galaxy. I: spectrum and chemical composition
In this paper we investigate the effect of stochasticity in the spatial and
temporal distribution of supernova remnants on the spectrum and chemical
composition of cosmic rays observed at Earth. The calculations are carried out
for different choices of the diffusion coefficient D(E) experienced by cosmic
rays during propagation in the Galaxy. In particular, at high energies we
assume that D(E)\sim E^{\delta}, with and being the
reference scenarios. The large scale distribution of supernova remnants in the
Galaxy is modeled following the distribution of pulsars, with and without
accounting for the spiral structure of the Galaxy. We find that the stochastic
fluctuations induced by the spatial and temporal distribution of supernovae,
together with the effect of spallation of nuclei, lead to mild but sensible
violations of the simple, leaky-box-inspired rule that the spectrum observed at
Earth is with , where
is the slope of the cosmic ray injection spectrum at the sources. Spallation of
nuclei, even with the small rates appropriate for He, may account for slight
differences in spectral slopes between different nuclei, providing a possible
explanation for the recent CREAM observations. For we find that
the slope of the proton and helium spectra are and
respectively at energies above 1 TeV (to be compared with the measured values
of and ). For the hardening of the He
spectra is not observed. We also comment on the effect of time dependence of
the escape of cosmic rays from supernova remnants, and of a possible clustering
of the sources in superbubbles. In a second paper we will discuss the
implications of these different scenarios for the anisotropy of cosmic rays.Comment: 28 pages, To appear in JCA
Sommerfeld Enhancement from Multiple Mediators
We study the Sommerfeld enhancement experienced by a scattering object that
couples to a tower of mediators. This can occur in, e.g., models of secluded
dark matter when the mediator scale is generated naturally by hidden-sector
confinement. Specializing to the case of a confining CFT, we show that
off-resonant values of the enhancement can be increased by ~ 20% for cases of
interest when (i) the (strongly-coupled) CFT admits a weakly-coupled dual
description and (ii) the conformal symmetry holds up to the Planck scale.
Larger enhancements are possible for lower UV scales due to an increase in the
coupling strength of the tower.Comment: 17p, 2 figures; v2 JHEP version (inconsequential typo fixed,
references added
Absolute electron and positron fluxes from PAMELA/Fermi and Dark Matter
We extract the positron and electron fluxes in the energy range 10 - 100 GeV
by combining the recent data from PAMELA and Fermi LAT. The {\it absolute
positron and electron} fluxes thus obtained are found to obey the power laws:
and respectively, which can be confirmed by the
upcoming data from PAMELA. The positron flux appears to indicate an excess at
energies E\gsim 50 GeV even if the uncertainty in the secondary positron flux
is added to the Galactic positron background. This leaves enough motivation for
considering new physics, such as annihilation or decay of dark matter, as the
origin of positron excess in the cosmic rays.Comment: Accepted by JCA
Secluded Dark Matter Coupled to a Hidden CFT
Models of secluded dark matter offer a variant on the standard WIMP picture
and can modify our expectations for hidden sector phenomenology and detection.
In this work we extend a minimal model of secluded dark matter, comprised of a
U(1)'-charged dark matter candidate, to include a confining hidden-sector CFT.
This provides a technically natural explanation for the hierarchically small
mediator-scale, with hidden-sector confinement generating m_{gamma'}>0.
Furthermore, the thermal history of the universe can differ markedly from the
WIMP picture due to (i) new annihilation channels, (ii) a (potentially) large
number of hidden-sector degrees of freedom, and (iii) a hidden-sector phase
transition at temperatures T << M_{dm} after freeze out. The mediator allows
both the dark matter and the Standard Model to communicate with the CFT, thus
modifying the low-energy phenomenology and cosmic-ray signals from the secluded
sector.Comment: ~50p, 8 figs; v2 JHEP versio
Planck Intermediate Results. IX. Detection of the Galactic haze with Planck
Using precise full-sky observations from Planck, and applying several methods
of component separation, we identify and characterize the emission from the
Galactic "haze" at microwave wavelengths. The haze is a distinct component of
diffuse Galactic emission, roughly centered on the Galactic centre, and extends
to |b| ~35 deg in Galactic latitude and |l| ~15 deg in longitude. By combining
the Planck data with observations from the WMAP we are able to determine the
spectrum of this emission to high accuracy, unhindered by the large systematic
biases present in previous analyses. The derived spectrum is consistent with
power-law emission with a spectral index of -2.55 +/- 0.05, thus excluding
free-free emission as the source and instead favouring hard-spectrum
synchrotron radiation from an electron population with a spectrum (number
density per energy) dN/dE ~ E^-2.1. At Galactic latitudes |b|<30 deg, the
microwave haze morphology is consistent with that of the Fermi gamma-ray "haze"
or "bubbles," indicating that we have a multi-wavelength view of a distinct
component of our Galaxy. Given both the very hard spectrum and the extended
nature of the emission, it is highly unlikely that the haze electrons result
from supernova shocks in the Galactic disk. Instead, a new mechanism for
cosmic-ray acceleration in the centre of our Galaxy is implied.Comment: 15 pages, 9 figures, submitted to Astronomy and Astrophysic
The Pierre Auger Observatory III: Other Astrophysical Observations
Astrophysical observations of ultra-high-energy cosmic rays with the Pierre
Auger ObservatoryComment: Contributions to the 32nd International Cosmic Ray Conference,
Beijing, China, August 201
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