120 research outputs found
A new look at the cosmic ray positron fraction
The positron fraction in cosmic rays was found to be a steadily increasing in
function of energy, above 10 GeV. This behaviour contradicts standard
astrophysical mechanisms, in which positrons are secondary particles, produced
in the interactions of primary cosmic rays during the propagation in the
interstellar medium. The observed anomaly in the positron fraction triggered a
lot of excitement, as it could be interpreted as an indirect signature of the
presence of dark matter species in the Galaxy. Alternatively, it could be
produced by nearby astrophysical sources, such as pulsars. Both hypotheses are
probed in this work in light of the latest AMS-02 positron fraction
measurements. The transport of the primary and secondary positrons in the
Galaxy is described using a semi-analytic two-zone model. MicrOMEGAs is used to
model the positron flux generated by dark matter species. The description of
the positron fraction from astrophysical sources is based on the pulsar
observations included in the ATNF catalogue. We find that the mass of the
favoured dark matter candidates is always larger than 500 GeV. The only dark
matter species that fulfils the numerous gamma ray and cosmic microwave
background bounds is a particle annihilating into four leptons through a light
scalar or vector mediator, with a mixture of tau (75%) and electron (25%)
channels, and a mass between 0.5 and 1 TeV. The positron anomaly can also be
explained by a single astrophysical source and a list of five pulsars from the
ATNF catalogue is given. Those results are obtained with the cosmic ray
transport parameters that best fit the B/C ratio. Uncertainties in the
propagation parameters turn out to be very significant. In the WIMP
annihilation cross section to mass plane for instance, they overshadow the
error contours derived from the positron data.Comment: 20 pages, 16 figures, accepted for publication in A&A, corresponds to
published versio
A Fixed-Target ExpeRiment at the LHC (AFTER@LHC) : luminosities, target polarisation and a selection of physics studies
We report on a future multi-purpose fixed-target experiment with the proton
or lead ion LHC beams extracted by a bent crystal. The multi-TeV LHC beams
allow for the most energetic fixed-target experiments ever performed. Such an
experiment, tentatively named AFTER for "A Fixed-Target ExperRiment", gives
access to new domains of particle and nuclear physics complementing that of
collider experiments, in particular at RHIC and at the EIC projects. The
instantaneous luminosity at AFTER using typical targets surpasses that of RHIC
by more than 3 orders of magnitude. Beam extraction by a bent crystal offers an
ideal way to obtain a clean and very collimated high-energy beam, without
decreasing the performance of the LHC. The fixed-target mode also has the
advantage of allowing for spin measurements with a polarised target and for an
access over the full backward rapidity domain up to xF ~ - 1. Here, we
elaborate on the reachable luminosities, the target polarisation and a
selection of measurements with hydrogen and deuterium targets.Comment: 6 pages. Proceedings of the Sixth International Conference on Quarks
and Nuclear Physics QNP2012 (16-20 April 2012, Ecole Polytechnique,
Palaiseau,France
Prospectives for A Fixed-Target ExpeRiment at the LHC: AFTER@LHC
We argue that the concept of a multi-purpose fixed-target experiment with the
proton or lead-ion LHC beams extracted by a bent crystal would offer a number
of ground-breaking precision-physics opportunities. The multi-TeV LHC beams
will allow for the most energetic fixed-target experiments ever performed. The
fixed-target mode has the advantage of allowing for high luminosities, spin
measurements with a polarised target, and access over the full backward
rapidity domain --uncharted until now-- up to x_F ~ -1.Comment: 6 pages, 1 table, LaTeX. Proceedings of the 36th International
Conference on High Energy Physics (ICHEP2012), 4-11 July 2012, Melbourne,
Australi
Spin physics at A Fixed-Target ExpeRiment at the LHC (AFTER@LHC)
We outline the opportunities for spin physics which are offered by a next
generation and multi-purpose fixed-target experiment exploiting the proton LHC
beam extracted by a bent crystal. In particular, we focus on the study of
single transverse spin asymetries with the polarisation of the target.Comment: Contributed to the 20th International Spin Physics Symposium,
SPIN2012, 17-22 September 2012, Dubna, Russia, 4 pages, LaTe
Holographic renormalization and supersymmetry
Holographic renormalization is a systematic procedure for regulating
divergences in observables in asymptotically locally AdS spacetimes. For dual
boundary field theories which are supersymmetric it is natural to ask whether
this defines a supersymmetric renormalization scheme. Recent results in
localization have brought this question into sharp focus: rigid supersymmetry
on a curved boundary requires specific geometric structures, and general
arguments imply that BPS observables, such as the partition function, are
invariant under certain deformations of these structures. One can then ask if
the dual holographic observables are similarly invariant. We study this
question in minimal N = 2 gauged supergravity in four and five dimensions. In
four dimensions we show that holographic renormalization precisely reproduces
the expected field theory results. In five dimensions we find that no choice of
standard holographic counterterms is compatible with supersymmetry, which leads
us to introduce novel finite boundary terms. For a class of solutions
satisfying certain topological assumptions we provide some independent tests of
these new boundary terms, in particular showing that they reproduce the
expected VEVs of conserved charges.Comment: 70 pages; corrected typo
PMm2: large photomultipliers and innovative electronics for the next-generation neutrino experiments
The next generation of proton decay and neutrino experiments, the
post-SuperKamiokande detectors as those that will take place in megaton size
water tanks, will require very large surfaces of photodetection and a large
volume of data. Even with large hemispherical photomultiplier tubes, the
expected number of channels should reach hundreds of thousands. A funded R&D
program to implement a solution is presented here. The very large surface of
photodetection is segmented in macro pixels made of 16 hemispherical (12
inches) photomultiplier tubes connected to an autonomous front-end which works
on a triggerless data acquisition mode. The expected data transmission rate is
5 Mb/s per cable, which can be achieved with existing techniques. This
architecture allows to reduce considerably the cost and facilitate the
industrialization. This document presents the simulations and measurements
which define the requirements for the photomultipliers and the electronics. A
proto-type of front-end electronics was successfully tested with 16
photomultiplier tubes supplied by a single high voltage, validating the
built-in gain adjustment and the calibration principle. The first tests and
calculations on the photomultiplier glass led to the study of a new package
optimized for a 10 bar pressure in order to sustain the high underwater
pressure.Comment: 1 pdf file, 4 pages, 4 figures, NDIP08, submitted to Nucl. Instr. and
Meth. Phys. Res.
Spin physics and TMD studies at A Fixed-Target ExpeRiment at the LHC (AFTER@LHC)
We report on the opportunities for spin physics and Transverse-Momentum
Dependent distribution (TMD) studies at a future multi-purpose fixed-target
experiment using the proton or lead ion LHC beams extracted by a bent crystal.
The LHC multi-TeV beams allow for the most energetic fixed-target experiments
ever performed, opening new domains of particle and nuclear physics and
complementing that of collider physics, in particular that of RHIC and the EIC
projects. The luminosity achievable with AFTER@LHC using typical targets would
surpass that of RHIC by more that 3 orders of magnitude in a similar energy
region. In unpolarised proton-proton collisions, AFTER@LHC allows for
measurements of TMDs such as the Boer-Mulders quark distributions, the
distribution of unpolarised and linearly polarised gluons in unpolarised
protons. Using the polarisation of hydrogen and nuclear targets, one can
measure transverse single-spin asymmetries of quark and gluon sensitive probes,
such as, respectively, Drell-Yan pair and quarkonium production. The
fixed-target mode has the advantage to allow for measurements in the
target-rapidity region, namely at large x^uparrow in the polarised nucleon.
Overall, this allows for an ambitious spin program which we outline here.Comment: 6 pages, 4 figures, 1 table, LaTeX. Proceedings of the Fourth
International Workshop on Transverse Polarisation Phenomena in Hard Processes
(Transversity 2014), 9-13 June, 2013, Chia, Ital
The High-Acceptance Dielectron Spectrometer HADES
HADES is a versatile magnetic spectrometer aimed at studying dielectron
production in pion, proton and heavy-ion induced collisions. Its main features
include a ring imaging gas Cherenkov detector for electron-hadron
discrimination, a tracking system consisting of a set of 6 superconducting
coils producing a toroidal field and drift chambers and a multiplicity and
electron trigger array for additional electron-hadron discrimination and event
characterization. A two-stage trigger system enhances events containing
electrons. The physics program is focused on the investigation of hadron
properties in nuclei and in the hot and dense hadronic matter. The detector
system is characterized by an 85% azimuthal coverage over a polar angle
interval from 18 to 85 degree, a single electron efficiency of 50% and a vector
meson mass resolution of 2.5%. Identification of pions, kaons and protons is
achieved combining time-of-flight and energy loss measurements over a large
momentum range. This paper describes the main features and the performance of
the detector system
Supercurrent anomalies in 4d SCFTs
We use holographic renormalization of minimal \mathcalN=2 gauged
supergravity in order to derive the general form of the quantum Ward identities
for 3d \mathcalN=2 and 4d \mathcalN=1 superconformal theories on
general curved backgrounds, including an arbitrary fermionic source for the
supercurrent. The Ward identities for 4d \mathcalN=1 theories contain both
bosonic and fermionic global anomalies, which we determine explicitly up to
quadratic order in the supercurrent source. The Ward identities we derive apply
to any superconformal theory, independently of whether it admits a holographic
dual, except for the specific values of the and anomaly coefficients,
which are equal due to our starting point of a two-derivative bulk supergravity
theory. In the case of 4d \mathcalN=1 superconformal theories, we show that
the fermionic anomalies lead to an anomalous transformation of the supercurrent
under rigid supersymmetry on backgrounds admitting Killing spinors, even if all
anomalies are numerically zero on such backgrounds. The anomalous
transformation of the supercurrent under rigid supersymmetry leads to an
obstruction to the -exactness of the stress tensor in supersymmetric vacua,
and may have implications for the applicability of localization techniques. We
use this obstruction to the -exactness of the stress tensor in order to
resolve a number of apparent paradoxes relating to the supersymmetric Casimir
energy, the BPS condition for supsersymmetric vacua, and the compatibility of
holographic renormalization with supersymmetry, that were presented in the
literature
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