803 research outputs found
Precision Studies of Light Mesons at COMPASS
The COMPASS experiment at CERN's SPS investigates the structure and
excitations of strongly interacting systems. Using reactions of 190 GeV/c pions
with protons and nuclear targets, mediated by the strong and electromagnetic
interaction, an unprecedented statistical precision has been reached allowing
new insight into the properties of light mesons. For the first time the
diffractively produced 3pi final state has been analyzed simultaneously in bins
of invariant mass and four-momentum transfer using a large set of 88 waves up
to a total angular momentum of 6. In addition to a precise determination of the
properties of known resonances and including a model-indepedent analysis of the
pi pi S-wave isobar, a new narrow axial-vector state coupling strongly to
f0(980)pi has been found in previously unchartered territory. By selecting
reactions with very small four-momentum transfer COMPASS is able to study
processes involving the exchange of quasi-real photons. These provide clean
access to low-energy quantities such as radiative couplings and
polarizabilities of mesons, and thus constitute a test of model predictions
such as chiral perturbation theory.Comment: Proceedings of the XV International Conference on Hadron Spectroscopy
(Hadron 2013). 9 pages, 5 figure
A Time Projection Chamber for High-Rate Experiments: Towards an Upgrade of the ALICE TPC
A Time Projection Chamber (TPC) is a powerful detector for 3-dimensional
tracking and particle identification for ultra-high multiplicity events. It is
the central tracking device of many experiments, e.g. the ALICE experiment at
CERN. The necessity of a switching electrostatic gate, which prevents ions
produced in the amplification region o MWPCs from entering the drift volume,
however, restricts its application to trigger rates of the order of 1 kHz.
Charge amplification by Gas Electron Multiplier (GEM) foils instead of
proportional wires offers an intrinsic suppression of the ion backflow,
although not to the same level as a gating grid. Detailed Monte Carlo
simulations have shown that the distortions due to residual space charge from
back-drifting ions can be limited to a few cm, and thus can be corrected using
standard calibration techniques. A prototype GEM-TPC has been built with the
largest active volume to date for a detector of this type. It has been
commissioned with cosmics and particle beams at the FOPI experiment at GSI, and
was employed for a physics measurement with pion beams.
For future operation of the ALICE TPC at the CERN LHC beyond 2019, where
Pb-Pb collision rates of 50 kHz are expected, it is planned to replace the
existing MWPCs by GEM detectors, operated in a continuous, triggerless readout
mode, thus allowing an increase in event rate by a factor of 100. As a first
step of the R&D program, a prototype of an Inner Readout Chamber was equipped
with large-size GEM foils and exposed to beams of protons, pions and electrons
from the CERN PS.
In this paper, new results are shown concerning ion backflow, spatial and
momentum resolution of the FOPI GEM-TPC, detector calibration, and dE/dx
resolution with both detector prototypes. The perspectives of a GEM-TPC for
ALICE with continuous readout will be discussed.Comment: Proceedings of the 13th Vienna Conference on Instrumentation (VCI
2013), submitted to Nuclear Instruments and Methods A, 4 pages, 10 figure
First Results of the PixelGEM Central Tracking System for COMPASS
For its physics program with a high-intensity hadron beam of up to 2e7
particles/s, the COMPASS experiment at CERN requires tracking of charged
particles scattered by very small angles with respect to the incident beam
direction. While good resolution in time and space is mandatory, the challenge
is imposed by the high beam intensity, requiring radiation-hard detectors which
add very little material to the beam path in order to minimize secondary
interactions.
To this end, a set of triple-GEM detectors with a hybrid readout structure
consisting of pixels in the beam region and 2-D strips in the periphery was
designed and built. Successful prototype tests proved the performance of this
new detector type, showing both extraordinary high rate capability and
detection efficiency. The amplitude information allowed to achieve spatial
resolutions about a factor of 10 smaller than the pitch and a time resolution
close to the theoretical limit imposed by the layout.
The PixelGEM central tracking system consisting of five detectors, slightly
improved with respect to the prototype, was completely installed in the COMPASS
spectrometer in spring 2008
RF-Separated Beam Project for the M2 Beam Line at CERN
Within the framework of the Physics Beyond Colliders initiative at CERN, discussions are underway on the feasibility of producing radio-frequency (RF) separated beams for Phase-2 of the AMBER experiment at the M2 beam line in the North experimental area of the CERN SPS. The technique of RF separation is applied to enrich the content of a certain particle type within a beam consisting of different species at the same momentum. It relies on the fact that each particle type has a different velocity, decreasing with rest mass. The successor of the COMPASS experiment, AMBER, requires for its Phase-2 measurements high-intensity, high-purity kaon (and antiproton) beams, which cannot be delivered with the currently existing conventional M2 beam line. The present contribution introduces the principle of RF separation and explains its dependence on different parameters of beam optics and hardware. The first examination of potential showstoppers for the RF-separated beam implementation is presented, based on the particle production rates, beam line transmission for specific optics settings, limitations for overall beam intensity and purity posed by beam line acceptance and radiation protection. Different beam optics settings have been examined, providing either focused or parallel beams inside the RF cavities. The separation and transmission capability of the different optics settings for realistic characteristics of RF cavities are discussed and the preliminary results of the potential purity and intensity of the RF-separated beam are presented. They illustrate the high importance of an RF-separated kaon beam for many of the AMBER Phase-2 data taking programs, such as spectroscopy, prompt-photon production, Primakoff reactions and kaon charge-radius measurement
Development of a GEM-TPC prototype
The use of GEM foils for the amplification stage of a TPC instead of a con-
ventional MWPC allows one to bypass the necessity of gating, as the backdrift
is suppressed thanks to the asymmetric field configuration. This way, a novel
continuously running TPC, which represents one option for the PANDA central
tracker, can be realized. A medium sized prototype with a diameter of 300 mm
and a length of 600 mm will be tested inside the FOPI spectrometer at GSI using
a carbon or lithium beam at intermediate energies (E = 1-3AGeV). This detector
test under realistic experimental conditions should allow us to verify the
spatial resolution for single tracks and the reconstruction capability for
displaced vertexes. A series of physics measurement implying pion beams is
scheduled with the FOPI spectrometer together with the GEM-TPC as well.Comment: 5 pages, 4 figures, Proceedings for 11th ICATTP conference in como
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Search for Axionlike and Scalar Particles with the NA64 Experiment
We carried out a model-independent search for light scalar (s) and
pseudoscalar axionlike (a) particles that couple to two photons by using the
high-energy CERN SPS H4 electron beam. The new particles, if they exist, could
be produced through the Primakoff effect in interactions of hard bremsstrahlung
photons generated by 100 GeV electrons in the NA64 active dump with virtual
photons provided by the nuclei of the dump. The a(s) would penetrate the
downstream HCAL module, serving as shielding, and would be observed either
through their decay in the rest of the HCAL detector or
as events with large missing energy if the a(s) decays downstream of the HCAL.
This method allows for the probing the a(s) parameter space, including those
from generic axion models, inaccessible to previous experiments. No evidence of
such processes has been found from the analysis of the data corresponding to
electrons on target allowing to set new limits on the
-coupling strength for a(s) masses below 55 MeV.Comment: This publication is dedicated to the memory of our colleague Danila
Tlisov. 7 pages, 5 figures, revised version accepted for publication in Phys.
Rev. Let
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