343 research outputs found
Evidence for -atoms with DIRAC-II
DIRAC-II is a fixed-target experiment at the CERN Proton Synchroton (PS) which has been designed to search for piK atoms, a bound state of a pi±K± pair, and measure their lifetime. These atoms are observed through an excess of low energetic piK pairs over the background, detected in the two spectrometer arms. This excess comes from the ionization of piK atoms in the target and can be related to their mean life. The piK S-wave scattering length combination |a1/2 - a3/2| (for isospin 1/2 and 3/2) can be related to the latter. The aim of the upgraded DIRAC-II experiment is a measurement of the scattering length combination |a1/2 - a3/2| with a precision of 5%. piK atoms have not been observed so far. The original DIRAC experiment was designed to measure the scattering lengths of pipi atoms. So far, close to 15 000 atoms have been detected, leading to a precision on |a0 - a2| which is better than 10%. In chiral perturbation theories (ChPT) the pipi scattering lengths have been calculated with 2% precision and are in good agreement with the DIRAC-II measurements. Predictions for the piK ones, which involve the s-quarks are obtained. DIRAC-II data will be very important to check ChPT extended to the s-quark. Previous measurements of a1/2 and a3/2 suffer from very large errors as they are extrapolated from high energy piK scattering. My first contribution was the development and construction of a Cherenkov threshold count er for the separation of kaons from pions, which is crucial for the detection of piK atoms. The momentum range (4-8 GeV/c) requires aerogel with refractive index n = 1.008 and n = 1.015 as Cerenkov radiator. Because of the strong light absorption in aerogel, the low indices and the very large size of the active area needed in the DIRAC-II experiment, new designs had to be invented and investigated. In this work new concepts of aerogel counters are presented e.g. introducing wavelength shifters in order to avoid the strong light absorption in aerogel. The second contribution was the analysis of the first run in 2007 and the search for piK atoms. A new tracking had to be developed to optimize the reconstruction of tracks using only detectors downstream of the spectrometer magnet, since no upstream detectors were operational at that time. This new method was carefully tested on the already published pipi 2001 data sample. From the 1137 ± 257 observed piK Coulomb pairs in the signal region where atoms are expected, one predicts that piK atoms must have been produced. Despite the low statistic collected during the first data run in 2007, we have already evidence for 173 ± 54 piK atoms with a significance of 3.2 sigma, which is in good agreement with expectation from the number of observed Coulomb pairs. A lower limit of 1.5 fs could be given for their mean life at 84.1% confidence level
The flux of secondary anti-deuterons and antihelium produced in the interstellar medium
Several measurements were performed to find antiprotons in the primary cosmic radiation. Because it is difficult to get completely separated secondarily produced antiprotons from primary ones, calculations based on accelerator results were performed for the flux of secondarily produced anti-deuterons and antihelium
A new CMS pixel detector for the LHC luminosity upgrade
The CMS inner pixel detector system is planned to be replaced during the
first phase of the LHC luminosity upgrade. The plans foresee an ultra low mass
system with four barrel layers and three disks on either end. With the expected
increase in particle rates, the electronic readout chain will be changed for
fast digital signals. An overview of the envisaged design options for the
upgraded CMS pixel detector is given, as well as estimates of the tracking and
vertexing performance.Comment: 5 pages, 8 figures, proceedings of 8th International Conference on
Radiation Effects on Semiconductor Materials Detectors and Device
Observation of πK -atoms with DIRAC-II
We present evidence for the first observation of electromagnetically bound π ± K  ∓ -pairs (πK-atoms) with the DIRAC-II experiment at the CERN-PS. The πK-atoms are produced by the 24 GeV/c proton beam in a thin Pt-target and the dissociated π ± and K  ∓ -mesons analyzed in a two-arm magnetic spectrometer. The observed enhancement at low relative momentum corresponds to the production of 173±54 πK-atoms. The mean life of πK-atoms is related to the s-wave πK-scattering lengths. From these first data we derive a lower limit for the mean life of 1.5fs at 84% confidence leve
A Compilation of High Energy Atmospheric Muon Data at Sea Level
We collect and combine all published data on the vertical atmospheric muon
flux and the muon charge ratio for muon momenta above 10 GeV. At sea level the
world average of the momentum spectra agrees with the flux calculated by E.V.
Bugaev et al. within 15%. The observed shape of the differential flux versus
momentum is slightly flatter than predicted in this calculation. The
experimental accuracy varies from 7% at 10 GeV to 17% at 1 TeV. The ratio of
fluxes of positive to negative muons is found to be constant, at a value of
1.268, with relative uncertainties increasing from approximately 1% at low
momenta to about 6% at 300 GeV
Scintillation efficiency of liquid argon in low energy neutron-argon scattering
Experiments searching for weak interacting massive particles with noble gases
such as liquid argon require very low detection thresholds for nuclear recoils.
A determination of the scintillation efficiency is crucial to quantify the
response of the detector at low energy. We report the results obtained with a
small liquid argon cell using a monoenergetic neutron beam produced by a
deuterium-deuterium fusion source. The light yield relative to electrons was
measured for six argon recoil energies between 11 and 120 keV at zero electric
drift field.Comment: 21 pages, 19 figures, 4 table
Radiation hardness of CMS pixel barrel modules
Pixel detectors are used in the innermost part of the multi purpose
experiments at LHC and are therefore exposed to the highest fluences of
ionising radiation, which in this part of the detectors consists mainly of
charged pions. The radiation hardness of all detector components has thoroughly
been tested up to the fluences expected at the LHC. In case of an LHC upgrade,
the fluence will be much higher and it is not yet clear how long the present
pixel modules will stay operative in such a harsh environment. The aim of this
study was to establish such a limit as a benchmark for other possible detector
concepts considered for the upgrade.
As the sensors and the readout chip are the parts most sensitive to radiation
damage, samples consisting of a small pixel sensor bump-bonded to a CMS-readout
chip (PSI46V2.1) have been irradiated with positive 200 MeV pions at PSI up to
6E14 Neq and with 21 GeV protons at CERN up to 5E15 Neq.
After irradiation the response of the system to beta particles from a Sr-90
source was measured to characterise the charge collection efficiency of the
sensor. Radiation induced changes in the readout chip were also measured. The
results show that the present pixel modules can be expected to be still
operational after a fluence of 2.8E15 Neq. Samples irradiated up to 5E15 Neq
still see the beta particles. However, further tests are needed to confirm
whether a stable operation with high particle detection efficiency is possible
after such a high fluence.Comment: Contribution to the 11th European Symposium on Semiconductor
Detectors June 7-11, 2009 Wildbad Kreuth, German
Measurements of integral muon intensity at large zenith angles
High-statistics data on near-horizontal muons collected with Russian-Italian
coordinate detector DECOR are analyzed. Precise measurements of muon angular
distributions in zenith angle interval from 60 to 90 degrees have been
performed. In total, more than 20 million muons are selected. Dependences of
the absolute integral muon intensity on zenith angle for several threshold
energies ranging from 1.7 GeV to 7.2 GeV are derived. Results for this region
of zenith angles and threshold energies have been obtained for the first time.
The dependence of integral intensity on zenith angle and threshold energy is
well fitted by a simple analytical formula.Comment: 4 pages, 4 figures, 1 tabl
Extraction of electric field in heavily irradiated silicon pixel sensors
A new method for the extraction of the electric field in the bulk of heavily
irradiated silicon pixel sensors is presented. It is based on the measurement
of the Lorentz deflection and mobility of electrons as a function of depth. The
measurements were made at the CERN H2 beam line, with the beam at a shallow
angle with respect to the pixel sensor surface. The extracted electric field is
used to simulate the charge collection and the Lorentz deflection in the pixel
sensor. The simulated charge collection and the Lorentz deflection is in good
agreement with the measurements both for non-irradiated and irradiated up to
1E15 neq/cm2 sensors.Comment: 6 pages, 11 figures, presented at the 13th International Workshop on
Vertex Detectors for High Energy Physics, September 13-18, 2004,
Menaggio-Como, Italy. Submitted to Nucl. Instr. Meth.
Simulation of Heavily Irradiated Silicon Pixel Detectors
We show that doubly peaked electric fields are necessary to describe
grazing-angle charge collection measurements of irradiated silicon pixel
sensors. A model of irradiated silicon based upon two defect levels with
opposite charge states and the trapping of charge carriers can be tuned to
produce a good description of the measured charge collection profiles in the
fluence range from 0.5x10^{14} Neq/cm^2 to 5.9x10^{14} Neq/cm^2. The model
correctly predicts the variation in the profiles as the temperature is changed
from -10C to -25C. The measured charge collection profiles are inconsistent
with the linearly-varying electric fields predicted by the usual description
based upon a uniform effective doping density. This observation calls into
question the practice of using effective doping densities to characterize
irradiated silicon. The model is now being used to calibrate pixel hit
reconstruction algorithms for CMS.Comment: Invited talk at International Symposium on the Development of
Detectors for Particle, AstroParticle and Synchrtron Radiation Experiments,
Stanford Ca (SNIC06) 8 pages, LaTeX, 11 eps figure
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