343 research outputs found
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
Signal height in silicon pixel detectors irradiated with pions and protons
Pixel detectors are used in the innermost part of multi purpose experiments
at the Large Hadron Collider (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 the detectors 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 up to which radii the
present pixel technology can be used. In order to establish such a limit, pixel
sensors of the size of one CMS pixel readout chip (PSI46V2.1) have been bump
bonded and irradiated with positive pions up to 6E14 Neq/cm^2 at PSI and with
protons up to 5E15 Neq/cm^2. The sensors were taken from production wafers of
the CMS barrel pixel detector. They use n-type DOFZ material with a resistance
of about 3.7kOhm cm and an n-side read out. As the performance of silicon
sensors is limited by trapping, the response to a Sr-90 source was
investigated. The highly energetic beta-particles represent a good
approximation to minimum ionising particles. The bias dependence of the signal
for a wide range of fluences will be presented.Comment: Contribution to the 7th International Conference on Radiation Effects
on Semiconductor Materials, Detectors and Devices October 15-17, 2008
Firenze, Ital
Inclusive b-hadron production cross section with muons in pp collisions at sâ=7TeV
A measurement of the b-hadron production cross section in proton-proton collisions at sâ=7TeVs=7TeV is presented. The dataset, corresponding to 85 nbâ1, was recorded with the CMS experiment at the LHC using a low-threshold single-muon trigger. Events are selected by the presence of a muon with transverse momentum pÎźT>6GeVpTÎź>6GeV with respect to the beam direction and pseudorapidity |Ρ Îź | < 2.1. The transverse momentum of the muon with respect to the closest jet discriminates events containing b hadrons from background. The inclusive b-hadron production cross section is presented as a function of muon transverse momentum and pseudorapidity. The measured total cross section in the kinematic acceptance is Ď(pp â b + X â Îź + Xâ˛) = 1.32 Âą 0.01(stat) Âą 0.30(syst) Âą 0.15(lumi)Îźb
Data acquisition software for the CMS strip tracker
The CMS silicon strip tracker, providing a sensitive area of approximately 200 m2 and comprising 10 million readout channels, has recently been completed at the tracker integration facility at CERN. The strip tracker community is currently working to develop and integrate the online and offline software frameworks, known as XDAQ and CMSSW respectively, for the purposes of data acquisition and detector commissioning and monitoring. Recent developments have seen the integration of many new services and tools within the online data acquisition system, such as event building, online distributed analysis, an online monitoring framework, and data storage management. We review the various software components that comprise the strip tracker data acquisition system, the software architectures used for stand-alone and global data-taking modes. Our experiences in commissioning and operating one of the largest ever silicon micro-strip tracking systems are also reviewed
Search for Dijet Resonances in 7 TeV pp Collisions at CMS
This is the publisher's version, also available electronically from http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.105.211801.A search for narrow resonances in the dijet mass spectrum is performed using data corresponding to an integrated luminosity of 2.9ââpb(â1) collected by the CMS experiment at the Large Hadron Collider. Upper limits at the 95% confidence level are presented on the product of the resonance cross section, branching fraction into dijets, and acceptance, separately for decays into quark-quark, quark-gluon, or gluon-gluon pairs. The data exclude new particles predicted in the following models at the 95% confidence level: string resonances, with mass less than 2.50 TeV, excited quarks, with mass less than 1.58 TeV, and axigluons, colorons, and E6 diquarks, in specific mass intervals. This extends previously published limits on these models
Transverse-momentum and pseudorapidity distributions of charged hadrons in pp collisions at sâ=0.9 and 2.36 TeV
This is the publisher's version, also available electronically from http://link.springer.com/article/10.1007%2FJHEP02%282010%29041.Measurements of inclusive charged-hadron transverse-momentum and pseudorapidity distributions are presented for proton-proton collisions at sâ=0.9 and 2.36 TeV. The data were collected with the CMS detector during the LHC commissioning in December 2009. For non-single-diffractive interactions, the average charged-hadron transverse momentum is measured to be 0.46 Âą 0.01 (stat.) Âą 0.01 (syst.) GeV/c at 0.9 TeV and 0.50 Âą 0.01 (stat.) Âą 0.01 (syst.) GeV/c at 2.36 TeV, for pseudorapidities between --2.4 and +2.4. At these energies, the measured pseudorapidity densities in the central region, dN (ch)/dΡ|(|Ρ|)<0.5, are 3:48 Âą 0:02 (stat.) Âą 0.13 (syst.) and 4:47 Âą 0:04 (stat.) Âą 0.16 (syst.), respectively. The results at 0.9 TeV are in agreement with previous measurements and confirm the expectation of near equal hadron production in p-bar p and pp collisions. The results at 2.36 TeV represent the highest-energy measurements at a particle collider to date
Observation of long-range, near-side angular correlations in proton-proton collisions at the LHC
This is the publisher's version, also available electronically from http://link.springer.com/article/10.1007%2FJHEP09%282010%29091.Results on two-particle angular correlations for charged particles emitted in proton-proton collisions at center-of-mass energies of 0.9, 2.36, and 7 TeV are presented, using data collected with the CMS detector over a broad range of pseudorapidity (Ρ) and azimuthal angle (Ď). Short-range correlations in ÎΡ, which are studied in minimum bias events, are characterized using a simple âindependent clusterâ parametrization in order to quantify their strength (cluster size) and their extent in Ρ (cluster decay width). Long-range azimuthal correlations are studied differentially as a function of charged particle multiplicity and particle transverse momentum using a 980 nb(â1) data set at 7 TeV. In high multiplicity events, a pronounced structure emerges in the two-dimensional correlation function for particle pairs with intermediate p T of 1â3 GeV/c, 2.0 < |ÎΡ| < 4.8 and ÎĎ â 0. This is the first observation of such a long-range, near-side feature in two-particle correlation functions in pp or pp-bar collisions
Azimuthal anisotropy of charged particles at high transverse momenta in PbPb collisions at sqrt(s[NN]) = 2.76 TeV
The azimuthal anisotropy of charged particles in PbPb collisions at
nucleon-nucleon center-of-mass energy of 2.76 TeV is measured with the CMS
detector at the LHC over an extended transverse momentum (pt) range up to
approximately 60 GeV. The data cover both the low-pt region associated with
hydrodynamic flow phenomena and the high-pt region where the anisotropies may
reflect the path-length dependence of parton energy loss in the created medium.
The anisotropy parameter (v2) of the particles is extracted by correlating
charged tracks with respect to the event-plane reconstructed by using the
energy deposited in forward-angle calorimeters. For the six bins of collision
centrality studied, spanning the range of 0-60% most-central events, the
observed v2 values are found to first increase with pt, reaching a maximum
around pt = 3 GeV, and then to gradually decrease to almost zero, with the
decline persisting up to at least pt = 40 GeV over the full centrality range
measured.Comment: Replaced with published version. Added journal reference and DO
Search for anomalous t t-bar production in the highly-boosted all-hadronic final state
A search is presented for a massive particle, generically referred to as a
Z', decaying into a t t-bar pair. The search focuses on Z' resonances that are
sufficiently massive to produce highly Lorentz-boosted top quarks, which yield
collimated decay products that are partially or fully merged into single jets.
The analysis uses new methods to analyze jet substructure, providing
suppression of the non-top multijet backgrounds. The analysis is based on a
data sample of proton-proton collisions at a center-of-mass energy of 7 TeV,
corresponding to an integrated luminosity of 5 inverse femtobarns. Upper limits
in the range of 1 pb are set on the product of the production cross section and
branching fraction for a topcolor Z' modeled for several widths, as well as for
a Randall--Sundrum Kaluza--Klein gluon. In addition, the results constrain any
enhancement in t t-bar production beyond expectations of the standard model for
t t-bar invariant masses larger than 1 TeV.Comment: Submitted to the Journal of High Energy Physics; this version
includes a minor typo correction that will be submitted as an erratu
X-ray emission from the Sombrero galaxy: discrete sources
We present a study of discrete X-ray sources in and around the
bulge-dominated, massive Sa galaxy, Sombrero (M104), based on new and archival
Chandra observations with a total exposure of ~200 ks. With a detection limit
of L_X = 1E37 erg/s and a field of view covering a galactocentric radius of ~30
kpc (11.5 arcminute), 383 sources are detected. Cross-correlation with Spitler
et al.'s catalogue of Sombrero globular clusters (GCs) identified from HST/ACS
observations reveals 41 X-rays sources in GCs, presumably low-mass X-ray
binaries (LMXBs). We quantify the differential luminosity functions (LFs) for
both the detected GC and field LMXBs, whose power-low indices (~1.1 for the
GC-LF and ~1.6 for field-LF) are consistent with previous studies for
elliptical galaxies. With precise sky positions of the GCs without a detected
X-ray source, we further quantify, through a fluctuation analysis, the GC LF at
fainter luminosities down to 1E35 erg/s. The derived index rules out a
faint-end slope flatter than 1.1 at a 2 sigma significance, contrary to recent
findings in several elliptical galaxies and the bulge of M31. On the other
hand, the 2-6 keV unresolved emission places a tight constraint on the field
LF, implying a flattened index of ~1.0 below 1E37 erg/s. We also detect 101
sources in the halo of Sombrero. The presence of these sources cannot be
interpreted as galactic LMXBs whose spatial distribution empirically follows
the starlight. Their number is also higher than the expected number of cosmic
AGNs (52+/-11 [1 sigma]) whose surface density is constrained by deep X-ray
surveys. We suggest that either the cosmic X-ray background is unusually high
in the direction of Sombrero, or a distinct population of X-ray sources is
present in the halo of Sombrero.Comment: 11 figures, 5 tables, ApJ in pres
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