3,785 research outputs found
Time resolution of the plastic scintillator strips with matrix photomultiplier readout for J-PET tomograph
Recent tests of a single module of the Jagiellonian Positron Emission
Tomography system (J-PET) consisting of 30 cm long plastic scintillator strips
have proven its applicability for the detection of annihilation quanta (0.511
MeV) with a coincidence resolving time (CRT) of 0.266 ns. The achieved
resolution is almost by a factor of two better with respect to the current
TOF-PET detectors and it can still be improved since, as it is shown in this
article, the intrinsic limit of time resolution for the determination of time
of the interaction of 0.511 MeV gamma quanta in plastic scintillators is much
lower. As the major point of the article, a method allowing to record
timestamps of several photons, at two ends of the scintillator strip, by means
of matrix of silicon photomultipliers (SiPM) is introduced. As a result of
simulations, conducted with the number of SiPM varying from 4 to 42, it is
shown that the improvement of timing resolution saturates with the growing
number of photomultipliers, and that the 2 x 5 configuration at two ends
allowing to read twenty timestamps, constitutes an optimal solution. The
conducted simulations accounted for the emission time distribution, photon
transport and absorption inside the scintillator, as well as quantum efficiency
and transit time spread of photosensors, and were checked based on the
experimental results. Application of the 2 x 5 matrix of SiPM allows for
achieving the coincidence resolving time in positron emission tomography of
0.170 ns for 15 cm axial field-of-view (AFOV) and 0.365 ns
for 100 cm AFOV. The results open perspectives for construction of a
cost-effective TOF-PET scanner with significantly better TOF resolution and
larger AFOV with respect to the current TOF-PET modalities.Comment: To be published in Phys. Med. Biol. (26 pages, 17 figures
Classification of singular Q-homology planes. I. Structure and singularities
A Q-homology plane is a normal complex algebraic surface having trivial
rational homology. We obtain a structure theorem for Q-homology planes with
smooth locus of non-general type. We show that if a Q-homology plane contains a
non-quotient singularity then it is a quotient of an affine cone over a
projective curve by an action of a finite group respecting the set of lines
through the vertex. In particular, it is contractible, has negative Kodaira
dimension and only one singular point. We describe minimal normal completions
of such planes.Comment: improved results from Ph.D. thesis (University of Warsaw, 2009), 25
pages, to appear in Israel J. Mat
Determination of the b quark mass at the M_Z scale with the DELPHI detector at LEP
An experimental study of the normalized three-jet rate of b quark events with
respect to light quarks events (light= \ell \equiv u,d,s) has been performed
using the CAMBRIDGE and DURHAM jet algorithms. The data used were collected by
the DELPHI experiment at LEP on the Z peak from 1994 to 2000. The results are
found to agree with theoretical predictions treating mass corrections at
next-to-leading order. Measurements of the b quark mass have also been
performed for both the b pole mass: M_b and the b running mass: m_b(M_Z). Data
are found to be better described when using the running mass. The measurement
yields: m_b(M_Z) = 2.85 +/- 0.18 (stat) +/- 0.13 (exp) +/- 0.19 (had) +/- 0.12
(theo) GeV/c^2 for the CAMBRIDGE algorithm. This result is the most precise
measurement of the b mass derived from a high energy process. When compared to
other b mass determinations by experiments at lower energy scales, this value
agrees with the prediction of Quantum Chromodynamics for the energy evolution
of the running mass. The mass measurement is equivalent to a test of the
flavour independence of the strong coupling constant with an accuracy of 7
permil.Comment: 24 pages, 10 figures, Accepted by Eur. Phys. J.
Observation of Two Resonant Structures in e+e- to pi+ pi- psi(2S) via Initial State Radiation at Belle
The cross section for e+e- to pi+ pi- psi(2S) between threshold and
\sqrt{s}=5.5 GeV is measured using 673 fb^{-1} of data on and off the
\Upsilon(4S) resonance collected with the Belle detector at KEKB. Two resonant
structures are observed in the pi+ pi- psi(2S) invariant mass distribution, one
at 4361\pm 9\pm 9 MeV/c2 with a width of 74\pm 15\pm 10 MeV/c2, and another at
4664\pm 11\pm 5 MeV/c2 with a width of 48\pm 15\pm 3 MeV/c2, if the mass
spectrum is parameterized with the coherent sum of two Breit-Wigner functions.
These values do not match those of any of the known charmonium states.Comment: 10 pages, 5 figures, 2 tables, version to appear in Phys. Rev. Let
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
In-Medium Effects on K0 Mesons in Relativistic Heavy-Ion Collisions
We present the transverse momentum spectra and rapidity distributions of
and K in Ar+KCl reactions at a beam kinetic energy of 1.756 A
GeV measured with the spectrometer HADES. The reconstructed K sample is
characterized by good event statistics for a wide range in momentum and
rapidity. We compare the experimental and K distributions to
predictions by the IQMD model. The model calculations show that K at low
tranverse momenta constitute a particularly well suited tool to investigate the
kaon in-medium potential. Our K data suggest a strong repulsive in-medium
K potential of about 40 MeV strength.Comment: 10 pages, 10 figures, accepted by Phys. Rev.
Measurement of e^+e^- to pi^+pi^-J/psi Cross Section via Initial State Radiation at Belle
The cross section for e^+e^- to pi^+pi^-J/psi between 3.8 and 5.5 GeV/c^2 is
measured using a 548 fb^{-1} data sample collected on or near the Upsilon(4S)
resonance with the Belle detector at KEKB. A peak near 4.25 GeV/c^2,
corresponding to the so called Y(4260), is observed. In addition, there is
another cluster of events at around 4.05 GeV/c^2. A fit using two interfering
Breit-Wigner shapes describes the data better than one that uses only the
Y(4260), especially for the lower mass side of the 4.25 GeV enhancement.Comment: 11 pages, 5 figures, 2 tables; version to appear in PR
Improved search for D^0 mixing using semileptonic decays at Belle
A search for mixing in the neutral D meson system has been performed using
semileptonic D0 -> K(*)- e+ \nu and D0 -> K(*)- \mu+ \nu decays. Neutral D
mesons from D*+ -> D0 \pi_s^+ decays are used and the flavor at production is
tagged by the charge of the slow pion. The measurement is performed using 492
fb^-1 of data recorded by the Belle detector. From the yield of right-sign and
wrong-sign decays arising from non-mixed and mixed events, respectively, we
measure the ratio of the time-integrated mixing rate to the unmixed rate to be
R_M = (1.3 +- 2.2 +- 2.0) x 10^-4. This corresponds to an upper limit of R_M <
6.1 x 10^-4 at the 90% C.L.Comment: 20 pages, 16 figures, 8 tables; accepted by PRD; changes from
previous version: minor rewording
Dilepton production in pp and CC collisions with HADES
Dilepton production has been measured with HADES, the "High Acceptance
DiElectron Spectrometer". In pp collisions at 2.2GeV kinetic beam energy,
exclusive eta production and the Dalitz decay eta -> gamma e+e- has been
reconstructed. The electromagnetic form factor is well in agreement with
existing data. In addition, an inclusive e+e- spectrum from the C+C reaction at
2AGeV is presented and compared with a thermal model.Comment: 11 pages, 3 figures, proceedings of the IVth International Conference
on Quarks and Nuclear Physics, Madrid, June 5th-10th, submitted to
Eur.Phys.J.
- …