26 research outputs found
Photon stimulated desorption and effect of cracking of condensed molecules in a cryogenic vacuum system.
The design of the Large Hadron Collider (LHC) vacuum system requires a complete understanding of all processes which may affect the residual gas density in the cold bore of the 1.9 K cryomagnets. A wealth of data has been obtained which may be used to predict the residual gas density inside a cold vacuum system exposed to synchrotron radiation. In this study, the effect of cracking of cryosorbed molecules by synchrotron radiation photons has been included. Cracking of the molecular species CO/sub 2/ and CH/sub 4/ has been observed in recent studies and these findings have been incorporated in a more detailed dynamic gas density model for the LHC. In this paper, we describe the relevant physical processes and the parameters required for a full evaluation. It is shown that the dominant gas species in the LHC vacuum system with its beam screen are H/sub 2/ and CO. The important result of this study is that, while the surface coverage of cryosorbed CH/sub 4/ and CO/sub 2/ molecules is limited due to cracking, the coverage of H/sub 2/ and CO molecules may increase steadily during the long-term operation of the machine. (7 refs)
Magnetic and electric field effect on the photoelectron emission from prototype LHC bean screen material.
This paper describes experimental studies of the effect of a dipole field on the photoelectron emission and on the photon reflectivities from LHC beam screen material. These studies were performed using synchrotron radiation from the VEPP-2M storage ring at BINP (Novosibirsk). The particular surface roughness and geometry of the prototype LHC beam screen material requires dedicated experimental measurements. The experiments were performed under conditions close to those expected in the LHC. An important result obtained is that a dipole magnetic field attenuates the photoelectron emission from surface by more than two orders of magnitude with the magnetic field aligned parallel to the surface. The measurements of photon reflectivities, forward scattered and diffuse, and the azimuthal distribution of emitted photoelectrons from the same material are reported. These experimental results are important input for the final design of the LHC beam screen
Measurement of main parameters of the \psi(2S) resonance
A high-precision determination of the main parameters of the \psi(2S)
resonance has been performed with the KEDR detector at the VEPP-4M e^{+}e^{-}
collider in three scans of the \psi(2S) -- \psi(3770) energy range. Fitting the
energy dependence of the multihadron cross section in the vicinity of the
\psi(2S) we obtained the mass value
M = 3686.114 +- 0.007 +- 0.011 ^{+0.002}_{-0.012} MeV and the product of the
electron partial width by the branching fraction into hadrons \Gamma_{ee}*B_{h}
= 2.233 +- 0.015 +- 0.037 +- 0.020 keV.
The third error quoted is an estimate of the model dependence of the result
due to assumptions on the interference effects in the cross section of the
single-photon e^{+}e^{-} annihilation to hadrons explicitly considered in this
work.
Implicitly, the same assumptions were employed to obtain the charmonium
leptonic width and the absolute branching fractions in many experiments.
Using the result presented and the world average values of the electron and
hadron branching fractions, one obtains the electron partial width and the
total width of the \psi(2S):
\Gamma_{ee} =2.282 +- 0.015 +- 0.038 +- 0.021 keV,
\Gamma = 296 +- 2 +- 8 +- 3 keV.
These results are consistent with and more than two times more precise than
any of the previous experiments
Search for narrow resonances in e+ e- annihilation between 1.85 and 3.1 GeV with the KEDR Detector
We report results of a search for narrow resonances in e+ e- annihilation at
center-of-mass energies between 1.85 and 3.1 GeV performed with the KEDR
detector at the VEPP-4M e+ e- collider. The upper limit on the leptonic width
of a narrow resonance Gamma(R -> ee) Br(R -> hadr) < 120 eV has been obtained
(at 90 % C.L.)
Measurement of \Gamma_{ee}(J/\psi)*Br(J/\psi->e^+e^-) and \Gamma_{ee}(J/\psi)*Br(J/\psi->\mu^+\mu^-)
The products of the electron width of the J/\psi meson and the branching
fraction of its decays to the lepton pairs were measured using data from the
KEDR experiment at the VEPP-4M electron-positron collider. The results are
\Gamma_{ee}(J/\psi)*Br(J/\psi->e^+e^-)=(0.3323\pm0.0064\pm0.0048) keV,
\Gamma_{ee}(J/\psi)*Br(J/\psi->\mu^+\mu^-)=(0.3318\pm0.0052\pm0.0063) keV.
Their combinations
\Gamma_{ee}\times(\Gamma_{ee}+\Gamma_{\mu\mu})/\Gamma=(0.6641\pm0.0082\pm0.0100)
keV,
\Gamma_{ee}/\Gamma_{\mu\mu}=1.002\pm0.021\pm0.013 can be used to improve
theaccuracy of the leptonic and full widths and test leptonic universality.
Assuming e\mu universality and using the world average value of the lepton
branching fraction, we also determine the leptonic \Gamma_{ll}=5.59\pm0.12 keV
and total \Gamma=94.1\pm2.7 keV widths of the J/\psi meson.Comment: 7 pages, 6 figure
No Sommerfeld resummation factor in e+e- -> ppbar ?
The Sommerfeld rescattering formula is compared to the e+e- -> ppbar BaBar
data at threshold and above. While there is the expected Coulomb enhancement at
threshold, two unexpected outcomes have been found: |G^p (4M_p^2)|= 1, like for
a pointlike fermion, and moreover data show that the resummation factor in the
Sommerfeld formula is not needed. Other e+e- -> baryon-antibaryon cross
sections show a similar behavior near threshold.Comment: 9 pages, 6 figure
New precise determination of the \tau lepton mass at KEDR detector
The status of the experiment on the precise lepton mass measurement
running at the VEPP-4M collider with the KEDR detector is reported. The mass
value is evaluated from the cross section behaviour around the
production threshold. The preliminary result based on 6.7 pb of data is
MeV. Using 0.8 pb of data
collected at the peak the preliminary result is also obtained:
eV.Comment: 6 pages, 8 figures; The 9th International Workshop on Tau-Lepton
Physics, Tau0
Measurement of and between 3.12 and 3.72 GeV at the KEDR detector
Using the KEDR detector at the VEPP-4M collider, we have measured
the values of and at seven points of the center-of-mass
energy between 3.12 and 3.72 GeV. The total achieved accuracy is about or
better than at most of energy points with a systematic uncertainty of
about . At the moment it is the most accurate measurement of in
this energy range
Study of KS KL Coupled Decays and KL -Be Interactions with the CMD-2 Detector at VEPP-2M Collider
The integrated luminosity about 4000 inverse nanobarn of around phi meson
mass ( 5 millions of phi mesons) has been collected with the CMD-2 detector at
the VEPP-2M collider. A latest analysis of the KS KL coupled decays based on 30
% of available data is presented in this paper.
The KS KL pairs from phi meson decays were reconstructed in the drift chamber
when both kaons decayed into two charged particles. From a sample of 1423
coupled decays a selection of candidates to the CP violating KL into pi+ pi-
decay was performed. CP violating decays were not identified because of the
domination of events with a KL regenerating at the Be beam pipe into KS and a
background from KL semileptonic decays.
The regeneration cross section of 110 MeV/c KL mesons was found to be 53 +-
17 mb in agreement with theoretical expectations. The angular distribution of
KS mesons after regeneration and the total cross section of KL for Be have been
measured.Comment: 14 pages, 8 figure
Investigation of the reaction in the energy domain near the -meson
The process was investigated
in the SND experiment at the VEPP-2M collider. A narrow energy interval near
the -meson was scanned. The observed cross-section reveals, at the level
of three standard deviation, the interference effect caused by decay. The cross-section parameters, as well as the real and
imaginary parts of the -meson related amplitude, were measured.Comment: 34 pages, 7 figures, LaTeX, uses FEYNMAN package. Erroneously
reported C_{12} coefficient corrected (an error discovered in
hep-ph/0603241). in the published version Nucl. Phys. B569(2000), 158 this
coefficient is reported incorrectl