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排序方式: 共有695条查询结果,搜索用时 31 毫秒
661.
A precise investigation of radiative pion decay (π+ → e+νγ) in a pion beam from the meson factory of the Paul Scherrer Institute (Switzerland) was performed by the PIBETA Collaboration with the aid of the PIBETA detector. This resulted in finding 41 601 events of radiative pion decay in three kinematical regions. The absolute values of the branching ratio for radiative pion decay were determined in each of these regions. To a precision approximately four times higher than that known previously, the ratio of the axial-vector to the vector form factor was found to be γ ≡ F A /F V = 0.443 (15), the latter being fixed at F V = 0.0259. The number of events found in the kinematical region specified by photon energies of Eγ > 55.6 MeV, positron energies of E e > 20.0 MeV, and angles of θγ, e > 40° between the momenta of the corresponding particles (B region) was 5233. In region B, the measured branching ratio for radiative pion decay, Rπ→evγ(expt) = 11.6(3) × 10?8, proved to be smaller by eight standard deviations than that which follows from the Standard Model, Rπ→evγ (theor) = 14.34(1) × 10?8. 相似文献
662.
Beer G Bragadireanu AM Cargnelli M Curceanu-Petrascu C Egger JP Fuhrmann H Guaraldo C Iliescu M Ishiwatari T Itahashi K Iwasaki M Kienle P Koike T Lauss B Lucherini V Ludhova L Marton J Mulhauser F Ponta T Schaller LA Seki R Sirghi DL Sirghi F Zmeskal J;DEAR Collaboration 《Physical review letters》2005,94(21):212302
The DEAR (DAPhiNE exotic atom research) experiment measured the energy of x rays emitted in the transitions to the ground state of kaonic hydrogen. The measured values for the shift epsilon and the width Gamma of the 1s state due to the K(-)p strong interaction are epsilon(1s)=-193 +/- 37 (stat) +/- 6 (syst) eV and Gamma(1s)=249 +/- 111 (stat) +/- 30 (syst) eV, the most precise values yet obtained. The pattern of the kaonic hydrogen K-series lines, K(alpha), K(beta), and K(gamma), was disentangled for the first time. 相似文献
663.
White CG Burnstein RA Chakravorty A Chan A Chen YC Choong WS Clark K Dukes EC Durandet C Felix J Gidal G Gu P Gustafson HR Ho C Holmstrom T Huang M James C Jenkins CM Kaplan DM Lederman LM Leros N Longo MJ Lopez F Lu LC Luebke W Luk KB Nelson KS Park HK Perroud JP Rajaram D Rubin HA Teng PK Volk J White SL Zyla P;HperCP Collaboration 《Physical review letters》2005,94(10):101804
A sensitive search for the rare decays Omega(-)--> Lambdapi(-) and Xi(0)--> ppi(-) has been performed using data from the 1997 run of the HyperCP (Fermilab E871) experiment. Limits on other such processes do not exclude the possibility of observable rates for |DeltaS| = 2 nonleptonic hyperon decays, provided the decays occur through parity-odd operators. We obtain the branching-fraction limits B(Omega(-)-->Lambdapi(-)) < 2.9 x 10(-6) and B(Xi(0)--> ppi(-)) < 8.2 x 10(-6), both at 90% confidence level. 相似文献
664.
Mibe T Chang WC Nakano T Ahn DS Ahn JK Akimune H Asano Y Daté S Ejiri H Fujimura H Fujiwara M Hicks K Hotta T Imai K Ishikawa T Iwata T Kawai H Kim ZY Kino K Kohri H Kumagai N Makino S Matsuda T Matsumura T Matsuoka N Miwa K Miyabe M Miyachi Y Morita M Muramatsu N Niiyama M Nomachi M Ohashi Y Ooba T Ohkuma H Oshuev DS Rangacharyulu C Sakaguchi A Sasaki T Shagin PM Shiino Y Shimizu H Sugaya Y Sumihama M Titov AI Toi Y Toyokawa H Wakai A Wang CW Wang SC Yonehara K Yorita T Yoshimura M Yosoi M 《Physical review letters》2005,95(18):182001
Photoproduction of a phi meson on protons was studied by means of linearly polarized photons at forward angles in the low-energy region from threshold to Egamma = 2.37 GeV. The differential cross sections at t = -|t|min do not increase smoothly as Egamma increases but show a local maximum at around 2.0 GeV. The angular distributions demonstrate that phi mesons are photoproduced predominantly by helicity-conserving processes, and the local maximum is not likely due to unnatural-parity processes. 相似文献
665.
Kinematically complete measurements of the
reaction were performed for beam energies in the range 292-298 MeV. We detected both protons in coincidence by using the large-acceptance COSY-TOF spectrometer and an external proton beam at COSY-JÃ&frac;lich; thus, we measured total and differential cross-sections and energy distributions. A strong enhancement was observed in the Dalitz plots due to the final-state interaction between the two outgoing protons; the data are well reproduced by Monte Carlo simulations using a standard scattering length of a0=-7.83 fm and effective range of r0=2.8 fm. Our measured total cross-sections are roughly
larger than those of recent internal-target experiments at IUCF and CELSIUS. This discrepancy may be due to the final-state interaction pushing some events into the very small-angle region which is almost inaccessible to an internal-target experiment. In the angular distributions we found only very slight deviations from isotropy. The analysis of our momentum distributions yielded a small fraction of
for Ps or Pp wave contributions.PACS:
13.75.Cs Nucleon-nucleon interactions (including antinucleons, deuterons, etc.) - 25.10.+s Nuclear reactions involving few-nucleon systems - 25.40.Ep Inelastic proton scattering - 29.20.Dh Storage rings 相似文献
666.
667.
The anomalous weak dipole moments of the
lepton are measured in a data sample collected by ALEPH from 1990 to 1995 corresponding to an integrated luminosity of 155 pb-1. Tau leptons produced in the reaction
at energies close to the
mass are studied using their semileptonic decays to
,
,
or
. The real and imaginary components of both the anomalous weak magnetic dipole moment and the CP-violating anomalous weak electric dipole moment,
,
,
and
, are measured simultaneously by means of a likelihood fit built from the full differential cross section. No evidence of new physics is found. The following bounds are obtained (95% CL):
,
,
, and
.Received: 25 September 2002, Published online: 29 August 2003 相似文献
668.
A search was performed for charginos with masses close to the mass of the lightest neutralino in
collisions at centre-of-mass energies of 189-209 GeV recorded by the OPAL detector at LEP. Events were selected if they had an observed high-energy photon from initial state radiation, reducing the dominant background from two-photon scattering to a negligible level. No significant excess over Standard Model expectations has been observed in the analysed data set corresponding to an integrated luminosity of 570 pb-1. Upper limits were derived on the chargino pair-production cross-section, and lower limits on the chargino mass were derived in the context of the Minimal Supersymmetric Extension of the Standard Model for the gravity- and anomaly-mediated Supersymmetry breaking scenarios.Received: 26 July 2002, Published online: 11 July 2003 相似文献
669.
The H Collaboration 《The European Physical Journal C - Particles and Fields》2003,29(4):497-513
Inclusive jet cross sections are measured in photoproduction at HERA using the H1 detector. The data sample of
events in the kinematic range of photon virtualities
and photon-proton centre-of-mass energies
represents an integrated luminosity of
. Jets are defined using the inclusive
algorithm. Single- and multi-differential cross sections are measured as functions of jet transverse energy ET jet and pseudorapidity
in the domain
and
. The cross sections are found to be in good agreement with next-to-leading order perturbative QCD calculations corrected for fragmentation and underlying event effects. The cross section differential in ET jet, which varies by six orders of magnitude over the measured range, is compared with similar distributions from
colliders at equal and higher energies.Received: 24 February 2003, Revised: 26 May 2003, Published online: 11 July 2003 相似文献
670.
Chung P Ajitanand NN Alexander JM Anderson M Best D Brady FP Case T Caskey W Cebra D Chance JL Cole B Crowe K Das AC Draper JE Gilkes ML Gushue S Heffner M Hirsch AS Hjort EL Holzmann W Huo L Issah M Justice M Kaplan M Keane D Kintner JC Klay J Krofcheck D Lacey RA Lauret J Lisa MA Liu H Liu YM Milan J McGrath R Milosevich Z Odyniec G Olson DL Panitkin S Porile NT Rai G Ritter HG Romero JL Scharenberg R Srivastava B Stone NT Symons TJ Taranenko A Whitfield J Wienold T Witt R Wood L Zhang WN 《Physical review letters》2003,91(20):202301
The yield for the multistrange Xi(-) hyperon has been measured in 6A GeV Au+Au collisions via reconstruction of its decay products pi(-) and Lambda, the latter also being reconstructed from its daughter tracks of pi(-) and p. The measurement is rather close to the threshold for Xi(-) production and therefore provides an important test of model predictions. The measured yield for Xi(-) and Lambda are compared for several centralities. In central collisions the Xi(-) yield is found to be in excellent agreement with statistical and transport model predictions, suggesting that multistrange hadron production approaches chemical equilibrium in high baryon density nuclear matter. 相似文献