2002 Vol. 26, No. 11
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We report the results of a search for the semileptonic decays D+→e+ν,D+→K－π+e+ν,D0→K－e+ν and D0→K－μ+ν using the BES detector at the BEPC e+e－ Collider. Using a data sample of 22.3pb－1 collected at the center of mass energy s=4.03GeV, we select 2472±87D 0 tag DX and 7628±149D 0 tag DX singly tagged D events, where the X could be γ, π 0 and/or π±. Based on the method of "combined D+and D0 single and double tags", we determine that 2471±218 D+X and 7629±251 D0 X are contained in the recoil side of the singly tagged neutral and charged D mesons, respectively. No events for the 4 semileptonic decay modes are observed in the recoil side of the tagged D mesons. The 90% confidence level upper limits of B(D+→e+ν)<1.38%,B(D+→K－π+e+ν)<2.01%,B(D0→K－e+ν)<0.53% and B(D0→K－μ+ν)<0.53% are determined from the search for the semileptonic decays of D+ and D0.
We study the different properties of the two jet like events in e+e－→Z0→hadrons under different color connections with the aid of the event generator JETSET. We find that the two kinds of colour connections at the interface of perturbative and nonperturbative QCD, i.e., colour neutral flow and colour separate singlet, lead to significant differences for some observables in the final states. Therefore, the measurements of these observables can be used to distinguish these two kinds of colour connections.
The 3 jet events produced in e+e－ collisions at 91.2GeV have been studied using Monte Carlo method. After applying two angular cuts the three angles between the jets are used to identify the individual jet in 3 jet events. The energy distributions of the three jets, the mean particle multiplicities, mean transverse momenta of the three jets in equal energy bins and their distributions have been analyzed. Comparing with the corresponding results from the quark jets in 2 jet events, a simple method to select gluon and quark jets from 3 jet events is obtained. The properties of the gluon and quark jets being selected using the introduced method are in qualitative agreement with the expectations of perturbative QCD. The ratio of the mean multiplicity between quark and gluon jets,
The reaction of 22Na(p,γ)23Mg has been investigated at the Radioactive Ion Beam Line in Lanzhou(RIBLL) using a radioactive beam 23Al. The β+ delayed proton decay spectrum has been measured with the TOF-ΔE method and zero degree detectors. The half life of 23Al is determined to be T1/2=(476±45)ms by using a time scaler combined with the precision pulse generator,which is consistent with the other group's result of (470±30)ms. The known β+ delayed protons at Er=0.216,0.278,0.438,0.479MeV have also observed in the experiment. The resonance strength of isospin analog state (IAS) is calculated. The resonance intensity of IAS is (11.4±6.0)meV. A new energy level of β+ delayed proton decay has been identified at Ex=8.916MeV,and their relative intensity also been obtained.
Spectral statistics for six particles in single j and two j model coupled with a deformed core are studied in the frames of particles rotor model and cranking shell model. The nearest neighbor distribution of energy levels and spectral rigidity are studied as a function of the spin or cranking frequency, respectively. The results of single j shell are compared with those in two j case. The system becomes more regular when single j space (i13/2) is replaced by two j shell (g7/2+d5/2),although the basis size of the configuration space is unchanged. However,the degree of chaoticity of the system changes slightly when configuration space is enlarged by extending single j shell(i13/2) to two j shell (i13/2+g9/2). Nuclear chaotic behavior is studied when we take a two body interaction as delta force and pairing interaction,respectively.
Within Brueckner-Hartree-Fock framework,the equation of state of nuclear matter has been investigated by adopting the charge dependent Argonne V18 two-body realistic force and the microscopic three-body forces based on a meson-exchange model. The contributions to the equation of state from the three-body forces due to different microscopic elementary processes have been calculated and discussed carefully. It is shown by comparing with the results from the relativistic Dirac-BHF approach that the main relativistic correction to nuclear equation of state can be-reproduced fairly well by the present Brueckner-Hartree-Fock calculations including only the three-body force originated from the scalar σ meson exchange coupling to the virtual excitation of a nucleon-antinucleon pair. However,the effects of the three body forces due to other processes can not be cancelled completely with each other,and even around the saturation density of nuclear matter,their net contribution is not neglegible.
An exactly solvable mean filed plus nearest orbit pairing model for describing the well deformed nuclei is adopted for study of the nuclei in rare earth and actinide regions. Binding energies and pairing excitation energies of 158—171Er,160—178Yb,170—183Hf,226—234Th,230—240U and 236—243Pu isotopes are calculated and compared with the corresponding experimental results.
The spallation neutron source induced by high energy protron nucleus interaction is an important link for acceralator driven system. The quantum molecular dynamics (QMD) model is applied to analize the high energy proton induced reactions on 208Pb. The QMD simulations of the double differential cross section of 208Pb (p,xn) reactions with incident energies of 590,800 and 1500MeV are in good agreement with the experimental data,and the results of QMD calculations are better than that of HETC and LAHET.
The pre-equilibrium correction for the nuclear reactions induced by fast neutron is considered as follows: the nuclear reaction processes with the exciton number equal to or larger than five can still be described by the statistical theory of the nuclear reaction. The particle emission processes, in which less than five excitons are involved, are calculated by means of the exciton model and the γ emission of one exciton state is calculated by the direct capture mechanism. For the three-exciton state, only the semidirect capture mechanism, which plays the main role in the researched energy region, is taken into account. The interference effect between the direct and semidirect capture is also considered. The radiative capture cross sections for 40Ca and 208Pb in the neutron incident energy region from 3 MeV to 20 MeV are calculated and a better coincidence with the experimental values is obtained. At the same time, the contribution to the (n, γ) reaction cross sections of the γ emissions before and after statistical equilibriums as well as the characteristics of the direct capture, semidirect capture and their interferece terms are discussed.
The data of multi particle production in SNN=130 GeV Au+Au collisions are analyzed by two source statistical model. It is found that in this reaction the two sources (inner source and outer source) are identical. They have the same temperature, volume, particle density and other thermodynamic quantities. Besides, the results of two source model are identical with that of single source model (the total volume of the two sources equals the volume of single source). The identities between two sources and between two models are due to the particles being measured in a limited central rapidity region. The SNN=130 GeV Au+Au collision has large rapidity region, while data are taken from a small pseudo-rapidity region, η<0.5. In the region, the particles are uniformly distributed as a single source. Comparing to the inner source in 158 A GeV/c Pb+Pb collision, which has a small and hot inner source surrounded by a larger and cooler outer source, the temperature is at least 15MeV higher and the volume is at least two times larger. The RHIC can provide a hot and large inner source that may be formed in the early stage of hadronization from QGP and may have important physical content behind. We suggest to make synchronous measurement in the outer region, e.g. 3<y<4, so as to make comparison.
Taking the interactions between quarks into account, we study thermodynamics of strange quark matter system in the quasi-particle approximation with the result that an extra term should be added to the thermodynamic potential density considering the self-consistent condition. The equation of states turns out to be "softened" when applying the above thermodynamics to strange quark matter in strange stars, and this is consistent with the results of the mass chemical potential-dependent model. However, the quasi-particle description model indicates the dependence of medium effects on the coupling constant of strong interaction.
The energy resolution and time resolution of two φ75×100 BGO (bithmus germanate) detectors for high energy γ ray newly made were measured with 137Cs and 60Co resources. The two characteristic γ rays of high energy emitted from the thermal neutron capture of germanium in BGO crystal were used for the energy calibration of γ spectra. The intrinsic photopeak efficiency, single escape probability and double escape probabilities of BGO detectors in photon energy range of 4-30MeV are numerically calculated with GEANT code. The real count response and count ratio of the uniformly distributed incident photons in energy range of 0-30MeV are also calculated. The distortion of γ spectra caused by the photon energy loss extension to lower energy in detection medium is discussed.
Some advanced techniques to optimize the emittance measurement devices have been studied, such as thin lens approximation, multi changing focusing strength, optimizing the position of the profile monitor and software improvement for accurate data acquisition and effective processing. By applying these techniques to the emittance measurement for electron beam of BEPC Linac, the satisfactory measurement results have been obtained.
The stress distortion and a cooling method of crystal of double crystal monochromator and its experimental result are reported. The crystal, cooled by this method and exposed to the heat load of total power of 210W and the peak power density of 1.28W/mm2, can be used for bending magnet beam line of Shanghai Synchrotron Radiation Facility.
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