Search Results (68 total)

The fragment yields from the multifragmentation of gold, lanthanum, and krypton nuclei obtained by the EOS Collaboration are examined in terms of Fisher’s droplet formalism modified to account for Coulomb energy. The critical exponents σ and τ and the surface energy coefficient c₀ are obtained. Estimates are made of the pressure-temperature and temperature-density coexistence curve of finite neutral nuclear matter as well as the location of the critical point.

Radiative decays of the radially excited charmonium resonance, ψ(2S), into ππ, KK̅ and ηη final states have been measured in a sample of 4.02×10⁶ ψ(2S) events collected by the BES Collaboration. The branching ratios B(ψ(2S)→γf₂(1270))=(2.12±0.19±0.32)×10^-4 and B(ψ(2S)→γf₀(1710))×B(f₀(1710)→K⁺K^-)=(3.02±0.45±0.66)×10^-5 are obtained. When compared to the corresponding radiative J/ψ decays, the observed ψ(2S) radiative decay rates into γf₂(1270) and γf₀(1710) are consistent with the prediction of the “12%” rule.

A systematic analysis of multifragmentation (MF) in fully reconstructed events from 1A GeV Au, La, and Kr collisions with C has been performed. These data are used to provide a definitive test of the variable volume version of the statistical multifragmentation model (SMM). A single set of SMM parameters directly determined by the data and the semi-empirical mass formula are used after the adjustable inverse level density parameter ε₀ is determined by the fragment distributions. The results from SMM for second stage multiplicity, size of the biggest fragment, and the intermediate mass fragments are in excellent agreement with the data. Multifragmentation thresholds have been obtained for all three systems using SMM prior to secondary decay. The data indicate that both thermal excitation energy E_th^* and the isotope ratio temperature T_He-DT decrease with increase in system size at the critical point. The breakup temperature obtained from SMM also shows the same trend as seen in the data. The SMM model is used to study the nature of the MF phase transition. The caloric curve for Kr exhibits back-bending (finite latent heat) while the caloric curves for Au and La are consistent with a continuous phase transition (nearly zero latent heat) and the values of the critical exponents τ, β, and γ, both from data and SMM, are close to those for a “liquid-gas” system for Au and La. We conclude that the larger Coulomb expansion energy in Au and La reduces the latent heat required for MF and changes the nature of the phase transition. Thus the Coulomb energy plays a major role in nuclear MF.

We report values of R  =  σ(e⁺e^-→hadrons)/σ(e⁺e^-→μ⁺μ^-) for 85 center-of-mass energies between 2 and 5 GeV measured with the upgraded Beijing Spectrometer at the Beijing Electron-Positron Collider.

The branching fraction of the ψ(2S) decay into τ⁺τ^- has been measured for the first time using the BES detector at the Beijing Electron-Positron Collider. The result is B_ττ=(2.71±0.43±0.55)×10^-3, where the first error is statistical and the second is systematic. This value, along with those for the branching fractions into e⁺e^- and μ⁺μ^- of this resonance, satisfy well the relation predicted by the sequential lepton hypothesis. Combining all these values with the leptonic width of the resonance, the total width of the ψ(2S) is determined to be (252±37) keV.

Multifragmentation MF results from 1A GeV Au on C have been compared with the Copenhagen statistical multifragmentation model (SMM). The complete charge, mass, and momentum reconstruction of the Au projectile was used to identify high momentum ejectiles leaving an excited remnant of mass A, charge Z, and excitation energy E^* which subsequently multifragments. Measurement of the magnitude and multiplicity (energy) dependence of the initial free volume and the breakup volume determines the variable volume parametrization of SMM. Very good agreement is obtained using SMM with the standard values of the SMM parameters. A large number of observables, including the fragment charge yield distributions, fragment multiplicity distributions, caloric curve, critical exponents, and the critical scaling function are explored in this comparison. The two stage structure of SMM is used to determine the effect of cooling of the primary hot fragments. Average fragment yields with Z>~3 are essentially unaffected when the excitation energy is ≤7 MeV/nucleon. SMM studies suggest that the experimental critical exponents are largely unaffected by cooling and event mixing. The nature of the phase transition in SMM is studied as a function of the remnant mass and charge using the microcanonical equation of state. For light remnants A<~100, backbending is observed indicating negative specific heat, while for A>~170 the effective latent heat approaches zero. Thus for heavier systems this transition can be identified as a continuous thermal phase transition where a large nucleus breaks up into a number of smaller nuclei with only a minimal release of constituent nucleons. Z<~2 particles are primarily emitted in the initial collision and after MF in the fragment deexcitation process.

A systematic analysis of the moments of the fragment size distribution has been carried out for the multifragmentation of 1A GeV Au, La, and Kr on carbon. The breakup of Au and La is consistent with a continuous thermal phase transition. The data indicate that the excitation energy per nucleon and isotopic temperature at the critical point decrease with increasing system size. This trend is attributed primarily to the increasing Coulomb energy with finite size effects playing a smaller role.

The transverse momenta (p_x,p_y) of projectile fragments produced by 1.0A GeV ¹⁹⁷Au nuclei incident on Au and C targets have been measured. The medium and heavy fragments have p_x and p_y distributions, which are wider than predicted by models. For the Au target the widths of the distributions are significantly larger than those for C, particularly for the heavy fragments. The C distributions show a different gross structure, which may be due to the target-projectile size difference.

A sample of 3.95M ψ(2S) decays registered in the BES detector are used to study final states containing pairs of octet and decuplet baryons. We report branching fractions for ψ(2S)→pp̅ , ΛΛ̅ , Σ⁰Σ̅ ⁰, Ξ^-Ξ̅ ⁺, Δ⁺⁺Δ̅ ^--, Σ⁺(1385)Σ̅ ^-(1385), Ξ⁰(1530)Ξ̅ ⁰(1530), and Ω^-Ω̅ ⁺. These results are compared to expectations based on the SU(3)-flavor symmetry, factorization, and perturbative QCD.

The cluster distributions of three different systems are examined to search for signatures of a continuous phase transition. In a system known to possess such a phase transition, both sensitive and insensitive signatures are present; while in systems known not to possess such a phase transition, only insensitive signatures are present. It is shown that nuclear multifragmentation results in cluster distributions belonging to the former category, suggesting that the fragments are the result of a continuous phase transition.

In a sample of 7.8 million J/ψ decays collected in the Beijing Spectrometer, the process J/ψ→γη_c is observed for five different η_c decay channels: K⁺K^-π⁺π^-, π⁺π^-π⁺π^-, K^±K_S⁰π^∓ (with K_S⁰→π⁺π^-), φφ (with φ→K⁺K^-) and K⁺K^-π⁰. From these signals, we determine the mass of η_c to be 2976.6±2.9±1.3 MeV. Combining this result with a previously reported result from a similar study using ψ(2S)→γη_c detected in the same spectrometer gives m_ηc=2976.3±2.3±1.2 MeV. For the combined samples, we obtain Γ_ηc=11.0±8.1±4.1 MeV.

It is shown that the Fisher droplet model, percolation, and nuclear multifragmentation share the common features of reducibility (stochasticity in multiplicity distributions) and thermal scaling (one-fragment production probabilities are Boltzmann factors). Barriers obtained, for cluster production on percolation lattices, from the Boltzmann factors show a power-law dependence on cluster size with an exponent of 0.42±0.02. The EOS Collaboration Au multifragmentation data yield barriers with a power-law exponent of 0.68±0.03. Values of the surface energy coefficient of a low density nuclear system are also extracted.

The first measurement of B(D⁰→φX⁰) and an upper limit for B(D⁺→φX⁺) are determined from 22.3 pb^-1 of e⁺e^- annihilation data at a c.m. energy of 4.03 GeV. The data were recorded by the Beijing Spectrometer (BES) at BEPC. A recoil charge method is applied to charm threshold data to determine the charge of the D meson in the recoil from 9054±309±416 reconstructed D⁰, D⁺ mesons. The branching fractions B(D⁰→φX⁰)=(1.71_-0.71^+0.76±0.17)%, and B(D⁺→φX⁺)<1.8% are determined from 10 events with a reconstructed D and a recoiling φ. In addition, a 90% C.L. upper limit of B(D⁺→φe⁺X⁰)<1.6% is determined from a search for semileptonic decays of the D⁺.

Multifragmentation in fully reconstructed events from 1A GeV Kr and La collisions with C has been studied. Results are compared with similar data for 1A GeV Au+C. The emitted charged particles and fragments are identified with emission from either a prompt first stage or a second stage in which the remnant resulting from the first stage breaks up. The nuclear charge, mass, and excitation energy distributions of the remnant are determined. The total charged multiplicity, as well as those of the first and second stages are obtained. Freeze-out temperatures and thermal excitation energy permit the determination of the caloric curve. The fragment charge distribution as well as the IMF multiplicity distribution and those of individual fragments are obtained. The various results are examined as to the extent of universal behavior when scaled for varying system size. Comparisons are made with intranuclear cascade and statistical multifragmentation model calculations.

Using a sample of 3.8 M ψ(2S) events accumulated with the BES detector, the process ψ(2S)→π⁺π^-J/ψ is studied. The angular distributions are compared with the general decay amplitude analysis of Cahn. We find that the dipion system requires some D wave amplitude, as well as S wave. On the other hand, the J/ψ-(π⁺π^-) relative angular momentum is consistent with being pure S wave. The decay distributions have been fit to heavy quarkonium models, including the Novikov-Shifman model. This model, which is written in terms of the parameter κ, predicts that D wave pions should be present. We determine κ=0.183±0.002±0.003 based on the joint m_ππ-cos θ_π^* distribution. The fraction of D wave amplitude as a function of m_ππ is found to decrease with increasing m_ππ, in agreement with the model. We have also fit the Mannel-Yan model, which is another model that allows D wave pions.

The cross section for charmed meson production at sqrt[s]=4.03 and 4.14 GeV has been measured with the Beijing Spectrometer. The measurement was made using 22.3 pb^-1 of e⁺e^- data collected at 4.03 GeV and 1.5 pb^-1 of e⁺e^- data collected at 4.14 GeV. Inclusive observed cross sections for the production of charged and neutral D mesons and momentum spectra are presented. Observed cross sections were radiatively corrected to obtain tree level cross sections. Measurements of the total hadronic cross section are obtained from the charmed meson cross section and an extrapolation of results from below the charm threshold.

The inclusive light fragment (Z<~7) yield data in Au+Au reactions, measured by the EOS Collaboration at the LBNL Bevalac, are presented as a function of multiplicity. Moving from central to peripheral collisions the measured charge distributions develop progressively according to a power law which can be fitted, within errors, by a single τ exponent independently of the bombarding energy except for the data at 250A MeV. In addition, the location of the maximum in the individual yields of different charged fragments, for a given beam energy, shifts towards lower multiplicity as the fragment charge increases from Z=3 to Z=7. This trend is common to all six measured beam energies. Moments of charge distribution are also reported. The universal features observed in the present Au + Au data are consistent with previous experimental findings in the Au + C multifragmentation reaction at 1A GeV.

Using the upgraded Beijing Spectrometer, we have measured the total cross section for e⁺e^- annihilation into hadronic final states at center-of-mass energies of 2.6, 3.2, 3.4, 3.55, 4.6, and 5.0 GeV. Values of R, σ(e⁺e^-→hadrons)/σ(e⁺e^-→μ⁺μ^-), are determined.

The properties of the remnant resulting from the emission of prompt particles in the interaction of 1A GeV ¹⁹⁷Au+C interactions have been compared with intranuclear cascade and Boltzmann-Uehling-Uhlenback transport calculations. The number of first-stage particles and the energy spectra of first-stage protons are also compared. Both models can fit the general but not the detailed features of the data.

Hadronic decays of the P-wave spin-triplet charmonium states χ_cJ(J=0,1,2) are studied using a sample of ψ(2S) decays collected by the BES detector operating at the BEPC storage ring. Branching fractions for the decays χ_c1→K_S⁰K⁺π^-+c.c., χ_c0→K_S⁰K_S⁰, χ_c2→K_S⁰K_S⁰, χ_c0→φφ, χ_c2→φφ and χ_cJ→K⁺K^-K⁺K^- are measured for the first time, and those for χ_cJ→π⁺π^-π⁺π^-, χ_cJ→π⁺π^-K⁺K^-, χ_cJ→π⁺π^-pp̅ and χ_cJ→3(π⁺π^-) are measured with improved precision. In addition, we determine the masses of the χ_c0 and η_c to be M_χc0=3414.1±0.6(stat)±0.8(syst) MeV and M_ηc=2975.8±3.9(stat)±1.2(syst) MeV.

A sample of 3.79×10⁶ ψ(2S) events is used to study the decays of charmonium to axial-vector plus pseudoscalar mesons. The branching fraction for the decay ψ(2S)→b₁^±(1235)π^∓ agrees with expectations based on scaling the corresponding J/ψ branching fraction. Flavor-SU(3)-violating K₁(1270)- K₁(1400) asymmetries with opposite character for ψ(2S) and J/ψ decays are observed. This contrasting behavior cannot be accommodated by adjustments of the singlet-triplet mixing angle.

Triple-differential cross sections for neutrons from high-multiplicity La-La collisions at 250 and 400 MeV per nucleon and Nb-Nb collisions at 400 MeV per nucleon were measured at several polar angles as a function of the azimuthal angle with respect to the reaction plane of the collision. The reaction plane was determined by a transverse-velocity method with the capability of identifying charged-particles with Z=1, Z=2, and Z>2. The flow of neutrons was extracted from the slope at mid-rapidity of the curve of the average in-plane momentum vs the center-of-mass rapidity. The squeeze-out of the participant neutrons was observed in a direction normal to the reaction plane in the normalized momentum coordinates in the center-of-mass system. Experimental results of the neutron squeeze-out were compared with BUU calculations. The polar-angle dependence of the maximum azimuthal anisotropy ratio r(θ) was found to be insensitive to the mass of the colliding nuclei and the beam energy. Comparison of the observed polar-angle dependence of the maximum azimuthal anisotropy ratio r(θ) with BUU calculations for free neutrons revealed that r(θ) is insensitive also to the incompressibility modulus in the nuclear equation of state.

The decays of the ψ(2S) into vector plus tensor meson final states have been studied for the first time using the BES detector. We determine upper limits on branching fractions for ψ(2S) decays into ωf₂, ρa₂, K^*0K̅ ₂^*0+c.c., and φf₂^′(1525) that are, in each case, significantly smaller than the corresponding branching fractions for the J/ψ meson, scaled according to the expectations of perturbative QCD.

Neutron fluences were measured from 435 MeV/nucleon Nb ions stopping in a Nb target and 272 MeV/nucleon Nb ions stopping in targets of Nb and Al for neutrons above 20 MeV and at laboratory angles between 3° and 80°. The resultant spectra were integrated over angles to produce neutron energy distributions and over energy to produce neutron angular distributions. The total neutron yields for each system were obtained by integrating over the angular distributions. The angular distributions from all three systems are peaked forward, and the energy distributions from all three systems show an appreciable yield of neutrons with velocities greater than the beam velocity. Comparison of the total neutron yields from the two Nb+Nb systems suggests that the average neutron multiplicity decreases with decreasing projectile energy. Comparison of the total yields from the two 272 MeV/nucleon systems suggests that the total yields show the same dependence on projectile and target mass number as do total inclusive neutron cross sections. The data are compared with Boltzmann-Uehling-Uhlenbeck model calculations.

The processes ψ(2S)→γπ⁺π^-, γK⁺K^-, and γpp̅ have been studied using a sample of 3.79×10⁶ ψ(2S) decays. We determine the total width of the χ_c0 to be Γ_χc0^tot  =  14.3±2.0±3.0 MeV. We present the first measurement of the branching fraction B(χ_c0→pp̅ )  =  (15.9±4.3±5.3)×10^-5, where the first error is statistical and the second one is systematic. Branching fractions of χ_c0,2→π⁺π^- and K⁺K^- are also reported.