Transverse momentum spectra in high-energy nucleus-nucleus, proton-nucleus and proton-proton collisions

The transverse momentum distributions of final-state particles produced in nucleus-nucleus (AA), proton-nucleus (pA), and proton-proton (pp) collisions at high energies are investigated using a multisource ideal gas model. Our calculated results show that the contribution of hard emission can be neglected in the study of transverse momentum spectra of charged pions and kaons produced in Cu-Cu collisions at √s_NN=22.5 GeV. And if we consider the contribution of hard emission, the transverse momentum spectra of p and p produced in Cu-Cu collisions at √s_NN=22.5 GeV, K_s⁰ produced in Pb-Pb collisions at 158 A GeV, J/ψ particles produced in p-Pb collisions at 400 GeV and π⁺ , K⁺, p produced in proton-proton collisions at √s=200 GeV, can be described by the model, especially in the tail part of spectra.     

Shanghai laser electron gamma source and its applications

Shanghai Laser Electron Gamma Source, a high intensity beam line of γ-ray, is expected to generate γ-rays up to the maximum energy of 22 MeV by Compton backscattering between a CO₂ laser and electrons in the 3.5 GeV storage ring of the Shanghai Synchrotron Radiation Facility. The luminosity of SLEGS γ-ray beam is estimated to be 7×10⁷ A^－1W^－1s^－1 in a optimized setup. Indirect measurement of cross section of the key nuclear-astrophysics reaction ¹²C(α,γ)¹⁶O and γ-ray-triggered transmutation of long-lived radioactive wastes are discussed based on the estimated SLEGS γ-ray beam properties.     

Recent results from the KEDR detector

We report results of experiments performed with the KEDR detector at the VEPP-4M e⁺e^－ collider. They include precise measurement of the D⁰ and D^± meson masses, determination of the ψ(3770) resonance parameters, and a search for narrow resonances in e⁺e^－ annihilation at center-of-mass energies between 1.85 and 3.1 GeV.     

Centrality dependence of pT spectra for identified hadrons in Au+Au and Cu+Cu collisions at √sNN= 200GeV

The centrality dependence of transverse momentum spectra for identified hadrons at midrapidity in Au+Au collisions at √s_NN= 200GeV is systematically studied in a quark combination model. The p_T spectra of π^±, K^±, p(p) and Λ(Λ) in different centrality bins and the nuclear modification factors (R_CP) for these hadrons are calculated. The centrality dependence of the average collective transverse velocity 〈β(r)〉 for the hot and dense quark matter is obtained in Au+Au collisions, and it is applied to a relative smaller Cu+Cu collision system. The centrality dependence of p_T spectra and the R_CP for π⁰, K_s⁰ and Λ in Cu+Cu collisions at √s_NN= 200GeV are well described. The results show that 〈β(r)〉 is only a function of the number of participants N_part and it is independent of the collision system.     

Luminosity determination for the pd reaction at 2.14 GeV with WASA-at-COSY

The luminosity for a WASA-at-COSY experiment involving the pd reaction at 2.14 GeV proton-beam energy is determined by the forward pd elastic scattering, which yields an average beam-on-target value of [5.2 ± 0.3(stat) ± 0.3syst] × 10³⁰s^-1cm^-2. In addition, the forward pd elastic-scattering angular distribution is obtained with four-momentum transfer squared -t between 0.16 (GeV/c)² and 0.78 (GeV/c)² at this beam energy, which is compared with other experimental data and the pd double scattering model.     

A simulation study on the measurement of D0-D0 mixing parameter y at BESⅢ

We established a method on measuring the D⁰-D⁰ mixing parameter y for BESⅢ experiment at the BEPCⅡ e⁺e^－ collider. In this method, the doubly tagged π(3770) →D⁰-D⁰ events, with one D decays to CP-eigenstates and the other D decays semileptonically, are used to reconstruct the signals. Since this analysis requires good e/π separation, a likelihood approach, which combines the dE/dx, time of flight and the electromagnetic shower detectors information, is used for particle identification. We estimate the sensitivity of the measurement of y to be 0.007 based on a 20fb^-1 fully simulated MC sample.     

Evidence for e+e-→γχc1,2 at center-of-mass energies from 4.009 to 4.360 GeV

Using data samples collected at center-of-mass energies of √s=4.009, 4.230, 4.260, and 4.360 GeV with the BES0 detector operating at the BEPC/ collider, we perform a search for the process e⁺e^-→γχ_c,j (J =0, 1, 2) and find evidence for e⁺e^-→γχ_c1 and e⁺e^-→γχ_c2 with statistical significances of 3.0σ and 3.4σ, respectively. The Born cross sections σ^B(e⁺e^-→γχ_c,j), as well as their upper limits at the 90% con dence level (C.L.) are determined at each center-of-mass energy.     

Study of diphoton decays of the lightest scalar Higgs boson in the Next-to-Minimal Supersymmetric Standard Model

The CMS and ATLAS experiments at the LHC have announced the discovery of a Higgs boson with mass at approximately 125 GeV/c² in the search for the Standard Model Higgs boson via, notably, the γγ and ZZ to four leptons final states. Considering the recent results of the Higgs boson searches from the LHC, we study the lightest scalar Higgs boson h₁ in the Next-to-Minimal Supersymmetric Standard Model by restricting the next-to-lightest scalar Higgs boson h₂ to be the observed to the 125 GeV/c² state. We perform a scan over the relevant NMSSM parameter space that is favoured by low fine-tuning considerations. Moreover, we also take the experimental constraints from direct searches, B-physics observables, relic density, and anomalous magnetic moment of the muon measurements, as well as the theoretical considerations, into account in our specific scan. We find that the signal rate in the two-photon final state for the NMSSM Higgs boson h₁ with the mass range from about 80 GeV/c² to about 122 GeV/c² can be enhanced by a factor of up to 3.5 when the Higgs boson h₂ is required to be compatible with the excess from latest LHC results. This motivates the extension of the search at the LHC for the Higgs boson h₁ in the diphoton final state down to masses of 80 GeV/c², particularly with the upcoming proton-proton collision data to be taken at center-of-mass energies of 13-14 TeV.     

China Spallation Neutron Source - an overview of application prospects

The China Spallation Neutron Source (CSNS) is an accelerator-based multidisciplinary user facility to be constructed in Dongguan, Guangdong, China. The CSNS complex consists of an H^－ linear accelerator, a rapid cycling synchrotron accelerating the beam to 1.6 GeV, a solid-tungsten target station, and instruments for spallation neutron applications. The facility operates at 25 Hz repetition rate with an initial design beam power of 120 kW and is     

Off-Shell Effect of Quarks in Bs→γe+e-

We evaluate the rare decays B_s→γe⁺e^- in the standard model, considering the off-shell effect of quarks in B_s meson as well as some contributions neglected in previous works. It is found that the quarks off-shell are large. With the predicted branching ratios at order of 10^-8, it is expected that the radiative dileptonic decays will be detected in the LHC-b experiments.     

利用RHIC-PHENIX寻找临界行为

Integrated two particle correlation functions have been extracted from charge particle multiplicity density fluctuations in pseudorapidity space by analyzing Au+Au collision events at s_NN=200GeV taken by RHIC-PHENIX. The correlation lengths as a function of the number of participants N_p indicate a non monotonic increase at around N_p=100 and the corresponding energy density based on the Bjorken picture is ε_Bjτ～2.5GeV.fm^－2. This could be a symptom of a critical behavior.     

Study of ππ production in two-photon collisions at Belle

We have measured the cross section for π⁺π^－ production in two-photon collisions using a data sample with an integrated luminosity of 85.9fb^－1 collected with the Belle detector. The f₀(980) resonance is observed as a peak in the energy spectrum of the cross section. We also report preliminary results for γγ→π⁰π⁰ with two-photon center-of-mass energies ranging from 0.6 to 4.0GeV, based on a 95fb^－1 data sample. We find at least four resonant structures including a peak from f₀(980). In addition, there is evidence for χ_c0 production. We also make a preliminary discussion of the angular dependence and cross section ratio of γγ→π⁺π^－ and γγ→π⁰π⁰.     

π-J/ψ correlation and elliptic flow parameter v2 of charmed mesons at RHIC energy

We study the correlation between the trigger π and the associated J/ψ on near and away sides in Au+Au collisions at √s_NN=200 GeV. In the region of trigger momentum p_t>4 GeV/c, the π spectrum is composed of thermal-shower and shower-shower recombinations in the frame work of the recombination model. We consider the azimuthal anisotropy in the quenched hard parton distribution and then calculate the elliptic flow parameter v₂ of charmed mesons (J/ψ, D⁰ and D_s) for different centralities.     

Hypertriton and light nuclei production at Λ-production subthreshold energy in heavy-ion collisions

High-energy heavy-ion collisions produce abundant hyperons and nucleons. A dynamical coalescence model coupled with the ART model is employed to study the production probabilities of light clusters, deuteron (d), triton (t), helion (³He), and hypertriton (_Λ³H) at subthreshold energy of Λ production (≈ 1 GeV per nucleon). We study the dependence on the reaction system size of the coalescence penalty factor per additional nucleon and entropy per nucleon. The Strangeness Population Factor (S₃ / (³He × (Λ/p))) shows an extra suppression of hypertriton comparing to light clusters of the same mass number. This model predicts a hypertriton production cross-section of a few μb in ³⁶Ar+³⁶Ar, ⁴⁰Ca+⁴⁰Ca and ⁵⁶Ni+⁵⁶Ni in 1 A GeV reactions. The production rate is as high as a few hypertritons per million collisions, which shows that the fixed-target heavy-ion collisions at CSR (Lanzhou/China) at Λ subthreshold energy are suitable for breaking new ground in hypernuclear physics.     

Spin distribution of evaporation residue cross section within a stochastic approach

The Langevin equation including particle emission was used to reproduce the recently measured spin distribution of evaporation residue cross sections in the reaction ¹⁶O+¹⁸⁴W at beam energies of 84, 92, 100, 108, 116 and 120MeV. By comparing the theoretical calculations with the experimental data, the validity of the stochastic approach to dissipative fission is verified. Moreover, a pre-saddle nuclear viscosity coefficient of 5×10²¹s^－1 is extracted.     

Cs2 Te photocathode fabrication system at Peking University

A cesium telluride (Cs₂Te) photocathode with a quantum efficiency of 13% at 253.7 nm (radiant incidence 200 μ/cm²) is fabricated by tellurium and cesium vapor deposition onto a stainless-steel substrate. The cesium telluride cathode will be used to provide a high-brightness electron beam source for the 3+1/2 photo-injector at Peking University. The design of the system, the fabrication procedures and the preliminary experimental results are presented in this paper.     

O(α_sv~2) correction to J/ψ plus η_c production in e~+e~- annihilation at √s=10.6 GeV

Based on the nonrelativistic QCD factorization approach, O(α_sv2) corrections to J/ψ plus η_c production in m e⁺e^- annihilation at √s=10.6 ≥v are calculated in this work. The numerical results show that the correction at α_sv2 order is only about a few percent of the total theoretical result. This indicates that the perturbative expansions become convergent and that a higher order correction will be smaller. The uncertainties from the long-distance matrix elements, renormalization scale and the measurement in the experiment are also discussed. Our result is in agreement with the previous result by Jia.     

Study of charmonium(-like) states via ISR at Belle

The cross sections for e⁺e－→π⁺π^－J/ψ, π⁺π^－ψ(2S), K⁺K^－J/ψ, DD, D⁰D^－+π⁺+c.c., D^*D+c.c., and D^*D^* are measured using data sample collected on or near the Υ(4S) resonance with the Belle detector at KEKB. A peak near 4.25GeV/c², corresponding to the so called Y(4260), is observed in π⁺π^－J/ψ final state. In addition, there is another cluster of events at around 4.05GeV/c². Two resonant structures are observed in the π⁺π^－ψ(2S) invariant mass distribution, one at 4361±9±9MeV/c² with a width of 74±15±10MeV/c², and another at 4664±11±5MeV/c² with a width of 48±15± 3MeV/c². The rich structures observed in all these final states indicate that our understanding of the vector charmonium states above the open charm threshold is still poor, let alone the other possible dynamics such as charmonium hybrids or final state re-scattering and so on.     

He and H sequential implantation induced surface blistering and exfoliation of Si covered with oxide layer

Si wafers with a 220 nm top oxide layer were sequentially implanted at room temperature with 40 keV He and 35 keV H ions at fluence of 5× 10¹⁶/cm² and 1× 10¹⁶/cm², respectively. Techniques of scanning electron microscopy, atomic force microscopy and cross-sectional transmission electron microscopy (XTEM) were used to characterize the thermal evolution of surface damage as well as defect microstructures. Surface blisters as well as the localized exfoliation (～ 0.42 μm in depth) have been observed for samples annealed at temperatures of 500 ℃ and above. XTEM observations reveal a variety of defect microstructures, including cavities, platelets, nanometer or micrometer sized cracks and dislocations. The platelets and cracks are mainly distributed at the depth of about 0.42 μm parallel to the sample surface, which are responsible for the occurrence of the observed surface features. The relations between surface damage and defect microstructures are described in detail.     

Optimization on Synchrotron Radiation Lattice of BEPCII

The Beijing Electron and Positron Collider II （BEPCII） is a double ring electron positron collider, which can also be used as a synchrotron radiation （SR） light source. Since the BEPCII will start commissioning with SR mode in November 2006, it is essential to have a satisfying SR lattice. Here we first perform frequency map analysis on the original BEPCII SR lattice. The result shows that the resonance is really severe in the lattice and the transverse motion of particles is unstable. Then a new SR lattice is designed to obtain a large dynamic aperture and much more stable transverse motion. The dynamic aperture of the new lattice including nonlinear wiggler effect is calculated to be very good. This new SR lattice can be used in the commissioning stage of the BEPCII.