SPIN INTERACTION EFFECTS ON MOMENTUM CORRELATIONS FOR IDENTICAL FERMIONS EMITTED IN RELATIVISTIC HEAVY-ION COLLISIONS
The Hanbury-Brown and Twiss (HBT) effects predict a Bose–Einstein enhancement of the two-particle momentum correlations of identical bosons at small relative momentum. However, the parallel momentum correlations between identical fermions are less argued. The momentum correlations can be altered by many factors, among which the spin interaction effects are discussed in this paper. It is found that the spin interaction plays an important role on the momentum correlations of identical fermions. For spin triplet state, a full Fermi–Dirac suppression represents as expected. On the contrary, a fake Bose–Einstein enhancement shows up for spin singlet state. The measured momentum correlations of fermions could hence provide some hints of spin interactions between them if all other factors such as Coulomb interactions were removed. Spin interactions make it more complicated to extract physical information from momentum correlations between fermions.

title: Evidence for the decay $X(3872)\rightarrow\psi(2S)\gamma$
abstract: Evidence for the decay mode $X(3872)\rightarrow\psi(2S)\gamma$ in $B^+\rightarrow X(3872)K^+$ decays is found with a significance of 4.4 standard deviations. The analysis is based on a data sample of proton-proton collisions, corresponding to an integrated luminosity of 3fb$^{-1}$ collected with the LHCb detector, at centre-of-mass energies of 7 and 8TeV. The ratio of the branching fraction of the $X(3872)\to\psi(2S)\gamma$ decay to that of the $X(3872)\rightarrow J/\psi \gamma$ decay is measured to be $$ \frac{Br(X(3872)\rightarrow\psi(2S)\gamma)}{Br(X(3872)\rightarrow J/\psi\gamma)} = 2.46\pm0.64\pm0.29, %\pm0.06,$$ where the first uncertainty is statistical and the second is systematic. The measured value agrees with expectations for a pure charmonium interpretation of the $X(3872)$ state and a mixture of charmonium and molecular interpretations. However, it does not support a pure $D\bar{D}^{*}$ molecular interpretation of the $X(3872)$state.

title: Measurement of the resonant and CP components in $\overline{B}^0\to J/\psi \pi^+\pi^-$ decays
abstract: The resonant structure of the reaction $\overline{B}^0\rightarrow J/\psi \pi^+\pi^-$ is studied using data from 3 fb$^{-1}$ of integrated luminosity collected by the LHCb experiment, one-third at 7 Tev center-of-mass energy and the remainder at 8 Tev. The invariant mass of the $\pi^+\pi^-$ pair and three decay angular distributions are used to determine the fractions of the resonant and non-resonant components. Six interfering $\pi^+\pi^-$ states: $\rho(770)$, $f_0(500)$, $f_2(1270)$, $\rho(1450)$, $\omega(782)$ and $\rho(1700)$ are required to give a good description of invariant mass spectra and decay angular distributions. The positive and negative CP fractions of each of the resonant final states are determined. The $f_0(980)$ meson is not seen and the upper limit on its presence, compared with the observed $f_0(500)$ rate, is inconsistent with a model of tetraquark substructure for these scalar mesons at the eight standard deviation level. In the $q\overline{q}$ model, the absolute value of the mixing angle between the $f_0(980)$ and the $f_0(500)$ scalar mesons is limited to be less than $17^{\circ}$ at 90% confidence level.

title: Thermal and nonthermal pion enhancements with chiral symmetry restoration
abstract: The pion production by sigma decay and its relation with chiral symmetry restoration in a hot and dense matter are investigated in the framework of the Nambu-Jona-Lasinio model. The decay rate for the process sigma -> 2pion to the lowest order in a 1/N_c expansion is calculated as a function of temperature T and chemical potential mu. The thermal and nonthermal enhancements of pions generated by the decay before and after the freeze-out present only in the crossover region of the chiral symmetry transition. The strongest nonthermal enhancement is located in the vicinity of the endpoint of the first-order transition.

title: Study of beauty hadron decays into pairs of charm hadrons
abstract: First observations of the decays Λb 0 → Λc +D(s) - are reported using data corresponding to an integrated luminosity of 3fb-1 collected at 7 and 8 TeV center-of-mass energies in proton-proton collisions with the LHCb detector. In addition, the most precise measurement of the branching fraction B(Bs 0→D+Ds -) is made and a search is performed for the decays B(s) 0→Λc +Λc -. The results obtained are B(Λb 0→Λc +D-)/ B(Λb 0→Λc +D s -)=0.042±0.003(stat)±0.003(syst), [B(Λb 0→Λc +D s -)/B(B0→D+D s -)]/[B(Λb 0→Λ c +π-)/B(B0→D +π-)]=0.96±0.02(stat)±0.06(syst),B(B s 0→D+Ds -)/ B(B0→D+Ds -)=0. 038±0.004(stat)±0.003(syst),B(B0→Λ c +Λc -)/B(B , B(Bs 0→Λc +Λ c -)/B(Bs 0→D+D s -)<0.30[95%C.L.]. Measurement of the mass of the Λb 0 baryon relative to the B0 meson gives M(Λb 0)-M(B0)=339. 72±0.24(stat)±0.18(syst)MeV/c2. This result provides the most precise measurement of the mass of the Λb 0 baryon to date.

title: Performance study of a GEM-TPC prototype using cosmic rays
abstract: Time projection chambers (TPCs) have been successfully used as the central tracking devices in a number of high-energy physics experiments. However, the performance requirements on TPCs for future high-energy physics experiments greatly exceed the abilities of traditional TPCs read out by multi-wire proportional chambers (MWPCs). Micro-pattern gas detectors (MPGDs), such as gas electron multipliers (GEMs) or micromegas, have great potential to improve TPC performance when used as readout detectors. In order to evaluate its feasibility, a GEM-based TPC prototype with a drift length up to 50 cm was designed. Measurements of the spatial resolution of cosmic-ray tracks without and with a magnetic field ( B =1 T) are presented. A very good performance is achieved, matching the analytic formula for the spatial resolution of a MPGD-readout TPC. A dedicated study shows that the increase of GEM detector gain can improve the TPC's spatial resolution.

title: Asymptotic Freedom and Dirichlet String Theory
abstract: Fixed angle scattering at high energy in a string theory with boundaries satisfying Dirichlet conditions (Dirichlet strings) in D = 4 is shown to have logarithmic dependence on energy, in addition to the power-like behavior known before. High temperature free energy also depends logarithmically on temperature. Such a result could provide a matching mechanism between strings at long distance and asymptotic freedom at short distance, which is necessary for the reformulation of large-N QCD as a string theory. In order to reformulate the large-N QCD  as a string theory, assuming it is possible at all, we must find a string theory that contains only massive excitations (for pure QCD) and shares the characteristic features of asymptotic freedom, such as partonic behavior and logarithmic violation of scaling at high energy. The critical string theories we are mostly familiar with fail to meet theses two requirements: they contain massless gravitons and massless Yang-Mill particles, and fixed angle scattering falls off exponentially with total energy. Liouville theory [2] has been argued to be irrelevant for the problem of QCD , meanwhile the effective string picture advocated in , which without doubt is correct for long strings, has problems at short distances. Rigid strings  at first sight seem to give the right behavior at high energy and high temperature, but they have the wrong sign of free energy at high temperature as compared with QCD [6], and furthermore there are problems with unitarity [7]. Another simple yet non-trivial modification of short distance behavior of strings is to add world-sheet boundaries satisfying Dirichlet boundary conditions – each boundary is mapped to a single point in space-time and the position is then integrated over. Such a theory was originally proposed as an off-shell extension of closed string amplitudes . Meanwhile the point-like states of the Dirichlet boundaries were proven to produce powerlike behavior of fixed angle scattering at high energy [9]. The point-like structure was then linked to partons. Developments of the Dirichlet string theory in modern language can be found in . Recently Green [11] points out that at high temperature the Dirichlet boundaries produce a free energy similar to that of the large-N QCD found by Polchinski[6]. Namely, in QCD the free energy of a long string per unit length goes as μQCD(β)/L 2 ∼ − (β)N π2β4 , (1)

title: A remark on the Higgs effect in presence of Chern-Simons terms
abstract: In D=3, a Higgs scalar is shown to transmute a non-dynamical Chern-Simon term into topologically massive, parity-violating, electrodynamics. A similar transmutation also occurs for gravity.

title: Effective Superstrings
abstract: We generalize the method of quantizing effective strings proposed by Polchinski and Strominger to superstrings. The Ramond-Neveu-Schwarz string is different from the Green-Schwarz string in non-critical dimensions. Both are anomaly-free and Poincare invariant. Some implications of the results are discussed. The formal analogy with 4D (super)gravity is pointed out.

title: A remark on conformal anomaly and extrinsic geometry of random surfaces
abstract: In low dimensions, conformal anomaly has a profound influence on the critical behavior of random surfaces with extrinsic curvature rigidity 1/ α . We illustrate this by making a small D expansion of rigid random surfaces, where a non-trivial infrared fixed point is shown to exist. We speculate on the renormalization group flow diagram in the (α, D ) plane. We argue that the qualitative behavior of numerical simulations in D =3,4 could be understood on the basis of the phase diagram.

title: Magnetic Fields and Passive Scalars in Polyakov's Conformal Turbulence
abstract: Polyakov has suggested that two-dimensional turbulence might be described by a minimal model of conformal field theory. However, there are many minimal models satisfying the same physical inputs as Polyakov's solution (2, 21). Dynamical magnetic fields and passive scalars pose different physical requirements. For large magnetic Reynolds number other minimal models arise. The simplest one, (2, 13), makes reasonable predictions that may be tested in the astrophysical context. In particular, the equipartition theorem between magnetic and kinetic energies does not hold: the magnetic one dominates at larger distances.