Fine structure in the azimuthal transverse momentum correlations at $\sqrt{s_{NN}}=200$ GeV using the event shape analysis

The experimental results on transverse momentum azimuthal hadron correlations at RHIC have opened a rich field for parton energy loss analysis in heavy-ion collisions. Recently, a considerable amount of work has been devoted to study the shapes of the “away-side” jet which exhibit an interesting and unexpected “double hump” structure not observed in the analogous treatment of the pp data. Driven by the possibility that the latter result might just mean that such a structure exists already in the case of pp collisions, but that its relative intensity could be small, here we use the Event Shape Analysis to show that it is possible to identify and select well defined event topologies in pp collisions, among which a double hump structure for the away-side jet emerges. Using two shape parameters, the sphericity in the transverse plane and the recoil to analyze a sample of PYTHIA generated pp collisions at  GeV, we show that this structure corresponds to two jets emitted in the backward hemisphere. Finally, we show that Q-PYTHIA qualitatively reproduces the decrease in the yield of dijet events and the increase of the double hump structure in the away side observed in heavy-ion collisions. The implications for the treatment of parton energy loss in heavy-ion collisions are discussed.     

A short review on jet identification

Jets can be used to probe the physical properties of the high energy density matter created in collisions at the Relativistic Heavy Ion Collider (RHIC). Measurements of strong suppression of inclusive hadron distributions and di-hadron correlations at high p _T have already provided evidence for partonic energy loss. However, these measurements suffer from well-known geometric biases due to the competition of energy loss and fragmentation. These biases can be avoided if the jets are reconstructed independently of their fragmentation details—quenched or unquenched. In this paper, we discuss modern jet reconstruction algorithms (cone and sequential recombination) and their corresponding background subtraction techniques required by the high multiplicities of heavy ion collisions. We review recent results from the STAR experiment at RHIC on direct jet reconstruction in central Au+Au collisions at  GeV.     

Investigations of electron beam characteristics using an accelerator based on a magnetron gun with a secondary-emission cathode

The reliability and service life of accelerating installations are substantially determined by the lifetime of electron sources. The accelerator under consideration has a magnetron gun with a channel-free cold secondary-emission cathode in crossed fields as an electron source [1, 2]. In the present work, the data of the electron beam parameters obtained in the accelerator based on the magnetron gun with a secondary-emission cathode are given, and possible secondary uses are suggested. The text was submitted by the authors in English.     

Meson cloud of the nucleon in polarized semi-inclusive deep-inelastic scattering

We investigate the possibility of identifying an explicit pionic component of the nucleon through measurements of polarized ⁺⁺ baryon fragments produced in deep-inelastic leptoproduction off polarized protons, which may help to identify the physical mechanism responsible for the breaking of the Gottfried sum rule. The pion-exchange model predicts highly correlated polarizations of the ⁺⁺ and target proton, in marked contrast with the competing diquark fragmentation process. Measurement of asymmetries in polarized production may also reveal the presence of a kaon cloud in the nucleon.Communicated by: W. Weise     

Identified hadron production at high transverse momenta in p+ p collisions at $\sqrt{s_{NN}}=200$ GeV in STAR

We report the transverse momentum (p _T ) distributions for identified charged pions, protons and anti-protons using events triggered by high deposit energy in the Barrel Electro-Magnetic Calorimeter (BEMC) from p+p collisions at  GeV. The spectra are measured around mid-rapidity (|y|<0.5) over the range of 3<p _T <15 GeV/c with particle identification (PID) by the relativistic ionization energy loss (rdE/dx) in the Time Projection Chamber (TPC) of the Solenoidal Tracker at RHIC (STAR). The charged pion, proton and anti-proton spectra at high p _T are compared with published results from minimum bias triggered events and the Next-Leading-Order perturbative quantum chromodynamic (NLO pQCD) calculations (DSS, KKP and AKK 2008). In addition, we present the particle ratios of π ⁻/π ⁺, , p/π ⁺ and in p+p collisions.     

Cross-sections of large-angle hadron production in proton– and pion–nucleus interactions II: beryllium nuclei and beam momenta from ±3 GeV/c to ±15 GeV/c

We report on double-differential inclusive cross-sections of the production of secondary protons and charged pions, in the interactions with a 5% λ _abs thick stationary beryllium target, of proton and pion beams with momentum from ±3 GeV/c to ±15 GeV/c. Results are given for secondary particles with production angles 20° <θ<125°.     

Cross-sections of large-angle hadron production in proton- and pion-nucleus interactions I: beryllium nuclei and beam momenta of +8.9 GeV/c and −8.0 GeV/c

We report on double-differential inclusive cross-sections of the production of secondary protons, deuterons, and charged pions and kaons, in the interactions with a 5% λ _abs thick stationary beryllium target, of a +8.9 GeV/c proton and pion beam, and a −8.0 GeV/c pion beam. Results are given for secondary particles with production angles 20^○<θ<125^○. On leave of absence at Ecole Polytechnique Fédérale, Lausanne, Switzerland.     

A QCD motivated model for soft interactions at high energies

In this paper we develop an approach to soft scattering processes at high energies which is based on two elements: the Good–Walker mechanism for low mass diffraction and multi-pomeron interactions for high mass diffraction. The principal idea, which allows us to specify the theory for pomeron interactions, is that the so called soft processes occur at rather short distances (r ² ∝1/〈p _t 〉² ∝ α′_ℙ≈0.01 GeV⁻²), where perturbative QCD is valid. The value of the pomeron slope α′_ℙ is obtained from a fit to the experimental data. Using this theoretical approach, we suggest a model that fits all soft data in the ISR-Tevatron energy range: total, elastic, single and double diffractive cross sections, as well as the t dependence of the differential elastic cross section, and the mass dependence of single diffraction. In this model we calculate the survival probability of diffractive Higgs production, and we obtain a value for this observable that is smaller than 1% at the LHC energy range.     

Study of B c → J / ψ π , η c π decays with?perturbative?QCD?approach

The B _c →J/ψ π, η _c π decays are studied with the perturbative QCD approach. It is found that the form factors and for the B _c →J/ψ, η _c transitions and the branching ratios are sensitive to the parameters ω, v, f _J/ψ and , where ω and v are the parameters of the charmonium wave functions for a Coulomb potential and the harmonic-oscillator potential, respectively, and f _J/ψ and are the decay constants of the J/ψ and η _c mesons, respectively. The large branching ratios and the clear signals of the final states make the B _c →J/ψ π, η _c π decays the prospective channels for measurements at the hadron colliders.     

Low-lying spectrum of the Y-string three-quark potential using hyper-spherical coordinates

We calculate the energies of three-quark states with definite permutation symmetry (i.e. of SU(6) multiplets) in the N=0, 1, 2 shells, confined by the Y-string three-quark potential. The exact Y-string potential consists of one term, the so-called three-string term, and three angle-dependent two-string terms. Due to this technical complication we treat the problem at three increasingly accurate levels of approximation: (1) the (approximate) three-string potential expanded to first order in trigonometric functions of hyper-spherical angles; (2) the (approximate) three-string potential to all orders in the power expansion in hyper-spherical harmonics, but without taking into account the transition(s) to two-string potentials; (3) the exact minimal-length string potential to all orders in a power expansion in the hyper-spherical harmonics, and taking into account the transition(s) to two-string potentials. We show the general trend of improvement of these approximations: the exact non-perturbative corrections to the total energy are of the order of one per cent, as compared with approximation (2), yet the exact energy differences between the [20,1⁺],[70,2⁺],[56,2⁺],[70,0⁺]-plets are shifted to 2:2:0.9, from the Bowler and Tynemouth separation rule 2:2:1, which is obeyed by approximation (2) at the one per cent level. The precise value of the energy separation of the first radial excitation (“Roper”) [56^′,0⁺]-plet from the [70,1⁻]-plet depends on the approximation, but does not become negative, i.e. the “Roper” remains heavier than the odd-parity [70,1⁻]-plet in all of our approximations.     

Jet production rates at LEP and the scale of αs

We discuss the scale dependence of _s in connection with jet multiplicities on theZ pole in the framework of perturbative QCD. Several scale defining procedures are applied to jet fractions and compared to recent measurements at LEP.Dedicated to Prof. Dr. H.J. Mang on the occasion of his 60th birthday     

Multi-jet events in the k T -factorisation scheme

A Markovian Monte Carlo algorithm for multi-parton production in the high-energy limit is proposed and the matching with unintegrated parton densities is discussed.     

Abelian 2-form gauge theory: superfield formalism

We derive the off-shell nilpotent Becchi–Rouet–Stora–Tyutin (BRST) and anti-BRST symmetry transformations for all the fields of a free Abelian 2-form gauge theory by exploiting the geometrical superfield approach to the BRST formalism. The above four (3+1)-dimensional (4D) theory is considered on a (4, 2)-dimensional supermanifold parameterized by the four even spacetime variables x ^μ (with μ=0,1,2,3) and a pair of odd Grassmannian variables θ and (with ). One of the salient features of our present investigation is that the above nilpotent (anti-) BRST symmetry transformations turn out to be absolutely anticommuting due to the presence of a Curci–Ferrari (CF) type of restriction. The latter condition emerges due to the application of our present superfield formalism. The actual CF condition, as is well known, is the hallmark of a 4D non-Abelian 1-form gauge theory. We demonstrate that our present 4D Abelian 2-form gauge theory imbibes some of the key signatures of the 4D non-Abelian 1-form gauge theory. We briefly comment on the generalization of our superfield approach to the case of Abelian 3-form gauge theory in four, (3+1), dimensions of spacetime.     

Hot quarks and gluons at an electron-ion collider

The nuclear wave-function is dominated at low- and medium-x by gluons. As the rapid growth of the gluon distribution towards low x, as derived from current theoretical estimates, would violate unitarity, there must be a mechanism that tames this explosive growth. This is most efficiently studied in colliders running in e+A mode, as the nucleus is an efficient amplifier of saturation effects occurring with high gluon densities. In fact, large A can lead to these effects manifesting themselves at energies a few orders of magnitude lower than in e+p collisions. In order to study these effects, there are proposals to build an e+A machine in the USA, operating over a large range of masses and energies. These studies will allow for an in-depth comparison to A+A collisions where results have given tantalising hints of a new state of matter with partonic degrees of freedom. In order to explain these results quantitatively, the gluons and their interactions must be understood fully as they are the dominant source of hard probes at both RHIC and LHC energies.     

The WKB method for the Dirac equation with the vector and scalar potentials

The WKB approximation is developed for the Dirac equation with the spherically symmetrical vector and scalar potentials. The relativistic wavefunctions are constructed, new quantization rule containing the spin-orbital interaction is obtained. For spherically symmetrical model of the Stark effect the quasi-classical spectrum of relativistic hydrogen-like atom is calculated. Application of the WKB method to the mass spectrum of the hydrogen-like quark systems was done.     

On the applicability of perturbative QCD to exclusive processes at currently available momentum transfer

We re-examine the problems connected with the end-point dominance in the calculation of exclusive processes in perturbative QCD. In a re-analysis we construct nucleon quark distribution amplitudes from the respective moments obtained from a QCD sum rule approach. These functions lead to acceptable values for the e.m. Dirac form factorsF ₁ ^p,n of the nucleon if effective gluon masses of ca. 300–600 MeV are included into the hard-scattering amplitude. In addition we also find a reasonable Q₂ — dependence of the proton form factor. The results point at the importance of the end-point k-dependence of distribution amplitude and hard-scattering amplitude in the calculation of exclusive processes.Work is supported by Deutsche Forschungsgemeinschaft (Ga 153-13-1) and partially by NATO (0581/87)     

Lowest-lying tetra-quark hadrons in anisotropic lattice QCD

We present a detailed study of the lowest-lying hadrons in quenched improved anisotropic lattice QCD. Using the π π and diquark–antidiquark local and smeared operators, we attempt to isolate the signal for I(J ^P )=0(0⁺),2(0⁺) and 1(1⁺) states in two flavour QCD. In the chiral limit of the light-quark mass region, the lowest scalar 4q state is found to have a mass, m _4q ^I=0=927(12) MeV, which is slightly lower than the experimentally observed f ₀(980). The results from our variational analysis do not indicate a signature of a tetraquark resonance in I=1 and I=2 channels. After the chiral extrapolation the lowest 1(1⁺) state is found to have a mass m _4q ^I=1=1358(28) MeV. We analysed the static 4q potential extracted from a tetraquark Wilson loop and illustrated the behaviour of the 4q state as a bound state, unbinding at some critical diquark separation. From our analysis we conclude that the scalar 4q system appears as a two-pion scattering state and that there is no spatially-localised 4q state in the light-quark mass region.