A first look at Au+Au collisions at RHIC energies using the PHOBOS detector

The PHOBOS detector has been used to study Au + Au collisions at√^sNN = 56,130, and 200 GeV Several global observables have been measured and the results are compared with theoretical models. These observables include the charged-particle multiplicity measured as a function of beam energy, pseudo-rapidity, and centrality of the collision. A unique feature of the PHOBOS detector is its almost complete angular coverage such that these quantities can be studied over a pseudo-rapidity interval of |η|≤5.4. This allows for an almost complete integration of the total charged particle yield, which is found to be about N _ch ^tot = 4200 ±470 at √^sNN = 130 GeV and N _ch ^tot = 5300 ±530 at √^sNN = 200 GeV. The ratio of anti-particles to particles emitted in the mid-rapidity region has also been measured using the PHOBOS magnetic spectrometer. Of particular interest is the ratio of anti-protons to protons in the mid-rapidity region, which was found to be (i.e.921-1) at √^sNN = 130 GeV. This high value suggests that an almost baryon-free region has been produced in the collisions.     

Study of the jet shape at 920 GeV/c in proton–nucleus interactions with HERA-B detector

We performed a measurement of differential and integral jet shapes in proton-carbon, proton-tungsten and proton-aluminium collisions at 920 GeV/c proton momentum with the HERA-B detector at HERA for the jet transverse energies in the range 4 GeV<E_T(jet)< 12 GeV. Jets were identified using the k_T-clustering algorithm. The measurements were performed for the hardest jet in the event, directed towards the opposite side with respect to the trigger direction. Jets become narrower with increasing transverse energy and measured distributions agree well with predictions of the PYTHIA 6.2 model. We do not observe any significant difference in the jet shape for the carbon and the aluminium targets. Nevertheless, the transverse energy flow at small and large radii for the tungsten sample is slightly less than for light nuclei. This observation indicates some influence of the nuclear environment on the formation of jets in heavy nuclei, especially at lower transverse energies, 5 GeV<E_T(jet)< 6 GeV. PACS 13.85.-t; 13.87.-a; 13.87.Fh     

z-scaling in heavy ion collisions at the RHIC

Experimental data on transverse particle spectra obtained by the STAR, PHENIX, PHOBOS, and BRAHMS collaborations at the RHIC are analyzed in the framework of the generalized concept of z-scaling. It was developed for analysis of inclusive particle production in proton-(anti)proton collisions at high p _T and high multiplicities. The general scheme of the approach based on the physical principles of self-similarity, locality, and fractality is reviewed. Independence of the scaling function ψ(z) from energy, multiplicity, and atomic weight for h ^±, π ^±,0, K _S ⁰ , and Λ hadrons produced in Au-Au and Cu-Cu collisions at √s = 130 and 200 GeV is discussed. Based on z-scaling, the multiplicity dependence of pion transverse spectra up to p _T = 25 GeV/c in Au-Au collisions at √s = 200 GeV for experiments at the RHIC is predicted. The text was submitted by the author in English.     

Experimental study of the reaction e + e − → K S K L in the energy range √S = 1.04−1.38 GeV

We present a measurement of the e ⁺ e ⁻ → K _S K _L cross section in the energy range √s = 1.04−1.38 GeV. For an energy of √s ≥ 1.2 GeV the cross section exceeds vector meson dominance model predictions with only ρ(770), ω(783), and ϕ(1020) mesons taken into account. The measured cross section agrees well with previous measurements. Published in Russian in Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2006, Vol. 130, No. 5, pp. 831–839. This article was translated by the authors.     

Pseudorapidity distributions of charged particles in Pb-Pb collisions at super proton synchrotron energies from the NA50 experiment

We present the measurements of charged particle pseudorapidity distributions dN_ch/dη performed by the NA50 experiment in Pb-Pb collisions at the CERN SPS. Measurements were done at incident energies of 40 GeV (√s = 8.77 GeV) and 158 GeV (√s = 17.3 GeV) per nucleon over a broad impact parameter range. The multiplicity distributions are studied as a function of centrality using the number of participating nucleons(N _part ), or the number of binary nucleon-nucleon collisions (N_coll). Their values at midrapidity exhibit a linear scaling withN _part at both energies. Particle yield increases approximately by a factor of 2 betweeny √s = 8.77 GeV and √s = 17.3 GeV.     

Integral energy spectrum of primary cosmic rays at high energies

The integral energy spectrum of primary cosmic rays has been obtained. In the energy range (2.4×10³−1.1×10⁵ GeV), the spectrum of all nuclei is consistent with a power law of indexγ=1.55±0.06 and the flux of all nuclei is:N(⩾E ₀)⋍(5.1±1.8)×10⁻¹×E ₀ ^−1.55 particles/cm² sterad. sec., whereE ₀ is in GeV. The spectrum of primaryα-particles in the energy range (4.4×10³−4.8×10⁴) GeV is also consistent with a power law of indexγ=1.71±0.12 and the flux is:N(⩾E ₀)=(4.2±1.4)×10⁻¹×E ₀ ^−1.71 , particles per cm² sterad. sec, whereE ₀ is in GeV.     

Jet energy density in hadron-hadron collisions at high energies

The average particle multiplicity density dN _ch/dη is the dynamical quantity that reflects some regularities of particle production in the low-p _T range. The quantity is an important ingredient of z scaling. Experimental results on charged particle density are available for pp, pA, and AA collisions, while experimental properties of the jet density are still an open question. The goal of this work is to find the variable that will reflect the main features of the jet production in a low transverse energy range and play the role of the scale factor for the scaling function ψ(z) and variable z in data z-presentation. The appropriate candidate is the variable we called “scaled jet energy density.” The scaled jet energy density is the probability of having a jet with defined E _T in defined x _T and pseudorapidity regions. The PYTHIA6.2 Monte Carlo generator is used for calculation of the scaled jet energy density in proton-proton collisions over a high energy range and at η = 0. The properties of the new variable are discussed and sensitivity to “physical scenarios” applied in the standard Monte Carlo generator is noted. The results of the calculations of scaled jet energy density at LHC energies are presented and compared with predictions based on z scaling. The text was submitted by the authors in English.     

Atmospheric gamma rays in the energy region 40–1000 GeV

A large stack of lead-emulsion sandwich detector assembly was flown over Hyderabad, India. High energy gamma rays at the float altitude were unambiguously identified from the cascades they induced, and their energies reliably determined by improved methods. From an analysis of 163 gamma rays of energy ≳ 30 GeV, it is found that the differential energy spectrum is represented by the power lawJ _r (E)= 129·4E ^{−2·62±0·12} photons m⁻² sr⁻¹sec⁻¹ GeV⁻¹ at an effective atmospheric depth of 14·3 g cm⁻²; this is the first reliable balloon measurement of atmospheric gamma rays in the energy range 40–1000 GeV. After correcting for the gamma rays radiated by the primary cosmic ray electrons, the production spectrum of gamma rays, resulting from the collisions of cosmic ray nuclei with air nuclei, at the top of the atmosphere isP _r (E, 0)=8·2 × 10⁻⁴ E^2.60±0.09 photons g⁻¹sr⁻¹sec⁻¹ GeV⁻¹. The atmospheric propagation of the electromagnetic component due to the cascade process is also derived from the gamma ray production spectrum.     

The total yields of K *(892)+, Σ+(1385), and Σ0 in neutrino-induced reactions at ? E v? ≈ 10 GeV

Using the data obtained with the SKAT bubble chamber, the total yields of K*(892)⁺, Σ⁺(1385), and Σ⁰ are estimated for the first time in neutrino-induced reactions at moderate energy 〈E _v〉 = 10.4 GeV. It is shown that the recently observed enhancement of the K ⁰ and Λ yields in vA interactions as compared to vN interactions is contributed only slightly by the K*(892)⁺ and Σ⁺(1385) production. The contribution of resonances to the K ⁰ and Λ yields is found to be in qualitative agreement with higher energy (〈E _v〉 ≳ 40 GeV) data. It is shown that the energy dependence of the K*(892)⁺ mean multiplicity in vN interactions is approximately linear in the range of 〈E _v〉 ≈ 10−60 GeV, while that for Σ⁰ in vA interactions (A = 20−21) is approximately logarithmic in the range of 〈E _v〉 ≈ = 10−150 GeV. The text was submitted by the authors in English.     

Reaction e + e ? → π + π ? π + π ? at energies of √ s ≤ 1 GeV

The e ⁺ e ⁻ → π ⁺ π ⁻ π ⁺ π ⁻ cross section is calculated for energies of 0.65 ≤ √s ≤ 1 GeV in the framework of the generalized hidden local symmetry model. The calculations are compared with the data of CMD-2 and BaBaR. It is shown that the inclusion of heavy isovector resonances ρ(1450) and ρ(1700) is necessary for reconciling calculations with the data. It is found that, at √s ≈ 1 GeV, the contributions of the above resonances are much larger, by a factor of 30, than the ρ(770) one, and amount to a considerable fraction ∼0.3–0.6 of the latter at √s ∼ m _ρ. The text was submitted by the authors in English.     

A search for high-energy cosmic gamma-ray bursts

Summary A search has been made for excesses of cosmic-ray counting rates at primary energiesE ₀₁>5 GeV andE ₀₄>2·10⁴ GeV over the time scalet=(1÷100) ms. The measurement was performed by means of a small extensive air shower array operating at mountain altitude (3500 m a.s.l.). During the running time 111 cosmic gamma-ray bursts were detected by satellites; 10 of them certainly (55 probably) above the horizon of the detector. No significant counting rate excess has been recorded out of the statistical fluctuations. Also the search for correlations with satellite events has given a negative result. The upper limit for high-energy cosmic gamma-ray flux in bursts isϕ ₁(E>E ₀₁)<4·10⁻⁵ (t = time scale in ms),ϕ ₄(E>E ₀₄)<1.6·10⁻⁵erg/cm². Paper presented at the 2^o Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984.     

The directed flow maximum near cs = 0

We investigate the excitation function of quark-gluon plasma formation and of directed in-plane flow of nucleons in the energy range of the BNL-AGS and for the E ^kin _Lab = 40A GeV Pb + Pb collisions performed recently at the CERN-SPS. We employ the three-fluid model with dynamical unification of kinetically equilibrated fluid elements. Within our model with first-order phase transition at high density, droplets of QGP coexisting with hadronic matter are produced already at BNL-AGS energies, E ^kin _Lab≃ 10A GeV. A substantial decrease of the isentropic velocity of sound, however, requires higher energies, E ^kin _Lab≃ 40A GeV. We show the effect on the flow of nucleons in the reaction plane. According to our model calculations, kinematic requirements and EoS effects work hand-in-hand at E ^kin _Lab = 40A GeV to allow the observation of the dropping velocity of sound via an increase of the directed flow around midrapidity as compared to top BNL-AGS energy. Received: 7 May 2000 / Accepted: 2 August 2000     

Dijet production in diffractive deep inelastic scattering at HERA

The production of dijets in diffractive deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 61 pb^-1. The dijet cross section has been measured for virtualities of the exchanged virtual photon, 5 < Q² < 100 GeV², and γ^*p centre-of-mass energies, 100 < W < 250 GeV. The jets, identified using the inclusive k_T algorithm in the γ^*p frame, were required to have a transverse energy E^* _T,jet > 4 GeV and the jet with the highest transverse energy was required to have E^* _T,jet > 5 GeV. All jets were required to be in the pseudorapidity range -3.5<η^* _jet<0. The differential cross sections are compared to leading-order predictions and next-to-leading-order QCD calculations based on recent diffractive parton densities extracted from inclusive diffractive deep inelastic scattering data.     

Measurement of event shape variables in deep-inelastic scattering at HERA

Deep-inelastic ep scattering data taken with the H1 detector at HERA and corresponding to an integrated luminosity of 106 pb^-1 are used to study the differential distributions of event shape variables. These include thrust, jet broadening, jet mass and the C-parameter. The four-momentum transfer Q is taken to be the relevant energy scale and ranges between 14 GeV and 200 GeV. The event shape distributions are compared with perturbative QCD predictions, which include resummed contributions and analytical power law corrections, the latter accounting for non-perturbative hadronisation effects. The data clearly exhibit the running of the strong coupling α_s(Q) and are consistent with a universal power correction parameter α₀ for all event shape variables. A combined QCD fit using all event shape variables yields α_s(m_Z)=0.1198±0.0013 ^+0.0056_-0.0043 and α₀=0.476±0.008 ^+0.018_-0.059.     

Interpretation of the recent Kolar events

We give plausible interpretations of the unusual events seen in the proton decay detector at Kolar Gold Fields indicating the existence of a massive (≳2GeV) long lived (10⁻⁸−10⁻⁹s) particle. We show that it is possible to accommodate the particle in the standard model as a fourth generation neutrino, or inE ₆ grand unified theory as a neutral fermion occurring in27 representation or in supersymmetric theory as a scalar neutrino. However, there is a difficulty in explaining the large production rate for the particle.     

Probing universal extra dimension at the international linear collider

We consider the UED scenario and study the detectability of the first KK electron-positron pair at the ILC. A few hundred GeV KK electron decays into a nearly degenerate KK photon, which carries away missing energy, and the standard electron. The mass splitting between the KK electron and KK photon is controlled by the bulk-and brane-induced radiative corrections. We look for the signal event e ⁺ e ⁻ + large missing energy for √s = 1 TeV and observe that with a few hundred fb⁻¹ luminosity the signal can be deciphered from the standard model background. We briefly outline how the UED signals may be distinguished from the supersymmetric signals. This talk is based on a work the author did with Paramita Dey, Anirban Kundu and Amitava Raychaudhuri [1].     

Photoproduction of πmesons off protons from the Δ(1232) region to Eγ = 3 GeV

Photoproduction of π⁰-mesons was studied with the Crystal-Barrel detector at ELSA for incident energies from 300MeV to 3GeV. Differential cross-sections dσ/dΩ, dσ/dt, and the total cross-section are presented. For E _γ < 3GeV, the angular distributions agree well with the SAID parametrization. At photon energies above 1.5GeV, a strong forward peaking indicates t-channel exchange to be the dominant process. The rapid variations of the cross-section with energy and angle indicate production of resonances. An interpretation of the data within the Bonn-Gatchina partial-wave analysis is briefly discussed.     

Measurement of prompt photons with associated jets in photoproduction at HERA

The photoproduction of prompt photons, together with an accompanying jet, has been studied in ep collisions at a centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 77 pb^-1. Cross sections were measured for the transverse energy of the photon and the jet larger than 5 and 6 GeV, respectively. The differential γ+jet cross sections were reconstructed as functions of the transverse energy, pseudorapidity and x_γ ^obs, the fraction of the incoming photon momentum taken by the photon-jet system. Predictions based on leading-logarithm parton-shower Monte Carlo models and next-to-leading-order (NLO) QCD generally underestimate the cross sections for the transverse energies of prompt photons below 7 GeV, while the k_T-factorisation QCD calculation agrees with the data better. When the minimum transverse energy of prompt photons is increased to 7 GeV, both NLO QCD and the k_T-factorisation calculations are in good agreement with the data.