Neutral transverse momentum spectra in 60 and 200A·GeV16O+nucleus and proton+nucleus reactions

Transverse momentum (p _T) distributions fo inclusive photons and neutral pions at midrapidity are measured with a lead glass calorimeter in 60 and are measured with a lead glass calorimeter in 60 and 200A·Gev¹⁶O+nucleus and and proton+nucleus reactions. Inclusive photon distributions are compared for central and peripheral reactions. The degree of centrality is determined either from the charged particle multiplicity or from the remaining projectile energy in the forward direction. Deviations from a nucleus+nucleus interaction model based upon linear extrapolation from p+p reactions are observed in central¹⁶O+Au data. The variation of theaverage transverse momentum is investigated as function of centrality. The target-mass and energy dependence of π⁰ p _T distributions are presented. For¹⁶O+Au a change of slope in these distributions is observed atp _t ≈0.8 GeV/c compatible with hydrodynamic expansion models.     

Charged hadron suppression in Au+Au collisions at high p T from STAR

We report high statistics measurements of inclusive charged hadron production in Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV and 130 GeV. A large, approximately constant hadron suppression is observed in central Au+Au collisions for 5 ‖p _T ‖ 12 GeV/c. The collision energy dependence of the yields and the centrality and p _T dependence of the suppression provide stringent constraints on theoretical models of suppression. Models incorporating initial-state gluon saturation or partonic energy loss in dense matter are largely consistent with observations. The p _T -dependent suppression expected from models incorporating jet attenuation in cold nuclear matter or absorption of fragmentation hadrons is not observed.     

A study of the reaction p + p → p + p + X at ISR energies

We have performed an experiment to investigate the reaction p + p → p₁ + p₂ + X at the CERN Intersecting Storage Rings (ISR). The dependence of the cross-section on the mass and rapidity of the system X, and on the momentum transfer squared t₁, t₂ to the outgoing protons is studied. We observe signals for the production of ?, ω, f, and A₂ mesons in the missing-mass spectra, and conclusions are made about the contribution of double-Pomeron exchange.     

Neutral kaon production in p+p and p+Nb collisions

The kaon nucleus (KN) interaction in dense nuclear matter is predicted to be repulsive and increasing with density. However, determined values for this potential are not yet consistent with each other (Benabderrahmane et al., Phys Rev Lett 102:182501, 2009; Agakishiev et?al., Phys Rev C 82:044907, 2010; Büscher et?al., Eur Phys J A 22:301–317, 2004). We analyze $K^0_S$ mesons identified with the HADES detector in p+p and p+?⁹³Nb reactions at 3.5?GeV kinetic beam energy. To determine the KN potential at normal nuclear density we propose to compare the $K^0_S$ differential distributions in p+?⁹³Nb and p+p collisions. High statistics of low p _t -kaons (p _t ?<?100?MeV/c) ensure the sensitivity of our measurements to the nuclear matter effects. We present the data analysis method and first results.     

Charge and energy flow in π+ p,K + p andpp interactions at 250 GeV/c

We present data on the flow of energy and charge in π⁺ p,K ⁺ p andpp interactions at 250 GeV/c. The energy and charge flow in the beam c.m. hemisphere is analysed in terms of the cluster-invariant Bialas-Ochs-Stodolsky variable λ=cot θ/E _jet. The profile functionsdQ/dλ anddE/dλ indicate a widening inp _T of jets between 32 and 250 GeV/c incident laboratory momentum, whereas the ratiodQ/dE is energy independent. The data are compared to μp deep-inelastic scattering resutls and to expectations from the single string LUND model, the Dual Parton Model and the two-string LUND model (FRITIOF).     

LowpT phenomena in A+A and p+A collisions at mid-rapidity

CERN experiment NA44 measures pion and kaon distributions atpT=0–320 MeV/c at midrapidity in p+A and A+A collisions at 450 and 200A GeV/c, respectively. Pion production from p+Pb and A+A is slightly enhanced at lowpT relative to p+Be, but less than observed at target rapidity. The lowpT behavior can be understood in the context of the RQMD model as arising from baryonic resonance decays.     

Di-jet correlation in Au+Au and Cu+Cu Collisions from PHENIX

PHENIX has measured the two-particle azimuth correlation in Au+Au at √s=200 GeV. Jet shape and yield at the away side are found to be strongly modified at intermediate and low p _T, and the modifications vary dramatically with p _T and centrality. At high p _T, away side jet peak reappears but the yield is suppressed. Similar jet strength is found for Au+Au and Cu+Cu collisions with similar number of participant nucleons.     

Inclusive prompt photon production inpp and\bar p p interactions

High transverse momentum (p _T ) production of pions and photons in proton-proton and proton-antiproton interactions are studied within the QCD framework and compared with the presently available experimental data. Scaling violation effects are included, and shown to be important at largep _T . Predictions are made for the γ/π⁰ ratio at collider energies. The possibility of extricating the gluon distribution function from the invariant cross section data for the process \(\bar p\) +p→γ+X is suggested.     

Comparing the p̅p → K − K + and p̅p → π−π+ reactions

A partial wave amplitude analysis of LEAR data on differential cross sections and analyzing powers of p?p → K ^? K ⁺ has been performed in the range p _lab = 360 ? 1000 MeV/c, and compared to the results from the same observables for p?p → π^?π⁺. Contrary to other analyses we see no compelling evidence of resonance behavior in our amplitudes. However, as we discuss, the resulting amplitudes are not unique since a third observable remains to be measured for both reactions. Another remarkable result is that for p?p → K ^? K ⁺ all our solutions in the momentum interval below 900 MeV/c, only partial wave amplitudes including J _max = 2 are necessary. This upper limit on the angular momentum is smaller than J _max = 3 which is required for the data of the reaction p?p → π^?π⁺ in the same momentum interval. This finding is consistent with a model analysis for data above 1 GeV/c. Annihilation models with short range baryon exchange give too small J ≥ 2 amplitudes and final state ππ and K?K interactions are presumably very important.     

Self-similarity of high-p T cumulative hadron production in p + A collisions at high energies at U70

New data on inclusive transverse momentum spectra of charged hadrons produced in p + A collisions at U70 are analyzed in the framework of the z-scaling approach. Self-similarity of the hadron production in the high-p _T cumualtive region is verified. Scaling function ψ(z) for C, Al, Cu, W nuclei is constructed. It is expressed via the invariant cross section and the average multiplicity density of charged particles. Results of analysis are compared with the data obtained by J. Cronin, R. Sulyaev and D.Jaffe groups. Self-similarity of the hadron production in p + A collisions over a wide kinematical range is verified. A microscopic scenario of p + A interactions in terms of momentum fractions x ₁, x ₂ is discussed. Indication on self-similarity of the high-p _T cumulative hadron production in p + A collisions over a wide kinematical range has been found. Based on the universality of the shape of the scaling function the inclusive cross sections of h ^? and h ⁺ hadrons produced in p + A collisions on C, Al, Cu, W targets in the high-p _T deep-cumulatice region are predicted.     

High transverse momentum Ξ baryon correlations in p+p collisions at √s=200 GeV

Angular correlations between produced high-p _TΞ^? baryons and unidentified charged and neutral particles are observed in high-tower triggered (on a large electromagnetic energy deposit) √s=200 GeV p+p collisions. This trigger favors events with higher average multiplicity than those in minimum bias. These events are likely to contain jets. The average multiplicity of a high-tower triggered event is similar to that of a minimum bias event containing a Ξ baryon, which implies that Ξ baryons are likely to be produced in jets. Ξ^? <p _T> is higher in the triggered data then in the minimum bias sample.     

A spark chamber measurement of the reactions π + p → ϱ + p at 15 GeV/c

A new method, using spark chambers, for the study of the reactions π^± + p → ?^± + p is described. The charged pion and both γ rays from the π^± decay are detected. Differential and integrated cross sections σ_π⁺=50 ± 9 μb, σ_π⁻=47 ± 9 μb) for 0.0 ?|t|?1. (GeV/c)² and a laboratory momentum (p_Lab) of 15 GeV/c are presented. The momentum dependence of σ_γ^± is well fitted from 2.7 to 16 GeV/c by σ = Kp_Lab⁻ with n_γ⁺ = 1.80 ± 0.80 and n_γ⁻ = 1.87 ± 0.15.     

Inclusive photon and π0 production inK + p interactions at 70 GeV/c

Inclusive and semi-inclusive cross sections and distributions of γ's and π⁰'s inK ⁺ p interactions at 70 GeV/c are presented. The results are compared to other experiments and to the Lund model for low-p _T hadron collisions.     

Interferometrical signature of mixed phase in A+A collisions

We have calculated the pion rapidity distribution, the transversal momentum spectrum and the pion correlator in outward, sideward and longitudinal directions for S+S collisions at high energy. We used a two-temperature model that allows us to explain the abundance of pions at smallp _T and highp _T . The first source, which is at high temperature, is in a quark-gluon-hadron mixed phase radiating pions due to a deflagration shock wave on the background of longitudinal expansion. The second source consists of the unburnt part of the matter in hadronic phase. The interferometrical analysis at differentp _T gives a signature for two expanding sources separated in time and allows the measurement of the lifetime and other parameters of each of them.     

Conformal approach to fluctuations in hadroproduction

We study two-photon final-states atep-colliders. In particular, we investigate the process γ+p →γ+γ+X at HERA and LEP/LHC energies with the intention of determining its measurability at these accelerators. We find that the transverse momentum distribution will be measured top _T=25 GeV/c at HERA andp _T=50 GeV/c at LEP/LHC. We find that the cross section of this process is too small for it to be a sensitive, direct probe of the gluon content of the proton and the photon. However, we find that it will supplement deep inelastic structure function investigations of the quark distributions within the proton and photon. Further, we predict that at very low transverse momenta, this process will allow the measurement of the box diagramg+g→γ+γ.     

Comparison of planar features of K−p interactions with e+e− annihilations and deep inelastic μp scattering

The planar features of hadronic final states in K^?p interactions at 4.5, 5.6 and 14.3 GeV c.m. energy are compared with data from e⁺e^? annihilation and deep inelastic μp scattering. A strong energy variation of the distributions (1/σ)dσ/dp_t², 〈p_t²〉_in and Σp²_{t in} is observed over this energy range. Striking agreement is found between the K^?p data at 14.3 c.m. energy and the leptonic data at similar hadronic c.m. energies. The behaviour of these distributions from K^?p data is not well described by the simple quark fragmentation model, but shows deviations similar to those attributed to hard gluon effects in leptonic interactions.     

Pion exchange current effects inv μ +d→μ− +p+p

The differential cross sectiondσ/dq ² for the processv _μ +d→μ ^?+p+p is studied using realistic deuteron wave functions and closure over the dinucleon states. The effect of meson exchange currents is estimated using one-pion exchange diagrams in the static limit. Implications of our present results on the determination ofF _A (q ²) from neutrinodeuteron reaction experiments are discussed.     

Femtoscopy in p+p vs. heavy ion collisions in STAR

The geometric substructure of the particle-emitting source has been characterized via two-particle interferometry by the STAR collaboration for several energies and colliding systems at RHIC. In heavy ion collisions the m_T dependence of femtoscopic radii has been observed for all particle types by several experiments at different collision energies. This dependence has been thought to arise from space-momentum correlations generated by collective flow. On the other hand, there are several reports of a similar m_T dependence by experiments measuring elementary particle collisions. Here, quite different physical mechanisms – including resonances, strings, and uncertainty arguments – have been proposed to explain the dependence. Determining the differences or similarities in the space-time physics driving the signal in heavy ion versus p+p collisions requires a direct comparison of m_T dependence of the radii. Such a comparison has, until now, been sorely lacking. STAR data allow, for the first time, such a direct comparison between A+A, d+A, and p+p collisions, at the same energy, measured in the same detector, and using the same analysis techniques. Surprisingly, our preliminary results indicate an m_T-independent scaling of the femtoscopic radii with overall system size. Possible physics implications of these observations will be discussed, and the importance of long-range non-femtoscopic correlations for low multiplicity collisions will be emphasized. PACS 25.75.Gz; 25.75.-q     

Measurement of Neutral pions in √NN=200 GeV and 62.4 GeV Au+Au collisions at RHIC-PHENIX

Neutral pions (π⁰) in √_NN=200 GeV and 62.4 GeV Au+Au collisions were measured at RHIC-PHENIX Year-4 Run. In √_NN=200 GeV, π⁰ spectra are measured up to p _T=20 GeV/c. A strong suppression by a factor of ～5 is observed and stays almost constant up to 20 GeV/c.     

Inclusive cross section ratios in highp T proton-proton scattering at ISR energies

Ratios of inclusive cross sections σ(π⁺)/σ(π⁺+K ⁺+p) and \(\sigma (\pi ^ - )/\sigma (\pi ^ - + K^ - + \bar p)\) were measured for proton-proton interactions with a highp _T hadron in the final state around c.m.s. scattering angles θ?20°, 20° and 45° at two ISR energies \(\sqrt s = 31\) Gev and 62 GeV. Results are shown as functions of transverse and longitudinal momentum and are compared with parton model predictions. The different dependences of positive and negative pion fractions atp _T ?2–3 GeV/c on longitudinal momenta is similar to that observed in soft hadronic interactions at low values ofp _T where the leading proton effect (diquark fragmentation) is known to contribute. The quantitative agreement of the data with diquark model predictions indicates the presence of diquark fragmentation also in highp _T jets.