An analysis of energetic cosmic ray interactions in graphite

Fifty two high energy cosmic ray interactions in graphite have been analysed. The analysis strongly suggests that some of the characteristics of high energy interactions are dependent on the total transverse momentum,Σp _t, of allγ-rays in an interaction. Out of the 52 events analysed, 29 haveΣp _t⩽2.5 GeV/c and the rest haveΣp _t>2.5 GeV/c; the former are called smallp _t events while the latter are designated as largep _t events. For these two types of events, the characteristics investigated are: (a) fractional energy distribution ofγ-rays, (b) the invariant mass ofγ-rays and (c) the energy distribution ofγ-rays in the emission system.     

Transverse momentum of charged hadrons produced in ν and\bar v deuterium charged current interactions

The transverse momentum properties of charged hadrons produced in charged current ν and \(\bar v\) deuterium interactions are studied. The variation of <P _t ² > withW ²,Q ²,x _F andz ² is analyzed. A study of transverse momentum balance shows that soft gluon emission and some quark primordial transverse momentum with <k _t ² >?(0.4 GeV/c)² are needed to balance highp _t particles. Distributions ofp _tin andP _tout in and out of the event plane are presented. A three jet structure is observed for events withW ²>50 (GeV²) and with a highp _t track, which is compatible with models taking into account hard gluon emission.     

Comparison of the diffraction-dissociation model for high-energy rho photoproduction with empirical data

We compare diffraction-dissociation models for two-pion photoproduction in thep-wave state with recent data of the SLAC-Berkeley-Tufts Collaboration. Two models for two-pion final-state interaction are considered. One is the model worked out earlier which leads to a monotonic modification of the π⁺π?p-wave spectrum with momentum transfer and has fort=0 the extra factor (m _p /m)⁴ compared to the usual Breit-Wigner factor. The other model has a zero in the spectrum aroundm=1.1 GeV which is shifted with momentum transfer.     

Quark fragmentation and trigger side momentum distributions in high-p T processes

It has been widely argued that the experimental evidence concerning the momentum accompanying highp T triggers is a grave problem for models which take the trigger hadron to be a quark fragment. It is claimed that the trigger hadron takes much too large a fraction (z _c ) of the jet momentum for the trigger side jet to be a quark. The jet momentum is not directly measured, but deduced from the derivative of the momentum (p _x ) accompanying the trigger with respect to the trigger transverse momentum-p _T ^t . This argument is shown to be unsafe. Using both an approximate analytic approach to illustrate the physics and subsequently a full numerical computation it is proved that the deduction of the fractional momentum accompanying the trigger, 1/z _c ?1, fromdp _x /dp _T ^t is not correct. Further we show that models—specifically that of Feynman and Field—which do take the trigger to be a quark fragment are essentially in agreement with the data on trigger side momentum distributions. A surprising prediction of our analysis is thatp _x should be approximately constant forp _T ^t ≧6GeV/c.     

Charged particle production at the CERN ISR as a function of transverse momentum

Experimental results are presented on the production of charged particles in inclusive reactions at ISR energies. The data have been taken as function of transverse momentum p_t at the fixed x-values of 0.08, 0.16 and 0.32. The behaviour of the p_t-distributions for the different particles is discussed.     

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.     

Observation of a dip-bump structure in differential cross section for pp → nn in the 700–760 MeV/c momentum range

Based on a sample of about 3500 events, we have measured the total and differential cross sections of $\text{p}\text{p}\text{→}\text{n}\text{n}$ in the 700–760 MeV/c incident momentum region. It is found that σ_CE = 10.7 ± 0.2 mb at the average momentum of 730 MeV/c. The differential angular distribution is characterised by a sharp peak and a dip in the forward direction followed by a secondary maximum. The position of the dip corresponds to |t| ≈ m_π². These results are compared with the predictions of the model of Bryan-Phillips. On the other hand, this dip-bump structure can be well understood on a simple picture involving a π exchange and a constant background (for |t| ? 3 m_π²).     

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.     

Momentum loop dynamics and random surfaces in QCD

A new approach to QCD - momentum loop dynamics - is proposed. The basic quantity is the local momentum p_μ(s) of quarks propagating along the closed loop in a gluonic vacuum. Using the equations of QCD at N = ∞, we derive the stochastic motion of p_μ(s) in extra proper time H. We propose an explicit method for computer simulation, preserving all the symmetries of the continuum theory. An advantage of this approach is the genuine reduction of degrees of freedom. The problems are related with cancellations in numerical simulations.     

Jets and correlations in large pT reactions

Recent experimental data on correlations in large transverse momentum reactions are analyzed in a two-jet picture. Scaling in jet fragmentation is shown to account well for the existing data. The formulae that are given can be used as a basis for a more detailed analysis of the dynamics. A particular result is that we predict that a two-jet structure can also be produced in events where it is not required that any single hadron has large p_T, with a cross section as large as $\text{1}\text{10}$ mb or more (at ISR energies with the total transverse momentum of each jet ?2.5 GeV/c).     

Inclusive single-particle distributions and transverse momenta of forward produced charged hadrons in μp scattering at 470 GeV

Using data from the Fermilab fixed target experiment E665, general properties of forward produced charged hadrons in μp interactions at a primary muon energy of 470 GeV are investigated. The normalized inclusive singleparticle distributions for Feynman-x D(xF ) and for the transverse momentum D(p ² _t , xF ) are measured as a function of W and Q ². The dependence of the average transverse momentum squared 〈p ² _t〉 on xF , W and Q ² is studied. The increasing contribution from diffractive production as Q ² decreases leads to a reduction of the average charged hadron multiplicities at low (positive) xF and an enhancement at large xF , for Q ² ? 10 GeV². It also reduces 〈p ² _t〉 for Q2 ? 5 GeV2 and 0.4 ? xF < 1.0.     

Transverse momenta of helium fragments in gold fragmentation at 10.6 GeV/nucleon

The transverse momentum (p_t) distributions of helium fragments from gold fragmentation on different nuclei of nuclear emulsion have been measured and a clear increase of average p _t with target mass is seen. The p _t distributions can be parameterized by a sum of three exponential functions of the form $\sim {\rm exp}(-p_{t}^{2}/B_{i})$ . The differences in p _t distributions in interactions on different targets can be explained by different contributions of the three exponential functions. These contributions depend on the projectile breakup in the collision, and for a given degree of the projectile breakup do not depend on the target mass.     

General characteristics of\bar pp interactions at low energies in terms of principal axis and impact parameter variables

The reaction configuration in momentum and position space has been investigated in detail from exclusive final states in \(\bar pp\) interactions at centre of mass energies 2.01 and 2.96 GeV. In momentum space one finds a tendency towards a planar configuration at low multiplicities. In position space, one sees the average impact parameter decreasing with increasing multiplicity. At large multiplicities, events become more phase-space like i.e., more central whereas at lower multiplicities the effect ofp _t damping can be seen.     

Study of gold nanocluster sputtering under 38-keV Au ion bombardment by a classical molecular dynamics method

The computer simulation of the interaction of 38-keV Au₁ ions with isolated spherical Au _N nanoclusters of diameters 2.6 and 18 nm is performed in the framework of the classical molecular dynamics (MD) method. The distribution of the absorbed energy ε per one atom of the irradiated cluster and the sputtering yields are analyzed for different ratios of the nanocluster diameter D to the average projective range R _p of the bombarding ion. It is established that the small values of the absorbed energy (ε ? ε_max = E/N) are most probable for D < R _p, and either small (ε ? ε_max) or the maximum possible (ε ～ ε_max) values are mainly realized for D ≤ R _p. It is shown that the total sputtering yield depends weakly on the impact parameter. It is demonstrated for the first time that the irradiated cluster, as a whole, can be ejected by direct impact with a probability of approximately 6–13%. Such events are realized in the cases where the bombarding ion causes secondary cluster-atom emission in the dominant direction to a substrate, with the result that an unsputtered cluster fraction acquires momentum in the opposite direction. This recoil effect can be one of the mechanisms for desorption of nanoclusters deposited on the surface under ion (or cluster) bombardment.     

Dissipation and fragmentation of low-Q 2 scattered partons in Au−Au collisions at RHIC

Two-particle correlations and event-wise fluctuations in transverse momentum p _tare reported for Au?Au collisions at √s _NN =62 and 200 GeV on pseudorapidity η and azimuth ?. Distributions of all pairs of particles (no leading trigger particle) reveal jet-like correlations, or peaks at pair-wise opening angles of order 1 radian or less. The width of this same-side correlation peak increases dramatically on pseudorapidity and decreases on azimuth for increasing collision centrality. Evolution of the same-side peak with centrality suggests dissipation of low-Q ² partons via strong couplign to an expanding bulk medium. p _t correlations, which provide access to temperature and/or velocity distributions in the colliding system, are also presented.     

A meson-theoretical explanation of the f 0 (980)-production puzzle in the reaction π− p → π0π0 n

A meson-theoretical model for the reaction π^? p → π⁰π⁰ n for large π^? momenta is developed which treats the final-state interaction between the two produced pions microscopically. For small momentum transfers, the squares of the S-wave amplitudes of the produced two-pion system show a dip for invariant twopion masses in the vicinity of the f ₀(980) meson, while for momentum transfers ?t>0.2(GeV/c)², the f ₀(980) meson appears as a bump. The model provides a microscopic explanation of the long-standing puzzle seen by both the GAMS and the BNL experimental collaborations.     

Study of data on the associated momentum on the trigger side in highP T hadron production

The British-French-Scandinavian collaboration has recently studied the non trigger charged mean momentum in different rapidity regions on the trigger hemisphere, 〈p _x〉, in the collision of two hadrons at the CERN Intersecting Storing Rings (ISR). In particular, they give for the rapidity regions |y|<0.5 and |y|<1 the values of the slope, α, of 〈p _x〉 with the trigger momentum,p _T ^t . Several authors have analysed those values of α in the framework of hard scattering models which predict values independent ofp _T ^t for 〈z _c〉, the longitudinal momentum fraction of the outgoing hard scattered system taken by the trigger. From this analysis they give estimates of 〈z _c〉 of very difficult reconcilliation with those calculated in the Feynman, Field and Fox hard scattering model or in the QCD treatment of highp _T hadron production. The authors of the present paper have looked for, and found, other data whose model independent analysis is more feasible than that of the data mentioned above. More specifically, we analyse in the framework of the hard scattering models, but otherwise model independently, data on 〈p _x〉 in two other rapidity regions (|y|<3, 2<|y|<3) and find that consistence of the average slopes, α, in these two regions is only achieved with mean values of 〈z _c〉 significantly, increasing withp _T ^t and close in value to those obtained by Feynman et al. [6, 7].     

Geometrical description of binary reactions and large p⊥ phenomena

We study consequences of defining the impact parameter as being the variable conjugate to transverse relative c.m. momentum, p_⊥. This coincides with a proposal originally made by Chang and Raman. The dynamical information carried by these are intimately connected to the behaviour of 90° scattering. In particular, impact parameter dynamics is a priori qualitatively different from direct channel angular momentum dynamics. A phenomenological study of elastic π^±p and pp data at fixed values of longitudinal momentum, p₆, leads to a generalized geometrical scaling hypothesis. In contrast with the power behaviour predicted by parton models, the data suggest exponential behaviour of the scaling amplitudes at fixed p₆ of the type e^Rp⊥ with R ? 1 fm. Correspondingly these amplitudes are described by a pair of complex conjugate singularities on the imaginary axis in the impact parameter plane.     

A comparative study of the dissociation of protons into low-mass pπ+π− systems in coherent and incoherent proton-nucleus collisions at 18.6 GeV/c

A decay angular moments analysis has been performed on 8500 pπ⁺π^? events produced by the interaction of high-energy protons with nuclear targets. The analysis was carried out separately for the low-t, coherent events and high-t, incoherent events. The moments of the two samples show significant differences and possible reasons for such differences are discussed.