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).     

Restoration of kT factorization for low pT hadron hadroproduction

We discuss the applicability of the k _T factorization theorem to low-p _T hadron production in hadron–hadron collision in a simple toy model, which involves only scalar particles and gluons. It has been shown that the k _T factorization for high-p _T hadron hadroproduction is broken by soft gluons in the Glauber region, which are exchanged among a transverse-momentum-dependent (TMD) parton density and other subprocesses of the collision. We explain that the contour of a loop momentum can be deformed away from the Glauber region at low p _T , so the above residual infrared divergence is factorized by means of the standard eikonal approximation. The k _T factorization is then restored in the sense that a TMD parton density maintains its universality. Because the resultant Glauber factor is independent of hadron flavors, experimental constraints on its behavior are possible. The k _T factorization can also be restored for the transverse single-spin asymmetry in hadron–hadron collision at low p _T in a similar way, with the residual infrared divergence being factorized into the same Glauber factor.     

Weak and electromagnetic effects in large pT proton-proton collisions

The weak and electromagnetic contributions to the inclusive cross section for hadron production at large p_T in proton ISR experiments are calculated using the parton model. When compared with extrapolations of recent strong interaction data, it is found that they overtake the strong cross section at large p_T for beam energies of several hundred GeV at a cross section which might be accessible (assuming a reasonable W boson mass in the weak case). Further, the parity violating effects of the weak interaction are found to be large and thus in principle could give an unambigous signature to weakly produced events, using polarised beams.     

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.     

Nuclear-like effects in proton–proton collisions at high energy

We show that several effects considered nuclear effects are not nuclear in the sense that they do not only occur in nucleus–nucleus and hadron–nucleus collisions but, as well, they are present in hadron–hadron (proton–proton) collisions. The matter creation mechanism in hh, hA and AA collision is always the same. The p _T suppression of particles produced in large multiplicity events compared to low multiplicity events, the elliptic flow and the Cronin effect are predicted to occur in pp collisions at LHC energies as a consequence of the high density partonic medium obtained.     

A measurement of high-pT correlations in 340 GeV/c pp and 280 GeV/cπ−p reactions

We have measured correlations between single high-p_T(1.5<p_T<3.5 GeV/c) trigger particles on one side of the beam line and groups of particles entering a calorimeter on the opposite side of the beam line. The mean transverse momentum measured in the calorimeter is found to increase with the trigger-particle transverse momentum. The coplanarity of the events increases with trigger-particle transverse momentum. We have compared our data with the predictions of a phenomenological four-jet model. To fit our data we find that we must give large (0.9 GeV/c) mean transverse momenta to the constituents of the initial hadrons.     

Forward particle production in π− p andK − p collisions at 58 GeV/c and comparison with quark models

We present single inclusive π^±, π⁰ andK ^± spectra in the forward fragmentation region (x>0.2,p _T <1.5 GeV/c) as well as correlations between two charged particles. The data were recorded in an unseparated negative hadron beam at the CERN SPS using a large acceptance forward spectrometer. Our maasurements are compared in detail with several models which emphasise the role of the beam valence quarks in this production process. The connection to measurements at largep _T is also investigated.     

Comparison of the k T-factorization approach and the QCD parton model for charm and beauty hadroproduction

We compare numerical predictions of the conventional QCD parton model and of the k _T-factorization approach (semihard theory) for heavy-quark production in high-energy hadron collisions. The total production cross sections and one-particle rapidity and p _T distributions, as well as two-particle correlations, are considered. The distinction between the predictions of the two approaches is not very large, while the shapes of the distributions are slightly different.     

Transverse momentum and angular distributions of hadrons in e+e− jets from QCD

Hadron jets in e⁺e^? annihilation will broaden at high energies due to gluon bremsstrahlung. With nonperturbative p_T effects dying out rapidly, the basic features of hadron jets can be calculated in perturbation theory. We examine the p_T distribution of secondarily produced hadrons. This is uniquely connected with the deviation from the 1 + cos²θ dependence of single particle inclusive distributions. We discuss what can be learned about the gluon fragmentation given the p_T and/or angular distributions. A sum rule is derived which establishes a relationship between the average p_T² and α_S.     

Analysis of transverse momentum distributions forpp interactions at 360 GeV/c in the framework of a quark-diquark fragmentation model

Using a quark-diquark fragmentation model, in which either the Field-Feynman or the Lund model is coupled with a quark-diquark distribution function, we study transverse momentum distributions,p _T, for the inclusive reactionspp→hadron +anything at 360 GeV/c. We find that a primordial mean transverse momentum 〈k _T〉?0.4 GeV/c can well reproduce thep _T ² distributions of charged hadrons, π⁰,K _s ⁰ , Λ⁰,K ^* and Σ^* and the Feynmanx?p _T correlations. We confirm that a diquark in a proton plays an important rôle in reproducing thex?p _T correlation of Λ⁰.     

Dual topology in large-pT reactions

We investigate a dual-string approach to the study of jets in large-p_T proton-proton collisions. The model is discussed for the dominant planar topology of valence quark-valence quark scattering where final hadrons may be viewed as fragments of two strings stretched between quarks and forward- or backward-moving diquarks. We assume that in the string c.m. frame, its fragments form a jet analogous to that observed in e⁺e^?→hadrons. Going back to the overall c.m. frame we recover the four-jet structure of large-p_T reactions. We discuss away side multiplicity distributions and show that the string approach which unifies the treatment of large-p_T and background secodaries, is equivalent to the naïve approach when the secondary momentum is large enough compared to its mass. Specific effects emerge for small enough momentum secondaries which result in widening of multiplicity rapidity distributions. Comparison with ISR data shows that this improves the agreement with experiment. Non-dominant contributions are also briefly considered.     

Largep T photoproduction and tests of QCD

A complete investigation of largep _T hadron photoproduction is carried out aiming to isolate and study QCD predictions concerning both direct coupling and anomalous photon components. Comparison with experimental data shows that these components may be disentangled from a large hadron-like background, provided cross-section differences or symmetric pair correlations are measured. The production of jets is also examined with emphasis on gluon jets.     

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.     

Probabilistic quark-model approach to proton fragmentation

A probabilistic approach is formulated to study the behaviour of the incident hadron quark flavours in the fragmentation process of high energy protons in lowp _T reactions. Analysis of available data onpp collisions, mainly on hyperon and antibaryon multiplicities, leads to estimates of the probabilities for the different ways in which the incident valence quarks recombine into final hadrons. We find that all three incident quarks emerge in one and the same outgoing nucleon (or nucleon resonance) with probabilityA ₃=0.35–0.4, that two of them emerge in one baryon and the third in another hadron (mostly a meson or meson resonance) with probabilityA ₂=0.6–0.5, and that they emerge in three distinct hadrons (mostly mesons or meson resonances) with probabilityA ₁=0.05–0.1. We find good support for a very simple probabilistic picture of the fragmentation process.     

Kaon and pion ratio probes of jet quenching in nuclear collisions

Non-Abelian energy loss in quark gluon plasmas is shown to lead to novel hadron ratio suppression patterns in ultrarelativistic nuclear collisions. Here we investigate pion and kaon production in pp and AA collisions in a perturbative QCD frame, suppression pattern and hadron ratios. The K^?/K⁺ and K⁺/π⁺ ratios are found to be most sensitive to the opacity (density) of the plasma. Experimental data indicate that the fragmentation dominated pQCD region will be reached only at higher p _T; in an intermediate p _Tregion other particle production mechanisms dominate the K/π ratios.     

On the transverse energy distributions in the central rapidity region of 16O+Pb collisions at 200 GeV per nucleon

We point out that the recent data obtained by the NA35 collaboration at the CERN SPS on E_T distributions in ¹⁶O + Pb collisions can be understood within the wounded nucleon model with a simple geometrical calculation of the number of interacting nucleons in ¹⁶O. Although the large E_T trigger increases the average E_T per final state hadron, we are unable to explain in this way the observed rather large value of this quantity.     

Polarized coincidence electroproduction scaling laws for the final state

We study the inclusive electroproduction of single hadrons off a polarized target. Bjorken scaling laws and the hadron azimuthal distribution are derived from the quark parton model.The polarization asymmetries scale when the target spin is along the direction of the virtual photon, and (apart from one significant exception) vanish for transverse spin. These results have a simple explanation; emphasis is given both to the general mathematical formalism and to intuitive physical reasoning.Through this framework we consider other cases: quarks with anomalous magnetic moment; renormalization group effects and asymptotic freedom; production of vector mesons (whose spin state is analysed by their decay); relation to large transverse momentum hadron production; and a covariant parton model calculation. We also look into spin-0 partons and Regge singularities.All of these cases (apart from the last two) modify the pattern of conclusions. Vector meson production shows polarization enhancements in the density matrix element ?₀₊; the renormalization group approach does not lead to any significant suppressions. They are also less severe in parton models for large p_T hadrons, and are not supported by the covariantly formulated calculation. The origins of these differences are isolated and used to exemplify the sensitivity that polarized hadron electroproduction has to delicate detail that is otherwise concealed.     

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.     

Measurement of the hadron structure function in hadron-hadron interactions

Previously published data on low-p _T π⁺/K ⁺/p?p interactions at 250 GeV/c are used to analyze the rapidity charge distribution and hadron structure function for the projectile hadrons. It is shown that the rapidity charge distribution for projectile hadrons can be approximated by a Gauss distribution, and their structure functions are found.     

The transverse momentum of partons in large pT processes

An approximate method is used to investigate the effects of parton transverse momentum in large p_T particle production within the framework of hard scattering models. We derive an approximate expression for the mean bias towards the trigger of each of the two participating partons and find that event one of the partons is biased more than the other, even with a 90° trigger. We treat the transverse momentum of partons and their closely related off-mass-shell behaviour as a perturbation in the equation for the single-particle inclusive cross section, and then expand in a Taylor series. We calculate the first non-zero correction term and find that to this order, the cross section is increased by parton transverse momentum effects by typically a factor of 1.7 for p_T = 3 GeV/c with 〈k_T〉 = 500 MeV/c, or 1.25 for p_T = 3 GeV/c with 〈k_T〉 = 300 MeV/c, and that the correction decreases rapidly with increasing p_T.