Fragmentation Region Research Articles

Centrality dependence of baryon and meson momentum distributions in proton-nucleus collisions

The proton and neutron inclusive distributions in the projectile fragmentation region of $pA$ collisions are studied in the valon model. Momentum degradation and flavor changes due to the nuclear medium are described at the valon level using two parameters. Particle production is treated by means of the recombination subprocess. The centrality dependences of the net proton and neutron spectra of the NA49 data are satisfactorily reproduced. The effective degradation length is determined to be 17 fm. Pion inclusive distributions can be calculated without any adjustable parameters.

T-odd fragmentation functions

We discuss the properties of fragmentation functions arising from the interference of two leading hadrons produced inside the same jet in the current fragmentation region of a hard process. For the case of semi-inclusive Deep Inelastic Scattering (DIS), a proper folding of the cross section, integrated over the azimuthal position of the detected hadrons, produces a factorized form that allows for the extraction of the quark transversity distribution at leading twist. Using an extended spectator model, explicit calculations are shown for the hadron pair being two pions with invariant mass inside the $\rho$ resonance width.

Single-spin asymmetry in deeply virtual Compton scattering: Fragmentation region of polarized lepton

For the kinematical region when a hard photon is emitted predominantly close to the direction of motion of a longitudinally polarized initial electron and relatively small momentum transfer to a proton we calculate the azimuthal asymmetry of photon emission. It arises from the interference of the Bethe-Heitler amplitude and those which are described by a heavy photon impact factor. Azimuthal asymmetry does not decrease in the limit of infinite cms energy. Lowest order expression for the impact factor of a heavy photon is presented.

Hyperon polarization in semi-inclusive deeply inelastic lepton-nucleon scattering at high energy

We calculate the polarizations for different octet hyperons produced in the current fragmentation regions of the deeply inelastic lepton-nucleon scatterings $\mu^-N \to \mu^- HX$ and $\nu_{\mu} N \to \mu^- HX$ at high energy using different models for spin transfer in fragmentation processes. The results show that measurements of those hyperon polarizations should provide useful information to distinguish between different models in particular the SU(6) and the DIS pictures used frequently in the literature. We found, in particular, that measuring the polarization of $\Sigma^+$ produced in these processes can give a better test to the validity of the different spin transfer models.

Charged-particle pseudorapidity density distributions from Au+Au collisions at sqrt[s(NN)]=130 GeV.

The charged-particle pseudorapidity density dN(ch)/d eta has been measured for Au+Au collisions at sqrt[s(NN)] = 130 GeV at RHIC, using the PHOBOS apparatus. The total number of charged particles produced for the 3% most-central Au+Au collisions for /eta/<or=5.4 is found to be 4200+/-470. The evolution of dN(ch)/d eta with centrality is discussed, and compared to model calculations and to data from proton-induced collisions. The data show an enhancement in charged-particle production at midrapidity, while in the fragmentation regions, the results are consistent with expectations from pp and pA scattering.

Parton densities and fragmentation functions from polarized production in semi-inclusive DIS

We consider the longitudinal polarization of Lambda and Lambda-bar produced in the current fragmentation region of polarized deep inelastic scattering. We show how the various cross sections can be used to test the underlying parton dynamics, and how one can extract information about certain parton densities which are poorly known, in particular the polarized strange density sum Delta-s(x) + Delta-s-bar(x), and about fragmentation functions which are totally unknown and which are difficult to access by other means. We show also how one can obtain information concerning the intriguing question as to whether s(x) = s-bar(x) and whether Delta-s(x) = Delta-s-bar(x).

Quark combinatorics and meson-production ratios in hadronic Z 0 decays

A vast body of experimental data accumulated in the past few years in hadronic Z 0 decays allows one to check quark-combinatorics relations for a new type of processes-namely, quark jets in the decays Z 0 → \(q\bar q\) → hadrons. In this paper, we review quark-combinatorics rules for the yields of vector and pseudoscalar mesons, V/P, in the central and fragmentation regions of hadronic Z 0 decays. It is emphasized that, in the central region, a direct verification of quark-combinatorics rules is rather problematic because of a considerable background associated with the decay of highly excited resonances; however, such a verification is possible in the fragmentation region, at x hadron∼0.5–1, where the contribution of resonance decays is suppressed owing to a fast decrease of the spectra with increasing x hadron. It is shown that, in the fragmentation region, experimental data on ρ0/π0 and p/π+ are in reasonable agreement with the predictions of quark combinatorics. The ratios of the heavy-meson yields, B*/B and D*/D, are also discussed: the data demonstrate good agreement with the quark combinatorial results. We analyze the structure of the suppression parameters for strange-and heavy-quark production in soft processes and estimate their orders of magnitude in multiperipheral processes; the ratio K ±/π± at x hadron∼0.5–0.8 and the production probabilities for \(c\bar c\) and \(b\bar b\) mesons are in good agreement with the estimates.

CASTOR: Centauro and strange object research in nucleus-nucleus collisions at the LHC

We present a phenomenological model which describes the formation of a Centauro fireball in the baryon-rich projectile fragmentation region in nucleus-nucleus interactions in the upper atmosphere and at the LHC, and its decay to non-strange baryons and Strangelets. Strangelets are assimilated to the “strongly penetrating component” frequently observed accompanying hadron-rich cosmic ray events. We describe the CASTOR subdetector for the ALICE experiment at the LHC. CASTOR will probe, in an event-by-event mode, the very forward, baryon-rich phase space 5.6 ≤ η ≤ 7.2 in 5.5 × A TeV central Pb + Pb collisions. It will look for events with pronounced imbalance between hadronic and photonic content and for deeply penetrating objects. We present results of simulations for the response of the CASTOR calorimeter to the passage of Strangelets.

Measurement of the polarization in charged current interactions in the NOMAD experiment

The Λ polarization in ν μ charged current interactions has been measured in the NOMAD experiment. The event sample (8087 reconstructed Λ 's) is more than an order of magnitude larger than that of previous bubble chamber experiments, while the quality of event reconstruction is comparable. We observe negative polarization along the W -boson direction which is enhanced in the target fragmentation region: P x (x F <0)=−0.21±0.04 (stat) ±0.02 (sys) . In the current fragmentation region we find P x (x F >0)=−0.09±0.06 (stat) ±0.03 (sys) . These results provide a test of different models describing the nucleon spin composition and the spin transfer mechanisms. A significant transverse polarization (in the direction orthogonal to the Λ production plane) has been observed for the first time in a neutrino experiment: P y =−0.22±0.03 (stat) ±0.01 (sys) . The dependence of the absolute value of P y on the Λ transverse momentum with respect to the hadronic jet direction is in qualitative agreement with the results from unpolarized hadron–hadron experiments.

Two-hadron interference fragmentation functions. I. General framework

We investigate the properties of interference fragmentation functions that can be extracted from the distribution of two hadrons produced in the same jet in the current fragmentation region of a hard process. We discuss the azimuthal angular dependences in the leading order cross section of two-hadron inclusive lepton-nucleon scattering as an example of how these interference fragmentation functions can be addressed separately.

Gluon fragmentation into glueballs and hybrid mesons

The constituent nature of candidate gluonic mesons can be studied by comparing their production in quark and gluon jets. The production rate for such mesons depends on the colour confinement processes at the end of the perturbative evolution. Whereas we expect enhanced production of hybrids in the fragmentation region of a gluon jet, the rate for glueballs depends on the relative importance of colour triplet and colour octet neutralization. These neutralization processes can be studied independently in events with large rapidity gaps. If octet processes turn out important the recently suggested lightest J PC =0 ++ glueball with mass around 1000 MeV should become visible already in the spectrum of the leading charged particle pairs.

A new scaling law for analyzing power in hadron production by transversely polarized baryons

Experimental data on analyzing power for inclusive meson and baryon production in hadron-proton(polarized) collisions have been analyzed. It is found that the existing data can be described by a simple function of collision energy, transverse momentum (Pt) and a new scaling variable Xa = E/Ebeam. At beam energies above 40 GeV and Pt above 1 GeV/c the analyzing power is described by a function of Xa and Pt only (AN = F(Pt)G(Xa)) for both polarized proton fragmentation and central regions of proton-hadron collision. Comparison of data from Fermilab and new IHEP data measured using 40 GeV/c polarized proton beam was most decisive for the revelation of the above regularities. This new scaling law allows one to predict the analyzing powers for kinematic regions, not yet explored in experiments and constrains models of strong interactions. The new scaling law allows one also to use some reactions as polarimeters for experiments with a polarized beam.

Fracture functions in the very forward limit

This talk gives a brief discussion of extended fracture functions, which parametrise the non-perturbative physics in the target fragmentation region of semi-inclusive DIS. In the forward limit z → 1, it can be seen that fracture functions can be identified with insertions of composite operators. This enables polarised fracture functions to be used to test a target-independence hypothesis of the “proton spin effect”.

Does the efficiency of high energy collisions depend on a hard scale?

The multiplicity of charged hadrons in the current fragmentation region of both the c.m.s. and the Breit frame of deep inelastic scattering is calculated and compared with the HERA data. The results are in agreeement with Yang's hypothesis that the efficiency of high energy processes increases at larger momentum transfer.

Negative heat capacity in the critical region of nuclear fragmentation: an experimental evidence of the liquid-gas phase transition

An experimental indication of negative heat capacity in excited nuclear systems is inferred from the event by event study of energy fluctuations in Au quasi-projectile sources formed in Au+Au collisions at 35 A.MeV. Equilibrated events are selected and the excited source configuration is reconstructed through a calorimetric analysis of its de-excitation products. Fragment partitions show signs of a critical behavior at about 4.5 A.MeV excitation energy. Around this value the heat capacity shows a negative branch providing a direct evidence of a first order liquid gas phase transition.

Directed and elliptic flow in 158 AGeV Pb+Pb collisions

Directed and elliptic flow of protons and positively charged pions has been studied in the target fragmentation region using the Plastic Ball detector in the WA98 experiment. The results exhibit a strong dependence on centrality, rapidity, and transverse momentum. The rapidity dependence can be described by a Gaussian distribution. The model comparisons reveal a large discrepancy of the flow strength obtained from the data and the simulations.

Erratum: Proof of factorization for diffractive hard scattering [Phys. Rev. D 57, 3051 (1998)

A proof is given that hard-scattering factorization is valid for deep-inelastic processes which are diffractive or which have some other condition imposed on the final state in the target fragmentation region.

Single-spin asymmetries for small-angle pion production in high-energy hadron collisions

Within the framework of a simple model, we study single-spin asymmetries for pion production in hadron–hadron collisions at high energies with one hadron polarized. The asymmetries are generated via a mechanism of final- (initial-) state interactions. For peripheral kinematics, we find nonzero asymmetries at the high-energy limit when the pion belongs to the fragmentation region of the polarized proton. Numerical results and comparison with existing experimental data are presented. We also discuss the relationship with Odderon exchange phenomenology.

Strange particle production and polarization of Λ hyperons in the HERMES experiment

Semi-inclusive and inclusive reactions with Λ, Λ , and K s in the final states are studied in the HERMES experiment using unpolarized gas targets and the polarized 27.5 GeV positron beam of the HERA collider. Semi-inclusive DIS kinematics of Λ production in current fragmentation region show a good agreement with LEPTO based Monte Carlo calculations for the accepted by the HERMES spectrometer events. Longitudinal Λ polarization for the semi-inclusive Λ production is measured. The false asymmetry resulting from the spectrometer acceptance is excluded by analyzing two data sets with opposite beam polarizations. Polarization transfer coefficient from the knocked out quark to the Λ is found to be 0.04 ± 0.17( stat) ± 0.09( syst). In inclusive case transverse polarizations of Λ and Λ are measured. Averaged over the range of the transverse Λ, Λ momentum 0 < p t < 1.7 GeV/c it is found P n = 0.066 ± 0.011( stat) ± 0.025( syst) for Λ and P n = −0.066 ± 0.037( stat) ± 0.025( syst) for Λ .

Hard exclusive electroproduction of two pions and their resonances

We study the hard exclusive production of two pions in the virtual photon fragmentation region with various invariant masses including the resonance region. The amplitude is expressed in terms of two-pion light cone distribution amplitudes (2πDA's). We derive dispersion relations for these amplitudes, which enable us to fix them completely in terms of ππ scattering phases and a few low-energy subtraction constants determined by the effective chiral Lagrangian. Quantitative estimates of the resonance as well as ππ background DA's at low normalization point are made. We also prove a certain new soft pion theorem relating two-pion DA's to the one-pion DA. Crossing relations between 2πDA's and parton distributions in the pion are discussed. We demonstrate that by studying the shape of the ππ mass spectra (not the absoloute cross section!) in diffractive electroproduction one can extract the deviation of the meson (π, π, etc.) wave functions from their asymptotic form 6 z (1 - z) and hence get important information about the structure of mesons. Also we discuss how the moments of the quark distributions in the pion can be measured in the hard diffraction. We suggest (as an alternative to Söding's) a parametrization of ππ spectra which is suitable for large photon virtuality.