Flavor asymmetry of polarized antiquark distributions and semi-inclusive DIS

The –expansion of QCD suggests large flavor asymmetries of the polarized antiquark distributions in the nucleon. This is confirmed by model calculations in the large– limit (chiral quark–soliton model), which give sizable results for and . We compute the contributions of these flavor asymmetries to the spin asymmetries in hadron production in semi-inclusive deep–inelastic scattering. We show that the large flavor asymmetries predicted by the chiral quark–soliton model are consistent with the recent HERMES data for spin asymmetries in charged hadron production. Received: 30 November 1999 / Published online: 17 March 2000     

Polarized antiquark flavor asymmetry in Drell–Yan pair production

We investigate the role of the flavor asymmetry of the nucleon's polarized antiquark distributions in Drell–Yan lepton pair production in polarized nucleon–nucleon collisions at HERA (fixed–target) and RHIC energies. It is shown that the large polarized antiquark flavor asymmetry predicted by model calculations in the large– limit (chiral quark–soliton model) has a dramatic effect on the double spin asymmetries in high mass lepton pair production, as well as on the single spin asymmetries in lepton pair production through –bosons at . Received: 31 May 2000 / Revised version: 1 December 2000 / Published online: 5 February 2001     

New target transverse spin dependent azimuthal asymmetries from COMPASS experiment

In general, eight target transverse spin-dependent azimuthal modulations are allowed in semi inclusive deep inelastic scattering of polarized leptons on a transversely polarized target. In the QCD parton model four of these asymmetries can be interpreted within the leading order approach. Two of them, namely Collins and Sivers effects were already measured by HERMES and COMPASS experiments. Other two leading twist and remaining four azimuthal asymmetries which can be interpreted as twist-three contributions have been measured for the first time in COMPASS using a 160 GeV/c longitudinally polarized (P _l }~ −0.8) muon beam and a transversely polarized ⁶LiD target. We present here the preliminary results from the 2002–2004 data. on behalf of the COMPASS collaboration     

g_1 at low x and low Q^2 with Polarized ep colliders

Measurements of at low x and low are expected to provide a sensitive probe of the transition from Regge to perturbative QCD dynamics, offering a new testing ground for models of small x physics. We discuss the potential of polarized ep colliders (Polarized HERA and eRHIC) to investigate this physics – varying between 0.01 and 1 GeV– and to constrain the high-energy part of the Drell-Hearn-Gerasimov sum-rule for polarized photoproduction. Received: 28 August 2000 / Revised version: 9 November 2000 / Published online: 21 December 2000     

Perturbative QCD and weak asymmetries in lepton pair production

The interference of electromagnetic and weak production mechanisms for lepton pair production may give rise to several effects which violate parity and charge symmetry. These effects are generally of the order of 1% for dilepton masses of 10 GeV. The theoretical calculations presented here show that experimental studies of these asymmetries may be useful. In particular, measurements of these asymmetries in collisions of pions with polarized protons may lead to a greatly enhanced understanding of the polarization distribution of quarks in a polarized proton. The polarization structure of the d quark is shown to be of special interest. Measurement of the parity-violating asymmetries in proton-polarized proton collisions may prove to be a sensitive probe of the flavor symmetry of the proton antiquark sea. Analysis of the parity conserving charge asymmetry (which is predicted to occur in collisions of unpolarized hadrons) allows a unique further test of the Drell-Yan model for lepton pair production, as well as of our understanding of weak interactions.The above asymmetries mentioned are computed within the framework of the parton model. The focus of this work is on the asymmetries calculated with the inclusion of first-order perturbative QCD effects. The asymmetries are calculated in a differential form for values of the dilepton transverse momentum large compared with the typical (～1 GeV) scale of nonperturbative effects, and also in a form in which this transverse momentum has been integrated over. Corrections to the parton model results, which can be large, show rather different structure for the various asymmetries.A parity-violating asymmetry which may occur in collisions of unpolarized hadrons is also discussed. This asymmetry is very sensitive to the nonperturbative structure of hadrons, and is estimated to be approximately 0.01% for dilepton masses of 10 GeV.     

Scaling violation and shadowing corrections at HERA

We study the value of shadowing corrections (SC) in the HERA kinematic region in the Glauber–Mueller approach. Since the Glauber–Mueller approach was proven in perturbative quantum chromodynamics (QCD) in the double-logarithmic approximation (DLA), we develop the DLA approach for the deep inelastic structure function which takes into account the SC. Our estimates show small SC for in the HERA kinematic region while they turn out to be sizable for the gluon structure function. We compare our estimates with those for gluon distribution in leading order (LO) and next to leading order (NLO) in the DGLAP evolution equations. Received: 21 September 1998 /Published online: 14 January 1999     

IR-renormalon contributions to the structure functions g 3 and g 5

We calculate the leading 1/N _f perturbative contributions to the polarized nonsinglet structure functions g ₃ and g ₅ to all orders in α_s. The contributions from the first renormalon pole are determined. It is a measure for the ambiguity of the perturbative calculation and is hypothetically assumed to dominate the power corrections. The corrections Δg ₃ and Δg ₅ are given as functions of the Bjorken variable x and turn out to be negligible. The anomalous dimensions of the leading twist operators are obtained in the next-to-leading order. Received: 10 June 1998 / Revised version: 25 September 1998     

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. Received: 11 May 1999 / Revised version: 22 July 1999 / Published online: 3 November 1999     

Probing transverse quark polarization via azimuthal asymmetries in leptoproduction

We consider the leading order result for polarized leptoproduction, putting emphasis on transverse momentum dependent effects appearing in azimuthal asymmetries.Measurements of weighted cross sections enable extraction of the distribution of transversely polarized quarks. We focus on the distribution in a longitudinally polarized hadron and estimate the expected asymmetries in leptoproduction.     

Phenomenological determination of polarized quark distributions in the nucleon

We present a fit to spin asymmetries which gives polarized quark distributions. These functions are closely related to the ones given by the Martin, Roberts and Stirling fit for unpolarized structure functions. The integrals of polarized distributions are discussed and compared with the corresponding quantities obtained from neutron and hyperonβ-decay data. We use the combination of proton, neutron and deuteron spin asymmetries in order to determine the coefficients of our polarized quark distributions. Our fit shows that phenomenologically there is no need for taking polarized gluons into account. Work supported in part by the KBN-Grant 2-P302-143-06     

Computations of ¶in the 2-d O(N) Non-Linear σ-Model

We review the various computations of the ratio of the mass gap M to the Λ-parameter entering the perturbative computations of amplitudes at high energies in the O(n) non-linear σ-model. In particular we reproduce from original notes of H. Lehmann his computation of this ratio in the (next–to) leading order of the 1/n expansion. Received: 23 March 2000 / Accepted: 10 April 2000     

Identification of a horizontal gauge boson in hadronic collisions

We compare distributions of leptons produced in hadronic collisions, by a horizontal neutral gauge boson, suggested by the model, to those produced by other theoretically motivated neutral gauge bosons occurring in left–right symmetric models and in superstring-inspired models. Forward–backward asymmetries in the gauge boson leptonic decay are found to be sensitive to specific forms of the couplings. The asymmetries are expected to be maximal for distinguishing it from the other Z's. Received: 17 September 2001 / Published online: 7 December 2001     

The production of large transverse momentum jets in plarized photon-photon collisions

A comprehensive survey is presented of the polarization structure of the leading order QCD processes for the production of jets at largep _t in photon-photon collisions which are the result of colliding positrons. Results are given for asymmetries. It is shown that by separating events with various jet topologies it is possible to investigate different aspects of polarization and QCD. For instance the four anf three jet process asymmetries are found to be sensitive to the relative magnitudes of the perturbative anf non-perturbative components of the photon structure function. A discussion is given of the usefulness of polarization in separating the two gluon jet subprocess. It is concluded that such experiments could be very useful in understanding QCD and the strong interactions.     

Quark sea structure functions of the nucleon in a statistical model

Within a statistical model of linear confined quarks we obtain the flavor asymmetry and corresponding structure function of the nucleon. The model parameters are fixed by the experimental available data. The temperature parameter is adjusted by the Gottfried sum rule violation and the chemical potentials by the corresponding up (u) and down (d) quark normalizations in the nucleon. The light antiquark and quark distributions in the proton, given by d̄/ū, d/u and d̄-ū, as well as the neutron to proton ratio of the structure functions, extracted from the experimental data, are well fitted by the model. As the quark-confining strengths should be flavor dependent, a mechanism is introduced in the model to adjust the corresponding distribution, in order to improve the comparison obtained for the sea-quark asymmetries in the nucleon with the available experimental analysis. PACS  11.30.Hv; 14.20.Dh; 12.39.Ki; 12.40.Ee; 11.55.Hx     

Global analysis of helicity parton densities and their uncertainties

We present a new analysis of the helicity parton distributions of the nucleon. The analysis takes into account the available data from inclusive and semi-inclusive polarized deep inelastic scattering, as well as from polarized proton-proton (p-p) scattering at RHIC. For the first time, all theoretical calculations are performed fully at next-to-leading order (NLO) of perturbative QCD, using a method that allows incorporation of the NLO corrections in a very fast and efficient way in the analysis. We find evidence for a rather small gluon polarization in the nucleon, over a limited region of momentum fraction, and for interesting flavor patterns in the polarized sea.     

Reconstructing the chargino system at e^+e^- linear colliders

In most supersymmetric theories charginos, , belong to the class of the lightest supersymmetric particles. The chargino system can be reconstructed completely in collider experiments: . By measuring the total cross sections and the asymmetries with polarized beams, the chargino masses and the gaugino–higgsino mixing angles of these states can be determined accurately. If only the lightest charginos are kinematically accessible in a first phase of the machine, transverse beam polarization or the measurement of chargino polarization in the final state is needed to determine the mixing angles. From these observables the fundamental SUSY parameters can be derived: the SU(2) gaugino mass , the modulus and the cosine of the CP–violating phase of the higgsino mass parameter , and , the ratio of the vacuum expectation values of the two neutral Higgs doublet fields. The remaining two–fold ambiguity of the phase can be resolved by measuring the normal polarization of the charginos. Sum rules of the cross sections can be exploited to investigate the closure of the two–chargino system. Received: 2 February 2000 / Published online: 17 March 2000     

Phenomenological determination of polarized quark distributions in the nucleon

We present a fit to spin asymmetries which gives polarized quark distributions. These functions are closely related to the ones given by the Martin, Roberts and Stirling fit for unpolarized structure functions. The integrals of polarized distributions are discussed and compared with the corresponding quantities obtained from neutron and hyperon??-decay data. We use the combination of proton, neutron and deuteron spin asymmetries in order to determine the coefficients of our polarized quark distributions. Our fit shows that phenomenologically there is no need for taking polarized gluons into account.     

Spin-dependent structure functions of real and virtual photons

The implications of the positivity constraint, , on the presently unknown spin–dependent structure function of real and virtual photons are studied at scales where longitudinally polarized photons dominate physically relevant cross sections. In particular it is shown how to implement the physical constraints of positivity and continuity at in NLO calculations which afford a nontrivial choice of suitable (DIS) factorization schemes related to and and appropriate boundary conditions for the polarized parton distributions of real and virtual photons. The predictions of two extreme ‘maximal’ and ‘minimal’ saturation scenarios are presented and compared with results obtained within the framework of a simple quark ‘box’ calculation expected to yield reasonable estimates in the not too small regions of x and . Received: 16 March 2001 / Published online: 18 May 2001     

Jet–jet and hadron–jet correlations in hadro- and electroproduction

We discuss, in the framework of perturbative QCD at next to leading order, two related observables which are usually considered to provide tests of the BFKL dynamics: jet–jet correlations at Tevatron energies and forward particle–jet correlations at HERA. In the first case we study the rapidity gap dependence of the azimuthal correlations and find slightly too strong correlations at large gap. In the second case we discuss the cross section as well as the azimuthal correlations over a rapidity gap range of 5 units. We find that the requirement of a forward particle imposes strong kinematical constraints which distort the distributions, notably at small rapidity gaps. We also show that the decorrelation is stronger in electroproduction than in hadron–hadron collisions. Unfortunately data are not yet available for comparison.