Direct probes of linearly polarized gluons inside unpolarized hadrons

We show that linearly polarized gluons inside unpolarized hadrons can be directly probed in jet or heavy quark pair production in electron-hadron collisions. We discuss the simplest cos2? asymmetries and estimate their maximal value, concluding that measurements of the unknown linearly polarized gluon distribution in the proton should be feasible in future Electron-Ion Collider or Large Hadron electron Collider experiments. Analogous asymmetries in hadron-hadron collisions suffer from factorization breaking contributions and would allow us to quantify the importance of initial- and final-state interactions.     

Transverse spin effects in diffractive hadron leptoproduction

We consider double spin asymmetries for longitudinally polarized leptons and transversely polarized protons in diffractive vector meson and production at high energies within the two-gluon model. The connection of the two-gluon approach with skewed gluon distributions is discussed. The asymmetry predicted for meson production is quite small. The asymmetry for production contains two independent terms which are large and can be used to obtain information on the polarized skewed gluon distributions in the proton. Received: 19 February 2002 / Revised version: 19 March 2002 / Published online: 22 May 2002     

Twist-3 single-spin asymmetries in semi-inclusive deep-inelastic scattering

The single-spin asymmetries for a longitudinally polarized lepton beam or a longitudinally polarized nucleon target in semi-inclusive deep-inelastic scattering are twist-3 observables. We study these asymmetries in a simple diquark spectator model of the nucleon. Analogous to the case of transverse target polarization, non-vanishing asymmetries are generated by gluon exchange between the struck quark and the target system. It is pointed out that the coupling of the virtual photon to the diquark is needed in order to preserve electromagnetic gauge invariance at the twist-3 level. The calculation indicates that previous analyses of these observables are incomplete.     

Longitudinal double-spin asymmetry for inclusive jet production in p[over -->] + p[over -->] collisions at sqrt[s]=200 GeV

We report a new STAR measurement of the longitudinal double-spin asymmetry A(LL) for inclusive jet production at midrapidity in polarized p + p collisions at a center-of-mass energy of sqrt[s]=200 GeV. The data, which cover jet transverse momenta 5相似文献   

Spin dependent gluon densities from largeP T photon and dimuon production

We investigate spin asymmetries in highp _T photon and dimuon production and show that the spin dependent gluon density can be extracted reliably frompp collisions with these final states. The statement remains true even when the balancing jet is not observed. Hence fixed target experiments can be used to measure the spin dependent gluon density. We also compute the asymmetries in \(p\bar p\) collisions, and show that the EMC measurement of the spin structure function can be verified and the spin dependent gluon density can be extracted from these experiments.     

Angular asymmetries and final state polarization effects ine^ - e^ + \to \gamma , {\rm Z}^0 \to q\bar qg

We discuss the forward-backward asymmetry and the polarization (mean helicity) of the quark, antiquark and gluon ine ^? e ⁺→γ, \(e^ - e^ + \to \gamma , {\rm Z}^0 \to q\bar qg\) , and of the inclusively produced hadrons (baryons or vector mesons) belonging to the most energetic jet. In the energy range of present interest we find substantial asymmetries (up to 30%) and polarizations (up to 15%), with partons (predominantly antiquarks) of positive helicity emitted preferentially in the forward direction.     

Jet production in polarized proton proton scattering

We propose a model of polarized partonic densities based on Regge behaviour, quark counting rules,U(1) _A Goldberger-Treiman relation and positivity constraints. The resulting polarized proton structure function is found to be in agreement with EMC and SLAC data. From this input we compute double helicity asymmetries for 1 and 2 jet inclusive production in polarized proton proton scattering at high energies. We obtain asymmetries of 10–20% at energies \(\sqrt s = 50 - 500\) GeV.     

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.     

Final-state interactions and single-spin asymmetries in semi-inclusive deep inelastic scattering

Recent measurements from the HERMES and SMC Collaborations show a remarkably large azimuthal single-spin asymmetries A_UL and A_UT of the proton in semi-inclusive pion leptoproduction γ^*(q)p→πX. We show that final-state interactions from gluon exchange between the outgoing quark and the target spectator system lead to single-spin asymmetries in deep inelastic lepton–proton scattering at leading twist in perturbative QCD; i.e., the rescattering corrections are not power-law suppressed at large photon virtuality Q² at fixed x_bj. The existence of such single-spin asymmetries requires a phase difference between two amplitudes coupling the proton target with J^z_p=±1/2 to the same final-state, the same amplitudes which are necessary to produce a nonzero proton anomalous magnetic moment. We show that the exchange of gauge particles between the outgoing quark and the proton spectators produces a Coulomb-like complex phase which depends on the angular momentum L^z of the proton's constituents and is thus distinct for different proton spin amplitudes. The single-spin asymmetry which arises from such final-state interactions does not factorize into a product of distribution function and fragmentation function, and it is not related to the transversity distribution δq(x,Q) which correlates transversely polarized quarks with the spin of the transversely polarized target nucleon.     

A new forward spectrometer for PHENIX

In the last decade PHENIX has accessed via longitudinal double spin asymmetries the gluon helicity inside the proton, started probing flavor dependent valence and sea quark helicity distributions and started to explore the proton transverse spin structure. A major upgrade is planned for the next decade that will provide an increased acceptance and additional tracking and electromagnetic plus hadronic calorimetry in the forward region. Here, the layout of the new forward detector system will be discussed, together with the new insights in the proton spin structure that the new forward PHENIX spectrometer will provide.     

A test for the gluon selfcoupling in the reactionse + e −→4 jets andZ 0→4 jets

We consider the reactionse ⁺ e ^?→γ^*→4 jets andZ ^o→4 jets with the 4 jets coming in two pairs of essentially back to back jets of high and low energy. We calculate the angular distribution of the low energy jet axis with respect to the high energy jet axis in QCD, in an abelian gluon model “QED” and a phase space model (PS). Using simple helicity arguments we show that our angular distribution is very sensitive to the triple gluon coupling in QCD. This is then confirmed by a complete calculation. Our correlation offers, therefore, a direct test for QCD as a non-abelian gauge theory.     

Exclusive photoproduction of hard dijets and magnetic susceptibility of the QCD vacuum

We argue that coherent production of hard dijets by linearly polarized real photons can provide direct evidence for chirality violation in hard processes, the first measurement of the magnetic susceptibility of the quark condensate and the photon distribution amplitude. It can also serve as a sensitive probe of the generalized gluon parton distribution. Numerical calculations are presented for HERA kinematics.     

Virtual photon structure

The structure functions of a real photon are calculable in QCD. The leading contribution is proportional to 1n Q², with a coefficient reflecting the gluon flux in a real photon. We investigate this leading term for non-zero target photon mass. In an appropriate limit the gluon content in a virtual photon is found to vanish. The gluon radiative corrections of QCD can thus be turned off by tuning the target photon mass.     

Proposal for direct measurement of a pure spin current by a polarized light beam

The photon helicity may be mapped to a spin-1/2, whereby we put forward an intrinsic interaction between a polarized light beam as a "photon spin current" and a pure spin current in a semiconductor, which arises from the spin-orbit coupling in valence bands as a pure relativity effect without involving the Rashba or the Dresselhaus effect due to inversion asymmetries. The interaction leads to linear and circular optical birefringence, which are similar to the Voigt effect and the Faraday rotation in magneto-optics but nevertheless involve no net magnetization. The birefringence effects provide a direct, nondemolition measurement of pure spin currents.     

The vacuum structure in QCD and hadron-hadron scattering

Standard ideas on the structure of the vacuum in QCD suggest it to be full of fluctuating color fields. We investigate the possible influence of the chromomagnetic vacuum fields on high energy hadron-hadron reactions. We suggest that high energy quarks traversing these fields will produce soft gluon and photon radiation analogous to synchrotron radiation from electrons and positrons in a storage ring. We argue that this radiation will lead to polarization phenomena for quarks in spin- and colorspace which in turn can explain theK-factor in the Drell-Yan reaction. We point out that jet production offers another way to study these polarization phenomena. We present then a calculation of the number of “synchrotron” photons which should be emitted inp?p collisions at high energies. Thus, we predict a sizable signal of prompt photons of nnergy less than a few hundred MeV with a characteristic frequency distribution. Observation of such photons would give strong support to our naive picture. Finally we point out a number of other phenomena like charmed particle decays where our “synchrotron” effect may be of importance.     

Spin asymmetries for large-pt jet production in two-photon processes

Relying on the hard scattering formalism we estimate cross sections and spin asymmetries for large-p_t jet production from two-photon processes in collisions of electrons and positrons of definite helicity states. We use distribution functions of polarized partons in polarized electrons obtained in a modified leading logarithmic approximation as well as spin-dependent parton cross sections according to lowest-order QCD calculations. Detecting at least one forward or backward hadronic jet, topologies of jets can be observed different from the 2 large p_t production.     

Mapping the proton unintegrated gluon distribution in dijets correlations in real and virtual photoproduction at HERA

We discuss how the dijet azimuthal correlations in DIS and real photoproduction at HERA probe the differential (unintegrated) gluon distribution in the proton. We find a strong dependence of the azimuthal correlation pattern on Bjorken-x, photon virtuality and the cut on the jet transverse momenta. A rise of the azimuthal decorrelations is observed with decreasing Bjorken-x due to the interplay of perturbative and nonperturbative effects. We predict a strong rise of the same-side jet rate with photon energy for real photoproduction. We discuss conditions for the correlation function to be dominated by hard perturbative gluons and ways of constraining the size of the nonperturbative soft component. We make some predictions for the THERA energy range. The analysis of the energy dependence of the isolated jet and two-jet cross sections in photoproduction would be a new way to study the not yet well constrained unintegrated gluon distributions and to explore the onset of the pQCD regime.     

Signatures of thes-leptons andW,Z spins in polarized γe collisions

We study the gaugino +s-electron,Wv andZe photoproductions off electrons with both beams polarized. We make a systematic calculation of the spin and angular asymmetries and obtain model independent results, which allow to identify the spin and/or charge of the scalars-leptons and of the electroweak gauge bosons, either in the high energy limit or at the threshold. For example, the photon helicity asymmetries of theW andZ total cross sections have the distinct value 1 at threshold, but it is negative for the reactions γe→spinor + scalar. In the high energy limit, the same asymmetry for the differential cross sections is equal to one for spin zero boson and negative forW andZ photoproductions and Compton scattering.     

Measurement of the proton spin polarizabilities at MAMI

The spin polarizabilities of the nucleon are fundamental structure constants which describe the response of the nucleon spin to an incident polarized photon. The most model-independent way to measure the nucleon spin polarizabilities is the Compton scattering with polarization degrees of freedom. Three Compton scattering asymmetries on the proton were measured in the Δ(1232) region using a polarized incident photon beam and a polarized (or unpolarized) proton target at the Mainz Microtron (MAMI). These asymmetries are sensitive to values of the spin polarizabilities. Fits to asymmetry data were performed using a dispersion model calculation, and a separation of all four proton spin-polarizabilities in the multipole basis was achieved. The values of the proton spin polarizabilities are presented.     

Azimuthal and spin asymmetries in DIS

In these lectures I want to discuss how the structure functions in deep inelastic scattering relate to quark and gluon correlation functions. In particular we will consider the issue of intrinsic transverse momenta of quarks, which becomes important in processes like 1-particle inclusive leptoproduction. Some examples of cross sections and asymmetries, in particular in polarized scattering processes are discussed. We also discuss the operator structure for azimuthal asymmetries and their evolution.