Polarized electron-proton scattering at highQ 2

We show that polarized electron-proton scattering is an excellent tool to measure helicity changing amplitudes. The asymmetry of polarized protons measured with longitudinally polarized electrons determines the ratio of Pauli (F ²) to Dirac (F ¹) form factors. For the leading Fock state the Pauli form factor originates only from helicity changing quarkgluon interactions which are zero for zero quark masses. Therefore at high momentum transfer the ratioF ²/F ¹ depends essentially on the up and down quark masses.     

Top quark polarization in polarized e+e− annihilation near threshold

Top quark polarization in e⁺e^{t -} annihilation into tt? is calculated for linearly polarized beams. The Green function formalism is applied to this reaction near threshold. The Lippmann—Schwinger equations for the S-wave and P-wave Green functions are solved numerically for the QCD chromostatic potential given by the two-loop formula for large momentum transfer and Richardson’s ansatz for intermediate and small momenta. S- P— wave interference contributes to all components of the top quark polarization vector. Rescattering of the decay products is considered. The mean values 〈nl〉 of the charged lepton four-momentum projections on appropriately chosen directions n in semileptonic top decays are proposed as experimentally observable quantities sensitive to top quark polarization. The results for 〈nl〉 are obtained including S- P— wave interference and rescattering of the decay products. It is demonstrated that for the longitudinally polarized electron beam a highly polarized sample of top quarks can be produced.     

Results from the forward G0 experiment

The G0 experiment is dedicated to the determination of the strange quark contribution to the electric and magnetic nucleon form factors for a large range of momentum transfers between 0.1 to 1(GeV/c)² . This information is provided by the asymmetries of cross-sections measured with longitudinally polarized electrons in elastic electron-proton scattering and quasi-elastic electron-deuteron scattering. A set of measurements at two different Q² will allow the complete separation of the electric and magnetic weak, as well as axial nucleon form factors. This report summarizes the physics case, gives details about the dedicated set-up used, and shows the results of the combination of the strange quark contribution in the electric and magnetic form factors of the protons. The experiment was performed at the Jefferson Laboratory, during years 2003 and 2004, and will be completed after backward-angle measurements in 2006, 2007.     

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     

Measurement of the polarized parton distribution functions at HERMES

The HERMES experiment at DESY has measured the inclusive and semi-inclusive double-spin asymmetries of polarized positrons scattering from polarized hydrogen and deuterium targets in the kinematic range of Bjørken-x 0.023 < x < 0.06 and 1 GeV ² < Q ² < 10 GeV². A RICH detector was installed for the deuterium running period and by providing the identification of charged pions and kaons has enabled the first measurement of charged kaon asymmetries. Based on the measured proton and deuterium asymmetries the polarized quark distributions have been extracted in leading-order pQCD.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.60.Hb Total and inclusive cross-sections (including deep-inelastic processes) - 13.88. + e Polarization in interactions and scatteringJ. Stewart: For the HERMES Collaboration     

The NLO Parton Distribution in the (x, Q 2)-Plane: A Relativistic Quark-Exchange Approach to A = 3 Mirror Nuclei

A next-to-leading order QCD analysis of polarized and unpolarized structure functions of the proton in the (x, Q²)-plane is discussed within the scheme of the radiation parton formalism. The valence quark distribution is obtained from the application of the relativistic quark-exchange model to A = 3 mirror nuclei, i.e., ³He and ³H. The sea quark and gluon distributions are calculated using the inverse Mellin technique in the NLO approximation. A comparison is made with the corresponding available experimental data. We find a good fit for F₂^p(x, Q²) to the data. It is shown that our new NLO calculation improves our previous works. We argue that the valence quark scenario at some μ₀² ≪ Q² is a reasonable assumption in the framework of the DGLAP evolution equation. In agreement with the data, it is demonstrated that the asymmetry A₁^p(x, Q²) has no significant Q²-dependence as we go to the small x, even at NLO limit. Finally we argue that for small x ≤ 0.2 it is a good approximation to consider ³He and ³H structure functions as those of neutrons and protons, respectively.     

Heavy Quark Hadron Mass Scale

Without the spin interactions the hadron masses within a multiplet are degenerate. The light quark hadron degenerate multiplet mass spectrum is extended from the 3 quark ground state multiplets at J^P = 0⁻, ½⁺, 1⁻ to include the excited states which follow the spinorial decomposition of SU(2) × SU(2). The mass scales for the 4, 5, 6,… quark hadrons are obtained from the degenerate multiplet mass m₀/M = n²/α with n = 4, 5, 6,… The 4, 5, 6,… quark hadron degenerate multiplet masses follow by splitting of the heavy quark mass scales according to the spinorial decomposition of SU(2) × SU(2).     

Polarized electron-nucleon scattering at TESLA

Measurements of polarized electron-nucleon scattering can be realized at the TESLA linear collider facility with projected luminosities that are about two orders of magnitude higher than those expected of other experiments at comparable energies. Longitudinally polarized electrons, accelerated as a small fraction of the total current in the e⁺ arm of TESLA, can be directed onto a solid state target that may be either longitudinally or transversely polarized. A large variety of polarized parton distribution and fragmentation functions can be determined with unprecedented accuracy, many of them for the first time. A main goal of the experiment is the precise measurement of the x- and Q²-dependence of the unknown transversity distributions that will provide us with the full information on the nucleon's quark spin structure as relevant for high energy processes. Comparing their Q²-evolution to that of the corresponding helicity distributions constitutes an important precision test of the predictive power of QCD in the spin sector. The additional possibilities of using unpolarized targets and of experiments with a real photon beam turn TESLA-N into a versatile next-generation facility at the intersection of particle and nuclear physics.     

Polarized quark distributions in bound nucleon and polarized EMC effect in Thermodynamical Bag Model

The polarized parton distribution functions (PDFs) and nuclear structure functions are evaluated by the phenomenological Thermodynamical Bag Model for nuclear media ⁷Li and ²⁷Al. The Fermi statistical distribution function which includes the spin degree of freedom is used in this statistical model. We predict a sizeable polarized EMC effect. The results of quark spin sum and axial coupling constant of bound nucleons are compared with theoretical predictions of modified Nambu–Jona-Lasinio (NJL) model by Bentz et al.     

Imaging the Transverse (Spin) Structure of Hadrons

Parton distributions in impact parameter space, which are obtained by Fourier transforming GPDs, exhibit a significant deviation from axial symmetry when the target and/or quark are transversely polarized. Connections between this deformation and transverse single-spin asymmetries as well as with quark–gluon correlations are discussed. The sign of transverse deformation of impact parameter dependent parton distributions in a transversely polarized target can be related to the sign of the contribution from that quark flavor to the nucleon anomalous magnetic moment. Therefore, the signs of the Sivers function for u and d quarks, as well as the signs of quark–gluon correlations embodied in the polarized structure function g ₂ can be understood in terms of the proton and neutron anomalous magnetic moments.     

The E166 experiment: Development of an undulator-based polarized positron source for the international linear collider

A longitudinal polarized positron beam is foreseen for the international linear collider (ILC). A proof-of-principle experiment has been performed in the final focus test beam at SLAC to demonstrate the production of polarized positrons for implementation at the ILC. The E166 experiment uses a 1 m long helical undulator in a 46.6 GeV electron beam to produce a few MeV photons with a high degree of circular polarization. These photons are then converted in a thin target to generate longitudinally polarized e ⁺ and e ⁻. The positron polarization is measured using a Compton transmission polarimeter. The data analysis has shown asymmetries in the expected vicinity of 3.4% and ∼1% for photons and positrons respectively and the expected positron longitudinal polarization is covering a range from 50% to 90%.      

Quark matter formation in dense stellar objects

It is expected that at very large densities and/or temperatures a quark-hadron phase transition takes place. Lattice QCD calculations at zero baryon density indicate that the transition occurs at T _c ∼ 150–170 MeV. The transition is likely to be second order or a cross over phenomenon. Although not much is known about the density at which the phase transition takes place at small temperatures, it is expected to occur around the nuclear densities of few times nuclear matter density. Also, there is a strong reason to believe that the quark matter formed after the phase transition is in colour superconducting phase. The matter densities in the interior of neutron stars being larger than the nuclear matter density, the neutron star cores may possibly consist of quark matter which may be formed during the collapse of supernova. Starting with the assumption that the quark matter, when formed consists of predominantly u and d quarks, we consider the evolution of s quarks by weak interactions in the present work. The reaction rates and time required to reach the chemical equilibrium are computed here. Our calculations show that the chemical equilibrium is reached in about 10⁻⁷ seconds. Further more during the equilibration process enormous amont of energy is released and copious numbers of neutrinos are produced. Implications of these on the evolution of supernovae will be discussed.     

Pion fluctuation in deep inelastic scattering

The forward neutron production in the ep collisions at 300GeV measured by H1 and ZEUS Collaborations at DESY has been used to estimate the total probability for proton fluctuation into nπ⁺ and pπ⁰. The probability found is on the order of 30%. This number is compared with the numbers obtained for the probability of quark fluctuation into π⁺ from several alternative DIS processes (Gottfried sum rule, polarized structure function) and the axial-vector coupling constant, where the pion fluctuation is believed to play an important role.     

$e^{+}e^{-}\to t\bar{t}H$ including decays: on the size of background contributions

We present results for the lowest order cross sections, calculated with the complete set of standard model Feynman diagrams, of all possible detection channels of the associated production of the top quark pair and the light Higgs boson, which may be used for determination of the top–Higgs Yukawa coupling at the future e ⁺ e ⁻ linear collider. We show that, for typical particle identification cuts, the background contributions are large. In particular, the QCD background contributions are much bigger than could be expected when taking into account a possibly low virtuality of exchanged gluons. Moreover, we include the initial state radiation effects and discuss the dependence of the cross sections on the Higgs boson and top quark masses.     

Radiative energy loss of high-energy quarks in finite-size nuclear matter and quark-gluon plasma

The induced gluon radiation of a high-energy quark in a finite-size QCD medium is studied. For a sufficiently energetic quark produced inside a medium we find the radiative energy loss ΔE _q∝L ², where L is the distance traveled by quark in the medium. It has a weak dependence on the initial quark energy E _q. The L ² dependence turns to L ¹ as the quark energy decreases. Numerical calculations are performed for a cold nuclear matter and a hot quark-gluon plasma. For a quark incident on a nucleus we predict ΔE _q ≈0.1E _q (L/10fm) ^β , with β close to unity. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 585–589 (25 April 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Spin physics at HERMES

The primary goal of the HERMES experiment is the study of the spin structure of the nucleon. Results on the measured inclusive and semi-inclusive hadron asymmetries using a polarized positron beam on polarized ³He, hydrogen and deuterium targets are here presented. In the covered kinematic range, 0.023<x _Bj <0.6 and 1 GeV²<Q ²<10GeV² the polarized quark distribution were determined for all up (u+?u) and down (d+?) quarks, and separately for valence and sea quarks. The up quark polarization is positive, and the down quark polarization is negative. The polarization of the sea is consistent with zero in the measured range. A first indication of a positive gluon polarization is presented, based on the measured spin asymmetry in the photo-production of hadron pairs with high transverse momentum p _T . This asymmetry is negative, which is in contrast to the measured positive asymmetry for inclusive experiments.     

Spin physics and more with HERMES

A review of recent results obtained by the HERMES experiment is given. Inclusive measurements on polarized and unpolarized targets provide precise information on the polarized structure functions g₁^d and g₁^p and the isoscalar unpolarized structure function ratio F₂^A/F₂^d. The geometrical acceptance of the HERMES detector and the good particle identification capabilities allow the study of semi-inclusive reactions as well. Using polarized targets, the polarized quark distribution functions can be extracted as well as a first indication of the transversity distributions . On unpolarized heavy targets, hadron formation in a nuclear environment is studied. Finally, data on Deeply Virtual Compton Scattering, the hard exclusive electroproduction of real photons, are presented, which are closely linked to the novel framework of Generalised Parton Distributions. For this reaction HERMES has measured the asymmetry main beam charge as well as in-beam spin.Received: 1 November 2002, Published online: 15 July 2003PACS: 13.60.Hb Total and inclusive cross-sections (including deep-inelastic processes) - 13.60.Le Meson production - 24.85.+p Quarks, gluons, and QCD in nuclei and nuclear processes - 25.30.Mr Muon scattering (including the EMC effect)B. Seitz: For the HERMES Collaboration     

Understanding the proton spin “puzzle” with a new “minimal” quark model

We investigate the spin structure of the nucleon in an extended Jaffe-Lipkin quark model. In addition to the conventional 3q structure, different (3q)(Q ) admixtures in the nucleon wave function are also taken into account. The contributions to the nucleon spin from various components of the nucleon wave function are discussed. The effect due to the Melosh-Wigner rotation is also studied. It is shown that the Jaffe-Lipkin term is only important when antiquarks are negatively polarized. We arrive at a new “minimal” quark model, which is close to the naive quark model, in order to understand the proton spin “puzzle”. Received: 4 November 2000 / Accepted: 23 October 2001     

Target mass correction on the 3He polarized structure function

Target mass correction (TMC) is employed to amend the polarized helium structure functions, ³He . The structure function can be obtained via the convolution of the light cone momentum distribution with the polarized structure of the proton and neutron. The calculation of the polarized structure function of the nucleon is based on the constituent quark model. The analytical result for ³He polarized structure function at low values of Q² is not in good agreement with the available experimental data. The reliability of calculations is increased using TMC effect. New comparison confirms a better agreement with the experimental data.