102 Annual Report 2008

Measurement of Parton Distributions of Strange Quarks from Charged-kaon Production in Deep-inelastic Scattering on the Deuteron* by W.Augustyniak, B.Maria Lski, A.Trzci Lski, P. aupra Lski [I.4]

Parton distribution functions (PDFs) form the basis to the kinematic requirements:Q 2#1GeV 2, for the description of the flavor structure of the W2#10GeV 2 and y $0.85, where -Q 2 is the squared nucleon. The spin-averaged parton distribution four momentum of the virtual photon, W is the functions q(x), of quarks and antiquarks of flavors invariant mass of the photon-nucleon system, and q=(u,d,s) describe the quark momentum distributions, y= */E with * being the photon energy and E the where x is the dimensionless Bjorken scaling variable positron beam energy. The Bjorken x range of representing the momentum fraction carried by the measurement was 0.02 "0.6. The spin averaged PDFs parton in a frame where the target has “infinite” for the strange quarks were determined from the shape longitudinal momentum. They are the sums of the and magnitude of the measured yield of charged kaons number densities of the quarks with the same and for the two beam-target polarization states. The results opposite helicity as that of the nucleon. The are presented in Fig. 1. differences or helicity distributions, denoted here by ! describe the flavor dependent contributions of the quark spins to the spin of the nucleon. The features of the parton distributions reflect the QCD dynamics of the constituents. Because strange quarks are objects which reflect directly properties of the nucleon sea, they are of special interest. Their distributions are also important because of their impact on quantitative calculations of certain key short-distance processes at hadron colliders, and their implications for the Fig. 1 The multiplicity (corrected for spectrometer acceptance) of charged kaons from a deuterium target, as a function of the Bjorken measurement of the Weinberg angle in deep-inelastic scaling variable x. The solid curve is the result of the HERMES scattering (DIS) of neutrinos. analysis. The dashed (dash-dotted) curve is the nonstrange (strange) Much of the information on properties of the quark contribution to the multiplicity for this fit. The dotted curve is the best fit obtained using previously assumed strange quark PDFs. helicity distribution of strange quarks is based on the The error bars are statistical. The band represents the systematic analysis of inclusive DIS and hyperon decay under the uncertainties. The values of (Q 2) for each x bin are shown in the assumption of SU(3) symmetry among the structures lower panel. of the octet baryons. Now for the first time the Hermes In the absence of experimental data for strange experiment has made a direct measurement of the quarks it has been customary to assume that the shape strange quark PDFs. To probe the flavor of of the strange quark PDFs is the average of that for the strangeness one detects, in coincidence with the up and down quarks. Attempts to fit the HERMES scattered lepton, charged K mesons which contain data using this assumption (see dotted curve in Fig. 1) strange and anti-strange quarks. do not provide a reasonable fit to the observed multiplicity curve. However, by taking the strange We report here on a new isoscalar extraction of quark PDF as an unknown, using known values for the s(x)+s "(x) and (s(x)+s "(x)) from polarized DIS on a up and down quark PDFs together with known values deuterium target. The data were recorded with a for the fragmentation functions which describe the longitudinally nuclear polarized gas target internal to conversion of the struck quark into the final kaon, it the E=27.6 GeV HERA positron storage ring at was possible to obtain a good fit to the data. The DESY. The self-induced beam polarization was improved fit (solid curve in Fig. 1) is an indication measured continuously by Compton back scattering of that the actual distribution is substantially different circularly polarized laser beams. The open-ended from those of the nonstrange quarks. target cell was fed by an atomic-beam source based on Stern-Gerlach separation with hyperfine transitions. The differences of the measured multiplicities for The nuclear polarization of the atoms was flipped at the two beam-target polarization states yielded the 90s time intervals, while both this polarization and the helicity distribution of the strange quarks. The results atomic fraction inside the target cell were indicate that, within experimental uncertainty, the continuously measured. The average value of the strange quarks are unpolarized and make no deuteron polarization was 0.845 with a fractional contribution to the spin of the proton. Furthermore, in systematic uncertainty of 3.5%. large measure the strange sea is generated by gluons conversion into quark-antiquark pairs. Consequently, Scattered beam leptons and coincident hadrons the HERMES result suggests that the gluons in the were detected by the HERMES spectrometer. Leptons proton are themselves not strongly polarized. were identified with an efficiency exceeding 98% and * a hadron contamination of less than 1%. A ring- The HERMES collaboration comprises 32 imaging Cerenkov (RICH) detector was used to institutions from 11 countries at the Deutsches identify charged kaons. Events were selected subject Elektronen Synchrotron (DESY) in Hamburg