358 research outputs found
Hyperon Polarization in the Constituent Quark Model
We consider mechanism for hyperon polarization in inclusive production. The
main role belongs to the orbital angular momentum and polarization of the
strange quark-antiquark pairs in the internal structure of the constituent
quarks. We consider a nucleon as a core consisting of the constituent quarks
embedded into quark condensate. The nonperturbative hadron structure is based
on the results of chiral quark models.Comment: 14 pages, LaTeX, 2 Figures, References adde
Different behaviour of the spin structure functions and at
We consider low- behaviour of the spin structure functions and
in the unitarized chiral quark model which combines ideas on the
constituent quark structure of hadrons with a geometrical scattering picture
and unitarity. A nondiffractive singular low- dependence of and
indicated by the recent SMC experimental data is described. A
diffractive type smooth behaviour of is predicted at small . The
expectations for the double-spin asymmetries in the low-mass Drell-Yan
production at RHIC in the central region are discussed alongside.Comment: LaTeX, 10 pages, 2 figure
The Polarised Valence Quark Distribution from semi-inclusive DIS
The semi-inclusive difference asymmetry A^{h^{+}-h^{-}} for hadrons of
opposite charge has been measured by the COMPASS experiment at CERN. The data
were collected in the years 2002-2004 using a 160 GeV polarised muon beam
scattered off a large polarised ^6LiD target and cover the range 0.006 < x <
0.7 and 1 < Q^2 < 100 (GeV/c)^2. In leading order QCD (LO) the asymmetry
A_d^{h^{+}-h^{-}} measures the valence quark polarisation and provides an
evaluation of the first moment of Delta u_v + Delta d_v which is found to be
equal to 0.40 +- 0.07 (stat.) +- 0.05 (syst.) over the measured range of x at
Q^2 = 10 (GeV/c)^2. When combined with the first moment of g_1^d previously
measured on the same data, this result favours a non-symmetric polarisation of
light quarks Delta u-bar = - Delta d-bar at a confidence level of two standard
deviations, in contrast to the often assumed symmetric scenario Delta u-bar =
Delta d-bar = Delta s-bar = Delta s.Comment: 7 pages, 3 figures, COMPASS, revised: details added, author list
update
Spin alignment and violation of the OZI rule in exclusive and production in pp collisions
Exclusive production of the isoscalar vector mesons and is
measured with a 190 GeV proton beam impinging on a liquid hydrogen target.
Cross section ratios are determined in three intervals of the Feynman variable
of the fast proton. A significant violation of the OZI rule is found,
confirming earlier findings. Its kinematic dependence on and on the
invariant mass of the system formed by fast proton
and vector meson is discussed in terms of diffractive
production of resonances in competition with central
production. The measurement of the spin density matrix element of
the vector mesons in different selected reference frames provides another
handle to distinguish the contributions of these two major reaction types.
Again, dependences of the alignment on and on are
found. Most of the observations can be traced back to the existence of several
excited baryon states contributing to production which are absent in
the case of the meson. Removing the low-mass resonant
region, the OZI rule is found to be violated by a factor of eight,
independently of .Comment: 23 pages, 13 figures and 5 table
Polarised Quark Distributions in the Nucleon from Semi-Inclusive Spin Asymmetries
We present a measurement of semi-inclusive spin asymmetries for positively
and negatively charged hadrons from deep inelastic scattering of polarised
muons on polarised protons and deuterons in the range 1
GeV. Compared to our previous publication on this subject, with the new
data the statistical errors have been reduced by nearly a factor of two.
From these asymmetries and our inclusive spin asymmetries we determine the
polarised quark distributions of valence quarks and non-strange sea quarks at
=10 GeV. The polarised valence quark distribution, , is positive and the polarisation increases with . The polarised
valence quark distribution, , is negative and the non-strange
sea distribution, , is consistent with zero over the measured
range of . We find for the first moments , and
, where we assumed
. We also determine for the first time the
second moments of the valence distributions .Comment: 17 page
Multiplicities of charged pions and unidentified charged hadrons from deep-inelastic scattering of muons off an isoscalar target
Multiplicities of charged pions and unidentified hadrons produced in
deep-inelastic scattering were measured in bins of the Bjorken scaling variable
, the relative virtual-photon energy and the relative hadron energy .
Data were obtained by the COMPASS Collaboration using a 160 GeV muon beam and
an isoscalar target (LiD). They cover the kinematic domain in the photon
virtuality > 1(GeV/c, , and . In addition, a leading-order pQCD analysis was performed using the
pion multiplicity results to extract quark fragmentation functions
The -dependence of the generalised Gerasimov-Drell-Hearn integral for the deuteron, proton and neutron
The Gerasimov-Drell-Hearn (GDH) sum rule connects the anomalous contribution
to the magnetic moment of the target nucleus with an energy-weighted integral
of the difference of the helicity-dependent photoabsorption cross sections. The
data collected by HERMES with a deuterium target are presented together with a
re-analysis of previous measurements on the proton. This provides a measurement
of the generalised GDH integral covering simultaneously the nucleon-resonance
and the deep inelastic scattering regions. The contribution of the
nucleon-resonance region is seen to decrease rapidly with increasing . The
DIS contribution is sizeable over the full measured range, even down to the
lowest measured . As expected, at higher the data are found to be in
agreement with previous measurements of the first moment of . From data on
the deuteron and proton, the GDH integral for the neutron has been derived and
the proton--neutron difference evaluated. This difference is found to satisfy
the fundamental Bjorken sum rule at GeV.Comment: 12 pages, 10 figure
Flavor decomposition of the sea quark helicity distributions in the nucleon from semi-inclusive deep-inelastic scattering
Double-spin asymmetries of semi-inclusive cross sections for the production
of identified pions and kaons have been measured in deep-inelastic scattering
of polarized positrons on a polarized deuterium target. Five helicity
distributions including those for three sea quark flavors were extracted from
these data together with re-analyzed previous data for identified pions from a
hydrogen target. These distributions are consistent with zero for all three sea
flavors. A recently predicted flavor asymmetry in the polarization of the light
quark sea appears to be disfavored by the data.Comment: 5 pages, 3 figure
Quark helicity distributions in the nucleon for up, down, and strange quarks from semi--inclusive deep--inelastic scattering
Polarized deep--inelastic scattering data on longitudinally polarized
hydrogen and deuterium targets have been used to determine double spin
asymmetries of cross sections. Inclusive and semi--inclusive asymmetries for
the production of positive and negative pions from hydrogen were obtained in a
re--analysis of previously published data. Inclusive and semi--inclusive
asymmetries for the production of negative and positive pions and kaons were
measured on a polarized deuterium target. The separate helicity densities for
the up and down quarks and the anti--up, anti--down, and strange sea quarks
were computed from these asymmetries in a ``leading order'' QCD analysis. The
polarization of the up--quark is positive and that of the down--quark is
negative. All extracted sea quark polarizations are consistent with zero, and
the light quark sea helicity densities are flavor symmetric within the
experimental uncertainties. First and second moments of the extracted quark
helicity densities in the measured range are consistent with fits of inclusive
data
First Measurement of the Tensor Structure Function of the Deuteron
The \Hermes experiment has investigated the tensor spin structure of the
deuteron using the 27.6 GeV/c positron beam of \Hera. The use of a tensor
polarized deuteron gas target with only a negligible residual vector
polarization enabled the first measurement of the tensor asymmetry \At and
the tensor structure function \bd for average values of the Bj{\o}rken
variable and of the squared four-momentum transfer . The quantities \At and \bd are found to be
non-zero. The rise of \bd for decreasing values of can be interpreted to
originate from the same mechanism that leads to nuclear shadowing in
unpolarized scattering
- …