17 research outputs found
Double hadron leptoproduction in the nuclear medium
First measurement of double-hadron production in deep-inelastic scattering
has been measured with the HERMES spectrometer at HERA using a 27.6 GeV
positron beam with deuterium, nitrogen, krypton and xenon targets. The
influence of the nuclear medium on the ratio of double-hadron to single-hadron
yields has been investigated. Nuclear effects are clearly observed but with
substantially smaller magnitude and reduced -dependence compared to
previously measured single-hadron multiplicity ratios. The data are in fair
agreement with models based on partonic or pre-hadronic energy loss, while they
seem to rule out a pure absorptive treatment of the final state interactions.
Thus, the double-hadron ratio provides an additional tool for studying
modifications of hadronization in nuclear matter
Evidence for a narrow |S|=1 baryon state at a mass of 1528 MeV in quasi-real photoproduction
Evidence for a narrow baryon state is found in quasi-real photoproduction on
a deuterium target through the decay channel p K^0_S --> p pi^+ pi^-. A peak is
observed in the p K^0_S invariant mass spectrum at 1528 +/- 2.6 (stat) +/-2.1
(syst) MeV. Depending on the background model,the naive statistical
significance of the peak is 4--6 standard deviations and its width may be
somewhat larger than the experimental resolution of sigma=4.3 -- 6.2 MeV. This
state may be interpreted as the predicted S=+1 exotic Theta^{+}(uuddbar(s))
pentaquark baryon. No signal for an hypothetical Theta^{++} baryon was observed
in the pK^+ invariant mass distribution. The absence of such a signal indicates
that an isotensor Theta is excluded and an isovector Theta is unlikely.Comment: 8 pages, 4 figure
Two-pion Bose-Einstein correlations in central Pb-Pb collisions at = 2.76 TeV
The first measurement of two-pion Bose-Einstein correlations in central Pb-Pb
collisions at TeV at the Large Hadron Collider is
presented. We observe a growing trend with energy now not only for the
longitudinal and the outward but also for the sideward pion source radius. The
pion homogeneity volume and the decoupling time are significantly larger than
those measured at RHIC.Comment: 17 pages, 5 captioned figures, 1 table, authors from page 12,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/388
Suppression of charged particle production at large transverse momentum in central Pb-Pb collisions at TeV
Inclusive transverse momentum spectra of primary charged particles in Pb-Pb
collisions at = 2.76 TeV have been measured by the ALICE
Collaboration at the LHC. The data are presented for central and peripheral
collisions, corresponding to 0-5% and 70-80% of the hadronic Pb-Pb cross
section. The measured charged particle spectra in and GeV/ are compared to the expectation in pp collisions at the same
, scaled by the number of underlying nucleon-nucleon
collisions. The comparison is expressed in terms of the nuclear modification
factor . The result indicates only weak medium effects ( 0.7) in peripheral collisions. In central collisions,
reaches a minimum of about 0.14 at -7GeV/ and increases
significantly at larger . The measured suppression of high- particles is stronger than that observed at lower collision energies,
indicating that a very dense medium is formed in central Pb-Pb collisions at
the LHC.Comment: 15 pages, 5 captioned figures, 3 tables, authors from page 10,
published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/98
Evidence for a Transverse Single-Spin Asymmetry in Leptoproduction of pi+ pi- Pairs
A single-spin asymmetry was measured in the azimuthal distribution of pi+pi-
pairs produced in semi-inclusive deep-inelastic scattering on a transversely
polarized hydrogen target. For the first time, evidence is found for a
correlation between the transverse target polarization and the azimuthal
orientation of the plane containing the two pions.The corresponding single-spin
asymmetry is expected to be related to the product of the little-known quark
transversity distribution function and an unknown naive-T-odd chiral-odd
dihadron fragmentation function.Comment: 18 pages, 4 figures, 1 table; minor revisions to content for clarit
Measurement of the tensor structure function b(1) of the deuteron
The Hermes experiment has investigated the tensor spin structure of the deuteron using the 27.6 GeV/c positron beam of DESY 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 A(zz)(d) and the tensor structure function b(1)(d) for average values of the Bjorken variable 0.01 < 5 GeV2. The quantities A(zz)(d) and b(1)(d) are found to be nonzero. The rise of b(1)(d) for decreasing values of x can be interpreted to originate from the same mechanism that leads to nuclear shadowing in unpolarized scattering
Beam-charge azimuthal asymmetry and deeply virtual Compton scattering
The first observation of an azimuthal cross section asymmetry with
respect to the charge of the incoming lepton beam is reported from a
study of hard exclusive electroproduction of real photons. The data
have been accumulated by the HERMES experiment at DESY, in which the
HERA 27.6 GeV electron or positron beam scattered off an unpolarized
hydrogen gas target. The observed asymmetry is attributed to the
interference between the Bethe-Heitler process and the deeply virtual
Compton scattering (DVCS) process. The interference term is sensitive
to DVCS amplitudes, which provide the most direct access to generalized
parton distributions
Precise determination of the spin structure function of the proton, deuteron and neutron
Precise measurements of the spin structure functions of the proton
and deuteron are presented over the kinematic
range and GeV GeV.
The data were collected at the HERMES experiment at DESY, in deep-inelastic
scattering of 27.6 GeV longitudinally polarized positrons off longitudinally
polarized hydrogen and deuterium gas targets internal to the HERA storage ring.
The neutron spin structure function is extracted by combining proton
and deuteron data. The integrals of at GeV are
evaluated over the measured range. Neglecting any possible contribution to
the integral from the region , a value of (evol.) is obtained for
the flavor-singlet axial charge in a leading-twist NNLO analysis