26 research outputs found
On Gauge-Invariant Decomposition of Nucleon Spin
We investigate the relation between the known decompositions of the nucleon
spin into its constituents, thereby clarifying in what respect they are common
and in what respect they are different essentially. The decomposition recently
proposed by Chen et al. can be thought of as a nontrivial generalization of the
gauge-variant Jaffe-Manohar decomposition so as to meet the gauge-invariance
requirement of each term of the decomposition. We however point out that there
is another gauge-invariant decomposition of the nucleon spin, which is closer
to the Ji decomposition, while allowing the decomposition of the gluon total
angular momentum into the spin and orbital parts. After clarifying the reason
why the gauge-invariant decomposition of the nucleon spin is not unique, we
discuss which decomposition is more preferable from the experimental viewpoint.Comment: The version to appear in Phys. Rev.
Transverse Momentum Dependent Parton Distribution/Fragmentation Functions at an Electron-Ion Collider
We present a summary of a recent workshop held at Duke University on Partonic
Transverse Momentum in Hadrons: Quark Spin-Orbit Correlations and Quark-Gluon
Interactions. The transverse momentum dependent parton distribution functions
(TMDs), parton-to-hadron fragmentation functions, and multi-parton correlation
functions, were discussed extensively at the Duke workshop. In this paper, we
summarize first the theoretical issues concerning the study of partonic
structure of hadrons at a future electron-ion collider (EIC) with emphasis on
the TMDs. We then present simulation results on experimental studies of TMDs
through measurements of single spin asymmetries (SSA) from semi-inclusive
deep-inelastic scattering (SIDIS) processes with an EIC, and discuss the
requirement of the detector for SIDIS measurements. The dynamics of parton
correlations in the nucleon is further explored via a study of SSA in D (`D)
production at large transverse momenta with the aim of accessing the unexplored
tri-gluon correlation functions. The workshop participants identified the SSA
measurements in SIDIS as a golden program to study TMDs in both the sea and
valence quark regions and to study the role of gluons, with the Sivers
asymmetry measurements as examples. Such measurements will lead to major
advancement in our understanding of TMDs in the valence quark region, and more
importantly also allow for the investigation of TMDs in the sea quark region
along with a study of their evolution.Comment: 44 pages 23 figures, summary of Duke EIC workshop on TMDs accepted by
EPJ
Understanding the proton's spin structure
We discuss the tremendous progress that has been towards an understanding of
how the spin of the proton is distributed on its quark and gluon constituents.
This is a problem that began in earnest twenty years ago with the discovery of
the proton ``spin crisis'' by the European Muon Collaboration. The discoveries
prompted by that original work have given us unprecedented insight into the
amount of spin carried by polarized gluons and the orbital angular momentum of
the quarks.Comment: Review article for J. Phys. G, 1 figure, 22 page
Bose-Einstein correlations in hadron-pairs from lepto-production on nuclei ranging from hydrogen to xenon
Bose-Einstein correlations of like-sign charged hadrons produced in
deep-inelastic electron and positron scattering are studied in the HERMES
experiment using nuclear targets of H, H, He, He, N, Ne, Kr,
and Xe. A Gaussian approach is used to parametrize a two-particle correlation
function determined from events with at least two charged hadrons of the same
sign charge. This correlation function is compared to two different empirical
distributions that do not include the Bose-Einstein correlations. One
distribution is derived from unlike-sign hadron pairs, and the second is
derived from mixing like-sign pairs from different events. The extraction
procedure used simulations incorporating the experimental setup in order to
correct the results for spectrometer acceptance effects, and was tested using
the distribution of unlike-sign hadron pairs. Clear signals of Bose-Einstein
correlations for all target nuclei without a significant variation with the
nuclear target mass are found. Also, no evidence for a dependence on the
invariant mass W of the photon-nucleon system is found when the results are
compared to those of previous experiments
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
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
Pentaquark search at HERMES
The earlier search at HERMES for narrow baryon states excited in quasi-real
photoproduction, decaying through the channel ,
has been extended with improved decay-particle reconstruction, more advanced
particle identification, and increased event samples. The structure observed
earlier at an invariant mass of 1528 MeV shifts to 1522 MeV and the statistical
significance drops to about 2 for data taken with a deuterium target.
The number of events above background is
. No such structure is observed
in the hydrogen data set
The HERMES Polarized Hydrogen and Deuterium Gas Target in the HERA Electron Storage Ring
The HERMES hydrogen and deuterium nuclear-polarized gas targets have been in
use since 1996 with the polarized electron beam of HERA at DESY to study the
spin structure of the nucleon. Polarized atoms from a Stern-Gerlach Atomic Beam
Source are injected into a storage cell internal to the HERA electron ring.
Atoms diffusing from the center of the storage cell into a side tube are
analyzed to determine the atomic fraction and the atomic polarizations. The
atoms have a nuclear polarization, the axis of which is defined by an external
magnetic holding field. The holding field was longitudinal during 1996-2000,
and was changed to transverse in 2001. The design of the target is described,
the method for analyzing the target polarization is outlined, and the
performance of the target in the various running periods is presented
The HERMES Recoil Detector
For the final running period of HERA, a recoil detector was installed at the HERMES experiment to improve measurements of hard exclusive processes in charged-lepton nucleon scattering. Here, deeply virtual Compton scattering is of particular interest as this process provides constraints on generalised parton distributions that give access to the total angular momenta of quarks within the nucleon. The HERMES recoil detector was designed to improve the selection of exclusive events by a direct measurement of the four-momentum of the recoiling particle. It consisted of three components: two layers of double-sided silicon strip sensors inside the HERA beam vacuum, a two-barrel scintillating fibre tracker, and a photon detector. All sub-detectors were located inside a solenoidal magnetic field with an integrated field strength of 1 T. The recoil detector was installed in late 2005. After the commissioning of all components was finished in September 2006, it operated stably until the end of data taking at HERA end of June 2007. The present paper gives a brief overview of the physics processes of interest and the general detector design. The recoil detector components, their calibration, the momentum reconstruction of charged particles, and the event selection are described in detail. The paper closes with a summary of the performance of the detection system
Passage Performance of two Cyprinids with Different Ecological Traits in a Fishway with Distinct Vertical Slot Configurations
The azimuthal cos{\phi} and cos2{\phi} modulations of the distribution of hadrons produced in unpolarized semi-inclusive deep-inelastic scattering of electrons and positrons off hydrogen and deuterium targets have been measured in the HERMES experiment. For the first time these modulations were determined in a four-dimensional kinematic space for positively and negatively charged pions and kaons separately, as well as for unidentified hadrons. These azimuthal dependences are sensitive to the transverse motion and polarization of the quarks within the nucleon via, e.g., the Cahn, Boer-Mulders and Collins effects