Overview of HERMES physics topics:
Polarized quarks and gluons
The fact that the nucleon has a large anomalous magnetic moment proves
that it is not a fundamental spin-1/2 Dirac particle. The approximate
agreement of the measured magnetic moments of the members of the
baryon multiplets with predictions of the SU(3)f x
SU(2) symmetric quark-parton model is an important confirmation
of the quark model approach. Additional predictions about the spin
structure of the nucleon were the Bjorken and Ellis-Jaffe sum rules,
which however were not experimentally accessible in the first two
decades of the quark model. In 1987 the EMC collaboration published a
measurement of the spin structure function
gp1(x) of the proton and of its first
moment, the Ellis-Jaffe sum. The observed violation questioned our
understanding of the spin structure of the nucleon in terms of the
quark-parton model. It caused intense discussion in the community and
demanded experimental and theoretical clarification.
Since then a series of new spin experiments has been performed at
SLAC, CERN and DESY. They became feasible by improved experimental
techniques to polarize beams and targets. At CERN and SLAC the
improved figure of merit, which is the product of luminosity and the
squared values of the polarization of beam and target, allowed for a
precision measurement of the inclusive spin structure functions. The
HERMES experiment uses a completely novel method, a polarized internal
storage cell target in a storage ring with longitudinally polarized
electrons. The HERMES storage cell is a 40 cm long elliptical tube
around the stored electron beam. Polarized atoms are continuously
injected into the storage cell and leave the cell only after an
average of a few hundred wall bounces. The main advantage of the
storage cell technique is the ability to use pure, highly polarized
atomic species (H, D, 3He) in contrast to solid state
targets where only a small fraction of the atomic species is
polarizable.
The HERMES experiment is dedicated to semi-inclusive measurements,
i.e. measurements which detect final state hadrons in coincidence with
the scattered lepton. By tagging certain final state hadrons with
different flavor content, the spin contributions Delta u,
Delta d, Delta s of the up, down and strange quark flavors can be
disentangled.
The inclusive as well as the semi-inclusive results show that only
a fraction of the nucleon spin is due to the spin contribution
Delta Sigma = Delta u + Delta d + Delta s of the quarks. The rest
is due to the contribution Delta G of the gluon spin and due
to angular momentum contributions Lq and
LG of quarks and gluons moving with high speed in
the nucleon. Current experiments show first evidence of a non-zero
contribution by the gluon spin, however further experiments are needed
in future to disentangle and understand all the contributions of the
nucleon's spin sNz which are
summarized in the helicity sum rule
sNz = 1/2 = 1/2 (Delta u + Delta d + Delta s) + Lq + Delta G + LG.
To improve our understanding of the spin structure of the nucleon,
in future not only the collinear spin contributions have to be
measured, but more emphasis has to be given to investigate transverse
spin components, twist-3 contributions, spin-dependent off-forward
parton distributions and spin effects in fragmentation.
The spin dependent part of the inclusive deep inelastic scattering
cross section e + N -> e' + X is characterized by two spin
structure functions g1(x,Q2) and
g2(x,Q2). In the quark-parton model the
Bjorken variable x is interpreted as the momentum fraction
carried by the struck quark and -Q2 is the squared
four-momentum of the exchanged virtual photon and is related to the
resolution of the scattering process. The first spin structure
function g1(x,Q2) is interpreted as
g1(x,Q2) = 1/2 sumf ( ef2 Delta qf(x,Q2))
with ef being the charge for a quark with
flavor f in units of the elementary charge and Delta
qf(x,Q2)
is the polarized quark distribution
function.
The spin structure function g1 is being
measured since many years, and an impressive, precise data set has
been collected by the experiments at SLAC, by SMC and by HERMES as
shown in the Figure below. At all three sites the spin structure
function g1(x,Q2) was measured for the
proton and the neutron. The neutron spin asymmetry is obtained either
from the subtraction of deuteron and proton data, or directly from a
3He target. In the 3He nucleus the spins of the
protons are anti-parallel and do not contribute to the measured
asymmetry (except for a small contribution which can be corrected).
All experimental results are consistent and agree with the
Q2 evolution as predicted by QCD.
Recent results for the
spin structure functions g1(x) at
Q2=5 GeV2?? for the
proton, deuteron and neutron.
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Publications:
Measurement of the Neutron Spin Structure Function g1(n) with a Polarised 3He Internal Target
K. Ackerstaff et al., Phys. Lett. B404 (1997) 383-389.
Eprint numbers: hep-ex/9703005
Measurement of the Proton Spin Structure Function g1p with a Pure Hydrogen Target
A. Airapetian et al, Phys. Lett. B442 (1998) 484-492.
Eprint numbers: hep-ex/9807015 and DESY-98-072
The Gerasimov-Drell-Hearn (GDH) sum rule, relates the polarization
dependent part of the total photoproduction cross section to the
anomalous magnetic moment of the nucleon. The GDH sum rule can be
generalized for electroproduction. Data have been recently published
by the HERMES collaboration.
Cross section difference for proton and neutron from the GDH analysis.
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Publication:
Determination of the Deep Inelastic Contribution to the Generalised Gerasimov-Drell-Hearn Integral for the Proton and Neutron
K. Ackerstaff et al, Phys. Lett. B444 (1998) 531-538.
Eprint numbers: hep-ex/9809015 and DESY-98-122
The observed violation of the Ellis-Jaffe sum rule in 1987 by EMC
made clear that the
spin structure of the nucleon is not understood. More detailed
experimental information is needed to disentangle the various possible
contributions of quarks and gluons to the spin of the nucleon.
Semi-inclusive data can be used to
measure the sea polarization directly and to test SU(3)f symmetry
by comparing the first moments of the flavor distributions to the
SU(3)f predictions. In addition, semi-inclusive polarized DIS
experiments can determine the separate spin contributions Delta qf
of quark and antiquark flavors f to the total spin of the nucleon
not only as a total integral but as a function of the Bjorken scaling
variable x.
The inclusive (a) and semi-inclusive asymmetries for positively (b)
and negatively (c) charged hadrons on the proton (top) and 3He
(bottom) target. The inclusive asymmetries are compared to SLAC
results for g1/F1 (open triangles). The
hadron asymmetries on
the proton are compared to SMC results (open squares) truncated to
the HERMES x-range. The data points are given for the measured mean Q2
at each value of x, which is different for the different experiments.
The error bars of the HERMES (SLAC and SMC)
data are statistical (total) uncertainties and the bands are
systematic uncertainties of the HERMES data.
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Publication:
see below (flavor decomposition).
New data from HERMES have been presented recently on the decomposition
of the proton spin into contributions from the different quark
flavors. The figure shows the polarization Delta q/q of the
quarks. The up flavor has a positive polarization which
reaches about 40% at large x whereas the
down flavor has a polarization opposite to the proton spin, in
excess of 20%. In the sea region at small x the up and down
polarizations do not vanish completely. The sea polarization itself is
compatible with zero as shown in the lower panel. The extraction of
the sea was done under the assumption that the polarization of the sea
quarks is independent of their flavor.
The polarization of quarks in the proton has been measured by HERMES
as a function of x, separately for the flavors up and down and for the
sea. The error bars shown are the statistical and the bands the
systematic uncertainties.
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Publication:
Flavor Decomposition of the Polarized Quark Distributions in the Nucleon from Inclusive and Semi-Inclusive Deep-inelastic Scattering
K. Ackerstaff et al, Phys. Lett. B (in press).
Eprint numbers: hep-ex/9906035 and DESY-99-048
The most wanted component of the nucleon spin is the polarization
of gluons, as they are probably responsible for the spin deficit of
the quarks. As the virtual photon does not couple directly to gluons,
a measurement of the gluon polarization was up to now only very
indirectly possible by using the QCD evolution equations which relate
the Q2-dependence of the quark distributions to the gluon
distribution. A QCD NLO analysis of recent data yields a gluon
contribution to the proton spin of approximately Delta G = 1.8 +/-
1.0.
For the first time a more direct measurement of the gluon
polarization has been
presented. By selecting events with two hadrons with
opposite charge and with large transverse momentum,
HERMES was able to accumulate a sample of
events which is enriched by photon-gluon fusion events. By requiring a
large transverse momentum of 1.5 (1) GeV/c for the first (second)
hadron, the sub-process where the gluon splits into two quarks has a
hard scale and can be treated pertubatively. HERMES estimates from
Monte-Carlo studies that the average squared transverse momentum of
the quarks is 2.1 (GeV/c)2. As long as the fragmentation process
is spin independent, the spin asymmetry in the production of the
quark-antiquark pair is the same as the spin asymmetry of the observed
final state. The measured asymmetry is however affected by background
processes. The unique signature of the HERMES result is the negative
sign of the asymmetry. All background processes have a positive
asymmetry, as long as they are dominated by the positive polarization
of the up-quarks in the proton. The observed negative asymmetry
can be explained by a significant positive gluon polarization. The
change of sign comes from the negative analyzing power of the
photon-gluon fusion diagram. Using a specific background Monte Carlo,
HERMES obtains a value of the gluon polarization
of Delta G/G = 0.41+/-0.18+/-0.03 at
a mean xG=0.17. The quantitative result depends however
critically on the detailed understanding of the background processes.
The figure shows the asymmetry together with Monte-Carlo
predictions using various assumptions for the gluon polarization
Delta G/G.
Spin asymmetry in the production
of hadron pairs with high pT and opposite charge. One hadron with
pT>1.5 GeV is required. The spin asymmetry is plotted as a
function of the pT of the second hadron and compared to
Monte-Carlo predictions using various assumptions for the gluon
polarization Delta G/G.
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Publication:
Measurement of the Spin Asymmetry in the Photoproduction of Pairs of High-pT Hadrons at HERMES
A. Airapetian et al, submitted to Phys. Rev. Lett.
Eprint numbers: hep-ex/9907020 and DESY-99-071
The next step in polarized DIS beyond the understanding of the
collinear part of the quark and gluon polarization in the nucleon is
the understanding of the transverse polarization components.
Single-spin asymmetries in polarized hadronic reactions are
interpreted as effects of
time-reversal-odd distribution
functions (Sivers mechanism) or time-reversal-odd fragmentation
functions (Collins mechanism).
SMC presented recently the first measurement of
semi-inclusive DIS hadron production on a transversely polarized
target. Leading hadron production has been analyzed in
terms of the Collins angle and indeed a non-zero asymmetry
AN=11%+/-6% has been found for positive hadrons, whereas the
negative hadrons yield -2%+/-6%.
A significant result has been reported by HERMES
on a related quantity. HERMES measured
the asymmetry of hadron production on a longitudinally polarized
target. Even in this case an asymmetry is expected in the azimuthal
angle between the plane which contains the produced pion and the
virtual photon and the plane which contains the scattered lepton and
the virtual photon. The figure shows this single-spin
asymmetry as a function of the azimuthal angle for positive and
negative pions. A sinusoidal fit yields an asymmetry of AN=2.0%+/-0.4% for the positive and AN=-0.1%+/-0.5% for the negative
pions.
Azimuthal dependence
of the single-spin asymmetry in the cross section for pi+
(top) and pi- (bottom). The error bars are statistical
uncertainties. The curves are sinusoidal fits to the data.
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Publication:
Azimuthual Single-Spin Asymmetries in Semi-Inclusive DIS from HERMES
H. Avakian,
7th International Workshop on Deep Inelastic Scattering and QCD (DIS 99), Zeuthen, Germany, April 19 - 23, 1999.
Due to the parity violation of the weak decay of Lambda
hyperons, the angular distribution of
the decay products can be used to extract the spin orientation of the
Lambda hyperon before its decay. This unique feature was used at
HERMES to extract two interesting
quantities.
The first one is the measurement of the polarization transfer in DIS
scattering of longitudinally polarized electrons off unpolarized
targets. A Lambda polarization of PLambda=0.03 +/- 0.06 +/- 0.03
was reported, a number which however is not precise enough to
distinguish between different predictions. The naive quark model
which assumes 100% polarization of s-quarks in Lambda hyperons
predicts PLambda=0.018, whereas a SU(3)f symmetric model from
Jaffe predicts PLambda=-0.057, based on the measured results for
the Ellis-Jaffe sum.
A much more precise result was reported concerning the transverse
polarization of Lambda hyperons in
quasi-photoproduction off an unpolarized
target:
gamma(*) p -> Lambda X.
The polarization was measured in respect to the plane perpendicular to
the Lambda production plane. The figure shows the
polarization as a function of the transverse momentum of the Lambda
and Anti-Lambda. A large positive polarization is observed for
Lambda hyperons, with the tendency to increase with its transverse
momentum. The Anti-Lambda hyperons show a negative
polarization. There is no straight-forward explanation of the
observed asymmetries in QCD; however, similar polarizations have been
found in hadronic collisions.
The squares (triangles) indicate the transverse polarization of
Lambda (Anti-Lambda) in unpolarized photoproduction as
function of the transverse momentum.
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Publication:
Strange Particle Production and Polarization of Lambda Hyperons in the
HERMES Experiment
S. Belostotski,
7th International Workshop on Deep Inelastic Scattering and QCD
(DIS 99), Zeuthen, Germany, April 19 - 23, 1999.
Last modified: Tue Oct 26 10:44:23 1999
by
Michael Düren