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Measurement of
in the low-x and lowregion at HERMES
Cite as: AIP Conference Proceedings 549, 709 (2000);
Published Online: 28 March 2001
Ralf Kaiser, and HERMES Collaboration
AIP Conference Proceedings 549, 709 (2000);
© 2000 American Institute of Physics.
549, 709
Measurement of g\ in the low-x and
low-Q2 Region at HERMES
Ralf Kaiser
on behalf of the HERMES Collaboration
DESY Zeuthen, Platanenallee 6, 15738 Zeuthen, Germany
Abstract. A new analysis has extended the kinematic range of the 1997 HERMES
data on the polarized structure function g% to include the region of 0.0021 < x < 0.021
and 0.1 GeV2 < Q2 < 0.8 GeV 2 . The preliminary results on g? in this region are
presented and the observation of scaling violations in polarized deep inelastic leptonnucleon scattering (DIS) is discussed.
The polarized proton structure function g[ has been studied for over a decade,
yet the experimental precision is still not satisfactory, in particular at small values
of Bjorken-x. New data at low values of x will reduce the extrapolation uncertainty
for the integral of g\, which in turn will improve the accuracy in the determination
of the quark contribution to the nucleon spin. Low x (x < 0.01) data in current experiments are typically measured at relatively small values of Q2 (Q2 « 1.0 GeV 2 ).
By studying scaling violations it may become possible to draw conclusions on the
validity of perturbative QCD (pQCD) at low photon virtualities in polarized DIS.
The HERMES experiment uses the polarized 27.6 GeV positron (or electron)
beam of the HERA accelerator at DESY. Spin rotators convert the transverse
beam polarization that arises due to the Sokolov-Ternov effect [1] into a longitudinal
polarization in the interaction region. The average beam polarization for the 1997
polarized hydrogen data was 0.55 ± 0.02 (syst.). The internal gas target uses a
cooled, open-ended storage cell inside the beam vacuum. The storage cell is fed
with polarized hydrogen by an atomic beam source. The average target polarization
was measured to be 0.88 ± 0.04 (syst.).
The HERMES detector [2] is an open-geometry forward spectrometer. A horizontal septum in the dipole magnet of the spectrometer shields the beam pipe against
the magnetic field. As a result, the detector system is split into two identical halves
above and below the beam pipe and the minimum vertical acceptance is limited
to ±40mrad. The maximum acceptances are ±140mrad vertically and ±170mrad
horizontally. The spectrometer has an angular resolution of SQ < 0.6 mrad and
CP549, Intersections of Particle and Nuclear Physics: 7th Conference, edited by Z. Parsa and W. J. Marciano
© 2000 American Institute of Physics 1-56396-978-5/007$ 17.00
a momentum resolution of Sp/p < 1.5%. An excellent hadron-electron separation
is provided by the combination of a transition radiation detector, a preshower hodoscope, an electromagnetic calorimeter and a gas threshold Cerenkov counter. The
positron identification efficiency is better than 97 % with a hadron contamination
of less than 1.0 % for each x-bin of the presented analysis. The DIS trigger consists
of a coincidence of signals from three hodoscope planes and the calorimeter with a
required energy deposit of at least 1.4 GeV.
The present analysis extends the kinematic range of the HERMES results on
g[(x,Q2) of the previous publication [3] (0.0212 < x < 0.85 and Q2 > 0.8 GeV2)
down to 0.0021 in x and 0.1 GeV2 in Q2. To make this possible, the upper limit
on t/, the fractional virtual photon energy, was increased from 0.85 to 0.91. This
required the detailed understanding of the detector and trigger performance in the
low momentum region close to the trigger threshold for the scattered positron. As a
result, seven new, preliminary data points for g\(x, Q2) were obtained in the low-x,
low-Q2 region. The polarized structure function g[(z, Q2) was determined from the
ratio 9l/Fl by multiplying with F^x.Q2) = F2(x,Q2)(l + ^2)/2x(l + R(x,Q2)).
The ratio g\/F\ has been calculated from the measured longitudinal cross section
asymmetry A\\. F<2 and R were parameterized according to [4] and [5]. More details
on the extraction of g\ are given in ref. [3].
The experimental uncertainties for the newly analyzed low-x region originate
mainly from normalization, background corrections, acceptance cut variations and
scattering angle systematics. The systematic uncertainties due to radiative and
smearing corrections have been minimized through an iterative procedure. The
total systematic uncertainty is about 14 % for the lowest or-value and about 9 % for
the other six points.
A comparison of HERMES and SMC data on the x-behaviour of the structure
function ratio g\/F\ (see ref. [6]) exhibits no Q2-dependence within the experimental uncertainties. This represents the first comparison of the SMC data at low x
to data with improved precision by HERMES. Figure 1 shows the x-dependence
of gi (x, Q2) at its measured average Q2-values in comparison to recent data from
E143 [7] and SMC [8]. The new low-x data points (0.0021 < x < 0.021) from HERMES are shown together with the previously published data [3] for x > 0.0212.
The higher beam energy of SMC compared to HERMES and El43 leads to values
of Q2 that are about one order of magnitude larger in every x-bin. This is shown in
the lower panel of figure 1. Hence, the comparison of the HERMES and SMC data
allows observations on the (^-dependence of 9i: Scaling violations are not visible
above x w 0.2, while they are significant for lower x, extending into the region of
the new data points. Further low-or data that cover a wide enough Q2-range are
necessary to confirm the latter observation and to test the validity range of pQCD
in the spin sector at low photon virtualities.
The author would like to thank all HERMES colleagues that contributed to this
analysis, in particular U.Stosslein, H.Bottcher, Y.Garber and W.-D.Nowak.
""?- i.o
o HERMES low-x preliminary
T E143
FIGURE 1. The HERMES polarized proton structure function g\(x, Q2) (upper panel) and the
measured average Q2-values (lower panel) in comparison to E143 [7] and SMC [8] data. The
error bars represent the statistical uncertainties. The four additional SMC points at even lower x
from ref. [9] have not been included here.
A.A. Sokolov, I.M. Ternov, Sov. Phys. Doklady 8, 1203 (1964)
HERMES Coll., K. Ackerstaff et al, Nucl. Instr. and Meth. A 417, 230 (1998).
HERMES Coll., A. Airapetian et a/., Phys. Lett. B 442, 484 (1998).
H. Abramowicz and A. Levy, hep-ph/9712415.
L. W. Withlow et a/., Phys. Lett. B 250, 193 (1990).
H. Bottcher, XlVth Rencontre de Physique de la Vallee d'Aoste, Italy, Feb. 2000
E143 Coll., K. Abe et al, Phys. Rev. D 58, 112003 (1998).
SMC Coll., B. Adeva et a/., Phys. Rev. D 58, 112001 (1998)
SMC Coll., B. Adeva et a/., Phys. Rev. D 60, 072004 (1999)
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