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The HERMES experiment
NUCLEAR PHYSICS A Nuclear Physics A622 (1997) 16c--30c
ELSEVIER
The HERMES Experiment
R.G. Milner Massachusetts Institute of Technology , Cambridge , MA 02139, USA and Deutsehes Elektronen-Synchrotron, Notkestrasse 85, 22603 Hamburg, Germany
On Behalf of the HERMES Collaboration
Abstract
HERMES, H._EERAMeasurement of Spin, is an experiment underway at DESY to study the spin structure of the nucleon by using polarized internal gas targets in the HERA 27.5 GeV electron storage ring. Inclusive and semi-inclusive spin dependent deep inelastic scattering are simultaneously measured in a forward spectrometer. In 1995 the experiment was installed and commissioned and data were taken on a polarized 3He target as well as on unpolarized targets of hydrogen and deuterium. In this paper, several aspects of the spin structure of the nucleon which can be probed by HERMES are discussed. The experiment is briefly described and some preliminary results from 1995 running are presented.
1
Introduction
The study of how the spin of the nucleon is constructed from its elementary constituents remains an i m p o r t a n t and fundamental problem. Twenty years of measurements at SLAC and CERN [1-4] have almost completely focussed on inclusive, spin-dependent deep inelastic scattering over the kinematic range 0.003 < x < 0.6. Here x = ~ is the usual Bjorken scaling variable. To date, emphasis has been placed on the determination of the "Ellis-Ja:ffe integrals", i.e. the first moments of the gl(x) spin structure function of the nucleon. Using an analysis based on the quark parton model with QCD corrections, the central conclusion of these polarized deep-inelastic scattering experiments is that the spin fraction A E carried by the quarks in the nucleon is 0.20 + 0.11 at Q2 = 10 (GeV/c) 2 [4]. This is much less than that predicted by models of nucleon --
0375-9474/97/$17.00 © 1997 - Elsevier Science B.V. All rights reserved. PII: S0375-9474(97)00329-1
--
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R. G. Milner /Nuclear Physics A622 (1997) 16c-30c
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spin structure. In addition, it is deduced that the strange quarks in the nucleon have significant negative polarization/ks = - 0.12 + 0.04. Many open questions remain: -
* • • • •
what is the what is the what is the how is spin what is the
contribution of the valence quarks? contribution of the sea quarks, in particular the strange quarks? role of orbital angular momentum? distributed in other hadrons, e.g. A? role of glue?
HERMES has been designed [5] to address many of the open questions concerning the spin structure of the nucleon using a new experimental technique to measure spindependent deep inelastic scattering. Simultaneously, both inclusive and semi-inclusive spin-dependent scattering are measured to high precision with good particle identification. Measurements are made on undiluted polarized targets of hydrogen, deuterium, and 3He for any target spin state. Semi-inclusive spin asymmetries resulting from hadron particle identification provide the means to investigate the spin structure of the nucleon in a new way.
2
T h e Spin S t r u c t u r e of t h e N u c l e o n
Rather than give a comprehensive review of the HERMES physics program (see [5]), I would like to illustrate the potential of the experiment to study the spin structure of the nucleon by providing a brief overview of spin dependent deep inelastic scattering in the context of the quark parton model. Then I will provide three examples of measurements which are in progress by HERMES.
2.1
The quark patton model
In the quark parton model the unpolarized structure function FI(x) and the spindependent structure function, gl(x) are expressed as
F,(x) = -~ ~:,
(1)
where the quark distribution functions, q/('~)(x), refer to quarks with flavor i and spin parallel (antiparallel) to the nucleon spin. Assuming the spin-dependent structure function g2(x) is small, as supported by recent data [6], the polarization asymmetry, A~(x), is formed from the spin-dependent cross-
R.G. Milner/Nuclear Physics A622 (1997) 16c-30c
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sections ~ri-* and c%,, and is related to
gl(x) and &(x) by
A~((x) _ °"t+a~+ +- a~°*~~ D(x, Y) gl(x) = D(x., y)A~ (x),
where the virtual photon depolarization factor y = ~ atJ s
D(x, y) =
y(2 - y) y2 + 2(1 - y)[1 +
(2)
D(x, y) is expressed in terms of x and
R(x,y)]"
(3)
R(x, y) is the ratio of cross sections for longitudinally and transversely polarized virtual photons. The first moment of gl(x) provides one constraint on the flavor dependence of the net quark helicities, Aq, = fl 5qi(x) = f~[q~(x) - q~(x)]dx:
/g~(x)dx = -~(-~Au 1 4 + ~Ad + 1As)(1 -- O(a~)).
(4)
Further constraints on the Aq,'s also come from applying isospin and SU(3) flavor symmetries [7] to neutron decay and semi-leptonic hyperon decay data: Au-Ad=F+D;
Au+Ad-2As=3F-D.
(5)
where F and D are the symmetric and anti-symmetric weak SU(3)/couplings, respectively, of the baryon octet. If one assumes in addition to SU(3)/ symmetry, that the strange sea is unpolarized [8], the matrix elements for neutron and hyperon decay alone fix the total quark spin in the nucleon, A u + A d + A s , at the value of ~ 0.6, the "canonical expectation". However, as noted above, experimental data indicate a value of only 0.2. The Bjorken sum rule [9], a fundaxaental relationship arising from current algebra between the difference in the first moments of the proton and the neutron, is confirmed by the data. Note that the inclusive results have also been interpreted [10] in terms of breaking of SU(3)/ symmetry with As = 0. Thus, in spite of careful and extensive efforts, the formulation of a model of nucleon spin structure, which accounts for all the spin in terms of the partonic constituents remains a clear challenge. Semi-inclusive deep inelastic scattering from the nucleon can also be understood in terms of the parton model [11]. The probability for a quark of flavor i to fragment into a hadron h is described by the fragmentation functions D~(z, Q2), where z = ~ is the fractional energy of the hadron. Assuming factorization and that scaling also holds for the fragmentation functions, the semi-inclusive cross-section for production of a hadron
R.G. Milner/Nuclear Physics A622 (1997) 16c-30c
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h on a nucleon N is given by: 1 do'h(3g, z) _ ~~,n=1 e~q,(x)Dh(z)
~,o,
dz
E,:I e~q~(~)
(6)
The spin asymmetry for a hadron h can be expressed in the quark model as
A~(x) = E'~=~e~Sq,(x)D~(z)
EL-, e ~ q , ( z ) D ~ ( z )
"
(7)
As an example, consider the spin asymmetry for polarized 7r- electroproduction. In the parton picture, because of the dominance of the ~ channel in fragmentation to n-(~d), the ~r- asymmetry should be sensitive to the sea polarization. Application of the standard parton model with the assumptions of charge conjugation and isospin invariance of the quark fragmentation functions [12,13] gives the following form for
A~~- 45uv(x) +riSdv(x) 4 5 ( 1 +ri)5~(x) +25s(x) Ap = 4uv(x) +ridv(x) +5(1 +ri)~(x)+2s(x)
(8)
Here 5uv, 5dv are the polarized valence quark distributions; 5~ indicates the polarized light sea with 5us = 6~s = 5d8 = 6d,; 5s is the polarized strange quark distribution; and uv, dy, -~, and s axe the analogous unpolarized counterparts, ri is the ratio of favored (struck quark) to unfavored fragmentation functions, l+z D(z) =riD(z) ; ri~ l - z "
(9)
The form, originally suggested by Feynman and Field [14], has been confirmed by the EMC collaboration [15]. A~- contains contributions from both valence and sea quarks. At small x, where the sea dominates, a substantial sea polarization will generate large observable r - asymmetries. A central thrust of the HERMES program is to isolate the different quark contributions as a function of x to the nucleon spin. This involves applying the quark parton model to the data set and hence invokes several assumptions, e.g. factorization, spin-independence of fragmentation, knowledge of fragmentation functions etc. However, these assumptions can be tested. For example, fragmentation functions can be extracted from the HERMES unpolarized data. Further, the spin dependence of the fragmentation process can be studied. It is anticipated that within several years the HERMES data set can be used to extract precise information on the quark contribution to nucleon spin within the kinematic range of the experiment.
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2.2
Decomposition of valence spin
The inclusive polarized measurements do not distinguish between valence and sea con tributions to the quark spin. To isolate the valence spin contribution it is necessary to detect in coincidence the leading meson/12,13]. Consider the measurement of spindependent semi-inclusive pion asymmetries. For different relative polarizations of the incident positron and the target the data can be combined to generate the experimental asymmetry:
A
=
+-
(10)
TC"b - - ~ r - -
N~(,) (x) is the difference between the number of leading 7r+ and rr- produced in a given bin of the Bjorken variable x for anti-parallel (parallel) target polarization. Note that in this quantity the non-leading contributions from resonances and vector meson decays are expected to cancel. This asymmetry can be used to determine the valence quark spin distributions 5uy(x) and 5dv(x) in the nucleon. The asymmetries expected for hydrogen, deuterium and 3He targets are:
45uy(x) - 5dr(x) 5uv(x) + 5dr(x) . A~ = 4 u v ( x ) - dy(x) ; A~d = u v ( x ) + d r ( x ) '
A3~H" =
- S u v ( x ) + 4~dv(x) 7uv(x) + 2dr(x) "
(11)
(12)
The asymmetries on the proton and deuteron have been measured for the first time by the SMC collaboration [16]. Data with relatively large statistical uncertainties and no pion identification yield results in agreement with expectations based on the inclusive asymmetry measurements. HERMES will yield higher precision data with clean pion identification over the next several years.
2.3
The polarized strange sea
The central result of the inclusive measurements, namely that the Ellis-Jaffe sum rule is violated, indicates that As = -0.12 4- 0.04 [4]. It is of great importance to measure 5s(x). One possible method is to combine both inclusive and semi-inclusive spindependent data [12,17]. The essential idea is to use combined 7r+ and rr- data as a means of tagging the scattering from u and d quarks. An appropriate subtraction of the semi-inclusive pion yields from the inclusive yields the effect of the strange quarks.
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Following [17], consider the total spin-dependent yields of zr+ + zr-:
~
×
~z
;
~
×
~
(13)
D(z) is defined as the probability for a u quark to fragment into either a 7r+ or 7r-, i.e. D(z) = D:++~-(z) = D-~++'-(z) = D'~++~-(z) = O~++'-(z).
(14)
Further, defining the quantities
c~ = nt+(x, z) + nit(x, z) - 2D(z) ; j3 = nt+(x, z) - nit(x, z),
(15)
Ss(x) can then be written as
~s(~) =
1 + ~ x A~(x) z × s(x). -Z A~(x)
(16)
Either 5s(x) can be determined for a given s(x) or the ratio ~ can be extracted. The favored and unfavored fragmentation functions must be known. The HERMES 95 and 96 data will allow such an analysis to extract 5s(x) for the neutron and proton. Further, the fragmentation functions can be determined experimentally from the HERMES data.
2.4
Nucleon spin and the A
A's are unique among light hadrons in that their polarization can be easily reconstructed experimentally from their dominant non-leptonic decay A --+ p~r. Other hadrons with spin do not preserve polarization information in their decay products because the decays preserve parity. Jaffe has argued [18] that the quark spin distributions in the A can be estimated by applying SU(3) symmetry to the distributions measured in the nucleon. The experiments at CERN and SLAC have obtained integrated u, d, and s distributions [4]: A u + A ~ ~ 0.80 ; A d + A ~
--0.47 ; As + A ~
--0.12.
(17)
Assuming SU(3) symmetry of the light-quark sea: A~ = A d = A~ = As, then [18] A u ~ 0.86 ; Ad ~ -0.41 ; As ~ --0.06.
(18)
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Applying SU(3) symmetry, Jaffe obtained for the :\ [18] AuA ~ --0.12 ;AdA ~ -0.12 ; AsA ~ 0.66.
(iv)
The conclusion is somewhat sensitive to different parametrizations of the sea quark in the A. A u i and Aria are equal because the A is an isosinglet. If this correlation is also true for the spin-dependent A fragmentation functions, A~A and AriA, then the spin transfer to the A can be large and the u-quark dominates. Assuming u-quark dominance, the polarization of the A, Ph, in terms of the beam polarization, Pb, is
PA = Pb ~(2 - y) l+(1-y)2
A~(z, Q~) ~(x, Q2)
/20)
At HERMES energies and running parameters about 4,000 A events in the current fragmentation region would be sensitive to ~UA as small as 0.05. Finally, Jaffe has pointed out [18] that if a significant longitudinal q --+ A spin transfer is observed in the current fragmentation region, then the next step would be to orient the target spin transverse to the (unpolarized) positron beam and look for a transverse A polarization. If observed, this would provide the first measurement of the u-transversity distribution in the nucleon. The above discussion focussed on the current fragmentation region. It has also been shown [19] that by detecting A's in the target fragmentation region, the polarization of the strange quarks can be probed. HERMES also has extracted A's in the target fragmentation region.
3
The HERMES Experiment
The HERMES experiment is located in the East Hall of the 6.4 km circumference HERA positron-proton collider. The experiment is arranged such that the 27.5 GeV longitudinally polarized positron beam interacts with internal polarized gas targets without interference from the proton beam. The beam is self-polarized transverse to the beam direction by the Sokolov-Ternov effect [20]. Spin rotators located at the entrance and exit of the East straight section [21] precess the spin direction from vertical to longitudinal at the target position and, following the target, back to the vertical position. The polarization of the beam with the rotators in place has been measured for both positrons and electrons. Longitudinal polarizations of about 50% are stable and reproducible and are measured using a transverse Compton backscattering polarimeter with a systematic uncertainty of better than +5%. The polarization time is about 25 minutes. As a result of the ease with which the direction of the target polarization can be changed, it will be possible to study parallel and perpendicular polarization asymmetries for all targets.
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In 1995 data were taken with a polarized 3He target and at present HERMES is running with a polarized hydrogen target. The HERMES target consists of an open-ended thin walled storage cell through which the circulating positron beam of the HERA accelerator passes. A magnetic holding field provides a quantization axis for the target polarization. The target densities are about 7.5(5) × 1013 atoms/cm 2 for hydrogen (deuterium) and 3.5 × 101. atoms/cm 2 for 3He. The typical polarizations of the target are 90% (50%) for the hydrogen/deuterium (3He) sources and are measured to better than 4-5% with polarimeters. Luminosities are in the range of 4 - 30 × 1031 nucleons cm-2sec -1. The gases are pure so that there is no target dilution. The extremely small target thickness and absence of end windows minimize external radiative corrections. Unpolarized gases of hydrogen and deuterium up to 101~ atoms/cm 2 are also used for data taking. The maximum target thickness is determined by the impact on the stored positron lifetime. The HERMES spectrometer was designed to detect scattered positrons and hadrons from deep inelastic scattering. Here I give a brief overview; for a more detailed description and a summary of the spectrometer performance see the paper of E. Kinney in these proceedings. The spectrometer consists of two identical halves above and below the positron ring plane. This provides two independent measurements of observables and thus a crosscheck on systematic uncertainties. The HERMES spectrometer contains a dipole magnet with a bending strength of 1.3 T.m to measure particle momenta and to reject background. A system of segmented hodoscopes and a fly's eye lead glass calorimeter provide a deep inelastic scattering trigger. A transition radiation detector provides strong discrimination against hadrons. Tracking chambers before the magnet, in the magnetic field, and behind the magnet provide charged particle tracking. A pair of threshold gas Cerenkov counters are used for identification of pions. Each Cerenkov unit is divided into 20 cells viewed by individual photomultipliers. In 1995 the threshold for pions to trigger the Cerenkov counter was 6 GeV/c. In 1996 the gas composition has been modified to reduce the threshold to 4 GeV/c. Relative luminosity is monitored with ~ 1% precision by the coincident detection in symmetrically placed bismuth tungstenate calorimeters of the positron-electron pair from symmetric Bhabha scattering off the target atomic electrons. The target empty luminosity rate is negligible. A collimator system prevents the large flux of synchrotron photons from impinging on the target cell. The angular acceptance of the experiment is 40 < /9 < 220 mrad. The kinematic range accessible is 0.02 < x < 0.8 and 0.2 < Q2 < 20 (GeV/c) 2.
4
Preliminary results from 1995 running
During the 1995 HERA running period, the HERMES experiment was commissioned from June until late August, by which time the polarized 3He target and spectrometer were operating reliably and adequate background conditions had been established for the experiment. Production data were taken from mid-August to late November which resulted in about 5 million inclusive deep-inelastic events from 3He and about 1 million
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Z~
M~P~ 2OO
,,o ~ J Ol*fl6
=
(7 =
1.1157
J
0.0021
~._%___,~ i
,
i
,
i
1.1
] 1.12
i
i
i
I 1.14
i
i
i
i 1.16
i
i
i
i 1.18
i
mass, G e V ~
Fig. 1. The 3He(e,d A) signal observed by HERMES in 1995. unpolarized DIS events on hydrogen and deuterium. The average target polarization during production data taking was 0.47 with a fractional uncertainty of 5%. The target spin orientation was reversed every 10 minutes. In addition, the target polarization was monitored by detecting the circular polarization of light due to excitation by the HERA beam. The average beam polarization during production data taking was 52% with a fractional uncertainty of 5.5%. Table 1 Approximate hadron yields from 5 million deep-inelastic scattering events on longitudinally polarized 3He. Particle
Approx. Yield Particle
Approx. Yield
h+
450k
~
~2k
h-
300k
¢
~2k
~+
53k
p0
10k
~-
38k
A°,A°
~2k
~o
lOOk
J/~
~50
K~
~2k
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The approximate particle yields obtained during the 1995 running are presented in Table 1. In addition to copious numbers of pions, a clean sample of p°'s has been reconstructed from the data (see the contribution of G. van der Steenhoven to these proceedings). Further, ,-~ 2,000 A's were produced. Figure 1 shows the yield of h's produced in 1995 running. In addition, a clean sample of J/k~ events has been extracted. A minimum momentum transfer of Q2 > 1.0 (GeV/c) 2 facilitates interpretation of the data in terms of the quark parton model. A cut on the invariant mass of W2>4 GeV ~ eliminates contributions from the resonance region. A cut on the energy loss of y<0.85 eliminates large radiative corrections. With these cuts measurements can be made over the range 0.02 < x < 0.8 A limited axaount of data was taken in 1995 with unpolarized hydrogen and deuterium targets to establish the reliability and precision of the HERMES spectrometer. A number of interesting observables can be extracted with these data. The structure function ratio F~/F~ of the neutron and proton can be measured directly from the ratio of the DIS yields for the proton and the deuteron. Instrumental efficiencies and acceptances cancel to first order. Only corrections for kinematic smearing, radiation, and nuclear binding effects need be considered. The results [24] agree well with measurements from NMC [25] and SLAC [26]. From the 1995 unpolarized semi-inclusive data on hydrogen and deuterium it is possible to extract the valence quark ratio, dv(x)/uy(x), for the proton [24]. This is the spinaveraged analog analysis of the valence spin extraction described above. Because the leading hadrons in fragmentation contain the struck quark, the pion multiplicities are sensitive to the quark flavor distributions. The ratio, dy(x)/uy(x), can be estimated from the measured pion ratio, R~(x):
N~+(x)-N~-(x) Nf(x).(l+ F~(x)]_l, g;-(~) N~+(~------~ F;(x)'
n~(x) = g;+(x)
(21)
where N~(x) = f dN'~dz and N ~+ is the scattered positron yield. The ratio of quark dz distributions can be calculated directly f r o m / ~ ( x ) :
dr(x) uv(x)
4R~(x) + 1
R,~(x) + 4
(22)
Preliminary results obtained for this ratio are presented in Figure 2 for two analyses. In one, the measured ratios result from an analysis using all hadrons and correcting [27] for the contamination of the pion sample by kaons and nucleons. In the second, only identified pions are used. The agreement with earlier experiments is excellent. Figure 2 provides a strong indication that the quark parton model can be correctly applied to semi-inclusive data in the HERMES kinematic regime. With the improved particle identification implemented in 1996, HERMES will provide new information on this ratio.
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R.G. Milner/Nuclear Physics A622 (1997) 16c-30c
• A
CDHS WA21/25
.o" 0.8
!
0.6 ,
0.4 0.2 0 -0.2 -0.4
!
i
I
©
i
A
HERMES EMC
q
I
(hadrons,
i
z > 0.1)
~ i i I
I
1
" 1.2 > "o
0.8 0.6 0.4 0.2 0 -0.2
-0.4
i J
•
o ,
HERMES
(plons, p>5.5)
EMC ,
,
,,
,
,I
10
-1
,
,
,
,
, ,I 1 X
Fig. 2. The valence quark distribution function ratio (statistical errors only for HERMES95 (preliminary), for WA21/25, EMC and CDHS stat. and sys. combined). The upper plot shows the HERMES95 (preliminary) and EMC result using all hadrons. The triangles are measurements from neutrino/anti-neutrino experiments. The line is the CTEQ3M parameterization for the valence quark distribution functions. The lower plot shows the same EMC data and the CTEQ fit but with preliminary HERMES 95 data using the pion PID, in which case the correction for the hadron fragmentation functions is not needed. The dashed lines indicate the naive QPM expectation with exact SU(6) symmetry.
A preliminary analysis of the data taken with a polarized 3He target has been carried out in order to extract the spin structure function g'((x) [31]. The inclusive spin asymmetries were extracted from the scattered positron yields using the luminosity monitor for relative normalization. Radiative corrections were made by means of the techniques of Akushevich and Shumeiko [28]. Instrumental smearing was confirmed to be small. Pending a more complete understanding of spectrometer instrumental effects and unresolved issues of analysis procedures, points at lowest and highest x were excluded from the results reported here. Stringent quality cuts eliminated from analysis over half of the ~ 5 million events accumulated. The preliminary results for g~ are presented in Figure 3. Only statistical errors are shown. The results are in good agreement with the data reported from SLAC experiment E-142 [1] also using a 3He target.
R. G. Milner /Nuclear Physics A622 (1997) 16c-30c
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0.4
•
HERMES (PRELIMINARY)
• E-142 ( P U B L I S H E D )
0.2
-0.2
i
-0.4
-0.6 10 -1
1 XBj
Fig. 3. The preliminary result for the spin structure function g~(x) obtained from the inclusive spin asymmetry on 3He measured in 1995 by HERMES [30]. Also shown are the published data from SLAC measurement E-142 [1].
The Ellis-Jaffe integral P'~ = f(¢ g'~(x)dx can be estimated from the data on g~(x) by evolving the points to a fixed value of Q2 = 3 (GeV/c) 2 assuming that the measured asymmetry A'~(x) is independent of Q2. Simple models employed in the analyses of previous experiments have been used to extrapolate g~(x) outside the measured region, giving g'~(x) =const for x < 0.03 [29] and g'~(x) ,~ (1 - x) 3 for x > 0.6 [30]. With these extrapolations, the integral of g'~(x) is [31] P~ = -0.032±0.013stat.+0.017~yst. The relatively large statistical and systematic errors reflect the preliminary character of the analysis. Preliminary semi-inclusive pion asymmetries from 3He have also been extracted [32]. Figure 4 shows the preliminary asymmetry from HERMES 95 running for semi-inclusive negative hadron production on polarized SHe (z > 0.2) compared to SMC data on the proton and deuteron. The asymmetry at low x is negative, as expected from the quark parton model and inclusive data on the neutron.
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I [I'G SUC - At(p)for h+ (Z,gt.O.2) D SMC - A l ( d ) for h+ (Z.gt.O.2) HERMESPREUMINARY- A1 (~4e) for h+
"0
(z.qt.o.2)
0.8
0.6
°"f 0.2 ,
iiJ,i 10-2
I
i
B i i,,,,I
, ~
1°-~
Xb]
Fig. 4. The preliminary asymmetry from HERMES 95 running for semi-inclusive negative hadron production on polarized 3He (z > 0.2) [31] compared to SMC data on the proton and deuteron [15].
5
Conclusions
The HERMES experiment started data taking in 1995. The new technique to measure spin-dependent deep inelastic scattering works well, as proposed. Inclusive and semi-inclusive processes are simultaneously measured, and backgrounds are low. Unpolarized structure function and charged pion multiplicity ratios have been extracted from data with unpolarized targets and are in good agreement with previous data. The preliminary result for g~ obtained with polarized 3He is consistent with data from earlier experiments. High yields of a range of hadrons are detected with good particle identification. In 1996 data taking is proceeding on a polarized hydrogen target.
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Acknowledgement
I wish to thank my colleagues in the HERMES collaboration. The extraction of 5s(x) is based on the work of D. DeSchepper and A. Mateos and also a conversation with A. Sch~fer. My understanding of the motivation for the A measurements benefited greatly from a visit of R. Jaffe to DESY. I thank H.E. Jackson for assistance in the preparation of this manuscript. The author's research is supported in part by the U.S. Department of Energy, Nuclear Physics Division under cooperative agreement No. DEFC01-94ER40818. I thank M I T and DESY for support while on research leave.
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[20] A. A. Sokolov and I. M. Ternov, Sov. Phys. Doklady 8, 1203 (1964). [21] J. Buon and K. Steffen, Nucl. Instr. Methods A245, 248 (1986). [22] P. Hoodbhoy, R. L. Jaffe, and A. Manohar, Nucl. Phys. B312, 571 (1989). [23] R. L. Jaffe and A. Manohar, Phys. Lett. B223, 218 (1989). [24] K. Ackerstaff, PhD thesis, Univ. Hamburg (1996). [25] P. Amaudruz et al., Nucl. Phys. B371, 3 (1992). [26] L. W. Whitlow et al., Phys. Lett. B282, 475 (1992). [27] J. Ashman et al., Forward produced hadrons in #p and #d scattering and investigation of the charge structure of the nucleon, CERN-PPE/91-53, March (1991). [28] I. V. Akushevich and N. M. Shumeiko, J. Phys. G: Nucl. Part. Phys. 20, 513 (1994). [29] J. Ellis and M. Karliner, Phys. Lett. B213, 73 (1988). [30] S. J. Brodsky et al., Nucl. Phys. B441, 197 (1995). [31] M.C. Vetterli, Plenary Talk at the 6th International Workshop on Deep Inelastic Scattering and Related Phenomena, Rome, April 15 20, 1996. [32] E.E.W. Bruins, contribution to the Proceedings of the 12th International Symposium on High Energy Spin Physics, Amsterdam, September 10 - 14, 1996.
ELSEVIER
The HERMES Experiment
R.G. Milner Massachusetts Institute of Technology , Cambridge , MA 02139, USA and Deutsehes Elektronen-Synchrotron, Notkestrasse 85, 22603 Hamburg, Germany
On Behalf of the HERMES Collaboration
Abstract
HERMES, H._EERAMeasurement of Spin, is an experiment underway at DESY to study the spin structure of the nucleon by using polarized internal gas targets in the HERA 27.5 GeV electron storage ring. Inclusive and semi-inclusive spin dependent deep inelastic scattering are simultaneously measured in a forward spectrometer. In 1995 the experiment was installed and commissioned and data were taken on a polarized 3He target as well as on unpolarized targets of hydrogen and deuterium. In this paper, several aspects of the spin structure of the nucleon which can be probed by HERMES are discussed. The experiment is briefly described and some preliminary results from 1995 running are presented.
1
Introduction
The study of how the spin of the nucleon is constructed from its elementary constituents remains an i m p o r t a n t and fundamental problem. Twenty years of measurements at SLAC and CERN [1-4] have almost completely focussed on inclusive, spin-dependent deep inelastic scattering over the kinematic range 0.003 < x < 0.6. Here x = ~ is the usual Bjorken scaling variable. To date, emphasis has been placed on the determination of the "Ellis-Ja:ffe integrals", i.e. the first moments of the gl(x) spin structure function of the nucleon. Using an analysis based on the quark parton model with QCD corrections, the central conclusion of these polarized deep-inelastic scattering experiments is that the spin fraction A E carried by the quarks in the nucleon is 0.20 + 0.11 at Q2 = 10 (GeV/c) 2 [4]. This is much less than that predicted by models of nucleon --
0375-9474/97/$17.00 © 1997 - Elsevier Science B.V. All rights reserved. PII: S0375-9474(97)00329-1
--
2rn~,
R. G. Milner /Nuclear Physics A622 (1997) 16c-30c
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spin structure. In addition, it is deduced that the strange quarks in the nucleon have significant negative polarization/ks = - 0.12 + 0.04. Many open questions remain: -
* • • • •
what is the what is the what is the how is spin what is the
contribution of the valence quarks? contribution of the sea quarks, in particular the strange quarks? role of orbital angular momentum? distributed in other hadrons, e.g. A? role of glue?
HERMES has been designed [5] to address many of the open questions concerning the spin structure of the nucleon using a new experimental technique to measure spindependent deep inelastic scattering. Simultaneously, both inclusive and semi-inclusive spin-dependent scattering are measured to high precision with good particle identification. Measurements are made on undiluted polarized targets of hydrogen, deuterium, and 3He for any target spin state. Semi-inclusive spin asymmetries resulting from hadron particle identification provide the means to investigate the spin structure of the nucleon in a new way.
2
T h e Spin S t r u c t u r e of t h e N u c l e o n
Rather than give a comprehensive review of the HERMES physics program (see [5]), I would like to illustrate the potential of the experiment to study the spin structure of the nucleon by providing a brief overview of spin dependent deep inelastic scattering in the context of the quark parton model. Then I will provide three examples of measurements which are in progress by HERMES.
2.1
The quark patton model
In the quark parton model the unpolarized structure function FI(x) and the spindependent structure function, gl(x) are expressed as
F,(x) = -~ ~:,
(1)
where the quark distribution functions, q/('~)(x), refer to quarks with flavor i and spin parallel (antiparallel) to the nucleon spin. Assuming the spin-dependent structure function g2(x) is small, as supported by recent data [6], the polarization asymmetry, A~(x), is formed from the spin-dependent cross-
R.G. Milner/Nuclear Physics A622 (1997) 16c-30c
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sections ~ri-* and c%,, and is related to
gl(x) and &(x) by
A~((x) _ °"t+a~+ +- a~°*~~ D(x, Y) gl(x) = D(x., y)A~ (x),
where the virtual photon depolarization factor y = ~ atJ s
D(x, y) =
y(2 - y) y2 + 2(1 - y)[1 +
(2)
D(x, y) is expressed in terms of x and
R(x,y)]"
(3)
R(x, y) is the ratio of cross sections for longitudinally and transversely polarized virtual photons. The first moment of gl(x) provides one constraint on the flavor dependence of the net quark helicities, Aq, = fl 5qi(x) = f~[q~(x) - q~(x)]dx:
/g~(x)dx = -~(-~Au 1 4 + ~Ad + 1As)(1 -- O(a~)).
(4)
Further constraints on the Aq,'s also come from applying isospin and SU(3) flavor symmetries [7] to neutron decay and semi-leptonic hyperon decay data: Au-Ad=F+D;
Au+Ad-2As=3F-D.
(5)
where F and D are the symmetric and anti-symmetric weak SU(3)/couplings, respectively, of the baryon octet. If one assumes in addition to SU(3)/ symmetry, that the strange sea is unpolarized [8], the matrix elements for neutron and hyperon decay alone fix the total quark spin in the nucleon, A u + A d + A s , at the value of ~ 0.6, the "canonical expectation". However, as noted above, experimental data indicate a value of only 0.2. The Bjorken sum rule [9], a fundaxaental relationship arising from current algebra between the difference in the first moments of the proton and the neutron, is confirmed by the data. Note that the inclusive results have also been interpreted [10] in terms of breaking of SU(3)/ symmetry with As = 0. Thus, in spite of careful and extensive efforts, the formulation of a model of nucleon spin structure, which accounts for all the spin in terms of the partonic constituents remains a clear challenge. Semi-inclusive deep inelastic scattering from the nucleon can also be understood in terms of the parton model [11]. The probability for a quark of flavor i to fragment into a hadron h is described by the fragmentation functions D~(z, Q2), where z = ~ is the fractional energy of the hadron. Assuming factorization and that scaling also holds for the fragmentation functions, the semi-inclusive cross-section for production of a hadron
R.G. Milner/Nuclear Physics A622 (1997) 16c-30c
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h on a nucleon N is given by: 1 do'h(3g, z) _ ~~,n=1 e~q,(x)Dh(z)
~,o,
dz
E,:I e~q~(~)
(6)
The spin asymmetry for a hadron h can be expressed in the quark model as
A~(x) = E'~=~e~Sq,(x)D~(z)
EL-, e ~ q , ( z ) D ~ ( z )
"
(7)
As an example, consider the spin asymmetry for polarized 7r- electroproduction. In the parton picture, because of the dominance of the ~ channel in fragmentation to n-(~d), the ~r- asymmetry should be sensitive to the sea polarization. Application of the standard parton model with the assumptions of charge conjugation and isospin invariance of the quark fragmentation functions [12,13] gives the following form for
A~~- 45uv(x) +riSdv(x) 4 5 ( 1 +ri)5~(x) +25s(x) Ap = 4uv(x) +ridv(x) +5(1 +ri)~(x)+2s(x)
(8)
Here 5uv, 5dv are the polarized valence quark distributions; 5~ indicates the polarized light sea with 5us = 6~s = 5d8 = 6d,; 5s is the polarized strange quark distribution; and uv, dy, -~, and s axe the analogous unpolarized counterparts, ri is the ratio of favored (struck quark) to unfavored fragmentation functions, l+z D(z) =riD(z) ; ri~ l - z "
(9)
The form, originally suggested by Feynman and Field [14], has been confirmed by the EMC collaboration [15]. A~- contains contributions from both valence and sea quarks. At small x, where the sea dominates, a substantial sea polarization will generate large observable r - asymmetries. A central thrust of the HERMES program is to isolate the different quark contributions as a function of x to the nucleon spin. This involves applying the quark parton model to the data set and hence invokes several assumptions, e.g. factorization, spin-independence of fragmentation, knowledge of fragmentation functions etc. However, these assumptions can be tested. For example, fragmentation functions can be extracted from the HERMES unpolarized data. Further, the spin dependence of the fragmentation process can be studied. It is anticipated that within several years the HERMES data set can be used to extract precise information on the quark contribution to nucleon spin within the kinematic range of the experiment.
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2.2
Decomposition of valence spin
The inclusive polarized measurements do not distinguish between valence and sea con tributions to the quark spin. To isolate the valence spin contribution it is necessary to detect in coincidence the leading meson/12,13]. Consider the measurement of spindependent semi-inclusive pion asymmetries. For different relative polarizations of the incident positron and the target the data can be combined to generate the experimental asymmetry:
A
=
+-
(10)
TC"b - - ~ r - -
N~(,) (x) is the difference between the number of leading 7r+ and rr- produced in a given bin of the Bjorken variable x for anti-parallel (parallel) target polarization. Note that in this quantity the non-leading contributions from resonances and vector meson decays are expected to cancel. This asymmetry can be used to determine the valence quark spin distributions 5uy(x) and 5dv(x) in the nucleon. The asymmetries expected for hydrogen, deuterium and 3He targets are:
45uy(x) - 5dr(x) 5uv(x) + 5dr(x) . A~ = 4 u v ( x ) - dy(x) ; A~d = u v ( x ) + d r ( x ) '
A3~H" =
- S u v ( x ) + 4~dv(x) 7uv(x) + 2dr(x) "
(11)
(12)
The asymmetries on the proton and deuteron have been measured for the first time by the SMC collaboration [16]. Data with relatively large statistical uncertainties and no pion identification yield results in agreement with expectations based on the inclusive asymmetry measurements. HERMES will yield higher precision data with clean pion identification over the next several years.
2.3
The polarized strange sea
The central result of the inclusive measurements, namely that the Ellis-Jaffe sum rule is violated, indicates that As = -0.12 4- 0.04 [4]. It is of great importance to measure 5s(x). One possible method is to combine both inclusive and semi-inclusive spindependent data [12,17]. The essential idea is to use combined 7r+ and rr- data as a means of tagging the scattering from u and d quarks. An appropriate subtraction of the semi-inclusive pion yields from the inclusive yields the effect of the strange quarks.
R. G. Milner /Nuclear Physics A622 (1997) 16c-30c
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Following [17], consider the total spin-dependent yields of zr+ + zr-:
~
×
~z
;
~
×
~
(13)
D(z) is defined as the probability for a u quark to fragment into either a 7r+ or 7r-, i.e. D(z) = D:++~-(z) = D-~++'-(z) = D'~++~-(z) = O~++'-(z).
(14)
Further, defining the quantities
c~ = nt+(x, z) + nit(x, z) - 2D(z) ; j3 = nt+(x, z) - nit(x, z),
(15)
Ss(x) can then be written as
~s(~) =
1 + ~ x A~(x) z × s(x). -Z A~(x)
(16)
Either 5s(x) can be determined for a given s(x) or the ratio ~ can be extracted. The favored and unfavored fragmentation functions must be known. The HERMES 95 and 96 data will allow such an analysis to extract 5s(x) for the neutron and proton. Further, the fragmentation functions can be determined experimentally from the HERMES data.
2.4
Nucleon spin and the A
A's are unique among light hadrons in that their polarization can be easily reconstructed experimentally from their dominant non-leptonic decay A --+ p~r. Other hadrons with spin do not preserve polarization information in their decay products because the decays preserve parity. Jaffe has argued [18] that the quark spin distributions in the A can be estimated by applying SU(3) symmetry to the distributions measured in the nucleon. The experiments at CERN and SLAC have obtained integrated u, d, and s distributions [4]: A u + A ~ ~ 0.80 ; A d + A ~
--0.47 ; As + A ~
--0.12.
(17)
Assuming SU(3) symmetry of the light-quark sea: A~ = A d = A~ = As, then [18] A u ~ 0.86 ; Ad ~ -0.41 ; As ~ --0.06.
(18)
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Applying SU(3) symmetry, Jaffe obtained for the :\ [18] AuA ~ --0.12 ;AdA ~ -0.12 ; AsA ~ 0.66.
(iv)
The conclusion is somewhat sensitive to different parametrizations of the sea quark in the A. A u i and Aria are equal because the A is an isosinglet. If this correlation is also true for the spin-dependent A fragmentation functions, A~A and AriA, then the spin transfer to the A can be large and the u-quark dominates. Assuming u-quark dominance, the polarization of the A, Ph, in terms of the beam polarization, Pb, is
PA = Pb ~(2 - y) l+(1-y)2
A~(z, Q~) ~(x, Q2)
/20)
At HERMES energies and running parameters about 4,000 A events in the current fragmentation region would be sensitive to ~UA as small as 0.05. Finally, Jaffe has pointed out [18] that if a significant longitudinal q --+ A spin transfer is observed in the current fragmentation region, then the next step would be to orient the target spin transverse to the (unpolarized) positron beam and look for a transverse A polarization. If observed, this would provide the first measurement of the u-transversity distribution in the nucleon. The above discussion focussed on the current fragmentation region. It has also been shown [19] that by detecting A's in the target fragmentation region, the polarization of the strange quarks can be probed. HERMES also has extracted A's in the target fragmentation region.
3
The HERMES Experiment
The HERMES experiment is located in the East Hall of the 6.4 km circumference HERA positron-proton collider. The experiment is arranged such that the 27.5 GeV longitudinally polarized positron beam interacts with internal polarized gas targets without interference from the proton beam. The beam is self-polarized transverse to the beam direction by the Sokolov-Ternov effect [20]. Spin rotators located at the entrance and exit of the East straight section [21] precess the spin direction from vertical to longitudinal at the target position and, following the target, back to the vertical position. The polarization of the beam with the rotators in place has been measured for both positrons and electrons. Longitudinal polarizations of about 50% are stable and reproducible and are measured using a transverse Compton backscattering polarimeter with a systematic uncertainty of better than +5%. The polarization time is about 25 minutes. As a result of the ease with which the direction of the target polarization can be changed, it will be possible to study parallel and perpendicular polarization asymmetries for all targets.
R. G. Milner /Nuclear Physics A622 (1997) 16c-30c
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In 1995 data were taken with a polarized 3He target and at present HERMES is running with a polarized hydrogen target. The HERMES target consists of an open-ended thin walled storage cell through which the circulating positron beam of the HERA accelerator passes. A magnetic holding field provides a quantization axis for the target polarization. The target densities are about 7.5(5) × 1013 atoms/cm 2 for hydrogen (deuterium) and 3.5 × 101. atoms/cm 2 for 3He. The typical polarizations of the target are 90% (50%) for the hydrogen/deuterium (3He) sources and are measured to better than 4-5% with polarimeters. Luminosities are in the range of 4 - 30 × 1031 nucleons cm-2sec -1. The gases are pure so that there is no target dilution. The extremely small target thickness and absence of end windows minimize external radiative corrections. Unpolarized gases of hydrogen and deuterium up to 101~ atoms/cm 2 are also used for data taking. The maximum target thickness is determined by the impact on the stored positron lifetime. The HERMES spectrometer was designed to detect scattered positrons and hadrons from deep inelastic scattering. Here I give a brief overview; for a more detailed description and a summary of the spectrometer performance see the paper of E. Kinney in these proceedings. The spectrometer consists of two identical halves above and below the positron ring plane. This provides two independent measurements of observables and thus a crosscheck on systematic uncertainties. The HERMES spectrometer contains a dipole magnet with a bending strength of 1.3 T.m to measure particle momenta and to reject background. A system of segmented hodoscopes and a fly's eye lead glass calorimeter provide a deep inelastic scattering trigger. A transition radiation detector provides strong discrimination against hadrons. Tracking chambers before the magnet, in the magnetic field, and behind the magnet provide charged particle tracking. A pair of threshold gas Cerenkov counters are used for identification of pions. Each Cerenkov unit is divided into 20 cells viewed by individual photomultipliers. In 1995 the threshold for pions to trigger the Cerenkov counter was 6 GeV/c. In 1996 the gas composition has been modified to reduce the threshold to 4 GeV/c. Relative luminosity is monitored with ~ 1% precision by the coincident detection in symmetrically placed bismuth tungstenate calorimeters of the positron-electron pair from symmetric Bhabha scattering off the target atomic electrons. The target empty luminosity rate is negligible. A collimator system prevents the large flux of synchrotron photons from impinging on the target cell. The angular acceptance of the experiment is 40 < /9 < 220 mrad. The kinematic range accessible is 0.02 < x < 0.8 and 0.2 < Q2 < 20 (GeV/c) 2.
4
Preliminary results from 1995 running
During the 1995 HERA running period, the HERMES experiment was commissioned from June until late August, by which time the polarized 3He target and spectrometer were operating reliably and adequate background conditions had been established for the experiment. Production data were taken from mid-August to late November which resulted in about 5 million inclusive deep-inelastic events from 3He and about 1 million
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Z~
M~P~ 2OO
,,o ~ J Ol*fl6
=
(7 =
1.1157
J
0.0021
~._%___,~ i
,
i
,
i
1.1
] 1.12
i
i
i
I 1.14
i
i
i
i 1.16
i
i
i
i 1.18
i
mass, G e V ~
Fig. 1. The 3He(e,d A) signal observed by HERMES in 1995. unpolarized DIS events on hydrogen and deuterium. The average target polarization during production data taking was 0.47 with a fractional uncertainty of 5%. The target spin orientation was reversed every 10 minutes. In addition, the target polarization was monitored by detecting the circular polarization of light due to excitation by the HERA beam. The average beam polarization during production data taking was 52% with a fractional uncertainty of 5.5%. Table 1 Approximate hadron yields from 5 million deep-inelastic scattering events on longitudinally polarized 3He. Particle
Approx. Yield Particle
Approx. Yield
h+
450k
~
~2k
h-
300k
¢
~2k
~+
53k
p0
10k
~-
38k
A°,A°
~2k
~o
lOOk
J/~
~50
K~
~2k
R. G. Milner/Nuclear Physics A622 (1997) 16c-30c
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The approximate particle yields obtained during the 1995 running are presented in Table 1. In addition to copious numbers of pions, a clean sample of p°'s has been reconstructed from the data (see the contribution of G. van der Steenhoven to these proceedings). Further, ,-~ 2,000 A's were produced. Figure 1 shows the yield of h's produced in 1995 running. In addition, a clean sample of J/k~ events has been extracted. A minimum momentum transfer of Q2 > 1.0 (GeV/c) 2 facilitates interpretation of the data in terms of the quark parton model. A cut on the invariant mass of W2>4 GeV ~ eliminates contributions from the resonance region. A cut on the energy loss of y<0.85 eliminates large radiative corrections. With these cuts measurements can be made over the range 0.02 < x < 0.8 A limited axaount of data was taken in 1995 with unpolarized hydrogen and deuterium targets to establish the reliability and precision of the HERMES spectrometer. A number of interesting observables can be extracted with these data. The structure function ratio F~/F~ of the neutron and proton can be measured directly from the ratio of the DIS yields for the proton and the deuteron. Instrumental efficiencies and acceptances cancel to first order. Only corrections for kinematic smearing, radiation, and nuclear binding effects need be considered. The results [24] agree well with measurements from NMC [25] and SLAC [26]. From the 1995 unpolarized semi-inclusive data on hydrogen and deuterium it is possible to extract the valence quark ratio, dv(x)/uy(x), for the proton [24]. This is the spinaveraged analog analysis of the valence spin extraction described above. Because the leading hadrons in fragmentation contain the struck quark, the pion multiplicities are sensitive to the quark flavor distributions. The ratio, dy(x)/uy(x), can be estimated from the measured pion ratio, R~(x):
N~+(x)-N~-(x) Nf(x).(l+ F~(x)]_l, g;-(~) N~+(~------~ F;(x)'
n~(x) = g;+(x)
(21)
where N~(x) = f dN'~dz and N ~+ is the scattered positron yield. The ratio of quark dz distributions can be calculated directly f r o m / ~ ( x ) :
dr(x) uv(x)
4R~(x) + 1
R,~(x) + 4
(22)
Preliminary results obtained for this ratio are presented in Figure 2 for two analyses. In one, the measured ratios result from an analysis using all hadrons and correcting [27] for the contamination of the pion sample by kaons and nucleons. In the second, only identified pions are used. The agreement with earlier experiments is excellent. Figure 2 provides a strong indication that the quark parton model can be correctly applied to semi-inclusive data in the HERMES kinematic regime. With the improved particle identification implemented in 1996, HERMES will provide new information on this ratio.
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R.G. Milner/Nuclear Physics A622 (1997) 16c-30c
• A
CDHS WA21/25
.o" 0.8
!
0.6 ,
0.4 0.2 0 -0.2 -0.4
!
i
I
©
i
A
HERMES EMC
q
I
(hadrons,
i
z > 0.1)
~ i i I
I
1
" 1.2 > "o
0.8 0.6 0.4 0.2 0 -0.2
-0.4
i J
•
o ,
HERMES
(plons, p>5.5)
EMC ,
,
,,
,
,I
10
-1
,
,
,
,
, ,I 1 X
Fig. 2. The valence quark distribution function ratio (statistical errors only for HERMES95 (preliminary), for WA21/25, EMC and CDHS stat. and sys. combined). The upper plot shows the HERMES95 (preliminary) and EMC result using all hadrons. The triangles are measurements from neutrino/anti-neutrino experiments. The line is the CTEQ3M parameterization for the valence quark distribution functions. The lower plot shows the same EMC data and the CTEQ fit but with preliminary HERMES 95 data using the pion PID, in which case the correction for the hadron fragmentation functions is not needed. The dashed lines indicate the naive QPM expectation with exact SU(6) symmetry.
A preliminary analysis of the data taken with a polarized 3He target has been carried out in order to extract the spin structure function g'((x) [31]. The inclusive spin asymmetries were extracted from the scattered positron yields using the luminosity monitor for relative normalization. Radiative corrections were made by means of the techniques of Akushevich and Shumeiko [28]. Instrumental smearing was confirmed to be small. Pending a more complete understanding of spectrometer instrumental effects and unresolved issues of analysis procedures, points at lowest and highest x were excluded from the results reported here. Stringent quality cuts eliminated from analysis over half of the ~ 5 million events accumulated. The preliminary results for g~ are presented in Figure 3. Only statistical errors are shown. The results are in good agreement with the data reported from SLAC experiment E-142 [1] also using a 3He target.
R. G. Milner /Nuclear Physics A622 (1997) 16c-30c
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0.4
•
HERMES (PRELIMINARY)
• E-142 ( P U B L I S H E D )
0.2
-0.2
i
-0.4
-0.6 10 -1
1 XBj
Fig. 3. The preliminary result for the spin structure function g~(x) obtained from the inclusive spin asymmetry on 3He measured in 1995 by HERMES [30]. Also shown are the published data from SLAC measurement E-142 [1].
The Ellis-Jaffe integral P'~ = f(¢ g'~(x)dx can be estimated from the data on g~(x) by evolving the points to a fixed value of Q2 = 3 (GeV/c) 2 assuming that the measured asymmetry A'~(x) is independent of Q2. Simple models employed in the analyses of previous experiments have been used to extrapolate g~(x) outside the measured region, giving g'~(x) =const for x < 0.03 [29] and g'~(x) ,~ (1 - x) 3 for x > 0.6 [30]. With these extrapolations, the integral of g'~(x) is [31] P~ = -0.032±0.013stat.+0.017~yst. The relatively large statistical and systematic errors reflect the preliminary character of the analysis. Preliminary semi-inclusive pion asymmetries from 3He have also been extracted [32]. Figure 4 shows the preliminary asymmetry from HERMES 95 running for semi-inclusive negative hadron production on polarized SHe (z > 0.2) compared to SMC data on the proton and deuteron. The asymmetry at low x is negative, as expected from the quark parton model and inclusive data on the neutron.
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I [I'G SUC - At(p)for h+ (Z,gt.O.2) D SMC - A l ( d ) for h+ (Z.gt.O.2) HERMESPREUMINARY- A1 (~4e) for h+
"0
(z.qt.o.2)
0.8
0.6
°"f 0.2 ,
iiJ,i 10-2
I
i
B i i,,,,I
, ~
1°-~
Xb]
Fig. 4. The preliminary asymmetry from HERMES 95 running for semi-inclusive negative hadron production on polarized 3He (z > 0.2) [31] compared to SMC data on the proton and deuteron [15].
5
Conclusions
The HERMES experiment started data taking in 1995. The new technique to measure spin-dependent deep inelastic scattering works well, as proposed. Inclusive and semi-inclusive processes are simultaneously measured, and backgrounds are low. Unpolarized structure function and charged pion multiplicity ratios have been extracted from data with unpolarized targets and are in good agreement with previous data. The preliminary result for g~ obtained with polarized 3He is consistent with data from earlier experiments. High yields of a range of hadrons are detected with good particle identification. In 1996 data taking is proceeding on a polarized hydrogen target.
R. G. Milner /Nuclear Physics A622 (1997) 16c-30c
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Acknowledgement
I wish to thank my colleagues in the HERMES collaboration. The extraction of 5s(x) is based on the work of D. DeSchepper and A. Mateos and also a conversation with A. Sch~fer. My understanding of the motivation for the A measurements benefited greatly from a visit of R. Jaffe to DESY. I thank H.E. Jackson for assistance in the preparation of this manuscript. The author's research is supported in part by the U.S. Department of Energy, Nuclear Physics Division under cooperative agreement No. DEFC01-94ER40818. I thank M I T and DESY for support while on research leave.
References
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