We discuss the potential of polarized HERA to measure the spin dependent part
of the total photoproduction cross-section at large $\sqrt{s_{\gamma p}}$.Comment: 7 pages, LaTeX, 1 figure generated as fig1.tex. To appear in Proc.
  Workshop on ``Physics with Polarized Protons at HERA'

Bass, S. D.

Brisudova, M. M.

De Roeck, A.

English

arXiv

The Drell-Hearn-Gerasimov sum-rule for spin dependent photoproduction relates the difference of the two cross-sections for the absorption of a real photon with spin anti-parallel {sigma}{sub A} and parallel {sigma}{sub P} to the target spin to the square of the anomalous magnetic moment of the target nucleon. The authors discuss the potential of polarized HERA to measure the spin dependent part of the total photoproduction cross section at large {radical}(s{sub {gamma}p})

Bass, S.D.

Brisudova, M.M.

UNT Digital Library

* Title: Authorfs): submitred to: Los Alamos THE DRELL-HEARN-GERASIMOV SUM-RULE AT POLARIZED HERA S.D. Bass, lnstitut fur Theoretiische Kernphysik, Universitat Bonn, Nussallee 14-16, D53115 Bonn, Germany M. M. Brisudova, T-5, MS 8283, Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545, USA A. De Roeck, DESY, Deutsches Elektronen Synchrotron, Notkestrasse 85,D-22607 Hamburg, Germany The Proceedings of the Workshop on Physics with Polarized Protons at CERN, European Laboratory for Particle Physics, Geneva, Switzerland, October 4-5, 1997. ; &  g-2 IT‘-- *= w - .  -. b- --- N A T I O N A L  L A B O R A T O R Y  I ~ DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, exprus or implid, or assumes any legal liability or responsibility for the accuracy, completeness, or use fulness of any information, apparatus, product, or process disclosod, or represents that its use would not infringe privately owned rights. Reference herein to any spc- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement, m m -  mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or.any agency thereof. The Drell-Hearn-Gerasirnov Sum-Rule at Polarized HERA S. D. Bass ', M. M. Brisudovd and A. De Roeck a Institut fur Theoretische Kernphysik, Universitat Bonn, Nussallee 14-16, 0-53115 Bonn, Germany Alamos, NM 87545, U.S.A. Theoretical Division, MSB283, Los Alamos National Laboratory, Los DESY, Deutsches Elektronen Synchrotron Notkestrasse 85, 0-22607 Hamburg, Germany Abstract We discuss the potential of polarised H E M  to measure dependent part of the total photoproduction cross-section 6. the spin at large The Drell-Hearn-Gerasimov sum-rule [?I (for reviews see [?, ?]) for spin dependent photoproduction relates the difference of the two cross-sections for the absorption of a real photon with spin anti-parallel OA and parallel ~p to the target spin to the square of the anomalous magnetic moment of the target nucleon, where u is the (LAB) energy of the exchanged photon, m is the nucleon mass and K is the anomalous magnetic moment. The first direct test of the Drell-Hearn-Gerasimov sum-rule will be made in experiments which are planned or underway at the ELSA, GRAAL, LEGS and MAMI facilities. These experiments will measure ( C A  - a p )  up to a photon LAB energy u = 3GeV (fi 2 2.5GeV). They will make a precise measurement of nucleon resonance contributions to (DHG), thus testing mul- tipole analyses of unpolarised single-pion photoproduction data [?I, as well as the contributions from strangeness production and vector meson dominance. There is no elastic contribution to (DHG). The high-energy part of ( (TA - a p )  is expected to be determined by Regge theory for fi 2 2.5GeV. In this note we briefly review possible Regge con- tributions to  (CTA - a p )  and summarise the present knowledge of these con- tributions from low Q2 ( N 0.45GeV2) inclusive photoproduction. Finally, we discuss the contribution that polarised HERA could make to the measure- ment of these high fi Regge contributions to the real-photoproduction DHG sum-rule. At large centre of mass energy squared (s = 2rnv+m2), soft Regge theory predicts [?, ?] Here a,, and af, are the intercepts of the isovector ul(1260) and isoscalar fl(1285) and fi (1420) Regge trajectories, which are usually taken between -0.5 and 0.0 [?I. ( We note, however, that the isotriplet part of the deep inelastic structure function g1 behaves like 5-0*5 in the range (0.01 < x < 0.12) in the SLAC data [?I, corresponding to an effective Regge intercept aal (Q2) N +0.5 at the relatively low deep inelastic Q2 N 3 - 5GeV2.) The In s/s term is induced by any vector component to the short range ex- change potential [?] - for example, two nonperturbative gluon exchange [?]. The 1/ ln2 s term represents any pomeron-pomeron cut contribution to ( (TA - ap). The coefficients N3, NO, Ng and N p p  in Equ.(2) are to be determined from experiment. To estimate the Regge contribution to (OA - a p )  at Q2 = 0 we take the low Q2 data from the E-143 [?] and SMC [?] experiments (0.25GeV2 < Q2 < 0.7GeV2) on This low Q2 data has the following features. First, the spin asymmetries AI; and Af show no significant Q2 dependence in the range of each experiment. Table 1: AI at large s and low Q2 Experiment ( Q 2 )  fi Af Ai' (Q2 5 0.7GeV2, fi 2 2.5GeV) E-143 0.45 3.5 0.077 f 0.016 +0.008 f 0.022 SMC 0.45 16.7 0.064 f 0.024 -0.013 f 0.020 Secondly, the isoscalar deuteron asymmetry At is consistent with zero in both the E-143 and SMC low Q2 bins. There is a clear positive proton asymmetry in the E-143 data, signalling a strong iso-triplet term in (oA - ap) at s N 12GeV2. The SMC Af data is less clear: combining the SMC low Q2 Af data yields a positive value for A;. However, the majority of these SMC points are consistent with zero. In Table 1 we combine the low Q2 data to obtain one point corresponding to each experiment. We impose the cut (Q2 5 0.7GeV2, fi 2 2.5GeV), so that the mean Q2 is less than 0.5GeV2 for each experiments, and so that our data set is well beyond the resonance region. We assume that the large fi AI is approximately independent of Q2 between Q2 = 0 and Q2 20.5 GeV2. For the total photoproduction cross- section we take (in units of pb), which is known to provide a good Regge fit for fi be- tween 2.5GeV and 250GeV [?]. (Here, the s+O*0808 contribution is associated with pomeron exchange and the s-0*4545 contribution is associated with the isoscalar w and isovector p trajectories.) Since the E-143 low Q2 data shows a clearly positive A? with the smallest experimental error, we choose to nor- malise to E-143. We estimate (UA - ap) 2 lOpb (Q2 = 0, fi = 3.5GeV) (5) The small isoscalar deuteron asymmetry A; indicates that the isoscalar contribution to A; in the E-143 data is unlikely to be more than 30%. In Fig. 1 we show the asymmetry A; as a function of fi between 2.5 and 250 GeV for the four different would-be Regge behaviours for (oA - op): that the high energy behaviour of ( o A  - op) is given 1. entirely by the ( a 1 , f l )  terms in Equ.(2) with Regge intercept either (a) -f (conventional) or (b) +; (motivated by the observed small x behaviour of sP-~) ) ,  2. by taking 2/3 isovector (conventional) a1 and 1/3 two non-perturbative gluon exchange contributions at  fi = 3.5GeV, 3. by taking 2/3 isovector (conventional) a1 and 1/3 pomeron-pomeron cut contributions at  fi = 3.5GeV. Looking to the future, the HERMES experiment expect to measure a low Q2 AT to the same accuracy as E-143 at fi N 7GeV. Photoproduction cross-sections can be measured in ep collisions at  HERA at high ,& energies. The dominant processes in ep collisions are yp inter- actions where the photon is on mass shell. The electron is scattered un- der approximately zero degrees with respect to the electron beam direction, which at HERA means that it remains in the beampipe. The energy of the scattered electron E: is however reduced to E: = E, - E,, with E, the inci- dent electron energy and E, the emitted photon energy. The HERA machine magnets in the beamline, which steer the beam into a closed orbit, act as a spectrometer on these off-momentum electrons. The experiments H1 and ZEUS have installed calorimeters to detected these kicked out electrons along the beamline. In case of H1 calorimeters (stations) are installed at  three lo- cations: at 8 m , 30 m and 44 m distance from the interaction point [?]. The stations accept (tag) electrons from different momentum ranges, which correspond to ,& ranges of 280-290 GeV, 150-250 GeV and 60-115 GeV respectively. At the central energy value of each region the acceptance of these devices amounts to 15%, 85% and 70% respectively. The locations of the stations may change as a result of the HERA luminosity upgrade, and are presently under study. Equ.?? shows that the yp cross section is large, of order of hundreds of microbarns. The photon energy spectrum emitted from an electron beam is given in the Weizskker-Williams approximation [?I. An integrated ep  luminosity at HERA of 1 pb-' can yield about N = 1500K yp events in each of the 30m and 44m stations, and about 10 times less in the 8 m station. Around the year 2000, spin rotators will be installed for the electrons, con- verting the natural transverse polarization of the electron beam into a physics wise more useful longitudinal one. Studies are being made to have also the proton beam at HERA polarize [?I which would enable polarized ep  and thus polarized yp collisions. Note however that the polarization of the pho- ton beam will be reduced by afactor D = y(2-y)/(y2+2(1-y)), the so called depolarization factor. Here y = syP/sep. For the three stations the measure- ments are at y = 0.09,0.44 and 0.90, leading to values of D = 0.094,0.52 and 0.98. The measured asymmetries at HERA are correspondingly reduced by this factor. If HERA is fully polarized, event samples of the order of 100 - 500 pb-' will be collected, with expected beam polarizations P, = Pp = 0.7. In the small asymmetry approximation the error on the asymmetries, SA1 can be calculated as l/(PePpfl). Due to data-taking bandwidths and trigger prob- lems presently not all tagged yp events are recorded. Assuming a data taking rate of 2 Hz for these events, also in future, leads to about 40 M events/year giving a reachable precision SA1 = l /(PePpfl)  = 0.0003. It is however not excluded that novel techniques in triggering, data-taking and data storage will become available and can be installed for the experiments, which would allow to collect all produced events, amounting to approximately 15,000M events in total for the three stations in a period of 3 to 5 years. This would lead to maximal reachable precisions of SA1 = 3.10-5 for a measurement at the 30m and the 44m station, and SA1 = for a measurement at the 8m station. Note that when compared with the true asymmetries as shown in Fig. 1, the depolarization factor D reduces the effective sensitivities with the numbers given above. Given the projected asymmetries, polarized HERA would be sensitive at 6 = 50GeV to a ( c r ~  - crp) falling no faster than s-l. Taking the conventional, a,, = -1 it would be sensitive to a two non-perturbative gluon exchange contribution which is not less than 30% in the E-143 data and to a pomeron-pomeron cut contribution which is not less than 3% in the E-143 data. At 6 = 250GeV, polarized HERA would be sensitive to ( ( J A  - up) which falls no faster than s-Oe6. and to a pomeron-pomeron The collider experiments record presently unpolarized ep  collisions. =I 2' cut contribution which is no less than 6% of the E143 A!. We note that a zero-result (no significant signal) would put an upper bound on the Regge contribution to the Drell-Hearn-Gerasimov integral. Support from the Alexander von Humboldt Foundation (SDB) and the United States Department of Energy (MMB) is gratefully acknowledged. References [l] S.D. Drell and A.C. Hearn, Phys. Rev. Lett. 162 (1966) 1520; S.B. Gerasimov, Yad. Fiz. 2 (1965) 839. [2] S. D. Bass, Mod. Phys. Lett. A12 (1997) 1051; Proc. Zeuthen Work- shop: Deep Inelastic Scattering OH Polarized Targets: Theory Meets Experiment [3] D. Drechsel, Prog. Part. Nucl. Phys. 34 (1995) 181. [4] I. Karliner, Phys. Rev. D7 (1973) 2717; R.L. Workman and R.A. Arndt, Phys. Rev. D45 (1992) 1789; A.M. Sandorfi, C.S. Whisnant and M. Khandaker, Phys. Rev. D50 (1994) R6681. [5] R.L. Heimann, Nucl. Phys. B64 (1973) 429; J. Ellis and M. Karliner, [6] S.D. Bass and M.M. Brisudovb, Bonn preprint TK-97-10 (1997). Phys. Lett. B213 (1988) 73. [7] F.E. Close and R.G. Roberts, Phys. Lett. B336 (1994) 257. [SI P.V. Landshoff, Proc. Zuoz Summer School, PSI Proceedings 94-01 (1994) 135, hegph/9410250. [9] S.D. Bass and P.V. Landshoff, Phys. Lett. B336 (1994) 537. [lo] The E-143 Collaboration (K. Abe et al.), Phys. Rev. Lett. 74 (1995) 346; The E-154 Collaboration (K. Abe et al.), Phys. Rev. Lett. 79 (1997) 26. [ll] The E-143 Collaboration (K. Abe et al.), Phys. Lett. B364 (1995) 61. [12] The Spin Muon Collaboration (D. Adams et al.), Phys. Lett. B396 (1997) 338; (B. Adeva et al.), CERN preprint CERN-PPE-97-118 (1997). [13] H1 Collab., I. Abt et al., Nucl. Instr. and Meth. A386 (1997) 310 and A386 (1997) 348. [14] C.F. Weizsikker, Z. Phys. 88 (1934) 612; E.J. Williams, Phys. Rev. 45 (1934) 729. [15] D.P. Barber et al., Proceedings of the Workshop Future Physics at HERA, Eds. G. Ingelman, A. De Roeck, R. Klanner, (1996) 1205. ? I I 1 0.1 0.01 A! 0.001 0.0001 10 100 fi Figure 1: The asyrnmetryAP as a function of fi for different Regge behaviours for ( ( T A - O P ) :  given entirely by (la) the ( a l ,  f1) terms in Equ.(2) with Regge intercept either -4 (conventional) or (lb) +k; (2) by 2/3 isovec- tor (conventional) a1 and 1/3 two non-perturbative gluon exchange contri- butions at fi = 3.5GeV; (3) by 2/3 isovector (conventional) a1 and 1/3 pomeron-pomeron cut contributions at fi = 3.5GeV. ,-- Publ. Date (1 1) Sponsor Code (1 8) -3u d E R  DOE 

The Drell-Hearn-Gerasimov sum-rule at polarized HERA

https://digital.library.unt.edu/ark:/67531/metadc694932/m2/1/high_res_d/645561.pdf

The Drell-Hearn-Gerasimov Sum-Rule at Polarized HERA

Abstract

Similar works

Full text

Available Versions

UNT Digital Library