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POLARIZATION PHENOMENA IN
ELECTROMAGNETIC INTERACTIONS AT INTERMEDIATE ENERGIES
V. Burkert
To cite this version:
V. Burkert. POLARIZATION PHENOMENA IN ELECTROMAGNETIC INTERACTIONS AT INTERMEDIATE ENERGIES. Journal de Physique Colloques, 1990, 51 (C6), pp.C6-283-C6-298.
�10.1051/jphyscol:1990623�. �jpa-00230889�
COLLOQUE DE PHYSIQUE
Colloque C6, suppl6ment au n 0 2 2 , Tome 51, 15 novembre 1990
POLARIZATION PHENOMENA IN ELECTROMAGNETIC INTERACTIONS AT INTERMEDIATE ENERGIES
V. BURKERT
Continuous Electron Beam Accelerator Facility, 12000 Jefferson Avenue, Newport News, Virginia 23606, U.S.A.
Resume. Des r6sultats r6cents de mesures de polarisation en interaction 6lectromagnCtique aux Cnergies intermidiaires sont discutbs. Les perspectives concernant les expCriences de po- larisation a u p r b des nouveaux accCl6rateurs d'dectrons it courant continu, ainsi qu'aupr2s de machines plus anciennes amkliories, sont bri&vement dkrites. En concfusion, on montre que Ies expCriences de polarisation vont jouer un r61e t r b important dans l'6tude de la structure du nucMon et des noyaux lkgers.
Abstract. Recent results of polarization measurements in electromagnetic interactions a t inter- mediate energies are discussed. Prospects of polarization experiments a t the new CW electron accelerators, as well as on upgraded older machines are outlined. I t is concluded that polariza- tion experiments will play a very important role in the study of the structure of the nucleon and of light nuclei.
I. INTRODUCTION.
In this talk I discuss some of the recent results in intermediate energy electromagnetic physics with polarized partides fiom an expenmentalist point of view. Polarized hydrogen and deuterium targets as well as recoil polarimeters have been developed over the past several decades. These have become reliable experimental tools, and are used in varies physics areas. Much of the recent experimental effort in electromagnetic physics has focussed on the nucleon and deuteron. With the development of =He as a polarized quasi-neutron target one may expect extensive experimental studies o{ the tnucieon system t o be carried out in the near term future. The physics possibilities using 3He as a target have been discussed in several presentations a t this conference. My focus here is on the physics with hydrogen and deuterium targets, and I want to discuss the major experimental trends which have emerged over the past few years, mainly in support of the physics programs for the new CW electron machines that are presently under construction.
Of course, in electron scattering experiments "polarizationn is always present, as a transverse polar- isation of the electric field vector of the virtual photon. To limit the scope of this talk, I will assume that the polarization of one of the external partides is measured.
JI. STRUCTURE OF THE NUCLEON.
The interest in the structure of the nudeon and its resonances, and their role in nudear properties has dramatically increased in recent years and is one of the major motivations in support of the planned experimental program a t the new electron machines. The structure of the nucleon may be probed in elastic electron nucleon scattering and in inelastic reactions induced by electrons or photons. These studies are of fundamental importance as they measure the charge and current distribution as well as the transition currents of the fundamental building blocks of matter. Knowledge of these quantities allows stringent tests of microscopic models applicable at low and intermediate energies, like QCD- based quark models, chiral bag models, Skyrme models, etc.. With increasing momentum transfer
Q2 the transition from the non-perturbative regime to the perturbative regime can be studied, where simple quark counting rules may apply1.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990623
C6-284 COLLOQUE DE PHYSIQUE
Figure 11.1 Result of the QCD analysis of the proton elantic formfaciorr' using A = 180MeV. The solid line reprerent8 the jit to the data.
11.1 Electromagnetic Formfactors.
In elastic electron nucleon scattering the hadronic current is specified by the electric and magnetic formfactors G E ( Q ~ ) and GM(Qa). These are related to the Dirac and Pauli formfactors F1 and F z
as:
GE
=
F1-
rF2, GM = F1+
F 2where T
= S.
The usual techniques for measuring the elastic formfactors is the Rosenbluth separation, where one makes use of the different angular dependence of the electric and the magnetic term in the unpolarized elastic cross section to separate lGEl and IGMI.This techniques ceases to be useful, when either G;
<
G&, or a t high values of Q2, where the magnetic contributions dominate both the angular dependent and the angular independent terms.Experimental information on the neutron electric formfactor GEn is rather poor for all non-zero Q'. In the absence of a free neutron target, we have to rely on scattering from the few-nucleon systems. Unlike for the proton, the Rosenbluth separation of GEn from GMn is diacult even at low Q2 because of the small size of GEn compared to GM,. At low Q2
,
GEn would be non-zero only if the charge distribution within the neutron were not uniform. The slope of GEn versus Qa, at low Q2 has been accurately measured, and it is positive, indicating that the neutron appeazs t o have a slightly positive core surrounded by a region of negative charge. For Qa<
1 GeVa G E ~ has been extracted from elastic electron-deuteron scattering data2t3. Using this procedure, however, i t is necessary to assume a model for the deuteron structure. The most recent analysis allows for a 15% systematic uncertainty in GEn forq2 <
O . S G ~ V ~ . For larger values of Q ~ , uncertainties in the deuteron wavefunction render the methods unreliable.The electric formfactor of the proton has been measured a t SLAC4 using Rosenbluth separation for Q' up to about 4 GeV2, with error bars of 20% at the highest momentum transfer. The proton magnetic formfactor, on the other hand, has been measured for Q2 up t o 31 GeV2. An analysis of the electromagnetic formfactors by Gari and Stefanis5 dowed extraction of the QCD scale parameter AQCD. In lowest order A Q c ~ can be defined in relation to the strong coupling constant a,:
where nf is the number of quarks with mass lower than Q'. Assuming a conventional meson picture at small Q2, and asymptotic QCD predictions a t high Q2
a fit to the electromagnetic formfactors (Figure 11.1) yielded values for hecD between 100
-
250MeV/c. The uncertainties are mainly due to the limited knowledge of GEp at Qa
2
2GeV2. A more precise determination of this parameter would be very interesting since AQcD is not well determined from other measurements.11.2 Polarization Techniques to Measure GE, and GSp
Two methods appear very promising in that they allow measurement of GB, in a model independent fashion. Both methods employ quasi elastic scattering of polarized electrons off deuterium. In the first case polarized electrons are scattered off polarized deuteriumd. In the second case an unpolarized deuterium target is used and the polarization of the recoil neutron is measured in a second scattering experiment7. Both methods are equivalent, and allow measurement of the polarization asymmetry
where z ~ ~ , v T , v & , V&& are known kinematic quantities. By varying 6, the angle between the nuclear spin and the direction of momentum transfer, it is possible to pick out the longitudinal and transverse pieces of the quasi elastic spin dependent cross section. In particular, if B = 90°, the asymmetry is proportional to GEn/GMn. Knowing GM, the electric formfactor can be determined.
T h e first method uses a polarized deuterium (or 3He) target, ather as a ultra thin gas targetare in an electron storage ring, or a solid state target (ND3)1°*11*12, or a dense 3He 13*14 gas target, in an external electron beam. In case the polarization asymmetry is measured using vector polarized deuterium, it will be necessary to measure the recoil neutron in coincidence with the scattered electron to veto against the much larger asymmetry effects from the polarized proton in the deuteron.
Model c a l c u l a t i ~ n s ~ ~ ~ ~ ~ show (Figure 11.2) that the polarization asymmetry is linearly dependent on GE,/GMn as long as the recoil neutron is
Figure 11.2 Doubk polarization asymmetry A.1 for v a t i o w wsumptionr on G.,, and different interac- tion modelrls.
Figure II.3 Pmjected data for a m e ~ r e m e n t of Gcn at CEBAP wing nail polakation techniquesm.
COLLOQUE DE PHYSIQUE
Figure 11.4 Electropdtlctwn of hadronic final ahtes via a-channel resonance decays. The 7NN' oerlez
W dea& by the photocoupling helicity amplitudes Allat Asp. Si/a-
-0.10
0 l 2 3
Q ' (CCV')
Figure 11.5 Compariaon of ezpm.mentaI data on the interference km R~(E~+M:+/IM:+I for the A(1232) with variow modcl cakulationr.
emitted a t small angles with respect to the direction of the virtual photon. In this region the influence of the deuteron wave function on the extracted value of GEn/GMn is almost absent. A quantitative check can be accomplished by measuring the proton asymmetry a t the same time, and by comparing it to the asymmetry obtained with a polarized hydrogen target (e.g. NHS). For I H e nudear corrections are not negligible1? and will have to be carefully studied when extracting GEn.
Employing state of the art polarirred target technology it appears feasible to measure GEn for Q' up to 0.5 GeV2 with a 1 GeV electron beam, and up to 2 or 3 GeVa using a 4 GeV beam. Projected error bars for a measurement a t CEBAF which employs the recoil polarization techniques are shown in Figure 11.3.
The polarization techniques discussed above can also be used to measure the electric formfactor GBp of the proton. Most promising in this respect appears to be the recoil polarization techniques using an unpolarized hydrogen targetl8, and the measurement using a polarized NHS target19. With a 4 GeV beam values of Q' up to 5 GeVz can be reached with either techniques. Recent achievements in polarized target technologyz0 may allow one to push this limit to even higher values.
11.3 Nucleon Resonance Transition Formfactors
A large number of resonances, attributed to the excitation of the nucleon have been observed in hadron scattering, the A(1232) being the most prominent one. Electroucitation of resonances on the free nucleon yields information on the 7, NN* vertex as a function of Q' (Figure 11.4). The transition into a specific excited state is described by three amplitudes AlI2(Q2), Aslr(Q2), SlIz(Q2), where A and S refer to transverse and scalar coupling, respectively, and the subscripts refer to the total helicity of the 7, N system (frequently, the electromagnetic multipoles El*, M,*, Sl* are used instead). Inclusive measurement of 7 N -( X, or eN --+ e x reveal a few broad bumps, clearly indicating the excitation of resonances in the mass region below 2 GeV. Their broad widths and close spacing makes it impossible to separate them in inclusive production reactions. By explicit measurement of the decay products such as xN,
f l ,
pN, xA, and others, it is possible to identify them according to their spin and isospin assignments.The physics issues one hopes to study are manifold, and address fundamental questions about the interaction of quarks and gluons in confined systems. Specifically, one would like to study how the transition between the 3-quark ground state and excited states is mediated. Most models assume that the excitation is due to a single quark transition. However, recent studieszl indicate that double quark transitions may be present at a non-negligible level. Measurement of the Q2 evolution
of the transition formfactors provides information about the wave function of the excited state. The helicity asymmetry
for the transition into some excited states, like the Dls(1520)
,
or Fls(1688), was found to be sensitive to the potential that confines the valence quarks inside the nucleona1. At high momentum transfer one may observe the transition fiom the non-perturbative to the perturbative regime, where power law rules for the helicity amplitudes are predicted to applyZZ such that:if logarithmic terms are neglected. Whether this regime can be reached a t the available energies (e.g. 4 GeV a t CEBAF) is an open question. None the less, it will be interesting to push studies of the helicity structure of nucleon resonance exdtations to the highest possible Q'.
The QCD motivated extension of the non-relativistic quark-modelaa predicts many states, in par- ticular a t higher masses, which have not been observed in r N reactions. Theoretical calculationsa4 indicate that the "missing" states tend to decouple from the r N channel, which would explain why they have not been seen in elastic r N scattering. Our picture of baryon structure could drastically change if these states did not exist. Several of these states are predicted to couple strongly to pho- tons (real or virtual) and may thus be searched for in photo- or electroproduction experiments. A complete program to study nucleon resonance transiti0n~~e.g. in 7. N -, r N , involves measurement of 6 (4 in photoproduction) complex, parity conserving amplitudes.
which makes i t necessary to measure at least 11 independent observables, not counting additional measurements to resolve quadrstic ambiguities. Experiments involving unpolarized paxticles only, allow measurement of only 4 response functions UT, CL, UTT, CAT:
Measurement of polarization observables yield information on many response functions (table)
I want to illustrate the significance of polarization measurements for this program with two examples, the rupA+(1232) and the 7upP&(1440) transitions.
C6-288 COLLOQUE DE PHYSIQUE
11.3.1 The Transition r a p -+ A+(1232) -+ N r .
In SU(6) symmetric quark models this transition is explained by a simple quark spin-fiip in the
L3, = 0 ground state, corresponding to a magnetic dipole transition MI+. The electric and scalar quadrupole transitions are predicted to be El+ = &+ = 0. In more elaborate QCD based models which include color magnetic interaction arising from the one-gluon exchange a t small distances, the A(1232) acquires an
LsQ =
2 component, leading to small electric and scalar quadrupole contribu- tions (e.g. IEl+/Ml+I c 0.01 a t Q2=
0). The ratio IEl+/Ml+I is predicted to be weakly dependent on Q2. At very high Q2 QCD predicts2s El+/Ml+ -+ 1, and Sl+/Ml+ -+ 0. Precise measurements of these contributions from Q2 = 0 to very large Q2 are obviously important for the development of realistic models of the nucleon. Present experimental information about El+ is shown in Figure 11.5, together with model predictions. The quality of the data is clearly not sufficient to discrimi- nate against any of the models. Experiments at MIT-Batesz6 and CEBAF27 are in preparation to measure the electric and scalar quadrupole transition over a large Qz range, nsing polarised electron beams and/or recoil polarimeters. In these experiments one obtains information about the termsThe imaginary parts of the biinear terms can only be accessed using polarization degrees of freedom.
They are particular sensitive to phase relations between the multipoles. If the multipoles were strictly in phase, these terms would vanish identically.
11.3.2 The Transition rap -P ~ A ( 1 4 4 0 ) --+ Nr
.
The Roper resonance Pll has a modestly strong photocoupling at Q2 = 0, but in electron scattering there is little evidence for its excitation both, in the inclusive cross section and in exclusive single pion production at Qz
<
lGeVZ. In an analysiszs of single pion production data at Q2=
l ~ e V ' , a small positive value was found for the A1/z(P~l) amplitude. At Q2 = 0 this value is larger and negative.One may speculate that the apparent quenching of the Roper a t Q'
>
0 might be accidental and related to a sero crossing of the transverse amplitude. The Pll may then be expected to "reappear"a t higher Q2. In fact, non-relativisti~~~ as well as relativisedzl quark models predict the Ali2(P11) amplitude to grow relative to the A(1232). In non-relativistic models A112(P11)/A1/2(A) Q', so that the Pll would dominate a t high
$.
W (GeV) W(GeV)
Figure 11.6 Predicted cross section for 7 . p + p* at Q' = lGeVa, E = 0.8, l?: = O., wing the amplifudes of G. Kroeseds. The sensitivity to the (small) amplitudes of the Roper resonance is shoum (left). Predicted target asymmetry 2'. for the same kinematics (tight). The sensitivity to the Roper and the SI1(1535) is shown. The dashed-dotted line is obtained when the nearby Sll resonance amplitudes a n rwitched o f f . Note that the dominant SII amplitude W known to better than 20 %.
It has been pointed out that the dominance of the Pll at high Qz will be a crucial test of the quark model classification as a N=2 radially excited statez9. Measurements with polarized proton targets19, or proton recoil polarization measurementss0 can provide a strong signature for the excitation of this resonance in electron scattering experiments. Figure 11.6 shows the sensitivity of the unpolarized cross section and the polarised target asymmetry
TT,
in y,p + pro to the&l
amplitudes. Clearly, measurement of polarization observables will have an important impact on the determination of the photocoupling amplitudes of the Roper.ZI.4 Meson Photoproduction from Nucleons 11.4.1 Photoproduction of T Mesons.
Single pion photoproduction experiments have a long history, and a large body of data has been generated over more than two decades of experiments a t various laboratories. The goal has been to extract the 4 complex helicity amplitudes HI,
..,
H4 that completely specify the single pion produc- tion process. Data are needed in different isospin channels, like yp + p#, 7p -+ n r + , and 7 n +m-.
Such a complete set of data would then allow one to determine the photocoupling amplitudes of the N* and A resonances in a model independent fashion. At least 9 independent measurements are needed a t each kinematic point (E,,
@g),
including 5 double polarization measurementss1. Much cross section data and single polarization data have been accumulated, and recent measurements have focussed on obtaining the missing information from the double polarization experiments. Fig- ure 11.7 and 11.8 show recentresult^^^*^^
from the Yerevan 4.5 GeV electron synchrotron, in the 2,d and 3,d nucleon resonance region. The sensitivity to the ingredients of various phenomenologicd analysis is shown as well. The same analysis that correctly predicted the pzO results, fails in the nx+channel, which demonstrates the importance of measurements in different isospin channels. Unfor- tunately, the impact of this data on an extraction of the resonant amplitudes in a phenomenologicd fit would likely be small, because of the large error bars.
11.4.2 Photoproduction of q Mesons.
At the Bonn stretcher ring ELSA preparations are under way to measure the target asymmetry in the reaction p(7,p)q in the region of the SlI(1535) nucleon resonances4. Only two of the known nucleon resonances, the s1(1535), and the P11(1710), have a significant branching ratio into the Nq channel.
The Pll(l?lO) has been proposed as a possible candidate for a hybrid stateg5 (three valence quarks, one valence gluon). Precise measurements of the photocoupling amplitudes for proton and neutron targets are needed to clarify the assigment of this state as a conventional Qs or an exotic QsG state. If the P11(1710) is a hybrid state then A;lz(Pxl) = 0, and A;,,(Pll)
#
0. An experimental program, including polarization measurements, for a complete measurement of this channel should prove extremely interesting, and would be far less complex than the corresponding program for the NT channel.II.4.3 Photoproductbn of Strange Particles.
At CEBAF a program to study the reaction 7p + K+A +
v-
is in preparation. This reaction has been studied very poorly in the past, with the result that the production mechanism is not understood. If one takes a diagrammatic approach, one can hope to extract information about the KAN and KAN* coupling constants. Coupling constants, extracted from existing photoproduction data disagree with the ones extracted trom hadronic processes. Recent c a l ~ u l a t i o n s ~ ~ indicate that measurement of the A recoil polarization, as well as measurement of the beam asymmetry, are very sensitive to specific ingredients of the model, and assumptions about the coupling constants.An efficient experimental program to study polarization degrees of freedom in the KA channel re- quires use of large acceptance detectors with nearly 4x solid angle coverage. At CEBAF such a detector is under construction for one of the three experimental endstations. With such an instru- ment, detailed polarization data can be obtained for this reaction. For example, the A polarization can be inferred from an analysis of its *-p decay. Using a longitudinally polarized electron beam, cir- cularly p_olarized bremsstrahlungs photons can be generated, and the polarization transfer reaction q p + KA can be studied as well.
C6-290 COLLOQUE DE PHYSIQUE
Figure 11.7 Double pohriration aaymmetry 0. at 0: Pigure 11.8 Double polarkatwn aaymmetry 0.for for the proceaa7p -P prr* in the region of the 2-r and yp
-
z+n. Data fmm Yemvanss. Same calculation 3.1 nucleon resonance. Data from ~ m v a n ~ ' . The aa in Figure II.7.curves are predictiow of phenomenologicd analynea.
Solid line: Metcdf and Walkern
11.4.4 Drell-Hearn Gerasimov Sum Rule.
The result of the polarized proton structure functions measurement has triggered new interest"^^^
in experimental tests of the sum rule of Drell-HearnS8 and GerasimovSB:
where v is the photon energy, Ul/r and Ua/r are the absorption cross sections for total helicity 1/2 and 3/2, and k is the anomalous magnetic moment of the target nudeon. The sum rule has been derived on rather general grounds but has never been tested experimentally, although an analysis of single pion production experiments allows to place some limits on by how much it may be violated4'.
Using a circularly polarized tagged photon beam, generated by longitudinally polarized electrons, and polarized N H S as target materid, the total absorption cross section difference
-
ua/r candirectly be measured as a function of the photon energy. The integral is weighted by l/v, therefore the lower energy regime, in particular the resonance region give the largest contributions. Existing or planned medium energy accelerators are the appropriate instruments for testing this s u m rule.
IIX. STRUCTURE OF THE DEUTERON.
IIX.1 Elastic Electron-Deuteron Scattering
In elastic eD scattering, the hadronic current is specified by 3 isoscalar electromagnetic formfactors, GC
,
Gg, GM, which are related to the charge, electric quadrupole moment, and magnetic dipole moment, respectively. The charge formfactor, in particular, is very sensitive t o the short range part of the N-N potential, and to the size of isoscalar meson exchange currents. Rosenbluth separation of the unpolarized elastic eD scattering cross section allow separation ofInformation about GC and Gq separately can only be obtained in measurements of polarization observables.
In the absence of a polarized electron _beam, one can either measure the tensor polarization compe nents of the recoil deuteron in D(e, elD), or alternatively measure the analyzing power in D(e, el)D.
Figure 111.1 Ezpcrimentd rerults on the analyzing power TZo for e D
-
eD. The data are compared with various model calculations. For a detailed discuaaion see M. Garconn.The first method was used in two measurements a t M I T - B a t e ~ ~ l g ~ ~ . The second method was em- ployed in experiments at the Novosibirsk VEPP-2 and VEPP-3 storage rings43*44 with a polarized Dz gas target. A Bonn group, working at the 2.5 GeV synchrotron used a dynamically polarized ND3 target4=. For a tensor polarized target the elastic scattering cross section is given by:
T2o appears to be most sensitive to GC. If the magnetic component can be neglected (at forward angles, and rG&
<
G:,
*G;)The interference term GcGQ makes Tzo a particularly suitable quantity to measure. Figure 111.1 shows the present experimental situation. Only the most recent results from MIT-Bates have suffi- cient sensitivity to allow discrimination between some of the models. Clearly, the asymptotic scaling model4" which predicted
a,
to asymptotically approach the value-4
is ruled out in the Q2 range where data are available.As we have heard during this conference4' new measurements are being planned a t Novosibirsk with 100 times higher tsrget densities. Also, polarized solid state target technology has advanced to a point where improvements of similar magnitude appear feasiblez0. These advances may allow to push the measurements to higher momentum transfer, as well as to improve the statistical significance a t lower Q'.
III.2 Disintegration of t h e D e u t e r o n 111.2.1 Photodisintegration
To a large degree recent polarization measurements have been prompted by the observation of an unexpectedly large proton recoil polarization seen in a Tokyo experiment47 near 550 MeV photon energy. The result was interpreted as an indication for the excitation of intermediate dibaryon states.
In the energy region of the A(1232) new measurement of the proton recoil polarization have become available from KharkovM (Figure III.2), which allow to test various model calculations. In general,
C6-292 COLLOQUE DE PHYSIQUE
Figure 111.2 Recent rcsullr on proton recoil poIarLatwn measurements i n deuteron photo disintegrationa, and po- larized neutron caplurc. Top: Non-relativistic cdcula- i i o w uring v a t i o w N N potentiab: Bonn (dotted), A*
gonne V14 (daahed-dotted), A v o n n e V28 (short daahes), RSC (long daahes), RSC/nSO (solid]. Bottom: Coupled channel calculations by Tanabe & OhtaS0 (dotted), and Leidemann & Arcnhove~* (IA: dwhed, IA+IC: daahed- dotted, IAi-IC+MEC: solid).
Figure III.3 Bonn d a d 1 on the andyn'ng power 4 in deukmn photodin'nkgmiion wing a pobr- k d NDs target. Non-rrldatioirtic cdcJotioru by Leidemonn k AmnhoveFe [dorhcd-doitcd), and relatioirtic -c bg Lag# [dorhed), and K a d ' . Noie thai the caicdaiwna agree on the position of the em ring.
non-relativistic calculations describe the data well, when IC and MEC are included, in a coupled channel approachqB, or in unitary theory4=, and there is no apparent need for including dibaryon states into the calculation.
Figure 111.3 shows results of a target asymmetry measurement carried out at the Bonn 2.5 GeV synchrotrons1 using a polarised NDa target. The data exhibit an energy dependence, which can presently not be explained in calculations using conventional models. A partial wave fit indicates that the angular distribution a t 550 MeV requires a J,,,,, = 2 angular momentum contribution, which could indicate the excitation of an intermediate J=2 state. However, in the analysis of the Tokyo data J,, = 4 yielded the best fit. It would clearly be desirable to collect good statistics data in smaller energy bins that would allow one t o study the energy dependence in more detail.
Also, analyses that make use of all existing data including angular distributions of the unpolarized cross section, could provide better constraints on exotic contributions to the process.
Polarized photon asymmetries have recently been measured a t the Yerevan synchrotron52. An energy spectra is shown in Figure 111.4. The 90° low energy data
(5
0.5GeV) can be described by conventional model calculations. An interesting feature is the apparent failure of the coupled channel c a l c u l a t i ~ n ~ ~ compared to the impulse approximation result in the same model. The data extend to lGeV photon energy, and there is a clear need for the development of theoretical models in the higher energy regime.I£
one displays the beam asymmetry differenceFigure 111.4 Beam asymmetry C for deuteron pholo- d ~ i n t e ~ o n . Data from Adamian et al.=". Caku- latwn by Leidemann & Annhovepe [IC: dotted,
full
CC: aolid), and LageP (daahed-doited). The liner that eztend to 0.6 Ge V are the result of a phenomeno- logical analyau that inciudea varioua dibaryon aiatca.
(dashed line: nonreaonant coniribuiiona only).
Figure 111.5 Beam orymnuiry difference Z(4S0)
-
C(13S0). Same calnJdiona or in Figure 1.I.I.
one is able to test isospin properties of the yD 4 pn amplitudes. The generalized Pauli principle requires
AX =
0 for pure isospin states. Non-zero values indicate interferences between isoscalar and isovector amplitudes. The data are shown in Figure 111.5. Some theoretical calculations that reproduce the beam asymmetry data4' a t 90° reasonably well, dearly fail to pass this isospin test.The calculation by Lagetsa that includes the Pll(1440) baryon resonance does comparatively better, which indicates that the excitation of higher mass resonances with isospin
$
(P11(1440), Slt(1533)) may play an important role in this process.II.2.2 "High Energy" Photodisintegration
Recently, there has been much speculation about the result of an experiment carried out a t SLACs4, where the photodisintegration cross section was measured for energies np to 1.55 GeV. A striking feature is that around 1 GeV the data appear to follow precisely the power law behaviour of
(Jii
is the cms energy) that was predicted by dimensional quark counting rules (Figure 111.6).Fi- 121.8 Deukronphotodirintegmtwn cmrrrection mul- Figure 131.7 Proton rccod pohrirdion in deuteron photo- tiplied by .l1. The data indicate dimendrional rcaling for dLinkgrdion d B;"L = 120..
energies above l GeV. Model calculation by T.S. L e P .
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Figure 111.8 Double polarization asymmetq for Figure 111.9 Beam asymmetry in coherent r" pro- deuteron dwintegmtion in quasi elastic eD rcat- duction from deuterium. Data from Adamian at tering. Predictions by ~ekalo" wing RIA. The d.d'. The calcdatwna include IA (dotted), and sensitivity to variow aaaumptiona alout the NN IA+RS (dashed). The solid line includes uatiow potentid is shown: Patis: rolid, RSC: d&d
,
dibaryon stales, e.g. 3F3(226~),' G4(2510).Buck-Grora: daah-dotted
A calculation based on conventional theoryK5 fails to reproduce this particular behaviour. We may ask the question, what we can learn about the process by looking a t polarization observable. At asymptotic energies QCD makes a simple prediction66 about the nudeon recoil polarization: P, --+ 0.
Proton recoil polarization data relevant for this discussion have in fact been measured a t K h a r k ~ v ~ ~ . Some of the data are shown in Figure 111.7. The trend is obviously quite different from the asymptotic QCD predictions; the data show nearly maximum recoil polarization a t high energies, P, -4 -1.
These results are obviously in serious disagreement with the simple counting rule interpretation of the cross section data.
III.2.3 Electrodisintegration
Unpolarized measurements of the electrodisintegration reaction have revealed the importance of mesonic exchange reactions near the threshold for deuteron beakupS8. Other measurements have revealed the importance of A isobar
contribution^^^.
The study of polarization observables is largely a matter of future experiment. At MIT-Bates and CEBAF experiments are under consideration to study this reaction using polarised electrons, proton recoil polarimeters, and vector polarized deuterium targets, in non-parallel kinematics.D(< ep)n (l), D(e',ep')n P ) , @e',ep)n (3)
The first process yields new information only when the measurement is carried out in out-of-plane kinematics, where the proton is measured out of the electron scattering plane. A number of theo- retical studies have been carried out 1S*16*w indicating that the reaction (1) is largely sensitive t o final-state interactions, whereas (2) and (3) may reveal properties of the deuteron wavefunction, and of meson exchange contributions. Some of the results are displayed in Figure 111.8, which shows the double polarisation asymmetry for the reaction (3) in quasi-elastic kinematics. With increasing Q2 the asymmetry becomes very sensitive to assumptions about the NN potential and MEC's.
III.3 Coherent re Photoproduction f r o m Deuterons
Another source of information about the two-nucleon system is coherent 7i-O production off deuterons y D -+
Dx".
The process is sensitive to the z N N dynamics in the two-nucleon system, and to possible excitations of dibaryon resonances. This reaction has recently been studied at Yerevan, using a linearly polarized coherent bremsstrahl~n~ beam6'. The results are shown in Figure 111.9.Theoretical models that include impulse approximation (IA) and rescattering terms (RS) describe the beam asymmetry I: qualitatively for energies up to about 600 MeV. At higher energies the theoretical description fails qualitatively, even when predicted dibaryon states are included in the calculation. Lack of knowledge of the deuteron wavefunction at short distances may, in part, be responsible for this discrepancy.
TV.
P H O T O P R O D U C T I O N OFF L I G H T NUCLEAR TARGETS.Data on polarization observables become very spares for nuclei other than the deuteron. Also, the interpretation of the data becomes more ambiguous as final state interactions become increasingly important. However, in some cases, one may be able to reduce effects from FSI by measuring polarization asymmetries. A Kharkov has studied the beam asymmetry for various nuclei in (7, X* ,) and (7, p) reactions, for energies in the region of the A(1232), where strong pion absorption usually prohibits the study of the initial photo-absorption mechanism, when pions are detected in the final state. When measuring the polarized beam asymmetry
FSI largely drop out in the ratio. The results for (7, X - ) are shown in Figure N.1. C is nearly independent of the target mass, which indicates that a quasi-free production mechanism dominates.
1.0
0.0 0 10 20 30 40 50
Target Mass Humber A Kharkov (7s-)
E,=350MeV
-
8,=90°
7 I
?.-=-P.h id+, - l, , P I ti
l , , , , l , # , , l , , , , l , , , , ~
Figure IV.l Beam asymmetry in the A(1233) region, for inclusive r- production from nuclei.
Data from Zybdov et d.4s.
V. P A R I T Y VIOLATION I N E L E C T R O N SCATTERING EXPERIMENTS.
In low and medium energy (Q2
<
M;.) neutral current interactions the parity violating contribu- tions arise trom the interference between the one-photon exchange and the neutral weak boson Ze exchange graphs (Figure V.l). In electron scattering the interaction contains an isoscalar as well as an isovector piece in both, the vector (V,) and the &a1 vector (A,) coupling. The relevant piece of the Lagrangian can be written likeCOLLOQUE DE PHYSIQUE
Figure V.1 Lowest order Feynman diagrams which Figure V.2 Diagram for the quark coupling con- contribute to parity non-comemation in electron s h n b C2.4 and CZr. The areas allowed by ihe scattering at low and intermediate energies. Maim and SLA C ezperimentr, and b y neutrino-
hadron scattering a n indicated.
where B,
p, 7, d
denote the electro-weak coupling constants which have to be determined experi- mentally. By choosing appropriate kinematical conditions for electron scattering from nucleons and nudear targets, the couplings can be determined by a set of four linearly independent measurements.The famous
SLAC
D(e,e9)X experimentas, in conjunction with atomic physics experiments enabled a model independent measurement of and7.
V.1 The Mains P a r i t y E x p e r i m e n t
Recently, an experiment at MainsM was completed that measured a t backward electron scattering angles, e m p h ~ i s i n g %different combination of the coupling constants which allowed extraction of the combiiation /3+0.046
=
0.0053~ 0.17. This result is displayed in Figure V.2 in the terms of the quark coupling constants Cza =(-B+8)
and= M+$),
where the results of the SLAC experiment and of neutrino hadron scattering experiments are shown as well. Although a factor of three irnpr~vement compared to the previous state of affairs has been achieved in _the axial vector coupling sector, the result still aU,ows fol a vanishing axial vector interaction=
6 = 0). Using results of the previous experiments/3
and 6 could be separated, and their respective values determined toAssuming the validity of the Standard Model the coupling constants are related to the weak mixing angle 8w in the following way:
With sinaOw = 0.232 f 0.004 i t is obvious that the result of the Mainz experiments is in agreement with this value. However, the precision of the experiment is not sufficient to have impact on the determination of sin28w.
V.2 The MIT-Bates P a r i t y Experiment
Another experiment to measure parity violation in 12C(e',e')'2C has recently been completed at Mm-Batesa5. This experiment measured directly 7, the isoscalar piece of the vector coupling with the result
7
= 0.136 f 0.032 f 0.009,
which is in agreement with the prediction of the Standard Model of
TsM
= 0.155.It appears the most important outcome of both experiments is the demonstration that polarised electron scattering experiments allow control of systematic errors much below the level.
V.3 T h e S A M P L E Experiment at MIT-Bates
A new experimentd5~m has been approved for MIT-Bates to measure parity violation in elastic electron-proton scattering a t backward angles. h this case the asymmetry depends on the elastic electromagnetic formfactors F:, F:, and the neutral weak formfactors F;, Ff. The latter one is related t o the proton anomalous moment coupling to the neutral weak current. In the Standard Model, and invoking strong isospin symmetry,
Ff
can be related to the electromagnetic formfactors like1 1
F: =
(Z -
s i n l B w ) F ~-
F,O,
where
F :
is a singlet piece which arises fiom equal contributions of all three quark flavors. Since it contains contributions from strange quarks it is sensitive to the possible presence of large s i contributions to the proton structured7. Large s i contributions have been suggested from two sources. Firstly, the analysisd8 of the pion-nucleon sigma term (CwN) suggests a 30% s i contribution to the proton rest mass. Secondly, the spin-dependent structure functions of the proton, measured by the EMC collaboration have been interpretedd0 to indicate that the strange sea is polarized opposite to the nucleon spin. The parity violating asymmetry in elastic ep scattering is given by:where
<
= F:'+ rG1 +
2rtg1!(F:+
F:)1. At Iarge electron scattering angles the first term domi- nates. The term with Gl is suppressed since (1-
4sin2Bw) is small. SinceF? <
F: the asymmetry is essentially proportional toF : ,
and the term of interest F; can immediately be extracted.VI.
C O N C L U S I O N S / O U T L O O KMeasurements of polarization observables in photoabsorption and in electron scattering processes have already generated information relevant for understanding the structure of the nucleon and deuteron. Interest in polarization aspects of intermediate energy physics has sharply risen in recent years as the physics possibilities have become more dearly visible. The experimental tools (po- larized gas and solid state targets, proton and neutron recoil polarimeters, and polarized electron sources), which are necessary to exploit these possibilities are under continued development. Their most efficient exploitation will be possible a t the new DC electron machines which are now under construction. In conjunction with the use of large acceptance detectors and high precision mag- netic spectrometers these instruments will help generate data which are essential for the study of fundamental properties of nudeons and nuclei.
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