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S. Page
To cite this version:
S. Page. PARITY VIOLATION IN HADRONIC SYSTEMS. Journal de Physique Colloques, 1990,
51 (C6), pp.C6-253-C6-264. �10.1051/jphyscol:1990621�. �jpa-00230887�
COLLOQUE DE PHYSIQUE
Colloque C6, suppl6ment au n022, Tome 51, 15 novembre 1990
PARITY VIOLATION IN HADRONIC SYSTEMS
S.A. PAGE
Department of Physics, University of Manitoba, Winnipeg, R3T 2N2, Canada
Rksumk - A l'heure actuelle notie comprkhension de la partie hadronique de l'interaction faible con- servant l'ktrangetk (AS=O) repose sur des mesures de grande prkcision, effectukes dans le systeme nuclkon-nuclkon ou dans les noyaux lkgers, qui isolent l'interaction faible par le signal provenant de la non-conservation de la paritk. L'interaction faible entre nuclkons B basse et moyenne knergie est dkcrite par un mod&le d'kchange de mksons, impliquant l'kchange des mksons n , p et W oh l'un des vertex est gouvernk par l'interaction forte, tandis que l'autre est par l'interaction faible. Les rbsultats expkrimentaux ont produit des informations importantes mais incomplktes concernant les constantes de couplages faibles entre m6sons et nuclkons, qui sont reproduites de fason qualitative par des estimations baskes sur le mod&le standard, bien que le couplage du pion se rkvele plus faible que prkvu. Les rksultats obtenus par des mesures du pouvoir d'analyze longitudinal (A,) dans le systkme p+p fournissent, pour l'instant, les plus importantes contraintes pour les constantes de couplage faible entre mksons et nuclkons; des nouveaux rbsultats B basse knergie ainsi que ceux en provenance d'une experience B moyenne knergie, en dkveloppement B TRIUMF, aurront un impact significatif sur notre connaissance de l'interaction faible entre nuclkons. L'ktat actuel de la thkorie et de l'expkrience seront examinks, plus particulikrement les techniques expkrimentales ainsi que l'interprktation dans le cadre du mod&le standard.
Abstract - Our present understanding of the AS=O hadronic weak interaction is based on a col- lection of high precision experiments in the two-nucleon system and light nuclei, which isolate the weak interaction via its parity-violating signature. The weak nucleon-nucleon interaction at low and intermediate energies is described in terms of a meson exchange model involving n, p and W
mesons, with one strong and one weak vertex. Experiments have yielded significant but incomplete information on weak meson-nucleon couplings, which are in qualitative agreement with predictions based on the standard model, although the pion coupling is much weaker than expected. Studies of the p+p system have provided the most significant constraints on weak meson-nucleon couplings to date, through measurements of the helicity dependence of elastic scattering (A,); new results at low energy, plus the outcome of an intermediate energy experiment in progress at TRIUMF, will have significant impact on our knowledge of the weak nucleon-nucleon interaction. The present status of theory and experiment will be reviewed, with emphasis on experimental techniques and interpretation in the context of the standard model.
1 - INTRODUCTION
Hadronic weak interactions are of fundamental importance, yet elusive to experiment, since the competing strong interaction dominates by a factor of order 10'. Experiments to study the weak interaction in the quark sector are necessarily restricted to observables which violate a symmetry respected by the strong interaction, such as parity violation or quark flavor changing. At low and intermediate energies, the parity- violating weak nucleon-nucleon (N-N) interaction can be successfully described in terms of an effective meson exchange model with one weak and one strong vertex. The strong meson-nucleon (M-N) couplings are taken from a conventional description of the strong N-N interaction (e.g. the Bonn potential). The
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990621
weak couplings have been predicted from the Weinberg-Salam model involving W and Z exchanges between quark constituents of the nucleon and meson, although with large uncertainties associated with the strong interaction. A consistent, complete set of weak meson-nucleon couplings remains to be unambiguously determined, despite over a decade of experimental and theoretical effort.
Recently, advances in high intensity polarized electron sources have made possible precise measurements of parity violation in electron-nucleus scattering, and experimental programmes are underway or planned at MIT-Bates, Mainz, and CEBAF; these experiments test the standard model of semileptonic.weak inter- actions, with small mesonic corrections due to the weak N-N interaction, and will not be discussed further here. Further discussion is given in a contribution to this conference on the MIT-Bates ''C experiment/l/, and many details are found in the proceedings of the 1990 workshop on "Parity Violation in Electron Scattering" held at Caltech/2/.
2 - THEORETICAL APPROACH: LOW AND INTERMEDIATE ENERGY
Below the pion production threshold, the weak N-N interaction has been successfully described in terms of the exchange of n, p and w mesons, neutral scalar mesons being excluded by C P conservation. In a quark model assuming strong SU(3) symmetry, the main contribution to the weak pion-nucleon coupling can be related to measured parity-violating decay amplitudes of the A and E hyperons. However, even a naive estimate of the weak vector meson couplings is considerably more difficult, as an analogous relationship to weak decays does not exist. Desplanques, Donoghue and Holstein/3/ (DDH) discussed this problem in a comprehensive paper in 1980, in which they employed a SU(6) quark model to calculate a complete set of six weak meson-nucleon couplings, denoted: (fk, h:, hi, h;, h:, h:) where the superscripts refer to isospin changes. Strong interaction effects were treated using renormalization group theory, giving rise to large ( ~ 3 0 0 % ) uncertainties in the predicted coupling constants. DDH evaluated these uncertainties and reported both 'best value' and 'reasonable range' estimates of the meson-nucleon couplings, which have been effectively adopted as a reference standard for predictions of parity-violating observables at low energy.
A similar quark model approach was taken more recently by Dubovic and Zenkin/4/ (DZ), whose results lie within the 'reasonable range' estimates of DDH, but are significantly smaller than the DDH 'best values'.
The predictions of DDH, DZ, and other recent works are summarized in Table 1. Noteworthy is the value of the isovector pion coupling
c,
which gives rise to the only long range component of the weak meson exchange interaction. The strength of the weak pion coupling is very sensitive to strong interaction uncer- tainties, and its value therefore constitutes a sensitive test of theoretical predictions. As will be discussed in a later section, the experimental value of fk is significantly smaller than the DDH prediction, 95% of which is attributed to neutral current contributions. Holstein/5/ has reexamined the DDH quark model calculations and found that the small measured value of f i requires the current algebra quark mass values to be increased by about afactor of 2 from the original Weinberg values, which tends t o suppress theoretical estimates of other hadronic weak interaction phenomena, e.g. the AI = rule (where the change is in the wrong direction for predicting a AI=$ enhancement!).An alternative approach to calculating weak meson-nucleon vertices is provided by a topological chiral soliton model, which approaches &CD in the large N, limit. Grach and Shmatikov/G/ (GS) have followed such an approach, finding fk 0 in contrast to quark model calculations, and closer to the experimental value, whereas their vector meson couplings (with the exception of h:) are roughly an order of magnitude larger than DDH, and are therefore somewhat difficult to reconcile with experimental data. Kaiser and Meissner/7/ (KM) have also used the nonlinear chiral effective Lagrangian method, simultaneously deter- mining both weak and strong M-N couplings, the latter being fitted t o nucleon properties. They find a nonzero value of
c,
intermediate between the DDH and DZ quark model predictions, and vector meson couplings which are comparable to DDH. In addition, ICM evaluate a fourth weak coupling constant for the p-meson, ha, which is 'magnetic' in character and comparable in magnitude to the other p couplings in their calculation. h: and the term it couples to in a parity-violating N-N potential have traditionally beenneglected in the interpretation of nuclear parity violation data. Holstein/8/ has commented that this cou- pling cannot be evaluated in a static quark model, and estimated its value to be negligible (z of the later I<M prediction), showing at the same time that a value smaller than _N 20 X 10-' would not alter the in- terpretation of nuclear parity violation data significantly if it were accounted for in theoretical calculations.
In a recent review, Adelberger and Haxton/9/ (AH) fitted the most significant nuclear parity violation data to a 2-parameter expression based on the quark model formalism of DDH. The fitted values of the weak M-N couplings were unfortunately only marginally better constrained than the 'reasonable range' estimates of DDH, due to a lack of independent, high-precision data. The results of the AH fitting procedure, which have been updated to include recent measurements of low-energy scattering, are also shown in Table 1.
Alternative models based on explicit quark degrees of freedom have been used to calculate parity violation in the p+p system/lO,ll/, but as yet have not been applied to a comprehensive analysis of nuclear parity violation data.
Table 1. Weak Meson-Nucleon Couplings
The values in this table are given in units of IO-?; calculations are discussed in the text.
3 - EXPERIMENTAL RESULTS - FINITE NUCLEI M-N
Coupling
C
h;
h:
h;
An excellent review of nuclear parity violation, both theory and experiment, was given by Adelberger and Haxton/9/ in 1985; only a brief summary and discussion of present and future work will be given here.
A quantitative understanding of the AS=O weak interaction at low energy requires sufficient experimental data that the meson exchange model can be tested and compared to alternate descriptions of the weak N-N interaction. Since the meson exchange model has been qualitatively successful to date, the following dis- cussion will be given in its context. If the additional coupling h: is neglected, as Holstein suggests, then a total of six weak meson-nucleon couplings need to be determined, requiring at least as many independent, high-precision experiments. It is important that the nuclear structure model dependence of theoretical predictions be minimized by a careful choice of experiments to avoid systematic error in evaluating the weak M-N couplings from the data. This requirement effectively restricts interpretable experiments to the two-nucleon system and light nuclei. When applied in conjunction with technical feasibility at the level of accuracy implied by predictions of the weak M-N couplings, this requirement is highly selective, and it has proved extremely difficult to find a complete set of experiments t o meet this goal.
1 1
-10.3 -+ 5.71
-1.91
-3.91
-101 1::: 1
-6.71
-1.9 -+ -0.8 -1.1 -2.2 -1.5 -1.0 -2.3 DDH/3/
Range 0 -t 11.4 -31 --+ 11.4
-0.38 + 0 -11.0 -+ -7.6
GS/6/
0 -91
0 -89 DDH/3/
Best Value 4.6 -11.4 -0.19 -9.5
KM/7/
0.19 -3.7 -0.1 -3.3 DZ/4/
1.3 -8.3 0.39 -6.7
AH/9/
Fit 2.1 -6.7 -0.21
-7.3
Despite a relative wealth of data, there are only 4 independent experimental constraints on the minimal 6 weak meson-nucleon couplings. These have been obtained from studies of y-decays of parity-mixed doublets in light nuclei, and from measurements of the helicity dependence of elastic proton scattering from various targets. The experimental situation is briefly summarized in Table 2. Experimenters face the difficult challenge of extracting a very small parity-violating signal from a relatively enormous parity conserving background, the ratio of weak to strong interaction effects being of order 10-' in the basic N-N interaction.
Parity-violating observables can be enhanced by nuclear structure effects in certain finite nuclei, but this is typically achieved at the expense of additional uncertainty in the nuclear wavefunctions which are needed to quantitatively interpret the data.
Table 2. Experimental Sensitivities to Weak Meson-Nucleon Couplings The sensitivities of selected experiments to different weak meson-nucleon couplings are indicated in the table. In general, parity violating observ- ables depend on a linear combination of weak meson-nucleon couplings - - with coefficients proportional to matrix elements of the weak N-N inter- action; this is indicated by an expression of the form: a h a
+
bhg,+ ...
inthe column labelled " Experimental Sensitivity", where a and b take on different values in the various cases cited. Unless otherwise indicated, values of a and b are taken from Ref. 9, with the modification that dif- ferences in predicted weak matrix elements at the 10% level are ignored to simplify the expressions. A superscript
'*'
indicates work in progress or proposed. Note that the analysis of p'S d (and to a lesser degree p'+&) scattering data is complicated by the angular dependence of parity vi- olating effects in both scattering and breakup channels as discussed in Ref. 24.Experimental Sensitivity/9/
f),
a f:
+
b(h;+h:+h;)(Note/l3/: a/b 2: 10) a f),
+
b(h:+
0.6he)h:+hi+h:/&
a(ht+h;)
+
b(h:+hi+h;/&) a f t+
bh:+
ch:+
d(hE+hL)Expt. Performed (or in progress) lsF: Py (1081 keV)/12/
*S + P : dpn,lX/14/
''F: A,(110 keV)/l5/
p'+ CY: A,(46 MeV)/16
"Ne: Py(2789 keV)/l7/
14N: AZ(O+; 8624 keV)/18/
*g+ p: A,(230 MeV)/19/
*g+ p: A, (13.6 MeV)/20/
g+
p: A, (15 MeV)/21/g+
p: A, (45 MeV)/22/p'+ d: A, (15 MeV)/23/
p'+
d:
A, (43 MeV)/24/Recently, some extremely large enhancements of weak interaction effects have been discovered in low en- ergy neutron resonances in nuclei of the rare earth region/25/, the largest effect being a parity-violating asymmetry of E 10% due to interference of S- and p-wave resonances at 0.734 eV in I3'La. This has in turn raised the possibility of using the parity-mixed neutron resonances to search for evidence of t-violation in either 3-fold (p-odd,t-odd) or 5-fold (p-even,t-odd) spin-momentum correlations, as discussed in the proceedings of the 1987 workshop on Tests of Time Reversal in Neutron Physics/26/. Currently, there are active programs at Dubna, LAMPF and KEK to study these low energy neutron scattering effects; further details are reported in contributed abstracts to this conference/27/.
In the mass-20 region, several extremely favourable cases exist in which the parity-violating observables are enhanced and the nuclear structure calculations are reliable. The common feature of these cases is a low- lying doublet of states with the same spin but opposite parity. The weak interaction gives rise to small wave function admixtures of opposite parity in both members of the doublet, which leads to circular polarization of their y-decays via El-M1 multipole mixing. A significant difference in the lifetimes of the two states enhances the circular polarization in decays of one state, and suppresses the circular polarization in decays of the other; the relative enhancement factor R is given by the square root of the lifetime ratio. The sensitivity to weak meson-nucleon couplings is selected by the isospin quantum numbers of the interfering states - either isovector, dominated by the long-range pion coupling f:, or isoscalar, determined by a short- range contribution from p and w exchanges: (h:
+
0.6 h:), or a combination of both. A summary of the favourable cases in light nuclei is shown in Figure 1.Figure 1: Parity Mixed Levels in Light Nuclei
For the parity mixed doublets, the enhancement factor R arising from nuclear structure which multiplies the basic weak N-N interaction is in- dicated. In the case of
'",
as discussed in the text, the opposite parity wavefunction impurity has contributions from several nearby levels, the reIative sign of each being both of crucial importance to the analysis and very difficult to predict.Nuclear matrix elements needed t o extract weak meson-nucleon couplings from the y-ray observables are notoriously sensitive to the radial shape of nuclear wavefunctions, short range correlations, and truncation of the model space, as discussed in detail by Haxton/28/. This problem can be largely circumvented in two cases, where a first-forbidden P-decay to one member of the doublet contains a significant contribution from pion exchange currents. The most favourable case is 18F, in which the nuclear matrix element re- sponsible for wavefunction admixtures within the doublet is related by an isospin rotation/29/ to the pion exchange contribution which actually dominates the first forbidden
P+
decay of 18Ne to the upper member of the lsF doublet. (A similar situation occurs for the isovector contribution to parity violation in "F.) A measurement of the forbidden /?-decay rate, in addition to the 18F y-ray circular polarization, enables the weak pion coupling fk to be evaluated with confidence; two independent measurements/l2/ of parity mixing in 18F are in agreement with each other and find f: to be 4 a smaller than the DDH Lbest value', although a value of zero is still consistent with the DDH Lreasonable range' of this parameter as well as with all other calculations (see Table 1).In constrast to the weak pion coupling, which can be isolated in the case of 18F, the weak isoscalar coupling which contributes to parity violation in light nuclei, (h:
+
0.6 h:), is not well determined. In principle, this isoscalar coupling can be determined by comparing circular polarization measurements in 21Ne and lgF, where it enters in linear combination with f: but with opposite sign. However, when considered together with the lsF result, these three cases do not determine consistent values of the isovector and isoscalar couplings. The discrepancy can reasonably be attributed/30/ to uncertainty in the shell model wave- functions used to describe 21Ne. In an attempt to establish an independent measurement of the isoscalar combination (h:+
0.6 h:), a measurement of parity violation in p'+13C scattering was undertaken by the University of Washington-Wisconsin collaboration. A positive result at the 1.5 a level was obtained in the experiment/lS/ Unfortunately, shell model calculations for the 14N system have been shown to have less predictive power than previously thought; the original calculations on which the 14N experiment was based lead to an isoscalar constraint of opposite sign t o the DDH prediction and in better agreement with 21Ne than ''F. A precise interpretation of this experiment therefore awaits improves shell model calculations of the 14N system which are currently in progress/30/.In addition t o the y-ray experiments, a classic test of parity violation is the search for parity-forbidden a-decays. One important case is the decay 160(2-, 8.87 MeV) + 12C(O+)+a, which is sensitive only to the isoscalar component of the weak N-N interaction, since initial and final states have isospin zero. A parity-forbidden decay width of (1.03 5 0 . 2 8 ) ~ 1 0 - ' ~ eV was reported/31/ following poulation of the 2- state via P-decay of 16N. Unfortunately, the decay width arises due t o the admixture of several nearby levels of opposite parity, rather than a single other level as in the cases of lsF, ''F and "Ne, and nuclear structure calculations must reliably predict both the amplitude and relative signs of the weak matrix elements for all interfering levels. To date, this difficulty has meant that the existing 160 measurement has not been used to constrain the weak isoscalar coupling (h:
+
0.6 h:). However, larger basis shell model calculations are in progress/30/ which may resolve the interpretation of parity mixing in 160, and a proposed experiment at TRIUMF/32/ using a radioactive 16N beam from the TISOL facility aims to provide a more precise measurement of the forbidden (2-) decay width. Another new proposal by the Giessen group, reported in the contributed papers to this conference/33/, aims to study isovector parity mixing in the (2-, 12.97 MeV) state of 160 via the 15N(5, a,)12C reaction,
which would in principle provide an independent constraint on fk.4 - EXPERIMENTAL RESULTS: 2 NUCLEON SYSTEM
The two nucleon system poses stringent experimental problems for measurements of parity violation, since there is no enhancement mechanism analogous to that provided by nuclear structure in the two-level mix- ing cases above. However, the two nucleon system offers a straightforward interpretation in terms of one or more parity-mixed partial wave amplitudes at low energy, which virtually eliminates the wavefunction uncertainties which complicate the analysis of nuclear data. A complete set of partial wave amplitudes would thus provide an ideal test case for models of the weak N-N interaction.
Several parity violation measurements have been reported in the n+p system a t low energy. Considerable interest was excited by the first measurement of circular polarization in thermal neutron capture by the Leningrad group/34/, P, = (-13 f 4.5) X 10-?, since the result was several orders of magnitude than predicted theoretically. Subsequently, a great deal of effort was expended t o design an improved apparatus for a new measurement of the effect. Systematic errors were greatly reduced, and an overall reduction of almost a factor of 3 was achieved in the statistical error. The most recent result/35/, P, = (1.8f 1 . 8 ) ~ 10-', is in agreement with the theoretical prediction of 0.6 x10-? (based on DDH 'best values'), but it is un- fortunately not sufficient t o significantly constrain predictions of the weak meson-nucleon couplings. The circular polarization dependence of the deuteron photodisintegration cross-section, a closely-related observ- able, was measured at Chalk River/36/. As in the earlier measurements, the result (A, = (27 f 28) xlO-?) is consistent with theory but falls significantly short of testing the meson exchange model prediction, which is less than 0.5 ~ 1 0 - ' . A complementary n+p observable is the y-ray asymmetry A, in polarized thermal neutron capture, which has been measured at Grenoble. The result is again consistent with zero/37/, A, = (-0.15 f 0.48) X lO-?, and would need to be improved significantly t o test the theoretical prediction of -0.5 xlO-?. The result was reported to be statistics-limited, and the collaboration has suggested that a higher flux of neutrons now availiable from the new cold neutron source at ILL would facilitate a mea- surement at the f 3 X lO-' level, which would be a remarkable achievement. A new experiment has been proposed/l4/ to measure a parity-violating neutron spin rotation in liquid parahydrogen at ILL, which is mainly sensitive to
fl,.
The collaboration has proposed a measurement at 4 a level with an expected effect of order 10-7 radians, which would provide a very important independent test of the small pion coupling deduced from "F.In contrast t o the n+p system, where after many years of work improvements are still needed in experi- mental precision to challenge theoretical predictions, the proton+proton system has provided significant constraints on weak N-N interaction models. The observable that has been studied is the helicity depen- dence of the total scattering cross section, A,. The longitudinal analyzing power A, = - may be expressed in a model-independent decomposition as a sum of parity-mixed partial wave scattering am- plitudes: (('So -3 Po),(3Pz
-'
D2),('D2 -3 Fz) ...). The shape of the angular distribution of each term is governed by the strong interaction, which is relatively well known at low and intermediate energy, while the relative strengths are set by the weak interaction which we seek to measure. The parity-mixed ('So -3 PO) amplitude is now well determined from several measurements below 50 MeV, dominated by a high precision measurement/22/ of A, a t 45 MeV, carried out at SIN: A,(45 MeV) = (-1.5 f 0.2) X 10-?. A new result at 13.6 MeV obtained by the Bonn group is reported in the contributed sessions to this conference/20/:this ongoing measurement' of A, has already reached a precision of f 0.5 X lO-?, and is in agreement with earlier data at low energy. These remarkable levels of precision are made possible by using integral counting techniques, together with run-by-run corrections for systematic errors which are carefully monitored and accounted for in the analysis.
Since the initial and final states in p+p scattering are isovector, parity mixing can arise via a weak N-N interaction that is a sum of isoscalar, isovector and isotensor components. Furthermore, no exchange can- not directly contribute without violating CP as well as parity, with the result that parity mixing in p+p scattering arises purely from short range components of the weak N-N interaction mediated by p and w mesons. There are therefore two constraints that can be obtained from p+p scattering, which determine effective p and w couplings summed over isospin: h:P =h:+hi+hz/&, and hLP =hz+hk. These can be evaluated directly from measurements of the two lowest partial wave contributions to A,: the ('So -3 PO) amplitude discussed above, and the (3Pz
-'
D2) amplitude which contributes significantly to A, above 100 MeV. It has been shown/38,39/ that the (3P2-'
D2) amplitude depends only on p exchange in this model, while p and w contribute to the ('So -3 PO) amplitude with approximately equal weight.Kloet, Silbar and Tjon/40/ have calculated an additional contribution to A, due to 2~ exchange via a NA intermediate state, where the weak interaction takes place at an NNn vertex. This contribution is proportional to the weak coupling f:. The mechanism is characterized by J=2, and therefore contributes to the (3P2 -l D2) amplitude. For the DDH 'best value' prediction of f:, the NA mechanism is a small fraction of the conventional p and w exchange contribution at low energy and becomes more significant at
higher energy, varying smoothly from a value -0.2 X 10-7 at 50 MeV to +0.5 X 10-7 at 900 MeV. The contribution is a factor of 4 smaller if the DDH 'best value' for f i is replaced by the experimental upper bound from ''F measurements. If the weak interaction takes place a t the NAn vertex rather than the NNn vertex, the contribution is strongly suppressed at low and intermediate energy by a kinematical factor q2/(4MN)2; Kaiser and Meissner/7/ have calculated the corresponding weak NAn vertex in their topolog- ical Lagrangian model and find a very small coupling: fN1 A?r = 0.2 X 10-7. Thus, in both cases, the weak 27r exchange contribution via an NA intermediate state is expected to be very small a t low and intermedi- ate energy, supporting the simple interpretation that the (3P2 -'Dz) amplitude is dominated by p-exchange.
5 - THE TRIUMF EXPERIMENT
The contribution of the two lowest partial waves to A,, as predicted in a meson-exchange calculation by Simonius/39/ using DDH 'best value' coupling constants as a reference, is shown in Figure 2, together with experimental data. It is clear that a unique situation exists at 230 MeV, where the ('So -3 PO) partial wave contribution vanishes. This arises from the cancellation of 'So and 3Po strong phase shifts and is completely independent of the weak interaction. This allows the (3P2 -l D2) partial wave contribution to be measured independently, which is the goal of an experiment presently underway a t TRIUMF/19/.
In practise, the angular distribution A,(B) contains contributions from both partial waves, and must be carefully weighted with the response functions of the detectors, accounting for finite apertures and target length. This has been simulated using Monte Carlo techniques, with the result that the beam energy at the center of the target must be 215 MeV to cancel the ('So -3 PO) contribution to A, in the TRIUMF experiment, or an incident beam energy of 229 MeV for a 40 cm target. For the range of variation in the strong 'So and 3Po phase shifts exhibited by state-of-the-art analyses (Arndt SP89, Saclay S260, Bonn Potential, Paris Potential), contamination of A, by the ('So -3 PO) amplitude is less than 4% of the domi- nant (3P2 -l D2) amplitude at the TRIUMF energy. The expected effect1411 is approximately 0.6 ~ 1 0 - ~ , and the planned experimental precision is f 0.2 X 10-' or better. Such a precision is required in order to discriminate between different model predictions of h:?' (see Table 1).
Figure 2: Partial wave contributions to A, in p'+ p scatteringl391.
incorpo- A thorough discussion of several recent calculations of A, has been given by Driscoll et al./41/, '
rating different treatments of the N-N interaction and weak meson-nucleon coupling strengths. For a fixed choice of weak couplings, the zero crossing energy for the total asymmetry A, varies from one calculation to another due to the interplay of a negative ('So -3 PO) and positive (3P2 -l D2) contribution, whose individual values depend on the choice of form factors for the p and w mesons. However, it is important to emphasize that the energy at which the ('So -3 PO) contribution alone vanishes is fixed by the behavior of the corresponding phase shifts which may be taken from experiment.
In the TRIUMF programme, two separate measurements of A, will be performed in transmission and scattering geometry. It is planned to complete the first measurements in transmission mode, with incident and transmitted protons from a 40 cm LH2 target detected by parallel plate ionization chambers oper- ated in current mode, similar to those used in an 800 MeV experiment at LAMPF/42/. A microsopic model has been developed which simulates the effects of scattered beam, &rays, space charge, and ion pair recombination, which has been used to optimize the design of the ionization chambers for the TRIUMF experiments. In the second phase of the programme, A, will be remeasured in scattering geometry with a large, cylindrically symmetric parallel plate ionization chamber to collect the scattered particles. These two complementary measurements of A, at the same beam energy will lend confidence to the validity of the experimental results.
The optically pumped polarized ion source at TRIUMF will provide the polarized beam; this source is in principle ideal for parity measurements, as the spin reversal mechanism involves changing the polarization and frequency of laser light, with the minimum possible changes to beam current and emittance. Spin rotation in two sets of solenoid-dipole magnet pairs will transform the vertical polarization of the beam from the cycltotron to longitudinal polarization at the location of the parity apparatus. The apparatus also contains high precision detectors to monitor beam position, intensity, and transverse polarization, which is necessary to control false parity-violating asymmetries which may arise if the beam properties are not identical in the two helicity states.
The approach used to design the experiment is to account for systematic effects in the following manner:
AA, =
xi
where xi is any beam property (e.g. current, position, transverse polarization ...) and Ax, is the helicity correlated change in that property. The main detectors are designed to minimize the sensitivitiese,
based on Monte Carlo calculations accounting for realistic beam properties. The beam monitoring devices are designed to achieve measurements of Az; to sufficient accuracy that the resulting false asymmetries are independently determined to an accuracy which exceeds that of the parity-violating asymmetry measured in the main detectors on a comparable timescale. The main detectors will be cali- brated to determine the sensitivities$$
by introducing known modulations of the beam properties xi in ancillary experiments.A dominant systematic error which must be accounted for is an apparent parity-violating asymmetry caused by residual transverse polarization components in the beam. These couple to a relatively large parity-allowed transverse analyzing power A, 2 10-l. If the beam properties are identical in the two he- licity states and the detection apparatus has perfect cylindrical symmtery about the beam axis, this effect vanishes. In practise, the false asymmetry can only be minimized, measured and corrected for. The false asymmetry is proportional to the first moment of transverse polarization of the beam,
<
xP,>,
which has two sources: (i) a net transverse polarization coupled with an offset of the beam from the symmetry axis of the detection apparatus; (ii) an intrinsic first moment of transverse polarization, which can arise from a finite energy spread in the beam and/or nonuniform magnetic fields in the beamline elements. Systematic error calculations predict a sensitivity of cx 2 X 10-S for a 100% transversely polarized beam offset by 1 mm from the symmetry axis (a first moment<
xP,>
= 1 mm). The net transverse polarization will be minimized by careful design of beam optics and control of the magnets used to rotate the polarization from transverse to longitudinal. Possibilities of a slow feedback loop to stabilize these beamline elements are being explored, with the aim of suppressing transverse polarization components below 10-3. Two scanning polarimeters with rotating blade targets will be used to measure the profile of residual transverse polarization in the longitudinally polarized beam during the experiment; these will be capable of measuring<
xP,>
to an accuracy o f f 5 x 10-2 mm in 1 second at 500 nA, as indicated by systematic error calculations.Error estimates based on a net transverse polarization of 0.01 have shown that the beam centroid must be stabilized at two points on the symmetry axis of the apparatus to within 10 pm. A dual-function multiwire beam profile/position centroid monitor has been developed, based on secondary electron emission from thin foils in vacuum which greatly reduces the amount of material in the beam as compared with conventional gas-filled monitors. Two of these nlonitors will measure the beam intensity profile, and couple to a fast feedback system which has demonstrated the ability to control random beam excurions to less than 3 pm at frequencies up to 1 kHz.
All of this apparatus has been designed, and a large fraction of it has been built and tested at TRIUMF or is currently under construction. Using these high-precision beam monitors, the sensitivity of the parity violation apparatus to transverse polarization components will be measured with a transversely polarized beam. A similar procedure will be followed to calibrate the sensitivity to beam position and intensity excursions, which will be independently measured during the final data-taking and corrected for in the data analysis.
6
-
RESULTS AT HIGHER ENERGYParity violation has also been studied in p+p scattering at higher energies, where a simple meson-exchange model loses most of its predictive power due to uncertainties in the treatment of inelastic processes. At Los Alamos/42/, A, was measured to be (2.4 f 1.1) X 10-' at 800 MeV, roughly a factor of two larger than the results below 50 MeV, and of opposite sign. The experimental value is in reasonable agreement with meson exchange calculations, which have been expanded to include inelasticities, although the latter bracket a range of possible values roughly 5 times as large as the experimental uncertainty.
At still higher energy, there exists one additional measurement of A, in p+p scattering. This was per- formed as a transmission experiment with a 5.1 GeV polarized proton beam incident on a water target at the ZGS. The result/43/, A, = (2.6 f 0.7) X 10-'j, is an order of magnitude larger than expected based on a variety of models ranging from meson-exchange to direct W and Z boson exchanges. A Glauber screen- ing calculation/44/ indicates the experimental result obtained with a water target should be enhanced a further factor of 1.7 for interpretation in the context of p+p scattering, further enhancing the discrepancy with predictions. The experiments are confident in their analysis of possible systematic errors, which are of similar origin to those that have been extensively studied at lower energy. An additional background asymmetry due to polarized protons resulting from decays of polarized hyperons produced in the target was eliminated by installing a magnetic spectrometer to transmit only protons of the appropriate beam momentum.
The large value of A, at 5 GeV has inspired alternative model calculations in the p+p system. Two in- dependent models have succeeded at reproducing the large effect measured in the experiment, although both have been the subject of debate in the literature. Nardulli and Preparata/45/ predict an effect that is independent of energy above 800 MeV and consistent with the 5 GeV result. Their model is based on intrinsic parity violation in the nucleon wavefunction. The energy dependence of the diquark model calculations of Goldman and Preston/46/ is markedly different - the asymmetry rises rapidly with energy by almost two orders of magnitude from 800 MeV to 12 GeV. It has been aruged/47/ that the former model implies results at low energy which are too large and inconsistent with existing data. The diquark model calculations have been criticized/48/ for implying an unphysical growth of the predicted effects at high energy, while the authors have argued/49/ that their model gives a firm prediction for the energy dependence of A, while taking a normalization from the measured 5 GeV result. Clearly, the enhanced parity-violating asymmetry at 5.1 GeV represents a serious challenge, both for experimental confirmation and renewed theoretical efforts.
Recent advances in acceleration of polarized proton beams, as demonstrated at the Brookhaven AGS, will make possible higher energy parity violation measurements in the future. Detailed studies for the planned 30 GeV KAON facility in Vancouver are being carried out to optimize the synchrotron lattice designs for minimum losses due to depolarizing resonances. At present, it is expected that over 10 pA of polarized
beam could be accelerated with 72% polarization a t 3 GeV from the Booster and 55% polarization or better surviving to 30 GeV/50/. Such a facility would make possible a wide range of polarization experiments up to 30 GeV.
7 - CONCLUSIONS
Studies of parity violation in nuclear systems, despite enormous technical and interpretive challenges, have yielded some significant and interesting physical results. A weak meson-exchange model provides a consis- tent framework for interpreting experimental data at low energy. Although independent measurements of all six weak meson-nucleon couplings have yet to be achieved, data from light nuclei and the p+p system are consistent with standard model predictions. The small experimental value of the weak pion coupling f$ has provided evidence of a dynamical neutral current suppression that was completely unexpected. While in many cases, the enhanced parity-violating effects in finite nuclei are too difficult t o analyze unambiguously due to nuclear structure uncertainties, the extremely large effect seen in neutron resonances hold promise for systematic studies using a statistical model approach and may allow tests of time reversal symmetry which might rival sensitivites reached in neutron electric dipole moment experiments. Probably the great- est promise for precise tests of standard model predictions lies in the two-nucleon system, where recent advances in technology and experience have made possible measurements a t the 10-' level with impressive accuracy. The tantalizingly large parity-violating effect measured at 5.1 GeV with a water target remains to be confirmed by an independent experiment and to be understood in a consistent framework with the low energy data.
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