POLARIZATION AND SYMMETRIES

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POLARIZATION AND SYMMETRIES

W. van Oers

To cite this version:

W. van Oers. POLARIZATION AND SYMMETRIES. Journal de Physique Colloques, 1990, 51 (C6),

pp.C6-505-C6-510. �10.1051/jphyscol:1990661�. �jpa-00230931�

POLARIZATION AND SYMMETRIES

W.T.H. van OERS

Laboratoire National SATURNE, F-91191 Gif sur Yvette Cedex. France and Department of Physics, University of Manitoba, Winnipeg, Manitoba.

R3T 2N2, Canada

1 - INTRODUCTION

Spin is a most important commodity with which t o test conservation laws or to measure symmetry violations to a high degree of precision. From an experimental point of view one cannot fail but t o remark the high degree of sophistication which is displayed in performing such symmetry violation experiments, not at least in those experiments reported on at this Symposium. The objectives have not been restricted to varifying the extent of the validity of a conservation law or symmetry principle but in many instances to make important inroads into understanding the short range hadronic interaction. The topics of discussion in an approximate classification scheme are as follows : charge symmetry violation due to a so called class IV interaction which preserves neither charge symmetry, nor charge independence, nor isospin ; parity violation in nuclear systems due to the flavor conserving weak hadronic interaction ; and C P violation or time-reversal invariance breaking assuming that CPT invariance is maintained.

2 - CHARGE SYMMETRY AND CHARGE INDEPENDENCE

Charge symmetry leads to the complete separation of the isoscalar and isovector components of the n - p interaction. This in turn leads to the equality of the differential cross sections for polarized neutrons scattering from unpolarized protons and vice versa. As a result the analysing powers A, E A,, where the subscript represents the polarized particle. A non-vanishing analysing power difference, AA A, - A,

#

0, is directly proportional t o the isospin singlet-triplet, spin triplet-singlet mixing amplitude and therefore direct evidence of a charge symmetry breaking class IV term in the n - p interaction.

Isospin conservation is broken by the electromagnetic interaction and therefore one expects effects of the order of the finestructure constant a.

An earlier measurement at TRIUMF of AA at the angle where the analysing power passes through zero at an incident neutron energy of 477 MeV /l/ yielded the first evidence of the existence of the class IV term in the n - p interaction AA = (47 f 22 % 8)

*

1 0 - ~ . At this Symposium the result of a similar IUCF experiment /2/ at an incident neutron energy of 183 MeV was presented : AA = (32.1% 6.1% 6)* 10-4, the average of the values for AA around the zero-crossing angle (82.2" - 116.1° c.m.) for which (A(0)) = 0.

This result differs from zero by nearly four standard deviations. In general one has difficulties in extracting an angular distribution of AA(0) since the difference in the asymmetries for beam and target polarized can be expressed as

E b - E t = AA(Pb

+

^Pt)/2

+

^{(A)(Pb - Pt)}

,

pointing to the need of calibration of the beam and target polarizations (Pb and P t ) with an accuracy unattainable a t present. In the analysis of the IUCF experiment this difficulty was overcome by adjusting the ratio of the polarizations (Pb/Pt) until1 the error-weighted rms value of AA(@) over the angular range of the experiment was minimized. In this manner the twelf point angular distribution, shown in Fig. 1, was obtained. The solid lines represents the theoretical prediction for AA(0) of Holzenkamp, Holinde and Thomas /3/.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990661

COLLOQUE DE PHYSIQUE

"AA(0)" (Minimal Variance). 1'" = 170 -193 MeV

O.O1°

7 - 1

0.005

-

-0.005

-

-

I ~ O I B I I (HHT). X' = 0.1 ( 7.1)

- . - . - . - fleicl (UW). 'X = 11.6 (10.0) -0.OIO -L-L-u-d

B0 80 100 120

Fig.1 - Twelf point angular distribution of AA(@) at 183 MeV. The AA(@) values may differ from the true AA(@) values by a constant times A(@) (whose zero crossing angle is indicated by the arrow), as a result of the experimental uncertainty in the ratio of the polarizations %/Pt.

The measured analysing power differences of the IUCF and TRIUMF experiments are well reproduced by theoretical predictions, based upon the meson exchange model, which include contributions from one photon exchange (the magnetic moment of the neutron interacting with the current of the proton), from the neutron proton mass difference effecting charged one and p exchange, and from p - ^W meson mixing (see Fig.2). Due to the dynamics of the interaction only the 183 MeV results are sensitive to the p - ^W meson mixing contribution. The agreement between experiment and meson-exchange model theoretical predictions points to the adequacy of the latter in parameterizing the underlying quark-quark interactions.

Note that it is the short range N - N interaction which is being scrutinized in these experiments. In a further contribution to this Symposium a new measurement at TRIUMF with greatly improved statistics (by a factor four) is being anticipated /4/, of great importance in delineating the various contributions.

IUCF. 183 MeV TRIUMF. 477 MeV

Fig.2 - Results for AA(0) averaged over angular regions straddling the zero-crossing of A ( @ ) , for which (A(@)) = 0. The left panel represents the 183 MeV IUCF data for the range 82.2'

5

,,@

5

116.1'. The righ panel represents the 477 MeV TRIUMF datum obtained from the difference in the zero-crossing angles. The calculations are from /3/, averaged over the same angular regions as the data.

N

-

N

precise measurements of n - p observables in the low energy region is underway a t TUNL, various results have been presented a t this Symposium (e.g. 151). These measurements, in particular of the analysing power Ay (see Fig.3), will aid greatly in establishing the differences in the p - p and n - p triplet P-wave phase shifts (3~o,1,2).

Fig.3 - Data for A,(O) in n - p elastic scattering at 12.0 MeV. The curves are predictions calculated from the Bonn potential (adjusted in the w N N coupling constant) - solid line, the Paris potential

-

^dotted

line, and the Nymegen

-

dashed line, and Arndt et al's (SP 89) phase shift analyses - dashed

-

^dotted

line.

3 - FLAVOR CONSERVING WEAK HADRONIC INTERACTION

Parity violation in hadronic systems is not studied to learn about the electroweak interaction but rather to learn about the non-leptonic weak interaction between hadrons. In particular note that the flavor conserving weak neutral current interaction can be studied only in hadronic systems. At low and in- termediate energies a meson exchange description involving one strong interaction vertex and one weak interaction vertex appears a most convenient approach. At the weak interaction vertex a weak meson- nucleon coupling constant is multiplying a weak hadronic matrix element. In the case of two nucleons initial and final state are described by strong interaction wave-functions obtained using a N

-

N potential and N - N phase shift information. There exist six weak meson-nucleon coupling constants correspond- ing to n, p, and w exchanges. The exchange of neutral scalar mesons is suppressed by CP conservation by a factor of a few times 103. The six weak meson-nucleon coupling constants are

f:,

h:, h i , h:, h:, h;, where the superscript indicates isospin changes. These weak meson-nucleon coupling constants have been calculated by Desplanques, Donoghue and Holstein and more recently by Dubovik and Zenkin /6/

using a SU(6)w model for the quarks and treating strong interaction effects in renormalization group theory. A complete determination of the six weak meson-nucleon coupling constants requires a t least six linearly independent pieces of experimental information.. As of t o date only four experimental constraints of significance exist. Consequently several new precision parity violation measurements are required.

The most precise measurement made to date is of the longitudinal analysing power A, in p'-p scattering at an energy of 45 MeV [A, = (-1.50 f 0.22)

*

1 0 ~ ~ 1 . /6/. It provides a constraint on a combination of hp and h, essentially involving only the first parity violating transition amplitude ('So ^-3 PO) in a partial wave decomposition of A,. At this Symposium a new measurement at 13.6 MeV was described being pursued along quite similar lines /7/. The interim result presented is A, = (-1.5 f 0.5)

*

^10-'.

It is hoped that the continuation of the measurement will result in a significant reduction of the present error eventually approaching the error of the PSI experiment quoted above. This will allow pinning down the combination of weak meson-nucleon coupling constants h$'

+

hcp, with h$p = h:

+

hi

+

h$/& and hLP = h:

+

h:. The energy dependence of the longitudinal analysing power A, in p'- p scattering in the low-energy region is shown in Fig.4.

COLLOQUE DE PHYSIQUE

Fig.4 - Energy dependence in the low energy region of the first two partial wave contributions ('So- 3Po) and 3 P 2 -l 0 2 ) to the longitudinal analysing power A, in @- p scattering.

4 - TESTS O F CP CONSERVATION AND TIME REVERSAL INVARIANCE

CP non-conservation has been established exclusively through measurements of E and E'/& from kaon decays. There appears to be at present a possible discrepancy between the CERN and FNAL results for

E'/&

.

Any non-zero result from any other system would be very important in helping to determine the

origin of CP non-conservation. Limits on the magnitude of CP non-conservation are less usefull, especially in nuclear systems where there are uncertainties in interpretation. There exist no direct evidence of time- reversal non-invariance but it is strongly implied by CPT invariance together with CP non-conservation.

The evidence of parity violation in hadronic systems is well established. In nuclear systems there exist large enhancement factors which favor experiment but which give an added difficulty to theoretical interpretation. Such enhancements factors occur very pronounced in epithermal neutron scattering, e.g. in 139La (0.734 eV resonance) /g/. Here a large helicity dependence of the total cross section (longitudinal analysing power) has been observed as the result of the mixing of close low lying S- and p-wave resonances as a result of the weak hadronic interaction. In a contribution to this Symposium /10/ a successful search for parity violation in 13'~a, 1 6 5 ~ o , 235U, and 238U has been reported. The objective is to study the nuclear structure magnification factor in order to apply it to a measurement of time-reversal non-invariance to possibly an accuracy a few times 1 0 - ~ using an epithermal polarized neutron beam and an aligned 165Ho target. The envisaged expeGmentjs then to measure the dependence of the total cross section on the P-even T-odd correlation (F.J x f l ( J . 8 , where the dynamical variables are neutron spin

a,

neutron momentum @and target spin

f.

The first results in producing an aligned 165Ho target have been presented in another contribution to this Symposium /11/.

Time reversal non-invariance can also be tested through the P-odd, T-odd correlation (f.F X

fl

in nuclear P-decay. Here the dynamical variables are

.f

the target spin, F the electron spin and @ t h e electron momentum. A new measurement is underw_ay at PSI. The experiment is based on the P-decay of polarized 8Li produced in the reaction 7~i(G,p)sLi. The interim results is a t the level o f f 0.04 1121.

Clearly to be of significance greatly improved accuracy must be obtained. From the upper limit on the electric dipole moment of the neutron one may deduce an upper limit of time reversal non-invariance in that system of 10-' - 10-~.

production process pp

-

AA. I t is to be noted that the latter also serves to accentuate the duality between boson exchange and quark model descriptions : the t-channel exchange of I<-mesons versus a SS quark pair creation. Unfortunately present experimental approaches have difficulties t o arrive at differences in the weak decay parameters a of significance in terms of C P violation. In a contribution /13/ to this Symposium a n interim result on the asymmetry parameter

has been presented A = -0.023 f 0.057, a result not inconsistent with A = 0. Here a and P are the decay asymmetry parameters and polarizations of the A-particle, respectively. It is possible that with enough efforts in improving the incident

p

beam and the detection system a level of accuracy of 1 0 - ~ may be reached. According present theoretical predictions an effect ranging from 10-4 to 2

*

^{10-5 is}

expected 1141. For a future advanced hadron facility, the exclusive production process pp

-

^{3 2 -}

constitutes a promising, unique case for the search for A S = 1 direct C P violation in hyperon decays. It would allow for measuring the hyperon decay products polarization differences, in addition to the decay products asymmetries.

5 - OTHER TESTS

According t o the Conserved Vector Current Hypothesis (CVC), the vector coupling constant C" is universal. CVC can be tested in P-decays between isospin T = 112 doublets. In the mixed (Fermi and Gamow-Teller) decays the measurement of the lifetime must be supplemented by a second measurement such as the decay asymmetry parameter A. in order to determine the vector and axial-vector coupling strengths separately. At the Symposium a new measurement of the P-asymmetry parameter A. for the decay of 3 5 ~ r to the groundstate of 35C1 was discussed 1151. Earlier results were either incompatible or of insufficient accuracy with regard to CVC. I t is anticipated that a final accuracy in A. of f 0.03 can be reached which corresponds t o an uncertainly of i 0.007 in the weak vector coupling constant

CV.

Some final comments concern new high energy measurements of the analysing power Ay at 24 GeV/c in p - p elastic scattering at momentum transfers corresponding to p% = 4.5, 5.5 and 6.9 ( G ~ V / C ) ~ , where P! = p~,sin28,, 1161. These measurements were made with a high intensity unpolarized proton beam (1.6 X 1013 proton per pulse, repetition rate 0.38 Hz) and a high cooling power polarized proton target (magnetic field of 5 T and a temperature of 1 K). The data show (see Fig.5) that spin dependent

Fig.5

-

The analyzing power Ay in p - p'scattering at 24 GeV obtained with the AGS. The data are plotted against also shown are previous data a t nearly energies.

.

-

^{24 GeV CERN}

28 GeV AGS

,-J - ^{This Exper. 24 GeV}

.2 -

.l -

-.l

-.2

2 4 6

P:(G~v/c)'

-

-

-

-

C6-510 COLLOQUE DE PHYSIQUE

effects are as significant a t 24 GeV as a t 200 MeV, (where A, has a maximum value of 0.33), in contradiction to what expected from simple energy considerations. Most theoretical models based on perturbative QCD suggest that all spin dependent effects decrease with increasing energy and large p:, in contradiction with experiment.

REFERENCES

/ l / R. Abegg et al., Phys. Rev.

m,

2464 (1989).

/2/ J. Sowinski et al., Contribution 15D t o this Symposium.

/3/ B. Holzenkamp, K. Holinde, and A.W. Thomas, Phys. Lett.

m,

121 (1989).

/4/ R. Abegg et al., Contribution 20D to this Symposium.

/5/ G.J. WeiseI et al., Contribution 16D t o this Symposium.

/6/ B. Desplanques, J.F. Donoghue, and B.R. Holstein, Ann. Phys. (N.Y.)

124,

449 (1980) ; V.M. Dubovik and S.V. Zenkin, Ann. Phys. (N.Y.)

172,

100 (1986).

/7/ S. Kistryn et al., Phys. Rev. Lett.

56,

1616 (1987).

/8/ P.D. Eversheim et al., Contribution 3D to this Symposium.

/g/ Y. Masuda et al., Nucl. Phys.

m,

269 (1989).

/10/ I. Penttila et al., Contribution 9D t o this Symposium.

/11/ Y. Masuda, Contribution 8D t o this Symposium.

/l21 J. Sromicki et al., Contribution 12D to this Symposium.

/13/ P.D. Barnes et al., Contribution 18D to this Symposium.

/14/ J.F. Donoghue and S. Pakvasa, Phys. Rev. Lett.

55,

162 (1985);

J.F. Donoghue, B.R. Holstein and G. Valencia, Phys. Lett.

m,

319 (1986).

/15/ M. Allet et al., Contribution I D to this Symposium.

1161 B. Vuaridel e t al., Contribution 19D to this Symposium.