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Some Polarized Target Experiments for Elementary Particle Physics

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Some Polarized Target Experiments for Elementary Particle Physics SOME POLARIZED TARGET EXPERIMENTS FOR ELEMENTARY PARTICLE PHYSICS R.H. DALITZ Department of Theoretical Physics, Oxford INTRODUCTION At the present stage in elementary particle physics, there are three major areas of phenomena where the use of polarized targets may lead to especially illuminating information. a. High energy scattering, in the energy range where the dominant contributions to the processes observed may arise from Reggion ex­ change. The features of particular interest in these situjtions will be discussed at this meeting by Dr R.J.N. Phillips 1 • b. Tests of time-reversal (T) invariance for electrom~gnetic pro­ cesses, following the suggestion by Bernstein et al 2) that the CF-violation observed in the weak decay of kaons arises from the existence of an electromagnetic interaction which strongly viola­ tes T-invariance. Evidence for this hypothetical T-violating elec­ tromagnetic interaction can be sought most directly in the study of electromagnetic processes from a polarized proton target, for example in the study of the electro-excitation process e + N-+ e + N* , as discussed by Christ and Lee 3J. The experi~ents proposed will be discussed at this meeting by Dr M. Jacob 4J. c. Hadron spectroscopy. Very many resonant states have been obser­ ved for masonic and baryonic states. These hadronic states are be­ lieved to correspond to the patterns appropriate to unitary multi­ plets of states, and these unitary multiplets appear to be grouped in supermultiplet patterns. In our attempt to classify and under­ stand all these hadronic states, our first need is for the deter­ mination of the spin and parity for each state. A general survey of the methods available for their determination will be given at 262 R.H. DALITZ this meeting by Dr M. Jacob 4). In this talk, we wish to discuss briefly some particular situations of interest, in order to illus­ trate the kinds of experiment which appear especially relevant at present. 2 RESONANCE FORMATION EXPERIMENTS The additional information made available by the use of polarized targets is especially valuable in the case of resonant states which may be formed by direct collisions. Such "resonance forma­ tion experiments" are possible only for resonance states for which there is an entrance channel corresponding to a conveniently long­ lived particle (n*, K~, K~, p, n or p) incident on a proton or neutron target. We shall uiscuss the situations briefly in turn. The nN system has already been much studied in the resonance re­ gion. Polarized target experiments have been reported for n+p and n-p elastic scattering up to N* mass about 2200 MeV, as summari­ zed by Dr o. Chamberlain 5) at this meeting. The nN scattering processes are described by two amplitudes (2.1) for total isospin I = 1/2 and 3/2, and it is possible to carry out analysis on the basis of these n:-p elastic scattering data alone. However it will also be desirable to have available the angular distribution and polarization data for the charge ex­ change process n-p _,. n°n, which is governed by the difference (S3/2-s1; 2 ), in order to ,11ow a unique phase shift analysis for the nN system. Lovelace 6) has recently emphasized that these charge-exchange experiments would be of more immediate value for this purpose than the more difficult spin correlation experiments, involving measurement of the Wolfenstein R and A parameters. Polarization studies of inelastic processes may also prove to be important, especially for those N* resonances which happen to ha­ ve small partial width for the nN channel. The reaction n-p-+ n~ is a rather convenient example, which may be studied at the same time as the charge-exchange process ; at least one resonance, the (1/2-) NfL 2 (1540) resonance which leads to the strong n~ threshold proauction, is known to have particularly large partial width for the n~ channel. Other reactions of the same kind are n-p ~ AK 0 , which also selects I = 1/2 N* states, and the various reactions nN __.. [K, especially the process n+p-+ E+K+ which se- EXPERIMENTS FOR ELEMENTARY PARTICLE PHYSICS 263 lects I = 3/2 N* states. The AK0 and L+K+ reactions just mentio­ ned are of particular interest in that a polarization analysis of the final state is readily available in consequence of the strong polarization dependence of the decay processes A -pn- and L+ _. pno. Some polarization analyses of these reactions have al­ ready been carried out on this basis, without the use of polari­ zed targets, but these have not been sufficient for a unique ana­ lysis of the reaction amplitudes. Marked oscillations in theAKO polarization angular distribution h~ve been reported in bubble­ chamber data analysed by Schwartz 7J, for pion momenta about 1500- 1700 MeV/c, which are probably associated with an Nf/ 2 resonance not otherwise known 8) ; however, the statistics available even from such large bubble-chamber experiments are not sufficient to determine the details of the phenomena with sufficient precision for a definite interpretation. An investigation of this reaction with a polarized target, together with polarization observations for the final A particle, would be of particular interest, in that this situation would allow a complete determination of the spin properties for this reaction. For the L +K+ reaction, pola­ rization effects have been reported which are associated with the resonance N3; 2 (1920), although the statistics in these bubble­ chamber experiments are r~latively limited, and these have been interpreted by Holladay 9) in terms of interference between reso­ nant and peripheral production amplitudes to give support for the assignment (772+) for the spin-parity of this state. A more ade­ quate study of the polarization pro~erties of this r+K+ reaction could be made also for the higher N3/2 states, with the use of polarized targets. For the KN systemt there are two elastic amplitudes , of the s0 s1 general form (2.1). The K-p elastic scattering amplitude is 1/2(So ! s ) ; the_ampl~tude for the charge exchange process 0 1 K-p ~ K n is 1/2(s1 - s 0 ). Hence, a complete analysis for the KN system requires angular distribution and polarization data for both the elastic and charge exchange processes. Here, the knowled­ ge of the charge exchange process is essential for. an adequate a­ nalysis, and this process has received very little attention to date ; there is some preliminary bubble-chamber data on angular distributions and total cross-sections, but no information at all on its polarization properties. The inelastic process K-p -..Ano is of particular interest, both because the final state has I = 1 and is therefore an indicator for If states and because of the polarization analysis possible for the A hyperon. Bubble-chamber studies of this reaction (or of the corresponding reaction K-n - An-) haye already yielded much information on the If resonance states 10). The inelastic reaction K-p -- A~ similarly has special interest, in that the final state has I = 0 and is an indicator for YO stat~s ; apart from the thres­ hold studies, which indicate the existence of a Y (1670) resonance with (1/2-), there is rather little data available0 on this reaction, 264 R.H. DALITZ essentially none on its polarization properties. These reaction amplitudes are not closely related with the elastic KN amplitu­ des, except for the form of the Breit-Wigner amplitude for the resonant state, since the unitarity relations are complicated, owing to the large number of other competing channels. However, with polarized target and polarization observations for the fi­ nal A particle, complete spin and partial-wave analyses are pos­ sible for these amplitudes alone. Of course, the study of these reactions with polarized targets is made difficult by the fact that the final mesons are neutral, unless targets are available with a very high proportion of polarized protons. For the KN system, K+p elastic scattering leads directly to the I = 1 amplitude S1 of the form (2.1 ). Polarized target studies are needed for a partial wave analysis for s 1 and experiments are being planned by several groups. At present, these will be of particular interest in the neighbourhood of K+ momentum 1250 MeVJc, where a small)bump has been found recently in the K+p to­ tal cross-section 11 • The I = 0 KN amplitude s0 is more difficult to reach. One possi­ bility involves the study of the charge exchange reaction K+n _..K~p, whose amplitude is (s1 - s0 )/2f2, and there has alrea­ dy been some study of its angular distrioution from charge ex­ change observations in K+d collisions 12). At present there is particular interest in the study of the I = 0 amplitude in the neighbourhood of K momentum 1150 MeV/c in consequence of a rather marked bump whic~ has been observed recently in the K+d total cross-section 11) and which must be attributed to the I= 0 KN interaction. Polarization information on the I = 0 KN interaction is therefore much desired, in order to assign this bump to a de­ finite spin-parity state for the KN system and to clarify its in­ terpretation. This could be done with a polarized deuterium tar­ get, since the neutron within the deuterium will have polariza­ tion (P+ 1 - P_1 ), where P denotes the percentage of deuterons with magnetic quantum num~er m along the polarization direction. The K? angular distribution observed requires rather substantial corrections at forward scattering angles for the effect of the Pauli principle, arising from the presence of two final protons in the reaction K+d -Kfpp (the differential cross-section neces­ sarily vanishes for 0° scattering with full energy) ; the corres­ ponding corrections to the polarization angular distribution would need to be looked into, since the Pauli principle effects are certainly spin dependent here.
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