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2024
ELECTROMAGNETIC EXPERIMENTS AT NAL R. J. Morrison University of California, Santa Barbara E. Engels, Jr. University of Pittsburgh M. Goitein Lawrence Radiation Laboratory E. A. Paschos The Rockefeller University P. Patel McGill University F. M. Pipkin and M. Tannenbaum Harvard University J. Tenenbaum Stanford Linear Accelerator Center J. S. Trefil University of Illinois and D. Yount University of Hawaii
ABSTRACT
A number of possibilities for studying electromagnetic interactions at NAL are considered. In this report the scope of these studies is given, and some of the pro posed experiments are discussed. Other experiments as well as questions concerning the electron -photon beam are considered in accompanying reports. We believe that a number of these experiments warrant early scheduling, and we recommend that NAL plan for the small but important modifications required for electron use of a high intensity beam.
1. INTRODUCTION
The photon, muon, electron, and K beams which can be made at NAL will pro vide the opportunity to cover a wide range of real and virtual photon physics. We note in particular that the tagged-photon beams which can be built 1 will have fluxes at
energies up to 80 GeV comparable to those available at electron synchrotrons, and with a significantly better duty cycle. The combination of known incident energy and high duty cycle will make experiments like the photon total cross section and vector meson production relatively straightforward. The electron fluxes are comparable to the expected muon -beam intensity so that deep-inelastic electron -scattering experi ments are feasible over the same kinematic range as the proposed muon experiments (described in 88-188 and 88-190). Under NAL conditions, inelastic Compton scatter ing should be possible as a complementary experiment in the deep-inelastic region. In addition, the high -energy 1T and K beams at NAL can be used with stationary elec tron targets to measure the charge radii of these particles. The studies which we have made fall under the following categories: A. Measurement of the charge radii of the pion and kaon The form factor for the pion has been well measured in the time -like, but not in the space -like, region. Pions of incident energies -80 GeV can give momentum transfers q 2 " 1. 5 f- 2 to a stationary-electron target. The radius can then be meas ured to a precision of - O. 03 f with cross -section measurements of 1% absolute accur acy. Compared with the possibilities at 8erpukhov the q 2 range is larger at NAL and the systematic errors can be more carefully studied since pion beams of both charges will be available. Comments concerning these experiments are given in 88 -165 and 88-169. B. Photon interactions in the diffracti ve region At least qualitatively it has become evident that photon scattering behaves much like pion scattering in the diffractive region. This is demonstrated by the behavior of elastic photon scattering Jthe photon total cross section, and vector-meson photo production. The phenomena are described qualitatively by the ITlodel of vector-meson dominance which is described in 8ection ll. The relevant NAL experiments are also described in that section. The goal of these experiments is to test the region of validity of the model and to see if the rho -like behavior of the photon persists at high energy. C. Deep-inelastic scattering The deep-inelastic electron scattering results from 8LAC 2 have stimulated a great deal of interest and may lead to a better understanding of the structure of the nucleon. The 8LAC work can be extended at NAL with muons as described in other reports or with electrons. In addition, the energy available at NAL and the high duty cycle will make possible the study of deep-inelastic Compton scattering, a process which has not yet been observed due to experimental problems at the lower-energy machines. This proc ess and the proposed experiment are described in 8ection Ill. D. New-particle searches
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In terms of reaction amplitudes, we can write
A(y+T -X) (^) (2)
where A(y + T - x) represents the amplitude for a photon to produce a final state X from the target T, and A(v + T - X) represents the amplitude for one of the vector mesons to do the same. An especially sensitive test of this idea can be made if one considers particu lar photon-induced reactions, namely
y+p-y+p (3) y+p-v+p (4) (5)
and
y+A-p+A, (6)
where A is a nucleus. [Note that the optical theorem connects Eq. (3) and Eq. (5) at
t = 0 so that two experiments are, in a sense, equivalent for our purposes.) For the
sake of simplicity of development, let us consider only the p meson in the sum over vector mesons above (the other mesons could, of course, be included in a more realistic discussion). Then, by the hypothesis of vector dominance, we write
A(y + P - Y + p) = f v 2 A(p + P _ P + p)
= f 2(i + '" ).£.. ° ,
(7) v P 41T pN
where the last equality holds at t 0, and ° pN is the p-nucleon total cross section. Similarly,
A(y + P - P + p) = fvA(p + p - p + p)
k^ (8)
= f)i + "'p) 4:;- °pN'
where, again, the last equality holds at t = O. Finally, from the production of p mesons on nuclei, one can, by well-known techniques, extract (J pN Taking the imaginary part of Eq. (8) at t = 0, we get
ImA(y+p-y+p) = f v (9)
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where Q'"p is the total photon-nucleon cross section. Thus. we see that if ~/pE:l·;.cnen·· tal data on any three of the equations (3-6) are availa;,lc , Eqs. (7-9jare '."ere" '.H':nil1',j. and a sensitive test of vector dominance can be made. ~n addition, Si!lLt:! ~itl T. photon beam would presumably exist at a wide range of energies up to rUl):le U:a&.n 100
GeV Ic, it becomes possible to make this type of test as a function of energ;' .( his lS
particularly useful in the evaluation of Eq. (6), since tlw systemaLic errClrs"iL 'h occur in the extraction of a pN from nuclear production data (due. for eX8iJlp1c. to llli certainties in the handling of the nuclear wave functions ,re not energy (i~pt"( "t, So that the energy dependence of a pN can probably be determined much mor", al (:uratdy than the precise value of a pN at a given energy. This comment would apply tv 'he other reactions as well so that the possibility of really testing vector domman',,, ',)'er a wide range of energies antJ comparing the energy dependence of tlit: d;]':ue l·e;:,ctions (a comparison which does not depend critically on comparing nornlalizaLions between different experiments) offers a program of photon physic" at NAL which is not avail able at other facilities. Finally, we note that the measurement of the photoproductiun of the p 'md the
q, meson at high energies will test the accepted explanation of one of the great puzzles
in photoproduction, namely, the question of why the production of the q, meson is so
small compared to the p, when the coupling constants in the SU(3) limit are in the ratio
f p^2 ; lq,,2 ~ 9 : 2.
From Eq. (8). neglecting the real parts of the photoproduction amplitudes, we see that at t ~ 0, (jt('1+P-q, +p) R
da
~ ~ (aq,N )
da dt (^) ('1+p-p+p) 9 apN
so that if a pN ~ a q,N' the 6 cross section should be 2/9 that of the rho, whereas in fact, the ratio is more like 0.03. The explanation for this lies in predicting the ratio a </>NI a pN from the quark model, where the rho and phi differ in that the former con tains only nonstrange quarks, while the latter contains only strange ones, Using the additive quark model, one can then relate a </>N and a pN to a 1TN and a (^) KN ,since the pion also contains only nonstrange quarks, and the kaon has a mixture of strange and non strange. The prediction then becomes 2 2(a (^) K - (^) n.;. aK.;.p - O",,+pl R
9( a,,+< (J,,_p )
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report shows that in a modest amount of time a high -statistics study of rho photopro duction can be made. The same apparatus can be used to study the rho photoproduction from hydrogen and from complex nuclei. At the same time, a search can be made for higher -mass vector mesons and the density-matrix elements for the rho could be nleasured as a function of energy and momentum trans fer. This would give a test of helicity conservation in the production reaction:
- Elastic Compton scattering. There is no proposal to do this experiment. There is, however, a letter of intent from a Santa Cruz -Berkeley group to study elastic Compton scattering. The details of this experiment are not given. It is presumed, however, that it is similar to the experiment outlined in the 1969 Summer Study (SS-801. The reader is referred to these reports for further details. This is a very important but difficult experiment
III. DEEP-INELASTIC COMPTON SC.".TTEIUNG E. '. Paschos anell{. J. Alorrison
A. Theoretical Motivation Recent experiments have shown that lepton -indue ed proc esses at large rTIomen tum transfers have point-like cross sections. This is true for the SLAC electro production experinlent 5 where
--2--^ dO'^ --4rr(>2^ II'. .((~^2 v) (at ,;l1\a11 ang.les I, clq dv^ Q2^2 ' \ lth v\V Z(Q 2 v) = 0.33 in the deep -inelastic linlit. Sirnilal'l:- the C E In', neutrino e:.'-periment b^ shows that the total neutrino cross section rises linearly with energy. with a slope very close to the slope predicted for a point cross section: 2 2
'7 tot^ (0^ .D~..•^ ~^ ()^1 c;) ~2" (~)AI'
There are further indications that point (TOSS sectlons ma.v also have been obser-ed in other interactions:
- There arc indications that rnassive lepton -pair production in proton proton' collisions has a point-like cross section. The process is C:l-en bj' p - P -~ fJ I~.l l' and it is consistent \viil1 a 1110del calculation that gi\e~' <.:: 2 C!'J) "-hro.;, dq - 3(~
'.vhere Q2 is the invariant mas~ of the 111uon pair, S is the 5quarc of the center -of-
Inass ener-sf)-, anu 0 is a forn1 factor of tJt'(.Ier unity in the intel'esting kinelnatic region.
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- Preliminary results on the electron-positron ring at Frascati^9 suggest the abundant production of hadrons in the range of 2.5 (Ge"j2 :s S:s 41GeV)2 All these observations suggest that tl,ese cross sections arise from the interaction of tnassive electl'0111agnetic (weak) currents with a point-like structure within the proton. It is very important to obtain additional evidence for the presence of point-like cross sections in other processes. It has been suggested that inelastic Compton scattering at high energies and large momentum transfers proceeds through the incoherent scattering from point-like constituents.^10 The process has been analyzed in detail, and it was found that it can be pj'edicted from the existing electroproduetion data through the relation:
da ) v^^2 (^ da^ )^ <^ (~4> ( drldE' (^) yp EE'^ drldE'^ ep--2-'
This rclation is sensitive to thc ratio of the mcan value of the fourth power of charge to the mean square charge. The most formidable difficulty in such an experiment is the background from the decay of IT 0, s. Studies of the background contributions to such an experiment indicate that performing this experiment at SLAC at presently available energies will very likely be inconclusive. This is shown in Fig. I, taken from Ref. 10, where the photon spectrum from the decay of lT^ o , s from 20-GeV bremsstrahlung is shown. The predicted inelastic Compton scattering is also shown under the assumption f 01. The situation changes dramatically at NAL eneqs-ies, however, as shown in Fig. 2, taken from Proposal 24, where the comparable spectra are shown with 40-GeV incident bremsstrahlung. In this case the sum of the two contributions shows a clear break at a cross section
A variation of this experirnent where a muon pair is produced has also been studied in detail. 11 The main background now comes from the Bethe-Heitler diagrams. Estimates of the events for the se two experiments at the Cornell Synchrotron 12 indi eate that the rates are small and the Bethe-Heitler is comparable to inelastic Compton. Such terms could perhaps be investigated in the W. Lee. et aI., proposal. 3 B. The Proposed Experiment (Proposal 24) In this proposal scattered photons are detected in a lead-glass stack at an angle of 6' The experinwnt is designed to cover the eange of q2 from 6-10 (GeV fe)2 and a cange of ' "" E - E I "rom 16-24 (Ge- l. The lead-glas5 array contains at least 40cQunters (presently existing equipment) and would subtend an angle of 6 x 10- 3 steradians. In order to maxinlize the data rate~ the bcarn is adjusted for 40-GeV electrons incident
N
E
QI
o c:> ~
U>
b " "oc: 10 1 "
k' GeV/c
Fig. 1. Photon yields per equivalent quantum expected with an inc ident bremsstrahlung spectrum of end-point energy 20 CeV. The background from ",0 decays is seen to be at least comparable to the inelastic Compton yield which has been estimated under the assumption < Q2> / < Q4> = 1.
-11- (^55) -
-31^ TT'0^ Decay^ Yields 10
Inelastic Compton u^ -32^ Yields^ Q^ =I (\J ......^^10
Eu C>CIl
~ UI
10
- 35 L-...J...-----!:-....L...---L....1....L-L..L---L.ll..----l._.L.-----a..-l (^10 4 8 12 16 20 24 )
Y Energy (GeV/c)
Fig. 2. Photon yields per equivalent quantum with an incident bremsstrahlung spec trum of end -point energy 40 GeV. At this energy there is a large kinematic region in which the estimated background of photons from lTo decays is small compared with the inelastic Compton yield.
