A Comparison of Electron-Proton. Positron-Proton Elastic Scattering at. Four-Momentum Transfers up to 5.0 (C%V/C)~ *

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1 to be submitte to Physical Review Letters SLACPm47 mp) A Comparison of ElectronProton an PositronProton Elastic Scattering at FourMomentum Transfers up to 5 (C%V/C)~ * JERRY MAR, BARRY C BARSH, JEROME PNE California nstitute of Technology an DAVD H COWARD, H DeSTAEBLER, J LT, A MNTEN, f RCHARD E TAYLOR Stanfor Linear Accelerator Center an M BREDEXBACH Massachusetts nstitute of Technolop$ (Receive May, 196) L * Work supporte in part by the U S Atomic Energy Commission f Present aress: CERN, Geneva 3, Stitzerlan

2 l, Measurements of the ratio (R) of positronproton an electronproton elastic scattering cross sections have been mae, with the square of the fourmomentum transfer (q) equal to, 69, 73, 154, 44, 37, 379, an 5 (Gev/~)~ The measurements, after raiative corrections, are consistent with R = 1, with stanar errors ranging from f 16 to 13 The results give limits for the size of the two photon effects Electron elastic scattering experiments to ate have been interprete using the Rosenbluth formula base on the single photon exchange moel A measurement of R is a test of this moel because a eviation, of R from 1 is an inication of the size of the real part of the two photon exchange amplitue 1) Because the interference between the single photon amplitue an the two photon amplitue occurs with opposite sign for electrons an positrons, one may write R x ReB/A, where ReB/A is the ratio of the real part of the two photon amplitue (ReB) to the single photon amplitue (A) Earlier measurements of R by other experimenters ) for the most part gave R z 1 Past theoretical estimates 394) either make no efinite preiction as to the size of R11, or preict it to be < A summary of previous investigations of R has recently been given by Pine') RESULTS The ratio R was measure for the laboratory scattering angle regions 15'5 < 35 an 6'5 _< 15 with incient electron (an positron) energies of 4 GeV an 1 GeV, respectively The high q

3 ata exten to higher q than earlier experiments, an the moerate q ata inclue measurements at smaller angles than previously explore The results are isplaye in Table, an a comparison with previous measurements is given in Figure 1 n the table, R is the correcte experimental ratio with its uncertainty The uncertainty in R is the square root of the sum of the squares of the statistical uncertainty an the estimate uncertainty ue to systematic errors, both of which are given in the table The systematic error is ominate by the beam monitor uncertainty The ifference in raiative corrections for ef an e scattering was calculate using the results of Meister an Yennie, 6) with exponentation The column labelle "RaCorr" is the net correction to R from raiative effects No uncertainty is assigne to the raiative corrections The column labelle "ReB/A" in the table gives the 95% confience limits for the quantity ReB/A efine earlier As can be seen in the table, all the elastic ata are consistent with R = 1 This result is in agreement with estimates by Drell, Ruerman, an others, 3) an supports the one photon approximation over an enlarge kinematical region The inelastic measurements in the table, labelle "N*(13)", give R for all scattere events in which the missing mass of the final state particles other than the recoiling electron lay between 111 MeV an 137 MeV About 7% of the cross section leas to N*(13) prouction The remainer of the scattering in this region can be attribute to nonresonant pion prouction an to the raiative tail for elastic scattering No raiative corrections were mae to tnese cross sections For these ata R is again consistent with 1

4 3 EXPERMZNTAL METHOD The positron an electron beams were mae by passing an electron beam, with energy about 55 GeV, into a inch thick watercoole copper raiator positione onethir of the way along the SLAC! accelerator The low energy electrons or positrons emerging from this raiator were accelera te to form the beams for the experiment 7) n this way for each ata point the positron an electron beams were similar with regar to transverse phase space, energy spectrum an intensity This technique was important in minimizing the effects of possible systematic errors The full energy sprea of the beams varie from 5% to 1% To increase intensity, the 1% with was use for most of the ata The + average intensity varie from 6 x 1' e/see to 4 x 1 11 e + /sec The incient beam irection was maintaine to better than + 1 mra The beam charge was measure with a toroi current transformer ) i ~ ~ an a Faraay cup 9) Two thinfoil seconary emission monitors were also use The ratio of positron to electron charge measure by the toroi iffere from the ratio measure by the Faraay cup by up to 15% Compari sons with the seconary emission monitors inicate that the Faraay cup was more likely to be in error than was the toroi Various arguments ten to support this conclusion, but the iscrepancy is not fully unerstoo As a consequence, the toroi was use as the stanar for etermining beam charge an a systematic error in R equal to the observe isagreement between Faraay cup an toroi was assigne for each ata point The SLAC GeV/ c magnetic spectrometer was use to analyze particles scattere from a 7 cm iameter vertical cyliner of liqui hyrogen For the small angles (6' an 5), the SLAC GeV/c

5 4 spectrometer was use with a 7 cm iameter target The soli angle acceptances into these systems were approximately msr an 6 msr, respectively The etection systems of both the GeV/c spectrometer 1) an the GeV/c spectrometer 11) have been escribe in earlier papers Both systems containe momentum (p) an angle (6) scintillation counter hooscopes an a lealucite total absorption shower counter for Xe iscrimination The energy loss (E/X) in a counter positione after 5 raiation lengths of lea was also use to improve 51e iscrimination forthe ata at so Pion contamination was reuce to less than 1% by requiring the pulse heig$,ts in the shower an E/X counters to be greater than certain minima R was etermine from the number of counts in a stanar area in the backgrounsubtracte p6 hooscope plane which containe the elastic peak The backgroun subtractions were approximately % an ha negligible effects upon the values of R Corrections were mae for small variations in incient energy an scattering angle as well as for electronic an computer losses, ACKNOWJ3DGME3TS We want to thank R A Early, H W Kenall, P N Kirk, S C Loken, an L W MO; an also E Taylor an other members of the spectrometer group, for their contributions to the experiment We are expecially grateful to the operating staff of the SLAC accelerator, particularly to Roger Miller, for proviing the positron beam

6 5 REFERENCES 1 D Yount an J Pine, Phys Rev 1, 14 (196) D Yount an J Pine, Phys Rev 1, 14 (196) A Browman, F Liu, an C Schaerf, Phys Rev 139, B179 (1965) R L Anerson et al, Phys Rev Letters l7, 47 (1966) Phys Rev 166, 1336 (196) G Cassiay et al, Phys Rev Letters l9, 1191 (1967) A DeHollan, E Engels, B Knapp, an L Han, Paper 6a Proceeings of the X nt Conf on High Energy Physics, Berkeley (1966) Bartel et al, Physics Letters 3, 4 (196) Bouquet et al, Physics Letters B, 17 (196) 3 Theories consiering enhancement ue to J PC = l*, * mesons: D Flame an W Kummer, Nuovo Cimento S, 33 (1963) S D Drell an J D Sullivan, Physics Letters l9, 516 (1965) M Gourin, Recontre e Morion sur les nteractions Electromagnetiques (Jan 714, 1966), Comptes Renus, Vol, p139, an Diffusion es electrons e haute energie(masson et Cie, Paris 1966) p51 4; Theories consiering enhancement ue to N*(13): S D Drell an M Ruerman, Phys Rev 16, 561 (1957) S D Drell an S, Fubini, Phys Rev 113, 741 (1959) N R Werthamer an M A Ruerman, Phys Rev 13, 15 (1961) 5 J Pine, Proceeings of 1967 nternational Symposium on Electron an Photon nteractions at High Energy, Sept 57, 1967, (SLAC)pl 6 N Meister an D R Yennie, Phys Rev 13, 11 (1963) A comparison of the raiative corrections to R using the results of Tsai, Phyt: Rev m, 19 (1961) agree with our corrections to better than 3%

7 6 \ 7 H Brechna, K E Breymayer, K G Carney, H DeStaebler, R H Helm, an C T Hoar, The Stanfor Two Mile Linear Accelerator, eitor R B Neal (to be publishe, W A Benjamin nc), p 545 JPine, Stanfor Linear Accelerator Center nternal Report No TN63, 1963 R S Larsen an D Horelick, Stanfor Linear Accelerator Center Publication No 39 (April 196) 9 D Your&, Nucl nstr Methos 5, 1 (1967) 1 D HCowar et al, Phys Rev Letters, 9 (196) R E Taylor, Proceeings of 1967 nternational Symposium on Electron an Photon nteractions at High Energy, Sept 59, 1967 (S&AC!), p, 7 11 A M Boyarski et al, Phys Rev Letters, 3 (196) B" Richter, Proceeings of 1967 nternational Symposium on Electron an Photon nteractions at High Energy, Sept 59, 1967 (SLAC), p 39

8 Table The final raiatively correcte ratios (R) of this experiment are shown together with the statistical an systematic errors The ratios of the real part of the two photon amplitue (ReB) to the single photon amplitue (A) are inclue

9 : 1 El 3 E * cfc z N c % z i z E * % b ffi cu cu CP 5 5 * ul E: w! G 3 ri =f: 3 ti rl G * % ri ;t cl tl 1 tz m s % 5 A ri Nl Ln l1 E w $ tl : Lo ti ri!i ll i A iii 4 WC Lfl cu co k w c; LG m zi Ln ri zl ; Lo

10 Figure 1 The ratios R (from Ref ) of positronproton to electronproton elastic scattering cross sections are shown plotte against fourmomentum transfer square (9') The new results from this experiment are shown as soli points

11 P N P= t * i i ta f C t +?