± fm and ± Th K meson result is provisional.

Transcription

1 305 MEASUREMENT O PON AND KAON ORM ACTORS AT NA7 (RASCAT, MLAN, PSA, TURN, TRESTE, ROYAL HOLLOWAY COLLEGE, SOUTHAMPTON) S. G.. RANK University of Southampton The electromagnetic form factors of and K mesons have been measured by scattering these par t icles from the electrons in a l iqu id hydrogen target. The var iat ion of the form factors with four momeqtum transfer gives the root mean square of the p on charge radiu s, <r > ± fm and ± Th K meson result is provisional. that of the K meson <r K > A measurement of the reaction T + e + T- + rr 0 + e- g ives a result in good agreement with PCAC with 3 colours. = =

2 306 The elastic scattering of pions on electrons is described by the following diagram: where q is the four momentum transfer in the collision due to the exchange of a virtual photon. The pion photon vertex involves a form factor which is a function of q only. The differential scattering cross-section is given by the expression: [ 1 - q qlmax where would be for a point charge. distribution is g iven by : J The mean square radius of the pion charge 6 d dq The pion form factor was measured u sing an incident en<ergy of 3 00 GeV. With thi s incident energy the maximum value of q is (Gev The portion of this c ) experiment covering the range < q < 0. 1 (Gev : has already been c ) ± fm. published 1 ). This gave the root mean square charge radius <r/, l We have now completed the analys is up to q of events is neglibl e. problems. The analysis for q Beyond this the number 0. 6 (Ge''-) c Gev 0. 1 <c:-) presents special > At lower q an accurate knowledge of the scattering angles of the two particles is sufficient to identify which traj ectory corresponds to the electron and which to the pion because the electron angle is larger than the largest n the neighbourhood of q = ( Gev ) c the pion and the electron have the same momentum and angle. n the region of scattering angle experienced by the pion < q < approximately, a knowl edge of the angles alone allows two possibilities for the value of q depending on which traj ectory is assumed

3 307 to be which particl e. This is call ed the ambiguity region. This can only be resolved by using addit ional information to identify the electron and the pion. This add it ional information is provided by electro-magnetic shower detectors which form part of the forward spectrometer. Each of these shower detector s is divided into a front portion of a thickness of 4 radiation lengths which is fol lowed by another radiation l engths. The identification of electrons and pions is improved considerably by the information of the longitudinal shower development provided by thi s subdiv i s ion. ig. shows the overall layout of the experiment and ig. shows the arrangement of the shower detector s. ig. 3 illustrates the way in which the information from the shower detector s is used. The vertical axis gives the ratio o f the shower energy absorbed by the front portion to the total energy absorbed by the whole shower detector. The horizontal axis gives the ratio of the total shower energy to the energy of the incident charged par t icle as measured by the magnets of the forward spectrometer. ig. 3a contains only particles which have already been unambiguously ident if ied as electrons by the scatter ing kinematics and ig. 3b contains only those particles which have similarly been ident ified as pions. The dotted l ines define very safe cuts for particle identification. Events between these cut s are ident if ied by a detailed examination of the particle trajector i e s. They correspond mainly t o electrons which strike the sides of the central holes of the shower detector s. t should b e borne i n mind that only those events are wrongly attr ibuted in which both par t icles have been misidentified. ig. 4 shows the complete result o f this experiment and an inset giving the results of the best previous measurement by an Amer ican Rus s ian collaboration ) who were the pioneers in this f ield. The result f i t s the parametr isation proposed by Dubnicka and Mart inovic is l inked to the known T T phase shift in the region of the p meson. has a x probability of 55%. l 3 ) which This fit t gives a pion charge radius of <r T > =0. 663±0. 006fm.

4 308 We also measured the kaon charge radius u sing 50 Gev kaons identified by a differential gas Cerenkov counter in the incident beam l ine. n every other respect this experiment was very similar to the 3 00 Gev pion experiment described above. The result, which is provisiona l, is shown in ig. 5 g iving a charge radius of r K >! = 0. 63±0. 05 fm. At the same time as data were taken for rr - e elastic catter ing at 3 00 Gev, we also looked for events of rr + e rr e This: work has been 4 ) published. Such event s were ident ified by looking at the missing mass from - + charged particle kinematics when the shower detectors d etected with the decay of a 11, ig. 6. y rays consistent The result is in good agreement with the predictions of PCAC with three colours (ig. 7 ). References () () (3 ) (4 ) S. R. Amendolia et al. Phys. Let t. l 46B (1 984) E. B. Dalby et al. Phys. Rev. Lett. 48 ('.1 98) 375 S. Dubnicka and L. Martinovic, M. Czech. J. Phys. B9 ( } S. R. Amendolia e t a l. Phys. Lett. l 55B (1 985) 457

5 309 Shower detector l!l[al V Y V) WP( H.!...+ Ji S5l \1 /rvl i l 1/ \ ig. 10 c < 10. i p,.... :: i - ig, k :;_ <,L.+-._ ig. f \ 4,j ' Radiation lengths,.._. 1, ' ' ll ig. 0 T bl l ll :; L TARCi(T Sh '. ront counters lflg} Scmt 1Uator 0. (G;v) ' ;:; Da l l y et al, ll ic.v1c11 alt HSSNG HASS NAL SAMPLE.10.,G.V/tl11 l l l'qlll lllla cl 10 'l.t.c --J r -,,.._, ig. 6 ' ll 00 i. 7 t..,