Volume 143B, number 4, 5, 6 PHYSICS LETTERS 16 August 1984 TEST OF QUANTUMCHROMODYNAMICS THROUGH PARTON-PARTON SCATTERING IN PHOTON-PHOTON COLLISIONS S.P. LI and H.C. LIU a Department of Physics, University of California at lrvine, Irvine, CA 92717, USA Received 14 February 1984 Revised manuscript received 20 April 1984 Various cross sections of jet and hadron productions through hard parton-parton scattering in photon-photon collisions are calculated by quantumchromodynamics. Their characteristics are significantly different from those of e+e - and pp collisions. Hard scattering processes between partons in pp and p~ collisions have been studied in great detail both experimentally and theoretically in the framework of perturbative QCD [1,2]. On the other hand, calculations based on QCD on the scattering of partons in e+e - collisions have also been car- ried out recently [3-5] ,x. Similar calculations on photon-photon collisions are much less extensive. It is our purpose here to carry out some QCD calculations in this area. As proposed, colliding e + e- beams with energy >~ 100 GeV could be obtained at linear accelera- tors designed in Stanford and Novosibirsk [6]. It is discussed in ref. [6] that, using such accelerators, one can obtain polarized colliding photon-photon beams with approximately the same energies and luminosities as those of the e+e - beams. This is achieved by using an incident laser beam on an electron beam. High energy photons are obtained by the backward Compton scattering process (as On leave of absence from the Department of Physics, Nankai University, Tienjin, China. ,a We know of two related references. In ref. [4] the process T3' -~ 4 jets is pointed out without detailed calculation. In ref. [3] formulas of "effective cross section" for a special case of the process YT -'* 4 jets, i.e. y + y -, q~l+qFq -~ qFqq~l, are given. Such effective cross section formulas are intro- duced as a constituent part of a numerical calculation for the process e +e --* 4 jets. Numerical and analytic calcula- tions for ee ~ YT -~" qq --~ qq (or gg) subprocesses are given there. 0370-2693/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) opposed to the usual Bremsstrahlung process from e+e beams). It is therefore conceivable that pre- cise tests of QCD will be carried out in photon beam experiments in the near future. The use of photon beams to test QCD has an advantage over pp (or p~) experiment: it is known that the contribution to the structure function of the point-like component of the photon can be calculated by QCD. The contribution of the vector meson component can, in general, be neglected for x >_ 0.1 at high energy. On the other hand, the structure function of the nucleon must be determined by experiments which hampers the QCD predictions on parton collisions. It is there- fore, in principle, better to test QCD through photon-photon collisions. The cross sections of the parton-parton scatter- ing process in e+e - collisions are small [7]. The cross sections of the same process in photon-pho- ton collisions are, in general, three orders of mag- nitude larger at the same se÷ e ,2, which for com- parable luminosity makes it easier to test QCD in photon beam experiments than in e+e experi- ments. ,2 This result can also be obtained by comparing fig. 4 in this paper with the corresponding figure in e+e - collision, i.e. fig. 14 in the 2nd paper of ref. [5]. This is the main reason why we believe that to test QCD for patton processes in a VV collider may be much better than that in e+e - colliders. 489