Volume 262, number 4 PHYSICS LETTERS B 27 June 1991 Very energetic photons at HERA A.C. Bawa ~ and Maria Krawczyk 2 Deutsches Elektronen-Synchrotron, DESY, W-2000 Hamburg, FRG Received 12 December 1990; revised manuscript received 16 April 1991 We show that very energeticphotons in the backwarddirection can be produced in deep inelastic Compton scattering at HERA. Assuminga fixed energy of 9 GeV for the initial photons and 820 GeV for the protons a high rate is found for the production of final photons with a transverse momentum equal to 5 GeV/c and energybetween 40 GeV and 300 GeV. These energeticphotons arise mainly from the scattering of the soft gluonic constituents of the initial photon with quarks from the proton. They are produced in the backward direction in coincidencewith a photon beam jet of energy ~ 9 GeV in the forward direction. The deep inelastic Compton (DIC) process ~{p~{X, ( 1 ) with the transverse momentum of the final-state pho- ton larger than say 5 GeV/c, is the simplest type of photoproduction which can be measured at the ep collider HERA. Several effects arising from the had- ronic interaction of photons in this process have been studied in the literature. The contributions contain- ing the structure and fragmentation functions of the photon have been included in analyses based on the order c~s calculation [ 1-4 ] as a background to the ba- sic subprocess 7q-~Tq (2) (see fig. 1 ). In ref. [ 5 ] these leading-logarithmic con- tributions have been considered with respect to a Supported by Ministry of Science and Higher Education, Re- search Problem CPBP 01.03. Permanent address: Department of Physics, University of Durban-Westville, Durban, South Africa. 2 On leave of absence from Institute of Theoretical Physics, Warsaw University, Hoza 69, PL-00-681 Warsaw, Poland. Fig. 1. The Feynman diagram for the 7q-"/q Born subprocess. study of the structure of the photon at HERA. A more detailed investigation which incorporates a study of the fragmentation of the final-state partons into pho- tons is presented in our recent papers [6,7 ]. The cross section for the DIC process (1) at the ep collider HERA has been calculated [6,7 ] with the simplify- ing assumption that the initial photon has a fixed en- ergy of 9 GeV and this leads to a 7P centre-of-mass energy squared of Svp---30 000 GeV 2. In addition, ref. [7] provides some estimates of the production rate for the DIC process in the ep collision based on a more realistic treatment of initial photons, based on the equivalent photon approximation. We studied the Born contribution (eq. (2) ), the O (c~s) corrections to the process and the c~c~ S and c~ 2 leading-logarith- mic subprocesses which arise if one of the initial or final photons or both photons in process ( 1 ) interact with the constituents of the proton not directly but via their constituents, i.e. like hadrons. The two most important subprocesses of this type are shown in figs. 2 and 3. The hadronic interaction of photons with the protons dominate at low and medium p~; that is for 5<p~< 15 GeV/c [6,7]. This is due to the fact that in this range ofpW the photon can be probed to very small xv, down to ~ 0.003 at pTr = 5 GeV/c and ~0.03 at p~= 15 GeV/c. Note that the proton is probed similarly in this region ofpTr. In this note we would like to discuss a particular feature of the DIC process at HERA, namely the pos- sibility of the production of very energetic photons 492 Elsevier Science Publishers B.V. (North-Holland)