Real and virtual photon structure Leif Jnsson University

Real and virtual photon structure Leif Jönsson University of Lund representing the H 1 and ZEUS collaborations Outline of the talk • Physics processes; di-jet events • Virtual photon structure • Real photon structure • Di-jet events with charm production • Conclusions 20/03/2002 Leif Jönsson

Physics Processes • • Electron-proton scattering proceeds via the exchange of a virtual photon – Photoproduction Q 2~0 Ge. V 2 – Deep inelastic scattering Q 2>>0 Ge. V 2 Pointlike photons (direct) Q 2>k. T 2 Resolved photons Q 2<k. T 2 Direct processes with k. T non-ordered parton emissions (CCFM) DGLAP evol. CCFM evol. sep e. X = dy fg/e(y, Q 2) sgp Y: electron momentum fraction taken by the photon fg. T/e dominating; fg. L/e contributes as y gets small Direct: sgp = i dxp fi/p(xp, mp) sig Resolved: sgp = ij dxg fj/p(xg, mg) dxp fi/p(xp, mp) sij xg: the fractional photon momentum entering the hard scattering xp: the fractional proton momentum taken by the interacting parton 20/03/2002 Leif Jönsson 2

xgobs = jets. ETe- /2 y. Ee A cut at xg around 0. 7 -0. 8 gives good separation between direct and resolved processes 20/03/2002 Leif Jönsson 3

Virtual photon structure Triple differential cross sections • Direct processes only describe data in the region Q 2>(ET)2 • In the region Q 2<(ET)2 the resolved processes become important 20/03/2002 Leif Jönsson 4

Virtual photon structure Including longitudinal photon polarisation • • Recently QCD parametrisation of longitudinally polarised photons has been implemented in Herwig comes much closer to data 20/03/2002 Leif Jönsson 5

Virtual photon structure Comparisons with Cascade (CCFM) • Cascade provides k. T non-ordered parton showers • Cascade with less degrees of freedom (no photon structure) describes data reasonably well 20/03/2002 Leif Jönsson 6

Structure of real photons ds/dxg. OBS compared to NLO calculations • NLO calculations give reasonable description of data • Only slight dependence on photon PDF’s 20/03/2002 Leif Jönsson 7

Structure of real photons ds/dcos * compared to NLO calculations Mjj>42 Ge. V xg< 0. 75 H 1 x g> 0. 75 cos *=|tanh( 1 - 2)/2 • NLO calculations give reasonable agreement with data 20/03/2002 Leif Jönsson 8

Cross section ratio of resolved and direct processes as a function of Q 2 • The cross section ratio decreases with increasing Q 2 as the contribution from resolved processes gets less important • Sa. S 1 D falls below the data 20/03/2002 Leif Jönsson 9

Q 2 dependence in charm production R=sgobs<0. 75)/sgobs>0. 75) vs Q 2 • Data are not able to distinguish between Q 2 suppression or not • Cascade (CCFM) gives good description of data • Aroma (DGLAP) falls below • Extrapolation to the full D* phase space confirms no Q 2 suppression • Two different scales come into play 20/03/2002 Leif Jönsson 10

ds/dxg. OBS vs xg. OBS in charm production Predictions by Cascade (CCFM) • In a significant fraction of the events the gluon is the hardest parton • Cascade on hadron level gives reasonable agreement with data 20/03/2002 Leif Jönsson 11

Conclusions Virtual photon structure • Direct photon processes only describes data in the kinematic region Q 2>ET 2 • The inclusion of resolved photon processes provides better agreement in the region Q 2<ET 2 • Considering also longitudinally polarized photons improves the agreement even more • CASCADE with k. T non-ordered parton emissions (CCFM) gives similar agreement over the full kinematic range; do we need resolved photons? Real photons • NLO calculations reproduce data reasonably well • The dependence on photon PDF’s seems small • The dominant error comes from NLO scale uncertainties Charm production • No Q 2 suppression observed in contrast to the case where no charm requirement is made • Suppression due to charm and Q 2 not independent 20/03/2002 Leif Jönsson 12