PROGRESS REPORT HaPHEEP Activity Report: Nordita 1 August 1996 - 31 July 1997 1 WORK DONE UNDER THE FUNDING OF CONTRACT For completed projects see the list of publications under point 5. The following work is in progress. P. Hoyer and S. Peigne: Projectile spectator effects in the hadroproduction of charmonia. E. Bagan, P. Ball and V. Braun: Radiative corrections to the light-cone sum rules for exclusive semileptonic and rare radiative B-decays. - Work in progress P. Gosdzinsky and N. Kivel: Resummation of running coupling effects in pion-gamma^*-gamma form factor. P. Ball, V. Braun, Yu. Koike and K. Tanaka: Rho-meson light-cone distribution amplitudes beyond leading twist. P. Hoyer has given talks on network related physics as follows. Second Elfe Workshop, St. Malo (23-27.9): 'Charmonium Production at Elfe Energies' (to be published in the proceedings). SLAC, Stanford University, USA (9.12): 'Physics at ELFE: Exploring Hadron and Nuclear Wave Functions with PQCD' (colloquium). Workshop on Deep Inelastic Structure of Nuclei, Jefferson Lab, USA (13-14.12): 'Nuclear Dependence of Structure Functions in Coordinate Space'. Joint DESY/GSI/NuPECC Workshop on Electron-Nucleon/ Nucleus Collisions, Seeheim, Frankfurt aM (3-4.3): 'Physics with ELFE@DESY (Theory)'. Helsinki Institute of Physics (18.3) 'Physics Opportunities at ELFE' (colloquium). SLAC, Stanford University, USA (11.7): 'Rescattering Effects in Charmonium Production' (seminar). M. V\"anttinen has given talks on network related physics as follows. Second Elfe Workshop, St. Malo (23-27.9): 'Coordinate-space Picture of Nuclear Structure Functions' (to be published in the proceedings). NORDITA Workshop on Dense Hadronic Matter, Nordita (9-10.4): 'Nuclear Dependence of Structure Functions in Coordinate Space'. The activities of S. Peigne are listed under point 3. 2 TRAINING ACTIVITIES The following postdocs have participated in network related research: P. Gosdzinsky (Nordita) N. Kivel (Petersburg Nuclear Physics Institute) S. Peigne (Nordita, paid by the network) M. V\"anttinen (Nordita) V. Braun, P. Hoyer and M. V\"anttinen arranged the 'Nordic School on High Energy Physics Phenomenology', Solvalla, Finland (31.8 - 6.9.), with approximately 50 European students. 3 SUMMARY OF THE ACTIVITY OF THE POSTDOC FUNDED BY THE NETWORK Postdoc: S. Peigne, NORDITA, Copenhagen. 1) Research activity (October 1996 - August 1997). My research activity focusses on specific features of J/Psi hadroproduction, the study of which is strongly motivated: -The charm quark mass is large enough to assume that a perturbative QCD calculation of J/Psi production rates might be relevant. -J/Psi production is a domain of QCD where the disagreements between theory and experiments are very large. -J/Psi suppression has been proposed as a quark-gluon plasma signal. The basic process usually considered in J/Psi hadroproduction consists of the fusion between two gluons (from the projectile and target hadrons) with emission of an additional gluon, required due to the negative charge conjugation of the J/Psi. However, data on hadron-nucleus collisions show the presence of soft rescatterings for the charmed quark-antiquark pair. This is seen in the medium-induced transverse momentum of the produced bound state, which is also observed to depend on the mass (J/Psi versus Upsilon). From these experimental facts several questions arise, which we have explored and solved in a simple model for rescattering in nuclear matter, allowing an exact calculation of production amplitudes induced by multiple scattering. 1. Can one of the soft rescattering gluons be used for fixing the J/Psi charge conjugation? No. Indeed, we show in our model that in the basic J/Psi production process induced by two scatterings of the incoming quark-antiquark pair, the two corresponding momentum transfers are hard. 2. What happens to the soft scatterings? For the production process induced by n scatterings, we show that exactly two of these scatterings are hard, the (n-2) others being soft. The soft transfers are shown to factorize from the hard ones in the cross section, which is a crucial property making the perturbative treatment reliable. 3. As a color-octet quark-antiquark pair should behave as a gluon in soft scatterings, how can we understand the dependence of the transverse momentum induced by these soft scatterings on the mass of the bound state? Our calculation actually yields a mass dependence and leads to a simple explanation of this experimental fact, which was not understood before. When weighting the differential cross section for soft scattering by the momentum squared, large transfers become important and the integral turns out to be sensitive (logarithmically) to the upper bound, given by the mass of the bound state. 2) Conferences and Seminars 1996 September 23-27: Workshop "Hadronic Physics" (ELFE project), St-Malo, France. Talk: "Radiative energy loss of energetic partons in nuclear matter and finite size effects". November 29-30: Workshop "Hadronic Physics" (ELFE project), Valencia, Spain. Talk: "A simple model for the perturbative calculation of the $J/\psi$ production rate". 1997 January 9-10: Meeting "A la recherche du plasma de quarks et de gluons", LPC Clermont-Ferrand, France. January 16-17: "Rencontre annuelle de theorie et phenomenologie des particules", Grenoble, France. Talk: "Higher-twist $J/\psi$ production". January 23-25: Meeting of the European Network Hadronic Physics with High Energy Electromagnetic Probes, "Exclusive and Semi-inclusive Reactions in Electron Scattering", Heidelberg, Germany. Talk: "Higher-twist $J/\psi$ production". February 19: SPhT Saclay, seminar: "Soft rescattering in heavy quarkonium production in QCD". March 21: IPNL Lyon, seminar: "Soft rescattering ...". March 22-29: Moriond Conference "QCD and Hadronic Interactions at High Energy". Talk: "Soft rescattering ...". April 03: Nantes, seminar: "Soft rescattering ...". April 22: Stockholm, seminar: "Radiative energy loss of jets in hot/cold nuclear matter" 4 COLLABORATIONS WITHIN THE NETWORK P. Hoyer and S. Peigne participated in EU/TMR network meeting in Valencia, Spain (28.11-1.12). V. Braun participated in Heidelberg meeting ?? V. Braun, P. Hoyer and S. Peigne are organizing the following workshop, intended for participants in the network, other experts as well as students: Workshop on Exclusive Processes in QCD, NORDITA, Copenhagen, September 11-13, 1997. The workshop will be devoted to reviews and discussion of hot topics in the theory and phenomenology of exclusive processes in QCD, in particular 1.Hard diffractive meson (\rho, J/\psi) production. Off-forward parton distributions. Virtual Compton scattering at small momentum transfer. 2.Hadron distribution amplitudes and exclusive processes beyond leading twist. Hadron transverse size distributions and Sudakov resummation effects. 5 PUBLICATIONS FUNDED IN PART OR ENTIRELY BY THE NETWORK P. Hoyer: "Physics Opportunities at ELFE", Talk presented at the meeting on 'Future Electron Accelerators and Free Electron Lasers', Uppsala, April 1996, NORDITA-96/76-P, hep-ph/9611436. \begin{abstract} I review some central physics opportunities at the $15\ldots 30$ GeV continuous beam electron accelerator \elf, proposed to be built in conjunction with the DESY linear collider. Our present detailed knowledge of single parton distributions in hadrons and nuclei needs to be supplemented by measurements of compact valence quark configurations, accessible through hard exclusive scattering, and of compact multiparton subsystems which contribute to semi-inclusive processes. Cumulative ($x>1,\ x_F>1$) processes in nuclei measure short-range correlations between partons belonging to different nucleons in the same nucleus. The same configurations may give rise to subthreshold production of light hadrons and charm. \end{abstract} P. Hoyer: "Charmonium Production at ELFE Energies", Talk presented at the 'Second ELFE Workshop', Saint Malo, September 1996, NORDITA-97-8 P, hep-ph/9702385. \begin{abstract} \noindent I discuss issues related to charmonium production, in view of physics possibilities at a 15 \ldots\ 30 GeV continuous beam electron facility. High energy photo- and hadroproduction of heavy quarkonia presents several challenges to QCD models concerning cross sections, polarization and nuclear target dependence. Theoretical approaches based on color evaporation as well as on color singlet and color octet mechanisms have met with both successes and failures, indicating that charmonium production is a sensitive probe of color dynamics. Experiments close to charm kinematic threshold will be sensitive also to target substructure since only unusual, compact target configurations contribute. In particular, subthreshold production on nuclei should identify nuclear hot spots of high energy density. At low energies, charmonium will form inside the target nucleus, allowing a determination of $\cpair$ bound state interactions in nuclear matter. \end{abstract} P. Hoyer: "Physics at ELFE", Based on talks presented at the meetings on 'Future Electron Accelerators and Free Electron Lasers', Uppsala, April 1996,, and the 'Second ELFE Workshop', Saint Malo, September 1996, NORDITA-97-25 P, hep-ph/9703462. \begin{abstract} I review some central physics opportunities at the $15\ldots 30$ GeV continuous beam electron accelerator \elf, proposed to be built in conjunction with the DESY linear collider. Our present detailed knowledge of single parton distributions in hadrons and nuclei needs too be supplemented by measurements of compact valence quark configurations, accessible through hard exclusive scattering, and of compact multiparton subsystems which contribute to semi-inclusive processes. Cumulative ($x>1,\ x_F>1$) processes in nuclei measure short-range correlations between partons belonging to different nucleons in the same nucleus. The same configurations may give rise to subthreshold production of light hadrons and charm. The challenges of understanding high energy charmonium production indicate that charmonium will be a sensitive probe of color dynamics at ELFE. At low energies, charmonium forms inside the target nucleus, allowing a determination of $c\bar c$ bound state interactions in nuclear matter. \end{abstract} P. Hoyer and S. Peigne: "Rescattering Effects in Quarkonium Production", NORDITA-97/37 P, hep-ph/9706486 (submitted to Phys. Rev. D). \begin{abstract} We study $\eta_c$ and $\jpsi$ hadroproduction induced by multiple scattering off fixed centres in the target. We determine the minimum number of hard scatterings required and show that additional soft scatterings may be factorized, at the level of the production amplitude for the $\eta_c$ and of the cross section for the $\jpsi$. The $\jpsi$ provides an interesting example of soft rescattering effects occurring inside a hard vertex. We also explain the qualitative difference between the transverse momentum broadening of the $\jpsi$ and of the $\Upsilon$ observed in collisions on nuclei. We point out that rescattering from spectators produced by beam and target parton evolution may have important effects in $\jpsi$ production. \end{abstract} P. Hoyer and M. V\"anttinen, Nuclear Dependence of Structure Functions in Coordinate Space, Z. Phys. C 74, 113-117 (1997) (NORDITA-96/20 P, hep-ph/9604305) \begin{abstract} \noindent The momentum distributions of partons in bound nucleons are known to depend significantly on the size of the nucleus. The Fourier transform of the momentum ($\xbj$) distribution measures the overlap between Fock components of the nucleon wave function which differ by a displacement of one parton along the light cone. The magnitude of the overlap thus determines the average range of mobility of the parton in the nucleon. By comparing the Fourier transforms of structure functions for several nuclei we study the dependence of quark mobility on nuclear size. We find a surprisingly small nuclear dependence ($<2\%$ for He, C and Ca) for displacements $t=z \lsim 2.5$ fm, after which a nuclear suppression due to shadowing sets in. The nuclear effects observed in momentum space for \mbox{$\xbj \lsim 0.4$} can be understood as a reflection of only the large distance shadowing in coordinate space. \end{abstract} Patricia Ball, V.M. Braun, Use and Misuse of QCD Sum Rules in Heavy to Light Transitions: The Decay b ---> rho e neutrino Reexamined. Phys.Rev.D55:5561-5576,1997. [HEP-PH 9701238]