Vector meson production at large t at HERA

1. HEP2005 International Europhysics Conference on High Energy Physics Lisboa, Portugal July 21-27, 2005 Vector meson production at large t at HERA Marta Ruspa, Univ. Piemonte Orientale & INFN-Torino, Italy On behalf of Vector meson production at large t at HERA

2. Diffractive vector meson production at HERA Exclusive Proton dissociative VM = vector meson (ρ, ω, φ,J/ψ, ... ) Q2 = (4-momentum exchanged at lepton vertex)2 W = c.m.energy of photon-proton system |t| = (4-momentum exchanged at proton vertex)2 • Experimentally clean process in wide kinematic range • Simultaneous control of different scales: Q2, t, MVM • Large Q2, t, MVM  pQCD applicable, transition soft-hard regime Vector meson production at large t at HERA

3. Proton dissociative VM production at high |t| VM = qqbar dipole, interacts with the proton through the exchange of 2 gluons or a gluon ladder standard DGLAP or BFKL evolution equations High |t| domain little explored so far Proton dissociative processes dominate at high |t| • pQCD predictions: • cross section t-dependence ~ |t|-n • steep rise of the cross section with W • possible SCHC violations Vector meson production at large t at HERA

4. J/ψ in photoproduction (ZEUS) Submitted paper n. 334 50 < W < 150 GeV |t| < 20 GeV2 Proton remnant mass MY < 30 GeV ρ0in photoproduction (H1) Submitted paper n. 640 75 < W < 95 GeV 1.5 < |t| < 10 GeV2 Proton remnant mass MY < 5 GeV • Cross section t-dependence • Spin density matrix elements Data samples Vector meson production at large t at HERA

5. Electron scattered at small angles (detected in 44 m tagger) Two oppositely charged tracks (pion candidates) Nothing else in LAr above 400 MeV Detailed description of p-diss Tuned to describe data t-dependence Reweighted to take into account s-channel helicity non conservation Also used to simulate ω, φ, ρ’ backgrounds ρ0 selection DATA DiffVM MC t = transverse momentum squared of ρ0 Vector meson production at large t at HERA

6. t-dependence of γ p  ρ0 Y cross section • t-dependence well described by BFKL model of Poludniowski et al. [R.Enberg et al., JHEP 0309 (2003); G.G.Poludniowski et al., JHEP 0312 (2003) Parameters tuned to ZEUS data EPJ C26 (2003)] • Fit to a power law distribution of the form |t|-n  obtained with MY < 25 GeV, also well described by the same model t-dependence sensitive to MY cut Vector meson production at large t at HERA

7. γ p  ρ0 Y: helicity analysis 3 angles defined in photon-proton c.m.frame: θ* φ* Φ Helicity=spin component along direction of the particle’s motion Decay angular distribution related to spin of γ* and VM through Spin Density Matrix Elements (SDME) • study of transition between γ* and VM helicity states • Photoproduction: can only measure θ*, φ* access to 3 of the 15 SDMEs • s-channel helicity conservation: vector meson retains photon helicity  all 3 SDMEs are predicted to be zero Vector meson production at large t at HERA

8. Production of longitudinally polarized ρ(0) from transversely polarized γ(+)(helicity “single flip”) SCHC: expected zero  small: production dominated by transversely polarised ρ mesons  Good agreement with ZEUS data  Data well described by BFKL model of Poludniowski et al. γ p  ρ0 Y: dependence on cosθ* Vector meson production at large t at HERA

9. SCHC: expected zero γ p  ρ0 Y:dependence on * Production of transversely polarized ρ(+/-) from oppositely polarized γ(-/+)(helicity “double flip”)  significantly different from zero: s-channel helicity non-conservation  Good agreement with ZEUS data  BFKL model of model of Poludniowski et al. qualitatively reproduces data ciabut too large at low values of |t| Vector meson production at large t at HERA

10. J/ψ in photoproduction (ZEUS) Submitted paper n. 334 50 < W < 150 GeV |t| < 20 GeV2 Proton remnant mass MY < 30 GeV ρ0in photoproduction (H1) Submitted paper n. 640 75 < W < 95 GeV 1.5 < |t| < 10 GeV2 Proton remnant mass MY < 5 GeV • Cross section t-dependence • Cross section W-dependence Data samples • Cross section t-dependence • Spin density matrix elements Vector meson production at large t at HERA

11. Electron undetected in CAL Two oppositely charged tracks (muon candidates) Nothing else in detector above 300 MeV Signal simulated with EPSOFTMC tuned to describe data t-dependence Non resonant background from γγfusion process ep e +-Ysimulated with GRAPE-DILEPTON MC J/ selection DATA t = transverse momentum squared of dimuon system Vector meson production at large t at HERA

12. t-dependence of γ p  J/  Y cross section • DGLAP and BFKL LL as well as BFKL nonL with fixed s describe general behaviour of data • BFKL nonL with running s too steep [BFKL: J. Bartels et al., PL B 375 (1996); J. R. Forshaw and G. Poludniowski, EPJ C26 (2003); R. Enberg et al., EPJ C26 (2002); hep-ph 0502045 DGLAP: E. Gotsman et al., PL B 532 (2002) Parameters tuned to ZEUS data EPJ C26 (2003)] Vector meson production at large t at HERA

13. W-dependence of γp  J/  Y cross section • Rise of cross section with W • Fit to a power law distribution of the form W  Extracted value of  increase with rising |t| • Regge fit to the 4 t-bins:  Consistent with previous H1 result [PL B568 (2003)] Regge theory: Vector meson production at large t at HERA

14. W-dependence of γp  J/  Y cross section • DGLAP LL not able to describe the data while BFKL LL qualitatively reproduces the rise of the cross section Vector meson production at large t at HERA

15. Summary VM production shows at large |t| features of a hard process: - hard t distributions • steep rise of the cross section with W • SCHC breaking • Photoproduction of ρ mesons measured up to |t|= 10 GeV2 - cross section power-like t-dependence well described by BFKL model of Poludniowski et al. • small measured single flip amplitude agrees well with BFKL model of Poludniowski; large measured double flip amplitude confirms s-channel helicity non-conservation Photoproduction of J/ mesons measured up to |t|= 20 GeV2 • cross section rises like W,suggesting onset of BFKL dynamics; • significant t-dependence of  Vector meson production at large t at HERA