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DVCS and meson production at HERA

DVCS and meson production at HERA. Introduction : HERA kinematics Vector meson production and pQCD Heavy VM production DVCS cross section measurements HERA kinematical coverage Cross section measurement strategy Results Future plan. Santorini GPD workshop

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DVCS and meson production at HERA

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  1. DVCS and meson production at HERA • Introduction : HERA kinematics • Vector meson production and pQCD • Heavy VM production • DVCS cross section measurements • HERA kinematical coverage • Cross section measurement strategy • Results • Future plan Santorini GPD workshop 2 Oct 2001, Nomikos Centre, Santorini, Greece Y. Yamazaki (KEK-IPNS) On behalf of the H1 and ZEUS collaborations DVCS and meson production at HERA, Santorini GPD workshop

  2. HERA kinematics for elastic vector meson/photon production proton-dissociative process DVCS and meson production at HERA, Santorini GPD workshop

  3. Soft VM production with no hard scale: VDM hadron-hadron collisions between g = VM and p Typical characteristics of the “Pomeron trajectory” Slow rise of the cross section “Shrinkage” in the forward peakb-slope is related to the impact parameter = size of the target/projectileReflecting the ‘blow-up’ of the interaction size with energy increase VM production at HERA (1) soft VDM+Regge DVCS and meson production at HERA, Santorini GPD workshop

  4. Soft VM production:  in photoproduction • Slow rise behaviour at high energy • Described by DL Pomeron+Reggeon(dashed curve) DVCS and meson production at HERA, Santorini GPD workshop

  5. VM production at HERA: pQCD framework • Hard VM production: expected to be explained by pQCD • Hard scale given by Q2, MV or t • Interaction is viewed in three steps • photon  qq dipole • interaction with proton • ‘fragment’ into a VM • Hard scatter of proton and qq via 2-gluon (+ higher order) exchange • non-perturbative input :VM wave function, gluon density • Fast rise of the cross sections in W : steep rise of g(x) towards small x, W2 1/x • Dipole (qq) size is small if a hard scale is available‘blow-up’ of the interaction size not visible  small shrinkage DVCS and meson production at HERA, Santorini GPD workshop

  6. QCD behaviour(1): cross section in W • r in photoproduction (PHP):no hard scale • cross section rise slow •  for Q2 >> L2 (DIS regime) • hard scale availablerise steeper in Was Q2 becomes larger • Similar rise in J/y PHP to at Q2  MV2 • hard scale from charm mass • VM probes gluons g(x, m2) atm2 = Q2 + MV2 ? • pQCD works ! DVCS and meson production at HERA, Santorini GPD workshop

  7. QCD behaviour(2) : shrinkage in J/y PHP • MV is expected to provide a hard scale • Shrinkage is small (but finite) • DL soft Pomeron is excluded success of pQCD in VM, another evidence DVCS and meson production at HERA, Santorini GPD workshop

  8. GPD in VM production: heavy VMs • GPD is important when a heavy object is involved in the interaction • High-Q2real photon : DVCS • real photon  heavy VM : J/y,  • Sensitive to the skewness in the gluon density(x1 – x2 ~ 10–2) •  in photoproduction measured at HERA DVCS and meson production at HERA, Santorini GPD workshop

  9. Effect of GPDs(?) in heavy VM production • Measured  cross sections: surprisingly large • Later found that the GPD is (one of) the reasonto enhance the cross section MRT: Martin, Ryskinand TeubnerFMS: Frankfurt, Mcdermott and Strikman • Theories: large factor from GPD (~ 2) • Others: real part of the amplitude, NLO, non-perturbative effect • Should be tested in a clean environment ratio (J/y)/ Theory: LLA, no GPD DVCS and meson production at HERA, Santorini GPD workshop

  10. DVCS at HERA: dominant diagrams • (Perhaps) 2-gluon exchange is dominant in low-x = HERA kinematic range • LO diagrams (Handbag): dominant in high-x(JLab and HERMES) x1 – x2 ~ 0.1 x1 x2 for high-Q2x1 – x2 ~ 10–3 – 10–4 Sensitive to the gluonic skewed parton density DVCS and meson production at HERA, Santorini GPD workshop

  11. QCD-based model predictions at low-x In the framework of FFS (Frankfurt, Freund and Strikman) model: • We want to extract h to access GPD The best way is to measure the cosr like the other experiment … (QED Compton [QEDC] = Bethe-Heitler [BH] ) DVCS and meson production at HERA, Santorini GPD workshop

  12. First objective at HERA: cross section • DVCS at HERA discovered in 1999 (not trivial !) • Current HERA status: establishing the existence of DVCSCross sections are measured [Today’s presentation] • Subtracting BH contributionThe interference term disappear by integrating over r • Benefit at HERA: clear separation betweenthe diffractive (=DVCS) and inelastic processes • High-energy: fully diffractive domain, clear large rapidity gapSmall background from e.g. non-diffractive 0 production • Next step: measuring interference DVCS+BH through cosr asymmetry Why not yet done ? Explanation later Outlook: at the end of the talk DVCS and meson production at HERA, Santorini GPD workshop

  13. DVCS: similar kinematical distribution as (diffractive) DIS most of the events are at low-y, low Q2  in central/forward of LAB frame e' : small-angle scattering BH: g and e' has similar energy e' could go forward Isolating DVCS from BH at HERA e p BH can be normalised using this sample Central-going g: DVCS-enriched sample DVCS and meson production at HERA, Santorini GPD workshop

  14. Reconstructing kinematical variables • No track in the event for DVCS-like sample(scattered e' : too low angle, no tracker) Z-position of the vertex not known • Q2, W can be reconstructed with restrictionUnfolding assuming the vertex distribution • No detector to measure the momentum of the outgoing proton • t, fr: need pT of electron & photon,resolution not very good • Therefore: ds/dcosr and ds/dt not yet  Challenging measuremnt, A future project DVCS and meson production at HERA, Santorini GPD workshop

  15. Photon energyin rear (ZEUS prel.)BH BH-enriched control sample Electron energyand angle (H1) • BH is QED calculable … used for controlingthe detector response • ZEUS: Normalisation of BH fixed using thissample (uncertaintyin proton-dissociative process) Both positron and photon properties are well describedby BH MC – OK to normalise BH DVCS and meson production at HERA, Santorini GPD workshop

  16. DVCS-enriched sample • Contains both DVCS and BH • Normalisation of BH determinedin the previous sample Clear excessover BH in low W(ZEUS prel.) BH After subtracting BH, DVCS describes the Q2 distributionof the data DVCS+BH describesphoton property well (H1) GenDVCS (FFS) DVCS and meson production at HERA, Santorini GPD workshop

  17. DVCS cross sections by H1 BandsshowB-slopeuncertainty( ds/dt ~ e Bt5 < B < 9 GeV–2 ) FFS: Frankfurt,Freund and StrikmanDD: Donnachie and Dosch • Good description by both pQCD-based models in Q2 and WBasic framework by pQCD seems OK ! • Providing a testing ground for models e.g. NLO (large at low Q2 ?) DVCS and meson production at HERA, Santorini GPD workshop

  18. DD model • Similar to the vector meson models • coupling of a dipole and a photon through wave functions • cross section of dipole-protonparameterised using pp data • the W dependence of the interaction is given bysoft+hard Pomeron model convolution of dipole and(real/virtual) photon wave functions Interaction viasoft+hard Pomeron DVCS and meson production at HERA, Santorini GPD workshop

  19. DVCS cross section from ZEUS (prel.) • Again in good agreement with pQCD(here: B-slope is assumed to be 4.5 GeV–2 data not corrected for the proton-dissociative, ~ 20 %) DVCS and meson production at HERA, Santorini GPD workshop

  20. Outlook(1): measuring interference • Challenge in experimental technique for measurements • Understanding detector resolution to measure fr out of pT balance, but we try with current data • Good news: Z-vertex position can be reconstructed by H1 BST (Backward Silicon Tracker), installed in 1997 • Sign change of the interference term • e+ and e– :Most of the HERA data before 2000 by e+ (~ 100pb–1), e– smaller (~ 16pb –1)Similar e– and e+ sample from Lumi-Upgraded HERA (HERA II)Start Dec/2001, expect 1fb–1 • PolarisationNo data so far, butAlmost all the data from HERAII will be polarised DVCS and meson production at HERA, Santorini GPD workshop

  21. Outlook(2): p' by upgraded H1 FPS • Very high tagging efficiency: essentially 100 %in the fiducial xP (= x1 + x2) range • t, f measurement gives complementary information to the (t, f ) reconstruction by the central detector : 0.4rad t: 0.1 GeV2 0 0.5 0 0.5 |t| (GeV2) |t| (GeV2) 4-5 bins in t, 5-6 bins in f DVCS and meson production at HERA, Santorini GPD workshop

  22. Summary • Vector meson production with a hard scale: described by pQCD • steep rise in W, small shrinkage etc. • Large excess of the  VM cross section over LO QCD • Effect of the skewed parton density ? • DVCS at HERA, cross section measured • Good agreement with theory, again the pQCD framework seems OKCross section will provide constrain to the models (e.g. NLO correction) • Next step: t and fr reconstruction DVCS and meson production at HERA, Santorini GPD workshop

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