1 / 30

Vector Meson Production and DVCS from H1 at HERA

Vector Meson Production and DVCS from H1 at HERA. K.Hiller, DESY - for the H1 Collaboration. Introduction r and J/ y in Photo & Electroproduction - W- dependence - t – dependence - effective Regge trajectories - Q 2 – dependence

addison-everett
Download Presentation

Vector Meson Production and DVCS from H1 at HERA

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Vector Meson Production and DVCS from H1 at HERA K.Hiller, DESY - for the H1 Collaboration • Introduction • r and J/y in Photo & Electroproduction - W- dependence - t – dependence - effective Regge trajectories - Q2 – dependence - helicity studies • Deeply Virtual Compton Scattering (DVCS) • Conclusions Lowx Meeting, Lisbon 2006

  2. Introduction - Vector Mesons at HERA (1) Vector mesons have JPC = 1 - - as photon  no quantum number exchange necessary  mainly produced in diffractive processes Elastic process, dominates at low |t| Proton-dissociation, mainly at high |t| Q2g* virtuality 0 < Q2 < 100 GeV2 Wgp energy of the g*p system 20 < Wgp < 200 GeV t 4-momentum tansfer at p-vertex 0 < |t| < 30 GeV2 VM Vector mesons r, w, f, J/y, y’, U HERA collides e-p at energies of 27 GeV – 920 GeV Large diffractive cross sections & wide kinematic range  HERA is a good place for VM studies Lowx Meeting, Lisbon 2006

  3. Introduction – Signatures (2) Photoproduction DIS Mainly elastic and p-dissociative processes , 2/3 track topologies Clean samples with less backgrounds J/y m+m- F  K+K- r  p+p- Lowx Meeting, Lisbon 2006

  4. Introduction – Theoretical Concepts (3) Regge Model pQCD Models hard processes by 2-gluons/ladder exchange soft processes by Pomeron exchange aP(t) = a0 + a’ t, a0 = 1.08, a’ = 0.25GeV-2 Leading order: s ~ [x g(x,Q2)]2 • slow increasing total cross section s(W) = W d , d = 4(a0-1-a’/b) • t-dependence s(t) = exp(-bt) with shrinkage b = b0 + 4a’ ln(W/W0) • s-channel helicity conservation (SCHC) • steeper W-dependecnce according to gluon density at low x • universal slope ds/dt ~ |t|-n , without shrinkage • violation of SCHC Lowx Meeting, Lisbon 2006

  5. r Photoproduction (1) New 2005 data !!! • Motivation: replace 1993 r data … 358 events • 2005 data: • Fast Track Trigger , 2+3 prong events • L = 570 nb-1, ~ 240.000 r candidates • kinematic range : Q2 < 4 GeV2, <Q2> ~ 0.01 GeV2 • 20 < W < 90 GeV • |t| < 3 GeV2 • Backgrounds: r’, w, f < 2 % • separation of elastic and p-dissociative processes using forward detectors Lowx Meeting, Lisbon 2006

  6. rPhotoproduction (2) Large r statistics allows 2-dimensional analysis in “W-t-bins” Fit W-dependence in each t-bin: s ~ (W/W0) 4[a – 1]  a(t) H1 fit in good agreement with measurements of ZEUS and OMEGA First a(t) measurement in a single experiment Lowx Meeting, Lisbon 2006

  7. r Photoproduction (3) • Fitting a in t-bins by a linear trajectory: • a(t) = a0 + a’ t a0 = 1.093 ± 0.003 ± 0.008/0.007 a’ = (0.116 ± 0.027 ± 0.036/0.046) GeV-2 Note: “ZEUS values” combine ZEUS & H1 &OMEGA a’ differs significantly from classical value a’ = 0.25 GeV-2 Lowx Meeting, Lisbon 2006

  8. r Electroproduction (1) What happens at larger “photon masses” ? 2000 data, L = 42 pb-1 8 < Q2 < 60 GeV2 40 < W < 180 GeV, |t| < 0.5 GeV2 s ~ W d 0.50 0.80 0.96 1.08 W-dependence steeper at large Q2 d(W) reflect soft  hard transition Lowx Meeting, Lisbon 2006

  9. r Electroproduction (2) What is the virtual photon polarization ? Using 4 production & decay angles to determine spin density matrix element • polar decay angle: • ds/dcosq*  • 1 – r0004 + (3r0004-1) cos2Q* R = sL/sT = r0004 / e (1-r0004) ( ε ~ 1, SCHC ) Matrix elements based on no/single/double flip amplitudes: M++,M -- / M+0, M-0 / M+-, M -+ describe angular spectra At larger Q2 longitudinal photon dominates pQCD model works Lowx Meeting, Lisbon 2006

  10. r Electroproduction (3) Is the photon helicity conserved ? (at large Q2 r0004 > 0 consistent with SCHC) r azimuthal production angle: ds/d f 1 + 2e(1+e) cosf(r005+2r115) – e cos2f (r001 + 2r111) Clear violation of SCHC Increase with |t| predicted by pQCD model* *) Ivanov/Kirchner Lowx Meeting, Lisbon 2006

  11. r Photoproduction at large |t| (1) Published 2006 Continues SCHC violation at higher |t| ? 2000 data, L = 20 pb-1 1.5 < |t| < 10 GeV2 Q2 < 0.01 GeV2 , 75 < W < 95 GeV r0004 Fitting r0004, r1004, r1-104 to s(Q*, f*) in bins of |t| r1-104 SCHC violation persists 2-gluon and BFKL models clearly inconsistent with data r1004 Lowx Meeting, Lisbon 2006

  12. J/y Photo & Electroproduction (1) Change the mass scale … 1999+2000 data , L=55pb-1 |t| < 1.2 GeV2, MY < 1.6 GeV p-diss. background ~ 14% σ~ Wδ, with δ ~ 0.75 at low and high Q2, J/y mass provides a hard scale for pQCD Lowx Meeting, Lisbon 2006

  13. J/y Photo & Electroproduction (2) Describing W-t dependencies by an “effective” Pomeron trajectory: s exp(b0t) W d with d = 4(a(t)-1), a(t) = a0 + a’ t <Q2> ~ 0 / <Q2> ~ 9 GeV2 a0 ~ 1.22 / 1.18 a’ ~ 0.16 / 0.02 GeV-2 trajectories similar (errors !) large α0~ 1.2 reflects steep W-dependence of hard / perturbative process Lowx Meeting, Lisbon 2006

  14. J/y Photo & Electroproduction (3) What about helicity conservation ? r0004 ~ 0 in photoproduction, but rises with increasing Q2 For SCHC: r1-11 = (1 – r0004)/2 Other spin density matrix elements ~ 0 SCHC works … Lowx Meeting, Lisbon 2006

  15. J/y Photoproduction at large |t| (1) Extension to higher |t| … 2 < |t| < 30 GeV2 1996 – 2000 data, L = 78 pb-1 d 0.77 1.29 1.30 s  |t|-n n = 3.00 ± 0.08 ±0.05 d ~ 1 confirms large |t| as hard scale, DGLAP / BFKL work in complementary |t| regions < / > MJ/y2 Lowx Meeting, Lisbon 2006

  16. J/y Photoproduction at large |t| (2) And helicity conservation … Untagged photoproduction: 2 angles  3 matrix elements: consistent with SCHC: r0004 = r1-104 = Re{r1004} = 0 (large errors at highest |t|) Lowx Meeting, Lisbon 2006

  17. How J/y compares to r ? J/y – meson r – meson • Low-Q2, low-|t| flat W-dependence  W0.2, exponential s ~ exp(-bt) a’ ~ 0.1 GeV-2, no universal Pomeron • High-Q2, low-|t| steeper W-dependence  W1, “effective” Pomeron a0 ~ 1.2 sL dominates at high Q2 violation of SCHC • Low-Q2, high-|t|: power law s |t|-n violation of SCHC persists • Low-Q2, low-|t| steeper W-dependence  W1 • High-Q2, low-|t| SCHC valid • Low-Q2, high-|t|: SCHC valid Lowx Meeting, Lisbon 2006

  18. Deeply Virtual Compton Scattering (1) What can we learn about parton correlation ? Ideal process: DVCS e + p  e + p + g massive g* needs skewness ξ, t-dependence for transverse motion • H (x, ξ, t, Q2) • generalized parton density functions (GPDs) LO x - ξ x + ξ NLO • simple process: similar to VM production, but • no wave function needed • - Bethe-Heitler background pure el.mag.process, precisely known Lowx Meeting, Lisbon 2006

  19. DVCS – Theoretical Tools (2) Color Dipole Models Generalized PDF Models • hard process: use factorized ansatz of hard scattering ME x GPDs • NLO leading twist calculations * using ansatz for gluon and q-singlet: Hg (x, ξ, t, μ2) = x g(x,μ2) exp (-b|t|) Hq (x, ξ, t, μ2) = q(x,μ2) exp (-b|t|) with PDFs MRST2001 / CTEQ6 • Q2 and x dependence by evolution - • g* fluctuates into qq pair • Color dipole interacts with proton • qq pair converts to real g - *) Freund, McDermott A =∫ d2Rdz YginsdipoleYgout sdipole: model input Lowx Meeting, Lisbon 2006

  20. DVCS – W & t dependences (3) HERA-2 data 6.5 < Q2 < 80 GeV2, 30 < W < 140 GeV, |t| < 1 GeV2 t - dependence W - dependence b ~ 6 GeV-2 Wd, with d ~ 1 Consistent with H1/HERA-1 data, ZEUS data at low W a bit higher …typical hard process at low |t| Lowx Meeting, Lisbon 2006

  21. DVCS – Models versus Q2 / W (4) GPD - model Dipole - model Band for b-range 5.26 < b < 6.4 GeV-2, no Q2 dependence of b Both models reproduce general trend quite well, smaller errors needed for model discrimination Lowx Meeting, Lisbon 2006

  22. Summary • New high-statistics r-measurement at Q2 ~ 0 • New publication of r-photoproduction at high |t| • Soft to hard transition visible s W0.2 s W1 • Regge parametrization alive, but no universal Pomeron • Violation of SCHC for r, not for J/y • pQCD models describe the trend, need improvements in details (… helicity at high |t|) Vector Mesons DVCS • Theoretical interest to extract generalized PDFs • First HERA-2 measurement performed, confirms HERA-1 data • Models based on GPDs and color dipoles reproduce trend Lowx Meeting, Lisbon 2006

  23. Spare slides … Lowx Meeting, Lisbon 2006

  24. r Photoproduction (2) Data & Monte Carlo • Generator DIFFVM: Pomeron exchange plus VM dominance • ds/dMY2~ MY-2(1+e) with e = 0.08 • low-|t|: ds/dt ~ exp(- b|t|) with b = 11.4 / 6.5 GeV-2 for elastic/p-dissociative • high-|t|: ds/dt ~ |t|-n with n = 23 / 8.5 for elastic /p-dissocitive Monte Carlo reproduces well-known features of diffractive vector meson production Lowx Meeting, Lisbon 2006

  25. r Photoproduction at large |t| (2) What about ds/d|t| ? Standard low-|t|exponential fails, ds/dt ~ |t|-n , with n ~ 4.3 QCD-Models: 2-gluon model fixed/running as fails BFKL / gluon ladders better, but undershoots at highest |t| High |t| range nice for tuning of QCD models Lowx Meeting, Lisbon 2006

  26. J/y Photo & Electroproduction (3) Slope of the exp(-b|t|) in dependence on W … b(W) = b0 + 4a’ ln(W/W0) Photoproduction a’ = (0.164 ± …) GeV-2 Electroproduction a’ = (0.019 ± …) GeV-2 Photoproduction: b increases with W … shrinkage Electroproduction: b ~ constant Lowx Meeting, Lisbon 2006

  27. J/y Photo & Electroproduction (5) Use r0004 to separate sL and sT … (M2 J/y + Q2) –n n = 2.486 ± 0.080 ± 0.068 stot = sL + sT at low Q2 dominates sT, at large Q2 more sL pQCD predictions*: at large Q2 sensitive to gluon PDFs *) Martin, Ryskin, Teubner Lowx Meeting, Lisbon 2006

  28. Vector Mesons & pQCD s [x g(x,Q2)]2 with x  1/W2 VM production sensitive to g(x,m2) At hard scales, large MVM or Q2 or |t|, pQCD reproduces general features Lowx Meeting, Lisbon 2006

  29. DVCS – Models versus W (4) GPD - model Dipole - model Band for b-range 5.26 < b < 6.4 GeV-2, no Q2 dependence of b Consistent with H1/HERA-1 data ZEUS data at low W a bit higher Both models reproduce trend, MRST2001 ~ 50% off Lowx Meeting, Lisbon 2006

  30. DVCS – Models versus Q2 (5) GPD - model Dipole - model Band for b-range 5.26 < b < 6.4 GeV-2, no Q2 dependence of b Again: both models give reasonable predictions, MRST2001 seems to overshoot the data Lowx Meeting, Lisbon 2006

More Related