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Diffraction and vector meson production

13 August 2007. Diffraction and vector meson production. A.Rostovsev (ITEP, Moscow). XVIII c. particles stay intact d s /dt ~ exp(bt); b ~ r 2 σ ~ s α -1 ; α > 1. Diffraction phenomena are interesting now?. 1977. γ + A → γ + A. Diffractive pattern. LP 2007.

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Diffraction and vector meson production

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  1. 13 August 2007 Diffraction and vector meson production A.Rostovsev (ITEP, Moscow)

  2. XVIII c • particles stay intact • ds/dt ~ exp(bt); b ~ r2 • σ ~ sα-1; α > 1 Diffraction phenomena are interesting now? 1977 γ + A →γ + A Diffractive pattern LP 2007 • Spectacular QCD phenomenon • Challenge for perturbative QCD • Unique tool : • nucleon tomography • nucleon angular momentum • trajectories / strong forces • Higgs boson production

  3. Deeply Virtual Compton Scattering (DVCS) |A(W,t,Q2)| pQCD pQCD 0 Q2 real photon Compton scattering gamma conversion DVCS H1, ZEUS ( x ~ 10-2 – 10-4 ) HERMES JLab ( x ~ 10-1 ) COMPASS • the theoretically cleanest process • information on GPDs

  4. Generalized Parton Distributions (GPDs) GPDs describe the correlation between two partons ( x1, x2 ) which differ by longitudinal (x1-x2) and transverse (t) momentum at the given Q2 GPDs could be reduced to PDFs : Form Factors : Nucleon tomography : Total angular momentum carried by partons in the nucleon : Ji’s sum rule (1997)

  5. ds(e-,n) – ds(e-,n) = P (Ju,Jd) Total Angular Momentum of u- and d- in Proton HERMES:transverse proton spin asymmetry with unpolarized e-beam is sensitive to Ju and Jd at high x JLAB:e-beam polarized cross section difference measured on deuteron and proton target. ds(e+,p↑) – ds(e+,p↓) = F (Ju,Jd) HERMES 99 Du = 0.57 ± 0.04 Dd = -0.25 ± 0.08 Ds = -0.01 ± 0.05 sums up to about 30% of nucleon spin Lqadds about 30% to the proton spin

  6. Tomography picture of proton Scanning of the nucleon at different x is a unique feature of diffractive reactions GPDs constrained to the Pauli and Dirac form factors of proton Tomography plots for u- and d- quarks at high x: d u proton polarization direction M. Diehl, T. Feldmann, R. Jakob and P. Kroll, Eur. Phys. J. C 39, 1 (2005).

  7. Proton tomography with DVCS at HERA A measurement of t-dependence of DVCS cross section at different values of Q2, W Approximated : dσ/dt ~ exp(bt) t-slope parameterb(Q2) decreases with increasing Q2 No W-dependence of t-slope observed. <rT> =√ b ≈ 0.65 fm dominated by sea and gluons (low-x @ HERA) experimental input to GPDs parameterization as function of t

  8. Diffractive Vector Meson Production Vector Meson (VM) = ρ0, ω, φ, Ψ, Υ DVCS e + p → e + p + VM Q2 + M2 Q2 Scale : No extra aem supression : σρ ~ 10∙σDVCS VMs have larger signal to background ratio than DVCS VM wave function involved : theoretically not that clean as DVCS

  9. DVCS Total g* + p→ V + p cross section as function of Q2 The cross sections were scaled by factors, according to the quark charge content of the vector meson Approximated with DVCS : n ≈ 1.5 VM : n ≈ 2.5 extra (Q2+M2)-1 from the Vm wave function Details: Fit to whole Q2 range gives bad 2/dof • striking universality in vector meson production. • The Q2 dependence of (*pρp) cannot be described by a simple propagator term.

  10. s(g*+p → V+p). Energy dependence. Cross section rises with energy. Photoproduction (Q2≈ 0) s(W) ~ W d the exponent is Q2+M2 scale dependent d = d0 + 0.25ln(Q2+M2) process becomes hard as Q2+M2 -scale becomes larger.

  11. s(g*+p → V+p). Energy dependence. Elastic Vm production c.s. rises fast Inclusive c.s. is defined by Structure Function s(g*+p → X) ~ (F2) ~ W2l s(g*+p → V+p) ~ (F2)2~ Wd F2(x,Q2) ~ (1/x)l = 4 ∙ Energy dependence is defined by Structure Function d(Q2+M2) = 4 ∙ l(Q2)

  12. VM@HERA help to improve PDF parameterizations High sensitivity to gluon density at low-x

  13. ds/dt(g*+p → V+p). t- dependence. bV bp - at low t, b=4aV+4ap≡bV+bp - at large t, (4 < n < 8) t- dependence is defined by Form Factors (FVFp) Slope b is a function of Q2+M2 scale Geometric picture: b = bp + bV Size of the scattered vector meson is getting smaller with Q2+M2 scale The exponential slope of the t distribution decreases with Q2 and levels off at about b = 5 GeV-2.

  14. ds/dt(g*+p → V+p). Interplay of t,Q2 Alternative approach: Take as a scale Q2 + M2 - t - at low t- values For n = 2.5 b0 = 5 GeV-2 effective slope b : If so, Real slope b0≈ 5 GeV-2 and The exponential slope of the t distribution does not change with Q2

  15. ds/dt(g*+p → V+p). Interplay of t,W Regge-type anzatz: Regge trajectories with a trajectory slope intercept a(0) and a’ are fundamental parameters represent basic features of strong interactions and hadron spectroscopy. pp, ISR a(0) determines the energy dependence of total and diffractive cross sections a’ determines the growth with energy of the transverseextension of the scattering systemand characterizes the confinement forces in QCD Interplay t and W “coded” in GPDs Access to a’ only in diffraction

  16. Effective Pomeron Trajectory γp → Vp Elastic r0 photoproduction (Q2+M2) = 0.6 GeV2 r0 a(0)(γp) ≈α(0)(pp) But…. α’(γp)≈ ½ α’(pp) Two different softPomeron trajectories? a' reflects the diffusion of partons in impact parameter, b_t, plane during the evolution in rapidity ~ln(s) pp, ISR Size of 2 proton system in pp scattering grows twice faster with s than a single proton in gp-scattering?

  17. Effective Pomeron Trajectories γp → Vp As the scale gets harder the intercept grows The available data do not allow to make a conclusion on the slope evolution with the scale. Need more data!

  18. Diffractive dissociation processes SD: one of colliding particle dissociates DD: t- measurement is not possible What is the diffraction pattern? SD DD DPE Dy Rapidity Gap  A class ofreactions with non-exponentially suppressed largerapidity gaps is operationallytermed diffractivereactions (Bjorken 1994)

  19. Extraction of Diffractive Parton Densities at HERA Diffractive x-sections. Diffractive PDFs Assuming a factorization one could define and measure parton densities inside an object (Pomeron) exchanged in the diffractive reaction

  20. Factorization breaking at HERA Once factorization is observed in DIS diffractive di-jet production it is broken in di-jet photoproduction

  21. T 2 Rapidity Gap Survival Probability Central production of high Mass or di-jets : M sincl(M) ∙S 2 = sexcl(M) T2 –suppression of strongbremsstrahlung when two colour charged gluons ‘annihilate’ into a heavyobject (Sudakov form factor) For higher M the suppression T2 is higher (calculated in pQCD) S2 - probablity that rapidity gaps survive against population bysecondary hadrons from soft rescattering (responsible for non-factorization). For higher collision energy the suppression S2 is higher (soft process, large uncertainty) LHC: for M ≈ 100 GeV ;

  22. S2 suppression at works R(SD/ND) R(DPE/SD) DSF from two/one gap: factorization restored! SD / ND DPE / SD Diffractive di-jets at CDF Diffractive fractions: (~10%) as expected from HERA (~1%) measured at TEVATRON - suppressed by S2 - non-suppressed First rapidity gap selects event topologies without soft rescatterings (suppression!) …but the next gap in the same event is for free!

  23. -jet gap gap H h p p -jet dipole dipole p’ p’ roman pots roman pots p + H + p papers pp → p + H + p First reliable estimates for exclusive Higgs production hep-ph/0006005. + Missing mass method and proton tagging for Higgs hep-ph/0009336. Missing mass with proton tagging: σLHC ~ 3 fb

  24. H b, W b, W Exclusive Higgs boson production at LHC • missing mass method • high mass resolution (2÷4 Gev) • suppression of bb-pair production • high S/B ratio Rapidity gap suppression Low cross section How large is the rapidity gap suppression? This is tested at TEVATRON: • exclusive ccproduction ( p + p  p + cc+ p ) • exclusive di-jet production ( p + p  p + jet-jet + p ) • exclusive ggproduction ( p + p  p + gg+ p ) cc

  25. Upper limit of 49 ± 18(stat) ± 39(syst) pb Central Exclusive cc production at CDF Use the decays cc J/Ψ(μμ)γwithin |y|<0.6 central detector 10 events J/Ψ+γ found in the CDF detector and nothing else OBSERVABLE If assume all 10 events are cc(0++) to be compared with prediction of 70 pb for |y|<0.6 (Khoze, Martin, Ryskin, 2001; uncertainty factor 2÷5) • small fraction of CDF statistics used in analysis • needs for account of the cc(2++) state Sinceσ ~ Ggg ,G(2++)/G(0++) ≈ 0.13 But BR(c J/Ψγ) : BR(2++)/BR(0++) ≈ 20 One expects about equal contributions from 0++ and 2++ states

  26. Central exclusive di-jet production at CDF Good agreement with the calculations up to the expected Higgs mass lends credence to the cal- culationfor exclusive Higgs production atLHC.

  27. r0 Central exclusive gg production at CDF What Else? Lumi = 532 pb-1 • Search for exclusive gg • ET(g) > 5 GeV and |h|<1 • 3 candidate events found • 1 (+2/-1) predicted from ExHuME MC • estimated ~1 bgd event from p0 p0 ,hh pp  pp+g+r0 Similarity to VM-production at HERA If assume 3 events are DPE gg Upper limit s < 410 fb

  28. FP420 project at LHC Accurate measurement of the scattered protons TOTEM on CMS side RP220 on ATLAS side FP420 For L=30 fb-1 one expects to observe ~30 events Hbb ~5 events HWW (leptonic modes)

  29. MSSM Higgs exclusive production at LHC For high tan(b) the WW channel is suppressed by Hbb, but the cross section is enhanced. need for clean environment to isolate b-jets exclusive production possibility to separate A and H,h bosons (A is suppressed in DPE) about 1000 proton tagged events are expected for 30 fb-1 luminosity Exclusive H production – discovery channel in certain MSSM scenarios

  30. Summary • Significant progress in the Experiment and Theory of diffractive processes during last years • The way to go: • Experiment: to make precision measurements (DVCS@high-x, a’@HERA, diffractive exclusive reactions @TEVATRON) • Theory: to provide a consistent model for diffraction at all scales • Diffraction is a powerful instrumental tool in particle physics • Good luck for FP420 initiative @LHC !

  31. Extraction of GPDs A sDVCS measurement : Interference term : azimuthal asymmetry measurement B GPD JLab x r

  32. DVCS. Beam Charge Asymmetry at HERA sDVCS measurement : Interference term : azimuthal asymmetry measurement H1 prelim:

  33. Effective Pomeron Trajectory γp → Vp J/Y r0 α’(γp)≈ ½ α’(pp) a(0)(γp) ≈α(0)(pp)

  34. e M2 Q2 e Double DVCS Deep Virtual Photon Lepton Pail Production e + p → e’ + p + e+e- Scale of the process - Q2 + M2

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