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Inclusive Diffraction at ZEUS

Inclusive Diffraction at ZEUS. Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua. Marcella Capua Calabria University and INFN Cosenza (Italy). on behalf of. Diffractive structure function and cross section results:

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Inclusive Diffraction at ZEUS

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  1. Inclusive Diffraction at ZEUS Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua Marcella Capua Calabria University and INFN Cosenza (Italy) on behalf of • Diffractive structure function and cross section results: • Hep-ex/0501060 (accepted by Nucl. Phys. B) • Eur. Phys. J. C. 38,43 (2004) and hep-ex/0408009 • Comparison with models • NLO QCD fits of diffractive data

  2. Diffraction at HERA Q2 W e’(k’) Q2 e (k) *(q)  X W xIP IP P (p) P’(p’) t Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua Standard Deep Inelastic Scattering x = fraction of proton’s momentum carried by struck quark  Q2/W2 W = photon-proton centre of mass energy DIS probes the partonic structure of the proton Diffraction: exchange of color singlet producing a rapidity GAP in the particle flow Diff DIS probes the partonic structure of colour singlet exchange

  3. Inclusive diffraction γ*p  Xp e’(k’) Q2 e (k) *(q)  X W xIP IP P (p) P’(p’) t Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua diffractiveγ*p cross section diffractive structure function fraction of the p momentumcarried by the IP: fraction of the IP momentum carried by the struck quark:  No activity in the forward direction  Proton intact after the collision

  4. Selection of events γ*p  Xp with LPS method Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua Leading Proton Spectrometer Used till the and of the HERA I data taking Diffractive peak • Direct t measurement • Free of p-diss background • Low acceptance  low statistics

  5. Selection of events γ*p  Xp with Mx method Non-diffr. Diffr. Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua • Forward Plug Calorimeter (FPC): • CAL acceptance extended by 1 unit in pseudorapidity from η=4 to η=5 • higher Mx (a factor 1.7) and lower W • if MN > 2.3 GeV deposits EFPC> 1GeV • recognized and rejected High acceptance t measurement not possible systematics from p-diss (epeXN) D, c, b from a fit to data - exponentially falling for decreasing Mx for non-diffractive events - flat vs ln Mx2for diffractive events

  6. Kinematic domains Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua Large overlapping but also complementary regions Mx method allow higher β bins and lower xIP , LPS method allow higher xIP 97 data 0.03<Q2<0.60 GeV2 (3.6pb-1) 2<Q2<100GeV2 (12.8pb-1) 25<W<280GeV 1.5<Mx<70GeV |t|<1GeV2 98-99 data (4.2pb-1) 2.2<Q2<80GeV2 37<W<245GeV 0.28<Mx<35GeV hep-ex/0501060(accepted by Nucl. Phys. B) Eur. Phys. J. C38, 43 (2004) and hep-ex/0408009

  7. t dependence of cross section (LPS method) Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua Exponential fit to t distribution: • agrees/improves previous results • no Q2 dependences Regge phenomenology predicts “shrinkage” of the diffractive peak (b rise as xIP→ 0):

  8. xIPdependence of F2D(3)(LPS method) Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua 97 LPS sample Common xIP dependence in all bins consistent with the assumption ofRegge factorization: with by fitting xIP dep. at fixed β,Q2:

  9. Comparison between methods Good agreement between LPS and MX method (x0.7 for MN< 2.3GeV) xIPdep. of F2D(3) equivalent to W dep. of dσ/dMx

  10. Cross section W dependence (MX method) Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua 98-99 FPC sample (Higher β region) • p-dissociation events with MN<2.3 GeVincluded (~30%) MX> 2 GeV: d/dMX rises rapidly with W Used for a power-like fit

  11. αIP(0) from diffractive and total γ*p scattering (Mx method) Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua fit to diffractive cross section data: from LPS data • IPdiff(0) higher than soft Pomeron • Evidence of a rise of IPdiff with Q2  mild Regge factorisation violation

  12. σdiff/ σtot W and Q2 dependence (Mx method) Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua • In DIS diff/tot independent of W • low MX :strong decrease of diff/tot with increasing Q2 • high MX :no Q2 dependence Same results from ZEUS-LPS highest W bin: Diffraction is a substantial part of the total cross section diff(MX<35 GeV)/tot ~ 16 %Q2= 4 GeV2  10 %Q2= 27 GeV2

  13. The colour dipole picture g* g* • Lifetime of dipoles very long  it is the dipole that interacts with the proton • Transverse size 1/  (Q2+ Mqq2) • This is why can do diffraction in ep collisions ! Transverse size of incoming hadron beam can be reduced at will. Can be so small that strong interaction with proton becomes perturbative (colour transparency) Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua Virtual photon fluctuates to states (colour dipoles) Provide a good description of the DIS diffractive data: BEKW (Bartels, Ellis, Kowalski and Wüsthoff) BGK (Bartels, Golec-Biernat, Kowalski) FS04(Forshaw & Shaw) CGC(Colour Glass Condensate)

  14. BEKW model Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua • For b <= 0.7 and x=b xIP < 0.002 xIP2FD(3) increases with increasing Q2 Positive scaling violations • For fixed b, Q2 dependence of xIPF2D(3) changes with xIP Transition to a constant cross section as Q20 (similar to total cross section ) Main features of the data described by BEKW parametrization (xIP<0.01) (Bartels, Ellis, Kowalski and Wüsthoff) energy dep. for both : (from fits to the data) FTqq ~β(1-β), weak Q2 dep. FTqqg ~(1- β)γ , ln (1+Q2/Q20), Q20=1GeV2 FLqq only at high β medium β small β qqg fluctuations dominant at low Q2

  15. F2D(3) Q2 dependence and BGK model (LPS method) sqq Saturation r pQCD npQCD Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua xIP < 0.01 (diff peak region) Will show QCD fits for this region only xIP > 0.01 (exchange of subleading trajectories important) Data well described by BGK (Bartels, Golec-Biernat, Kowalski)saturation model (xIP<0.01) Positive scaling violation at all values of xIP: lots of gluons!

  16. Comparison with Colour Dipole Model - I • Forshaw & Shaw(FS04) model  Refer to hep-ph/0411337Regge dipole model with/without saturation • Iancu, Itakura, MunierColour Glass Condensate (CGC) model  Refer to hep/0310338 • ZEUS LPS97 FS04(nosat) Q2=2.4 GeV2 F2 Fit F2 and then predict xIPF2D(3) CGC FS04(sat) F2 Q2=39 GeV2 Q2=39 GeV2 xIP b=0.007 b=0.03 b=0.13 b=0.48 x (Heuijin Lim DIS05)

  17. Comparison with Colour Dipole Model - II Low Q2 from ZEUS MX 98-99 (x 1./0.7) MX=30 GeV MX=20 GeV MX=11 GeV MX=6 GeV MX=3 GeV MX=1.2 GeV xIP  Predictions of model are corrected by 1/0.7 for the MN<2.3 GeV of ZEUS MX method. (Heuijin Lim DIS05)

  18. Comparison with Colour Dipole Model - III High Q2 from ZEUS MX 98-99 (x 1./0.7) MX=30 GeV MX=20 GeV MX=11 GeV MX=6 GeV MX=3 GeV MX=1.2 GeV xIP • CGC and FS04(sat) are able simultaneously to describe F2 and xIPF2D(3). • Forshaw & Shaw have not been able to find a good fit which does not invoke saturation. (Heuijin Lim DIS05)

  19. (Diffractive) hard scattering factorisation Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua Diffractive DIS, like inclusive DIS, is factorisable into a hard part and a soft part [QCD Hard Scattering factorization:Trentadue, Veneziano; Berera, Soper; Collins…]: pq/p(xIP,t,x,Q2): probability to find, with probe of resolution Q2, in a proton, parton q with momentum fraction x, under the condition that proton remains intact and emerges with small energy loss, xIP, and momentum transfer t Additional assumption: REGGE FACTORISATION Regge motivated IP flux: Shape of diffractive PDFs, independent on xIP and t universal hard scattering cross section (same as in inclusive DIS) Diffractive Deep Inelastic Scattering probes the diffractive PDFs of the proton relevant when the vacuum quantum numbers are exchanged

  20. NLO QCD fit on LPS+charm data Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua • NLO DGLAP fit, QCDNUM • MX>2 GeV, xIP<0.01, Q2>2 GeV2 • Regge factorisation assumption • DL flux: • F2D(3) (xIP,ß,Q2)=ΦIP(xIP) F2IP(ß,Q2) • initial scale Q02=2 GeV2 • Quark flavour singlet and gluon • parameterized according to: • zf(z)=(a1+a2z+a3z2)(1-z)a4 QCD fit describes data fractional gluon momentum is Similar to H1: 75 ±15% (exp+theor) Higher than Mx method data: ~55% (see A. Levy DIS05) [F2D (3)cc from DESY-03-094]

  21. Summary Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua • Recent data from ZEUS with improved precision and extended kinematic range • Slope of dσ/d|t| is measured: it is indipendent of Q2 but decreases with xIP • Indication that αIP increases with Q2 • Similar W and Q2dep. of diffractive and total cross section • Diffraction shows evidence for pQCD evolution with Q2 • Data described by dipole models (BEKW, GBW, FS04, CGC) • Diffractive PDFs extracted from DGLAP fits dominated by gluons: • LPS+charm data suggest momentum fraction carried by gluons 82% (consistent with H1 results)

  22. Reserve

  23. F2D(3) /F2Q2 dependence (LPS method) Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua The ratio is largely Q2-indipendent

  24. Comparison with H1 Low x - International Workshop, SINAIA, ROMANIA, June 29 - July 2 2005 - M. Capua H1 Large rapidity gap selection: MY<1.6 GeV and |t|<1 GeV2 LPS proton selection: MY= mp extrapolated to |t|<1 GeV2 • constrain IR contribution at high xIP H1 LRG / LPS ratio: • p-dissociation contribution ~10% Good agreement between two methods and two experiments ZEUS Mx / LPS ratio: p-dissociation contribution ~30%

  25. Low x 2005 - International Workshop, SINAIA, ROMANIA, June 29 - July 2 - 2005 F2D(3) β dependence Different β dep. at low and high xIP Data well described by BGK saturation model (xIP<0.01)

  26. NLO QCD fits to the ZEUS data • Donnachie and Landshoff pomeron flux • QCDNUM (Sasha);CTEQ (Tel Aviv) • u=d=s=ubar=dbar=sbar (Sasha);s=sbar=0 (Tel Aviv) • Fits to F2D (Sasha); reduced cross section, ie including FL(Tel A.) • Only statistical errors (Sasha);stat+syst in quadrature (Tel Aviv) • Initial scale=2 GeV2 (Sasha);3 GeV2 (Tel Aviv) • None of these differences significant. Given same input, same fits are obtained. • PARAMETRISATIONS of PDFs at initial scale: • A) zf(z)= AzB (1-z)C • B) zf(z)= H1 parametrisation=S[CjPj(2z-1)]2exp[a/(z-1)] , • Pj=Chebychev polynomials; a=0.01; 3 terms kept in sum • C) zf(z)= (a1+a2z+a3z2) (1-z)a4 • D) zf(z)= (a1+a2z+a3z2) (1-z) • Fits restricted to MX>2 GeV, xIP<0.01, Q2>2 GeV2 (Sasha), Q2>3 GeV2 (Tel Aviv)

  27. LPS vs H1 PDFs Results similar to H1’s S. Proskuryakov

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