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Diffrazione ad HERA IFAE 2004 Torino 13–15 aprile 2004 Diffrazione inclusiva ad HERA → DiffPDF Produzione di mesoni vettori DVCS Marcella Capua – INFN e Universita’ della Calabria

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 2 X W Q2Q2 e (k) e’(k’) P (p) P’(p’) t IP x IP * (q) Diffraction: exchange of color singlet producing a rapidity GAP in the particle flow Q2Q2 W Standard Deep Inelastic Scattering in a frame in which the proton is very fast (Breit frame): x = fraction of proton’s momentum carried by struck quark Q 2 /W 2 W = photon-proton centre of mass energy fraction of the p momentum carried by the IP: fraction of the IP momentum carried by the struck quark: Diffractive γ*p interactions at HERA

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 3 Flux of Pomerons “Pomeron structure function” F 2 D(4) f IP (x IP,t) F 2 IP ( ,Q 2 ) Naively, if IP were particle: diffractive γ * p cross section: diffractive structure function (assumes ): Inclusive diffraction γ*p Xp DIS probes the partonic structure of the proton Structure function: X W Q2Q2 e (k) e’(k’) P (p) P’(p’) t IP x IP * (q) Q2Q2 W

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 4 Large Rapidiy Gap method X system and e’ measured System Y not measured, some theoretical and experimental uncertanties Integrate over t<1GeV 2 and M Y <1.6 GeV High acceptance p tag method Measurement of t Free of p-diss background Higher M X range Lower acceptance M X method High acceptance t measurement not possible systematics from p-diss Diffractive peak NB: if scattered proton not detected, background from proton dissociative events

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 5 H1 Diffractive Structure Function vs Diffraction Proton x F2F2 x IP F 2 D(3) x F2F2 x Weak dependence – not a “normal” hadron !

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 6 Positive scaling violations Diffraction x IP F 2 D(3) Proton F2F2 Q2Q2 Diffractive Structure Function vs Q 2 Q2Q2

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 7 (Diffractive) hard scattering factorisation f i/p D (x,Q 2,x IP,t): probability to find, with probe of resolution Q 2, in a proton, parton i with momentum fraction x, under the condition that proton remains intact, emerging with small energy loss and momentum transfer given by x IP, t A new type of PDFs, with same dignity as standard PDFs. Applies when vacuum quantum numbers are exchanged. Diffractive DIS, like inclusive DIS, is factorisable into a hard part and a soft part [QCD Hard Scattering factorization:Trentadue, Veneziano; Berera, Soper; Collins…]: hard partonic cross sectionAt fixed x IP and t diffractive parton distribution functions evolve according to DGLAP

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 8 Regge phenomenology - “ resolved IP model ” There is no evidence of the or Q 2 dependence when x IP changes → Regge factorization Regge motivated pomeron flux Shape of diffractive pdfs independent of x IP and t

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 9 NLO DGLAP QCD fits to the H1 data Gluon momentum fraction 75 ±15 % at Q 2 = 10 GeV 2 and remains large up to high Q 2 -Regge factorization -Singlet Σ and gluon g with Q 2 0 =3GeV 2 -NLO DGLAP evolution -Fit medium Q 2 (6 < Q 2 < 120 GeV 2 ) -Experimental and theor incertainties Diff PDF’s: Extended to large z Gluon dominate Σ well constrained

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 10 Test of QCD factorization NLO Comparisons with Diff DIS Jets Good agreement with prediction with NLO fit at medium Q 2 Use diff PDFs to predict the rate of diffractive dijet production: Normalisation and shape of data described ok Same conclusion for charm production jet Hard scattering factorisation works in diffractive DIS at HERA

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 11 Diff PDFs crucial e.g. to estimate rates of diffractive processes at LHC/Fermilab (eg diffractive Higgs) F D JJ (= F 2 D ) ? ( =x IP ) CDF data Predictions based on rescattering assuming HERA diffractive PDFs F2DF2D Violation of factorisation understood in terms of (soft) rescattering corrections of the spectator partons (Kaidalov, Khoze, Martin, Ryskin): Test factorisation in pp events NB several other important approaches: o) Bjorken (1993) o) Gotsman, Levin, Maor (1993) o) Goulianos (1995) o) Buchmueller, Gehrmann, Hebecker (1997) o) Cox, Forshaw, Loennblad (1999) o) Enberg, Ingelman, Timneanu (2000) o) Erhan, Schlein (2000) o) Bialas, Peschanski (2002) o)...

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 12 QCD fit describes data fractional gluon momentum is M X >2 GeV, x IP 2 GeV 2 NLO DGLAP fit Regge factorisation assumption possible for this small data set initial scale Q o 2 =2 GeV 2 NLO DGLAP QCD fits to the ZEUS data Similar to H1!

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 13 Lifetime of dipoles very long it is the dipole that interacts with the proton Transverse size 1/ (Q 2 + M qq 2 ) This is why can do diffraction in ep collisions ! Virtual photon fluctuates to qq, qqg states (colour dipoles) Transverse size of incoming hadron beam can be reduced at will. Can be so small that strong interaction with proton becomes perturbative (colour transparency) ! The colour dipole picture ** ** Two models : Saturation Model (Golec-Biernat and M. Wusthoff) BEKW model (Bartels, Ellis, Kowalski and Wüsthoff)

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 14 Saturation vs data (LPS) (Bartels, Golec-Biernat, Kowalski) Q2Q2 x IP F 2 D(3) F2F2 Inclusive diffraction: Inclusive DIS: Data well described by BGK saturation model

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 15 Main features of the data described by BEKW parametrization (x IP <0.01) BEKW vs data (LPS) (Bartels, Ellis, Kowalski and Wüsthoff) Transition to a constant cross section as Q 2 0 (similar to total cross section) qqg fluctuations dominant at low Q 2 medium β small β

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 16 Vector Meson production (J PC =1 -- ): J IP pp V 2-gluon exchange: LO realisation of vacuum quantum numbers in QCD p p V Cross section proportional to probability of finding 2 gluons in the proton: growth with decreasing x (increasing W) at large Q 2 + M V 2 reflecting large gluon density at low x Gluon density in the proton ! [Ryskin (1993), Nikolaev et al (1994), Brodsky et al (1994),...] W Large variety of processes to study dynamics versus scales: M V 2, Q 2, t

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 17 W x MVMV VM: sensitivity to gluons in proton xg(x) x W 0.2 W 0.8 W 1.7 ( p Vp), Q 2 =0 (not the most recent) Fit: ~ W with P (0) -1) At small M V (M V 2 1 GeV 2 ): Incoming dipole behaves like a normal-size hadron. Flat vs W reflects flat gluon distribution for Q 2 0 At large M V :Fast growth of with W reflects growth of gluon distribution with decreasing x

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 18 ( p Vp), Q 2 =0 W x MVMV LLA High precision data Large M V supplies a scale for hard processes apply QCD models Now able to dicriminates theory models VM: sensitivity to models

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 19 Deeply Virtual Compton Scattering p p Similar to elastic VM production, but instead of VM in final state No VM wavefunction involved Again rapid increase of cross section with W – a reflection of the large gluon density at low x

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 20 GPD-based calculations, NLO QCD(!) - Freund & with 2 sets of GPDFs Colour dipole models Donnachie &, Favart & Sensitivity to GPDs (so far) in DVCS (large Q 2 ) and production A field in its infancy. Holds the promise of mapping parton-parton correlations and transverse distribution of partons in the proton Important to find how to extract GPDs from data DVCS vs GPDs Both theoretical approaches consistent with measurements

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 21 Conclusioni E’ possibile discutere la diffrazione in termini di pQCD Sappiamo determinare le PDF diffrattive confermando la fattorizzazione in diffrazione ad HERA. L’oggetto scambiato e’ essenzialmente gluonico (alta’ densita’ gluonica nel protone 75 ±15 % ) L’ipotesi del rescattering offre la possibilita’ di interpretare anche i dati del Tevatron e quindi di parlare di universalita’ delle PDF. Il color dipole model e’ una finestra nella regione di transizione dal perturbativo al non perturbativo. Processi di VM offrono la possibilita’ di misurare le densita’ gluoniche e la precisione dei nostri dati e’ in grado di discriminare tra modelli teorici di pQCD. Sensibilita’ alle GPDs e quindi alle correlazioni tra partoni.

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 22 x1x1 x2x2 p p In general, x 1 x 2 : Hard diffraction sensitive to parton-parton correlations in the proton Generalised PDFs: sensitive to parton-parton correlations in the proton Hard diffraction sensitive to proton structure and calculable in QCD Hard diffraction sensitive to parton correlations and transverse distribution of partons in proton via GPDs Ingredient for estimating diffractive cross sections at LHC GPD

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 23 - large kinematic range and cross section: ~ 10 % of DIS is diffractive - 4 acceptance: important for * diss system - point-like probe - wide Q 2 range: to access the transition region from soft to hard processes Three different selections method Diffractive γ*p interactions at HERA No activity in the forward direction Proton almost intact after the collision

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IFAE04 Torino aprile 2004 Marcella Capua INFN e Universita' della Calabria 24 e.g. BEKW model (Bartels, Ellis, Kowalski and Wüsthoff) e.g. Saturation Model (Golec-Biernat and M. Wusthoff) F T qq β(1- β), weak Q 2 dep. F T qqg (1- β) γ, ln (1+Q 2 /Q 2 0 ), Q 2 0 =1GeV 2 F L qq only at high β energy dep. for both : from fits to the data Color dipole models qq r Saturation npQCD pQCD pQCD : s qq r 2 1/Q 2 (colour transparency) As Q 2 0, s qq violation of unitarity Growth by s qq saturating at s qq s(rp)

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