Difrakce v e-p interakcích Co víme o difrakci na HERA? Alice Valkárová

Strong Interactions We master QCD only when perturbative methods can be applied i.e. small distance (or equivalently, hard scale: ) processes We are unable to use QCD to compute the bulk of hadronic interactions, i.e. the “soft” (or large distance) cross sections and Only ∼ (0.01)% of all events understood in terms of perturbative QCD!! There were two partly successful attempts undertaken: Regge phenomenology (60’s) QCD (80’s)

Dominance of Soft Collisions (mb)

Foreword Confinement Traditionally: study the binding forces between quarks described in terms of interquark potential ⇨ calculate static properties of hadrons, like masses Traditionally: study the binding forces between quarks described in terms of interquark potential ⇨ calculate static properties of hadrons, like masses High energy hadronic scattering ⇒ hard diffraction: class of events in which an initial state hadron may emerge intact. confinement wins over strong forces which tend to breakup hadrons ⇨ hope to learn about fundamental properties of the binding forces High energy hadronic scattering ⇒ hard diffraction: class of events in which an initial state hadron may emerge intact. confinement wins over strong forces which tend to breakup hadrons ⇨ hope to learn about fundamental properties of the binding forces

beam particle emerge intact (elastic) or dissociate into low mass states X, Y (M X, M Y ≪ √s) there is a t-channel exchange of a colourless object emerging systems hadronize independently ⇨ Large Rapidity Gap (LRG) if s is large enough: Large fractions of events ( ∼ 30% of ) in which: Diffractive scattering

Regge model: analytic model of HADRONIC scattering Exchange of collective states: linear trajectories in the spin- energy (α,t) plane, Regge model: analytic model of HADRONIC scattering Exchange of collective states: linear trajectories in the spin- energy (α,t) plane, The Hadronic Level: Regge Model Dirac 58: Singularities in l (poles) correspond to bound states or resonances Experimental observations in diffractive scattering: weak energy dependence of the cross section: very small scattering angles  exponential dependence of the exchanged 4-momentum |t|: B increases with energy

Regge phenomenology: hadronic σ tot To describe the rise of σ tot ⇒ pomeron (P) trajectory with α P (0)>1 (not associated to any real particle) Great success for Regge phenomenology: asymptotic behaviour of all hadronic σ tot (pp, πp, Kp, γp) described by the same α P (0) ⇒ Donnachie-Landshoff fit for “soft” Pomeron: α P (0) = 1.08, α ’ P =0.25 α R (0) ∼ 0.55 α P (0) ∼ 1.08 pp: 21.7s s

The Pomeron Candidate for pomeron is glueball observed by experiment WA91 with mass MeV

The parameters α P (0) and α‘ P

Hard diffraction at HERA diffraction in DIS is much simpler than in hadron-hadron, only one large (~ 1 fm) non-pert. object (hadron) present virtual γ provides varying resolution power: Q 2 : →10 5 GeV 2 (corresponding to probing distances Δr: 1 → fm which allows to study the transition between soft and hard regimes excellent acceptance for diffractive dissociated system: asymmetric beams (E e+- = 27.5 GeV, E p =820 (920) GeV) open up γ * -hemisphere clean channels for Odderon search → no Pomeron background in the reactions γp→ (Odderon – partner of Pomeron with odd parity P and C!) About 10% of the DIS events at HERA at small x ⇨ diffractive Several advantages:

Hard diffraction at HERA β= fraction of exchanged singlet (pomeron) momentum carried by struck quark x P = fraction of proton momentum carried by singlet (pomeron)

Two systems X and Y well separated in phase space with low masses M X,M Y << W System Y : proton or p-dissociation carries most of the hadronic energy System X : vector meson, photon or photon-dissociation Signatures of Diffraction non-diffractive event diffractive event no visible forward activity Exchange of colourless object, Pomeron, with low momentum fraction x P Pomeron

)Large Rapidity Gap / H1 2) M X – Method / ZEUS 3) Proton Tagging / H1, ZEUS Fit excess above exponential fall-off FPS / LPS & beam line optics Typical cut: 0 max < ~ 3. *) Selection Methods  = -ln tan (  / 2) ln M X

The diffractive structure functions Integrate over t when proton is not tagged → σ R D(3) (β,Q 2,x P ) → diffractive reduced cross section at low y if

Hard Diffraction in QCD QCD hard factorization in diffractive DIS: At Q 2 large enough (Collins 1998): where ξ is the fraction of the proton momentum carried by diffr.parton i, is the universal partonic cross (the same as as this for DIS) section and the Diffractive Parton Distributions (DPD) for parton i For fixed x P and t the DGLAP equations applicable (like inclusive DIS) with evolution in β and Q 2

Models for hard diffraction

Models for hard diffraction

Regge factorisation –Resolved pomeron model Regge factorisation is an additional assumption, there is no PROOF!! Pomeron with partonic structure (Ingelman,Schlein 1984) Regge motivated pomeron flux Shape of diffractive pdf’s independent of x P and t (Breit frame)

Use Ingelman&Schlein resolved Pomeron ansatz: σ diff = flux(x P ) · object (β,Q 2 ) For large x P > 0.01 add Reggeon exchange : with flux in Regge limit: DDIS: x P -Dependence & α P (0) Free parameteres in fit: α P (0), A P (β,Q 2 ), A R (β,Q 2 ) in each (β,Q 2 )bin → higher than 1.08 for the soft pomeron !

DDIS: QCD analysis QCD fit model: Use QCD hard factorisation Use Regge factorisation (supported by data) ⇒ shape of diffr. PDFs is independent of x P Parton ansatz for exchange : Pomeron = ∑q(z)+q(z) + g(z) z is the momentum fraction of the parton entering the hard subprocess with respect to diffr.exchange α=1.173 is taken from the fit Q 2 > Q 0 2 = 3 GeV 2 Use NLO DGLAP to evolve diffr. PDFs to the diffr. PDFs are parameterised using Chebychev polynomials

Flat up to high β, no x P dependence  Regge factorization works strong positive scaling violations up to high $  large gluon component DDIS:  and Q 2 -Dependences (1) Fit region: 6 < Q 2 < 120 GeV 2

Diffractive Parton Distributions NLO & LO DGLAP fit Gluon momentum fraction 75 ±15 % at Q 2 = 10 GeV 2 and remains large up to high Q 2 Notice: inclusive measurements not particularly sensitive to gluons at large z P (or β). Jets measurements do much better!

Gluons in DIS and diffr. DIS Momentum carried by gluons: DISDiffr.inclusive DIS Pomeron: slowly decreasing with Q 2, 〈 z g 〉 = 0.75 Proton: 〈 x g 〉 increasing with Q 2, 〈 x g 〉 =0.4,…0.55

Are the pdf’s universal? How to test? Diffractive pdf’s (Pomeron pdf) extracted from DGLAP NLO fit of inclusive DIS events HERA cross sections of Jet/HQ production (sensitive to gluons!) compared to the calculation using the pdf’s. assume factorizable Pomeron with partonic structure (Regge factorization) pdf’s universal at HERA? pdf’s generally universal? TEVATRON cross section of Jet production compared to the calculation using HERA pdf’s

Pdf’s universal at HERA? Jet and heavy flavour production ⇒ high sensitivity to diffractive gluon distribution! momentum fraction of diffractive exchange entering hard process z P : : mass of two jets  high p T jet production  c→ D * Meson production

Diffractive DIS Dijets LO calculations too low size of NLO correction on average factor ∼ 2 (due to low jet p T ) NLO,corrected for hadronization: reasonable description in shape and normalization

~ 260 D*, 1.5 < Q 2 < 200 GeV 2 2) Colour dipole 2 gluon exchange Open charm production very sensitive to the 1) Resolved Pomeron - Boson-gluon fusion Final States : Open Charm in DDIS _ Resolved Pomeron : - NLO fit Alvero & 2-gluon exchange qq+g: - Golec-Biernat & - Bartels & All models agree with data for x P < 0.01 x P < 0.01 gluon/Pomeron component:  *  c c _

Pdf’s generally universal? Due to presence of second hadron in initial state? Serious breakdown of factorization observed if HERA pdf’s transported to TEVATRON: Predictions based on H1 pdf’s one order of magnitude above CDF data! Spectator interactions break up antiproton, ”rapidity gap survival probability” CDF Tevatron data:

HERA DIS & photoproduction TEVATRON IDEA: Almost real photon may develop hadronic structure → similar to pp Dijets in diffraction Does QCD factorization also work in diffractive photoproduction (although not proven)?

New 2002 fit describes direct and resolved contribution Direct comparison DIS & γp: No suppression of γp w.r.t. DIS diffractive jets!! Diffractive pdf’s implemented to MC RAPGAP Photon: LO GRV pdf Dijets in photoproduction

HERA II Higher luminosity (3-5x), e polarization, factor 10 in statistics (2007) →1fb -1 Tag and measure the scattered proton at HERA II with large acceptance at low x P and down to lowest t Precision studies of ep→ epX New tool for HERA II: H1 Very Forward Proton Spectrometer - VFPS

VFPS Location VFPS location is optimised for acceptance  220m NL Proton beam is approached horizontally (use HERA bend) Bypass is needed to re-route the cold beam line x IP = 0.01 H1 ZEUS VFPS HERMES HERA-B present FPSVFPS

VFPS Detectors VFPS detectors similar to FPS: –2 “Roman Pot” stations equipped with 2 scintillating fibre detectors each –1 fibre detector measures both u - and v - co-ordinates 5 fibres/light guide  8.2 photo- electrons  99.4% detection efficiency Staggered fibres properties: Diameter 480μm Pitch 340μm Theoretical resolution 63μm Prototype test resolution 94μm

VFPS installation Installed during HERA shutdown Operation started at September 2003

Diffraction: H1 VFPS lowest |t| for low x IP  r D for x IP =0.017 integrated over |t|< 0.8 GeV 2 measure x =  x IP and Q 2 in central detector simulation for 350 pb -1 exclusive final states statistics limited get high statistics for all proton elastics: HERA 2 H1 VFPS at z = 220 m

 For the first time HERA allows the partonic content of the Pomeron to be probed and thus to shed new light on the nature of diffractive phenomena  Diffractive pdf’s are universal within DIS and photoproduction at HERA  Both inclusive measurements and the properties of the hadronic final states require a high gluonic content of the Pomeron,  Breakdown of the factorization at Tevatron pp interactions (by factor ∼ 10) is observed  Perturbative QCD can describe VM data for large Q 2,|t| or M 2 vm.  Regge model still alive: two pomerons? two-pole structure? Conclusions

Basic questions 1) What is the precise relation between hard and soft diffraction? 2) Are there several pomerons (as some believe) or is there only one (as others advocate)? 3) Is the notion of pomeron meaningful at all? 4) What does QCD tell us about diffraction and the pomeron?