Alice Valkárová, ÚČJF Difrakce na experimentu H1 a detektor VFPS 1.část

Difrakce – hadronová fyzika Hadronová fyzika : Difrakce je jev, kdy částice (nebo soubor částic) po interakci má stejná kvantová čísla jako počáteční částice. dσ/dt ≃ dσ/dt| (t=0) (1-B|t|), |t| ∝ θ²při vys.energiích, B ∝ R²,kde R je poloměr hadronu terčíku. Pojem difrakční hadronové fyziky se objevil v 50-tých letech (Landau, Pomerančuk, Feinberg a dal.) a většina interakcí, která se tehdy a brzo poté studovala, byla difrakčního typu. To ale není ta fyzika,co nás zajímá!!!! Podobný obrazec rozptylu jako v optice Analogie s optikou není však zdaleka úplná...

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

From hadrons to partons So far, we discussed hadron degrees of freedom, the soft interactions. We need to describe phenomena in terms of hadronic subcomponents and quantum field theories, i.e. in terms of QCD ⇒ need hard scale to apply perturbative methods! 1984 – hard diffraction predicted by Ingelman& Schlein G. Ingelman,P.Schlein,Phys.Lett.B152,256(1985), 583 citations!!!!! 1987 – beginning of the age of hard diffraction – UA8 experiment – first measurement of diffractive jet production UA8 collaboration,Evidence for transverse jets in high mass diffraction Phys.Lett.B211,239,(1988), 163 citations Since that time – hard diffraction measured by Tevatron and HERA!

HERA II: rok 2005 ∼ 115 pb -1

Kinematics of ep diffraction x P = fraction of proton momentum carried by singlet (pomeron) β= fraction of exchanged singlet (pomeron) momentum carried by struck quark Photoproduction – Q 2 ≃ 0, DIS scattering Q 2 >5 GeV 2

Experimental Techniques We are sure - it is the diffraction! Some diffractive dissociation background can be still present

Models for hard diffraction

QCD factorisation inclusive dijet hard scattering QCD matrix element, perturbatively calculated, process dependent Universal diffractive parton densities identical for all processes get PDFs from inclusive diffraction ⇨ predict cross sections for exclusive diffraction universal hard scattering cross section (same as in inclusive DIS) diffractive parton distribution function s → obey DGLAP universal for diffractive ep DIS (inclusive, di-jets, charm) proven for DIS (J.Collins (1998)) not proven for photoproduction!

σ diff = flux(x P ) · object (β,Q 2 ) Results from inclusive diffraction β Q2Q2 Reduced cross section from inclusive diffractive data get diffractive PDFs from a NLO (LO) DGLAP QCD Fit to inclusive data from 6.5 GeV 2 to 120 GeV 2 extrapolation of the Fit to lower Q 2 to higher Q 2 gives a reasonably good description of inclusive data from ∼ 3.5 GeV 2 –1600 GeV 2 Regge factorisation is an additional assumption, there is no PROOF!! pomeron flux factor pomeron PDF

Diffractive Parton Densities determined from NLO QCD analysis of diffractive structure function more sensitive to quarks gluons from scaling violation, poorer constraint gluon carries about 75% of pome- ron momentum large uncertainty at large z P Assuming factorisation holds, the jet and HQ cross sections give better constraint on the gluon density

Jet and HQ production Hard scale is E T of the jet or HQ mass Direct access to gluon density Can reconstruct z P in dijet events tests of universality of PDF’s (=QCD factorisation) test of DGLAP evolution

Charm cross section (DIS) NLO calculations HVQDIS (Harris & Smith) Good agreement within experimental & theoretical uncertainties. Good description of diffractive D* production in DIS (2GeV 2 <Q 2 <100 GeV 2 ) NLO calculations with PDFs from inclusive diffraction Factorisation holds !

Dijets in DIS NLO calculations = diffractive extension of DISENT Catani&Seymour (Nucl.Phys.B485 (1997) 29), interfaced to diffr.PDFs of H1 Hadronisation corrections – RAPGAP MC

Dijets in DIS NLO corrections to LO are significant – factor 1.9 excess at high x γ is kinematically connected with the lack of events with η lab of jets < -0.4 in comparison with NLO Good agreement with NLO within exp.&theor. uncertainties Factorisation holds!

pp  γ*p CDF Tevatron data: At Tevatron HERA PDF’s do not work….???? Dijet cross section factor 5-10 lower than the QCD calculation using HERA PDFs ? Breakdown of factorisation! Exporting PDFs from HERA to the Tevatron

Direct and resolved processes at HERA x γ - fraction of photon’s momentum in hard subprocess DIS (Q 2 >5GeV 2 ) and direct photoproduction (Q 2 ≃ 0): photon directly involved in hard scattering x γ =1 Resolved photoproduction: photon fluctuates into hadronic system, which takes part in hadronic scattering dominant at Q 2 ≃ 0 x γ <1 unsuppressed! suppressed! ? ?

Photoproduction as hadronic process resolved contribution expected to be suppressed by factor 0.34 (Kaidalov,Khoze,Martin,Ryskin:Phys.Lett.B567 (2003),61) Typical models that describe suppression at Tevatron assume secondary interactions of spectators as the cause: HERA resolved photoproduction Secondary interactions between spectators Jets in photoproduction thought to be ideal testing ground for rescattering

Dijets in photoproduction The same kinematical region as for DIS NLO overestimates the cross section by factor ∼ 2 both direct and resolved are suppressed RAPGAP LO – good description

Ratio:data over NLO prediction no suppression observed for DIS overall suppression factor of about 2 observed for both resolved and direct components in photoproduction suppression is independent of the cms energy W

Summary of 1 st part  Dijets in DIS & D* cross section: agree with the NLO prediction with the H diffractive pDFs factorisation holds (assuming PDF is correct)  Dijets in photoproduction: to investigate the puzzle of disagreement of HERA/Tevatron data (expectation: resolved will be suppressed and direct not) data are half of NLO prediction – both resolved and direct are suppressed ⇨ conflict with the theoretical expectation  More ideas? Hádanka zatím nerozřešena