Difrakce na experimentu H1 a detektor VFPS

1. Difrakce na experimentu H1a detektor VFPS 1.část Alice Valkárová, ÚČJF

2. 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. Podobný obrazec rozptylu jako v optice Analogie s optikou není však zdaleka úplná... 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!!!!

3. Diffractive scattering • • beam particle emerge intact (elastic) or dissociate into low • mass states X, Y (MX, MY ≪ √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:

4. 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!

5. HERA II: rok 2005 ∼ 115 pb-1

6. Kinematics of ep diffraction Photoproduction – Q2≃0, DIS scattering Q2>5 GeV2 xP= fraction of proton momentumcarried by singlet (pomeron) β= fraction of exchanged singlet(pomeron) momentum carried bystruck quark

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

8. Models for hard diffraction

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

10. Results from inclusive diffraction Q2 β Regge factorisation is an additional assumption, there is no PROOF!! pomeron flux factor pomeron PDF σdiff = flux(xP) · object (β,Q2) Reduced cross section from inclusive diffractive data • get diffractive PDFs from a NLO (LO) • DGLAP QCD Fit to inclusive data from • 6.5 GeV2 to 120 GeV2 • extrapolation of the Fit • to lower Q2 • to higher Q2 • gives a reasonably good description of • inclusive data from ∼3.5 GeV2 –1600 GeV2

11. 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 zP Assuming factorisation holds, the jet and HQ cross sections give better constraint on the gluon density

12. Jet and HQ production Hard scale is ET of the jet or HQ mass Direct access to gluon density Can reconstruct zPin dijet events • tests of universality of PDF’s • (=QCD factorisation) • test of DGLAP evolution

13. Charm cross section (DIS) Good agreementwithin experimental & theoretical uncertainties. Factorisation holds ! NLO calculations with PDFs from inclusive diffraction NLO calculations HVQDIS (Harris&Smith) Good description of diffractive D* production in DIS (2GeV2 <Q2<100 GeV2)

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

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

16. CDF Tevatron data:  γ*p pp Exporting PDFs from HERA to the Tevatron......... 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! ?

17. Direct and resolved processes at HERA xγ - fraction of photon’s momentum inhard subprocess • DIS (Q2>5GeV2) and direct photoproduction (Q2≃0): • photon directly involved in hard scattering • xγ=1 ? unsuppressed! • Resolved photoproduction: • photon fluctuates into hadronic system, which • takespart in hadronic scattering • dominant at Q2≃0 • xγ<1 ? suppressed!

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

19. 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

20. 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

21. Summary of 1st part • Dijets in DIS & D* cross section: • agree with the NLO prediction with the H1 2002 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..........