1.12.2005

Alice Valkárová, ÚČJF

Difrakce na experimentu H1a detektor VFPS

1.část

1.12.2005

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...

1.12.2005

• 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:tot

)(ln21 2XMsy

Diffractive scattering

1.12.2005

From hadrons to partons

So far, we discussed hadron degrees of freedom, the soft interactions.We need to describe phenomena in terms of hadronic subcomponentsand 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!

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• HERA II: rok 2005 ∼ 115 pb-1

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Kinematics of ep diffraction

1'

p

pxF

222 )'( eeqQ pqQpqW 2)( 222

2)'( ppt

22

22

QW

QMx X

P

xP= fraction of proton momentum carried by singlet (pomeron)

22

2

22

22

)'(2 QM

Q

tQM

Q

qpp

Q

XX

β= fraction of exchanged singlet (pomeron) momentum carried by struck quark

Photoproduction – Q2 ≃0, DIS scattering Q2>5 GeV2

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Experimental Techniques

We are sure - it is the diffraction!Some diffractive dissociation backgroundcan be still present

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Models for hard diffraction

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QCD factorisation

inclusive dijethard scattering QCD matrix element, perturbativelycalculated, process dependent

Universal diffractive parton densitiesidentical for all processes

get PDFs from inclusive diffraction ⇨ predict cross sections for exclusive diffraction

),(),,,()( 2*2

_

* QxtxQxfXpp iIP

D

ipartoni

D i*

Difuniversal hard scattering cross section (same as in inclusive DIS)diffractive parton distribution functions → obey DGLAPuniversal for diffractive ep DIS (inclusive, di-jets, charm)

• proven for DIS (J.Collins (1

998))

• not proven for photoproduction!

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σdiff = flux(xP) · object (β,Q2)

Results from inclusive diffractionβ Q2

Reduced cross section frominclusive 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 GeV∼ 2 –1600 GeV2

Regge factorisation is an additional assumption, there is no PROOF!!

),/(),(),,,( 2/

2 QxxftxftxQxf IPIPiIPpIPIP

Di

pomeron flux factor pomeron PDF

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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 givebetter constraint on the gluon density

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Jet and HQ production

Hard scale is ET of the jet or HQ massDirect access to gluon densityCan reconstruct zP in dijet events

• tests of universality of PDF’s (=QCD factorisation)

• test of DGLAP evolution

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Charm cross section (DIS)

NLO calculations HVQDIS(Harris & Smith)

Good agreement within experimental & theoreticaluncertainties.

Good description of diffractive D* production in DIS (2GeV2 <Q2<100 GeV2)

NLO calculations with PDFs from inclusive diffraction

Factoris

ation holds !

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Dijets in DIS

NLO calculations = diffractive extension of DISENT Catani&Seymour (Nucl.Phys.B485 (1997) 29),

interfaced to diffr.PDFs of H1Hadronisation corrections – RAPGAP MC

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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

Good agreement withNLO within exp.&theor.uncertainties

Fact

oris

atio

n hol

ds!

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pp

γ*p

CDF Tevatron data:At Tevatron HERA PDF’sdo not work….????

Dijet cross section factor 5-10 lowerthan the QCD calculation using HERAPDFs

?

Break

down o

f fac

toris

atio

n!

Exporting PDFs from HERA tothe Tevatron.........

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Direct and resolved processes at HERA

xγ - fraction of photon’s momentum in hard subprocess

DIS (Q2>5GeV2) and direct photoproduction (Q2 0):≃• photon directly involved in hard scattering

• xγ=1

Resolved photoproduction:• photon fluctuates into hadronic system, which takes part in hadronic scattering• dominant at Q2 0≃

• xγ<1

unsuppressed!

suppressed!

hadronsz

jetszOBS

pE

pExx

)(

)(

?

?

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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 secondaryinteractions of spectators as the cause:

HERA resolved photoproduction

Secondary interactionsbetween spectators

Jets in photoproductio

n thought to

be

ideal testin

g ground for re

scatterin

g

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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

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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

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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..........