5.2.2004 1

Difrakce v e-p interakcíchCo víme o difrakci na HERA?

Alice Valkárová

5.2.2004 2

Strong Interactions

We master QCD only when perturbative methods can be applied i.e. small distance (or equivalently, hard scale: ) processes,...., 22 QpT

We are unable to use QCD to compute the bulk of hadronicinteractions, i.e. the “soft” (or large distance) cross sections and elastictotal ,

diffr

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)

5.2.2004 3

Dominance of Soft Collisions

(mb)

5.2.2004 4

Foreword

Confinement

Traditionally:study the binding forces between

quarks described in terms ofinterquark potential

⇨ calculate static properties ofhadrons, like masses

Traditionally:study the binding forces between

quarks described in terms ofinterquark potential

⇨ calculate static properties ofhadrons, 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

5.2.2004 5

• 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

5.2.2004 6

Regge model: analytic model ofHADRONIC scattering

Exchange of collective states:linear trajectories in the spin-

energy (α,t) plane,

Regge model: analytic model ofHADRONIC scattering

Exchange of collective states:linear trajectories in the spin-

energy (α,t) plane,

),,()0()( ' RPjtt jjj

The Hadronic Level: Regge Model

Dirac 58: Singularities in l (poles) correspond to bound states or resonances

16.0 sExperimental 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

)( tBe

5.2.2004 7

Regge phenomenology: hadronic σtot

To describe the rise of σtot pomeron (P) trajectory with ⇒ αP(0)>1(not associated to any real particle)

Great success for Regge phenomenology: asymptotic behaviour of all hadronic σtot (pp, πp, Kp, γp) described by the same αP(0) ⇒

Donnachie-Landshoff fit for “soft” Pomeron: αP(0) = 1.08, α’P=0.25

αR(0) 0.55∼

αP(0) ∼1.08pp: 21.7s0.0808+56.08s-0.4525

1)0()0,(Im1 stsfstot

5.2.2004 8

The Pomeron

2Candidate for pomeron is glueball observed by experiment WA91 with mass MeV121918

2

5.2.2004 9

The parameters αP(0) and α‘P

5.2.2004 10

5.2.2004 11

5.2.2004 12

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: Q2: 10-8→105 GeV2 (corresponding to probing distances Δr: 1 → 10-3 fm

which allows to study the transition between soft and hard regimes• excellent acceptance for diffractive dissociated system: asymmetric beams (Ee+-= 27.5 GeV, Ep=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:

,,0 ,...),,( p

5.2.2004 13

Hard diffraction at HERA222 )'( eeqQ

2)'( ppt

pqQpqW 2)( 222

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

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

1||||

max p

pxF

pepqy

5.2.2004 14

Two systems X and Y well separated in phase space with low masses MX ,MY << 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 visibleforwardactivity

Exchange of colourless object, Pomeron, with low momentum fraction xP

Pomeron

5.2.2004 15

1) Large Rapidity Gap / H1 2) MX– Method / ZEUS 3) Proton Tagging / H1, ZEUS

Fit excess above exponential fall-off

FPS / LPS & beam line opticsTypical cut: max < ~ 3. *)

Selection Methods

= -ln tan (/ 2)

ln MX2-2 0 2 4 6 8

5.2.2004 16

5.2.2004 17

The diffractive structure functions

Integrate over t when proton is not tagged → σRD(3)(β,Q2,xP)

),,,()2

1(4)( 2)4(

2

4

2

2

4

txQy

yQdtdxdQd

eXpepdP

DR

em

P

)4(DR → diffractive reduced cross section

)4(2

2)4(

2)4(

)2

1(2

DL

DDR F

yy

yF

)4(2

)4( DDR F at low y

)4(2

)4( DDR F

0)4( DLF

if

5.2.2004 18

Hard Diffraction in QCD

QCD hard factorization in diffractive DIS:

At Q2 large enough (Collins 1998):

),,,(),,(ˆ),,,( 22

0

22*

*

txQfQxddtdx

txQxdP

Di

i

i

x

P

XYpP

P

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

i*

ˆ D

if

For fixed xP and t the DGLAP equations applicable(like inclusive DIS) with evolution in β and Q2

5.2.2004 19

Models for hard diffraction

5.2.2004 20

Models for hard diffraction

5.2.2004 21

Regge factorisation –Resolved pomeron model

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

Pomeron with partonic structure (Ingelman,Schlein 1984)

),(ˆ),(),()( 22** QQptxfXpp

ppqIPpIP

1)(2/ ),( tIP

Bt

IPpIP xe

txf

Regge motivated pomeron flux Shape of diffractive pdf’sindependent of xP and t

(Breit frame)

5.2.2004 22

Use Ingelman&Schlein resolved Pomeron ansatz:

σdiff = flux(xP) · object (β,Q2)

For large xP > 0.01 add Reggeon exchange :

with flux in Regge limit:

DDIS: xP-Dependence & αP(0)

Free parameteres in fit: αP(0), AP(β,Q2), AR(β,Q2) in each (β,Q2)bin

063.0034.0.)(017.0.)(018.0173.1)0(

syststatP

→ higher than 1.08 for the soft pomeron !

5.2.2004 23

DDIS: QCD analysis

QCD fit model:

• Use QCD hard factorisation• Use Regge factorisation (supported by data) ⇒ shape of diffr. PDFs is independent of xP

• 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

Q2 > Q02 = 3 GeV2• Use NLO DGLAP to evolve diffr. PDFs to

• the diffr. PDFs are parameterised using Chebychev polynomials

5.2.2004 24

Flat up to high β, no xP dependence Regge factorization works

strong positive scaling violations up to high large gluon component

DDIS: and Q2-Dependences (1)Fit region: 6 < Q2 < 120 GeV2

5.2.2004 25

Diffractive Parton Distributions

NLO & LO DGLAP fit

Gluon momentum fraction 75 ±15 % at Q2 = 10 GeV2

and remains large up to high Q2

Notice: inclusive measurements not particularly sensitive to gluons at large zP (or β). Jets measurements do much better!

5.2.2004 26

Gluons in DIS and diffr. DISMomentum carried by gluons:

DISDiffr.inclusive DIS

Pomeron:

slowly decreasing with Q2, ⟨zg = 0.75⟩

Proton:

⟨xg increasing with Q⟩ 2, x⟨ g =0.4,…0.55⟩

5.2.2004 27

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

?

?

5.2.2004 28

Pdf’s universal at HERA?

Jet and heavy flavour production high sensitivity to⇒ diffractive gluon distribution!

22

212

2

XP MQ

MQz

momentum fraction of diffractive exchangeentering hard process zP:

sM ˆ12 : mass of two jets

high pT jet production c→ D* Meson production

5.2.2004 29

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

5.2.2004 30

~ 260 D* , 1.5 < Q2 < 200 GeV2

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 xP < 0.01

xP < 0.01

gluon/Pomeron component: * c c

_

5.2.2004 31

Pdf’s generally universal?

Due to presence of second hadronin initial state?

Serious breakdown of factorizationobserved if HERA pdf’s transportedto TEVATRON:

Predictions based on H1 pdf’s oneorder of magnitude above CDFdata!

Spectator interactions break upantiproton, ”rapidity gap survivalprobability”

CDF Tevatron data:

5.2.2004 32

HERA DIS & photoproduction TEVATRON

IDEA:

Almost real photon may develop hadronic structure

→ similar to pp

Dijets in diffraction

Does QCD factorization also work in diffractivephotoproduction (although not proven)?

5.2.2004 33

New 2002 fit describes direct and resolved contribution

Direct comparison DIS & γp: .)(exp30.025.1)(

)(

DIS

p

DataModelData

Model

No suppression of γp w.r.t. DIS diffractive jets!!

Diffractive pdf’s implemented to MC RAPGAP

Photon: LO GRV pdf

Dijets in photoproduction

5.2.2004 34

5.2.2004 35

5.2.2004 36

5.2.2004 37

5.2.2004 38

5.2.2004 39

5.2.2004 40

HERA II

Higher luminosity (3-5x), e polarization,factor 10 in statistics (2007) →1fb-1

Tag and measure the scattered proton at HERA II withlarge acceptance at low xP and down to lowest t

Precision studies of ep→ epX

New tool for HERA II: H1 Very Forward Proton Spectrometer - VFPS

5.2.2004 41

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

xxIPIP = 0.01 = 0.01H1

ZEUS

VFPS

HERMES

HERA-B

present FPS VFPS

5.2.2004 42

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 480 µm pitch 340 µm cladding 30 µm theoretical resolution 63 µm prototype test resolution 94 µm

Diameter 480μm

Pitch 340μm

Theoretical resolution 63μm

Prototype test resolution 94μm

5.2.2004 43

VFPS installation

Installed during HERA shutdown

Operation started at September 2003

5.2.2004 44

Diffraction: H1 VFPS

lowest |t| for low xIP

rD for xIP=0.017 integrated over |t|< 0.8 GeV2

measure x = xIP and Q2 in central detectorsimulation for 350 pb-1

HERA 2H1 VFPS at z = 220 m

5.2.2004 45

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, )%1575(

Breakdown of the factorization at Tevatron pp interactions (by factor 10)∼ is observed

Perturbative QCD can describe VM data for large Q2,|t| or M2vm.

Regge model still alive: two pomerons? two-pole structure?

Conclusions

5.2.2004 46

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?