leading baryons at hera

Leading Baryons Leading Baryons at HERAat HERA

Graziano BruniGraziano Bruni

INFN – BolognaINFN – Bologna

On behalf of H1 and ZEUS On behalf of H1 and ZEUS CollaborationsCollaborations

Lowx08, July 6-10, 2008 2Graziano Bruni - INFN (Bologna)

OutlineOutline

Physics casePhysics case Recent and new results on:Recent and new results on:

Leading protons (new)Leading protons (new) Leading neutronsLeading neutrons Comparison with modelsComparison with models Leading neutron + 2 jets in Leading neutron + 2 jets in p p

SummarySummary


Semi-inclusive reactionSemi-inclusive reaction

ep e Xh h = LB = p, n

X = hadronic state

Current jetCurrent jet

Proton Proton remnant remnant

Q2 > 1 GeV2 : deep-inelastic scattering (DIS)

Q2 < 1 GeV2 : photo-production p

DIS regimeDIS regimeScale for secondary particle production decreases from Q2 (current region) to a soft hadronic scale (proton fragmentation region)

Photoproduction (Photoproduction (p) p) regimeregimeHadronic component of the photon.

Can re-introduce hard scale (e.g. requiring high-pT jets)


Some approachesSome approaches

Fracture Functions ( L.Trentadue, G.Veneziano Phys. Lett. B323, 201 (1994) )

- Not discussed here

MC fragmentation models

Dynamical particle-exchange models


Production modelsProduction models

Hadronisation of proton Hadronisation of proton remnantremnant

Herwig (cluster model)Herwig (cluster model) MEPS (parton shower,SCI)MEPS (parton shower,SCI) Ariadne (CDM)Ariadne (CDM)

Exchange of virtual particlesExchange of virtual particles

leading protonsleading protons: : 00, IR, IP , IR, IP (isoscalar + isovector)(isoscalar + isovector)

leading neutronsleading neutrons: : ++, , ++, , … (isovector)… (isovector)

LBLBxL=ELB/Ep

pT2

t=(p-p’)2

Q2, xBj, y, W

1 L

xx

x

p p’


e.g. LN production via e.g. LN production via -exchange-exchange

Neutron absorption through rescatteringNeutron absorption through rescattering D’Alesio and Pirner D’Alesio and Pirner (EPJ A7(2000) 109)(EPJ A7(2000) 109)

Neutron rescatters on Neutron rescatters on hadronic component. hadronic component.Absorption enhanced when Absorption enhanced when -n system size -n system size rrnn small small

w.r.to the w.r.to the : : rn ~ 1/pT . .

Dependence on the pion-flux from xDependence on the pion-flux from xL:L: <r <rnn> >

increases with xincreases with xLL more absorption at low more absorption at low xxLL . .

- n kicked to lower x- n kicked to lower xLL & higer p & higer pTT ( (migrationmigration) )

- may escape detection (may escape detection (absorption lossabsorption loss))

more absorption in photoproduction than DISmore absorption in photoproduction than DIS (( “size” larger) “size” larger)

p

DIS

Rescattering and Rescattering and absorptionabsorption


Nikolaev,Speth and ZakharovNikolaev,Speth and Zakharov (NSZ)(NSZ)(hep-ph/9708290)(hep-ph/9708290)

Re-scattering processes via additional Re-scattering processes via additional pomeron exchanges + Optical Theorempomeron exchanges + Optical Theorem

( ( Uncertainties in Uncertainties in structure function structure function

extraction extraction ))

(Kaidalov,) Khoze, Martin, Ryskin (KKMR)(Kaidalov,) Khoze, Martin, Ryskin (KKMR) (hep-ph/0602215, hep-ph/0606213)(hep-ph/0602215, hep-ph/0606213)

Enhanced absorptive corrections, Enhanced absorptive corrections, calculation calculation of migrationsof migrations, include also , include also and aand a22 exchange exchange

(different x(different xLL & p & pTT dependences) dependences)

Measure xMeasure xL L and pand pTT22

distributionsdistributions

Study dependence on Study dependence on QQ22

Compare Compare p and p and DISDIS

Look for effects due to Look for effects due to absorptionabsorption


Experimental toolsExperimental tools


ZEUS Leading Proton Spectrometer (LPS)ZEUS Leading Proton Spectrometer (LPS) 6 stations6 stations each made by each made by 6 Silicon-detector6 Silicon-detector planes planes Stations inserted at 10Stations inserted at 10beambeam from the proton beam during data taking from the proton beam during data taking xxLL

< 1%, < 1%, ppTT22 ~ ~ few MeVfew MeV2 2 ((better than p-beam spread better than p-beam spread ~ 50 - 100 MeV~ 50 - 100 MeV))

H1 Forward Neutron Calorimeter (FNC)H1 Forward Neutron Calorimeter (FNC) Lead-scintillator calorimeterLead-scintillator calorimeter @ 107m from I.P. + @ 107m from I.P. + veto hodoscopesveto hodoscopes (E)/E(E)/E≈20%, neutron detection eff. 93±5%≈20%, neutron detection eff. 93±5%

ZEUS Forward Neutron Calorimeter (FNC) + Forward Neutron Tracker (FNT)ZEUS Forward Neutron Calorimeter (FNC) + Forward Neutron Tracker (FNT) 1010 lead-scintillator sandwich lead-scintillator sandwich σσ/E =0.65/√E, Energy scale 2%/E =0.65/√E, Energy scale 2% Acceptance Acceptance nn<0.8 mrad, azimuthal coverage 30%<0.8 mrad, azimuthal coverage 30% FNT: FNT: Scint. hodoscope @ 1Scint. hodoscope @ 1λλintint, , σσx,yx,y=0.23 cm, =0.23 cm, σσθθ=22 =22 μμradrad

20 m107 m

Lower xL accessible

H1 Forward Proton Spectrometer (FPS)H1 Forward Proton Spectrometer (FPS) 2 stations 2 stations each made by each made by 4 planes of sci-fiber hodoscopes4 planes of sci-fiber hodoscopes + + trigger scintillatorstrigger scintillators xx = = yy = 100 = 100 m, m, (E) < 8 GeV, |E(E) < 8 GeV, |Escalescale| = 10 GeV, <| = 10 GeV, <tracktrack> ~ 50%> ~ 50%


Leading Protons Leading Protons (DIS-(DIS-regime)regime)

22 2

22( , ) ( , )

dK x Q F x Q

dxdQ

Fully inclusive: ep Fully inclusive: ep eX eX

42 (4) 2 2

22 2( , ) ( , , , )LP

L TL T

dK x Q F x Q x p

dxdQ dx dp

Semi-inclusive: ep Semi-inclusive: ep epX epX

(3) 2 (2) 22 2( , , ) , ( , ) LP LP

LF x Q x F x Q


xxL L cross-sectioncross-section NEW ZEUS Results

12.8 pb-1

Cross section normalised to Cross section normalised to inclusive DIS cross sectioninclusive DIS cross section

Flat below diffractive peakFlat below diffractive peak

DIS and DIS and low Qlow Q22 data are data are compatiblecompatible

Non diffractive region

pT2<0.5 GeV2


PPTT22 cross-section in bins of x cross-section in bins of xLL

Exponential behaviourExponential behaviour

Fit to ~ exp(-b pFit to ~ exp(-b pTT22) )

No strong dependence No strong dependence of b on xof b on xLL

-26.8 GeVb

Different Q2


Rates to inclusive DISRates to inclusive DIS

),(

),,(),,(

22

2)3(22)3(

QxF

xQxFxQxr L

LP

LLP

Structure function ratio

Measurement for 0.32<xL<0.92(diffractive peak excluded)

rrLP(3)LP(3) almost independent almost independent of x and Qof x and Q2 2 with average with average value value ~~0.40.4

xBj

Q2

9.6x10-5 – 3x10-3

4.2 – 237 GeV2


),(

),(2

2

2)2(2)2(

QxF

QxFr

LPLP

~~24% of DIS events have a LP 24% of DIS events have a LP with 0.32<xwith 0.32<xLL<0.92, almost <0.92, almost

independently of x and Qindependently of x and Q22Low Q2

0.32 < xL < 0.92

0.60 < xL < 0.97

Slight increase with QSlight increase with Q22 not not excludedexcluded

(2)LPr


Leading neutronsLeading neutrons

*

2(1 )Ls x W

One Pion Exchange is the leading contribution at large xL

(Regge)-factorization of the cross section *p nX

2 2/ *( , ) ((1 ) , )p L L

L

df x t x W Q

dx dt

Flux-factor

Elementary cross-section

(notice xL dependence)

Limiting Fragmentation (in the proton target region the production of particles is

independent on the incident particle) Vertex Factorization

2 2 22 ( , ) ( , )L T

L T

dg x p G W Q

dx dp

Broken if absorption


•LN yield decreases for xLN yield decreases for xLL1 1 due to kinematic limitdue to kinematic limit

• Below xBelow xLL ~ 0.7 the yield drops ~ 0.7 the yield drops

due to decreasing pdue to decreasing pTT22 range: range:

ppTT22 < < 0.476 0.476 xxLL

22

Phase space coverage

xxL L spectrumspectrum

DIS regime•ppTT22 < 0.476 x < 0.476 xLL

22


DIS cross section vs pDIS cross section vs pTT22 in in

bins of xbins of xLL

22

( )2

1( ) L Tb x pLN

Linc L T

da x e

dx dp

Good description through a single exponential in each xL bin

b ~ 0 for xL < 0.3

Falling with xL

Plateau 0.4 < xL < 0.8


QQ22 – dependence: – dependence: xxLL spectraspectra

Yield increases with QYield increases with Q22: : large increase from large increase from ppto to DISDIS

Smaller QSmaller Q22 dependence dependence at intermediate valuesat intermediate values

Violation of vertex Violation of vertex factorizationfactorization

p

3 Q2 bins + p


QQ22 – dependence: – dependence: xxLL spectra spectraComparison Comparison p vs DIS p vs DIS

Ratio Ratio p/DISp/DIS

~ 70% intermediate x~ 70% intermediate xLL

Rises above 1 as xRises above 1 as xLL 1 1

Consistent with Consistent with absorption:absorption:

- separation separation -n -n decreases at low xdecreases at low xLL

- smaller separation smaller separation more absorption at low xmore absorption at low xLL

p2T < 0.476

x2L

Ratio Ratio p/DISp/DIS


QQ2 2 – dependence: – dependence: pp22TT

slopesslopes

DIS – slopes:DIS – slopes:

~ no Q2 dependence

pp – slopes: – slopes:

higher in 0.6 < xhigher in 0.6 < xLL < < 0.90.9

p


QQ2 2 – dependence: – dependence: pp22TT slopes. slopes.

Comparison Comparison p vs DISp vs DISNormalized to 1

DIS

p

Consistent with vertex factorization Consistent with vertex factorization violation from absorption:violation from absorption:

- more absorption at small r- more absorption at small rn n large large ppTT

- loss of LN at high pT larger slope

bp > bDIS for 0.6 < xL < 0.9

Depletion at large pT

p-p-DISDIS


Comparison LP - LNComparison LP - LN

Pure isovector exchange: LP = ½ LN Other IR contributions in LP Similar slopes in 0.7 < xL < 0.85

Same pT2 range: pT

2 < 0.04 GeV2

LN: rise with xL

LP: ~ flat

LP

LN


Comparison to Comparison to modelsmodels


Leading ProtonsLeading Protons

Reasonable description by Reasonable description by , IR, IP , IR, IP exchange modelexchange model

Standard fragmentation MC models Standard fragmentation MC models fail to describe the datafail to describe the data

DJANGOH+SCI+MEPS ~ ok b(xDJANGOH+SCI+MEPS ~ ok b(xLL) ) shapeshape

IPIR


Leading neutronsLeading neutrons

MC models generally fail to fully reproduce the data

Shapes ~ ok by Shapes ~ ok by RAPGAPRAPGAP with with standard fragmentationstandard fragmentation + + -exch.-exch.

LEPTO+SCILEPTO+SCI: x: xLL shapes ~ shapes ~ ok, not slopesok, not slopes


22 ( , ) ( )ep enX

L T e eL T

df x p s

dx dp

1 2 ( ) 22 (1 ) [ ( , )]

( )t

L L

tf x F x t

t m

Models differ by and F

Reasonable Reasonable agreement in agreement in shape, shape, notnot in rate in rate

One-Pion Exchange ModelsOne-Pion Exchange Models


(K)KMR model(K)KMR model

Good description of data Good description of data when when additional IR additional IR includedincluded

Absorption & migration included in the predictions: not sufficient to describe the slopes

Slopes - DISKKMR model

( – exchange)

KMR model + secondary IR (,a2)


XL -p

KKMR model

( – exchange)

KMR model + secondary IR (,a2)

Pure Pure -xch. + -xch. + inclusion migrations inclusion migrations in xin xLL and p and p22

TT after after rescattering rescattering

Reasonable Reasonable description of shape description of shape and normaliz.and normaliz.

Add (Add (aa22)-xch: again )-xch: again reasonable reasonable agreement in agreement in pp


Comparison DIS - Comparison DIS - pp

p / DISModels: OPE + absorptionModels: OPE + absorption

2[(1 ) ]L ps x W

*

0.13(1 ) (1 )L Lx x

Good agreement with the Good agreement with the datadata

( different cms energy dependence)


Leading n in Leading n in p + dijetsp + dijets

1 2

ηT

jet ,obsγ

z had

E e

x(E p )

Photon momentum fraction thatenters in the hard scattering

x = 1 direct PHP, DIS

x < 1 resolved PHP

( hadron-like photon )

( )

JetJet

JetJet

Introduce a Introduce a hard scale in hard scale in pp

EETT > 7.5 GeV > 7.5 GeV

EETT > 6.5 GeV > 6.5 GeV

Absorption effects seen going from hard soft scale

High Q2 Low Q2 p

Still absorption?Still absorption?


Factorization testsFactorization tests

ZEUS-DATAZEUS-DATA - Nucl.Phys.B596,3(2001) - Nucl.Phys.B596,3(2001)

RAPGAP / HERWIG MIRAPGAP / HERWIG MI

H1-DATAH1-DATA - ( - (Eur. Phys. J. C41 (2005) 273-Eur. Phys. J. C41 (2005) 273-286 286 ) )

RAPGAP/PYTHIA-MIRAPGAP/PYTHIA-MI

Not yet conclusions on factorization breaking in resolved Not yet conclusions on factorization breaking in resolved p p (x(xγγ<1)<1)

2

2

j n

j

Ratio almost independent on ETjet: factorization

Strong dependence on x: breaking of factorization

Fewer neutrons in the resolved region


LN+jj p vs LN (DIS)

b-slopes similar in magnitude b-slopes similar in magnitude and shape in DIS and and shape in DIS and p+dijetp+dijet

Same production mechanismSame production mechanism

p p withoutwithout jets: suppression at jets: suppression at low xlow xLL

p p withwith jets: suppression at high jets: suppression at high xxLL

Not yet firm conclusions on Not yet firm conclusions on absorptionabsorption

DISDIS

pp


SummarySummary

New data from HERA on LB production and properties, new ZEUS LP results

Observation of factorization breaking effects going from hard to soft scales, mostly in LN

p + hard jets: more work needed LP needs isoscalar-IR contributions to explain the data Standard fragmentation MC-models fail to describe the data Improvement in MC models with particle-exchange

implemented LN: pure exchange not sufficient (slopes not described) Recent calculations with exchange and

absorption/migration effects improve the agreement in magnitude and shape of the xL spectra

Additional exchanges (, a2) improve further


Additional slidesAdditional slides


Fracture Functions Fracture Functions approachapproachL.Trentadue, G.Veneziano

Phys. Lett. B323, 201 (1994)

Dynamical models allow to Dynamical models allow to link them to concepts like link them to concepts like structure-functions of structure-functions of exchanged objects (e.g. IR exchanged objects (e.g. IR trajectories)trajectories)

Current jetCurrent jet

Proton Proton remnant remnant

Production in the current current region

/ /( ) ( )current q q p h q Lq

c f x D x hxL

- F.F.F.F. are non-perturbative universal functions

- Q2 dependence governed by evolution equations

current current + + targettarget

arg , /(1 ) [ , (1 ) ]t et q q h p Lq

c x M x x x Production in the targettarget region: F.F.F.F.

h

xL

Fragmentation function

Fracture function

leading baryons at hera

Documents

xl spectracomparison

neutron detection

smaller q2 dependence

xl reggefactorization

kinematic limitbelow

neutron rescatters

modelsleading neutron

cross section g