leading baryon production in ep collision lorenzo rinaldi on behalf of zeus collaboration

LEADING BARYON PRODUCTION LEADING BARYON PRODUCTION IN ep COLLISIONIN ep COLLISION

Lorenzo RinaldiLorenzo RinaldiOn behalf of ZEUS CollaborationOn behalf of ZEUS Collaboration

Introduction and experimental setupIntroduction and experimental setup Leading Proton (LP) in DIS Leading Proton (LP) in DIS (EPS03)(EPS03) Leading Neutron (LN) pLeading Neutron (LN) pTT

22 distribution in DIS and distribution in DIS and

photoproduction, rescattering and absorption photoproduction, rescattering and absorption (NEW)(NEW) Leading Neutron tagged D* production Leading Neutron tagged D* production (ZEUS Coll., (ZEUS Coll., Phys. Phys. Lett. B Lett. B 590590 (2004) 143 (2004) 143) Conclusions Conclusions

Low-x Workshop 01/07/2005

2L.Rinaldi - Leading Baryons Production

IntroductionIntroduction Large fraction of events with a Leading Baryon (LB) in final stateLarge fraction of events with a Leading Baryon (LB) in final state Production mechanism still not clearProduction mechanism still not clear Model in use:Model in use:

,IR,IP

N,PN,P

Lepton variablesQ2, W, x, y

LB variables:pT

2, xL=ELB/Ep

t=(p-p’)2

Vertex factorization

Virtual particle exchange LP: neutral iso-scalar iso-vector (,IR,IP) LN: charged iso-vector(+,+,...) LB also from p fragmentation in double dissociative diffraction

Standard fragmentation LB from hadronization of p remnant Implemented in MC models (Cluster, Lund strings...)

p’p’



ZEUS leading baryon detectorsZEUS leading baryon detectors

Leading Proton Spectrometer (LPS)Leading Proton Spectrometer (LPS) 6 stations each made by 6 Silicon-detector planes 6 stations each made by 6 Silicon-detector 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))



ZEUS leading baryon detectors 2ZEUS leading baryon detectors 2Forward Neutron Calorimeter (FNC)Forward Neutron Calorimeter (FNC) 10 10 II Pb-sci. sandwich Pb-sci. sandwich /E = 0.65%//E = 0.65%/√E√E ppTT

22 ~ MeV ~ MeV22 (dominated by p-beam (dominated by p-beam

spread)spread)Forward Neutron Tracker (FNT)Forward Neutron Tracker (FNT) Scintillator hodoscope at 1Scintillator hodoscope at 1I I

x,yx,y= 0.23 cm = 0.23 cm = 22 = 22 radrad

Kinematic regions covered by the Kinematic regions covered by the angular acceptance of FNCangular acceptance of FNC

Beampipe hole

FNT

Acceptance window

(n<0.75 mrad)L

LpnL

L

T

x

xmmx

x

pt

))(1( 222



Leading proton: cross section vs xLeading proton: cross section vs xLL

MEPS: (1-xL)1.0

Ariadne: (1-xL)1.4

Herwig (1-xL)1.0

~ Flat distribution below the diffractive peak

MonteCarlo models fail to describe MonteCarlo models fail to describe the measured leading baryon xthe measured leading baryon xLL

spectrum. spectrum.

Montecarlo samples (standard Montecarlo samples (standard fragmentation):fragmentation): Herwig (cluster model)Herwig (cluster model) MEPS (parton shower,SCI)MEPS (parton shower,SCI) Ariadne (CDM)Ariadne (CDM)

DIS{



Leading proton: cross section vs pLeading proton: cross section vs pTT22

Data distribution Data distribution ~ exponential~ exponential

Fit to exponential in each xFit to exponential in each xLL bin: bin:

2)(2

2TL pxb

TL

edpdx

d



Leading proton: b-slopesLeading proton: b-slopes

No strong b-slope dependence No strong b-slope dependence of xof xL L observedobserved

Also the pAlso the pTT22 is not well is not well

simulated. simulated.



Leading proton summaryLeading proton summary

leading proton features have been measured with high leading proton features have been measured with high precision precision

~ flat cross section vs x~ flat cross section vs xLL below the diffractive peak below the diffractive peak approximate exponential fall of papproximate exponential fall of pTT

22 cross section cross section No visible dependence of pNo visible dependence of pTT

22 slopes vs x slopes vs xLL

Differences found with models predictions Differences found with models predictions Accurate measurement available for MC tuningsAccurate measurement available for MC tunings

And what about the neutrons? And what about the neutrons?



O.P.E. explains features of leading O.P.E. explains features of leading neutron data:neutron data: production rateproduction rate xxLL spectrum spectrum factorizationfactorization

),)1((),(),,,( 22

*/

22

QWxtxfdtdx

txQWdLLp

L

LeXnep

),()1()(

),( 2)(22/ txFx

mt

ttxf L

tLLp

(t)(t) and and FF22(x(xLL,t),t) model dependent model dependent

ppTT22 slopes discriminate between production models (flux) slopes discriminate between production models (flux)

Leading Neutron: Leading Neutron: One-Pion-Exchange modelOne-Pion-Exchange model



Leading Neutron cross section vs pLeading Neutron cross section vs pTT22 in x in xLL bins bins

Cross sections normalized to inclusive processCross sections normalized to inclusive process Fit to exponential in xFit to exponential in xLL bins bins



Leading Neutron b-slopes vs xLeading Neutron b-slopes vs xLL

OPE models:OPE models: Dominant at 0.6<xDominant at 0.6<xLL<0.9<0.9 (non-) Reggeized flux, different form (non-) Reggeized flux, different form factors with different parametersfactors with different parameters none of the models seem to decribe none of the models seem to decribe well the datawell the data

Best models:Best models: FMS-mono FMS-mono

[Frankfurt, Mankiewicz, Strickman, [Frankfurt, Mankiewicz, Strickman, Zeit. Phys.Zeit. Phys. A334A334 (1989) 343] (1989) 343]

FMS-dipole FMS-dipole [Frankfurt, Mankiewicz, Strickman,[Frankfurt, Mankiewicz, Strickman, Zeit. Phys.Zeit. Phys. A334A334 (1989) 343] (1989) 343]

GKS GKS [Golec-Biernat, Kwiecinski, Szczurek,[Golec-Biernat, Kwiecinski, Szczurek,Phys. Rew.Phys. Rew. D56D56 (1997) 3955] (1997) 3955]

Bishari0 Bishari0 [Bishari, [Bishari, Phys. Lett.Phys. Lett. B38B38 (1972) 510] (1972) 510]



Rescattering model and absorptionRescattering model and absorption

Model 1: (D’Alesio and Pirner, EPJ A7(2000) 109)Model 1: (D’Alesio and Pirner, EPJ A7(2000) 109)

Model 2: (Nikolaev,Speth and Zakharov, hep-ph/9708290)Model 2: (Nikolaev,Speth and Zakharov, hep-ph/9708290)

absorption from additional absorption from additional pomeron exchangepomeron exchange implies large uncertainties to implies large uncertainties to pion pdf descriptionpion pdf description

a=e,p

more absorption when photon size more absorption when photon size larger (small Qlarger (small Q22) ) less neutrons less neutrons detected in photoproductiondetected in photoproduction more absorption when mean more absorption when mean -n -n system size (system size (‹‹rrnn››) smaller at low x smaller at low xLL

less neutrons detected at low x less neutrons detected at low xLL

more absorption more absorption fewer neutrons fewer neutrons detected with higher pdetected with higher pTT

22 larger b- larger b-

slope expected in photoproductionslope expected in photoproduction



Comparison DIS and Comparison DIS and photoproduction:photoproduction: LN in photoproduction LN in photoproduction cross section smaller than cross section smaller than DIS (factorization violation)DIS (factorization violation) At lower xAt lower xLL less neutrons less neutrons

observed in photoproduction observed in photoproduction Same trend in rescattering Same trend in rescattering (absorption) of D’Alesio and (absorption) of D’Alesio and PirnerPirner

Rescattering model: measurementsRescattering model: measurements



Rescattering model: DIS & photoproductionRescattering model: DIS & photoproduction

bb>0 : Measurement >0 : Measurement consistent with predictionsconsistent with predictions

Fit to exponential

PHP

DIS



Leading Neutron tagged D* photoproductionLeading Neutron tagged D* photoproduction

n

Hard scalepT>1.9 GeV

soft scalepT<0.3 GeV

epepe’D*e’D*++nX nX

D*D*++(D(D00KK--ss++ and c.c. and c.c.

Information on interplay between Information on interplay between soft and hard scalessoft and hard scales

=2.08=2.08++0.22(stat.)0.22(stat.)+0.12+0.12 --0.180.18

(syst.)(syst.)++0.05(B.R) nb0.05(B.R) nb

Kinematic region:Kinematic region: QQ22<1 GeV<1 GeV22

130<W<280 GeV130<W<280 GeV ||(D*)|<1.5(D*)|<1.5 ppTT(D*)>1.9 GeV(D*)>1.9 GeV xxLL>0.2>0.2 nn<0.8<0.8



Leading Neutron tagged D* photoproductionLeading Neutron tagged D* photoproduction

Fragmentation models also fail to Fragmentation models also fail to describe leading neutron describe leading neutron OPE gives a good description of OPE gives a good description of datadata OPE model sensitive to pion flux OPE model sensitive to pion flux parametrisationparametrisation

Ratio to inclusive processesRatio to inclusive processes

rrPHPPHP<r<rDISDIS≈≈rrD*D* rescattering suppressed by the precence of a hard scale rescattering suppressed by the precence of a hard scale

LN production modelsLN production models

Ratio LN tagged to inclusive D* production rD*=8.85+0.93(stat.)+0.48-0.61(syst.) %

Ratio LN tagged to inclusive DIS rDIS=8.0+0.5 % Ratio LN tagged to inclusive PHP rPHP=5.7+0.4 %

fi = parametrization of flux factor

Standard Fragmentation

OPE



Conclusion and outlookConclusion and outlook

High precision measurements on leading baryon availableHigh precision measurements on leading baryon available Leading proton features well measured Leading proton features well measured High precision measurement of leading neutron pHigh precision measurement of leading neutron pTT

22

slopes both in DIS and photoproductionslopes both in DIS and photoproduction Absorption effects visible in data Absorption effects visible in data

Leading particles production mechanism is still not Leading particles production mechanism is still not completely understoodcompletely understood

Theoretical predictions differ from measurementsTheoretical predictions differ from measurements Need to improve leading baryon production modelsNeed to improve leading baryon production models New results input to tune the models New results input to tune the models

leading baryon production in ep collision lorenzo rinaldi on behalf of zeus collaboration

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leading baryon production

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flat cross section vs

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