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LEADING BARYON PRODUCTION IN ep COLLISION Lorenzo Rinaldi On behalf of ZEUS Collaboration. Introduction and experimental setup Leading Proton (LP) in DIS (EPS03) Leading Neutron (LN) p T 2 distribution in DIS and photoproduction, rescattering and absorption (NEW) - PowerPoint PPT Presentation
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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’
Low-x Workshop 01/07/2005
3L.Rinaldi - Leading Baryons Production
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))
Low-x Workshop 01/07/2005
4L.Rinaldi - Leading Baryons Production
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
Low-x Workshop 01/07/2005
5L.Rinaldi - Leading Baryons Production
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{
Low-x Workshop 01/07/2005
6L.Rinaldi - Leading Baryons Production
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
Low-x Workshop 01/07/2005
7L.Rinaldi - Leading Baryons Production
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.
Low-x Workshop 01/07/2005
8L.Rinaldi - Leading Baryons Production
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?
Low-x Workshop 01/07/2005
9L.Rinaldi - Leading Baryons Production
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
Low-x Workshop 01/07/2005
10L.Rinaldi - Leading Baryons Production
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
Low-x Workshop 01/07/2005
11L.Rinaldi - Leading Baryons Production
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]
Low-x Workshop 01/07/2005
12L.Rinaldi - Leading Baryons Production
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
Low-x Workshop 01/07/2005
13L.Rinaldi - Leading Baryons Production
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
Low-x Workshop 01/07/2005
14L.Rinaldi - Leading Baryons Production
Rescattering model: DIS & photoproductionRescattering model: DIS & photoproduction
bb>0 : Measurement >0 : Measurement consistent with predictionsconsistent with predictions
Fit to exponential
PHP
DIS
Low-x Workshop 01/07/2005
15L.Rinaldi - Leading Baryons Production
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
Low-x Workshop 01/07/2005
16L.Rinaldi - Leading Baryons Production
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
Low-x Workshop 01/07/2005
17L.Rinaldi - Leading Baryons Production
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