leading baryon production at hera lorenzo rinaldi on behalf of h1 and zeus collaborations

LEADING BARYON PRODUCTION LEADING BARYON PRODUCTION at HERAat HERA

Lorenzo RinaldiLorenzo RinaldiOn behalf of H1 and ZEUS CollaborationsOn behalf of H1 and ZEUS Collaborations

MotivationsMotivations Large fraction of events with a Leading Baryon (LB) in final Large fraction of events with a Leading Baryon (LB) in final state carrying high fraction of the proton beam momentumstate carrying high fraction of the proton beam momentum LB produced at small angle in forward direction: difficult LB produced at small angle in forward direction: difficult detection detection Production mechanism still not clear Production mechanism still not clear Interest in LB study for next experiments @ LHC Interest in LB study for next experiments @ LHC (absorptive corrections for diffractive Higgs, pile-up (absorptive corrections for diffractive Higgs, pile-up background...)background...)

Experimental results discussed in this talk:Experimental results discussed in this talk: Leading Proton (LP) spectra in DIS Leading Proton (LP) spectra in DIS Leading Neutron (LN) spectra in DIS and Leading Neutron (LN) spectra in DIS and photoproductionphotoproduction Dijet production with a Leading NeutronDijet production with a Leading Neutron Latest developments in theoryLatest developments in theory Comparison with modelsComparison with models

Diffraction06 05-10/09/2006

3L.Rinaldi - Leading Baryons @ HERA

Leading baryon production in ep collisionsLeading baryon production in ep collisions

,IR,IP

N,PN,P

Lepton variablesQ2, W, x, y

LB variables:pT

2, xL=ELB/Ep

t=(p-p’)2

Vertex factorization

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

p’p’

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

LB production affected by absorption and rescattering effects: evidences of vertex factorization violation



Leading baryon detectorsLeading baryon detectors

ZEUS Leading Proton Spectrometer (LPS)ZEUS 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 Forward Neutron Tracker (FNT) ZEUS Forward Neutron Tracker (FNT) Scint. hodoscope @ 1Scint. hodoscope @ 1λλint, int, σσx,yx,y=0.23cm, =0.23cm, σσθθ=22=22μμradrad

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

ZEUS Forward Neutron Calorimeter (FNC)ZEUS Forward Neutron Calorimeter (FNC) 1010 lead-scintillator sandwich lead-scintillator sandwich σσ/E =0.65/√E, ∆Eabs=2%/E =0.65/√E, ∆Eabs=2% Acceptance Acceptance nn<0.8 mrad, azimuthal coverage 30%<0.8 mrad, azimuthal coverage 30%



Leading Proton: cross section vs xLeading Proton: cross section vs xLL

MEPS

Ariadne

Herwig

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)Bad description of xBad description of xLL spectrum spectrum

Flat below diff. peak Flat below diff. peak (1-x(1-xLL)), , ~0~0 Good description by reggeon-Good description by reggeon-exchange modelexchange model



LP: cross section vs pLP: 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



LP: b-slopes vs xLP: b-slopes vs xLL

No strong dependence observed on xNo strong dependence observed on xLL

observed fluctuations due to fit rangeobserved fluctuations due to fit range Good agreement with prediction from Good agreement with prediction from Reggeon-exchange modelReggeon-exchange model

• Different slopes in LEPTODifferent slopes in LEPTO• Better HERWIGBetter HERWIG• b-slope not well simulated by b-slope not well simulated by fragmentation modelsfragmentation models



LP summaryLP summary

leading proton quantities have been measured with high leading proton quantities 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

Good description by reggeon-exchange modelGood description by reggeon-exchange model Fragmentation models fail to describe LP production Fragmentation models fail to describe LP production Accurate measurements available for MC tuningsAccurate measurements available for MC tunings

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



O.P.E. partially explains the LN O.P.E. partially explains the LN production production

),)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

Longitudinal momentum spectrum and pLongitudinal momentum spectrum and pTT22 slopes discriminate slopes discriminate

between different parametrizations of fluxesbetween different parametrizations of fluxes

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



Rescattering model and absorption 1Rescattering model and absorption 1

Model 1: One pion exchange in the Model 1: One pion exchange in the framework of triple-Regge formalism framework of triple-Regge formalism

Nikolaev,Speth & ZakharovNikolaev,Speth & Zakharov

Re-scattering processes via Re-scattering processes via additional pomeron exchanges additional pomeron exchanges (Optical Theorem)(Optical Theorem)

(hep-ph/9708290)(hep-ph/9708290)

((Kaidalov,) Khoze, Martin, Kaidalov,) Khoze, Martin, Ryskin (KKMR)Ryskin (KKMR)

Enhanced absorptive corrections Enhanced absorptive corrections (( exclusive Higgs @ LHC), exclusive Higgs @ LHC), calculation of calculation of migrations, include migrations, include also also and aand a22 exchange exchange

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

(hep-ph/0602215, hep-ph/0606213)(hep-ph/0602215, hep-ph/0606213)



Rescattering model and absorption 2Rescattering model and absorption 2

Model 2: calculations from D’Alesio and Model 2: calculations from D’Alesio and Pirner in the framework of target Pirner in the framework of target fragmentation fragmentation (EPJ A7(2000) 109)(EPJ A7(2000) 109)

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



LN: longitudinal momentum spectrumLN: longitudinal momentum spectrum

• LN yield increases with xLN yield increases with xLL due to due to

increase in phase space: increase in phase space: ppTT

22 < < 0.476 0.476 xxLL22

• LN yield decreases for xLN yield decreases for xLL1 due to 1 due to

kinematic limitkinematic limit

DISDIS PHPPHPDISDIS

~W~W, , ((pp) ) (( *p*p) )

WW22=(1-x=(1-xLL)W)Wpp

22 (1-x (1-xLL) ) -0.1-0.1

• LN yield in PHP < yield in DIS LN yield in PHP < yield in DIS factorization violation factorization violation

• Models in agreement with data !Models in agreement with data !

D’Alesio & Pirner:D’Alesio & Pirner:



LN: longitudinal momentum spectrum 2LN: longitudinal momentum spectrum 2

KKMR predictions: Including KKMR predictions: Including migrations and other iso-vector migrations and other iso-vector exchanges, like exchanges, like and a and a22

• Pure Pure exchange too high exchange too high• Absorption and migrations effects Absorption and migrations effects

reduces the LN yield and fit the reduces the LN yield and fit the data betterdata better

• Additional Additional and a and a22 exchanges exchanges

enhance the LN yieldenhance the LN yield

PHPPHP



LN: cross section vs pLN: cross section vs pTT22 in x in xLL bins bins

• Exponential behavior with slope bExponential behavior with slope b

• Intercept and exponential slope fully characterize the pIntercept and exponential slope fully characterize the pTT22 spectra spectra

DISDIS



LN: intercepts and b-slopes vs xLN: intercepts and b-slopes vs xLL

• intercept ~ cross section intercept ~ cross section

integrated over all pintegrated over all pTT22

rise towards x rise towards xLL~0~0

• LN slope increases up to xLN slope increases up to xLL~0.8~0.8

• LP slope almost flatLP slope almost flat• Similar values in the range 0.7<xSimilar values in the range 0.7<xLL<0.85 <0.85

when when exchange dominates exchange dominates



LN b-slopes vs xLN b-slopes vs xLL: Models: Models

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 the data wellthe data well

KKMR model:KKMR model: good description of the data good description of the data considering absorption effects and considering absorption effects and ,a,a22 exchange contributions exchange contributions



LN b-slopes: DIS & photoproductionLN b-slopes: DIS & photoproduction

Fit to exponential

PHP

DISslopes different in PHP and DISslopes different in PHP and DISin general agreement with expectation from in general agreement with expectation from absorption absorption more absorption more absorption @@ small n- small n-sizesize depletion depletion @@ large p large pTT

steeper slope in PHPsteeper slope in PHP

a2



LN b-slopes: DIS & photoproduction from KKMR modelLN b-slopes: DIS & photoproduction from KKMR model

a2

Other exchanges flatten the pOther exchanges flatten the pTT22

distributions in both, a bit more distributions in both, a bit more in PHP than in DISin PHP than in DIS



• Lepto+MEPSLepto+MEPS best for xbest for xLL spectra, but flat b-slope, spectra, but flat b-slope,

• Lepto+Ariadne,Lepto+Ariadne, RAPGAPRAPGAP in standard modein standard mode, , CASCADECASCADE cannot describe any of cannot describe any of the distributions: too few neutrons, too low xthe distributions: too few neutrons, too low xLL, b-slopes too flat., b-slopes too flat.

Assuming the leading baryon production proceeds via the standard Assuming the leading baryon production proceeds via the standard fragmentation process fragmentation process do standard MC generators describe the data ? do standard MC generators describe the data ?

LN production compared to MC modelsLN production compared to MC models



xobs

Re-scattering processes expected for resolved PHP: photon acts hadron-like Re-scattering processes expected for resolved PHP: photon acts hadron-like additional interactions between remnants and scattered partons additional interactions between remnants and scattered partons

Relevant variable: Relevant variable:

momentum fraction of the photon entering the hard sub-process. momentum fraction of the photon entering the hard sub-process.

DIS DIS direct PHPdirect PHP resolved PHPresolved PHP

hadz

,jet

ηT

obsγ )p(E

eE

x

21

Dijet production with a LNDijet production with a LN



Dijet production with a LNDijet production with a LN

Lower values @ higher xLower values @ higher xLL

● DIS○ pjjXn

● DIS○ pjjXn

Comparable slopesComparable slopes



No possibility to decide on factorization breaking due to re-scattering No possibility to decide on factorization breaking due to re-scattering processes in resolved PHP (xprocesses in resolved PHP (x<1) with hadron-like photon.<1) with hadron-like photon.

ZEUS: RAPGAP/HERWIG-MI ( ) ZEUS: RAPGAP/HERWIG-MI ( ) (Nucl.Phys.B596,3(2001))(Nucl.Phys.B596,3(2001))

H1: RAPGAP/PYTHIA-MI ( ) H1: RAPGAP/PYTHIA-MI ( ) ((Eur. Phys. J. C41 (2005) 273-286 Eur. Phys. J. C41 (2005) 273-286 ))

Dijet production with a LN: Dijet production with a LN: jj+njj+n//jjjj

ok

Not okok

Not ok



SummarySummary

• High precision measurements of leading baryon production available High precision measurements of leading baryon production available from HERAfrom HERA

• LP spectra measured and well described by Reggeon-exchange LP spectra measured and well described by Reggeon-exchange modelmodel

• LN production characterized by rescattering effects (observed also in LN production characterized by rescattering effects (observed also in LP, not shown here): available models give a general good LP, not shown here): available models give a general good description of the datadescription of the data

• MC generators in general fail to reproduce the measured quantities MC generators in general fail to reproduce the measured quantities need to understand the process of leading baryon production and the need to understand the process of leading baryon production and the implementation of its mechanism in the generators.implementation of its mechanism in the generators.

• theory provides now a lot of predictions, experimental measurements theory provides now a lot of predictions, experimental measurements fundamental for model tuningfundamental for model tuning

Leading baryon study remains an important topic in HEP with a direct Leading baryon study remains an important topic in HEP with a direct impact on next experiments @ LHCimpact on next experiments @ LHC



LP absorption effectsLP absorption effects

leading baryon production at hera lorenzo rinaldi on behalf of h1 and zeus collaborations

Documents

rinaldi leading baryons

leading neutrons

leading neutron ln spectra

leading proton lp spectra

bslopes vs xl

neutron detection

ln production

flat cross section vs