Leading Baryon Production at HERA

• Recent Measurements

• Developments in Theory

• Comparisons to Monte Carlo Simulations

Kerstin Borras ICHEP 2006 RAS Moscow

(DESY)

HERA

Motivation

Standard target fragmentation:

• complex final-state with p or n in proton remnant

• or p stays intact

Particle exchange (à la Regge):

p: iso-scalar, iso-vector

(IR, ρ, a2 …)

n: iso-vector (π, ρ, a2 …)

• Significant fraction of ep scattering events contain a leading baryon in the final state carrying a substantial proportion of the energy of the incoming proton

• Production models not yet completely understood

p´,n p´,n

´,nxL

Kinematics and Vertex Factorization

Photon vertex:

lepton variables

Q2, x , W , yProton vertex:

leading baryon variables

xL = Ep´,n / Ep

t = (p – p´)2 - pT2 / xL

Cross section dependence on leading baryon variables

independent of kinematics at photon vertex

Violation of factorization in LB production similar as in diffraction: re-scattering processes can lead to absorption and migrations

W

Theoretical models (I)

p,n

Production of leading neutrons or protons during the fragmentation process.

check, if standard settings of the MC generators reproduce the measured xL

and pT2 distributions (see plots at the end of the

talk).

Theory: two absorption models available

1) Calculations from D’Alesio & Pirner: (EPJ A7(2000)109)

DIS: small (point-like) photon no re-scattering

PHP: large (hadron-like) photon re-scattering n

kicked to lower xL and higher pT (migration)

might escape detection (absorption)

Theoretical models (II)

2) One pion exchange in the framework of triple-Regge formalism

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

Nikolaev,Speth & Zakharov(hep-ph/9708290)

Enhanced absorptive corrections ( exclusive Higgs @ LHC), calculation of migrations, include also ρ and a2 exchange

(different xL & pT dependences)

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

Leading Baryon Detection

Forward Neutron Calorimeter (FNC) 10 λint Pb-Sc sandwich, σ/E

=0.65/√E, ∆Eabs=2%

Forward Neutron Tracker (FNT) Sc hodoscope @ 1λint, σx,y=0.23cm,

σθ=22μrad

Leading Proton Spectrometer (LPS) Six stations with silicon μ-strip

detectors σxL < 1%, σpT

5 MeV

H1: similar devices

Data samples:

neutron: 0.2<xL<1, θn<0.75mrad pT2 < 0.476 xL

2 GeV2

proton: 0.56<xL, pT2 varying with xL between pT

2 <0.15 and <0.5 GeV2

• DIS (ep Xn): 40pb-1, Q2>2GeV2

• PHP (γp Xn): 6pb-1, Q2<0.02 GeV2

• di-jets in PHP (γp jjXn): 40pb-1, Q2<1GeV2, 130<W<280GeV,

ETj1>7.5GeV, ET

j2>6.5, -1.5<ηj1,2<2.5

pT resolution

limited by beam spread: 50 MeV horizontal, 100MeV vertical

Leading neutrons: xL spectrum

• LN yield increases with xL due to increase in phase space: pT

2 < 0.476 xL2

• LN yield decreases for xL1 due to kinematic limit

DIS

• LN yield in PHP < yield in DIS factorization violation

• Models in agreement with data !

D’Alesio & Pirner:

σ~Wα, α(σγp) α(σ γ*p) &

Wπ2=(1-xL)Wp

2 (1-xL) -0.1

PHP DIS

Leading Neutrons: pT2 spectra in DIS

•Exponential behavior with slope b

•Intercept and exponential slope fully characterize the pT2

spectra

DIS

Leading Neutrons: pT2 spectra intercept

•intercept ~ cross section integrated over all pT

2

rise towards low xL

pT2=0

DIS

• Iso-spin Clebsch-Gordan for pure iso-vector exchange:

expect rLP=1/2 rLN

• but rLP > rLN other additional

exchanges needed in LP

pT2<0.04

DIS

Leading Neutrons: b - slopes

b-slopes in data not described by the b-slopes in the models.

´,nxL

One Pion Exchange Model:

s´= (cm energy of eπ system )2

fπ/p= pion flux factor

exponential fit

π

Leading Neutrons: b-slopes in PHP & DIS

•slopes different in PHP and DIS

•in general agreement with expectation from absorption more absorption @ small rnπ

depletion @ large pT

steeper slope in PHP

normalized @ pT2=0

Leading Neutrons & Protons: b-slopes

• slopes almost flat for LP (different additional exchanges)

• visible decrease for xL1 in LP can be due

to changing acceptance of LPS in pT2 for

increasing xL (for

small pT2 slope seems steeper as for larger pT

2)

Predictions from Theory (KMR)

Including other iso-vector exchanges, like ρ and a2,

• additional exchanges increase the cross section (e.g. for xL)

• but they decrease the b-slopes

difficult to describe all dependencies simultaneously

a2

Predictions from Theory (KMR)

In summary: adjustment of all available parameters with the precise leading baryon data gives not only valuable information for the absorptive effects at work in exclusive Higgs-production @ LHC,

but also determines the fluxes and the measurement of F2π.

Other exchanges flatten the pT2

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

Leading Neutrons: Comparisons to MC

• Lepto+MEPS best for xL spectra, but flat b-slope,

• Lepto+Ariadne, RAPGAP in standard mode, CASCADE cannot describe any of the distributions: too few neutrons, too low xL, b-slopes too flat.

Same situation for leading protons !

But: good MC description crucial

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

Factorization in PHP di-jets with LN ?

xobs

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

absorption (see also diffractive di-jets in PHP, previous talk)

Relevant variable: xγ

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

DIS direct PHP resolved PHP

hadz

jet1,2

ηT

obsγ )p(E

eE

x

Factorization in PHP di-jets with LN ?

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

not ok

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

ok

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

not ok ok

Summary

• a lot more, new and precise, leading baryon data available from HERA

• precise input for the determination the pion-flux and the pion structure function

• indications for absorption/migration observed

• theory provides now a lot of predictions can be used to further tune the absorption factors expected for the discovery channel of exclusive Higgs production @ LHC

• MC generators in general not describing the data need to understand the process of leading baryon production and the implementation of its mechanism in the generators.

Even in this old and traditional high energy physics topic a lot remains still to be understood which has direct impact

on the physics @ LHC