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Leading Baryon Production at HERA

Leading Baryon Production at HERA. HERA. Recent Measurements Developments in Theory Comparisons to Monte Carlo Simulations. Kerstin Borras ICHEP 2006 RAS Moscow (DESY). Motivation.

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Leading Baryon Production at HERA

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  1. Leading Baryon Production at HERA HERA • Recent Measurements • Developments in Theory • Comparisons to Monte Carlo Simulations Kerstin Borras ICHEP 2006 RAS Moscow (DESY)

  2. Motivation • 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 • 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 …)

  3. xL ´,n Kinematics and Vertex Factorization Photon vertex: lepton variables Q2, x , W , y Proton 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 W Violation of factorization in LB production similar as in diffraction: re-scattering processes can lead to absorption and migrations

  4. 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 kickedto lower xL and higher pT (migration)  might escape detection (absorption) Theoretical models (I) 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). p,n

  5. 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) Theoretical models (II) 2) One pion exchange in the framework of triple-Regge formalism

  6. 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 pT resolution limited by beam spread: 50 MeV horizontal, 100MeV vertical • Data samples: • neutron: 0.2<xL<1, θn<0.75mrad  pT2 < 0.476 xL2 GeV2 • proton: 0.56<xL, pT2 varying with xL between pT2 <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, ETj2>6.5, -1.5<ηj1,2<2.5

  7. D’Alesio & Pirner: PHP DIS σ~Wα, α(σγp) α(σγ*p) & Wπ2=(1-xL)Wp2 (1-xL) -0.1 • LN yield in PHP < yield in DIS  factorization violation • Models in agreement with data ! Leading neutrons: xL spectrum DIS • LN yield increases with xL due to increase in phase space: pT2 < 0.476 xL2 • LN yield decreases for xL1 due to kinematic limit

  8. Leading Neutrons: pT2 spectra in DIS DIS • Exponential behavior with slope b • Intercept and exponential slope fully characterize the pT2 spectra

  9. pT2<0.04 DIS • Iso-spin Clebsch-Gordan for pure iso-vector exchange: expect rLP=1/2 rLN • but rLP > rLN other additional exchanges needed in LP Leading Neutrons: pT2 spectra intercept pT2=0 DIS • intercept ~ cross section integrated over all pT2 rise towards low xL

  10. Leading Neutrons: b - slopes One Pion Exchange Model: xL π ´,n s´= (cm energy of eπ system )2 fπ/p= pion flux factor exponential fit b-slopes in data not described by the b-slopes in the models.

  11. slopes different in PHP and DIS • in general agreement with expectation from absorption  more absorption @ small rnπ  depletion @ large pT  steeper slope in PHP Leading Neutrons: b-slopes in PHP & DIS normalized @ pT2=0

  12. Leading Neutrons & Protons: b-slopes • slopes almost flat for LP (different additional exchanges) • visible decrease for xL1 in LP can be due to changing acceptance of LPS in pT2 for increasing xL(for small pT2 slope seems steeper as for larger pT2)

  13. 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

  14. Predictions from Theory (KMR) p p,r,a2 Other exchanges flatten the pT2 distributions in both, a bit more in PHP than in DIS 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.

  15. Lepto+MEPS best for xL spectra, but flat b-slope, • Lepto+Ariadne, RAPGAPin 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  Leading Neutrons: Comparisons to MC Assuming the leading baryon production proceeds via the standard fragmentation process  do standard MC generators describe the data ?

  16. Factorization in PHP di-jets with LN ? resolved PHP DIS direct PHP xobs 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.

  17. H1: RAPGAP/PYTHIA-MI (Eur. Phys. J. C41 (2005) 273-286 ) not ok ok Factorization in PHP di-jets with LN ? ZEUS: RAPGAP/HERWIG-MI (Nucl.Phys.B596,3(2001)) not ok ok No possibility to decide on factorization breaking due to re-scattering processes in resolved PHP (xγ<1) with hadron-like photon.

  18. 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 

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