Hadronization in DIS at Valeria Muccifora (on behalf of the HERMES collaboration) SQM2003 Atlantic Beach 12-17/03/03 Medium modification: Parton Distributions and Fragmentation Functions. Models for hadronization in DIS. Nuclear attenuation measurements. Conclusions and outlook

Medium Modification of Parton Distributions Inclusive DIS on nucleons -> Parton Distributions Inclusive DIS on nuclei -> EMC effect: Reduction of the structure function of a nucleus in the valence quark region (Bjorken variable x>0.4) compared to that of free nucleon. G.A.Miller and J.R.Smith P.R.C 65 (2002)015211: …the depletion of DIS structure function observed in the valence quark is due to some interesting effect involving dynamics beyond the conventional nucleon-meson treatment of nuclear physics. V.Muccifora SQM2003

Parton Fragmentation Semi-Inclusive DIS (SIDIS) of high energy leptons on nucleons: =E-E’ x= Q 2 /2M Z =E h / d  h (z)  f q f (x)·d  f ·D f h (z)  DF FF FFs are measured in e + e - and follow pQCD evolution like DFs. SIDIS multiplicities are also good measurements of FFs: V.Muccifora SQM2003

Multiplicities vs FFs HERMES Eur. Phys. J. C21 (2001) 599 Factorization holds in the HERMES kinematics. Isospin invariance in the production of charged and neutral pions Agreement with NLO QCD evolution of FFs measured at LEP. V.Muccifora SQM2003

Medium Modification of Fragmentation Nucleus acts as an ensemble of targets: reduction of multiplicity of fast hadrons due to both hard partonic and soft hadronic interactions. Nuclear Attenuation: Medium modification of FF D f h (z) Hadron formation time (  f =l f /c) =E-E’ x= Q 2 /2M Z =E h / V.Muccifora SQM2003

Formation times   dependence of the dilepton mass spectrum produced in heavy ion collision at GSI. P.L.B 411 (1997) 187. E.P.J. C5 (1998) 349.  p dependence of antiproton yield produced in p+A reaction at AGS. Nucl. Phys. A707 (2002) 224. V.Muccifora SQM2003

Models for nuclear attenuation Direct production: One-time scale  f. Bialas Chmaj, P.L.B 133 (1983) 241 String model: Two-time scale model  c,  f.  f -  c =cE h /k=cz /k. (string constant k  1 GeV/fm). Bialas Chmaj, P.L.B 291 (1987) 793 V.Muccifora SQM2003

FF Modification Model Guo, Wang PRL 85 (2000) 3591 FF and their QCD evolution are described in the framework of multiple parton scattering and induced radiation. The emitted g and the leading q propagate coherently  Landau-Pomeranchuk-Midgal interference effects. Different modification of quark and antiquark FF. Rescattering without g-radiation: p t -broadening. Rescattering with another q : mix of q and g FF. g-rescattering including g-radiation: dominant contribution in QCD evolution of FF.

FF Modification Model D q  h (z h,Q 2,A) ´ =D q  h (z h,Q 2 )+  D q  h (z h,Q 2 )  D q  h (z h,Q 2 ) ' C A  s 2 /N c C/Q 2 x 2 A  d q  h (z h,x B,x A,Q 2 ). Equivalent to rescale the FF for the fraction of the parton energy loss  E/E: D(z)  Wang, Guo N.P. A 696 (2001) 788Arleo JHEP 11 (2002) 44. Quark energy loss shift its energy: and leads to a z shift: V.Muccifora SQM2003

Rescaling Model In the nucleus:  A >  N  Q 2 rescaling q f A (x,Q 2 )=q f (x,  A (Q 2 )Q 2 ) D f h|A (z,Q 2 )=D f h (x,  A (Q 2 )Q 2 ) Scale factor related to deconfinement scale: Q 2 appears scaled either by  (mass scale parameter) or by  2 (low momentum cutoff): 1/  2   2 Accardi, Muccifora, Pirner, nucl-th , NPA in print. Multiplicity appears with rescaled DF and rescaled FF: (Supplemented by nuclear absorption in terms of string model) )(2 )( )( Q Q Q s A N A              

HERMES at DESY HERMES studies the spin structure of the nucleon … and not only.. V.Muccifora SQM2003 E=27.5 GeV e + (e - ) I ~ 30 mA p beam of 920 GeV, not used by HERMES Internal target polarized and unpolarized gas. Last part of the fill:high-density unpolarized runs.

HERMES Spectrometer e + identification: 99% efficiency and <1% of contamination. Particle identification: Cerenkov (RICH ’98), TRD, Preshower, e.m. Calorimeter. 14 N: Cerenkov  4< P  <14 GeV. 84 Kr: RICH  , K, p, p separation 2.5<P<15 GeV. - V.Muccifora SQM2003

Hadron attenuation vs transfer energy HERMES kinematics optimal to study medium effect in FF. Increase with consistent with the EMC data at higher energy. Discrepancy with SLAC mainly due to the EMC effect HERMES, EPJ C 20 (2001) 479. HERMES, hep-ex/ EMC Coll. Z.Phys. C52 (1991) 1. SLAC PRL 40 (1978) V.Muccifora SQM2003

Hadron attenuation vs z=E h / HERMES data provide information in the unexplored region z>0.8. V.Muccifora SQM2003 EMC SLAC

Charged hadron/pion on 14 N No difference between  + and  - Difference between h + (  +,K +,p) and h - (  -,K -,p-).  different t h for different hadrons  RICH to clarify the differences for different h V.Muccifora SQM2003  different FF modification for different hadrons

Attenuation on 84 Kr vs with hadron ID RICH ID:  /K ( GeV), p/p (4-15 GeV). - +  +  - and    K - K + > K - p> p and p>  /K - V.Muccifora SQM2003

Attenuation on 84 Kr vs z with hadron ID Possible interpretation: Different FF modification for q and q. - Different t f for mesons and baryons. Different hadronic cross sections:   + =   -   20 mb.   +   17 mb   -   23 mb.  p   40 mb  p   60 mb - V.Muccifora SQM2003

Hadron attenuation vs FF modification (parton scattering and induced radiation without hadron- rescattering) E.Wang, X.N. Wang PRL 89 (2002) First evidence of the quadratic nuclear size dependence [A 2/3 ] th due to QCD radiation spectrum and LPM interference. 1 free parameter C  quark-gluon correlation strength in nuclei. From 14 N data C= GeV 2 : V.Muccifora SQM2003

Gluon density by dE/dL E.Wang, X.N. Wang PRL 89 (2002)  E sta   0 R A 2 ;  0 gluon density and R A  6 fm  exp   E sta (2  0 /R A );  0 initial formation time of dense medium PHENIX: hot, expanding system. HERMES: cold, static system.   V.Muccifora SQM2003 dE/dL PHENIX  Au predictions determined by using C= GeV 2 from HERMES data.  0.5 GeV/fm for 10-GeV quark in Au. Gluon density in the initial hot stage of Au+Au is  15 times higher than in cold nuclear matter.

F. Arleo JHEP 11 (2002) 044.   s Q L 2 Transport coefficient Q =0.75 GeV/fm 2 derived from Drell-Yan  Hadron attenuation vs FF modification dE/dL= /L  dE/dL=0.62 GeV/fm for L  5 fm. V.Muccifora SQM2003

Q 2 -rescaling + nuclear absorption charge and flavor dependence Different cross sections for different hadrons (isospin averaged) Q 2 -rescaling: upper curves Q 2 -rescaling + Nuclear absorption (LUND string model for hadron formation and interaction): lower curves Nice agreement for  +,  - and K +. K - data show higher absorption. V.Muccifora SQM2003

Q 2 -rescaling + nuclear absorption charge and flavor dependence Discrepancy between data and predictions for K - suggests a stronger absorption mechanism for particles that do not contain any valence quark. V.Muccifora SQM2003 Shorter hadron formation length since rank 1 hadrons would not partecipate in the K - formation in the model: K - and K + (p and p) data can provide new insight about formation length -

In the lepton-nucleon scattering neither multiple scattering of the incident particle nor interaction of its constituents complicate the interpretation.  Clean and reliable information on quark transport in ‘cold’ nuclear matter.  Attenuation vs p t 2 (Cronin effect ) The hard component of the incoherent parton scattering becomes dominant at p t  1-2 GeV. The higher p t enhancement in Kr suggest A-dependence for Cronin in DIS. EMCHERMES V.Muccifora SQM2003

Direct measurements of multiparton correlation functions  broadening depends on multiparton correlation function T qF A and on nuclear size  A 1/3 :  p t -broadening in SIDIS HERMES data, on 5 nuclei and for , K, p and p, will provide infos on the A- and flavor-dependence of. - V.Muccifora SQM RHIC: different for , K, p and p.

 Conclusions & Outlook  First observation of hadron-type dependence of the attenuation.  Cronin effect has been observed. The HERMES measurements are providing information on: Nuclear modification of the quark Fragmentation Functions, Hadronization times, Partonic energy loss and Cronin effect. GOAL: To obtain unambiguos informations on hadron and quark transport in Cold Nuclear Matter Near Future:  p t -broadening for , K, p and p.  Collected data on 4 He, 20 Ne  New insights on the A-dependence. -  Nuclear attenuation in a new region of kinematic plane for 14 N and 84 Kr and for the first time for identified  +,  -, K +, K -, p, p. -

Falter et al. nucl-th/ FSI by means of a BUU transport model. Input parameter  f  Hadron FSI and formation times R M h very sensitive to the interaction cross section of the propagating leading hadrons. Average  f >0.3 fm/c compatible with the values extracted from p+A reactions at AGS. V.Muccifora SQM2003