1. Recent multiplicity studies at ZEUS, Michele Rosin U. WisconsinHadron Structure 2004, Sept. 1st 2004 1 University of Wisconsin, Madison on behalf of the ZEUS Collaboration Hadron Structure 2004 September 1, 2004 Recent results on multiplicity from ZEUS Michele Rosin

2. Recent multiplicity studies at ZEUS, Michele Rosin U. WisconsinHadron Structure 2004, Sept. 1st 2004 2 HERA description & DIS kinematics 920 GeV p + (820 GeV before 1998) 27.5 GeV e - or e + 318 GeV cms (300 GeV) Equivalent to a 50 TeV Fixed Target DIS Kinematics: DESY Hamburg, Germany ZEUS H1 remnant e(k) e’(k’) p(P)   (q) q’ Virtuality of photon Inelasticity 0 ≤ y ≤ 1 Fraction of p momentum carried by struck parton

3. Recent multiplicity studies at ZEUS, Michele Rosin U. WisconsinHadron Structure 2004, Sept. 1st 2004 3 Multiplicity e + e - and pp  Agreement between e + e - and pp plotted vs. pp invariant mass e+e-e+e- pp Invariant mass of pp e+e-:e+e-: pp vs. √q 2 had pp vs. √s pp

4. Recent multiplicity studies at ZEUS, Michele Rosin U. WisconsinHadron Structure 2004, Sept. 1st 2004 4 e + e - & ep : Breit Frame DIS event Lab Frame Breit Frame  Breit Frame definition:  “Brick wall frame” incoming quark scatters off photon and returns along same axis.  Current region of Breit Frame is analogous to e + e -. PTPT PLPL

5. Recent multiplicity studies at ZEUS, Michele Rosin U. WisconsinHadron Structure 2004, Sept. 1st 2004 5 Measurement vs. Q  Consistent with e+e- data for high Q 2  Disagreement at low Q 2 may be attributed to gluon radiation  Idea of current analysis: Understand current and target multiplicity and compare to e+e- European Physics Journal C11 (1999) 251-270

6. Recent multiplicity studies at ZEUS, Michele Rosin U. WisconsinHadron Structure 2004, Sept. 1st 2004 6 Multiplicity: ep vs. e + e - (1)  e+e-: boson with virtuality √s produces 2 quarks & hadronization is between 2 colored objects q and q  ep: boson with virtuality Q produces 1 quark: the 2 nd quark comes from the interaction between the photon and the proton  current region of Breit frame for ep similar to one hemisphere of e+e-  If we use Q as scale to compare to e+e-, multiply hadrons by 2

7. Recent multiplicity studies at ZEUS, Michele Rosin U. WisconsinHadron Structure 2004, Sept. 1st 2004 7 Multiplicity: ep vs. e + e - (2)  ep: Split into Current and Target Region – one string two segments.  In ep we have a color field between 2 colored objects the struck quark and the proton remnant  When we use Q2 as a scale we are assuming the configuration is as symmetric as it is in e+e-, but it isn’t  This asymmetric configuration leads to migration of particles from the current region to the target region Breit Frame diagram

8. Recent multiplicity studies at ZEUS, Michele Rosin U. WisconsinHadron Structure 2004, Sept. 1st 2004 8 Gluon radiation, Q, and 2*E Breit q q g q q g Soft ContributionHard Contribution QCD Compton  In hard and soft processes gluon radiation occurs  These gluons can migrate to target region  Total energy in the current region of Breit frame and multiplicity are decreased due to these migrations (Q 2 is not)  Effect is more pronounced for low Q 2 : more low energy gluons  Must use 2*E Breit and 2*N ch for comparing with e+e- No migrations: With migrations: N < N expected

9. Recent multiplicity studies at ZEUS, Michele Rosin U. WisconsinHadron Structure 2004, Sept. 1st 2004 9 Multiplicity vs. 2*E breit  Measure multiplicity dependence on 2*E Breit and compare to previous ZEUS measurement vs. Q, and to e+e-  Points agree with the e + e - points  This approximation of invariant mass partially takes into account the real distribution of the particles.  Current region understood, would like to use some energy scale to compare target region for ep to e+e-..

10. Recent multiplicity studies at ZEUS, Michele Rosin U. WisconsinHadron Structure 2004, Sept. 1st 2004 10 Visible multiplicity in Breit and hadronic center of mass (HCM) frames  Breit Frame: 90% of hadrons in current region visible in detector, only 30% of target region hadrons are visible  HCM Frame: Particles in proton hemisphere unseen in detector. Photon region dominated by contribution from target region of Breit frame (~80% of visible hadrons) Visible Part All Hadrons Current Region Breit Frame Proton Region HCM Frame Photon Region HCM Frame Visible Part Proton remnant

11. Recent multiplicity studies at ZEUS, Michele Rosin U. WisconsinHadron Structure 2004, Sept. 1st 2004 11 Moving to the hadronic center of mass (HCM) frame Using HCM frame is intuitive because all interactions in e+e- and pp happen in the center of mass frame. Photon region of HCM frame has BIG contribution from target region of Breit frame To be consistent with Breit frame measurement we measure 2 vs. 2*E of correspondent hadronic system

12. Recent multiplicity studies at ZEUS, Michele Rosin U. WisconsinHadron Structure 2004, Sept. 1st 2004 12 Multiplicity vs. W in HCM

13. Recent multiplicity studies at ZEUS, Michele Rosin U. WisconsinHadron Structure 2004, Sept. 1st 2004 13 Multiplicity in current region of Breit and HCM frames compared to e+e- and pp  Measurement in current region of Breit frame agree with e+e- and pp points.  Measurement in photon hemisphere of HCM frame agrees with LEP points. Request for preliminary

14. Recent multiplicity studies at ZEUS, Michele Rosin U. WisconsinHadron Structure 2004, Sept. 1st 2004 14 Summary  The hadronic final state has been investigated in DIS ep scattering in terms of the mean charged multiplicity and respective invariant mass of the charged and neutral particles, M eff  Measurement in current region of the Breit frame show same dependence as e+e- if 2*E current is used as the scale  Similar dependence observed in multiplicity for visible part of the current and target regions of the Breit frame as a function of their respective effective mass  Lab frame measurements show no strong dependence on x or Q 2  Full comparison of current and target region is still underway, the main problem being that visible part of the target region is only 30% of the total, and MC studies expect that the multiplicity behavior changes when we go closer to the proton remnant.