Recent results on multiplicity from ZEUS

1. Recent results on multiplicity from ZEUS Michele Rosin University of Wisconsin, Madison on behalf of the ZEUS Collaboration Hadron Structure 2004 September 1, 2004

2. DESY Hamburg, Germany H1 ZEUS e’(k’) e(k)  (q) remnant p(P) q’ 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: Inelasticity 0 ≤ y ≤ 1 Virtuality of photon Fraction of p momentum carried by struck parton

3. e+e- & ep : Breit Frame DIS event • 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-. Lab Frame Breit Frame PT Breit Frame PL

4. Measurement vs. Q • Consistent with e+e- data for high Q2 • Disagreement at low Q2 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

5. 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 2nd 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-

6. 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

7. q q q q g g Gluon radiation, Q, and 2*EBreit Soft Contribution Hard 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 (Q2 is not) • Effect is more pronounced for low Q2 : more low energy gluons • Must use 2*EBreit and 2*Nch for comparing with e+e- N < N expected With migrations: No migrations:

8. <nch> vs. 2*Ecurrent • Measurement of multiplicity dependence on 2*Ecurrent compared to previous ZEUS measurement vs. Q • 2*E gives better description of multiplicity at lower energy • This approximation of invariant mass partially takes into account the real distribution of the particles. • Lepto prediction slightly above Ariadne • Current region understood, would like to use some energy scale to compare target region for ep to e+e-..

9. Visible multiplicity in Breit and hadronic center of mass (HCM) frames Visible Part • Breit Frame: 90% of hadrons in current region visible in detector, only 30% of target region hadrons are visible • Need some other way to investigate these particles All Hadrons Current Region Breit Frame Proton remnant

10. Ecms =W E E p g Nphoton region Nproton region Ecms/2 Ecms/2 Nphoton region vs W Hadronic center of mass frame

11. Visible multiplicity in Breit and hadronic center of mass (HCM) frames • HCM Frame: We see a bigger number of particle in the detector than in the current region of the Breit frame • Photon region dominated by contribution from target region of Breit frame (~80% of visible hadrons) • Proton region unseen in detector • Using HCM is intuitive because all interactions in e+e- and pp happen in center of mass frame Visible Part All Hadrons Current Region Breit Frame Proton remnant Visible Part All Hadrons Photon Region HCM Frame Proton Region HCM Frame

12. Multiplicity vs. W in HCM • Measurement of Multiplicity in photon region of HCM frame vs. W. • Lepto prediction slightly above that for Ariadne. • Interesting to multiply the multiplicity measured in both the current region of the Breit frame and the photon region of the HCM frame by 2 (there are 2 hemispheres) and compare to e+e-

13. Multiplicity in current region of Breit and HCM frames compared to e+e- and pp • Measurements in current region of Breit frame and photon region of HCM frame multiplied by 2. • There is agreement with e+e- at low and high energy and with the pp results which are plotted vs. a scale that takes into account the leading particles. • Would also like to make a measurement vs. invariant mass, where we measure visible multiplicity and plot it vs. visible energyless model dependence.

14. Study: <nch> vs. Meff CAL within the CTD acceptance CTD Charged Hadrons & Effective Mass:experimental method • Measure hadronic final state within  for best acceptance in the central tracking detector (CTD) • Measure # charged tracks, reconstruct number of charged hadrons • Measure invariant mass of the system (Meff) in corresponding delta eta region. • Energy is measured in the Calorimeter (CAL)

15. Lab frame: <nch> vs. Meff in x bins • Lab frame multiplicity vs. Meff, shown in 4 xbins, with Ariadne predictions. • x range split into similar bins as in previous multiplicity paper. • weak x dependence in both data and MC observed. • Q2 dependence? => next page

16. Lab frame: x and Q2 bins • Data described by ARIADNE • LEPTO slightly above data • No Q2dependence observed

17. Summary • The mean charged multiplicity has been investigated in DIS ep scattering • Measurement in current region of the Breit frame show similar dependence to e+e- if 2*Ecurrent is used as the scale, the same dependence is obs for the pho reg of hcm fra vs W. • The mean ch multi in the lab frame has been also meas vs. inv mass of resp hadronic system. There is a weak x depen, and almost no Q dep were observed.