Charged Particle Multiplicity in DIS

1. Charged Particle Multiplicity in DIS Request for Preliminary M. Rosin, D. Kçira, and A. Savin University of Wisconsin L. Shcheglova Moscow State University, Institute of Nuclear Physics July 5th, 2004 http://www-zeus.desy.de/~sumstine/multip

2. Introduction • Details: http://www-zeus.desy.de/~sumstine/multip • short introduction to the method • new measurement in Breit Frame • Plots for preliminary

3. Study: <nch> vs. Meff CAL within the CTD acceptance CTD Charged Hadrons & Effective Mass • Measure HFS within  for best CTD acceptance • 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 CAL

4. The use of Meff as energy scale • For ep in lab frame, measure visible part of <nch> vs. visible part of energy available for hadronization: Meff Whad Meff Lab Frame Visible part Meff: HFS measured in the detector where the tracking efficiency is maximized

5. 1996-97 Data sample • Event Selection • Scattered positron found with E > 12 GeV • A reconstructed vertex with |Zvtx| < 50 cm • Scattered positron position cut: radius > 25cm • 40 GeV < E-pz < 60 GeV • Diffractive contribution excluded by requiring ηmax> 3.2 • Track Selection • Tracks associated with primary vertex • || < 1.75 • pT > 150 MeV • Physics and Kinematic Requirement • Q2 da > 25 GeV2 • y el < 0.95 • y JB > 0.04 • 70 GeV < W < 225 GeV ( W2 = (q + p)2 ) 735,007 events after all cuts (38.58 pb-1)

6. Event simulation • Ariadne ’96-’97 4.08 • Matrix elements at LO pQCD O(s) • Parton showers: CDM • Hadronization: String Model • Proton PDF’s: CTEQ-4D • Lepto 6.5.1 • Including SCI • Also used Lepto without SCI Luminosity of MC : 36.5 pb-1

7. Kinematic Variables, Meff and Tracks • 96-97 data compared to ARIADNE and LEPTO w/SCI • Both ARIADNE and LEPTO show good agreement for kinematics, Meff & tracks

8. Comparison of e+e-, pp, & ep • agreement between e+e- and pp for <nch> vs. invariant mass. • <nch> * 2 vs. Q for ep measured in the current region of the Breit Frame (CRBF) agree with e+e- and pp for higher Q • Lab frame points lie above e+e- and pp, and above the previous ZEUS measurement in CRBF. • ARIADNE describes data dependence, LEPTO higher than the data, LEPTO w/ SCI even higher. Used both for correcting data: systematic errors small • Agree with 1995 preliminary measurement of <nch> vs. Meff in lab frame • Request for preliminary

9. Lab frame: <nch> vs. Meff in x bins • Check if ep vs. e+e- and pp difference is due to quark and gluon distributions: study x and Q2 dependence • x range split into similar bins as in previous multiplicity paper. • weak x dependence in both data and MC observed not sufficient to explain difference • Request for Preliminary • Q2 dependence? => next page

10. Lab frame: x and Q2 bins • Data described by ARIADNE • LEPTO above data • No Q2dependence observed • Request for preliminary

11. Breit Frame - Change to scale Meff • Using Meff agreement between: • Lab frame • Breit Frame Current Region • Breit Frame Target Region • Number of charged hadrons plotted vs. Meff shows similar behavior for the lab frame, and current & target region of Breit frame • Request for Preliminary

12. PT PL Invariant Mass Invariant Mass in the Current Region Current Hemisphere

13. E θ = π θ = 0 E θ E E E E Invariant Mass Calculation With the same energy of the particles, one gets very different invariant mass of the system depending on the system configurations.

14. E θ = π θ = 0 E θ E E E E Invariant Mass Calculation With the same energy of the particles, one gets very different invariant mass of the system depending on the system configurations.

15. “Mirrored” invariant mass Invariant Mass Construction Current region Breit Frame • Assume system is collimated • Expect inv. Mass of CRBF to change significantly if we “mirror” each particle, with another particle with the same energy but opposite momentum. • M2 = (Eobs+Emirrired)2-(pobs-pmirrored)2 = 2E • Total number of particles increased by 2 collimated

16. Breit Frame - Change to scale Meff • Points lie slightly above the e+e- points. • This approximation of invariant mass partially takes into account the real distribution of the particles. • Similar studies for target region are hampered by the fact that the analogy between our target region and the target region in pp is a bit more complex. • Request for Preliminary

17. Correlated & Uncorrelated Systematics

18. Comparison to 2nd analysis • Agreement between 1st and 2nd analysis within 1% for all preliminary plots: • Lab Frame • Current Region Breit Frame • Target Region Breit Frame • X bins • X and Q2 bins • Current Region Breit Frame vs. 2*E

19. Summary Measured mean charged multiplicity in the Lab and Breit frame. Compared to predictions, to other ZEUS data, and e+e- Difference ep vs. e+e- and pp: • not due to experimental measurement. • not due to the choice of frame (Lab Breit). • not due to quark/gluon distributions. • is due to choice of scale (Q vs. Meff) Plans for the future • Finish checks in the Breit frame. • Run on Lepto without SCI • Make results preliminary: soon!

20. Invariant Mass Calculation Isotropic configuration Configuration with leading particles

21. Multiplicity e+e- and pp e+e- e+e-: pp vs. √q2had pp vs. √spp pp Invariant mass of pp • Agreement between e+e- and pp plotted vs. pp invariant mass