Charged Particle Multiplicity in DIS QCD Group Update M. Rosin, D. Kçira, and A. Savin University of Wisconsin L. Shcheglova Moscow State University, Institute of Nuclear Physics April 22, 2004

Outline Data selection & simulation Control Plots (LEPTO vs. ARIADNE) Track acceptance vs. pT & η New systematics: track pT Correction methods: Bin by bin (modified) Matrix Summary and plan

1996-97 Data sample 701,270 events after all cuts (38.46 pb-1) 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 ) For this initial study, I have looked at part of the 1996 data, And I used the following criteria to select my events. EVENT SELECTION: To ensure that the kinematic variables can be reconstructed with sufficient accuracy, I required a good positron be found coming from the vertex, I used a box cut to ensure that the positron is fully contained in the detector, And required the total E-pz to be near 55 Gev TRACK SELECTION: Trks associated with primary vtx, <+/-1.75, ensure tracks are in a region of high CTD acceptance, where detector response & systematics are well understood Pt>150, to ensure the track makes it thru the layers of the CTD KINEMATIC: Yel<.95 removes photoproduction Yjb>0.04 to guarentee accuracy for DA method (which is used to reconstruct Q2) The hadron angle depends on yjb, and the measurement of yjb is distorted bu Uranium noise in the cal for low yjb NOTES: E-pz. For e+ E=27.5, pz = -27.5 since E2-p2=m2 and m=0, so 27.5- - 27.5 = 55, for the proton, Ep=920, p=920, 920-920 =0 Total e-pz = 55 GeV E-pz removes photoproduction (scattered e is missing, q2 is low=0, electron goes in beam pipe, Ee=0.) removes evnts w/ large radiative corrections (electron looses energy by radiating a big photon, so Ee is less than 27.5) E-pz calculated from cells Q2da calculated from cells Yjb calculated from cells 701,270 events after all cuts (38.46 pb-1)

Event simulation Ariadne ’97 6v2.4 Matrix elements at LO pQCD O(s) Parton showers: CDM Hadronization: String Model Proton PDF’s: CTEQ-4D (Simulates both ’96 and ’97 data; no changes in detector) Proton PDF’s are parameterized from experimental data Detector simulation besed on JAY-AUNT Luminosity of MC : 36.5 pb-1

1996-97 Data vs. ARIADNE & LEPTO LEPTO & ARIADNE show a similar level of agreement to kinematic variables.

Meff & Tracks LEPTO better agreement with Meff ARIADNE better agreement with tracks Further study needed : is the ratio of generated charged to neutral particles different for the two MC’s?

Effect on final results <2% LEPTO vs. ARIADNE Effect on final results <2%

Track acceptance vs. η and pT Eta range for analysis within region of high track acceptance Track pT cut where acceptance is high and well behaved

Systematics for track pT Track pT cut varied from 0.12 GeV to 0.20 GeV Effect on final results less than 1%

Correction to hadron level: bin by bin Part one: correct to hadron level using only hadrons generated with pT > 0.15 GeV Part two: correct for hadrons with lower pT, using ratio of <gen> with pT cut to <gen> no pT cut in each bin.

Correction to hadron level: bin by bin Correction for detector effects Correction of hadrons of pT > 0.15 to all hadrons

Correction to hadron level: matrix No. of events with p tracks generated when o tracks were observed No. events with o tracks observed Mp,o = The matrix relates the observed to the generated distributions in each bin of Meff by:

Comparison of correction methods Agree within 5% Now we look at a previous ZEUS investigation of the degree…, A possible explanation is initial gluon radiation at HERA . Further study in this area could lead to better understanding… NOTES: The obtained results can be considered as possible evidence that in the inclusive production about 85% of final state hadrons are produced by non-perturbative mechanism (confinement) while about 15% of the mean charged multiplicity is contributed by the hard perturbative mechanism of color charge radiation

<nch> vs. Meff Good agreement with 95 prelim. points Lydia’s points are a global systematic check that the results don’t depend on the trigger selection (She uses DIS01 & DIS03 – we use only DIS03) Now we look at a previous ZEUS investigation of the degree…, A possible explanation is initial gluon radiation at HERA . Further study in this area could lead to better understanding… NOTES: The obtained results can be considered as possible evidence that in the inclusive production about 85% of final state hadrons are produced by non-perturbative mechanism (confinement) while about 15% of the mean charged multiplicity is contributed by the hard perturbative mechanism of color charge radiation

Summary Plan LEPTO & ARIADNE simulate data well LEPTO: ~5% systematic error at high Meff LEPTO shows better agreement to Meff ARIADNE shows better agreement to tracks More study needed (ratio of generated charged to neutral particles different?) Agreement between correction methods less than 2% Agreement between 1st and 2nd analyses less than 1% <nch> vs. Meff agrees with 1995 ZEUS preliminary results Plan Make preliminary Look at Breit frame for consistency check Study diffractive events; combine ARIADNE & RAPGAP Including all 96 Data will increase my statistics an order of magnitude Total luminosity for my study 0.06 pb-1; total for 1996 ~12pb-1 Diffraction: Events with low momentum transferred to the hadron.