1. 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

2. 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

3. 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 = since E2-p2=m2 and m=0, so = 55, for the proton, Ep=920, p=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)

4. 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

5. 1996-97 Data vs. ARIADNE & LEPTO
LEPTO & ARIADNE show a similar level of agreement to kinematic variables.

6. 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?

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

8. 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

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

10. 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.

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

12. 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:

13. 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

14. <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

15. 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.