1. Charged Particle Multiplicity in DIS
ZEUS Collaboration Meeting
M. Rosin, D. Kçira, and A. Savin
University of Wisconsin
L. Shcheglova
Moscow State University, Institute of Nuclear Physics
March 4, 2004

2. Outline
Universal dependence of mean charged multiplicity, <nch>, on effective energy going into particle production, Whad, for e+e-, pp, and ep.
Introduction of Meff and motivation for its use as an energy scale
Data selection & simulation
Resolutions and systematics
Measurements of <nch> vs. effective mass
Comparison to second analysis
Trigger studies
Summary and plan

3. Early experimental evidence for universality
The current region in the Breit frame is analogous to a single hemisphere of e+e- annihilation; Q for ep reactions corresponds to Whad for e+e- reactions
Mean charged multiplicity, <nch> , vs. Q shows logarithmic dependence for both e+e- and ep on the effective energy going into hadronization
Universal dependence of <nch> observed in e+e- and ep reactions in Breit frame.
Now move to lab frame; see the effects of target region
e+e- ep
Whad is root s for e+e-
In e+e- interaction the entire energy of the colliding particles goes into the prod of final state hadrons
In order to determine the energy used in hadronization for ep and hh the energy of the leading particles should be subtracted from the cms energy of the incident particles.
In the current region of the Breit frame Q at hera corresponds to Whad. (the effective energy going into hadronization)

4. Motivation for the use of Meff as energy scale
Similarity of particle production at e+e- and ep colliders
Whad
Similarity of Whad dependence on <nch> has been observed
A common Whad dependence on <nch> implies the production of secondary particles is similar in the different interactions
Study the dependence of <nch> of the observed part of the produced HFS on it’s total invariant mass, Meff
Lab Frame
Whad
If you look at hadron production in e+e-, ep and ppbar, what you see is there is a section of the process where the hadrons get produced through a similar process.
If you compare e+e- and ep, you see the difference is that in ep I have the proton remnant to contend with.
I can test how similar the process is by studying the universality. This can be done by measuring…
energy scale for the analysis is Meff, then Can look at just part of the string: assumed to give final state particles proportional to logarithm of mass
Meff : the effective mass of a part of the produced multi-hadronic final state measured in the detector, unlike Whad, which corresponds to the hadronic final state measurement in full phase space
Meff
Whad: HFS measured in full phase space
Meff: HFS measured in the detector where the tracking efficiency is maximized

5. 1996-97 Data sample Event Selection Track Selection
Scattered positron found with E > 12 GeV
A reconstructed vertex with |Zvtx| < 50 cm
scattered positron position cut: |x| > 15 cm or |y| > 15cm (in RCAL) “Box cut”
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
705,381 events
after all cuts
(38 pb-1)

6. 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 : 2.48 pb-1

7. Validation of analysis method
Can we look at just the observed part of the HFS?
Study η dependence using generated events
The dependence of <nch> on the Meff of the produced system for ep generated events is the same in different regions of phase space
Can use the observed part of the produced HFS, with good tracking (|h| < 1.75) for studying this dependence

8. Resolutions of kinematic variables
Resolutions well behaved
Use standard deviations for excursions in systematic studies
s = 22 %
s = 15.5 cm
s = 10 %
s = 6.8 %

9. Correlated & Uncorrelated Systematics
Change
% Difference in Meff bins
Bin 1
Bin 2
Bin 3
Bin 4
Bin 5
Ee’
± 1 GeV
< 0.5%
0.8 %
1.2%
Box Cut
± 1cm
0.67%
0.62% Q2
± 2.25 GeV2
3.35%
2.08%
2.17%
0.91%
yJB
± .008
yel
± .05
Zvtx
± 15 cm
0.53%
0.75%
W (upper)
± 15 GeV
2.0%
W (lower)
± 7 Gev
E - pz
± 2 GeV |
CAL energy scale
± 3 %
1.1%
1.4%
1.3%
< 0.5%

10. 1995 ZEUS measurement in lab frame
ZEUS ’95 PRELIMINARY RESULT (never published)
Compare <nch> vs. Meff dependence in e+e-, pp, and ep (ZEUS).
<nch> proportional to log Meff
<nch> 15% above corresponding e+e-
Suggestion: difference due to ep color dynamics at the pre- hadronization stage.
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

11. <nch> vs. Meff : ’96-’97 vs. ’95
This analysis compared to 1995 study
Data corrected to hadron level
Full error bars: statistical & systematic uncertainties added in quadrature
Inner error bars: statistical uncertainties
Reasonable agreement with ZEUS preliminary result

12. Comparison to 2nd analysis
Main difference:
M.R. using ORANGE D.K. is not
Good agreement after much work!
tracks: D.K. using vceaze
cell energy: M.R. calculating imbal. after corrections
1% diff. caused by Ee’
MicheleRosin
<nch>
Dorian Kçira

13. Trigger studies by L. Shcheglova
Group
Run Range
#Runs
Non-prescaled DIS01
DIS03 1
445
Yes, 12x6
14x14 2
795 - 3 11
427
r >25 4
196
2500
Lydia has investigated the possibility to go to lower Q2.
Because of changing prescales for DIS01 and changing radius for DIS03, must use a weighting scheme
Created a mixed sample of DIS01 & DIS03 to get agreement with MC
The weighting scheme is described in detail here:

14. Results of reweighting
Good agreement between data and MC down to Q2 = 15 or 10 GeV2
Currently Q2 > 25, but lowering Q2 cut can increase the kinematic lever arm

15. Summary
The dependence of <nch> on the Meff of the produced system for ep generated events is the same for restricted eta regions
Systematic errors are small, dominated by CAL energy scale & Q2
Trigger study shows possibility of going to lower Q2
Agreement between 1st and 2nd analyses less than 1%
<nch> vs. Meff agrees with 1995 ZEUS preliminary results
Plan
Increase statistics of ARIADNE MC.
Study systematic effect of using different MC (LEPTO)
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.