1. Charged Particle Multiplicity in DIS
QCD Group Review Meeting
M. Rosin, D. Kçira,
A. Savin, and L. Shcheglova
University of Wisconsin
Feb. 5, 2004

2. 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
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

3. 1996 & 1997 Data Selection DIS 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
Track Selection
Tracks associated with primary vertex
|| < 1.75
pT > 150 MeV
Physics and Kinematic Requirement
25 GeV2 < Q2 da < 1200 GeV2
y el < 0.95
y JB > 0.04
70 GeV < W < 260 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, high track reconstruction efficiency
KINEMATIC:
Yel<.95 removes photoproduction
Yjb>0.04 to guarantee 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

4. Monte Carlo Study Lab frame
Dependence of multiplicity on Meff is the same for restricted regions of phase space

5. Comparison to 2nd analysis: Q2
2nd Analysis: Dorian Kçira
Small data sample
Gen level MC
Perfect agreement
Det level MC
Good agreement
Data
Some small disagreement

6. Comparison to 2nd analysis: Meff
Small data sample
Gen level MC
Perfect agreement
Det level MC
Good agreement
Data
Some small disagreement

7. Comparison to 2nd analysis: <nch>
Small data sample
Gen level MC
Perfect agreement
Det level MC
Good agreement
Data
Some small disagreement

8. Comparison with second analysis
General agreement within statistical errors

9. Uncorrelated systematics
Change
% difference in Meff bins
Bin 1
Bin 2
Bin 3
Bin 4
Bin 5
Ee’
± 1 GeV
+ 1.4% %
Box cut
± 1cm
Q2da
± 2.5 GeV2
- 0.9%
yel
± 0.05
+1.2% %
yjb
± 0.01
+ 0.7%
E - pz
± 2 GeV
|Zvtx|
± 5 cm

10. 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 investigsated 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:

11. Results
Full 96 & 97 data sample
Same dependence as data

12. Results of reweighting
Good agreement between data and MC up to Q2 15 or 10 GeV2
Currently Q2 >25, but it should be possible to go lower

13. Summary Plans MC studies Started systematics
2nd analysis is on the way; first look: general agreement
Trigger study by Lydia Shcheglova
Plans
Make preliminary
work on small differences between 1st and 2nd analysis
finish systematics