1. Charged Particle Multiplicity in DIS
Progress Report
ZEUS Collaboration Week
M. Rosin, D. Kçira, and A. Savin
University of Wisconsin
L. Shcheglova
Moscow State University, Institute of Nuclear Physics
June 24th, 2004

2. Outline Introduction and motivation Data selection & simulation
Control plots
Correction methods
Measurement of <nch> vs. Meff
Comparison of analyses
Systematics
Checks in the Breit frame
Summary and plan

3. Multiplicity e+e- and pp
pp vs. √q2had
pp vs. √spp pp
Invariant mass of pp
Start with mean multiplicity for e+e- and pp. Here is e+e- annhilation, and a plot of multiplicity for ee vs. the scale is sqrt s, (inv. Mass of ee collisions). In pp tried sqrt of s using whole energy of incoming protons, plotted number charged particles vs this scale, for three different values of sqrt s. Points are below ee data. So, tried to correct for leading effects: A certain amt of energy doesn’t contribute, and it is taken out by leading protons. Q2 had as shown here corresponds to inv. Mass of system that was created within the detector, (one should keep in mind the sqrt s for e+e- is also inv. Mass). And when multiplicity for pp is plotted against q2had, the data lie on top pf the e+e- points So based on this study it was suggested that hadronization has universal behvior for the inv. Mass (energy). To check: is it still true in ep collision.
Agreement between e+e- and pp plotted vs. pp invariant mass

4. Multiplicity: ep vs. e+e- (1)
Breit Current region of ep similar to one hemisphere of e+e-
Use Q as scale, multiply hadrons by 2
Where does other end of the string connect?

5. Multiplicity: ep vs. e+e- (2)
ep hadronization compared to e+e-.
ep: Split into Current and Target Region – one string two segments.
Use invariant masses of the string (Lab) and segments (Breit).
Breit Frame =>
Uniform string?

6. The use of Meff as energy scale
Analogous to the pp study, is natural to measure dependence of <nch> of on it’s total invariant mass. (Energy available for hadronization)
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
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: HFS measured in the detector where the tracking efficiency is maximized

7. e+e-, pp, ep, 4 questions
Differences between ep and e+e- and between Breit and Lab
Is difference due to:
Analysis method?
use of frame (Lab, Breit)?
quark / gluon distributions?
Choice of scale?
New analysis seeks to answer these questions
=> Analysis description

8. 1996-97 Data sample 735,007 events after all cuts (38.58 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
735,007 events
after all cuts
(38.58 pb-1)

9. Event simulation Ariadne ’96-’97 4.08 Lepto 6.5.1
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 generated Lepto without SCI
Proton PDF’s are parameterized from experimental data
Detector simulation besed on JAY-AUNT
Luminosity of MC : 36.5 pb-1

10. Kinematic Variables
96-97 data compared to ARIADNE and LEPTO for kinematic variables
Both ARIADNE and LEPTO show good agreement for kinematic variables

11. Lab analysis: Meff & Tracks
LEPTO and ARIADNE comparisons to tracks and Meff
Tracks better described by ARIADNE
Meff better described by LEPTO
Will compare also with LEPTO without SCI

12. 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.

13. Correction to hadron level: bin by bin
Correction for detector effects
Ariadne - Lepto very similar (see later)
Correction of hadrons of pT > 0.15 to all hadrons

14. 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 of tracks in each bin of Meff by:
Matrix corrects tracks to hadron level
ρ corrects phase space to hadron level
Hadrons passing gen level cuts
ρ =
Hadrons passing det level cuts

15. Comparison of bin-by-bin and matrix methods
Methods agree better than 2% in all Meff bins
use bin-by-bin for results
Matrix sensitive to statistics
Use matrix method considered as a systematic check

16. Correlated & Uncorrelated Systematics
Change
% Difference in Meff bins
Bin 1
Bin 2
Bin 3
Bin 4
Bin 5
matrix instead of bin-by-bin
1.3%
0.9%
0.6%
1.2%
LEPTO instead of ARIADNE
< 0.5%
Ee’
± 1 GeV
Radius Cut
± 1cm Q2
± 1.6 GeV2
yJB
± .0006
Track pT
-.03/+.05GeV
Zvtx
± 5 cm W
± 19 GeV
E - pz
± 5 GeV |
CAL energy scale
± 3 %
1.1%
1.2%
0.8%
0.5%

17. Check with 2nd analysis: total, Q2 and x bins
Agreement between 1st and 2nd analysis within 1% for total and in bins of x and Q2
Full kinematic range: agreement better than 1% for both correction methods.
Meff

18. New measurement in the LAB
New Lab Data
Good agreement with prelim. points, with smaller statistical, systematic errors
Confirm difference ep vs. e+e- and pp.
ARIADNE describes data dependence, LEPTO higher than the data

19. 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
Q2 dependence? => next page

20. Lab frame: x and Q2 bins Data described by ARIADNE LEPTO above data
No Q2 dependence observed
Difference not due to quark / gluon distributions in the proton

21. Breit Frame: Current Region Analysis
New analysis agrees with previously published ZEUS result in the Current Region of the Breit Frame for scale Q.
Previous ZEUS publication: Eur. Phys. J.C 11, (1999)
Previous ZEUS publ.
96-97 – Current Breit

22. Breit Frame - Change to scale Meff
Using Meff agreement between:
Lab frame
Breit Frame Current Region
Breit Frame Target Region
=> String is “uniform”

23. 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!
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.