1. Analysis Update: Mean Charged Multiplicity in DIS
M. Rosin, D. Kçira, and A. Savin
University of Wisconsin
L. Shcheglova
Moscow State University, Institute of Nuclear Physics
ZEUS Collaboration Meeting
Padova, Italy
October 20th, 2004

2. Comparing ep and e+e- using current region of Breit Frame
e+e-: boson with virtuality √s produces (in L.O.) 2 quarks & hadronization is between 2 colored objects q and q to produce final state
ep: boson with virtuality Q produces 1 quark: the 2nd quark comes from the interaction between the photon and the proton
current region of Breit frame for ep similar to one hemisphere of e+e-
If we use Q as scale to compare to e+e-, multiply hadrons by 2
Here you see the diagram that we use to describe the current region of BF to see correspondence between e+e- and ep. What happens after e+e- annihilation is that a virtual boson is produced with virtuality sqrt s that produces, in LO, 2 quarks and the hadronization takes place between 2 colored objects, q and qbar, to produce the final state. In ep have a virtual boson with vituality Q, that produces 1 quark, because the 2nd q is coming from the interaction between photon and proton. The current region of the Breit frame for ep is similar to one hemisphere if e+e-, so if we are using Q as a scale for comparing to e+e- we multiply the multiplicity by 2

3. Previous ZEUS Measurement <nch> vs. Q compared to e+e-
multiplicity vs. center of mass energy (√s) for different e+e- experiments: data agree with one another
Attempt to compare multiplicity in current region of Breit frame vs. Q, for ep, to multiplicity vs. √s for e+e-.
Multiplicity for e+e- has been scaled down by factor of 2
Consistent with e+e- data for high Q2
Disagreement at low Q2 may be attributed to gluon radiation
Idea of current analysis: Understand current and target multiplicity and compare to e+e-
If you plot all e+e- data together as multiplicity vs cm energy(e+e-) then they nicely lie on top of each other. And assuming that current region of the BF is half of e+e-, ZEUS points can be plotted as twice the multiplicity vs. Q (thou in this plot vice-vesra). The main feature is the following; it looks as though ep multiplicity agrees with e+e- for middle region of Q but disagrees at lower values. Currently studies of multiplicity at ZEUS are underway and just presented in ICHEP which are devoted to studies of the current and target regions and the comparisons between them. The first strp toward that end is thinking if the q2 is the proper scale to compare ep and e+e- data.
European Physics Journal C11 (1999)

4. Closer look at current region
ep: Split into Current and Target Region – one string two segments.
In ep we have a color field between 2 colored objects the struck quark and the proton remnant
When we use Q2 as a scale we are assuming the configuration is as symmetric as it is in e+e-, but it isn’t
This asymmetric configuration leads to migration of particles from the current region to the target region
Here you see FD with the gluon ladder, this is the current region in ep and this is the target region in ep. In other words, we have a color field between 2 colored objects, the struck quark and the proton remnant that is shown here. When we use q2 as a scale we assume the configuration is as symmetric as in e+e-, but we can see that that is not true. This assumption doesn’t work because the assymetric configuration leads to migration of particles from current region into target region. The simple example if such migration can be demonstrated here.
Breit Frame diagram

5. Gluon radiation, Q, and 2*EBreit
Soft Contribution
Hard Contribution
QCD Compton
In hard and soft processes gluon radiation occurs
These gluons can migrate to target region
Total energy in the current region of Breit frame and multiplicity are decreased due to these migrations (Q2 is not)
Effect is more pronounced for low Q2 : more low energy gluons
Must use 2*EBreit and 2*Nch for comparing with e+e-
We can imagine 2 major contribution of migration of particles out of the CR as comp to e+e-. 1st soft contribution because of the parton shower mechanism where the gluons in the shower are produced with certain pt, because of this pt dependence on energy of the initial quark, some partons can escape the current region, as you can see in the figure it is not a problem for e+e- where we measure the whole sphere and the total number of particles will be the same but it is certainly a problem for ep because the number of particles will decrease while the scale, q, stays the same. Another type of migration is because of hard scattering in ep cascade (the typical diagram for this is QCD compton) where we have a gluon with energy comparable to the energy of the initial quark and this is shown here, so we can here also have migrations of particles out of CR. So it means that instead of scale q we need to use a scale that describes, not the expected, but the real energy on the current region of the Breit frame. So we can use the energy of all the particles that were found in the current region, and use it as a scale to investigate our multiplicity. So if there would be no migrations E=q over 2, but in case of migrations the energy will always be smaller than that. So to make comparisons between ep and e+e- we would need to plot 2*number of particles (again because ep is only ½ hemisphere) vs. 2* E in the current region of the Breit frame. It is also important to note that 2*E is Lorentz invariant, exactly as sqrt s, or q.
N < N expected
No migrations:
With migrations:

6. Multiplicity vs. 2*Ebreit
Measure multiplicity dependence on 2*EBreit and compare to previous ZEUS measurement vs. Q, and to e+e-
Points agree with the e+e- points
This approximation of invariant mass partially takes into account the real distribution of the particles.
Current region understood, would like to use some energy scale to compare target region for ep to e+e-
Start by looking at comparison of e+e- and pp where agreement in the multiplicity dependence has been observed
Now we see that our points agree with e+e- points. This approx is much better than used previously in ‘99 publication where they used q, because it partially takes into account the real distribution of the particles. Now that we have understood the current region of ep collision and have good agreement with e+e-, we would like to use some energy scale to compare target region for ep to e+e-. Let me remind you that previously it was observed that the multiplicity in e+e- and PP collision exhibit the same energy behavior if measured using proper energy scale.

7. Breit frame vs Meff
Similar dependence in multiplicity for visible parts of current and target regions of the Breit frame as a function of their respective effective mass
Target multiplicity is higher than current multiplicity and can reach higher values of Meff
combined current and target region (black points) show same behavior as target region
Multiplicity and Meff are Lorentz invariant so can move to the lab frame
See here is comparison between multiplicity of current and target regions of the Breit frame vs. effective mass.1st bullet. The target multiplicity go to higher values and it would go even farther if we would be able to see whole tr but this is only the part that we observe in our detector. Clearly, the combined cr and tr multiplicity (black points) behaves in the same way. And the combined multiplicity ismainly dominated by the target, shown here as black dots. Now we can use the fact that the number of charged hadrons and eff mass are both Larence invariant. to move to the lab frame because the number of hadrons vs eff mass should behave in same way, ( visible breit is equal visible lab) now we try to investigate the behavior of the multiplicity in the lab frame in terms of x and q2.

8. Previous results in lab frame: <nch> vs
Previous results in lab frame: <nch> vs. Meff compared to e+e- and pp
1995 preliminary results
<nch> plotted vs. Meff for ep, vs. qhad for pp and vs. √s for e+e-.
<nch> for ep is ~15% higher than that for e+e- and pp.
Current analysis consistent with previous results
Checked x and Q2 dependence: no Q2 dependence, weak x dependence not enough to explain difference
Investigate this difference: look at visible charged multiplicity vs. Meff for current and target regions of the Breit frame.
Keep in mind that I have just shown that the current region of the Breit frame for ep reproduces the e+e- multiplicity well. We are trying to understand what makes the difference in multiplicity between ep and e+e- & pp when we plot using Meff as a scale. So lets look at visible charged multiplicity vs. effective mass for current and target regions of the Breit frame.

9. Check: Compare to e+e- & pp using ep dijet events
Boson-Gluon Fusion
QCD Compton
Try to mimic final state of e+e- by looking at dijet events
Require events with only 2 jets, one in the current region of the Breit Frame and one in the Target region
pp collision
e+e- annihilation
remnant
remnant

10. <nch> vs. Meff for events with 2 jets
First look at dijet events
Multiplicity decreasing with increasing cut on jet ET
For events with 2 jets of Et>10 Gev, shows reasonable agreement with pp and e+e-
Second analysis on these points underway
next: Look at hadronic center of mass frame

11. Summary
Measurement in current region of the Breit frame show same dependence as e+e- if 2*Ecurrent is used as the scale
Similar dependence observed in multiplicity for visible part of the current and target regions of the Breit frame as a function of their respective effective mass
Lab frame measurements 15% above e+e- and pp & show no strong dependence on x or Q2
Study using dijet events to better mimic pp collision is underway and shows decreased multiplicity with increasing dijet ET
Plans:
working in γP canter of mass frame
paper before end of year