Study of Multiplicity and Event Shapes using ZEUS detector at HERA Michele Rosin University of Wisconsin, Madison on behalf of the ZEUS Collaboration QFTHEP 2004 June 17th

Outline Introduction and motivation

HERA description & DIS kinematics 920 GeV p+ (820 GeV before 1999) 27.5 GeV e- or e+ 318 GeV cms (300 GeV) Equivalent to a 50 TeV Fixed Target DIS Kinematics: DESY Hamburg, Germany ZEUS H1 remnant e(k) e’(k’) p(P)   (q) q’ Virtuality of photon Fraction of p momentum carried by struck parton Inelasticity 0 ≤ y ≤ 1

e+e- & ep : Breit Frame DIS event Breit Frame definition: “Brickwall frame” incoming quark scatters off photon and returns along same axis. The hadronization products are in the current region Current region of Breit Frame is analogous to e+e-. Lab Frame Breit Frame For testing the universality, I need a frame where I can compare e+e- and ep directly So move to Breit frame… Z=0 separates current from target region, Longitudinal momentum = 0 ZEUS data must be multiplied by 2 when comparing to e+e-, because in e+e- there are two quarks hadronizing, in ep just 1. So my current region in BF is same as either hemisphere for e+e-. (in e+e- both hemispheres are same) Breit Frame

Hard and soft processes ??? Maybe don’t need. Don’t have only hard production havea good device to investigate features of soft prod. Like frag & hadroniz canactuall study by analyzing the energy flow in the det multiplicity & e.s. Investigating QCD using Multiplicity and Event shapes

Mean multiplicity: e+e- and pp To be done in ee and pp one observed this universal behavior of multiplicity explain how did they go, in ee 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. Points are below ee data. So, tried to correct for leading effects: certain amt of energy doesn’t contribute to hadronizatation, taken out by leading protons. Q2 hard = sqrt e init – e out,^2. 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). so based on ee study it was suggested that hadronization has universal behvior for their inv. Mass (energy) was inport. To check: is it still true in ep collision.

Motivation for the use of Meff as energy scale Analogous to the pp study: want to measure the dependence of <nch> of on the total invariant mass of the system Boost in proton direction => proton remnant & fraction of string escape down the beam pipe Can measure only a fraction of string: assume <nch> vs. energy available for hadronization is universal, can compare to pp data Use Meff as scale for comparing to pp (and e+e-) Meff Lab Frame Then it would be natural to repeat this measurement. in zeus det., one should keep in mind because of the high boost in the proton direction, the proton remnant and some part of the string will escape down the beam pipe undetected. We can determine only a fraction of the string, but assuming that nch vs. energy allowed for hadronization should be universal, we can be allowed to compare to pp data . Method has advantage:. We are working with visible part of the string, and if something happens not immedialtley near the proton it should be universal. -Then it would be natural to repeat this measurement for ep using the ZEUS detector. -Analagous to the pp study, we would want to measure the dependence of the mean charged multiplicity on it’s total invariant mass. -Because of the high boost in the proton direction, the proton remnant and a fraction of the string escape down the beam pipe. So we can’t measure the TOTAL invariant mass of the ep system, We can measure the energy available for hadronization, (which for pp collisions is the tot. inv. Mass) We measure the energy available for hadronization in our detector where tracking efficiency is maximized using this formula and refer to it as the Effective Mass. -So If you assume that mean charged multiplicity vs. the energy available for hadronization is universal then we can use the Effective Mass as a scale for comparing to pp and e+e- data. Meff: HFS measured in the detector where the tracking efficiency is maximized

Comparison of multiplicity for ep, with e+e- & pp mean charged multiplicity, <nch>, vs. energy available for hadronization for e+e- (√s), pp (√q2) and ep (Meff) Excess in <nch> observed for ep data Possible explanations: Different contributions from gluons (HERA can reach smaller x than pp) <nch> vs. Q for ep in current region of Breit frame agrees with e+e- and pp data, for high Q Show 1995 measurement, now it is observed that number of charged hadrons for same amount of energy is higher than for ee and pp, there might be few explanations for excess: for example, diff contribution from gluons, HERA can reach a smaller x than pp. So then it would be interesting to compare to LEP data (high energy) which should have contribution from gluons.

Compare to LEP data LEP data at higher energy: should have contribution form gluons Can’t conclude from this plot, it seems both ep and pp data could meet LEP points Working on improving this measurement using more statistics, and spitting data into x and Q2 bins New results should be ready for ICHEP 2004; might lead to a better conclusion Lep points are added, and based on this data can’t say exactlly, seems both could meet lep data, other hand, zeus did meas in cr bf. Think that the ene that goes into the cr is prop to the q , nch prod in cf plotted a fun of q lies on top of ee and pp. same meas showed that in temrs of q, for the same events higher multiplicity in the target region then in the cr.conc, that s why it is imp to continue study with higher stat and to show new results at ichep, aiming to show here, but too early.

Study Hadronization using Event Shapes Event shape variables measure aspects of the topology an event’s hadronic final state Event shapes in DIS should allow investigation of QCD over a wide range of energy scales, but hadronization corrections are large for these variables Power Correction model seeks to describe the effect of hadronization on these variables Event shape analysis is done in Breit Frame Measure event shapes in the Breit frame, where you have the larges possible separation between proton and scattered quark.

Power corrections: an analytical approach Power correction reduces hadronization corrections for any infrared safe event shape variable, F Mean event shape variables are sum of perturbative and non-perturbative parts The non-perturbative power corrrection is based on two parameters, α0 and αs Used to determine the hadronization corrections Idea behind pc mean shape varibales is sum of mean es variables at lev of pertab (parton level) + hadronization caclculated analytically with this for. 2 parameters inthis model, ao as, where ao is non pert free papameter “non perturbative parameter” -(Dokshitzer, Weber Phys. Lett. B 352(1995)451)

Event Shape Variables Thrust Thrust: longitudinal momentum sum n for TT axis Photon axis = n : T Thrust Thrust: longitudinal momentum sum Broadening: transverse momentum sum Both measured with set to the Thrust axis and Photon axis Another way es studies, dis of energy, introduce variables, thrust, broadening, cparam, and jet mass. Thrust: Linear columnation of hadronic system along a specific(“thrust”) axis. 4 thrusts in DIS Tz, TM, Tm, and Tc depending on the axis. Broadening: Broadening of particles in transverse momentum w.r.t. thrust axis. Can range from 0<B<0.5, where for columnated events values of B tend toward min, and for isotropic events B tends toward max. Two types of broadening: Bw, Bt Jet Mass and C parameter are correlations of pairs of particles, and are indepandant of the axis. C parameter varies between 0 and 1 and like broadening tends toward 0 for colimated events and towards the max for isotropic events. Jet Mass and C parameter: correlations of pairs of particles Sum over all momenta in Current Region of Breit Frame

Mean event shape variables NLO + Power correction fits to means plotted in bins of X and Q2 High x points (open circles) not fitted All variables fitted with a good χ2 Photon axis variables (1-Tγ) show large x-dependence 1-Tγ correction very small and negative Model describes data well Take a plot in bins of x and q2, to get eff & pur of method at certain level, in each bin the pred was fittted..kink: artificial, two lines, two x regions, other ways overload the plot, because there you have two points that differ, not a fit, every point was corrected simultan. Now fitting all points simul for diff variable sget a0 and as, show data presented to dis04,

Extraction of αs and α0 from NLO + PC fits to means Not all variables give same αs and αo. 1 – Tγ fit poorly defined -large systematic errors Show as famous plot, ask adam for title, concl. Not all variab give same as and a0 have some vars not well described.

Differential distributions NLO+PC Fits to Differential Distributions Try to improv our unders using dv. Show sh slide 7, and explain in words, Fit differential distributions over a limited range Usable range increases with Q2, Try to improve our understanding using differential distributions Power correction is interpreted as a ‘shift’ in the NLO distribution

Extraction of αs and α0 from fits to differential distributions Photon axis variables fit with high αs, but other variables consistent in αs and αo Fits αo somewhat high compared to that from means Method a little unstable, try adding NNLO effects- resummations As and ao once more, exracted usng diff dist, page 10 od stevens talk, makes a big diff, a0 went down by fact of 1.5 -2 somehow this methof isnt stable, need to add nnlo effects, resummations, so we tried to fit dd+nnlo+resummat+pc

Differential distributions: with resummation NNLO+NLO+PC Fits to Differential Distributions Page 12 of sh, helps describe beter the behavior of nlo, make the hope that the fitting procedure is better, enlarged range of fit, Behavior of NLO better described, possible fitting procedure is better Enlarged range of fit

Extraction of αs and α0 from fits to differential distributions C is consistent in αs but low in αo. C result very sensitive to fitted range: under investigation Results consistent with αs = 0.118 and suggest α0 ≈ 0.5. Sh slide 15, as and ao again, now ao comes backto where it was, as is unique, some pron with c param under investigation,

Summary Showed results for two methods of investigating hadronization: Multiplicity: Mean charged multiplicity vs. effective mass was measured for ep and compared to e+e- and pp. Multiplicity shows excess in data for ep. Current study aiming for higher precision using new data Event Shapes: NLO + power correction has been fitted to the mean event shape data, αs ≈ 0.12, α0 ≈ 0.45. Consistent with published results. Photon axis variables poorly determined NNLO Resummed calculations give better results than NLO + power correction only, with αs ≈ 0. 118, α0≈0.5. Resummation gives consistent αs,αo for all event shape variables, but C fit dependant on range Current investigation of new event shape variables & new methods. (Kout for events with 2 or more jets, 2 jets can fix the NLO predictions better