Energy Dependence of the Mean Charged Multiplicity in Deep Inelastic Scattering with ZEUS at HERA

1. Energy Dependence of the Mean Charged Multiplicity in Deep Inelastic Scattering with ZEUS at HERA Michele Rosin University of Wisconsin, Madison Thesis Defense, Madison WI Jan. 27th, 2006

2. Standard Model Fermions • Matter made of fermions: • quarks or leptons • Each particle has anti-particle with opposite quantum numbers • Quarks carry color “charge” • Four fundamental forces • Electromagentic (EM) force • Weak force • Strong force • Gravity

3. Standard Model (II) Bosons • Strength of forces determined by coupling constant (αEM and αs) • forces mediated by exchange of bosons: γ, W±, Z0,g • Gravity described at macroscopic scale by general relativity. • very weak, neglected in high energy particle physics • Quantum Electrodynamics (QED): theory of EM, combined with weak  Electro-weak theory • Quantum Chromodynamics (QCD): theory of strong interaction • Combined theories  Standard Model

4. probe lepton Particle Scattering • Study structure of proton • Scattering via probe • Wavelength • Deep Inelastic Scattering (DIS) – Q2 large • High energy lepton transfers momentum to a nucleon via probe h : Plank’s Constant Q2: related to momentum of probe Size of proton ~ 1 fm HERA Q2 Range ~ 40,000GeV2 HERA can probe to ~ 0.001 fm

5. e’(k’) Virtuality of exchanged photon e(k)  (q) remnant p(P) Inelasticity: 0  y  1 q’ = Center of mass energy of the ep system Center of mass energy of the *P system Only two independent quantities Kinematic Variables Fraction of proton momentum carried by struck parton 0 x  1

6. History of DIS, QPM

7. QCD Theory • QCD Quantum Chromodynamics • Strong force couples to color and is mediated by the gluon • Gluon permits momentum exchange between quarks • Gluons create quarks through pair production • Color confinement: free partons are not observed, only colorless objects -hadrons • Parton distribution function (PDF) gives probability of finding parton with given momentum within proton (experimentally determined)

8. QCD Scale • Running of aS • As scale m increases, aS(m) decreases (m = ET or Q) • Perturbative QCD (pQCD) • Small aS(m) (hard scale) • Series expansion used to calculate observables • Nonperturbative QCD • Large aS(m) (soft scale) • Series not convergent Leading Order (LO) Next to Leading Order (NLO) A = A0 + A1aS + A2aS2 +... HERA DIS Data: Running of aS(m)

9. pQCD

10. From Partons to Hadrons hard scattering  parton showers  hadronization • Hard scattering: large scale (short distance) perturbative process • Parton showers: initial QCD radiation of partons from initial partons • Hadronization: colorless hadrons produced from colored partons                          soft process (large distance) - not perturbatively calculable          phenomenological models and experimental input

11. Multiplicity and Energy Flow • The hard scattering process determines the initial distribution of energy • Parton Shower + Hadronization determine the number of charged particles produced • Measure mean number of charged particles produced, (mean charged multiplicity, <nch>), versus the energy available for production of final state hadrons, study the mechanisms of hard scattering, parton showers and hadronization • Universality of the hadronization process can be tested by comparison with data from e+e- and hadron colliders.

12. Ecms =W E E p g* Nphoton region Nproton region Ecms/2 Ecms/2 Nphoton region vs W Hadronic center of mass (HCM) frame Hadronic center of mass energy is W Definition of HCM frame • Forward moving particles: photon hemisphere • Backward moving particles: proton hemisphere • Incoming photon and proton E= W/2 • Final state: both hemispheres E=W/2

13. Breit Frame • “Brickwall” frame: incoming quark scatters off photon and returns along same axis • Breit Frame definition: • pZ<0: current region, pZ>0: target region • Current region of Breit Frame is analogous to single hemisphere e+e-: diagrams are similar above dashed line • In e+e- pair of quarks produced back to back with E=√s/2 each of them equiv. to the struck quark of E=Q/2 in DIS. Mean charged multiplicity has been measured for various initial state interactions, e+e-, pp, ep DIS, and fixed target DIS, in both Breit and HCM frames

14. Previous Measurements: Multiplicity in e+e- and pp e+e- e+e-: pp vs. √q2had pp vs. √spp pp Il Nuovo Cimento 65A N3 (1981) 404 Energy available for particle production • Agreement between e+e- and pp plotted vs.

15. Previous Measurements: Multiplicity vs. Q in Breit frame ep DIS • Current region Breit frame multiplicity vs. Q2 (hemisphere) shown along with e+e- data (whole sphere divided by 2) • Consistent with e+e- data for high Q2 disagreement at Q2< 80 GeV2 • ep has gluon radiation whereas e+e- does not– possible source of disagreement at low Q2 ?? European Physics Journal C11 (1999) 251-270

16. Measurement vs. W in HCM frame <n> Zeitschrift für Physik C72 (1996) 573 • ep DIS vs. W compared to fixed target DIS experiments and e+e- prediction similar rate of increase with W for ep and e+e-

17. Present Analysis • Study energy dependence of <nch> in • photon region of HCM frame • compared to e+e-, pp and previous DIS • Breit Frame: current and target regions • expected to behave similarly to one hemisphere of e+e-: previous results show disagreement at low energies: we use total energy in current region of Breit frame as a scale for comparison with e+e- • Laboratory frame: in bins of X and Q2 • Propose an alternative energy scale, the effective mass of hadronic system, Meff • compared <nch> in different regions of Breit and HCM frames for ep DIS

18. HERA Description • 920 GeV p+ • (820 GeV before 1999) • 27.5 GeV e- or e+ • 318 GeV cms • Equivalent to a 50 TeV Fixed Target • Instantaneous luminosity max: 1.8 x 1031 cm-2s-1 • 220 bunches • 96 ns crossing time • IP~90mA p • Ie~40mA e+ H1 ZEUS DESY Hamburg, Germany Unique opportunity to study hadron-lepton collisions

19. HERA Data Luminosity upgrade • 5x increase in Luminosity  expect 1 fb-1 by end of 2006

20. HERA Kinematic Range Q2 = sxy 0.1 < Q2 < 20000 GeV2 10-6 < x < 0.9 Equivalent to a 50 TeV Fixed Target Experiment

21. ZEUS Detector Electron 27.5GeV • General Purpose Detector • Almost hermetic • Measure ep final state particles: energy, particle type and direction

22. Central Tracking Detector e p • Drift Chamber inside 1.43 T Solenoid • Can resolve up to 500 charged tracks • Average event has ~20-40 charged tracks • Determine interaction vertex of the event • Measure number of charged particles (tracks) • Region of good acceptance: -1.75 < η < 1.75 View Along Beam Pipe Side View

23. Uranium-Scintillator Calorimeter  = 0.0  = 90.0o  = 1.1  = 36.7o  = -0.75  = 129.1o • alternating uranium and scintillator plates (sandwich calorimeter)  = 3.0  = 5.7o  = -3.0  = 174.3o Positrons 27.5 GeV Protons 820 GeV • compensating - equal signal from hadrons and electromagnetic particles of same energy - e/h = 1 • Energy resolution  e/Ee= 18% / Eh/Eh= 35% / E , E in GeV • Depth of FCAL > RCAL due to Ep > Ee • Used for measuring energy flow of particles. • covers 99.6% of the solid angle

24. ZEUS Trigger 107 Hz Crossing Rate,105 Hz Background Rate, 10 Hz Physics Rate First Level • Dedicated custom hardware • Pipelined without deadtime • Global and regional energy sums • Isolated m and e+ recognition • Track quality information Second Level • “Commodity” Transputers • Calorimeter timing cuts • E - pz cuts • Vertex information • Simple physics filters Third Level • Commodity processor farm • Full event info available • Refined Jet and electron finding • Advanced physics filters

25. Modeling DIS with Monte Carlo • Hadronization Models • String Fragmentation (Lund) • Cluster Model • Event generators use algorithms based on QCD and phenomenological models to simulate DIS events • Hard subprocess: pQCD • Parton Cascade • Hadronization • Detector Simulation • correct for detector effects: finite efficiency, resolutions & acceptances Next slide Parton Level Hadron Level Detector Simulation • Parton Cascades • LO Matrix Element + Parton Showers (MEPS) • Color Dipole Model (CDM) Next slide PDFs

26. Monte Carlo models: parton cascades and hadronization Models for parton cascades: Color Dipole Model: Parton Shower Model: • Gluons are emitted from the color field between quark-antiquark pairs, supplemented with BGF processes. • cascade of partons with decreasing virtuality continuing until a cut-off LEPTO HERWIG ARIADNE Hadronization models: Lund String Model: Cluster Fragmentation Model: • color "string" stretched between q and q moving apart, • string breaks to form 2 color singlet strings, and so on untilonly on-mass-shell hadrons. • color-singlet clusters of neighboring partons formed • Clusters decay into hadrons LEPTO HERWIG ARIADNE

27. 1996-97 Data sample • 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 )

28. Analysis Method This analysis: Look at final state mean charged multiplicity, <nch>, and compare to other experiments to study similarities in particle production from different initial state particles.

29. Analysis Methods: Breit Frame Investigate cause of disagreement between ep vs. Q and e+e- at low energies look more closely at comparison of one hemisphere e+e- and current region Breit frame • 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 Breit Frame diagram

30. q q q q g g Current region Breit Frame Q and 2*EBreit Soft Contribution Hard Contribution e+e-: whole sphere ep: shaded region: current region of Breit frame 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 instead of Q for comparing with e+e- N < N expected With migrations: No migrations:

31. Monte Carlo study of 2*E/Q as a function of Q. Lower energies: Q doesn’t accurately reflect actual energy in hemisphere. Use 2*E as a scale for comparing to e+e-

32. Analysis Methods: photon hemishpere HCM frame 2*E/W as function of W Difference is negligable Want to measure dependence of <nch> on Minv as was done in Breit frame, Migrations here are small, so 2*Ephoton = W so use Minv = W as scale

33. Study: <nch> vs. Meff CAL within the CTD acceptance CTD Invariant Mass of Hadronic System • Measure hadronic final state within  for best acceptance in the central tracking detector (CTD) • Measure # charged tracks, reconstruct number of charged hadrons • Measure invariant mass of the system (Meff) in corresponding Δη region. • Energy is measured in the Calorimeter (CAL) Used as a scale to compare: current and target regions of Breit framecurrent region Briet frame to photon region HCM

34. Correction to hadron level: matrix Step 1: Correction Matrix: The matrix relates the observed to the generated distributions by: Step 2: Correction for acceptance of event selection cuts in the bins ρGEN :distribution with GEN level cuts ρDET :distribution with GEN level cuts • Matrix corrects tracks to hadron level • ρ corrects phase space to hadron level

35. Correction to hadron level: matrix Example: current region of Breit frame vs. 2*E To illustrate average size of 1st part of correction: Mean matrix correction factor: range: 0.98-1.78 average: ~1.2 Mean of C distributions: range: 0.92-2.05 average: ~1.02

36. Correction to hadron level:modified bin-by-bin Part one: correct to hadron level using only hadrons generated with pT > 0.15 GeV (Detector Effects) Part two: correct for hadrons with lower pT, using ratio of <gen> with pT cut to <gen> no pT cut in each bin. number distribution

37. Correction to hadron level:modified bin-by-bin Example: lab frame vs. Meff Average correction for detector effects: <C1,i> range: 0.96-1.84 average: ~1.1 Correction of hadrons of pT > 0.15 GeV to all hadrons Invariant Mass correction: normal bin-by-bin method: Average less than 2% range: 1.02-1.15 average: ~1.05

38. Acceptance correction: Current region of Breit frame: Visible Part • Breit Frame: 95% of hadrons in current region visible in detector, only 30% of target region hadrons are visible All Hadrons Current Region Breit Frame Proton remnant Generated in visible part Multiplied by <nch>

39. Acceptance correction:Photon region HCM • HCM Frame: 60-80% Photon region HCM frame contained in visible part of detector.  larger corrections. Visible Part All Hadrons Photon Region HCM Frame Proton Region HCM Frame Correction factors for <nch> vs. W in photon region HCM frame: 1.78, 1.42, 1.26 Additional correction needed for Meff in HCM:calculated in bins of W, similar to hadron acceptance correction: Correction factors: 2.38, 1.73, 1.45Applied on an event by event basis

40. Results <nch>

41. Comparison to other <nch> measurements at HERA <nch>

42. Comparison of ep DIS <nch> to other experiments <nch> • Measurement of multiplicity dependence on 2*Ecurrent compared to previous ZEUS measurement vs. Q, and to e+e- and pp data (<nch> is multiplied by 2 for comparison) • 2*E gives better description of multiplicity at lower energy • Current region understood, would also like to compare the target region of ep to e+e- but…

43. bins

45. Correlated & Uncorrelated Systematics Main systematics here

46. Summary and Conclusions HFS investigated in NC ep DIS in range 25<q2 and 70< W< 225 in terms of <nch> , the center of mass energy, and the invariant mass, Meff 1st time, lower energy data of cr Breit frame shown to agree with e+e- and pp by using 2*E as energy scale <nch> in photon region HCM agree with e+e- Total energy region of analysis from 2 to 200 New energy variable used for comparison between diff e regions of ep HFS <nch> scales with Meff in the same way as 2*E in cr Breit frame, (and therefore also same as e+e-), <nch> in photon region HCM rises faster as a function of Meff than <nch> in current region BF. <nch> in photon region HCM show no dep. On x or Q