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Jet fragmentation results at CDF

Jet fragmentation results at CDF. Andrey Korytov (for CDF Collaboration). 1.2 fb -1 on tape. CDF. p-pbar collisions sqrt(s) = 1.96 TeV peak L = 10 32 cm -2 s -1. D0. Jet Fragmentation vs Analytical pQCD. Momentum distributions of particles in jets PRD 68 (2003) 12003

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Jet fragmentation results at CDF

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  1. Jet fragmentation results at CDF Andrey Korytov (for CDF Collaboration) 1.2 fb-1 on tape CDF p-pbar collisions sqrt(s) = 1.96 TeV peak L = 1032 cm-2s-1 D0

  2. Jet Fragmentation vs Analytical pQCD • Momentum distributions of particles in jets • PRD 68 (2003) 12003 • Multiplicity of particles in jets (including g/q-jet differences) • PRL 94 (2005) 17802 • PRL V87 No.21 (2001) 211804 • kT-distribution of particles in jets (kT with respect to jet axis) • Momentum correlations of particles in jets • Global Event Shapes

  3. NLLA + LPHD • Partons: NLL resummation • Mueller (1983) • Dokshitzer, Troyan (1984) • Malaza, Webber (1984) • plus subsequent corrections e.g., momentum distribution: dN/dx = D(Y, x) x = ln(1/x) = ln(Ejet/pparton) Y = ln(Q/Qeff), Q = Ejetqcone Qeff = Qcutoff = LQCD • From partons to hadrons: Local Parton-Hadron Duality Hypothesis • Azimov, Dokshitzer, Khoze, Troyan (1985) Hadron observables follow patterns predicted for partons • Nhadrons ~ Npartons ? • Momentum distributions? • Momentum correlations? • …

  4. Particle momenta & multiplicity CDF cone opening angle q=0.47 • Momentum distribution • of charged particles in jets • dijet events with well-balanced ET • 15-30 cone around dijet axis • Two parameter fit (MLLA+LPHD) • works surprisingly well in a wide range of dijet masses • MLLA Qeff = 23040 MeV kT-cutoff can be set as low as LQCD • KLPHD( ) = 0.56  0.10 Nhadrons  Npartons

  5. Multiplicity of Particles: Gluon vs Quark Jets • LLA and NLLA: r = Ngluon / Nquark = 9/4 = 2.25 “n”NLLA: r = 1.4-1.8, depending on the energy scale • di-jet and g-jet events have very different fractions of q/g-jets  multiplicities in quark and gluon jets can be resolved • Results agree with “n”NLLA r = 1.64  0.17 at Q~20 GeV • Most recent LEP result (OPAL) r = 1.51  0.04 at Q~90 GeV Ratio = Ng-jet / Nq-jet Q = Ejet qcone

  6. kT-distribution of particles in jets: theory jet • Perez-Ramos and Machet (2006), MLLA-based calculations • Shape of dN/dkT (kT>1 GeV) is practically parameter-free • nearly no dependence on Qeff • weak dependence on jet’s origin (quark or gluon) • energy scale Q=Ejetq basically defines cutoff for max kT kT particle’s k

  7. kT-distribution: data vs theory • Data lack high kT particles • Data approach the theory with increasing energy scale Q • Is the difference due to incomplete pQCD or intervention of hadronization?

  8. Two-particle momentum correlations • all particle pairs in cone q=0.5 around jet axis • theory: C. Fong and B. Webber (1990) • update for R just became available: Perez-Ramos (2006); no binominal… • to decouple momentum correlations from multiplicity fluctuations, we use distributions normalized to unity and binominal moments

  9. Two-particle momentum correlations Theory: Expansion around the peak: Dx=x-x0 c1(Ejet) is always positive c2(Ejet) is always negative Ridge-like structure: - particles with similar momenta correlate - correlation is stronger for soft particles Distribution normalized to unity Dx=x-x0

  10. Momentum correlations: results (1) • hadron correlations follow the pattern expected for partons

  11. Momentum correlations: results (2) • hadron correlations follow the pattern expected for partons

  12. Momentum correlations: results (3) Q = Ejet qcone (GeV)

  13. Momentum correlations: results (4) • c0 coefficient is inconsistent with the theory, it is too small • However, it does not show much trend with energy, as expected Q = Ejet qcone (GeV)

  14. Global Event Shapes (e+e- and DIS) • Measurements of as • CA and CF fits • MC tuning • Analytical 1/Q non-perturbative corrections (shift) • equate to hadronization corrections? • parameterize with universal infrared finite a0?

  15. Global Event Shapes: Tevatron vs e+e- • Four-prong color antenna with rich underlying color structure (qqqq, qqgg, gggg) • Complication: forward direction experimentally inaccessible  indirectly global Event Shape = central Event Shape + Recoil Term, where both central ES and Recoil are calculated via central region observables Tevatron LEP example to be used in this talk

  16. Global Event Shapes at Tevatron MC at parton level = Theory Theory: A. Banfi, G. Salam, G. Zanderighi (preliminary 2005) MC at hadron level = MC at parton level • true for both Central ES and Recoil Term • true event-by-event, too • Hmm… Where is the hadronization shift? • At least, MC does not have it!

  17. CFD II Preliminary CFD II Preliminary Global Event Shapes at Tevatron Central Event Shapes: Detector Sim = MC at hadron level • true for tracks and calorimeter towers • true on event-by-event basis, too • approx. no instrumental effects to unfold Recoil Term: Detector Sim ≠ MC at hadron level • instrumental effects to be unfolded…

  18. Global Event Shapes at Tevatron: results Central Event Shapes: Data ≈ Detector Sim + Shift • true for tracks and calorimeter towers •  the shift is real and not reproduced in MC! • Is it 1/Q? Waiting for final analytical results… Recoil Term: Data = Detector Sim • no surprises

  19. Summary • Momentum distributions of particles in jets • consistent with the parton distribution predicted in pQCD (MLLA) • Qeff = 230±40 MeV for the entire range Mjj=80-600 GeV/c2 • Multiplicity of charged particles in jets: • particle multiplicity is proportional to that of partons: Nhadrons = KLPHD Npartons • KLPHD=0.56±0.10 for the entire range Mjj=80-600 GeV/c2 • ratio of multiplicities in gluon and quark jets rLPHD=1.6±0.2 is consistent with pQCD • Momentum correlations of particles in jets • particle momentum correlations follow pQCD predictions for partons • Qeff = 140±80 MeV for parabolic and linear terms • the constant (momentum-independent) term is much smaller than predicted… • kT-distribution of particles • data at low kT seem to follow theory… • but then sag at larger kT. Is it due to incomplete pQCD or hadronization? • the discrepancy gets smaller for larger Q • Global Event Shapes • data disagree with MC! • we are waiting for theory to catch up… • this branch of studies promises very interesting results

  20. P.S. Fragmentation = QCD + Hadronization • Jet fragmentation is one of the best manifestations of the rich structure of QCD • The more we learn, the less the role of phenomenological hadronization in shaping the jet structure seems to be… HADRONIZATION

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