Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 1 Quark and Gluon Jet Fragmentation Differences Abstracts covered in this talk Fragmentation differences of quark and gluon jets at the Tevatron (CDF) ‏ Charged particle multiplicities in 3-jet events and two-gluon systems (DELPHI) ‏ Andrey Korytov

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 2 Multiplicity in Gluon and Quark Jets: Theory CDF k T =1 GeV/c kTkT  Parton shower stage—challenge for pQCD calculations in very soft domain (k T < 1 GeV): resummation at LLA and NLLA precision: - start out from color singlet - small opening angles around jet direction - r = N g /N q = C A /C F = 9/4 = 2.25 continues flow of papers since mid-1980 to include corrections going beyond the NLLA accuracy most recent r = N g /N q = (Q= GeV) ‏  Hadronization stage—still a mystery: Believed to be local, independent of jet origin: N hadrons = K  N partons (Local Parton-Hadron Duality) ‏

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 3  Proved to be a hard measurement: continuous flow of papers from e + e - experiments (15+ papers over last 15 years): r = N g /N q ranged from 1 to 1.5 Multiplicity in Gluon and Quark Jets: Experiment

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 4 Gluon vs. Quark Jets: Historical Perspective r-measurement history Year r = Ng / Nq HRS OPAL SLD ALEPH DELPHI CLEO CDF NLL extensions C A /C F =9/4 (LLA, NLLA) ‏ Ratio r = N ch (gluon jet) / N ch (quark jet)‏

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 5 DELPHI: Theory behind the new analysis Eden et al., 1998: New look at relationship between N gg and N qq multiplicities - Predict N gg from N qq (e + e - ) data. - Match Integration constant to reproduce experimental N ch multiplicity in Υ -decays. Eden et al., 1999: Multiplicity in 3-jet events (e + e - data) can be written as a sum of a) restricted multiplicity in qq-system (two energy scales) ‏ b) half of unrestricted multiplicity in gg-system (single energy scale) ‏  Eden “A” No consensus on proper scales!  Eden “B” Various scales appearing in formulae above:

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 6 DELPHI: Multiplicity in 3-jet events DATA  s=91 GeV Events clustered in 3 jets Two independent variables:  1 and  3 q- and g-jets are not identified on event-by-event basis THEORY (with experimental input): Eden “A” Eden “B” 11 Jet 1 Jet 3 Jet 2 33

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 7 DELPHI: N gg from 3-jet events Use preferred Eden “A”: N gg (k Le ) is obtained from 1) N qqg = N 3-jet (  1,  3 ): measured multiplicity 2) Constrained N qq (L qq, k Lu ): evaluated from unconstrained multiplicity in e + e - data, using: Theory curve for N gg is based on N qq (s) data and normalized on this point

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 8 DELPHI: r = (N gg from 3-jet events) / N qq

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 9 CDF: Data analysis strategy Dijet and  -jet events with M jj and M  j ~ GeV  tracks are not used in jet reconstruction Dijet events (~60% gluon jets) and  -jet events (~80% quark jets) ‏  small uncertainties in energy range used (well known range of PDFs) ‏ Dijet or  -jet center of mass frame: E jet = ½M jj or ½M  j N ch multiplicity in cones with opening angle  from ~0.3 to ~0.5 rad Energy scale Q = 2E jet tan(  cone /2)  E jet  cone  Q=11-25 GeV (no controversy on proper scales for small opening angles) ‏ Some leading-order diagrams for dijet events Some leading-order diagrams for  -jet events Jet 1 Jet 2  cone p p

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 10 CDF: N ch multiplicities in gluon and quark jets  Multiplicities measured in range of Q=11-25 GeV  Tevatron and e + e - data (Q>10 GeV) agree  E  -scaling observed  Tevatron and e + e - data (Q>10 GeV) follow 3NLL trends  Multiplicities measured in range of Q=11-25 GeV  Tevatron and e + e - data (Q>10 GeV) agree  E  -scaling observed  Tevatron and e + e - data (Q>10 GeV) follow 3NLL trends two-parameter fit to CLEO (10 GeV) and OPAL (80 GeV) data e + e - model-dependent results

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 11 CDF: ratio r = N g / N q  Ratio measured in range of Q=11-25 GeV: e.g., r=1.64  0.17 at Q=19 GeV  E  -scaling observed  Data agrees with the recent NLL extensions  Ratio measured in range of Q=11-25 GeV: e.g., r=1.64  0.17 at Q=19 GeV  E  -scaling observed  Data agrees with the recent NLL extensions

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 12 CDF: Data vs. Monte Carlo Gluon Jets: Herwig 5.6 & Pythia reproduce multiplicity in gluon jets fairly well Quark Jets: Herwig 5.6 & Pythia over-estimate multiplicity in quark jets by ~30% Side Note on Pythia vs Herwig: Pythia gives ~3-4% higher multiplicity in comparison to Herwig

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 13 CDF: momentum distributions of particles dN/d  Gluon jets: Herwig and Pythia are in reasonable agreement with data Quark jets: Herwig and Pythia disagree with data x = p/E jet =

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 14 CDF: momentum-dependent ratio r(  )‏ Momentum distributions of charged particles in gluon and quark jets  Ratio reaches max and flattens for soft part of spectrum at ~1.8  0.2  Same pattern was observed at LEP x = p/E jet =

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 15 Summary CDF:  Multiplicities in gluon and quark jets and their ratio are measured for energy scales Q=11-25 GeV (analysis is model-independent): r = 1.64  0.17 at Q=19 GeV  E jet  cone -scaling is observed  Multiplicities and their ratio follow recent pQCD calculations pushing resummation precision beyond the Next-to-Leading Log Approximation DELPHI:  Multiplicities in gluon jets are measured for energy scales Q=13-53 GeV (analysis is model- dependent): r ~ 1.35 ( about 1% error) at Q=20 GeV  Ratio of multiplicities in gluon and quark jets agrees with recent pQCD calculations at Q>20 GeV and is systematically lower at Q<20 GeV  Multiplicities of charged particles in 3-jet events may help to sort out uncertainties with scales to be used for describing events of complicated topologies