Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 1

Quark and Gluon Jet Fragmentation Differences

Abstracts covered in this talk• 5-0081 Fragmentation differences of quark and gluon jets at the Tevatron (CDF)

• 5-0437 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

kT=1 GeV/c

kT

Parton shower stage—challenge for pQCD calculations in very soft domain (kT < 1 GeV):

• resummation at LLA and NLLA precision: - start out from color singlet- small opening angles around jet direction- r = Ng/Nq = CA/CF = 9/4 = 2.25

• continues flow of papers since mid-1980 to include corrections going beyond the NLLA accuracy most recent r = Ng/Nq = 1.4-1.7 (Q=20-100 GeV)

Hadronization stage—still a mystery:

• Believed to be local, independent of jet origin:Nhadrons = K Npartons (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 = Ng/Nq 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

0.0

0.5

1.0

1.5

2.0

2.5

1980 1985 1990 1995 2000 2005

Year

r = N

g / N

q HRSOPALSLDALEPHDELPHICLEOCDF

NLL extensionsCA /CF =9/4 (LLA, NLLA)

Ratio r = Nch(gluon jet) / Nch(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 Ngg and Nqq

multiplicities - Predict Ngg from Nqq (e+e-) data.

- Match Integration constant to reproduce experimental Nch multiplicity in Υ-decays.

Eden et al., 1999: Multiplicity in 3-jet events (e+e- data) can be written as a sum ofa) 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

Jet 1

Jet 3

Jet 2

3

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 7

DELPHI: Ngg from 3-jet events

Use preferred Eden “A”:

Ngg(kLe) is obtained from

1) Nqqg = N3-jet(1, 3): measured multiplicity

2) Constrained Nqq(Lqq, kLu): evaluated from unconstrained multiplicity in e+e- data, using:

Theory curve for Nggis based on Nqq(s) dataand normalized on this point

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 8

DELPHI: r = (Ngg from 3-jet events) / Nqq

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 9

CDF: Data analysis strategyDijet and -jet events with Mjj and Mj ~80-100 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: Ejet = ½Mjj or ½Mj

Nch multiplicity in cones with opening angle from ~0.3 to ~0.5 rad

Energy scale Q = 2Ejettan(cone/2) Ejetcone

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

conep p

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 10

CDF: Nch 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

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 = Ng / Nq

Ratio measured in range of Q=11-25 GeV: e.g., r=1.640.17 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 6.115 reproduce multiplicity in gluon jets fairly well

Quark Jets:Herwig 5.6 & Pythia 6.115 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 dataQuark jets: Herwig and Pythia disagree with data

x = p/Ejet = 1 0.5 0.1 0.05

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 ~1.80.2

Same pattern was observed at LEP

x = p/Ejet = 1 0.5 0.1 0.05

Andrey Korytov, University of Florida ICHEP2004 August 15-22, 2004, Beijing 15

SummaryCDF:

Multiplicities in gluon and quark jets and their ratio are measured for energy scales Q=11-25 GeV (analysis is model-independent): r = 1.640.17 at Q=19 GeV

Ejetcone-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