Studies of the hadronic decaysof Z bosons at LEP

Alessandro De AngelisUniversity of Udine and INFN Trieste

ICHEP 2000 Osaka, July 2000

105) OPAL: Charge multiplicities in Z decays to u,d,s pairs163) ALEPH: Inclusive Production of the ω(782)161) ALEPH: Inclusive Production of π0, η, η'(958), K0

and Λ in 2- and 3-Jet Events641) DELPHI: Rapidity-rank structure of p-pbar pairs

~ 4MZ / experiment recorded in 1990-95

A. De Angelis - ICHEP 002/13

z Experimental problem: tagthe decays into light quarksy Leading particle

z Calculation of themultiplicity in MLLA => αs

for light flavors

105) OPAL: Charged multiplicities inu-ubar, d-dbar, s-sbar events

z QCD: flavour symmetryy Investigate the multiplicity for Z

decays into different flavoursy Already studied in the case of the

decay into b-bbar (mass effectdemonstrated)

u,d,s

u-bar,d-bar,s-bar

A. De Angelis - ICHEP 003/13

OPAL: Charged multiplicities in u-ubar, d-dbar, s-sbar eventsLight flavour tagging: high-x K0

S, K+, charged particles

z Leading K0: tags a primary s (and to a < extent a d)z Leading K+: tags a primary s (and to a < extent a u)

–Fast leading charged particle:antitags c, b

xp

K0s

s

d-bar

d

d

s-bar

s

~ 3

A. De Angelis - ICHEP 004/13

OPAL: Charged multiplicities in u-ubar, d-dbar, s-sbar eventsResults and conclusions

z 3 independent equations (one for each sample)z Cross-checked for double tagsz To reduce the bias, study of the hemisphere

opposite to the one used for tagging

z Systematics from fragmentation, nheavy

0.861.20

1.461.17

0.390.37

17.77 0.52

21.44 0.69 ( , ) 0.89

20.02 0.14

u

d u d

s

n

n C n n

n

+−

+−

+−

= ±

= ± = −

= ±

nu/nd consistent with 1 in 1.8σσ, ns/nd in 0.9σσ, ns/nu in 1.5σσ=> ααs values also consistent at precisions ~ 5% to 9%

<nch>

A. De Angelis - ICHEP 005/13

163) ALEPH: Inclusive Production of the ω (782)

z Identified particles: a key testfor hadronization modelsy in particular vector mesons,

closer to the main event

z Fit in 6 xp intervalsy 0.05 - 0.10 (not used for η)y 0.10 - 0.15y 0.15 - 0.20y 0.20 - 0.30y 0.30 - 0.50y 0.50 - 1.00

23%)~(BR 0

0

0.17~eff.

89%)~(BR

π−π+π→η

−+ πππ→ω

A. De Angelis - ICHEP 006/13

z Most precise measurement of <ω>, accuracy on <η> comparable with L3 & OPAL

ALEPH: Inclusive Production of the ω (782)Results

z The observation of a peak of 18 events in ω -> µ+µ- allows to estimate

5( ) (9.0 2.9 1.1) 10BR ω µ µ+ − −→ = ± ± ×

improving the present upper limit, and consistentwith theory (Van Royen and Weisskopf 67)

1.06 0.03 0.06 0.08 (PDG2000 :0.95 0.07)

JETSET OK, HERWIG ~ TOO HIGH & STEEP

1.00 0.03 0.05 0.02 (PDG2000 :1.08 0.09)

BOTH JETSET & HERWIG TOO HIGH

η

ω

< >= ± ± ± ±

< >= ± ± ± ±

A. De Angelis - ICHEP 007/13

161) ALEPH: Inclusive Production of π0, η, η'(958), K0 and Λ in 2- and 3-Jet Events

z ALEPH: selection of 3-jet events withDURHAM, ycut = 0.01y 64% of the events are 2-jety 31% of the events are 3-jet

z Jets ordered by energy, and jet 3 isthe gluon (OK with 71% probability)

z Isoscalars (η, η’, ω, φ) could be enhanced in gluon compared to quark jetsy Peterson & Walsh 80: isoscalars produced directly in g fragmentationy Hwa & al. 77,81; Migneron & al. 82y Ball, Frère & Tytgat 96: significant gg component in η’y Fritzsch 97: large coupling of η’ to g

z ARGUS 96, Crystal Ball 91 (~ 10 GeV); JADE 85 (~ 34 GeV) don’t seeany effect; L3 96 sees a η rate OK in 2-jet events, but a harderproduction than in QCD-inspired models for 3-jet events

A. De Angelis - ICHEP 008/13

ALEPH: Production of π0, η, η'(958), K0 and Λ in 2- and 3-Jet EventsReconstruction of π0, η, η'(958)

0.3-0.1 ~ eff.

44%)(BR )958(' =πηπ→η −+

0.5-0.2 ~ eff.

%)93(BR

%)99(BR 0

=γγ→η=γγ→π

A. De Angelis - ICHEP 009/13

ALEPH: Production of π0, η, η'(958), K0 and Λ in 2- and 3-Jet EventsReconstruction of K0

s and Λ

0.5 ~ eff.

69%)(BR K0S =ππ→ −+

0.5 ~ eff.

64%)(BR p =π→Λ −

A. De Angelis - ICHEP 0010/13

ALEPH: Production of π0, η, η'(958), K0 and Λ in 2- and 3-Jet EventsConclusions

z K0 and Λ spectra: OKin JETSET, ARIADNE;not in HERWIG(best determination ofK0 and Λ multiplicity)

K0sΛΛ

z Isoscalar sectorconsistent with JETSETfor 2- and 3-jet (η’ : extra“ad hoc” suppression of0.275)y Too steep x dependence

predicted by HERWIG5.8 & 5.9 in 2-jets

ηη

A. De Angelis - ICHEP 0011/13

641) DELPHI: Rapidity-rank structure of p-pbar pairs

z Meson fragmentation within thestring model: quark-antiquark pairscreated from the string potential

z Baryons come from the creation ofdiquark-antidiquark pairs

z Popcorn mechanism: mesons canpossibly “pop up” in an effectivediquark-antidiquark pair

y Free parameter in Jetset, relatedto the probability of B-M-Bbar

z The baryon sector in the QCD-inspired Monte Carlos is critical.Neither the cluster nor the stringmodels are fully OK, but thelatter give a better picture

A. De Angelis - ICHEP 0012/13

DELPHI: Rapidity-rank structure of p-pbar pairsThe technique of the rapidity ordering

z The popcorn probability influencesthe ∆y distribution of baryon-antibaryon pairs

z Previous studies of the ∆ydistribution of Λ−Λbar pairs(OPAL93, DELPHI93, ALEPH94)indicate that popcorn can beimportant (~50-80%)

z DELPHI 2000 proposes a moredirect test: the study of therapidity ordering of p-pbar pairs

A. De Angelis - ICHEP 0013/13

z Observable: relative amount of thep-M-pbar configuration wrt ∆ymin

z Selection: ~27,000 events with onep and one pbar in a hemispherey purity 63%

Background subtracted (pp)

z Consistency with no popcorn !Popcorn < 15% @ 90% CL

z The results contradict the ΛΛ case,pointing to inconsistencies in thebaryon sector of the Lund model

DELPHI: Rapidity-rank structure of p-pbar pairsConclusion: no need for the popcorn mechanism !

R( ∆ymin )100% popcorn

no popcorn