We present measurements by the Mark II experiment of the ratios of the leptonic partial widths of the Z boson to the hadronic partial width. The results are Γ_(ee)/Γ_(had)=0.037_(-0.012^()+0.016),Γ_(µµ)/Γ_(had)=0.053-_(0.015)^(+0.020), and Γ_(ττ)/Γ_(had)=0.066_(-0.017)^(+0.021), in good agreement with the standard-model prediction of 0.048. From the average leptonic width result, Γ_(ll)/Γ_(had)=0.053_(-0.009)^(+0.010), we derive Γ_(had)=1.56_(-0.24)^(+0.28) GeV. We find for the vector coupling constants of the tau and muon v_τ^2=0.31±0.31_(-0.30)^(+0.43) and v_μ^2=0.05±0.30_(-0.23)^(+0.34)

Abrams, G. S.

Barish, B. C.

Hawkes, C. M.

Kuhlen, M.

McKenna, J. A.

Milliken, B. D.

Peck, C.

Porter, F. C.

Soderstrom, E.

Stroynowski, R.

Weinstein, Alan J.

Weir, A. J.

Wicklund, E.

Wu, D. Y.

English

Caltech Authors - Main

VOLUME 63, NUMBER 26 PHYSICAL REVIEW LETTERS 25 DECEMBER 1989 
Measurement of Z Decays into Lepton Pairs 
G. S. Abrams, (I) C. E. Adolphsen, <2> D. Averill, (3) J. BaHam, <4> B. C. Barish, <5> T. Barklow, <4> B. A. 
Barnett, <6> J. Bartelt, <4 > S. Bethke, (I) D. Blockus, (3) G. Bonvicini, <7> A. Boyarski, <4> B. Brabson, <3> 
A. Breakstone, <8> F. Bulos, <4> P. R. Burchat, <2> D. L. Burke, <4> R. J. Cence, <8> J. Chapman, <7 > 
M. Chmeissani, <7> D. Cords, <4> D. P. Coupal, <4 > P. Dauncey, <6> H. C. DeStaebler, <4> D. E. Dorfan, <2> 
J. M. Dorfan, <4> D. C. Drewer, <6> R. Elia, <4> G. J. Feldman, <4> D. Fernandes, <4 > R. C. Field, <4> W. T. 
Ford, <9> C. Fordham, <4 > R. Frey, <7> D. Fujino, <4> K. K. Gao, <4> E. Gero, <7> G. Gidal, (I) T. Glanzman, <4 > 
G. Goldhaber, (I) J. J. Gomez Cadenas, <2> G. Gratta, <2> G. Grfndhammer, <4> P. Grosse-Wiesmann, <4 > 
G. Hanson, <4> R. Harr, Ol B. Harral, <6> F. A. Harris, <8> C. M. Hawkes, <5> K. Hayes, <4 > C. Hearty, (I) C. A. 
Heusch, <2> M. D. Hildreth, <4> T. Himel, <4> D. A. Hinshaw, <9> S. J. Hong,(?) D. Hutchinson, <4> J. Hylen, <6> 
W. R. Innes, <4> R. G. Jacobsen, <4> J. A. Jaros, <4> C. K. Jung, <4> J. A. Kadyk, (I) J. Kent, <2> M. King, <2> 
S. R. Klein, <4 > D. S. Koetke, <4> S. Komamiya, <4> W. Koska, <7> L. A. Kowalski, <4> W. Kozanecki, <4> J. F. 
Kral, (I) M. Kuhlen, <5> L. Labarga, <2> A. J. Lankford, <4> R. R. Larsen, <4> F. Le Diberder, <4> M. E. Levi, (I) 
A. M. Litke, <2> X. C. Lou, (3) V. Liith, <4> J. A. McKenna, <5> J. A. J. Matthews, <6> T. Mattison, <4> B. D. 
Milliken, <5> K. C. Moffeit, <4> C. T. Munger, <4 > W. N. Murray, <3> J. Nash, <4> H. Ogren, <3> K. F. 
O'Shaughnessy, <4> S. I. Parker, <s> C. Peck, <5> M. L. Perl, <4> F. Perrier, (4 ) M. Petradza, (4 ) R. Pitthan, (4 ) 
F. C. Porter, <5> P. Rankin, <9> K. Riles, <4> F. R. Rouse, <4> D. R. Rust, <3) H. F. W. Sadrozinski, <2> M. W. 
Schaad, (I) B. A. Schumm, (I) A. Seiden, <2> J. G. Smith, <9 > A. Snyder, <3> E. Soderstrom, <5> D.P. Stoker, <6> 
R. Stroynowski, <5> M. Swartz, <4 > R. Thun, <7> G. H. Trilling, (I) R. Van Kooten, <4> P. Voruganti, <4> S. R. 
Wagner, <4 > S. Watson, <2> P. Weber, <9> A. Weigend, <4> A. J. Weinstein, <5> A. J. Weir, <5> E. Wicklund, <5> 
M. Woods, <4 > D. Y. Wu, <5> M. Yurko, (3) C. Zaccardelli, <2> and C. von Zanthier<2> 
(!)Lawrence Berkeley Laboratory and Department of Physics, University of California, Berkeley, California 94720 
(2)University of California, Santa Cruz, Santa Cruz, California 95064 
<J> Indiana University, Bloomington, Indiana 47 405 
<4> Stanford Linear Accelerator Center, Stanford University, Stanford, California 94309 
<s>california Institute of Technology, Pasadena, California 91125 
<6>Johns Hopkins University, Baltimore, Maryland 21218 
<1luniversity of Michigan, Ann Arbor, Michigan 48109 
<s>university of Hawaii, Honolulu, Hawaii 96822 
<9 >University of Colorado, Boulder, Colorado 80309 
(Received 24 October 1989) 
We present measurements by the Mark II experiment of the ratios of the leptonic partial widths of the 
Z boson to the hadronic partial width. The results are r •• !rhad -0.037 ±8:81t r,.,./rhad -0.053±8:8rg, and 
r ,,/rhad -0.066 ±8:8H, in good agreement with the standard-model prediction of 0.048. From the average 
leptonic-width result, ru/rhad -0.053 ±8:863, we derive rhad -1.56±8:~~ GeV. We find for the vector cou-
pling constants of the tau and muon v'! =0.31 ± 0.31 ±8:1il and v} -0.05 ± 0.30±8:H. 
PACS numbers: 13.38.+c, 12.15.Ji 
In this Letter we present the first study of Z-boson de-
cays into lepton pairs using the Mark II detector at the 
SLAC Linear Collider. We present the ratio of the par-
tial decay width into lepton pairs to the partial decay 
width into hadrons, 
drift chamber in the angular range I cos9l < 0.92, where 
9 is the angle with respect to the beam axis. The elec-
tromagnetic calorimeters consist of a barrel covering the 
angular range I cos9l < 0. 72 and end caps extending the 
coverage to I cos9l < 0.96. The Mark II data-
acquisition system is triggered by either two or more 
charged tracks within I cos9l < 0. 7 6, or by an elec-
tromagnetic shower with localized energy deposition 
greater than 3.3 GeV in the barrel or greater than 2.2 
GeV in the end-cap calorimeters. 
~ _ B (Z ____,. II) 
rhad B(Z ____,. hadrons) ' 
which is independent of the center-of-mass energy. In 
the standard model, this ratio is expected 1 to be 0.048. 
Data were collected in the e + e - center-of-mass ener-
gy (Ec.m) range of 89.2 to 93.0 GeV and correspond to 
an integrated luminosity of 19 nb -I. A detailed descrip-
tion of the Mark II apparatus can be found in Ref. 2. 
This analysis uses information mainly from the central 
drift chamber and the electromagnetic calorimeters. 
Tracks from charged particles are reconstructed in the 
In order to study the leptonic decays of the Z, we 
select electron-pair, .u-pair, r-pair, and hadronic-event 
candidates, and then correct the observed number of 
events for detection inefficiency and for backgrounds 
from beam-gas interactions, cosmic rays, two-photon in-
teractions, and misclassified Z decays. 
Charged tracks used in the event selection must origi-
2780 © 1989 The American Physical Society 
VOLUME 63, NUMBER 26 PHYSICAL REVIEW LETTERS 25 DECEMBER 1989 
nate from a cylindrical volume of radius 1 em and half-
length 3 em around the beam line, centered at the in-
teraction point. To ensure efficient track reconstruction, 
the tracks must lie in the angular range I cosO I < 0.82 
and have a transverse momentum with respect to the 
beam axis greater than 0.15 Ge VI c. Calorimeter energy 
depositions greater than 1 GeV that are not associated 
with a charged track are used together with charged 
tracks to calculate the visible energy of the event. 
Events from Z decays are required to have visible energy 
larger than 15% of E c.m. and charged energy larger than 
5% of E c.m. to minimize contributions from two-photon 
interactions and from beam-gas interactions. 
Hadronic and leptonic decays are separated on the 
basis of the number of tracks and the event thrust. Ha-
dronic events are selected by requiring at least seven 
charged tracks coming from the interaction region, and 
leptonic-event candidates are required to have fewer than 
seven charged tracks. In addition, lepton candidates 
must have at least one charged track in each event hemi-
sphere, defined by the plane perpendicular to the thrust 
axis calculated from the charged particles, and no tracks 
outside I coso! < 0.82. 
The separation of the leptonic Z decays into e, J.l., and 
-r pairs requires additional criteria. Electron events are 
expected to deposit the full available center-of-mass en-
ergy in the calorimeter, and they are separated from J.1. 
and -r events by requiring that the total calorimeter ener-
gy Ecal is larger than 80% of Ec.m.· The missing momen-
tum carried away by neutrinos for -r events is used to 
divide the remaining two-prong sample into J.1. and -r can-
didates. Events are taken as muon candidates if both 
tracks have momentum larger than 60% of the beam 
momentum. Otherwise they are classified as -r events. 
The effect of these two cuts is illustrated in Figs. 1 and 2. 
In addition, muon events are restricted to the fiducial 
volume of the barrel calorimeter limited by I cosO I 
< 0.68, and the shower energies associated with the 
20 
• Data 
-eMC 
;g 15 
--- ll MC 
ci ........ 1:MC 
iJ5 
f- 10 Cut z 
L.U 
> L.U 
5 
0.2 0.4 0.6 0.8 1.0 1.2 
Ecai/Ecm 
FIG. I. Calorimeter energy divided by Ec.m. for data and 
Monte Carlo predictions for electron (solid), muon (dashed), 
and , (dotted) pairs. The arrow indicates the cut at 0.8 to 
separate electron from muon and • events. 
tracks have to be smaller than 15 GeV. Lepton candi-
dates with more than one track in either thrust hemi-
sphere are classified as -r pairs if the effective mass of the 
charged tracks in that hemisphere is smaller than 2.2 
GeV /c 2 and the event thrust is greater than 0.99. This 
cut suppresses background from hadronic events. 
The detection efficiencies and contributions from other 
physics processes are calculated using Monte Carlo 
simulations. We use the LUND (version 6.3) parton-
shower model 3 to simulate hadronic decays of the Z. 
For the J.l.· and -r-pair production we use the KORALZ 
generator, 4 which includes initial- and final-state radia-
tion effects. Electron-pair production and two-photon in-
teractions are simulated using the generators described 
in Refs. 5 and 6, respectively. Monte Carlo-generated 
events are used to simulate raw data events from the 
Mark II detector. They are combined with data from 
random beam cross triggers to ensure that the effects of 
beam-related noise on the reconstruction of events are 
taken into account. The simulated events are then 
reconstructed in the same way as the real data. 
The detection efficiencies, which include acceptance 
corrections, for the different classes of events with the 
selection criteria described above are 88% for hadronic 
events, 66% for -r pairs, 54% for J.1. pairs. The efficiency 
for electron pairs produced at the Z peak within I cosO I 
< 0.82 is 93%. There are 397 hadronic events, 13 J.1. 
pairs, 21 -r pairs, and 18 electron pairs which satisfy the 
selection criteria. 
For the -r sample the estimated background from qij 
production is 0.6 event, from electron-pair production 
0.2 event, and from J.l. pairs 0.5 event. For the J.1. sample 
the background from electron and -r pairs and from ha-
dronic events is less than 0.1 event. The background for 
the electron sample from all other physics processes is 
less than 0.15 event. Backgrounds from cosmic rays and 
10 
• Data 
Jj 8 -eMC 0 --- ~-tMC ~ 0 6 ······· 1:MC U5 f- Cut z I rf l -: L.U 4 > L.U 2 
r--1 I __ 
0.2 0.4 0.6 0.8 1.0 1.2 
Pmin/Pbeam 
FIG. 2. Ratio of Pmin to the beam momentum for two-track 
events after the cut to remove electron events. Pmin is the 
smaller of the two momenta in the event. The data are shown 
with a Monte Carlo prediction for electron (solid), muon 
(dashed), and • (dotted) pairs. A cut at 0.6 is used to separate 
muon and • events. 
2781 
VOLUME 63, NUMBER 26 PHYSICAL REVIEW LETTERS 25 DECEMBER 1989 
beam-gas interactions are estimated by displacing the 
volume which defines the interaction region along the 
beam direction. We then look for events originating 
from the displaced volume which otherwise would fulfill 
the event-selection criteria. From this procedure we esti-
mate a background of 0.15 event for the J.l sample, 0.3 
event for the -r sample, and < 0.4 event for the hadronic 
sample. The remaining backgrounds in the qij and lep-
tonic samples are negligible. 
The electron sample has a sizable contribution from 
QED and weak-electromagnetic interference. To reduce 
this contribution we make the additional demand that 
the positron scattering angle 9 be in the range -0.82 
< cos(}< 0.68, eliminating the forward region where the 
QED contribution is largest. This cut leaves twelve 
events with a background of 1.4 ± 0.6 events due to QED 
and weak-electromagnetic interference. The overall 
efficiency after all cuts is 62%. 
The systematic errors include the uncertainty of the 
calorimeter energy scale and the trigger efficiency, the 
estimate of the effect of beam-related noise and the mod-
el dependence of the Monte Carlo generation of hadronic 
events; in all cases they are much smaller than the sta-
tistical errors. 
After inclusion of all systematic and statistical uncer-
tainties 7 the resulting ratios of the partial decay widths 
are 8 
reelrhad =0.037 :!::8:81~' 
ri' . Jrhad -o.o53 :!:8:8r~, 
r .. lrhad =0.066 :!:8:8r~. 
These results are consistent with each other and agree 
well with the standard-model prediction of 0.048. Under 
the assumption of lepton universality the combined lep-
ton sample yields ru/rhad =0.053 :!:8:8b3. 
The hadronic width of the Z depends on the quark 
couplings and may have additional contributions from 
new-particle production. Since the lepton couplings are 
experimentally better constrained than the quark cou-
plings, we assume a standard-model width for the lep-
tons, r 11 =0.083, GeV, and derive from our measurement 
the hadronic width, rhad -1.562:8j~ GeV. The predic-
tion from the standard-model quark couplings is rhad 
-1.73 GeV, in agreement with our measurement. 
The number of leptonic events can also be used to pro-
vide measurements of the leptonic couplings. At the Z, 
the cross section is proportional to (ae2 +ve2 Ha?+v/), 
where a and v denote the axial-vector and vector cou-
plings of the incoming electrons and the final-state lep-
tons. We compare the measured number of J.l-pair and 
-r-pair events with the expectation from our luminosity 
measurements and the cross section with Vt as a parame-
ter. We use the cross-section formula given in Ref. 9 to-
gether with our measured values for the Z mass and 
width, 10 Mz=-91.14±0.12 GeV/c 2 and r=2.42:!:8:13 
2782 
GeV. If we fix the experimentally well-constrained 
quantities ae, ve, and at to their standard-model values, 
we find v; =0.31 ± 0.31 :!::8:16 and v} =0.05 ± 0.30 :!::8:~~­
The first error represents the statistical error, and the 
second the systematic error, which is entirely dominated 
by our uncertainty in the Z width. These results can be 
compared with previous measurements, v,- - 1.04 
± 1.25, 11 vi'= -0.24 ± 0.32, 12 and the standard-model 
value vt- -9.01. 
The forward-backward charge asymmetry AFB for lep-
tons in e + e - annihilation is defined as the difference of 
the integrated differential cross sections for the positively 
charged lepton emitted into the forward and backward 
hemispheres, where the forward hemisphere is defined by 
the direction of the positron beam. For the combined 
lepton sample of 9 muon and 15 -r events from the region 
around the Z pole, 90.6 Ge V < E c.m. < 91.5 Ge V, we 
measure AFB ==0.05 ± 0.22, where a radiative correction 
of +0.02 has been applied. 13 For our data set we expect 
AFa-0.026 for Mz -91.14 GeV/c 2, in good agreement 
with the measurement, but the result is not precise 
enough to place useful limits on standard-model parame-
ters. 
In conclusion, we have measured the ratios of partial 
widths of the Z decaying into electrons, muons, and -r 
leptons to the hadronic partial width. We have derived 
the hadronic partial width and values for the vector cou-
pling constants of the J.l and -r leptons. These measure-
ments confirm within their statistical precision the weak 
couplings of the leptons and quarks to the Z boson as 
predicted by the standard model. 
One of us (M. Kuhlen) wishes to acknowledge the 
Alexander von Humboldt Foundation for its support. 
This work was funded in part by Department of Energy, 
Contracts No. DE-AC03-81-ER40050 (California Insti-
tute of Technology), No. DE-AM03-76SF00010 (Uni-
versity of California, Santa Cruz), No. DE-AC02-
86ER40253 (University of Colorado), No. DE-AC03-
83ER40103 (University of Hawaii), No. DE-AC02-
84ER40125 (Indiana University), No. DE-AC03-
76SF00098 (LBL), No. DE-AC02-76ER01112 (Univer-
sity of Michigan), and No. DE-AC03-76SF00515 
(SLAC), and by the National Science Foundation 
(Johns Hopkins University). 
1W. F. L. Hollik, DESY Report No. DESY 88-188, 1988 
(unpublished). A top-quark mass of 100 GeV/c 2 is assumed, 
and a QCD correction of 5% has been applied for the hadronic 
width. 
2G. S. Abrams et a/., Nucl. Instrum. Methods Phys. Res., 
Sect. A 281, 55 (1989). 
3T. Sjostrand, Comput. Phys. Commun. 39, 347 (1986). 
This model was found to describe well the general featues of 
hadronic Z decays; see G. S. Abrams et a/., Phys. Rev. Lett. 
63, 1558 (1989). 
4S. Jadach and Z. Was, Comput. Phys. Commun. 36, 197 
VOLUME 63, NUMBER 26 PHYSICAL REVIEW LETTERS 25 DECEMBER 1989 
(1985). 
5F. A. Berends, W. Hollik, and R. Kleiss, Nucl. Phys. B304, 
712 0988). 
6F. A. Berends, P. H. Daverveldt, and R. Kleiss, Comput. 
Phys. Commun. 40, 309 (1986). 
7The statistical errors are calculated as 68.3%-C.L. intervals 
using the technique of F. James and M. Roos, Nucl. Phys. 
Bl72, 475 (1980). 
8The overall detection efficiencies for hadronic, electron, 
muon, and -r events are 0.88 ± 0.04, 0.62 ± 0.04, 0.54 ± 0.03, 
and 0.66 ± 0.04, respectively. The corrected numbers of events 
are 450 ± 30 hadronic events, 16.7 ± 5.6 e events 23.7 ± 6.8 Jl 
events, and 29.5 ± 7.2 -r events. 
9R. N. Cahn, Phys. Rev. D 36, 2666 (1987). 
lOG. S. Abrams eta/., Phys. Rev. Lett. 63,2173 (1989). 
llW. T. Ford eta/., Phys. Rev. D 36, 1971 (1987). 
12A. Argento eta/., Phys. Lett. llOB, 245 (1983). This mea-
surement assumes standard-model couplings for the quarks. 
13J. E. Campagne and R. Zitoun, Phys. Lett. B 222, 497 
(1989). 
2783 


Measurement of Z Decays into Lepton Pairs

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Measurement of Z Decays into Lepton Pairs

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