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Fermi National Accelerator Laboratory
FERMILAEi-Pub-94/301-E
A Direct Measurement of the W Boson Width I? (W)
F.Abe et. al The CDF Collaboration
Fermi National Accelerator Laboratory P.O. Box 500, Batavia, Illinois 60510
September 1994
Submitted to Physical Review Letters September 6,1994
a Operated by Universities Research Association Inc. under Contract No. DE-AC02-76CH03000 with the United States Department of Energy
Disclaimer
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
CDF/ANAL’ELECTROWEAWCDFR/2642
FERMILAB-PUB-94/301-E Version 4.0
A Direct Measurement of the WBoson Width r(W)
F. Abe,13 M. G. Albrow, D. Amide&l6 J. Antos,*s C. Anway-Wiese,4 G. Apollinari,*s H. Ared,’ M. Ata~,~ P. Auchincloss,*5 F. Azfar,*r P. Azzi,*o N. Bacchetta,ls W. Badgett,lG
M. W. Bailey,18 J. B~o,~~ P. de Barbaro,*s A. Barbaro-Galtieri,14V. E. Barnes,*4 B. A. Barnett,l* P. Bartalini,23 G. Bauer,ls T. Baumann,s F. Bedeschi,*s S. Behrends,s
S. Belforte,*” G. Bellettini,*s J. Bellinger,3s D. Benjamin,32 J. Benlloch,ls J. Bensinger,s D. Benton,*’ A. Berehas,7 J. P. Berge,’ S. Bertolucci,* A. Bhatti,zs K. Biery,tr M. BinkIey,r F. Blrd,2g D. Bisello,*o R. E. Blair ,r C. Blocker,*9 A. Bodek,zs W. Bokharl,15 V. Bolognesi,*s D. Bortoletto,24 C. Boswell,l* T. Bou10s,~~ G. Brandenburg,g E. Buckley-Geer,7 H. S. Budd,*s K. Burke&l6 G. Busetto,20 A Byon-Wagner,7 K. L Byrt~m,~ J. Cammerata,12 C. Campagnari,7
M. Campbell,16 A. Caner,7 W. Carithers,t4 D. Carlsmith J. Chapman,16 M.-T.
,33 A Ca.~tro,~~ Y. Cen,21 F. Cervelli,*3 Cheng, 28 G. Chiarelli,* T. Chikamatsq31 S. Cihangir,’ A. G. Cla~-k,*~
M. Cobal, M. Contreraq5 J. Conway,27 J. Cooper,7 M. Cordelli,* D. Crane,1 J. D. Cunningham,3 T. Danielsls F. DeJongh? S. Delchamps,’ S. Dell’Agnello,23 M. Dell’Orso,*3
L Demorder,26 B. Denby S. Donati,23 R B.
,23 M. Deninno,* P. F. Denver&l6 T. Devlin,*’ M. Dickson,*5 Drucker, l4 A. Dunn,16 K. Einsweller,14 J. E Elhq7 R Ely,14
E Engels, Jr.,** S. I3t0,~ D. Brrede,lo S. Errede,to Q, Fan,25 B. Farhat,ls I. Fiori,* B. Flaugher,’ G. W. Foster ,7 M. Franklin,g M. Frautschi,l* J. Freeman,’ J. Frk~Iman,~~
H. Frisch,5 A. Fry,zg T. A. Fuess,l Y. Fukui,ls S. Funaki,sr G. Gagliardi,*s S. Galeotd,zs M. Gallinaro,20 A. F. Garfinkel,24 S. Geer,7 D. W. Gerdes,r6 P. Giannetd,23 N. Giokaris,*b
P. Giromini,a L Gladney,*r D. Glenzinski,12 M. Gold,‘* J. Gonzalez,” A. Gordon,9 A. T. Goshaw,c K. Goulianos,26 H. Grassmann,6A. Grewal ,*l G. Griec~,*~ L Groer,*’ C. Grosso-
Pilcher,s C. Haber,l* S. R. Hahn,7 R. Hamilton,9 R Handler,33 R M. Hans,34 K. Hat-a,31 B. Hart~I,~l R. M. Harris,’ S. A. H auger,6 J. Hauser,’ C. Hawk,27 J. Heinrkh,21 D. Cronin-
Hennessy,6 R Hollebeek,*l L Holloway,10 A. Holscher,lt S. Hong16 G. Houk,*l P. Hu,** B. T. H~ffman,~~ R H~ghes,~~ P. Hurqg J. Huston,17 J. H~th,~ J. Hyleq7 M. Incagli,23
J. Incandela,’ H. 1~0,s~ H. Jensen ,I C P. Jessop,g U. Joshi, R W. Kadel,” E Kajfasz,7a
R. D. T. Kamoq30 T. Kaneko,31 D. A. Kardelis,lo H. Kasha,s4 Y. Kato,ls L Keeble,so Kennedy, 27 R Kephart,7 P. Kesten ,14 D. Kestenbaum,g R M. Keup,l” H. Keutelkq7
F. Keyvan,’ D. H. I&q7 H. S. Kim,” S. B. Kim,16 S. H. Klm,31 Y. K. Kim,14 L Kirsch,s P. Koehn,2s K. Kondo,31 J. Konigsberg,s S. Kopp,s K. Kordas,ll W. Koska,7 E Kovacs,ra
W. Kowald,6 M. Krasberg,‘” J. Kroll ,’ M. Kruse,*‘S. E Kuhlmann,’ E Kuns,*7 A. T. Iaasanen,24 N. Labanca,23 S. Lamn~I,~ J. I. Lamoureu~,~ T. LeCompte,lo S. Leone,23 J. D. Lewi~,~ P. Limon? M. Lindgren,’ T. M. Liss,lO N. Lockyer,*l C. Loomis,*7 0. Long,*’ M. Loreti,*O E H. Low,*l J. Lu,~ D. Lucchesi,*s C. B. Luchini,lo P. Lukens,’
K. Maeshima,7 A P. Maas,
Maghakian, 26 P. Maksimovi~,~~ M. Mangan~,*~ J. Mansour,17 M. Mariotti,*s J. P. Marriner,’ A Martin,r” J. A J. Matthews,” R Matdngly,rs P. McIntyqso P. MeIese,26 A Menzione,23 E Meschi,zs C. Mkhail,g S. Mikamo,~3 M. MilIer,s R Miller,17 T. Mhnashi,sr
S. Miscetti,8 M. Mishina,lj H. Mitsushlo,31 S. Miyashita,st Y. Mot-&a,13 S. MouIding,zs J. Mueller,*’ A Mukherjee,’ T. Muller? P. Musgrave,ll L F. Nakae,2g I. Nakano,31 C Nelson,7
D. Neuberger,* C. Newman-H~lmes,~ L Noduhnar~,~ S. Oga~a,~l S. H. Oh,6K. E Oh1,34 R Oishi,31 T. Okusawa,19 C. Pagliarone,zs R Paoletti,*J V. Papadimitrioy7 S. Park,7
J. Patrick,’ G. Pauletta,23 M. PauIini,14 L Pescar~~~ M. D. Peterqr4 T. J. Phillips,6 G. Piacendno,* M. Pillai,25 R Plunkett,’ L Pondrom, 33 N. Prod~it,‘~ J. Proudfoot, F. Ptohos,g
G. Punzi,23 K. Ragan,ll F. Rimondi,2 L Bistorl,23 M. Roach-BeIlino,32 W. J. Robertson,6 T. Rodrigo,’ J. Romano,s L Rosenson,ls W. K. Sak~moto,*~ D. Salt~berg,~ A Sansoni,s
V. Scarpine,so A Schindler,l’ P. Schlabach ,g G. F. Sciacca,zs A !kribano,~3 S.
E E Schmidt,7 M. P. Schmidt,” 0. Schneider,14 Segler ,’ S. Seidei18 Y. Seiya,3t G. Sganos,lt A Sgolacchia,z
M. Shapiro,‘” N. M. Shaw,24 Q, Shen,*4 P. F. Shepard,** M. Shimojima,31 M. Shochet,5 J. Siegrist,2g A SiR7a P. Sinervo ,11 P. Singh,22 J. Skarha,l* K. !%~a,~~ D. A Smith,*3 F. D. Snider,12 L Song7 T. Son&l6 J. Spalding,7 L Spiegel,7 P. Sphkas,ls A Spies,12
Submitted to Physical Review Letters September 6, 1994
L Stanc~,~~ J. Steele,33 A. Stefanini,23 K. Strahl,” J. Strait,7 K. Sumorok,ls R. L Swam, Jr.,lO T
D. Stuart,’ C. Sullivan,s . Takahashi ,lg K. Takikawa,si F. Tartarelli,23 W. Taylor,”
Y. Teramoto,lg S. Tether,t5 D. Theriot,‘J. Thomas,2g T. L T~oIMs,~~ R. Thun,l6 M. Tlmko 32 P. Tipton, A. Titov,26 S. Tkaczyk,’ K. Tollefson,25 A. Tollestrup,7 J. Tonnison 24 ’
J. F. de Troconiqa J. Tseng,l2 M. Turcotte,2g N. Turini, N. Uemurq3r F. Ukegawa,zr G. Unal,21 S. van den Brink,22 S. Vejcik, IILl R. Vidal,’ M. Vondracek,lo R. G. Wagner,1
R. L Wagner,’ N. Wainer ,’ R. C. Walker,25 G. Wang,23 J. Wang,s M. J. Wang,28 Q F. Wang,26 A. Warburton, G. Watts,2s T. Watts, 27 R. Webb,30 C. Wendt,33 H. Wenzel,14
W. C. Wester III,l4 T. Westhusing,lo A, B. Wicklund,t E. Wicklund, R. Wilkinson,2i H. H. Williams,21 P. Wilson,5 B. L Winer,25 J. Wolinski,30 D. Y. Wu,16 X. WU,~~ J. WYSS,~~
A. Yagii,’ W. Yao,14 K. Yasuoka, 31 Y. Ye,lr G. P. Yeh,’ P. Yeh,28 M. Yin,‘j J. Yoh,’ T. Yoshida,lg D. Yovanovitch,71. Yu,~~ J. C Yun,’ A. Zanetti,23 F. Zetti,23 L Zhang,33 S. Zhang,16
W. Zhang,21 and S. Zucchelli2
(CDF Collaboration)
lArgonne National Laboratory, Argonne, Illimis 60439 2IstUu~ Naa-di Plska Nuleare, University of Bobgua, 140126 Bologna, ItAy
3Brandeis University, Walthm. Massachusetts 02254 4univ~ly of chmmia at Los Angeles, Los Angeles, California 90024
5university ofchlcyo, chkag* Iullois 60637 ‘Duke Unlverslty, Durham, North Guolina 27708
‘Fenni National AcceIeratm Labwatory, Batavia, IlUds 60510 8hbontod Nazhali di FrascaU, Istituto Nazicmale dI FIska Nudeare, I-OoWd Frascati, Italy
91ianwdunlverslty, Cambrklge, Massachusetts 02138 lo Unh’ersity of Ullnois, Urbana, Ill&h 61801
’ ’ hstit~te Qf lbtkle F%~~cs, McGffl University, MonUeaI H3A 2T6, and U-ty of Toronto, Toronto MSS 1 A’, Canada 12Tbe Johna Hopkins University, Baltlnmre, Maryland 21218
13Nathul Islb0ntm-y for High Energy Physics (KEK), Tsukuba. Ibarakl305, Japan %awre!ue Berkeley Iabuaw, Berkdey, Califcxnia 94720
%iamchusetts Institute of Technology, CiuhMge, -usetts 02139 16UnhmrsltyofMkh&an,AnnArbor,Mkh@n48109
“b-&an state University, hst Lamin& Idida@n 48824 %nhrsItyofNaHexko,AIbuqucrpuqNew-87131
“Osaka City University, Osaka 588, Japan 29mhrenleadiPadovl,~UtutoNsrzbllsrlediFlslcPNudePre,~~dtPdov;SI-3513lPadwa,I~
21university of Fennsyhfan& Pbhb@h@ Femsylvarda 1910)
22univarslty of Pkbur& Pktsburg& Penusyfvcmlp 15260 23Istltllt0N~IllledlFUolNudeP1P,UniversityludScudpNormalcS~~~ofPfs,E56100PLsa.Italy
2+IuThle Unlvdty, west Iahyeno Indiana 47907 2bdversityof Rochter, Rubeater, New York 14627 26B* Univdty, New York New York 10021 27RuQess University, Plscabway, New Jm 08854
‘hcadeda SJnka, Taiwan 11529, Rep&l&z of U&m 2~~~ndu~ Super cdllder Lab~ratoty. Dallas, Texas 75237
Texas A&M University, Coik+ge Station, T~JGU 77843 31UnhmrsityofTsukuhe,Tsukubn,Ibarakt305, Japan
j2Tuftl uldversily, Medford -uaetb 02 155 =I.hhmity Qfwiscm h4adisQa wkonsin 53706
3%leIJllhmity,NewHaveqcoMecucut06511 %dlKw
2 September 1994
2
Abstract
This letter describes a direct measurement of the W boson decay width, r(W),
using the high-mass tail of the transverse mass spectrum of W+ev decays recorded
by the CDF experiment. We find r(W) = 2.11 f 0.28(s&t)f O.l6(sys.) GeV, and
compare this direct measurement with indirect means of obtaining the width.
PACS numbers: 13.38.+c, 12.15.Ff, 11.80.Dq, 14..50.h-
The W boson width, T(W), is a fundamental parameter that is well-predicted in
the Standard Model. The W decays with approximately equal probability to each of *
three lepton and two quark families kinematically available (with a color factor of
three on the number of quark families). Hence, its branching ratio into (C,Q) is
approximately i. The quark decays are enhanced by as(MW)/lr to first order in QCDso
that the leptonic branching ratio isI 0.1084 f 0.0002, where the 0.0002 reflects the
uncertainty in the value of as at @=I$,$ . The predicted W width may then be
predicted by dividing the leptonic partial width r(W+!vJ - (G&)(&f& /6z) by the
branching ratio. Using MW - 80.14 f 0.27 GeV/cZ,E*] Rosner et al.111 find
Zi%!I = 2.067 f 0.021 GeV where the uncertainty in the prediction is dominated by
the uncertainty in M++c
The W width has been obtained experimentally[31 by an indirect method using
the ratio R I ~B(p~~W~~/~~~p~j~~~~~. A measurement of R, together with
a calculatior#] of the ratio of production cross sections o(p~+W)/c~{p~+zo) and the
LEP measurement&] of r(zo) and Q@-&+t), is used to obtain a measurement of the
W leptonic branching ratio r(W+&j/r(W). A calculation of r(W+&j is then used to
obtain the W width. With this method a precision of 85 MeV has been obtained. As
Rosner et aZ.[l] have noted, however, loops at the W-fermion vertex involving a @ or
3
scalar Higgs @,I61 could alter the effective W-fermion coupling, and hence T(W), but
not affect the ratio r(W+t’v)/r(W). Thus, a measurement of r(W) from
T(W+t’v)/T(W) assumes that the W coupling to leptons is given by the Standard
Model. While in principle such non-standard couplings would also alter the W
production cross section and thus affect the value of I’(W-+k’v)/r(W) extracted from
R, a direct measurement of the full width r(W) is desirable so that these radiative
corrections to r(W) can be observed.
Direct measurements of T(W) have been reported by the UA1[71 and I~~2181
Collaborations. Including systematic uncertainties, they obtain the values
r(w) = 2.8 _‘;1:: f 1.3 GeVand r(W) < 7.0 GeV (9096 CL) respectively. These direct
measurements result from fits of the W transverse mass distribution for the best
values of Mw and r(W). The transverse mass is defined as MT P ?/ 2Pj++( 1-cos(A#)),
where P’T and q are the transverse momenta191 of the electron and neutrino and A#
is the azimuthal angle between them. The fits were performed over a limited range
in MTnear the Mwpeak.
The tail of the transverse mass distribution of the W contains information on
T(W). Events with MT > MW can arise due to the non-zero W width or due to the
calorimeter resolution. The resolution, furthermore, degrades for large values of the
W transverse momentum, Py However, a precise measurement of T(W) from the
high-mass tail is possibleI be cause the q distribution is sufficiently well-known
and because far above MW the Breit-Wigner tail dominates over the gaussian
resolution of the detector. In this analysis the W width is determined from a binned
log-likelihood fit to the transverse mass distribution in the region MT > 110 GeV/c2,
where a Monte Carlo study indicates there is good sensitivity to the width and the
4
systematic uncertainties are small. Possible non-gaussian tails to the resolution are
discussed below.
This measurement is made with 19.7 f 0.7 pb-1 of data collected by the
Collider Detector at Fermilab (CDF) during the 1992-1993 run of the Fermilab Tevatron
Collider. The Tevatron produces pF collisions at 4 = 1.8 TeV. Detailed descriptions of
the detector can be found elsewhere.[lll Th e portions of the detector relevant to this
measurement are (i) electromagnetic and hadronic calorimeters covering the
pseudorapidity[l*] range 1~1 < 4.2 and arranged in a projective tower geometry; (ii) a
drift chamber (CTC) immersed in a 1.4 T solenoidal magnetic field for tracking
charged particles in the range 1~1 < 1.4; (iii) a time-projection chamber (VTX) for
vertex finding; and (iv) two arrays of scintillator hodoscopes located on either side of
the detector for triggering.
To select candidate events we require an electron in the central barrel region
of the detector (I@ < 1.05) with calorimeter transverse energy ET > 30 ~&lo] and
CTC transverse momentum PT > 13 GeV/c. We require the electron track to be
isolated in the CTC, requiring Iso < 5 GeV/c, where Iso is defmed as the
scalar sum of the PT of all tracks except the electron track within a region in q--#
space of -\, (A#)* + (A@* = 0.25 centered on the electron. The ratio of energy in the
hadron (Had) and electromagnetic (EM) calorimeter towers of the electron cluster is
required to satisfy Had/EM < 0.055 + 0.045 x A transverse momentum
imbalance is required to signal the presence of the neutrino. We require
ET s 30 GeV, where the missing transverse momentum, or ET, is defined as the
vector sum of the ET in all calorimeter towers with lql < 3.6. Finally, the pjF
interaction point, which is distributed by an approximate gaussian of width
az = 26 cm along the beam direction, is required to satisfy lZind c 60 cm, and the
5
total accidental calorimeter energy not in time with the pj7 collision is required to be
< 100 GeV. There are 10845 events passing these cuts. Of these, 93 events with second
isolated, high-@ tracks pointing to electromagnetic clusters are removed from the W
sample as @ candidates, and 226 events with clusters of tracks in the CTC which point
at the ET vector in # and at calorimeter cracks in Q are removed as mismeasured QCD
jet events (“dijets”), leaving 10526 events.
Several processes can also mimic the W signal. The process W+z~+evvv has a
signature similar to W+ev decays, but at lower MT The process @+e+e-, where one
electron is detected and the other is lost because it falls into an uninstrumented
region of the detector, can produce the signal of an electron and ET, as can QCD dijet
events where one jet is lost and the other passes our electron selection criteria, We
use the ENJET Monte Carlo program[l3] and a detector simulation to estimate the
number of @+e+e’ and W+w decays contaminating the W sample. We find that the
number of Zo events remaining in the W sample is 50 f 15 events and the
background from W-av is 150 f 45 events. The WD dijet background is estimated by
studying a data sample of events with an “electron” + ET in which the “electron” has
Iso(trk) > 6 GeV/c. These events are presumably mismeasured dijet events. We
study the efficiency of our dijet removal cuts on this sample and normalize to the
number (226) of events in the W sample removed using these cuts. We estimate that
the number of dijets left in the sample is 241 f 40 events. We observe 124 events
with MT > 110 GeV/c*. We expect that 3.5 f 0.5 events are due to Zo and W-av
events and that 53 f 8 events are from QCD dijets .
The W transverse mass spectrum is modeled using a Monte Carlo program that
generates zeroth order diagrams of W production, qq+W, according to an energy-
dependent Breit-Wigner distribution:[141
6
a(i) - 6 Hi - M&J2 + iQ’~M$ ]
where 6 s IS the (possibly off-shell) eV mass. This form of the cross section includes
vacuum polarization contributions to the W propagator. We use the MRSIT’ structure
functions.[lfi] The effects of higher order diagrams for W production are mimicked
by giving the bosons PT according to a previous measurementfl61 of the Py
spectrum. The lepton momenta are passed through a simulation of the detector
response. The same kinematic and geometric cuts as in the data are applied
A cut of Py < 20 GeV/c is imposed in order to suppress many of the expected
backgrounds in the fit region (MT > 110 GeV/d) and in order to suppress the
broader calorimeter resolution which arises at large Pr With this cut, the number
of W+ev candidates is reduced from 10526 to 9701 and the total background is reduced
from 441 f 62 events to 224 f 44 events. The background in the fit region is
reduced to w 1096 of the 58 events observed. Furthermore, Monte Carlo studies
indicate that the uncertainty in r(W) due to the #shape is reduced from 700 MeV to
120 MeV. Figure 1 shows the transverse mass distribution of the 9701 candidate
events after the flT < 20 GeV/c cut, along with the expected background.
To obtain F(W) a binned log-likelihood fit to the W MT distribution, binned in
1 Ge V/c2 bins, is performed for MT > 110 GeV/cZ. Monte Carlo templates are
generated with values of r(W) between 0.667 and 3.667 GeV at 200 MeVintervals. In
the templates the MT shapes are the sum of W Monte Carlo and of backgrounds, where
the background is normalized to 224 events and the W Monte Carlo is normalized to
9701- 224 = 9477 events. The data are fit to each template and a likelihood curve vs.
r(W) is made. Figure 2 shows the likelihood curve for the data. The most likely value
7
is r(W) = 2.04 f 0.28 GeK The uncertainty is purely statistical and is determined by
the point where the likelihood decreases by 0.5.
As a check of the fitting technique, we generate an ensemble of Monte Carlo
“experiments” of 9477 W events each, add 224 background events, and fit to the Monte
Carlo templates above. The mean returned value of QJm> = 210 f 0.02 GeVis
obtained from the ensemble of “experiments,” to be compared to the input value of
2.07 GeK Furthermore, the r.m.s. = 250 f 17 MeV of the fitted values is consistent
with the 280 MeVstatistical uncertainty estimated from the data.
The systematic uncertainties in this determination of the W width are due to
effects that alter the shape of the transverse mass distribution. They are: the Py
distribution, the electron resolution, the neutrino resolution, the backgrounds, and
the electron energy scale. To estimate the uncertainties due to these effects, we allow
these parameters to vary in our Monte Carlo program and then refit the Monte Carlo
transverse mass spectrum with the varied input parameters to the original Monte
Carlo templates.
The Monte Carlo model of the ET resolution is compared to the ET observed in
@+e+e- decays, where any ET in the event is due to the energy response of the
calorimeter and not due to neutrinos. Varying the gaussian width of the ET
resolution by the amount allowed by the Zo data leads to a 4.9% variation in T(W).
Possible non-gaussian tails in the calorimeter ET resolution, which have been
checked using minimum bias data, lead to negligible variations in r(W). The
calorimeter electron energy scale and resolution are determined using the mass peak
position and width of i?+e+e’ decays. Allowing the scale and resolution to vary in
the Monte Carlo leads to 2.0% and 0.6% variations in T(W), respectively. Allowing the
background to vary within its estimated uncertainty causes 0.8% variations in r(W).
Distorting the input $ distribution in the Monte Carlo within its uncertainty
and fitting the Monte Carlo with the distorted q to the nominal templates, we find a
variation of 696, or 120 MeV, in r(W). A different kind of Py uncertainty arises
because of the theoretical expectationfl71 that the Py distribution varies
logarithmically, - aw; ‘+ log( 5 ), with the mass M of the tV pair. We have checked T T
this possible source of bias in two ways: first, we fit a Monte Carlo sample with this
new PW T distribution to the Monte Carlo templates generated with the nominal
distribution and observe a shift of 23 MeVin r(W). Second, we have performed the
entire analysis with a cut of q < 10 GeV/c, observing a shift of 0 MeV. The
contribution of these shifts, when combined in quadrature with the 120 MeV
uncertainty from the input W-spectrum, is negligible.
Finally, we apply a shift to r(W) due to the effect of radiative decays of the W,
Wevx not accounted for in our Monte Carlo. While most of the radiated 7 ‘s are
collinear with the electron and, hence, are clustered in with the electron energy,
some of the photons are radiated at wide angles. These wide angle photons carry
away some of the W mass, and, hence, shift the MT distribution downward. We have
used a Monte Carlo programfl81 to estimate that we must shift our value for r(W) up
by 70 f 28 MeV to correct for radiative decays.
Assembling the results, the final result for the Wwidth is:
r(W) 3: 2.11 f0.28(stat.) f O.lG(sys.) GeK
As a check, we instead fit over the region MT > 120 GeV/cZ, and obtain a value of
r(W) = 2.15 f 0.34(srat.) f O.O9(sys.) GeV. Here the systematic uncertainty is
smaller because the cut-off for the fit is farther from the falling edge of the
resolution. Note that the MT > 110 GeV/cz sample has 58 events and the
MT > 120 GeV/d sample has 35 events.
In conclusion, we have reported on a direct measurement of the Wboson decay
width, r(W) = 2.11 f 0.28( staf.) f O.lG(sys.) GeV, using the tail of the transverse
mass distribution of W+ev decays in 19.7 ~6-1 of m collisions at & = 1800 Get!
With the combined data set of 200 pb-1 from both the CDF and Dj$ detectors
anticipated in the next year, a 100 MeV measurement may be anticipated. This may
be compared with the expected uncertainty1191 from the LEP-200 experiments of
200 MeV, which will be obtained after five years of data-taking. With future runs of
the Fermilab collider, a 30 MeV measurement is possible, which approaches the level
of the radiative corrections to the width.
We thank the Fermilab staff and the technical staffs of the participating
institutions for their vital contributions. We also thank James Cronin and Jonathan
Rosner for helpful discussions. This work was supported by the U.S. Department of
Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica
Nucleare; the Ministry of Education, Science and Culture of Japan; the Natural
Sciences and Engineering Research Council of Canada; the National Science Council
of the Republic of China; the A. P. Sloan Foundation; and the Alexander von
Humboldt-Stiftung.
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PI @I
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The transverse momentum is defined as PT- Px sine, where 8 is the angle with
respect to the proton beam direction. Similarly, ET = Ex sine, where E is
measured in the calorimeter.
[lOI S. Kopp, Ph.D. Thesis, The University of Chicago (1994), unpublished.
U1l F. Abe et al., Nucl. Instr. and Meth. A 271, 387 (1988).
fl21 At CDF cylindrical coordinates r, 9, and z, are used, where Q is the azimuthal
angle and z points in the proton beam direction and is zero at the center of the
detector. The pseudorapidity, q, is defined as q = ln(cot(tV2)~.
[131 F. Paige and S. D. Protopopescu, ISAJET Monte Carlo program, BNL Report No.
BNL38034,1986 (unpubJished).
11
1141 F. A. Rerends, “Z tine Shape,” in “Z Physics at LEP I,” CERN-89-08.
1151 A.D. Martin, R.G. Roberts, and W.J. Stirling, Phys. Lett. 306 B, 145 (1993);
Erratum Phys. Lett. 309 B, 492 (1993).
1161 F. Abe et. al, Phys. Rev. Lett. 66,295l (1991)
[l’l P. Arnold and R Kauffman, Nucl. Phys. B349, 381 ( 1991).
1181 R. G. Wagner (unpublished), based on calculations by F. Berends et al., Z. Phys.
C27,155 (1985) and F. Berends and R Kleiss, Z. Phys. C27,365 (1985).
[191 P. Roudeau et. al., in the “EPCA Workshop on LFP200,” CERN-87-08, edited by A.
B&m and W. Hoogland, p. 62.
12
Figure Captions
Figure 1: Transverse mass distribution of the 9701 W candidates surviving a cut of
Py < 20 GeV/c, along with the expectation for the background and the expected W
shape from the Monte Carlo program. In the Monte Carlo, r(W) = 2.067 GeVwas
used.
Figure 2: Results of the log likelihood fit of the data to Monte Carlo templates of
different W widths. Each point represents a log-likelihood fit performed over the
range MT > 110 GeV/cZ. The curve is the best fit of the likelihood points to a cubic
polynomial. The most likely value is at r(W) = 2.04 GeK
13
10:
“0 \ > Q) lo2
0 *
\" m c 10
F W
1
1
L
-
m
m
I
I
I
m
I
.
.
.
r
ta (19.7 pb-‘)
80
+ev Monte Carlo
Bockgrounds
160 280
Transverse Mass (GeV/c2)
-90
-92
-94
0 L -96
I cT, -98
0
-100
-102
-104 111, ,111 ,, l,,,,l,,,,(,,,*~*,,,~, 0,s 1.2 1.6 2 2.4 2.8 3.2 3.6 4
we (W