Article
Reference
Measurement of the W+W−γ cross section and direct limits on anomalous quartic gauge boson couplings at LEP
L3 Collaboration
ACHARD, Pablo (Collab.), et al.
Abstract
The process e+e−→W+W−γ is analysed using the data collected with the L3 detector at LEP at a centre-of-mass energy of 188.6 GeV, corresponding to an integrated luminosity of 176.8 pb−1. Based on a sample of 42 selected W+W− candidates containing an isolated hard photon, the W+W−γ cross section, defined within phase-space cuts, is measured to be:
σWWγ=290±80±16 fb, consistent with the Standard Model expectation. Including the process e+e-→vvγγ , limits are derived on anomalous contributions to the Standard Model quartic vertices W+W−γγ and W+W−Zγ at 95% CL: -0.043 GeV-2 < αo/Δ2< 0.043 GeV-2, -0.08 GeV
L3 Collaboration, ACHARD, Pablo (Collab.), et al . Measurement of the W+W−γ cross section and direct limits on anomalous quartic gauge boson couplings at LEP. Physics Letters. B , 2000, vol. 490, no. 3-4, p. 187-195
DOI : 10.1016/S0370-2693(00)00993-X
Available at:
http://archive-ouverte.unige.ch/unige:47348
Disclaimer: layout of this document may differ from the published version.
1 / 1
Ž . Physics Letters B 490 2000 187–195
www.elsevier.nlrlocaternpe
Measurement of the W
qW
yg cross section and direct limits on anomalous quartic gauge boson couplings at LEP
L3 Collaboration
M. Acciarri
z, P. Achard
s, O. Adriani
p, M. Aguilar-Benitez
y, J. Alcaraz
y, G. Alemanni
v, J. Allaby
q, A. Aloisio
ab, M.G. Alviggi
ab, G. Ambrosi
s, H. Anderhub
av, V.P. Andreev
f,aj, T. Angelescu
l, F. Anselmo
i, A. Arefiev
aa, T. Azemoon
c, T. Aziz
j, P. Bagnaia
ai, A. Bajo
y, L. Baksay
aq, A. Balandras
d,
S.V. Baldew
b, S. Banerjee
j, Sw. Banerjee
j, A. Barczyk
av,at, R. Barillere `
q, P. Bartalini
v, M. Basile
i, R. Battiston
af, A. Bay
v, F. Becattini
p, U. Becker
n, F. Behner
av, L. Bellucci
p, R. Berbeco
c, J. Berdugo
y, P. Berges
n, B. Bertucci
af,
B.L. Betev
av, S. Bhattacharya
j, M. Biasini
af, A. Biland
av, J.J. Blaising
d, S.C. Blyth
ag, G.J. Bobbink
b, A. Bohm ¨
a, L. Boldizsar
m, B. Borgia
ai, D. Bourilkov
av, M. Bourquin
s, S. Braccini
s, J.G. Branson
am, F. Brochu
d, A. Buffini
p, A. Buijs
ar, J.D. Burger
n, W.J. Burger
af, X.D. Cai
n, M. Capell
n,
G. Cara Romeo
i, G. Carlino
ab, A.M. Cartacci
p, J. Casaus
y, G. Castellini
p, F. Cavallari
ai, N. Cavallo
ak, C. Cecchi
af, M. Cerrada
y, F. Cesaroni
w, M. Chamizo
s, Y.H. Chang
ax, U.K. Chaturvedi
r, M. Chemarin
x, A. Chen
ax, G. Chen
g, G.M. Chen
g,
H.F. Chen
t, H.S. Chen
g, G. Chiefari
ab, L. Cifarelli
al, F. Cindolo
i, C. Civinini
p, I. Clare
n, R. Clare
n, G. Coignet
d, N. Colino
y, S. Costantini
e, F. Cotorobai
l,
B. de la Cruz
y, A. Csilling
m, S. Cucciarelli
af, T.S. Dai
n, J.A. van Dalen
ad, R. D’Alessandro
p, R. de Asmundis
ab, P. Deglon ´
s, A. Degre ´
d, K. Deiters
at, D. della Volpe
ab, E. Delmeire
s, P. Denes
ah, F. DeNotaristefani
ai, A. De Salvo
av, M. Diemoz
ai, M. Dierckxsens
b, D. van Dierendonck
b, C. Dionisi
ai, M. Dittmar
av,
A. Dominguez
am, A. Doria
ab, M.T. Dova
r,5, D. Duchesneau
d, D. Dufournaud
d, P. Duinker
b, I. Duran
an, H. El Mamouni
x, A. Engler
ag, F.J. Eppling
n, F.C. Erne ´
b,
P. Extermann
s, M. Fabre
at, M.A. Falagan
y, S. Falciano
q,ai, A. Favara
q, J. Fay
x, O. Fedin
aj, M. Felcini
av, T. Ferguson
ag, H. Fesefeldt
a, E. Fiandrini
af, J.H. Field
s, F. Filthaut
q, P.H. Fisher
n, I. Fisk
am, G. Forconi
n, K. Freudenreich
av, C. Furetta
z, Yu. Galaktionov
aa,n, S.N. Ganguli
j, P. Garcia-Abia
e, M. Gataullin
ae, S.S. Gau
k, S. Gentile
ai,q, N. Gheordanescu
l, S. Giagu
ai, Z.F. Gong
t, G. Grenier
x, O. Grimm
av,
0370-2693r00r$ - see front matterq2000 Elsevier Science B.V. All rights reserved.
Ž .
PII: S 0 3 7 0 - 2 6 9 3 0 0 0 0 9 9 3 - X
( ) M. Acciarri et al.rPhysics Letters B 490 2000 187–195 188
M.W. Gruenewald
h, M. Guida
al, R. van Gulik
b, V.K. Gupta
ah, A. Gurtu
j, L.J. Gutay
as, D. Haas
e, A. Hasan
ac, D. Hatzifotiadou
i, T. Hebbeker
h, A. Herve ´
q, P. Hidas
m, J. Hirschfelder
ag, H. Hofer
av, G. Holzner
av, H. Hoorani
ag, S.R. Hou
ax, Y. Hu
ad, I. Iashvili
au, B.N. Jin
g, L.W. Jones
c, P. de Jong
b, I. Josa-Mutuberrıa ´
y, R.A. Khan
r, M. Kaur
r,6, M.N. Kienzle-Focacci
s, D. Kim
ai, J.K. Kim
ap, J. Kirkby
q,
D. Kiss
m, W. Kittel
ad, A. Klimentov
n,aa, A.C. Konig ¨
ad, A. Kopp
au, V. Koutsenko
n,aa, M. Kraber ¨
av, R.W. Kraemer
ag, W. Krenz
a, A. Kruger ¨
au, A. Kunin
n,aa, P. Ladron de Guevara
y, I. Laktineh
x, G. Landi
p, M. Lebeau
q, A. Lebedev
n, P. Lebrun
x, P. Lecomte
av, P. Lecoq
q, P. Le Coultre
av, H.J. Lee
h, J.M. Le Goff
q, R. Leiste
au, P. Levtchenko
aj, C. Li
t, S. Likhoded
au, C.H. Lin
ax,
W.T. Lin
ax, F.L. Linde
b, L. Lista
ab, Z.A. Liu
g, W. Lohmann
au, E. Longo
ai, Y.S. Lu
g, K. Lubelsmeyer ¨
a, C. Luci
q,ai, D. Luckey
n, L. Lugnier
x, L. Luminari
ai,
W. Lustermann
av, W.G. Ma
t, M. Maity
j, L. Malgeri
q, A. Malinin
q, C. Mana ˜
y, D. Mangeol
ad, J. Mans
ah, G. Marian
o, J.P. Martin
x, F. Marzano
ai, K. Mazumdar
j,
R.R. McNeil
f, S. Mele
q, L. Merola
ab, M. Meschini
p, W.J. Metzger
ad, M. von der Mey
a, A. Mihul
l, H. Milcent
q, G. Mirabelli
ai, J. Mnich
q, G.B. Mohanty
j, T. Moulik
j, G.S. Muanza
x, A.J.M. Muijs
b, B. Musicar
am, M. Musy
ai, M. Napolitano
ab, F. Nessi-Tedaldi
av, H. Newman
ae, T. Niessen
a,
A. Nisati
ai, H. Nowak
au, R. Ofierzynski
av, G. Organtini
ai, A. Oulianov
aa, C. Palomares
y, D. Pandoulas
a, S. Paoletti
q,ai, P. Paolucci
ab, R. Paramatti
ai, H.K. Park
ag, I.H. Park
ap, G. Passaleva
q, S. Patricelli
ab, T. Paul
k, M. Pauluzzi
af, C. Paus
q, F. Pauss
av, M. Pedace
ai, S. Pensotti
z, D. Perret-Gallix
d, B. Petersen
ad,
D. Piccolo
ab, F. Pierella
i, M. Pieri
p, P.A. Piroue ´
ah, E. Pistolesi
z, V. Plyaskin
aa, M. Pohl
s, V. Pojidaev
aa,p, H. Postema
n, J. Pothier
q, D.O. Prokofiev
as, D. Prokofiev
aj, J. Quartieri
al, G. Rahal-Callot
av,q, M.A. Rahaman
j, P. Raics
o, N. Raja
j, R. Ramelli
av, P.G. Rancoita
z, R. Ranieri
p, A. Raspereza
au, G. Raven
am,
P. Razis
ac, D. Ren
av, M. Rescigno
ai, S. Reucroft
k, S. Riemann
au, K. Riles
c, J. Rodin
aq, B.P. Roe
c, L. Romero
y, A. Rosca
h, S. Rosier-Lees
d, J.A. Rubio
q, G. Ruggiero
p, H. Rykaczewski
av, S. Saremi
f, S. Sarkar
ai, J. Salicio
q, E. Sanchez
q, M.P. Sanders
ad, M.E. Sarakinos
u, C. Schafer ¨
q, V. Schegelsky
aj, S. Schmidt-Kaerst
a,
D. Schmitz
a, H. Schopper
aw, D.J. Schotanus
ad, G. Schwering
a, C. Sciacca
ab, A. Seganti
i, L. Servoli
af, S. Shevchenko
ae, N. Shivarov
ao, V. Shoutko
aa, E. Shumilov
aa, A. Shvorob
ae, T. Siedenburg
a, D. Son
ap, B. Smith
ag, P. Spillantini
p,
M. Steuer
n, D.P. Stickland
ah, A. Stone
f, B. Stoyanov
ao, A. Straessner
a, K. Sudhakar
j, G. Sultanov
r, L.Z. Sun
t, H. Suter
av, J.D. Swain
r, Z. Szillasi
aq,3,
T. Sztaricskai
aq,3, X.W. Tang
g, L. Tauscher
e, L. Taylor
k, B. Tellili
x,
C. Timmermans
ad, Samuel C.C. Ting
n, S.M. Ting
n, S.C. Tonwar
j, J. Toth ´
m,
C. Tully
q, K.L. Tung
g, Y. Uchida
n, J. Ulbricht
av, E. Valente
ai, G. Vesztergombi
m,
I. Vetlitsky
aa, D. Vicinanza
al, G. Viertel
av, S. Villa
k, P. Violini
q, M. Vivargent
d, S. Vlachos
e, I. Vodopianov
aj, H. Vogel
ag, H. Vogt
au, I. Vorobiev
aa,
A.A. Vorobyov
aj, A. Vorvolakos
ac, M. Wadhwa
e, W. Wallraff
a, M. Wang
n, X.L. Wang
t, Z.M. Wang
t, A. Weber
a, M. Weber
a, P. Wienemann
a, H. Wilkens
ad,
S.X. Wu
n, S. Wynhoff
q, L. Xia
ae, Z.Z. Xu
t, J. Yamamoto
c, B.Z. Yang
t, C.G. Yang
g, H.J. Yang
g, M. Yang
g, J.B. Ye
t, S.C. Yeh
ay, An. Zalite
aj, Yu. Zalite
aj, Z.P. Zhang
t, G.Y. Zhu
g, R.Y. Zhu
ae, A. Zichichi
i,q,r, G. Zilizi
aq,3,
B. Zimmermann
av, M. Zoller ¨
aaI. Physikalisches Institut, RWTH, D-52056 Aachen, Germany, and III. Physikalisches Institut, RWTH, D-52056 Aachen, Germany1
bNational Institute for High Energy Physics, NIKHEF, and UniÕersity of Amsterdam, NL-1009 DB Amsterdam, The Netherlands
cUniÕersity of Michigan, Ann Arbor, MI 48109, USA
dLaboratoire d’Annecy-le-Vieux de Physique des Particules, LAPP, IN2P3-CNRS, BP 110, F-74941 Annecy-le-Vieux CEDEX, France
eInstitute of Physics, UniÕersity of Basel, CH-4056 Basel, Switzerland
fLouisiana State UniÕersity, Baton Rouge, LA 70803, USA
gInstitute of High Energy Physics, IHEP, 100039 Beijing, China7
hHumboldt UniÕersity, D-10099 Berlin, Germany1
iUniÕersity of Bologna and INFN-Sezione di Bologna, I-40126 Bologna, Italy
jTata Institute of Fundamental Research, Bombay 400 005, India
kNortheastern UniÕersity, Boston, MA 02115, USA
lInstitute of Atomic Physics and UniÕersity of Bucharest, R-76900 Bucharest, Romania
m Central Research Institute for Physics of the Hungarian Academy of Sciences, H-1525 Budapest 114, Hungary2
nMassachusetts Institute of Technology, Cambridge, MA 02139, USA
oKLTE-ATOMKI, H-4010 Debrecen, Hungary3
pINFN Sezione di Firenze and UniÕersity of Florence, I-50125 Florence, Italy
qEuropean Laboratory for Particle Physics, CERN, CH-1211 GeneÕa 23, Switzerland
rWorld Laboratory, FBLJA Project, CH-1211 GeneÕa 23, Switzerland
sUniÕersity of GeneÕa, CH-1211 GeneÕa 4, Switzerland
t Chinese UniÕersity of Science and Technology, USTC, Hefei, Anhui 230 029, China7
uSEFT, Research Institute for High Energy Physics, P.O. Box 9, SF-00014 Helsinki, Finland
vUniÕersity of Lausanne, CH-1015 Lausanne, Switzerland
wINFN-Sezione di Lecce and UniÕersita Degli Studi di Lecce, I-73100 Lecce, Italy´
xInstitut de Physique Nucleaire de Lyon, IN2P3-CNRS, UniÕersite Claude Bernard, F-69622 Villeurbanne, France´ ´
yCentro de InÕestigaciones Energeticas, Medioambientales y Tecnologıcas, CIEMAT, E-28040 Madrid, Spain´ ´ 4
zINFN-Sezione di Milano, I-20133 Milan, Italy
aaInstitute of Theoretical and Experimental Physics, ITEP, Moscow, Russia
abINFN-Sezione di Napoli and UniÕersity of Naples, I-80125 Naples, Italy
ac Department of Natural Sciences, UniÕersity of Cyprus, Nicosia, Cyprus
adUniÕersity of Nijmegen and NIKHEF, NL-6525 ED Nijmegen, The Netherlands
aeCalifornia Institute of Technology, Pasadena, CA 91125, USA
afINFN-Sezione di Perugia and UniÕersita Degli Studi di Perugia, I-06100 Perugia, Italy´
agCarnegie Mellon UniÕersity, Pittsburgh, PA 15213, USA
ahPrinceton UniÕersity, Princeton, NJ 08544, USA
aiINFN-Sezione di Roma and UniÕersity of Rome,ALa SapienzaB, I-00185 Rome, Italy
ajNuclear Physics Institute, St. Petersburg, Russia
akINFN-Sezione di Napoli and UniÕersity of Potenza, I-85100 Potenza, Italy
alUniÕersity and INFN, Salerno, I-84100 Salerno, Italy
amUniÕersity of California, San Diego, CA 92093, USA
anDept. de Fisica de Particulas Elementales, UniÕ. de Santiago, E-15706 Santiago de Compostela, Spain
aoBulgarian Academy of Sciences, Central Lab. of Mechatronics and Instrumentation, BU-1113 Sofia, Bulgaria
apLaboratory of High Energy Physics, Kyungpook National UniÕersity, 702-701 Taegu, South Korea
aqUniÕersity of Alabama, Tuscaloosa, AL 35486, USA
arUtrecht UniÕersity and NIKHEF, NL-3584 CB Utrecht, The Netherlands
asPurdue UniÕersity, West Lafayette, IN 47907, USA
( ) M. Acciarri et al.rPhysics Letters B 490 2000 187–195 190
atPaul Scherrer Institut, PSI, CH-5232 Villigen, Switzerland
auDESY, D-15738 Zeuthen, Germany
avEidgenossische Technische Hochschule, ETH Zurich, CH-8093 Zurich, Switzerland¨ ¨ ¨
awUniÕersity of Hamburg, D-22761 Hamburg, Germany
axNational Central UniÕersity, Chung-Li, Taiwan, ROC
ayDepartment of Physics, National Tsing Hua UniÕersity, Taiwan, ROC
Received 19 July 2000; accepted 11 August 2000 Editor: K. Winter
Abstract
The process eqey™WqWyg is analysed using the data collected with the L3 detector at LEP at a centre-of-mass energy of 188.6 GeV, corresponding to an integrated luminosity of 176.8 pby1. Based on a sample of 42 selected WqWy candidates containing an isolated hard photon, the WqWyg cross section, defined within phase-space cuts, is measured to
q y
be:sWWgs290"80"16 fb, consistent with the Standard Model expectation. Including the process e e ™nngg, limits are derived on anomalous contributions to the Standard Model quartic vertices WqWygg and WqWyZg at 95% CL:
y0.043 GeVy2-a0rL2-0.043 GeVy2, y0.08 GeVy2-acrL2-0.13 GeVy2, y0.41 GeVy2-anrL2-0.37 GeVy2. q2000 Elsevier Science B.V. All rights reserved.
1. Introduction
The LEP centre-of-mass energy for eqey colli- sions is now well above the kinematic threshold for W-pair production allowing for the study of radiative W-pair production, eqey™WqWyg. The Standard
Ž . w x
Model SM 1,2 predicts the existence of quartic
Ž . q y
gauge couplings QGCs , leading to W W g pro- duction via s-channel exchange of ag or Z boson as shown in Fig. 1a.
As the contribution of these two quartic Feynman diagrams with respect to the other competing dia- grams, mainly initial-state radiation, is negligible at the LEP centre-of-mass energies, the process leading
1Supported by the German Bundesministerium fur Bildung,¨ Wissenschaft, Forschung und Technologie.
2Supported by the Hungarian OTKA fund under contract num- bers T019181, F023259 and T024011.
3Also supported by the Hungarian OTKA fund under contract numbers T22238 and T026178.
4Supported also by the Comision Interministerial de Ciencia y´ Tecnologıa.´
5Also supported by CONICET and Universidad Nacional de La Plata, CC 67, 1900 La Plata, Argentina.
6Also supported by Panjab University, Chandigarh-160014, India.
7Supported by the National Natural Science Foundation of China.
to the WqWyg final state could thus be sensitive to anomalous contributions to the SM quartic gauge-bo- son vertices WqWyg g and WqWyZg.
Ž .
The existence of Anomalous QGCs AQGCs
q y
would also affect the e e ™n n gge e process via the WqWy fusion Feynman diagram containing the
q y w x Ž .
W W gg vertex 3 see Fig. 1b . In the SM the
q y
reaction e e ™nngg proceeds predominantly through s-channel Z exchange and t-channel W exchange, with the two photons coming from initial state radiation, whereas the SM contribution from the WqWy fusion is negligible at LEP. AQGCs would enhance thenngg production rate, especially for the hard tail of the photon energy distribution and for photons produced at large angles with respect to the beam direction.
Here we describe the cross section measurement for the process eqey™WqWyg and the determina- tion of AQGCs using the data collected in 1998 with
'
w x Ž
the L3 detector 4 at ss188.6 GeV denoted as
'
ss189 GeV hereafter corresponding to an inte-. grated luminosity of 176.8 pby1. AQGCs are also independently determined using acoplanar photon- pair events with missing energy. This analysis is performed using the data at'
ss189 GeV and at'
ss182.7 GeV collected in 1997 denoted asŽ'
ss 183 GeV hereafter. corresponding to a total inte- grated luminosity of 231.7 pby1. The results derivedFig. 1. Feynman diagrams containing a four-boson vertex leading
q y
Ž . Ž .
to the a W W gand to the b n n gge e final states.
q y
on AQGCs from the W W g and nngg channels are finally combined.
The search for anomalous contributions to the SM quartic couplings is performed within the theoretical
w x
framework of Refs. 5,6 . Recently, experimental measurements for these couplings have already been performed on final states with three vector bosons
q y w x w x W W g 7 and Zgg 8 .
2. WHWIg final state and signal definition There are 14 Feynman diagrams at the tree level leading to the WqWyg final state, and many other diagrams corresponding to photons from the decay products of hadronic or leptonic W’s. We are inter- ested only in two of these, the quartic diagrams. The other diagrams leading to the same final state are
Ž .
initial state radiation ISR , final state radiation ŽFSR , and radiation from the W boson itself..
The Monte Carlo used for the WqWyg cross section determination is KORALW 9 . This genera-w x tor does not include the quartic coupling diagrams.
Initial state multi-photon radiation is implemented in KORALW in the full photon phase space. FSR from
charged leptons in the event up to double w x bremsstrahlung is included using the PHOTOS 10 package. Fragmentation processes of quarks into
w x hadrons are made according to the JETSET 11 algorithm including photons in the parton shower.
For the WqWyg cross section measurement, this modelling is sufficient since the contribution of all the other diagrams is very small. The background
q y q y
Ž .
processes such as e e ™Zrg™qqg and e e
Ž . w x
™ZZ™4 f g are simulated using PYTHIA 12 . The L3 detector response is simulated by the pro-
w x gram GEANT 13 .
In this analysis, the WqWyg signal is defined by the following phase-space cuts:
v Eg) 5 GeV, where Eg is the energy of the photon,
v ug) 208, where ug is the angle between the photon and the beam axis,
v ag) 208, where ag is the angle between the direction of the photon and that of the closest charged lepton or jet.
These cuts are mainly chosen for experimental reasons, to optimise the photon identification and the background suppression. They also largely avoid any infrared and collinear singularities in the calculation of the signal cross section.
The theoretically predicted WqWyg cross sec-
Ž .
tion from KORALW corresponds to 272 " 4 stat fb. In the EEWWG program 6 the effect of unde-w x tected additional ISR collinear to the beam pipe is
w x
included 14 by implementing the EXCALIBUR w15 collinear radiator function. The effect of thex higher order radiative corrections is to move the effective centre-of-mass energy towards lower val- ues, reducing the expected signal cross section by about 18%. The resulting EEWWG cross section
Ž .
corresponds to 233 " 12 theor fb. This is used as the SM expectation in the anomalous coupling analy- sis, which leads to less stringent constraints on
w x
AQGCs. The theoretical uncertainty 14 is propa- gated to the AQGC determination. Consistent results
w x
are obtained with the YFSWW3 16 MC which
Ž .
predicts 224 " 6 stat fb. Differences of this order in the predicted cross section with high transverse
w x
momentum photons are expected 17 between pure leading-log and leading-log plus matrix-element based calculations.
( ) M. Acciarri et al.rPhysics Letters B 490 2000 187–195 192
3. WHWIg event selection and cross section
The W-pair event selections used here are similar w x
to those reported in Ref. 18 . Only the semileptonic and fully hadronic W-pair decay modes are consid- ered. The number of selected data events and the expected number of signal and background events are shown in Table 1.
The photon selection in WqWy events is opti- mised for each four-fermion final state. Photons are selected by requiring energy deposition in the elec- tromagnetic calorimeter not associated with any track in the central detector, and low hadronic activity in a cone of half-opening angle of 78around the electro- magnetic cluster. The profile of the shower must be consistent with that of an electromagnetic particle. In addition, the highest energy photon has to satisfy the signal definition requirements: Eg) 5 GeV, ug) 208, and ag) 208. Fig. 2 shows the distributions of E ,g ug, and ag, where for the last variable the direction of the reconstructed hadronic jet is assumed as the direction of the quark in the final state. In general, good agreement between the data and the SM expectation is observed.
Table 2 summarises the results for the applied selection criteria. In total 42 WqWyg events are selected, where 37.8"0.6 is the Monte Carlo expec- tation. The efficiency´WWg is defined as the number of selected KORALW events Žregardless of any phase-space cuts divided by the number of gener-. ated MC events satisfying the signal definition. It accounts for small possible migration effects of events from outside the signal region into the se- lected sample due to the finite detector resolution.
Table 1
Number of observed events, selection efficiencies with statistical uncertainties, expected total number of events and background estimates for the various WqWydecay channels according to the SM prediction. The efficiencies shown here include the contribu- tion of cross efficiencies from the other W-pair decay modes.
exp exp
Decay channel Nobs ´WW NTOT NBkgr
qqene 355 0.768"0.005 361 19 qqmnm 364 0.834"0.002 375 19 qqtnt 313 0.605"0.003 300 42 qqqq 1514 0.892"0.006 1486 296
Fig. 2. Differential distributions, for the semileptonic and fully
q y
Ž . Ž .
hadronic W W gdecay modes, of a the photon energy, b the angle of the photon to the beam axis and c angle of the photonŽ . to the closest charged lepton or jet. The hatched area is the background component from ZZ, Zee, and qqŽ .g events. Final state radiation includes the contribution of photons radiated off the charged fermions and photons originating from isolated meson decays. In the upper plot, the distribution corresponding to a non-zero value of the anomalous coupling anrL2 is shown as a dotted line.
The WqWyg cross section is evaluated channel by channel and then combined according to the SM W-pair branching fractions. The result is:
sWWgs290"80"16 fb ,
where the first error is statistical and the second systematic. The measurement is in good agreement
Table 2
Number of observed events, selection efficiencies with statistical uncertainties and expected number of total and background events including final state radiation.
exp exp exp
Decay channel Nobs ´WWg NTOT NFSRqNBkgr
qqen ge 6 0.483"0.025 5.85"0.26 2.26"0.14 qqmn gm 5 0.547"0.027 6.87"0.28 2.88"0.17 qqtn gt 7 0.351"0.018 4.63"0.22 2.05"0.14 qqqqg 24 0.504"0.016 20.4"0.38 9.23"0.26 total 42 – 37.8"0.6 16.4"0.4
with both the KORALW and EEWWG SM expecta- tions.
Fig. 3 shows the result obtained together with the predicted total WqWyg cross section from the EEWWG Monte Carlo as a function of the centre- of-mass energy.
The systematic uncertainties arising in the inclu- w x
sive W-pair event selections 18 are propagated to the final measurement and correspond to an uncer- tainty of" 6.3 fb. Other possible systematic biases due to detector effects such as electro-magnetic clus- ter resolution, angular resolution, and calorimetric energy scale uncertainty are found to have a negligi- ble effect on the final result.
The total systematic uncertainty is dominated by the JETSET modelling of photons from meson de-
Ž 0 .
cays p ,h . To estimate this effect, a data sample
y1
'
of 3.9 pb collected in 1998 at ss 91 GeV is studied. The same photon identification criteria are applied to the selected Z™qq events. An overall
Ž .
excess of 20"10 % in the photon rate is found in data with respect to the PYTHIA Monte Carlo which uses the same JETSET fragmentation algorithm and particle decays as KORALW. A correction factor given by the ratio of photon production rates in data and MC is determined as a function of the photon energy. This correction is applied to the background component of qqg MC events as well as to the
q y q y
'
Fig. 3. Measured cross section for e e ™W W g at ss
Ž .
189 GeV point compared to the SM cross section as a function
Ž .
of the centre-of-mass energy solid line as predicted by the EEWWG Monte Carlo within the indicated phase-space cuts. The shaded band corresponds to the theoretical uncertainty of"5%.
The three dashed lines correspond to the cross section for non-
2 Ž y2
vanishing values of the anomalous coupling anrL in GeV units ..
hadronic side of the WqWyg MC events. The uncertainty on this correction is propagated to the measurement as a systematic uncertainty on the WqWyg cross section which corresponds to "15 fb.
4. Determination of anomalous quartic gauge cou- plings
The selected WqWyg events allow us to con- strain anomalous contributions to the SM quartic gauge boson vertices. In the framework of Refs.
w5,6 , the extended Lagrangian includes new dimen-x sion-6 operators,
e2 a0
mn a
L
L0s y16 L2 F F Wmn PWa
e2 ac m a b L
Lcs y16 L2 F F Wm b PWa
ie2 an Ži.
Žj. Žk.a mn
L
Lns y 2 ei jkWm aWn W F , 16 L
where a0rL2, acrL2, and anrL2 are the AQGCs, and L represents the energy scale for new physics.
The two parameters a0rL2 and acrL2, which are separately C and P conserving, generate anomalous WqWyg g and ZZgg vertices. The term anrL2, which is CP violating, gives rise to an anomalous contribution to WqWyZg. Although there are al-
w x w x 2
ready direct 7,8 and indirect 19 limits on a0rL and acrL2, only the study of WqWyg events allows for a direct measurement of the anomalous coupling anrL2 through the WqWyZg vertex.
The EEWWG program implements the effects of the AQGCs through the extended SM Lagrangian.
Fig. 3 shows how the anomalous coupling anrL2 manifests itself through a deviation of the total cross section.
The anomalous component from the above opera- tors is linear in the photon energy at the matrix element level 6 . This implies that also the shape ofw x the photon spectrum is affected by AQGCs, in par- ticular, the hard part of the energy distribution seeŽ Fig. 2a . The expected distribution for any value of. the three AQGCs is obtained by reweighting each
Ž .
KORALW MC event with the ratio WW E , a , a , ag 0 c n
( ) M. Acciarri et al.rPhysics Letters B 490 2000 187–195 194
of the known differential distributions of E at gen-g
erator level:
W
W
Ž
E , a , a , ag 0 c n.
dsEEWWG dsKO RALW
s Ža , a , a0 c n. .
dEg dEg
The reweighting procedure is applied only to the ISR component of the MC selected sample, while the
Ž .
FSR from KORALW and the background compo- nents of accepted events are kept fixed. The possible dependence of the selection efficiency on the photon polar angle and on the angular separation from the charged fermions in the event is found to be negligi- ble.
Both the shape and the normalisation of the ob- served photon spectrum in the range from 5 GeV to 30 GeV are used in a maximum-likelihood fit to each of the anomalous couplings a0rL2, acrL2 and anrL2, fixing the other two to zero. The effects of the same systematic uncertainties described for the cross section measurement are included, yielding the 68% CL intervals:
y0.028 GeVy2 - a0rL2 - 0.028 GeVy2
y2 2 y2
y0.04 GeV - acrL - 0.09 GeV
y2 2 y2
y0.26 GeV - anrL - 0.23 GeV .
The results are in good agreement with the SM value of zero for each of the anomalous quartic gauge couplings. The 1-parameter limits at 95% CL are:
y0.045 GeVy2 - a0rL2 - 0.045 GeVy2
y2 2 y2
y0.08 GeV - acrL - 0.13 GeV
y2 2 y2
y0.41 GeV - anrL - 0.37 GeV .
H I
5. The e e ™n n g g process
The selection of acoplanar multi-photon events is w x
identical to that described in Ref. 20 . At least two photons with energies greater than 5 GeV and 1 GeV are required, with polar angles between 148 and
w x w x
1668. The KORALZ 21 and NUNUGPV 22 Monte
q y
Carlo generators are used to model the e e ™nngg process according to the SM. The effects of the
AQGCs a0rL2 and acrL2 are simulated using the EENUNUGGANO program w x3 . Note that nngg production is not sensitive to the anrL2 coupling.
We select 14 events at
'
ss183 GeV and 21 events at'
ss189 GeV compared to a SM expecta- tion of 13.3 and 36.2 events respectively.The EENUNUGGANO program does not de- scribe the effects of the SM s-channel Z exchange diagrams and the interference between these dia- grams and the WqWy fusion diagram containing the WqWyg g vertex. Therefore additional cuts are ap- plied to suppress the SM contribution. The energy of both photons must be greater than 10 GeV. If both
Ž< < . photons are in the barrel region cosu -0.7 , either the recoil mass must be less than 80 GeV or the sum of the photon energies must be greater than 100 GeV. If one or two photons are in the endcaps, where the SM contribution is larger, the recoil mass must be less than 75 GeV. After applying these cuts no data event is selected, consistent with the SM expectation of 0.15 events.
The expected number of events for any AQGC value is calculated based on a sample of ten thou- sand simulated EENUNUGGANO events generated for several values of a0rL2 and acrL2. Its matrix element is used to reweight the events to any AQGC value required, testing the procedure by comparing the reweighted distributions to those from samples generated at various values of AQGCs. In all cases good agreement is observed.
Since the program does not include higher order corrections due to ISR, these effects are estimated by
w x
implementing the EXCALIBUR 15 collinear radia- tor function. The cross section is reduced by about 16% which is used in the following. The remaining
w x
theoretical uncertainty of 5% 14 is taken into ac- count in the AQGC limits. The systematic uncer-
w x
tainty on the selection efficiency 20 gives a much smaller contribution.
The 95% CL upper limit on the number of ex- pected events from the AQGC signal is obtained taking into account the systematic error on the ac- cepted cross section; this corresponds to the follow- ing 1-parameter limits at 95% CL:
y0.067 GeVy2 - a0rL2 - 0.066 GeVy2
y2 2 y2
y0.18 GeV - acrL - 0.18 GeV .
6. Conclusion
All results obtained results are in a good agree- ment with the SM expectation of zero for each anomalous quartic gauge boson couplings.
Combining the results on a0rL2 and acrL2from
q y
our analyses of W W g and nngg production, we derive the following 1-parameter 95% CL limits:
y0.043 GeVy2 - a0rL2 - 0.043 GeVy2
y2 2 y2
y0.08 GeV - acrL - 0.13 GeV
y2 2 y2
y0.41 GeV - anrL - 0.37 GeV .
Acknowledgements
We thank J.W. Stirling and A. Werthenbach for providing us with the eqey™WqWyg and the
q y
e e ™nngg analytical calculations including AQGCs. We wish to express our gratitude to the CERN accelerator divisions for the superb perfor- mance and the continuous and successful upgrade of the LEP machine. We acknowledge the contributions of the engineers and technicians who have partici- pated in the construction and maintenance of this experiment.
References
w x1 S.L. Glashow, Nucl. Phys. 22 1961 579; A. Salam, in: N.Ž .
Ž .
Svartholm Ed. , Elementary Particle Theory, Almqvist and Wiksell, Stockholm, 1968, p. 367; S. Weinberg, Phys. Rev.
Ž .
Lett. 19 1967 1264.
w x2 M. Veltman, Nucl. Phys. B 7 1968 637; G.M.’t Hooft,Ž .
Ž .
Nucl. Phys. B 35 1971 167; G.M.’t Hooft, M. Veltman,
Ž . Ž .
Nucl. Phys. B 44 1972 189; Nucl. Phys. B 50 1972 318.
w x3 J.W. Stirling, A. Werthenbach, Phys. Lett. B 466 1999 369.Ž . w x4 L3 Collaboration, B. Adeva et al., Nucl. Instr. Meth. A 289 Ž1990 35; J.A. Bakken et al., Nucl. Instr. Meth. A 275. Ž1989 81; O. Adriani et al., Nucl. Instr. Meth. A 302 1991. Ž .
Ž .
53; B. Adeva et al., Nucl. Instr. Meth. A 323 1992 109; K.
Ž .
Deiters et al., Nucl. Instr. Meth. A 323 1992 162; M.
Ž .
Chemarin et al., Nucl. Instr. Meth. A 349 1994 345; M.
Ž .
Acciarri et al., Nucl. Instr. Meth. A 351 1994 300; G. Basti
Ž .
et al., Nucl. Instr. Meth. A 374 1996 293; A. Adam et al.,
Ž .
Nucl. Instr. Meth. A 383 1996 342.
w x5 G. Belanger, F. Boudjema, Nucl. Phys. B 288 1992 201.Ž . w x6 J.W. Stirling, A. Werthenbach, Eur. Phys. J. C 14 2000Ž .
103.
w x7 Opal Collaboration, G. Abbiendi et al., Phys. Lett. B 471 Ž1999 293..
w x8 L3 Collaboration, M. Acciarri et al., Phys. Lett. B 478 2000Ž . 34.
w x9 KORALW V1.42, M. Skrzypek et al., Comp. Phys. Comm.
Ž . Ž .
94 1996 216; Phys. Lett. B 372 1996 289.
w10 E. Barberio, B. van Eijk, Z. Was, Comp. Phys. Comm. 79x Ž1994 291..
w11 T. Sjostrand, H.U. Bengtsson, Comp. Phys. Comm. 46 1987x ¨ Ž . 43.
w12 T. Sjostrand, Comp. Phys. Comm. 82 1994 74.x ¨ Ž .
w13 The L3 detector simulation is based on GEANT Versionx 3.15. See R. Brun et al., GEANT 3, CERN DDrEEr84-1 ŽRevised , September 1987. The GHEISHA program. ŽH.
Ž ..
Fesefeldt, RWTH Aachen Report PITHA 85r02 1985 is used to simulate hadronic interactions.
w14 J.W. Stirling, A. Werthenbach, private communication.x w15 F.A. Berends, R. Pittau, R. Kleiss, Comp. Phys. Comm. 85x
Ž1995 437..
w16 S. Jadach et al., preprint UTHEP-00-0101, to appear.x w17 S. Jadach, private communication.x
w18 L3 Collaboration, M. Acciarri et al., Preprint CERN-EP-x
Ž .
2000–104 2000 .
w19 O.J.P. Eboli, M.C. Gonzales–Garcia, Phys. Lett. B 411x Ž1994 381..
w20 L3 Collaboration, M. Acciarri et al., Phys. Lett. B 444 1998x Ž .
Ž .
503; Phys. Lett. B 470 1999 268.
w21 The KORALZ version 4.03 is used. S. Jadach, B.F.L. Ward,x
Ž .
Z. Was, Comp. Phys Comm. 79 1994 503.
w22 G. Montagna et al., Nucl. Phys. B 541 1999 31.x Ž .
