Article
Reference
Search for anomalous four-jet events in e
+e
−annihilation at √s = 130−172 GeV
L3 Collaboration
AMBROSI, Giovanni (Collab.), et al.
Abstract
A study of hadronic events with high jet-multiplicity is performed using the data sample collected by the L3 experiment at LEP at √s = 130-172 GeV. The observed event rates agree with the Standard Model predictions and upper limits are set on the production cross section of pairs of heavy particles that decay hadronically.
L3 Collaboration, AMBROSI, Giovanni (Collab.), et al . Search for anomalous four-jet events in e
+
e
−annihilation at √s = 130−172 GeV. Physics Letters. B , 1997, vol. 411, p. 330-338
DOI : 10.1016/S0370-2693(97)01001-0
Available at:
http://archive-ouverte.unige.ch/unige:113032
Disclaimer: layout of this document may differ from the published version.
1 / 1
16 October 1997
PHYSICS LETTERS B Physics Letters B 411 (1997) 330-338
Search for anomalous four-jet events in e +e- annihilation at 6 = 130-172 GeV
L3 Collaboration
M. Acciarri ac, 0. Adriani r, M. Aguilar-Benitez ab, S. Ahlen ‘, J. Alcaraz ab, G. Alemanni ‘, J. Allaby ‘, A. Aloisio ae, G. Alverson m, M.G. Alviggi ae, G. Ambrosi ‘, H. Anderhub ay, V.P. Andreev g,an, T. Angelescu “, F. Anselmo j, A. Arefiev ad, T. Azemoon ‘, T. Aziz k, P. Bagnaia am, L. Baksay at, S. Banerjee k,
SW. Banerjee k, K. Banicz av, A. Barczyk ay,aw, R. Barillere ‘, L. Barone am, P. Bartalini aj, A. Baschirotto ac, M. Basile j, R. Battiston aj, A. Bay ‘, F. Becattini r,
U. Becker q, F. Behner ay, J. Berdugo ab, P. Berges q, B. Bertucci aj, B.L. Betev ay, S. Bhattacharya k, M. Biasini ‘, A. Biland ay, G.M. Bilei q, J.J. Blaising d, S.C. Blyth ak, G.J. Bobbink b, R. Bock a, A. Bijhm a, L. Boldizsar O, B. Borgia am,
D. Bourilkov ay, M. Bourquin ‘, S. Braccini ‘, J.G. Branson ap, V. Brigljevic ay, I.C. Brock *, A. Buffini r, A. Buijs au, J.D. Burger q, W.J. Burger ‘, J. Busenitz at,
A. Button ‘, X.D. Cai q, M. Campanelli ay, M. Cape11 q, G. Cara Romeo j, G. Carlino ae, A.M. Cartacci r, J. Casaus ab, G. Castellini r, F. Cavallari am,
N. Cavallo ae, C. Cecchi ‘, M. Cerrada ab, F. Cesaroni y, M. Chamizo ab, Y.H. Chang ba, U.K. Chaturvedi t, S.V. Chekanov ag, M. Chemarin aa, A. Chen ba,
G. Chen h, G.M. Chen h, H.F. Chen “, H.S. Chen h, X. Chereau d, G. Chiefari ae, C.Y. Chien e, L. Cifarelli ao, F. Cindolo j, C. Civinini r, I. Clare q, R. Clare q,
H.O. Cohn ah, G. Coignet d, A.P. Colijn b, N. Colin0 ab, V. Commichau a, S. Costantini ‘, F. Cotorobai n, B. de la Cruz ab, A. Csilling O, T.S. Dai q, R. D’Alessandro r, R. de Asmundis ae, A. Degre d, K. Deiters aw, D. della Volpe ae,
P. Denes a’, F. DeNotaristefani am, D. DiBitonto at, M. Diemoz am, D. van Dierendonck b, F. Di Lodovico ay, C. Dionisi am, M. Dittmar ay, A. Dominguez ap, A. Doria ae, M.T. Dova t,l, D. Duchesneau d, P. Duinker b, I. Duran aq, S. Dutta k, S. Easo aj, Yu. Efremenko ah, H. El Mamouni aa, A. Engler A,
’ Also supported by CONICET and Universidad National de La Plats, CC 67, 1900 La Plats, Argentina.
0370.2693/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved HZ SO370-2693(97)01001-O
M. Acciarri et al. /Physics Letters B 411 (1997) 330-338 331
F.J. Eppling 9, F.C. Em6 b, J.P. Emenwein aa, P. Extermann “, M. Fabre aw, R. Faccini am, S. Falciano am, A. Favara r, J. Fay =, 0. Fedin an, M. Felcini ay, B. Fenyi at, T. Ferguson *, F. Ferroni am, H. Fesefeldt a, E. Fiandrini aj, J.H. Field “,
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M.N. Kienzle-Focacci “, D. Kim am, D.H. Kim as, J.K. Kim as, SC. Kim as, Y.G. Kim as, W.W. Kinnison ‘, A. Kirkby ai, D. Kirkby ai, J. Kirkby ‘, D. Kiss O,
W. Kittel ag, A. Klimentov q,ad, A.C. Kijnig ag, A. Kopp ax, I. Korolko ad, V. Koutsenko @, R.W. Kraemer *, W. Krenz a, A. Kunin q7ad,
P. Ladron de Guevara ab, I. Laktineh aa, G. Landi r, C. Lapoint q, K. Lassila-Perini ay, P. Laurikainen w, M. Lebeau ‘, A. Lebedev q, P. Lebrun aa, P. Lecomte ay, P. Lecoq ‘,
P. Le Coultre ay, J.M. Le Goff ‘, R. Leiste =, E. Leonardi am, P. Levtchenko an, C. Li “, C.H. Lin ba, W.T. Lin ba, F.L. Linde b3s, L. Lista ae, Z.A. Liu h, W. Lohmann =, E. Longo am, W. Lu ai, Y.S. Lu h, K. Lubelsmeyer a, C. Luci am,
D. Luckey q, L. Luminari am, W. Lustermann aw, W.G. Ma “, M. Maity k, G. Majumder k, L. Malgeri am, A. Malinin ad, C. Maiia ab, D. Mange01 ag, S. Mangla k, P. Marchesini ay, A. Marin ‘, J.P. Martin aa, F. Marzano am, G.G.G. Massaro b, D. McNally ‘, R.R. McNeil g, S. Mele ae, L. Merola ae,
M. Meschini r, W.J. Metzger ag, M. von der Mey a, Y. Mi ‘, A. Mihul n, A.J.W. van Mil ag, G. Mirabelli am, J. Mnich ‘, P. Molnar i, B. Monteleoni r, R. Moore ‘, S. Morganti am, T. Moulik k, R. Mount ‘, S. Miller a, F. Muheim ‘,
A.J.M. Muijs b, S. Nahn q, M. Napolitano ae, F. Nessi-Tedaldi ay, H. Newman ‘, T. Niessen a, A. Nippe a, A. Nisati am, H. Nowak =, Y.D. Oh as, H. Opitz a, G. Organtini am, R. Ostonen w, C. Palomares ab, D. Pandoulas a, S. Paoletti am,
P. Paolucci ae, H.K. Park *, I.H. Park as, G. Pascale am, G. Passaleva r, S. Patricelli ae, T. Paul m, M. Pauluzzi 4, C. Paus a, F. Pauss ay, D. Peach s, Y.J. Pei a, S. Pensotti ac, D. Perret-Gallix d, B. Petersen ag, S. Petrak i, A. Pevsner e,
D. Piccolo ae, M. Pier-i r, J.C. Pinto *, P.A. Piroue ‘, E. Pistolesi ac, V. Plyaskin ad, M. Pohl ay, V. Pojidaev ad9r, H. Postema q, N. Produit ‘, D. Prokofiev an,
2 Also supported by Panjab University, Chandigarh-160014, India
332 M. Acciarri et al./ Physics Letters B 411 (1997) 330-338
G. Rahal-Callot ay, N. Raja k, P.G. Rancoita ac, M. Rattaggi ac, G. Raven ap, P. Razis af, K. Read ah, D. Ren ay, M. Rescigno am, S. Reucroft m, T. van Rhee au,
S. Riemann ax, K. Riles ‘, A. Robohm ay, J. Rodin ¶, B.P. Roe ‘, L. Romero ab, S. Rosier-Lees d, Ph. Rosselet ‘, W. van Rossum a”, S. Roth a, J.A. Rubio ‘, D. Ruschmeier i, H. Rykaczewski ay, J. Salicio ‘, E. Sanchez ab, M.P. Sanders ag,
M.E. Sarakinos w, S. Sarkar k, M. Sassowsky a, C. Schafer a, V. Schegelsky an, S. Schmidt-Kaerst a, D. Schmitz a, P. Schmitz a, N. Scholz ay, H. Schopper az, D.J. Schotanus ag, J. Schwenke a, G. Schwering a, C. Sciacca ae, D. Sciarrino “,
L. Servoli %, S. Shevchenko ai, N. Shivarov ar, V. Shoutko ad, J. Shukla ‘, E. Shumilov ad, A. Shvorob ai, T. Siedenburg a, D. Son as, A. Sopczak ax, B. Smith q,
P. Spillantini r, M. Steuer q, D.P. Stickland a1, A. Stone g, H. Stone a’, B. Stoyanov ar, A. Straessner a, K. Strauch p, K. Sudhakar k, G. Sultanov t, L.Z. Sun “, G.F. Susinno “, H. Suter ay, J.D. Swain t, X.W. Tang h, L. Tauscher f,
L. Taylor m, Samuel C.C. Ting q, S.M. Ting q, M. Tonutti a, S.C. Tonwar k, J. T&h O, C. Tully a1, H. Tuchscherer at, K.L. Tung h, Y. Uchida q, J. Ulbricht ay, U. Uwer ‘, E. Valente am, R.T. Van de Walle ag, G. Vesztergombi O, I. Vetlitsky ad, G. Viertel ay, M. Vivargent d, R. Viilkert =, H. Vogel *, H. Vogt ax, I. Vorobiev ad,
A.A. Vorobyov an, A. Vorvolakos af, M. Wadhwa f, W. Wallraff a, J.C. Wang q, X.L. Wang “, Z.M. Wang “, A. Weber a, F. Wittgenstein ‘, S.X. Wu t, S. Wynhoff a,
J. Xu i, Z.Z. Xu “, B.Z. Yang “, CC. Yang h, X.Y. Yao h, J.B. Ye “, S.C. Yeh ba, J.M. You *, An. Zalite an, Yu. Zalite an, P. Zemp ay, Y. Zeng a, Z. Zhang h, Z.P. Zhang “, B. Zhou ‘, G.Y. Zhu h, R.Y. Zhu ‘, A. Zichichi j3s,t, F. Ziegler ax
a 1. Physikalisches Institut, RWTH, D-52056 Aachen, FRG 3 III. Physikalisches Institut, RWTH, D-52056 Aachen, FRG ’
b National Institute for High Energy Physics, NIKHEF< and University of Amsterdam, NL-1009 DB Amsterdam. The Netherlands
’ University of Michigan, Ann Arbor, MI 48109, USA
’ Laboratoire d’Annecy-le-Vieux de Physique des Particules, LAPP,INZP3CNRS, BP 110, F-74941 Annecy-le-Vieux CEDEX, France e Johns Hopkins University, Baltimore, MD 21218, USA
’ Institute of Physics, University of Base& CH-4056 Basel, Switzerland g Louisiana State University, Baton Rouge, LA 70803. USA h Institute of High Energy Physics, IHEP, 100039 Beijing, China 4
’ Humboldt University, D-10099 Berlin. FRG ’
’ University of Bologna and INFN-Sezione di Bologna, I-40126 Bologna, Italy
’ Tata Institute of Fundamental Research, Bombay 400 005. India
’ Boston University, Boston, MA 02215, USA m Northeastern University, Boston, MA 02115, USA
n Institute of Atomic Physics and University of Bucharest, R-76900 Bucharest, Romania
’ Central Research Institute for Physics of the Hungarian Academy of Sciences, H-1525 Budapest 114, Hungary 5
’ Harvard University, Cambridge, MA 02139, USA
’ Massachusetts Institute of Technology, Cambridge, MA 02139, USA r INFN Sezione di Firenze and University of Florence, t-50125 Florence, Italy
’ European Laboratory for Particle Physics, CERN, CH-1211 Geneva 23, Switzerland
3 Supported by the German Bundesministerium fir Bildung, Wissenschaft. Forschung und Technologie.
4 Supported by the National Natural Science Foundation of China.
M. Acciani et al./ Physics Letters B 411 (1997) 330-338 333
’ World Laboratory, FBLJA Project, CH-1211 Geneva 23, Switzerland
’ University of Geneva, CH-1211 Geneva 4, Switzerland
” Chinese University of Science and Technology, VSTC, Hefei, Anhui 230 029, China 4 w SEFT, Research Institute for High Energy Physics, P.O. Box 9, SF-o0014 Helsinki, Finland
’ University of Lausanne, CH-1015 L.ausanne, Switzerland
y INFN-Sezione di Lecce and Vniversitri Degli Studi di Lecce. I-73100 Lucce, Italy
’ Los Alanws National Laboratory, Los Alamos. NM 87544, USA
aa lnstitut de Physique Nucliaire de Lyon, IN2P3-CNRS, Vniversite’ Claude Bernard, F-49622 Villeurbanne, France ab Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas, CIEMAT, E-28040 Madrid, Spain 6
ac INFN-Sezione di Milano, I-20133 Milan, Italy
ad Institute of Theoretical and Experimental Physics, ITEP, Moscow, Russia ae INFN-Sezione di Napoli and University of Naples, l-80125 Naples, Italy
af Department of Natural Sciences, University of Cyprus, Nicosia, Cyprus ag Vniversiry of Nijmegen and NIKHEF, NL-6525 ED Nijmegen, The Netherlands
ah Oak Ridge National Laboratory, Oak Ridge. TN 37831, USA
’ California Institute of Technology, Pasadenn, CA 91125, USA
aJ INFN-Sezione di Perugia and Vniversitri Degli Studi di Perugia, I-06100 Perugia, Italy p Carnegie Mellon University, Pittsburgh, PA 15213, USA
a’ Princeton University, Princeton, NJ 08544, USA
am INFN-Sezione di Roma and University of Rome, “La Sapienza”, I-00185 Rome, Italy an Nuclear Physics Institute, St. Petersburg, Russia
” University and INFN, Salerno, l-84100 Salerno, Italy ap Vniversiry of California, San Diego, CA 92093, USA
q Dept. de Fisica de Particulas Elementales, Univ. de Santiago, E-15706 Santiago de Compostela, Spain ar Bulgarian Academy of Sciences, Central Lab. of Mechatronics and Instrumentation, BV-1113 Sofa, Bulgaria as Center for High Energy Physics, Korea Adu. Inst. of Sciences and Technology, 305-701 Taejon, South Korea
81 Vniversiry of Alabama, Tuscaloosa, AL 35486, USA au Vtrecht University and NIKHEF, NL-3584 CB Vtrecht, The Netherlands
ay Purdue University, West Lafnyette, IN 47907, USA aw Paul Scherrer Institut. PSI, CH-5232 Villigen, Switzerland ax DESY-lnstitut fir Hochenergiephysik, D-15738 Zeuthen, FRG
ay Eidgenassische Technische Hochschule, ETH Ziirich, CH-8093 Ziirich, Switzerland a University of Hamburg, D-22761 Hamburg, FRG
‘a High Energy Physics Group, Taiwan, ROC Received 30 May 1991
Editor: K. Winter
Abstract
A study of hadronic events with high jet-multiplicity is performed using the data sample collected by the L3 experiment at LEP at & = 130-172 GeV. The observed event rates agree with the Standard Model predictions and upper limits are set on the production cross section of pairs of heavy particles that decay hadronically. 0 1997 Elsevier Science B.V.
1. Introduction
The increase in energy of the LEP accelerator opens new possibilities to investigate physics beyond
the Standard Model. Several of these possibilities, like supersymmetric Higgs production and R-parity violation processes in Supersymmetry [ 11, are charac- terized by topologies with a high number of jets and no missing energy. The report from the ALEPH
5 Supported by the Hungarian OTKA fund under contract num- hers T14459 and T24011.
6 Supported also by the Comisi6n Interministerial de Ciencia y Technologia.
collaboration [2] of anomalous four-jet production has given additional motivation for the search in these channels.
In this paper we report on a search for anomalous
334 M. Acciarri et al./ Physics Letters B 411 (1997) 330-338
four-jet events at fi = 130-172 GeV. The associ- ated production of hadronically decaying particles (that is the process e+ e- --) XY + qijqij) would show up as a resonance in the distribution of the invariant mass of the two pairs of jets. The choice among the three possible pairings of the jets depends on the signal searched for. In the hypothesis Mx = Mr the combination where the difference between the two reconstructed masses is the smallest is considered, while in case the two masses are allowed to be different the other combinations are also used.
then fully simulated in the L3 detector using the GEANT library [6].
3. Analysis
3.1. Selection of four-jet events
2. Data reconstruction and Monte Carlo samples
The data collected by the L3 experiment during the three runs of LEP at !& = 130- 140 GeV (herein referred to as 133 GeV run) in 1995, 161 GeV and 170-172 GeV (herein referred to as 172 GeV run) in 1996 corresponding to an integrated luminosity of p= 5.0 pb-I, 10.9 pb-’ and 10.3 pb-’ respectively
are analysed.
The main sources of production of four-jet events in the Standard Model are hard gluon emission or jet misreconstruction in hadronic events and, at & 2
161 GeV, W and Z pair production. The separate contributions at the different energies are shown in Table 1. The selection of four-jet events is divided in the following steps:
- High Multiplicity Requirement: Events are re- quired to have at least 10 tracks and 30 calori- metric clusters. In addition the visible energy is required to be greater than 70% of the center-of- mass energy.
A detailed description of the L3 detector and its performance is given in Ref. [3]. Hadronic events are reconstructed using information from all subdetec- tom. In particular the energy of the event is obtained taking into account the energy deposition in the electromagnetic and hadronic calorimeters and the charged particles momenta measured by the central tracking chamber and the muon chambers.
In order to evaluate the expectations from Stan- dard Model processes, Monte Carlo (MC) event samples of 44(y), e+e-qq, Z/y *Z/y * were pro- duced using PYTHIA [4] and W+ W- using KO- RALW [5]. As a reference reaction to evaluate the efficiency on a possible signal, a sample of MC events e’e- --) hA -+ bbbb was generated -- with
PYTHIA for several Higgs bosons h and A masses in the range from 30 to 130 GeV. All events were
- Four-Jet Criteria: Events consistent with radiative return to the Z (e’e-+ Zy -+ qqy) are rejected by requiring the effective center-of-mass energy (G> to be greater than 0.87& . If the radiative photon is not detected in the calorimeters fi is computed from the jet angles with the assumption that one photon has escaped in the beam pipe.
Jets are then reconstructed using the Durham [7]
algorithm and events are rejected if they have less than four jets for the jet resolution parameter Ye,,, = 0.008. In addition none of the jets must contain electromagnetic energy in excess of 85%
of the jet energy. Events satisfying these require- ments are reconstructed again but with a variable Y CUt chosen in order to have exactly four jets.
A kinematic fit that requires four momentum conservation is applied to improve the energy and angular resolution. This constraint induces a nega-
Table 1
Cross sections (pb) of processes contributing to four-jet events production.
process e+e-* q2j e+e-+ WW e+e-AZ/y’Z/y’
6 = 130GeV 336 0.07 0.50
J& = 136 GeV 280 0.11 0.47
& = 161GeV 149 3.24 0.46
fi = 172GeV 124 12.2 0.43
M. Acciarri et al. /Physics Letters 3 41 I (19971330-338 335
tive correlation between the two jet pair masses such that the sum of the masses has a better resolution than the difference. Thus in the follow- ing the sum of the reconstructed masses for the pairing that has the smallest (ZM,) and the sec- ond smallest (CM,) mass difference are shown.
-
Jet Quality Requirements: Events are rejected if any of the four jets has an energy smaller than 5%fi, has no tracks or is within an 8” cone around the beam pipe. After applying these crite- ria mainly QCD four-jet events and hadronic W decays are expected from Standard Model pro- cesses.
-
Kinematic Requirements: QCD four-jet events predominantly consist of two nearly back to back energetic jets and two low energy jets. These events are therefore characterized by a high dif- ference between the most and the least energetic jet (A, = E,,, - Emin) and a maximum angle be- tween jets co,,,) close to 180”. The minimum invariant mass of jet pairs (M,,) is small. Events are therefore required to have A, < 0.36, Mmin
> O.lSfi and, for the 161 GeV data, COS(~,,,,,>
> -0.95.
*
W Pair Rejection: W pair events are identified using a kinematic fit with the assumption that for one possible pairing of the four jets both di-jet invariant masses are equal to the W mass. Events are rejected if the x2 of this fit is small.
Table 2
Fig. 1. Distribution of the sum of the masses for the smallest difference (EM,) at different stages of the analysis: (a) after the requirement of four jets, (b) after the requirements on kinematics and (c) after ail cuts. Histograms are the expectations from the Standard Model while the dots are the data collected at 6 = 130- 172 GeV.
The number of events expected from Standard Model processes, the effkiency on the signal, for the mass points M,, = MA = 5.5 or 60 GeV at 133 and
Number of data events observed (h&J, number of events expected from Standard Model processes (N,,) and efficiency for an e+e- --) hA signal at different stages of the analysis. The quoted errors are due to MC statistics only
& (GeV) Set of cuts N data N,, e&%1
130-140 High Multiplicity 1032 934*4 96.7 + 0.6
Four Jets 24 23.4 f. 1.0 58.6 + 1.6
Jet Quality 19 19.3 0.9 f 57.6 f 1.6
Kinematics 14 9.6 5 0.6 48.6 + 1.7
161 High Multiplicity 827 790+2 93.6 f 0.5
Four Jets 28 32.9 + 0.5 56.5 f 1.6
Jet Quality 24 29.2 * 0.4 55.1 f 1.5
Kinematics 7 10.0 0.3 f 35.5 * 1.5
WW rejection 6 7.9 f 0.2 33.8 + 1.5
170-172 High Multiplicity 623 610 f3 92.1 f0.9
Four Jets 49 54.8 f 0.4 52.8 &- 1.6
Jet Quality 48 49.8 f 0.4 50.7 f 1.6
Kinematics 33 36.4 i 0.2 40.8 + 1.6
WW rejection 14 14.0 0.2 f 31.1 f 1.4
M. Acciarri et al. / Physics Letters B 41 I (I 997) 330-338
l
data
-MC
cM,&CM, (GeV)
Fig. 2. Distribution of the sum of the masses for the two smallest combinations (SM, and PM,) together (two entries per event) after the selection. The whole data set collected a& 6 = 130- 172 GeV is used.
161-172 GeV respectively, and the number of data events observed are shown in Table 2. Agreement between data and MC is found at all the stages of the analysis as shown in Fig. 1. In order to search for signals where the two particles produced do not have the same but similar masses both ZM, and ZM, are plotted in Fig. 2. No significant deviation from the Standard Model expectations is found in these distri- butions.
3.2. Experimental checks
Several studies using control data samples are carried out to verify the detector performance.
- Return to the Z events: A sample of radiative return to the Z events with an energetic, isolated photon in the electromagnetic calorimeter is se- lected. The measured photon energy is required to be within +5 GeV of the expected one. These events are reconstructed imposing a three body constraint, assuming that the photon direction is known and leaving its energy free. The measured mass of the hadronic system peaks at the mass of the Z (Fig. 3a) and the width of the distribution in the data is 3.0 + 0.2 GeV. There is agreement
between data and MC and no shift is observed:
the difference between the mean values is 0.2 +_
0.2 GeV. The consistency between data and MC confirms that the evaluation of the sensitivity to the signal using MC samples is correct.
- MC simulation of e+e- -+ hA: In order to evalu- ate the mass resolution for a four-jet signal, the MC sample with MA = M,, = 55 GeV at 133 GeV and MA = Mh = 60 GeV at 161 and 172 GeV respectively were considered. The fit to the distri- bution of the sum of the masses CM, (Fig. 3b) gives a resolution of 2.2 Ifr 0.2 GeV at & = 133 GeV, 2.5 k 0.2 at & = 161 GeV and 3.2 k 0.2 GeV at & = 172GeV. The mean values of the fits are consistent with the generated mass values within 0.5 GeV.
40
Mhad (GeV)
EM, (GeV)
Fig. 3. (a) Distribution of the reconstructed Z mass in qqy events for data and MC at 130 < fi < 172 GeV. (b) Spectrum of ZM, for an e’ e- + hA MC sample with MA = Mh = 55 GeV at 6 = 133 GeV.
M. Acciani et al. / Physics Letters B 411 (1597) 330-338 337 15
10
5
0
L3
l
data [lww
I 50
CM,&EM, (GeV)
Fig. 4. Distribution of ZM,and ZM, at fi= 172 GeV before the WW rejection is applied.
. WW
events at fi = 172 GeV: The distribution of the sum of the di-jet invariant masses XM, and CM, at fi = 172 GeV before the WW rejection
8
% a6
j4 9
0
50 100 150
ZM, (GeV)
-3
b
l
10q
g5 E
80 50 100 150
~M,&ZM, (GeV)
Fig. 5. (a) Distribution of 8M, and (b) of 8M, and ZM, with two entries per event obtained in the complementary analysis descibed in Section 3.2. The data from fi = 130 to 172 GeV are combined.
Table 3
Number of data events observed (Ndatn), number of
events ex- pected
from the Standard Model (N,,f and efficiency for the signal e+ e- + hA (e,) using a selection that f0110WS the one published by ALEPH. Errors are due to MC statistics only.fi (GeV) N data N,, r,(a)
130-140 12 8.lkO.6 44.8k1.7
161 7 8.050.2 32.2+ 1.5
170-172 14 13.5 *0.2 25.7 f 1.4
(Fig. 4) shows a peak around 160 GeV due to W pair production. As a check of the analysis we extract the e+e- --) W+ W- cross section in a log-likelihood fit to this distribution assuming the Standard Model branching ratio of the W into hadrons. The result gww = (10.9 f 2.4(stat.))pb is in agreement with the Standard Model predic- tion of 12.2 pb.
Complementary analyses: In addition several analyses with different energy flows and kine- matic resealings have been performed yielding results similar to the one described in Section 3.1.
In particular an analysis close to the one pub- lished by the ALEPH collaboration [2] with simi- lar efficiencies and resolutions was performed.
The number of data events selected, the number of events expected from Standard Model pro- cesses and the efficiency on a possible signal are reported in Table 3. The differences between this analysis and the one described in Section 3.1 are in the jet reconstruction, in the d&jet quality requirements and in the WW rejection. For this analysis Fig. 5 shows the combined plot of CM, and ZM, for the three running periods after all cuts. Also for this analysis we find good agree- ment between data and Standard Model expecta- tions.
4. Results
Good agreement is found between our data and the expectations from the Standard Model both in the rate of four-jet events and in their mass spectrum.
The DELPHI and OPAL collaborations reported on similar studies on 130 - 140 GeV data [8].
Upper limits on the production cross section of
338 M. Acciarri et al./Physics Letters B 411 (1997) 330-338
I I I I
100 120 140 100 120 140
JZM,(GeV)
Fig. 6. Maximum cross section (cram) of a possible signal compat- ible with observations at 95% C.L. as a function of the sum of the masses BM,: (a) &= 133GeV, (b) h= 161GeV. (cl &=
172GeV and (d) combination of the three center-of-mass energies assuming a cross section constant with energy. Three different hypothesis are considered: the difference between the masses of the two bosom produced being 0, 10 or 20 GeV.
possible new physics in this channel are derived using the events that satisfy the criteria described in Section 3.1. The efficiencies -- calculated from the e+e- + hA + bbbb MC are assumed. The number of events predicted by the Standard Model is com- puted in ZM, windows with a half width corre- sponding to approximately twice the resolution. The central ZM, values of these windows are moved from 90 to 150 GeV in 1 GeV steps. In each window the maximum number of signal events compatible at 95% C.L. with our observation is calculated and converted into a cross section upper limit (slim) at each center-of-mass energy (Fig. 6a-cl. The combi- nation of data at different center-of-mass energies is also made, assuming constant signal cross section.
Removing the constraint that the two hadronically decaying particles produced have equal masses, the same procedure is carried out for the sum of the masses selecting jet pairs with mass differences clos-
est to 10 and 20 GeV. As shown in Fig. 6d the cross section upper limit ulim ranges between 0.7 and 1.6 pb for CM, < 140 GeV.
Acknowledgements
We wish to congratulate the CERN accelerator divisions for the successful upgrade of the LEP machine and to express our gratitude for the good performance of the machine. We acknowledge with appreciation the effort of all engineers, technicians and support staff who have participated in the con- struction and maintenance of this experiment. Those of us who are not from member states thank CERN for its hospitality and help.
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