Article
Reference
Search for neutral Higgs bosons of the minimal supersymmetric standard model in e
+e
−interactions at √s=192–202 GeV
L3 Collaboration
ACHARD, Pablo (Collab.), et al.
Abstract
A search for the lightest neutral CP-even and the neutral CP-odd Higgs bosons of the Minimal Supersymmetric Standard Model is performed using 233.2 pb−1 of integrated luminosity collected with the L3 detector at LEP at centre-of-mass energies 192–202 GeV. No signal is observed and lower mass limits are given as a function of tanβ for two scalar top mixing hypotheses. For tanβ greater than 0.8, they are mh>83.4 GeV and mA>83.8 GeV at 95%
confidence level.
L3 Collaboration, ACHARD, Pablo (Collab.), et al . Search for neutral Higgs bosons of the minimal supersymmetric standard model in e
+e
−interactions at √s=192–202 GeV. Physics Letters. B , 2001, vol. 503, no. 1-2, p. 21-33
DOI : 10.1016/S0370-2693(01)00192-7
Available at:
http://archive-ouverte.unige.ch/unige:44040
Disclaimer: layout of this document may differ from the published version.
1 / 1
Physics Letters B 503 (2001) 21–33
www.elsevier.nl/locate/npe
Search for neutral Higgs bosons of the minimal supersymmetric standard model in e + e − interactions at √
s = 192–202 GeV
L3 Collaboration
M. Acciarri
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s, O. Adriani
p, M. Aguilar-Benitez
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0370-2693/01/$ – see front matter 2001 Published by Elsevier Science B.V.
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ai, Yu. Zalite
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t, G.Y. Zhu
g, R.Y. Zhu
ad, A. Zichichi
i,q,r,
G. Zilizi
ap,3, B. Zimmermann
au, M. Zöller
aaI. Physikalisches Institut, RWTH, D-52056 Aachen, Germany III. Physikalisches Institut, RWTH, D-52056 Aachen, Germany4
bNational Institute for High Energy Physics, NIKHEF, and University of Amsterdam, NL-1009 DB Amsterdam, The Netherlands cUniversity 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, University of Basel, CH-4056 Basel, Switzerland
fLouisiana State University, Baton Rouge, LA 70803, USA gInstitute of High Energy Physics, IHEP, 100039 Beijing, PR China5
hHumboldt University, D-10099 Berlin, Germany4
iUniversity of Bologna and INFN-Sezione di Bologna, I-40126 Bologna, Italy jTata Institute of Fundamental Research, Bombay 400 005, India
kNortheastern University, Boston, MA 02115, USA
lInstitute of Atomic Physics and University of Bucharest, R-76900 Bucharest, Romania
mCentral Research Institute for Physics of the Hungarian Academy of Sciences, H-1525 Budapest 114, Hungary6 nMassachusetts Institute of Technology, Cambridge, MA 02139, USA
oKLTE-ATOMKI, H-4010 Debrecen, Hungary3
pINFN Sezione di Firenze and University of Florence, I-50125 Florence, Italy qEuropean Laboratory for Particle Physics, CERN, CH-1211 Geneva 23, Switzerland
rWorld Laboratory, FBLJA Project, CH-1211 Geneva 23, Switzerland sUniversity of Geneva, CH-1211 Geneva 4, Switzerland
tChinese University of Science and Technology, USTC, Hefei, Anhui 230 029, PR China5 uUniversity of Lausanne, CH-1015 Lausanne, Switzerland
vINFN-Sezione di Lecce and Università Degli Studi di Lecce, I-73100 Lecce, Italy
wInstitut de Physique Nucléaire de Lyon, IN2P3-CNRS, Université Claude Bernard, F-69622 Villeurbanne, France xCentro de Investigaciones Energéticas, Medioambientales y Tecnologícas, CIEMAT, E-28040 Madrid, Spain7
yINFN-Sezione di Milano, I-20133 Milan, Italy
zInstitute of Theoretical and Experimental Physics, ITEP, Moscow, Russia aaINFN-Sezione di Napoli and University of Naples, I-80125 Naples, Italy abDepartment of Natural Sciences, University of Cyprus, Nicosia, Cyprus acUniversity of Nijmegen and NIKHEF, NL-6525 ED Nijmegen, The Netherlands
adCalifornia Institute of Technology, Pasadena, CA 91125, USA
aeINFN-Sezione di Perugia and Università Degli Studi di Perugia, I-06100 Perugia, Italy afCarnegie Mellon University, Pittsburgh, PA 15213, USA
agPrinceton University, Princeton, NJ 08544, USA
ahINFN-Sezione di Roma and University of Rome, “La Sapienza”, I-00185 Rome, Italy aiNuclear Physics Institute, St. Petersburg, Russia
ajINFN-Sezione di Napoli and University of Potenza, I-85100 Potenza, Italy akUniversity of Californa, Riverside, CA 92521, USA
alUniversity and INFN, Salerno, I-84100 Salerno, Italy am University of California, San Diego, CA 92093, USA
anBulgarian Academy of Sciences, Central Lab. of Mechatronics and Instrumentation, BU-1113 Sofia, Bulgaria aoLaboratory of High Energy Physics, Kyungpook National University, 702-701 Taegu, South Korea
apUniversity of Alabama, Tuscaloosa, AL 35486, USA aqUtrecht University and NIKHEF, NL-3584 CB Utrecht, The Netherlands
arPurdue University, West Lafayette, IN 47907, USA asPaul Scherrer Institut, PSI, CH-5232 Villigen, Switzerland
atDESY, D-15738 Zeuthen, Germany
auEidgenössische Technische Hochschule, ETH Zürich, CH-8093 Zürich, Switzerland avUniversity of Hamburg, D-22761 Hamburg, Germany
awNational Central University, Chung-Li, Taiwan, ROC axDepartment of Physics, National Tsing Hua University, Taiwan, ROC
Received 27 November 2000; accepted 26 January 2001 Editor: K. Winter
Abstract
A search for the lightest neutral CP-even and the neutral CP-odd Higgs bosons of the Minimal Supersymmetric Standard Model is performed using 233.2 pb−1of integrated luminosity collected with the L3 detector at LEP at centre-of-mass energies 192–202 GeV. No signal is observed and lower mass limits are given as a function of tanβfor two scalar top mixing hypotheses.
For tanβgreater than 0.8, they aremh>83.4 GeV andmA>83.8 GeV at 95% confidence level.2001 Published by Elsevier Science B.V.
1. Introduction
The Minimal Supersymmetric Standard Model (MSSM) [1] requires two Higgs doublets. This gives rise to five Higgs bosons: a charged scalar pair, two neutral CP-even, the lightest of which is called h, and a neutral CP-odd, A. The two most important produc- tion mechanisms in e+e−collisions are:
(1) e+e−→Z∗→hZ,
(2) e+e−→Z∗→hA.
The cross section of the process (1) is smaller than that for the similar production of the Higgs boson in the Standard Model. This process is dominant at low values of tanβ (tanβ5), where tanβ is the ratio of the two Higgs vacuum expectation values. The pair- production of Higgs bosons (2) takes over at high values of tanβ.
Previous searches for the h and A bosons were re- ported by L3 [2] and other experiments [3]. In this paper, we present the results of the search for the h
1 Also supported by CONICET and Universidad Nacional de La Plata, CC 67, 1900 La Plata, Argentina.
2 Also supported by Panjab University, Chandigarh-160014, India.
3 Also supported by the Hungarian OTKA fund under contract numbers T22238 and T026178.
4 Supported by the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie.
5 Supported by the National Natural Science Foundation of China.
6 Supported by the Hungarian OTKA fund under contract numbers T019181, F023259 and T024011.
7 Supported also by the Comisión Interministerial de Ciencia y Tecnología.
and A bosons using the data collected with the L3 detector [4] at centre-of-mass energies√
s=191.6–
201.7 GeV, corresponding to 233.2 pb−1 of total in- tegrated luminosity. The sensitivity to the production of MSSM neutral Higgs bosons is improved by com- bining the results of these analyses with our previous searches.
2. Data and Monte Carlo samples
In 1999 the L3 detector collected data at LEP at average centre-of-mass energies √
s =191.6 GeV, 195.5 GeV, 199.5 GeV and 201.7 GeV, corresponding to the integrated luminosities 29.7 pb−1, 83.7 pb−1, 82.8 pb−1and 37.0 pb−1respectively.
The cross sections of processes (1) and (2) and the decay branching ratios of h and A are calculated using the HZHA generator [5]. For efficiency studies, Monte Carlo samples of Higgs events are generated using PYTHIA [6] and HZHA. 2000 Monte Carlo events are generated for each mass hypothesis. For hA samples the masses,mhandmA, of the h and A bosons range from 50 to 95 GeV in steps of 5 GeV. For hZ samples mhis chosen in steps of 5 GeV from 50 to 95 GeV and in steps of 1 GeV from 95 to 110 GeV. For the back- ground studies, the following Monte Carlo programs are used: PYTHIA (e+e−→qq(γ ), e¯ +e−→ZZ and e+e−→Ze+e−), KORALW [7] (e+e−→W+W−), KORALZ [8] (e+e−→ τ+τ−). Hadron production in two-photon interactions is simulated with PYTHIA and PHOJET [9]. EXCALIBUR [10] is used for other four fermion final states. The number of simulated background events for the most important background channels is more than 100 times the corresponding number of expected events.
The L3 detector response is simulated using the GEANT program [11], which models the effects of energy loss, multiple scattering and showering in the detector. The GHEISHA program [12] is used to simulate hadronic interactions in the detector. Time dependent inefficiencies are also taken into account.
3. Event selection
For the hA production, the following decay modes are considered: hA → bbb¯ b, hA¯ → bbτ¯ +τ− and hA→τ+τ−b¯b. In the case of hZ, four event topolo- gies covering approximately 98% of possible final states, are considered: qqq¯ q, q¯ qν¯ ν¯, qql¯+l−(l=e, µ, τ ) andτ+τ−qq. The searches in channels with hadronic¯ decays of the h boson are optimised for the dominant h→bb decay channel. The analyses q¯ qν¯ ν¯and qql¯+l− (l=e, µ) are the same as devised for the Standard Model Higgs search [13].
A common selection is applied to both the hA and hZ searches. In the four-jet channel, it mainly reduces the two-photon interaction background while keeping the signal efficiency high, then a neural network is used to build a discriminating variable. In the tau channel, first a selection is devised, then an optimal variable based on a likelihood approach is defined.
3.1. hA→bbb¯ b and hZ¯ →bbq¯ q selection¯
The signature of both the hA→bbb¯ b and hZ¯ → bbq¯ q final states is four hadronic jets and the presence¯ of b-hadrons. The dominant backgrounds come from qq(γ )¯ production and hadronic decays of W and Z pairs.
High multiplicity events are selected and their visi- ble energy,Evis, is required to be greater than 0.6√
s and less that 1.4√
s. Events with perpendicular imbal- ance greater than 0.35Evisor with a lepton whose en-
Table 1
Number of events observed and expected in the four-jet channels, after the selection and after cuts on the discriminating variables, NNhAand NNhZ. The hA signal efficiencies are quoted formA=mh=85 GeV. The hZ signal efficiencies correspond tomh=100 GeV at√s=192–
196 GeV andmh=105 GeV at√s=200–202 GeV
192 GeV 196 GeV
Cut Selection NNhA>0.9 NNhZ>0.5 Selection NNhA>0.9 NNhZ>0.5
Data 430 2 4 1186 3 33
MC 433.6 1.0 9.5 1197.5 3.2 26.0
qq¯ 201.2 0.4 3.0 531.3 1.2 7.1
W+W− 217.9 0.2 4.2 622.3 0.7 11.9
ZZ 14.5 0.4 2.3 43.9 1.3 7.0
εhA 95.0% 42.0% 65.9% 95.0% 42.0% 65.9%
εhZ 93.7% 12.6% 52.7% 93.7% 12.6% 52.7%
200 GeV 202 GeV
Cut Selection NNhA>0.9 NNhZ>0.5 Selection NNhA>0.9 NNhZ>0.5
Data 1198 2 9 506 1 6
MC 1118.4 2.0 9.1 507.8 1.0 4.8
qq¯ 478.3 0.7 2.5 215.0 0.3 1.5
W+W− 595.4 0.3 3.3 271.8 0.2 1.7
ZZ 44.7 1.0 3.3 21.0 0.5 1.6
εhA 94.1% 37.9% 37.9% 95.9% 38.3% 40.4%
εhZ 91.0% 5.7% 40.9% 92.3% 5.0% 36.3%
ergy exceeds 65 GeV are rejected to suppress semilep- tonic W pair decays. Initial state radiation events are further suppressed by requiringPmisL /(mvis−mZ) <
0.4, where PmisL is the longitudinal component of the missing momentum,mvis is the visible mass and mZ the Z boson mass. The remaining events are then forced into four jets using the DURHAM algo- rithm [14] and a kinematic fit requiring energy and momentum conservation (4C) is performed. The num- ber of expected and observed events in the data, to- gether with the signal efficiencies for the selection cuts are listed in Table 1.
After the selection, discriminating variables are combined into a feed forward neural network [15], with one hidden layer and three output nodes. The inputs include the probability that the jets contain b-quarks [16], hereafter termed BTag, event shape variables and mass information. The information on the event shape includes the event sphericity, the value of the DURHAM jet resolution parameter for which the event is resolved from three to four jets, the longitudinal component of the missing momentum and the event thrust. The mass information is summarised in aχ2variable defined as:
Fig. 1. Distributions of (a)BTag, (b) logarithm of theχ2variable, (c) cosine of the polar angleΘof the Higgs boson and (d) sphericity after the four-jet selection. The points represent the data at√
s=192–202 GeV, the open histograms are the expected Standard Model backgrounds, and the hatched histograms are the expected hA→b¯bb¯b signal scaled by a factor 50, formh=mA=85 GeV at tanβ=30.
χ2(mX, mh)=
Σijmin−(mX+mh)2
wΣ
+ (3)
∆minij − |mX−mh|2 w∆,
wheremX is eithermZormA. The pairing used gives the minimum value for the difference squared(∆ij−
|mX−mh|)2between the measured and expected dijet mass differences; Σijmin is the corresponding sum of the dijet masses. The weights,wΣandw∆, are derived from the mass resolutions, and their ratio is 3/5. In this way, the shape of the neural network output is made almost independent of the mass hypothesis. The polar angle of the Higgs boson, Θ, is also used as input for the hA analysis. It gives additional separation
between the hA signal and W+W−background, due to the different spins of the W and the Higgs bosons. The distributions of the discriminating input variables are shown in Fig. 1.
The Neural Network has three output variables which correspond to the signal,OHiggs, the qq(γ )¯ final state, Oqq, and the W+W− final state hypotheses, OWW. The discriminating variable is then obtained from the combination NN =OHiggs×(1−Oqq)× (1−OWW). Two different neural networks are used for the hA and hZ analyses, and the events are classified as hA or hZ according to the largest value of the discriminating variable. The neural network for the
Fig. 2. Distributions of the discriminating variable in the four jet channel for (a) hA→b¯bb¯b and (b) hZ→b¯bqq. The points show the data¯ collected at√s=192–202 GeV, the open histograms are the expected Standard Model backgrounds and the hatched histograms are the expected signals scaled by a factor 10. The discriminating variables are constructed assuming equal mass hypothesismh=mA=85 GeV at tanβ=30 in (a) andmh=100 GeV at tanβ=1 in (b).
Fig. 3. Distributions of (a) the sum of the reconstructed dijet masses in the hA→b¯bb¯b channel after the cut NN>0.9, (b) the sum of the dijet and the ditau masses in the hA→b¯bτ+τ−channel after a cut on theBTag. The points represent the data collected at√
s=192–202 GeV, the open histogram is the expected Standard Model background and the hatched histogram is the expected signal formh=mA=85 GeV at tanβ=30.
hZ is optimised for mh=100 GeV at √
s=192–
196 GeV andmh=105 GeV at√
s=200–202 GeV.
For the hA search, the neural network is optimised for mh=mA=85 GeV. Examples of the discriminating variable distributions of the two analyses are shown in Fig. 2. Good agreement is observed between the data and the expected background. An independent analysis based on a likelihood approach [17] validates the present results.
The highest signal sensitivity region corresponds to large values of NN. For illustrative purposes, in Table 1 we report the number of the observed and expected events selected after the cuts NN>0.9 for
the hA analysis, and NN>0.5 for the hZ analysis.
The mass distributions for the hA search after the cut NN>0.9 is shown in Fig. 3(a).
3.2. hA→bbτ¯ +τ−, hZ→b¯bτ+τ−and hZ→τ+τ− qq selections¯
The signatures of hA→bbτ¯ +τ−,8hZ→bbτ¯ +τ− or hZ→τ+τ−qq events are a pair of taus accompa-¯ nied by two hadronic jets. For each of the channels
8 Both the decay modes (h→b¯b, A→τ+τ−) and (h→τ+τ−, A→b¯b) are considered.
Table 2
Number of events observed and expected in the tau selection. Efficiencies for the hA signal are quoted formA=mh=85 GeV. For the hZ signal, they are quoted formh=100 GeV at√
s=192–196 GeV and formh=105 GeV at√
s=200–202 GeV
192 GeV 196 GeV 200 GeV 202 GeV
hA hZ hA hZ hA hZ hA hZ
Data 2 2 6 7 5 7 3 3
MC 2.6 3.0 7.3 8.4 7.2 7.6 3.2 3.4
e+e−→qq¯ 0.3 0.5 0.8 1.4 0.9 0.8 0.4 0.3
e+e−→W+W− 1.8 1.8 5.0 5.0 4.6 4.7 2.0 2.1
e+e−→ZZ 0.5 0.6 1.4 1.7 1.6 1.8 0.7 0.8
e+e−→Ze+e− 0.0 0.1 0.1 0.2 0.1 0.3 0.0 0.1
ε(hA→b¯bτ+τ−) 36% 36% 36% 36% 33% 34% 33% 34%
ε(hZ→b¯bτ+τ−) 21% 29% 21% 29% 17% 29% 17% 29%
ε(hZ→τ+τ−qq)¯ 20% 31% 20% 31% 20% 29% 20% 29%
hA and hZ an analysis is optimised based either on the tau identification or on the event topology by re- quiring four jets with two of them being narrow and of low multiplicity. The main background results from W-pair decays containing taus.
The hZ analysis is similar to the one described in detail in Ref. [18]. The hA selection is optimised for lower Higgs masses by omitting the cuts on the opening angles of the jets and tau pairs and on the invariant mass of the tau pair,mτ τ. The invariant mass of the hadronic jets,mqq, must be between 5 GeV and 125 GeV. The ratio of the sum of the energies of the tau decay products over the sum of the energies of the jets is required to be less than one and the value of the missing momentum vector in the rest frame of the Higgs must be less than 40 GeV. Finally, the cosine of the polar angle of the Higgs boson, |cosΘ|, has to be less than 0.8. The number of events observed, the number expected from background processes, and the signal efficiency for the two selections are listed in Table 2.
For each event class j (ZZ, W+W−, qq, Ze¯ +e−, hA, hZ), a probability function, fji, is constructed, wherei denotes the variables considered. These are theBTagfor each hadronic jet,mqqandmτ τ. They are presented in Fig. 4. The probability,pji, of an event to belong to classj, based on the value of the variablei,
is then defined as
(4) pij= fji
kfki.
Finally, the probabilities for the individual variables are combined by calculating the likelihood that the event belongs to the either signal class:
(5) FhA=
ipihA
k
ipik and FhZ=
ipihZ
k
ipki.
Events retained by both the hA and the hZ selec- tions, are classified according to highest value of the likelihood, as hA or hZ candidates. An example of the distribution of the discriminating variable for the hA search is shown in Fig. 5. Good agreement between the observed data and the expected background is found.
The mass distribution for the hA search after an addi- tional cut on theBTagis shown in Fig. 3(b).
4. Results and interpretation
A good agreement between data and expected background, both in the total number of events and in the shape of the distributions, is observed in all the channels analysed. The mass distributions of the events in the highest sensitivity region are not compatible with a signal for any mass hypothesis.
Fig. 4. The distributions for the hA→b¯bτ+τ− search of (a) the BTag for the most energetic hadronic jet and (b) the least energetic hadronic jet, (c) the reconstructed mass for the hadronic system, and (d) the reconstructed mass for the leptonic system. The points are the √
s=192–202 GeV data, the open histograms the expected Standard Model background and the hatched histograms the expected hA→b¯bτ+τ−signal formh=mA=85 GeV at tanβ=30.
Therefore, no evidence of the production of the h and A bosons is found.
The results of the search for the hA and the hZ pro- duction are interpreted in the framework of the con- strained MSSM (CMSSM) assuming unification of the scalar fermion masses, unification of the gaugino masses and unification of the trilinear scalar couplings at the GUT scale. This choice has little impact on the phenomenology of the Higgs bosons but reduces sig-
nificantly the number of free parameters. The remain- ing free parameters are tanβ,mA, the gaugino mass parameter,M2, the universal scalar fermion mass,m0, the common scalar quark trilinear coupling,A, and the Higgs mixing parameter,µ.
Two benchmark scenarios [19] are considered. In the first one, termed “maximal mixing”, the CMSSM parameters are chosen such that mh acquires its maximal value for any given value ofmA and tanβ.
Fig. 5. Distribution of the discriminating variable of the hA→b¯bτ+τ−selection in the hypothesismh=mA=85 GeV at tanβ=30. The points are the data collected at√
s=192–202 GeV, the open histogram is the expected Standard Model background and the hatched histogram is the expected signal for the hA search multiplied by a factor 10.
The second scenario corresponds to vanishing mixing in the scalar top sector and is referred to as “minimal mixing”.
The CMSSM parameters are chosen as follows:
m0=1 TeV, µ= −200 GeV, M2=200 GeV. The mass of the top quark is fixed to 175 GeV. The maximal mixing scenario is realised at Xt =A− µcotβ=√
6 TeV, whereXt is the parameter which controls the mixing in the scalar top sector. The minimal mixing corresponds to Xt =0. Keeping these values fixed, a scan over the two remaining independent parameters, tanβ andmA, is performed in each mixing scheme in the ranges: 0.5tanβ30 and 10 GeVmA1 TeV.
To set exclusion limits on the CMSSM parameters, the confidence level, CL, that the expected signal is absent in the data, is calculated [20] for each point (tanβ, mA) of the scan. The full distributions of the discriminating variables, NN and F, are used in this calculation.
Systematic and statistical uncertainties on the sig- nal and background are evaluated using the same pro- cedure as in the Standard Model Higgs search [13].
The main sources of systematic uncertainties are de- tector resolution, selection procedures, theoretical un- certainties and Monte Carlo statistics. The overall sys- tematic uncertainties are estimated to be 4% on the predictions for the expected signal and 10% for the
background events. Bins of the final variables with a signal-over-background ratio in the Monte Carlo of less than 0.05 are not considered in the calculation of the CL. This cut is chosen to minimise the ef- fect of systematic uncertainties on the average CL as calculated from a large set of Monte Carlo experi- ments.
The results of the MSSM Higgs search at lower
√s [2] are combined with those presented in this paper. Fig. 6 shows the region of the(tanβ, mh)plane and(tanβ, mA)plane excluded by L3 for the maximal mixing and minimal mixing scenarios. The regions not allowed by theory and those excluded from an analysis of Z-peak data [21] are also presented.
For the CMSSM parameters considered and assum- ing tanβ greater than 0.8, this results in lower mass limits at the 95% CL of:
mh>83.4 GeV, mA>83.8 GeV,
which compare to the median expected limits in the absence of a signal of mh >85.6 GeV and mA >
85.7 GeV.
The exclusion plots for the minimal mixing scenario present a small unexcluded area in the low tanβregion at low values of mA where the decay h→ AA is allowed but is not investigated among the signatures described above.
Fig. 6. Exclusion plots in the(tanβ, mh)and(tanβ, mA)planes at the 95% CL for the minimal and maximal mixing scenarios. The hatched area represents the exclusion and the crossed area is not allowed by the theory. The horizontal hatched area corresponds tomA<10 GeV and was previously excluded at LEP [21].
For 0.8<tanβ <1.8 values ofmAup to 1 TeV are ruled out for any mixing scenario allowing to exclude this tanβ region in the CMSSM, for the top mass 175 GeV.
Acknowledgements
We acknowledge the efforts of the engineers and technicians who have participated in the construction
and maintenance of L3 and express our gratitude to the CERN accelerator divisions for the superb performance of LEP.
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