Z pole results and (non-SUSY) exotics at LEP

ELSEVIER

Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 57-66

PROCEEDINGS SUPPLEMENTS

Z Pole Results and (non-SUSY) Exotics at LEP L. Moneta a a C E R N - P P E Division 1211 Geneve 23 Results from the LEP experiments at the Z pole are presented. The very precise measurements allows to perform a very stringent test of the Standard Model, proving the existence of electroweak radiative corrections. All measurements are consistent with the Standard Model predictions. Searches for new exotic physics in the high energy run of LEP are presented, in particular in the framework of observing deviations from the Standard Model expectations. The 4-jet events observed by ALEPH are the only puzzle.

1. I n t r o d u c t i o n Between 1989 and 1995, the four LEP experiments, ALEPH, DELPHI, L3 and OPAL have collected an impressive amount of data around the Z peak, in total about 15 million Z hadronic decays and 1.6 million leptonic decays. The LEP1 statistics collected by each experiment is shown in table 1.

I

I

qq

e+g-

II A I ) l

L I 0 ILEPI

'90-'91 451 357 416 454 1678 '92 680 697 678 733 2788 '93 prel. 6 4 0 677 646 646 2609 '94 prel. 1654 1241 1307 1524 5726 '95 prel. 739 584 311 1634 total i4164 3556 3358 3357 14435 '90-'91 55 36 40 58 189 '92 82 70 58 88 298 '93 prel. 78 74 64 82 298 '94 prel. 190 129 127 184 630 '95 prel. 80 67 28 42 217 total 485 376 317 454 1632

Table 1

The LEP statistics in units of 10 3 events used for the analysis of the Z lineshape and lepton forward-backward asymmetries.

This large amount of data allows a very precise test of the electroweak interactions at the Z pole. 0920-5632/98/$19.00 © 1998 Elsevier Science B.V. All fights reserved. PII S0920-5632(97)00643-9

From 1995 the beam energy of L E P has been increased above the Z pole reaching first a center of mass energy of about 133 GeV and in 1996 reaching initially the W W production threshold and afterwards 172 GeV. Each experiment collected about 5 pb -1 at v/S ~-. 133 GeV, 10 pb -1 at v ~ = 161 GeV and 10 pb -1 at vG = 172 GeV. A new window for discoveries is opened and searches for new exotic phenomena are performed. This paper is divided in two parts, in the first one the measurements performed at the Z pole are described with particular emphasis on the new Rb measurements which are now in closer agreement with the Standard Model prediction. The interpretation of all the electroweak measurements within the Standard Model is also presented. In the second part searches for new particles, outside the supersymmetric (SUSY) framework, are described. The intriguing four-jet events seen by ALEPH [1] are presented in combination with the searches for such topologies performed by the other LEP experiments.

2. Z l i n e s h a p e m e a s u r e m e n t s The large number of data collected by the LEP experiments allows to measure with great precision the Z lineshape parameters. The experiments measure around the Z pole the e+e cross section for various final states and forwardbackward and polarization asymmetries. Around the resonance, the Z s-channel dominates the cross section which can be written for a generic

L. Moneta/Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 57-66

58

fermion anti-fermion final state:

sFeF!

12r

40

af = M } ( s _ M}) 2 + s 2 ~ "

L3

M~

[] 1990-92

e+e" --* hadrons 30

FI is the Z partial width in fermion-antifermion pairs and can be expressed in term of the vector and axial coupling, gy and gA as: GFMz 3 , 2 -

tgvs

+ g s)

where GF is the Fermi constant. The cross section is measured for the hadronic final state (q@) and for the three different charged leptons (e, #, T). The experiments also measure the forwardbackward asymmetry for the leptonic final states, which at the pole can be written as:

APOle

FB =

~AeAt

A

•

i',~

•

1994

i

i

20

lO

1.01

0.9!

, I .

88

,

,

I

.

,

.

I

90 92 "/s [GeV]

I

I

i

94

At = ~ g_VtgAt

- ,.~2 ..t_ ,,~2

From the measurements of the Z partial width and the asymmetries it is therefore possible to derive the Z coupling to the fermions gy and gA. The data are collected mainly with a center of mass energy equal to the Z mass peak, but also in order to determine the Z width, some fraction of the data is collected at about 2 GeV below and above the Z peak, as shown in Fig. 1. In order to measure precisely the mass and width of the Z the energy of the beam has to be known with a very good accuracy. Using a beam depolarization technique, the beam energy can be measured to about 0.2 MeV in the the calibration run [2]. However, a much larger uncertainty comes from the extrapolation from the calibration runs to all the physical runs. In this extrapolation various factors have to be taken into account such as temperature dependence or spurious currents caused by trains passing in the Geneva area. At the end the contribution of the beam energy to the Z mass is 1.5 MeV and 1.7 Mev to the Z width [3]. From a combined fit to the cross sections and the asymmetries each experiment extracts 9 parameters: • the mass and the with of the Z;

Figure 1. L3 hadronic cross section as a function

of v/s (top) and comparison with the lineshape fit (bottom).

• the hadronic pole cross section, a°; • the ratios between the Z hadronic width and the leptonic partial widths, Rt - ['had/rt, for the three leptons; • the lepton pole asymmetries. The average results are shown in Table 2. A more detailed list of results can be found in [4]. The beam uncertainty dominates the m z measurement. However, this is, a very precise measurement, at the 10 -5 level. The mass of the Z together, the Fermi constant, GF, and the electromagnetic coupling, a, are used as input parameters to calculate Standard Model predictions for physical observables which can be tested with the data. The accuracy of e F is a factor of 10 better than m z [5], but c~ at the Z pole is known only at the 7 x 10 -4 level [6].

L. Moneta/Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 57-66

Parameter

Average Value

mz(GeV)

91.18635=0.0019 2.49475=0.0026 41.489+0.055 20.7565=0.057 20.7955=0.039 20.8315=0.054 0.01614-0.0025 0.0165±0.0014 0.02045=0.0018

rz(GeV) a°(nb) Re R, R~ O,e

AFB O,Iz AFB Off" AFB

With Lepton Universality: Re 20.7835=0.029 0,g

AFB

59

In particular for the charm, given its intermediate property, the isolation is difficult, and one has to rely on the reconstruction of charm hadrons. The sensitivity to sin 2 Ow is greater for the heavy quark forward-backward asymmetry, in particular for the b, given the high value of .Ab, as shown in table 3.

fermion type ]

0.01775=0.0010

gv gA Af 5 A f / 5 sin '~ Ow

g -0.04 -1/2 0.16 -7.9

u 1/2 1/2 1 0

d -0.35 -1/2 0.94 -0.6

u 0.19 1/2 0.67 -3.5

Table 2

Table 3

Average line shape and asymmetry parameters from the four LEP experiments.

Values of neutral current couplings and sensitivity to sin 2 Ow for the four types of fermions.

The other lineshape measurements are used to test the Standard model predictions. From the measurement of the hadronic cross section and assuming from the Standard Model the ratio of the leptonic to the neutrino partial width the number of light generations of neutrino is extracted: N~ = 2.992 5= 0.011. Alternatively, assuming that N~ --- 3 it is possible to limit the invisible decay width of the Z to AFinv < 2.9 MeV at the 95% CL. It is interesting to note that the theoretical error on the luminosity of 0.11 7o dominates the measurement of a °, and thus of

N.. The other building block of the electroweak measurements consists of asymmetries. They provide a measurement of the effective mixing angle, sin 2 0w, which is defined as a function of the neutral current leptonic couplings: sin 2 Ow

= 1(14

9Y.___Lt). gAg.

At LEP one can measure forward-backward asymmetry for the leptonic channels, testing therefore the universality of the Z lepton couplings, and for the heavy quark c and b. In this latter case a tagging method is required to isolate the heavy quark final state from the light quark.

In the case of the T lepton, it is possible to measure the polarization of the final state fermion from a kinematic study of the T decays. The average polarization is "P~ = - A ~ . From a measurement of the polarization as a function of the polar angle it is possible to derive not only A~ but also Ae: 2cosO

Pr (cosO) ~_

l + 2cosO ~

A

eA r

"

Further measurements are possible when the Iongitudinal polarization of the beam is available as in the case of the SLD experiment. At SLD one can measure the left-right polarization asymmetry, A L R -- O'L -- tTR ~ ,rite a L --}-GR

and a forward-backward polarized asymmetry, dPOl FB = ¼A]. The left-right asymmetry from SLD is very sensitive to sin 2 0w allowing SLD to have a determination of sin 2 8w with comparable accuracy with LEP even with a much smaller statistics [7]. The compilation of sin 2 0w measurements is shown in Table 4. Fig. 2 shows the comparison with the Standard Model prediction.

60

L. Moneta/Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 57-66

It is interesting to note a difference in the value of sin 2 0w of 2.8 a between the SLD ALR result and the LEP average.

PRELIMINARY 0.233

LEP/SLC/CDFIDOMarch 1997

z(mr=)=ll128.1B

m

SM mt=175.6 + 5.5 60
0.2325

Measurements

sin 2 0w 0.232

0.23068 0.23240 0.23264 0.23235 0.23155 0.2322 0.23192

A~ A~ AO,b

FB

Ao,c

FB < QFB > LEP average x2/d.o.f. = 7.3/5

::k 0.00055 ± 0.00085 ::k 0.00096 ± 0.00040 ± 0.00111 ± 0.0010 =t: 0.00027

sin20~pt 0.2315

"I 0.231

i - -

68~,

- -

95% C.L.

0.23058;.4 . . . 83.6 . . . . . 83.8 . . . . . .84. . . . 84.2 .

ALR (SLD)

0.23055 ± 0.00041

World average x2/d.o.f. = 15.1/6

0.23151 ± 0.00022

84.4

Fle~ (MeV)

Table 4

Comparison of several determinations of sin 2 8w from asymmetries measured at LEP and at SLD.

3. M e a s u r e m e n t o f

Figure 2. Average L E P / S L D estimate of sin 2 0w

versus Ft and Standard Model prediction as a function of mt and mH. The arrows shows the variation when a(m2z) is changed by one standard deviation.

Rb

Rb is defined as the ratio of the Z --+ bb partial width to the total hadronic width. It deserves a particular interest because provides a very powerful test of the Standard model, since the radiative corrections which depend on a, as and MH cancel in the ratio. The only remaining correction is the Z -~ bb vertex correction which depends on mr. Since mt is measured at the Tevatron [8], Rb gives a powerful test of the Standard Model and is sensitive to all the new physics which affects the Z --~ bb vertex, such as supersymmetry. In particular a great interest in Rb was triggered by the 1995 result which shows a discrepancy with the Standard model at the 3 a level [9]. However, the systematic errors dominate the measurements and they cannot treated as gaussians errors. A precise measurement of Rb is very difficult because it requires a tag, which

isolates the bb events from the hadronic sample. The b tags exploit the different physical properties of the b with respect to the udsc quarks, such as a longer lifetime and an higher mass. At the Z pole these translate in the presence of tracks which miss the primary vertex (lifetime tag), different event shape (mass and event shape tags) and presence of leptons with high transverse momentum (lepton tag). The lifetime tag is by far the most powerful. They can be built using the information of the track impact parameters or the decay length of reconstructed secondary vertices. ALEPH and D E L P H I employ the first method while OPAL and SLD choose the second one. In order to be independent of the large uncertainty in the knowledge of the production and decay of

61

L. Moneta/Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 57-66

the heavy quark, the analyses are designed to be self-calibrated with the data. This is achieved with the double tag method, using the fact that the b quarks are produced in pairs. After dividing the event in two hemispheres, one can count the events which have a single hemisphere tagged or both hemispheres tagged. This gives two equations which relate the fraction of single tagged hemispheres, ~-~, and of double tagged events, .Wd, to Rb and the b tag efficiency eb:

•~ s

:

£bRb +

ecRc +

£udsRuds;

Solving the two equations, Rb and eb can be determined. The other parameters are the efficiency to tag c, e~ and uds quarks, Cuds, and the efficiency correlation, p. Here come the difficulties because these quantities have to be estimated with the MOnte Carlo simulation and the corresponding uncertainties lead to a systematic error in the Rb measurement. For the lifetime tag, the uncertainties in the background efficiency arise mainly from the simulation of the c production and decay, from the fraction of gluon splitting to heavy quark in non-b events and from the knowledge of the detector resolution. The correlation estimate is more problematic and depends on quantities that affect the tag efficiency and are correlated between the two hemispheres. Possible sources in the case of the lifetime tag are the polar angle acceptance, hard gluon emission and the common reconstructed primary vertex. In order to profit of the high LEP1 statistics, the analyses have been greatly improved since 1995 [4,10]. The major improvements are: • The introduction of a new tag by ALEPH and SLD, which combine the lifetime with the mass information in order to reduce the charm contamination. ALEPH has achieved a reduction of e~ by a factor of 3 for the same eb. Hence this reduces the uncertainty in Rb due to the c physics. • The use of separate primary vertices for each hemisphere to reduce the uncertainty in the hemisphere correlation (ALEPH and

DELPHI). The correlation related to a common primary vertex is large and is difficult to estimate reliably from the simulation. This reduces the uncertainty in Rb due to the b fragmentation and the mean b decay multiplicity to a negligible level. The introduction of multitags analyses. OPAL combines a lifetime tag based on the decay length with the high Pt leptons. ALEPH uses a new method based on 5 exclusive tags based on lifetime, event shape and lepton information. Using more information one achieves not only a reduction of the statistical error of the measurement, but designing appropriately the cuts, also a further reduction of the systematic error associated with the background contamination.

F b ~ h ed

ALEPH m ~ t 1¢92.95 DELPHI mult

0.2159 ± 0.0009 ± 0.~11 -t--

0.2205 ± 0.0014 + 0.0018 0.2180 ± 0.0028 ± 0.0033

1.,3 impact pl~'~ l.~ shaper

-A

OPAL muir

0.2223 ± 0.0030 ± 0.0064 0.2178 ± 0.0014 ± 0.0017

SLD vtx mass 1~3-95

-'-'4

0.2152 ± 0 . ~

LEP l e p ~ s

±0.0016

0.2217 ± 0.0023 ± 0.0020

LEP~LC

0.2177 ± 0.0011 0.0~03 Texchange corr. added

0.21

0.22

FJFb~ for UJFh~ = 0.1"~

Figure 3. L E P / S L D measurements of Rb and the Standard Model prediction as a function of mr.

L. Moneta/Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 57-66

62

In Fig. 3 the measurements are shown. The ALEPH multi-tags measurement [11] is the most precise and dominates the world average. It relies heavily on the lifetime-mass tag and, given the high correlation with the single tag measurement, it is the official ALEPH number. The new average of Rb is Rb = 0.2177 • 0.0011, a value -~ 2a higher than the Standard Model prediction of 0.2158 ± 0.0002 (for m~ = 175.6 :t=5.5 GeV), but it is interesting to note that the new measurements of ALEPH, OPAL and SLD are in perfect agreement with the Standard Model. The dominant common systematic error is due to the fraction of g ~ bb events, which is not measured and the uncertainty is assumed from theory calculations to be +30%. Therefore, a further improvement of the measurement will require this fraction to be measured.

0.19

'

i

,

measured at lower energy. The new world average [4] is Rc = 0.1722 ± 0.0053 and is in perfect agreement with the Standad Model prediction of 0.172. Fig. 4 shows the new results of Rb and Re. 4. S t a n d a r d M o d e l fit t o t h e d a t a A global fit to all the electroweak measurements is performed, including not only the Z pole results, but also the new measurements of the W mass at LEP2 (row = 80.38 ± O.14GeV [4]) and the results from the hadron collider and the neutrino experiments. The only free parameters of the fit are the Higgs mass and as. The fit pulls for the various measurements are shown in Fig. 5 and the result of the fit is given in Table 5 [4].

mt [GeV] mH [GeV]

i

99% CL

R©

x2/d.o.f.

LEP (including LEP-II m w )

all data

155_+

172.7 ~ 5.4 -{-127 127_72 0.120 ± 0.OO3

70+147 -40

0.121 ± 0.003 10/9

21/15

0.18

Table 5 Results of the fits to L E P data alone and to MI data including the direct determinations of mt and m w (Tevatron and LEP-II).

EM 0.17

0.16

0.214

0.216

0.218

0.22

0.222 Rb

Figure 4. Probability contours of Rb versus R~ and Standard Model prediction.

New measurements of Rc have been performed by the LEP experiments with improved techniques which rely less on charm physics quantities

The data are in good agreement with the Standard Model apart from a small discrepancy in Rb and in the different measurements of sin 2 8w at LEP from the b quark forward-backward asymmetry and the SLD polarization asymmetry. Assuming that the SLD measurement is right and gives a measurement of A~, one observes a discrepancy of 3 a between Ab and the Standard Model prediction. Hower, some discrepancies are expected from so many measurements, the fit X2 is reasonable (Table 5). The data proves at the 3 a level the presence of the electroweak radiative corrections, which depend on mt and log mH , as shown in Fig. 6.

L. Moneta/Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 57-66

63

PRELIMINARY Measurement

Pull

Pull -p -2 -3

m z [GeV]

91.1863_+ 0 . 0 0 1 9

.06

Fz [GeV]

2 . 4 9 4 7 _+ 0 . 0 0 2 6

-.75

~hadr0[nb]

41.489_+ 0.055

.46

R~

20.783 _+0.029

A°t;~

0.0177 _+0.0010

1.57

As

0.1401 _+0.0067

-.98

Ae

0.1382 + 0.0076

-1.11

0.2322 _+0.0010

.63

• 2 lept

sin Oe~

m w [GeV]

80.38 _+0.14

1.75

Rc A°~;b

0.1722 + 0.0053 0.0985 + 0.0022

-.01 -1.96

A~t;c

0.0735 + 0.0048

.01

Ab

0.897 + 0.047

-.80

1 --

0.623 _+0.085

-.53

0.23055 + 0.00041

-2.48

0.2244_

0.0042

~

3

m

0.008

1•1•.006

i

O

% 0.004

m

0.002

0

0.005

0,01

Ap=e 1

n J

.28

m w [GeV]

80.37 + 0.10

.04

m t [GeV]

175.6 + 5.5

.52

128.894 + 0,090

-.16

1/o~

1

LEP/SLC-JCOFf0Q/vN March 1997

~M.)=1/128.B9

.10

0.2177 _+0.0011

sin20~pt

,

.80

Rb

Ac

0.01

Figure 6. Radiative correction parameters (~3 vs el [12]) extracted from the data. -3-2-1o

1 2 3

Figure 5. Input values to the global Standard

Model fit and resulting pulls.

The complete data fit yields log(mH) = ln+°'3° . ~ v _ 0 . 3 6 and gives an upper limit of m ~ < 465 GeV at 95% CL. The X2 of the fit as a function of m~/is shown in Fig. 7. In conclusion the electroweak measurements provide a very strong test of the Standard Model at the 10 -3 level. The data start to give an indirect measurement of the Higgs mass. New results are expected for the asymmetries measurements, in particular from SLD and on the W mass from LEP2. In order to improve the indirect determination of the Higgs mass, a reduction of the uncertainty of the electromagnetic constant at the Z pole, a(m2z), becomes necessary.

5. N o n - S U S Y searches at L E P 2 From November 1995 LEP has increased the center of mass energy above the Z resonance. This has triggered direct searches for new particles. I will concentrate on two phenomena outside the supersymmetric framework: new physics contributing to the hadronic cross section, such as contact interaction or leptoquark exchange in the t-channel and the A L E P H 4-jet events. I apologize for omitting many exotic searches performed at LEP, such as searches for heavy or exited leptons. 5.1. C o n t a c t i n t e r a c t i o n s The interest in the contact interactions has been triggered by the reported excess of events with high x and q2 at HERA. At LEP one can study contact interaction observing deviations in the measured hadronic cross section from the Standard Model prediction. In a general frame-

L. Moneta/Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 57-66

64

6

ij LL RR LR RL

4'

ij

2

0 10

10

2

10 3

LL RR LR RL

mR [GeV]

ij Figure 7. Fit X 2 as a function of mH. The shading along the curve is the theoretical uncertainty in the fit.

work [13] the contact interactions are described by the Lagrangian:

LL RR LR RL

coupling to one d-quark (~ = +1) (7/= -1) h(WeV) A(TeV) 2.4 1.0 2.1 1.2 1.7 1.4 1.6 1.5 coupling to one u-quark = +1) (~/= -1)

A(WeV)

h(WeV)

1.1 1.4 1.5 1.7

2.4 1.7 1.6 1.4

coupling to all flavours (~ = +1) (~ = -1) A(TeV) A(TeV) 2.8 2.1 2.3 2.5 2.4 2.4 2.0 2.9

Table 6 Contact interactions limit for various case of contact interactions. It is assumed that only one helicity yij dominates.

g i,j=L,R

where A is the unknown energy scale. The coupling is absorbed in A, assuming that g 2 / 4 r = 1. The ~ij are chiral coefficients which depend on the fermion helicity. If the interaction is chirally invariant, ~?ij -- -4-1. A deviation arises from the contribution of the pure contact term, which scales as 1/A 4 and the interference between the contact interaction and the Standard model which scales as 1/A 2. OPAL has produced a set of limits assuming that only one helicity term dominates the interaction and in the case that the contact interaction couples only to one quark flavour or to all flavours (Table 6) [14]. 5.2. L e p t o q u a r k A possible deviation from the Standard Model cross section arises from the exchange of a Lep-

toquark in the t channel. This is equivalent to a contact interaction in the limit that the leptoquark mass is much greater that the center of mass energy. Various limits are set by OPAL in the case the Leptoquark couples to all u-type or dtype quarks, using all the hadronic events, or for a Leptoquark coupling only to the b quark or only to the uds quarks, selecting either b events or uds with a lifetime tag [14]. These limits are reported in Fig. 8. In the HERA region, (mLQ ~ 200 GeV), the limits are between A < 0.24 (coupling only to b) and A < 0.4 (coupling to all d-type quarks) at 95% C.L. 6. A L E P H

4ojet e v e n t s

An ALEPH analysis designed to search for the reaction e+e - ~ h A ~ j j j j , shows in the data

L. Moneta/Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 57-66

.~ 0.7 0.6

OALP ,

t

I

I

. . . .

w-typ,~,#,g ~

. . . .

.

~

r

. J

preliminary

'

'

l

. . . .

~

~

..... " I

'

' A ~

I

,

,

0.5 0.4 0.3 0.2

65

the same as the old one, with only the addition of a cut against the additional W W background. The new cut rejects di-jet combinations with high di-jet mass sum values, compatible with the W W case. In the new data 8 additional events are seen in the peak when 3 are expected. The combination of all data is shown in Fig. 9. The number of events in the peak is 17 when 4 are expected and the mass value is 106 + 5 G e V / c 2. A striking signal, but what about the other L E P experiments?

0.1 O t ~ _ ,

,

100

,1

. . . .

150

I

. . . .

200

l

. . . .

250

I

. . . .

300

,

,

dov,l~,pe ~

,.~ 0.7

OLO

0.6 0.5

~/s=133-172 G e V

350 400 M (GeV/c:)

(b)

!

:~

0.4 ~

:

N,~p=90.2 Nd,w=93

0.3 0.2 0.1 0 100

H1

allflavors ........ b-m8

150

2041

250

300

ZM for minimum ~M

350 400 M (Gev/c~)

ALEPH

tt

N a --30.3

NdP=37

Figure 8. OPAL limits on couplings of the Lepto-

quark to quarks of u-type (top) and d-type (bottom).

collected at 130 GeV an excess of events containing four jets compared to the expectations from Standard Model processes [1]. However, these events do not appear to have any b enrichment, as expected in the case of hA production. The enhancement appears in the sum of the di-jet mass around 105 GeV, where 9 events are seen when only 1 is expected. In the previous conference the excess observed in the data collected at around 130 GeV was reported [15]. Here, I will therefore concentrate on the new data collected in 1996 at 161 and 172 GeV and on the observations of the other LEP experiments. The selection performed at 161 and 172 GeV is

20

411 611 M IW 120 140 IN IIIo 2tl0 T_,M for minimum 5 M

Figure 9. Sum of the di-jet mass for 4-jet events selected in all high energy data taking the combination with minimum mass difference for DEL-

PHI, L3 and OPAL combined (top) and ALEPH (bottom).

DELPHI, L3 and OPAL have performed similar analyses to look for 4-jet events, but they all find a number of events, in the mass window defined by ALEPH, compatible with the expectations from standard processes (Fig. 9 and Ta-

66

L. Moneta/Nuclear Physics B (Proc. Suppl.) 62A-C (1998) 57-66

133 161 172

9 (1.0) 5 (1.2) 3 (1.8)

4 (1.0) 2 (1.8) 1 (2.4)

2 (1.1) 1 (1.2) 2 (1.8)

1 (0.8) 0 (1.0) 0 (1.8)

Table 7 Number of events observed by the four L E P experiments in the peak region of ~ M .~ 106 4- 5 GeV. In brackets are the number expected from standard processes.

ble 7). In order to investigate further this effect a common LEP working group of the four experiments has been formed. Studies with simulated events and with radiative qqg' events in the data show that the experiments have comparable resolution and sensitivities to the 4-jet signal. Also each single ALEPH 4-jet event is passed to the experiment simulation and reconstruction and the efficiency and the di-jet mass sum resolution result similar. The conclusion of the group is that all the four experiments are able to see the 4-jet events. In the case we assume there is a signal, the discrepancy between the experiments is quantified with a probability of 7 x 10 -4 according to Poisson statistics. On the contrary, the probability that the background in one of the four experiment has fluctuated up is 8 x 10 -4. It is interesting that the probabilities that the effect is due to a signal or to a background fluctuations are comparable. Hence, the effect remains a mystery. We hope that taking more data in 1997 the puzzle will be solved. Otherwise, in the ultimated case, a run at an energy of about 130 GeV, repeating exactly the same experiments, could be considered. Acknowledgements

I would like to thank the organising committe for their invitation and for hosting and well organising a so interisting conference. My spe-

cial thanks are for the LEP Electroweak Working group for providing me a big part of the material and also to A. Blondel, F. Cerutti, P. Giacomelli and P. Janot. I also give my apologies to the authors of many interesting paper on exotic searches which for time and space limitation I had to skip. REFERENCES

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