- Email: [email protected]
Search for a narrow resonance in e+e− collisions between ECM = 58 and 60 GeV
Physics Letters B 304 (1993) 373-380 North-Holland
PHYSICS LETTERS B
Search for a n a r r o w r e s o n a n c e in e + e - c o l l i s i o n s b e t w e e n ECM = 58 a n d 60 G e V TOPAZ Collaboration K. Abe a, I. Adachi b, S. Awa c, M. Aoki d, R. Belusevic b, K. Emi a, R. Enomoto b, H. Fujii b, K. Fujii b, T. Fujii e, J. Fujimoto b, K. Fujita f, N. Fujiwara c, H. Hayashii c, B. Howell g, N. Iida c, H. Ikeda b, R. Itoh b, H. Iwasaki b, M. Iwasaki c, R. Kajikawa d, S. Kato h, S. Kawabata b, H. Kichimi b, T. Kishida e, M. Kobayashi h, D. Koltick g, I. Levine g, K. Miyabayashi d, A. Miyamoto b, K. Muramatsu c, K. Nagai i, T. Nagira c, K. Nakabayashi d, E. Nakano d, O. Nitoh a, S. Noguchi c, F. Ochiai j, Y. Ohnishi d, H. Okuno h, T. Okusawa f, K. Shimozawa d, T. Shinohara a, A. Sugiyama d, N. Sugiyama k, S. Suzuki d, K. Takahashi a, T. Takahashi f, M. Takemoto c, T. Tanimori k, T. Tauchi h, F. Teramae d, y. Teramoto f, N. Toomi c, T. Toyama d, T. Tsukamoto b, S. Uno b, y. Watanabe k, A. Yamaguchi c, A. Yamamoto b, S. Yamamoto e and M. Yamauchi b a Department of Applied Physics, Tokyo University of Agriculture and Technology, Tokyo 184, Japan b KEK, National Laboratory for High Energy Physics, Ibaraki 305, Japan c Department of Physics, Nara Women's University, Nara 630, Japan d Department of Physics, Nagoya University, Nagoya 464, Japan e Department of Physics, University of Tokyo, Tokyo 113, Japan f Department of Physics, Osaka City University, Osaka 558, Japan g Department of Physics, Purdue University, West Lafayette, IN 47907, USA h Institute for Nuclear Study, University of Tokyo, Tokyo 158, Japan i The Graduate School of Science and Technology, Kobe University, Kobe 657, Japan J Faculty of Liberal Arts, Tezukayama Gakuin University, Nara 631, Japan k Department of Physics, Tokyo Institute of Technology, Tokyo 152, Japan Received 1 March 1993
We carded out the energy scan between ECM = 58 and 60 GeV at the TRISTAN e + e - collider to search for the possible narrow resonance suggested by the L3 experiment at LEP. The total cross sections are measured for ~,7, multihadron, e + e - and /~+/z- productions at ten energy points coveting this energy range almost uniformly. The results are in good agreement with the Standard Model predictions, and 95% confidence level upper limits are set to Fee x BR of the hypothetical scalar and tensor resonances.
1. Introduction It was r e p o r t e d recently that the L3 e x p e r i m e n t at L E P o b s e r v e d a cluster o f 7Y i n v a r i a n t mass a r o u n d 59 G e V in the ~ + ~ - 7 7 final states o f Z ° decays, w h i c h is not e x p e c t e d f r o m the higher o r d e r correction in the S t a n d a r d M o d e l [ 1 ]. T h e i n t e r p r e t a t i o n o f this cluster needs m o r e statistics; however, it is possible that this cluster indicates a new n a r r o w r e s o n a n c e o f a mass a r o u n d 59 G e V . T h i s h y p o t h e t i c a l new resonance, d e n o t e d as X in this report, m a y h a v e any spin o t h e r t h a n one, since it decays to 77. A n d also, the b r a n c h i n g ratio B R ( X --, 77) should be large, since the L E P e x p e r i m e n t s r e p o r t e d no excess in o t h e r channels o f Z ° decay. I f the leptonic w i d t h o f X , Fee, is larger t h a n a few keV, there m a y be an observable e n h a n c e m e n t in e + e - --* Y7 and f f total cross sections Elsevier Science Publishers B.V.
3 73
PHYSICS LETTERS B
Volume 304, number 3,4
29Apdl1993
Table 1 ECM , its spread (rms) and the luminosity at each of ten energy points. The error in the integrated luminosity is a quadratic sum of the statistical and systematic errors. ECM (GeV)
ECM spread (MeV)
Integrated luminosity (pb - l )
57.37 57.77 57.97 58.22 58.47 58.72 58.97 59.22 59.47 59.84
95 96 95 98 98 99 100 100 102 88
1.018 78.1 1.249 1.326 1.319 1.760 1.343 1.132 0.902 2.97
+ 0.030 ± 2.0 a) + 0.036 -4- 0.038 -4- 0.038 + 0.049 + 0.038 ± 0.032 ± 0.027 i 0.37
a) Only 20.3-4-0.64 pb - l is used for Y7 analysis at this energy. around 59 GeV. To search for this resonance, T R I S T A N performed the energy scan between 58 and 60 GeV in the center o f mass energy. In this article, the results o f the energy scan are reported with respect to the total cross sections for yy production, multihadron production, Bhabha scattering a n d / ~ + / t - production, using the data taken by the T O P A Z detector.
2. Energy scan The results reported in this article are based on the data taken between 1988 and 1992 at ECM = 58 and 60 GeV, and the data in 1992 at the eight energies between them. The detailed description of the T O P A Z detector can be found elsewhere [2]. The energy points and the integrated luminosity at each point are summarized in table 1. The energies listed in this table are estimated by the field strength o f the bending magnets of the accelerator taking into account the energy shift due to the frequency shift of the accelerating RF. The spread o f the center of mass energy is 96 MeV at ECM = 58 GeV at T R I S T A N under the condition of this experiment, and this increases linearly as ECM. The energy step o f the scan is chosen to be approximately 250 MeV, which is determined from the beam energy spread to cover the entire energy range almost uniformly. The luminosity used in this analysis is measured only by the forward Bhabha scattering between 63 and 237 m r a d with respect to the beam axis to avoid the possible effect of the resonance in the large angle region. F o r this reason the cross sections reported in this article are slightly different from the previously published values at ECM = 58 and 60 GeV [3-5 ], where the luminosity is measured by the two calorimeters covering up to 462 mrad. The estimated error in the luminosity measurement includes typically 1.5% statistical and 2.5% c o m m o n systematic errors.
3. yy production The e + e - --. yy events are selected by requiring two back-to-back energy clusters in the barrel lead glass calorimeter, and no tracks in the inner drift chamber to exclude Bhabha events. The total energy deposit of 7's is required to be larger than 50% o f ECM, and the acollinearity angle between two ~,'s is required to be smaller than 10 °. The event selection efficiency of these requirements is estimated by Monte Carlo simulation to be 0.85 for Q E D events in the angular range [cos 0l < 0.77, where 0 is an angle of ~, with respect to the electron beam 374
Volume 304, number 3,4
PHYSICS LETTERS B
80 70 I (a) •
•
i
•
•
-
i
.
.
.
.
i
.
.
.
29 April 1993
.
i
•
e+e-->?'7
• ~
60 V
o
~"
10.0 5.0
1.00~" 0.50~
(b) ~
7 o~ 1.0 X ~ 0.5 r.
0.10 0,05
0.1
,
i
. . . .
57
i
. . . .
L
. . . .
58 59 Ecu or Mx (GeV)
i
,
60
Fig. 1. (a) Total cross section for e+e - ~ 77 in the angular range Icos0L < 0.77. The smooth curve is the QED prediction. (b) The 95% CL upper limits to Fee x BR(X --* 77) for narrow scalar (left axis) and tensor (right axis) resonances. axis. The more details o f 77 analysis are described in ref. [4]. The contamination o f Bhabha events and other sources is negligibly small. In fig. l a the total 77 cross sections in this angular range are plotted with the prediction of the lowest order QED. The cross sections are in good agreement with the Q E D prediction in this range o f the center o f mass energy. I f a narrow resonance X exists, this causes the following enhancement to the total cross section at the measurement point including the resonance mass Mx: f a r , ( W ) d W = ( 2 J + 1)~-~x " 2n22 Ee e B R ( X ~ 7 7 ) ,
(1)
where J = 0 or 2 for scalar and tensor assumptions, respectively. The acceptance for the 73' pairs from X decay is calculated from the angular coverage o f the detector and the event selection efficiency. F o r a scalar resonance, the angular distribution is uniform and the event selection efficiency is estimated to be 0.87 without assumption. On the other h a n d for the tensor case, the lagrangian for the tensor couplings to photons and fermions are assumed as follows [6]: 2.WT
gl,q[-~l,q~,u(Dv~lQ,q)
-
(Dv~lll,q)yp~tQ,q]
+ e
hFu~F;
(2)
,
l,q
where gt.q and h are the coupling constants to fermions and photons, respectively, and Fu,, is the photon field tensor. Here the couplings to left- and right-handed fermions are assumed to be identical. F r o m this lagrangian, the differential cross section for 77 production is expressed as follows: de
dO-
dO'QED dO
cz2(l
gehs2(M~-s)
+ S k 4 (mZx - S)2 + m2xl-r2
(1 + c o s / O ) +
1
g2eh2s4
64 (m2x - S)2 + m2xFr2
sinZO(l+cos20)~
(3)
]'
where s is the center o f mass energy squared and 1-'r is the total decay width of the resonance. In this expression, the terms which include the transverse b e a m polarization are not included, since they vanish in the total cross section. A n d also, the interference term can be ignored for a narrow resonance. The event selection efficiency is calculated from this angular distribution to be 0.90 for narrow tensor resonance. 375
Volume 304, number 3,4
PHYSICS LETTERS B
29 April 1993
The effect o f the initial state radiation is calculated as discussed in ref. [7], where the cross section including the initial state radiation is given only for scalar resonance. However, it can be shown that the error is less than a few percent when it is applied to tensor resonance, which is negligibly small compared to the experimental errors. Taking these and the spread o f the center o f mass energy into account, the 95% confidence level (CL) upper limits to Fee × BR (X ~ y y) are set as plotted in fig. 1b for narrow resonances. As a definition of 95% CL limit, we adopt the method given by the Particle Data Group [8]. The upper limits shown in fig. l b are applicable only to a narrow resonance of Fr << aecM, where trECM is the spread o f the center o f mass energy (_~ 100 MeV). F o r broader resonance the upper limit is similarly obtained by comparing the expected resonance shape (3) to the observed cross section. We obtained the following upper limits (95% CL) for resonances o f F r = 1 GeV in the mass range 58 < M x < 60 GeV: Fee × B R ( X ~ yy) < 8.9 keV < 0.05 keV
(for scalar resonance),
(4)
(for tensor resonance).
(5)
Here the limit to the tensor resonance is much smaller than the one for the scalar because of the large contribution o f the interference term in eq. (3), which is proportional to ~ .
4. Hadron production The multihadron events are selected by requiring the following conditions: (1) more than or equal to five charged tracks with Pr > 0.15 G e V / c with respect to the beam axis are found in I cos01 < 0.866, (2) the total visible energy is greater than 50% of EcM, (3) the m o m e n t u m imbalance along the beam axis is smaller than 0.4, and (4) the larger one of the jet masses is larger than 2.5 GeV. F r o m the number o f events the effective Born cross section is obtained using the radiative correction factor calculated with K O R A L Z program [9 ], where only photonic correction is included. This cross section has the same definition as the total hadronic cross section on Z ° pole reported by the experiments at LEP in ref. [ 10]. The total hadronic cross sections are plotted in fig. 2a with the prediction of the Standard Model for the effective Born cross section. In this calculation o f the theoretical prediction, the following parameters are used: sin 2 0w = 0.230, M z = 91.17 GeV and as (m 2) = 0.120. The measured cross sections are in good agreement with the Standard Model prediction. The effect o f the narrow resonance in this cross section is expected to be similar to eq. (1), where BR (X ---* YT) is replaced by B R ( X ~ hadrons). To set a limit to Fee × B R ( X ~ hadrons), the event selection efficiency for q~ decays of X is calculated by Monte Carlo program based on LUND6.3 [ 11 ] and the detailed detector simulation to be 0.86 and 0.76 for scalar and tensor assumptions, respectively. In this calculation the differential cross section for q~ production is assumed as follows for tensor X, which is also obtained from the lagrangian (2):
d~ dg2
-
dosM
g~ g~
s3
d-----~-+ ----------~ 8192~z ( M J - s ) 2 + M 2 F 2[(1 + c ° s 0 ) 2 ( 1 - 2 c ° s 0 ) 2
+ (interference t e r m ) .
+ (1-c°s0)2(1
+ 2c°s0)2] (6)
Here the interference term vanishes in the total cross section. The expected line shape o f the resonance is also calculated by the energy spread and the initial state radiation described in ref. [7]. The 95% CL limits to Fee >( B R ( X ~ hadrons) are plotted in fig. 2b for scalar and tensor assumptions. We obtained the following limits (95%CL) for the resonance of Fr = 1 GeV (58 < M x < 60 GeV): 376
Volume 304, n u m b e r 3,4
PHYSICS LETTERS B
200
.
#
-
(a)
180
29 April 1993
e+e - - > h a d r o n s
16o ~.
14o
b
120 100 :
~'~
50.0
I~
i0.0 5.0
~"
mo
1.o 0.5
:
:
. . . .
I
. . . .
I
. . . .
]
,
10.o 5.0 "~ 1.0~ 0.5 .
,
.
.
.
.
57
i
.
.
.
.
,
58
.
.
.
.
59
,
.
60
Ec~or Mx (GeV) Fig. 2. (a) Effective Born cross section for e + e - -~ hadrons. The smooth curve is the Standard Model prediction. (b) The 95% C L u p p e r limits to -Fee x B R ( X ~ h a d r o n s ) for n a r r o w scalar (left axis) a n d t e n s o r (right axis) resonances.
Fee x B R ( X ~ hadrons) < 11.6 keY < 2.6 keV
(for scalar resonance),
(7)
(for tensor resonance).
(8)
It should be noted that the multihadron final state by the gluonic decay of X is not taken into account in this analysis, and therefore, the limits given here are conservative.
5. Bhabha scattering The e + e - ---, e + e - events are selected by requiring back-to-back energy clusters in the barrel calorimeter and charged tracks in the inner drift chamber. The charges o f the tracks are not considered. The total cross section in the angular range [cos 01 < 0.768 is obtained from the number o f the events as the lowest order Q E D cross section. In fig. 3a the cross section at each energy point is plotted with the QED prediction. The result is in good agreement with QED. The effect expected from a narrow resonance is different in this reaction because o f the resonance contribution o f t-channel exchange o f X. The expected differential cross section is expressed as follows for scalar and tensor assumptions from the lagrangian (2) [6,12]: da dg-2
dO'QED
dQ
geM~ (s 2 2 + 3-N-Y M2x) 2 + M k r ? 2
+
t
--g~]
+
. s ( t - M~)2
2 a t - M J + ---ffO-~- ] J s
2
+ gl M } st(s - M2x) 80n [ ( s - M 2 ) 2 + M 2 r ] l ( t - M 2
)
(for scalar resonance),
(9)
377
Volume 304, number 3,4
PHYSICS LETTERS B
550
500
-
,
. . . .
,
. . . .
,
(a)
29 April 1993
. . . .
,
-
e+e-->e+e-
~
"¢ 400 ~b ¢9 "~
a5o 300
1.03
102
+
101
I
m i0 0 m
', ,
i
.
.
57
.
.
,: l
.
.
.
.
l
.
5B 59 Ecu or Mx (GeV)
.
.
.
l
,
60
Fig. 3. (a) Total cross section for e + e - ~ e + e - in the angular range [cos0[ < 0.768. The smooth curve is the standard model prediction. (b) The 9 5 % C L upper limit to Fee x B R ( X ~ e + e - ) for narrow scalar (leftaxis) and tensor (right axis) resonances.
da df2 +
dO'QED d.O
gEe [ gZeS[
-T-~-~(1 + x ) 2 ( 2 x -
1
)2
+ (1 - x ) Z ( 2 x + 1
)2
+ (1 + x ) 2 ( 2 x -
1)(7 +
, s - M2x~ X)t_--Z--~r )
%
+41ta
+
]
(s-M2)(l+x)2(2x-l)+4~zas-M2x(1-x)2(2x+l)s
g2e {ge2S + 5--i-~5~z/--~-( 1 \[
+ x)Z(7 + x) 2 + 4(5 + 3x) 2 ] + 2 h a ( 7 + x ) ( 1 +
+ 2a'o~[(7
+
+ X ) ( 1
X) 1 + 4 ( 5 +
3X)] -t - M - 2)
( t - S2 Mx2) 2
x)zt"
(s-M2)2+FT2M 2
i
S
(for tensor resonance).
(10)
Here x = cos 0. The upper limit to Fee is obtained by comparing the measured total cross section to these expressions integrated numerically over I cos01 < 0.768. In this calculation, the initial state radiation and the spread of the center of mass energy are considered only for the terms of s-channel exchange of X. The event acceptance for e + e - pair from the resonance decay is also taken into account using the angular distributions (9) and (10). The results are shown in fig. 3b for narrow resonances, and for broad resonances with Fr = 1 GeV, we obtain the following limits (58 < Mx < 60 GeV): Fee × B R ( X --* e+e - ) < 22.9 keV
(for scalar resonance),
(11)
< 13.2 k e y
(for tensor resonance).
(12)
6. /4+~ - production
The e+e - -~ g+/~- events are selected by requiring two charged tracks of the momenta larger than ECM/6 with an acollinearity angle smaller than I0 ° in ]cos01 < 0.75, and no large energy deposit in the calorimeters. The 378
Volume 304, number 3,4
PHYSICS LETTERS B
60 50
~,
4o
,~
30
-
i
i
.
.
.
(a)
.
i
29 April 1993
e .
.
.
.
i
•
+e- - >/~+p.-
20 b
lO
1. 0 0"~ x&0.0.15:".:.:.:.:,".:.:.:.:,".:.:.....0.10 57 58 59 60 5.00
io.o
g"
+::£
5.0
0.50
i ~ 1.0
IIl
0.05
EcM or M x (CeV)
Fig. 4. (a) Total cross section for e+e - -+ ~+/a-. The cross section has been translated to the lowest order electroweak cross section. The smooth curve is the standard model prediction. (b) The 95% CL upper limits to Fee x BR(X --+ #+/1- ) for narrow scalar (left axis) and tensor (fight axis) resonances. difference o f the two T O F times is also required to be smaller than 4 ns to exclude cosmic rays. After the event selection, the residual backgrounds are estimated to be 2% mainly from z + r - events and two photon processes. The more detail o f the/1+/~ - event analysis is given in ref. [5]. The total cross section is then calculated from the n u m b e r o f events as the lowest order electroweak cross section, using the radiative correction [ 13 ] and the acceptance calculated by the Monte Carlo program. The trigger efficiency is estimated by cosmic ray events to be 95.7%. The total cross section is plotted in fig. 4a with the Standard Model prediction, where the measurement is also in good agreement with the theory. F r o m this cross section, the upper limit to Fee x B R ( X -+/~+/1- ) is calculated in the same way as described in the previous section for the hadronic cross section. The obtained upper limits are plotted for narrow scalar and tensor resonances in fig. 4b. F o r broad resonance with Fr= 1 GeV, we obtained the following upper limits (58 < Mx < 60 GeV): Fee × B R ( X -+/~+/~- ) < 5.3 keY
(for scalar r e s o n a n c e ) ,
(13)
< 1,4 keV
(for tensor resonance).
(14)
7. Conclusion We carried out the energy scan between 58 and 60 GeV in the center o f mass energy to search for the possible narrow resonance indicated in the analyses o f e + e - 7 7 final states of Z ° decays by L3. We accumulated more than l pb-~ o f the integrated luminosity at each o f the ten energy points spaced by approximately 250 MeV, which cover this energy range almost uniformly, considering the spread o f the center o f mass energy. The total cross sections for 77, multihadron, e + e - a n d / ~ + / ~ - productions are measured at each point. They are all consistent with the predictions o f the Standard Model within the measurement errors, and the 95% CL upper limits are set to Fee × BR (X -+ y ) ' s o f the hypothetical new resonance X, where y is one o f the final states mentioned above. F o r this calculation o f the limits, X is assumed to be either scalar or tensor.
379
Volume 304, number 3,4
PHYSICS LETTERS B
29 April 1993
Acknowledgement We are indebted to the T R I S T A N accelerator division for the efficient and stable operation of the collider especially during the energy scan. We also thank K. Hagiwara, S. Matsumoto and M. Tanaka for helpful discussions and suggestions.
References [1] L3 Collab., O. Adriani et al., Phys. Lett. B 295 (1992) 337. [2] T. Kamae et al., Nucl. Instrum. Methods A 252 (1986) 423; S. Kawabata et al., Nucl. Instrum. Methods A 270 (1988) 11; J. Fujimoto et al., Nucl. Instrum. Methods A 256 (1987) 449. [3] TOPAZ Collab., I. Adachi et al., Phys. Lett. B 234 (1990) 535. [4] TOPAZ Collab., K. Shimozawa et al., Phys. Lett. B 284 (1992) 144; K. Shimozawa, Ph.D. Thesis, Nagoya University preprint DPNU-92-44 (1992). [5] TOPAZ Collab., B. Howell et al., Phys. Lett. B 291 (1992) 206. [6] K. Hagiwara, S. Matsumoto and M. Tanaka, KEK preprint 92-202 (1993). [7] VENUS Collab., S. Odaka et al., J. Phys. Soc. Japan 58 (1989) 3037. [8] Particle Data Group, K. Hikasa et al., Review of particle properties, Phys. Rev. D 45 (1992), Section 2.4.2, p. 111.39. [9] S. Jadach et al., Comput. Phys. Commun. 66 (1991) 276. [10] ALEPH Collab., D. Decamp et al., Z. Phys. C 48 (1990) 365; DELPHI Collab., P. Abreu et al., Phys. Lett. B 260 (1991) 240; L3 Collab., B. Adeva et al., Z. Phys. C 51 (1991) 179; OPAL Collab., G. Alexander et al., Z. Phys. C 52 (1991) 175. [11 ] T. Sj6strand, Comput. Phys. Commun. 39 (1986) 347; T. Sj6strand and M. Bengtsson, Comput. Phys. Commun. 43 (1987) 367. [12] W. Hollik, F. Schrempp and B. Schrempp, Phys. Lett. B 140 (1984) 424. [13] J. Fujimoto et al., Prog. Theor. Phys. Suppl. I00 (1990) 1.
380
PHYSICS LETTERS B
Search for a n a r r o w r e s o n a n c e in e + e - c o l l i s i o n s b e t w e e n ECM = 58 a n d 60 G e V TOPAZ Collaboration K. Abe a, I. Adachi b, S. Awa c, M. Aoki d, R. Belusevic b, K. Emi a, R. Enomoto b, H. Fujii b, K. Fujii b, T. Fujii e, J. Fujimoto b, K. Fujita f, N. Fujiwara c, H. Hayashii c, B. Howell g, N. Iida c, H. Ikeda b, R. Itoh b, H. Iwasaki b, M. Iwasaki c, R. Kajikawa d, S. Kato h, S. Kawabata b, H. Kichimi b, T. Kishida e, M. Kobayashi h, D. Koltick g, I. Levine g, K. Miyabayashi d, A. Miyamoto b, K. Muramatsu c, K. Nagai i, T. Nagira c, K. Nakabayashi d, E. Nakano d, O. Nitoh a, S. Noguchi c, F. Ochiai j, Y. Ohnishi d, H. Okuno h, T. Okusawa f, K. Shimozawa d, T. Shinohara a, A. Sugiyama d, N. Sugiyama k, S. Suzuki d, K. Takahashi a, T. Takahashi f, M. Takemoto c, T. Tanimori k, T. Tauchi h, F. Teramae d, y. Teramoto f, N. Toomi c, T. Toyama d, T. Tsukamoto b, S. Uno b, y. Watanabe k, A. Yamaguchi c, A. Yamamoto b, S. Yamamoto e and M. Yamauchi b a Department of Applied Physics, Tokyo University of Agriculture and Technology, Tokyo 184, Japan b KEK, National Laboratory for High Energy Physics, Ibaraki 305, Japan c Department of Physics, Nara Women's University, Nara 630, Japan d Department of Physics, Nagoya University, Nagoya 464, Japan e Department of Physics, University of Tokyo, Tokyo 113, Japan f Department of Physics, Osaka City University, Osaka 558, Japan g Department of Physics, Purdue University, West Lafayette, IN 47907, USA h Institute for Nuclear Study, University of Tokyo, Tokyo 158, Japan i The Graduate School of Science and Technology, Kobe University, Kobe 657, Japan J Faculty of Liberal Arts, Tezukayama Gakuin University, Nara 631, Japan k Department of Physics, Tokyo Institute of Technology, Tokyo 152, Japan Received 1 March 1993
We carded out the energy scan between ECM = 58 and 60 GeV at the TRISTAN e + e - collider to search for the possible narrow resonance suggested by the L3 experiment at LEP. The total cross sections are measured for ~,7, multihadron, e + e - and /~+/z- productions at ten energy points coveting this energy range almost uniformly. The results are in good agreement with the Standard Model predictions, and 95% confidence level upper limits are set to Fee x BR of the hypothetical scalar and tensor resonances.
1. Introduction It was r e p o r t e d recently that the L3 e x p e r i m e n t at L E P o b s e r v e d a cluster o f 7Y i n v a r i a n t mass a r o u n d 59 G e V in the ~ + ~ - 7 7 final states o f Z ° decays, w h i c h is not e x p e c t e d f r o m the higher o r d e r correction in the S t a n d a r d M o d e l [ 1 ]. T h e i n t e r p r e t a t i o n o f this cluster needs m o r e statistics; however, it is possible that this cluster indicates a new n a r r o w r e s o n a n c e o f a mass a r o u n d 59 G e V . T h i s h y p o t h e t i c a l new resonance, d e n o t e d as X in this report, m a y h a v e any spin o t h e r t h a n one, since it decays to 77. A n d also, the b r a n c h i n g ratio B R ( X --, 77) should be large, since the L E P e x p e r i m e n t s r e p o r t e d no excess in o t h e r channels o f Z ° decay. I f the leptonic w i d t h o f X , Fee, is larger t h a n a few keV, there m a y be an observable e n h a n c e m e n t in e + e - --* Y7 and f f total cross sections Elsevier Science Publishers B.V.
3 73
PHYSICS LETTERS B
Volume 304, number 3,4
29Apdl1993
Table 1 ECM , its spread (rms) and the luminosity at each of ten energy points. The error in the integrated luminosity is a quadratic sum of the statistical and systematic errors. ECM (GeV)
ECM spread (MeV)
Integrated luminosity (pb - l )
57.37 57.77 57.97 58.22 58.47 58.72 58.97 59.22 59.47 59.84
95 96 95 98 98 99 100 100 102 88
1.018 78.1 1.249 1.326 1.319 1.760 1.343 1.132 0.902 2.97
+ 0.030 ± 2.0 a) + 0.036 -4- 0.038 -4- 0.038 + 0.049 + 0.038 ± 0.032 ± 0.027 i 0.37
a) Only 20.3-4-0.64 pb - l is used for Y7 analysis at this energy. around 59 GeV. To search for this resonance, T R I S T A N performed the energy scan between 58 and 60 GeV in the center o f mass energy. In this article, the results o f the energy scan are reported with respect to the total cross sections for yy production, multihadron production, Bhabha scattering a n d / ~ + / t - production, using the data taken by the T O P A Z detector.
2. Energy scan The results reported in this article are based on the data taken between 1988 and 1992 at ECM = 58 and 60 GeV, and the data in 1992 at the eight energies between them. The detailed description of the T O P A Z detector can be found elsewhere [2]. The energy points and the integrated luminosity at each point are summarized in table 1. The energies listed in this table are estimated by the field strength o f the bending magnets of the accelerator taking into account the energy shift due to the frequency shift of the accelerating RF. The spread o f the center of mass energy is 96 MeV at ECM = 58 GeV at T R I S T A N under the condition of this experiment, and this increases linearly as ECM. The energy step o f the scan is chosen to be approximately 250 MeV, which is determined from the beam energy spread to cover the entire energy range almost uniformly. The luminosity used in this analysis is measured only by the forward Bhabha scattering between 63 and 237 m r a d with respect to the beam axis to avoid the possible effect of the resonance in the large angle region. F o r this reason the cross sections reported in this article are slightly different from the previously published values at ECM = 58 and 60 GeV [3-5 ], where the luminosity is measured by the two calorimeters covering up to 462 mrad. The estimated error in the luminosity measurement includes typically 1.5% statistical and 2.5% c o m m o n systematic errors.
3. yy production The e + e - --. yy events are selected by requiring two back-to-back energy clusters in the barrel lead glass calorimeter, and no tracks in the inner drift chamber to exclude Bhabha events. The total energy deposit of 7's is required to be larger than 50% o f ECM, and the acollinearity angle between two ~,'s is required to be smaller than 10 °. The event selection efficiency of these requirements is estimated by Monte Carlo simulation to be 0.85 for Q E D events in the angular range [cos 0l < 0.77, where 0 is an angle of ~, with respect to the electron beam 374
Volume 304, number 3,4
PHYSICS LETTERS B
80 70 I (a) •
•
i
•
•
-
i
.
.
.
.
i
.
.
.
29 April 1993
.
i
•
e+e-->?'7
• ~
60 V
o
~"
10.0 5.0
1.00~" 0.50~
(b) ~
7 o~ 1.0 X ~ 0.5 r.
0.10 0,05
0.1
,
i
. . . .
57
i
. . . .
L
. . . .
58 59 Ecu or Mx (GeV)
i
,
60
Fig. 1. (a) Total cross section for e+e - ~ 77 in the angular range Icos0L < 0.77. The smooth curve is the QED prediction. (b) The 95% CL upper limits to Fee x BR(X --* 77) for narrow scalar (left axis) and tensor (right axis) resonances. axis. The more details o f 77 analysis are described in ref. [4]. The contamination o f Bhabha events and other sources is negligibly small. In fig. l a the total 77 cross sections in this angular range are plotted with the prediction of the lowest order QED. The cross sections are in good agreement with the Q E D prediction in this range o f the center o f mass energy. I f a narrow resonance X exists, this causes the following enhancement to the total cross section at the measurement point including the resonance mass Mx: f a r , ( W ) d W = ( 2 J + 1)~-~x " 2n22 Ee e B R ( X ~ 7 7 ) ,
(1)
where J = 0 or 2 for scalar and tensor assumptions, respectively. The acceptance for the 73' pairs from X decay is calculated from the angular coverage o f the detector and the event selection efficiency. F o r a scalar resonance, the angular distribution is uniform and the event selection efficiency is estimated to be 0.87 without assumption. On the other h a n d for the tensor case, the lagrangian for the tensor couplings to photons and fermions are assumed as follows [6]: 2.WT
gl,q[-~l,q~,u(Dv~lQ,q)
-
(Dv~lll,q)yp~tQ,q]
+ e
hFu~F;
(2)
,
l,q
where gt.q and h are the coupling constants to fermions and photons, respectively, and Fu,, is the photon field tensor. Here the couplings to left- and right-handed fermions are assumed to be identical. F r o m this lagrangian, the differential cross section for 77 production is expressed as follows: de
dO-
dO'QED dO
cz2(l
gehs2(M~-s)
+ S k 4 (mZx - S)2 + m2xl-r2
(1 + c o s / O ) +
1
g2eh2s4
64 (m2x - S)2 + m2xFr2
sinZO(l+cos20)~
(3)
]'
where s is the center o f mass energy squared and 1-'r is the total decay width of the resonance. In this expression, the terms which include the transverse b e a m polarization are not included, since they vanish in the total cross section. A n d also, the interference term can be ignored for a narrow resonance. The event selection efficiency is calculated from this angular distribution to be 0.90 for narrow tensor resonance. 375
Volume 304, number 3,4
PHYSICS LETTERS B
29 April 1993
The effect o f the initial state radiation is calculated as discussed in ref. [7], where the cross section including the initial state radiation is given only for scalar resonance. However, it can be shown that the error is less than a few percent when it is applied to tensor resonance, which is negligibly small compared to the experimental errors. Taking these and the spread o f the center o f mass energy into account, the 95% confidence level (CL) upper limits to Fee × BR (X ~ y y) are set as plotted in fig. 1b for narrow resonances. As a definition of 95% CL limit, we adopt the method given by the Particle Data Group [8]. The upper limits shown in fig. l b are applicable only to a narrow resonance of Fr << aecM, where trECM is the spread o f the center o f mass energy (_~ 100 MeV). F o r broader resonance the upper limit is similarly obtained by comparing the expected resonance shape (3) to the observed cross section. We obtained the following upper limits (95% CL) for resonances o f F r = 1 GeV in the mass range 58 < M x < 60 GeV: Fee × B R ( X ~ yy) < 8.9 keV < 0.05 keV
(for scalar resonance),
(4)
(for tensor resonance).
(5)
Here the limit to the tensor resonance is much smaller than the one for the scalar because of the large contribution o f the interference term in eq. (3), which is proportional to ~ .
4. Hadron production The multihadron events are selected by requiring the following conditions: (1) more than or equal to five charged tracks with Pr > 0.15 G e V / c with respect to the beam axis are found in I cos01 < 0.866, (2) the total visible energy is greater than 50% of EcM, (3) the m o m e n t u m imbalance along the beam axis is smaller than 0.4, and (4) the larger one of the jet masses is larger than 2.5 GeV. F r o m the number o f events the effective Born cross section is obtained using the radiative correction factor calculated with K O R A L Z program [9 ], where only photonic correction is included. This cross section has the same definition as the total hadronic cross section on Z ° pole reported by the experiments at LEP in ref. [ 10]. The total hadronic cross sections are plotted in fig. 2a with the prediction of the Standard Model for the effective Born cross section. In this calculation o f the theoretical prediction, the following parameters are used: sin 2 0w = 0.230, M z = 91.17 GeV and as (m 2) = 0.120. The measured cross sections are in good agreement with the Standard Model prediction. The effect o f the narrow resonance in this cross section is expected to be similar to eq. (1), where BR (X ---* YT) is replaced by B R ( X ~ hadrons). To set a limit to Fee × B R ( X ~ hadrons), the event selection efficiency for q~ decays of X is calculated by Monte Carlo program based on LUND6.3 [ 11 ] and the detailed detector simulation to be 0.86 and 0.76 for scalar and tensor assumptions, respectively. In this calculation the differential cross section for q~ production is assumed as follows for tensor X, which is also obtained from the lagrangian (2):
d~ dg2
-
dosM
g~ g~
s3
d-----~-+ ----------~ 8192~z ( M J - s ) 2 + M 2 F 2[(1 + c ° s 0 ) 2 ( 1 - 2 c ° s 0 ) 2
+ (interference t e r m ) .
+ (1-c°s0)2(1
+ 2c°s0)2] (6)
Here the interference term vanishes in the total cross section. The expected line shape o f the resonance is also calculated by the energy spread and the initial state radiation described in ref. [7]. The 95% CL limits to Fee >( B R ( X ~ hadrons) are plotted in fig. 2b for scalar and tensor assumptions. We obtained the following limits (95%CL) for the resonance of Fr = 1 GeV (58 < M x < 60 GeV): 376
Volume 304, n u m b e r 3,4
PHYSICS LETTERS B
200
.
#
-
(a)
180
29 April 1993
e+e - - > h a d r o n s
16o ~.
14o
b
120 100 :
~'~
50.0
I~
i0.0 5.0
~"
mo
1.o 0.5
:
:
. . . .
I
. . . .
I
. . . .
]
,
10.o 5.0 "~ 1.0~ 0.5 .
,
.
.
.
.
57
i
.
.
.
.
,
58
.
.
.
.
59
,
.
60
Ec~or Mx (GeV) Fig. 2. (a) Effective Born cross section for e + e - -~ hadrons. The smooth curve is the Standard Model prediction. (b) The 95% C L u p p e r limits to -Fee x B R ( X ~ h a d r o n s ) for n a r r o w scalar (left axis) a n d t e n s o r (right axis) resonances.
Fee x B R ( X ~ hadrons) < 11.6 keY < 2.6 keV
(for scalar resonance),
(7)
(for tensor resonance).
(8)
It should be noted that the multihadron final state by the gluonic decay of X is not taken into account in this analysis, and therefore, the limits given here are conservative.
5. Bhabha scattering The e + e - ---, e + e - events are selected by requiring back-to-back energy clusters in the barrel calorimeter and charged tracks in the inner drift chamber. The charges o f the tracks are not considered. The total cross section in the angular range [cos 01 < 0.768 is obtained from the number o f the events as the lowest order Q E D cross section. In fig. 3a the cross section at each energy point is plotted with the QED prediction. The result is in good agreement with QED. The effect expected from a narrow resonance is different in this reaction because o f the resonance contribution o f t-channel exchange o f X. The expected differential cross section is expressed as follows for scalar and tensor assumptions from the lagrangian (2) [6,12]: da dg-2
dO'QED
dQ
geM~ (s 2 2 + 3-N-Y M2x) 2 + M k r ? 2
+
t
--g~]
+
. s ( t - M~)2
2 a t - M J + ---ffO-~- ] J s
2
+ gl M } st(s - M2x) 80n [ ( s - M 2 ) 2 + M 2 r ] l ( t - M 2
)
(for scalar resonance),
(9)
377
Volume 304, number 3,4
PHYSICS LETTERS B
550
500
-
,
. . . .
,
. . . .
,
(a)
29 April 1993
. . . .
,
-
e+e-->e+e-
~
"¢ 400 ~b ¢9 "~
a5o 300
1.03
102
+
101
I
m i0 0 m
', ,
i
.
.
57
.
.
,: l
.
.
.
.
l
.
5B 59 Ecu or Mx (GeV)
.
.
.
l
,
60
Fig. 3. (a) Total cross section for e + e - ~ e + e - in the angular range [cos0[ < 0.768. The smooth curve is the standard model prediction. (b) The 9 5 % C L upper limit to Fee x B R ( X ~ e + e - ) for narrow scalar (leftaxis) and tensor (right axis) resonances.
da df2 +
dO'QED d.O
gEe [ gZeS[
-T-~-~(1 + x ) 2 ( 2 x -
1
)2
+ (1 - x ) Z ( 2 x + 1
)2
+ (1 + x ) 2 ( 2 x -
1)(7 +
, s - M2x~ X)t_--Z--~r )
%
+41ta
+
]
(s-M2)(l+x)2(2x-l)+4~zas-M2x(1-x)2(2x+l)s
g2e {ge2S + 5--i-~5~z/--~-( 1 \[
+ x)Z(7 + x) 2 + 4(5 + 3x) 2 ] + 2 h a ( 7 + x ) ( 1 +
+ 2a'o~[(7
+
+ X ) ( 1
X) 1 + 4 ( 5 +
3X)] -t - M - 2)
( t - S2 Mx2) 2
x)zt"
(s-M2)2+FT2M 2
i
S
(for tensor resonance).
(10)
Here x = cos 0. The upper limit to Fee is obtained by comparing the measured total cross section to these expressions integrated numerically over I cos01 < 0.768. In this calculation, the initial state radiation and the spread of the center of mass energy are considered only for the terms of s-channel exchange of X. The event acceptance for e + e - pair from the resonance decay is also taken into account using the angular distributions (9) and (10). The results are shown in fig. 3b for narrow resonances, and for broad resonances with Fr = 1 GeV, we obtain the following limits (58 < Mx < 60 GeV): Fee × B R ( X --* e+e - ) < 22.9 keV
(for scalar resonance),
(11)
< 13.2 k e y
(for tensor resonance).
(12)
6. /4+~ - production
The e+e - -~ g+/~- events are selected by requiring two charged tracks of the momenta larger than ECM/6 with an acollinearity angle smaller than I0 ° in ]cos01 < 0.75, and no large energy deposit in the calorimeters. The 378
Volume 304, number 3,4
PHYSICS LETTERS B
60 50
~,
4o
,~
30
-
i
i
.
.
.
(a)
.
i
29 April 1993
e .
.
.
.
i
•
+e- - >/~+p.-
20 b
lO
1. 0 0"~ x&0.0.15:".:.:.:.:,".:.:.:.:,".:.:.....0.10 57 58 59 60 5.00
io.o
g"
+::£
5.0
0.50
i ~ 1.0
IIl
0.05
EcM or M x (CeV)
Fig. 4. (a) Total cross section for e+e - -+ ~+/a-. The cross section has been translated to the lowest order electroweak cross section. The smooth curve is the standard model prediction. (b) The 95% CL upper limits to Fee x BR(X --+ #+/1- ) for narrow scalar (left axis) and tensor (fight axis) resonances. difference o f the two T O F times is also required to be smaller than 4 ns to exclude cosmic rays. After the event selection, the residual backgrounds are estimated to be 2% mainly from z + r - events and two photon processes. The more detail o f the/1+/~ - event analysis is given in ref. [5]. The total cross section is then calculated from the n u m b e r o f events as the lowest order electroweak cross section, using the radiative correction [ 13 ] and the acceptance calculated by the Monte Carlo program. The trigger efficiency is estimated by cosmic ray events to be 95.7%. The total cross section is plotted in fig. 4a with the Standard Model prediction, where the measurement is also in good agreement with the theory. F r o m this cross section, the upper limit to Fee x B R ( X -+/~+/1- ) is calculated in the same way as described in the previous section for the hadronic cross section. The obtained upper limits are plotted for narrow scalar and tensor resonances in fig. 4b. F o r broad resonance with Fr= 1 GeV, we obtained the following upper limits (58 < Mx < 60 GeV): Fee × B R ( X -+/~+/~- ) < 5.3 keY
(for scalar r e s o n a n c e ) ,
(13)
< 1,4 keV
(for tensor resonance).
(14)
7. Conclusion We carried out the energy scan between 58 and 60 GeV in the center o f mass energy to search for the possible narrow resonance indicated in the analyses o f e + e - 7 7 final states of Z ° decays by L3. We accumulated more than l pb-~ o f the integrated luminosity at each o f the ten energy points spaced by approximately 250 MeV, which cover this energy range almost uniformly, considering the spread o f the center o f mass energy. The total cross sections for 77, multihadron, e + e - a n d / ~ + / ~ - productions are measured at each point. They are all consistent with the predictions o f the Standard Model within the measurement errors, and the 95% CL upper limits are set to Fee × BR (X -+ y ) ' s o f the hypothetical new resonance X, where y is one o f the final states mentioned above. F o r this calculation o f the limits, X is assumed to be either scalar or tensor.
379
Volume 304, number 3,4
PHYSICS LETTERS B
29 April 1993
Acknowledgement We are indebted to the T R I S T A N accelerator division for the efficient and stable operation of the collider especially during the energy scan. We also thank K. Hagiwara, S. Matsumoto and M. Tanaka for helpful discussions and suggestions.
References [1] L3 Collab., O. Adriani et al., Phys. Lett. B 295 (1992) 337. [2] T. Kamae et al., Nucl. Instrum. Methods A 252 (1986) 423; S. Kawabata et al., Nucl. Instrum. Methods A 270 (1988) 11; J. Fujimoto et al., Nucl. Instrum. Methods A 256 (1987) 449. [3] TOPAZ Collab., I. Adachi et al., Phys. Lett. B 234 (1990) 535. [4] TOPAZ Collab., K. Shimozawa et al., Phys. Lett. B 284 (1992) 144; K. Shimozawa, Ph.D. Thesis, Nagoya University preprint DPNU-92-44 (1992). [5] TOPAZ Collab., B. Howell et al., Phys. Lett. B 291 (1992) 206. [6] K. Hagiwara, S. Matsumoto and M. Tanaka, KEK preprint 92-202 (1993). [7] VENUS Collab., S. Odaka et al., J. Phys. Soc. Japan 58 (1989) 3037. [8] Particle Data Group, K. Hikasa et al., Review of particle properties, Phys. Rev. D 45 (1992), Section 2.4.2, p. 111.39. [9] S. Jadach et al., Comput. Phys. Commun. 66 (1991) 276. [10] ALEPH Collab., D. Decamp et al., Z. Phys. C 48 (1990) 365; DELPHI Collab., P. Abreu et al., Phys. Lett. B 260 (1991) 240; L3 Collab., B. Adeva et al., Z. Phys. C 51 (1991) 179; OPAL Collab., G. Alexander et al., Z. Phys. C 52 (1991) 175. [11 ] T. Sj6strand, Comput. Phys. Commun. 39 (1986) 347; T. Sj6strand and M. Bengtsson, Comput. Phys. Commun. 43 (1987) 367. [12] W. Hollik, F. Schrempp and B. Schrempp, Phys. Lett. B 140 (1984) 424. [13] J. Fujimoto et al., Prog. Theor. Phys. Suppl. I00 (1990) 1.
380