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Observation of short-lived particles produced in high energy neutrino interactions
Volume 89B, number 3,4
PHYSICS LETTERS
28 January 1980
OBSERVATION OF SHORT-LIVED PARTICLES PRODUCED IN HIGH ENERGY NEUTRINO INTERACTIONS H.C. BALLAGH, H.H. BINGHAM, W.B. FRETTER, T. LAWRY, G.R. LYNCH, J. LYS, J. ORTHEL, M.D. SOKOLOFF, M.L. STEVENSON and G.P. YOST Department of Physics and Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720, USA B. CHRISMAN, D. GEE, G. H A R I G E L 1, F.R. HUSON, E. SCHMIDT, W. SMART, E. TREADWELL and J. WOLFSON Fermi National Accelerator Laboratory, Batavia, 1L 60510, USA R.J. CENCE, F.A. HARRIS, M.D. JONES, S.I. PARKER, M.W. PETERS, V.Z. PETERSON and V.J. STENGER University of Hawaii at Manoa, Honolulu, HI 96822, USA T.H. BURNETT 2, L. FLURI, D. HOLMGREN, H.J. LUBATTI, K. MORIYASU, D. REES, H. RUDNICKA, G.M. SWlDER and E. WOLIN Visual Techniques Laboratory, Department of Physics, University of Washington, Seattle, WA 98195, USA and R. BENADA, U. CAMERINI, M. DUFFY, W. FRY, R.J. LOVELESS, P. McCABE, M. NGAI and D.D. REEDER University of Wisconsin, Madison, W153706, USA Received 14 August 1979
Examination of a sample of 89 high-energy, neutrino-induced dilepton events produced in the Fermilab 15 ft bubble chamber has resulted in the direct observation of the production and visible semi-leptonic decay of shortqived particles• One charged decay, two neutral decays, and one decay candidate of undetermined charge are found• Assumin~ these events and the remaining dilepton signal result from D meson decays, we obtain rD+ = 2 • 5 -+- 1 3.5 X 10-13 s and rDo = 3"5 -+- 13.5 .5 .7 × 10-13 s, using a maximum likelihood analysis•
In this letter we report the first direct observation of the semi-leptonic decays o f short-lived particles in neutrino-induced dilepton (/J/a and/ae) events. Our dilepton events come from a 326 000 picture exposure of the Fermilab 15 ft bubble chamber to the quadrupole triplet neutrino beam. The two-plane external muon identifier (EMI) [1 ] is used to identify muons with Pu > 4 GeV/c for a total of I0 300 v and 1800 charged-current events. Experimental details and preI Present address: CERN, 1211 Geneva 23, Switzerland. 2 A.P. Sloan Foundation Research Fellow.
liminary results on the dimuon sample have been reported [ 1 - 3 ] . For the/ae events we require that the electrons have Pe > 0.8 GeV/c. The average v (if) event energy, 90 (60) GeV, considerably higher than previous wideband beam bubble chamber experiments, in an important advantage for direct observation of short-lived particles * 1 ~:1 A narrow-band experiment in BEBC [4] with an average v (~ event energy of 95 (75) GeV has reported dilepton events; however, they have only 1260 charged-current events, with 12 t~t~ and 6 #e events. 423
Volume 89B, number 3,4
PHYSICS LETTERS
The complete sample of 54/a+/a - events (of which about 20 are background due mostly to 7r-/g decays) and a partial sample o f 35/ae events have been examined under very high magnification by physicsts for possible evidence of short-lived particles. Two neutral decays (N1 and N2), one charged decay (C1), and one decay candidate (U1) of undetermined charge are observed. All four events are in the/ae sample and have e + with momenta < 4 GeV/e. In two events, the e + does not extrapolate to the vertex. (The extrapolation of the e+ in N1 misses the primary vertex by 0.17 + 0.06 cm, in the direction parallel to the magnetic field.) Different m o m e n t u m cuts for/~+ (4 GeV/c) and e ÷ (0.8 GeV/c) affect detection efficiency. For example, our charm decay Monte Carlo shows that 30% of the e + but only 5% of the/~+ should have an angle with respect to the charmed particle direction greater than 5 °. For events N1, C1 and N2, we have attempted to determine the position o f the primary and decay vertices in two ways. The first method is corresponding point reconstruction. In order to reduce the possible bias in the measurer's judgment o f the vertex location, each event has been measured several times by different measurers. The decay lengths thus ob-
28 January 1980
tained are averaged and given in table 1. The quoted errors are the root-mean-square (rms) deviations. The second method uses only measurements of outgoing trakcs at a vertex and calculates the intersection o f the tracks, as discussed below. Events N1 (fig. 1) and N2 have a neutral secondary vertex with an outgoing e + and h - . For event N1, the extrapolation of the e + backward through the apparent decay vertex significantly misses the primary vertex. The angle between the e+ and h - is 13 ° , and the average separation of the vertices obtained from 14 determinations by track intersections is 0.54 cm with a statistical uncertainty of 0.04 cm and an uncertainty due to multiple scattering of 0.05 cm. Therefore, the decay vertex determined by this method is significantly downstream o f the primary vertex and is consistent with the result from the corresponding point reconstruction. Event C1 under high magnification appears to be a charged particle decaying into an e + and two oppositely-charged hadrons (h + and h - ) . This is based primarily upon the clear visual evidence, on the original film, of an abrupt increase in track density at the decay ver-
Table 1 Kinematic and decay quantities for the observed decay candidates. The invariant masses for event U1 result from combining the e+ with the two negatively charged hadrons in the forward jet.
Pvis" 0beam (GeV/c) Pt~ (GeV/c) (GeV/c) Plmiss (GeV/c) Pe+ (GeV/c) Me+n- (GeV/e 2) Me+K- (GeV/c 2) decay length (cm) PD (GeV/e) decay time (10 -12 s) Q2 (GeV2/c 2) x y W (GeV/c ~)
C1 (16040581)
N1 (16441094)
N2 (14651459)
U1 (15131206)
233 16.1 +- 0.3 2.9 0.5 3±1 0.58 ± 0.14 0.75 ± 0.11 0.62 ± 0.08 111-237 0.16 < t < 0.35 123 0.28 0.93 12
39 21.6 ± 0.5 3.8 2.4 1.1 +- 0.1 0.650 ± 0.041 0.824 ± 0.034 0.67 ± 0.12 8-40 1.04 < t < 5.20 27 0.77 0.45 6.2
74 12.4 ± 0.2 1.1 0.3 2.9 ± 0.4
82 53.7 ± 1.0 1.2 0.7 1.3 ± 0.2 0.947 ± 0.074 0.824 ± 0.072 1.097 ± 0.067 1.114 ± 0.068 0.3-0.6 2.3 0.04 0.35
0.852 ± 0.065 0.984 ± 0.057 0.87 ± 0.16 38-135 0.40 < t < 1.42 7.2 0.06 0.83 8.6
424
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Volume 89B, number 3,4
PHYSICS LETTERS
28 January 1980
Fig. 1. Event N1 (high magnification of vertex in insert). Tracks 7 and 8 appear to come from the decay of a neutral particle. Another view (not shown) reveals that track 2 is clearly not associated with the vertex from which tracks 7 and 8 emerge. Track 8 has a momentum determined from curvature of 6.9 GeV/c and undergoes an interaction that produces a K~. tex. Measurements of gap density in other tracks of the same event show that the probability for the three outgoing tracks to appear as one incoming track over the decay distance is less than 10 - 4 . Event N2 appears, from the difference in track density (by visual inspection), to have a neutral decay; but the apparent decay vertex is obscured by a different track in each view. The angle between the e + and h - is small (about 5°), and there is turbulence in this region of the chamber. Thus the rms deviation of the decay length measurements probably underestimates the real uncertainty. We treat it as an unseen decay vertex in the lifetime determination described below. Event U1 has a forward jet of several tracks (including t h e / 1 - ) from which an e + emerges at a 20 ° angle and clearly downstream of the primary vertex. The tracks of the jet are too closely spaced to see a decay vertex or determine which tracks, if any, also come from the decay. The e+, when extrapolated backwards, intersects the ~t- track at a distance of 0.35 -+ 0.03 cm downstream of the primary vertex. The prob-
ability that this is due to a scatter of a primary e + is less than 10 - 6 . We have calculated the possible background processes which could simulate a decay vertex from which an e+ emerges in a 5 m m interval near the primary vertex. The decay, K 0 -~ e+Tr-v, is ruled out for events N1, N2 and U1 because the e+Tr- invariant masses (see table 1) are significantly greater than the K 0 mass. For event C1, backgrounds from K 0 or A decay superimposed on a primary e + are excluded by invariant masses of the hadrons. Background from a neutral hadron interaction superimposed on a primary e + is negligible. The largest background results from asymmetric 7 conversions with an undetected e - (less than 5 MeV) occurring very near one of the hadron tracks and thereby simulating a decay vertex. We conclude that the sum of all backgrounds is less than 0.14 (0.11) events which would resemble a neutral (charged) decay. We assume that we are observing charmed particle decays and estimate the decay time from the observed 425
Volume 89B, number 3,4
PHYSICS LETTERS
decay distances. The D o meson is the only known (or expected) weakly-decaying, neutral charmed particle. The invariant masses (see table 1) o f the charged particles from the neutral decays are consistent with decay of a D O meson. The D + and F + mesons and the + A c baryon are all candidates for the charged decay in event C1. Event C1 is consistent with D + ~ e+K-Tr+v
(Me+K_ r÷ = 1.28 -+ 0.11 GeV)
and barely with +
A c ~ e+rr-Tr+Av
( M e + - + = 1.20 + 0.12),
but not with F+ ~ e + K - K + v
(Me+K-K+
=
2.49 + 0 . 2 1 ) ,
nor with Ac+ ~ e + K - p v
(Me+K_ p = 4.37 + 0 . 4 0 ) .
Assuming the observed events are examples of D O e + K - v and D + ~ e + K - zr+v, we calculate the minimum and maximum D momentum (table 1) consistent with the measured momenta of the charged particles. Note that the corresponding range of decay times given in table 1 does not rely on the poorly determined D direction. We attempt to determine the lifetimes of the D O and D ÷ mesons b y assuming that our sample of semileptonic decays is an equal mixture of D O and D + decays and that the D + decays to one charged particle are unobserved. Only the two events C1 and N1 that have a good measure of the decay distance were considered as observed decays for this calculation. The lifetime calculation is essentially the calculation of the lifetimes for which we could expect to see one D O and one D + decay in our experiment. The actual calculation was done by maximizing the likelihood functions that express the products of the probabilities (Ps) for the seen decays (N1 and C1)and the probabilities (Pu) for not detecting decays in events where the vertex is not observed. The maximum decay distance (/max) for an unseen decay is obtained b y judging how far from the primary vertex a decay into at least two charged particles would have to occur so that it is evident that
426
28 January 1980
not all tracks originate at the primary vertex. The D m o m e n t u m spectrum is obtained from our charm decay Monte Carlo using the parametrization of ref. [5]. The most probable values for the D ÷ and D O lifetimes thus obtained are ~-D÷ = 2,5+__3i5 × 10 -13 s and rDO = 3.5+3l'.5 × 10 -13 s, where the errors represent l o variations. The 2o limits are 6 X 10 -14 < rD÷ < 1.3 X 10 -12 s and 1.0 X 10 -13 < rDO< 1.2 × 10 -12 s. Our quoted errors take into account the sensitivity of the likelihood functions to the esitmates of lmax and the D momenta for the unseen decays and to the uncertainties in the decay distances and D m o m e n t a for the observed decays. They do not include the possibility that our sample may have other charmed particle decays than D mesons or that the D O and D ÷ decays may not be equally represented in our dilepton sample. These limits are consistent with upper limits obtained by bubble chamber experiments [ 6 - 8 ] with the decay times for the charged decays seen in emulsion experiments [9,10]. We gratefully acknowledge the efforts of the many people at Fermilab and the other institutions who have contributed to this experiment, Special thanks are given to the scanners and measurers for their dedication and perseverance. This work is supported in part by the U.S. Department of Energy and the National Science Foundation.
References [1] M.L. Stevenson, Proc. Oxford Conf. on Neutrino physics at accelerators (Oxford, England, 1978) p. 362. [2] F.A. Harris, Proc. Oxford Conf. on Neutrino physics at accelerators (Oxford, England, 1978) p. 209. [3] V.Z. Peterson, Proc. XIXth Intern. Conf. on High energy physics (Tokyo, Japan, 1978) p. 352. [4] P. Bosetti et al., Phys. Lett. 73B (1978) 380. [5] C.H. Lai, Phys. Rev. D18 (1978) 1422. [6] L. Pape, Proe. Oxford Conf. on Neutrino physics at accelerators (Oxford, England, 1978) p. 167. [7] A. Blondel, Proc. Oxford Conf. on Neutrino physics at accelerators (Oxford, England, 1978) p. 217. [8] O. Erriquez et al., Phys. Lett. 77B (1978) 227. [9] E.H.S. Burhop et al., Phys. Lett. 65B (1976) 299. [10] C. Angelini et al., Phys. Lett. 80B (1979) 428; 84B (1979) 150.
PHYSICS LETTERS
28 January 1980
OBSERVATION OF SHORT-LIVED PARTICLES PRODUCED IN HIGH ENERGY NEUTRINO INTERACTIONS H.C. BALLAGH, H.H. BINGHAM, W.B. FRETTER, T. LAWRY, G.R. LYNCH, J. LYS, J. ORTHEL, M.D. SOKOLOFF, M.L. STEVENSON and G.P. YOST Department of Physics and Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720, USA B. CHRISMAN, D. GEE, G. H A R I G E L 1, F.R. HUSON, E. SCHMIDT, W. SMART, E. TREADWELL and J. WOLFSON Fermi National Accelerator Laboratory, Batavia, 1L 60510, USA R.J. CENCE, F.A. HARRIS, M.D. JONES, S.I. PARKER, M.W. PETERS, V.Z. PETERSON and V.J. STENGER University of Hawaii at Manoa, Honolulu, HI 96822, USA T.H. BURNETT 2, L. FLURI, D. HOLMGREN, H.J. LUBATTI, K. MORIYASU, D. REES, H. RUDNICKA, G.M. SWlDER and E. WOLIN Visual Techniques Laboratory, Department of Physics, University of Washington, Seattle, WA 98195, USA and R. BENADA, U. CAMERINI, M. DUFFY, W. FRY, R.J. LOVELESS, P. McCABE, M. NGAI and D.D. REEDER University of Wisconsin, Madison, W153706, USA Received 14 August 1979
Examination of a sample of 89 high-energy, neutrino-induced dilepton events produced in the Fermilab 15 ft bubble chamber has resulted in the direct observation of the production and visible semi-leptonic decay of shortqived particles• One charged decay, two neutral decays, and one decay candidate of undetermined charge are found• Assumin~ these events and the remaining dilepton signal result from D meson decays, we obtain rD+ = 2 • 5 -+- 1 3.5 X 10-13 s and rDo = 3"5 -+- 13.5 .5 .7 × 10-13 s, using a maximum likelihood analysis•
In this letter we report the first direct observation of the semi-leptonic decays o f short-lived particles in neutrino-induced dilepton (/J/a and/ae) events. Our dilepton events come from a 326 000 picture exposure of the Fermilab 15 ft bubble chamber to the quadrupole triplet neutrino beam. The two-plane external muon identifier (EMI) [1 ] is used to identify muons with Pu > 4 GeV/c for a total of I0 300 v and 1800 charged-current events. Experimental details and preI Present address: CERN, 1211 Geneva 23, Switzerland. 2 A.P. Sloan Foundation Research Fellow.
liminary results on the dimuon sample have been reported [ 1 - 3 ] . For the/ae events we require that the electrons have Pe > 0.8 GeV/c. The average v (if) event energy, 90 (60) GeV, considerably higher than previous wideband beam bubble chamber experiments, in an important advantage for direct observation of short-lived particles * 1 ~:1 A narrow-band experiment in BEBC [4] with an average v (~ event energy of 95 (75) GeV has reported dilepton events; however, they have only 1260 charged-current events, with 12 t~t~ and 6 #e events. 423
Volume 89B, number 3,4
PHYSICS LETTERS
The complete sample of 54/a+/a - events (of which about 20 are background due mostly to 7r-/g decays) and a partial sample o f 35/ae events have been examined under very high magnification by physicsts for possible evidence of short-lived particles. Two neutral decays (N1 and N2), one charged decay (C1), and one decay candidate (U1) of undetermined charge are observed. All four events are in the/ae sample and have e + with momenta < 4 GeV/e. In two events, the e + does not extrapolate to the vertex. (The extrapolation of the e+ in N1 misses the primary vertex by 0.17 + 0.06 cm, in the direction parallel to the magnetic field.) Different m o m e n t u m cuts for/~+ (4 GeV/c) and e ÷ (0.8 GeV/c) affect detection efficiency. For example, our charm decay Monte Carlo shows that 30% of the e + but only 5% of the/~+ should have an angle with respect to the charmed particle direction greater than 5 °. For events N1, C1 and N2, we have attempted to determine the position o f the primary and decay vertices in two ways. The first method is corresponding point reconstruction. In order to reduce the possible bias in the measurer's judgment o f the vertex location, each event has been measured several times by different measurers. The decay lengths thus ob-
28 January 1980
tained are averaged and given in table 1. The quoted errors are the root-mean-square (rms) deviations. The second method uses only measurements of outgoing trakcs at a vertex and calculates the intersection o f the tracks, as discussed below. Events N1 (fig. 1) and N2 have a neutral secondary vertex with an outgoing e + and h - . For event N1, the extrapolation of the e + backward through the apparent decay vertex significantly misses the primary vertex. The angle between the e+ and h - is 13 ° , and the average separation of the vertices obtained from 14 determinations by track intersections is 0.54 cm with a statistical uncertainty of 0.04 cm and an uncertainty due to multiple scattering of 0.05 cm. Therefore, the decay vertex determined by this method is significantly downstream o f the primary vertex and is consistent with the result from the corresponding point reconstruction. Event C1 under high magnification appears to be a charged particle decaying into an e + and two oppositely-charged hadrons (h + and h - ) . This is based primarily upon the clear visual evidence, on the original film, of an abrupt increase in track density at the decay ver-
Table 1 Kinematic and decay quantities for the observed decay candidates. The invariant masses for event U1 result from combining the e+ with the two negatively charged hadrons in the forward jet.
Pvis" 0beam (GeV/c) Pt~ (GeV/c) (GeV/c) Plmiss (GeV/c) Pe+ (GeV/c) Me+n- (GeV/e 2) Me+K- (GeV/c 2) decay length (cm) PD (GeV/e) decay time (10 -12 s) Q2 (GeV2/c 2) x y W (GeV/c ~)
C1 (16040581)
N1 (16441094)
N2 (14651459)
U1 (15131206)
233 16.1 +- 0.3 2.9 0.5 3±1 0.58 ± 0.14 0.75 ± 0.11 0.62 ± 0.08 111-237 0.16 < t < 0.35 123 0.28 0.93 12
39 21.6 ± 0.5 3.8 2.4 1.1 +- 0.1 0.650 ± 0.041 0.824 ± 0.034 0.67 ± 0.12 8-40 1.04 < t < 5.20 27 0.77 0.45 6.2
74 12.4 ± 0.2 1.1 0.3 2.9 ± 0.4
82 53.7 ± 1.0 1.2 0.7 1.3 ± 0.2 0.947 ± 0.074 0.824 ± 0.072 1.097 ± 0.067 1.114 ± 0.068 0.3-0.6 2.3 0.04 0.35
0.852 ± 0.065 0.984 ± 0.057 0.87 ± 0.16 38-135 0.40 < t < 1.42 7.2 0.06 0.83 8.6
424
7.9
Volume 89B, number 3,4
PHYSICS LETTERS
28 January 1980
Fig. 1. Event N1 (high magnification of vertex in insert). Tracks 7 and 8 appear to come from the decay of a neutral particle. Another view (not shown) reveals that track 2 is clearly not associated with the vertex from which tracks 7 and 8 emerge. Track 8 has a momentum determined from curvature of 6.9 GeV/c and undergoes an interaction that produces a K~. tex. Measurements of gap density in other tracks of the same event show that the probability for the three outgoing tracks to appear as one incoming track over the decay distance is less than 10 - 4 . Event N2 appears, from the difference in track density (by visual inspection), to have a neutral decay; but the apparent decay vertex is obscured by a different track in each view. The angle between the e + and h - is small (about 5°), and there is turbulence in this region of the chamber. Thus the rms deviation of the decay length measurements probably underestimates the real uncertainty. We treat it as an unseen decay vertex in the lifetime determination described below. Event U1 has a forward jet of several tracks (including t h e / 1 - ) from which an e + emerges at a 20 ° angle and clearly downstream of the primary vertex. The tracks of the jet are too closely spaced to see a decay vertex or determine which tracks, if any, also come from the decay. The e+, when extrapolated backwards, intersects the ~t- track at a distance of 0.35 -+ 0.03 cm downstream of the primary vertex. The prob-
ability that this is due to a scatter of a primary e + is less than 10 - 6 . We have calculated the possible background processes which could simulate a decay vertex from which an e+ emerges in a 5 m m interval near the primary vertex. The decay, K 0 -~ e+Tr-v, is ruled out for events N1, N2 and U1 because the e+Tr- invariant masses (see table 1) are significantly greater than the K 0 mass. For event C1, backgrounds from K 0 or A decay superimposed on a primary e + are excluded by invariant masses of the hadrons. Background from a neutral hadron interaction superimposed on a primary e + is negligible. The largest background results from asymmetric 7 conversions with an undetected e - (less than 5 MeV) occurring very near one of the hadron tracks and thereby simulating a decay vertex. We conclude that the sum of all backgrounds is less than 0.14 (0.11) events which would resemble a neutral (charged) decay. We assume that we are observing charmed particle decays and estimate the decay time from the observed 425
Volume 89B, number 3,4
PHYSICS LETTERS
decay distances. The D o meson is the only known (or expected) weakly-decaying, neutral charmed particle. The invariant masses (see table 1) o f the charged particles from the neutral decays are consistent with decay of a D O meson. The D + and F + mesons and the + A c baryon are all candidates for the charged decay in event C1. Event C1 is consistent with D + ~ e+K-Tr+v
(Me+K_ r÷ = 1.28 -+ 0.11 GeV)
and barely with +
A c ~ e+rr-Tr+Av
( M e + - + = 1.20 + 0.12),
but not with F+ ~ e + K - K + v
(Me+K-K+
=
2.49 + 0 . 2 1 ) ,
nor with Ac+ ~ e + K - p v
(Me+K_ p = 4.37 + 0 . 4 0 ) .
Assuming the observed events are examples of D O e + K - v and D + ~ e + K - zr+v, we calculate the minimum and maximum D momentum (table 1) consistent with the measured momenta of the charged particles. Note that the corresponding range of decay times given in table 1 does not rely on the poorly determined D direction. We attempt to determine the lifetimes of the D O and D ÷ mesons b y assuming that our sample of semileptonic decays is an equal mixture of D O and D + decays and that the D + decays to one charged particle are unobserved. Only the two events C1 and N1 that have a good measure of the decay distance were considered as observed decays for this calculation. The lifetime calculation is essentially the calculation of the lifetimes for which we could expect to see one D O and one D + decay in our experiment. The actual calculation was done by maximizing the likelihood functions that express the products of the probabilities (Ps) for the seen decays (N1 and C1)and the probabilities (Pu) for not detecting decays in events where the vertex is not observed. The maximum decay distance (/max) for an unseen decay is obtained b y judging how far from the primary vertex a decay into at least two charged particles would have to occur so that it is evident that
426
28 January 1980
not all tracks originate at the primary vertex. The D m o m e n t u m spectrum is obtained from our charm decay Monte Carlo using the parametrization of ref. [5]. The most probable values for the D ÷ and D O lifetimes thus obtained are ~-D÷ = 2,5+__3i5 × 10 -13 s and rDO = 3.5+3l'.5 × 10 -13 s, where the errors represent l o variations. The 2o limits are 6 X 10 -14 < rD÷ < 1.3 X 10 -12 s and 1.0 X 10 -13 < rDO< 1.2 × 10 -12 s. Our quoted errors take into account the sensitivity of the likelihood functions to the esitmates of lmax and the D momenta for the unseen decays and to the uncertainties in the decay distances and D m o m e n t a for the observed decays. They do not include the possibility that our sample may have other charmed particle decays than D mesons or that the D O and D ÷ decays may not be equally represented in our dilepton sample. These limits are consistent with upper limits obtained by bubble chamber experiments [ 6 - 8 ] with the decay times for the charged decays seen in emulsion experiments [9,10]. We gratefully acknowledge the efforts of the many people at Fermilab and the other institutions who have contributed to this experiment, Special thanks are given to the scanners and measurers for their dedication and perseverance. This work is supported in part by the U.S. Department of Energy and the National Science Foundation.
References [1] M.L. Stevenson, Proc. Oxford Conf. on Neutrino physics at accelerators (Oxford, England, 1978) p. 362. [2] F.A. Harris, Proc. Oxford Conf. on Neutrino physics at accelerators (Oxford, England, 1978) p. 209. [3] V.Z. Peterson, Proc. XIXth Intern. Conf. on High energy physics (Tokyo, Japan, 1978) p. 352. [4] P. Bosetti et al., Phys. Lett. 73B (1978) 380. [5] C.H. Lai, Phys. Rev. D18 (1978) 1422. [6] L. Pape, Proe. Oxford Conf. on Neutrino physics at accelerators (Oxford, England, 1978) p. 167. [7] A. Blondel, Proc. Oxford Conf. on Neutrino physics at accelerators (Oxford, England, 1978) p. 217. [8] O. Erriquez et al., Phys. Lett. 77B (1978) 227. [9] E.H.S. Burhop et al., Phys. Lett. 65B (1976) 299. [10] C. Angelini et al., Phys. Lett. 80B (1979) 428; 84B (1979) 150.