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Experimental limits on nucleon lifetime for lepton+meson decay modes
Volume 220, number 1,2
PHYSICS LETTERS B
30 March 1989
EXPERIMENTAL LIMITS ON NUCLEON LIFETIME FOR L E P T O N + M E S O N DECAY M O D E S K.S. H I R A T A , T. K A J I T A , T. K I F U N E , K. K I H A R A , M. N A K A H A T A , K. N A K A M U R A , S. O H A R A , Y. O Y A M A , N. SATO, M. T A K I T A , Y. T O T S U K A , Y. Y A G I N U M A Institute for Cosmic Ray Research, University of Tokyo, Tokyo 188, Japan M. M O R I , A. S U Z U K I , K. T A K A H A S H I , T. T A N I M O R I , M. Y A M A D A l National Laboratory for High Energy Physics (KEK), lbaraki 305, Japan
M. K O S H I B A Tokai University, Tokyo 151, Japan T. S U D A Department of Physics, Kobe University, Hyogo 657, Japan K. M I Y A N O , H. MIYATA, H. T A K E I Niigata University, Niigata 950-21, Japan K. K A N E Y U K I , Y. N A G A S H I M A , Y. S U Z U K I Department of Physics, Osaka University, Osaka 560, Japan E.W. BEIER, L.R. F E L D S C H E R , E.D. F R A N K , W. F R A T I , S.B. KIM, A.K. M A N N , F.M. N E W C O M E R , R. VAN B E R G a n d W. Z H A N G Department of Physics, University of Pennsylvania, Philadelphia, PA 19104, USA Received 13 January 1989
We have searched for nucleon decay signals using data from the KAMIOKANDE-II detector. No evidence for nucleon decay has been found. Limits on the nucleon partial lifetime for various decay modes are obtained combining KAMIOKANDE-I and -II data (3.76 kt yr in total). The background subtracted limits at 90% CL range from 0.1 × 1032yrto 2.6 × 1032yrdepending on the decay modes. For the decay modes p--,e+n°, p~gK + and n--,gK°, the limits are 2.6>( 1032yr, 1.0>( 1032yr and 0.9)< 1032yr, respectively.
The p r e d i c t i o n o f nucleon decay by grand unified theories ~ stimulated the current experimental search for nucleon decay by d e d i c a t e d u n d e r g r o u n d detectors. After several years o f experimental searches ~2, a clear nucleon decay signal has not been observed, On leave from Niigata University, Niigata 950-21, Japan. ~ See ref. [1] for a review, ,2 See ref. [ 2 ] for recent reviews. 308
although there are several c a n d i d a t e events which are not easily explained by a t m o s p h e r i c neutrino interactions. This p a p e r reports the search for nucleon decay using the K A M I O K A N D E - I I ( K A M - I I ) detector a n d presents the c o m b i n e d results from K A M I O K A N D E - I ( K A M - I ) and KAM-II. In o r d e r to i m p r o v e the signal-to-background ratio in the search for nucleon decay, the K A M - I [ 3 ] water (2erenkov detector was upgraded to K A M - I I . The re-
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Volume 220, number 1,2
PHYSICS LETTERSB
suits from KAM-I have been already reported [3,4 ] previously. The KAM-II detector [ 5 ] and the data reduction process [ 6 ] have also been described. The characteristics of the KAM-II detector which are relevant here are ( 1 ); the inner detector is a cylindrical steel tank containing 3000 t of water viewed by 948 twenty-inch-diameter photomultiplier tubes ( P M T ) covering 20% of the inner surface, and (2); a 4n anticounter, which is also a water Cerenkov counter viewed by the 123 PMT surrounding the inner detector. Any events which fire more than 20 PMT in the inner detector within 100 ns are stored on magnetic tapes. The present trigger threshold corresponds to 6.8 MeV/c for electrons and 163 MeV/c for muons at 50% efficiency. The charge and the timing of each PMT are read by a multi-buffer charge and time digitizing system [ 5 ]. The set of fully contained events which should include any nucleon decay candidates is selected according to the following criteria; (i) total number of photoelectrons (p.e.) of the inner detector being between 110 and 4500; (ii) maximum number ofp.e. per PMT being less than 200; (iii) (maximum number of p.e.) / (total number of p.e.) being less than 0.3; (iv) total number of p.e. of the anti-counter being less than 20, or hit multiplicity of the anti-counter PMT being less than 6; and (v) vertex position being in a fiducial volume. The p.e. range of criterion (i) corresponds to 30-1330 MeV/c for electrons and 205-1500 MeV/c for muons, and is efficient for nucleon decay events. Criteria (ii) and (iii) are useful for removing events with penetrating particles. The fiducial region for the present analysis is defined to be 1.5 m inside from the PMT planes and contains 1040 t of water. The vertex position of an event is determined mainly by using the Cerenkov photon arrival time on each PMT. The resolution of the vertex position for p--,e+n °, for example, is estimated to be less than 20 cm at 1a. We have observed 215 fully contained events which satisfy the above criteria during 2.46 kt yr of exposure of the KAM-II detector. Of these, 151 events have single Cerenkov rings and 64 have two or more Qerenkov rings. The number of Cerenkov rings in an event is counted by physicists using an interactive graphic display. The momentum of an electron or a muon is determined from the total p.e. number within a 70 ° half
30 March 1989
angle cone relative to the track direction with the origin at the vertex position. The estimated momentum resolutions for electrons and muons are 4%/ v / E and ~ 4%, respectively, where E is the electron energy in GeV. The momentum of a pion is determined both from the total p.e. number within the 70 o half angle cone and from the opening angle of the ~erenkov cone. The momentum resolution of pions is, in general, worse than that of electrons or muons due to nuclear interactions in the water. The use of the Cerenkov opening angle improves the momentum resolution at low pion momentum ( ~<400 MeV/ c). The absolute energy normalization is determined using the energy deposition of penetrating muons, of stopping muons versus the measured range (from entrance point to decay electron vertex position), of electrons from the decay of stopping muons and the invariant mass of n o produced by the neutral current interactions of atmospheric neutrinos in the detector. These four data sets, which cover the energy range from 30 MeV to 4 GeV, are used to fix the absolute energy scale of the Monte Carlo. The Monte Carlo reproduces the energy scale for each of these data sets within + 3%. The particle identification separates electrons and photons from muons, charged pions and protons using the spatial distribution of Cerenkov photons. Cerenkov rings produced by electrons or photons exhibit more multiple scattering than those produced by muons, pions or protons. The reconstructed opening angle of the Cerenkov cone and the timing information are supplementarily used. The probability of misidentifying the particle species of single-ring events was estimated to be 2% using Monte Carlo simulated atmospheric neutrino events [6]. However, the application of this method to multi-ring events is, in general, not so efficient as for single-ring events, because of overlapping Cerenkov rings. For example, a Monte Carlo study of the p--.e+n°(eyy) mode with three fully reconstructed rings shows that 40% of the "/are correctly determined to be electron or y-like, while 4% are misidentified and the remaining 56% are ambiguous in particle species due to overlapping of the rings. However, the identification of the electron rings, which are well isolated from the two y rings, is as good as for single-ring electron events. The particle identification is used in the selection of nucleon decay as described below. 309
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PHYSICS LETTERS B
Selection of nucleon decay candidates for each dec a y m o d e is d o n e u s i n g t o t a l p.e. n u m b e r , n u m b e r o f (2erenkov rings, number of observed la-decay signals, particle identification, momentum
imbalance, total
invariant mass, meson momentum
and meson mass.
The criteria for charged lepton+ meson neutrino+meson
modes are summarized
and antiin tables 1
30 March 1989
CUt i n t e r v a l s a r e d e t e r m i n e d
from the Monte Carlo
studies of the nucleon decay events. The selection criteria are quite similar to those adopted in KAM-I
[3,4], except for improvements
in kinematical reconstruction, momentum
i.e., c u t i n t e r v a l s o n
imbalance, total invariant mass and me-
son mass are narrower than in KAM-I for most of the
a n d 2, r e s p e c t i v e l y . W e r e f e r t o a n o n - i n t e r a c t i n g
d e c a y m o d e s . A l s o t h e p a r t i c l e i d e n t i f i c a t i o n is a p -
neutral, massless particle as an "anti-neutrino".
p l i e d f o r all ( ~ e r e n k o v r i n g s a f t e r s p a t i a l r e c o n s t r u c -
The
Table 1 Selection criteria for nucleon decay into charged lepton + mesons in KAM-II. Modes
Total a) p,e.
Number of (~erenkov rings
Number of It-decay
Particle b) identification
AP and &/totc) tight
e+n ° e+n(yr) (37t°) e+to(yn °) (3n) e+p ° e+K°(n+n - ) (2n °) e+K*°(3n °) ( n + ~ - n °) (x-It+v) la+n ° It +rl(y'/) (3n °) It+co(yn°) (3n) ~t+p° It +K° (n+rt - ) (2n °) e+n e÷p It+nIt+p-
2900-3500 2900-3500 2900-3500 2900-3500
1100-1700 2900-3500 2900-3500 200-800 2200-2900 2200-2900 2200-2900 2400-3000
400-900 2200-2900 1500-2500 800-1800 -
2,3 3 i>4 3,4 3-5 2,3 2,3 3-5 />4 3-5 3 2,3 3 >_-4 2,3 2-4 2,3 2,3 3-5 2 2-4 2 2-4
0 0 0 0 0, 1 0, 1 0, 1 0 0 0, 1 1 1 1 1 1 I, 2 1, 2 1, 2 1 0 0 1 1
SS(S) SSS SSS... SSS(S) SMS.. SM(M) SM(M) SSS.. SSS.. SMS.. SMM MS(S) MSS MSS.. SS(S) MS.. MM(M) MM(M) MSS.. SM S... MM M...
d)
Notes
loose
o o o o
90-180 480-620 450-650 700-870 600-1000 600-900 440-550 400-590 700-1000 600-1000
e) f) g) g)
e,i) f,j) g) g)
o
90-180 480-620 450-650 700-870 600-1000 600-900 440-550 400-590 600-900
o
600-900
f,g)
o o o o o o o o -
M .....
o -
o
o o o
e)
f) h) e)
f,g)
o
a ~Total p.e. corresponds to electron energy by the relation 3.4 p.e. = 1 MeV. h~ M = (p.- or x-like), and S = (e- or 7-like). See the text. c) For decay modes indicated by "tight", total invariant mass, M,o, and momentum imbalance, AP, must satisfy, 830-1040 MeV/c 2 and < 250 MeV/c, respectively. For decay modes indicated by "loose", M,o~ and AP must satisfy 800-1100 M e V / c 2 and < 350 MeV/c, respectively. d~ Requirement on meson mass (MeV/c2). ~ 7t° mass (90-180 M e V / c 2 ) is required for three-ring events (en °, Itn ° and ~tco(Tn °) ) or for four-ring events (eco(yn°) ). r) If77 are assigned to any of two rings, the n ° mass (90-180 M e V / c 2) is required. g) If the observed number of Qerenkov rings is 2 and the opening angle between the two rings is larger than 130 °, then one missing particle (it or x) is assumed, and the momentum of the missing particle is assigned to minimize eq. ( 1 ). In this case, Mtot and APare calculated including the missing particle. After this procedure the criteria listed in the table are imposed on the event with the assumed one invisible particle. h~ p~,= 215-255 MeV/c is additionally required. ~ Meson momentum, Pn...... < 250 M e V / c is additionally required. J~ Pm.... < 350 MeV/c is additionally required, 310
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PHYSICS LETTERS B
Table 2 Selection criteria for nucleon decay into anti-neutrino + mesons in KAM-II. Modes
Total p.e.
Number of ~erenkov rings
Number of p-decay
Particle a) identification
9n + 9p + 9K+(p+v) (n+n °) 9K*°(n°n°n + ) (n+n+n - ) (rc°p+v) 9no 9q(~ 0 (3n °) 9to(Tn °) (3n) 9p0 9K°(~+n - ) (2n °) 9K*° (3n °) ( n + n - n °) (n-p+v)
1060-2230 740-1040 1660-2440 110-580 920-1560 1330-2000 1850-2510 1850-2510 2450-3010 680-1440 110-990 110-510 1740-2430 2570-3460 860-1660 130-470
1 2,3 1 2,3 3-5 2,3 2,3 2 2 /> 3 2,3 2-4 2 2 3,4 I> 3 2-4 2
0, 1 0, 1 1
M MS(S) M SS(M) MSS.. MM(M) MS(S) SS SS SSS.. SS(S) MS.. MM MM SSS(S) SSS.. MS.. MM
0, 1 0-2 1 0 0 0 0 0, 1 0, 1 0, 1 0 0 0, 1 1
Mmesonb)
P.....
600-900 440-550 600-1000 600-1000 90-180 480-620 450-650 700-870 600-1000 600-900 440-550 400-590 700-1000 600-1000 -
340-580 40-400 0-50 0-400 0-400 340-580 200-430 200-430 60-300 0-400 40-400 240-450 240-450 0-300 0-400 -
c
~
Notes
d) e) d,f)
d,g)
h) d)
d) g)
a) M = (p- or n-like), and S = (e- or ),-like). See the text. b~ Requirement on meson mass (MeV/c2). "Meson mass" corresponds to the total invariant mass of an event in these modes. ) Requirement on meson momentum (MeV/c). "Meson momentum" corresponds to the total momentum imbalance of an event in these modes. d) If T)' are assigned to any of two rings, then a n ° mass (90-180 MeV/c 2) is required. e~ pp = 215-255 MeV/c is additionally required. r) For two-ring events, M~ = 90-180 MeV/c 2 and 1 p decay are required and M . . . . . and Pme~onare not required. For three-ring events, the requirement on p decay is 0 or 1. s> Po = 215-255 MeV/c and 0~, > 90 ° are additionally required. ") n ° mass (90-180 MeV/c 2) is required for three-ring events.
t i o n . T h e p a r t i c l e t y p e o f e a c h ring, i f successfully identified, must not be incompatible with those s h o w n in t h e f i f t h c o l u m n o f t a b l e s 1 a n d 2. I f p a r t i c l e s p e c i e s is n o t d e t e r m i n e d u n i q u e l y f o r a n y ring, t h e n all p o s s i b l e p a r t i c l e s p e c i e s for t h e r i n g a r e t r i e d t o test t h e v a r i o u s k i n e m a t i c a l q u a n t i t i e s s u c h as m o mentum imbalance. T h e s e l e c t i o n c r i t e r i a f o r t h e d e c a y m o d e , p ~ K +, n e e d special e x p l a n a t i o n , b e c a u s e m o s t o f t h e K ÷ stop before they decay and only the decay products ofK ÷ are o b s e r v e d . F o r e x a m p l e , t h e Ix+ f r o m K + - p + v h a s a monochromatic momentum of 236 MeV/c. Therefore a n a c c u m u l a t i o n o f m u o n - l i k e s i n g l e - r i n g e v e n t s is e x p e c t e d at t h i s m o m e n t u m . Fig. 1 s h o w s t h e m o m e n t u m d i s t r i b u t i o n for m u o n - l i k e single-ring e v e n t s w h i c h h a v e o n e p - d e c a y signal. B o t h K A M - I a n d KAM-II data are combined and shown by histog r a m s . N o s i g n i f i c a n t a c c u m u l a t i o n o f real e v e n t s is
1
,~ 6 ~ 5 t~ ~ 4
T
T
>
= 3 09 ,,>, 2 o _=e 0
200
300
P(MeV/c)
400
Fig. 1. The momentum distribution for muon-like single-ring events associated with one ~t-decay signal. The expected background distribution is normalized to the total observed number of muon-like events, cut interval of nucleon decay search and the expected signal for z/B = 0.74 × 1032yr which corresponds to the 90% CL lower limit for this channel are also shown. 311
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PHYSICS LETTERS B
seen in the cut interval. The final limit on the nucleon partial lifetime for the p-+gK + mode, 1.0X 103:yr, is obtained by adding the contribution from the channel K + - ) n + n °, which has no significant background. Three kinds o f data sets, the real data, the simulated nucleon decay events and the simulated atmospheric neutrino interactions passed through the same selection criteria in order to obtain the n u m b e r o f nucleon decay candidates, the detection efficiency and the neutrino background rate. In calculating the detection efficiency, various nuclear effects such as the Fermi motion of nucleons in oxygen nuclei and the subsequent nuclear interactions of various mesons in oxygen nuclei and in water are considered, as detailed in ref. [7]. It should be noted that, in a water detector, 20% o f the proton decay are from free protons and are free from these nuclear effects in the nuclei. A dominant systematic error in the detection efficiency, which depends on the produced mesons and their m o m e n t u m , comes from these nuclear effects in oxygen nuclei. For pions, a systematic error of _+30% is estimated for the cross section in the nuclei. This error translates to an uncertainty of the detection efficiency of typically + 10% o f the efficiency quoted (p-~e+n °) or maximally _+35% (n-~gp°). For K mesons, on the other hand, the uncertainty is estimated to be + 1%, because o f the small interaction probability of 3% in oxygen nuclei. There is also a statistical error in the detection efficiency coming from a number of analyzed nucleon decay events. For each decay mode the detection efficiency is estimated using 30-140 simulated events. Therefore the overall uncertainty in the efficiency ranges from _+4% (3.7%; statistical fluctuation o f the number of events which satisfy the criteria, and 1%; systematic error) in the quoted efficiency for p ~ g K + ( I x + v ) to + 5 0 % (35%; statistical, and 35%; systematic) for n ~ g p °. Simulated events of atmospheric neutrino interactions in the detector are generated based on the calculated neutrino flux [ 8 ] and on the existing data on neutrino interaction cross sections. The simulation includes charged-current elastic scattering, chargedcurrent single- and multipion production and neutral-current single- and multipion production processes. The program was checked by comparing the distribution o f several kinematical quantities of the 312
30 March 1989
simulated events with those from data from accelerator neutrino experiments, and good agreement was obtained. The simulated neutrino data are also compared with the KAM-I and II data. Various (ve/v, independent) quantities such as the total visible energy distribution or the number of Cerenkov ring distributions are reasonably reproduced. For more details, see ref. [ 7 ]. As an illustration of how the various kinematical quantities are distributed, we show in fig. 2 the mom e n t u m imbalance, AP, versus total invariant mass, Mtot, plot of multi-ring events for (a) the data, (b) the simulated neutrino interactions ~3 and (c) the stimulated nucleon decay p ~ e + n °. These kinematical quantities are not always determined uniquely due to the ambiguity in the particle identification. In such a case, a point which gives the m i n i m u m o f (M, ot - 9 3 8 M e V / c 2) 2 + ( A P - 150 M e V / c ) 2 . 0 ( A P - 150 M e V / c ) ,
( 1)
where 0 is the step function, is selected and plotted for (a) and (b). The last factor, 0 ( A P - 150 M e V / c), is included considering the Fermi motion distribution o f nucleons and AP and Mto, cut defined in table 1. It is seen that the distribution of the data is similar to that of the simulated neutrino interactions. Fig. 3 shows the number of surviving events per kt yr for each step of the reduction for both the data and the simulated atmospheric neutrino events of 105 kt yr equivalent. The decay modes considered in this figure are p-*e+n °, e÷q(yy, n°n°n°), e+KO(nOnO), e+(o(Yx°), Ix +n°, IX+)](TT, n°n°n°) and ~t+K°(n°n°). This figure shows that the background estimation is done reasonably well. Possible systematic errors in the background estimation include the uncertainties in the absolute atmospheric neutrino fluxes at ~ l GeV ( + 2 0 % ) , in ))3
One may argue, looking at fig. 2b, that the background already dominates at 11.4 kt yr. We however note that the events in the allowed region in AP and Mto~are further subject to various selections such as meson mass, number of It decay, etc. They reduce the number of remaining events approximately by a factor of 2. The remaining events are backgrounds of e+p°, e+p-, e+o~(n+non-), e+K.O(n+non-), It+pO and lt+p -. It should also be noted that the decay modes which have high background rates (/> 1 event per 11.4 kt yr) involvep, to or K*. For decay modes involving n or K, the background rates are much lower. See tables 3 and 4.
Volume 220, number 1,2
PHYSICS LETTERS B
absolute neutrino cross sections for the final states with multi-t~erenkov rings ( ~ _+20%) and in the cross sections of pion interactions in the oxygen nu1500
I
30 March 1989
o Neutrino M.C. • Data
%
10 2 -
I
10 ~
(a)
o~
co
o o
n
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\ffl 1 ..tr-Oo o
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o
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o
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o'~ .°8 ..o,, o
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o",• • a• o,~o,.
<~ 5 o o 1 - ~ , ~ _ . ~ , : , . ° ° |o
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% ;0 o,
o
~.- ~
o
°
1000
>
(b) o
o
: I°
•
Io~ t i
'
'
(c)
Fig. 3. N u m b e r of neutrino events per kt yr for each step of the nucleon decay selection: ( 1 ) Fully contained event (see the text). Total p.e. cut; 2200-2900 for one ~t decay. 2900-3500 for zero I1 decay. (3) N u m b e r o f ~ e r e n k o v rings >/2. (4) Particle identification cut. (5) A P a n d M, ot cut. (6) Meson mass cut. The details of the selection criteria are shown in table 1. Combined data from both KAM-I and -II are also shown for the criteria ( 1 ) - ( 3 ) , because there is no serious quantitative difference between the two data samples as far as the criteria ( 1 ) - ( 3 ) are concerned. After the fourth criterion, only the KAM-II data are used.
¢J
>\
1000
~)
n
<~ 500
o OOo
o
,
0
500
1000
M T o T ( M e V / c ~)
1500
Fig. 2. AP versus M~o, of multi-ring events after reconstruction for (a) the KAM-II data, (b) the simulated neutrino interactions of I 1.4 kt yr exposure equivalent and (c) the simulated nucleon decay into e+n °. o and • represent events which have no and at least one observed la decay, respectively. In plotting the points in (c), various nuclear effects such as the Fermi motion of nucleons, and pion nuclear interactions in the oxygen nucleus are included. Events shown in (c) are after the application of the criteria of the total number of p.e., number of ~erenkov ring, number o f ~ decay and particle identity cuts. Therefore, in (c), most of the events whose n ° undergo nuclear interaction are removed before plotting the points. The broken and solid lines show the cut interval on m o m e n t u m imbalance and total invariant mass, see footnote c of table 1.
313
Volume 220, number 1,2
PHYSICS LETTERS B
cleus ( ~ + 3 0 % ) . T h e + 3 0 % u n c e r t a i n t y i n t h e p i o n cross section changes the number of multi-ring neut r i n o e v e n t s b y + 10%. F r o m t h e s e c o n s i d e r a t i o n s we estimate the systematic uncertainty in the backg r o u n d e s t i m a t i o n to b e ~ + 30%. T h e c o r r e s p o n d i n g u n c e r t a i n t y f o r p ~ g n + a n d p - , g K ° ( p . + v ) is est i m a t e d t o b e + 10%. T h i s is b e c a u s e t h e b a c k g r o u n d r a t e is n o r m a l i z e d t o t h e t o t a l n u m b e r o f s i n g l e - r i n g m u o n - l i k e e v e n t s . ~4. T h e r e is also a n e r r o r i n t h e
~4 Because of a deficit of the number of single-ring muon-like events relative to the Monte Carlo calculation (0.60 _+0.06, ref. [6 ] ), the estimated background numbers presented in fig. 1 and table 4 for 9K ÷ (It+v) and 9n + are >(0.6 of the calculated background without any deficit.
30 March 1989
background estimation coming from the statistical fluctuation of the number of analyzed neutrino e v e n t s . B e t w e e n 0 - 1 8 s i m u l a t e d n e u t r i n o e v e n t s survive within the nucleon decay criteria for the decay m o d e s w i t h m u l t i - r i n g f i n a l states, t h u s t h e s t a t i s t i c a l error of the estimated number of neutrino backg r o u n d r a n g e s f r o m + 2 4 % to > + 100% d e p e n d i n g on the decay mode, Tables 3 and 4 summarize the detection efficiency m u l t i p l i e d b y t h e m e s o n b r a n c h i n g r a t i o , e'Bn, t h e number of candidate events and the number of the e x p e c t e d n e u t r i n o b a c k g r o u n d e v e n t s ~4 i n K A M - I I . T h e c o r r e s p o n d i n g n u m b e r s in K A M - I are also s h o w n i n t h e s a m e t a b l e s . B e c a u s e o f t h e g o o d e n e r g y resol u t i o n o f e, 7 a n d p, t h e e x p e c t e d b a c k g r o u n d s a r e
Table 3 Results of the nucleon decay search for charged lepton+ meson modes. Detection efficiency multiplied by the meson branching ratio e.Bm, number of observed candidates and number of expected neutrino backgrounds in KAM-II and KAM-I are shown in the second to fourth and the fifth to seventh columns, respectively. The background subtracted limits on "riBat 90% CL are shown in the eighth column. Both the KAM-I and -II data are combined in deriving these numbers. Background unsubtracted limits are also shown in the last column. Modes
KAM-II (2.46 kt yr)
~'Bm e+n ° e+q(~) (3n °) e+co(yn°) (3n) e+p ° e+K°(n+n - ) (2n °) e+K*°(3n °)
(n+n-n °) (n-it+v)
It+n° It+'q(~7) (3n °) t.t+co(yn °) (3n) It+pO It+K°(n+n - ) (2n °) e+n e+p It+nIt+p-
0.45 0.14 0.10 0.03 0.08 0.24 0.14 0.11 0.01 0.03 0.09 0.43 0.13 0.08 0.04 0.09 0.22 0.20 0.10 0.28 0.15 0.23 0.06
number of event
b a, c
b
KAM-I ( 1.30 kt yr)
v BG
e'Bm
<0.02 a) <0.02 a) <0.02 a) <0.02 a) 0.5 1.8 0.1 <0.02 ~) <0.02 a) 0.5 0.2 <0.02 a) <0.02 a) <0.02 ~) <0.1 0.7 1.0 0.1 0.02 ~ <0.1 1.2 <0.1 0.9
0.53 0.17 0.11 0.04 0.17 0.18 0.18 0.13 0.02 0.04 0.08 0.40 0.16 0.06 0.04 0.17 0.18 0.20 0.11 0.32 0.10 0.25 0.08
number of event
B
B
A
C, E D
C
KAM-I + II(3.76 kt yr) T/B limit [yr] v BG
BG subtracted
BG unsubtracted
<0.02 <0.02 0.05 0.05 0.9 0.9 <0.1 0.05 0.05 0.7 0.1 0.05 <0.02 0.02 <0.1 1.2 0.7 0.3 0.02 0.1 0.7 <0.1 0.9
26 Xl031 14 Xl031
26 Xl031 14 X1031
4.5)< 1031
3.4X 1031
7.5X 1031 15 )<103j
5.2)< 103j 15 )<1031
5.2)< 1031
3.1 x 103j
23 )<1031 6.9×1031
23 )<1031 6.9>(103t
5.7)< 10 31
3 . 7 X 1031
11 )<103j 12 ×1031
11 )<1031 9.8>(1031
13 )<10 31 5.8X 103t 10 x l 0 3! 2.3)< 1031
13 X I 0 31 5.8X 103t 10 )<10 31 1.7X 103n
a) Neutrino background rates are estimated from Monte Carlo events equivalent to 105 kt yr for the decay modes denoted by (a) and to 22 kt yr for the other decay modes (KAM-II). 314
30 March 1989
PHYSICS LETTERS B
Volume 220, number 1,2 Table 4 Same as table 3 for anti-neutrino + meson modes. Modes
97t+ 9p + 9K+ (p.+v) (n+n °) 9K *+ (nonon+) (n+TC+~ -
(n%+v) Vffo
vq(~,7) (3n °) 9o)( 7n ° ) (3n) 9p° 9K°(n+n - ) (2n °) 9K*° (3nO)
KAM-II (2.46 kt yr)
e'Bm
number of events
v BG
0.24 0.15 0.51 0.10 0.03 0.02 0.02 0.28 0.15 0.10 0.03
22 1 5
23.5 0.5 4.5 0.5 0.3 0.5 0.2 2.0 0.2 0.3 <0.1 0.6 1.5 0.2 0.6 0.1 0.6 0.1
1
1 1 1
0.11
1
0.06 0.07 0.10 0.01
2
(n+n-n°)
0.01
1
(n-~t+v)
0.06
KAM-I + II (3.76 ktyr) z/Blimit [yr]
KAM-I ( 1.30 kt yr)
/~' B m
number of events
v BG
BG subtracted
BG unsubtracted
0.14 0.16 0.41 0.10 0.07 0.03 0.05 0.31 0.15 0.08 0.05 0.21 0.08 0.12 0.13 0.02 0.03 0.06
10 4 4
9.3 1.0 1.9 0.4 1.3 0.3 0.4 1.0 0.1 0.3 <0.1 2.1 2.1 0.9 0.7 0.1 1.6 0.1
2.5X 1031 2.7X 1031 10 X1031
0.6X 1031 2.1 X 1031 5.1XI03t
2.0X 1031
1.3)< 1031
10 X1031 5.4X 1031
7.5X 103t 4.6X 1031
small for the d e c a y m o d e s which i n v o l v e only these particles in the final state. O n the o t h e r h a n d , the search is l i m i t e d for s o m e o f the d e c a y m o d e s w h i c h i n v o l v e charged p i o n s in the final state. T h e r e are several c a n d i d a t e events, w h i c h h o w e v e r are, in m o s t cases, c o n s i s t e n t with those e x p e c t e d f r o m the neut r i n o b a c k g r o u n d ~5. F r o m these results, we c o n c l u d e that we h a v e n o t o b s e r v e d e v i d e n c e for n u c l e o n d e c a y in K A M - I I . T h e r e f o r e we calculate l o w e r l i m i t s o n the n u c l e o n partial lifetime, z/B w h e r e B is the b r a n c h i n g ratio o f n u c l e o n decay for the r e l e v a n t d e c a y m o d e , at 98% C L c o m b i n i n g K A M - I a n d -II d a t a a n d subtracting the a t m o s p h e r i c n e u t r i n o b a c k g r o u n d . T h e 90% C L u p p e r l i m i t for the n u m b e r o f n u c l e o n d e c a y candidates is c a l c u l a t e d by integrating the l i k e l i h o o d funct i o n to the 90 0Y0p r o b a b i l i t y level:
~5 Event A, which is a candidate of p--.~t+Tl(yr), observed in KAM-I might be a possible exception, because the expected background is ~<0.02. However, the observed momentum imbalance, 396 _+71 MeV/c, is too large to conclude that this single event is evidence for nucleon decay.
2 2
2 2
2 1
4.3X
1 0 31
2.7×
1 0 31
1.3X 103~ 8.6× 1031
0.8X 1031 8.6× 1031
2.1X103t
1.1XI031
Aqimit
+ y;~,[eB,.(j).r(j)l x
dx
+ ~,=l[eBm(j).T(j)]x
)1 dx }-I
=0.90, w h e r e X, mit is the n u m b e r to be o b t a i n e d (i.e, the upper l i m i t for the n u m b e r o f n u c l e o n d e c a y candid a t e s ) , n is the n u m b e r o f i n d e p e n d e n t o b s e r v a t i o n s (for e x a m p l e , n = 4 for the p - - . g K + m o d e , because there are two K + d e c a y m o d e s , ~t+v a n d n + n °, a n d two e x p e r i m e n t a l phases, K A M - I a n d - I I ) , P(N, x) is the p r o b a b i l i t y f u n c t i o n o f the P o i s s o n statistics, Nobs(i) is the n u m b e r o f o b s e r v e d c a n d i d a t e s in the Rh o b s e r v a t i o n , XB~(i) the n u m b e r o f n e u t r i n o b a c k g r o u n d e v e n t s for the ith o b s e r v a t i o n a n d T ( i ) 315
Volume 220, number 1,2
PHYSICS LETTERS B
the detector exposure in the ith observation (kt yr). T h e n the r/B limit is o b t a i n e d by
~ >
!
[eBm(J')'T(j)]
NN,
where NN is the n u m b e r o f protons or neutrons in 1 kt o f water. The detector exposure is 3.76 kt yr in total. These results are shown in the last two columns o f tables 3 a n d 4 for charged lepton + meson and for anti-neutrino + meson decay modes, respectively. The b a c k g r o u n d subtracted limits on v/B range from 0 . 1 × 103~ to 2.6)< 1032yr d e p e n d i n g on the decay modes. The present limits are the most stringent ones for most o f the decay m o d e s [ 9,10 ] ~6. F o r the decay m o d e p~e+Tt °, the limit on z/B is 2.6X 1032yr. This result should be compared with 4X 1029+-°-7(Mx/2X10J4GeV)4yr, M x = t 2n +.2v"_ll~. o j X 1014 G e V which is a theoretically predicted value [ 11 ] based on the m i n i m a l S U ( 5 ) m o d e l [ 12]. On the other hand, s u p e r s y m m e t r i c G U T models [ 13 ] predict nucleon decay into anti-neutrino + K - m e s o n s to be d o m i n a n t ones [ 14 ]. T h e r e is no evidence for nucleon decay for these decay modes. N o c a n d i d a t e events have been observed for 9K+(Tt+~°), 9 K ° ( n + n - ), a n d 9 K ° ( v ° n ° ) . The n u m b e r o f candidate events for 9 K + (Ix+v) is well explained by the a t m o s p h e r i c neutrino background. The limits on 3/ B are 1.0× 1032yr and 0.9X 1032yr for p--,gK + a n d n ~ g K °, respectively. These n u m b e r s constraint sup e r s y m m e t r i c G U T models [ 15 ]. We acknowledge the generous cooperation o f the K a m i o k a Mining a n d Smelting Co. This work was s u p p o r t e d by the Japanese Ministry o f Education, Science and Culture, by the U n i t e d States D e p a r t ~6 See also Frejus collaboration, results summarized in ref. [2].
316
30 March 1989
ment o f Energy, and by the U n i v e r s i t y o f Pennsylvania Research Fund. A part o f this work was done using the F A C O M M 7 8 0 / M 3 8 0 computer system o f the Institute for N u c l e a r Study, University o f Tokyo.
References [ 1] P. Langacker, Phys. Rep. 72 ( 1981 ) 185. [ 2 ] Y. Totsuka, Proc. 1985 Intern. Symp. on Lepton and photon interactions at high energies (Kyoto, Japan, August 1985) p. 120; R. Barloutaud, Proc. Eighth Workshop on Grand unification (Syracuse University, New York, April 1987 ) p. 5; preprint DPhPE 88-15 (August 1988). [3] K. Arisaka et al., J. Phys. Soc. Jpn. 54 (1985) 3213. [4 ] T. Kajita et al., J. Phys. Soc. Jpn. 55 ( 1986 ) 711. [5] K. Hirata et al., Phys. Rev. Lett. 58 (1987) 1490; K.S. Hirata et al., Phys. Rev. D 38 (1988) 448. [6] K.S. Hirata et al., Phys. Lett. B 205 (1988) 416. [7] M. Nakahata et al., J. Phys. Soc. Jpn. 55 (1986) 3786. [ 8 ] T.K. Gaisser, T. Stanev and G. Barr, Phys. Rev. D 38 ( 1988 ) 85; T.K. Gaisser, private communication. [9] T.J. Haines et al., Phys. Rev. Lett. 57 (1986) 1986; S. Seidel et al., Phys. Rev. Lett. 61 (1988) 2522. [ 10 ] Frejus Collab., Ch. Berger et al., Nucl. Phys. B 313 (1989) 509. [11] W.J. Marciano, Proc. Eighth Workshop on Grand unification (Syracuse University, New York, April 1987) p. 185; P. Langacker, DESY preprint DESY 88-076 (June 1988 ). [12] H. Georgi and S.L Glashow, Phys. Rev. Lett. 32 (1974) 438. [ 13 ] S. Dimopoulos, S. Raby and F. Wilczek, Phys. Rev. D 24 (1981) 1681; S. Dimopoulos and H. Georgi, Nucl. Phys. B 193 (1981) 150; N. Sakai, Z. Phys. C 11 (1981) 153. [ 14] N. Sakai and T. Yanagida, Nucl. Phys. B 197 (1982) 533; S. Weinberg, Phys. Rev. D 26 (1982) 287. [ 15 ] P. Nath and R. Arnowin, Phys. Rev. D 38 (1988) 1479.
PHYSICS LETTERS B
30 March 1989
EXPERIMENTAL LIMITS ON NUCLEON LIFETIME FOR L E P T O N + M E S O N DECAY M O D E S K.S. H I R A T A , T. K A J I T A , T. K I F U N E , K. K I H A R A , M. N A K A H A T A , K. N A K A M U R A , S. O H A R A , Y. O Y A M A , N. SATO, M. T A K I T A , Y. T O T S U K A , Y. Y A G I N U M A Institute for Cosmic Ray Research, University of Tokyo, Tokyo 188, Japan M. M O R I , A. S U Z U K I , K. T A K A H A S H I , T. T A N I M O R I , M. Y A M A D A l National Laboratory for High Energy Physics (KEK), lbaraki 305, Japan
M. K O S H I B A Tokai University, Tokyo 151, Japan T. S U D A Department of Physics, Kobe University, Hyogo 657, Japan K. M I Y A N O , H. MIYATA, H. T A K E I Niigata University, Niigata 950-21, Japan K. K A N E Y U K I , Y. N A G A S H I M A , Y. S U Z U K I Department of Physics, Osaka University, Osaka 560, Japan E.W. BEIER, L.R. F E L D S C H E R , E.D. F R A N K , W. F R A T I , S.B. KIM, A.K. M A N N , F.M. N E W C O M E R , R. VAN B E R G a n d W. Z H A N G Department of Physics, University of Pennsylvania, Philadelphia, PA 19104, USA Received 13 January 1989
We have searched for nucleon decay signals using data from the KAMIOKANDE-II detector. No evidence for nucleon decay has been found. Limits on the nucleon partial lifetime for various decay modes are obtained combining KAMIOKANDE-I and -II data (3.76 kt yr in total). The background subtracted limits at 90% CL range from 0.1 × 1032yrto 2.6 × 1032yrdepending on the decay modes. For the decay modes p--,e+n°, p~gK + and n--,gK°, the limits are 2.6>( 1032yr, 1.0>( 1032yr and 0.9)< 1032yr, respectively.
The p r e d i c t i o n o f nucleon decay by grand unified theories ~ stimulated the current experimental search for nucleon decay by d e d i c a t e d u n d e r g r o u n d detectors. After several years o f experimental searches ~2, a clear nucleon decay signal has not been observed, On leave from Niigata University, Niigata 950-21, Japan. ~ See ref. [1] for a review, ,2 See ref. [ 2 ] for recent reviews. 308
although there are several c a n d i d a t e events which are not easily explained by a t m o s p h e r i c neutrino interactions. This p a p e r reports the search for nucleon decay using the K A M I O K A N D E - I I ( K A M - I I ) detector a n d presents the c o m b i n e d results from K A M I O K A N D E - I ( K A M - I ) and KAM-II. In o r d e r to i m p r o v e the signal-to-background ratio in the search for nucleon decay, the K A M - I [ 3 ] water (2erenkov detector was upgraded to K A M - I I . The re-
0370-2693/89/$ 03.50 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing D i v i s i o n )
Volume 220, number 1,2
PHYSICS LETTERSB
suits from KAM-I have been already reported [3,4 ] previously. The KAM-II detector [ 5 ] and the data reduction process [ 6 ] have also been described. The characteristics of the KAM-II detector which are relevant here are ( 1 ); the inner detector is a cylindrical steel tank containing 3000 t of water viewed by 948 twenty-inch-diameter photomultiplier tubes ( P M T ) covering 20% of the inner surface, and (2); a 4n anticounter, which is also a water Cerenkov counter viewed by the 123 PMT surrounding the inner detector. Any events which fire more than 20 PMT in the inner detector within 100 ns are stored on magnetic tapes. The present trigger threshold corresponds to 6.8 MeV/c for electrons and 163 MeV/c for muons at 50% efficiency. The charge and the timing of each PMT are read by a multi-buffer charge and time digitizing system [ 5 ]. The set of fully contained events which should include any nucleon decay candidates is selected according to the following criteria; (i) total number of photoelectrons (p.e.) of the inner detector being between 110 and 4500; (ii) maximum number ofp.e. per PMT being less than 200; (iii) (maximum number of p.e.) / (total number of p.e.) being less than 0.3; (iv) total number of p.e. of the anti-counter being less than 20, or hit multiplicity of the anti-counter PMT being less than 6; and (v) vertex position being in a fiducial volume. The p.e. range of criterion (i) corresponds to 30-1330 MeV/c for electrons and 205-1500 MeV/c for muons, and is efficient for nucleon decay events. Criteria (ii) and (iii) are useful for removing events with penetrating particles. The fiducial region for the present analysis is defined to be 1.5 m inside from the PMT planes and contains 1040 t of water. The vertex position of an event is determined mainly by using the Cerenkov photon arrival time on each PMT. The resolution of the vertex position for p--,e+n °, for example, is estimated to be less than 20 cm at 1a. We have observed 215 fully contained events which satisfy the above criteria during 2.46 kt yr of exposure of the KAM-II detector. Of these, 151 events have single Cerenkov rings and 64 have two or more Qerenkov rings. The number of Cerenkov rings in an event is counted by physicists using an interactive graphic display. The momentum of an electron or a muon is determined from the total p.e. number within a 70 ° half
30 March 1989
angle cone relative to the track direction with the origin at the vertex position. The estimated momentum resolutions for electrons and muons are 4%/ v / E and ~ 4%, respectively, where E is the electron energy in GeV. The momentum of a pion is determined both from the total p.e. number within the 70 o half angle cone and from the opening angle of the ~erenkov cone. The momentum resolution of pions is, in general, worse than that of electrons or muons due to nuclear interactions in the water. The use of the Cerenkov opening angle improves the momentum resolution at low pion momentum ( ~<400 MeV/ c). The absolute energy normalization is determined using the energy deposition of penetrating muons, of stopping muons versus the measured range (from entrance point to decay electron vertex position), of electrons from the decay of stopping muons and the invariant mass of n o produced by the neutral current interactions of atmospheric neutrinos in the detector. These four data sets, which cover the energy range from 30 MeV to 4 GeV, are used to fix the absolute energy scale of the Monte Carlo. The Monte Carlo reproduces the energy scale for each of these data sets within + 3%. The particle identification separates electrons and photons from muons, charged pions and protons using the spatial distribution of Cerenkov photons. Cerenkov rings produced by electrons or photons exhibit more multiple scattering than those produced by muons, pions or protons. The reconstructed opening angle of the Cerenkov cone and the timing information are supplementarily used. The probability of misidentifying the particle species of single-ring events was estimated to be 2% using Monte Carlo simulated atmospheric neutrino events [6]. However, the application of this method to multi-ring events is, in general, not so efficient as for single-ring events, because of overlapping Cerenkov rings. For example, a Monte Carlo study of the p--.e+n°(eyy) mode with three fully reconstructed rings shows that 40% of the "/are correctly determined to be electron or y-like, while 4% are misidentified and the remaining 56% are ambiguous in particle species due to overlapping of the rings. However, the identification of the electron rings, which are well isolated from the two y rings, is as good as for single-ring electron events. The particle identification is used in the selection of nucleon decay as described below. 309
Volume 220, number 1,2
PHYSICS LETTERS B
Selection of nucleon decay candidates for each dec a y m o d e is d o n e u s i n g t o t a l p.e. n u m b e r , n u m b e r o f (2erenkov rings, number of observed la-decay signals, particle identification, momentum
imbalance, total
invariant mass, meson momentum
and meson mass.
The criteria for charged lepton+ meson neutrino+meson
modes are summarized
and antiin tables 1
30 March 1989
CUt i n t e r v a l s a r e d e t e r m i n e d
from the Monte Carlo
studies of the nucleon decay events. The selection criteria are quite similar to those adopted in KAM-I
[3,4], except for improvements
in kinematical reconstruction, momentum
i.e., c u t i n t e r v a l s o n
imbalance, total invariant mass and me-
son mass are narrower than in KAM-I for most of the
a n d 2, r e s p e c t i v e l y . W e r e f e r t o a n o n - i n t e r a c t i n g
d e c a y m o d e s . A l s o t h e p a r t i c l e i d e n t i f i c a t i o n is a p -
neutral, massless particle as an "anti-neutrino".
p l i e d f o r all ( ~ e r e n k o v r i n g s a f t e r s p a t i a l r e c o n s t r u c -
The
Table 1 Selection criteria for nucleon decay into charged lepton + mesons in KAM-II. Modes
Total a) p,e.
Number of (~erenkov rings
Number of It-decay
Particle b) identification
AP and &/totc) tight
e+n ° e+n(yr) (37t°) e+to(yn °) (3n) e+p ° e+K°(n+n - ) (2n °) e+K*°(3n °) ( n + ~ - n °) (x-It+v) la+n ° It +rl(y'/) (3n °) It+co(yn°) (3n) ~t+p° It +K° (n+rt - ) (2n °) e+n e÷p It+nIt+p-
2900-3500 2900-3500 2900-3500 2900-3500
1100-1700 2900-3500 2900-3500 200-800 2200-2900 2200-2900 2200-2900 2400-3000
400-900 2200-2900 1500-2500 800-1800 -
2,3 3 i>4 3,4 3-5 2,3 2,3 3-5 />4 3-5 3 2,3 3 >_-4 2,3 2-4 2,3 2,3 3-5 2 2-4 2 2-4
0 0 0 0 0, 1 0, 1 0, 1 0 0 0, 1 1 1 1 1 1 I, 2 1, 2 1, 2 1 0 0 1 1
SS(S) SSS SSS... SSS(S) SMS.. SM(M) SM(M) SSS.. SSS.. SMS.. SMM MS(S) MSS MSS.. SS(S) MS.. MM(M) MM(M) MSS.. SM S... MM M...
d)
Notes
loose
o o o o
90-180 480-620 450-650 700-870 600-1000 600-900 440-550 400-590 700-1000 600-1000
e) f) g) g)
e,i) f,j) g) g)
o
90-180 480-620 450-650 700-870 600-1000 600-900 440-550 400-590 600-900
o
600-900
f,g)
o o o o o o o o -
M .....
o -
o
o o o
e)
f) h) e)
f,g)
o
a ~Total p.e. corresponds to electron energy by the relation 3.4 p.e. = 1 MeV. h~ M = (p.- or x-like), and S = (e- or 7-like). See the text. c) For decay modes indicated by "tight", total invariant mass, M,o, and momentum imbalance, AP, must satisfy, 830-1040 MeV/c 2 and < 250 MeV/c, respectively. For decay modes indicated by "loose", M,o~ and AP must satisfy 800-1100 M e V / c 2 and < 350 MeV/c, respectively. d~ Requirement on meson mass (MeV/c2). ~ 7t° mass (90-180 M e V / c 2 ) is required for three-ring events (en °, Itn ° and ~tco(Tn °) ) or for four-ring events (eco(yn°) ). r) If77 are assigned to any of two rings, the n ° mass (90-180 M e V / c 2) is required. g) If the observed number of Qerenkov rings is 2 and the opening angle between the two rings is larger than 130 °, then one missing particle (it or x) is assumed, and the momentum of the missing particle is assigned to minimize eq. ( 1 ). In this case, Mtot and APare calculated including the missing particle. After this procedure the criteria listed in the table are imposed on the event with the assumed one invisible particle. h~ p~,= 215-255 MeV/c is additionally required. ~ Meson momentum, Pn...... < 250 M e V / c is additionally required. J~ Pm.... < 350 MeV/c is additionally required, 310
Volume 220, number 1,2
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PHYSICS LETTERS B
Table 2 Selection criteria for nucleon decay into anti-neutrino + mesons in KAM-II. Modes
Total p.e.
Number of ~erenkov rings
Number of p-decay
Particle a) identification
9n + 9p + 9K+(p+v) (n+n °) 9K*°(n°n°n + ) (n+n+n - ) (rc°p+v) 9no 9q(~ 0 (3n °) 9to(Tn °) (3n) 9p0 9K°(~+n - ) (2n °) 9K*° (3n °) ( n + n - n °) (n-p+v)
1060-2230 740-1040 1660-2440 110-580 920-1560 1330-2000 1850-2510 1850-2510 2450-3010 680-1440 110-990 110-510 1740-2430 2570-3460 860-1660 130-470
1 2,3 1 2,3 3-5 2,3 2,3 2 2 /> 3 2,3 2-4 2 2 3,4 I> 3 2-4 2
0, 1 0, 1 1
M MS(S) M SS(M) MSS.. MM(M) MS(S) SS SS SSS.. SS(S) MS.. MM MM SSS(S) SSS.. MS.. MM
0, 1 0-2 1 0 0 0 0 0, 1 0, 1 0, 1 0 0 0, 1 1
Mmesonb)
P.....
600-900 440-550 600-1000 600-1000 90-180 480-620 450-650 700-870 600-1000 600-900 440-550 400-590 700-1000 600-1000 -
340-580 40-400 0-50 0-400 0-400 340-580 200-430 200-430 60-300 0-400 40-400 240-450 240-450 0-300 0-400 -
c
~
Notes
d) e) d,f)
d,g)
h) d)
d) g)
a) M = (p- or n-like), and S = (e- or ),-like). See the text. b~ Requirement on meson mass (MeV/c2). "Meson mass" corresponds to the total invariant mass of an event in these modes. ) Requirement on meson momentum (MeV/c). "Meson momentum" corresponds to the total momentum imbalance of an event in these modes. d) If T)' are assigned to any of two rings, then a n ° mass (90-180 MeV/c 2) is required. e~ pp = 215-255 MeV/c is additionally required. r) For two-ring events, M~ = 90-180 MeV/c 2 and 1 p decay are required and M . . . . . and Pme~onare not required. For three-ring events, the requirement on p decay is 0 or 1. s> Po = 215-255 MeV/c and 0~, > 90 ° are additionally required. ") n ° mass (90-180 MeV/c 2) is required for three-ring events.
t i o n . T h e p a r t i c l e t y p e o f e a c h ring, i f successfully identified, must not be incompatible with those s h o w n in t h e f i f t h c o l u m n o f t a b l e s 1 a n d 2. I f p a r t i c l e s p e c i e s is n o t d e t e r m i n e d u n i q u e l y f o r a n y ring, t h e n all p o s s i b l e p a r t i c l e s p e c i e s for t h e r i n g a r e t r i e d t o test t h e v a r i o u s k i n e m a t i c a l q u a n t i t i e s s u c h as m o mentum imbalance. T h e s e l e c t i o n c r i t e r i a f o r t h e d e c a y m o d e , p ~ K +, n e e d special e x p l a n a t i o n , b e c a u s e m o s t o f t h e K ÷ stop before they decay and only the decay products ofK ÷ are o b s e r v e d . F o r e x a m p l e , t h e Ix+ f r o m K + - p + v h a s a monochromatic momentum of 236 MeV/c. Therefore a n a c c u m u l a t i o n o f m u o n - l i k e s i n g l e - r i n g e v e n t s is e x p e c t e d at t h i s m o m e n t u m . Fig. 1 s h o w s t h e m o m e n t u m d i s t r i b u t i o n for m u o n - l i k e single-ring e v e n t s w h i c h h a v e o n e p - d e c a y signal. B o t h K A M - I a n d KAM-II data are combined and shown by histog r a m s . N o s i g n i f i c a n t a c c u m u l a t i o n o f real e v e n t s is
1
,~ 6 ~ 5 t~ ~ 4
T
T
>
= 3 09 ,,>, 2 o _=e 0
200
300
P(MeV/c)
400
Fig. 1. The momentum distribution for muon-like single-ring events associated with one ~t-decay signal. The expected background distribution is normalized to the total observed number of muon-like events, cut interval of nucleon decay search and the expected signal for z/B = 0.74 × 1032yr which corresponds to the 90% CL lower limit for this channel are also shown. 311
Volume 220, number 1,2
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seen in the cut interval. The final limit on the nucleon partial lifetime for the p-+gK + mode, 1.0X 103:yr, is obtained by adding the contribution from the channel K + - ) n + n °, which has no significant background. Three kinds o f data sets, the real data, the simulated nucleon decay events and the simulated atmospheric neutrino interactions passed through the same selection criteria in order to obtain the n u m b e r o f nucleon decay candidates, the detection efficiency and the neutrino background rate. In calculating the detection efficiency, various nuclear effects such as the Fermi motion of nucleons in oxygen nuclei and the subsequent nuclear interactions of various mesons in oxygen nuclei and in water are considered, as detailed in ref. [7]. It should be noted that, in a water detector, 20% o f the proton decay are from free protons and are free from these nuclear effects in the nuclei. A dominant systematic error in the detection efficiency, which depends on the produced mesons and their m o m e n t u m , comes from these nuclear effects in oxygen nuclei. For pions, a systematic error of _+30% is estimated for the cross section in the nuclei. This error translates to an uncertainty of the detection efficiency of typically + 10% o f the efficiency quoted (p-~e+n °) or maximally _+35% (n-~gp°). For K mesons, on the other hand, the uncertainty is estimated to be + 1%, because o f the small interaction probability of 3% in oxygen nuclei. There is also a statistical error in the detection efficiency coming from a number of analyzed nucleon decay events. For each decay mode the detection efficiency is estimated using 30-140 simulated events. Therefore the overall uncertainty in the efficiency ranges from _+4% (3.7%; statistical fluctuation o f the number of events which satisfy the criteria, and 1%; systematic error) in the quoted efficiency for p ~ g K + ( I x + v ) to + 5 0 % (35%; statistical, and 35%; systematic) for n ~ g p °. Simulated events of atmospheric neutrino interactions in the detector are generated based on the calculated neutrino flux [ 8 ] and on the existing data on neutrino interaction cross sections. The simulation includes charged-current elastic scattering, chargedcurrent single- and multipion production and neutral-current single- and multipion production processes. The program was checked by comparing the distribution o f several kinematical quantities of the 312
30 March 1989
simulated events with those from data from accelerator neutrino experiments, and good agreement was obtained. The simulated neutrino data are also compared with the KAM-I and II data. Various (ve/v, independent) quantities such as the total visible energy distribution or the number of Cerenkov ring distributions are reasonably reproduced. For more details, see ref. [ 7 ]. As an illustration of how the various kinematical quantities are distributed, we show in fig. 2 the mom e n t u m imbalance, AP, versus total invariant mass, Mtot, plot of multi-ring events for (a) the data, (b) the simulated neutrino interactions ~3 and (c) the stimulated nucleon decay p ~ e + n °. These kinematical quantities are not always determined uniquely due to the ambiguity in the particle identification. In such a case, a point which gives the m i n i m u m o f (M, ot - 9 3 8 M e V / c 2) 2 + ( A P - 150 M e V / c ) 2 . 0 ( A P - 150 M e V / c ) ,
( 1)
where 0 is the step function, is selected and plotted for (a) and (b). The last factor, 0 ( A P - 150 M e V / c), is included considering the Fermi motion distribution o f nucleons and AP and Mto, cut defined in table 1. It is seen that the distribution of the data is similar to that of the simulated neutrino interactions. Fig. 3 shows the number of surviving events per kt yr for each step of the reduction for both the data and the simulated atmospheric neutrino events of 105 kt yr equivalent. The decay modes considered in this figure are p-*e+n °, e÷q(yy, n°n°n°), e+KO(nOnO), e+(o(Yx°), Ix +n°, IX+)](TT, n°n°n°) and ~t+K°(n°n°). This figure shows that the background estimation is done reasonably well. Possible systematic errors in the background estimation include the uncertainties in the absolute atmospheric neutrino fluxes at ~ l GeV ( + 2 0 % ) , in ))3
One may argue, looking at fig. 2b, that the background already dominates at 11.4 kt yr. We however note that the events in the allowed region in AP and Mto~are further subject to various selections such as meson mass, number of It decay, etc. They reduce the number of remaining events approximately by a factor of 2. The remaining events are backgrounds of e+p°, e+p-, e+o~(n+non-), e+K.O(n+non-), It+pO and lt+p -. It should also be noted that the decay modes which have high background rates (/> 1 event per 11.4 kt yr) involvep, to or K*. For decay modes involving n or K, the background rates are much lower. See tables 3 and 4.
Volume 220, number 1,2
PHYSICS LETTERS B
absolute neutrino cross sections for the final states with multi-t~erenkov rings ( ~ _+20%) and in the cross sections of pion interactions in the oxygen nu1500
I
30 March 1989
o Neutrino M.C. • Data
%
10 2 -
I
10 ~
(a)
o~
co
o o
n
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\ffl 1 ..tr-Oo o
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o
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o
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n
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o
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o
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"b, 0#
o ~o
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o
%
ioo%o •
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o o0
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o•
.
•
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v
•
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.
o
.
.~
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oo
0
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re) ~I- tO v
v
v
tD
,
• ,~.8--at,-~
o'~ .°8 ..o,, o
,o
o",• • a• o,~o,.
<~ 5 o o 1 - ~ , ~ _ . ~ , : , . ° ° |o
g--~ ~--
% ;0 o,
o
~.- ~
o
°
1000
>
(b) o
o
: I°
•
Io~ t i
'
'
(c)
Fig. 3. N u m b e r of neutrino events per kt yr for each step of the nucleon decay selection: ( 1 ) Fully contained event (see the text). Total p.e. cut; 2200-2900 for one ~t decay. 2900-3500 for zero I1 decay. (3) N u m b e r o f ~ e r e n k o v rings >/2. (4) Particle identification cut. (5) A P a n d M, ot cut. (6) Meson mass cut. The details of the selection criteria are shown in table 1. Combined data from both KAM-I and -II are also shown for the criteria ( 1 ) - ( 3 ) , because there is no serious quantitative difference between the two data samples as far as the criteria ( 1 ) - ( 3 ) are concerned. After the fourth criterion, only the KAM-II data are used.
¢J
>\
1000
~)
n
<~ 500
o OOo
o
,
0
500
1000
M T o T ( M e V / c ~)
1500
Fig. 2. AP versus M~o, of multi-ring events after reconstruction for (a) the KAM-II data, (b) the simulated neutrino interactions of I 1.4 kt yr exposure equivalent and (c) the simulated nucleon decay into e+n °. o and • represent events which have no and at least one observed la decay, respectively. In plotting the points in (c), various nuclear effects such as the Fermi motion of nucleons, and pion nuclear interactions in the oxygen nucleus are included. Events shown in (c) are after the application of the criteria of the total number of p.e., number of ~erenkov ring, number o f ~ decay and particle identity cuts. Therefore, in (c), most of the events whose n ° undergo nuclear interaction are removed before plotting the points. The broken and solid lines show the cut interval on m o m e n t u m imbalance and total invariant mass, see footnote c of table 1.
313
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cleus ( ~ + 3 0 % ) . T h e + 3 0 % u n c e r t a i n t y i n t h e p i o n cross section changes the number of multi-ring neut r i n o e v e n t s b y + 10%. F r o m t h e s e c o n s i d e r a t i o n s we estimate the systematic uncertainty in the backg r o u n d e s t i m a t i o n to b e ~ + 30%. T h e c o r r e s p o n d i n g u n c e r t a i n t y f o r p ~ g n + a n d p - , g K ° ( p . + v ) is est i m a t e d t o b e + 10%. T h i s is b e c a u s e t h e b a c k g r o u n d r a t e is n o r m a l i z e d t o t h e t o t a l n u m b e r o f s i n g l e - r i n g m u o n - l i k e e v e n t s . ~4. T h e r e is also a n e r r o r i n t h e
~4 Because of a deficit of the number of single-ring muon-like events relative to the Monte Carlo calculation (0.60 _+0.06, ref. [6 ] ), the estimated background numbers presented in fig. 1 and table 4 for 9K ÷ (It+v) and 9n + are >(0.6 of the calculated background without any deficit.
30 March 1989
background estimation coming from the statistical fluctuation of the number of analyzed neutrino e v e n t s . B e t w e e n 0 - 1 8 s i m u l a t e d n e u t r i n o e v e n t s survive within the nucleon decay criteria for the decay m o d e s w i t h m u l t i - r i n g f i n a l states, t h u s t h e s t a t i s t i c a l error of the estimated number of neutrino backg r o u n d r a n g e s f r o m + 2 4 % to > + 100% d e p e n d i n g on the decay mode, Tables 3 and 4 summarize the detection efficiency m u l t i p l i e d b y t h e m e s o n b r a n c h i n g r a t i o , e'Bn, t h e number of candidate events and the number of the e x p e c t e d n e u t r i n o b a c k g r o u n d e v e n t s ~4 i n K A M - I I . T h e c o r r e s p o n d i n g n u m b e r s in K A M - I are also s h o w n i n t h e s a m e t a b l e s . B e c a u s e o f t h e g o o d e n e r g y resol u t i o n o f e, 7 a n d p, t h e e x p e c t e d b a c k g r o u n d s a r e
Table 3 Results of the nucleon decay search for charged lepton+ meson modes. Detection efficiency multiplied by the meson branching ratio e.Bm, number of observed candidates and number of expected neutrino backgrounds in KAM-II and KAM-I are shown in the second to fourth and the fifth to seventh columns, respectively. The background subtracted limits on "riBat 90% CL are shown in the eighth column. Both the KAM-I and -II data are combined in deriving these numbers. Background unsubtracted limits are also shown in the last column. Modes
KAM-II (2.46 kt yr)
~'Bm e+n ° e+q(~) (3n °) e+co(yn°) (3n) e+p ° e+K°(n+n - ) (2n °) e+K*°(3n °)
(n+n-n °) (n-it+v)
It+n° It+'q(~7) (3n °) t.t+co(yn °) (3n) It+pO It+K°(n+n - ) (2n °) e+n e+p It+nIt+p-
0.45 0.14 0.10 0.03 0.08 0.24 0.14 0.11 0.01 0.03 0.09 0.43 0.13 0.08 0.04 0.09 0.22 0.20 0.10 0.28 0.15 0.23 0.06
number of event
b a, c
b
KAM-I ( 1.30 kt yr)
v BG
e'Bm
<0.02 a) <0.02 a) <0.02 a) <0.02 a) 0.5 1.8 0.1 <0.02 ~) <0.02 a) 0.5 0.2 <0.02 a) <0.02 a) <0.02 ~) <0.1 0.7 1.0 0.1 0.02 ~ <0.1 1.2 <0.1 0.9
0.53 0.17 0.11 0.04 0.17 0.18 0.18 0.13 0.02 0.04 0.08 0.40 0.16 0.06 0.04 0.17 0.18 0.20 0.11 0.32 0.10 0.25 0.08
number of event
B
B
A
C, E D
C
KAM-I + II(3.76 kt yr) T/B limit [yr] v BG
BG subtracted
BG unsubtracted
<0.02 <0.02 0.05 0.05 0.9 0.9 <0.1 0.05 0.05 0.7 0.1 0.05 <0.02 0.02 <0.1 1.2 0.7 0.3 0.02 0.1 0.7 <0.1 0.9
26 Xl031 14 Xl031
26 Xl031 14 X1031
4.5)< 1031
3.4X 1031
7.5X 1031 15 )<103j
5.2)< 103j 15 )<1031
5.2)< 1031
3.1 x 103j
23 )<1031 6.9×1031
23 )<1031 6.9>(103t
5.7)< 10 31
3 . 7 X 1031
11 )<103j 12 ×1031
11 )<1031 9.8>(1031
13 )<10 31 5.8X 103t 10 x l 0 3! 2.3)< 1031
13 X I 0 31 5.8X 103t 10 )<10 31 1.7X 103n
a) Neutrino background rates are estimated from Monte Carlo events equivalent to 105 kt yr for the decay modes denoted by (a) and to 22 kt yr for the other decay modes (KAM-II). 314
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Volume 220, number 1,2 Table 4 Same as table 3 for anti-neutrino + meson modes. Modes
97t+ 9p + 9K+ (p.+v) (n+n °) 9K *+ (nonon+) (n+TC+~ -
(n%+v) Vffo
vq(~,7) (3n °) 9o)( 7n ° ) (3n) 9p° 9K°(n+n - ) (2n °) 9K*° (3nO)
KAM-II (2.46 kt yr)
e'Bm
number of events
v BG
0.24 0.15 0.51 0.10 0.03 0.02 0.02 0.28 0.15 0.10 0.03
22 1 5
23.5 0.5 4.5 0.5 0.3 0.5 0.2 2.0 0.2 0.3 <0.1 0.6 1.5 0.2 0.6 0.1 0.6 0.1
1
1 1 1
0.11
1
0.06 0.07 0.10 0.01
2
(n+n-n°)
0.01
1
(n-~t+v)
0.06
KAM-I + II (3.76 ktyr) z/Blimit [yr]
KAM-I ( 1.30 kt yr)
/~' B m
number of events
v BG
BG subtracted
BG unsubtracted
0.14 0.16 0.41 0.10 0.07 0.03 0.05 0.31 0.15 0.08 0.05 0.21 0.08 0.12 0.13 0.02 0.03 0.06
10 4 4
9.3 1.0 1.9 0.4 1.3 0.3 0.4 1.0 0.1 0.3 <0.1 2.1 2.1 0.9 0.7 0.1 1.6 0.1
2.5X 1031 2.7X 1031 10 X1031
0.6X 1031 2.1 X 1031 5.1XI03t
2.0X 1031
1.3)< 1031
10 X1031 5.4X 1031
7.5X 103t 4.6X 1031
small for the d e c a y m o d e s which i n v o l v e only these particles in the final state. O n the o t h e r h a n d , the search is l i m i t e d for s o m e o f the d e c a y m o d e s w h i c h i n v o l v e charged p i o n s in the final state. T h e r e are several c a n d i d a t e events, w h i c h h o w e v e r are, in m o s t cases, c o n s i s t e n t with those e x p e c t e d f r o m the neut r i n o b a c k g r o u n d ~5. F r o m these results, we c o n c l u d e that we h a v e n o t o b s e r v e d e v i d e n c e for n u c l e o n d e c a y in K A M - I I . T h e r e f o r e we calculate l o w e r l i m i t s o n the n u c l e o n partial lifetime, z/B w h e r e B is the b r a n c h i n g ratio o f n u c l e o n decay for the r e l e v a n t d e c a y m o d e , at 98% C L c o m b i n i n g K A M - I a n d -II d a t a a n d subtracting the a t m o s p h e r i c n e u t r i n o b a c k g r o u n d . T h e 90% C L u p p e r l i m i t for the n u m b e r o f n u c l e o n d e c a y candidates is c a l c u l a t e d by integrating the l i k e l i h o o d funct i o n to the 90 0Y0p r o b a b i l i t y level:
~5 Event A, which is a candidate of p--.~t+Tl(yr), observed in KAM-I might be a possible exception, because the expected background is ~<0.02. However, the observed momentum imbalance, 396 _+71 MeV/c, is too large to conclude that this single event is evidence for nucleon decay.
2 2
2 2
2 1
4.3X
1 0 31
2.7×
1 0 31
1.3X 103~ 8.6× 1031
0.8X 1031 8.6× 1031
2.1X103t
1.1XI031
Aqimit
+ y;~,[eB,.(j).r(j)l x
dx
+ ~,=l[eBm(j).T(j)]x
)1 dx }-I
=0.90, w h e r e X, mit is the n u m b e r to be o b t a i n e d (i.e, the upper l i m i t for the n u m b e r o f n u c l e o n d e c a y candid a t e s ) , n is the n u m b e r o f i n d e p e n d e n t o b s e r v a t i o n s (for e x a m p l e , n = 4 for the p - - . g K + m o d e , because there are two K + d e c a y m o d e s , ~t+v a n d n + n °, a n d two e x p e r i m e n t a l phases, K A M - I a n d - I I ) , P(N, x) is the p r o b a b i l i t y f u n c t i o n o f the P o i s s o n statistics, Nobs(i) is the n u m b e r o f o b s e r v e d c a n d i d a t e s in the Rh o b s e r v a t i o n , XB~(i) the n u m b e r o f n e u t r i n o b a c k g r o u n d e v e n t s for the ith o b s e r v a t i o n a n d T ( i ) 315
Volume 220, number 1,2
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the detector exposure in the ith observation (kt yr). T h e n the r/B limit is o b t a i n e d by
~ >
!
[eBm(J')'T(j)]
NN,
where NN is the n u m b e r o f protons or neutrons in 1 kt o f water. The detector exposure is 3.76 kt yr in total. These results are shown in the last two columns o f tables 3 a n d 4 for charged lepton + meson and for anti-neutrino + meson decay modes, respectively. The b a c k g r o u n d subtracted limits on v/B range from 0 . 1 × 103~ to 2.6)< 1032yr d e p e n d i n g on the decay modes. The present limits are the most stringent ones for most o f the decay m o d e s [ 9,10 ] ~6. F o r the decay m o d e p~e+Tt °, the limit on z/B is 2.6X 1032yr. This result should be compared with 4X 1029+-°-7(Mx/2X10J4GeV)4yr, M x = t 2n +.2v"_ll~. o j X 1014 G e V which is a theoretically predicted value [ 11 ] based on the m i n i m a l S U ( 5 ) m o d e l [ 12]. On the other hand, s u p e r s y m m e t r i c G U T models [ 13 ] predict nucleon decay into anti-neutrino + K - m e s o n s to be d o m i n a n t ones [ 14 ]. T h e r e is no evidence for nucleon decay for these decay modes. N o c a n d i d a t e events have been observed for 9K+(Tt+~°), 9 K ° ( n + n - ), a n d 9 K ° ( v ° n ° ) . The n u m b e r o f candidate events for 9 K + (Ix+v) is well explained by the a t m o s p h e r i c neutrino background. The limits on 3/ B are 1.0× 1032yr and 0.9X 1032yr for p--,gK + a n d n ~ g K °, respectively. These n u m b e r s constraint sup e r s y m m e t r i c G U T models [ 15 ]. We acknowledge the generous cooperation o f the K a m i o k a Mining a n d Smelting Co. This work was s u p p o r t e d by the Japanese Ministry o f Education, Science and Culture, by the U n i t e d States D e p a r t ~6 See also Frejus collaboration, results summarized in ref. [2].
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ment o f Energy, and by the U n i v e r s i t y o f Pennsylvania Research Fund. A part o f this work was done using the F A C O M M 7 8 0 / M 3 8 0 computer system o f the Institute for N u c l e a r Study, University o f Tokyo.
References [ 1] P. Langacker, Phys. Rep. 72 ( 1981 ) 185. [ 2 ] Y. Totsuka, Proc. 1985 Intern. Symp. on Lepton and photon interactions at high energies (Kyoto, Japan, August 1985) p. 120; R. Barloutaud, Proc. Eighth Workshop on Grand unification (Syracuse University, New York, April 1987 ) p. 5; preprint DPhPE 88-15 (August 1988). [3] K. Arisaka et al., J. Phys. Soc. Jpn. 54 (1985) 3213. [4 ] T. Kajita et al., J. Phys. Soc. Jpn. 55 ( 1986 ) 711. [5] K. Hirata et al., Phys. Rev. Lett. 58 (1987) 1490; K.S. Hirata et al., Phys. Rev. D 38 (1988) 448. [6] K.S. Hirata et al., Phys. Lett. B 205 (1988) 416. [7] M. Nakahata et al., J. Phys. Soc. Jpn. 55 (1986) 3786. [ 8 ] T.K. Gaisser, T. Stanev and G. Barr, Phys. Rev. D 38 ( 1988 ) 85; T.K. Gaisser, private communication. [9] T.J. Haines et al., Phys. Rev. Lett. 57 (1986) 1986; S. Seidel et al., Phys. Rev. Lett. 61 (1988) 2522. [ 10 ] Frejus Collab., Ch. Berger et al., Nucl. Phys. B 313 (1989) 509. [11] W.J. Marciano, Proc. Eighth Workshop on Grand unification (Syracuse University, New York, April 1987) p. 185; P. Langacker, DESY preprint DESY 88-076 (June 1988 ). [12] H. Georgi and S.L Glashow, Phys. Rev. Lett. 32 (1974) 438. [ 13 ] S. Dimopoulos, S. Raby and F. Wilczek, Phys. Rev. D 24 (1981) 1681; S. Dimopoulos and H. Georgi, Nucl. Phys. B 193 (1981) 150; N. Sakai, Z. Phys. C 11 (1981) 153. [ 14] N. Sakai and T. Yanagida, Nucl. Phys. B 197 (1982) 533; S. Weinberg, Phys. Rev. D 26 (1982) 287. [ 15 ] P. Nath and R. Arnowin, Phys. Rev. D 38 (1988) 1479.