- Email: [email protected]
Study of ν backgrounds to nucleon decay searches using K2K 1KT detector data
I g [ l l [ l l l ~ ",ig ]lr~'1[Ikl "!
ELSEVIER
Nuclear PhysicsB 0aroc. Suppl.) 112 (2002) 154-158
PROCEEDINGS SUPPLEMENTS www.elsevier.com/locale/npe
Study of backgrounds to nucleon decay searches using K2K 1KT detector data Shun'ichi Mine a aDepartment of Physics and Astronomy, University of California, Irvine, Irvine, CA, 92697-4575, USA A study of neutrino backgrounds to nucleon decay searches using K2K's one kiloton water Cherenkov detector v beam data is described.
1. I n t r o d u c t i o n
Background for nucleon decay arises from interactions of muons and neutrinos produced by cosmic-ray interactions in the upper atmosphere. By locating the detectors underground, cosmicray muon background can be reduced to a manageable level, but neutrino background is unavoidable. The vast majority Of atmospheric neutrino interactions bear little resemblance to nucleon decay, but a small fraction are indistinguishable (based on topology and kinematic parameters) from the signal. Recently, data from a scaled down version of Super-Kamiokande[SK] installed in the neutrino beam line at KEK (K2K lkton water Cherenkov detector[1KT]) has allowed a high-statistics study of these backgrounds in a controlled environment, and will permit afar more precise estimation of their incidence once fully analyzed. The p-~e~r° is less model-dependent and one of the dominant modes expected by GUTs. The SK experiment has obtained the world's best limits, >5.0×1033 years partial lifetime (90% CL), on this mode [1,2]. The simplest SUSY predicted p--+eTr° partial lifetimes are few times 1034 to 1035 [3], which could be measured by future megaton size water Cherenkov detector [4]. 1.1. N u c l e o n d e c a y s e a r c h e s at S K
The total momentum (Ptot = ] ~,~u r~ngs ffi i where iffi is reconstructed momentum vector of i-th ring) and total invariant mass (Mtot = ~/E~ot - P~ot where total energy Etot = ~-~u ring8 ]ffil) distributions of proton decay MC
(top), atmospheric neutrino MC (center), and SK data (bottom) for ~the p--~e~r° search is shown in Figure 1. The SK detector currently expects an atmospheric neutrino background of approximately 0.2 events in this ~mode, and finds no candidate events. Since the number of candidate events is zero and the number of expected background is also almost zero, we can neglect uncertainty in the background in the current SK analysis. However, for one megaton detector we naively expect two background events every one year. Even assuming a relatively optimistic proton lifetime, the rate of nucleon decay events will be similar to the expected rate of background events and so a precise background estimation is crucial for the megaton detector experiment. Figure 2 shows the upper limit of the experimentally accessible lifetime as a function of the precision of the background determination assuming a background rate of about 2 event/Mt.yr and exposures 1, 5, and 10 Mt.yr. Not surprisingly, a precise background determination is more important for higher exposures. For example, to achieve a proton decay lifetime of 1 x 1035 yr for a 10 Mt.yr exposure, a precision of 30% or less is required. The atmospheric neutrino background to the p~err ° search comes mostly from CC ve (or lye) interactions where an electron (or positron) and a ~r° are produced in the final state [2]. Therefore, it is important to understand the kinematics of electrons and ~r°s from veN-+ez'°X interactions similar to the proton decay signature.
0920-5632/02/$ - see frontmatter© 2002 Publishedby ElsevierScienceB.V. PII S0920-5632(02)01753-X
S. Mine~Nuclear Physics B (Proc. Suppl.) 112 (2002) 154-158
The proton decay mode p-~ PK + is another dominant modes predicted by SUSY-GUTs. While there are a number of methods being used to search for the p ~ PK + at SK, the best lower limit of the lifetime is obtained with the so-called '~prompt 7" method [5]. The atmospheric neutrino interaction up --+ vAK+ will become the limiting background on this mode with an expected rate of about 1 event/Mt.yr. 1.2; C o m p a r i s o n w i t h p r e v i o u s v b e a m tests The statistics of the 1KT data is higher than any other previous similar beam experiments [6] by a few orders of magnitude and enables us to check the neutrino interactions in detail. This is the first data taken from a neutrinobeam with an avarage energy of 1 GeV on a water target. 1.3. Goal of this study At this stage, it is most important to evaluate the (muon) neutrino M C used for evaluating the sensitivity of nucleon decay search limits using the 1 K T data in a well controlled environment. W e will determine the level of background for p~elr ° search at the megaton detector with a precision of <30% by taking into account differences from the 1KT data such as neutrino energy, detector performance, and so on. The background of some other interesting nucleon decay modes, especially p--~ p K + mode, will be studied once the p-~er ° mode is well understood, since the cross section of kaon production in the 1KT is relatively small. In this paper, we show some preliminary results of the 1KT data analysis.
155
gion of 1-3 GeV, the 1KT data for 1020 protons on target [pot] corresponds to about 10 Mt.yr of atmospheric v~ data. The number of charged current events expected at 1KT which generate Ir° (kaon) in the final state is the order of 10,000 (100) for 1020 pot. Although the atmospheric neutrino flux contalus all the neutrino flavors, the atmospheric neutrino backgrounds can be Checked by studying P~N -~ #~r°X interactions produced in the K2K beam, because the dominant uncertainty on the background comes from the uncertainty of the neutrino cross section in the hadron sector and its secondary pion interaction in oxygen nucleus [8]. The main difference between SK and ! K T is the overall size. The 20 in. PMT's and their arrangement (40% photo-coverage) used in the 1KT are the same as in SK [1]. The standard fiducial volumes are 22.5 kt and 50 t, respectively. The basic detector performance of the 1KT, such as momentum or angular resolution for single ring events is almost same as that of SK [9,10]. The total momentum and total invariant mass for p-~ #It ° MC at 1KT is shown in Figure 4 (top), and demonstrates that the muon and 7r° from free protons are correctly reconstructed in the proton decay signal box. The absorption, charge exchange, and scattering of Ir°'s in the oxygen nucleus are the dominant contribution to the detection inefficiency. The neutrino interaction simulator used for the 1KT is the same as that used in SK analysis [8]. The analysis algorithms [1] are also common. Therefore, we may directly check the validity of the modeling of neutrino interactions in the atmospheric neutrino MC using 1KT data. 3. 1 K T data analysis
2. C o m p a r i s o n b e t w e e n SK and 1 K T The K2K neutrino beam [7] is a nearly pure v~ beam (98.2% v~,1.3% v~, and 0.5% f , ) with average energy -.~1.3 GeV (See Figure 3.) Fortunately, the energy of atmospheric neutrinos in the nucleon decay backgrounds is wellmatched by the energy of K2K's neutrino beam. If we simply compare the number of v~ interactions taken at SK and the 1KT in the energy re-
The data collected between January and March 2000 and between January and July 2001 corresponding to ~ 3 x 101° pot have been studied. 3.1. Event selection The fiducial volume is defined by a 4 m diameter, 4 m long cylinder (~50 t) along the beam axis. The pit° candidate events at 1 K T were selected with essentially the same cuts used in the
156
S. Mine~Nuclear Physics B (Proc. Suppl.) 112 (2002) 154-158
SK p-~ p~r° analysis: (A) fully contained, (B) the number of rings is 2 or 3, (C) one "muon"Iike ring and one or two "electron'-like ring (s), (D) 85 MeV/c2< r ° invariant mass<215 MeV/c 2 (3 ring sample). An event display of a typical v~N ~ p~r°X interaction (MC) is shown in Figure 5. Three rings can be seen, one muon and 2 7's from ~r° reconstructed as '~nuon"-like (cyan) and "electron"like (red) rings, respectively. 3.2. C o m p a r i s o n b e t w e e n d a t a a n d M C The total momentum and total invariant mass distributions of the/~Ir ° candidate events in the 1KT data (bottom) and the K2K muon neutrino MC (center) are shown in Figure 4. 1 To make the comparison clear, a variable "L" is defined to be the distance between each event point and a line in the proton decay signal region. The distribution of L for data and MC is shown in Figure 6. It is clearly seen that the data distribution is well reproduced by the muon neutrino MC including the signal box region (L_<,~lS0). Note that almost all the MC events in/near the signal box come from the v~N~ #Tr°X interactions and are correctly reconstructed. 4. S u m m a r y This is the first attempt to evaluate the atmospheric neutrino background for nucleon decay searches with data taken using the same water Cherenkov technique as SK in a well understood neutrino beam. By studying v ~ N --+ plr°X interactions in 1KT data corresponding to ,~10 Mt.yr atmospheric v~, equiv., we will determine the level of background for the p~eTr ° search at SK and the megaton water Cherenkov detector with a precision of less than 30 %. The p-+/~Ir ° selection used in SK analysis was applied to 1KT data and MC. Currently, detailed studies are being performed to check the details of the agreement between 1KT data and MC for 1There are no events around the origin in either figuredue to requiring >~1,000 pe collected photo:electronsin the 1KT analysis in order to select one neutrino interaction per beam spill. This does not affectthe backgroundstudy around the signal box.
various distributions, but Figure 6 clearly demonstrates that our ability to model nucleon decay background interactions is well supported by the data so far in hand. REFERENCES
1. M.Shiozawa et al., Phys. Rev. Lett. 81, 33193323 (1998) 2. M.Shiozawa, "Hyper-Kamiokande project and CP sensitivity with JHF", APS meeting Snowmass, 2001 July 3. "Physics Potential and Feasibility of UNO", http://superk.physics.sunysb.edu/ 4. NNN02 workshop ~ CEtLN, 2002 Jan, See //http://muonstoragerings.cern.ch/NuWorkshop02/welcome.html 5. Y.Hayato et al., Phys. Rev. Lett. 83, 1529 (1999) 6. G.Battistoni etal., Nucl. Instr. Meth. A219, 300 (1984), M.Derrick, et al., Phys. Rev. D30, 1605 (1984), W.A.Mann, et al., Phys. Rev. D34, 2545 (1986), C.Berger et at, Nucl. Instr. Meth. A302, 406 (1991) 7. S.H.Ahn et al., Phys.Lett.B511, 178-184 (2001) 8. NuInt01 workshop @ KEK, 2001 Dec See http://neutrino.kek.jp/nuint01 9. M.Shiozawa, "Search for proton decay via p -+ e+~r° in a large water Cherenkov detector", Doctor Thesis, University of Tokyo (1999). 10. S.Mine, "Experimental determination of the neutrino backgrounds to the nucleon decay searches using K2K lkton detector data", APS meeting ~ Snowmass, 2001 July 11. B.Viren, Super-Kamiokande Report No.9803, 1998, (unpublished but available from http://wwwsk.icrr.u-tokyo.ac.jp/doc/sk/pub)
157
S. Mine~Nuclear Physics B (Proc. Suppl.) 112 (2002) 154-158
p-.- e/i;°MC
1000
p--~e~° (90%CL)
.° °
J|°'
.o ,
i"
SO0
400 _300 100 00
atm.V MC
lOOO
i
1Mtyr
200 400 BOO N0 1000 1~00
10
lyltlmltl¢ error of ~
d
(~)
Figure 2. Lifetime sensitivityversus the uncertainty in the background estimation for 1,5, and I0 Mt.yr exposures. The ei~ciencyand background used in the current S K analysishave been assumed. The Bayes' theorem [11]was used to obtain this.
eoo
lOO 00
m~r~t~ rm,(mN~ SK DATA~ . . ,
K2K V beam
1000 ~ ~ , . ~ . ~ , . ~ .
xlOe
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!
.
~ b~;;-.. -.., .. ¼"':
F .~:,:
300
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m
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~* •
i"
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200 400 eO0 800 1000 1200
Figure 1. The total momentum and total invariant mass distributions for 3 samples: (top) p-~elr ° Monte Carlo, (center) atmospheric neutrino Monte Carlo corresponding to N1.6 Mt.yr, (bottom) SK data corresponding to N0.08 Mt.yr. The boxed region in each figure shows the signal region.
~~99,Ve/~ % ............
f~
e
O.S
1
L$
2
2.5
$
3.$
4
4.S
E~(e,ev)
Figure 3. Energy spectrum of the K2K y beam. The hatch is contamination of re.
158
S. Mine~Nuclear Physics B (Proc. Suppl.) 112 (2002) 154-158
p--~/~° MC 1KT Preliminm
i
'
'
'
1KT MC
. ~ , protons
efre~ protons
~
t.
,~ lk,t ~
OD ~
..
'
Kii!J
,o
.,.
.'~:?:!
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chargo(rodlue) vertm( p like ring • like ring
N~: J': ........
~.~I.~~ ,.',:. ~,:. :..
i m
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i iiiiiii ii!!it
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m
%
,o.
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K2K V~ MC Figure 5. Typical event display of vaN --~j~Ir°X interaction (1KT MC). The result of event reconstruction (vertex fit, ring-counting, and PID) is overlaid in the figure.
1KT preliminary •
totalImmdentmm (MeV/,~)
10
0ATA
'
E]MC
%'% L
1 KT DATA (-3x10~' pol)
10
10 total l e m e d m d m u m OlbV/et)
10
L (arbitrary)
) total I n w d a n t m m
(MeV/c j)
Figure 4. The total momentum and total invariant mass distributions for 3 samples which satisfy the criteria (A)-(D) (See text.): (top) p--+ #~r° Monte Carlo, (center) K2K muon neutrino Monte Carlo corresponding to ,-,3 × 1019 pot, (bottom) 1KT data corresponding to ,,,3× 1019 pot. The boxed region in each figure shows the proton decay signal region used in the SK a n a l y s i s .
Figure 61 Distance "L" distributions in the mass vs. momentum plot (Figure 4) for the 1KT data and the K2K v~ MC which satisfy the criteria (A)-(D) (See text.) The MC histogram is normalized to the area of the data histogram. In the upper right figure, the "L" is defined as a distance between each event point and the line in the proton decay signal region.
ELSEVIER
Nuclear PhysicsB 0aroc. Suppl.) 112 (2002) 154-158
PROCEEDINGS SUPPLEMENTS www.elsevier.com/locale/npe
Study of backgrounds to nucleon decay searches using K2K 1KT detector data Shun'ichi Mine a aDepartment of Physics and Astronomy, University of California, Irvine, Irvine, CA, 92697-4575, USA A study of neutrino backgrounds to nucleon decay searches using K2K's one kiloton water Cherenkov detector v beam data is described.
1. I n t r o d u c t i o n
Background for nucleon decay arises from interactions of muons and neutrinos produced by cosmic-ray interactions in the upper atmosphere. By locating the detectors underground, cosmicray muon background can be reduced to a manageable level, but neutrino background is unavoidable. The vast majority Of atmospheric neutrino interactions bear little resemblance to nucleon decay, but a small fraction are indistinguishable (based on topology and kinematic parameters) from the signal. Recently, data from a scaled down version of Super-Kamiokande[SK] installed in the neutrino beam line at KEK (K2K lkton water Cherenkov detector[1KT]) has allowed a high-statistics study of these backgrounds in a controlled environment, and will permit afar more precise estimation of their incidence once fully analyzed. The p-~e~r° is less model-dependent and one of the dominant modes expected by GUTs. The SK experiment has obtained the world's best limits, >5.0×1033 years partial lifetime (90% CL), on this mode [1,2]. The simplest SUSY predicted p--+eTr° partial lifetimes are few times 1034 to 1035 [3], which could be measured by future megaton size water Cherenkov detector [4]. 1.1. N u c l e o n d e c a y s e a r c h e s at S K
The total momentum (Ptot = ] ~,~u r~ngs ffi i where iffi is reconstructed momentum vector of i-th ring) and total invariant mass (Mtot = ~/E~ot - P~ot where total energy Etot = ~-~u ring8 ]ffil) distributions of proton decay MC
(top), atmospheric neutrino MC (center), and SK data (bottom) for ~the p--~e~r° search is shown in Figure 1. The SK detector currently expects an atmospheric neutrino background of approximately 0.2 events in this ~mode, and finds no candidate events. Since the number of candidate events is zero and the number of expected background is also almost zero, we can neglect uncertainty in the background in the current SK analysis. However, for one megaton detector we naively expect two background events every one year. Even assuming a relatively optimistic proton lifetime, the rate of nucleon decay events will be similar to the expected rate of background events and so a precise background estimation is crucial for the megaton detector experiment. Figure 2 shows the upper limit of the experimentally accessible lifetime as a function of the precision of the background determination assuming a background rate of about 2 event/Mt.yr and exposures 1, 5, and 10 Mt.yr. Not surprisingly, a precise background determination is more important for higher exposures. For example, to achieve a proton decay lifetime of 1 x 1035 yr for a 10 Mt.yr exposure, a precision of 30% or less is required. The atmospheric neutrino background to the p~err ° search comes mostly from CC ve (or lye) interactions where an electron (or positron) and a ~r° are produced in the final state [2]. Therefore, it is important to understand the kinematics of electrons and ~r°s from veN-+ez'°X interactions similar to the proton decay signature.
0920-5632/02/$ - see frontmatter© 2002 Publishedby ElsevierScienceB.V. PII S0920-5632(02)01753-X
S. Mine~Nuclear Physics B (Proc. Suppl.) 112 (2002) 154-158
The proton decay mode p-~ PK + is another dominant modes predicted by SUSY-GUTs. While there are a number of methods being used to search for the p ~ PK + at SK, the best lower limit of the lifetime is obtained with the so-called '~prompt 7" method [5]. The atmospheric neutrino interaction up --+ vAK+ will become the limiting background on this mode with an expected rate of about 1 event/Mt.yr. 1.2; C o m p a r i s o n w i t h p r e v i o u s v b e a m tests The statistics of the 1KT data is higher than any other previous similar beam experiments [6] by a few orders of magnitude and enables us to check the neutrino interactions in detail. This is the first data taken from a neutrinobeam with an avarage energy of 1 GeV on a water target. 1.3. Goal of this study At this stage, it is most important to evaluate the (muon) neutrino M C used for evaluating the sensitivity of nucleon decay search limits using the 1 K T data in a well controlled environment. W e will determine the level of background for p~elr ° search at the megaton detector with a precision of <30% by taking into account differences from the 1KT data such as neutrino energy, detector performance, and so on. The background of some other interesting nucleon decay modes, especially p--~ p K + mode, will be studied once the p-~er ° mode is well understood, since the cross section of kaon production in the 1KT is relatively small. In this paper, we show some preliminary results of the 1KT data analysis.
155
gion of 1-3 GeV, the 1KT data for 1020 protons on target [pot] corresponds to about 10 Mt.yr of atmospheric v~ data. The number of charged current events expected at 1KT which generate Ir° (kaon) in the final state is the order of 10,000 (100) for 1020 pot. Although the atmospheric neutrino flux contalus all the neutrino flavors, the atmospheric neutrino backgrounds can be Checked by studying P~N -~ #~r°X interactions produced in the K2K beam, because the dominant uncertainty on the background comes from the uncertainty of the neutrino cross section in the hadron sector and its secondary pion interaction in oxygen nucleus [8]. The main difference between SK and ! K T is the overall size. The 20 in. PMT's and their arrangement (40% photo-coverage) used in the 1KT are the same as in SK [1]. The standard fiducial volumes are 22.5 kt and 50 t, respectively. The basic detector performance of the 1KT, such as momentum or angular resolution for single ring events is almost same as that of SK [9,10]. The total momentum and total invariant mass for p-~ #It ° MC at 1KT is shown in Figure 4 (top), and demonstrates that the muon and 7r° from free protons are correctly reconstructed in the proton decay signal box. The absorption, charge exchange, and scattering of Ir°'s in the oxygen nucleus are the dominant contribution to the detection inefficiency. The neutrino interaction simulator used for the 1KT is the same as that used in SK analysis [8]. The analysis algorithms [1] are also common. Therefore, we may directly check the validity of the modeling of neutrino interactions in the atmospheric neutrino MC using 1KT data. 3. 1 K T data analysis
2. C o m p a r i s o n b e t w e e n SK and 1 K T The K2K neutrino beam [7] is a nearly pure v~ beam (98.2% v~,1.3% v~, and 0.5% f , ) with average energy -.~1.3 GeV (See Figure 3.) Fortunately, the energy of atmospheric neutrinos in the nucleon decay backgrounds is wellmatched by the energy of K2K's neutrino beam. If we simply compare the number of v~ interactions taken at SK and the 1KT in the energy re-
The data collected between January and March 2000 and between January and July 2001 corresponding to ~ 3 x 101° pot have been studied. 3.1. Event selection The fiducial volume is defined by a 4 m diameter, 4 m long cylinder (~50 t) along the beam axis. The pit° candidate events at 1 K T were selected with essentially the same cuts used in the
156
S. Mine~Nuclear Physics B (Proc. Suppl.) 112 (2002) 154-158
SK p-~ p~r° analysis: (A) fully contained, (B) the number of rings is 2 or 3, (C) one "muon"Iike ring and one or two "electron'-like ring (s), (D) 85 MeV/c2< r ° invariant mass<215 MeV/c 2 (3 ring sample). An event display of a typical v~N ~ p~r°X interaction (MC) is shown in Figure 5. Three rings can be seen, one muon and 2 7's from ~r° reconstructed as '~nuon"-like (cyan) and "electron"like (red) rings, respectively. 3.2. C o m p a r i s o n b e t w e e n d a t a a n d M C The total momentum and total invariant mass distributions of the/~Ir ° candidate events in the 1KT data (bottom) and the K2K muon neutrino MC (center) are shown in Figure 4. 1 To make the comparison clear, a variable "L" is defined to be the distance between each event point and a line in the proton decay signal region. The distribution of L for data and MC is shown in Figure 6. It is clearly seen that the data distribution is well reproduced by the muon neutrino MC including the signal box region (L_<,~lS0). Note that almost all the MC events in/near the signal box come from the v~N~ #Tr°X interactions and are correctly reconstructed. 4. S u m m a r y This is the first attempt to evaluate the atmospheric neutrino background for nucleon decay searches with data taken using the same water Cherenkov technique as SK in a well understood neutrino beam. By studying v ~ N --+ plr°X interactions in 1KT data corresponding to ,~10 Mt.yr atmospheric v~, equiv., we will determine the level of background for the p~eTr ° search at SK and the megaton water Cherenkov detector with a precision of less than 30 %. The p-+/~Ir ° selection used in SK analysis was applied to 1KT data and MC. Currently, detailed studies are being performed to check the details of the agreement between 1KT data and MC for 1There are no events around the origin in either figuredue to requiring >~1,000 pe collected photo:electronsin the 1KT analysis in order to select one neutrino interaction per beam spill. This does not affectthe backgroundstudy around the signal box.
various distributions, but Figure 6 clearly demonstrates that our ability to model nucleon decay background interactions is well supported by the data so far in hand. REFERENCES
1. M.Shiozawa et al., Phys. Rev. Lett. 81, 33193323 (1998) 2. M.Shiozawa, "Hyper-Kamiokande project and CP sensitivity with JHF", APS meeting Snowmass, 2001 July 3. "Physics Potential and Feasibility of UNO", http://superk.physics.sunysb.edu/ 4. NNN02 workshop ~ CEtLN, 2002 Jan, See //http://muonstoragerings.cern.ch/NuWorkshop02/welcome.html 5. Y.Hayato et al., Phys. Rev. Lett. 83, 1529 (1999) 6. G.Battistoni etal., Nucl. Instr. Meth. A219, 300 (1984), M.Derrick, et al., Phys. Rev. D30, 1605 (1984), W.A.Mann, et al., Phys. Rev. D34, 2545 (1986), C.Berger et at, Nucl. Instr. Meth. A302, 406 (1991) 7. S.H.Ahn et al., Phys.Lett.B511, 178-184 (2001) 8. NuInt01 workshop @ KEK, 2001 Dec See http://neutrino.kek.jp/nuint01 9. M.Shiozawa, "Search for proton decay via p -+ e+~r° in a large water Cherenkov detector", Doctor Thesis, University of Tokyo (1999). 10. S.Mine, "Experimental determination of the neutrino backgrounds to the nucleon decay searches using K2K lkton detector data", APS meeting ~ Snowmass, 2001 July 11. B.Viren, Super-Kamiokande Report No.9803, 1998, (unpublished but available from http://wwwsk.icrr.u-tokyo.ac.jp/doc/sk/pub)
157
S. Mine~Nuclear Physics B (Proc. Suppl.) 112 (2002) 154-158
p-.- e/i;°MC
1000
p--~e~° (90%CL)
.° °
J|°'
.o ,
i"
SO0
400 _300 100 00
atm.V MC
lOOO
i
1Mtyr
200 400 BOO N0 1000 1~00
10
lyltlmltl¢ error of ~
d
(~)
Figure 2. Lifetime sensitivityversus the uncertainty in the background estimation for 1,5, and I0 Mt.yr exposures. The ei~ciencyand background used in the current S K analysishave been assumed. The Bayes' theorem [11]was used to obtain this.
eoo
lOO 00
m~r~t~ rm,(mN~ SK DATA~ . . ,
K2K V beam
1000 ~ ~ , . ~ . ~ , . ~ .
xlOe
•~ F :':~:,.:'..:"';
!
.
~ b~;;-.. -.., .. ¼"':
F .~:,:
300
O0
m
*"* . 2"
~* •
i"
" .
200 400 eO0 800 1000 1200
Figure 1. The total momentum and total invariant mass distributions for 3 samples: (top) p-~elr ° Monte Carlo, (center) atmospheric neutrino Monte Carlo corresponding to N1.6 Mt.yr, (bottom) SK data corresponding to N0.08 Mt.yr. The boxed region in each figure shows the signal region.
~~99,Ve/~ % ............
f~
e
O.S
1
L$
2
2.5
$
3.$
4
4.S
E~(e,ev)
Figure 3. Energy spectrum of the K2K y beam. The hatch is contamination of re.
158
S. Mine~Nuclear Physics B (Proc. Suppl.) 112 (2002) 154-158
p--~/~° MC 1KT Preliminm
i
'
'
'
1KT MC
. ~ , protons
efre~ protons
~
t.
,~ lk,t ~
OD ~
..
'
Kii!J
,o
.,.
.'~:?:!
o.o o.o
0 •
chargo(rodlue) vertm( p like ring • like ring
N~: J': ........
~.~I.~~ ,.',:. ~,:. :..
i m
m.7 a~t N, m, ~ . s
.~.m, ... ~u,
i iiiiiii ii!!it
°..
m
%
,o.
,,-,
totalInmtent ma~ (BleV/~
K2K V~ MC Figure 5. Typical event display of vaN --~j~Ir°X interaction (1KT MC). The result of event reconstruction (vertex fit, ring-counting, and PID) is overlaid in the figure.
1KT preliminary •
totalImmdentmm (MeV/,~)
10
0ATA
'
E]MC
%'% L
1 KT DATA (-3x10~' pol)
10
10 total l e m e d m d m u m OlbV/et)
10
L (arbitrary)
) total I n w d a n t m m
(MeV/c j)
Figure 4. The total momentum and total invariant mass distributions for 3 samples which satisfy the criteria (A)-(D) (See text.): (top) p--+ #~r° Monte Carlo, (center) K2K muon neutrino Monte Carlo corresponding to ,-,3 × 1019 pot, (bottom) 1KT data corresponding to ,,,3× 1019 pot. The boxed region in each figure shows the proton decay signal region used in the SK a n a l y s i s .
Figure 61 Distance "L" distributions in the mass vs. momentum plot (Figure 4) for the 1KT data and the K2K v~ MC which satisfy the criteria (A)-(D) (See text.) The MC histogram is normalized to the area of the data histogram. In the upper right figure, the "L" is defined as a distance between each event point and the line in the proton decay signal region.