- Email: [email protected]
A search for νμ − ντ oscillation with CHORUS at CERN
PROCEEDINGS SUPPLEMENTS ELSEVIER
A search for
Nuclear Physics B (Proc. Suppl.) 59 (1997) 277-282
- ur oscillation with C H O R U S at C E R N
The CHORUS Collaboration presented by Hiroshi Shibuya * ~Department of Physics, Toho University, Funabashi, Japan, E-mail: [email protected], [email protected] A search for v , - ~ r oscillation with the CHORUS detector at CERN is in progress. The first data taking in 1994/1995 was successfully completed with 320 O00 recorded ~, CC events in the emulsion target and half of the second data taking period 1996/1997 has lead to 230 000 additional u~ CC events. A pilot analysis of the 1994 data for the muonic r decay search by using the fully automatic scanning systems, resulting in an upper limit sin2(28~,r) < 0.01 at 90% C.L. for large Am ~, confirms that the proposed sensitivity (sin2(28~,r) <: 3 × 10-~) is within reach in two years.
1. I N T R O D U C T I O N
2. T H E C H O R U S E X P E R I M E N T
The problem of neutrino mass and mixing is one of the most interesting subjects in modern particle physics. Among the three neutrino species, ~'r is predicted to be by far the heaviest on general arguments based on see-saw mechanism and it could have a mass of a few x 10 eV [1]. If it is the case, the ur might be an important constituent of cosmological dark matter in the universe. The ur mass is thus particularly interesting not only for particle physics but also for astrophysics and cosmology. The most sensitive way to find an evidence of finite neutrino masses is to observe neutrino oscillations, which also confirms the existence of neutrino mixings. The WA95/CHORUS at CERN is a dedicated u~, - ~,~ oscillation search experiment looking for the appearance of the inclusive reaction u r N "-* r - X using a nearly pure ~ beam. The reaction is detected by directly observing the characteristic decay topology of the short-lived r lepton in a nuclear emulsion target. The CHORUS experiment aims at improving the sensitivity by more than one order of magnitude over the existing limits [2-4]. As the sensitivity is almost propotional to the number of u~, interactions analyzed, fast and efficient scanning in the emulsion is crucial. It is being done by newly developed automatic scanning systems.
Detailed descriptions of the CHORUS experiment appear elsewhere [5,6], and are summarized here.
0920-5632/97/$17.00 © Elsevier Science B.E All rights mserved. PII S0920-5632(97)00452-0
2.1. T h e C E R N Wide-Band Neutrino Beam The West Area Neutrino Facility(WANF) of the CERN SPS provides a beam which is mainly composed of v~ with an average energy of 26.9 GeV, sufficiently above the 3.5 GeV threshold for a vr charged-current (CC) interaction. 450 GeV protons are extracted from the SPS on to a beryllium target twice in a cycle of 14.4 s (,,~ 2 × 1013 protons/cycle), producing mostly pions and kaons. Neutrinos are then produced from their decays in flight. Each spill has a length of 6 ms, separated by 2.7 s from the other. The distance from the neutrino source to the CHORUS detector is 400 -~ 800 m. Relative abundances and mean energies of all neutrino species are summarized in Table 1. The t'r contamination of the beam is expected to be at the negligible level of urCC/~,#CC ,,, 3 × 10 -6, corresponding to ,,~ 0.1 background event in the 4-year data taking.
2.2. T h e E x p e r i m e n t a l S e t u p The CHORUS detector is a hybrid apparatus composed of a nuclear emulsion target and an electronic detector.
H. Shibuya/Nuclear PhysicsB (Proc. Suppl.) 59 (1997) 277-282
278
Table 1 Composition of the CERN wide-band neutrino beam at the CHORUS experiment v species abundance (%) < Ev > (GeV) v~, 100 26.9
~,
5.6
21.7
Ve Pe
0.7 0.17
47.9 35.3
The nuclear emulsion target, with an excellent spatial resolution of 1 pro, is used to detect directly the production and decay of the shortlived r - from the charged-current (CC) interaction ~ h r ---* r - X , shown schematically in Figure 1 for the case of a muonic r decay.
TAIHiI[T IMULIII~i OTACK
(is er,,uw,~ j~tmd
Iq~R T I t ~
tou.noutrlno (V.r.I
in emulsion
zoom or Vertex
.Yxt._.. hl0erootlen
I
\ -"~...-
decay
~'r
I
-,,~ z,~,
Figure 1. The production and decay of the shortlived r - from the vr CC interaction in the emulsion target.
The emulsion target of 770 kg, almost 4 radiation lengths and 0.32 interaction length in total, is divided into 4 stacks each having an area of 1.4 × 1.4 m 2 and a thickness of 2.8 cm. Each stack is further subdivided into 8 sectors of an area 0.71 × 0.36 m 2, each consisting of 36 emulsion plates placed almost perpendicularly to the beam. A
plate is composed of a 90 pm thick plastic base with 350 pm thick emulsion layers on both sides. Three interface emulsion sheets, one special sheet (SS) and two changeable sheets (CS), are placed between each emulsion stack and the electronic detector. Each interface sheet consists of an 800 pm thick acrylic plate coated on both sides with 100 ~m thick emulsion, which allows a precise angle to be determined (a ~, 1.5 mrad). The electronic detector is used to find the neutrino interactions in the emulsion target, to select r-like events by kinematical cuts and to distinguish v~ CC interactions from various background events. It consists of a scintillating fiber tracker system, scintillator trigger hodoscopes, an air-core magnet, a lead/scintillator calorimeter, and a muon spectrometer, as shown in Figure 2. The fiber tracker system with good spatial resolution and good two-track separation provides event reconstruction and track prediction to the most downstream interface sheet (CS). Accuracy of the prediction from the fiber tracker system at the changeable sheet is currently a ~ 200pro in position and a ~, 3 mrad in angle. Together with the air-core magnet, the fiber tracker also measures the charge and the momentum of traversing particles (Ap/p ~, 0.3 at 5 GeV/c). Energies and directions of the hadronic and electromagnetic showers are measured by the calorimeter. (Its intrinsic resolutions are AE/E = 3 2 % / ~ for hadrons, and AE/E = 14~o/~ for electrons.) The charge and the momentum of muons are measured by the muon spectrometer system (Ap/p ,~, 1 0 - 15%). The scintillator trigger hodoscopes E, T, H, V and A, shown in Figure 2, are used to select neutrino interactions in the target and to reject background from cosmic rays, beam muons and neutrino interactions outside the target. A neutrino trigger in the target region is defined by a hit coincidence in E, T and H, which is consistent with a particle trajectory with tan 0 < 0.25 w.r.t, the neutrino beam axis. The measured rate of neutrino interactions is 0.5 event per 1013 protons on the target, corresponding to an effective mass
H. Shibuya/Nuclear Physics B (Proc. Suppl.) 59 (1997) 277-282
COOLBOX at
5"C
279
CHORUS
EMULSION TARGETS AND FIBER TRACKERS HIGH RESOLUTION CALORPaiETER HEXAGONAL] ST MAGNET I IY H T/.c J . T I\,t
TM
ST
DC
!
// BEAM
!
tt --+....
I! SPECTROMETER 15m
Figure 2. Schematic diagram of the CHORUS detector (side view) of 1600 kg. About 50 % of the triggered events originates in the emulsion target. Further improvements of the CHORUS detector have been introduced in 1996: the streamer tube tracker (ST) in front of the calorimeter has been replaced by a more accurate honeycomb tracker (HC). An emulsion tracker (ET) has been added to measure the momentum and charge of hadrons from r candidate events with better accuracy. These are also shown in Figure 2.
2.3. Status o f Data Taking The Run 1 (1994/1995) of the CHORUS experiment began in May 1994 and successfully ended in October 1995. At the middle of the run 1, 1/4 of the emulsion target (1 stack) was taken out, and replaced. It was processed immediately and used to start a pilot analysis described below. The total recorded sample corresponds to an estimate of 320 000 ~,~ CC events in the emulsion target. In October 1995, all emulsion stacks were taken out and processed. The processing
was completed in February 1996. In April 1996, the CHORUS run 2 (1996/1997) began with four new stacks. Two additional trackers, ET and HC, have been installed in April and August 1996, respectively. The sample collected in the completed half of run 2 corresponds to 230 000 CC events in the emulsion. A summary of the CHORUS data taking in 1994-1996 is given in Table 2, showing the number of protons delivered to the beryllium target to produce the neutrino beam, the CHORUS detector efficiency, the number of neutrino triggers recorded, and the expected number of ~, CC events in the emulsion target. 3. T H E P I L O T A N A L Y S I S 3.1. S t r a t e g y A pilot analysis for 1"- ---+ p - P ~ T has been performed without applying any kinematical preselection. The purpose of this pilot analysis is to exercise the whole analysis chain of the experi-
280
H. Shibuya/Nuclear Physics B (Proc. Suppl.) 59 (1997) 277-282
Table 2 Summary of the C H O R U S runs in 1994-1996 1994 1995 1996 p on target (/1019) 0.76 1.20 1.38 % on tape 77% 88% 94% neutrino triggers 400K 547K 638K CC events 120K 200K 230K
ment on a small but unbiased data sample. The signal of the muonic r decay is characterized by a short flight kink of which daughter is a p - and no other lepton originates from the primary vertex. To reduce the number of events to be scanned by 20 %, only events with p~, smaller than 30 GeV/c are considered. Semimuonic decays of charm particles produced in P~ CC (or ~e CC) interactions could be a main background source if the positive muon (or the e +) coming from the primary vertex is missed. It is estimated to be a few x l 0 -s and can therefore be neglected at the present level of statistics.
better accuracy to the special sheet. A similar scanning is then performed in a smaller area of the special sheet. 3) L o c a t i o n of v i n t e r a c t i o n vertices in t h e emulsion target The found track is further followed back in the successive plates of the emulsion target until it disappears. A neutrino interaction vertex should exist at some upstream position in the last emulsion plate in which the track is seen to enter. In this so-called vertex plate, the location of the neutrino interaction vertex is confirmed by looking for all other tracks predicted by the tracking detectors. The matching accuracy for the followed track is sufficiently high, to pick up the correct track, as a ~ 5 p m in position and ~ ~ 4 mrad in angle. Figure 3 shows the distribution of v interaction vertex positions predicted by the fiber tracker system. Distribution of actually located vertices out of the predicted vertices is also shown with a hatched histogram.
3.2. Analysis p r o c e d u r e s The analysis is done as follows. 1) R e c o n s t r u c t i o n o f e v e n t s a n d t r a c k prediction Based on the data recorded in the electronic part of the hybrid detector, neutrino events are reconstructed. If a muon is observed in the muon spectrometer, and its momentum is smaller than 30 GeV/c, track positions and slopes are predicted at the most downstream interface emulsion sheet (CS).
..q 400 350
i~
Located . . . prec.ctecl
~300 ~250
..... .
• fO.q
~
! i
")b:
~
Located p r s o. ~. o .
..... m I0,/"
"70
n~.......~ Predict. d Located
150
v
lOO 50
2) Scanning o f p r e d i c t e d tracks in t h e interface sheets At least one predicted track per event is searched for in the downstream changeable sheet. Track finding efficiency of a fully automatic scanning system, so-called "track selector", is measured to be ,,~ 98 % for tracks with 0 < 0.4. The scanning time by the "track selector" is 2.7 s per view (160 × 120 pro2), currently ,~ 4 minutes per track including the stage movement. If the track is found, it is automatically extrapolated upstream with
-°I*O '
o
lO
20
30
Bulk Emulsion - 3cm
4o* ' '50 plate Number :~
Figure 3. The distribution of ~, interaction vertex positions predicted by the fiber tracker system. Also shown hatched is distribution of located vertex positions out of the predicted vertices.
H. Shibuya/Nuclear PhysicsB (Proc. Suppl.) 59 (1997) 277-282 Non-uniformity of the predicted vertex positions along the beam direction is seen. The reconstruction inefficiency in the upstream side of the emulsion stacks is mainly due to the development of electromagnetic showers in the emulsion stacks, and also due to multiple scattering in the emulsion. However, the vertex finding efficiency (ratio of events located to those predicted) in the upstream half of the emulsion stack is the same as that in the downstream half of the stack, which means that events are not lost during the track following to the vertex. 4) r-kink search A 1"-kink is searched for by two methods, an impact parameter method for "short flight decays" and a pc-kink method for "long flight decays". ~Short flight decay" search by the impact parameter method: To check the possibility whether the # - is a daughter of a r decay inside the vertex plate, the impact parameter of the muon to the primary vertex is calculated. ~Lon9 flight decay" search by the p~.kink method: To check the possibility of a r - decay downstream of the vertex plate, a p,-kink value is determined by the product of the measured muon momentum Pt, and the difference of the track slope measured at the vertex plate and that measured at the special sheet (or the fiber trackers). (p,-kink = PtJ" sin([OvTx - OSS[).) If either the measured muon impact parameter is larger than 10 pm or the value of pc-kink is larger than 250 MeV/c, the event is retained for further analysis. The scanning procedures from 2) to 4) are done fully automatically with computer controlled microscopes.
250 MeV/c, the p- track is measured more precisely by the eye-assisted manual scanning within 5 plates downstream of the vertex plate to give a better estimate on pt-kink. 3.3. Check o f t h e whole analysis b y d i m u o n sample The whole analysis described above and its efficiency can be tested by applying the same method to a sample of events which have two reconstructed muons (a dimuon sample). Some events in the sample should be due to semimuonic decays of charm particles produced in u~ CC interactions. A systematic analysis of the dimuon sample is well under way. Some candidates of charm semimuonic decays have already been found. 3.4. R e s u l t s At present (October, 1996), a total of 7637 CC events with p~ smaller than 30 GeV/c has been located in the emulsion target. For this sample, the full analysis including the eye-assisted manual scanning and the application of the pt-kink cut has been finished. There is no r - candidate left. If a two-neutrino mixing parameterization is used, the probability is expressed as: P(uu ---* u~) = sin2(20~,j, sin 2 \
4E
(1)
with mixing angle 9~,r, difference of mass squared Am 2, source-detector distance L. In the case of large Am 2, the above equation (1) simplifies to: 1 P ( u , ~ uT) = sin~(28,~) • ~-
(2)
Experimentally, this probability is determined by the ratio of observed vr CC events and observed ~,~, CC events N~b*/N°: * (= 2.3/7637 for a 90 % C. L. upper limit) with a correction factor C:
5) Eye-assisted manual scanning At the last step, an eye-assisted manual scanning is done for the small number of events selected by the above procedure and for events with only one found track, for which no impact parameter selection can be applied. If the measured value of pt-kink is larger than
281
/Nob'~ No" . c _.
-
=
i:.r
J
(3)
The correction factor C depends on the average cross section ratio a~/a~. (= 1.89), the muonic decay branching fraction BR ( - 0.18 [7]), the acceptance ratio including reconstruction and ver-
H. Shibuya/Nuclear Physics B (Proc. Suppl.) 59 (1997) 277-282
282
tex finding efficiencies Av,,/Av. (~ 0.93), and the kink finding efficiency ¢~i,~ (~ 0.61):
\~,,/
<4,
\A,,]
The last two factors were estimated by Monte Carlo simulations. By using equations (2), (3) and (4), an upper limit of v~, - v~ oscillation for large Am 2 : sin2(20,,) <_ 0.01 (90% C.L.)
(5)
is obtained. This limit is indicated by an arrow in the exclusion plot together with results from previous experiments|2-4,8] in Figure 4.
(1994/1995) data and is only searching for the muonic r decay. A search for hadronic decay modes should also be accomplished, by using a kinematical preselection if necessary. The improvement in statistics will therefore be a factor of more than 30. The scanning of the whole run 1 data can be performed in about one year by the present 10 automatic microscope systems. The scanning power is further being improved by developing new faster automatic systems. Therefore, in the case of no r signal observed, the sensitivity of the CHORUS proposal sin2(20~,r) _< 3 × 10-4 for large Am s seems to be within reach in two years. 5. C O N C L U S I O N S
The whole analysis of the C H O R U S experiment has been successfullyexercised with the pilot analysisfor the muonic r decay, using the fully automatic emulsion scanning for the firsttime. It has turned out that the proposed sensitivityof the C H O R U S can be obtained.
pilot analysis /
10 s
" ...... I
' '!'"27__
E~I! ':.l,
........
|
~ ......
10 2
10
REFERENCES
~~
194-971
CDHS i ~
-1
lO 10 10
4
t
i
lit|H|
10 4
t
10 n
t
|tlttl|
!
10.2
sln22e~
t
i|lltl[
10.1
i
i
|I|H
1
Figure 4. Exclusion plot in the Am 2 -sin2(20,r) plane. The limit from the pilot analysis is indicated by an arrow.
4. O U T L O O K The pilot analysis described above has been done on a small fraction (~ 14% ) of the run 1
I. H. Harari, Phys. Lett. B216 (1989) 413. 2. N. Ushida et al. (E531 Collaboration), Phys. Rev. Lett. 57 (1986) 2897. 3. K. S. McFarland et al. ( C C F R Collaboration), Phys. Rev. Lett. 75 (1995) 3993. 4. M. Gruwe et al. ( C H A R M II Collaboration), Phys. Lett. B 309 (1993) 463. 5. M. de Jong et al. ( C H O R U S Collaboration), CERN-PPE/93-131, (1993). 6. E. Eskut et al. (CHORUS Collaboration), to be submitted to Nucl. Instr. and Meth. 7. R.M. Barnett et al. (Particle Data Group), Phys. Rev. D 54 (1996) 1. 8. F. Dydak et al. (CDHS Collaboration), Phys. Lett. B 134 (1984) 281.
A search for
Nuclear Physics B (Proc. Suppl.) 59 (1997) 277-282
- ur oscillation with C H O R U S at C E R N
The CHORUS Collaboration presented by Hiroshi Shibuya * ~Department of Physics, Toho University, Funabashi, Japan, E-mail: [email protected], [email protected] A search for v , - ~ r oscillation with the CHORUS detector at CERN is in progress. The first data taking in 1994/1995 was successfully completed with 320 O00 recorded ~, CC events in the emulsion target and half of the second data taking period 1996/1997 has lead to 230 000 additional u~ CC events. A pilot analysis of the 1994 data for the muonic r decay search by using the fully automatic scanning systems, resulting in an upper limit sin2(28~,r) < 0.01 at 90% C.L. for large Am ~, confirms that the proposed sensitivity (sin2(28~,r) <: 3 × 10-~) is within reach in two years.
1. I N T R O D U C T I O N
2. T H E C H O R U S E X P E R I M E N T
The problem of neutrino mass and mixing is one of the most interesting subjects in modern particle physics. Among the three neutrino species, ~'r is predicted to be by far the heaviest on general arguments based on see-saw mechanism and it could have a mass of a few x 10 eV [1]. If it is the case, the ur might be an important constituent of cosmological dark matter in the universe. The ur mass is thus particularly interesting not only for particle physics but also for astrophysics and cosmology. The most sensitive way to find an evidence of finite neutrino masses is to observe neutrino oscillations, which also confirms the existence of neutrino mixings. The WA95/CHORUS at CERN is a dedicated u~, - ~,~ oscillation search experiment looking for the appearance of the inclusive reaction u r N "-* r - X using a nearly pure ~ beam. The reaction is detected by directly observing the characteristic decay topology of the short-lived r lepton in a nuclear emulsion target. The CHORUS experiment aims at improving the sensitivity by more than one order of magnitude over the existing limits [2-4]. As the sensitivity is almost propotional to the number of u~, interactions analyzed, fast and efficient scanning in the emulsion is crucial. It is being done by newly developed automatic scanning systems.
Detailed descriptions of the CHORUS experiment appear elsewhere [5,6], and are summarized here.
0920-5632/97/$17.00 © Elsevier Science B.E All rights mserved. PII S0920-5632(97)00452-0
2.1. T h e C E R N Wide-Band Neutrino Beam The West Area Neutrino Facility(WANF) of the CERN SPS provides a beam which is mainly composed of v~ with an average energy of 26.9 GeV, sufficiently above the 3.5 GeV threshold for a vr charged-current (CC) interaction. 450 GeV protons are extracted from the SPS on to a beryllium target twice in a cycle of 14.4 s (,,~ 2 × 1013 protons/cycle), producing mostly pions and kaons. Neutrinos are then produced from their decays in flight. Each spill has a length of 6 ms, separated by 2.7 s from the other. The distance from the neutrino source to the CHORUS detector is 400 -~ 800 m. Relative abundances and mean energies of all neutrino species are summarized in Table 1. The t'r contamination of the beam is expected to be at the negligible level of urCC/~,#CC ,,, 3 × 10 -6, corresponding to ,,~ 0.1 background event in the 4-year data taking.
2.2. T h e E x p e r i m e n t a l S e t u p The CHORUS detector is a hybrid apparatus composed of a nuclear emulsion target and an electronic detector.
H. Shibuya/Nuclear PhysicsB (Proc. Suppl.) 59 (1997) 277-282
278
Table 1 Composition of the CERN wide-band neutrino beam at the CHORUS experiment v species abundance (%) < Ev > (GeV) v~, 100 26.9
~,
5.6
21.7
Ve Pe
0.7 0.17
47.9 35.3
The nuclear emulsion target, with an excellent spatial resolution of 1 pro, is used to detect directly the production and decay of the shortlived r - from the charged-current (CC) interaction ~ h r ---* r - X , shown schematically in Figure 1 for the case of a muonic r decay.
TAIHiI[T IMULIII~i OTACK
(is er,,uw,~ j~tmd
Iq~R T I t ~
tou.noutrlno (V.r.I
in emulsion
zoom or Vertex
.Yxt._.. hl0erootlen
I
\ -"~...-
decay
~'r
I
-,,~ z,~,
Figure 1. The production and decay of the shortlived r - from the vr CC interaction in the emulsion target.
The emulsion target of 770 kg, almost 4 radiation lengths and 0.32 interaction length in total, is divided into 4 stacks each having an area of 1.4 × 1.4 m 2 and a thickness of 2.8 cm. Each stack is further subdivided into 8 sectors of an area 0.71 × 0.36 m 2, each consisting of 36 emulsion plates placed almost perpendicularly to the beam. A
plate is composed of a 90 pm thick plastic base with 350 pm thick emulsion layers on both sides. Three interface emulsion sheets, one special sheet (SS) and two changeable sheets (CS), are placed between each emulsion stack and the electronic detector. Each interface sheet consists of an 800 pm thick acrylic plate coated on both sides with 100 ~m thick emulsion, which allows a precise angle to be determined (a ~, 1.5 mrad). The electronic detector is used to find the neutrino interactions in the emulsion target, to select r-like events by kinematical cuts and to distinguish v~ CC interactions from various background events. It consists of a scintillating fiber tracker system, scintillator trigger hodoscopes, an air-core magnet, a lead/scintillator calorimeter, and a muon spectrometer, as shown in Figure 2. The fiber tracker system with good spatial resolution and good two-track separation provides event reconstruction and track prediction to the most downstream interface sheet (CS). Accuracy of the prediction from the fiber tracker system at the changeable sheet is currently a ~ 200pro in position and a ~, 3 mrad in angle. Together with the air-core magnet, the fiber tracker also measures the charge and the momentum of traversing particles (Ap/p ~, 0.3 at 5 GeV/c). Energies and directions of the hadronic and electromagnetic showers are measured by the calorimeter. (Its intrinsic resolutions are AE/E = 3 2 % / ~ for hadrons, and AE/E = 14~o/~ for electrons.) The charge and the momentum of muons are measured by the muon spectrometer system (Ap/p ,~, 1 0 - 15%). The scintillator trigger hodoscopes E, T, H, V and A, shown in Figure 2, are used to select neutrino interactions in the target and to reject background from cosmic rays, beam muons and neutrino interactions outside the target. A neutrino trigger in the target region is defined by a hit coincidence in E, T and H, which is consistent with a particle trajectory with tan 0 < 0.25 w.r.t, the neutrino beam axis. The measured rate of neutrino interactions is 0.5 event per 1013 protons on the target, corresponding to an effective mass
H. Shibuya/Nuclear Physics B (Proc. Suppl.) 59 (1997) 277-282
COOLBOX at
5"C
279
CHORUS
EMULSION TARGETS AND FIBER TRACKERS HIGH RESOLUTION CALORPaiETER HEXAGONAL] ST MAGNET I IY H T/.c J . T I\,t
TM
ST
DC
!
// BEAM
!
tt --+....
I! SPECTROMETER 15m
Figure 2. Schematic diagram of the CHORUS detector (side view) of 1600 kg. About 50 % of the triggered events originates in the emulsion target. Further improvements of the CHORUS detector have been introduced in 1996: the streamer tube tracker (ST) in front of the calorimeter has been replaced by a more accurate honeycomb tracker (HC). An emulsion tracker (ET) has been added to measure the momentum and charge of hadrons from r candidate events with better accuracy. These are also shown in Figure 2.
2.3. Status o f Data Taking The Run 1 (1994/1995) of the CHORUS experiment began in May 1994 and successfully ended in October 1995. At the middle of the run 1, 1/4 of the emulsion target (1 stack) was taken out, and replaced. It was processed immediately and used to start a pilot analysis described below. The total recorded sample corresponds to an estimate of 320 000 ~,~ CC events in the emulsion target. In October 1995, all emulsion stacks were taken out and processed. The processing
was completed in February 1996. In April 1996, the CHORUS run 2 (1996/1997) began with four new stacks. Two additional trackers, ET and HC, have been installed in April and August 1996, respectively. The sample collected in the completed half of run 2 corresponds to 230 000 CC events in the emulsion. A summary of the CHORUS data taking in 1994-1996 is given in Table 2, showing the number of protons delivered to the beryllium target to produce the neutrino beam, the CHORUS detector efficiency, the number of neutrino triggers recorded, and the expected number of ~, CC events in the emulsion target. 3. T H E P I L O T A N A L Y S I S 3.1. S t r a t e g y A pilot analysis for 1"- ---+ p - P ~ T has been performed without applying any kinematical preselection. The purpose of this pilot analysis is to exercise the whole analysis chain of the experi-
280
H. Shibuya/Nuclear Physics B (Proc. Suppl.) 59 (1997) 277-282
Table 2 Summary of the C H O R U S runs in 1994-1996 1994 1995 1996 p on target (/1019) 0.76 1.20 1.38 % on tape 77% 88% 94% neutrino triggers 400K 547K 638K CC events 120K 200K 230K
ment on a small but unbiased data sample. The signal of the muonic r decay is characterized by a short flight kink of which daughter is a p - and no other lepton originates from the primary vertex. To reduce the number of events to be scanned by 20 %, only events with p~, smaller than 30 GeV/c are considered. Semimuonic decays of charm particles produced in P~ CC (or ~e CC) interactions could be a main background source if the positive muon (or the e +) coming from the primary vertex is missed. It is estimated to be a few x l 0 -s and can therefore be neglected at the present level of statistics.
better accuracy to the special sheet. A similar scanning is then performed in a smaller area of the special sheet. 3) L o c a t i o n of v i n t e r a c t i o n vertices in t h e emulsion target The found track is further followed back in the successive plates of the emulsion target until it disappears. A neutrino interaction vertex should exist at some upstream position in the last emulsion plate in which the track is seen to enter. In this so-called vertex plate, the location of the neutrino interaction vertex is confirmed by looking for all other tracks predicted by the tracking detectors. The matching accuracy for the followed track is sufficiently high, to pick up the correct track, as a ~ 5 p m in position and ~ ~ 4 mrad in angle. Figure 3 shows the distribution of v interaction vertex positions predicted by the fiber tracker system. Distribution of actually located vertices out of the predicted vertices is also shown with a hatched histogram.
3.2. Analysis p r o c e d u r e s The analysis is done as follows. 1) R e c o n s t r u c t i o n o f e v e n t s a n d t r a c k prediction Based on the data recorded in the electronic part of the hybrid detector, neutrino events are reconstructed. If a muon is observed in the muon spectrometer, and its momentum is smaller than 30 GeV/c, track positions and slopes are predicted at the most downstream interface emulsion sheet (CS).
..q 400 350
i~
Located . . . prec.ctecl
~300 ~250
..... .
• fO.q
~
! i
")b:
~
Located p r s o. ~. o .
..... m I0,/"
"70
n~.......~ Predict. d Located
150
v
lOO 50
2) Scanning o f p r e d i c t e d tracks in t h e interface sheets At least one predicted track per event is searched for in the downstream changeable sheet. Track finding efficiency of a fully automatic scanning system, so-called "track selector", is measured to be ,,~ 98 % for tracks with 0 < 0.4. The scanning time by the "track selector" is 2.7 s per view (160 × 120 pro2), currently ,~ 4 minutes per track including the stage movement. If the track is found, it is automatically extrapolated upstream with
-°I*O '
o
lO
20
30
Bulk Emulsion - 3cm
4o* ' '50 plate Number :~
Figure 3. The distribution of ~, interaction vertex positions predicted by the fiber tracker system. Also shown hatched is distribution of located vertex positions out of the predicted vertices.
H. Shibuya/Nuclear PhysicsB (Proc. Suppl.) 59 (1997) 277-282 Non-uniformity of the predicted vertex positions along the beam direction is seen. The reconstruction inefficiency in the upstream side of the emulsion stacks is mainly due to the development of electromagnetic showers in the emulsion stacks, and also due to multiple scattering in the emulsion. However, the vertex finding efficiency (ratio of events located to those predicted) in the upstream half of the emulsion stack is the same as that in the downstream half of the stack, which means that events are not lost during the track following to the vertex. 4) r-kink search A 1"-kink is searched for by two methods, an impact parameter method for "short flight decays" and a pc-kink method for "long flight decays". ~Short flight decay" search by the impact parameter method: To check the possibility whether the # - is a daughter of a r decay inside the vertex plate, the impact parameter of the muon to the primary vertex is calculated. ~Lon9 flight decay" search by the p~.kink method: To check the possibility of a r - decay downstream of the vertex plate, a p,-kink value is determined by the product of the measured muon momentum Pt, and the difference of the track slope measured at the vertex plate and that measured at the special sheet (or the fiber trackers). (p,-kink = PtJ" sin([OvTx - OSS[).) If either the measured muon impact parameter is larger than 10 pm or the value of pc-kink is larger than 250 MeV/c, the event is retained for further analysis. The scanning procedures from 2) to 4) are done fully automatically with computer controlled microscopes.
250 MeV/c, the p- track is measured more precisely by the eye-assisted manual scanning within 5 plates downstream of the vertex plate to give a better estimate on pt-kink. 3.3. Check o f t h e whole analysis b y d i m u o n sample The whole analysis described above and its efficiency can be tested by applying the same method to a sample of events which have two reconstructed muons (a dimuon sample). Some events in the sample should be due to semimuonic decays of charm particles produced in u~ CC interactions. A systematic analysis of the dimuon sample is well under way. Some candidates of charm semimuonic decays have already been found. 3.4. R e s u l t s At present (October, 1996), a total of 7637 CC events with p~ smaller than 30 GeV/c has been located in the emulsion target. For this sample, the full analysis including the eye-assisted manual scanning and the application of the pt-kink cut has been finished. There is no r - candidate left. If a two-neutrino mixing parameterization is used, the probability is expressed as: P(uu ---* u~) = sin2(20~,j, sin 2 \
4E
(1)
with mixing angle 9~,r, difference of mass squared Am 2, source-detector distance L. In the case of large Am 2, the above equation (1) simplifies to: 1 P ( u , ~ uT) = sin~(28,~) • ~-
(2)
Experimentally, this probability is determined by the ratio of observed vr CC events and observed ~,~, CC events N~b*/N°: * (= 2.3/7637 for a 90 % C. L. upper limit) with a correction factor C:
5) Eye-assisted manual scanning At the last step, an eye-assisted manual scanning is done for the small number of events selected by the above procedure and for events with only one found track, for which no impact parameter selection can be applied. If the measured value of pt-kink is larger than
281
/Nob'~ No" . c _.
-
=
i:.r
J
(3)
The correction factor C depends on the average cross section ratio a~/a~. (= 1.89), the muonic decay branching fraction BR ( - 0.18 [7]), the acceptance ratio including reconstruction and ver-
H. Shibuya/Nuclear Physics B (Proc. Suppl.) 59 (1997) 277-282
282
tex finding efficiencies Av,,/Av. (~ 0.93), and the kink finding efficiency ¢~i,~ (~ 0.61):
\~,,/
<4,
\A,,]
The last two factors were estimated by Monte Carlo simulations. By using equations (2), (3) and (4), an upper limit of v~, - v~ oscillation for large Am 2 : sin2(20,,) <_ 0.01 (90% C.L.)
(5)
is obtained. This limit is indicated by an arrow in the exclusion plot together with results from previous experiments|2-4,8] in Figure 4.
(1994/1995) data and is only searching for the muonic r decay. A search for hadronic decay modes should also be accomplished, by using a kinematical preselection if necessary. The improvement in statistics will therefore be a factor of more than 30. The scanning of the whole run 1 data can be performed in about one year by the present 10 automatic microscope systems. The scanning power is further being improved by developing new faster automatic systems. Therefore, in the case of no r signal observed, the sensitivity of the CHORUS proposal sin2(20~,r) _< 3 × 10-4 for large Am s seems to be within reach in two years. 5. C O N C L U S I O N S
The whole analysis of the C H O R U S experiment has been successfullyexercised with the pilot analysisfor the muonic r decay, using the fully automatic emulsion scanning for the firsttime. It has turned out that the proposed sensitivityof the C H O R U S can be obtained.
pilot analysis /
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Figure 4. Exclusion plot in the Am 2 -sin2(20,r) plane. The limit from the pilot analysis is indicated by an arrow.
4. O U T L O O K The pilot analysis described above has been done on a small fraction (~ 14% ) of the run 1
I. H. Harari, Phys. Lett. B216 (1989) 413. 2. N. Ushida et al. (E531 Collaboration), Phys. Rev. Lett. 57 (1986) 2897. 3. K. S. McFarland et al. ( C C F R Collaboration), Phys. Rev. Lett. 75 (1995) 3993. 4. M. Gruwe et al. ( C H A R M II Collaboration), Phys. Lett. B 309 (1993) 463. 5. M. de Jong et al. ( C H O R U S Collaboration), CERN-PPE/93-131, (1993). 6. E. Eskut et al. (CHORUS Collaboration), to be submitted to Nucl. Instr. and Meth. 7. R.M. Barnett et al. (Particle Data Group), Phys. Rev. D 54 (1996) 1. 8. F. Dydak et al. (CDHS Collaboration), Phys. Lett. B 134 (1984) 281.