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E872 — The direct observation of the ντ
ELSEVIER
Nuclear Physics B (Proc. Suppl.) 70 (1999) 204-206
PROCEEDINGS SUPPLEMENTS
E872 - The direct observation of the u, T.Kafka* for the E872 Collaboration (Aichi, Kobe, Nagoya, Osaka, Toho, and Utsunomiya from Japan; California (Davis), Kansas State, Minnesota, Pittsburgh, South Carolina, and Tufts from the U.S.; Athens, Greece; College de France, Paris, France; Gyeongsang, Changwon, Kon-Kult and Korean National from South Korea) The E872 experiment a.k.a. DONUT (Direct Observation of NU Tau) was a part of the recently concluded Fermilab fixed target run. The tau neutrinos were created by 800-GeV/c Tevatron protons interacting in a beam dump, and neutrino interactions were detected in a nuclear emulsion aided by a downstream spectrometer. In an exposure corresponding to 4.5 x 10” protons on the dump, we estimate 84 f 30 charged-current tau neutrino interactions in the emulsion. A second run of E8’72 is being planned for 1999.
1.
INTRODUCTION
Ever since the discovery of the third family of quarks (bottom) and leptons (tau) in the 1970’s, the tau neutrino has been one of the building blocks of the Standard Model. Nobody doubts its existence, despite the fact that we know it only by its absence, as missing energy in tau lepton decay. u, interactions have not been observed yet because it is not easy to build a v, beam or to prove that you indeed see a v, interaction once you have one. DONUT is an experiment to ob serve the first I+ interactions using the highest energy proton beam available to create the tau neutrinos, and a detector with the best spatial resolution available to detect the interaction. We hope to show over the next year that the combination of Tevatron beam and nuclear emulsion
target can accomplish the task on hand. 2. THE
BEAM
AND
THE
DETECTOR
DONUT is a beam dump experiment. GeV/c protons from Tevatron are stopped l-m-long tungsten beam dump. Among the duced secondary particles are D. mesons that cay into the ru, final state 4.6% of the time, the r decay adds another I+. We calculate 86% of u, interacting in our target originate *High Energy Physics, ford, MA 02155, USA
Tufts University,
800in a prodeand that thii
4 Colby St., Med-
0920-5632/98/$19.00 0 1998 Elsevier Science B.V. PI1 SO920-5632(98)00417-4
All rights reserved.
way, while additional u, are created in B meson decays, D* decays, Drell-Yen events, and in decays of charm produced in secondary x interactions. There is a significant uncertainty in the overall predicted number of u, interactions in the emulsion target, about 30%. We also have to bear in mind that despite the fact that most of the secondary pions interact, few will decay, and we expect lO-‘p in the forward hemisphere per proton in the dump, or 5 x 101op per typical accelerator spill. The experiment is designed to detect u, charged current interactions and identify the final-state tau lepton. Very good spatial resolution is needed because the r lepton lifetime is a fraction of a picosecond, and cm = 87.2pm. Nuclear emulsion is the only detector with sufficient resolution, 1 pm (4 pm) in the transverse (longitudinal) direction. However, its disadvantage is that it is “always on” - it integrates all charged tracks passing through it. We thus have two contradicting requirements to maximise the acceptance, the emulsion target must be as close to the beam dump as possible, but that in itself maximises the flux of muons from the dump through the emulsion. A calculation shows that the DONUT neutrino beamline must be designed to attenuate the muon flux by a factor of lo5 in order to achieve acceptable muon density, < 5 x lo5 crnsa integrated over the life time of the emulsion, in a 50 x 50 cm2 emulsion
T. Kafka/Nuclear Physics B (Proc. Suppl.) 70 (1999) 204-206
target at a distance of 36 m from the beam dump. The beamline consists of two sweeping magnets and a large amount of passive shielding. The first 7 m long sweeping magnet, SELMA, with 3 T magnetic field, is followed by a 5 m long magnet operated at 2 T, and by 18 m of steel and lead shielding (more than 500 tons of steel). The beam dump and the first magnet are completely surrounded by concrete blocks. The steel stack immediately upstream of the emulsion target has notches in its side walls big enough to let the muons from the sweeping magnets pass without interacting. This system provides an adequate “muon shadow” for the emulsion target. In addition, a beam interlock was put in place which disabled the proton beam whenever flux through the trigger counters exceeded a safe level. However, besides the muons, there is a lowenergy background present, consisting of neutrons, gammas and electrons, with a detrimental effect on the emulsion. To minimiie this background, the beam dump area was ventilated away from the experimental hall, a concrete wall was built downstream of the second magnet to separate the beam dump area from the target and detector area, and the emulsion target structure was covered by lead shielding. These measures allowed us to keep the low energy electron track density to < 7 x lo5 cm-‘. The detector system includes an upstream veto wall of two rows of verticrd scintillation counters, and three planes of trigger counters, the first two placed within the emulsion target structure, and the third at the upstream end of the analyring magnet. The emulsion target structure has space for four 7 cm thick emulsion modules, each containing between 50 and 80 individual sheets. Downstream of every emulsion module are several planes of scintillating fibers, each bundle viewed by a Hamamatsu image intensifier enclosed in a soft iron magnetic shielding. The system contains a total of 85,000 500~pm-thick fibers organised into 44 planes. The analyming magnet provides enough of a pi kick to enable momentum measurement of a 15-GeV/c track with 3% momentum resolution. Two sets of jet chambers are located within the magnet aperture. At the upstream end are three
205
100 x 70 cm2 Vertex Drift Chambers, with wire directions vertical, and 4~5’ off vertical, respectively. Two additional chambers are placed in the middle of the magnet, one with horisontal wires and the other with vertical wires. Three large (330 x 160 cm’) drift chambers, with four planes each, are placed downstream of the magnet. Resolution of the scintillation fibers and the vertex chambers is presently 200 pm, and it is 350 pm for the large drift chambers. The spectrometer calibration and alignment analysis is in progress. The next element downstream is the Electromagnetic Calorimeter containing 400 lead glass and scintillating glass blocks. The central region contains scintillating glass, 100 7.5 x 7.5 x 89 cm3 blocks, and 74 15 x 15 x 89 cm3 blocks, all 21 radiation lengths and 2 nuclear absorption lengths deep. The outer region contains 224 15 x 15 x 41.5 cm3 lead-glass blocks. The last part of the spectrometer, for muon identification, consists of three steel absorber walls, the first one 42 cm thick, and the other two 91 cm thick each. Each wall is followed by a detector plane containing 4-cm wide proportional tubes over most of the area, individual lengths ranging from 1.3 m to 6.25 m. The high-muonflux areas are covered by scintillator hodoscopes built from 4 cm wide strips to match the resolution of the proportional tubes, with lengths between 100 and 230 cm. The system contains a total of 992 proportional tubes and 448 scintillator strips.
3. THE TIAL
EMULSION ANALYSIS
TARGET
AND
INI-
In a typical emulsion target experiment, a track is followed back from a coarser resolution medium into one with finer resolution: from drift chamber to vertex chamber to scintillating fiber planes - but even there the resolution is not sufficient to pinpoint a track in the emulsion stack without ambiguity. To achieve that, every emulsion stack has a special changeable sheet on both surfaces. These sheets were changed once a week to keep the track density low enough to achieve unambiguous extrapolation, about one track per 200 x 200 pm field of view.
206
I? Kafka/Nuclear Physics B (Proc. Suppl.) 70 (1999) 204-206
Table 1 ES72 emulsion Module 1 2 3 4 5 6 7 Total
target
modules. Emul.
mass
Type
Mass
lhl
[%I
ECC ECC ECC/Bulk ECC/Bulk ECC/Bulk ECC/Bulk Bulk
98.4 98.4 70.9 67.9 66.6 69.4 54.8
10.1 10.1 55.4 39.5 35.3 24.2 94.3
In previous experiments, such as E531, E653, or CHORUS, only one type of emulsion module was used, the bulk emulsion, while E872 added a new emulsion stack design, Emulsion Cloud Chamber (ECC). A bulk emulsion sheet is a 90 pm thick plastic sheet covered by 320 pm thick layer of emulsion on each side. The analysis of a bulk emulsion sheet stack is conceptually straightforward as practically all vertices and full track lengths are visible. The ECC design alternates 1 mm thick stainless steel sheets and thin emulsion sheets, 200 pm thick plastic sheets with a 100 pm thick emulsion layer on each side. Therefore, emulsion forms only 10 % of the total mass, the cost is substantially reduced, and the design of massive emulsion targets suitable for use in long baseline experiments becomes feasible. Besides its main purpose of discovering u, interactions, E872 also serves as a proving ground for thii novel target design. A new and different analysis will be necessary, as most interaction vertices wilI occur within the steel plates, and the emulsion will serve only as a sampling detector for the tracks. During the experimental run, E872 used a total of seven emulsion target modules, for two reasons: (i) to limit the background track density in individual modules, and (ii) to test various module designs. Information on all modules can be found in Table 1 which lists the module type, total mass, emulsion mass fraction, the corresponding beam dump exposure, and numbers of neutrino interactions (I+,, ve, Ye) and of I+ interactions estimated without detection efficiencies taken into account.
p.0.t.
x 1o17 2.57 1.17 1.89 3.35 3.35 1.87 1.87 4.55
u int. est. 395 180 210 356 349 203 161 1854
v, int. est. 18 8 10 16 16 9 7 84
Emulsion module number 2 was the first to be taken out and developed, and its analysis is under way. While a new automatic emulsion scanning machine was being brought into operation in Nagoya, analysis of the spectrometer data began at Fermilab. First, the raw data were subject to a software filter (“stripping”) requiring that one of the following conditions be satisfied: (i) One or more tracks in the drift chambers pointing back to within 50 cm of the fiber tracker; (ii) Vertex in the emulsion reconstructed in the u-view of the fiber tracker; (iii) More than 30 GeV deposited in the Electromagnetic Calorimeter. In this step, the amount of data was reduced by a factor of 200. 95% of v7 CC Monte Carlo events pass these criteria. The resulting data set was then visually scanned by physicists with the aid of an event display program using the following criteria: (i) Require two or more tracks with a vertex in an emulsion module; (ii)Require no track in upstream fibers pointing to the vertex to eliminate interactions of incoming charged particles; (iii) Require more than 2 GeV energy in the spectrometer. The amount of data reported here corresponds to 4.5 x 1016 p.o.t. We calculate that 50 neutrino interactions in module 2 correspond to this exposure, which should yield 44 events after requisite efficiencies are taken into account. Our first scan yielded 51 events. The first event was located in an Emulsion Module at the beginning of October. The Collaboration plans to complete the scan of all available data by the end of 1998.
Nuclear Physics B (Proc. Suppl.) 70 (1999) 204-206
PROCEEDINGS SUPPLEMENTS
E872 - The direct observation of the u, T.Kafka* for the E872 Collaboration (Aichi, Kobe, Nagoya, Osaka, Toho, and Utsunomiya from Japan; California (Davis), Kansas State, Minnesota, Pittsburgh, South Carolina, and Tufts from the U.S.; Athens, Greece; College de France, Paris, France; Gyeongsang, Changwon, Kon-Kult and Korean National from South Korea) The E872 experiment a.k.a. DONUT (Direct Observation of NU Tau) was a part of the recently concluded Fermilab fixed target run. The tau neutrinos were created by 800-GeV/c Tevatron protons interacting in a beam dump, and neutrino interactions were detected in a nuclear emulsion aided by a downstream spectrometer. In an exposure corresponding to 4.5 x 10” protons on the dump, we estimate 84 f 30 charged-current tau neutrino interactions in the emulsion. A second run of E8’72 is being planned for 1999.
1.
INTRODUCTION
Ever since the discovery of the third family of quarks (bottom) and leptons (tau) in the 1970’s, the tau neutrino has been one of the building blocks of the Standard Model. Nobody doubts its existence, despite the fact that we know it only by its absence, as missing energy in tau lepton decay. u, interactions have not been observed yet because it is not easy to build a v, beam or to prove that you indeed see a v, interaction once you have one. DONUT is an experiment to ob serve the first I+ interactions using the highest energy proton beam available to create the tau neutrinos, and a detector with the best spatial resolution available to detect the interaction. We hope to show over the next year that the combination of Tevatron beam and nuclear emulsion
target can accomplish the task on hand. 2. THE
BEAM
AND
THE
DETECTOR
DONUT is a beam dump experiment. GeV/c protons from Tevatron are stopped l-m-long tungsten beam dump. Among the duced secondary particles are D. mesons that cay into the ru, final state 4.6% of the time, the r decay adds another I+. We calculate 86% of u, interacting in our target originate *High Energy Physics, ford, MA 02155, USA
Tufts University,
800in a prodeand that thii
4 Colby St., Med-
0920-5632/98/$19.00 0 1998 Elsevier Science B.V. PI1 SO920-5632(98)00417-4
All rights reserved.
way, while additional u, are created in B meson decays, D* decays, Drell-Yen events, and in decays of charm produced in secondary x interactions. There is a significant uncertainty in the overall predicted number of u, interactions in the emulsion target, about 30%. We also have to bear in mind that despite the fact that most of the secondary pions interact, few will decay, and we expect lO-‘p in the forward hemisphere per proton in the dump, or 5 x 101op per typical accelerator spill. The experiment is designed to detect u, charged current interactions and identify the final-state tau lepton. Very good spatial resolution is needed because the r lepton lifetime is a fraction of a picosecond, and cm = 87.2pm. Nuclear emulsion is the only detector with sufficient resolution, 1 pm (4 pm) in the transverse (longitudinal) direction. However, its disadvantage is that it is “always on” - it integrates all charged tracks passing through it. We thus have two contradicting requirements to maximise the acceptance, the emulsion target must be as close to the beam dump as possible, but that in itself maximises the flux of muons from the dump through the emulsion. A calculation shows that the DONUT neutrino beamline must be designed to attenuate the muon flux by a factor of lo5 in order to achieve acceptable muon density, < 5 x lo5 crnsa integrated over the life time of the emulsion, in a 50 x 50 cm2 emulsion
T. Kafka/Nuclear Physics B (Proc. Suppl.) 70 (1999) 204-206
target at a distance of 36 m from the beam dump. The beamline consists of two sweeping magnets and a large amount of passive shielding. The first 7 m long sweeping magnet, SELMA, with 3 T magnetic field, is followed by a 5 m long magnet operated at 2 T, and by 18 m of steel and lead shielding (more than 500 tons of steel). The beam dump and the first magnet are completely surrounded by concrete blocks. The steel stack immediately upstream of the emulsion target has notches in its side walls big enough to let the muons from the sweeping magnets pass without interacting. This system provides an adequate “muon shadow” for the emulsion target. In addition, a beam interlock was put in place which disabled the proton beam whenever flux through the trigger counters exceeded a safe level. However, besides the muons, there is a lowenergy background present, consisting of neutrons, gammas and electrons, with a detrimental effect on the emulsion. To minimiie this background, the beam dump area was ventilated away from the experimental hall, a concrete wall was built downstream of the second magnet to separate the beam dump area from the target and detector area, and the emulsion target structure was covered by lead shielding. These measures allowed us to keep the low energy electron track density to < 7 x lo5 cm-‘. The detector system includes an upstream veto wall of two rows of verticrd scintillation counters, and three planes of trigger counters, the first two placed within the emulsion target structure, and the third at the upstream end of the analyring magnet. The emulsion target structure has space for four 7 cm thick emulsion modules, each containing between 50 and 80 individual sheets. Downstream of every emulsion module are several planes of scintillating fibers, each bundle viewed by a Hamamatsu image intensifier enclosed in a soft iron magnetic shielding. The system contains a total of 85,000 500~pm-thick fibers organised into 44 planes. The analyming magnet provides enough of a pi kick to enable momentum measurement of a 15-GeV/c track with 3% momentum resolution. Two sets of jet chambers are located within the magnet aperture. At the upstream end are three
205
100 x 70 cm2 Vertex Drift Chambers, with wire directions vertical, and 4~5’ off vertical, respectively. Two additional chambers are placed in the middle of the magnet, one with horisontal wires and the other with vertical wires. Three large (330 x 160 cm’) drift chambers, with four planes each, are placed downstream of the magnet. Resolution of the scintillation fibers and the vertex chambers is presently 200 pm, and it is 350 pm for the large drift chambers. The spectrometer calibration and alignment analysis is in progress. The next element downstream is the Electromagnetic Calorimeter containing 400 lead glass and scintillating glass blocks. The central region contains scintillating glass, 100 7.5 x 7.5 x 89 cm3 blocks, and 74 15 x 15 x 89 cm3 blocks, all 21 radiation lengths and 2 nuclear absorption lengths deep. The outer region contains 224 15 x 15 x 41.5 cm3 lead-glass blocks. The last part of the spectrometer, for muon identification, consists of three steel absorber walls, the first one 42 cm thick, and the other two 91 cm thick each. Each wall is followed by a detector plane containing 4-cm wide proportional tubes over most of the area, individual lengths ranging from 1.3 m to 6.25 m. The high-muonflux areas are covered by scintillator hodoscopes built from 4 cm wide strips to match the resolution of the proportional tubes, with lengths between 100 and 230 cm. The system contains a total of 992 proportional tubes and 448 scintillator strips.
3. THE TIAL
EMULSION ANALYSIS
TARGET
AND
INI-
In a typical emulsion target experiment, a track is followed back from a coarser resolution medium into one with finer resolution: from drift chamber to vertex chamber to scintillating fiber planes - but even there the resolution is not sufficient to pinpoint a track in the emulsion stack without ambiguity. To achieve that, every emulsion stack has a special changeable sheet on both surfaces. These sheets were changed once a week to keep the track density low enough to achieve unambiguous extrapolation, about one track per 200 x 200 pm field of view.
206
I? Kafka/Nuclear Physics B (Proc. Suppl.) 70 (1999) 204-206
Table 1 ES72 emulsion Module 1 2 3 4 5 6 7 Total
target
modules. Emul.
mass
Type
Mass
lhl
[%I
ECC ECC ECC/Bulk ECC/Bulk ECC/Bulk ECC/Bulk Bulk
98.4 98.4 70.9 67.9 66.6 69.4 54.8
10.1 10.1 55.4 39.5 35.3 24.2 94.3
In previous experiments, such as E531, E653, or CHORUS, only one type of emulsion module was used, the bulk emulsion, while E872 added a new emulsion stack design, Emulsion Cloud Chamber (ECC). A bulk emulsion sheet is a 90 pm thick plastic sheet covered by 320 pm thick layer of emulsion on each side. The analysis of a bulk emulsion sheet stack is conceptually straightforward as practically all vertices and full track lengths are visible. The ECC design alternates 1 mm thick stainless steel sheets and thin emulsion sheets, 200 pm thick plastic sheets with a 100 pm thick emulsion layer on each side. Therefore, emulsion forms only 10 % of the total mass, the cost is substantially reduced, and the design of massive emulsion targets suitable for use in long baseline experiments becomes feasible. Besides its main purpose of discovering u, interactions, E872 also serves as a proving ground for thii novel target design. A new and different analysis will be necessary, as most interaction vertices wilI occur within the steel plates, and the emulsion will serve only as a sampling detector for the tracks. During the experimental run, E872 used a total of seven emulsion target modules, for two reasons: (i) to limit the background track density in individual modules, and (ii) to test various module designs. Information on all modules can be found in Table 1 which lists the module type, total mass, emulsion mass fraction, the corresponding beam dump exposure, and numbers of neutrino interactions (I+,, ve, Ye) and of I+ interactions estimated without detection efficiencies taken into account.
p.0.t.
x 1o17 2.57 1.17 1.89 3.35 3.35 1.87 1.87 4.55
u int. est. 395 180 210 356 349 203 161 1854
v, int. est. 18 8 10 16 16 9 7 84
Emulsion module number 2 was the first to be taken out and developed, and its analysis is under way. While a new automatic emulsion scanning machine was being brought into operation in Nagoya, analysis of the spectrometer data began at Fermilab. First, the raw data were subject to a software filter (“stripping”) requiring that one of the following conditions be satisfied: (i) One or more tracks in the drift chambers pointing back to within 50 cm of the fiber tracker; (ii) Vertex in the emulsion reconstructed in the u-view of the fiber tracker; (iii) More than 30 GeV deposited in the Electromagnetic Calorimeter. In this step, the amount of data was reduced by a factor of 200. 95% of v7 CC Monte Carlo events pass these criteria. The resulting data set was then visually scanned by physicists with the aid of an event display program using the following criteria: (i) Require two or more tracks with a vertex in an emulsion module; (ii)Require no track in upstream fibers pointing to the vertex to eliminate interactions of incoming charged particles; (iii) Require more than 2 GeV energy in the spectrometer. The amount of data reported here corresponds to 4.5 x 1016 p.o.t. We calculate that 50 neutrino interactions in module 2 correspond to this exposure, which should yield 44 events after requisite efficiencies are taken into account. Our first scan yielded 51 events. The first event was located in an Emulsion Module at the beginning of October. The Collaboration plans to complete the scan of all available data by the end of 1998.