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Observation of a first ντ candidate event in the OPERA experiment in the CNGS beam
Available online at www.sciencedirect.com
Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 38–42 www.elsevier.com/locate/npbps
Observation of a first ντ candidate event in the OPERA experiment in the CNGS beam O.Sato (Nagoya University) For the OPERA Collaboration
Abstract The OPERA experiment is aimed to prove the existence of νμ → ντ oscillation through the observation of the tau decay topology in ντ CC interactions. The physics run started in 2008 and, since then, the lead-emulsion target has been exposed to the CNGS neutrino beam every year, from spring to autum. Untill now, 1921 neutrino interaction vertices have been located and the decay search analysis has been completed for 1088 of them. One tau neutrino candidate has been found. Here, the event location and decay search procedures will be described and the tau neutrino candidate is presented. Keywords: tau neutrino, neutrino oscillation, appearance method
1. introduction The oscillation between neutrinos and antineutrinos was introduced by Pontecorvo [1]. The phenomenon of neutrino oscillation, the transition from a neutrino flavour to another, was first discussed by Maki,Nakagawa,Sakata in 1962 [2]. Two types of experimental methods can be used to detect such oscillations : observe the appearance of a neutrino flavour initially absent in the beam or measure the disappearance rate of the initial flavour. OPERA is aimed to detect νμ → ντ oscillation through the appearance method, by observing, in an alsmot pure νμ beam, the production and subsequent decay of tau leptons in ντ charged-current (CC) interactions. The disappearance of muonic neutrinos have been convincingly observed in different experiments [3] [4]. In the SNO[5] solar neutrino experiment, the measured rate of neutral-current (NC) interactions was shown to be compatible with the rate expected from total solar neutrino flux, supporting the idea that all flavour transitions occur among the three active flavours of Standard Model. However, there is no direct evidence yet of neutrino oscillation by the appearance method where the new flavour is identified by the identification of the 0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.09.006
charged lepton produced in its CC interaction.
2. The beam The CNGS(CERN Neutrino to Gran Sasso) νμ beam produced by the CERN-SPS is directed towards the OPERA detector at LNGS, 730 km away. It is a wide band beam, with an averaged energy of 17 GeV, optimized for the observation of oscillated ντ CC interactions. The νμ beam purity is about 97% with small contaminations of ν¯μ (2.1%) and νe +ν¯e (below 1.%) in terms of interaction rates. The prompt ντ component is negligibly small. The goal is to accumulate a statistics of neutrino interactions correspomding to 22.5 ×1019 proton on target (POT) in 5 years. The 2008 and 2009 runs achieved a total intensity of 1.78 ×1019 and 3.52 ×1019 POT respectively. The 2010 run is still in progress and the current total intensity is 0.81 ×1019 POT (@2010 Jun06) 1 .
1 At the time of writing, The achieved intensity is 4.04×1019 POT at the end of 2010 run.
O. Sato / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 38–42
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3. The detector The OPERA detector installed in the underground laboratory of LNGS has 3 main components (Fig.1).
Figure 2: Emultion Cloud Chamber structure
Figure 1: The OPERA detector in the LNGS hall C, at a depth corresponding to 3,100 m water equivalent overburden. The CNGS neutrino beam comes from left. The person on the left carries one of the 150,000 ECC bricks. The total target mass is 1.25kton.
The first one is an active target, consisting of 150,000 Emulsion Cloud Chamber (ECC) modules called ”bricks”. A brick is a sandwitch structure of 57 nuclear emulsion films and 56 1 mm thick lead plates (Fig.2). The lead plates serve as neutrino interaction target and the emulsion films as 3-dementional tracking detector providing track coordinates with a sub-micron accuracy and track angles with a few mrad accuracy. The material of a brick along the beam direction corresponds to about 10 radiation length and 0.33 interaction lengths. The brick size is 10cm × 12.5cm × 8cm and weigh about 8.3 kg. The total active target mass is thus 1250 tons. The target is subdivided in two identical units, each consisting of 31 walls of bricks. The second main component of the detector is the target tracker (TT) system, a set of scintilator planes interleaved with the brick walls. Each TT plane consists of 256 2.6cm wide plastic strips. The sandwitch structure of the brick walls and the TT planes is illustrated in Fig.3. The third detector component consists in two muon spectorometers following the two target units. Equipped with RPC and high precision drift chambers, they allow determining the muon momentum with better than 20% accuracy up to 30 GeV/c. 4. Neutrino interaction location From the energy deposited in the TT scintillator strips, it is possible to estimate the origin of each neutrino interaction and to define the brick which most probably contains the neutrino interaction vertex. The
Figure 3: Arrangement of the ECC brick walls and the TT scintillator planes.
accuracy of the TT reconstruction is however not much smaller than the brick size and a non negligible fraction of the predicted brick do not contain the neutrino vertex. To solve this problem, an interface device (so called Changeable Sheet) is attached on the downstream face of each brick. It is made of two emulsion films prepared and packed together in the underground LNGS laboratory in order to be exposed to a minimum ammount of cosmic rays. So, tracks found in the CS will mainly be related to the neutrino beam. For each event, the brick predicted as the most probable one is removed from the detector. Its CS is detached and developped for scanning. Two stations, at Nagoya [6] and at LNGS [7], share the CS scanning load. The scanning area is about 16cm2 for events with a reconstructed muon track and 50 to 120cm2 otherwise. If tracks are found in the CS films, the corresponding brick
40
O. Sato / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 38–42
films are developed after a short exposure to cosmic rays for alignment purposes. If, on the contrary, no track is found in the CS, the brick is put back in the detector with a new CS and the brick with second to highest probability to contain the neutrino vertex is extracted. The same procedure is repeated until a validated brick is found. The automated scanning microscopes can treat about 100 CS per week in each of the two stations. After development, the brick films are dispatched to scanning laboratories in Japan and in Europ to complete the event location in the brick. The tracks found in the CS are extrapolated to the most downstream film of the brick with an accuracy about 100 μm. When found in this film, the tracks are followed upstream from film to film. This scan-backprocedure is quite fast thanks to the good alignment accuracy between films. The procedure is stopped when no track candidate is found in two continuous films and the lead plate just upstream the last detected track segment is defined as the vertex plate. Around the estimated vertex position in this plate, a scanning volume is defined with a length 10 films and a transverse area of 1 × 1cm2 . All track segments in this volume are collected and analysed. After rejection of the passing through cosmic rays and of the tracks due to low energy particles, the tracks produced by the neutrino interaction can be selected and reconstructed. The neutrino vertex position can then be defined with a few micron precision. 5. Decay search The main signature of a secondary activity (decay or nuclear inetarction) is the observation of a track with a significant impact parameter (IP) relative to the neutrino interaction vertex. The expected tau daughter’s IP distribution is shown on the left plot of Fig.4. Its mean value is about 100 μm. We may recall here that 85 % of the taus decay into a 1-prong topology and 15 % into 3 prongs. For tracks emitted at the primary vertex, the IP distribution reflects the track measurement accuracy except for low momentum tracks where Multiple Coulomb Scattering (MCS) in the lead plate plays a dominant role. The right plot of Fig.4 shows the expected and observed IP distributions for primary tracks of momentum higher than 1 GeV/c. Their mean values are well bellow 10 μm. In practice, the momentum of all tracks reconstructed as described in the previous section was estimated from observed differences of the connected track segments. After filtering out the low momentum tracks, the tracks with an IP between 10 μm and 500 μm were selected and checked by visual inspection.
Figure 4: Left: Simulated tau daughter’s impact parameter distribution. Right: Comparison with the impact parameter distribution expected for tracks emitted at the primary vertex. The observed distribution(black dots) dose not include the tau and charm decay candidates discussed in the text.
A total of 1088 events were analysed up to now. Among the 187 events with no identified muon, one event has a track with a significant IP and is presented in the next section as a tau decay candidate. In the CC like sample (901 events), 20 charm decay candidates were observed. For the analysed sample, the Monte Carlo simulation expectations are 0.54 ± 0.13 detected tau decays (at Δ m223 = 2.5 × 10−3 eV 2 and full mixing) and 16 ± 2.9 detected cham decays plus 2 backgroud events for charm decays. 6. The first candidate event In this section, the first tau neutrino candidate [8] will be described. The location procedure explained in section 4 yielded a neutrino interaction vertex with 7 tracks of which one exhibits a visible kink with an angular change of 41 ± 2 mrad. The kink daughter momentum is estimated to 12+6 −3 GeV/c by MCS measurement and its momentum transverse to the parent direction is 470+230 −120 MeV/c. Fig.5 shows a schematic side view of the vertex topology. The daughter track has been followed in the downstream bricks and, after crossing 7 walls, it is seen to generate a hadronic interaction. Hence, the daughter particle is identified as a hadron. The other tracks from the neutrino interaction were also followed until they stop or interact. Three of them are identified as due to hadrons and the probability that one of the 3 other tracks is left by a muon is estimated to less than 0.1%. None of them is compatible with being that of an electron. A dedicated search for converted gamma rays in the vicinity of the interaction yielded two gamma candidates called γ1 and γ2 . The energy of γ1 is 5.6±1.0(stat.)±1.7(syst.) GeV and it is clearly pointing to the decay vertex and not to the neutrino interaction
O. Sato / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 38–42
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rection larger than 300 MeV/c. At the primary vertex, the main selection cut is that the angle φ in the transverse plane between the tau candidate track and the hadronic shower momentum direction must be larger than 90 degrees. The measured φ angle for the candidate event is 173 ± 2 degree, strongly favoring the ντ CC hypothesis. The invariant mass of the two observed gammas is 120 ± 20(stat.)±35(syst.), supporting the hypothesis that they are emitted in a π0 decay. The invariant mass of the charged decay daughter assumed to be a π− and the 2 +100 gammas is calculated to be 640+125 −80 (stst.)−90 (syst.) consistent with the ρ(770) mass. So the decay mode of the candidate is consistent with the hypothesis τ− → ρ− +ντ .
Figure 5: The tau neutrino candidate schematic side view
vertex. γ2 has an energy of 1.2 ± 0.4(stat.) ±0.4(syst.) GeV and is compatible with pointing to either vertex but with a much higher probability to the decay vertex. Fig.6 shows a longitudinal view of the event display while, in Fig.7, the view is transeverse to the beam direction. The white segments in these displays are the track segments measured in the emulsion films. All the selection cuts discussed in the OPERA proposal [9] are well satisfiled by this event : the kink angle is larger than 20 mrad, the daughter momentum larger than 2 GeV/c and its transverse component to parent di-
Figure 6: Display of the tau neutrino candidate (side view)
Figure 7: Display of the tau neutrino candidate (view transverse to the beam direction)
The two main sources of background on the hadronic tau decay mode were investigated. The first one is the charm production in νμ CC interactions in which the muon track is not identified. The second one is the production in a νμ NC event of a hadron making after a short flight path an interaction which mimics a tau decay. The Monte Carlo expectation for the first background source is 0.007 ± 0.004(syst.) event and, for the second background source, 0.011 ± 0.006(syst.) event. Two kinds of experimental validation for these Monte Carlo estimation were performed. The first one is the study of the inelastic interactions produced by a π beam in an ECC brick. So far, 119 inetarctions were analysed. The prong multiplicity and the Pt distribution are in good agreement with the Monte Carlo expectations. The fraction of intercations which mimic a tau decay by kink toplology is 7.4 ± 1.1% for the data and 7.6 ± 0.3% for Monte Carlo simulation.
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O. Sato / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 38–42
The other check is the study of interactions produced by the charged particles emitted in the neutrino interactions collected so far by OPERA. The tracks were followed over long distances, well above a possible decay range. The total track length for this study was 8.6m and no secondary interaction was found which could mimic a tau decay in the transverse momeentum signal region (Pt> 0.3 GeV/c). At lower Pt, 7 interactions were found with a possible decay toplology (all with Pt< 0.2 GeV/c) while 8 are predicted by the Monte Carlo simulation. The total background on the 1-prong hadronic tau decay mode is then 0.018 ± 0.007(syst.) event which corresponds to a signal significance of 2.36 σ for the observation of one tau candidate. Since not only 1-prong hadronic mode but also other modes (muonic and electronic and 3 prond decays) were searched for, the total estimated background increases to 0.045 ± 0.023(syst.) event and the significance of the observed candidate is reduced to 2.01 σ. 7. Summary Since 2008, OPERA is collecting neutrino ineteractions in the ECC targets. Up to now (June 2010), 1921 neutrino vertices have been precisely located and the decay search analysis has been completed for a subsample of 1088 events. A first tau-neutrino candidate has been found with an estimated background of 0.045 ± 0.020(syst.) event. This observation has a significance of not being a background fluctuation as 2.36 σ for analysis of the 1-prong hadronic decay mode where the candidate event was found and 2.01 σ for including all other decay modes where no candidate event was found. The 2010 physics run 2 is in progress and the experiment is forseen to continue for two aditional beam years.
References [1] B.Pontecorvo, Sov.Phys.JETP 6, 429(1957) B.Pontecorvo, Sov.Phys.JETP 7, 172(1958) [2] Z.Maki,M.Nakagawa,S.Sakata Prog.Theor.Phys.28:870-880,1962 [3] SUPER KAMIOKANDE collaboration, Y.Fukuda et al., Phys.Rev.Lett.81(1998)1562; SUPER KAMIOKANDE collaboration, K.Ave et al., Phys.Rev.Lett.97(2006)171801;
2 At the time of writing, The 2010 run beam exposure was successfully finished with the achieved intensity 4.04×1019 POT.
[4] MINOS collaboration, D.G.Michael et al., Phys.Rev.Lett.97(2006)191801; MINOS collaboration, P.Adamson et al., Phys.Rev.Lett.101(2008)221804; [5] SNO collaboration, Q.R.Ahmad et al., Phys.Rev.Lett.89(2002)011301 [6] K.Morishima and T.Nakano, JINST(2010) 5 P0411 [7] N.Armenise et al., Nucl.Instrum. Meth.A 551 (2005) 261; M.De Serio et al., Nucl.Instrum. Meth.A 554 (2005) 247; L.Arrabito et al., Nucl.Instrum. Meth.A 568 (2006) 578; [8] Observation of a first ντ candidate in the OPERA experiment in the CNGS beam. OPERA collaboration Phys.Lett.B691:138145,2010 [9] A.Ereditato, K.Niwa and P.Strolin, The emulsion technique for short, medium and long baseline νμ → ντ oscillation experiments, 423,INFN-AE-97-06,DPNU-97-07; OPERA collaboration, M.Guler et al., An appearance experiment to search for νμ → ντ oscillations in the CNGS beam: experimental proposal,CERN-SPSC-2000-028,LNGS P25/2000 OPERA collaboration, M.Guler et al., Status Report on the OPERA experiment,CERN/SPSC 2001-025,LNGS-EXP 30/2001 add.1/01
Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 38–42 www.elsevier.com/locate/npbps
Observation of a first ντ candidate event in the OPERA experiment in the CNGS beam O.Sato (Nagoya University) For the OPERA Collaboration
Abstract The OPERA experiment is aimed to prove the existence of νμ → ντ oscillation through the observation of the tau decay topology in ντ CC interactions. The physics run started in 2008 and, since then, the lead-emulsion target has been exposed to the CNGS neutrino beam every year, from spring to autum. Untill now, 1921 neutrino interaction vertices have been located and the decay search analysis has been completed for 1088 of them. One tau neutrino candidate has been found. Here, the event location and decay search procedures will be described and the tau neutrino candidate is presented. Keywords: tau neutrino, neutrino oscillation, appearance method
1. introduction The oscillation between neutrinos and antineutrinos was introduced by Pontecorvo [1]. The phenomenon of neutrino oscillation, the transition from a neutrino flavour to another, was first discussed by Maki,Nakagawa,Sakata in 1962 [2]. Two types of experimental methods can be used to detect such oscillations : observe the appearance of a neutrino flavour initially absent in the beam or measure the disappearance rate of the initial flavour. OPERA is aimed to detect νμ → ντ oscillation through the appearance method, by observing, in an alsmot pure νμ beam, the production and subsequent decay of tau leptons in ντ charged-current (CC) interactions. The disappearance of muonic neutrinos have been convincingly observed in different experiments [3] [4]. In the SNO[5] solar neutrino experiment, the measured rate of neutral-current (NC) interactions was shown to be compatible with the rate expected from total solar neutrino flux, supporting the idea that all flavour transitions occur among the three active flavours of Standard Model. However, there is no direct evidence yet of neutrino oscillation by the appearance method where the new flavour is identified by the identification of the 0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.09.006
charged lepton produced in its CC interaction.
2. The beam The CNGS(CERN Neutrino to Gran Sasso) νμ beam produced by the CERN-SPS is directed towards the OPERA detector at LNGS, 730 km away. It is a wide band beam, with an averaged energy of 17 GeV, optimized for the observation of oscillated ντ CC interactions. The νμ beam purity is about 97% with small contaminations of ν¯μ (2.1%) and νe +ν¯e (below 1.%) in terms of interaction rates. The prompt ντ component is negligibly small. The goal is to accumulate a statistics of neutrino interactions correspomding to 22.5 ×1019 proton on target (POT) in 5 years. The 2008 and 2009 runs achieved a total intensity of 1.78 ×1019 and 3.52 ×1019 POT respectively. The 2010 run is still in progress and the current total intensity is 0.81 ×1019 POT (@2010 Jun06) 1 .
1 At the time of writing, The achieved intensity is 4.04×1019 POT at the end of 2010 run.
O. Sato / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 38–42
39
3. The detector The OPERA detector installed in the underground laboratory of LNGS has 3 main components (Fig.1).
Figure 2: Emultion Cloud Chamber structure
Figure 1: The OPERA detector in the LNGS hall C, at a depth corresponding to 3,100 m water equivalent overburden. The CNGS neutrino beam comes from left. The person on the left carries one of the 150,000 ECC bricks. The total target mass is 1.25kton.
The first one is an active target, consisting of 150,000 Emulsion Cloud Chamber (ECC) modules called ”bricks”. A brick is a sandwitch structure of 57 nuclear emulsion films and 56 1 mm thick lead plates (Fig.2). The lead plates serve as neutrino interaction target and the emulsion films as 3-dementional tracking detector providing track coordinates with a sub-micron accuracy and track angles with a few mrad accuracy. The material of a brick along the beam direction corresponds to about 10 radiation length and 0.33 interaction lengths. The brick size is 10cm × 12.5cm × 8cm and weigh about 8.3 kg. The total active target mass is thus 1250 tons. The target is subdivided in two identical units, each consisting of 31 walls of bricks. The second main component of the detector is the target tracker (TT) system, a set of scintilator planes interleaved with the brick walls. Each TT plane consists of 256 2.6cm wide plastic strips. The sandwitch structure of the brick walls and the TT planes is illustrated in Fig.3. The third detector component consists in two muon spectorometers following the two target units. Equipped with RPC and high precision drift chambers, they allow determining the muon momentum with better than 20% accuracy up to 30 GeV/c. 4. Neutrino interaction location From the energy deposited in the TT scintillator strips, it is possible to estimate the origin of each neutrino interaction and to define the brick which most probably contains the neutrino interaction vertex. The
Figure 3: Arrangement of the ECC brick walls and the TT scintillator planes.
accuracy of the TT reconstruction is however not much smaller than the brick size and a non negligible fraction of the predicted brick do not contain the neutrino vertex. To solve this problem, an interface device (so called Changeable Sheet) is attached on the downstream face of each brick. It is made of two emulsion films prepared and packed together in the underground LNGS laboratory in order to be exposed to a minimum ammount of cosmic rays. So, tracks found in the CS will mainly be related to the neutrino beam. For each event, the brick predicted as the most probable one is removed from the detector. Its CS is detached and developped for scanning. Two stations, at Nagoya [6] and at LNGS [7], share the CS scanning load. The scanning area is about 16cm2 for events with a reconstructed muon track and 50 to 120cm2 otherwise. If tracks are found in the CS films, the corresponding brick
40
O. Sato / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 38–42
films are developed after a short exposure to cosmic rays for alignment purposes. If, on the contrary, no track is found in the CS, the brick is put back in the detector with a new CS and the brick with second to highest probability to contain the neutrino vertex is extracted. The same procedure is repeated until a validated brick is found. The automated scanning microscopes can treat about 100 CS per week in each of the two stations. After development, the brick films are dispatched to scanning laboratories in Japan and in Europ to complete the event location in the brick. The tracks found in the CS are extrapolated to the most downstream film of the brick with an accuracy about 100 μm. When found in this film, the tracks are followed upstream from film to film. This scan-backprocedure is quite fast thanks to the good alignment accuracy between films. The procedure is stopped when no track candidate is found in two continuous films and the lead plate just upstream the last detected track segment is defined as the vertex plate. Around the estimated vertex position in this plate, a scanning volume is defined with a length 10 films and a transverse area of 1 × 1cm2 . All track segments in this volume are collected and analysed. After rejection of the passing through cosmic rays and of the tracks due to low energy particles, the tracks produced by the neutrino interaction can be selected and reconstructed. The neutrino vertex position can then be defined with a few micron precision. 5. Decay search The main signature of a secondary activity (decay or nuclear inetarction) is the observation of a track with a significant impact parameter (IP) relative to the neutrino interaction vertex. The expected tau daughter’s IP distribution is shown on the left plot of Fig.4. Its mean value is about 100 μm. We may recall here that 85 % of the taus decay into a 1-prong topology and 15 % into 3 prongs. For tracks emitted at the primary vertex, the IP distribution reflects the track measurement accuracy except for low momentum tracks where Multiple Coulomb Scattering (MCS) in the lead plate plays a dominant role. The right plot of Fig.4 shows the expected and observed IP distributions for primary tracks of momentum higher than 1 GeV/c. Their mean values are well bellow 10 μm. In practice, the momentum of all tracks reconstructed as described in the previous section was estimated from observed differences of the connected track segments. After filtering out the low momentum tracks, the tracks with an IP between 10 μm and 500 μm were selected and checked by visual inspection.
Figure 4: Left: Simulated tau daughter’s impact parameter distribution. Right: Comparison with the impact parameter distribution expected for tracks emitted at the primary vertex. The observed distribution(black dots) dose not include the tau and charm decay candidates discussed in the text.
A total of 1088 events were analysed up to now. Among the 187 events with no identified muon, one event has a track with a significant IP and is presented in the next section as a tau decay candidate. In the CC like sample (901 events), 20 charm decay candidates were observed. For the analysed sample, the Monte Carlo simulation expectations are 0.54 ± 0.13 detected tau decays (at Δ m223 = 2.5 × 10−3 eV 2 and full mixing) and 16 ± 2.9 detected cham decays plus 2 backgroud events for charm decays. 6. The first candidate event In this section, the first tau neutrino candidate [8] will be described. The location procedure explained in section 4 yielded a neutrino interaction vertex with 7 tracks of which one exhibits a visible kink with an angular change of 41 ± 2 mrad. The kink daughter momentum is estimated to 12+6 −3 GeV/c by MCS measurement and its momentum transverse to the parent direction is 470+230 −120 MeV/c. Fig.5 shows a schematic side view of the vertex topology. The daughter track has been followed in the downstream bricks and, after crossing 7 walls, it is seen to generate a hadronic interaction. Hence, the daughter particle is identified as a hadron. The other tracks from the neutrino interaction were also followed until they stop or interact. Three of them are identified as due to hadrons and the probability that one of the 3 other tracks is left by a muon is estimated to less than 0.1%. None of them is compatible with being that of an electron. A dedicated search for converted gamma rays in the vicinity of the interaction yielded two gamma candidates called γ1 and γ2 . The energy of γ1 is 5.6±1.0(stat.)±1.7(syst.) GeV and it is clearly pointing to the decay vertex and not to the neutrino interaction
O. Sato / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 38–42
41
rection larger than 300 MeV/c. At the primary vertex, the main selection cut is that the angle φ in the transverse plane between the tau candidate track and the hadronic shower momentum direction must be larger than 90 degrees. The measured φ angle for the candidate event is 173 ± 2 degree, strongly favoring the ντ CC hypothesis. The invariant mass of the two observed gammas is 120 ± 20(stat.)±35(syst.), supporting the hypothesis that they are emitted in a π0 decay. The invariant mass of the charged decay daughter assumed to be a π− and the 2 +100 gammas is calculated to be 640+125 −80 (stst.)−90 (syst.) consistent with the ρ(770) mass. So the decay mode of the candidate is consistent with the hypothesis τ− → ρ− +ντ .
Figure 5: The tau neutrino candidate schematic side view
vertex. γ2 has an energy of 1.2 ± 0.4(stat.) ±0.4(syst.) GeV and is compatible with pointing to either vertex but with a much higher probability to the decay vertex. Fig.6 shows a longitudinal view of the event display while, in Fig.7, the view is transeverse to the beam direction. The white segments in these displays are the track segments measured in the emulsion films. All the selection cuts discussed in the OPERA proposal [9] are well satisfiled by this event : the kink angle is larger than 20 mrad, the daughter momentum larger than 2 GeV/c and its transverse component to parent di-
Figure 6: Display of the tau neutrino candidate (side view)
Figure 7: Display of the tau neutrino candidate (view transverse to the beam direction)
The two main sources of background on the hadronic tau decay mode were investigated. The first one is the charm production in νμ CC interactions in which the muon track is not identified. The second one is the production in a νμ NC event of a hadron making after a short flight path an interaction which mimics a tau decay. The Monte Carlo expectation for the first background source is 0.007 ± 0.004(syst.) event and, for the second background source, 0.011 ± 0.006(syst.) event. Two kinds of experimental validation for these Monte Carlo estimation were performed. The first one is the study of the inelastic interactions produced by a π beam in an ECC brick. So far, 119 inetarctions were analysed. The prong multiplicity and the Pt distribution are in good agreement with the Monte Carlo expectations. The fraction of intercations which mimic a tau decay by kink toplology is 7.4 ± 1.1% for the data and 7.6 ± 0.3% for Monte Carlo simulation.
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O. Sato / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 38–42
The other check is the study of interactions produced by the charged particles emitted in the neutrino interactions collected so far by OPERA. The tracks were followed over long distances, well above a possible decay range. The total track length for this study was 8.6m and no secondary interaction was found which could mimic a tau decay in the transverse momeentum signal region (Pt> 0.3 GeV/c). At lower Pt, 7 interactions were found with a possible decay toplology (all with Pt< 0.2 GeV/c) while 8 are predicted by the Monte Carlo simulation. The total background on the 1-prong hadronic tau decay mode is then 0.018 ± 0.007(syst.) event which corresponds to a signal significance of 2.36 σ for the observation of one tau candidate. Since not only 1-prong hadronic mode but also other modes (muonic and electronic and 3 prond decays) were searched for, the total estimated background increases to 0.045 ± 0.023(syst.) event and the significance of the observed candidate is reduced to 2.01 σ. 7. Summary Since 2008, OPERA is collecting neutrino ineteractions in the ECC targets. Up to now (June 2010), 1921 neutrino vertices have been precisely located and the decay search analysis has been completed for a subsample of 1088 events. A first tau-neutrino candidate has been found with an estimated background of 0.045 ± 0.020(syst.) event. This observation has a significance of not being a background fluctuation as 2.36 σ for analysis of the 1-prong hadronic decay mode where the candidate event was found and 2.01 σ for including all other decay modes where no candidate event was found. The 2010 physics run 2 is in progress and the experiment is forseen to continue for two aditional beam years.
References [1] B.Pontecorvo, Sov.Phys.JETP 6, 429(1957) B.Pontecorvo, Sov.Phys.JETP 7, 172(1958) [2] Z.Maki,M.Nakagawa,S.Sakata Prog.Theor.Phys.28:870-880,1962 [3] SUPER KAMIOKANDE collaboration, Y.Fukuda et al., Phys.Rev.Lett.81(1998)1562; SUPER KAMIOKANDE collaboration, K.Ave et al., Phys.Rev.Lett.97(2006)171801;
2 At the time of writing, The 2010 run beam exposure was successfully finished with the achieved intensity 4.04×1019 POT.
[4] MINOS collaboration, D.G.Michael et al., Phys.Rev.Lett.97(2006)191801; MINOS collaboration, P.Adamson et al., Phys.Rev.Lett.101(2008)221804; [5] SNO collaboration, Q.R.Ahmad et al., Phys.Rev.Lett.89(2002)011301 [6] K.Morishima and T.Nakano, JINST(2010) 5 P0411 [7] N.Armenise et al., Nucl.Instrum. Meth.A 551 (2005) 261; M.De Serio et al., Nucl.Instrum. Meth.A 554 (2005) 247; L.Arrabito et al., Nucl.Instrum. Meth.A 568 (2006) 578; [8] Observation of a first ντ candidate in the OPERA experiment in the CNGS beam. OPERA collaboration Phys.Lett.B691:138145,2010 [9] A.Ereditato, K.Niwa and P.Strolin, The emulsion technique for short, medium and long baseline νμ → ντ oscillation experiments, 423,INFN-AE-97-06,DPNU-97-07; OPERA collaboration, M.Guler et al., An appearance experiment to search for νμ → ντ oscillations in the CNGS beam: experimental proposal,CERN-SPSC-2000-028,LNGS P25/2000 OPERA collaboration, M.Guler et al., Status Report on the OPERA experiment,CERN/SPSC 2001-025,LNGS-EXP 30/2001 add.1/01