Study of hadron interactions in OPERA-like bricks Aim: background study of   h, hhh decays 4 th September 2012 H. Shibuya, H. Ishida, T. Matsuo, T. Fukuda, Y. Tsuchida, T. Kajiwara, J. Sudo, T. Suzuki, T. Makino, S. Nishimura, S. Fukunaga, S. Ogawa, S. Mikado, TOHO University Group 0) Motivation of this study 1)Summary of past results (multiplicity, p T vs. p, nuclear fragments) 2)Search for gamma rays from hadron interactions 3)MC study for large angle M. I. P. and nuclear fragments

2 Motivation of this study Strategy for OPERA to get the 4 sigma significance observation of  appearance 1.More statistics 2.BG reduction 3.More sophisticated analysis From Sato’s slide

3 Summary of past results (multiplicity, p T vs. p, nuclear fragments) Track length followed in the brick for each beam: 1.KEK 4 GeV/c : 87.0 m 2.CERN 2 GeV/c : 8.5 m, 4 GeV/c : 12.6 m, 10 GeV/c : 38.9 m Total track followed : 147 m, 632 events were found. Beam Exposure: OPERA-like bricks, the same structure as the OPERA brick, were exposed to hadron beams, 4 GeV/c,  - and 2, 4, 10 GeV/c,  - at KEK and CERN, respectively. Hadron Interactions: Beam tracks are reconstructed in 3 films and followed until not found in 3 consecutive films or scattered with  >20 mrad. The stopping point is considered as the signature for an interaction. It is then confirmed by visual inspection.

4 Interaction length of hadron beam in ECC Interaction length versus beam momentum: comparison between experimental (black) and simulated (magenta) data. Momentum (GeV/c) Interaction length (mm)

5 Secondary tracks emitted from each interaction were searched for under the following conditions: Minimum distance < 10  m x depth, which is the same condition as in OPERA. tan  < 0.6 which is the same condition as in OPERA. Base track exists in ≧ 3 out of 6 downstream films in order to reject fake tracks. Secondary tracks (MIP)

10 GeV/c 6 Comparison between data and MC ~ Topological analysis ~ 10GeV/c 4GeV/c 2GeV/c Multiplicity Kink angle (1-prong) Error bars : Experimental data Histogram : Simulated data

7 For  → h decay candidates, the selection criteria: Momentum of the daughter particle > 2 GeV/c, transverse momentum > 0.6 GeV/c are required. As in OPERA, momentum of each secondary particle was measured by the multiple Coulomb scattering (MCS) method in the ECC brick. We estimated momentum by the coordinate method. Kinematical analysis

8 8 P T vs. P distribution of secondary particle 10GeV/c 4GeV/c 2GeV/c 1-prong events 10 tracks 7 tracks 12 tracks 53 tracks (24 events) In signal region : 1/53 (1.9 %) No data 3-prong events

9 Nuclear fragment search The hadron interaction background can be further reduced by increasing the detection efficiency of highly ionizing particles. Highly ionizing particles are emitted at large angle. Large field of view (352 x 282  m 2 ) scanning system was introduced to pick them up. Scanned area : 3.5 x 2.5 mm 2, upstream and downstream films of the vertex Angular acceptance : tan  < 3.0 Impact parameter < 50  m x depth Number of background tracks was estimated to be tracks/event Highly Ionizing particle Association of highly ionizing particles is evidence of a hadron interaction.

Black track search for CERN 10 GeV/c Slope distribution Multiplicity 43 in 67 events(64± 6%) have black tracks. Combine 1, 3 – prong events ForwardBackward cos 

Black track emission probability Momentum [GeV/c] ratio Bern group data At least one highly ionizing particle was associated with about 50% of the events for beam momentum greater than 8 GeV/c. It provides an additional background reduction. Data : Black dots with error bars MC : Black track Magenta :  < 0.4 Blue :  < 0.3 Cyan :  < 0.25 Our data

12 Conclusions of previous results Hadron interactions have been studied as a background source by using OPERA-like bricks exposed to the 2, 4, 10 GeV/c  - beams. Experimental data and simulated data show fair agreement. The hadron interaction background is under control. Hadron interaction background can be further reduced by about 50% with the detection of highly ionizing particles.

Search for  rays from hadron interactions H. Ishida et al., TOHO University Group

14 Search for  rays from hadron interactions About 70% of  -> h “1-prong” decay modes have  0 (s). In fact, the 1 st  candidate event has 2  rays. What is the fraction of “1-prong” hadron interactions associated with  rays? 14 P daughter (GeV/c) E  1(GeV) 5.6±1.0±1.7 E  2(GeV) 1.2±0.4±0.4  --->  -  0  1 st  candidate event P parent particle > 10 GeV/c 2 11 3 2 daughter PDG

The  ray search Gamma search method It is almost the same method with the same parameters to those used for secondary track search, but it requires that no base track exist in the vtx film. MD < 10  m *dz (from one of 2 or 3 downstream films) tan  < 0.6 (using S-UTS data) manual check to confirm that no track exists in the vertex film, a track exists in downstream film(s). analyzed for 1-prong events Probability of gamma rays conversion in 3 downstream films : 23 % Pb  e+e+ e-e- × ○ Beam track 2ry track This method can find 2 gamma rays of 1 st  event, if the search is extended downstream. (P = (1-exp(-2.5/7.2)) - (1-exp(-0.5/7.2), where conversion length in Pb is 7.2 mm, the average position of the vertex is assumed (dz = 0.5 mm).)

Results of the  ray search Momentum [GeV/c]1042 Number of interactions Number of 1-prong events Number of selected tracks252 Number of   rays 141 Associated ratio3.8 %13.8 %3.0 % preliminary Manual check Momentum [GeV/c]1042 e-pairs1 pair2 pairs0 Electron single track02 tracks1 track Passing-through track111 We will extend the analysis to the downstream films.

An example of observed gamma rays Upstream film (2 tracks overlapped.) Pb Beam track e-pair Downstream film (2 tracks observed) film 2ry track

18 Conclusions and Outlook We have started search for  rays from hadron interactions. We have tried, for 1-prong events, almost the same method with the same parameters to those used for secondary track search, but it requires that no base track exist in the vtx film. We applied this method to the first   events. It turned out that it can detect the 2  rays which are already known to exist. A total of 6  rays (electron pairs) have been observed in a search of first 3 films for 88 1-prong interactions. We will extend the analysis to the downstream films. We will also try to apply the analysis for multi-prong events.

MC simulation study of hadron kinks with a large angle MIP/ nuclear fragments Percentage of 1-prong hadronic events in the signal region associated with a large angle MIP and nuclear fragments T. Matsuo, et al., TOHO University Group

Motivation Some daughter tracks of 1-prong hadron interactions come into the OPERA signal region. So we checked if these events have a large angle MIP or nuclear fragments. If so, we can reject these evens as background. proton pion kaon muon Σ±Σ± Monte Carlo simulation OPERA signal region p > 2.0 GeV/c p T > 0.6 GeV/c

Procedures 1. Ignore all electrons 2. Find all 1-prong vertices 3. Check whether MIP or fragments exist or not Tracks are classified into MIP/grey/fragment by the following conditions: a) MIP: 0.6 < tan θ < 3.0, 0.7<β b) Gray track: 0.0 < tan θ < 3.0, 0.3<β<0.7 c) Black track: 0.0 < tan θ < 3.0, β<0.3 (nuclear fragment) Events were generated by the FLUKA-based Monte Carlo code (A. Sheshukov ) Procedures and track classification

Scatter plot of p T vs. p of kink daughters (MC) ←10GeV/c Out of kink daughters, some are associated with a large angle MIP or nuclear fragments. These events are shown with colours Two thirds of hadronic interactions have another black, grey or thin track of tanθ<3.0 Black fwd Grey back Black back Grey fwd Thin fwd Thin back No track associated

23 Percentage of MIP and fragment association *) In case of 2 GeV/c, there are only 6 events in the signal region.

24 The rate of hadron kink rejection The ratio of vertices associated with either thin, grey or black track. The effect of 1. grey+black search only 2. additional large angle MIP search. By scanning large angle MIP, we can reject additional 10% of 1-prong vertices as BG.

25 * According to MC data of grey or black tracks up to tanθ 4.0GeV/c, we can eliminate roughly % of the hadronic vertices. * Large angle MIPs can eliminate about 10% of the vertices additionally. Summary of MC simulation study