Study of hadron interactions in OPERA-like bricks

Aim: background study of t h, hhh decays

4th September 2012H. 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 study1) Summary of past results (multiplicity, pT vs. p, nuclear fragments)2) Search for gamma rays from hadron interactions3) MC study for large angle M. I. P. and nuclear fragments

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Motivation of this studyStrategy for OPERA to get the 4 sigma significance observation of nt appearance1. More statistics2. BG reduction3. More sophisticated analysis

From Sato’s slide

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Summary of past results (multiplicity, pT vs. p, nuclear fragments)

Track length followed in the brick for each beam:1. KEK 4 GeV/c : 87.0 m2. 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, p- and 2, 4, 10 GeV/c, p-

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 q >20 mrad. The stopping point is considered as the signature for an interaction. It is then confirmed by visual inspection.

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Interaction length of hadron beam in ECC

Interaction length versus beam momentum: comparison between experimental (black) and simulated (magenta) data.

Momentum (GeV/c)

Inte

ract

ion

len

gth

(m

m)

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Secondary tracks emitted from each interaction were searched for under the following conditions:

•Minimum distance < 10 mm + 0.01 x depth, which is the same condition as in OPERA.• tan q < 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

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Comparison between data and MC~ Topological analysis ~

10GeV/c

4GeV/c 2GeV/cMultiplicity

Kink angle (1-prong)

Error bars : Experimental dataHistogram : Simulated data

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For t → 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

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PT vs. P distribution of secondary particle10GeV/

c4GeV/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

No data

3-prong events

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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 mm2) scanning system was

introduced to pick them up.• Scanned area : 3.5 x 2.5 mm2, upstream and downstream films of

the vertex• Angular acceptance : tan q < 3.0• Impact parameter < 50 mm + 0.01 x depth• Number of background tracks was estimated to be 0.035

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

cosq

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 : b < 0.4 Blue : b < 0.3 Cyan : b < 0.25

Our data

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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 p- 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 g rays from hadron interactions

H. Ishida et al., TOHO University Group

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Search for g rays from hadron interactions

• About 70% of t -> h “1-prong” decay modes have p0(s).• In fact, the 1st nt candidate event has 2 g rays.• What is the fraction of “1-prong” hadron interactions associated with g rays?

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P daughter (GeV/c) 12 +6-3

Eg1(GeV) 5.6±1.0±1.7

Eg2(GeV) 1.2±0.4±0.4

t ---> p- p0 nt

1st nt candidate event

P parent particle > 10 GeV/c

2

g1

3 g2

7

51

68 daughter

PDG

The g 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 mm + 0.01*dz (from one of 2 or 3 downstream films)

• tanq < 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

g

e+

e-

×○

Beam track

2ry track

This method can find 2 gamma rays of 1st nt 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 g ray search

Momentum [GeV/c] 10 4 2

Number of interactions 173 68 77

Number of 1-prong events 26 29 33

Number of selected tracks

2 5 2

Number of g rays 1 4 1

Associated ratio 3.8 % 13.8 % 3.0 %

preliminary

Manual check

Momentum [GeV/c] 10 4 2

e-pairs 1 pair 2 pairs 0

Electron single track 0 2 tracks 1 track

Passing-through track 1 1 1

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

film

2ry track

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Conclusions and Outlook• We have started search for g 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 nt events. It turned out that it can detect the 2 g rays which are already known to exist.

• A total of 6 g 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

pionkaonmuonΣ±

Monte Carlo simulation

OPERA signal regionp > 2.0 GeV/cpT > 0.6 GeV/c

Procedures1. Ignore all electrons 2. Find all 1-prong vertices3. Check whether MIP or fragments exist or not

Tracks are classified into MIP/grey/fragmentby the following conditions:

a) MIP: 0.6 < tan θ < 3.0, 0.7<βb) Gray track: 0.0 < tan θ < 3.0, 0.3<β<0.7c) 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 pT vs. p of kink daughters (MC)

←10GeV/c

Out of kink daughters,some are associatedwith a large angle MIPor nuclear fragments.These events are shownwith colours

Two thirds of hadronic interactions have another black, grey or thin track of tanθ<3.0

Black fwdGrey backBlack back

Grey fwdThin fwd Thin back

No track associated

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Percentage of MIP and fragment association

*) In case of 2 GeV/c, there are only 6 events in the signal region.

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The rate of hadron kink rejection

1 2 3 4 5 6 7 8 9 10 110

5

10

15

20

25

30

35

40

45

50

Track attachment ratio

Thin(%)

Grey(%)

Black(%)

Beam momentum (GeV)

atta

ched

ratio

(%)

1 2 3 4 5 6 7 8 9 10 110

10

20

30

40

50

60

70

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The effect of large angle MIP

thin

g+b

total

Beam momentum (GeV/c)

ratio

of k

ink

reje

ctio

n

The ratio of vertices associated with eitherthin, grey or black track.

The effect of 1. grey+black search only2. additional large angle MIP search.By scanning large angle MIP,we can reject additional 10%of 1-prong vertices as BG.

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* According to MC data of grey or black tracks up to tanθ < 3.0, with beam momentum >4.0GeV/c, we can eliminate roughly 50 - 60% of the hadronic vertices.

* Large angle MIPs can eliminate about 10% of the vertices additionally.

Summary of MC simulation study