Liquid Argon technique: a pizero Liquid Argon technique: a pizero identification analysis identification analysis forfor T2K T2K

beam beam

Paweł Przewłocki, Paweł Przewłocki, A. Soltan Institute for Nuclear Studies, WarsawA. Soltan Institute for Nuclear Studies, Warsaw

Warsaw Neutrino Group, 2005Warsaw Neutrino Group, 2005

Liquid Argon Technique

• LAr technique:– Precisely reconstructable

topology of events– High granularity: 3x3x0.6mm3

– Particle ID capability– Very good calorimetry

• Applications:– ICARUS (Gran Sasso, Italy)– Proposed small LAr detector in the intermediate

station of T2K experiment (Tokai, Japan)

More on it in Anselmo Meregaglia’s talk!

The outline of my presentation

• Pi identification studies overview

• Energy loss pattern study

• Vertex visibility study - my analysis– „Three wires” visibility criterion– Visibility in lower energies

• Conclusions

What would we like to do?

• Separate e events from other,

topologically similar

• It is essential in experiments looking for e

appearance (like T2K)

• Main background: NC events with single

0 production (0 decays into two gammas)νμ NC 0νe CC

• Three methods– Looking for two showers pointing at one

location (supposedly a vertex)(still remains to be done)

– Examining dE/dx energy loss pattern if we see one shower only(ICARUS analysis)

– Or looking for some particle (proton or charged pion) which marks the primary vertex of interaction, which is at some distance (‘gap’) from visible shower(the analysis which I present below)

... And how?

Energy loss analysis

• Performed by scientists from ICARUS Collaboration

• Based on dE/dx measurement (single mip signal of an electron vs. double signal of e+e- pair)

• First wires of a track (before showering starts) are studied

Energy loss analysis continued

• Main source of misidentification: compton scattering of gammas coming from pizero decay make the shower look like electron (single electron instead of a pair)

Compton

The energy loss analysis shows that:

• With this method 96.3%

pions are rejected for 90% electron efficiency (for 1GeV pions)

• For 250MeV pions (these will be most frequent in T2K) 93.5% are rejected

• The quality of rejection increases with energy – different from Cherenkov detectors (overlapping of two gamma induced rings)

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The third method

• Pizero can be visually recognized if the distance between the vertex and the point of gamma conversion is more than 1cm

• Conversion length of a photon in LAr is 18cm; hence, we get 5.4% of pions confused with electrons

• If we combine this method with the previous one we can get 0.2% for 1GeV (0.35% for 250MeV) pion misidentification, but…

• We have to know where the vertex is, and this information is only available when some other particle (apart from showering gammas) is visible at the vertex

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My analysis

• My analysis is based on the gap method, its aim is to evaluate the efficiency of finding pizero events this way

• It is done with the T2K-LAr detector in mind (the detector’s geometry, T2K neutrino energy spectrum)

• Events generated using Nuance 3.006 simulation software

• Nuance was originally created (by Dave Casper) for water Cherenkov detectors, but recent version (3.006) allows the user to specify any medium needed (Argon in particular)

My analysis

• Two stages of the study– First, a conservative approach: let’s see how

many pi zero events we can spot, when we assume that a particle in the vertex is visible if it lit up at least three wires

– Then we can try to look at the events which were recognized as invisible and see if it’s really true – we have to scan the events

How to see a particle (in LAr)• We assume that for a particle to

be seen we have to observe a signal on three consecutive wires (the conservative assumption)

• We take into account every charged particle track (protons, charged pions, muons, electrons, charged kaons), and calculate length of its track using known energy-range dependence for each type of particle

• We project it onto the wire plane • We project it again, this time onto

2 directions perpendicular to the wires’ directions (45deg, -45deg)

• If greater of these exceeds 9mm (3 wires times 3mm [wire pitch]) we conclude that the track is visible

zy

•Exclusion of y coordinate•Rotation by 45deg. •x and z give us needed lengths

x

General info

• Data simulated by Nuance 3.006• T2K beam @ 2km• Two samples: w/o and with FSI (final state

interactions [intranuclear cascade] in Ar nucleus)• Muon neutrinos, ca 25.000 NC events generated• No detector simulation• Simple C++ code used instead, to see which

tracks are visible

Protons and pions – with and without FSI (momentum)

Dashed line – w/o FSI

Solid line – with FSI

application of FSI gives us many low momentum protonsapplication of FSI gives us many low momentum protons

MeV/c MeV/c

Can we trust FSI model in Nuance?

• NUX+Fluka simulation as a cross-check• A thoroughly tested simulation for nuclear and particle physics• Resonant channels, energy of neutrinos @ 1750MeV• With and without rescattering in nucleus• A number of particles of each type per event and average

momentum are givenNuance no rescattering (Ar)

Nuance with rescattering (Ar)

NUX+Fluka with rescattering (Ar)

pi 0 0.36/evt 0.43GeV

0.31/evt0.35GeV

0.32/evt0.38GeV

pi plus 0.75/evt0.44GeV

0.52/evt0.41GeV

0.54/evt0.41GeV

pi minus 0.06/evt0.36GeV

0.10/evt0.26GeV

0.06/evt0.32GeV

n 0.31/evt0.81GeV

2.27/evt0.30GeV

3.13/evt0.21GeV

p 0.84/evt0.79GeV

2.95/evt0.36GeV

2.34/evt0.39GeV

There is a rough agreement between both generators when it comes to number of nucleons

Protons and pions - visibility

Black – all, red – invisible

MeV/c

Visible protons and pions – with and without FSI (momentum)

Dashed line – w/o FSI

Solid line – with FSI

MeV/cMeV/c

Visibility - summary

particlew/o FSI with FSI

visible invisible visible invisible

protons10472

79.8%

2646

20.2%

17199

33.4%

34301

66.6%

charged п6973

99.8%

16

0.2%

6237

98.8%

78

1.2%

charged K119

99.2%

1

0.8%

103

97.2%

3

2.8%

Vertex visibility and pizeros

NC evts w/o FSI with FSI

#evts 2588925889 2563625636

#evts with 1 or more pizeros

63346334 ( (24%24%)) 43014301 ( (17%17%))

#pizero events with 1 #pizero events with 1 or more visible trackor more visible track

33253325 ( (13%13% of all, of all, 53%53% of all pizero evts) of all pizero evts)

27622762 ( (11%11% of all, of all, 64%64% of all pizero evts) of all pizero evts)

#unidentifiable #unidentifiable pizerospizeros

30093009 ( (48%48% of all of all pizero events)pizero events)

15391539 ( (36%36% of all of all pizero events)pizero events)

FSI: Less pizeros, but more of them identifiable!FSI: Less pizeros, but more of them identifiable!

Relaxing the cuts...

• Are those low energy protons really invisible? Might it be that we can see them even if the signal is on two or even a single wire?

• To investigate it, one has to make use of detector simulation and visually scan the events

• Such analysis has been carried out. Several events with low-energy protons from Geant4 T2K-LAr simulation were scanned

• The analysis is in progress, the results presented here should be considered preliminary, but they can serve as a hint

Test sample of low energy protons

• 60 NC events• At least one pi zero

produced• One proton with kinetic

energy less than 20MeV (corresponds to ~200MeV/c in momentum)

• Any number of neutrons GeV/c

Proton momentum

Sample events

• Two weak cascades

Induction

Collection

Sample events

• Two showers overlapping

Sample events

• Neutron noise

Sample events

• ‘Total chaos’:-)

Identifiability criteria

• Vertex is considered visible, if:– Two gammas point at one location and there

is a visible hit there– Two gammas point at a distant location and

there can be more than one hit around this point

– There is at least one particle, giving signal on at least two wires and there is only one cascade visible and pointing at it (no other visible hits around)

Visibility in the test sample

• Red histogram shows protons from events that cannot be identified as pizero events

• We can see that many low energy protons are in fact visible, which allows us to identify pizero events

Black – all, red – invisible

GeV/c

Conclusions

• Analysis still in progress• Gap study:

– Electronic noise has to be considered– More events, more detailed scanning

• Independent analysis needed for ‘two visible showers pointing at one location’ case

• Further study neccesary, including electron events (visual scanning)

• Combining dE/dx with my results in conservative case gives 1.5% (1GeV pions) and 2.6% (0.25GeV) pion misidentification

• Eventually one should have the possibility to combine the three different analyses

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