Events in Magnetized GAr Tom Junk DUNE ND Meeting January 11, 2018 - - PowerPoint PPT Presentation

Electron Sign Determination in GENIE ve Events in Magnetized GAr

Tom Junk DUNE ND Meeting January 11, 2018

The question: What fraction of electrons in veCC events have their signs correctly identified? Approximate answer: We expect this to be very good for a GArTPC in a magnetic field. But very short tracks are hard to measure well.

Setup: Simulation

• I'm using LArSoft with the 1x2x6 TPC geometry (7.2m x 6m x 15 m) too big!
• Set the density of LAr to that of GAr.
• Set the magnetic field to 0.4 T
• Run GENIE with nominal task force flux file:

gsimple_dune10kt_v1_1x2x6_g4lbne_v3r4p2_QGSP_BERT_run15_12388_80GeV_neu trino_*.root

• Flux is not oscillated, so flavor and energy composition is close (modulo geometric

acceptance effects)

• Generate only ve and ve events (CC and NC). NC will be not representative because the

ve spectrum is harder than v𝜈 . Sample size = 2000 events.

• NC background is small in this study (but I'm being naive)
• GEANT4 simulation of GENIE particles
• No detector response simulation – looking at MCParticle trajectory steps. Bounded by

voxel size of 0.3 mm. Also containment isn't right as the 1x2x6 APA detector is too big by far. Can always apply fiducial cuts in the ND.

Jan 11, 2018 Tom Junk | GArTPC Electron Sign ID 2

Setup: "Reconstruction" and Analysis

• Cheat the primary vertex using the MCTruth Neutrino position (it's the

end of the neutrino's trajectory).

• Cheat electron identification by looking at PDG codes
• Pick the highest-energy electron or positron which starts within dcut of

the primary vertex (whether or not it is the electron from a charged- current interaction or something else)

• Make plots as function of the true electron energy
• Select trajectory points that are isolated from other particles' trajectory
• points. To be considered for selection, a trajectory point must be at

least rcut in 3D from any trajectory point on any other charged particle, including delta rays. But if a hit is within 1mm of the track, consider it

• indistinguishable. G4 makes lots of short delta rays with LArSoft

settings.

Jan 11, 2018 Tom Junk | GArTPC Electron Sign ID 3

Short Delta Rays in GEANT

• With LArSoft settings, GEANT4 conservatively creates lots of

very short delta rays (thousands per event). Most are a few hundred microns in length.

Jan 11, 2018 Tom Junk | GArTPC Electron Sign ID 4

Electron Track Length (cm) Electron Track Length (cm) Linear and log scales. Track lengths are just endpoint – start point distance; Affected by the magnetic field (but not the X component).

Setup: Analysis cont'd

• From the list of isolated trajectory points, compute the sagitta

incrementally:

• Start from the beginning of the track and for each point i, calculate the maximum

perpendicular distance of a track point from the line between the beginning to point i

• Calculate sagitta in the (y,z) plane. B is along x.
• If any sagitta calculated exceeds scut in absolute value, use the signed sagitta

(computed with a cross product) to determine the sign of the track. Keep stepping through points i until we run out. Provides protection against looping tracks.

• The values of dcut, rcut, and scut depend on detector parameters and hide a lot
• f ignorance in the procedure.

Jan 11, 2018 Tom Junk | GArTPC Electron Sign ID 5

Fiducial Volume Cuts

• New since Jan 5.
• Primary vertex must be 30 cm away from the APA plane, 30 cm

away from the CPA's, and 1.5m away from the downstream field-cage wall. 30 cm from other field-cage walls.

• Previous iteration had some electrons showering in the steel

frames of the APA's and in the cryostat material

Jan 11, 2018 Tom Junk | GArTPC Electron Sign ID 6

• Initial guess: dcut = rcut = scut = 0.5 cm. Maybe a little small, due

to the 7mm pixel dimension along one axis.

• Needed a log scale to show small contributions
• Track length is an important

determinant of whether we can measure the sign

• Here, |𝛦x| < 2.5m, |𝛦yz|< 2.5 m
• Very small chance to not be

able to tell what the sign is

• Allowing tracks to be 5m long

in both dimensions makes the "missing" component go away

Results Example

Jan 11, 2018 Tom Junk | GArTPC Electron Sign ID 7

An example Correct-Sign Event

Jan 11, 2018 Tom Junk | GArTPC Electron Sign ID 8

Magenta: Electrons Blue: Protons Red: Pions Black: Muons Event display centered on primary vertex Material interactions not classified (APA's cryostat) ROOT/X drawing bug puts magenta pixels on protons. Electron goes backwards a bit but still is measured properly.

An example Incorrect-Sign Event

Jan 11, 2018 Tom Junk | GArTPC Electron Sign ID 9

Magenta: Electrons Blue: Protons Red: Pions Black: Muons Event display centered on primary vertex Material interactions not classified (APA's cryostat) This one fails the fiducial cuts now

Shorter Tracks: 1 m long

• Tried looking at only the first 1m in YZ and 1m in X of tracks.
• Keep dcut = rcut = scut = 0.5 cm
• Makes a big difference!
• A small detector with poor

sagitta resolution struggles with the electron sign

• Kind of a test to make sure

the program isn't crazy

Jan 11, 2018 Tom Junk | GArTPC Electron Sign ID 10

ALICE

• ALICE has 5m diameter, and a space point resolution perpendicular to

the B and E fields and the beam of about 1 mm (between 0.4 and 2mm). B=0.5T in ALICE. Up to 159 space points per track

Jan 11, 2018 Tom Junk | GArTPC Electron Sign ID 11

• C. Lippmann, Physics Procedia 37 (2012) 434-441

Allow Smaller Sagittas

• YZ length < 1m and X length < 1m
• dcut = rcut = 0.5 cm, scut = 0.1 cm

Jan 11, 2018 Tom Junk | GArTPC Electron Sign ID 12

Even Shorter Tracks

• YZ length < 50 cm and X length < 50 cm
• dcut = rcut = 0.5 cm, scut = 0.1 cm

Jan 11, 2018 Tom Junk | GArTPC Electron Sign ID 13

Sagitta cut down to 100 microns

• Still with a YZ track length cut of 50 cm and X track length of 50

cm

• ALICE gets up

to 159 points to fit. Averaging is strong.

• But ALICE tracks

are 2.5 m long

• Our primary vertices

are distributed through the detector. A 50 cm track will not have lots of hits for averaging.

Jan 11, 2018 Tom Junk | GArTPC Electron Sign ID 14

Not done yet, but summary of conclusions

• Biggest effect is from making sure that an electron has a measurable

bend in the detector volume

• Strong enough field – what is the minimum requirement?
• Long enough track
• Space-point resolution
• Multiple scattering seems not to be a problem
• otherwise we'd have more incorrect-sign assignments at low sagittas
• Neutral Current BG seems not to troublesome
• but I cheated the primary vertex location
• no pileup.
• would need to topologically identify conversions which are displaced from

NC events but which might look like their own events or pileup. Two electrons, plus pointing to a hadronic recoil.

Jan 11, 2018 Tom Junk | GArTPC Electron Sign ID 15

Two NC events with electrons

Jan 11, 2018 Tom Junk | GArTPC Electron Sign ID 16

Distances are marked off in cm