Neutron TOF in the MPD ECAL Rahul Sahay Chris Marshall Lawrence - - PowerPoint PPT Presentation

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Neutron TOF in the MPD ECAL Rahul Sahay Chris Marshall Lawrence - - PowerPoint PPT Presentation

Dec 30, 2022 1.21k likes •1.69k views

Neutron TOF in the MPD ECAL Rahul Sahay Chris Marshall Lawrence Berkeley National Laboratory 18 September, 2019 Motivation Neutron kinetic energy is generally not visible in LAr TPCs Small (~20%) fraction of neutron KE shows up in

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SLIDE 1

Neutron TOF in the MPD ECAL

Rahul Sahay Chris Marshall Lawrence Berkeley National Laboratory 18 September, 2019

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Chris Marshall 2

Motivation

  • Neutron kinetic energy is generally not visible in LAr

TPCs

  • Small (~20%) fraction of neutron KE shows up in detector

via neutron re-interactions

  • Neutrons in the 10s to 100s MeV are a significant

source of neutrino energy misreconstruction

  • Neutron production in ν-Ar scattering is highly

uncertain → Measuring neutron energy spectrum in ND could constrain our missing energy corrections at FD

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Chris Marshall 3

Reminder: basic premise

  • Assuming neutron comes from

primary vertex, start and end positions are measured

  • Vertex time comes from charged

particle hits in ECAL, correcting for TOF back to vertex

  • Use neutron TOF to determine

its momentum

  • This works in any detector with

fast timing and 3D position reconstruction, i.e. MPD ECAL

  • r 3DST

ν μ n p Measure interaction vertex time from muon hits in ECAL

x0,t0 x1,t1

Measure neutron “endpoint” from scatter, i.e. n+12C→p+X

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Chris Marshall 4

Advantages of MPD ECAL vs. 3DST

  • Feasibility of neutron TOF

measurement has been demonstrated in 3DST

  • Two main advantages of

pursuing neutron TOF using MPD ECAL

  • Neutrons produced in ν-Ar

interactions → directly applicable to ν-Ar modeling of FD

  • Low density of gas TPC → lever

arm of several meters, compared to O(1m) scattering length in 3DST → improved energy resolution

ν μ n p ν n μ p

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Chris Marshall 5

Disadvantages of ECAL vs. 3DST

  • Often miss neutron scatters that
  • ccur in passive absorber of

ECAL → poor energy reco

  • Long lever arm → long TOF →

more beam pile-up problems

ν μ n p ν n μ p

2mm Cu 5mm CH

n

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Chris Marshall 6

Simulation details

  • Detector hall consists of rock, LAr TPC, Gas TPC + ~300t ECAL + 100t

cylindrical magnet (geometry created by Eldwain with NDGGD)

  • Guess on the rock location: 2m gap from rock to front of LAr TPC, rock right

below and ~4m above detectors, no side rock as hall will be wide in the x dimension

Rock events only Everything but rock

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Chris Marshall 7

Signal and background

  • Signal is νμ CC interaction in gas TPC, with a fiducial

volume 50cm from the edge of the active region

  • Overlay background events ±1μs from signal, and

reconstruct entire spill, with hit timing resolution in the ECAL of ±0.7 ns

  • 770 rock and 120 detector hall ν interactions per spill at

1.2 MW FHC, simulated separately and overlaid

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Chris Marshall 8

Almost-real reconstruction

  • Hits in active ECAL elements are formed, including

scintillator quenching effects

  • Ionization hits from charged particles originating in gas

TPC or entering the ECAL from the outside are excluded, but any hit with a neutral ancestor (neutron

  • r photon) is considered
  • Selection cut for neutrons uses both topological and

energy information

  • Basically neutrons are single-cell, high-energy hits, while

photons are typically multi-cell, more uniform energy

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Chris Marshall 9

Energy resolution

  • Very good energy resolution when reconstructed neutron

scatter is the first one

  • But due to the high passive fraction, ~50% of the events are

rescatters

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Chris Marshall 10

Energy resolution

  • At higher energies, resolution gets somewhat worse, up to ~40% for

first scatter

  • Fraction of rescatter events plateaus at ~60% at high energy
  • Could be improved by increasing CH/passive ratio
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Chris Marshall 11

Rock background: how much rock?

  • Simulated 2m thick rock
  • n top and bottom of

hall, and 4m upstream, no downstream rock

  • Plot shows all vertex

positions – note the beam divergence is non- negligible over this region

  • Where are the vertices

that produce neutron scatters in the ECAL?

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Chris Marshall 12

How much rock is enough? ν vertices producing ECAL activity

  • Most of the vertices

that produce ECAL neutrons are very near the detector hall

  • Expected, as ~1m

rock will attenuate neutrons

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Chris Marshall 13

How thick rock do we need to worry about?

  • Distance between neutrino interaction vertex that produces neutron hits in ECAL

and edge of hall

  • 2m on sides, and 4m upstream, is sufficient, maybe we underestimate by few %
  • Integrating, we expect ~10 neutron hits in the ECAL per spill, i.e. 1 per μs – this is

going to be sub-dominant

Upstream rock Top/bottom rock

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Chris Marshall 14

Hall-originating event vertices

  • First, position of all

interactions in detector hall

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Chris Marshall 15

Hall-originating event vertices

  • Position of neutrino

interaction for events that produce neutron candidates in ECAL

  • Predominant

background source is ECAL itself

  • Second is the magnet,

especially upstream

  • Most downstream parts
  • f LAr also contribute
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SLIDE 16

Chris Marshall 16

Intrinsic background

μ π

  • Backgrounds can
  • riginate in gas TPC

from neutrons produced by charged particle interactions

  • Often, the scatter
  • ccurs in the ECAL,

and the neutron is spatially near the TPC track vector, and can be rejected

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Chris Marshall 17

Intrinsic background

μ π

  • But when interaction
  • ccurs in the TPC, it

is very hard to distinguish from primary neutrons

  • This is the most

challenging background

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Chris Marshall 18

External (pile-up) background

μ π

  • Pile-up can produce

neutrons that accidentally coincide in time with a GAr TPC event

  • It is possible to apply

a veto when ECAL activity is observed just before a gas TPC interaction, which may produce neutrons

μ

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SLIDE 19

Chris Marshall 19

External (pile-up) background

μ π

  • But neutrons which

traverse the GAr are hard to veto – the TOF is 10s to 100s ns, and the rate is too high to veto these events

μ

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Chris Marshall 20

Out of the box

  • Pile-up backgrounds

are flat in Δt, so they tend to very low reco kinetic energy

  • Basically we can't

measure 20 MeV neutrons at full intensity because their time of flight is ~50 ns

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Chris Marshall 21

Zoom in

  • At high energy,

dominant background is from non-primary neutrons produced in the signal neutrino interaction

  • At low energy,

dominant background is accidental activity

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Chris Marshall 22

Charged particle trajectory cut

μ π

  • Project charged

tracks into the ECAL

  • Look at the distance

between a neutron candidate and the nearest charged track trajectory

  • Backgrounds from

charged particle interactions in the ECAL will be close

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Chris Marshall 23

Cut #1: charged particle distance

  • Plot shows all

candidates >5 MeV neutron energy

  • Cut at 80 cm
  • Rejects neutron

candidates from the signal interaction

  • Does not remove

accidentals

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Chris Marshall 24

ECAL activity veto

μ π

  • For each neutron

candidate, determine the time and distance to other ECAL activity

  • Exclude (-5, +10)ns

window around GAr vertex, where ECAL activity will be due to the signal interaction

μ

ECAL veto Δx, Δt

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Chris Marshall 25

Distance and Δt to ECAL veto

  • Signal is flat in Δt to random ECAL activity, peak around 6m is because most

pile-up is upstream-entering, and most signal neutrons are downstream, and thus ~6m apart

  • Background is generally close in time and space to other ECAL activity and

can be vetoed with almost no signal loss

Signal Pile-up

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Chris Marshall 26

Projected onto distance axis

  • Pile-up neutrons that

don't go all the way through gas TPC are easily rejected by cut at 2m

  • Pile-up for neutrons

that scatter far from where they are produced is not rejected – the veto is too long and would reject too much signal

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Chris Marshall 27

Isolation from other ECAL clusters

  • Largely redundant

with cut on entering charged tracks

  • But can remove

some additional background where track does not come from gas TPC

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Chris Marshall 28

Reco KE distribution

  • No cuts
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Chris Marshall 29

Reco KE distribution

  • Charged TPC track

cut

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Chris Marshall 30

Reco KE distribution

  • Charged TPC track

cut

  • ECAL activity veto
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Chris Marshall 31

Reco KE distribution

  • Charged TPC track

cut

  • ECAL activity veto
  • Isolation from other

clusters

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Chris Marshall 32

Reco KE distribution

  • Charged TPC track

cut

  • ECAL activity veto
  • Isolation from other

clusters

  • Forward neutrons
  • nly
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Chris Marshall 33

Selection efficiency

  • Efficiency for all

neutrons is ~40-50% with loose selection cuts

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Chris Marshall 34

Sample purity

  • But purity for all

neutrons is only ~40%

  • ~20% due to pile-up

background, the rest due to neutrons (or photons) from the gas TPC event

  • Better photon

rejection in progress, should help purity at high reconstructed KE

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Chris Marshall 35

Sample purity (forward)

  • Slightly better at

high energy when you only consider forward neutrons

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Chris Marshall 36

Conclusions

  • Neutron reconstruction in MPD is challenging
  • Can achieve ~50% efficiency with ~50% purity for

forward neutrons

  • Energy resolution is poor and biased above ~100 MeV

due to missing the initial scatter, could be improved by

  • ptimizing active fraction (more CH)
  • Non-primary neutrons are a major background
  • Backgrounds from rock are minimal – magnet, ECAL,

and LAr TPC are major sources

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Chris Marshall 37

Next steps

  • Improve n/γ cut by voxylizing hits in each layer and

looking at transverse quantities

  • Look at RHC
  • Less pile-up background
  • Less non-primary background on average due to lower

energy hadrons

  • More energetic primary neutrons
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Chris Marshall 38

Backup

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Chris Marshall 39

Distance to ECAL activity (>50 MeV reco only)

  • Most pile-up is

reconstructed at very low energy

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Chris Marshall 40

Kink track angle

  • Maximum kink angle
  • Some gas-induced

non-primary neutrons are correlated with interactions in the TPC which produce large kinks

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Chris Marshall 41

Purity for leading neutron only

  • All angles
  • Slightly higher purity
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Chris Marshall 42

Eff for leading neutron only

  • All angles
  • Similar efficiency to

considering all neutrons

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Chris Marshall 43

Backward neutrons

  • More pile-up, less

signal

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Chris Marshall 44

Tighter pile-up cut

  • Nominal cut only excludes neutrons within 2m of ECAL

activity

  • But can also exclude entire ECAL for fast-coincident activity

Signal Pile-up

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Chris Marshall 45

Tighter pile-up cut

  • Reduced efficiency, but only slightly improved purity
  • Not a good cut – vetoing on activity far away from the

neutron candidate removes too much signal