Spallation Tagging Techniques in Super-Kamiokande Scott Locke Super-Kamiokande Collaboration TAUP 2019 2019/09/13
Super Kamiokande • 50kton water Cherenkov • Ultrapure Water Detector • 22.5 kton fiducial volume (32 kton inner detector volume) • 11, 129 20-inch PMTs (inner), 1885 of 8-inch PMTs (outer) • Phase Period inner PMTs coverage • SK-I 1996-2001 11,146 40% • SK-II 2002-2005 5,182 19% • SK-III 2006-2008 11,129 40% • SK-IV 2008-2018 (same as SK-III with new 41.4 m electronics) • SK-V 2019-2020 • SK-Gd 2020- • ~400 kt-years exposure • Detects neutrinos from many sources 1000 m • T2K far detector 39.3 m 2019/09/13 Scott Locke - University of California, Irvine 2
Introduction to Muons in SK 5.6 Mpe ≈ 1 TeV deposited within detector • ~2 Hz muon rate • Muons typically deposit a few GeV in the detector • Even though in many instances muons behave as a MIP, it isn’t always the case • Large amounts of energy can be deposited, especially in the form of a shower • These showers may cause spallation Picture of Control Room Event Display 2019/09/13 Scott Locke - University of California, Irvine 3
Spallation • Spallation – Muons initiate showers of secondary particles (e, n, γ , π , …) which capture on/break apart nuclei, creating unstable isotopes, and those isotopes decay feigning a desired signal Spallation • Lifetime can be short ( τ ~ O(.001s)) to relatively Daughter Decay Particles long-lived ( τ ~ O(10s)) • Troublesome when trying to perform specific analyses False Signature Y. Zhang et et al. (Super-Kamiokande Collaboration),Phys. Rev. D 93, 012004 (2016) 2019/09/13 Scott Locke - University of California, Irvine 4
DNSB spallation efficiencies Why do we care? • Inhibits us from lowering the threshold for Diffuse Supernova Neutrino Phys.Rev. D85 (2012) 052007 Background (DSNB) analysis and is a lingering background • At ~1σ excess from last time analysis was done • Largest remaining background in the solar neutrino spectrum • Although roughly 90% is cut, it accrues ~20% deadtime as a result • Background in Day/Night analysis • Reducing the background mitigates this background shaping • In other words, pretty much in all the continuous Low Energy analyses • Not much is known in terms of the actual physics of spallation production • What is happening in those showers? 2019/09/13 Scott Locke - University of California, Irvine 5 Personal Work
Showering Muons • The muon energy loss rate is: dE/dx = α(E) + β(E)E • α term corresponds to continuous ionization energy loss • β term corresponds to the radiative processes • Usual two types of showers: • EM and Hadronic showers • Hadronic showers still have large EM component due to π 0 decay into γγ Downward going muons through center • Currently use this light to try and identify of the detector showering muons S. W. Li and J. F. Beacom, Phys. Rev. C 89, 045801(2014) • Calculate likelihoods to determine if spallation 2019/09/13 Scott Locke - University of California, Irvine 6
Improvements and Modifications • Results presented are with respect to the solar spallation tagging • Would need separate treatment for DSNB search, but basic principles still apply • Using Wideband Intelligent Trigger (WIT) system to tag hadronic showers after muons, specifically look for neutrons • Using multiple events to tag spallation • Revisiting likelihoods set in SK-I, and updating • New likelihoods? 2019/09/13 Scott Locke - University of California, Irvine 7
Defining Parameters Muon Track • Transverse distance (lt): x (distance along track) • Distance of closest approach of event to track lt (transverse distance) In all plots, lt is plotted as lt 2 for flat phase space • Neutron/Spallation • Longitudinal distance (ln): Candidate • Distance along track in reference to another point • Taken as (x i – x avg ) for neutrons • Time difference (dt) • Time from muon to candidate • Time ordering dictates signal vs BG • Muon before candidate → Signal • Candidate before Muon → BG • Multiplicity: • Number of candidate events for a muon (neutrons or spallation candidates) • Residual Charge (resq): • Excess light from muon, above minimum ionization • E tot – (E MIP per cm)*(track length) • cos( Θ sun ) • Cos of the angle between reconstructed vertex direction and path from the sun 5/9/2019 Scott Locke - University of California, Irvine 8
Neutron Captures • Neutrons from hadronic showers capture on H and emit a 2.2 MeV γ , but detection efficiency is very low (~7 detected photons) • Use WIT to look for these events • Events are below 3.49 MeV Kinetic, which is the trigger threshold for SK • WIT has lower trigger threshold, allowing for chance to tag neutron captures • AFT trigger (DSNB search) disabled for muons to save CPU time • Not the case for WIT • Look for events within 500 μs of muon, and 5m of muon track • Efficiency is still not great, WIT triggers on 11 hits above dark-noise and event reconstruction is less reliable at this low of energy 9/11/2019 Scott Locke - Spallation Studies in Super Kamiokande 9
Neutrons after Muons • When making an initial cut on event reconstruction goodness, definite cloud can be seen in lt vs ln • Reminder: ln is (x i – x avg ) for neutrons • For a clear dt distribution, an additional cut on lt (<1.5m) was made • Just for trying to have good fit to the neutron signal, reduce BG • τ = 208.2 +/ - 1.84 • A little over 1σ from AmBe measurement 1 5-19 Neutron 2 20-39 Multiplicity 3-4 40+ PRELIMINARY 2019/09/13 Scott Locke - University of California, Irvine 10
Making a Cut • Use MC to make a more intelligent quality cut on neutrons • Simulate 2.2 MeV γ’s in SK, compare well to poorly reconstructed events • 2 + 1 cut on clouds • 2+ events within 500μs/5m, 1+ events making quality cuts • Only parameterize cloud with “good” events • Taking shape of neutron cloud into PRELIMINARY consideration • Shift coordinate for cloud, use muon direction as z (muon track) z-axis, and project cloud back to track • Cylindrical coordinates y • Cut for 60s (30s for 2 neutron showers) Muon • Cut on cloud size as function of multiplicity Centered • Have big cuts in short time x • 7.5m sphere for 200ms, 5m sphere for 2s • Downside: No WIT data available for this analysis before late Oct 2016 11 2019/09/13 Scott Locke - University of California, Irvine
Multiple Spallation • A little more novel approach • Do not expect multiple solar neutrinos within the same area in a short time frame • Cut events within a small timeframe and small area of each other • 4m and 60s • Only use events that would make final sample, without spallation cut and patlik cut, and above 5.49 MeV kinetic • Patlik is more sensitive to spallation • Can cut ~45% of spallation this way, with minimal deadtime • Great, because it can be applied retroactively 2019/09/13 Scott Locke - University of California, Irvine 12
Revisit Likelihoods • Normal solar spallation cut is done by Frequency log likelihood in lt, residual charge, and New dt New Old • Updating muon-fitter Old • Former muon fitter struggled to fit high energy muons, and had worse track correlation for spallation • Refit for SK-IV data and the change in fitter • Different fitter will have different Fraction Sig distribution, cover any changes since 0-100 ms BG early days of SK-I 100ms-3s • Big difference in resq likelihood 3s-30s • Change lt → lt 2 • More intuitive with flat phase space • Find there to be a dt dependence in lt 2 • lt 2 likelihood binned in time and charge PRELIMINARY • dt has minimal difference • Fit for 7 major spallation isotopes 2019/09/13 Scott Locke - University of California, Irvine 13
Spallation Effectiveness During WIT Period Bringing it All Together • For time with WIT available, applying neutron cloud, All multiple spallation, updated likelihood: OLD • Maintain efficiency, minimize deadtime NEW • Less than half the deadtime! Spalike only • For time without WIT, apply multiple spallation and Cloud only updated likelihood (change likelihood cut value) • ~250 days added exposure for SK-IV alone Mult only • Plots shown for >5.99 MeV kinetic OLD Accidentals Spalike Accidentals • Solid line is signal, dashed is background (backwards dt) Cloud Accidentals Method Efficiency (%) Deadtime All SK-IV Remaining Events (%) OLD Old OLD 89.97 19.61 NEW New Updated spalike Cloud 54.34 1.18 and multi cut Multiple 46.48 1.27 Spal-like 81.08 7.70 Cl + Mu + Like 90.05 9.65 PRELIMINARY 2019/09/13 Scott Locke - University of California, Irvine 14
SK-IV Updated Peak (New spallation Cut) 9.6% More events PERSONAL WORK 5.3% Relative Statistical Error reduction 2019/09/13 Scott Locke - University of California, Irvine 15
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