Nucleon Decay Searches in DUNE Viktor P , The University of - PowerPoint PPT Presentation

Nucleon Decay Searches in DUNE Viktor P ěč , The University of Sheffield for the DUNE collaboration BLV 2019, Madrid October 22nd, 2019

Deep Underground Neutrino Experiment (DUNE) • Location • SURF, 1.5 km underground, Lead, South Dakota, • 1300 km from source • Neutrino source - beam @ Fermilab, Chicago, Illinois - powerful new beam of neutrinos 2

Experimental Halls at SURF • 4 modules, 17.5 kt/10 kt fiducial LAr each • Modules - Cryostats 18.9 m (W) x 17.8 m (H) x 65.8 m (L), 17.5 kt of LAr 3

DUNE Physics Programme • Neutrino oscillation (CP violation and mass hierarchy, mixing parameters) • Supernova neutrinos ← • Nucleon decay and nn ̅ oscillations this talk • Planned to start with 1st module in 2026, remaining 3 modules will be added sequentially 4

Liquid Argon Time Projection Chamber — LArTPC Basics of operation - electric field applied across drift volume - ionising particles create free charge — electrons drift Sense Wires X C U V Y towards anode planes V wire plane waveforms Liquid Argon TPC - multiple anode planes with readout wires with different orientation → position in transversal plane Charged Particles γ - drift time + wire plane location → 3D reconstruction of γ Cathode energy depositions γ Plane - signal proportional to deposited energy → dE/dx o n i r t u γ e measurement — particle ID N γ g n i m o c n I Anode planes - induction plane wires — electrons pass through Edrift t - collection plane wires — last plane, all ionisation C X Y wire plane waveforms electrons collected DUNE - collection plane (C) wires vertical - induction plane (U,V) wires 37.5º from vertical - wire pitch 5 mm 5

Advantage of LArTPC • Can reconstruct tracks • Sees dE/dx profile • Can identify K in nucleon-to-K decays • Can classify different event topologies C p → K + + ¯ ν MC Simulation K + → μ + + ν μ Signal μ + → e + + ¯ Drift time ν μ U • Example of crisp proton-decay event display • in 3 wire views V e + µ + K + Wire number 6

Nucleon Decays • Potential of DUNE for some nucleon decays investigated: ν n → e − K + p → e + π 0 p → K + ¯ - , , • Backgrounds: atmospheric neutrino CC and NC interactions p → K 𝛏̅ Key features - K Bragg peak near its decay - K-decay particles create unique tracks Difficulties - proton decays in Ar → K may undergo Final State Interactions (FSI) inside the nucleus - K loses energy and it is more difficult to reconstruct 7

Effect of FSI on K Track 800 Tracking Efficiency 1 + 700 Primary K + Final State K 600 0.8 500 Events 0.6 400 300 0.4 200 0.2 100 0 0 0 50 100 150 200 250 300 0 20 40 60 80 100 120 140 160 180 200 Kaon Kinetic Energy (MeV) Kaon Kinetic Energy (MeV) • Left: kinetic energies of kaons leaving Ar nucleus without and with FSI • Right: current tracking efficiency of kaons: reconstruction switches on only at about 40 MeV • Room for improvement 8

p → K 𝛏̅ Background Events MC Simulation Low scoring High scoring Atm.neutrinos 𝛏 e + n → e - + p + 𝜌 0 𝛏 𝜈 + n → 𝜈 - + p Drift time Drift time e - µ - p Wire number Wire number • Example of potential background events — atmospheric neutrino CC interaction • BDT multi-variate analysis used to classify events: - Left: well discriminated by the classifier (low score) - Right: poorly discriminated (high score) 9

Sensitivity to p → K 𝛏̅ • Current analysis predicts signal efficiency Arbitrary Units signal 15% with background suppression of 3x10 -6 background (about 1 bg event per Mton-year or 25 years of data taking) • Current K tracking efficiency only at 58% • Visual scanning of signal and background events suggests 80% K tracking eff. achievable 0.35 0.4 0.45 0.5 0.55 0.6 0.65 BDT response • If combined with improvements in K/p separation, signal efficiency 15% → 30% Systematics: Sensitivity: • contribution of FSI effect unknown → 2% • if no signal observed in 10 years, in full 40 kt configuration uncertainty on signal efficiency • limit of 1.3 x 10 34 years (90%CL) on partial • atmospheric neutrino flux and cross-section proton lifetime in p → K 𝛏 channel uncertainties → 20% uncertainty in backgrounds 10

n → e - K + • Similar analysis to p → K 𝛏̅ decay • Additional electron shower • Invariant mass ~1 GeV • Background: atmospheric neutrinos • Signal efficiency expected 47% with 15 bg events per Mton-year • Limit 1.1 x 10 34 years in 400 kt-year exposure with 6 background events • → >2 x improvement of current limits 11

p → e + 𝜌 0 • Signature: 3 EM showers, invariant mass ~1 GeV • Background: atmospheric neutrinos • Preliminary analysis based on MC truth • Reconstruction only approximated • 8.7x10 33 years to 1.1x10 34 for exposure of 400 kt-year • dependent on reco. approximation (energy smearing) • Doubling the exposure would allow reaching current SK limit 12

nn ̅ • Were nn ̅ oscillations possible, neutrons would transform into antineutron and quickly annihilate with surrounding nucleons • Oscillation time heavily suppressed for neutrons bound in nucleus • Effective conversion time T n − ¯ relates to free neutron oscillation n time : τ n ¯ n n = T n − ¯ τ 2 n n − ¯ R ff erent nuclei. This R = 0.666 × 10 23 s − 1 • Suppression factor calculated for iron [1] [1] Phys. Rev. D78 (2008) 016002 13

nn ̅ MC Simulation • Annihilation produces multiple Signal pions • FSI can yield nucleons • Typical star-like signal • Invariant mass ~2 GeV • Vanishing total momentum 14

nn ̅ Backgrounds MC Simulation • Atmospheric neutrino NC interactions Atm.neutrinos Low scoring High scoring 15

Nn ̅ Oscillation Time Limits • Analysis uses similar multi-variate Arbitrary Units signal methods to nucleon decay background searches • Bound neutron : 6.45 x 10 32 years @ 90% CL with 400 kt-year exposure (~10 years in full 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 configuration) BDT response e 6.9: Boosted Decision Tree response for ¯ oscillation for signal (blue) and backgrou • After conversion to free neutron oscillation time: - 5.53 x 10 8 s • 2x improvement over the current limits 16

Summary • LArTPC new technology for nucleon decay searches • DUNE will be the largest LArTPC with sensitivities complementary to large water Cherenkov detectors • p → K 𝛏̅ — potential improvement of current limits • n → e - K + — factor >2 improvement expected • p → e + 𝜌 0 — preliminary study suggests current limits reached only after double the exposure • nn ̅ — factor 2 improvement on free neutron oscillation time • Observation 1 event can constitute compelling evidence 17