Event Topologies K + has two main decay modes + (BR = 64%) - PowerPoint PPT Presentation

Selecting p → ത ν K Events Through K → μν and K → π + π 0 Decay Chains Dan Pershey Feb 6, 2019

Event Topologies ❑ K + has two main decay modes • μ + ν (BR = 64%) – monochromatic muon • π + π 0 (BR = 21%) – totally reconstructable – can construct K + invariant mass ❑ Both have same topology – two Bragg peaks facing in the same direction • Track is monochromatic, while a π 0 comes out opposite the π + in the π + π 0 case e + γ γ μ + K + K + π + μ + e + 2

Reconstructing GLOT’s ❑ K + → μ + → e + (or K + → π + → μ + → e + ) events produce three contiguous tracks ❑ Aim to “glue” together a set of ordered, reconstructed tracks (from pmtrack) • Separated into longest track, and activity in front of or behind the longest track • I’ve been referring to them as GLued Oriented Tracks (GLOT’s) “Post” segment “Pre” segment 3

dEdx Traces from GLOT’s ❑ Developed an analysis just focusing on the dEdx traces • Currently only using hits reconstructed from the induction plane – room for improvement dEdx (TPC charge) Pre segment Post L is negative, counting segment down from L = 0 Longest track Track Length from Reco K + Decay L = 0 4

Overall Event χ 2 ❑ Since we only have three particles, and one is monochromatic, all events should line up nicely ❑ We can make signal templates from MC, with L=0 at kink between the longest and pre portions, then make signal templates from MC ❑ Calculate a χ 2 /dof for each event by dotting its dE trace into these templates K → π + π 0 sample K → μν sample Track Length from Reco K + Decay Track Length from Reco K + Decay 5

Mass Fitting for GLOT Track Segments ❑ Bethe-Bloch formula for ionization energy loss is parameterized by charged particle mass and charge ❑ Fitting to observed dE/dx gives you a best fit mass of the track • Can run the fitter both “forward” and “backward” along the track • Can get a sense of which direction the particle was tracked by the Δχ 2 in each of these two scenarios • Important! For signal events, K and μ / π Bragg peaks are in the same direction Fitting a hand- selected signal K + track 6

Bethe-Bloch Fit – Signal (K-> μν ) Pre segment: true K Pre Segment Longest Track m pre = 363 MeV m long = 102 MeV m pre = 387 MeV m long = 10 MeV Δχ 2 = +87.5 Δχ 2 = +21.6 7

Bethe-Bloch Fit – ν μ CC Background Pre segment: true proton Pre Segment Longest Track m long = 115 MeV m long = 83 MeV Δχ 2 = +92.3 m pre = 905 MeV m pre = 774 MeV Δχ 2 = -55.7 8

Selecting Proton Decay Events – Fitted track masses ❑ To start with a preselection, require that the longest track and pre portions have a reconstructed mass between 10 and 2000 MeV • Gives a 48% efficiency hit – mostly boils down to requiring 2+ tracks in the event ❑ Require each of these sections reconstruct near physical values • 50 < mass long < 300 MeV • 100 < mass pre < 800 MeV 9

Selecting Proton Decay Events – Fitted track direction ❑ Again, put cuts on the longest and pre portions ❑ The longest portion doesn’t give much discriminating power, but cut on it anyway – it’s physical requiring the two to be similarly -directed ❑ Require • Δχ 2 long > 0 and Δχ 2 pre > 0 10

Selecting Proton Decay Events – μ LL ❑ Measure the “likeness” of the event to the dE trace to the MC prediction for signal ❑ Split the sample into two signal samples (for K → μν and K → π + π 0 ): • μ LL < 1.3 • μ LL > 1.3 && ππ LL<2.3 μ sample ππ sample Goes to ππ LL cut 11

Proton Decay Sample ❑ We’re left with two samples proton decay events ❑ Only atmospheric neutrino bkg included – very likely to dominate • Only one bkg event selected (trkl = 21cm) – bkg prediction here is the track length after mass long , mass pre , and μ LL/ ππ LL cuts, but normalized to give the total expected bkg K → π + π 0 sample K → μν sample Tot signal: 2.46 Tot signal: 0.41 Tot bkg: 1.5 Tot bkg: 0 12

Calculating a Sensitivity ❑ In the K → μν sample, optimize a signal range by optimizing S/sqrt(B) • 48 < Track Length < 58 cm • 1.939 signal events and 0.145 bkg events • Efficiency: 17.4% for K → μν , or 11.1% of all p → ത ν K events are selected in signal region ❑ In the K → π + π 0 sample, no bkg passed our cuts due to limited MC, so take • 26 < Track Length < 36 cm • 0.219 signal events • Efficiency: 6.0% for K → π + π 0 , or 1.3% of all p → ത ν K events are selected in signal region ❑ Define sensitivity as the maximum half life that we would exclude to 90%, assuming we see 0 events in 400 kton-years of data • Pois(0 | N( τ )) = e -N( τ ) = 0.1 → N( τ ) = ln(10) = 2.303 • Notice above quoted numbers add up to 2.303, were calculated at our sensitivity ❑ Sensitivity: τ p /Br(p → ത ν K) > 8.98e33 years (> 1e34 years at 446 kton-years) • Or, τ p /Br(p → ത ν K) > 8.14e33 just using K → μν sample 13

Summary ν K decays through K → μν and K → π + π 0 topologies ❑ We can select p → ത • Selects 12.4% of p → ത ν K decays ❑ Most effective discriminator we have is the forward-backward Δχ 2 fit to the Bethe-Bloch prediction for dE/dx for the K candidate ❑ Cutflow has some nice properties • Cuts are all physically motivated – helpful for understanding systs. on our efficiency • Track length not wrapped into the selection – largely responsible for why we can select both μ and ππ topologies ❑ Definitely room for improvement • Incorporate induction plane hits into analysis • Look for events where • Look for ππ events where the K has a longer track than the π • Tag K’s from ππ events by the invariant ππ mass in cases where the K isn’t tracked 14

Single Selected Background ❑ Only one bkg event remains, has a track length of 21 cm μ LL 15

More Typical Bkg μ LL 16

More Typical Bkg 17