Study of Halo ! " $% Background in the KOTO experiment Yuya - PowerPoint PPT Presentation

Study of Halo ! " → $% Background in the KOTO experiment Yuya Noichi Year End Presentation 2019 2019/12/23 2019 Year End Presentation 1

KOTO Experiment • Search for ! " → $ % &' & decay () undetectable & Signal 2+ � Nothing (4 , veto ) veto counter ) Au Target K L CsI Calorimeter Proton ' & 2019/12/23 2019 Year End Presentation 2

Halo ! " → $% Background • If beam halo K L decay for 2 ) , it may be confused with & ' → / 0 1* 1 signal. ! " → , - .* . halo ! " → $% ) ) K L ) ) K L z z + + * CsI CsI • P T of Halo K L must be measured correctly because it may cause Halo & ' → 2) background. 2019/12/23 2019 Year End Presentation 3

COE(Center of Energy) • COE can be calculated from CsI information; Hit position and energy of each gamma. !"#$ = ∑(()*! + , + ) ∑(()*. + , + ) ."#$ = ∑ , + ∑ , + • This can be indicator of halo K L because COE represents the arrival point of K L for the case angle between P T and P Z is small. / COE 3 4 z 012 2019/12/23 2019 Year End Presentation 4

Motivation • Conventional reconstruction method assumes mass of each 2gamma is ! " # and vertex position is on z axis. • Transverse position of K L was determined by the interpolation between target and COE. • However, assumption that K L fly from target to COE may not be correct because K L can scatter at the downstream of target. We want to develop new method without using this assumption. 2019/12/23 2019 Year End Presentation 5

New Method Decay position of K L may be determined by minimizing χ 2 of following formula. " " " , -,.,/ 01 23 , -,.,/ 01 23 , -,.,/ 01 23 * + " * , " * 6 " ! " ($, &, ') = + + 4 + 4 , 4 6 7 8 7 8 ($, &, ') 7 8 z K L Target I checked the precision of this new method by using toy simulation. 2019/12/23 2019 Year End Presentation 6

! " → $% & Toy Simulation θ is fixed, KL does not go CsI through the beam hole 0 1 → 33 4 decay - + + / - + , 400 mm � P = ( Pt, Pz ) BH width θ : Fixed z K L (150 mm) (Pt - Pz angle) | P |: spectrum measured at the beam exit z[m] 2019/12/23 7

Event Selection Criteria • 6 gamma positions are all inside the fiducial region of the CsI • Minimum gamma energy > 150 MeV • Minimum 2 gamma distance > 150 mm • 0 < Decay z < CsI (6168 mm) Using this toy simulation, I checked whether this new method can reconstruct the decay position. 2019/12/23 2019 Year End Presentation 8

True information of Toy Simulation True X,Y position hist1 hist1 600 True vertex Y [mm] True z position Y position (mm) Entries Entries 10000 10000 # of Events 14 - - hist9 hist9 Mean x Mean x 0.6744 0.6744 350 Entries Entries 10000 10000 Mean y Mean y 0.8278 0.8278 400 Mean Mean 4072 4072 Std Dev x 223.2 Std Dev x 223.2 Std Dev 824.6 Std Dev 824.6 12 300 Std Dev y 223.4 Std Dev y 223.4 200 10 250 200 8 0 150 6 - 200 100 4 50 - 400 2 0 1000 2000 3000 4000 5000 6000 - 600 0 - - - Z position (mm) 600 400 200 0 200 400 600 True vertex Z [mm] X position (mm) True vertex X [mm] We want to reconstruct these vertex positions 2019/12/23 2019 Year End Presentation 9

Reconstructed Vertex position(1) • Energy and position resolution of CsI... neither considered new reconstructed X,Y new reconstructed Z - true Z new reconstructed r - true r Rec. vertex Y [mm] hist2 hist2 hist3 hist3 # of Events hist11 hist11 # of Events 600 16 NEW Entries Entries 10000 10000 Entries Entries 10000 10000 Entries 10000 Entries 10000 NEW 10000 NEW - - 9000 - - 0 0 Mean x Mean x 0.8824 0.8824 Mean Mean 5.841e 5.841e 16 16 Mean Mean 14 - - Mean y Mean y 0.5082 0.5082 Std Dev Std Dev 1.513e 1.513e 13 13 Std Dev 0 Std Dev 0 400 8000 Std Dev x 194.3 Std Dev x 194.3 Std Dev y 193.5 Std Dev y 193.5 8000 12 7000 200 10 6000 6000 5000 0 8 4000 4000 6 - 200 3000 good 4 2000 2000 - reconstruction 400 2 1000 - 600 0 0 0 - - - - - - - - - - - - 600 400 200 0 200 400 600 200 150 100 50 0 50 100 150 200 100 80 60 40 20 0 20 40 60 80 100 Rec R – True R [mm] Rec. vertex X [mm] Rec Z – True Z [mm] Rec. vertex Y [mm] # of Events # of Events # of Events # of Events conventional conventional conventional conventional conventional conventional Rec R – True R [mm] Rec Z – True Z [mm] Rec. vertex X [mm] 2019/12/23 2019 Year End Presentation 10

Comparison of new and conventional • Energy and position resolution of CsI... both considered Rec. vertex Y [mm] # of Events # of Events NEW NEW NEW can not see the larger image difference Rec R – True R [mm] Rec Z – True Z [mm] Rec. vertex X [mm] Rec. vertex Y [mm] # of Events # of Events conventional conventional conventional Rec R – True R [mm] Rec Z – True Z [mm] Rec. vertex X [mm] 2019/12/23 2019 Year End Presentation 11

Chi2 distribution 1 sigma(ΔChi2 = 2.3) contour (for example 1 event) Rec vertex Y [mm] position resolution ~300mm ... not small 1σ Rec vertex X [mm] • The failure of reconstruction using new method is due to this bad position resolution. 2019/12/23 2019 Year End Presentation 12

Reason for bad resolution • Constraint from 6 gamma looks like sphere, and sum of them become too shallow to minimize. CsI ! ! Constraint ! surface ! ! ! z ! ! 2019/12/23 2019 Year End Presentation

Summary and Next Halo K L cause ! " → 2% background, so we need to measure • halo K L correctly. • I tried to develop new reconstruction method minimizing chi2 function of reconstructed mass of & ' . • Position resolution of new method was not enough to reconstruct vertex position correctly. • To know whether we need to develop more reconstruction method, I’m checking discrepancy of COE between data and MC. 2019/12/23 2019 Year End Presentation 14

Backup 2019/12/23 2019 Year End Presentation 15

Reconstructed Vertex position(1) • Energy and position resolution of CsI... neither considered new reconstructed X,Y new reconstructed Z - true Z new reconstructed r - true r Rec. vertex Y [mm] hist2 hist2 hist3 hist3 # of Events hist11 hist11 # of Events 600 16 NEW Entries Entries 10000 10000 Entries Entries 10000 10000 Entries 10000 Entries 10000 NEW 10000 NEW - - 9000 - - 0 0 Mean x Mean x 0.8824 0.8824 Mean Mean 5.841e 5.841e 16 16 Mean Mean 14 - - Mean y Mean y 0.5082 0.5082 Std Dev Std Dev 1.513e 1.513e 13 13 Std Dev 0 Std Dev 0 400 8000 Std Dev x 194.3 Std Dev x 194.3 Std Dev y 193.5 Std Dev y 193.5 8000 12 7000 200 10 6000 6000 5000 0 8 4000 4000 6 - 200 3000 good 4 2000 2000 - reconstruction 400 2 1000 - 600 0 0 0 - - - - - - - - - - - - 600 400 200 0 200 400 600 200 150 100 50 0 50 100 150 200 100 80 60 40 20 0 20 40 60 80 100 Rec R – True R [mm] Rec. vertex X [mm] Rec Z – True Z [mm] Rec. vertex Y [mm] # of Events # of Events # of Events # of Events conventional conventional conventional conventional conventional conventional Rec R – True R [mm] Rec Z – True Z [mm] Rec. vertex X [mm] 2019/12/23 2019 Year End Presentation 16

Reconstructed Vertex position(2) • Energy and position resolution of CsI... both considered Rec. vertex Y [mm] # of Events # of Events NEW NEW NEW Rec R – True R [mm] Rec Z – True Z [mm] Rec. vertex X [mm] Rec. vertex Y [mm] # of Events # of Events conventional conventional conventional Rec R – True R [mm] Rec Z – True Z [mm] Rec. vertex X [mm] 2019/12/23 2019 Year End Presentation 17

Sigma of M 2 #$ % (' ( ,* ( ,+ ( ,' % ,* % ,+ % ) #$ % (' ( ,* ( ,+ ( ,' % ,* % ,+ % ) #$ % (' ( ,* ( ,+ ( ,' % ,* % ,+ % ) ! M2 = ! ' ( + ! * ( + ... + ! + % #' ( #* ( #+ % = $ % ' ( -. ,* ( ,+ ( ,' % ,* % ,+ % /$ % ' ( ,* ( ,+ ( ,' % ,* % ,+ % #$ % (' ( ,* ( ,+ ( ,' % ,* % ,+ % ) #' ( . 2019/12/23 2019 Year End Presentation 18

Chi2 distribution 2019/12/23 2019 Year End Presentation 19

[1]Nakagiri-san’s Dr.thesis https://www-he.scphys.kyoto-u.ac.jp/theses/doctor/nakagiri_dt.pdf KL Generation in Toy Simulation(2) CsI Decay Position ' + % ) ' % & � P = ( Pt, Pz ) BH width z K L Generation | P |= [1] z[m] 2019/12/23 2019 Year End Presentation 20

KL Generation in Toy Simulation(3) CsI Decay Position � P BH width z K L Flying Distance = &'() ∗ − log(012345[0,1]) z[m] 2019/12/23 2019 Year End Presentation 21

MINUIT Detail used “MIGRAD” minimizer (also checked HESSE,MINOS) • used some limits in order to prevent the parameter • from taking on unphysical values e.g.) 0 < z < CsI, -1000< x,y <1000 fitting step width of (x,y,z)... (10,10,100) • max call ... 1000 times (500~ not chenged) • 2019/12/23 2019 Year End Presentation 22

Problem of Conventional Reconstruction Method Reconstructed vertex point True '() vertex point z ! "#$%#& True Target Scatter point 23

Resolution of CsI 24

Only Position Resolution of CsI 25

Only Energy Resolution of CsI 26