# Presentation: 726839

## Search for KL→π0νν

- Takao Shinkawa
- National Defense Academy
- Particle Physics Seminar@BNL
- August 7, 2008

## Overview

- Physics of KL→π0ννdecay
- KEK E391a at 12-GeV PSData taking was done in 2004-2005.Run II data analysis has been recently completed. PRL 100, 201802(2008.5)
- E14  at J-PARCStage 2 recommendation by PAC in July 2007

## Physics Motivation

-   KL→π0νν
- FCNC 	loop diagramss → d
- Direct CP violationCP  -  → +
-  Standard Model Prediction
- BR= (2.49±0.39) x 10-11
- Theoretical uncertainty 1-2%
-  Imaginary part of V**td ****(η)**** **Different **η** between  K and B signifies new physics beyond SM.

- s

- d

- ν

- ν

- t

- W

- Z0

- K0L→π0νν

**ρ**

**η**

- K+→π+νν

- 1

**V****td**

**unitarity triangle**

## Beyond the Standard Model

- Grossman-Nir limit
-   model independent limit

- F. Mescia, CKM 2006

**K****L****→π****0****νν  is one of most promising  process in a search for new physics**

- Grossman-Nir limit model independent limit

- Br(KL→π0νν<6.5x10-10

**Notes:**
Au: trilinear scalar couplings in the soft SUSY breaking terms

## KL→π0νν search before E391a

- Br( KL→π0νν )SM = (2.49±0.39)x10-11
- L.S.Littenberg  extracted a limit from KL→π0π0 decay experiment.      Br(KL→π0νν)< 7.6x10-3(90%C.L.) (1989)
- KTeV at Fermilab    searched for KL→π0+nothing by two ways.
-  π0→e+e-γ (Br=1.2%)Br( KL→π0νν) < 5.9x10-7(90%C.L.) (2000)
-  π0→γγ   one day dataBr(KL→π0νν ) < 1.6x10-6 (90%C.L.) (1999)

##  E391a collaboration

-  E391a collaboration

- Started by Takao Inagaki in 1995 with 7 other members.
- Now
-    12 institutes, ~50 members
- Dept. of Physics, Pusan National Univ.
- Dept. of Physics, Saga Univ.
- Joint Institute for Nuclear Research
- Dept. of Physics, National Taiwan Univ.
- Dept. of Physics and Astronomy,    Arizona State Univ.
- KEK & SOKENDAI
- Dept. of Physics, Osaka Univ.
- Dept. of Physics, Yamagata Univ.
- Enrico Fermi Institute, Univ. of Chicago
- National Defense Academy
- Dept. of Physics, Kyoto Univ.
- Research Center for Nuclear Physics, Osaka Univ.
- 5 countries: Japan, the US, Taiwan, South Korea, and Russia

## Experimental Method

- KL→π0νν decay         →γγtwo photons w/o any other observable charged or neutral particles.
- Constraint of two photon effective mass,  mγγ= mπ0 gives distance to the decay point.

- K0L

- γ

- photon detector

- hermetic photon veto

- γ1

- γ2

- distance

-  two photonenergies and positions

## Narrow (pencil) beam

- With pencil beam, decay vertex Zvtx is defined from beam direction and vertex distance.
- Transverse component of kinematical component PT can be estimated.

- Eγ

-  decay point  Zvtx

- KL beampencil beam

- PTγ

- distance

- PTπ0  = PTγ1 + PTγ2

- Most important constraints for KL→π0νν

## Experimental Setup

## Neutral beam line

- Protons  12-GeV PS       2 x 1012 POT/4s (duty 50%)
- Neutral Beam
- production angle        4 degreescore neutron/KL         40~60
- solid angle                 12.6 μstr (θ=2 m radian)pencil beam               6cmΦ at the middle of the detector
- PKL                            peak at 2 GeV/c (mean 2.6 GeV/c)
- Halo/core                   10-4
-                                                                           NIM A 545, pp.542-553(2005)

- The E391a Detector

## Features of E391a apparatus

- Decay region
- High vacuum: 10-5 Pa
- to suppress the backgroundfrom interactions w/ residual gas
- Detector components
- Set in the vacuum:  0.1 Pa
- separating the decay regionfrom the detector regionwith “membrane”: 0.2mmt film

- CsI calorimeter

- Charged Veto (CV)

- Main Barrel (MB)

- Front Barrel (FB)

**Notes:**
detector region is also evacuated

## Electromagnetic Calorimeter

- Undoped CsI crystal
-     7x7x30 cm3(16X0) 496 blocks
-     specially shaped      80 blocks
-     sandwich type          24 blocks
- Charged veto(CV) 6mmtplastic scintillation countersin front of CsI and beam side

- 190cm

-  beam hole 12cmx12cm

## Photon Veto

- Sampling counter
- Around the decay regionMain Barrel (MB)  13.5 X0  5.5  m   Front Barrel (FB)  17.5 X0   2.75 m
- Around the beamCollar counter CC00,CC02-07
- Beam region BA,        (alternating layers of lead, quartz, and scintillator)

## Electronics and DAQ

- Number of channels
- CsI calorimeter: ~600ch
- Veto counters:  ~400ch
- “AmpDiscri” Module
- Discrimination for TDC
- Set near the detector
- low noise; min. threshold: ~0.5 mV  (ex. ~0.7MeV for CsI)
- 8ch sum for the trigger
- Trigger
- Logic
- CsI hardware clustering (N>1)         (thres. 80MeV) + Veto (20-100MeV)
- ~300 events / 2 sec spill = 150Hz
- DAQ live time
- ~90%

## Run Summary

- Running conditions
-           K0 beamline in the East Counter Hall of KEK 12 GeV PS
- Intensity:
- 2 x 1012 protons on target (POT) per 2sec spill, 4sec cycle
- production angle: 4°, KL peak momentum 2GeV/c, n/KL ratio: 60~40
- Physics and calibration runs
- Run I:  February to July of 2004  w/o Be
- ~3 x1018 POT    problematic
- Run II: February to April of 2005 w/ Be
- ~2 x1018 POT      good condition
- Run III: October - December of 2005 mostly w/ Be
- ~1.4 x 1018 POT  good condition

**Notes:**
we appreciate the accelerator people’s effort for the operation of the PS

## Result from Run-I  

- Using 10% of Run-I data (Run-I one week)
- set new limit
- Br < 2.1x10-7(@90%C.L.)(PRD 74:051105, 2006)a factor of 3 improvementto KTeV result.

- CV

- CC02

## A serious problem in Run-I

- Beam core hits the membrane, which are accidentally drooped into the beam-line
-     -core neutron background-

- This problem was fixed before Run-II

## Analysis

## Run IIanalysis overview

- KL reconstruction
- KL flux calculation
- 6γ: KL→π0π0π0 (Br=19.56%)
- 4γ: KL→π0π0         (Br=8.69x10-4)
- 2γ: KL→γγ         (Br=5.48x10-4)
- Normalization by MC
- Systematics
- KL→π0νν search
- Backgrounds
- Results

## KL reconstruction

- π0(KL) reconstruction w/ 2 photons
- KL reconstruction w/ KL→2π0, 3π0
- Take the best **χ**2 for the vertex distribution in paring
- Cuts
- Photon Vetoes: typically O(1) MeV
- Kinematic cuts
- Photon quality cuts

## Channels used for normalization (KL flux estimation) and tuning of MC simulation 

**K****L****→3π****0**

- Signal region

- CV

- Photon Veto

-  data   MC

## Invariant mass of 2π0

- PV only

- Invariant mass of 2π0

- PV + kinematics

- 4γ invariant mass distribution in particular the tail due to KL→3π0
- (2 out of 6 photons were undetected) was well understood by MC.

- M4γ(GeV/c2)

## Summary of KL flux

| Mode | Signal Events
(Full Data Set) | Acceptance
(with Accidental Loss) | Flux 
(w/ systematic errors) | Discrepancy
(X - π0π0) /π0π0 |
| --- | --- | --- | --- | --- |
| K L→ γγ | 20,685 |  (0.697 ± 0.004Stat)% | (5.41 ± 0.37) x 109  | 5.0% |
| KL → π0π0 | 1494.9 (1500 - 5.1)
(π0π0π0 contribution) |  (3.35 ± 0.03Stat) x 10-4 | (5.13 ± 0.40) x 109  | 0% |
| KL → π0π0π0 | 70,054 |  (7.13 ± 0.06Stat) x 10-5 | (5.02 ± 0.35) x 109 | -1.9% |

## Systematic Errors on the flux

| CsI Veto | 6.1% |
| --- | --- |
| Decay Vertex (Z) Spectrum | 2.3% |
| Decay Vertex (Radial) Spectrum | 1.8% |
| Charged Veto | 1.3% |
| Fusion Neural-Network | 1.3% |
| Photon Hit Position (CsI Position) | 1.2% |
| 23 Others (Total) | 2.9% |
| TOTAL (In Quadrature) | 7.7% |

## Search for KL→π0νν

## KL→π0νν search

- Blind analysis was taken to remove bias.
- Hide signal region (+ Control region)
- The blind “Box”: on PT - Z plot
- All backgrounds are estimatedw/o looking into the Box
- After completion of BG estimation,the Box was opened

## Kaon backgrounds

- BG from Kaon decays
- KL→π0π0→γγγγ
- missing 2 photons
- Main backgroundin Kaon decays
- KL→γγ
- by PT mis-measurement

- PT

- Z

- KL→2π0

- KL→2γ

**signal region**

## Backgrounds from KL→π0π0 decays 

**Estimated by MC simulations, relying on the good reproduction of the π****0****π****0 ****distribution at low mass by ****K****L****→π****0****π****0****π****0**** MC. The missing 2-photon feature is similar.   ****MC data = 11xdata ****BG=0.11 events****.**

## Backgrounds from the other KL-decays 

- Z

**K****L****→2****γ**

- Energy mis-measurement or response tail can make higher PT events, but the a-coplanar angle cut is quite effective to reduce them.

**K****L****→ charged**
-  (**Ke3, Kμ3 and charged-Kπ3**)

**Backgrounds from all these decays are negligibly small**

**P****T**

- KL→γγ MC No cuts

## Halo neutron backgrounds

- Tails of 2**γ**produced by halo neutrons at the detectors
**“****CC02”**
- Tail of γ energy measurement due to leakage or mis-measurement
**“****CV”**
- Multi-**π****0**** **+ odd combination
** ****η **(548MeV) :  shift of vertex due to the assumption of M2γ = Mπ0

## CC02 background

- S/N

- Upstream edge of the signal region

**Estimated from the special run data, which were normalized to the CC02 events. **
**Upstream edge of the signal region was determined by S/N.**

- Al plate run

- CC02

- Signal region

- mγγ

- π0

- η

- CC02 BG 0.16±0.05 ev

## CV-π0 background

**     ****Estimated from the data using a bifurcation method.**
**     ****We also checked the reproduction of CV events by a halo neutron MC.**

- data: 17 events,
- MC: 18.2±6.1 events

- Cut sets
**set-up cuts**
- upstream veto detectors,
- CsI, and π0 kinematics
**set A**
- downstream veto detectors
**set B**
- gamma selection

- Signal region

- CV-π0 BG 0.08±0.04 ev

- CV region

- NAB=NABxR_B

## CV-η background

**     ****Estimated from MC with 200 times statistics, after carefully checked the cross section, momentum and PT distributions using the target run data.**

- Al plate run

- 16eventsCV-η BG0.06±0.02 events

- ηregion

- mγγ

## Background summary

- (1) 300-340cm : 1.9±0.2
- CC02: 1.9±0.2
**observed: 3 events**** **
- (2) 340-400cm: 0.15±0.05
- CC02: 0.11±0.04
- CV-η: 0.04±0.02
- (3) 400-500cm: 0.26±0.11
- CC02: 0.05±0.03
- CV-η: 0.02±0.01
- CV-π0: 0.08±0.04
- KL→π0π0: 0.11±0.09
-  0.41+0.11 for signal region
- (4) 300-500cm, Pt<0.12 GeV/c
- CC02: 0.26±0.07
- CV-η: 0.04±0.01
- CV-π0: 0.09±0.04
- total: 0.39±0.08
**observed: 2 event**

- Data w/ all the cuts

- CC02

- CV

## Open the signal region

- No event observed!!

## Result of Run II

- Acceptance:  A = 0.67%
- Flux:              NKL =  5.1 x 109
- S.E.S = 1 / (A∙NKL)          = (2.9 ± 0.3) x 10-8
- Upper Limit
- 0 event observed2.3 events  w/ Poisson stat.
- Br(KL→π0νν) < 6.7 x 10-8                           (@90% C.L.)
**                       **
**                             ****PRL 100, 201802(2008.5)**
**RunIIIhas similar quality.**
** ****70%of RunIIdata.**
**Analysis is going on.**

## J-PARC

-  High intensity proton accelerator at Tokai         56 km from KEK         beam intensity  x100 12-GeV PS
-  Accelerator construction will be completed in this year.

## Step by Step approach

- To event observation
- KEK E391a
- The first experimentdedicated to KL→π0νν
- To establishthe experimental method
- J-PARC E14
- Step1
- Prompt start
- The E391a Detector
- Common beamline
- The first event observation
- goal: ~3 events
- Step2
- Longer decay volume and larger detector
- Higher intensity beamline
- goal: ~100 events

## CsI  
7x7x30cm3→ 2.5x2.5x50cm3 , 5x5x50cm3 (from KTeV)
Reduce leakage
Better positioning
Readout Electronics
Wave-form digitization 
New Detectors
Beam Hole Photon Veto
Full active CC02
MB liner
New CV

- 7x7x30cm3→ 2.5x2.5x50cm3 , 5x5x50cm3 (from KTeV)
- Reduce leakage
- Better positioning
- Readout Electronics
- Wave-form digitization
- New Detectors
- Beam Hole Photon Veto
- Full active CC02
- MB liner
- New CV

- E14 at J-PARC

## E14 tentative schedule

- 2009 Beam line construction         Beam survey         KL flux measurement      with KL→π+π-π0  decay.
-         Flux might be three times more.
- 2010 CsI stacking
-              Engineering Run
- 2011  First run   1 event/year

## Summary

- KEK E391a – the first dedicated experiment for KL→π0νν decay.Br(KL→π0νν)< 6.7x10-8 (90%C.L.)RunIIIdata analysis is going on.
- J-PARC produces 100 times more intenseproton beam than KEK 12 GeV PS.
-    E14 is taking  step by step approach to precise measurement of the branching ratio of KL→π0ννdecay.

## Backup Slides

## KL beam 

- Beam profiles at the beam exitby MC.

- KL momentum distribution at the beam exit by MC.

## Vetoes

| counter | threshold (MeV) | comments |
| --- | --- | --- |
| FB | 1.0 | take sum of inner and outer channels |
| CC02 | 1.0 |  |
| MB | inner: 1.0
outer: 1.0 | E = sqrt( Eup * Edn), x0.95 for data |
| CV | outer : 0.3 
inner : 0.7 | convert gamma position to crystal number,
then check corresponding CV plane |
| CsI |   d < 17 cm  :       10.0
17< d < 25 cm : 5.0→2.0 d > 25 cm  :         2.0 | require the local flag
d: distance from the gamma position to the crystal
intermediate region (17-25cm):  threshold = 5.0 - (3.0/8.0) * ( d-17) |
| Sandwich | 2.0 |  |
| CC03 | 2.0 |  |
| CC04 | calorimeter : 2.0
scintillator : 0.7 |  |
| CC05 | calorimeter : 3.0
scintillator : 0.7 |  |
| BA | scintillator sum : 20.0
quarts : 0.5 MIPs | take AND of both conditions |
| BCV | 0.75 | E = sqrt( Eup * Edn) |
| CC06 | 10.0 |  |
| CC07 | 10.0 |  |
| BHCV | 0.1 |  |
| CC00 | 2.0 |  |

## gamma, π0 cuts

|  | minimum | maximum | comments |
| --- | --- | --- | --- |
| signal box z-vertex | 340 cm | 500 cm |  |
| signal box Pt | 0.12 GeV/c | 0.24 GeV/c |  |
| gamma time difference | -3.4000 | 2.8922 |  |
| gamma energy low | 0.25 GeV |  |  |
| gamma energy high | 0.15 GeV |  |  |
| gamma size 5MeV | 3 |  |  |
| gamma size 1MeV | 5 |  |  |
| gamma energy ratio | 0.88 |  |  |
| gamma TDI |  | 2.0 |  |
| gamma RMS |  | 4.0 |  |
| gamma energy balance |  | 0.75 | (Ehigh - Elow) / (Ehigh + Elow) |
| gamma distance | 15 cm |  |  |
| gamma hit position | 18 cm |  | square region |
| gamma hit radii |  | 88 cm | circle |
| acoplanarity angle | 45  degrees |  |  |
| π0 energy |  | 2.0 GeV |  |
| χ2θ |  | 1.0 | χ2 = Σ ( (θr1- θrec)2 / σθr12  ) |
| projection R
(π0 kinematics cut) |  |  | in the (z, Pt/Pz) plane, take the inside of
(300, 0.1), (400, 0.1), (500, 0.15), (500, 0.34), (300, 0.2) |
| reconstructed  KL momentum |  | 2.0 GeV | assuming the invariant mass of nu-nubar system is 0:
 two body decay |