# Presentation: 948959

## Final BNL E949 results on the search for the rare decay + → +

**Joss Ives**University of British Columbia / TRIUMF

## There’s new physics beyond the Standard Model, but we don’t know exactly what it is (yet)

**Dark matter and dark energy**

**Neutrino mass****and oscillations**

**over antimatter**

**Dominance of matter **

**Notes:**
(SETTING + ROLE ) x 4

## To find physics beyond the Standard Model, we need to break the Standard Model

- Energy frontier

- Precision/Intensity Frontier

- overcompensating.com

- LHC

- Rare decays such as***+* → ***+*** (E949/E787 @ BNL)

**Notes:**
(SETTING + ROLE ) x 4

## Quarks cannot decay to another quark of the same charge by a first-order process

****

****

**Notes:**
(SETTING + ROLE ) x 4

## + → +  proceeds very slowly through second-order weak processes

- The decay proceeds through second-orderprocesses due to massive top quark

***t***

***u***

**Notes:**
(SETTING + ROLE ) x 4

## Observing the rare decay + → +  is an enormous experimental challenge

- Observing the rare decay ***+** **→ ****+***  is an enormous experimental challenge

- Standard Model branching ratio is (0.84  0.7)****1010

*********+*********

*********+************ *****0**

**10****-11**

**10****-9**

**10****-6**

**10****-3**

-  **10****0**

*********+*********

*********+************0*********

**Branching Ratio**

**CEX**

*********+************ *****-*****e******+*********

*********+************ *****0********

**Beam**

- Experimental signature is*** *+ + "nothing"

- That’s 1 in 12.5 billion decays***!!!***

**Notes:**
Imbalance

## A precision measurement of the + → + decay rate probes for new physics

- Curve extracted from D.Bryman *et al*., IJMPA **21**, 487 (2006)

- Curve assumes a Minimal Flavor Violation (MFV) model

**Notes:**
balance

## The E949 experimental goal is to suppress backgrounds to below the sensitivity level of the signal

**Total****Background**

**Signal****Sensitivity**

**Notes:**
solution

## The rest of this talk will look at the E949 experiment in four sections

**Final****results**

**The****Experiment**

**Backgrounds**

**Numerical****details**

## In this section I will discuss the experimental challenge and the blind analysis method

**The****Experiment**

**Numerical****details**

**Final****results**

**Backgrounds**

## The experimental challenge is to observe  + + "nothing"

***K*******

********* *******

******

- Neutrinos...not so easy to detect

## The   0 decay splits the signal region in two:PNN1 and PNN2

- Standard Model *** *+ momentumfrom *** +* → *** +***

## Things get ugly in the PNN2 region

## E949 & E787 have previously observed four candidate events showing that it can be done

- Refs: PRD**70**, 037102 (2004), PRD**77**, 052003 (2008)

## How to observe + → +  in three easy steps

***K********* ****goes in**

********* ********* ****comes out**

- 

- 

- 

**No additional particles**

- 

## 700 MeV/c kaon enters and decays-at-rest in the scintillator-fiber target

- Beam instrumentation is used to identify a single *K*+ from the beam

- Kaon comes to rest in the target and we wait at least 2 ns for *K*+ to decay

## Decay pion comes to rest in the range-stack and undergoes the   e  decay sequence

** ** *e*  decay sequence is observed in the range-stack scintillator

**  range and energy are measured target and range-stack

**  momentum is measured in the drift chamber

## Veto the photons and any addition charged particle activity

- The photon veto gives full 4 sr coverage

- Pattern recognition is used in the target, drift chamber and range-stack to identify additional charged tracks

## The blind analysis technique can be summed up as eliminating everything that is not + → +  without directly examining the signal region

*“**How often have I said to you that when **you have eliminated the impossible,*
*whatever remains, however improbable, **must be the truth?”*
- - Sherlock Holmes to Watson
- In The Sign of Four

- *Thanks Benji

## Backgrounds that can mimic + → +  are identified a priori

## The bifurcation method uses two uncorrelated high-rejection cuts to estimate each background using information outside of the signal region

**A**

**B**

**C**

**D**

**CUT1**

**CUT2**

- Measure B:Invert CUT1 and apply CUT2

- Measure C+D:Invert CUT2

- A is the signalregion

- 

- 

- 

- 

- Estimate A:A = BC/D

- Measure D:Invert CUT2 andapply CUT1

**Background estimate = A!**

## Background estimates are performed using data different from those used to develop cuts

**Entire****Data Set**

**Cut****Development**

**Background****Estimates**

**1/3**

**2/3**

**Compare background estimates**

## Signal region is only examined once all background estimates have been finalized

- “Open the box”

## In this section I will detail the suppression of various backgrounds and reveal the total background

**Numerical****details**

**Final****results**

**The****Experiment**

**Backgrounds**

## The number one enemy is   0 when the   scatters in the target

********* ********** *****Target Scatter**

********* *******

***K*******

******

******

*** * and *** *0 are back to back so photons are directed to efficient part of the photon veto

- When the *** * scatters in the target it loses energy and the photons lose directional correlation with the *** *

**Target**

**Target**

## This background is suppressed by the photon veto and identification of   scattering

********* *******

***K*******

- A scatter can be identified by finding a kink in the *** * track

- A scatter can also be identified by large energy deposits in a pion fiber or hidden energy in a kaon fiber

**Target**

- KaonPionCombined

## The bifurcation cuts used for  + target-scatters are the photon veto and the target-scatter cuts

- The bifurcation cuts used for *** *+ target-scatters are the photon veto and the target-scatter cuts

**A**

**B**

**C**

**D**

**Target-scatter ID**

**Photon Veto**

**B**

**C**

**C+D**

**Background****A = BC/D**

- Photon veto is inverted (black curve) and the target cuts are applied (blue curve)

- Target-scatter cuts are inverted (black curve) and the photon veto is applied (red curve)

## In practice the three   0 backgrounds have to be disentangled

- The + target scatter sample made by inverting the photon veto has contamination from + range-stack scatter and *K* + → ** + ** 0  processes

## The    e  decay can mimic signal when the   and e have low kinetic energies

********* *****+**** momentum (MeV/*****c*****)**

********* ********* ******** *****e****** *********  ****kinetic energy (MeV)**

**Simulated  *****K********* ******** *********** ********* *********** ********* *****e****** ********* ********** **

**Signal****Region**

## Use target pattern recognition to isolate sample and estimate rejection power using simulation

*K*  *** * *** * *e***  sample is isolated using target pattern recognition

- Estimate the rejection power of the target pattern recognition using simulated data supplemented by the measured **  energy deposit in scintillator

## The charge exchange background is dominated by the neutral kaon decay  KL     l   

## KS is suppressed by K decay time condition and KL by gaps between the K and   tracks

*K*S lifetime is only 0.1 ns so the *K* decay time condition suppresses *K*S decays

*K*L lifetime is 50 ns so target pattern recognition is used to identify gaps between the *K*+ and **+ tracks

- Additional suppression is provided by detecting the negatively charged lepton

## Due to kinematic constraints, the troublesome   decays are the multi-body ones

- Not a big deal in the PNN2 region

***!***

## Muon processes are suppressed kinematically and by identification of the   decay

- Pulse shape in the range-stackstopping counter

- decay sequence: *** *** *e* 

- Momentum

- Range in plastic scintillator

- Muons have different kinematic signature in the detector than pions

## Beam backgrounds come from events other than a single kaon decaying at rest in the target

**Single-beam**

**Double-beam**

## Compared to the E787-PNN2 analysis, our total background was decreased by 24% and total acceptance increased by 63%

**0**

**0.2**

**0.4**

**0.6**

**0.8**

**1.0**

**1.2**

**1.4**

**Charge****Exchange**

*********+************ *****-*****e******+*********

**Beam**

*********+************ *****0********

*********+************ *****0**
**Target-****Scatters**

*********+************ *****0**
**Range-Stack****Scatters**

**Muon**

**Total**

**E787****Total**

**Background Estimate (Events)**

- _	__	__E787-PNN2 __	__This Analysis_ Total kaons	1.73  1012 	1.70  1012Total acceptance	0.84  103 	1.37  103

- Only 20% of the approved beam time

- 

## In this section I will discuss some last numerical details before getting to the grand opening

**Numerical****details**

**Final****results**

**The****Experiment**

**Backgrounds**

## Outside-the-box studies compare background estimates to direct measurements just outside the signal region

- Signal region is masked out; the background estimate and direct measurement for region A are compared directly

- If CUT1 and CUT2  are uncorrelated, the two values will agree

## The outside-the-box results demonstrate that the systematic uncertainties assigned to the backgrounds were reasonable

## By further tightening four of the cuts, the signal region was divided into nine cells

## The relative signal-to-background levels of the nine cells varied by about a factor of 4

**0.007**

**0.027**

**0.059**

**0.243**

**0.005**

**0.019**

**0.038**

**0.379**

**0.152**

**0**

**0.1**

**0.2**

**0.3**

**0.4**

**0.5**

**0.6**

**0.7**

**0.8**

**0.9**

**1**

**Cell**

**Signal-to-Background**

## Now it’s time to open the box...

## ...and three events were observed

**Notes:**
David’s infoB(K+ → π + ν ν ) =
 ̄
(7.89+9.26 ) × 10−10
−5.10
The probability of all 3
events to be due to
background only is 0.037.
SM expectation: B =
(0.85 ± 0.07) × 10−10

## The combined E787/E949 branching ratio is twice as high, but consistent with Standard Model expectation

- Despite the sizes of the boxesPNN1 analyses are 4.2 times more sensitive than PNN2 analyses

## The E787/E949 collaboration a.k.a. the fine folks behind these results

## The next measurements of + → +  are just around the corner

- Future experiments

- Fermilab Project X?

- Ref: FlaviaNet - http://www.lnf.infn.it/wg/vus/

## Thank you...questions?

- Thank you to David Jaffe for the resources that made this talk possible