# Presentation: 573792

## Rare Kaon Decay Results From E949 At BNL: K+ 

- Shaomin CHEN
- TRIUMF, Canada

- ICHEP2004, Beijing

- 8/16/2004

## Motivation is clear and simple

**(sin2****************sin********J/********Ks**

-   **Use ****K****+******** **************  **and** **K****0******** **************  t**o measure the CPV related elements**

**To construct the K unitarity triangle  and confirm the “golden” relation which is valid in the SM and MFV.**

- A.J. Buras *et.al* hep-ph/0405132

## Experiment is challenging

**Weapons needed to suppress background by 10****10 ****in order to see the signal.  **** **

| Process | Events |
| --- | --- |
| K |   6343000000 |
| K |   2113000000 |
| K |       55000000 |
| Beam bkg | 25000000 |
| Knp, Kl- | 46000 |
| K |                     1  |

**Where can we find such a  “WMD”? **

*Standard Model*: **Br(****K****+******** **************** 10****-10**

## BNL-E949 detector                 -- Rejecting beam backgrounds

****

****

********

************

**e****+******

**Beam backgrounds include pion scattering,  kaon decay-in-flight, and charge-exchange reactions.**

****** ****cerenkov**

**K cerenkov**

- 

**B4**

- 

**K**

**Top half of side view**

**Charge exchange**

**Beam 1**

**Beam 2**

**K cluster**

- ******cluster**

**K decay**

**Target fibres**

## BNL-E949 detector     -- Powerful and redundant particle ID 

**Resolutions:********P/P ~ 1.1%; ********R/R~ 3.0%; ******** ******
**Rejections************for********  ****~************for photon with 4******** sr coverage.**

- Top half of end view

******-ID from its decay chain.**

**dE/dx**

**R/P**

**Photon veto**

**E787**

**E949**

## This is a team effort!

** ****16 institutes and universities in 4 countries.**

**...****more than 60 physicists from 6 countries. **

**BNL/FNAL/SBU/UNM, U.S.A**
**IHEP/INR, Russia **
**Fukui/KEK/Kyoto/NDA/Osaka, Japan **
**TRIUMF/UA/UBC, Canada**

## Many thanks to BNL-AGS people

**Before data taking**

**After data taking**

**Twice the instantaneous intensity**** **** **

**Platinum target used in 2002**

## Unique Strategy for data analysis

- Full data set

- 1/3 data

- 2/3 data

- 

- 

- Beam bkg

**Background **
**study**

**Background**
** ****estimate**

- Step 1

- Step 2

- Step 3

- Step 4

**Open the box**

- Step 5

**E787/E949: ****we bought a one-way ticket,-no way back after opening the box!**

**Cuts tuning**

**E787: <0.1 event**
**E949: <0.5 event**

## Background suppression

|          Cut
Bkg | Kinematics cuts(p/R/E)  | Particle ID | Photon veto | Timing cuts |
| --- | --- | --- | --- | --- |
| K |  |  |  |  |
| K |  |  |  |  |
| Beams |  |  |  |  |
| Knp, 
Kl- |  |  |  |  |

## How to achieve a reliable background estimate?    

**Blind analysis in tuning the cuts.**
**Bifurcated analysis in estimating backgrounds.  **

- Form two independent cuts.
- Estimate the background.
- Check the correlation.

## Backgrounds inside the box 

| Items | E949 | E787 |
| --- | --- | --- |
| Nk(1012) | 1.8 | 5.9 |
| K | 0.068 ± 0.011 | 0.062 ± 0.045 |
| K | 0.216 ± 0.023 | 0.034 ± 0.007 |
| Beams | 0.009 ± 0.003 | 0.025 ± 0.016 |
| Knp, 
Kl- | 0.005 ± 0.001 | 0.025 ± 0.008 |
| Total bkg (evts) | 0.298 ± 0.026 | 0.146 ± 0.049 |

**Errors are statistical only.**

## What could be our nightmares?

**Correlation between two bifurcated cuts.**

**A 0.3 event background estimated, but more than 3 events observed!**

**Appearance of pathology backgrounds.**

**Underestimating background because...**

**For example...**

**It can happen if an observable is used in both CUT1 and CUT2 categories, spoiling the independence of the bifurcated analysis. **

**It rarely happens but can appear thru a loophole. Hand-scanning**** the events outside the box is the only way to spot the problem.**

## Example1: Correlation between two cuts

| Loosen PV x KIN  | Obs. | Pred. |
| --- | --- | --- |
| 20 x 20 | 3 | 1.8 |
| 20 x 50 | 8 | 3.0 |
| 50 x 50 | 22 | 11.1 |

| Loosen PV x KIN  | Obs. | Pred. |
| --- | --- | --- |
| 20 x 20 | 4 | 4.9 |
| 20 x 50 | 9 | 12.4 |
| 50 x 50 | 22 | 31.1 |

**A new geometrical cut that gained 10% more rejection again ********0**** was found to **

- 

- 

- 

- 

**cause trouble in the 1/3 outside-the-box study: ****N(obs.)~****2.0 ****N(pred.)**

- 

**retune PV**

**Reject any event with a photon in opposite side.**

**Reason:**** ****a new cut was developed that used the direction of the ********+**** to place a tighter requirement again photons **
** ****Remedy:**** ****remove this cut & retune PV.**

## Example 2: Pathology background

- 

- K+

- 

- K+

- B4

- Target

**Pions scattered back into the target overlaying a non-decaying kaon can make a fake signal.**

- 

- 

- x

- y

- z

- r

**Reason:**** ****track reconstruction only down to the kaon decay vertex.**
**Remedy:**** ****check the hits in the opposite direction of the track.**

**Readout fibre**

## Backgrounds outside the box

**Motivations:**
**To check the correlations;**
**To estimate systematic errors.**

| Bkg. | C  | Prob. |
| --- | --- | --- |
|  | 0.85 | 0.17 |
|  | 1.15 | 0.67 |
|  | 1.06 | 0.40 |

**Fit N*****obs*****. = *****c***** N*****pred*****.**

**N*****pred*****.**

**N*****obs*****.**

****
****

****
****

****
****

**Deviations of *****c***** ****from unity give the systematic errors.**

## Acceptance

**Br(**********************************
**P.D.G value is ****0.211****±0.001**

- Phase space and nuclear interaction effects from Monte Carlo.
- Acceptance loss due to PID, photon veto, timing and  other kinematics cuts directly measured from data.

**E949 acceptance: 0.0022 ****±**** 0.002**
**E787 acceptance: 0.0020 ****±**** 0.002**

- Verification from B(measurements.

## Open the box and guess what?

**We see one new event!**

- E949

## Branching Ratio Result

- To make best use of our knowledge of the signal and the  background distributions inside the box, we define cells inside the box.

|  | E949 | E787 |  |
| --- | --- | --- | --- |
| Candidate | E949A | E787A | E787C |
| S/b | 0.9 | 50 | 7 |
| W | 0.48 | 0.98 | 0.88 |
| Br  | ( 1.47  |       | )10-10 |

*B**i*: background of cell containing candidate.
*S**i *: Br  A*i*  NK.
- NK: Stopped K+’s.
- A*i*: acceptance in a cell.
- Br: branching ratio.
*W*:* **S*/(*S + b*).

****
****

## Impact on CKM matrix

**More data needed for reducing the uncertainty.**
**New neutral kaon experiments (for example KOPIO) expected for further justification.   **

**Many thanks to A. H****ö****cker & J. Ocariz *****et al***
**The CKM fitter group, hep-ph/0406184**

## Summary

**A successful run of E949 with a factor of two more beam intensity as in E787.**
**A new candidate event observed for K****+******** ****************, leading to a combined result from  E787/E949, ** 

**Br(****K****+******** **************** 10****-10**

**~2********Br(****K****+******** ****************SM,**** if** **(sin2****************sin********J/********Ks. **

***Statistical fluctuation or evidence of new physics?***

****
****

**hep-ex/0403036**