# Presentation: 624012

## New results for K+→+ at low+ momentum from BNL E949

- Artur Shaykhiev
- (for E949 collaboration)
- Institute for Nuclear Research
- Moscow

- New trends in high-energy physics (experiment, phenomenology, theory),
- Yalta, Crimea, Ukraine, September 27 - October 4, 2008

## Outline

- Motivation
- Experiment E949
- Backgrounds: suppression and estimation
- Results
- Future

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Motivation

**Motivation**

-  _Goal:_ Independently define
-  the Unitarity triangle/ CKM matrix (A, λ, η, ρ
-  Wolfenstein parameters)

- A better determination of Vtd from *K**+**** **π**+**νν*
- will provide a sensitive test of the SM by comparing
- the results from the K and B sector and probe new
- physics

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Sensitive probe to new physics

- J. Brod and M. Gorbahn
- arXiv:0805.4119

- Because of the strong suppression of the *s****d* short-distance amplitude in the SM, rare *K *decays are the most sensitive probes of new physics.

- SM prediction

- Excluded by E787/E949 results

- (*K**L****0***)4.4(*K**+****+***)
- Grossman – Nir Bound

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Previous results

|  | PNN1 | PNN2 |
| --- | --- | --- |
| P(+)(MeV/c) | [211,229] | [140,195] |
| Years | 1995-98(E787) and 2002(E949) | 1996-97(E787) |
| Stopped K+ | 7.71012 | 1.71012 |
| Candidates | 3 | 1 |
| Background | 0.450.06 | 1.220.24 |
| BR(K+ + ) |                                 (68%CL)       | < 2210-10 (90%CL) |

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## K

***K*******

********* *******

******

- Momentum spectra of charged particles from K+ decays in
- the rest frame

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## E949 Detector

-  Incoming ~700 MeV/c beam K slowed down
- by BeO and AD
-  K+ stops and decays at rest in scintillating
- fiber target – measure delay (at least 2 ns)
-  Outgoing + momentum is measured in UTC,
- energy & range in RS and target
-  + stops & decays in RS – detect
- ++e+ chain
-  Photons veto in BV – BVL, RS, EC, CO,
- USPV, DSPV

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## E949 analysis strategy

_*A priori*_* *identification of background sources
- Use at least two uncorrelated cuts to estimate each background
-     using information outside of the signal region
- Set cuts using 1/3 of data then measure backgrounds with
-     remaining 2/3 sample
- Verify backgrounds estimate by loosening cuts and comparing
-     observed and predicted rates
- “Blind” analysis: _DON’T_ examine signal region until all backgrounds
-     verified

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Backgrounds

| Background | BR (·10-3) |
| --- | --- |
| K+→π+πo (Kπ2) | 212 |
| Beam particles | - |
| K+→π+π-e+νe (Ke4) | 0.039 |
| K+→π+πoγ (Kπ2γ) | 0.275 |
| K+→πoμ+νμ (Kμ3) | 31.8 |
| K+→μ+νμγ (Kμνγ) | 5.5 |
| CEX (K+n→Kop, 
KoL → π+μ-νμ    or 
KoL → π+e-νe ) | Prob=0.0015
          0.135
          0.194 |

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Main background is K++0

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

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

***K*******

******

******

**Target**

**Target**

********* *******

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

*** * scatters in the target, loses energy and the photons lose directional correlation with the *** *

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Suppression of K2- scatter events

- Photon veto of ***0**** *. **’s may go to beam region => photon detection in this region is important
- Identification of ***+* scattering in the target:
- kink in the pattern of the target fibers
***+* track that doesn’t point back to the *K**+* decay point
- energy deposits inconsistent with an outgoing ***+*

***K*******

******

******

**Target**

********* *******

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Identification of + scattering 

- Kink in the pattern of target fibers

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Two cuts used for suppression +-scatter events are photon veto and target-scatter cuts 

**C**

**C+D**

**B**

- photon veto is inverted,
- target cuts are applied.

- target cuts are inverted,
- photon veto is applied.

**A**

**B**

**C**

**D**

- Signal region

**Photon veto**

**target**

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

- Sample “B” also contains *K****2**-*scatter in range stack and *K**+****+****0**** *events.

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## K++-e+(Ke4) background

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

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

*K**e4** *process forms a background when the ***-* and *e**+* interact in the target without leaving a detectable trace
*K**e4 *background could not be distinguished from the larger *K****2*-scatter background based solely on the ***+** *track, and we cannot make a purely data-based background estimate

- Use both data and Monte Carlo to estimate this background

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## K++-e+(Ke4) background

- Use target pattern recognition to isolate *K**e4** *sample

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

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Charge exchange (CEX) background

- This background is suppressed by *K**+** *decay time condition (*K**s** *lifetime is only 0.1 ns ) and target pattern recognition to identify gaps between *K**+** *and ***+** *tracks.  Additional suppression is provided by detecting the negative lepton.

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Muon background

- Due to kinematics constrain only multi-body decays *K**+****+****0****** *and *K**+****+******** *are important

- This backgrounds are suppressed kinematically and by identification of ***+* decay

- Momentum (MeV/c)

- Range (cm)

- Decay chain:

***+*

***+*

*e**+*

- Muons have different kinematics parameters than pions

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Beam backgrounds

- Single-beam

- Double-beam

- Suppressed by time delay

- Suppressed by looking for any extra activities that are coincident with the charged track in the beam instrumentation

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Total background in pnn2

- Compared to E787 pnn2 analysis, our total background was decreased by 24% and total acceptance was increased by 63%

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Division of signal region

- Use four cuts to divide the signal region into nine cells with differing the relative signal-to-background levels

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Examine the signal region

- Solid line represents signal region, dashed shows tightened kinematics cuts. Gray points are simulated *K**+****+***

_**The probability of all 3 events to be due to background only is 3.7%**_

- From these three new candidates alone

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Combined E787/E949 results

- _The probability that all seven events__were due to background only is 0.1%_

- Consistent with SM prediction

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

##  The future

- NA62 (formerly NA48/3) in preparation at CERN. The use of kaon decay-in-flight to measure *K**+****+**** *has not been attempted before.
- NA62 proposes to observe  65 *K**+****+*** events
- There is a letter of intent for a stopped kaon decay experiment in Japan.
- Analyses of E949/E787 for other *K**+* decay modes still continue.

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4

## Thank you! Questions?

- Special thanks to David Jaffe and Joss Ives for the resources that made this talk possible

- A.Shaykhiev, INR

- Crimea08, Sept 27 - Oct 4