# Presentation: 371581

## Progress on rare K and π decays from BNL E949

- For the E949 collaboration:
- Ilektra A. Christidi
- SUNY at Stony Brook

- April APS meeting 2005
- Apr 19 2005

-  K+ π+νν with pπ<195Mev/c (PNN2)
-  πo  νν

## K+ π+νν

**K****+******** ****π****+****νν**

## Theory-motivation

- Goal: Independently define the Unitarity triangle/ CKM matrix (A, λ, η, ρ  Wolfenstein parameters)
- Vud Vub* + Vcd Vcb* + Vtd Vtb* = 0

**Theory-motivation**

- Rare K & π decays from E949
- Ilektra A. Christidi

- 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

## The SM K+ π+νν BR

-  Neutral Flavor Changing Process  all diagrams 2nd order
-  Main contribution of t in the loop
-  Very theoretically “clean” calculation (precision < 5%, uncertainties mainly from c sector)

- ** ****ellipse in ****ρ-η**** plane**

**The SM K****+******** ****π****+****νν ****BR**

- Rare K & π decays from E949
- Ilektra A. Christidi

## Current situation

**Current situation**

|  | PNN1 |  | PNN2 |  |  |
| --- | --- | --- | --- | --- | --- |
| Pπ (MeV/c) | [211,229] |  | [140,195] |  |  |
| Years  | 1995-98 (E787) | 2002 (E949) | 1996-97 (E787) |  | 2002 (E949) |
| Stopped K+  | 5.91012 | 1.81012 | 1.71012 |  | 1.81012 |
| Candidates  | 2 | 1 | 1 | S/b = (BR·SES)/b  0.1 | GOAL: ~ 1 |
| Background  | 0.150.05 | 0.300.03 | 1.220.24 |  |  |
| BR(K+ π+νν)  | (68% CL) |  | < 2210-10 (90% CL) |  | ? |

- Rare K & π decays from E949
- Ilektra A. Christidi

## The measurement w/ E949 detector

**The measurement w/ E949 detector**

- Rare K & π decays from E949
- Ilektra A. Christidi

-  Incoming 700MeV/c beam K slowed down by BeO and AD

-  K+ stops & decays at rest in scintillating fiber target – measure delay (2ns)

-  Outgoing π+ momentum measured in UTC, energy & range in RS and target

-  π+ stops & decays in RS – detect π+μ+e+ chain

-  Photons vetoed hermetically in BV-BVL, RS, EC, CO, USPV, DSPV

## The measurement w/ E949 detector

-  Incoming 700MeV/c beam K slowed down by BeO and AD
-  K+ stops & decays at rest in scintillating fiber target – measure delay (2ns)
-  Outgoing π+ momentum measured in UTC, energy & range in RS and target
-  π+ stops & decays in RS – detect π+μ+e+ chain
-  Photons vetoed hermetically in BV-BVL, RS, EC, CO, USPV, DSPV
-  New/upgraded elements

**The measurement w/ E949 detector**

- Rare K & π decays from E949
- Ilektra A. Christidi

## Backgrounds

-  3-body decay w/ 2 missing particles: 0  pπ+  227 MeV/c 
- 	Signal: π+ + nothing , backgrounds (bg) vetoed ~ 10-11 !
-  Need
-  particle identification (PID)
-  all other charged particles vetoed < 10-3
-  redundant precise kinematic measurements

| Background | BR (·10-3) | PID | VETO | KIN. |
| --- | --- | --- | --- | --- |
| K+→π+πo (Kπ2) | 212 | - | √ (2γ) | √ |
| Beam particles | - | √ | √(cp) | √(t) |
| K+→π+π-e+νe (Ke4) | 0.039 | - | √(2cp) | - |
| K+→π+πoγ (Kπ2γ) | 0.275 | - | √ (3γ) | - |
| K+→πoμ+νμ (Kμ3) | 31.8 | √ | √ (2γ) | - |
| K+→μ+νμγ (Kμνγ) | 5.5 | √ | √(1γ) | - |
| CEX (K+n→Kop, 
KoL → π+μ-νμ    or 
KoL → π+e-νe ) | Prob=0.0015
          0.135
          0.194 | - | √ | - |

- Rare K & π decays from E949
- Ilektra A. Christidi

**Backgrounds**

## PNN2 analysis

**PNN2 analysis**

- Main bg mechanism: K+π+πο with π+ scatter in target 
-  Simultaneous shift in range AND momentum
-  Photons head near beam direction, the weakest PV region of the detector
-  *Goal: sensitivity equal to PNN1, s/b = 1* 
- 	2  acceptance and 5  rejection
-  Improved PV: new detectors at central region (BVL) and small angles
-  Improved algorithms to identify π+ scatters in target

- Rare K & π decays from E949
- Ilektra A. Christidi

## Photon Veto improvement

**Photon Veto improvement**

- In the Kπ2 kinematic box:
- ~ *2 **** better rejection* at nominal PNN1 acceptance (80%) or
- ~ *5% more acceptance* with E787 rejection !
-  Good news for PNN2 as well...
- Extra tools for PNN2:
-  Active degrader (PV detector near beam)
-  Account for “*veto blindness*” (i.e. when an early accidental masks the γ pulse, by using double-pulse information and early-time loose veto.

- Rare K & π decays from E949
- Ilektra A. Christidi

- E787, E949

## πo  νν

**π****o**** ******** ****νν**
**analysis by K.Mizouchi, Kyoto University, Japan**

## Theoretical motivation

-  Decay prohibited by helicity for massless ν
-  For massive ν,
-   therefore, for the max mντ = 18.2 MeV/c, BR < 2.110-9
-  From cosmology: cooling through the pion pole γγπονν gives BR < 2.910-13 , but it’s model dependent
-  Existing limits: BR < 8.310-7 (90% CL) from E787
-  Inclusive measurement: πο  anything invisible  BSM

**Theoretical motivation**

- Rare K & π decays from E949
- Ilektra A. Christidi

## Background & measurement

-  Tag πο from Kπ2 (K+→π+πo) decay  pπo = pπ+ = 205 MeV/c
- 	Signal: π+ + nothing , but with fixed 2-body kinematics
-  Two steps in selection/bg calculation:
-  Kπ2 tag, non-Kπ2 bg calculated from data w/ blind analysis: 3.07 events
-  Apply tightest PV & look in the box. Conservative limit: all events due to missed photons from detector inefficiencies.

| Background | BR | Bg |
| --- | --- | --- |
| K+→μ+νμ(γ) (Kμ2(γ)) | 0.63 | 3.6610-1 |
| Beam particles:
Single beam
Double beam | - | 
3.2710-2
2.67 |
| Total  |  | 3.07 |

- Rare K & π decays from E949
- Ilektra A. Christidi

**Background & measurement**

- Poisson 90% CL

- # of Kaons

- Kπ2 acceptance

- Correction factors

## Analysis & result

-  PV cuts tuned on 1/3 of the sample, randomly selected, with optimization procedure.
-  1/3 sample discarded, to maintain blindness, and measurement is done with the remaining 2/3.
-  Open the 2/3 sample box:              99 events  99 - 3.07 = 95.93 good Kπ2 tags  N90 = 110
-  NK = 3.019109
-  AKπ2 = 0.1173
-  CdisCacc = 1.136
- So,

- Rare K & π decays from E949
- Ilektra A. Christidi

**Analysis & result**

## Conclusions

**Conclusions**

-  E787 upgrade into E949 worked as expected: x2 better PV rejection at PNN1 acceptance, x10 at low acceptances
-  PNN2 analysis under way, showing signs of improved bg suppression
-  Updated pi0->nunu BR limit, x3 better than before: 2.73x10-7 from 8.3

- Rare K & π decays from E949
- Ilektra A. Christidi

## Extras

**Extras**

## Previous (E787) results (2)

**Previous (E787) results (2)**

- Candidate E787A

- Candidate E787C

## stuff

**stuff**

|  | PNN1 |  | PNN2 |  |
| --- | --- | --- | --- | --- |
| Pπ (MeV/c) | [211,229] |  | [140,195] |  |
| Years  | 1995-98 (E787) | 2002 (E949) | 1996-97 (E787) | Goals for 2002 (E949) |
| Stopped K+  | 5.91012 | 1.81012 | 1.71012 | 1.81012 |
| Acceptance | (20.2) 10-3 | (2.20.2) 10-3 | (0.8370.003) 10-3 |  |
| S.E.S. | 0.810-10 | 2.610-10 | 6.910-10 |  |
| Candidates  | 2 | 1 | 1 | ? |
| Background  | 0.150.05 | 0.300.03 | 1.220.24 | < 1 |
| BR(K+ π+νν)  | (68% CL) |  | < 2210-10 (90% CL) | ? |

## Delayed coincidence

**Delayed coincidence**

## π+ μ+ e+ identification

- Eμ = 4.1 MeV, Rμ ~ 1 mm, τπ =26 ns
- Ee < 53 MeV, τμ = 2.2 μs

**π****+******** ****μ****+******** e****+**** identification**

## Toy MC for Junk code

**Toy MC for Junk code**

- BR dependence on s/b of cell where event is found: