# Presentation: 823717

## 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

## Outline

-  The decay K+ π+νν with pπ<195Mev/c (PNN2)
-  Theoretical motivation
-  Current situation: PNN1 & PNN2
-  Backgrounds & PNN2 specifics
-  The detector/measurement
-  Progress & improvements
-  The decay πo  νν
-  Theoretical motivation
-  Backgrounds & measurement
-  Result (preliminary)

**Outline**

## 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 (u & c almost cancel by GIM mechanism)
-  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

## 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

## 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

**Backgrounds**

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

## The measurement w/ E949 detector

-  Incoming 700MeV/c beam K+: identified by ckov, WC, scint. hodoscope (B4). Slowed down by BeO and AD

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

-  Outgoing π+ : verified by IC, VC, T counter. Momentum measured in UTC, energy & range in RS and target           (1T magnetic field parallel to beam)

-  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**

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

## The measurement w/ E949 detector

-  Incoming 700MeV/c beam K+: identified by ckov, WC, scint. hodoscope (B4). Slowed down by BeO and AD
-  K+ stops & decays at rest in scintillating fiber target – measure delay (2ns)
-  Outgoing π+ : verified by IC, VC, T counter. Momentum measured in UTC, energy & range in RS and target 	    (1T magnetic field parallel to beam)
-  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

## PNN2 analysis (1)

**PNN2 analysis (1)**

- 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 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**

- ~ 2  better rejection at nominal PNN1 acceptance (80%) or
- ~ 5% more acceptance with E787 rejection !
- * Good news for PNN2 as well...
- E787, E949

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

## πo  νν

**π****o**** ******** ****νν**
**studies 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 < 10-9
-  From cosmology: cooling through the pion pole γγπονν gives BR < 2.910-13 , but it’s model dependent
-  recent research shows much higher πονν rates in dense medium  could invalidate cooling model
-  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, bg calculated from data w/ blind analysis: 3.07 events
-  Apply tightest PV & look in the box (*not blind analysis*). Conservative limit: all events due to missed photons from detector inefficiencies.

| Background | BR (·10-3) | PID | VETO | KIN. |
| --- | --- | --- | --- | --- |
| K+→μ+νμ (Kμ2) | 630 | √ | - | √ |
| Beam particles:
Single beam
Double beam | - | 
√
- | 
-
√(cp) | 
√(t)
- |
| K+→πoμ+νμ (Kμ3) | 31.8 | √ | √ (2γ) | √ |
| K+→μ+νμγ (Kμνγ) | 5.5 | √ | √(1γ) | √ |

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

**Background & measurement**

- Poisson 90% CL

- # of Kaons

- Kπ2 acceptance

- Correction factors

## Optimization & result

-  Open the 2/3 sample box:           99 events  99 - 3.07 = 95.93 good Kπ2 tags  N90 = 110
-  NK = 3.019109
-  AKπ2 =
-  Cdis = 1.136
-  Cacc = 0.1173
- So,

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

**Optimization & result**

## Conclusions

**Conclusions**

-  E787 upgrade into E949 worked as expected
-  One 		   candidate event observed, bringing the BR to					, which is still consistent with the SM
-  Additional running needed for more influencial results
-  PNN2 analysis is under way

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

## Extras

**Extras**

## Previous (E787) results (2)

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

- Candidate E787A

- Candidate E787C

|  | 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) | ? |

## The E949 detector

**The E949 detector**

## 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:

## Effect of systematics on BR

**Effect of systematics on BR**

- Combined (E787 & E949) 84% upper and lower limits and central value of BR for single simulated events in the 2002 data set, with variations of the assumed Kp2 bg component of 30%