Progress on rare K and π decays from BNL E949

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

KK++ ππ++νννν

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

Vud Vub* + Vcd Vcb

* + Vtd Vtb* = 0

Theory-motivationTheory-motivation

Rare K & π decays from E949Ilektra 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

B R K V V x V V x

B R

cs cd c ts td tl e

o

( ) ( ) ( )

. . . ( ) ( )

/

* *

, ,

2

2 2

2

21

1 2

• 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 KThe SM K++ ππ++νν νν BRBR

B R Kth ( ) ( . . ) 0 7 7 0 11 1 0 1 0


Current situationCurrent situationPNN1 PNN2

Pπ (MeV/c) [211,229] [140,195]

Years 1995-98 (E787) 2002 (E949) 1996-97 (E787) 2002 (E949)

Stopped K+ 5.91012 1.81012 1.71012 1.81012

Candidates 2 1 1 S/b = (BR·SES)/b 0.1

GOAL: ~ 1

Background 0.150.05 0.300.03 1.220.24

BR(K+ π+νν) (68% CL) < 2210-10 (90% CL) ?

( . )..1 4 7 1 00 8 9

1 3 0 1 0


“PN

N1”

“PN

N2”

The measurement w/ E949 detectorThe 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

• 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 detectorThe measurement w/ E949 detector


“PN

N1”

“PN

N2”

• 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

- √ -PNN2


BackgroundsBackgrounds

PNN2 analysisPNN2 analysisMain 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


Photon Veto improvementPhoton Veto improvementIn 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.


E787, E949

ππoo ννννanalysis by K.Mizouchi, Kyoto University, Japananalysis by K.Mizouchi, Kyoto University, Japan

• Decay prohibited by helicity for massless ν

• For massive ν,

therefore, for the max mντ = 18.2 MeV/c, BR < 2.110-9

• From cosmology: cooling through the pion pole γγπονν gives BR < 2.910-13 , but it’s model dependent

• Existing limits: BR < 8.310-7 (90% CL) from E787

• Inclusive measurement: πο anything invisible BSM

Theoretical motivationTheoretical motivation


B Rm

m

m

mo

o

oo o

( )( )

( ).

/

1 2 1 0 147

22

2

1 2

• 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 3.6610-1

Beam particles:

Single beam

Double beam

-

3.2710-2

2.67

Total 3.07


Background & measurementBackground & measurement

B RN

N B R K A C Co

Ko

K d is a cc

( )( )

9 0

2

Poisson 90% CL

# of Kaons Kπ2 acceptance

Correction factors

• 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 = 3.019109

• AKπ2 = 0.1173

• CdisCacc = 1.136

So,


Analysis & resultAnalysis & result

B R o( ) . 2 7 3 1 0 7

ConclusionsConclusions

• 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


ExtrasExtras


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

Candidate E787A Candidate E787C


stuffstuff

PNN1 PNN2

Pπ (MeV/c) [211,229] [140,195]

Years 1995-98 (E787) 2002 (E949) 1996-97 (E787) Goals for 2002 (E949)

Stopped K+ 5.91012 1.81012 1.71012 1.81012

Acceptance (20.2) 10-3 (2.20.2) 10-

3

(0.8370.003) 10-3

S.E.S. 0.810-10 2.610-10 6.910-10

Candidates 2 1 1 ?

Background 0.150.05 0.300.03 1.220.24 < 1

BR(K+ π+νν) (68% CL) < 2210-10 (90% CL) ?


Delayed coincidenceDelayed coincidence


Eμ = 4.1 MeV, Rμ ~ 1 mm, τπ =26 ns

Ee < 53 MeV, τμ = 2.2 μs

ππ++ μμ++ e e++ identification identification


Toy MC for Junk codeToy MC for Junk code

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

Progress on rare K and π decays from BNL E949

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