Γ(Ke2)/ Γ(K µ2) Ratio Using Stopped Positive Kaons at...

Precision Measurement of

Γ (Ke2)/Γ (Kµ2) Ratio

Using

Stopped Positive Kaons

at J-PARC

S. Shimizu

Osaka university

for the TREK collaboration

July 28, 2011

S. Shimizu ([email protected]) PANIC2011, MIT, July 28, 2011

OUTLINE

� Lepton Flavor Universality Test in K+ �l+ ν (Kl2)

� Physics motivation

� Experimental method

� Detector

� Monte Carlo simulation

� Sensitivity

� Schedule

� Summary


Lepton universality in Kl2 and ππππl2 decays .

Standard Model:

� By forming ratio of the Γ(Ke2) to the Γ(Kµ2),

hadronic form factors are cancelled out and

the RKSM is highly precise.

� Strong helicity suppression of the electronic

channel enhances sensitivity to effects

beyond the SM.

Uncertainty is ∆RK/RK ~ 0.05%

Helicity suppression

due to V−A structure

Radiative correction

due to the Internal

bremss. part of the

K�lνγ process

RKSM= ( 2.477±0.001)x 10−5

RπSM= (12.352±0.001)x 10−5

K+

s

u

W

-

e+

νe

,µ+

,νµ

PL99(2007)231801

Kl2 decays beyond the SM

Contribution from MSSM

A charged Higgs-mediated SUSY LFV contribution, can

be strongly enhanced by emitting a τ neutrino.

Using

∆13=5x10−4, tanβ=40, MH=500GeV/c2

RKLFV=RK

SM(1±0.013)

PRD74(2005)11701

K+

s

u

H+

(Higgs)

∆13

~

-

ν

(Slepton)

(Sneutrino)

~

e+

ντ

B~

(Bino)

0

10

20

30

40

50

60

70

80

90

100 200 300 400 500 600 700 800 9001000

MH (GeV/c

2)

tan

βblack:red :blue :

∆=1 x10-4

∆=5 x10-4

∆=1 x10-3

allowed

excluded





Using

∆13=5x10−4, tanβ=40, MH=500GeV/c2

RKLFV=RK

SM(1±0.013)

Important!

PRD74(2005)11701

K+

s

u

H+

(Higgs)

∆13

~

-

ν

(Slepton)

(Sneutrino)

~

e+

ντ

B~

(Bino)

0

10

20

30

40

50

60

70

80

90

100 200 300 400 500 600 700 800 9001000

MH (GeV/c

2)

tan

βblack:red :blue :

∆=1 x10-4

∆=5 x10-4

∆=1 x10-3

allowed

excluded





Using

∆13=5x10−4, tanβ=40, MH=500GeV/c2

RKLFV=RK

SM(1±0.013)

Important!

Analogous SUSY effect in

π decay is suppressed by

a factor (mπ/MK)4.

PRD74(2005)11701

K+

s

u

H+

(Higgs)

∆13

~

-

ν

(Slepton)

(Sneutrino)

~

e+

ντ

B~

(Bino)

0

10

20

30

40

50

60

70

80

90

100 200 300 400 500 600 700 800 9001000

MH (GeV/c

2)

tan

βblack:red :blue :

∆=1 x10-4

∆=5 x10-4

∆=1 x10-3

allowed

excluded

Experimental status

Previous experiment (in-flight K method)

� KLOE

RK=(2.493±0.025±0.019)x10−5

� NA62

RK=(2.487±0.011±0.007)x10−5

� Error weighted average

RK=(2.488±0.012)x10−5

∆RK/RK=0.5%

Deviation from the SM at 0.9σ level.

Eur.Phys.J.C64(2009)627

Strong motivation to perform the RK exp. with an accuracy of

∆RK/RK=0.2% at J-PARC comes to our mind.

arXiv:hepex1008.1219

2.4

2.42

2.44

2.46

2.48

2.5

2.52

2.54

2.56

2.58

2.6

2007 2008 2009 2010 2011 2012 2013

Year

RK

(x

10-5

)

PDG

KLOE

NA62Avr.

SM

Experimental setup (upgrade of the E246 system)

� Stopped K+ decay

� K1.1BR beamline will be used.

� momentum measurement

SC Toroidal spectrometer

� e+,µ+ identification

TOF and Cherenkov

� γ measurement :CsI(Tl)

� µ+ polarimeter

0

f

o

r

w

a

r

d

0

b

a

c

k

w

a

r

d

A = NCW - NCCW

NCW + NCCW COILGAP

IRON POLE

+ STOPPER

e+ COUNTER

TARGET

CsI(Tl)

End View

π

µ

π

0

.

5 1

M

C

r

y

o

s

t

a

t

Iron Pole

L

e

a

d

S

h

i

e

l

d

+

C

2

C

3

C

4

T

O

F2

M

u

o

n

D

e

g

r

a

d

e

r

Muon Stopper

e+

C

o

u

n

t

e

r

T

a

r

g

e

t

F

i

b

e

r

TOF1, Aerogel Cherenkov

D

e

g

r

a

d

e

r

B

0

K+

C

h

e

r

e

n

k

o

v

C

1

C

s

I

(

T

l

)

0 0.5 1.0 m

Side View

µe+

γ

GP Counter

Experimental setup (newly made)

� C1 GEM

� Aerogel Cherenkov

� TOF

� CsI(Tl) readout

0

f

o

r

w

a

r

d

0

b

a

c

k

w

a

r

d

A = NCW - NCCW

NCW + NCCW COILGAP

IRON POLE

+ STOPPER

e+ COUNTER

TARGET

CsI(Tl)

End View

π

µ

π

0

.

5 1

M

C

r

y

o

s

t

a

t

Iron Pole

L

e

a

d

S

h

i

e

l

d

+

C

2

C

3

C

4

T

O

F2

M

u

o

n

D

e

g

r

a

d

e

r

Muon Stopper

e+

C

o

u

n

t

e

r

T

a

r

g

e

t

F

i

b

e

r

TOF1, Aerogel Cherenkov

D

e

g

r

a

d

e

r

B

0

K+

C

h

e

r

e

n

k

o

v

C

1

C

s

I

(

T

l

)

0 0.5 1.0 m

Side View

µe+

γ

GP Counter

Overview of the measurement

Number of accepted events

RK is basically obtained as,

N(Ke2)= N(Ke2) + N(Ke2γ)

N(Kµ2)= N(Kµ2) + N(Kµ2γ)

Requirement

� Momentum measurement

by the spectrometer

P(e)>Pmax(Ke3)=228MeV/c

� Particle identification by TOF

and Cherenkov

� N(γ) = 0 or 1

Exp.

data

MC

data

0γ 1γ

µ+e+ µ+e+

D0e D1e D0µ D1µ

~

~

~~

~

~

~

~~~

~~~ ~

~

~

e/µµµµ identification

� TOF

Time of flight measurement between TOF1 and TOF2

� e+ trigger counter by aerogel Cherenkov detector.

β(Ke2 e+) ~1

β(Kµ2 µ+) ~ 0.92=1/1.087

Estimated eff.= 99.2 ± 0.2%

PID performance and detector efficiency will be directly

measured by using the experimental data.

⊗ beambeamTarget

n=1.05

Fresnel mirror

A.Toyoda et. al.

Expected spectra obtained by MC

(1) Ke2 with and without external bremss. photon

(2) Kµ2

(3) Radiative Kl2 decays

CsI(Tl)

target

e+external

bremss. γ

IB γ

SD γ

µ+

0

500

1000

1500

2000

2500

100 150 200 250

e+ momentum (MeV/c)

counts

/bin

0

0.05

0.1

0.15

0.2

0.25

0.3

0 2 4 6

target path length (cm)

Bre

mss. pro

b.

1

10

102

103

0 20 40

E γ (MeV)

counts

/bin

1

10

102

103

0 5 10 15 20

θ γ (deg)

counts

/bin

(d) (c)

(b) (a)



(2) Kµ2


CsI(Tl)

target

e+external

bremss. γ

IB γ

SD γ

µ+

0

500

1000

1500

2000

2500

3000

3500

100 120 140 160 180 200 220 240 260

µ+ momentum (MeV/c)

co

un

ts/b

in

µ+ momentum and polarization

will be measured.



(2) Kµ2


CsI(Tl)

target

e+external

bremss. γ

IB γ

SD γ

µ+

Black: Structure dependent

Red: Internal bremss.

0

50

100

150

200

250

100 150 200 250

e+ momentum (MeV/c)

co

un

ts/b

in

(a)

0

25

50

75

100

125

150

175

200

225

0 50 100 150

θγ (deg)

co

un

ts/b

in(b)

0

10

20

30

40

50

60

0 50 100 150 200 250

Eγ (MeV)

co

un

ts/b

in

(c)

0

20

40

60

80

100

120

140

160

180

0 50 100 150 200 250

Eγ (MeV)

θ γ (

de

g)

(d)

Momentum 800 MeV/c

Momentum bite ±3%

Acceptance 4.5 msr % ∆p/p

K+ intensity 2 ×105 /s

K/π ratio > 2

Beam spot 1.0 ×1.0 cm

(FWHM)

Final focus achromatic

K0.8 ( K1.1-BR)

K1.1

K0

.8

proton

K1.1/ K0.8

(S-type)

K0.8/ K1.1

(C-type)

KLHigh-p

Final Focus

Single stage DC separator

Vertical focuses before and

after the separator

Horizontal focus just after

the second D-magnet

Beamline for K+ extraction

K+



Sensitivity estimation of RK

∆RK/RK=0.2%Statistical error

3x1050.07Detector acc.

4x1061.55x 10−5Branching ratio of Ke2

Statistics

3x10110.25Kaon stopping eff.

1x10121500kW・day

(= 30kW・50days)

Beam

source ∆RK/RK

Detector acceptance 0.05%

PID performance 0.11%

Background 0.04%

Total systematic error 0.13%

Statistical sensitivity

Systematic errors

2.4

2.42

2.44

2.46

2.48

2.5

2.52

2.54

2.56

2.58

2.6

2007 2008 2009 2010 2011 2012 2013

Year

RK

(x

10-5

)

PDG

KLOE

NA62TREKAvr.

SM

Summary of systematic uncertainties

We estimated the systematic uncertainty to be ∆RK/RK=0.13%

∆RK=0.003 x 10-5

∆RK=0.005 x 10-5

� Since the J-PARC facility was seriously damage by the earthquake, it is very

difficulty to say accurate schedule.

� According to the J-PARC director, the delay is estimated to be 1 year.

Schedule

Collaboration

� Canada U. Saskatchewan

TRIUMF

UBC

U. Montreal

U. Manitoba

� USA Hampton U.

MIT

U. South Carolina

Iowa State U.

� Russia INR

� Vietnam National Science U.

� Japan KEK

Tohoku U.

Osaka U.

TITech

U. of Tokyo

� Beamline

� Target

� Cherenkov and TOF

� CsI(Tl) readout

� γ detectors

� DAQ

� GEM chambers

� Tracking upgrade


Summary

� RK is a very sensitive prove of SUSY LFV contribution, which

can be strongly enhanced by emitting a τ neutrino.

� We propose a J-PARC experiment with low-intensity beam

such as during the 30kW operation to pursue

∆RK/RK =0.2%

� We will use K1.1BR beamline.

� The experiment will be performed using upgraded E246

detector (a part of TREK detector)

� The proposed experiment can be done on our way to the final

TREK configuration.


Backup Slides

Overview of the measurementNumber of accepted events

RK is basically obtained as,

N(Ke2)= N(Ke2) + N(Ke2g)

N(Kµ2)= N(Kµ2) + N(Kµ2γ)~

~~~

~

~

~

~~~

~~

� Most of the systematic effects are reduced by forming the ratio of N(Ke2) and N(Kµ2) and their acceptances.

� Several items will not be cancelled out perfectly, which introduces a systematic error in the RK measurement.

� Dominant contributions to the RK uncertainties can be considered as three groups: [G1]. Misunderstanding of the detector acceptances[G2]. Imperfect PID performance [G3]. Background contaminations

� Comparison of various Ke2 and Kµ2 spectra with the MC.� Ke3, Kµ3, and Kπ2 spectra are also compared with the MC.

� These studies have already been performed in the E246/470 experiment with the level of 1%.

� This uncertainty should be improved with increasing the numbers of Ke3, Kµ3, and Kπ2 up to 106 level, which is 100 times higher statistics than E246/470.

� We aim at achieving this uncertainty better than∆RK/RK<0.1%

PL B513(2001)311

PL B495(2000)33

[G1] Imperfect reproducibility of the experimental spectra

solid: exp.dotted: MC

Kµ3

Kπ2Kπ2

Kµ3

Ke3Ke3

E246/470 data

χ2=1.07χ2=0.96

χ2=1.12 χ2=0.94

χ2=1.10 χ2=1.07

[G1] High energy external bremss. emission� The probability of the bremsstrahlung

emission is proportional to the flight path in the target .

� Uncertainty of the path length in the target of σ=1.4 mm introduces the systematic uncertainty.∆RK/RK=0.02%

� Also, offset of the path length distribution is taken into account.

target

high energy

ext. bremss. γ

e+

e+ momentum

Ke2Ke3 endpoint

[G1] Effect from efficiency difference between e+s and µ+s for particle trackers

� The efficiencies are directly measured by using experimental data.

� By changing spectrometer field, sufficient Ke3 and Kµ3 events are collected within one hour.

� Particle identification will be performed by the AC counter and the TOF system.

� From the statistical uncertainty of the efficiency measurement,

∆RK/RK=0.035%

C1

spectrometerC4

C3

C2

AC

TOF2

[G1] Misunderstanding of the SD component in D0

� SD measurement using 1 gamma events. � Statistical error for the subtraction of the SD component is

adopted as the systematic uncertainty.∆RK/RK=0.036%

CsI(Tl)

target

e+

external bremss. γ

IB γ

SD γ

IB:Internal bremss. part

of the K�lνγ process

SD: Structure dependent

part of the K�lνγ process

0

50

100

150

200

250

100 150 200 250

e+ momentum (MeV/c)

co

un

ts/b

in

(a)

0

25

50

75

100

125

150

175

200

225

0 50 100 150

θγ (deg)

co

un

ts/b

in

(b)

0

10

20

30

40

50

60

0 50 100 150 200 250

Eγ (MeV)

co

un

ts/b

in

(c)

0

20

40

60

80

100

120

140

160

180

0 50 100 150 200 250

Eγ (MeV)

θ γ (

de

g)

(d)

Black: SD

Red: IB

[G2] Effects from particle mis-identification by TOF and AC

� Direct measurement of the mis-identification probability with the help of an additional Pb-glass Cherenkov counter.

� Particle identification is performed using two of the three and signal in the other detector is checked.

� Statistical error of this measurement is adopted as a systematic uncertainty of the mis-identification effect.

∆RK/RK=0.035% (I)AC

(III)PGC

(II)TOF

[G2] Events with in-flight µ+ decay (µ+�e+νν) from Kµ2

� We can put AC counter as close as possible to the K+ stopper.

� Uncertainty of muon decay inside the AC introduces∆RK/RK=0.025%

[G2] e± creation from a radiated photon in Kµ2γ decay

� Kµ2γ events with e± generation through electro-magnetic interaction in the target would be mis-identified as Ke2

� Charged particle mass from the TOF measurement has muon rest mass. ∆RK/RK=0.02%

target

eKµ2γ µ

+

AC

γ

e± production prob. Eγ in Kµ2γ

[G3] Mis-identification of beam particles hitting to the CsI(Tl) calorimeter as photons from the target.

� Beam particle hitting the calorimeter can be mis-identified as γ.� However, these backgrounds are accidental events, and they

are common for Ke2 and Kµ2.� Taking into account realistic K+ intensity, π/K ratio, and duty

factor, the event loss probability is assumed to be 5% in the TREK experiment.

� Event loss by these backgrounds is regarded as uncertainty from this effect. ∆RK/RK=0.04%

[G3] Mis-identification of beam π+ as e+ by the aerogelCherenkov counter.

� π+ particles in the beam can generate signals in the AC counter and are identified as e+.

� B0 counter rejection (99%) for incident particles during K+

decay. π/K ratio is assumed to be <1.� Charged particle mass from the TOF measurement has muon

rest mass. ∆RK/RK=0.002% <0.01%

External bremss. distribution in E470

C3 C4

CsI(Tl)Calorimeter

IronPole

K+

CerenkovCounter

Lead Shield

C2

TOF2

0 0.5 1m

Target

TOF1

µ+

µ+

Pb-plastic Detecto r

GEMe+

e+

External bremss. distribution in E470 (2)

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

-100 0 100

θ e+

GPcos

(a)

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

-100 0 100

cos

γ m

ax

(b)

(deg) θ e+

GP (deg)

θ G

P

γ m

ax

θ G

P

exp. MC

Property of SD and IB

CsI(Tl)

target

e+external

bremss. γ

IB γ

SD γ

µ+

SD/IB spectra for Kl2g D1

0

50

100

150

200

250

300

210 220 230 240 250


counts

/bin

0

10

20

30

40

50

60

0 50 100 150

θγ (deg)

counts

/bin

0

5

10

15

20

25

30

0 50 100 150 200 250

Eγ (MeV)

counts

/bin

(c)

0

20

40

60

80

100

120

140

160

180

0 50 100 150 200 250

Eγ (MeV)

θγ (

deg)

(d)

(a) (b)

0

50

100

150

200

250

100 150 200 250

e+ Momentum (MeV/c)

coun

ts/b

in

(a)

0

25

50

75

100

125

150

175

200

225

0 50 100 150

θγ (deg)

coun

ts/b

in

(b)

0

10

20

30

40

50

60

0 50 100 150 200 250

Eγ (MeV)

coun

ts/b

in(c)

0

20

40

60

80

100

120

140

160

180

0 50 100 150 200 250

Eγ (MeV)

θ γ (de

g)

(d)

Km2g SD/IB D1 sample Ke2g SD/IB D1 sample

SD/IB spectra (2) Up:e+, Dn:µ+

0

100

200

300

400

500

600

700

800

100 150 200 250

(a)

e+ momentum (MeV/c)

coun

ts/b

in

0

200

400

600

800

1000

1200

1400

1600

1800

0 10 20 30 40

(b)

Eγ (MeV)

coun

ts/b

in

0

200

400

600

800

1000

1200

1400

1600

100 150 200 250

(c)


coun

ts/b

in

0

500

1000

1500

2000

2500

0 10 20 30 40

Eγ (MeV)

coun

ts/b

in

(d)

0

20

40

60

80

100

120

150 175 200 225 250

Pe+ (MeV)

coun

ts/b

in

(a)

0

5

10

15

20

25

30

0 50 100 150 200 250

Eγ (MeV)

coun

ts/b

in

(b)

0

5

10

15

20

25

30

35

150 175 200 225 250

Pµ+ (MeV)

coun

ts/b

in

(c)

0

2

4

6

8

10

12

0 50 100 150 200 250

Eγ (MeV)

coun

ts/b

in

(d)

SD for Kl2g

Black:D1

Red: D0

IB for Kl2g

Summary of RK exp

Check the validity of simulation

We confirmed with use of K+→π+π0 decay.

χ2 = 114/107 χ2 = 183/191

π + (MeV/

co

un

ts /

bin

co

un

ts /

bin

cos θπ+π

0

(b)(a)

momentum c)

Exp.

MC

Exp.

MC

Reproducibility of Charged particle

• black line : experimental data

• red line: MC

co

un

ts /

bin

C2Z (cm)

C4R (cm)

Target Z (cm)

C3R (cm)

co

un

ts /

bin

co

un

ts /

bin

co

un

ts /

bin

(c)

(b)(a)

(d)

counts

/ b

in

Tracking

Target distance (cm)

Ring distance (cm)χ 2

counts

/ b

in

counts

/ b

in

(a) (b)

(c)

hit position in tracking elementstracking status

K+→π+π0 decay.

γ and π0 reproducibility


• red line: MC

(MeV)

counts

/ b

in

ME

E1 2

E

γγ γγ

γγ

(MeV)(MeV)

(MeV)

(a)

(c)

(b)

(d)

counts

/ b

incounts

/ b

in

counts

/ b

in cos θπ+π

0

counts

/ b

in

cos θ γγ

(a) (b)

counts

/ b

in

K+→π+π0 decay.

Ke3 and Kµ3 spectra


• red line: MC

C4R<105 cm 105<C4R<115 cm

115<C4R<125 cm 125<C4R cm

(a)

co

un

ts /

bin

x 10 3

M (MeV 4 2

c ) / 2

TOF

(b)

co

un

ts /

bin

x 10 3

M (MeV 4 2

c ) / 2

TOF

(c)

co

un

ts /

bin

x 10 3

M (MeV 4 2

c ) / 2

TOF

(d)

co

un

ts /

bin

x 10 3

M (MeV 4 2

c ) / 2

TOF

(deg)θ π

(a)

co

un

ts /

bin

µ

(b)

P (MeV/c)µ

co

un

ts /

bin

(d)

co

un

ts /

bin

P (MeV/c)e

(c)

θeπ

co

un

ts /

bin

(deg)

Kµ3

Ke3

TOF spectra gated C4R

e

µ

Measurement of Γ（Kμ3）/Γ（Ke3) ratio

• Physics motivation

� Evaluation of ChPT theory

� µ-e universality

• Measurement

co

un

ts /

bi

n

x 10 3

µ

+

+

e

M (MeV 4 2

c ) / 2

TOF

Exp MC

K. Horie et al.

Phys. Lett. B513(2001)311

Basic method is

similar to Γ(Kµ2)/Γ(Ke2)

Spectroscopy of

Ke3 decay

Spectroscopy of

K+� e+π0π0ν

BR=3x10-5 ~ BR(Ke2)-1 -0.5 0 0.5 1

cos θ π0 π

0

counts

/bin

counts

/bin

counts

/bin

0

20

40

0

20

40

-0.5 0 0.5

q2/(4 m

π 2)

Spectroscopy of

K+�π+π0γ

treatment

for both of IB and SD

(MeV/c)pπcos πθ

cos πθ E (MeV)

+

0

+

γ 3γ

3

γ 3

counts

/ b

incounts

/ b

in

counts

/ b

incounts

/ b

in

Exp

MC

MC DE

Exp Exp

Exp

MC

MC DE

MC

MC DE

MC

MC DE

Spectroscopy of K+�π0µ+γν

radiative decay

J-PARC K1.1BR beamline

� The beamline K1.1BR was completed in summer 2010.

� The beam tuning was carried out in Oct. and Nov. 2010 by the TREK group.

� A reasonable K+ intensity with fairly well π/K ratio ~1 could be confirmed.

� There might be still room for improvement.

� However, a better π/K ratio (~0.3) can be only achieved by upgrading

electro-static separator.

charge exchange reaction K+ � K0

• cross section of K+ � K0 is ~ 5mb• considering degrader (BeO) length , reaction

probability is • rou/A x 6*1023 *30 x sigma

= 3/25x6*1023 x 30 x 5*10-27 =0.01

Γ(Ke2)/ Γ(K µ2) Ratio Using Stopped Positive Kaons at...

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