Precision Measurement of the π+→e+νe Branching Ratio in the PIENU Experiment

Kuno-Group D3 Shintaro Ito

1

Outline

•π+→e+νe Decay

•Measurement Method

•Detector

•Analysis

•Status and Uncertainties

•Summary

2

π+→e+νe Decay•π branching ratio in the SM !

!

•Previous experiment at TRIUMF REXP=[1.2265±0.0034(stat)±0.0044(syst)]×10-4

•Precise measurement of R -Electron-muon universality violation (ge≠gµ) -Helicity unsuppressed pseudoscalar interaction ➡0.1% allows access new physics up to 1000 TeV.

•The PIENU experiment (TRIUMF): aims <0.1%.

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V.Cirigliano, I.Rosell, PRL 99 231801 (2007)

π

Measurement Method

R=NPIE/NPIMU×(1+ε)

π+→µ+→e+ NPIMU/(τµ-τπ)×(e-t/τ -e-t/τ ) π+→e+νe (NPIE/τπ)e-t/τ

Raw branching ratio R’

Corrections (e.g: low energy tail)

Decay positron time (MC)

4

Calorimeter

π+

μ+

e+ e+

Ee:0.5~52.8 MeV Ee:69.8 MeV

Target

Decay positron energy (MC)

π+→e+νe low energy tail

π+→µ+→e+ π+→e+νe

πµ

π

(×104)

PIENU Beam Line & Detector

WC:Wire Chamber S:Silicon Strips B1,B2,Tg,T1,T2:Plastic scintillator

48cm

48cm

TRIUMF M13 Beam line •Beam rate:~70 kHz •Beam momentum:75±1 MeV/c •π+:μ+:e+=85:14:1

PIENU detector

5

Zoom up

Data taking in 2009~2012.

0.5～ 52.8MeV

V3

Analysis: Raw Branching Ratio Extraction

π+→e+νeπ+→µ+→e+

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!•Data set: November in 2010. •4.0×105 clean π+→e+νe events. • Time=tT1-tB1[ns] • Fit both time spectra simultaneously. •Raw branching ratio was blinded

52 MeV

R’=(1.1972±0.0022±0.0005)×10-4

Time window: -300<t<540 ns

Tail Correction•Low energy π→eν buried under the π→µ→e distributions.

•Estimated by 2 methods.

- Suppressed dominant π→µ→e events.

- Special data set using mono-energetic beam positron.

Tail fraction <52 MeV=3.07±0.12%

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[MeV]NaI+CsIE0 10 20 30 40 50 60 70 80 90 100

Cou

nts

1

10

210

310

410

510

Time 0.9916Energy loss 0.2947

Kink 0.1542

S3 0.1457

Pulse fit 0.1441

Tail measurement using e+ beam.

Low energy tail

Photo nuclear effect NIMA,621,188-191 (2010)

π→µ→e suppressed spectrum.

Acceptance Correction

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•The correction of acceptance difference between π+→e+νe and π+→µ+→e+. ➡ Positrons energy dependence of interactions.

•Track of positron is reconstructed by S3,WC3. •Rely on MC to study various systematic effects. - π stopping position. - Displacement of detectors. - Thickness, etc...

Acceptance correction: 0.9991 ± 0.0003

Schematic of the tracking.

NaI

CsI

CsI

Tg S3 WC3

π

e

e

μ

Branching Ratio and Uncertainties•The initial analysis was completed.

- No systematic dependence.

-R=[1.2344±0.0023(stat)±0.0019(syst)]×10-4 (0.24%) -gµ/ge=1.0004±0.0012⇨ Most precise result!!

9 Branching ratio vs Radius cut.Sliced radius at WC3[mm]

20 30 40 50 60 70 80 90 100

)-7

10×N

orm

aliz

ed b

ranc

hing

ratio

(

-120

-100

-80

-60

-40

-20

0

20

40

60

80

Raw branching ratio

DIF correctionsµTail +

DIF + Acceptance correctionsµTail +

Phys. Rev. Lett., 115, 071801 (2015)

•The dominant source was statistics. ➡×10 statistics for full data set

•Systematic uncertainty was dominated by tail correction ➡Tail depends on the statistics.

•Remaining data is being analyzed.

Uncertainties

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2011 data set is about twice statistics. 2011 data set is still blinded.

Error Previous Experiment

PIENU (2010)

PIENU (2010+2011) Preliminaly

Goal of PIENU

Statistic 0.28% 0.18% 0.11% 0.05%

Time spectrum 0.19% 0.03% 0.03% 0.03%

Tail Correction 0.25% 0.12% 0.08% 0.03%

Acceptance Correction 0.11% 0.03% 0.03% 0.03%

Other 0.11% 0.05% 0.03% 0.03%

合計 0.47% 0.24% ~0.15% <0.1%

Summary•Precise π+ branching ratio measurement is sensitive to new physics beyond the SM. •PIENU aims to measure π+ branching ratio to <0.1% level. •Finished data taking in 2012. •The initial analysis was completed:0.24%. •Analyzing the remaining data. - Systematic study for 2011 data set was finished. - Precision level: ~0.15%, improved by a factor of 3.

•Final result using full data set will be finished next year.

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Thank you for your attention!!

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Backup

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Pseudoscalar Interaction

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e+u

d νe

π+W+

e+u

d νe

π+~1/Λ2+

SM New interaction

π+→e+νe Decay•π+ branching ratio R in SM !

!

!

!!

- ge=gµ: lepton universality. - π+→e+νe is disfavored (V-A). ➡ Helicity suppression.

•Experimental result

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R =Γ((π→µν)+(π→µνγ))Γ((π→eν)+(π→eνγ))

SM

=ggme (m -m )

µ m 2π

(m -m )πµ

e e

µ

22

2

2

2 2

2 2

2 (1+δ)(1+ε)Radiative Correction

= (1.2352±0.0002)×10-4 (0.02%)

R = (1.230±0.004)×10-4 (0.4%)Exp

W+

e+,µ+u

d νe,νµ

π+

ge,gµ

Couplings

History of R measurement.

V.Cirigliano, I.Rosell, PRL 99 231801 (2007)Expected by New Measurement

CzapekBrittonBryman

DiCapuaAnderson

Theoretical region

Universality Test & Beyond SM

•Lepton universality violation. - π+ R is one of the most

precise measurement.

- Improve R measurement.

➡0.05% in gµ/ge.

•New pseudo-scalar interaction without helicity suppression.

- 0.1% level allows to access new physics up to 1000 TeV/c2.

- R-parity violation SUSY, leptoquarks etc...

• PIENU experiment at TRIUMF → aims at <0.1% level.

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Decay Mode g /g Yearτ→µ/τ→e 1.0018±0.0014 2010π→µ/π→e 1.0021±0.0016 1994K→µ/K→e 0.996±0.005 2011K→πµ/K→πe 1.002±0.002 2007W→µ/W→e 0.997±0.010 2008

Current experimental results of g /g

µ e

µ e

R-Parity Violation SUSY

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Current constraints

Expected by PIENU

Future measurement of proton weak charge.

λ’i1k

M.J.Ramsey-Musolf, S.Su & S.Tulin, PRD 76 095017 (2007).

Previous TRIUMF Experiment

•E248 experiment at TRIUMF in 80’s. •R=(1.2265±0.0034±0.0044)×10-4 •1.5×105 π+→e+νe events. •Dominant sources of errors - Small acceptance: ~2% ➡ Low statistics.

- Many unsuppressed π-DIF in low energy region : ~20% ➡ Largest systematic uncertainty.

E248 detector

Energy spectrum after π+→µ+→e+ suppression

Many π-DIF in low energy region

(stat) (syst)

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Experiments of Universality Test

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•Γ(K+→e+νe(γ))/Γ(K+→µ+νµ(γ)): NA62, TREK etc

➡gµ/ge=0.996±0.005

•Γ(τ-→µ-νµντ)/Γ(τ-→e-νeντ): Bhabha etc

➡gµ/ge=1.0018±0.0014

•Γ(π+→e+νe(γ))/Γ(π+→µ+νµ(γ)): PIENU(TRIUMF), PEN(PSI)

➡Ongoing. Expected to improve gµ/ge <0.05%.

•Γ(B+→K+µ+µ-)/Γ(B+→K+e+e-):LHCb

➡0.745 (stat) ± 0.036(syst) (2.6σ from SM)+0.090-0.074

PIENU Detectors & Beam Line

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NaI 1 CsI crystal Scint + Si Strip WC3

PIENU detector

π+ beam

Trigger Logic

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Analysis: Blind Technique•Need to avoid bias in precision measurement. • The procedure shown below is done before analysis. - Central value of R should be blinded. - π+→e+νe/π+→µ+→e+ events are suppressed. - Inefficiency factor is produced by random number (~1%). - R should be blinded until all systematic errors are estimated.

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Case1: π+→e+νe suppression. Case2: π+→µ+→e+ suppression.

Initial Analysis: Event Selection Cuts•Data set: taken in Nov~Dec 2010. ➡~10% of full data.

• Event selection cuts. - Beam profile by WC1,2. - Pion selection by dE/dx in B1,B2. - Single hit requirement in B1,B2,T1,T2 - Acceptance cut.

23Beam profile by WC1 dE/dx in B2 Reconstructed radius

at WC3.

0.5～ 52.8MeV

V3

Tail Analysis

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πDIF

Analysis: Tail Correction1•π+→e+νe low energy tail due to shower leakage buried under π+→µ+→e+.

• Suppress π+→µ+→e+ by time window, target energy, beam angle, etc ➡π+→µ+→e+ were suppressed by a factor of ~105. ➡Estimated tail fraction: N<52MeV/NAll=1.48±0.10%.

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π+→µ+→e+π+→e+νe

π+→µ+→e+ suppression

Target energy

π μ eπDIF

πDAR

θ

Beam angle

Time window: 5<t<35 ns

Analysis: Tail Correction•Assume that all events below i(0~52 MeV) are coming from remaining unsuppressed π+→µ+→e+ backgrounds. • The amount of tail is estimated by shapes of suppressed spectrum and Michel spectrum from data. !

•This procedure gives only lower bound of tail. N<52MeV/NAll = (2.95 ± 0.12)%

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The amount of π+→e+νe tail = A - a[i]×B/b[i]

π+→µ+→e+ suppressed spectrum

A

a[i]

iMichel spectrum from data

b[i]

B

i

Tail Analysis

27

Zoom

Analysis: Tail Correction1•Some π+→e+νe events were removed by target energy cut. •Most events with large Target energy events are due to Bhabha scattering in Target. •MC correction was studied.

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π+→e+νe events with/without Target cut by MC study.

•Tail fraction before correction. N<52MeV/NAll=1.48±0.10%.

•After correction of lower bound N<52MeV/NAll=2.95±0.12%.

Analysis: Tail Correction2•Tail measurement using mono-energetic positron beam.

•Rotate the crystals for different entrance angle measurement.

•Possibility of low momentum positron beam contamination.

➡Upper bound: N<52MeV/NAll = (3.19 ± 0.09)%

29Positron beam measurement. Rotation of crystals Tail measurement at 0 degrees.

Low energy tail

Photo nuclear effect NIMA,621,188-191 (2010)

Tail Analysis -NaI Response-

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MC without hadronic interactionMC with hadronic interactionData

•First observation of photo-nuclear effect.

•Peaks are consistent with neutrons escaping.

•Good agreement between data and MC

A.Aguilar-Arevalo et al Nucl. Inst. and Methods A (2010)

Tail Analysis

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Analysis: Combination of Tail Correction•Lower bound: 2.95±0.12% Upper bound: 3.19±0.09%

•Assume both distribute Gaussian functions (Left).

•Combine the 2 error functions and get combined value (Right).

Combined tail fraction:3.07±0.12%

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Lower bound Upper bound

Acceptance Correction

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Example of MC study: Pion beam stopping position.

Radius at WC3[mm]20 30 40 50 60 70 80 90 100

Rat

io o

f acc

epta

nce

0.995

0.996

0.997

0.998

0.999

1

1.001

Usual+0.1mm-0.1mm+0.2mm-0.2mm

Radius at WC3[mm]58.5 59 59.5 60 60.5 61 61.5

Rat

io o

f acc

epta

nce

0.998

0.9982

0.9984

0.9986

0.9988

0.999

0.9992

0.9994

Energy Dependence in T1

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B1

B3

T1

e+: 0.5~52.8 MeVµ+

Energy Dependence in T1

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Energy in NaI+CsI[MeV]0 10 20 30 40 50

t0 ti

me[

ns]

1.46

1.48

1.5

1.52

1.54

1.56

/ ndf 2χ 12.72 / 8Intercept 0.01013± 1.514 Slope 0.0003104± -0.0002794

/ ndf 2χ 12.72 / 8Intercept 0.01013± 1.514 Slope 0.0003104± -0.0002794

Correction value: 1.0004±0.0005