First Measurement of the 𝜏 Lepton Polarization∗ −in the Decay 𝐵 → 𝐷 𝜏 𝜈𝜏 at BelleS. Hirose et al. (Belle Collaboration),
arXiv:1612.00529 (submitted to Phys. Rev. Lett.)∗ −The decay 𝐵 → 𝐷 𝜏 𝜈𝜏 is predicted to be sensitive to new physics including a non-universal coupling over the three generation. We report
the first measurement of the 𝜏 polarization 𝑃𝜏 (𝐷 ∗ ) and a new measurement of the ratio of the branching fractions
𝑅 𝐷 ∗ = 𝐵𝐹(𝐵 → 𝐷 ∗ 𝜏 − 𝜈𝜏 )/𝐵𝐹(𝐵 → 𝐷 ∗ 𝑙 − 𝜈𝑙 ) using the full data sample containing 7.72 ± 0.11 × 108 𝐵𝐵 pairs accumulated at the Belleexperiment. We reconstruct signal events from 𝜏 − → 𝜋 − 𝜈𝜏 and 𝜌− 𝜈𝜏 . Our measurement results in 𝑃𝜏 𝐷∗ = −0.38 ± 0.51 stat. +0.21−0.16 (syst. ) and+0.028∗𝑅 𝐷 = 0.270 ± 0.035(stat. ) −0.025 (syst. ). These are consistent with the SM prediction.

Shigeki Hirose (Nagoya University, Japan), Toru Iijima (Nagoya U.) and the Belle Collaboration■Belle Experiment+−B-factory at the 𝒆 𝒆 Collider KEKB
KL/μ detector

𝜏−World Average

Scalar+ −• 𝑒 𝑒 collision at 10.58 GeV,− , 𝐻−𝜈𝜏𝑊
where 𝐵 mesons are
Electromagnetic
VectorCalorimeter(SM-like)
Tensorproduced through 𝛶 4𝑆 →𝑏𝑐+SM𝒆𝐵𝐵Vector∗(3.5 GeV)
𝐷𝐵(Right-handed) Clean environment as𝑢, 𝑑𝑢, 𝑑Time Of Flightthere is no particle
Counter
except for two B mesons • Sensitive to new physics (NP) having
Aerogel Cherenkov
Definition
anenhanced
couplingtothethird∗𝜏 −𝜈 )• KEKB holds a world-record
𝐵𝐹(𝐵→𝐷Γ−ΓCounter𝜏RL∗∗𝑅 𝐷 =𝑃𝜏 𝐷 =∗𝑙−𝜈 )generation
fermions
Γ+Γ
𝐵𝐹(𝐵→𝐷peakluminosityofRL𝑙Central Drift−−−
34
2ChamberUsing𝜏→𝜋𝜈and𝜌𝜈𝜏 ,•3.3σ
deviation
fromthestandard
2×10 /cm /s𝜏∗• Measurement of 𝑃𝜏 (𝐷 )model (SM) prediction has been Belle has recorded∗• Measurement of 𝑅 𝐷
observeddata containing
independent of the past meas.
7.72 ± 0.11 × 108 𝐵𝐵 • Belle has measured this mode with
Collaboration, PRD 92, 072014 (2015)
are possible! Belle
leptonic 𝜏 decays
Belle Collaboration, PRD 94, 072007 (2016)

R(D*): S. Fajfer et al., PRD 85, 094025 (2012)
Pτ(D*): M. Tanaka and R. Watanabe, PRD 87,034028 (2013)

Superconducting
Magnet𝒆Based on the calculation in M. Tanaka andR. Watanabe, PRD 87, 034028 (2013)∗−■𝐵 → 𝐷 𝜏 𝜈𝜏−(8 GeV)
Silicon Vertex
Detector■𝝉 Polarization Analysis Method
Principle of the Measurement
𝜏-rest frame

We can obtain this value experimentally By the rotation symmetry, the frame
2𝐸𝜏 𝐸𝑑 − 𝑚𝜏2 − 𝑚𝑑2
can be boosted along any direction toWe need to measure cos𝜃hel , but cannotcos𝜃𝜏𝑑 = Obtain a correct cos𝜃hel
2|𝒑𝝉 ||𝒑𝒅 |obtain the complete 𝜏 momentum due toinsufficient constraint
 Use 𝑊 rest frameParameterization• Use two bins of cos𝜃hel• Signal yields in [0, 1] and [-1, 0] are,respectively,
𝑁𝛼
∗
𝑁𝐹 =
1 + 𝑃𝜏 (𝐷 )cos𝜃hel
22
𝑁
𝛼
𝑁𝐵 =
1 − 𝑃𝜏 (𝐷 ∗ )cos𝜃hel
22• 𝑃𝜏 (𝐷 ∗ ) is parameterized as

Full reconstruction and obtain𝑝𝑊 = 𝑝sig − 𝑝𝐷∗ Boost along 𝑝𝑊1 𝑑𝛤
1=
1 + 𝛼𝑃𝜏 (𝐷 ∗ )cos𝜃hel
𝛤 𝑑cos𝜃hel21for 𝜏 − → 𝜋 − 𝜈𝜏
𝛼 = (𝑚2 − 𝑚2 )/(𝑚2 + 𝑚2 ) for 𝜏 − → 𝜌− 𝜈𝜏𝜏𝜌𝜏𝜌𝑊-rest frame𝑃𝜏 𝐷■ Signal Extraction■ Hadronic B Background Calibration Calibration Method• 𝐸ECL is a linear energy sum of the remaining
clusters in ECL• This is the best variable in terms of- good background discrimination∗- very small correlation to 𝑃𝜏 (𝐷 )• Important background component:- similar event topology to the signal- huge uncertainty due to low energy
hadronization process• Strategy for the yield determination- Calibrate composition of hadronic B decays
modes using data
- Determine the yield in the final fit● Signal (𝑃𝜏 𝐷∗ = −0.5; SM)● Signal (𝑃𝜏 𝐷∗ = +0.5)● Background

Background(example)

Signal■ Result• Fully reconstruct seven specific B decay modes• Compare the yields between the MC and the data
 Take the yield ratio• The ratio is used as a calibration factor for the yield
The observed discrepancy isin the MC
𝐵 → 𝐷∗ 𝜋 − 𝜋 − 𝜋 +Best-fit
SM𝑃𝜏 𝐷 ∗ = −0.38 ± 0.51 stat. +0.21−0.16 (syst. )+0.028∗𝑅 𝐷 = 0.270 ± 0.035(stat. ) −0.025 (syst. )First 𝑃𝜏 𝐷 ∗ measurement!
Results consistent with the SMcalibrated by the yield ratio
extracted from the comparison● Data■ MC (true 𝐵)□ MC (fake 𝐵)

Undetected particles
 missing momentumWA∗

2 𝑁𝐹 − 𝑁𝐵=𝛼 𝑁𝐹 + 𝑁𝐵

■ Summary∗∆𝐸 ≡ 𝐸𝐵∗ − 𝐸beam(GeV)

Using hadronic 𝜏 decays 𝜏 − → 𝜋 − 𝜈𝜏 and 𝜌− 𝜈𝜏 , wemeasured 𝑃𝜏 𝐷 ∗ as well as 𝑅 𝐷 ∗ . One of the difficulties in∗
the 𝑃𝜏 𝐷 measurement was that the full 𝜏 momentum∗
could not be obtained. We have established the 𝑃𝜏 𝐷
measurement method using the rest frame of 𝑊 and thesymmetry in the decay kinematics. To cope withbackground from hadronic 𝐵 decays, we have calibrated thecomposition of the decay modes using the calibration data
samples. Our measurement results in+0.21∗
𝑃𝜏 𝐷 = −0.38 ± 0.51 stat. −0.16 syst. ,+0.028∗𝑅 𝐷 = 0.270 ± 0.035 stat. −0.025 syst. ,consistent with the SM prediction. Our study has
demonstrated the polarization measurement in 𝐵 →𝐷 ∗ 𝜏 − 𝜈𝜏 , that gives an additional dimension in the NP
searches with the semitauonic 𝐵 meson decays.