Krakow summer

Posted 2024-06-01

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J. Mędrala, R. Torres, A. Bożek, and J. Wiechczyński, Polarization measurement in B meson decays, 9th IFJ PAN Particle Physics Summer Student Programme, PL (2021-07), talk, plaintext version below

Polarization measurement in B meson decays

• Ralph Torres
  Ateneo de Manila University (PHL)
• Justyna Mędrala
  AGH University of Science and Technology (POL)
• Supervisors: dr hab. Andrzej Bożek & dr Jarosław Wiechczyński
  Department of Leptonic Interactions, IFJ PAN (POL)

Overview

• Introduction
• Methods
• Results
• Conclusions

Intro

Motivation

• Standard Model

  • most promising elementary particle theory, but experimentally challenging i.e. no new physics found after Higgs discovery
  • there should be hints of NP as there are physical phenomena that we are yet to explain
  • hints → deviations in experimental results when compared to theoretical predictions in SM
  • where to search?
  • what to search?

• b quark sector

  • why?
    • (1) lots of different decays since it’s heavy → more opportunities to find something, especially via rare decays
    • (2) we have efficient b quark sources i.e. B factories → hadrons containing b: B mesons (B0, B*, B_s), etc.

SuperKEKB & Belle II

• Located in Tsukuba, Japan
• A next-gen b factory experiment: lots of B mesons to probe!
• Collides electrons and positrons with an asymmetric energy ratio of 7 GeV : 4 GeV, respectively
• Expected to have 40 times instantaneous luminosity over the original KEKB accelerator
• Designed to make precise measurements of weak interaction parameters and find NP beyond SM

Polarization

• B0 → D* ⍴
  • tree-dominant decay
  • bottom to charm quark
  • scalar → vector vector (SVV)
    • compare to SVS decay
    • e.g. B0 → D* π
  • accounts for 1% of B decays

Polarization measurement

Motivation (revisited)

• hints → deviations
  • where to search? b quark sector (via b factories)
  • what to search? polarization
• polarization
  • why? (1) deviation from theoretical predictions of SM based on various experimental results →
    • f_L = longitudinal polarization fraction
    • f_L ~ 0.5 for penguin dominant
    • f_L ~ 1.0 for tree dominant
  • (2) still need to decouple QCD from weak int. → understanding the results may give insight
• thus, polarization measurement in B meson decays among decay modes, we choose the well-known ones
  • B -> D* π (SVS)
  • B -> D* ⍴ (SVV)

Methods

Overview

Configuring polarization

Event interpretation

Boosting momenta vectors

Kinematic variables

Results

Conclusions

• We learned modern analysis tools employed in modern particle physics: Monte Carlo event generation, detector simulation, final reconstruction, and analysis.
• We were able to reconstruct the polarization of B meson decays: B0 → D* π and B0 → D* ρ.
• Upon measuring the polarization, we observe that the corresponding plots have different features as discussed:
  • For π, the polarization tends to be longitudinal since the minimum is at cosθ = 0 while the maximum is at both cosθ = {-1, 1}. This confirms what is expected.
  • For ⍴, the polarization tends to be a mix of both transversal and longitudinal in the standard generated case. In particular, its longitudinal component is more dominant than the transversal.
  • For the case of ⍴ with transversal polarization, we observe a behavior that is exclusively opposite to that of longitudinal polarization, that is minimum at both cosθ = {-1, 1} and maximum at cosθ = 0.
• For both π and ⍴, the polarization plots reveal an asymmetric feature, see cosθ = {-1, 1}. This is due to the variation of reconstruction efficiency as the angle varies