Krakow summer present

Posted 2021-05-30

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)

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.

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

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)

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