1. Measurement of CP violation with the LHCb experiment at CERN
Annabelle Chuinard
McGill University
WIPC 2013
WIPC 2013

2. The LHC experiment Key facts Physics goalsb b
Physics goals
Key facts
b-physics: study of particles containing a (anti-)beauty quark!
Branching fraction of B meson decay
 Feynman diagram
CP asymmetry
 CKM matrix terms
Physics beyond the SM
760 people, 14 countries
4500 t. apparatus, 100 m underground
single arm forward (≠ ATLAS)
WIPC 2013

3. The LHCb detector Magnets Trackers Electronic CALorimeter Hadronic
Muons detector
(MWPC)
-271.3°C
RICH 2
-5 to 5°C
RICH 1
VErtex LOcator
-25°C
Proton beam
pp collision
WIPC 2013

4. My job: measure CP asymmetry for
Tree
Radiative B decay : A1
Amplitudes
Penguin (loop) A2
There are 2 main Feynman contributions to this decay.
WIPC 2013

5. CKM terms and couplings
Cabbibo-Kobayashi-Maskawa CKM) matrix describes quark mixing in weak interactions.
WIPC 2013

6. Asymmetry
Conjugate process (CP)
WIPC 2013

7. Charge asymmetry Asymmetry due to production rate of B+ et B-
Detection asymmetry
between K+ et K-
(found compatible with 0)
Measured value
from experimental rates
Corrections

8. How to determine raw asymmetry?
Distribution of the
invariant mass η’K+
Blue line: Multiparameter fit to maximize the extended likelihood function
Distribution of the
invariant mass η’K-

9. Use this as a boundary condition to fit ρ as of function of pK+.
Detection asymmetry
Detection efficiency is a function of the cross-section of with matter.
At high energy, K+ and K- indistinguishable = same cross section
Test sample:
(no production asymmetry)
Map ρ and use MINUIT tool to minimize χ2
Use this as a boundary condition to fit ρ as of function of pK+.

10. Detection asymmetry computation
Integrate over the distribution to have number of events
Distribution of
as a function
of the momentum
Distribution of the
number of events
as a function of
Distribution of the
number of events
as a function of

11. SUMMARY Calculated using rates from mass distributions
Calculated using new method based on kaons cross-sections
Compatible with PDG world average :
Improvements :
Increase luminosity = more statistics

12. Appendix 1: 2D Likelihood fit
Unbinned limit (either 0 or 1 event per bin)
Poisson’s distr.
mB and mη’ are uncorrelated

13. Appendix 2: Detection asymmetry – Mapping process