Rare B meson decays - mathieu trocmé

III- Analysis Method 19
III.2.2- Selection variables:
III.2.2.1- Event shape variables:
Two event shape variables are used in this analysis: the thrust angle (or more precisely
its cosine) and the Cornelius Fisher discriminant. They both aim at rejecting the continuum
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background events coming from qq random production (with q=u,d,c,s). The B / B mesons
being produced almost at rest in the centre-of-mass frame, they decay isotropically
(spherically) in this frame. In contrast, continuum events are very jet-like. Therefore, if a
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B / B candidate appears to have a jet-like decay, it is more likely to have been reconstructed
from random continuum events. This feature provides a powerful tool to discrimate between
(real) signal and continuum background events.
Practically, the thrust angle is defined as the angle between the direction of thrust of the
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three particles constituting the decayed B / B and the direction of thrust of all the other
events (in the centre-of-mass frame) Taking the cosine of this angle leads to a flat distribution
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for real B / B and a very peaked one near cos(θt)=±1 (θt≈±π) for dummy candidates.
Concerning the Cornelius Fisher discriminant (‘fisherCrn’), its physical representation
is hazier. It combines several event shape variables together. Namely it includes the summed
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energy of the aforementioned rest of events in nine cones around the thrust axis of the B / B
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candidate, as well as the cosine of the B / B thrust axis with respect to the beam axis and
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the cosine of the B / B decay axis with respect to the beam axis [18].
As shown hereafter on Monte Carlo simulated data, typical values for these cuts are:
. cos( θ t ) < 0.
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. fisherCrn
< −0.
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