Violation in Mixing

82 Ã Ë reconstruction and efficiency studies
modes efficiency (%) # Ã Ë efficiency (%) # Ã Ë
block 1 per event block 2 per event
Monte Carlo (b1 MC) Monte Carlo (b2 MC)
� � 93.9 0.52 94.9 0.52
� � 94.0 0.46 95.3 0.46
�� 94.0 0.49 95.1 0.49
77.8 0.43 77.1 0.43
Ù�× 67.5 0.27 63.9 0.27
Table 3-2. Event selection efficiency of the hadronic selection in generic Monte Carlo events used in this
analysis and the number of Ã Ë per event after the hadronic selection from the Monte Carlo truth.
resolution on Monte Carlo is better than on data. In figure 3-5, data show a � dependence of the number of
reconstructed Ã Ë which is also well reproduced by Monte Carlo.
Then, the reconstructed decay length of the Ã Ë is considered both on MC and on real data samples. Defining
the two-dimensional decay length:
�Ö �
Õ ÜÃ× Ü���Ñ ÝÃ× Ý���Ñ
(where ÜÃ× and ÝÃ× are the reconstructed Ü and Ý position of the Ã Ë decay vertex and Ü���Ñ and Ý���Ñ come
from the reconstructed impact point from Bhabhas), figures 3-6 show the invariant mass and the resolution
obtained as functions of �Ö. Monte Carlo and data have similar structures with �Ö � , but they look pretty
different within the beam pipe (�Ö � cm): the Monte Carlo does not show any slope, while the data do.
This behaviour is still under investigation.
In the range of �Ö between and � cm, less and less material gets traversed and therefore the fact that the
data used for the analysis store the helix parameters at the origin causes an increase in the measured mass:
the energy loss correction is applied although the material has not been traversed by the tracks.
Another drop is visible towards the end of the SVT: this might be due to the transition between DCH only
and SVT only tracks, but the real source of the effect is not yet understood.
Another variable that has been considered in this analysis is the reconstructed momentum of the Ã Ë candidate,
see figures 3-7. Data show a flat distribution, while Monte Carlo presents a positive slope at low
momenta.
MARCELLA BONA