Malcolm John - 08 Jun 2006
This page describes the study of how to use the T-stations to [quickly] confirm a L0 trigger in the HCAL.
The starting point: L0 cluster
The highest pT cluster in the HCAL is stored at /Event/Trig/L0/Calo in the form of the data object L0CaloCandidate. This object defines the 2x2 group-of-cells in which the high pT particle is expected. The energy-weighted bary-centre of the cluster is calculated from the CaloDigits of these four cells.
Improving the HCAL position precision: S-curve correction
As the granularity of the HCAL is larger than the average hadron shower, the bary-centre calculation is always biased towards the centre of the dominant cell. This effect can be partially corrected for by applying a simple S-curve correction. This correction, which is taken from Monte Carlo of 350000 minimum bias events paramterises the correction as:
d_CORR = A * tan ( B * d_REC ) + C
where d_REC is the position of the bary-centre w.r.t. the centre of the 2x2 cell group and A,B,C are the parameters taken from the Monte Carlo. The same parameterisation is valid in both X and Y. The paramters used are recorded in the following table:
__ | parameter A | parameter B | parameter C | ||
inner HCAL | X: | left | 45 | 0.015 | -14 |
right | 41 | 0.016 | 8 | ||
Y: | upper | 54 | 0.014 | -10 | |
lower | 47 | 0.015 | 8 | ||
outer HCAL | X: | left | 43 | 0.0096 | -20 |
right | 48 | 0.0092 | 16 | ||
Y: | upper | 61 | 0.0086 | -18 | |
lower | 67 | 0.0082 | 18 |
The before and after the S-curve correction are plotted here:
The benefit of this correction on the position precision is small but non-negligible:
Use the ECAL as well
In approximately two-thirds of L0 HCAL triggers, the shower starts in the ECAL. The higher granularity of the ECAL means these deposits can provide additional position information on the track.
Starting from the best position estimate in the HCAL (assumed to be z=13690mm) a X-Y hit point for our track is predicted in the ECAL (assumed at z=12830mm). This point will fall in the middle of an ECAL cell. The nearest cell edge is then taken and the "reference" in both X and Y. The cross-hair of these two edges forms the centre of a 2x2 cell from which the CaloDigits are used to calculate a barycentre of the proto-hadron shower in the ECAL.
On the use of S-curve corrections in the ECAL
First a decision has to be made on the theshhold above which we assume the energy deposit in the ECAL is associated to the hadron shower. As a guide, we look at the S-curves in the ECAL as a function of energy (not E_T!) cut-off. Where no cut-off is applied, additional energy is seen occuring on only one cell. Visually this 'noise' is represented as tails away from the S-curve. These tails are seen to reduce with large cut on the ECAL energy deposit considered.