| | 25 | >>> Showers are not produced in Delphes as the particle momenta are simply smeared |
| | 26 | >>> according to the relevant calorimeter resolution. The discreteness of the calorimeter |
| | 27 | >>> has simply an impact on the (eta,phi) resolution of the final observables. |
| | 28 | >>> As it is, the pessimistic spatial resolution assumed in ECAL only affects photons, |
| | 29 | >>> since for electrons we have the track information and we therefore assume infinite resolution. |
| | 30 | >>> If there was a consistent difference in the performances due the this calorimeter simplification, |
| | 31 | >>> it would first appear in the calorimeter resolutions, which seems to be correctly reproduced. |
| | 32 | |
| | 85 | |
| | 86 | |
| | 87 | >>> (michele suggested section 2.2) |
| | 88 | |
| | 89 | After propagating in the magnetic field, long-lived particles reach the calorimeters. The electromagnetic calorimeter, ECAL, is responsible for measuring the energy of electrons and photons, while the hadronic calorimeter, HCAL, measures the energy of strongly interacting particles. |
| | 90 | |
| | 91 | In \DELPHES, the calorimeters have a finite segmentation in pseudo-rapidity and azimuthal angle ($\eta$,$\phi$). The size of the elementary cells can be defined in the configuration file. For simplicity the segmentation is uniform and for computational reasons we assume the same granularity for ECAL and HCAL. The coordinate of the resulting calorimeter object, the tower, is computed as the geometrical center of the cell. |
| | 92 | |
| | 93 | Long-lived particles reaching the calorimeters deposit a fixed fraction of their energy in the corresponding ECAL ($f_{ECAL}$) and HCAL ($f_{HCAL}$) cells. |
| | 94 | Electrons, photons and neutral pions leave all their energy in ECAL, while charged pions and other neutral hadrons deposit all their energy in HCAL. |
| | 95 | Long-lived particle such as kaons, pions and $\Lambda$'s are considered stable by most event generators. |
| | 96 | In \DELPHES, such particles are assumed to deposit a fixed fraction of their energy both in ECAL and HCAL. |
| | 97 | By default, $f_{ECAL}$ is set to $30\%$ and $f_{HCAL}$ to $70\%$ --- according to their expected decay products --- but this can be tuned by the user. |
| | 98 | Finally, muons, neutrinos and neutralinos, do not deposit anything in the calorimeters ($f_{ECAL}=f_{HCAL}=0$). |
| | 99 | |
| | 100 | The resolution of the calorimeters is parametrised as a function of the particle energy and the pseudo-rapidity: |
| | 101 | \begin{equation} |
| | 102 | \left(\frac{\sigma}{E}\right)^2 = \left(\frac{S(\eta)}{\sqrt{E}}\right)^2 |
| | 103 | + \left(\frac{N(\eta)}{E}\right)^2 |
| | 104 | + C(\eta)^2\,, |
| | 105 | \label{eq:calores} |
| | 106 | \end{equation} |
| | 107 | where $S$, $N$ and $C$ are respectively the \textit{stochastic}, \textit{noise} and \textit{constant} terms. The electromagnetic and hadronic energy deposits are independently smeared by a log-normal distribution with variance $\sigma$. The final tower energy is then computed as: |
| | 108 | \begin{equation} |
| | 109 | E_{Tower} = \sum_{particles}\text{ln}\mathcal{N}\left(f_{ECAL} \cdot E,\sigma_{ECAL}(E,\eta)\right) +\text{ln}\mathcal{N}\left(f_{HCAL} \cdot E,\sigma_{HCAL}(E,\eta)\right) . |
| | 110 | \label{eq:etow} |
| | 111 | \end{equation} |
| | 112 | where the sum runs over all particles that reach the given tower, and $\text{ln}\mathcal{N}(m,s)$ is the log-normal distribution with mean $m$ and variance $s$. A calorimeter tower is also characterized by its position in the ($\eta$,$\phi$) plane, given by the geometrical center of the corresponding cell. In order to avoid having to deal with discrete tower positions, an additional uniform smearing of the position over the cell range is performed. |
| | 113 | |
| | 114 | Calorimeter towers are, along with tracks, crucial ingredients for reconstructing isolated electrons and photons, as well as high-level objects such as jets and missing transverse energy. |
| | 115 | |
| | 116 | |
