# Reconstruction

## General

The reconstruction take the detector hits as input and interpret them as the physics objects, with measured quantities such as energy, position, momentum, charge, vertex information, etc. The input detector hits, can be both generated from simulation-digitization and from acquisition from experimental devices. These two kinds of input should be in principle indistinguishable from for the reconstruction.

The reconstruction at CEPC is realized in three steps. The sub-detector level, the PFA level and the Physics objects level.

The first step is sub-detector level reconstruction. In the CEPC software, this step refers explicitly to the Tracking procedure. (In fact, the sub-detector level reconstruction also consists of the calorimeter hit clustering module – which is included in the PFA reconstruction as in the second step). The tracking system of the CEPC conceptual detector is composed of several sub-detector system, thus the tracking process is also composed of track-fragmentation reconstruction and the global track reconstruction. The final tracks are characterized by a 5-parameter helix: the curvature Omega that represents the Pt of the track, the theta and phi angle that describes the track direction, and the impact parameters D0 and Z0. A dedicated tracking performance description (i.e., tracking reconstruction efficiencies at different condition and the corresponding resolutions) for CEPC detector models, and the corresponding diagnosis tools could be found in Link//.

The Second step, the PFA level reconstruction is the core of CEPC reconstruction. It takes in the tracks and Calorimeter hits, and builds the reconstructed particles accordingly. Each reconstructed particle is described by its momentum, charge, and particle type. Operationally, the PFA algorithm at CEPC, Arbor, is consists of three sub-modules.

1st, the Calorimeter Clustering algorithm, creates local calorimeter clusters from calorimeter hits.
2nd, the Final state particle reconstruction, matches the tracks and calorimeter clusters, building neutral and charged final state particles.
3rd, the final state particle identification, the charged particle are classified into two kinds of leptons (mu+mu- and e+e-) and three kinds of hadron (pion, kaon and proton) via the LICH algorithm [XX, XX]. The neutral ones are classified into photons and neutral hadrons [XX].


The Third step is the physics object reconstruction. The output of the PFA level reconstruction, the reconstructed and identified final state particles provide a uniform bases for the reconstruction of all the physics objects. In terms of the key objective measurements at the CEPC, the physics objects includes: 1st, Single particle physics objects: i.e, the leptons, photons and kaons. These physics objects is directly established from the reconstructed particle list 2nd, Composited Physics Objects: i.e. the pi0, the converted photons, the taus, the jets and the missing energy.

The reconstruction of composited physics objects depends on dedicated Object finding algorithm. The jet reconstruction quote the Fast simulation algorithm. A dedicated convereted photon finding algorithm have been developed. Dedicated tau finding algorithm have been developed at the Br(H->tautau) measurement. No official algorithms had been provided for the pi0 reconstion yet.

## Tracking

Some detailed introduction for tracking performance can be found in (somelink).