CPV cut: Charged Particle Veto (CPV) defines a reconstructed particle as a neutral if there is no charged
track in the vicinity of the EMC reconstructed point (CPV cut). This neutral isolation criterion is defined by the distance between the EMC rec. point and the projection of a charged track onto the EMC surface. As far as we rely only on the CPV information about charged tracks, we calculate the projection of the CPV rec. point into the EMC along the straight line coming from the IP. The worst distance is that one for hadrons. The plots Delta-x and Delta-z show the RMS of the Gaussian fit of the EMC-CPV distance along the x-axis (i.e. across the magnetic field) and z-axis (along the beam) for the charged pions produced with the uniform p_T distribution vs. the deposited energy.. These Delta-x and Delta-z define the neutral isolation rectangular: the EMC rec. point is identified as a neutral particle if there is no charged track projection within (Delta-x,Delta-z) rectangular."TOF cut: The Time Of Flight (TOF) is the time between collision and arrival to PHOS. At low energies, it takes more time to arrive to PHOS to heavy particles than to photons (see figure with TOF of Photons, Pions and Nucleons), so a cut in TOF has been applied. Different time resolutions has been applied to the TOF selection and the effect on the particle rejection is shown in this figure, where HIJING spectra of photons, neutrons and antineutrons, accepted with the TOF is shown. PCA cut: Principal Components Analysis is a method based on the TPrincipal class. This method is used to reduce the number of correlated parameters to new uncorrelated parameters but fewer in number. In our case we have 7 correlated parameters: lambda0, lambda1, dispersion, sphericity (related to the topology of the shower ), digits multiplicity, maximal cluster energy/energy and cluster core energy. All this parameters are reduced to 2 uncorrelated ones, which are given by this method. If this two parameters are plotted in a 2 dimensional graph can be observed that photons correspond to particles inside a given ellipse, and outside this ellipse are the rest of the particles. Unfortunately this is not a clean ellipse and there are some contaminations. The ellipses used to fit the distributions are gaussian ellipses:
The Principal Components are x and y, and all the parameters that define the ellipse are a , b , c , x0 ,and y0, which have been studied in function of the energy:
Hence, depending on the energy we have a different ellipse which contains the pair of PCA parameters of the candidates to be a photon. The chosen ellipse is the one whose value is 1, 2, or 3 standard deviations (s.d.) of the functions above presented. Examples of the PCA ellipse cuts applied at some energies for single photons can be seen, also with the 3 ellipses each one corresponding to 1-2-3 s. d. : 1 GeV , 2 GeV , 3 GeV , 4 GeV, 30 GeV , 50 GeV , 70 GeV and 90 GeV .
High Efficiency - Low Purity: CPV: 1 sigma rectangle TOF: Photons are those particles who arrive at the calorimeter later than 1.7e-8 s PCA: 3 sigma ellipse Medium Efficiency - Medium Purity CPV: 2 sigma rectangle TOF: Photons are those particles who arrive at the calorimeter later than 1.7e-8 s
PCA: 2 sigma ellipseLow Efficiency - High Purity CPV: 3 sigma rectangle TOF: Photons are those particles who arrive at the calorimeter later than 1.65e-8 s
PCA: 1 sigma ellipse
Soft Photons SpectrumIn the experiment the occupancy of particles in the detector will be very high, for this reason it is necessary to study the identification in a high multiplicity environment. HIJING like events have been simulated and reconstructed with our PID.
The following particles in the HIJING environment are studied ( HIJING simulated spectrum available): photon, neutron, antineutron, proton, antiproton and pi+. The efficiency of reconstruction through the different criteria is represented in the following ratios of the pictures. These are the ratios of reconstructed particles energetic spectrum , identified as a photon by the different identification cuts (or any combination of them) and generated by one of the primary particles above mentioned, to the spectrum of all reconstructed particles originated by one of these primary particles.
The association of the primary particle that mainly generated the reconstructed particle was not an easy task, taken into account that not all primary particles generate a reconstructed particle and that a reconstructed particle can have contributions from several primaries ( see this figure were the number of primaries associated to a reconstructed particle is represented). Finally, the association was done as follows: the primary particle which deposits more energy in the cell with the maximum amplitude of the reconstructed point is taken as the most probable primary particle associated with the reconstructed point.
High energy single photons, the ones used for the PID efficiency with single Photons, are merged with the previous HIJING simulations, and the efficiency of reconstructing high energy photons in heavy ion environment is studied. As in the previous section, the study is made following the different cuts. This are the results:PID with CPV identification cut
High purity PID with CPV Medium purity PID with CPV Low purity PID with CPV PID with TOF identification cut
High purity PID with TOF Medium purity PID with TOF Low purity PID with TOF PID with PCA identification cut
High purity PID with PCA Medium purity PID with PCA Low purity PID with PCA PID with CPV + TOF identification cuts
High purity PID with CPV + TOF Medium purity PID with CPV + TOF Low purity PID with CPV + TOF PID with CPV + PCA identification cuts
High purity PID with CPV + PCA Medium purity PID with CPV + PCA Low purity PID with CPV + PCA PID with TOF + PCA identification cuts
High purity PID with TOF + PCA Medium purity PID with TOF + PCA Low purity PID with TOF + PCA PID with CPV + TOF + PCA identification cuts
High purity PID with CPV + TOF + PCA Medium purity PID with CPV + TOF + PCA Low purity PID with CPV + TOF + PCA This is a different case from the pictures above, photon identification purity and contamination of the other particles are shown. Purity is defined as the ratio between the spectrum of particles identified as photon by the 3 cuts who were generated from a primary photon, and the spectrum of all particles identified as photon. Contamination is the same but the primary who generates the reconstructed particle is not a photon.Photon purity and contaminations
High purity Medium purity Low purity
Hard Photons Spectrum