Summary of PHOS Internal Notes(part I)

Rafael Diaz Valdes

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Table of Content

• PHOS Detector Description• Reconstruction Algorithm• Geometrical Acceptance• EMC Performance

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PHOS Detector Description

PHOS: ALICE PHOton Spectrometer designed to measurement photons and light neutral mesons (pi0, eta) and measurement of global event observable like electromagnetic transverse energy and multiplicity.

5 EMC modules with CPV

One EMC module 56

64

PbWO4 scintillator crystals

x

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Reconstruction Algorithm

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Reconstruction Algorithm Steps1- Clustering neighbour digits (cristals) Starting from a seed selected as the digit with E > Ethre this Ethre is a value above the energy deposited by the minimum-ionizing particles Ethre

ion = 30 MeV. Clusters with more than one maximum are unfolded.

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E = eidigits

∑

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x =

x iwidigits

∑

widigits

∑

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wi =max[0, p+ log(ei /E)]Cluster position (x,z) as the center of gravity. Same equation for z coordinate

ei digit energyE cluster energy

p is a parameter, empiric value = 4.5

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Cluster Parameters

(a) Digit multiplicity(b) Lateral dispersion d in (x,z) plane measured in the EMC surface as:

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(c) The shape of the surface interaction of the cone containing the shower with the font plane of the calorimeter. This surface is expressed as a covariance matrix

The diagonalization of this covariance matrix defines the main axis of the shower surface λ1 and λ2 as the square root of the eigen vectors of the covariance matrix.

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(d) The Sphericity parameter defined from the major axes λ1 and λ2

Z

X

λ2 λ1

(e) The largest fraction of the cluster energy deposited in the crystal(f) The core energy: the summed digits energy around the largest digit in a radius R=0.3 (Ecore)(g) Time of the flight (TOF) selected as the shortest time among the digits making the cluster

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2- Reconstruction of track segment:A track segment in PHOS is made up by at most two reconstructed points, one in CPV and one in EMC or EMC alone.First the algorithm search for all possible pair in a distance less than 10 cm. A CPV reconstructed point can be used once in a track segment.

CPV

EMC

3-Particle Identification :Every track segment is a seed for a reconstructed particle. The criteria used to identify particles in PHOS are: i- charged particle rejection with CPV and EMC ii- TOF (time elapsed between the collision time and the impact of the particle in the calorimeter) iii- the shape of the shower that develops the particle in the calorimter

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Geometrical Acceptance

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The photon acceptance is defined as a probability that a photon radiated from the interaction point within |η| < 0.5 and 0 < φ < 2π

The acceptance for π0 or η mesons detected by two photons invariant mass is defined as the probability that the two photons hit PHOS if the π0 was radiated from the interaction point within |η| < 0.5 and 0 < φ < 2π

A lot of corrections in the market http://indico.cern.ch/getFile.py/access?contribId=1&resId=1&materialId=slides&confId=38827

PHOS EMC Performance

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1- Energy Resolution

Electron beam test

Electrons energy resolution

a: contribution from electronic noiseb: stochastic termc: another constant

Simulations of mono energetic photons impinging of the center of PHOS

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2- Position Resolution

The impact position on PHOS (x,z) is corrected for the incident direction of the impinging photon as:

xin xrec

α