Neutral Pions identification in PHOS
PHOS Geometrical Acceptance
The acceptance of the detector ( 5 modules) to the two decay photons of
Pi0 and Eta's has been studied. Pi0 and Eta's has been simulated from the
ALICE interacting point, with angles 0 < Phi < 360 and rapidity's
-0.5 < y < 0.5. Both particles are represented in two different
energy ranges. The result is shown in this figure.
(As the detector will have 4 modules, it has to be redone )
Invariant Mass Analysis
The Pi0 identification efficiency with the invariant mass analysis is presented.
A Pi0's flat spectrum of energies from 0.5 to 100 GeV has been simulated
over the PHOS modules (only Pi0's that decay the two photons in PHOS are
simulated). In the real experiment some cuts will be applied to identify
correctly the Pi0. This cuts has been applied although they have few sense
in flat distribution but useful if we want to compare with the real case
distribution (HIJING) , that will be done later.
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Pi0 Reconstruction
Efficiency: The cuts are applied one after the other and are the following:
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No cuts
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Fidutial cut: the rec. points near the borders are not taken into
account.
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2 sigma cut: only pairs whose invariant mass is inside 2 standard
deviations of the gaussian invariant mass. All reconstructed Pi0 give an
invariant mass who belongs to a gaussian distribution around the mass of
the Pi0. This has been studied depending on the energy and the dependence
can be seen in this figure)
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Asymmetry cut: only pairs with asymmetry slower than 0.8 are taken
into account. Greater asymmetries means photons very close and one with
the great part of the energy, so difficult to resolve in real life.
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Pi0 Reconstruction
Efficiency + PID cut: only pairs that both photons are HE-LP or ME-MP
or LE-HP (see Photon ID)
Invariant Mass Analysis in HIJING
Invariant mass analysis has been applied to HIJING events.
First, the energy
asymmetry A=|E1-E2|/(E1+E2) is shown for any reconstructed pairs
in the Hijing events at various p_T ranges. The asymmetry between
the two photons from the pi0 decay is flat, and the deviation from
the flat distribution results from the combinations of the uncorrelated
pairs. At lower p_T the asymmetry has more often small values because these
pairs cam from the hadrons with small deposited energy (around MIPS), and
many pairs of soft particles have small and almost equal reconstructed
energy. At higher p_T the high peak at large A>0.7 is observed which is
explained mainly by the combinations of a hard photon with either a soft
photon or an hadron. Therefore the limit A<0.75 would eliminate
such false combinations leaving only photon pairs from the pi0 decay. The
validity of such asymmetry cut is demonstrated by the shown invariant
mass spectra of any reconstructed particle pairs with asymmetry A<0.75.
It is also shown the p_T
spectrum of reconstructed pi0 per HIJING event.
Hard Pi0 identification
Hard Pi0 in Photon PCA
The same Pi0 simulation as in the previous paragraph is again analyzed,
but now we only look for one reconstructed particle, the two decay photons
are very close (mainly great part of the particles with E > 30 GeV) and
they can not be resolved separately. These simulated events with both photons
overlapped as one reconstructed particle were passed through the photon
PCA criterion. PCA plots
for reconstructed energy of 30, 50, 70 and 90 GeV Pi0's are shown. The
3 photon ellipses can be seen in the figures
and it represents where are supposed to be the photons in this PCA analysis.
At not very hard energies this ellipse is not including the Pi0, but at
greater energies, they overlap too much.
Hard Pi0 PCA
To estimate the misidentification probability of photons as pi0 at high
energy, one has to define PCA for the pi0's. A new PCA analysis has been
done. Now the particles to be identified are the Hard energy Pi0. The procedure
is the same as in the photons case.
For the hard Pi0's, the ellipse
parameters dependence on the energy is shown. Also available some examples
with the 3 fitted ellipses
to the PCA analysis of the Pi0's. Photon contamination to the pi0 spectrum
can be defined by passing the photons through the pi0's PCA. Figure Photons
in PCA for Pi0's shows the principal components of the photon events
calculated with the PCA of the pi0. Three ellipses show the pi0 principal
components with 1-, 2- and 3-sigma.
Identification Efficiency of Pi0 and Photon in both
PCA
Both PCA analysis (the one for photons and the one for pi0's) have
been applied for hard photons and pi0's. In the following pictures is shown
the ratio defined by the pi0 or photon spectrum reconstructed as one reconstructed
point and identified as a photon or pi0 (by the 3 ellipses cuts), to the
spectrum of all pi0's or photons reconstructed as one reconstructed point.
The increase of the pi0 misidentification, observed in the figures, is
explained by the fact that high energy pi0 have a very similar shower shape
to that of a single photon. The best photon / pi0 separation will be achieved
when the ratio of the pi0 misidentification probability to the photon true
identification probability is minimized. This leads to the necessity to
decrease the photon identification efficiency at high p_T in order to increase
the pi0 suppression power. The smooth degrading of the PCA ellipse from
1 sigma at 50 GeV to, e.g. A*sigma at 100 GeV (A<1) will do it.
Identification Efficiency of Pi0 in HIJING
High energy pi0's, flat energy distribution, the ones used previously,
are merged with HIJING simulations,
and the efficiency of reconstructing high energy neutral pions in heavy
ion environment is studied. As with the photons
section, the study is made following the different cuts, and with both
PCA analysis. This are the results:
In ALICE/PHOS internal note "Predictions
for high Pt and Pi0 production ALICE/LHC" (ALICE reference number:
ALICE-INT-2002-02 V 1.0), there is a prediction of the Pi0 spectrum to
be observed in PHOS/ALICE for a full year of ALICE running, in the 10%
most central Pb+Pb collisions at 5.5 TeV, which were derived from
binary scaling of p+p collisions. This spectrum is shown in the
following figure, corrected
by the High Purity efficiency
detection by our identification procedure.
Go to Photons Identification