Download TOF (and Global) PID

TOF (and Global) PID
F. Pierella
for the TOF-Offline Group
INFN & Bologna University
PPR Meeting, January 2003
Summary and Highlights
TOF-PID Probability approach has
been implemented (on time-of-flight
basis), so TOF is ready to provide its
own probability;
 A more general approach to TOF PID
(based on various particle bands)
has been considered;
 Global (TOF+TPC+ITS) PID is “in
fieri” and results are on the way;

Definition of Probability for
(from) TOF


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From track length and momentum (given by
reconstruction), and after a mass hypothesis
for the current track, it is possible to derive
the corresponding (“a priori”) time-of-flight;
A gaussian is generated
 around the measured time-of-flight,
 with a (fixed for each track) sigma equal
to to the current measurement of the
(mean) double-stack MRPC resolution;
Probabilities (after normalization) are derived
from the gaussian in the previously
calculated “a priori” times-of-flight
i.e.
2
 mi 
t i  l  
 1
 p 
l  reconstruc ted track length
p  reconstruc ted momentum
mi  mass assumption
General Constraint on Probability
Space from previous definition

Positivness and unitarity
Distribution of Probabilities for
Pions (100 HIJING ev, B=0.4T)
Distribution of Probabilities for
Kaons
Distribution of Probabilities for
Protons
Expected anticorrelation on
Probability space
TOF PID
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TOF PID “stand alone” is based (at least for
kaons and protons) on particle bands in a
given scatter plot (usually m vs p);
Contour cuts are introduced to define
regions where a particle is said to be of a
certain type (or not to be of a certain other
type);
The contour cuts themselves are arbitrary
(in the sense that they depend on the
physical problem);
TOF PID


Moreover, PID based on contour cuts (2D,
for the time being) is not the unique way
to do PID (see e.g. the probability
approach with which a “combination” of
different detectors is possible, no cuts at
all needed)
In any case, it is worth to introduce some
generalization on particle bands (and not
limit ourselves exclusively on
reconstructed mass vs momentum scatter
plot)
(e.g.) Time of Flight Spectra
(250 HIJING ev., B=0.4T)
Momentum vs Time-of-Flight:
separation of the “bands” at high
momentum (low particle statistics)
 1/Momentum vs Time-of-Flight:
separation of the “bands” at low
momentum (large particle statistics)

More on Bands for TOF PID
(as in PHENIX)
Momentum vs Mass
 Momentum vs Square Mass
 Momentum vs Time-of-Flight
difference from electron
 1/Momentum vs Time-of-Flight
difference from electron
 1/() vs Momentum

Mass Hypothesis and time
of flight
Back step to probability approach
based on times of flight;
 Measured time-of-flight and à priori
time-of-flight difference (mass
hypothesis: pion, kaon, proton);
 Bands for different particle types
appear also there.

Example of Global PID

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dE/dX from ITS-TPC
Reconstructed Momentum
Mass from Time-of-Flight
Notice that different combinations are
possible (see the section on ‘bands’)
Moreover, if dE/dX is profitable for TOF (it
provides a separation at low momentum
–large statistics-, where TOF PID
contamination is “high” for protons and
kaons), the reverse is also true (TOF
provides a separation in another
“direction”, “mass direction” in this case).