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### 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

- 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

General Constraint on Probability Space from previous definition

- Positivness and unitarity

TOF PID

- 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

- 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).

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