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Time of flight in kaon identification.

The time of flight cut is used to separate $K^+, K^-$ from $p, \bar{p}$ respectively in the strong and weak field setting of the spectrometer at both arm angles. Since only particles with momenta within a narrow range are accepted by the spectrometer, velocity

$$v = \frac{p}{\sqrt{m^2+p^2}}$$ (31)

becomes a measure of particle's mass $m$. $v$ itself is measured by time interval between the interaction (provided by beam counter ) and the scintillation hodoscope H3. $K/p$ separation is therefore achieved by a TOF cut. A good cut is slat dependent as velocity and slat number are correlated by virtue of the spectrometer's analyzing power. The problem of this cut's cleanliness/efficiency becomes noticeable only in the strong momentum setting. By performance evaluation I mean a procedure which allows me to answer two questions:

• how many real particles get lost due to the cut?
• how many unwanted ones get misidentified ?

The TOF distribution is Gaussian and therefore easy to analyze. The algorithm consists of the following steps:

1. Select a sample of tracks of desired PID, using all cuts except for the hodoscope cut.
2. For this sample, fill separate TOF histograms for every slat.
3. Fit each slice with a sum of two Gaussians; assume that TOF distribution is independent of PID and take the sigmas to be the same.
4. For the two Gaussians with known separation, find the contamination and the loss due to the cut.

The corrections can be found in Table 4.10.