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Collimator-related uncertainties

Due to the crucial role played by the single-channel Cherenkov devices, the charged track multiplicity has to be kept low. In preparation for the $Pb$ beam, the need of limiting the spectrometer's acceptance in the low $p_T$ setting was realized. The problem was solved by installing a tungsten alloy (90% W, 6% Ni, 4% Cu) collimator (``jaws'') in the magnetic channel of the spectrometer, between D1 and Q1 (see Fig.  3.1). Orientation of the collimator corresponds to the focusing setting and is horizontal in the singles data. In order to cut off the tails of the particle distribution where the particles momenta might be affected by interaction with the collimator, a cut on the vertical component of momentum $p_y$ has been applied. The TURTLE[40]-based Monte Carlo simulation available during my work on the singles analysis used to deviate from the experimental data in its prediction of the peak position in $p_y$, shifting it in the positive direction by $\approx$ 2.5 MeV/c. I studied dependence of the acceptance corrected $\,dN/\,dy$ of various particles on the position of a narrow $p_y$ window, in which the samples were collected. The sliding window cut study showed that, as the window slid towards the negative $p_y$, the $\,dN/\,dy$ would increase due to the ``emptiness'' of MC acceptance, not balanced out by the data. For the positive $p_y$, an abnormal decrease would be observed. In between these two regimes, a narrow zone of relative independence of $\,dN/\,dy$ on the $p_y$ cut position would be found.

Figure: Understanding the collimator-related uncertainty in the acceptance-corrected pion $\,dN/\,dy$. The horizontal bars show the extent of the fiducial $p_y$ window used. In this plot, other corrections were fixed at the values they had when the study was undertaken.

Such a behaviour (see Fig. 4.1) is characteristic of situations with a misplaced acceptance. The remedy is to avoid any of the edges, by limiting the acceptance to the middle zone. The $p_y$ ranges so chosen are listed in Tables 4.7 and 4.9.