We are examining many tracking issues linked to the performance of both the COT and the silicon. In particular, we are concerned with the efficiency and purity of the association of silicon hits to tracks, and the precision of the final track fit.
Two useful calibration signals are the Z boson, and the J/psi resonance which decay to two muons. This talk (24th Oct 01) describes some preliminary results on work which is ongoing.
COT systematic effects
High energy muons from cosmic rays pass through the detector, and give the appearance of two tracks originating close to the beam. Being one track, the track parameters for the upper and lower legs ought to be the same, once corrections have been made for the t0 which is uncorrelated with the beam crossing and the fact that the upper leg appears to go backwards in time.
- Since it is one continuous track, the COT should measure the same d0 (distance of closest approach to the origin) for both legs. Summing the d0 for both legs should give a Gaussian distribution centred on zero whose width is root(2) times the intrinsic impact parameter resolution. However, as can be seen, it is offset by 200 microns. The resolution of 460 microns implies an average impact parameter from the COT alone of 325 microns.
- The two legs of the cosmic should be back-to-back. However, an offset of 0.5 mrad is observed. A pointing resolution of 0.85mrad is implied from the width of the distribution.
- The curvature measurement is much less biased. An offset of 0.0000007 is observed, an order of magnitude smaller than the resolution.
- Cotan-theta is offset by 3 mrad. The resolution implied by the width of the distribution is 5.5mrad.
- z0 has no large offset, at least within the rather broad resolution of 4mm.
COT systematic deformations
Talk on 11th Sept 2002 with detailed information from Aseet on subtle mechanical effects which cause systematic offsets in the tracking.
The effects of different initial COT wire positions on COT track residuals have been studied. The following plots show the hit residuals (y axis) against wire number. The wires are ordered in increasing radius; all wires (ie. all phi) at a given radius are plotted together.
- Track residuals with 4.7.1 code default COT wire positions.
- Track residuals with a newer version of Aseet's wire positions. Note that the stereo superlayers are much improved.
- Track residuals found when the pure COT CMM wire positions are used. Note the similarity in behaviour with respect to Aseet's wire positions.
- Track residuals found using the CMM survey and a) a circular model, b) a quadratic model, c) a cubic model of endplate deformation. Note that these models of endplate deformation do not appear to affect residuals.
- Track residuals obtained using the CMM survey and rotating the superlayer wire slots by1 mradand 3 mradrespectively. As expected, all residual slopes move by the same amount in the same direction.
Finally, track parameters for tracks obtained using these different COT wire positions are compared. We take the same track that has been fitted assuming each set of wire positions in turn, and compare the impact parameter (d0) for each to that obtained using only the CMM survey positions. Positive and negative curvatures are considered separately. The resulting distributions are fitted to Gaussians. The mean and sigma from these fits are used to compare the effects of different wire positions.
- Summary of results. Mean difference in d0 is shown on the y axis, for each comparison on the x axis: compared to the CMM survey; compared to CMM+circ. deformation; compared to CMM+cubic deformation; compared to CMM+quad. deformation; compared to CMM+1 mrad deformation; compared to CMM+3 mrad deformation; Aseet positions; 4.7.1 positions. Positive (negative) tracks are shown in black (red). The error on d0 difference is given by the Gaussian fit in each case. Note that a rotation of slots appears to offset d0 in the same direction for positive and negative tracks. Note also that these models of endplate deformation appear to offset d0 in different directions for positive and negative tracks.
- Individual fits for the above are shown here.
Knowledge of the beamspot position, size and stability is vital for many physics analyses which depend on measuring displaced vertices.
Martin T. has performed a study of the size of the beamspot for a number of fills taken in early 2002. Two interesting conclusions can be drawn. Firstly, the size appears to be about 37 microns rather than 25 microns which the beam experts expect. Secondly, the beam has an hour-glass shape, narrower in the centre than at the ends. Furthermore the profile in x and y is different, which may be due to the fact that the focussing is performed separately in each dimension. Discussions are ongoing with both the machine physicists and in the tracking group to resolve these issues.