Estimating momentum resolution with fast simulator
Understanding, parametrizing, and correcting for momentum resolution
is important for HBT in E895, as shown by
Here, we use RQMD events through the fast simulator to get the
p- momentum resolution.
Obviously, saying it is "1%" or something is way too simplistic.
It will vary with the track rigidity, and with the number of padrows
the track passes over in the detector.
Also, it is important to get the resolution for each component!
There is no reason the momentum resolution along the beam direction, in the
drift direction, and in the bend direction must be the same percentage of
the total momentum.
Here, we use only tracks with small DCA ( < 1.0 cm because the fastsim
gives better DCA than the data), and look at the reconstructed vs. input
momentum for negative pions. We do this for windows on |p| and on
# padrows crossed. For each cut, I have checked, and the difference
distribution for each component looks (roughly) Gaussian, so I characterize
the resolution with a RMS.
Another point, in these simulations the target is OUT. I
wanted to see what the effects other than MCS and straggling in target
was. I take care of them seperately.
Here are the results. Black stars are when the vertex refit is on, and
red is when it is off. (Thanks to Chris for bringing this back to life!)
Plotted is the RMS of (R'i-Ri)/Ri, where
R' is reconstructed, and R is input, and i=x,y,z. This means, for example
that in the plot for Rx, the resolution for long tracks
goes from ~ 1.3 percent
of the total momentum to ~0.5% of the total momentum
starting at about 0.5 GeV.
So, when we correct the correlation functions using the parameterized resolution,
how do the fit parameters compare with our
previous estimates using a 1% and 3.5% tracking resolution?
Here are the plots. The point at "0" is raw (uncorrected), and the point at "-1" is using the