The problem of spatial digitization in 2DH

Suggestion: Go directly to the conclusions and list of figures

Problem?.... What Problem?

At the July collaboration meeting, the point was made by several people, notably the SUNY crowd, that the hit position coming from the 2D hitfinder is "discrete" in y. That is, it is clustered about the center of the time bucket position. This presents a problem in making t0 tables from the data, and in topological searches. It has also been known that the resolution (as measured by RMS of xdev_fit and ydev_fit) is worse for 2DH than for 1DH.

What's going on?

As is well known, in 2DH we make 2 passes at the data. In the first pass, the position of the hit is guessed at, and in the second, that guess may be refined, and the dE is calculated using information from the tracker.

The upshot is that if our initial guess has "digitization" in it, then the final positions will be digitized as well.


In 2DH, the initial guess at position (in 1st pass) was just the weighted mean in pad and time of the "peak pixel" and its 8 closest neighbors in 2 dimensions. This led to digitization in x and y, but mostly in y. These plots were made with a 2 A GeV event.

Conclusions (prior to fixing the problem):

  • 2DH+BTF uses about 15% more hits in this event than 1DH+BTF. This is consistent with previous studies.
  • 2DH gives about 10% better resolution in x than 1DH.
  • 2DH gives about 25% worst resolution in y than 1DH.
  • Both 2DH and 1DH show significant digitization in x, of very different structure.
  • 2DH has much worse digitization in y than 1DH.


    The initial guess has been made a little more sophisticated, so that a 3-point fit to the y-projection is made, using the shaper response function

    P(t) = ((t-t0)/tau)**N * exp(-(t-t0)/tau)

    where N=4 for E895.

    Also, a 3-point fit to a Gaussian is made to the x-projection. This gives just about the same result as a weighted mean.

    Especially the first change makes a *big* difference. See the plots here. Again, you may want to compare with how the 1DH does.



    The spatial digitization problem in 2DH has been fixed.
    Using 1DH as a benchmark:

    The worse y-resolution as compared to 1DH will be studied more (it may be due to using more hits). But my conclusion is that the code is fixed enough to say it is "ready."


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