User name A. Deur
Log entry time 11:31:12 on August20,2003
Entry number 112093
keyword=2-step background for E=1.147 GeV
I analyzed the empty ref cell runs of E=1.147 GeV.
It appears that when collimators are on, the windows are very well cut away. Furthermore, there is essentially no pions for all the kinematics. So all we see in empty ref cell runs is 2-step process background, i.e events that undergo radiative elastic scattering at the beam pipe Be-window at very forward angles (step 1) and then cross the He3 cell wall with sharp angles and undergo there non-radiative elastic scattering (step 2). So these events appear as coming from the cell itself even if this one is empty.
In fig 1, I plot the yield (arbitrary units) vs the energy loss (in GeV). We clearly see the exponential rise with nu predicted by the simulation (see technote www.jlab.org/~deurpam/e97110/tail_bg/stragtail.ps.gz ).
The first point at nu=0.07 is abnormal (we can see that with fig2).
In fig 2, I plot the charge normalized y_t spectrum for each energy loss. The highest spectra are the ones at large nu. We can see here also the exponential increase of yield with nu (fig 2 is in log scale ).
The 2 lower spectra are at the same momentum. That's why they overlap.
The abnormal spectra corresponding to nu=0.07 is the black line with large wings. It should have been the lower spectrum. The much larger wings tell that the run condition was different (beam drift ?, beam angle ?, something wrong with reconstruction for this run ?). These large wings are not coming from the cell glass window (I checked with no-collimator empty ref cell run 3524) but further away.
We note also that the maximum of the spectra moves toward lower yt values (i.e. upstream window) with increasing nu. This is making sense: The yield is determined by 3 factors:
*the cross section at the Be window
*the amount of energy radiated before to cross the cell wall
*the amount of glass crossed i.e the angle with which the e- cross the glass
When we are at low nu, the first 2 factors are relatively small so it is the amount of glass crossed that makes the yield to be big i.e we tend to see events closer to the downstream window (smaller angles). The yield increases of course linearly with the amount of glass crossed. When we are at large nu, the two first factors increase and the yield increases exponentially with each of these factors, so they dominate at large nu and we tend to be peaked at the middle of the yt acceptance since the 3rd factor is not driving the yield anymore.