2007.07.25 Revised gamma/pi0 algorithm in 2006 p+p collisions at sqrt(s)=200 GeV



Revised gamma/pi0 algorithm in 2006 p+p collisions at sqrt(s)=200 GeV


The class


computes the maximal sided residual of the SMD response in the u- and v-plane for gamma candidates. It is based on C++ code developed by Jason Webb from the original code by Les Bland who got the idea from CDF (?) The algorithm follows the steps below:

  1. The SMD response, which is SMD strips with hits in MeV, in each plane (U and V) is stored in histogram hU and hV.
  2. Fit functions fU and fV are created. The functional form of the SMD peak is a double-Gaussian with common mean and fixed widths. The widths were obtained by the SMD response of single photons from the EEMC slow simulator. As such, the only free parameters are the common mean and the total yield. The actual formula used is: [0]*(0.69*exp(-0.5*((x-[1])/0.87)**2)/(sqrt(2*pi)*0.87)+0.31*exp(-0.5*((x-[1])/3.3)**2)/(sqrt(2*pi)*3.3))
    • [0] = yield
    • [1] = mean
  3. The mean is fixed to the strip with maximum energy and the yield is adjusted so the height of the fit matches that of the mean.
  4. The residual for each side of the peak is calculated by subtracting the fit from the data (residual = data - fit) from 2 strips beyond the mean out to 40 strips.
  5. The maximal sided residual is the greater residual of each side.


Candidates selection

  • 2006 p+p at 200 GeV dataset from Sivers analysis (from Jan Balewski)
  • Gamma candidate from gamma maker: 3x3 clusters with pt > 5 GeV
  • No track pointing to cluster
  • Minimum of 3 SMD hits in each plane
  • Cuts from Jan & Naresh electron analysis:
    • Preshower 1 energy > 0.5 MeV
    • Preshower 2 energy > 2.0 MeV
    • Postshower energy < 0.5 MeV
  • The triggers caption in the PDF files shows the trigger id's satisfied by the event. A red trigger id is a L2-gamma trigger. I observe that generally the L2-gamma triggered event are a bit cleaner. Also shown is the pt and energy of the cluster.

Raw SMD response

  1. No additional cuts
  2. Pick only L2-gamma triggers
  3. Pick only L2-gamma triggers but no jet patch trigger
  4. Make isolation cut (see below)

The parameters of the isolation cut were suggested by Steve Vigor:

Hi Pibero,

  In general, I believe people have used smaller cone radii for isolation
cuts than for jet reconstruction (where the emphasis is on trying to
recover full jet energy).  So you might try something like requiring
that no more than 10 or 20% of the candidate cluster E_T appears
in scalar sum p_T for tracks and towers within a cone radius of
0.3 surrounding the gamma candidate centroid, excluding the
considered cluster energy.  The cluster may already contain energy
from other jet fragments, but that should be within the purview of
the gamma/pi0 discrimination algo to sort out.  For comparison, Les
used a cone radius of 0.26 for isolation cuts in his original simulations
of gamma/pi0 discrimination with the endcap.  Using much larger
cone radii may lead to accidental removal of too many valid gammas.


Pibero Djawotho
Last updated Wed Jul 25 10:07:07 EDT 2007