1. ISSUES AND SYSTEMATICS ASSOCIATED WITH FIDING LOW-ENERGY PHOTONS WHEN RECONSTRUCTING  0 s Luke Winstrom Al Eisner Bruce Schumm

2. Preliminary issue: How do we know when a  0 ’s LE photon is “found”?? In fact, what we really want to know is when some neutral object is reconstructed with properties close enough to that of the LE photon that, when combined with the HE photon, it falls within the  0 mass window. For now, as a proxy, call a LE photon “found” if it has a “proximity match” (a neutral object close in both energy and angle to the true LE photon). Can perhaps confirm/correct this by looking at “best  0 mass” distributions for “found” and “missed” photons (underway).

3. First study: How many LE photons miss CAL? How many go in endcap? Procedure: 1) Make list of all HE gamma candidates that pass all photon quiality cuts except  0 consistency. 2) Select those that are truth-matched to a photon from a  0 3) Identify the other (true) daughter of each truth-matched HE, and study its properties. Based on point of impact at the CAL surface: In Barrel 88.8% In EndCap 6.8% Missed Cal 4.4% For the rest of this presentation, we’ll consider only the Barrel.

4. What happens to the 88% of LE photons that are headed towards the barrel? Below 30 MeV Good Photons Loose cut * (“lost”) 13.9% Interact before DIRC (“lost”)5.0% Pass through CAL (“lost”)0.8% Interact in DIRC, but no prox match (“lost”) 5.0% Interact in DIRC, with prox match (“found”) 8.0% Convert in CAL (some “lost”, some “found”) 67.3% TOTAL100% of the 88% in Barrel * Apply hard cut on MCTruth; ignore reconstruction in smearing

5. So, 67.3% of LE photons that are headed towards the barrel convert in the barrel. What happens to those depends upon the degree of isolation. Isolation measured relative to true photons (beam background photons don’t count) Degree of IsolationFraction of Total Efficiency for proximity match No photon w/in 40cm26.5%95.8% HE  w/in 40cm 21.0%90.8% Other  w/in 40cm 8.4%76.1% HE and other  w/in 40cm 11.4%79.5% ALL67.3%89.0% * * Had been ~91% in prior studies, which had used cut (40 MeV) well above 30 MeV CalorNeutral requirement

6. This allows us to assess what we’re up against: how well do we understand each of these effects? All in all, efficiency for finding LE gamma, including the inefficiency for LE gammas that miss CAL, is about 63%. How well do we know this number?  In what follows, “size of effect” is amount by which this 63% of found LE photons would increase if effect disappeared. “Accuracy requirement” is relative accuracy to which effect must be known in order to not contribute more than 1% sys error to b  s  rate meaurement. In the Accuracy figure, I use an estimate that a 1% change LE gamma efficiency will produce a 1.5% change in the background subtraction (a LE photon that unwittingly gets transferred from the measured to unmeasures region both decreases the subtraction and increases the background!)

7. EffectSizeRequirement Misses CAL7%10% Energy too low22%3% Converts before DIRC8% “Lost” in DIRC8% Isolated LE  inefficiency * 4%15% Confusion and overlap 6%10% * Compensating for effect of energy resolution at 30 MeV boundary, but including photons that pass through CAL as inefficiency. Again, “requirement” is relative accuracy to which effect must be known in order to not contribute more than 1% sys error. CHALLENGING COMPONENTS