p 0 analysis

1. p0 analysis I.Larin PrimEx Collaboration meeting

2. First step: data sample selection Runs with unclear detector status or suspicious beam conditions were rejected for this stage of analysis. Estimated total flux for selected 39 runs (with beam trips cut off) is ~704.3B I.Larin PrimEx Collaboration meeting

3. Second step: alignment, etc. Important for analysis parameters have been checked and tuned: • Timing: “tdif” has been aligned for each particular T-channel using spectrum for elastic p0s • Beam position has been checked using “ALIgnment with Single Arm Compton” (stability) and p0 alignment (once per all group of runs), no correction was required. • Preliminary p0-calibration has been applied (might be done better with LMS-correction) I.Larin PrimEx Collaboration meeting

4. Third step: kinematical fit • Kinematical fits were applied: • Mass constrain: to improve p0 energy resolution • Elasticity constrain: to improve p0 mass resolution and to reject background p0s • Both constrains are to be applied to improve production angular resolution (needs tests and simulations). I.Larin PrimEx Collaboration meeting

5. Fourth step: event selection • Timing cut (tdif) ±4ns • Eg > 0.5GeV • Area behind absorber (4x4 modules) was excluded • p0mass cut 100MeV was applied for pre-selected event sample (skim files) I.Larin PrimEx Collaboration meeting

6. p0 analysis: variables • Variables to be analyzed for selected events: • R – elasticity (all p0s in analysis supposed to be elastic with known beam energy (properly tagged). Deviation of parameter R from 1 comes from energy resolution, misidentified whether p0 or beam or inelastic p0 . • M – invariant mass of two clusters (any deviations from 134.976(6)MeV are from Hycal resolution or non-p0 (mostly Compton) background. • q– production angle. This variable allows to separate different p0 production mechanisms and extract Primakoff part of cross-section. Experimental resolution on this variable is important as well as for example precision flux measurement. I.Larin PrimEx Collaboration meeting

7. p0 analysis: methods R M q Method #1: • Select signal and few background bands on variable R (elasticity) • Then make slices on variable q(production angle) • Do the fit of M distributions (invariant mass) for each R band and for each q slice • Construct dN/dq distributions for number of p0s in the mass peak for each R band • Subtract from signal R band angular distribution properly scaled sideband R distribution (it is necessary to fit elasticity distribution given by number of p0 entries to get a proper scaling factor) This works fine. Potential problem is that dN/dq-distribution for background under the signal and background in the sideband region may be a bit different. Thus subtraction procedure will disturb obtained shape of the signal production angle I.Larin PrimEx Collaboration meeting

8. p0 analysis: methods R M q Method #2: • First make slices on variable q(production angle) • Then make slices on variable R (elasticity) as narrow as possible to have enough statistics in each slice • Do the fit of M (invariant mass) distributions for each (R, q) slice • Then fit R distributions given by number of p0 entries for each q slice Switching the order of the analyzed distributions gives less systematcis for q-spectrum, but statistics for each elasticity distribution is much less. That gives additional problems with Signal / Background separation and associated with it systematcis I.Larin PrimEx Collaboration meeting

9. Contributions to error bars of dN/dq distribution: Contaminated by Compton area Each bin of this distribution is a result of mass plot fit Area of the maximal uncertainties from sideband subtraction I.Larin PrimEx Collaboration meeting

10. p0 analysis: combining variables (elasticity fit or mass constrain) MC Examples of mass and elasticity distributions for PWO R M or X RM I.Larin PrimEx Collaboration meeting

11. After combining variable we can introduce Method #3: R X M MC MC q • First make slices on variable q(production angle) • Then make slices on variable MC(mass with elasticity constrain) • Do the fit of MC distributions for each q slice • Variable MC in principle can be replaced by RM (elasticity after mass constrain) in this method • Working with only 2 distributions (less systematics) • Mass resolution is improved (better signal / background rejection) • Inelastic p0 and accidentals are rejected (less problems with sideband subtraction) Needs simulations for more systematics checking I.Larin PrimEx Collaboration meeting

12. p0 analysis: cross-section • Preliminary result, further analysis and simulations are to be done ds/dqmeas. x efficiency (qmeas.) All Hycal PWO-only I.Larin PrimEx Collaboration meeting

13. Applying efficiency curve and fit: • Ashot’s results of the fit to p0 width Used efficiency curve: ds/dq 0.040 binning: ds/dq 0.020 binning: I.Larin PrimEx Collaboration meeting

14. p0 analysis: what’s next? One of important issues for current stage of analysis is background subtraction Here are some of background sources in our data: • When we select event for analysis it may happen that other beam particle also produced signal in HYCAL . • We may pick up accidental event: Hycal totalsum signal may accidentally coincide with MOR. • We may pick up for event reconstruction wrong candidate from Tagger hit bank (we normally have many Tagger candidates even within narrow timing window). • Unlike Compton, p0 also has inelastic "branch". Even if we have a real coincidence between Hycal and Tagger. • Background from other physical processes of beam interaction. I.Larin PrimEx Collaboration meeting

15. Summary • After consistency check with the Compton data we are focusing on p0 analysis • We are using kinematical fit in our analysis • There is a preliminary agreement between current p0 width value and results of the data fit (~7.5 eV) • This work is in progress. The most important item for this stage of analysis is background simulation. • Combinatorial background from wrong beam candidates have not been subtracted yet. This will be done after simulations of this effect. I.Larin PrimEx Collaboration meeting

16. Spare slide: p0 (with elasticity constrain) mass plot shape • The shape of the background on the "regular mass plot" comes from physical effects and looks quite natural. For "mass with elasticity constrain" the situation is a bit different. Procedure of elasticity constrain assumes that pi0 is elastic and tagged (i.e. its energy is equal to tagger measured beam energy). The Primex experiment is taking such pi0's for analysis. Rough estimation shows that about 10% of all recorded pi0's on Primex are elastic and tagged. It means that about 90% pi0s on "normal mass plot" are not elastic and tagged. They are not good for lifetime analysis so we can say "we don't care about them" or "we don't care about proper calculation of their momenta, mass etc". What kinematic constrain procedure really does?: It improves reconstructed mass (or momentum) for "good" pi0s as much as we can only see now effect of coordinate resolution, affecting mass resolution. For other pi0's this procedure wrong, since we're assigning wrong energy. Most of them have energy less (or much less) than beam energy. The energy spectrum could be seen on the elasticity plots. Kinematical constrain procedure increases their energy up to beam energy (which is wrong now). The invariant mass is proportional to square root of their energies. So now their masses are greater than table value. In other words: inelastic and mistagged pi0's from "regular pi0 mass" peak "go" to the background (which is now mostly on the right part of mass plot). We can see only tagged elastic pi0's in the mass peak now (with improved resolution). Also number of pi0's in the peak on the "mass plot with tagger energy constrain" is much less than on the "regular mass plot". I.Larin PrimEx Collaboration meeting