Report on p 0 decay width: analysis updates

1. Report on p0 decay width:analysis updates

2. Analysis updates: • Clock frequency correction . Flux is ~2.4% up • Tagger energy corrections were introduced • More statistics added: 130nA and 110nA runs were added to previously used 100nA runs. Statistics increased by factor of 2 and stat. err dropped from 0.28eV to 0.18eV

3. Analysis updates: • Error budget was updated • Efficiency was recalculated (non-resonant gg background was added) • Background from w and r± decays was subtracted from p0 yield • To make sure that skim data are not biased, raw data files were analyzed. p0 yield fit parameters including lifetime are stable

4. Response function of Hycal: Observed elasticity (i.e. EHycal/ETAGGER ratio) during snake scan when beam was centered on W2016 What do we measure during snake scan ? Shape of experimentally observed response function: *) resp. function itself *) accidentals (with false Tagger hits) *) electronics noise *) “leakage”

5. How to distinguish (tail and zero) of response function from the background. (Can we do it in the future run to decrease our systematics ?) Put TAC behind Hycal during snake scan

6. Response function (MC) with and w/o TAC during snake Hycal only Hycal with TAC TAC itself has energy resolution (MC) ~ 7%/Ö E

7. To study response function it was split into elasticity regions: X = min – 0.2 X = 0.2 – 0.5 X = 0.5 – 0.8 X = 0.8 – 0.9

8. Fraction of events in elasticity tails was estimated for runs with different beam current: 20, 30, 50, 60, 95 pA Hycal resp. function itself is assumed to be independent from beam current. Asymptotic values from the following simple fit were used as a most close to the real values: Y = c1/x + c0

9. What does MC reproduce for response function: Snake scan Centered beam

10. Data VS MC (percentage of events in the tail of response function): Difference between data and MC ~ 0.5 – 0.9% 1.3GeV 4.4GeV

11. Snake scan data might be used as a unique measure of wrapping thickness between modules.Any other measurements of this thickness could not have enough accuracy. Tail of response function comes mostly from wrapping:

12. “tagging ratio” during snake, MC: “tagging ratio” means number of events with enough energy and position close to beam impact point divided by number of total events with normal beam Elasticity cut 0.7 Elasticity cut 0.4 edges / wrapping edges / wrapping

13. “tagging ratio” during snake, data: Dips near edges may slightly vary from one module to another Elasticity cut 0.4 Elasticity cut 0.7 edges / wrapping

14. Estimated contribution from response function to error budget • Systematic error comes from inconsistency between data and MC response functions (not from response function shape itself). • Since MC may underestimate tails of response function up to 0.9%, we have to introduce correction factor of 0.995 for MC efficiency and 0.5% sys. error comes from this item. • More precise measurement of Hycal response function would decrease this uncertainty

15. Lifetime sensitivity to single gamma energy cut value Method 1: we have to vary cut value and check how result (with no additional correction for efficiency) depends on it. 10% variation from cut value (0.5GeV) was used as a probe. This cut variation is changing result by 0.2% Cut variation Used cut

16. Lifetime sensitivity to single gamma energy cut value Method 2: vary cut value and check how result (with corrected efficiency) depends on it. Final result shows 0.2% fluctuations from the mean value (it is probably statistical only fluctuations)

17. Lifetime sensitivity to single gamma energy cut value Conclusion: error budget contribution from this item is £0.2%

18. Contribution from misalignment: • Beam VS Hycal position uncertainty: 0.2mm uncertainty value gives negligible syst. Error • Beam slope stability: 0.12mrad uncertainty value gives syst. error £ 0.1% • Beam width stability: varying beam parameters according to superharp scan data gives 0.3% syst. error • Hycal Z uncertainty: 1.5cm uncertainty value gives 0.4% syst. error

19. w and r± sources of p0 background were subtracted

20. The following sources of quasielastic p0s were studied: • Coherent w: g A -> w A • Incoherent w: g A -> w N A’ g A -> wD A’ • Incoherent r±: g A -> r± N A’ g A -> r±D A’ With consequent decays: w/r0 -> p0g; w -> p0p-p+; r± -> p0p±

21. Simulations:t’ distributions for w and r production used t’ = |t – tmin|

22. Simulations:production angle distributions for w and r used

23. Simulations:cos(qGJ) distributions for w and r used 1+x2 1-x2

24. Distributions obtained from MC were scaled and subtracted from the data Scale factor for MC: Kscale = F data / FMC FMC = Nevent NC / (L Br s) L – number of atoms per sq. unit Br – decay branching ratio s – production cross section

25. p0 elasticity distribution observed (crystal part of Hycal) with simulated background from w and r+ - w and r+ - contribution r+ - only contribution

26. p0 elasticity distribution with subtracted background from w and r+ - Elastic peak parameters improved

27. dN/dq for original data and with background subtraction (on the level of gg invariant mass spectrum, bin by bin basis) original rad. width changes by ~ -0.8%; more calculations will be performed Subtracted part

28. Non-resonant 2g background was added to the MC • Old method: number of reconstructed p0 events was calculated in mass window • New method: background with linear shape and line parameters obtained from the data (individually for each T-counter and each q-bin) was added to MC distributions and the same fitting procedure was performed as for the data data MC

29. Non-resonant 2g background affects on efficiency New efficiency curve for 5.2GeV Difference between new and old efficiencies Rad. Width goes +1%

30. New formfactors (with more advanced calculations) were introduced into analysis

31. Imaginary part of formfactors were introduced: ds/dq interference = Ö [ cos(f)· ( Re(Prim)· Re(Coh) + Im(Prim)· Im(Coh) ) +sin(f) · ( Re(Prim) · Im(Coh) + Im(Prim)· Re(Coh) ) ] Interference term was modified: sin(f) appeared

32. c2 was estimated for Primakoff region (0o…0.5o) separately to make sure quality of the fit in most sensitive region. Reanalyzing lifetime with all corrections applied: Old formfactors: (Primakoff region (0o-0.5o) c2 = 1.12 New formfactors: (Primakoff region (0o-0.5o) c2 = 1.05 Better agreement

33. Error budget table • Target (thickness and density) ……... ………. 0.1% • Target material (impurity) …………... ………. -0.4% • Photon beam flux ……………………. ……… 1.1% • Trigger efficiency ……………………... ………. +0.1% • ADC channels status during run time ………. negligible • Photon beam energy uncertainty ……..... 0.3% • Photon beam flux distribution within E-counters ……………………... ….…… 0.1% • p0 branching ratio …………………….. ………. Negligible • Energy cut for single g ……………….. ………. 0.2% • Energy cut for p0 ……………………... ………. negligible • p0 production angle resolution uncertainty ………. 0.25%

34. Error budget table • MC eff. simulations (statistical) accuracy …… 0.3% • Selection of “best in time” beam candidates .. 0.3% • Hycal z uncertainty ……………................ 0.4% • Beam position uncertainty ………....................... negligible • Beam direction uncertainty ………..................... +0.1% • Beam divergence uncertainty ……….................. 0.3% • Timing cut (tdiff) ………................ 1.0% • Hycal response function ………................ 0.5% • Background separation: (currently used value) . 1.0% • Uncertainty in theoretical parameters ………. ?

35. Summary • Sources of background were taken into account (most of them) • Error budget is close to be completed • Rad. width (to be updated): 8.18eV ± 2.2% stat. ± 2.1% syst.

36. Plans: • Update incoherent amplitude: • ds/dq shape update • Include Fermi motion (production angle and energy spread) ? • Introduce different efficiency for primakoff /coherent and incoherent production • Check lifetime stability to variations of formfactor parameters. More formfactor updates will be performed by Sergey • Use of LG part of Hycal for better incoherent estimation

37. Spare slides

38. Fermi motion angular smearing(p0 production ): Simulated smearing for q = 0.25o, 1o and 2o dw/dp (Carbon)