A Precision Measurement of the Neutral Pion Lifetime

A Precision Measurement of the Neutral Pion Lifetime via the Primakoff Effect Jefferson Lab Experiment E 99 -014 Prim. Ex Collaboration Eric R. I. Clinton Rory Miskimen, Ph. D. , Advisor Given at NNPSS, June 15, 2005

The goal of the Prim. Ex Collaboration n Make a high precision measurement of the po to gg decay rate dominated by the axial anomaly. n This will be done using the Primakoff Effect, photopion production in the Coulomb field of the nucleus. n Using the tagged photon facility in Hall B, a state of the art Hybrid Calorimeter, and well understood nuclear targets, the collaboration hopes to measure the Primakoff cross section to 1. 5% or better.

The Chiral Anomaly

What is the physics motivation? n In the Chiral limit where the u and d quark masses vanish, we can exactly compute the decay amplitude. Agg = a. Nc / ( 3 pfp ) n And we know the decay width of the po to gg. GCL= mp 3|Agg|2 / ( 64 p ) = 7. 725 +- 0. 044 e. V

More physics motivation n Real world quark masses are not 0 Me. V n n Adler and Bardeen n Non-renormalization theorem gives quark mass correction. The po mixing with h and h’ -- 2 -3% increase in the decay width. Smaller corrections (~ 0. 001) n n Mass of u, d quarks on order of 5 -7 Me. V. Saturation of the chiral anomaly by heavier mesons. Thus, G(po to gg) is the most accurate prediction in QCD, depending only on the number of colors.

Recent Theory Developments n Joity, Bernstein, Donoghue, and Holstein n n Calculated NLO corrections to p 0 width. Result: GNLO = 8. 10 ± 0. 081 e. V GCL = 7. 725 +- 0. 044 e. V GPDG = 7. 84 ± 0. 556 e. V n Mousaallam et. al. have a very similar result.

Other measurements of the po decay. -There are three reactions that allow us to conveniently measure the po decay width. n n n “Direct measurement” through time of flight data gg collisions The Primakoff Effect.

The Primakoff Effect n The Primakoff Effect is photopion production in the Coulomb field of a high Z nucleus.

Previous Primakoff Experiments n Browman et al. n G = 8. 02 ± 0. 42 e. V n n n h decay widths measured using a similar set-up and analysis yielded questionable results. DESY n n n (no luminousity or detector efficiency uncertainties included). 1. 0 Ge. V beam---- G = 9. 02 ±. 95 e. V 1. 5 -2. 0 Ge. V beam----G = 11. 7 ± 1. 2 e. V. Tomsk n 1. 1 Ge. V beam---- G = 7. 32 ± 0. 5 e. V.

Prim. Ex at Jefferson Lab n Tagged Photon Facility at JLab n n n Increase precision and get cleaner kinematics. The invariant pion mass will be deduced by detecting the neutral pion decay g’s. Good neutral pion detector n n Identify nuclear coherent and multi-photon background contributions Provide good energy and angular resolution.

Prim. Ex, continued n n The Primakoff cross section for unpolarized photons is d 3 sp/d. W = (Ggg 8 a. Z 2 b 3 E 4|Fem(Q)|2/m 3 Q 4)sin 2 qp Strong maximum in the Primakoff CS at q ~ mp 2 /(2 Ep 2) n n Falls off rapidly at higher angles. That’s not the whole story n Nuclear coherent and incoherent processes and a cross term between NC and Primakoff effect contribute to the cross section.

Error budget n n n n Statistical 0. 4% Target Thickness Photon Flux po detector acceptance and misalignment Background subtraction Beam energy Distorted form factor calculation errors 0. 3% Total Error 0. 7% 1. 0% 0. 4% 0. 2% 1. 4%

Conceptual Prim. Ex set-up

Photon Tagger

The Hybrid Calorimeter n Hy. Cal is a highly segmented array of lead tungstate and lead glass crystals n n 7. 5 meters downstream of the targets. The interior array of crystals n n The outer array n n 1152 lead tungstate modules, each 2. 05 x 2. 05. 18 cm 3, 20 Xo, and 2. 0 cm Moliere radius. 576 lead glass modules, each 3. 84 x 3. 84. 45 cm 3 and 17 Xo. The small size of the interior detectors means very fine angular resolution (better than 0. 02 o) and energy resolution (~3. 5 Mev).

Final Thoughts n The Prim. Ex took date from September 2004 to November 2004. n We see lots of pi-zeros. n Good statistics on Carbon, fair statistics on Lead. n n Preliminary analysis, calibration, and bookkeeping stage. Then, about 1. 5 years from now, I graduate. n I’m crossing my fingers…