A Precision Measurement of the Neutral Pion Lifetime via the Primakoff Effect Eric R. I. Clinton University of Massachusetts Amherst On behalf of PrimEx Collaboration (Jefferson Lab Experiment E99-014) Duke University Seminar May 31, 3007

Outline Where I’m going, and how I intend to get there. Who we are and a brief history of PrimEx Physics Theory and Motivation Some of World Data Photo-nuclear and other physics measured by PrimEx Compton, pair production as calibration reactions Experimental Set-up I’m an experimentalist. Go figure. Data Analysis Calibration reaction results How other analysis groups looked at the pion data My analysis Simulation Acceptance correction, resolution effects Results Radiative width Error Evaluation Summary of collaboration  0 width measurements

The PrimEx Collaboration

History of the PrimEx collaboration January of 1999, the TJNAF PAC approved #E with an “A-” rating. #E is more commonly known as PrimEx. In June of 2002, the PAC upgraded #E from and “A-” to “A” rating. PrimEx's main goal The measurement of the neutral pion decay width. Data was collected in Fall 2004 on 208 Pb and 12 C targets 12 C results will be shown. Measure Primakoff cross section to 1.5% or better.

PrimEx Pions for Dummies This includes me.  0 discovered by Bjorklund, Crandall, Moyer and York at Berkeley Cyclotron in Pre-QCD,   0→  = 0 Primakoff Effect Predicted Henry Primakoff (Phys. Rev 81, 899, (1951)) The Primakoff Effect: Photo-pion production in the Coulomb field of a high Z nucleus

Physics Motivation Adler, Bell, Jackiw and Bardeen discover triangle diagrams that alter PCAC predictions for   decay Post QCD, O(p 4 ) anomalous Lagrangian is constructed by Wess, Zumino and Witten. This permits transitions between even and odd numbers of pseudo-scalars The (zero order, m u,d = 0 ) decay width of the  o →   Real world quark masses are not -0- MeV Mass of u,d quarks on order of 5-7 MeV. p k1k1 k2k2

Corrections Zero-eth order width = eV Adler and Bardeen -- Phys. Rev. 182, (1969) Non-renormalization theorem gives quark mass correction Bijens & Prades, Z.Phys. C64 (1994), 475  and  ’ mixing % increase in width Goity, Bernstein, & Holstein -- Phys. Rev. D66, , 1-10 (2002) NLO Theory Calculation ± eV. For comparision, PDG width = 7.84 eV. Thus,  (  o →  ) is the most accurate prediction in QCD, depending only on the number of colors.

World Data

Photo-nuclear processes in PrimEx Primakoff Coherent Incoherent Interference See last term in Total CS Total Cross Section -- --F e.m.(Q) pion electromagnetic form factor, F N (Q) is the nuclear matter dist. form factor (both corrected for FSI). Csin 2   is the isospin and spin dependent part of  0 photoproduction in a single nucleon, 1-G(Q) reduces cross section at small momentum transfer (Pauli principle), and dsH/dW is p0 photoproduction on a single nucleon.

Compton and Pair Production Compton cross section Klein-Nishina, radiative corrections, all in hand “Calibration reaction” Use HyCal to detect and measure photons and electrons Significant result in unto itself New 5 GeV Compton measurement Pair production Measure pair production during physics runs Pair production rate is a monitor of relative photon flux Another “Calibration reaction” Use HyCal to detect e - and e + co-incidences

Error budget Statistical0.4% Target Thickness0.7% Photon Flux 1.0%  o detector acceptance and misalignment0.4% Background subtraction0.2% Beam energy0.2% Distorted form factor calculation errors0.3% Total Error1.4%

Conceptual PrimEx set-up

Photon Tagger Post bremmstrahlung electron momentum analyzer 384 “E- counters” provide energy information Accessing only the highest energy photons ( GeV) Energy resolution is 0.1% for GeV photon beam. 61 “T-counters”, provide timing information Only the highest energy 11 T-counters and 56 E- counters active. Sub nanosecond timing resolution 1% or better uncertainty photon flux

The Bullseye What are we shooting all these photons at? 12 Carbon Target Highly Order Vacuum Deposited Pyrolytic Carbon Mouthful for VERY homogeneous, pure 12 Carbon At UMass, PrimEx targets were measured Carbon (1in x 1in x.996cm) Micrometer and water displacement density measured Outsourced elemental analysis Result -- Pyrolytic Carbon is VERY homogenous, pure 12 carbon Error on # atoms/cm 2 = 0.04 % The Carbon targets Electron micrograph of carbon target

Pair Spectrometer Pair Spectrometer constructed for PrimEx and Hall B The PS must have a flat acceptance curve, and be placed with overlapping momentum acceptance. Dipole magnet will sweep pairs from the physics targets into the PS Pair production rates will be used to calibrate relative photon flux. Total Absorption Counter for absolute flux measurement

The Hybrid Calorimeter Highly segmented array of lead tungstate and lead glass crystals 7.3 meters downstream of the targets. The interior array of crystals 1152 lead tungstate modules 2.05x2.05.x18 cm 3, 20 X o, and 2.0 cm Moliere radius. The outer array 576 lead glass modules 3.84x cm 3 and 17 X o. Small detector size Fine angular resolution (~ 0.02 o ) Energy resolution (~3.5 Mev). Identify multi-photon backgrounds Charge Particle Veto Counters Offline removal of charged events

Data Analysis We've had data for 2 years now... And all I'm getting is this lousy Ph.D Let’s define a few key terms... Photon Flux Number of Photons on physics target Tagger Energy Energy of photon inferred from bremm'ed electron Cluster Energy Energy deposited in/measured by HyCal in event. Invariant Mass  Invariant mass from reconstructed 4-vectors of decay photons Elasticity ratio a cluster pair energy sum and tagger energy Hybrid Mass Projection of 2-D elasticity and invariant mass onto new axis

Photon flux Since it’s the largest bit of our error budget

Compton Signal and radiative Effects RADIATIVE EFFECTS IN THE DATA COMPTON SIGNAL

Compton Preliminary results with radiative corrections Radiative corrections: I. virtual loops, and soft double Compton scattering, Brown and Feynman; II. hard double Compton scattering Mork, and Mandl and Skyrme Summary of Compton analysis Agreement with theory at the level of few % Work in progress to reduce the systematic errors to 1 – 2 % level

Pair production Signal and differential cross section Theory by Alexandr Korchin Summary of pair analysis Agreement with theory ~3.8 % Work in progress to reduce the systematic errors to 1 – 2 % level

I. Larin’s Approach --Kinematic fitting to the condition Elasticity =1

D. McNulty’s approach --Elastic and Inelasltic yield extraction To get elastic yield, project data below onto Invariant Mass axis a function of  0 production angle Then extract the Inelastic yield by fitting slices in elasticity and plotting extracted yield

My Method Project events onto axis perpendicular to the kinematic correlation between Elasticity and M  for elastic events Fit peak in mass distribution

Sample "Hybrid Mass" fits

Preliminary  0 Cross Sections Efficiency/Acceptance Uncorrected

Monte Carlo Gambling time MC used efficiency calculations Theoretical lineshapes thrown at simulated HyCal Primakoff, nuclear coherent and incoherent, and interference MC data "conditioned" to look like physical data Energy smearing, electronic noise, resolutions built in Then, the data got the "full treatment" Run thru event selection, same cuts... Geometric and Reconstruction Efficiencies As function of photo-nuclear process and  0 angle

Ta-da!!  0 radiative witdth = eV ± 2.0% (Stat)

Efficiency Corrected Differential Cross Sections Mine, not the other guys'

Larin and McNulty’s results I. Larin’s Results No Parking. This space reserved for D. McNulty.

Error on radiative width... Systematic Errors (major contributors) Photon Flux1.1 % Lineshape uncertainty1.0 % Integration cutoffs1.1 % Cluster position finding0.33 % Veto counter conversion0.25 % Incoherent background shape*1.5 % Total2.4 % *Estimate from I. Larin’s analysis. Work in progress to minimize this error

Summary  (  0  ) = 7.93eV  2.1% (stat)  2.0% (syst) Lifetime: (8.20±0.24)x10-17 sec PDB average: (8.4±0.6)x10-17 sec

Future work towards a final result and publication Incoherent process Discussion regarding shape of incoherent Determine final result Evolve cross section to single photon energy Finish writing and defend dissertation

The 400 lb. gorilla would like to extend Thanks to… Rory Miskimen David Lawrence Mike Wood

Support provided by the DOE and Jefferson Lab in part by NSF MRI grant PHY in part by RFBR Grant