Session LJ: Mini-Symposium: Light Meson Decays III

Precision Measurement of Neutral Pion Lifetime

As the lightest and the simplest hadronic particle, π⁰ plays a crucial role in understanding the symmetries of QCD at low-energy. The π⁰→γγ decay width offers a fundamental test of the QCD predictions based on the chiral anomaly and spontaneous chiral symmetry breaking. The theoretical calculations over the past two decades have reached 1% precision in the decay amplitude of the π⁰ into two photons. The experimental measurement of this parameter with a comparable accuracy will provide a stringent test of QCD. The PrimEx collaboration at Jefferson Lab has developed and performed two experiments (PrimEx I &II) to measure the π⁰ radiative decay width via the Primakoff effect. The published final result achieved  a 1.5% total uncertainty on the neutral pion lifetime, the most precise result for this fundamental parameter up to date. The experimental result and its comparison with the theoretical predictions will be discussed.

    

Measurement of the Charged- and Neutral-Pion Polarizabilities using the GlueX Detector in Hall-D at JLab

A central problem of modern physics research is the solution to QCD in the non-perturbative regime.  One method of testing QCD in this low-energy region is by measuring certain structure constants of hadrons — called polarizabilities — that show particular promise of allowing a direct connection to the underlying quark/gluon dynamics through comparison to modern QCD-inspired model calculations, and to solutions of QCD done computationally on the lattice.  This talk will give a status report on our measurement of cross sections for γγ→π⁺π^- and γγ→π⁰π⁰ via the Primakoff effect using the GlueX detector in Hall D at Jefferson Lab, with the end goal of precise measurements of the pion polarizability α_π - β_π for both charged and neutral pions.   

Pion Polarizability 2022 Update

 The electric α_π and magnetic β_π charged and neutral pion polarizabilities characterize the induced oscillating dipole moments of the pion during γπ Compton scattering via the interaction of the γ’s electromagnetic field with the quark substructure of the pion. In particular, α_π is the proportionality constant between the γ’s electric field and the electric dipole moment, while β_π is similarly related to the γ’s magnetic field and the induced magnetic dipole moment. Pion polarizabilities affect the shape of the γπ Compton scattering angular distribution. By crossing symmetry, the γπ→γπ  amplitudes are related to the γγ→ππ amplitudes. The polarizabilities are basic characteristics of the pion, and are therefore of fundamental interest in the low-energy sector of quantum chromodynamics. A stringent test of chiral perturbation theory (ChPT) is possible based on comparisons of precision experimental pion polarizabilities with ChPT predictions. The combination (α_π-β_π) has been measured by: (1) radiative pion Primakoff scattering (Bremsstrahlung of 190 GeV/c negative pions) in the nuclear Coulomb field of the Ni nucleus: π^- Ni → π^- Ni γ, (2) two-photon fusion production of pion pairs γγ→ππ via the e⁺e⁻ → e⁺e⁻π⁺π⁻  reaction at SLAC Mark-II, (3) radiative pion photoproduction from the proton γp→ γπn at MAMI in Mainz. Only the CERN COMPASS charged pion polarizability measurement has acceptably small uncertainties. The COMPASS polarizabilities are in good agreement with ChPT predictions; and by Dispersion Relations with DESY Crystal Ball γγ → π⁰π⁰ data; strengthening the identification of the pion with the Goldstone boson of chiral symmetry breaking in QCD. This status report follows the review by Moinester and Scherer at IJMPA 34 (2019) 1930008, and includes a description of ongoing and planned pion polarizability experiments (COMPASS at CERN, BESIII at Beijing, JLab at Newport News).

    

Status of the PrimEx-Eta Experiment in Hall D at Jefferson Lab

The η→γγ decay proceeds primarily through the chiral anomaly. A precise measurement of this decay width will provide critical input to extract the η-η' mixing angle and the light quark mass ratio, while also improving the precision on all other η-meson partial decay widths. Previous measurements of this decay width through e⁺e^- collider experiments differ from the result of the sole Primakoff experiment by more than 4σ. To address this discrepancy and to reduce the total uncertainty for the decay width, the PrimEx-Eta fixed-target experiment in Hall D at Jefferson Lab (E12-10-011\footnote{Spokespersons: A. Gasparian (contact), L. Gan, I. Larin, A. Somov}) will perform a precision measurement of the η→γγ decay width using the Primakoff method. The first two runs of the experiment were completed in Spring 2019 and Fall 2021, with the third and final run scheduled to finish this Fall. In this talk we will discuss the status of the experiment and the steps toward extracting the radiative decay width. Simultaneous with the η decay width measurement, the total cross section for Compton scattering off the atomic electrons is being measured. This well-understood QED process is used to verify the overall systematic uncertainties associated with a cross section measurment in the forward direction. In this talk we will also present preliminary results for the integrated total Compton scattering cross section.   

Pion and Kaon Form Factor Measurements at the EIC

Developing our understanding of hadronic mass generation mechanisms is one of the three key physics questions for the upcoming Electron-Ion Collider (EIC). From the little that we do understand, we know that mass generation is intricately connected to the internal structure of hadronic systems. Somewhat counter intuitively, it is some of the lightest hadronic objects, the charged pion and kaon, that may be able to fill in the missing piece of the puzzle. One potential window into the internal structure of the charged pion and kaon is their elastic electromagnetic form factors, F_π(Q²) and F_K(Q²).  Electromagnetic form factors are fundamental quantities which describe the spatial distribution of partons within a hadron. Determining these form factors, as well as how they vary with Q² is an important step on our road to understanding the internal structure of these objects. The EIC opens up the possibility of studying F_π(Q²) andF_K(Q²) to very high Q². The Q² reach of these measurements is deep into unexplored territory, these cutting edge measurements could help disentangle the mass generation puzzle of QCD. In this talk, I will outline the opportunities and challenges of pion and kaon form factor measurements at the EIC.    

Measurement of the electromagnetic π0 transition form factor at space-like virtualities at MAMI

A measurement of the electromagnetic transition form factor (TFF) of the neutral pion in the space-like doubly-virtual regime via single-pion electroproduction is planned at the MAMI electron accelerator facility at Mainz, Germany. Measuring the electroproduction cross section off a highly charged target at low momentum transfer gives access to the so-called virtual Primakoff contribution, i.e. the meson production on the Coulomb field of the nucleus, which is proportional to the π⁰ TFF. The scattered electrons and the π⁰ decay photons will be detected with an electromagnetic calorimeter (EMC) placed at forward scattering angles, in order to exclusively reconstruct π⁰ production events. The experiment will be set up at the MAMI A1 electron scattering facility, upgraded within the phase 0 of the FAIR project with an adaptation of the backward EMC designed for the PANDA spectrometer. An overview of feasibility studies, both simulations and test measurements, will be given, and the current status of the experiment preparation reported.   

Precision measurement of the neutral pion transition form factor at low Q2 at Jefferson Lab

The neutral pion transition form factor (TFF) plays an important role in tests of low energy QCD, and in the determination of the hadronic-light-by-light (HLbL) scattering contribution to the muon anomalous magnetic moment, (g-2). Several measurements of this form factor exist in the large space-like Q² region, but the low Q2 space-like region remains largely unexplored. I will present the details and impact of the proposed precision measurement of the TFF to be carried out in Hall B at Jefferson Lab. This experiment will use a 10.6 GeV electron beam, silicon target, two GEM detectors, and a low background beamline in Hall B to measure the virtual Primakoff cross-section for the neutral pion electro-production. These data will be used to extract the pion TFF in the Q² range of 0.003-0.3 GeV², and constrain calculations for the HLbL contribution to muon (g-2) at low Q².

    

Neutral Pion Transition Form Factors Measured Through the Primakoff Mechanism on Atomic Electrons at very low Q2 range.

One of the largest uncertainties in the Standard Model prediction for the muon g-2 magnetic moment is coming from the hadronic light-by-light scattering mechanism. The uncertainty on this contribution critically depends on knowledge of the neutral pion transition form factors at low Q² region. Arguably, to measure this process with an accuracy that is appropriate for the task is using the electroproduction of neutral pions on atomic electrons via the Primakoff mechanism. Simultaneous detection of the scattered and recoil electrons together with two photons from the neutral pion decay process in the experiment will provide a full control of the reaction kinematics and, most importantly, will completely remove the hadronic contributions that are the main background sources in all other Primakoff type of experiments. The initial Monte Carlo simulation results to optimize this experiment will be presented and discussed in this talk.