Measuring the charged pion polarizability in the γγ → π^{+} π^{-} reaction

Abstract

Development has begun of a new experiment to measure the charged pion polarizability α π − β π . The charged pion polarizability ranks among the most important tests of low-energy QCD presently unresolved by experiment. Analogous to precision measurements of π • → γγ that test the intrinsic odd-parity (anomalous) sector of QCD, the pion polarizability tests the intrinsic evenparity sector of QCD. The measurement will be performed using the γγ → π + π − cross section accessed via the Primakoff mechanism on nuclear targets using the GlueX detector in Hall D at Jefferson Lab. The linearly polarized photon source in Hall-D will be utilized to separate the Primakoff cross-section from coherent ρ • production.

Hadron Spectroscopy in Photoproduction

“…An upgrade to the inner portion of the forward calorimeter from lead glass to lead tungstate crystals, yielding better energy and position resolution, is expected to be performed in 2023. Several shorter measurements focusing on other hadronic physics topics, such as η Primakoff production [29], pion polarizabilities [28], and physics with nuclear targets [30], have been scheduled in between the spectroscopy runs. Additional running with an intense (≈ 10 4 /s) K L beam [31].…”

Pion polarizabilities from a γγ→ππ analysis

“…In e + e − colliders, z is typically 0.6-0.7 for charged pions and 0.8 for π 0 π 0 . The GlueX experiment will produce good angular coverage for 40 o < θ < 140 o according to [13], so z = 0.77. Consequently, the differential cross-sections are integrated up to cos θ = z to give σ c,n (s, z) with uncertainties ∆σ nc,n (s, z).…”

“…Table 2: To determine each polarizability listed with an uncertainty of 100%, the corresponding (charged or neutral pion) cross-section for γγ → ππ has to be measured at 450 MeV (as an example) to the accuracy tabulated for z = 0.77 for charged and neutral pions, where GlueX is expected to have good angular coverage [13]. At other energies the percentage accuracies can be read off from the graphs in Fig.…”

“…Recently a proposal has been accepted to study polarizabilities by measuring low energy γγ → π + π − [13] in Hall D at Jefferson Lab. The issue is then how well do such measurements determine the pion polarizability: reliability and accuracy.…”

Dispersion relations for γ∗γ∗ → ππ: helicity amplitudes, subtractions, and anomalous thresholds

“…While an extraction of the charged-pion polarizability via radiative pion production off the nucleon [22] had been interpreted as a potential tension with ChPT [21]-despite the model-dependence from the extrapolation to the pion pole-the most recent Primakoff measurement [10] confirmed the chiral prediction. In addition to a future update from COMPASS [23], further low-energy measurements that would entail additional information on the charged-pion polarizabilities are planned at Hall D at Jefferson Lab via the Primakoff process with an incident photon [24].…”