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Scalar radius of the pion and zeros in the form factor  Jose A. Oller
; Luis Roca
;  Date: 
31 Mar 2007  Subject:  hepph (High Energy Physics  Phenomenology)  Abstract:  The quadratic pion scalar radius, <r^2>_s^pi, plays an important role for
present precise determinations of pipi scattering. Recently, it has been
object of controversy between Yndur’ain, on the one hand, and Ananthanarayan,
Caprini, Colangelo, Gasser and Leutwyler (ACCGL), on the other. While the
former, based on an Omn`es representation of the scalar form factor, obtains
<r^2>^pi_s=0.75pm 0.07 fm^2, the latter, from a solution of the
MuskhelishviliOmn`es equations with pipi and Kar{K} as coupled channels,
end with <r^2>^pi_s=0.61pm 0.04 fm^2. A large discrepancy between both
values, given the precision, then results. We reanalyze Yndur’ain’s method and
show that for some Swave null isospin (I) Tmatrices the scalar form factor
has a zero previously overlooked. Once this is accounted for, the resulting
<r^2>_s^pi is compatible with the value of ACCGL. Following the corrected
version of Yndur’ain’s approach we perform a new determination and obtain
<r^2>_s^pi=0.63pm 0.05 fm^2. The main source of error is present experimental
uncertainties in low energy Swave I=0 pipi phase shifts. Another important
contribution to the error is the not yet settled asymptotic phase of the scalar
form factor from QCD.  Source:  arXiv, arxiv.0704.0039  Services:  Forum  Review  PDF  Favorites 


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