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Physics  1999 

Perturbative Pions and the Effective Range Expansion

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Abstract:

In this talk the $Q$ counting scheme to implement effective field theory is discussed. It is pointed out that there are two small mass scales in the problem $m_\pi$ and $1/a$ with $1/a \ll m_\pi$. It is argued that while the expansion based on $1/a$ being small compared to the underlying short distance scales works well, the chiral expansion may not. The coefficients of the effective range expansion are sensitive to the chiral physics and are very poorly described in $Q$ counting at lowest nontrivial order. A ``shape function'' is introduced which again is sensitive to pionic physics and insensitive to fitting procedures. It is also poorly described in $Q$ counting.

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