Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Rare decay \pi^0 \to e^+e^- as a Test of Standard Model   Experimental and theoretical progress concerning the rare decay \pi^0 \to
e^+e^- is briefly reviewed. It includes the latest data from KTeV and a new
model independent estimate of the decay branching which show the deviation
between experiment and theory at the level of $3.3\sigma$. The predictions for
\eta and \eta' decays into lepton pair are presented. We also comment on the
impact on the pion rare decay estimate of the BABAR collaboration on the pion
transition form factor at large momentum transfer.

19
The categories of this paper are: eess
21
The categories of this paper are: adap-org
14
The categories of this paper are: math-ph
25
The categories of this paper are: patt-sol
13
The categories of this paper are: quant-ph
9
The categories of this paper are: nlin
18
The categories of this paper are: econ
20
The categories of this paper are: acc-phys
24
The categories of this paper are: dg-ga
22
The categories of this paper are: chao-dyn
10
The categories of this paper are: q-bio
33
The categories of this paper are: plasm-ph
23
The categories of this paper are: solv-int
37
The categories of this paper are: supr-con
32
The categories of this paper are: atom-ph
5
The categories of this paper are: gr-qc