# Changes between Version 2 and Version 3 of QandA

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Timestamp:
04/19/12 09:34:58 (8 years ago)
Comment:

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Unmodified
 v2 of the Standard Model (violated by weak interactions) it should be included. The problem is that one would then expect, from naturalness argument to have $\theta$ of order one. Experimental bounds coming mainly from the limits on the neutron electric dipole moment gives %$||\theta||<10^{-10}$. This gives the so-called "strong CP problem". The Peccei-Quinn mechanism ([http://prola.aps.org/abstract/PRL/v38/i25/p1440_1 Phys.Rev.Lett. 38 (1977), 1440]) is a famous attempt to solve this problem by introducing a dynamical field, which gives rise to the axion. See the PDG section on [http://pdg.lbl.gov/2008/reviews/axion_s029.pdf axion searches] for  an up-to-date report on axion searches. $||\theta||<10^{-10}$. This gives the so-called "strong CP problem". The Peccei-Quinn mechanism ([http://prola.aps.org/abstract/PRL/v38/i25/p1440_1 Phys.Rev.Lett. 38 (1977), 1440]) is a famous attempt to solve this problem by introducing a dynamical field, which gives rise to the axion. See the PDG section on [http://pdg.lbl.gov/2008/reviews/axion_s029.pdf axion searches] for  an up-to-date report on axion searches. ====== The parton distribution functions are non-perturbative quantities. Is there any solid lattice result?  ====== There are several : the properties (energy and angular distributions) of the muon decay %$\mu^- \to e^- \bar \nu_e \nu_\mu$, Beta decay, and pion decays $\pi^- \to l^- \bar \nu_l$. $\mu^- \to e^- \bar \nu_e \nu_\mu$, Beta decay, and pion decays $\pi^- \to l^- \bar \nu_l$. A concise list and discussion can be found, for instance, in the older version of the lectures on the SM by A. Pich ([http://www.slac.stanford.edu/spires/find/hep/www?eprint=hep-ph/9412274 hep-ph/9412274]).