# Changes between Version 1 and Version 2 of QandA

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Timestamp:
04/06/12 16:33:02 (7 years ago)
Comment:

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Unmodified
 v1 where %$\tilde G$% is the dual field tensor. This term violates parity %$P$% and time reversal %$T$%, but since both are not symmetries where $\tilde G$ is the dual field tensor. This term violates parity $P$ and time reversal $T$, but since both are not symmetries 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. 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. ====== The parton distribution functions are non-perturbative quantities. Is there any solid lattice result?  ====== In practice I am not aware of any result competitive with the most common global fits from CTEQ, MRST, NNPDF,... ====== In a MC approach we always start from the high %$Q^2$% interaction and then apply forward (time-like) evolution to connect it with the final state and backward (space-like) evolution to connect it with the initial state. Why don't we start from the initial state and go all the way to the final one? ====== ====== In a MC approach we always start from the high $Q^2$ interaction and then apply forward (time-like) evolution to connect it with the final state and backward (space-like) evolution to connect it with the initial state. Why don't we start from the initial state and go all the way to the final one? ====== The main reason is just efficiency. Consider for example Higgs production from gg or Drell-Yan. If I start a forward shower from the incoming protons I will only very very rarely hit the right %$Q^2$% (of order of the boson masses) in correspondence with the right x's (%$x_1 x_2 S=M^2$%). protons I will only very very rarely hit the right $Q^2$ (of order of the boson masses) in correspondence with the right x's ($x_1 x_2 S=M^2$). === Lecture 2 === 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]). If you do so, then you can consider neutrino masses as our first solid indication of physics beyond the Standard Model. This is nice for many reasons. My preferred one is as follows. Considering the Standard Model as an effective field theory, one can start including the effects of new physics as described by higher dimensional operators. The largest effects should be coming from the lower dimensional operators. Now it turns out that you can write dozens of operators of dimension 6 and beyond. However, there is only one operator of dimension 5, of the type (schematically) %$( L \phi) (\phi L)/M$%,  that of operators of dimension 6 and beyond. However, there is only one operator of dimension 5, of the type (schematically) $( L \phi) (\phi L)/M$,  that describes Majorana neutrino masses. Isn't that great?