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 The enter of value of helicity of the second incoming particle.
 This option is available if process in symbolic session was 
 specified in the corresponding manner with the % symbol after 
 particle name. Note that helicity is defined as a projection 
 of spin on the direction of motion. So,the input value is 
 restricted by +/- particle spin.  Left SM fermion have 
helicity  -0.5. Helicity of left unti-fermion is +0.5