We study the effect on leptogenesis due to B - L cosmic strings of a U ( 1 ) _ { B - L } extension of the Standard Model .
The disappearance of closed loops of B - L cosmic strings can produce heavy right handed neutrinos , N _ { R } ’ s , whose CP -asymmetric decay in out-of-thermal equilibrium condition can give rise to a net lepton ( L ) asymmetry which is then converted , due to sphaleron transitions , to a Baryon ( B ) asymmetry .
This is studied by using the relevant Boltzmann equations and including the effects of both thermal and string generated non-thermal N _ { R } ’ s .
We explore the parameter region spanned by the effective light neutrino mass parameter \tilde { m } _ { 1 } , the mass M _ { 1 } of the lightest of the heavy right-handed neutrinos ( or equivalently the Yukawa coupling h _ { 1 } ) and the scale of B - L symmetry breaking , \eta _ { B - L } , and show that there exist ranges of values of these parameters , in particular with \eta _ { B - L } > 10 ^ { 11 } { GeV } and h _ { 1 } \mbox { \raisebox { -2.58 pt } { ~ { } $ \stackrel { > } { \sim } $~ { } } } 0.01 , for which the cosmic string generated non-thermal N _ { R } ’ s can give the dominant contribution to , and indeed produce , the observed Baryon Asymmetry of the Universe when the purely thermal leptogenesis mechanism is not sufficient .
We also discuss how , depending on the values of \eta _ { B - L } , \tilde { m } _ { 1 } and h _ { 1 } , our results lead to upper bounds on \sin \delta , where \delta is the the CP violating phase that determines the CP asymmetry in the decay of the heavy right handed neutrino responsible for generating the L -asymmetry .