Interpretation of the ``fDs puzzle'' in the standard model and beyond

The recent measurement on the decay constant of D_s shows a discrepancy between theory and experiment. We study the leptonic and semileptonic decays of D and D_s simultaneously within the standard model by employing a light-front quark model. There is space by tuning phenomenological parameters which can explain the “f_Ds puzzle” and do not contradict other experiments on the semileptonic decays. We also investigate the leptonic decays of D and D_s with a new physics scenario, unparticle physics. The unparticle effects induce a constructive interference with the standard model contribution. The nontrivial phase in unparticle physics could produce direct CP violation which may distinguish it from other new physics scenarios.