Helicity amplitudes for the γ * N → ∆ transition are calculated using the cloudy bag model. A correction for center-of-mass motion is carried out using a modified Peierls-Thouless projection method. This reduces the magnitude of the transition amplitudes at small momentum transfer and enhances them at modest momentum transfers. Our calculation shows that the pion cloud contributes substantially...

Bootstrap percolation is a general representation of some networked activation process, which has found applications in explaining many important social phenomena, such as the propagation of information. Inspired by some recent findings on spatial structure of online social networks, here we study bootstrap percolation on undirected spatial networks, with the probability density function of lon...

We consider the directed percolation process as a prototype of systems displaying a nonequilibrium phase transition into an absorbing state. The model is in a critical state when the activation probability is adjusted at some precise value p(c). Criticality is lost as soon as the probability to activate sites at the first attempt, p(1), is changed. We show here that criticality can be restored ...