When a neutron decays via the weak interaction, one of the possible processes is the emission of a W+ boson. The W+ boson is already a massive particle before it is emitted in the decay. It does not require a further Higgs mechanism step to acquire its mass.
In the Standard Model of particle physics, the W+ and W- bosons, along with the Z boson, acquire their masses through the Higgs mechanism. The Higgs mechanism involves the interaction of the W and Z bosons with the Higgs field, which gives them their masses. However, it is important to note that the W and Z bosons themselves are already massive before they are involved in the Higgs mechanism.
The masses of the W and Z bosons are experimentally determined quantities and are known to be approximately 80.4 GeV/c² and 91.2 GeV/c², respectively. These masses were measured independently of the Higgs boson discovery, which confirmed the existence of the Higgs field and its associated mechanism.
In the neutron decay process, the W+ boson is created as a result of the weak interaction. Since the W bosons are already massive particles, they do not require any further step of the Higgs mechanism to become massive. The Higgs mechanism is responsible for the masses of the W and Z bosons themselves but does not play a role in the process of their emission during particle decays.
To summarize, the W+ boson created in a neutron decay is already massive. The W bosons acquire their masses through the Higgs mechanism, but this is independent of the process in which they are emitted, such as in neutron decay.