the W and Z bosons
Every time a nucleus decays by beta radiation, every time the Sun burns hydrogen into helium in its core, an invisible heavyweight is doing the work: the W boson. Together with its neutral sibling the Z, it is the carrier of the weak nuclear force, the only force that can change one kind of quark or lepton into another. Unlike the massless photon that can travel forever, these carriers are ponderously heavy, and that heaviness is the reason the weak force is both weak and short-ranged, it can barely reach across a proton.
The W and Z are the massive spin-1 gauge bosons of the electroweak interaction. The W comes in two charged states, W+ and W-, with a mass of about 80.4 GeV; the Z is neutral with a mass of about 91.2 GeV, each roughly ninety times heavier than a proton. The charged W mediates 'charged-current' processes that change particle identity: in beta decay a down quark emits a W- and becomes an up quark, and the W- then materializes as an electron and an antineutrino. The neutral Z mediates 'neutral-current' scattering, in which particles exchange momentum and energy without changing identity. Their large mass makes the effective interaction extremely short-ranged, of order 10^-18 m, since a virtual carrier of mass M can reach only about hbar/(M c).
Where does that mass come from? Gauge symmetry forbids putting it in by hand, so the W and Z acquire mass by absorbing the would-be Goldstone bosons of the spontaneously broken electroweak symmetry, the Higgs mechanism, while the photon stays exactly massless. Their masses were predicted by electroweak theory years before the particles were found at CERN's proton-antiproton collider in 1983, a triumph that earned Rubbia and van der Meer the Nobel Prize. The precise ratio of the W and Z masses is set by the electroweak mixing angle and remains one of the most stringent tests of the Standard Model.
In ordinary beta-minus decay, a neutron becomes a proton by having one of its down quarks emit a virtual W- boson; because the W is far heavier than the tiny energy released, it exists only fleetingly as a virtual particle and immediately turns into the emitted electron and antineutrino.
The weak force's feebleness in beta decay is a direct consequence of the enormous mass of the W boson it must borrow.
The weak force is not weak because its intrinsic coupling is small, it is comparable to electromagnetism, but because the W and Z are so heavy that their virtual exchange is strongly suppressed at low energies.