loopinduced

Loopinduced refers to processes in quantum field theory that occur only through loop diagrams and do not proceed at tree level. In perturbation theory, interactions are organized by order in coupling constants; tree-level diagrams represent the lowest order, while loop diagrams involve virtual particles circulating in closed circuits. Loop-induced processes are generally more suppressed than tree-level ones due to additional powers of coupling constants and the effects of loop integrals, but they can be crucial probes of the theory.

In the Standard Model, several important processes are loop-induced. Examples include the decay of the Higgs boson into two photons, H -> gamma gamma, which proceeds via virtual W boson and top-quark loops; and the production of the Higgs boson by gluon fusion, gg -> H, which occurs through a top-quark loop. Other loop-induced phenomena include H -> Z gamma and flavor-changing neutral currents such as b -> s gamma, which do not occur at tree level in the SM and are therefore sensitive to contributions from new particles.

The calculation of loop-induced amplitudes involves evaluating loop integrals, applying regularization and renormalization to handle divergences, and often employing effective field theory when heavy degrees of freedom are integrated out. Loop effects can reveal or constrain physics beyond the Standard Model, since new particles or interactions can modify loop amplitudes even if they are too heavy to produce directly.

Experimentally, loop-induced processes are studied through precise measurements of rates, distributions, and asymmetries. Deviations from Standard Model predictions can indicate new dynamics, guiding searches for new particles or interactions and informing theoretical models.

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