2p2h

2p2h stands for two-particle-two-hole excitations. In many-body physics, a 2p2h configuration refers to a state in which two particles are promoted from occupied (hole) states below the Fermi surface to unoccupied (particle) states above it, leaving two holes behind. This is in contrast to simpler 1p1h (one particle, one hole) excitations and becomes important when correlations beyond mean-field are significant.

In nuclear physics, 2p2h configurations arise from two-body interactions and correlations that mix different excitations. They contribute to the fragmentation and damping of collective modes, such as giant resonances, by spreading strength over a wider energy range. They also open pathways for multi-nucleon emission in reactions. The inclusion of 2p2h states is a feature of extended many-body approaches, such as second random phase approximation (SRPA), extended RPA, and particle-vibration coupling schemes, as well as certain time-dependent and configuration-interaction methods.

In scattering experiments, 2p2h processes can lead to final states with two nucleons ejected and can affect measured cross sections and response functions. In electron- and neutrino-nucleus scattering, two-nucleon currents, including meson-exchange currents, can populate 2p2h configurations, contributing notably in the few-hundred MeV to GeV energy region and helping to reconcile discrepancies with simpler models.

Overall, 2p2h excitations encode correlations and complex reaction dynamics beyond single-particle pictures, and they are essential for accurately describing spectra, damping, and multi-particle emission in many-body systems.