Collisional shift and broadening of the transition lines in pionic helium

TL;DR

This paper calculates how density and temperature affect the spectral lines of pionic helium in gaseous helium, providing data to improve laser spectroscopy experiments and data interpretation.

Contribution

It offers the first detailed evaluation of density-dependent shifts and broadenings of specific pionic helium transition lines at low temperatures.

Findings

Blue shifts observed for certain unfavored transitions.

Red shift observed for the favored transition.

Linear dependence of shift and broadening on density.

Abstract

We calculate the density shift and broadening of selected dipole transition lines of pionic helium in gaseous helium at low temperatures up to T=12 K and pressure up to a few bar. In the approximation of binary collisions the shift and broadening depend linearly on the density; we evaluate the slope of this linear dependence for a few spectral lines of known experimental interest, and also investigate its temperature dependence. We find a blue shift of the resonance frequencies of the $(n,l)=(16,15) \rightarrow (16,14)$, $(17,16) \rightarrow (17,15)$, and $(16,15)\rightarrow(17,14)$ unfavored transitions, and a red shift for the favored one $(17,16) \rightarrow (16,15)$. The results are intended to significantly increase the efficiency of the laser spectroscopy investigations of pionic helium and help the interpretation of the experimental data.