In a new paper published in Physical Review C, Axel Schmidt and collaborators from MIT explored the onset of the short-range correlated regime for protons inside nuclei. At any given moment, most protons inside nuclei feel strong attraction from many nearby nucleons (other protons and neutrons). However, some protons happen to be very close to a nearby neighbor nucleon, enter a short-range correlated configuration, in which the forces between the paired nucleons are much much stronger than the forces from the rest of the nucleus. These correlations produce states of very high relative momentum, which is typically how we identify them in electron scattering experiments. One such approach is to examine the momentum of electrons after they have scattered from a nucleus. If an electron leaves the collision with significantly more momentum than would be expected, given their scattering angle (high Bjorken-x, xB), then it is likely the electron hit a high-momentum nucleon participating in a short range correlation.
In this paper, we consider a slightly different technique: knocking out a proton from the nucleus. The additional information from the proton momentum vector allowed us to reject collisions with additional undetected particles, which are a background hiding our short-range correlations signal. We showed that this technique works by verifying the scaling in Bjorken-x seen in the carbon-to-deuterium cross section ratio; only we see it over the full range of x, shown in the figure above.
Detecting the knocked out proton has an additional advantage: an extra handle on what the proton’s momentum was prior to the collision, a variable we call missing momentum or pmiss. By selecting clean proton knock-out using missing mass, we could dial in different proton momentum regions using missing momentum. This allowed us to get an estimate of the width of the transition region between the uncorrelated and correlated regimes, shown in the figure above.
Future experiments, such as those in inverse kinematics, or those detecting low-energy recoiling nucleons (such as the recently approved ALERT experiment at Jefferson Lab) will be able to enhance the precision of our understanding of this transition region, and on how short-range correlations form inside the nucleus.