[Prex] Update on PREX-2

[Paschke, Kent Dieter (kdp2c)]

Greetings to our fellow PREX collaborators,

The road to the neutron radius of 208Pb has been long, but the experiment has proven itself ready to accumulate statistics and achieve its precision goals. 

At the time of this writing, PREX-II has well surpassed the precision of PREX-I, having accumulated about 15% of the intended statistics. The total detected rate is around 4 GHz, divided into measurements at 30Hz each with a precision of about 96 parts per million. The residual sources of noise are identified and well understood. The results so far display good statistical convergence, and the beam modulation correction appears to be able to handle the modest beam asymmetries. The targets are performing well and the collimator and vacuum system are stable. Radiation is about as expected, and manageable. The Moller polarimeter has been commissioned and shown to be capable of providing a robust and precise measurement of polarization. In summary, this is excellent: the experiment can run at the intended luminosity, and beam quality and apparatus performance are as we planned for. 

The experiment has faced significant challenges to get to this point.  A subtle leak in the brazing of the water target led to significant lost time due to intermittent vacuum issues. Once this was repaired, the experiment was only up for about 10 minutes before a site-wide power outage brought the accelerator down for a week (and damaged the Compton polarimeter laser system). This was followed by another week of accelerator issues with sparse beam delivery. The experiment completed commissioning during this period,  “on-time” in terms of the number of beam hours used, and was ready to start production when regular beam became available.

Although vigilant attention to systematic uncertainties is required, the chief challenge facing the experiment now is running time.  With about 1/3 of the scheduled beam time remaining, the experiment can only hope to reach about 50% of its expected statistics in the best of circumstances. As you know, the value of PREX is predicated on reaching the statistical error goal, which (if we do it right) will far outweigh the systematic uncertainty. The experiment has shown itself ready to achieve full precision. We remain hopeful that an extension of runtime will replace the lost beam time and allow PREX the opportunity to make a precise measurement with a lasting impact.

All the best,
Kent (for the PREX-II co-spokespersons)