[Prex] [EXTERNAL] PREX-2 Concludes

[Paschke, Kent Dieter (kdp2c)]

Dear Fellow Neutron-skin Enthusiasts - 

The PREX-2 run ended Monday morning at 8am, and I was glad to reflect on what we have accomplished this summer. 

We came into this run sweating a pretty wide range of issues, e.g. acceptance, radiation, resolution, noise… and came out looking great. Even in that long, rocky startup, in amongst the beam issues and the vacuum leak, our commissioning data clearly showed us that the apparatus was well designed and fundamentally capable of achieving the proposal precision. When the accelerator was able to get on track after the power outage, the run was characterized by an incremental process of refining precision and analysis quality, an epic battle with polarimetry, and a relentless drive toward high efficiency. 

And how did it turn out?  By the end of the experiment, we were measuring a total rate of about 5 GHz. After the run time of about 3 calendar months, the statistical uncertainty is now just slightly over 14 ppb, about 3% of the expected Apv.  Beam asymmetries appear to have averaged to be very small, near to or less than 1 nm for each dimension including our energy monitor. Throughout the run, results have maintained good statistical consistency. Under the watchful eye of our “WACs", the data has been carefully sorted, cataloged, and aggregated. We commissioned the new Moller polarimeter, with an improved understanding of optics and systematic uncertainties. We also recovered the damaged Compton polarimeter, and it appears we will be able to provide well-normalized polarimetry data for the second half of the run.  We gathered extensive cross-checks for optics, kinematics normalization, and background systematics. In short, this amazingly precise data set looks very clean, and we are well positioned to complete this analysis within a year. 

For context: the Qweak experiment, with 16 meters of detector, measured a rate of about 7 GHz, and ran in parts of three calendar years to achieve a raw precision of about 6.4ppb (3.6%). The position/energy sensitivities for the heavy 208Pb nucleus were fully 10x higher than those for the Qweak measurement, so in terms of absolute asymmetry measurement I would argue that our PREX-2 result will be in a class of its own. 

Congratulations to our collaboration!  We are now planning a post-run celebration to express our gratitude to those in the accelerator and physics divisions that helped us make this such a success. We also aim for a collaboration meeting perhaps in early October, to review the PREX data and direct preparations for CREX, which will be starting in mid-November. 

Cheers,
Kent (for the PREX-2 spokespersons)