Suggestions from PRL

Last month, I commented about the high proportion of articles in the nuclear physics section of Physical Review Letters that are in the sub-field of relativistic heavy ion collisions (RHIC) and how they form the preponderance of articles promoted by the editors to appear on the journal front page.   In the last month, the trend has continued, and only RHIC papers have been highlighted.  So, let me highlight here a paper which was published last month, and has been picked up by a few news websites:

The paper is called "Direct measurement of the mass difference of ¹⁶³Ho and ¹⁶³Dy solves the Q-value puzzle for the neutrino mass determination."  The physics motivation behind the work is that the nuclear reaction:

¹⁶³Ho + e^– → ¹⁶³Dy + ν_e

can be used as a model-independnt (i.e. not relying on assumptions about the structure of the nucleus that may not be correct) mans of determining the mass of the electron neutrino -- something that is not pinned-down at all well.  The mass of the neutrino is known to be extremely small, but not zero.  By observing the above reaction, and using conservation of energy, one can work out the energy balance (the "Q-value" of the title) from the observed particles, and deduce the energy carried off by the (unobserved) neutrino, and hence its mass.  To be able to do this, the masses of the two nuclear isotopes in the equation: Holmium–163 and Dysprosium–163 must be known to very high accuracy.  Actually -- it is only the difference between the two masses that needs to be known, and there is disagreement in the published data what this difference is, to a level that swamps the tiny neutrino mass.

The experimenters here used a Penning trap to store ions of each of the isotopes.  For a given particle of mass m and charge q moving in a trap with magnetic field B, the particle will move in a circle with frequency f=qB/(2πm).  Measuring the frequency allows a value of the mass to be determined.  They put the two different ions in the same trap at the same time, but in separate bunches, reducing systematic effects to do with e.g. variations in the magnetic field.  They concluded that their value for the mass difference is at variance with the "accepted" value (that published in the Atomic Mass Evalution) by more than 7 times the size of their error bar.  Thus, the error in determining the neutrino mass should be able to be reduced in future experiments looking at the reaction above.