As someone who has spent several years thinking hard about how to detect “hidden particles” — ones that are not affected by the three forces of the Standard Model, the electromagnetic force and the strong and weak nuclear forces — I am pleased to see the result that just appeared from the APEX experiment, at the Jefferson Laboratory in the US state of Virginia:
http://arxiv.org/PS_cache/arxiv/pdf/1108/1108.2750v1.pdf.
Quoting from [and modifying for non-experts, in italics] the abstract:
- We present a search at Jefferson Laboratory for new particles, lighter than protons, that are carriers of a new force and that can decay to electron-positron pairs [recall positron = anti-electron]. Such a particle can be produced by slamming an electron beam into ordinary matter... Using APEX data, we searched … and found no evidence of such a particle in a certain range of masses and force strengths. Our findings demonstrate that experiments of this type can explore a new, wide, and important range of masses and strengths for sub-proton-mass forces.
What they’re doing is looking for a particle that is similar to a Z particle, one of the carriers of the weak nuclear force, but that on the one hand (a) exerts a smaller force on ordinary matter than does the weak nuclear force, and on the other hand (b) is lighter in mass than the Z, so the force it exerts dies off more slowly with distance than does the weak nuclear force. If such a particle were found, it would represent the first new force of nature discovered in many decades!
A competitor experiment from Mainz (called MAMI) reported results a few months ago. The two experiments are sensitive to a comparable force strength and to slightly different mass ranges. You can see their results in the figure, which shows the APEX search region in blue and the MAMI search region in green, and other experiments in variants of grey. These two experiments are probing weaker forces — looking for force carrier particles that are more hidden — than has ever been possible before.
Both of these experiments are still in the very early days; their results come from short, preliminary runs of data-taking. They both plan to have a much longer run, which should allow them much greater sensitivity to weaker forces and to force carriers of a wider range of masses. In particular, the unshaded region in the figure, which hasn’t been searched yet, should be entirely covered soon.
I am also pleased because among the leaders on the APEX experiment are three young theorists, Rouven Essig, (a former Rutgers student of my colleague Scott Thomas, and now a Stonybrook faculty member), Natalia Toro and Philip Schuster (two of my co-authors on a recent paper, and faculty members at the Perimeter Institute), and an experimentalist and professor at nearby New York University, the ubiquitous Kyle Cranmer (who also is on ATLAS and presented that experiment’s result on the search for the Higgs particle a few weeks ago.)
Congratulations, APEX!!!
