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Two "hot topics" from particle physics -- read the actual papers! Answered

I've noticed that two recent particle physics "discoveries" (what I'd call analysis results) are beginning to get some media coverage, at least in the science press. Below, I provide links to the actual papers (the preprints in arXiv), and a bit of my own opinion. I encourage the curious to read, or at least skim, the papers --- there will probably be a lot you don't understand (these are technical research papers, after all!), so don't hesitate to ask questions.

Dark matter "observation" by PAMELA satellite -- excess of high energy positrons in cosmic rays

The PAMELA data are high-statistics, and they've done an excellent job with their systematics and
detector calibration. Their low energy data are entirely consistent with previous results, which makes the excess and plateau seen above 10 GeV rather compelling. I don't think we know enough about conventional astrophysical sources of positrons to have any confidence ascribing this result to "new physics."

"Ghost particles" in Fermilab p-antip collisions -- excess of high impact parameters muons

Charged particles (like muons) are identified in big detectors such as CDF (and BaBar) by picking up signals from their occasional interactions with low density material, and fitting a trajectory in space to those "hits." You can't put active detector material all the way down to where the beams collide, so an extrapolation from the detector region to the "interaction point" is required. There can be many complicated and non-obvious systematic uncertainties affecting those fits. The fact that only about 1/3 of CDF was willing to sign this paper suggests to me that the systematics in this analysis are not really under control yet, and any interpretation of the result should be taken with a grain of salt.


Lithium Rain

10 years ago

Very interesting reading-thanks for sharing these! On the first one, I thought that dark matter was supposed to make up much more than 23% of the "universe's energy budget". Or is that incorrect?

kelseymhLithium Rain

Reply 10 years ago

Good question. The current "concordance model" of cosmology has best fit values of 74% dark energy (or cosmological constant), 23% dark matter, and 4.5% baryonic matter.

The numbers don't necessarily add up to 100% because they can be determined independently -- the deviation from unitarity is a test of the model itself. I haven't quoted uncertainties on those values, either. Please read the Wikipedia article for details :-)