Friday 1 May 2009

All eyes on Denver

Tomorrow (Saturday) morning, the FERMI/GLAST collaboration is going to announce their first results at the APS meeting in Denver. FERMI/GLAST is a satellite cosmic gamma-ray observatory, but it also has capabilities to measure the electron+positron spectrum. The latter is eagerly awaited by the particle physics community. Last year, the measurements of the cosmic ray positron fraction by PAMELA and of the combined electron+positron flux by ATIC have sparked some 150 theory papers and one paparazzi affair. Recall that PAMELA sees an excess of positrons over the background (whatever the background means) in the 10-100 GeV range, while ATIC claims there is a clear bump in the spectrum at around 700 GeV. One tantalizing interpretation of these data is that the excess positrons originate from annihilation or decay of TeV scale dark matter particles.

If you can't wait till tomorrow have a look at the plot extracted from a theory paper of two weeks ago. The solid black line on that plot by sheer accident reproduces pretty well the FERMI/GLAST data to come. The sexy ATIC bump is gone and is replaced with milder features: a shallow deep around 100 GeV followed by a mild rise toward 800 GeV, and then a steep decline consistent with the earlier HESS measurements. These new results neither exclude (there's still an excess) nor significantly support the dark matter cause (there's no smoking gun features). Next week arXiv will be flooded with papers refitting the earlier theoretical models to the new data.

Update: FERMI/GLAST has revealed the new measurement of the e+e- cosmic ray spectrum but the plot is not available yet - it will be published coming Monday. According to those who saw Denver's talk, the spectrum is indeed similar to the one plotted above, although the low energy (20-80 GeV) data points lie slightly below the background curve and the dip is even less pronounced. Also, FERMI's data stop at 1 TeV so the high-energy decline cannot be clearly seen. So at this point everything is clear: it's either dark matter or a pulsar or an alien civilization or maybe the galactic propagation model needs refining ;-). More insight should come from FERMI's measurement of the diffuse photon spectrum which is expected by late summer.

Update 2: and here is the original plot from Fermi's today paper on arXiv:
Also HESS got its foot in the door and just published new results for the electron+positron flux above 340 GeV, consistent with those of FERMI and inconsistent with ATIC.


a quantum diaries survivor said...

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Anonymous said...

thank you for keeping us up with the news.

Goodbye dark matter :)