More Conflict in the Universe

As I predicted, the debate over dark matter detection has already heated up. (See my last post for the background details)

The three experiments with positive dark matter signals (DAMA, CDMS-II, and CoGent) are standing by their results, and at least two of these groups have raised serious concerns about the XENON experiment. And probably with good reason.

The three prior experiments had long runs before publishing results, while XENON has run for approximately 11 days. They will run for longer in future, but in my opinion it is too early to publish - I would have completed at least one or two more runs before making the bold claim that the well studied and peer reviewed results of other groups are false. I have a feeling that with so much previous data stacked against XENON, their reputation could suffer for this. And that would be sad, because it is a very good experiment in principle.

I am not an experimentalist, and have never claimed to be an expert on these large scale detectors. However my suspicion is that XENON may have made two mistakes in their analysis.
The first is in the background exclusion. For the non-technical readers, all experiments have a background signal which contains false positives caused by anything from radiation in the surrounding soil to cosmic rays, and this must be removed in such a way as to not remove real dark matter signals. XENON is supposed to have a very low background signal, so it is possible that some form of exotic dark matter has been lost in their processing of the data. Although that is pure speculation on my part, and not based on any analysis yet completed.
The second possibility, and perhaps the stronger argument, is that XENON may not be as sensitive to certain types of dark matter. For several years there has been interest in low mass dark matter candidates which fit the data of DAMA and CDMS, and perhaps CoGent as well. As I understand the XENON analysis, they assume that their detector is as sensitive to low mass dark matter as it is to heavier particles - but this is not true of any previous experiment and so is unlikely at this one. In fact, all previous experiments are known to be insensitive to very light particles. The question is where this cutoff is for XENON. My feeling from reviewing the detector proposals is that it can detect light dark matter but at a suppressed rate - and that would explain the lack of a signal in the small data set analyzed.

With such a short experimental run of 11 days, it is even possible that the Earth was travelling through a low dark matter bubble at the time! Contrary to popular belief, dark matter is not necessarily uniform in the galaxy and it could be that the Earth travels through clumps and voids at different times of the year (and even different years through the century). If XENON collected data in a void it could explain the missing signal.

In the end, only time and further reviews will resolve this conflict. I just hope that a good experiment like XENON doesn't lose its reputation by publishing too quickly.