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About the DAMA-LIBRA result April 17, 2008

Posted by dorigo in astronomy, cosmology, news, personal, physics, science.

The new result of the DAMA-LIBRA collaboration, which finds a yearly-modulated signal of interactions in their NaI crystals which is compatible with dark matter in our galactic halo, has caused some ripples in the web. I was surprised to find that a comment I had posted yesterday on the matter was linked by Symmetry magazine as well as by Peter Woit (who was however the true originator of my remark).

I must say I do not particularly enjoy to always sound skeptical. A scientist should keep an open mind, and if the DAMA-LIBRA signal has a cross-section which is apparently already excluded by the CDMS result, as well as orders of magnitude above the estimates for mainstream dark matter candidate models, one should wait before taking a step back, and rather consider questioning the exclusions rather than the signal. Indeed, establishing a signal comes before challenging it with specific models, and the comparison of DAMA’s result with CDMS exclusion contours belongs firmly to the second category. CDMS excludes specific models, while DAMA establishes a yearly frequency in its signal yield which could be due to particles we have still not even conceived.

So, why not focusing on the establishment of the signal ? I tried to build an opinion on the solidity of the DAMA result this morning by talking to a few people who attend the conference, and in particular with a young researcher who now works for CUORE and has been in the DAMA collaboration in the past. She in fact worked at the analysis of data, trying to interpret the modulation in photomultiplier counts with different density models of dark matter in the milky way halo as well as components from our close satellites, the magellanic clouds.

It looks like the phase of the cosine oscillation in signal yield, which is something I had doubts about, is indeed compatible with being zero for a time of the year compatible with June 2nd, which is the date when the Earth travels in the direction of the Sun’s motion in the galaxy (if one forgets about the inclination of the Earth orbit with respect to the Sun’s line of motion). Not perfectly matching June 2nd, but compatible with it.

Another issue is of course the one which was raised at yesterday’s talk: signal efficiency is steeply rising in the region between 2 and 6 keV where the modulated signal is observed. It turns out that the efficiency is not full but it is quite stable – stability is checked weekly – and the reason why it is not 100% is due to specific cuts that are made to exclude a background contamination. This, of course, might make the signal yield dependent on subtleties of the shape of the PMT response (the cuts are made on signal shape form factors), but I see no reason to doubt that the stability is well under control, although extraordinary claims require extraordinary evidence, and the latter implies going after the subtlest of possible non-exotic explanations.

One thing I would have done if I had designed the experiment myself would have been to prepare a mock-up of the active region, wrap it with black paper, and instrument with similar photomultipliers and the accessory set-up, the overpressure, the temperature controls. This would have allowed to have a real-time comparison between PMT counts from the real NaI detector and PMT counts in a fake one. It would dismiss any claim that PMTs are varying their response seasonally without appeal, but unfortunately we do not have such luxury available in the DAMA-LIBRA setup.

So the question remains. If the signal is strong and significant, and if it is not due to instrumental nuisances -I am sure about the former, less so about the latter- what is its source ? Can we get a model of dark matter particles which produce a similar flux ? It should not be too difficult. Maybe the ball is in the theorists’ court in fact. As for me, I keep cool. I still think there is no new particles to be found with these cunning but a bit overoptimistic endeavours.


1. Not Even Wrong » Blog Archive » Dark Matter - April 17, 2008

[…] Yet more from the New York Times and Tommaso on this […]

2. Nick - April 17, 2008

Just found your blog! I’m a physics major at NDSU, looking forward to reading more about things that I can’t understand yet. 🙂

Can’t wait until the day I do, i’d say that’ll be a good one.


3. dorigo - April 17, 2008

Hi Nick, welcome!

This blog is for people like you, who like to read about physics. I try to keep this blog to a level lower than yours, but you will find a lot of information here.


4. changcho - April 17, 2008

I knew you’d write something about this. I first read about this from CIP’s blog (http://capitalistimperialistpig.blogspot.com): something about italian particle particle physicists finding possible evidence for WIMPs). So, you don’t sound too convinced?

5. dorigo - April 17, 2008

Hi Changcho,

if you have these intuition often, please let me know the next time… Sometimes it would save me time to know what I’d be writing about later 🙂

No, I am not too convinced. The signal is genuine, but I have reasons for concern. The main weakness of the experiment is that whatever they find they cannot determine what it is…


6. Kea - April 17, 2008

Very interesting post, thanks. I think the ball is in both the theorists and experimentalists’ courts. Firstly, surely the experiment needs to be duplicated by an independent group. Secondly, I wouldn’t put any money on any of the standard DM candidates, so it is clear that theorists have a lot of work to do.

7. Luboš Motl - April 18, 2008

Dear Tommaso, “symmetry” is spelled with “y”, not with “i”, unless you are Russian.

8. dorigo - April 18, 2008

Hi Lubos,

no, as far as I know I am not Russian (but one can never be completely sure). In any case, thank you, fixed – It is a mistake I tend to repeat, along with a few others.


9. Erik - April 18, 2008

Couldn’t radiation originating from the sun have a similar modulation? Neutrinos coming through the earth would have different angles at summer and winter. I could not find if there was any 24h modulation, have they made such a plot?

10. Erik - April 18, 2008

Thinking a bit more.. Comparing plots from both 24h and 23.934h windows could discriminate very nicely between solar and galactic origin.

11. DB - April 18, 2008

New Scientist issued a report on this yesterday; the massive scepticism which greeted DAMA’s previous announcements over the last eight years still seems to linger, but the tone seems to have moderated:


12. dorigo - April 18, 2008

Hi DB,

yes – the general feeling is that one cannot dismiss this result, but take it seriously and try to verify it with an independent experiment. Rubbia today suggested to take the whole apparatus to the southern emisphere and re-do the experiment – the logic being that a galactic signal would not change sign while a seasonal one would. We will see what happens.


13. dorigo - April 18, 2008

Hi Erik,

indeed, as I noted in #12, there are ways to confirm the galactic origin of the signal. I think the check you propose has been done already by DAMA-LIBRA, but the one that Carlo Rubbia suggested today at the conference of course hasn’t yet…


14. World of Science News : Blog Archive : links for 2008-04-20 [Uncertain Principles] - April 20, 2008

[…] About the DAMA-LIBRA result « A Quantum Diaries Survivor A closer look at a controversial claim to have detected dark matter (tags: physics astronomy science blogs experiment theory) Read the comments on this post… […]

15. Uncle Al - April 20, 2008

Collar, JI, et al., Science 319(5865) 933 (2007)
“Spin-Dependent WIMP Limits from a Bubble Chamber”
(no WIMPs)

Sensitivity, selectivity, modest cost, and active medium Enviro-whiner hissy fit CF3I. Results from large scale apparatus using perfluorobutane will be flat out exciting.

Like 100,000 gallons of perchloroethylene buried 4900 feet deep, Collar’s setup lacks the looming visceral thrill of accidently dropping a hex nut and blasting South Dakota into space. Uncle Al recommends a Media darling active medium: iodine heptafluoride, IF7.

bp -32.5°C, d = 2.36 g/ml
@ 25°C, d = 2.06 g/ml liquid
29.092 wt-% F, 64.777 wt-% I

mp = 4.5 C d = 2.7 g/cm^3 (solid)
bp = 4.77 C, colorless
51.171 wt-% F, 48.829 wt-% I

Incompatible with Pyrex, fused silica (Teflon AF windows?), and most metals (Monel?); expensive, corrosive, toxic, malodorous. IF7 has it all!

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17. And I thought I had been harsh… « A Quantum Diaries Survivor - April 21, 2008

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18. dorigo - April 28, 2008

Uncle Al, your proposal would be interesting to submit. Maybe the US congress would get excited about it… I know many would vote to fund it just to see what happens.


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[…] the cosmological inputs have difficulty with the standard WIMP scenario. DAMA, Pamela, Atic are recently evidenced anomalies that do not fit with our simplest-minded picture. […]

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