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SUSY more unlikely by the new CDMS II results March 5, 2008

Posted by dorigo in physics.

A nice feature of a new physics model with a thousand free parameters is that once you marry it, you will not need to change it for a lifetime. Regardless of how deep experimental searches cut into the flesh and blood of the model, thus setting more and more stringent limits on the existence of the phenomena predicted by it, you can sit back and relax: the phase space of the thousand parameters shrinks, but stays wide enough to park your tenure truck in it.

What I state above is the main reason for my dislike of Supersymmetry, an otherwise quite cunning theory – maybe the only really neat idea produced in the last thirty-five years on how to extend the Standard Model to mend its shortcomings. I really hate it when I have to buy something without being able to look inside the package, but worse still is the feeling of being cheated when you are purposely prevented from doing so -the exact sensation that the mechanism of SUSY mass breaking gives me.

What is SUSY mass breaking? Basically, we are taught that yes, for each particle there is a super-particle, with opposite spin-statistics of ordinary particles: if quarks and leptons have half-integer spin, then squarks and sleptons will have integer ones; and the reverse holds for vector bosons. Fine; that is in fact a nice, symmetrical feature of the model. Then, we are also explained that superparticles are unstable, and they decay to the lightest one. That is also not in contradiction with the intuition we built on years of studies of particle physics.

But then, we learn that the lightest one is neutral, and interacts only very weakly with non-SUSY matter… Darn: this means we cannot detect it by ordinary means, and that the consequences of its existence on the observable universe are quite indirect… We are caressed by the thought that SUSY has been designed to be out of our reach. But surely, our accelerators can produce other sparticles and confirm the theory ? Well, no: because, you see, SUSY is a higher symmetry of nature than the one the Standard Model is based on, but something broke it, and made all sparticles much heavier than ordinary particles.

Ok, how heavy ? Hmmm, it turns out that present-day accelerators cannot yet produce them. So we wait for a more powerful one. In the meantime, SUSY theorists flourish, string theorists speculate, and we wonder whether we have bought a lemon.

After this detailed and exhaustive account of supersymmetric extensions of the standard model, let me tell you what are the news from CDMS II, a detector designed to detect weak interacting massive particles. WIMPs, as they are called, could be the cause of observed disagreement between the amount of luminous mass in the universe and the motion we observe: Dark Matter. It is to be noted that strictly speaking WIMPs do not only belong to SUSY, although a SUSY WIMP would nicely kill two big birds with one small stone - a nice feature for an Ockhamist.

If WIMPs are the source of missing mass in our universe, gazillions of them should traverse our body day in and day out, just as the much less exotic neutrinos do. The WI in WIMP explains why we would not feel the bombardment: they are  Weakly Interacting. A typical cross section between a WIMP and a nucleon could be of the order of a few billionths of a picobarn, or approximately the apparent size the dot on this i would have if observed from the other side of our galaxy. 

So, enter CDMS II. CDMS stands for Cryogenic Dark Matter Search. It consists in 19 germanium and 11 silicon solid-state detectors, for a total mass of about 6 kilograms, kept at a temperature of 50 millikelvin (that’s right: a fraction of a degree above absolute zero) in a mine deep underground, at Soudan, Minnesota. These solid-state detectors are shaped in 3″ disks (see picture on the left), and they can detect the energy released by the interaction between a WIMP and the crystal in the form of phonons at the surface of the crystal, from a change in the resistance of the superconducting material.

The energy released by a WIMP is expected to be very small, between 10 and 30 keV: that explains the need for a ultra-sensitive, ultra-background-free detector and environment. The placement at large depth underground allows to reduce the background from cosmic rays, and the ultracold temperature enables the detection of faint phonon signals. Radioactivity is the main concern, especially neutrons induced by radioactive processes or residual cosmic rays interacting near the apparatus, but they can be reduced by an active muon veto surrounding the apparatus.

The CDMS-II collaboration has just released the analysis of two runs taken between October 2006 and July 2007. Twice as much data is still in the process of being analyzed, but the results are already quite interesting. In fact, they observed zero events in their search region – one which reduces by millions of times the backgrounds: the expectation from non-WIMP events was a total of 0.6 pm 0.5 events in the whole sample.

By the usual see-no-events-set-a-limit procedure, a 90% confidence level limit curve is thus extracted on the cross section for WIMP-nucleon as a function of the hypothetical WIMP mass. The curve (the black line in the graph below) sizably cuts into the region of masses and cross sections allowed by different Supersymmetric models.


As I said right at the start, the parameter space of these models is so wide that a chunk always remains untouched. But, for those of us who did not believe in SUSY in the first place, this is just a nice confirmation. I should add that I would be much, much happier man if we did observe a signal of SUSY one of these days… But if I have to choose, I prefer to be in the loop and so I hope it will be LHC’s business!

More information about the result by CDMS can be found in this talk (given at Dark Matter 2008 at UCLA two weeks ago) or the relevant paper.

Update: I just discovered -better late than never- that my friend John wrote about the same result at Cosmic Variance….

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1. Chase - March 5, 2008

Really enjoyed this post… especially the “tenure truck.” It is natural that as a science becomes mature it is harder and harder to come up with new ideas that aren’t already eliminated by experiment. But maybe we need to give more credit to people whose new theories are shown to be wrong. The skill of designing slippery theories that evade failure is not one to be encouraged!

2. Myke - March 5, 2008

Hi Tomasso, my SUSY question still stands:

How would current state-of-the-art detectors handle the
appearance of sub-GeV neutral scalars?


3. superweak - March 5, 2008

I’m hardly a SUSY apologist in general – I’m that one guy in the corner who keeps mumbling about composite Higgs – but you’re being more than a bit harsh here; CDMS is roughly an order of magnitude in cross-section away from testing the bulk of the preferred region. By your logic I could say that the Tevatron experiments have excluded the SM Higgs up to 200 GeV. Sit back, relax, and enjoy the fact that there’s a preferred region on that plot at all (unlike other BSM models I could mention).

4. dorigo - March 5, 2008

Hi Superweak,

I have to disagree. My understanding of what is shown by the green and blue and grey areas is that they are not just different likelihoods of one particular model (although the way they are labeled suggests so), but rather, different models: to me, a SUSY model where four critical parameters are changed together is not the same one model, because the phenomenology becomes very different as you vary them. In fact, it is the very essence of what drove me to the initial statement in the post – we will hardly ever kill SUSY at all, for a long while more. Please see here the paper by Baltz and Gondolo (JHEP 0410 (2004)) for a reference: they vary M0, M1/2, A0, and tan(beta) freely, and apply constraints by WMAP.

So, what you call “the bulk”, is just a concentration of points, each of which is not more likely than those already ruled out just because it is neighbored by others lying around it.

Many of those points in the parameter space, in any case, are already being ruled out by the experiment. That is what I was referring to when I said SUSY is less likely today than it was three weeks ago. Even if we take MSUGRA as one single model, once you kill a quarter of the phase space, that does tell us something. We have to continue moving the bar up, saying that experimental searches have been unlucky. It is not quite as saying the Tevatron has excluded the Higgs at 200: it is about to exclude it at 160 – which is a tiny bit of the phase space yet.

Further, please note that the bulk is less than one order of magnitude away from the black curve. And indeed, the final sensitivity of CDMS will be at the bottom of the plot (black thick line shown near the x axis). It will reach there one day…


5. superweak - March 5, 2008

Of course these are distinct models – that wasn’t my point. If and when CDMS improves by an order of magnitude and still doesn’t see anything, there will be a lot of worrying; the thing is while the current results are far from being the death of SUSY, such a fate _could_ happen from experiments like this showing that reasonable WIMPs are difficult to obtain. This current result is nice, but in terms of excluding parameter space it has nothing on b -> s gamma, or the KS/KL mass difference, or EDM limits, or (insert favorite precision measurement here).

6. dorigo - March 6, 2008

Hi Chase, that is right. I think the system pays off more to those whose theories survive the longest, though, because it is based on the amount of citations one’s work receives.

Myke, the question stands but I have no answer to offer to you, because it is too vague. Saying a particle is neutral (no e.m. interactions) and a scalar (no polarization) is too little information to characterize it, even if you supply a mass value. Is it strong-interacting ? Is it weak-interacting ? With what couplings ? What model do you have in mind, in a word ?

Hi Superweak, ok – I think we have isolated the disagreement to a narrower area. I agree that the CDMS-II result says nothing about some of the phenomenology of SUSY models. It does, however, rule out a certain part of the MSUGRA space of parameters. And I agree: eventually, SUSY is testable fully. My point was that when it will be excluded, it will have been a very longeve theory, taking half a century to kill. That because of the number of parameters and distinct phenomenological implications of their variation.

Cheers all,

7. apetrov - March 6, 2008

Hmm… I think there still is plenty of parameter space for SUSY DM. If I remember it right, the lower bound on neutralino-nucleon cross-section is 10^(-46) cm^(-2) and mass of the order 150-500 GeV is still fine, (which I’d prefer for DM)… SUSY might not be IT, but it has a nice DM candidate and does solve a number of problems of the Standard Model… And it really has nothing to do with strings, even though most string theories use it. It’s like saying that Newtonial mechanics is bad since many silly theories of gravity reduce to it in the nonrelativistic limit…

My regards from La Thuille!


8. dorigo - March 6, 2008

Hi Alexey,

sure, SUSY is alive and well. Darn, there is just too much phase space open. The conclusion still holds, though: some parts of it are ruled out…
As for string theories, I have to agree: I should not blame it on SUSY whether stringers are so eager to see it confirmed…

Cheers, and have fun in La Thuile… I was there three years ago. Time for another visit next year.

9. Count Iblis - March 7, 2008

There are alternative ways to stabilize the electroweak scale, see e.g. here.

10. carlbrannen - March 7, 2008

I’d like to know why measures of area are smaller when observed across the galaxy than when observed at short distances. I realize that measures of angular area change with distance, but not area. Maybe it’s meant, “in comparison with the size of this dot seen at a distance of 1 meter.”

11. dorigo - March 7, 2008

Dear Count Iblis,

I am aware of the main alternatives. My focus on SUSY had the purpose of expressing my suspicions on theories with too many parameters and too wide variety of phenomenological implications. If string theory sucks because it does not make predictions, SUSY does because it makes too many!


12. dorigo - March 7, 2008

Hello Carl,

of course I was thinking at the dot on the i in my computer screen as seen from the other side of the galaxy as if it was there. Mine was a pictorial way of expressing the smallness of the cross section… Indeed, it has units of area, but it is not really an area after all.


13. Count Iblis - March 7, 2008

Dorigo, yes, I agree 100%. My point was just meant to make clear that one can address some of the points SUSY addresses. So, if SUSY becomes more unlikely, you would expect more research to be done in these alternative ideas. Unfortunately that is not happening…

14. island - March 9, 2008

And it won’t happen as long as there’s a chance that the LHC will give credence to any part of any theoretical extension. To me, the real question is what will convince theorists look inward.

Persnoally, I think that Dr HongSheng Zhao of the University of St Andrews in Scotland has it nearly wrapped up as two faces of the same coin, where dark energy arise from a single “dark” fluid when dark matter condensed into tangibility from the “aether”.

This works well with general relativity, but I don’t think that Dr. Zhao has it quite right, just yet. I do think that he’s correct to say that dark matter is a twin phenomenon of dark energy, which means that it will not show up at the LHC, but can been seen in galaxy clusters.

But Zhao compares his dark fluid to Earth’s atmosphere, where atmospheric pressure causes air to expand, while part of the air can collapse to form clouds. He thinks the dark fluid might generally expand, but it also could collect around galaxies to help hold them together, and I agree with this, except that the effect that drives expansion is different when you condense matter from a finite vacuum.

Course, I’ve been prejudiced*… ;) … but this also ties the “ether field” to the higgs expectation, so I’m hoping for a null result on all fronts, and I see this as the most wonderful thing that could happen for science, because it greatly simplifes the whole mess, which is exactly what the scientific method says that scientists are *supposed* to see as a good thing.

If a simpler model (with a single word) can explain all the data, then cosmologists will gladly accept it.
-Christian Boehmer of the University College London .

Sure they will, when the SM fails at the higgs scale and “beyond”, but not before their theoretically righteous selves are forced to it by this much desparation, and this makes no sense to me.

*My prejudice:

15. dorigo - March 10, 2008

Hi Island,

I have no idea what is more beautiful, whether a complex theory whose inner simplicity we as of yet do not see, or a simpler theory which explains the complexity with fewer parameters. As scientists we seek simplicity, but as human beings we appreciate the beauty of complexity…


16. Not Even Wrong » Blog Archive » HEP and Politics News - March 10, 2008

[...] not very convincing deviations from the Standard model in B-mixing and charm decays, and stringent new limits on WIMPs that make SUSY more [...]

17. Coin - March 11, 2008

Perhaps a bit of a late question but…

Dorigo, you say that “It is to be noted that strictly speaking WIMPs do not only belong to SUSY” but your analysis of the damage done by the partial WIMP exclusion only focuses on the consequences for SUSY. To be clear, do these negative results mean that WIMP dark matter itself is less likely? Or does the “90%” only refer to excluding a WIMP that happens to also be the Lightest Supersymmetric Partner?

Meanwhile, my understanding is that recent astronomical results (e.g. the bullet cluster observation) are considered to have proven that dark matter is a form of physical matter which just happens to not take part in “normal” interactions. So if we did somehow manage to exclude the WIMP… what would that tell us? Would the conclusion be that whatever dark matter is, it can only interact gravitationally? (Would these be… what, GIMPs?) Or at some point would we be forced to conclude the bullet cluster observation was somehow mistaken?

18. dorigo - March 13, 2008

Hi Coin,

sure, if one excludes nuclear interactions in CDMS one is excluding wimps, not SUSY. The latter is affected, but the result is strictly on the former. If CDMS increases its sensitivity by an order of magnitude, it will still not reach down to the lowest bound on the predicted interaction cross section (I think this is the main point made by Alexei above, which concerns SUSY but applies to generic wimps as well). In any case, a particle-physics measurement will never be able to prove that a cosmological measurement is mistaken: it will only point to other causes of what is observed.


19. Jimbo - March 13, 2008

From my view, first, the RickyHatton body-blow when the B->S switch rate was found to be at significant variance with SUSY predix.
Now, a FloydMayweather left check-hook to the jaw, has knocked SUSY down for the count, as Dorgio details….Things do not look good for her.
Nonetheless, many still champion this failed paradigm of nature because of her longevity, and life/death role in superstring/Mtheory.
These hardcore believers will no doubt still be helplessly hoping several years hence, when the LHC delivers the KO.
Fellows, its time to wakeup & smell the coffee & in SUSY’s case, the embalming fluid.

20. Coin - March 13, 2008

dorigo, I see, thanks.

21. About the DAMA-LIBRA result « A Quantum Diaries Survivor - April 17, 2008

[...] 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, [...]

22. Highlights from the morning talks at PPC08 « A Quantum Diaries Survivor - May 19, 2008

[...] another direct search for dark matter candidates. I also discussed the results of their work in a recent post (I am surprised to be able to say that and rather proud of it), so I will also not indulge in the [...]

23. Some posts you might have missed in 2008 « A Quantum Diaries Survivor - January 5, 2009

[...] Mar 5: This is a discussion of the CDMS dark matter search results, and the implications for Supersymmetry and its parameter space. [...]

24. A Quantum Diaries Survivor - January 12, 2009

[...] detector we cook up unscathed. Despite that, we of course are looking for such things, with CDMS, DAMA, and other dark-matter-dedicated [...]

25. Has Dark Matter Finally Been Detected? « Not Even Wrong - December 18, 2009

[...] Lykken seems to be ignoring the fact that the new CDMS results, two events and all, rule out yet more of the supersymmetry parameter space. For an explanation of this, written when the last CDMS results came out, already causing problems for supersymmetry, see Tommaso Dorigo’s posting SUSY more unlikely by the new CDMS II results. [...]

26. Tom - December 19, 2009

Well, if you read arXiv 0812.0980,0903.4409 & 0909.4088 you will see that CDMS has hardly made a dent in the SUSY parameter space so this claim in your title is really misleading. Maybe you can make such a claim after the limit goes down by another 2-3 orders of magnitude without seeing anyhing, but it is likely that by then the LHC will tell us one way or the other in an unambiguous fashion.

27. Matti Pitkänen - December 19, 2009

Dear Tommaso,

the title of the posting is somewhat misleading. The results eat the parameter space of SUSYs only if one identifies neutralino with dark matter. Otherwise the bound on cross section is much higher.

See this.

28. Matti Pitkänen - December 19, 2009

Sorry the link given in the previous posting was not active for some reason. I try again.

29. D R Lunsford - December 20, 2009

I wonder when people will realize that all this dark matter nonsense is based on a simple misunderstanding of GR? Namely – as with fluid flow, the non-linearity is essential – you cannot treat an extended distribution of matter in the same way as test particles in an external field! Cooperstock has shown the way, but no one listens.

In some ways, the dark matter scenario is even more depressing than string theory, because it’s just based on a lack of understanding an existing theory. Profoundly depressing, because what can you do to convince anyone when they will not read the papers?


30. 健康网 - December 20, 2009


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