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Dark matter candidates, what chances would you give them?

by EL
Tags: candidates, chances, dark, matter
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EL
#55
Mar12-06, 04:34 PM
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Quote Quote by Garth
I was pulling your leg!
Yeah, I noticed that, just had to defend my case anyway...My capital letters maybe were too much, or at least I should have added a smiley in the end...so I'll do it now instead...

But the serious point is that we need to identify the DM particle in the laboratory, measure its properties and prove they are concordant with the cosmological constraints and then and only then will we know what we are talking about.
Of course. But that's the way it goes for every suggested solution to the problem: We have to find a way to measure its validity!

It is this lack of confirmation that continues to render the standard [itex]\Lambda CDM[/itex] model provisional.
Yepp, as well as all others...
EL
#56
Mar12-06, 05:18 PM
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Quote Quote by turbo-1
There has been a tremendous expenditure of resources of all types (including peoples' entire careers) thrown at this problem, but I have a question. Why are we building higher-energy colliders to look for the LSP?
Well, the LSP is just one out of plenty of reasons why we build colliders like LHC. Maybe the most interesting to come out of LHC is something we had not even thought about. But anyway, let's move on:

The concentration should be on the building of detectors, because if the the LSPs exist, they should be everywhere.
Sure, and there are a lot of experiments running and being developed for direct detection. These will probably reach enough sensitivity to scan at least some part of the LSP parameter space in a few years. Besides particle physics constraints on the cross section for interaction with the detector, the chance of succeeding also highly depends on astrophysical conditions; we're not exaclty sure of what local density of DM particles to expect. If we're lucky we'll find some evidence for DM through direct detection even before LHC, but due to that the LSP probably interacts very tiny with ordinary matter I personally doubt that.
However, once (and if!) we find the LSP at CERN we of course also need to confirm that it really makes up the DM, so direct detection is of course needed for the complete confirmation.

If lightest supersymmetric particle is truly the lightest, there is no lighter supersymmetrical particle that it can decay to, meaning that if they exist, the universe should be teeming with them already. Every one ever produced still exists - they are immortal.
You're are right to a certain extent. The LSP is forbidden to decay into other particles through a multiplicative quantum number called R-parity which is conserved in supersymmetric theories. (Well, actually there are SUSY theories without conserved R-parity, but for several reasons those are not as intersesting, but let's keep this simple.) Ordninary particles have R-parity (1) while their supersymmetric partners carry (-1). That's why a single LSP cannot decay into anything less massive. However, two particles with R-parity (-1) each, that is a total of (-1)*(-1)=(1) can annihilate into a standard model pair, like two gammas. Observing these gammas (for example from the dense region in the galactic centre) in fact is a way to indirectly detect the LSP, and much work has been put down on clearing such things out too. However it's not clear wheter this signal will drown into the background from other astrophysical sources.

The fact that LSPs have not been detected already should be sobering to the guys building and equipping the collidors.
The missing signal from direct detection of course puts limits on what properties the LSP might have, but the parameter space where the LSP can be is still HUGE, and direct detection need to become much more efficient before it can rule out the LSP as a DM candidate.

Has this been discussed in the literature, EL?
Oh yeah, in hundreds, maybe more, papers over the years. A good one to start with is maybe Jungman et al:
http://www.arxiv.org/abs/hep-ph/9506380
It's certainly not up to date, but the concepts are nicely explained.
turbo
#57
Mar12-06, 06:08 PM
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Please allow me to clarify, EL. When I asked if "this has been discussed on the literature", I was refering not to "LSP as Dark Matter", which is self-evident, but to the concept that the current non-detection of LSP is a problem for the Standard Model.

I have a very compelling reason to believe that there is no Graviton, no Higgs Boson, no SUSY particles, etc, which I cannot elucidate here due to forum rules, so this is an important subject for me.
EL
#58
Mar12-06, 06:17 PM
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Quote Quote by turbo-1
When I asked if "this has been discussed on the literature", I was refering not to "LSP as Dark Matter", which is self-evident, but to the concept that the current non-detection of LSP is a problem for the Standard Model.
But it is not a problem for the Standard Model that the LSP has not been detected yet. Why should it be?
turbo
#59
Mar12-06, 06:50 PM
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Quote Quote by EL
But it is not a problem for the Standard Model that the LSP has not been detected yet. Why should it be?
Because the LSP (if it exists) should permeate the Universe, and even if it is extremely weakly interactive, the odds are that we should have seen some hints that they exist. So far, none yet.
Chronos
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Mar12-06, 07:39 PM
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The lack of detection is not an issue in the minds of most theorists. Historical, some ideas take longer than others to meet the burden of proof. The atom is a good example. First conceived by Democritus in 460 BC, their detection was not achieved until around 1803: when Dalton conducted experiments suggesting matter was indeed composed of elementary, tiny particles [atoms]. Even so, it was another century before Rutherford and Wilson achieved the first real 'proof' of the atom.

Neutrinos [the other dark matter] also proved elusive. Pauli predicted their existence 1931. First detection was not achieved until 1959 by Cowan and Reines, and the elusive tau neutrino was not detected until 2000. Researchers did not give up on the neutrino for the same reasons they have not given up on their more introverted DM relatives.
turbo
#61
Mar12-06, 09:01 PM
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Quote Quote by Chronos
The lack of detection is not an issue in the minds of most theorists.
This is a problem. If your cosmology is hanging on something that has never been detected (even tentatively) then the theorists need to study obervations for a bit and come up with some new speculations. If theorists are allowed to frame the question and reject all observations that conflict with their assumptions, we are in a very unhealthy situation.
EL
#62
Mar13-06, 03:54 AM
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Quote Quote by turbo-1
Because the LSP (if it exists) should permeate the Universe, and even if it is extremely weakly interactive, the odds are that we should have seen some hints that they exist. So far, none yet.
What you are are saying is simply not correct.
Theory of supersymmetry, together with simulations telling us what local density of DM we could expect, indeed favours the situation that we have not yet detected the LSP! With current experiments we have just started to scratch the surface of the huge parameter space where the LSP could live.
However, with the LHC together with future direct detection experiments, a major part of the parameter space can be searched trough.
You can read about all this in the Jungman link I gave you.
EL
#63
Mar13-06, 03:59 AM
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Quote Quote by turbo-1
If theorists are allowed to frame the question and reject all observations that conflict with their assumptions, we are in a very unhealthy situation.
But theorists don't reject any observations, where have you got that from?
Instead all observations and experiments put limits on the theory, i.e. they reduce the parameter space of the LSP.
(Here are some recent results: http://www.arxiv.org/abs/hep-ph/0602028 )
For every new experiment/observation the parameter space is cut down by another small amount, but there's still a huge piece left over.
Again, you can also read about this in Jungman.
turbo
#64
Mar13-06, 07:56 AM
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EL, you posted a link to Jungman's table of contents, not to the paper, and the embedded URLs in the abstract do not work either. I tried Googling on "Supersymmetric Dark Matter" and got over 40,000 hits - too many to wade through. Do you have a link to the full PDF?
EL
#65
Mar13-06, 11:26 AM
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Quote Quote by turbo-1
EL, you posted a link to Jungman's table of contents, not to the paper, and the embedded URLs in the abstract do not work either. I tried Googling on "Supersymmetric Dark Matter" and got over 40,000 hits - too many to wade through. Do you have a link to the full PDF?
Oops, sorry for that . No I don't have a link to the PDF, but I managed to download it from Physics Reports. However if you don't have access to that journal it may be hard to find it for free (leagaly).

Try this paper by Bergstrom instead:
http://www.arxiv.org/abs/hep-ph/0002126
It's not as detailed as Jungman, but instead easier to follow, and somewhat more up to date (although a lot has happened during the last years). Check out chapter 8-9 in specific.
SpaceTiger
#66
Mar13-06, 12:03 PM
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Quote Quote by turbo-1
Because the LSP (if it exists) should permeate the Universe, and even if it is extremely weakly interactive, the odds are that we should have seen some hints that they exist.
Oh really? Could you please show us this calculation?
turbo
#67
Mar13-06, 01:49 PM
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Quote Quote by SpaceTiger
Oh really? Could you please show us this calculation?
http://arxiv.org/PS_cache/astro-ph/pdf/0504/0504241.pdf
Quote Quote by paper
For WIMPs with masses of approximately 100 GeV/c2 (the mass of a A=100 nucleus, we will see later the motivation for this example), the local density is 3000 WIMP per cubic meter, and a flux of 6x104 WIMPs is traversing each cm2 of our body every second. Another important aspect is that the average kinetic energy of these WIMPs is 20 keV.
If the LSP can self-annihilate in pairs, the energy released in such annihilation should be pretty significant, with a "signature" energy or energies (depending on the nature of the decay particles). If the LSP is truly "weakly interactive" it cannot be excluded from the detectors of the accelerators around the world, yet such an annihilation event has never been observed (or at least none have been recognized and reported, to my knowledge). With such a high WIMP flux impinging on detectors at accelerators, shouldn't we have observed WIMP annihilation serendipitously by now? It would appear as if the decay particles arose spontaneously, with excess energy carried of as gamma rays.
EL
#68
Mar13-06, 03:47 PM
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Quote Quote by turbo-1
If the LSP can self-annihilate in pairs, the energy released in such annihilation should be pretty significant, with a "signature" energy or energies (depending on the nature of the decay particles).
People have investigated what signatures to expect. See for example:
http://www.arxiv.org/abs/hep-ph/0507229
However, it's not clear wheter it will completely drown into the background from other astrophysical sources or not.

If the LSP is truly "weakly interactive" it cannot be excluded from the detectors of the accelerators around the world, yet such an annihilation event has never been observed (or at least none have been recognized and reported, to my knowledge). With such a high WIMP flux impinging on detectors at accelerators, shouldn't we have observed WIMP annihilation serendipitously by now? It would appear as if the decay particles arose spontaneously, with excess energy carried of as gamma rays.
First of all: The DM direct detection detectors are not part of any accelerators. Detectors in accelerators are design to detect what's produced in the accelerator. Direct detection detectors are designed somewhat similar as neutrino detectors. For example Edelwise (which is mensioned in your quoted paper) is situated several kilometers inside a mountain, at the border between France and Italy (actually I recently visited Edelwise), in order to reduce the background.

For the second: Have you even taken your time to read the paper you're citing? In section 4.5 it says:
"As current experiments are more than four order of magnitude away from a full coverage of the bulk of supersymmetric predictions, the coming years may reveal that the ultimate sensitivity can only be reached by detector techniques that are now in a very early development stage."
And in the conclusions it clearly states:
"there is still a lot of development in progress on the road to the 10^−8 pb sensitivity of current projects to the ultimate 10^−10 pb sensitivity necessary to cover most of the MSSM domain."
That is, we need to get to 10^-10 pb sensitivity before most of the LSP parameter space can be covered by direct detection experiments.
At the moment we are, as said before, just scratching the surface.

For the third: Were is the calculation Space Tiger asked for? All your cited paper succeeded with was to completely debunk your own claims.

Suggested reading is still the Bergstrom paper I linked. Please feel free to ask about things you don't find clear.
SpaceTiger
#69
Mar13-06, 03:50 PM
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An excellent link, thank you turbo-1. Unfortunately, you seem to have seriously misinterpreted the paper. Skip ahead to section 3.4, titled "Current status of direct searches":

Quote Quote by Gascon 2005
At present, the most competitive direct searches have reached sensitivities close to 10-6 pb. This starts to explore the domain of optimistic supersymmetric models.
In other words, the majority of WIMP candidates from theories of supersymmetry are undetectable by these experiments.
turbo
#70
Mar13-06, 04:26 PM
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Maybe I wasn't clear. Direct detection of LSP may be some time away, but given the flux density of this proposed DM candidate, should we not have seen (serendipitously) the spontaneous production of the decay products of the LSP in at least some accelerator detectors by now? If the decay products and the energy released falls in the mass range of the LSP, that would be a very good indirect detection, and help nail down the sensitivities needed for direct detection. If the LSP is indeed weakly interactive, there is no way to exclude those particles from the detector chambers. Certainly the people monitoring that equipment have an idea what such a serendipitous observation should look like, including a range of energies that might be released.

We shouldn't have to concentrate on trying to make these WIMPS if they are as plentiful as expected. We should simply start watching for pair-decay. If the Standard Model is correct, the Universe has already produced all the LSPs we need and we can use existing detectors to watch for the decay of the LSPs, which will appear to us as if the decay products simply popped into existence with an accompanying realease of energy.
EL
#71
Mar13-06, 04:54 PM
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Quote Quote by turbo-1
Maybe I wasn't clear. Direct detection of LSP may be some time away, but given the flux density of this proposed DM candidate, should we not have seen (serendipitously) the spontaneous production of the decay products of the LSP in at least some accelerator detectors by now?
Well, the LSP cannot decay, but I suppose what you mean is annihilation products produced by collisions between LSP's.
Think of this: Interactions between WIMPs and ordinary matter in the direct detection experiments are not frequent enough to be detected. The density of ordinary matter in a detector is way higher than the expected local WIMP density. Which event should occur more often: WIMPs interacting with the detector, or WIMPs interacting with WIMPs? What conclusion can be drawn?
turbo
#72
Mar13-06, 06:30 PM
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From what I have read (and I will qualify this by saying that I have a physical revulsion to tacking over a hundred new dimensionless parameters on the standard model to extend it with MSSM, so I have not been a real fan of any brand of SUSY) LSPs (in pairs) can decay in pairs into lighter baryonic particles plus gamma rays. Given the predicted flux of LSPs, shouldn't we have observed at least one such decay by now? If not, why not? Indirect detections of LSP seem a whole lot more likely than direct detections.


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