Does Dark Matter Really Exist?

[kenel]

There is a very interesting article on the credibility of the theory about Dark Matter on slashdot.org.

 

Check it out here:

http://slashdot.org/article.pl?sid=04/02/09/128223

[Gampin]

kenel,

 

check this out.

[ydoaPs]

dark matter is probably planets, asteroid belts, moons, comets, ect.

[MacM]

I personally doubt it does. At least not as some exotic matter but only as some minor unobserved components. The galatic rotational velocity issues has an alternative gravity explanation.

 

See:

 

http://www.scienceforums.net/forums/showthread.php?t=4934

 

This is my own idea so take it for what it is worth.

[Ubermensch]

dark matter is probably planets, asteroid belts, moons, comets, ect.

Uhm, no.

 

By observation, the Sun = 98% of the mass in this solar system. Jupiter = 1%, the assorted debris = 1% total.

 

Thus, the stellar mass of the universe = 98% of the non-Dark mass of the universe.

 

Gravitational calculations result in Dark Matter being as much as 75% of the mass of the universe.

 

Thus, all the stars and black holes and white dwarves of the universe = 24% of the universe; the "planets, comets, assorted debris" is about 1%; and the Dark Matter is the rest.

[Firedragon52]

Is it possible that Dark Matter could be connected to the Multiverse?

[Severian]

Dark matter is probably a neutralino [math]\tilde \chi_1^0[/math].

 

(I am giving a talk on this at the 5th International Workshop on the Identification of Dark Matter, tomorrow.)

[drz]

it is the secret hidden dimension where all of our missing socks go.

[Thales]

One problem I have with the neutrino explaination is that don't these particles move near the speed of light and doesn't that mean they would very quickly be radiated into intergalactic space where there is very little evidence for dark matter (the opposite is dominate infact). Hot dark matter has neversat well with me but if you have the time Severian, I would really appreciate an explanation to how a particle travelling at near c can pertain a gravitational influence on the rotational curves of galaxies given that they would be off the scene so quickly.

[Martin]

One problem I have with the neutrino explaination is that don't these particles move near the speed of light and doesn't that mean they would very quickly be radiated into intergalactic space where there is very little evidence for dark matter (the opposite is dominate infact). Hot dark matter has neversat well with me but if you have the time Severian, I would really appreciate an explanation to how a particle travelling at near c can pertain a gravitational influence on the rotational curves of galaxies given that they would be off the scene so quickly.

 

Apparently the neutralino in question has a mass between

50 and 100 GeV, which is very heavy

(proton is only around 1 GeV)

 

and in theory it is stable, so if a lot of them were produced at big bang time they would still be drifting around

 

they are not like the neutrinos we have heard about which go almost the speed of light and have very small mass on order of just a few eV.

these things are billion times more massive

 

it would be good if Severian could explain to us what is the neutralino so we do not confuse it with neutrino

 

Thales here is a relevant paper, if you want to look at something about it

hep-ph/0408102

[Severian]

Thales talked about a neutrino, rather than a neutralino, and he is correct - the neutrino would not explain the dark matter sitting in the halo of galaxies. They could only explain the part of dark matter which is needed to explain the WMAP data (which is a lot more than galactic halos or MACHO's can explain).

 

Neutralinos on the other hand are (as Martin said) heavy. They are actually supersymmetric particles. If supersymmetry is true then every particle gets a partner which differs by spin 1/2. So the spin 1 gauge bosons (the W, Z, gluon and photon) all get partners which are spin 1/2 called gauginos, and the Higgs bosons get partners which are spin 1/2 called higgsinos.

 

Now the neutral gauginos and higgsinos all have the smae quantum numbers so what you see in reality(?) is a mixture of them called neutralinos -- part higgsino and part gaugino. In minimal supersymmetry, there are 4 neutralinos: 2 gauginos (partners of the Z and the photon, and two higgsinos, partners of the Higgs bosons h and H).

 

In minimal supergravity, the lightest neutralino is mainly the gaugino partner of the photon (to be more precise it is usually mainly the partner of the U(1) hypercharge boson, called the bino).

 

Supersymmetry has an odd property called R-parity, which is needed to stop protons decaying, R-parity says that if a supersymmetric particle decays, it must produce another supersymmetric particle. Since the lightest neutralino is often the lightest supersymmetric particle (LSP), it cannot decay and is stable. This is what makes it a good (cold) dark matter candidate.

 

The paper that Martin quotes is not a very good one for reading up on this because it is about a non-minimal model called the NMSSM (Next-to-Minimal-Supersymmetric-Standard-Model). In that model, there is an extra Higgs boson and its susy partner (a higgsino) is the LSP. A higgsino LSP has different properties from a bino like LSP, so this is not very standard. It is an interesting model though. It also has problems with domain walls in the early universe. You can read up on this theory in a slightly more pedagogic form in hep-ph/0304049 and hep-ph/0407209.

 

Alternatively you can read up about Cold Dark Matter at the LHC in hep-ph/0406147 and hep-ph/0403047.

[Martin]

Dark matter is probably a neutralino [math]\tilde \chi_1^0[/math].

 

(I am giving a talk on this at the 5th International Workshop on the Identification of Dark Matter' date=' tomorrow.)[/quote']

 

Severian, I am wondering what the reasons are for saying "probably" dark matter is this neutralino of mass 50-100 GeV

[math]\tilde \chi_1^0[/math].

 

I know very little about this particle -----it it correct to say that it is a hypothetical particle depending on a supersymmetric extension of the standard model---as yet unobserved?

How sure can one be that the mass is what they say: between 50 and 100 GeV?

 

Hope your talk went well at the Edinburgh conference!

[Severian]

Severian' date=' I am wondering what the reasons are for saying "probably" dark matter is this neutralino of mass 50-100 GeV

[math']\tilde \chi_1^0[/math].

 

I know very little about this particle -----it it correct to say that it is a hypothetical particle depending on a supersymmetric extension of the standard model---as yet unobserved?

 

Yes - that is true. It is unobserved

 

The reason I believe it to be dark matter is mainly because I believe supersymmetry exists. And if supersymmetry with R-parity exists, then the LSP will contribute to dark matter. The fact that a neutralino of a 100GeV or so would provide almost exactly the right amount of dark matter to explain the WMAP data is extra evidence. Can this just be coincidence?

 

 

How sure can one be that the mass is what they say: between 50 and 100 GeV?

 

The lower bound is easy - since we have been looking for it and not found it, it must be heavier than 50 GeV or so. If it was lighter than this we would have seen it at LEP.

 

The upper bound is more tricky to explain. Firstly there is the WMAP data: if the neutralino is too heavy, then it gives too much mass to the universe and disagrees with the WMAP 'relic density' measurements. But if I recall correctly, this is not 100GeV, but quite a bit more (you can play tricks like having the neutralino be half the mass of a heavy Higgs boson, to increase its annihilation rate, and thus its contribution to dark matter densities), maybe 200GeV or so.

 

Then there are theoretical prejudices. For example, if one believes in unification of masses at the GUT scale (very high energies), then the LSP neutralino becomes quite light in comparison to other susy particles (this is because it is mainly a bino, so doesn't feel the SU(3) or SU(2) forces which would push the mass up). Then, if you want supersymmetry to solve phenomenological problems at low energies, there is an upper limit on the susy masses, placing an upper limit on the neutralino mass. As I said though, these are prejudices and don't have to be so.

 

Where did you get the 100 GeV figure?

 

Hope your talk went well at the Edinburgh conference!

 

Hmm.. I wasn't 100% happy about it, but it went OK I suppose.

[Martin]

Apparently the neutralino in question has a mass between

50 and 100 GeV' date=' ...

... a relevant paper, if you want to look at something about it

hep-ph/0408102[/quote']

 

You asked about the estimate 50-100 GeV, especially the upper bound 100.

I just took that from a recent (7 Aug 2004) paper by D.Cerdeño.

"Theoretical predictions for the direct detection of neutralino dark matter in the NMSSM"

 

I am glad to be able to talk with someone who has been to that conference.

Also glad your talk went OK.

Also I would like to know if anybody else's talks were especially interesting.

can there be observational signatures in some kind of cosmic ray or gamma ray that show existence of particles which one cannot yet prove using accelerators?

What observational talks seemed good to you? I am eager to hear a little more.

[Severian]

OK - I understand the 100 GeV bound now. This is for the NMSSM, not the MSSM.

 

In next-to-minimal models the Higgs boson can be considerably lighter than the LEP experimental 'bounds' because the coupling to the Z boson can be much less. This is because the main LEP channel for producing a Higgs would be [math]e^+e^- \to Z^* \to ZH[/math]. By reducing the ZZH coupling, you reduce the the cross-section of the above reaction (known as Higgs-strahlung) and therefore LEP would not have seen it.

 

Coincidentally, I pointed this out first in hep-ph/0403137 (so now you know who I am). I can't believe these idiots didn't cite my paper - they definitely know about it because Hugonie has cited it before - they are just being jerks.

 

Anyway, politics aside, they are saying that in these 'interesting scenarios' where the Higgs in so light the lightest neutralino is in the mass range 50-100GeV. In other words, they are not saying that it is an upper bound on the nuetralino - just that it has to be light if you want such a light Higgs boson (this is basically because the it is the scale [math]\mu[/math] which sets both masses).

[Martin]

 

Coincidentally' date=' I pointed this out first in hep-ph/0403137 .[/quote']

 

congratulations on the paper! (one of many)