We almost know that its mass should be dominated by a new light particle species that is heavy enough so that it moves rather slowly relatively to the speed of light ("cold" dark matter). Because dark matter isn't gone yet, such a particle must be stable or almost exactly stable – lifetime in billions of years, to say the least.
The lightest particle carrying a "new type of charge" is the best explanation why it's stable. By far the most popular clarification what this new charge is is the R-parity, a new "sign" introduced by SUSY. All the known particles in the Standard Model have the R-parity equal to \(+1\) which is why the Standard Model interactions never produce individual superpartners whose R-parity is \(-1\). For the Standard Model particles, the R-parity may be written as \((-1)^{3B-3L+2J}\) where the odd coefficients may be replaced by any odd integers (although some values may be more correct for the exotic particles).
The stable particle is then the lightest particle with the R-parity equal to \(-1\), the lightest superpartner, the so-called LSP: it's almost all the supersymmetric models' candidate for the WIMP, the new dark matter particle. If you look just superficially, it may be the superpartner of any elementary particle in the Standard Model. The R-parity has just two possible values (signs) so it forms a group isomorphic to \(\ZZ_2\). The number of R-parity-odd particles in a physical object is therefore conserved modulo two. In particular, they may be pair-created at the colliders and they're expected to pair-annihilate in the outer space.
However, if the LSP were a superpartner of a charged particle, it would be charged, too. And charged particles interact with the electromagnetic field. They may be seen. They're not dark. I am sort of convinced that one may construct viable theories of the observed data with non-dark matter as well – with "superheavy hydrogen atoms" that have a different charged particle as the nucleus, for example, pretend to be hydrogen, but store much more mass – but let's assume that the dark matter is dark, indeed.
In the literature, most models assume or conclude that the LSP is either the gravitino – the spin-3/2 superpartner of the graviton (in that case, the R-parity may be broken because the gravitino is still nearly stable due to the gravitational i.e. very weak character of its interactions) – or, much more frequently, the lightest neutralino. The neutralino is the spin-1/2 superpartner of an electrically neutral Standard Model elementary particles. Because there are several such particles (components of the W-boson, photon, and the Higgs field), there are several neutralinos. The mass of such neutralinos is given by a matrix and the "sharply defined mass" eigenstates are general superpositions of the photino, zino, and higgsinos – four neutralinos.
Physical technicalities usually force us to adopt the mass of hundreds of gigaelectronvolts for such a neutralino if it is the LSP. Such a "relatively heavy" neutralino couldn't explain the positive hints in the dark-matter direct search experiments that indicate the existence of a sub-\(10\GeV\) WIMP. The SUSY phenomenological literature is really obsessed by the idea that the LSP has to be a neutralino. It almost looks like a Christian dogma and different cliques seem to fight whether the neutralino LSP is mostly a wino (Catholic) or a bino (Protestant) or a higgsino (Orthodox). I am afraid that the certainty that the LSP has to be a neutralino is one of the not quite justified assumptions that are adopted because of group think, not because of solid evidence.
Now, building on various previous papers such as this 2006 paper and the authors' 2012 paper (PRD), Ki-Young Choi (Korea) and Osamu Seto (Japan) propose an attractive label for this new hypothetical \(8.6\GeV\) or so dark matter particle: a light Dirac right-handed sneutrino.
Light Dirac right-handed sneutrino dark matterWhat does it mean? The adjective "light" means that its mass is supposed to be much smaller than hundreds of gigaelectronvolts. The word "sneutrino" says that it's the superpartner of a neutrino; the prefix "s-" may be interpreted not just as "the superpartner of" but also as "scalar" because these superpartners of spin-1/2 fermions are scalar fields and particles, i.e. spin-0 particles. The adjective "right-handed" means that it's the superpartner of a right-handed neutrino – all the neutrinos we directly observe today are left-handed (while the antineutrinos are right-handed but we are not talking about any antineutrinos with \(L=-1\) here at all). Finally, the adjective "Dirac" means that the neutrino is assumed to have mostly Dirac (and not Majorana) masses, coming from the bilinear product of the left-handed and right-handed neutrino components. Note that the adjective "Dirac" says nothing about the sneutrino per se; we first say that the neutrino is Dirac and then add the "s-" prefix. How the neutrinos get their mass is important for the sneutrinos, too.
The Asian authors claim that they can write down a model of this sort that is pretty much consistent with all the data – positive and perhaps even negative (XENON100) direct search experiments; experiments measuring the invisible width of the Z-boson and the Higgs boson (including the "effective number of neutrino species").
It would be sort of fair if the superpartner of the most invisible Standard Model particle – a neutrino – became the first visible superpartner, albeit still a "dark one". ;-)
Don Lincoln of the Fermilab attempted to explain supersymmetry in 6 minutes today. See the video above. By the way, 10,000 papers on SUSY is probably a huge underestimate. It depends what you count but over 100,000 papers may be mentioning supersymmetry.
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