Longevity of Stau (supersymmetric particle)

[shelanachium]

In the New Scientist of 5 July 2008 is an article 'Crucible of Creation' on pp 28 - 31. Part of it concerns the ratio of the isotopes Li-6 and Li-7 which formed just after the Big Bang. They discuss the work of Maxim Pospelov of the Perimeter Institute in Waterloo, Ontario.

Pospelov proposes that the Li-6/Li-7 ratio was affected by the presence of the stau, supersymmetric partner of the tau lepton, which catalysed fusion reactions involving beryllium-7 with protons (thereby preventing much Be-7 from decaying to Li-7, instead converting it to He-4 via B-8); and later of helium-4 with deuterons to give lithium-6. This latter reaction is said to have taken place THREE HOURS after the Big Bang.

Therefore the stau must have a half-life of the order of hours, far longer than that of any other unstable particle. Why is this thought possible or likely? To what would the stau decay?

Such a long-lived particle must surely form in energetic cosmic-ray reactions. It should be far more penetrating than the similarly charged muon, because of its great mass. So why hasn't it already been found? - for example in experiments in deep mines looking for proton-decay or cosmic neutrinos.

 

[mathman]

To the best of my knowledge, no supersymmetric particles have been found as yet.

 

[humanino]

I found two possible references :
Particle Physics Catalysis of Thermal Big Bang Nucleosynthesis, Maxim Pospelov, Phys. Rev. Lett. 98, 231301 (2007)

If the gravitino mass is in the region from a few GeV to a few 10's GeV, the scalar lepton X such as stau is most likely the next lightest supersymmetry particle. The negatively charged and long-lived X⁻ may form a Coulomb bound state (AX⁻) with a nucleus A and may affect the big-bang nucleosynthesis through catalyzed fusion process. We calculate a production cross section of ⁶Li from the catalyzed fusion $$^{4}\text{He}X^{-}+d\rightarrow \,^{6}\text{Li}+X^{-}$$ by solving the Schrödinger equation exactly for three-body system of ⁴He, d and X. [...]

Stau-catalyzed ⁶Li production in big-bang nucleosynthesis, K. Hamaguchia, T. Hatsudaa, M. Kamimurab, Y. Kinoc and T.T. Yanagidaa, Physics Letters B Volume 650, Issue 4, 5 July 2007, Pages 268-274

We point out that the existence of metastable, $$\tau>10^{3}$$ s, negatively charged electroweak-scale particles ($$X^{-}$$) alters the predictions for lithium and other primordial elemental abundances for A>4 via the formation of bound states with nuclei during big bang nucleosynthesis. In particular, we show that the bound states of $$X^{-}$$ with helium, formed at temperatures of about T=10⁸ K, lead to the catalytic enhancement of ⁶Li production, which is 8 orders of magnitude more efficient than the standard channel. [...]

Interesting.

Therefore the stau must have a half-life of the order of hours, far longer than that of any other unstable particle. Why is this thought possible or likely? To what would the stau decay?

No. The neutron is unstable in a free state but stable in a nuclei. The stau in a bound state can live for as long as three hours. This is explicity mentionned. But on the other hand, this is just a model, which for once has an interesting property of leading to observable predictions.

 

[shelanachium]

Thanks for references, Humanino! I did know about free neutrons, though minutes rather than hours. My main point was that if the long-lived stau does exist, it should already have been found in secondary cosmic rays. It hasn't, so probably does not exist.

 

[Vanadium 50]

Such a particle would probably be less penetrating than a muon - take a look at the Bethe-Block curve: as a particle gets heavier, it slows down, moves to the left where the ionization is larger. In fact, since the typical muon sits very close to the minimum of that curve, pretty much anything will ionize more.

But suppose you do make them. How would you tell that you have a stau instead of something else in a cosmic ray experiment? Tracks don't come with a label on them saying "this track was made by a proton". PS What is the boundary between HENPP and BSM? I would have thought that a post about something that is supersymmetric would clearly be BSM, but the BSM section seems to be only about stringery and alternatives.

 

[blechman]

the stau is often a candidate for the LSP (lightest susy particle) or the NLSP, in many models; for example in models of "gaugino mediation" - and can therefore potentially be very long lived. that's why some people favor this possibility.

PS What is the boundary between HENPP and BSM? I would have thought that a post about something that is supersymmetric would clearly be BSM, but the BSM section seems to be only about stringery and alternatives.

There never was a clear boundary. unfortunately, SUSY has gained enough notoriety that it is sometimes not given its full "speculative" value. but i would think anything that could be even remotely relevant for the LHC, including SUSY and its BSM cousins, have a home here!