## Wednesday, August 18, 2010 ... //

### Alpinekat on sneutrino inflation

Nude Socialist has finally published an article that may be imperfect and the model it promotes is probably untrue: but it made me smile in approval. The smile began with the title and the name of the author. ;-)

Kate McAlpine wrote an article called

Closing in on the inflaton, mother of the universe.
As you should know, McAlpine is the author of the renowned LHC rap video which has attracted 6 million visitors to YouTube:

If you need a version with a singer you're familiar with, here it is. ;-) Well, I admit that I have recorded it at a lower speed (by a factor of nearly two) and sped it up artificially later: rap is too fast a genre for me, and not only in English. :-)

She highlights two papers on inflation in supersymmetric models:

Antusch et al.:
Gauge Non-Singlet Inflation in SUSY GUTs (March 2010)
Sneutrino Hybrid Inflation and Nonthermal Leptogenesis (July 2010)
Both of them have 0 citations at this point but the citation counting didn't have a terribly good record to localize quality articles recently, so let's ignore this not too relevant sociological point.

The cosmic microwave background has a nearly uniform temperature. However, deviations of order 10^{-5} - i.e. 1/1,000 of a percent - exist in its temperature. These non-uniformities or anisotropies, as they're more accurately called, are being observed by probes such as WMAP. Inflation naturally produces a uniform Universe. However, it could be too uniform if the typical mass scale associated with the inflaton were too low.

The measured number 10^{-5} links the inflaton to the vicinity of the GUT scale, about 2-3 orders of magnitude beneath the Planck scale. Is there something near the grand unified scale that could play the role of the inflaton - so that we don't have to find "new" particles and instead, we may "efficiently" use something that we already have?

Well, the particle responsible for inflation has to be electrically neutral (I hope but I am actually not so sure) and it has to be a spinless boson. Supersymmetry offers you lots of new spinless bosons - they're the superpartners of the known spin-1/2 fermions.

Because the inflaton probably (?) has to be electrically neutral, we have to talk about superpartners of neutral elementary fermions: and the neutrinos are the only examples. In other words, the neutral elementary fermions are called neutrinos. However, the neutrinos are very light and assuming a low-energy supersymmetry breaking, the masses of the sneutrinos should be pretty close i.e. also low, only around a TeV or so.

So we must talk about particles whose masses are close to the GUT scale. The right-handed neutrinos - whose heavy mass produces the light mass of their left-handed cousins via the seesaw mechanism ("mLIGHT = mELECTROWEAK^2 / mHEAVY", which comes from the lighter eigenvalue of a matrix with 3 entries) - match the criteria. Their mass should be near the GUT scale. The same should hold for their superpartners, the heavy sneutrinos.

The models with the heavy sneutrinos as inflatons have to be models of "hybrid inflation" (inflation with several scalar fields - the normal inflaton and additional scalars that trigger the end of inflation) - the first indication that the assumptions may force us to make things contrived - and the authors state that the inflaton gets it big mass from a "waterfall" phase transition. The baryon asymmetry is produced via non-thermal leptogenesis. They have similar conceivable answers to all these basic questions of creation.

Those models may look (and be) stretched and people may give you some memorized criticisms why these models can't work. Well, the main type of a problem with your favorite inflaton is that its potential is unlikely to be flat enough to admit slow-roll inflation if you choose too well-known a particle for the inflaton. I hope that the main arguments will be presented by someone in the comments.

However, the authors present some quasi-solutions to the slow-roll problem. They are apparently driven by the wishful thinking that the inflation - which is usually linked to inaccessible high scales - could also be mixed with some low-energy physics, namely the physics of supersymmetry that should be observable by the LHC. The seesaw could help the LHC to swing to the very high, nearly fundamental, energy scales.

While I hate theories driven purely by the desire to predict something for the experiments in the near future (and to increase the chance of the authors to get famous soon, regardless of the question whether there is actually reason why they should be - whether the theory is right), I must admit that this efficient representation of the inflaton is kind of attractive.

On the other hand, I do believe that one can naturally get many - or at least several - scalar fields with masses around the GUT scale from promising stringy models so the argument based on "efficiency" (minimization of the number of particle species) could be wrong. Well, it could also be right. A vacuum selection mechanism we don't yet understand could favor a minimization of the number of scalar fields at the GUT scale or below it.

We will see. Or the generations in a very distant future will see. ;-)