Saturday, 21 April 2007

Caught in a Twister

CERN Cosmo Coffe is a tough venue for speakers. The audience is smart and inquisitive. Discussions are fierce. Not everyone survives...

Last Wednesday, Francesco Sannino was talking about dark matter abundance predicted by a certain technicolor model of his own. Francesco's research is devoted to understanding technicolor - a mechanism of electroweak symmetry breaking by new strong forces. Recently, he has been advocating a particular model that, according to him, is able to pass tight experimental constraints. This model has also a dark matter candidate. In a subsequent paper he discussed the amount of dark matter generated during
the evolution of the universe. This was the topic of the last Cosmo Coffee.

Things looked quite interesting. In the presence of the new strong forces, new particles with masses around 1 TeV appear, in a similar fashion as baryons (like the proton and the neutron) arise in QCD. These new particles are called technibaryons. There is also a global U(1) symmetry under which technibaryons are charged. The new global symmetry leads to a new conserved quantum number which, in analogy to the baryon number in QCD, is called the technibaryon number. The lightest technibaryon is stable and can be promoted to a dark matter candidate. Furthermore, the technibaryon number is violated by quantum anomalies; again, just like the baryon number in QCD. When the universe was very hot, the processes changing baryons into technibaryons (and vice-versa) were operative. The model thus relates the quanity of dark matter and visible matter. Francesco's claim was that the observed ratio of 5 between the dark and visible matter abundances could be naturally obtained.

Alas, the audience didn't buy this claim. Francesco's rather involved computation was questioned. Instead, a simple argument based on a conservation of a certain combination of baryon and technibaryon numbers was presented. This argument suggests that the today number densities of baryons and technibaryons predicted by Francesco's model are comparable. Since the technibaryons are about 1000 times heavier than the proton, the predicted dark matter density is too big by a factor of hundred. Bang, bang, you're dead...

The coming Thursday Francesco will give another talk on the related subject. We will learn if things can be saved. I'll let you know.

8 comments:

Anonymous said...

The (self proclaimed) very smart audience... seems to have hard times performing simple integrals.

The computations presented by Sannino are quite standard ones and can be found in the paper by Harvey and Turner Phys.Rev.D42:3344-3349,1990. Klebnhikov and Shaposhnikov have made some refined computations which do not alter the general results.

The claim made in the related blog is wrong. In general, the technibaryon number is not equal to the baryon number but is exponentially suppressed by a Boltzmann exponential factor. This happens if the TB-B is violated at some point after inflation and it is near conserved at the electroweak scale.

Besides, as also clearly explained by Sannino, Technibaryons, as possible dark matter candidates, are quite standard in the literature.

Last Thursday, Sannino was able to provide the complete picture and show how to construct natural DM candidates via technibaryons.

Jester said...

From what i've heard, the controversial point about dark matter was not really addressed on Thursday. Francesco only made a comment that everybody now agrees with the conclusions of his paper. Well...maybe...i haven't spoken to everybody yet ;-)
It would be good to understand what changed between that fatal Wednesday and last Thursday. You say that the combination of B and TB, which was supposedly conserved on Wednesday, is not conserved at higher energies after all. Did the model change? Besides, there was a claim that Turner's paper is incorrect. Where's the truth?

Anonymous said...

If TB-B is conserved after inflation, theory is dead. If it isn't, theory isn't dead. The audience got the impression that it was TB-B conservation that set the dark matter/baryon ratio but in his paper from august 2006 it isn't. Misunderstanding might therefore be due to bad presentation or he doesn't understand his own paper. Either way, theory may not be dead.

Francesco Sannino said...

can I make a comment?

I understand my papers perfectly, but is seems to me that there was a very limited will to understand that "fatal" Wednesday.

During my talk on Thursday I have "clearly" stated and once more explained the main points reguarding DM from technicolor. Please "jester" try to 1) read the papers and 2) pay attention to the lectures rather than asking your colleagues.

I am very happy to re-explain the main points to anyone truly interested in the scientific case rather than gossip.

Francesco Sannino

Anonymous said...

I was in both talks of Sannino and I think I know where the confusion is coming from. The audience's argument was that if TB-B is conserved all the way up to the Planck scale or inflation, TB should be of the same order with B. I am not an expert in technicolor, but I know that in order to give masses to the Standard Model particles through technicolor you need to have Extended technicolor at a higher scale than electroweak. I think that Sannino mentioned that. Since you couple quarks with techniquarks, Extended technicolor does not have to conserve TB-B. So this was (I believe) the point of Sannino. You need to have TB-B conserved close to the electroweak scale and you don't have to assume that TB-B is conserved up to inflation. He did not provide a mechanism for technibaryogenesis, since he does not know the extended technicolor, but he said that. So I don't see any obvious problem with the model. As for the Harvey paper, it is considered pretty standard and correct (modulo epsilon corrections).

Jester said...

Thanks to all who contributed so far to the discussion. Several points are clearer now. But one thing in the last comment still confuses me. IF the interactions that break TB-B are not specified (as they belong to the extended technicolor) and IF that very interactions are crucial for getting the correct amount of dark matter, how can the observed 1/5 BM/DM ratio be predicted?

Anonymous said...

Since I made the last comment before Jester, let me clarify why I think despite the ignorance of the extended technicolor model, the TB/B ratio can be predicted. By now we all agree that if TB-B is conserved all the way up to inflation, TB is forced to be equal to B and therefore the ratio is 1 and the model is ruled out. However, if we accept that TB-B was violated at some point, let's assume that once TB-B becomes conserved, TB-B is not zero but TB-B=c, where c is a number depending on the extended technicolor that we don't know. Then the ratio is TB/B = 1 + c/B. And i think this is the whole point. Through sphalerons B can be violated. The thermal equilibration of the particles drives B in such a way that no matter what is the "initial condition" c, 1+c/B gets always the "right value" ~10^(-3). So from this point of view the ratio does not depend on c, as long as c is not zero. Maybe Sannino can explain this better, but this is the impression I got from his talk.

Anonymous said...

Right, OK, this is clearer, thankyou. (if it is correct, and it sounds close to what I understand)

but, what is zero? After inflation, c is going to be EXTREMELY small, OK, maybe not zero, but is there not a way of coming up with a working definition of zero for the purposes of this calculation? for example, more than 1 per horizon? If the universe reheats at 1e10 GeV then the horizon size will be 1/(10GeV) so we would then have a number density of 1e-60 per planck volume. Running back through 60 efolds of inflation then the number density of c would be 10^18 per planck volume, unless c was created after inflation? Could this be a problem?