Wednesday, June 16, 2010

Steven Weinberg on LHC, Higgs, SUSY, dark matter

Half a year ago, Steven Weinberg gave a nice talk about the LHC and related stuff.



The introduction by a woman takes the first minute. Around 4:30, Weinberg finally learns how to speak to the microphone. ;-) But don't get too optimistic prematurely: he unlearns it before 8:00 again. At 22:00, he uses the microphone as a pointer. But he's re-taught the microphone lesson at 25:40.

Weinberg talks about the Standard Model - Weinberg gave it the name, besides completing the content of theory. :-) The SU(2) x U(1) Higgs mechanism is Weinberg's personal contribution to the electroweak theory; it was made independently by Abdus Salam.

The group-independent discovery of the Higgs mechanism is due to the winners of the 2010 Sakurai Prize, namely Robert Brout, Francois Englert, Gerald Guralnik, C. R. Hagen, Peter Higgs, and Tom Kibble. Tom Kibble therefore uses an awkward name for the Higgs mechanism.

Near 17:20, we learn that if the Congress hadn't have the imbecility to cancel the SSC, the Higgs would have been discovered here in Texas. :-)




Around 20:00, Weinberg discusses technicolor that he attributes to himself and Leonard Susskind. He thinks it's unlikely to be seen because the theory has to be adjusted to agree with the data and it looks like the Ptolemaic epicycles as the result.

At 21:00, he jumps on the best motivated scenario for new physics at the LHC, supersymmetry. Some Russian and Western history of SUSY is outlined. The new symmetry acts horizontally in his table - linking different spins.

Around 23:30, we hear that years ago, Gell-Mann and Weinberg debated what the naming scheme for superparticles is. Gell-Mann argued it was a slanguage while Weinberg thought it was a languino. :-)

After 24:00, SUSY is presented as a moral counterexample to the (wrongly generalized) Coleman-Mandula theorem. A minute later, the gauge coupling unification strengthens the case for SUSY. The SUSY improvement to the coupling unification is attributed to Savas Dimopoulos and Howard Georgi.



Before 28:00, it's explained that unlike the normal fermions and gauge bosons, their superpartners - well, at least the scalars - may get massive without a Higgs. He gets to the reasons why SUSY helps to solve the hierarchy problem. The Higgsinos may have a "chiral" reason to be light - and the Higgs must therefore follow.

After 31:00, Weinberg states that there's no unique "canonical" model of SUSY. Nevertheless, he says that SUSY is the main target of the LHC, ahead of the Higgs - and explains why physicists are terrified that the LHC will only see the Higgs and nothing else.

A minute later, SUSY is also presented as the donor of the dark matter particles. A cosmological discussion about the dark matter and its density follows. Galactic rotational curves were replaced by hot gas and gravitational lensing in the role of the best proxy to measure the density. Another woman leaves at 41:18 but this one actually returns at 43:30.

After 45:00, he says that dark matter has to be made out of stable particles: it's still there. But it's good if it annihilates in pairs - because its amount would otherwise be much higher than it is. SUSY's neutralinos with the odd R-parity are perfect guys to satisfy the constraints.

At 47:30, Weinberg mixes "photinos, zinos, and winos" to get the dark matter candidates. Dr Weinberg, you better not mix the charged winos into the mixture: higgsinos could be a better ingredient, thank you. ;-)

The talks ends up at 48:48. Tom Siegfried of Science News - who has provoked the discussion about the defense of statistical methods - slightly distorted what Weinberg's talk was about - he didn't mention any SUSY - and asked about Weinberg's opinions on string theory. Weinberg said that the theory has developed mathematically and theoretically but not to the point to produce a unique model, a fact that Weinberg finds disappointing.

The LHC could potentially provide a clue to something we're missing in string theory; SUSY is the most natural option. Weinberg mentions the large landscape and low-energy SUSY as a pretty generic property of the vacua. Weinberg explains that it is unlikely for the LHC to probe D-branes or quantum gravity. Clues are inevitably indirect. In the 1980s, Weinberg moved from strings to cosmology because it became as hot as particle physics 10-20 years earlier - convergence of theory and experiments.

Weinberg still thinks that string theory is the best hope we have, at 52:52. He mentions his recent work on asymptotic safety but although he's worked on it, he finds string theory more attractive but not attractive enough. :-)

Well, this excitement about cosmology was deserved and cosmology has incredibly become another precision science. On the other hand, I think it's equally fair to say that the last really nontrivial findings in cosmology occurred 5 years ago or so and I don't see significant reasons to expect that this revolution in cosmology will be revived or will continue.

The second question asks whether we should see the light created by pair annihilation of dark matter from two colliding clusters. Yes but the amount of light is tiny. Only if one finds closer clusters of dark matter, he has a chance to see something.

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