Finally, after some frantic waiting filled with rumors, we heard the truth from people at CERN. And we discovered that rumors were just right. Evidence is mounting for a Higgs particle at around 120-130 GeV, after new data were accounted for. All these evidences point toward a Standard Model Higgs. But some caution words are needed (see Matt Strassler’s post) as a discovery cannot be claimed yet. ATLAS sees a 3.6 sigma overall evidence but, accounting for look elsewhere effect, this go down to 2.5 sigma while CMS has a similar 2.6 sigma going down to 1.9 with look elsewhere effect. This is not enough to rule out a fluctuations but, anyhow, a strong indication where to point researchers attention for the near future. All the matter will be pinned down later next year. From my side, I just note a possible contradiction between the two experiments as ATLAS keeps on claiming an excess around 500-600 GeV, also with increasing number of data and indeed evidence now goes beyond 2 sigma, while, as for today, CMS claims this range ruled out. It is possible that this is another glimpse for a Higgs multiplet as required by supersymmetry. I think that also this matter will be fixed soon next year.
The conference raised a lot of enthusiasm (see here) to some caution (see here) or skepticism (see here).
Fabiola Gianotti, Rolf Heuer and Guido Tonelli
What makes these hints striking is the fact that both experiments see the excess in the same region where the particle was expected and with the proper rates. It should also be said that, with these data and energy, people at CERN have done an excellent work with the analysis of them. But, of course, it is still possible that we are coping with a fluctuation and the particle is hiding elsewhere or is something else. For sure, next year the puzzle will be completed and also this part of the Standard Model will be part of our textbooks in the right way. What we have here is a completely new situation holding the premises for a clear understanding of one of the greatest question of mankind ever. So, when a child will ask to you: “Mom, what are we made of?” this question will have an answer, an answer arising from the work of a lot of smart people running one of the greatest technological achievement of our history: LHC.
While CERN is calming down rumors (see here), research activity on Yang-Mills theories keeps on going on. A few days ago, a paper by Axel Weber appeared on arxiv (see here). As my readers know, having discussed this at length, in these last years there has been a hot debate between the proponents of the so called “scaling solution” and the “decoupling solution” for the propagators and the running coupling of a pure Yang-Mills theory in the infrared limit. Scaling solution describes a scenario with the gluon propagator reaching zero with lowering momenta, a ghost propagator enhanced with respect to the tree level one and the running coupling reaching a finite non zero value in the same limit. Decoupling solution instead is given by a gluon propagator reaching a finite non-zero value at lower momenta, a ghost propagator behaving like the one of a free particle (tree level) and the running coupling going to zero in this limit.. It is quite easy to recognize in the decoupling solution all the chrisms of a trivial infrared fixed point for a pure Yang-Mills theory against common wisdom that pervaded the community for a lot of years. So, for some years, having lattice computations unable to tell which solution was the right one, scaling solution seemed the only one to be physically viable and almost accepted by a large part of the community.
Things started to change after the Lattice Conference in Regensburg on 2007 when some groups where able to display lattice computations on very huge volumes. The striking result was that lattice computations confirmed the decoupling solution against common wisdom. What was really shocking here is that the gluon becomes massive at the expenses of the BRST sysmmetry that seems now to acquire an even more relevant role in the understanding of Yang-Mills theory.
The idea of Axel Weber is to perform an -expansion for the Yang-Mills Lagrangian with a massive term to fix the scale. The striking result he gets is that both the scaling and the decoupling solutions are there but the former is unstable with respect to the renormalization group flow in dimensions greater than 2. So, this computation confirms again the scenario that I and other authors were able to devise.
Today, we have reached a deep understanding of the infrared physics of a Yang-Mills field theory. Scientific community is urged to take a look to the work of these people that could accelerate progress in a large body of physics research.
Axel Weber (2011). Epsilon expansion for infrared Yang-Mills theory in Landau gauge arXiv arXiv: 1112.1157v1
Marco Frasca (2007). Infrared Gluon and Ghost Propagators Phys.Lett.B670:73-77,2008 arXiv: 0709.2042v6
It is now officially published the agenda of the meeting of the Scientific Policy Commitee of CERN (see here) on 12 December. On 13 December it is scheduled a seminar by ATLAS and CMS about Higgs search (see here) by the spokepersons of these experiments: Fabiola Gianotti and Guido Tonelli. As usual, you can follow an ongoing discussion at Philip Gibbs’ blog. Philip promised further combined graphs in real time. Just stay tuned!