A fair account of the matter for once March 18, 2007Posted by dorigo in language, news, physics, science.
I read a honest account of the status of things with the Higgs search on an italian newspaper today, “Il Sole – 24 Ore“. Here is a quick-and-dirty translation (my excuses to the author for lost or distorted meaning), for the benefit of those of you who have no access to italian press or no understanding of the language:
In search of the God Particle
The Higgs boson – the particle hypothesized by Peter Higgs in 1964 and considered crucial for the consistency of the extremely successful theory of fundamental interactions known as the “standard model” – is still at large, and chances are that it will stay there for a few more years.
A signal of decay of the Higgs particle, whose phenomenology is perfectly known within the standard model but whose mass is still unknown, has been sought with no success between 1996 and 2000 in electron-positron collisions provided by the LEP collider at CERN. The negative result of the four LEP experiments allowed to understand that if the Higgs exists, its mass is with all probability above 114 GeV – equivalent to the mass of 120 protons.
Meanwhile, the global analysis of several measurable quantities of the standard model made more and more strict the margin of agreement between all other experimental results and the absence of the Higgs boson. If the standard model is correct, that particle cannot be much heavier than the inferior limit found by LEP.
With the de-commissioning of LEP in 2000 the spotlights have passed to the Tevatron accelerator at Fermilab, where the proton-antiproton collisions grant some sensitivity to heavier Higgs particles. The CDF and D0 experiments that analyze the Tevatron collisions will collect data at least until 2009, and it is possible that they manage by then to obtain a first evidence of the existence of the elusive boson. If that is not the case, the definitive crowning of the standard model will become a task for the new super-powerful proton accelerator about to be completed at CERN, the Large Hadron Collider (LHC), and of the CMS and ATLAS experiments that will study its collisions from 2008 onwards.
And what if the standard model were not correct ? The question is a legitimate one, since there is consensus within the experts in maintaining that the theory, which has by now lived through forty years of more and more precise experimental verification, is at most an “effective” theory, that is one valid at low energy – just like Newtonian mechanics is a particular case of Einstein’s relativistic mechanics, which is valid for speed small with respect to that of light.
On what there may be beyond the standard model opinions are not univocal, however. Many hold that the most enticing and theoretically motivated extension is supersymmetry, a construction implying the existence, for every existing elementary particle, of a “super-partner” with similar characteristics but a different value of the intrinsic angular momentum – “spin”, a quantum number which distinguishes the matter constituents, quarks and leptons, from the carriers of the four fundamental forces, like the photon and the vector bosons.
If supersymmetry were the correct theory, there should exist more Higgs bosons: not one, but at least five distinct particles! And in that case, the Tevatron would have more chances to observe a first evidence, since one would then expect a larger production rate for these “exotic” Higgses.
In the meantime, the CDF and D0 experiments patiently continue to collect data, and with six-month cadence they publish newpreliminary analyses, after scrutinizing with infinite attention every detail of the obtained results. The last CDF result in the search for supersymmetric Higgs particles shows a mild excess of events, which could represent a first signal of decay of a Higgs with mass equal to about 160 GeV in pairs of tau leptons. It could, if it was not due – something infinitely more probable – to a normal statistical fluctuation. The competitor experiment, D0, is showing as a matter of fact a deficit of events for the same mass value, making it even more clear that it is much, much too early to place the champagne in the refrigerator.