Yesterday the Italian newspaper "Il Manifesto" featured two pieces written by yours truly on the discovery of the Higgs boson. I was delighted to have a chance to write for that newspaper, which has an illustrious history and is totally independent (and on the verge of being shut down). By the way, I must thank Peter Woit who suggested the reporters of the newspaper to contact me for the piece.

The articles are in Italian, but I can make an effort at translating them for you here.

First of all the links: the web version of the main piece is here, and the pdf scan of the paper version (the full newspaper, so you need to browse pages 2 and 3) is here.

Now the translation. Quick and dirty because I am on a rush today (backlog of stuff impending, among which two important articles on the Atlas Higgs search and on other issues - wait for them today or tomorrow...).


And there it is. Hypothesized almost in a low voice 48 years ago to solve with elegant maths a thorny enigma, and then little by little risen to the role of keystone of our present understanding of the subnuclear physical world, the Higgs boson has finally been pictured by the very high-energy collisions of proton beams of the Large Hadron Collider, CERN's jewel, the "dream machine" of particle physicists. And the picture is surprisingly well defined.

The histograms that the spokespersons of the CMS and ATLAS collaborations Joe Incandela and Fabiola Gianotti show in front of the packed CERN auditorium speak loud and clear. The experimental data are impossible to interpret if one does not allow that there in the midst, at a mass of 125 GeV (equal to the weight of an entire Iodine atom), the histograms contain a significant lot of events due to the decay of a new particle, which has all the predicted characteristics. How significant the signal is, numbers will tell you: the probability that the data are observed as they are, in the absence of the new particle, is lower than a part in three millions; in other terms, it is more risky to jump on a plane than to bet one's life on the existence of the particle seen at CERN. Rather than referring to plane flights, experimental physicists usually describe these effects in numbers of "standard deviations", units of measure that quantify the dispersion of a measurement from the expected value: well, both the CMS and the ATLAs data are incompatible with the absence of a signal at the level of five standard deviations.
Either of these observations, alone, would suffice to declare a discovery; the fact of having two experiments confirming one another is pleonastic by now.

The experimental evidence is strong, and contrarily to predictions of the vigil, the CERN management does not even attempt at dampening the enthusiasm. Maybe the general director Rolf Heuer, remembering the problems of last fall with the announcement later retracted of superluminal neutrinos, would have wanted to wait more; but CERN's line is no longer one of maximum caution: today Heuer, at the end of the seminar with which Incandela and Gianotti describe their results, turns to the audience and says "I think we have it, do you agree?", and the audience applauds enthusiastic.

This is science in real time: the decision on whether a result is worth a discovery occurs by acclamation, as it should be. The scientific community is convinced, and these double 5 standard deviations are worth therefore much, much more than the illusory 6 standard deviations of superluminal neutrinos, which had convinced nobody.

We talk about it with Guido Tonelli, former spokesperson of the CMS experiment, who on December 13th last year stepped on the same stage and showed a more blurred version of the same picture. Six months ago there was no unanimous consensus on the discovery, and in fact one spoke only about "evidence": the intensity of the signal was smaller but Tonelli, and yours truly with him, was among those already convinced that this was really a Higgs boson signal, and not an ephemeral fluctuation.

"When two experiments show excesses of such statistical significance in the same mass region it is very hard to keep a line of excessive prudence. I am not disclosing any secret if I say that even the most sceptical colleagues came in procession to my office these days to confess that they are now convinced that we've found it", explains Guido.

To say it all, there still are perplexities among the insiders. But not on the discovery: that this is a new particle there's no doubt, and that it is really a Higgs boson appears just as clear. The doubt -or better, the interest- concentrates now on the interpretation: the standard model, the collection of theories that explains the phenomenology of subatomic particles, dictates with what frequency the Higgs bosons must be observed in each of the "final states" which are studied, and this now is the point.

The final state is the collection of all particles that are observed in the detector: these are either stable (protons, electrons, photons) or unstable ones but ones living a time sufficient to cross the sensitive components of the experimental apparatus (muons or light hadrons), where they leave an identifiable signal. It is with the particles observed in the detector that experimental physicists reconstruct backwards the chain of processes that took place in the collision, finally getting to the first produced particle, which disintegrated in a time fantastically short, a time that is to a second as a second is to a thousand times the lifetime of the universe. The Higgs boson has thus been observed both in its decays to photon pairs (photons are light quanta) and in its decays to Z boson pairs, as well as in other final states, as predicted by the standard model.
However, if one compares what is observed with the expectations, one sees that both ATLAS and CMS observe a modest excess of decays of the Higgs boson in two photons with respect to model predictions, and a corresponding deficit in decays in other final states.

Let us get this straight: these faint anomalies are well interpreted as statistical fluctuations. And nevertheless they are already enough to foster interesting speculations on what we will want to study in more detail in the near future. Because there exist extensions of the standard model which predict a different schema of the decay modes of the Higgs boson, or even the existence of replicas of the phantom particle. For instance, in supersymmetric theories that extend the standard model and repair a few of its apparent inconsistencies, there exist not just one but at least five Higgs bosons, and each of them has different characteristics.

Yes, supersymmetry. Sold ever since the eighties as a sure thing by a growing group of enthusiast theoretical physicists, supersymmetry offers automatic solution to some unsolved enigmas in fundamental physics, and to boot offers a possible explanation to the problem of dark matter in the universe. In supersymmetric theories one hypothesizes that for each existing elementary particle described in the standard model (quarks, leptons, vector bosons) there is a supersymmetric replica of similar characteristics. Each of these new particles must however by force have a very large mass, lest it would have already been seen: one must therefore hypothesize that this symmetry between ordinary and supersymmetric matter be "broken" by some external mechanism.

The doubling in one single stroke of the number of subatomic particles solves a couple of theoretical problems of the standard model; furthermore, the neutralino -the lightest of supersymmetric particles, bound to live forever by the absence of lighter particles into which to decay- it is a natural candidate to explain what makes up the 80% of matter in the universe, which we now exists from a multitude of gravitational indicia, but which does not form stars and is therefore obscure. Notwithstanding these attractive features, the need to introduce a large number of new free parameters (new particles, new characteristics, a considerable complication of the physical reality) makes the realization of the theory less elegant than its theoretical formulation. Occam's razor is seized by sceptics to "cut" this non-necessary multiplication of entities: "entia non sunt multiplicanda praeter necessitatem", as the franciscan friar William admonished seven hundred years ago.

Will it be a regular Higgs boson the one discovered today ? Or is it maybe supersymmetric ? Intriguing and legitimate question, which could keep us busy for quite a while longer. Undoubtedly a question of great importance for physics, even if maybe not sufficient to justify the building of a new super-accelerator which collides muons instead than protons, a "muon collider", a sort of factory of Higgs bosons: a project already in advanced state, but one which represents a titanic effort and a dreadful technological challenge.

In any case, yesterday's discovery guarantees to particle physicists years of very interesting studies, to eviscerate the properties of the new particle, and to theoretical physicists a fertile and solid ground to grow possible alternatives to the by now boringly precise and predictive standard model. After all, the LHC collider at CERN was built to discover the Higgs boson, but by now everybody hopes that it will lead us in terra incognita, toward new particles, new dimensions of space-time, or maybe, why not, toward the discovery of things up to now not even imagined by human mind.

Because, on an afterthought, Nature has shown so far to be even too predictable; I would say obedient, as probably will feel in his bones the 83-years-old Peter Higgs, now applauded with a standing ovation by the discoverers of "his" boson in the CERN auditorium.