Le LHC2 peut-il tester SUSY2?

Le retour d'un super-héro quantique français : Pierre Fayet 

Toujours à l'affût des derniers développements sur le front de la recherche physique en général, et sur celui de la physique quantique en particulier, le blogueur interroge régulièrement les moteurs de recherche de ses sites favoris comme arxiv.org avec soit des mots clés qui empruntent au vocabulaire technique, soit des noms propres de physiciens (et de quelques mathématiciens) parce qu'il aime suivre leurs travaux ou parce qu'ils appartiennent à l'Olympe des scientifiques (ou à son Panthéon personnel) et qu'il est toujours intéressant de savoir sur quoi ils travaillent. Il s'agit là soit-dit en passant d'une attitude propre aux physiciens sur lequel on reviendra sûrement un autre jour. Quelle n'est pas sa surprise donc et sa joie de découvrir un article tout récent (déposé le 24 mars) de Pierre Fayet, l'un des pères fondateurs de la supersymétrie (et tout récemment élu à l'académie des Sciences) dont il avait pour la première fois vu le nom grâce à un blog de qualité dans le texte suivant :

... there was some sort of sadness coming out of the talks of particle physicists and string theorists. We heard the usual claims about string theory being "the only theory of quantum gravity" (Costas Bachas), or the "only game in town" on the unification side (John Ellis). This message seemed to be carried like an old reflex, without any enthusiasm, by people who have been waiting for too long without seeing any thing really new and interesting happening in their field. However, the talk of Pierre Fayet about supersymmetry standed out thanks to its remarkable honesty. In fact, Pierre Fayet was so honest about the various problems plaguing SUSY theories that I ended up being even more sceptical than I was before his talk ! Answering some question in the audience, John Ellis made the interesting comment that if SUSY is not seen at the LHC, he will certainly lose interest in the theory.

Fabien Besnard, Billet Frontiers of Fondamental Physics 11, Blog Mathéphysique, 10/07/2010

Avant de revenir à Fayet signalons que John Ellis dont il est aussi question dans cet extrait est également un scientifique de renom à qui on doit - outre un vaste corpus de travaux sur les conséquences phénoménologiques de la supersymétrie - les premiers efforts et incitations pour une recherche effective du boson de Higgs comme le rapelle la conférence Nobel de ce dernier. Remarquons aussi que malgré l'absence de signes de supersymétrie dans le premier round de fonctionnement du LHC John Ellis s'intéresse toujours à cette dernière dans l'attente des résultats du second round (LHC2) qui ne commencera au plus tôt que l'année prochaine (2015) :

 I still think that supersymmetry is the best-motivated extension of the Standard Model, and look forward to the next phases of LHC operation at higher energy and luminosity.

John Ellis, Summary of the Nobel Symposium on LHC Results, 13/09/2013

Défendre les recherches [de particules] {sur les théories} supersymétriques au vue des résultats du LHC1 (dans la perspective du LHC2) 
Revenons maintenant à Pierre Fayet et voyons dans quelle mesure il partage l'opinion de John Ellis. Dans son nouvel article il dessine les perspectives d'une famille de théories supersymétriques qui s'accommoderait des derniers résultats expérimentaux récoltés par le LHC jusqu'à une énergie de 8 TeV et qui pourrait fournir des prédictions testables lors du second round d'expériences à 14 TeV.

The electroweak scale is the most natural one where to search for supersymmetric particles, with supersymmetry usually expected to show up a not-too-high scale, now typically considered to be < a few TeV at most. But no supersymmetric particle has shown up yet, with lower limits on squarks and gluinos often reaching ≃ TeV scale. Still we keep hoping that the next round of LHC experiments will lead to a direct discovery of the new superpartners. In between, the finding at CERN of a new particle likely to be a spin-0 BE-Higgs boson seems to indicate that we are indeed on the right track, in contrast with other anticipations about the nature of electroweak breaking.
At which energy scale we may expect to find supersymmetric particles depends on the magnitude of the parameters describing supersymmetry breaking, in connection with the amount of breaking of the continuous R symmetry ... What should be the mass scale for the new particles, if not ∼ TeV scale as was commonly expected, remains an open question. 
As supersymmetry breaking and electroweak breaking are in general two independent phenomena, we should take seriously the possibility that their breaking scales be of different magnitudes. The supersymmetry-breaking scale could then be significantly larger, especially if a new physical phenomenon not directly related to electroweak breaking is involved in this process, such as the compactification of an extra dimension.
This could fix the supersymmetry-breaking scale in terms of the compactification scale ... We may then face the eventuality that superpartner masses be considerably larger than the presently accessible ≈ TeV scale, especially if the compactification of extra dimensions also sets the scale for grand-unification breaking. This may tell us that supersymmetry should only show up manifestly through the presence of R-odd superpartners at the compactification scale, i.e.  

m(R-odd superpartners) ≈ compactification scale ?  

This one is not necessarily directly tied to the electroweak scale, especially as the electroweak breaking can be directly formulated in the higher 5-or-6 dimensional spacetime (where it leaves an electrostrong symmetry unbroken), independently of the compactification scale. It may be quite high, especially if two similar compactification scales determine both the supersymmetry and grand-unification scales ... This would imply  

m(R-odd superpartners) ≈ GUT scale ??  

with the further possibility that the GUT scale be lower than usually considered, in connection with the possible stability of the proton associated with GUT-parity.

Pierre Fayet,  THE SUPERSYMMETRIC STANDARD MODEL with a Brout-Englert-Higgs boson as spin-0 partner of the Z, 24/03/2014

On peut à la lecture de cet extrait avoir un aperçu, je crois, de la rigueur et l'honnêteté intellectuelle dont parlait F. Besnard précédemment, celle d'un constructeur de modèles qui ne cache pas les difficultés auxquelles est confrontée toute théorie ambitieuse qui se confronte au tribunal du réel, celle d'un physicien courageux qui continue à proposer des solutions hypothétiques mais qui met bien en exergue les questions qui restent ouvertes dans le cadre de ses spéculations.

Le boson de Higgs est-il le signe d'une supersymétrie au carré !?
Avant de terminer il faut tout de même noter cette audacieuse proposition faite par Fayet dans le même article :

The new boson found at CERN with a mass close to 125 GeV/c², believed to a Brout-Englerts-Higgs boson associated with the electroweak breaking, may well also be interpreted, in general up to a mixing angle as we shall see, as a spin-0 partner of the 91 GeV/c2 Z boson under two infinitesimal supersymmetry transformations ... 
The observation of a new spin-0 particle ... thus appears as an important indication in favor of this symmetry. According to this gauge-Higgs unification (already within N = 1 theories in 4 dimensions), BEH bosons naturally appear as extra spin-0 states of massive spin-1 gauge bosons. And this, in spite of the fact that they have different gauge-symmetry properties – thanks to the spontaneous breaking of the electroweak symmetry. 

Le nouveau boson découvert au CERN avec une masse proche de 125 GeV/c², supposé être un boson de Brout-Englert-Higgs associée à la brisure électrofaible, pourrait bien aussi s'interpréter comme [super]partenaire [au carré?] de spin 0 du boson Z de 91 GeV/c² moyennant deux transformations de supersymétrie infinitésimales et un  certain angle de mélange comme nous allons le voir ... 
L'observation d'une nouvelle particule de spin nul ... apparaît ainsi comme une indication importante en faveur de cette symétrie. Selon cette unification entre bosons de jauge et de Higgs (déjà présente dans les théories à 4 dimensions avec N = 1), les bosons BEH apparaissent naturellement comme [super]partenaires [au carré] de spin 0 des bosons de jauge massifs de spin 1. Et ce, en dépit du fait qu'ils ont des propriétés vis-à-vis des symétries de jauge différentes - grâce à la brisure spontanée de la symétrie électrofaible.

ibid. 

Je laisse au lecteur meilleur physicien que moi apprécier la pertinence de cette vision des choses, pour ma part je m'interroge encore pour savoir si derrière cette interprétation audacieuse il n'y a pas un peu ... de malice.

//léger travail de correction et traduction effectués le 29/03/14
Le texte entre crochet dans la traduction de l'article de Fayet est un ajout de ma part qui traduit une formulation personnelle et pas celle de P. Fayet.

La rumeur d'un signe expérimental de l'inflation cosmologique (peut-elle faire autre chose qu')enfle(r?)

Vous avez dit ondes gravitationnelles primordiales?

Ce week-end la blogosphère scientifique bruisse d'une rumeur excitante car elle tourne autour d'une possible découverte majeure en astrophysique : la première mise en évidence expérimentale de la trace d'ondes gravitationnelles primordiales dans la polarisation du fond diffus cosmologique. Pour être précis remarquons que la rumeur porte a priori sur le contenu de la "découverte majeure en astrophysique" car cette dernière est déjà une annonce publique faite par une institution américaine sérieuse donc a priori une vrai promesse et demain à 18 heure un fait avéré :

March 17th Press Conference on Major Discovery at Harvard-Smithsonian Center for Astrophysics
Press Release Source: Harvard-Smithsonian Center for AstrophysicsPosted Wednesday, March 12, 2014
The Harvard-Smithsonian Center for Astrophysics (CfA) will host a press conference at 12:00 noon EDT (16:00 UTC) on Monday, March 17th, to announce a major discovery.
Video of the press conference will be streamed live beginning at 11:55 a.m. EDT from the link at http://www.cfa.harvard.edu/news/news_conferences.html

Pour savoir ce que pense la communauté des scientifiques professionnels qui partagent sur la toile leurs réactions et commentaire à propos de cette future nouvelle discutée aussi dans la presse grand publique anglo-saxonne (The Guardian) voici une liste non exhaustive : 

• The Trenches of Discovery: "A major discovery", BICEP2 and B-modes (by Shaun Hotchkiss)
• Richard Easther's ExcursionSet.com: The Smoking Gnu
• Bruce Bassett's writing: Should you hold your breath for B-modes?
• Résonaances: Plot for Weekend: flexing biceps (Adam Falkowski)
• viXra: Primordial Gravitational Waves? (Phil Gibbs)
• Blank on the map: B-modes, rumours, and inflation (by Sesh Nadathur)
• The Reference Frame: Rumor: inflation-related primordial B-modes to be announced on Monday

Derrières les ondes gravitationnelles, la question de l'inflation et de la cosmologie quantique...

We do not know the history of the observable Universe before the epoch of nucleosynthesis, but it is widely believed that there was an early era of cosmological inflation. During this era, the Universe was filled with a homogeneous scalar field φ, called the inflaton field, and essentially nothing else. The potential V(φ) dominated the energy density of the Universe, decreasing slowly with time as φ rolled slowly down the slope of V. 

The attraction of this paradigm is that it can set the initial conditions for the subsequent hot big bang, which otherwise have to be imposed by hand. One of these is that there by no unwanted relics (particles or topological defects which survive to the present and contradict observation). Another is that the initial density parameter should have the value Ω = 1 to very high accuracy, to ensure that its present value has at least roughly this value. There is also the requirement that the Universe be homogeneous and isotropic to high accuracy. 

All of these virtues of inflation were noted when it was first proposed by Guth in 1981 and very soon a more dramatic one was also noticed. Starting with a Universe which is absolutely homogeneous and isotropic at the classical level, the inflationary expansion of the Universe will ‘freeze in’ the vacuum fluctuation of the inflaton field so that it becomes an essentially classical quantity. On each comoving scale, this happens soon after horizon exit. Associated with this vacuum fluctuation is a primordial energy density perturbation, which survives after inflation and may be the origin of all structure in  the Universe. In particular, it may be responsible for the observed cosmic microwave background  (cmb) anisotropy and for the large-scale distribution of galaxies and dark matter.  Inflation also generates primordial gravitational waves as a vacuum fluctuation, which may contribute to the low multipoles of the cmb anisotropy. 

When it was first proposed in 1982, this remarkable paradigm received comparatively  little attention. For one thing observational tests were weak, and for another the inflationary density perturbation was not the only candidate for the origin of structure. In particular,  it seemed as if cosmic strings or other topological defects might do the job instead. This situation changed dramatically in 1992, when COBE measured the cmb anisotropy on large angular scales, and another dramatic change is now in progress with the advent of smaller scale measurement. Subject to confirmation of the latter, it seems that the paradigm of slow-roll inflation is the only one not in conflict with observation.

David H. Lyth et Antonio Riotto, Particle Physics Models of Inlfation and the Cosmological Density Perturbation, 16/03/1999

Le paradigme de l'inflation trouvera-t-il demain son meilleur modèle d'inflaton?

Is it possible to show convincingly that inflation is responsible for the large-scale homogeneity, isotropy and flatness of the universe, and the primordial spectrum of metric fluctuations that seeded galaxy formation and sourced the temperature and polarization variations in the cosmic microwave background (CMB)? Some would claim no, because there is too much freedom in constructing inflationary models. Even if one shows that the observations are consistent with the predictions of a particular inflationary model, this is unconvincing because, for virtually any given combination of observations, one can design many inflationary models that reproduce them. If a theory allows everything, it has no predictive power. In this paper, though, we describe how to combine observations into a sequence of “bootstrap tests” that, if any one of them is passed, will be the most direct confirmation possible that the universe underwent a brief period of acceleration followed by a long period of deceleration.

Latham Boyle et Paul J. Steinhardt, Testing Inflation: A Bootstrap Approach, 8/10/2010 

Un peu plus près du Big-Bang et de l'énergie de Planck (aux erreurs systématiques près)

ça-y-est ! L'article de la collaboration BICEP2 - à qui on doit à tout le moins vraisemblablement la naissance de l'astrophysique expérimentale des ondes gravitationnelles primordiales - est maintenant disponible :

BICEP2 I: DETECTION OF B-mode POLARIZATION AT DEGREE ANGULAR SCALES

La rumeur était donc vraie; voici les premiers échos sur la blogosphère :

At a presentation from the Harvard-Smithsonian the BICEP2 team have announced that they have the ”first direct evidence of cosmic inflation”. As rumoured they have detected what they believe to be primordial gravitational waves with a ratio or tensor to scalar modes of r=0.2 (+0.07 -0.05) which is 5 sigma over the null hypothesis. This is a game-changing result for inflationary cosmology and possibly for quantum gravity research because the result indicates that the scale of the inflation is only about a factor of 100 below the Planck scale.

Philip Gibbs, “first direct evidence of cosmic inflation” BICEP2 results, 17/03/2014

Until this measurement/discovery is confirmed by another experiment, you should consider it provisional. Although this is too large a signal to be likely to be due to a pure statistical fluke, it could still be due to a mistake or problem. The history of science is littered with examples; remember the 2011 measurement by OPERA that showed neutrinos moving faster than the speed of light was far too large to be a statistical fluke. Fortunately there will be other experiments coming and so we’ll have a chance for various experiments to either agree or disagree with each other in the very near future.

If this measurement is correct, and if indeed it reflects gravitational waves from inflation in the most conventional way, then it would tell us that inflation occurred with a dark energy per unit volume (i.e. dark energy density) that is comparable to the energy scales associated for decades with the energy and distance scale at which all the known non-gravitational forces would naively have about the same strength — the so-called “unification of coupling constants”, sometimes extended to “grand unification” in which the various forces actually turn out to be manifestations of just a single force. This would be very remarkable, but not necessarily evidence for unification. There are other ways to get the same scale, which is about 100 times lower in energy (100,000,000 times lower in energy per unit volume) than the scale of quantum gravity (the Planck scale, which, roughly is the energy density required to make the smallest possible black hole.)

Matt Strassler, BICEP2: New Evidence Of Cosmic Inflation!, 17/03/2014

Ajout du 15/05/2014

Pour découvrir des développements plus récents sur ce sujet voir ce billet.

"Règle de trois" heuristique dans le champ des possibles physiques: "plus de particules, un nouvel éther, ou une meilleur modélisation de l'espace"

Les physiciens des hautes énergies n'aiment pas le bouillon que la nature leur sert...
La blogosphère se fait l'écho ces temps-ci des réflexions autour de l'opportunité de poursuivre la course aux hautes énergies avec la construction éventuelle d'un accélérateur de 100 TeV au regard du "peu" de nouvelles particules élémentaires découvertes lors du premier round de fonctionnement du LHC - le plus grand accélérateur actuel - qui a déjà atteint 8 TeV et devrait fonctionner à 14 TeV à partir de 2015. Matt Strassler est le dernier en date à ma connaissance à discuter du sujet de manière détaillée et simple sur son blog. On peut aussi le voir à la fin de cette vidéo débattre de la question en termes techniques avec entre autre Nima Arkani Hamed, David Gross et Nathan Seiberg, trois éminents physiciens théoriciens et spécialistes de la théorie des cordes (une retranscription du débat accompagnée du commentaire d'un blogueur qui connait bien ces physiciens et la théorie des cordes se trouve ici). 

...les physiciens des solides y ont pourtant déjà mis un grain de sel qui rend le bouillon moins amère...

Or donc la soupe de particules produites par les collisions proton-proton à 8 TeV du LHC est trop claire aux yeux des physiciens des hautes énergies et certains s'inquiètent de l'impossibilité théorique d'enrichir leur soupe en récoltant des preuves expérimentales de particules supersymétriques (sans parler d'indices pour des dimensions supplémentaires de l'espace-temps) à l'échelle de la dizaine de TeV (accessible dans un futur proche au LHC). Alors la physique des particules ou du moins celle des accélérateurs géants risque-t-elle de boire le bouillon pour parler familièrement? Le spectre de l'abandon du programme de construction du SSC américain hante les mémoires des plus avertis...  D'aucuns à ce sujet se complaisent parfois à souligner le rôle qu'a pu jouer un éminent spécialiste de la physique des solides, Philip W. Anderson, qui plaida contre la construction de cet accélérateur qui aurait dû pouvoir atteindre 40 TeV. Le mieux pour dépasser un éventuel ressentiment est sûrement de lire une très intéressante entrevue du courrier du CERN avec un (le plus?) éminent spécialiste de physique des particules, Steven Weinberg (faite à l'occasion d'une conférence sur les 50 ans de la théorie BCS) : I remember that Phil Anderson and I testified in the same Senate committee hearing on the issue, he against the SSC and I for it. His testimony was so scrupulously honest that I think it helped the SSC more than it hurt it.

Mais ce qui est vraiment intéressant pour le sujet qui nous intéresse c'est ce que dit Weinberg un peu plus loin dans l'entrevue déjà citée:

During the SSC debate, Anderson and other condensed-matter physicists repeatedly made the point that the knowledge gained in elementary-particle physics would be unlikely to help them to understand emergent phenomena like superconductivity. This is certainly true, but I think beside the point, because that is not why we are studying elementary particles; our aim is to push back the reductive frontier, to get closer to whatever simple and general theory accounts for everything in nature. It could be said equally that the knowledge gained by condensed-matter physics is unlikely to give us any direct help in constructing more fundamental theories of nature.

...

Condensed-matter physics and particle physics are relevant to each other, despite everything I have said. This is because, although the knowledge gained in elementary-particle physics is not likely to be useful to condensed-matter physicists, or vice versa, experience shows that the ideas developed in one field can prove very useful in the other. Sometimes these ideas become transformed in translation, so that they even pick up a renewed value to the field in which they were first conceived. 

The example that concerns me is an idea that elementary-particle physicists learnt from condensed-matter theory – specifically from the BCS theory. It is the idea of spontaneous symmetry breaking.

Steven Weinberg, From BCS to the LHC, 21/01/2008 

Où l'on voit que Weinberg souligne la différence de nature autant que de culture entre physiciens du solide à la recherche de lois émergentes et physiciens des particules en quette de réductionnisme, les premiers manipulant des théories aux approximations sinon mal définies, souvent peu contrôlées quand les autres s'enorgueillissent de calculs perturbatifs à la précision diabolique. Mais Weinberg insiste surtout sur le rôle crucial des réflexions sur la phénoménologie de la physique du solide pour l'élaboration conceptuelle du mécanisme de brisure spontanée de la symétrie électrofaible par des physiciens comme le désormais célèbre Peter Higgs. Ce dernier l'explique aussi parfaitement dans sa récente conférence Noble citant en particulier Nambu et Goldstone mais aussi Anderson, lequel prophétisait au début des années 2000:

Strings make it possible to understand the Bekenstein limit on the entropy of a black hole, which ... suggests that quantum gravity is, in some sense, able to create ist own "convergence factor" : to prevent the infinite subdivision of space and time. But between here and the Planck limit there is still the probability (rather than a mere "possibility") of enormous surprises. The great variety of unexpectedly large or small parameters in the Standard Model (the neutrino mass the latest to appear) makes it almost certain that there are several further stages in the hierarchy. The first of these will appear in the next decade, certainly, when we will probably discover unexpected complexity in the Higgs phenomenon. (A personal guess: but no supersymmetry). 

Philip W. Anderson, 21st Century Physics in More is Different.

Jusqu'à aujourd'hui on peut dire que le physicien des solides est dans le vrai, si ce n'est qu'en matière de complexité inattendue du phénomène de Higgs, il faut plutôt parler de simplicité effective du boson de Higgs dans la mesure où ses propriétés semblent en accord avec le modèle le plus simple prévu celui du Modèle Standard et de l'extrême non naturalité  de ses paramètres qui fait consensus ou presque... 

Et si quelques physiciens hardis et mathématiciens savaient voir non pas un bouillon à boire mais une soupe à manger pour grandir?

... à suivre (où l'on verra que quelques physiciens tentent de voir dans le boson de Higgs un pont qui mènerait très loin, à l'époque de l'inflation cosmologique en fait, et permettrait de monter très haut sur l'échelle des énergies, pas très loin du barreau de Planck)

Retours vers le passé en attendant la prochaine percée expérimentale ...

 ... qui fera avancer la théorie [des cordes], {de la gravitation quantique à boucle} (ou une autre) 

One often hears that the present day situation in fundamental physics (string theory, loop gravity) has some similarity with the early years of quantum theory, before the great breakthrough – mostly by a young generation – in 1925-26. I find this analogy totally wrong. Without the precision experiments by the Berlin group (Kurlbaum, Rubens, etc.) and the difficult measurements of the specific heat of molecular hydrogen and other diatomic gases at low temperatures, that demonstrated the freezing out of the rotational degrees of freedom, as well as the low temperature measurements of the specific heat of solids by Nernst, Lindemann and others, it is hard to imagine that quantum theory could have been developed. This is, of course, not new, but it may not be inappropriate to be recalled in an article ...”

Norbert Straumann, On the first Solvay Congress in 1911, 10/2011

Évaporation des données dans le trou noir du buzz mediatique

... mais distillation* de l'information grâce aux blogs scientifiques ;-)

http://backreaction.blogspot.fr/2014/01/if-it-quacks-like-black-hole.html

http://profmattstrassler.com/2014/01/30/did-hawking-say-there-are-no-black-holes/

//ajout du 01/02/14 et édition du 02/02/14
Sans oublier la part des anges (attention l'un d'eux sait aussi faire la bête hélas ;-)

*Il faudrait préciser distillation  fractionnée car il y a plusieurs plateaux ou niveaux dans l'analyse des enjeux autour de la diffusion et la médiatisation grand public d'un bref mémo de Steven Hawking sur le problème du devenir de l'information quantique lors de l'évaporation d'un trou noir.

Pour continuer à filer la métaphore, disons qu'après la double distillation précédente, vient la maturation mais aussi l'évaporation, dont le fruit est ce qu'on appelle la part des anges, celle avec laquelle on s'enivre le plus de discussion physique pointue, argumentée mais aussi - et là est le risque - spéculative : cette part des anges donc se trouve peut-être bien là :

http://motls.blogspot.fr/2014/02/reactions-to-hawkings-black-hole-denial.html

Qu'est-ce que la science (post)moderne ?

Sans commentaire // ou presque (20^ème épisode)

//Commençons par une célèbre formule qu'on ne doit peut-être pas à qui on croit...

Measure what is measurable, and make measurable what is not so.

Galileo Galilei (folklore?)  

Although Galileo is quoted with these words in a large number of publications, the authenticity of the sentence is highly dubious because no one has ever provided a precise bibliographical reference for where to find it in Galileo’s works. Galileo’s alleged rule about measurement can be traced back to the works of two nineteenth century French scholars. This phrase was subsequently picked up by some internationally renowned scientists, who were responsible for its dissemination in German and English books and articles. The two English versions of the measurement quotation published by Hermann Weyl in the late forties of the last century strongly contributed to its worldwide diffusion. The sentence was even re-translated into German and French, and in recent scientific textbooks it is frequently used in order to characterize the methods of modern science. Notwithstanding its increasing popularity, referring to this expression as a quotation from Galileo is a striking example of academic sloppiness.

Kleinert Andreas, Der messende Luchs : Zwei verbreitete Fehler in der Galilei-Literatur 2009

//Où l'on voit que la pression pour publier et communiquer de façon attrayante pousse parfois des gens a priori sérieux et rigoureux à faire des citations non étayées ...

//Passons à une très récente illustration possible de la formule précédente qui est supposée caractériser la Science Moderne; celle-là même qui naît avec Galilée et rend compréhensible le réel empirique par des concepts a priori impossibles (le principe d'inertie) validés par des expériences incroyables (dans le vide un marteau tombe à la même vitesse qu'une plume) mais difficiles à réaliser.

One of the striking and longstanding problems of fundamental physics is the irreconcilability among the two main theories of last century, General Relativity and Quantum Theory. A manifestation of this tension is the value that quantum field theory attributes to the vacuum energy density, enormously larger than the value constrained from General Relativity by considering the radius of our universe. This problem, known as the cosmological constant problem, has been faced over the last decades with deep theoretical investigations, following also the evolution of the most important quantum gravity theories, like string theories, loop quantum gravity and many others. None of the theoretical efforts has so far succeeded in finding a consensual solution, so that it is still questionable whether vacuum energy does interact with gravity, and what is its contribution to the cosmological constan. In spite of the common belief by the scientific community in the existence of an interaction between vacuum energy and gravity, not a single experimental test of this interaction exists. About a decade ago, it was pointed out that a possible way to verify the interaction of vacuum fluctuations with gravity was to weight a (suitably realized, layered) rigid Casimir cavity. At that time it was yet unclear whether Casimir energy could be modulated in a rigid cavity. Furthermore, the most important macroscopic detectors of exceedingly small forces, the gravitational wave detectors with which we compared our force, were still under construction. Nowadays, thanks to many activities in the various fields mentioned, the situation has been remarkably improved so that it is possible to step from the initial idealistic experiment to a road map towards the measurement of the effect. 

Enrico Calloni et al,  Towards measuring the Archimedes force of vacuum, 27/01/2014

//Il reste à vérifier que le contenu de cet article n'est pas lui aussi un exemple de négligence académique ;-)
(ce billet a été légèrement remanié le 01/02/2014)

Est-ce bien naturel de demander à une théorie physique d'être ... naturelle ?

Rubrique dévissage 

Vous avez dit naturel ?

Le billet précédent exposait deux critiques de la pertinence du concept de naturalité pour l'élaboration de théories physiques plus fondamentales. Celui-ci évoquera au contraire quelques éléments d'une argumentation très précise en faveur de ce concept par James D. Wells, un physicien qui a beaucoup réfléchi sur cette question récemment. Commençons par le début avec une définition qui distingue deux aspects du concept de naturalité :

Formulating the question of whether Naturalness is a useful concept suffers from imprecision if we do not define the term explicitly, yet broadly enough to capture all its needed usage. Let us begin with what we call Absolute Naturalness. A theory has Absolute Naturalness if all dimensionless parameters and the ratio of any dimensionful parameters have explanation in terms of O(1) parameters. A parameter possesses Absolute Naturalness with respect to the rest of the theory if it can be explained by O(1) parameters. There can be difference of opinion about the precise range of values O(1) implies, but when it is helpful to think precisely about the value let us say O(1), or its inverse, can be as low as 10⁻³. An example of a parameter that is Absolutely Natural is Λ_QCD in quantum chromodynamics. Λ_QCD∼1GeV is Natural in this sense despite it being eighteen orders of magnitude smaller than the Planck scale (M_Pl∼10¹⁸GeV) because it can be explained by an O(1) parameter (strong coupling constant) near M_Pl, which upon renormalization group flow to the infrared diverges at E∼Λ_QCD. [...]

There is another notion of Naturalness that has been articulated from at least the early 1980’s, which is called Technical Naturalness, or sometimes ‘t Hooft Naturalness (‘t Hooft 1980). A theory has Technical Naturalness even if some parameters are small, if an enhanced symmetry develops when the small parameter is taken to zero. For example, a very light fermion mass is Technically Natural since an enhanced chiral symmetry emergences when the mass is taken to zero. In quantum field theory this protects the small parameter from any large quantum correction, and the small value is technically stable.

James D. Wells, The Utility of Naturalness, and how its Application to Quantum Electrodynamics envisages the Standard Model and Higgs Boson, 15/05/13

Mais c'est bien naturel !
On poursuit ici avec une illustration de l'efficacité du concept de naturalité au sens technique du terme tirée d'un exemple historiquement reconnu.

Assuming Naturalness as a law of nature imposes very strong constraints on model building. In the case of the Higgs boson, it leads not only to ideas like supersymmetry, which protects the Higgs boson from having a large mass, but the devotion one has to strict Naturalness leads to radically different hypothesized superpartner spectra.

In the past, Technical Naturalness has been used to understand experimental results that have already been measured. [...]

For example, the masses of the pions, proton and neutron are understood well from symmetries and Technical Naturalness. We know from asymptotic freedom of quantum chromodynamics (QCD) that the perturbative gauge coupling in the ultraviolet flows to strong value at the low scale and confinement happens at Λ_QCD∼1 GeV. This gives the characteristic scale of the hadrons in the theory, and the proton and neutron obtain mass approximately equal to this scale. However, the pion masses are much lower, and can be understood as the Goldstone bosons of SU(2)_L×SU(2)_R → SU(2)_V flavor symmetry breaking. The mass is exactly zero when there are no explicit quark masses in the theory, and this “hierarchy” is very well understood. Furthermore, few are distressed that the proton mass mp ∼ 1 GeV is much less than the Planck mass M_Pl∼10¹⁹ GeV = (G_N)^-1/2. The reason is as stated earlier: an O(1) number, namely the QCD gauge coupling, is an input at some high scale that through renormalization group flow generates an exponentially suppressed scale through dimensional transmutation. This is not concerning because no very big or very small numbers were needed as input. In other words, there is an explanatory theory that possesses Absolute Naturalness.

id. 

(Aurait-on pu) aller au delà de l'électrodynamique quantique en cherchant à expliquer pourquoi la masse de l'électron est naturellement faible ?

Voilà maintenant une présentation plus audacieuse du pouvoir heuristique de la naturalité à travers une réflexion épistémologique originale sur l'électrodynamique quantique vue dans le cadre plus général du Modèle Standard.

I will outline a series of Realistic Intellectual Leaps (RILs) that I think would be achieved in short order, and were ripe to do so at the time, if only researchers were committed to Naturalness and were unwaveringly tenacious at trying to explain the smallness of the electron mass. [...] the argument here is not reliant on historical analysis, but of conceptual inevitabilities once Naturalness is declared a primary goal. [...] Let us begin with the first RIL that would set it all in motion.

RIL #1: The value of m_e is closer to zero than it is to M_Fermi.

On the surface this may look like a strange statement, but we hear such statements frequently in many different contexts, and they have been part of physics discussions time immemorial. From the earliest days of Newtonian mechanics when considering orbits of “massless” planets, to present day collider physics where b quark is “massless” in comparison to the characteristic energies of the collisions, researchers have realized that for all practical purposes objects sometimes have a mass “closer to zero” than the other characteristic scales or masses of the problem. [...] Inspection of the Quantum Electro-Dynamics (QED) lagrangian tells us immediately that the problem is the existence of the gauge-invariant and Lorentz invariant operator . This mass operator is dimension three and so to round out the dimensions to four one needs a massive coupling, which Absolute Naturalness demands should be similar to M_Fermi. A reasonable starting path is to somehow recast the theory to make ψψ not an invariant, in the spirit of recognizing that m_e is “closer to zero than it is to M_Fermi.” If adherence to Absolute Naturalness is our primary concern, no ugliness or complexity should stop us from finding a way to banish this offending operator ψψ . In time it would be inevitable that theorists would recognize that there is something special about that operator compared to others. This leads to the next RIL.

RIL #2: The representation structure of the Lorentz group allows us to write QED in two-component spinors, which is seen to demonstrate a qualitatively unique feature to the mass operator.

We see that the mass term mixes the right and left handed components of the spinor, whereas the kinetic term and gauge interactions do not. [...]Thus, there is a qualitative difference in the Lorentz structure of the mass term compared to the kinetic and gauge potential interaction terms. Now that we have established this unique Lorentz structure of the mass term, the next leap is to recognize that the ψψ term can now be banished.  

RIL #3: The electron mass can be forbidden by assigning ψ_L different properties than ψ_R, thereby disallowing  ψ^†_Lψ_R + ψ^†_Rψ_L as an invariant of the theory.  

The first thought would be to give ψ_L and ψ_R different electric charge, but that violates experiment badly since there are not two different electric charges to be seen. The electric charge for both we must keep at -1, but we can assign different charges for each under a new symmetry G. [...] 

The principle of Naturalness cannot be derived from first principles, and its invocation in science is more of a product of intuition against the likelihood of large numbers conspiring together to give small numbers than it is on rigorous deduction. Nevertheless, the concept bears fruit and is satisfied with respect to our QED example here and other examples to be found in the literature. It remains to be seen if the hard pursuit of Naturalness for the Higgs boson sector has correctly predicted new physics, yet to be seen, that is near the Higgs boson mass scale.

 id.