Le boson de Higgs ne semble pas connaître la supersymétrie à l'échelle du TeV (Chapitre 5)

Qu'est-ce que le boson de Higgs sait que nous ne sav(i)ons pas? 

Voyons aujourd'hui ce que la découverte du boson de Higgs peut avoir comme conséquence sur la recherche de la supersymétrie. A tout seigneur tout honneur, donnons d'abord la parole à une grande figure de la physique théorique au CERN qui, avant d'être un fervent supporter de la supersymétrie, est d'abord l'un des premiers théoriciens qui encouragea les expérimentateurs à chercher un boson scalaire massif (si l'on en croit Peter Higgs):
In addition to all the traditional arguments for supersymmetry based on naturalness, uni cation, string, dark matter, etc., the LHC discovery of a Higgs boson has provided more. Not only could supersymmetry stabilize the electroweak vacuum, but it predicted successfully that the Higgs mass should be < 130 GeV in simple models ... and it also predicted successfully that its couplings should be within a few % of the Standard Model values ... Global fits to supersymmetric model parameters incorporate inputs from precision electroweak observables, flavour physics, the anomalous magnetic moment of the muon (gμ - 2), the Higgs mass, the dark matter density and searches for astrophysical dark matter, and LHC constraints from searches for missing-energy (MET) events [57] and heavy Higgs bosons [36]. The Higgs mass and MET searches push strongly-interacting sparticle masses above 1 TeV, which makes gμ - 2 difficult to explain within simple models. ... Gluino masses below about 3 TeV should be accessible to the high-luminosity upgrade of the LHC (HL-LHC), but gluino masses above that would probably be accessible only to a higher-energy collider.
John Ellis, Summary of the Nobel Symposium on LHC Results, 13/09/2013

On voit donc que si la particule de Higgs est bien au rendez-vous du LHC, la rencontre très attendue avec des particules supersymétriques n'a pas (encore?) eu lieu et elle n'est peut-être pas pour demain ... Mais ne cédons par au découragement et s'il n'y a rien à tirer pour le moment de la physique expérimentale alors découvrons en attendant une possible leçon d'épistémologie qui est aussi un vibrant hommage aux plus jeunes pour renouveler la phénoménologie de la supersymétrie :
... the march of null results suggests that we were mostly wrong about precisely how supersymmetry would appear at the LHC. To me, this suggests immense opportunity to step back and re-evaluate the criteria that led us to this point. In particular, we built our expectations for supersymmetry at the LHC on the twin pillars of parsimony and naturalness. The null results at the LHC suggest that those two pillars were perhaps not the right foundations. Consequently, there is now tremendous opportunity to figure out the correct theory of the universe, with much room for it to be supersymmetric in some form.
... it's a splendid opportunity for young physicists to inject new ideas and take SUSY phenomenology in new directions. The motivation is still strong, but data has told us that our pre-LHC criteria were perhaps not correct, and new criteria (such as the Higgs mass and couplings) provide suggestive avenues for further development. Thankfully, this is a more or less falsifiable proposition - if we turn on the LHC at 13 TeV and have still discovered no indication of new physics within a few years of starting Run II, even convoluted models of weak-scale supersymmetry will be strongly disfavored. Of course, this says nothing about models of supersymmetry above the weak scale, perhaps motivated by gauge coupling unification - but that's a matter for another day...
Nathaniel Craig, The State of Supersymmetry after Run I of the LHC02/09/2013

L'unification des interactions de jauge n'est pas le sujet d'aujourd'hui pour reprendre les derniers mots de l'extrait précédent mais celui du septième et dernier billet de la série. Avant cela nous parlerons d'un autre type d'extension possible du modèle standard, celui associé non à la supersymétrie mais à la symétrie conforme. Mais avant de nous quitter aujourd'hui, terminons par un résumé de la situation et une note optimiste pour les futurs expérimentateurs du prochain grand accélérateur qui viendra après le LHC, celui dont la tâche sera d'analyser beaucoup plus précisément le premier boson scalaire découvert:
So far, what we've seen looks minimal. Later, a CERN announcement made in March 2013 said it is a Higgs boson. Indeed, the newly discovered particle looks very much like the Standard Model Higgs boson. We've been after this particle ever since 1933 when Fermi wrote his theory of nuclear beta decay. There, he introduced a constant ... which we now call the Fermi constant GF. It corresponds to the energy scale GF-1/2 ≃ 300 GeV, and we learned from him that something is going on at this energy scale. It took a whopping eighty years to come to the point where we now have a UV-complete theory of strong, weak, and electromagnetic forces with all of the parameters measured. In fact, it is a renormalizable and consistent theory that may be valid all the way up to the Planck scale. Coincidentally, even cosmology looks minimal given the Planck data..., which suggests a minimal single-field inflation. Maybe the year 2013 will be remembered in history as the year of elementary scalars...
On the other hand, the Standard Model does not tell us why the Higgs boson condenses in our Universe. This is not only artificial, it is unsatisfying...
Then, what should we do? Of course, we should study this intruder as much as we can! If we look closely enough, maybe we can tell it it has siblings or relatives. We may find it has a finite size. Or we may bring it back to spin in our dimensions. Fortunately, the observed mass of 125 GeV is the best case scenario. It allows us to measure branching fractions to b b̄, WW* , ZZ* , gg, τ+τ -, cc̄, γγ, Zγ, possibly even μ+μ -. Some of them would not be accessible if the Higgs were lighter or heavier by just a few tens of GeV. It is actually a dream case for experiments! ... If the history is any guide, the future precision measurement of the top and Higgs sector would tell us the next energy scale we should go after.
Hitoshi Murayama, Future Experimental Programs, 06/01/2014


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