mercredi 3 décembre 2014

Pourquoi le blogueur Jester (physicien des particules) est-il si (raisonnablement) "méchant" (avec ses collègues astrophysiciens)?

Hypothèse 1 : parce qu'il sait que toute preuve expérimentale est probablement fausse (sans estimation correcte de son incertitude) jusqu'à preuve du contraire (sa reproductibilité)
There indeed seems to be an excess in the 2-4 GeV region. However, given the size of the error bars and of the systematic uncertainties, not to mention how badly we understand the astrophysical processes in the galactic center, one can safely say that there is nothing to be excited about for the moment. 

It is well known that sigmas come in varieties: there or more significant 3 sigmas, less significant 3 sigmas, and astrophysical 3 sigmas. 

Notice that different observations of the helium abundance are not quite consistent with each other, but that's normal in astrophysics; the rule of thumb is that 3 sigma uncertainty in astrophysics is equivalent to 2 sigma in conventional physics. 

Although the natural reaction here is a resounding "are you kidding me", the claim is that the excess near 3.56 keV ...  over the background model is very significant, at 4-5 astrophysical sigma. It is difficult to assign this excess to any known emission lines from usual atomic transitions. If the excess is interpreted as a signal of new physics, one compelling (though not unique) explanation is in terms of sterile neutrino dark matter. In that case, the measured energy and intensity of the line correspond to the the neutrino mass 7.1 keV and the mixing angle of order 5*10^-5, see the red star in the plot. This is allowed by other constraints and, by twiddling with the lepton asymmetry in the neutrino sector, consistent with the observed dark matter relic density.
Clearly, a lot could possibly go wrong with this kind of analysis. For one thing, the suspected dark matter line doesn't stand alone in the spectrum. The background mentioned above consists not only of continuous X-ray emission but also of monochromatic lines from known atomic transitions. Indeed, the 2-10 keV range where the search was performed is pooped with emission lines: the authors fit 28 separate lines to the observed spectrum before finding the unexpected residue at 3.56 keV. The results depend on whether these other emission lines are modeled properly. Moreover, the known Ar XVII dielectronic recombination line happens to be nearby at 3.62 keV. The significance of the signal decreases when the flux from that line is allowed to be larger than predicted by models. So this analysis needs to be confirmed by other groups and by more data before we can safely get excited.

Hypothèse 2 : Parce qu'il est un peu las de ne pas trouver dans son laboratoire la nouvelle physique (que les autres voient déjà dans leur observatoire)
There is no evidence of new physics from accelerator experiments (except, perhaps, for the 3-3.5 σ discrepancy of the muon (g-2) 7a, 7b, 8)). Most of the experimental evidence for new physics comes from the sky like for Dark Energy, Dark Matter, baryogenesis and also neutrino oscillations (that were first observed in solar and atmospheric neutrinos). One expected new physics at the electroweak scale based on a ”natural” solution of the hierarchy problem 4). The absence so far of new physics signals casts doubts on the relevance of our concept of naturalness. 
(Submitted on 8 Jul 2014 (v1), last revised 17 Jul 2014 (this version, v2))

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Cher-ère lecteur-trice, le blogueur espère que ce billet vous a sinon interessé-e du moins interpellé-e donc, si le coeur vous en dit, osez partager avec les autres internautes comme moi vos commentaires éclairés !
Dear reader, the blogger hopes you have been interested by his post or have noticed something (ir)relevant, then if you are in the mood, do not hesitate to share with other internauts like me your enlightened opinion !