- Welcome. There is no doubt that since the 4th of July we have a new particle, a boson with mass near 125 GeV. There is little doubt that this particle is a Higgs boson. True, the discovery relies to a large extent on observing a resonance in the diphoton spectrum, which could also be produced by another spin-0 or even a spin-2 particle that has nothing to do with electroweak symmetry breaking. What convinces us of the higgsy nature of the new particle is the signal in the ZZ and WW final states. Indeed, the coupling [h V V], allegedly responsible for the decays to W and Z bosons, is a watermark signature of a Higgs boson, as it is central to its mission of giving mass to gauge bosons.
- Farewell. Welcoming the Higgs, we need to clean the room of some old toys we've got used to. First of all, Higgsless technicolor for obvious reasons goes into the trash bin of history. So does the unhiggs or the whole class of stealthy Higgs theories where the Higgs was supposed to escape detection by decaying into complicated final states. Quite robustly, the 4th generation of chiral fermions is now excluded because, if it existed, the Higgs production rate would be many times larger than observed. Finally, a simple and neat theory of dark matter that annihilates or scatters via a Higgs exchange, the so-called Higgs portal dark matter, is getting disfavored because Higgs would have a large invisible branching fraction, and thus a suppressed rate of visible decays.
- Law. The other Pauli principle: that fermions are discovered in the US, while bosons are discovered Europe has been spectacularly confirmed. Note it was a very non-trivial prediction in this particular case. Higgs would have been discovered at the SSC if the US congress did not intervene to scrap the entire program. Furthermore, Higgs would have been discovered at the Tevatron if the Fermilab management didn't intervene to scrap some crucial Run-II detector upgrades, ensuring the Tevatron discovery potential stops just short of a 3 sigma significance. This only shows how powerful the other Pauli principle is. Don't you think it deserves, if not a Nobel prize, at least the ig-Nobel prize? ;-)
- Hope. The Higgs data from ATLAS and CMS match well the Standard Model prediction with one exception: the diphoton event rate is 50-100% too large, with the significance of about 2 sigma. These are most likely statistical fluctuations, but if the enhancement persists when more data is collected it may become the first clear evidence of new physics. If that is the case, the most plausible interpretation of the current data is that the enhancement is due a light 100 GeV-ish scalar or fermion that carries electric charge but no color. This way the loop contributions of that particle could affect the Higgs decays into photons without messing up the gluon fusion production mode. Furthermore, the new scalar or fermion needs to have a large coupling to the Higgs boson, but its mass has to come dominantly from another source (otherwise it would actually decrease the diphoton rate). If it were confirmed, it would be a particle that apparently no one ordered. On the other hand, theoretically cherished particles (stops, little Higgs top partners, staus) all require a serious tuning and some conspiracy to fit the available Higgs data.
- Nightmare. Despite what I said above, one cannot help noticing that the data are indecently consistent with the simplest Higgs boson of the Standard Model. Overall, adding the 8 TeV data improved the consistency, eradicating some of the hints of non-standard behavior we had last year. It's been often stressed that the Higgs boson is the special one, a particle different from all the others, a type of matter never observed before. Yet it appears in front of us exactly as described in detail over the last 40 years. This is a great triumph of particle theory, but at the same time it's very disappointing to those whose future existence depends on new physics, that is to a large majority of particle theorists.
Monday, 23 July 2012
H-day: the morning after
No, I don't mean I've slept for almost 3 weeks ;-) I mean this particular state of mind of waking up with a vague memory of a crazy party last night, but at the same time unwilling to open your eyes for the fear that the person lying next to you is really the one you think it is.