So, what's new since last week?
- Finally, CMS came out with their long awaited update of the Higgs→γγ search. It is clear why they have been so shy: new calibration of the electromagnetic calorimeter completed after last summer forced them to revise the significance of the Higgs signal in the diphoton channel. The current significance is merely 3.2 sigma, and from that it's easy to deduce that they wouldn't have been able to claim a formal 5 sigma discovery on the 4th July had they done their sums back then. But that's of course irrelevant now, as there's no shadow of a doubt the Higgs is there at 125 GeV. The real game changer is the (related) fact the signal strength relative to the SM one measured by CMS is μ=0.8±0.3 and, unlike the one in the ATLAS and previous CMS measurements, does not show any excess over the standard model prediction. Naively combining the signal strength measured by ATLAS and CMS one gets the disappointing μ=1.2±0.2. Move on folks, nothing to see here. See Matt's blog for a more in-depth discussion of the CMS diphoton update.
- One visible consequence of the CMS updated is that the preference for negative Yukawa couplings, displayed by the Higgs data before Moriond, completely vanished. In the plot, borrowed from here, is a section of the Higgs coupling space where one freely varies the coupling to W and Z bosons (denoted as "a"), and, independently, the coupling to the standard model fermions (denoted as "c"). Previously, the Higgs data somewhat preferred the bizarre region near a=1, c=-1 over the standard-model-like region near a=c=1, as the former was leading to an enhancement of the Higgs→γγ rate. This is no more, only the standard-model-like island remains.
- Another important update last week was from ATLAS in the WW→2l2ν channel. Again no surprises here: the signal significance almost 4 sigma, the rate μ=1.0±0.3.
- To say that the Higgs is standard-model-like is an understatement. This bastard screams and spits standard model. After the Moriond updates the standard model gives an absolutely perfect fit to the combined data (previously it was disfavored at 80% confidence level, mostly due to the late diphoton excess). Not even a single cliffhanger to makes us wait for the next episode..... If there's anything non-standard about the Higgs couplings to matter it is hiding very well and will be tricky to uncover at the LHC, even after the energy upgrade.
- It's worth stressing again that is has been firmly established that the Higgs couples to mass of W and Z bosons, the statement which can be reinterpreted that the Higgs gives mass to W and Z bosons. After Moriond the evidence for that is strengthened even further. The LHC Higgs data alone (blue in the plot) imply this coupling should be within ~30% of the standard-model value at 95% confidence level. If one also takes into account precision observables from LEP (red in the plot) the allowed range shrinks to ~10% (although the latter number is a bit less robust, as fine-tuned new physics effects could alter the conclusions). Although announcing the Higgs is a Higgs at this particular moment of history was to a large extent arbitrary, it was by no means premature.
- We see the Higgs, of which the corollary is that we don't see the invisible Higgs. The invisible decays could arise for example if the Higgs couples to dark matter, which is the case in a large class of well-motivated models. In the simplest situation, when the Higgs couplings to known particles take the standard model values, an invisible width would lead to a universal reduction of the rates in all decay channels. That is severely constrained by global fits, as seen in the plot (solid blue): the invisible branching fraction more than about 15-20% is strongly disfavored. Note that for the sake of this analysis "invisible" is "anything not measured", thus any non-standard decay channel is constrained this way. But that doesn't mean there's no point looking for exotic Higgs decays. In the end, we've already observed much more rare decays: the branching fraction to 2 photons is around 0.2%, and to 4 leptons - around 0.01%. So 10% of invisible or some other freak decays seems like a goal well within reach of experimentalists.
- A fun fact for dessert. In CMS the measured central values of the Higgs rates in the 3 most sensitive channel all fall slightly below 1, while in ATLAS they are all slightly above one. The new motto for ATLAS: we try harder ;-)