Tuesday 19 March 2013

Higgs: more of the same

Everything's clear now.  The most important Higgs search channels have been updated by both ATLAS and CMS (the only interesting channel that has not is the decay to b-quarks).  We're not going to learn much new for the next 2 years.  The CERN director, heeding to Résonaances' callannounced Habemus Higgsam.

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 ;-)


cb said...

And just as you said also : "let's turn our eyes to heaven" ... looking for a smoking gun from tenebris materia or counting how many wimpibili in the sky ?

Anonymous said...

You write:
"To say that the Higgs is standard-model-like is an understatement. This bastard screams and spits standard model."

whereas CERN announces to the public on March 14:
"It remains an open question, however, whether this is the Higgs boson of the Standard Model of particle physics..."

What is this very different evaluation due to?

They further state:
"To determine if this is the Standard Model Higgs boson, the collaborations have, for example, to measure precisely the rate at which the boson decays into other particles and compare the results to the predictions."

making it appear to the outsider
(who cannot appreciate the subtle meaning of "precisely") as if none of these comparisons have already been done.

Could it be that CERN misrepresents the results for the public to avoid a critical politician's question
"If you found what u were looking for and nothing else, why can't we close CERN down?"

Anonymous said...

Looks like won't need to keep our eyes on the CERN chimney for the white smoke signaling habemus BSM, at least for a few years. With HEP folks in denial or depression, where is this going? A field of this size seems highly unsustainable unless we get some kind of a miracle in the second half of the decade.
Well, the signs for this to happen were there (cosmological constant problem is also not solved by new physics at the 10^-3 eV scale; or precision tests, flavor, etc)... Let's see what Planck says this week.

Jester said...

Anon-2: I don't think there's a contradiction. I'm stressing that, at this point, it looks exactly as the SM Higgs without a single hint of anything unusual. CERN emphasizes that in the future things may change. The current precision of Higgs coupling measurements is 20-50%, depending on the coupling. It is not excluded that when they go down to 10-20% precision during the next run, some deviations of the Higgs couplings from the SM predictions will be revealed.
Anon-1: From the BSM point of view the Planck data are utterly depressing :-)

Anon-2 said...

> I don't think there's a contradiction.

You are right there isn't.
Their statement:

"It remains an open question
if it is a SM Higgs"

and your statement:

"It screams: "I'm a SM Higgs"."

are not in logical contradiction.
But they are different.
Theirs is a statement about the
situation July 4th, 2012, yours
is a statement about the news.
They did not report the news.

What is missing in their statement
is a sentence like:
"13 of its properties were measured with limited precision and they are all SM like, i.e. it seems to be the SM Higgs boson we were looking for."
Why is it missing?

It summarizes the real situation, and is understandable to journalists and politicians.
Why did they omit it?
(Don't tell me because they
forgot ;-)).

Because it is missing the general
newspapers (which mostly just copy
the CERN statements) did not
inform the public about the
real news.

Jester said...

I'd insist it's just a small difference of emphasis. I'm a BSM physicist, and for me it's somewhat disappointing that the Higgs does not give me an edge to hang on to, especially given the early promises and that the current situation will be frozen for another 2 years. CERN on the other hand is stressing that there's still a long way to go. Both points of view are perfectly legitimate imo.

Anonymous said...

CERN said "... It remains an open question, however, whether this is ... the lightest of several bosons predicted in some theories that go beyond the Standard Model. ...".

Does that "give [you] and edge to hang on to" as "a BSM physicist" ?


Anonymous said...

Jester, I forgot which announcement it was that referred to possibility of the 125 GeV Higgs being the lightest one of a Higgs multiplet. Any comments on that (of course if it's not a thing of my imagination)?

Jester said...

Currently there are no serious experimental hints that other Higgs bosons exist, but it is certainly a possibility. In any case, from the current LHC data it seems that even if the 125 GeV particle is merely *a* Higgs boson it is the most important Higgs boson. The heavier partners would couple less strongly to W and Z bosons, and thus would contribute less to their masses.

Marco Frasca said...

I would just rise a flag on the measurement, very precise apparently, by CMS of the WW decay. This hints toward a lower value than one and agrees (but here errors are larger) with ZZ decay. This seems BSM peeping out. I think we need to wait for improvements here to see the emerging of an evidence and ATLAS is still well beyond that values. If the values of the rates are lower than expected from SM, Brazil bands figures should be somewhat tuned again and some surprises would come out about higher mass Higgs bosons that now appear just a fluctuating background.

chris said...

Marco Frasca,

you'b better learn some basics of statistics before making such a claim.

Flakmeister said...


I have been a long time lurker. Thanks for all the great posts....

I am no longer in the field (a long story) but I have been following the Higgs since my introduction to it in 1988 as part of coursework for my Ph.D. And I am proud to say that I made more than a few significant contributions to the hunt along the way...

Well, call me gobsmacked, a fundamental scalar exists at mass which is clearly suggesting to us that the next new physics threshold may be forever beyond our reach...

At Snowmass 2001, you would have garnered strange looks if you asked what if SM and nothing else at the LHC. Looking up the chimney and seeing nothing but blue sky so to speak. It would seem the famous "No Lose Theorem" of a TeV scale collider paid off with the lowest possible jackpot...

Anyway, what I would give to share a beer with the Higgs Hunters Guide now...

Any chance that we can dust off the TESLA TDR?

Jester said...

Well, TESLA merged into the ILC project. Time will tell whether and when it will come into existence...

Anonymous said...

Jester, thanks! (Anon 3). Such heavier Higgses, if found would be BSM, correct? Can we expect to have this question closed by Run 2? And also would it favour (or not) some BSM theories like SUSY?

Alex said...

"Such heavier Higgses, if found would be BSM, correct? Can we expect to have this question closed by Run 2? And also would it favour (or not) some BSM theories like SUSY?"

They would be called BSM, but it could be very minimal BSM, namely it could simply be that the SM is realized with two Higgs doublets instead of one (for example one giving mass to up-fermions and one to down-fermions like in the MSSM) without there being other BSM physics within reach. So people would certainly not hesitate to claim that finding such Higgses would be a hint towards the MSSM, but this need not be so at all.