Tuesday, 23 August 2011

Higgs won't come out of the closet, part II

After a short summer break we're back to Higgs hunting. The LHC continues to exceed all expectations with regard to the machine performance as it continues to disappoint (or to test our patience, if you prefer) with regard to discoveries. The latest Higgs search results based on about 2 inverse femtobarns of data were presented by ATLAS and CMS yesterday at the Lepton-Photon conference in Mumbai (though properly it should be called Lepton-Photon-Jet-and-Missing-Energy). The last status update: still no Higgs in sight.











Nothing new at first sight, so what's new?
  • Within the framework of Standard Model the Higgs boson is excluded by at least one experiment in the mass range 145-466 GeV, except for a small 288-296 GeV window that probably would also be excluded if ATLAS and CMS results were combined. Furthermore, the Standard Model Higgs heavier than 466 GeV is by far excluded by precision electroweak observables, mostly by the precise measurement of the W and Z boson masses to which Higgs contributes at the quantum level. This leaves 115-145 GeV as the most likely hiding place. That range shrinked only by a few GeV compared to the limits presented at EPS a month ago.
  • CMS updated several Higgs search channels with 1.5-1.7 fb-1 of data. ATLAS, on the other hand, updated only the 2 channels which provide most of the steam : H→WW→2l2ν and H→2Z→4l, although throwing in a bit more data than CMS. That is because ATLAS is more dependent on European workforce which in August retreats en masse to the seaside.
  • After the EPS conference there was a reasonable hope that an evidence for the Higgs could emerge this summer. The previous LHC results were suggestive of a 140-ish GeV Higgs boson producing a broad excess in the H→WW→2l2ν channel. Now it seems that a 140 GeV Higgs is not preferred by the latest data, even if it's not formally excluded: as Tommaso explains in these two posts, if the Higgs has indeed 140 GeV we would expect a larger excess by now. A lighter Higgs, 115-130 GeV, remains perfectly consistent with the data, in the sense that we would not expect to see it just yet.
  • The sample of the "golden-channel" final state with 2 Z bosons decaying to 2 leptons each is growing in size but nothing glitters here. This channel is the leading one for the heavy Higgs, and it retains some sensitivity for intermediate masses above 140 GeV. Unfortunately, the shape of the ZZ invariant mass spectrum that emerges has no significant bumps and nicely follows the background continuum. The di-photon sample, whose sensitivity is approaching the Standard Model cross section for a light Higgs, shows no interesting bumps either (the plot below).
  • It is somewhat surprising that the LHC didn't show the combination of 1fb-1 ATLAS and CMS data, contrary to what they promised. Probably they decided it would be confusing as the excess seen in the earlier data is not being confirmed by the newer data. Another hypothesis is that they didn't show it because the plot turned out identical to the one on viXra log ;-)
  • One should not forget that the LHC limits refer to the Standard Model Higgs. Beyond the Standard Model the Higgs may have a reduced cross section, larger width, invisible or more pesky decays, and so on. Any of these modifications may invalidate the Standard Model limits and make the search more challenging. For the moment the standard Higgs is the priority but we'll think more seriously about the alternatives in case no evidence is seen in 5fb-1. Furthermore, going beyond the Standard Model, a very heavy Higgs above 450 GeV becomes formally allowed provided some other particles mess up into our precision observables.
  • Finally, one can't help but notice that the Higgs, if it exists in the Standard-Model-like avatar, chose its own mass so as to maximize the difficulty of discovering it. If it's a god particle it's Loki rather than Thor.
The next major Higgs update will probably wait until this year's LHC run is completed, that is until November. Is there anything else we should expect during Lepton-Photon? According to the Bollywood rules of the genre there must be a happy ending with everybody dancing in the last scene. Actually, there is a persistent rumor among theorists that LHCb, whose presentation is scheduled for Saturday, is sitting on an interesting result. Is this true and, if so, will they share it in Mumbai? Experience shows you should not trust theorist-driven rumors but, regardless, it may be worth to wake up early on Saturday :-)

21 comments:

Kea said...

An LHCb rumour? Cool! But could they possibly have enough data already for that nice dimuon anomaly?

Robert L. Oldershaw said...

Something is seriously wrong with our "standard" [general cosmology/particle] paradigm.

Anybody want a hint?

We have rigidly assumed, but never tested, the idea that the gravitational coupling factor is the same on the different major scales [..., atomic, stellar, galactic, ...] of nature's self-evident hierarchical organization.

If one recasts general relativity so as to incorporate discrete cosmological self-similarity, then the gravitational coupling factor is different on each scale by a factor of 10^38 and we get an accurate model of nature instead of untestable postmodern pseudo-science.

RLO
Fractal Cosmology

Matti Pitkanen said...

I think that we must gradually be ready to open our mind so that instead of dividing ourselves to fairy-fielders and non-fairy-fielders we can make the question "What it is if it is not the Higgs?". This is just what already Hegel taught and thought us to do;-).

In Quantum diaries blog at CERN this question was already posed. Perhaps because those belonging to inner circles are already realizing the implications of the results of ATLAS. Maybe also blog scientists should be ready to do so;-).

I really *strongly* encourage to spend 10 minutes by studying the *third graph* in this posting appearing also in in Tommaso Dorigo's blog.

The downwards tip for the probability that data are compatible with background in a theory with Higgs plotted as a function of Higgs mass is compared to the theoretical curve. The curve has a sharp downwards tip at about 145 GeV -the mass of the CDF bump - but the depth of the tip is by more than a factor ten smaller than required by Higgs. If one believes that the results are accurate enough, the interpretation is clear: something is there but it is not Higgs.

For one possible answer to the question "What it is?" see my blog posting.

Anonymous said...

I really think the important point that needs to be emphasized for the present data is that the low-energy region 115-130 GeV is precisely where the lightest CP-even Higgs mass should be in the MSSM. So, what the data that Atlas and CMS have collected so far is perfectly and completely consistent with what it should be if the MSSM is the correct description of low-energy physics. For those claiming (or at least implying) that the existence of the Higgs has been ruled-out, the fact is actually the opposite.

Tony Smith said...

With respect to the Golden Channel ATLAS plot that you show, you say "... the shape ... nicely follows the background continuum ...."
but
it appears to me from the graph that the shape actually "nicely follows" the histogram of the sum of both:
Total background (light purple)
and mH = 220 GeV 1xSM (dark purple).

Does that not indicate that the Golden Channel is seeing some indication of Higgs roughly around 220 GeV ?

By indication I mean something that looks like the Higgs and clearly merits further study with more luminosity even though it does not have formal statistical significance of evidence (3 sigma) or discovery (5 sigma).
In my view, such an "indication" should trump such things as a composite 90 per cent CL Brazil Band exclusion.

Tony

Jester said...

4 events observed vs 2 expected that's a 1 sigma fluke. Besides it seems that 220 GeV SM Higgs is safely excluded by ATLAS.

chris said...

just last week i heard from an LHCb guy that they are desperately looking for new ideas of where to look for deviations from the SM. he explicitly said that they are frustrated by not seeing any hint of new physics at all.

Robert L. Oldershaw said...

Update from Lepton-Photon 2011.

No SUSY.

Gosh, who would have guessed?

RLO
Fractal Cosmology

ThePeSla said...

Was it Dorigo who asked if we saw a particle how would we know it was the Higgs or not or something else?

Of course we have found new physics and as they said good colliding! while the rest of us goes back to the pencil and paper.

Oldershaw- there are places and objects where the physics is neither fractal nor holographic.

Well, I posted at pesla.blogspot.com on this very question on a level where there is already a lot of dulled pencils and running out of paper with some of the best ideas of all of you.

There is much more physics we can reason out beyond the SUSY.

ThePeSla

Anonymous said...

I also heard from an LHCb guy that they are seeing no deviation in the like sign dimuon asymmetry, completely contradicting D0.

Phil Gibbs said...

LOL. I would like to know why they really did not show the combo though. They should not select what they show depending on what the results look like.

Vortices said...

I completely agree with Phil here, its not right that they promised the combination but then didnt show it. Even if it was contradicting what the newer results were showing and perhaps showed up the inadequacies of the data the machine is collecting. But they should also trust the physicists to look at the data and understand the statistical uncertainties that lie therein. Anyhoo, i have a feeling the data is being carefully disseminated and that there are in fact bigger hints of things which they are keeping back until they collect more data so that they dont do a tevatron and announce dodgy results. But at least by the end of the year we should have a pretty darn good idea whether this pesky higgs exists or not.

Robert L. Oldershaw said...

Want to make some empirical and theoretical contact with nature.

On 8/24/11 watch the NOVA program on PBS entitled:

"Hunting The Hidden Dimension"

9 PM EST

(Can also be viewed at PBS website anytime)

Anonymous said...

phil and vortices,

they didn't do the combo because it's freakin summer vacation! i heard that there were about 24 hours between unblinding and talk at the EPS meeting and with dada comming in that fast it's a hell of a job to show what they have shown.

Anonymous said...

For people who have waited for decades for the LHC the impatience of many looks just silly.

Jester said...

Phil is referring to the 1fb-1 combo which was ready on time but shelved.

Robert L. Oldershaw said...

You can wait decades more and the answers will still be the same.

It's a pile of Ptolemaic pseudo-science.

String /brane theory, SUSY, the WIMP fiction and QCD are the archetypal examples.

Theoretical physics is lost in the cosmos without a reliable physical principle to guide it.

Those who are unwilling to think anew will continue to be lost.

Anonymous said...

"String /brane theory, SUSY, the WIMP fiction and QCD are the archetypal examples."

Oh, boy! QCD too! How about Newtonian mechanics, thermodynamics and electromagnetism? Those are fictions as well?!?

Anonymous said...

If you are losing a chess game with nature, you should not kick the table over.

You should learn from the master.

Anonymous said...

Funny to see so many speculaitons about the absence of combined results.Also nice to see the experiments crowd becoming responsible enough to not rush, for a change.

Anonymous said...

Probably there is no good particle. Now stop worrying and enjoy your life.