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There are a lot of articles being posted in the wake of a CERN announcement that they have not observed the Higgs boson in the range of energies so far searched (between 145 and 466 billion eV), e.g. this Scientific American blog post.

How can this be? Wasn't the existence of the Higgs an almost foregone conclusion. Can someone who looked at the details at CERN explain if they are hoping to run the LHC for more time to find it, or is the search becoming hopeless (or perhaps its energy was estimated wrong).

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The LHC has produced 2.5/fb of data, whereas they need at least 6/fb to get a 3-sigma signal for the the most difficult 115Gev Higgs, if it's there. There is a good chance of seeing a signal by the end of October if things go well, but we will know for sure by the end of 2012 physics.stackexchange.com/questions/9319/… –  John McVirgo Aug 31 '11 at 15:26
There is still room in the 114--140 GeV range. Further Higgs at about 115 GeV has been a favorite with quite a few people in the last decade or so. They've put this result out there not because it is Earth-shattering, but because it shows that they are achieving the kind of physics results that were expected and that they have their analysis tools in train. That kind of thing is just part of the culture in particle physics (one of my experiments is pushing hard to get a "early" paper out right now, it won't set the world on fire either) –  dmckee Aug 31 '11 at 15:32
Prof. Strassler explains in this post profmattstrassler.com/articles-and-posts/the-higgs-particle/… why it is too early to draw definitive conclusions about the higgs. –  Dilaton Aug 31 '11 at 15:40
I've been considering asking a question as to whether or not the upgrade that was talked about will matter. If I understand, they're doing like 3+3=7 TeV or something as opposed to 7+7=14 TeV energies, which would still give some results (as published now), but wouldn't be quite as good. I'm interested to know if that difference mattered or not. Maybe they now need the upgrade to find the Higgs? Like we're just chasing the lower limit? –  Alan Rominger Aug 31 '11 at 17:54

3 Answers 3

up vote 6 down vote accepted

I wouldn't call the existence of the Higgs boson a foregone conclusion. It got people excited because we have a very elegant theory that predicts pretty much everything else correctly, and also predicts the existence of a Higgs boson, but the theory could certainly be wrong. Elegance is no guarantee of success.

Anyway, I recently made a blog post about that that you might want to read. The gist of it is that the signal observed by the LHC detectors - in other words, the difference between what they actually saw and what they expected to see - has been getting smaller the more data they collect, over the past month or so. But it's too soon to say anything with certainty. The LHC will continue running, and if the Higgs boson is there to be discovered, they hope to do so by the end of this year. Otherwise, they expect to be able to exclude it by the end of next year.

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Is it then possible to explain success of SM at LHC with fitting parameters being too numerous in SM? –  Vladimir Kalitvianski Aug 31 '11 at 19:23
@Vladimir: there are plenty of nontrivial predictions of the standard model. GSW theory predicted a Z boson, for example, which is pretty unnecessary unless you're breaking SU(2)xU(1). Fitting billions of data points to 50-odd parameters is very very nontrivial. –  Jerry Schirmer Sep 1 '11 at 14:57
Those billions of data points are not random, Jerry. I can compare this situation with an attempt to describe, say, a 3D curve with help of 13D basis with stuffing it with fitting constrains. On the other hand, it still looks like an attempt to describe a 3D curve in 2D space with help of "running units" of this insufficient 2D space. –  Vladimir Kalitvianski Sep 1 '11 at 15:20

The signal for a Higgs boson is still alive and kicking in the data.

There were some reports that is had "faded" in strength since the EPS conference in July but that is not true. CMS had changed the analysis method from MVA-based to Cut-based in the WW channel which dominates much of the mass regions. This made it look like the hints of a signal reported at EPS had diminished by a lot, but that is not true.

In fact the combined data from all experiments globally is now sufficient to expect to exclude a Higgs boson with any mass below 500 GeV if it does not exist. However a strong possibility for Higgs boson remains in the range 115 GeV to 145 GeV. This is certainly not something that should lead anyone to say that the Higgs boson does not exist. On the contrary, evidence for a light Higgs or even more than one of them is mounting.

The mass range that remains to be fully tested is the hardest to probe because a light Higgs in this range decays in ways that are harder to separate from the background. It is also worth saying that minimal supersymmetry favours a Higgs below 128 GeV and that the standard model favours the range above 130 GeV for a stable vacuum, and below 180 GeV to avoid the Landau pole. So the region left is the one with the most prior interest. There is no reason to think that excluding most of the wide mass range being searched over by the experiments reduces its chance of existence.

The scientific American blog post linked to is full of ignorant statements such as this "Congress may feel that even though its 1993 decision to cancel the American alternative to CERN—the Superconducting Super Collider—was generally met with chagrin by the American physics community, it may have been the right move one after all: to spend billions of taxpayer dollars in search of a particle that likely does not exist would have been wasteful." Anybody even vaguely acquainted with the physics knows that ruling out the Higgs boson would also be an enormous discovery. Journalists have no excuse for not knowing this because the director of CERN has emphasized just this point repeatedly in recent press conferences.

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Yeah, no Higgs is way more exciting than if the LHC finds a Higgs and nothing else. –  Jerry Schirmer Sep 1 '11 at 15:00
Also, the SSC would have probed energies ten times as high as the LHC. It was a much more ambitious experiment that was nearly ready to be fired up. –  Jerry Schirmer Sep 1 '11 at 15:01
hang on, you say we have enough evidence to exclude <500 GeV but evidence for ~130 GeV... this sounds contradictory! –  Nic Sep 1 '11 at 15:22
I did not say we have enough evidence to exclude < 500 GeV. I said we have enough data to expect to exclude the Higgs < 500 GeV if it is not there. I hope you see the difference. –  Philip Gibbs Sep 1 '11 at 16:23

They've published results using 1 to 1,7/fb of data by now. Discovering the most difficult Standard Model Higgs with a mass of 115 GeV would take an average of 17/fb, although there would be hints long before. By now they've gathered 2,5/fb for both CMS and Atlas. By the end of this year's running (end of October) they will have gathered 5-7/fb each. These can be combined for a total of 10-14/fb which should be enough to give us a strong hint of whether the Standard Model Higgs is there or not.

LEP excluded everything up to 114 GeV and the Tevatron excluded everything above 158 Gev. This allowed range of 114 - 158 GeV has now shrunk to 114 - 145 Gev so about 30% of the allowed range has been cut by the LHC. Add to this that indirect measurements show us the Higgs is more likely to be on the light end of this interval than the heavy end.

Also note that all this is specific to the Standard Model Higgs while what you call an "almost foregone conclusion" would be (I think) that there is some Higgs-like mechanism to be found. The mathematics require that something fixes the Standard Model at these energies: the probabilities no longer add up to one.

The most popular alternatives, different flavors of supersymmetry, have several Higgses, usually with a lower cross section than the Standard Model one. That means you need more data to find them. IIRC, the worst case for the simplest supersymmetric model, MSSM (Minimal Supersymmetric Standard Model) is that you need 300/fb for a discovery. Although that's more than we'll get by the end of 2012 (after which there's a break for the upgrade to 14 TeV) it still means we would have a small 2-3 sigma bump by then.

If there's no Standard Model Higgs we'll know by the end of this year. Many, if not most, physicists would call this the most likely outcome.

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