So, no Higgs boson then?

Question

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).

Answer 1

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.

Answer 2

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.

Answer 3

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.