(See the sidebar below for background info on the Higgs boson.)
Hadron Collider Physics Syposium 2011 was held in Paris Nov. 14 through Nov. 18. At that meeting, the results of a preliminary analysis of CMS and ATLAS combined data sets was released (not including any LHC data gathered since the summer). The analysis of 2 inverse femtobarns of ATLAS and CMS data reveals that the Standard Model Higgs is excluded at 95% confidence level for all masses between 141 GeV and 476 GeV. Even more stringent limits (99% exclusion) apply for the 146-443 GeV range (with some gaps). There is a 90% exclusion confidence level reaching down to 132 GeV.
What this means is that the Standard Model Higgs boson can only possibly be found now in the 114-132 GeV region. But, keep in mind the fact that not all available data has yet been analyzed (and rumors from insiders indicate that they are seeing no signal for a Higgs greater than 2 sigma in the preliminary results for the remaining data). This remaining band is actually the favored place for interactions producing a Higgs boson to be found, but this is problematic in that it is also a rather noisy portion of the mass spectrum, making it exceptionally difficult to pull a signal out of the background noise.
“Riiiiight. So what’s an inverse femtobarn?”
During World War II, American physicists working on the Manhattan Project jokingly referred to the uranium nucleus as being “as big as a barn” compared to other nuclei. Hoping that the terminology would obscure the nature of their research, “barn” came to be used as a unit of area equal to 10-28 m2. A femtobarn is thus equal to 10-43 m2. An inverse femtobarn (fb-1) is a measure of collisions per femtobarn. Given the tiny cross-sectional area of proton-proton collisions such as those at the LHC, 1 fb-1 s corresponds to roughly 16,000,000 collision events recorded. A related figure in accelerator physics is luminosity, which corresponds to the number of collisions per cm2 and per second.
Preliminary analysis of the full dataset might possibly be announced by the time of the CERN Council week in mid December. All channels should analyzed by the Moriond conference at the beginning of March. LHC x3 data collected will be analyzed before the 2012 Winter Conferences, and the now-defunct Tevatron will provide final results on their full 10 fb-1 dataset by the 2012 Summer Conferences. Around the same time, there is expected to be a combination of the available LHC and Tevatron datasets.
What are the consequences if the Standard Model Higgs Boson is not found? Well, Matt Strassler explains the potential scenarios much better than I, but here they are in a nutshell:
- There is a hitherto-unknown interaction suppressing production of the Higgs boson. Weeee! New physics!
- There are actually multiple Higgs bosons, but we are looking in the wrong places for them. There are several potential extensions to the Standard Model which predict this, but it will take a long time to test them. (Some candidate models are based upon Supersymmetry, but the latest LHC and Tevatron results don’t bode well for most flavors of Supersymmetry.) Still, weee! New physics!
Either way, explaining the failure to detect the Standard Model Higgs boson requires extending the Standard Model. Given that we already know that the Standard Model is incomplete, this would not come as a great surprise.
Notice that the absence of a Higgs field altogether doesn’t really enter into the picture. The absence of a Higgs boson does not automatically preclude the existence of the Higgs field, and we know that the Higgs field exists (or at least something very much like it), otherwise, there would be no such thing as mass. And such a universe
would be dramatically different (and devoid of life as we know it).
“ATLAS and CMS combine summer ’11 search limits on the Standard Model Higgs” ATLAS Experiment Blog (November 18, 2011)
Kétévi Adiklè Assamagan, “Joining Forces in the search for the Higgs“. ATLAS Experiment Blog (November 18, 2011)
“No Higgs is good Higgs!“. Quantum Diaries, October 16, 2011.
“Collision Course: What will scientists do if they fail to find the Higgs boson?“
Nature Editorial. Nature 479, 6 (03 November 2011) doi:10.1038/479006a Published online 02 November 2011
Geoff Brumfiel, “Higgs hunt enters endgame“, Nature 479, 456–457 (21 November 2011) doi:10.1038/479456a
symmetry breaking » Blog Archive » Favored Higgs hiding spot remains after most complete search yet
Kathryn Grim, SymmetryBreaking, Nov. 18, 2011
“Where do we stand on the Higgs boson search?“, Quantum Diaries, Nov. 23, 2011
Background info on the Higgs:
An Idiosyncratic Introduction to the Higgs
A diagrammatic hint of masses from the Higgs
Higgs and the vacuum: Viva la “vev”
Helicity, Chirality, Mass, and the Higgs
Who ate the Higgs?
Why do we expect a Higgs boson? Part I: Electroweak Symmetry Breaking
The Higgs FAQ 1.0 | Of Particular Significance
Implications of Higgs Searches (as of 9/2011) | Of Particular Significance
Why The Hints of Higgs Currently Rest on Uncertain Ground | Of Particular Significance
Presentation on the hunt for the Higgs. from QMUL – NExT, Nov. 9, 2011
Note that Higg’s paper was simply the first to present a relativistic version of a model proposed earlier by Anderson (applying concepts from the BCS theory of superconductivity) and others:
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- Ph. Anderson: “Plasmons, gauge invariance and mass.” In: Physical Review. 130, pp. 439–442 (1963).
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- Guralnik, G. S., Hagen, C. R. & Kibble, T. W. B. “Global Conservation Laws and Massless Particles“, Phys. Rev. Lett. 13, 585–587 (1964). doi:10.1103/PhysRevLett.13.585.
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- S. Weinberg, “A model of leptons“, Phys. Rev. Lett. 19: 1264-1266 (1967). Steven Weinberg’s Electroweak Theory, which incorporated the Higgs mechanism.
- Salam, A. in Proc. 8th Nobel Symp. (ed. Svartholm, N.) 367–377 (Almqvist & Wiksell, Stockholm,1968).
- ‘t Hooft, G. “Renormalizable lagrangians for massive Yang-Mills fields“, Nucl. Phys. B 35, 167–188 (1971).
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[hep-th/9802142] Spontaneous Symmetry Breaking in Gauge Theories: a Historical Survey
Peter Higgs : A Brief History of the Higgs Mechanism
[0907.3466] The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles
Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism – Scholarpedia
Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism (history) – Scholarpedia
J. Bernstein, “A question of mass“, American Journal of Physics, 79, Issue 1, pp. 25 (January 2011).
L. Álvarez-Gaumé and J. Ellis, “Eyes on a prize particle“, Nature Physics 7, 2-3 (2011) doi:10.1038/nphys1874
Why 10 Years To Be Sure There’s No Higgs Particle(s)? | Of Particular Significance
Lots of Misleading Higgs Stories | Of Particular Significance
Why We Need the Higgs, or Something Like It | Cosmic Variance | Discover Magazine
Hidden symmetries | Cosmic Variance | Discover Magazine
How Difficult is it to Find the Higgs? | Quantum Diaries
At CERN, waiting. Here’s what to watch for on Tuesday. | Of Particular Significance
3 Articles on Standard Model Higgs: #1 Production; #2 Decays; #3 Search and Study | Of Particular Significance