Search for the Higgs boson
[2] This confirmed answer proved the existence of the hypothetical Higgs field—a field of immense significance that is hypothesised as the source of electroweak symmetry breaking and the means by which elementary particles acquire mass.
[Note 1] Symmetry breaking is considered proven but confirming exactly how this occurs in nature is a major unanswered question in physics.
[4] Despite their importance, the search and the proof were extremely difficult and took decades, because direct production, detection and verification of the Higgs boson on the scale needed to confirm the discovery and learn its properties required a very large experimental project and huge computing resources.
Ultimately the search led to the construction of the Large Hadron Collider (LHC) in Geneva, Switzerland, the largest particle accelerator in the world, designed especially for this and other high-energy tests of the Standard Model.
Since the Higgs boson, if it existed, could have any mass in a very wide range, a number of very advanced facilities were eventually required for the search.
These included very powerful particle accelerator and detectors (in order to create Higgs bosons and detect their decay, if possible), and processing and analysis of vast amounts of data,[9] requiring very large worldwide computing facilities.
[9][10][11] Experimental techniques included examination of a wide range of possible masses (often quoted in GeV) in order to gradually narrow down the search area and rule out possible masses where the Higgs was unlikely, statistical analysis, and operation of multiple experiments and teams in order to see if the results from all were in agreement.
[12] However, the prospect of actually finding the particle were not very good; the authors of one of the first articles on Higgs phenomenology warned: We should perhaps finish our paper with an apology and a caution.
[17] But in the end the data was inconclusive and insufficient to justify another run after the winter break and the difficult decision was made to shut down and dismantle LEP to make room for the new Large Hadron Collider in November 2000.
[20] The Superconducting Super Collider was to accelerate protons in an underground 87.1 km circular tunnel just outside Dallas, Texas to energies of 20 TeV each.
However, hadron colliders also provide another way producing a Higgs boson through the collision of two gluons mediated by a triangle of heavy (top or bottom) quarks.
After run 1 (1992–1996), in which the collider had discovered the top quark, Tevatron had shut down for significant upgrades focused on improving the potential for finding the Higgs boson; the energies of the protons and antiprotons was bumped up to 0.98 TeV, and the number of collisions per second was increased by an order of magnitude (with further increases planned as the run continued).
If it were too light (<140 GeV), then the Higgs would predominantly decay to pairs of bottom quarks—a signal that would be swamped by background events, and the Tevatron would not produce enough collisions to filter out the statistics.
[24] In their final analyses, the collaborations of the two detectors at Tevatron (CDF and DØ) report that based on their data they can exclude the possibility of a Higgs boson with a mass between 100 GeV/c2 and 103 GeV/c2 and between 147 GeV/c2 and 180 GeV/c2 at a 95% confidence level.
[32] Preliminary results from the ATLAS and CMS experiments at the LHC as of July 2011 excluded a Standard Model Higgs boson in the mass range 155-190 GeV/c2[33] and 149-206 GeV/c2,[34] respectively, at 95% CL.
[49][50] Speculation escalated to a "fevered" pitch when reports emerged that Peter Higgs, who proposed the particle, was to be attending the seminar.
[53][54] Using the combined analysis of two decay modes (known as 'channels'), both experiments reached a local significance of 5 sigma — or less than a 1 in one million chance of a statistical fluctuation being that strong.
[45] The two teams had been working independent from each other, meaning they did not discuss their results with each other, providing additional certainty that any common finding was genuine validation of a particle.
Moreover, the production rates and branching ratios for the observed channels match the predictions by the Standard Model within the experimental uncertainties.
[58] Further confirmation required more precise data on some of the characteristic of the new particle, including its other decay channels and various quantum numbers such as its parity.
To allow for further data gathering, the LHC proton-proton collision run had been extended by seven weeks, postponing the planned long shutdown for upgrades in 2013.
[59] In November 2012, in a conference in Tokyo researchers said evidence gathered since July was falling into line with the basic Standard Model more than its alternatives, with a range of results for several interactions matching that theory's predictions.
Numerous statements by the discoverers at CERN and other experts since July 2012 had reiterated that a particle was discovered but it was not yet confirmed to be a Higgs boson.
