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Before the LHC: Past searches for the Higgs boson
Physicists have been searching for the Higgs boson since the late 1980s. Physicists report their knowledge about the Higgs boson by ruling out possible masses for the Higgs boson.
Particle Physicists measure the mass of subatomic particles such as the Higgs boson in units of GeV/c2. 1 GeV/c2 is equivalent to 1.79 x 10-27 kg, or roughly the mass of the proton. Here we present four historical searches for the Higgs boson.
- The NA31 collaboration set a limit of mH > 0.015 GeV/c2 in 1989.
- Four experiments at the LEP collider set a limit of mH > 114.4 GeV/c2 in 2000.
- The Tevatron collider ruled out Higgs boson masses between 156 and 177 GeV/c2 in summer 2011.
- Indirect searches, using precision measurements from LEP and the Tevatron, suggest mH< 161 GeV/c2.
First Searches: NA31
The first experimental results on the Higgs boson was published in 1989 by NA31 collaboration based at CERN, of which Edinburgh was a member. NA31's main purpose was to look for differences between matter and anti-matter. However, using the data it was already collecting, NA31 may have been able to observe the Higgs boson, provided it had a very small mass.
NA31 looked for a Higgs boson produced from the decay of another particle (called a neutral kaon) with the Higgs boson subsequently decaying to an electron and an anti-electron. NA31 observed just three events with this experimental signature.
However three background events, from other physics processes not related to the Higgs boson were also expected to have the same signature. NA31 therefore observed no evidence for a very low mass Higgs boson.
Using this observation NA31 were able to set the first definitive limits on the mass of the Higgs boson: the mass of the Higgs boson must be greater than 15 MeV/c2 or 0.015 GeV/c2.
The LEP Era
During the 1990s Edinburgh was a member of the ALEPH collaboration at the CERN Large Electron Positron Collider (LEP). LEP collided electrons and positrons (anti-electrons) at energies ranging from 87 GeV to 209 GeV in the same 27km underground tunnel now used for the LHC.
ALEPH was one of four experiments at LEP looking for a Higgs boson produced in association with a Z0 boson, and then the Higgs boson decaying into a pair of bottom quarks. As for NA31 the number of events observed was compatible with the background expectations.
Hence, in 2000, after taking and analysing data for 10 years, the LEP experiments were able to set a lower limit on the mass of the Higgs boson of 114.4 GeV/c2.
The Tevatron Collider at Fermilab near Chicago collided protons and anti-protons from 1987, and was shutdown finally this year. The Tevatron have searched for the Higgs boson created directly as a result of a proton -- anti-proton collision, with the Higgs boson decaying subsequently to a pair of W-bosons.
Using around 80% of the total data collected, the two experiments at the Tevatron, CDF and DØ, ruled out the existence of a Higgs boson with a mass between 156 and 177 GeV/c2, and between 100 and 108 GeV/c2.
Although LEP did not find any direct evidence for the Higgs, measurements from the four LEP experiments, along with measurements from the SLC and Tevatron colliders in the USA, have found circumstantial, or indirect evidence for the Higgs boson. In particle physics, all the fundamental particles except the massless photon interact with the Higgs boson. Therefore almost all measurements in particle physics are senstive, to a small degree, to the Higgs boson. These measurements, collectively, suggest that the mass of the Higgs boson is less than 161 GeV/c2.
- The NA31 experiment at CERN
- NA31's results on searches for the Higgs boson
- The LEP Collider at CERN
- Higgs boson search results from the LEP experiments
- Publication of Higgs boson seaches at LEP
- The Tevatron collider
- Higgs Results from the Tevatron
- Indirect Measurements of the Higgs boson mass: