Early limits During the early 1970s there were only few constraints on the existence of the Higgs boson. The limits that did exist came from the absence of the observation of Higgs related effects in
nuclear physics,
neutron stars, and
neutron scattering experiments. This resulted in the conclusion that the Higgs—if it existed—was heavier than . Thereafter the search for the Higgs boson became firmly established within the LEP program. 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. The inconclusive results of the direct search for the Higgs boson at LEP resulted in a final lower bound of the Higgs mass at the 95%
confidence level. In parallel to the direct search program, LEP made precision measurements of many observables of the weak interactions. These observables are sensitive to the value of the Higgs mass through contributions of processes containing loops of
virtual Higgs bosons. This allowed for the first time a direct estimate of the Higgs mass of about . The
Superconducting Super Collider was to accelerate
protons in an underground circular tunnel just outside
Dallas, Texas to energies of each. One of the primary goals of this megaproject was finding the Higgs boson. On 22 December 2011, the DØ collaboration also reported limitations on the Higgs boson within the Minimal Supersymmetric Standard Model, an extension to the Standard Model. Proton-
antiproton (p) collisions with a centre-of-mass energy of 1.96 TeV had allowed them to set an upper limit for Higgs boson production within MSSM ranging from 90 to 300 GeV, and excluding > 20–30 for masses of the Higgs boson below 180 GeV ( is the ratio of the two Higgs doublet vacuum expectation values). following a
magnet quench event nine days after its inaugural tests that damaged over 50 superconducting magnets and contaminated the vacuum system. The quench was traced to a faulty electrical connection and repairs took several months; electrical fault detection and rapid quench-handling systems were also upgraded. Data collection and analysis in search of Higgs intensified from 30 March 2010 when the LHC began operating at 7 Tev . 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- and 149-, respectively, at 95% CL. All of the above confidence intervals were derived using the
CLs method. As of December 2011 the search had narrowed to the approximate region to 115–130 GeV, with a specific focus around 125 GeV, where both the ATLAS and CMS experiments had independently reported an excess of events, meaning that a higher than expected number of particle patterns compatible with the decay of a Higgs boson were detected in this energy range. The data was insufficient to show whether or not these excesses were due to background fluctuations (i.e. random chance or other causes), and its statistical significance was not large enough to draw conclusions yet or even formally to count as an "observation", but the fact that two independent experiments had both shown excesses at around the same mass led to considerable excitement in the particle physics community. At the end of December 2011, it was therefore widely expected that the LHC would provide sufficient data to either exclude or confirm the existence of the Standard Model Higgs boson by the end of 2012, when their 2012 collision data (at energies of 8 TeV) had been examined. Updates from the two LHC teams continued during the first part of 2012, with the tentative December 2011 data largely being confirmed and developed further. On the same date, the DØ and CDF collaborations announced further analysis that increased their confidence. The significance of the excesses at energies between 115 and 140 GeV was now quantified as 2.9
standard deviations, corresponding to a 1 in 550 probability of being due to a statistical fluctuation. However, this still fell short of the 5 sigma confidence, therefore the results of the LHC experiments were necessary to establish a discovery. They excluded Higgs mass ranges at 100–103 and 147–180 GeV.
Discovery of new boson On 22 June 2012 CERN announced an upcoming seminar covering tentative findings for 2012, and shortly afterwards rumours began to spread in the media that this would include a major announcement, but it was unclear whether this would be a stronger signal or a formal discovery. Speculation escalated to a "fevered" pitch when reports emerged that
Peter Higgs, who proposed the particle, was to be attending the seminar. On 4 July 2012 CMS announced the discovery of a previously unknown boson with mass 125.3 ± 0.6 GeV/
c2 and ATLAS of a boson with mass 126.5 GeV/
c2. 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. When additional channels were taken into account, the CMS significance was 4.9 sigma. On July 31, the ATLAS collaboration presented further data analysis, including a third channel. They improved the significance to 5.9 sigma, and described it as an "observation of a new particle" with mass . Also CMS improved the significance to 5 sigma with the boson's mass at . On 14 March 2013 CERN confirmed that: : "CMS and ATLAS have compared a number of options for the spin-parity of this particle, and these all prefer no spin and even parity [two fundamental criteria of a Higgs boson consistent with the Standard Model]. This, coupled with the measured interactions of the new particle with other particles, strongly indicates that it is a Higgs boson." ==Events in 2012==