Search for the Higgs boson

Status as of March 2011. Coloured sections have been ruled out to the stated confidence intervals either by indirect measurements and LEP experiments (green) or by Tevatron experiments (orange).

Like other massive particles (e.g. the top quark and W and Z bosons), Higgs bosons decay to other particles almost immediately, long before they can be observed directly. However, the Standard Model precisely predicts the possible modes of decay and their probabilities. This allows the creation and decay of a Higgs boson to be shown by careful examination of the decay products of collisions. The experimental search therefore commenced in the 1980s with the opening of particle accelerators sufficiently powerful to provide evidence related to the Higgs boson.

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, requiring very large worldwide computing facilities. Ultimately over 300 trillion (3 x 10¹⁴) proton-proton collisions at the LHC were analysed in confirming the July 2012 particle's discovery. 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.

Exclusion of possible ranges

Prior to the year 2000, data gathered at the Large Electron–Positron Collider (LEP) at CERN had allowed an experimental lower bound to be set for the mass of the Standard Model Higgs boson of 114.4 GeV/c² at the 95% confidence level (CL). The same experiment produced a small number of events that could be interpreted as resulting from Higgs bosons with a mass just above this cut off—around 115 GeV—but the number of events was insufficient to draw definite conclusions. The LEP was shut down in 2000 due to construction of its successor, the Large Hadron Collider (LHC). This approach of narrowing down and excluding possible ranges continued under the Tevatron and LHC programs.

Tevatron and Large Hadron Collider

Full operation at the LHC was delayed for 14 months from its initial successful tests, on 10 September 2008, until mid-November 2009, 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.

At the Fermilab Tevatron, there were also ongoing experiments searching for the Higgs boson. As of July 2010, combined data from CDF and DØ experiments at the Tevatron were sufficient to exclude the Higgs boson in the range 158-175 GeV/c² at 95% CL. Preliminary results as of July 2011 extended the excluded region to the range 156-177 GeV/c² at 95% CL.

Data collection and analysis in search of Higgs intensified from 30 March 2010 when the LHC began operating at 3.5 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 155-190 GeV/c² and 149-206 GeV/c², 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 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.

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 (pp) 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 tanβ > 20–30 for masses of the Higgs boson below 180 GeV (tanβ is the ratio of the two Higgs doublet vacuum expectation values).

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. Updates were also available from the team analysing the final data from the Tevatron. All of these continued to highlight and narrow down the 125 GeV region as showing interesting features.

On 2 July 2012, the ATLAS collaboration published additional analyses of their 2011 data, excluding boson mass ranges of 111.4 GeV to 116.6 GeV, 119.4 GeV to 122.1 GeV, and 129.2 GeV to 541 GeV. They observed an excess of events corresponding to the Higgs boson mass hypotheses around 126 GeV with a local significance of 2.9 sigma. On the same date, the DØ and CDF collaborations announced further analysis that increased their confidence. The significance of the excesses at energies between 115–140 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

Feynman diagrams showing the cleanest channels associated with the Low-Mass, ~125GeV, Higgs Candidate observed by the CMS at the LHC. The dominant production mechanism at this mass involves two gluons from each proton fusing to a Top-quark Loop, which couples strongly to the Higgs Field to produce a Higgs Boson. Left: Diphoton Channel: Boson subsequently decays into 2 gamma ray photons by virtual interaction with a W Boson Loop or Top-quark Loop. Right: 4-Lepton "Golden Channel" Boson emits 2 Z bosons, which each decay into 2 leptons (electrons,muons). Experimental Analysis of these channels reached a significance of 5 sigma . The analysis of additional vector boson fusion channels brought the CMS significance to 4.9 sigma.

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 interaction types (known as 'channels'), both experiments reached a local significance of 5 sigma — or less than a 1 in one million chance of error. When additional channels were taken into account, the CMS significance was 4.9 sigma.

The two teams had been working 'blinded' from each other for some time, meaning they did not discuss their results with each other, providing additional certainty that any common finding was genuine validation of a particle. This level of evidence, confirmed independently by two separate teams and experiments, meets the formal level of proof required to announce a confirmed discovery. CERN have been cautious, and stated only that the new particle is "consistent with" the Higgs boson, but scientists have not positively identified it as being the Higgs boson, pending further data collection and analysis.

On July 31, the ATLAS collaboration presented further data analysis, including a third channel. They improved the significance to 5.9sigma, and described it as an "observation of a new particle" with mass 126 ± 0.4 (stat.) ± 0.4 (sys) GeV/c². Also CMS improved the significance to 5sigma with the boson's mass at 125.3 ± 0.4 (stat) ± 0.5 (sys) GeV/c². Even though the data is consistent with the Higgs boson, further analysis is necessary.

This announcement means that observations show the newly discovered boson could be a Higgs boson, and it is widely believed by scientists to be very likely a Higgs boson, but further study of this particle, now that its existence is proven, will still be required to place beyond doubt the question whether the particle is in fact confirmed as a Higgs boson.

Timeline of experimental evidence

All results refer to the Standard Model Higgs boson, unless otherwise stated.

• 2000–2004 – using data collected before 2000, in 2003–2004 Large Electron–Positron Collider experiments published papers which set a lower bound for the Higgs boson of 114.4 GeV/c² at the 95% confidence level (CL), with a small number of events around 115 GeV.
• July 2010 – data from CDF (Fermilab) and DØ (Tevatron) experiments exclude the Higgs boson in the range 158–175 GeV/c² at 95% CL.
• 24 April 2011 – media reports "rumors" of a find; these were debunked by May 2011. They had not been a hoax, but were based on unofficial, unreviewed results.
• 24 July 2011 – the LHC reported possible signs of the particle, the ATLAS Note concluding: "In the low mass range (c. 120–140 GeV) an excess of events with a significance of approximately 2.8 sigma above the background expectation is observed" and the BBC reporting that "interesting particle events at a mass of between 140 and 145 GeV" were found. These findings were repeated shortly thereafter by researchers at the Tevatron with a spokesman stating that: "There are some intriguing things going on around a mass of 140GeV." On 22 August 2011 it was reported that these anomalous results had become insignificant on the inclusion of more data from ATLAS and CMS and that the non-existence of the particle had been confirmed by LHC collisions to 95% certainty between 145–466 GeV (except for a few small islands around 250 GeV).
• 23–24 July 2011 – Preliminary LHC results exclude the ranges 155–190 GeV/c² (ATLAS) and 149–206 GeV/c² (CMS) at 95% CL.
• 27 July 2011 – preliminary CDF/DØ results extend the excluded range to 156–177 GeV/c² at 95% CL.
• 18 November 2011 – a combined analysis of ATLAS and CMS data further narrowed the window for the allowed values of the Higgs boson mass to 114–141 GeV.
• 13 December 2011 – experimental results were announced from the ATLAS and CMS experiments, indicating that if the Higgs boson exists, its mass is limited to the range 116–130 GeV (ATLAS) or 115–127 GeV (CMS), with other masses excluded at 95% CL. Observed excesses of events at around 124 GeV (CMS) and 125–126 GeV (ATLAS) are consistent with the presence of a Higgs boson signal, but also consistent with fluctuations in the background. The global statistical significances of the excesses are 1.9 sigma (CMS) and 2.6 sigma (ATLAS) after correction for the look elsewhere effect.
• 22 December 2011 – the DØ collaboration also sets limits on Higgs boson masses within the Minimal Supersymmetric Standard Model (an extension of the Standard Model), with an upper limit for production ranging from 90 to 300 GeV, and excluding tanβ>20–30 for Higgs boson masses below 180 GeV at 95% CL.
• 7 February 2012 – updating the December results, the ATLAS and CMS experiments constrain the Standard Model Higgs boson, if it exists, to the range 116–131 GeV and 115–127 GeV, respectively, with the same statistical significance as before.
• 7 March 2012 – the DØ and CDF collaborations announced that they found excesses that might be interpreted as coming from a Higgs boson with a mass in the region of 115 to 135 GeV/c² in the full sample of data from Tevatron. The significance of the excesses is quantified as 2.2 standard deviations, corresponding to a 1 in 250 probability of being due to a statistical fluctuation. This is a lower significance, but consistent with and independent of the ATLAS and CMS data at the LHC. This new result also extends the range of Higgs-mass values excluded by the Tevatron experiments at 95% CL, which becomes 147-179 GeV/c².
• 2 July 2012 – the ATLAS collaboration further analysed their 2011 data, excluding Higgs mass ranges of 111.4 GeV to 116.6 GeV, 119.4 GeV to 122.1 GeV, and 129.2 GeV to 541 GeV. Higgs bosons are probably located at 126 GeV with significance of 2.9 sigma. On the same day, the DØ and CDF collaborations also announced further analysis, increasing their confidence that the data between 115–140 GeV is corresponding to a Higgs boson to 2.9 sigma, excluding mass ranges at 100–103 and 147–180 GeV.
• 4 July 2012 – the CMS collaboration announced the discovery of a boson with mass 125.3 ± 0.6 GeV/c² within 4.9 σ (sigma) (up to 5 sigma depending on the analysed channel), and the ATLAS collaboration a boson with mass of ∼126.5 GeV/c².

• 31 July 2012  – the ATLAS collaboration further improved their analysis and announced the discovery of a boson with mass 126 ± 0.4 (stat.) ± 0.4 (sys) GeV/c². Also CMS improved the significance to 5sigma with the boson's mass at 125.3 ± 0.4 (stat) ± 0.5 (sys) GeV/c².

These findings meet the formal level required to announce a new particle which is "consistent with" the Higgs boson, but scientists have not positively identified it as being the Higgs boson, pending further analysis.