July 4, 2012 In a joint seminar today (July 4, 2012) at CERN and the "ICHEP 2012" conference in Melbourne, researchers with the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) presented their preliminary results on the search for the standard model (SM) Higgs boson in their data recorded up to June 2012.
CMS observes an excess of events at a mass of approximately 125 GeV with a statistical significance of five standard deviations (5 sigma) above background expectations. The probability of the background alone fluctuating up by this amount or more is about one in three million. The evidence is strongest in the two final states with the best mass resolution: first the two-photon final state and second the final state with two pairs of charged leptons (electrons or muons). The interpret this to be due to the production of a previously unobserved particle with a mass of around 125 GeV.
The CMS data also rule out the existence of the SM Higgs boson in the ranges 110-122.5 GeV and 127-600 GeV with 95% confidence level -- lower masses were already excluded by CERN's LEP collider at the same confidence level.
Within the statistical and systematic uncertainties, results obtained in the various search channels are consistent with the expectations for the SM Higgs boson. However, more data are needed to establish whether this new particle has all the properties of the SM Higgs boson or whether some do not match, implying new physics beyond the standard model.
The LHC continues to deliver new data at an impressive rate. By the end of 2012, CMS hopes to have more than triple its total current data sample. These data will enable CMS to elucidate further the nature of this newly observed particle. They will also allow CMS to extend the reach of their many other searches for new physics.
CMS Search Strategy
CMS analysed the full data sample of proton-proton collisions collected in all of 2011 and in 2012, up until June 18. These data amount to up to 5.1 fb−1 of integrated luminosity, at a centre-of-mass energy of 7 TeV in 2011 and up to 5.3 fb−1 at 8 TeV in 2012.
The standard model predicts that the Higgs boson lasts for only a very short time before it breaks up, or "decays," into other well-known particles. CMS studied five main Higgs boson decay channels. Three channels result in pairs of bosonic particles (γγ, ZZ or WW) and two channels result in pairs of fermionic particles (bb or ττ), where γ denotes a photon, Z and W denote the force carriers of the weak interaction, b denotes a bottom quark, and τ denotes a tau lepton. The γγ, ZZ and WW channels are equally sensitive in the search for a Higgs boson around 125 GeV and all are more sensitive than the bb and ττ channels.
The γγ and ZZ channels are especially important as they both allow the mass of the new particle to be measured with precision. In the γγ channel the mass is determined from the energies and directions of two high-energy photons measured by the CMS crystal electromagnetic calorimeter (ECAL). In the ZZ channel the mass is determined from the decays of the two Zs to two pairs of electrons, or two pairs of muons, or a pair of electrons and a pair of muons. These are measured in the ECAL, inner tracking and muon detectors.
The WW channel is more complex. Each W is identified through its decay to an electron and a neutrino or a muon and a neutrino. The neutrinos pass through the CMS detectors undetected, so the SM Higgs boson in the WW channel would manifest itself as a broad excess in the mass distribution, rather than a narrow peak. The bb channel has large backgrounds from standard model processes, so the analysis searches for events in which a Higgs boson is produced in association with a W or Z, which then decays to electron(s) or muon(s). The ττ channel is measured by observing τ decays to electrons, muons and hadrons.
CMS Search Results
The CMS data sample should be sensitive enough to completely exclude the mass range 110-600 GeV at 95% confidence level, if the SM Higgs does not exist. In fact, the CMS data do rule out the existence of the SM Higgs boson in two broad mass ranges of 110-122.5 GeV and 127-600 GeV with 95% confidence level.
The range of 122.5-127 GeV cannot be excluded because the scientists see an excess of events in three of the five channels analysed:
- γγ channel: the γγ mass distribution. There is an excess of events above background with a significance of 4.1 sigma, at a mass near 125 GeV. The observation of the two-photon final state implies that the new particle is a boson, not a fermion, and that it cannot be a "spin 1" particle.
- ZZ channel: the mass distribution for the four leptons (two pairs of electrons, or two pairs of muons, or the pair of electrons and the pair of muons). Accounting also for the decay angle characteristics, it yields an excess of 3.2 sigma above background at a mass near 125 GeV.
- WW channel: a broad excess in the mass distribution of 1.5 sigma is observed.
- bb and ττ channels: no excess is observed.
The statistical significance of the signal, from a combined fit to all five channels, is 4.9 sigma above background. A combined fit to just the two most sensitive and high-resolution channels (γγ and ZZ) yields a statistical significance of 5.0 sigma. The probability of the background alone fluctuating up by this amount or more is about one in three million.
The mass of the new particle is determined to be 125.3 ± 0.6 GeV, independent of any assumptions about the expected relative yields of the decay channels. The measured production rate (σDAT) of this new particle is consistent with the predicted rate (σSM) for the SM Higgs boson: σDAT/σSM = 0.80 ± 0.22.
Great care has also been taken to understand numerous details of the detector performance, the event selection, background determinations and other possible sources of systematic and statistical uncertainties. The 2011 analysis showed an excess of events at about 125 GeV. Therefore, to avoid a potential bias in the choice of selection criteria for the 2012 data that might artificially enhance this excess, the 2012 data analysis was performed "blind," meaning that the region of interest was not examined until after all the analysis criteria had been fully scrutinized and approved.
As a general cross-check, the analyses were performed by at least two independent teams. A number of other features reinforce confidence in the results:
- The excess is seen at around 125 GeV in both the 2011 data sample (7 TeV) and the 2012 data sample (8 TeV);
- The excess is seen at the same mass in both the high-resolution channels (γγ and ZZ);
- The excess seen in the WW is consistent with one that would arise from a particle at 125 GeV;
- The excess is seen in a range of final states involving photons, electrons, muons and hadrons.
The preliminary results presented today will be refined, with the aim of submitting them for publication towards the end of the summer.
The new particle observed at about 125 GeV is compatible, within the limited statistical accuracy, with being the SM Higgs boson. However, more data are required to measure its properties such as decay rates in the various channels (γγ, ZZ, WW, bb and ττ) and ultimately its spin and parity, and hence ascertain whether it is indeed the SM Higgs boson or the result of new physics beyond the standard model.
The LHC continues to perform extremely well. By the end of 2012, CMS expects to more than triple its total data sample, and hence to probe further the nature of this new particle. If this particle is indeed the SM Higgs boson, its properties and implications for the standard model will be studied in detail. If it is not the SM Higgs boson, CMS will explore the nature of the new physics that it implies, which may include additional particles that are observable at the LHC. In either case, searches will also continue for other new particles or forces that can be observed in future runs of the LHC at higher beam energies and intensities.
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