This is from a paper prepared by the CERN Scientific Policy Committee to provide background information for members of CERN Council on the scientific significance of the possible exclusion regions for the Standard Model Higgs boson expected by the time of the 2011 EPS-HEP conference in July 2011. Italic font indicates more technical details.
- Although in the popular imagination the goal of the LHC is to discover the Higgs boson, the scope of the LHC is really much broader: it is the scientific exploration of the TeV scale. A major part of this enterprise is to answer one of the most intriguing open questions in physics: “Why is it that the W and Z-bosons and the top quark have masses of around 100 times that of the proton, whereas the photon is massless?” In technical terms this is “to discover the mechanism of electroweak symmetry breaking”. Discovering a light Higgs boson as predicted by the Standard Model is just one of the possible outcomes of this endeavour. What is finally discovered will depend ultimately on how Nature has chosen to resolve this intriguing puzzle. However, general arguments based on fundamental principles such as the conservation of total probability (in technical terms: unitarity) make us confident that by exploring the TeV scale the LHC will give us the answer one way or another.
- Information on the existence or non-existence of the Higgs boson predicted by the Standard Model has so far been obtained in two ways. Direct searches for the production and decay of Higgs bosons at the LEP and Tevatron colliders have demonstrated that the Standard Model Higgs boson is statistically unlikely to have a mass of less than 114 GeV or lie within the mass range 158 to 173 GeV. These limits are quoted at 95% confidence level. Indirect evidence for the existence of the Standard Model Higgs boson comes from taking into account the expected effect of processes involving “virtual Higgs” bosons on a large number of precisely measured physical processes at the LEP, SLC and Tevatron colliders. This analysis suggests that, if the Standard Model is the correct theory in the TeV region, the Higgs boson is statistically unlikely to have a mass of more than 158 GeV. This limit is quoted at 95% confidence level. Technical note on the interpretation of a 95% confidence level interval: if the Standard Model Higgs boson were actually to exist and have a particular mass, there would be a less than 5% probability that statistical fluctuations in the experimental data (or “bad luck”) would have led to that mass being nevertheless excluded at 95% confidence level.
- Finding the Standard Model Higgs boson would be a very important discovery indeed, for which the commonly accepted “gold standard” of statistical significance is that there be much less than one in a million chance that any observed signal be the result of a statistical fluctuation rather than a genuine discovery. In technical terms this is referred to as “a significance of 5 standard deviations”. However, ruling out the Standard Model Higgs boson in its otherwise allowed mass range, and with it the Standard Model in its current formulation, would be an even more important discovery, for which the same stringent requirement on statistical significance (5 standard deviations) should be applied.
- With the amount of data expected at both the Tevatron and the LHC by the end of 2012 (around 10 fb-1 of integrated luminosity per experiment for each of ATLAS, CMS, CDF and DØ) we shall achieve a combined 5 standard deviation sensitivity for a Standard Model-like Higgs boson for masses between 114 GeV and 600 GeV. Technical note: a number of relatively modest possible extensions to the Standard Model could reconcile the indirect evidence from precision measurements with the discovery of a Higgs boson having a mass above 173 GeV. On the other hand, a Higgs mass greater than around 600 GeV would imply the replacement of the Standard Model with a more complete and strongly interacting theory already at the TeV scale, which would be accessible to the LHC. Information from the Tevatron and the LHC is complementary in more than one respect. Some technical detail: The most challenging region for the combined Tevatron and LHC Higgs boson search is the region just above 114 GeV, which happens also to be the one most favoured by current data. In this region the dominant decay mode of the Standard Model Higgs is to a b quark-antiquark pair. At the LHC this decay mode is particularly difficult to observe, because of the backgrounds from other Standard Model processes containing b quarks. The most sensitive channel at the LHC in this mass region is provided by the decay of the Higgs boson to two photons. This decay provides a very clean experimental signature, but within the Standard Model it has a very low branching fraction. In contrast, at the Tevatron the dominant b quark decay of the Standard Model Higgs boson is visible above the background, in events in which the Higgs boson is produced in association with a W or Z vector boson. With 10 fb-1 per experiment, the Tevatron is expected to have 95% confidence level exclusion sensitivity down to 0.7 times the SM cross-section, the LHC down to 0.5 times the SM cross-section. Combining the Tevatron and LHC results will yield the required 5 standard deviation sensitivity.
- By the time of the 2011 EPS-HEP conference the LHC experiments will have analysed a data set (about 1 fb-1 of integrated luminosity) of around one tenth of that expected by the end of 2012. The combined Tevatron plus LHC sensitivity will therefore be far from the required 5 standard deviation level over much of the range of possible Higgs boson mass. Summer 2011 will therefore very much represent a "work in progress", with definitive answers much more likely to be available by summer 2012. Some detail: At the statistically much less stringent level of 95% confidence level sensitivity, it is likely that the region of Higgs boson mass around 114 GeV may start to be covered, in addition to the region between about 130 GeV and 300 GeV. Although not definitive, in the sense of reaching the required 5 standard deviation sensitivity over the entire range of masses, such results would nevertheless be very interesting.
Looking further into the future, if a light Standard Model Higgs boson were eventually excluded at the 5 standard deviation level, direct searches should continue, looking for Higgs particles or other signals foreseen in models alternative to the Standard Model. For example, many models such as supersymmetry or extra dimensions predict the existence of particles that would look like a Standard Model Higgs boson, but produce a lower rate of experimentally detectable signals at the Tevatron and LHC. This would call for extending the search to larger samples of LHC data, in which these more subtle signals could be detectable. In addition, it would be crucial to study the scattering of pairs of massive vector bosons (WW, WZ, ZZ). Of particular relevance are the “longitudinal” components of these vector bosons. These are the components that arise due to their non-zero mass and thus due to their coupling to the electroweak symmetry breaking sector. Given sufficient collision energy and luminosity, massive vector boson scattering can give information on what replaces the Standard Model Higgs in ensuring that total probability is conserved (more technically, in preventing the weak coupling amplitudes for WW, WZ and ZZ scattering from violating unitarity). Existing analyses suggest that such a study would require several years of operation of the LHC at its design energy of 14 TeV. Of course, all of this talk of exclusion limits should not make us forget that the discovery of a state consistent with the standard model Higgs boson would be a great triumph, which would require extensive further experimental investigation at the LHC to confirm that the observed state had the couplings expected of a Higgs boson in the Standard Model.