In 2012 the ATLAS and CMS collaborations discovered a new particle that – within the present theoretical and experimental uncertainties – is consistent with the predictions for the Higgs boson of the Standard Model (SM) at a mass of about 125 GeV. However, it is also compatible with the predictions of a wide variety of extensions of the SM. While no conclusive signs of physics beyond the SM (BSM) have been found so far at the LHC, both the measurements of the properties of the discovered state at 125 GeV (its couplings to gauge bosons and the heaviest fermions are known up to an experimental precision of roughly 10%) and the existing limits from the searches for new particles leave significant room for interpretations in models of physics beyond the SM.

Many BSM models feature extended Higgs-boson sectors. Such extended Higgs sectors may provide the Dark Matter content of the universe. They can also feature a strong first-order electroweak phase transition in the early universe, which is a requirement for possible explanations of the matter-antimatter asymmetry of the universe. In contrast to the minimal Higgs sector of the SM, models with extended scalar sectors predict the existence of more than one scalar particle. Consequently, one of the main tasks of the LHC Run 3 and beyond will be to investigate whether the observed scalar boson forms part of the Higgs sector of an extended model.

Additional Higgs bosons of extended Higgs-boson sectors may have escaped the searches that were carried out so far. This can be the case if the additional Higgs bosons are much heavier than the observed Higgs boson at 125 GeV since their higher masses reduce the rates with which they are produced at particle colliders. On the other hand, an additional Higgs boson may also be lighter than the one at 125 GeV if its couplings to SM particles are significantly smaller than for the observed Higgs boson. Accordingly, the search for light additional Higgs bosons is of crucial importance for exploring the mechanism of electroweak symmetry breaking giving rise to the masses of elementary particles.

A search channel for a Higgs boson below 125 GeV that works particularly well at the LHC regarding the discrimination of a potential signal from the backgrounds utilises the final state consisting of two photons. CMS reported their results from searches for scalar resonances decaying into two photons that were performed at 8 TeV and 13 TeV. Interestingly, the results based on the 8 TeV plus the full Run 2 data set at 13 TeV showed a local excess of 2.9 σ at a mass value of 95.4 GeV [1]. The measured signal strength, or μ value, defined as the ratio where the observed cross-section times branching ratio is divided by the SM expectation for a Higgs boson at this mass, is given by μ_CMS = 0.33 +0.19 -0.12.

More recently, also ATLAS presented the result based on their full Run 2 data set [2]. ATLAS finds an excess with a local significance of 1.7 σ at precisely the same mass value as the one that was previously reported by CMS, namely at 95.4 GeV. The ATLAS μ value was found to be μ_ATLAS = 0.18 ± 0.10. The two results are indicated in the left plot of Fig. 1. The expected and observed cross-section limits obtained by CMS as a function of the hypothetical Higgs-boson mass are indicated by the black dashed and solid lines, respectively, and the 1σ and 2 σ uncertainty intervals are indicated by the green and yellow bands, respectively. Overlaid in red are the expected and observed limits from ATLAS. The two μ values with their respective uncertainties are shown in black (CMS) and red (ATLAS). It is clearly visible that the two excesses are well compatible with each other, while for the rest of the analysed mass range there is no mass value for which both ATLAS and CMS observe a local excess above the 1σ level.

Regarding the interpretation of the results from ATLAS and CMS, it is important to note that a possible signal at about 95 GeV giving rise to a relatively small number of events would occur on top of a much larger fluctuating background. Therefore, one cannot necessarily expect that the excesses should occur with exactly the same signal strength, and the fact that both collaborations report their most significant excess at precisely the same mass value has to be seen in this context as a certain level of coincidence, even if the origin of the excesses is a new particle.

Limits on cross-section times branching ratio for the channel pp → h → γγ as a function of the mass of the hypothetical new particle, m_h (see text). The expected and observed limits obtained by CMS as a function of the hypothetical Higgs-boson mass are indicated by the black dashed and solid lines, respectively, and the 1σ and 2 σ uncertainty intervals are indicated by the green and yellow bands, respectively. Also shown in red are the expected (dashed) and observed (solid) limits from ATLAS. The plots show the μ values (see text) with their respective uncertainties for CMS (black, left plot), ATLAS (red, left plot) and combined (cyan, right plot). The blue and orange points show the result of a parameter scan in a particular BSM model (S2HDM). Taken from [3]

Neither of the two experiments reached individually the 3 σ level, which is often regarded as the level where there is some first ”evidence” for a possible signal. However, neglecting possible correlations, it is possible to combine the two signal strengths. We find μ_CMS+ATLAS = 0.24 +0.09 -0.08 corresponding to an excess of 3.1 σ. This is shown in cyan in the right plot of Fig. 1 [3].

If the origin of the di-photon excesses at 95.4 GeV is a new particle, the question arises whether it is also detectable in other search channels at the LHC. Furthermore, the new particle could have been produced already in small numbers in previous experiments. In this regard, it is interesting to note that LEP had reported a local 2.3 σ excess in the e⁺e⁻ → Z(h → bb) searches [4], which would be consistent with a scalar resonance with a mass of about 95 GeV.

Given that the excesses discussed above occurred at a similar mass, they might arise from the production of a single new particle – which would be a first sign of BSM physics in the Higgs-boson sector. Indicated in Fig. 1 by the blue and orange points is the result from a parameter scan in a particular BSM model that is investigated regarding a possible description of the observed excesses. The model under consideration is the SM augmented by a second Higgs doublet and by a complex Higgs singlet (S2HDM). Besides the experimental results of the searches in the di-photon channel also all other relevant theoretical and experimental constraints have been taken into account in the analysis. The latter comprise in particular the rate measurements of the 125 GeV Higgs boson at the LHC, as well as the limits from BSM Higgs boson searches at the LHC and LEP. The two colors represent two different versions of the S2HDM which differ in the couplings of the Higgs bosons to fermions. It can be seen that this class of models is indeed suitable for describing the di-photon excess at the LHC (and also the bb excess at LEP).

The simultaneous observation of excesses in the di-photon channel at the same mass value of 95.4 GeV both at CMS and ATLAS (together with the other observed excess compatible with this mass value) gives rise to the intriguing possibility that a particle that cannot be accommodated by the SM of particle physics could be discovered in the near future at the LHC.

References

[1] CMS-PAS-HIG-20-002.

[2] ATLAS-CONF-2023-035.

[3] T. Biekötter, S. Heinemeyer and G. Weiglein, Phys. Rev. D 109 (2024) no.3, 035005 [arXiv:2306.03889 [hep-ph]].

[4] LEP Working Group for Higgs boson searches, ALEPH, DELPHI, L3, OPAL collaboration, Phys. Lett.B 565 (2003) 61 [hep-ex/0306033].