A fraction of a second after the Big Bang, the Universe experienced a phase transition into a state of minimum energy, where matter particles interacted with the Higgs field to acquire mass. We have been living in this energy state ever since.

In the post-Higgs discovery era, scientists at the Large Hadron Collider (LHC) have been hard at work studying the Higgs boson’s properties. One property that remains to be experimentally verified is whether the Higgs boson can couple to itself (self-coupling). Such an interaction would contribute to the production of a pair of Higgs bosons, and would define the shape of the Higgs potential. If the Higgs boson’s self-coupling differs significantly from the Standard Model prediction, the Universe might be able to transition into a lower energy state where the laws that govern the interactions of matter could take on a very different shape.

The ATLAS Collaboration has released a new result which aims to address this question by searching for pairs of Higgs bosons (HH). This process is incredibly rare in the Standard Model – more than 1000 times rarer than the production of one Higgs boson! Physicists looked for the most common HH production processes that should be present in LHC collisions, both illustrated in the Feynman diagrams in Figure 1. Only the triangle diagram includes the Higgs self-coupling, and it contributes mainly to the production of Higgs pairs at low mass (shown in pink in Figure 1). If new physics is at play, it could change the Higgs self-coupling and ATLAS would see many more pairs of Higgs bosons than expected (i.e. a higher cross section).

The most powerful HH decay channel in the important low-mass region is the two bottom quark plus two photon channel, HH → bbɣɣ. ATLAS physicists developed new analysis techniques to search for this rare process. First, they divided events into low and high mass regions to better target the Higgs self-coupling. Then, they used a multivariate discriminant (Boosted Decision Tree) to separate the events that look like the HH → bbɣɣ process from those that don’t.

Due to the development of these new analysis techniques, ATLAS’ latest result is more than twice as powerful as the previous ATLAS result in the same channel! Figure 2 shows limits on the HH production cross section (σ) as a function of the ratio of the Higgs self-coupling to its Standard Model value (κ_λ). The allowed range for the Higgs self-coupling is shown by the intersection of the observed limit with the theoretical prediction, between -1.5 and 6.7 times the Standard Model prediction. Physicists were able to set a limit on the HH production cross section of 4.1 times the Standard Model prediction. Limits are also set on HH production via the decay of a hypothetical new scalar particle.

Although this result sets the world’s best limits on the size of the Higgs self-coupling, the work is not done. Much more data is needed to precisely measure the Higgs self-coupling and to see whether it agrees with the Standard Model prediction. The High-Luminosity upgrade of the LHC plans to deliver a dataset 20 times larger than the one used here. If HH production behaves as predicted by the Standard Model, it will be observed in this huge dataset – allowing LHC researchers to make a more quantitative statement on the size of the Higgs self-coupling and the nature of the Higgs potential.

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