CERN Accelerating science

With the basic properties of the Higgs boson established, it might appear that what remains to be done is just a few more coupling measurements to complete the table and that’s it. But there’s much more to it than that.

The Higgs boson is a mysterious particle, different in several ways from all particles seen so far, and connected to many unanswered questions in physics. The most well known connection is with the Brout-Englert-Higgs (BEH) mechanism that gives mass to the W and Z force carrier particles. Physicists have now seen that the Higgs field is also responsible for the masses of the heaviest of the three generations of matter particles, but whether it also gives mass to the other two remains to be seen.

The simple version of the BEH mechanism, with just a single Higgs boson doing all the work, doesn’t necessarily have to be the one realised in Nature. In many extensions of the Standard Model there is a whole “Higgs sector” of particles stemming from some more fundamental principles.

The BEH mechanism can also have a bigger impact than “just” generating mass. At the heart of the mechanism is the spontaneous breaking of the electroweak symmetry, an event that occurred right after the Big Bang, transforming the universe from a symmetrical state with massless particles to the state that we see today. But how did that happen? Was it a gradual change or was it more akin to a pot of boiling water, with “bubbles” of broken symmetry popping into existence in different places? In certain cases, this phase transition could have been the source of the matter–antimatter asymmetry seen today in the universe.

To learn about this, researchers are going to be looking for Higgs boson interactions with second generation matter particles, the muon and the charm quark, and also looking for extra Higgs-like particles.

Even the mass of the Higgs boson, seemingly just an ordinary property of the particle, has far-reaching implications. The observed value of this mass is, from the theoretical point of view, unnaturally small, suggesting that either the Higgs boson is a more complicated object (for example a composite particle) or that the theory needs new symmetry or some other mechanism that would stabilise the Higgs boson mass.

The mass also relates to whether the present state of the universe is stable. All observations so far indicate a stable universe, but for certain values of the Higgs boson and top quark masses, theory predicts a meta-stable universe, which can transition to a lower energy state. Precise measurement of the Higgs boson mass is going to tell us if this is the case. If yes, we need to find a whole new mechanism to stabilise the vacuum, otherwise the theory would not agree with reality.

Another unknown is where the Higgs mass originates from. If it results from the interaction with the Higgs field, it should be possible to observe the Higgs boson “self-interaction”, with the production of pairs of Higgs bosons being the signal to look for. Observing and measuring this process is the “holy grail” of the Higgs research programme, with the potential to shed light on the nature of the electroweak symmetry breaking mechanism itself. Being extremely rare, it is most likely going to require the upgraded High Luminosity LHC to be observed, and an entirely new future accelerator to be studied fully.

Discovering a new particle means it has to be produced in a collision or have some other indirect effect on known phenomena. For this to happen, the new particle has to be able to interact with known particles. But for new particles that don’t feel the electromagnetic, strong or weak force, such interaction would be almost nonexistent, making them invisible and inaccessible in practice. This could for example be the case for particles of dark matter.

These particles could however interact with the Higgs boson. The Higgs boson could then decay into pairs of such particles, that would then leave the detector without interacting, which is the subject of searches for so-called “invisible Higgs decays”.

The interaction between the Higgs boson and the top quark is also very sensitive to the influence of new particles. This makes the experimental measurement of the Higgs coupling to the top quark one of the more promising ways of searching for new phenomena. More exotic possibilities are also being studied, for example new heavy particles decaying into a Higgs boson and a different particle, or Higgs boson decays that would be forbidden in the Standard Model – for example into a top quark and a muon.

The Higgs boson is a fantastic laboratory for new physics searches. The journey is only just beginning.