New particles means new forces and so it follows that if the Higgs boson interacts with new heavy particles they will generate a new force between the Higgs and itself. If so, hardly an SM-like Higgs! We must do better and, through much more precise coupling measurements at the 0.2% level, a future Higgs factory like the FCC-ee could determine if the Higgs is point-like as far down as the 6% level.ĭoes the Higgs find itself attractive? Yes, according to the SM. It could still be very fuzzy indeed! As fuzzy as a pion. How fuzzy is it? Present LHC Higgs coupling measurements suggest it is effectively point-like down to a length scale merely a factor three below the electroweak scale. If it vanishes, the Higgs is entirely point-like. For your interest, the operator that can capture these properties is written (∂μ|H|2)2. Alternatively, it could be point-like but probing it closely may reveal the telltale clues of a cloud of new particles that it interacts with. How "fuzzy" is it? Is it point-like down to the smallest distance scales or is it, like the pion, made up of other as-yet-unidentified new particles? In the latter case, much as for the pions and their constituent quarks and gluons, directly observing the new stuff would require going to higher energies. Let's consider two hand-picked examples and investigate just how SM-like the Higgs is. If all of these extra operators vanish, the Higgs is SM-like. Similarly, the non-SM Higgs operators can capture the long-distance leftover effects of the microscopic innards of the Higgs, but not reveal their full glory in detail. By eye, the astronaut may be able to make out some features on Earth and surmise that there may be mountains, but they couldn't actually estimate the students' elevation gain. Whatever's going on in there, at energies near enough to mh, it is effectively described as a point particle with a handful of additional "operators," which are essentially new particle interactions that aren't contained in the SM (don't feature on that mug or T-shirt) but do involve SM particles. At shorter distances, getting in amongst the electrons, this description fails entirely.ĭitto the Higgs. Far from a neutral atom, it effectively appears as a point-like particle with some leftover multipolar interactions with photons. For summer students hiking to Le Reculet, it is not. To astronauts, Earth is very effectively described as a smooth sphere. We may quantify an answer through the language of effective field theory, which is a mathematical manifestation of the notion that the most effective way to describe an object depends on the length scale you're viewing it from. Really? Are we sure? Whether or not the Higgs is SM-like is a question that will shape the experimental future of Higgs research. On 4 July 2022, enjoying the many beautiful presentations at the symposium, a phrase kept ringing in my ears: "Compatible with Standard Model (SM) predictions." Alarm bells were ringing.
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