Twice the Higgs, Twice the Problem
A fraction of a second after the Large Bang, the Universe skilled a section transition to a state of minimal power, the place particles of matter interacted with the Higgs area to accumulate mass. Since then, we’ve been residing on this energetic state.
Within the post-Higgs period of discovery, scientists on the Massive Hadron Collider (LHC) have labored arduous to check the properties of the Higgs boson. A property that continues to be to be verified experimentally is whether or not the Higgs boson can couple to itself (self-coupling). Such an interplay would contribute to the manufacturing of a pair of Higgs bosons and outline the form of the Higgs potential. If the Higgs boson self-coupling differs considerably from the Commonplace Mannequin prediction, the Universe may be capable of transfer to a decrease power state the place the legal guidelines that govern the interactions of matter may take a really totally different.
The ATLAS collaboration revealed a new outcome which goals to reply this query by in search of pairs of Higgs bosons (HH). This course of is extremely uncommon within the Commonplace Mannequin – over 1000 instances rarer than producing a Higgs boson! Physicists have researched the most typical HH manufacturing processes which are anticipated to be current in LHC collisions, each illustrated within the Feynman diagrams in Determine 1. Solely the triangle diagram contains Higgs autocoupling, and it primarily contributes to the manufacturing of low mass Higgs pairs (proven in pink in Determine 1). If some new physics is concerned, it may change the Higgs auto-coupling, and ATLAS would see many extra Higgs boson pairs than anticipated (i.e. a better cross part).
The strongest HH decay channel within the massive low mass area is the decrease two quark plus two photon channel, HH → bbɣɣ. ATLAS physicists have developed new analytical strategies to analysis this uncommon course of. First, they divided the occasions into high and low mass areas to higher goal Higgs self-coupling. Then, they used a multivariate discriminant (Improved resolution tree) to separate occasions that appear to be the HH → bbɣɣ course of from people who do not.
The most recent results of the ATLAS collaboration is greater than twice as highly effective as their earlier end in the identical channel!
Because of the improvement of those new evaluation strategies, the most recent ATLAS result’s greater than twice as highly effective because the earlier ATLAS outcome in the identical channel! Determine 2 reveals the boundaries of the family manufacturing cross part (σ) as a perform of the ratio of the Higgs autocoupling to its customary mannequin worth (κλ). The allowable vary for Higgs autocoupling is indicated by the intersection of the noticed restrict with the theoretical prediction, between -1.5 and 6.7 instances the Commonplace Mannequin prediction. Physicists had been capable of set a restrict for the HH manufacturing cross part of 4.1 instances the Commonplace Mannequin prediction. Limits are additionally set on the manufacturing of HH through the decay of a hypothetical new scalar particle.
Whereas this outcome units the world’s finest limits on the scale of the Higgs autocoupling, the job is just not completed. It takes much more information to precisely measure the Higgs auto-coupling and see if it agrees with the Commonplace Mannequin prediction. The high-brightness improve of the LHC plans to ship a knowledge set 20 instances bigger than that used right here. If the HH manufacturing behaves as anticipated by the usual mannequin, it is going to be noticed on this big dataset – enable LHC researchers to make a extra quantitative assertion on the scale of the Higgs autocoupling and the character of the Higgs potential.
About occasion show: Candidate occasion HH → bbɣɣ in ATLAS information taken in 2017. The charged particle tracks are displayed in inexperienced, the 2 candidate b-jets are represented by purple cones and the 2 candidate photons are represented by cyan turns.