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New Data for Detecting the Higgs Boson at the Large Hadron Collider

CMS and ATLAS have teamed up to present their new results on the novel signatures for identifying the Higgs boson at CERN’s Large Hadron Collider.

Collision events recorded by ATLAS (left) and CMS (right), used in the search for rare Higgs boson transformations. Image Credit: CERN.

These comprise the quest for a second particle and unusual changes of the Higgs boson into a Z boson—which happens to be a carrier of one of the underlying forces of nature.

While such transformations have been predicted to be rare, their observation and analysis help improve one’s interpretation of particle physics and may also point the way to novel physics if there are variations between the predictions and observations.

The outcomes also included the quest for signs of rare transformations of the Higgs boson into “invisible” particles, which could shed new light on possible dark-matter particles. Such studies involved virtually 140 inverse femtobarns of data, or about 10 million billion collisions between protons, documented between 2015 and 2018.

The CMS and ATLAS detectors cannot directly visualize a Higgs boson: as an ephemeral particle, it changes, or “decays”, into lighter particles nearly instantly after being generated in the collisions between protons, and these lighter particles leave distinct signs in the detectors.

But other Standard-Model processes could also produce analogous signatures. Hence, researchers must initially detect the separate pieces that correspond with this signature and subsequently build up sufficient statistical proof to demonstrate that Higgs bosons were indeed produced by these collisions.

When Higgs boson was identified in 2012, it was mainly visualized in transformations into pairs of photons or pairs of Z bosons. These ostensible “decay channels” can be detected more easily because they have comparatively clean signatures, and they have also been visualized at the LHC. While other changes have been predicted to take place, they would occur very rarely, or likely to have an indistinct signature, and hence are difficult to detect.

ATLAS presented the new results At LHCP, relating to the quests for one such unusual process, where a Higgs boson changes into a photon (γ) and a Z boson. The Z boson thus produced, being unstable itself, changes into pairs of leptons, either muons or electrons, leaving a telltale signature of a photon and two leptons in the detector.

Considering the low chances of visualizing the transformation of a Higgs boson to Zγ with the amount of data examined, ATLAS was able to discard the possibility that over 0.55% of Higgs bosons generated in the LHC would change into Zγ.

With this analysis, we can show that our experimental sensitivity for this signature has now reached close to the Standard Model’s prediction.

Karl Jakobs, Spokesperson, ATLAS Collaboration

The most optimal value extracted for the H→Zγ signal strength, defined as the ratio of the visualized to the estimated Standard-Model signal yield, is noted to be 2.0+1.0−0.9.

The outcomes of the first quest for Higgs transformations were presented by CMS. These transformations also involved a Z boson but were accompanied by a φ (phi) or ρ (rho) meson. Once again, the Z boson changes into pairs of leptons, whereas the second particle changes into pairs of kaons (KK) in the case of the φ and into pairs of pions (ππ) in the case of the ρ.

These transformations are extremely rare, and are not expected to be observed at the LHC unless physics from beyond the Standard Model is involved.

Roberto Carlin, Spokesperson, CMS Collaboration

The analyzed data-enabled CMS to discard the theory that over 0.6% may change into Zφ and over approximately 1.9% of Higgs bosons may change into Zρ. Even though such limitations are relatively greater than the predictions made by the Standard Model, they certainly demonstrate the potential of the detectors to pave the way to the quest for physics apart from the Standard Model.

The supposed “dark sector” comprises hypothetical particles that may constitute dark matter—the elusive element responsible for over five times the mass of regular matter in the universe.

According to researchers, the Higgs boson may hold clues regarding the nature of dark-matter particles, because certain extensions of the Standard Model have suggested that a Higgs boson may change into dark-matter particles.

Such particles would not be able to interact with the CMS and ATLAS detectors, that is, they will continue to remain “invisible” to them. This would enable the particles to evade direct detection and exist as “missing energy” in the collision event.

At LHCP, ATLAS demonstrated the new upper limit—of 13%—on the likelihood that a Higgs boson may change into invisible particles called weakly interacting massive particles, or WIMPs for short, whereas CMS presented the outcomes from the latest search into the transformations of Higgs boson into four leptons through at least a single intermediate “dark photon.” This also presents limitations on the chances of such a transformation taking place at the LHC.

The Higgs boson continues to be quite useful, especially in supporting researchers to test the Standard Model of particle physics and explore physics that probably lies beyond. These are just some of the many outcomes regarding the Higgs boson presented at LHCP.

Technical Note

In case data volumes are not sufficiently high to claim a clear observation of a specific process, physicists can estimate the limitations that they assume to place on the process.

With regard to the transformations of Higgs boson, such limitations depend on the product of two terms—the speed at which a Higgs boson is created in collisions between protons (production cross-section), and the speed at which it will experience a specific conversion to lighter particles (branching fraction).

If such a transformation was absent, ATLAS was looking forward to place an upper limit of 1.7 times the Standard Model expectation for the process involving the transformations of Higgs boson to a photon (H→Zγ) and a Z boson.

The ATLAS and CMS collaboration successfully placed an upper limit of 3.6 times this value, reaching the sensitivity to the predictions of the Standard Model.

The CMS quest was for a relatively rarer process, estimated by the Standard Model to take place only once in every million transformations of the Higgs boson, and the collaboration successfully set the upper limits of around 1000 times the expectations of the Standard Model for the H→Zφ and H→Zρ processes.


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