Using the Higgs boson to search for dark photons

The seek for new physics is without doubt one of the essential objectives of the Atlas collaboration. With the invention of the Higgs boson in 2012, the Commonplace Mannequin gained a key aspect towards understanding elementary particles and their interactions – however it will probably’t be thought of full but. The character of darkish matter, which makes up about 27% of the universe, stays a serious open query. Darkish matter could also be a part of a posh “darkish sector” of particles outdoors the Commonplace Mannequin, which has an inside symmetry construction and interactions. Amongst these “darkish particles”, the “darkish photon” is an anticipated intermediate particle for interactions on this new sector. If darkish photons work together with Commonplace Mannequin particles, they are often produced within the high-energy proton-proton collision on the LHC and could be detected by the ATLAS experiment.

Physics, atlas
Determine 1: Photon and darkish photon system cross-sectional block (y axis) for Commonplace Mannequin processes (stuffed histograms), indicators (dashed strains) and information (black dots). This amount is straight associated to the mass of the Higgs boson (125 GeV), from which the photon and darkish photon originated. The decrease panel reveals the ratio between the info and commonplace mannequin predictions, and the dashed bands present the full uncertainties. The info returns are according to the anticipated variety of commonplace mannequin occasions. (Picture: ATLAS Collaboration/CERN)

The ATLAS Collaboration group is in search of indicators of darkish photons in information collected by the experiment throughout LHC Run 2 (2015-2018). The most recent analysis purpose, for the primary time at ATLAS, is to supply the Higgs boson with the Z boson, with the following decay of the Higgs right into a photon and a darkish photon.

The darkish photon won’t go away any seen indicators within the ATLAS detector, as it’s anticipated to work together very weakly with Commonplace Mannequin particles. Nevertheless, it manifests itself within the type of an imbalance within the complete power within the transverse airplane of the detector ( misplaced transverse power). Of their new analysis, the physicists chosen collision occasions with two leptons (originating from the decay of the Z-boson), a single photon and a lacking tangential power. A number of Commonplace Kind operations may end up in an identical signature; These fashions are constructed as precisely as doable utilizing simulations and data-driven estimates.

As well as, the researchers used a machine studying approach (Boosted Determination Tree, BDT) to tell apart between occasions which can be extra prone to be brought on by a darkish photon (extremely rated in BDT) than those who come up from Commonplace Mannequin processes (ranked decrease than BDT). If a darkish photon is produced, it is going to be noticed as an extra of occasions with a excessive BDT rating, with respect to occasions anticipated from Commonplace Mannequin processes. The physicists looked for darkish photons with a variety of potential lots, starting from massless to 40 gigaelectronvolts. No extra was noticed. This allowed the researchers to set exclusion limits on the speed of decay of the Higgs boson right into a photon and a darkish photon: the outcomes confirmed that the utmost permissible charge ranged from 2.3% for a massless darkish photon to 2.5% for the opposite photon studied. lots.

The ATLAS Collaboration set the perfect exclusion limits on the LHC to seek for Higgs bosons that decay right into a photon and a darkish photon.

Related outcomes have been obtained with a beforehand printed evaluation, looking for a similar Higgs-boson decay, however in a unique Higgs manufacturing mode (Vector Boson Fusion, that includes a better cross-section). This second evaluation excluded the decay of the Higgs boson into massless darkish photons with charges above 1.8%. The info was additionally used to put robust constraints on eventualities with further Higgs-like bosons that decay in the identical means.

Physics, atlas
Determine 2: Exclusion limits on the decay charge of the Higgs-Boson to a photon and a darkish photon (%), as a perform of the darkish photon mass. The continual line reveals the noticed restrict (primarily based on the info), whereas the anticipated restrict (in a simulated commonplace mannequin situation solely) is proven by a dashed line, with 1 sigma and a pair of sigma uncertainty within the inexperienced and yellow bands. (Picture: ATLAS Collaboration/CERN)

In conclusion, the ATLAS Collaboration set the perfect exclusion boundary on the LHC to seek for Higgs bosons that decay into photon and darkish photon. These outstanding outcomes are made doable by the superb efficiency of the ATLAS experiment in the course of the second run of the LHC, and by superior and extremely correct information evaluation methods developed by the ATLAS members. The outcomes, together with these from different accelerators or astrophysics experiments, are of nice curiosity to theorists to validate their fashions and enhance their predictions. ATLAS researchers are trying ahead to the bigger dataset anticipated from LHC Run 3, which can enable them to make extra correct measurements.

About occasion shows: The filter occasion is proven within the seek for darkish photons, arising from the decay of the Higgs boson co-produced with the Z boson. The Higgs boson will decay right into a photon (inexperienced and yellow bars not related to a inexperienced line) and a darkish photon (leading to a tangential power signature lacking, indicated by purple dashed strains), whereas the Z boson decays into two electrons (left occasion show, inexperienced strains) or muons (proper occasion show, pink strains). (Picture: ATLAS Collaboration/CERN)

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