Measurement of the Higgs boson mass in the di-photon decay channel with $140 {fb^{-1}}$ of pp collisions data at $\sqrt{s}=13$ TeVcollected by the ATLAS detector.

In this communication the latest Higgs boson mass measurement in the di-photon channel with the ATLAS detector is presented; the analysis uses proton-proton collisions data from the Large Hadron Collider at a centre-of-mass energy of 13 TeV for a total integrated luminosity of $140 {fb^{-1}}$. To measure the Higgs boson mass, the di-photon and four leptons decays are the best channels as all of the decay particles can be reconstructed by the ATLAS detector with high precision. The Higgs mass in the $\gamma\gamma$ channel is evaluated from the position of the narrow resonant peak in the di-photon invariant mass distribution over a large, monotonically decreasing, continuum background. Despite the higher branching ratio with respect to the $4l$ decay, the $\gamma\gamma$ channel exhibits larger background and systematic uncertainties from photon calibration procedures: for this reason the new measurement exploits the most updated knowledge of the detector performance and calibrations and uses an event categorisation optimized to reduce the impact of the total uncertainty.