Shedding light on physics beyond the Standard Model at the LHC Run 2 using photons – PhotonPortal

The project associates analyses searching for physics beyond the SM, with the precision measurements of the properties of an already established particle. These two domains – searches for NP, precision measurements – are traditionally developed within high-energy-physics collaborations by separate communities, motivated by the different needs of precision and understanding of the objects in the final state. In this respect this project proposes an innovative approach, where the same effort needed to measure with the ultimate precision the properties of the Higgs boson (e.g. the Higgs boson mass with the ?? decays, strongly depending on the precise understanding of the photon energy scale calibration) will at the same time benefit to searches for NP phenomena with a similar final state (e.g. new diphoton resonances, or the Higgs boson in more complex decay chains). For this reason, we propose to group analogous activities from the two domains, in order to gain from the common work needed to understand events with similar final states.

The project explores the potential improvements brought to physics analyses by the use of modern machine learning techniques. The connection between the precision measurement of the Higgs boson properties and the search for NP is made by means of Effective Field Theory interpretation of the measurement results, capable of putting stringent limits on NP phenomena.

In 2018, analyzes based on data set pp 2015-2017at 13 TeV (80 fb-1), presented at the winter and summer 2018 conferences (Moriond 2019, LHCP 2019, ICHEP 2018).

Analyses based on full LHC pp Run 2 dataset at 13 TeV (deadline: 2019-2020)

ATLAS Collaboration, Observation of Higgs boson production in association with a top quark pair at the LHC with the ATLAS detector, arXiv:1806.00425 [hep-ex].

ATLAS Collaboration, Measurement of the Higgs boson mass in the H->ZZ* ->4 l and H ->?? channels with sqrt(s)=13 TeV pp collisions using the ATLAS detector, arXiv:1806.00242 [hep-ex].

ATLAS Collaboration, Measurements of Higgs boson properties in the diphoton decay channel with 36 fb-1 of pp collision data at sqrt(s) = 13 TeV with the ATLAS detector, arXiv:1802.04146 [hep-ex].

ATLAS Collaboration, Search for dark matter in association with a Higgs boson decaying to two photons at sqrt(s) = 13 TeV with the ATLAS detector, Phys. Rev. D 96 (2017) 112004, arXiv:1706.03948 [hep-ex].

ATLAS Collaboration, Searches for the Z? decay mode of the Higgs boson and for new high-mass resonances in pp collisions at sqrt(s) = 13 TeV with the ATLAS detector, JHEP 1710 (2017) 112, arXiv:1708.00212 [hep-ex].

ATLAS Collaboration, Search for new phenomena in high-mass diphoton final states using 37 fb-1 of proton--proton collisions collected at sqrt(s)=13 TeV with the ATLAS detector, Phys. Lett. B775 (2017) 105--125,arXiv:1707.04147 [hep-ex].

ATLAS Collaboration, Search for top quark decays t->qH, with H->??, in sqrt(s)=13 TeV pp collisions using the ATLAS detector, JHEP 10 (2017) 129, arXiv:1707.01404 [hep-ex].

ATLAS Collaboration, Search for photonic signatures of gauge-mediated supersymmetry in 13 TeV pp collisions with the ATLAS detector, Phys. Rev. D97 (2018) 092006, arXiv:1802.03158 [hep-ex].

The discovery of a Higgs boson at the LHC Run 1 represents a milestone of particle physics. On the other hand, no clear sign of new phenomena beyond the Standard Model was found in the proton-proton collisions collected at 7 and 8 TeV at the LHC. The LHC Run 2 started in 2015, and ATLAS already collected and analyzed 3.2 fb$^{-1}$ of pp collisions at 13 TeV. The increased center of mass energy with respect to Run~1 has allowed ATLAS to search for new physics directly by looking for new particles, and will permit, in the years to come, to reveal NP through precision studies in the Higgs boson sector.

In this context, final states involving photons will play an crucial role: di-photon events will be used to search additional Higgs-boson-like particles, while topologies with non-resonant photon pairs or single photons in presence of missing transverse energy will probe more exotic scenarios. In parallel, the study of the production of the Higgs boson associated with other objects represents an excellent opportunity to discover new physics: by exploiting the di-photon decay channel it will be possible to cleanly isolate the Higgs boson signal in events with additional objects, and to explore the possibility of its resonant or non-resonant production in association with new particles. In December 2015, ATLAS presented preliminary results on the search for diphoton final states, and these results were recently updated. A modest but intriguing excess of diphoton events with respect to the SM expectations was observed, corresponding to a mass of about 750 GeV, and the CMS collaboration also reported a similar excess. With the data collected in 2016, 8-to-10 times more than in 2015, it will be possible to establish the origin of the excess, while with the full Run 2 dataset, about 100 fb$^{-1}$ of pp collisions at $\sqrt{s}$ = 13 TeV, the consortium plans to complete a rich and diverse program of searches and precision measurement exploiting photons in the final state as unifying element.

This project is organised around four collaborative axis, each one addressing one physics line, and three transverse tasks to optimise the performance of reconstructed objects.