## Journal Club

Seminar Room

If you want to propose a paper, you can contact **Supratim Das Bakshi** (*sdb AT ugr.es*)

### Monday 25th of February, 2019

# Higgs Physics at the HL-LHC and HE-LHC

(Submitted on 31 Jan 2019)

The discovery of the Higgs boson in 2012, by the ATLAS and CMS experiments, was a success achieved with only a percent of the entire dataset foreseen for the LHC. It opened a landscape of possibilities in the study of Higgs boson properties, Electroweak Symmetry breaking and the Standard Model in general, as well as new avenues in probing new physics beyond the Standard Model. Six years after the discovery, with a conspicuously larger dataset collected during LHC Run 2 at a 13 TeV centre-of-mass energy, the theory and experimental particle physics communities have started a meticulous exploration of the potential for precision measurements of its properties. This includes studies of Higgs boson production and decays processes, the search for rare decays and production modes, high energy observables, and searches for an extended electroweak symmetry breaking sector. This report summarises the potential reach and opportunities in Higgs physics during the High Luminosity phase of the LHC, with an expected dataset of pp collisions at 14 TeV, corresponding to an integrated luminosity of 3~ab−1 . These studies are performed in light of the most recent analyses from LHC collaborations and the latest theoretical developments. The potential of an LHC upgrade, colliding protons at a centre-of-mass energy of 27 TeV and producing a dataset corresponding to an integrated luminosity of 15~ab−1 , is also discussed.

presented by P. Kozow

# Bayesian and frequentist approaches to resonance searches

(Submitted on 8 Feb 2019)

We investigate Bayesian and frequentist approaches to resonance searches using a toy model based on an ATLAS search for the Higgs boson in the diphoton channel. We draw pseudo-data from the background only model and background plus signal model at multiple luminosities, from about 0 to 106 /fb. We chart the change in the Bayesian posterior of the background only model and the global p-value. We find that, as anticipated, the posterior converges to certainty about the model as luminosity increases. The p-value, on the other hand, randomly walks between 0 and 1 under the background only model. After briefly commenting on the frequentist properties of the posterior, we make a direct comparison of the significances obtained in Bayesian and frequentist frameworks. We find that the well-known look-elsewhere effect reduces local significances by about 1σ . We furthermore find a previously unknown effect: significances from our Bayesian framework are typically about 1 to 2σ smaller than the global significances. This suggests that even global significances could significantly overstate the evidence against the background only model. This effect - the Bayes effect - could radically change our interpretation of the evidence for new physics in resonance searches. We checked that the effect was robust with respect to thirteen choices of prior.

Presented by M. Pérez-Victoria

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