Measurement of the Brout-Englert-Higgs boson couplings in its diphoton decay channel with the ATLAS detector at the LHC

After the Higgs boson discovery in the first LHC data, the focus is now on its properties measurement. Among these properties, its couplings are of particular importance since any deviation from the expected value can be an indication of new physics, beyond the \ms. This thesis is oriented towards the Higgs couplings measurements with the ATLAS experiment, using the diphoton decay channel. Selected diphoton events are classified into different categories to disentangle the five Higgs production modes by tagging the objects produced in association with the Higgs boson: two jets for the $VBF$ production mode, lepton and missing transverse energy for the higgsstrahlung ($WH$ and $ZH$), $b$-jets for $ttH$, the remaining events being mostly produced via the dominant production mode $ggH$. The impact of the Higgs $p_T$ modelling in the $ggH$ production mode is also investigated. Theoretical developments provide predictions of the $p_T$ shape at NNLO+NNLL accuracy, including top and bottom mass effects in the loop up to NLO+NLL, implemented in the \hres program. A reweighting technique to take into account these latest theoretical improvements is derived, taking into consideration the correlation with the number of jets. Its impact on the final measurement is estimated to be of the order of a few percent. The final couplings results based on $4.5$ fb$^{-1}$ at $7$ TeV and $20.3$ fb$^{-1}$ at $8$ TeV do not show any significant deviations from the \ms. The couplings, measured at Higgs mass measured by the combination of the \hgg and \hql channels in ATLAS ($m_H = 125.4 \pm 0.4$~GeV) do not show any statistically significant deviation from the \ms: The observed signal strength $\mu = \frac{\sigma^{obs}}{\sigma^{exp}}$ is found to be: $$ \mu = 1.17^{+0.27}_{-0.27} = 1.15^{+0.23}_{-0.23} \text{ (stat.)}~ ^{+0.10}_{-0.08} \text{ (syst.)} ~^{+0.12}_{-0.08} \text{ (theory)} $$ The ratio of the observed number of events in each production mode to the expected ones are measured at: \[ \mbox{ $\displaystyle \begin{split} \mu_{ggH} &= 1.32 \ \pm 0.32 \ \mathrm{(stat.)} \ ^{+0.13}_{-0.09} \ \mathrm{(syst.)} \ ^{+0.19}_{-0.11} \ \mathrm{(theory)}\\ \mu_{VBF} &= 0.8 \ \pm 0.7 \ \mathrm{(stat.)} \ ^{+0.2}_{-0.1} \ \mathrm{(syst.)} \ ^{+0.2}_{-0.3} \ \mathrm{(theory)}\\ \mu_{WH} &= 1.0 \pm 1.5 \ \mathrm{(stat.)} \ ^{+0.3}_{-0.1} \ \mathrm{(syst.)} \ ^{+0.2}_{-0.1} \ \mathrm{(theory)} \\ \mu_{ZH} &= 0.1 \ ^{+3.6}_{-0.1} \ \mathrm{(stat.)} \ ^{+0.7}_{-0.0} \ \mathrm{(syst.)} \ ^{+0.1}_{-0.0} \ \mathrm{(theory)}\\ \mu_{ttH} &= 1.6 \ ^{+2.6}_{-1.8} \ \mathrm{(stat.)} \ ^{+0.6}_{-0.4} \ \mathrm{(syst.)} \ ^{+0.5}_{-0.2} \ \mathrm{(theory)} \end{split} $} \]