Search for Scalar Diphoton Resonances in the Mass Range $65-600$ GeV with the ATLAS Detector in $pp$ Collision Data at $\sqrt{s}$ = 8 $TeV$ (1407.6583v2)

Abstract: A search for scalar particles decaying via narrow resonances into two photons in the mass range $65-600$ GeV is performed using 20.3 fb$^{-1}$ of $\sqrt{s}$ = 8 TeV $pp$ collision data collected with the ATLAS detector at the Large Hadron Collider. The recently discovered Higgs boson is treated as a background. No significant evidence for an additional signal is observed. The results are presented as limits at the 95 % confidence level on the production cross-section of a scalar boson times branching ratio into two photons, in a fiducial volume where the reconstruction efficiency is approximately independent of the event topology. The upper limits set extend over a considerably wider mass range than previous searches.

• The paper presents model-independent upper limits on scalar resonance production by analyzing the diphoton invariant mass spectrum in 8 TeV pp collisions.
• It employs a dual-analysis strategy that distinguishes low- and high-mass regions and categorizes events by photon conversion to optimize signal sensitivity.
• These findings constrain beyond-Standard Model scenarios by providing stringent exclusion limits that guide future high-energy physics investigations.

Essay on Search for Scalar Diphoton Resonances in pp Collision Data with the ATLAS Detector

This paper details a comprehensive search conducted by the ATLAS Collaboration for scalar particles decaying via narrow resonances into two photons across a broad mass range of 65--600 GeV. The analysis utilizes data from proton-proton collisions at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb$^{-1}$, gathered by the ATLAS detector at the Large Hadron Collider (LHC). Significantly, this paper treats the previously discovered Higgs boson as a background component and does not consider model-dependent interference effects between the resonance and the continuum diphoton background.

This investigation explores the analytical fit of signal and background distributions to the observed diphoton invariant mass spectrum. By extending the methodology employed for the Higgs boson coupling measurements in the $\gamma\gamma$ channel, the analysis presents model-independent upper limits at the 95% confidence level on the production cross-section of a scalar boson times its branching ratio into two photons. These upper limits are particularly noteworthy as they span a considerably wider mass range than former studies.

Quantitatively, the limits set for a potential additional scalar resonance stretch from 90 fb at a mass of 65 GeV to 1 fb at 600 GeV. The search strategy is divided into two distinct mass regions with separate analysis tactics: a low-mass analysis (65--110 GeV) and a high-mass analysis (110--600 GeV). Each mass region uses selection criteria aimed at augmenting the signal sensitivity while considering the dominating backgrounds, such as Drell-Yan processes in the low-mass range, where electrons are reconstructed as photons, and the irreducible $H \to \gamma\gamma$ background for the high-mass range.

The low-mass analysis is further divided into three categories based on the conversion status of the photon pairs, accommodating the differential reconstruction efficiency and background characteristics. This stratification illustrates methodical diligence in addressing the signal-to-background ratio disparities inherent in such a complex search.

This work has profound implications for the field of particle physics, especially in the context of extensions to the Standard Model that postulate additional Higgs-like scalar fields. One practical implication of these findings is the stringent exclusion limits it provides for theoretical models predicting new scalar resonances within the surveyed mass range. The results underscore the LHC's capability to interrogate and constrain new physics scenarios using high-energy collisions.

For future endeavors, this analysis exemplifies the meticulous approach needed to disentangle a potential new-physics signal from overwhelming background noise. Moreover, the methodology and findings solidify the foundation to explore other potential new physics signatures, such as scalar resonances predicted by beyond-the-Standard-Model theories. Additionally, ongoing and forthcoming LHC runs with higher luminosities and energies will enable similar analyses to further sharpen sensitivity and extend the frontier of high-energy physics research.