Search for new phenomena in events with at least three photons collected in $pp$ collisions at $\sqrt{s}$ = 8 TeV with the ATLAS detector

TL;DR

The ATLAS collaboration conducts a comprehensive search for new phenomena in events with at least three photons using 20.3 fb^{-1} of 8 TeV pp collision data. The analysis combines a model-independent inclusive 3γ search with targeted interpretations for h→aa→4γ, Z′→aγ→3γ, and a rare Z→3γ decay, employing both simulation-based and data-driven background estimates. No significant deviation from the Standard Model is observed; the study sets stringent upper limits on signal yields, fiducial cross sections, and branching ratios, notably BR(Z→3γ) < 2.2×10^{-6}, five times stronger than LEP. These results provide the first model-independent three-photon search and substantially constrain non-SM Higgs-like and vector-boson scenarios in multi-photon final states.

Abstract

Results of a search for new phenomena in events with at least three photons are reported. Data from proton--proton collisions at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb$^{-1}$, were collected with the ATLAS detector at the LHC. The observed data are well described by the Standard Model. Limits at the 95% confidence level on new phenomena are presented based on the rate of events in an inclusive signal region and a restricted signal region targeting the rare decay $Z\to 3γ$, as well as di-photon and tri-photon resonance searches. For a Standard Model Higgs boson decaying to four photons via a pair of intermediate pseudoscalar particles ($a$), limits are found to be $σ\times {\text {BR}}(h \rightarrow aa) \times {\text {BR}}(a \rightarrow γγ)^{2} < 10^{-3} σ_{\text{SM}}$ for 10 GeV $< m_{a} <$ 62 GeV. Limits are also presented for Higgs boson-like scalars ($H$) for $m_{H} > $ 125 GeV, and for a $Z'$ decaying to three photons via $Z' \rightarrow a+γ\rightarrow 3γ$. Additionally, the observed limit on the branching ratio of the $Z$ boson decay to three photons is found to be BR$(Z \rightarrow 3γ) < 2.2 \times 10^{-6}$, a result five times stronger than the previous result from LEP.

Figures (5)

• Figure 1: Feynman diagrams for possible beyond-the-Standard Model (top) and rare Standard Model (bottom) scenarios that result in final states with at least three photons.
• Figure 2: Observed and expected yields in signal and control regions for the full mass range (left) and the restricted range of 80 GeV $< m_{3\gamma} <$ 100 GeV (right), for events where all three photon candidates satisfy the tight photon identification criteria. The bins along the horizontal axis correspond to orthogonal subsets of events where each subset is categorised by whether the three photons --- ordered from largest to smallest values of --- passed ("P") or failed ("F") the isolation criterion. The leftmost bin is the signal region, composed of events satisfying PPP, and the other bins are the different control regions, where at least one of the photon candidates failed the isolation criterion. The red hatched band, in the signal region bin, is the combination of statistical and systematic uncertainties, while the black hatched bands represent statistical uncertainties. As a result of the data-driven jet background estimate, the statistical uncertainty in each bin is partially correlated with the uncertainty on the data in that bin.
• Figure 3: Observed spectra of $m_{12}$, $m_{13}$, and $m_{23}$, where the 1, 2, and 3 refer to three -ordered photons, as well as $m_{3\gamma}$. For illustration purposes only, also shown is the expected background per bin, determined via unbinned sideband fits to the data as a part of the resonance search, for a hypothesised resonance mass defined by the centre of the bin, as well as the signal expectation for a few mass points for the BSM scenarios considered here. The lower panels show the significance, in units of standard deviations of a Gaussian function, of the observation in each bin, taking into account the fractional uncertainty on the background as a result of the sideband fit. This significance is derived from the $p$-value for the background-only hypothesis for each bin, calculated using a frequentist binomial parameter test 2008NIMPA.595..480C2006sppp.conf..112C2003sppp.conf...35L. The signal distributions used for the $m_{2\gamma}$ resonance searches have two components, a narrow Gaussian core for correctly paired two-photon combinations and a wide distribution for incorrectly paired combinations that is well described by the polynomial used to simultaneously model the background shape for the resonance search described in Section \ref{['sec:ressearch']}.
• Figure 4: Left: Local $p$-values for the background-only hypothesis as a result of a resonance search with respect to the BSM process $h/H \rightarrow aa \rightarrow 4\gamma$, for $m_{h}$ = 125 GeV (top row) and $m_{H}$ = 600 GeV (bottom row), as a function of $m_{a}$, determined via a search for local excesses in the $m_{23}$ spectrum. Right: Upper limits, at the 95% C.L., on $(\sigma / \sigma_{\text{SM}}) \times {\text{BR}}(h \rightarrow aa) \times {\text{BR}}(a \rightarrow \gamma\gamma)^{2}$ (top row) and $\sigma_{H} \times {\text{BR}}(H \rightarrow aa) \times {\text{BR}}(a \rightarrow \gamma\gamma)^{2}$ (bottom row). Also shown are the $\pm 1$ and $2 \sigma$ uncertainty bands resulting from the resonance search hypothesis tests, taking into account the statistical and systematic uncertainties from simulated signal samples which are used to determine signal efficiency and Gaussian resonance width due to detector resolution for each mass hypothesis.
• Figure 5: Left: Local $p$-values for the background-only hypothesis as a result of a resonance search with respect to the production of a new vector gauge boson $Z'$ as a function of $m_{Z'}$, determined via a search for local excesses in the $m_{3\gamma}$ spectrum, using a narrow-width approximation to the $Z'$ resonance width. The smallest local $p$-value is found to be 0.0003 ($3.4 \sigma$) which corresponds to a global $p$-value of 0.087 ($1.4 \sigma$). Right: Upper limits, at the 95% C.L., on $\sigma_{Z'} \times {\text{BR}}(Z' \rightarrow a+\gamma) \times {\text{BR}}(a \rightarrow \gamma\gamma)$. Also shown are the $\pm 1$ and $2 \sigma$ uncertainty bands resulting from the resonance search hypothesis tests, taking into account the statistical and systematic uncertainties from simulated signal samples which are used to determine signal efficiency and Gaussian resonance width due to detector resolution for each mass hypothesis.