Probing the light radion through diphotons at the Large Hadron Collider

A radion in a scenario with a warped extra dimension can be lighter than the Higgs boson, even if the Kaluza-Klein excitation modes of the graviton turn out to be in the multi-TeV region. The discovery of such a light radion would be a gateway to new physics. We show how the two-photon mode of decay can enable us to probe a radion in the mass range 60–110 GeV. We take into account the diphoton background, including fragmentation effects, and include cuts designed to suppress the background to the maximum possible extent. Our conclusion is that, with an integrated luminosity of $3000{\mathrm{fb}}^{-1}$ or less, the next run of the Large Hadron Collider should be able to detect a radion in this mass range, with a significance of 5 standard deviations or more.

Figure 1

(a) Production cross section of radion via gluon fusion versus $m_{\phi }$ for 13 and 14 TeV center of mass energies at the LHC. (b) Branching ratios for the radion decay modes as functions of its mass $m_{\phi }$.

Figure 2

Normalized distribution of $p_T^{\gamma }$ for two sample masses of radion, diphoton background and signal photon background. (a) Normalized distribution of ${\mathrm{p}}_{\mathrm{T}}^{\gamma ,\text{leading}}$ for $m_{\phi }=60\mathrm{GeV}$; (b) Normalized distribution of ${\mathrm{p}}_{\mathrm{T}}^{\gamma ,\text{subleading}}$ for $m_{\phi }=60\mathrm{GeV}$; (c) Normalized distribution of ${\mathrm{p}}_{\mathrm{T}}^{\gamma ,\text{leading}}$ for $m_{\phi }=100\mathrm{GeV}$; (d) Normalized distribution of ${\mathrm{p}}_{\mathrm{T}}^{\gamma ,\text{subleading}}$ for $m_{\phi }=100\mathrm{GeV}$.

Figure 3

(a) Luminosity required for $5\sigma$ discovery of radion with $m_{\phi }$ with ${\mathrm{\Lambda }}_{\phi }=2\mathrm{TeV}$. (b) Maximum ${\mathrm{\Lambda }}_{\phi }$ for a radion to be discovered at $5\sigma$ with $m_{\phi }$.

Figure 4

Invariant mass peak of the signal against the background, for $m_{\phi }=60\mathrm{GeV}$.