750 GeV diphoton excess

We explore several perturbative scenarios in which the diphoton excess at 750 GeV can potentially be explained: a scalar singlet, a two Higgs doublet model (2HDM), a 2HDM with an extra singlet, and the decays of heavier resonances, both vector and scalar. We draw the following conclusions: (i) due to gauge invariance a 750 GeV scalar singlet can accommodate the observed excess more readily than a scalar $SU(2{)}_L$ doublet; (ii) scalar singlet production via gluon fusion is one option, however, vector boson fusion can also provide a large enough rate, (iii) 2HDMs with an extra singlet and no extra fermions can only give a signal in a severely tuned region of the parameter space; (iv) decays of heavier resonances can give a large enough diphoton signal at 750 GeV, while simultaneously explaining the absence of a signal at 8 TeV.

Figure 1

Values of ${\lambda }_{\gamma }$ required to obtain the ATLAS central value of the observed diphoton excess, 10 fb (red solid contours). The black dashed contours indicate the total width of the scalar $S$, from top to bottom $100/10/1/0.1/0.01\mathrm{GeV}$. The brown shaded region is excluded by LHC dijet resonance searches.

Figure 2

Regions in the ${\lambda }_g\text{−}{\lambda }_{\gamma }$ plane that can be reached in two example models with vectorlike fermions. Top panel: Two flavor $SU(3)$ triplets with charge $2/3$ together with two flavor $SU(3)$ singlets with charge 1. Bottom panel: One $SU(3)$ triplet with charge $5/3$ together with one $SU(3)$ singlet with charge 1. Setting the couplings of the fermions with the scalar $c_f=1$, the mass of the triplet/singlet in GeV is indicated by the numbers in red/blue, while red/blue lines indicate contours of fixed triplet/singlet masses. The $1\sigma$ ($2\sigma$) range of the ATLAS excess (1) gives the brown (gray) shaded region, setting $\mathrm{\Delta }\mathrm{\Gamma }=0$.

Figure 3

Predicted signals for $WW$, $ZZ$, and $Z\gamma$ resonances at invariant mass of 750 GeV as a function of the $\gamma \gamma$ signal in the scalar singlet model. The $1\sigma$ ($2\sigma$) range of the ATLAS excess (1) is shown in brown (gray). Two example choices of heavy vectorlike fermions that induce the coupling to electroweak gauge bosons are shown. Red solid lines show the case of fermions in $SU(2{)}_L$ multiplets with zero hypercharge; blue dashed lines the case of $SU(2{)}_L$ singlets.

Figure 4

The brown (gray) shaded region in $({\lambda }_W,{\lambda }_B)$ parameter space that gives the $1\sigma$ ($2\sigma$) range for the diphoton signal (1) exclusively from vector boson fusion. The red shaded regions are exclusions due to 8 TeV searches in $ZZ$ [96, 97], $Z\gamma$ [98] channels (lighter and darker shades, respectively). The solid blue lines show the branching ratio to two photons, while the dashed blue lines show total decay width in GeV, assuming only $WW$, $ZZ$, $Z\gamma$, $\gamma \gamma$ open channels.

Figure 5

Contours of constant cross section (in fb) for $\sigma (gg\to H)\times \mathrm{BR}(H\to \gamma \gamma )+\sigma (gg\to A)\times \mathrm{BR}(A\to \gamma \gamma )$ in the $m\text{−}{Y}_2$ plane for the case of six flavors of charge $5/3$ color triplets. The value of $\tan \beta$ is 50 and the coupling $Y_1$ is set to 0.4. The shaded region corresponds to the region best fitting the ATLAS excess of events. The region on the left of the dashed black line is excluded by LHC measurements of the SM Higgs couplings to photons and gluons.

Figure 6

The mass of $N_f=1$, 3 copies of color octet vectorlike fermions coupling to $S_2$ resonance with $g_{2,f}=1$ strength, such that a diphoton signal (1) is obtained from $pp\to S_2\to S_1{S}_1$ decay chain, where $S_1$ decays to photons through loops of fermions that carry only hypercharge.

Figure 7

Top: The blue solid lines show $g_V=g_{R,u}^{W^{\prime }}$ required to obtain the diphoton signal cross section of 10 fb as a function of $W^{\prime }$ mass, assuming combined branching ratios $\mathrm{Br}(W^{\prime }\to {\pi }^{+}\mathcal{S}(\to \gamma \gamma ))=10^{-1}/10^{-2}/10^{-3}/10^{-4}$ (bottom to top), and setting to zero the other couplings of SM fermions to $W^{\prime }$. The grey (red) shaded regions show dijet exclusions from ATLAS [109] (CMS [91, 110]) 8 TeV data assuming $\mathrm{Br}(W^{\prime }\to jj)=100\%)$ (10% for darker regions). Bottom: The same as the top but assuming $W^{\prime }$ only couples to the second generation right-handed quarks.