Probing doubly charged Higgs bosons at the LHC through photon initiated processes

We show that the photon-photon fusion process contributes significantly to the pair production of doubly charged Higgs bosons at the LHC at a level comparable to the Drell-Yan production. We reinterpret the ATLAS lower limit of 570 GeV (420 GeV) on the mass of ${\mathrm{\Delta }}_L^{\pm \pm }$ (${\mathrm{\Delta }}_R^{\pm \pm }$) arising from an $SU(2{)}_L$ triplet (singlet) scalar by including the photon-initiated process, and we derive a new lower limit of 748 GeV (570 GeV), assuming that ${\mathrm{\Delta }}^{\pm \pm }$ decays into $e^{\pm }{e}^{\pm }$ 100% of the time. We have also shown that the $5\sigma$ discovery reach for ${\mathrm{\Delta }}_L^{\pm \pm }$ (${\mathrm{\Delta }}_R^{\pm \pm }$) is 846 GeV (783 GeV) with $100{\mathrm{fb}}^{-1}$ luminosity at the 13 TeV LHC. We derive a somewhat more stringent limit on the mass when the doubly charged scalar arises from higher-dimensional representations of $SU(2{)}_L$.

Figure 1

Feynman diagram for the pair production of ${\mathrm{\Delta }}^{\pm \pm }$ ($pp\to {\mathrm{\Delta }}^{\pm \pm }{\mathrm{\Delta }}^{\mp \mp }X$) via the Drell-Yan process, with subsequent decays of ${\mathrm{\Delta }}^{\pm \pm }$ into same-sign dileptons.

Figure 2

Feynman diagrams for the pair production of ${\mathrm{\Delta }}^{\pm \pm }$ ($pp\to {\mathrm{\Delta }}^{\pm \pm }{\mathrm{\Delta }}^{\mp \mp }X$) via photon-photon fusion, with subsequent decays of ${\mathrm{\Delta }}^{\pm \pm }$ into same-sign dileptons. Left segment: Elastic scattering subprocesses. Middle segment: Semielastic scattering subprocesses. Right segment: Inelastic scattering subprocesses.

Figure 3

Contour plot for the branching ratio $\mathrm{Br}({\mathrm{\Delta }}_L^{\pm \pm }\to l^{\pm }{l}^{\pm })$ in the $v_{{\mathrm{\Delta }}_L}-M_{{\mathrm{\Delta }}_L^{\pm \pm }}$ plane in the type-II seesaw model. The branching ratio scale is shown on the right side of the figure. The red shaded zone corresponds to $\mathrm{Br}({\mathrm{\Delta }}_L^{\pm \pm }\to l^{\pm }{l}^{\pm })=100\%$.

Figure 4

Left: The ratio of ${\sigma }_{\gamma \gamma }$ and leading-order ${\sigma }_{DY}$ for doubly charged Higgs pair production at the LHC for different energies using NNPDF parton distribution functions and assuming ${\mathrm{\Delta }}^{\pm \pm }$ belongs to an $SU(2{)}_L$ triplet. Right: Comparison of DY and photon fusion production cross section for ${\mathrm{\Delta }}_L^{\pm \pm }$ at 13 TeV LHC using NNPDF parton distribution functions (red lines) and MRST PDFs (blue lines). The dashed lines are for the DY pair-production cross section, whereas the solid lines are for the pair-production cross section via photon fusion.

Figure 5

Upper limits at 95% C.L. on the cross section as a function of the dilepton invariant mass for the production of ${\mathrm{\Delta }}^{\pm \pm }$ decaying into (a) $e^{\pm }{e}^{\pm }$ (top left), (b) $e^{\pm }{\mu }^{\pm }$ (top right), and (c) ${\mu }^{\pm }{\mu }^{\pm }$ pairs (bottom) with a branching ratio of 100% using ATLAS results at $\sqrt{s}=8\mathrm{TeV}$ with $20.3{\mathrm{fb}}^{-1}$ integrated luminosity. The green and yellow regions correspond to the $1\sigma$ and $2\sigma$ bands on the expected limits, respectively. The red (brown) solid lines are for pair production $pp\to {\mathrm{\Delta }}_L^{\pm \pm }{\mathrm{\Delta }}_L^{\mp \mp }$ via Drell-Yan and photon fusion processes (only DY process). The purple (blue) solid lines are for pair production $pp\to {\mathrm{\Delta }}_R^{\pm \pm }{\mathrm{\Delta }}_R^{\mp \mp }$ via Drell-Yan and photon fusion processes (only DY process).

Figure 6

The observed and expected 95% C.L. upper limits of the production cross section [$\sigma (pp\to {\mathrm{\Delta }}_{L,R}^{\pm \pm }{\mathrm{\Delta }}_{L,R}^{\mp \mp })$] as a function of $M_{{\mathrm{\Delta }}_{L,R}^{\pm \pm }}$ using ATLAS results at $\sqrt{s}=13\mathrm{TeV}$ with $13.9{\mathrm{fb}}^{-1}$ integrated luminosity. Left: The limits derived under the assumption that $\mathrm{BR}({\mathrm{\Delta }}_{L,R}^{\pm \pm }\to e^{\pm }{e}^{\pm })=100\%$. Right: The limits derived under the assumption that $\mathrm{BR}({\mathrm{\Delta }}_{L,R}^{\pm \pm }\to e^{\pm }{e}^{\pm })=50\%$. The green and yellow regions correspond to the $1\sigma$ and $2\sigma$ bands on the expected limits, respectively. The red (brown) solid lines are for pair production $pp\to {\mathrm{\Delta }}_L^{\pm \pm }{\mathrm{\Delta }}_L^{\mp \mp }$ via Drell-Yan and photon fusion processes (only DY process). The purple (blue) solid lines are for pair production $pp\to {\mathrm{\Delta }}_R^{\pm \pm }{\mathrm{\Delta }}_R^{\mp \mp }$ via Drell-Yan and photon fusion processes (only DY process).

Figure 7

Upper limits at 95% C.L. on the cross section as a function of the dilepton invariant mass for the production of ${\mathrm{\Delta }}^{\pm \pm }$ decaying into (a) $e^{\pm }{e}^{\pm }$ (top left), (b) $e^{\pm }{\mu }^{\pm }$ (top right), and (c) ${\mu }^{\pm }{\mu }^{\pm }$ pairs (bottom) with a branching ratio 100% using CMS results at $\sqrt{s}=7\mathrm{TeV}$ with $4.9{\mathrm{fb}}^{-1}$ integrated luminosity. The red (brown) solid lines are for pair production $pp\to {\mathrm{\Delta }}_L^{\pm \pm }{\mathrm{\Delta }}_L^{\mp \mp }$ via Drell-Yan and photon fusion processes (only DY process). The purple (blue) solid lines are for pair production $pp\to {\mathrm{\Delta }}_R^{\pm \pm }{\mathrm{\Delta }}_R^{\mp \mp }$ via Drell-Yan and photon fusion processes (only DY process).

Figure 8

95% C.L. exclusion limits on $M_{{\mathrm{\Delta }}_{L,}^{\pm \pm }}$ using ATLAS results at $\sqrt{s}=13\mathrm{TeV}$ with $13.9{\mathrm{fb}}^{-1}$ integrated luminosity. Black solid line: Observed limit. Blue dotted line: Expected limit. Red solid line: Production cross section considering both DY and photon fusion processes, using parton distribution function NNPDF. Blue solid line: Production cross section considering both DY and photon fusion processes, using parton distribution function MRST. The limit is derived under the assumption that the BR to same-sign dielectrons is 100%.

Figure 9

Left: Significance versus ${\mathrm{M}}_{{\mathrm{\Delta }}_L^{\pm \pm }}$ plot assuming $\mathrm{BR}({\mathrm{\Delta }}_L^{\pm \pm }\to l^{\pm }{l}^{\pm })=100\%$ at 13 TeV LHC for $30{\mathrm{fb}}^{-1}$, $50{\mathrm{fb}}^{-1}$, and $100{\mathrm{fb}}^{-1}$ luminosities. The left part of the dashed black line is excluded by the current experimental limit as derived earlier. Here the doubly charged scalar is from an $SU(2{)}_L$ triplet. Right: Significance versus ${\mathrm{M}}_{{\mathrm{\Delta }}_R^{\pm \pm }}$ plot assuming $\mathrm{BR}({\mathrm{\Delta }}_R^{\pm \pm }\to l^{\pm }{l}^{\pm })=100\%$ at 13 TeV LHC for $30{\mathrm{fb}}^{-1}$, $50{\mathrm{fb}}^{-1}$, and $100{\mathrm{fb}}^{-1}$ luminosities. The left part of the dashed black line is excluded by the current experimental limit as derived earlier. Here the doubly charged scalar is an $SU(2{)}_L$ singlet.

Figure 10

The ratio between ${\sigma }_{\gamma \gamma }$ and leading-order ${\sigma }_{DY}$ for doubly charged Higgs pair production at the 13 TeV LHC for different choices of $SU(2{)}_L$ multiplets. From top to bottom, ${\mathrm{\Delta }}^{\pm \pm }$ belongs to a singlet (blue), doublet (purple), triplet (red), quadruplet (green), and quintuplet (gray).

Figure 11

95% C.L. exclusion limits on $M_{{\mathrm{\Delta }}_{L,}^{\pm \pm }}$ using ATLAS results at $\sqrt{s}=13\mathrm{TeV}$ with $13.9{\mathrm{fb}}^{-1}$ integrated luminosity. Black solid line: Observed limit. Blue dotted line: Expected limit. From top to bottom, the brown, cyan, red, purple, and blue solid lines are model-predicted cross sections, when ${\mathrm{\Delta }}^{\pm \pm }$ belongs to a quintuplet, quadruplet, triplet, doublet, and singlet, respectively. The limit is derived under the assumption that the BR to same-sign dielectrons is 100%.