Diphoton excess at 750 GeV and LHC constraints in models with vectorlike particles

Junichiro Kawamura and Yuji Omura

Phys. Rev. D 93, 115011 – Published 8 June 2016

Abstract

Recently, the ATLAS and CMS collaborations report excesses around 750 GeV in the diphoton channels. This might be the evidence which reveals new physics beyond the Standard Model. In this paper, we consider models with a 750 GeV scalar and vectorlike particles, which couple each other through Yukawa couplings. The decay of the scalar to diphoton is given by the loop diagrams involving the extra colored particles. We investigate not only the setup required by the excesses, but also the LHC constraints, especially concerned with the vectorlike particles. In our scenario, the extra colored particles decay to quarks and a dark matter (DM) via Yukawa couplings. Then, the signals from the vectorlike particles are dijet, $b \bar{b}$ and/or $t \bar{t}$ with large missing energy. We discuss two possibilities for the setups: One is a model with vectorlike fermions and a scalar DM, and the other is a model with vectorlike scalars and a fermionic DM. We suggest the parameter region favored by the excess in each case, and study the constraints based on the latest LHC results at $\sqrt{s} = 8 TeV$ and 13 TeV. We conclude that the favored region is almost excluded by the LHC bounds, especially when the 750 GeV scalar dominantly decays to DMs. The mass differences between the vectorlike particles and the DM should be less than $O (100) GeV$ [ $O (10) GeV$ ] to realize the large diphoton signal and the large decay width, if the extra colored particle only decays to a top (bottom) quark and a dark matter. Otherwise, these scenarios are already excluded by the latest LHC results.

Received 2 March 2016

DOI:https://doi.org/10.1103/PhysRevD.93.115011

Physics Subject Headings (PhySH)

Extensions of fermion sector Hypothetical particle physics models Particle dark matter

Particles & Fields

Authors & Affiliations

Junichiro Kawamura¹ and Yuji Omura²

¹Department of Physics, Waseda University, Tokyo 169-8555, Japan
²Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University, Nagoya 464-8602, Japan

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Issue

Vol. 93, Iss. 11 — 1 June 2016

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Images

Figure 1
$m_{U}$ vs the diphoton signal via the gluon-gluon fusion at $\sqrt{s} = 13 TeV$ , in the type-I model with $N_{d} = 0$ (left) and $N_{u} y_{U} = N_{d} y_{D}$ (right). The total decay width is 10 GeV. Each line corresponds to the prediction of the 750 GeV $S$ , at $N_{u} y_{U} = 3$ , 6, 9, respectively. The thick (dashed) gray line is the exclusion line from the diphoton search at the ATLAS (CMS) experiment [27, 28].
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Figure 2
${\tilde{m}}_{U}$ vs the diphoton signal via the gluon-gluon fusion at $\sqrt{s} = 13 TeV$ , in the type-II model with ${\tilde{N}}_{d} = 0$ (left) and ${\tilde{N}}_{u} {\tilde{y}}_{U} = {\tilde{N}}_{d} {\tilde{y}}_{D}$ (right). ${\tilde{y}}_{U}$ ( ${\tilde{y}}_{D}$ ) is defined as ${\tilde{y}}_{U} = A_{U} / {\tilde{m}}_{U}$ ( ${\tilde{y}}_{D} = A_{D} / {\tilde{m}}_{D}$ ). The total decay width is 10 GeV. Each line corresponds to the prediction of the 750 GeV $S$ , at ${\tilde{N}}_{u} {\tilde{y}}_{U} = 10$ , 20, 30, respectively. The thick (dashed) gray line is the exclusion line from the diphoton search at the ATLAS (CMS) experiment [27, 28].
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Figure 3
Values of $Γ_{tot}$ and comparison with the ATLAS monojet search in the type-I model with $N_{u} y_{U} = N_{d} y_{D} = 3$ (left) and $N_{u} y_{U} = N_{d} y_{D} = 6$ (right). Black lines represent the distance against the lower bound on the mass parameter: $Δ M_{*} \equiv M_{*} - M_{*}^{\exp}$ . $M_{*}$ is defined in Eq. (18). Blue dashed line indicates that the correct relic abundance of dark matter is realized with an assumption that $\tilde{X}$ couples to SM particles only through the scalar $S$ . Background colors depict the total decay width of the scalar resonance and attached numbers are its values in the unit of GeV.
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Figure 4
Exclusion limits from $j j + E_{T}^{miss}$ search. The green thick, solid, and dashed lines represent the exclusion limits for the case with $N_{u} = N_{d} = 1$ , 2, 3, respectively. The decay of the vectorlike quarks to the dark matter $\tilde{X}$ is kinematically forbidden above the black line $m_{U} = m_{\tilde{X}}$ . The gray line corresponds to $m_{\tilde{X}} = 375 GeV$ which is the threshold whether the scalar resonance can decay into $\tilde{X}$ .
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Figure 5
Exclusion limits from $b b + E_{T}^{miss}$ search. The blue thick, solid, and dashed lines represent the exclusion limits for the case with $N_{d} = 1$ , 2, 3, respectively. The decay of the vectorlike quarks to the dark matter $\tilde{X}$ is kinematically forbidden above the black line $m_{D} = m_{b} + m_{\tilde{X}}$ . The gray line corresponds to $m_{\tilde{X}} = 375 GeV$ which is the threshold whether the scalar resonance can decay into $\tilde{X}$ .
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Figure 6
Exclusion limits from $t t + E_{T}^{miss}$ search. The red thick, solid, and dashed lines represent the exclusion limits for the case with $N_{u} = 1$ , 2, 3, respectively. The two-body (three) decay of the vectorlike quarks to the dark matter $\tilde{X}$ and the SM particles, $U \to t \tilde{X}$ ( $U \to b W^{+} \tilde{X}$ ) is kinematically forbidden above the upper (lower) black line. The gray line corresponds to $m_{\tilde{X}} = 375 GeV$ which is the threshold whether the scalar resonance can decay into $\tilde{X}$ .
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