Enhancing scalar productions with leptoquarks at the LHC

The Standard Model (SM), when extended with a leptoquark (LQ) and right-handed neutrinos, can have interesting new implications for Higgs physics. We show that sterile neutrinos can induce a boost to the down-type quark Yukawa interactions through a diagonal coupling associated with the quarks and a scalar LQ of electromagnetic charge $1/3$. The relative change is moderately larger in the case of the first two generations of quarks, as they have vanishingly small Yukawa couplings in the SM. The enhancement in the couplings would also lead to a non-negligible contribution from the quark fusion process to the production of the 125 GeV Higgs scalar in the SM, though the gluon fusion always dominates. However, this may not be true for a general scalar. As an example, we consider a scenario with a SM-gauge-singlet scalar $\varphi$ where an $\mathcal{O}(1)$ coupling between $\varphi$ and the LQ is allowed. The $\varphi q\overline{q}$ Yukawa couplings can be generated radiatively only through a loop of LQ and sterile neutrinos. Here, the quark fusion process can have a significant cross section, especially for a light $\varphi$. It can even supersede the normally dominant gluon fusion process for a moderate to large value of the LQ mass. This model can be tested/constrained at the high luminosity run of the LHC through a potentially large branching fraction of the scalar to two jets.

Figure 1

Feynman diagrams showing the SM-like Higgs ($h_{125}$) decaying to (a)–(c) down quarks and (d), (e) gluon pairs through loop diagrams mediated by $S_1$ and chiral neutrinos. Only in (a) and (c) does the Higgs couple to $\nu$, whereas it couples to $S_1$ in all the other diagrams. The couplings $g_L=y_1^{LL}$ and $g_R=y_1^{\overline{RR}}$ [see Eq. (7)]. The diagrams for $s$- and $b$-quarks are similar to the last two diagrams. Note that we absorb a factor of $1/\sqrt{2}$ in the definition of Yukawa couplings in the mass basis, i.e., we write $y_{\nu }$ instead of $y_{\nu }/\sqrt{2}$.

Figure 2

Feynman diagrams showing the (a) $O(v^0)$ and (b) $O(v^2)$ corrections to the quark propagator from loop diagrams mediated by $S_1$ and chiral neutrinos. The couplings $g_L=y_1^{LL}$ and $g_R=y_1^{\overline{RR}}$ [see Eq. (7)]. The diagrams for $s$- and $b$-quarks are similar. These corrections are independent of the external momentum ($p$) and hence contribute as mass corrections.

Figure 3

Variation of the coupling modifiers (a) ${\kappa }_d$, (b) ${\kappa }_s$, and (c) ${\kappa }_b$ [defined in Eq. (2)] with $M_{S_1}$ for different values of $M_{{\nu }_R}$. Here we set $g^2{y}_{\nu }=1$ for all three generations and keep $\lambda =1$.

Figure 4

Variation of $\text{BR}(h\to ii)$ with $M_{S_1}$ for $i=d$, $s$, $b$, and gluon and two values of $M_{{\nu }_R}$: (a) 600 GeV and (b) 2100 GeV. We have set $g^2{y}_{\nu }=1$ for all generations and taken $\lambda =1$.

Figure 5

(a) The normalized production cross section of $h_{125}$ as a function of $M_{S_1}$ for $M_{{\nu }_R}=1.1\mathrm{TeV}$. Here also, we take $g^2{y}_{\nu }=1$ for all the generations and $\lambda =1$. (b) Relative production factor (${\mathrm{R}}_{\mathrm{h}}$) (defined in the text) for the $\mathrm{SM}+\mathrm{LQ}$ scenario as a function of $M_{S_1}$ for $M_{{\nu }_R}=1.1\mathrm{TeV}$. (c) Relative production factor ${\mathrm{R}}_{\mathrm{h}}^{\mathrm{BSM}}$ as a function of $M_{S_1}$ for $M_{{\nu }_R}=1.1\mathrm{TeV}$ when the $hq\overline{q}$ ($hgg$) coupling at leading order in the SM is assumed to be zero.

Figure 6

The singlet scalar, $\varphi$ decaying to down-type quarks.

Figure 7

Variation of $\varphi q\overline{q}$ coupling as a function of $M_{S_1}$ for $M_{{\nu }_R}=1\mathrm{TeV}$ and 2 TeV, $g^2{y}_{\nu }=1$, ${\lambda }^{\prime }=2\mathrm{TeV}$, and $M_{\varphi }=500\mathrm{GeV}$.

Figure 8

Variation of BR $(\varphi \to q\overline{q})$, BR $(\varphi \to gg)$, and BR $(\varphi \to \gamma \gamma )$ as a function of (a) $M_{S_1}$ and (b) $M_{\varphi }$ for $g^2{y}_{\nu }=1$ and $M_{{\nu }_R}=1\mathrm{TeV}$. The ratios are independent of ${\lambda }^{\prime }$. We set ${\lambda }^{\prime }=2\text{TeV}$ to compute the partial decay widths of $\varphi$.

Figure 9

Variation of the production cross section of $\varphi$ times the branching ratios as functions of (a) $M_{S_1}$ and (b) $M_{\varphi }$ for $g^2{y}_{\nu }=1$, ${\lambda }^{\prime }=2\mathrm{TeV}$, and $M_{{\nu }_R}=1\mathrm{TeV}$ at the 14 TeV LHC.

Figure 10

Variation of $R_{\varphi }(ii\to \varphi )$ [Eq. (44)] with (a) $M_{S_1}$ and (b) $M_{\varphi }$ for $g^2{y}_{\nu }=1$, ${\lambda }^{\prime }=2\mathrm{TeV}$, and $M_{{\nu }_R}=1\mathrm{TeV}$.