Boosted Higgs bosons from chromomagnetic $b$’s: $b\overline{b}h$ at high luminosity

This paper examines detection prospects and constraints on the chromomagnetic dipole operator for the bottom quark. This operator has a flavor, chirality and Lorentz structure that is distinct from other dimension-6 operators considered in Higgs coupling studies. Its nonstandard Lorentz structure results in boosted $b\overline{b}h$ events, providing a rate-independent signal of new physics. To date, we find this operator is unconstrained by $pp\to h+\mathrm{jets}$ and $pp\to \overline{b}b$ searches: for order-1 couplings the permitted cutoff $\mathrm{\Lambda }$ for this operator can be as low as $\mathrm{\Lambda }\sim 1\mathrm{TeV}$. We show how to improve this bound with collider cuts that allow a $b$-tagged Higgs-plus-dijet search in the Higgs-to-diphoton decay channel to exclude cutoffs as high as $\sim 6\mathrm{TeV}$ at $2\sigma$ with $3{\mathrm{ab}}^{-1}$ of luminosity at the 14 TeV LHC. Cuts on the $p_T$ of the Higgs are key to this search, because the chromomagnetic dipole yields a nonstandard fraction of boosted Higgses.

Figure 1

(A) These diagrams illustrate that the interference term between SM and chromomagnetic dipole $b\overline{b}$ production will require a chirality flip. This flip suppresses the bottom-quark chromomagnetic dipole contribution to bottom-diquark production (relative to the top quark, which receives substantial corrections to $t\overline{t}$ production from its chromomagnetic dipole) by a factor of the bottom Yukawa coupling, $y_b$. (B) These diagrams show $s$-channel $b\overline{b}h$ processes from ${\mathcal{O}}_{ghd}$.

Figure 2

This diagram shows $s$-channel gluon production of $b\overline{b}h$. Note that, in contrast to SM processes where the Higgs must be radiated from a fermion, here the amplitude is proportional to the gluon momentum, which scales directly with the momentum of the Higgs: ${\mathbf{p}}_{\mathbf{g}}={\mathbf{p}}_{\mathbf{b}}+{\mathbf{p}}_{\overline{\mathbf{b}}}+{\mathbf{p}}_{\mathbf{h}}$. The process $pp\to b\overline{b}$ proceeds through the same diagram, with the Higgs set to its vacuum expectation value; there ${\mathbf{p}}_{\mathbf{g}}={\mathbf{p}}_{\mathbf{b}}+{\mathbf{p}}_{\overline{\mathbf{b}}}$.

Figure 3

The parton-level transverse momentum of the Higgs boson in $b\overline{b}h$ final states (at leading order) is shown for five different values of the coupling $c_{ghd}$. The black, red, blue, dashed green, green, and yellow lines, listed from bottom to top, show the Higgs $p_T$ for $c_{ghd}=0,1,3,+5,-5$, and 10, respectively. These plots are made assuming an equal number of events for each distinct value of $c_{ghd}$, hence the kinematic differences arise independently of new physics alterations to the total cross section for $pp\to b\overline{b}h$. The left plot shows the distributions for 8 TeV, and the right plot contains the 14 TeV distributions. Both sets of curves were generated with CTEQ6L parton distribution functions, default scale choices, and all parton-level cuts set to default MadGraph5 aMC@NLO values.

Figure 4

The rate of parton-level, leading-order $pp\to b\overline{b}h$ events with Higgs $p_T$ above a set cut value $p_{T,\text{cut}}$ as a function of $p_{T,\text{cut}}$. The number is normalized to the SM rate after applying the same cut criteria. A number of different $c_{ghd}$ values are shown, using the same coloring scheme as Fig. 3 (from bottom to top, the curves are for $c_{ghd}=0,1,3,+5,-5$, and 10, respectively).

Figure 5

This diagram shows one contribution of the Higgs-gluon kinetic coupling term, ${\mathcal{O}}_{GH}$, to $b\overline{b}h$ processes at the LHC.

Figure 6

This plot shows the ratio of new physics to Standard Model $pp\to b\overline{b}$ cross sections (${\mu }^{\text{parton}}=\frac{\sigma }{{\sigma }_{\mathrm{SM}}}$) plotted against the $b$-quarks’ dijet invariant mass, for chromomagnetic dipole couplings ($c_{ghd}$) as indicated, and for $\sqrt{s}=8\mathrm{TeV}$ proton collisions. Additional cuts applied included requiring two or more jets with $p_T>30\mathrm{GeV}$), with $|{\eta }_j|<2.5$, and $m_{jj}>1\mathrm{TeV}$. Measurements of the area normalized cross section vs $b$-tagged dijet invariant mass [71] indicate that $c_{ghd}=40$ is ruled out, while $c_{ghd}=20$, 10 are allowed at $\sim 95\%$ confidence.

Figure 7

This diagram shows one contribution of the bottom-quark chromodipole operator (4) to Higgs boson production at the LHC. Note that the chirality flip in the quark loop will introduce a suppression proportional to the bottom Yukawa $y_b$ for the case of a $b$-quark chromomagnetic dipole contributing to Higgs boson production.