Associated Production Evidence against Higgs Impostors and Anomalous Couplings

TL;DR

The paper investigates whether the 125 GeV particle X is the SM-like $J^P = 0^+$ Higgs or a non-SM impostor by studying the energy dependence of associated production with vector bosons ($V+X$) and vector-boson fusion (VBF). It formalizes HVV couplings, including minimal $0^+$, pseudoscalar $0^-$, graviton-like $2^+$, and dimension-six operators, and defines energy-growth observables $R_{AP}(X)$ and $R_{VBF}$. Using Tevatron V+X signals and LHC upper limits, it shows that the spin-2 hypothesis would predict too-large cross sections and is disfavored, while the data are compatible with a minimal $0^+$ scenario; comparable conclusions hold when examining $0^-$ and certain non-minimal couplings, with VBF data providing weaker but supportive constraints. The results bolster the interpretation of X as a $J^P = 0^+$ Higgs with SM-like couplings and highlight how future 14 TeV data could sharpen limits on non-minimal HVV interactions through energy-dependent observables.

Abstract

There is still no proof that the new particle $X$ recently discovered by the ATLAS and CMS Collaborations indeed has spin zero and positive parity, as confidently expected. We show here that the energy dependence of associated $W/Z + X$ production would be much less for a $J^P = 0^+$ boson with minimal couplings, such as the Higgs boson of the Standard Model, than for a spin-two particle with graviton-like couplings or a spin-zero boson with non-minimal couplings. The $W/Z + (X \to {\bar b}b)$ signal apparently observed by the CDF and D0 Collaborations can be used to predict the cross section for the same signal at the LHC that should be measured under the spin-two and different spin-zero hypotheses. The spin-two prediction exceeds by an order of magnitude the upper limits established by the ATLAS and CMS Collaborations, which are consistent with the minimal $0^+$ prediction, thereby providing {\it secunda facie} evidence against spin-two Higgs impostors. Similar analyses of energy dependences provide evidence against $0^-$ impostors, non-minimal scalar boson couplings, including the best LHC limits on dimension-six operators. Comparing the LHC vector boson fusion cross sections at 7 and 8 TeV in the centre of mass provides additional but weaker evidence in favour of the identification of the $X$ particle as a $J^P = 0^+$ boson with minimal couplings.

Figures (6)

• Figure 1: The energy dependences of (left) the associated production cross sections for $W^\pm + X$ (red line) and $Z^0 + X$ (black line) and (right) the 0-, 1- and 2-lepton signals (blue, red and black lines, respectively) after experimental cuts at different LHC energies relative to the corresponding signals at the TeVatron, as expressed via the double ratio ${\cal R}\; \equiv \; \left(\frac{\sigma^\text{Spin~2}_\text{LHC~8}}{\sigma^\text{Spin~2}_\text{TeVatron}} \right) / \left(\frac{\sigma^{0^+}_\text{LHC~8}}{\sigma^{0^+}_\text{TeVatron}} \right)$.
• Figure 2: The energy dependence of the cross section at the LHC relative to the cross section at the TeVatron for production of $X$ in association with a $Z$ boson decaying via the 2-lepton channel, under different hypotheses for the $X$ particle: $0^+$ with minimal coupling (black), $0^+$ with $\epsilon_{W}=1$ (blue), $0^+$ with $\epsilon_{WW}=1$ (blue-dashed), $2^+$ (red) and $0^-$ (green).
• Figure 3: The likelihood for the ratio ${\cal R}_\text{data} = \mu_\text{LHC~8}/\mu_\text{TeVatron}$ extracted from the experimental data at 8 TeV (blue) and 7 TeV (green). The most conservative spin-two expectations ${\cal R}_\text{Spin~2} = 5.9$ and $7.4$ for 7 and 8 TeV, respectively, are excluded, and the $0^-$ expectations ${\cal R}_{0^-} = 1.48$ and $1.56$ for 7 and 8 TeV, respectively, are highly disfavoured, whereas the $0^+$ expectation ${\cal R}_{0^+} = 1$ is quite consistent with the data.
• Figure 4: The effect of non-minimal couplings in the double ratio ${\cal R}$. We show the effect of $\epsilon_{W}$ (left) and $\epsilon_{B}$ (right) in $\Re$, with the bands of 1(2)$\sigma$ in green (yellow).
• Figure 5: The energy dependence of the double ratio of cross sections for VBF production of $X$, ${\cal R}_{VBF} \; \equiv \; \left(\frac{\sigma^{J^P}_{VBF}(E_\text{CM})}{\sigma^{J^P}_{VBF}(\text{8~TeV})} \right) / \left(\frac{\sigma^{0^+}_{VBF}(E_\text{CM})}{\sigma^{0^+}_{VBF}(8~TeV)} \right)$ for $J^P = 2^+$ (black) and $0^-$ (red).