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Register a new userInadequacy of zero-width approximation for a light Higgs boson signal
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In the Higgs search at the LHC, a light Higgs boson (115 GeV <~ M_H <~ 130 GeV) is not excluded by experimental data. In this mass range, the width of the Standard Model Higgs boson is more than four orders of magnitude smaller than its mass. The zero-width approximation is hence expected to be an excellent approximation. We show that this is not always the case. The inclusion of off-shell contributions is essential to obtain an accurate Higgs signal normalisation at the 1% precision level. For gg (-> H) -> VV, V= W,Z, O(10%) corrections occur due to an enhanced Higgs signal in the region M_VV > 2 M_V, where also sizable Higgs-continuum interference occurs. We discuss how experimental selection cuts can be used to exclude this region in search channels where the Higgs invariant mass cannot be reconstructed. We note that the H -> VV decay modes in weak boson fusion are similarly affected.
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The zero-width approximation (ZWA) restricts the intermediate unstable particle state to the mass shell and, when combined with the decorrelation approximation, fully factorizes the production and decay of unstable particles. The ZWA uncertainty is expected to be of O(Gamma/M), where M and Gamma are the mass and width of the unstable particle. We review the ZWA and demonstrate that errors can be much larger than expected if a significant modification of the Breit-Wigner lineshape occurs. A thorough examination of the recently discovered candidate Standard Model Higgs boson is in progress. For M_H ~ 125 GeV, one has Gamma_H/M_H < 10^(-4), which suggests an excellent accuracy of the ZWA. We show that this is not always the case. The inclusion of off-shell contributions is essential to obtain an accurate Higgs signal normalization at the 1% precision level. For gg -> H -> VV, V = W,Z, O(5-10%) corrections occur due to an enhanced Higgs signal in the region M_VV > 2 M_V, where also sizable Higgs-continuum interference occurs. We discuss how experimental selection cuts can be used to suppress this region in search channels where the Higgs mass cannot be reconstructed. We note that H -> VV decay modes in non-gluon-fusion channels are similarly affected.
We demonstrate that the multi-top productions efficiently probe the CP-property of top-Higgs interaction and the Higgs-boson width at the LHC. The four top-quark production alone can exclude a purely CP-odd top-quark Yukawa coupling at the 13~TeV LHC with an integrated luminosity of $430~{rm fb}^{-1}$, regardless the size of the Yukawa coupling. Combining the single Higgs-boson production, the $tbar{t}H$ associated production and the four top-quark production, we show that the CP-phase of the top-quark Yukawa coupling and the Higgs-boson width can be stringently bounded at the LHC with integrated luminosities of $300~{rm fb}^{-1}$ and $3000~{rm fb}^{-1}$.
As one of the key properties of the Higgs boson, the Higgs total width is sensitive to global profile of the Higgs boson couplings, and thus new physics would modify the Higgs width. We investigate the total width in various new physics models, including various scalar extension, composite Higgs models, and fraternal twin Higgs model. Typically the Higgs width is smaller than the standard model value due to mixture with other scalar if the Higgs is elementary, or curved Higgs field space for the composite Higgs. On the other hand, except the possible invisible decay mode, the enhanced Yukawa coupling in the two Higgs doublet model or the exotic fermion embeddings in the composite Higgs, could enhance the Higgs width greatly. The precision measurement of the Higgs total width at the high-luminosity LHC can be used to discriminate certain new physics models.
Natural Next-to-Minimal Supersymmetric Standard Model (nNMSSM) is featured by predicting one CP-even Higgs boson satisfying $m_{h_1} lesssim 120 ,{rm GeV}$ and Higgsinos lighter than about 300 GeV, and consequently the cross section for DM-nucleon scattering in this scenario is usually quite large. We study the diphoton signal of the light Higgs boson in nNMSSM by considering the tight constraints from the latest LUX and PandaX-II experiments, and we conclude that the optimal value of the signal rate at 8 TeV LHC is greatly reduced in comparison with earlier predictions. For example, previous studies indicated that the rate may exceed $120 ,{rm fb}$ for $m_{h_1} simeq 80 ,{rm GeV}$, while it is at most $25 ,{rm fb}$ if the lightest neutralino in the scenario is fully responsible for the measured DM relic density. We also investigate the case of $m_{h_1} simeq 98 ,{rm GeV}$ which is hinted by the excesses of the LEP analysis on $Z bar{b} b$ signal and the CMS analysis on the diphoton signal. We conclude that nNMSSM can explain simultaneously the excesses at $1sigma$ level without violating any known constraints.
We study the Higgs boson $(h)$ decay to two light jets at the 14 TeV High-Luminosity-LHC (HL-LHC), where a light jet ($j$) represents any non-flavor tagged jet from the observational point of view. The decay mode $hto gg$ is chosen as the benchmark since it is the dominant channel in the Standard Model (SM), but the bound obtained is also applicable to the light quarks $(j=u,d,s)$. We estimate the achievable bounds on the decay branching fractions through the associated production $Vh (V=W^pm,Z)$. Events of the Higgs boson decaying into heavy (tagged) or light (un-tagged) jets are correlatively analyzed. We find that with 3000 fb$^{-1}$ data at the HL-LHC, we should expect approximately $1sigma$ statistical significance on the SM $Vh(gg)$ signal in this channel. This corresponds to a reachable upper bound ${rm BR}(hto jj) leq 4~ {rm BR}^{SM}(hto gg)$ at $95%$ confidence level. A consistency fit also leads to an upper bound ${rm BR}(hto cc) < 15~ {rm BR}^{SM}(hto cc)$ at $95%$ confidence level. The estimated bound may be further strengthened by adopting multiple variable analyses, or adding other production channels.
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