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CMS-PAS-FTR-18-019

Prospects for HH measurements at the HL-LHC

CMS Collaboration

December 2018

Abstract: The prospects for the study of Higgs boson pair production at the High-Luminosity LHC with the CMS detector are presented. Five decay channels, bbbb, bbWW, bb$\tau\tau$, bb$\gamma\gamma$, and bbZZ, are studied. Analyses are developed using a parametric simulation of the upgraded detector response and optimised for a projected integrated luminosity of 3000 fb$^{-1}$. The statistical combination of the five decay channels results in an expected significance for the standard model HH signal of 2.6$\sigma$. Projections are also presented for the measurement of the Higgs boson self-coupling $\lambda_\text{HHH}$. The expected 68 and 95% confidence level intervals for the coupling modifier $\kappa_\lambda = \lambda_\text{HHH} / \lambda_\text{HHH}^\text{SM}$ are $[0.35, 1.9]$ and $[-0.18, 3.6]$, respectively.

Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ;

Figures
png pdf	Figure 1: BDT output distribution for the signal and background processes considered in the bbbb resolved search.
png pdf	Figure 2: $ {m_{\text {JJ}}} $ distributions for the estimated multijet background and the SM (blue) and shape benchmark 2 (red) signals. The distributions on the left are for the 3b and those on the right are for the 4b subjet b-tagged categories. Both signals are normalised to the SM HH production cross section for visualisation.
png pdf	Figure 2-a: $ {m_{\text {JJ}}} $ distribution for the estimated multijet background and the SM (blue) and shape benchmark 2 (red) signals, in the 3b subjet b-tagged category. Both signals are normalised to the SM HH production cross section for visualisation.
png pdf	Figure 2-b: $ {m_{\text {JJ}}} $ distribution for the estimated multijet background and the SM (blue) and shape benchmark 2 (red) signals, in the 4b subjet b-tagged category. Both signals are normalised to the SM HH production cross section for visualisation.
png pdf	Figure 3: Loss of sensitivity of the $ HH \to {{\mathrm {b}} {\mathrm {b}} {\mathrm {b}} {\mathrm {b}}} $ resolved search as a function of the minimal jet $ {p_{\mathrm {T}}} $ threshold (left) and as a function of the uncertainty assumed on high S/B bins for the QCD multijet background (right). In each curve, only the quantity shown on the horizontal axis is varied while the other are kept fixed to the nominal values assumed. The "loss'' quantity plotted on the ordinate is defined $1 - Z / Z^0$, where $Z$ denotes the significance of the HH signal in the hypothesis considered and $Z^0$ the significance for the cases of a 45 GeV ${p_{\mathrm {T}}}$ threshold (left) and of no uncertainty considered (right).
png pdf	Figure 3-a: Loss of sensitivity of the $ HH \to {{\mathrm {b}} {\mathrm {b}} {\mathrm {b}} {\mathrm {b}}} $ resolved search as a function of the minimal jet $ {p_{\mathrm {T}}} $ threshold. Only the quantity shown on the horizontal axis is varied while the others are kept fixed to the nominal values assumed. The "loss'' quantity plotted on the ordinate is defined $1 - Z / Z^0$, where $Z$ denotes the significance of the HH signal in the hypothesis considered and $Z^0$ the significance for the case of a 45 GeV ${p_{\mathrm {T}}}$ threshold.
png pdf	Figure 3-b: Loss of sensitivity of the $ HH \to {{\mathrm {b}} {\mathrm {b}} {\mathrm {b}} {\mathrm {b}}} $ resolved search as a function of the uncertainty assumed on high S/B bins for the QCD multijet background (right). Only the quantity shown on the horizontal axis is varied while the others are kept fixed to the nominal values assumed. The "loss'' quantity plotted on the ordinate is defined $1 - Z / Z^0$, where $Z$ denotes the significance of the HH signal in the hypothesis considered and $Z^0$ the significance for the case of no uncertainty considered.
png pdf	Figure 4: The expected upper limits for non-resonant HH production in the standard model and other shape benchmarks (1-12). The inner (green) and the outer (yellow) bands indicate the regions containing the 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis.
png pdf	Figure 5: Example distributions for some of the features of the signal and background processes. Low-level features in the $\mu {{\tau} _\mathrm {h}} $ final-state: (a) Transverse mass of the muon (i.e. $\tau _{\mu}$), as defined in xxxxx, (b) Transverse momentum of the $ {{\tau} _\mathrm {h}} $. Higgs-candidate masses for all final states together: (c) ${{\mathrm {H}} _{{\tau} {\tau}}}$ mass, (d) ${{\mathrm {H}} _{{{\mathrm {b}} {\overline {\mathrm {b}}}}}}$ mass. High-level features for all final-states channels together: (e) The stransverse mass $M_{T2}$, (f) $s_{\mathrm{T}}$ (defined as the scalar sum of lepton $ {p_{\mathrm {T}}}$, ${p_{\mathrm {T}}}$ of both b-jets and $ {{\tau} _\mathrm {h}} $, and ${{p_{\mathrm {T}}} ^\text {miss}}$). Distributions are normalized to unit areas for signal and background, separately.
png pdf	Figure 5-a: Distribution of the transverse mass of the muon (i.e. $\tau _{\mu}$), as defined in 2, normalized to unit area for signal and background, separately.
png pdf	Figure 5-b: Distribution of the transverse momentum of the $ {{\tau} _\mathrm {h}} $, normalized to unit area for signal and background, separately.
png pdf	Figure 5-c: Distribution of the ${{\mathrm {H}} _{{\tau} {\tau}}}$ mass, normalized to unit area for signal and background, separately.
png pdf	Figure 5-d: Distribution of the ${{\mathrm {H}} _{{{\mathrm {b}} {\overline {\mathrm {b}}}}}}$ mass, normalized to unit area for signal and background, separately.
png pdf	Figure 5-e: Distribution of the stransverse mass $M_{T2}$, normalized to unit area for signal and background, separately.
png pdf	Figure 5-f: Distribution of $s_{\mathrm{T}}$ (defined as the scalar sum of lepton $ {p_{\mathrm {T}}}$, ${p_{\mathrm {T}}}$ of both b-jets and $ {{\tau} _\mathrm {h}} $, and ${{p_{\mathrm {T}}} ^\text {miss}}$), normalized to unit area for signal and background, separately.
png pdf	Figure 6: The output of the NN after the selections, evaluated in the $ {\mathrm {e}}^{+} {\mathrm {e}}^{-}$ (top left), $\mu ^{+}\mu ^{-}$ (top right), $ {\mathrm {e}}^{\pm}\mu ^{\mp}$ (bottom) channels.
png pdf	Figure 6-a: The output of the NN after the selections, evaluated in the $ {\mathrm {e}}^{+} {\mathrm {e}}^{-}$ channel.
png pdf	Figure 6-b: The output of the NN after the selections, evaluated in the $\mu ^{+}\mu ^{-}$ channel.
png pdf	Figure 6-c: The output of the NN after the selections, evaluated in the ${\mathrm {e}}^{\pm}\mu ^{\mp}$ channel.
png pdf	Figure 7: Expected distribution of events in the photon (left column) and jet (right column) pair invariant mass. The full circles denote pseudo-data obtained from the expected events yields for the sum of the signal and background processes for 3000 fb$^{-1} $. Only the most sensitive high purity category is shown.
png pdf	Figure 7-a: Expected distribution of events in the photon pair invariant mass, in the high mass category. The full circles denote pseudo-data obtained from the expected events yields for the sum of the signal and background processes for 3000 fb$^{-1} $. Only the most sensitive high purity category is shown.
png pdf	Figure 7-b: Expected distribution of events in the jet pair invariant mass, in the high mass category. The full circles denote pseudo-data obtained from the expected events yields for the sum of the signal and background processes for 3000 fb$^{-1} $. Only the most sensitive high purity category is shown.
png pdf	Figure 7-c: Expected distribution of events in the photon pair invariant mass, in the medium mass category. The full circles denote pseudo-data obtained from the expected events yields for the sum of the signal and background processes for 3000 fb$^{-1} $. Only the most sensitive high purity category is shown.
png pdf	Figure 7-d: Expected distribution of events in the jet pair invariant mass, in the medium mass category. The full circles denote pseudo-data obtained from the expected events yields for the sum of the signal and background processes for 3000 fb$^{-1} $. Only the most sensitive high purity category is shown.
png pdf	Figure 7-e: Expected distribution of events in the photon pair invariant mass, in the low mass category. The full circles denote pseudo-data obtained from the expected events yields for the sum of the signal and background processes for 3000 fb$^{-1} $. Only the most sensitive high purity category is shown.
png pdf	Figure 7-f: Expected distribution of events in the jet pair invariant mass, in the low mass category. The full circles denote pseudo-data obtained from the expected events yields for the sum of the signal and background processes for 3000 fb$^{-1} $. Only the most sensitive high purity category is shown.
png pdf	Figure 8: Invariant mass distribution of the four leptons selected at the end of the analysis for the $ {{\mathrm {b}} {\overline {\mathrm {b}}}} 4\ell $ final state.
png pdf	Figure 9: Upper limit at the 95% CL on the HH production cross section as a function of $ {\kappa _\lambda} = {\lambda _{{\mathrm {H}} {\mathrm {H}} {\mathrm {H}}}} / {\lambda _{{\mathrm {H}} {\mathrm {H}} {\mathrm {H}}}} ^\text {SM}$ for the five decays channels investigated and their combination. The red band indicated the theoretical production cross section.
png pdf	Figure 10: Expected likelihood scan as a function of $ {\kappa _\lambda} = {\lambda _{{\mathrm {H}} {\mathrm {H}} {\mathrm {H}}}} / {\lambda _{{\mathrm {H}} {\mathrm {H}} {\mathrm {H}}}} ^\text {SM}$. The functions are shown separately for the five decay channels studied and for their combination.

Tables
png pdf	Table 1: Branching fraction of the five decay channels considered in this document. The symbol $\ell $ denotes either a muon or an electron. In the bbWW decay channel, $\ell $ from the intermediate production of a $\tau $ lepton are also considered in the branching fraction.
png pdf	Table 2: Kinematic requirements ($ {p_{\mathrm {T}}} $, $\eta $, and isolation) of electrons, muons, and hadronic taus. The hadronic tau requirements are listed according to the final states considered.
png pdf	Table 3: Object definitions and event selections requirements.
png pdf	Table 4: Photon and jet kinematic selections.
png pdf	Table 5: Categorisation applied for events selected in the bb${\gamma} {\gamma}$ analysis. The symbols MP and HP denote, respectively, the medium and high purity categories based on the BDT output.
png pdf	Table 6: Upper limit at the 95% confidence level, significance, projected measurement at 68% confidence level of the Higgs boson self coupling $ {\lambda _{{\mathrm {H}} {\mathrm {H}} {\mathrm {H}}}} $ for the five channels studied and their combination. Systematic and statistical uncertainties are considered.

Summary

Prospects for the search of Higgs boson pair (HH) production and for the measurement of the Higgs boson self-coupling (${\lambda_{\mathrm{H}\mathrm{H}\mathrm{H}}} $) at the High-Luminosity LHC (HL-LHC) are presented. The study is performed using the five decay channels of the HH system to bbbb, bb$\tau\tau$, bbWW (with both W decaying leptonically), bb$\gamma\gamma$, and bbZZ (with both Z decaying to a pair of electrons or muons). The response of the upgraded CMS detector is studied with a parametric simulation that accounts for an average of 200 pp interactions per bunch crossing, and simulates the performance in the reconstruction and identification of physics objects. Assuming that no HH signal exists, a 95% confidence level (CL) upper limit on its cross section can be set to 0.77 times the SM prediction. Assuming that a HH signal exists with the properties predicted by the SM, we expect a combined significance of 2.6$\sigma$ and a determination of the ${\lambda_{\mathrm{H}\mathrm{H}\mathrm{H}}} $ coupling corresponding to the interval $[0.35, 1.9]$ at the 68% CL and to $[-0.18, 3.6]$ at the 95% CL.

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