Test of CP invariance in vector-boson fusion production of the Higgs boson in the $H\rightarrow\tau\tau$ channel in proton$-$proton collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

The ATLAS collaboration
Phys.Lett. B805 (2020) 135426, 2020.

Abstract (data abstract)
A test of CP invariance in Higgs boson production via vector-boson fusion in the $H\rightarrow\tau\tau$ decay mode is presented. This test uses the Optimal Observable method and is carried out using $36.1\,\mathrm{fb}^{-1}$ of $\sqrt{s}=13\,\mathrm{TeV}$ proton-proton collision data collected by the ATLAS experiment at the LHC. Contributions from CP-violating interactions between the Higgs boson and electroweak gauge bosons are described by an effective field theory, in which the parameter $\tilde{d}$ governs the strength of CP violation. No sign of CP violation is observed in the distributions of the Optimal Observable, and $\tilde{d}$ is constrained to the interval $[-0.090, 0.035]$ at the 68% confidence level (CL), compared to an expected interval of $\tilde{d} \in [-0.035, 0.033]$ based upon the Standard Model prediction. No constraints can be set on $\tilde{d}$ at 95% CL, while an expected 95% CL interval of $\tilde{d} \in [-0.21,0.15]$ for the Standard Model hypothesis was expected. Object definition:- Electron candidates are required to pass the medium likelihood-based identification selection, to have transverse momentum $p_T > 15$ GeV and $|\eta| < 2.47$, excluding the region $1.37< |\eta| < 1.52$. - The muon candidates are required to have $p_T > 10$ GeV and $|\eta|$ <2.5 and to pass the medium muon identification requirements. - The tau candidates are required to have $p_T > 20$ GeV, one or three associated tracks, an absolute electric charge of one and $|\eta| < 2.47$, excluding the region $1.37< |\eta| < 1.52$. They have to pass the medium identification requirement in the $\tau_{\mathrm{lep}}\tau_{\mathrm{had}}$ channel and the tight identification requirement in the $\tau_{\mathrm{had}}\tau_{\mathrm{had}}$ channel. - Jets are reconstructed from topological clusters in the calorimeter using the anti-Kt algorithm with a radius parameter value R = 0.4, and are required to have $|\eta| < 4.9$. The VBF inclusive region is characterized by the presence of two jets with $p_T > 30$ GeV. In addition, the two jets are required to be in opposite hemispheres of the detector with a large pseudorapidity separation of $|\Delta \eta_{jj}| > 3$ and their invariant mass $m_{jj}$ is required to be larger than 300 GeV. To construct a signal region enriched in VBF signal events, BDTs trained to discriminate between the VBF signal and backgrounds are used in all channels. Kinematic variables used in the BDT training can be categorized as follows: properties of the Higgs boson which discriminate against all background processes without a Higgs boson, properties of a resonant di-tau decay which discriminate against processes with mis-identified tau-decay candidates, and properties of the VBF topology. The most important variables in the training are $m_{\tau\tau}^{\mathrm{MMC}}$, $m_{jj}$ and $C_{jj}(\tau)$. A threshold on the BDT score is used to define the final signal region in each channel. This threshold is chosen to yield a high signal significance.

  • Table 1

    Figure 2 (a)

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    Post-fit BDT distributions after the VBF event selection for the $\tau_{\mathrm{lep}}\tau_{\mathrm{lep}}$ SF analysis channel. The VBF signal is shown for...

  • Table 2

    Figure 2 (b)

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    Post-fit BDT distributions after the VBF event selection for the $\tau_{\mathrm{lep}}\tau_{\mathrm{lep}}$ DF analysis channel. The VBF signal is shown for...

  • Table 3

    Figure 2 (c)

    10.17182/hepdata.91678.v1/t3

    Post-fit BDT distributions after the VBF event selection for the $\tau_{\mathrm{lep}}\tau_{\mathrm{had}}$ analysis channel. The VBF signal is shown for $\mu...

  • Table 4

    Figure 2 (d)

    10.17182/hepdata.91678.v1/t4

    Post-fit BDT distributions after the VBF event selection for the $\tau_{\mathrm{had}}\tau_{\mathrm{had}}$ analysis channel. The VBF signal is shown for $\mu...

  • Table 5

    Figure 3 (a)

    10.17182/hepdata.91678.v1/t5

    Post-fit $m_{\tau\tau}^{\mathrm{MMC}}$ distributions in the low BDT score CR for the $\tau_{\mathrm{lep}}\tau_{\mathrm{lep}}$ SF analysis channel. ''Other bkg'' denotes all background...

  • Table 6

    Figure 3 (b)

    10.17182/hepdata.91678.v1/t6

    Post-fit $m_{\tau\tau}^{\mathrm{MMC}}$ distributions in the low BDT score CR for the $\tau_{\mathrm{lep}}\tau_{\mathrm{lep}}$ DF analysis channel. ''Other bkg'' denotes all background...

  • Table 7

    Figure 3 (c)

    10.17182/hepdata.91678.v1/t7

    Post-fit $m_{\tau\tau}^{\mathrm{MMC}}$ distributions in the low BDT score CR for the $\tau_{\mathrm{lep}}\tau_{\mathrm{had}}$ analysis channel. ''Other bkg'' denotes all background contributions...

  • Table 8

    Figure 3 (d)

    10.17182/hepdata.91678.v1/t8

    Post-fit $m_{\tau\tau}^{\mathrm{MMC}}$ distributions in the low BDT score CR for the $\tau_{\mathrm{had}}\tau_{\mathrm{had}}$ analysis channel. ''Other bkg'' denotes all background contributions...

  • Table 9

    Figure 4 (a)

    10.17182/hepdata.91678.v1/t9

    Post-fit Optimal Observable distributions in the low BDT score CR for the $\tau_{\mathrm{lep}}\tau_{\mathrm{lep}}$ SF analysis channel. ''Other bkg'' denotes all...

  • Table 10

    Figure 4 (b)

    10.17182/hepdata.91678.v1/t10

    Post-fit Optimal Observable distributions in the low BDT score CR for the $\tau_{\mathrm{lep}}\tau_{\mathrm{lep}}$ DF analysis channel. ''Other bkg'' denotes all...

  • Table 11

    Figure 4 (c)

    10.17182/hepdata.91678.v1/t11

    Post-fit Optimal Observable distributions in the low BDT score CR for the $\tau_{\mathrm{lep}}\tau_{\mathrm{had}}$ analysis channel. ''Other bkg'' denotes all background...

  • Table 12

    Figure 4 (d)

    10.17182/hepdata.91678.v1/t12

    Post-fit Optimal Observable distributions in the low BDT score CR for the $\tau_{\mathrm{had}}\tau_{\mathrm{had}}$ analysis channel. ''Other bkg'' denotes all background...

  • Table 13

    Figure 5 (a)

    10.17182/hepdata.91678.v1/t13

    Post-fit distributions of the event yields as a function of the Optimal Observable in the SR for the $\tau_{\mathrm{lep}}\tau_{\mathrm{lep}}$ SF...

  • Table 14

    Figure 5 (b)

    10.17182/hepdata.91678.v1/t14

    Post-fit distributions of the event yields as a function of the Optimal Observable in the SR for the $\tau_{\mathrm{lep}}\tau_{\mathrm{lep}}$ DF...

  • Table 15

    Figure 5 (c)

    10.17182/hepdata.91678.v1/t15

    Post-fit distributions of the event yields as a function of the Optimal Observable in the SR for the $\tau_{\mathrm{lep}}\tau_{\mathrm{had}}$ analysis...

  • Table 16

    Figure 5 (d)

    10.17182/hepdata.91678.v1/t16

    Post-fit distributions of the event yields as a function of the Optimal Observable in the SR for the $\tau_{\mathrm{had}}\tau_{\mathrm{had}}$ analysis...

  • Table 17

    Figure 6 (a)

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    The observed $\Delta\mathrm{NLL}$ curve as a function of $\tilde d$ values. For comparison, expected $\Delta\mathrm{NLL}$ curves are also shown. The...

  • Table 18

    Figure 6 (b)

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    The expected $\Delta\mathrm{NLL}$ curves comparing the sensitivity of the fit with and without systematic uncertainties. For comparison, other curves are...

  • Table 19

    Figure 6 (c)

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    The observed $\Delta\mathrm{NLL}$ curves for each analysis channel as a function of $\tilde d$, compared to the combined result. For...

  • Table 20

    Figure 7 (a)

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    Post-fit BDT distributions in the top-quark CR for the $\tau_{\mathrm{lep}}\tau_{\mathrm{lep}}$ SF channel. The size of the combined statistical, experimental, and...

  • Table 21

    Figure 7 (b)

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    Post-fit BDT distributions in the top-quark CR for the $\tau_{\mathrm{lep}}\tau_{\mathrm{lep}}$ DF channel. The size of the combined statistical, experimental, and...

  • Table 22

    Figure 7 (c)

    10.17182/hepdata.91678.v1/t22

    Post-fit BDT distributions in the $Z\to \ell\ell$ CR for the $\tau_{\mathrm{lep}}\tau_{\mathrm{lep}}$ SF analysis channel. The size of the combined statistical,...

  • Table 23

    Figure 8 (a)

    10.17182/hepdata.91678.v1/t23

    Post-fit Optimal Observable distributions in the top-quark CR for the $\tau_{\mathrm{lep}}\tau_{\mathrm{lep}}$ SF channel. The size of the combined statistical, experimental,...

  • Table 24

    Figure 8 (b)

    10.17182/hepdata.91678.v1/t24

    Post-fit Optimal Observable distributions in the top-quark CR for the $\tau_{\mathrm{lep}}\tau_{\mathrm{lep}}$ DF channel. The size of the combined statistical, experimental,...

  • Table 25

    Figure 8 (c)

    10.17182/hepdata.91678.v1/t25

    Post-fit Optimal Observable distributions in the $Z\to \ell\ell$ CR for the $\tau_{\mathrm{lep}}\tau_{\mathrm{lep}}$ SF analysis channel. The size of the combined...

  • Table 26

    Figure 9

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    Post-fit distribution of weighted event yields as a function of the Optimal Observable for all four SRs combined. The contributions...

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