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Search for $H^0 \rightarrow b \bar{b}$ or $c \bar{c}$ in association with a $W$ or $Z$ boson in the forward region of $pp$ collisions

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Abstract

The LHCb dataset consisting of proton--proton collisions recorded at $\sqrt{s}=8$ $\mathrm{TeV}$, corresponding to an integrated luminosity of 1.98 $\pm$ 0.02 $fb^{-1}$, is used to search for a Higgs boson with a mass of 125 $\mathrm{GeV}$, produced in association with a $W$ or $Z$ boson and decaying to a $b \bar{b}$ or $c \bar{c}$ pair. The final state considered is a pair of heavy flavour tagged jets and one or two high $p_T$ leptons (electrons or muons). No excess over the background expectation is found and upper limits on the product of cross section times branching fraction, with two heavy quarks from $H^0$ and one lepton from $W$/$Z$ in the LHCb acceptance $2 < \eta < 5$, are set at 95% CL: \sigma(pp $\rightarrow$ W/Z + H^0 ) \times \mathrm{ $\cal B$ }(H^0 $\rightarrow$ b \bar{b}) < 1.6 \mathrm{pb}, \sigma(pp $\rightarrow$ W/Z + H^0 ) \times \mathrm{ $\cal B$ }(H^0 $\rightarrow$ c \bar{c}) < 9.4 \mathrm{pb}.

Figures and captions

SV-tagger algorithm $\mathrm{BDT}_{b|c}$ for the highest jet $p_{\rm T}$ versus $\mathrm{BDT}_{b|c}$ for second highest jet $p_{\rm T}$ distributions. Distributions are obtained from $ W / Z + H ^0 ( b \overline b )$ (blue) and $ W / Z + H ^0 ( c \overline c )$ (red) simulation. The areas of the rectangles are proportional to the number of entries which is equal to the SM expectation after the selection cuts.

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a) Dijet mass, b) uGB ( $ W$ / $ Z$ + $ H ^0$ vs $W+b\bar{b}$) and c) uGB ( $ W$ / $ Z$ + $ H ^0$ vs $t\bar{t}$) distributions of selected data events with dijet mass greater than 70 $\mathrm{\,GeV} $ for the muon sample used in the $ H ^0 \rightarrow b \bar{b}$ limit. The SM signal and background prediction is also shown. The $ H ^0 \rightarrow b \bar{b}$ yield (in magenta) is multiplied by a factor 50. The other background components include $Z+ b \overline b $, $Z+ c \overline c $, $ZZ$, $WZ$, $Z$ and $W$+jets and several QCD processes. The compatibility between the observed data and the expected theory yield is below 2 Gaussian standard deviations.

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higgs_bb_mu.pdf

a) Dijet mass, b) uGB ( $ W$ / $ Z$ + $ H ^0$ vs $W+b\bar{b}$) and c) uGB ( $ W$ / $ Z$ + $ H ^0$ vs $t\bar{t}$) distributions of selected data events with dijet mass greater than 70 $\mathrm{\,GeV} $ for the electron sample used in the $ H ^0 \rightarrow b \bar{b}$ limit. The SM signal and background prediction is also shown. The $ H ^0 \rightarrow b \bar{b}$ yield (in magenta) is multiplied by a factor 50. The other background components include $Z+ b \overline b $, $Z+ c \overline c $, $ZZ$, $WZ$, $Z$ and $W$+jets and several QCD processes. The data is compatible with the background expectation.

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higgs_bb_e.pdf

SM prediction of a) Dijet mass, b) uGB ( $ W$ / $ Z$ + $ H ^0$ vs $W+b\bar{b}$) and c) uGB ( $ W$ / $ Z$ + $ H ^0$ vs $t\bar{t}$) distributions of events with dijet mass greater than 70 $\mathrm{\,GeV} $ for the muon sample used in the $ H ^0 \rightarrow c \bar{c}$ limit. The $ H ^0 \rightarrow c \bar{c}$ yield (in magenta) is multiplied by a factor of 1000. The other background components include $ W + b \overline b $ , $ t \overline t $ , $Z+ b \overline b $, $Z+ c \overline c $, $ZZ$, $WZ$, $Z$ and $W$+jets and several QCD processes. There are no data events passing the selection criteria for this sample.

higgs_[..].pdf [17 KiB]
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higgs_cc_mu.pdf

SM prediction of a) Dijet mass, b) uGB ( $ W$ / $ Z$ + $ H ^0$ vs $W+b\bar{b}$) and c) uGB ( $ W$ / $ Z$ + $ H ^0$ vs $t\bar{t}$) distributions of events with dijet mass greater than 70 $\mathrm{\,GeV} $ for the electron sample used in the $ H ^0 \rightarrow c \bar{c}$ limit. The $ H ^0 \rightarrow c \bar{c}$ yield (in magenta) is multiplied by a factor of 1000. The other background components include $ W + b \overline b $ , $ t \overline t $ , $Z+ b \overline b $, $Z+ c \overline c $, $ZZ$, $WZ$, $Z$ and $W$+jets and several QCD processes. There are no data events passing the selection criteria for this sample.

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higgs_cc_e.pdf

Distributions of $ W / Z + H ^0 ( b \overline b )$ , $Wb\bar{b}$ and $t \bar{t}$ simulated events in the transformed uGB space.

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higgs_bb_mu_uGBs.pdf

Observed and expected $CL_s$ and $95\%$ CL upper limit for the $ H ^0 \rightarrow b \bar{b}$. The 0.05 $CL_s$ level is indicated by the red horizontal line.

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limit_bb.pdf

Observed and expected $CL_s$ and $95\%$ CL upper limit for the $ H ^0 \rightarrow c \bar{c}$. The 0.05 $CL_s$ level is indicated by the red horizontal line.

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limit_cc.pdf

Animated gif made out of all figures.

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Tables and captions

Bin--per--bin weighted average variation of the histograms used in the $ H ^0 \rightarrow b \bar{b}$ limit computation for the main uncertainty sources and for signal and background in the muon and electron samples.

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Table_1.pdf

Created on 09 December 2018.Citation count from INSPIRE on 09 December 2018.