The inclusive $b \bar{b}$ and $c \bar{c}$dijet production crosssections in the forward region of $pp$ collisions are measured using a data sample collected with the LHCb detector at a centreofmass energy of 13 TeV in 2016. The data sample corresponds to an integrated luminosity of 1.6 fb$^{1}$. Differential crosssections are measured as a function of the transverse momentum and of the pseudorapidity of the leading jet, of the rapidity difference between the jets, and of the dijet invariant mass. A fiducial region for the measurement is defined by requiring that the two jets originating from the two $b$ or $c$ quarks are emitted with transverse momentum greater than 20 GeV$/c$, pseudorapidity in the range $2.2 < \eta < 4.2$, and with a difference in the azimuthal angle between the two jets greater than 1.5. The integrated $b \bar{b}$dijet crosssection is measured to be $53.0 \pm 9.7$ nb, and the total $c \bar{c}$dijet crosssection is measured to be $73 \pm 16$ nb. The ratio between $c \bar{c}$ and $b \bar{b}$dijet crosssections is also measured and found to be $1.37 \pm 0.27$. The results are in agreement with theoretical predictions at nexttoleading order.
Fit templates for (left) $t_0$ and (right) $t_1$ projections for the sameflavour and differentflavour processes, summed over the $[ p_{\mathrm{T}} (j_0), p_{\mathrm{T}} (j_1)]$ bins, where $t_0$ and $t_1$ are the linear combinations of the tagging observables of both jets. The templates are normalised to unit area. To simplify the visualisation, the $qb$ and $bq$ samples ($qc$ and $cq$) are merged in the plot. 
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The observables $t_0$ and $t_1$ obtained by summing the fitted yields over the $\eta(j_0)$ and $[ p_{\mathrm{T}} (j_0), p_{\mathrm{T}} (j_1)]$ bins, where $t_0$ and $t_1$ are the linear combinations of the tagging observables of both jets. The fit is compared against the data (black points). The statistical uncertainty on data is small and not visible in the plot. The stacked histograms show the contribution from: (red) $b\bar{b}$, (blue) $c\bar{c}$, (lavender) $bq$ and $qb$, (light blue) $cq$ and $qc$. The $qq'$ component is not displayed since its yield is negligible. The dashed grey areas represent the total uncertainty (statistical and systematic) on the fit result. 
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Differential $b\bar{b}$ and $c\bar{c}$dijet crosssections as a function of the (top left) leading jet $\eta$, (top right) the leading jet $p_{\mathrm{T}}$, (bottom left) $\Delta y^*$ and (bottom right) $m_{jj}$. The error bars represent the total uncertainties, that are almost fully correlated across the bins. The nexttoleadingorder predictions obtained with Madgraph5 aMC@NLO + Pythia are shown. The prediction uncertainty is dominated by the renormalisation and factorisation scale uncertainty. The leadingorder prediction obtained with Pythia is also shown. 
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Measured $c\bar{c}$ to $b \bar{b}$ crosssection ratio as a function of (top left) the leading jet $\eta$, (top right) the leading jet $p_{\mathrm{T}}$, (bottom left) $\Delta y^*$ and (bottom right) $m_{jj}$. The error bars represent the total uncertainties, that are almost fully correlated across the bins. The nexttoleadingorder predictions obtained with Madgraph5 aMC@NLO + Pythia are shown. The prediction uncertainty is dominated by the renormalisation and factorisation scale uncertainty. The leadingorder prediction obtained with Pythia is also shown. 
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Measured differential $b\bar{b}$ and $c\bar{c}$dihet crosssections as a function of the (left) leading jet $p_{\mathrm{T}}$ and (right) $m_{jj}$ on a linear scale. The error bars represent the total uncertainties, that are almost fully correlated across the bins. The nexttoleadingorder predictions obtained with Madgraph5 aMC@NLO + Pythia are shown. The prediction uncertainty is dominated by the renormalisation and factorisation scale uncertainty. The leadingorder prediction obtained with Pythia is also shown. 
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Animated gif made out of all figures. 
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List of fiducial requirements on jet transverse momentum, pseudorapidity and the azimuthal angle between the jets. 
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Mean relative uncertainties in percent on the crosssections calculated averaging over the leading jet pseudorapidity intervals for $b\bar{b}$ and $c\bar{c}$ events. For the total uncertainty individual contributions are added in quadrature. 
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The total $b \bar{b}$dijet and $c \bar{c}$dijet crosssections and their ratio in the fiducial region, compared with the NLO predictions. The first uncertainty on the measurement is the combined statistical and systematic uncertainty and the second is the uncertainty from the luminosity. For the measurement of $R$ the luminosity uncertainty cancels in the ratio. The statistical uncertainty for the crosssection and $R$ measurements is also reported. For the predictions the first uncertainty corresponds to the scale uncertainty, the second to the PDF uncertainty. 
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Numerical results of $b\bar{b}$ and $c \bar{c}$dijet crosssections, $c\bar{c}/b\bar{b}$ dijet crosssection ratios and their total uncertainties as a function of the leading jet $\eta$. 
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Numerical results of $b\bar{b}$ and $c \bar{c}$dijet crosssections, $c\bar{c}/b\bar{b}$ dijet crosssection ratios and their total uncertainties as a function of $\Delta y^*$. 
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Numerical results of $b\bar{b}$ and $c \bar{c}$dijet crosssections, $c\bar{c}/b\bar{b}$ dijet crosssection ratios and their total uncertainties as a function of the leading jet $ p_{\mathrm{T}} $. 
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Numerical results of $b\bar{b}$ and $c \bar{c}$dijet crosssections, $c\bar{c}/b\bar{b}$ dijet crosssection ratios and their total uncertainties as a function of $m_{jj}$. 
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Covariance matrix, corresponding to the total uncertainties, obtained between the leading jet $\eta$ intervals of the $b\bar{b}$dijet differential cross sections. The unit of all the elements of the matrix is nb$^2$. 
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Covariance matrix, corresponding to the total uncertainties, obtained between the leading jet $\eta$ intervals of the $c\bar{c}$dijet differential cross sections. The unit of all the elements of the matrix is nb$^2$. 
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Covariance matrix, corresponding to the total uncertainties, obtained between the leading jet $\eta$ intervals of the $b\bar{b}$ (horizontal) and $c\bar{c}$ (vertical) differential cross sections. The unit of all the elements of the matrix is nb$^2$. 
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Covariance matrix, corresponding to the total uncertainties, obtained between the $\Delta y^*$ intervals of the $b\bar{b}$dijet differential cross sections. The unit of all the elements of the matrix is nb$^2$. 
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Covariance matrix, corresponding to the total uncertainties, obtained between the $\Delta y^*$ intervals of the $c\bar{c}$dijet differential cross sections. The unit of all the elements of the matrix is nb$^2$. 
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Covariance matrix, corresponding to the total uncertainties, obtained between the $\Delta y^*$ intervals of the $b\bar{b}$ (horizontal) and $c\bar{c}$ (vertical) differential cross sections. The unit of all the elements of the matrix is nb$^2$. 
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Covariance matrix, corresponding to the total uncertainties, obtained between the leading jet $ p_{\mathrm{T}} $ intervals of the $b\bar{b}$dijet differential cross sections. The unit of all the elements of the matrix is $\left( \frac{\mathrm{nb}}{\text{ Ge V /}c } \right)^2$ and the $ p_{\mathrm{T}} $ intervals are given in $\text{ Ge V /}c$ . 
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Covariance matrix, corresponding to the total uncertainties, obtained between the leading jet $ p_{\mathrm{T}} $ intervals of the $c\bar{c}$dijet differential cross sections. The unit of all the elements of the matrix is $\left( \frac{\mathrm{nb}}{\text{ Ge V /}c } \right)^2$ and the $ p_{\mathrm{T}} $ intervals are given in $\text{ Ge V /}c$ . 
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Covariance matrix, corresponding to the total uncertainties, obtained between the leading jet $ p_{\mathrm{T}} $ intervals of the $b\bar{b}$ (horizontal) and $c\bar{c}$ (vertical) differential cross sections. The unit of all the elements of the matrix is $\left( \frac{\mathrm{nb}}{\text{ Ge V /}c } \right)^2$ and the $ p_{\mathrm{T}} $ intervals are given in $\text{ Ge V /}c$ . 
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Covariance matrix, corresponding to the total uncertainties, obtained between the $m_{jj}$ intervals of the $b\bar{b}$dijet differential cross sections. The unit of all the elements of the matrix is $\left( \frac{\mathrm{nb}}{\text{ Ge V} } \right)^2$ and the mass intervals are given in $\text{ Ge V}$ . 
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Covariance matrix, corresponding to the total uncertainties, obtained between the $m_{jj}$ intervals of the $c\bar{c}$dijet differential cross sections. The unit of all the elements of the matrix is $\left( \frac{\mathrm{nb}}{\text{ Ge V /}c^2 } \right)^2$ and the mass intervals are given in $\text{ Ge V /}c^2$ . 
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Covariance matrix, corresponding to the total uncertainties, obtained between the $m_{jj}$ intervals of the $b\bar{b}$ (horizontal) and $c\bar{c}$ (vertical) differential cross sections. The unit of all the elements of the matrix is $\left( \frac{\mathrm{nb}}{\text{ Ge V /}c^2 } \right)^2$ and the mass intervals are given in $\text{ Ge V /}c^2$ . 
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Created on 20 February 2021.