Measuring the $b$-jet tagging efficiency on $c$-jets containing $D^{*+}$ mesons with ATLAS data

The identification of jets originating from $b$-quarks ($b$-tagging) is a crucial tool for the LHC physics program, both for precision measurements and in searches for new particles. One important ingredient when using $b$-tagging in physics analyses is the determination of the probability to mistakenly $b$-tag a jet originating from a $c$-quark ($c$-tag efficiency). The optimal sample for the measurements of the $c$-tag efficiency would be a clean sample of jets originating from $c$-quarks. The sample of jets associated to reconstructed $D^{*+}$ mesons (through the decay chain $D^{*+}\to D^0\pi^+\to K^-\pi^+\pi^+$) is quite close to this ideal case since most $D^{*+}$ mesons originate from $c$-quark hadronization. The $c$-tag efficiency has been measured by comparing the reduction in the yield of $D^{*+}$ mesons before and after the $b$-tagging requirement of the associated jet. The contamination from $D^{*+}$ mesons that originate from $b$-hadron decays has been taken into account using a fit to the $D^0$ pseudo-proper time distribution.