Forward electrons

The replacement of the Inner Detector by the all-silicon ITk, together with the timing information from the HGTD, opens the forward region to track-based electron reconstruction for the first time in ATLAS. The work presented here exploited these new handles to develop a dedicated calibration and identification chain for forward electrons in the high-pile-up environment of the HL-LHC, extending the coverage out to $\lvert\eta\rvert= 4.0$.

The $p_\mathrm{T}$-calibration was addressed with a DNN regression combining the cluster moments, the layer energy fractions, the tracker hits, the track–calorimeter matching variables, and the HGTD timing into a single calibrated estimator $p_\mathrm{T}^\mathrm{DNN}$. The track-only measurement is unusable in the forward region owing to the very short lever arm, while the calorimeter-only and combined measurements suffer from a sizeable bias and broad tails, in particular in the forward calorimeter region. The DNN instead recovers a response centred on unity in all three $\lvert\eta\rvert$ bins, removing the bias, with a resolution equal to that of the calorimeter-based methods in the two inner bins and markedly better in the forward calorimeter region, where their tails are largest.

For the identification, a second DNN classifier was trained together with a $p_\mathrm{T}$-decorrelation technique, in which the cluster moments are replaced by the residuals of a linear regression against $p_\mathrm{T}^\mathrm{DNN}$. This provides working points with a flat signal efficiency across the full $p_\mathrm{T}$ range and a substantially better background rejection than the legacy LH and cluster-moment selections, which were not designed for the stringent conditions expected in Run-4. The decorrelation further enables a smooth and bounded extrapolation of the working-point thresholds beyond the training range, down to $5\,\mathrm{GeV}$ and up to $500\,\mathrm{GeV}$. The complete chain, comprising the calibration, the identification, and the working-point definition, was implemented in Athena.

More: electron reconstruction and calibration in ATLAS