Particle Identification

Figure: Specific ionization energy loss relative to that of a minimally ionizing particle, plotted against momentum, for a sample of $D^*$ meson candidate events which also contain a proton candidate (taken from [51]). The curves indicate parameterizations of the most probable responses of the H1 drift chambers for pions, kaons and protons, respectively.

Particle identification is often used in order to enhance the contribution from heavy hadrons and/or to reduce the combinatorial backgrounds. For the reconstruction of invariant mass spectra it is useful to identify charged pions, kaons and protons, such that particles that are clearly identified as not coming from the heavy hadron decay can be removed from the list of particles used for the mass reconstruction. Although not optimized for this purpose, the central drift chambers can be used to determine the specific energy loss $dE/dx$ for a given particle. This energy loss depends on the velocity of the particle. The measurement of the summed charge of the hits of a drift chamber track, together with the measurement of the particle momentum thus allows to discriminate between $\pi$, $K$ and $p$ (see fig.12). The H1 and ZEUS drift chambers provide a relative uncertainty for the charge measurement of typically $8\%$, leading to a $K$-$p$ separation of $\sim 1 \sigma$ at 2 GeV.

The identification of leptons originating from semi-leptonic decays of heavy hadrons is a very useful means for the selection of heavy quark event candidates. Furthermore, in charmonium decays into leptons, the lepton identification allows to remove the largest part of the combinatorial background for the reconstruction of the charmonium invariant mass.

The $\mu$-identification is particularly simple as the instrumented iron return yokes are available for the reconstruction of muon tracks. The HERA experiments have full acceptance for the muons to be measured in the instrumented iron at transverse energies of 2.0 GeV and above. The muon identification can be enhanced by reconstruction of isolated energy deposits in calorimeter cells close to the extrapolated muon track, which are consistent with the amount of energy deposited by a minimally ionizing particle. The longitudinal segmentation of the H1 and ZEUS calorimeters allows to reconstruct quantities, such as the length of the track inside the calorimeter or the energy deposited in a narrow cone along the projected muon track, which provide for a muon fake probability of 1-2% at muon momenta of 1-2 GeV [145].

The separation between $\pi$ and $e$ in the calorimeter is important to suppress fake background in samples of semi-electronic decays. A detailed study of electron identification in the calorimeter in a dense hadronic environment has been given e.g.in [146].