Separation of Pions and Kaons with the ATLAS TRT


The ATLAS (A Toroidal Lhc ApparatuS) detector at CERN has been build to investigate several questions in physics. Among these are the search for the Higgs-Boson and the new particles, for example predicted by the string theory.

The TRT (Transition Radiation Tracker) was build only for tracking of charged particles and identification of electrons using transition radiation.[1]

The thesis investigated, if the TRT can be used for particle identification by energy loss.

For particles of a momentum from 1GeV to 10GeV, the Bethe formula describes the energy loss as a function of the particle momentum. But the slope in this energy range is very small, therefore only a weak particle identification strength is expected.[2]

Despite this, a statistical separation of the few kaons from the large number of pions has the advantage of improving the signal to background ratio.


The Standard Model of particle physics predicts only a very small CP violation for the Bs meson. But if unknown particles are present in the mixing between Bs and its anti particle, this CP violation would be significantly increased. The decay of the Bs meson and its anti particle is therefore a sensor for new physics.[3]

The kaons appear in the decay channel of Bs to J/ψ and ϕ, and ϕ to K+K-. There are always pairs of kaons produced, which have to be identified.


The simulation of the ATLAS detector is implemented in the Geant Framework. This framework simulates the entire detector for a large number of events. The necessary simplifications include simplifications of the simulation of the TRT.

Therefore a new simulation for individual TRT straws has been developed. The simulation consists of several steps:

  • Energy loss by ionization of the gas
  • Drift of the electrons in the electric field and electron avalanches
  • Signal propagation and signal reflection
  • Simulation of the analog electronics, the ASDBLR (A Shaper Discriminator with Base Line Restoration)
  • Simulation of the digital electronics, the DTMROC (Digital Time Measurement and Read Out Chip)

For the calculation of the primary ionisation, HEED++ (High Energy Electrodynamics) was used. Magboltz calculated the electron avalanches which are initiated by the primary electrons. These two libraries were united by Garfield++.

The signal propagation was simplified to attenuation and retardation. The reflection is here the addition of two signals with half amplitude with different path length.

The HSpice simulation for the ASDBLR could not be used due to legal issues. Therefore a new simulation was developed, based on the data of the HSpice simulation. This simulation consisted of several parts to take the non linear parts of the ASDBLR into account.

The digital electronics was simulated using a dynamic threshold. This threshold was compared to the input signal in fixed time intervals. This gives integer numbers for ToTs (Time over Threshold).

The simulation of the simplified TRT uses the results of the simulation for the single straws. This allows a Monte Carlo simulation of the energy loss for the entire detector along a random path.


The simulation gives distributions of ToT-Sums as a function of energy and momentum for pions and kaons separately. These distributions for pions and kaons with identical momentum are similar, but have different means. The simulation could show a particle separation of about 1σ.


[1] E Barberio. Studies and optimisation of the transition radiation performance for the atlas trt with an improved detector simulation. Technical Report ATL-INDET-2003-013, CERN, Geneva, Dec 2002.
[2] K.Nakamura et. al (Particle Data group). Review of particle physics. Journal of Physics G, 37, 2010.
[3] Alastair. A study of Bs → J/ψϕ and Bd → J/ψK0* decays with the ATLAS experiment. PhD thesis.