Tipos de enzimas

The production and detection of transition radiation (TR) is one of the unique features of the TRT detector. Transition radiation may be produced whenever a highly relativistic charged particle crosses a boundary between materials with differing dielectric constants. The TR photons are generally soft X-rays (energy of 1-30 keV for the TRT) and are emitted with a rate proportional to γ and an angle 1/γ with respect to the particle trajectory. Because electrons have a mass which is roughly 250 times smaller than pions, electrons will have a much higher rate of TR emission than pions of the same energy. Therefore, the detection of these TR photons allows for electron/pion discrimination.

However, the probability of emitting a TR photon at each boundary is small, so many transitions are needed in order to produce a detectable signal. Therefore, the space between the TRT straws is filled with radiator materials which have been optimized for production of transition radiation. In the straw, xenon was chosen as the primary active gas due to its large absorption cross section for TR photons (the carbon dioxide and oxygen are added for stability). Absorption of these TR photon results in a large number of primary electrons and consequently produces a much larger signal than that of a minimally ionizing particle, passing the high threshold of the ASDBLR. Therefore, the presence of high threshold hits on a track are evidence of transition radiation.

The fraction of high threshold (HT) hits on track is the primary TRT observable used for electron/pion discrimination.1 _{The HT hit probability, measured as the ratio of HT hits to}

in order to validate the modeling of transition radiation production and detection. This is important as mismodeling of the HT probability would lead to mismodeling of the electron selection efficiency because the HT fraction is used in the electron selection criteria later in this analysis.

A high purity sample of electron candidates (>95% in simulation) is selected fromZ→ee decays and photon conversions, covering the range γ ∼ 103₋₁₀5_{, using a tag and probe}

technique. In the tag and probe technique, a tag electron is selected using strict selection criteria and the probe, which is selected using looser criteria, is used to measure HT probability such that the measurement is unbiased by the candidate selection. If both electrons in an event pass the tag criteria, then both are used as probe candidates. For this measurement, the both tag and probe electrons are required to to pass the calorimeter based “medium” [62].

Pion candidates are selected from a minimum bias selection of tracks. A veto is applied to tracks coming from photon conversion candidates to suppress electrons and a requirement of dE/dx>1.6 MeVg−1_cm−2 _{as measured in the Pixel detector is applied in order to suppress}

protons (and to a lesser extent kaons). The main background sources in the pion sample are protons and kaons and the purity of the pion sample varies from 95% atγ∼100_{down to 60%}

at γ∼103 _{as estimated in the simulation [62].}

The high threshold probability as a function of γ in the TRT barrel region is shown in Figure3.5for both the data and simulation using these selected samples of electron and pion candidates. For the pion candidates at lowγ, the average HT hit probability is roughly 0.05, which arises due to large energy deposits from the tails of the Landau dE/dx distribution which produce HT hits. A small increase in HT probability with γ is also observed for the pions, which is due to the increasing average dE/dx with γ. A clear turn-on is observed in

1_{One may also use the Time over Threshold (ToT) for particle identification purposes, particularly at low}

3. The ATLAS Detector factor γ 1 10 ₁₀2 ₁₀3 ₁₀4 ₁₀5 ₁₀6 High-threshold probability 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Electron momentum [GeV]

1 10 ₁₀2

Pion momentum [GeV]

1 10 = 7 TeV) s Data 2010 ( |<0.625 η | from Z ± Data, e γ from ± Data, e ± π Data, from Z ± Simulation, e γ from ± Simulation, e ± π Simulation, ATLAS Preliminary High-threshold fraction 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Entries [normalized to unity]

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Electron candidates Pion candidates Data 2010 TRT barrel 4 < p < 20 GeV ATLAS Preliminary

Figure 3.5: The plot on the left shows the probability to observe a high threshold hit as a function of the particle’s Lorentz factorγfor electron and pion candidates in the TRT barrel region using√s= 7 TeV collision data and simulation. The plot on the right shows the distribution of the fraction of high threshold hits on track for the same candidates, showing good separation between electrons and pions [62].

the range γ∼103₋₁₀4_{, where the HT probability rapidly increases from 0.05 to 0.2. Above}

γ∼104_{, the HT probability plateaus due to detector saturation effects. Figure3.5also shows}

the HT fraction for the electron and pion candidates, showing good separation [62].