DESARROLLO COMUNICATIVO

The purpose of the cylindrical ATLAS calorimeter system [243–245] is to measure the energy of a particle by analysing the particle shower it created. Since particles like electrons and photons interact pre- dominantly electromagnetically, while hadrons interact also through the strong force, two different calorimeters are needed. First in line is the electromagnetic calorimeter, which is nested inside the larger hadronic calorimeter. Both are sampling calorimeters of the non- compensating type11

and designed to have a large angular coverage 11

In a sampling calorimeter the lay- ers of passive and active materials which create and detect the shower respectively, are placed alternatingly. A compensating calorimeter has an equally strong response for electromag- netic and hadronic particles of the same incident energy

to minimise the chance of particles escaping the detector outside its acceptance. This allows an excellent reconstruction of missing en- ergy, which hints at the presence of neutrinos or other invisible par- ticles in an event. A schematic view of the ATLAS calorimeter system is given in figure3.16.

In the dense passive layers of the electromagnetic (EM) calorime- ter, charged particles are subject to bremsstrahlung, i.e. the emittance of a photon, while highly energetic photons on the other hand can undergo a pair creation process in the presence of a heavy nucleus. Since the two processes feed each other as depicted in figure 3.17a, an electromagnetic shower is formed which terminates only when the available energy is too small to keep it alive. The total charge produced in such a shower is proportional to the energy of the initial particle. It is measured from the active layers. The denser the passive material, the faster the shower forms and the smaller the necessary

dimensions of the system. Figure

3_.16_{: Schematic view of the} calorimeter system and the position of calorimeters within the detector. The figure was adapted from [219_]. The EM calorimeter consists of a barrel with acceptance range

|η| < 1.475 and an end-cap on each side extending the pseudora-

pidity range to 1.375 < |η| < 3.2. Liquid argon functions as the

active material12

, while lead is used as the passive absorption ma-

12

which is why the ATLAS electromag- netic calorimeter is sometimes only re- ferred to as the "Liquid argon" or LAr. terial. To prevent ’dead towers’ in the detector, its layers have an

accordion shape (figure 3.18) with axial foldings in the barrel and radial foldings in the end-caps. The resolution varies with η and

reaches a granularity of∆η×∆φ = 0.025×0.025 in the barrel and

∆η×∆φ=0.1×0.1 in the end-cap region. The three barrel layers of

the EM calorimeter are at least 22 radiation lengths thick. In the end- cap region the thickness increases to 24 interaction lengths. Mea- sured particle energies have to be corrected for losses experienced while the particle traversed the ID, the cryostat and the solenoid magnet before reaching the calorimeter. To aid this, a presampler was installed in the range|η|<1.8. The design energy resolution of

the EM calorimeter is given by Figure3_.17_{: (a) shows the development}

of an electromagnetic shower with the definition of one radiation length. (b) shows how the shower from a photon is contained within one detector block of a fictional calorimeter tile. The figure was adapted from [246].

σ(E) E = 10 %√GeV p E[GeV] M 0.7 %√GeV , (3.4)

where the first term is called the stochastic term, the second one the constant term and the symbol ’L

’ denotes the quadratic sum. Measurements with test beams [193] and cosmic muons [247] have shown that the ATLAS EM calorimeter achieves this resolution.

Figure 3.18: Section of the LAr elec- tromagnetic calorimeter to reveal the accordion structure. Figure taken from [219_].

In the hadronic calorimeter showers are created through strong interactions of the particle in the passive layers. While the mecha- nisms of shower formation are by nature more complicated than in the EM calorimeter, the working principle remains the same. The hadronic calorimeter has a granularity of ∆η×∆φ = 0.1×0.1 in

the barrel which consists of a barrel segment with|η| <1.0 and an

extended barrel with 0.8 < |η| < 1.7. Active layers of scintillating

tiles are alternated with passive iron layers. In the end-cap region 1.5 < |η| < 3.2 the hadronic end-cap calorimeter achieves a gran-

ularity of ∆η×∆φ = 0.2×0.2. Here again liquid argon is used

with interjacent lead absorption layers. In the very forward region of 3.1< |η| < 4.9 the liquid argon forward calorimeter (FCal) con-

sisting of concentric copper and tungsten tubes extends the coverage. The maximum thickness of the hadronic calorimeter are eleven inter- action lengths, which is enough to prevent a punch-through into the Muon Spectrometer and contains all hadronic-shower end-products well within the calorimeter system, while only allowing muons, neu- trinos and other hypothetical long-lived particles to pass.

Figure3.19: Schematic of a scintillating wedge of the barrel tile calorimeter. Fig- ure taken from [248].

Measurements with pion test-beams have shown the energy reso- lution of the tile calorimeter is given by [193,249]

σ(E) E = (56.4±0.4)%√GeV p E[GeV] M (5.5±0.1)%√GeV (3.5)

while the resolution

σ(E) E = (70.6±1.5)%√GeV p E[GeV] M (5.8±0.2)%√GeV (3.6)

is stated for the hadronic end-caps [250].

At a radial distance of up to 4.2 m the tile calorimeter provides a very good timing measurement. In this study it is used to perform time-of-flight measurements to calculate the propagation velocity of a traversing particle. One scintillating wedge containing many tiles is sketched in figure3.19.