The data were collected during October 2018 at the H2 beam line, [11], located in the EHN1 experimental hall of the CERN-SPS North Area. Secondary beams of hadrons or positrons up to a maximum momentum of 400 GeV/c can be transported in the
experi-mental areas with variable purity (10∼99%). These secondary beams are produced by the interaction of the primary proton beam impinging on a thin beryllium plate primary target, designated ’T2’. The H2 beamline transports the produced particles over a length of approximately 600 m, with the last 250 m being inside the surface EHN1 experimental hall. The H2-beam line is a magnetic spectrometer consisting of dipole magnets, a num-ber of quadrupoles and collimators. The H2 dipole spectrometer principle of operation is shown in Figure. 4.4.
Figure 4.4: H2 beamline flexible magnetic spectrometer.
The primary beam from the SPS impinges on a Beryllium target and produces the secondary particles. They are subsequently momentum-analyzed and selected with the use of two series of vertical dipole magnets (depicted as triangles) and collimators depicted as orange rectangles.
The beamline selects the secondary particles produced at the target within a
∆p / p = 0.2 ∼ 2% acceptance, depending on the upstream collimator settings. The currents of the first and second sets of dipoles define the beam momentum emerging from the last dipole of the spectrometer located 250 m upstream the HGCAL prototype. The final beam is achromatic in first order, with a fixed momentum spread defined by the collimators. Optics-wise, only second-order chromatic aberrations introduce a negligible
However, for positrons, Synchrotron Radiation (SR) losses induce an additional beam spread that is particularly important for energies above 100 GeV. The total beam spread is given by the convolution of these two effects and is shown for positrons in Table 2. In the case of positrons, SR losses in the dipole magnets also lead to a final beam momentum that is lower than the nominal one.
Nominal Momentum (GeV) Final Momentum (GeV)
Table 2: H2 beamline positron beam momentum and momentum spreads.
The H2 beamline and HGCAL-prototype experimental setup are shown in Fig. 4.5.
The beamline section from the last spectrometer dipole to the face of the calorimeter, in-cludes sections of air, beam windows and beam counters like delay wire chambers (DWC) and Cerenkov detectors accounting for about 0.2X0 of material.
Figure 4.5: H2 beamline and HGCAL-prototype experimental setup.
The DWC is most commonly used supporting detector in the HGCAL beam tests at
H2 beam line. The DWC is built in an aluminium case of 220 × 220 × 56 (mm), with two kapton windows of 110 × 110 (mm) and a thickness of 25 µm. The picture of DWC is shown in Figure 4.6
Figure 4.6: Delay wire chamber and supporting frame.
The supporting frame and the DWC is shown. The functioning part is labeled by the red frame.
The DWC is a gaseous detector, when a particle passes through, the ionized gas will be accelerated by the high voltage between the thin cathode and anodes. The DWC has 4 direction TDC(time to digital converter) readouts (top, bottom, left and right), by cal-culating the delay between x or y, the point where the particle penetrates is retrieved. By using multiple DWCs in the beam line, the track of the particle can be reconstructed. The
normal trajectory. The resolution of the DWC in the H2 beam line is 0.2 mm, which is enough for the cell size of the sensor (∼1cm).
4.2.1 HGCAL Prototype
The HGCAL prototype tested in October 2018 comprised a CE-E and a CE-H section.
The CE-E section consisted of 28 sampling layers of hexagonal modules with hexagonal Si pads (≈1 cm2) interleaved with alternating copper plus copper-tungsten absorbers or lead plus stainless steel absorbers. For the 2018 beam test, 28 hexagonal modules were assembled from a stack of a copper-tungsten baseplate, a polyimide foil, a silicon sensor and a readout PCB (referred to as Hexaboard).
The modules followed the same design as the prototypes tested in 2016, albeit with better grounding to improve the electronic noise and take advantage of the new Skiroc2-CMS front end chip [12]. The Skiroc2-Skiroc2-CMS ASIC was specifically designed for HGCAL requirements. It offered a broad range of energy measurement thanks to a dual-gain am-plification and an additional time-over-threshold technique (TOT) to cope with very high energy deposits where the low gain channel saturates. For the low and high gain chain, signal pulses collected from each Si pad were amplified, shaped, sampled with a 25n sam-pling frequency and finally stored in a 13 Switch Capacitor Array rolling analog memory (SCA). The chip also provided a time-of-arrival measurement (TOA) to allow shower time development studies. These changes aimed to get closer to the HGCAL ultimate performance and validate the simulation on which all high level analyses rely on.
The prototype tested in 2018 offered a variety of three configurations, among which the two first had identical CE-E section of 28 instrumented layers, close to the HGCAL design. The layout of the CE-E section is shown in Fig. 4.7. The results presented in this work were based on the data taken with these two configurations. Complete description of the prototype tested in 2018 CERN beam tests can be found in [13], including the CE-H section.
Figure 4.7: Layout for the CE-E prototype.
The CE-E prototype is built of 14 cassettes, where one cassette carries two hexagonal modules. In a hexagonal module, the Si sensor (dark blue) is located betwen the
cupper-tungsten plate and the Hexaboard. A cassette starts with a lead plus stainless steel absorber (light blue and pink) and ends with the Hexaboard (green) of the second
module. The lead plus stainless steel absorber of the second cassette is also shown.
Figure 4.8: Test beam CE-E and CE-H setup.
4.2.2 Trigger and DAQ systems
The Data AcQuisition system [5] consisted of DAQ crates with back-end components and a DAQ computer. Seven Hexaboards were connected via seven interposer boards to the readout cards. The latter were controlled by a Synchronising board delivering a 40 MHz clock, TTL Trigger and busy signals. The readout cards and the synchronising board were hosted in the DAQ crates. For the CE-E prototype, four readout cards were used to read the 28 layers. The DAQ computer running the EUDAQ software [14] collected the data from the readout cards as well as the data from the completmentary beam detectors via a Gigabit Ethernet server. Scintillators were used to trigger the acquisition. A low-level electron trigger was obtained online, requiring the coincidence of 2 scintillators upstream the CE-E prototype and the veto of a scintillator downstream the CE-H prototype.