Abstract
The physics reach and feasibility of the Future Circular Collider (FCC) with center of mass energies up to 100 TeV and unprecedented luminosity is currently under investigation. This energy regime opens new opportunities for the discovery of new heavy particles (new gauge bosons), as well as new precise measurements in the Higgs sector (self-coupling, rare decays). However, high mass gauge bosons or high \(\hbox {p}_{\mathrm{T}}\) vector bosons decaying to pairs of hadrons require an efficient reconstruction of very high \(\hbox {p}_{\mathrm{T}}\) jets. The reconstruction of these boosted jets (\(\thicksim \)5–20 TeV), with a large fraction of highly energetic hadrons, sets the requirements on the calorimetry: excellent energy resolution (especially low constant term), containment of highly energetic hadron showers, and high transversal granularity to provide sufficient distinction of close by objects. Additionally the FCC detectors have to meet the challenge of a very high pile-up environment.
We will present the preliminary results of the ongoing performance studies, discuss the feasibility and potential of the technologies under test, while addressing the needs of the physics benchmarks of the FCC-hh experiment for the calorimeters.
1 Introduction
The collaboration around the Future Circular Collider (FCC) project studies the next generation of circular colliders for electron-electron (ee), hadron-hadron (hh) and electron-hadron (eh) collisions with desired center-of-mass energies of up to 100 TeV. Therefore, the construction of a 100 km tunnel in the Geneva area is under investigation. Hereby CERN functions as the host laboratory and provides necessary infrastructure. The high energy and intensity require new developments of magnets and RF structures for the accelerator, as well as new experiment and detector designs.
Within this paper we will elaborate on the key requirements on the detectors, with the focus on the calorimeters, and introduce the current design proposal that is used as a reference for the conceptional design report in preparation for the FCC study.
2 FCC-hh Experiment
The new energy regime of a FCC-hh experiment opens new possibilities to probe the Standard model. An interesting study is the Higgs production in vector boson fusion (VBF) processes which give access to a range of Higgs properties and allows for dark matter searches in invisible Higgs decays [1]. However, the associated jets will carry low transverse momentum and thus appear at high pseudo-rapidities in the detector. This sets the requirements of the detector to cover \(\upeta \) ranges of up to 6, which corresponds to a distance from the beam-pipe of 2 cm at a distance of 10 m to the interaction point. Therefore a forward tracking system is proposed to cover \(\upeta > 2.5\) to \(\upeta = 6\), see Fig. 1. This requires two additional solenoids, which provide a magnetic field of 4 T.
The ultimate running scenario reaches a luminosity of \(30\times 10^{34}\) cm\(^{-2}\)s\(^{-1}\); with a bunch spacing of 25 ns, and an average of 1,000 proton-proton collisions. After 25 years of operation this sums up to approx. 30 ab\(^{-1}\).
One of the challenges arising from these conditions is the need to resolve the pile-up events that are on average expected to be separated by 170 \(\upmu \)m in space and 0.5 ps in time. This sets the requirements on the spacial and timing resolution of the tracking system and becomes especially challenging for \(\upeta >2.5\) [3].
Another challenge are the large data rates, produced by the large amount of channels of the detector systems and the expected high occupancy. Investigations on how to deal with hundreds of TB/s started within the collaboration.
Additionally, most of the instruments have to withstand extreme radiation. The expected radiation levels are shown in terms of 1 MeV neutron equivalent fluence in Fig. 2, which illustrates the need for a 50 times radiation harder inner pixel detector than planned for the High-Luminosity upgrade of the LHC.
2.1 FCC-hh Calorimeters
The FCC-hh reference calorimeter system consists of electromagnetic and hadronic sections, in the Barrel and extended Barrel (B and EB) \(\upeta < 1.5\), Endcap (EC) \(1.5< \upeta < 2.5\), and Forward (F) \(\upeta > 2.5\) regions. In the areas most strongly exposed to radiation, the reference detector is equipped with Liquid Argon (LAr) calorimeters (based on lead absorbers and LAr as active material) [7], while the outer barrel regions are covered by a Scintillator-Stainless Steel hadron calorimeter based on the ATLAS TileCal design [7].
This combined system has proven good performance in the ATLAS experiment, however for the new energy frontier reachable with the FCC, the calorimeter technologies have to be further developed to provide the necessary performance in jet reconstruction.
This requires higher transverse and longitudinal granularity for particle shower separation, reconstruction algorithms e.g. particle flow, and pile-up rejection.
Additionally, the depth in terms of nuclear interaction lengths has to be sufficient to provide full containment of hadrons with energies up to 10 TeV [8].
To achieve the best possible energy resolutions for single particles and jets at very high energies, the constant term \(\hbox {b}\) of the energy resolution, following Eq. 1, has to be kept as small as possible.
The constant term is dominated by shower leakage and uncertainties on the calibration, while the stochastic term a is dominated by the sampling fluctuations of a calorimeter and additionally increase in hadronic calorimeters in case of non-compensation and the fluctuations in the invisible shower components.
For full simulations of the FCC-hh reference detector a new software framework is in development [9], which currently includes the tracking and calorimeter system in the Barrel and Endcap regions. The material budget is shown in Fig. 3 and shows the \(\upeta \) coverage up to 2.5. Figure 3(b) illustrates that a coverage of 11 \(\uplambda \) is achieved, except for around \(\upeta \thicksim 1.7\). In order to overcome this depth deficit an increase in z of the length of the HCAL EB of 50 cm is needed and will be implemented in the new detector layout. The number of radiation lengths, see Fig. 3(a) are especially important for the muon detection outside the solenoids. The degradation of the muon momentum measurement by multiple scattering in the calorimeters is currently under investigation.
2.2 Electromagnetic Calorimeter
In terms of energy resolution, the electromagnetic calorimeter is required to resolve electrons and photons with at least a precision of \(\frac{10\,\%}{\sqrt{\hbox {E}}}\oplus 1\,\%\) to achieve a precise Higgs mass measurement for \(\hbox {H}\rightarrow \upgamma \upgamma \). Additionally, the non-linearities of the calorimeter have to be on the sub-percent level to sufficiently suppress the systematic uncertainties on the physics searches.
The reference LAr ECAL Barrel calorimeter is tested in simulation using lead plates as absorbers. The geometry of the ECAL layers is simplified compared to the ATLAS design by arranging the active layers with an inclination angle of \(30^\circ \). By this the longitudinal segmentation can be increased, which results in an increase of the sampling fraction with the radius and requires a correction within the calibration. After the calibration and a correction of the lost energy within the cryostat providing the cooled Argon, the single electron reconstruction achieves and exceeds the required energy resolution, see Fig. 4(a). However, the effects of electronic noise and noise originating from the large number of pile-up events is not yet included in the simulations.
2.3 Hadronic Calorimeter
The hadronic calorimeter for a FCC-hh experiment has to fulfil three major tasks; First it has to reconstruct jets in forward regions up to \(\upeta =6\). Second, it needs to contain the high-energy hadron showers, which requires a thickness of the whole calorimeter system of \({\approx }11\uplambda \). And third, the transverse and longitudinal granularity has to be high enough to distinguish between close by, boosted objects.
The reference hadron calorimeter is a Scintillator-Steel sampling calorimeter, based on the ATLAS design, with an increased granularity to \(\Delta \upeta \times \Delta \upvarphi = 0.025\times 0.025\) and 10 longitudinal layers. This design is currently under test within full simulations and achieves an energy resolution (with a depth of \(9.3\,\uplambda \) at \(\upeta =0.36\)) for single pions up to 10 TeV of \(\frac{\upsigma _{\mathrm{E}}}{\hbox {E}}=\frac{42.4\,\%}{\hbox {E}}\oplus 3.3\,\%\). This calorimeter is non-compensating (\(\hbox {e}/\hbox {h}=1.23\)) which intrinsically degrades the energy resolution of hadronic showers, but it also motivates the application of algorithms that re-weight the calorimeter cells to optimise the calorimeter resolution. Thus further improvement can be expected using so-called software compensating algorithms.
The combined calorimeter system has been tested for the energy reconstruction of single pions with both systems calibrated to the EM scale. The energy resolution and non-linearity, without further corrections for the dead material between the calorimeters nor for the non-compensation, are shown in Fig. 4(b) and compared to the performance of the HCAL only. The large non-linearity is expected due to the energy losses in the cryostat of the ECAL and can be eliminated by additional corrections applied during the energy reconstruction.
2.4 Technological Options
Additional technological options for the calorimeters are currently taken under consideration.
For the ECAL parts of the FCC-hh detector a Silicon-Lead sampling calorimeters following the CMS Phase II Endcap upgrade design [10] is planned to be further tested in simulations and the experiences and performance results within the CMS collaboration will be followed with high interest. Another option for the ECAL is also based on Silicon-Lead technology, however considering digital signal information of Monolithic Active Pixel Sensors (MAPS). This technology is also considered for the forward calorimeter upgrade for the ALICE experiment and first simulation studies for the FCC-hh experiment have started and show promising results. However, these Silicon based technologies can only be considered in the barrel regions of the FCC-hh detector due to radiation tolerances.
An option for the hadronic calorimeter in the barrel region is the Scintillator-Steel sampling calorimeter following the CALICE Analogue Hadron Calorimeter (AHCAL) design, that has a classical sampling geometry with an integrated readout. This technology has been developed for the future electron-positron collider ILC, but is currently being adjusted for the CMS experiment which again rises the attention within the FCC-hh collaboration.
3 Outlook
The FCC-hh detector design is based on the experiences from the running LHC experiments, including adjustments to the new energy regime reachable with the FCC machine. The current efforts focus on the completion of the conception design report to provide the input for the next European strategy update in 2019.
The simulation of the reference calorimeter system of the FCC-hh experiment has proven good performance for single particle reconstruction. Further optimisation studies concerning material, total depth, \(\upeta \) coverage and geometry are on-going.
Finally the system has to demonstrate good performance in the high pile-up environment and in jet reconstruction, especially for boosted topologies.
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Neubüser, C., on behalf of the FCC-hh Detector Working Group. (2018). Performance Studies and Requirements on the Calorimeters for a FCC-hh Experiment. In: Liu, ZA. (eds) Proceedings of International Conference on Technology and Instrumentation in Particle Physics 2017. TIPP 2017. Springer Proceedings in Physics, vol 213. Springer, Singapore. https://doi.org/10.1007/978-981-13-1316-5_7
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