Experiments get ready for the HL-LHC.
Le programme d’amélioration pour ATLAS et CMS passe à la phase suivante
Le projet LHC haute luminosité permettra également d’étendre le programme de physique des expériences. Des éléments clés des détecteurs devront être remplacés pour que ces instruments puissent traiter l’empilement d’interactions proton-proton – 140 à 200 en moyenne par croisement de paquets. En octobre 2015, le Comité d’examen des ressources du CERN a confirmé que les collaborations peuvent à présent élaborer des rapports de conception technique (TDR). Le franchissement de cette première étape du processus d’approbation est un grand succès pour les expériences ATLAS et CMS.
At the end of the third operational period in 2023, the LHC will have delivered 300 fb–1, and the final focussing magnets, installed at the collision points at each of four interaction regions in the LHC, will need to be replaced. By redesigning these magnets and improving the beam optics, the luminosity can be greatly increased. The High Luminosity LHC (HL-LHC) project aims to deliver 10 times the original design integrated luminosity (number of collisions) of the LHC (CERN Courier December 2015 p7). This will extend the physics programme and open a new window of discovery. But key components of the experiments will also have to be replaced to cope with the pile-up of 140–200 proton–proton interactions occurring, on average, per bunch crossing of the beams. In October 2015, the ATLAS and CMS collaborations met a major milestone in preparing these so-called Phase II detector upgrades for operation at the HL-LHC, when it was agreed at the CERN Resource Review Board that they proceed to prepare Technical Designs Reports.
New physics at the HL-LHC
The headline result of the first operation period of the LHC was the observation of a new boson in 2012. With the present data set, this boson is fully consistent with being the Higgs boson of the Standard Model of particle physics. Its couplings (interaction strengths) with other particles in the dominant decay modes are measured with an uncertainty of 15–30% by each experiment, and scale with mass as predicted (see figure 1). With the full 3000 fb–1 of the HL-LHC, the dominant couplings can be measured with a precision of 2–5%; this potential improvement is also shown in figure 1. What’s more, rare production processes and decay modes can be observed. Of particular interest is to find evidence for the production of a pair of Higgs bosons, which depends on the strength of the interaction between the Higgs bosons themselves. This will be complemented by precise measurements of other Standard Model processes and any deviations from the theoretical predictions will be indirect evidence for a new type of physics.
In parallel, the direct search for physics beyond the Standard Model will continue. The theory of supersymmetry (SUSY) introduces a heavy partner for each ordinary particle. This is very attractive in that it solves the problem of how the Higgs boson can remain relatively light, with a mass of 125 GeV, despite its interactions with heavy particles; in particular, SUSY can cancel the large corrections to the Higgs mass from the 173 GeV top quark. According to SUSY, the contributions from ordinary particles are cancelled by the contributions from the supersymmetric partners. The presence of the lightest SUSY particle can also explain the dark matter in the universe. Figure 2 compares the results achievable at the LHC and HL-LHC in a search for electroweak SUSY particles. Electroweak SUSY production has a relatively low rate, and benefits from the factor 10 increase in luminosity. Particles decaying via a W boson and a Z boson give final states with three leptons and missing transverse momentum.
Other “exotic” models will also be accessible, including those that introduce extra dimensions to explain why gravity is so weak compared with the other fundamental forces.
If a signal for new particles or new interactions begins to emerge – and this might happen in the second ongoing period of the LHC operation, which is running at higher energy compared with the first period – the experiments will have to be able to measure them precisely at the HL-LHC to distinguish between different theoretical explanations.
To achieve the physics goals, ATLAS and CMS must continue to be able to reconstruct all of the final-state particles with high efficiency and low fake rates, and to identify which ones come from the collision of interest and which come from the 140–200 additional events in the same bunch crossing. Along with this greatly increased event complexity, at the HL-LHC the detectors will suffer from unprecedented instantaneous particle flows and integrated radiation doses.
Detailed simulations of these effects were carried out to identify the sub-systems that will either not survive the high luminosity environment or not function efficiently because of the increased data rates. Entirely new tracking systems to measure charged particles will be required at the centre of the detectors, and the energy-measuring calorimeters will also need partial replacement, in the endcap region for CMS and possibly in the more forward region for ATLAS.
The possibility of efficiently selecting good events and the ability to record higher rates of data demand new triggering and data-acquisition capabilities. The main innovation will be to implement tracking information at the hardware level of the trigger decision, to provide sufficient rejection of the background signals. The new tracking devices will use silicon-sensor technology, with strips at the outer radii and pixels closer to the interaction point. The crucial role of the tracker systems in matching signals to the different collisions is illustrated in figure 3, where the event display shows the reconstruction of an interaction producing a pair of top quarks among 200 other collisions. The granularity will be increased by about a factor of five to produce a similar level of occupancies as with the current detectors and operating conditions. With reduced pixel sizes and strip pitches, the detector resolution will be improved. New thinner sensor techniques and deeper submicron technologies for the front-end read-out chips will be used to sustain the high radiation doses. And to further improve the measurements, the quantity and mass of the materials will be substantially reduced by employing lighter mechanical structures and materials, as well as new techniques for the cooling and powering schemes. The forward regions of the experiments suffer most from the high pile-up of collisions, and the tracker coverage will therefore be extended to better match the calorimetry measurements. Associating energy deposits in the calorimeters with the charged tracks over the full coverage will substantially improve jet identification and missing transverse energy measurements. The event display in figure 4 shows the example of a Higgs boson produced by the vector boson fusion (VBF) process and decaying to a pair of τ leptons.
The calorimeters in ATLAS and CMS use different technologies and require different upgrades. ATLAS is considering replacing the liquid-argon forward calorimeter with a similar detector, but with higher granularity. For further mitigation of pile-up effects, a high-granularity timing detector with a precision of a few tens of picoseconds may be added in front of the endcap LAr calorimeters. In CMS, a new high-granularity endcap calorimeter will be implemented. The detector will comprise 40 layers of silicon-pad sensors interleaved with W/Cu and brass or steel absorber to form the electromagnetic and hadronic sections, respectively. The hadronic-energy measurement will be completed with a scintillating tile section similar to the current detector. This high-granularity design introduces shower-pointing ability and high timing precision. Additionally, CMS is investigating the potential benefits of a system that is able to measure precisely the arrival time of minimum ionising particles to further improve the vertex identification for all physics objects.
The muon detectors in ATLAS and CMS are expected to survive the full HL-LHC period; however, new chambers and read-out electronics will be added to improve the trigger capabilities and to increase the robustness of the existing systems. ATLAS will add new resistive plate chambers (RPC) and small monitored drift tube chambers to the innermost layer of the barrel. The endcap trigger chambers will be replaced with small-strip thin gap chambers. CMS will complete the coverage of the current RPCs in the endcaps with high-rate capability chambers in gas electron multipliers in the front stations and RPCs in the last ones. Both experiments will install a muon tagger to benefit from the extended tracker coverage.
The trigger systems will require increased latency to allow sufficient time for the hardware track reconstruction and will also have larger throughput capability. This will require the replacement of front-end and back-end electronics for several of the calorimeter and/or muon systems that will otherwise not be replaced. Additionally, these upgrades will allow the full granularity of the detector information to be exploited at the first stage of the event selection.
Towards Technical Design Reports
To reach the first milestone in the approval process agreed with the CERN scientific committees, ATLAS and CMS prepared detailed documentation describing the entire Phase II “reference” upgrade scope and the preliminary planning and cost evaluations. This documentation includes scientific motivations for the upgrades, demonstrated through studies of the performance reach for several physics benchmarks and examined in 140 and 200 collision pile-up conditions. The performance degradation with two scenarios of reduced cost, where the upgrades are descoped or downgraded, was also investigated. After reviewing this material, the CERN LHC Committee and the Upgrade Cost Group reported to the CERN Research Board and the Resource Review Board, concluding that: “For both experiments, the reference scenario provides well-performing detectors capable of addressing the physics at the HL-LHC.”
The success of this first step of the approval process was declared, and the ATLAS and CMS collaborations are now eager to proceed with the necessary R&D and detector designs to prepare Technical Design Reports over the next two years.
• For further details, visit https://cds.cern.ch/record/2055248, https://cds.cern.ch/record/2020886 and https://cds.cern.ch/record/2055167/files/LHCC-G-165.pdf.