Next generation detectors to look beyond the standard model The discovery of the Higgs Boson at the Large Hadron Collider (LHC) at CERN represented an exciting step forward in particle physics, now further experimentation is being conducted and searches made for new physics beyond the standard model. That will require improved detectors, explains Professor Sir Tejinder Virdee A high-energy physics experiment is typically shaped like a cylindrical onion, with four principal layers, which together allow researchers to measure the energy, direction and identity of the particles produced in a collision. “The particles go through these layers: the first one, submerged in a magnetic field, measures the curvature of charged tracks and the next two measure the energy of particles. The first one of the energy measuring layers is called the electromagnetic calorimeter, the second layer is the hadronic calorimeter,” explains Professor Sir Tejinder Virdee. “The particles that go through these – like electrons, photons and some other charged and neutral particles – create showers of secondary particles by interacting inside the dense material”. Based at Imperial College in the UK, Professor Virdee is the Principal Investigator of an ERC-backed project developing a novel approach to calorimetry. “We propose an approach to energy measurement that combines state of the art techniques so far only used independently, either in charged particle tracking or in conventional calorimeters,” he outlines. The proposed calorimeter is based on large-scale use of silicon sensors with fine cell size. In addition to the measurement of the energy of high-energy particles it is envisaged to use information about their precise timing and the path they follow to provide sufficient information to be able to cope with the extreme rates associated with the High Luminosity LHC (HL-LHC). Amongst the technologies to be advanced are powerful electronics in emerging fine-feature size technologies; low-cost silicon sensors in 8” wafer technology; high performance and fast decision making logic using new more powerful Field Programmable Gate Arrays (FPGA’s), all to be produced at an industrial scale.
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One hexagonal silicon sensor module comprising a silicon sensor mounted on a copper/tungsten backing plate topped with the PCB that has mounted on it the front-end electronics. In this test the silicon sensor is cut out from a 6” diameter wafer. Particles usually deposit energy by either exciting or ionising the atoms in the traversed material. “Excited atoms emit light upon de-excitation, and the total amount of light picked up is proportional to the energy of the incident particle,” says Professor Virdee. In ionisation, the electron breaks free from the atomic bond. “As the ionisation electrons move they induce a current in the detecting medium, which is picked up by a very sensitive amplifier which amplifies the signal. That signal is again proportional to the energy of the incident particle,” explains Professor Virdee. “We’re developing a highperformance calorimeter that will also be suitable for the next generation of High Energy Physics (HEP) experiments.”
New physics This work has been prompted to a large degree by the search for new physics beyond the standard model (SM) of particle physics. Earlier in his career Professor Virdee was instrumental in developing the lead tungstate scintillating crystal
electromagnetic calorimeter used in the CMS experiment at CERN, in which the Higgs Boson was experimentally discovered. “The Higgs Boson can be detected when it decays into two photons, and these photons deposit energy in the electromagnetic calorimeter,” he explains. This discovery has opened up a window into new physics which researchers are keen to explore. When a new particle is discovered the next step is to study it and learn more about the context in which nature has presented it. “For that, we really need to make the particular particle in very large numbers,” continues Professor Virdee. “In order to do that, in 2015 the energy of the LHC accelerator was increased, so that we now make twice as many Higgs bsoons per proton-proton interaction. At the HL-LHC it is foreseen to increase the interaction rate by a factor five and the total number of interactions examined by a factor of ten compared with the original plan.” Despite its immense success in describing all current measurements at the LHC, it is known that the SM is incomplete;
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