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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At a glance Full Project Title Exploring the Terascale at LHC with Novel Highly Granular Calorimeters (Novel Calorimetry)
amongst the questions for which answers are being sought are: What constitutes dark matter? Are there extra dimensions of space? Why is the universe composed of matter and not antimatter? Professor Virdee says that elements of the existing experiment will not be able to function effectively in the new environment. “The replacement elements will have to function with a five-times larger flux of particles going through, and ten times higher radiation levels,” he outlines. The volume of information gathered by the detector is another major consideration. “We need to be able to separate the interesting collisions from the less interesting collisions,” says Professor Virdee. “For every interesting collision there are typically
Next generation detectors These high-performance calorimeters will play a key role in the next generation of detectors. A more powerful instrument will allow researchers to analyse collisions in greater detail, and gain new insights into major questions in physics, including those around dark matter. While much has been achieved over the years at the LHC, Professor Virdee is a strong advocate of continued development, which is central to further experimental advances. “As experimentalists we keep an open mind,” he says. “The hope is to find something completely new, that enhances our knowledge, and guides future research.”
The particles that go through detectors – like electrons, photons and some other charged particles – create showers of other particles by interacting inside the material about 140 less interesting ones superposed.” “When you design new instruments you start from the science you wish to explore and sort out the basic concepts, allowing a degree of design flexibility so that you can benefit from technological advances,” explains Professor Virdee. One area being closely monitored is the development of FPGAs, a commercially available integrated circuit technology that Professor Virdee says could be used to help identify interesting particle collisions. “The performance of these FPGAs is improving. We would like to use the latest generation of FPGAs when we actually build the calorimeter” he says. This is still a few years away. Currently researchers are looking more to prototype the key technologies to be used in the calorimeter, while also remaining open to further potential improvements.
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There are many unanswered questions in physics, including around the fundamental theoretical foundations. The two pillars of modern physics are Einstein’s general theory of relativity and quantum mechanics, yet they cannot co-exist in extreme conditions. “That means we don’t have a unified understanding of nature, where all the forces of nature are considered to be unified,” explains Professor Virdee. Many different conjectures and theories have been put forward and sophisticated technology is essential to experimentally verify or refute them, underlining the importance of the continued development of research infrastructure. “This instrument that we are developing will be able to see things that the current generation in the LHC can’t,” stresses Professor Virdee.
Project Objectives This ERC proposal deals with a novel approach to calorimetry that combines state of the art techniques so far only used independently either in charged particle tracking or conventional calorimeters. New technologies are being developed, including powerful, radiation hard electronics using feature sizes of 130 nm or 65 nm; low-cost silicon sensors using 8” silicon wafers; environmentallyfriendly cooling technologies using liquid C02; high performance and fast decision making logic using new more powerful FPGA’s, all to be produced at an industrial scale. The approach has been chosen by the LHC-CMS experiment for its upgrade of endcap calorimeters. Project Funding ERC ADG Advanced Grant Project Partners • CERN with staff contributions Contact Details Professor Sir Tejinder Virdee Imperial College of Science, Technology and Medicine, South Kensington Campus, Exhibition Road, London SW7 2AZ T: +44 20 7594 7804 E: t.virdee@imperial.ac.uk W: http://cordis.europa.eu/project/ rcn/198709_en.html The Compact Muon Solenod Phase II Upgrade, Technical Proposal, CERN-LHCC-2015-010. First beam tests of prototype silicon modules for the CMS high granularity endcap calorimeter, paper in preparation. The CMS HGCAL detector for HL-LHC Upgrade, presentation by A. Martelli at the 2017 LHCP conference, Shanghai.
Professor Sir Tejinder Virdee
Sir Tejinder Virdee is Professor of Physics at Imperial College, London. He is one of the two founding fathers of the Compact Muon Solenoid experiment at the LHC. He pioneered some of the techniques used in its calorimeters (for the measurement of energies of particles) crucial for the discovery of a Higgs boson announced by the CMS experiment in July 2012, along with the sister experiment ATLAS. Virdee’s is currently developing a novel calorimetric technique for very high luminosity LHC running, due to start in mid-2020’s. He has won numerous prizes from the UK, European and American Physical Societies.
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