View of the Detector ATLAS open. © CERN
Phi meson candidates in the search for Higgs decays to a phi meson and a photon. Published in JHEP 07 (2018) 127 (arXiv:1712.02758).
How does the Higgs boson couple to lighter particles? The Higgs boson was discovered in 2012, now researchers in the ExclusiveHiggs project are looking at how it couples to light quarks. We spoke to Professor Kostas Nikolopoulos about their work in analysing particle collisions, research which could help elucidate the properties of the Higgs boson and guide the search for new physics. The Higgs boson was discovered in 2012, confirming decades-old theoretical predictions of its existence, yet much remains to be learned about its properties. As the Principal Investigator of the ExclusiveHiggs project, Professor Kostas Nikolopoulos is investigating how the Higgs boson interacts with light quarks. “We are trying to understand the properties of the Higgs boson, and in particular how it interacts with lighter particles,” he explains. The heavier a particle, the more likely it is to interact with the Higgs boson, so investigating its interactions with lighter particles is a challenging goal. “The elementary matter particles are divided into leptons, like the electron, and quarks, like the up and down that are the constituents of the proton and the neutron. These particles are organised in a family, with a first, second and third generation,” outlines Professor Nikolopoulos. “The three generations are identical to each other. But as you go from 24
the first generation to the second, the particles become heavier, and as you go from the second to the third, the particles become heavier again.” Researchers have observed the Higgs boson interacting with some of the heavier particles, such as the top quark, the bottom quark, and the tau lepton. However, there is less evidence of its interactions with the four light quarks – up, down, charm and strange – which is where Professor Nikolopoulos and his colleagues in the project come in. “At the time of the discovery, there was really no prospect of studying these interactions. We are working at the Large Hadron Collider in the project, as it’s the only place that can produce a Higgs boson. We are using the ATLAS detector in a way that is beyond what it was designed for,” he says. Around 40 million particle collisions occur every second when the LHC operates, with researchers aiming to identify the
most interesting in terms of the project’s wider goals. “We have developed a selection mechanism to identify interesting collisions. ATLAS records roughly 1,000 collisions a second, but we use less than 1 percent to collect events for exclusive decay searches,” continues Professor Nikolopoulos. The aim in the project is to sift through these data to find evidence of the Higgs boson interacting with light quarks through observations of its decay products. The Higgs boson itself can decay into a quark and anti-quark pair, yet Professor Nikolopoulos says this is not easy to distinguish from other processes. “A lot of other processes give signals that look exactly like this, so we need something that is very distinctive,” he explains. This is why attention is focused on exclusive decays, for example of the Higgs boson decaying into a phi meson. “One of the strange and anti-strange quarks radiates a photon, and then together with the other
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they make a phi meson. That’s a signal that we can look for,” says Professor Nikolopoulos. “Another way is to look directly for the production of a W or Z boson together with a Higgs boson, that subsequently decays to a charm and an anti-charm quark. These travel away from their production point before they decay, and this creates an interesting experimental signature.” This latter approach has been used to observe Higgs boson decays to bottomquarks, yet it’s less sensitive for charm quarks that travel a shorter distance before they decay. While progress has been made in terms of identifying lighter particles, at this point no single approach seems able to provide all the answers researchers are seeking in the
holds wider relevance in these terms. “The outputs of our project will help to plan these future experiments to maximise the scientific output,” says Professor Nikolopoulos. More immediately, research continues into the interactions of lighter particles with the Higgs boson. “With these studies we cover new ground in our understanding of the Higgs boson and evaluate their future potential, at the same time we explore ideas that were not in the initial proposal,” continues Professor Nikolopoulos. “For example, we are investigating what is called associated production.”
Associated production This involves looking for evidence of a Higgs boson being produced together with a charm quark. The initial collision here might involve a gluon and a charm quark, but in the final state a charm quark and a Higgs boson decay are observed in the detector. “These two are produced in association. We aim to gain new insights into the interaction of the Higgs boson with the charm quark,” explains Professor Nikolopoulos. Significant progress has been made over the course of the project, and Professor Nikolopoulos plans to investigate a couple of other ideas over the remainder of the funding term.
We are trying to understand the properties of the Higgs boson, and in particular how it interacts with
lighter matter particles. project, so Professor Nikolopoulos and his colleagues are looking to combine different approaches. “We try to investigate as many of these ideas as possible,” he outlines. This may not necessarily provide the answers to all of the questions around different particle couplings, but Professor Nikolopoulos hopes it will help researchers map a path forward. “Even for the most successful of these methods, we will need a lot of data in order to probe the standard model, because the processes we are investigating are very rare,” he says. “We will need more data to consider observing these processes. The high luminosity upgrade of the LHC will help in this respect.” The upgrade of the LHC is expected to be operational in 2027, and will open up new possibilities in terms of observing rare decay processes. Beyond that, there is also an ongoing debate about the next generation of particle colliders - the European Strategy for Particle Physics was recently updated, looking decades ahead towards the needs of future researchers, and the project’s research
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“We are also looking at how to apply these techniques in the search for new physics, for additional Higgs bosons,” he continues. “Some of the techniques that we have developed in the project can be used to search for additional Higgs bosons. For example, we have recently searched for possible Higgs Boson decays to a Z boson and a light scalar particle.” A large number of Higgs bosons need to be produced in the first place in order to pursue this research and observe these rare decays. For example, the decay of the Higgs boson to a phi meson and a photon is expected to happen only twice for every million bosons. “We need to produce huge numbers of Higgs bosons, because these are very rare processes,” stresses Professor Nikolopoulos. This is a technically challenging area of research, yet that is a large part of the attraction for Professor Nikolopoulos and his colleagues. “We want to learn as much as we can from the data that we have available, and then that will help pave the way for future experiments,” he says.
ExclusiveHiggs Search for New Physics in First and Second Generation Quark Yukawa Couplings through Rare Exclusive Decays of the Observed Higgs Boson Project Objectives
This project tackles for the first time, in a systematic and comprehensive way, the experimentally most unconstrained sector of the SM: the couplings of the light-quarks (up, down, charm and strange) to the Higgs boson, including possible flavour-violating interactions. Searches for the rare exclusive Higgs boson decays to a meson and a photon or Z boson, a novel and unique approach, are performed with the ATLAS detector at the CERN Large Hadron Collider. At the same time, an extensive set of measurements of analogous rare exclusive decays of the W and Z bosons is performed, further enhancing the scientific value of the proposed research programme. Moreover, the project scope has expanded to investigate further new and innovative approaches within this new field of study. These include direct searches for associated Higgs boson production with a W or Z boson, with subsequent Higgs boson decay to a charm—anti-charm quark pair, and associated production of a Higgs boson with a charm quark.
Project Funding
This project is funded by a European Research Council Starting Grant under H2020-EU.1.1. - EXCELLENT SCIENCE
Contact Details
Project Coordinator, Kostas Nikolopoulos Professor of Physics School of Physics and Astronomy University of Birmingham Edgbaston, Birmingham B15 2TT, United Kingdom T: +44 (0) 121 414 4627 E: k.nikolopoulos@bham.ac.uk W: http://exclusivehiggs.eu Professor Kostas Nikolopoulos
Kostas Nikolopoulos is Professor of Physics at the University of Birmingham. He is an experimental particle physicist, focusing on the study of electroweak symmetry breaking and the Higgs sector. Recently, his research interests have expanded to another major open question in physics: the nature of Dark Matter, through both direct and collider-based searches. Professor Nikolopoulos was recently honoured with the first ERC Public Engagement with Research award. One of three recipients of the inaugural award, Professor Nikolopoulos was honoured for his work in public outreach, which included art exhibitions, dance performances and workshops with students.
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