The simultaneous production of a Higgs boson and a top/antitop quark pair has just been observed for the first time. For physicists, observing this affinity is an important advance in understanding the origins of mass. Christophe Delaere, FNRS research professor and professor at the Institute for Research in Mathematics and Physics (IRMP), who participated in the discovery, helps us understand.
The Higgs, the quark and mass: three seemingly distinct realities connected by a strong affinity. Let’s take a quick tour of all three.
We all have an idea of what an object’s mass (as opposed to its weight) is, simply through association with the inertia that must be overcome to move the object. For physicists, however, many questions about mass remain. Where does it from? Is it the simple sum of the mass of particles that make up an object? Does it change when the object moves or its particles move?
We owe the answers to some of these questions to Albert Einstein, who in 1905 stated that mass corresponds to the internal energy of particles and does not vary with their speed. This is his famous equivalence between the mass and energy of a body at rest, translated into the well-known equation E = mc2. The equation has little impact on our daily lives but a lot on particles. For example, at our scale, calculating the total mass of an object with different constituents is straightforward (the mass of a full beer bottle is equal to the mass of the empty bottle plus that of the beer), but this is not the case when considering atoms or particles. The energies of the system’s various internal constituents must also be taken into account. Thus a hydrogen atom’s mass is made up of the masses of its constituents (mass of the proton plus mass of the electron) minus the binding energy between them, thus the equation MH = mp + me – binding energy. Hydrogen has a mass less than the sum of its constituents, which is also the reason for its stability. Is this ‘correction’ large? It amounts to about 1/100,000,000 of the sum of the masses of the constituents. In the case of hydrogen, this represents energy of only 13.6 electronvolt, which is nothing to write home about, except that it represents all the energy released, for example, during a chemical reaction.
Let’s ‘descend’ in order of magnitude from atoms to nuclei. The mass of a nucleus will be equal to the sum of the masses of its protons and neutrons minus the binding energies. The difference is these energies no longer represent a correction of a hundred-millionth but rather of a thousandth. They are therefore 100,000 times greater than in the case of the atom. Hence we’ve gone from chemical energy to nuclear energy, and we’re well aware of the latter’s potential destructiveness.
Mass and the quark
Let’s ‘descend’ again and see what happens in the proton. It’s composed of three elementary particles, all quarks (two up quarks and one down quark). These three quarks are linked by a very bizarre strong interaction that increases with distance – the farther apart we separate quarks, the more they attract each other. So it’s impossible to isolate a quark for the purpose of accurately measuring its mass. Yet physicists have managed to measure it via theoretical calculations. The verdict: the sum of the masses of quarks that make up the proton is only about 5% of the proton’s mass. Nearly all of the remaining 95% comes from the interaction (movement) – between quarks and the massless mediators known as gluons – that binds quarks together.
Mass, quark and Higgs
Thus energy makes up most of the mass of elementary particles – but, as implied above, not all of it. Theoretical models, including those of Peter Higgs and our compatriots Robert Brout and François Englert, had postulated the existence of another particle, a boson, which since its 2012 discovery by European Organization for Nuclear Research (CERN) teams has been known as the Brout-Englert-Higgs boson. Its discovery led to confirming the existence of a corresponding field (in the dual quantum particle/wave world, a field is associated with a particle and a magnetic field is associated with a photon, etc.). And it’s by interacting with this field that particles acquire (or do not acquire) their mass. ‘This is a fundamental mechanism’, explains Prof. Delaere. ‘Without the Higgs field, other mass mediators, such as the W boson responsible for beta decay in stars, would not exist, in which case our sun would have disappeared a long time ago. And an electron with zero mass obviously couldn’t revolve around a proton and therefore could never form an atom: all the particles would move at the speed of light and we would have a “gas” of particles that interact very little. The creation of atoms, stars – all matter – are based on the mechanism that gives their mass to elementary particles.’
Still, very little is known about the Higgs boson; getting to know it better is one of the missions of the Large Hadron Collider (LHC), CERN’s particle accelerator. The heavier a particle, the more it will interact with the Higgs field. It was therefore tempting to try to study the Higgs boson using the heaviest particle, the top quark. But the particle is so heavy that it’s impossible for the Higgs boson to disintegrate into two top quarks. What’s the alternative? Provoking the appearance of Higgs bosons in combination with two top quarks (or rather a top quark and an antitop quark), a phenomenon called ttH production that was predicted by theory but is unfortunately very rare in practice, as only 1% of Higgs bosons are produced this way. Thus scientists of CERN’s CMS and ATLAS collaborations smashed together protons – a lot of protons, in more than 150 billion collisions – and presto: they observed the simultaneous production of the Higgs boson and two quarks as well as their ‘coupling’, or the interaction between the two types of particles(1).
‘This tells us about the force between quark and Higgs boson’, Prof. Delaere explains. ‘Our goal is to try to understand how the two interact, in order to get information about possible, even heavier, particles that we are unable to observe at the moment and that could also connect to the boson.’ The ultimate goal is to find what’s faulty in the Standard Model and embark on a path toward a new physics. This is not yet the case: the measurements and observations published on 4 June fit perfectly into the Standard Model.
And now? ‘We’ve measured the probability of ttH production with an accuracy of 30%,’ Prof. Delaere says, ‘but we continue to collect data and hope to be able to significantly reduce the measurement’s uncertainty in a few years to really be sure of the agreement with the Standard Model. Thirty per cent uncertainty remains a big unknown and doesn’t yet allow us to say very specific things about the mechanism of mass and the role of the Higgs. So we’re pushing the limits, but we’re not closing the door to new physics.
UCL has a group of about 30 people involved in the CMS collaboration. ‘This group has a particular feature’, Prof. Delaere explains. ‘It’s composed of both theorists and experimentalists who are engaged in an ongoing dialogue.’ Thus Prof. Fabio Maltoni is credited with having proposed, some 15 years ago, exploring the path of ttH production. For his part, Andrea Giammanco has coordinated all of the top quark physics for several years.
Meanwhile, Giacomo Bruno, Vincent Lemaître and Prof. Delaere are interested in the physics of Higgs bosons and particularly the production of pairs of bosons, which allows them to understand the coupling of the boson with itself. They also see to the detector’s operation, particularly the construction of the trajectography, which is at the centre of the CMS and allows for reconstructing the trajectories of charged particles released by collisions.
(1)Observation of ttH production, CMS collaboration, Physical Review Letters, 2018.
A glance at Christophe Delaere's bio
2017: FNRS Senior Research Associate
2009-17: FNRS Research Associate
2009: Professor, Institute for Research in Mathematics and Physics (IRMP), UCL
2008-09: FNRS Postdoctoral Researcher
2006-08 : Scientific Fellow, European Organization for Nuclear Research (CERN), Geneva
2005-06: Postdoctorate, Institut de Physique nucléaire (FYNU), UCL
2002: Belgian Physical Society Award
2001-05: PhD, FNRS Candidate, Institut de Physique nucléaire (FYNU), UCL,
thesis on the decay of the Higgs boson of the standard model in both the ALEPH and CMS experiments.