Beauty and Strangeness in Particle Physics

Professor Cristina Lazzeroni grew up in a small, walled town in Tuscany, where a schoolteacher told her she would ‘never be good at science’. Today, far from the confines of Lucca’s historic ramparts, she leads the UK teams on two CERN experiments that could reveal new physics and unlock some of the remaining mysteries of the Universe.

Just this month it was announced that one of these experiments, the Large Hadron Collider beauty (LHCb), had discovered three new ‘exotic’ particles (so-called because they are made of four quarks rather than the usual two or three) and the confirmation of the existence of a fourth one in data from the LHC. These new particles, or tetraquarks, are highly unstable, decaying almost immediately into other particles.

Cristina, who is Professor in Particle Physics, brims with excitement at the prospect of further, even more momentous revelations – despite being something of an old hand at scientific discovery.

‘The experiment I joined immediately after coming to the UK – called NA48 –discovered direct CP-violation (one aspect of the mechanism that breaks the symmetry between the behaviour of matter and antimatter), which was amazing,’ she recalls. ‘In the following years, we measured the rarest decay of a strange particle at that time. So I’ve developed a bit of a taste for discoveries! And now we are going to measure one ever rarer.’

The focus of Cristina’s current research – which she outlined in her recent Inaugural Lecture, ‘Beauty and Strangeness in Particle Physics’ – is the study of the behaviour of particles containing the beauty and strange quarks in the LHCb and NA62 experiments at CERN. She and her UK team were involved in building part of the NA62 detector and are carrying out data analysis from both experiments.

‘My research is about trying to understand how the Universe is made from the building blocks of Nature. The Standard Model (SM) of Particle Physics describes particles’ behaviour and their interactions, and has been very successful in matching experimental observations. But there are several shortcomings; for example, the difference in behaviour between matter and antimatter is far too small to explain the prevalence of matter in the Universe; and there is no explanation for dark matter.

‘So the two CERN experiments are investigating how particles containing the beauty and the strange quarks behave, and what we can learn from them about matter-antimatter asymmetry and the possible existence of new particles never seen before. These are exciting times, when experiments are pushing hard towards extraordinary levels of precision and sensitivity that make new discoveries possible. Hopefully we will get insights that allow us to resolve some of the shortcomings in our understanding of the particle world.’

Cristina’s journey into Particle Physics began at school in Italy nearly 30 years ago.

‘It’s difficult to say exactly why I decided to study Physics at university, but certainly there were two key factors,’ she recalls. ‘Lucca, where I grew up, is a beautiful town with big walls, but one that I felt didn’t have much push for novelty. From when I was young, I wanted to see what was beyond those walls. I felt that studying Physics would help my need to explore the world and pursue progress and novelty. Also, I had a teacher – a woman – who told me I was never going to be good at science. I am known for not doing what I’m told unless there’s a good reason – I will do the opposite.’

In her third year at the University of Pisa, a professor asked if Cristina would be interested in doing a PhD in Particle Physics. As soon as she completed it, she left Italy for the UK, taking up her first post-doctorial position at Edinburgh. After two years working on an antimatter experiment, she moved to Cambridge. During her seven years there, Cristina was awarded a Royal Society University Fellowship, which in 2007 she brought with her to Birmingham.

As well as continuing with antimatter research, Cristina joined the ALICE (A Large Ion Collider Experiment) at CERN. ‘But as soon as I arrived there, I knew that what I wanted to do was to start a new experiment. I initiated the UK enterprise in NA62, which is a follow-up experiment in strange quarks. It was a long process and we got a large grant from the EU to build a part of the detector, which is the UK’s responsibility. We are still leading that. We also lead the data analysis, and we have other grants to progress with that until 2019, when we will apply for more funding.’

Cristina’s ultimate ambition is to discover new physics – and she believes that the best chance of finding it is with a multi-layered approach.

‘What makes the news headlines is mostly the LHC, which looks directly for new particles. There is reason to believe that there are new particles – dark matter we don’t know about; that the SM can’t explain,’ she says. ‘Although ATLAS found the Higgs boson, this was long predicted, so it’s not really given us any stunning new physics yet. This is why a multiple approach is a good idea.

‘The LHCb looks for quite subtle deviation from the SM, so you’re looking at a small deviation in rare processes. This allows you to access a much higher energy indirectly. That’s why LHCb and ATLAS are complementary. To have a wide scan of the horizon is useful. It’s also very important, especially now when everybody is sure there is new physics, but no one knows where it is or what it is.

‘Even after we find something, we have to investigate exactly what it is, so the nature of this new physics needs a multiple approach. When you’re facing something new but unknown, you need to cast your net wide.’

The main aim of the NA62 experiment is to study rare kaon decays. Understanding these decays will help physicists to check some of the predictions the SM makes about short-distance interactions.

‘We plan to run NA62 until 2018, which should give us enough data to see a particular process known as the “golden channel” for new physics,’ explains Cristina. ‘Several models for new physics predict there’s going to be a deviation from the SM, so we should be able to give an answer on that. But we are also looking for something completely exotic – something that isn’t foreseen at all in the SM.

‘Both LHCb and NA62 are very promising. It would be absolutely amazing if we could see something new.’