B-physics on the grid – a view from the frontline
Serena Oggero explains how grid computing is important for her PhD research about a particle called the beauty quark.
If you ask “hey man, shall we go for a beer later?,” how many of your friends reply with something like: “maybe tomorrow, I really have to babysit my ntuples tonight”?
...I really have to babysit my ntuples tonight.
Welcome to the happy and slightly geeky particle physics community! Happy, because I think we are a species of truly fortunate people, despite our constant scepticism and restlessness. And most of us were already slightly geeky anyway, even before starting to babysit ntuples.
In particle physics, an ntuple is a standard way of storing data. Only variables useful for a certain analysis enter the ntuple, as ‘columns’ of a table, while events are listed as rows. Producing ntuples is one of the obvious steps towards analysis optimisation and we need distributed computing resources specifically to reduce the large (large!) amount of data produced by our experiments. This is where the grid becomes essential to us – it’s where we sit, we type, we run jobs, we send and retrieve stuff, we type again and we check, we wait, send again, and wait again. In a word: where we babysit.
Above are cows, sunflowers and vineyards… Below are giant machines to accelerate protons and heavy ions almost to the speed of light.
My job-on-the-grid babysitting task is related to the search for evidence of B-meson to muon decays (Bs → µµ) at the LHCb detector at the Large Hadron Collider (LHC) – the biggest scientific experiment ever attempted, to quote Brian 'rock-star physicist' Cox. The LHC is a 27 km long circular tunnel, excavated under the Swiss-French border nearby Geneva. Above are cows, sunflowers and vineyards, awesome climbing spots and gentle ski-slopes. Below are giant machines, unique works of art and the technology we need to accelerate protons and heavy ions almost to the speed of light.
The accelerator complex that culminates with the LHC is in fact more complicated than a single tunnel. Everything starts from a tiny hydrogen bottle and develops into multiple accelerating segments. The proton beams circulate in opposite directions, moving inside vacuum and guided by superconducting magnets. They meet at only four points. Here is where the collisions happen and our eyes go sharply on focus!
Every collision creates new particles. The detectors used to look at particles for the four big CERN experiments are placed at these collision points. The LHCb experiment is named after the ‘beauty’ or b-quark and it was designed to detect the decays of B mesons – particles made of a quark (either ‘up’, ‘down’, ‘strange’ or ‘charm’) and an antiquark, namely an anti-b quark.