In 2010, the brand-spanking-new CMS and ATLAS detectors started taking data for the first time. But the question physicists asked was not, “Where is the Higgs boson?” but rather “Do these things actually work?”
“Each detector is its own prototype,” says UCLA physicist Greg Rakness, run coordinator for the CMS experiment. “We don’t get trial runs with the LHC. As soon as the accelerator fires up, we’re collecting data.”
So LHC physicists searched for a few old friends: previously discovered particles.
“We can’t say we found a new particle unless we find all the old ones first,” says Fermilab senior scientist Dan Green. “Well, you can, but you would be wrong.”
Rediscovering 50 years' worth of particle physics research allowed LHC scientists to calibrate their rookie detectors and appraise their experiments’ reliability.
The CMS collaboration produced this graph using data from the first million LHC particle collisions identified as interesting by the experiment's trigger. It represents the instances in which the detector saw a pair of muons.
Muons are heavier versions of electrons. The LHC can produce muons in its particle collisions. It can also produce heavier particles that decay into muon pairs.
On the x-axis of the graph is the combined mass of two muons that appeared simultaneously in the aftermath of a high-energy LHC collision. On the y-axis is the number of times scientists saw each muon+muon mass combination.
On top of a large and raggedy-looking half-parabola, six sharp peaks emerge.
“Each peak represents a parent particle, which was produced during the collision and then spat out two muons during its decay,” Green says.
When muon pairs appear at a particular mass more often than random chance can explain, scientists can deduce that there must some other process tipping the scale. This is how scientists find new particles and processes—by looking for an imbalance in the data and then teasing out the reason why.
Each of the six peaks on this graph can be traced back to a well-known particle that decays to two muons.