Q&A With Scientists From the World’s Largest Science Experiment

A Higgs boson decaying into 2 tau leptons (azure cones), which subsequently decay into either an electron (blue line) or a muon (red line). Credit: ATLAS/CERN

A Higgs boson decaying into 2 tau leptons (azure cones), which subsequently decay into either an electron (blue line) or a muon (red line). Credit: ATLAS/CERN

Scientists from the world’s largest science experiment participated in an ‘Ask Me Anything’ on reddit on May 28. They were engineers and physicists working on the Large Hadron Collider, a giant particle accelerator located underground beneath the border of France and Switzerland, and managed by the European Centre for Nuclear Research, CERN. The LHC accelerates and smashes protons together to recreate conditions from the early universe.

It was famously the site of the discovery of the Higgs boson in July 2012, an elementary particle that’s responsible for the mass of other elementary particles. The discovery got two physicists who had hypothesised its existence in 1964 the Nobel Prize for physics in 2013. After two years of upgrades, the LHC restarted in early April 2015, smashing protons to produce a record 13 tera-electron-volts of energy.

Within minutes of opening, the AMA was flooded with over 2,000 questions, a number that quickly climbed to over 5,000. Here’s a selection edited for clarity.

How powerful are typical collisions and how powerful is 13 TeV?

Clap your hands together. Congratulations, you’ve made a collision with more energy than the LHC.

The difference though, is in the energy density. Stick a thumbtack against one of your palms, and clap again. Notice the difference? By concentrating the collision point, even with the same total energy we get a more intense collision. Protons are really, really, really tiny, so when they collide the energy density is huge, making the results a lot more interesting than a hand clap.

Is there a certain energy “goal” you are aiming for? Or simply trying to increase it as much as you can?

The energy goal is 7 TeV per beam. This number is fixed by the magnetic field our magnets can achieved “by design” and by the circumference of LHC. So these are design limits, it is not possible to overcome them. To overcome the limits we would need magnets able to stand more than 12000 amps of current or an LHC-like with bigger circumference than LHC, i.e. more than 27 km. In fact the next proton-proton accelerator design is based on a 100 km circumference to get 50 TeV per beam (but this is a project under study). Some more details can be found here.

How do the collisions create new particles?

The things being collided have stored in them enough energy to recreate conditions closer to the beginning of the universe when some particles roamed around freely. As the universe cooled down, those particles decayed into other particles, and so on and so forth. So, the particles are new in the sense that they are being created now. But they are not new, in the sense that the same kind of particles existed close to the beginning of the universe.

What difference does it make to add more energy into the collisions? What new results are expected to be seen?

It makes a difference because the higher collision energy may provide access to new phenomenas, some of which cannot be explained within the standard model so the discovery potential ranges from finding particles that could constitute dark matter to establishing the origin of the asymmetry between matter and anti-matter and reproducing primordial matter.

Apart from the amount of energy used, do you run the experiments in the same way every time? The reason I ask is, if you are running the same experiment, wouldn’t you expect to find the same results each time?

Now, when we repeat an experiment, we never expect the exact same result. That is actually quite rare. Part of the reason is that colliding protons means actually looking at interactions between the quarks and gluons inside, which have different energies every time. But even if you would get the same quarks or gluons at the same energies, these are quantum mechanical processes, so the outcome is probabilistic and some times you may get one outcome, some others another.

Theoretically, if a human were to sit inside the LHC as it’s on, what would happen to them?

Well, it depends on the place. Some places are quite safe, for example equipment in arcs is expected to get 100 Gy (5 Gy is lethal for a human) in 10 years of operation. Paper. Other places likes Point 3 and Point 7 (collimators cleaning the beam) or beam dumps have dangerous radiation levels and unless there’s a need nobody goes there.

What are collimators for?

Collimators are used to clean the beam of stray particles that go too far from the main part of the beam. It’s better to stop them in known places (Points 3 and 7) than to lose them somewhere else.

If the LHC was transparent, what would we see when the collisions occur?

“Seeing” is done by large detectors with several layers of materials that each help measure a different property of the particles created in the collision.

  1. A tracker helps identify the trajectory of any charged particles produced
  2. Calorimeters measure the energy of the particles
  3. Muon detectors observe these “heavier cousins of the electron”

Putting together all this information from various sub-detectors presents us with a single picture of what happened at the collision point. (This is a bit of an oversimplification, but I hope it makes sense…)

See the full AMA here.