The Large Hadron Collider at CERN is waking up after a 2 year shutdown for major upgrades. My piece “The Pause that Refreshes: CERN LHC 2-year shut-down” from 2 years ago explains that these upgrades were necessary to double the collision energy. Chances for new physics discoveries increase with collision energy. The upgrades have been completed. LHC is now essentially a new machine.
In the “new LHC” the center-of-mass energy of the proton-proton collisions will be 13 Tev but unfortunately only about 1-2 Tev of that energy will be available for particle creation.
Remember, proton is a composite particle. Proton is composed of quarks and gluons. Physics inside a proton is very complicated. When a 6.5 Tev proton collides with another 6.5 Tev proton the quarks and the gluons inside these protons interact violently. Most of the 13 Tev energy go into tearing the protons apart and only about 1-2 Tev energy go into quark-antiquark, quark-gluon or gluon-gluon collisions which may result in the creation of particles theorized but never observed before.
Remember, for particle creation to occur we need to create high energy density in a small volume of space. If the concentrated energy in that spot where the collision takes place is higher than the mass-energy of the theorized particle then that particle may show up if Nature indeed has such a particle. Everyone is hoping for sparticles – particles predicted by the theories based on supersymmetry – to show up in this run of LHC but from what I have been reading it will be very difficult to find sparticles in the debris of 13 Tev proton-proton collisions.
Particle creation is more efficient when we collide particles with their antiparticles directly because particle and its antiparticle annihilate each other. It is better to design a machine that collides electrons and positrons. Everyone agrees on this but the technology of a 5-10 Tev electron-positron collider is very challenging. For the foreseeable future we will be stuck at this energy frontier and have to live with the messy data from 13 Tev proton-proton collisions (1-2 Tev available for particle creation).
One good news is that the interaction probabilities – to produce Standard Model particles at 13 TeV – will be between 1.5 and 2 times higher than those at 8 TeV. But again, we have no idea what the discovery potential is at the 13 Tev center-of-mass proton-proton collisions. We are hoping for surprises.
In the “new LHC” the spacing between proton bunches will be 25 ns (nanoseconds) which is approximately 7 meters. This means that more collisions will take place at the centers of CMS and ATLAS detectors. This does not increase the discovery potential directly but it improves the statistics and therefore aids the discovery process indirectly.
We are talking about colliding protons head-on. Do you know how difficult that is? We don’t have the technology to aim the individual protons against each other precisely. We aim the proton beams against each other.
What is a proton beam? Beam is a collection of protons. In LHC there are 2 proton beams circulating in opposite directions in different beam pipes. Beams are forced to cross each other in the locations where the detectors (CMS, ATLAS, ALICE, LHCb) are located. Proton beams are prepared in bunches (pulses). See the representative pictures below. In each counter-circulating LHC beam there are 2808 bunches.
Each bunch is few cm long and a mm wide when they are far from a collision point. As they approach the collision points, they are squeezed to about 16 micron (a human hair is about 50 micron thick) to allow for a greater chance of proton-proton collisions.
Few parameters of the LHC
- Circumference = 26 659 meters
- Dipole operating temperature = -271.3 degrees Celcius (colder than outer space)
- Number of magnets = 9593
- Number of main dipole magnets (bending the protons into orbit) = 1232
- Number of main quadrupole magnets (focusing the proton beams) = 392
- Number of RF cavities (maintaining beam energy) = 8 per beam
- Peak magnetic dipole field strength = 8.33 Tesla
- Minimum distance between proton bunches = 7 meters (25 ns in units of time)
- Number of bunches per proton beam = 2808
- Number of protons per bunch (at start) = 1.1 x 10^11
- Number of turns per second = 11 245
- Number of proton-proton collisions per second at the crossing points = 600 million
If you do the math you will see that only 1 in 50 million protons end up in a collision with another proton.
Beam is inherently unstable because same-charge particles repel each other. Protons carry 1 unit of positive electrical charge. They repel each other. They would hit the walls of the beam pipe if they were not focused by very strong quadrupole magnets. There are also dipole magnets in LHC. The dipole magnets bend the charged particle beams and force them to circulate in the circular beams pipes 27 km long. The beam pipe has extereme vacuum. Air molecules and any other type of molecules are pumped out. We don’t want the circulating protons collide with the air molecules. Those protons would be knocked out of the orbit and hit the walls of the beam pipe causing x-rays. Nothing is perfect. Some protons hit the beam pipe and even penetrate through the outer layers (superconducting magnet components) and hit the tunnel walls too. This is why no human is allowed in the tunnels of the accelerators during beam operations. There are elaborate safety procedures that have been established in the last 100 years based on our knowledge of charged particle beams.
The PDF document prepared by CERN titled “CERN FAQ: LHC: the guide” is a very informative document.
Here’s the LHC schedule.
Here’s the latest news from beam commissioning.