Tag Archives: Quark

LHC To Restart This Weekend?

CERN is reporting that the Large Hadron Collider is ready to resume running. There was a minor problem with an electrical connection that has been fixed, so hopefully everything is ready for collisions at a center of mass energy of 13 TeV, just below the design energy of 14 TeV.

The beam is expected to start this weekend, as long as no other problems arise. Physics beam won’t happen quite yet though. It sounds like there is going to be an extended period of beam tuning, with actual collisions at full beam energy starting in June.

Higher energy allows for a great deal of new work to be done. In some important analyses, such as Higgs searches, the background can fall faster with energy than the signal, boosting the overall statistical power of results. Furthermore, higher energy increases the possible reach of searches for new physics. Many hope for new physics at the TeV scale in order to avoid fine-tuning in the electroweak sector of the Standard Model (such as from heavy quark loop corrections to the Higgs mass). In collisions at these energies, the protons can’t be treated as individual particles, but rather than collections of quarks and gluons (collectively called partons in this context). While the center of mass energy of a proton-proton collision will be 13 TeV, the actual amount of energy accessible in a collision will be smaller. If, for example, we take two typical quarks with about 30% of the total momentum, the center of mass energy would instead be around 4 TeV. Gluons typically carry even less energy (the actual momentum distributions of quarks and gluons in a proton are called “parton distribution functions”). So, in order to access TeV-scale physics, it would be useful to have an overall energy around an order of magnitude higher.


Bryan Fischer: The Strong Force is Jesus

Ed Brayton has a post today pointing to a particularly ignorant monologue by noted bigot Bryan Fischer of the American Family Association. In it Fischer says that

  1. The strong nuclear force is what holds atomic nuclei together (true),
  2. scientists don’t understand it (more or less false for decades), so therefore
  3. the strong nuclear force is Jesus.

This seems to be a version of the “god of the gaps” argument, arguing that things we don’t understand must be due to divine intervention. This is recognized by most people as a logical fallacy. Obviously, when this is applied to things that we actually do understand, it looks bad for religion. It’s also a dangerous argument for science because it encourages people to be incurious about the world. If we ascribe a supernatural origin to everything we don’t understand then there is no need for science; we already have the explanation for any problem.

Just in case anyone wants a brief explanation of the strong force: 

Nuclei are made of protons and neutrons. Neutrons have no electric charge while protons are all positively charged. So, the electromagnetic forces between protons tend to try to push them apart – to cause the nucleus to break apart. The nucleus is held together because there is another, stronger force (unimaginatively called the strong force) that pulls the protons together more than electromagnetism pushes them apart. In quantum mechanics (the nucleons are nonrelativistic), this can be roughly modeled as a deep short-range square potential well that replaces the usual 1/r Coulomb potential from electrostatics. The potential is generated by the other nucleons in a nucleus, so this is only a very simple approximation.

In more advanced (but still not fundamental models) the forces between nucleons can be modeled as an exchange of mesons (typically pions), similarly to how electromagnetic interactions are caused by photon exchange between particles. The fundamental interaction comes from the local SU(3) color symmetry of quark fields in quantum chromodynamics. There are 3 colors and 8 bosons (called gluons) that allow for exchange of color charge between particles. The strong force is also what holds the nucleons together – they are made of quarks and gluons, which are all in turn believed to be elementary particles. Calculations of the properties of hadrons (protons, neutrons, pions, etc) from first principles requires the use of the world’s most powerful supercomputers. That is a field called Lattice QCD.

Nuclei are complicated objects made of complicated composite particles, so we can’t feasibly calculate anything we want to arbitrary precision. The strong force is also difficult to deal with it because of it’s large coupling constant (becoming nonperturbative in many problems) and it’s non-Abelian nature (a math term related to the properties of SU(3) that here means that gluons can interact directly with other gluons). We do still have a pretty good understanding of how they work. Fischer only had to look up the nuclear force on Wikipedia if he wanted to get some idea of what we know.