Rediscovering the Standard Model

In the past week, three milestones have been passed:

• the first π⁰ particles have been reconstructed from their decay products, shown at the public LHC week 1 conference (by CMS and LHCb);
• the LHC has officially become the world’s highest-energy collider, by colliding protons at 2.36 TeV (above the Fermilab Tevatron’s record of 1.96 TeV);
• the first paper based on LHC data has been submitted to the arXiv (by ALICE).

The π⁰ observation represents the first step in “rediscovering the Standard Model” as part of the detector commissioning.  It’s like a walk through history, where this step is at about 1950, when the π⁰ was first discovered in cyclotrons and cosmic rays.

The above plot shows invariant mass distributions of pairs of photons observed in CMS and LHCb.  From every pair of photons, you assume that they came from the decay of a particle and plot what the mass of that particle must have been.  For many pairs, the assumption is false, so you get a combinatoric background of random photons, but for photons that actually came from π⁰ → γγ, you get a peak at the π⁰ mass.  The combined distribution is a peak on a smooth background.  Most particles in the Standard Model are known only through their decay products, and this is the first example to be seen at the LHC.

Since we already know a lot about π⁰s, we now use them to calibrate the photon detectors.

Needless to say, these algorithms are not just being developed now— they’ve been in the works for years.  That explains how ALICE was able to put together and internally approve a paper based on the first collisions in one week.  For a brand new analysis, that would take many months at least!