Rediscovering the Standard Model

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In the past week, three milestones have been passed:

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.

Plots of pi0 peaks

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!

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5 Responses to “Rediscovering the Standard Model”

  1. S.C. Kavassalis Says:

    Great summary of the LHC highlights.

  2. The particle body-count « The Everything Seminar Says:

    [...] neutral pion (π0) was seen in the first 900 GeV LHC collisions this November.  Most of the charged particles produced in proton collisions are also pions (π+ [...]

  3. Retro particle physics at CERN « Antimatter Says:

    [...] neutral pion (π0) was seen in the first 900 GeV LHC collisions this November.  Most of the charged particles produced in proton collisions are also pions (π+ [...]

  4. free math worksheets Says:

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  5. Камеры Видеонаблюдения Says:

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