LHC is running

A rather large group of phenomenologists at Harvard, led by Nima Arkani-Hamed, spends one hour every Wednesday by analyzing the data from the LHC.



Today, they (or we) were making sure that the spins of the particles are what they should be. How can you tell that the Z boson - that has clearly been seen at the LHC, according to the graphs - is a spin one particle, as opposed to spin zero particle?

Well, the spin zero particles have a uniform angular distribution. The spin one particles, on the other hand, have different distributions as a function of the angles.

When the LHC creates a Z, it's almost never the "longitudinal" polarization parallel to the incoming momenta - it's because you must combine the opposite helicities from the two quarks contained in the proton. Consequently, the spin of the Z boson is always aligned with the quarks - their spin can never cancel. You can see the parabolic profile of the angular distribution even if you assume that 50% of the Z-bosons have one spin, and 50% have the other spin.

Analogously, we have also found some Z' (Z prime) particles, showing that they're spin one.

The next task was to identify some superpartners that the LHC has created for us - roughly 250 events. How can you say that the stop has spin zero? In fact, there are many differences between the bosons and the fermions.

The pair-production of the stop squarks has roughly 5 times larger cross section than for two top quarks - you may imagine that the fermions "repel" by the Pauli exclusion principle, and they correspondingly have a lower effective phase space.

Also, right above the production threshold, the cross section for producing the fermions goes like "beta" while the cross section for producing the bosons goes like "beta cubed", because of some extra derivative couplings for the boson.

The graphs showed relatively clearly that the stop squark was a spin 1/2 particle, and it decayed to the top quark plus a bino - which seems to be the lightest supersymmetric particle, as far as everyone sees. Some things are very easy to see - note that the LHC is producing roughly 6 jets per event, and their opening angle is roughly 10 degrees.

There have been some other graphs without much structure. They suck because it's hard to analyze what particles were exactly produced in these events.

Summary: the phenomenologists at Harvard are ahead of the experimentalists - I guess that the experimentalists won't be able to finish their first analyses of the LHC data earlier than 2007. ;-)