Quarks Galore

Here’s a nice example of some particle physics from the LHCb experiment at the Large Hadron Collider at CERN.


(taken from the LHCb news page at http://lhcb-public.web.cern.ch/lhcb-public/ )

The picture shows an unfortunately-named Bs particle (for Beauty Strange), produced as a result of a 7 TeV proton-proton collision. This particle doesn’t live long; it decays into a Ds+ particle, a mu- lepton and a neutrino. The Ds+ isn’t a long-living entity either, and soon decays into three more stable particles, namely K+, K- and  pi+ mesons.

What’s particularly nice about this example, is that we have five flavours of quarks in it.  The Bs can be considered as being a combination of a b (beauty, or bottom) quark, and an anti-s (s for strange) quark. The Ds+ is a combination of a c (charm) and an anti-s.  The K+ is a u (for up) anti-s pair, the K- an s anti-u pair, and the boring boring pi+ is a   u anti-d pair. (Here is our fifth quark, d is for down.) So we have five of our six quarks in one picture, the down and up (which comprise most of matter – e.g. 2 ups and a down make a proton, 2 downs and an up make a neutron), the strange and charm, and the beauty (or bottom).  The one missing is the ‘top’ quark.

Isn’t particle physics easy? 

Of course, you can rightfully ask the question "So what?".  What does this collision tell us about the universe?  On its own, probably not a great deal, but the LHC is steadily compiling many millions of collisions.  (www.twitter.com/cern reported half a billion by mid-May). The statistics of these is likely to give a good deal of information. For example, the LHCb experiment, from which this example comes, is looking particularly for the processes that would have occurred soon after the big bang. A specific question that begs an answer is ‘why is there so much more matter than anti-matter in the universe?’  If we go back to the example, note that the Bs, the Ds+, and the K+ / K- / pi+ trio all have the same number of matter quarks as anti-matter quarks. And that’s normal.  Assuming the Big Bang created equal numbers of quarks and anti-quarks (which seems a pretty natural assumption), where did all the anti-quarks go? That’s the LHCb experiment.

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