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Christine Nattrass | USLHC | USA

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ALICE’s second paper!

ALICE’s second paper has been submitted!  If you’ve been following carefully, you’ll have heard that it’ll take at least a couple of years to get enough statistics to see the Higgs (if it’s there) – but we don’t have to wait that long for other results.  This paper presents a measurement of the number of charged particles produced in proton-proton collisions at center of mass energies of 0.9 TeV and 2.36 TeV.  (CMS actually published their paper on the same subject first.)  Proton-proton collisions are actually pretty complicated and we still don’t understand everything about them.  Protons are made up of quarks and gluons, so when we slam them together we get a combination of quark-quark, quark-gluon, and gluon-gluon interactions.  We can describe the products of these interactions pretty well when both particles hit each other hard, but not if they barely interact.  They can also interact multiple times.  So proton-proton collisions are really difficult to model.  Theorists have come up with models that try to describe proton-proton collisions, but these models still need improvement.  Counting the number of particles produced in a collision is a relatively straight forward measurement.  (Not to say it’s easy – there’s still a lot of work that has to be done for this, but there’s even more work needed for other measurements.)  And this measurement gives us data we can compare to models.   The models seem to be underestimating the number of particles produced – only by a few percent, but they’re still not quite right.

ALICE and CMS’s measurements also agree.  This is very important.  These measurements are very complicated and many things can go wrong.  Since two experiments did the same measurement with very different detectors, different methods, different code, different people, etc. and still agree, this gives us greater confidence in the measurement.  This is one of the reasons for having multiple collaborations doing the same measurement.

You won’t read about these results in the newspapers because there have been no dramatic paradigm shifts in our understanding of proton-proton collisions, but these are very important basic measurements that improve our understanding incrementally and they have to be done before we can hope to discover new physics.

Update April 21 – the third ALICE paper, on the 7 TeV data, was submitted today!

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