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Christine Nattrass | USLHC | USA

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Exciting new results from CMS

I’m giddy today because CMS just came out with some very exciting results.  I don’t think we understand what they mean at all – and as a scientist, there is nothing I love better than shocking data, data that challenge what we think we understand.  (For the technical audience, the slides from the talk at CERN are here and the paper is here.)  I might be biased because this topic is very closely related to my doctoral thesis, but I think it’s safe to say this is the first surprising result from the LHC, something that changes our paradigm.

In heavy ion collisions at the relativistic heavy ion collider we observed something called the ridge (from this paper):

We more or less understand the peak – called the “jet-like correlation” – but we don’t understand the broad structure the peak is sitting on.  This broad structure is called the ridge.  What I mean when I say we don’t understand the ridge is that we haven’t settled in the field how this structure is formed, where it comes from.  We have a lot of models that can produce something similar, but they can’t describe the ridge quantitatively.

Here’s what CMS saw:

It’s a slightly different type of measurement – I’ve put a box around the part with the ridge.  We see the same peak as we saw before – again, we pretty much understand where this comes from.  But there’s a broad structure beneath this peak.  It’s smaller than what we saw in heavy ion collisions above, but it’s there – the fact that it’s there is surprising.

In the models we have from heavy ion collisions the ridge is from:

  • A high energy quark or gluon losing energy in the Quark Gluon Plasma,
  • Collective motion of particles in the Quark Gluon Plasma, or
  • Remnants of the initial state (meaning the incoming particles)

In our current understanding of what goes on in a proton-proton collision, there is no Quark Gluon Plasma – so the conservative interpretation of these data would mean that the ridge is somehow some remnant of the initial state. Even conservatively, this would severely constrain our models.  Some physicists, such as Mike Lisa at Ohio State University, have proposed that there may be collective motion of particles in proton-proton collisions, similar to what we see in heavy ion collisions.  This would imply that we also see a medium in proton-proton collisions.  That would be a huge discovery.  (Just to be clear, CMS is not making this claim, at least at this point.)  It will take a while for the community to debate the meaning of these data and come to a consensus on what they mean.  But these data are definitely very exciting – this is the most exciting day for me since the first collisions!

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