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Peter Steinberg | USLHC | USA

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The First Discovery at the LHC

In the recent mini-wave of “What Will We Find at the LHC” posts, no-one mentioned that the first actual measurements at the LHC will certainly not be of anything as exotic as the Higgs boson, supersymmetry, or large extra dimensions. This is not for any reason as prosaic as the fact that it’ll take time to get to the design energy and luminosity, which is true. If we define the very notion of “finding” something as being “measuring a quantity that we could not predict with current tools”, then the very first measurements at the LHC will count as discoveries of great interest to those not just focussed on what typically counts as exotic phenomena.


To boil it down to something concrete, consider the number of particles produced in a typical collision at the LHC. And to make things more straightforward, only consider the number of charged particles, the ones that leave curling tracks when moving through a magnetic field.
These are neat looking events, and they happen each and every time protons collide, at every energy, since the dawn of the accelerator era (you need a few GeV to even make a bunch of pions!). Thus, these particles are the “grass” that one sees in lego plots showing two huge jets, or the steel wool amidst which the two high energy muons emerge after a Higgs particle decays. For most people, this part of the collision is a background that needs to be cut away to see the interesting physics.

Here’s the rub: while it’s moderately easy to count the number of particles in each event, no-one has ever managed to come up with a bottom-up theoretical scheme by which one can predict this number. This is mainly due to the somewhat-scandalous situation that we know what protons “do” when they get close to each other (and can propagate that information into very precise predictions for the production of high pT particles, etc — the bread and butter of the LHC), but we don’t really get “why” they do it. Thus, we don’t have a very solid means to extrapolate our current knowledge into the LHC era, even if the Tevatron is only a factor of 7 lower in energy.

Thus, I promise that the first things you will see coming out of the LHC program are a bunch of measurements pertaining to “minimum bias” events, i.e. the events 99% of the experiment want to throw away so they can look for the needle in the haystack. Some of us (which include many of us directly interested in the heavy ion program at the LHC) want to see how the grass grows when those first collisions appear. We’ll count the particles emerging near 90 degrees, turn it into a “particle density” (by restricting the angle over which we count them), and put it on a plot with the rest of the data — probably with a few curves reflecting our favorite predictions. And everyone wants the first paper from the LHC, so it’ll be a real race, and so the results will appear almost as soon as real collision data is written to tape (which may well be this fall!)

For entertainment value, here’s my take.

This is a pretty straightforward application of the ancient (and bizarre) Landau hydrodynamical model to p+p minimum bias collisions and heavy ion collisions. It assumes that the two protons dump all of their energy into the collision as they overlap, and the system expands collectively like a relativistic fluid after that (sound familiar?). It describes the multiplicity (linear with the entropy) weirdly well for heavy ions (the top curve) above 20 GeV or so, and predicts a very high density at the LHC. It is pretty scratchy for p+p (the bottom curve — which may or may not be related to trigger bias issues — we’ll have to discuss that soon, too) but at least predicts something quite a bit higher than most popular models. But if this model has anything non-trivial to say about proton-proton collisions (something suggested by Landau and Fermi in the 1950’s, but which became controversial and even “heretical” during the rise of QCD, something I wrote about a few years ago), then we may have to start to take seriously the possibility that even small systems have “medium”-like aspects similar to what people already say about the QGP at RHIC. And how fun would that be?

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