Tomorrow at 2PM two back-to-back talks by Jim Olsen and Marumi Kado at CERN will disclose the newest results of the CMS and ATLAS experiments, which will be based on the analysis of 2.5 to 4 inverse femtobarns of 13 TeV proton-proton collisions acquired by the experiments this year.
Why should you be interested ? Of course, because the year could end with a boom! Maybe the experiments have found evidence for something totally unexpected in their data. After all, 13 TeV is 63% more energy than 8 TeV.
Mind you - if you thought that a particle X of, say, 9 TeV could be spotted in this year's data but be impossible to see in the 2011-2012 collision data taken at lower energy you would be a bit naive. Such a heavy X particle might have a *very* large natural width. This means that the true mass of produced X particles would not always be 9.0 TeV, but be sometimes much smaller and sometimes higher. In that case, we might have caught it by its low-energy "tail" already years ago...
In any case, 9 TeV are already over a half of the total energy of the proton-proton collision. Now, the collision that gives rise to a new particle is one between proton constituents - quarks or gluons. As there are many such bodies inside the proton, it almost never happens that one constituent carries half of the energy of its parent. Hence 13 TeV proton-proton collisions releasing energies of 6 or 7 TeV are extremely rare. I believe CMS has just one event with a dijet mass of just above 6 TeV, and ATLAS has one 7 TeV dijet event. Hence if a new particle will be claimed tomorrow, its mass will hardly be above 5 TeV.
But can it be a much lower-mass body ? Again, that's difficult, for a different reason. The 2012 data amounted to 20 inverse femtobarns of collisions, so 5 to 8 times more than what has been analyzed by ATLAS and CMS so far. A particle with a mass of, say, 700 GeV would be certainly produced more copiously in the 13 TeV collisions of this year, but not five times more so (see the luminosity ratio at the bottom for the precise number). As we have not discovered anything like that in the past few years, the chance we can do so tomorrow is not great.
Maybe because of the above, the most exciting region where to look is around 2 TeV. Sure: there's where the two experiments have spotted excesses in their 8 TeV dijet data. Why, they got as far as combining their datasets, maybe to raise interest in tomorrow's event. You can find the results of the combination of ATLAS and CERN results on the 2 TeV bump, produced by some CERN scientists, in the graph below.
Above: unofficial combination of 2-TeV excesses in resonance searches by ATLAS and CMS. (This is just one of several such graphs in the paper).
As you can see, the combined data shows an effect that, under the background-only hypothesis, has a probability of less than one ten-thousandth of occurring. Does that mean we already have a new particle in our hands ? No, hold your horses there. Although the fact that the two experiments see the same effect (at slightly different masses, okay, but that might be an accident of energy calibrations) is suggestive, that's no cigar yet. For CMS and ATLAS have studied dozens of different mass distributions, and a bump could have appeared in a thousand places. I believe the bump is just a fluctuation - the best fluctuation we have in CERN data so far, but still a fluke.
Above: ratio between the luminosity (vertical axis) needed at 8 and 13 TeV in order to produce in same amounts a particle of mass Mx (in abscissa) by the three production mechanisms of gluon-gluon, gluon-quark, and quark-antiquark interaction.
So, tomorrow we will definitely see if I am right about the 2 TeV thing - the 13 TeV data should see significantly more of that excess if it is due to a new particle; 2 TeV is a mass region where the production of a resonance by gluon-gluon interaction is 10 times more frequent at 13 TeV than 8, so the 13 TeV data of this year is quite relevant for this particular search.
And what else ? Of course, new particle searches are fun, but we could discover new physics also by studying old guys like the top quark or the Higgs boson. Indeed. But again, the luminosity ratio plot above tells you that for such low-lying fruits of masses of just a few hundred GeV the new data will not be as ground-breaking as we would like it to be....
In any case, stay tuned. The two talks tomorrow will certainly be quite interesting. As for me, I will be blogging in real time about those results from here, if I have a chance.
The New Discoveries You Won't Hear About At Tomorrow's CERN Jamboree