Today I was reading a superb paper by the D0 Collaboration on the Measurements of inclusive W+jets production rates (arXiv:1106.1457, 7-June-2011) when I was struck by a sentence in the introduction:
Therefore, it is crucial to make precision measurements of W+Jets production at the Fermilab Tevatron Colllider and the CERN Large Hadron Collider…
At this point in time, the particle physics community is blessed by results from two complementary colliders: ppbar at 2 TeV and pp at 7 TeV. So if you want to look at some important process from two angles, so to speak, now is the time to do it.
It is well known that the analyses done by the LHC collaborations follow the lead, at least in part, of prior analyses done at the Tevatron. It is also known, perhaps less well, that the performance of the LHC detectors is so superb that some major analysis problems at the Tevatron turn out not to be so horrible at the LHC. Some obvious examples include the calibration of the calorimeters, the tails of the missing energy distribution, and the hermeticity of the detectors (i.e., how few “holes” they have in their coverage).
But if we set aside the question of evolution and competition, a third point emerges which the authors of the D0 paper plainly see: due to the differences in the machines, the same process when studied at both the Tevatron and the LHC is richer and can teach us more. I am wondering whether there might be some opportunities that we might stand in danger of missing.
Take a simple example: the cross section for the production of W and Z bosons. This is a bread-and-butter measurement, and experimenters at CDF, D0, CMS, ATLAS and LHCb know well how to make an accurate measurement. This is a real art which is now in the Nth generation, and the methods are very refined.
The point is: the same refined measurements performed at the Tevatron and at the LHC tell us somewhat different things about the beam particle (protons), especially about the momentum distributions of quarks and anti-quarks inside. At the Tevatron, most Ws and Zs are produced by the annihilation of a quark in the proton and an anti-quark in the anti-proton, and in fact these are “valance” (anti-)quarks which carry a significant fraction of the beam particle’s energy. At the LHC, by contrast, there are no anti-protons, so the anti-quark must come from the “sea” quarks (in contradistinction to the valence quarks) which we know will carry only a small fraction of the beam particle’s energy. This means that the two measurements of the same process (W and Z production) probe different parts of the proton, and that can be quite interesting.
Another example is the sample of top – anti-top quark pairs produced. There is an intriguing result from CDF showing that the forward-backward asymmetry of the top and the anti-top quark is much larger in magnitude than expected. At the LHC, top quarks are produced copiously, but through a different mechanism, so it is not obvious that the LHC experiments can make an immediate contribution to the question of what might be going on with the ttbar asymmetry.
The Tevatron experiments CDF and D0 have an impressive history of achievement across decades of high-energy collisions. The LHC experiments CMS, ATLAS and LHCb have practically exploded with new physics results starting already at quite a high level of sophistication. The overlap of the two sets of physics results should be viewed in stereo and one should perhaps look for points at which measurements do not quite seem to “jive”.