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Michael Schmitt | USLHC | USA

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A time when the Tevatron and the LHC Overlap

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”.

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5 Responses to “A time when the Tevatron and the LHC Overlap”

  1. I sure hope someone over at Tevatron & LHC is trying to make a deal to sell our assets to CERN in return for not losing the ability to keep tevatron running. It makes no sense to me that our politicians stupidity and shortsightedness should get in the way of *someone* reaping the rewards of all the hard work that’s done. Tevatron has already shown usefulness to the LHC for testing stuff, and I’m sure overflow projects. alone would keep them busy. I’m sure that our congressmen would sell the assets for real cheap (EU/CERN wins!) and get back to pounding rocks and or sand or whatever it is they’re doing and let humanity get on with the discoveries of the universe.

  2. paolo says:

    “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”

    just in case d0 sees no excess around 150Gev, who’s going to belive mentioned large asymmetry? I mean the only stereo view we are left with is the following:
    LHC will have to reject all those politically inspired “evidences” reported by disappointed experimenatlists at CDF (disappointed by lack of funding)
    Isn’t it that the “new experimental era” enthusiasm is not justifiable -> Potential of LHC is overwhelmed by emerging lack of profesionalism at CDF.

  3. Michael Schmitt says:

    Paolo,

    we can’t trash the entire scientific output of CDF due to any one result. And the CDF Anomaly deserved to be published so that other experiments, such as D0, could make a formal cross-check with independent data. Even if the CDF anomaly at 150 GeV turns out to be a systematic error, it is better that the results are formally recorded than to leave them in a student Ph.D. thesis. Time will tell – fairly quickly – whether the CDF anomaly is new physics or not.

    Michael

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