Archive for May 28, 2006


It would be great to observe a set of mono-jet events at the Tevatron!

Recently the CDF Collaboration published an account of their search for mono-jet events in hep-ex/0605101. (Note: As a member of the CDF Collaboration, my name appears on this paper, but I had little to do with this analysis. I'm not posting comments on my own work today!) It is a well-written paper on a well-motivated search for new physics.

Here are the basics: You trigger on events with at least one energetic jet, and then tighten the jet criteria in your offline analysis to be sure your trigger efficiency is easy to understand. In this case, the jet must have ET>150 GeV, fall in the central calorimeter, and contain at least a couple of charged tracks. Next, demand some missing energy, at least 120 GeV, which would be rare in ordinary multi-jet events. Now you have a set of "interesting" events.

This event sample will contain some events from Standard Model processes. For example, a Z-boson decaying to neutrinos produced together with an energetic jet (a gluon radiated in the initial state) will amount to a perfect mono-jet event. A W-boson decaying leptonically with the charged lepton missed by the reconstruction amounts to the same thing. There will also be some messier backgrounds coming from cosmic rays and from muons flying along the beam direction and passing through the calorimeters. The authors of this analysis demonstrated clearly that they understand the SM production of W and Z-boson production in association with an energetic jet. They also dicuss a clear and straight forward method for estimating the contribution from di-jet events in which one jet is lost or badly mis-measured. The dominant contribution to their sample comes from a Z-boson and a jet.

The agreement of the data wth their prediction from SM and instrumental backgrounds is extremely good: expect 265±30 and observe 263. The shape of their missing energy distribution also matches the prediction beautifully: CDF MET distribution in the mono-jet sample
Given no signs of an anomalous contribution to this event sample, the authors derive an upper limit of 67 signal events at 95%CL (systematic uncertainties play a major role here) which they use to place constraints on a popular scenario for extra-dimensional phenomenology. The constraint is somewhat weaker than a direct search for deviant graviational forces when assuming a small number of extra dimensions, but is quite a bit stronger when one assumes a larger number of extra dimensions.

For more details, and to find out who did this very nice analysis, go to the CDF public web page for this particular analysis.

So, is that it? No extra dimensions and we need to add more data or simply wait for the LHC?

Is this just a big yawn?

Certainly not! First, the analysis is very well done. The cuts are uncomplicated, the sample is quite clean and the estimate of its composition is robust. We should hope that the equivalent analysis at the LHC will share these qualities. In fact, I expect this kind of search at the LHC to be quite hard, since the SM sources of a jet and missing energy will be much larger (for example, from di-boson and top quark production), and the resolution on the missing energy will be difficult to understand due to the underlying event and multiple interactions in each beam crossing. There may even be some confusion from multiple new physics channels contributing to this particular channel, though it would be perverse to complain about that… I hope that people who plan to explore this channel at the LHC have read carefully the corresponding studies with Tevatron data.

Second, the authors chose to focus on models with extra dimensions, but there are other models which might produce a signal in this channel. It might have been interesting to consider an invisible Higgs, not because this is popular or likely, but rather to see how well suited this kind of analysis might be. One might even expect some non-negligible acceptance for certain SUSY channels, such as a pair of squarks decaying to q+chi01. The authors did not explore such channels in detail because the paper would have become too heavy, and in any case a real attempt to constrain, say, R-parity conserving SUSY would involve many channels.

Third, this is an important analysis to do. We must examine events with large missing energy because we hope that new physics will turn up there. This is a benchmark search along with several others without which the collider program would not be a complete success. We don't know which channel will reveal new physics (and it may be an odd, peculiar, obscure channel) so we must look in as many places as possible. The only requirement is to do a good job, as has been done here.

Finally, I would like to ask why this analysis and others like it do not receive much attention from the community. We can read in a dozen places that the LHC will surely find new physics at the TeV scale, and it will likely show up in a missing-energy channel. So why aren't people waiting in anticipation for precisely these results from the Tevatron? It seems that people have written off the Tevatron because they would rather wait for the LHC. And worse, in some quarters it seems that people feel that the LHC will hold no major surprises, so we should go ahead and start building the ILC so that we can really start to answer the big questions of particle physics.

I wish there were a way to restore a more general and more genuine interest in the kind of work exemplified by this particular analysis. It is true that we have all been waiting many years for signs of new physics. But since none of turned up yet, I think we need to look all the more intensively and creatively at the data we already have…


May 28, 2006 at 6:45 pm 2 comments

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