A puzzle concerning the underlying event
I love this period of high energy physics. Of course we are all hoping for a wonderful discovery that ushers in a new era in particle physics, etc. etc. Also, the challenge to SUSY is interesting at least for those of us who had positive subjective attitudes toward it in the past.
But there’s more! There is something worth your attention in nearly every subject you can think of. Today’s post is meant to be an example.
The underlying event is the unwanted stuff that comes along with the main interaction. On the one hand, it is a nuisance. On the other, it is very rich – and we don’t know how to describe it in detail. While advances in this area will not elucidate electroweak symmetry breaking or dark matter, the problems are knotty and you should “get your hands dirty” by working on them – or at least making the effort to pay attention.
Look at this plot produced by the ATLAS Collaboration (arXiv:1103.1816, 9-March-2011):
It shows the particle density as a function of the angle with respect to a moderately high-pT particle, called the lead particle. The choice of colors and dash/dot lines make the plot hard to interpret, but if you make an effort, you will notice that none of the theoretical curves match the data points. (The blue-grey boxes around the points represent the systematic uncertainties which are not important in this situation.)
I have written about plots like this before and I am sure that Tommaso Dorigo has written about the underlying event too. The peak in the middle comes form the quasi-jet to which the leading particle belongs. Think of a quark flying out at Δφ=0, leading to a spray (“jet”) of hadrons in a narrow cone – the leading particle happens to be the most energetic of them. The broad hump at ±π is just the recoil, ie, the quark or gluon on the other side.
The interesting region is in between – what we call the “transverse” region. What should go at right angles to the main jet-jet axis (given by Δφ=0 and π)? Evidently, this is hard to say – witness the wide range of predictions. The solid black line is the favorite choice of the ATLAS Collaboration. It does a good job in the jet-jet region, but it is too low in the transverse region, despite extensive tuning on other distributions and observables. The funny thing is that it is good in the transverse region and not so good in the jet-jet region if the leading particle is less energetic. So this bull-horn behavior changes as a function of the transverse momentum of the leading particle, and the relative successes of the models vary, too.
Think about what we’re talking about here. The jet-jet axis part seems clear enough – two partons fly out back-to-back in produce two sprays of particles, one narrower than the other. But – what is the origin of particles in the transverse region? In a naive, texbook view with arrows and Feynman diagrams, there are no partons flying out in that direction. Could the particles simply be produced by the hadronization of the main process – i.e., by the color string wiggling and breaking or by the color dipoles radiating? Not according to the models. Apparently these particles are not produced by the main process – they have some other origin. Our best guess are other partons scattering when the protons collide, or something like that. This sounds pretty murky and it is – hence the difficulty in modeling it successfully. Maybe this is a good opportunity for someone to clarify and shed light on the topic?
Here is another observable – the mean pT2 as a function of the pT of the leading particle, in the transverse region.
If you trace the individual predictions, you will see that some of the fail completely, while others fail only for part of the range. Somehow, getting the right answer is not easy.
These measurements have been made before. The first measurements at the LHC were done by CMS (Eur. Phys. C70 555) and afterward ATLAS published a paper too (arXiv:1012.0791). These two analysis are based on charged tracks while the new ATLAS paper is based on calorimeter energy clusters in addition to charged tracks. While this may seem like a minor difference, in fact is is important as the ability to resolve fine structure with the ATLAS calorimeter is very good, with some advantages over charged tracks. Thus there is a technical improvement, of sorts, in the new ATLAS paper with respect to the old.
So – can anyone explain the transverse region in detail? Is this an eternal mystery? Maybe someone in an alternate universe managed it.
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