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Answer to Fred: tiny signal and huge backgrounds March 15, 2006

Posted by dorigo in computers, physics, science.

Fred left a comment to my post below (“Programming mood” ) and I feel tidy enough to answer him in a separate post, where my effort can be more readily available to anybody else interested…
Fred writes:

My question of the day: In layman’s terms, what is the purpose of needing to determine the effectiveness of a selection aimed at isolating a tiny signal in a huge background of QCD events? What would be a practical application attained from the results of this endeavor?

The CMS experiment at the LHC supercollider will only start taking physics data in 2008. We want to be ready for it, and we need to justify our existence as software specialists while our fellow hardwarists work their butts out in trying to put together these battleship-sized, multi-million-electronic-channel, billion-dollar detectors.

Therefore, we study what we can do with the data, even before we collect it, by simulating interesting processes and the relative concurring backgrounds with the aid of sophisticated computer programs that use our theoretical knowledge of the physics of subatomic particles, together with a detailed blueprint of the would-be detector and its response to passage of radiation, to yield an output as similar to the one we will get when we start colliding real protons. A simulation.

Now for the answer to your question. Tiny signals are the most interesting, because they are the ones not yet unearthed by previous experiments: because previous experiments were colliding particles at lower energy, and fewer of them. Both higher collision energy and higher number of collisions work together to allow us to study rarer and rarer processes, where yet unknown particles are produced.

Take the top quark as an example.

The top quark was sought by the CERN SppS collider experiments in the eighties, when the total collision energy was 630 GeV (about 680 proton masses) and the number of collisions per second was in the tens of thousands. The experiments may well have produced a few top quarks, but for sure they could not detect them: too few to make a significant signal, buried in a large background. Two top quarks (they are produced in pairs) make a total mass of 350 GeV, and thus require that the collision does almost nothing else but producing them: very improbable, very rare, very few events. No discovery.

Then came the Tevatron: it started to collide protons and antiprotons in 1987, at a three times higher energy (1800 GeV). Top pairs are less rare here! Moreover, the collision rate was in the hundreds of thousands per second. But still, no discovery in a year of data taking… It took three more years of data, collected between 1992 and 1994, to get a significant signal (a handful of top pair production events) and claim discovery.

Still, it was a tiny signal in a huge background! When CDF announced the first evidence for top in 1994, it was based on about ten events, selected amidst a dataset of several millions.

To find a new particle, you have to be smarter and smarter the smaller the signal is. And the more we progress in our understanding of particle physics, the deeper we want to go – so the Higgs boson, the long sought particle thought responsible of the mass of all others, is produced at the Tevatron with the rate of one per hour (if it exists and has the characteristics we foresee), while concurring processes are produced at a rate of 3 millions per second!

Now, the specific selection I was talking about in my previous post was not about finding the Higgs at the Tevatron, but about the much simpler task of selecting top pair production events at the LHC – there, the top quark is not so rare (well, still sort of – one pair produced every 100,000,000 collisions). Top pairs are worth collecting even at the LHC, which will have million-event samples of top decays, because associated with the top quark we can sometimes find a Higgs boson! It is sort of like digging ore from a gold mine, with the aim of extracting the metal with some chemical or mechanical process thereafter.

I hope I have somehow answered Fred’s question here… If any or all of the above is confusing, please let me know…


1. Peter Woods - November 11, 2009

This is a most helpful piece of work. I am a fascinated Theologian (not the Doomsday kind) that relishes the mysticism of this discipline. In my field we have documented cases of mystics who have devoted entire lifetimes to meditation/prayer based on one encounter/observation that stood out from the background. So, yeah, I get it.

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