Casting a wide net

To search for unanticipated physics, you need to make sure you do searches that make the minimum number of assumptions of what new physical phenomena might look like.

I give a lot of public lectures. I speak about DZero’s attempts to find the Higgs boson or the search for supersymmetry. Most of the time, the talks go off without a hitch. But every so often, a hand will go up, and I’ll get hit by a thoughtful question. The words change from person to person, but essentially the question is “Excuse me Dr. Lincoln. You’re telling us about all these particles you guys are looking for. But what if the thing to be discovered is something nobody’s ever thought of? How do you know you’re not overlooking something totally unexpected?”

And this is a most excellent question. The history of science is replete with observations that weren’t predicted. As Isaac Asimov, the American author and biochemist, is reported to have said, “The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’ but rather ‘That’s funny…’”

Luckily, this insightful question has an answer. DZero completed a Model Independent Search, guided by a single assumption, which is that new phenomena will most likely be found in the most violent collisions. In essence, we made thousands of plots, consisting of 117 different combinations of electrons, muons, jets, etc., and compared them to the predictions of the Standard Model.

The idea is that if there were new physics to be found, it would show up in one or more of these plots. To verify that the idea works, we even “re-found” the top quark, by comparing our data to Standard Model calculations from which top quarks were removed.

This approach is generally not as effective at any specific search as one motivated by a theoretical prediction. However, it casts a wide net, to ensure no new physics fish slips through. If we see something, a more focused study is in order.

Of the 117 different combinations of particles, only two exhibited any significant difference from the Standard Model. Because these two combinations are known to be difficult to model correctly, we cannot claim we’ve discovered something. But we have provided a direction for more directed studies. If there are any fish to be found, DZero will catch them.

—Don Lincoln

These physicists performed this analysis.
Horst Wahl of Florida State University chairs a committee that ensures that DZero physics results are represented at the many conferences held each year across the globe.