Top quarks and photons playing together

The plot shows four different measured cross sections (points) compared to the hashed lines, which are the theoretical expectation. The top three are control samples that are in excellent agreement with theory. The lower most point is the presented measurement that is also in excellent agreement with Standard Model expectation.

Physicists test the Standard Model to see if the pieces in place, or those they think might be missing, can tell them anything new. One important test is to measure the properties of when the particles collide head on. This allows physicists to study interactions at the shortest distances to see whether our observations agree with prediction.

When our results agree with the Standard Model, we gain confidence in this theory. Disagreement with the Standard Model could point to new physics. A few things that physicists look at in each physics process is whether the rates of particle production agree with prediction and whether the kinematic distributions (how the physics objects in question physically distribute themselves in the detector) behave as we expect.

In this analysis, we search for events that contain a lepton (electron or muon) with significant momentum perpendicular to the beam indicating a “head-on” type collision, a photon, and a heavy flavor jet (b-jet) and an energy imbalance hinting at a possible neutrino being present. In these searches, we check each collision to see if we observed the above quantities. To verify that we have identified our object correctly and to ensure that what we are calling a photon is actually a photon, we consider various physics control samples. In this case, one check resulted in 84 events when 86 events, with a plus or minus nine error range, were predicted by the Standard Model in excellent agreement.

An interesting subset of this search consists of a top quark plus an antitop quark pair plus a high-energy photon. Physicists predict this process is smaller by a factor of 100 than current top and antitop quark production.

There are two important ingredients to this measurement. First a lot of collisions need to take place. Second, there has to be a very clear event signature.

The Tevatron has produced many collisions (10 inverse femtobarns). This result is based on 5.7 inverse femtobarns, which is approximately 100 times the number of collisions that were necessary in Run I to discover the top quark. Now, rather than just looking for top quarks and antitop quarks produced together, we’re looking for events that also contain a high-energy photon.

In this search, CDF observed 30 candidate events with this ttbar+photon signature. We expected to see 27 events ± 3. Thus, CDF scientist performed the first measurement of the ratio between top quarks produced in association with a photon to those in which only top quarks are produced. Using a nearly identical event selection between the two signals the ratio is measured to be 0.013 ± 0.005, which agrees well with the theoretical prediction of 0.010 ± 0.002.

More information.

— edited by Andy Beretvas

First row from left: Benjamin Auerbach and Paul Tipton, Yale. Second row from left: Andrey Loginov, Yale; Henry Frisch, University of Chicago; and Irina Shreyber, ITEP/CEA Saclay, France.