Jet Flow for Tagging High-pT Top QuarksDavid Nisson

UC Davis Boosted Top Quark Group

Phy 130B Final Project

Exotic Particles

The LHC may produce extremely massive hypothetical particles (1-5 TeV)

These decay to high-pT top quarks Energy to top mass ratio is high – makes

overlapping jets. Our goal is to disentangle them.

Top quark W

bott

om

jet

Motivation

The Particle Flow Algorithm attempts to identify the particles in events in the CMS from all subsystem outputs.

• We try to take advantage of the performance of the algorithm to find boosted tops.

• Current techniques identify only 45-50% of boosted tops.

• So, we designed an algorithm, called “Jet Flow”, that attempts to improve top tagging.

Particle Flow and Detectors

Tracker(measures trajectory)

Calorimeter(absorbs particle and measures energy) Particle Flow

Particle Flow takes the raw outputs of the tracker and calorimeter and reconstructs the original particles.

γe

Jet Flow

• We expect that each jet should have a nicely peaked distribution of energy in space.

• By fitting a sum of such distributions, we should resolve overlapping jets coming from different quarks.

The algorithm is therefore based on fitting to the energy distribution of a large jet, whereas conventional algorithms are based on clustering.

How it works

• First, we assume a single jet.

• We minimize the error in the model (chisquare)

• If the chisquare is too big, there must be more jets.

Chisquare too big!Add more jets

Jet Flow direction resolution

Very good! But…

Jet Flow energy resolution

What’s going on? The energy is so poor that we can’t even resolve the top mass yet!

Top mass resolution

We need to correct for the energy problems, otherwise this is all we’ll see with Jet Flow.

Conclusion

Jet Flow promises to tag boosted tops very efficiently. If so, it could be a good tool for tagging boosted objects in general. The Boosted Top group is confident that we will solve the algorithm’s remaining problems.

Acknowledgments

Thanks to professor John Conway for supervising my work.

Thanks to the UC Davis Boosted Top Quark Group, in particular Ricardo Vasquez Sierra.