Finding ultra-high energy B-hadrons in high energy physics

Prof. Todd Huffman, Prof. Jeff Tseng, and project student Charles Jackson of the Oxford ATLAS Exotics Group have recently published a paper on a new method of tagging ultra-high energy B hadrons in jets. The paper featured on the cover of the Journal of Physics G: Nuclear and Particle Physics. The new method would appear to maintain its efficiency even at momenta so high that the B hadron traverses several detector layers before finally decaying.

(A general introduction and link to the full paper can be found at JPhysG: Nuclear and Particle Physics)

In the summer of 2015 the centre of mass energy of the Large Hadron Collider (LHC) at CERN increased to 13 TeV from 8 TeV (1 TeV is 1012 electron-volts of energy). Prof. Huffman’s and the Oxford Exotics group’s research is to search for massive states of matter that decay into two quarks or gluons producing exceptionally high energy jets of particles in their wake. One such event is shown in Figure 1 where you can easily see two back-to-back jets of particles streaming from the collision point at the centre of the detector.

There are many theories that allow for the existence of such massive particles, but our primary interest is driven by experiment. Regardless of any theory, a detector like the ATLAS detector shown above is constructed such that we would see such massive particles if they do in fact exist.

The problem with such exceptionally high speeds, and slow decay times, is that the B hadrons can traverse the inner layers of the detector before they decay. This is illustrated below where a particle comes in from below, crosses a detector which registers a hit, and then decays between detector layers; so the next detector further out registers an increased number of hits from the many decay products of the B hadron decay.

Traditional b-tagging methods try to use the hits obtained to reconstruct the particle tracks, but if the B hadron decayed after the first layer or two there will be missing layers or even worse, the tracks will use false hits from inner layers which were not actually from the correct particle. As a result of this effect and also other relativistic effects, the efficiency of track-based b-tagging starts to drop as the energy increases.

Shown schematically here is a particle traversing a pixel layer from the lower left and decaying before the next layer, causing multiple hits to appear. For this technique to be effective the particle should decay into many daughter particles. B hadrons h

The idea was to see if, rather than even trying to form tracks from the dense environment within a high energy jet, one could identify B hadrons by only looking to see if there is an increase in hit pixels between detector layers. The group's preliminary findings indicate that a method based upon this idea has merit and warrants further, more detailed investigation.

This study is on-going and the group is hoping to prove this method has sufficient usefulness to be included in the suite of b-tagging techniques used by the collaboration to increase the sensitivity of the detector to ultra-high energy B hadrons dramatically.