Z Boson Decay into Electrons and Muons
Finding the Mass of the Z Boson
Skye Gagnon


The purpose of this project is to find the mass of the Z boson by using the combined mass of two particular fermion-antifermion pairs that a typical Z boson will decay into. I obtained this information through the CMS e-Lab website and compared the graphs available there to the recognized value of the Z boson mass. Upon observing the data I was able to see the average mass of a Z boson is 90.00 GeV/c^2. These results fit nearly perfectly with the calculated Z boson mass of Beringer and his particle data group with a slight discrepancy in the electron-position data. This suggests that perhaps there was a sight calibration error in the CMS experiment.



The Z boson is one of the elementary particles included in the standard model. There are two main types of particles in this model: the fermions or matter particles and the bosons or force particles. The Z boson, along with the W boson, communicates the weak force. This force is one of the four fundamental forces and is responsible for beta decay and nuclear fusion processes. It is a force much like the electromagnetic force but its range is much smaller. The Z boson is an electrically neutral particle that is its own anti particle. It is a very massive particle that is nearly ninety times as large as a proton. The Z boson was a very important particle involved in the unification the electromagnetic and the weak force into the electroweak force. For the sake of consistency during the development of this theory it was found that a new force particle must exists to unite this these forces. This particle was named the Z particle. Before the proposed Z particle was brought into existence it was well understood that the W particle mediated an interaction with neutrinos that transformed that into electrons that allowed them to interact with matter, a process called charged current interaction. With the presence of the Z particle, however, it was believed that a neutrino could interact without transforming itself. This process of non-transformation was called neural current interaction. In 1973 experiments using bubble chambers were used to find evidence of this interaction, which provided physical proof for electroweak unification. Also from these experiments, using the ratios of the neutral and the charged currents, it became possible for the first time to estimate the mass of the W and Z particle.



Using data collected from CERN located on the CMS e-Lab website, and data from Beringer's particle data group, my goal is to conduct a calibration study of the CMS detector. To do this I will collect data on Z boson decay into muon and anti-muon pairs as well as electron and positron pairs. By adding the kinetic energy produced by one of these particle-antiparticle pairs immediately after their production from a parent Z particle, I can accurately collect data of the appropriate mass of a Z boson. For greater accuracy I will collect several cases of these decays from each of these particle-anti-particle pairs and make two separate graphs of the resulting data. From these graphs I will be able to find the most common mass and ultimately find the most accurate mass of the Z boson.



From the graphs you can see that the average mass of the muon-antimuon pair was between 90.00 and 91.00 GeV/c^2. The average mass of the electron-positron pair turned out to be between 89.00 and 90.00 GeV/c^2.


Discussions & Conclusions

From these results, drawn by the data from the CMS e-Lab, one can see that the average mass of a Z boson is 90.00 GeV/c^2. This resulting average mass is very close to the calculated mass that Beringer and his team found. Although there appears to be a slight discrepancy concerning the electron-positron graph and the value that Beringer’s particle data group calculated. According to the results presented by this team, the data from the CMS e-Lab for the electron-positron pair is nearly 2.18 GeV/c^2 off. This slight but apparent discrepancy may be the result of a skewed set of data or possibly a slight fault in the calibration of the CMS detector. To check this discrepancy I would suggests more tests on the decays of Z particles into electron-positron pairs should be conducted. An increase in data may raise the mass average towards Beringer’s particle data group calculation of 91.18 GeV/c^2. After these tests have been completed the data observation units that I used on the CMS e-Lab website should be used again to interpret the new data and should once again be compared to the data collected by Beringer and his team.



"CERN Accelerating Science." The Z Boson. N.p., n.d. Web. 12 Feb. 2014. "Gauge & Higgs Bosons." PdgLive. N.p., n.d. Web. 17 Feb. 2014. Giudice, Gian F. A Zeptospace Odyssey. New York: Oxford, 2010. Print. "First Z Bosons Detected by CMS in Heavy-ion Collisions | CMS Experiment." First Z Bosons Detected by CMS in Heavy-ion Collisions | CMS Experiment. N.p., n.d. Web. 17 Feb. 2014.