Z boson 40th Anniversary

Guest post by: Raffaella Tramontano

It has been 40 years since the discovery of the Z boson, and it is such an important aspect of physics how we know it and an integral part of how research is conducted at CMS. Raffaella Tramonano is a postdoc in high energy physics at Universität Zürich working with CMS and here explains a little bit more about the Z boson.

Firstly, what is the Z Boson?

The Z boson is a heavy neutral particle. Together with its charged cousin, the W boson, it mediates the weak force, which is one of the 4 fundamental interactions and has a very short range. Z bosons are tossed back and forth between two particles undergoing a weak interaction as a way to transport information. What type of information? The Z boson mediates weak processes where the total electric charge is conserved, while the energy of the particle involved changes - they are therefore carriers of "neutral currents". 

How important is the Z boson really? What would the universe look like without it?

The Z boson and its neutral currents are pieces to a precise mechanism that perfectly describes the electromagnetic and weak interactions as two faces of the same coin. This mechanism gives us massless photons (through which we can see the world) and a very weak weak force. In the absence of the Z boson, we might end up with a weak force that's not that weak anymore or photons with mass. if the weak force wasn't weak, neutrino-to-matter interactions would happen way more often, and complex states of matter (such as atoms, molecules, bacteria, and humans) might not be able to hold together as they do now. Equally, if the photon was not massless, it would decay just like the Z and W bosons do - nothing to light up the universe. 

When and where was it discovered?

The first hints of the Z boson were found in measurements of neutrino interactions with matter in the Gargamelle experiment in 1973. Sudden tracks appeared seemingly from nowhere in the huge Gargamelle heavy liquid chamber: they were footprints of a proton or neutron suddenly gaining kinetic energy after having interacted with a neutrino in an elastic scattering via the exchange of a Z boson.

The particle was directly observed by both the UA1 and UA2 experiments, operating on the CERN Super Proton Synchrotron in 1983. 

In terms of the CMS experiment, how important is the Z boson to the current and future research being done here?

The Z boson doesn't stick around very long - it decays really quickly (3 x 10^-25 seconds) into a variety of things: electrons, quarks, neutrinos... 

Its decays have allowed us to measure its properties with great precision. Since we know we can measure them with such high precision, we can use the Z boson to validate new tools and algorithms used to identify particles in the CMS detector. 

The decay of the Higgs boson into two  Z bosons (decaying in two leptons each) was one of the ‘golden channels’ for the Higgs boson discovery. 

How does CMS use the Z boson for calibrations?

The Z boson is used in CMS electron reconstruction calibration. Electrons and positrons likely coming from Z boson decays are selected and combined to reconstruct Z bosons. If the mass of the Z boson obtained in this way does not add up precisely to the known mass, it is most probably because the electron reconstruction needs further adjustments. By comparing the true value of the mass and the reconstructed one, electron reconstruction properties are corrected, enhancing the performance of CMS. 

How does CMS use the Z boson to look for new physics?

The Z boson is used as an intermediate state to many new physics searches in which a very heavy particle decays to an arbitrary number of Z bosons, which then decay in diverse final states. The Higgs boson itself was observed in a process in which it decayed into two Z bosons, each decaying into two leptons. Precise knowledge of its properties allows us also to search for new physics which would behave very similarly to the boson itself, as we can spot even the slightest deviation from its predicted mass or lifetime. 


Disclaimer: The views expressed in CMS blogs are personal views of the author and do not necessarily represent official views of the CMS collaboration.