﻿ International Physics Masterclasses

## Graviton

It is expected that a hypothetical force particle, the graviton (G), is the mediator of gravity, in a similar way that the electromagnetic, weak and strong forces are expressed through the exchange of gauge bosons: the photon, the W and Z weak bosons, and the gluons, respectively.

Some new particle physics scenarios beyond the Standard Model, based on extra space dimensions, predict the existence of graviton excitations. Should they exist, such heavy resonances are expected to be produced at the LHC and to show up in the invariant mass of decay products. Given that the graviton couples to energy-momentum (and not to mass as Newton's theory suggests!) it would decay in pairs of leptons, quarks and bosons, including photon-pairs and gluon pairs, although the photon and gluon are massless.

Of particular interest for the Z-Path are the decays of the graviton into final states with di-leptons, di-photons and 4-leptons (ZZ), which were used to measure the Z boson, discover the Higgs boson, and search for the hypothetical Z' gauge boson.

We recall that the identification of the Z and Z' bosons is similar, although they have very distinct masses; both decay into di-leptons and none decays into di-photons. The Higgs boson decays into di-photons and ZZ (4-leptons); although it is observed to decay into a pair of heavy leptons (tau), the decay into di-electrons and di-muons is heavily suppressed because these leptons have tiny masses and the Higgs couples to mass, contrary to the graviton!

Wait, this is getting confusing. The invariant mass tells me that I have identified known particles or discovered new ones, but how do I know what I have discovered?

This is where we need to rely, in addition to electric charge and mass, on an intrinsic fundamental property of particles, which you may have heard about, used and taken advantage of - spin.

The photon, the Z and the Z' have spin 1, the Higgs has spin 0 and the graviton has spin 2. If you are worried about why the graviton has spin 2 and the photon spin 1, think of the difference between the two forces. Gravity, in contrast to electromagnetism, is only attractive.

The theory of spin is governed (together with other properties we have not met yet) by Quantum Mechanics. Particle decays satisfy certain conservation laws, such as the conservation of electric charge, energy and momentum. The conservation of angular momentum and spin come in addition, and impose constraints on allowed particle decays. The Higgs boson can decay to 2 spin-1 or 2 spin-1/2 particles (remember the Higgs decay to electron or muon pairs is not observed because these lepton masses are tiny). The Z and Z' bosons can decay to 2 spin-1/2 but not to 2 spin-1 particles. The graviton can decay to 2 spin-1/2 or 2 spin-1 particles.