# What abut the Graviton?

1. Jul 29, 2014

### joeyb9

Why is there seemingly so little interest among particle physicists about discovering the graviton? Is it because it is believed to be too difficult, even hopeless, to find, or some other reason?

2. Jul 29, 2014

### Staff: Mentor

I think its because the graviton comes from the General Relativity world and theories trying to bridge between Quantum Mechanics and General Relativity are still too tenuous that it isn't seriously looked for.

Also, once gravity waves are found then they will dig deeper.

http://en.wikipedia.org/wiki/Graviton

but I'm sure if they ever do find a spin-2 massless particle there will be a lot of excitement in the Physics world...

3. Jul 29, 2014

### The_Duck

Detecting gravitons experimentally is hopelessly infeasible, because they pass through everything with very high probability. This is similar to the problem of detecting neutrinos, but many many orders of magnitude worse. See e.g. http://arxiv.org/pdf/gr-qc/0601043v3.pdf

4. Aug 2, 2014

### lpetrich

The graviton Lagrangian is very roughly
$$L = \frac{1}{2G_N} |\nabla h|^2 - h \cdot T$$
for Newtonian gravitational constant GN, metric perturbation h, and energy-momentum tensor T. Going over to quantum-mechanical units,
$$G_N = \frac{1}{m_{Pl}{}^2}$$
where mPl is the Planck mass.

Let's rescale h by multiplying it by the Planck mass:
$$h = \frac{1}{m_{Pl}}{\tilde h}$$
That makes the Lagrangian
$$L = \frac12 |\nabla {\tilde h}|^2 - \frac{1}{m_{Pl}}{\tilde h} \cdot T$$

That makes interaction matrix elements proportional to the reciprocal of the Planck mass, and total process rates proportional to the reciprocal of the square of it. So,
$$\text{(process rate)} \sim \text{("normal" process rate)} \left( \frac{E}{m_{Pl}} \right)^2$$

At the LHC's energies of about 1 TeV/parton, that's about 10-32. Thus, it will be VERY difficult to see evidence of individual gravitons at the LHC, as opposed to macroscopic, classical-limit gravity.

5. Aug 2, 2014

### Staff: Mentor

The "normal" graviton will be impossible to find with the LHC, but some models (in particular, models with extra dimensions) predict massive graviton-like particles that could be detectable.
CERN made a webpage for it, and searching for "graviton LHC" gives several publications about it.