From Should we worry about what the LHC is not finding? on NewScientist:
What? Gravitons are supposed to be massless. What are they referring to?
Without a more detailed explanation, I’m honestly not sure what they’re talking about. The one possibility I can think of is that they’re obliquely referring to a “brane-world” scenario, where (roughly speaking) gravitons above a certain energy can “leak” into extra dimensions of spacetime. If this scenario was true, the LHC should supposed to be able to detect this “leakage”, at least if the leakage happens below such-and-such energy scale. If this is what they’re referring to, then they’re saying that gravitons with energies below 2 TeV do not leak into these extra dimensions (if said dimensions in fact exist.) Note that in these models, gravitons are still “massless”, more or less; it’s just that they behave differently when they have different amounts of energy.
These models, BTW, have very little to do with supersymmetry, which is the main thrust of the article. So if my interpretation is right, it’s a badly-phrased throwaway sentence in a popular-science article — not the first time such a beast has been observed in the wild.
They could also be referring to what might be called various polarization states of the graviton. In higher numbers of dimensions, you can end up with many different polarization states, representing oscillations not just in this direction or that direction, but in various combinations of the extra dimensions. In our world, these polarization states would manifest as separate particles, and some of those particles might be effectively massive.
Thanks.
(Sometimes it’s a fun puzzle to figure out what the scientist-who-knows-what-he’s-talking-about said that led to what the reporter-who-hasn’t-a-clue wrote.)
I’ll admit that I am not good with this stuff (and may be what I’m about to say is irrelevant) but your quote makes it sound like it’s talking of gravitons having a mass of 2 TeV, when the full article seems to be referring to the momentum from the collision.
Of course, for all I know, those two things are one and the same.
Does no one want to touch on my comment? I find using the same units for different concepts to be confusing, so I’d love to know if I understood it correctly.
Oh, sorry, didn’t realize there was a question there. Physicists often use energy, mass, and momentum interchangeably; just multiply or divide by c the appropriate number of times until the dimensions work out right. For example, that 2 TeV figure mentioned above is actually an energy, equivalent to about 3 x 10[sup]-7[/sup] joules. Divide that by c and you get a momentum of 1 x 10[sup]-15[/sup] kg m/s. (Note that energy = momentum * speed of light for a massless particle, so if a graviton is massless and has an energy of 2 TeV, that would be its momentum.) Once more and you get a mass of 3.5 x 10[sup]-24[/sup] kg.
The reason physicists do this is just because factors of c pop up so frequently in particle physics that it’s more convenient to just express things in units where c = 1. It’s one of those things that’s confusing as hell the first time you see it, but becomes second nature pretty quickly.
Okay, so if 2 TeV is the amount of energy, can you explain to me the OP’s question? That’s what’s confusing me. It doesn’t seem to me that the article is talking about the graviton itself having mass.
Or is that just the only way it could be detected?
MikeS is on target. These are searches for graviton excitations predicted by the Randall-Sundrum model. The RS model attempts to solve the hierarchy problem by having gravity live in a higher-dimensional space than our usual 3+1. A consequence of the model is the presence of observable graviton excitations (i.e., new particles). If you work out what properties the extra dimension(s) and the interactions all have to have to “solve” the hierarchy problem, you find that the observable gravitons should have a mass spectrum with TeV-scale splittings and weak-scale couplings to other particles.
Because the RS model is relatively clean and predicts distinct new observables (for instance, the new particles should have spin-2), and because these new predictions are in the range that the LHC can probe, there has been much buzz around the idea. If the RS model happened to predict 10[sup]5[/sup] TeV masses for the lowest graviton excitations, people would be less interested. (To be sure, the TeV scale is special beyond its arbitrary relation to the LHC’s energy. In particular, the weak interaction is also roughly TeV scale.)
I can’t speak for ZenBeam, but I believe he interpreted the article as saying that “gravitons have a rest mass whose equivalent energy is 2 TeV”. Physicists use this kind of shorthand all the time — you’ll hear, for example, people say “the top quark has an energy of 172 GeV” when they really mean “the top quark has a mass whose equivalent energy is 172 GeV”. So if you see a sentence like this in an article, and you’re used to this kind of physics shorthand, then confusion can arise.