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Monday, 15 August 2016

The Search for the Graviton

What holds the world together? It sounds like a simple question - but it isn’t.We know of four fundamental forces of nature that seem to be doing the job, but have only found three exchange carrier particles (particles which give rise to the forces between other particles). Physicists like symmetry, and we like to think that the universe does too. Faced with the scenario of more forces than carriers, it is natural for a physicist to assume we’re missing one.

So far, we have found photons, which carry the electromagnetic force (which describes the interactions between charged particles); W and Z bosons, which carry the weak nuclear force (responsible for radioactive decay); and gluons, which carry the strong nuclear force (which holds the nuclei of atoms together). But what about gravity?

A diagram outlining the interactions between fundamental particles according to the Standard Model.
Not all types of elementary particles interact with all other types, and only some fundamental particles interact with particles of the same type. For example, photons do not (directly) interact with other photons, or with gluons. Image credit: Public domain

This idea is summarised nicely in an article by science writer Brian Koberlein: in quantum field theory … [you] start with a wave form and then ‘quantize’ it, you break the wave down into the smallest amounts of it that can exist (for example, photons are quanta of light). This has been used with photons, and doing so with gravitational waves leads to the idea of gravitons.

However, this approach encounters some problems when tackling gravity. General relativity tells us that that gravity and the curvature of spacetime are intricately linked. As matter travels through spacetime it causes spacetime to curve around it, and as spacetime curves, this tells matter where to move. It is this motion, caused by the curvature of spacetime, that can be considered ‘gravitation’ [8].

Artist’s impression of the curvature of spacetime by the Earth.
Artist’s impression of the curvature of spacetime by the Earth. You can observe the same effect with a taut sheet and a paperweight. Ideally you need the sheet taut, horizontal, and not on the floor - friends or chairs can help with this. Put the paperweight in the centre, and the sheet will dip - representing the curvature of spacetime. Image Licensed under CC BY-SA 3.0 via Wikimedia Commons

Because gravity is so closely linked to spacetime, in order to quantise gravity you first need to quantise spacetime itself. No one’s really sure how to do this, but that doesn’t mean gravitons don’t exist. In fact, one researcher I spoke to said that he was not aware of any serious physicist who believes that gravitons don’t exist. Even though we don’t really know how to quantise spacetime, the main theories used to explain quantum gravity still predict gravitons with the same properties we see in the simple ‘quantised wave’ approach. A quick summary of this, is that even though we aren’t sure, in a mathematical sense, how to break gravitational waves down into gravitons, we’re pretty confident that the gravitons don’t mind and will exist anyway.

It should be noted that, even though we’ve detected gravitational waves, and lots of people think gravitons exist, that doesn’t make it true. It’s even possible that gravitational waves aren’t made of gravitons (but we’ll get to that later)! Now - on to the search!

Who is looking for the Graviton?

There are two groups of scientists (called collaborations) looking for evidence of gravitons in proton-proton collision experiments at the Large Hadron Collider at CERN. Once a graviton has been created, it’s expected to decay in one of a few possible ways - and it’s evidence of these decays that the collaborations are looking for.

Diagram of a Higgs boson decay process into two Z bosons, each decaying in to two leptons.
When a particle decays, it transforms into other particles (called decay products). Image credit: Public domain.

Although both the ATLAS and CMS collaborations are searching for several of these possible decays, I’ll just focus on two: the ATLAS search for evidence that the gravitons decays into two photons[2], and the CMS search for evidence that the graviton decays into two jets (bursts) of hadrons (a particular class of particle).

How are scientists searching for Gravitons?

Beams of protons collide with each other in the Large Hadron Collider causing lots of new particles to be created. The collaborations sift through the results to try to find evidence that some previously undiscovered particles have been created. Often, the undiscovered particles decay very quickly - so instead of trying to look for the particles themselves, scientists will look for the decay products instead. To do this, you have to determine what your proposed particle could decay into, and what energies these decay products would have. Once you have this information, you can begin studying the data from collisions.

When particles collide at the LHC, the results can seem very messy and it would be incredibly difficult to search through all the data looking for signals by hand. Instead, both collaborations use computers to recreate and identify the particles involved in events[3], and to determine possible candidates from energy deposited in the detectors[2].

They have to be careful of “background events”. Even if you find an event that contains the right decay products with the right energies, that doesn’t always mean you’ve found your undiscovered particle. It is always possible that some other process will have happened under just the right circumstances to produce the results you’re looking for, without being an indication of your undiscovered particle. If you don’t account for these background events, you risk false positive results.

Simulated proton-proton collision at the LHC. The lines represent theoretical particle tracks.
Simulated proton-proton collision at the LHC. The lines represent theoretical particle tracks. Image credit: Lucas Taylor / CERN (CC BY-SA 3.0)

Have the teams at CERN found anything?

Instead of using what could be described as a “normal” unit of mass (like grams, for instance) particle physicists actually use electronvolts (denoted eV). Now, it should be noted that electron volts aren’t traditionally a unit of mass - they’re a unit of energy related to the electron! They are able to use eV because at the speeds that the particles are travelling, Einstein’s famous mass-energy equivalence can be brought into play: E = mc2. This lets them use units of energy to discuss masses, and they do this to make it easier for themselves: it’s much easier to talk about a proton mass of 938 MeV than 1.67x10-27 kg.

Photograph of Einstein
Einstein’s mass-energy equivalence allows scientists to use units of energy to describe masses. Image credit: Photograph by Doris Ulmann (public domain).

Both collaborations have released publications[2][3] detailing their progress so far. The CMS collaboration has been studying data taken from a set of collisions observed in 2015, and running computer simulations. But so far, they’ve found no evidence for new particles.

The ATLAS Collaboration have also been studying data taken during 2015. They looked for particles with masses between 500 and 3500 GeV, and the data they studied agreed with their background-only hypothesis[2]. This is essentially their estimate of how many background events they will see in their data, and means they avoid the risk of rather serious false positive results.

However, although most of their data is considered background, the ATLAS collaboration have observed some deviations from the background-only theory, the largest of which is around 750 GeV[2]. Despite some excitement surrounding this deviation, CERN have now been announced that the deviation is not present in a larger, more recent data set and should therefore be considered to be the result of random chance in the 2015 data run.

So, no - scientists haven’t found the graviton yet, but they’re trying.

Are we sure gravitons can be found?

It is quite possible that the graviton will never be found. This could be be because they don’t exist, or it could be because, regardless of their existence, we are simply unable to detect them. In 2012, the renowned theoretical physicist Freeman Dyson gave his PoincarĂ© Prize lecture on this subject, discussing whether gravitons are detectable in principle. The agreed conclusion is possibly, but probably not[6].

The only scenario where we don’t find the graviton, but still form a definite conclusion, is rather unlikely. We’d have to unanimously agree that gravitons didn’t exist, based on the idea that we would definitely have found them if they did. This is very unlikely since detecting gravitons is so difficult. However, in this case, new and alternative theories of gravity might be true - if we can think of them.

A fallen giant
If a tree falls in the forest, and no-one’s around to hear it, does is still make a sound? Our question is similar in nature, but with a slightly higher propensity for causing headaches. If we can never prove whether something exists or not, what do we do next? Image credit: Stanislav Sedov (via Flickr CC BY 2.0)

This leads us to the question - what next? If we are entirely unable to detect gravitons, then should we continue to act as though they exist, or should we formulate new theories of gravity that don’t depend on them? As American physicists Rothman and Boughn ask, is it meaningful to talk about gravitons as physical, or do they become metaphysical entities? Maybe, as Dyson suggests, instead of being made of gravitons, gravitational fields are a statistical concept like temperature - valid only for effects over large regions, rather than over individual particles[5].

So we haven’t found the graviton, and don’t even know if we could, but in the spirit of scientific discovery we’re still going to try.

This article was written by TWDK's 2016 physics writing intern Alice Wayne, from Royal Holloway University of London.

why don't all references have links?

[1] Blokhintsev, D. I., and Gal’perin, F. M. “Gipoteza neitrino i zakon sokhraneniya energii”, Pod Znamenem Marxisma, vol(6), p.147-157, 1934.

[2] The ATLAS Collaboration. “Search for resonances in diphoton events with the ATLAS detector at √s = 13 TeV.” ATLAS Conference Note, 2016.

[3] Khachatryan, V. et al. “Search for Narrow Resonances Decaying to Dijets in Proton-Proton Collisions at √s = 13 TeV.” Physical Review Letters, vol.116(7), p.071801, 2016. DOI: 10.1103/PhysRevLett..116.071801.

[4] The ATLAS Collaboration. “Search for resonances decaying to photon pairs in 3.2 fb-1 of pp collisions at √s = 13 TeV with the ATLAS detector.” ATLAS Conference Note, 2015.

[5] Dyson, F. “Is a Graviton Detectable?” Poincare Prize Lecture, 2012.

[6] Rothman, T., and Boughn, S. “Can Gravitons Be Detected?” Foundations of Physics, vol.36(12), p.1801-1825, 2006. doi:10.1007/s10701-006-9081-9

[8] Wallace, D. “The Quantization of Gravity - an introduction.” arXiv preprint gr-qc/0004005 (2000). [9] Misner, C., Thorne, K., and Wheeler, J. Gravitation. W. H. Freeman and Company, 1973.