Thursday, December 13, 2018

New experiment cannot reproduce long-standing dark matter anomaly

Close-up of the COSINE detector  [Credits: COSINE collaboration]
To correctly fit observations, physicists’ best current theory for the universe needs a new type of matter, the so-called “dark matter.” According to this theory, our galaxy – as most other galaxies – is contained in a spherical cloud of this dark stuff. Exactly what dark matter is made of, however, we still don’t know.

The more hopeful physicists believe that dark matter interacts with normal matter, albeit rarely. If they are right, we might get lucky and see one of those interactions by closely watching samples of normal matter for the occasional bump. Dozens of experiments have looked for such interactions with the putative dark matter particles. They found nothing.

The one exception is the DAMA experiment. DAMA is located below the Gran Sasso mountains in Italy, and it has detected something starting in 1995. Unfortunately, it has remained unclear just what that something is.

For many years, the collaboration has reported excess-hits to their detector. The signal has meanwhile reached a significance of 8.9σ, well above the 5σ standard for discovery. The number of those still unexplained events varies periodically during the year, which is consistent with the change that physicists expect due to our planet’s motion around the Sun and the Sun’s motion around the galactic center. The DAMA collaboration claims their measurements cannot be explained by interactions with already known particles.

DAMA data with best-bit modulation curve.
Figure 1 from arXiv:1301.6243

The problem with the DAMA experiment, however, is that the results are incompatible with the null-results of other dark matter searches. If what DAMA sees was really dark matter, then other experiments should also have seen it, which is not the case.

Most physicists seem to assume that what DAMA measures is really some normal particle, just that the collaboration does not correctly account for signals that come, eg, from radioactive decays in the surrounding mountains, cosmic rays, or neutrinos. An annual modulation could come about by other means than our motion through a dark matter halo. Many variables change throughout the year, such as the temperature and our distance to the sun. And while DAMA claims, of course, that they have taken into account all that, their results have been met with great skepticism.

I will admit I have always been fond of the DAMA anomaly. Not only because of its high significance, but because the peak of the annual modulation fits with the idea of us flying through dark matter. It’s not all that simple to find another signal that looks like that.

So far, there has been a loophole in the argument that the DAMA-signal cannot be a dark matter particle. The DAMA detector differs from all other experiments in one important point. DAMA uses thallium-doped sodium iodide crystals, while the conflicting results come from detectors using other targets, such as Xenon or Germanium. A dark matter particle which preferably couples to specific types of atoms could trigger the DAMA detector, but not trigger the other detectors. This is not a popular idea, but it would be compatible with observation.

To test whether this is what is going on, another experiment, COSINE, set out to repeat the measurement using the same material as DAMA. COSINE is located in South Korea and has begun operation in 2016. They just published the results from the first 60 days of their measurements. COSINE did not see excess events.

Figure 2 from Nature 564, 83–86 (2018)
Data is consistent with expected background

60 days of data is not enough to look for an annual modulation, and the annual modulation will greatly improve the statistical significance of the COSINE results. So it’s too early to entirely abandon hope. But that’s certainly a disappointment.

Friday, December 07, 2018

No, negative masses have not revolutionized cosmology

Figure from arXiv:1712.07962
A lot of people have asked me to comment on a paper by Jamie Farnes, titled
Farnes is a postdoc fellow at the Oxford e-Research center and has previously worked on observational astrophysics. A few days ago, Oxford University published a press-release celebrating the publication of Farnes’ paper. This press-release was then picked up by and spread from there to a few other outlets. I have since gotten various inquiries by readers and journalists asking for comments.

In his paper, Farnes has a go at cosmology with negative gravitational masses. He wants these masses further to also have negative inertial masses, so that the equivalence principle is maintained. It’s a nice idea. I, as I am sure many other people in the field, have toyed with it. Problem is, it works really badly.

General Relativity is a wonderful theory. It tells you how masses move under the pull of gravity. You do not get to choose how they move; it follows from Einstein’s equations. These equations tell you that like masses attract and unlike masses repel. We don’t normally talk about this because for all we know there are no negative gravitational masses, but you can see what happens in the Newtonian limit. It’s the same as for the electromagnetic force, just with electric charges exchanged for masses, and – importantly – with a flipped sign.

The deeper reason for this is that the gravitational interaction is exchanged by a spin-2 field, whereas the electromagnetic force is exchanged by a spin-1 field. Note that for this to be the case, you do not need to speak about the messenger particle that is associated with the force if you quantize it (gravitons or photons). It’s simply a statement about the type of interaction, not about the quantization. Again, you don’t get to choose this behavior. Once you work with General Relativity, you are stuck with the spin-2 field and you conclude: like charges attract and unlike charges repel.

Farnes in his paper instead wants negative gravitational masses to mutually repel each other. But general relativity won’t let you do this. He notices that in section 2.3.3. where he goes on about the “counterintuitive” finding that the negative masses don’t actually seem to mutually repel.

He doesn’t say in his paper how he did the N-body simulation in which the negative mass particles mutually repel (you can tell they do just by looking at the images). Some inquiry by email revealed that he does not actually derive the Newtonian limit from the field equations, he just encodes the repulsive interaction the way he thinks it should be.

Farnes also introduces a creation term for the negative masses so he gets something akin dark energy. A creation term is basically a magic fix by which you can explain everything and anything. Once you have that, you can either go and postulate an equation of motion that is consistent with the constant creation (or whatever else you want), or you don’t, in which case you just violate energy conservation. Either way, it doesn’t explain anything. And if you are okay with introducing fancy fluids with uncommon equations of motion you may as well stick with dark energy and dark matter.

There’s a more general point to be made here. The primary reason that we use dark matter and dark energy to explain cosmological observations is that they are simple. Occam’s razor vetoes any explanation you can come up with that is more complicated than that, and Farnes’ approach certainly is not a simple explanation. Furthermore, while it is okay to introduce negative gravitational masses, it’s highly problematic to introduce negative inertial masses because this means the vacuum becomes unstable. If you do this, you can produce particle pairs from a net energy of zero in infinitely large amounts. This fits badly with our observations.

Now, look. It may be that what I am saying is wrong. Maybe the Newtonian limit is more complicated that it seems. Maybe gravity is not a spin-2 interaction. Maybe you can have mutually repulsive negative masses in general relativity after all. I would totally be in favor of that, as I have written a paper about repulsive gravity myself (it’s quoted in Farnes’ paper). I believe that negative gravitational masses are the only known solution to the (real) cosmological constant problem. But any approach that attempts to work with negative masses needs to explain how it overcomes the above mentioned problems. Farnes’ paper falls short of this.

In summary, the solution proposed by Farnes creates more problems than it solves.

Thursday, December 06, 2018

CERN produces marketing video for new collider and it’s full of lies

The Large Hadron Collider (LHC) just completed its second run. Besides a few anomalies, there’s nothing new in the data. After the discovery of the Higgs-boson, there is also no good reason for why there should be something else to find, neither at the LHC nor at higher energies, not up until 15 orders of magnitude higher than what we can reach now.

But of course there may be something, whether there’s a good reason or not. You never know before you look. And so, particle physicists are lobbying for the next larger collider.

Illustration of FCC tunnel. Screenshot from this video.

Proposals have been floating around for some while.

The Japanese, for example, like the idea of a linear collider of 20-30 miles length that would collide electrons and positrons, tentatively dubbed the International Linear Collider (ILC). The committee tasked with formulating the proposal seems to expect that the Japanese Ministry of Science and Technology will “take a pessimistic view of the project.”

Some years ago, the Chinese expressed interest in building a circular electron-positron collider (CEPC) of 50 miles circumference. Nima Arkani-Hamed was so supportive of this option that I heard it being nicknamed the Nimatron. The Chinese work in 5-year plans, but CEPC evidently did not make it on the 2016 plan.

CERN meanwhile has its own plan, which is a machine called the Future Circular Collider (FCC). Three different variants are presently under discussion, depending on whether the collisions are between hadrons (FCC-hh), electron-positions (FCC-ee), or a mixture of both (FCC-he). The plan for the FCC-hh is now subject of a study carried out in a €4 million EU-project.

This project comes with a promotional video:

The video advertises the FCC as “the world’s biggest scientific instrument” that will address the following questions:

What is 96% of the universe made of?

This presumably refers to the 96% that are dark matter and dark energy combined. While it is conceivably possible that dark matter is made of heavy particles that the FCC can produce, this is not the case for dark energy. Particle colliders don’t probe dark energy. Dark energy is a low-energy, long-distance phenomenon, the entire opposite from high-energy physics. What the FCC will reliably probe are the other 4%, the same 4% that we have probed for the past 50 years.

What is dark matter?

We have done dozens of experiments that search for dark matter particles, and none has seen anything. It is not impossible that we get lucky and the FCC will produce a particle that fits the bill, but there is no knowing it will be the case.

Why is there no more antimatter?

Because if there was, you wouldn’t be here to ask the question. Presumably this item refers to the baryon asymmetry. This is a fine-tuning problem which simply may not have an answer. And even if it has, the FCC may not answer it.

How did the universe begin?

The FCC would not tell us how the universe began. Collisions of large ions produce little blobs of quark gluon plasma, and this plasma almost certainly was also present in the early universe. But what the FCC can produce has a density some 70 orders of magnitude below the density at the beginning of the universe. And even that blob of plasma finds itself in a very different situation at the FCC than it would encounter in the early universe, because in a collider it expands into empty space, whereas in the early universe the plasma filled the whole universe while space expanded.

On the accompanying website, I further learned that the FCC “is a bold leap into completely uncharted territory that would probe… the puzzling masses of neutrinos.”

The neutrino-masses are a problem in the Standard Model because either you need right-handed neutrinos which have never been seen, or because the neutrinos are different from the other fermions, by being “Majorana-particles” (I explained this here).

In the latter case, you’re not going to find out with a particle collider; there are other experiments for that (quick summary here). In the former case, the simplest model has the masses of the right-handed neutrinos at the Planck scale, so the FCC would never see them. You can of course formulate models in which the masses are at lower energies and happen to fall into the FCC range. I am sure you can. That particle physicists can fumble together models that predict all and everything is why I no longer trust their predictions. Again, it’s not impossible the FCC would find something, but there is no good reason for why that should happen.

I am not opposed to building a larger collider. Particle colliders that reach higher energies than we probed before are the cleanest and most reliable way to search for new physics. But I am strongly opposed to misleading the public about the prospects of such costly experiments. We presently have no reliable prediction for new physics at any energy below the Planck energy. A next larger collider may find nothing new. That may be depressing, but it’s true.

Correction: The video in question was produced by the FCC study group at CERN and is hosted on the CERN website, but was not produced by CERN.