Did the LUX Dark Matter Experiment Fail to Detect Dark Matter?

[Garth]

The negative findings of the Large Underground Xenon (LUX) dark matter experiment, which is a 370 kg liquid xenon time-projection chamber that aims to directly detect galactic dark matter and which were published at the international dark matter conference in Sheffield, UK, raises questions about the nature of DM.

The http://lux.brown.edu/LUX_dark_matter/Talks_files/LUX_NewDarkMatterSearchResult_332LiveDays_IDM2016_160721.pdf show that

LUX has delivered the world’s best search sensitivity since its first run in 2013,” said Rick Gaitskell, professor of physics at Brown University and co-spokesperson for the LUX experiment. “With this final result from the 2014 to 2016 search, the scientists of the LUX Collaboration have pushed the sensitivity of the instrument to a final performance level that is four times better than the original project goals. It would have been marvelous if the improved sensitivity had also delivered a clear dark matter signal. However, what we have observed is consistent with background alone.

(quoted from here.)

One alternative possibility was suggested in an eprint on the physics ArXiv: Can Dark Matter be a Scalar Field?.

Garth

 

[jerromyjon]

Not that my opinion is worth much, but I have been thinking this way about it for some time now. I haven't gotten far enough with my math yet and I have only read the heading of the paper you linked which I'm hoping to read tomorrow, but on a lighter, hypothetical note... how would the Hubble constant itself fit on a scalar field basis? Would it be as simple as a linear relationship to the "z" redshift? I think what I'm asking is does the acceleration of the expansion fit the profile of a scalar field as well?

 

[MathematicalPhysicist]

Or we should modify our theories; like MOND.

 

[Garth]

Or we should modify our theories; like MOND.

We may indeed have to modify the theory; one unexplained feature of the standard $\Lambda$CDM model is the presence of several puzzling coincidences.

The energy density of the cosmological constant is of the same order of magnitude as the density of matter today: $\Omega_M \sim \Omega_\Lambda$, when the DE density parameter is constantly increasing.

The age of universe is equal to Hubble time to within observational error bars: $t_0 = H^{-1}$,

and one that doesn't seem to be commented on very much; when $\Omega_\Lambda$ is 10^-120, or smaller, than the quantum expectation of zero point energy, and when the DM and the baryonic matter density parameters could be absolutely anything in GR, why on Earth should, (to within observational error bars,)
$\Omega_m + \Omega_{DM} + \Omega_\Lambda = 1$?

None of these relationships are predicted by GR and if the standard theory is the final word then they are all just extraordinary coincidences.

Garth

 

[mathman]

Not that my opinion is worth much, but I have been thinking this way about it for some time now. I haven't gotten far enough with my math yet and I have only read the heading of the paper you linked which I'm hoping to read tomorrow, but on a lighter, hypothetical note... how would the Hubble constant itself fit on a scalar field basis? Would it be as simple as a linear relationship to the "z" redshift? I think what I'm asking is does the acceleration of the expansion fit the profile of a scalar field as well?

Your comment seems to be about dark energy (acceleration of expansion). The only connection to dark matter is "dark".

 

[PeterDonis]

None of these relationships are predicted by GR and if the standard theory is the final word then they are all just extraordinary coincidences.

AFAIK nobody is claiming that the standard theory is "the final word". I would say we don't currently have a good explanation of these relationships; but that doesn't mean we will necessarily have to modify GR. It's simply an open question at this point.

 

[rootone]

Nevertheless the complete failure to detect DM as some kind of particle, despite the detecting equipment performing excellently and beyond, is significant in a way.
What's the next best candidate that is testable?

 

[DaveC426913]

I confess I didn't read up on the experiment. How do they test for a particle whose nature they do not know?

What if they're looking for 'red' particles but DM is really 'blue'?

I guess the one thing we know is that they do interact gravitationally. So if they found no presence of gravitational interaction where they could have expected it, they can safely say there can't be any DM there. ?

 

[PeterDonis]

if they found no presence of gravitational interaction where they could have expected it

I'm not sure how this could be done with an Earthbound experiment, because any dark matter "halo" attached to our galaxy would be expected to be homogeneous in our vicinity--i.e., same density everywhere. That would mean it would have no local gravitational effects.

 

[Orodruin]

Nevertheless the complete failure to detect DM as some kind of particle, despite the detecting equipment performing excellently and beyond, is significant in a way.
What's the next best candidate that is testable?

We have essentially tested one type of particle that has been very popular for its many attractive features and not even ruled out the entire parameter space for it. There are many other particle candidates that LUX and others simply do not possesses the power to test.

The latest I hearf from MOND was that it does not fit observations very well but that it can be made to ... If you assume the existence of additional unseen matter.

 

[guywithdoubts]

I confess I didn't read up on the experiment. How do they test for a particle whose nature they do not know?

What if they're looking for 'red' particles but DM is really 'blue'?

They are basically trying to find limits on how much DM interacts with regular matter. So now we have even weaker limits on how weakly it might interact. As of it nature, who knows so far.

 

[DaveC426913]

I'm not sure how this could be done with an Earthbound experiment, because any dark matter "halo" attached to our galaxy would be expected to be homogeneous in our vicinity--i.e., same density everywhere. That would mean it would have no local gravitational effects.

Again, I'm speculating from ignorance, but what I'm assuming is that, if they can very accurately determine how much mass is in a volume, and then determine how much gravitational force/curvature is observed, they would detect zero discrepancy. i.e. all gravitationally-interacting particles are accounted for by known, visible particles.

 

[DaveC426913]

We have essentially tested one type of particle that has been very popular for its many attractive features and not even ruled out the entire parameter space for it. There are many other particle candidates that LUX and others simply do not possesses the power to test.

The latest I hearf from MOND was that it does not fit observations very well but that it can be made to ... If you assume the existence of additional unseen matter.

Yes, this was my pessimistic thought (as opposed to the optimistic one, above).

"We thought it might have been X. We found no X. That rules out X, but not Y, Z - or A through W".

 

[Orodruin]

If you look at plots like this one

(From http://arxiv.org/abs/arXiv:1310.8642)
The WIMP is only one tiny region in the mass vs cross section plane. There are many other candidates remaining and I think this graph is not complete either.

 

[tionis]

Wait, they were expecting black holes to show-up in the detectors

 

[Orodruin]

Wait, they were expecting black holes to show-up in the detectors

No. You are misinterpreting. This is a graph of candidates and, as I said, the detector was constructed explicitly to look for WIMPs (the brown region). My entire point was that there are many other candidates that would not show up in the detector.

 

[tionis]

No. You are misinterpreting. This is a graph of candidates and, as I said, the detector was constructed explicitly to look for WIMPs (the brown region). My entire point was that there are many other candidates that would not show up in the detector.

Ah, ok. I thought maybe micro-black holes or something lol.

 

[Redbelly98]

I guess the one thing we know is that they do interact gravitationally. So if they found no presence of gravitational interaction where they could have expected it, they can safely say there can't be any DM there. ?

This experiment was not set up to detect a gravitational interaction, it was set up to detect the weak interaction.

 

[Chalnoth]

If you look at plots like this one

(From http://arxiv.org/abs/arXiv:1310.8642)
The WIMP is only one tiny region in the mass vs cross section plane. There are many other candidates remaining and I think this graph is not complete either.

For comparison, the LUX experiment could have detected dark matter with cross section potentially as low as about $10^{-45}$cm$^2$, in a mass range from roughly 1GeV to 1000GeV. That barely touches the top left of that brown rectangle.

 

[rootone]

... If you assume the existence of additional unseen matter.

To me this seems likely.

 

[Chalnoth]

To me this seems likely.

The problem is that for this particular solution, they have both a more complicated theory of gravity than General Relativity (in this example tensor-vector-scalar gravity) and a form of dark matter (though at a smaller mass fraction). Occam's Razor tends to favor just having the dark matter alone.

 

[Chronos]

As Sherlock Holmes once noted 'Once you eliminate the impossible whatever remain, however improbable, must be the truth.' We still have an abundance of DM candidates that have not been ruled out. Even the hapless WIMP, despite its shrinking parameter space, still has some wriggle room. I'm still not convinced a single particle species is necessarily the only answer to the DM riddle

 

[ohwilleke]

The WIMP parameter space that LUX has ruled out is very important because while there are an infinite number of possible dark matter candidates, the ones that LUX is testing for are the one predicted generically by the most popular beyond the Standard Model theories such as supersymmetry (SUSY). Also, the LUX constraints can't be taken in isolation. Old hat collider experiments like LEP strongly disfavor weakly interacting particles that are lighter than those excluded by LUX. And, there are also significant astronomy based limits on dark matter particle candidates that are "relics" or in which dark matter particles can annihilate. So, the global constraints of dark matter particles actually approach "over constrained" and have relatively few viable alternatives that aren't really baroque.

The fact that dark matter phenomena can be explained in models with just one to three degrees of freedom or so also favors theories in which the dominant contributors to observed effects are few and simple.

This makes modifications to gravity (obviously not non-relativistic toy models like basic MOND) still attractive.

Some of the recent battlegrounds for the dark matter particle v. gravity modification efforts in terms of astronomy evidence have been to look at the dynamics of Milky Way stars outside the plane of he main galactic disk and to look at the scatter predicted in relationships like Tully-Fischer in a dark matter particle v. gravity modification scenario.

 

[Chalnoth]

As Sherlock Holmes once noted 'Once you eliminate the impossible whatever remain, however improbable, must be the truth.' We still have an abundance of DM candidates that have not been ruled out. Even the hapless WIMP, despite its shrinking parameter space, still has some wriggle room. I'm still not convinced a single particle species is necessarily the only answer to the DM riddle

Multiple particle species are definitely a possibility, but it is pretty hard for two different species with different properties to have similar densities.

 

[Orodruin]

Multiple particle species are definitely a possibility, but it is pretty hard for two different species with different properties to have similar densities.

Electrons and protons. Of course, there are properties linking these two, but that might be true in the dark sector as well.

 

[Chalnoth]

Electrons and protons. Of course, there are properties linking these two, but that might be true in the dark sector as well.

The two are still off in terms of matter density by a factor of more than 2000, which is more than sufficient to ensure that it is the protons/neutrons alone that determine almost all of the normal matter density.

 

[Orodruin]

The two are still off in terms of matter density by a factor of more than 2000, which is more than sufficient to ensure that it is the protons/neutrons alone that determine almost all of the normal matter density.

In mass density yes, in number density no. If you want density, then take protons and neutrons. With a 25% by mass helium fraction, you get about 1 neutron per 7 protons.

In addition, the dark matter density is very close (factor of five or so) to the luminous matter density. In the case of WIMP dark matter, this is called the WIMP miracle, but there are other theories, such as asymmetric dark matter, that link the abundances of baryons to that of the dark matter.

 

[Chalnoth]

In density yes, in number density no. If you want density, then take protons and neutrons. With a 25% by mass helium fraction, you get about 1 neutron per 7 protons.

Yes, but protons and neutrons interact strongly with one another, making such a thing much easier. If it weren't for the fact that neutrons are stable when bound within certain atomic nuclei, all of the neutrons would have decayed in the very early universe, long before the CMB was emitted, and almost all of the normal matter density in our universe would be made up by protons.

In addition, the dark matter density is very close (factor of five or so) to the luminous matter density. In the case of WIMP dark matter, this is called the WIMP miracle, but there are other theories, such as asymmetric dark matter, that link the abundances of baryons to that of the dark matter.

Sure, but adding a second significant species of dark matter creates yet another coincidence that would have to be explained.

I'm not saying it's impossible. Just that it's not very likely.

 

[Orodruin]

Sure, but adding a second significant species of dark matter creates yet another coincidence that would have to be explained.

It is not a coincidence if there is a dynamic at work such as the interactions between protons and neutrons or a thermal creation of quasi-degenerate states with similar cross sections in the early universe. I do not think it very unlikely that a dark sector could exhibit features such as these. After all - the visible sector does. To be honest, I think explaining why the baryon and the DM densities are similar is more of a challenge.

 

[Chalnoth]

It is not a coincidence if there is a dynamic at work such as the interactions between protons and neutrons or a thermal creation of quasi-degenerate states with similar cross sections in the early universe. I do not think it very unlikely that a dark sector could exhibit features such as these. After all - the visible sector does. To be honest, I think explaining why the baryon and the DM densities are similar is more of a challenge.

That would be the potential explanation for such a coincidence.

But it still can't be an interaction mechanism anything like the one between protons and neutrons that would keep their densities similar, as self-interaction within the dark sector is fairly tightly-constrained. Because they can't really react strongly either with themselves or with other matter in order to explain the cosmological observations, it's not easy to moderate the density in such a way that both would come out with similar numbers (e.g. within a factor of 100).

With the most common models for dark matter, what you'd need is two different particles which have nearly identical cross-sections and masses. So far as I'm aware, in the visible sector this only happens between particles that are related by some sort of broken symmetry. The problem is, if they are so similar, then one should decay into the other unless they have exactly identical masses (at which point I'd wonder whether we should consider them different particles at all).

 

[rootone]

Is there any theory which posits the DM could be assemblages of different particles into units, just as regular atoms are?
Then though I guess the LUX experiment might have produced a result of that was the case.

 

[Orodruin]

The problem is, if they are so similar, then one should decay into the other unless they have exactly identical masses (at which point I'd wonder whether we should consider them different particles at all).

... or have no or a very supressed coupling to particles with masses smaller than the mass gap. It is not difficult to imagine such a scenario. The free neutron decays to a proton only because the electron (plus neutrino) are lighter than the proton-neutron mass gap. If the electron would have been much heavier (well, not that much) neutrons would not decay.

Is there any theory which posits the DM could be assemblages of different particles into units, just as regular atoms are?

Yes, but more like protons and neutrons are made from quarks. The words you are searching for are "composite dark matter".

 

[rootone]

The words you are searching for are "composite dark matter".

Thanks for that, it lead me to this:
http://arxiv.org/pdf/1512.01081v1.pdf
Which as a non-professional I found it to be comprehensible enough.
So dark analogs of atoms are candidates, but they would be very massive compared with ordinary matter.
The main problem though seems to be that in order for such a thing to form, we could expect abundances of isotopes in normal matter to be different to what they actually are.
A bonus though!. this theory apparently requires no rethinking of currently accepted physics.
It does require though, a single strange particle which is described in the paper as O−−

 

[Orodruin]

Thanks for that, it lead me to this:
http://arxiv.org/pdf/1512.01081v1.pdf
Which as a non-professional I found it to be comprehensible enough.
So dark analogs of atoms are candidates, but they would be very massive compared with ordinary matter.
The main problem though seems to be that in order for such a thing to form, we could expect abundances of isotopes in normal matter to be different to what they actually are.
A bonus though!. this theory apparently requires no rethinking of currently accepted physics.
It does require though, a single strange particle which is described in the paper as O−−

Be very careful of taking one arXiv paper as representative. There are other forms of composite dark matter that that paper does not cover.

 

[Traruh Synred]

QM will of course modify GR when and if we ever figure out how to make them consistent with each other. String/Brane theory claims to do this, but so far not in a very useful way. It conceivable that GR+QM will explain why the cosmological constant is so small, yet not zero, or may be not!
Q