The Searches For Dark Matter & A Map Of The Galaxies

My pleasure. If it’s any consolation, Fermilab shut down the Tevatron. The muon g-2 measurement and neutrino physics are the main active programs, as far as I can tell. CERN is it for collider experiments. Figuring out the dark matter puzzle could take awhile. By analogy, I think it was Bruno who first thought there would be planets around other stars. It didn’t end well for him and it took us nearly 500 years to find them.

Rad Antonov

Thanks for all that information. I figured that competent people have been considering ways of verifying the existence of DM, but I am out of that loop. Such is life outside of research. I wish that I had applied for an electrical engineering position at Fermilab when I got out of college, but I didn't feel competent at that point, now it's too late.

As to your question: 1. More powerful collider, if money is no object 2. More neutrino oscillation experiments that either rule out or confirm the existence of sterile neutrinos 3. Mach-Zehnder interferometer type experiments like the ones being performed by Kasevich’s group at Stanford: https://www.stanford.edu/group/kasevich/cgi-bin/wordpress/?page_id=71

Rad Antonov

Your logic is fine, though Pauli exclusion is not really necessary. In fact, the PandaX result that was cited is checking for something similar. They are looking for elastic collisions between dark matter that are accelerated by cosmic rays and nucleons in their detector mass. The facility is not new, nor is the result. This is the fourth iteration of a detector that seems to me was built with the primary purpose of looking at neutrinos. It's not very surprising they see nothing but noise. The other search that is checking for an electric dipole moment in neutrons relies on the assumption that an axion dark matter particle can interact via the strong force, not exactly dark. Again, the DM search is of secondary importance to checking if neutrons have an electric dipole. Similar dipole moment studies are done for electrons and there was a recent result improving the precision. The main point of these experiments is note a search for dark matter.

Rad Antonov

Thank you for that explanation. I had the feeling that you were getting at that but wasn't sure. My logic is this: 1. The standard model fails to provide any information as to what a dark matter particle could be, one that is not affected by the strong, weak, or electromagnetic forces, only gravity (Higgs field?). 2. Gravity is the only force affecting the dark matter particle, therefore gravity's relative weakness to the others is irrelevant, it is the strongest in context of the dark matter particle. 3. If dark matter particles are fermions then they must follow the Pauli Exclusion Principle with regards to the quantum state of mass, the others not being attributes of dark matter particles, and so it cannot exist in the same quantum state with another positive mass. 4. Therefore, a collision between an electron, in an electron beam, and a dark matter particle, just passing through the electron beam, is possible with the change in momentum of the electron involved in the collision being recorded in the detector. I understand the potential cross section problem, long duration, low probability, difficulty in measurement, but at the moment we have nothing. So why not just run a beam with no target but the potential dark matter particle? That's as systemic as any liquid scintillator, IMO at the moment. What experiment would you suggest?

The reason I am saying it is because that is what would be required to detect a dark matter particle by accounting for the momentum and finding out some of it is missing, which was described in the CERN paper you mentioned. I recognize you have something different in mind, which in principle is not incorrect. A dark matter particle will exert a gravitational force by virtue of its mass and thus deflect an incoming projectile. The technical question is what’s the cross section https://en.wikipedia.org/wiki/Cross_section_(physics)? At first glance we could say zero because these particles are point like by definition. Then we ask how much bigger they “appear” on account of their gravitational influence? I don’t know, no one else really does either. We don’t have a theory of gravity at the elementary particle level. We could ask, can we detect Earth’s gravity by running the beam? ATLAS claims their detector can resolve tracks 25 μm apart. For a free falling relativistic particle I find that we need 667 km to resolve the deflection. No problem you say, we put it in a ring, send it around and let it drift vertically and we should see it. Maybe, but now we want something much harder to see. A tiny variation of the gravitational background by the presence of a dark matter particle. You say, we’ll just get really close to it and 1/r^2 will get it done. Unfortunately, we have no target to aim at, we don’t even know why any of these particles would be floating around instead of condensing on the bottom of the beam pipe. Let’s say you spotted a pixel fire where you didn’t expect it to? How do you reproduce it to convince yourself this wasn’t just background, cosmic rays or even air that leaks in. It seems practically very difficult to build enough statistics to convince yourself, much less the rest of the world that you are scattering off dark matter particles. Hence, the more systematic approach of looking for the missing momentum becomes the method of choice.

Rad Antonov

OK. You keep saying that the collider might not have enough energy to PRODUCE a dark matter particle. I'm not even remotely suggesting that a dark matter particle is to be produced. My only point in that quote is about momentum, from the collision of two masses. I don't care how feeble gravity is compared to the other forces, gravity warps space-time, the others don't appear to. I am saying that an electron may collide with a dark matter particle like billiard balls due to them having mass and thus the electron would be detected because it was driven from the beam by the collision, a dark matter particle collision changed the momentum of the electron. You have yet to prove to me that this mass to mass collision, like billiard balls colliding on a table, is not possible. If there are two masses that must obey the Pauli Exclusion Principle, then they should interact, collide, via their masses.

The CERN paper is exactly what Tracey mentioned and I said the collider might not have enough energy to produce a dark matter particle in the proton collision. To be clear, this is very different from what you are describing. There is no dark matter target that deflects the proton. The dark matter is created in the collision but cannot be detected and so some momentum goes missing, which is how neutrinos were initially spotted. As for showing how feeble gravity is compared to the other forces, take a look at the magnitude of the coupling constants in the Wikipedia links.

Rad Antonov

I'm not a quantum physicist so you'll have to prove to me that a collision between two particles that have mass would be too feeble to register in a detector. While gravity is a weaker force than the others, there is nothing that I can think of that would indicate that an electron colliding with a dark matter particle that also has mass would not result in the electron being driven off-beam path such that it would register in a detector. I searched for and found the following that aligns with my idea (at https://home.cern/news/series/lhc-physics-ten/breaking-new-ground-search-dark-matter) but using protons: "So how has the LHC been looking for signs of dark-matter production in proton collisions? The main signature of the presence of a dark-matter particle in such collisions is the so-called missing transverse momentum. To look for this signature, researchers add up the momenta of the particles that the LHC detectors can see – more precisely the momenta at right angles to the colliding beams of protons – and identify any missing momentum needed to reach the total momentum before the collision. The total momentum should be zero because the protons travel along the direction of the beams before they collide. But if the total momentum after the collision is not zero, the missing momentum needed to make it zero could have been carried away by an undetected dark-matter particle."

It has everything to do with it. There are no billiard balls in particle accelerators. The collision, or rather scattering, to use the technical term, of an electron from a nucleon is mediated by the exchange of virtual photons. The physics is encapsulated in the diagrams that made Feynman famous. Pauli exclusion has nothing to do with the scattering. These are not identical particles that you are trying to jam in the same state. What’s the point of me laying all this out? It’s to show that an electron scattering off a dark matter particle would be an extremely feeble interaction, since the only force both particles can feel is gravity. That makes it hard to detect.

Rad Antonov

How an electron interacts with a nucleus isn't the issue. Up and down quarks are charged but dark matter particles are not, they are proposed to have only mass, from what I've read. Therefore, the only way that I can conceive of to detect them is to see if an electron traveling in a confined path through a vacuum is otherwise detected outside that confined path as a result of a collision with an uncharged object that has mass. So, like billiard balls, if masses at that scale obey the Pauli Exclusion Principle as I currently assume that they do.

How does the electron interact with the nucleus? What is the physics that actually takes place when we say an electron has crashed into a nucleon, proton or neutron, whichever you prefer.

Rad Antonov

"The only way these point masses know of each other’s existence is via their gravitational tug, which would be infinitesimally small." I don't think so. In normal operation, the beam is comprised of moving electrons that crash into a target, such as a nucleus. The electron isn't pulled into the nucleus by gravity or charge. In this case, a moving electron would collide with a dark matter particle and show up in the detector as being deflected from the beam path.

Except these are not billiard balls. The only way these point masses know of each other’s existence is via their gravitational tug, which would be infinitesimally small. At least on the rare occasion a neutrino collides with a nucleus via the weak force, it produces charged particles that radiate Cherenkov light.

Rad Antonov

I would guess any deflection would be noteworthy. If there is a stream of electrons with intermittent electron signals showing up in the detector, then it could be surmised that electrons are colliding with things in the vacuum that are not showing up. I don't see this as a "gravitational" collision, but an actual impact between two masses caused by their paths crossing.

Yes and what would determine how much the motion of the incoming projectile mass is perturbed by the gravitational collision with the dark matter mass?

Rad Antonov

That dark matter interacts via gravity means that it has mass and thus can collide with another mass. It can be treated as an invisible ball passing through an electron beam while we watch for electrons that happen to collide with it. No virtual anythings are required.

I understand, but by definition, the dark matter particle cannot “collide” with an electron via any known force except gravity. If it were to undergo elastic scattering by exchanging virtual photons, i.e. electromagnetic interaction, it would not be dark.

Rad Antonov

"A purely gravitational interaction would be imperceptible, even ignoring obvious questions like how do you even know where to aim the beam." I'm not calling for a gravitational interaction, only to look for an elastic collision between an electron and a dark matter particle.

There are 20 orders of magnitude difference between the respective force constants per unit mass/charge (https://en.wikipedia.org/wiki/Gravitational_constant vs https://en.wikipedia.org/wiki/Coulomb_constant). The LHC tops out at 7 TeV, which is 23 orders below a kg. A purely gravitational interaction would be imperceptible, even ignoring obvious questions like how do you even know where to aim the beam. Your main point stands. There is a puzzle here that will only be resolved with a clever measurement.

Rad Antonov

I'm describing a passive experiment, not trying to produce any "new" particle. It seems that the quantum zoo has run its course but there appears to be something unknown that interacts only via gravity. It has mass but no other useful attributes. So, try a simply experiment in which one looks for an electron collision with something invisible with no particle shower. The question is just how much energy is necessary to produce a reasonable deflection of two point masses?

There is a reason for colliders, but it’s unlikely an existing collider would have enough energy to produce the requisite particle and detect the type of interaction you describe. A more promising path is to search for missing energy, as Tracey described, or anomalies in certain reactions that are well understood in the standard model. For example, decays of b-quarks into leptons should be indifferent between muons and electrons, but there is growing evidence that’s not the case. More than anything, physicists need data that can guide the theorists. That could come from colliders, or neutrino detectors (there is speculation that neutrino oscillations could be sensitive to a minimum length from quantum gravity: https://arxiv.org/abs/1111.2341) or CMB polarization, but absent an experimental discovery, the field is lost.

Rad Antonov

I understand. However, as MOND doesn't have an answer to the gravitational lensing question, then it seems that there is still a reason for colliders, in this case to see if an electron gets deflected by something that isn't itself detectable except by either lensing or such a collision. As detecting neutrinos requires a huge statistical advantage, such as huge reservoirs of clear liquid surrounded by large numbers of photodetectors that operate for years, so too might detecting dark matter particles, such as a large number of electrons traveling in one direction through detectors over a period of years. It might end up not costing that much nor needing the high energy colliders. If dark matter interacts only via gravity, then such an experiment may be the only way to verify their existence at that level.

thank you

hb

The LHC was the second best option after the SSC was canceled in the early 90s. That was the collider that particle physicists were pinning their hopes on and making promises for. The collision energy was going to be 40 TeV vs 14 TeV at the LHC. There was even doubt that the LHC would have enough energy to find the Higgs. Doesn’t matter anymore. The money isn’t there to build anything else, so particle physicists are in a deep existential crisis.

Rad Antonov

1. There is more than enough water to make hydrogen for fuel without straining any potable freshwater sources. And as water is recreated in use of a fuel cell, the water is returned to the cycle and so it is net zero. No water is ever lost. So, losing water as a source of hydrogen is similar to its loss to evaporation. Of course, drought conditions would play a role in determining production viability . 2. "...what it should find were not based in reality, but rather based on unsupported assumptions." I'm glad that you said that because I had the same feeling, but without the requisite physics background. I 3. "..but it looks like from the abstract in the 2nd linked paper on fermionic dark matter that dark matter scattering off of nuclei has already been searched for and likely has come up negative." I can't find a link to that 2nd paper but in the first they discuss what may be the same issue in "5.1 Direct Detection: Direct detection experiments measure the properties of WIMPs as they elastically scatter with atomic nuclei." But in my mind if we know nothing about these proposed particles, WIMPS, then perhaps seeing no scattering of nuclei will prove to be valid simply because the nuclei are far more massive than the WIMP as there is no way to "see" a scattered WIMP. Therefore, the only valid substitute in my mind would be electrons because they're far less massive. However, the cross section issue makes that more like finding neutrinos as in more time and more electrons improve the statistics.

About the abundance of water, it is important to distinguish between freshwater (perhaps specifically potable freshwater) and saltwater. Freshwater is a valuable commodity that is abused and mistreated and contaminated at a pretty high rate making it scarce in many areas where it used to be abundant and making a lot of people worried that, especially due to the side effects of climate change, we may run into serious global freshwater supply problems. Desalination of the abundant saltwater reserve seems to be an obvious thing to do, but its current implementation is pretty destructive towards the local ecosystem around the desalination plants. I'm not arguing for or against EBR II reactors or producing hydrogen with them or running closed-loop systems, I'm just trying to clarify Jason's statement about water being a scarce resource. I think the 2008 dark matter at the LHC paper and your questions exactly outline Sabine's frustrations leading to her first book. The LHC was built with all of these expectations that it would find so much stuff and solve so many problems, but the arguments behind what it should find were not based in reality, but rather based on unsupported assumptions. I'm not up on all the different ways that the different hypothetical forms of dark matter can be tested in colliders, but it looks like from the abstract in the 2nd linked paper on fermionic dark matter that dark matter scattering off of nuclei has already been searched for and likely has come up negative. If one tried the collision/scattering with an electron beam, since the cross section for collisions is much smaller for electrons than for nuclei, the resulting scattering signal would also be much smaller and even more difficult to detect.

Thanks Tracey and Happy Friday! I didn't see your reply before I added 3a to my original post as it was "hidden" by that "See other replies" control. In that paper it is stated: "In the next few years, the Large Hadron Collider (LHC) shows the promise of producing these elusive particles and possibly measuring their microscopic properties [1,2]." and then in 3.1 Dark Matter as a Thermal Relic: "When combined with cosmological observations – we expect new physics at the electroweak scale." But what if that expectation, electroweak, of the dark matter particle is simply wrong? What if the idea of SUSY is simply wrong? What if it is only mass, gravity (quantum gravity?)? That could mean that dark matter by nature would not be capable of being produced in the LHC but it could still be detected via dark matter mass to electron mass collisions, correct?

1. Water is an abundant resource and unlike any other energy source the overall supply is maintained as when it combines with oxygen it reenters the system as water! A closed loop system. Hydrogen is understood by energy companies as the energy source for the future (https://www.powermag.com/six-major-electric-utilities-join-forces-to-pursue-a-southeastern-hydrogen-hub/) but we need to generate it from water, not fossil fuels as is done now, and from fast breeders at various locations as shown in the "Hub" article so that we don't have to transport it very far and regions can be self sufficient in that regard. That should have been the goal since 1986 when the EBR II proved itself safe and capable. 2. There MUST be an alternate explanation of gravitational lensing, either from MOND or something else. I've seen none, but I'm not an astrophysicist.

I don't know what the point of the PRL comment was, but for anyone who took it to imply that the most prestigious physics journal is shutting out alternatives to CDM, rest assured that it also publishes MOND papers, such as this recent advance that can account for the CMB spectrum: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.161302 (pre-print here: https://arxiv.org/pdf/2007.00082.pdf). It's a torturous exercise to get through it. Suffice it to say that to fit the data, the authors need two new fields, one scalar, the other a vector, which of course no one has ever seen. To hang your hat on that, while "explaining" that other approaches are like monkeys typing behind typewriters is...well, you decide.

Rad Antonov

Hi Jeffery, regarding your 3rd point, this kind of dark matter should still carry energy and so one could search for missing energy or momentum from collider experiments -- but just saying this is not helpful. This paper from 2008 (https://arxiv.org/pdf/0807.2244.pdf) set out some high hopes for finding dark matter with the LHC that have not come to fruition. Regarding point 2, there's just so much wide-ranging evidence for dark matter that it is hard to get around -- I just read today that the Shapiro delays for the gravitational waves and the gamma rays from GW170817 are yet more evidence for LambdaCDM over MOND-like models. I like the idea of a dark matter that transitions to superfluid behavior on galactic scales so that LambdaCDM is retained on cosmological scales and MOND-like behavior is retained on galactic scales, but I do wonder if "fixing" galactic scale behavior with a superfluid breaks the other, lesser bits of evidence like these Shapiro delays.

1. Regarding producing hydrogen from water, water is also a scarce resource. Some of that new hydrogen may find its way back into new water when energy is more abundant than old water. 2. An opinion of dark matter is that it is a convenient placeholder for something that is missing. The mathematics can abstract the missing physical observations with variables. Each model's mathematics's unobserved variables provide clues to observing new phenomena. Each search for new observations to add to science can only follow so many clues, if the dark matter content of a model is not acceptable, pick a different model and use its findings to fill in the gaps of the other models.

Jason Bolton

1. We should be producing hydrogen from water with an industrial scale EBR II. That would end the need for fossil fuels altogether. 2. Regarding dark matter, while MOND seems to work for galactic clusters it doesn't seem to provide an answer to gravitational lensing. Any new ideas as to what would cause that without the need for fundamental particles that interact only via gravity? 3. Regarding dark matter part 2, IF dark matter is a real fundamental particle that can be detected only via gravity, then isn't it a fact that it wouldn't be found with any collider because they only detect composite particles, such as the unstable delta baryons, and fundamental particles that make up composite particles, such as quarks. If dark matter particles do not interact then there's no reason to consider the collider as the tool for their discovery, correct? 3a. I was thinking yesterday, after I had shut down, that perhaps colliders could find dark matter. It seems that as the electron has mass as does dark matter then that would be the "charge" that would enable interaction. Therefore, couldn't running an electron beam alone and then looking for electron scattering from an electron colliding with a dark matter particle, versus those with neutrinos, be a valid test?

The underlying question with respect to dark matter searches is how many more of these experiments need be performed (and funded) before the overwhelming results are accepted? Dark matter isn't there. Period. If your favorite mathematical models need dark matter to work properly then they are inconsistent with the scientific realm of empirical reality; they are wrong.

Super cool is right! I haven’t been to the AMNH in years and this calls for a visit. Thanks Tracey.

Rad Antonov

12 years ago, the American Museum of Natural History produced a fly-through (zoom out and then back in) of basically the data set represented in the map of the observable universe, https://www.youtube.com/watch?v=17jymDn0W6U. The map on that web page has more data due to the spate of sky surveys in the intervening time, but the video is super cool too.