It appears we might have the first genuine detection candidate for a DM particle in one of our liquid xenon detectors:

Berkeley Lab News Center

LZ Sees Surprising Result in Search for Dark Matter

Of course, a single detection event - if it is confirmed to be real - does not yet constitute a discovery, but it is interesting.

3 hours ago, Markus Hanke said:

It appears we might have the first genuine detection candidate for a DM particle in one of our liquid xenon detectors:

Berkeley Lab News Center

LZ Sees Surprising Result in Search for Dark Matter

Of course, a single detection event - if it is confirmed to be real - does not yet constitute a discovery, but it is interesting.

I know next to nothing about the physics proposed for WIMPs (if they exist). But if they are not subject to the EM interaction, in what sense can a WIMP "collide" with an atom of Xe? Something to do with the Strong Force? Or something else? And why would this create a cascade of electrons?

4 hours ago, exchemist said:

I know next to nothing about the physics proposed for WIMPs (if they exist). But if they are not subject to the EM interaction, in what sense can a WIMP "collide" with an atom of Xe? Something to do with the Strong Force? Or something else? And why would this create a cascade of electrons?

The weak interaction, perhaps.

If there is momentum transfer to the nucleus, you can leave electrons behind.

The process of detection is based on the direct hit and recoil of a WIMP with the Xenon atomic nucleus.
These events are exceedingly rare, and so, require very large amounts of pure liquified Xenon.
IOW, what Swansont said.

1 hour ago, MigL said:

The process of detection is based on the direct hit and recoil of a WIMP with the Xenon atomic nucleus.
These events are exceedingly rare, and so, require very large amounts of pure liquified Xenon.
IOW, what Swansont said.

Would the Weak interaction produce a force of mutual repulsion then?

I think I read that the new Nancy Grace telescope is hoped to be effective at studying Dark Matter.

So,fingers crossed.I think it will take a few months before it reaches its "mooring position"

3 hours ago, exchemist said:

Would the Weak interaction produce a force of mutual repulsion then?

Not as far as I know; it is more of a collision, producing a recoil of the xenon nucleus.

"WIMPs would be expected to interact exclusively with the liquid xenon nuclei, resulting in nuclear recoils that would appear very similar to neutron collisions. In order to single out WIMP interactions, neutron events must be minimized, through shielding and ultra-quiet building materials."

From Large Underground Xenon experiment - Wikipedia

Some excitement here - we live about 25 miles from the detector, located at the SURF Lab, in the old Homestake Gold Mine in Lead, SD. DK if they still have Neutrino Days*, but that was an annual weekend up there when you could visit the lab. Possibly the only nerd tourist attraction in the Black Hills. OK, there's a geology museum in Rapid City, too. Sort of.

* WIMP Days was also considered but for some odd reason didn't make the final cut.

1 hour ago, MigL said:

Not as far as I know; it is more of a collision, producing a recoil of the xenon nucleus.

"WIMPs would be expected to interact exclusively with the liquid xenon nuclei, resulting in nuclear recoils that would appear very similar to neutron collisions. In order to single out WIMP interactions, neutron events must be minimized, through shielding and ultra-quiet building materials."

From Large Underground Xenon experiment - Wikipedia

A collision must imply a force of repulsion, surely? These are QM entities, after all, not billiard balls.

1 minute ago, exchemist said:

A collision must imply a force of repulsion, surely? These are QM entities, after all, not billiard balls.

Or attraction. Celestial bits and bodies can impart momentum to each other without actually touching.

In QM it’s an exchange of a virtual particle, and that can be attractive or repulsive

1 hour ago, exchemist said:

A collision must imply a force of repulsion, surely? These are QM entities, after all, not billiard balls.

Sorry @exchemist
Didn't really clue in as to what you were asking.
As far as I know there is no repulsion or attraction, as with EM forces, and I would assume the interaction involved is the weak, as WIMPs must interact via the weak interaction.
There is obviously an 'exchange force' because momentum is transferred ( similarly to a collision of billiard balls ).
It is this momentum transfer, and resultant recoil of the nucleus, which leads to the electron effects and their electroluminescence ( which is detected )

20 hours ago, exchemist said:

But if they are not subject to the EM interaction, in what sense can a WIMP "collide" with an atom of Xe?

I readily admit that the technical details of the actual paper are way above my pay grade. However, from what I understand, they studied two different interaction mechanisms; the more probable one is based on the notion that both the particular type of WIMP in question and the atomic nucleus have a magnetic moment. When they come close enough, these interact, leading to transfer of momentum; since the WIMP is much heavier, it is then the nucleus that experiences a recoil.

To make this work, the WIMP would need to be electrically neutral and have a mass on the order of ~1 TeV. That’s a whole order of magnitude above anything the Standard Model offers right now.

Note that this data is also compatible with some proposed SUSY particles, particularly some versions of the Higgsino.

2 hours ago, Markus Hanke said:

I readily admit that the technical details of the actual paper are way above my pay grade. However, from what I understand, they studied two different interaction mechanisms; the more probable one is based on the notion that both the particular type of WIMP in question and the atomic nucleus have a magnetic moment. When they come close enough, these interact, leading to transfer of momentum; since the WIMP is much heavier, it is then the nucleus that experiences a recoil.

To make this work, the WIMP would need to be electrically neutral and have a mass on the order of ~1 TeV. That’s a whole order of magnitude above anything the Standard Model offers right now.

Note that this data is also compatible with some proposed SUSY particles, particularly some versions of the Higgsino.

Oh now that is interesting. I had thought Dark Matter was said not to take part in EM interactions. But if WIMPs can have a magnetic moment that clearly can't be right. So we have two possible mechanisms now: momentum transfer via the Weak Interaction, or via a magnetic interaction.

But I would have thought if these things have a magnetic moment they might be subject to EM absorption and emission via magnetic dipole interactions with EM radiation. So they would not be 100% dark!

Although.....thinking aloud here......unless there was something to create space quantisation, i.e. so that the energy of the state depended on spin orientation, I suppose there would be no absorption or emission.

Edited Friday at 08:08 AM5 days by exchemist

20 hours ago, exchemist said:

I had thought Dark Matter was said not to take part in EM interactions.

My understanding was that this means specifically that DM does not absorb or emit EM radiation, so that it can’t be detected in the EM spectrum. That wouldn’t preclude it having a magnetic moment. But maybe I’m wrong? Anyone?

21 hours ago, exchemist said:

momentum transfer via the Weak Interaction

How would that work?

3 hours ago, Markus Hanke said:

My understanding was that this means specifically that DM does not absorb or emit EM radiation, so that it can’t be detected in the EM spectrum. That wouldn’t preclude it having a magnetic moment. But maybe I’m wrong? Anyone?

How would that work?

I don't know how the weak interaction works. I'm just repeating what @swansont and @MigL suggested as a means for a WIMP to interact with an atom. But if the constituents of Dark Matter have magnetic properties I can imagine that could in principle have observable consequences.

7 hours ago, Markus Hanke said:

  On 9/4/2026 at 4:01 AM, exchemist said:

momentum transfer via the Weak Interaction

How would that work?

I'm not at all a valid authority on the weak force, and am just making some ( educated ? ) guesses.

The weak force interacts with the quarks in protons/neutrons by the exchange of W^+/- and Z bosons. These are massive 90 GeV bosons, with a limited 10^-24 sec lifetime ( because of HUP ) and limited range ( because of c ), so, heavier than a proton but with a range less than a proton diameter. The interaction is governed by QuantumFlavorDynamics, and exchanges the flavor of the quarks, and various types of neutrinos are always involved.

The one type of momentum transfer I am aware of, occurs with neutrinos

"The second type is called the "neutral-current interaction" because the weakly interacting fermions form a current with total electric charge of zero. It is responsible for the (rare) deflection of neutrinos."

From Weak interaction - Wikipedia

Whether this mechanism could account for the recoil of a Xenon nucleus is the question.
There do seem to be results to a google search of "momentum transfer via the weak interaction" but I don't have time to slog through them.

Remember that the W and Z are the exchange particles, not the WIMP. Their mass dictates the limited range of the interaction, as noted. That short range suggests a small cross-section for scatter, i.e. (describing in classical terms) you need a direct hit; the particles “look” extremely small in that analogy.

A massive particle could knock a Xe around a bit, just not very often

15 hours ago, MigL said:

The interaction is governed by QuantumFlavorDynamics, and exchanges the flavor of the quarks

Ok, but…my understanding is that WIMPs are thought to be a new type of elementary particle, so it’s no constituted of quarks that could interact with the quarks in the nucleus.

But I think we’re getting a bit ahead of ourselves here - there is still a non-negligible chance that the observed event is just a rare type of otherwise bog-standard ordinary physics; it could even just be a statistical fluke in the data. Remember, confidence sits at just around 3 sigma, not enough to be sure. So I think more data is needed to draw more definite conclusions.

9 hours ago, Markus Hanke said:

But I think we’re getting a bit ahead of ourselves here - there is still a non-negligible chance that the observed event is just a rare type of otherwise bog-standard ordinary physics; it could even just be a statistical fluke in the data. Remember, confidence sits at just around 3 sigma, not enough to be sure. So I think more data is needed to draw more definite conclusions.

Reminiscent of the Valentine’s Day monopole event from ~ half a century ago. Not a lot of definitive physics with a one-off event.

Still ...
It would be interesting to know why WIMPs are expected to recoil Xenon nucleii ( even if in extremely rare cases ).
There are too many such detection experiments for it to be a WAG, and 'hope for the best'.

31 minutes ago, MigL said:

Still ...
It would be interesting to know why WIMPs are expected to recoil Xenon nucleii ( even if in extremely rare cases ).
There are too many such detection experiments for it to be a WAG, and 'hope for the best'.

I suppose we need to look out for them to publish a paper on their findings.

7 hours ago, MigL said:

Still ...
It would be interesting to know why WIMPs are expected to recoil Xenon nucleii ( even if in extremely rare cases ).
There are too many such detection experiments for it to be a WAG, and 'hope for the best'.

Why wouldn’t a collision result in a recoil? Neutrinos undergo scattering interactions

1 hour ago, swansont said:

Why wouldn’t a collision result in a recoil? Neutrinos undergo scattering interactions

I agree, and that's what I've been claiming.

But what is 'special' about Xenon ?
If the particular electron luminescence effect needs the full outer orbital of the noble gases, why not one with a lighter nucleus, which would experience a greater recoil, such as Neon, Argon, or Krypton ?
( I discounted Helium because of Quantum effects when liquified )

1 hour ago, MigL said:

I agree, and that's what I've been claiming.

But what is 'special' about Xenon ?
If the particular electron luminescence effect needs the full outer orbital of the noble gases, why not one with a lighter nucleus, which would experience a greater recoil, such as Neon, Argon, or Krypton ?
( I discounted Helium because of Quantum effects when liquified )

https://lz.ac.uk/lz-experiment/how-it-works/

“[Xe] offers an extremely low background environment to search for rare interactions. Xenon has no problematic long-lived radioisotopes: any radioactivity produced by cosmic-rays, for example, decays away relatively quickly. Being a noble gas, it can be purified to exquisite levels, so that most other contaminants can be removed. In fact, the only backgrounds we really worry about come from other noble elements, such as argon, krypton and radon. In the liquid form, xenon is very dense indeed – aluminium floats in it! – and a large mass will fit within a relatively small cryostat”

I think the density matters because each nucleon would be considered a potential target. A larger recoil likely matters less than having more potential targets, as interactions would be rare.

Thanks.