Monday, 28 May 2018


After today's update from the XENON1T experiment, the situation on the front of direct detection of WIMP dark matter is as follows

WIMP can be loosely defined as a dark matter particle with mass in the 1 GeV - 10 TeV range and significant interactions with ordinary matter. Historically, WIMP searches have stimulated enormous interest because this type of dark matter can be easily realized in models with low scale supersymmetry. Now that we are older and wiser, many physicists would rather put their money on other realizations, such as axions, MeV dark matter, or primordial black holes. Nevertheless, WIMPs remain a viable possibility that should be further explored.
To detect WIMPs heavier than a few GeV, currently the most successful strategy is to use huge detectors filled with xenon atoms, hoping one of them is hit by a passing dark matter particle. Xenon1T beats the competition from the LUX and Panda-X experiments because it has a bigger gun tank. Technologically speaking, we have come a long way in the last 30 years. XENON1T is now sensitive to 40 GeV WIMPs interacting with nucleons with the cross section of 40 yoctobarn (1 yb = 10^-12 pb = 10^-48 cm^2). This is 6 orders of magnitude better than what the first direct detection experiment in the Homestake mine could achieve back in the 80s. Compared to the last year, the  limit is better by a factor of two at the most sensitive mass point. At high mass the improvement is somewhat smaller than expected due to a small excess of events observed by XENON1T, which is probably just a 1 sigma upward fluctuation of the background.

What we are learning about WIMPs is how they can (or cannot) interact with us. Of course, at this point in the game we don't see qualitative progress, but rather incremental quantitative improvements. One possible scenario is that WIMPs experience one of the Standard Model forces,  such as the weak or the Higgs force. The former option is strongly constrained by now. If WIMPs had interacted in the same way as our neutrino does, that is by exchanging a Z boson,  it would have been found in the Homestake experiment. Xenon1T is probing models where the dark matter coupling to the Z boson is suppressed by a factor cχ ~ 10^-3 - 10^-4 compared to that of an active neutrino. On the other hand, dark matter could be participating in weak interactions only by exchanging W bosons, which can happen for example when it is a part of an SU(2) triplet. In the plot you can see that XENON1T is approaching but not yet excluding this interesting possibility. As for models using the Higgs force, XENON1T is probing the (subjectively) most natural parameter space where WIMPs couple with order one strength to the Higgs field. 

And the arms race continues. The search in XENON1T will go on until the end of this year, although at this point a discovery is extremely unlikely. Further progress is expected on a timescale of a few years thanks to the next generation xenon detectors XENONnT and LUX-ZEPLIN, which should achieve yoctobarn sensitivity. DARWIN may be the ultimate experiment along these lines, in the sense that there is no prefix smaller than yocto it will reach the irreducible background from atmospheric neutrinos, after which new detection techniques will be needed.  For dark matter mass closer to 1 GeV, several orders of magnitude of pristine parameter space will be covered by the SuperCDMS experiment. Until then we are kept in suspense. Is dark matter made of WIMPs? And if yes, does it stick above the neutrino sea?


Cédric Bardot said...

The "kind of detectors that made great serendipitous discoveries : neutrinos from SN1987A, neutrino oscillations" were first built to look for proton decay ... that "has not been observed to date, and who knows what future proton-decay detectors may uncover" wrote Alvaro de Rujula in

Dark matter direct detectors have not yet produced any serendipitous discoveries ... but what if the next hint of physics beyond the standard model was waiting for us in the neutrino background of dark matter detectors?

Mad Hatter said...

I think this is unlikely. We already measured these neutrinos quite well by other means (superKamiokande, Borexino, ... ), and to my understanding we won't learn anything new about them from coherent scattering in future xenon detectors. If I'm missing something I'll be glad to learn :)

Anonymous said...

How come no one talks about dark matter of the same mass range but only interacts via gravity. Isn't that a reasonable proposition even in supersymmetry/string models?

Mad Hatter said...

That's a possibility. It is plausible to assume that dark matter couples to ordinary matter because it ensures the former is produced by thermal processes in the early universe. But it's also easy to construct models where the dark matter production mechanism is non-thermal, and there is no observable coupling between the dark and ordinary matter. This would be the 'nightmare scenario of cosmology'...

Cédric Bardot said...

@ Mad Hatter

If one could guess the kind of fish waiting for Dark Matter trawlers in the neutrino sea that would not be serendipitous discovery anymore.

I cannot believe there is no unexpected interesting physics that could "emerge" from probing such an amount of stuff under such a scrutiny with such brilliant falks (even with a potential null result ;-)

As far as astrophysics is concerned there seems to be mitigated hope to gain info from xenon dark matter detectors:

"XENON1T (XENONnT and LZ; DARWIN) experiments will be sensitive to a supernova burst up to 25 (35; 65) kpc from Earth at a significance of more than 5 sigma, observing approximately 35 (123; 704) events from a 27 Msun supernova progenitor at 10 kpc. Moreover, it will be possible to measure the average neutrino energy of all flavours, to constrain the total explosion energy, and to reconstruct the supernova neutrino light curve. Our results suggest that a large xenon detector such as DARWIN will be competitive with dedicated neutrino telescopes, while providing complementary information that is not otherwise accessible". (

Anonymous said...

What about non-standard interactions? If we didn't get success for standard spin-dependent and spin-independent interactions, would it be right to say that this is the picture with non-standard interactions also? I am curious to know.

Mad Hatter said...

That's a possibility too... The simplest example is the so-called inelastic dark matter, where the dark matter particle transforms into another state after interacting with matter. Not the most plausible one imo - it's more likely that dark matter is simply not a WIMP. Unfortunately, the possibilities are endless at this point...

Anonymous said...

WIMPS could of course be much heavier than the 10 TeV you mention in the article.
In Type 1 Seesaw Models, the Majorana mass of the right-handed neutrino is on the order of 10^10 GeV. (10,000,000,000 GeV)
To both explain leptogenesis and the small mass of the left-handed neutrino, the Seesaw Majorana mass needs to be on the order of 4*10^10 GeV.

While this number is well above the energy at the LHC, if Type I Seesaw is the correct model, there should be indirect signs at LHC. For example, in the Seesaw model, there is still a Yuakawa coupling of the left-handed neutrinos with the Higgs field (and right handed anti-neutrinos.) This would show up as an invisible decay of the Higgs bosons into neutrino pairs.

To explain leptogenesis and the small mass of the left-handed neutrino, the Higgs boson should decay into tau-like neutrinos roughly with the same branching ratio as it decays into tau leptons. ATLAS and CMS both constrain decay into invisible particles like neutrinos, but so far, the constraints do not rule out these Type I Seesaw models.

What are your thoughts on Type I Seesaw Models with Majorana Masses near 10^10 GeV?

RBS said...

Jester, what is the rationale for dark matter interacting with regular matter at all (other than gravitationally)? Are there any indications for that theoretical or empirical, other than it's something that can be probed?

Mad Hatter said...

Currently there is no robust experimental indications for non-gravitational interactions between dark and ordinary matter. Theoretically it seems plausible: given that the amounts of dark and baryonic matter in the universe are similar, it is natural to think that the two could talk to each other in the early epoch of the universe when their abundances were set. But it doesn't have to be so, and one can easily construct models where baryonic and dark matter are produced by completely independent mechanisms.

Mad Hatter said...

Thanks Cédric, yes, I agree, that's at least one interesting potential use of the future WIMP detectors.

andrew said...

"How come no one talks about dark matter of the same mass range but only interacts via gravity. Isn't that a reasonable proposition even in supersymmetry/string models?"

Because any truly "sterile" dark matter can't be detected in a direct dark matter detection model, and such particles don't really have a place in the most straightforward supersymmetry/string models, which generally utilize the weak interaction or something like it to cause all but the lightest supersymmetric particle to decay rapidly with the LSP remaining as a dark matter candidate due to R-symmetry.

"WIMPS could of course be much heavier than the 10 TeV you mention in the article."

The astronomy data strongly disfavor heavy dark matter candidates as they would give rise to to much small scale structure (e.g. at the scale of satellite galaxies and sub-halos of galaxies) which is not observed. The sweet spot from the astronomy data perspective is in the warm dark matter keV mass range. Strictly speaking, the question is average dark matter particle velocity rather than dark matter particle mass, but the two are tightly correlated in most scenarios.

andrew said...

@RBS "Jester, what is the rationale for dark matter interacting with regular matter at all (other than gravitationally)? Are there any indications for that theoretical or empirical, other than it's something that can be probed?"

A couple of arguments regarding this question are the following, both of which are admittedly currently only pre-prints:

Lin Wang, Da-Ming Chen, Ran Li "The total density profile of DM halos fitted from strong lensing" (July 31, 2017). Abstract:

"In cosmological N-body simulations, the baryon effects on the cold dark matter (CDM) halos can be used to solve the small scale problems in ΛCDM cosmology, such as cusp-core problem and missing satellites problem. It turns out that the resultant total density profiles (baryons plus CDM), for halos with mass ranges from dwarf galaxies to galaxy clusters, can match the observations of the rotation curves better than NFW profile. In our previous work, however, we found that such density profiles fail to match the most recent strong gravitational lensing observations. In this paper, we do the converse: we fit the most recent strong lensing observations with the predicted lensing probabilities based on the so-called (α,β,γ) double power-law profile, and use the best-fit parameters (α=3.04,β=1.39,γ=1.88) to calculate the rotation curves. We find that, at outer parts for a typical galaxy, the rotation curve calculated with our fitted density profile is much lower than observations and those based on simulations, including the NFW profile. This again verifies and strengthen the conclusions in our previous works: in ΛCDM paradigm, it is difficult to reconcile the contradictions between the observations for rotation curves and strong gravitational lensing."

As the body text explains:

"It is now well established that, whatever the manners the baryon effects are included in the collisionless CDM N-body cosmological simulations, if the resultant density pro- files can match the observations of rotation curves, they cannot simultaneously predict the observations of strong gravitational lensing (under- or over-predict). And for the case of typical galaxies, the reverse is also true, namely, the SIS profile preferred by strong lensing cannot be supported by the observations of rotation curves near the centers of galaxies."

* Paolo Salucci and Nicola Turini, "Evidences for Collisional Dark Matter In Galaxies?" (July 4, 2017). Abstract:

"The more we go deep into the knowledge of the dark component which embeds the stellar component of galaxies, the more we realize the profound interconnection between them. We show that the scaling laws among the structural properties of the dark and luminous matter in galaxies are too complex to derive from two inert components that just share the same gravitational field. In this paper we review the 30 years old paradigm of collisionless dark matter in galaxies. We found that their dynamical properties show strong indications that the dark and luminous components have interacted in a more direct way over a Hubble Time. The proofs for this are the presence of central cored regions with constant DM density in which their size is related with the disk length scales. Moreover we find that the quantity ρDM(r,L,RD)ρ⋆(r,L,RD) shows, in all objects, peculiarities very hardly explained in a collisionless DM scenario."

andrew said...

More problems for dark matter models:

* Dark matter distributions have to closely track baryon distributions, even though there is no viable mechanism to do so: Edo van Uitert, et al., "Halo ellipticity of GAMA galaxy groups from KiDS weak lensing" (October 13, 2016).

* Cold dark matter models don't explain the astronomy data.

"Evidence that Cold Dark Matter (ΛCDM), CDM+ baryons and its proposed tailored cures do not work in galaxies is staggering, and the CDM wimps (DM particles heavier than 1 GeV) are strongly disfavoured combining theory with galaxy astronomical observations."

* Baryon effects can't save cold dark matter models.

andrew said...

Leptophillic dark matter also have a seriously constrained parameter space:

"Dark matter and neutrinos provide the two most compelling pieces of evidence for new physics beyond the Standard Model of Particle Physics but they are often treated as two different sectors. The aim of this paper is to determine whether there are viable particle physics frameworks in which dark matter can be coupled to active neutrinos.

We use a simplified model approach to determine all possible renormalizable scenarios where there is such a coupling, and study their astrophysical and cosmological signatures.

We find that dark matter-neutrino interactions have an impact on structure formation and lead to indirect detection signatures when the coupling between dark matter and neutrinos is sufficiently large. This can be used to exclude a large fraction of the parameter space.
In most cases, dark matter masses up to a few MeV and mediator masses up to a few GeV are ruled out. The exclusion region can be further extended when dark matter is coupled to a spin-1 mediator or when the dark matter particle and the mediator are degenerate in mass if the mediator is a spin-0 or spin-1/2 particle."

Andres Olivares-Del Campo, et al., "Dark matter-neutrino interactions through the lens of their cosmological implications" (November 14 2017).

theonh55 said...

The DM riddle would be easy cake for 1.8 eV neutrinos. They are "strongly ruled out", but soon Katrin will do the absolute test.
For lensing by the galaxy clusters A1689 and A1835, thermal pseudo-Dirac neutrinos perform well, unlike other proposals like NFW, isothermal, thermal bosons, MACHOs made of axions or not, or primordial black holes.
arXiv:1710.01375 Subjecting dark matter candidates to the cluster test

Likewise, MOND, f(R) and so on meet troubles in clusters, arXiv:1610.01543
How Zwicky already ruled out modified gravity theories without dark matter

Galactic DM should then be baryonic, zillions of Earth mass Machos, also "ruled out".

RBS said...
This comment has been removed by the author.
Psychedelic fathomer said...

Cold dark maatter is the simplest explanation for the mass discrepancy in astrophysics and cosmology. According to our understanding of physics and our prejudices. However as nature has shown us many times before, she does not give a damn about our understanding of her nor our prejudices.

RBS said...

@andrew, thanks for these links so if my understanding is correct there may be indications that just gravitational interaction with DM wouldn't be sufficient to explain cosmological observations like galaxy dynamics (or cooler than expected gas in the early Universe?). I'd like to understand though what would be the confidence level of these arguments, even on a scale from "perhaps" to "should"?

Xezlec said...

I've always thought the next prefix ought to be "xacto-", like the knife.

Glad you're back, even if physics is over.