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Signal of neutrino dark matter

By Dr Adam Falkowski (Résonaances; Orsay, France)

The title of this post is purposely over-optimistic in order to increase the traffic. A more accurate statement is that a recent analysis

Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters by Esra Bulbul and 5 co-authors (NASA/Harvard-Smithsonian)
of X-ray spectrum of galactic clusters claims the presence of a monochromatic \(3.5\keV\) photon line which can be interpreted as a signal of a\[

\large{m_{\nu({\rm ster})} = 7\keV }

\]sterile neutrino dark matter candidate decaying into a photon and an ordinary neutrino. It's a long way before this claim may become a well-established signal. Nevertheless, in my opinion, it's not the least believable hint of dark matter coming from astrophysics in recent years.

First, let me explain why one would anyone dirty their hands to study X-ray spectra. In the most popular scenario the dark matter particle is a WIMP — a particle in the \(\GeV\)-\(\TeV\) mass ballpark that has weak-strength interactions with the ordinary matter. This scenario may predict signals in gamma rays, high-energy anti-protons, electrons etc, and these are being searched high and low by several Earth-based and satellite experiments.

But in principle the mass of the dark matter particle could be anywhere between \(10^{-30}\) and \(10^{50}\GeV\), and there are many other models of dark matter on the market. One serious alternative to WIMPs is a \(\keV\)-mass sterile neutrino. In general, neutrinos are dark matter: they are stable, electrically neutral, and are produced in the early universe. However we know that the 3 neutrinos from the Standard Model constitute only a small fraction of dark matter, as otherwise they would affect the large-scale structure of the universe in a way that is inconsistent with observations. The story is different if the 3 "active" neutrinos have partners from beyond the Standard Model that do not interact with W- and Z-bosons — the so-called "sterile" neutrinos. In fact, the simplest UV-complete models that generate masses for the active neutrinos require introducing at least 2 sterile neutrinos, so there are good reasons to believe that these guys exist. A sterile neutrino is a good dark matter candidate if its mass is larger than \(1\keV\) (because of the constraints from the large-scale structure) and if its lifetime is longer than the age of the universe.

How can we see if this is the right model? Dark matter that has no interactions with the visible matter seems hopeless. Fortunately, sterile neutrino dark matter is expected to decay and produce a smoking-gun signal in the form of a monochromatic photon line. This is because, in order to be produced in the early universe, the sterile neutrino should mix slightly with the active ones. In that case, oscillations of the active neutrinos into sterile ones in the primordial plasma can populate the number density of sterile neutrinos, and by this mechanism it is possible to explain the observed relic density of dark matter. But the same mixing will make the sterile neutrino decay, as shown in the diagrams here. If the sterile neutrino is light enough and/or the mixing is small enough then its lifetime can be much longer than the age of the universe, and then it remains a viable dark matter candidate.

The tree-level decay into 3 ordinary neutrinos is undetectable, but the 2-body loop decay into a photon and and a neutrino results in production of photons with the energy\[

\large{E=\frac{m_{\rm DM}}{2}.}

\] Such a monochromatic photon line can potentially be observed. In fact, in the simplest models sterile neutrino dark matter heavier than \(\approx 50\keV\) would produce a too large photon flux and is excluded. Thus the favored mass range for dark matter is between \(1\) and \(50\keV\). Then the photon line is predicted to fall into the X-ray domain that can be studied using X-ray satellites like XMM-Newton, Chandra, or Suzaku.

Until last week these searches were only providing lower limits on the lifetime of sterile neutrino dark matter. This paper claims they may have hit the jackpot. The paper use the XMM-Newton data to analyze the stacked X-ray spectra of many galaxy clusters where dark matter is lurking. After subtracting the background they see is this:

Although the natural reaction here is a loud "are you kidding me", the claim is that the excess near \(3.56\keV\) (red data points) over the background model is very significant, at 4-5 astrophysical sigma. It is difficult to assign this excess to any know emission lines from usual atomic transitions. If interpreted as the signal of sterile neutrino dark matter, the measured energy and the flux corresponds to the red star in the plot, with the mass \(7.1\keV\) and the mixing angle of order \(5\times 10^{-5}\). This is allowed by other constraints and, by twiddling with the lepton asymmetry in the neutrino sector, consistent with the observed dark matter relic density.

Clearly, a lot could go wrong with this analysis. For one thing, the suspected dark matter line doesn't stand alone in the spectrum. The background mentioned above consists not only of continuous X-ray emission but also of monochromatic lines from known atomic transitions. Indeed, the \(2\)-\(10\keV\) range where the search was performed is pooped with emission lines: the authors fit 28 separate lines to the observed spectrum before finding the unexpected residue at \(3.56\keV\). The results depend on whether these other emission lines are modeled properly. Moreover, the known argon XVII dielectronic recombination line happens to be nearby at \(3.62\keV\). The significance of the signal decreases when the flux from that line is allowed to be larger than predicted by models. So this analysis needs to be confirmed by other groups and by more data before we really get excited.

Decay diagrams borrowed from this review. For more up-to-date limits on sterile neutrino DM see this paper, or this plot. Update: another independent analysis of XMM-Newton data observes the anomalous 3.5 keV line in the Andromeda and the Perseus cluster. The text was reposted from Adam's blog with his permission...

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reader Shannon said...

Any anthem is patriotic... Ours is brutal in words but we French don't take it "at face value".... at least since the Revolution :-).

reader CIPig said...

We know that dark matter is gravitationally bound to galaxies and galaxy clusters. How could this be true for a low mass particle unless it is implausibly cold? That would seem to require decoupling from the particle soup at rather late times. And 7 Kev is really cold.

reader Uncle Al said...

1) arXiv:1306.3983, 1306.5534.
2) bullsh*t on sterile neutrinos.
3) arXiv:1310.4009; and Milgrom acceleration can be empirically sourced in existing apparatus, on a bench top, within 90 days.

reader Shannon said...

The FN doesn't care where you are born. It is your passport that tells which nationality you are.
Anyway you've been so brainwashed... I can only give you a look of exhaustion and ever so slight amusement:

reader Luboš Motl said...

Dear Pig, you are confused. 7 keV is still a huge energy relatively to the room temperature kT. 7 keV divided by the Boltzmann constant is about 80 million Celsius degrees:

So of course that such neutrinos are still cold dark matter.

reader Eugene S said...

Glad to amuse you, but "FN doesn't care where you are born": it's not easy to square that with the information found on Wikipedia:

FN doesn't care where you are born

In the early years of the FN, immigration policy was only a minor
issue for the party, although it did call for immigration to be reduced.[34]
Themes of exclusion of non-European immigrants was largely brought into
the party in 1978, with the arrival of Jean-Pierre Stirbois and his
"solidarist" group. The topic subsequently became increasingly important
in the early 1980s.[130]

In more recent popular and even academic press, the party's program has often been reduced to the single issue of immigration.[127] The party opposes immigration, particularly Muslim immigration from North Africa, West Africa, and the Middle East. In a standardised pamphlet delivered to all French electors in the 1995 presidential election, Le Pen proposed to send "three million non-Europeans" out of France by "humane and dignified means."[131]
Over the years, and especially since the 1999 split, the FN has
cultivated a more moderate image on issues of immigration and Islam, at
least compared to some of the proposals of Mégret's MNR or Philippe de Villiers's Movement for France.[132]
It does no longer expressly support the systematic repatriation of
legal immigrants, although it supports the deportation of illegal,
criminal, and unemployed immigrants.[132]

Since becoming leader of the party in 2011, Marine Le Pen has focused mostly on the perceived threat against the secular value system of the French Republic. She has criticised Muslims, for what she sees as their intents to impose their own values on the country.[133] Following the rebellions in several Arab countries, she has been active in campaigning on halting the migration to Europe of Tunisian and Libyan immigrants.[134]

reader CIPig said...

Thx for link

reader imho said...

They lost me with: "the authors fit 28 separate lines to the observed spectrum"... Plus the density of states is large in that region??? I'm not very hopeful.

reader Eugene S said...

in principle the mass of the dark matter particle could be anywhere between 10^-30 and 10^50 GeV

Eh? The mass of the moon, expressed in gigaelectronvolts, is 4.1207x10^49 GeV.
So a single "particle" of dark matter could be more than twice as massive as the moon?

reader Shannon said...

Dear Eugene, Jean-Marie Le Pen is the godfather of about 30 persons! He has never refused when it was asked genuinely. It doesn't "cost" anything. So when Dieudonné asked him he accepted, they are not sectarian at the FN.

reader Eugene S said...

Sorry Shannon, but that is extreme naivete at best. Not buying it.

I'll sum up my thoughts and then bow out of this conversation.

Front National is not racist, fascist or Nazi, although some of their members surely are, but that is true for all the political parties. They may call themselves "patriotic" but it means nothing, as the other parties (with the possible exception of the communists) are equally entitled to it.

In France, they are properly designated national-populist, but in English political language they are nativists.

"Les français d'abord" (The French come first) was the title of one of Jean-Marie Le Pen's books, and his daughter has recently re-used the slogan. That is nativism in a nutshell, even if it is slightly modified to include naturalized citizens.

Again, I do not consider nativists "beyond the pale", I believe they should not be excluded from political discourse and their legitimate aims deserve to be represented in the political arena. If FN can further purge itself of antisemites and racists, they will move closer to the center (and the center closer to them) and their influence will grow. If they cannot, it won't.

reader lucretius said...

Actually very few languages recognise this concept. The Wikipedia has just 10 different language entries, mostly very short ones, one of them (the Serbian( being actually a mistake - the link takes one to the entry for psychological “nativism”. Curiously the Croat version (the only one in a Slavic language) is fairly long but most of it seems to be devoted to what might be perhaps called “regional nativism” (in this case native Zagrebians vs. migrants).

On the other hand, “nationalism” has lots of entries (of very uneven quality ). The concept of “nativism” appears to be generally unknown outside countries with recent immigration problems. Of course there are also countries, particularly in the former eastern block that have old and unresolved minority problems (most of all with Hungarians since as the result of being on the wrong side in both world wars Hungary lost 30% of its territory and 3 million Hungarians found themselves living in countries that have been either traditional subjects or traditional enemies (usually both). But nobody uses the word “nativism” in this context, it’s just a traditional clash of competing nationalism.

I don’t have the time write now to write on these complex subjects. I have to say, I have a very low regard for the French National Front, its leaders and its supporters (sorry Shannon). But I am not afraid of them in any sense directly. However, I remember well what the late James Goldsmith ( ), who was a very perceptive men (and a strong opponent of the EU): “The French believe they can ride the German stallion but they will discover that they are only the stable boy”. I believe that the main achievement of the NF, if they win power, will be to make the French discover this a little earlier. I worry a little more what will happen when the Germans are finally forced to openly display they power and find that it is pleasant experience.

The late grate worrier and write Ernst Junger, who was a fierce German nationalist (but an anti-Nazi one) finally “repented” of his views when he was about 100 years old and decided to support the EU. In reference to his old nationalist views he only said “our nationalism was only an imitation of the French. We always imitate them. ” (I am paraphrasing now because I have no time to look up the exact words, although I have the book on my shelf).

The only anti-immigration movement in Europe that I strongly supported, the only politician who expressed my views almos tin their entirety was the late Pim Fortuyn. Not surprisingly, he refused to have anything at all to do with Le Pen, he did not think they could even collaborate on a limited scale. I am very much in favour of making broad coalitions - for example, I have no problems with allying myself with American fundamentalist Christian, or evangelicals or with the more enlightened factions of the Christian right or even with Russian nationalists of Alexei Navalny’s type, but National Front both in its older and present from I would not have anything to do with.

reader Tom said...

lucretius, wonderfully sage quote on riding the German stallion. You very likely have come across this British military aphorism: He who has not fought the Germans does not know war.

reader Tom Moran said...

Ok, wind and currents then.

reader Tom Moran said...

Of note:
Read the highlighted photo below and then watch Ics1956 animation above to see the multiyear sea ice being pushed out of the Arctic thru the Straits of Fram. Wind and currents move the ice out of the Arctic.

reader Rehbock said... says that only less than 3/4 of those questioned believed that earth circles the sun.

reader Luboš Motl said...

No, I am sure that Adam wanted to include 10^0 GeV = 1 GeV in the interval, after all, that's where the WIMP-like masses are. Whether 10^50 GeV was something he meant seriously, I am not sure. It is comparable to the Earth mass.

Dark matter may be composed of such superheavy Earth-mass-like objects, especially if they were elementary particles, as far as a naive substitution of parameters goes. As a theorist, I would say it is nonsense - virtually all elementary objects above the Planck mass 10^{19} GeV have to be black holes.

reader Luboš Motl said...

Well, right, it seems like a lot of room for errors and random flukes. But if they mattered too much, the deviations would probably show at many places, sort of scattered everywhere, so "your" explanation doesn't seem to be in a terribly good shape, either.

reader Alexander Ač said...

Tom, try to explain me, how can the total amount of ice in the Arctic decline, if the radiation balance of that area is stable. (I. e. even if the currents and winds are pushing ice out of the place (which I agree), why would the open water not re-freeze?)

AND, try to explain me, how can the amount if ice NOT decline, when the average temeprature is increasing.




reader Vladimir Kalitvianski said...

Hi Lubosh, I think that your bet amount (10 000) with Adam is in a bad taste. If you are a noble person, you have to cancel this bet, in my opinion.

reader Albert Zotkin said...

It is required an imperative paradigm shift in theoretical physics right now. We must be courageous in junking to the trash some well-established ideas (theories) that have become mainstream physics, because they are doing a lot of damage to the effective and successful progress of science. To think there are dark matter particles is one of them, because you are wasting much money in null result experiments.

reader Luboš Motl said...

Glad to hear that my nobility is opposite to yours. If he agreed, I would be happy to quadruple the bet.

reader TomVonk said...

These answers are really trivial.
On the first question it is simply the bathtub problem. Of course that arctic refreezes in winter. Every winter since millions of years and will still refreeze during millions of years.
So the only question is if "melt in summer + pushed out by wind" is greater or smaller than "refreezed in winter".
Both signs can obviously exist.
Btw this has nothing to do with radiation balance which is anyway not "stable" - in winter incoming radiation = 0.
The second question is clearly a question of how average temperatures compare to 0°C. If the average temperature is far below 0°C, it doesn't matter much whether it increases or decreases. What matters is whether the bathtub empties faster or slower than the filling rate - see above.

reader Vladimir Kalitvianski said...

No, Lubosh, it's a serious matter. Adam is deeply depressed, not because of your bet, of course, but generally. He needs positive emotions. If he kills himself, it would be a tragedy. Just cancel the bet and say him a word of love and appreciation.

reader Luboš Motl said...

It's just nonsense. He is frustrated because not much is happening on the phenomenological/experiment interface he finds so crucial and this reaction is completely rational, I think.

But I assure you that ideas about new experiments or collisions deciding the fate of new physics at the scale of our bet is one of the things that returns him energy.

He is also in no shortage of money and it would be refreshing for him if he had to pay the modest amount of money for the new physics. He would ultimately be happy about the new physics as well, of course, it's his nature.

reader Dilaton said...

Hi Lumo,

do we really need such aggressive spampots here on TRF ... :-( ?

This seems to be a perfect textbook example of a blacklist candidate ;-)

reader Luboš Motl said...

Thanks, Dilaton, blacklisted. ;-) Is it OK to keep the comments already posted?

reader Dilaton said...

Hm yes, I thought too that he must be very sad when reading his previous post over there.

But I think Lumo's reminding him of their funny bet might be a good way to cheer him up and implicitly giving him the advice to be patient and take things not that serious. The game of fundamental physics is not over, even if "low-energy" experiments find no new robust enough hints of new physics in the very near future.

Maybe he also likes that his nice and well explained article got reposted here (I have already read it on Resonaances, I have not tested if I am still banned over there), so he can get additional comments to read :-).

Of course, trolling spambots, such as the one I criticized above, do probably not exactly help ...

reader Dilaton said...

Yes, it is such a nice perfect textbook example of an aggressive trolling spambot ... ;-)

This comment contains such a high density of ignorance and aggressivity; it makes me wonder why it does not autocollapes to a black hole because of these almost singular values anyway ;-P


reader Dilaton said...

Duh, they are talking quite big ... is this really justified?

You know, the issue of such big claims needing extraordinary evidence instead of wishful thinking etc ...

reader Vladimir Kalitvianski said...

Maybe you are right, but depression is very dangerous thing. What Adam writes might not be all and is not reduced to physics.

reader Luboš Motl said...

LOL, nope, it's not really justified. There is no solid evidence against the conventional cosmology with dark matter. MOND may still be right but the paper above doesn't prove it in any way, it's just another paper on MOND.

reader Luboš Motl said...

An answer from Adam:

Ah, I've got my own comment section to tend to, and a pile of old neutrino papers to read :)

For the 10^50 number i had the same question in my blog: this bound comes from MACHO dark matter searches using microlensing. Dark matter made of macroscopic objects (mini black holes, or planetary size objects) is of course very disfavored, but afaik never fomally killed. How they fit these 28 lines i don't even wanna know ;)

reader Luboš Motl said...

An answer from Adam F:

Ah, I've got my own comment section to tend to, and a pile of old neutrino papers to read :)

For the 10^50 number i had the same question in my blog: this bound comes from MACHO dark matter searches using microlensing. Dark matter made of macroscopic objects (mini black holes, or planetary size objects) is of course very disfavored, but afaik never fomally killed. How they fit these 28 lines i don't even wanna know ;)

reader imho said...

You said "the deviations would probably be scattered everywhere"

Perhaps or perhaps not... It's easy to create localized effects with 28 lines... think of an expansion of a delta sequence using a complete set of functions. Hell, you don't even need completeness to achieve locality. It could probably be done in under 20 lines! Your statement really depends on the details. But, i'm not curious enough to investigate further so I do not know.

reader Luboš Motl said...

You may get a delta by carefully adjusting the 28 coefficients, but if you're claiming that this is what happened, you are really accusing them of intentional fraud because the delta just won't arise randomly, will it?

reader imho said...

Who said anything about fraud??? Perhaps they were attempting to fit data in the region in question. As I said... I have no idea.

reader Luboš Motl said...

OK, I was not trying to accuse you from anything. It is hard to understand what you are suggesting if it does not make sense one way or another.

As I already tried to point out, but the message has not reached you, if one honestly fits 28 lines at various places, the residual has no reason to look localized, so your original explanation seems unlikely.

reader Michael said...

It seems that there's an overwhelming evidence that climate change is occuring and that anthropogenic CO2 emissions are increasing .see . Until you give me an evidence that this is not true ,I'll consider you as a crank .

reader ducdorleans said...

re. California drought ...

let's not forget Polanski's Chinatown ... Jack Nicholson nose was severed because Los Angeles was actually a desert ...

re. the rest ...

I agree with Lubos' "we don't really know" ...

reader yonason said...

I don't know how accurate this is, but it looks impressive.,90.27,577

reader yonason said...

p.s. here's a more pedestrian, but imo a more credible one by someone who does this for a living.

"La Nina winters are typically synonymous with harsh conditions across
the northern tier of the United States and drier-than-normal conditions
throughout the southern tier."