
X17 Particle - miobrien
https://en.wikipedia.org/wiki/X17_particle
======
andyljones
There is some skepticism around this to say the least[1]. The core of it is
that this group has found new bosons twice before:

 _The Atomki group has produced three previous papers on their beryllium-8
experiments — conference proceedings in 2008, 2012 and 2015. The first paper
claimed evidence of a new boson of mass 12 MeV, and the second described an
anomaly corresponding to a 13.45-MeV boson. (The third was a preliminary
version of the Physical Review Letters paper.) The first two bumps have
disappeared in the latest data, collected with an improved experimental setup.
“The new claim now is [a] boson with a mass of 16.7 MeV,” Naviliat-Cuncic
said. “But they don’t say anything about what went wrong in their previous
claims and why we should not take those claims seriously.”_

[1] [https://www.quantamagazine.org/new-boson-claim-faces-
scrutin...](https://www.quantamagazine.org/new-boson-claim-faces-
scrutiny-20160607/)

~~~
ajkjk
Here is another highly critical article:

[https://www.science20.com/tommaso_dorigo/the_17_mev_anomaly_...](https://www.science20.com/tommaso_dorigo/the_17_mev_anomaly_that_would_not_die-243528)

------
knzhou
It's important to keep in mind that there are _always_ plenty of outstanding
experimental anomalies in physics. At the moment, this is one of ~40 roughly
equally credible hints towards new physics, and it's more likely than not that
all of those hints will fade away over time. That isn't anybody's fault
either: it has always been like this, and it happens because experiments are
difficult and subtle.

Personally, I still find this extremely exciting, even though history tells us
it has less than a 1% chance of panning out. A 1% chance of revolution is
still meaningful. But don't be too surprised if we land in the 99%.

~~~
beaner
Any chance you know of a list of such anomalies?

~~~
PhantomGremlin
One of my favorite "anomalies" is dark matter. Not much explanation for it so
far. One of my favorite quips on HN put it thusly:

 _GR only matches observation because you added in 20x more stuff that is
undetectable other than as a deviation from the predictions of GR._

[https://news.ycombinator.com/item?id=19537192](https://news.ycombinator.com/item?id=19537192)

~~~
knzhou
That's a witty quip, but it's witty because it's unfair.

We don't grade theories based on what fraction of the stuff is obviously
visible. If that were true, even electromagnetism would be a terrible theory
because the vast majority of the spectrum we consider is invisible to human
eyes. What always matters is how well the theory predicts given how simple its
assumptions are, and dark matter is great at that.

~~~
ryacko
Dark matter is a fudge factor.

~~~
dogma1138
Isn’t the cosmological constant the OG fudge factor in GR?

AFAIK we need dark matter to explain any theory of gravity, the observations
don’t match what we would expect form classical mechanics either which is
still the go to theory for things like galaxies.

MOND was introduced initially to fix the observations mostly in regards to how
we understand Newtonian mechanics, the relativistic versions of MOND can be
used as an alternative to GR but these have been more or less debunked with
the observations of gravitational waves since afaik all the versions of MOND
have “instant gravity” just like Newtonian mechanics.

~~~
raattgift
"The relativistic versions of MOND" are just adding fields to General
Relativity. Thus making them _relativistic_ theories.

Famaey and McGaugh §7.
[https://arxiv.org/abs/1112.3960](https://arxiv.org/abs/1112.3960)

Consequently the rest of your last paragraph is incorrect.

"we need dark matter to explain any theory of gravity"

No. MOND _is_ a theory of gravitation, but it's not relativistic, and it does
not work at scales larger than that of galaxies (it is notably wrong with
respect to the peculiar motions of galaxies in massive clusters, and a
residual mass term must be added _ibid._ §6.6.4 : essentially, MOND still
needs dark matter at galaxy cluster scales, even if it were to correctly
describe all the individual galaxies in the massive cluster).

Additionally, General Relativity does not require dark matter any more than
Newton's F=ma requires dark matter. The issue is that General Relativity's G=T
like F=ma does not tell you about the initial trajectories, you plug those in
by hand. If you start with the trajectories learned by observing galaxies (as
Vera Rubin did), you can work out a stress-energy tensor that satisfies those
trajectories -- and the majority of it has to be electromagnetically-
uncharged, interacting very weakly or even only gravitationally (i.e., they
can't clump or diffuse on the scales of mere millions of years), and slow-
moving. If neutrinos weren't so inclined to zip around at speeds very close to
that of light, they'd fit nearly perfectly; unfortunately, we're left trying
to find the microscopic details of the unknown parts of the stress-energy
tensor. Milgrom's MONDian approach to Rubin's discoveries that the orbits of
"surface" stars are non-Keplerian was to turn "a" in F=ma into a function that
depends on the radial distance from a galaxy's core; the function was found
empirically, comparable to how the stress-energy distribution was found.
However, it's the dependence on a _coordinate_ distance that makes this
approach non-relativistic.

(For terseness, in the paragraph above I've discarded some factors, set the
constants c and G to unity, and omitted the greek-letter indices on the
stress-energy and Einstein tensors (T resp. G). In the MOND context, F=ma is
more appropriately written as in the first paragraph of §6 of Famaey &
McGaugh.)

~~~
dogma1138
I'm not sure if I've either explained myself too simplistic or that you
haven't understood my post.

My points were:

1) DM isn't a fudge factor for GR or Newtonian Mechanics, (Classical)
Newtonian Mechanics doesn't have one, PPR does but PPR isn't a theory. GR has
a fudge factor built-in into the theory - the Cosmological Constant that gives
you a variable that can adjust the predictions to match observations as it
wasn't known at the time if the universe is static or not. It's not a perfect
fudge factor since it can't deal with accelerated expansion easily but it's a
fudge factor nonetheless.

2) MOND was based on classical mechanics and as such isn't a replacement for
GR, I'm not sure if I agree with the assertion that TeVeS is an extension of
GR. And yes all theories of gravity need "Dark Matter".

Dark Matter isn't a fudge factor it's a placeholder for missing mass needed to
align predictions with observations doesn't matter if those predictions are
derived from GR or Classical Mechanics or as you've mentioned even MOND.

And yes MOND still requires "Dark Matter" or to be exact some additional mass
however it requires much less of it and it requires it to be concentrated in
the center of galaxies which means it can be much more easily explained for
through known mechanisms and forms of matter (e.g. the black holes in the
center of a galaxy are more massive, higher density of interstellar medium and
gas etc.), hence it can be described as a theory that "solves" the problem of
Dark Matter, because it doesn't implicitly require new forms of matter or
complex explanations for missing mass.

However like you've mentioned MOND is a flawed theory, it doesn't even work on
galaxies that well since while it can describe their movement today, it has
difficulties aligning with observations of various clusters and more
importantly it doesn't work well when you start to wind back the arrow of
time, and you don't even have to go as far back as to the formation of
galaxies (which aren't possible under vanilla MOND) rather it can fail as
quickly as by rewinding the clock half a billion years in some cases.

~~~
raattgift
1/ I was restricting my comments to dark matter, sorry if you read a comment
about dark energy into my reply; probably I should have made it explicit that
I wasn't touching your "fudge factor" comment. Coincidentally, Sabine
Hossenfelder has just published this in the past day or so:
[http://backreaction.blogspot.com/2019/11/what-is-dark-
energy...](http://backreaction.blogspot.com/2019/11/what-is-dark-energy.html)
and you might want to take your comment about "fudge factor" there, although
I'd bet a doughnut without looking that at least two other assiduous
commentators there have already done so. Hopefully someone in there will
discuss Jeans instability and the Raychadhuri focusing theorem if it gets
sufficiently technical: the latter was a later discovery which (if he had been
aware of it) would have led Einstein (and practically everyone else) more
quickly away from a steady-state cosmology and towards something like
Einstein-deSitter or Einstein-Friedmann as a way of solving the problems
arising in the former. One should also bear in mind that until 1922 M31 (then
the "Great Andromeda Nebula") was not known to be comparable in size to the
Milky Way nor at distances of more than a few thousand light years (as opposed
to the modern figure of ~2.5 million), and that it wasn't until several years
later that it became known that there are lots of galaxies in the sky -- this
was all after the cosmological constant was introduced by Einstein in 1917.

2/ I will restrict myself to just:

"I'm not sure if I agree with the assertion that TeVeS is an extension of GR."

Bekenstein's tensor-vector-scalar gravitation (TeVeS) is a theory of
gravitation, like MOND and GR. You're right that as originally formulated it's
not laying a field on top of GR, but like numerous theories of gravitation it
was found to be inconsistent with observation. In particular TeVeS does not
allow for long-lived stars, and those appear in our sky in abundance. We also
have lots of Einstein lensing data that conflicts strongly with TeVeS
predictions. Amusingly one fix proposed for TeVeS for the "cosmic shear" weak
gravitational lensing is to add a hidden mass term in galaxy clusters, with a
specific proposal for a WIMP. (This doesn't fix TeVeS's other difficulties).

This is dealt with in Famaey & McGaugh §7.4 (wherein there is a delightful
summary of a generalization of TeVeS: "... a tensor-vector-scalar theory with
an Einstein-like metric, an Einstein-Aether-like unit-norm vector field, and a
k-essence-like scalar field", which is close to saying it's "just GR with two
extra fields that you are free to place on either side of the Einstein Field
Equations" as you'll likely read from such non-particle-DM academics), and on
textbook treatments of the Paramaterized Post-Newtonian (PPN) formalism (a
convenient table is here
[https://en.wikipedia.org/wiki/Alternatives_to_general_relati...](https://en.wikipedia.org/wiki/Alternatives_to_general_relativity#PPN_parameters_for_a_range_of_theories)
and surrounding text). Many of the PPN parameters are chosen so that they can
be individually tested; hackernews user ISL does that for a living!

2(b)/ restricting to "the black holes in the center of a galaxy are more
massive, higher density of interstellar medium and gas"

The work of
[http://www.astro.ucla.edu/~ghezgroup/gc/](http://www.astro.ucla.edu/~ghezgroup/gc/)
and others put strong limits on the stress-energy in the central parsec, and
more broadly in the core: your approach does not work in the Milky Way at all,
and yet observations are increasingly consistent with a dark matter halo (see
[https://en.wikipedia.org/wiki/Dark_matter_halo#cite_note-32](https://en.wikipedia.org/wiki/Dark_matter_halo#cite_note-32)
and cite note 33 ibid., which review the Milky Way's rotation curve)

2(c)/ "you've mentioned MOND is a flawed theory". The theory itself is fine,
it's just not a good match for extragalactic observations. I have zero problem
in using pure Milgromian MOND in studies of the doppler shifts of molecular
gas clouds in LSB elliptical galaxis, for instance. But it doesn't work at all
in cosmology, and is grossly wrong in the solar system and in mergers of
massive compact objects, and "fixes" for those regimes are even more unwieldy
than the GR-based linearizations and other GR-based post-Newtonian expansion
techniques already in use.

Finally,

"wind back the arrow of time"

What does that mean?

From context I think you are just saying that large scale structure formation
is not adequately explained by theories that lack some form of dark matter,
but "the arrow of time" means something to cosmologists (and physicists
generally).

------
beezle
Find it really hard to believe they find something at 17 MeV that was never
seen at Tevatron, RHIC, CEBAF, HERA, LEP, SLAC, etc.

~~~
noobermin
You also need a beryllium nuclei. It comes a nuclear physics experiment.

------
RandomTisk
I find the E8 lattice incredibly interesting, if for no other reason to find
out if it's truly representative of particle physics. It predicts more
particles we haven't observed yet.

[0]
[https://en.wikipedia.org/wiki/An_Exceptionally_Simple_Theory...](https://en.wikipedia.org/wiki/An_Exceptionally_Simple_Theory_of_Everything)
[1] [https://science.howstuffworks.com/science-vs-
myth/everyday-m...](https://science.howstuffworks.com/science-vs-
myth/everyday-myths/theory-of-everything2.htm)

~~~
K0SM0S
Honestly, even if the E8 lattice is nothing but imagination, it's so beautiful
that we need to make a para-verse like that someday. You know, when we've
mastered creating universes and stuff, at least when we're able to live in
virtual fluff. The beings in such a simulated universe would be so in awe of
'everything' as they uncover the theory thereof.

~~~
pavel_lishin
If you haven't yet, you should read some of Greg Egan's books.

~~~
arrow7000
Greg Egan is indeed a goldmine for this kind of material. For this particular
topic I would highly recommend his book Diaspora.

------
dang
Related thread:
[https://news.ycombinator.com/item?id=21616381](https://news.ycombinator.com/item?id=21616381)

------
isoprophlex
Someone knowledgeable, please tell us why this is just runaway science
journalism before we get too excited!

This is potentially the most groundbreaking stuff in decades, right?

~~~
ddavis
We're going to need an independent experiment to see this "X17" before anyone
truly believes it's a new particle. Some example major particle discoveries
over the last few decades (and the independent experiments which observed
them): Higgs (ATLAS & CMS at CERN); Top quark (CDF & D0 Fermilab); W & Z
Bosons (UA1 and UA2 at CERN); J/Psi (Richter's team at SLAC & Ting's team at
MIT). Some other fundamental particles were observed by a single experiment
(the bottom quark, for example) and the community believed it, but in those
cases it's always because a theory has already predicted the particle, and
we've always been able to study them with future experiments anyway. So this
won't gain traction unless another experiment can observe it.

------
mo-1
What, you say>

Nobody should argue that advancements haven't been made since 1988, but
nobody's manufacturing organic human organs yet --- chemical pieces of:
probably.

------
2bitencryption
could someone explain something--

I have a faint understanding of the "Standard Model" of physics, which lays
out a pattern of particles and forces that has so far held up to
experimentation i.e. the Higgs boson fit neatly into the model, exactly where
the model predicted it would go.

The standard model predicted the Higgs, and the Higgs was found exactly how
the model predicted, right? But I've never heard of this X17 particle before.
Is this something the standard model has predicted? Does it go against the
standard model, strengthen it, or neither?

~~~
pdonis
_> Is this something the standard model has predicted?_

No.

 _> Does it go against the standard model, strengthen it, or neither?_

I would say neither.

------
make3
this is extremely exciting. makes me hope that we will see a unified theory of
physics in our lifetime

~~~
lokimedes
Usually discovering new fundamental forces will move us away from unification,
not towards it.

~~~
semiotagonal
Discovering new physics doesn't move us away from a unified theory, it just
means we were wrong about how much farther we have to go.

