17
yeah it's been around for a while i
think if it's around at all
i'd rather read about it i think a
couple of uh 2016
was the first time i came across it this
work done by these
nuclear physicists in hungary they were
looking at
beryllium
the the nucleus of beryllium it's got
four protons and four neutrons in it
you can excite this nucleus we know with
atoms and the electrons right if you
excite the electrons in an atom they
raise up an energy level
and then
they don't want to stay there we're all
lazy really right we want to go back to
what we call our ground state and that's
how we produce light and so we're all
familiar with the consequences of that
but in fact you can do it with a nucleus
as well what what they do is um they
create an excited beryllium nucleus by
they have they start with a lithium
nucleus and then they
attach a proton to it so of a very
specific energy so here's my lithium
nucleus and in comes a proton with a
very specific energy and it then forms a
beryllium nucleus which is already at an
excited state so and that excited state
is compared to its natural ground state
it doesn't want to stay like this right
it and so
what actually usually happens is it just
disintegrates again it just goes back to
the proton goes out and you're left with
the lithium but every now and again what
actually happens is as it
drops back down to its ground state is
it emits a virtual photon
and that virtual photon just goes and
disappears off on you then every now and
again even less likely is the is that it
emits the photon and then the photon
itself creates a pair of electron
electron positron pair an e plus e minus
pair
now what usually happens is in those
situations
the photons coming out and so the the
two particles the electron and positron
pair they're very light particles so
when they're created they're almost
co-linear with the with the outgoing
photon is so photons coming out towards
you brady and then the electron positron
pair are created and and so the angle
between them this is the crucial thing
is it usually remains small
and so as that angle increases the
number of electron positron pairs that
come out at larger and larger angles
decreases because it it doesn't want to
do it it wants to create come out
because of conservation of momentum
easier for it to come out straight
forward
now what these uh people noticed was
that
um at an angle of about 140 degrees so
let's see this is zero
uh is that 90
and so 140 i mean is is out here they
found a bump they found an increase in
the number of uh
electron positron pairs coming out
and we know from our higgs days
that if you've got an increase
in
number of events then some it's it's a
potential sign that something's funny
going on though you would normally have
expected this number coming out as you
increase the angle just to drop off
monotonically but it suddenly it went up
what could be causing this
one interpretation is it's it's a
particle that's being produced that then
decays itself into the e plus e minus
and this is the x particle and when they
work out given the angle that the e plus
e minus pair come out at 140 degrees you
can work out what the corresponding mass
of the particle must be and it's 17
million electron volts 17 med and that's
where the x-17 comes from it's that it's
the the mass of this particle
in comes the initial proton comes out in
from outside the lithium hits the
lithium converts it into a beryllium
nucleus which has now got four protons
and four neutrons by fine-tuning the
incoming proton energy you can raise the
energy level of this nuclei of the
beryllium to actually it's i think it's
18.16
mev million electron volts
and that's an excited state it's not the
ground state it then decays as it decays
rather than creating the photon which
then creates the electron positron pair
it creates this
x particle so that's when the x-17
happens during the
relaxation of the excited yeah right it
creates this but it's a massive particle
so it's coming out now with with
relatively low momentum
that then means that when it when it
decays when it does when it doesn't
split it does it split to create the
electron positron pair they can come out
at a large angle ah
whereas a photon couldn't because i
thought it's got high momentum is going
to then
want to go directly into the e plus c
minus in the same direction so because
the x 17 is so sluggish yeah and has
less momentum yeah it creates this
ability to do a wider angle angle split
and it and so that was the
interpretation and they they wrote this
up it didn't do a lot for a lit for a
while for a few months it sort of
laid there but i remember reading a
paper by jonathan feng
and his collaborators at irvine
where they were looking at this
experiment and trying to interpret this
result in terms of some model and so it
was then that i i remember reading it
and thinking in order to explain this
result they're having to do something
some
twists and turns in the particle physics
beyond what you would normally expect
from the standard model of particle
physics in order to get
these
this is a it's a this x particle is a
boson it's got spin one and there have
been experiments looking for these
particles the na48 experiment is one at
cern which had been looking for virtual
bosons like these and haven't found any
and in fact it would naively have ruled
out this experiment of the beryllium
from their own results and in order to
sort of make sure the two were
compatible
fang and collaborators realized that one
way of doing it would be to effectively
change some of the interactions
that the protons were having with this x
particle
such that it would be excess it would be
allowed for the beryllium experiment to
do what it was doing but it would
account for the fact that the na48
experiment was not seeing anything
it's generous a lot of interest i mean i
think there's like 140
papers have been written citing this
original work but it was back in 2016.
i think the thing that has caused the
renewed interest the same group in
hungary produced new results this time
not using beryllium
but using a helium nucleus they did the
same sort of experiment
and they found that once again they
there was a bump when they were looking
at the the creation of the electron
positron pair they found a bump not 140
degrees which was the result for the
beryllium but i think 115 degrees
and when they were then worked backwards
and took into account the fact this was
a helium nucleus and not a beryllium
nucleus they once again found that the
corresponding mass of the particle that
if you understood it as from a decay of
a particle
was i think it's 16.7
million electron volts again comparable
to what they had before that sounds like
a smoking gun it does sound like a
smoking gun it's quite intriguing but in
the meantime
there have been
the the initial results um that they
they obtained in 2016 has of course
got people thinking about this and
the experimentalists at cern
have been there's an experiment called i
think it's na64
which published its results in 2006 2018
so about a year ago
where they went looking for the original
x particle right they went looking for a
for a gauge boson which had a mass of
about 16.7 million electron volts and
the way they did it was they used the
super protons synchrotron collider at
cern they had a beam of electrons of
about 100
gv
that's a giga electron volts
they collided into a target so it's
called a beam dump they collided into a
target because if this particle was
there what would happen is that
electrons that were being stopped as it
hit the target could interact with
actually z-bosons said vector bosons in
the target and they in principle could
produce this x particle
and then the x particle being
with its interactions could actually
leave the beam dump
and it would then decay further down the
chain
and in a set of detectors creating the e
plus e minus pair and so what they were
looking for were these two sort of
almost simultaneous events not quite
simultaneous where the beam dump loses
an amount of energy but that same amount
of energy is found further down the line
and the interpretation would be the
electron had
as it hit the z boson would have created
this particle which then moved out from
the beam dump further down into the
detector and decayed
and they didn't see it they saw no
evidence of it and so that has allowed
them to rule out regions of the
parameter space
that the original paper was was
exploring
but this new result which is with the
helium
sort of is it sounds like a smoking gun
in some ways it is because it's a new
nuclei
but what's it's really really crying out
for is another experiment along the same
lines as the beryllium one but done by a
different team
right so why did we why did people so
readily accept the higgs well i think
there were two major reasons one was
mathematically it was expected to be in
the ballpark where it was found that's
why they built the lhc with the energy
they did on the size it did so the
mathematics of the standard model and
beyond the standard model had suggested
there should be something there
and then
two detectors totally separate detectors
found it effectively simultaneously they
were finding evidence for the higgs
that's the cms detector and atlas we're
finding it
simultaneously in this case there's one
experiment
and it's found it once it's there's a
bit of a track record where they have
found events before that have that have
disappeared found evidence for these
bosons of different masses that have
disappeared they're arguing this is a
stronger result
but they've found it
for beryllium and now for helium that
that's positive but what it's really
requiring is another experiment along
those lines to come along and say yes
and confirm it
have other teams try to are there other
teams around the world who've done
similar experiments and said we haven't
found that bump at those angles i'm not
aware of other experiments doing exactly
what the beryllium
the hungarian guys are doing because
they're doing a proper nuclear physics
experiment and so this is this
interesting interface between nuclear
physics and actual particle physics the
the experiments that i'm aware of that
are testing this are particle physics
experiments colliding electrons at a
high energy into a beam dump
and looking
for different types of detectors
looking for dark matter detectors i mean
one of the exciting things about this
result
if it was to hold
is that
there are dark matter models which would
have a particle of around
10 20
million electron volts these light
particles could be there
and and these
there are models where there are these
vector bosons
when i talked about the why why did we
believe the higgs i said well because
the standard model was suggesting and
and beyond it
suggesting that it should be there it
was in the mathematics
one of the things that has had they've
had to do in this particular case from
the theoretical standpoint
is you we've had to do some quite
severe twists and turns to the particle
physics to make sure we can accommodate
this result in terms of a particle
whilst also accepting na48 didn't find
it at cern it's not an easy fit it's not
an easy fit and in fact you've basically
had to introduce a new charge that that
accounts for why this x particle doesn't
interact with the proton like it would
with the electron and in fact it's it
it's called protophobic
it doesn't want to interact with the
proton it will happily now interact with
the neutron but not with the proton and
that's unusual photons normally will
interact with the with the with the
proton and you're forcing this to do it
and so you need to
look for for examples why this is not
happening
is there another logical explanation
doing the rounds are there other
physicists who are saying oh no i can
explain those bumps at those angles it
could just be you know yeah xyz well i
think that
if you want it to be a so one
possibility is as i said it it is one
possibility that's a mistake oh yeah yes
so we should point that out of course
right that um
the that's another reason why you need
to do this experiment with another group
that there are
maybe they've got their systematics
wrong they perhaps this bump is uh
i mean the i
the word you hear is this is like a
seven sigma detection well seven sigma
means it's one
one chance in a hundred billion that
it's a random event i mean that so but
if you've missed some
big systematic that that could all go
away
so i'm not aware of any
sort of
realistic accepted particle physics
explanations as to what this is other
than people coming in with this idea of
a particle or the possibility that
it's it's it's actually not there at all
we we've just got to go and revisit the
systematics of the experiment
if we imagine the x-17 particle is legit
it is real right yeah what can you tell
me about it can you give me an idea can
you give me an idea and it's like how
big that is and what charge it is so
it's light so an an electron which we we
actually tend to think of as being
almost massless right it's half an mev
so this is 34 times the mass of the
electron the proton
is roughly a thousand mev but that
you've you've
as usual you've come up with a key
question
which which is that um
it's not particularly the pro the
particle that's important here
and that why is that
the particle in in in particle physics
is is symptomatic of the existence of
something else it's symptomatic of the
existence of a quantum field
so when we think of the photon in
particle physics the associated feels
the electromagnetic field when we think
of the um
the higgs particle that was discovered
why people were so excited was the
existence of the higgs field
and
what the particles are excitations of
those fields very strong excitations
the existence of a field means there
there's a force
between there's an interaction between
particles
and so what you are finding here is if
this particle really exists there's an
associated x field
exists and we have a fifth force
and the mass of this field is such that
this could be a long range fifth force
and and then you've got all sorts of
interesting possibilities coming in
about why we not detected it
what are what are its subsequent
properties and what other influences
could it have on the on forces on nature
we're always looking for fifth forces i
mean
well some people would say it's a sixth
force let me we'd say there's
electromagnetism there's the weak
interactions the strong interactions
gravity the discovery of the higgs is
regarded as some as
providing us with another interaction
which is the interaction of the higgs
field with particles
and but
this
new interaction is has not been detected
before if it's there
and it could open the you know there are
many searches on for examples of either
these long-range interactions with
regard to the dark energy driving the
acceleration of the universe with all
with regard to their presence of the
particles themselves as dark matter
candidates there there's a big push for
these light
dark matter candidates at the moment
because we're not we haven't seen any
evidence of supersymmetry in the large
hadron collider which is usually
associated with the existence of wimps
so the wimp dark matter candidate hasn't
shown up yet and so people are naturally
beginning to sort of think well perhaps
we should need to look further afield
could x-17 be dark matter well there's
one possibility there are dark matter
candidates that are in that regime in
terms of their mass range now i i don't
know enough about the interaction
cross-section of this x-17 particle with
other particles
the people have looked in are looking in
this mass range with these called
directed
detectors that direct matter detectors
and they haven't seen anything but
whether or not that in its own right yet
says anything about these x17 remains to
be seen
x17 has no charge like it's not negative
like an electron or positive like a
proton um
no i don't think so because it decays
producing an e plus e minus so overall
electric charge is zero yeah
if you told me brady go and find me an
electron i think i know where to look
that you know if you said brady find me
a proton or a neutron i know where you
said find me a photon
they're quite easy to find oh yeah where
are the x-17s if they exist where are
they living so the if the if they're
there in the the
they're um
it turns out that they're not in that
decay of the beryllium for example
where remember what i said beryllium
naturally just decays back to the
lithium it just breaks up again there's
then then the next level of decays via
these virtual photons which then
themselves
decay into e plus e minus
one in a million of them roughly would
be the
production of the x particle but that's
a rare event that's collision that's
like a that's a manufacturer
so right now there's no x-17s if they
exist right now there's no exception in
you yeah
um i don't that's a good question i i
don't think i'm any of well i often i'm
quite excited and i do get excited but i
don't know i think of an individual
uh
healing i mean i've got helium in me and
i don't think of and i must have some
beryllium in me i guess too i don't
think it's that that excited energy
state that then decays if it does by the
way
um it's pretty hard to find i think
their lifetime is around 10 to the minus
12 seconds so it could have found and
they just disappear so ed if particles
are excitations of a field including
electrons and protons and things like
that how come electrons and protons are
so abundant in this room
and so long-lived and yet this x-17
which is heavier than an electron and
larger than a proton doesn't have this
ability to manifest itself and then hang
around
so that's that's a that's a tough
question
but so the electron is the so the
electromagnetic field is pervading this
room and the the electrons would be
associated with well the photons would
be associated with the excitations of
that as would be the electrons which
obey electromagnetic interactions
the
the protons are a bit more unusual that
the majority of the mass of the proton
is not in the quarks
the majority of the mass of the proton i
mean if you if you add up the mass of
the quarks each quark is has a
rest mass of about
3 mev so their total mass is about 10
mev and i've just said the mass of a
proton is a
1000 mev
so the majority of the mass of the
protein is actually in the binding
energy of the of the of the quarks
of the gluons sorry they're that are
propagating around it all the time so
it's not quite as straightforward as
sort of turning on the quarks and saying
there's your proton
so but so i think it's it's but you it
is a case of the the the the
the relative
excitation in the field is the thing
that determines the underlying um
mass of these objects but the proton
itself is is this combination of the
quarks and the gluons which makes it a
bit less
clear how you're going to generate them
so the x-17 field if it exists yeah is
one that just doesn't get excited very
often in our world i guess that's true i
mean it seems to be the product of the
decay of a
a
more excited
beryllium but
you've got me thinking i'm not quite
sure because it doesn't require a huge
amount of energy these are not massive
energy scales compared to what you what
you get
but i think the the issue is is it's not
just that
it does depend on the interaction these
the x-17 particles have what what type
of interactions it has with the other
force other matter particles and we've
seen that in order to explain its
existence we have to already do
something rather unusual we have to make
that interaction one that's
proton protophobic
in order which means that it's not
interacting like the usual particles are
interacting so that could have an impact
on how likely it is to be formed out of
some excitation of the field
are you an x-17 skeptic or i still
remain a little bit of an x-17 skeptic
yeah i think
it does require
um a it requires more particle physics
explanation for why it
might be there in in a in a coherent
framework
and and b
above all it needs
more experimental
evidence coming from a similar type of
experiment to verify
whether the bump that they're seeing is
actually real or not
shoot it through the air it's absorbed
in the air so to measure the x-ray
absorption of our compound which is
sensitive to the air we need to be able
to remove the air and encase it so we
use beryllium to make a magic box around
our sample that the x-rays can come into