Can you see this?
Yes.
So then let's get going. So yeah, thanks everyone for dialing in on your Saturday.
And I hope everyone is doing as well as possible given their challenging
circumstances and so on. So I was asked to tell you a bit about some recent or less
recent theory developments in dark sectors. And so that's what we're going to
be doing. So you can start by asking a question, what a Dark Sector is. And if
you're doing the first thing, which is to Google this, you'll find a remarkably
unhelpful answer in the sense that Wikipedia says that it's a collection of
unobserved quantum fields and their corresponding hypothetical particles. So
that's not it gets a little bit more specific often done, but it's not really
particularly helpful for what we want to do. So I'm going to operate on the
following definition where a Dark Sector here means an extension to the stone
model, which does not carry stone model of quantum numbers. causes you to be cross
you want. So in this Venn diagram, you see the space of obvious and theories, where
the gluing electroweak, you know, and so on are stuff like our visible corner of
quantum numbers from this time model that are visible. So this is not Dark Sector
and everything else counts as a Dark Sector in this definition. So the Dark
Sector may or may not contain a dark matter candidate, but that's not
mandatory. And sort of for the purpose of focus, I will talk only about accelerator
applications of these models. There's a lot of cool astrophysics cosmology that
won't be addressing at all today. Even in this with these caveats. This is of
course, a huge chunk of literature and I'll be neglecting large chunks of it. So
my apologies in advance for that. So this is a bit of a cherry picked set of things.
So first, I thought it'd be useful to maybe elaborate a little bit on like, what
are the two schools of thoughts in my mind that are In the theory community when we
think about these things, so the first set of theory priors you can adopt is that of
minimalism. So in the sense that you take the same model, and you try to extend it
with a smaller set of particles as possible, maybe one or two new particles,
but a very small set of couplings. And so minimalism is really what drives your
motivation here. So in theorists talk about simplified models, or portals, this
is usually the type of thing you're talking about. The other school of thought
is, is a realism, as I would call it, which is where you're really trying to
address a particular problem in the summit model, like a hierarchy problem, talk
matter or whatnot. And so these models tend to be much more elaborate and much
more intricate and they tell some sort of story, rather than just like a set of
particles. So at the risk of beating a dead horse, I thought it would be useful
to very briefly remind you of the pros and cons of each of these two approaches,
right. So in minimalist scenario You have a very few a fairly limited set of options
that you could be pursuing. Because there's only so many ways you can extend
this model with one or two particles. The models tend to be simple, very low
dimensional parameter space. And that makes them really great for benchmarking
and for comparing one experiment against another. So that that's, that's, of
course, a plus. But it's also I put it also in the negative column in the sense
that it's very easy to get carried away a little bit with this if it's not careful,
because of the convenience of projecting everything on two dimensional parameter
space. So this is something we always have to be mindful of when we do these things.
And then there is the obvious downside is that there is no a priori, deep motivation
as to why these things have to be there. It's just something that you can consider.
For the realist models, the theory of motivation is there. So there is like a
specific problem that you're trying to solve and that means that these models
have much more predictive power. Dan, the minimalist model in the center, so
specific question that you're seeking the answer to And that gives you a prediction
on the parameter space. The downsides are that there's a huge number of these things
like order a hundreds of these each year of being published. And so it's very, it's
somewhat unmanageable in that sense. And often in these models the parameter space
as relatively high dimensional, and that means that it can be quite difficult to
rigorously falsify them. So my personal view on this is that a model independent
approach to dark sectors because it's such a nebulous concept, is is probably not
possible. And if it is, it's very difficult. And in that light, I think it's
important that we balance these two schools of thought and relying too
strongly on one of them is can be dangerous and we could be missing things.
So here is a cartoon of what a journey through the Dark Sector could look like
for us. So we live on the standard model, which is this continent on the on the on
the bottom left, that's really nice. There's all our stuff is there. We Have a
good time, and lots of interesting things to explore. But we want to see what is
beyond. And so if you're interested in minimalist models, so I think the analogy
is that these are things that live in the shallow water around our own island in
some ways. And so here are a few examples of models that you see show up and
glitcher. Very often, the more realist models that we compare with, like islands
that we see in the distance, and they require a boat to go there. And to explore
in some detail, there's a bit more rich structure on these things. But they're a
bit more often a distance and we're basically relying we can come up with
anything. And so I'll be discussing a little bit both of these. So let's start
with a minimum of models. So you will and the upper right corner you always see the
art depiction of what I'm talking about. So you can see which class models I'm
currently discussing. So usually a single sort of archetypical a minimalist model is
just one single new particle that you connect to the standard model. And so
there's a production mode and a decay mode that comes with that. And typically the
couplings of these things are fairly small just because of experimental constraints
that already exist. And so, that means because these couplings are very small,
your production modes that are most promising are either exotic decays or very
narrow Standard Model particles will elaborate on or situations where you have
a very, very high part on luminosity function for this initial state over here.
The decay is also quite intricate and is quite interested in the sense here you see
sort of simplified formula of what the K the K width of a particular particle might
look like or achieve some coupling constant m is the mass of the particle in
question and m is some heavy mass scale. And then m here is an integer that this
positive in the church depends on the particular model in question and so So
from this structure, all the key amplitudes will will look like this. You
see that if this mass scale over here is fairly heavy because we haven't observed
any of these new particles yet. So this is of the order of a few TV or so that means
that if this light particle over here, the thing that you're actually looking for is
in the roughly gv mass range, you can you generically expect that this this weight
is very small and that you have long with particles in this context. And that really
informs the way we look for these these models, at least in this low mass range.
So let me Oh, and then yes, because we are in sort of minimalist mindset, you get
bonus points if your particular model predicted decay and production vertex
which are the same, so if it's the same vertex, then that's considered better. So
let's first show you one example of where you have very high Barca luminosity which
could be in So this is a nice result that was updated earlier this week by athletes.
It's the light by light scattering and ultra virtual heavy ion collisions. And so
here, the photon photon luminosity function is so enormous because of the
high charge of the ions over here, that you can actually fuse two field photons
into a light action like particle which can indicate back into two photons. And so
that here are some limits produced by both Atlas and CMS. On the mass of this is the
mass of the particle. And this is a coupling constant, you can probe pretty
small coupling constants, and the world's best limits on this scenario are currently
coming from heavy ion collisions, which is kind of interesting. sort of more
bicurious scenario is where you're producing one of these light states
through the gay of a standard model particle which is very, very narrow. And
so I picked this particular example from any 62 because it's recently came out a
few weeks ago. We are looking for a heavy neutral laptop. And so this is really a
neutral fermion which makes us with a standard neutrino. And so with below the K
on mass, you can produce it in this in this particular decay over here. And so
really what they're taking advantage of here is that the K on itself is an
extremely narrow particle in the standard model. So that means even a very small
coupling like you can see here down to 10 to the minus nine, you can still have an
appreciable production rate for this hidden set and neutral laptop. And,
of course, if you're Atlas and CMS, the last thing you want to do probably is or
LCD is to pick a fight with any 62. When it comes to a key on the case, however,
you have some heavier particles at your disposal, which are also very narrow,
which are the beam as ons. And as tight multix. I will say very little about the
Higgs because I'm not going to talk about that in the next session. So I will be
largely silent about this. But just to give you some motivation as to why we
carry here you see the number of particles produced in the house. luminosity Elysee
versus future Collider, like the ILC. In some optimistic scenario or belt two, you
see for all the heaviest particle particles in this model, the high
luminosity, by far has the largest production rates, many orders of magnitude
in some case. And so again, because the Higgs and the B are so narrow, they're
really excellent targets for us to look for hidden sector particles. And so
really, the challenge is, under what conditions can we overcome the trigger on
background challenges to the extent that we can leverage this this addition this
increased production rate over some of these more cleaner experiments like a
future lepton collider or dumbbell to experiment? And so that's really the
challenge that we have to be thinking about. So one great advance in this
direction, I think is the concept of data scouting. So what you're seeing here is
The sort of famous diamond invariant mass plot, that every experiment in history so
far has been made as made in some way one way or the other. But really what you're
what's special about this CMS one over here is that this was done with a data
scouting data set. And the threshold for the neurons was was only three gv so this
is basically low as you can go for the neurons to still reach to your chamber.
And so I think this type of analysis has a lot of potential to look for hidden sector
particles. On the same node, there is the whole data data parking concept where you
save some of the data for later reconstruction. And so in particular, CMS
has already attempted a 10 unbias beam as on tape. And so that's something that we
should be looking into. But for both of these cases, it's very important that some
plan is in place before the data has been taken. Of course So I think this is where
we have the theory community should engage more with our experimental colleagues to
really try to understand what is possible in the experimental side? And what are
things that theorists would think we would be looking for. We weren't looking for in
these trigger level analyses or in this data set. Now, so when I'm talking about
online event reconstruction, I would be, it would be outrageous, if I wouldn't
mention lacp. As specific specifically, there are new online event reconstruction
or in the next upgrade, it will basically eliminate the level one hardware trigger,
and do everything in software. And so here are some really nice studies where they
showed that in the particular context of this dark photon parameter space, they'll
be able to cover very large chunks off of the parameter space. And this is really
thanks to this innovation where they can do things on the software trigger.
Specifically, they can cover what I would call this the triangle of doom in the
sense that that These upper gray areas are excluded by Muslim factories. Well, this
is the triangle over here, the gray area is from beaten up experiments. And so this
is a majority difficult part of parameter space because for a beef factory does the
production rate is not high enough, while four being an experiment, the lifetime is
too short. And so this thing decays before it hits your detector. And so lacp is
going to clean up this parameter space. putting us in a bit of broader context. If
you extend this plot all the way to lower couplings. You see basically that once we
have the lacp result, we will know fairly robustly that there are no dark photons
below a mass of roughly hundred meters, because this just keeps going. All these
other constraints keep coming in. And so this will be very nice and very robust. So
another thing, just focusing on a slightly different model and focusing on CMS that
you can try doing this, you can With the new CMS level one track trigger that will
be implemented for high luminosity Elysee, it might be possible to look for these
dark hex scenarios where you have a light Higgs a light scalar field phi here, which
makes us with a standard model Higgs. And so it picks up a you call a coupling, in
this case to the top you column. And you produce it in these
v penguin decays basically down to the case of neurons in the displace vertex. So
when you're rehearsing with that study last year, are we trying to estimate what
the potential signal would be for CMS for this type of model, and you can find we
can see that it's roughly in the same ballpark of what lacp could record in
terms of the number of events. So this year with Jared Evans have been looking a
little bit into what would be the offline sort of offline implications for this and
you see that you can really potentially improve the reach for this type of model
by by quite a lot. If CMS is To do this now, so this is more or less all I want to
say about the sort of minimal model scenario. So there is a really nice review
that we worked on last year led by a guy on friendship maximum possible off, which
I think to the day, that's really the most comprehensive overview of minimalist
models, especially in the context of both in the context of experiment and theory,
where a lot of things were compared. Now there's a very fast moving field. So that
means that some of the things were outdated already in months after the
review was put out. But this is a very good starting point for anyone who's
interested in these things. I also want to advertise a small virtual workshop we'll
have in a few weeks, where many of these things will be addressed in detail, so it
does interest you and I encourage you to sign up for the workshop. Now I will now
switch gears and talk more about these more complete models which are attempting
to address one of the users We'll be questions that are plaguing us with a
standard model, what is the dark matter biogenesis, and so on. I think you're all
familiar with what this says. So let's get started on like these three particular
examples is, of course, many more, but I had to make some some choices. So let's
talk about how symmetric dark matter and biogenesis. So the really underlying
question is, why is there more matter than antimatter in the universe? And so the
question, this question was answered in a sort of, pragmatic point of view by soccer
off in the 60s is like, if you want to do get Baron, a symmetry that's large enough,
you need some sort of CP violation, some out of equilibrium process and some amount
of barrier number violation. Now, it turns out the standard model has all of these
things. We have a phase in the CPM matrix, we have an electric phase transition and
there are electrics failure on processes. However, if you do the estimate, you can
show that none of these things are enough really to create a large environment
symmetry in In the universe that has all the right ingredients, but unfortunately,
in the details, it doesn't really work.
So this means
when you think about hidden sectors, you put all your hopes and dreams in the
hidden sector, which means that you can really realize all the things that the
standard model has the potential for but didn't actually really achieve. And so
here to to understand how this works, we need to take a look at the history of the
universe as a function of time and temperature. So we always start very hot
as usual. And as the universe cools on time, all side you first go through an
electric phase transition. All the W m z bosons are getting a mass essentially.
Then around one gv there's a QC phase transition where you start condensing
gluons and so on into metals, and around BBN, you BBN around one and the V you
start forming nuclei. On the Dark Sector, something somewhat analogous happens if
you want to realize this scenario. So first You have a dark phase transition,
which tends to happen at a temperature higher than the electric phase transition.
And you want this to be a very strong face position. So you can generate lots of
large enough barium, barium number in the Dark Sector. So the next thing that
happens is that that barium number in the Dark Sector must be somehow transferred to
the standard model. And then finally, you're left over with a lot of symmetric
degrees of freedom in the Dark Sector. And so they have to annihilate away to the
standard model. And ideally, that's happened before the onset of Big Bang
liquid census, otherwise, you end up in a boatload of trouble. And so these last two
steps are very interesting for us because it means that there is some mandatory
coupling between the Dark Sector and a standard model for this to work for this
cosmology to work. And so these are these are exactly the couplings that we're going
to try to use in order to discover these things and experiment. So here's just a
recent example from the group the group at UC Berkeley. So they considered like a
confining Dark Sector, where you have a dark neutron with a MOS of roughly a gv,
which acts as a dark matter candidate. And so since they are dark neutrons, they end
up with a lot of dark pi zeros as well. And so those guys you have to get rid off
before BBN stars, otherwise you will, you will mess up the predictions from Big Bang
nucleosynthesis. And so it is works and their model is this dark pion will decay
to dark photons. And then the dark photons themselves will decay back to the standard
model. And so you can see the parameter space here on the right, so this is the
same plot that we showed before the dark photon mass versus the mixing angle. With
the Standard Model photon This is the triangle of doom that I talked about
earlier. And in their model, they don't have a specific prediction of where where
you should live in this parameter space, but they just tell you that well, we have
to live to the left of this blue line. And to the above this, this red line over
here. So that means that the the model is definitely discoverable, but like it's not
strictly Speaking falsifiable at least not in this channel in the sense that you
could, in principle be a magnet scenario, look over here. And this is sort of
inherently a generic feature of these more complicated models. That tends to be
always like some way out. I think I need to speed up a little bit. So the other
problem that is interesting is the actual quality problem. So if you're interested
in the strong CP problem, this works really well. If there's no, so there's a
global symmetry of pq symmetry, something's broken the actual fixed
potential by contributions from QC D, and you want to sit in his minimum over here,
but even very small deviations from for instance, gravity will mess this up and
you're slightly off the center of the potential which ruins the setup. In this
plot, you're seeing the mass of the axiom versus the coupling constant and so this
line over here, as you can see the axial line above this line, you have an axiom
question. The problem below this line you end up with, with the data quality problem
that you need to solve. So there exists UV solutions for this some additional model
building is needing it. But there is another interesting question is like, Can
you make the axiom heavy enough so that you live in this space you don't have, you
can see the colliders but you don't have this quality problem. And so this is where
a doctor comes in. So if you have an extra Dark Sector, in addition to the standard
model, and it's connected with the axiom, in this way, the axiom derives its miles
both from the standard model and from the Dark Sector. And it's still parametrically
lighter than than the standard model. However, if you now make the Dark Sector
much, much heavier than the stone model, you can pull up the mass of the action in
this way, and solve the oxygen quality problem. Of course, as always, there's
some cleverness that's needed in the sense you have to set this up very, very
carefully. So you don't reintroduce a strong safety problem. But there's a set
of nice papers over here which do it so This
experimentally, consequences were studied recently in these two papers over here,
which I encourage you to take a look at. So this is the parameter space, the axial
inverse of the coupling constant. And here are some projections for instance for lacp
dye photon search, which are many different channels that are of interest.
The generic thing to remember is that if you want to solve a strong CP problem, you
must have a coupling to gluons. So, there is some way of producing these particles.
And you're also very likely to couple to photon So, photon jets, which is the study
assumption, this study is a very well motivated signature for this scenario. So,
the final thing I wanted to talk about is the hierarchy problem, specifically class
of models are called twin Hicks models. And so this is motivated by the fact that
the status of the hierarchy problem thanks to the excellent work for our colleagues,
Atlas and CMS, is a little bit bleak these days in the sense that your stop is
excluded down below roughly 1300 gV or so while you're really wanting to lose Around
500 feet. And so the question is, can you still have top partners that are living in
the sort of more preferred parameter space. So this is addressed in so called
twin x models where you have an additional sector dice to the standard model, which
is more or less a symmetric copy of the standard model. And in particular, it also
has a top Quark and, and really what's happening is that the top Quark in the
Twin sector serves to partially cancel the divergence off the Standard Model top and
this reduces the hierarchy problem to some degree. Again, of course, some cleverness
is required. And you can see this was achieved in this paper and then when you
follow papers, since. So I think the citation history of this particular paper
is actually interesting because it's a very nice metaphor for the psychology of
theoretical physics and in the last couple in the last decade or so. So one The paper
came out like this is well before the Elysee, people were quite confident that
this is a nice model and so on. So we give you some citations, but Susie will
definitely show up at the Elysee. So not to concern that we discovered a Higgs. And
then you see here around 25 members of the barn data being collected as this spike,
this threshold of citations are suddenly reality was starting to set in. And the
sort of more exotic possibilities became suddenly much more attractive for us to
study. But jokes aside, just in terms of a pragmatic point of view, what are the
implications of these type of models? So twin Higgs models
it's almost up. So
I'm I this is my second to last slide. So Twin Peaks model is is an example of a
Hidden Valley which means there is some confining dynamics there and some
complicated things can happen. So for instance, what can happen is that you have
quarks and gluons in the Dark Sector so you can create some dark shower Large
multiplicity of dark metals, which then decay back to the standard model,
sometimes their displays are too short. So you got this very goofy signature over
here where you have a jet with a bunch of this place where it shows. The general
question here is how do we build a set of maximally inclusive searches that can
capture all these types of signatures with a minimal amount of searches on the
experimental side. And so this is a very complicated problem, a lot of theory work
is needed and is in progress for this. But the current status has been summarized in
the Longwood article white paper that came out last year. And so it's a very nice
subject in the sense that it combines things you're doing along with particle
searches with jet substructure and precision PCT calculations as well as new
tools like machine learning and so on. Alright, so this is why I will will
conclude so the one of the points I wanted to make is that the dark sectors if you
want to call it or like it's really like the jack of all trades and stuff, bsm
model, it just shows up everywhere because it's such a incredibly broad framework in
some way. And there's a lot of flexibility in terms of doing things, which makes it
also difficult to really pin it down. There's a lot of great work going on in
all different types of frontier. So Atlas and CMS we talked about, we didn't mention
much about intensity from fear, but even neutrino experiments are contributing a
lot to this field now. And on the theory side, I talked about model building. But
in the appendix of this talk, what are some nice results and proof calculations?
Also, production decay rates have so many scenarios, we didn't even touch cosmology.
Now, what is there to do for experiments for experimentalists? I think the
challenges are really how do we maximize our sensitivity to exotic Higgs the case
it's like been a case of high luminosity Elysee to make sure that we really squeeze
the most out of this. And I'm a sort of more long term perspective is like how do
we preserve our data for future generations because given the complexity
of these models, and the inherent level of uncertainty is quite clear. possible that
in a few decades from now, someone will come up with a completely plausible new
model. And we really need to find a way of making sure that that does not get lost at
that point. On the theory side, I would say that we need to engage our
experimental friends more, but things like triggers and scouting techniques and so
on. So they're really maximize, maximize the efficacy of these very cool methods.
There is a question about what our comprehensive coverage is possible. And if
so, is that what does that even really concretely mean? How do we define this is
a debate we can have? And maybe on a more philosophical level is like in this space
of theories, what are we mean when we are trying to when we're saying we're making
predictions? Do we insist on discoverability falsifiability? Or do we
do we are we insisting on need or if the theory solves a problem, are we satisfied
with it? So these are some open questions. So I will finish I thank you all for
listening. I hope everyone stays healthy and unwell in these difficult times I
think the agar Eagle especially for for helping me think through some of the
things that were discussed in this talk. And if anyone wants to discuss more at the
end of the session if your zoom meeting ID and a password will be present after the
last talk in this session.
Okay, thank you very much, Simon for this nice overview talk. So we have now 92%
participants, I'm sure there should be questions. At the moment, I don't see any.
So let me ask you about precision in this area. What is the status here? I mean, do
you are you at one loop level to low level QC electric?
It really depends on on the specific scenario. So for example, I can I can show
maybe like, these are some two results on the theory side that I think I wanted to
Highlighted enough time for. So if you have a dark Higgs, which is what's
happening on the left hand side, this is your you're trying to calculate the decay
width of this particle mixing with the Higgs, but it's around a few gv. So it's
very challenging to actually do this calculation and so you need to use all
these unitarity methods and so on. And so this this resolved by Martin Winkler is
very nice. The theory uncertainty on this and the few gv regime is fairly large, I
mean, it's still or one and it will probably remain, remain like that in the
foreseeable future in the lower masses in the higher models, the uncertainties are
fairly small. And then on the right hand side is the same thing for accident like
particle again, the future here is very challenging.
Okay, I see. Thank you very much. I have now one question. So you can talk now.
Think of unmuting yourself. Water.
Yeah. Very nice Talk. Thank you very much. I have perhaps a little bit Viet question
we had just recently, a very nice seminar by Madison. center. And he brought forward
the big question that how we may test that actually quantum field theory is the right
theory. Because if I did all your models correctly, all the models rely on this
very, very fundamental assumption that we are working with a quantum field theory,
right?
That's correct. Yeah.
Yeah. And and I found this intriguing question, if we have now this tremendous
data and this tremendous position, and you were mentioning position, choose leaf as
an example. Would you see Is there any way forward also, this respect to possibility
and this?
This is a this is a very, very challenging question. I spent a little bit of time
thinking about this at some line, or Well, let's put it this way. I had useful
discussions with people who were thinking about mostly at the Institute for Advanced
Study like the one and EMA and so on and Yeah, I mean, like so one thing that you
know, if if you're suppose you were measuring a distribution, let's say the
beauty spectrum of TT by production or something like that, and for some reason
your your spectrum with some would be a discontinuity, right, like so. So one
thing one robust prediction of quantum field theory is that you should see a very
well behaved behaved continuous spectrum and so on. So if you would see, you know,
discontinuities like, like delta functions, or just like things falling off
cliffs, that would be strong evidence that something very major is wrong. Now, that
being said, we don't know how to calculate anything, and that's an area right so so
you might be able to consistently do a tree level calculation. But then at one
loop, if you're violating things like locality and so on, it becomes very
difficult to really understand what it is that you're doing when you're trying to
calculate particular observables. But yes, there if you would see very strange things
in the data. Like discontinuities, I think that would be my shortstop, my first
guess.
Thank you. So this was presented at the winds that we may need from the theories
that say, just look into this. And this
is really helpful, perhaps. Thank you.
Thanks a lot. So we are running out of time, but maybe you can ask very quickly
on this question from the chat. So the question is, what's your point of view
about the x 17? As you mentioned, the current exclusion limit on gamma t mas
less than 100 MDB.
So if you repeat the question, is it related?
So the question is, what's your point of view about the x 17 x 17.
So, this is the are you referring to the
the the x is in the Hungarian experiment at 17. MEP or is this
was like it was extended? Then you resonance? Yes. Yes. Then you resonance?
Yeah. So There are there are two, I want to be careful about this the theory model
as models that you have to build in order to accommodate this access exist. But
that's, but they're very complicated and quite contrived. So they don't give you a
lot of confidence in my opinion, but this is this is real, at least from a theory
prior point of view. And then yeah, this is not my opinion, necessarily, but I hear
from experimental colleagues that the observation itself, the experimental
measurement is tricky and there might be there might be questionable, but this I'm
not I don't want to elaborate on because I'm not qualified to weigh in on that very
accurately.
Okay, thank you very much. I'm sorry, but we are running out of time. People can
join you in the Zoom Room afterwards. Thank you for giving your talk and I hand
over now to Catalina Yes,
move on to the