And the first talk is by Kate Pachal, who is talking about the dark matter searches
at Atlas and CMS.
Just checking Caterina you can hear me and see my slides?
And
also, if I fullscreen them just a second
full screen, you can still see them.
Yeah, everything works. And five minutes from the end of your talk, I will be signing a warning.
Thanks a lot.
Alright, um, first I just wanted to say thanks a lot to the organizers. You've
done a lot of work to put together a really interesting online conference that
I've gotten a lot out of. And thanks also to all of the participants who connected
on a Saturday to listen to us talk about dark matter. That's much appreciated. So
as Cate rina says, I'm here to give the experimental overview of some dark matter
searches at Atlas and CMS. So I'm starting with a very, very baseline overview of
what we're doing. Obviously, Simon's talk set the stage for this much, much better.
But here's the doodle that you all will have seen a million times about dark
matter searches and the different ways that we can do them. So if we assume we
have some dark matter particle, which we will just say there's only one for now,
nothing else, and that there is some form of interaction which connects it to the
standard model, then there are typically three ways that we go about looking for
this. So the first way would be to look for dark matter particles interacting and
producing Standard Model particles. This is indirect detection, and is what you see
at experiments like ice cube and super K. Next, you have dark matter and Standard
Model particle scattering, which would be the direct detection experiments like Lux
and Xenon. But there's of course, one more dimension, you can read this diagram,
which would be standard model particles interacting in some way to produce dark
matter particles. And this is what we have at colliders such as Atlas and CMS. So
what you can see is that colliders really do make up a key part of this picture. All
of these searches can access slightly different kinds of models, slightly
different spaces. And so we do need all three components to build up a full search
picture. Today, I'm just going to be talking about this one here, which is dark
matter searches at Atlas and CMS. And I should put a further warning, a ton of
different things can constitute Dark Matter searches, I'm only talking about a
small fraction of those, there's going to be a ton more things that would be
relevant here. But really, we can't talk about all of them today. So let's start by
thinking about what Dark Matter could really actually look like at the LHC. So
essentially, this blob from the previous slide, what is that? This depends a ton on
our assumptions. And again, we've just had an entire talk about this. But to take it
down to how we've been addressing this in the experiments, at very low energies, you
really don't need a clearer picture than that. You can get away with just leaving
the blob blurry and have an effective field theory that treats this as a four
point interaction. And you're pretty much done. And this is valid as long as you
have situations where the momentum transfer is small compared to the fundamental process.
So for example, for direct and indirect detection experiments, that's valid, but
at the LHC most of the time, there's one exception later in this talk, but most of the time,
our high energies mean we really need a more complete picture than that. And so then
we have to ask ourselves, what should that be? There's a ton of different models
available, as you've seen with different levels of completeness, and
simplification, and each of them has different dominant signatures
experimentally, which is where this comes into becoming a really important question
for us. So in this talk, I'm going to discuss Dark Matter by a few very
simplified models, all fitting into the minimalist version of Simon's landscape,
and highlighting the analyses that best constrain each and I'll be focusing on new
results with full Run Two data as often as possible in the two experiments. So this
is a picture that is a very useful diagram from the LHC dark matter working group
that sort of clusters together these different models into regions, I guess
where this over here is basically our E F T s. And it's not really what we do at
the LHC anymore. In the middle here with simplified minimal Dark Matter models
where we have just like maybe one or two mediators or one dark matter particle,
everything relatively simple. This is pretty much where we're at right now
today. And so I'll be using a couple of these models to frame the discussion.
We're going to have in a minute.
But then of course, there are all of these much more complete models as well. These
were mentioned by Simon, they're a little beyond today's scope. One of the ones that
we obviously have a lot of experimental limits in terms of is supersymmetric
models. But we've had a lot of different talks on those in this conference. So we
don't need another one right now. I want to start by talking about mono x
signatures. So this is a really key final state in all of our dark matter searches.
And what is it. Basically you have missing energy plus a physical object. So we have
something usually a jet, but you could radiate anything off of an incoming Quark
in our mystery diagram. And this gives us both something we can trigger on, and the
momentum that we can measure which is important because then we can also have
missing energy via an unobserved system of particles on the other side of our event.
If we look for momentum balance, we see that it isn't there. This is our missing
energy. This tells us there were invisible particles. The great thing about this, as
you can see is that it still doesn't matter what the blob is. That's really
model independent, no matter what your interaction is between the dark matter and
the Standard Model, you can still get this signature because all you need is for one
of your incoming quarks to radiate something and then to make some dark matter.
What's interesting though, is that once you assume a model, a lot of other
signatures become powerful as well. So today, we're actually going to go beyond
the Mono jet signature, we're not going to talk about mono jet. And we're going to
showcase instead, recent Atlas and CMS results in the areas of their own unique
strengths. So let's start out by talking about an S channel Z prime model and the
mediator searches that you get from that. So let's consider that we just stretch out
our blob and we have a single interacting mediator x. This could be all kinds of
things. It could be vector, scalar, pseudo scalar, whatever you like. But
fundamentally, we're just going to have one massive mediator, one dark matter
particle and two allowed vertices. For this set of comparisons we also then pick a
vector, axial vector, Zed prime cause that has higher cross sections and it's a little
easier for us to handle. And when you think about this model, you can see that
there are two key signatures. The first one is the mono x that we just talked about,
where you have a decay of your mediator to the dark matter particles. And then you
additionally have a radiated something that you can trigger on and look at and
this gives us a mono x signature. You additionally have this one down here where
you could have some coupling from your Zed prime to fermions. And you just have a
pair of fermions in the final state. This is a visible channel. So this is really,
really useful for us because we don't have to worry about the missing energy. And it
means that we have a resonance we can directly reconstruct by looking at those
two fermions that come out. So I also want to highlight because we're going to be
using this model a lot throughout this talk. There's only two new couplings in
it. There's the coupling, in this case to quarks but could be leptons as well
between your Zed prime and whatever fermions you have and there's the coupling
of the Zed prime to dark matter. What this means is that the values we choose for
these couplings strongly affect the limits we present. So we can have a large amount
or a small amount of exclusion in this model pretty much by tuning these
parameters.
This is something to bear in mind when you look at any limit plots in it.
Okay, from there, let's get on to our first experimental result, we're going to
be talking about dijet resonances. And I want to highlight this really nice, new
paper from CMS here. So dijet resonance is, in some sense, the absolute simplest
of the mediator searches. If you have qq going to Zed prime as an allowed vertex,
then obviously you can have Zed prime back to qq as your decay channel, all you
need to do is take your two jets, find the invariant mass and look in it for some
sort of new resonance. If you have a background made entirely of Q C D jets, then
you expect this mass spectrum will be smoothly falling. As you can see this line
of black dots in the right hand side plot here, where signals will appear as peaks
that you will be able to pick up on top of that background. Now traditionally, the
way that we've done these searches is we use an approach to the background
estimation where we essentially fit a functional form that is smoothly falling
like we expect the Q C D distribution to parameterize the background. This new
analysis from CMS has done something really fun they've actually used the data
which has jets going really forward in the detector to predict the shape of the
spectrum in the center, when you have jets in the center of the detector, you can see
in the bottom here that there's actually really good agreement between the two
methods, which are the red and the green leftovers. But the fact that they're using
this method instead means that you can have much wider signals and still
constrain them compared to what was possible with the fit only search. And
this allows us to set limits up to much, much higher couplings than were done in
the past because larger couplings tend to give you a wider width of this resonance.
So this means that the, the CMS result was actually able to extend limits on this
model all the way up to cases where you have a width to mass ratio up to 50%, which
for us is really quite novel and you can see this really nice limit plot here. The
next thing I want to talk about is if you have a lepto-philic Zed prime. If you
allow a coupling from the mediator to leptons, then you can have a dilepton
final state as well and this is a very clean powerful decay channel which has
lower backgrounds than dijets has and can also be a very important sort of
mediator search. So you can see this new Atlas result here, we have electron pair
channel on the left hand side and muons on the right hand side. One thing you'll
notice here is that because lepton triggers have much lower thresholds, we're
able to access much lower masses here than the dijet search could get to. So
these axes go down a lot lower. The other thing is that the resolution is better in
the e e channel. And this means that we have these really fine peaks, which are, again,
a big improvement on what we can do in dijets. So in the situation where you
might have a coupling to leptons this is a really great signature to look at as
mediators. You can also of course, assume that you have heavy quarks instead of
regular quarks and we can look for a TT bar resonance. This is a new Atlas full
run 2 result that came out quite recently. In this case, you would want to
identify the hadronically decaying tops, which turn into large radius jets and
have various substructure information in high mass that can make them identifiable
as coming from top Quarks. So we would tag two top like jets and look at their
invariant mass as well. Now, in order to sort of assist with this, the analysis was
looking at the tracks that are contained within these high radius, or large radius
jets to identify when there's a B jet inside each of them. Ideally, you would
expect there to be one B jet within each of your top candidates. There's also
often another situation that we fail to tag one adequately. And so there are two
signal regions for this analysis, one with just one B tag required, where you can see a
smaller signal contribution, and one with two b's required where you can actually
see a fairly dominant signal shape, but then taking the two in combination gives a
stronger limit. And so you can see that nice limit obtained from the statistical
combination of the one and two b signal regions right here. I want to highlight
that these three curves here are different theoretical cross sections, of course,
where the theory cross section crosses the black line, that's the mass limit being
set on this model by the analysis. But you can see here the difference between
the blue dotted line and the red dotted line is the difference in the width of the
mediator that you assume. And so that can really, really change the the actual
exclusion that's obtained.
Now, I mentioned that low mass is one of the advantages of the dilepton analysis.
What if we want to go even lower in any object that we have, then the simple
resonance searches we've seen a moment ago? Well, it's really challenging to
search for low mass mediators, because of our triggers. This is something that Simon
mentioned a moment ago, but essentially, the decay products from our resonance have
to have high enough momentum to pass a trigger so that we save the event. And
in ATLAS and CMS, we have two main methods to work around this. One of them is to
trigger on associated objects, which are produced with the resonance that you're
looking for, and the other is to perform the analysis at the trigger level. So I'm
just going to start by talking about triggering on initial state radiation.
This is on some associated object. Imagine x is the resonance that we want to find
here. It's going to decay to two jets but the jets are too soft to pass a trigger.
If we have a hard radiation of an additional jet or a photon or something else, we can
use that to trigger and then look in the rest of the event for the thing that we
want to reconstruct. Now, if this is really, really quite light, then these
decay products will end up being boosted. And this is the CMS result that I want to
talk about at this moment. In this case, the decay products of the boosted Zed
prime are so close together that they become a single large radius jet. You can
then look at the distribution of energy within the large radius jet in the same
way as we top tag to select for topology that looks like there are sort of
two separate energy clusters. And this is something we can use to reject Q C D and
identify the signal. This then gives you something where you can estimate the
background using events which failed the large-R jet tagging selections. And you can see
the really nice estimate here and this is then used to push a resonant search all
the way down to around 50 GeV,
which is pretty fantastic.
Atlas recently put out an additional full Run 2 result which is looking for the
same kind of Zed prime mediator but where you have initial state radiation of a W or
Zed instead. And so this is essentially looking for dijet production in
association with something that gives you a high p T lepton and the lepton is used for
triggering. Here again, you can see that it's possible to push the dijet spectrum
down to around 250 GeV, which for such a search is really quite low. And once
again, fit this smooth distribution that goes all the way up to around 7 TeV, and then
search in it for excesses. As with the other things that we've been showing here,
you can see there aren't any, but it's pretty neat to be able to access this
range when we have a look. Once again, here, the fit is sent to the combined e
mu channels, and you can see a limit plot on the right hand side, you'll note
that the intersection of the theory curve with the observed limit is at quite a bit
of a lower mass here than the previous limits we've seen in this model. And
that's essentially because of the lower probability when you expect your radiated
W or Zed in the initial state as well. So the next thing is bringing us back to the
trigger level which is really quite an exciting topic. So we've got two cool new
results
to highlight right here.
I can see that I switched up my links by mistake. This is obviously a CMS result. Ah
yes, CMS result is down here, Atlas result is up there. Both have done
trigger level jet analyses. But the one I want to highlight right here is an
interesting combination of the initial state radiation and the trigger level
approaches. So this is a new result from CMS, which is using three jets to have
enough combined sum momentum to pass a trigger. And then saving only the calorimeter
jet information and nothing else so that by saving a minimal amount of data for a
large number of events, you can overcome the trigger rate reduction, which would
otherwise happen. They then only take the two hardest jets to make the invariant
mass and assume the third one was just initial state radiation. But when doing
this,
what you can see is that there's a really
fairly high statistics distribution that again, can get down to around 300 GeV which
can be used to search for excesses with the signal shapes that you can see here.
There's also a new result I think is really interesting, which is using the
same trigger level strategy, so again, saving a lot of data, or saving a small
amount of data for a large number of events, but doing this for the first time
for muons, and then doing a muon dilepton invariant mass search,
similar to the one that we did before. So you can see right here the distribution of
the invariant mass of muons in this CMS analysis, you can see a nice Zed peak
here. And then as you go downwards, around 50 GeV, the red line here, which is from
the standard triggers, drops off as trigger pre sales kick in, whereas the green line
here represents what's gained by doing the scouting or trigger level analysis and
taking the muons just at the trigger level. And you can see that at the lowest
end here, it's about a two order of magnitude gain, which is really, really
cool. And so this region is now able to be searched for new excesses, in a much more
sensitive way than we had before. The last thing I want to talk about in the resonance
search section is diboson resonances. This is a little bit different from the
simplified model we've been talking about so far, but in some models, you can
essentially allow a Zed prime to couple to W and Z bosons. And then depending on the
other couplings, you have various available production modes. For that you
could have gluon-gluon fusion, Drell-Yan, and V B F. It really depends. So this is a
recent Atlas search, which has a huge number of different signal regions and
plots. I've only picked out one here just to sort of highlight, but they're looking
at 0, 1 and two lepton final states, assuming one of the vector bosons goes to
q q and you can get another one of these large radius jets, and then taking a look
at either the invariant mass distribution or the transverse mass depending on how
much energy is lost to neutrinos, and searching in that distribution for
resonance peaks. So you can see one such distribution up here. This is the W W
with one lepton invariant mass distribution, assuming gluon-gluon
fusion and selecting for that. And as a result, the limit on Z prime to W W, which
is being set down here in the bottom plot. Okay, this brings me to the next section
of this talk. We're going to move away now from that s channel, Zed prime
simplified model, and we're going to have a brief interlude to talk about invisible
decays of the Higgs. This is another form of simplified model connection
between the standard model and dark matter, which is a really powerful
benchmark for us to be checking out. So in this case, we're going to assume that we
have one new dark matter particle and then it couples the Standard Model only via the
Higgs. Now if we assume that we understand the total Higgs production cross section,
it's natural to ask how much room is there for BSM decays within that. All of the
numbers that you see here I've taken from some ATLAS documentation, which is in the
link in the next slide. But roughly speaking, the combination of the observed
Higgs decays that we have now sets an upper limit on the remaining possible
branching fraction of the Higgs to invisible or BSM decays at around 30%.
This is a larger number than I expected. My understanding is that this is because
there's quite a bit of wiggle room in the theoretical production cross section, but
maybe
If anyone has more details on that, feel free to put it in after the talk.
Regardless, this actually leaves us quite a bit of wiggle room for having some BSM
Higgs decays, even though we've had as many measurements as we have on the Higgs.
So the natural question asked is does this include invisible decays to dark matter?
These are really challenging searches. The goal of them is to set an upper limit on
the possible Higgs to invisible branching ratio. For comparison, the Standard Model,
of course, predicts some Higgs to invisible decays, but on the order of point 1%, so
we really don't have to worry about that. Now, here's the nice new result that's
just come out of Atlas. This is looking for VBF production of a Higgs with the
Higgs decaying to dark matter or other invisible particles. The topology that's
useful for picking this out is that you have very forward jets. This helps you to
reject just V plus jets production. There's an obvious remaining challenging
background from the production of VBF Zed going to invisible. So the approach to
this analysis is to trigger on the missing energy. Look for QCD dijets that are back
to back in eta, but not in phi. There are multiple control regions defined which
allow the constraint of the W and Z based backgrounds. You can see these along the
side here. And then a data driven fit is done to predict the multijet background in
these signal regions right here. And what you can see is this red line here
represents the maximum amount of Higgs to invisible decay that we could be having
without having observed it in the data. And this corresponds to a upper limit
on the branching ratio of 13%, both observed and expected, which is a really
nice improvement over what we've had before and well below what we can do by
just looking at the visible channels. There's also this plot included which is a
really nice comparison to the direct detection limits. In this case, an EFT
framework documented at the link there is used to translate the results into limits
on the wimp-nucleon cross section, which is what direct detection experiments tend
to use. And this is valid essentially, because the relevant mass scale here is
the Higgs which is quite a bit lighter than any sort of additional processes that
would be happening in this model. You can also see there's two different lines which
are being set as limits by this analysis which correspond to the
differences in the relationship between Higgs to invisible branching fraction and
this cross section, depending on whether your dark matter is scalar or fermion.
And you can see that the limits end at half the mass of the Higgs as we would
expect. Now, the last thing I want to talk about is to 2 HDMa models, and it's the
dominant signatures that come with that. So if we move to a simplified model, where
we think about a two Higgs doublet model, plus a pseudo scalar - again, this is just one of
many options, but it's one that we've done a number of interpretations in - then like
the Zed prime model, you can search in a met plus x signature, we have our typical
monojet signature over here, or you can look for visible mediator decays,
something like this. But these, you'll note, all of these situations here prioritize
third generation couplings and signatures which have vector bosons and Higgses. So
this is a bit different than the Z prime situation. Here we have TT bar, BB bar plus
met or all heavy flavor. And over here we have mono Z H signatures which have a
special role. So the first thing I want to talk about here is the heavy flavor plus
missing energy analysis. So this would be this Feynamn diagram here where you have a
TT bar and then invisible decay of this mediator. So the thing to do here is to
search for Dark Matter basically looking for two tops and high missing momentum
final state. In this case, this nice new analysis from Atlas selects a one lepton
decay to sort of balance high stats with suppression of QCD and then requires
at least two b tags to keep the top-like events. The signal region then is defined
here optimized for dark matter and you can see a multi-bin fit is performed in this
distribution, which is the angular, angular difference between the missing ET and the
lepton. You can see unsurprisingly, this is dominated by ttZ where the Z decays
invisibly and that is in this case constrained with a dedicated lepton-based
control region. You can see the nice exclusion plot which comes up here. So
this is a very new and very cool analysis. We have now mono V or Higgs which as we
pointed out earlier also has a special sensitivity to these 2 HDM plus X models.
This is a CMS result which took a bunch of existing limits and added in a W W and Zed
Zed pair of channels for this. So you can see here this plot is for the Higss to W W,
where they actually trained a BDT on the momenta, transverse masses, angular
variables and so on. And you can see this nice distinguishing between the signal and
the background. For the Zed Zed, it was just a typical missing energy
distribution. But the combination of the two has gone into this really nice limit plot
here where you can see the the dominant channel across all of this is still really
Higgs to bb bar, but that the combination has actually pushed down and has set
exclusions across a very wide range. This is really nice to see.
You can also of course, look for physical mediator decays. So if you assume that the
A can decay visibly, if it's very light, you would actually reconstruct even a, an A
decay to two gluons as a single small radius jet. So this cool Atlas analysis
leans quite heavily on a machine learning based estimation of this. So the first
step then was to train on a single small radius jet in signal to look for that two
pronged substructure and learn the resonance mass from that. A second step then took
that learned mass and various substructure variables and trained a selection neural
net. And then for events which pass the selection neural net, you take your Zed to LL
candidate, and this single, small radius jet, you make the invariant mass of those
three things. And if you have a new resonance, it should add up to the mass of
the Higgs. And so this little window around the Higgs mass is then used as a
signal region and searched for any excess. Okay, this brings me to the end of the
specific analyses that I wanted to show today. But I just want to remind you how
many more things there are that we do that are also Dark Matter searches, even if we
don't cover them in a talk like this. So this is just a reminder that
supersymmetry can also be a dark matter model. When we look for R-parity
conserving SUSY, what this means is that we require there's an even number of SUSY
particles in each interaction. And that in turn means that the lightest SUSY particle
has to be stable.
And this makes it a good Dark Matter candidate.
Depending on various other parameters that you tune. The quality of the Dark Matter
candidate will depend on your other assumptions, because various parameter
choices can give you something with too high relic density and so on. But this
is one of the powerful, more complicated models that we should be thinking of as
part of our dark matter search program. And so if you want more details on this,
I've put a link to one of the other talks from LHCP right here, go and check it out. I want
just to use my last couple of minutes to talk about putting all of these together
into a single big picture. It's really hard to keep track of all of these
scattered analyses, if we don't have these simplified models that let us put them all
in a single plot and take a look at how they all interrelate. So this is the first
plot that I want to show for this. This is taking all of our dijet-like resonance
searches and putting them onto a plot where we have a coupling to quarks on the
y axis, we
have the mass
of the mediator that we're searching for on the x axis, and everything sort of
above the solid lines here is something that we've excluded. This has been nicely
updated. I think this is the most fresh off the press result I'm showing today
since we just made this public yesterday. But you can see here the new limits from
the full Run 2 dijet and di-b and how those are really strongly constraining us
down towards this low coupling and high mass corner. The other thing that we can do, of
course, is put this more clearly into the rest of the Dark Matter picture for these Zed prime
models. And these are some summary plots you'll've seen a lot probably from Atlas
and CMS where we have in this case, dark matter mass on the y axis, mediator mass
again on the x axis, and we have to specify all of the couplings rather than setting
limits on those directly. What you can see here is Mono X limits from things like
mono jet fill in this corner, and then you have pretty much independence of the
Dark Matter mass when you look at the resonance searches. This is also new this
week from CMS and includes now the full Run 2 dijet analysis that we just looked
at earlier, and it was so nice. I highlight the couplings here because I
want to point out that if you change them, you get a completely different picture.
And so it's very important to keep in mind what you're including when making this
kind of comparison. This is my last slide of content. This is just reminding you all
that we should also be paying close attention to putting all of this in
context with the non-collider world. It's important that we also keep track of how
our results interplay with the direct and indirect detection results from other
experiments. I'm showing here just two versions of this. One of these is limits
using an axial vector Zed prime with couplings specified here, you can see this
is a spin dependent limit. On this side, we have a spin independent Dark Matter
nucleon interaction cross section limit for a vector mediator, you can see how
very different these two pictures are once again, where we have power, where we don't.
But making these plots and continuing to try to introduce these is something that we
want to do to make sure we keep up as active a conversation as possible. This
brings me to my conclusions. Sorry, I realize I'm a minute or two over. We
explored the wide range of analyses here today, which can constrain dark matter at
the LHC and tried to highlight the unique contributions of each of them. This
included in this talk Higgs to invisible searches, resonance searches and missing
energy based searches, which are all key contributors to our dark matter search
program. There's more full Run 2 results in progress now, so stay tuned for
some exciting updates. I know we have a few things aiming for ICHEP, so should be
fun. And as a final reminder, our dark matter possibilities are really super
diverse. There's a lot of broader Dark Sector searches, there's Susy searches, there's
long lived particle searches, all of these can be handles on dark matter. So I've put
down here a collection of talks around this week that you can go back and look at.
There's still more, but it's a place to start.
Thanks a lot.
Thank you very much for the nice overview talk. It wasn't, ah, it was almost on time.
It was closer to on time than not. So do we have any questions for the talk?
At the moment it doesn't look like there's any audience questions. So
maybe I will ask one?
So where would you see the most exciting Run Three developments that could be
coming up in this kind of analysis?
Could be coming up?
I guess it's just a personal question. It's not like there's a right or wrong
answer anywhere else. But what woud you aim for
in Run Three? I'd be interested to see us... I'd be interested to see us extending what
we're doing with trigger level analyses. I know this is piggybacking a little bit on
what Simon was just talking about, but I really do think we've only begun to
scratch the surface there of what we could possibly do and that at the moment, we're
being a little bit limited by the number of objects that we're actually using there.
Jets and now newly muons from CMS, but we could be thinking more about, if we are
able to, what other analyses like this could benefit from that approach to have a
better look at the low mass limits. The other thing I think we need to do is think
more about the simplified models that we're interested in using for our
summaries. It's too easy to make a plot like this, and look at it and see no holes
and say that we were done. And so I think we need to be pushing ourselves to examine
more thoroughly, more realistic scenarios where we're making more clear to ourselves
our limitations, and that should help us also to figure out what we need to focus
on.
Thank you.