Okay, so my name is Doug Schaffer from the University of Chicago. I'll be talking on
behalf of Atlas and CMS for searches for the Higgs boson decaying to invisible
particles. So probably like you, I've been spending a lot of time on the couch
recently, staying at home. But when we were able to get out and investigate the
world, we saw quite a few indications that the standard model is not complete,
including things like flat Flat rotation curves, remnants from the cosmic microwave
background, and images like the bullet cluster in which you had slower gases and
faster gases from those that interacted electromagnetically. So we have this
strong evidence that there's a lot of matter out there that we currently are not
observing. And so one possible way at the LSE to look for this is Using the Higgs
boson. So we know that the Higgs boson couples to various particles and gives
them mass. For example, the leptons acquire their mass through Cal coupling.
So potentially some similar interaction is occurring also for these dark matter
particles. And so usually we would just measure the Higgs width. And we would know
whether or not it was decaying to additional invisible particles. But
unfortunately, the Higgs intrinsic width is much smaller than the detector
resolutions that listen CMS. So then there are various models to predict how if the
dark matter is less than half of the Higgs mass, if it's a scalar, vector thermionic
Dark Matter ways in which you could couple. So this is how we might observe
the Higgs decay, how it's produced. Then how we actually search for it, because
we're not going to measure any of these dark matter particles in the ATLAS
detector assuming you know it's not interacting. So the production mechanisms
that we'll focus on here are mostly the vector both on fusion, z, H, and th these
are lower cross sections, then were used, like GG F to discover the Higgs, but they
come with lower backgrounds, especially at higher net. So, missing transverse
momentum, as you know, one of the important things on both the Atlas and CMS
in order to state that we have if we found something Oh, and so I thought it was
worthwhile to look at a couple comparisons of how the ETS is constructed in the two
experiments, and I think the the main driving difference that you see between
the two is is mostly in the soft terms. So the soft terms are, you know, in Your jets
or leptons, your photons, they're soft signatures that we're not entirely sure
which vertex they come from. And also, they are much lower energy, so it's less
clear how to calibrate them. So on Atlas, the approaches taken to use only primary
vertex tracks and to remove all of the associated clusters, the soft clusters on
CMS, there's a different approach used to recreate these clusters by various factors
to correct for the amounts of pileup. Now, there have been advances in many of these,
but these are the methods that are definitions that are used in the analyses
that I'll describe later. Okay, moving on to some of the searches. So, Higgs to
invisible searches, as I showed come from quite a few different Higgs production
mechanisms, and all of these are combined to give one ultimate limit and also the
run one and run two analyses were combined to have an even better limit. And in
addition to these, you can also beyond just these direct searches, you can use
Higgs coupling measurements, so use all the visible channels as well as this input
to put additional tighter limits get down to around 16% expected 30% observed in the
Atlas adn versus empty barn, couplings measurement, similar results from CMS and
Atlas using 36% of arms.
Okay, so both Atlas and CMS with up to 36 and verse two bar and set limits of 26 and
19%. What are the channels that feed into these? Well, the most sensitive is the BBF
followed by the Z decay electronically plus Higgs boson through Higgs strong, and
then the channel VH, where the B goes hydraulically. So I'll summarize through
these first few in order of sensitivity. So the first one is the DBF analysis. So
this channel looks for two jets that are well separated in the detector and have
large and great mass and then a large missing transverse momentum that's
balancing these two jets. So the Atlas analysis with 139% of Barnes requires this
large met this large invariant mass and then the name of the game is you come with
large backgrounds and you need some way to control those. So the way that it's done
In the Atlas analysis is that the Z decay to visible leptons is a control region
used to constrain z decay to neutrinos and then the W, where you have a visible
leptons, and you've identified it is used to constrain the normalization of the W or
you've lost one. So you can see the control regions in the upper right. And
you can see the prefix and postfix agreement in the lower right as a function
of digit and variant mass. And you can see there very good agreement, because of this
ATLAS experiment proceeded to set limits on the Higgs branching ratio to invisible
and set a limit of 13% to be compared to the 36 and first from the barn analysis
with 28%. Okay, so what changed between these analyses the 3600 hours And the 139
inverse empty barn. So there's re optimization to recover around 50% more
signal on the same dataset. They improved lepton detail, reduce the wl new
background from 48% to 33%. Increased acceptance for the Z to Ll control region.
Also optimizing electron selection recovered 34% more. So this is really
increasing your precision on that dominant z to new new background. In addition,
simulation statistics were an issue. So there was quite a bit of development to
generate three times the amount of simulated cross section with the same CPU
using matrix element filters. And so then you can see the change in the
categorization as opposed to just a three been MJ fit. There's now 11 bins and you
can see the various increase incense signal the background as you go toward a
higher m JJ and lo d phi JJ. Okay, the CMS analysis started CMS experiment also did a
similar study, currently with 36 members from the barn. As opposed to the Atlas
results. The main differences are that there is no third jet Ito. So this allows
for more sensitivity to ggf Higgs production, there's no explicit Tao veto,
and then slightly lower mgh a threshold also increases the GTF contribution
contributions. And then the CMS result uses the shape fit. So the previous result
has a control region for each signal region been so it's not a shape fit. They
actually use the mtj shape to better constrain the backgrounds and so this
leads to CMS having a 25% expected limit with 36% barn it'd be exciting to see with
the full data set.
The next most sensitive channel is the CH where the Z indicates left tonically. So
the dominant background here is debose on z to two leptons and the other one going
to neutrinos. And this as well as the W z. backgrounds are both modeled using
simulation. And you can see on the right the end result of the Met distribution.
And there's some slight excess, although still well within the uncertainties. And
so there there's an expected 39% at 95% confidence level and observed of 67%. One
exciting channel from CMS that was not included in the previous combination Is TT
h. So this is 36 inverse dumpster barn results that was reinterpreted from a Suzy
top partner search and it combines 01 and to leptons channels you can see in the
lower right and then looks in various categories of met. And so the zero and one
are about are the most sensitive expected channels, but the combined result is 48%
expected and 46% observed so it's a really nice addition to results. So just to
summarize all of the current results starting on the bottom with the run one
plus run to 36% of barn results you can see that there are expected of around 15
and 17% for Atlas CMS and then this th channel, then is a strong addition to
their currently combined channels. And then you can see comparing to just the 36%
of our and this is this is going to add quite a bit. And then the updated full run
to Atlas results at 13% now sets the best limit on Higgs branching to invisible. So
then one thing that we can do with the searches is to try to compare to direct
detection experiments. It always comes with some copy some caveats, but it seems
that we can in some decay channel or in some wimp models provide complementarity
to some of the direct detection experiments especially at low went best
and so you can see it Below, around 10 G, you can add sensitivity to lowers wimps.
Okay, so then the last analysis I wanted to summarize is a bit different than the
standard ones. So this is semi visible case to case. So this is just a simplified
model trying to make metabolics assumptions and, and so this one looks for
a dark photon, so a very light massless photon coupling to the Higgs and the way
this is produced is with the photon and in addition to the dark photon So, you'll,
you'll search for met plus photon and the search from CMS is done in the Z h z
decaying WEP tonically channel and is put out with 137 interest rates To barns, and
excludes hates branching ratio to gamma gamma dark of less than 5%. Okay, and then
just some conclusions. So the Standard Model, Higgs production of invisible
decays is only around point 1%. And so we can look to see for extensions of this.
Beyond the Standard Model physics, as you can see, there's a rich regime of possible
ways to search for Higgs to invisible with DBF leading the way and exciting new
channels like tgh to be included, and then we'll continue to see more full run to
results coming out. That should further push sensitivity. And I just wanted to end
on one projection from CMS about the potential sensitivity with
well first to be 300% the barn that we hope to get Soon, in addition to longer
scale, HL hc projections that get down to around 4% sensitivity from the CMS
experiment. So it'll be exciting to see with more data to look more closely it
Higgs to invisible to case. That's it. Thanks.
Thank you very much, Doug. This was a very nice and complete overview with a very
nice result.
Any questions, comments?
I have one, maybe if I can.
Hey, jack. Thanks. This was a really interesting talk. I was wondering if there
are plans to reinterpret the Atlas? TT plus met Susie search that john presented
in this model, in the same way as a CMS result that you mentioned?
Yes, sir. Yeah, there is there are plans to include similar analyses For Atlas, but
you know, obviously this takes time. So I think this will come out relatively soon.
Lots
more questions.
Maybe I have one.
So the new result the vdf.
So you mentioned a few improvements. Did you check where you would be with the
expected limit without those improvements?
Ah, yeah. So as ever call, I think that it is roughly around scaling with a factor of
two decrease from 28%. You would expect something like around 17 18% you have to
keep in mind that there's increased pileup, and so then this results in some
more pernicious backgrounds like Multijet. So it doesn't perfectly scale like the
factor of two that you'd hoped from the 28%. But add something more around 17 18%
Yeah, sorry, I don't have an exact number for you, but roughly that's
so when the So, mainly This is a improve the acceptance and the categorization. So,
which one is more
more to the improvement?
So, I think actually the things that contribute the most are actually this a
decrease in the W background. And then I would say similarly, increasing the Z to
Ll control region. So that is the precision on your largest uncertainty. So
the more Zika Ll you have to constrain z to new new, the better. The analysis is
going to do. Those, those two are two of the biggest improvements. And then if you
want the additional, what was the biggest limiting factor in the previous analysis
that was MC stats. So generating the specter of three was also a large
contribution. In terms of the breakdown. I think the simulation if you would have
stayed with the same amount of simulation would have been the biggest problem. And
then if you just want pure analysis changes, then these leptons acceptances of
the next biggest. Thank you.
Any more questions coming?
I have a question. That from Fadi.
Yeah. Oh.
So I'm curious about the gamma, the semi visible Higgs. So the question is, is this
motivated by particular model or because I would, so it's the gamma gamma two per mil
in the standard model, and Gamma dark is? Well, I mean, a couple of ships with, you
know, epsilon charge to the top. And, you know, so any model of something like this
would be super suppressed, but maybe I don't have something in mind that would
give a big did you have something in mind that that is motivated by a particular
model? Maybe? Sorry?
I yeah, I must admit, I don't, I am pretty sure that there is a model and I think I
can look it up and send it to you. I think that there's one referenced in the paper.
Okay. I believe that there's the there are some papers that show that this could be
large, which was the motivation to look for it. But yeah, I'm sorry, I don't
remember the exact details. No, no, it's okay.
You have the reference for the paper. Thank you.
Any more questions? So you mean you're basically out of time, but maybe One more
question to any of the speakers of the session.
If not, thank you very much, Doug, and thank you to our speakers. It was a very
nice session, I think and see you at the next part of our sessions.