So, an outline
of this presentation, I will first tell you something about the Higgs boson self
coupling, then move to rare decays and beyond standard model physics, and then if time allows a
very brief outlook towards high-luminosity LHC. I will focus only on a selection of more recent
results. There's many many results especially from the early LHC Run2 data
that you can find in the public pages of ATLAS and CMS and also in the parallel
talks. So for the self coupling, well it has been introduced already in the
previous presentations, in the standard model in the potential we have lambda phi
to the fourth power, and after the electroweak symmetry breaking, when we look
at the physical Higgs, we have a term that gives us a three Higgs vertex and also a four
Higgs vertex, but the four Higgs vertex, we will never be able to probe
at the LHC. So,
talking about the
so the trilinear vertex, this we can probe in double Higgs production and the
dominant production cross section at the LHC is gluon fusion, which has two
contributions, as we have essentially production of a single Higgs boson that so
goes dominantly through the top quark loop, followed by the self coupling to
produce two Higgses, or we have contributions like the square that only
depends on the couplings of the single Higgs boson to standard model particles.
There is a destructive interference between the two that gives us this very
small cross section of 31 femtobarns and causes the cross section to depend in a
non trivial way from the self coupling. In fact, if you go to zero self coupling, the
cross section increases instead of decreasing. So, in terms of results on double Higgs
boson production, the state of the art is still the results based on the early LHC
Run2 data. So it's mostly 2016 data. These, however, are fairly old results that
have been around for a couple of years, often, so I will not describe them there,
I just wanted to give you more or less the ballpark sensitivity. This is driven by
three modes, the bb tau tau that has put here in green for the two experiments, bb gamma
gamma in blue, and four b in red and the sensitivity also depends on how
advanced the analysis was on one side or on the other side of the LHC ring. But
combining all of them, the two experiments get to about 10 times a limit on the
double Higgs section that is about 10 times the standard model prediction. So
there is still a long way to go. But this was just with a partial data set. We have a
couple of results, and in particular one from ATLAS, that goes to analyze the full
LHC Run2 data but this is in the final state for double Higgs production to b
bbar WW fully leptonic, which is not one of the most sensitive the three most
sensitive modes that I was describing earlier. Still, this has, the analysis of
this has been a lot improved compared to the earlier searches. It uses advanced
multi class deep neural network to separate the signal. Here you can see in
the plot in black from the two main backgrounds that are ttbar in blue and
the Zed to ll and Zed to tau tau in different shades of orange and then
the discriminator that is produced from this neural network is used with some selection
cuts to define some signal regions that you see plotted here in the bottom right.
For same flavor leptons, where you have some backgrounds from the Zed and ttbar
and then opposite flavor leptons, so emu, that is dominated by ttbar. And you can see if
you look carefully the green, light green outline of the signal. This is however
scaled by a factor 20. So the sensitivity of this analysis in the end is about 30
times the standard model. This is a large improvement compared to the first searches
in this final state, a factor eight or three compared to the older searches by
ATLAS and CMS. It's not yet competitive with the best modes but it shows you
that hopefully also for those modes, we will get large improvements when we go to
new analyses on the full data set. Then we have another search for double Higgs boson
production, but targeting VBF. So VBF has an even smaller cross section than gluon
fusion for double Higgs production, it's less than two femtobarns. But it is
interesting because it has in the Standard Model three contributions. One, first, that
is again, production of a single Higgs boson, virtual, that then through the self
coupling gives two Higgs bosons in the final state; we have, as before, a process,
a contribution where only the coupling between the Higgs boson and the Standard
Model fields appears but just two times and you get two Higgs bosons with no self coupling,
but then there is this Kappa two V
diagram where we have a quartic vertex with two vector bosons that scatter and
produce two Higgs bosons. As it was, so, in the Standard Model or in the
Standard Model effective theory, the Higgs is part of an SU two doublet, and, more in
the way it was explained in the previous talk, the symmetry is linearly realized.
and these two diagrams are tightly connected. Here is Kappa two v, has to be
the square of Kappa v. And this creates a cancellation. However, if you go in more
general extensions of the Standard Model, like the electroweak chiral Lagrangian that
was described before where the Higgs is a generic scalar, you do not have this cancellation.
And you would have potentially a very large increase in the VBF cross section
because you will essentially violate unitarity in the scattering. So, ATLAS has
deployed the search to look for this, based on the entire Run 2 data set, except the
very early, very early part of it, focusing on the four b final state to have
a large branching ratio. And targeting this kinematic that you get when the
Kappa 2V parameter is different from one so you don't have the cancellation.
Here the kinematics becomes much harder compared to a normal VBF double Higgs
production. And so the analysis can afford very tight selection criteria to select
VBF like events with one TeV invariant mass of the two jets, and then, once you are in
this VBF phase space, you have in the central part of the detector double Higgs to four
b, and so, the strategy of the earlier double Higgs to four b search from ATLAS
on the partial data set is applied here. And so signal regions are
defined in the invariant mass of the pairs of the two bbbar pairs, and then you
draw a ring around it as a sideband that has been used to estimate the background
from QCD production that would contaminate, would be below your signal, and
in addition, to make the analysis more sensitive compared to the older searches,
a b jet energy regression is used to sharpen the resolution of the jets and so make the two
b invariant mass have a narrower peak for the signal. And still the
extraction of the signal is focused on this scenario as I was saying of Kappa two V
different from one and so the other couplings, including the self coupling are
set to unity. And in the bottom right here, you can see the sensitivity of the
analysis. So the cross section that the analysis can exclude,
that
becomes much worse if you go really to the Standard Model point because the
kinematics is less extreme but that improves as you move away in kappa 2V.
And the opposite behavior is for the cross section that is minimal around the Standard
Model point and then increases. So at the Standard Model with this analysis, you
would never see anything, this limit is at 500 times the Standard Model cross section.
But, thanks to these two features, you can instead set the first limits on this Kappa 2V
parameter at the LHC between minus half and three apsproximately.
So finally, the other topic about the self coupling is, as described by the previous
speaker, we can also constrain it from single Higgs production by looking at next
to leading order contributions where the self coupling appears. These are small
however, the, for example, for inclusive cross sections the largest effect is for
pT Higgs varying the self Higgs coupling by plus or minus 100%. Compared to the standard
model, the increase in the cross section is just about three and a half percent.
These can be enhanced a bit if you use kinematical information like it's shown
here, but only by a factor two, factor three, not much more. So you need good
accuracy on single Higgs boson measurements. This was tested by the two
collaborations in their latest single Higgs combinations. And here you can see
the plots setting limits on the self coupling divided by the Standard Model value.
And let me draw your attention to the bottom right one from ATLAS. This is just
the expected sensitivity but just because it's more useful to make comparisons because
you can see in blue here the constraints that you can put from single Higgs boson
production and in black the ones that you can put from double Higgs boson production.
And, well, the luminosity in the two cases is different, but still you can see they
are comparable.
This is good. However, there is a big
disclaimer in the sense that for the double Higgs, what we're probing is double
Higgs, for the single Higgs these couplings, these constraints are true if
the only thing that changes compared to Standard Model is the self coupling and
all the other couplings are fixed to Standard Model. You can relax a little bit and
assign one scale factor to fermions or one scale factor to vector bosons, but as soon
as you float both you will wash out the entire sensitivity at least with the
present analysis. So we may get to this for in a more realistic fit, but this will
take a long while and also more work from theory predictions in order to get all
corrections and all next to leading order predictions as function of this self coupling
and also all the EFT operators. Still, as summarized in this slide 12, the
constraints that we have on the self coupling and you can see that, depending
on the inputs you use, the experiment, and so on, we are between minus three
minus five times Standard Model to a bit more than plus ten. And, so, not yet so
close to it. This was for the self coupling. Let me now skip to the next topic that are
rare and forbidden and beyond Standard Model decays. I've made the list of new
results from 2020. There is one however, the Higgs to invisible, that is motivated by
dark matter and so, I will skip entirely from this presentation and you will see it
in the plenary session in the talk by Katherine in the session about dark
matter. However, before jumping to these decays I want to say very quickly
something about second generation fermion couplings. Luca has shown you that third
generation fermion couplings are all observant and well measured. Second
generation couplings are smaller, and so they are harder to probe. There are no new
results compared to late last summer. And in the case of the muons, the state of
the art was the ATLAS analysis from summer of last year based on the full data set.
Here experimentally this is a simple analysis, but the branching ratio is so
small that the off-shell tail of the Z above the signal is a factor of 1000 bigger than
the Higgs to mumu signal. One can try to improve this by using smart categorization
to select VBF like events but it remains a quite challenging analysis. And
the result from ATLAS here you can see in the data minus background panel, that
perhaps there is a very little tiny start of an excess. This is less than one sigma
in terms of statistical significance. Also, because the signal strength measured
is a half standard model with an uncertain go plus or minus 0.7, it's up to you to
choose if it's a full half or empty half. Now, for cc bar similarly, we don't
have new results to see from this year, the state of the art was the search from
CMS on a partial dataset that was released again in the summer. Here the
branching ratio is not so small, but it's still a factor 20 smaller than Higgs to
b bbar that wasn't an easy search to begin with. And so this is much harder
than that. CMS deployed a quite advanced analysis targeting vector boson
associated production in all the three final states, Z to ll, W to l nu and Z
to two neutrinos. And even with the combination of a resolved analysis that is
like the Higgs to b bbar with the Higgs decaying in two jets, with a boosted
analysis similar to what Luca described for boosted Higgs to b bbar just recently,
and despite all this, the sensitivity that CMS can get is only around 40 times the
Standard Model with them setting an upper limit at about 70 times the Standard Model. So for the muons,
we'll probably get there fairly often, like in Run 3, for Higgs to c cbar
it's gonna be more a long,
a long way.
Now jumping to new results, instead, there is one from, that was just shown for the
first time in this conference, from CMS, trying to look, probe couplings
To even lighter quarks than the charm. Of course, in the Standard Model, these
are vanishingly small. However, in extensions of the Standard Model motivated
probably by flavour, these can be largely enhanced. And so you could have in this case
production of a Zed boson plus a rho or a phi meson, so to the light q qbar states,
through diagrams of this form that could be largely enhanced. And this search uses clean
final states of rho going to pi pi or phi going to K K and so you have these two
tracks that have to be isolated, close by and at least one of the tracks have to be of
high momentum so that you reject the otherwise overwhelming background of Z plus
jets. And once you do this, since everything is very well measured in the
detector, you can build a resonant mass of the signal at 125 GeV and the background can
be parameterized with some analytic functions. And this allows to set limits
on this kind of decays. And for this search they are between point two and sorry
point three and 2% depending on the resonance, and also the polarization that
you assume that gives you different acceptance. These are the first limits in
this final state, but they still correspond to about 1000 times the
Standard Model. Now moving to another example of something that has a small
branching ratio in the Standard Model, I want to discuss Higgs to Zed gamma that was
released by ATLAS and presented at a CERN seminar just a couple of weeks ago.
This is the only diboson decay of the Higgs that we haven't observed then, in
fact, all the others have been seen since quite a long while. But the problem is
that this has a small branching ratio, comparable to Higgs to four leptons, but, once
you take a look at leptonic Z's, but a larger background from Standard Model Zed plus
gamma. However it's quite important to try to probe this because if you go in
extensions of the Standard Model typically symmetries tie together these four
vertices Higgs to WW, Higgs to ZZ Higgs to gamma, gamma and Higgs to Zed gamma. So, new physics
should give you a correlated deviation across these if it respects this symmetries.
So, for example, in the in the Standard model effective field theory you should
see a correlated deviations again in these, if you measure only three you cannot
resolve it but if you measure all four you could. And this is again a quite hallenging
search, similarly to Higgs to mu mu you have to try to get your signal peak as narrow as possible
and your signal to background as good as possible. For example, selecting VBF
events.
And I have put here a plot
[you have five minutes left]
Yes, with all the categories and in the background subtracted window you can start
to see a bit larger excess compared to Higgs to mu mu. Now that was two
sigma sorry one sigma, this is two sigma. So, not yet evidence, and this in part is
helped by the fact that a positive fluctuation is seen, so the signal strength is
two plus or minus one, but still we may be seeing hints of the first Higgs to Zed gamma signal. Now, let
me move instead to BSM physics and searches for light pseudoscalars
from extensions of say the MSSM and starting again with a new paper from ATLAS on
production of this light pseudoscalar in association with the Zed for light masses
of this particle. So we look for something that is less than four GeV and decays
hadronically, this can only be seen, this can only be reconstructed as a jet because if
you go for an inclusive thing there is not much more than you can do. But you can use
a multilayer perceptron, so essentially a neural network, to try to regress what is the
mass of the particle that gives origin to the jet and you can see that from this plot
that for light particles like half a GeV you can really separate them from heavier
particles and from the continuum background. And you can also use the same
strategy to produce a discriminator that selects signal and rejects the background
from Z plus jets. With that, you can enhance your purity and then try to reconstruct a
signal peak in the lepton lepton jet mass and build your signal region.
Unfortunately, a very good data to Monte Carlo agreement is seen not just in control
regions, but also in the signal region. And so there is no evidence for these new
particles. Limits can be set depending on the decay modes that you assume, for
example, here it is for a going to a pair of gluons. And for light particles
where the sensitivity is better, you can set, ATLAS can set branching ratio limits
that go as low as about 30%. Still for light particles there is a new paper also
submitted a couple of days ago from CMS for now for pair production of particles in a
exclusive final state now, one a going to a pair of muons and one a going into a
pair of taus. Again, we're looking at light particles. And so there is a
challenge that the decay products of the two taus will start to overlap. So CMS
deployed dedicated reconstruction, tagging first a muon, removing it and then
trying to reconstruct, a muon from a decay of a tau of course, and then
trying to reconstruct the hadronically decaying tau from the from the remaining
decay products and this allow to boost the efficiency by 50% for masses of say
five to ten GeV. And then you, one can use the fact that the dimuon mass is
measured very precisely and the signal should be resonant there and then also
fitted simultaneously in two dimensions not fit with the invariant mass of the
whole system that should resonate at the Higgs mass. And this allows to set quite
stringent limits on this final state, between point .02 and .08%. That's
a function of the mass with a little bit worse sensitivity around the Upsilon
peak of course. There is another search for pair produced light scalars but
instead on hadronically decays, hadronic decays, submitted by Atlas
yesterday. Now, if you look for Higgs to a a to to four b's, and especially at low mass,
you could not, you cannot trigger these events at the LHC at the moment. So the
search is done requiring a Zed associated production with the Zed going leptonically
to select the events. And then in order to catch both cases where the b bbar give
rise to two jets or the b bbar are captured by a single jet when using
standard reconstruction, a wider cone is used to re cluster the standard jets in
order to make sure to catch one of the, each of the a bosons into a single jet.
And then a fancy substructure and flavor tagger can be used to discriminate between
this process compared to just a jet that contains a single b quark. In addition,
one can set kinematic constraints that the two a bosons have have the same mass
and that the a pair has to have the mass of the Higgs.
Signal regions and control regions can be made depending on the number of loose and
tight tags of the two a legs and and then this allows to set limits in the mass
range between 15 and 30 GeV at high mass at some point the standard Higgs to aa
to four b analysis takes over. So let me get to the last search. [So Giovanni
your time is is actually up].
Now, okay, I'll be extremely quick on this. It's just a search for Zed Zed to..
Zed dark Zed dark or Zed Zed dark to four leptons. And for the Zed plus Zed dark
you can find the Zed requiring four leptons to be on the Higgs peak and then scan the
mass of the light there and set limits down to 10 to the minus four. And for
Zed dark Zed dark is a similar strategy but you want to find two light like mass
pairs and so you scan a box around the diagonal of the masses of two leptons. And
since there is very little events there you can set extremely tight limits about
10 to the minus six for this branching ratio. This search on MSSM A to tau tau I think,
at this moment I will just skip then, it's a very classical search for MSSM Higgs to tau tau that has
been done in the past, it's just updated to the full luminosity. So it has better
sensitivity than what was done up to now and one can get limits on tangent beta
as low as eight for even one TeV tau tau masses. For the high luminosity LHC
we don't have really new new results since European strategy updates. So back then we
had shown that we can set single Higgs boson observables we can probe them to
better than 2 few% level: cross sections, branching ratios, couplings and
differential distributions, and we could we predict to be able to observe when
combining ATLAS and CMS double Higgs production or at least get close to it
with four sigma and measure the self coupling to plus or minus 50%.
So, concluding, for the Higgs boson self coupling we have
new developments, we're improving analysis, we're starting to probe VBF double Higgs
production to test new couplings. We expect to have much stronger results when
the LHC full Run 2 data set will be analyzed in the channels that have higher
sensitivity like bb gamma gamma or bb tau tau that hasn't happened yet. And then we have quite many
searches that probe into the unknown: rare decays, or decays into BSM particles.
There are no big excesses or evidences for signals yet, but there is many more
searches to try and many searches to extend to the full Run 2 data set and
beyond towards the high luminosity LHC. Thank you.
Great, thanks a lot, Giovanni.
Please, if you have any questions, go ahead and raise your hands.
All right, I don't see any hands. I had a curiosity. It was back. When you were
talking about the background for the
the Higgs to phi I thing.
When you were showing this it looked like you said that there was a simple
polynomial fit. It looked like the background shape was somewhat complex. I
was curious if you could say a little bit about the background fit. So
now it's the shape is an analytic function. Now I don't remember exactly
what analytic function. It has a turn on. That's why we ended up with not having a
lot of sideband on the left in order to not have to model this, this turn on. In this
region here it's described by fairly smooth analytic functions.
If this is the.
Yeah, I was just curious how you validated it because it looks like as you're saying
you're you're avoiding a turn on but it also looks like the shape changes somewhat
during the range. I was just curious what
that is. The usual procedures that are done for like gamma gamma, I mean like bias
studies to test different shapes that still are able to fit the data and see
whether they would produce a spurious signal or not.
Okay, thanks. Any other questions?
All right, if I don't see any more, then Christophe, if you want to
go ahead and wrap up. I wanted to thank the four speaker that did a complete
overview, and also all the attendees. And as I said in the chat, I mean, at
least Maggie and Luca are waiting for you in the Zoom room. You can connect if you
want to discuss more with them.