ICHEP is the big International Conference
on High Energy Physics and this year it was
supposed to take place in Prague, but given
the situation with the coronavirus, the committee
has decided to have it as a full online conference
so it was the first edition of an online-only
conference at that scale, at this large scale.
They had something like 2700 registered participants
so it's really a big conference. So at this
conference the ATLAS Collaboration did report
many new results based on the complete dataset,
which was collected in the second experimental
phase at the LHC. We have many new results on many
areas of physics starting from the Higgs boson,
test of the Standard Model, going to use the
LHC for the first time as a photon-photon
collider. And as well on searches for new
particles, searches for new physics.
ATLAS presented new insights on the Higgs
boson physics. One of them is the constraint
on the case of the Higgs boson into invisible
particles. We addressed another rare process
which was the decay of the Higgs boson to
a Z boson and a photon. And then, as well,
very important, we presented final results
based on the LHC Run 2 on the possible decay
of the Higgs boson into fermions of the second
generation.
This I think is a very important result.
Because so far we have seen the Higgs
boson decays into bosons – this was the discovery
channel of 2012, which is the W, the Z, and
then also via loops into photons.
And then over time, we got, for example in 2018
clear evidence and observation of the Higgs boson decaying
into b quarks and also coupling to top quarks.
And now for the first time we get sensitivity
to the second generation. We cannot claim
evidence yet for this. However, we have a
strong excess at the level of two sigma to
see an excess at the right mass of the Higgs boson.
For the Higgs the large dataset allowed us also
to study the properties of the Higgs particle,
because an important question is, is it really
the Standard Model Higgs or do the properties
deviate in some way from the predictions of
the standard theory.
And here we have increased the precision.
For example, one measure
is to look for the couplings, the strengths
of the interaction of the Higgs with the standard
particles. And here, what is usually done is
to introduce a scale factor, which allows
for deviations of these coupling from the
Standard Model value. And these scale factors
are constraine d now to be standard-model like
with a precision at the level of 5 to 6%
for the W and Z bosons, and at the level of
14 to 15% for the fermions of the third generation.
Which is, I think, a big step forward compared
to the precision, which we achieved in Run 1.
One study we did is to look for invisible
decays of the Higgs boson. Now you may ask,
how do you see an invisible decay of the Higgs
boson or in particles that do not leave any
traces in your detector? So there we have
to use an analysis strategy where the Higgs
boson is produced in association with another
particle. So for example, there's one very
nice study done in ATLAS and presented, where
you have the Higgs boson produced in association
with a jet. Or with two jets. There's one
Higgs boson production process which is called
the vector boson fusion, where you have two accompanying jets going in the forward direction of your
experiment. And then what you see is missing,
energy. And this missing energy is a sign
that something escaped detection. And then
you look, if your events where you have such
a signature can be described by Standard Model
processes. And this is exactly the case so
we'd see no excess beyond the expectation from
production of Standard Model processes, and
this then allows us to constrain and set limits on the
contributions of Higgs decays to invisible particles.
We look more generically for missing energy events. For example, if we see only
one jet in the event, this is what we call the mono jet signature.
And this mono jet signature is very striking.
You see, for example, we have a nice event recorded where you have an 800 GeV jet going
in one direction of the experiment, and nothing
on the other side you obviously know there's
missing energy. Now, this could be dark matter
going away on the other side. But of course
it could also be background, which you expect
in the Standard Model. For example, a Z boson
produced in associated with a jet, and the
Z boson decays to neutrinos would give
you a background event. And now we have estimated
the background events and came to the conclusion
that our data can be described by the standard
background contributions. So there's no room
for a large contribution of dark matter and
this allows us to put up constraints, both
on the Higgs boson decay, and also on the
direct production of dark matter/
Another major highlight, which ATLAS presented
at ICHEP was first results and observations
of interesting processes, based on light by
light scattering at the LHC. This is a process
which should not happen in classical electrodynamics,
Light should not interact with itself. However,
in quantum electrodynamics, you have a possibility. It's one of the early predictions of quantum electrodynamics
that you can have interactions light by light,
and this was seen for the first time at the
LHC at the high energy in 2019 and published
by ATLAS. Now we can use the light-by-light
scattering to produce massive electroweak
gauge bosons, namely a pair of W+ and W-
and this process was observed now
with a high significance
above the discovery threshold by ATLAS.
So you can ask why is
this process so interesting. It's first of
all interesting because you have here in the hadron collider environment where you have
protons, quarks and gluons, you basically can
isolate an electroweak process you have a
radiation of the photons from the
protons and these photons then interact
and they interact directly in the self coupling
of the gauge bosons. And this, in a very
clean, process without what we call an underlying
event, because you do not break up the proton,
you do not perturb any of the quarks
inside the protons. The protons remain intact.
And then that's when you see the electromagnetic
process. It reminds you a little bit to the
cleanliness of the interaction processes which
you see normally, or are used to see in
e+e- colliders. And since the proton remains
intact here, there is another experimental
technique you can exploit, you can basically
look, whether you can detect these protons,
which have radiated the photons. And for this,
the ATLAS experiment has designed a special
spectrometer we call it the AFP,
the ATLAS Forward Proton spectrometer.
In a different process, not with W pairs in
the final state, but with lepton pairs
in the final state, we have seen the
first physics result which is based on detecting
as well the protons in these forward spectrometers.
So the light-by-light photon collider aspect of the LHC
has been used now in two observations
by ATLAS, the observation of the W pairs,
and the observation of lepton pairs with
the accompanying detection of the forward protons and I think these are very nice results
and shown for the first time.
ATLAS has for the first time looked into
Lepton Flavour Universality.
In the Standard Model, you basically predict that the
interaction strength between the various types of the
leptons is identical. It doesn't depend on
the lepton type, it doesn't depend on the mass.
So the electron strength, the muon strength, the tau strength should be identical, and
this is called Lepton Flavour Universality.
There was a result published about 20 years
ago by LEP, the combined result, where there
was a 2.7 sigma tension on the interaction
strengths that the W decays differently to
tau compared to muons. And we have checked
now on this result. We have used here the
large number of top pairs, which are produced
at the LHC. The LHC is really a top factory.
And this large number gives us access as
well to Ws because the top nearly to 100%
decays into a W and a b quark. And then we can
use these W's to look at their decays into
muons and taus. And this is a new analysis
technique which was designed now by ATLAS,
and we have got a very nice measurement and
namely a result that the ratio between the
W decays to muon and tau is respecting unitarity
we find the result which is within 1.3% very
close to unity. And the amazing thing
is that the precision reach there at the LHC is
now a factor of two better than what
was reached at LEP. So this was a very nice
first batch of results that we released now
at this ICHEP conference. But we are not done yet
with the analysis of our large Run 2
dataset. We will continue to look
in searches in more complex areas of parameter space.
And then there is the area of precision
measurements, I just mentioned, and there we are
definitely not done, these analyses are complex,
they need a careful assessment of systematic
uncertainties and the work there will continue,
and I'm sure ATLAS will present another nice
suit of results at the next conferences.
