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MARKUS KLUTE: Welcome
back to 8.701.

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So in this lecture, we
have a very brief view

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at the current status
of Higgs boson research.

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And I have to tell you
that we could spend

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an entire week discussing this.

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I just give you the
high-level overview, maybe

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the 30,000 feet kind
of overview of what

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we know about the Higgs boson.

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On the Canvas page,
you'll find a reference

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to a summary report,
which gives you

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a little bit more information
than I give you here.

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But I do think that there's
a few things to highlight.

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And those are the ones which
I'm going to talk about.

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The first part is that we
have discovered the Higgs

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boson in decays to photons and
also in decays to the Z bosons.

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Where the Z boson itself
decays into a pair of leptons,

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electrons and muons.

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And the detectors
we have available--

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and here is an example
for ATLAS and for CMS--

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they are very good in measuring
with precision the energy

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or momenta of photons,
electrons, and muons.

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So this allows us
to then reconstruct

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the mass of the Higgs boson
as it goes through the decay.

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And you can see
this here, this is

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125 GeV reconstructed
two-photon final state.

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So we have Higgs--

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two photons from ATLAS.

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You see that there
is a slight bump

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over an enormous background.

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So those are other
sources of diphoton events

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which are produced
in a hadron collider.

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But when you subtract those two
spectra which is shown here,

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data minus background, you see
this beautiful peak of Higgs

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to gamma gamma events.

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Similarly, Higgs to ZZ--

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and I put a little star here,
because one of the Z bosons

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has to be off-shell.

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Z boson mass is 91 GeV, the
mass of the Higgs boson 125 GeV.

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So 91 plus 91 is 128.

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So one has to be a
little off its mass peak.

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And then we look at decays
into e plus e minus,

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and mu plus mu minus,
and combination of those,

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for example for muon events and
for electron events as well.

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Also shown here, and again,
you have this beautiful peak

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consistent with the Higgs
boson at a mass of 125 GeV.

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And again, you have
other processes

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which contribute to
this final state,

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namely the one where you
have the four leptons coming

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from the Z boson itself and/or
from a pair of Z bosons,

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as is shown here.

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There's also
processes which mimic

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the leptons in the detector.

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Those have to be
evaluated as well.

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And they're typically shown
here in this plot here in green.

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OK.

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So as you can imagine, we can
measure the cross-section very

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precisely, because we can
identify those particles well.

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But we can also
measure the mass.

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And this is shown here.

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This is a summary of
measurements from ATLAS and CMS

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using those to final
states, one with two photons

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or the one with four leptons.

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And you see that measurements
are generally in agreement,

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and they can be combined to
this measurement of 125 GeV.

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And the best, the combined
value is shown here.

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So this is a
precision measurement.

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And since the Higgs
boson mass now

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is the only unknown
parameter of the Higgs

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in the standard
model, we know and can

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check all other properties.

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And one is the coupling strength
to the bosons, which we just

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looked at already, or
the coupling strength

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to the fermions.

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So this is two
examples, one that's

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showing Higgs to
tau tau events--

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tau plus tau minus.

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And you see again, there's a
lot of background processes

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which mimic-- have the
same signature, the signal.

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But we can find,
when you subtract

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the background from the
data, the nexus of event,

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again at 125 GeV.

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So taus themselves decay, and
they decay into neutrinos,

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which cannot be measured
at these detectors.

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Therefore, the
mass reconstruction

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is much harder than in the
final stage we discussed before.

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That is easier in
this final state,

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where we have a Higgs into
two muons, mu plus mu minus.

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The issue here is that
the weight is very small.

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The number of Higgs
boson decaying to mu mu

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is very small.

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But there's also a large
amount of background.

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So again, very similar to
the Higgs to gamma gamma

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final state we looked at before.

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We have a huge
amount of background.

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And here by eye, you
don't even see the bump.

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You see the bump
a little bit when

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you do data minus background.

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It's a fraction.

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You see this small
axis here, which

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is consistent with 3
standard deviations.

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So the likelihood
of the background

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to fluctuate without the
presence of the Higgs signal

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is about 3 standard deviations.

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All right.

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This information
then can be used

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to extract information
about the couplings itself.

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And this is a
beautiful plot here,

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which shows the Higgs
coupling on one axis,

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either to the
fermions or the boson,

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versus the mass of the particle.

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And so you find your favorite
particle, the top, the W, Z,

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the bottom, the
tau, and the muon.

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Those are the ones where we can
actually measure the coupling.

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And you see whether or
not this is in agreement

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with the standard model, which
is this blue dotted line here.

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And you see it is.

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And that plot here tells
me that the Higgs mechanism

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is responsible for the mass
generation of this particle.

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So it might be that there
is a mechanism which

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is very similar to the Higgs
boson which results in the very

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same observations
in nature, but it's

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not quite what we have
in the standard model.

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In order to make
those statements,

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one has to improve the size
of the error bars here,

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or the statistical significance
or the significance

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of the measurements.

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That is part of our program.

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That's what we're trying to do.

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All right.

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In summary-- again, this is
a very high-level summary--

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we measured the mass, we
measured the spin and CP

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of Higgs boson, we
measured the coupling

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to the Z, the W, the top, the
bottom, the tau, and the muon.

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We have not been able to measure
the coupling to lighter quark

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masses--

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strange, charm, up, and down.

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We have not been able to measure
the coupling to the electron.

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That would be spectacular.

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And we have not measured
the coupling to itself.

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So in the standard
model, the Higgs boson

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can couple to itself.

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So we have diagrams which look
like this, Higgs, Higgs Higgs.

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And we have not been
able to measure this.

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This is not free in
the standard model,

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and this would be closing
the argument that the Higgs

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boson requires mass--

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or the Higgs boson
is the particle

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predicted in the standard
model giving mass

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to the W and Z boson.

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If you want, you can take
the strength of the coupling

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here as this lambda.

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This is the lambda
term in our potential.

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So that's what we are trying
to measure in the future.

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But there's more open questions.

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Maybe there's more
than one Higgs boson.

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The Higgs-- adding one
duplet to the standard model

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with this potential is
one possible solution

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to the problem of
generating mass.

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But you could have very well
added two, or m, or triplets,

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or more complicated things.

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And so the question is, are
those maybe realized in nature

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or not?

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Will more precision
tell us something

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about the Higgs boson?

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Are there decays of the Higgs
bosons of nonstandard model

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particles, for example the Higgs
boson decaying into those guys

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here, which could be
evidence for that matter?

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We have looked.

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We have not seen this.

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But more precision might
give us a different answer.

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That's basically the status
of the summary of where

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we are with the Higgs boson.

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Again, much more can
be said about that.

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And you will see this
every now and then

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in seminars at MIT
or other places.