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A few other people are joining but I think
we can start

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Welcome to the second level
physics session and today we will talk

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about CP violation, CKM metrology and other form factors. Let us start

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with this CP violation in charm and for this we
have Guillaume Pietrzyk who will give this overview.

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Hi everyone and thanks to the
organizers for setting up such a nice

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online conference. I will talk to you
about CP violation in charm at LHCb. A few words

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on CP violation in the
charm sector. You may know that the charm

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sector encompasses the only up type quark decays 
in which we can probe CPV in 

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neutral mesons, CPV in the Standard Model
is predicted to be to be

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quite small at the level of 10-4
, or 10-3. 

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So there's really room for new physics
enhancements there. But you have to know

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that these predictions are have large
uncertainties because if you check the two

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drawings at the bottom, you can see the
short distance contributions with W exchange.

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This is heavily suppressed 
in charm decays. Whereas if you check

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the long distance contributions with the
exchange of pions and kaons. These have

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large uncertainties. By making precise
measurements, we can really improve these predictions.

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What's really nice
for instance at LHCb is the fact that we have

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a lot of charm decays. We have about 1
billion D0 decays that are ready

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to be analyzed at LHCb. If you take
into account the full Run 1 and Run 2

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data sample. There are three
types of CP violation the charm sector. In any

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sector but also in the charm sector. They
are in two families. You have

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the direct sector where we have CP violation in the
decay so it means that the amplitude of a D0

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going to a final state is not the same as
the one of D0bar going to the anti-final state.

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This is where we saw a CP violation
last year. I will talk

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later about it. You also have the
indirect sector, where you have CPV in the

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mixing. It means that the
the amplitude of a D0 oscillating to a

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D0bar is not the same as the D0bar
oscillating to a D0. You also

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have the interference between mixing and
decay. And this can be seen as the fact

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that the CP-violating phase  is
different from zero up to up to a phase.

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This is the indirect family where we still have
no evidence of CP violation for now. 

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the LHCb detector could be could be almost
called the LHCc detector because it's

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perfectly made to study charm decays. 
We have the vertex locator at the pp

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collision point where we have a very good
time resolution of 45 fs and a very

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good IP resolution of about 20
micrometers. Before and after the 40 Tm

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dipole magnets, we have
two tracking stations that have a good

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momentum resolution of about 0.5% at high
momenta. Also with the rich system,

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the calorimeters and the muon chambers,
we have very good particle identification,

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especially when it comes to kaons
and pions. And so this is this is all very

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good for for charm decays. So first I'll
start with CPV in the decays. 

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You may all have heard of the delta ACP
measurement where we saw CP violation.

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I'll talk a bit about it. So it was done
using the full Run 2 dataset of 6 fb-1

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It was really a comparison
between the two Cabibbo-suppressed decays,

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The D0->KK and the D0->pipi. 
There are two production modes.

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There's the prompt mode where a D*+

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is produced directly at the PV. It has a very
short lifetime. So it immediately decays

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to a D0 and a soft pion (called a soft pion
because it has a low momentum). 

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The charge of this pion is used for flavor
tagging. If you have a pi+, you

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have a D0 ; if you have a pi- you
have a D0bar. ThereÕs also the

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semi-leptonic B decays production
mode. In this case, at the PV a B is

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produced which has a significant
lifetime. It flies a bit, then it

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decays to D0 in other particles
and in these other particles, you have the

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the muon, and this is useful for flavor
tagging. I'll talk a bit about the

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yields. We have a bit more data when it
comes to prompt decays with respect to semileptonic

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B decays. For instance, if you
check the mass distributions on the left,

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you have 44 million D0->KK decays whereas in
the semileptonic B decays you have 9

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million signal events.

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So, the delta ACP measurement is basically
the subtraction of two CP asymmetries. 

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But at LHCb when we measure CP
asymmetries we donÕt directly observe CP

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asymmetries, we have raw asymmetries which are
the difference of the yields of D0

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versus D0bar that you observe and
so that what differs Araw from ACP,

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ACP being this physical asymmetries, are
the experimental asymmetries. You have

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two main sources of experimental 
asymmetries. You have the detection asymmetries

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that lead to an asymmetry in the
detection of the tagging tracks so you

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do not really detect pi+ and pi- in
the same way.

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You have the production of
asymmetry which is the asymmetry of

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production of the D*+ versus D*- 
for prompt decays and Bbar versus B

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for semileptonic B decays.
In the end what is left is this

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CP asymmetry. But if we were to measure
not delta ACP, but just ACP, then A_D

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and A_P would be very challenging to
assess. Really the strategy that is

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used in this is to
equalize the A_D and A_p for KK and pipi.

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To do that what was done was a series of fiducial
cuts that remove the regions of high

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detection asymmetry. But also a reweighting
procedure to match the kinematics of the

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KK and pipi. You can see that at the
bottom right. And once you do this, the

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delta ACP which is the difference of raw asymmetries
becomes the difference

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of CP symmetries. And this is what we are
looking for. So, this is not equal to zero

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because of SU(3) breaking that tells you
that ACP(KK)=-ACP(pipi).

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So I go to the to the Run 2
results. So we see we see a significant

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departure from zero in the case of prompt decays. 
We do not really see it in

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the semileptonic B decays (linked to the
fact that we have less statistics). But if

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you combine these two Run 2 measurements with
the ones that were obtained with the two

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production modes of Run 1, you see
delta ACP=(-15.4 +- 2.9)*10^-4.

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The first observation of CP violation
in charm decays at the level of 5.3 sigma.

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I switch to the latest ACP measurements that
were measured using charged decays.

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The goal was to measure the ACP using 2015-2017 data 
using three Cabibbo-supressed decay modes.

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You have Ds+->Kspi+, D+->KsK+ and D+->phi pi+.

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To do this analysis,
we made use of three Cabibbo-favored 

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control samples and we know that
in these samples the ACPs must be compatible with zero

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at the current level of precision. And
so, the signal modes are in red

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and the control samples are in blue. And
you can see at the bottom the

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mass distributions of all of
these decays. So, to measure the CP asymmetries,

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we make use of the raw
asymmetries of the control samples. So, if

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you check, for instance, the ACP of the
Ds+->Ks pi+ it is approximately

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equal to the raw asymmetry of the
Ds+->Ks pi+ minus the raw

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asymmetry of one of the control samples. And
so, this was this was done for all of the

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three decays in red and it was
possible then to measure the CP

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asymmetries for all of these decays.
We see no deviation from the CP

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conservation hypothesis at this current
level of precision. On the right you can

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see the systematic uncertainties where you
can see that the main one that comes from

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the fit models. There is some foreseen
reduction by constraining the

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shapes to simulation. Now, I switch
to indirect CP violation searches.

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First I will talk a bit about
charm-mixing. Flavour-mixing as you

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Know leads to the distinction between
the mass eigenstates and the flavor

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eigenstates. And the oscillations in charm
depend on two parameters that are

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x and y (they are displayed on the
on the left). So, if you have x and y

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equal to zero, you have no mixing and you
can see that in the current situation, we

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are very far away from that. So charm
mixing is well established at more than 5 sigma.

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To look for indirect CPV,
we have some CP-violating mixing

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parameters that are called xCP, delta x, yCP and delta y.
They depend on x, phi, |q/p| and y.

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In the case of no CP
violation. In the case where phi is

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equal to zero and |q/p| is equal to
one, what you have is the fact that xCP is

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equal to x that delta x is equal to zero ; yCP is
equal to y and delta y is equal to zero. 

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This is what we use to see if we have CP
Violation in the indirect sector. 

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There was a nice analysis that was done to
measure these parameters in D0->Kspipi 

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by using two production modes.

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There is one, the prompt mode, with 2012
data at 2fb-1, and the semi-leptonic mode

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using full run 1.
You can see the two distributions below.

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So you have a bit more data in the prompt
sample than in the semi-leptonic one and

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a bit more background in the semi-leptonic
sample ; but what you need to

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know is the fact that D0->Kspipi 
has a rich resonance structure.

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And so, this is really a challenge when it
comes to measuring these CP parameters in these

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decays. There was a very nice method that
was developed that is called the bin-flip

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method. It avoids the need of a fit
of the decay amplitudes. So what is done

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basically, the data is binned in Dalitz coordinates 
where the binning scheme

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is chosen to have constant strong
phases differences. These regions are

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shown on the Dalitz plots at the bottom
right. The yields are shown on the

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left where you have a bit more data at the
bottom and on the top left because of more

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Cabibbo-favoured decay modes at the bottom
right. And so, to measure these CP

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parameters, there was a simultaneous fit
of the yield of the ratio between the

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events at the bottom right with
respect to the one at the top left:

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the +b and the -b regions. This was
done as a function of decay time and this

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gives access to the wanted CP parameters.
And so, thanks to that, we measured some

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some value for xCP, delta x, yCP and delta y, then it was possible to

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derive the mixing parameters x, y, |q/p|
and phi. And then you can compare

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the different values to see if you have
some CP violation. If you check the value

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of x that was obtained and you combine it
with the word average value, you have a

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departure of x from zero. And this is the
first evidence as mass difference between

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neutral charm-meson eigenstates. So that is
a very nice result. And at the bottom

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right, you can see the improvements of
this analysis when it comes to the

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parameter x, y, |q/p| and phi. So there was
really a nice improvement with respect to

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the word average value. And I can say that
the run 2 measurements are underway both

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in the prompt modes and in the semi-leptonic ones, 
so it is going to be even

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more precise. And last, the search
for time dependent CPV in D0->KK and

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D0->pipi decays. So it is basically a
measurements of the raw asymmetry as a

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function of decay time. This time
the goal is to measure a parameter called

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Agamma and a departure of Agamma from
zero is a signal of CP violation. In this

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measurement, you have some experimental
asymmetries that depend on time

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and you have to deal with that. They are
the flavor identification asymmetries and the

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production asymmetries. So to have this
measurement, there was a measurement of 

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Agamma done to D0->Kpi because we
know that Agamma must be compatible with

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zero at the level
of 10-5, 10-6.

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And so, D0->Kpi is really used
to cross-check the analysis procedure.

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I can show you the results
that were obtained using prompt

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decays from 2015 and 2016. This is still 
a preliminary result. 

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It was also done using the semileptonic
B decays from 2016 to 2018. Agamma is the

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inverse of the slope but you can see on
this graph and you can also see that there

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is no strong departure of Agamma from
zero despite the huge precision that we

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can achieve. You can see now the
current status of the measurements of Agamma.

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The last two results in blue are
the KK and pipi combinations from the

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results that I showed you in the previous
slides, and thanks to all of these

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measurements that were done by LHCb and
all of the other experiments, it was possible

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to compute a non-official (or homemade)
world average value, which is very close

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to 1*10-4, so we have a nice

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precision, when it comes to Agamma. The
Standard Model prediction is very low, it is

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at the level of 2*10-5.

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We are going to improve a bit this world-average 
value because the 2017 and 2018

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prompt results are coming soon. And I can
show you on the at the bottom right the

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prospects for Run 4 and Run 5
at LHCb. And we can see that at

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the end of run 4 / run 5, we might
be able to reach the standard

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model prediction. So we really need a
huge amount of data to see a signal.

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These results are already
very nice. So in summary,

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I can tell you (I mean you
already know) that CP violation has been

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measured for the first time in
charm decays, we are making some

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very precise measurements and we are
looking for additional sign of CP

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violation, especially in the indirect
sector. We are all working very hard to

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really finalize these
measurements, using all of the run 2 dataset

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that we have, but bare in mind that
the results are limited by statistics. So,

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we are really looking forward to having
additional data with run 3, run 4 and run 5.

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Thanks a lot for your
attention.

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Thank you for the nice overview.
Are there any questions?

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So, to ask a question, please raise your
hands and you will be allowed to ask your

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question. Given that I do not see any
hands so far, maybe I can start. With the

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first measurement, you presented the
delta ACP measurements if you go around

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slide 8 or something. So of course,
you can keep the systematics, much lower

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than the statistical error because you are
measuring a difference. Right? So,

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but you can measure ACP(KK) and ACP(pipi)
for example. Yeah.

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Yeah. ACP was measured using run 1
date. We are looking forward to

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getting the run 2 value. But of course,
it is going to be to be maybe less precise

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than the than this disparity as far as I
know.

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Okay, do you have any idea if we will still
be statistically dominated or the

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systematics will start to become big? For
example, for the KK sample?

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I am not an expert, but I guess yes.
We are going to be limited by statistics

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in this regard.

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Okay,

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So if somebody in the audience does not
agree, please tell me. 

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And can I ask a question?

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Yes.

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I have a curiosity on page 11. You can go
up. Could you

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explain a bit of the combination to obtain
the signal asymmetry here, I had not even

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expected that, you should simply
subtract from the raw asymmetry, the

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corresponding raw symmetry of the Cabibbo
favored control sample where you have

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exactly the same kind of spacing. So you
want to subtract basically the detector

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components. So you should have the same

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particles in the final state. So, I assume
that basically you should have the same

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raw symmetry. Instead you use a
combination that is not the one you use. You subtract

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a different channel. 
So, in these asymmetries, what was not

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mentioned directly are the asymmetries linked
to the K0 because they are

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implicitly present in these
asymmetries. So, for instance in all

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Of the first two lines, you
also had to add to this the asymmetry

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linked to the K0 but these I think
if I am not mistaken, this is taken

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for from additional measurements.

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Okay,

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I have to think about it.

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and then you have another curiosity about
the on page number 8. 

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Here on the bottom left,
you have these different zones that

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you want to remove. So, could you repeat/elaborate the reason.

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So basically you have an angular dependence
On the detector. 

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So if you take into account these regions


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of high detection of symmetry then this
would really bias your measurements. So,

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you really have to average regions where you
have a very low asymmetries. What is

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exactly the question? Is it linked to
whether or not they cancel out for KK and pipi?

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No, it was just very very naive. So, I
think that depends on the way in which

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is been to the detail.

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Exactly. So, yeah you want to you want to
be as far as possible from the boundaries.

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Not too close to the beam pipe.

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In the acceptance you really want to be away
from the boundaries. So, this is

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the idea. 
Thank you.

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I also had a question about slide 19.

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Next one.

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Yeah, sorry

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For run 3 and run 4. But
it was looking at your systematic

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uncertainty which is comparable to the
standard model that we see here, so what

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is the main uncertainty that you
have? Do you expect to be able to reuse

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them? so much?

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That is a big question. So
really the goal of run 3, 4 and 5 is really

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to decrease the statistical
uncertainty uncertainty and

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we really want to keep the systematic as
low as possible.

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So

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so this is really going to be the
challenge that we are going to be facing

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too. So yeah, I guess these are linked
to the statistical uncertainty. So the

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systematics are going to be a
challenge when we go to 10-5

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Okay

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okay.

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I see one question or comment, Maurizio.

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Yes. Thank you. That is Maurizio Martinelli. I just
wanted to add on this point that yes,

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these extrapolations are made based on the
statistics, statistical power that you

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expect for the upgrade. But I wanted to
add that most of the systematic

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uncertainties are still measured on data
using control samples for example. So this

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should scale with statistics. Other
systematic uncertainties, we

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reuse the one of run 1 and run 2
to understand whether our selection will

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introduce in some systematic uncertainties
or trigger level for example, and that we

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could avoid them. And of course, for the
run three onwards, we would be working to

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reduce those as much as possible as well.
So, of course, there is some optimism,

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but we know what we should do in
order to reduce that it is not that there

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will be anything that will be clearly
limiting ourselves.

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Okay, thank you. Yes the data or usually
add for this.

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Okay.

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So if there are no more questions or
Comments. Thanks Guillaume!

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Thanks a lot!