A few other people are joining but I think we can start
Welcome to the second level physics session and today we will talk
about CP violation, CKM metrology and other form factors. Let us start
with this CP violation in charm and for this we have Guillaume Pietrzyk who will give this overview.
Hi everyone and thanks to the organizers for setting up such a nice
online conference. I will talk to you about CP violation in charm at LHCb. A few words
on CP violation in the charm sector. You may know that the charm
sector encompasses the only up type quark decays in which we can probe CPV in
neutral mesons, CPV in the Standard Model is predicted to be to be
quite small at the level of 10-4 , or 10-3.
So there's really room for new physics enhancements there. But you have to know
that these predictions are have large uncertainties because if you check the two
drawings at the bottom, you can see the short distance contributions with W exchange.
This is heavily suppressed in charm decays. Whereas if you check
the long distance contributions with the exchange of pions and kaons. These have
large uncertainties. By making precise measurements, we can really improve these predictions.
What's really nice for instance at LHCb is the fact that we have
a lot of charm decays. We have about 1 billion D0 decays that are ready
to be analyzed at LHCb. If you take into account the full Run 1 and Run 2
data sample. There are three types of CP violation the charm sector. In any
sector but also in the charm sector. They are in two families. You have
the direct sector where we have CP violation in the decay so it means that the amplitude of a D0
going to a final state is not the same as the one of D0bar going to the anti-final state.
This is where we saw a CP violation last year. I will talk
later about it. You also have the indirect sector, where you have CPV in the
mixing. It means that the the amplitude of a D0 oscillating to a
D0bar is not the same as the D0bar oscillating to a D0. You also
have the interference between mixing and decay. And this can be seen as the fact
that the CP-violating phase is different from zero up to up to a phase.
This is the indirect family where we still have no evidence of CP violation for now.
the LHCb detector could be could be almost called the LHCc detector because it's
perfectly made to study charm decays. We have the vertex locator at the pp
collision point where we have a very good time resolution of 45 fs and a very
good IP resolution of about 20 micrometers. Before and after the 40 Tm
dipole magnets, we have two tracking stations that have a good
momentum resolution of about 0.5% at high momenta. Also with the rich system,
the calorimeters and the muon chambers, we have very good particle identification,
especially when it comes to kaons and pions. And so this is this is all very
good for for charm decays. So first I'll start with CPV in the decays.
You may all have heard of the delta ACP measurement where we saw CP violation.
I'll talk a bit about it. So it was done using the full Run 2 dataset of 6 fb-1
It was really a comparison between the two Cabibbo-suppressed decays,
The D0->KK and the D0->pipi. There are two production modes.
There's the prompt mode where a D*+
is produced directly at the PV. It has a very short lifetime. So it immediately decays
to a D0 and a soft pion (called a soft pion because it has a low momentum).
The charge of this pion is used for flavor tagging. If you have a pi+, you
have a D0 ; if you have a pi- you have a D0bar. ThereÕs also the
semi-leptonic B decays production mode. In this case, at the PV a B is
produced which has a significant lifetime. It flies a bit, then it
decays to D0 in other particles and in these other particles, you have the
the muon, and this is useful for flavor tagging. I'll talk a bit about the
yields. We have a bit more data when it comes to prompt decays with respect to semileptonic
B decays. For instance, if you check the mass distributions on the left,
you have 44 million D0->KK decays whereas in the semileptonic B decays you have 9
million signal events.
So, the delta ACP measurement is basically the subtraction of two CP asymmetries.
But at LHCb when we measure CP asymmetries we donÕt directly observe CP
asymmetries, we have raw asymmetries which are the difference of the yields of D0
versus D0bar that you observe and so that what differs Araw from ACP,
ACP being this physical asymmetries, are the experimental asymmetries. You have
two main sources of experimental asymmetries. You have the detection asymmetries
that lead to an asymmetry in the detection of the tagging tracks so you
do not really detect pi+ and pi- in the same way.
You have the production of asymmetry which is the asymmetry of
production of the D*+ versus D*- for prompt decays and Bbar versus B
for semileptonic B decays. In the end what is left is this
CP asymmetry. But if we were to measure not delta ACP, but just ACP, then A_D
and A_P would be very challenging to assess. Really the strategy that is
used in this is to equalize the A_D and A_p for KK and pipi.
To do that what was done was a series of fiducial cuts that remove the regions of high
detection asymmetry. But also a reweighting procedure to match the kinematics of the
KK and pipi. You can see that at the bottom right. And once you do this, the
delta ACP which is the difference of raw asymmetries becomes the difference
of CP symmetries. And this is what we are looking for. So, this is not equal to zero
because of SU(3) breaking that tells you that ACP(KK)=-ACP(pipi).
So I go to the to the Run 2 results. So we see we see a significant
departure from zero in the case of prompt decays. We do not really see it in
the semileptonic B decays (linked to the fact that we have less statistics). But if
you combine these two Run 2 measurements with the ones that were obtained with the two
production modes of Run 1, you see delta ACP=(-15.4 +- 2.9)*10^-4.
The first observation of CP violation in charm decays at the level of 5.3 sigma.
I switch to the latest ACP measurements that were measured using charged decays.
The goal was to measure the ACP using 2015-2017 data using three Cabibbo-supressed decay modes.
You have Ds+->Kspi+, D+->KsK+ and D+->phi pi+.
To do this analysis, we made use of three Cabibbo-favored
control samples and we know that in these samples the ACPs must be compatible with zero
at the current level of precision. And so, the signal modes are in red
and the control samples are in blue. And you can see at the bottom the
mass distributions of all of these decays. So, to measure the CP asymmetries,
we make use of the raw asymmetries of the control samples. So, if
you check, for instance, the ACP of the Ds+->Ks pi+ it is approximately
equal to the raw asymmetry of the Ds+->Ks pi+ minus the raw
asymmetry of one of the control samples. And so, this was this was done for all of the
three decays in red and it was possible then to measure the CP
asymmetries for all of these decays. We see no deviation from the CP
conservation hypothesis at this current level of precision. On the right you can
see the systematic uncertainties where you can see that the main one that comes from
the fit models. There is some foreseen reduction by constraining the
shapes to simulation. Now, I switch to indirect CP violation searches.
First I will talk a bit about charm-mixing. Flavour-mixing as you
Know leads to the distinction between the mass eigenstates and the flavor
eigenstates. And the oscillations in charm depend on two parameters that are
x and y (they are displayed on the on the left). So, if you have x and y
equal to zero, you have no mixing and you can see that in the current situation, we
are very far away from that. So charm mixing is well established at more than 5 sigma.
To look for indirect CPV, we have some CP-violating mixing
parameters that are called xCP, delta x, yCP and delta y. They depend on x, phi, |q/p| and y.
In the case of no CP violation. In the case where phi is
equal to zero and |q/p| is equal to one, what you have is the fact that xCP is
equal to x that delta x is equal to zero ; yCP is equal to y and delta y is equal to zero.
This is what we use to see if we have CP Violation in the indirect sector.
There was a nice analysis that was done to measure these parameters in D0->Kspipi
by using two production modes.
There is one, the prompt mode, with 2012 data at 2fb-1, and the semi-leptonic mode
using full run 1. You can see the two distributions below.
So you have a bit more data in the prompt sample than in the semi-leptonic one and
a bit more background in the semi-leptonic sample ; but what you need to
know is the fact that D0->Kspipi has a rich resonance structure.
And so, this is really a challenge when it comes to measuring these CP parameters in these
decays. There was a very nice method that was developed that is called the bin-flip
method. It avoids the need of a fit of the decay amplitudes. So what is done
basically, the data is binned in Dalitz coordinates where the binning scheme
is chosen to have constant strong phases differences. These regions are
shown on the Dalitz plots at the bottom right. The yields are shown on the
left where you have a bit more data at the bottom and on the top left because of more
Cabibbo-favoured decay modes at the bottom right. And so, to measure these CP
parameters, there was a simultaneous fit of the yield of the ratio between the
events at the bottom right with respect to the one at the top left:
the +b and the -b regions. This was done as a function of decay time and this
gives access to the wanted CP parameters. And so, thanks to that, we measured some
some value for xCP, delta x, yCP and delta y, then it was possible to
derive the mixing parameters x, y, |q/p| and phi. And then you can compare
the different values to see if you have some CP violation. If you check the value
of x that was obtained and you combine it with the word average value, you have a
departure of x from zero. And this is the first evidence as mass difference between
neutral charm-meson eigenstates. So that is a very nice result. And at the bottom
right, you can see the improvements of this analysis when it comes to the
parameter x, y, |q/p| and phi. So there was really a nice improvement with respect to
the word average value. And I can say that the run 2 measurements are underway both
in the prompt modes and in the semi-leptonic ones, so it is going to be even
more precise. And last, the search for time dependent CPV in D0->KK and
D0->pipi decays. So it is basically a measurements of the raw asymmetry as a
function of decay time. This time the goal is to measure a parameter called
Agamma and a departure of Agamma from zero is a signal of CP violation. In this
measurement, you have some experimental asymmetries that depend on time
and you have to deal with that. They are the flavor identification asymmetries and the
production asymmetries. So to have this measurement, there was a measurement of
Agamma done to D0->Kpi because we know that Agamma must be compatible with
zero at the level of 10-5, 10-6.
And so, D0->Kpi is really used to cross-check the analysis procedure.
I can show you the results that were obtained using prompt
decays from 2015 and 2016. This is still a preliminary result.
It was also done using the semileptonic B decays from 2016 to 2018. Agamma is the
inverse of the slope but you can see on this graph and you can also see that there
is no strong departure of Agamma from zero despite the huge precision that we
can achieve. You can see now the current status of the measurements of Agamma.
The last two results in blue are the KK and pipi combinations from the
results that I showed you in the previous slides, and thanks to all of these
measurements that were done by LHCb and all of the other experiments, it was possible
to compute a non-official (or homemade) world average value, which is very close
to 1*10-4, so we have a nice
precision, when it comes to Agamma. The Standard Model prediction is very low, it is
at the level of 2*10-5.
We are going to improve a bit this world-average value because the 2017 and 2018
prompt results are coming soon. And I can show you on the at the bottom right the
prospects for Run 4 and Run 5 at LHCb. And we can see that at
the end of run 4 / run 5, we might be able to reach the standard
model prediction. So we really need a huge amount of data to see a signal.
These results are already very nice. So in summary,
I can tell you (I mean you already know) that CP violation has been
measured for the first time in charm decays, we are making some
very precise measurements and we are looking for additional sign of CP
violation, especially in the indirect sector. We are all working very hard to
really finalize these measurements, using all of the run 2 dataset
that we have, but bare in mind that the results are limited by statistics. So,
we are really looking forward to having additional data with run 3, run 4 and run 5.
Thanks a lot for your attention.
Thank you for the nice overview. Are there any questions?
So, to ask a question, please raise your hands and you will be allowed to ask your
question. Given that I do not see any hands so far, maybe I can start. With the
first measurement, you presented the delta ACP measurements if you go around
slide 8 or something. So of course, you can keep the systematics, much lower
than the statistical error because you are measuring a difference. Right? So,
but you can measure ACP(KK) and ACP(pipi) for example. Yeah.
Yeah. ACP was measured using run 1 date. We are looking forward to
getting the run 2 value. But of course, it is going to be to be maybe less precise
than the than this disparity as far as I know.
Okay, do you have any idea if we will still be statistically dominated or the
systematics will start to become big? For example, for the KK sample?
I am not an expert, but I guess yes. We are going to be limited by statistics
in this regard.
Okay,
So if somebody in the audience does not agree, please tell me.
And can I ask a question?
Yes.
I have a curiosity on page 11. You can go up. Could you
explain a bit of the combination to obtain the signal asymmetry here, I had not even
expected that, you should simply subtract from the raw asymmetry, the
corresponding raw symmetry of the Cabibbo favored control sample where you have
exactly the same kind of spacing. So you want to subtract basically the detector
components. So you should have the same
particles in the final state. So, I assume that basically you should have the same
raw symmetry. Instead you use a combination that is not the one you use. You subtract
a different channel. So, in these asymmetries, what was not
mentioned directly are the asymmetries linked to the K0 because they are
implicitly present in these asymmetries. So, for instance in all
Of the first two lines, you also had to add to this the asymmetry
linked to the K0 but these I think if I am not mistaken, this is taken
for from additional measurements.
Okay,
I have to think about it.
and then you have another curiosity about the on page number 8.
Here on the bottom left, you have these different zones that
you want to remove. So, could you repeat/elaborate the reason.
So basically you have an angular dependence On the detector.
So if you take into account these regions
of high detection of symmetry then this would really bias your measurements. So,
you really have to average regions where you have a very low asymmetries. What is
exactly the question? Is it linked to whether or not they cancel out for KK and pipi?
No, it was just very very naive. So, I think that depends on the way in which
is been to the detail.
Exactly. So, yeah you want to you want to be as far as possible from the boundaries.
Not too close to the beam pipe.
In the acceptance you really want to be away from the boundaries. So, this is
the idea. Thank you.
I also had a question about slide 19.
Next one.
Yeah, sorry
For run 3 and run 4. But it was looking at your systematic
uncertainty which is comparable to the standard model that we see here, so what
is the main uncertainty that you have? Do you expect to be able to reuse
them? so much?
That is a big question. So really the goal of run 3, 4 and 5 is really
to decrease the statistical uncertainty uncertainty and
we really want to keep the systematic as low as possible.
So
so this is really going to be the challenge that we are going to be facing
too. So yeah, I guess these are linked to the statistical uncertainty. So the
systematics are going to be a challenge when we go to 10-5
Okay
okay.
I see one question or comment, Maurizio.
Yes. Thank you. That is Maurizio Martinelli. I just wanted to add on this point that yes,
these extrapolations are made based on the statistics, statistical power that you
expect for the upgrade. But I wanted to add that most of the systematic
uncertainties are still measured on data using control samples for example. So this
should scale with statistics. Other systematic uncertainties, we
reuse the one of run 1 and run 2 to understand whether our selection will
introduce in some systematic uncertainties or trigger level for example, and that we
could avoid them. And of course, for the run three onwards, we would be working to
reduce those as much as possible as well. So, of course, there is some optimism,
but we know what we should do in order to reduce that it is not that there
will be anything that will be clearly limiting ourselves.
Okay, thank you. Yes the data or usually add for this.
Okay.
So if there are no more questions or Comments. Thanks Guillaume!
Thanks a lot!