Yes. Hi. It's a pleasure to talk to you about the latest results from
last March actually, on electroweak penguin decays from LHCb. So on slide 2,
I thought it would be nice to just include a small picture of myself with this remote
setup that we have. So this is me trying to teach quantum mechanics at the
university. And so I was hoping to stand in front of you like this, maybe with a
nicer shirt, but it is what it is. So my outline is that I'll start with a
brief introduction to electroweak penguins, and detector and angular decompositions
because those are the ingredients that we need to explain these two new new papers
that came out in March from LHCb, on the B to K*mumu angular analysis update and
the limits on the B to ee branching ratio. On slide four, what would be considered
as electroweak penguins are the B to sll transitions. So I'm not talking about
where the gamma is on shell, I'm talking about the diagram shown on the right here.
These are flavour changing and neutral. So these are forbidden at tree level in the
Standard Model. So they are very suppressed, and called rare decays. And
because of that, sensitive to new physics in the loops, or even at tree
level, if that's the form that new physics takes. We can categorise these penguins
in three classes: the ratios of different flavours of leptons, which we call R(X),
but that's the subject of the talk by Miriam, which is right after mine. And the
measurements of branching fractions and angular observables. We measure these
as a function of the Q-squared which is the invariant mass of, in this case, the
dilepton pair. Because as you can see on the bottom plot as a function of Q-
squared, there's a lot of different physics going on depending on where you
are. Also the form-factor calculations or theory calculations are done in a
different way. At high Q-squared, it comes from lattice QCD, at low Q-squared
typically from light-cone sum rules, and in the middle, there are some effective
parameterisations which we'll come back to at the end. On slide five, just briefly,
the LHCb detector, as this is the first in the session. Important for these two analyses
is the high vertex resolution that we have to separate combinatorial background from
our signals. Good tracking or momentum resolution for the narrow mass peaks also
for signal to background ratio, particle identification to reject hadronic decays
in favour of our leptons and then the calorimetry to trigger on the electrons,
and the muon system which is highly efficient in triggering on muons. Slide
six. Over the past few years, a lot of wonderful results came out of this
beautiful detector. So what you see here are for different b2sll transitions, the
branching ratio as a function of the Q- squared. And you can see that
consistently, the data points in black are below the theory predictions. Not so much,
maybe around one sigma, maybe a bit more. So by themselves, they're not that
spectacular, but combined in an effective model independent approach, which I'll
come back to later, this goes up to four sigma or higher away from the Standard
Model. So should we get excited? Maybe we should start to get excited by about
now. Yes. On slide seven. Besides these branching ratios, we can measure
angular observables. The K* is a spin- one vector, and decays to a kaon and a
pion. This kaon-pion pair has different polarisation amplitudes.
These depend on the Wilson coefficients, which describe the type of physics that
can happen at the vertex. And they also depend on the form factors. So, this is
the theory input with some uncertainty associated to it. Now, in our
fits to the data, which is a function of Q-squared, and with these three angles,
that you can see in the figure on the right, we have eight observables. We can
describe them in different bases. The Si observables are the CP averaged
observables. We can combine those to make the Pi observables. These are
constructed in such a way to minimise the theory uncertainties. And for this the P5
prime - for which the calculation is shown on the bottom right - is this famous
observable that has this big discrepancy with the Standard
Model expectation.
On slide eight: why am I talking so enthusiastically about this K*mumu
decay? Well, the previous analysis, which was using Run-1 data only, so
three inverse femtobarn for LHCb, showed some reasonably significant
discrepancies with the Standard Model in this P5-prime observable. So in
certain bins of Q squared, that goes up to about three sigma away. And this was a
source for a lot of interest and discussion, of course. Now, this analysis
that I will talk about today, it effectively doubles this dataset as you
can see on the on the bottom. So it's the same amount of decays we roughly
add to this. On slide nine, I just briefly want to mention the steps we do in
this analysis. We trigger on the muons, which is highly efficient. We demand
a high vertex quality and impact parameter criteria, shown on the figure
on the right. We demand particle identification to reject against what we
call ‘peaking backgrounds’, which mimic the signal. These mostly come from Lambda-b
decays and Bs decays, where we miss- identify a proton or kaon as a pion.
Then we use multivariate techniques to suppress further combinatorial
background. Then, we define eight bins in Q-squared, in the green regions on the
bottom right plot. We avoid the cc bar resonances because they come from a
different process. But we do use these, the J/psi and the psi(2S), as control
channels to completely check all of our analysis steps before we unblind the data,
so before we look at the final results. Then we perform this maximum
likelihood fit in these three angles and Q squared, where the signal parameters, the
Si or the Pi’s are shared between all our datasets, because the physics should be
the same. On slide 10, just a word on the angular response. The fact that
LHCb has a very peculiar shape, optimised for B-hadron decay detection, the
acceptance and also the the selection and reconstruction, affect how our data
looks in Q-squared and the three angles. So for instance, on the right, you see the
cosine of one of those angles, for different Q-squared bins. It's definitely
not flat, and it's also different between the bins. So we need to understand that,
and this we study with high statistics, simulated samples. We fit the shapes
using Legendre polynomials in these four dimensions. And finally, as you can see on
the right, there's also spin zero components in the K-pi system, which we
also need to include in our angular description, which we do. This
modelling of the acceptance shape is actually the largest source of systematic
error, as you can see on the table here. It depends a bit on which which observable
you look at. But still, this is a lot smaller than the statistical error that we
have. So we're still limited by the amount of data that we have, by a factor of four
to 20, even. On slide 11 there are the new results. I show
you the P5-prime versus Q-squared again. There are actually eight
angular observables, not just this one. But this is the most popular one to look
at. What I can say is we have a very good agreement between our two data sets.
That means we are internally consistent. This is shown in the plot of S5 versus
Q-squared, on the bottom here. I just picked one, so that's black versus red.
The local tension in these two Q-squared bins in P5-prime remained about the
same, but it's somewhat less. Even though our statistical error, of course, from the
experimental side, decreased a bit. This has a reason, because the theory
predictions also changed a bit. The form factor uncertainties actually
decreased. But the sub-leading corrections to the interpolation, it
actually became more conservative. Together with a small shift in central
value of the observation, this results in just a slight decrease in discrepancy with
the Standard Model. But the behaviour of all of our angular observables is now more
consistent with a single change in C9 which is the vector boson coefficient.
Speaking of Wilson coefficients, just one slide on effective theory. This is a
tool that helps us write the process as a sum over effective operators, that
describe the physics happening at this effective four point interaction on the
bottom right block,
and their Wilson coefficients. These parameterise the amount of which kind of
physics happens at this vertex. The idea is that this allows for a model
independent interpretation of which kind of new physics is happening here. But also
it allows to combine different measurements of different processes which
are sensitive to a subset of the same Wilson coefficients. Now, in this Q-
squared region where these bins deviate, the C9 and the C10 - the vector and
axial vector - are actually the most important ones. When we allow a freedom in C9
and C10 in a fit, you can see on the bottom left that we have about 2.8
sigma deviation from the Standard Model. But mostly this appears to be in the C9
direction, so the vector direction. If we only allow to vary C9,
you see the middle plot, we are actually 3.3 standard deviations away with just
this K*mumu measurement. And then on the right, for comparison, you see
when we change C9 to minus one, which seems to be this preferred value, the
Standard Model and the data - the theory and the data -
match better by eye. So this is going from the solid line or the blue blocks to these
blue dashed lines. Slide 14. I mentioned combinations of different
measurements contributing to the same Wilson coefficients, so it's a natural
moment to move on to the second analysis I wanted to highlight, which is the
measurement of B to ee. This is also a b2sll transition, but it has a different
decay topology, to leptons only. And because of helicity suppression, this is
even more suppressed. If you see this equation in the branching ratio here, the
helicity suppression occurs in front of this C10 term, which is the same C10
as we saw for this angular analysis. But there are also scalar and pseudo-scalar
contributions which were not there. They contribute to the
overall picture of the coefficients, of course, but the B to ee analysis is even
more suppressed in C10. So this is a very good probe of scalar new physics. And even
one observation, one event, would actually be a very significant hint for new physics
already. In slide 15, the B to mumu results. Aiden already talked about it. So, I will
not mention that now. But on the bottom left, you can see our results for the
search of B to ee, which resulted in no observation, but a limit that is 30
times better than the previous CDF one. The global picture of those
coefficients will help constrain which kinds of new physics
might explain our other observations.
In slide 16, I'm done with the LHCb measurements. But I just wanted
to mention that there are many independent groups who do these models-independent
fits to a lot of these measurements combined: in the order of 170. And they
find somewhat consistently that C9 only, deviates from zero by four sigma to
five sigma, depending on your assumptions and which group you ask. But
it all seems to be consistent with a single change in a Wilson coefficient, this
factor seems to be minus one, maybe. So this is a very interesting result, how all
of these measurements somehow seem to point to a single explanation. I think
that's fascinating. So any kind of vector new physics is very hot at the
moment, of course, because of this. On slide 17, my conclusion. I showed you two
new exciting results on b2sll transitions from LHCb: the angular analysis K*mumu,
and this new rare decay, B to ee, where we have a limit. The discrepancy in this angular
analysis with the Standard Model still stands, although it did not increase
because of various reasons. And all of these results are not done with the full
dataset yet: we’re still analysing the remaining data that we took in Run-2, so we
can expect the what we call ‘Run-1+2 legacy results’ soon. This also
goes for the new B to mumu analysis, which is coming out, hopefully,
very soon. The B to ee analysis, and on top of that, we actually have many other analysis
ongoing. These are R(X) analyses that Miriam will talk about after me, but also we
finally have enough statistics to measure the angular analysis of K*ee
actually. So all kinds of very interesting new results will hopefully come out soon
with that more data.
And that's it. Thank you. Thank you for your attention.
Question to Jacco, please raise your hand
Yeah, my chemical yosity just started. Sure.
So, you mentioned second edition and China is that you can start of course not but
there is one missing I remember the visa are quite old the result from the bar
about B plus C two three plus down that was about two or something time 10 to the
minus three
is quite interesting. I think c said
should be sensitive to the effects massing and the the tweaks doubled more than
So, do you think it is something that can be feasible in Italy should be a thing. So
since you use already final stage without?
Yeah, so
I will I will not say it's not possible? Absolutely not. We've made an attempt at B
to dow which is even more challenging. So it's definitely something we we studied,
we can study I should say. But it is tricky at the LSE. Right? So these these
measurements with without, you have to make it takes time. I should say that,
yeah, I would expect the result like that coming from LGB soon, but it doesn't mean
it's not something maybe we can all do. And it adds to the to the full picture. So
it's an interesting channel, I think.
Thank you very much.
Other question
seems not so we can move to the third