No, let me share my screen.
Can you see my screen? Yeah, it works. Go ahead.
Okay, so yeah, ehm today I'm going to be talking about recent ttH measurement with
the Atlas experiment.
And, oops,
okay, we'll start with a brief motivation of why we're interested with ttH
measurements. As we know all the Higgs property measurements so far are consistent with
standard model, and one of the information from measurements are the couplings of
the fermions, in particular the top Yukawa coupling, which is the goal of this ttH
analysis, and we can directly measure it via ttH cross section
measurements. Since the top Yukawa coupling is the largest in standard model as it's
shown on the top right plot, it has a major impact on the theory, such as the
impact on the effective potential of the Higgs field and which the variations on
coupling can lead to second minima that has an impact on the understanding of the
metastability of the universe as can be seen on the bottom right plot, and
also has an impact on self coupling of the Higgs which is still to be measured and
a major milestone of the future LHC experiments. Another interesting point is
CP nature of this coupling, where the standard model predicts CP even Higgs boson,
but on the other hand, the CP odd is not excluded yet. And on bottom plot, you
can see the total invariant mass distribution ending on the mixing angle, where
difference can change the peak of mass distribution of ttH. And this talk is
actually given by another session in this in this conference. So, the ttH
measurements at ATLAS experiment utilizes the broad range of final states
originating from ttbar process and exploit Higgs decay mode. Therefore the
measurements are categorized depending on the decay modes of Higgs boson, on bottom
table I'm giving you showing you those categories. First category is the Higgs to bb
channel. This channel has much high rate, suffering from the enormous
QCD background and the latest publication is with 36 inverse femtobarn. And the
second group is called a ttH multilepton group, where it focuses on WZ boson
and tau decays of Higgs boson. And the rate is reasonable but the
background estimation is also challenging due to non prompt backgrounds,
and the latest publications is with 18 inverse femtobarn. The last category is ZZ
and gamma gamma decays where the decay rates are quite small, with respect to
others. However, they have a clean signal without backgrounds, and the latest
publication for both of them is with full run two data set. And the final observed
finally observation of the ttH measurement announced by ATLAS in 2018 where
only the gamma gamma channels was using eighty inverse femtobarns while the
other were using 36 inverse femtobarns.
In next slide I'm starting with Higgs to bb channel and I shouldn't go so much in details since it was a
it was presented many times in this conference and and on this channel we are
currently working on the updating the check the results with the full run two
data, just to give some brief information this channel is most common on other
channel categories, but it has a challenging final step as I said, and for
this situation, to handle the situation the various control regions are defined to
constrain those backgrounds, and analysis regions are split in two with one
lepton and dilepton, and also a boosted one lepton channel is exploited, with the signal
and control regions are defined depending on the number of jets and the b-tagging working point.
An example sketch us is given on the right bottom and the y x and y axes
represent a combination of different working points for the b-jets and the
tightest will be the highest purity in ttH. And as it loosen, the control regions
are defined. And the main challenge is the modelling of the ttbar jet backgrounds
since it's inferred indirectly with the tt Higgs to bb final state as it can
be seen from the given Feynman diagram here. And the control regions are depending on
the flavor of the jets. And furthermore, numbers of the uncertainties are implemented
to cover the modeling differences between different schemes and the generators. And
also the analysis make use of sophisticated multivariate techniques to
extract the signal region in which I'm going to talk about in the next slide. So, two
steps were employed to discriminate the signal from background. What's
called intermediate techniques are focusing on the reconstruction of the tt Higgs
system, such as the reconstruction.. extraction of boosted decision tree to get the best Higgs
and top candidates in the event. And the likelihood discriminant considers the kinematic
such as the possible jet particle assignments and calculates the discriminant for each
event again, and the matrix element method also provides discrimination built from
first principles. On the bottom plot, left plot, the proper discrimination on the
reconstruction of the Higgs boson mass is given. And finally, those outputs from the methods
are combined with an additional information in the second stage in which is
called the classification BDT which will will be used to extract the signal
strength and on the bottom right plot one of the signal region in this channel is
shown, where the classification BDT is used and obviously, discrimination is
quite successful. Coming to the fit setup tt Higgs bb uses the profile likelihood fit to extract
mu ttH in 10 control region and nine signal regions. The classification BDT is
used for signal region, and for the control regions usually a single bin or kinematic
variable such as some of the pT of the objects. And the major backgrounds are tt plus
jets, are, are free floating in the fit to control better those backgrounds. And the summary
of the distributions are given on the right side of the slides and showing the
comparison of expected and observed yields and overall very good agreement with data
is observed and the results for each channel is given on the table bottom left.
And the significance is 1.4 sigma observed with 1.6 sigma expected. And the major
backgrounds are arising from tt plus jets modeling and in the from the Monte Carlo
statistics. And obviously the analysis is dominated by systematic uncertainties. So,
I now I move to the second analysis categories called ttH multilepton. And this analysis is
focusing on the Higgs to WW, ZZ and tau tau decay modes, together with ttbar system, requiring
at least one lepton. The baseline selection of these analysis channels are done
depending on the charge and the flavor of the leptons. There are six channels are
used, three of them with and without hadronic taus in the final states, as shown on the
table right, and the most sensitive channels are two leptons, same sign, zero tau
and the three leptons zero taus, are also further splitted into control regions to constrain
the different different sources of the backgrounds. The main backgrounds are
standard model processes such as the ttW, ttZ, diboson and also the non prompt
backgrounds, mainly coming from ttbar process, such as leptons arising
from semileptonic b decays or the conversions or electrons where
one of the electron has charged misassigned and for to to further reduce those
background contributions there is special object selections are used on top of
the baseline selections. At the BDT for instance, BDT isolation is employed to
reduce the leptons from heavy decay flavors, flavor decays, and a charge misid BDT
also provided by performance group is used, and variables to reject the material and gamma star
conversions are also benefited.
Just to be a bit more concrete, in this slide, I'm showing you the region
definition on most sensitive channels in ttH multilepton. And as well as I will give some
details about non prompt background estimation. And regions are further divided
into high and low N jet regions. In high N jet regions the multi dimensional BDT
space is exploit for signal and control region, and also further split on depending
on the charge and the flavour of the leptons to constrain different source of
backgrounds again, and it contains again signal regions in high BDT and control
regions. And the low N jet regions are more dedicated to control the non prompt
backgrounds and also the ttW standard model process and there are some kinematic
variables such as HT lepton, N b jet distribution and also events yield is used.
And on the right plot I'm showing you one of these regions in this category. There's
sum of the pT of the leptons, here the orange contribution is non prompt muons and
is populated in the first two bins. It gives a good control of this background.
And the estimation of light background, light lepton background, is based on the
Monte Carlo shapes. And the normalizations are obtained from simultaneous fit to
signal strength. And hum here I'm giving you these processes. Heavy flavor semileptonic
decay, where an isolated lepton can be reconstructed within the heavy flavor and
sorry, and dedicated normalization factors for muons and
electrons are defined.
The second is the electron from material conversion, where the gamma interact with
detector material and resulting in one is isolated lepton and the last one is
electron from internal conversions, where the virtual gamma star decays into two
leptons, again, one being isolated. And it can occur in at a parton showel, shower or
the matrix element depending on generator. On slide number nine I want to give more
details about ttW measurements within the ttH multilepton channel. The standard
model's ttW process is free floated in the simultaneous fit due to the high
contamination in both control and signal regions. Ehm three decorrelated normalization
factors are assigned: two lepton same sign low N jets, high N jet, and three lepton. And
the reason why we divide the normalization factors is that the different phase space
yields different values and on the right plot inclusive num ehm N jet distribution for
for two leptons same sign is shown. Here the dashed lines on the ratio plot in
indicates the pre-fit ratio. And it's clear that the first two bins needs a
more ttW than the rest to improve the data Monte Carlo agreement. However, these
results give us a more ttW than the standard model expected. These values are given on the
table here and those numbers are also including a missing higher order
corrections to account for NLO ttW plus jets due to the opening up in the
quark ehm gluon quark initiated process and also the electroweak corrections where the
NLO tree term is dominating due to the tt tW scattering through the Higgs boson and
also were not included into the yellow report four and as well as the nominal
sample of us and impacts on them can be non negligible. And finally, this the
mismodelings are seen observed in a two b plus plus region. And this difference is added
in the fit model as a shape uncertainty for the ttW phase space. And the
motivation behind this is purely experimental. But actually the charges
method is known for a minimum for the ttW process. And also, there's very
interesting talk in this
session about the ttW modelling.
On the last slide in this channel is for the fit set up. As usual profile likelihood used
for 17 control regions and eight signal region as shown in the bottom plot and the
results per channel is given on bottom right plot. And the overall significance
is 1.8 sigma observed with 3.1 sigma expected and the major schematics are JES
an tt ll modelling and ttW modeling and overall good data Monte Carlo agreement is observed. The
next search that I'm going to cover today is tt Higgs to tt gamma gamma. This
channel is using already full run two data. And regions are divided into hadronic
where there is one lepton veto there is lepton veto, and leptonic where there is one or
more leptons in the final state, and of course with two well isolated photons. Ehm the
background estimation is taken from sidebands in data where the photon
requirement is reversed and also they make use of dedicated control samples to
define the function to fit it. And after certain cuts on BDT to reject the highest
possible background contamination, the bed regions are further divided depending
on the purity of the signal. And signal extraction is performed in invariant mass of
diphoton distribution by using double sided Crystal Ball function. And here on top
right I'm showing you all the regions included in the fit and bottom post shows
you the fit to the invariant mass of diphoton system in all categories, where
the events are weighted depending on the purity of the category. The significance was found -
you have two minutes - to be, okay. The significance is found to be 4.9 sigmas observed which
means this channel has already almost reached the observation alone. And this
result as we expect is stat limited. The last channel I'm going to cover today is
the ttH production where the Higgs decays to ZZ and subsequently to four l.
And in this relevant paper in fact, the cross sections are measured for main production
modes in several exclusive regions of the Higgs boson production phase space, but
today I will just talk about only the ttH production mode. In the analysis is used also make
use of full run two statistics. And they focus on the Higgs mass window between
115 and 130 for the signal regions, which is quite low, then the background estimations
are done from the dedicated sidebands and the analysis regions are classified depending on
the ttbar decays. The event yield is used for the ttH, for the signal
extraction in ttH lepton enriched region, and for the ttH hadronic region
sophisticated neural network algorithms are exploited which are trained against
the back main backgrounds. And as a result, two indepedendent outputs are used to
extract in this region, and the distributions are given on the bottom plot. Then the
extracted signal is 1.7 with big statistical uncertainty as expected, but three
candidates in total are observed in this channel which is a good success with such
a smaller channel. And finally, I'm coming back to a combination of ttH
measurements. There is no combination is performed yet for by exploiting the
latest results that I've shown today. However, the observation of ttH measurement
was already performed using very using partially run two data. And here on the
left plot I'm showing you data and theory comparison of results with the
first point from run one and the second point from partially run two data and on
right plot there is also all channel contribute to that observation which is
shown and 5.8 sigma is observed. And also the the combined with
this result with full with run one and and have the 6.3 sigma was observe it
takes six sigma observed
Okay so then in conclusion, I showed today the latest studies related to tt Higgs
measurements. Atlas has established observation of ttH production by
combining various Higgs decay channels and results are all compatible standard
model. And some individual channels have updated their results with more data. Observation,
for example, observation, almost observation with ttH production in gamma
gamma channel, and also the three events are observed with ttH to four l channel.
And for, and for the other channels there's the challenging background is there always
and requires better understanding of the background modellings, such as ttW,
tt bb, and currently the improvements are expected with full run two data. And also the
full run two data will allow us a new opportunities and studies including
differential measurements and also the property measures. Thank you.
Thank you very much. Any questions?
Yeah, I see Marco has a question. Go ahead.
Yeah, thank you for this very nice talk. So, I think in the context of this session
in particular your slide nine is, is very interesting. So, maybe could could you
take the chance to add some details on the justification for this say decorrelation,
whether you have done some studies also on the say generator level phase space, because,
I mean, naively one could wonder whether this full decorrelation of these
three reco level say phase spaces isn't too conservative from our point of view, and
then a second part, I mean, to the same question could be whether you checked
weather you have, for instance, ttW events that end up confusing with the ttH ones
even at higher jet multiplicities, I mean here the last bin you show is
the inclusive six jet bin, well, I would suspect that even higher jet
multiplicities you could play an important role as a background for ttH in say high
S over B regions. Thank you.
Okay, let me try to get her all. For the second part I
will say for the ttH ttW
we don't expect to have such events because I mean in the end, we have the
correlation matrix in after the fit that we have. We have such not that higher ttW
and ttH uhm correlations in both normalization factors. Uhm and for the first
point is I'm not sure I really understood your
question is, I think it was about
is it too conservative to have decorrelated normalization factors, I guess?
So that the question is for instance, why do you decorrelate three lepton
from two lepton low jet that in principle, I would suspect they're populated by the
same say kind of ttW events in a generator level phase space apart from from the
decay, right.
Okay. So, there was a, we had a dedicated study for that, in fact,
ehm, while
for example, for the
control region only fit into a three lepton, and we have also the same
normalization factor of ttW and a similar one, where we have only one
normalization factor. This three three l region is is really lowering down in
the I mean its normalisation factor is going really low and then in fact
in the ttH measurements and so we found that decorrelation of three of them is
basically is more reasonable.
Okay, are there any other questions?
You can raise your hands if yes.
Okay, I don't see anything else. Okay, thank you very much Merve and we move