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And this will combine the results from ATLAS and CMS.
So just start when you're
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ready, Barbara.
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Thank you very much. So, I´m Barbara Alvarez
from University of Oviedo, and yes, as Andrea
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said, this is the topic of my talk. So, as already presented,
at hadron colliders, top
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quarks are produced mainly in pairs via
the strong interaction, but also it can be
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produced singly through the electroweak interaction
as shown in these Feynman
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diagrams on the on the right, it could be
produced in three different production
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mechanisms. So T channel, tW and s channel.
So, single production was first observed
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in two thousand nine at the Tevatron, by the CDF and
D zero experiments, and then later
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observed in two thousand eleven, by ATLAS and CMS.
So why is this single production interesting?
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Because it's directly sensitive to the
TB CKM matrix element, it's also sensitive to
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parton distribution functions and help us
to better understand the inner structure
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of the proton, can study this interesting
effects of the TW channel and interference
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with TT bar. And given the higher
statistics and the higher cross sections
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at the LHC we are allowed to prove
also differentially these processes and some
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measurements are already available. There
are plenty of results, but only most of
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the recent results are covered in this talk.
So, starting from from the
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achievement of the combination of the
ATLAS and the CMS inclusive cross sections of
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run one, the run one data set. So at seven and
eight TeV I will follow these results with the
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CMS results of the inclusive and
differential TW and T channel production
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cross sections of thirteen TeV with also ATLAS
results from the singly and double resonance
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productions and the TZQ production. All
these are being published already
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public and published. So first I start from the
combination. So we have an ATLAS and CMS
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combination per center of mass energy,
at seven and eight and per production
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mechanism with the three different
production modes. This table summarizes
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the published results with the total the
central cross section and the total
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uncertainties. The combination has been
performed with the best linear unbiased estimator
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method, with BLUE, and the results are
in this extra table in red and blue and so
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on. The total uncertainty of these results
presents the most precise measurements up
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to date. The dominant uncertainties in
this case are the theory modeling and the
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jet related systematics, also data
statistics in the case of the seven TeV so
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for example, I have a breakdown
uncertainty table that shows one of the
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combination measurements for the T channel
at eight TeV with all the different
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contributions of the different uncertainties
and highlighting the dominant ones which
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is, as I mentioned the theory, the theory
modeling and jet related uncertainties. In
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this new slide we have again the
combination a with a plot that shows
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plenty of things. So we have in blue the
ATLAS results and in red the CMS results
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for the different center of mass energies
and for the different production
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mechanisms, T channel, TW and S channel with
the different cross sections of course,
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and in in black, the combination. Also the
thirteen TeV results for the for the different
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collaborations are also shown but there is
no combination performed yet given that
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these analyses at thirteen TeV don't have the
full run two data set. So there is new
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results coming and all these results have
been are compared with the predictions at
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next to leading order and next to next to leading log and also next to next to leading
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order in the case of T Channel. In the
gray boxes you can see what's the
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precision of these measurements. So,
we have eight to seven per cent for T channel, twenty five to
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sixteen per cent for TW and thirteen per cent for S channel at eight TeV.
These combinations also give the combined
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results of the TB CKM matrix element, which
is the ratio which is performed as the
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ratio of the measured cross sections to the
theoretical cross sections that are presented
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on the table on the left, with first the individual
combinations of the V_TB in
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T channel, TW and S channel and then the full
correlation, the full combination of the
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V_TB at T channel seven and eight TeV for ATLAS and
CMS, the TW at eight and seven TeV for ATLAS and CMS
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and then the S channel at eight TeV for the ATLAS
results. There is also a talk on the
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measurements of top properties in CMS
by Joscha that shows also new CKM
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matrix element results in this in
this plot and also we have the correlation
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matrix of all the combination which each
bin corresponds to the measurement on a
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given production and experiment and so on.
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So, the next result is the TW
inclusive cross section thirteen TeV.
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This measurement at CMS is performed in
the dilepton analysis of one electron and
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one muon and a multivariate discriminant
is used in this case boosted decision
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trees is used to separate the signal over
background on these plots you can see
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two of the most discriminated variables of
the input variables of the BDT in the
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signal region and see how the challenge,
one of the challenges of these
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measurement is is the measurement over
this huge overwhelming TTbar background. For
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the analysis to extract the inclusive cross
section, three distributions are used one
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BDT in the one jet one B signal region, one
BDT distribution in the two jet one B
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region and the sub-leading jet p T
distribution in the two jet two B and the
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cross section is extracted is in
agreement with a standard model with the
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dominant systematic
uncertainties coming from experimental
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uncertainties in particular the lepton
efficiency, jet energy scale and pile-up. Also the integrated
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luminosity for this analysis which is
performed in
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partial run two data set
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is dominated.
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So, follow the inclusive we also have a very
new result, which is this TW differential
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cross section analysis at thirteen TeV using also
this partial data set, the event selection
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is similar to the one just presented. So,
two leptons with one electron and one
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muon and few cuts in the P T. Exactly one
jet and one b jet is the signal region.
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And then an extra cut where the lower
energetic jets are veto in order to reduce
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the background contribution as highlighted
in in this plot of the number of loose
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jets only zero loose jets are used. The
differential production cross sections are
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normalized to this to the fiducial cross
section and this fiducial region is
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equivalent to the reco level region but applied
to particle level objects. So, the results
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are performed or the differential
cross sections are performed as a function of
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six different observables. Here in the
slide I brought two, the leading lepton P T
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and the jet P T on the on the left you see
the normalized differential cross
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sections where the signal is extracted
substructing the MonteCarlo contribution
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to the data and then compared also with
different predictions in this case
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Powheg with the diagram removal and Powheg
with the diagram substruction
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and Madgraph aMC@NLO with with a good
agreement within the uncertainties
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and the main uncertainties are also shown
on the right plot and one can see how these
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uncertainties come from jet energy resolution,
jet energy scale and the theoretical modeling
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mainly due to the fact of this analysis, as
I mentioned, it's also overwhelmed by the
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TTbar background. The next analysis is
T channel inclusive cross section at thirteen TeV.
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In this case, the event selection is
performed with one electron or one
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muon and classified depending on the
number of jets and B jets as shown in
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this plot on the left, where we have two
jets one tag, three jets one tag, and three
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jets two tags, and that is done in
electrons and muons as well and also in
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plus or minus charges. This analysis
also uses multivariate discriminators
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one per category. So, different BDT are
trained in this analysis and one of the most
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discriminating variables or input variables
of this BDT is this dijet mass
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distribution shown here with a signal
contribution over all the backgrounds. The QCD
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is extracted from a maximum likelihood fit in
the case of the muons to the transverse
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W mass, in the case of the
electrons to the missing P T distribution.
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As I mentioned BDTs are used, one BDT
per category here they are three examples,
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we have one per category per lepton charge,
and the results are
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extracted for top and antitop separately
with the ratio and also the total and
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the total cross sections, and the V_TB
result is also extracted in terms of with
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the experimental and theoretical
uncertainties. This analysis
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is systematically dominated and
mainly dominated by the signal modeling as
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I'm showing in the next slide. And so,
here we have the uncertainty table of the
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ratio result of the top and anti top and
also the individual top and anti top cross
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section one can see highlighted in red,
the non profiled uncertainties are the
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dominant and those correspond to
the signal modeling of scales and PDFs, but
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they are much reduced once the ratio is
applied. So that's a good improvement with
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respect to other results. But this
basically reflects the fact that more
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precise signal modeling uncertainties
or signal modeling descriptions need
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to be
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applied, need to be studied. The rest
of the profiled uncertainties are much
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smaller than, than these others. The ratio
can also be seen in these plot on the
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right where we have the ratio with the
statistical and the total systematic
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uncertainties and it's compared with the
next to leading order prediction for
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different PDF sets. And this help us
understanding the inner structure of the
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proton, given that it's, the result is
already sensitive to the different parton
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distribution functions that we have in
the market. Next is the T channel,
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differentially at thirteen TeV also
contains also one electron and muon
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the signal extraction it's a bit more
complicated and that's why I have a
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dedicated backup slide on this extraction. The
cross section are determined as a
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function of six observables and
here I brought just one of them which is
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the top quark P T. One can see the first
plot is the signal enreached region and
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with the signal contribution and the
background and then the differential cross
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sections as a function at the parton
level and the particle level of as well
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with predictions of Powheg and aMC@NLO.
All of them in in certainly good good
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agreement. This analysis also brings into
an additional measurement which is the
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ratio or the charge ratio that has been
measured as a function of also other
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variables top quark, as a function of top
quark, charge lepton and W boson
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kinematic observables. Again, at parton
and particle levels. The ratio is
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I mean, the ratio is defined as the
differential top over the sum of top at
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anti top, and it's compared with a set of
different PDFs, all in good agreement, so
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compatible with all these predictions that
are compared with. This result is
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particularly important because it's again
sensitive to the up and down content of
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the proton. And a, an extra result in in
this analysis, we have the measurement of
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the top quark spin asymmetry that has
been measured using the differential cross
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section in this case as a function of the
top quark polarization angle, as shown in
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these well shows the definition and also
the two plots in the background and the
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signal region, which, by the way, have a
remarkable agreement in here and we also
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have the relation on how this
normalized differential cross section is
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related with this spin asymmetry and what's
the product of it on what's the result of
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this value that it's in good agreement
with a standard Standard Model prediction.
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So we have from ATLAS this analysis of
proving the quantum interference of the
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single and double doubly resonance top
quark. In the Feynman diagrams, we see the
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doubly resonance and singly resonance
resonant in red, the top quarks are
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presented and the results are compared
with different predictions from simulation
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using various strategies of the
interference and this is the idea of
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comparing the six existing
interference models. And check was the one
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that performs the best diagram removal,
diagram substruction of or all
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with two B jets and two leptons in the
final state. For this case, we have
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selected events of two leptons and two B tags
and then special would say different
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variable called the minimax. And this is a
variable defined of the minimum of two
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maximums of the invariant mass masses of
the combination of the B jets and leptons
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in the final state. So here you see the
stack plot of the backgrounds and the
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signal, the signal represented with the
diagram removal and diagram
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substruction definitions. And on the
sketch you see how this separation is
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reached from the TTbar which is the
dominant contribution and from the signal
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let's say from the other
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leg which is the TW and how this
minimax reaches the discrimination and the
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separation of these two contributions.
This minimax variable is used normalized
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in differentially also to extract the
conclusions and the comparison with the
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different models and we can conclude that
these two these Powheg Box with two B jets
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and two leptons in the final state is in
good agreement in the bulk and also a in
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the tails of the distribution. The
other models, the diagram removal, the diagram substruction
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are in good agreement up to
two hundred fifty and then diverges in the tails and
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this diagram to model clearly shows
a mismodeling in the tail of
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the distribution. The statistics and the
test statistics are also shown in this table
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for the whole distribution in all bins,
and also for the distribution
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higher than hundred sixty GeV comparing the
data and the predictions.
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you have still three minutes left.
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Thank you This is
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the last part of my talk just basically
saying that there are other results
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related with these topics, single top
plus extra contributions and in
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this case, for example, I'm bringing one of the
latest results from ATLAS which
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is this TZQ observation at thirteen TeV that
was also observed by CMS. Both results are
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in good agreement with Standard Model and
within within themselves. And also to
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mention that there is this CMS evidence
for the T gamma at thirteen TeV also consistent with
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the Standard Model. But these results will be
covered in details in other talks
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related to rare decays and rare productions
and so on. So to conclude, these results that
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I presented here today demonstrate a
good understanding of the electroweak
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production mechanism of single top quarks. The
achievement of the combinations between
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ATLAS and CMS results done using these
run one published results represent the
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most precise measurements up to date. So
far, all measurements are consistent with
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the Standard Model predictions by basically
providing important constraints on the
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interference mode and a better understanding
of the structure of the proton. But as I
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mentioned, some of these thirteen TeV results are
done with partial data sets, in particular
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for these run two data sets. So there are
more exciting results come in, and stay
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tuned. Thank you very much.
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Thank you very much, Barbara, for this very
nice overview and comparison between the
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two different experiments. I'm sure there
are some questions, so please raise your
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hands if you have any.
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Okay, so far, I don't see any. Um, there
was I had a question about your, about the
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S channel result from CMS at eight TeV.
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So is there
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I see that there's some discrepancy,
obviously, I mean, within the uncertainty,
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it's a probably that is still compatible with
the Standard Model. But is there any
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reason why the uncertainty is so much
larger than the one from ATLAS?
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Actually, I'm not sure if anyone in the
room
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knows the details of the of the
measurement. But I'm I'm sorry, I will
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have to check it out because I don't know.
Since you, you're asking exactly one of
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the analysis that I didn't cover in this
talk.
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Can I speak, sorry? Yes.
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Okay. So and this actually, there has been
a difference, there has been two main
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differences between ATLAS and CMS analyses.
So one one is that ATLAS was using
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essentially a matrix elements
discriminator which might have impacts on
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the improvement of the results. And
second, concerning the uncertainties in
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the CMS analysis almost none of the
uncertainties were profiled. So even for
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the jet energy scale and so on, we have
externalized them and and estimated ways to
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do experiments while via profiling
there is a possibility, I mean, since
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correlations are taken into account on a
spot, I mean, there's a possibility that
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you'll see some cancellations and
constraints, thanks to the data. So these
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are the two main differences that are that
are
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present between the two analyses.
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Okay, yeah, that makes sense. I forgot
that in ATLAS, we use the matrix element
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method and that you didn't do profiling.
Okay. Thank you for the explanation. Yeah.
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Yes. Thanks.
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Are there any further questions?
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If not, we still have a few minutes before
the next plenary starts. So in case you
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have questions to one of the previous
presentations that I had to cut a little
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bit short, because it looked like we were
running very late. You can ask them also.
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now I know Francesco, you still had a
couple of questions that I cut off.
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I am very I had only one question on the
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previous presentation by a CMS colleague,
Javier, and on slide thirteen, he was I just wanted
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to know what the definition was for the
uncertainties fpr parton and shower and matrix
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element, the hard scattering, is it always
Powheg versus Pythia? And is the hard
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scattering? How is that estimated it is
with varying the matching scale? Or is it
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with a comparison with a two point systematic?
I don't think that CMS does it. I think
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that it's just using the internal
variation but I just want the confirmation.
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I don't remember this.
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Hello. Yes, I think so we are varying
the parameters on the
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MonteCarlo and then propagating to the to the
final measurement. Let's say in the
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usual way.