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