Okay, so today I'm talking about precision QCD the measurements on behalf of these
four, fine collaborations. And so of course, we know that the LHC is a QCD
factory, and that pretty much every event we see has some element of QCD within it.
And indeed, QCD is at the center of the standard model. And by studying it, we
understand nature itself better. Of course, it's crucial as well to understand
QCD if we're going to understand the rest of what we see at the LHC. And so today,
I'm going to look at a variety of measurements that probe QCD at exciting
levels of precision, and I'll take a look at a wide variety of different energy
scales and I've got these different processes that I've been asked to cover.
And I'll try and touch on on some of these today. So let's start by looking at Drell-Yan
physics and vector boson plus jet physics. So I thought I'd start with this
lovely analysis from CMS, where they take the measurements of W and Z boson
production cross sections of the Drell-Yan process that were made using the run one
datasets. You can actually see these results from these two plots as the horizontal
bands in gray. And you can use these to extract values of alpha_s. As I say, these
happen to pick out the W and Z boson cross sections at 8 TeV: W plus boson I should say
there are other plots like this. And then you've got a joint probability
distribution function for the different PDF sets, showing the one sigma
favoured regions. And of course, you can do this and then say, what is the favoured
value of alpha s and on the left hand plots, you've got the curves for different
PDF sets considered
and the analysis takes onboard
And uses CT and mmht14 - it claims that these are the most robust in the context of this
analysis. And indeed you can see the value of alpha s just in these measurements is
extracted, which is really quite precise. This is an impressive achievement. We can
also ask, what do these W and Z boson events look like. So CMS have recently
published this run two analysis considering rapidity, PT, distributions of Z bosons
and more, and comparing these to a wide variety of theory predictions. On the left
hand side of the plot here, I show the ratio of different predictions to data,
including the low PT region where there is clearly a continuing challenge in terms of
modeling. And then you've got fixed order predictions on the right hand side,
you've got FEWZ and so on, and indeed again these show reasonable agreement
across a variety of PT scales. And ATLAS have measured the Z boson
PT distribution, I show here (they also measure far more in this this paper that's
recently accepted in EPJC)
you can see again that
pythia eight is here doing well at low PT but obviously being leading order
struggles at higher PT, Sherpa does well a high PT and indeed radish, NNLO plus N-cubed-LL
describes the entire spectrum well, which is pleasing to see.
And CMS have put out recently this very nice
analysis where they consider the 13 TeV data set and make a measurement that is
a double differential cross section measurement
in terms of rapidity and PT. And again picking out the different left
handed and right handed helicities of the W boson. And this is a high precision
study of QCD. It gives key sensitivity to and ability to constrain PDFs. But of
course, it's also a crucial step towards an MW measurement it's its own right
and I'm sure we'll hear more about this in the electroweak plenary. Overall, we see
reasonable consistency between data and predictions from QCD once you account for
correlations between the different bins in data. And CMS have also produced this new
measurement, which I'm delighted to be able to show, with differential cross-sections
of Z plus jet and photon plus jet. The measurement considers the
differential cross sections for Z plus jet, photon plus jet. It also considers
the boosted topologies, so here we see separation of the Z and the closest jet
for high PT jets. And you can see that this is a nice measurement, extending this idea
of making precision measurements of standard model processes, but it extends
this idea into extreme regions of phase space. So that's that's also very pleasing
to see. And indeed, good agreement is seen with theoretical predictions throughout
throughout this analysis. And CMS have also done a dedicated study of a triple
differential cross section for isolated photon plus jet using the 8 TeV
data set. And this analysis gives sensitivity to the gluon PDF. And indeed,
good agreement is seen with predictions at next to leading order. And ATLAS have probed
photon plus two jet and this measurement picks out a variety of regions in phase
base where you can pick out sensitivity to different processes, direct processes,
fragmentation processes, and many different differential distributions have
been considered. And in all of them good agreement is seen with theoretical
predictions from QCD, except potentially high dijet masses, as you see
on the right most plot, or indeed where the, the, there is significant separation.
And so in the middle plot, you can see the jet-jet separation, the data somewhat
undershoots the theoretical predictions. So, it's these more extreme
regions of phase space where we still need to understand what's going on. And CMS
have produced the first measurements of associated vector boson plus heavy flavor jets
at 13 TeV we have from the LHC. It's really nice to see these these analyses. And
again, they've considered b jets, c jets and the ratio and throughout there is
reasonable agreement with the next leading order predictions. And ATLAS have
considered Z plus b jets and they've considered both Z plus one b jet
inclusive and Z plus two b jet inclusive. And they show here, essentially
a plot: the first plot is from the one b jet case, and the plot
on the right hand side is from the two b jet case. And the analysis is a tour de force
it considers a large number of different predictions, including this sherpa fusing
scheme that enables predictions to be combined using the four flavour number
scheme and the five flavour number scheme. And broadly the next leading order
predictions using the five flavour number scheme seem to describe the data well. And
indeed the fourth flavour number scheme predictions do agree with the data in the
two b jets case so if you look at the right hand plot you see the four flavour number
scheme does does well here. So that's kind of a whirlwind tour of some of the
analysis that has been done looking at vector boson final states. I'm now going
to look more at physics that can be done using the jets themselves either using
looking at events that contain jets or then going on and looking at jet
substructure and and what happens the evolution inside the jet.
So let's start by looking at event shapes. So this is a new measurement from ATLAS of
event shape observables. This is done in bins of jet multiplicity and ht2. It probes
multi jet energy flow, at TeV scales, and there are a variety of different
observables the analysis considers. Here I'm just going to pick out this tau
perpendicular variable which is essentially related to the transverse
thrust within the event.
And what you can see, if we look at these plots,
we take the top plot.
This is a bin of ht2 between one and 1.5 TeV, the bottom is ht2 above two TeV,
but then you've got the different lines comparing to predictions of the
three jet case, the four jet case, the five jack case and so on. And you can see that
for lower jet multiplicities the shape seems to not be perfect in Monte Carlo - it seems to
not be perfectly describing the data. As you move to the higher multiplicity, so
these are the lower plots that you see, we find that the shape seems to now be picked
out reasonably, but we see discrepancies in the normalization still. Moving on, we
can consider inclusive production from ALICE, it's nice to show this, this result
from ALICE. Here ALICE study the dependence of the inclusive production
cross section on the distance parameter are used within the anti kT algorithm,
ie the extent of pase space, you're going to end up clustering into jets. Now
there's much more within the paper as well, of course, this is a study performed
using p p data and lead lead collisions. And what I'm showing here are the ratio of
these cross sections for values of this anti kT R between point one and point
six. And you can see that the data and the different theoretical
predictions are all right on top of each other. These next to leading order predictions,
which show that the jet shapes, the size of the jets that dependence of the cross sections on that seems to be reasonably well understood. Excuse me. A similar
study has been made by CMS using 13 TeV data here taking the ratio of the
inclusive cross sections measured in steps of this radius parameter between point one
and 1.2. And here CMS report that they see much better agreement with NLO
predictions than leading order predictions. We look at this bottom plot
on the right hand side, we can see as well that we also see that non perturbative
effects are also crucial. That's also clear on the top plot on the right hand
side that it really is the the non perturbative effects as well that
really lead to the data prediction agreement. And CMS and totem have also
looked at dijet events where there's a rapidity gap between the two leading jets,
and also potentially where there's a gap and then there's also you see a
forward proton. And this is sensitive to hard collar singleness exchange. And the
fraction of these dijet events that you see that have this
gap is plotted here, you can see that the data as a function of the azimuthal
separation of the dijet system shows reasonable agreement with with these
predictions. The BFKL predictions. The middle plot is interesting it shows that
there is some evolution with energy we see with this. You can see the results, CDF and
DZero at different energies in red and then green and then coming down to the
LHC, but actually the results at the LHC from CMS at 7 and 13 TeV are broadly
similar. And again, on the right hand plots, you've got the Totem results, we
use Totem is shown in red, and that's just picking out where you pick up this gap and
then a forward proton. We can also consider jet substructure using the soft drop
algorithm, which is essentially a clustering technique that removes soft and
wide angle radiation in the jet, and really enables you to find the hardest
one to two splitting within the jet. ATLAS study here, the jet mass, the momentum carried
by subjets and much, much more. I'm just showing here essentially, the mass of this
soft-dropped jet system, normalized, it's shown here on the right hand side and there's
good agreement between predictions and the unfolded measurements, and indeed, this
is a common feature in this this recent paper.
We can also consider the evolution of jets
as characterized by the Lund plane. So you've got these kind of variables
where you can use similar clustering techniques to follow kind of
proto jets and emissions and track back through the, through the evolution of the
jet, how the the emissions occurred. And you can look and see, well what fraction
of the momentum was carried by the emission, what was left in the core, what
was the separation of the emission from the core. And you can plot these out and
you, you map out this Lund plane as shown schematically here on the right hand
side.
And you can pick out here,
some measurements, you can, in different bins, you can pick up different bins with
these measurements. And so you can see the schematic you've got your triangle shown
on both of these plots, and this is showing the relevant bin. So on the left
hand side, we've got this bin in separation that we're picking out and then
we're showing multiple bins in this z variable, which is how much of the PT
is carried out by the emitted particle. On the right hand side we're then binning in that;
we've picked out one bin in that variable. And then we're considering bins.
The plot itself shows the Delta R of the emission and the core. And again, there are
comparisons, so all of these different predictions that that we've just heard
some information about. And here we find that herwig 7.13 angular ordering gives,
broadly speaking a very good description across the Lund plane.
And another result from ALICE.
We know that emissions are suppressed within an angle of roughly mq over EQ. And
this is going to be more significant for charm and beauty jets where you have the quark
mass. So ALICE consider these D0 tagged jets and inclusive jets and they treat
them as similarly as possible. And you can see here essentially the ratio of cross
sections you see as a function of this angle of the core and the radiator. You can
see that as one over theta increases, you get the suppression of the D zero
tagged jets relative to just general inclusive jets. In other words, this is an
observation of this dead cone effect, which is very nice to see. And the effect
becomes more pronounced with hard cuts on kT, this is just the difference between
the blue the green and the red. The idea is that these are then introducing an
ability to reduce the impact of non perturbative physics such as hadronisation
effects or reducing the impact of a decay of your D zero, but you can see that
the dead code effect is clearly there. And LHCb have considered jet fragmentation
and hadronisation. So LHCb considers we've got a jet that's recoiling against
Zed boson can we measure the content of that jet? What are the charged hadrons in
the jet, what, what's the fraction of the jet momentum carried by
these, these charged hadrons. So this is shown on the left hand side, we're
comparing LHCb against pythia8 data, and in the bulk of the distribution
it's broadly compatible. But as you see, as you get to high values of z, or
indeed low values, instead, you start to see the data overshoot the the Monte Carlo
prediction. And on the right hand side, I'm comparing this measurement from LHCb
to an earlier measurement of inclusive jets from ATLAS. And indeed, you see that
there are differences in this distribution. And it is hypothesised that
this comes from the fact that the LHCB Z plus jet sample is dominated by light quark
jets, whereas this ATLAS sample is potentially dominated by gluon jets.
ATLAS, Have I said before done this measurement at 8 TeV, this is now the thirteen TeV
measurement considering similar properties. On the left hand plot, we've
got the expected number of charged particles, on the right hand plot we've got
momentum fraction carried by those charged particles. And you can see that the data
theory agreement in the jet multiplicity and the energy composition of the jet is
pretty reasonable. Between them pythia 8, herwig plus plus and Sherpa are all
accurate to about 20%. So leaving jets to one side now: having looked at vector boson,
vector boson plus jets, jets, evolution of jets, and how we go through the event,
we can also consider what about the production of hadrons. So ALICE have
measured the D meson production cross section. This is a measurement that is
sensitive to the low x gluon content of the proton. So it's picking out the low x
gluon PDF. And ALICE compare the data to a wide range of theoretical predictions
with reasonable agreement. But another item in the paper - I've not
shown it here - is that ALICE also compare their results to the LHCb results in a
different rapidity interval. It's very nice to see so it means that you've got
very similar measurements so they're just picking out different regions of rapidity,
different values of x and really getting some nice complimentary physics analysis
going on there.
ATLAS have also considered
production of J/psi and pi(2S)
we see here,
this is done a high pt. So this is for pts of 60 gev and above, and measuring prompt and
non prompt contributions. And actually something that's quite interesting is that
the non prompt fraction of J/psi is relatively flat as a function of PT and
rapidity. And then you can take that non prompt contribution and compare it to the
fonll model. And indeed you see agreement in the low PT region. But actually, the
prediction ends up overshooting the data at higher PT. And so those are just a
couple of recent hadronic production cross section measurements that I just wanted to
pick out. And so I thought I'd spend the last few few moments of the talk just
covering hydronic spectroscopy. So LHCb has looked at the lambda_c k minus mass
spectrum in run two data and it sees various structures over a smooth
background. And reports new states and makes precise mass and width
measurements and of these the two lowest mass states are definitively observed for
the first time. The higher mass state requires further study to confirm it is separate
to the already known cascade_c 2970 but it's nice to see these these new
resonances seen.
Will, you have three minutes.
The LHCb also looks at
new excited omega b resonances. And this uses the entire LHCb data set. And we see
here on the bottom plot, the contribution that you would get from (excuse me a second), you
would get from the combination of k plus and if you took the k minus you get the top
plot, you recover this peaking structure. And I say these are the local
significances I've put on the top plot. And these are between 3.6 and 7.2 sigma.
And then when you account for the look elsewhere effect, the two lowest mistakes end up
with a significance below three sigma. But the two high mistakes have significance as
above five sigma still. LHCb and CMS have both reported in the last year new excited
lambda_b resonances, measuring the lambda_b pi plus pi minus mass spectrum,
three states have been observed. These are separate analyses I should be clear from
CMS and LHCb. And these analyses make use of different decay modes. But the key
feature is that the excesses are seen at both experiments. And again, you can see
the pi plus pi minus combination at the top with the pi plus pi plus and pi minus pi
minus blow and you only see these excited estates in the pi plus pi minus
combination. And there's a suggested interpretation, a doublet of states that
correspond to the lambda_b (1D). And then you've got the lambda (2S) observed
as well. There's also a measurement from LHCb of the BC mass using multiple decay
modes. And what's quite nice here is that there's an improvement on the current world
average by a factor of roughly two. And indeed, this is already far more precise
than the prediction from unquenched lattice QCD. And then the final result I
want to share is another new result. This one from LHCb. This is the first
measurement that the X(3872) has nonzero width. It's the most precise determination
of mass and binding energy to date from this channel. There are actually two analyses
here that are both in preparation: one uses an inclusive selection, one uses an
exclusive selection. And the analyses make an extensive study of branching fractions
and the line shape of the state considering modeling using flatte and
Breit-Wigner distributions. And indeed, the first pole of the complex amplitudes
found within this extensive study of line shape implies results consistent with
bound dates. So just to conclude, I've kind of done a rapid whirlwind tour of
studies of QCD at the LHC, which covers scales, from high momenta to lower momenta
and probes a range of phenomena. There are stunning levels of precision achieved by
ALICE, ATLAS, CMS and LHCb with new and exciting measurements in the last year.
And of course, I've had to sum up the entirety of a parallel session in one
plenary talk. So do connect to the parallel sessions to learn more, or if you just
want to ask some more questions or chat to me, then I can be found at this link after
the plenary sessions are over today. Thanks for listening.
Thanks a lot Will for this impressive summary of research results from the
LHC experiments. Questions.
Bogdan?
Okay.
Hello. Do you hear me? Yes I hear you. Yes. Hello. Very nice. Very nice talk. I have a
question/comment. Two measurements from ALICE and from CMS
concerning the ratio between jet cross-sections for different jet sizes. So I
would like to to know how the authors comment about the two important
systematics: the modeling system metrics that are involved in the calibration when
extrapolating between different jet sizes, and also how the correlations of the
uncertainties are therefore taken into account when computing those ratios.
So unfortunately, I'm not a member of either of those collaborations so I can't
give an extensive comment on this.
More for someone's from CMS or from ALICE, I mean, if they are, yeah, if someone wants
to speak up, please feel free.
I don't see any hand: if you want to speak up, please raise your hand.
I would also be happy to follow up on this topic offline if someone is available.
Yeah, it's clearly interesting to know the degree which you can expect
systematics to cancel in this kind of measurement; to what degree as you change
your jet size, your systematic effects actually do cancel or to what
extent they are clearly correlated with the collimation of the jet.
My concern.
Yeah, sorry. My concern is mainly related to the modeling dependence. I mean, this
extrapolation and I think it is quite large. So this is what I wanted to hear
about.
Yeah, this probably really needs the experts. So maybe if there is an expert in
the audience, you can join the chat room of Will afterwards after the plenary
session.
We have time for one more question.
No hands up. Okay. Then we move to the last talk of today. So we move to the low
scales.