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In my house, talking about recent results
on call activity and small systems from

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the Atlas collaboration.

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And let's see if we'll go forward. There
we go. So there are a wealth of Atlas

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results on small systems and I listed a
number of the recent ones down below. But

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in these 15 minutes, I want to focus on
just one recent measurement. And that's

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the transverse momentum and process
dependent azimuth and I saw trapeze in p

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lead collisions published recently in this
article, and that's the thesis work of

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Kurt Hill which you can read in full
detail linked here.

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So key features of heavy ion Coalition's
we often think of the Hydra dynamically

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described collectivity of the bulk, you
can argue about which PT separates that

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region, but you might think sort of below
five six gv and the jet quenching

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phenomenon, the suppression of high Pt
parkins jets and hey drones in the high PT

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region.

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Maybe above 10 to 12 GDP. And if you think
of the hydrodynamics, you have the initial

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state where you have steeper density
gradients along the x axis in this diagram

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on the left. And so, due to the steeper
density gradients, when you couple that

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with hydrodynamics, you get more bulk
particles along the x axis. In the case of

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jet quenching, you have that same
geometry. But in this case, these high PT

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part cons and the resulting jets have a
shorter path out of the medium along the x

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axis. And so, you expect more jets and
protons at high PT to survive on the x

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axis and to be more suppressed in vertical
or Y axis. And so that means that due to

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these two different phenomena, low PT bulk
particles are asked to be fully correlated

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with high PT jet particles because they
both result from the same underlying

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geometry that impacts them

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And that's exactly what's seen in lead
lead collisions. Here is the vn for

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moments two through seven and the
different colors versus transverse

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momentum. And in sort of the low PT region
that's shaded in blue might think of this

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as being from hydrodynamics. In high PT
region, you still see a significant v2,

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that's often interpreted as being again
from jet quenching. Again, remember, it's

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a correlation between high PT and low PT
is the way these VMs are measured. So just

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like I discussed in the last slide, and
then in purple, this is some transition

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region where you have a mixture of these
effects. And in p led Coalition's are the

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same plot over here. And as discussed in
the previous talks, there's sort of

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similar anisotropy is in the low PT region
in blue, maybe that's hydrodynamics at

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high PT from previous Atlas publication is
really just

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one data point in this higher PT region.
And maybe that's a hint about this

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question of jet quenching or some other
phenomena and a large region of sort of

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purple in the middle. And so the goal was,
how do we fill out that high PT region in

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led collections.

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And so this Atlas analysis uses the 2016 p
loaded ATV data, we're going to use three

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different samples, I'm only going to show
two of them in this talk, but all three

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are in the paper, there's a minimum bias
triggered sample and there's a sample

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triggered by events that have jet with PT
greater than 100 Gp, and I'll skip the

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detector details since they're pretty
familiar. So what we're doing is a p led

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to particle correlation. And what's shown
on the left is the two particle

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correlation and high multiplicity p light
events in the case where it's the minimum

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bias event trigger and a standard

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template method is used to remove the non
flow, and you see the dominance of the v2

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coefficient in the lower panel. However,
if you look at events where there's a jet

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greater than 100, gv, you see that the non
flow contribution is a lot bigger, right?

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sort of imagine, here's the blue flow
coefficient potentially. And there's this

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very large jet correlation, which is
expected because these are events from

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jets. And in fact, you can see that
basically, there's a distortion here in

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the template subtracting procedure,
because effectively the signal to

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background is so small. And so in order to
do this kind of measurement reliably, we

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need to reduce that non flow background.
And so the technique that's used is

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restricting associated particles in the
events that have 100 gV jets. So we do the

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standard thing of the associated particle
must be

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Have a delta theta greater than two from
the trigger particle, as we've heard in

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previous talks. But now In addition, we're
going to restrict the associated particle

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to be delta Ada greater than one relative
to all jets greater than 15. gv in the

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event. So the trigger particle can come
from a jet. That's what you might expect

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that high PT. But the associated particle,
we want to reduce its contribution coming

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from jets. And that was has a large enough
acceptance that even an event with

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multiple jets, there's still quite a bit
of acceptance to get statistics for these

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correlations.

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And so this just displays how this works.
The left plot is the one that I already

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showed you a couple slides ago. These are
p led high multiplicity events with 100 gV

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jet, so that has a low flow signal to jet
background, this big peak here, and then

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after we apply this jet restriction

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So we try to eliminate associated
particles that are in jet cones, you see

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that a dramatic improvement in the signal
to background. And we now have what looks

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like a pretty successful template fit to
basically remove the non flow. And so the

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improvement in the signal to background
really reduces the sensitivity to the

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template method assumptions. And that's a
key in this measurement.

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And so here are the results. And so we're
plotting v2 as a function of P t of the

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associated particle. And there are two
different sets of points. So the black

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points are in minimum bias events where we
do not have to apply this jet rejection.

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And we see this familiar pattern where the
v2 is rising up.

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It's then falling and now we have enough
statistics to actually have a couple of

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points at PT great

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Than 10, gv that seems to confirm a v2 of
order a couple percent even in that

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region. And then what's really
dramatically different is now we can look

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in these events with a jet trigger greater
than 100, gv. And of course, this trigger

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sample, the full p lead, luminosity
delivered. And so we get the rise the fall

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and now we have very, very good statistics
all the way out to 50 gV in PT. And we see

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that that two to 3% v2 remains up to the
highest pts that we measure. And so, in

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the blue region, you can think again of
the low PT bulk we can confirm the rising

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v2.

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Now in the high PT region, getting all the
way up to 50. We really see that there is

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a correlation and acid myth between the
low PT bulk particles and the high p TP

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articles and in a Coalition's This is
interpreted in terms of jet quenching.

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And then there's a transition region in
purple in the middle. And there's a

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difference between the two trigger
samples, which I'm not going to have time

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to talk about in this talk. But if people
have questions, you can take a look in the

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paper or ask at the end.

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One thing that's quite striking is if you
overlay the v two results versus PT with

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previously published Atlas led led data
with the P led scale by a single factor of

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1.5, they actually look quite similar. And
again, I think there's a lot of

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speculation that there's a common
hydrodynamic explanation for sort of the

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lower PT. And I guess the question is, is
that our common explanation for the high

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PT correlation?

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You know, in the last few minutes, talk
about a couple of different pictures. So

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what if we think of it as jet quenching in
pea lead? Well, in fact, this is

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Some old paper from Zhang and Lau, where
they model a small q GP droplet in p led

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Coalition's and they actually calculated
v2 and the two different dashed curves are

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two different size q GP droplets. And what
you see is that actually, their

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description of the P lead v2 is
qualitatively not so bad, but in the same

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paper in the same model. They also predict
in the right hand plot the RP lead the

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nuclear suppression factor and P lead and
they predicted nuclear suppression factor

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of almost a factor of point five at low PT
rising very slowly up to 50 gV, but still

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being a 30% suppression. And even though
there are, you know, challenges in

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measuring RP lead in central events, their
results from Atlas and Elise, basically

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strongly disfavor that level.

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hole of suppression. And so just like in a
collisions, you need a pretty large

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modification to the PT spectra in order to
generate a differential jet quenching. And

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so this cannot describe both azimuthal and
isotropy. And the lack of change in PT

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spectra. And that's a pretty fundamental
challenge, I think, to any jet quenching

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model, not just this specific one.

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The other interpretation I want to walk
through which was discussed in the paper

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is the initial state effect model. And I
want to remind people that a key check for

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the original pee pee rich correlation
observed by CMS was checking in pythia.

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And so here are a couple plots from the
original sort of seminal CMS paper. The

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left was the pee pee rich in real data,
and the right was the result from pythia

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where no Ridge was seen.

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But pythia sort of has a revival.

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ENTJ In fact, in the left plot, this is
pythia, eight in soft to CD non

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diffractive mode. And as CMS observed in
their paper, there's no long range ridge.

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But actually, if you run pythia, eight in
Hart q CD mode with a PT hat men have 15,

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gv, and then basically requiring a jet in
the event as we're sort of doing in our

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analysis. There's actually a long range
rich in pythia. And actually, we've traced

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that down to an implementation and pythia
of initial state radiation in space shower

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routine, and confirm that with the
author's it's a radiation pattern from

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color coherence considerations. And just
to emphasize, since some of you will be

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familiar, this is not the string shoving
option described in this paper. This is

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really not due to interaction. It's due to
a coherence effect. However,

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The effect is biggest in low multiplicity
events, and is watered down in high

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multiplicity events. It's a correlation
amongst only a subset of the particles

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from this radiation pattern. And so in
fact, what happens is kind of interesting.

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If you run the pythia. In this case, it's
argued inter. So it's p lead. If you

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actually run it through our full analysis
procedure, what you get is there's

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actually a near side.

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Rich that comes out, but because it's
stronger in low multiplicity events than

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in high multiplicity, when you apply the
template subtraction procedure, it

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actually predicts a negative correlation
and high multiplicity events. So of

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course, the V to two predicted due to this
long range rich and pythia actually is

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negative. So we plotted this v two two
because normally you

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You'd have to take a square root, so you'd
get an imaginary vi to me. I mean, it's,

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yep, thank you. And I think this is a
problem with any explanation in the

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initial state that involves lowercase n
less than n total particles, which is that

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as you go to higher and higher
multiplicity events, this kind of initial

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state effect gets watered down gets
smaller and smaller. And that's the

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opposite of what we see in experimental
data. So sort of both of these theoretical

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approaches face major challenges in
describing experimental data.

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And so just sort of on my last slide here,
want to show the result again, in p led

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collisions in black and then orange for
the events with a jet greater than 100. gv

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overlaid with the lead lead events. And so
this new Atlas results really extend

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These v2 measurements up to very high PT.
The paper also shows v3. It's similar

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trend but statistically a little bit
challenged at the highest PT. So I think

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this sets out that there's a definitive
correlation of high and low PT particles

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and P lead events where low PTs and youth
land isotropy is typically explained by

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hydrodynamics and geometry. So that
implies if you follow that logic that

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these high PT particles are correlated
with the geometry. But there are

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fundamental conflicts with that
explanation in terms of jet quenching, and

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also in terms of initial state
explanations. And so I think this is an

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excellent puzzle to be solved in the
future. Thank you for your attention.

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Thank you, Jeremy, for this very
interesting presentation. And yeah, so now

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we have time for questions. if people have
questions, please

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force a raise hand button.

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I don't see any hands.

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Hi Ivana. This is Alina I cannot raise
hand because in being host I cannot. But a

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Hello Jamie

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Hello is a when you mention the results
from btn

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taking calm to a these radiation DC, a
mentioning that has nothing to do with a

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string showing. Uh huh. What do you mean?
I mean, it's just to mean domain that has

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nothing to do with color reconnection.
Well, what I meant is there's a specific

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implementation of pythia

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from Christian beer Lucia.

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All that has strings shoving, and air,
they do see a long range Ridge because

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they're basically coupling the string
interactions to geometry. And I just

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wanted to make very clear that the effect
that we're seeing here is not the string

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shoving, implementing. Okay, it's really
from a different effect. Okay, yeah. Thank

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you. Thank you very much.

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Thank you, Elena. Do we have more
questions to James?

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Okay, I don't see any but I have 111

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where you are discussing the this model
for jet quenching in proton. So how do we

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how do they decide define sizes? So
perhaps you said that but somehow I missed

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it. What's the difference between a and b?
Yeah. So it's a rather old

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paper, I think they just are basically
making different assumptions that relate

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the multiplicity of the event to the size
and temperature of the DGP droplet. So

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they provided both predictions. I don't
know whether one could sort of think of

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that as a systematic uncertainty in their
picture on the size of the droplet.

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Okay, thank you. That explains.

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Okay, I don't see more questions to this
specific presentation. But right now,

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given that we have a discussion with all
speakers of the session, so first of all,

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I'd like to invite all speakers to be
available to answer questions. And of

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course, I'd like to invite all
participants to ask questions to either to

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specific speakers or to all of them. So
please, if you have a question to add to

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our speakers of the session, please push

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And I can button.

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Hello Ivana. This is la nafa radio. Yeah.
Hello, please go. Hello. I had a question

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for a Lexus. A concerning a Alex's. Can
you hear me? Yes. Yeah. A I like very much

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your talk. I had a question when you were
estimating the kinetic time

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a is proportional to the number of
creative particles divided by the

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transverse transverse area. My question
would be, how do you do you have an

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asymmetric Colosseum? Because your example
I think was different is life saving

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A your examples were for proton proton or
nucleus nucleus coalition.

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Yes. So in that case, it was

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of course a simplification, but we'll just
take a typical area of the system. So, in

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a symmetric collisions of Ah sorry, you
meant not peripheral, but yes, yes yeah,

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no, no no I mentioning a set for
collisions that a proton nucleus Yeah. So,

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you have to consider the mid rapidity.

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Particle multiplicities and where I mean
of course, lots of mismo participants will

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not not participate, you know. So, you can
think of P let collisions also as with

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some transfer Syria, of course, it's very
maybe crude approximation, but we didn't

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know

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Specific so the description I was

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talking about is well suited for, of
course, symmetric, large system

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collisions. It's just a way of how you
could, from that understanding try to make

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predictions. For smaller systems, it
doesn't have inherently a symmetric

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symmetry. So it's just more a question of
a, you know, once you have proton

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colliding on on on lead, can you also just
assume that the system has a small

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transfers area and just forget about the
rest of Yeah, so like, proton? A, okay.

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Yeah.

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Yeah. Okay. Okay. Thank you very much.
Thank you.

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Thank you. Do we have more questions?

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Okay, so maybe I will start actually, I
have a question to all experimental

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speakers.

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So, how low in drachma TP PCT do we see
collectivity in proton proton collisions?

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So what's your current understanding?

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So can we start with Zana? Elise from
Elise? I think actually, this was a

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question from, from your motivations like,
yeah, this is actually well from if you

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remember the blog that I showed for VM
coefficients, I can also share it again,

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but maybe you remember this points for BB
collisions where we see even the results

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for higher full harmonics actually speed
to six for example, staying there for up

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collisions up to the very down very low
multiplicity sexually the lowest

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multiplicity that is possible there MVP.
So what's the lowest multiplicity? I think

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it's around 10 tracks or so. And tracks.

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Yeah, okay.

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Can

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CMS? Yeah. A find that a very scary
question.

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Okay, could you comment?

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Yeah, I

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think I think my honest answer is that I
don't know. Hey, okay. A

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we can, you can. I'm confident above 100.
And then

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hey, yeah, you were going to do a
multivariate kind of look at this overall.

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And I, so I think it would be something to
combine. I mean, it would be nice to

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combine the results of all the experiments
and try to really nail this nail this

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down.

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I think that's a wonderful question. It's
just I just find it very hard.

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Maybe just a comment from the Atlas side
in the Atlas and CMS

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data down to the lowest multiplicities,
you know, or 10 or even lower are in

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agreement. But in the Atlas paper, they
extract the two that continues is pretty

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flat into that region. Whereas CMS
extracts a v2 that's basically really

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dropping off. And that's just based on two
different assumptions in the non flow

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subtraction method. And as far as I could,
so it's not an experimental difference.

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It's a difference in the assumption used
in the non flow subtraction. And as far as

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I know, no one has really figured out a
way to validate or invalidate those

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particular assumptions. And I think that's
a general problem at both Rick and the Lh

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see is when you have a very large non flow
contribution, you're very susceptible

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To the assumptions of the non flow
subtraction, if it's a pretty modest

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subtraction, then it's a small, systematic
and it doesn't matter so much. So I think

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that's still quite open.

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That's Jimmy's right? That's part of why
we were trying to do theism cumulants with

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with different sub event, but I certainly
think we probably need theoretical health.

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Because the data is very good. And it's
and it's not contradictory between the

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experiments.

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Okay, Alexa, has something to say. I'm
wondering if it's related to this

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question, or is it a separate question, a
different question. It's maybe related,

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but not exactly, um, is in order to
provide more maybe fear understanding. So

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what were challenges of, you know,
applying our typical models of heavy hand

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collisions into very small systems. Of
course, we have

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Breaking manual of assumptions in many of
these models. And there's one particular

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aspect that how we do conduct how do we
produce particles at a final state from a

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fluid is by doing a Cooper fry freeze out.
So, we do a

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particle ization and often it relies from
you know, the fact that system is

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sufficiently close to thermal equilibrium
that we know what kind of hardware

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distributions we should use. However, if a
system is really freezing out without

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reaching equilibrium in very unusual
traffic state, there is no general

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prescription how to convert such a matter
into into particles. However, what we

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could maybe be better at constraining is
what is the energy flow in respective law

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what is the conversion to matter, but that
will require knowing the flow in terms of

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energies than particles. So, my question
to experimental This is what is there any

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fundamental difference in our

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Calculating the particle of correlations
and Particle Flow or energy correlation

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and energy flow.

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So you I think that's a very good point
and he sounds like Mike Tannenbaum.

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They're always telling us to look at the
flow of energy. I think experiments could

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certainly do that, which is a combination
of energy. It's a good idea.

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I mean, I guess one thing is the the PT
cutoff of the experiments,

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if you want the total energy, and isotropy
is the limitation.

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But I think other than that, I don't see a
particular

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limitation. I think it's the opposite
because the energy is less sensitive to

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cut than particle yield, because you have
an extra power of PT

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Right, but what we measure, like I showed
in my talk, we're measuring, say v2 versus

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PT. So we show the v2 over the PT range
that we measure. But if you wanted a v2,

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integrated overall PT or integrated
overall energy, you have to integrate down

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to zero. So there's a region where all
experiments at some point don't measure.

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I don't know how large an effect that
would would be

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in terms of an uncertainty.

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Mitchell, I haven't seen such a such
measurement. Is it correct or

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it seems I'm not the first one to have
suggested? I think if you have the like

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from a least the particle spectra and the
v2 versus PT, for the different p IDs, you

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can calculate it from that this Yeah, you
should be able to just just

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I think it's implicit in those two.

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parabolas. Yeah.

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All right. Okay. Do we have do you do you
have other questions? I think there was a

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question from a Christian before. I'm
wondering if he still wants to

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ask his question.

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It was pretty much settled previously.
Thank you. Okay. Thank you.

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Anyone else wants to ask a question.

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Okay, there's a question from you.

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Instead of questions more like a common
rule will be discussed, which is very

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nice. And for the how low democracy we can
go to may flow collectivity. But I'm

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really afraid already for the ongoing
discussion forum for these templates

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versus the prefer subtraction which is
unclear as

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was already mentioned by Jimmy, but by I
varied is the method of the peripheral

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subject in itself, because, if you mean
this method already assume there's no flow

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at the lower symmetries of the region. So,
if you apply this method depends on how

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low and you treat as no flow region and
then you will not get flow by default. So

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for CMS, if you treat authenticity below
20 as lowest democracy division and you

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don't expect any flow you will get zero.
So I am afraid the conclusion might

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depends on the method you are applying.

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Yes, we're definitely trying to go beyond
that.

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With with

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Essentially multiple observables

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as a function multiplicity

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we will we will keep trying to, to hack at
this, look at this.

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But I agree, you kind of just say that
there's no flow by default to you. It's

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better to

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measure up measure all your variables as a
function of multiplicity and then look at

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the total picture.

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Okay, thank you any federal question or
input to the discussion, comment?

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Okay, I don't see any raised hands. So I
think it's time to conclude. Okay, so I'd

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like to thank you

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speakers for inspiring talks given in this
session, and I'd like to thank everyone

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who has contributed to our discussions.
Also, I'd like to remind you that

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tomorrow there is dedicated john session
between the heavier loads and the QC D on

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initial state nuclear PDFs and other
topics. So if you are interested, please

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00:31:25,499 --> 00:31:34,379
join Friday at 1230 same time. Okay, I
think we are done. So thank you everyone,

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and the session is closed. Thank you to
the organizers. Thank you organizers

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00:31:39,629 --> 00:31:42,419
Alison Lena aunty Thank you much