In my house, talking about recent results on call activity and small systems from
the Atlas collaboration.
And let's see if we'll go forward. There we go. So there are a wealth of Atlas
results on small systems and I listed a number of the recent ones down below. But
in these 15 minutes, I want to focus on just one recent measurement. And that's
the transverse momentum and process dependent azimuth and I saw trapeze in p
lead collisions published recently in this article, and that's the thesis work of
Kurt Hill which you can read in full detail linked here.
So key features of heavy ion Coalition's we often think of the Hydra dynamically
described collectivity of the bulk, you can argue about which PT separates that
region, but you might think sort of below five six gv and the jet quenching
phenomenon, the suppression of high Pt parkins jets and hey drones in the high PT
region.
Maybe above 10 to 12 GDP. And if you think of the hydrodynamics, you have the initial
state where you have steeper density gradients along the x axis in this diagram
on the left. And so, due to the steeper density gradients, when you couple that
with hydrodynamics, you get more bulk particles along the x axis. In the case of
jet quenching, you have that same geometry. But in this case, these high PT
part cons and the resulting jets have a shorter path out of the medium along the x
axis. And so, you expect more jets and protons at high PT to survive on the x
axis and to be more suppressed in vertical or Y axis. And so that means that due to
these two different phenomena, low PT bulk particles are asked to be fully correlated
with high PT jet particles because they both result from the same underlying
geometry that impacts them
And that's exactly what's seen in lead lead collisions. Here is the vn for
moments two through seven and the different colors versus transverse
momentum. And in sort of the low PT region that's shaded in blue might think of this
as being from hydrodynamics. In high PT region, you still see a significant v2,
that's often interpreted as being again from jet quenching. Again, remember, it's
a correlation between high PT and low PT is the way these VMs are measured. So just
like I discussed in the last slide, and then in purple, this is some transition
region where you have a mixture of these effects. And in p led Coalition's are the
same plot over here. And as discussed in the previous talks, there's sort of
similar anisotropy is in the low PT region in blue, maybe that's hydrodynamics at
high PT from previous Atlas publication is really just
one data point in this higher PT region. And maybe that's a hint about this
question of jet quenching or some other phenomena and a large region of sort of
purple in the middle. And so the goal was, how do we fill out that high PT region in
led collections.
And so this Atlas analysis uses the 2016 p loaded ATV data, we're going to use three
different samples, I'm only going to show two of them in this talk, but all three
are in the paper, there's a minimum bias triggered sample and there's a sample
triggered by events that have jet with PT greater than 100 Gp, and I'll skip the
detector details since they're pretty familiar. So what we're doing is a p led
to particle correlation. And what's shown on the left is the two particle
correlation and high multiplicity p light events in the case where it's the minimum
bias event trigger and a standard
template method is used to remove the non flow, and you see the dominance of the v2
coefficient in the lower panel. However, if you look at events where there's a jet
greater than 100, gv, you see that the non flow contribution is a lot bigger, right?
sort of imagine, here's the blue flow coefficient potentially. And there's this
very large jet correlation, which is expected because these are events from
jets. And in fact, you can see that basically, there's a distortion here in
the template subtracting procedure, because effectively the signal to
background is so small. And so in order to do this kind of measurement reliably, we
need to reduce that non flow background. And so the technique that's used is
restricting associated particles in the events that have 100 gV jets. So we do the
standard thing of the associated particle must be
Have a delta theta greater than two from the trigger particle, as we've heard in
previous talks. But now In addition, we're going to restrict the associated particle
to be delta Ada greater than one relative to all jets greater than 15. gv in the
event. So the trigger particle can come from a jet. That's what you might expect
that high PT. But the associated particle, we want to reduce its contribution coming
from jets. And that was has a large enough acceptance that even an event with
multiple jets, there's still quite a bit of acceptance to get statistics for these
correlations.
And so this just displays how this works. The left plot is the one that I already
showed you a couple slides ago. These are p led high multiplicity events with 100 gV
jet, so that has a low flow signal to jet background, this big peak here, and then
after we apply this jet restriction
So we try to eliminate associated particles that are in jet cones, you see
that a dramatic improvement in the signal to background. And we now have what looks
like a pretty successful template fit to basically remove the non flow. And so the
improvement in the signal to background really reduces the sensitivity to the
template method assumptions. And that's a key in this measurement.
And so here are the results. And so we're plotting v2 as a function of P t of the
associated particle. And there are two different sets of points. So the black
points are in minimum bias events where we do not have to apply this jet rejection.
And we see this familiar pattern where the v2 is rising up.
It's then falling and now we have enough statistics to actually have a couple of
points at PT great
Than 10, gv that seems to confirm a v2 of order a couple percent even in that
region. And then what's really dramatically different is now we can look
in these events with a jet trigger greater than 100, gv. And of course, this trigger
sample, the full p lead, luminosity delivered. And so we get the rise the fall
and now we have very, very good statistics all the way out to 50 gV in PT. And we see
that that two to 3% v2 remains up to the highest pts that we measure. And so, in
the blue region, you can think again of the low PT bulk we can confirm the rising
v2.
Now in the high PT region, getting all the way up to 50. We really see that there is
a correlation and acid myth between the low PT bulk particles and the high p TP
articles and in a Coalition's This is interpreted in terms of jet quenching.
And then there's a transition region in purple in the middle. And there's a
difference between the two trigger samples, which I'm not going to have time
to talk about in this talk. But if people have questions, you can take a look in the
paper or ask at the end.
One thing that's quite striking is if you overlay the v two results versus PT with
previously published Atlas led led data with the P led scale by a single factor of
1.5, they actually look quite similar. And again, I think there's a lot of
speculation that there's a common hydrodynamic explanation for sort of the
lower PT. And I guess the question is, is that our common explanation for the high
PT correlation?
You know, in the last few minutes, talk about a couple of different pictures. So
what if we think of it as jet quenching in pea lead? Well, in fact, this is
Some old paper from Zhang and Lau, where they model a small q GP droplet in p led
Coalition's and they actually calculated v2 and the two different dashed curves are
two different size q GP droplets. And what you see is that actually, their
description of the P lead v2 is qualitatively not so bad, but in the same
paper in the same model. They also predict in the right hand plot the RP lead the
nuclear suppression factor and P lead and they predicted nuclear suppression factor
of almost a factor of point five at low PT rising very slowly up to 50 gV, but still
being a 30% suppression. And even though there are, you know, challenges in
measuring RP lead in central events, their results from Atlas and Elise, basically
strongly disfavor that level.
hole of suppression. And so just like in a collisions, you need a pretty large
modification to the PT spectra in order to generate a differential jet quenching. And
so this cannot describe both azimuthal and isotropy. And the lack of change in PT
spectra. And that's a pretty fundamental challenge, I think, to any jet quenching
model, not just this specific one.
The other interpretation I want to walk through which was discussed in the paper
is the initial state effect model. And I want to remind people that a key check for
the original pee pee rich correlation observed by CMS was checking in pythia.
And so here are a couple plots from the original sort of seminal CMS paper. The
left was the pee pee rich in real data, and the right was the result from pythia
where no Ridge was seen.
But pythia sort of has a revival.
ENTJ In fact, in the left plot, this is pythia, eight in soft to CD non
diffractive mode. And as CMS observed in their paper, there's no long range ridge.
But actually, if you run pythia, eight in Hart q CD mode with a PT hat men have 15,
gv, and then basically requiring a jet in the event as we're sort of doing in our
analysis. There's actually a long range rich in pythia. And actually, we've traced
that down to an implementation and pythia of initial state radiation in space shower
routine, and confirm that with the author's it's a radiation pattern from
color coherence considerations. And just to emphasize, since some of you will be
familiar, this is not the string shoving option described in this paper. This is
really not due to interaction. It's due to a coherence effect. However,
The effect is biggest in low multiplicity events, and is watered down in high
multiplicity events. It's a correlation amongst only a subset of the particles
from this radiation pattern. And so in fact, what happens is kind of interesting.
If you run the pythia. In this case, it's argued inter. So it's p lead. If you
actually run it through our full analysis procedure, what you get is there's
actually a near side.
Rich that comes out, but because it's stronger in low multiplicity events than
in high multiplicity, when you apply the template subtraction procedure, it
actually predicts a negative correlation and high multiplicity events. So of
course, the V to two predicted due to this long range rich and pythia actually is
negative. So we plotted this v two two because normally you
You'd have to take a square root, so you'd get an imaginary vi to me. I mean, it's,
yep, thank you. And I think this is a problem with any explanation in the
initial state that involves lowercase n less than n total particles, which is that
as you go to higher and higher multiplicity events, this kind of initial
state effect gets watered down gets smaller and smaller. And that's the
opposite of what we see in experimental data. So sort of both of these theoretical
approaches face major challenges in describing experimental data.
And so just sort of on my last slide here, want to show the result again, in p led
collisions in black and then orange for the events with a jet greater than 100. gv
overlaid with the lead lead events. And so this new Atlas results really extend
These v2 measurements up to very high PT. The paper also shows v3. It's similar
trend but statistically a little bit challenged at the highest PT. So I think
this sets out that there's a definitive correlation of high and low PT particles
and P lead events where low PTs and youth land isotropy is typically explained by
hydrodynamics and geometry. So that implies if you follow that logic that
these high PT particles are correlated with the geometry. But there are
fundamental conflicts with that explanation in terms of jet quenching, and
also in terms of initial state explanations. And so I think this is an
excellent puzzle to be solved in the future. Thank you for your attention.
Thank you, Jeremy, for this very interesting presentation. And yeah, so now
we have time for questions. if people have questions, please
force a raise hand button.
I don't see any hands.
Hi Ivana. This is Alina I cannot raise hand because in being host I cannot. But a
Hello Jamie
Hello is a when you mention the results from btn
taking calm to a these radiation DC, a mentioning that has nothing to do with a
string showing. Uh huh. What do you mean? I mean, it's just to mean domain that has
nothing to do with color reconnection. Well, what I meant is there's a specific
implementation of pythia
from Christian beer Lucia.
All that has strings shoving, and air, they do see a long range Ridge because
they're basically coupling the string interactions to geometry. And I just
wanted to make very clear that the effect that we're seeing here is not the string
shoving, implementing. Okay, it's really from a different effect. Okay, yeah. Thank
you. Thank you very much.
Thank you, Elena. Do we have more questions to James?
Okay, I don't see any but I have 111
where you are discussing the this model for jet quenching in proton. So how do we
how do they decide define sizes? So perhaps you said that but somehow I missed
it. What's the difference between a and b? Yeah. So it's a rather old
paper, I think they just are basically making different assumptions that relate
the multiplicity of the event to the size and temperature of the DGP droplet. So
they provided both predictions. I don't know whether one could sort of think of
that as a systematic uncertainty in their picture on the size of the droplet.
Okay, thank you. That explains.
Okay, I don't see more questions to this specific presentation. But right now,
given that we have a discussion with all speakers of the session, so first of all,
I'd like to invite all speakers to be available to answer questions. And of
course, I'd like to invite all participants to ask questions to either to
specific speakers or to all of them. So please, if you have a question to add to
our speakers of the session, please push
And I can button.
Hello Ivana. This is la nafa radio. Yeah. Hello, please go. Hello. I had a question
for a Lexus. A concerning a Alex's. Can you hear me? Yes. Yeah. A I like very much
your talk. I had a question when you were estimating the kinetic time
a is proportional to the number of creative particles divided by the
transverse transverse area. My question would be, how do you do you have an
asymmetric Colosseum? Because your example I think was different is life saving
A your examples were for proton proton or nucleus nucleus coalition.
Yes. So in that case, it was
of course a simplification, but we'll just take a typical area of the system. So, in
a symmetric collisions of Ah sorry, you meant not peripheral, but yes, yes yeah,
no, no no I mentioning a set for collisions that a proton nucleus Yeah. So,
you have to consider the mid rapidity.
Particle multiplicities and where I mean of course, lots of mismo participants will
not not participate, you know. So, you can think of P let collisions also as with
some transfer Syria, of course, it's very maybe crude approximation, but we didn't
know
Specific so the description I was
talking about is well suited for, of course, symmetric, large system
collisions. It's just a way of how you could, from that understanding try to make
predictions. For smaller systems, it doesn't have inherently a symmetric
symmetry. So it's just more a question of a, you know, once you have proton
colliding on on on lead, can you also just assume that the system has a small
transfers area and just forget about the rest of Yeah, so like, proton? A, okay.
Yeah.
Yeah. Okay. Okay. Thank you very much. Thank you.
Thank you. Do we have more questions?
Okay, so maybe I will start actually, I have a question to all experimental
speakers.
So, how low in drachma TP PCT do we see collectivity in proton proton collisions?
So what's your current understanding?
So can we start with Zana? Elise from Elise? I think actually, this was a
question from, from your motivations like, yeah, this is actually well from if you
remember the blog that I showed for VM coefficients, I can also share it again,
but maybe you remember this points for BB collisions where we see even the results
for higher full harmonics actually speed to six for example, staying there for up
collisions up to the very down very low multiplicity sexually the lowest
multiplicity that is possible there MVP. So what's the lowest multiplicity? I think
it's around 10 tracks or so. And tracks.
Yeah, okay.
Can
CMS? Yeah. A find that a very scary question.
Okay, could you comment?
Yeah, I
think I think my honest answer is that I don't know. Hey, okay. A
we can, you can. I'm confident above 100. And then
hey, yeah, you were going to do a multivariate kind of look at this overall.
And I, so I think it would be something to combine. I mean, it would be nice to
combine the results of all the experiments and try to really nail this nail this
down.
I think that's a wonderful question. It's just I just find it very hard.
Maybe just a comment from the Atlas side in the Atlas and CMS
data down to the lowest multiplicities, you know, or 10 or even lower are in
agreement. But in the Atlas paper, they extract the two that continues is pretty
flat into that region. Whereas CMS extracts a v2 that's basically really
dropping off. And that's just based on two different assumptions in the non flow
subtraction method. And as far as I could, so it's not an experimental difference.
It's a difference in the assumption used in the non flow subtraction. And as far as
I know, no one has really figured out a way to validate or invalidate those
particular assumptions. And I think that's a general problem at both Rick and the Lh
see is when you have a very large non flow contribution, you're very susceptible
To the assumptions of the non flow subtraction, if it's a pretty modest
subtraction, then it's a small, systematic and it doesn't matter so much. So I think
that's still quite open.
That's Jimmy's right? That's part of why we were trying to do theism cumulants with
with different sub event, but I certainly think we probably need theoretical health.
Because the data is very good. And it's and it's not contradictory between the
experiments.
Okay, Alexa, has something to say. I'm wondering if it's related to this
question, or is it a separate question, a different question. It's maybe related,
but not exactly, um, is in order to provide more maybe fear understanding. So
what were challenges of, you know, applying our typical models of heavy hand
collisions into very small systems. Of course, we have
Breaking manual of assumptions in many of these models. And there's one particular
aspect that how we do conduct how do we produce particles at a final state from a
fluid is by doing a Cooper fry freeze out. So, we do a
particle ization and often it relies from you know, the fact that system is
sufficiently close to thermal equilibrium that we know what kind of hardware
distributions we should use. However, if a system is really freezing out without
reaching equilibrium in very unusual traffic state, there is no general
prescription how to convert such a matter into into particles. However, what we
could maybe be better at constraining is what is the energy flow in respective law
what is the conversion to matter, but that will require knowing the flow in terms of
energies than particles. So, my question to experimental This is what is there any
fundamental difference in our
Calculating the particle of correlations and Particle Flow or energy correlation
and energy flow.
So you I think that's a very good point and he sounds like Mike Tannenbaum.
They're always telling us to look at the flow of energy. I think experiments could
certainly do that, which is a combination of energy. It's a good idea.
I mean, I guess one thing is the the PT cutoff of the experiments,
if you want the total energy, and isotropy is the limitation.
But I think other than that, I don't see a particular
limitation. I think it's the opposite because the energy is less sensitive to
cut than particle yield, because you have an extra power of PT
Right, but what we measure, like I showed in my talk, we're measuring, say v2 versus
PT. So we show the v2 over the PT range that we measure. But if you wanted a v2,
integrated overall PT or integrated overall energy, you have to integrate down
to zero. So there's a region where all experiments at some point don't measure.
I don't know how large an effect that would would be
in terms of an uncertainty.
Mitchell, I haven't seen such a such measurement. Is it correct or
it seems I'm not the first one to have suggested? I think if you have the like
from a least the particle spectra and the v2 versus PT, for the different p IDs, you
can calculate it from that this Yeah, you should be able to just just
I think it's implicit in those two.
parabolas. Yeah.
All right. Okay. Do we have do you do you have other questions? I think there was a
question from a Christian before. I'm wondering if he still wants to
ask his question.
It was pretty much settled previously. Thank you. Okay. Thank you.
Anyone else wants to ask a question.
Okay, there's a question from you.
Instead of questions more like a common rule will be discussed, which is very
nice. And for the how low democracy we can go to may flow collectivity. But I'm
really afraid already for the ongoing discussion forum for these templates
versus the prefer subtraction which is unclear as
was already mentioned by Jimmy, but by I varied is the method of the peripheral
subject in itself, because, if you mean this method already assume there's no flow
at the lower symmetries of the region. So, if you apply this method depends on how
low and you treat as no flow region and then you will not get flow by default. So
for CMS, if you treat authenticity below 20 as lowest democracy division and you
don't expect any flow you will get zero. So I am afraid the conclusion might
depends on the method you are applying.
Yes, we're definitely trying to go beyond that.
With with
Essentially multiple observables
as a function multiplicity
we will we will keep trying to, to hack at this, look at this.
But I agree, you kind of just say that there's no flow by default to you. It's
better to
measure up measure all your variables as a function of multiplicity and then look at
the total picture.
Okay, thank you any federal question or input to the discussion, comment?
Okay, I don't see any raised hands. So I think it's time to conclude. Okay, so I'd
like to thank you
speakers for inspiring talks given in this session, and I'd like to thank everyone
who has contributed to our discussions. Also, I'd like to remind you that
tomorrow there is dedicated john session between the heavier loads and the QC D on
initial state nuclear PDFs and other topics. So if you are interested, please
join Friday at 1230 same time. Okay, I think we are done. So thank you everyone,
and the session is closed. Thank you to the organizers. Thank you organizers
Alison Lena aunty Thank you much