Yes, hello, can you see my slides?
Yes, I can see them and I can hear you very well.
It's full screen, yes, please start. Right. Hi, everyone.
It's my pleasure to talk about the recent results on collectivity in small
systems on behalf of the ALICE Collaboration. Let me start from a short reminder on
collective effects. So as you know, the main goal of ALICE experiment is to study
the properties of quark-gluon plasma, or QGP, which is created in heavy-ion collisions.
And so far we've been quite successful in describing the QGP evolution by
hydrodynamics, according to which, as you can see in the sketch on the right,
QGP expansion is driven by pressure gradients that push matter outwards with
collective transverse velocity that we call radial flow. And this is the first effect
that we distinguish when talking about collective effects. Now, the second
one, anisotropic flow, is caused by the anisotropy in the initial geometry of the collision,
which is transformed via low-viscosity QGP into the anisotropy in the
momentum distribution of the final state particles. So these collective effects
are reflected in the measurements of our observables. For example, in the angular
correlation function of the particle pairs that you can see here for lead-lead collisions,
where we observe a famous near-side ridge structure as well as the modulation on
the away side that are attributed to the anisotropic flow. Now, we've been also looking
into the small systems, proton-nucleus and proton-proton collisions, where QGP was
not expected to form, in order to disentangle the cold nuclear matter
effects from QGP-related effects. However, recent observations of collectivity
signs also in high-multiplicity pp and p-Pb collisions led to the change of
this paradigm and raised new questions for us, such as are these observations a
manifestation of collectivity, where the working experimental definition of the
collectivity that we use is "long-range multi-particle correlations", and I will
come back to this later in my slides. If yes, what is the origin of these effects
in small systems and down to which multiplicity do we see it. So, so far we
have few theoretical approaches that try to explain where these effects can come
from, first of them being based on initial state effects, and arguing that collectivity
is present already in the very early stages of the collisions and is
given by initial momentum correlations, at nucleonic or sub-nucleonic level.
The second one is based on the final state effects and states that the collectivity
builds up later in the collision and is given either by the QGP formation or some
other microscopic mechanisms such as string recombination. There are also
hybrid models that combine both of these stages. But so far we don't have a clear
answer on what kind of combination can give a full description of our data in
small systems. So, as experimentalists, we can provide more measurements to constrain
these models and in this talk, I will cover the recent results from ALICE that can
help in this, first focusing on observables sensitive to the anisotropic flow and
then moving to the ones sensitive to the radial flow. So, let me first continue with
the studies of the ridge. Just to remind you, usually we investigate particle
correlations at mid-rapidity using central barrel detectors of ALICE, TPC and ITS.
However, in one of the recent analyses, we also looked at the forward and backward
ranges of the rapidity using forward multiplicity detector FMD. And this way,
one can explore the ridge evolution up to high delta eta values around eight, which is a
unique measurement at the LHC. Now in the bottom plots, you can see the results for
high-multiplicity p-Pb collisions on the left and high-multiplicity pp
collisions on the right, that show the per-trigger yield distributions corrected
for non-flow in both cases, given by FMD-FMD correlations. And, as you can see, the
ridge structure extends up to eight in delta eta in p-Pb case and six in pp
collisions. In order to get more of a quantitative feeling of these results, we
can also extract v2 coefficients as a function of eta, which you can see in
this plot for p-Pb collisions, for four multiplicity classes from zero to five and
up to 20 to 40%, where the data are compared to predictions from AMPT
model with string melting. So, as you can see, we observe an asymmetry in the data,
which is similar to the asymmetry in the charged particle multiplicity distribution
as a function of eta, which is expected due to the asymmetric system. And this
asymmetry seems to be quite well reproduced by AMPT while the multiplicity dependence
that is seen in the results seems to be not that well described by the model.
In this plot, you can see the similar result extracted very recently for pp
collisions as well, in very high multiplicity and with these three curves
you can see the results that correspond to the different non-flow subtraction
methods. But as you can see, the trend remains the same for all of them. Now, to
continue with the latest results on flow coefficients, last year ALICE has published
quite a comprehensive set of measurements not only of v2, but also higher flow
harmonics and in this plot, you can see the results for v2, v3 and v4 obtained
with two-particle cumulants as a function of multiplicity compared in different
systems, Pb-Pb, Xe-Xe, p-Pb and pp collisions and also to predictions from hydro model with
initial state for different systems and PYTHIA8 predictions for pp collisions. Now, the fact
that the data are obtained with the separation in eta means that we observe
long-range two-particle correlations in these results, and this covers the first
part of the definition of the collectivity that I mentioned in first slide. I will
come to the second part in a second. Let me first focus on a few important points
from this plot. So as you can see, we observe quite a pronounced multiplicity
dependence in all vn results for large systems, Pb-Pb and Xe-Xe, which is
however not seen in the small systems, but it's interesting to notice that the
values of all vn are compatible in small and large systems at low multiplicities.
It's also interesting to see that the pp data cannot be described solely by non-
flow effects, which are dominating in PYTHIA8, but it cannot be either described by
the hydro model which does, however, quite a good job for the Pb-Pb and Xe-Xe and is also
able to get qualitatively the data in p-Pb collisions. So, so far, we don't
have a good description of the pp data from the theory side for these results.
Now, coming back to the definition of the collectivity, in order to get this second
part, multi-particle correlations, one can look at the results with multi-particle
cumulants and here you can see v2 results with four, six and eight particle
cumulants. Again, for different systems as a function of multiplicity. What is
important here is that we were able to observe real value of v2{4} for the
first time in pp collisions, which was possible to extract with this correct
non-flow subtraction using the three- eta subevent method. And we also see
non-zero values not only of v2{4}, but also be v2{6} in both pp and p-Pb
collisions in these results, that allows to confirm that we observe not only a
long-range, but also multi-particle correlations. So indeed, this confirms the
observation of collectivity in small systems. However, it's hard to conclude if
the origin of these correlations is the same as in the hevay-ion collisions based only
on vn measurements still. So one can also look at other observables that are
sensitive to the radial flow, one of them being the balance function, which is given
as the combination of charge-dependent per-trigger yields as you can see here,
where the definition of the per- trigger yield can be seen on the
right. So, as you can see, the like-sign part of the correlation is subtracted,
which allows to keep only the charge- dependent part and this means that anisotropic
flow effects are removed, but radial flow effects are still present. On
the right, you can see the example of the two-dimensional balance function distribution
for Pb-Pb collisions in most central class. What we do is we project this on Delta
eta and delta phi and we look at the one- dimensional projections as a function of,
in different centrality or multiplicity classes, and we extract the width of these
projections as a function of multiplicity, so, sigma delta eta and sigma delta phi. Now,
for the purpose of this talk, I will focus only on Sigma Delta eta.
You can see the result of this observable plotted as a function of multiplicity for
Pb-Pb collisions here for charged particles in low-pT range. And what we observe is that we
see a narrowing of the balance function width with increasing multiplicity, which
was mainly attributed to the larger radial flow in central collisions with respect
to peripheral collisions in the Run1 analysis. Now, interestingly, we also
observed the same (qualitatively) behavior in small systems, pp and p-Pb collisions, which
is consistent with the idea of collective phenomena present in these systems as well.
The data in pp collisions were also compared to predictions from PYTHIA8 with
and without color connection. And keep in mind that in this plot, on the x axis,
we see the multiplicity class instead of the multiplicity so the trend is reversed.
Now, you can see here that the tune with color connection is able to get the
trend in the data while the tune without color connection fails to do so. And in
the new analysis these results (this measurement) is extended to the level of
identified hadrons where we expect to see more pronounced narrowing for heavier
particles, if this narrowing is driven by collective phenomena in small systems, due
to the more significant response of the heavier particles to the boost from
collective effects. In this plot, you can see the results, this time for pions and
protons, compared to predictions from PYTHIA8 with and without color
reconnection again, in pp collisions. And it's interesting to notice that the model
with color connection indeed predicts more pronounced narrowing for protons
than for pions, while in the data we see a different trend: while pions show clear
narrowing with increasing multiplicity, protons rather stay flat and this
suggests that there might be some other effects playing a role apart from
collective phenomena. And we can also say that for this particular observable, the
results disfavor color reconnection mechanism implemented in PYTHIA8 at the
level of identified hadrons. Another observable that one can look into
is the pT spectra of identified hadrons. In this plot, you can see the
latest result for p-Pb collisions at 8 TeV for pions, kaons and protons, and these
results, of course, complement the previous studies that ALICE has done for
p-Pb at lower energies and also for pp collisions. What we observe is the behavior that
is qualitatively similar to the one well known from Pb-Pb collisions, where it's
driven by the radial flow. So basically the hardening of pT spectra at higher
multiplicity with respect to lower multiplicity class, which is more
pronounced for heavier particles seen by this more significant flattening at low
pT. And, in order to study better the multiplicity dependence of these results
it's more convenient to use ratios of the particle yields, in particular baryon over
meson ratio. Last year, we published a comprehensive set of measurements for
these ratios in light flavor sector. Here you can see the results (the example) of
the proton over pion ratio compared in small systems and Pb-Pb collisions in two
multiplicity classes of zero to five and 60 to 80%. And if you look at the Pb-Pb results,
you can see an enhancement at intermediate pT which is more pronounced
for central collisions and this is again consistent with the radial flow idea, but
also with quark coalescence at hadronization. And the origin of this
enhancement is still debated...(Alright, thank you.) But it's important to notice that we see
a striking similarity in the trend in these ratios also in the small systems. However,
since these multiplicity classes correspond to very different multiplicities in these
systems, we can compare the values of these ratios and the same multiplicity in
specific pT bins and this is what is done in the bottom plot. You see the ratio
plotted as a function of multiplicity for the low pT range on the left,
intermediate pT range in the middle and high pT range on the right. So, what we
see is the smooth multiplicity evolution from pp up to Pb-Pb collisions of
these ratio seen in all pT ranges and these results suggest common mechanism
driving the multiplicity dependence. And it also further supports the idea of collective
effects present in small systems. We can also check what models have to say about
that: you can see the same ratio
here in the low pT and intermediate pT for pp collisions compared to predictions from
different models. It's easy to see that PYTHIA8 for example is successful in
describing the qualitative features of these data only if color connection is
enabled. HERWIG is not able to reproduce the trend, but this model is being
improved. DIPSY is able to get only the qualitative trend, but not the absolute
values and EPOS-LHC getst the data in low pT, but overestimates it in the intermediate
pT. So these results and also the previous results that I showed in the
slides before provide an important input for improving our models. And since this
brings me almost to the end of my talk, I would like to mention the talk of Livio that you
can check for more interesting details and results on a similar topic. And in the
last few seconds I would like to flesh also the recent results on these ratios in
heavy flavor sector. You can see a lambda_c over D0 ratio plotted on top of lambda
over K0S here for pp collisions for low and high multiplicity classes, and
what is seen here is that we observe a similar trend not only in light flavor
sector but also in heavy flavor sector seen, actually, to be very compatible between
each other at the same multiplicities, and this is a very interesting observation,
but for more details on that, such as the comparison to the models, for example, and
interpretation of this results, I refer you to the previous plenary and parallel
talks in a hard probes section. And this brings me to my conclusions. We see a
similar behavior attributed to collectivity in heavy-ion collisions in many
observables in small systems, which persists down to very low multiplicities.
But most of the times, existing models do not fully reproduce our data in small
systems. So it seems that understanding the origin of collectivity there still
remains a challenging task. Thank you very much.
Thank you Zhanna for this very nice presentation. So now we have time for
discussion. if people have questions, please push, rise hand button.
Okay. I don't see any
slack. Zhanna. Yes.
Could you explain what what this color reconnectivity. Color reconnection
you mean? Yes. Yeah, so this effect basically resembles the radial flow at
microscopic level, it's based on Lund string fragmentation model. So it's basically
strings combining together. and then, since this is somehow connected to the
amount of MPI (due to the fact that there is more MPI's in central
collisions, this leads also to the higher tension between the strings. And in the
end, this will kind of resemble the radial flow mechanism. So you will have more
correlation in the central collisions with respect to peripheral ones, for example,
I'm talking now about the particular example of this balance function results
that I showed here.
Okay, okay. That's that's really helpful. Thank you.
So, given that we are on this slide, I was wondering for the data point points, they
are basically in terms of the precision. This is driven by systematic
uncertainties, right. So what is the main contribution to the systematic uncertainty
on this data points? Let me think.
I'm not sure, now I don't remember this off the top of my head, what was the main
contribution here, because this is the previous analysis that we have done, but
I check this later and let you know maybe offline.
Yeah, I think that's reasonable. Okay. Thank you. I don't see any other
questions. So thank you Zhanna. So now we will switch to a discussion of small
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