Which will be an overview of ALICE status from Andrea Dainese.
So whenever you're ready to share your screen, go ahead and you can start
whenever you like.
Hello, good afternoon. I'm sharing my screen.
So I tried to open my camera but didn't manage to so I flash here a picture of
myself so that you know who I am.
And could you please confirm that you can see my screen? Yes.
Okay. So good morning, everyone. It is my pleasure to present the highlights and
perspectives from ALICE. And of course, we thank the organizers for the
invitation. And I
also thank the collaboration for preparing so many
nice results that I can show you today. So we have in ALICE, more than 1000 signing
authors from over 39 countries and in terms of papers we reached around 300 today with
eight new papers and we have seven more in the pipeline. So in total 15 papers for
this conference and hard probes next week. The table shows our harvest from run
one and two. And of course, we are grateful to the LHC team for always
providing high success run. So I don't have to introduce too much our detector to
this audience I guess. We have a central barrel with tracking hadron electron
photon and jet reconstruction and the forward muon spectrometer and this is how
the central barrel looked like when we opened the magnet doors at the beginning of
LS two to start our major upgrade. Now, I will start now by presenting our
recent highlights, so I start from our core business so to speak, lead lead collisions
and the study of the deconfined phase of QCD matter, the quark gluon plasma, which forms
when the energy density is larger than oh point five GeV per fermi cube. So in LHC
in LHC lead lead collisions we reach well eyond that over a large volume.
So I will present the results following a bit the time evolution of the
collision. I start with a macroscopic property, vorticity, that we started
recently to study. So in case of non central collisions, the system has a large
angular momentum, and spin orbit interactions may polarize the quarks and
lead to a visible spin alignment for the vector mesons. And if you see here, in
red, we show an effect at low pT in non centralKcollisions for the k star which is
a spin one, and this is the deviation with respect to the one third the value of this
row 00 variable, which we find as a good cross check in pp collisions, and for the K
zero, which is spin one. Now, moving ahead, one of the characterizing features of the
QGP is of course color deconfinement and there the prime tool to study are
quarkonia, so, heavy quark bound states. So, at the LHC, we are in a regime in
which the melting due to color screening, color dissociation which is pictured
here
on the left is in part compensated by regeneration in case of charm because
there is a high density of charm quarks in the system. So, here I show our latest
results for the nuclear modification factor of the J/psi in central lead lead
collisions. So, this is the lead lead yield over the pp yield the scaled by the number
of binary nuclear nuclear collisions. So, at high pT you see we have a suppression
of a factor five which is described by models in terms of color screening or
energy loss of the J/psi. And then when we go to low pT, and this is something
that has been seen only at the LHC and not at lot lower energies, we see that the
suppression is is lifted, there is an increase. And this is the effect of
regeneration that I mentioned, it is described by models, and it is also
supported by the fact that that when we go to central rapidity, so from red to black,
we see more of that, and this is consistent with a higher density of
C quarks at central rapidity. So moving on parton interactions in the system are a
powerful tool to understand its properties. And also to understand how
parton dynamics works in a high density deconfined system. So there the main
tools are jets. I start by showing the jet quenching in terms of nuclear modification
factor of the jet yield, So how much is the jet yield
reduced
and this is again central lead lead collisions divided by proton proton. So in
the central plot I show our recently published result for R equal Oh point
two where we started from 40 GeV. And in the right hand side plot, I show a new
result where we explore machine learning techniques to separate the larger
underlying event energy fluctuations from the jet components. And this allows us to
go down to 40 GeV also for a larger resolution parameter of 0.4 with a caveat
that this may be somehow biased because we train on Pythia. But overall, we see a
reduction of a factor three to four of the jet yield down to 40 GeV, so this suggests
that the lost energy when we have medium induced gluon radiation is not
recovered in the jet cone. So, this radiation occurs predominantly at larger
angles. Now to study more the dynamics of jet interactions in the medium, we
have started a broad set of jet substructure measurements. Here I will
just show one example. So, we use the grooming technique to go and look for the
first hard splitting. And essentially we measure the angular
distribution between the two prongs. And in the figure you see at large angles
close to one we see a suppression in central dijet with respect to pp collisions,
which suggests that the jet core is more collimated in lead lead collisions with
respect to pp. Now, let me make a small QCD interlude where I will show a couple
of proton proton results. So, we use this substructure
technology so to speak,
to carry out measure many measurements. And in particular here I show how we
dissect the soft charm jets using the grooming technique again and we count
the number of hard splittings in the jet fragmentation with momentum radiation
larger than 10% and here in the figure so here we use jets tagged with the
D mesons with pT larger than five GeV and here we plot the number of these
splittings and we see that for charm jets this is shifted towards lower
splittings by about one unit with respect to inclusive jets. And this is
connected to the harder fragmentation of heavy with respect to light quarks and
gluons. In pp collisions we have also recently exposed for the first
time the dead cone effect in in for heavy quarks. So this effects is predicted to
reduce gluon radiation at small angles from heavy quarks and here we use iterative
declustering and the Lund plane analysis to compare the D0 jets and the inclusive
jets as you see here in the smaller plots. And when we translate this into a
measurement as a function of radiation angle, and we make a ratio of charm over
inclusive, we see a suppression at small angles which is what is predicted by by
the jet cone mechanisms and it is actually well described by Pythia for example. So the
jet cone in the case of lead lead collisions and medium induced gluon radiation has been
predicted almost 20 years ago to lead the to a reduction of energy loss from
b works with respect to c quarks, due to the larger mass, and now we have a
measurement that is sensitive to that. We measured the nuclear modification
factor of non prompt D0 mesons in blue compared to that of prompt
D0 mesons. So, they probe b and c quarks respectively. And we see that for,
for the case of b quarks there is less suppression, which is well described by
the models that implement radiative energy loss
with the dead cone effect.
Now, moving ahead with the evolution of the system, it is extremely interesting to
study the expansion dynamics, which is well described by hydrodynamic simulations and
also allow us to extract characterizing processes of the QGP. So, here it is
useful to consider non central collisions, in which there is an elongated
geometry from the beginning. And then during the expansion via pressure
gradients, this maps into an azimuthal modulation in momentum space that we
analyzed with Fourier decomposition and the second coefficient, the v2,
is called elliptic flow, it quantifies the dominant elliptic modulation. So, here
you see our latest results on the left for six different hadron species,
we see at low momentum circled in yellow an ordering the mass, and that if you look
on the right we even see when we take light nuclear with A equals two and A equals
three that we measured recently. So, this suggests that particles participate in the
collective expansion. There is a common flow velocity that imparts to hadrons
an additional momentum, a momentum shift, which is proportional to their
mass. Now instead at intermediate momenta here in green, we see a clear
separation between baryons and mesons. And this suggests that in this
momentum range, the flow is carried by quarks and it is then propagated to
hadrons via an hadronisation process, at least in part by recombination similar
to what I mentioned for the J/psi. So, now these very high precise data are used
by theory groups with sophisticated statistical techniques like Bayesian
procedures to really extract the properties of the system. And this, for
example, uses the hydrodynamic simulation that I showed at the beginning. So,
here I focus on one of the really characterising properties of the QGP, the
shear viscosity versus temperature. And you see it is now determined with the
precision of 20% and it is found to be 10 times
smaller than for any other form of matter.
So, today we put out our final full run two results for also flow measurements
involving heavy works, both charm and beauty. So, these are compiled in this in this
figure, that you see, and we see that D mesons, in orange, and J/psi, in red,
exhibit a large flow.
They are, they are ordered and both are both
both are lower than that of pions in in black. So, this is consistent with a
scenario which is implemented in model calculations that describe the data in
which both light and charm quarks participate in the expansion of the system,
they carry some some flow, and that by the charm quarks to a smaller extent than
light quarks. And then at least in this momentum range they form these hadrons
via recombination. So we also see that B mesons are sensitive to the
elongated shape of the system. These are the purple points, while the very heavy
Upsilon 1S doesn't show a significant flow signal in blue. And this
is consistent with its very large mass and also with a small regeneration probability
in case of beauty. Now, in the second part of the highlights, I will show a selection
of results that give you an idea of the very comprehensive QCD program that we
have beyond QGP studies. So this really sparked with LHC surprises and
new findings and especially with the high luminosity pp and p lead samples of run two
this this these measurements have been developed very significantly. So, I
start from small systems and it is already well known since some time that we
see in pp and p lead collisions at high particle multiplicity effects that look
like what we see in lead lead collisions. So, here I show some updates. On the left,
you see
the strangeness over pion
plot as a function of multiplicity which we published for pp collisions in Nature
Physics in 2017. Now, it contains four different colliding systems including
xenon xenon, and also different center of mass energies and we see a
smooth path across the systems. So it seems to be driven by multiplicity and
also independent of square root of s and there is a steeper rise in within the small
systems for increased strangeness content and at high multiplicity in small
systems values similar to those in lead lead are reached. On the right, I show
also a plot that we published last year in which we compiled elliptic flow
measurements in the four colliding systems that we had. And also in this case, we see
in high multiplicity pp and p lead collisions, clear elliptic flow signal, so v2 larger
than zero, and similar to lead lead at the same multiplicity. So now, here I show a new
result that we have just
prepared for this conference,
which is related to elliptic flow in pp collisions, we have measured it over the
maximum possible delta eta separation. So, we use hadron pairs separated by six units in
in eta, as you see in the sketch. And in high multiplicity collisions we
clearly see up to this high separation,
the so called near side,
near side ridge, which is a characteristic of this flow modulation. And we see it up to
this delta eta which supports the idea that this is truly collective correlations between
between hadrones and when we represent the v2 parameter for different colliding
systems as you see here, on the right at similar multiplicity, we find that that it
has a similar magnitude. Now, in pp collisions, it is also interesting to look
for low mass dielectron production and this we have done using our samples
that we collected with lower magnetic field in the central barrel to have higher
acceptance at very low momentum. And we see a hint of an excess here over the
hadron cocktail at low mass and low pT. And this is quite interesting because it
is reminiscent of an effect an excess that was seen for real and virtual photons in
the 80s. And also interestingly this cannot be described by calculations either
using hadronic bremsstrahlung or even radiation from a mini QGP. So this
clearly deserves some more investigation.
Andrea, just slightly less than three minutes. Thank you.
So, in proton proton we have also several measurements concerning hadronization of
charm. Here I show baryon over meson ratios. So, on the left the lambda c was
already shown to be increased with respect to e+ e- expectations and
now we see that this increase depends on multiplicity in pp. And we also measure
the sigma c and xi c which also exhibit large increases. And for example, the
sigma c is not described by any tune of Pythia even with the color reconnection. Now,
the LHC is not only a nucleus collider but also an anti nuclear nuclear factory. We
measured them by using our TPC and time of flight. The production mechanism is not
yet
fully understood.
It can be described in terms of nucleon coalescence or statistical
hadronisatino. But today I want to focus on an aspect that is
a strong implication on the dark matter searches
in space, for example, from a neutralino annihilation to anti deuteron anti helium
three. Now these searches are affected by large uncertainties due to the limited
knowledge of the ordinary hadron production by cosmic, so anti nuclei, and also their
absorption in space. And so today we provide new measurements that give
insights in both directions. On the left, you see our deuteron over proton ratio as a
function of multiplicity and we just published the data in yellow, pp at 13 TeV.
So, this clearly constraints the production models. And on the right, we
measured for the first time at low momentum, the anti deuteron absorption
cross section in the ALICE material. And this is very interesting because at
low momentum this is larger than the state of the art modelling. So the last result
that I want to highlight is a study of QCD interactionos among hadrons using
essentially proton hyperon pairs, and their momentum correlation is sensitive to the
strong interaction
potential. This is a field that, an
interaction potential that is not well measured and high statistics data from the LHC
are really a breakthrough. So, here we present the proton xi minus and proton
omega minus correlation, which now precisely show an attractive strong
interaction in addition to the Coulomb interaction. So, in the last couple of
minutes, I will show the perspectives, we are preparing for run three and run four.
Our major upgrade is taking shape, it will be discussed in detail later by
Piotr. So, overall, we aim at improving the tracking resolution at low pT and
increasing by a factor of 50 the readout capabilities. So the impact of the
COVID-19 at present is quantified into a delay of 4.5 months, which means that we
will be ready for begin commissioning at the end of August of next year. So here I
just flash a couple of major highlights. The TPC is already instrumented with
GEM chambers, and, while the old ITS is serving in the ALICE exhibition, the new
one together with the muon forward tracker fully made of pixels are ready for
installation. So we are also proposing upgrades for LS three. On the left, a
forward electromagnetic calorimeter with high granularity readout to
measure photons in p lead, down to 10 to the minus five in x which are sensitive
to the gluon density in a region where it is essentially unconstrained and where
gluon saturation may set in, and at central rapidity, we are considering and proposing
to replace the inner barrel of the ITS with three truly cylindrical maps layers
with much better performance. And you see we are already testing the curved
sensors. Now pushing farther this idea of ultra thin filling silicon detectors we
are preparing a proposal for a silicon only, so to speak, heavy ion experiment for
beyond run four, which would be fast, ultra thin and provide essentially
ultimate performance for several probes like multi heavy flavor, thermal radiation, and
soft, very soft hadron production. And those who could exploit the higher LHC
luminosity that the LHC could be provided with nuclei lighter than lead.
So to conclude we have a large number of results based on full run two samples.
Not only they provide insight on the QGP workings and properties but also on a much
broader QCD research program. Our major upgrade is on track to be ready for pp
collisions at the end of August next year. And we are getting up for LS three
upgrades and planning for run five and beyond. So in thanking you I inviting you
to enjoy the 29 talks and posters with ALICE presenters that we have at the
conference. Thank you.
Okay, thank you, Andrea.
This nice summary. And we have a question.
Marek. Hold on.
I can allow. Okay, Marek should be able to unmute.
Ah, so two questions. One is, is there any data about pT, transverse momentum
dependence, of the anti deuteron production? This is very relevant for some
issues regarding exotic hadrons where the deuteron is a well understood system.
Yes, yes, we have also measurement of transverse momentum spectra of deuteron, and
anti deuteron and even more light nuclei. So, I don't have backup figures here, but
you can find them in our papers, for example the one that I linked here.
Okay, thank you. And the second question is, you mentioned measuring interactions
between,
interactions between hadrons
by measuring correlations.
Can you look at correlations between protons and sigma c baryons?
Well not with the statistics that we have from run two, we we have been able to
measure sigma c production but likely there is not enough statistics to study
correlations, but definitely this is something that we could consider
for run three.
It's highly relevant for understanding the nature of the hidden charm pentaquark
that was discovered by LHCb.
Okay, thank you. Thank you. Thank you for the suggestion. We will, we will look into
it.
Thank you. Okay.
So I think if there are no other questions, we can thank Andrea again. Thank
you. Thank you