Heavy-Ion Physics

[00:00:02.11] – Ivan Kisel
Yes. Yes, Aihong, please start. You can share the screen and start, please. Yes, something like that. Yes. If you make full screen, please. Yes, very good. Yes.

[00:00:53.10] – Aihong Tang
What about now? Can you hear me now? Sorry, I forgot to on our microphone. Yeah, first of all, I would like to thank you, Professor Ivan Kisel, for inviting us. I really appreciate the hospitality. And upon a golden year, I also’m very pleased to learn that the Frankfurt group has evolved from doing the pattern recognition and tracking and now involved into more complicated parking, recognition of AI and go beyond even physics. I think that this is really nice to see this expanding. I was asked to give this lecture, and I was told that some of us has a background in computer science, and not everybody has background in physics. I arranged my in such a way that I hope everybody can follow. But for those of you who do have a physics background, you may find that my material may be oversimplified for you. But if it’s that case, please bear with me and we can always discuss offline if you have questions. Okay, so let me start.

[00:02:25.11] – Aihong Tang
The introduction of here, we are in physics. Let’s see. Let’s start with the building block of the universe. Let’s start with our body. We know our human body, maybe 60 or 70% are from the water. You have the water molecule. Then if you are looking deeper inside the water molecule, you see this is hydrogen and oxygen autum. And if you magnify the autum, you see the nucleus. This is by a few orders of magnitude, this 10 to the minus 15 to 10 to the 14 meters. So you see the nucleus is made of protons and the neutrons, and you will further magnify, we call them nucleons. You will see quarks and the gluance inside the gluance, and this is a dynamic picture, and they are fluctuating around. They have this confinement, confined within what is called a hadron. Now, we say that hadron, we imply those are boundaries. This boundary actually can be broken, just like you will have an ice, and when you increase the temperature, you will get water, and you continue to heat the water, you got to steam. That’s the same substance, but reveal itself in different phases. Similarly, for normal hydrons, if we put either the pressure or you hit it, you will see the boundary starts to resolve and the quarks and the blue-ons are set free and they’re really rolling around forming what is called the cropped long pass.

[00:04:33.21] – Aihong Tang
I would like to use this very nice video. I think I would like to show you this video to be your some introduction or about the face boundary and the cropped long pass. Let me play this video, I hope. Can we give the sound? No.

[00:04:56.23] – Aihong Tang
Can we For example, water can be in a solid, liquid, or gaseous state. Heating or cooling liquid, you can get solid matter, ice, or gaseous matter, steam. The transition of matter from one state to another is called a phase transition. In order for a phase transition to occur, it is necessary to change the temperature of a substance, its internal energy. But under these transformations, water molecules themselves do not change. From the school course of physics, you know that the boiling point of water, that is the temperature of the phase transition, depends on pressure. For example, the water in a pot on the mountaintop will boil at a temperature lower than 100 degrees celsius. The dependence of the state of matter on pressure and temperature is usually illustrated by graphs called phase diagrams. The phase diagram of water shows that at the atmospheric pressure, typical at the surface of the Earth, we can first melt the ice and turn it into a liquid, and then heat the liquid to its boiling point and turn it into steam. However, on Mars, ice will not melt with rising temperature, but immediately turn into steam. In addition to the gaseous liquid and solid state of matter, there is one more state, plasma.

[00:06:33.20] – Aihong Tang
The ordinary substance consists of atoms and molecules. High pressures and temperatures destroy molecules, and atoms lose external electrons. This produces plasma consisting of negatively charged electrons and positively charged atomic nuclei. For example, our sun is a substance in a state of plasma. At even higher the pressures and temperatures that existed in the early universe, corks and gluons, of which photons and neutrons now consist, were in a state that is commonly called cork-luon plasma. At present, cork-luon plasma may exist in the center of neutron stars and black holes. The process that results in formation of protons and neutrons from quirks and gluons, as well as the process of transition of protons and neutrons back to forkluon plasma, is called a phase transition. Scientists believe that the conditions necessary for the creation of forkluon plasma can be reached by colliding nuclei moving with a speed close to the speed of light and possessing energies of tens and hundreds of billions of electron volts. In order to obtain such nuclear collisions, unique physical setups, colliders, are constructed.

[00:08:06.08] – Aihong Tang
All right, let me continue. Can you hear me fine? Okay. On this slide, I show this nice video, but this is also supported by theoretical calculations, in particular the QCD, that’s the theory for describing the strong interaction. They have this analysis calculation. They shown the energy density as a function of temperature. When you see after you increase the temperature, suddenly you can pump into a lot of energy into the system. You see this jump at one. What does it mean? It means that you have more of freedom to accommodate those additional kinetic energy. That means you are reaching a different level of degree of freedom. In this case, the clock level and the ground level degree of freedom is at a plate. This is shown by theoretical calculation. And from this, we learn that the critical temperature for it to happen is around 165 MeV. In Also we know the critical density for this to happen is around one jump per femicule. And here we use this energy in terms of the weight or whatever, so to express the density, similarly for temperature. Now, this is a little brief history of how the story started.

[00:09:59.02] – Aihong Tang
Actually, the The story started in 1974. There was a workshop on Jeff and nuclear collisions of heavy ions. At this workshop, Chen Da Li, the Nobel Prize winner in 1957, he proposed that you can investigate some bulk of phenomena by distributing high energy over a relatively large volume. It started from this workshop, the communities started wanted to put in serious effort to try to make this happen. Then in 1984, the SPS started. It ended in 2003. And in 1986, the AGS at the Royal Kevin in In long-run in the United States. It started, it ran for decades and more, and it ended in 2000. Then it was followed by Rick in The RIEC program is expected to end next year. And in 2010, the LHTC started. There are also other experiments that are in preparation, like a file experiment and the NECA. Of course, you are familiar with file. Then also Nicar program. I would like to have a slide to pay tribute to TD Nee. As some of you know that he passed away just last week. He’s really one of the significant founder of this field. He had a tremendous influence of this heavy on conditions, his direction Also, he shaped the development of the Bohkai Lin Lab.

[00:11:50.08] – Aihong Tang
I think his contribution will be memorized. Why we’re starting heavy energy conditions? We want to answer one big question. So how did the universe started and then evolved? We know that the universe started with a big band. And at the beginning of the big band, it is widely expected that the system is in the quark-rump plasma state. And then the system pushed down and it started to form a hard runs. And then from there, you form the class of clouds and you form a galaxy. And at the it involved into a universe as we observed today. Well, we cannot play with the big band because we have only one big band. We cannot control it. We can only possibly observe it and we We could not even play with it. But the little band, the relativistic heavy-eye collisions, we can play with it. It’s a controlled experiment. From there, the environment achieved in little band is very to the environment at the beginning of the universe. In the little band, you also start with a collision and a tremendous amount of energy is deposit, you concentrate in the tiny volume Then in the form of quark-gluan plasma, it has gone QGP phase.

[00:13:21.04] – Aihong Tang
Then as the system continue to evolve, it will cool down and quark and the gluan start to haggernize. At the end, it will form all bunch of particles and freeze out and reach our detectors. But through this process, we mimic the beginning of the universe and see how either the universe or the particle is dissolved in the QGP and the hadronize. So we can learn and study the universe. The other mission of And the second way I’m curious is to study the QCD phase diagram. You already see the video and we know that when you heat or press hadrons, it will form a quark-rump plasma. But under what condition that happens, that was a big question for us to answer. So Rick and the LHC are part of that effort. We can increase the energy, we can change the system size, and we select a down in lobs, and by Doing that, we see how the system respond to those downing, and we can learn maybe there’s a face boundary somewhere and we can try to locate the face boundary. Then the other mission of heavy iron coalitions is to study the dynamical properties of QCT matter.

[00:14:50.20] – Aihong Tang
By now we know that the substance that we created at both LHC and the rig, that the hottest matter. I will talk more about this. We also know that the matter that we created is close to perfect liquid. It has the least viscosity in the world. Recently, we We’re going to learn that this substance is spinning ultra fast. It has most vertical multiplicity. We also know that this matter is strongly fluctuating, is in a strongly fluctuating the environment. So this is good relative. It’s heavy on conditions in the ideal test run for QCD. And you can see what you would expect and those extreme conditions and compare those to its QCD expectations. The heavy on experiments, now you know the two large facilities currently in operation. As we’re speaking, the Wrig is running. I believe LHC is also running. At WIC, this is situated in Long Island. It’s about maybe 60 miles east of the New York City, for Kevin National Laboratory. It used to have two large-scale experiment. One is called Phoenix, the other one is called Star. The Phoenix experiment is stopped operation a few years ago, but now it’s upgraded to a new experiment.

[00:16:30.21] – Aihong Tang
It’s called S. Phoenix, and the star is also still currently in operation. At LGC, you see, the ring is bigger. It’s much, much bigger than ring. The Hevian experiment as these two big facilities, I already said, it’s Phoenix and the star. Here I show a cartoon for star. The dedicated Hevian experiment at LGC is called ALICE. So again, at those experiments, the time evolution of the lead biome is common. You start with the initial collision and then you have a pre-collision state. Then it’s gone through plasma transformation, then freeze out and arrive at the detector as we see. To give you an example, so how have you an experiment operates? And if you haven’t been to rig or Now, as you see, I would like to give you a snapshot of everything. Let’s see.

[00:17:38.05] – Speaker
The night sky. And I would wonder. Where I grew up in the mountains outside of Portland, Oregon, I could look up in the sky and see thousands and thousands of stars in the night sky. And I would wonder, where did all this come from? I find it amazing that I can rewind the clock and go back and look at what existed in the universe before any of those stars were even created. We can recreate the conditions of the early universe, and we can study the force that holds together that matter, as well as all the matter that exists in the visible universe today. I’m Paul Sorenson. We’re in the main control room of the relativistic heavy ion collider, an atom smasher at Brookhaven National Lab. From this room, we steer ions around a 2.5-mile circular track and smash them together at nearly the speed of light. When we glide the islands, it creates a fireball that only exists for one billionth of one billionth of one millionth of a second, a very, very brief amount of time. The fireball is only one billionth of one millionth of a meter across. This is a very short-lived and a very small speck of matter.

[00:18:49.08] – Speaker
But with the detectors that we have, we can look at all of the remnants that come flying out from those collisions, and we can trace them back, and then look at the patterns of how they come out to try to understand what the matter was like that created all of them. One of the most amazing things that we’ve discovered at Rick is that this matter that we recreate is very much like a liquid as opposed to a gas, which many people thought it would behave like before. It’s amazing that this matter, which is 250,000 times hotter than the center of the sun, actually behaves a lot like a liquid. This is what the early universe was behaving like, so we’re really peering back and looking at that. This fundamental research brings together some of the smartest people in the world to try to solve some very difficult technological problems. How do you accelerate these gold nuclei, for example, up to such high energies? How do you detect what’s coming out from all of the collisions? How do you analyze all the data that’s coming out? This leads to advances in superconduct magnets. It leads to advances in detector technology, and it leads to advances in computing.

[00:19:52.23] – Speaker
This is also where the next generation of scientists will be trained, and who knows what they’re going to go off and do and discover with the knowledge knowledge that they’ve gained at Ritz.

[00:20:09.13] – Aihong Tang
All right, so that’s the not a show. Paul Sorel He was used to be my colleague at the Prokip. He later move on to DOE. I think in this short video, he really give you a description of everything, including how the rig operated and the computing. In the following slide, I will focus on physics meanings. On this slide, you see how the collision started. It started with booster and then the got injected. Into the AGS. And then the beam was injecting into a once clock called a yellow, the other one is called a blue ring. They are running in one clockwise, the other one is counterclockwise. Then there are few collision points. They’re not perfectly wrong. So in order for them to collect, you have to make them… You have to steal them at some place so they can collect. At a star in the Phoenix side, they are collied and the data are collected. Compared to alimentary collisions, the heavy iron collections are more complicated. This cartoon shows compared to E plus, E minus collection, PP, proton, iron, and iron-ion collections. You see the system in iron-ion collections, the system is more elongated in time and it has gone through a few distinct faces.

[00:21:44.19] – Aihong Tang
What you are doing is to try to collect the remnants for all those things and trying to reconstruct what happened at the beginning. This is a very complicated task. I need to put a string in the requirement for detectors. On this slide show, you need a good particle identification. You need to reconstruct topologies or particle decay, and you need a a load of a particle identification and different momentum. You also need, say, some dedicated detector, say, detector photons or just high p-reunions or happy flavor fragments. At the end, you really And try to take advantage of the state of order with a detected capability in either current stage. It has to be large acceptance, high efficiency, high resolution, and excellent particle identification capability to do that. So this cartoon shows the of the stars detector complex. Well, I use stars as an example. Similar thing you will see at least. I use stars because it’s convenient for me to assemble those materials. Okay, so in the center, you see a beam line. You see a single in there, then you see a beam line passing through the detector. And then you see the TPC is surrounded by a barrel, electrical magnetic calorimeter, the BMC.

[00:23:30.13] – Aihong Tang
Then you also see the toff outside the TBC. There are other detector components like a beam counter and the EMC. I also put besides the heavy flavor truck and the HRT. But HRD is an independent infrastructure, not putting besides the detector. It’s sitting in our data acquisition room. I will talk about it more tomorrow. This is the animation of the detector complex just to give you some taste of the scale of the detector itself. You see you have three racks of platforms, and on those platforms we have the electronic creators to have the front-end electronics. Then you see this detector has two end caps, and inside the end of the cap, you have the TPC. This is a real picture of the time projection chamber seen from view from the one end cap and look into the center of… Well, look at the beam line. So you see the beam line going through the detector. You view from the beam line and into the detector itself. You see one end cap, the beam going the detector center and the collision happens at the center. Well, on this picture is covered, so you do not see what it look like inside.

[00:25:09.13] – Aihong Tang
But anyway, this is a cross-section. Tell you what is inside. You see that it has a central membrane at the center. That’s the catheter. Then at the two ends of the cylinder, you see two end place. That’s the annul. We have created an electrical field pointing from the two end place to the center. And also, surrounded the TPC, there’s a point that can generate the magnetic field, pointing one direction. On this carton, it’s from right to the left. So to the right, I’ll give you an example. Say you have a charged particle passing through the gas in the TPC. This is a P10 gas. I will call the P10 gas. It’s a 90% argand plus 10% of the method. This particular mixture of gas makes it easier for a charged particle to produce an electron cluster. And this cluster once it’s produced and the influence of the electrical field and the magnetic field, you are drift towards the end place. Well, at the end place, the end place is consist of a ray, a two dimensional array of copper paths, and those copper paths are connected to the electrical board, and well, the signal can be collected.

[00:26:40.19] – Aihong Tang
And above those copper paths, there was an array of annular wires, and it’s sitting 5 millimeter above path. Then, say, if you have a charged particle flying through the TPC gas, it will a cluster of liberated electrons. Then the electrical field, it will drift towards the end path, where it can produce an electron avalanche. Those avalanche will produce signal on the copper pads, and those signal can be collected by the electronics. That will produce some logical signal, and you’ve got to digitize, and those digitize signal will be sent in to tape and recorded for analyze. The TPC is functioning more like a multi-pixel 3D camera, which is here I show you two example of the picture taken by the TPC. One is the end of view, the other one is the cross-section view. You will see those tracks. Well, you special attention, you probably will see those tracks are curved. The reason they are curved is because the whole TPC is immersed in magnetic field. We know a charge particle cutting through the magnetic flux, it will get burned by the magnetic field. That is the reason why it got a curvature. That’s good for us because we can take the the technology of this curvature and we can infer the momentum because the heavier the momentum, the straight the track.

[00:28:39.08] – Aihong Tang
The smaller the momentum, you will see it is more curving around. You can also combine this momentum information with the energy loss information. In previous slide, I talked about you can collect the avalanche signal by those copper pads. Those local pads is proportional to the energy loss of each individual electron cluster. So you can calculate this energy loss as a function momentum. This is the characteristic of TPC gas. So this energy loss is supposed to be only related to the velocity or charge particle passing through a volume. Now we know that different particles, for the same momentum, different particles, because because they have a different mass. That means at the same momentum, they have different velocity. As a consequence, different velocity means different energy loss. If we plot the signal, say the energy loss as a function of momentum, we see different particle species are separated from each other. So by doing that, we can achieve the particle separation. There are other ways to do the particle identification, like the time of light detector. This detector is made of multi-gap with this plate chamber. It’s a sandwich structure. It has fishing lines, the glass, then there’s a plate, then followed by another fishing line.

[00:30:25.00] – Aihong Tang
And it’s very good at detecting the time it costs for the particle to arrive our detector. The principle for the time of light detector to identify particles is similar to the TPCD, the energy loss, because at the same momentum, Here, different particle time has different velocities. That’s different travel time as shown by this situation. So if you plot the inverse velocity or other velocity, it doesn’t matter. And if we plot it function of particles momentum, you will see different particles are separated from each other. If we plot it just as a function velocity, they collapse in a single curve. But luckily, because in the detector, we measure particles momentum, not velocity. So by that, we can separate one particle species from the other. There are also other dedicated particle identification devices like the Barrow Electromagneter Calorimeter. This device is a sandwich structure. It’s between scintillator tile and lead plate. What is good at it is it’s good at it to detect the energy deposited by electrons and the photons. The principle for doing that is for electrons, it’s momentum, this ratio of momentum of energy is But this number will be very different for hydrons.

[00:32:05.09] – Aihong Tang
By cutting this ratio, because we can measure the energy, we can measure momentum. By cutting this ratio, you can separate the electrons from other hadrons. Each detector has its own advantage in some aspects. Before I move on, I give you some basic picture of collision geometry and kinematics. So usually you are here, heavy on thesis, talking about the transverse momentum. This is just a momentum. See in the carton, you have a beam direction passing through the center of the cylinder Then you have a plane that is perpendicular to this beam direction in Z. And transverse momentum is just momentum projected into that transverse plane. You And also hear people talk about rapidity. Put it as simply as rapidity, you can just regard that as the velocity in the Z direction. But this is different than the velocity. The reason you use rapidity rapidity instead velocity is because when you put that in rapidity, the spectral shape is preserved under Lorentz transformation. So that means you imagine a spectral shape in rapidity, you remain the same shape even you add more rapidity. But that is not so if you put it in the velocity. Sometimes we also use the pseudo rapid for lighter particles, say PIAS.

[00:33:54.17] – Aihong Tang
And when the mass is very small, like a PIAS is rapid and the pseudo rapid is is very close to each other, but it is… So the pseudo-repeated and provide some convenient way in some cases. You can also achieve particle identification via topology and the environment mass reconstruction. I think this group is very good at identifying decay topologies, so I don’t think I should spend much more time on this. Anyway, this is taking advantage of the environment mass. The advantage is the environment. The environment, you can regard it as… Because it exists a particular particle, there will be a particular correlation between the mass and the momentum. By doing that, if there’s a real particle, it you will see enhancement in the environment. For the weak decay, you can also see it’s decay topology detector and you write some sophisticated pattern recondition program, then you can reconstruct have those decay topologies like the K-particle. The centrality is important concept in heavy-run conditions. Before I move on, I have to introduce a little bit So for central collision, that means the two ions are colliding with each other, almost a complete head-on-head. Then, opposed to that, you have peripheral collations.

[00:35:27.16] – Aihong Tang
In peripheral collations, that means two heavy ions are value-grazing each other. And in peripheral collations, what we call the impact parameter with the connecting line between the centroid of the two ions is largest. And in central, this impact parameter is the minimum. So in peripheral collisions, you would expect a small overlap, large impact parameters that will produce the least number of particles. And the system size would be small. In central collisions, it will produce the maximum number of particles, and it will create a large and a whole to medium. So let’s have those chronology described and let’s characterize QGP. First, we know that it’s very hot. It’s how hot is hundreds of thousands of times hotter than the core of the sun. How do we know that? We just You want to just believe it by words, right? How do we know that we started the temperature by starting the Pt slope of the structure in transverse momentum? So this is shown the spectrum in Pt, you see this exponential shape. How to translate that into temperature? Well, simply speaking, when the temperature high, you have more energy to deposit in each degree of freedom.

[00:37:03.09] – Aihong Tang
That means you have more chance to pump those pump of kinetic energies into those particles, and they tend to have more particles has a larger PT. So that means this slope will be flat and your temperature is high, and this slope will be steep, your temperature is low. So by fitting this temperature with this formula, say, approximately this exponential formula, you can extract the temperature. From that, we can extract that the temperature for central collisions is 240 MeV. In one of the previous slide, I’ve shown that the critical temperature from the QCD theoretical calculation is about 165 MeV. So the 240 already surpassed that critical condition. The temperature has been measured at both rig and the LHC experiment. So this shows the temperature as a function of energy measured at a rig by Phoenix experiment. Also the ALICE experiment at rig, they all surprised with a critical temperature, which is about 165 MeV. And you see also this temperature increase as a function of time. So the critical condition for QGP is satisfied. This in terms of temperature. What about the density? We know it’s very dense. How dense? Just imagine you pack the entire Earth inside a stadium.

[00:38:44.05] – Aihong Tang
That’s really hard to imagine. Actually, here I plot the stadium. In reality, on this picture of the Earth, the stadium is probably no more than a picture size on this JAPAC picture. So that’s very, very dense. How do we know? Well, we can tell the energy density by starting the particle area. This is the one example. Say you can study the particle density as a function of centrality. The bottom part I show as a function of centrality. To the left side is pretty broke collision, and to the right side is a central collision. From there, you can also figure out the overlap region. You You know the overlap region, you know the energy, you measure it, then you can calculate the energy density. From there, you can see that energy density in front from the experiment is 4.9 job per femicule. In one of the previous slide, I showed the for QCT calculation, for QCT expectation, the critical density is about one J for Fermi cube. So now 4.9 J for Fermi cube, and that surpassed requirement for QGP to be formed. To put this energy density and the temperature in the broader context, here I show you the temperature versus particle density for different materials.

[00:40:15.06] – Aihong Tang
So at the bottom left, you have the ultra-cold lithium. It has a very low density and very low temperature. Then you have, say, the usual cup of coffee is sit somewhere in in the middle of this diagram, and you have a giant planet that’s at a slightly higher temperature. Then you have the sun sitting at, say, Close to the top right corner. And then you have a supernova, you call the white gloves and the cold neutral stuff. Then the quark-blonde plasma, which I highlight by the red circle in the top right corner. That’s where quark-blonde plasma sees. You can see it’s really, really extremely high temperature and high density. And we also know that the material created at both LHTC and the rig is at a chemical and a thermal equilibrium. The chemical and the thermal equilibrium means First, the chemical equilibrium means that the particle composition that constantly bring, destroy, and form. And the rate that they got to destroy is equivalent to the rate that they got to form so that they reach a equilibrium. At that point, the relative particle ratio will remain the same. And you can also have some structure tell you whether the system is at a thermal equilibrium This plot shows the mid-rappidity hadron ratios for caons, caons, protons, and another bunch of particles.

[00:42:10.11] – Aihong Tang
And then you have the measurement showing a red circles. Then you compare to the theoretical model that has this chemical and the thermal equilibrium invoked. You see it grift reasonably good with theory. And this tells you particle ratios are Very likely in chemical and thermal equilibrium. This is a similar study at the LHCC energies. You see that they all even expanded it to helium, anti-helium four, et cetera. Now we know the system is also well-thermonized in equilibrium. You can also study this feature of this QGP by studying how the temperature We’re against this collective velocity to see how the system evolved from pre-floor to central collision. We know that the system expands faster and become cooler when reaching the Kinematic results of going from pre-fro to central collisions. We also know that the system behaves like a perfect liquid. Why? Because we can have a way to figure out this How to tell? Before we talk about the risk, let’s first talk about some basics. This is about a reaction plane and the flow of the solvables. The reaction plane is defined by the beam of the nine connecting two colliding nuclei. So that a form of plane.

[00:43:51.11] – Aihong Tang
And in the middle, you have this ominous shape, the fireball and the expand. And in this expansion, you This pressure in the implant direction, in the exit direction is larger than the pressure in the Y direction. That’s obvious because you have less resistance in the exit direction because the nuclei are passing by each other quickly and leaving nothing in the way in the expansion. So that means this initial asymmetry in the coordinate space will be converted into an anisotropy in the momentum space. So we use We use the eccentric to quantify the asymmetry in coordinate space, and we use another parameter called the V2 to quantify the anisotropy in momentum space. V2 and the eccentric They are just a dual version of each other. One describes the anisotropy in coordinate space, the other one describes the anisotropy in a momentum space. However, the ratio of V2 over encentricity describes how efficient the system convert initial and isotomy in coordinate space into momentum space. If it’s well converted, then it’s more the system behaves more like a liquid. So indeed, people started the V2 function centralities. So to the right side is a central collision, left side is a peripheral collision.

[00:45:28.16] – Aihong Tang
The data points are shown by black circles. And if you compare to hydrocalculation, the hydro assume the system is behaving a fluid in light. You see in central collisions, the hydrocalculation begin to agree with data. That is the first indication that the system looks like a perfect liquid. While you can also quantify that, it’s not just reaching hydro behavior, It has also people can figure out the viscosity of entropy density. Well, we got about this entropy density. Just think about the viscosity in a general sense. On the plot to the right, we plot this viscosity density for helium, the nitrogen gas or the water, the normal water showing in the right circles. And then you see the QGP as a single point at the bottom. So you see as you reach critical temperature from left to right, you see rig is really at the bottom of this viscosity density. So when you see that, you know that the system really behaves like a liquid. We also learned that the system is part of and change. How do we know that? How do we know this part of degree of freedom at work? There are multiple indications, but Let me just give you a simple one, and I hope this one is easy to grasp.

[00:47:23.21] – Aihong Tang
We know the hadrons, there are two types of hadrons. One is called the massons. V2, consisting of two quarks, and the other one is variance, consisting of three quacks. Now, if you can study the V2. I show that what a V2 means. It’s just a quantity to describe the asymmetry in the momentum space. If you study the V2 for massons and the variance, so if the massons here, I have a K short, and the variance, I have lambda plus anti-namda and plot as a function of transverse momentum. You see basically they’re different. You see number is lower than K short and a low Pt, but higher than K short and a larger Pt. However, if you replot this plot by dividing V2 by the constituted block, say basically this, if it’s number divided by three, it’s a K short divided by two. Similarly, For the momentum, say the momentum divided either by three or by two, you see these two curves convert, collect into a single curve. What does it mean? That means the part of the degree of Cork level degrees of freedom is at work because you divide either by two or by three by their constitution of the clock.

[00:48:55.11] – Aihong Tang
This is just for two particle species. There are many species. This has has been tested over many species and it holds to be true, well, to some level. But I think to the first order, this agreement is very impressive and amazing. That’s one of the evidence that we think the pathological degree of freedom is at work. We also know that the material is a pipe. How do we tell? Because we do see jet quenching. What is a jet? Jet is a high energy part of shower, so it’s directly produced. It’s not, say, formed by coalescence or some low PT phenomenon. Why we think that JET are quenched? Well, you can study this phenomenon by study what is called RAA. This RAA is really a ratio of the yield in proton-proton collections normally. Sorry, the ratio of yield in gold-gold, so in ion collections, normalized by the yield in proton-proton collisions with some normalization factor. If this RAA is smaller than one, that means you have a suppression. You have a suppression means the jet get a destroy in the medium and the yield is suppressed. If it’s equal to one, that means there’s no medium.

[00:50:24.01] – Aihong Tang
What you see in heavy iron collisions is just a simple imposition of proton-proton If it’s greater than one, that means you see an enhancement. Now, this plot to the right shows this RAA for photons and for pi-ons. You can see for photons, there’s no suppression because that’s the reason the photon is produced initially and they escape very fast. Nobody can stop it. For Pi-ons, you see a significant suppression at larger P P. That means just a quench. The similar updated study with LHTC data and they extend it to a larger Pt, you see through the whole P. D. Range, you see this our AL is less than one, indicating that Gets are suppressed. That means the medium is okay. You can study that from a different analysis, what we call the two-particle correlation. On this slide, you can take two particles and study the relative angle between them. If there are zero angle, that means the two particles going together. If it’s a Pi angle, that means they are back to back. So zero is going together. Pi means two particles are going back to back. And on this part, you can see, well, you have a gold-gold collision showing in black.

[00:52:01.17] – Aihong Tang
Then the blue stars is gold-gold collision. You see gold-gold collision at zero degree angle, that means they’re going together. Gold-gold is just similar to PP collisions. However, if you move to the 180 degree at a Pi direction, means two particles going back to back. You see the gold-gold collision, the correlation in gold-gold collision is significantly suppressed than PP. The PP is still there and also D gold is both there, but in gold, gold, they are at zero. That means those particles that is going inside the medium, they got to be destroyed. And only the particles going outside the medium, they survive. And that explains that the 180 degree configuration are suppressed. That’s another evidence that the jet are Recently, we also begin to realize that the whole system is spinning very fast. We call it, we have seen the most vertical flow. To explain that, let’s go back to the configuration at the beginning. So you have the two nuclear fragments passing by each other. In the middle is created that twisting motion. This twisting has a very large global angular momentum. It’s turned to the three or 10 to the seven H bar. And also it created a large magnetic field, which I will talk about it later.

[00:53:44.03] – Aihong Tang
And because of this twisting, this very strong twisting, it will create some… I wouldn’t say it’s a local effect because it’s known many years ago. Why it’s called a Balning effect, the other one is called the Einstein deHussle. But for the Balning effect, you just take an object and you rotate it. If you rotate it fast enough, you can magnetize it. This is very interesting because the rotation is a classical mechanical concept, but the magnetize, meaning you change the spin of the particles inside of that material. But the spin is a quantum mechanical concept. Usually a concept in classical mechanics has nothing to do concept in quantum mechanics. They’re just two totally different work. But this is a very interesting example. Say, I just play some tricks in everyday life. I just rotate something. I can influence the behavior in quantum mechanical world. In that sense, this is a very interesting example. The dual Another version of this is if we have a polarization in the material, that will cause a rotation under the influence on some external field. So this example, if you have You put some magnetic field there, it will cause a rotation.

[00:55:19.24] – Aihong Tang
That’s because the material is polarized. So these two are very good example to illustrate the bridge between classical world and the quantum world. Why I’m talking about this? Well, the relativistic Hevian conditions can be just be regarded as a burn in effect by the extreme conditions. Because once you have a huge rotation, it will polarize the whole medium. So what we call this global polarization. How can we tell the system is global polarized, we can study whether the system is globally polarized by starting the parity violating weak decay of heparons like lambda. How? Because the lambda is known to decay by preferentially emit the proton direction according to the lambda’s spin direction. To put it a simple word, the proton carries the spin of the lambda direction. The daughter proton carries the spin of its current lambda direction. So by studying the decay topology of the lambda, you can infer the polarization of the lambda. By studying the polarization of many namdas in the event, then you know the global polarization of that event. That’s the logic. The study experiment have started what is called this This pH number denotes the global polarization of the system.

[00:57:06.12] – Aihong Tang
The study experiment has started this pH as function of energy. We do see at the lower energies you see a significant the PH number. You can study this PH for lambda and anti-namda separately. From that, you can also infer what is called the vulticity of the system. From there, you can see the the velocity is 10 to the 22 per second. So it spin 20 to the 22 runs per second. That’s really, really fast. How fast we have to put that into a broader context, right? So in ocean flows, we know the velocity is 10 to the minus 5 per second. And for terrestrial atmosphere, like the The hurricane is like a 10 to the minus 4, and the core of a supercell tornado is 10 to the minus 1. Then the solar substance flow is 10 to the minus 6. The giant great red spot on the Jupiter that has exist for hundreds years is 10 to the minus 4. The experiment, the highest the voltage you can reach is in nanoglap superfluid. Helium is 10 to the six. However, at RIEC, it is found that you can reach the velocity as high as 10 to the 22 per second.

[00:58:53.11] – Aihong Tang
That’s the most the vertical flow. It has many other consequences because of this velocity, but that is beyond the scope of this talk. We also got to know that the system is very electromagnetic vibrate. So it has generally the electromagnetic field stronger even than the magnet. Again, we have to put that in the broader context. We know the magnetic field on Earth that’s used to tell you the direction, that is the half Gauss. And it’s on the order of half Gauss and one Gauss for everyday gadget like the speaker, the magnet in the speaker’s bottle, also half Gauss I think. So for a nitrolyt on Earth, it’s turned to third or fourth Gauss. The most known, the strong magnetic field is expected to add a surface of Lutron style, what people call the magnet It’s 10 to the 14th Gauss. However, in heavy iron collations, it can reach as high as 10 to the 18 Gauss. That’s a few orders of magnitude higher, even higher than the magnet top. And how do we see that? We can actually see that by how this is very simple. This is classical electrical magnetic example. So you know the particle, a charged particle, when it passes through the magnetic field, it will go to bend.

[01:00:33.11] – Aihong Tang
It’s the same for the same reason. You will say the Z axis here is the beam axis, then you have the axis, and the axis is coming out of your screen is the Y axis. That’s the X and Y axis consist of this transfer thing. So you have particles coming out, traveling through in the beam direction because it’s cutting through the on a magnetic flux, it will get burned in one way. It depends on the direction it’s traveling. Now we know there’s another effect called a Faraday effect. That is basically in the medium, it will create the resistance to the change of the magnetic field. So that’s a variety of effect that tends to move the particle in the opposite direction, and these two effect competes each other. At the end, you will see that the pattern, the flowing pattern, we call it the flowing pattern, basically is just imagine just particles are deflecting in different ways for particles with opposite charge. So here we show this pattern. This is splitting in pattern between positive and negative charge as a function of centrality for different particles species. This phenomena probably is hard for me to explain in details why there’s this pattern, but you can just trust me for a moment.

[01:02:04.01] – Aihong Tang
And this rise and the fall feature, and in particular the fall in feature, when you reach the pre-flow conditions for protons and the chaos that has to be explained by the existence of the electromagnetic field. So we do see this strong field. Previously, we have a long history trying to see the trace of the EM field in heavy-run conditions. And because this effect is really elusive, it hasn’t been much successful until recently, you will really begin to realize that indeed, you can go back to a simple setup and you see that there was a splitting pattern between positively charged particles and the negatively charged particles. Okay, so instead of giving you a In summary, I really only want to mention a few key takeaways. First, I want to have this impression that in heavy-end conditions, we really recreate the extreme conditions similar to the Big Bang. The croc-goulon plasma is a state of matter where crocs and goulons are deep confined, providing insights into the early universe and serve a test ground for QCD. That’s the major goals, why we’re doing heavy-earned coalitions. And through this process, we have advanced many theoretical methods also driving the technologies, including this pattern, we can recognize algorithm.

[01:03:49.17] – Aihong Tang
We have this dynamic and exploration to study this characteristic of QGP, and that really help us to understand our usual matter and also have our standard universe. The ultimate goal is to uncover the fundamental building blocks of matter and the forces that govern their interactions, enhancing our understanding of the universe. I will stop here.