1
00:00:00,880 --> 00:00:16,870
George. Thank you so much for the introduction.

2
00:00:16,920 --> 00:00:25,590
So, yes, my name is Jasmine Brewer, and I'd like to thank you all for coming today and Saturday to join me,

3
00:00:25,920 --> 00:00:34,410
talk a bit about my research and in particular to try to explain what I mean by this liquid states of quarks and gluons.

4
00:00:36,370 --> 00:00:41,950
So the model that we see around us is often sort of a very complex composition of much simpler objects.

5
00:00:41,960 --> 00:00:49,800
So the chair that you're sitting on, maybe it's made of wood. So at the basis of that is some, you know, strings of cellulose molecules.

6
00:00:49,810 --> 00:00:54,850
Those molecules are themselves composed of atoms like oxygen and carbon.

7
00:00:55,900 --> 00:01:02,110
And at the centre of those atoms in the nucleus are particles like protons and neutrons.

8
00:01:03,130 --> 00:01:08,700
And inside protons and neutrons are the particles which we actually consider fundamental, which are quarks and gluons.

9
00:01:10,210 --> 00:01:16,070
And one of the way that you can think about heating up matter is that as the temperature gets higher and higher, sort of.

10
00:01:16,480 --> 00:01:20,230
More and more complex objects become unstable.

11
00:01:20,920 --> 00:01:29,440
So if you imagine heating up this chair, at some point, these long strings of molecules would start to break down into something simpler, like carbon.

12
00:01:30,910 --> 00:01:35,319
And in the sun, for example, it's so hot. So this is kind of the hottest place in our universe.

13
00:01:35,320 --> 00:01:40,330
It's so hot that only sort of the simplest atoms are stable, like helium and hydrogen.

14
00:01:40,660 --> 00:01:45,129
And actually, if you go to the centre of the sun, these are all sun also lost their electrons.

15
00:01:45,130 --> 00:01:47,200
And so they're essentially protons and neutrons.

16
00:01:48,700 --> 00:01:56,469
And so one of the kind of fundamental questions that you can ask from this picture is if the hottest place in our sun or in our universe is the sun,

17
00:01:56,470 --> 00:02:03,160
you know, what would it actually take to melt these protons and neutrons into their fundamental constituents is quarks and gluons.

18
00:02:05,320 --> 00:02:13,090
And it turns out that there actually was a point in the universe where all matter was in the form of

19
00:02:13,090 --> 00:02:17,800
where protons and neutrons weren't stable and all matter was in the form of these quarks and gluons.

20
00:02:18,310 --> 00:02:25,930
So here I'm showing a picture of the evolution of the universe from the time of the Big Bang all the way to the present day.

21
00:02:27,100 --> 00:02:30,400
And actually in the first microseconds after the Big Bang.

22
00:02:30,640 --> 00:02:37,930
Protons and neutrons weren't yet stable. And so all of the matter in the universe was actually in this liquid state of quarks and gluons,

23
00:02:38,530 --> 00:02:41,340
and that the universe was expanding and cooling very quickly.

24
00:02:41,380 --> 00:02:46,600
So after a microsecond, these these quarks and gluons turn back into protons and neutrons.

25
00:02:47,080 --> 00:02:53,770
After 380,000 years, their cooling, it happens enough that the simplest atoms could form like hydrogen.

26
00:02:54,130 --> 00:03:01,150
And today we know that we experience in our daily lives much more complex atoms and also molecules.

27
00:03:02,360 --> 00:03:07,700
And so today, the only way that we can actually recreate these very extreme conditions where

28
00:03:07,700 --> 00:03:12,620
protons and neutrons aren't stable is actually in high energy collider experiments.

29
00:03:15,100 --> 00:03:22,000
So the large Large Hadron Collider, which is based in Geneva, is the biggest the biggest particle accelerator in the world.

30
00:03:22,090 --> 00:03:29,350
It's about 27 kilometres around and most of the time at the Large Hadron Collider, they're colliding protons.

31
00:03:30,250 --> 00:03:37,930
And so you can imagine these protons go around in this 27 kilometre ring in tunnels that look like this, which are buried underground.

32
00:03:39,410 --> 00:03:46,580
And so at some special points along the ring, they take protons going one way and protons going the other way, and they collide them.

33
00:03:47,030 --> 00:03:51,019
And the places that they do this are in these four special interaction points where

34
00:03:51,020 --> 00:03:55,370
there are experiments which are designed to measure the output of these collisions.

35
00:03:57,830 --> 00:04:03,270
And so when you collide protons at high energies, you can kind of think of them essentially as as breaking apart.

36
00:04:03,290 --> 00:04:08,510
So what it looks like when you collide a proton with another proton is that basically you collide,

37
00:04:08,960 --> 00:04:13,430
say, a quark from one of the protons with a gluon and the other one.

38
00:04:15,530 --> 00:04:22,760
And so essentially, you can think of these interactions as a collision between a quark one from one proton and a Parker one from another proton.

39
00:04:23,270 --> 00:04:28,810
And these collisions can produce a wide variety of different types of particles.

40
00:04:28,820 --> 00:04:31,040
So both, you know, quarks and gluons,

41
00:04:31,040 --> 00:04:37,790
but also electrons and then even more exotic particles that you might have heard of, like the Higgs boson or the W boson.

42
00:04:38,510 --> 00:04:48,169
And so one of the big goals of this program is to sort of both produce these particles and also study how they interact with

43
00:04:48,170 --> 00:04:54,230
one another so that we can really confront our theoretical understanding of these types of particles and their interaction.

44
00:04:55,750 --> 00:04:58,600
But even though we can produce all of these types of particles,

45
00:05:00,100 --> 00:05:09,339
kind of the by far the largest contribution to the particles which are produced since protons are themselves sort of composed of quarks and gluons,

46
00:05:09,340 --> 00:05:12,970
most of the time we're producing also quarks and gluons.

47
00:05:13,510 --> 00:05:23,440
And so unless you ask for something specific in general, you'll see a lot of production of these particles which are associated with the strong force.

48
00:05:23,440 --> 00:05:30,160
And so I'll be mostly talking today about the production of quarks and ones and the strong force.

49
00:05:32,370 --> 00:05:36,540
But protons aren't the only thing that we collide at the Large Hadron Collider.

50
00:05:36,930 --> 00:05:41,210
You can also collide heavy nuclei, for example, lead.

51
00:05:42,420 --> 00:05:49,890
And for this purpose you can essentially think of lead is just being a giant bag of 82 protons and 126 neutrons.

52
00:05:50,550 --> 00:05:55,500
And so when you collide to light atoms, you essentially get a huge.

53
00:05:57,340 --> 00:06:05,080
As a huge number of on proton collisions, which all happen in a very small region of space, essentially at the same time.

54
00:06:06,400 --> 00:06:10,150
And because you have so many of them all at the same place,

55
00:06:10,150 --> 00:06:16,330
actually something kind of fundamentally different happens in these collisions than what happens in proton proton collisions.

56
00:06:16,840 --> 00:06:25,959
So in particular, the energy density or the temperature in that tiny region of space is so high that actually protons and neutrons do become unstable.

57
00:06:25,960 --> 00:06:30,790
And so we produce in that region of space the hottest temperatures in the universe,

58
00:06:31,630 --> 00:06:36,550
which essentially produce this unique state of matter where quarks and guns are actually free.

59
00:06:37,390 --> 00:06:40,480
And this is this state of matter is called the quark long plasma,

60
00:06:40,480 --> 00:06:45,040
which is what I'll be kind of talking about today and what a lot of my research focuses on.

61
00:06:45,730 --> 00:06:49,150
And just to give you a sense of the types of scales that we're talking about,

62
00:06:49,150 --> 00:06:55,389
the temperature which is created in this interaction region of these nuclei is 250,000 times

63
00:06:55,390 --> 00:07:00,520
the temperature at the core of the sun in a size which is 10,000 times smaller than an atom.

64
00:07:00,820 --> 00:07:06,520
So these are really, really extreme environment where we can produce this unique material.

65
00:07:08,810 --> 00:07:15,830
So one of the reasons that the quite warm plasma is so fascinating is that Clarkson gluons are very strange particles.

66
00:07:16,610 --> 00:07:25,250
So for the ordinary types of forces that you experience around you in general, the strength of the interaction will decrease with distance.

67
00:07:25,280 --> 00:07:33,859
So, for example, if you think about electricity magnetism, if you hold two magnets close together, then you'll feel them attracting a lot.

68
00:07:33,860 --> 00:07:38,180
And then as you pull them apart, the attraction between them will start to decrease.

69
00:07:39,150 --> 00:07:42,390
And this is completely the opposite for quarks and gluons.

70
00:07:42,420 --> 00:07:49,320
So if you could hold two quarks in your hands when they were close together, you wouldn't feel much interaction between them.

71
00:07:49,920 --> 00:07:53,700
But as you pull them apart, the interaction gets stronger and stronger and stronger.

72
00:07:54,570 --> 00:07:58,560
And in fact, it gets so strong that you actually can't pull them apart at all.

73
00:07:59,220 --> 00:08:08,010
And so this is what leads to them becoming confined in bound states like protons and neutrons, before you can really pull them apart.

74
00:08:08,020 --> 00:08:13,830
So this is the essential reason why we don't observe free quarks or gluons in the world around us.

75
00:08:15,310 --> 00:08:18,340
And so because quarks and gluons have such strange interactions,

76
00:08:18,340 --> 00:08:25,720
it's really fascinating to try to understand what would be the properties of a material which is composed of many of these quarks and gluons.

77
00:08:26,000 --> 00:08:29,470
You know, would they interact strongly with one another? Would they interact weakly?

78
00:08:30,520 --> 00:08:38,590
And in fact, because quarks and ones interact weakly when their distance separation is small,

79
00:08:38,890 --> 00:08:43,840
many people thought that when you create this dense material in a small space,

80
00:08:44,140 --> 00:08:48,880
there would actually be essentially a gas where those particles would interact weakly with one another.

81
00:08:49,720 --> 00:08:57,740
But in fact, one of the kind of fascinating outcomes of the heavy in physics program has been that it's actually completely the opposite.

82
00:08:57,760 --> 00:09:04,510
So this material which we produce is actually the most strongly interacting fluid which has ever been observed.

83
00:09:06,850 --> 00:09:13,060
And so this is a really sort of exciting avenue to kind of study this completely novel state of matter.

84
00:09:15,050 --> 00:09:18,200
So I hope I've convinced you that the quote from plasma is interesting.

85
00:09:18,410 --> 00:09:22,220
But one of the issues is it's also very challenging to study.

86
00:09:23,000 --> 00:09:29,540
And the reason that you can kind of imagine this is that it's produced in a very small place and for a very short time.

87
00:09:29,960 --> 00:09:32,000
And so very quickly after the collision,

88
00:09:32,000 --> 00:09:39,380
these quarks and gluons turn back into bound states like protons and neutrons and other stuff, pions and chaos.

89
00:09:39,710 --> 00:09:44,870
And then those particles are what, you know, flies along for a while until we can measure it.

90
00:09:45,260 --> 00:09:52,440
And the detector. And because we have so many proton proton collisions occurring at the same time,

91
00:09:52,770 --> 00:09:57,630
the energy density and the temperatures so high, we also produce a huge number of particles.

92
00:09:58,110 --> 00:10:03,660
So what I'm showing here is an event display from one of the experiments at CERN.

93
00:10:04,500 --> 00:10:12,090
For a single collision of two nuclei and what and all of these sort of rainbow

94
00:10:12,090 --> 00:10:15,990
colours in the central individual particles which are produced in this event.

95
00:10:16,590 --> 00:10:25,410
So you can imagine that digging out some detailed understanding about this material from that huge collection of particles is not always easy.

96
00:10:26,130 --> 00:10:31,560
So when I tell you that this is a strongly interacting liquid, how do we actually know that?

97
00:10:32,190 --> 00:10:44,379
How have we measured it? So the essential kind of in some sense we get lucky, which is that's actually kind of the dynamics of a of a liquid.

98
00:10:44,380 --> 00:10:50,320
Tell us quite a lot about the underlying interactions between the constituents that form that liquid.

99
00:10:51,010 --> 00:10:56,139
So to try to explain that a bit, imagine that you have some liquid, for example,

100
00:10:56,140 --> 00:11:01,810
the quite one plasma that can be anything which is has a squished geometry.

101
00:11:01,810 --> 00:11:04,900
So it's shorter along one direction than along the others.

102
00:11:06,010 --> 00:11:10,240
Then as these particles in this material aren't really interacting much with each other,

103
00:11:10,480 --> 00:11:14,950
then the way that this is going to expand will be equally quickly in all directions.

104
00:11:15,280 --> 00:11:18,460
So this is kind of a characteristic feature of gases.

105
00:11:19,270 --> 00:11:23,410
On the other hand, if these particles do interact strongly with one another,

106
00:11:23,650 --> 00:11:27,729
then actually this will generate and I saw entropy in the pressure where there will be

107
00:11:27,730 --> 00:11:32,740
higher pressure along the short direction and lower pressure along the long direction.

108
00:11:32,740 --> 00:11:39,840
And this will drive the system to expand more quickly along the short direction than it does along the long direction.

109
00:11:41,230 --> 00:11:49,090
And actually, it turns out that in most cases and heavy ion collisions, we actually do produce a curriculum plasma, which is spatially deformed.

110
00:11:49,600 --> 00:11:53,259
And you can see this because you can imagine if you take nuclei which share

111
00:11:53,260 --> 00:11:58,060
represented as circles and they collided a little bit off centre with each other.

112
00:11:58,570 --> 00:12:06,280
Then the region where they overlap, which is what's shown in pink, actually has this kind of almond shape where it's, it's a bit asymmetric.

113
00:12:07,720 --> 00:12:12,820
And so then we can actually measure the particles which are produced in these collisions,

114
00:12:12,820 --> 00:12:19,360
and we can actually find that the the correlations between particles produced in these collisions

115
00:12:19,360 --> 00:12:24,700
indicate that they're actually expanding more quickly in one direction than in others.

116
00:12:24,700 --> 00:12:34,450
And this allows us to to understand that this is a fluid undergoing this pressure driven expansion, indicating strong interactions.

117
00:12:36,790 --> 00:12:44,830
But to get more quantitative, we can actually also access the viscosity of the cyclone plasma from experiments.

118
00:12:45,370 --> 00:12:53,199
So viscosity is a really key feature of fluids which basically tells you how efficiently it flows around objects.

119
00:12:53,200 --> 00:12:56,859
So the quintessential example is that honey has a large viscosity.

120
00:12:56,860 --> 00:13:08,559
Well, something like water would have a low viscosity. And one of the kind of curious or perhaps non-intuitive features of viscosity is that

121
00:13:08,560 --> 00:13:13,420
actually as you have stronger interactions between the constituents in a fluid,

122
00:13:13,420 --> 00:13:15,970
actually the viscosity decreases.

123
00:13:17,320 --> 00:13:23,799
And so the way that we can try to measure how viscosity of the quark long plasma is by running hydrodynamic simulations of

124
00:13:23,800 --> 00:13:32,050
the evolution of the iron collision and then of viscosity is the parameter which goes into these hydrodynamic simulations.

125
00:13:32,050 --> 00:13:42,100
And then by comparing the output of these simulations to particles which we measure experimentally, we can constrain the values of that parameter.

126
00:13:43,600 --> 00:13:48,610
And so here I'm showing the plot of the sheer viscosity, the ratio,

127
00:13:48,730 --> 00:13:54,910
the entropy density for the quark, long plasma extracted in this way as a function of temperature.

128
00:13:56,560 --> 00:14:02,800
And so by itself, you know, maybe it's not completely clear what you should take away from this number.

129
00:14:03,580 --> 00:14:06,610
So now I want to show this same curve.

130
00:14:06,940 --> 00:14:09,280
So this curve, which we're looking at now in blue,

131
00:14:09,550 --> 00:14:16,720
shows up now in orange on this picture in comparison to the sheer viscosity of many other common liquids.

132
00:14:17,740 --> 00:14:23,980
So here you see the sheer viscosity of water in blue and superfluid, helium in green.

133
00:14:24,610 --> 00:14:31,989
And what's kind of remarkable is that the viscosity of the quark loan plasma is much lower than the viscosity of any of these other liquids.

134
00:14:31,990 --> 00:14:34,990
And in fact, than any other liquid which has ever been observed.

135
00:14:36,430 --> 00:14:47,290
And in fact, so quantum mechanics, it turns out, actually predicts a lower bound on the shear viscosity for any physical quantum fluid.

136
00:14:47,560 --> 00:14:50,470
And that's what's shown here in this dashed black line.

137
00:14:50,830 --> 00:14:57,430
And so what we see is that not only is the quark one plasma lower viscosity than any other fluid,

138
00:14:57,430 --> 00:15:03,010
but in fact it's essentially the lowest viscosity liquid that's allowed by quantum mechanics.

139
00:15:04,450 --> 00:15:10,150
And so this kind of also means since low viscosity liquids have strong interactions,

140
00:15:10,510 --> 00:15:18,310
that these quarks and gluons interacting really almost as strongly as they can in this liquid consistent with quantum mechanics.

141
00:15:22,430 --> 00:15:32,940
So, you know, I've been talking so far about about the viscosity, but viscosity is not the only parameter that we might care about, about the plasma.

142
00:15:33,290 --> 00:15:41,900
So hydrodynamics is an effective theory which tells us how systems behave when they're relatively close to equilibrium.

143
00:15:42,860 --> 00:15:46,730
But recently, people have become very interested in also understanding how.

144
00:15:48,680 --> 00:15:52,980
You know how this the system behaves also when it's further out of equilibrium.

145
00:15:53,000 --> 00:15:57,829
So here I am showing again sort of the time evolution of a heavy iron collision.

146
00:15:57,830 --> 00:16:05,090
But now there's a more detailed view of what of sort of the process which leads to the formation of the quark one plasma itself.

147
00:16:05,570 --> 00:16:08,720
So here you can see the nuclei collide.

148
00:16:09,830 --> 00:16:11,750
And then as I was telling you at the beginning,

149
00:16:11,750 --> 00:16:18,440
you kind of have a case where you have a bunch of proton proton collisions which happen essentially independently from one another.

150
00:16:18,680 --> 00:16:22,309
And so if you look at the energy density for that state, it's really,

151
00:16:22,310 --> 00:16:27,500
really spiky because you just have a bunch of a bunch of collisions which are relatively uncorrelated.

152
00:16:28,460 --> 00:16:31,910
But then these quarks and gluons, they interact a lot and they radiate a lot.

153
00:16:32,960 --> 00:16:42,320
And so very quickly, these very spiky energy density starts to smooth out and you get to a state which is sort of approaching equilibrium.

154
00:16:42,800 --> 00:16:50,390
And then at some point, it's sort of close enough to equilibrium that we can describe sort of the subsequent evolution with hydrodynamics.

155
00:16:51,830 --> 00:16:58,100
And this sort of phase, the process of equilibration, which leads to the formation of the leak of this liquid,

156
00:16:58,670 --> 00:17:03,950
also holds a lot of information about what are the underlying dynamics inside the liquid.

157
00:17:04,370 --> 00:17:09,530
So, for example, it was shown that if quarks and gluons interact with one another very weakly,

158
00:17:10,430 --> 00:17:14,510
then actually the process which leads to formalisation happens through turbulence.

159
00:17:15,170 --> 00:17:21,440
And there's been a lot of interest in understanding in the more realistic case that these interactions are stronger.

160
00:17:21,680 --> 00:17:27,230
What are kind of the general features of this process of equilibration that would hold?

161
00:17:30,730 --> 00:17:39,430
Okay, So I started on this topic by saying, you know, okay, we have this plasma, which is very,

162
00:17:39,730 --> 00:17:45,430
very small, and it's challenging to actually find ways to measure its properties.

163
00:17:45,760 --> 00:17:50,829
And so we got a little bit lucky here and we said, okay, you know, because of some properties of liquids,

164
00:17:50,830 --> 00:17:58,780
we can measure correlations of these relatively low momentum particles and actually access to the liquid properties of the quark long plasma.

165
00:17:59,710 --> 00:18:05,430
But the question is, you know, what other information, how what other strategies might we use to try to measure it?

166
00:18:07,740 --> 00:18:15,809
So historically, a very kind of important way to try to measure the properties of a material is essentially by shooting

167
00:18:15,810 --> 00:18:22,080
high energy particles on it and seeing how they deflect through their interactions with the plasma.

168
00:18:22,440 --> 00:18:27,839
So a famous example of this that you might have be familiar with is that, you know,

169
00:18:27,840 --> 00:18:33,240
when they were trying to understand the structure of atoms, there were kind of two competing models,

170
00:18:33,630 --> 00:18:41,700
either sort of the plum pudding model where all of this matter was essentially uniformly distributed inside of the atoms or the Rutherford model,

171
00:18:41,700 --> 00:18:46,140
where all of that matter was essentially concentrated in this dense nucleus.

172
00:18:46,950 --> 00:18:51,480
And so the way that they tried to address this is shoot high energy particles at this thing.

173
00:18:51,690 --> 00:18:56,040
And then what they measured is that these high energy particles actually had large deflections,

174
00:18:56,700 --> 00:19:03,600
which weren't consistent with this kind of plum pudding model. And this led to the discovery of the nucleus inside of the atom.

175
00:19:05,070 --> 00:19:08,379
So ideally, we would like to do something similar with acquired Gluon plasma,

176
00:19:08,380 --> 00:19:12,570
and we would like to shoot high energy particles at it and use their deflection

177
00:19:12,570 --> 00:19:17,820
to study how plaques and gluons are actually oriented inside of this material.

178
00:19:19,280 --> 00:19:24,889
But the problem is that this curriculum plasma is much too short lived to actually

179
00:19:24,890 --> 00:19:28,910
achieve this using any sort of external particles to the collision itself.

180
00:19:29,300 --> 00:19:34,670
Because the total lifetime of the cartoon plasma is a thousand times shorter than

181
00:19:34,670 --> 00:19:38,780
the fastest laser pulses that might provide particles that we could study it with.

182
00:19:41,670 --> 00:19:45,470
But fortunately, we do have some recourse, which is that.

183
00:19:46,010 --> 00:19:52,070
So Russian proton collisions when they happen, you very often, as I was describing at the beginning,

184
00:19:52,370 --> 00:19:59,000
have a very high energy interaction, say, between one quark or one and one proton and a Parker one and the other proton.

185
00:19:59,720 --> 00:20:07,100
And if these interaction is very high energy, it produces essentially sprays of high energy particles, which we call jets.

186
00:20:07,580 --> 00:20:15,559
And so these, you know, in an in detector jets might look like these blue towers of column rated sprays of

187
00:20:15,560 --> 00:20:21,650
particles which are kind of back to back with one another and then having ion collisions.

188
00:20:21,650 --> 00:20:23,000
As I said, you have many, many,

189
00:20:23,000 --> 00:20:32,000
many proton proton collisions and a lot of these are relatively low momentum and those are kind of mainly contributing to forming this plasma itself.

190
00:20:32,690 --> 00:20:36,979
But you can also have occasional interactions in the same way you do in proton proton

191
00:20:36,980 --> 00:20:41,840
collisions where you have a very high energy transfer and this produces jets.

192
00:20:42,740 --> 00:20:49,340
And the difference between jets and heavy iron collisions and Janson program proton collisions is that in heavy ion collisions,

193
00:20:49,340 --> 00:20:56,270
these jets are produced inside of the plasma and then they have to essentially plough out through it before you can measure them.

194
00:20:57,830 --> 00:21:05,450
And so then by measuring essentially how jets are modified in having collisions compared to proton proton collisions,

195
00:21:05,450 --> 00:21:13,370
we can try to access sort of the properties of the medium through their interaction with jets that leads to this modification.

196
00:21:16,450 --> 00:21:23,370
So just to kind of summarise again, what we wanted to do is to shoot high energy particles at it, but since that's not possible,

197
00:21:23,380 --> 00:21:27,400
instead we use these high energy sprays of particles which are produced inside,

198
00:21:28,450 --> 00:21:34,990
and this provides a really key signature for the formation of the quark one plasma and also an avenue to study it.

199
00:21:36,250 --> 00:21:38,530
So in fact, this effect is fairly large,

200
00:21:38,530 --> 00:21:46,630
so there's about half as many jets and heavier inclusions of a particular energy as as you would expect from proton proton collisions.

201
00:21:46,870 --> 00:21:55,000
And then they also have other modifications, for example, increased imbalance in the momentum between jets which are produced back to back.

202
00:21:56,230 --> 00:22:02,680
And so this is really exciting because it means that these actually do interact substantially with the correct one plasma,

203
00:22:02,680 --> 00:22:08,940
and therefore that they really give us an avenue to try to study the pathway on plasma using these objects.

204
00:22:12,950 --> 00:22:19,700
So in order to kind of dig a little bit deeper and try to understand the correct one plasma and a bit more detail,

205
00:22:19,700 --> 00:22:21,860
we also have to really understand us.

206
00:22:22,250 --> 00:22:27,319
And the reason that you can understand this is if you're trying to study the atom by shooting particles through it,

207
00:22:27,320 --> 00:22:30,800
you would really like to know what you're shooting at it and what energy they have.

208
00:22:31,280 --> 00:22:37,640
And since these are produced inside the collision, we have much less control over that than we would in an ordinary type of experiment.

209
00:22:39,920 --> 00:22:43,430
So jets are complex objects which are built out of quarks and gluons.

210
00:22:44,000 --> 00:22:48,590
But at a basic level, building a jet is fairly simple.

211
00:22:48,710 --> 00:22:52,010
There's only three things that can happen to quarks and gluons.

212
00:22:52,400 --> 00:22:54,900
You can have a quark which splits to a quark, and again,

213
00:22:55,100 --> 00:22:59,720
you can have a gluon which splits the two gluons and you can have a gun that splits the two quarks.

214
00:23:00,950 --> 00:23:05,990
And jets are essentially a composition of these basic building blocks.

215
00:23:06,290 --> 00:23:12,800
So here I'm showing just one example of how a high energy quark might split into other quarks and gluons.

216
00:23:13,610 --> 00:23:22,160
And once they reach sort of low enough scales, those quarks and gluons will again become bound and hadrons, protons and neutrons and pions.

217
00:23:22,430 --> 00:23:26,590
And those are what ends up as as these sprays of particles in the detector.

218
00:23:28,040 --> 00:23:31,519
But you can already see from here is that even with these three building blocks,

219
00:23:31,520 --> 00:23:38,120
there's a huge variety of jets that you can actually composed by assembling these different building blocks.

220
00:23:39,050 --> 00:23:44,900
And this is both kind of an advantage, but also a challenge because, you know,

221
00:23:44,900 --> 00:23:49,040
as you can imagine, the car from plasma is just a mass of quarks and gluons.

222
00:23:50,280 --> 00:23:55,650
And so each of these different types of jets, depending on how the quarks and guns are oriented inside of it,

223
00:23:55,920 --> 00:24:00,900
can interact differently with with this quite warm plasma than each other.

224
00:24:01,920 --> 00:24:07,260
And so the more information that we can control on what a jet actually looks like,

225
00:24:07,950 --> 00:24:13,530
the easier it's going to be for us to use that to study sort of detailed properties of this medium.

226
00:24:17,010 --> 00:24:20,100
So as I already kind of alluded to on the last slide, in general,

227
00:24:20,100 --> 00:24:24,480
we expect that a quark and a gluon will interact differently with the quark long plasma.

228
00:24:24,780 --> 00:24:33,960
The basic reason is that gluons have larger charge than quarks, but by exactly and so they'll interact more.

229
00:24:34,380 --> 00:24:39,450
But the exact amount more which they interact is kind of an important property of the medium.

230
00:24:39,450 --> 00:24:44,279
And the reason you can think of this is that that charge can be screened inside of the medium.

231
00:24:44,280 --> 00:24:51,960
And so we're not guaranteed that the ratio of the charges of quarks and gluons is actually the ratio with which they are interacting.

232
00:24:52,260 --> 00:24:56,910
So we'd really like to to study this part of the medium.

233
00:24:59,470 --> 00:25:06,790
But one of the challenges has I iterated already before is that once these quarks and gluons all turn back into hadrons,

234
00:25:06,790 --> 00:25:12,640
you don't have any information anymore about what was the way in which they were actually produced.

235
00:25:14,120 --> 00:25:20,090
But it turns out that there's kind of a cute way where you can anyway actually separate jets,

236
00:25:20,390 --> 00:25:24,140
which started as a quirk from those that started with a gluon.

237
00:25:24,950 --> 00:25:25,960
Despite this.

238
00:25:25,970 --> 00:25:36,140
So the idea is that you can kind of essentially imagine that you just classify a jet as whether it was initiated by a quark or initiated by gluon.

239
00:25:36,140 --> 00:25:40,880
And then you can think of all events as just being a big bag of jets.

240
00:25:41,270 --> 00:25:46,850
And you have, you know, that they were all initiated by a quicker gluon, but you don't know which one is which.

241
00:25:47,720 --> 00:25:54,860
And so this actually kind of ends up being sort of a classical kind of machine learning problem where you can try to distinguish, you know,

242
00:25:54,860 --> 00:26:01,129
a bag of jets with one ratio of quark and go on composition to a bag of jets with a different ratio of parking going

243
00:26:01,130 --> 00:26:09,200
composition and understand from that's basically this separate modification of quark initiated jets and Gluon initiated jets.

244
00:26:09,230 --> 00:26:13,670
So this was done first impression proton collisions and then with with collaborators.

245
00:26:13,670 --> 00:26:23,150
I did this also in heavy ion collisions, which gives us really a method to access the different modification of these types of jets.

246
00:26:23,410 --> 00:26:35,200
And apart from plasma. So whether a jet is initiated by a quark or glue on his very important feature of its interactions.

247
00:26:35,440 --> 00:26:41,800
And the reason you can think about that is that it's what sets the charge of the hold just because of charge conservation.

248
00:26:43,870 --> 00:26:50,080
But of course, you can have many different types of charts which start as a quark or which start as a gluon.

249
00:26:50,110 --> 00:26:55,420
So these categories still include many different types of jets.

250
00:26:55,660 --> 00:27:02,290
And as I've said before, the challenge is that you really can't tell these jets apart once you only have hadrons anymore.

251
00:27:03,610 --> 00:27:09,460
But there's one special type of exception to this, which is for particular types of particles.

252
00:27:10,090 --> 00:27:18,020
For example, quarks that have a large mass. Actually, they can't be produced during the hydrogen ization process.

253
00:27:18,040 --> 00:27:24,100
They're too heavy. And so that means that we can kind of essentially that we can trace.

254
00:27:24,370 --> 00:27:31,689
So if we find hadrons, which contain these heavy quarks inside of jets because they can't have been produced in transition,

255
00:27:31,690 --> 00:27:35,020
we know that they came from the jet itself or from the shower.

256
00:27:35,470 --> 00:27:41,560
And so these kind of give us a unique opportunity to actually trace back the history of how a jet formed

257
00:27:41,830 --> 00:27:48,070
and really understand which were all of the building blocks that went together and its composition.

258
00:27:48,080 --> 00:27:56,770
And so this process is something that I've been working on in several things recently to understand sort of how we can leverage

259
00:27:56,770 --> 00:28:03,130
this unique control that we have in these types of cases to really gain a better understanding of the quark flowing plasma.

260
00:28:05,630 --> 00:28:11,090
So finally, I want to just kind of take a step back and try to come a little bit full circle

261
00:28:11,390 --> 00:28:15,140
and also give you a bit of a sense of some of the direction of the field.

262
00:28:15,710 --> 00:28:23,630
So in the first part of the talk, I discussed sort of how we can use measurements of correlations of flow momentum particles

263
00:28:23,630 --> 00:28:29,390
to access sort of the hydrodynamic behaviour like the viscosity of the quark long plasma.

264
00:28:30,050 --> 00:28:37,820
And then in the second part of the talk, I kind of transition to another type of strategy to study the cartoon plasma,

265
00:28:37,820 --> 00:28:42,889
where we use these high energy sprays of particles to kind of probe it.

266
00:28:42,890 --> 00:28:50,150
And these are sort of two of the classical kind of strategies which the community has been using to study the quark on plasma.

267
00:28:50,930 --> 00:28:53,480
But recently, it's become appreciated that, you know,

268
00:28:54,560 --> 00:29:03,260
a key aspect of understanding the cyclone plasma in detail is also understanding how it behaves in far from equilibrium situations.

269
00:29:03,920 --> 00:29:08,770
And so this involves both understanding kind of how it forms after the collision,

270
00:29:08,810 --> 00:29:13,190
how it comes to be described by hydrodynamics and this process of equilibration.

271
00:29:13,730 --> 00:29:18,290
And it turns out that it's also really key for understanding Jad measurements.

272
00:29:18,500 --> 00:29:25,399
And the reason that you can kind of think of this is that we've been talking so far as if you have this high energy spray of particles,

273
00:29:25,400 --> 00:29:29,510
which is just going through a static thermal bath of stuff.

274
00:29:30,290 --> 00:29:33,380
But of course, as you can imagine, if you shoot a bullet through water,

275
00:29:33,680 --> 00:29:40,400
you know you're going to have a major non-equilibrium response of the water in the wake of that bullet.

276
00:29:40,670 --> 00:29:46,549
And so we actually have this process also in having collisions that when jets go through the quite glowing plasma,

277
00:29:46,550 --> 00:29:53,360
they also leave a lot of non-equilibrium material in the wake, which we aim to understand.

278
00:29:53,360 --> 00:29:58,370
And so there's a lot of theoretical effort and also new experiments which are

279
00:29:58,370 --> 00:30:03,979
being designed to try to address sort of this intersection of hydrodynamics,

280
00:30:03,980 --> 00:30:08,180
equilibration, and also jets in heavy ion collisions.

281
00:30:10,650 --> 00:30:16,710
And so there's many, I think, kind of open questions which are helping us to sort of drive the directions of the future.

282
00:30:16,830 --> 00:30:22,530
So one of the major questions which we aim to address with jets is to understand how we can try to

283
00:30:22,530 --> 00:30:30,240
access this microscopic structure of the of the quite warm plasma using these high energy processes.

284
00:30:31,380 --> 00:30:33,810
And as I tried to argue to you today,

285
00:30:33,810 --> 00:30:41,910
this really necessitates that we have excellent control over the Jets themselves in order to really study the medium using them.

286
00:30:43,170 --> 00:30:48,540
And rare processes can hold unique insights for this.

287
00:30:48,620 --> 00:30:59,310
This is one thing for the example which I gave in this talk, where we can use these heavy quarks to kind of pin down the type of jet which we had.

288
00:31:01,260 --> 00:31:05,969
And there are also some other types of cases where you can use other particles produced in the event,

289
00:31:05,970 --> 00:31:12,960
for example, zebra ones or photons to to gain more information about the event than you would have otherwise.

290
00:31:13,680 --> 00:31:19,400
But the disadvantage is that these processes tend to be rare, and so you need a lot of collisions to observe them.

291
00:31:19,410 --> 00:31:23,670
So for example, with the, you know, you might observe.

292
00:31:24,630 --> 00:31:31,170
500 sort of normal jobs for every one jet with, you know, to have equal arcs inside of it.

293
00:31:31,860 --> 00:31:41,489
And so this is part of what sort of drives a program for extensive more data collection at the Large Hadron Collider so that we can

294
00:31:41,490 --> 00:31:50,220
gain the statistics which will allow us to use these more rare but also more informative processes to kind of drive our understanding.

295
00:31:52,160 --> 00:31:53,840
In addition is I kind of motivated.

296
00:31:53,840 --> 00:31:59,840
We're very interested in understanding these processes of equilibration and the formation of the quark on plasma itself.

297
00:32:01,520 --> 00:32:08,810
And one of the challenges historically for doing this is that in having collisions, which is what I've been talking about this whole time,

298
00:32:09,680 --> 00:32:13,100
you do have a significant phase where equilibration is happening,

299
00:32:13,310 --> 00:32:19,040
but there's also a really long time where the system is essentially described by hydrodynamics.

300
00:32:21,020 --> 00:32:26,010
And so there's been a lot of interest recently and also understanding collisions of smaller nuclei.

301
00:32:26,030 --> 00:32:26,659
For example,

302
00:32:26,660 --> 00:32:36,410
this is a picture of an oxygen collision where the process of of sort of the whole lifetime of the system is shorter because it's smaller.

303
00:32:37,130 --> 00:32:41,450
And so the relative importance of this phase of equilibration compared to the phase of

304
00:32:41,450 --> 00:32:48,049
hydrodynamics is more and so it gives us kind of more emphasis and sort of another eye into this,

305
00:32:48,050 --> 00:32:49,600
this process of equilibration.

306
00:32:49,600 --> 00:32:58,610
And so there's a lot of upcoming excitement and new experiments on these other types of collisions where we can try to access this information.

307
00:33:00,480 --> 00:33:09,900
And so there's sort of several kind of upcoming experiments, both at the LHC and the smaller running experiments in Long Island,

308
00:33:10,320 --> 00:33:15,730
really trying to pin down a lot of these effects and gain also higher statistics.

309
00:33:15,750 --> 00:33:21,500
So I'm looking forward to an exciting progress in the coming years in this area.

310
00:33:21,510 --> 00:33:23,760
So thank you very much for your attention.