1
00:00:01,100 --> 00:00:11,380
Sort of like.

2
00:00:11,380 --> 00:00:16,420
So any of the three talks this morning?

3
00:00:16,420 --> 00:00:23,920
Mine is going to be more of a general overview of the subject of particle physics,

4
00:00:23,920 --> 00:00:30,370
but with a perspective on it that is perhaps somewhat different from the one you normally hear.

5
00:00:30,370 --> 00:00:33,370
So in particle physics, you often hear about big unanswered questions.

6
00:00:33,370 --> 00:00:40,900
You see these in the newspapers, in the media every now and then the fundamental particles and their interactions.

7
00:00:40,900 --> 00:00:46,210
But a lot of the emphasis is on dark matter while searching for things like supersymmetry,

8
00:00:46,210 --> 00:00:50,680
understanding dark energy, understanding the hierarchy of scales.

9
00:00:50,680 --> 00:00:56,570
Why do we live with scales of the order of a G.V. when the fundamental scale or

10
00:00:56,570 --> 00:01:01,000
unified where you unify gravity with all the other forces is a 10 to the 19 G.V.?

11
00:01:01,000 --> 00:01:06,100
What's the origin of that gap? A factor of 10 to the 19?

12
00:01:06,100 --> 00:01:13,010
These are big questions, important questions. But we don't know how quite to get the answers.

13
00:01:13,010 --> 00:01:22,040
We know of places where we can look, but we don't know if we'll get the answers anytime in our lifetime of this century.

14
00:01:22,040 --> 00:01:27,640
So meanwhile, you've got to do something.

15
00:01:27,640 --> 00:01:35,050
And luckily, there are some big answerable questions, questions that it turns out we can't get answers right now.

16
00:01:35,050 --> 00:01:40,300
And that's what this talk and to some extent the next two talks are going to be about.

17
00:01:40,300 --> 00:01:50,120
But what these questions are, which is mostly my talk and some of the aspects about how we go about answering them.

18
00:01:50,120 --> 00:01:56,560
So particle physics talks often show the particles.

19
00:01:56,560 --> 00:02:04,180
We've got the folks here, we've got the leptons like the electron me on in the tau neutrinos and the force carriers,

20
00:02:04,180 --> 00:02:10,280
plus the latest arrival on the block, the Higgs boson.

21
00:02:10,280 --> 00:02:15,970
You've probably all heard about the Higgs boson discovery.

22
00:02:15,970 --> 00:02:24,910
That's a plot that shows what people mean when they discover the Higgs boson they take for electrons, when you look at them in the centre of mass.

23
00:02:24,910 --> 00:02:32,560
Add up their energies and then they plot the number of events that they get as a function of the energy.

24
00:02:32,560 --> 00:02:40,360
And you see that there's a cluster of events here or with about the same energy or invariant mass more technically.

25
00:02:40,360 --> 00:02:44,350
And that is the Higgs boson. That's the evidence that we have for the Higgs boson.

26
00:02:44,350 --> 00:02:53,790
It's pretty strong evidence now. This plot shows more recent data than when the discovery took place.

27
00:02:53,790 --> 00:02:58,980
So people often say success, the standard model is complete,

28
00:02:58,980 --> 00:03:10,170
you'll hear many people particle physicists included saying that, and then the next step is, Oh dear.

29
00:03:10,170 --> 00:03:26,830
And this is not just particle physicist saying this. The New York Times says it's a cloud hanging over the physics community.

30
00:03:26,830 --> 00:03:33,340
So what is the standard model? Well, it's a bunch of particles, great. But.

31
00:03:33,340 --> 00:03:37,990
It's worth thinking a second. What would life be like if we just had the particles?

32
00:03:37,990 --> 00:03:44,230
We're made of electrons and up quarks and down quarks.

33
00:03:44,230 --> 00:03:50,550
Now, if before we had with the up quarks, down quarks and electrons, they'd be flying around. We wouldn't exist.

34
00:03:50,550 --> 00:03:58,040
It's the interactions between them. That make the world as it is, the strong force holds the Upwork's and uncorks together into protons,

35
00:03:58,040 --> 00:04:05,480
neutrons and from their nuclei, electromagnetism holds the nuclei together with electrons.

36
00:04:05,480 --> 00:04:11,760
And gives us all of molecular physics, chemistry, biology and so forth.

37
00:04:11,760 --> 00:04:18,790
The interactions are what matters. And if you go to sin, you can go to there.

38
00:04:18,790 --> 00:04:31,960
They have a great visitor centre. You can go to their shop. But I doubt their most famous product is a T-shirt.

39
00:04:31,960 --> 00:04:40,420
This is John Ellis who wrote this out, and he even got it knitted onto a jumper for himself.

40
00:04:40,420 --> 00:04:49,790
Well, somebody did it for him. This T-shirt is about the interactions, you can get it in all sizes.

41
00:04:49,790 --> 00:05:05,630
My son loves it, too. Now, when you buy the T-shirt, it comes with a little card.

42
00:05:05,630 --> 00:05:11,630
And that crowd does two things. The original version of the T-shirt actually had something that many people feel is a typo.

43
00:05:11,630 --> 00:05:16,230
It was next to H.S. there. So the court sort of gives you the version without the tape.

44
00:05:16,230 --> 00:05:25,690
They printed all the T-shirts already. And if you buy the kids ones, they don't sell as many. So the Typekit still on that one.

45
00:05:25,690 --> 00:05:31,120
But what I want to concentrate on a second is this sentence here.

46
00:05:31,120 --> 00:05:40,510
This equation neatly sums up our understanding of fundamental particles and forces the two together that make up the standard model.

47
00:05:40,510 --> 00:05:45,990
And understanding is a very funny word. Why?

48
00:05:45,990 --> 00:05:52,610
Well, is it knowledge? Is it what we think is there?

49
00:05:52,610 --> 00:06:03,240
Which is it? So what I'm going to do during this talk is try to analyse this.

50
00:06:03,240 --> 00:06:08,580
Might look a little bit forbidding if you expand it out with the details.

51
00:06:08,580 --> 00:06:13,640
That's what it looks like.

52
00:06:13,640 --> 00:06:25,930
I'm not going to go through all of this, but I'll try to give you a flavour of some of what is in this so we can think about what this T-shirt means.

53
00:06:25,930 --> 00:06:37,980
Now, the first part I want to talk about is up here. Basically, what this is is the quantum formulation of Maxwell's equations.

54
00:06:37,980 --> 00:06:47,990
Not just for electromagnetism, but their analogues, for the strong force and for the for the weak interactions.

55
00:06:47,990 --> 00:06:57,480
Now, if there's one thing to pull out of this kind of formula and something that I will be reusing throughout the lecture.

56
00:06:57,480 --> 00:07:09,090
It's trying to understand what this term means. So first of all, what is CI so indicates a firming up, for example, an electron or a coke?

57
00:07:09,090 --> 00:07:14,640
What specifically the field for it. The presence of electrons excitations D.

58
00:07:14,640 --> 00:07:22,600
Well, it's a derivative plus a bunch of other things. And what we're going to care about is that it contains a photon filled.

59
00:07:22,600 --> 00:07:28,620
So when you multiply three times, oops, something went wrong, that button.

60
00:07:28,620 --> 00:07:34,770
Well. OK, we'll get there.

61
00:07:34,770 --> 00:07:43,480
So when you multiply three times, the vote on field climbs aside, what you've got is an interaction.

62
00:07:43,480 --> 00:07:51,670
This is a fundamental interaction, vertex and particle physics. For example, it can be an electron coming in, going out and in the process,

63
00:07:51,670 --> 00:08:00,890
it's a a photon or it can be a coke going in and it's emitted a gluon or it can be a cork and it's emitted AWB,

64
00:08:00,890 --> 00:08:05,420
or which is the origin of radioactive decay.

65
00:08:05,420 --> 00:08:16,170
So this term tells you there have been electron, there are electron proton interactions and all the other interactions of that kind.

66
00:08:16,170 --> 00:08:23,520
Well, when people tell you and you may have heard this, the standard model has been probed and found to be successful over and over again.

67
00:08:23,520 --> 00:08:29,670
What they're essentially telling you is that they've tested this. We've been testing this since the 1860s in a certain sense.

68
00:08:29,670 --> 00:08:34,200
This is Maxwell's equations and many variants, many important.

69
00:08:34,200 --> 00:08:43,690
It's much more than just Maxwell's equations, but the same idea of what is called a gauge interaction is at the basis of this.

70
00:08:43,690 --> 00:08:51,750
And this really works. Works after high precision, the best tests go up to one part in 10 to the eight.

71
00:08:51,750 --> 00:08:59,300
We have a lot of confidence in this. The part.

72
00:08:59,300 --> 00:09:03,770
That I'm going to talk about the rest of this lecture, is this one?

73
00:09:03,770 --> 00:09:09,780
You see a five here, five means the Higgs field, and we're going to have that coming back over and over again,

74
00:09:09,780 --> 00:09:18,060
and until seven years ago, no one had ever observed any of these terms directly.

75
00:09:18,060 --> 00:09:29,640
So let's start with the Higgs field. What is it? Well, the Higgs field has a potential turn, and we set at the minimum of a potential,

76
00:09:29,640 --> 00:09:33,150
this potential is actually much more to it than just this one dimension.

77
00:09:33,150 --> 00:09:38,280
There's four dimensions of the Higgs field, but we're going to simplify it and just look at one dimension.

78
00:09:38,280 --> 00:09:45,470
It's what a minimum here in a minimum there. The very special thing about the Higgs field and.

79
00:09:45,470 --> 00:09:52,890
This is a hypothesis. The very special thing about the Higgs field is that its minimum is not a zero.

80
00:09:52,890 --> 00:09:57,120
The way the the potential is arranged is that at zero you go down because of that

81
00:09:57,120 --> 00:10:04,910
minus sign and then you come up because the plus sign multiplying five to the fourth.

82
00:10:04,910 --> 00:10:11,210
So you've got this potential at every point in space. All around us.

83
00:10:11,210 --> 00:10:20,890
And all around us, we're sitting at something called the vacuum expectation value.

84
00:10:20,890 --> 00:10:23,090
Which has got this value in terms of these constants.

85
00:10:23,090 --> 00:10:29,090
The exact value doesn't matter, what matters is that you've got some, some non-zero value for the field.

86
00:10:29,090 --> 00:10:34,040
So what does the Higgs boson? The Higgs boson is an exhortation of this field.

87
00:10:34,040 --> 00:10:39,220
So this illustrates the field oscillating back and forth.

88
00:10:39,220 --> 00:10:44,770
And. You can turn this round minus the glitch.

89
00:10:44,770 --> 00:10:49,480
This field exists at every point in space, so the Higgs boson at a given point in space,

90
00:10:49,480 --> 00:11:03,340
the Higgs boson here is this field going up and down at this point in space here.

91
00:11:03,340 --> 00:11:11,750
That there is a Higgs boson is established. That's what was established in 2012 with the discovery.

92
00:11:11,750 --> 00:11:16,970
That you have this particular form of the potential is a hypothesis.

93
00:11:16,970 --> 00:11:24,040
There's no direct evidence of that whatsoever. We know that there's we know that there's a minimum.

94
00:11:24,040 --> 00:11:34,600
We know we can produce oscillations around it, but we don't know. Whether this form here is right or wrong.

95
00:11:34,600 --> 00:11:38,830
Theorists will tell you I'm a serious I should be telling you,

96
00:11:38,830 --> 00:11:43,720
that's the only form you can write down in four dimensions for a scalar field a field without spin.

97
00:11:43,720 --> 00:11:57,820
That's the only option. Now, that's great. But physicists job is not to listen to theorists or let me rephrase that.

98
00:11:57,820 --> 00:12:07,540
It's great to listen to theorists, but you need to establish what's actually out there in nature.

99
00:12:07,540 --> 00:12:15,110
What are the kinds of terms do you have in there? Well, let's take this defy.

100
00:12:15,110 --> 00:12:22,160
This is the this was actually the origin of thinking about this kind of problem of a Higgs field.

101
00:12:22,160 --> 00:12:30,590
When you expand it out, you get two kinds of term. You get something some bunch of constants which multiply two fields.

102
00:12:30,590 --> 00:12:36,550
And whenever you see that in particle physics, that tells you there's a mass.

103
00:12:36,550 --> 00:12:40,830
You multiply to two fields. That's the field going from one place to another place.

104
00:12:40,830 --> 00:12:45,450
And there's something that happens in that process and it involves a mess.

105
00:12:45,450 --> 00:12:54,540
That's what that constant tells you. Then there's another term which involves three fields.

106
00:12:54,540 --> 00:12:59,590
Two sides in this case. Plus the Higgs. What's happened this FY?

107
00:12:59,590 --> 00:13:03,030
The fire got expanded to five zero plus h.

108
00:13:03,030 --> 00:13:13,620
And because of that expansion, here is the excitation of the Higgs field, and five zero is the value at rest if you like the vacuum.

109
00:13:13,620 --> 00:13:18,140
So you always find a combination of five zero plus h appearing everywhere.

110
00:13:18,140 --> 00:13:22,460
And this term gives you a mouse, and this term gives you an interaction,

111
00:13:22,460 --> 00:13:26,840
and the special characteristic of this of the Higgs mechanism is that you always

112
00:13:26,840 --> 00:13:37,130
find these two things pet an origin for mass at an associated interaction.

113
00:13:37,130 --> 00:13:43,020
So this particular interaction was actually one of the interactions through which the Higgs was discovered.

114
00:13:43,020 --> 00:13:48,420
So this block that I showed you here shows the Higgs decaying to a z to two sets.

115
00:13:48,420 --> 00:13:54,150
So the Higgs interacting with two sets, that's how people discover the Higgs.

116
00:13:54,150 --> 00:14:03,270
And that's what led to the 2013 Nobel prise for the brute Anglo Higgs mechanism.

117
00:14:03,270 --> 00:14:12,730
Showing that the mass of Z related directly to the interaction with the Higgs boson established that mechanism.

118
00:14:12,730 --> 00:14:23,340
That's not the only thing that's that that was that was the origin for introducing the Higgs concept, try to get masses for four force carriers.

119
00:14:23,340 --> 00:14:32,400
No one had figured out how to do that consistently beforehand. But then once you had this field around.

120
00:14:32,400 --> 00:14:42,460
Others came along and said, well, we can write down all the terms. This term here involves fermions.

121
00:14:42,460 --> 00:14:47,740
And when you expand it, you find there's a constant multiplying of professions.

122
00:14:47,740 --> 00:14:56,950
And that's a fair Myanmar's. And you find there's a constant multiplying a Higgs exhortation and two famines.

123
00:14:56,950 --> 00:15:03,750
And that's an interaction between the Higgs and those two families, the Higgs boson and those two famines.

124
00:15:03,750 --> 00:15:10,140
And people said, Oh, we could use this to explain, explain, maybe an inverted commas,

125
00:15:10,140 --> 00:15:17,340
the masses of all the fermions we know, or at least the the quirks and the charge leptons.

126
00:15:17,340 --> 00:15:23,810
Just by putting in different values for these wives, this constant arbitrary values.

127
00:15:23,810 --> 00:15:29,840
We don't know where they come from, but if we stick in some values for these wise. These are the values you stick in.

128
00:15:29,840 --> 00:15:37,060
We don't know what to do with neutrinos here. You'll explain the masses of all the particles.

129
00:15:37,060 --> 00:15:51,180
All the all the metaphorical for the families. Now, that's a very simple idea, and it's a step towards understanding perhaps where masses come from.

130
00:15:51,180 --> 00:15:56,230
We don't know whether masses of particles come from. But if this is the mechanism,

131
00:15:56,230 --> 00:16:04,850
we know that what we need to do to understand it is to explain to understand the origin of these blue couplings as they're called.

132
00:16:04,850 --> 00:16:10,490
The thing is, this is insane in a part, in part of hypotheses, too.

133
00:16:10,490 --> 00:16:20,150
We don't actually know if this is what's going on to explain masses of particles. So what's special about that?

134
00:16:20,150 --> 00:16:25,600
Well, it's not just that it generates firmly on message, but it's a very.

135
00:16:25,600 --> 00:16:33,490
Unusual interaction. It's the first fundamental infraction, or apparently fundamental interaction that we see at the quantum level,

136
00:16:33,490 --> 00:16:36,460
where the interaction strength is not quantised.

137
00:16:36,460 --> 00:16:42,530
So if you take electromagnetism, everything is quantised in terms of the electric, the electric charge.

138
00:16:42,530 --> 00:16:45,770
You have a chart of one child of two kind of a chart of one third.

139
00:16:45,770 --> 00:16:52,500
But you can't have a chart of point zero three six five four three two, whatever doesn't happen.

140
00:16:52,500 --> 00:16:57,710
With this, you call our interactions. You can have an arbitrary value, apparently.

141
00:16:57,710 --> 00:17:00,480
That's qualitatively very different.

142
00:17:00,480 --> 00:17:07,860
The first time we see something like that in a fundamental interaction, gravity is sort of like that in a loose sense.

143
00:17:07,860 --> 00:17:15,460
But this is not gravity is not something we've probed at the quantum level.

144
00:17:15,460 --> 00:17:20,600
So let's take a step backwards. Why do we care about matters?

145
00:17:20,600 --> 00:17:29,930
Let's. There are many illustrations that I could give you, but here's one.

146
00:17:29,930 --> 00:17:42,870
Let's take the cooks up, cooks and cooks. Now on Upwork has a massive of about two MTV, a down caucus, a mass of about four and a half MTV.

147
00:17:42,870 --> 00:17:49,140
What's a proton? It's a two hops and one down. So let's.

148
00:17:49,140 --> 00:17:55,350
Take the up and the up in the down. That's two of these lighter things the Upwork's and one heavier one.

149
00:17:55,350 --> 00:18:01,080
And we take the neutron. It's an up and down on a down one lighter and two heavier.

150
00:18:01,080 --> 00:18:06,720
And the result of that is that the proton is a bit lighter than the neutron.

151
00:18:06,720 --> 00:18:10,410
Now you might genuinely and with good reason ask, what about those three dots?

152
00:18:10,410 --> 00:18:16,330
I mean, this is silly. That's 900 and something missing here, and you're telling me about the two.

153
00:18:16,330 --> 00:18:29,450
Well. The nine hundred and something here all comes from a strong force which doesn't really care about the masses of the quirks too much.

154
00:18:29,450 --> 00:18:34,280
There's a little bit extra coming from associated with the QED affects the charge of voting,

155
00:18:34,280 --> 00:18:37,790
but basically all of these three dots don't change the underlying picture,

156
00:18:37,790 --> 00:18:43,420
which is that these masses make the proton a little bit lighter from the neutron.

157
00:18:43,420 --> 00:18:54,650
Now, if a particle is lighter than another one, it's stable, so the proton is stable, the neutron is not, the neutron can decay into protons.

158
00:18:54,650 --> 00:19:02,090
If it weren't for these matters, if we had no UCAR interactions, if we had no masses for the Hawks,

159
00:19:02,090 --> 00:19:06,560
then you wouldn't have this pattern to the protons light of a neutron. It would be the other way around.

160
00:19:06,560 --> 00:19:10,640
The neutron would be like two protons would decay into neutrons.

161
00:19:10,640 --> 00:19:17,000
After a few minutes, there would be no hydrogen.

162
00:19:17,000 --> 00:19:21,950
Now, if you really want to be sophisticated, you can play the game and ask, OK, but I have nuclear physics. Could I create nuclei?

163
00:19:21,950 --> 00:19:25,660
And it gets a bit more subtle than maybe you can have deuterium.

164
00:19:25,660 --> 00:19:32,370
But the world will be a very different place if we only had deuterium versus hydrogen.

165
00:19:32,370 --> 00:19:38,390
So. This pattern of coke masses, this hypothesis that they come from the UK,

166
00:19:38,390 --> 00:19:47,110
our interactions is what gives us the hydrogen atom and chemistry and biology.

167
00:19:47,110 --> 00:19:59,880
So it'd be nice to know if this is true. There's one other recent example that we can give, which is for the electron.

168
00:19:59,880 --> 00:20:07,500
Now the radius of an atom, the border radius basically goes one over the electron mass.

169
00:20:07,500 --> 00:20:11,310
You can write it in different ways, depending on which constants you prefer.

170
00:20:11,310 --> 00:20:18,720
At the end of the day, if the electron mass is proportional to the ukwa coupling the Higgs field, then this is one over the you call a company.

171
00:20:18,720 --> 00:20:25,890
So this these kinds of couplings also set the size of atoms and sets the energy levels of all chemical reactions.

172
00:20:25,890 --> 00:20:35,600
It's OK, be nice to know if it's true. So how do we go about that?

173
00:20:35,600 --> 00:20:44,070
Well. The first generation has a low mass because these interactions with the Higgs field are very weak.

174
00:20:44,070 --> 00:20:51,640
10 to the minus six. The problem is that when the interaction is very weak, it's very hard to measure.

175
00:20:51,640 --> 00:20:58,300
So essentially today, no one has a clue about how to establish whether it really is interactions with

176
00:20:58,300 --> 00:21:03,580
the Higgs field or responsible for the mass of this part of these particles.

177
00:21:03,580 --> 00:21:12,250
But if you take the third generation, the masses are quite large, which translates to large interactions with the Higgs field.

178
00:21:12,250 --> 00:21:19,090
And so there you can potentially test it.

179
00:21:19,090 --> 00:21:28,390
And that's where the LHC comes in. The LHC, the Large Hadron Collider at CERN in Geneva, is the machine that discovered the Higgs boson.

180
00:21:28,390 --> 00:21:32,650
And it was built in order to discover the Higgs boson, amongst other things,

181
00:21:32,650 --> 00:21:38,270
but it didn't just discover the Higgs boson and stop and stop it's carried on.

182
00:21:38,270 --> 00:21:46,170
So what does it do? Well, it takes protons and sends them around a series of rings.

183
00:21:46,170 --> 00:21:52,860
Which accelerate them to successively higher energies, let's twenty five give either.

184
00:21:52,860 --> 00:22:00,760
Then up to four hundred and fifty give. And then it sends them into the big 27km ring.

185
00:22:00,760 --> 00:22:11,290
Well, it's got some TV proton. Since then around this tunnel with the complete with a large of.

186
00:22:11,290 --> 00:22:19,360
Crosses the border many times a second. And that there's a proton with three quarks and lots more.

187
00:22:19,360 --> 00:22:27,490
But listen, we'll tell you about later. And protons come from both directions and collide.

188
00:22:27,490 --> 00:22:34,260
And four big experiments called Atlas CMS and HCB and Alex.

189
00:22:34,260 --> 00:22:45,390
And produce hundreds of particles in each collision.

190
00:22:45,390 --> 00:22:54,180
So this happens well, it happens 40 million times a second each time there are collisions, it's actually not one collision,

191
00:22:54,180 --> 00:23:05,650
but about 30, 40, 50 collisions happening at the same time, slightly lower rates that are actually being released.

192
00:23:05,650 --> 00:23:14,870
And the question you might ask is, well, OK, what can we do with it? What underlying processes tell us about what's going on?

193
00:23:14,870 --> 00:23:24,330
So are we going to do is we're going to look. At a very idealised version of particle collisions and pollution and Fabrizio later on,

194
00:23:24,330 --> 00:23:31,310
we'll tell you a little less idealised version of particle collisions.

195
00:23:31,310 --> 00:23:36,770
What we going to do is we're going to talk not about protons colliding, but about gluons colliding.

196
00:23:36,770 --> 00:23:47,810
You've got gluons inside protons. You'll find out more in that. Two gluons collide and they produce.

197
00:23:47,810 --> 00:23:56,810
A virtual top, a top that bursts is a top tier top quarter that bursts into existence for a tiny fraction of a second.

198
00:23:56,810 --> 00:24:03,970
But and then annihilates again to produce a Higgs boson.

199
00:24:03,970 --> 00:24:12,670
If the Higgs couples to the top, if the Yukawa hypothesis that the that the Higgs field is responsible for the top mass,

200
00:24:12,670 --> 00:24:17,830
then there's an interaction in the top and the Higgs expectations, the.

201
00:24:17,830 --> 00:24:24,340
And you expect to see that happen once for every two billion proton proton collisions.

202
00:24:24,340 --> 00:24:30,180
Now you've got two billion collisions a second. So that's actually happening quite often.

203
00:24:30,180 --> 00:24:35,610
That was the LHC data was consistent with that already of discovery in 2012.

204
00:24:35,610 --> 00:24:41,130
So let's look at that a little more detail. You have to gluons coming in. They don't interact with the Higgs field this punchier,

205
00:24:41,130 --> 00:24:51,530
but they produce talks which do interact with the Higgs field today and leave an expectation of the Higgs field.

206
00:24:51,530 --> 00:24:59,730
And after a while, that creates another top there. Which then annihilate and produce two photons.

207
00:24:59,730 --> 00:25:06,580
So this is the thing that happens two billion times once every two billion collisions.

208
00:25:06,580 --> 00:25:15,560
And what you can do is you can look at the invariant mass of the two photons, some of their energies in the rest frame of that system.

209
00:25:15,560 --> 00:25:20,420
And, well, there are lots of other processes that produce pairs of photons,

210
00:25:20,420 --> 00:25:27,050
and you can plot how many events you get as a function of the energy of the inherent mass of that photon pair,

211
00:25:27,050 --> 00:25:32,790
and you've got a background here from all the other events that produce them. And you have a little bump here.

212
00:25:32,790 --> 00:25:38,720
And that little bump was the other evidence for the Higgs discovery. I showed you a peak from Z.Z before.

213
00:25:38,720 --> 00:25:47,050
That's from the photon. And that gives quite compelling, indirect evidence that there.

214
00:25:47,050 --> 00:25:56,970
That there is something happening here that is like what you expect. The difficulty is that you don't know what's happened in this doctor.

215
00:25:56,970 --> 00:26:01,980
I can show you an animation of what's going on, but you don't actually see that in real life.

216
00:26:01,980 --> 00:26:08,370
All you see is protons coming in. Photons coming out.

217
00:26:08,370 --> 00:26:16,670
So the question is, is, can you do something that actually convinces you that it really is interacting with a top?

218
00:26:16,670 --> 00:26:22,970
Well, the answer is, if you can actually produce a top that you see, not just a virtual excitation of a top power,

219
00:26:22,970 --> 00:26:30,180
nothing that flits in and out of existence, but something that lasts for a while.

220
00:26:30,180 --> 00:26:34,800
Well, once you produce the top, it's much more likely that you can actually produce eggs.

221
00:26:34,800 --> 00:26:41,180
Very unlikely thing in the diagram I showed you a few slides ago is to actually produce the top in the first place.

222
00:26:41,180 --> 00:26:47,660
But if you can actually show that you've produced a top. Then you expect to see one Higgs.

223
00:26:47,660 --> 00:26:53,050
Not every two billion collisions, but every six hundred.

224
00:26:53,050 --> 00:26:58,810
It's much more likely because of the strong interaction of the Higgs with the top. So what does that look like?

225
00:26:58,810 --> 00:27:06,300
Well. You've got the gluon coming in, producing a top pair, which excites the Higgs field at the same time,

226
00:27:06,300 --> 00:27:11,660
the top just go off now gotten to be something to be seen somewhere new detector.

227
00:27:11,660 --> 00:27:20,620
So you tag the presence of the tops. And then in the events where you've tied the presence of the tops,

228
00:27:20,620 --> 00:27:29,620
you again count the number of events per bin of invariant mass of the two photons and you see a peak.

229
00:27:29,620 --> 00:27:38,780
And you see a peak that is at a rate that is consistent with one for every six top pairs.

230
00:27:38,780 --> 00:27:46,670
That's the plot from Atlas from about a year ago. This is the same plot from CMS around the same time,

231
00:27:46,670 --> 00:27:54,380
the this plot is much more readable because you see a physical cognitive impairment mass of two photons.

232
00:27:54,380 --> 00:28:04,140
CMS has done a more has shown a different kind of fiscal analysis, but it contains the same message.

233
00:28:04,140 --> 00:28:12,630
So this establishes this hypothesis that interactions of the Top Rock with the Higgs field.

234
00:28:12,630 --> 00:28:19,200
Are responsible for the mass of the top quark to within 20 percent.

235
00:28:19,200 --> 00:28:30,290
So that's one sort of one of the nine quarks and leptons we want to figure out if this is true or not.

236
00:28:30,290 --> 00:28:37,840
That. The other thing you can do is look at takes the case, for example, a Higgs can decay to a tile plus or minus.

237
00:28:37,840 --> 00:28:45,070
This is the third generation Lepton, and within the standard model Higgs style, you have a coupling hypothesis.

238
00:28:45,070 --> 00:28:49,090
This happens for one in every 16 Higgs boson decays let out last time.

239
00:28:49,090 --> 00:28:59,950
Minus times are difficult objects to play with, but they managed, and this time CMS has the slightly more compelling plot.

240
00:28:59,950 --> 00:29:07,450
And you see here a peak in the mass of the Tao Plus Tao, minus that it coincides roughly with the Higgs mass.

241
00:29:07,450 --> 00:29:11,200
You can't measure tiles very well, so it's not a not a super precise thing,

242
00:29:11,200 --> 00:29:16,630
but we don't know of any other particles around that mass and the rate is consistent with what you'd expect.

243
00:29:16,630 --> 00:29:24,770
So that tells you you've got you've established this as the origin for the tail mass.

244
00:29:24,770 --> 00:29:34,580
How about the other third generation part of the B? This is about 58 percent of Higgs boson, such dictator BS, because the B is the mass,

245
00:29:34,580 --> 00:29:43,990
the heaviest particle to which the Higgs boson can actually decay without violating energy conservation, essentially.

246
00:29:43,990 --> 00:29:50,260
Well, this was actually one of the toughest to see of the ones that are being seen so far when people were about to turn on the U.S.,

247
00:29:50,260 --> 00:29:58,490
they thought this would be impossible. In the meantime, ideas have come about.

248
00:29:58,490 --> 00:30:06,260
We contributed to that. Back in 2008 about how to actually go about seeing this Higgs to be OK.

249
00:30:06,260 --> 00:30:10,940
On a huge background of reproduction that don't like us.

250
00:30:10,940 --> 00:30:20,660
And you see from both Atlas and CMS again, a peek, a pretty broad peek because you don't measure the mass of the system very well.

251
00:30:20,660 --> 00:30:30,040
A pretty broad peak around 125 of the Higgs mass of events that are consistent with Higgs decays.

252
00:30:30,040 --> 00:30:38,700
So what does this mean? Well. The Five Sigma Statistical Golden Grail,

253
00:30:38,700 --> 00:30:48,420
the holy grail of particle physics observation of these three processes takes Higgs to kowtow and has to be independently by two experiments.

254
00:30:48,420 --> 00:31:00,060
This is really the standard you need firmly established existence of a new kind of fundamental interaction.

255
00:31:00,060 --> 00:31:05,160
The interactions are important because, first of all, because they qualitatively unlike anything we've seen before.

256
00:31:05,160 --> 00:31:10,740
They're not what I called gauge interactions. They don't come with an underlying unit of charge.

257
00:31:10,740 --> 00:31:18,330
And they hypothesise to be responsible for the stability of hydrogen, the size of atoms and a whole lot more.

258
00:31:18,330 --> 00:31:30,180
Establishing the pattern of nuclear couplings across the four remaining set of fermions is one of the big challenges of particle physics today.

259
00:31:30,180 --> 00:31:33,810
And you could ask yourself, is this any less important than actually discovering things going on in the first place?

260
00:31:33,810 --> 00:31:43,440
And I would say no, because interactions are just as important as the particles, the particles without the interactions are very boring.

261
00:31:43,440 --> 00:31:50,030
Another thing you could say about this is that it's a fifth force. Because the Higgs carries a force between fragments.

262
00:31:50,030 --> 00:31:59,550
If you've got two top pairs, two tops coming in, they interact because they feel a strong force between because of the Higgs.

263
00:31:59,550 --> 00:32:08,810
It's up to you to decide whether you prefer to talk about new interactions or new forces. But either way, it is qualitatively new.

264
00:32:08,810 --> 00:32:14,660
OK, so let's look at the guy in that table of particles of fermions.

265
00:32:14,660 --> 00:32:32,360
And what we've done, we what Allison seems to have done with some help from theorists is establish the newcomers here to within roughly 20 percent.

266
00:32:32,360 --> 00:32:36,730
With the LHC, there's one more of these that can be done.

267
00:32:36,730 --> 00:32:43,340
Which is the me. There's no evidence for this today. It's quite a rare process.

268
00:32:43,340 --> 00:32:49,440
Don't you only produce millions and one one in 4000 decays roughly.

269
00:32:49,440 --> 00:32:56,740
But with more data to come along, you can see this.

270
00:32:56,740 --> 00:33:04,990
Trump, there's no evidence. We'll come back to that in a second. But there's we know how we could get there with the regulator and the other three,

271
00:33:04,990 --> 00:33:09,310
the other four, which together account for about one in four thousand decades.

272
00:33:09,310 --> 00:33:21,960
We just don't know how to actually establish whether the Higgs hypothesis that you call a hypothesis is true for this.

273
00:33:21,960 --> 00:33:27,750
There's also something which has to do with the Higgs width. I'll come back to that in a second.

274
00:33:27,750 --> 00:33:38,010
Well, there's lots more that you can do with the Higgs, far more than you probably would like to hear.

275
00:33:38,010 --> 00:33:42,540
Let's have a brief interlude. What about the big questions? Why aren't we seeing answers?

276
00:33:42,540 --> 00:33:54,430
We were promised answers. Why haven't we gotten? Well, let's talk about one in particular, which is dark matter.

277
00:33:54,430 --> 00:34:01,510
Somebody a little incautious. Well, it's a challenge.

278
00:34:01,510 --> 00:34:11,810
It remains a challenge. There was a lot of talk of this finding dark matter at the LHC.

279
00:34:11,810 --> 00:34:18,200
Now, we suspect dark matter exists, there's very strong evidence from galaxies and how they rotate,

280
00:34:18,200 --> 00:34:21,110
this is how you'd expect them to rotate without dark matter,

281
00:34:21,110 --> 00:34:28,010
and the points here show how they actually rotate because many other sources of evidence for something like dark matter.

282
00:34:28,010 --> 00:34:37,130
And there were compelling, theoretically elegant and compelling theories that suggested dark matter might be within reach of the LHC.

283
00:34:37,130 --> 00:34:41,270
So the LHC has searched for it and has been very successful at excluding certain

284
00:34:41,270 --> 00:34:45,470
scenarios of dark matter and other kinds of experiment called direct detection,

285
00:34:45,470 --> 00:34:52,640
actually big tanks sitting deep underground, waiting for dark matter to come and hit and an atom or a nucleus.

286
00:34:52,640 --> 00:34:59,890
I've also searched and been very successful in covering potential face space, but haven't seen anything.

287
00:34:59,890 --> 00:35:06,900
But. This is one variant of dark matter, and there are many variants possible of dark matter.

288
00:35:06,900 --> 00:35:14,460
This is a plot from 2013 showing all the variants of dark matter that people had in mind at the time.

289
00:35:14,460 --> 00:35:24,840
This is sort of a map of what people call theory space. This here shows a plot of mass and interaction strength.

290
00:35:24,840 --> 00:35:32,260
Now you can't read it. I can't read it, even though I'm standing right next to the screen.

291
00:35:32,260 --> 00:35:37,880
But basically, every tick here is a factor of 10 to the three in mass.

292
00:35:37,880 --> 00:35:47,720
I think or maybe even 10 to the six and the bit that the LHC is sensitive to is this little bit here.

293
00:35:47,720 --> 00:35:53,280
Now, this little bit here is is in some sense, theoretically compelling, but it's not the only option.

294
00:35:53,280 --> 00:35:57,150
There's a huge space over which one could search for dark matter.

295
00:35:57,150 --> 00:36:04,650
Over which dog, much of my life, for that matter, dark matter might not interact anything other than gravitationally.

296
00:36:04,650 --> 00:36:07,140
So you need to search for it. You want a search.

297
00:36:07,140 --> 00:36:18,630
We want to find out what it is, but we have to keep in mind that this is not a problem that you will necessarily answer tomorrow or solve tomorrow.

298
00:36:18,630 --> 00:36:22,830
So what do we do? Well, we carry on looking for it.

299
00:36:22,830 --> 00:36:26,730
But in the meantime, we figure out what else we can do.

300
00:36:26,730 --> 00:36:39,750
And, for example, amongst approved plans, the LHC will collect between 40 and 100 times more data than than was used for the plots that I showed you.

301
00:36:39,750 --> 00:36:46,510
At mostly similar energy. And that's a programme called the high luminosity LFC.

302
00:36:46,510 --> 00:36:53,710
One of the axes of this programme is that you collect more statistics for the things you've already observed.

303
00:36:53,710 --> 00:37:02,620
For example, Higgs decay into gamma gamma. You can use gamma gamma as the the ratio of the observed rate of production of

304
00:37:02,620 --> 00:37:06,670
Higgs going to gamma gamut of what you expect from theoretical predictions.

305
00:37:06,670 --> 00:37:17,530
And this is a plot showing that my extrapolation of the precision you expect as a function of how many collisions are collected by the LHC.

306
00:37:17,530 --> 00:37:20,410
So this was at the time, shortly after discovery.

307
00:37:20,410 --> 00:37:28,980
This is what has been analysed today, roughly one investment about 10 to the 14 collisions, 100 trillion collisions.

308
00:37:28,980 --> 00:37:37,620
And by the end of the LHC, it will have collected almost well, a third of a quintillion collisions.

309
00:37:37,620 --> 00:37:48,420
By 2036 ish roughly. From a statistical point of view that has the potential to bring the precision down to a few percent.

310
00:37:48,420 --> 00:37:54,920
A couple of one or two percent of. Why do you care about that?

311
00:37:54,920 --> 00:37:58,640
Well, you've got a new interaction, a new kind of force.

312
00:37:58,640 --> 00:38:05,170
You wouldn't say you've established electromagnetism if you'd only tested it within 20 percent.

313
00:38:05,170 --> 00:38:08,550
You want us to go beyond that.

314
00:38:08,550 --> 00:38:15,720
And the point of part of the point about all of this high luminosity, Alexie, is that it can deliver one to two percent.

315
00:38:15,720 --> 00:38:27,080
Hopefully for a range of couplings with the caveat. If the theoretical interpretations can be made sufficiently accurate, let's just take one example.

316
00:38:27,080 --> 00:38:30,500
This shows the expected uncertainty on a particular interaction with the bees.

317
00:38:30,500 --> 00:38:37,210
Peacock's broken up into different parts the statistical part.

318
00:38:37,210 --> 00:38:47,980
Some uncertainty related to the detectors and an uncertainty related to our ability to connect the underlying theory with what happens at colliders.

319
00:38:47,980 --> 00:38:54,750
And in this case, this dominates almost three times larger than the other uncertainties.

320
00:38:54,750 --> 00:38:58,740
And so really, being able to exploit the AFC and understand what's going on depends,

321
00:38:58,740 --> 00:39:06,570
crucially on our theoretical understanding of the processes that happen at large at the Hadron Collider.

322
00:39:06,570 --> 00:39:13,080
It's not the case at all for everything, for example, for the muon. It's not the theory that dominates at this stage.

323
00:39:13,080 --> 00:39:18,080
It's other things. It's mainly statistics.

324
00:39:18,080 --> 00:39:28,040
And this will be a very important milestone for that to see, because this be the first establishment of a second generation you call interaction.

325
00:39:28,040 --> 00:39:37,640
What about beyond that? Well, people today are putting on paper plans for colliders beyond their legacy plans in China.

326
00:39:37,640 --> 00:39:42,680
There are plans in Japan and there are plans in Europe at some.

327
00:39:42,680 --> 00:39:48,480
There are many different kinds of flyers electron, positron, proton, proton, electron, proton.

328
00:39:48,480 --> 00:39:54,530
And I'll just give you a very brief overview.

329
00:39:54,530 --> 00:40:00,650
Essentially, let's start with Electrum gliders. They're a great acquisition.

330
00:40:00,650 --> 00:40:06,680
They're also very clean. They can give you the charm quite nicely.

331
00:40:06,680 --> 00:40:10,250
Another second generation coupling.

332
00:40:10,250 --> 00:40:16,400
And the other thing is, they can tell you, does the Higgs stick to anything else, things that you can't easily detect?

333
00:40:16,400 --> 00:40:21,720
They have a very powerful way of figuring out everything that the Higgs goes to goes to.

334
00:40:21,720 --> 00:40:32,660
Which you can't do without close. Proton colliders, high energy proton climbers can produce huge numbers, hugely enhanced numbers of interactions,

335
00:40:32,660 --> 00:40:39,460
and the one that I think is most important is the interaction of the Higgs with itself.

336
00:40:39,460 --> 00:40:49,980
In a harmonic potential and harmonic well. Some people just five squared.

337
00:40:49,980 --> 00:40:56,500
You have an oscillation that goes on forever. But the next potential is not just five squares, five squared plus five to the fourth.

338
00:40:56,500 --> 00:41:05,160
It has high derivatives and harmony and harmonic potential. And when you have an harmonic potential modes and talk to each other so you can

339
00:41:05,160 --> 00:41:11,890
go for an exhortation with one Higgs to next station with two Higgs boson.

340
00:41:11,890 --> 00:41:18,750
That's illustrated here, you have gluons colliding. They produce a top infinitesimal amount of time.

341
00:41:18,750 --> 00:41:26,190
Which annihilate and produces a very strong excitation of the Higgs field, which then splits into two Higgs boson.

342
00:41:26,190 --> 00:41:31,530
And by measuring that.

343
00:41:31,530 --> 00:41:42,940
You can figure out just how and harmonicas potential is, is it really five square percent of the force and you need a lot of exits to do that.

344
00:41:42,940 --> 00:41:50,590
But you can get at a future collider, a future hadron collider. You could get up to in the five to 10 percent range.

345
00:41:50,590 --> 00:41:55,150
And this is important because the Higgs potential is the keystone.

346
00:41:55,150 --> 00:41:56,950
It's what holds everything else together.

347
00:41:56,950 --> 00:42:04,330
It's what causes the Higgs field to have a non-zero expectation that it means that the Higgs field is non-zero everywhere around us,

348
00:42:04,330 --> 00:42:08,750
which is what gives mass to particles at the end. Well,

349
00:42:08,750 --> 00:42:20,090
there's a big exercise going on called the European Strategy Update for particle physics to plan within Europe what we think the priorities should be.

350
00:42:20,090 --> 00:42:24,980
And actually, next week, one of the main meetings of that is going to take place in Spain.

351
00:42:24,980 --> 00:42:30,630
And the kind of plans we're talking about are long 70 years.

352
00:42:30,630 --> 00:42:38,790
On the other hand, the LHC programme, together with its predecessor, Collider collapse, lasted 55 years.

353
00:42:38,790 --> 00:42:46,870
These are the timescales we're talking about. To get answers to some of the most fundamental questions that we have.

354
00:42:46,870 --> 00:42:55,690
So to come to a close. The Hague sector is unlike anything that we've heard before in particle physics.

355
00:42:55,690 --> 00:43:01,360
And much of it remains to be established and explored, and in a way,

356
00:43:01,360 --> 00:43:06,280
we are remarkably fortunate that we can answer some of these questions with the collider

357
00:43:06,280 --> 00:43:13,210
that we have today and with the Collider's we can envisage building in the coming decades.

358
00:43:13,210 --> 00:43:18,600
Unlike Dark Matter, where we're not sure. There's many questions such as accessing Hicks.

359
00:43:18,600 --> 00:43:25,020
You can be on the third generation or understanding the hicks potential.

360
00:43:25,020 --> 00:43:27,240
As well as turning this from just discovering,

361
00:43:27,240 --> 00:43:32,910
discovering these interactions to actually pinning them down and seeing if they're really at the five percent level,

362
00:43:32,910 --> 00:43:38,850
what we expect in the meantime, the search for new physics continues,

363
00:43:38,850 --> 00:43:42,870
and there is much scope here for being ingenious about how you think about searching for new physics,

364
00:43:42,870 --> 00:43:48,120
what kind of experiments you do with use, the data from the big experiments and the ways you plan to use it.

365
00:43:48,120 --> 00:43:52,050
The data from the big experiments in ways you hadn't planned build a little add ons to big

366
00:43:52,050 --> 00:43:56,520
experiments and suddenly open up new bits of face space where you can search for things.

367
00:43:56,520 --> 00:44:01,840
There's lots of ideas. The field is teeming with ideas about things to do you.

368
00:44:01,840 --> 00:44:08,200
And that's something that the searchers and the Higgs and other physics colitis share in common,

369
00:44:08,200 --> 00:44:11,980
which is the need to think about how we relate the underlying physics underlying

370
00:44:11,980 --> 00:44:18,630
the grandjean to the observations we make of 10 to the 16 high energy collisions.

371
00:44:18,630 --> 00:44:24,150
How do you make the connexion between? What you put on a T-shirt?

372
00:44:24,150 --> 00:44:30,900
And what comes out of a collider that will be Lucian on February two, strokes after the break?

373
00:44:30,900 --> 00:44:51,123
Thank you.