Okay. Good afternoon, everybody. So welcome to this week's colloquium.
And it's a great pleasure today to introduce Daniela, both Neto, who's going to talk to us about the Higgs boson.
Now, Daniela did an undergraduate degree in Pavia, and then she moved to the United States as a graduate student in Syracuse.
And that started a long, long and successful career, firstly doing physics on a number of different experiments,
and then more recently in pursuit of the Higgs boson,
culminating, of course, in being a member of an important member of the CMS collaboration,
which actually discovered the Higgs boson a couple of years ago, years ago.
So so and one of the effects of particle physics over the over that time is, of course,
that the centre of gravity for experimental energy, frontier particle physics has very definitely moved to third.
And so for a long time, Daniela was in Purdue, which, as you all know, is quite a long way from CERN.
And we were extremely, extremely fortunate to observe is a great deal closer.
And so we were able to induce her to move here in 2013.
So it's it's a very great pleasure to have her today to to talk to us.
And without further ado, I'll hand over. Talk to us about Higgs boson.
Okay. Okay.
I want to talk to you today about the Higgs, the LHC, its status and its plans for the future.
And the story of the Higgs actually starting in the sixties with this paper from
a variety of people and low browed Higgs and key book surrounding and Eggen.
And if you look at the phenomenology of the Higgs, when we started,
we were we had this paper in 1975 from Alice Gaillard in Annapolis, in which our Fenomeno just told us, you know, the Higgs actually,
you know, we were looking at the Higgs of the movie level,
and the phenomenology told us that they were not very sure about escaping to other particle particles, except that they were probably all very small.
And for this reason, we do not want to encourage because the search searches for the Higgs boson.
But we do feel that people performing experiments vulnerable to the Higgs boson should know how it may turn up.
So as you can see, we didn't listen at all to this advise and we went to a very big experiment us and the conclusion of this search,
in a sense was a culmination, I would say not a conclusion was in 2012,
on July the fourth, 2012, in the auditorium at CERN, where people were crying, people were smiling,
and it was a great triumph of theoretical and expanding meant of particle physics that took a long time.
And again, as you know, we did the cover of Physics Let US Be I'm told from the years that when they published this paper,
the ranking of Xavier went by a factor of two to a factor of six.
They tripled the the effect of of observer, the impact factor.
And and then we were also, you know, you know, the breakthrough of the year.
And and, of course, again, another major achievement was a Nobel Prize to England, an exam in 2007.
So what I'm telling you today, you know, first of all, you know,
I want to talk to you about the LHC and the experiment, the standard model and the Higgs.
And I would like to give you the almost final results on the first round of the LHC.
This round started in 2010 and ended in February of 2013, you know, two years ago.
The machine then is gone through a lot of refurbishing so that we can operate at higher energy.
I will speak a little bit about that, but the results are still coming up.
But, you know, it takes time to get to the final results.
And and we are still sort of waiting for the combination of some of the result that is going to come from Atlas and CMS, and they will come up soon.
And also, I would like to finish with what we expect for round two and from the I luminosity LHC.
So here is a picture of the standard model.
So you have quarks and the lap dances at this being one off particles and you have bosons that are spin one vector,
both of which are exchanging the forces. And then you have the Higgs that is a spin zero particle.
So again, the first family of quarks and leptons makes up everything that we see around now.
And now as this that of families, you can find, you know, elements that are particles that are found in cosmic rain.
And we can produce them in accelerators. And we think that they exist, you know, in the universe.
At universe, just that there's a big bang. And then we have also the force is electromagnetic interaction, which is mediated by the photon.
We have the gluons that mediate the strong interaction,
and we have the W and Z bosons that mediate the weak interaction which is responsible of radioactive decay of atoms.
And then we had the Higgs, which is a little bit, you know, the foundation really say the structure that all together as a standard model,
okay we have the reach Z structure, which is if you want, is a table of element of of particle physics.
We have a lot of accelerators that have operated over, you know, in the last 100 years, let's say.
And the first accelerator was in 1929.
You can see Lawrence here holding in its hand an 80 KV accelerator.
So accelerator were very small at that time. Here you can see CERN in 1954 when they were sort of holding current 18 at the first accelerator at CERN.
And you can see that again. It was really, really.
A small and now you know you have to cycle around does the LHC because it has a circumference of 27 kilometres so it's really much bigger.
But what we are really interested in, in what we do, it's really in the energy.
And energy has really increased by about 1 billion in the last 18 years.
So it's really a very large change in what we can do.
And so and the as to our energy for us are important because a, we can look deeper into the matter.
We can reach a level of ten to the -19 centimetre.
We can discover heavier particles because each is equal to a square.
We can probe the early condition of the universe because e and temperature, energy and temperature are related.
Okay? And so we really are benefiting by the ion energy on a variety of levels.
And again, accelerator are such that we collide more and more particles so that we can probe events that are more and more rare.
And again, for example, at the LHC, we are bound crossing almost every ten to the seven bunch crossing per second in any collision happens every year.
We have ten to the nine protons per seconds and therefore we are able to explore
events that are extremely be mirrored like the production of the Higgs boson.
As we will see and again, I will speak a lot. You know, you could speak about, you know,
the number of events that you observe in terms of the luminosity expressed in one of us is centimetre square,
coldly made out of your beam if you want times a cross-section and centimetre square.
But we actually we use that luminosity expressing inverse function and the conceptual in fact open.
And I remind you that one bomb is more or less the area of uranium nucleus.
So it was a unit that was is useful to measure nuclear and particle physics cross-sections.
So the LHC is really is a machine that has really allowed us to look at the Higgs.
And the main challenge for the LHC was really the construction of these superconducting dipole magnets.
And at the talk that was here before Llewellyn Smith from Oxford was the head of CERN at the time,
told us that he chose the colour of this magnetic and if you know it's are Oxford blue okay not surprisingly so these are very challenging
because in Germany that if you want to steer the proton in this race track because that is at the LHC and to have a 70 the beam in the LHC,
you need eight Tesla for this and it's not easy to build them.
They are superconducting.
So you have a cable that is built by Dynamo, Niobium Tin and it has a current of 12 clamp and it operates at 1.9 degree kelvins,
which is a colder than, you know, empty space in the universe.
And that is also known as a big refrigerator of a plant.
And it has about 120 tons of superfluid helium to refrigerate the LHC.
So what about electric performance? And actually, performance has been outstanding.
So as you can see, the legacy has delivered about certainly in both found a bunch of data.
It's between seven and eight TV. And you can see here in 2010 is in green.
You cannot even see it in this scale because the energy luminosity increased a lot in 2011 and then in 2012 again.
And that luminosity that we have now is that we have analysed about three times what we had for July 12, 2012.
So the collision rate, so this increasing collision rate.
It's good that because we create more volume but is also very challenging for the experiment.
So I just want to see you to show you some picture and you can see here that, for example, in 2010 we had about two overlapping collisions per event.
So we saw two event, overlapping event we call the pileup.
And the interbank spacing was actually 150 nano seconds.
In 2007, the pileup was already 5 to 10.
So it was much more problematic.
And in 2012, when we had almost a full amount of protons that we can put in the machine, we had like 1380 bunches with 10 to 11 protons.
They pileup, but colliding every 50 nanosecond.
The pileup you see was of the order of 2030.
And again, as you can imagine, it becomes a little bit more challenging to see which track belong to the event of interest.
So it becomes very, very challenging. The experiments at Absolute allowed us to look at this and Coal Atlas and CMC stands for Compact,
the Muon Spectrometer, and it's compact in the sense that is smaller than an atlas.
But like us, this is really huge. Okay. And it says Atlas is about half of the size of Notre Dame in Paris.
So it's a really big thing. And to give you a scale is this is a human being, you know, standing there.
Okay. So the overall diameter of ourselves is 25 metre lens, 45 metres and 7000 tons.
You see CMS is smaller and but is considerably heavier and these are digital camera.
So they record the position of particles and they are really cathedral.
So when you go to CMS and Atlas, I think that everybody that enters at all remains like like that.
It's really overwhelming.
It's really, you know, if you can go to Geneva, just go to see to see them if you can, because it's really an amazing success of humanity.
So why is this so big? I'll try to tell you a little bit.
You know how we detect particles. So here you can imagine that you have your beam pipe.
So the protons are coming this way and the are protons is this way and that it close to the interaction region.
You have detectors which measures the position of particle very, very precisely.
Okay, usually silicone or gases detector and then then you have what are called the calorimeter.
And so a calorimeter is built by technology where a photon or an electron almost loses all
of its energy in the electromagnetic part of the calorimeter and then a in a drone loses.
All of these are like the protons and neutrons,
loses its energy and suddenly calorimeter and then to nuance as a particles that can go through the whole detector and you know,
they give you a signal from your chambers that are located, you know, after the old detectors, usually gas chambers.
So here is a view of this detector. If you were standing on the platform and you were doing some repair to the detector, let's say,
and you can see here that the movement of I don't know if you see them very well, you can see he has a new chamber up here.
Then you can see the training calorimeter here, which is made of iron and simply later or a copper and the liquid argon in there for one region.
And then you can see the electromagnetic calorimeter here, not not much of the detail,
which again is the high granularity liquid argon calorimeter and we've led absorber and then the tracking system very close of here,
which is made by a gases detector which is called a straw chamber, which is of course a TFT and SILICO detector strip and pixels.
And again, you can see the path of the particles that are emerging from this interaction through this detector here.
So let me show you a picture, a little bit of the pixel detector and the strip detector.
So here is a pixel detector that was built for CMS, the forward pixel detector.
So it's a pixel detector with 60 million pixel. Each size of the pixel is 100 times 150 microns or.
Really. There's is a precision of of the particle.
Very, very nicely, let's say, exquisite looking nicely. But these detectors take a long time to be built.
Okay. Just to give you a scale with respect to me.
I start out in D when Francesca, my daughter was losing our tooth oak and does a detector,
was installed in CMS when Francesca looked like that and she was, you know, going to, you know, junior proms and she had other issues and losing.
And the tooth fairy that's so and so it was really, you know, it takes really a long time, you know, to build this detector.
So I feel like, you know, this detector is sort of my child and this is my adopted child because actually was building in Oxford.
And I'm sure that my colleague at CNN are picture of their children and, you know,
or or whatever when when they build it, because it took a lot of effort.
And this is a strip detector instead of being a pixel detector.
So if you look at Atlas and CMS, you know, the technology that these two detector I've chosen are actually very different.
But the performance is, in fact about the same. So I'm not going to go through this list.
Okay. I just want to point out that, you know, maybe, you know, they this CMS tracker is a little bit better than ATLAS.
But for example, the electromagnetic and calorimeter bottles are a little bit better than CMS.
Okay. So there is a little bit of different choices.
One of the biggest choice between the two experiment is where you put your magnetic field, your solenoid.
You're measuring the momentum of track because they curl in a magnetic field.
Okay. And in the case of of of CMS, you have the big magnetic field.
And so all of the detector is inside this big solenoid of the almost for Tesla in the case instead of
Atlas only the track is inside the solenoid and the value of the B field is a little bit smaller.
So what? Why are these detectors so big when you want to detect something that is so small?
And again, the idea is that you want to you know, these particles that we want to detect are extremely energetic.
And if you want to detect, for example, a one TB electrons, you know,
you need like 18 centimetre of lead in order for the electron to stop to measure its energy.
Similarly, if you want to measure one TV ad drones, you need like two metre of ions.
So, so you need something big. And to measure the momentum of one TV muons, you know, you need a big magnetic field and a big LeBaron.
So you need the big B and a big L because they to error on the value of the momentum goes like one over B and square.
So different technology, but very similar result again.
So if you look at this collaboration in order to build something so, so challenging, you need a lot of people and not a lot of money.
And so you have to build, you know, very big international collaboration.
And again, the size of the two experiment are all of the two collaboration are also very similar in
the amount of how many countries and online institutions participate is very similar.
So let us go back to the Higgs. So we end up to to the standard model.
So we started at the beginnings of the standard model. We had like the strong interaction.
We had the electroweak interaction. And in the standard model, we really we know what to do.
We know that the that that the strong interaction is represented by the cell free groups and the electroweak
interaction is represented by the S2 two times U one group where that's au1 group represents a weak aperture.
So again, we know also that the lateral magnetic and the weak interaction are are unified because we have done a lot of experiments,
for example, Akira, where we have collided electron with protons.
And in these turns, in this kind of interaction, you can exchange photons, W and Z,
and so you can measure the magnitude, if you want, of the electromagnetic force and the magnitude of the weak force.
And you can see that the interaction when you go to higher energy at lower energy, the electromagnetic force is much bigger than the weak force.
But when you reach energy of the order of 100 GB, you can see that the two in fact are rather similar.
So again, you know, you can build this theory and you can build a theory which is a lot of renewables,
a ball engaging variant that out which you want, but you have to have all of the gauge balls on to be massless.
Okay. And you know already that they want us to exist mediate the weak interaction are not massless.
So for example say that Boston has a mass of 91 GV and Z debut as a mass of HGV.
So how do you do that? And it's really here that you need the mechanism to provide masses to this particle without putting them directly in.
So the idea that was used to do that is the idea of the Higgs mechanism.
And just to illustrate the idea, consider if you have a pin and you can rotate this pin and this is a de la Grande Jones.
ET cetera. Present is interaction is going to be a rotational invariant.
But again, if you put this pin badly about the critical, let's say, a value of the force, the pin is going to buckle.
And so, in a sense, the system loses is they say the the the equilibrium state of the system is no longer rotational invariant.
You have chosen left or right or your choose. Choosing any of this possible point was is being can rotate you can can bend so that what we do
really in terms of the Higgs mechanism so we introduce a single physical beam boson particle,
which is a Higgs boson, and we introduce this potential that as a term you square as a as a value of the
Higgs field square plus in terms of like the Higgs field to the fourth power.
And if according to the value of MU, if new is positive or negative, this field will look like a standard parabolic sort of function.
Or you will have what is called often the Mexican hat, the sort of potential of the bottle of the bottom of the wine bottle kind of potential.
And you can see here then if you have a new negative, you can see that the Zen Z is that you have this kind of potential here.
And then the minimum of zero of your potential in fact happens at the value V, which is equal to music.
What mu divided by the square root of lambda and we can call it we call these the
vacuum expectation value of the Higgs boson and it has a value of 246 GV Okay,
so then we assign in this Lagrangian this factor to be the mass of the Higgs of you becomes mass of the Higgs,
and lambda is equal to the Higgs self coupling.
And again, so we think that early in the universe the value of music was such that we were in this situation, okay?
And so all the WS, a Z and the photon were all massless, and as a universe down we add the phase transition,
the value of new change sign for some reason and the symmetry was broken.
So it's a universe chose one of these possible vacuum and we chose this it chose this one for example.
And then because of these transitions that W and Z became massive and the photon remained massless.
So once you have the Higgs in the standard model, and this is one of the easiest ways that you can.
Break his symmetry introducing these five to the force potential than deferral masses can be given by what is called actions.
A You have a mechanism and therefore they are related to this vacuum exploitation value by some you cover coefficient and
the w masses also depend from these from the vacuum application value and then you can also have itself coupling and they.
S production for example, is a final state which will depend on the mass of the Higgs, for example, and that they can expectation value.
So that looks very predictive. So the standard model is very predictive, but we don't know really why, you know,
the Higgs potential as a shape that it has or if it has even that shape.
But you know why there is a non-zero vacuum expectation value.
And also in a sense, we still know that the a top core which has a large mass is going to have a, you know, a coupling that is close to one one.
Instead, a an electron which has a mass of alpha may be is going to have a coupling to the expose on that is of the order of ten to the minus six.
So again, we have we we know how to put the masses in Lagrangian, but we don't know why that it's coupling at the values that they have.
So and again, you know, we are solving the problem of mass, but most of the mass of the universe,
for example, the mass of the proton is not due to the Higgs mechanism,
but is due to the to the kinetic energy of the constituent of of the of the proton, for example, you know, the gluons or quarks, etc.
So why did it take so long to find the Higgs, even at the LHC?
So here is a plot of the cross-section of the X that best is a square root of energy.
So here was a Tevatron, a previous machine, and here is LHC.
So you see that there's a cross-section for that, but for the scattering of the proton.
Proton is up here and the cross-section of the Higgs is down here.
So it means that only one in 10 to 10 events are going to produce, in fact, a Higgs boson.
So that's why you have to have a high rate of interaction.
And again, it's a little bit like to look for a needle in the haystack.
So here is a plot of the production of the Higgs versus the mass of the Higgs.
So we now know that is about here. Okay. The Higgs really coupled to mass.
So if we wanted to produce a Higgs boson, the best way to do so will be to interact, you know, to scatter together a beam of top quarks.
That, of course, you cannot do that. The top decays almost immediately after the generated.
Okay. So the best thing that you can do is conclude on fusion, where you have a virtual top loop that gives rise to the production of the top.
And you see that the Z is really the highest production mechanism for the Higgs of any mass.
Then you have what is called vector boson fusion where a core can meet the debut and
Z and then you have the produces that Higgs and then you have what is called X.
The W is that strong. So the Higgs is coming out of all the it w is that and then you have a or b b production where you have gluon interaction,
you exchange a top cork and then the Higgs is coming out here.
So again, you can see that again, all of this is a logarithmic scale.
So you are going order of magnitude between each of these production models.
So there is a big, big jump in the cross-section. How does a Higgs decay while the Higgs actually decay?
When I'm then 25 of GD Higgs decays mainly to bottom quarks.
But the way that we discovered the Higgs on July 12 was to looking at the decay of the Higgs to two photon and the decay of the Higgs to Z Z.
So we discovered actually the Higgs by looking at some sliver of this pie chart.
And why did we do that? The branching duration is high, but the signal to background to look for the Higgs in p b bar is really very poor.
And again, instead that for Z Z's branching ratio it's Mauk.
And for Gamma, Gamma is at the level of about two per meal.
But again, you know, the signal to background is much better.
And moreover, looking at this decay mode, it gives you two very,
very nice peaks because you can measure the energy and of the photon and electron very well in the detector that we are building
partially the electromagnetic calorimeter of both the mass and that stars were chosen so that you could do x to gamma gamma decays.
Okay. So you can see here a simulated x two gamma gamma and here I simulated x2zz with four muons.
And you can see that what you exact aspect is.
In the case of X two, Gamma Gamma is a sharp peak over a big background, and so is a crucial, crucial issue for Gamma.
Gamma is to maintain this very good mass resolution.
And this is issue is this huge irreducible background.
And in the case of X to Z said that you have low background, but the cross-section is three times the branching ratio.
It's really tiny. Okay.
And so really and some of your lab, Don, at very low transverse momentum is the transversal mentum is in the range between five and 20 G.
So is that important that you maintain very good efficiency for low momentum left?
So what does happen after July the fourth? Well, we have tried to answer this kind of question.
Is the standard model is the standard model Higgs boson, the particles that we have observed?
And to do that, we have to find out if it captures two fermions and bosons proportional to the masses as it is expected on the standard model.
Does it have to be zero and positive parity? Is it elementary?
In this case would be the first.
You know, Scala Elementary School out there found escaping, capping the date for the standard model expectation that, for example,
there could be new physics loops that enter in these in these gluon fusion loops,
or are they decaying to go into photons or that it decay to invisible particles so as they are more exposed, for example.
So we really try to answer some of this question,
but we are really at the beginning to find some of these answers because the data that we have is still quite limited.
And again, what we have been doing is like to measure all the expected production rate and
all the expected decay rate of the Higgs that we can with the data that we have.
And so, for example, we don't only try to measure X, two, Z, Z in gluon fusion,
but we are also measuring in vector bottom fusion when we expect us to set objects together with the Higgs that we have produced.
So just to tell you, to show you a little bit what we are doing,
July is a force of 4x2 gamma gamma yet was a result for our class and here was the result for CMS.
So you see that the observed significance of the signal was 4.4 around four sigma for each of the experiment and the signal strands of this,
the production production was much larger than what we expected from the standard model.
This is the ratio of the two. So if the event was happening according to the standard model, this MU should have been equal to one.
And you see that it was almost closer to two for Atlas,
but we felt huge error and again that was already stimulated a lot of theoretical
interest because again if you look at the loop of the Higgs to decay,
two photons of photon are massless. So again, the Higgs cap was to default on the decay again to a top loop or super w loop.
Okay. What if you had new physics? Maybe you could have new physics that made these Z's deviation from the standard model.
Okay, so it was a lot of interest. What do we have now?
Well, in the case of you can see that now, say each of the experiment as a Five Sigma significance just in this channel alone.
But the signal strength now has gone down and it is still a big error.
So it's still consistent with one, but is gone closer to the expectation of the standard model.
What about A to Z? Z? I told you these are very relevant to both ATLA and seems a bit about certainty of disadvantage.
So the Higgs is here and here is a z reality that goes on digging into four leptons.
Okay, so you can see here and you can see that the probabilities that the peak is from the.