Tuesday evening lecture with Stan Bentvelsen
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Transcript: English(auto-generated)
00:00
The big question is we have two particles tonight the glass is it's half empty or half full and Stone Benfelzen will tell us all about it We move on to a to a second particle That I'd like to present to you. I'm very happy that I am
00:22
Able here to present to you the results that we obtained With our detector the atlas detector And I'll be speaking on behalf of our atlas group and what you see here on the cover is Is the well the observation of the Higgs particle?
00:41
I put some question mark there because we are not so sure maybe as a previous speaker that this is the The particle that you are looking for but the evidence that was also alluded to is indeed something which is close to six sigmas and not five and
01:01
But I think here it'd be indeed play a completely different kind of game This is indeed large science Last year we were celebrating the 20th birthday of the atlas collaboration and imagine that you had to have the vision in
01:21
1992 to make the letter of intent for atlas it was approved by the council of CERN in 1994 We started installing some 10 years ago or so and it was done in 2008 and then we took data, and I'd like to share with you some of the things that we saw in that data
01:41
But before doing that I also like to present The group of people from the Netherlands that are involved in this kind of research and really experimental particle physics is teamwork I'm here on behalf of my team, but the people that I that are really doing the hard work I would like to refer to the tomorrow sessions focus session Higgs into focus and the parallel session
02:06
subatomic physics because there you can hear all the details and all the nitty-bitty Kind of hard work that has been going on in order to achieve this This fact so we are a collaboration of from Nick F The about University and and the University of Amsterdam, so let's start by doing some particle physics and particle physics
02:26
I mean particles without any size without any form factor without just point like particles that can be described by mathematical points and We have to dive deep inside the the the nucleon to
02:42
To encounter the quarks here the up and the down quark as you well know and the leptons the neutrino and the electron But those are not the only particles that we know of because in the in the last many years decades via all sorts of accelerators we know that this has to be extended by by
03:03
new families of particles that have been discovered over the years for example the charm and the strains and the top and the bottom the Top was only discovered in 1995 at the Tevatron in Chicago and the the town neutrino was only made visible Five years later, so but these are then the basic constituents of of matter
03:23
interesting, but maybe much more interesting is how they interact with each other and We know that there are four fundamental forces playing a role in in physics. I listed them here from you for you Electromagnetism weak interactions strong interactions and gravitation and over the years again. We have
03:46
created quantum field theories to in order to describe them and for example 1948 by Feynman and and others Tom Naga and swinger managed to write down a quantum field theory of Electromagnetism where you see that the messenger for the force is the photon
04:04
Well known much later Also, the strong interaction was made in a quantum field theory where the gluon also massless is mediating the strong interaction But the weak interaction posed more problems and it was only in In 1967 or so that glass how Weinberg and Salah managed to write down a theory describing in one go
04:26
Electromagnetism and weak interaction by introducing the W and Z bosons and the W and Z bosons They are not so light. They are actually pretty massive 1891 times the photon mass or so and And that caused the problems the problems are partly solved. No, let me first
04:47
Discuss with you That this this model is actually the standard model. We call it the standard model and you can write it down on a cup of tea and and it very elegantly Summarizes all these very fundamental forces based on symmetries on on gate symmetries and
05:05
The model was made consistent around 1970 or so By the renormalization program of Feldman and at often they rightfully were awarded the Nobel Prize in 1999 and
05:21
When they were doing The Renormalization program they discovered that they can that can easily be done not easily but could be done for massless particles But not for the massive particles. So the W and Z bosons being massive were causing problems But then it was noted that somewhat earlier already in 1964
05:46
Another way of dynamically generating mass was on the market described by Peter Hicks Francois Englert and Robert Brauth and That was put in the standard model in order to give the W's and the Z bosons and also the fermions later on
06:03
actually a mass and By the discovery of the Z and the W bosons in 1983 That standard model as it was then called was made into a big success and also there was a Nobel Prize given to a Dutch colleague by Simon van de Meer and
06:22
also Carlo Rubia So what is then this Hicks idea and the idea is but I told this based on symmetries and the idea is that you need the gate symmetry in order to preserve Renormalization and that we call in an unbroken face
06:41
But in an unbroken symmetric face the particles cannot have a mass and that would mean that They would move with the speed of light every particle that we know of in an empty vacuum The idea is then In a broken face in the vacuum there can exist fields at the Higgs field here
07:03
visualized by this somewhat wavy kind of of background field and particles do actually interact with this background field and by Interacting with this background field they act effectively acquire a mass and one particle is Interacting a bit stronger with this background field than the other one and thereby
07:22
Effectively acquiring a different mass than the other one so masses are described now in a completely different way Masses are described as interactions interactions with an omnipresent Higgs field but in order to see if this description makes sense because here you have now a
07:40
description with which you can solve the problems of renormalization in the standard model, but The consequence of this description is that also the vacuum itself can fluctuate and This is then the smoking gun for this whole description which means that there is an excitation of this vacuum field and this is what we call the Higgs particle and
08:01
The Higgs particle is actually the only particle because it's a scalar It's a boson that does have a mass in in the standard model, but the trouble is that we don't know the mass It's not predicted by theory So that means that we basically have to look everywhere in order to see if this whole picture that I present here Does make sense or not?
08:20
So in order to do that You're well aware and it was also alluded to previously already. We have this magnificent accelerator being constructed many years ago already with 27 problems in circumference and and the big Detectors Visualizing and detecting the collisions the atlas detector and the counterpart the CMS detector because we do want to be pretty
08:47
Sure, if you have a measurement that it's also measured independently by an independent group of scientists in our case the CMS people and There are two big challenges in the construction of this of this
09:03
Accelerator the first challenge is really to get ultimate high energies in order to create Heavy new resonances heavy particles and indeed. It's in the TV range The design is 7,000 to 7,000 GV photon to photon collisions and in order to do so you have to really
09:24
Keep the particles in their path in the circular path and therefore you need two magnets and here you see one of the dipole Magnets that is doing that one of the twelve hundred and thirty four Dipole magnets that are installed in the accelerator It will be or they're cooled to 1.9
09:43
Kelvin with the liquid helium and they ultimately give a magnetic field of 8.3 Tesla and to impress you a little bit one-third of the helium supply of of the whole world is actually in this machine and Here you see on the on the right downside you see the the two little holes something like
10:07
five centimeters in diameter one with a magnetic field going upward the other one with a magnetic field going downward so that the protons can move in opposite directions through each of these
10:20
tubes and Then comes the second challenge and a second challenge is really to get an awful lot of collisions and We call that luminosity the brightness of the beam the amount of collisions that that we'd like to observe You have to imagine that if you collide two protons head-on the the chances of actually producing an Higgs particle
10:44
There is ten orders of magnitude below having a cross-section at all So it's really looking for a needle in something like 10 million haystacks or something and So in order to increase in the luminosity The particles are actually
11:02
Accelerated in bunches each bunch consisting of 10 to 11 protons and the bunch Is followed by another bunch some 50 nanoseconds or something like 16 meters or so further down And so in this machine a whole train of bunches are going clockwise and a whole train of bunches are going anti-clockwise
11:22
and the bunches of protons they actually they Go past each other having a chance of a proton for one bunch hitting a proton of the other bunch and the rest continuing to make a new chance in the next round so What you do when you operate such a machine?
11:42
It's you operate it 24 hours seven days a week Between March and December it means you have to fill the machine at low energy at injection energy You have to ramp it up to the energy the high ultimate energy you start the collisions And that is a stable configuration of something like 10 hours or so and after the 10 hours the the the intensity of the beams
12:06
Has degraded a little bit due to the lifetime the products has just disappeared And then you recycle and the recycling takes typically a few hours, and then you restart the operation again and And all the time the atlas detector is taking data taking collisions
12:22
whenever LHC gives you a call saying hey listen we have a stable beam here and So what what you're doing then is Looking at at the end of the day individual collisions between protons, but protons are of course Build out of quarks and and partons and the
12:43
Interesting physics that we obtain here is when actually two quarks or two Partons of this proton make the collision that makes the heart collision where some new physics is is hopefully Happening and you see on the right side The plot where the time goes to the right how we visualize the two protons
13:04
Hitting each other producing new physics Now you have to detect the decay products of the collision and For that we build the atlas detector and here you see a part of the atlas detector with the beam perpendicular his
13:24
perpendicular to the beam pipe at the beam pipe at the downside of this picture and like onion rings various sub detectors Subsequently putting one after the other so in the very heart of the detector you find the tacking chambers Which measuring the direction and the momentum of the charged particles?
13:43
Because they are meted in a magnetic field Then the chlorine meters where you stop the particles like electrons and photons in order to measure their energy And then there is one particular particle the muon 200 times as heavy as an electron Basically that is able to penetrate through the chlorine meters and hitting at the very outside of the detector the muon chambers
14:05
now this is simulate or this is a Picture of the atlas detector you have to mention 44 meters long and 22 meters high Detector where you see here a simulation of a of an event. This is not a real event and
14:20
The Involvement of of Nick F in this project is both in industry in Instrumentation in R&D for developing new ways of detecting particles We we are heavily involved in electronics in the calibration Now we are heavily involved in the analysis in the software development in the computing etc
14:43
So there's a whole bunch of activities that we are actually doing in this detector And if you make it more concrete, it means that we have built part of the muon precision chambers. We have The end cap towards from the industry we have an semiconducting tracker end cap
15:02
Made it in Amsterdam and put it downstairs In Geneva involved in the trigger and the data acquisition tracking and in an analysis wise we're involved in a supersymmetry Researchers Hicks physics as I will tell you about and top quark physics, so Ten years ago or so
15:21
We were in the face that we could really enter the the cavern and start building the thing and here is a very Quick movie on this period so here you see the the barrel toys being built up that this is a part of the muon
15:45
system And that gives that very characteristic a view of the Atlas detector where you inside see the calorimeter being installed And there's some movements up and down in order to be able to put everything in together here you see the
16:03
calorimeter inside the The magnet being put in so that's a quite a large Effort and also as I told you at Nick F. We have done our Not only fair share But actually quite a lot which you see here some impression on the on the people that have been doing that for
16:23
Some some time and I couldn't resist showing this plot of first danko Which is one of our psd students that are actually installing one of the calibration systems for the for the muon chambers so at one point in 2008 or so the
16:40
Detector and I'm seeing were all done, and then we had this this famous discovery or Not a discovery it is a disaster One of the you know it's pretty serious actually the the two of these long dipole magnets they are joined up by
17:03
by electric by by an electric joint or yeah, and There's something went wrong and was a spark developing and the spark actually Started to blow up or melt down part of the pipe where the liquid helium was in and liquid helium started to escape
17:21
And it was a pressure wave inside the collider itself and something like 400 meters were of the accelerator were destroyed and It took us a year or so to recover from this some hundred dipole magnets were really effective affected by this by this incident
17:41
That means that after a year when we were fully recovered The LHC is not running at the design energy of 7,000 GV per beam but half of its energy three and a half thousand and in this year or sorry in 2012 4,000 GV and We will be stopping at a program next month in order to
18:03
install safety valves and to make more adjustments in order to Restart the machine at the end of 2014 with a full energy, so we're going in a shutdown period for a while Nevertheless 2011 it was extremely successful the the
18:21
The expectations for the luminosity were exceeded by a factor of five or so and again last year in 2012 the luminosity was exceeded by by expectations. It means that in total we have something now Like 1.8 times 10 to the 15 collisions
18:40
Viewed by the Atlas detector the detector did an excellent Was performing excellently data taking efficiencies up to 94% 96% of the channels operational these are all the things the nitty-gritty details that we are worrying about and What makes this so successful and and and look at the harsh environment here you see
19:07
actually an overlapping Bunch crossing I was telling you that in the bunch crossing you have a chance of one proton hitting another proton But that is actually not the case at all Here you see an overlapping
19:21
20 protons or 27 collisions in one bunch crossing and one of them is actually doing some interesting physics in one of them Z boson is being produced, but now you have to figure out what it is and you have to get rid of the underlying physics So what we do when we analyze our data is the following we have something like 20 to 20 million collisions a second
19:44
This these are observed with the Atlas detector We have an online triggering system which spits out at a rate of 600 Hertz Events that are selected by one of the many many trigger menus over a thousand trigger Possibilities that we have and that means that we are storing something like a gigabyte of data per second
20:04
This data is then spread out over the world in ten centers one of them being Sarah a Nick F in Amsterdam And so that we can then look at the data from there and do our analysis
20:20
And this is what the PhD students typically are doing when they make the analysis Now here's the here's an example This is actually quite a cute example because here you see in the detector if you see the Atlas detector As I told you the muons they are penetrating through coming Completely out of the detector. So these are represented by the headlines in this in this event display. This is real data and
20:45
You see that the two muons Actually are a signature for Z boson So here's the Z boson produced and what you see is if you calculate the invariant mass of the two muons You actually get a mass very close to the Z boson and so
21:02
That's a nice observation, but we're not so interested in Z bosons We know that the Z boson is there and we have found it back But we wanted to look for the Higgs particle now, so here's the status of of the Higgs Particle before we started doing collisions on the line below you see somewhere between 0 and 500 GV
21:23
Let's say the range in which we expect the Higgs particle to exist indicating the mass of the proton the mass of the W and the Z boson the gold atom and Also the red area the area which has been excluded already before LHC started up to 114 GV or so
21:41
so When you make Higgs particles or when you search for Higgs particles you first have to produce Higgs particles if it's there now the prediction with the data for 2012 is Depending on its mass, which we don't know something like 200,000 or 77,000 Of those Higgs particles out in this 10 to the 18 million collisions
22:03
Now the Higgs particle once you have created one doesn't exist for a long time. It actually decays and It has various possibilities of decaying so one of the promising golden channels is the decay to 2 Z particles that
22:20
Subsequently decay to four leptons and you see here on the bottom side you see let's say the sensitivity Of this channel as a function of the Higgs mass So if it's somewhere in the blue curve on the blue line We will be sure that we find the Higgs particle if it has a mass in in that area But that's not the only channel another possibility is when the Higgs decays into two photons and
22:44
You see that that there's a completely different sensitivity actually what we call a low mass sensitivity around 114 hundred and twenty or so and then a third channel, which is where Higgs decays into two W bosons decaying into two leptons and there is a bit of a problem
23:00
Because the W's also decay into neutrinos and that you cannot measure so we measure the neutrinos only through missing energy Now you see also the nikka flags for the channels that we have put Emphasis on within our group. So the naming and group is very active. For example, so in the Higgs to the WW and Higgs to CC as well
23:22
Now in the first round of data in 2011 we were able to exclude the existence of this Higgs particle for a very long range of its possible mass before we started off somewhere between 1441 up to something like 470 GEV we knew from the data that we took in
23:41
2011 that it was not there so that leaves only a tiny gap Let's say in between hundred and fourteen and hundred and forty one to see if there is a new particle now Here's an example of a display where you see a Higgs to a WW Going to two leptons in this case an electron and a muon and two neutrinos which you cannot see
24:02
So that is what we call missing energy and from the here you have to reconstruct back the kinematics in order to see if you Can find back the Higgs mass not so trivial Much more straightforward is when the Higgs is decaying into two z bosons and the two z bosons subsequently decay to two four Leptons or four muons and here you see a beautiful example actually of a collision where indeed you see four of those muons in
24:26
the final state and When you calculate the invariant mass of these four muons you will find that this adds up to hundred and twenty five point one GEV But this is one event and that's not Significant enough as you can imagine, but we found others. Here's another one where you have again four of those muons
24:45
in in the final state and it's in this case the mass of the four muons add up to hundred and twenty four point six GEV or so so Having Having all this data in your hands you start to make
25:02
Distributions, of course and you know that we are looking in a certain window for masses. So Here is then the plot of the invariant mass on the horizontal axis of the four leptons I gave you two examples and here is the plot that is covering the range between zero and 250
25:22
GEV and What we do is we blind our data first not in order to to To be too biased so what you see here is the blinded data before unblinding What you see here is the data itself and the red histogram that you see is
25:47
Calculations that we did There we also have four muons in the final state of four leptons I should say in the final state, but they are not coming from a Higgs particle at all They are the background So this is the backgrounds for continuous ZZ background and you see some sort of a little Feynman diagram on the top there
26:05
Which which shows you that you can have also contributions, but there's no Higgs at all involved in the game But you still have the same signature in your detector. So you have to disentangle the two and At one point we started to unblind and then low and behold we found something which you can say is a small signal and
26:25
Significance of three point six Sigma you see that it basically consists of something like eleven events that are above the red background, which is Which is significant three point six, but we couldn't claim a discovery
26:46
We were looking also at other channels Here is a channel where you have a Higgs decaying into two of these photons and these photons they are They don't carry any charge. So what they do is they leave their signature in the calorimeter these are the yellow blobs that you see in this in this
27:03
event display and the With the red circles and what you see is the the yellow blobs you can measure the energy of the photons by this absorption in the calorimeter and Because you know the direction you can calculate the event mass again and what you find is here a mass of 126
27:23
GEV this is not at all trivial because there are many processes that also lead to Two photons in the final state we background again and are basically two important ways of background Fake photons, for example a pion if you know what a pion is a pi zero can decay into two photons
27:41
That are very collinear and it's not so easy to disentangle a pump photon from from a decaying pine Also, you have pump photons again, not from Higgs decays as I show him in the plot here down below so effectively what you do if if you calculate the event mass of the whole set you get a
28:05
Different behavior that was about a show before in a few four muons But again as a function of the Higgs mass you see now a continuum, but we're looking for peaks here and indeed if you fit the continuum and subtract it from
28:22
From the observation you will see a little peak here with the significance of something like four and a half Sigma Now You can combine all this information at this one and the previous channels and then at some point You can calculate the data significance for each mass
28:42
And this is the situation of July 4th in 2012 where you see the data significance as a function of the Higgs mass all the range between 110 GV to 500 something GV The the p0 value. I think you know that And you see a dip there a dip which is
29:02
reaching something like six Sigma and the dip means that you cannot describe the data without having any anything new in it and This is what we then call a discovery five Sigma but by claiming a truth Discovery we also have to look at our friends from the CMS collaboration and on that date. We were extremely
29:24
Curious to what they were showing and indeed they were also having at the very same spot 126 or so GV a significance of Five Sigma so that led to this to this observation of the CERN director-general
29:40
I think we have it you agree For me it's Really an incredible thing that it's happened in my lifetime
30:04
I know Peter Higgs a little bit that he was completely cute Flabberg acid by the fact that he found this particle so soon and also in in the Netherlands
30:22
We had our we had our fair share of celebrating and we have to suddenly deal with media Which we didn't do so much before So what happened since July 4th now the question is is it the standard model Higgs We're now in the in the phase in which we are really in very much detail
30:42
Go and find if if the predictions of the standard model once we know the mass are actually valid So what you see here is a plot. It's difficult to read maybe but you see the different decay channels and Hicks to gamma gamma and Hicks to ZZ and and The expectation from the standard model is given by the
31:03
Vertical line and you see that the data is not on the vertical line very nicely So we are now in an intermediate states Like is it something that we don't understand or do we just need more collisions? Realize that we have something like two-thirds of the total data set unblinded
31:20
So we still have new data on the shelves that we haven't looked at something else very intriguing What we found actually not so long ago is if you look at the masses of the Higgs particle when it decays into two photons or the mass of the Higgs particle when it decays to four leptons, we don't find the same mass back and The mass difference is something like three three and a half GV a two and a something Sigma
31:47
Is that significant enough to claim that maybe we are not dealing with a simple standard model Higgs something more complex We don't know yet, but this is clearly something that's for which we need more data. I think that
32:02
This this discovery is actually one of the most important discoveries in particle physics for the last 42 years or so We have definitely observed a particle But if it's really Hicks, we don't know completely for sure yet But in the whole LHC program
32:20
Imagine that only 1% of the data has been delivered at this moment The program continues for many more years to come in a near future. The energy will be doubled We will be doing in-depth Higgs studies and we will search for beyond the standard model physics and in the far future our ambitions to Is to going for very high luminosity at the LHC and maybe doing high precision physics in an e plus e minus Collider
32:48
So that leads me to my very last slide I think that this Discovery was really a major leap in understanding, but there are new questions that are now Getting very urgent. We really have to find out what are the quantum numbers of these particles either more Higgs particles?
33:06
Or are there any other particles? Is it elementary or is it the composite? How does it couple to itself and and what we also don't know because it's a scalar particle What actually screens it must form being at the Planck scale and there's all there's a whole connection to cosmology and maybe
33:23
It's nice to quote what Feldman always used to tell in his in his lectures on on Higgs physics You know, you have this Higgs vacuum had his Higgs field, which also contains energy and if you start calculating What is the curvature of this of this
33:41
Effect of having energy in our universe it will mean that the universe would shrink to the size of a football and It clearly means that we lack understanding of what's really going on at this very fundamental level. So thank you very much