• John
  • Felde
  • University of Maryland
  • USA

Latest Posts

  • USLHC
  • USLHC
  • USA

  • James
  • Doherty
  • Open University
  • United Kingdom

Latest Posts

  • Andrea
  • Signori
  • Nikhef
  • Netherlands

Latest Posts

  • CERN
  • Geneva
  • Switzerland

Latest Posts

  • Aidan
  • Randle-Conde
  • Université Libre de Bruxelles
  • Belgium

Latest Posts

  • TRIUMF
  • Vancouver, BC
  • Canada

Latest Posts

  • Laura
  • Gladstone
  • MIT
  • USA

Latest Posts

  • Steven
  • Goldfarb
  • University of Michigan

Latest Posts

  • Fermilab
  • Batavia, IL
  • USA

Latest Posts

  • Seth
  • Zenz
  • Imperial College London
  • UK

Latest Posts

  • Nhan
  • Tran
  • Fermilab
  • USA

Latest Posts

  • Alex
  • Millar
  • University of Melbourne
  • Australia

Latest Posts

  • Ken
  • Bloom
  • USLHC
  • USA

Latest Posts

Posts Tagged ‘calorimeter’

Versão em Português abaixo…

Passed the crazy week of the Higgs finding, or, if we are to keep the complete scientific correctness, the weird different particle which is most likely the one that we have been searching for “just” 30 years, well, after that I feel like it is time to explain the different pieces that contributed to such historic finding. The whole thing depends, as is often said, in a number of different factors which we will never be able to put in a few pages of a blog or anything of the sort. Still, I’d like to urge you to hang on a bit and hopefully, you will find as I do, lots of interesting little details. I will try to make a little weekly series that should tell the history of the parts of the ATLAS detector which I happen to be closer to : The ATLAS Calorimeter and Trigger systems.

Let’s start right at the moment when the huge energy accumulated by a proton (this little system of 3 massive particles called quarks and nobody knows how many gluons) is concentrated in a very small volume of space. This energy, following the famous E=mc2, crystalizes in the form of different types of particles. Many of the collisions happen at the so-called parton (a quark or a gluon) level. That means that most likely a shower of particles called a jet will come out of the collision. Well, actually, two jets will usually be produced (things must always balance!). Very rarely, however, other processes take the role and make something completely different. For instance, sometimes, they will produce a Z or W boson. Both are, in the kingdom of particles that we call the Standard Model, what we could call the heavy weights, having lots of mass (one can say more than 80 GeV – Giga-electron-Volts). These guys, when formed, have a very short life (around 3×10-25s), but don’t waste your time thinking on how many zeros do you have to write, just keep in mind that no Z’s or W’s will ever leave the small pipes that bring the protons into collision. Much before that happens, a Z, for instance, will decay, producing a pair of particles that take the energy unfrozen (if you want) in their mass as speed. So, we talk about Z->ee. The Z particle has a mass around 91 GeV and the electrons will have, on average, half of that in “speed energy”. One interesting thing that is always present in physics (check my other colleagues in this blog) is that many properties must be conserved. For instance, the Z particle has no electric charge, but the electron has a negative charge. So, actually, what we get is not a pair of electrons, but, rather, an electron-positron pair, the positron being the positive charged version of the electron, or as we call the electron antiparticle. So, if I wanted to be more rigorous, I should have written Z->e+ e, meaning that a zero charged particle results in a positive and negative charges : the sum is zero again!

The particles (and the anti-particles!) will invade the detector coming from the center (the beam pipe) crossing layers of detectors in the way and will finish their journey in the calorimeters. These devices were developed during many years and now, only in one of the ATLAS calorimeters, we are around 300 people working together!!. For now I will stop here. In the next week, I will explain what happens when each of the electrons enter in the calorimeter and how we use this information to detect the electron and make physics out of it!

To give you a quick taste of what is to come, I call your attention to two videos available in youtube. In the first one, you see the chain of accelerators with increasing size and proton energy. When we get to the LHC, the image zooms inside the tunnel and you will see the equations of the Standard Model of particles in the walls (like we would do that..) The proton will cross the French/Switzerland border in a complete illegal form (no passports!!!) and you will see the colored quarks inside the proton until they meet inside the detector. In the second collision, you will see the Z->ee event. After the collision, the software marks the two blue tracks left be the electron-position pair in the tracking detector and “illuminates” a few of the calorimeter cells represented in green in the movie. We will discuss what happens and how we can see these cells in the next postings. And, later on, you will understand the relation between detecting a Z and detecting a Higgs…
First video : http://www.youtube.com/watch?v=NhXMXiXOWAA
Second video : http://www.youtube.com/watch?v=RdYvtm4CIAE

Portugaise version :

Como Funciona um Detetor de Partículas!!

Passada a semana louca da descoberta do Higgs, ou se quisermos manter a imparcialidade e a retidão científica, a estranha e diferente partícula que muito provavelmente é aquela que estivemos procurando por “apenas” 30 anos, penso que é hora de explicar um pouco todas as peças que contribuíram para essa descoberta histórica. A coisa toda depende numa multitude de fatores os quais nunca poderemos colocar em algumas páginas de um blog. Ainda assim, peço que vocês agüentem firme e, quem sabe, vocês encontraram o mesmo prazer que eu em compreender os pequenos detalhes que fazem o sucesso dessa incrível experiência. Tentarei manter um fluxo de episódios semanais explicando como funciona a parte do ATLAS que conheço mais de perto : O Calorímetro e o Sistema de seleção do ATLAS.

Comecemos exatamente no momento em que a enorme energia acumulada por um próton (esse pequeno sistema de três partículas massivas e não sabemos quantos glúons) se concentra num pequeno volume de espaço. Seguindo o famoso E=mc2, essa energia “se cristaliza” na forma de diferentes tipos de partícula. A maior parte das colisões ocorre entre partons (quarks ou glúons), resultando numa cascata de diferentes partículas, à qual damos o nome de “jato”. Normalmente, como a experiência tem um certo balanço a respeitar, temos dois jatos sendo produzidos com energias bastante similares. Muito raramente, entretanto, outros processos acontecem e algo completamente diferente pode surgir. Por exemplo, algumas vezes, tais processos podem produzir um bóson Z ou W. Ambos são, no reinado das partículas que chamamos de Modelo Padrão, o que podemos chamar de Pesos Pesados (pode-se dizer falar de uma massa maior que 80 GeV – Giga-elétron-Volts). Tais partículas têm uma vida muito curta de 3×10-25s, mas nem perca tempo pensando em quantos zeros se deve colocar depois da virgula. Saiba apenas que um Z formado não chega jamais a tocar o tubo que traz os prótons até o ponto de colisão. Um Z decai, produzindo, um par de partículas que levam a energia contida na massa do Z. Assim, falamos de Z->ee. Como o Z tem uma massa próxima a 91 GeV, os elétrons vão carregar média metade desse valor em “energia do movimento”. Outra coisa interessante (pesquise um pouco os artigos de meus colegas nesse blog) e que é sempre importante em física é que muitas quantidades devem ser conservadas. Assim, como o Z não tem carga elétrica e o elétron tem uma carga negativa, um dos elétrons é, na verdade, um pósitron, a anti-partícula do elétron com carga positiva. Assim, o Z sem carga resulta em uma carga positiva e uma negativa : a soma é zero! Se eu quiser ser realmente rigoroso, tenho que escrever Z->e+ e.

As partículas (e as anti-partículas!) invadem o detetor vindo do centro (onde está o tubo com os feixes) atravessando camadas de detetores e terminando sua viagem nos calorímetros. Esses aparelhos foram desenvolvidos em muitos anos de estudo e, hoje em dia, apenas um dos calorímetros do ATLAS ainda precisa de 300 pessoas trabalhando continuamente!! Por agora, eu vou parar por aqui. Na próxima semana, vou explicar o que acontece quando cada um dos elétrons entra no calorímetro e como usamos essa informação para detectar o elétron e “fazer física”!

Para dar um gostinho do que está por vir, gostaria de chamar atenção de vocês pra dois vídeos disponíveis no youtube. No primeiro, vocês podem ver toda a seqüência de aceleradores com tamanho e energia cada vez maiores. Quando chegamos no LHC, a imagem entra no túnel, em cuja parede, podemos ver as equações do Modelo Padrão de partículas (como se fosse verdade!). O próton que seguimos atravessa “ilegalmente” (alguém já viu um próton com passaporte?!) a fronteira da França com a Suíça e vocês podem ver os quarks viajando dentro do próton até a colisão dentro do detetor. Na segunda colisão, vocês podem ver um evento Z->ee se formando. Depois da colisão, o programa identifica os traços deixados pelo par elétron-pósitron no detetor de traços com linhas azuis. O par também “ilumina” algumas células do calorímetro representadas em verde no filme. Vamos discutir na semana que vem o que acontece e como podemos ver essas células no próximo blog… E, mais tarde, vamos entender qual a diferença entre detectar um Z e um Higgs…

Primeiro vídeo : http://www.youtube.com/watch?v=NhXMXiXOWAA
Segundo vídeo : http://www.youtube.com/watch?v=RdYvtm4CIAE
Canal ATLAS/Brasil : http://webcast.web.cern.ch/webcast/play.php?type=permanent&event=12

Share

Stop right there, particle!

Tuesday, July 26th, 2011

Looking back over my previous posts, I noticed that I forgot to describe the calorimeter and muon systems before jumping straight to the trigger. The subject of today’s post will thus be the calorimeters and my next post will probably be about the muon system.

So what is a calorimeter? I vaguely remember that in high school chemistry, we performed a calorimetry experiment to measure the energy change in a chemical reaction by measuring the heat released (for those who are enjoy their etymology, calorimeter derives from the Latin word, calor, which means heat).

It is slightly different in particle physics, where the main function of the calorimeter detector subsystems is to measure the energy of produced particles. The materials and techniques vary, however the basic principle of all calorimeter systems is the same: to stop particles in the detector and measure how much energy is produced through interactions with the detector material.

On my very first post, I mentioned that LHCb contains two calorimeters; the electromagnetic calorimeter is responsible for measuring the energy of electrons and photons, while the hadron calorimeter samples the energy of protons, neutrons and other particles containing quarks. The calorimeters provide the main way of identifying particles that possess no electrical charge, such as photons and neutrons.

Both calorimeters have a sandwich-like structure, with alternating layers of metal and plastic plates. The metal plates are to stop particles, while the plastic plates are to measure the energy released. More technically, when particles hit the metal plates, they produce showers of secondary particles. These, in turn, excite molecules within the plastic plates, emitting ultraviolet light, which is then guided to photomultiplier detectors. The amount of light produced is proportional to the energy of the particles entering the calorimeter.

Above is a photo of the two calorimeters, the one labeled with LHCb ECAL is unsurprisingly the electromagnetic one, while the hadronic one is behind it. It is a little hard to get a sense of scale from the photo, but the electromagnetic calorimeter wall is approximately 6.3 metres by 7.8 metres and 0.5 metres thick, while the hadronic calorimeter wall is around 8.4 metres by 6.8 metres and 1.7 metres thick.

I think that’s all I have to say about the LHCb calorimeters, except to leave you all with this random fact. The specific design of the electromagnetic calorimeter, its alternate layers of scintillator and lead, readout by plastic fibres which run parallel to the plates, is called shashlik, which is also a type of shish kebab… mmm…

Share

Making Improvements

Monday, May 10th, 2010

The LHC has only had collisions for a little over a month now, and I’m as excited as the next scientist about the new data that is coming in.  With it we will hopefully be able to push existing boundaries in new ways.  The detectors are running well and I think that is a testament to all of the years that went into their development.  (Even if some of those years weren’t planned.)

For my first US LHC blog post, I want to write about something I’ve been working on.  Even though things are looking rosy right now, I’m in the business of improvement.  I work on the hadronic calorimeter (HCAL for short) for the CMS detector.  It is a large heavy detector system charged with trying to stop any hadrons (pions, kaons, protons, neutrons, etc.) from the collisions and measure their energy.  The CMS calorimeter is a sandwich of brass and plastic scintillator planes.  We measure the energy of the hadrons based on the amount of light we collect from the scintillator material.

Here is where the improvement comes in.  The HCAL design was essentially finalized in 1997.  That’s right 13 years ago.  And this was after several years of R&D to come up with a good design.  It then had to be manufactured and installed to be ready for what has been a very exciting commencement to data taking.

In the years since the HCAL was specified and built, new and exciting technologies have emerged that could potentially improve the performance of our calorimeter.  One of these is the silicon photomultiplier.  This device could allow us to better measure the light from the scintillators thereby improving our measurement of the hadron energy.  However, because it took 13+ years to get the original HCAL to a fully integrated system, it will probably take several years for the new upgrade to be designed, specified, prototyped and produced and then it must be integrated into the existing CMS detector.  All this means that although beam operations have been going on for months, upgrade plans have been going on for years.

I’m excited about what we can learn from the data being taken now with CMS and the other LHC detectors, and I looking forward to improvements that are coming in the future years to better exploit the discovery potential of this remarkable machine.

Share

How Tracking Works

Tuesday, November 25th, 2008

Author’s note: I didn’t mean for this to end up so complicated that it had equations, figures, and footnotes, but that’s how it turned out. I do apologize for the inconvenience, and if it’s any compensation I can assure you that about half the footnotes are funny.

I’ve written before about how a pixel detector works, but at the time I left as a “topic for another day” the broader question of what a pixel detector is for.  I’m going to answer one part of that question today, and discuss the tracking system, of which a pixel detector is one possible component.1 I’ll have to leave the question of the specific advantages of using pixels, as opposed to other tracking technologies, for another other day.

Regardless of the technology used, the basic idea of a tracker is to put together a bunch of stuff that measures the path a charged particle has taken.   The “stuff” could be silicon, in which electron-hole pairs are separated as the charged particle passes through, and can be used to produce a current, as I explained in my pixel detector entry.  It could also be gas, in which case electron-ion pairs are separated and produce a current in wires; this is the technology used in the ATLAS Transition Radiation Tracker.  If you want to “track” a baseball through the stands, the “stuff” is people: even if you can’t see the baseball in the crowd on other side of the stadium, you can see where it’s gone by who stands up or jumps down and starts grabbing under the seats.  An individual jumping person, or silicon pixel producing a current, is what we call a hit.

Our primary interest actually isn’t in how particles move through the detector, even though that’s what we directly measure.   So let me take a step back now and describe what we are measuring, first and foremost: momentum.

Momentum: What It’s Really All About

The best way I can think of to describe momentum in a few words is to quote Newton and call it the “quantity of motion.”2 It reflects not just the speed and direction (i.e. velocity) of an object, but also the amount of stuff (i.e. mass) that makes up that object.  In ordinary life, if you double the mass then you double the momentum, and if you double the velocity you get double the momentum too; in other words:

  • p = mv

where m is the mass, v is the velocity, and p is the momentum.3 Unfortunately, things get a little more complicated when the particle goes really fast, which they usually do in our detectors; then the equation doesn’t work anymore.  We’ll get to one that does in a minute.

Momentum intuitively seems the same as energy of motion, but technically the ideas aren’t exactly the same, and it just so happens that the difference is important to how the LHC detectors work.  One way to think of the energy of a particle is as follows: if you slammed the particle into a big block of metal and then extracted all the ensuing vibrations of the metal’s atoms4 and put them in a usable form, it’s the amount of mechanical work you could do.  In fact, that’s exactly what a detector’s calorimeter does, up to a point.  It’s made of big blocks of metal that absorb the particle’s energy, and then it samples that energy and turns it into an electrical current — not so we can do any kind of work with it, but just so we know how much energy there was in the first place.  So the calorimeter is the piece of ATLAS or CMS that measures the energy of particles and absorbs them; the tracker, by contrast, measures the momentum of particles and lets them pass through.   These two pieces of information are related by the following equation:

  • E2 = p2c2 + m2c4

where p and m are still momentum and mass, E is the energy, and c is the speed of light.  The intuitive understanding of this equation is that the energy of a particle is partially due to its motion and partially due to the intrinsic energy of its mass.  The application to particle detectors is that if you know the mass of a particular particle, or if it’s going so fast that its energy and momentum are both huge so that the mass can be roughly ignored, then knowing the energy tells you the momentum and vice versa — and knowing at least one of the two is critical for analyzing where a particle might have come from and understanding the collision as a whole.  We have both kinds of systems because they have different strengths — for example, some kinds of particles don’t get absorbed by the calorimeter, and some kinds of particles (the uncharged ones) can’t be seen in the tracker — and together, they cover almost everything.

(By the way, the second equation is relativistic; that is, it’s compatible with Einstein’s Theory of Relativity.  That means it always works for any particle at any speed — it might assume that space is reasonably flat or that time really exists, but these are very reasonable assumptions for experimental physicists working on Earth.  For those who haven’t seen the equation before and enjoy algebra problems: what famous equation do you get if you take the special case of a particle that isn’t moving, i.e. with a momentum of zero?)

Particle Motion and Momentum

The next ingredient you need to understand what a tracker does is something I haven’t mentioned yet: the whole thing is enclosed in a huge solenoid magnet, which produces a more-or-less uniform magnetic field pointing along the direction of the LHC beam.  As a charged particle moves through a magnetic field, the force exerted on it by the field works at a right angle to both the direction of motion and the field — I tried to illustrate this in figure 1, where the magnetic field is pointing into your screen if you assume the particle is positively charged.5 This means that as the charged particle flies from the center of the detector, it curves (figure 2).  The amount it curves by is inversely proportional to the momentum, which means that higher-momentum particles curve less.  Along its path, it leaves hits in the detecting material, as I discussed above (red dots, figure 3).  Finally, in a process called track reconstruction, our software “connects the dots” and produces a track — which is just our name for “where we think the particle went” (figure 4).

You’ll notice that figure 2 looks a lot like figure 4, but the conceptual difference is a very important one.  The red line in figure 2 is the actual path followed by the particle, which we don’t see directly, while the black line in figure 4 is our track as determined by detector hits.  If we do our job right, the red line and black line should be almost exactly the same, but that job is complex indeed — literally thousands of person-years have been put into it, including two or three Seth-years6 spent on detector calibration and writing automated tools for making sure the tracking software works properly.

The detector is shown here with only three layers.  Although this would be enough to find a particle’s path in ideal circumstances, we actually have many more: this allows us to still make good measurements even when one layer somehow doesn’t see the particle, and to get a final result for the path that’s more accurate.  And don’t forget that there will actually be many particles passing through the detector at the same time — so we need lots of measurements to be sure that we’re seeing real tracks and not just a bunch of “dots” that happen to “line up”…!

More Than Just Momentum

If you measure the path of a particle, you can do more than just find its momentum; you can also see where it came from, or at least whether it could have come from the same place as another particle.  Pixel detectors excel at making accurate measurements to figure out this kind of thing, but as I said already, to do that subject justice will require another entry.

So there you have it.  In a very broad sense, that’s what I’m working toward when I talk about calibrating the pixel detector.  Tracking provides critical basic information about every charged particle that passes through our detector; combined with data from the calorimeter and the muon systems, this information is what will let ATLAS and CMS measure the properties of the new particles that we hope the LHC will produce.


1 Both ATLAS and CMS have one, but many other detectors at colliders do not, because the technology is complex, relatively new, and expensive.
2 See Corollary III here for what he says about it, if you like your science extra-opaque.
3 I’m really not sure why we always use p for momentum, although a good guess seems to be that it’s related to impetus or impulse.
4 A friend of mine, who has the mysterious superpower of understanding how bulk matter works rather than just mucking about with individual particles, looked at a draft of this and was very concerned that I’m implying that all the energy from such a happening would end up as atomic vibrations. So let the record show that this probably isn’t true. And now, if you’d be so kind, can we pretend it is true? It will make illustrating my point very much easier. Thanks!
5 The particle is definitely not actual size, and don’t ask me why it’s green.
6 A Seth-year doesn’t make nearly as big a contribution as a year of work by any of our real experts, but they do happen to be of particular interest to me.

(Back to top)

Share

99.9%

Thursday, June 12th, 2008

The ATLAS Liquid Argon Calorimeter has 183,296 separate channels, meaning it makes that many independent measurements of energy simultaneously all around the point where the protons collide in the LHC. It has to be able to reliably make these measurements millions of times per second for many years. Since we are expecting collisions very soon, the detector has now been closed up and we won’t have access to it again until December for any repairs.

Last week here at CERN was a Liquid Argon Week. These are weeks that happen about 4 times per year, and they are when the whole community (hundreds of people) get together to discuss the current status of the Liquid Argon Calorimeter project. There were presentations and discussions of how the calorimeter is working now that it is in the state it will be in for the eagerly awaited first proton-proton collisions.

It has taken a long time to get to this point. According to the article in Nuclear Instruments and Methods A 558 Issue 2, 15 March 2006, p 388:

The first studies of liquid argon calorimetry for LHC date back to 1990…the choice of the liquid argon technology by the ATLAS collaboration for its electromagnetic calorimetry [was made in] in 1995…The fabrication of some of the elements of the calorimeter…started in the beginning of 2000.

In order for the calorimeter to work, everything from design to construction to installation has to go right. The temperature and purity of the liquid argon must be maintained, the front end crates that receive the deposited energy and turn it into digital data cannot fail, the trigger and data-acquisition systems have to get the data to permanent storage, and of course there is plenty of other infrastructure including the high voltage and low voltage power, many custom-designed electronics boards, and cabling between all the systems. Basically, everything from this:

Part of the ATLAS Liquid Argon Calorimeter being installed

to this:

and beyond has to be working correctly.

So last week there was a tallying of “dead” channels; those that didn’t make it through this whole process and probably can’t ever be used. The great news is that more than 99.9% of all the channels are working and ready to find that Higgs boson, or whatever else awaits. This is a pretty impressive achievement!

Share