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Archive for August, 2012

About three months ago, I wrote a blog post about science communication in which I bemoaned the disconnect between the American public and scientific consensus on many fundamental principles (such as evolution); suggested that Americans are exposed to science in a variety of media (movies, television news, online), but that much of this exposure contains inaccurate, partisan, and/or sensationalized information; and noted (without surprise) that science education has a positive impact on scientific literacy, so we should continue encouraging it — in a rational manner. Although I’ll be the first to admit that the post bordered on rant, it was based on a fair amount of personal experience and research, and seemed pretty reasonable overall. (I hope.) I had meant to quickly follow it up with a more pointed critique of the overall inability of scientists to effectively communicate with the public, as well as proffer some ways to improve the status quo.

Evidently, I was much delayed… for two reasons: 1) I realized that I was wading out into deep and choppy intellectual waters, so I should really do more research before opening my big mouth again, and 2) I finished writing my dissertation, defended it, and graduated with a PhD in Physics. So, after a line of research that led me far afield (into science education, cognition, social sciences, …), I am back. 🙂

First of all, I’d like to clear up some common misconceptions:

Gross domestic expenditures on R&D by the United States, EU, and selected other countries: 1981–2009. (NSF S&E Indicators 2012)

  • Americans are anti-science. Consistently, scientists are held in very high esteem (falling just behind firefighters and just above doctors) and are considered the most trusted sources of information (above religious institutions, news organizations, etc.). Public interest in science is high, as reflected most recently by the wide media coverage of the Higgs (sorry, “new”) boson discovery last month, not to mention all the friends and family who went out of their way to tell me how totally cool that was. Investment in scientific research and development has consistently increased over the last fifty years, and in total dollar amounts is higher than any other country in the world. Yes, the U.S. is pro-science!
  • Distrust of scientific findings stems from a lack of knowledge, so reducing that knowledge deficit will shift public opinion in favor of the science. This is the basic premise of the classic “deficit model” of science communication, which appeals to people trained to base their conclusions on evidence alone (i.e. scientists). However, research has proven it false. People aren’t just blank slates, waiting to be imprinted with scientific knowledge; they filter new information through cultural/religious/political perspectives, and when making decisions, these other considerations often trump pure facts. Counterintuitively, more knowledge may result in less support, and the hardening of opposition to the science. Take, for example, what happened in March when a bevy of distinguished climatologists presented overwhelming scientific evidence on climate change before Congress. No… it didn’t go well.
  • Scientists should be more assertive in public about advocating policy prescriptions based on scientific results. To me, this seems like a good idea, and it’s one that I have long supported. Unfortunately, research shows that when scientists talk about policy rather than just science, there are negative consequences: reductions in the percentage of people trusting what the scientist said, in the overall percentage of scientists that can be trusted, in the perception of the science itself, and so on. While Americans strongly believe that science should inform policy, they seem to prefer that scientists stick to the science, about which they have a credible voice. Note that I’m talking specifically in the context of communicating with the public, and not about the very good work that many science advocacy organizations do in collaboration with the public and the government. They key word, it turns out, is collaboration.

When Science Meets Politics: A Tale of Three Nations. ("In Science We Trust," Scientific American)

Science and technology are increasingly important in America, affecting aspects of our lives both small (morning routines, making social plans) and large (long-term economic prospects, health care), as well as the development and leadership of the entire country. The aforementioned disconnect with the public on both established and emerging scientific issues is problematic for a number of reasons, not least of which because it facilitates poor personal and national decisions (if memory serves, the Founding Fathers had quite a bit to say about the necessity of “a well-informed citizenry”). Of course, most people aren’t scientists, so they must rely on someone else to share salient and useful scientific knowledge with them, and in turn, make better-informed decisions. This is the role of science communication.

The problem that science communication currently faces is not a prevailing anti-science sentiment, nor a lack of activism on the part of scientists, nor insufficient knowledge of the public alone. I’ll explain next time.

Yes, I’m ending on a cliffhanger. 🙂

— Burton


I’ve mentioned before that measuring CP violation is important in understanding why we have a matter dominated universe. So far, CP violation has been observed in the decay and mixing of neutral mesons containing strange, charm and bottom quarks and most measurements have been consistent with theory.

However, there is one measurement which has found evidence for significant CP violation in the decays of neutral B mesons, beyond what is expected from theory. In 2010, with an update in 2011, reported an interesting observation: that the number of events containing two positively charged muons is lower than the number of events containing two negatively charged muons. Like-sign dimuons can be produced from the decays of pairs of neutral B mesons, since they can mix between their particle and antiparticle states. A difference between the number of positive and negative dimuons is an indication of CP violation. The observed difference was close to 1% and 3.9σ away from the theory prediction. The analysis could not distinguish between the two different neutral B mesons, \(B^0_d\) and \(B^0_s\), so the difference had to be expressed in terms of two asymmetries: \(a^d_{sl}\), the semileptonic asymmetry of \(B^0_d\) mesons, and \(a^s_{sl}\), the semileptonic asymmetry of \(B^0_s\) mesons.

At ICHEP, DØ presented direct measurements of \(a^d_{sl}\) and \(a^s_{sl}\), by looking at the decays, \(B^0_d \rightarrow D^{(*)\pm}\mu^\mp X\) and \(B^0_s \rightarrow D_s^\pm\mu^\mp X\).

On the left, I have made a plot of these three results, comparing them to the Standard Model predictions. You can see that all three results are somewhat inconsistent with the prediction, which could indicate a contribution from new physics.

But of course, DØ isn’t the only experiment that is able to measure these asymmetries…


\(a^d_{sl}\) has been previously measured by both Belle and BaBar using \(B^0_d\) meson pairs produced by the decay of the \(\Upsilon(4S)\) meson and the results combined by the Heavy Flavour Averaging Group (HFAG).

And… LHCb released a preliminary result for ICHEP, measuring \(a^s_{sl}\) using \(B^0_s \rightarrow D_s^\pm\mu^\mp X\) decays.

On the right, I’ve added these results to the DØ ones, and now you can see that the situation now isn’t as compelling for new physics, with the BaBar, Belle and LHCb results all being compatible with the theory.

However, all experimental results are still compatible within two standard deviations, so new results are needed to definitively resolve the issue… Stay tuned to see if this is where evidence of new physics is found!


Portuguese version below…

So, I am finally back from vacation, with the email list almost cleaned up and a list of tasks ready to start piling up before I can do anything.. In a nutshell : the usual working life. The last time I posted something, the idea was to explain how a particle, like an electron, a photon or a neutron enters in the detector called calorimeter, hits the material named the absorber and gets a part of its original energy (speed!) converted into a little shower of particles. Part of this energy however gets sampled by a material called (see how physicists can be very creative sometimes) the sampling material. So, what happens in the sampling material of the two ATLAS calorimeters and how we can use this information to “measure” the final energy of the incoming particle?

The sampling material uses some basic physics process to convert the energy it receives into some other physical quantity. The smart thing physicists try to do is to make this “other physical quantity” something easy to measure. Let’s start with a very simple example. In your home, you probably still have either in your wall or in your medical box an old style thermometer. I mean, not a digital one. It is built with a small scale and a little pipe containing either artificially colored alcohol (the wall thermometer) or mercury (the medical box one). When it receives heat from air or a person’s body, the atoms of such “sampling” materials get very agitated. Due to that, the same number of molecules now occupy a much larger space, increasing the necessary volume to contain them. This translates into a longer column which we can easily readout.

In here, we can identify all the elements of the sampling process. First, the heat modifies some inner property of the sampling material considered (their internal heat or movement of the material). Then, the material responds with some global change in a larger scale (its volume increases) and, in consequence, you can now measure with light and some light receiver (your eyes) the shift in the top position of the alcohol or mercury column.

Now, let’s see the case of the liquid argon (electromagnetic calorimeter) of ATLAS. You might want to review the discussion about the absorption process on the previous post. Again, to illustrate, let’s see an extract of the ATLAS Episode II movie, that you can see completely in youtube (part 1 and part 2). There, we followed an electron as it enters the electromagnetic calorimeter and looses its energy by producing a huge shower of particles thanks to the lead absorbers. When the electron is not in the absorber material, it is walking through argon cooled to -185 oC in order to be kept liquid. The electron, or the particles that come out of it, crosses many atoms of Argon, giving so much energy to the argon electrons, that many of of them get free from the argon nuclei in a process called ionization (see the white little dots in the movie!). The argon electron is now a little negative free charge and the rest of the atom is a positive free charge (yellow). Remember that between one accordion plate and the next one, there are copper plates, the electrodes. Between the electrodes, there is a very high voltage (2000V – almost 10 times the voltage in an electric wall plug in an European home). Such high voltage in a very small gap (~2mm) and a bunch of free electrons is a shocking combination, a bit like throwing a small wire on an electric fence. Usually, the high voltage would not cause electrons to travel from one plate to the other, but now that they were freed by the particle’s energy, they will be collected by the positive electrode, generating an electric current. The intensity of this current relates to the energy that the particle lost in the region around the electrode (this region, we call a cell). The current sparks quite quickly (around 400 ns – you will later see that this is actually not as quick as we would love to!) and this is the “other physical quantity” that we can measure. Using this information, we can calculate back the energy of the original particle that entered the calorimeter.

Those interested in seeing a nice, live example of the ionization process are invited to check how to build a cosmic ray detector (sorry for the “idiot”!) and a little video I recorded during a conference (CHEP2010) of a little device called Cloud Chamber (no relation to Harry Potter!).

I would also like to show an example of a particle entering the hadronic calorimeter of ATLAS called Tile Calorimeter. We will use another extract of the ATLAS Episode II Movie to see it. In this video, we start by seeing the iron absorbers (gray) and the plastic scintillating plates (the “Tiles”, drawn in Violet). The absorption part is very similar to the Liquid Argon calorimeter (particle hits, particle looses energy, shower forms) with the difference that particles hit the nuclei and NOT the atoms as in the EM calorimeter case. The sampling material is an special plastic. The particles passing inside this plastic excite the electrons of the atoms that compose it. Not enough to ionize it as in the argon (the electron stays attached to the nucleus but a bit farther away than its usual “orbit”). When the electron return to its natural position, the energy is released back in the form of photons, or, to simplify, light of a very specific frequency (or a specific color, coincidence : Violet). Some of these photons will be collected by an optical fiber and send to a special device that converts light into an electrical signal, a photomultiplier (some of you may have played with that if your school ever had a project to build cosmic ray detectors!). See a photo of Cíbran Santamarina Rios, a colleague from Galicia beside a plastic scintillator detector (the photomultiplier I hope I am saying this right, is in the bottom of the plastic plate – protected by a black cover!).

Photo with Ph.D Cibran Santamarina Rios assembling a cosmic rays detector

Photo with Ph.D Cibran Santamarina Rios assembling a cosmic rays detector

In the next post, I will discuss a bit the outcome of the sampling process : the electrical signal and how it can be used to calculate the energy of a particle, specially when multiple particles hit the calorimeter in different collisions. This concludes our session on “How the detector works”. Then, we will discuss how the trigger works to select particles for a discovery!!!!

Portuguese version :

Finalmente de volta das férias, com uma lista de emails quase limpa e uma lista de tarefas começando a crescer antes que eu possa tentar fazer algo… Em resumo, uma semana normal de trabalho. Na última vez que postei algo, a idéia era explicar como uma partícula tal como um elétron, um fóton ou um nêutron entra num detetor chamado de calorímetro, bate num material denominado absorvedor e perde parte de sua energia, que estava acumulada na forma de velocidade, na forma de uma cascata de partículas. Parte dessa energia, entretanto, sensibiliza um material chamado “material de amostragem” (o termo vem de amostra, parte de um todo que o representa). Assim sendo, como podemos usar o material de amostragem no caso dos dois calorímetros do ATLAS para medir a energia das partículas que entram nestes detetores?

O material de amostragem usa alguns princípios básicos de física para converter a energia que recebe em algum outro valor com significado físico. A esperteza está em tentar fazer esse “outro valor” ser algo fácil de medir. Comecemos com um exemplo bem simples. Em sua casa, Vocês ainda devem ter em sua parede ou na caixinha de remédios um termômetro antigo (não digital). Esse termômetro é constituído de uma pequena escala com as temperaturas e um tubinho contendo álcool colorido (termômetro de parede) ou mercúrio (o da caixinha de remédios). Quando o termômetro recebe calor do ar ou do corpo de uma pessoa, os átomos desse material de “amostragem” ficam muito agitados. Desta forma, um dado número de moléculas passa a ocupar um volume bem maior. Isso se traduz numa coluna mais longa que pode ser facilmente lida.

Neste exemplo, podemos identificar todos os elementos do processo de amostragem. Primeiro, o calor modifica uma propriedade interna do material de amostragem (o seu “calor interno” ou o movimento de seus átomos). Depois, o material responde com uma mudança global numa escala mais larga (seu volume cresce) e, em conseqüência, pode-se medir com luz e um receptor de luz (os olhos do observador) a variação da posição do topo da coluna de mercúrio ou álcool.

Agora vejamos o caso do Argônio líquido usado no calorímetro eletromagnético do ATLAS. Talvez valha a pena rever a discussão sobre o processo de absorção no último post. De novo, para ilustrar, vejamos um exemplo do filme Episódio II do ATLAS, que pode ser visto na íntegra no youtube (parte 1 e parte 2). Naquele exemplo, seguimos um elétron que entra no calorímetro eletromagnético e perde sua energia se transformando numa enorme cascata de partícula graças aos absorvedores de chumbo. Quando o elétron não está no material absorvedor, ele está atravessando o Argônio refrigerado a -185 oC para se manter líquido. O elétron, ou as partículas produzidas por ele, atravessam muitos átomos de Argônio, dando tanta energia aos elétrons destes átomos que muitos deles se liberam de seus núcleos atômicos num processo chamado de ionização (veja os pontos brancos no filme!). O elétron do Argônio é agora uma carga elétrica negativa livre e o resto do átomo uma carga positiva (em amarelo). Lembre-se que entre uma placa do acordeão e a próxima, existem placas de cobre chamadas de eletrodos e que entre estes eletrodos há uma altíssima voltagem (2000 V – quase 10 vezes a voltagem de uma tomada na Europa). Essa voltagem aplicada entre placas tão próximas umas das outras (cerca de 2mm de distância) com um grupo de elétrons livres no meio é uma combinação “chocante”! O que acontece não é muito diferente do que se vê quando um pequeno fio de metal é jogado numa cerca eletrificada. Usualmente, a alta voltagem não causa o “passeio” de elétrons, mas quando eles são libertados pela energia da partícula, eles podem ser coletados pelo eletrodo positivo gerando uma corrente elétrica. A intensidade da corrente se relaciona com a energia que a partícula perdeu na região vizinha ao eletrodo, que chamamos de célula. A corrente cria uma centelha que desaparece rapidamente (cerca de 400ns – mais tarde veremos que esse tempo não é tão curto quanto gostaríamos!) e esse é o “outro valor” com significado físico que podemos medir. Usando essa informação, podemos calcular a energia da partícula inicial que entrou no calorímetro.

Aqueles interessados num exemplo vivo sobre o processo de ionização são convidados a ver como construir um detetor de raios cósmicos e um pequeno vídeo que gravei durante uma conferência (CHEP2010) de uma câmera de núvens.

Eu também queria mostrar um exemplo de uma partícula entrando no calorímetro hadrônico do ATLAS, chamado de calorímetro de Telhas (Tile Calorimeter). Vamos usar outro pequeno filme Episódio II para entender o que acontece. Nesse vídeo, vemos os absorvedores de ferro (cinza) e as placas cintiladoras (as “Telhas”, desenhadas em Violeta). A parte referente à absorção é muito parecida com o que vimos para o calorímetro eletromagnético (partícula bate, partícula perde energia, cascata de partículas se forma) com a diferença que as partículas agora batem nos núcleos dos átomos e não os átomos em si como no calorímetro de Argônio. O material de amostragem, neste caso, é um plástico especial. As partículas passando dentro deste plástico perturbam os elétrons que compõe o mesmo. Não o suficiente para ionizá-lo como acontece com o Argônio : o elétron continua ligado ao núcleo mas fica um pouco mais distante que sua “órbita” usual. Quando o elétron volta a sua posição natural, a energia é devolvida na forma de fótons, ou, para simplificar, luz de uma freqüência específica (de cor, coincidência : Violeta!). Alguns destes fótons são coletados por um sistema de fibras óticas e enviados para um aparelho especial que converte luz em sinal elétrico, uma fotomultiplicadora (alguns de vocês podem ter brincado com tal aparelho se sua escola tentou realizar um projeto para construção de detetores de raios cósmicos!). Veja uma foto de Cíbran Santamarina Rios, um amigo da Galicia ao lado de um detetor plástico cintilador (a fotomultiplicadora aparece embaixo da placa cintiladora protegida por um protetor negro).

Foto do Ph.D Cibran Santamarina Rios montando um detetor de raios cósmicos

Foto do Ph.D Cibran Santamarina Rios montando um detetor de raios cósmicos

No próximo post, pretendo discutir um pouco o resultado do processo de amostragem : o sinal elétrico produzido e que pode ser usado para se calcular a energia da partícula, especialmente quando a partícula atinge o calorímetro em diferentes colisões. Assim, vamos concluir a seção sobre “como funciona um detetor de partículas”. Depois, discutiremos como funciona o sistema de seleção de eventos, tão importante para as descobertas!

Ah! E quase tinha esquecido. O canal ATLAS/Brasil agora tem uma nova pagina e formatação :


On Tuesday, CMS and ATLAS submitted their papers on their observation of a new boson to the journal Physics Letters B. These are surely the most significant publications of the LHC experiments to date, and, without airing too much internal laundry, you can imagine that the content and the phrasing of the papers was very thoroughly discussed within the collaborations. Within CMS, the length of all the comments submitted during collaboration review was longer than the paper itself. You will also notice that CMS and ATLAS came up with slightly different titles; one says that a boson was observed, the other says that a particle (spin unspecified) was observed in a search for the Higgs boson. And for sure neither one says that what is observed is the Higgs boson; as has been discussed in many other posts, we’re very far away from being able to make any confident statements about that.

We can expect that these papers will soon be accepted for publication (in fact, sooner than you might think), and then go on to be fixtures of the scientific literature of particle physics, cited many times over in future papers. Which got me thinking — what are the most highly cited papers in particle physics, and where might the “Higgs” observation papers end up in that list? (Note how he takes pains to put “Higgs” in quotation marks!)

Now, you’ve heard me sing the praises of the Particle Data Group before, but now let me put in a word for the people at INSPIRE, which has recently succeeded SLAC’s SPIRES database as the repository of publication information in our field. I wouldn’t be able to put my CV together or brag about my crazy-big h-index without them. Not only do they track every paper by author, they also keep track of paper citations. How often a paper is cited is a measure of the impact of the paper on the field.

It’s not hard to generate a list of the most cited papers tracked by INSPIRE. And the results may surprise you! A few observations:

  1. The most cited papers are theory papers, not papers that describe measurements. The number one paper, with 8414 citations, is by Juan Maldacena, describing a major breakthrough in string theory. (Don’t ask me to explain it, though!) This paper is only 14 years old. Number two, at 7820, is Steven Weinberg’s paper that was among the first to lay out the electroweak theory. It’s from 1967, predating the Maldacena paper by more than thirty years. And number three, at 6784, is by Kobayashi and Maskawa, explaining how a third generation of quarks could straightforwardly accommodate the phenomenon of CP violation; it’s from 1973.
  2. That famous paper by Peter Higgs? Only #95, with 2043 citations.
  3. The first experimental paper that shows up, at #4, is actually an astrophysics paper, the first results from the WMAP satellite, which among other things really nailed down the age of the universe for the first time. There are in fact many highly-cited papers on experimental results on cosmology. This is of course partly a function of the kind of papers that INSPIRE tracks.
  4. The first experimental papers that show up are actually compendia of results, from the PDG. They release a new review every two years, so many of them are on the list.
  5. The most-cited paper on a single experimental measurement is at #27, with 3769 citations. It’s the Super-Kamiokande paper from 1998 that showed the first evidence of the oscillation of atmospheric neutrinos.

So while it’s true that these observation papers will be among the most highly cited from the LHC experiments, the evidence already suggests that they will be pikers compared to many other publications in the literature. (So was it worth all that effort on what the title should be?) It will be interesting to watch…if nothing else, it will surely be one of the most cited papers that I am an author on, and it is definitely an achievement that we can be proud of.