Quantum Diaries http://www.quantumdiaries.org Thoughts on work and life from particle physicists from around the world. Mon, 05 Oct 2015 14:25:33 +0000 en-US hourly 1 http://wordpress.org/?v=4.2.5 Nobel Week 2015 http://www.quantumdiaries.org/2015/10/05/nobel-week-2015/ http://www.quantumdiaries.org/2015/10/05/nobel-week-2015/#comments Mon, 05 Oct 2015 14:10:42 +0000 http://www.quantumdiaries.org/?p=36172 So, once again, the Nobel week is upon us. And one of the topics of conversations for the “water cooler chat” in physics departments around the world is speculations on who (besides the infamous Hungarian “physicist” — sorry for the insider joke, I can elaborate on that if asked) would get the Nobel Prize in physics this year. What is your prediction?

With invention of various metrics for “measuring scientific performance” one can make some educated guesses — and even put the predictions on the industrial footage — see Thomson Reuters predictions based on a number of citations (they did get the Englert-Higgs prize right, but are almost always off). Or even try your luck with on-line betting (sorry, no link here — I don’t encourage this). So there is a variety of ways to make you interested.

My predictions for 2015: Vera Rubin for Dark Matter or Deborah Jin for fermionic condensates. But you must remember that my record is no better than that of Thomson Reuters.

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Neutrinoless Double Beta Decay and the Quest for Majorana Neutrinos http://www.quantumdiaries.org/2015/09/22/majorananeutrinos-0vbb/ http://www.quantumdiaries.org/2015/09/22/majorananeutrinos-0vbb/#comments Tue, 22 Sep 2015 11:00:27 +0000 http://www.quantumdiaries.org/?p=36134 Neutrinos have mass but are they their own antimatter partner?

The fortunate thing about international flights in and out of the US is that, likely, it is long enough for me to slip in a quick post. Today’s article is about the search for Majorana neutrinos.


Mexico City Airport. Credit: R. Ruiz

Neutrinos are a class of elementary particles that do not carry a color charge or electric charge, meaning that they do not interact with the strong nuclear force or electromagnetism. Though they are known to possess mass, their masses are so small experimentalists have not yet measured them. We are certain that they have mass because of neutrino oscillation data.

Words. Credit: Particle Zoo

Neutrinos in their mass eigenstates, which are a combination of their flavor (orange, yellow, red) eigenstates. Credit: Particle Zoo

This history of neutrinos is rich. They were first proposed as a solution to the mystery of nuclear beta (β)-decay, a type of radioactive decay. Radioactive decay is the spontaneous and random disintegration of an unstable nucleus in an atom into two or more longer-lived, or more stable, nuclei. A free neutron (which is made up of two down-type quarks, one up-type quark, and lots of gluons holding everything together) is unstable and will eventually undergo radioactive decay. Its half-life is about 15 minutes, meaning that given a pile of free neutrons, roughly half will decay by the end of those 15 minutes. A neutron in a bound system, for example in a nucleus, is much more stable. When a neutron decays, a down quark will become an up-type quark by radiating a (virtual) W- boson. Two up-type quarks and a down-type quark are what make a proton, so when a neutron decays, it turns into a proton and a (virtual) W- boson. Due to conservation of energy, the boson is very restricted into what it can decay; the only choice is an electron and an antineutrino (the antiparticle partner of a neutrino). The image below represents how neutrons decay.

Since neutrinos are so light, and interact very weakly with other matter, when neutron decay was first observed, only the outgoing electron and proton (trapped inside of a nucleus) were ever observed. As electrons were historically called β-rays (β as in the Greek letter beta), this type of process is known as nuclear beta-decay (or β-decay). Observing only the outgoing electron and transmuted atom but not the neutrino caused much confusion at first. The process

Nucleus A → Nucleus B + electron

predicts, by conservation of energy and linear momentum, that the electron carries the same fixed amount of energy in each and every decay. However, outgoing electrons in β-decay do not always have the same energy: very often they come out with little energy, but other times they come out with a lot of energy. The plot below is an example distribution of how often (vertical axis) an electron in β-decay will be emitted carrying away a particular amount of energy (horizontal axis).

Electron spectrum in beta decay: Number of electrons/beta-particles (vertical axis) versus energy/kinetic energy (KE) or electrons (horizontal axis). Credit: R. Church

Scientists at the time, including Wolfgang Pauli, noted that the distribution was similar to the decay process where a nucleus decays into three particles instead of two:

Nucleus A → Nucleus B + electron + a third particle.

Furthermore, if the third particle had no mass, or at least an immeasurably small mass, then the energy spectrum of nuclear β-decay could be explained. This mysterious third particle is what we now call the neutrino.

One reason for neutrinos being so interesting is that they are chargeless. This is partially why neutrinos interact very weakly with other matter. However, since they carry no charge, they are actually nearly indistinguishable from their antiparticle partners. Antiparticles carry equal but opposite charges of their partners. For example: Antielectrons (or positrons) carry a +1 electric charge whereas the electron carries a -1 electric charge. Antiprotons carry a -1 electric charge were as protons carry a +1 electric charge. Etc. Neutrinos carry zero charge, so the charges of antineutrinos are still zero. Neutrinos and antineutrinos may in fact differ thanks to some charge that they both possess, but this has not been verified experimentally. Hence, it is possible that neutrinos and antineutrinos are actually the same particle. Such particles are called Majorana particles, named after the physicist Ettore Majorana, who first studied the possibility of neutrinos being their own antiparticles.

The Majorana nature of neutrinos is an open question in particle physics. We do not yet know the answer, but this possibility is actively being studied. One consequence of light Majorana neutrinos is the phenomenon called neutrinoless double β-decay (or 0νββ-decay). In the same spirit as nuclear β-decay (discussed above), double β-decay is when two β-decays occur simultaneously, releasing two electrons and two antineutrinos. Double β-decay proceeds through the following diagram (left):

Double beta decay (L) and neutrinoless double beta decay (R). Credit: CANDLES experiment

Neutrinoless double β-decay is a special process that can only occur if neutrinos are Majorana. In this case, neutrinos and antineutrinos are the same and we can connect the two outgoing neutrino lines in the double β-decay diagram, as shown above. In 0νββ-decay, a neutrino/antineutrino is exchanged between the two decaying neutrons instead of escaping like the electrons.

Having only four particles in the final state for 0νββ-decay (two protons and two electrons) instead of six in double β-decay (two protons, two electrons, and two neutrinos) has an important effect on the kinematics, or motion, of the electrons, i.e., the energy and momentum distributions. In double β-decay:

Nucleus A → Nucleus B + electron + electrons + neutrino + neutrino

the two protons are so heavy compared to the energy released by the decaying neutrons that there is hardly any energy to give them a kick. So for the most part, the protons remain at rest. The neutrinos and electrons then shoot off in various directions and various energies. In neutrinoless double β-decay:

Nucleus A → Nucleus B + electron + electrons

since the remnant nucleus are still roughly at rest, the electron pair take away all the remaining energy allowed by energy conservation. There are no neutrinos to take energy away from the electrons and broaden their distribution. This difference between ββ-decay and 0νββ-decay is stark, particularly in the likelihood of how often (vertical axis) the electrons in β-decay will be emitted carrying away a particular amount of energy (horizontal axis). As seen below, the electron energy distribution in double β-decay is very wide and is centered around smaller energies, whereas the 0νββ-decay is very narrow and is peaked at the maximum of the 2νββ-decay curve.

For double beta decay (blue) and neutrinoless double beta decay (red peak), the electron spectrum in beta decay: Number of electrons/beta-particles (vertical axis) versus energy/kinetic energy (KE) or electrons (horizontal axis). Credit: COBRA experiment

Unfortunately, searches for 0νββ-decay have not yielded any evidence for Majorana neutrinos. This could be because neutrinos are not their own antiparticle, in which case we will never observe the decay. Alternatively, it could be the case that current experiments are simply not yet sensitive to how rarely 0νββ-decay occurs. The rate at which the decay occurs is proportional to the mass of the intermediate neutrino: a zero neutrino mass implies a zero 0νββ-decay rate.

Experiments such as KATRIN hope to measure the mass of neutrinos in the next coming years. If a mass measurement is obtained, it would be a very impressive and impacting result. Furthermore, definitive predictions for 0νββ-decay can be made, at which point the current generation of experiments, such as MAJORANA, COURE, and EXO will be in a mad dash for testing whether or not neutrinos are indeed their own antiparticle.


Lower view of CUORE Cryostat. Credit: CUORE Experiment


Inside view of CUORE Cryostat. Credit: CUORE Experiment

Happy Hunting and Happy Colliding,

Richard Ruiz (@BraveLittleMuon)

PS Much gratitude to Yury Malyshkin,  Susanne Mertens, Gastón Moreno, and Martti Nirkko for discussions and inspiration for this post. Cheers!

Update 2015 September 25: Photos of the Cryogenic Underground Observatory for Rare Events (CUORE) experiment have been added. Much appreciate to QD-er Laura Gladstone.

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About to start a Physics degree? Hold on tight… http://www.quantumdiaries.org/2015/09/08/about-to-start-a-physics-degree-hold-on-tight/ http://www.quantumdiaries.org/2015/09/08/about-to-start-a-physics-degree-hold-on-tight/#comments Tue, 08 Sep 2015 10:22:32 +0000 http://www.quantumdiaries.org/?p=36113 It’s the beginning of September, which means two things. 1- I haven’t managed to write a blog in 6 months (turns out second year is busy!), and 2- it’s nearly the start of the academic year. These two facts have inspired me to write a post aimed for all those fresh-faced 18 year olds about to embark on the adventure of university. Or college for the Americans– a weird concept to us Brits, as college is in fact where 16 year olds go to do mainly non-academic courses like “Travel and Tourism” and “Hair and Beauty”. Right now in the UK, thousands and thousands of teenagers are probably getting increasingly nervous as their start dates in September and October loom nearer and nearer. I thought I would write something for anyone who is about to attempt a physics degree at university, with none of that prospectus fluff.

I think the most succinct way to sum up my undergraduate degree is “play hard, work harder”! It wasn’t easy, some of the time it felt downright impossible but at the end of the day I had fun, I made friends, I got a masters degree and finally a PhD place. What more can you ask for from university?

Other Physicists
So, first things first, before you do any physics, you are going to see and meet some of your coursemates. I’m going to be brutally honest here: physicists are weird. They just are. To dedicate yourself to a subject like physics you just need to have a little bit of weirdness in you, that seems to be a fundamental law. This is not always a bad thing. A disclaimer here, some of my best friends are physicists. My colleagues at UCL are a brilliant, funny and sociable bunch. My boyfriend, although now having sold his soul to the actual law, I met studying the laws of physics at uni. There is nothing wrong with physicists as a whole, but a lot of them are a little strange. You may encounter people who are painfully socially awkward, wear fedoras or suits to lectures, LARPers, guys with LOTS of hair (both on their head and faces), posh kids buying Grey Goose, poor kids living on supernoodles and beans, international kids, gamers, heavy drinkers, those that are politically driven, or lazy with questionable hygiene, and on rare occasions, women.
I joke, I think my course was ~10% girls by the masters year, so not too rare. It’s getting better, and I think we are better off than computer sciences, but we are still in the minority. If you are so lucky to be female amongst the physicists, be warned, they may stare, and they will probably know who you are when you can’t possibly be expected to remember all those generic male faces! I can’t count the times I was approached by strangers, usually in clubs but once on a train, with the line ‘you’re that girl who does physics!’.
I had the good luck of having another physicist in my halls of residence – a rather normal one, who played guitar and drunk a lot. We were flatmates for many years, and we stuck together in lectures and labs as much as we could. In my first year, I remember having a very strong aversion to making any other friends on my course. They’re all awkward and weird and nerdy, I said. I don’t want to hang out with them, they wont be fun, I said. The important thing here is I WAS WRONG. After I realised my flatmate was leaving after third year with a bachelors, I made a bit of effort to meet people, and made some extremely good friends in physics, who I still see often. And guess what? They aren’t weird and no fun. They are really great guys, and I wish I had made friends with them earlier.
So don’t write anyone off immediately. Be sociable, chat to people. People will be shy (I was, and still am) and awkward, but give everyone a chance. You wont get on with everyone, but you may be surprised at who you do end up friends with. Physicists are usually a little bit odd – but they are also often a lot of fun.

The Actual Physics
A very important thing to understand when you start a course like physics (or maths, or any science really) is that unless you are some supreme genius, there will be people cleverer than you. Lots of them. If you are going to a top uni with very high entry grades (Warwick at the time was AAB, A*s weren’t available yet) then chances are you are going to feel a little bit inferior. I went from being the top of my physics A-level class to somewhere in the upper quartile, and at first it was a little disconcerting. But don’t worry – there are more qualities to a person than their grades!

Doing stuff like this in the library will make you feel clever.

Doing stuff like this in the library will make you feel clever.

For my first few weeks at Warwick, I was in a bit of a panic. We had a course called “Physics Foundations” which you may think sounds like some nice gentle introductory course. Wrong. We were thrown in the deep end. It bared almost no resemblance to A-level physics (there was, thank God, a mechanics course that did, but it came after Special Relativity, which also had me panicking a fair bit) and involved all sorts of notation and nomenclature I’d never even heard of (like ’tilde’?!). I also had not done further maths, and whilst they brought us up to speed in maths quite quickly I did feel a little disadvantaged by my lack of knowledge on imaginary numbers. I genuinely spent the first few weeks thinking I chose the wrong course. What had felt so right at A-level, so naturally the thing I was best at, was now giving me an identity crisis. I began wondering how I would explain to my friends that I had failed.

And then, a miracle happened. I talked to other people. I talked to the students in my tutor group and my seminar groups. And guess what – they were all just as confused as me. This was a wonderful realisation. I also noticed my problem sheet marks were actually not so bad. I didn’t always understand what I was doing but I seemed to be doing it the right way. This is another important thing to note – do not expect to understand your lectures. I didn’t understand much at all until I did problems, past papers and proper revision – often in the third term! Do not panic early on if things aren’t going in. Do not think problem sheets aren’t important. They help, seriously.

You’re going to need to do some work. You’re going to notice your hallmates doing humanities having only 8 hours a week of contact time, whereas you’re closer to 25. You will be swamped every week with problems and lab reports and will have problems classes on top of your lectures. You WILL hate labs – I am yet to speak to anyone who really enjoyed them. But it is all essential to your development as a competent physicist (honestly..) and you will be glad of it in the long run.

Start of degree vs end of degree. Proof that blondes don’t have more fun? 

Living Conditions
Now this is an interesting one. You are probably going to be in student halls. Brace yourself. Here are some things that WILL happen:
– If you drink, you will vomit (probably multiple times).
– Again, if you drink, you will be forced to down a dirty pint (the worst I ever saw contained whisky, milk, garlic and and beer). And then you will probably vomit.
– You will really hate 9am lectures. Especially if you’re hungover/still drunk
– Everything will be a mess. All the time. No one will wash up
– You will encounter the panicked rush far too early to sort out a house for your second year, and you may end up not even liking the people you are gong to live with by the end of term
– The toilet will be covered in all manners of disgusting bodily fluids on multiple occasions.
– You will get freshers flu and feel ill for weeks and your lecture halls will be filled with the sound of coughing.
– Someone will not understand how to use a washing machine (and there may even be someone who takes their laundry home to their mum)
– You will inevitably fall out with someone who was initially your best friend
– There will be some romance and some drama. Some couples will last, others will not. Inevitably, people will start breaking up with home boyfriends/girlfriends.
– You wont change your sheets for an unholy amount of time.
– There will be people in your halls you didn’t even know existed until you awkwardly encounter them in the corridor at the end of term or cooking in the middle of the night.
– You will feel sad and miss your parents, your pets and your home friends, no matter how much fun you’re having.
Sound fun? Unfortunately, this seems to be what it takes to get yourself a physics degree. Things might improve when you move off campus into a house, but this is heavily dependent on your choice of housemates. Really, you have to work out what works best for you in order to survive student living. Maybe you wont mind the mess and the mould. What I will say though is please, please wash your sheets at least once a term. It’s gross.

There’s going to be blood, sweat and tears. Literally. There’s going to be fights and drama and emotional and intellectual struggles. There’s going to be regret and awful hangovers. There will be late nights writing lab reports or finishing problems. You will want to tear your hair out over the electromagnetic field of an infinite charged plane, or a pulley with mass, or second order differential equations, or whether or not γμ is a four vector (spoiler: it isn’t). You will hate some lecturers – worst are the ones that pick people out to answer questions, some will send you to sleep and others you will love and respect. You are going to hate physics, you’re going to love physics, and you’re going to question yourself why the hell you chose it. But in the end, if you make it out with your degree, you’ve done something incredible, and a lot of doors will be open to you. I always knew I wanted to stay with physics and my four years at Warwick left me still enjoying physics and well prepared for a PhD.

If you’re about to start your degree – it’s going to be a wild ride, but it may just be some of the best years of your life. Good luck!

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Double time http://www.quantumdiaries.org/2015/08/27/double-time/ http://www.quantumdiaries.org/2015/08/27/double-time/#comments Fri, 28 Aug 2015 03:32:22 +0000 http://www.quantumdiaries.org/?p=36102 In particle physics, we’re often looking for very rare phenomena, which are highly unlikely to happen in any given particle interaction. Thus, at the LHC, we want to have the greatest possible proton collision rates; the more collisions, the greater the chance that something unusual will actually happen. What are the tools that we have to increase collision rates?

Remember that the proton beams are “bunched” — there isn’t a continuous current current of protons in a beam, but a series of smaller bunches of protons, each only a few centimeters long, with gaps of many centimeters between each bunch.  The beams are then timed so that bunches from each beam pass through each other (“cross”) inside one of the big detectors.  A given bunch can have 10E11 protons in it, and when two bunches cross, perhaps tens of the protons in each bunch — a tiny fraction! — will interact.  This bunching is actually quite important for the operation of the detectors — we can know when bunches are crossing, and thus when collisions happen, and then we know when the detectors should really be “on” to record the data.

If one were to have a fixed number of protons in the machine (and thus a fixed total amount of beam current), you could imagine two ways to create the same number of collisions: have N bunches per beam, each with M protons, or 2N bunches per beam with M/sqrt(2) protons.  The more bunches in the beam, the more closely spaced they would have to be, but that can be done.  From the perspective of the detectors, the second scenario is much preferred.  That’s because you get fewer proton collisions per bunch crossing, and thus fewer particles streaming through the detectors.  The collisions are much easier to interpret if you have fewer collisions per crossing; among other things, you need less computer processing time to reconstruct each event, and you will have fewer mistakes in the event reconstruction because there aren’t so many particles all on top of each other.

In the previous LHC run (2010-12), the accelerator had “50 ns spacing” between proton bunches, i.e. bunch crossings took place every 50 ns.  But over the past few weeks, the LHC has been working on running with “25 ns spacing,” which would allow the beam to be segmented into twice as many bunches, with fewer protons per bunch.  It’s a new operational mode for the machine, and thus some amount of commissioning and tuning and so forth are required.  A particular concern is “electron cloud” effects due to stray particles in the beampipe striking the walls and ejecting more particles, which is a larger effect with smaller bunch spacing.  But from where I sit as one of the experimenters, it looks like good progress has been made so far, and as we go through the rest of this year and into next year, 25 ns spacing should be the default mode of operation.  Stay tuned for what physics we’re going to be learning from all of this!

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The Tesla experiment http://www.quantumdiaries.org/2015/08/27/the-tesla-experiment/ http://www.quantumdiaries.org/2015/08/27/the-tesla-experiment/#comments Thu, 27 Aug 2015 13:40:55 +0000 http://www.quantumdiaries.org/?p=36095 CMS scientist Bo Jayatilaka assumes the driver seat in a Tesla Model S P85D as part of a two-day road trip experiment. Photo: Sam Paakkonen

CMS scientist Bo Jayatilaka assumes the driver seat in a Tesla Model S P85D as part of a two-day road trip experiment. Photo: Sam Paakkonen

On May 31, about 50 miles from the Canadian border, an electric car struggled up steep hills, driving along at 40 miles per hour. The sun was coming up and rain was coming down. Things were looking bleak. The car, which usually plotted the route to the nearest charging station, refused to give directions.

“It didn’t even say turn around and go back,” said Bo Jayatilaka, who was driving the car. “It gave up and said, ‘You’re not going to make it.’ The plot disappeared.”

Rewind to a few weeks earlier: Tom Rammer, a Chicago attorney, had just won two days with a Tesla at a silent cell phone auction for the American Cancer Society. He recruited Mike Kirby, a Fermilab physicist, to figure out how to get the most out of those 48 hours.

Rammer and Kirby agreed that the answer was a road trip. Their initial plan was a one-way trip to New Orleans. Another involved driving to Phoenix and crossing the border to Mexico for a concert. Tesla politely vetoed these options. Ultimately, Rammer and Kirby decided on an 867-mile drive from Chicago to Boston. Their goal was to pick up Jayatilaka, a physicist working on the CMS experiment, and bring him back to Fermilab. To document their antics, the group hired a film crew of six to follow them on their wild voyage from the Windy City to Beantown.

Jayatilaka joked that he didn’t trust Rammer and Kirby to arrange the trip on their own, so they also drafted Jen Raaf, a Fermilab physicist on the MicroBooNE experiment, whose organizational skills would balance their otherwise chaotic approach.

“There was no preparing. Every time I brought it up Tom said, ‘Eh, it’ll get done,’” Raaf laughed. Jayatilaka added that shortly after Raaf came on board they started seeing spreadsheets sent around and itineraries being put together.

“I had also made contingency plans in case we couldn’t make it to Boston,” Raaf said, with a hint of foreshadowing.

The Tesla plots the return trip to Chicago, locating the nearest charging station. Photo: Sam Paakkonen

The Tesla plots the return trip to Chicago, locating the nearest charging station. Photo: Sam Paakkonen

On May 29, Rammer, Kirby and Raaf picked up the Tesla and embarked on their journey. The car’s name was Barbara. She was a black Model S P85D, top of the line, and she could go from zero to 60 in 3.2 seconds.

“I think the physics of it is really interesting,” Jayatilaka said. “The reason it’s so fast is that the motor is directly attached to wheels. With cars we normally drive there is a very complicated mechanical apparatus that converts small explosions into something that turns far away from where the explosions are. And this thing just goes. You press the button and it goes.”

The trip started out on flat terrain, making for smooth, easy driving. But eventually the group hit mountains, which ate up Barbara’s battery capacity. In the spirit of science, these physicists pushed the boundaries of what they knew, testing Barbara’s limits as they braved undulating roads, encounters with speed-hungry Porsches and Canadian border patrol.

“If you have something and it’s automated, you need to know the limitations of that algorithm. The computer does a great job of calculating the range for a given charge, but we do much better knowing the terrain and what’s going to happen. We need to figure out what we are better at and what the algorithm is better at,” Kirby said. “The trip was about learning the car. The algorithm is going to get better because of all of the experiences of all of the drivers.”

The result of the experiment was that Barbara didn’t make it all the way to Boston. As they approached the east coast, it became clear to Kirby and Raaf that they wouldn’t have made it back in time to drop off the car. Although Rammer was determined to see the trip through to the end, he eventually gave in somewhere in New Jersey, and they decided to cut the trip short. Jayatilaka met the group in a parking lot in Springfield, Massachusetts, and they plotted the quickest route back to Chicago.

Flash forward to that bleak moment on May 31. After crossing the border, just as things were looking hopeless, Barbara’s systems suddenly came back to life. She directed the group to a charging station in chilly Kingston, Ontario. Around 6:30 in the morning, they rolled into the station. The battery level: zero percent. After a long charge and another full day of driving, they pulled into the Tesla dealership in Chicago around 8:55 p.m., minutes before their time with Barbara was up.

“The car was just alien technology to us when we started,” Jayatilaka said. “It was completely unfamiliar. We all came away from it thinking that we could have done this road trip so much better with those two days of experience. We felt like we actually understood.”

Ali Sundermier

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Pour une physique nucléaire accessible à tous http://www.quantumdiaries.org/2015/08/18/pour-une-physique-nucleaire-accessible-a-tous/ http://www.quantumdiaries.org/2015/08/18/pour-une-physique-nucleaire-accessible-a-tous/#comments Tue, 18 Aug 2015 14:39:16 +0000 http://www.quantumdiaries.org/?p=36087 Grégoire Besse, doctorant au CNRS en physique nucléaire théorique, nous confie son intérêt pour la médiation des sciences.

C’est en débutant ma thèse que je me suis aperçu du lien inéluctable entre la recherche et la vulgarisation. J’ai donc progressivement choisi de me lancer dans cette démarche afin d’expliquer mes recherches et de les rendre plus « limpides » pour le commun des mortels. Ma thèse porte sur la physique nucléaire théorique et s’intitule  « Description théorique de la dynamique nucléaire lors des collisions d’ions lourds et ses implications astrophysiques ». Elle se déroule au laboratoire Subatech à Nantes. Je travaille sur la description dynamique d’un système nucléaire, c’est-à-dire des noyaux en collision ou en réseau. Pour cela, le groupe de recherche dont je fais partie a élaboré un code de simulation nommé Dynamical Yavelets in Nuclei (DYWAN). Ce code est déjà opérationnel mais reste en phase d’optimisation.

Exemple de collision à basse énergie entre deux noyaux. On observe que les noyaux se déforment sous l’effet de la force nucléaire pour se coller, jusqu’à atteindre une fusion.

Exemple de collision à basse énergie entre deux noyaux. On observe que les noyaux se déforment sous l’effet de la force nucléaire pour se coller, jusqu’à atteindre une fusion.

La physique nucléaire s’intéresse aux noyaux et aux comportements de la force nucléaire. La force nucléaire, ou interaction forte résiduelle, est l’effet de l’interaction forte (quarks-gluons) à l’échelle nucléaire. Il s’agit de l’interaction nucléon-nucléon. Bien moins médiatisée que la physique des hautes énergies (celle du LHC et du boson de Higgs), la physique nucléaire reste néanmoins un maillon essentiel pour comprendre la matière. De plus, ses applications sont immédiates, comme par exemple avec la radioactivité, la fission, la fusion ou la production de radio-isotopes.

Ma passion au service de mon travail

Aperçu de l’environnement 3D en OpenGL. Il est visitable comme un jeu-vidéo avec clavier-souris. Les noyaux (bleu-rouge et cyan-rose), déjà mêlés, sont représentés par des objets mathématiques : les états cohérents (les boules avec des nuages de points).

Aperçu de l’environnement 3D en OpenGL. Il est visitable comme un jeu-vidéo avec clavier-souris. Les noyaux (bleu-rouge et cyan-rose), déjà mêlés, sont représentés par des objets mathématiques : les états cohérents (les boules avec des nuages de points).

Le but de ma thèse est de fournir un code de simulation puissant capable de reproduire des données et des comportements observés expérimentalement puis de prédire des réactions. Nous nous focalisons sur la collision d’ions lourds qui permettent de produire des systèmes nucléaires très exotiques tels que de la matière très riche en neutrons. D’autres groupes de recherche du laboratoire s’intéressent plutôt aux études de la radioactivité, de la durée de vie et du comportement des noyaux isolés. Ceci me rappelle la métaphore d’Albert Einstein qui expliquait que pour comprendre le fonctionnement d’une montre sans l’ouvrir, vous avez deux solutions : l’observation (écoute, regard, prise de données et émission d’hypothèses) ou l’expérimentation (vous lancez la montre contre un mur, vous regardez les pièces qui sortent et vous essayer de tout remettre en ordre). Nous utilisons plutôt cette deuxième méthode.

Parallèlement à ma thèse, j’essaie de mettre au point un logiciel  alliant recherche et nouvelles technologies (j’en suis arrivé à un environnement 3D visitable avec clavier-souris). Je suis très intéressé par la réalité virtuelle et la réalité augmentée : je pense que ces outils permettront de nouvelles approches dans la recherche, un nouveau point de vue pour une nouvelle théorie. Et cela a déjà fait ses preuves : nous avons débusqué des erreurs sur DYWAN grâce à mon logiciel !

L’oiseau bleu, ami de la recherche

Mon arrivée sur Twitter n’est pas très ancienne, mais très vite j’ai compris que ce réseau social est un outil formidable pour la recherche. Cette dernière est un monde actif en constante évolution, il paraît alors légitime de se tenir informé des avancées car cela fait normalement partie de notre travail. Par ailleurs, Twitter permet un aperçu rapide (- de 140 caractères) des informations importantes.

J’ai découvert le compte @EnDirectDuLabo par hasard : chaque semaine, un scientifique en prend les rênes pour partager son quotidien avec les abonnés. Avec un public potentiel de plus de 2 000 personnes, l’expérience peut être intimidante. Mais finalement, lorsque ce fut mon tour, tout s’est bien passé et j’ai eu des échanges avec un public varié : chercheurs, doctorants, journalistes, community managers, amateurs et autres curieux.

Au final, cette expérience m’a aidé à mieux cerner mon sujet de thèse. De plus, ces « relations » sont très enrichissantes au quotidien : une photo, une phrase, un article, un blog, vive la curiosité et le partage 2.0 !

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MicroBooNE sees first cosmic muons http://www.quantumdiaries.org/2015/08/12/microboone-sees-first-cosmic-muons/ http://www.quantumdiaries.org/2015/08/12/microboone-sees-first-cosmic-muons/#comments Wed, 12 Aug 2015 14:20:13 +0000 http://www.quantumdiaries.org/?p=36081 This article appeared in Fermilab Today on Aug. 12, 2015.
This image shows the first cosmic ray event recorded in the MicroBooNE TPC on Aug. 6. Image: MicroBooNE

This image shows the first cosmic ray event recorded in the MicroBooNE TPC on Aug. 6. Image: MicroBooNE

A school bus-sized detector packed with 170 tons of liquid argon has seen its first particle footprints.

On Aug. 6, MicroBooNE, a liquid-argon time projection chamber, or LArTPC, recorded images of the tracks of cosmic muons, particles that shower down on Earth when cosmic rays collide with nuclei in our atmosphere.

“This is the first detector of this size and scale we’ve ever launched in the U.S. for use in a neutrino beam, so it’s a very important milestone for the future of neutrino physics,” said Sam Zeller, co-spokesperson for the MicroBooNE collaboration.

Picking up cosmic muons is just one brief stop during MicroBooNE’s expedition into particle physics. The centerpiece of the three detectors planned for Fermilab’s Short-Baseline Neutrino program, or SBN, MicroBooNE will pursue the much more elusive neutrino, taking data about this weakly interacting particle for about three years. When beam starts up in October, it will travel 470 meters and then traverse the liquid argon in MicroBooNE, where neutrino interactions will result in tracks that the detector can convert into precise three-dimensional images. Scientists will use these images to investigate anomalies seen in an earlier experiment called MiniBooNE, with the aim to determine whether the excess of low-energy events that MiniBooNE saw was due to a new source of background photons or if there could be additional types of neutrinos beyond the three established flavors.

One of MicroBooNE’s goals is to measure how often a neutrino that interacts with an argon atom will produce certain types of particles. A second goal is to conduct R&D for future large-scale LArTPCs. MicroBooNE will carry signals up to two and a half meters across the detector, the longest drift ever for a LArTPC in a neutrino beam. This requires a very high voltage and very pure liquid argon. It is also the first time a detector will operate with its electronics submerged in liquid argon on such a large scale. All of these characteristics will be important for future experiments such as the Deep Underground Neutrino Experiment, or DUNE, which plans to use similar technology to probe neutrinos.

“The entire particle physics community worldwide has identified neutrino physics as one of the key lines of research that could help us understand better how to go beyond what we know now,” said Matt Toups, run coordinator and co-commissioner for MicroBooNE with Fermilab Scientist Bruce Baller. “Those questions that are driving the field, we hope to answer with a very large LArTPC detector.”

Another benefit of the experiment, Zeller said, is training the next generation of LArTPC experts for future programs and experiments. MicroBooNE is a collaborative effort of 25 institutions, with 55 students and postdocs working tirelessly to perfect the technology. Collaborators are keeping their eyes on the road toward the future of neutrino physics and liquid-argon technology.

“It’s been a long haul,” said Bonnie Fleming, MicroBooNE co-spokesperson. “Eight and a half years ago liquid argon was a total underdog. I used to joke that no one would sit next to me at the lunch table. And it’s a world of difference now. The field has chosen liquid argon as its future technology, and all eyes are on us to see if our detector will work.”

Ali Sundermier

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Prototype of Mu2e solenoid passes tests with flying colors http://www.quantumdiaries.org/2015/08/11/prototype-of-mu2e-solenoid-passes-tests-with-flying-colors/ http://www.quantumdiaries.org/2015/08/11/prototype-of-mu2e-solenoid-passes-tests-with-flying-colors/#comments Tue, 11 Aug 2015 20:27:23 +0000 http://www.quantumdiaries.org/?p=36078 This article appeared in Fermilab Today on Aug. 11, 2015.

This prototype represents one of 27 modules that will make up a critical section of the Mu2e experiment, the transport solenoid. Photo: Reidar Hahn

This prototype represents one of 27 modules that will make up a critical section of the Mu2e experiment, the transport solenoid. Photo: Reidar Hahn

If you’ve ever looked at a graphic of Fermilab’s future Mu2e experiment, you’ve likely noticed its distinctive, center s-shaped section. Tall and wide enough for a person to fit inside it, this large, curving series of magnets, called the transport solenoid, is perhaps the experiment’s most technologically demanding piece to build.

Last month a group in the Fermilab Technical Division aced three tests — for alignment, current and temperature — of a prototype transport solenoid module built by magnet experts at Fermilab’s Technical Division and INFN-Genoa in Italy.

The triple milestone means that Fermilab can now order the full set for production — 27 modules.

“The results were excellent,” said Magnet Systems Department scientist Mau Lopes, who is leading the effort.

There’s not much wiggle room when it comes to the transport solenoid, a crucial component for the ultrasensitive Mu2e experiment. Mu2e will look for a predicted but never observed phenomenon, the conversion of a muon into its much lighter, more familiar cousin, the electron, without the usual accompanying neutrinos. To do this, it will send muons into a detector where scientists will look for particular signatures of the rare process.

The transport solenoid generates a magnetic field that deftly separates muons based on their momentum and charge and directs slow muons to the center of the Mu2e detector. The maneuver requires some fairly precisely designed details, not the least of which is a good fit.

When put together, the 27 wedge-shaped modules will form a tube with the snake-like profile. Muons will travel down this vacuum tube. To guide them along the right path to the detector, the solenoid units must align with each other to within 0.2 degrees. The Magnet Systems team exceeded expectation: The prototype was aligned with 100 times greater precision.

The team achieved not just the right shape, but the right current. The electrical current running through the solenoid coil creates the magnetic field. The Mu2e team exceeded the nominal current of 1,730 amps, reaching 2,200 amps. As a bonus, while that amount of current has the potential to create a slight deformation in the module’s shape, the Mu2e team measured no change in the structure.

Nor was there much change in the model’s temperature, which must be very low. The team delivered 2.5 watts of power to the coil — well above what the coils will see when running. The module proved robust: The temperature changed by a mere whisker — 150 millikelvin, or 0.27 degrees Fahrenheit. The coils will be at 5 Kelvin when operating. The prototype sustained the nominal current at up to 8 Kelvin.

Fermilab has selected a vendor to produce the modules. Lopes expects that it will be two and a half years until all modules are complete.

“We thank all the smart people at INFN Genoa, the Fermilab Test and Instrumentation Department, the Magnet Systems Department and the Accelerator Division Cryogenics Department for this achievement,” Lopes said. “These seven months of hard work have paid off tremendously. Our project continues at full steam ahead.”

Leah Hesla

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The short-baseline detectives and the mysterious case of the sterile neutrino http://www.quantumdiaries.org/2015/08/10/the-short-baseline-detectives-and-the-mysterious-case-of-the-sterile-neutrino/ http://www.quantumdiaries.org/2015/08/10/the-short-baseline-detectives-and-the-mysterious-case-of-the-sterile-neutrino/#comments Mon, 10 Aug 2015 14:56:41 +0000 http://www.quantumdiaries.org/?p=36074 This article appeared in Fermilab Today on Aug. 10, 2015.

The Fermilab Short-Baseline Neutrino program will use three detectors: SBND, MicroBooNE (shown here) and ICARUS. Photo: Reidar Hahn

The Fermilab Short-Baseline Neutrino program will use three detectors: SBND, MicroBooNE (shown here) and ICARUS. Photo: Reidar Hahn

In 1995, physicists working on the Liquid Scintillator Neutrino Detector, or LSND, at Los Alamos National Laboratory stumbled upon some curious results.

The experiment, whose goal was to investigate oscillations between the three different flavors of the elusive neutrino, saw evidence that there might be at least one additional flavor of neutrino lurking just out of reach. In 2002, an experiment at Fermilab called MiniBooNE started collecting data to explore this anomaly, but the results were inconclusive: some data seemed to refute the possibility of a fourth neutrino, but other data seemed to indicate particle interactions that couldn’t be explained with conventional three-neutrino models. The possibility of a mysterious, fourth neutrino remained alive.

“It’s a question that’s been first lingering with the anomalies from LSND and then MiniBooNE,” said Bonnie Fleming, co-spokesperson of a new neutrino experiment at Fermilab called MicroBooNE. “There’s now a worldwide campaign to address whether these short-baseline oscillations and hints from other experiments are indicating new physics.”

Scientists from Fermilab and more than 45 institutions around the world have teamed up to design a program to catch this hypothetical neutrino in the act. The program, called the Short-Baseline Neutrino (SBN) program, makes use of a trio of detectors positioned along one of Fermilab’s neutrino beams. Although there are other reactor and source-based experiments in the world that actively seek a fourth neutrino, also called a sterile neutrino, SBN is the only program that uses a particle accelerator to produce neutrinos and multiple neutrino detectors for this search.

“No one else is doing an experiment like this,” said Peter Wilson, coordinator for the SBN program. “There are no other experiments on this energy scale using the concept of a near detector and a far detector.”

Determining whether there are more than three neutrino flavors would affect how scientists interpret data from experiments like the planned Deep Underground Neutrino Experiment, which is expected to make transformative discoveries about neutrinos, and perhaps other aspects of the universe, in the future. Solving the mystery of the anomalies seen at LSND and MiniBooNE, however, will not be easy. Because the sterile neutrino would not interact through the weak nuclear force as the other three do (hence the name “sterile”), detecting this particle would be like chasing the shadow of a ghost.

It begins at the Fermilab Booster, where protons are accelerated to 8 GeV and smashed into a target, creating new particles. Charged particles are bent forward by a magnetic focusing device into a tunnel where most decay to produce muon neutrinos. The three detectors — named the Short-Baseline Near Detector, or SBND, MicroBooNE and ICARUS — will be spread out over a distance of 600 meters. SBND, 100 meters from the target, will take data close to the source to reduce systematic uncertainties by measuring the initial characteristics of the muon neutrino beam. Four hundred meters beyond the planned site for SBND is MicroBooNE, which is already installed. ICARUS will be located 110 meters past MicroBooNE. ICARUS is an existing detector from a previous experiment at the Italian INFN laboratory at Gran Sasso that is currently being refurbished at CERN. It will have a massive chamber holding 760 tons of liquid argon to beat down statistical uncertainties in the experiment.

All three of the detectors are time projection chambers, a type of detector that allows physicists to analyze particle collisions in three dimensions. For these particular TPCs, scientists use liquid argon because its relatively heavy mass ensures a higher rate of interactions.

MicroBooNE received its last fill of liquid argon in July and recently began taking data. Scientists are expecting to break ground on buildings for both ICARUS and SBND by this fall. In 2017, ICARUS will be fully refurbished and delivered to Fermilab. Scientists hope to complete building SBND that same year.

Since experimenters won’t be able to directly detect the sterile neutrino, they will search for clues in the trails of particles the three known neutrino flavors leave behind in the liquid argon after they interact. If the experiments, expected to begin running in 2018, see deviations in the expected neutrino oscillation pattern, scientists will know that they’re on the right track in their hunt for this fugitive particle. If not, they will be able to put the mystery of the sterile neutrino to rest.

“If we design a strong enough experiment, which I believe we have, then one of two things will happen when we start taking data,” said David Schmitz, co-spokesperson for SBND. “Either we will rule out the earlier hints, or we make, frankly, the most exciting discovery in particle physics in some time.”

Ali Sundermier

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Trois mots pour résumer une conférence http://www.quantumdiaries.org/2015/07/29/trois-mots-pour-resumer-une-conference/ http://www.quantumdiaries.org/2015/07/29/trois-mots-pour-resumer-une-conference/#comments Wed, 29 Jul 2015 13:18:32 +0000 http://www.quantumdiaries.org/?p=36067 Impressionnant, excitant et plein de nouvelles perspectives. Cela résume mon impression alors que se termine aujourd’hui la conférence de physique des particules de la Société européenne de physique (EPS) à Vienne.

Nous avons été exposés à une quantité impressionnante de nouvelles données. Non seulement les expériences du Grand collisionneur de hadrons (LHC) du CERN ont finalisé la plupart de leurs analyses sur l’ensemble des données recueillies avant l’arrêt début 2013, mais elles ont aussi déjà commencé à analyser les nouvelles données. Ceci confirme que tout, des détecteurs aux logiciels de reconstruction, fonctionne parfaitement après le vaste programme d’améliorations et de réparations.


Souper de clôture de la conférence au magnifique palais Schönbrunn à Vienne (Photo: Gertrud Konrad)

Tous les outils nécessaires aux analyses de physique – simulations, systèmes d’acquisition de données, trigger, calibrations et algorithmes d’analyse – produisent déjà des résultats de haute qualité avec les données des collisions à une énergie de 13 TeV. Les expériences sont clairement en mesure de reprendre les analyses là où elles les avaient laissées avec les données collectées à 8 TeV. Bien sûr, il n’y a encore aucuns signes de nouveaux phénomènes mais les expériences LHCb, CMS et ATLAS ont toutes de petites anomalies qui devraient être élucidées avec les nouvelles données du LHC.

Durant cette conférence, on a pu apprécié aussi la variété des expériences en place et les nouveaux résultats qui commencent déjà à arriver sur la matière sombre et l’énergie sombre. De nouvelles avenues sont aussi explorées pour élargir les recherches dans l’espoir de découvrir les 95 % du contenu de l’Univers qui manquent toujours à l’appel. Les expériences ont fait des pas de géants et on s’attend à des percées majeures d’ici à peine quelques années. On peut aussi espérer des développements dans le secteur des neutrinos, un domaine de recherche prolifique mais aussi un des plus déconcertants et embrouillants depuis de nombreuses années.

Comme l’a souligné Pierre Binetruy, un théoricien travaillant en cosmologie : « Les découvertes simultanées du boson de Higgs et la confirmation de quelques unes des caractéristiques de l’inflation (la période marquée par une expansion fulgurante juste après le Big Bang) a ouvert une nouvelle ère dans la compréhension commune de la cosmologie et de la physique des particules ». Nous sommes clairement à la veille de percées majeures et de nouvelles découvertes dans plusieurs domaines. La prochaine conférence sera sans aucun doute un événement à ne pas manquer.

Pauline Gagnon

Pour recevoir un avis lors de la parution de nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou par e-mail en ajoutant votre nom à cette liste de distribution ou consultez mon site web.

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Three words to summarize a conference http://www.quantumdiaries.org/2015/07/29/three-words-to-summarize-a-conference/ http://www.quantumdiaries.org/2015/07/29/three-words-to-summarize-a-conference/#comments Wed, 29 Jul 2015 13:12:31 +0000 http://www.quantumdiaries.org/?p=36064 Impressive, exciting and eye-opening. This is how I would summarize the European Physics Society (EPS) particle physics conference that is ending today in Vienna.

The participants were treated to an impressive amount of new data. Not only had the Large Hadron Collider (LHC) experiments at CERN finalised most of their analyses on the entire set of data collected prior to the long shutdown of the last two years, but they had also already started analysing the new data. This confirms that everything, from hardware to software, is up and running after extensive upgrades, repairs and improvements.

All the tools for physics analysis – simulations, data acquisition systems, trigger menus, calibration and analysis algorithms – are already performing beautifully at the new collision energy of 13 TeV. The experiments are clearly in a position to take up the analyses where they had left them with the 8 TeV data. True, there are no signs for new physics anywhere yet but LHCb, CMS and ATLAS all have little hints that will soon be elucidated with the new data.


Conference dinner in the beautiful Schönbrunn castle in Vienna (Credit: Gertrud Konrad)

A wealth of new experiments and results were also presented at the conference on dark matter and dark energy. New avenues are also explored to broaden the searches in the hope of accounting for the 95% of the content of the Universe that is still completely unknown. Giant steps have already been taken and major breakthroughs are expected in the very near future. Developments are also expected in the neutrino sector, a prolific research domain that has been most puzzling and confusing for many years.

As stated by Pierre Binetruy, a theorist working on cosmology: “The simultaneous discovery of the Higgs and confirmation of some of the basic features of inflation (the rapid expansion that followed the Big Bang) has opened a new era in the common understanding of cosmology and particle physics“. It is clear that we are on the eve of major advances and discoveries. The next conference is sure to be an event not to be missed.

Pauline Gagnon

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La route cahoteuse menant aux découvertes http://www.quantumdiaries.org/2015/07/28/la-route-cahoteuse-menant-aux-decouvertes/ http://www.quantumdiaries.org/2015/07/28/la-route-cahoteuse-menant-aux-decouvertes/#comments Tue, 28 Jul 2015 13:32:16 +0000 http://www.quantumdiaries.org/?p=36050 La conférence de physique des particules de la Société de Physique Européenne (EPS) se poursuit à Vienne, les sessions parallèles ayant cédé la place aux sessions plénières. Les présentateurs et présentatrices ont maintenant la dure tâche de récapituler les centaines de résultats présentés jusqu’ici à la conférence et d’en tirer une vue d’ensemble.

Durant les deux dernières années, le Grand Collisionneur de Hadrons (LHC) a subi des améliorations majeures. Les expérimentalistes en ont profité pour examiner sous toutes les coutures (et même plus!) l’ensemble des données accumulées avant l’arrêt. Avec les calibrations finales et des algorithmes améliorés, presque toutes les analyses incluent maintenant la totalité des données récoltées à une énergie de 8 TeV. Dans la plupart des cas, ces mois de travail acharné effectué par des centaines de personnes n’auront produit qu’une légère amélioration dans la précision des résultats. Ces récents résultats, bien que solides comme le roc, n’ont malheureusement rien révélé de nouveau.

C’est la mauvaise nouvelle. La bonne nouvelle : on s’attend à quatre fois plus de données dans l’année qui vient et à plus haute énergie, ce qui rendra de nouveaux phénomènes accessibles.

En voici un exemple. Les expériences CMS et ATLAS cherchent, entre autres, des particules lourdes mais encore hypothétiques qui se désintègreraient en deux bosons connus, à savoir des photons, ou des bosons Z, W ou de Higgs. Les trois derniers bosons peuvent à leur tour se désintégrer en jets de particules légères faites de quarks.

La désintégration d’une particule s’apparente à faire la monnaie pour une grosse pièce de monnaie : la pièce de monnaie initiale ne contient pas de petites pièces, mais peut être échangée pour des pièces de valeur égale, comme sur le diagramme ci-dessous. Les quatre pièces de 50 centimes pourraient provenir d’une pièce de deux euros ou de deux pièces de un euro. De même, dans nos détecteurs, quand nous trouvons quatre jets de particules, ils peuvent provenir de deux bosons produits indépendamment (dans l’exemple ci-dessus, deux bosons Z), ou venir de quatre quarks produits directement. Tout ceci constitue le bruit de fond, tandis que le signal correspond dans ce cas au nouveau boson, celui qui s’est désintégré en deux bosons.

pieces de monnaie

La désintégration d’une particule s’apparente à faire la monnaie pour une pièce.

Une pièce de monnaie n’a qu’une valeur mais une particule possède à la fois masse et énergie. Quand on échange une grosse pièce pour de la monnaie, la valeur initiale est conservée. Avec des particules, nous devons prendre en compte la masse et l’énergie de tous les produits de désintégration pour calculer la masse combinée de la particule originale. Dernier détail : si la particule qui se désintègre est beaucoup plus lourde que les deux bosons qu’elle produit, les jets venant de ces bosons seront à peine séparés. Ils se déplaceront côte à côte. On n’observera alors non pas quatre jets, mais seulement deux jets plus évasés.

Si ces deux larges jets proviennent de deux Z bosons produits indépendamment, la valeur totale de leur masse combinée sera aléatoire, comme si nous additionnions la valeur de la monnaie au fond de nos poches. Si des milliers de personnes notaient sur un graphe la valeur de leur petite monnaie, nous obtiendrions une distribution comme celle de la ligne bleue ci-dessous. La majorité des gens ne traîne qu’un peu de monnaie, mais certaines personnes trimbalent une petite fortune en pièces de monnaie.


Un excès d’évènement trouvés ayant une masse de 2 TeV trouvés par ATLAS

L’axe horizontal donne la valeur de la masse combinée des deux jets pour chaque événement récolté par la Collaboration d’ATLAS qui en contenait deux. L’axe vertical montre combien d’événements ont été trouvés avec une valeur de masse particulière. La ligne bleue montre les contributions du bruit de fond et les autres lignes colorées correspondent à diverses hypothèses théoriques. Les points noirs représentent les données réelles et devraient être distribués de façon similaire à la ligne bleu en l’absence de nouvelles particules.

Une petite bosse est visible autour d’une valeur de masse de 2 TeV : il y a plus d’événements dans les données que ce à quoi on s’attend venant de sources connues. Mais il y a toujours un certain flou dans toute mesure à cause des erreurs expérimentales. Si on répétait la même mesure mille fois, au moins une de ces mesures aurait un écart semblable. Il est donc beaucoup trop tôt pour dire qu’il pourrait s’agir des premiers signes de la présence d’une nouvelle particule, comme un boson W’ hypothétique par exemple. Mais ce sera à suivre dans les nouvelles données.


Des évènements intrigants trouvés par CMS dans les nouvelles (à gauche) et les anciennes données (à droite)

La Collaboration CMS a aussi quelques événements intrigants, comme celui ci-dessus à gauche trouvé parmi les toutes nouvelles données recueillies depuis la reprise du LHC à 13 TeV. Les deux jets ont une masse combinée d’environ 5,0 TeV. Un évènement semblable ayant une masse combinée de 5,15 TeV (droite) a aussi été trouvé dans les données accumulées à 8 TeV. Il y a 500 fois moins de données à 13 TeV qu’à 8 TeV, mais les expériences peuvent déjà poursuivre les analyses effectuées à 8 TeV.

Il est beaucoup trop tôt pour dire quoi que ce soit. Un peu comme si nous regardions à distance, par un jour brumeux et à la tombée de la nuit, essayant de voir si le train s’en vient. La forme floue aperçue au loin est-elle réelle ou juste une illusion ? Personne ne le sait, il faut attendre que le train se rapproche. Mais pas pour longtemps puisque le LHC est déjà en marche. Les expériences CMS et ATLAS devraient bientôt avoir suffisamment de nouvelles données pour pouvoir trancher. Et là, attachez bien vos tuques, ça va devenir excitant!

Pauline Gagnon

Pour recevoir un avis lors de la parution de nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou par e-mail en ajoutant votre nom à cette liste de distribution ou consultez mon site web.

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The bumpy road to discoveries http://www.quantumdiaries.org/2015/07/28/the-bumpy-road-to-discoveries/ http://www.quantumdiaries.org/2015/07/28/the-bumpy-road-to-discoveries/#comments Tue, 28 Jul 2015 13:21:33 +0000 http://www.quantumdiaries.org/?p=36045 Yesterday, at the European Physics Society (EPS) Particle Physics conference in Vienna, we moved from parallel sessions to plenary sessions. The tasks of the speakers is now to summarize the hundreds of results presented so far at the conference, and draw the big picture.

For the past two years, the Large Hadron Collider underwent major upgrade work. Experimentalists have used this downtime to look at all collected data from all possible angles (and a few more!). With final calibrations and improved algorithms everywhere, nearly all analyses now included all data collected at 8 TeV. In most cases, months of hard work for hundreds of people only slightly improved the resolution. But these rock solid results have unfortunately not revealed new discoveries.

That’s the bad news. The good news is that four times more data is expected in the coming year at higher energy, making new phenomena accessible.

Here is one example. Both the CMS and ATLAS experiments are looking for heavy hypothetical particles that would decay into two of the known bosons, namely photons, Z, W or Higgs bosons. In turns, the last three bosons could decay into jets of light particles made of quarks.

A particle decay is very similar to making change for a large coin: the initial coin does not contain the smaller coins but can be exchanged for smaller coins of equal value, like on the diagram below. The four pieces of 50 centimes could come either from a two euro coin or from two coins of one euro. Likewise in our detectors, when we find four jets of particles, they can come from two independently produced Z, W or H bosons, or simply from four quarks produced directly. All this is called the background while the signal in this case would be a new boson that first decayed into two bosons.


A particle decay is like making small change for a large coin.

A coin only has one value but a particle carries both mass and energy. When one breaks a large coin, its total value is conserved. With particles, we must take into account the mass and the energy of all the decay products to calculate the combined mass of the original particle. One last detail: when the initial decaying particle is much heavier than the two bosons it produces, the jets coming from these bosons will hardly be separated. They will fly along side each other. In the end, we will not see four jets but rather two broader jets.

If the two broad jets come from two unrelated Z bosons, their total combined mass will be random, just as if we were to sum up the values of the small coins we carry in our pocket. If thousands of people told us the value of their small change, we would get a distribution like the one shown below by the blue line. Most people have only a little change, but some carry a small fortune in coins.

ATLAS-bumpThe horizontal axis gives the combined mass value of each event containing two broad jets found by the ATLAS Collaboration. The vertical axis shows (on a logarithmic scale) how many events were found with a particular value. The blue line shows what is expected from various backgrounds and the other colourful lines correspond to a few hypotheses. The black dots represent the real data and would look similar to the blue line if nothing new were there.

A small bump shows up around a mass value of 2 TeV, that is, more events are seen in data than what is predicted. The excess is 3.4 σ. Since there is always a spread in measured values due to the experimental errors, such a difference would occur at least once if we were to measure this quantity 1000 times. Hence, it is to early to say this could be the first sign of something new like a hypothetical boson denoted W’.


Intriguing events found by CMS with a mass around 5 TeV in the new (left) and old (right) data.

The CMS Collaboration also showed a few intriguing events. One is found in the newest data collected at 13 TeV after the restart of the LHC. The two jets combined mass is 5 TeV (left figure). The second event comes from the data collected earlier at 8 TeV and has a mass of 5.15 TeV. With 500 times less data at 13 TeV than 8 TeV, the experiments are already extending the analyses started with the 8 TeV data.

At this stage, it is way too early to tell. This is similar to looking in the distance on a foggy day, at dusk, trying to see if the train is coming. A faint shape is visible but is this real or just a mirage? No one knows, we must wait for the train to come closer. But not for long since the LHC is on track. Both experiments should soon have enough new data to be more definitive. And then, hold on to your hat, it’s going to get really exciting.

Pauline Gagnon

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Trop passionnant pour ne pas partager http://www.quantumdiaries.org/2015/07/27/trop-passionnant-pour-ne-pas-partager/ http://www.quantumdiaries.org/2015/07/27/trop-passionnant-pour-ne-pas-partager/#comments Mon, 27 Jul 2015 12:11:39 +0000 http://www.quantumdiaries.org/?p=36042 La plupart des physiciens et physiciennes sont d’accord: la physique est bien trop passionnante pour la réserver seulement aux scientifiques. Et pour la première fois, la Société Européenne de Physique (EPS) y a consacré une session entière samedi lors de sa conférence de physique des particules en cours à Vienne. Plusieurs y ont rapporté des initiatives variées visant à partager le meilleur de la physique des particules avec le grand public.

La plupart des activités décrites visaient des étudiants et étudiantes de tous âges, venant de pays développés ou en développement. Kate Shaw, chercheure au Centre International de Physique Théorique (ICTP) de Trieste en Italie, a souligné comment la science peut aider à résoudre divers problèmes d’environnement et de développement. Le monde a besoin de plus de scientifiques, a déclaré Kate. Investir dans l’éducation, ainsi que dans les institutions technologiques et culturelles jouent un rôle-clé dans le développement d’une économie basée sur la connaissance. La recherche fondamentale stimule les sciences appliquées par l’innovation, la technologie et l’ingénierie. Elle a aussi souligné l’importance d’inclure toutes les minorités et les jeunes issus de familles à faible revenu.

Kate a fondé le programme “Physique sans Frontières” au ICTP et organisé des “Masterclasses” (voir ci-dessous) et autres activités en Palestine, en Égypte, au Népal, au Liban, au Viêt-Nam et en Algérie. Non seulement elle inspire les jeunes à entreprendre des études en science, mais elle les assiste aussi, les aidant à accéder à des programmes de maîtrise et de doctorat. Kate a reçu aujourd’hui le Outreach Award de l’EPS « pour son travail de dissémination de la physique des particules dans des pays qui n’ont pas de programmes bien établis ».


Etudiantes participant à une Masterclasse en Palestine dans le cadre du programme “Physique sans Frontières”

Une Masterclasse consiste en une journée entière d’activités interactives conçues pour des élèves. Des scientifiques décrivent d’abord la physique des particules et l’expérience à laquelle ils ou elles participent. Un repas pris en commun facilite les échanges avant de se lancer dans de vraies analyses avec de vraies données. Chaque année, une masterclasse internationale réunit environ 10 000 élèves de 42 pays. Ils et elles rejoignent des scientifiques de 200 universités ou laboratoires voisins, pour effectuer de véritables mesures de physique en collaboration internationale avec les autres élèves. Pourquoi ne pas participer à une Masterclasse?

Ces élèves ainsi que d’autres groupes peuvent aussi prendre part à une visite virtuelle d’une expérience de physique. Un ou une scientifique sur place au laboratoire interagit avec le groupe, avant de leur faire visiter les installations à l’aide d’une connexion vidéo en direct.

Vous cherchez une activité inspirante qui soit simple, bon marché et accessible pour un événement spécial, une conférence ou un groupe? Invitez-les à une visite virtuelle au CERN (ATLAS ou CMS). Ainsi en janvier, 500 élèves de Mumbai ont profité de leur “visite” de l’expérience IceCube située à 12 000 km au pôle sud, pour bombarder les scientifiques avec leurs questions.

Le Teacher Programme du CERN a déjà accueilli un millier de personnes. Les enseignants et enseignantes du niveau secondaire venus de partout dans le monde s’en font mettre plein la vue pendant plusieurs semaines afin de s’assurer qu’ils partageront leur enthousiasme avec leurs élèves à leur retour.

Les présentations publiques et les livres de vulgarisation scientifique visent un public plus général. Beaucoup de scientifiques, moi y compris, se feront un plaisir de venir donner une conférence près de chez vous. Il suffit de demander.

Pauline Gagnon

Pour recevoir un avis lors de la parution de nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou par e-mail en ajoutant votre nom à cette liste de distribution ou consultez mon site web.


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Too exciting to leave it only to physicists http://www.quantumdiaries.org/2015/07/27/too-exciting-to-leave-it-only-to-physicists/ http://www.quantumdiaries.org/2015/07/27/too-exciting-to-leave-it-only-to-physicists/#comments Mon, 27 Jul 2015 12:04:18 +0000 http://www.quantumdiaries.org/?p=36038 Most physicists agree: physics is too interesting to leave it only to physicists. For the first time, the European Physics Society (EPS) dedicated a whole session to Outreach this year at its ongoing Particle Physics conference in Vienna. The participants reported on a wealth of creative initiatives undertaken by individuals or institutions to share the best of particle physics with the general public.

Most activities described aimed at students of all ages, in developed and developing countries. Kate Shaw, a researcher from the International Centre for Theoretical Physics (ICTP) in Trieste, Italy, stressed how science can help solve various environmental and developmental problems. The world needs more scientists, Kate stated, and investing in education, technology and cultural institutions plays a key-role in developing a knowledge-based economy. Fundamental research stimulates applied sciences through innovation, technology and engineering. She also mentioned the importance of reaching out to all minorities and low-income students everywhere.

Kate initiated the Program “Physics without Frontiers” at ICTP and conducted “Masterclasses” (see below) in the Palestinian Territories, Egypt, Lebanon, Nepal, Vietnam and Algeria. Not only does she inspire students to study in science, but she also mentors them to help them access Masters and PhD programs. Kate received today the EPS Outreach Prize “for bringing particle physics to countries with no strong tradition in particle physics”.


Students taking part in a Masterclass in Palestine sponsored by “Physics without Frontiers”

Masterclasses refer to a full-day of interactive activities designed for high-school and undergraduate students. Physicists first describe their fields and their experiment. Then the students can interact with them over lunch before launching into real analysis with real data. Every year, an international Masterclass brings together some 10000 students from 42 countries. They join scientists at 200 nearby universities or research centres, measuring meaningful quantities in collaboration with the other international students. You too could participate in a Masterclass.

Masterclasses participants and other groups are also often treated to a virtual visit of a top-notch experiment. A scientist located at the laboratory interacts with the group, then “walks” them through the facilities using a live video connection.

Are you looking for an inspiring activity that is simple, cheap and accessible to all for a special event, conference or group? Treat them to a virtual visit to CERN (ATLAS or CMS). In January, 500 students from Mumbai “visited” the IceCube. experiment at the South Pole 12,000 km away, flooding the scientists with questions.

The CERN’s Teacher Programme is also thriving, with one thousand participants so far. High-school teachers from all over the world are treated to unforgettable experiences to make sure they will share their enthusiasm and excitement with their students when they return home.

Public lectures and popular science books aim at more general audiences. Many scientists worldwide, including myself, will be happy to come give a public lecture in your area upon request. Just ask.

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline  or sign-up on this mailing list to receive an e-mail notification. You can also visit my website



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