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Detecting something with nothing

Tuesday, March 3rd, 2015

This article appeared in Fermilab Today on March 3, 2015.

From left: Jason Bono (Rice University), Dan Ambrose (University of Minnesota) and Richie Bonventre (Lawrence Berkeley National Laboratory) work on the Mu2e straw chamber tracker unit at Lab 3. Photo: Reidar Hahn

From left: Jason Bono (Rice University), Dan Ambrose (University of Minnesota) and Richie Bonventre (Lawrence Berkeley National Laboratory) work on the Mu2e straw chamber tracker unit at Lab 3. Photo: Reidar Hahn

Researchers are one step closer to finding new physics with the completion of a harp-shaped prototype detector element for the Mu2e experiment.

Mu2e will look for the conversion of a muon to only an electron (with no other particles emitted) — something predicted but never before seen. This experiment will help scientists better understand how these heavy cousins of the electron decay. A successful sighting would bring us nearer to a unifying theory of the four forces of nature.

The experiment will be 10,000 times as sensitive as other experiments looking for this conversion, and a crucial part is the detector that will track the whizzing electrons. Researchers want to find one whose sole signature is its energy of 105 MeV, indicating that it is the product of the elusive muon decay.

In order to measure the electron, scientists track the helical path it takes through the detector. But there’s a catch. Every interaction with detector material skews the path of the electron slightly, disturbing the measurement. The challenge for Mu2e designers is thus to make a detector with as little material as possible, says Mu2e scientist Vadim Rusu.

“You want to detect the electron with nothing — and this is as close to nothing as we can get,” he said.

So how to detect the invisible using as little as possible? That’s where the Mu2e tracker design comes in. Panels made of thin straws of metalized Mylar, each only 15 microns thick, will sit inside a cylindrical magnet. Rusu says that these are the thinnest straws that people have ever used in a particle physics experiment.

These straws, filled with a combination of argon and carbon dioxide gas and threaded with a thin wire, will wait in vacuum for the electrons. Circuit boards placed on both ends of the straws will gather the electrical signal produced when electrons hit the gas inside the straw. Scientists will measure the arrival times at each end of the wire to help accurately plot the electron’s overall trajectory.

“This is another tricky thing that very few have attempted in the past,” Rusu said.

The group working on the Mu2e tracker electronics have also created the tiny, low-power circuit boards that will sit at the end of each straw. With limited space to run cooling lines, necessary features that whisk away heat that would otherwise sit in the vacuum, the electronics needed to be as cool and small as possible.

“We actually spent a lot of time designing very low-power electronics,” Rusu said.

This first prototype, which researchers began putting together in October, gives scientists a chance to work out kinks, improve design and assembly procedures, and develop the necessary components.

One lesson already learned? Machining curved metal with elongated holes that can properly hold the straws is difficult and expensive. The solution? Using 3-D printing to make a high-tech, transparent plastic version instead.

Researchers also came up with a system to properly stretch the straws into place. While running a current through the straw, they use a magnet to pluck the straw — just like strumming a guitar string — and measure the vibration. This lets them set the proper tension that will keep the straw straight throughout the lifetime of the experiment.

Although the first prototype of the tracker is complete, scientists are already hard at work on a second version (using the 3D-printed plastic), which should be ready in June or July. The prototype will then be tested for leaks and to see if the electronics pick up and transmit signals properly.

A recent review of Mu2e went well, and Rusu expects work on the tracker construction to begin in 2016.

Lauren Biron


1. Some people think that physics is exciting.



2. They say “There’s nothing like the thrill of discovery”.

(ALICE Masterclass)

(ALICE Masterclass)

3. But that feeling won’t prepare you for the real world.



4. Discoveries only happen once. Do you really want to be in the room when they happen?



5. It’s not as though people queue overnight for the big discoveries.



6. CERN’s one of the biggest labs in the world. It’s like Disneyland, but for physicists.

7. The machines are among the most complex in the world.

(Francois Becler)

(Francois Becler)

8. Seriously, don’t mess with those machines.



9. They’re not even nice to look at.

(Michael Hoch, Maximilien Brice)

(Michael Hoch, Maximilien Brice)

10. The machines are so big you have to drive through the French countryside to get from one side to the other.

11. There’s nothing beautiful about the French countryside.

12. And there’s nothing cool about working on the world’s biggest computing grid with some of most powerful supercomputers ever created.



13. A dataset so big you can’t fit it all in one place? Please.



14. So you can do your analysis from anywhere in the world? Lame!

(CERN Courier)

(CERN Courier)

15. And our conferences always take place in strange places.

16. Who has time to travel?

17. Some people even take time away from the lab to go skiing.



18. Physicists have been working on this stuff for decades. Nobody remembers any of these people:



19. But particle physics is only about understanding the universe on the most fundamental level.

20. We don’t even have a well stocked library to help us when things get tough.

21. Or professors and experts to explain things to us.

22. And the public don’t care about what we do.



23. Even the press don’t pay any attention.

(Sean Treacy)

(Sean Treacy)

24. And who wants to contribute to the sum of human knowledge anyway?



25. There’s nothing exciting about being on shift in the Control Room either.



26. Or travelling the world to collaborate.

27. Or meeting hundreds of people, each with their own story and background.

28. You never get to meet any interesting people.



29. And physicists have no sense of humour.

30. Honestly, who would want to be a physicist?




  • http://www.atlas.ch/news/2008/first-beam-and-event.html
  • http://opendata.cern.ch/collection/ALICE-Learning-Resources
  • http://cds.cern.ch/record/1406060?ln=en
  • http://cds.cern.ch/record/1459634
  • http://cds.cern.ch/record/1459503?ln=en
  • http://cds.cern.ch/record/1474902/files/
  • https://cds.cern.ch/record/1643071/
  • http://cds.cern.ch/record/1436153?ln=en
  • http://home.web.cern.ch/about/computing
  • http://home.web.cern.ch/about/computing/grid-software-middleware-hardware
  • http://cerncourier.com/cws/article/cern/52744
  • http://lhcb.web.cern.ch/lhcb/fun/FunNewPage/album-crozet-jan2012/index.html
  • http://en.wikipedia.org/wiki/Solvay_Conference
  • http://home.web.cern.ch/about/updates/2014/05/cern-celebrates-its-anniversary-its-neighbours
  • https://atlas-service-enews.web.cern.ch/atlas-service-enews/2009/news_09/news_beam09.php
  • http://the-sieve.com/2012/07/06/higgsmania/
  • http://www.stfc.ac.uk/imagelibrary/displayImage.aspx?p=593
  • http://press.highenergyphysicsmedia.com/ichep-2012-cern-announcment.html
  • http://cds.cern.ch/record/1965972?ln=en
  • http://cds.cern.ch/record/1363014/

Twitter, Planck et les supernovae

Thursday, February 26th, 2015

Matthieu Roman est un jeune chercheur CNRS à Paris, tout à, fait novice sur la twittosphère. Il nous raconte comment il est en pourtant arrivé à twitter « en direct de son labo » pendant une semaine. Au programme : des échanges à bâton rompu à propos de l’expérience Planck, des supernovae ou l’énergie noire, avec un public passionné et assidu. Peut-être le début d’une vocation en médiation scientifique ?

Mais comment en suis-je arrivé là ? Tout a commencé pendant ma thèse de doctorat en cosmologie au Laboratoire Astroparticule et Cosmologie (APC, CNRS/Paris Diderot), sous la direction de Jacques Delabrouille, entre 2011et 2014. Cette thèse m’a amené à faire partie de la grande collaboration scientifique autour du satellite Planck, et en particulier de son instrument à hautes fréquences plus connu sous son acronyme anglais HFI. Je me suis intéressé au cours de ces trois années à l’étude pour la cosmologie des amas de galaxies détectés par Planck à l’aide de « l’effet Sunyaev-Zel’dovich » (interaction des photons du fond diffus cosmologique avec les électrons piégés au sein des amas de galaxies). En mars 2013, j’étais donc aux premières loges au moment de la livraison des données en température de Planck qui ont donné lieu à un emballement médiatique impressionnant. Les résultats démontraient la solidité du modèle cosmologique actuel composé de matière noire froide et d’énergie noire.

A-t-on découvert les ondes gravitationnelles primordiales ?
Puis quelques mois plus tard, les américains de l’expérience BICEP2, située au Pôle Sud, ont convoqué les médias du monde entier afin d’annoncer la découverte des ondes gravitationnelles primordiales grâce à leurs données polarisées. Ils venaient simplement nous apporter le Graal des cosmologistes ! Nouvelle excitation, experts en tous genres invités sur les plateaux télés, dans les journaux pour expliquer que l’on avait détecté ce qu’avait prédit Einstein un siècle plus tôt.

Mais dans la collaboration Planck, nombreux étaient les sceptiques. Nous n’avions pas encore les moyens de répondre à BICEP2 car les données polarisées n’étaient pas encore analysées, mais nous sentions qu’une partie importante du signal polarisé de la poussière galactique n’était pas pris en compte.

Les derniers résultats ont montré une carte de poussière galactique sur laquelle a été rajoutée la direction du champ magnétique galactique. Je la trouve particulièrement belle ! Crédits : ESA - collaboration Planck

Les derniers résultats ont montré une carte de poussière galactique sur laquelle a été rajoutée la direction du champ magnétique galactique. Je lui trouve un aspect particulièrement artistique ! Crédits : ESA- collaboration Planck

Et voilà : depuis quelques jours, c’est officiel ! Planck, dans une étude conjointe avec BICEP2 et Keck, fixe une limite supérieure sur la quantité d’ondes gravitationnelles primordiales, et par conséquent pas de détection. En somme, retour à la case départ, mais avec beaucoup d’informations supplémentaires. Les futures missions spatiales, ou expériences au sol ou en ballon visant à détecter avec une grande précision la polarisation du fond diffus à grande échelle, dont l’intérêt aurait pu être remis en question si BICEP2 avait eu raison, viennent de prendre à nouveau tout leur sens. Car il faudra bien aller les chercher, ces ondes gravitationnelles primordiales, avec un nombre de détecteurs embarqués de plus en plus grand afin d’augmenter la sensibilité, et la capacité de confirmer à coup sûr l’origine cosmologique de tout signal détecté !

De la poussière galactique aux explosions d’étoiles
Entre temps, j’ai eu l’opportunité de prolonger mon activité de recherche pendant trois années supplémentaires avec un post-doctorat au Laboratoire de physique nucléaire et des hautes énergies (CNRS, Université Pierre et Marie Curie et Université Paris Diderot) sur un sujet complètement nouveau à mes yeux : les supernovae, ces étoiles en fin de vie dont l’explosion est très lumineuse. On les étudie dans le but ultime de connaître précisément la nature de l’énergie noire, tenue responsable de l’expansion accélérée de l’Univers. Au temps de la preuve de l’existence de l’énergie noire obtenue à l’aide des supernovae (1999), on imaginait que leur courbe de lumière était assez peu variable. On a pris d’ailleurs l’habitude de les appeler « chandelles standard ».

Sur cette  image de la galaxie M101 on peut voir distinctement une supernova qui a explosé en 2011 : c'est le gros point blanc en haut à droite. Crédit T.A. Rector (University of Alaska Anchorage), H. Schweiker & S. Pakzad NOAO/AURA/NSF

Sur cette image de la galaxie M101 on peut voir distinctement une supernova qui a explosé en 2011 : c’est le gros point blanc en haut à droite. Celle-ci se situe dans l’un des bras spiraux, mais ne brillerait pas de la même façon si elle était au centre. Crédit T.A. Rector (University of Alaska Anchorage), H. Schweiker & S. Pakzad NOAO/AURA/NSF

Avec l’affinement des méthodes de détection, on se rend compte que les supernovae ne sont pas vraiment les chandelles standard que l’on croit, ce qui relance complètement l’intérêt du domaine. En particulier, le type de galaxie dans laquelle explose une supernova peut créer des variations de luminosité, et ainsi affecter la mesure du paramètre décrivant la nature de l’énergie noire. C’est le projet dans lequel je me suis lancé au sein de la (petite) collaboration du Supernova Legacy Survey (SNLS). En espérant un jour pouvoir étudier ces objets sous la forme d’autres projets scientifiques, avec des détecteurs encore plus puissants comme Subaru ou LSST.

Twitter en direct de mon labo…
En fait c’est une amie, Agnès, qui m’a fait découvrir Twitter et m’a encouragé à raconter mon travail au jour le jour et pendant une semaine via le compte @EnDirectDuLabo. Il s’agissait d’un monde nouveau pour moi, qui n’était pas du tout actif sur ce que l’on appelle « la twittosphère ». C’est malheureusement le cas pour de nombreux chercheurs en France. Expérience très enrichissante s’il en est, puisqu’elle semble susciter l’intérêt de nombreux twittos, et a permis de porter le nombre d’abonnés à plus de 2000. Cela m’a permis par exemple d’expliquer les bases de l’électromagnétisme nécessaires en astronomie, des détails plus techniques sur les performances de l’expérience dans laquelle je travaille ou encore ma vie au quotidien dans mon laboratoire.

Ce fut très amusant de livrer mon travail quotidien au grand public, mais aussi très chronophage ! J’ai toujours été convaincu par l’importance de la médiation scientifique, sans jamais oser me lancer. Il était peut-être temps…

Matthieu Roman est actuellement post-doctorant au Laboratoire de physique nucléaire et de hautes énergies (CNRS, Université Pierre et Marie Curie et Université Paris Diderot)


Ten unusual detector materials

Tuesday, February 17th, 2015

This article appeared in symmetry on Feb. 17, 2015.

The past century has generated some creative ideas for tracking particles. Image: Sandbox Studio

The past century has generated some creative ideas for tracking particles. Image: Sandbox Studio

Hans had been waiting in the darkened room for 45 minutes. It was a dull part of his day, but acclimating his eyes was a necessary part of his experiment—counting faint sparkles of light caused by alpha particles deflecting off a thin metal foil.

The experiment was part of a series organized by Ernest Rutherford in 1908, and it led to the discovery of the atomic nucleus. Rutherford’s assistant, physicist Hans Geiger, would share credit for the discovery.

Their experiment was particle physics in its infancy.

Studying particle physics requires revealing the smallest bits of matter. This work might involve hurling billions of accelerated particles at a target and watching for the flash of energy that results from the crash. It might involve setting up a detector to wait for particles created in nature to pass through.

Over the years, electronics and mainframe computers have taken over Rutherford and Geiger’s painstaking particle-counting duties. And physicists have used a host of materials other than foil to lure those particles—including hard-to-catch neutrinos—into view.

1. Dry cleaning fluid.

Physicist Ray Davis had either a terrific idea for a particle detector or a tremendous load of laundry. In a few years leading up to 1966, he obtained 600 tons of a common dry cleaning solvent, perchloroethylene, and deposited it nearly a mile beneath the Black Hills of South Dakota in a detector stationed in the Homestake gold mine. He hoped to count solar neutrinos, which trigger a detectable chemical reaction when they pass through this fluid. Davis’ perchloroethylene-filled particle detector was a success, even though he tallied only a third of the neutrinos he was expecting. Revelations that neutrinos change form as they travel were soon to follow.

2. Soviet-era artillery shells.

In the 1940s, the Russian navy armed its vessels with a grade of brass specifically designed to hold its shape under extreme stress and for long periods of time. More than 50 years later, the CMS particle detector under construction at the Large Hadron Collider at CERN required brass with the same high standards. It needed to be able to withstand a bombardment of particles with unflinching consistency over its lifetime. The lab struck a deal with Russian officials to melt down old, unused shells for the CMS hadron calorimeter, a part of the detector that measures the energy of particles produced in collisions in the LHC.

3. 2.5 million gallons of mineral oil.

Fermilab’s 14,000-ton NOvA neutrino detector in northern Minnesota, possibly the largest freestanding plastic structure in the world, is filled with a liquid substance that is 95 percent mineral oil. That single raw material took up 108 rail cars and a barge as it left a refinery in southwest Louisiana for a facility 1000 miles away near Chicago, where it was blended with the remaining ingredients 110,000 gallons at time. The result was a liquid scintillator, which releases measurable light as a result of collisions between neutrinos and particles in the liquid.

4. Lead bricks wrapped in foil by robots.

The OPERA detector at Gran Sasso National Laboratory catches neutrinos with something a bit more, as they say in Italy, duro—a wall of 150,000 18-pound bricks. The bricks themselves are stacks of lead sheets and radiation-sensitive film, wrapped in reflective aluminum tape and sealed in an airtight container. When neutrinos collide with the lead, they create other particles that streak across the film and leave tracks that can be analyzed after the film is developed. The 11 robots of Gran Sasso’s brick-assembly machine, otherwise known as BAM, cranked out 750 bricks per day, faster and with much less complaining than an army of graduate students.

5. Smartphones. Yep, there’s an app for that.

Actually, there are at least two. A physicist at the University of Wisconsin, Madison, and a director of citizen science at the LA Makerspace are working on one called DECO, an educational app that records speedy cosmic-ray particles that your phone’s camera accidentally detects. Two more physicists, one from University of California, Irvine, and the other from University of California, Davis, are at work on a similar app called CRAYFIS. Their objective: gather enough users to create a functional cosmic ray detector from a massive network of devices.

6. A crystal ball.

No, SLAC National Accelerator Laboratory did not enlist a psychic medium to locate subatomic particles when they built the Crystal Ball detector in the late ’70s. They did, however, arrange more than 600 sodium iodide crystals into a sphere 13 ½ feet around to detect neutral particles at the SPEAR particle collider. The crystals work in similar fashion to the liquid inside the NOvA detector (see No. 2 in this list), converting energy from particle collisions to measurable light. The detector is still in use, currently at Johannes Gutenberg University in Mainz, Germany. Its future, ironically, is uncertain.

7. Antarctica, from below.

When penguins look down, there’s a chance they might discover one of 86 holes drilled more than a mile deep into the Antarctic ice for the IceCube experiment. When turbocharged cosmic neutrinos collide with ice, the resulting particle shrapnel creates a blue flash of light otherwise known as Cherenkov light. Scientists survey the ice sheet for that light with an array of more than 5000 separate, bauble-like detectors strung on wires running down each hole.

8. Antarctica, from above.

Should penguins look up instead, they may spot the Antarctic Impulsive Transient Antenna, or ANITA, floating above them, suspended from a massive scientific balloon. ANITA listens for radio waves emanating from the ice below. The pure, polar ice makes an unbelievably clear medium for the Askaryan effect, discovered only in 2000, in which cosmic neutrinos similar to the ones that produce light for the IceCube experiment generate a signature radio signal. The floating antenna is so sensitive that it can detect a handheld radio up to 400 miles away.

9. A breath of fresh Martian air.

Our descendants may well enjoy a beautiful sunset on Mars—if we can engineer its atmosphere to warm the planet from its current average temperature of about minus 60 degrees Celsius to something more friendly to vacationing humans. For such a project, some researchers have singled out the compound octofluoropropane as the greenhouse gas of choice. In the meantime, researchers on the PICO experiment at underground Canadian laboratory SNOLAB are using octofluoropropane in its liquid state to detect dark matter. If a particle of dark matter can knock one fluorine nucleus hard enough, it will cause the superheated liquid to boil and form a telltale bubble in the chamber.

10. Dry ice, alcohol and a fish tank.

This one you can build yourself. The cloud chamber earned its inventor the 1927 Nobel Prize in physics, and variations of it—including No. 9 on this list— have a long history of use in particle physics labs. But many DIY varieties exist online, too. The gist is usually to create a thick vapor (of alcohol) that is cooled (by dry ice). Be patient, and you’ll catch a passing particle such as a cosmic muon as it bumps into vapor molecules and triggers a cloudy streak of condensation through the chamber (a.k.a. fish tank).


Troy Rummler


This article appeared in Fermilab Today on Feb. 11, 2015.

Fermilab is developing superconducting accelerating cavities similar to this one for SLAC's Linac Coherent Light Source II. Photo: Reidar Hahn

Fermilab is developing superconducting accelerating cavities similar to this one for SLAC’s Linac Coherent Light Source II. Photo: Reidar Hahn

Now one year into its five-year construction plan, the Linac Coherent Light Source II, an electron accelerator project at SLAC, will produce a high-power free-electron laser for cutting-edge scientific explorations ranging from refined observations of molecules and cellular interactions to innovative materials engineering. Cornell University as well as Argonne National Laboratory, Lawrence Berkeley National Laboratory, Fermilab and Thomas Jefferson National Accelerator Facility are partners in the SLAC-directed project.

“We at the laboratories are all developing close ties,” said Richard Stanek, Fermilab LCLS-II team leader. “The DOE science lab complex will be stronger for this collaboration.”

In 2015, Fermilab will intensify its LCLS-II contribution in the overlapping areas of superconducting radio-frequency (SRF) accelerator technology and cryogenics, critical components that distinguish LCLS-II from SLAC’s current LCLS facility, whose laser production has enabled noted scientific investigations in cancer treatment and other important areas.

SLAC physicist Marc Ross, LCLS-II cryogenics systems manager, said LCLS cannot keep up with scientists’ requests for use. The existing LCLS facility and LCLS-II combined will offer researchers laser X-rays with a wide range of properties.

“This new approach will transform the repetition rate of LCLS — from 120 pulses per second to up to 1 million per second,” Ross said. “This will allow a completely new class of experiments and, eventually, a much larger number of experimental stations operated in parallel.”

Fermilab Technical Division physicists Hasan Padamsee, division head, and Anna Grassellino and their team are working on SRF technology for LCLS-II, in particular on implementing Fermilab’s two recent findings to reduce the needed cryogenic power. In one innovation, known as nitrogen doping, Grassellino found that infusing a small amount of nitrogen gas when preparing the superconducting cavities — the structures through which beam is accelerated — reduces two main causes of the usually expected resistance to radio-frequency currents.

“It is exciting to see our discovery becoming an enabling technology for LCLS-II,” Grassellino said.

Grassellino’s high-Q team has also found that the cavities’ cooling dynamics significantly helps expel magnetic flux, another major source of cavity power dissipation. The Fermilab high-Q team, together with Cornell University and Jefferson Lab, are currently working on calibrating the cooling thermogradient for LCLS-II.

Stanek said Fermilab is advancing its SRF work with its LCLS-II participation.

“I see this project taking us from an R&D phase of SRF technology, which is where we have been the past six to eight years, and moving our expertise into production,” Stanek said. “This is a big step forward.”

Fermilab and Jefferson Lab are working closely together on the cooling systems that enable the cavities’ superconductivity. Fermilab scientist Camille Ginsburg leads LCLS-II cryomodule production at Fermilab, and Fermilab engineer Arkadiy Klebaner manages the LCLS-II cryomodules distribution system.

“To build a high-energy beam using SRF technology, LCLS-II needed expertise in cryogenics,” Klebaner said. “So Jefferson Lab and Fermilab, who both have special expertise in this, were ready to help out.”

A cryogenic plant generating the refrigeration, a cryogenic distribution system for transporting the refrigeration into cryomodules and the cryomodules themselves make up the LCLS-II cryogenics. Jefferson Lab will provide the cryogenic plant, and Fermilab is in charge of developing the cryogenic distribution system. Jefferson Lab and Fermilab are jointly developing LCLS-II’s 35 cryomodules, each one about 10 meters long.

Fermilab’s contribution draws on the Tevatron’s cryogenics and on SRF research begun for the proposed International Linear Collider. The lab’s LCLS-II experience will also help with developing its planned PIP-II accelerator.

“So when we build the next accelerator for Fermilab, PIP-II, then we will have already gotten a lap around the production race course,” Padamsee said.

All labs have something special to contribute to LCLS-II, Ross said.

“The Fermilab team have figured out a way to make this kind of accelerator much better operating in the cold temperature that superconducting technology requires,” Ross said. “It is worthy of special recognition.”

Rich Blaustein

Well, that was fun!

At 8 PM ET on February 5, 2015, Quantum Diaries ran a post that was tied to “The Troll Manifestation”, an episode of “The Big Bang Theory” (TBBT) that was being aired at exactly that time.  This was generated in partnership with the show’s writers, staff and advisers. What happens when you couple a niche-interest website to one of the most popular TV shows in the United States? The QD bloggers and support staff had a great time getting ready for this synergistic event and tracking what happened next.  Here’s the story behind the story.

I’ve mentioned previously, in my largely unheralded essay about the coffee culture at CERN, that I have known David Saltzberg, UCLA faculty member and science adviser to TBBT, for a very long time, since we were both students in the CDF group at The University of Chicago.  On January 14, David contacted me (and fellow QD blogger Michael DuVernois) to say that Quantum Diaries was going to be mentioned in an episode of the show that was going to be taped in the coming week.  David wanted to know if I could sign a release form allowing them to use the name of the blog.

I couldn’t — the blog is not mine, but is operated by the InterAction Collaboration, which is an effort of the communications organizations of the world’s particle physics laboratories.  (They signed the release form.)  But I did come up with an idea.  David had said that the show would refer to a Quantum Diaries blog post about a paper that Leonard and Sheldon had written.  Why not actually write such a post and put it up on the site?  A real blog post about a fake paper by fake scientists.  David was intrigued; he discussed it with the TBBT producers, and they liked the idea too.  The show was to air on February 5.  Game on!

David shared the shooting script with me, and explained that this was one of the rare TBBT episodes in which he didn’t just add in some science, but also had an impact on the plot.  He had described his own experience of talking about something with a theorist colleague, and getting the response, “That’s an interesting idea — we should write a paper about it together!”  I myself wouldn’t know where to get started in that situation.  This gave me the idea for how to write about the episode.  The script had enough information about Leonard and Sheldon’s paper for me to say something intelligible about it.  The fun for me in writing the post was in figuring out how to point to the show without giving it all away too quickly.  I ran my text by David, who passed it on to the show’s producers, and everyone enjoyed it.  We knew there was some possibility that the show’s social media team would promote the QD site through their channels; their Facebook page has 33 million likes and their Twitter account has 3.1 million followers.

Meanwhile, the Quantum Diaries team sprung into action.  Kelen Tuttle, the QD webmaster, told the other bloggers for the site about our opportunity to gain national recognition for the blog, and encouraged everyone to generate some exciting new content.  Regular QD readers might have noticed all the bloggers becoming very voluble in the past week!  Kevin Munday and his team at Xeno Media prepared the site for the possible onslaught of visitors  — remember, twenty million people watch TBBT each week! — by migrating the site to the CloudFlare content delivery network, with 30 data centers worldwide, and protecting the site against possible security issues.

We all crossed our fingers for Thursday night.  I spent Thursday at Fermilab, and was flying back to Lincoln in the afternoon, scheduled to land at 6:43 PM, a few minutes before the 7 PM air time.  When I got home, I started keeping an eye on the computer.  The blog post was up, but was TBBT going to say anything about it?  Alas, their Twitter feed was quiet during the show.  (No, I didn’t watch — I have to admit that we watch so little television that we couldn’t figure out which channel it might be on!)

All of us involved were a bit disappointed that evening.  But David took up the case again with the CBS interactive team the next day, and was told that they’d put out a tweet as long as we changed our blog post to link to the archive of the show.  We did that, and then at 12:45 Central Time, we got the shout-out that we were hoping for:

So what happens when a TV audience of around 20 million people hear a website (which may or may not be real) mentioned in a show?  Or when 3.1 million people who are fans of a TV show receive a tweet pointing to a blog post?  The Quantum Diaries traffic metrics tell the tale. Here is a plot of the number of visitors to the site during the past four weeks, including the February 5 air date and the February 6 tweet date:
When the blog was mentioned on the air, there was a definite spike in activity, and an even bigger spike on the day after, when the tweet went out. Traffic on the site was up by a factor of four thanks to TBBT!

However, the plot doesn’t show the absolute scale. On February 6, the site had about 4600 visitors, compared to a typical level of 800-1000 visitors. This means that only 0.1% of people who saw the TBBT tweet actually went and clicked on the link that took them to QD. This is nowhere near the level of activity we saw when the Higgs boson was discovered. TBBT may be a great TV show, but it’s no fundamental scientific discovery.

However, the story did have some pretty strong legs in Nebraska.  My employer, the University of Nebraska-Lincoln, graciously wrote a story about my involvement in the show and promoted it pretty heavily through social media.  This led to a couple of appearances on some news programs that enjoy making local links to national stories (if you could call this a national story).  I found it a bit surreal and was reminded that I need to get a haircut and clean my desk.

Thank you to David Salzberg for making this possible, and to the TBBT producers and writers who were supportive, and of course to all of my colleagues at Quantum Diaries who did a lot of writing and technical preparation last week.  (A special shout-out to Kelen Tuttle, who left QD for a new position at Invitae this week; at least we sent her off with a Big Bang!) And if you have discovered this blog because of “The Troll Manifestation”, I hope you stay for a while!  These are great times for particle physics — the Large Hadron Collider starts up again this year, we’re planning an exciting international program of neutrino physics that will be hosted in the United States, and we’re scanning the skies for the secrets of cosmology.  We particle physicists are excited about what we do and want to share some of our passion with you.  And besides, now we know that Stephen Hawking reads Quantum Diaries — shouldn’t you read it too?


Sonic Copper Cleaning

Saturday, February 7th, 2015

IMG_7979Today we cleaned parts to go into the detector using a sci-fi piece of machinery called a “sonic bath”.

On CUORE, we’re looking for a faint signal of radioactivity. That means we can’t let anything swamp that signal: we have to clean away the normal low-level of dirt present in the atmosphere and biological systems. Even something as normal as a banana has so much naturally-occurring radiation that the “banana-year” is a (someone irreverent and imprecise) unit of measurement for backgrounds of dark matter experiments.

The parts we’re cleaning will be guide tubes for a calibration system. Through them, we’ll place wires close to the detector, then remove them again when it’s time for the main data taking. The calibration wires have a measured amount of radioactivity, and we use that known signal to calibrate the other signals within CUORE.

We used a sonic bath to clean the parts: they’re in a bag with soap water, inside a larger tub filled with tap water. To agitate everything (like the dasher in a clothes washer) the machine uses sound. It’s a bit like the little machines that some people use to clean their contact lenses, but larger: about the size of a laundry room sink, or a restaurant kitchen sink.

IMG_8002My favorite part of the process was the warning on the side: running with an empty bath could cause burnout of the ultrasonic coupler. “The ultrasonic coupler” sound like something out of science fiction: like a combination of “sonic screwdriver” and “flux capacitor”. But it’s not fiction– this is just what we need to do for our daily work!

The noise it makes sounds a bit like an electric fly zapper: a low level electric buzz and cackle, with a faint hiss hinting that there’s something higher pitched above that. It’s practically impossible to hear the main frequency because it’s pitched so much higher than human hearing: the noise is at 30-40kHz, and a child can usually hear as high as 20kHz. Some of the lower resonances fall into an audible range, which is what makes it sound like there’s more going on than I can hear.

In the smaller machine (about the size of a bathroom sink), the agitation noise was more audible, almost headache-inducing in long doses. Since I just watched the fourth Harry Potter movie, it reminded me of the recorded mermaid message: you can only hear it when you’re underwater. If you’re in air, it sounds like a screech instead of a message. Knowing the line between science fiction and fact, I didn’t actually stick my ear in the water (and we wore earplugs in the lab).

IMG_7992There’s a funny effect with some of the bubbles in the tub. They get caught in vibrational nodes within the water, so even though they’re clearly made of air, they don’t rise to the top. It’s like an atom trap made of lasers holding a single atom in place, except this works at a macroscopic level so it’s more intuitive. Seeing the modes in action is a little reward for having worked through all those Jackson problem sets where we deconvolved arbitrary functions in various ways.

When the parts come out at the end, and after we repeat the process with some citric acid (like what you find in lemon juice) and then rinse everything, the rods are a completely different color. They’ve gone from a dead-leaf brown to a peachy pink, all shiny and bright and hopeful. It’s a clean start for a new detector. We preserved the clean exteriors by sealing them in vacuum bags,  and told the chem lab supervisor we were done for the day.


Regular readers of Quantum Diaries will know that in the world of particle physics, there is a clear divide between the theorists and the experimentalists. While we are all interested in the same big questions — what is the fundamental nature of our world, what is everything made of and how does it interact, how did the universe come to be and how might it end — we have very different approaches and tools. The theorists develop new models of elementary particle interactions, and apply formidable mathematical machinery to develop predictions that experimenters can test. The experimenters develop novel instruments, deploy them on grand scales, and organize large teams of researchers to collect data from particle accelerators and the skies, and then turn those data into measurements that test the theorists’ models. Our work is intertwined, but ultimately lives in different spheres. I admire what theorists do, but I also know that I am much happier being an experimentalist!

But sometimes scientists from the two sides of particle physics come together, and the results can be intriguing. For instance, I recently came across a new paper by two up-and-coming physicists at Caltech. One, S. Cooper, has been a noted prodigy in theoretical pursuits such as string theory. The other, L. Hofstadter, is an experimental particle physicist who has been developing a detector that uses superfluid liquid helium as an active element. Superfluids have many remarkable properties, such as friction-free flow, that can make them very challenging to work with in particle detectors.

Hofstadter’s experience in working with a superfluid in the lab gave him new ideas about how it could be used as a physical model for space-time. There have already been a number of papers that posit a theory of the vacuum as having properties similar to that of a superfluid. But the new paper by Cooper and Hofstadter take this theory in a different direction, positing that the universe actually lives on the surface of such a superfluid, and that the negative energy density that we observe in the universe could be explained by the surface tension. The authors have difficulty generating any other testable hypotheses from this new theory, but it is inspiring to see how scientists from the two sides of physics can come together to generate promising new ideas.

If you want to learn more about this paper, watch “The Big Bang Theory” tonight, February 5, 2015, on CBS. And Leonard and Sheldon, if you are reading this post — don’t look at the comments. It will only be trouble.

In case you missed the episode, you can watch it here.

Like what you see here? Read more Quantum Diaries on our homepage, subscribe to our RSS feed, follow us on Twitter, or befriend us on Facebook!


Physics + wine = plasma + fun

Wednesday, February 4th, 2015

Ever fancied making your own particle accelerator? Fermilab posted a great blog entry last month (here) showing how anyone can make a particle detector for viewing cosmic rays. In this post, I will explain how particle accelerators can also be hacked so that you can make your very own cathode ray tube (CRT).

I came across this experiment when attending an accelerator school at the Australian Synchrotron last year. To read more about my adventures down under please see Accelerating Down Under and If you can’t stand the heat, get into the Synchrotron!.

What is a cathode ray tube?

Good question. It consists of a vacuum chamber containing some electrodes between which a high voltage is applied. Electrons are accelerated from the negatively charged cathode to the positively charged anode. But some electrons fly past the anode to hit a glass wall. CRTs were utilised in old television sets to form images on a fluorescent screen.


You will need:

  • - a clear wine bottle
  • - a vacuum pump
  • - a rubber hose
  • - epoxy resin
  • - mini chrome-plated metal doorknob
  • - a piece of steel brake line
  • - a piece of steel wire several centimetres long
Empty wine bottles at the Australian Synchrotron.

Experimental preparation at the Australian Synchrotron: GRAPE 1, 2 & 3. Image credit: Ralph Steinhagen.


A detailed method for this experiment may be found (here) but I summarise the main steps below:

  1. Drink a bottle of wine. Wash out the wine bottle with warm soapy water and remove all labelling from the exterior.
  2. Drill a hole about 1/2 way down the wine bottle which is big enough to fit the metal wire through. This will act as the mount for the anode. If your bottle cracks, throw it away and return to step 1.
  3. Drill a hole through the metal doorknob. Use epoxy to attach the break line to the doorknob’s screw mount. This will act both as the cathode and vacuum port. Apply epoxy to the rim of the mouth of the wine bottle and attach the cathode to form an airtight seal.
  4. Bend the steel wire into a C-shape and thread it through the hole you drilled in the wall of the wine bottle. This is your anode. Orient it so that all points on it are equidistant from your cathode. Secure it with epoxy and ensure it is airtight.
  5. Attach the rubber hose to your anode and the other end to the vacuum pump. Attach the anode and cathode to a high voltage power supply. Turn on the power supply and vacuum pump and enjoy!



The GRAPE 2 experiment: a vacuum pump is connected to the experiment via the rubber tube to the right of the bottle. The anode and cathode, which are connected to a high voltage supply, are seen to glow. Image credit: Ralph Steinhagen.


A word of warning: using high voltages, creating vacuums and drilling holes in glass bottles are all inherently dangerous activities. If you attempt this experiment please observe all safety advice. In particular, wear protective clothing and safety glasses, don’t use cracked bottles for the experiment – you risk implosion – and apply the voltage for a maximum of 30/40 seconds.

And please leave adequate time between consuming the wine and carrying out the experiment to sober up.


The video below shows what happened when the switch was flicked on the GRAPE 2 experiment at the Australian Synchrotron:


Initially there is a clear purple electric discharge between the anode and cathode. This discharge excites the atoms in the gas in the bottle causing a burst of liberated free electrons. The electrons are travelling much faster than the positive ions they leave behind and so diffuse to the cathode and bottle walls. Thus a plasma (or ionised gas) is created.

The plasma stabilises as more ionisation occurs, then begins to glow as electrons and ions recombine and emit photons. This process of ionisation and recombination is continuous. The instabilities or fluctuations observed indicate that different proportions of the remaining gas are being excited as the experiment proceeds. Can you think of why this happens? If so, please comment below.

When a magnet is placed near the bottle the plasma is visibly distorted. This phenomenon is known as magnetic deflection and is described by the Lorentz force law. The plasma’s charged particles experience a force when they travel through the magnetic field which is perpendicular both to the path they follow and to the applied magnetic field, that is the magnet causes the particles to follow a curved path. This effect is used in circular particle accelerators, such as the Large Hadron Collider, where strong dipole magnets are used to steer the particles around the machine.

A cross section of the LHC showing the dipole magnets which are used to bend the path followed by protons.

A cross section of the LHC showing the dipole magnets which are used to bend the path followed by protons. The magnets may be seen flanking the left-hand beam pipe. Image credit: James Doherty

What are you waiting for?

Particle physics is not a game that only elite scientists at well-funded institutions can play. With a little effort, determination and ingenuity, it is possible to make your own particle accelerator or detector. So what are you waiting for? Give it a go and let us know how you get on in the chat box below. Good luck!

The GRAPE 2 experiment was carried out by Kaitlin Cook, Paul Bennetto and Tom Lucas under the supervision of Ralph Steinhagen at the 2014 Australian Synchrotron Accelerator School. The above photos and video are courtesy of Ralph Steinhagen.


The dilution refrigerator is the coldest cooling stage for the CUORE detector. It keeps the crystals cold enough that the heat can be detected from a single radioactive decay. The purpose of CUORE is to study the energy spectrum of these decays, so it’s vital that the surroundings be cold. Here’s how it works.

The CUORE cryostat dilution unit

The CUORE cryostat dilution unit

What CUORE Does

CUORE is looking for a kind of radioactive decay that’s extremely rare if indeed it happens at all. It’s never been observed before. It’s called “neutrinoless double beta decay:” a decay emitting two electrons but no neutrinos. Lots of radioactive elements undergo beta decay and emit electrons. Some emit two at once, in a double interaction. That’s accompanied by two neutrinos. The special case that CUORE investigates is the theoretical possibility that the neutrinos annihilate each other before the interaction is completed, so no neutrinos come out.

This can happen only if neutrinos are their own antiparticles, which is an amazingly interesting possibility. Whether or not neutrinos are their own antiparticles is one of the great open questions in neutrino physics today. In the process of investigating this, we also will learn about absolute neutrino masses, two neutrino double beta decay, and a whole host of experimental techniques.

How Cold Helps

Making the detector profoundly cold makes this search possible. Heat is the main signal, so any extra heat floating around is background noise. But additionally, the signal itself gets stronger at lower temperatures. The heat capacity is a strong function of temperature: the colder you go, the less heat it takes to create a change in temperature. So by making the detector colder, the amount of heat deposited by a single decay creates a larger change in temperature, making it more distinguishable from background noise.

The temperatures we’re considering here are measured in millikelvin. For perspective, let’s look at some other cold things. At the South Pole (where my grad school experiment IceCube is) the outdoor temperature ranges between 0 and -100 Fahrenheit, or 255 to 200 Kelvin (K). Liquid argon boils at 87K. Liquid nitrogen boils off at 77K. The cosmic microwave background is at 2.7K. The temperature we’re hoping to use is 10mK or less, which makes CUORE the coldest cubic meter in the universe. The advancement CUORE represents isn’t simply the temperature but also the volume. It might not be the coldest place in the universe, but it’s the coldest place as big as a cubic meter.

A Mixture of Helium Isotopes

There’s a trick we use to cool down our helium so much. Helium comes in two isotopes: 3He and 4He. When they get cold enough, both of them become superfluids, but at different temperatures: 2.2K for 4He and 1mK for 3He. The refrigerator operates between these two, so the 4He is superfluid but the 3He is only a regular liquid. By simple 3He evaporation, we can get it down to about 300mK. The key to our trick is that when you mix the two isotopes together, the mixture can become even colder than either would be individually. It splits into two phases, and by making one phase change into the other, we can pump out more heat.

When you cool a mixture of 3He and 4He to very nearly zero (below 867mK), the mixture separates into two different phases. One phase contains more 3He, so we call it the concentrated phase. The other contains less 3He, so we call it the dilute phase. We have a tube going down into the dilute phase and pumping away 3He, shifting the balance of the concentrations in the two phases. As we pump away 3He from the dilute phase, more 3He changes phase from the concentrated phase to take its place and maintain an equilibrium. As each atom changes phase, it absorbs heat because of the lower enthalpy in the dilute phase. The faster we pump out 3He, and the more 3He changes phase, the more cooling power the system has. The power of the cooling engine in limited by the interface area between the two phases, so large area makes more power.

The process of cooling something by pumping away from the dilute phase shows up in another, more familiar context: it’s the same process as when we cool tea by blowing away the steam. More tea can evaporate, cooling what remains.

Throughout this process, we follow the 3He for “dilute” and “concentrated” naming conventions for a couple of reasons. First, the stuff that’s circulating through the system is nearly all 3He and only a little 4He. The 4He stays in its superfluid state within the mixing chamber while the 3He is pumped through condensing lines. There can be a tiny tiny bit of 4He that creeps along the tubes up the pumps (yes, “creep” is the technical term to use here, literally), where it can evaporate and be pumped along with the 3He, but that’s usually less than 1%. The second reason we follow the 3He is that it’s extremely expensive and rare, but that’s a different topic, and it involves international politics in the nuclear age.

[1] Image from F. Pobell: Matter and Methods at Low Temperature, 2nd ed., Springer-Verlag, New York (1995), via G. Ventura and L. Risegari: The Art of Cryogenics Low-Temperature Experimental Techniques, Elsevier, Oxford (2008).