## Hot Topics

### The PhD defense

The defense: perhaps the most stressful, intimidating right of passage in the PhD process. It’s the culmination of years of work, all wrapped up in a single meeting. This week, Quantum Diarist Laura Gladstone gives us an overview of what it was like to finish her PhD. It’s something that quite a few of our diarists have survived!

### Graduating, part 1: the defense

By Laura Gladstone | November 25, 2014
It’s been a crazy 3 weeks since I officially finished my PhD. I’m in the transition from being a grad student slowly approaching insanity to a postdoc who has everything figured out, and it’s a rocky transition.

### PhD candidate, at last

By Flip Tanedo | May 6, 2010
As of last Tuesday I am officially a PhD candidate… which doesn’t really change much, but it means that my department “officially” acknowledges that I’m on track for a doctorate degree some time in the future.

### From PhD to postdoc

By Emily Thompson | June 26, 2013
As a new US LHC blogger, I thought I would spend this first post talking a little about my background and how I decided what kind of postdoc position to look for. Lately, quite a few of my friends in their last year of graduate school have been asking about the latter.

## Latest Posts

### Graduating, part 2: Final Thesis Revisions

Wednesday, November 26th, 2014

The doorway to the registrar’s office where the final thesis check takes place

I took an entire month between defending my thesis and depositing it with the grad school. During that month, I mostly revised my thesis, but also I took care of a bunch of logistical things I had been putting off until after the defense: subletting the apartment, selling the car, engaging movers, starting to pack… and of course putting comments into the thesis from the committee. I wrote back to my (now current) new boss who said we should chat again after I “come up for air” (which is a pretty accurate way of describing it). I went grocery shopping, and for the first time in months it was fun to walk around the store imagining and planning the things I could make in my kitchen. I had spare creative energy again!

Partly I needed a full month to revise the thesis because I was making changes to the analysis within the thesis right up to the day before I defended, and I changed the wording on the concluding sentences literally 20 minutes before I presented. I didn’t have time to polish the writing because the analysis was changing so much. The professor who gave me the most detailed comments was justifiably annoyed that he didn’t have sufficient time to read the whole dissertation before the defense. It worked out in the end, because the time he needed to finish reading was a time when I didn’t want to think about my thesis in any way. I even left town and visited friends in Chicago, just to break up the routine that had become so stressful. There’s nothing quite as nice as waking up to a cooked breakfast when you’ve forgotten that cooked breakfasts are an option.

There were still thesis revisions to implement. Some major comments reflected the fact that, while some chapters had been edited within a peer group, no one had read it cover-to-cover until after the defense. The professor who had the most detailed comments wrote a 12-page email detailing his suggestions, many of which were word substitutions and thus easy to implement. Apparently I have some tics in my formal writing style.

I use slightly too many (~1.2) semicolons per page of text; this reflects my inclination to use compound sentences but also avoid parentheses in formal writing. As my high school teacher, Perryman, taught me: if you have to use parentheses you’re not being confidently declarative, and if you ever want to use nested parentheses in a formal setting, figure out what you really want to say and just say it! (subtext: or figure out why you don’t want to say it, and don’t say it. No amount of parenthesis can make a statement disappear.) Anyway, I’d rather have too many semicolons than too many parentheses; I’d rather be seen as too formal than too tentative. It’s the same argument, to me, that I’d rather wear too much black than too much pink. So, many of the semicolons stayed in despite the comments. Somehow, in the thesis haze, I didn’t think of the option of many simple single-clause sentences. Single-clause sentences are hard.

I also used the word “setup” over 100 times as a catch-all word to encompass all of the following: apparatus, configuration, software, procedure, hypothesis. I hadn’t noticed that, and I have no good reason for it, so now my thesis doesn’t use the word “setup” at all. I think. And if it does, it’s too late to change it now!

And of course there was the matter of completing the concluding paragraph so it matched the conclusion I presented in my defense seminar. That took some work. I also tried to produce some numbers to complete the description of my analysis in more detail than I needed for the defense seminar, just for archival completeness. But by the time I had fixed everything else, it was only a few hours until my deposit margin-check appointment (and also 2:30am), so I gave up on getting those numbers.

The deposit appointment was all of 5 minutes long, but marked the line between “almost done” and “DONE!!!”. The reviewing administrator realized this. She shook my hand three times in those 5 minutes. When it was done, I went outside and there were birds singing. I bought celebratory coffee and a new Wisconsin shirt. And then started packing up my apartment for the movers arriving the next morning.

I’d gotten into a grove of watching a couple episodes each night of long-running TV shows with emotionally predictable episodic plot lines. Star Trek and various murder mysteries were big. The last series was “House, MD” with Hugh Laurie. By coincidence, when I defended my thesis and my stress level starting deflating, I was almost exactly at the point in the series where they ran out of mysteries from the original book it was based on, and started going more into a soap-opera style character drama. By the time I wasn’t interested in the soap opera aspects anymore, it was time to start reengaging with my real-life friends.

A few days after I moved away from Madison, when I was staying with my parents, I picked up my high school routine of reading the local paper over breakfast, starting with the comics, then local editorials. I found (or rather, my dad found) myself criticizing the writing from the point of view of a dissertator. It takes more than a few days to get out of thesis-writing mode. The little nagging conscience doesn’t go away, still telling me that the difference between ok writing and great writing is important, more so now than at any point so far in my career. For the last edits of a PhD, it might be important to criticize at that level of detail. But for a local paper, pretty much anything is useful to the community.

At lunch Saturday in a little restaurant in the medieval part of the Italian village of Assergi, I found the antidote. When I can’t read any of the articles and posters on the walls, when I can’t carry on a conversation with more than 3-word sentences, it doesn’t matter anymore if the paragraphs have a clear and concise topic sentence. I need simple text. I’m happy if I can understand the general meaning. The humility of starting over again with Italian is the antidote for the anxiety of a thesis. It’s ok to look like a fool in some ways, because I am a certified non-fool in one small part of physics.

It’s not perfect of course: there’s still a lot of anxiety inherent in living in a country without speaking the language (well enough to get by without english-speaking help). I’ll write more about the cultural transition in another post, since I have so many posts to catch up on from while I was in the thesis-hole, and this post is definitely long enough. But for now, the thesis is over.

### Scintillator extruded at Fermilab detects particles around the globe

Wednesday, November 26th, 2014

The plastic scintillator extrusion line, shown here, produces detector material for export to experiments around the world. Photo: Reidar Hahn

Small, clear pellets of polystyrene can do a lot. They can help measure cosmic muons at the Pierre Auger Observatory, search for CP violation at KEK in Japan or observe neutrino oscillation at Fermilab. But in order to do any of these they have to go through Lab 5, located in the Fermilab Village, where the Scintillation Detector Development Group, in collaboration with the Northern Illinois Center for Accelerator and Detector Design (NICADD), manufactures the exclusive source of extruded plastic scintillator.

Like vinyl siding on a house, long thin blocks of plastic scintillator cover the surfaces of certain particle detectors. The plastic absorbs energy from collisions and releases it as measurable flashes of light. Fermilab’s Alan Bross and Anna Pla-Dalmau first partnered with local vendors to develop the concept and produce cost-effective scintillator material for the MINOS neutrino oscillation experiment. Later, with NIU’s Gerald Blazey, they built the in-house facility that has now exported high-quality extruded scintillator to experiments worldwide.

“It was clear that extruded scintillator would have a big impact on large neutrino detectors,” Bross said, “but its widespread application was not foreseen.”

Industrially manufactured polystyrene scintillators can be costly — requiring a labor-intensive process of casting purified materials individually in molds that have to be cleaned constantly. Producing the number of pieces needed for large-scale projects such as MINOS through casting would have been prohibitively expensive.

Extrusion, in contrast, presses melted plastic pellets through a die to create a continuous noodle of scintillator (typically about four centimeters wide by two centimeters tall) at a much lower cost. The first step in the production line mixes into the melted plastic two additives that enhance polystyrene’s natural scintillating property. As the material reaches the die, it receives a white, highly reflective coating that holds in scintillation light. Two cold water tanks respectively bathe and shower the scintillator strip before it is cool enough to handle. A puller controls its speed, and a robotic saw finally cuts it to length. The final product contains either a groove or a hole meant for a wavelength-shifting fiber that captures the scintillation light and sends the signal to electronics in the most useful form possible.

Bross had been working on various aspects of the scintillator cost problem since 1989, and he and Pla-Dalmau successfully extruded experiment-quality plastic scintillator with their vendors just in time to make MINOS a reality. In 2003, NICADD purchased and located at Lab 5 many of the machines needed to form an in-house production line.

“The investment made by Blazey and NICADD opened extruded scintillators to numerous experiments,” Pla-Dalmau said. “Without this contribution from NIU, who knows if this equipment would have ever been available to Fermilab and the rest of the physics community?”

Blazey agreed that collaboration was an important part of the plastic scintillator development.

“Together the two institutions had the capacity to build the resources necessary to develop state-of-the-art scintillator detector elements for numerous experiments inside and outside high-energy physics,” Blazey said. “The two institutions remain strong collaborators.”

Between their other responsibilities at Fermilab, the SDD group continues to study ways to make their scintillator more efficient. One task ahead, according to Bross, is to work modern, glass wavelength-shifting fibers into their final product.

“Incorporation of the fibers into the extrusions has always been a tedious part of the process,” he said. “We would like to change that.”

Troy Rummler

### Geometry and interactions

Tuesday, November 25th, 2014

Or, how do we mathematically describe the interaction of particles?

In my previous post, I addressed some questions concerning the nature of the wavefunction, the most truthful mathematical representation of a particle. Now let us make this simple idea more complete, getting closer to the deep mathematical structure of particle physics. This post is a bit more “mathematical” than the last, and will likely make the most sense to those who have taken a calculus course. But if you bear with me, you may also come to discover that this makes particle interactions even more attractive!

The field theory approach considers wavefunctions as fields. In the same way as the temperature field $$T(x,t)$$ gives the value of the temperature in a room at space $$x$$ and time $$t$$, the wavefunction $$\phi (x,t)$$ quantifies the probability of presence of a particle at space point $$x$$ and time $$t$$.
Cool! But if this sounds too abstract to you, then you should remember what Max Planck said concerning the rise of quantum physics: “The increasing distance between the image of the physical world and our common-sense perception of it simply indicates that we are gradually getting closer to reality”.

Almost all current studies in particle physics focus on interactions and decays of particles. How does the concept of interaction fit into the mathematical scheme?

The mother of all the properties of particles is called the Lagrangian function. Through this object a lot of properties of the theory can be computed. Here let’s consider the Lagrangian function for a complex scalar field without mass (one of the simplest available), representing particles with electric charge and no spin:

$$L(x) = \partial_\mu \phi(x)^* \partial^\mu \phi(x)$$.

Mmm… Is it just a bunch of derivatives of fields? Not really. What do we mean when we read $$\phi(x)$$? Mathematically, we are considering $$\phi$$ as a vector living in a vector space “attached” to the space-time point $$x$$. For the nerds of geometry, we are dealing with fiber bundles, structures that can be represented pictorially in this way:

Click on image for larger version

The important consequence is that, if $$x$$ and $$y$$ are two different space-time points, a field $$\phi(x)$$ lives in a different vector space (fiber) with respect to $$\phi(y)$$! For this reason, we are not allowed to perform operations with them, like taking their sum or difference (it’s like comparing a pear with an apple… either sum two apples or two pears, please). This feature is highly non-trivial, because it changes the way we need to think about derivatives.

In the $$L$$ function we have terms containing derivatives of the field $$\phi(x)$$. Doing this, we are actually taking the difference of the value of the field at two different space-time points. But … we just outlined that we are not allowed to do it! How can we solve this issue?

If we want to compare fields pertaining to the same vector space, we need to slightly modify the notion of derivative introducing the covariant derivative $$D$$:

$$D_\mu = \partial_\mu + ig A_\mu(x)$$.

Here, on top of the derivative $$\partial$$, there is the action of the “connection” $$A(x)$$, a structure which takes care of “moving” all the fields in the same vector space, and eventually allows us to compare apples with apples and pears with pears.
So, a better way to write down the Lagrangian function is:

$$L(x) = D_\mu \phi(x)^* D^\mu \phi(x)$$.

If we expand $$D$$ in terms of the derivative and the connection, $$L$$ reads:

$$L(x) = \partial_\mu \phi(x)^* \partial^\mu \phi(x) +ig A_\mu (\partial^\mu \phi^* \phi – \phi^* \partial^\mu \phi) + g^2 A^2 \phi^* \phi$$.

Do you recognize the role of these three terms? The first one represents the propagation of the field $$\phi$$. The last two are responsible for the interactions between the fields $$\phi, \phi^*$$ and the $$A$$ field, referred to as the “photon” in this context.

Click on image for larger version

This slightly hand-waving argument involving fields and space-time is a simple handle to understand how the interactions among particles emerge as a geometric feature of the theory.

If we consider more sophisticated fields with spin and color charges, the argument doesn’t change. We need to consider a more refined “connection” $$A$$, and we could see the physical interactions among quarks and gluons (namely QCD, Quantum Chromo Dynamics) emerging just from the mathematics.

Probably the professor of geometry in my undergrad course would call this explanation “Spaghetti Mathematics”, but I think it can give you a flavor of the mathematical subtleties involved in the theory of particle physics.

### Graduating, part 1: The Defense

Tuesday, November 25th, 2014

It’s been a crazy 3 weeks since I officially finished my PhD. I’m in the transition from being a grad student slowly approaching insanity to a postdoc who has everything figured out, and it’s a rocky transition.

The end of the PhD at Wisconsin has two steps. The first is the defense, which is a formal presentation of my research to the professors and committee, our colleagues, and very few friends and family. The second is actually turning the completed dissertation to the grad school, with the accompanying “margin check” appointment with the grad school. In between, the professors can send me comments about the thesis. I’ve heard so many stories of different universities setting up the end of a degree differently, it’s pretty much not worth going into the details. If you or someone you know is going through this process, you don’t need a comparison of how it works at different schools, you just need a lot of support and coping mechanisms. All the coping mechanisms you can think of, you need them. It’s ok, it’s a limited time, don’t feel guilty, just get through it. There is an end, and you will reach it.

The days surrounding the defense were planned out fairly carefully, including a practice talk with my colleagues, again with my parents (who visited for the defense), and delivery burritos. I ordered coffee and doughnuts for the defense from the places where you get those, and I realized why such an important day has such a surprisingly small variety of foods: because deviating from the traditional food is so very far down my list of priorities when there’s the physics to think about, and the committee, and the writing. The doughnuts just aren’t worth messing with. Plus, the traditional place to get doughnuts is already really good.

We even upheld a tradition the night before the defense. It’s not really a tradition per se, but I’ve seen it once and performed it once, so that makes it a tradition. If you find it useful, you can call it an even stronger tradition! We played an entire soundtrack and sung along, with laptops open working on defense slides. When my friend was defending, we watched “Chicago” the musical, and I was a little hoarse the next day. When I was defending, we listened to Leonard Bernstein’s version of Voltaire’s “Candide,” which has some wonderful wordplay and beautiful writing for choruses. The closing message was the comforting thought that it’s not going to be perfect, but life will go on.

“We’re neither wise nor pure nor good, we’ll do the best we know. We’ll build our house, and chop our wood, and make our garden grow.”

Hearing that at the apex of thesis stress, I think it will always make me cry. By contrast, there’s also a scene in Candide depicting the absurd juxtaposition of a fun-filled fair centered around a religious inquisition and hanging. Every time someone said they were looking forward to seeing my defense, I thought of this hanging-festival scene. I wonder if Pangloss had to provide his own doughnuts.

The defense itself went about as I expected it would. The arguments I presented had been polished over the last year, the slides over the last couple weeks, and the wording over a few days. My outfit was chosen well in advance to be comfortable, professional, and otherwise unremarkable (and keep my hair out my way). The seminar itself was scheduled for the time when we usually have lab group meetings, so the audience was the regular lab group albeit with a higher attendance-efficiency factor. The committee members were all present, even though one had to switch to a 6am flight into Madison to avoid impending flight cancellations. The questions from the committee mostly focused on understanding the implications of my results for other IceCube results, which I took to mean that my own work was presented well enough to not need further explanation.

It surprised me, in retrospect, how quickly the whole process went. The preparation took so long, but the defense itself went so quickly. From watching other people’s defenses, I knew to expect a few key moments: an introduction from my advisor, handshakes from many people at the end of the public session, the moment of walking out from the closed session to friends waiting in the hallway, and finally the first committee member coming out smiling to tell me they decided to pass me. I knew to look for these moments, and they went by so much faster in my own defense than I remember from my friends. Even though it went by so quickly, it still makes a difference having friends waiting in the hallway.

People asked me if it was a weight off my shoulders when I finally defended my thesis. It was, in a way, but even more it felt like cement shoes off my feet. Towards the end of the process, for the last year or so, a central part of myself felt professionally qualified, happy, and competent. I tried desperately to make that the main part. But until the PhD was finished, that part wasn’t the exterior truth. When I finished, I felt like the qualifications I had on paper matched how qualified I felt about myself. I’m still not an expert on many things, but I do know the dirty details of IceCube software and programing. I have my little corner of expertise, and no one can take that away. Degrees are different from job qualifications that way: if you stop working towards a PhD several years in, it doesn’t count as a fractional part of a degree; it’s just quitting. But if you work at almost any other job for a few years, you can more or less call it a few years of experience. A month before my defense, part of me knew I was so so so close to being done, but that didn’t mean I could take a break.

And now, I can take a break.

### Neutrinos, claymation and ‘Doctor Who’ at this year’s physics slam

Monday, November 24th, 2014

Wes Ketchum of the MicroBooNE collaboration is the Physics Slam III champion. Ketchum’s slam was on the detection of particles using liquid argon. Photo: Cindy Arnold

On Nov. 21, for the third year in a row, the Fermilab Lecture Series invited five scientists to battle it out in an event called a physics slam. And for the third year in a row, the slam proved wildly popular, selling out Ramsey Auditorium more than a month in advance.

More than 800 people braved the cold to watch this year’s contest, in which the participants took on large and intricate concepts such as dark energy, exploding supernovae, neutrino detection and the overwhelming tide of big data. Each scientist was given 10 minutes to discuss a chosen topic in the most engaging and entertaining way possible, with the winner decided by audience applause.

Michael Hildreth of the University of Notre Dame kicked things off by humorously illustrating the importance of preserving data — not just the results of experiments, but the processes used to obtain those results. Marcelle Soares-Santos of the Fermilab Center for Particle Astrophysics took the stage dressed as the Doctor from “Doctor Who,” complete with a sonic screwdriver and a model TARDIS, to explore the effects of dark energy through time.

Joseph Zennamo of the University of Chicago brought the audience along on a high-energy journey through the “Weird and Wonderful World of Neutrinos,” as his talk was called. And Vic Gehman of Los Alamos National Laboratory blew minds with a presentation about supernova bursts and the creation of everything and everyone in the universe.

The slammers at this year’s Fermilab Physics Slam were, Michael Hildreth, University of Notre Dame (far left); Marcelle Soares-Santos, Fermilab (second from left); Vic Gehman, Los Alamos National Laboratory (third from left); Wes Ketchum, Fermilab (second from right); Joseph Zennamo, University of Chicago. Fermilab Director Nigel Lockyer (third from right) congratulated all the participants. Photo: Cindy Arnold

The winner was Fermilab’s Wes Ketchum, a member of the MicroBooNE collaboration. Ketchum’s work-intensive presentation used claymation to show how different particles interact inside a liquid-argon particle detector, depicting them as multicolored monsters bumping into one another and creating electrons for the detector’s sensors to pick up. Audience members won’t soon forget the sight of a large oxygen monster eating red-blob electrons.

After the slam, the five scientists took questions from the audience, including one about dark matter and neutrinos from an eight-year-old boy, sparking much discussion. Chris Miller, speech professor at the College of DuPage, made his third appearance as master of ceremonies for the Physics Slam, and thanked the audience — particularly the younger attendees — for making the trek to Fermilab on a Friday night to learn more about science.

Video of this year’s Physics Slam is available on Fermilab’s YouTube channel.

Andre Salles

### Stanley Wojcicki awarded 2015 Panofsky Prize

Tuesday, November 18th, 2014

Stanley Wojcicki

In late October, the American Physical Society Division of Particles and Fields announced that Stanford University professor emeritus of physics and Fermilab collaborator Stanley Wojcicki has been selected as the 2015 recipient of the W.K.H. Panofsky Prize in experimental particle physics. Panofsky, who died in 2007, was SLAC National Accelerator Laboratory’s first director, holding that position from 1961 to 1984.

“I knew Pief Panovsky for about 40 years, and I think he was a great man not only as a scientist, but also as a statesman and as a human being,” said Wojcicki, referring to Panofsky by his nickname. “So it doubles my pleasure and satisfaction in receiving an award that bears his name.”

Wojcicki was given the prestigious award “for his leadership and innovative contributions to experiments probing the flavor structure of quarks and leptons, in particular for his seminal role in the success of the MINOS long-baseline neutrino experiment.”

Wojcicki is a founding member of MINOS. He served as spokesperson from 1999 to 2004 and as co-spokesperson from 2004 to 2010.

“I feel a little embarrassed being singled out because, in high-energy physics, there is always a large number of individuals who have contributed and are absolutely essential to the success of the experiment,” he said. “This is certainly true of MINOS, where we had and have a number of excellent people.”

Wojcicki recalls the leadership of Caltech physicist Doug Michael, former MINOS co-spokesperson, who died in 2005.

“I always regret that Doug did not have a chance to see the results of an experiment that he very much contributed to,” Wojcicki said.

In 2006, MINOS measured an important parameter related to the mass difference between two neutrino types.

Fermilab physicist Doug Glenzinski chaired the Panofsky Prize review committee and says that the committee was impressed by Wojcicki’s work on flavor physics, which focuses on how particles change from one type to another, and his numerous contributions over decades of research.

“He is largely credited with making MINOS happen, with thinking about ways to advance neutrino measurements and with playing an active role in all aspects of the experiment from start to finish,” Glenzinski said.

More than 30 years ago, Wojcicki collaborated on charm quark research at Fermilab, later joining Fermilab’s neutrino explorations. Early on Wojcicki served on the Fermilab Users Executive Committee from 1969-71 and on the Program Advisory Committee from 1972-74. He has since been on many important committees, including serving as chair of the High-Energy Physics Advisory Panel for six years and as member of the P5 committee from 2005-08. He now continues his involvement in neutrino physics, participating in the NOvA and MINOS+ experiments.

“I feel really fortunate to have been connected with Fermilab since its inception,” Wojcicki said. “I think Fermilab is a great lab, and I hope it will continue as such for many years to come.”

Rich Blaustein

### Dark Matters: Creation from Annihilation

Thursday, November 13th, 2014

Hanging around a pool table might seem like an odd place to learn physics, but a couple of hours on our department’s slanted table could teach you a few things about asymmetry. The third time a pool ball flew off the table and hit the far wall I knew something was broken. The pool table’s refusal to obey the laws of physics gives aspiring physicists a healthy distrust of the simplified mechanics they learnt in undergrad. Whether in explaining why pool balls bounce sideways off lumpy cushions or why galaxies exist, asymmetries are vital to understanding the world around us. Looking at dark matter theories that interact asymmetrically with visible matter can give us new clues as to why matter exists.

Alternatives to the classic WIMP (weakly interacting massive particles) dark matter scenario are becoming increasingly important. Natural supersymmetry is looking less and less likely, and could be ruled out in 2015 by the Large Hadron Collider. Asymmetric dark matter theories provide new avenues to search for dark matter and help explain where the material in our universe comes from -baryogenesis. Baryogenesis is in some ways a more important cosmological problem than dark matter. The Standard Model of particle physics describes all the matter that you are familiar with, from trees to stars, but fails to explain how this matter came to be. In fact, the Standard Model predicts a sparsely populated universe, where most of the matter and antimatter has long since annihilated each another. In particle colliders, whenever a particle of matter is created, an opposing particle of antimatter is also created. Antimatter is matter with all its charges reversed, like a photo negative. While it is often said that opposites attract, in the particle physics world opposites annihilate. But when we look at the universe around us, all we see is matter. There are no antistars and antiplanets, no antihumans living on some distant world. So if matter and antimatter are always created together, how did this happen? If there were equal amounts of matter and antimatter, each would annihilate the other in the first fractions of a second and our universe would be stillborn. The creation of this asymmetry between matter and antimatter is known as baryogenesis, and is one of the strongest cosmological confirmations of physics beyond the Standard Model. The exact amount of asymmetry determines how much matter, and consequently how many stars and galaxies, exists now.

And what about the other 85% of matter in the universe? This dark matter has only shown itself through gravitational interactions, but it has shaped the evolution of the universe. Dark matter keeps galaxies from tearing themselves apart, and outnumbers visible matter five to one. Five to one is a curious ratio. If dark and visible matter were entirely different substances with a completely independent history, you would not expect almost the same amount of dark and normal matter. This is like counting the number of trees in the world and finding that it’s the same as the number of pebbles. While we know that dark and visible matter are not the same substance (the Standard Model does not include any dark matter candidates), this similarity cannot be ignored. The similarity in abundances between dark and visible matter implies that they were caused by the same mechanism, created in the same way. As the abundance of matter is determined by the asymmetry between antimatter and matter, this leads us to a relationship between baryogenesis and dark matter.

Asymmetric dark matter theories have attracted significant attention in the last few years, and are now studied by physicists across the world. This has give us a cornucopia of asymmetric dark matter theories. Despite this, there are several common threads and predictions that allow us to test many of them at once. In asymmetric dark matter theories baryogenesis is caused by interactions between dark and normal matter. By having dark matter interact differently with matter and antimatter, we can get marginally more matter in the universe then antimatter. After the matter and antimatter annihilate each other, there is some minuscule amount of matter left standing. These leftovers go on to become the universe you know. Typically, a similar asymmetry in dark matter and its antiparticle is also made, so there is a similar amount of dark matter left over as well. This promotes dark matter from being a necessary, yet boring spectator in the cosmic tango to an active participant, saving our universe from desolation. Asymmetric dark matter also provides new ways to search for dark matter, such as neutrinos generated from dark matter in the sun. As asymmetric dark matter interacts with normal matter, large bodies like the sun and the earth can capture a reservoir of dark matter, sitting at their core. This can generate ghostlike neutrinos, or provide an obstacle for dark matter in direct detection experiments. Asymmetric dark matter theories can also tell us where we do not expect to see dark matter. A large effort has been made to see tell-tale signs of dark matter annihilating with its antiparticle throughout the universe, but it is yet to meet with success. While experiments like the Fermi space telescope have found potential signals (such as a 130 GeV line in 2012), these signals are ambiguous or fail to survive the test of time. The majority of asymmetric dark matter theories predict that there is no signal, as all the anti dark matter has long since been destroyed.

As on the pool table, even little asymmetries can have a profound effect on what we see. While much progress is made from finding new symmetries, we can’t forget the importance of imperfections in science. Asymmetric dark matter can explain where the matter in our universe came from, and gives dark and normal matter a common origin. Dark matter is no longer a passive observer in the evolution of our universe; it plays a pivotal role in the world around us.

### Fermilab’s Oliver Gutsche keeps LHC community computing

Wednesday, November 12th, 2014

Fermilab’s Oliver Gutsche leads worldwide computing operations for the CMS experiment. Photo: Reidar Hahn

Since he was a graduate student in Germany, Oliver Gutsche wanted to combine research in particle physics with computing for the large experiments that probe the building blocks of matter.

“When I started working on the physics data coming from one of the experiments at DESY, I was equally interested in everything that had to do with large-scale computing,” said Gutsche of his time at the German laboratory. Gutsche now works at DOE’s Fermi National Accelerator Laboratory. “So I also began working on the computing side of particle physics. For me that was always the combination I wanted to do.”

Gutsche’s desire to merge the two focuses has paid off. For the past four years Gutsche has been in charge of worldwide computing operations of the Large Hadron Collider’s CMS experiment, one of two experiments credited with the 2012 Higgs boson discovery. In December he was awarded the CMS Collaboration Award for his contributions to the global CMS computing system. And more recently, he has been promoted to assistant head of the Scientific Computing Division at Fermilab.

As head of CMS Computing Operations, Gutsche orchestrates data processing, simulations, data analysis and transfers and manages infrastructure and many more central tasks. Monte Carlo simulations of particle interactions, for example, are a key deliverable of the CMS Computing Operations group. Monte Carlo simulations employ randomness to simulate the collisions of the LHC and their products in a statistical way.

“You have to simulate the randomness of nature,” explained Gutsche. “We need Monte Carlo collisions to make sure we understand the data recorded by the CMS experiment and to compare them to the theory.”

When Gutsche received his Ph.D. from the University of Hamburg in 2005, he was looking for a job to combine LHC work, large-scale computing and a U.S. postdoc experience.

“Fermilab was an ideal place to do LHC physics research and LHC computing at the same time,” he said. His postdoc work led to his appointment as an application physicist at Fermilab and as the CMS Computing Operations lead.

Today Gutsche interacts regularly with people at universities and laboratories across the United States and at CERN, host laboratory of the LHC, often starting the day at 7 a.m. for transatlantic or transcontinental meetings.

“I try to talk physics and computing with everyone involved, even those in different time zones, from CERN to the west coast,” he said. Late afternoon in the United States is a good time for writing code. “That’s when everything quiets down and Europe is asleep.”

Gutsche expects to further enhance the cooperation between U.S. particle physicists and their international colleagues, mostly in Europe, by using the new premier U.S. Department of Energy’s Energy Sciences Network recently announced in anticipation of the LHC’s restart in spring 2015 at higher energy.

Helping connect the research done by particle physicists around the world, Gutsche finds excitement in all the work he does.

“Of course the Higgs boson discovery was very exciting,” Gutsche said. “But in CMS Computing Operations everything is exciting because we prepare the basis for hundreds of physics analyses so far and many more to come, not only for the major discoveries.”

Rich Blaustein

### Interstellar: Bring tissues, and maybe a general relativity textbook

Monday, November 10th, 2014

On Saturday, I went to see Interstellar at the London BFI IMAX. I wouldn’t usually be so extravagant; my usual cinema trips are on 2-for-1 Orange Wednesdays. But I felt Interstellar was worth seeing in all its IMAX high resolution glory – and it definitely was.  The film, directed by Christopher Nolan, is an epic masterpiece describing the journey of Cooper (Matthew McConaughey), a pilot-turned farmer-turned pilot again out of our galaxy in search of a new home for humans. The Earth is blighted by, well, blight, and the human race is starving. It paints a grim picture of our potential future here on Earth, and it seems entirely plausible.

The black hole featured in Interstellar. Equations from physicist Kip Thorne were used to model the lensing of light around the black hole.

Interstellar did an excellent job of using physics. This should be expected – the filmmakers worked with prominent theorist Kip Thorne. Thorne has worked on all those delicious-sounding areas of cosmology that I wish I had the brains for – black holes, wormholes, quantum gravity, gravitational waves, relativistic stars, etc. Genuine equations were used to visualise the black hole and wormhole featured in Interstellar – a fact that the majority of the audience would not know or particularly care about, but satisfies physicists. There is something to be said for not straying too far from real science in films when we live in a world plagued by quack-science that has tarred the word quantum (for examples, just search for quantum healing, or note that searching for quantum crystals brings up a site for buying “quantum balance crystals” before it brings up anything related to quantum mechanics and solid-state physics).

I was surprised to find posts on the internet “explaining” Interstellar. The film wrapped everything up nicely in my opinion. But most people haven’t done a course in General Relativity! I don’t pretend to be any sort of expert, it was 2 years ago and I can barely remember the maths but I do know the basic concept of gravitational time dilation. It is this, it seems, that was confusing people – how time was passing at a different rate for those on Earth and those in space.

I don’t want to give away any spoilers, but Cooper ends up close to a black hole (aptly named “Gargantua”), where time runs much more slowly for him than anyone further away. This is a strange and frightening thought for us humans who spend our lives moving consistently forwards in time at the same rate (at least within our perception). In reality, you are ageing slightly faster at the top of a skyscraper than you are at the Earth’s surface – but the effect is too small to notice. It is, however, definitely there. General and special relativity are used to correct the time given by GPS satellites – they are in a weaker gravitational field than we are down on Earth, and so their clocks run slightly faster. Without this correction, GPS would not work.

To understand relativity, you need to remember that time is just another dimension, a fact that becomes important later in the film. Like our 3D space is warped around a massive object, so is time. The foundations of general relativity lie in something called the “equivalence principle”. Einstein himself wrote this as:

“A little reflection will show that the law of the equality of the inertial and gravitational mass is equivalent to the assertion that the acceleration imparted to a body by a gravitational field is independent of the nature of the body.

A geodesic is a path minimising line connecting two points on a sphere. Everything falling under gravity is following one of these lines in curved space-time.

What this means is that under gravity, all things will accelerate at the same rate, independent of their mass. We see this on Earth, where that rate of acceleration is 9.81m/s. A feather and a rock dropped together reach the ground at the same moment (ignoring air resistance!).

Next, Einstein deduced that an object in “free-fall”, i.e. an object with only gravity acting upon it, is not actually accelerating - there is no force of gravity. This was actually one of those rare “my mind is blown” moments I had during my degree.  An object in free-fall is not accelerating – it is simply following a geodesic in curved space-time. A geodesic is the analogy to a straight line within curved space – think if it as the shortest path between two points on a sphere.

I’m going a bit off tangent here, but general relativity is a fascinating subject! What I wanted to get to is the time dilation part. Why does time run slower for someone in a strong gravitational field? It actually comes back to special relativity, general relativity’s less scary little brother. I was taught special relativity in the first year of my degree, and it was the first time I felt like I was learning real exciting physics. The postulates of SR are:

• The laws of physics are the same in all inertial frames of reference.
• The speed of light in free space has the same value c in all inertial frames of reference.

A beam of light in an accelerating rocket appears curved to an outside observer. This would be the same for a free-falling laboratory.

Combining the principle of a freely-falling (i.e. travelling on a geodesic in a gravitational field) laboratory and applying special relativity introduces time dilation. The Pound and Rebka experiment is helpful to read up on for understanding this. By the laws of SR, both an observer inside the laboratory and one outside should measure the speed of light as c. Imagine a beam of light in the laboratory, the observer outside sees the path of light bend as the laboratory falls, whilst the observer inside sees a straight line as they are in an inertial frame. This means that for the outside observer, the light has travelled a longer (curved) path. As light always travels at c, the observer will deduce that more time has passed inside the laboratory than the person inside will measure. The stronger the gravitational field, the faster the free-fall, and the more the light will appear curved to the outside observer – so the time dilation factor increases with the field strength.

The strength of the black hole’s field in Interstellar means that minutes for Cooper become years for those outside. He is “free-falling” at an incredible rate, so his clock is running thousands of times slower than the ones on Earth, but it feels totally normal to him. He sees his own clock running at a normal speed, but he knows that the ones on Earth are running much faster. Emotions run high as every second he spends on his mission could be years he is missing of his children’s’ lives.  I came out of the film an emotional wreck – I’d shed many tears and my chest felt tight, it was genuinely traumatic. Don’t get me wrong, I cry at a lot of films (I even cried when Gandalf died in Lord of the Rings, even though I’d read the books and so knew he was fine.) but I’ve never left one still feeling so upset. But that helps make it a brilliant film – not just the special effects, the beautiful images of space and stars and black holes, but the human reality; at the end of the day, it is just a father fighting to save his children.

I strongly recommend you go see this film, whether you are a physicist or not. I could go on for a lot longer and discuss the paradoxes some of the wormhole travel introduces as well as some other puzzles, but that would reveal spoilers, so instead I’ll just stop here!  Interstellar is heartbreaking, but also breathtaking, and also warns us to take care of our planet. It’s not so easy to find another, and for god’s sake don’t stop investing in science! Make sure you bring tissues.

### From wavefunctions to detectors: how to think about particles

Sunday, November 9th, 2014

This blog is all about particle physics and particle physicists. We can all agree, I suppose, on the notion of the particle physicist, right? There are even plenty of nice pictures up here! But do we know or are we aware of what a particle really is? This fundamental question tantalized me from the very beginning of my studies and before addressing more involved topics I think it is worth spending some time on this concept. Through the years I probably changed my opinion several times, according to the philosophy underlying the topic that I was investigating. Moreover, there’s probably not a single answer to this question.

1. The Standard Model: from geometry to detectors

The human mind conceived the Standard Model of Particle Physics to give a shape on the blackboard to the basic ingredients of particle physics: it is a field theory, with quantization rules, namely a quantum field theory and its roots go deep down to differential geometry.
But we know that “particles” like the Higgs boson have been discovered through complex detectors, relying on sophisticated electronic systems, tons of Monte Carlo simulations and data analysis. Quite far away from geometry, isn’t it?
So the question is: how do we fill this gap between theory and experiment? What do theoreticians think about and experimentalists see through the detectors? Furthermore, does a particle’s essence change from its creation to its detection?

1. Essence and representation: the wavefunction

Let’s start with simple objects, like an electron. Can we imagine it as a tiny thing floating here and there? Mmm. Quantum mechanics already taught us that it is something more: it does not rotate around an atomic nucleus like the Earth around the Sun (see, e.g., Bohr’s model). The electron is more like a delocalized “presence” around the nucleus quantified by its “wavefunction”, a mathematical function that gives the probability of finding the electron at a certain place and time.
Let’s think about it: I just wrote that the electron is not a localized entity but it is spread in space and time through its wavefunction. Fine, but I still did not say what an electron is.

I have had long and intensive discussions about this question. In particular I remember one with my housemate (another theoretical physicist) that was about to end badly, with the waving of frying pans at each other. It’s not still clear to me if we agreed or not, but we still live together, at least.

Back to the electron, we could agree on considering its essence as its abstract definition, namely being one of the leptons in the Standard Model. But the impossibility of directly accessing it forces me to identify it with its most trustful representation, namely the wavefunction. I know its essence, but I cannot directly (i.e. with my senses) experience it. My human powers stop to the physical manifestation of its mathematical representation: I cannot go further.
Renè Magritte represented the difference between the representation of an object and the object itself in a famous painting “The treachery of images”:

“Ceci n’est pas une pipe”, it says, namely “This is not a pipe”. He is right, the picture is its representation. The pipe is defined as “A device for smoking, consisting of a tube of wood, clay, or other material with a small bowl at one end” and we can directly experience it. So its representation is not the pipe itself.

As I explained, this is somehow different in the case of the electron or other particles, where experience stops to the representation. So, according to my “humanity”, the electron is its wavefunction. But, to be consistent with what I just claimed: can we directly feel its wavefunction? Yes, we can. For example we can see its trace in a cloud chamber, or more elaborate detectors. Moreover, electricity and magnetism are (partly) manifestations of electron clouds in matter, and we experience those in everyday life.

You may wonder why I go through all these mental wanderings: just write down your formulas, calculate and be happy with (hopefully!) discoveries.

I do it because philosophy matters. And is nice. And now that we are a bit more aware of the essence of things that we are investigating, we can move a step forward and start addressing Quantum Chromo Dynamics (QCD), from its basic foundations to the latest results released by the community. I hope to have sufficiently stimulated your curiosity to follow me during the next steps!

Again, I want to stress that this is my own perspective, and maybe someone else would answer these questions in a different way. For example, what do you think?