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## Archive for January, 2008

### Burst my bubble!

Thursday, January 31st, 2008

I used to like to think that I wasn’t a geeky scientist, but my bubble burst yesterday when I destroyed my first shirt via an uncapped red pen – maybe I really do need that pocket protector!

### No Mere Cog

Wednesday, January 30th, 2008

This is a post that writes itself, since I already spent an inordinate amount of time writing it for publication. My friends at Symmetry Magazine (which is really one of the best — and best-looking — sources of information about nuclear and particle physics that you can explain to your grandmother — in fact, I just signed up my grandmother for a subscription!) asked me to write a short essay about this blog in the context of the run-up to the LHC turn-on (what does one call compounds like “run-up” and “turn-on” anyway?…) Have a look if you have a minute.

As an aside (to which I’m clearly no stranger), this isn’t my first contribution to Symmetry. This is, although that time I was more of a subject, or at least my Dad’s old “relativator” was (and don’t think I made that Wikipedia entry — I just found it. Gee.)

### The Closing

Tuesday, January 29th, 2008

Despite its size, ATLAS is not a completely stationary detector. It has a stationary infrastructure but it does have some parts that can move around, allowing for access to the internal elements of the detector. (By contrast CMS is very mobile–it is like a gigantic Yule log where each section can be separated from each other.) On each side of ATLAS, there are three big moveable parts as seen in this picture. The muon wheels shown by the turquoise arrow can be moved back. This allows for the end-cap magnets indicated by the red arrow to be pulled out. Once the end-cap is moved, we can gain access to the extended barrel calorimeters (such as TileCal). The extended barrels shown by the blue arrow can also move forward, allowing for access to the calorimeter barrel as well as the inner detector. But space is limited so it is not possible to access all places at once. And so in typical ATLAS fashion, there are committees in place to organize who gets access when.

But the end of April is ‘the closing’. Or perhaps I should say THE CLOSING. Or maybe even, THE CLOSING.

Where all the moveable parts are put into their final location. And the installation of the beam pipe and the beam pipe shielding begins.

The bad news is that we will lose access to the detector. The good news is that it means the beam is coming.

The closing is one of those things that can simultaneously fill you with ecstatic excitement and absolute panic. And it really is a simultaneous feeling. On one hand it is like sitting in a Ferrari waiting for the keys. Feeling the pressure of the seat against your back, the feel of the wheel under your hands, anticipating the roar of the engine when you hit the gas and thinking, ‘Alright. Show me what 0 to 60 really means.’ And yet at absolutely the same moment, it is like the night before your last final exam. Staring intently at a pile of books, willing the information to leap out of the pages and into your brain, lamenting, ‘why didn’t I get to this sooner’, but knowing that it was impossible in any case because you had other exams which took precedent.

But heedless of anyone’s feelings on the subject, the closing approaches. And I will bet that as it continues to approach the two most common phrases in the halls will be either ‘We are closing soon, the beam will be here!’ or ‘We are closing soon, and it is the end of January already. How did it get to be the end of January already?!’.

The end of April, it is so far and yet so close.

### Some pretty big questions

Saturday, January 26th, 2008

I have readers who aren’t my relatives! asking some pretty big questions. First thing that comes to mind is the Rodgers and Hart lyric “if they asked me, I could write a book”. Indeed, regarding what we might find at the LHC there are books, but for a quick understanding for non-physicists (and a refreshing look at our world from the outside for us) I’d start with the fresh-off-the-press Scientific American articles about the LHC and Particle Physics in general. (Thanks to Dan Green for passing these on). The people who wrote these spent much more time on it than I can afford, and strove to make it accessible to the general public. My nutshell is:

• The final piece of the Standard Model is the so called Higgs Boson. The Standard Model has been proven right over and over, but there’s this piece which is used to give mass to particles in a consistent fashion, and it looks very elegant but we haven’t had any direct proof that it is true. The LHC should be able to give the definitive answer, yea or nay, on the Standard Model’s Higgs.
• Beyond the Standard Model: Even if we find a simple Higgs, there are a few problems with the Standard Model that need resolution. Turns out that with the simplest Higgs solution, there are really very large corrections both positive and negative which have to cancel out just right in order to have a consistent theory. That seems a bit too contrived to sit with many physicists, so they are looking for a way for these things to cancel out naturally by inventing a new set of fundamental particles which are symmetric to the ones we (think we) know. So wonderfully symmetric that they call the theory “super symmetry” which is commonly called SUSY. However, the problem with supersymmetry is that there is a proliferation of new particles and parameters, so our quest to come up with a elegant theory takes a bit of a left turn. It may seem aesthetic, but now we have this nice compact “twelve things which make up everything else” and it is a little disconcerting to suddenly need a bunch more which don’t do anything except fix this one theoretical problem. However, somewhere in here we stray from physics to metaphysics, so I’ll just leave it at that. If SUSY with a certain set of parameters turns out to be true, it is a contender to be the first new insight out of the LHC.
• Non-Discovery physics: this is somewhat heretical, because the LHC is a discovery machine, but there is quite a bit of solid non-discovery physics to be done there as well. The top quark, for instance, is produced at the Tevatron, and they (I’m using the CDF collaboration results as my source, D0 of course has results too, but I worked on CDF, so I know what I’m talking about there) can measure its cross section, mass, properties, and even look for single top production. However, they do all these analyses (probably 50 or so in those pages, and 20 abstracts submitted to the American Physical Society conference recently) with only a few hundred top quark events identified. The LHC will be a “top quark factory” and be able to very quickly increase the statistics of all these analyses by orders of magnitude. Why is it important to pursue “known physics”? Two reasons
1. Sometimes you can see the unknown by precisely measuring the known physics. The canonical example of this is the limits on the Higgs mass coming from the measuring electoweak parameters, especially the top mass and the W boson mass (not surprising, since the higgs couples to mass and these are two of the heaviest fundamental particles we know about). If you plot the W mass versus the top mass, then overlay the potential values for the Higgs mass which keep the theory consistent, you get:
The blobs are what we can say with 68% confidence about the values of the W mass and top mass – there’s a 2/3 chance that the real values lie within the blob. The red blob is what we knew before the Tevatron from other experiments – pretty good with Ws, not so great with top. The blue is adding the Tevatron measurements and the measurements from LEP 2, which focused on W production. The dramatic reduction in area tells you that our knowledge of these two parameters increased dramatically – there are fewer possible pairs of values which make the theory hold water. More interesting is the grey band – what it is saying is “these are the possible values for the top and W mass given that the higgs mass is a certain value, where the top of the band is a Higgs mass of 114 GeV, the current lower limit, and the bottom of the band is a Higgs mass of 1 TeV. The interesting bit is that the blue blob and the grey band don’t overlap! At face value, this means with 68% confidence we say there is no pair of values of (top mass, W mass) consistent with a Higgs mass more than what we know is the lower limit. Pretty cool for just measuring non-Discovery physics.
BUT, my fellow physicists will take me to task for being a sensationalist. First, 68% is not big at all – how many times do you roll a dice and get a 5 or a 6? So we’re not in a pickle yet. In addition, this is just the simplest Higgs model, it could be more complicated, which would modify the picture. But, it does amply demonstrate that you can learn quite a bit about the unknown from accurately measuring the known, which was the point.
2. The other reason for pursuing better measurements of known quantities in addition to “Discovery physics” is the nature of the unknown – you don’t know it! The existence of “Dark Energy” comes to mind – noone was looking for Dark Energy, which apparently makes up 70% or the known energy in the universe, but the WMAP experiment set out to measure more accurately the Comic Microwave Background and suddenly confronted a consequence they had not anticipated-the Universe was accelerating its expansion, which was a surprise to most everyone in the field. This effect was highlighted to me in a recent talk by Professor Ting of MIT, who’s been deeply involved in the field since the mid 1960s, where he just listed several of the seminal discoveries of Particle Physics, and what they were “suppossed to do” which was almost always something quite different. The problem with “Surprise Physics” is it is by nature impossible to forsee and therefore hard to justify, especially to the public and agencies. But it is often the case that when looking for something you find something else that calls into question your fundamental assumptions, and that is where you really gain some understanding.

Whew – so much for the nutshell. Ok, Ken asks about SPIN with capital letters. First, I must say this looks a little crazy. The LHC itself will not rotate, no. However, spin is a tricky subject. First, there’s the normal meaning: a spinning top, or the canonical figure skater, or the Black Hole, is undergoing a rotation about an axis. When they spin that big wheel on The Price is Right (I am a child of the 70s), the $1.00 spot changes its orientation, first on the left, then up, the on the right, then down, but never changes its distance from the center of the wheel. That is the canonical understanding of spin, and I believe applies to Spinning Black Holes, although I am not the expert there. Particle spin is completely foreign to common notions, however. It’s not easy to grasp even with a physics background, but I’ll give the explanation a shot. Consider that we treat an electron as a point particle with no extent, and completely featureless – so if something has no extent, and no “$1.00 marker” to distinguish its orientation, how can we say it has “spin”? What we really mean by “spin” is that we can distinguish apparently two identical particles by an intrinsic quantity having nothing to do with its position or velocity or any given axis but behaves like spin, or more specifically angular momentum. Wait, what does “behave” mean? Well, if I look at the Price is Right wheel from the other side, instead of seeing the \$1.00 marker do “left, up, right, down” I see it go “right, up, left, down”. The thing is doing the same motion, but how I describe it depends on how I look at it, and it changes in a specific way. That’s what we mean by “behave”. So Angular Momentum behaves in a certain way under transformations, and interacts in certain specified ways with for example magnetic fields, and experimentally this other quantity behaves like angular momentum, so we call it a type of angular momentum and label it “spin”.

But wait, it gets more strange. Quantum mechanics here jumps in and tells us that particles can only have certain allowed values of spin, and we divide them up into two groups – those with “half integer” spin, like spin =1/2, 3/2, 5/2, … are called fermions (for Mr. Fermi), and those with integer spin, like spin= 0,1,2,3…are called bosons (for Mr. Bose). But there isn’t any particle with spin = 3/4 for instance. So it isn’t a continuous variable, but it is quantized. Now, for some reason known as the “Pauli exclusion principle” (a principle is something we know is true but don’t know why) fermions cannot share the same state, i.e. two electrons cannot be in exactly the same spin and orbit around a nucleus, whereas two bosons can. Is this important? As my Mom would say, “you bet your boots”. It is exactly this principle which gives us the Periodic table structure. Here’s a picture:

Hydrogen has one proton and one electron, Helium has two protons and two electrons in the same orbit but with opposite spin states. But an electron only has 2 possible spin states, so you couldn’t put another electron in that orbit. That means you have to open a new “shell”, which means next row in the periodic table. In the new shell you continue building with Lithium and Berryllium, one electron in each state for this orbit (as well as for the previous shell) but now you can introduce a new “suborbit” which has 3 potential configurations times two electons for each configuration, giving you 6 new possibilites – that would be Boron, Carbon, Nitrogen, Oxygen, Fluourine, and Neon. This pattern repeats for Sodium through Argon, but in the next level you open up yet another suborbit for Scandium through Zinc, etc. Spin gives you the Periodic table, which gives you Chemisty, which leads to Biology…at least that’s the way we Physicists like to think of it!

Which brings us back to SUSY, from above. The hypothesis behind SUSY is that for every type of fundamental fermion (leptons and quarks) there is a supersymmetric partner which has integer spin, i.e a boson, and for every known type of fundamental boson there is a supersymmetric partner with half integer spin. Given what spin has done for science, something like this could rock the world pretty significantly. So while we won’t spin the LHC, the LHC may very well have something to say about spin.

PS: in true blog fashion, this is stream of consciousness pretty much, so fellow physicists please pardon perceived lapses of reason or explication, or at least try it yourself before you complain!

### In the Maze

Wednesday, January 23rd, 2008

Ah, the ATLAS counting rooms. The destination of many of the cables coming from the detector. It is quite a nice series of rooms. All with raised flooring so that cables can be run underneath. There are a large number of racks with water cooling for each sub-detector. Most of my time underground is spent in the counting room also known as USA15 (which does not stand for United States of America). I can be found usually near the Tile racks on the first level or the Level-one trigger racks on the second level (which is where Tile’s trigger cables are).

But I am not the only person working in USA15. And sometimes the work patterns of the other sub-systems can turn the counting room into a human maze. Suddenly just getting from point A to point B becomes an amusement-park challenge.

Let’s take this particular day last week for example. It went something like this…

Finish tests on the upper level and start lugging heavy oscilloscope back downstairs to the Tile racks. Lo and behold, people are running cable and they have removed a piece of the floor near the door.

No problem. Will use other door on this side. Work self through TRT racks to other door. More people working. Naturally.

Will take back route. Go back through TRT racks, past stairs with intention to go behind elevator wall. Hm. See many level-one people frantically cabling on both sides of the racks. Do not look like they wish to be disturbed.

Go back upstairs. Can’t we get a lighter oscilloscope? Cross the upper level, take the outside stairs down to hallway on the lower level. People on other side of door have one section of the floor removed. Is this a joke? Observe that people behind door are in the same cabling group as those on the right-hand side door. Obviously they are running cable under the hallway. Should have seen this when I first tried to exit on the right-hand side door.

Retrace steps. Back upstairs, then downstairs to level-one cabling people. Apologize profusely as I interrupt them to pass. Stagger to Tile racks to put down scope. Right arm is now completely dead. Will be one-hand typing for the rest of day.

Now must get to elevator. Head back to the level-one racks, interrupt a different set of level-one cablers. Again apologize profusely. Back upstairs, across the upper level, and downstairs. Notice that hallway cablers are done. Both doors now clear. Typical. Head to elevator. Success! Have conquered the maze again!

### Kaffee Klatch (or Cafe Clash?) at BNL

Wednesday, January 16th, 2008

Pam’s recent post reminded me to post about a fantastic development at BNL: our own dedicated “Brookhaven Cafe”, expressly intended to import as much of the CERN model as we can muster on this side of the pond. While it may take years, generations even, to self-generate even a fraction of the atmosphere of drinking coffee and chatting at one of the CERN cafe’s (Cafeteria 1 remains my favorite, although I’ve done serious time in Bat. 40, not having an office at CERN…), it’s already changing the work landscape of lots of people I’ve spoken to (of course as I was sitting there with my laptop, latte at my side). It’s a widely known, but poorly documented, phenomenon that scientists get their best ideas when they stop working for a minute, sit down with a cup of coffee etc, bump into someone else doing the same thing, and chat — leading to new thoughts. Go figure. In this light, a cafe is most likely a good investment in a lab’s future.

What amazes a lot of us is that the whole thing sprang from the minds of actual lab users (at my last official meeting of the RHIC/AGS users group), developed into a serious presentation (which I strung together, and my colleague Carla presented to management), resulted in a labwide poll — and 1.5 years later, we have real espresso and a nice place to sit and drink it.

And it has a little fountain that gurgles in the background. Surprisingly relaxing — and unique to BNL, as far as I know. Hey, these Cafe’s can use some competition, right?– even if they are separated by thousands of miles. From Kaffee Klatsch to Cafe Clash, eh?

### Last piece of the puzzle

Wednesday, January 16th, 2008

We’re nearing the lowering of the final piece of the detector, should happen next week. Today I was out at the experiment, but wasn’t able to get to the cavern (my access expired sometime during my absence without my realizing it – there’s no warning, you’re just supposed to remember I guess) so I got to watch the YE-1 endcap move toward the end of the hall, where it will be lowered down. It is quite impressive to watch a mostly iron disk that’s something like 7 m in radius actually move – there isn’t much else that large that you actually see moving.

Anyway, a quickie, but just another reminder that the time to turn on the beam is looming!

### Doing Physics

Wednesday, January 16th, 2008

Call me a geek but I do believe that anyone who thinks that doing physics isn’t cool has never actually seen physics being done.

The purpose of the Tile Calorimeter in ATLAS is simple: estimate the energy of particles entering the calorimeter. These energies are then used to identify the potentially interesting particles (such as the Higgs) produced at the beam’s collision point. It is such a simple statement and yet so hard to achieve in reality. For this purpose, the Tile Calorimeter is designed as a ‘sampling calorimeter’ composed of scintillating tiles sandwiched between plates of iron. In some ways a sampling calorimeter is a lot like trying to watch a dance through a series of photographic snapshots. The dancers move from one picture to the next and although you didn’t directly observe the movement you can derive what the movements were. The faster the snapshots, the more lifelike the dance.

TileCal is similar except we try to measure a particle’s energy not a dancer’s movement and we do that by measuring the amount of light in the scintillating tiles. The scintillator/iron structure is clearly shown in this picture. For a sense of scale, this is only part of one wedge of the calorimeter and there are a total of 256 wedges. The dark lines between the backlit scintillating tiles are layers of iron. Energetic particles coming from beam’s collision will be slowed down in the iron and will produce light in the scintillator. Without the iron, the particles would not slow down very quickly and the calorimeter would have to be even more humungous than it already is. The small circles at the bottom are where the photomultiplier tubes (PMT) sit. Optical fibers are run from each scintillator to a PMT and the PMT measures the total light.

A wedge with all the fibers connected looks like this. The upper row of fibers is just a template for the technicians to know which fiber goes to which PMT. If the fibers are run to the wrong location, we will be confused as to which scintillator tile is being lit by the particles passing through.

When asked what it means for me to ‘do physics’, I could say, ‘we estimate the energy of particles entering the calorimeter’ or I could show these pictures. They both have the same meaning but I think they leave very different impressions. To me the former sounds pretty boring but the latter looks very cool.

### Look out Congress

Tuesday, January 15th, 2008

Hah! Mom’s on the job!. This may have more implications than one might think. Mom is not only the most energetic person I know (having 8 kids might require that) but she has never been shy about speaking her mind. When I was growing up she was for some time the president of the Wisconsin chapter of Common Cause, a lobbying group for the common man focused on progressive issues in the spirit of “Fighting Bob” Lafollette and Bill Proxmire and his famous Golden Fleece awards, including a study on “How to buy Worcestershire sauce” and money to fund “The Great Wall of Bedford, Indiana”. Anyway, she’s probably got Russ Feingold on her speed-dial. Go get ’em Mom. Any readers (related or not) are also encouraged to contact their congressmen. It’s easy.
In other news, we’ve had blogger-blogger interactions! I spotted Monica in the cafeteria yesterday, and discussed via email with Pam the nuances of the new video conferencing machines out at the experiment. Not sure if Peter is around, but four-blogger scattering is not so improbable.
Being back after a while is good – things are just starting to take off in terms of connecting the tracker, after which we’ll start to test everything out. It isn’t exactly “plug and play”, but after some iteration to get all the saftey, cooling, power and readout systems cooperating, we’ll get there. This stage is always hard because problems pop up all the time which require immediate firefighting, so the immediate plan is always changing. Imagine trying to plan a 10 hour drive when the routes along the way randomly open and close on a ten minute time scale. Not sure exactly how long it will take to get there, but eventually we will. It just takes hard work and considerable patience.

### Driving

Friday, January 11th, 2008

When I came here two years ago I brought my Volkswagen from the states. It was cheaper to bring my German made (one of the last ones) car here than buy a new one. I like my car, it had low miles, was paid for, and was familiar to me, which made adapting to French and Swiss driving habits easier. I also knew I could get it repaired here without any trouble.

When I lived several years ago in Hamburg, Germany, I didn’t need a car. And I wish I could have done the same thing here. There I could get around by public transport or bicycle and not have to worry about insurance, maintenance and repairs. It is a lot cheaper, and if I was in a hurry, I could always take a taxi.

I can’t do that here. I work on 3 different sites at CERN. The experiment is at the red CMS dot on the picture below. Meetings are most often at the main site (next to the red ATLAS dot). CMS is the furthest away from the main CERN site. The other site I visit, Prevessin, is somewhere in between. The LHC ring is 27 km in circumference. In the course of a day, I often have to switch sites. This is a minimum of 8.5 km (I will measure this the next time I do it with my car’s trip computer!). And I take a short cut through the countryside…not the longer truck or bus route! Not that there is a bus.

So some days, when things are not well planned, or something crops up, I make 4 trips. Each one takes about 25 minutes (if I am lucky I make it in 20), unless I get stuck behind a farmer on his or her tractor, or I have to wait for the cows to be moved to the next pasture. Some days, I feel like all I am doing is driving. Those days I have to remind myself what a nice countryside it is, since I am spending about 1.5 hours driving in it, almost always on the same roads, staring at the same cows.

But being the nature loving type, I occasionally get rewarded with a glimpse of something special. Regularly I see grey herons and large birds-of-prey in the fields, an odd fox, and tremendous views of the Alps as I just leave CMS on my way to CERN. Then I am glad I made the drive that day.

A la prochain…