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Karen Andeen | Karlsruhe Institute of Technology |

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What do you see, what do you see?!?!

Sunday, April 7th, 2013

This is an age old question, always asked (and always fervently!) of the person with the better vantage point: the older sibling peering into the next room through the keyhole; the watchman scanning the horizon from the ship’s crows nest; and now us, AMS, taking our first glance out over the universe from the space station. What do you see, what do you see?!?!

The answer, as we squint through our sights, trying to make out shapes of unknown and unexpected things, is also age-old: “I’m not sure yet, gimme more time!”

The first AMS-02 results– the positron fraction–were announced this week. Sam Ting, our spokesperson and PI, gave the talk here at CERN as per an agreement with the CERN directorate and, now that the result is public we have all been inundated with that same question. And the answer, of course, is: “We aren’t sure yet… just give us more time!”

So I thought I’d come here and give a few answers to some obvious (or not-so-obvious) questions.

If we can’t tell what we see, why are we publishing?

Well I guess I shouldn’t be so glib: we can tell you exactly what we see and how precisely we see it. We just don’t know exactly what it means yet. It’s the scientific equivalent of saying “I see this hazy thing in the fog…it’s shaped like a person, but I can’t tell for sure if it’s a man or a woman. But I can tell that he or she is wearing a green coat, has short hair, and is 5 and a half feet tall.” As experimentalists, it’s our job to say: “we see this thing, it looks like this”. Then we give that information to theorists, who help explain our observations. This happens back and forth: an experiment has results and publishes them; the theorists look at the results and publish new ideas that might explain the results; the experimentalists then improve their observations to test the new theories and publish their new results; the theorists revise their ideas or come up with new ones to explain the new results… and so on as we refine our understanding of what we see compared to the physical principles we believe to be true.

With AMS-02 we are at the first moment where we are ready to report what we see, and it may or may not point to new physics. So now we publish our results so the theorists can have a look and think about what this might mean. In a few years we hope to publish more results that will probe higher in energy (but it will take awhile to collect enough data to be able to do this reliably), and between now and then the theorists may have new ideas for other things we should look for.

What exactly is it we’re looking for?

Well, the latest round of similar experiments—FERMI and PAMELA—strongly indicated an excess in the number of positrons flying around the universe. This excess might be called “mysterious”, but we do have theories about where it could come from. Many of these theories invoke something with an even more mysterious sounding name: Dark Matter.

Dark Matter is so called because we don’t know what it is. Based on our observations of the universe, we are pretty certain that there’s some type of matter out there that we haven’t been able to identify yet, and we think it makes up around 25% of the matter in the universe. (I’ll let you read about this on your own—how stuff works has a pretty good summary )

The trick is that we can’t see it because this Dark Matter doesn’t interact like normal “bright” matter that we see every day (“bright” matter would be like your hand, the desk, the wall, the sun, the stars, etc.–basically anything that glows on its own or anything that you can see by shining light on it). We can’t see the dark matter just by turning on the lights, but we hope that we can see it indirectly—this means that instead of seeing the dark matter itself, we might see its byproducts. It’s like when you go white-water rafting: you can’t see the boulders hidden under the water, but you can avoid danger by watching for the ripples on the surface. Because of the ripples, you know the big boulder’s there, even though you can’t see it. And if you’re a really experienced paddler you might be able to tell me something about the boulder’s shape and size based on the pattern of the ripples. So what AMS-02 does is look for these ripples in the water, and from our collective experience we try to figure out what’s beneath them.

We do this by looking for things that we think should act in a certain way for normal bright matter, and trying to detect something anomalous about the way these things actually behave. In this case, we are looking at the fraction of positrons vs. the total number of positrons + electrons. We think that if dark matter doesn’t exist this fraction should be around 10% and decrease slightly with energy. (For those of you who aren’t familiar with this terminology, energy can essentially be seen as how fast a particle is going. For particles that are all traveling at “normal” speeds, about 10% of them should be positrons. And of the particles moving twice as fast, maybe only 8% should be positrons.) However, according to many theories, if dark matter does exist, it will cause an increase in the number of positrons—which we would see as a bump at a certain energy in the fraction of positrons (where exactly the bump occurs depends on which theory is your favorite). Here are some predictions made two years ago about what AMS “would” be able to see after both 3 years and 20 years of data collection compared to the predictions from a few different Dark Matter models.

So the short answer is, we are looking for a bump in the positron fraction energy spectrum because it’s possible that the bump’s location and shape could indicate the presence of dark matter. Though it might indicate something else entirely…it’s tough to say for sure. Either way, a bump would indicate something different from what is currently expected.

So what do you see, what do you see!?!?

Well, you asked for it: We aren’t sure yet… just give us more time! But, to be serious, what we see is an increase in the positron spectrum at high energies. This plot shows our main result (we’re the red circles):

Our new measurement is similar to experiments past, but extends the energy range of the observations, and measures the positron fraction much more accurately (you can tell this because the little vertical bars which indicate the measurement error are smaller). Notice that most of the experiments agree: first you see a decrease in the fraction (on the left side of the plot), which is what’s expected. When you get to the right side, the fraction begins increasing again. If there was a bump–due to dark matter or something else–then you’d see the fraction start to fall again.  So if there is a bump, it’s right half appears to be somewhere off of the right edge of our plot… but on the other hand, maybe that’s not a bump–maybe the fraction will just keep increasing. To really see what’s going on here, we need to continue to take more data, which will allow us to plot the positron fraction out to higher energies (ie extend the right side of the plot).  To get the full picture, though, we will need to look at many more things–not just the positron fraction.  It will simply take some time before we can give you a good answer.  So far, we have only strong hints of something unexpected happening that is not well understood.

Did you actually get to work on this?

Yes, in fact.  I was really lucky to join AMS at exactly the right time: they let me help with the very first results.  My role was somewhat minor, but it might be interesting to relay how this result was achieved.  You see, most experiments have a cross-check–either another experiment, or two independent analyses.  Well, this experiment doesn’t have a matching experiment to cross-check (like ATLAS and CMS at the LHC) so instead we split our collaboration into two teams: A, and Alpha.  The teams were split by country to be approximately equal in size: A-Team was Italy, Germany, Portugal and Turkey while Alpha was the rest of the world.  Since I work for a young investigator group at Karlsruhe, in Germany, I was on the A-Team (fulfilling a childhood dream).  The teams didn’t communicate with each other at all until collaboration meetings once every month or two, when we would compete to try to show the best new results.  At that point, we’d see the other team’s progress, and then try to catch up or somehow be more clever.  Alpha took the strategy of putting all focus on one analysis, while A-Team separated into groups and did the analysis using five different analyses techniques.  (My analysis was one of these.)  When all of us were finished (after many sleepless nights), we discovered that all of our analyses matched.  This was a huge relief, since when you do something six different ways, you’re never sure you’re going to get six matching results.  This is how we gained the confidence required to publish our findings.

Why aren’t we publishing more points at higher energies?

If you are asking this question, you’re in good company. Almost every question from the physicists in the audience at this week’s announcement was some incarnation of this very question. The answer is that these data points are still a bit foggy. This explanation clearly didn’t satisfy the curiosity of the physicists, who seemed to ask in unison, “But don’t you have some idea? We know you must have data out there at higher energies, why won’t you give us even a preliminary result out there?”

I’ll leave you with Sam Ting’s answer: “It took us 18 years to build this detector. I think in the next 20 years [that it will be running on the space station], no one will be foolish enough to repeat what we’ve done. We want to do it correctly.”

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particle physics…in space?

Monday, March 18th, 2013

I’m thinking that for the first post on this blog, maybe I should tell you a story.  Fortunately I just picked up a new little snippet about how AMS and NASA first joined forces.

The other night I was sitting here on shift in the AMS control room (called the POCC—Project Operations Control Center…I think).  I was wearing a big warm sweater and trying not to fall asleep, much like I am now (note the dark circles—yep, it’s 5AM!):

Much to my surprise at 2 AM someone walks in the door.  Normally (provided nothing horrible is happening) this place is a ghost town at 2 AM…it’s me and one other person quietly clicking away on our laptops.  So anyway, this guy walks in and I realize he’s one of the NASA guys from Houston whom we have a video conference with every Wednesday at 5PM our time.  Turns out he’s just arrived from the US and is totally jet-lagged.  He couldn’t sleep, so he came to the control room to see what’s going on.  I introduced myself, and he wanted to know how I’d come to be here (since I’m quite new).  Through the chit-chat it comes out that he’s been part of this project from very near the beginning—1994, I think it was.  And he happened to be in the meeting where Sam Ting first brought the idea of AMS to NASA.  Now, I had always assumed that this project was some sort of mutually beneficial agreement from the beginning, but it turns out it wasn’t quite that easy. He said it went like this.

Here they all were at NASA, and in comes Sam Ting (now the AMS spokesperson), and he says: I have this great idea.  I want to build an incredibly precise, amazingly delicate, super awesome particle detector.  (Emphasis on precise and delicate–he was trying to sell it, remember, and in particle physics those attributes are considered a bonus.)  And you guys are going to launch it into space and put it on the station for me.

OK, now I don’t know if you realize this, but if you’ve ever watched videos from the launches or if you ever saw a movie with a space shuttle taking off (Apollo-13 comes to mind ) you’ll know that launching is not a cake walk.  It’s complicated, stuff sometimes goes wrong and, moreover, there is a ton (actually ~2000 tons) of shaking, vibrating, jostling…it might be like your average airplane landing in a hurricane, with hail, times 1000.  Here is how two astronauts described it:

In the space shuttle, astronauts are strapped in on their backs a few hours before launch. As the main engines light, the whole vehicle rumbles and strains to lift off the launch pad. Seven seconds after the main engines light, the solid rocket motors ignite and this feels like a huge kick from behind. The vehicle shakes a lot and the ride is rough for the first two minutes as you are pressed back into your seats with twice your weight. When the solid rocket motors burn out there is a big flash of light as they separate from the big fuel tank the shuttle is strapped to. Then the ride [smooths] out. As you get higher into the thinning atmosphere and burn off most of the fuel, the vehicle accelerates faster and you are pressed back into your seat with three times your weight for the last two and a half minutes of the ride. This two and a half g’s feels like a giant gorilla is sitting on your chest making it more difficult to breathe. Eight and a half total minutes after liftoff, the main engines stop and immediately you go from the being squashed by the gorilla to being weightless.

In the Soyuz (Russian Space Capsule): Shortly before the time of launch you start hearing different noises below you and you know things are getting ready to happen. Then, it is as if a giant beast is waking up. You hear and feel the thumping and bumping of valves opening and closing as engine systems are pressurized. When the first engines light there is a terrific low frequency rumbling and things start to shake. Then the main engine lights and the rumbling and shaking get even louder. Slowly, slowly you begin to move up and away from the launch pad. But, very quickly you build up speed and the g-load, or the force of gravity or acceleration on a body, increases. You shake and rattle along and then there is a bang when the rescue system is jettisoned, another bang when the four strap on boosters separate, and another bang when the nose faring comes off. Now the windows are uncovered and you can see light coming in. At the second stage separation there is another bang and the g-load drops immediately. You go from about four and a half g’s down to about one and a half or two g’s. Then the third stage engine lights; you have a big push forward and the g-load builds again.  Eight and a half minutes after launch there is a loud bang and jerk and the last section of the rocket is jettisoned from the Soyuz spacecraft. And just like that, you are there–in space. It feels like you are hanging upside down in your shoulder harness. This is simply because there is nothing pushing you back into your seat anymore. Everything floats, including you.

So now Professor Ting wants to take this extremely precise and delicate (and also quite expensive) equipment…and launch it?

And that’s just getting there.  Once you’re there, you have to deal with changing conditions all the time.  The most shocking example is the temperature: if there were no temperature control, the difference between the sunny side to the dark side of the station would be up to 500 degrees Fahrenheit!  (Remember, space itself is cold, but the sun without any atmosphere for shielding—super hot!  (You can read about how they keep the astronauts from becoming fried eggs.)  Particle detectors do not like temperature changes.  Any temperature changes.  At all.  500 degrees?!?!?

Not surprisingly, the NASA folks thought this idea was nuts (to put it politely).

But somehow, throughout history, the best physics ideas have always sounded crazy, so keep this in mind next time some poor physicist comes begging for money or attention, sporting a big ego and bartering an idea that sounds “impractical”, “unfeasible” or just downright “impossible”.  Be careful, because if you tell that person what you think of their idea, you may very well find yourself eating your words.

Fortunately, NASA clearly has some experience dealing with bizarre-o ideas.  They played it well…here they were, thinking this project was absolutely crazy, never going to work.  But, you can’t very well tell someone who’s got a Nobel Prize in Physics that you think their idea is…well, not quite what you might call…possible.  So they told him he’d have to build his detector within all kinds of really stringent (but 100% necessary) specifications, and if he managed to do this, then maybe…if the money worked out…they would consider launching it.  He said OK and, somehow, after 19 years of sleepless nights and days spent arguing and sweating for an idea that everyone thought was impractical, unfeasible and just downright impossible, the dream has been accomplished: we’re up and running…in space.

Hello world, this is AMS, reporting for duty.

(I just bet Sam Ting goes to sleep with a little smirk on his face every night:  Ha, told you so.)

So there you have it.  If you want a good story, go sit in a control room.  Control rooms may look awesome with all the flashing lights and monitors, but they tend to be rather dull places (that is, until something horrible happens and panic ensues).  This means that people have time to kill and out come the old stories.  (Seriously, who needs a campfire anymore when you’ve got a control room?)  I think it’s important to capture these stories—the stories of how the great experiments came to be, the trials and tribulations of the generations before…they remind us that science is more than just a numbers game—we are, all of us, human, and subject to all of the follies and foibles that that implies.

Let me leave you with my favorite NASA video, which I think is a good reminder of our humanity—especially if you notice how thin the atmosphere really is!



 

 

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