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.”