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Zachary Marshall | USLHC | USA

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Meta-Blogs and New Physics (?)

Monday, May 24th, 2010

Hi there!

First, hello to the MIT Tech Review who made me famous for about 15 seconds this month. That’s one of my Facebook comment replies Singhal is pointing at (below) – and hi there Facebook users! It struck me when I saw that how strange the spread of information is, and how much information we read in a heavily digested format in a typical day…

Meanwhile, the LHC is running as normal. No news is good news, for the moment. If you want to hear about some (possibly) exciting physics, I highly recommend the NYT article on a possible hint of new physics from the Tevatron. They do a pretty good job explaining things there, so I won’t confuse you any further. There does seem to be something to it – and now the race is on to find what’s out there at the LHC!! Physicists or the very brave can read the full article here. Notice that their result is “evidence,” not “discovery” (or “observation”). That means they’re 99.7% sure the result is new physics – to claim discovery, they’d have to be 99.9999% sure (ok, there needs to be a little more mathematical definition of “sure” to make that precise, but I hope it gives you some idea of what I mean).

Note: Fixed my evidence / observation mix up. Thanks for the careful reading…



LHC Physics News

Monday, May 17th, 2010

By special request, here is an update on how the physics and machine are doing. Hope it helps!

Any results or new physics hints?

We don’t yet have enough data to see much new physics. Unless it’s something really strange, we expect only one “interesting” event for every thousand million collisions we see. We’ve seen hundreds of millions of collisions now, but it’s unlikely that we’ll say anything definitive until we’ve seen quite a bit. You can see my previous posts about being careful and errors for some of the reasons why.

As for the results we have seen, we’re progressing well. I liked the way one of my collaborators put it: a march through the history of physics. We’ve gone through the ’50’s, ’60’s, and some of the 70’s. We’ve gotten some of the ’80’s done as well. We probably need ten or one hundred times more data to get through the ’90’s. After that, the sky is the limit…

Why is the machine going so slow?

Truth is, the LHC is doing great. It feels as though every week we collect as much data as we had collected up to that point. Of course, the exponential growth won’t go on forever. But there are quite a few things that we can still do to get a lot more data out of the machine. In numbers: we are around one half of one thousandth of one percent of the “maximum” collision rate in the LHC. We know very well how to get thousands of times more collisions per second – but we want to go slowly. This machine has to be around for 20 years – it won’t do to get hasty and have an accident at this point! We can’t just go buy another one!!!

If you watch the LHC Page 1 obsessively, then you’ll see a lot of down time. It turns out that, if you’re in Europe particularly, it’s self-selecting. That is, when the experts are there during the day, we’ll spend a lot of time doing tests. At night, they often have stable runs (by “night” I mean midnight to 8am). But it’s those tests (during which we don’t have “stable beam,” and sometimes we don’t even have beam) that give us the confidence to raise the collision rate by quite a bit. Tonight, more tests – so that we can get a higher collision rate!

So when will we know more??

The collision rate will continue to increase, and I hope we hit the “magic number” – 100 inverse picobarns of data – around the end of the year. I say “magic number” because that’s around the time when we’ll start to really beat previous machines like the Tevatron in Chicago. To some, that’s when the fun starts – when we start looking for new physics, and we have reach well beyond any other machine in history.

We release a lot of results at conferences. The summer has several key conferences, so I fully expect several results from each experiment at every conference. They probably won’t be “discovery” results, but they will be the first key physics results. They’re exciting to some physicists, but, frankly, many will consider most of these “ho-hum” results (unless one of us does find something new!). Everyone recognizes how important it is to get some results, and what attitude is being expressed. We’re releasing physics results less than six months after the machine started! There are experiments that have taken years with their data before releasing any results!!

And as the exciting results roll in, we’ll keep you up-to-date here, as much as we can!



I Wrote That!

Tuesday, May 11th, 2010

This is one of the things most confusing to folks outside of high energy particle physics. Our papers have 3000 authors. The ATLAS author list is about 17 pages long, depending on the formatting. Sometimes there are even fewer than 3000 words in the paper – surely we aren’t suggesting that different people cross t’s and dot i’s?

Well, what does it mean to be an “author” of a paper? In our case, it means that you made a contribution to the work described in the paper. Of course, if you built part of the detector, and that part of the detector is used in the analysis, then you should be a co-author on the paper! And if you were responsible for running that part of the detector during one of the critical times, or calibrating it so it would work, then you should be a co-author! And if you wrote some of the software (ATLAS software, at least) that was used during the analysis, then you should be a co-author on the paper! And if you derived some of the numbers that they used, even though you haven’t written a paper on your numbers yet, you should be a co-author on the paper! ….. The list grows pretty fast! In practice, it’d be almost impossible to try to select the correct several hundred people for an authors list. So instead, ATLAS keeps a running list of all the people who have contributed substantially to the experiment (roughly the list of people who have been members for more than one year) and uses that as our authors list. There are some stickier points to this – trying to draw a line between software we “develop” and software we “use,” for example.

Here I should add one category that is, perhaps, less obvious: graduate student advisors. As a graduate student, you don’t have a reputation in the field yet. Your advisor’s signature asserts that they vouch for the correctness and appeal of the work you’ve done. So I would say an advisor should always sign their students’ work – even if they have not been deeply involved in the paper.

This mess can lead to all kinds of confusing conversations. You keep a list of papers on which you were listed as an author and another list of papers to which you “significantly contributed.” And if you show those lists to anyone outside physics, they’ll say, “Wait, you didn’t contribute to these papers, but you’re an author????” I must admit that I have not read every word of all the papers that I’m a “co-author” of!

Because there are different practices in different fields (or even different experiments!), we can get into tough spots as well. Some fields list “major authors” first. Or they list graduate students first. Or they rotate the authors’ list alphabetically, so that one person is the “first author” on every paper. We don’t tend to do that in ATLAS, so it’s tough to tell who really wrote the paper.

It can be frustrating when you’re applying for a job – imagine having to explain which of the papers that you “co-authored” you actually wrote! – but I confess it’s kinda fun to have a list of publications as long as your arm…

One other note. Everyone has probably heard of the “six degrees of separation” game. You can play the same game with paper co-authors, and thanks to this notion of authorship the connections spread fast! In mathematics, it’s called the Erdos number after Paul Erdos. With physicists, there’s some debate over who should be the Erdos equivalent (my nominee is John Ellis). You can also play the game with academic genealogy. In fact, I can trace the Italian side of my academic genealogy further back (1860’s) than the Italian side of my family genealogy (1880’s), and they both go though Pisa! I leave you with the XKCD take:



Errors Not in Your Favor

Sunday, May 9th, 2010

It’s something we physicists don’t talk about much: what happens if we’re wrong? I told you a little bit about being careful about what we say and why it takes so long to say anything before.

Success Error?

Almost no complete analysis is done perfectly on the first try. Physicists make mistakes just like anybody else, whether it’s simply a bug in our code or something more serious. The problems that we encounter are usually the most interesting part of the work, though! When I’m grappling with understanding some physics, I feel less like a code monkey.

We show each other results constantly. I probably show various people several dozen plots each week. Some of them make sense, and I’m showing the plots to share information. Others I show because I don’t understand what’s going on. Inevitably, one or two of them are because of my own mistake. Sometimes it’s something more than that.

If you’re lucky, you get to catch a problem that the entire collaboration has missed so far. That doesn’t mean people were lazy!! It means you looked in a particular spot that (perhaps) hadn’t been looked at before, or you looked more carefully, or another issue which had prevented others from looking until that point was solved. Great! You have earned the difficult task of convincing everyone that you found a real problem (it has to be confirmed by others), and once you’re done everybody will benefit from the solution.

Once a result is ready for publication, the real grilling begins. You get a lot more attention, and the smart folks in the collaboration may point out issues that you hadn’t thought of yet. It’s great as a student to find out what the more experienced members of the collaboration see, and what they consider serious. A large number of problems still get caught at this stage, when you look at things you hadn’t looked at before.

Next comes writing up the paper with your results. Often there isn’t much room in the paper for explaining problems that you might have had. I think it’s a shame, because the problems in the analysis are sometimes the most interesting part! And those are the things that people in the future can really learn from. Often, some of the problems will be explained in a talk about the analysis at a conference, or in a student’s thesis. But those are a bit harder to track down for the next person who comes along.

Once your paper is all set, it’s sent to a journal, and the journal asks a few people to review the paper. In the days when collaborations were much smaller, this was critical – it was the way to do quality control for all the physics results on earth. With 3000 member collaborations like those at the LHC, I’m not so sure that it’s as important. Still, sometimes there are real concrete questions raised during the review. Once that’s all done, there’s still a little time before the paper is really published.

Finally the thing goes out, and people start reading it. If one of the readers find a problem (which could happen), then there are a few approaches to fixing it. Most journals accept “errata.” Those are short notes saying, “this thing we wrote about wasn’t quite right.” One big problem with errata is that people search for, find, and cite the original articles – and they don’t always find the errata!! So they might be reading or citing the wrong version of your paper. Some electronic journals will accept a new revision, so that you can make the change in place (a bit safer). And for papers on the arXiv, you can upload a new revision.

These are frustrating, though! The paper’s been reviewed so many times, catching a problem after all that is really disappointing.

Then there are the hard, philosophical ones, that I will leave you to think about. Let’s say you are relying on someone else’s number. Some group somewhere has calculated something useful to your work, and you used their calculation in the paper. And let’s say they find out their calculation was wrong. Now you know your results are wrong as well, but it’s not your fault!! Is it worth publishing an erratum? Putting a new version of the paper together? Holding the publication while you go back and fix things? Even if it was in its final stages? Tough choices, particularly when you want that paper out….




Saturday, April 17th, 2010

Hi there!

Thought I’d put up today’s PhD comic, about research budgets in the US.

Budget Comic

Often we’re asked to justify the cost of our research, but it really is a pretty small fraction of the Federal budget. Although we often hear things like “The LHC cost $5 billion US”, that cost is spread over many years and is split between many countries. Anyway, thanks Jorge for the nice demonstration!



Rediscovery! (kinda)

Saturday, April 10th, 2010

You’ve heard from us a few times that the first thing ATLAS (or CMS) will have to do is “rediscover” the physics that we’re pretty sure is there. ATLAS made a big step in that direction this week with the identification of the first W boson candidate events:

First candidate W boson decaying to a muon and neutrino

First candidate W boson decaying to a muon and neutrino

First W boson candidate decaying to an electron and neutrino

First W boson candidate decaying to an electron and neutrino

This is a pretty big deal for us. I think Flip will tell you more about the weak force in his next blog, but here’s the very quick version. A W boson can decay to an electron and a neutrino, or a muon and a neutrino, among other things (we have one of each!!). The electrons are those things that orbit a nucleus in an atom. A muon is, basically, a cosmic ray. They go through your body constantly (probably several thousand will have gone through you by the time you finish reading this sentence), and are one of the things that help evolution by kicking around your DNA. Neutrinos can’t be detected by ATLAS or CMS. They fly right out of the detector, completely unnoticed. Actually, they keep flying off into space. Neutrinos are produced by the billions in the sun, and several million will go through your body as you read this. They don’t do any harm – they basically do not interact with your body at all.

So what does a W boson look like?? We don’t see it directly – it decays too fast. We see the electron or muon in our detector, and we can measure that thing’s momentum. We see most of the other ‘stuff’ in the event, and add it all up. Once we’ve done that, the event doesn’t balance. The next trick comes from Newton: for every action there is an equal and opposite reaction. The protons come into ATLAS going East-West. If something comes out going North, then there must be something that comes out going South as well! That’s how we can “see” the neutrino. We look for what’s missing when we add up the rest of the event.

In both of these events, there is some missing piece (the red dotted line) and an electron (in yellow) or muon (in red). We know that’s what the W boson looks like – they’ve been seen many times at LEP and the Tevatron. So if we guess that the missing piece is a neutrino, and that the neutrino and electron/muon came from the same particle, we can check what the mass of the particle was. And if that mass comes out close to the mass of the W boson, then we can say that this was probably an event with a W boson in it. It could have been something else – we can’t be positive – which is why we call it a “candidate.”

Why is this a big deal? Well, we only expect one W boson for every million events!! So that we managed to pick this up so quickly is a great sign for the way our detector is behaving!! At the very least, it’s a great first step!

The next thing on the list is the Z boson (I’ll leave that to Flip), and then the top quark after that. And probably in the meantime we’ll find some good high energy “jets” (quarks and gluons). Once we have all of those down (or maybe even a little before), you may start to see limits – or even discovery – of new physics!!!



Being Careful

Saturday, April 10th, 2010

Physicists try to be very clear about what they say (believe it or not!). If we claim to have “discovered” something, then millions, or even billions, of dollars could be put towards studying it. We’d better be sure!

Here are a couple of nice pictures we can talk about. Both are taken from the Particle Data Group. First is the neutron lifetime – how long it takes a neutron (with protons, neutrons make up the nuclei in every atom in the universe) to decay. Second is the W-boson mass – a boson that is a part of the “weak force” that controls some decays of nuclei, for example. And both of these are measurements as they have evolved with time.

Neutron Lifetime

Lifetime of the neutron

W boson mass

The mass of the W boson

You can see why I picked these two. It looks like the first measurements of the neutron lifetime were way off! And, contrarily, it looks like the W-boson mass measurements might even be too good! Either way, it is satisfying to see error bars on all these measurements. That part is really important! It allows the possibility that you’re wrong.

It’s a little easier to talk about this in terms of politics, since we’re all pretty familiar with “polls.” Take the latest Zogby poll that reported that “48.8% +/- 1.7%” (read 48.8 plus or minus 1.7 percent) of likely voters approve of the job President Obama is doing. In polls, that 1.7% error usually only reflects statistics (polling 10 people is less accurate than polling 1000). There are also systematic errors, like the differences in population between those responding to your poll and those voting, possibilities that people misunderstand the question or even lie when polled, and so on. Those are really important to include, although they’re really hard to justify sometimes – how much do you trust your work? But that “48.8 +/- 1.7%” really means one of two things, depending on your philosophy:

  • If you repeated this poll 100 times, 65 times you would get a number between 47.1% and 50.2%.
  • We are 65% confident that the “real” answer is between 47.1% and 50.2%.

When ever we physicists claim to discover a new particle, for example, we require that it be outside the expected error bar by at least five times the error bar’s width (called five standard deviations or five “sigma” – one “sigma” is 1.7% in this case). In other words, we would only have “discovered” something with this poll if we had predicted approval above 57.3% or below 40.3%. Three sigma is often called “observation,” two sigma is often called “evidence.” And we usually choose to consider something new “excluded” if it is ruled out by three sigma. In the case of finding a new particle, for example, we might expect to see 6 events that look such-and-such a way, and we could claim “discovery” only if we find more than 36+/-6.

This sounds complicated, but it’s all to ensure that we are very confident about what we’ve seen before announcing to the world that we have discovered a new particle! If you trust your error bars completely, five sigma means the chance we’re wrong is 0.00006%!! And this is also what we spend a huge amount of our time on: making sure those error bars are honest!

Next time I’ll talk a bit about what happens if we’re wrong!


Hurry Up!!

Tuesday, April 6th, 2010

Preface: the folks over at the ATLAS control room blog are doing a great job too, so don’t miss their posts if you want more up-to-the-minute LHC news!

A lot of people ask me when we’ll start announcing discoveries of new physics (or exclusions of new physics). It could take a while. Even I don’t like the idea of waiting long before the really interesting new physics at the LHC gets published. So why does it take so long?? Well, for fun, let’s say ATLAS has found a new particle. And let’s say that you are the one who gets to say when the result is made public (in reality, that responsibility is shared among several people).

It's not quite that fast...

We have to write a paper about the discovery!! This is our chance to explain, clearly and carefully, what makes us think we have found something new. When the entire collaboration is happy with the paper, we send it off to a journal. The article we’ve written is then reviewed and, we hope, accepted in the journal. Finally, if all went well, the journal publishes the paper. If everything went perfectly, the review would all take a couple of months. No problem!

But it rarely goes perfectly. Usually, we find problems along the way, and those problems have to be corrected. And because it takes so long, the results are often presented in some “preliminary” form at a conference before they have been published. That’s one of the reasons we like going to conferences so much (and we get to go to some pretty sweet places). All those results get marked “PRELIMINARY” in big bold capital letters, because they aren’t published yet. There might still be some problems that shake out in the review of the work.

So now comes your job…

  • How long before work is ready to be published do you allow it to be seen outside the collaboration? If you wait too long, someone else might announce the discovery before you do! But if you move to quickly, then you might announce the discovery of a particle that doesn’t exist!! Both have happened a few times. It’s also hard to keep secrets (physicists like to gossip!!), so you may tip off someone else on where to look before you’re ready to announce your findings!
  • Who makes the announcement? Is it the person who did the analysis, and so knows the details best? Often that’s a graduate student or post-doc, and we like to protect them from the fallout if it turns out the announcement is wrong. Does the spokesperson make the announcement? Does the “physics coordinator”? The coordinator of the physics group that included the work?
  • In what journal do you publish the work? One of the most famous in physics is Physical Review Letters, but articles are not allowed to be more than four pages long. You could publish in a less well-known journal so that the page limit goes away – but do you need to publish a long article first?
  • The result might be more interesting if you can present it in terms of some particular theory. But do you want to share the information with a person outside of the collaboration? The information might get out – and long before you are ready for it to! (Of course, in ATLAS, we have quite a few experts in various theories, so this may not be an issue…).

In the past, there was some added pressure to publish first so that you could be recognized with a Nobel Prize. In many previous experiments, there was some person taking the lead who could be recognized (though in some cases that is debatable – or even resulted in picking the wrong person). Maybe I’m wrong, but I don’t know of any obvious experimentalist to get it for a discovery at the LHC. How can you leave out the people who designed and built the machine? Or the people who designed and built the detector? Or the people who did the analysis? And no more than three people may receive the prize!! So the Nobel Committee will have its work cut out for them…

Good luck!!

Note: I’ll try to follow this with a blog about what happens when we’re wrong…


Discovery and the Ocean of New Physics

Thursday, March 25th, 2010

Hi there!

The LHC is about to start colliding protons at 7 TeV (3.5 TeV per beam) or with about three times more energy than has ever been achieved by man. This is really exciting stuff! We’ll have a big media day on Tuesday to make sure everyone has a front row seat to the event!

Last night I was asked an interesting question – interesting enough that I thought I’d share the answer.

Will we collide the beams at many energies, or only at 7 TeV?

The LHC is really a discovery machine. Imagine that you’re back in 1490, and we’ve built a new ship that is capable of going seven times further than any previous ship in history before it needs to land (to refresh its stores, etc). The first thing we want to do with this is take it as far as it will go to see what’s out there! It could be that we’ll find a whole mess of new particles – that would be wonderful! It could be that we find nothing at all. That really would be like Columbus sailing for the new world and coming to the edge of the earth! It seems impossible and would fly in the face of everything we know – for physicists, it would be almost as interesting as finding a load of new particles! And, of course, when you’re sailing that far, you might pass some interesting things along the way…

End Of The Earth Images

Once you’ve searched for new high energy physics, you might want to try collisions at a few different energies. That would be something like looking for islands in the Atlantic. They might not be as exciting as a new continent, but they’re worth searching for all the same. Technically, just like if we were to sail out, we have no choice but to pass all the energies in between. But we do so for only a moment, and don’t really pause there to do any significant search in the middle. Side note: the middle energies aren’t quite as exciting at the LHC as they would be at, say LEP, because protons are “composite”, rather than single objects (as one professor put it, it’s like colliding two garbage cans). You can get some sense of what’s going on at lower energies from the higher energy collisions.

So on Tuesday, you should expect to see the beams go up from 450 GeV each (when they go into the machine) to 3500 GeV each (when they are colliding) without stopping in the middle – unless they plan something I don’t know about, of course!

One fun (very) technical note. Computers have a clock that keeps them in time (your computer is probably 2 GHz, for example). The whole LHC acts like a giant set of computers, all of which are timed together. It’s as though the entire thing has a single heart beat, around 40 MHz. We actually keep the heartbeat going at just the right pace to always have collisions “on time.” But the protons are changing energy from when they are injected at 450 GeV to when they collide at 3.5 TeV!! That means the entire heartbeat of the machine speeds up just a little bit to keep up (because of relativity, it’s only a fraction of a percent, but it is noticeable!!). To make sure things are safe, ATLAS usually stops collecting data while the heartbeat is actually changing – it’s a delicate operation, and we don’t want to have to stop to fix something right before the data arrives! So we may or may not actually see any of the collisions at energies between 900 GeV and 7 TeV!




The Final Countdown

Tuesday, March 23rd, 2010

Hi there!

This morning there’s been a press release from CERN announcing March 30th as the first attempt for collisions at 7 TeV! You can still follow CERN or ATLAS on Twitter for all the action…

A little reminder – these collisions will be 3.5 times higher energy than our competitors at the Tevatron, and 3 times higher than we reached at the end of last year (when the last record was set). Because of the higher energy, we should be able to quickly match the Tevatron’s sensitivity to “new physics” – what ever that might turn out to mean…

If all goes well, this will mark the start of a very exciting year in physics!!

Will things ever be the same again?? It’s the Final Countdown!!