Blogs / Cosmic Variance

09:37 PST: Like many of my colleagues, I’ve been eagerly awaiting word that the LHC has successfully threaded the proton beam around the whole ring. In recent days they have gotten it half way around the 27 km circumference, and within hours, they should be able to circulate it and I assume “capture” it with the RF, which creates stable bunches in the synchrotron. Everything has gone very smoothly to this point, so I expect success shortly!

Once beam has circulated stably in both rings, some time next week the LHC team will attempt to collide protons at the injection energy of 450 GeV (a total center of mass energy of 900 GeV). While this is much less than the Tevatron is colliding presently, it could provide some sorely needed initial data for the detectors to do timing and calibration of the various subsystems. There will even hopefully be a few collision events recorded with clear “dijet” structure – collisions where quarks and/or gluons inside the protons hit head on and effectively bounce sideways into the detector, giving two back-to-back collimated sprays of particles. Pictures of such events will be great to see, at long last!

You can follow progress live on twitter: http://twitter.com/cern and I will update this post as I learn more.

10:32 PST: The LHC has gotten beam around clockwise, to Point 6! Woo hoo!

10:45 PST: Magnet quench – should be recovered soon…

11:25 PST: Beam has reached Point 7!

11:30 PST: Point 8! Next beam will be sent past Point 1 where ATLAS is…

11:39 PST Beam all the way around the ring! WOO HOO!! It’s baaaaaack! The LHC Page 1 display shows that the injection probe beam made it more than once around the machine:

11:54 PST: Next goals: do the same with the counterclockwise beam. Will they attempt RF capture tonight? Trying to find out…

13:11 PST: Turns out (no pun intended) they decided to go for RF capture of the clockwise beam rather than probe counterclockwise. They are up to 10 million turns with the RF on! Fantastic!

13:30 PST: Having captured the beam for several minutes, the LHC will now switch to counterclockwise.

14:53 PST: About to go for a full orbit of the counterclockwise beam…done!! Now to RF capture!

15:30 PST: Counterclockwise beam is RF captured! The LHC is operational…colliding beams within a week? Stay tuned.

You know, other people talk a lot about time, too — it’s not just me. Here’s a great video from Nature, featuring a conversation between David Gross and Itzhak Fouxon about the existence of time. (Via Sarah Kavassalis.) Itzhak plays the role of the starry-eyed young researcher — he opens the video by telling us how he originally went into physics to impress girls, although apparently he has stuck with it for other reasons. Gross, of course, shared a Nobel Prize for asymptotic freedom, and has become one of the most influential string theorists around. David plays the role of the avuncular elder statesman (I’ve seen him be somewhat more acerbic in his criticisms) — but he’s one of the smartest people in physics, and his admonitions are well worth listening to. He gives some practical advice, but also advises young people to think big.

Unfortunately the video doesn’t seem to be embeddable, but you can go to the video page and click on the “David Gross” entry. (The others are good, too!)

You all know my perspective here — time probably exists, and we should try to understand it rather than replace it. But I’ll agree with David — let’s not ignore more “practical” problems, but not be afraid to tackle the big ideas!

I had that somewhat rare experience two days ago, getting the message from Physical Review Letters that our paper had finally been published online. In our field it can take quite a long time to get a paper all the way to publication; this one took longer than usual…

The paper describes the results of our search for Higgs bosons predicted in supersymmetric theories, in the CDF experiment at Fermilab. Alas, we didn’t see any evidence for Higgs boson production, despite earlier hints that something might lurk in our sample, and so we were able to rule out some regions of supersymmetric parameter space. We first obtained a preliminary result from this analysis in late 2007. A group of us from Rutgers University, University of Valencia, and UC Davis all worked directly on it, within our several-hundred-member collaboration, using Tevatron data recorded up through mid-2007. Since then we’ve more than tripled our data sample, but this result stuck with the data sample on which it was based and has finally reached publication.

The analysis was the topic of the Ph.D. thesis of my student Cris Cuenca, who was formally a student of my former postdoc Juan Valls at University of Valencia. Cris was a visitor in our group at UC Davis, and I was effectively his thesis advisor. After we got the preliminary results, Cris focused on writing his thesis, eventually defending it in Valencia in April 2008. One nice effect of that was that I got to visit Valencia for the first time: what a fantastic city!

Once the thesis was done, it was clear we needed to publish the result formally. In fact, we should have been already writing the paper but as usual it’s hard to find time to get started on writing projects. Here is a place we could have saved some time, though…

Anyway, I wrote a draft after the birth of our son Ian in June 2008, while helping with baby care at home. In our collaboration there is a very formal review process before submitting a paper for publication. The spokespeople of the collaboration “godparents” who perform a full internal review of the analysis and the draft of the paper. Without naming names, we got some very knowledgeable godparents who asked us hard questions. Some of these questions took weeks to answer, because more data analysis had to be performed. And some of them led to even more questions. This review process is a good thing — it ensures that the quality of the final paper is very high, and that the result is correct to the best of our ability. In fact, in the course of the review process we found that there was a minor software bug, and we repeated the full end stage of the analysis. As it turned out, the bug had very little effect on the final result but we needed to be sure. (At present there are only unknown bugs in the analysis software!)

Whenever there is a change to an analysis like this, it needs to be re-approved in our physics analysis group meetings, with two presentations: a “pre-blessing”, and then a “blessing” two weeks later. (Hey, I’m not responsible for the pseudo-religious jargon used in this process…) This eventually happened in March 2008.

With the result final and the godparents happy with the paper draft, it was time for the general collaboration review. The collaboration gets two weeks to comment on each draft. Then the authors go through comment by comment and reply to the commenters, modifying the draft as needed. The godparents reviewed our replies and then we arrive at the next draft. This part of the process can take many weeks depending on how much time the authors have two devote to the paper. Once the final draft stage is reached, a “paper reading” is scheduled at the weekly general collaboration meeting. Following the presentation of the result, the collaboration has 48 hours before the paper is submitted to the journal. For us this happened, finally, in June of this year.

We heard back from PRL in late July, with blind referee comments to address. There then ensued a back-and-forth between us and the referees, answering questions, making changes to our submission, and eventually reaching agreement that the paper would be published in PRL. This happened a few weeks ago, and our paper has now appeared in what I think is still considered to be the most prestigious journal in our field, though Nature possibly tops it. (That might inspire a comment flame war but I hope not…)

Maybe this is an extreme example, and I certainly will endeavor to bring results to publication much more quickly in the future. (I always say that.) Certain results, if they are “hot”, can be published on a fast track in CDF, within weeks, but that is quite rare.

Many will, no doubt, argue that print media of almost every form is on the way out. Will this happen to print science journals? I do think there is a strong need for blindly refereed publication of scientific results, even though many scientists have reviewed these papers by the time they are submitted.

And clearly once the LHC experiments have physics results to publish, we will need a very rapid means of getting them into print. For any striking new discovery we’ll want to have a paper submitted for publication when we announce the result…this is in contrast to the case of not-very-striking results, where we announce the results at conferences first and publish later. The reason is that the experiments will try to establish scientific priority by publishing striking results before the other one does, but I have to wonder, in the modern age of electronic media and collaborations with thousands of members, whether simply announcing or presenting the the result in public doesn’t accomplish that anyway. If ATLAS says they see a resonance in muon pairs at 1.5 TeV mass and so does CMS, the same week, will we really say “ATLAS found it first” or “ATLAS was the one to discover it and CDF confirmed it?” I hope the science mainstream media don’t present such a thing that way…but more than that I just hope this is a problem we will actually face!

Lead paragraph from the Times Online UK about the latest LHC snafu:

The rehabilitation of the beleaguered Large Hadron Collider was on hold tonight after the failure of one of its powerful cooling units caused by an errant chunk of baguette.

Speaking of successful NASA/DOE collaborations, there’s an interesting new paper on astro-ph claiming that the Fermi gamma-ray satellite has found evidence for a gamma-ray excess in the vicinity of the galactic center — similar to what you might expect from high-energy electrons produced by annihilations or decays of dark matter.

The Fermi Haze: A Gamma-Ray Counterpart to the Microwave Haze
Authors: Gregory Dobler, Douglas P. Finkbeiner, Ilias Cholis, Tracy R. Slatyer, Neal Weiner

Abstract: The Fermi Gamma-Ray Space Telescope reveals a diffuse inverse Compton signal in the inner Galaxy with the same spatial morphology as the microwave haze observed by WMAP, confirming the synchrotron origin of the microwaves. Using spatial templates, we regress out pi0 gammas, as well as ICS and bremsstrahlung components associated with known soft-synchrotron counterparts. We find a significant gamma-ray excess towards the Galactic center with a spectrum that is significantly harder than other sky components and is most consistent with ICS from a hard population of electrons. The morphology and spectrum are consistent with it being the ICS counterpart to the electrons which generate the microwave haze seen at WMAP frequencies. In addition to confirming that the microwave haze is indeed synchrotron, the distinct spatial morphology and very hard spectrum of the ICS are evidence that the electrons responsible for the microwave and gamma-ray haze originate from a harder source than supernova shocks. We describe the full sky Fermi maps used in this analysis and make them available for download.

In English: if the dark matter is a weakly-interacting massive particle (WIMP), individual WIMPs should occsasionally annihilate with other WIMPs, giving off a bunch of particles, including electron/positron pairs as well as high-energy photons (gamma rays). Indeed, searching for such gamma rays was one of the primary motivations behind the Fermi mission (formerly GLAST). And it makes sense to look where the dark matter is most dense, in the center of the galaxy. But it’s a very hard problem, for a simple reason — there’s lots of radiation coming from the center of the galaxy, most of which has nothing to do with dark matter. Subtracting off these “backgrounds” (which would be very interesting in their own right to galactic astronomers) is the name of the game in this business.

But Doug Finkbeiner at Harvard has for a while now been suggesting that there was already evidence for something interesting going on near the galactic center — not in the form of high-energy photons, but in the form of low-energy photons. The so-called WMAP haze is alleged to be radiation emitted when high-energy electrons are being accelerated by magnetic fields, leading to low-energy photons (synchrotron radiation). And Finkbeiner and collaborators claim that a careful analysis of data from WMAP (whose primary mission was to observe the cosmic microwave background) reveals exactly the kind of radiation you would expect from annihilations near the galactic center.

If that model is right, it gives us some guidance about what to look for in the gamma rays themselves, which Fermi is now observing. And according to this new paper, this is what we see.

That’s one of many images, and has been extensively processed; see paper for details. The new paper claims that there is an excess of gamma rays, and that it has just the right properties to be arising from the same population of electrons that gave rise to the WMAP haze. These much higher-energy photons arise from inverse Compton scattering — electrons bumping into photons and pushing them to higher energies — rather than synchrotron emission. So we’re not talking about gammas that are produced by dark-matter annihilations, but ones that might arise from electrons and positrons that are produced by such annihilations. The authors pointedly do not claim that what we see must arise from dark matter, or even delve very deeply into that possibility.

There have been speculations that the microwave haze could indicate new physics, such as the decay or annihilation of dark matter, or new astrophysics. We do not speculate in this paper on the origin of the haze electrons, other than to make the general observation that the roughly spherical morphology of the haze makes it difficult to explain with any population of disk objects, such as pulsars. The search for new physics – or an improved understanding of conventional astrophysics – will be the topic of future work.

That’s as it should be; whether or not the gamma-ray haze is real is a separate question from whether dark matter is the culprit. But on a blog we can speculate just a bit. Therefore I’m going to go out on a limb and say: maybe it is! Or maybe not. But a wide variety of promising experimental techniques are attacking the problem of detecting the dark matter, and we’ll be hearing a lot more in the days to come.

It’s well known that dark energy is a mystery — both for scientists, and apparently for funding agencies who are trying to figure out how best to learn more about this stuff that makes up about 73% of the energy of the universe. I haven’t been paying close attention to the ins-and-outs of this saga (there are more rewarding ways to give yourself an ulcer), but last I had heard the National Academy of Sciences had given very high priority to a satellite observatory meant to pin down the properties of dark energy. This was the JDEM idea — Joint Dark Energy Mission, where “joint” indicates a partnership between NASA and the Department of Energy. (They don’t always play well together, but the Fermi satellite is a notable recent success.)

Now, via Dan Vergano’s Twitter feed, I see a story in Nature News to the effect that things have become murky once again. The proposals got too expensive, so NASA turned to the European Space Agency for help, but ended up giving away things the DOE thought were in their domain, so they threatened to take their toys and go home, giving up on the idea of a satellite altogether.

The story is complicated by disagreement over how important it is to measure the dark energy equation-of-state parameter, the number characterizing how quickly the energy density changes (if at all). It’s frequently said that “we know nothing” about dark energy, but that’s not true; we know that it’s smoothly distributed and nearly-constant in density through time. We even have a very natural candidate for what it is: the vacuum energy. There is of course the problem that the vacuum energy is much smaller than it should be, but that problem is there whether it’s strictly zero or just really small. Other models still have that problem, and tend to add other fine-tunings on top. It would be great, and we would certainly learn a lot, if the dark energy were not simply vacuum energy; but right now we have no compelling reason to think it’s not, so it’s a bit of a long shot.

This past weekend saw the first beam particles in the LHC since the magnet quench incident of September 2008. Protons and lead ions were threaded in two directions around part of the ring before being dumped, and everything worked without a hitch. The graphs show the ion beam spot entering Collision Point 2 before being dumped.

The LHC machine commissioning will pick up where it left off more than a year ago, and the plan is, if all goes well, to collide beams of protons in the experiments at a center of mass energy of 7 TeV (3.5 TeV per beam) before the end of the year. The luminosity will not be large at first, but should increase steadily with time until next fall, when the long shutdown to retrofit the remaining magnets with new quench detection and helium pressure relief systems begins. By that point the experiments hope to have accumulated upwards of 200 pb^-1 of integrated luminosity. This initial data sample is sorely needed to shake down the detectors and start tuning up the event reconstruction and analysis. And who knows, maybe we’ll see something totally unexpected. (Please, no black hole comments!)

The next main milestone will be beam circulating around the whole ring and captured by the RF system. That should happen by mid-November. Fingers crossed!

I am just back from Princeton where we held the annual meeting of the GRE Physics Committee of Examiners, a group of six, ahem, distinguished professors (we have grey hair) who sit around a conference table working through hundreds of potential and actual Physics GRE problem. Each year new exam forms are completed, new questions added to the pool, statistics reviewed, and a good time is generally had by all.

This was my last meeting – I have served on the committee for six years. The membership rotates roughly every two years. I had been an external reviewer and problem writer for a couple years, and was then asked to serve on the committee. I am sworn to secrecy about a lot of the details, for good reason, but let me try to tell you from my perspective as an exam writer how to study for this dreaded event in your physics education.

Firstly, there’s the format. The exam is 100 questions long, and you have 170 minutes to do it. This is, therefore, different from just about every other physics exam you have had in college, where you have, say, four to six problems in an hour-long exam. The GRE Physics problems (or “items” in assessment world jargon) are short, to-the-point questions, and just about all of them are short calculations, if any, and take little time once you see what to do. Writing such questions is a difficult thing to do, let me tell you. We are continually amazed how, after about six levels of review, we can find issues of clarity, reasoning, and even sometimes basic physics correctness in the items submitted to the pool. All the committee members spend a lot of time each year reviewing hundreds of problems, looking for flaws, but more often than you would think the face-to-face meeting in Princeton with the ETS folks reveals something previously overlooked. It’s a really interesting process.
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I’m in the middle of jetting hither and yon, talking to people about the arrow of time. (Wouldn’t it be great if I had a book to sell them?) Right now, as prophesyed, I’m at the Quantum To Cosmos Festival at the Perimeter Institute. They’re extremely on the ball over here, so every event is being recorded by the ultra-professional folks at TVO, and instantly available on the web. So here is the talk I gave on Saturday night — a public-level discussion of entropy and how it connects to the history of our universe.

Yes, that’s a pretty suave picture of me on the image capture. What can I say? I’m just one of those lucky folks with an effortless magic in front of the camera.

If you prefer to get your talks about entropy unadulterated by voice and motion, and don’t mind a more technical presentation, I’ve put the slides from my recent Caltech colloquium online. These are aimed basically at grad students in physics, so there is an equation or two, and the caveats are spelled out more clearly. But the punchline is the same.

A recent essay in the New York Times by Dennis Overbye has managed to attract quite a bit of attention around the internets — most of it not very positive. It concerns a recent paper by Holger Nielsen and Masao Ninomiya (and some earlier work) discussing a seemingly crazy-sounding proposal — that we should randomly choose a card from a million-card deck and, on the basis of which card we get, decide whether to go forward with the Large Hadron Collider. Responses have ranged from eye-rolling and heavy sighs to cries of outrage, clutching at pearls, and grim warnings that the postmodernists have finally infiltrated the scientific/journalistic establishment, this could be the straw that breaks the back of the Enlightenment camel, and worse.

Since I am quoted (in a rather non-committal way) in the essay, it’s my responsibility to dig into the papers and report back. And my message is: relax! Western civilization will survive. The theory is undeniably crazy — but not crackpot, which is a distinction worth drawing. And an occasional fun essay about speculative science in the Times is not going to send us back to the Dark Ages, or even rank among the top ten thousand dangers along those lines.

The standard Newtonian way of thinking about the laws of physics is in terms of an initial-value problem. You specify the state of the system (positions and velocities) at one moment, then the laws of physics tell you how it will evolve into the future. But there is a completely equivalent alternative, which casts the laws of physics in terms of an action principle. In this formulation, we assign a number — the action — to every possible history of the system throughout time. (The choice of what action to assign is simply the choice of what laws of physics are operative.) Then the allowed histories, the ones that “obey the laws of physics,” are those for which the action is the smallest. That’s the “principle of least action,” and it’s a standard undergraduate exercise to show that it’s utterly equivalent to the initial-value formulation of dynamics.

In quantum mechanics, as you may have heard, things change a tiny bit. Instead of only allowing histories that minimize the action, quantum mechanics (as reformulated by Feynman) tells us to add up the contributions from every possible history, but give larger weight to those with smaller actions. In effect, we blur out the allowed trajectories around the one with absolutely smallest action.

Nielsen and Ninomiya (NN) pull an absolutely speculative idea out of their hats: they ask us to consider what would happen if the action were a complex number, rather than just a real number. Then there would be an imaginary part of the action, in addition to the real part. (This is the square-root-of-minus-one sense of “imaginary,” not the LSD-hallucination sense of “imaginary.”) No real justification — or if there is, it’s sufficiently lost in the mists that I can’t discern it from the recent papers. That’s okay; it’s just the traditional hypothesis-testing that has served science well for a few centuries now. Propose an idea, see where it leads, toss it out if it conflicts with the data, build on it if it seems promising. We don’t know all the laws of physics, so there’s no reason to stand pat.

NN argue that the effect of the imaginary action is to highly suppress the probabilities associated with certain trajectories, even if those trajectories minimize the real action. But it does so in a way that appears nonlocal in spacetime — it’s really the entire trajectory through time that seems to matter, not just what is happening in our local neighborhood. That’s a crucial difference between their version of quantum mechanics and the conventional formulation. But it’s not completely bizarre or unprecedented. Plenty of hints we have about quantum gravity indicate that it really is nonlocal. More prosaically, in everyday statistical mechanics we don’t assign equal weight to every possible trajectory consistent with our current knowledge of the universe; by hypothesis, we only allow those trajectories that have a low entropy in the past. (As readers of this blog should well know by now; and if you don’t, I have a book you should definitely read.)

To make progress with this idea, you have to make a choice for what the imaginary part of the action is supposed to be. Here, in the eyes of this not-quite-expert, NN seem to cheat a little bit. They basically want the imaginary action to look very similar to the real action, but it turns out that this choice is naively ruled out. So they jump through some hoops until they get a more palatable choice of model, with the property that it is basically impotent except where the Higgs boson is concerned. (The Higgs, as a fundamental scalar, interacts differently than other particles, so this isn’t completely ad hoc — just a little bit.) Because they are not actually crackpots, they even admit what they’re doing — in their own words, “Our model with an imaginary part of the action begins with a series of not completely convincing, but still suggestive, assumptions.”

Having invoked the tooth fairy twice — contemplating an imaginary part of the action, then choosing its form so as to only be relevant where the Higgs is concerned — they consider consequences. Remember that the effect of the imaginary action is non-local in time — it depends on what happens throughout the history of the universe, not just here and now. In particular, given their assumptions, it provides a large suppression to any history in which large numbers of Higgs bosons are produced, even if they won’t be produced until some time in the future.

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