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## Archive for March, 2008

### Doomsday, in the Court

Thursday, March 27th, 2008

And speaking of black holes, right on cue, it’s 1999 all over again, from MSNBC’s Cosmic Log (via Slashdot…):

Some folks outside the scientific mainstream have asked darker questions as well: Could the collider create mini-black holes that last long enough and get big enough to turn into a matter-sucking maelstrom? Could exotic particles known as magnetic monopoles throw atomic nuclei out of whack? Could quarks recombine into “strangelets” that would turn the whole Earth into one big lump of exotic matter?

Former nuclear safety officer Walter Wagner has been raising such questions for years – first about an earlier-generation “big bang machine” known as the Relativistic Heavy-Ion Collider, and more recently about the LHC.

Last Friday, Wagner and another critic of the LHC’s safety measures, Luis Sancho, filed a lawsuit in Hawaii’s U.S. District Court. The suit calls on the U.S. Department of Energy, Fermilab, the National Science Foundation and CERN to ease up on their LHC preparations for several months while the collider’s safety was reassessed.

“We’re going to need a minimum of four months to review whatever they’re putting out,” Wagner told me on Monday. The suit seeks a temporary restraining order that would put the LHC on hold, pending the release and review of an updated CERN safety assessment. It also calls on the U.S. government to do a full environmental review addressing the LHC project, including the debate over the doomsday scenario.

It’s an obvious question to ask who is going to get to review the situation in the next four months. All, and literally all, of the people nominally qualified to evaluate this kind of thing still aren’t even slightly more afraid of this than they were in 1999, after studying billions and billions of collision events at RHIC. And Walter Wagner (“the founder of a botanical garden in Hawaii”, according to Robert Crease, commenting on a letter Wagner wrote to Scientific American in 1999 — the one which started the avalanche) has been through this before, to no effect. But it’s nice to learn a few new things in this piece:

1. Someone thinks the “inner workings” of ATLAS is what I always thought was the outside. I should be nicer about this, but it’s a little funny. While a cheap shot, I admit, I consider this lapse fair game, since the phrase “inner workings” certainly was meant to have a sinister ring in this context.
2. But speaking of (not being) funny, physicists’ attempts at being wry often misfire. Michio Kaku, whom Boyle seems to have used as a source, provides a reasonable, if blustery, dismissal of strangelets — “We see no evidence of this bizarre theory” — but then trips up: “Once in a while, we trot it out to scare the pants off people. But it’s not serious.” Unfortunately, this comes across as insulting to people who are seriously concerned about the effect of science on the environment, and does nothing to inspire their trust in us. If we keep making “jokes” like this to reporters, then we deserve to waste all of the energy that we do fending off folks like Wagner. So let’s stop intentionally scaring people, even in jest.
3. I’ve always complained that these same folks haven’t updated the conceptual basis for their paranoia (b.t.w. there is literally no factual basis, not even a hint — we’d be shouting it to the rooftops if there was merely a hint of a hint, believe me). But CERN did make a good faith attempt to update things in 2002-3, two years into the RHIC era — and I’m embarassed to admit that I’ve never seen it (but that said, no-one has ever brought it to my attention, and it certainly doesn’t percolate up to Google’s notice — but “CERN doomsday” does yields up this gem.) Someone asked me recently to check out the “Safety Concerns” section of the Wikipedia article on the LHC, and…well, I was busy. Live and learn

Anyway, I’ll be following this closely on the physiblogosphere. Stay tuned.

### Happy Easter

Wednesday, March 26th, 2008

So, it’s MITs spring break, which gives me a chance to zip over to CERN and see how the CMS tracker checkout is going (my students are heavily involved, and doing excellent work – I’m like a proud poppa!). But I arrived on Easter Sunday, and Friday and Monday were holidays too, so it has been pretty quiet actually. Nothing like arriving in France with no restaurants or stores open for two days! The Europeans take their holidays seriously, about which I am totally hypocritical – on one hand, sometimes it seems that things take infinitely longer due to people taking off all the time, but on the other, I wonder if this isn’t just my work-aholic American upbringing blinding me to the fact that there is more to life than just work, and maybe the Europeans have the right idea. I can argue either side, and frequently do. But, when push comes to shove, what I usually do is think about how much money it costs to run the accelerator – if my piece of the detector isn’t working, that is money down the drain, and we’re talking $100 million per year, or about$10 per second – that’s pretty expensive, so we need to be sure that none of that beam time is wasted. If we can be confident that we’ll get all there is to get out of the beam, all the extra hours and weekend time are worth it. Just don’t tell my wife.

### Blooming Dipoles

Tuesday, March 25th, 2008

The Geneva area is really quite ideal in terms of climate. During the winter the many mountains in the area get tons of snow but there is rarely snow in the city or surrounding towns. As a result you have the best of both worlds: great skiing very nearby without the drudgery of constantly digging out your car. But occasionally we do get snow storms in the lower altitudes, such as this past weekend. For me this serves as a reminder of why I don’t live in places like Buffalo, NY, for example. Although, I do rather enjoy the occasional digging out of my car from a foot or so of soft, fluffy snow. It is quite therapeutic. It is not, however, therapeutic to dig out my car from 4 feet of very compact snow, dumped by the snowplow directly in front of the car. This being the situation I found myself in this morning. Honestly, snowplow person? Did you not see my car there?

Those frustrations aside, as a native Californian I was raised with the belief that the end of March is Spring (go ahead, laugh. But ask any Californian when Spring is and you will get a similar response. This is because Californians deep down believe that seasons are really just fictional, made up by Northerners and East-coasters to discourage us from vacationing there). So as April looms, I expect to wake up to my garden flowers blooming, not to my front-door stairs becoming a ramp of snow.

But apparently the LHC magnets are responding to the call of Spring. Over the past few weeks, magnets have been popping up everywhere. In the center of round-a-bouts, outside supermarkets, and several places around CERN, such as this superconducting dipole magnet which is just outside my office building.

All of these magnets are being displayed in anticipation of CERN’s ‘open days’, which take place during the first weekend of April. During the open days, all access points including the beam tunnel and all experiments are open to the public for tours. If you are in town, go! It is a great opportunity to see the guts of the LHC and the detectors.

This dipole magnet shown in the picture is what one of the main dipole magnets used in the accelerator ring looks like. Of which there are 1232 in total. Actually there are almost 9600 different magnets used in the LHC. This fact guide (linked at the bottom of the page) gives a description of the purpose of the many different magnet types.

And of course if you are entering a round-a-bout and happen to see one of these huge magnets in the center, don’t worry about your car being sucked into it, these are just shells. But anyone can see the real ones during the open day.

### Talkin’ Black Hole Blues

Monday, March 24th, 2008

Interesting week last, especially last Monday when I and a theory colleague were asked to chat with a documentary producer for the National Geographic Channel. The producer (who has previously worked on a series about exploring time) is working on a show about black holes, with more focus on what space-time looks like around (and even inside) a black hole. He had heard something about the connections between RHIC physics and black holes and wanted to see whether we had anything to offer. Given our spotty past in explaining these kind of things without freaking out people who would otherwise be interested, my colleague and I actually managed to put together an interesting story (none of which implying any danger to anyone not living in a higher dimensional space). Turns out that there are three almost-distinct pictures of “black holes” being used in connection with high energy nuclear collisions (and at the LHC, it’ll only get higher!):

1. “Real” gravitational black holes – it’s been argued many times over that, for normal gravitational physics, energy densities at RHIC aren’t capable of producing enough matter in a small enough space to induce gravitational collapse, and subsequent decay via Hawking radiation. However, the presence of large extra dimensions has the effect of dramatically lowering the Planck scale and allowing this sort of phenomenon to occur, leading to spectacular isotropic decays of high mass black holes into the kitchen sink of particles from the standard model and beyond. I have relatively little experience bantering about this (i.e. one should check out Backreaction for more details on blackholology), but this is certainly the most “popular” conception of black holes at colliders these days.
2. Hawking-Unruh radiation – my colleague in this interview, Dima Kharzeev, and collaborators have put forth an interesting analogy between “minimum bias” particle production, i.e. the many low energy particles produced in essentially every proton proton collision and, scaled up, every nucleus-nucleus collision. In this scenario, the process of the incoming projectiles “stopping” each other, and thus slowing down, by construction leads to acceleration (well, deceleration in this case). Einstein’s equivalence principle (remember that you can’t tell the difference between an elevator accelerating upwards at 9.8m/s^2 and the Earth pulling you down by gravity at 9.8m/s^2…) allows them to connect this slowing down to the Unruh effect in a gravitational field, which predicts the quantum tunneling of particles with an effective temperature of T=a/2Pi. When numbers are put in, out pops the famous freezeout (or Hagedorn) temperature we measure at RHIC (and in proton-proton collisions for years). So in effect, all strong interactions measured in the laboratory make a “black hole”, but not one resulting from gravitational collapse. As an onlooker, I find this connection curious, but not isolated — over the years I’ve noticed many authors make a variety of connections between gravitational physics and strong interactions, but they always feel mysterious, and thus it’s unclear where to go next.
3. “Dual Black Holes” – this is something I and many others (both amateurs and pros) have found intrinsically exciting for a few years. The famous AdS/CFT conjecture suggests that strong interactions involving strongly-coupled quarks and gluons are really better (and more easily) described as a theory living on the boundary of a 10 dimensional gravity theory, with 5 extended and 5 compact dimensions. In this picture, again, every collision involves a black hole, which controls it’s microscopic properties (e.g. the viscosity), but one that lives in a larger dimensional space, and is thus again not the result of gravitational collapse in 4 dimensions. As people who have followed this thread (e.g. via my various blogs) over the years may be aware, this connection is allowing the development a striking number of techniques relevant to actual heavy ion phenomenology — and carries no risk to the 4-dimensional world (which someone should have told the BBC in early 2005…). Of course, we’re all hoping that the extra dimensions actually have some ontological status beyond being a mere mathematical trick, but time will tell.

Anyway, there we had it: three kinds of black hole physics, all of which are probably connected in some way, and all of which are potentially connected to RHIC or the LHC. (Recheck this post soon for more links…it’s bedtime – ok it’s wednesday, but done!)

### Turning them away

Saturday, March 22nd, 2008

Another sign of the apocalypse: It was the week of the Open House for prospective graduate students at MIT, and I met about a dozen of the best and brightest undergraduates interested in Experimental Particle Physics. At this time when there is a peak in interest in the LHC, we are hitting a nadir in funding necessary to bring the new students on board and I had to tell them that were in not for the funding situation I would be happy to bring them into the group, but because of funding I have to turn the majority of them away. I hope they find a spot in another top notch institution and become my collaborators anyway, but am not confident that given the overall situation in Fundamental Research at the frontiers of knowledge that all institutions are not in the same boat, more or less. At a recent meeting of the MIT faculty of the School of Science, MIT President Hockfield (who is working hard to make things better in this direction on our behalf, which I appreciate greatly) opened the discussion with “I don’t need to tell you why we need to increase Graduate Fellowships” which tells me that this problem is endemic.

### Particle Interactions

Thursday, March 20th, 2008

Why is this detector so complicated? I often find myself asking that question. It is usually coupled with the exasperated ‘Why isn’t this code compiling?’ or ‘Why is the DAQ not configuring?’ or ‘Why has the front-end electronics stopped sending data?’

As I was meandering through interactions.org (which is a great resource for fancy physics pictures), I came across this nice picture of particle interactions in ATLAS. Which does in a simple way start to address the question of why ATLAS is as complicated as it is.

What is shown here is a pie-slice of ATLAS from the perspective of looking down the beam-pipe. The white circle at the bottom is the beam pipe, in the center of which the proton-proton collisions occur. One of ATLAS’s design goals is to detect new particles such as the Higgs or Supersymmetric particles. But that is a bit misleading because we don’t really detect these particles themselves, we detect their decay products. By measuring those decay products, we can reconstruct any new particle’s properties, such as its mass.

So the particles we actually observe in the detectors are mostly just the ordinary things like electrons, photons, muons, protons, pions and the like. Different detector types are better at measuring say, an electron, than a muon, therefore in ATLAS we use many different detector technologies so that we can be sure we don’t miss anything.

This picture nicely shows which sub-detectors within ATLAS are better at measuring what. The closest detectors to the interaction are the pixel detector, the semiconducting tracker (SCT) detector and the Transition Radiation Tracker (TRT). Collectively known as the ‘trackers’ or ‘inner detector’, these detectors aim to track the trajectory of charged particles. The charge particles are bent by the magnetic field provided by a solenoid magnet. From the direction and magnitude of the curvature, we can determine the charge and momentum of the particle.

The next layer, the calorimeters, measure the particles’ energies. The first layer of calorimetry, the electromagnetic calorimeter measures the energy from photons and electrons whereas proton and neutron energies are largely measured in the second layer of calorimetry, the hadronic calorimeter. AKA Tile Cal. Muons are hard to stop and generally exit the detector completely. Similar to the inner detector, the Muon system is a series of tracking chambers to measure the trajectory of the muons. Here there is a second magnetic field (not shown in the figure), the toroid magnetic which again is used to bend the muon’s path (and is where the T’ in ATLAS comes from). Particles like neutrinos are completely invisible to ATLAS. We can only infer their existence by measuring the missing energy’—the energy that the neutrino takes with it as it leaves the interaction and the detector.

In that light, if you have ever wondered, ‘are all those sub-systems really necessary’, the answer is definitely, ‘yes!’.

### Talk like a physicist day

Friday, March 14th, 2008

I just want you all to know that today is not only Einstein’s Birthday and Pi day (3.14) but it is also Talk like a Physicist Day which my wife just pointed out to me. Fortunately for me, I don’t have to do anything different!

### Cosmics vs Beam

Friday, March 14th, 2008

In response to my last posting about the ‘6th Milestone week’, the following questions were posed by Jacques.

Once you are satisfied with the results of this test, or any subsequent test that might be decided using cosmic rays, will you “just” have to wait for the LHC to start beaming, at which time you can immediately start gathering and exploiting the data from proton collisions, or does another heavy test campaign begin then? For how long?

Outside the difference in the data frequency/volume(is this not a 1 to 1 million ratio?), are there limitations in the current ATLAS tests due to the nature of cosmic rays? Or will the most volatile particles created by the collision decay so quickly that you can only track the results of such decays for which cosmic rays are a good subrogate to calibrate the various detectors and check they deliver consistent “tracks” whenever a given particle crosses them?
How about the calibration of the Level One Trigger in this context?

For starters, ‘satisfied with the results of this test’ is a constantly changing criterion. A year ago, satisfaction was getting just two sub-systems to run together. Today satisfaction is running all sub-systems with a level-one trigger rate of 10kHz. Next month, satisfaction will be running at 100kHz (which is the level-one rate we want to have during beam running). At the start of each ‘M-week’, we have a whole list of problems that we experienced in the previous ‘M-week’. At the end of the week, we have an entirely new list of problems which are generally more complicated and therefore more difficult to solve. So I suppose forward progress is defined as finding harder and harder problems.

We will never be in a position where we ‘just wait’ for the beam. Nor when the beam is running will we just be waiting for the data to roll in. It is a cultural trait of high energy particle physics to push the system. If we are stable with a level-one trigger rate of 100kHz, someone will suggest an idea to push that rate to 120kHz. There is a saying, ‘If it was easy, it would have been discovered already’. Thus in order to make the big discovery, we have to be willing to take risks and push the detector to its design limits and if possible beyond. And pushing the limits is all the fun!

In the period before the beam, cosmic rays aren’t a great way of testing ATLAS’ limitations. But it is all we have. Using the muon trigger chambers, the cosmic ray rate is about 100Hz. The beam will be 40 MHz which is roughly a factor of a million greater. We try to push the rate during cosmic running by using a high rate ‘random’ trigger but there are no physics events associated with these triggers. Additionally most cosmic events tend to be a single muon slicing through the detector. Whereas with the beam running there will be hundreds to thousands of particles in the detector.

Cosmic muons are very useful to study tracking in the inner detector and muon chambers. For the calorimeters, we can use them as a preliminary cross-check of our energy calibration. They are also helpful to establish the relative timing between sub-systems (which needs to be known on the nanosecond level). While cosmic muons are helpful for calibrating the part of the level-one trigger that looks at muons, it doesn’t help us much with calorimeter-based level-one trigger. The reason is that the calorimeters measure energies that are typically associated with “jets” of many particles, not a single track like those from cosmic muons.

The bottom line is cosmics are all good and fun. But if given the choice of cosmics vs beam. Give us beam!

### Running!

Friday, March 14th, 2008

Well, not that kind. When I was at CDF, I was a member of the Run Too running club, which started as some CDFers running around the “Ring Road” above the Tevatron ring during lunch, as a way to train for the Chicago Marathon, and eventually expanded to other races, other experiments/divisions, other activities (Dragonboating, biking, etc). On the page you’ll see I am the “Ambassador to Cheeseland and Beantown”. Anyway, I haven’t run much, but recently ran a 1:50:18 in the Hyannis Half Marathon following the “Greg Feild training program” (Greg is a buddy who has a habit of running races with very little training). Here’s me, finishing strong:

Both feet in the air! It’s a two-fer! Just to say that we don’t only do physics.

### Eat your heart out, Al Gore

Friday, March 14th, 2008

So, I was walking through the hallway in Building 1 at CERN on my way to the bus and I came upon this nice plaque commemorating one of CERNs most ubiquitous contributions to society.

Unfortunately I didn’t get the text verbatim, and my phone camera doesn’t have the best resolution, but the upshot is that it was in this hallway and in nearby building 31 that Mr. (now Sir) Tim Berners-Lee and compadres developed the web – they are responsible for the terms “www”, “http”, “url”, “html” etc. Consider the impact that has made!