Coffeeshop Physics

Hello RSS subscribers!

It has been over a year since my last post, and since then, I have started developing my own blog. (I was just a guest author here.) It is called Coffeeshop Physics, and it is a relaxed presentation of physics topics that I think are interesting. The name was inspired by my experiences with Cafe Scientifique, a series of coffeeshop presentations about the sciences— I want to replicate that kind of atmosphere online.

Whereas the Everything Seminar was intended for mathematical audiences, Coffeeshop Physics is for general audiences, much like Cafe Sci. Therefore, I don’t assume that the readers know what a derivative is (relevant for yesterday’s article), but you might find it interesting anyway. Many of the topics that I’m writing about are things that I struggled to understand as an undergrad and even a grad student, the intuition behind the mathematical formalism.

For instance, my favorite article so far is about curved surfaces and gravitation. When I studied general relativity, I could push Christoffel symbols around, but I was frustrated by the fact that I couldn’t visualize the problems that we were working on.

I got a better appreciation for curved surfaces by learning to sew, and after making about a dozen little models, the picture came into focus. Here is a photo of a model of space-time at the surface of the earth, in which we can see that a freefall is a shorter path through space-time than just standing on the ground. It doesn’t have Minkowski structure, so it is not quantitatively accurate (it should open up at the top, not the bottom), but it is a picture to keep at the back of one’s mind.

I’ve also turned the spectrum of resonances in electron-positron collisions into a sound, so that we can hear what it sounds like when the collide, and found a nice demonstration of entropy in a story about a leprechaun tying ribbons on trees in a forest. If you enjoyed the What Killed Madame Curie? detective serial that I started on this blog, I am expanding it into a novel, with links on the site.

Cheers,
— Jim

My last LHC status update

I haven’t said much since this year’s start-up of the LHC, but there have been some interesting developments, so I’ll add one last update.  If you haven’t been following the LHC status, it has been exponentially increasing in collision rate while maintaining a fixed collision energy (about 3 pb^-1 of 7 TeV collisions have been collected by the LHC experiments, which is a few thousand times less than the Tevatron’s 9 fb^-1 of 1.96 TeV collisions, collected since 2001).  My “particle body counts” are now completely obsolete: nearly all known particles have been re-discovered in the LHC experiments.  And today, the first unexpected effect has been presented by an LHC experiment: “Long-range, near-side angular correlations,” which is presented in detail on the CMS public page.  Below the cut here, I’ll explain what this means.

7 TeV collisions tonight…

… if all goes well. The LHC has been circulating two 3.5 TeV beams off and on for the past week, and tonight they plan to turn off the separators keeping them apart. We should then see the first 7 TeV collisions ever produced in a laboratory.  If there are any undiscovered particles with a mass in the new energy range that is being opened up, it would now become possible for them to be spontaneously produced in a detector where we can see them (as opposed to cosmic ray air showers).  Our view of particle physics is about to become three and a half times larger than it has ever been.

The plan at first is just to let the beams collide without focusing them, so the luminosity will be low, and the rate at which new particles could be produced would be correspondingly low.  As time goes on, the beams will be focused and the intensity will be raised, which increases the rate of collisions and therefore the probability of seeing new stuff.  This is the beginning of an 18–24 month period of continuous data-taking and open-ended exploration.

Tonight I’ll be following this from the Fermilab control room (the LHC is in Switzerland— this is a remote control room).  I’ll post any interesting updates as comments to this article (they won’t come up in RSS feeds).  Here are other sources of information, all more direct than this blog (I mostly try to avoid repeating them):

• CERN twitter (from the LHC control room)
• ATLAS control room blog
• CMS e-commentary
• LHC page 1: live update of machine status.  When I last looked, the energy was 3.5 TeV and the beam intensities were 1.5e10 (higher than the past few weeks).  The red and blue lines are intensities of the clockwise and counter-clockwise beams versus time.
• CMS data aquisition: live update of CMS data collection.  The main plot is data accumulated versus time; it’s a constant slope for cosmic rays (no LHC beam), but could jump up if we get a lot of events from the beams.
• CMS event display: pictures of the events as we see them (in three projections: face-on, side view, and 3D).  Yellow lines are particle trajectories, red and blue bars are calorimeter energy deposits.  If a yellow line goes beyond the calorimeters, it’s a muon!  Right now, I think it’s re-playing events from the low-energy collisions of 2009; that will change sometime tonight.

In my timezone, the sun is setting. Happy Passover!

Particle body-count 2

As a result of today’s talks, here’s the updated body-count (all four experiments with a lot of overlap):

Particle	Original discovery	Method of observation in the LHC experiments
Electron/positron	1896 (e–), 1932 (e+)	Peak at 1.0 in calorimeter energy to track momentum ratio, also observed in pairs from photon conversions in matter (X γ → X  e+e– where X is a nucleus)
Photon	1900 (Planck’s quanta)	Photon conversions and π0→ γγ
Proton	1911	Energy loss charged particle’s trajectory (dE/dx)
Deuteron	1931	Also seen in dE/dx
Muon	1936	Specialized muon detectors
Pion	1950 (π0)	Neutral pion in π0→ γγ, charged pions in dE/dx
Eta meson	1961	η → γγ
Kaon	1947 (KS)	Neutral kaon in KS → π+π–, charged kaons (K+ and K–) in dE/dx and ring-imaging Cerenkov detectors
Phi meson	1962	φ → K+ K–
Lambda	1947 (Λ0)	Λ0→ π+p– / Λ0→ π–p+
Xi baryon	1964	Ξ → π Λ0
Dark matter WIMPs	not yet	two candidates in the unblinded signal region of the Cryogenic Dark Matter Search (CDMS) (not an LHC experiment)

–

The last entry is for yesterday’s CDMS paper, which shows two candidate events surviving all analysis cuts, set prior to looking at the result (unblinding).  The probability for background fluctuating up to account for these two events is 20-23%, so no one is calling it a signal.  Both are close to the edges of the analysis cuts, so even if the observed events had significantly exceeded the background estimates, there would be room for doubt.  This may be the tip of the iceberg for direct dark matter detection, but then again, it may not.

The particle body-count

Earlier today, the LHC finished its 2009 run.  They did everything they said they were going to do: provide physics-quality 900 GeV collisions and break the world record by colliding protons with a combined energy of 2.36 TeV (that happened Monday), as well as many other studies to make sure that everything will work for 7 TeV collisions next year.  We’ve been busily finding the familiar particles of the Standard Model— I wrote two weeks ago about the re-discovery of the π⁰; since then new particles been dropping in almost daily.  I’ll explain some of the already-public results below the cut, but first I want to point out that there will be another LHC Report this Friday at 12:15 (European Central Time = 6:15 AM Eastern U.S. = 3:15 AM Pacific) on CERN’s webcast site.  This is where all of the LHC experiments will present their results and probably make a few more public.

Also, in case you haven’t heard, there have been a lot of rumors that the Cryogenic Dark Matter Search (CDMS) has discovered something interesting.  They’ll be presenting whatever it is tomorrow with a paper on the arXiv, a Fermilab presentation at 4:00 PM Central U.S. (webcast here), and a SLAC presentation at the same time, 2:00 PM Pacific (webcast here).  It might be the direct detection of dark matter particles, which would be incredibly exciting.

Erratum: no high-energy collisions yet

Yesterday, when I said,

The LHC has officially become the world’s highest-energy collider, by colliding protons at 2.36 TeV (above the Fermilab Tevatron’s record of 1.96 TeV),

I misunderstood a point in the press release that wasn’t heavily stressed.  The LHC has become the world’s highest-energy accelerator, reaching counter-rotating energies of 1.18 TeV each, but the beams were not collided at this high energy yet.  Last week, they were collided at low energy, and this week, they have been accelerated to high energy, but not collided.  The two beams passed by each other in the interaction region, held apart by electric fields.  A few protons on the fuzzy outer edge of the distribution might have collided, but the big collisions are yet to come.

Small steps, yet very fast from one step to the next.

Rediscovering the Standard Model

In the past week, three milestones have been passed:

• the first π⁰ particles have been reconstructed from their decay products, shown at the public LHC week 1 conference (by CMS and LHCb);
• the LHC has officially become the world’s highest-energy collider, by colliding protons at 2.36 TeV (above the Fermilab Tevatron’s record of 1.96 TeV);
• the first paper based on LHC data has been submitted to the arXiv (by ALICE).

The π⁰ observation represents the first step in “rediscovering the Standard Model” as part of the detector commissioning.  It’s like a walk through history, where this step is at about 1950, when the π⁰ was first discovered in cyclotrons and cosmic rays.

The above plot shows invariant mass distributions of pairs of photons observed in CMS and LHCb.  From every pair of photons, you assume that they came from the decay of a particle and plot what the mass of that particle must have been.  For many pairs, the assumption is false, so you get a combinatoric background of random photons, but for photons that actually came from π⁰ → γγ, you get a peak at the π⁰ mass.  The combined distribution is a peak on a smooth background.  Most particles in the Standard Model are known only through their decay products, and this is the first example to be seen at the LHC.

Since we already know a lot about π⁰s, we now use them to calibrate the photon detectors.

Needless to say, these algorithms are not just being developed now— they’ve been in the works for years.  That explains how ALICE was able to put together and internally approve a paper based on the first collisions in one week.  For a brand new analysis, that would take many months at least!

Protons have *collided* in the LHC

At least it looks like it from this CMS event display:

See the CMS e-commentary for hourly updates and more information.  (That’s how I know which results are public. :))

The yellow boxes are silicon strips that detected the passage of particles (most likely pions in this case) and the green lines radiating from the center are tracks reconstructed from those hits.  They’re not constrained to meet at the center: that’s an indication that these particles actually originated where the beams collide.  Beyond that, the red and blue bars show how much energy was collected in the electromagnetic calorimeter (electrons, photons, and hadrons) and the hadronic calorimeter (hadrons only), respectively.  No activity can be seen in the muon detector (red boxes).

This is all consistent with what one should expect from the collision of two protons— a strong (QCD) interaction between the quarks and gluons producing a handful of strongly-interacting hadrons, rather than photons, electrons, muons, or taus, which are insensitive to the strong force.  An electroweak interaction between the quarks and gluons, producing possible Higgs bosons or any of a number of other exciting possibilities * * * * * * * *…,  are more rare, and will require collecting and sifting through huge numbers of collisions.

The CERN twitter site says that all four experiments saw collision-like events.  It’s finally happening!

Protons have orbited the LHC!

The beam went three times around the LHC ring: see CMS’s e-commentary and CERN’s twitter.  This is the milestone that was a big media event last year (September 10, 2008).

Update: now it’s 500 times around the ring (about 0.05 seconds).  Last year’s record was about 9 minutes of continuous beam.

Update: up to 9 seconds, 30 seconds (50k events seen by CMS)…

Update: and now a beam in the other direction has made a full orbit.  (All you gotta do is smack ’em together!)

Update (Nov 21): on Saturday, we got hours of stable beam (single beams, not colliding).  This has never been done before with the LHC: from here on, it’s all new territory.  Now I’ve got to get to work on the offline data, which should be great for detector alignment…

“Beam-splashes” to arrive at CMS soon, possibly this weekend

See CMS e-commentary for live updates.

“Beam-splashes” are when a beam is threaded part-way through the LHC ring, then deliberately collided with an absorbing block of tungsten to stop it, upstream of a detector. Many particles are created in this collision, most of them are absorbed, with the exception of the muons and neutrinos. CMS can detect muons, and what it sees is a huge splash of activity, shown in this event display from September, 2008.

The blue bars indicate huge deposits of energy in the calorimeters. They seem to project from the center of the detector, but this is an artifact of the software, which was designed to visualize collisions from the center. The calorimeter cells measure energy, not direction, so when it sees energy coming from a flood of particles arriving from the right, it draws them as though they came from the center.

You can also see little parallel lines surrounding the central burst like a school of fish. These are individual muons seen by the barrel muon detectors, which do measure direction.

Update: here it is, the first CMS beam-splash of 2009 (from the e-commentary page)!

The little red lines are reconstructed muon tracks, blue dots are raw hits, and the yellow/blue starburst in the center is the calorimeter energy.  You can tell that the beam is coming from the right-hand side of the detector (“LHC beam-1”, the clockwise direction around the ring).