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)

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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).

The Beam is Back

A little over a year after the highly publicized start-up and break-down of the LHC, the damage has been repaired, new protection systems are in place, and all sectors are cold and ready for beam. Yesterday, the first injection test of 2009 was completed— beams of protons and heavy ions were successfully threaded into the LHC beampipe from its predecessor, the Super Proton Synchrotron (SPS). The beams were allowed to flow as far as the first experiments in both directions, ALICE on the clockwise side, LHCb on the other.

What killed Madame Curie? (Part 4)

New York City, 1956

Leaning on a Chinese restaurant at a busy street corner in Greenwich Village, I crossed my legs, tipped my hat low, and quietly panicked. This case is turning into a nightmare: dozens of suspects, growing daily, and they all seem to swap places when you’re not looking. A pion couldda done it; pions seem to be some kind of front for the nuclear force that Madame Curie was playing with before she died. But leave a pion to itself and it disintegrates into a muon and a neutrino, neither of which claims to have ever heard of nuclear forces. Radiation in the form of muons and neutrinos has been raining down on us since the beginning of time, and it’s never even hurt. If pions are just glowing with nuclearness, where does the nuclearness go when they die?

For that matter, what is a particle, anyway? I have to admit, I wasn’t suspicious when I first heard the word— I thought they were talking about little rocks or marbles or something. But rocks don’t just change into different kinds of minerals on their own, except for Curie’s rocks, that is. What are these particles? The physicists themselves don’t seem to know: everyone I ask gives a different answer. They seem to be some shadowy energy-clouds, sometimes insubstantial and sometimes infinitely hard. What kind of world are we living in, anyway?

I felt a crumpled slip of paper in my pocket. Pulling it out, I read the well-worn handwriting under my breath, “Seek the Dragon Lady.” I scanned the crowd. I’d bet none of them knew the half of what’s going on, right under their noses! Well, not just their noses, but everywhere in fact. “Any of you folks know a Dragon Lady?”

“Are you looking for Madame Wu?” The young man startled me. From the high-necked sweater and the pipe in the corner of his mouth, I’d reckon he was a student.

What killed Madame Curie? (Part 3)

Ithaca, NY, 1948

After a wrong turn in Albuquerque, I caught up with Bugs Bunny, alias Richard Feynman, somewhere near the ends of the earth.  Up to my elbows in snow-drifts, I spied on the little window to his office, in which he seemed to be doing normal professor-things, plus wild gesticulations.  I decided on a particularly frozen morning that I would have to risk visibility if I was to get answers, so I enrolled at Cornell, posing as a G.I. bill student.  In Professor Feynman’s introductory physics lectures, I could see that there was something remarkable happening here.  People researching physics is about as natural as fish studying water: it’s the very stuff we’re made of.  He had a knack of getting down to the ground floor, asking the basic questions, just as much in a block sliding down a plane as in neutrinos.

His teaching assistant, a quirky bow-tied Brit by the name of Freeman Dyson, knew the man personally, so I inquired.  “Oh, he’s working on something, yes.  The trouble is he just won’t publish, no matter how much I cajole him.  He says he’s depressed, but Dick depressed is just a little more cheerful than any other person exuberant.  It’s the Bomb, I think, and of course Arlene, his poor wife who died in New Mexico.  I probably shouldn’t be telling you this, but Dick and Arlene got married knowing she hadn’t long to live, she having T.B.  Bit like Dick to give it a go anyway.”

“What do you suppose he’s up to?”

“Well, he’s got his own private quantum theory for starters.  Quantum theory, that’s the theory of the atom and electrons.  Until recently, no one’s been able to make it work with Einstein’s relativity; it’s riddled with infinities, you know.  Schwinger’s done some remarkable work reconciling the two— all operator theory and renormalization, I’m still trying to understand it.  Somehow, there’s a way to replace the infinities with experimental measurements, then the beast is well behaved and gives very nice results.  Dick manages calculate the same thing with these funny little pictures, and he puts plus signs between them like they were real mathematical formulae.  Quite a ball at conferences: squiggle plus squiggle equals whatever.  I mean to pick his brain about it before he flies off to Brazil.”

“Brazil?!?”

“Yes, he’s taking a visiting professorship.  Says he hates the cold.”

On my way home that evening, I saw a shadow linger on my doorstep, then dart away.  I broke into a run to pursue it, but not a trace was left, not even footprints in the snow.  With one exception, that is: crinkled under the door and sodden with melt-water was a little envelope.  Inside was a note, which read,

“The killer is left-handed.

—an Insider”