## Posts Tagged ‘LHC’

### The AdS/QCD correspondence: delivery failure

Thursday, November 3rd, 2011

As described previously here, there are good theoretical reasons to think that the so called AdS/QCD correspondence should provide a poor description of the collisions of strongly interacting particles like the proton, or their internal quarks and gluons.  The idea for the correspondence was inspired by string theory where is can be shown that special (strongly interacting, supersymmetric, scale invariant)  theories with gluons can be simply described by calculations on a curved 5D space called anti-de Sitter space, and abbreviated as AdS.  While the theory of quantum chromodynamics (QCD) does contain gluons, it is not supersymmetric, not scale invariant, and, it turns out, not strongly interacting enough for the correspondence to work. The problem can be seen fairly easily in collisions.  In QCD collisions of quarks and gluons tend to produce narrow sprays of particles, known as jets, that look something like this:

Jets of particles. The length of the line shows the energy of the particle.

While in AdS theories the produced particle spread out uniformily in all directions like this:

spherical spray of particles

Some theorists have shrugged their shoulders about this problem and tried to apply the AdS/QCD correspondence to heavy ion collision data pointing out that some particular measurements happened to agree with the AdS/QCD prediction.

The BackReaction blog points out that the latest data from the LHC again points to the inadequacies of the AdS/QCD correspondence.

The ratio of the probability of finding a jet in lead-lead collisions to the same probability in proton-proton collisions as a function of the momentum of the jet away from the beam line (aka transverse momemtum P_T). Image from Thorsten Renk, Slide 17 of this presentention

The data most closely follow a model of ordinary QCD jet  production, labelled YaJEM for Yet another Jet Energy-loss Model, rather than the AdS calculation.  For the experts: while the jetty description of QCD continues to work at large number of colors, $N$, the AdS description requires both $N$ and the coupling times the number of colors, $\alpha N$, to be large, and it is the latter condition that fails in the real world.

• Systematics of the charged-hadron $P_T$ spectrum and the nuclear suppression factor in heavy-ion collisions from $\sqrt{s}=200$ GeV to $\sqrt{s} =2.76$ TeV
arxiv.org/abs/1103.5308v2
• Pathlength dependence of energy loss within in-medium showers
arxiv.org/abs/1010.4116
• The AdS/QCD Correspondence: Still Undelivered
arxiv.org/abs/0811.3001

### Seeing Protons at the LHC

Wednesday, March 17th, 2010

Last week at the Moriond conference Jorg Wenninger gave an interesting presentation on the status of the LHC. One of his slides showed the spot of light that is emitted by the proton beam.

Spot of light produced inside the LHC.

When a charged particle, like a proton, is accelerated it emits some photons. Depending on the strength of the acceleration the photons can having different energies corresponding to radio waves, visible light, or even X-rays. At the LHC the protons go around a 27 kilometer (16.8 mile) ring at nearly the speed of light. The protons are kept on track by a series of 1232 bending magnets (dipoles). When you go around a corner in a fast car you can feel you body being flung outward, this is because the car is being accelerated inward. (Otherwise the car would keep going in a straight line!)  Each time a proton goes around the LHC ring it looses one billionth of its energy. With the planned operating energy in 2010 and 2011, a proton while going through each dipole will loose about one trillionth of its energy, which would be about 7 electron-Volts (the energy a single electron picks up going through a 7 Volt battery). A video camera positioned at the end of the dipole magnet can pick up this light, which can be used to monitor the size and position of the beam.

This is really the first time a beam of protons has been directly seen using light in the lab. The technical term for this light is synchrotron radiation. In previous machines the synchrotron radiation from protons has been too feeble to see. The amount of synchrotron radiation goes inversely with the mass of the particle squared; this is the reason that the highest energy machines use protons rather than electrons. Electrons are 2000 times lighter than protons, so they would have 4 million times as much synchrotron radiation. If electrons were being used instead of protons, the electricity bill would at least 4 million times larger just to keep the electrons up to speed!

### Varieties of Particle Jets

Friday, September 18th, 2009

A simulation of a string repeatedly breaking looks similar to the jets of particles found in collisions of quarks and gluons.

“Jets” is the name given to sprays of particles, headed in roughly the same direction, that appear when quarks or gluons collide. Jets turn out to be a useful way to relate experimental results on quarks and gluons with the theory of Quantum Chromodynamics (QCD) which describes the interactions of quarks and gluons. Their usefulness arises partly because the jets can be seen to emerge in a simple way.  The primary particles involved in the scattering have a small probability to emit a new gluon which, it turns out, is most likely to head in the direction of the particle that emitted it.  The new gluons have a small probability to emit further gluons, and so on. Iterating this process a few times gives you a jet of quarks and gluons. The gluon emission probability is small because the QCD interaction strength is fairly small in high-energy processes.

It is somewhat surprising that we can also see jets emerge in an entirely different way.  It is known that QCD becomes much simpler if we imagine that the number of “colors” of quarks is a large number, N, rather than the small number, 3, that we find in our Universe.  For large N, the allowed configurations have a flux-tube, or string, connecting every quark to an anti-quark (to make a meson) or have the strings of N quarks meeting at a point (to make a baryon). This “large N approximation” actually does a pretty good job of describing our world, leading to the oft repeated quasi-joke that 3 is a large number.

A baryon, like a proton, consists of three quarks connected by three strings which meet at a junction.

Of course such strings can break.  This occurs when a quark and anti-quark are created at some point along the string.  The energy required to produce the quark and the anti-quark can be offset by the broken string contracting.  In this way we can imagine a heavy meson or baryon with a very long (excited) string decaying into lighter “daughter” mesons and baryons made of shorter strings.

The string in baryon can break to form a new baryon and a new meson.

Imagine producing a quark and an anti-quark in a high energy collision. The quark and the anti-quark would be flying apart in opposite directions with a string stretching between them. Starting with this very excited “meson,”  we could simulate the repeated breaking of the string and see what comes out.  This is a little tricky, the quarks and strings are moving in a complicated way due to all this breaking, but we were able to do it (mainly thanks to Matt Reece’s programming skills).  The result is that the string tends to break into relatively short bits, which therefore have little rest mass (the mass grows like the string length) and thus lots of kinetic energy, since the total energy has to add up to the initial energy. Interestingly the string bits end up mostly going in the directions of the initial quark and anti-quark. This is because in the rest frame of one of the daughter mesons, the subsequent “grand-daughters” are equally likely to go in any direction, but in the rest frame of the lab, the daughter meson was moving rapidly in the direction of the original quark or anti-quark, and the grand-daughters are “thrown” forward in this direction. So we get something that looks very much like a jet.  This is just what is shown in the picture at the top of this page.

This is very different from what happens in theories where the interaction strength and N are both large.  Such theories can be approximately scale invariant in which case they are called conformal field theories (or CFT’s).  CFT’s are thought to be described by almost non-interacting particles moving in a five dimensional anti-de Sitter (AdS) space.  This is the basis of the AdS/CFT correspondence. It is fairly easy to work out what happens in this case, either using CFT methods or direct simulation in AdS.  Each time an excited meson decays into two lighter mesons, most of the initial energy goes into the rest mass of the daughter particles, so they have very little kinetic energy.  This means that there is very little difference between the rest frame of the daughter particle and the rest frame of the lab, so the grand-daughters are equally likely to go in any direction. The result of this type of process is shown below.

When the interaction strength is large enough the jets broaden so much that the events look spherical.

This raises some interesting prospects for the large hadron collider.  First we need fairly precise estimates of standard QCD jets, especially those containing b quarks, so that we can separate out the “old” physics from the new physics, and the stringy picture may be helpful for improving these calculations.  Second, in some scenarios the new physics does look like a CFT, in which case the standard types of analysis will not be helpful in teasing out the underlying information. In that case we will need some new ideas in order uncover the new physics.

(Technical note: in the top and bottom graphic, the length of each lines is proportional to the energy of the particle moving in that direction.)

### LHC Cooldown Begins Again

Friday, September 11th, 2009

The cooldown has begun for the final section of the Large Hadron Collider ring.  Five of the eight sections are already in the few degree Kelvin range, which is already colder than outer space! The temperature of the magnets in the final section  (known as sector 6-7, since it lies between the cleverly named points 6 and 7 on the ring) is shown above. This image should automatically update if you reload the page.  Temperatures for all the sectors can be seen here.

### LHC Explosion

Thursday, March 5th, 2009

Pictured left is the assembly of a dipole magnet junction at the LHC. To the right is what is left of a connector after a 1000 amp arc from a short circuit.

After the LHC startup, there was a major malfunction, a bad electrical connection led to a 1000 amp arc of current which completely vaporized the surrounding metal.  This also released the liquid helium which immediately evaporated with explosive force.  1 ton magnets were knocked off of their supports and soot filled large parts of the vacuum chamber.  Hopefully everything will be put back together by next fall.  The current plan is to run through next winter without a shutdown, in order to make up for lost time.

Photos taken from Roger Bailey’s talk at the Aspen Winter Conference

### LHC Startup

Thursday, September 11th, 2008

a CMS event from a the LHC startup. A proton has collided with a strip of metal inserted into the beam.

The LHC successfully started up in Geneva yesterday and as expected the world did not end. A creative film-maker did however come up with a fun little clip of the LHC being sucked into a black hole. Locally swissnex, the Swiss knowledge exchange program hosted a celebration in downtown San Francisco where particle physicists from Northern California congregated.

Wired magazine was there and you might recognize some familiar faces…

### LHC VR Tour

Wednesday, September 3rd, 2008

portion of a virtual reality page showing the ATLAS detector construction

The Large Hadron Collider will be starting up on Sept. 10, and the excitement (of physicists at least) is starting to build. Photographer Peter McCready has a set of virtual reality tours of that were taken during the construction phase of the project in case you didn’t get to see it in person.

The start-up will also be broadcast live.  You can check it out if you have satellite TV access, the schedule is here.

### Aspen LHC Workshop

Wednesday, August 13th, 2008

Lian-Tao Wang at Aspen

I was just at the Aspen Center for Physics to attend a workshop called “LHC: Beyond the Standard Model Signals in a QCD Environment.” I saw some interesting talks by Johan Alwall on merging of parton showers and matrix elements, Graham Kribs on the R-symmetric supersymmetric standard model, Jesse Thaler on generating QCD events analytically, Spencer Chang on dark matter and the DAMA signal, and Lian-Tao Wang on identifying highly boosted tops. There was also time to work on some new ideas as well as do a little biking and hiking.

### LHC Beam Transfer test

Wednesday, August 13th, 2008

the yellow spot indicates a bunch of protons arriving after traveling one eighth of the way around the Large Hadron Collider tunnel

Last weekend the Large Hadron Collider was tested by injecting a small number of protons into the beam line and steering them part way around the ring. Another test beam will be sent in the opposite direction on Aug. 22.

### Zuoz Summer School

Monday, August 11th, 2008

the view of Zuoz from the Lyceum Alpinum

I recently spent a week lecturing at a summer school in Zuoz Switzerland. This was finally my chance to stay at a Swiss Boarding School: the Lyceum Alpinum. My lectures were on strong interactions: Quantum Chromodynamics, Seiberg duality, the anti-de Sitter/conformal field theory correspondence and various models like quirks, higgsless, and hidden valleys, There were also lectures on black holes, little Higgs models, and the Large Hadron collider.  Slides of all the lectures can be found here.