Posts Tagged ‘physics’

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.

data for jets in lead-lead collisions

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.


Further reading:


Read or Written a good Ph.D. Thesis Lately?

Monday, July 25th, 2011

It might be eligible for an award:

Dissertation Award in Theoretical Particle Physics

Starting this year, the Division of Particles and Fields has
established a Dissertation Award in Theoretical Particle Physics.
The Award recognizes exceptional young scientists who have
performed original doctoral thesis work of outstanding scientific
quality and achievement in the area of theoretical particle physics.
The annual Award consists of $1,500, a certificate citing the
accomplishments of the recipient, and an allowance of up to $1,000
for travel to attend a meeting of the DPF or APS, where the Award
will be presented.

Nominations will be accepted for any doctoral student studying at
a college or university in the United States or in an education
abroad program of a college or university in the United States for
dissertation research carried out in the field of theoretical
particle physics. The work to be considered must have been
completed as part of the requirements for a doctoral degree.
Nominees for the 2012 Award must have passed their thesis defense
between September 16, 2010 and September 15, 2011.

The deadline for submission of nominations for the 2012 prize is
October 1, 2011. For detailed guidelines and to submit a nomination,

Mystery bump at Tevatron

Monday, June 13th, 2011

With great fanfare the CDF experiment released a paper on Wed. April 6, 2011 saying that they had found a significant discrepany from the Standard Model of particle physics just months before the Tevatron is scheduled to shut down. Is it a new particle, a statistical fluke, or a “budgetron”?

The media was given access to the discovery before the physics community, so there was already a New York TImes article on April 5. Another example of the disturbing trend of science by press release.

FermiLab Physicists May Have Found New Particle
The results, if they hold up, could be a spectacular last hurrah for Fermilab’s Tevatron, once the world’s most powerful particle accelerator and now slated to go dark forever in September or earlier, whenever Fermilab runs out of money to operate it.
[1104.0699] Invariant Mass Distribution of Jet Pairs Produced in Association with a W boson in ppbar Collisions at sqrt(s) = 1.96 TeV
Apr 4, 2011 … arXiv . org > hep-ex > arXiv : 1104.0699 . Search or Article-id. (Help | Advanced search) … Link back to: arXiv , form interface, contact.
The bump appears in the combined mass of two QCD jets produced alongside a W boson.
The Tevatron goes bump | Jon Butterworth, Life & physics | Science |
This week the CDF experiment at the Tevatron proton-antiproton collider caused a stir with this paper and a special seminar. This is a common situation in scientific research. The result is (a) really important if confirmed and (b) in need of confirmation before everyone is certain of it.
Anomalies at Fermilab | Cosmic Variance | Discover Magazine
The Tevatron accelerator at Fermilab is shutting down soon, for some unavoidable reasons (the LHC is taking over) and some frustrating ones (we’re out of money). But there may be life in the old beast yet; a couple of intriguing anomalies have particle theorists raising their eyebrows in charmingly understated excitement.
Mystery Atom Discovery Has Physicists Abuzz
In a development physicists are calling "huge," "tantalizing" and "unexpected," researchers have measured a signal that could herald a new kind of particle or force of nature. Yet the finding is not yet conclusive, and leaves many researchers skeptical. The discovery comes from an atom smasher called the Tevatron at the Fermilab physics laboratory in Batavia, Ill.

It was quickly pointed out that knowing the energy of the jets is extremely important, and an error in this step could be the explanation of the effect.  An animated gif by Tommaso Tabarelli de Fatis dramatically shows the how big this effect could be.

The Jet Energy Scale As An Explanation Of The CDF Signal
In my post about the new CDF signal of a mysterious new resonance decaying to jet pairs, there is an active comments thread. I posted there a graph crafted by Tommaso Tabarelli de Fatis, a CMS collaborator, who picked the CDF data and simulation and

May 20, The CDF collaboration blesses an updated analysis with more data but the results are withheld from other physicists until it can reviewed by lab director Pierre Odone.

May 30, In a talk by Giovanni Punzi at the Blois conference, CDF announces that with added data the significance of the bump has increased to 4.8 sigma, which is almost enough to proclaim a discovery.
RÉSONAANCES: CDF: Wjj bump almost 5 sigma!!!
Today at the conference Rencontres de Blois the CDF collaboration presented an update on the invariant mass of 2 jets produced in association with a W boson. Recall that 2 months ago CDF posted a paper based of 4.3 fb-1 of data claiming that this observable displays an unexpected bump near 150 GeV with a significance of 3.2 sigma.
June 10, The D0 (pronounced D-Zero) experiment at the Tevatron reveals their independent analysis of their own data.  They find that the hypothetical CDF particle is ruled out at 99.999% confidence level.
Fermilab’s D0 detector shows no sign of the CDF "bump" that could have signalled physics beyond the Standard Model:
June 10, 2011
Tevatron teams clash over new physics : Nature News
Nature – the world’s best science and medicine on your desktop

A task force is set up to get to the bottom of the discrepancy. The task force will include Fermilab theorists Estia Eichten and Keith Ellis.

#Fermilab finds no new particle in 2nd experiment, forming task force to figure out what’s up:
June 10, 2011
Fermilab Today
Two months ago, CDF scientists reported an unexpected excess of proton-antiproton collisions that produce a W boson accompanied by two jets of particles. One possible explanation for the excess could be the existence of a new, unanticipated particle. Now the DZero collaboration has finished an independent analysis that tests the CDF result.
Panel probes new particle results
The head of the US’ biggest particle physics lab has appointed an expert committee to establish whether or not a new, unanticipated sub-atomic particle has been detected by scientists. Such a discovery, hinted at by experts in April, would mark one of the most radical changes to physics in years.
Sept. 30, The Tevatron is shutdown for good. So far the panel has not made any progress, each of the experiments blames the other. maybe only the LHC can resolve the story…

Neutrinos, Antarctica, and People

Sunday, March 28th, 2010

Another highlight from the Moriond conference: Abby Veiregg gave a presentation on the ANITA experiment. ANITA is basically a set of radio antennas put on a balloon that circles the Antarctic for several weeks at a time.  The experiment is looking for evidence of extremely high-energy cosmic ray neutrinos. When these high-energy neutrinos graze the Antarctic ice, they are traveling faster than light can travel in the ice, which produces the analogue of the shock wave a supersonic aircraft makes. This electromagnetic shock wave, called Cherenkov radiation, includes some radio waves that can be detected by ANITA. Unfortunately this round of the experiment was not sensitive enough to see any neutrinos; an upgraded version is currently being built. One of the reasons that the experimentalists had a difficult time is that there was a lot of radio wave background from human sources. Apparently there are scientists all over Antarctica!

Radio waves detected by the ANITA experiment provide a "heat" map of Antarctica.

Radio waves detected by the ANITA experiment provide a "heat" map of Antarctica.

The dark blue region is the area scanned by ANITA, the other colored areas represent places where radio waves were found.

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, image courtesy of Jorg Wenninger.

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
jets of particles

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.

three quarks connected by three strings

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.

string in baryon breaking

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.

spherical spray of particles

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.

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.

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…


Wednesday, September 3rd, 2008

portion of a virtual reality page showing the ATLAS detector construction

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.