The good folks at the LHC have not been shy about sharing their results. Indeed, at the end of last year, the bigwigs at CERN called a press conference to announce that they hadn't found the Higgs boson yet, but they were starting to see some signals that might be the Higgs. If only all of us in research could get away with progress reports like that.

OK, that was a very cynical opening to a story that shows the benefits of such openness. The signal seen by the LHC's CMS and ATLAS detectors hinted at a Higgs Boson with a mass in the range of 124-126GeV. But buried in the details are some numbers that, if they hold up, will be impossible to accommodate in the standard model of physics. What does any good theoretical physicist do in these circumstances? Plug the numbers into their favorite model to see if it is still in the running. Something that could not be done had CERN not been so open about its preliminary results.

Get off that branch before it breaks

The details of obtaining the Higgs' mass range contains a huge amount of statistics and modeling of particle production. It is not just that these collisions produce huge numbers of different particles, but that these particles can decay to different particles, and collisions between particles can produce different collision products. You can think of each collision as a measurement on a quantum system, where there is more than one possible result. But the probabilities of each result are governed by the underlying details of the collision.

Unluckily (or, perhaps, luckily), the detectors don't see any of these intermediate particles. Instead, they only detect the relatively stable end products—basically, the LHC detects electrons, positrons, muons, and radiation. It is then a case of figuring out, from large numbers of collisions, what paths were involved in creating the particles we do see.

Each particle could have arrived by a number of different pathways through intermediate particles. Some pathways are more common than others, so we end up with what are referred to as branching ratios. Adding Higgs production to the mix will enhance some branching ratios and suppress others. Luckily, the standard model of physics tells us how to calculate these changes.

This is where the results from CERN are important. The mass of the Higgs Boson fits quite nicely with the standard model, but the branching ratios, according to Cheung and Yuan, are going to be difficult to accommodate. What the CMS results show is that one particular branch must be enhanced by Higgs production, and two others are suppressed. But the standard model suggests otherwise (though it should be pointed out that the data is not certain enough to be clear that the standard model is wrong).

New Physics

This may actually come as a relief to many, because nothing new has been turned up by the LHC so far. Physicists have many proposals for physics beyond the standard model—all motivated by the desire to resolve conflicts between general relativity and quantum electrodynamics. And now everyone is waiting for data from the LHC to help decide which models best reflect the world.

The most popular of these models involves giving every particle a heavy partner to satisfy certain symmetries—the model is called supersymmetry. It turns out that there are a few ways to make supersymmetric models, but physicists have generally favored the simplest. Except that if that model were right, the LHC should have started to see signs of the lightest particles predicted by supersymmetry. Which it hasn't.

So the field appears to be rather open at the moment, with every new data point eliminating someone's favorite model while providing tantalizing hints that someone else's might be right. In this case, the model that's still in the running is a relative of supersymmetry, involving one extra dimension and a lot of new, heavier particles. Now, the production of one particle, called the radion, would have the effect of simultaneously enhancing one branching ratio while suppressing others, in agreement with the LHC data.

This paper can't really come to any clear conclusions because the data from the LHC is not certain enough to support anything definitive. But what this points to is the difficulty in understanding and interpreting data from modern particle accelerators. Even if, in the next year, the LHC pins the Higgs down to 125GeV, it is unlikely that the data will be clear enough to pick a single model for physics beyond the standard model—if, indeed, it provides any support for such a model at all.

I also think that particle physicists get to use the coolest names for their particles.

ScienceDaily (Apr. 27, 2012) — Physicists from the University of Zurich have discovered a previously unknown particle composed of three quarks in the Large Hadron Collider (LHC) particle accelerator. A new baryon could thus be detected for the first time at the LHC. The baryon known as Xi_b^* confirms fundamental assumptions of physics regarding the binding of quarks.

In particle physics, the baryon family refers to particles that are made up of three quarks. Quarks form a group of six particles that differ in their masses and charges. The two lightest quarks, the so-called "up" and "down" quarks, form the two atomic components, protons and neutrons. All baryons that are composed of the three lightest quarks ("up," "down" and "strange" quarks) are known. Only very few baryons with heavy quarks have been observed to date. They can only be generated artificially in particle accelerators as they are heavy and very unstable.

In the course of proton collisions in the LHC at CERN, physicists Claude Amsler, Vincenzo Chiochia and Ernest Aguiló from the University of Zurich's Physics Institute managed to detect a baryon with one light and two heavy quarks. The particle Xi_b^* comprises one "up," one "strange" and one "bottom" quark (usb), is electrically neutral and has a spin of 3/2 (1.5). Its mass is comparable to that of a lithium atom. The new discovery means that two of the three baryons predicted in the usb composition by theory have now been observed.

The discovery was based on data gathered in the CMS detector, which the University of Zurich was involved in developing. The new particle cannot be detected directly as it is too unstable to be registered by the detector. However, Xi_b^* breaks up in a known cascade of decay products. Ernest Aguiló, a postdoctoral student from Professor Amsler's group, identified traces of the respective decay products in the measurement data and was able to reconstruct the decay cascades starting from Xi_b^* decays.

The calculations are based on data from proton-proton collisions at an energy of seven Tera electron volts (TeV) collected by the CMS detector between April and November 2011. A total of 21 Xi_b^* baryon decays were discovered -- statistically sufficient to rule out a statistical fluctuation.

The discovery of the new particle confirms the theory of how quarks bind and therefore helps to understand the strong interaction, one of the four basic forces of physics which determines the structure of matter.

The University of Zurich is involved in the LHC at CERN with three research groups. Professor Amsler's and Professor Chiochia's groups are working on the CMS experiment; Professor Straumann's group is involved in the LHCb experiment.

CMS detector

The CMS detector is designed to measure the energy and momentum of photons, electrons, muons and other charged particles with a high degree of accuracy. Various measuring instruments are arranged in layers in the 12,500-ton detector, with which traces of the particles resulting from the collisions can be recorded. 179 institutions worldwide were involved in developing CMS. In Switzerland, these are the University of Zurich, ETH Zurich and the Paul Scherrer Institute.