This week I’ve decided to venture a bit away from my areas of expertise and examine a couple of papers from the field of High Energy Physics. I chose these two papers because they present an important negative result (not always worthy of publication, but definitely in this case), and because between these two papers there are 1200 authors! The sheer number of authors on these papers is often ridiculed, but it reflects the enormous effort needed to run massive particle accelerators such as the Tevatron at Fermilab. The picture at the left shows the ring of the Tevatron (top), along with the main injector ring (bottom), within which protons and antiprotons are accelerated to speeds ridiculously close to the speed of light. The Tevatron at Fermilab is currently the most powerful particle accelerator in the world, until the Large Hadron Collider at CERN is ready sometime in 2007.
The two papers I’m reviewing here present results from a search for the Higgs Boson on two enormous particle detectors along the Tevatron ring, the D0 and the CDF. The Higgs Boson is a hypothetical elementary particle (like neutrinos, electrons, or quarks) that plays an extremely important role in our universe; we believe that it gives all other particles mass. To “see” the Higgs Boson, the Tevatron smashes protons and antiprotons into each other with enough energy that some of the quarks that make up the protons fuse together. This fusion product is unstable and decays into a number of particles, including the Higgs. These collisions take place in the Tevatron only where the proton ring and the antiproton rings cross, inside the D0 and CDF detectors.
Citations:
(check out the complete author lists online at CiteULike.org, see bracketed links)
- CDF Collaboration (2006). Search for Neutral Higgs Bosons of the Minimal Supersymmetric Standard Model Decaying to tau Pairs in Proton-Antiproton Collisions at sqrt(s) = 1.96 TeV. Physical Review Letters 96(1). [online]
- D0 Collaboration (2006). Search for the Higgs Boson in H –> WW[sup (*)] Decays in Proton-Antiproton Collisions at sqrt(s) = 1.96 TeV. Physical Review Letters 96(1).[online]
Synopsis:
(follow link to their websites, both groups have plain english summaries of their research, an excellent outreach tool, CDF Collaboration, D0 Collaboration)
Previous studies have indicated that the Higgs Boson particle mass must be at least 114.4 GeV/c2 (a unit of mass used for elementary particles, a proton mass is approximately 1 GeV/c2). This determination was made by the non-detection of the Higgs Boson particle. The ability to detect a particle depends on the number of particles that are created in the accelerator, which depends on how many proton-antiproton collisions there are and at what energy those collisions occur. More massive particles require more energy to be created, and limitations of the sensitivity of detectors means that not all particles that are created can be detected.
The Tevatron at Fermilab has enough energy to produce Higgs Bosons, according to current interpretations of the Standard Model of particle physics, however the intensity of its proton-antiproton beams is thought not to produce enough Higgs Bosons for a detection to occur. So, during the most recent two-year run of the Tevatron from 2002-2004, physicists were optimistic but not expectant of a Higgs Boson detection. In fact, according to the D0 paper, fewer than 1 Higgs Boson particle was expected to be produced in the mass range 100-200 GeV/c2. Lighter Higgs particles would occur more often than heavier ones due to the energies in the accelerator.
The CDF group (whose detector is shown above on the right), was specifically looking for pairs of neutrinos that would signal the decay of a Higgs particle. There is a certain background rate of detection of these neutrino pairs that means that even if one or two Higgs particles were detected, their signature may be overwhelmed by the presence of noise in the detector. This same background noise problem is present in the D0 detector as well, in which the decay products of Higgs Boson were being searched for as well. A large chunk of the D0 group collaborators are shown in the picture on the left gathered for a conference in July 2002.
Neither group detected the products of a Higgs Boson to any degree of certainty. However, they used this non-detection to constrain the minimum mass of the Higgs as a function of the number of proton-antiproton collisions. This “negative result” was expected as mentioned above, and helps to rule out certain variants of the Standard Model theories.
Context:
The Tevatron experiment has run several times at the Fermilab. During a run in the mid-90s, the CDF and D0 detectors announced the discovery of the Top Quark, what is currently still the heaviest of the basic particles in physics. Since then, the CERN laboratory on the France/Switzerland border has committed to an upgrade that will make it the most powerful and most intense particle accelerator in the world. The Large Hadron Collider (LHC) is probably the place where the Higgs Boson will be confirmed, if indeed it exists. Presumably, the experiments that will prove the existence of the Higgs and measure its mass will be run and published by around 2010. Because a new particle accelerator is fantastically expensive, the Fermilab opted for upgrades to its existing Tevatron so that it could better measure the properties of many of the fundamental particles. It was hoped that perhaps a few Higgs Bosons could be detected as well, but the lack of their detection is certainly not a blow to the Standard Model.
General Explanations:
1100 authors on 2 papers? What’s going on here? Well, in experimental sciences, authorship is given to those who have contributed a significant amount of original analysis and preparation to the final results. While I don’t know the details of the D0 and CDF collaborations, it is likely that all 1100 authors spent some significant amount of time directly working on results that led to the final analyses in these papers. These detectors collect a fantastic amount of data. In fact, the LHC at CERN has necessitated the construction of the first truly worldwide supercomputing grid in order to process the petabytes of data that each run generates. So, it’s not hard to believe that while probably only 5-10 people actually wrote the final paper, all 1100 deserve credit for the work. So, follow those links over to CiteULike.org and browse through the list of collaborators!
Sorry, I don’t have too many explanations to offer on the physics here. The best place to go is Wikipedia, or failing that, Physics World. My physics knowledge is primarily confined to Astrophysics, and this paper required a major stretch of my credentials.
Cool.
Wow, Anthony, how do you find the time to even browse into something like this (pretty far afield), let alone create a rather detailed summary? I keep up with a lot of stuff but most of it at the “general info” level.
Interesting work, and interesting too that even a list of the authors would be a small database!
-Bruce
To be honest I’ve visited the Fermi lab and been given a tour by the MSU head of the D0 collaboration. Also, I was a member of the Society of Physics Students for a number of years and went to a few lectures dealing with the search for the Higgs as well as the efforts of the various National Laboratories to push forward the boundaries of particle physics. So, even though it’s not my field, I knew enough about this topic to not make a complete idiot of myself.
Plus, it was a rather slow research week in the big journals, so I had to dig into my “second tier” for an appropriate topic. It’s unfortunate that this week there are at least three or four unrelated articles that I would like to review!