More than 80 physicists from all over the world met in January in the ancient Portuguese town of Coimbra to take part in the International Workshop on Top Quark Physics, TOP2006.
Coimbra, in central Portugal, was the country’s capital from 1143 to 1255 and in historical importance ranks behind only Lisbon and Oporto. Its university was founded in 1290 and was the only one in Portugal until the beginning of the 20th century. Its ancient setting contrasted well with the central theme of TOP2006: the top quark, discovered only in 1995 in experiments at Fermilab’s Tevatron.
The workshop itself grew from the idea of developing a strong collaboration between theorists and experimentalists who are interested in studying the properties of the top quark. The first properties of this unique particle were measured during Run I of the Tevatron by the CDF and D0 experiments; with Run II more data are now becoming available. Though not yet sufficient to perform the precision tests required to challenge (once again) the Standard Model, the data acquired so far are already providing valuable information on top-quark physics. The knowledge of the physics of the top quark will then enter a totally new phase – the precision era – with the start-up of the Large Hadron Collider (LHC) at CERN, foreseen towards the end of 2007.
The top quark is the heaviest quark found (mt = 172.5±2.3 GeV/c2) and is still believed to be a fundamental particle. It completes the third-generation structure of the Standard Model, as the isospin partner of the b (bottom) quark. Why it is so heavy and why its Yukawa coupling to the Higgs field (after spontaneous symmetry breaking) is of the order of 1 is a mystery. Its solution requires an answer to the question: does the top quark play a special role in the electroweak symmetry-breaking mechanism of the Standard Model?
Although mainly produced via the strong interaction at particle colliders (double production via gluon–gluon fusion or qqbar annihilation), the top quark decays through the weak force to a b quark and a W boson with a branching ratio of almost 100%. Because of their large mass and decay rate (Γ = 1.42 GeV at next-to-leading order), top quarks, unlike any other quark, are produced and decay as free particles. With a very short lifetime (around 10–25 s), the top quark decays before hadronization can take place. For the same reason no toponium bound states with sharp binding energy are expected in the Standard Model; any evidence of a ttbar bound state would be a sign of physics beyond the model. The flavour-changing neutral-current decays of the top quark are also highly suppressed in the Standard Model, with branching ratios at the level of around 10–12 to 10–14; any evidence of decays such as t → qZ, qγ or qg would therefore constitute a sign of new physics.
Top-quark properties
The first day of the workshop was dedicated to the current theoretical and experimental status of top-quark physics, in the morning and afternoon sessions, respectively. C P Yuan of Michigan State University recalled the need for a precise measurement of the top-quark mass to constrain the Higgs mass when combined with the measurement of the W mass. Within the context of current theoretical knowledge, the day also covered the importance of the rate of single top production at colliders (not yet observed) as a probe for the element Vtb in the Cabibbo–Kobayashi–Maskawa matrix. He also stressed the fact that the different channels (s, t and Wt) that contribute to single top production are important processes for the search for physics beyond the Standard Model.
Aurelio Juste from Fermilab reviewed the current experimental status of the top quark starting from the total cross-section measurement at the Tevatron, with a relative precision of around 25% in Run I, dominated essentially by statistics. In Run II, with a luminosity of 2 fb–1, this error is expected to be reduced to about 10%. The mass is by far the most precisely measured property of the top quark, with a relative error less than 2%. The top charge, anomalous couplings and single top production were also discussed.
The second day examined the experimental methods used to select top quarks at colliders, and the leading-order and next-to-leading-order generators and theoretical methods available for understanding the data. Evelyn Thomson of the University of Pennsylvania presented the experimental methods that are used in the selection and analysis of top-quark decays at hadron colliders. In particular, she discussed the importance of the trigger, the difficult question of the background rejection and estimation (as W+jets and Z+jets), the need for a detailed calibration and determination of the jet energy scale (a major source of systematic error), and b-tagging, a key tool to reduce the background. She stressed the need to fine-tune the available Monte Carlo to reproduce data accurately. Available top-selection tools involve multivariate analysis and different statistical techniques.
Werner Bernreuther, of RWTH (Rheinisch-Westfälische Technische Hochschule) Aachen, described spin effects in hadronic top-pair production and polarized top decays, ttbar spin correlations (which are transferred to the decay products), and the possible existence of heavy ttbar resonances. As the top polarization is reliably calculable, it is well suited for experimental checks of the predictions of the Standard Model and its extensions. Bernreuther concluded that the top-quark physics is an excellent probe to test electroweak symmetry breaking and that it provides powerful observations to determine the structure of the tbW vertex. Sergey Slabospitsky of the Institute for High Energy Physics, Protvino, and Borut Kersevan of the Josef Stefan Institute presented the status of the important event generators that are being developed and used at the Tevatron and LHC to simulate top production and decays.
Top prospects
The prospects for top physics on the up-coming colliders were discussed on the third day of the workshop. In the morning, Dominique Pallin of Blaise Pascal University presented the expected performance of the LHC as a top factory. In particular, he showed the work going on for early top-quark studies, such as the measurement of the ttbar production cross-section and the top mass, as well as the determination of the W and top polarizations, in the lepton+jets channel. The top quark is a very useful calibration tool for early data (for the jet energy scale, b-tagging, trigger etc), which can also be used to check detector performance. With the increase of luminosity at the LHC many precision measurements of top-quark properties will be possible.
In the afternoon, Lynne Orr of the University of Rochester gave a talk about top physics at the LHC and a future International Linear Collider (ILC). She described the electroweak symmetry breaking mechanism and the hierarchy problem. She also discussed top-quark physics in models beyond the Standard Model, which are possible solutions to this problem: supersymmetry, little Higgs, technicolour and its descendents, and modified space–time models with extra dimensions. Finally, the sensitivity of different top-quark couplings at the LHC and ILC was reviewed. Brian Foster of Oxford University presented the status of the ILC.
Finally John Womersley, of the CCLRC, Rutherford Appleton Laboratory, presented a lively and appealing workshop summary talk. He also covered the status and the open questions in particle and astroparticle physics. All in all, the workshop was a fruitful opportunity for interesting discussions on the exciting subject of top-quark physics. The participants are looking forward to the next workshop, which will probably take place two years from now, where the latest results of the Tevatron’s Run II and the first results from the LHC in top-quark physics will be presented and discussed, and new challenges to the Standard Model will be tested.
