My research and the 2008 Nobel Prize in Physics

Research that I carried out, together with members of my group in the Department of Physics of Liverpool University, is linked to the Nobel Prize in Physics 2008 awarded on October 7th to Yoichiro Nambu "for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics," and to Makoto Kobayashi and Toshihide Maskawa "for the discovery of the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature." The Nobel Foundation mentions the BABAR and Belle experiments which confirmed key predictions by Kobayashi and Maskawa, and references our publication:
B. Aubert et al. [BABAR Collaboration], Measurement of the CP-violating Asymmetry Amplitude sin2beta, Phys. Rev. Lett. 89 (2002) 201802
Discovery of CP Violation in the B system and the Nobel Prize
My personal contributions
Next stage: search for neutrino CP Violation

Introduction: CP Violation, the Kobayashi-Maskawa theory, BABAR, my group at Liverpool

For each known type of particle there is an anti-particle (with identical mass and opposite electric charge). When a particle-antiparticle pair (e.g. an electron and a positron) come close they annihilate: they both disappear and their mass becomes energy (photons, i.e. light). The Universe at early stages contained equal amounts of matter and antimatter. That Universe was very violent, with annihilations happening everywhere and huge amounts of energy being released constantly. If all was symmetric, today there would be little matter, little antimatter, and lots of photons. Life as we know it would have been impossible.

The development of our matter-dominated, suitable-for-life Universe was only possible because at some early point antimatter disappeared completely: out of every 10 billions of particles and antiparticles only 1 particle survived. Understanding the mechanisms that created this matter-antimatter asymmetry is essential if we want to know “why are we here”.

Until 1964 it was assumed that if we do any experiment with particles and the same experiment in a mirror (exchanging left-right and forward-backward) with antiparticles the results would be exactly identical: Nature was believed to obey “CP symmetry”. In 1964 Christenson, Cronin, Fitz, and Turlay at the Brookhaven AGS accelerator found that this symmetry is violated in the decays of Kaons and antiKaons (mesons containing a strange quark or antiquark). The unexpected discovery earned two of them the physics Nobel Prize in 1980. Many theories were developed to explain this phenomenon (CP Violation), some assuming the existence of new, unknown, forces or particles. In 1972 Kobayashi and Maskawa were able to explain CP violation assuming the existence of at least three more quarks (to add to the three known at the time). Their work extended earlier work by Nicola Cabibbo resulting in the so-called “CKM model”.

The extra quarks (charm, beauty, top) were discovered between 1974 and 1994, but room for alternative theories of CP violation remained. CKM predicted that CP violation should also appear in B mesons (heavier cousins of kaons containing a beauty instead of a strange quark) and by the mid-80s an accurate prediction of the size of the effect was available. Discovery of CP violation in the B system and comparison of its size to this prediction would be the acid test to confirm CKM and invalidate alternatives developed since 1964. This motivated the construction of two specially-built facilities (B Factories): The SLAC B Factory consists of the PEP-II accelerator and the BABAR detector at Stanford, California, and the KEK B Factory consists of the KEK-B accelerator and the Belle detector at KEK, Tsukuba Japan.

BABAR is a $300M experiment carried out by 600 researchers from 75 institutes in 11 countries at SLAC, near Palo Alto, California. BABAR collected data containing half a billion B-antiB meson pairs between 1999 and 2008. Liverpool physicists were founding members of the collaboration in 1994, constructed part of the detector in the Department of Physics, and led many physics analyses since operations started in 1999. I joined the group in 1997 and I became head of the group (Principle Investigator) in 2001. The group includes Professor Erwin Gabathuler FRS OBE, Dr John Fry, Dr Raymond Gamet, Dr David Hutchcroft, and 7 postdocs over the years (in post in 2008: Dr David Payne, Dr Carlos Chavez, Dr James Burke).

CP Violation in the B meson system and the 2008 Nobel Prize

BABAR (and Belle) started collecting data in summer 1999 and presented first results in summer 2000. In July 2001 BABAR measured a non-zero value for the parameter sin2beta and established CP Violation in the B meson system with a probability of 99.9997%. We published our result in Phys. Rev. Lett. 87, 091801 (2001). This result proved the CKM model. Belle confirmed our result one month later. Subsequent measurements with improved precision from both experiments found this parameter in excellent agreement with the CKM expectations. Later we also measured the CP Violation parameters sin2alpha and gamma, both in agreement with CKM. Following our measurements it is now accepted beyond any doubt that the CKM mechanism is the one describing the known CP Violation in Nature and alternative theories developed in the last 40 years are finally put to rest. Our 2001 discovery was reported widely in the international press (including the New York Times and the Economist).

Personal contributions

In 1996 I proposed to measure sin2b in BABAR and that proposal earned me a five-year PPARC Advanced Fellowship. I joined the experiment in 1997 at Liverpool University and I developed event reconstruction and selection methods for one of the “golden decays” (B to J/psi Kshort with the Kshort going to two neutral pions). I also demonstrated the reconstruction of the decay mode B to J/psi Klong which was then pursued further by other collaborators in BABAR. In 1999-2001 I lived at Palo Alto working full-time on the BABAR experiment. The events selected by my analysis were part of the sample that led to the 2001 discovery and subsequent measurements. In 2001-2003 I was co-convener of the Charmonium Analysis Working Group in BABAR, that analyzed decays of B mesons to final states containing charmonium (J/psi, psi prime, eta_c) which are the golden modes for measuring sin2b. In 2001-2004 I was either editor or reviewer of all BABAR publications in this area. In July 2001 I did the first presentation of our discovery at an international meeting on behalf of the collaboration (European Physical Society conference in Budapest.

In 2003 I initiated, together with colleagues at Saclay, the measurement of sin2a using the B to rho+ rho- decay channel, which at the time had not been yet observed. We developed event reconstruction and selection and advanced fitting with multi-dimensional maximum likelihood fitting implemented on the RooFitTools toolkit. In 2004 we presented our results which became a highlight in that year’s particle physics conferences, as this proved to be the most sensitive method for the measurement of sin2a, contrary to prior expectations. Our result was, again, in agreement with the CKM model.

Matter dominance in the Universe, neutrinos, and the way forward

In 1967 Andrei Sakharov stated that in order for our matter-dominated universe to develop there had to be a period in its early history when three conditions were simultaneously fulfilled: CP violation, Baryon number violation, and the universe out of thermal equilibrium. The known CP Violation in the quark sector (described above) is too weak by 10 orders of magnitude, based in comparisons of our measurements and astronomical observations (amount of Helium and study of the photon spectrum in the Universe). So why are we here?

CP Violation appears in systems made of quarks. This is well known but too weak to play this role in cosmology. The other option is CP Violation in leptons, and in particular neutrinos. Neutrinos are the least interacting particles known. Billions pass through our bodies every minute but not much happens. This makes them difficult to detect and study. Neutrinos, like quarks, come in three families, and as we know in the last ten years they can also oscillate, i.e. change from one type to another. This quantum mechanical effect (flavour mixing) allows with three families the possibility that CP is violated in this system. If that happens, and it happens more than in the quark sector, then this could have been the CP Violation in Sakharov’s conditions!

Neutrino oscillation is now established but not well studied. In particular one of the three possible oscillations (the one we call 1-3) has not been detected yet. The discovery of this oscillation is the main objective of the T2K experiment which will use the highest power pulsed proton beam in the world from the JPARC accelerator facility in Japan and the SuperKamiokande 50,000ton detector 300km away. An international collaboration of 400 scientists, T2K is also constructing a near detector, which will study the neutrino beam near its origin, before any oscillation takes place. I am involved in this project as head of the Liverpool group which I initiated in 2003, responsible for the electromagnetic calorimeter for the Near Detector, Project Manager of the T2K-UK £15M construction project, and deputy chair of the T2K Global Analysis Group. T2K will dominate neutrino oscillation measurements in the world for the next five years, and we are already planning the next phase where we will use the beam with more power and we will build a huge far detector (which could be a cryogenic 100,000ton Liquid Argon Tracker) which will be sensitive to neutrino CP Violation. I am currently initiating in the UK an R&D programme in this direction.