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
Introduction
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).=
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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.