Annual Report 2006

Annual report FOR THE YEAR 2006

The scientific activities of the Theory Unit (PH-TH) of CERN’s Department of Physics cover a
wide spectrum of research, reaching from phenomenological topics related to the physics potential of the
LHC to formal aspects of string theory. Let us, before highlighting some of the achievements that were
obtained in 2006, give a brief statistical overview of the publications (showing also the corrsponding
numbers for 2005 in parentheses): 273 (269) papers appeared with a registered CERN-PH-TH number,
and 264 (303) publications with at least one author showing a CERN affiliation were submitted to the
e-print archives. The distribution over the individual archives reads as follows: 134 (172) hep-ph, 82
(66) hep-th, 27 (28) astro-ph, 15 (18) hep-lat, 3 (10) gr-qc, 2 (8) nucl-th, and 1 (1) quant-ph. If we group
these papers by their main subjects, we obtain the following statistics: 33% (37%) Standard Model (SM)
phenomenology, 18% (22%) physics beyond the SM, 29% (22%) string theory and formal aspects of
field theory, 14% (13%) particle astrophysics and cosmology, and 6% (6%) field theories on the lattice.
There is a significant overlap between these categories, which contain also some more specific research
topics. The “SM phenomenology” area consists, for instance, also of heavy-ion and flavour physics,
while “beyond the SM” includes papers with a pronounced phenomenological cut and others dealing
with more field-theoretical aspects of the extensions of the SM. It should also be emphasised that several
papers of the SM category are actually driven by the search for new physics (NP), which requires a solid
understanding of the SM picture.
We shall now have a closer look at the activities of the Theory Unit in 2006, where the quickly
approaching start of the LHC is triggering a lot of excitement. The focus of the collider physics group
has continued to be the preparations for this new machine. Since the LHC is a hadron collider, every
collision of interest is initiated by coloured partons, and all processes suffer large corrections from QCD
effects which are blunting our sensitivity to any NP that may lie hidden in the corresponding observables.
The Theory Unit has continued to play a leading rˆole in pushing back these uncertainties on
several fronts. With the need for higher order perturbative calculations for a wide variety of processes
firmly established, much progress was made towards fully automated next-to-leading order calculations.
Several different methods for numerically evaluating the necessary loop amplitudes with many external
legs were proposed and studied. Progress was also made towards combining these loop integrations with
the phase-space integrals needed for real emission contributions. Ground-breaking results for two-loop
amplitudes are starting to bear fruit for cross section calculation, for example for Higgs boson production
via gluon fusion. New methods for the numerical implementation of next-to-next-to-leading order jet calculations
were also proposed. Our long-term event generator projects have continued as well, with the
existing version of HERWIG effectively frozen but still receiving a high level of user support and physics
exploitation, particularly through the MC@NLO package, which combines the strengths of HERWIG’s
parton shower algorithm with the accuracy of a next-to-leading order (NLO) cross section calculation.
A new and improved generation of programs, ALPGEN and Herwig++ is under active development.
These Monte Carlo tools were also used to make important phenomenological studies in preparation for
the LHC, for example of spin correlations in Higgs + two jet events, and of the effect of different jet
algorithms on the top mass measurement. Finally, more theoretical studies of QCD in extreme kinematic
regimes produced the first application of the NLO BFKL equation to jet production in the limit of very
high energy and uncovered an unexpected tower of superleading logarithms in the limit of a very hard
rapidity veto. Furthermore, a workshop (MC4LHC) was organized, which brought together most of the
world experts in Monte Carlo event generators for hadronic collisions, who spent two weeks at CERN
discussing the recent developments, and planning the work for future progress.
In the phenomenology of heavy-ion collisions, members of the Theory Unit have continued to
focus on the novel theoretical challenges arising from the 30 times higher centre-of-mass energy of LHC
compared to previous measurements at RHIC. One focus of the research was on “hard probes” such
as jets, open charm and beauty and their sensitivity to the properties of the dense state produced in
the collision. In particular, the group explored how jet structures can be characterized within the high
multiplicity environment of a heavy-ion collision, and how they are modified characteristically dependent
on the properties of the medium. In a different line of approach, string theory techniques were used for
the first time to address the in-medium propagation of quarks and quarkonium bound states. While
these are not QCD calculations, they reveal many generic features expected to carry over to QCD, and
they promise novel ways of gaining insight into the physics of viscous hot QCD matter, jet quenching
and quarkonium dissociation. Preparation of the theoretical tools for the most versatile understanding of
LHC data continued also in the soft physics section, where work focussed on understanding the range of
validity of hydrodynamical modelling of the collision dynamics, and signatures of dissipative properties.
Also in 2006, members of the group had close contact with experimentalists working on the LHC heavyion
programme of ALICE, ATLAS and CMS.
The Theory Unit hosts also a group, which studies the non-perturbative regime of strong interactions
with the help of lattice QCD. This technique allows, in principle, the calculation of the basic
properties of the light hadrons using numerical simulations. Algorithmic advances made in the past few
years (the introduction of domain-decomposition methods in particular) allow numerical simulations of
lattice QCD to be performed at significantly smaller lattice spacings and quark masses than was possible
before. The spontaneous breaking of chiral symmetry, for example, can now be studied in great
detail. A rather striking outcome of the recent simulations is that leading-order chiral perturbation theory
describes the dependence of the pion mass on the light-quark masses very well up to meson masses of
about 500 MeV. Another topic is the exploration of the
I = 1/2 rule in the hadronic kaon decays,
which has always been difficult to explain. Using numerical simulations and a form of lattice QCD
which preserves chiral symmetry, the relevant transition matrix elements can be computed as a function
of the quark masses. As it turns out, a large
I = 1/2 enhancement is already present in the Glashow–
Iliopoulos–Maiani (GIM) limit, where the charm and strange quark masses are set to the light-quark
masses. Whether the decoupling of the charm quark increases the enhancement to its physical value is
currently being investigated. When calculating such matrix elements, non-perturbative renormalization
is indispensable, as otherwise it is not possible to fix the normalization ambiguities in a reliable manner.
The use of lattice formulations that preserve chiral symmetry brings in some extra theoretical complications
in this context. It has now been shown, however, that these can be circumvented either by using the
Wilson formulation as an intermediate scheme or an adapted definition of the Schr¨odinger functional in
which the chiral symmetry remains unbroken away from the space-time boundaries.
Studies of non-perturbative parameters through lattice QCD play also an important rˆole for the
testing of the Kobayashi–Maskawa mechanism of CP violation, which allows us to accommodate the
phenomenon of CP violation in the SM. The associated topic of quark-flavour physics is another line of
research at the Theory Unit. In this field, the highlight of last year was the observation of B0
s –¯B0
s mixing
and the measurement of the mass difference
Ms of the corresponding mass eigenstates at the Tevatron.
Since B0
s –¯B0
s mixing is a strongly suppressed loop process in the SM, it offers a powerful NP probe. The
experimental value of
Ms has received considerable attention from the theoretical community, also at
PH-TH, where the Bs-meson system is a key target of research for many years, as its physics potential
can be fully exploited at the LHC. A detailed discussion of the interpretation of the
Ms measurement in
terms of NP parameters was given and illustrated in specific scenarios for physics beyond the SM, with
the main result that the new data still leave a lot of space for CP-violating NP contributions to B0
s –¯B 0
s
mixing, which could be detected at the LHC with the help of the B0
s ! J/ channel. Besides these
studies, research of strongly suppressed “rare” B decays, which are absent at the tree level of the SM,
was performed, and phenomenological studies with a strong interplay between theory and the B-factory
data were further pursued, with an emphasis on puzzling patterns in the data that may be first indications
of new sources of CP violation. In 2006, also the CERN workshop on “Flavour in the era of the LHC”,
which was initiated at the Theory Unit in November 2005 and explores the synergy between flavour and
high-Q physics at the LHC, was successfully continued through three working-group meetings at CERN,
which will be followed by the final plenary meeting in March 2007.
The flavour physics of the charged lepton and neutrino sectors is another important line of resarch
at PH-TH. Since the established neutrino oscillations point to physics beyond the SM, this field
receives also a lot of attention in the particle physics community for many years. In 2006, further work
was devoted at the Theory Unit to models of neutrino masses and mixings. Moreover, CP violation in
the neutrino sector and the prospects of observing it in future neutrino oscillation experiments were explored,
and studies of “leptogenesis” performed, offering an attractive framework for the generation of
the baryon asymmetry of the Universe. Here in particular the possibility of having all the required CP
violation in the neutrino mixing matrix was investigated in detail.
Concerning work on physics beyond the SM, the phenomenological studies of supersymmetric
theories remained a core activity at the Theory Unit in 2006. Electroweak precision measurements, as
well as cosmological data from the new three year results of NASA’s WMAP satellite, were extensively
used to extract indirect information about the scale of supersymmetry breaking. It was stressed that
the actual lack of direct experimental evidence for supersymmetry reduces the huge parameter space of
the models to a small region, which may be selected by anthropic evolution, and is close to the critical
line separating the broken and unbroken phases of the electroweak symmetry. Models were constructed
where the supersymmetry breaking scale is stabilized parametrically above the electroweak scale thanks
to accidental global symmetries of the Higgs sector. The construction of phenomenological models with
extra space-time dimensions has also driven a lot of work. Some tools were developed to deconstruct
5D gauge theories in curved anti-de Sitter (AdS) background and a connection was established with the
Migdal approach and the Pade approximation to compute correlation functions in large-N gauge theories.
A field-theory description of the ubiquitous stringy multi-throat backgrounds was obtained and used for
several phenomenological applications. Many possibilities for NP feature non-standard Higgs sectors,
which may also provide (as an alternative to the leptogenesis scenario discussed above) the new sources
of CP violation that are needed to accommodate the observed dominance of matter over anti-matter in our
Universe. A comprehensive study of the most relevant theories beyond the Standard Model was carried
out, with a focus on the possible experimental signatures at the LHC and other colliders. Concerning
the latter aspect, there were also various collaborations and common activities with experimentalists. An
example of these efforts is the “LHC Olympics”, which was again hosted at CERN in February 2006.
On the more theoretical side, it was discovered that there exist IR obstructions to UV completion
of perfectly sensible low-energy effective field theories, despite the evidence for an enormous landscape
of string vacua in string theory. Some new consistency conditions were derived, for instance on the
signs of certain higher-dimensional operators. The violation of these conditions allows us to construct
small fluctuations around translationally invariant backgrounds that propagate superluminally, making it
impossible to define a Lorentz-invariant time-ordering of events. Conversely, any experimental evidence
of a violation of these positivity constraints would signal a crisis for the usual rules of macroscopic
locality and causality and, almost incidentally, falsify perturbative string theory.
As in previous years, a substantial part of the research done at the Theory Unit was centred at
string theory and related areas, including supergravity, black holes and mathematical physics. It comprised
all major resarch topics in this field. Particular emphasis was put on the following key areas. First,
on the more phenomenological side, many aspects of string compactifications, in particular ones involving
fluxes, were investigated. A main topic in this context was moduli stabilization in pseudo-realistic
models, which is one of the most important open issues in string model building. Related fields that
were investigated are the landscape of string vacua and its population from the viewpoint of cosmology.
Moreover, string models based on D-branes enjoyed continuing popularity. Another important and fascinating
field, which has been a focus of research since many years, is the AdS/CFT correspondence,
which postulates a duality between string theory involving gravity, and gauge theory. Specifically, work
was done on the relation to integrable models and spin chains, as well as on magnon kinematics and
so-called giant gravitons. In a similar vein, a series of papers have been written dealing with the gravity
description of certain Sasaki–Einstein backgrounds. Another important line of research, ongoing since
a few years, too, concerns theoretical aspects of black holes in supergravity theories; here the analysis
of the so-called attractor equations has played a central rˆole. A fourth major line of research deals with
more mathematical aspects of D-branes and topological string theories; some emphasis was put on the
matrix description of D-branes in relation to mirror symmetry. Apart from this, there has been work
on a large number of quite different areas, such as string cosmology, and a possible connection between
certain scaling phenomena in heavy-ion collisions, as noted above, and black holes.
Another research branch at PH-TH is particle astrophysics and cosmology, which ranges from the
exploration of gamma-ray bursts and cosmic rays to the most challenging cosmological problems, showing
typically also a significant overlap with studies of the physics beyond the SM. To be more specific, in
2006, it was stressed that future gravitational wave detectors like LISA could provide useful hints on the
dynamics of the electroweak phase transition, and could even give information complementary to collider
data, like the first estimates of the Higgs self-couplings. Another activity was the proposal of a model in
which a link between dark matter properties and neutrino masses emerges naturally, while the predicted
MeV dark matter particle thereof appears as a fortunate candidate to explain the mysterious emission of
511 keV photons in the centre of our galaxy. Moreover, a minimal extension of the SM by three righthanded
neutrinos and by the inflaton field was constructed. In addition to inflation, the resulting theory
can explain simultaneously the dark matter and the baryon asymmetry of the Universe, and is consistent
with experiments on neutrino oscillations and with all astrophysical and cosmological constraints on the
sterile neutrino as a dark matter candidate. The mysterious dark matter was also addressed in more detail:
a strategy of searching for dark matter particles possessing a radiative decay channel was studied, and
constraints on their parameters from observations of X-rays from our own Galaxy and its dwarf satellites
were derived. When applied to the sterile neutrinos in the keV mass range, it allows a significant
improvement of restrictions to its parameters, as compared with previous works. Finally, the effect of an
extended local foreground on the low-l anomalies found in the Cosmic Microwave Background (CMB)
was considered. It was shown that the Rees–Sciama effect of local foregrounds can indeed induce the
observed anomaly in the CMBs.
In 2006, the Theory Unit has continued to play a leading rˆole in the research areas described
above. Since we are now entering the LHC era, the PH-TH visitors programme was restructured through
the launch of a “Theory Institute”, with focused programmes in high-energy physics and related areas.
This programme was received very well by the particle physics community. For 2007, already three programmes
were approved, dealing with heavy-ion collisions at the LHC, the interplay between cosmology
and the LHC, and new physics at the LHC. Furthermore, members of PH-TH showed a high visibility
at various international conferences and workshops, and were again involved in the work of the Particle
Data Group, as well as in several teaching activities, including the Academic Training, the Summer
Student and High-School Teacher programmes, as well as the European and Latin American Schools of
High-Energy Physics and the CERN–Fermilab Hadron Collider Physics Summer School. The Theory
Unit has also successfully continued the PhD doctoral student programme within the European Commission
6th framework project EUROTHEPHY. In addition to the workshop activities described above,
the Theory Unit is involved in the INTAS programme and the following research networks: “Integrated
Large Infrastructures for Astroparticle Science” (ILIAS), “Quest for Unification”, “Monte Carlo Event
Generators for High-Energy Particle Physics” (MCnet), “Tools and Precision Calculations for Physics
Discoveries at Colliders” (HEPtools), and “The Origin of our Universe: Seeking Links Between Fundamental
Physics and Cosmology” (UniverseNet), and hosted the Winter School on Supergravity, Strings
and Gauge Theories, associated to the EC-RTN network “Constituents, Fundamental Forces and Symmetries
of the Universe”. Finally, CERN theorists continued to contribute to the study groups for CLIC
and the physics opportunities for future proton accelerators.