Space, time, matter and forces studying the fundamental properties of nature
(Curator Allen Caldwell)
At a Glance. The next five years will bring a wealth of results from new experiments in
particle physics on subjects ranging from the search for dark matter, the exploration of
neutrino properties, the understanding of missing antimatter, the search for extra dimensions
and the possible production of mini black holes. These experimental efforts are matched by
intense theoretical work on precision calculations within the standard model and exciting
new theories such as superstring theory.
Particle physics is concerned with the basic constituents of matter and their interactions,
and with the fundamental properties of space and time. First experiments in what can be
classified as particle physics started one century ago with radioactive sources and the
observation of cosmic rays. These experiments led to our understanding of the structure of
atomic matter and to the discovery of new particles beyond the particles making up stable
matter (the electron, proton and neutron). The development of particle accelerators in the
th
20 century allowed for high precision experiments to be performed at ever increasing
energy scales, and therefore ever smaller distances. This history is depicted grpahically in
the fIgure below. The use of more and more powerful particle accelerators led to the
discovery of a wealth of new particles, including quarks and gluons, the fundamental
particles involved in the strong interactions, and the W and Z bosons responsible for the
weak interactions, and laid the foundation for the development of the most precise theory the
world has known -the Standard Model of particle physics. The Standard Model is built on a
unification of two of the four forces, the weak and electromagnetic forces, and also
incorporates the strong nuclear force in a rigorous mathematical framework. Physical
quantities ranging from magnetic moments of fundamental particles such as the muon to the
production rates of heavy particles at the highest energy colliders can be now be accurately
predicted. To date, all predictions which could be carried out within the framework of the
Standard Model have been borne out by the data.
Despite these successes, particle physicists are convinced that the Standard Model is not
the end of the story. There are many fundamental issues in particle physics which are not
addressed or resolved by the Standard Model, both from an experimental as well as from a
theoretical perspective. Well-known examples are the nature of ‘dark energy’ and ‘dark
matter’, neither of which fit within the Standard Model. A likely source of dark matter is a new
type of weakly interacting particle pervading all space. New technologies based on crystals
operated at milliKelvin temperatures have been pioneered by members of the MPP and have
led to supremely sensitive experiments searching for this type of particle [1]. Liquid noble
gases are another complimentary new technique with excellent sensitivity to dark matter
particles which is pursued at the MPIK. The sensitivity limits of such direct dark matter
search experiments are now reaching levels where a discovery could be imminent. The next
decade will likely see a consolidation of the wide range of ongoing experiments into fewer,
larger-scale, experiments, and the MPG will continue its leadership role in the experimental
efforts.
The search for dark matter is one area where particle physics, astrophysics and
astronomy join forces to further our understanding of fundamental physics. A new area of
research, particle astrophysics, has been created out of the synergy between these fields
where, for example, high energy gamma rays are used to probe the nature of space and
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Figure 1. Evolution of effective accelerator energies over time.
time in the vacuum, and to look for a signature of the annihilation of dark matter. Research
groups at the MPIK and MPP are world-leaders in this new field, operating large area
telescopes observing light produced by high energy gamma rays showering in the
atmosphere [2, 3]. The current experiments are in a mature state and have demonstrated
the scientific potential of this approach, and the next five years should see the development
of a new, much larger scale, experiment based on the successes of the current efforts.
A possible realization of dark matter could be in the form of particles from an extension of
the Standard Model known as Supersymmetry, a theory co-discovered by Julius Wess
(formerly of the MPP). Supersymmetry extends and completes the symmetry groups
incorporated in the Standard Model and predicts a wide range of new particles, some of
which could be the source of the dark matter. Should Supersymmetry be realized in nature,
then its effects should be seen not only in the dark matter search experiments but also in
high energy particle colliers. The MPP is at the heart of theoretical efforts to work out the
predictions of this theory for future experiments such as the ATLAS experiment at the LHC
[4]. There exist many other theoretical extensions beyond the Standard Model which explain
the nature of Dark Matter and the origin of electro-weak symmetry breaking in different
ways, which are studied both at MPIK and MPP. On the experimental side, a large research
team at the MPP is involved in the ATLAS experiment, one of the two main collider
experiments at the LHC (see topic 11, ‘Big questions, big projects’). A primary goal of the
LHC is the observation of the Higgs boson, an integral part of the Standard Model that has
eluded detection until today. The observation of the Higgs particle at the LHC will not be
straightforward as it will need to be dug out from a mass of underlying processes. Precise
theoretical predictions for all processes will be needed for a reliable extraction as well as
intense experimental efforts. The theory group at the MPP is developing the mathematical
and computational tools needed for the theoretical predictions, while the experimental group
develops the necessary experimental and data analysis tools. This joint effort provides an
excellent base for discoveries at the LHC.
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Additional topics of research at the LHC will include the search for exciting extensions to
the Standard Model such as extra spatial dimensions. The possibility of extra spatial
dimensions, motivated by string theory, has generated tremendous excitement in the physics
community. String theory is our best candidate theory to unify gravity with the three forces
incorporated in the Standard Model. The MPP and AEI are very active in the mathematical
underpinnings of string theory, in the elucidation and enumeration of classes of string
theories, and in attempting to whittle down the large number of possible theoretical scenarios
to those which could lead to our observed universe. Of great interest are predictions for the
LHC which result from special compactification schemes used for the extra dimensions [6].
These extra dimensions clearly need to be compactified since they are not seen with our
current observational tools (colliders). However, they need not be unobservedly small. An
exciting scenario posits large (large enough to be observable) extra dimensions which could
already be observable at the LHC. The observation of large extra dimensions would bring
about a revolution in our understanding of space and time not seen since the introduction of
the theories of relativity and quantum mechanics. A related topic is the possible production
of black holes at the LHC [7], which would also produce a revolution in particle physics. It is
clear that some type of paradigm shift is needed -we currently do not know how to unify the
theories of gravity and quantum mechanics. MPG scientists are at the forefront of theoretical
efforts to do just that.
There are other fundamental questions to which particle physicists need answers and
which go beyond the physics of the Standard Model. For example, why is there an
imbalance between matter and anti-matter in our observed universe ? When P.A.M Dirac
postulated the existence of anti-matter 80 years ago, this was considered a preposterous
idea. Today, we routinely produce equal amounts of matter and anti-matter in our particle
accelerators, and we confidently claim that the Big Bang produced equal amounts of matter
and anti-matter. Yet when we look out in the universe around us, we do not see the antimatter. The question today has been turned around -where is the missing anti-matter ?
Various theoretical possibilities have been proposed to address this question, and research
teams at the MPP and MPIK are investigating scenarios for the disappearance of the
antimatter. Precision experiments at the LHC with the LHC-b experiment, with participation
of a group at the MPIK, and at an accelerator facility in Japan (Belle II) with a strong group
from the MPP will study decays of particles made of b type quarks which should shed light
on this question in the next years. These experiments approach the question to physics
beyond the Standard Model in a manner complementary to that pursued by the LHC. Instead
of searching for new particles
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Figure 2. Matter at its most fundamental level may be based on strings.
at the highest energies, deviations from the Standard Model are searched for through
comparison of extremely precise data with equally precise predictions. The presence of new
types of particles should be visible through unexpected rare decays or tiny deviations from
predictions. The experiment at the KEK laboratory in Japan will rely on technology
developed at, and only available from, the Halb-Leiter-Labor of the MPG. The specific
technology, thin pixel detectors, will give a tremendous boost to the physics potential of this
experiment.
The answer to the question of the missing antimatter could well be linked to the nature of
the neutrino, and whether the neutrino is its own anti-particle. The most exciting results in
particle physics over the last decade came from the study of neutrinos. The discovery that
neutrinos oscillate from one type to another proved that neutrinos have mass. The Standard
Model assumed massless neutrinos, and the observation that neutrinos are massive opened
a broad vista of theoretical possibilities, including the possibility that the neutrino is involved
in the matter anti-matter asymmetry. Both neutrino masses and mixings came as a surprise
and a strong team at MPIK studies the phenomenological consequences and theoretical
implications for particle and astroparticle physics and connections to cosmology [8]. The
MPIK group are also leading efforts in a new experiment [9] to measure a critical parameter
describing how neutrinos oscillate. Meanwhile, large research teams at the MPK and MPP
are leading the
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Figure 3. The evolution of the universe since the big bang.
GERDA experiment [10], an experiment to directly test whether the neutrino is its own antiparticle. The GERDA experiment studies very rare decays of a specific isotope of
Germanium using a novel concept. Both the Double CHOOZ and GERDA experiments are
very close to starting data taking, such that the next years are expected to bring about major
advances in the understanding of neutrinos. Both experiments have the sensitivity to make
ground-breaking discoveries in the physics of neutrinos, which would have far reaching
consequences.
The future direction of particle physics research will clearly depend on the results from the
LHC and from the precision neutrino, dark matter search, and other experiments pursued by
the particle physics community. The International Linear Collider is currently considered to
be the next large scale accelerator project (see Figure 1), and MPP scientists are involved in
research and development towards the realization of this project. The linear collider would
collide electrons rather than protons. Electrons have the important experimental advantage
that they have no substructure, as opposed to protons, so that the analysis of the data is
much more straightforward. Another important aspect of the point-like nature is that the full
energy delivered by the accelerator is used in the collisions. However, electrons have the
disadvantage that they cannot be accelerated to high energies in a circular accelerator since
they radiate away their energy when forced to follow a curved trajectory. The linear collider
solves this problem by accelerating the electrons in a very long straight accelerator. This
very long accelerator will however be very costly. It is clear that the construction of ever
larger and costlier accelerator facilities will eventually reach an end, and that new
technologies will be needed to push the energy frontier. A new effort at the MPP has been
started to develop a novel acceleration technique based on plasma wake fields which could
drastically lower the cost of building an electron accelerator [11]. In the approach pursued at
the MPP, high energy proton beams, such as those from the LHC, could be used to
accelerate electron beam to high energies in a plasma. Experimental tests of this idea are
planned for the next years, and success in this endeavor could point the way to a new
generation of particle physics accelerators. Another possibility to go beyond the current
accelerator concepts is to accelerate and collide muons, a particle very much like the
electron by 200 times more massive. A muon has the advantages of an electron in that it has
no substructure, while its heavier mass suppresses the radiation such that high energies can
be reached in a circular accelerator. However, muons are unstable and decay in two
microseconds, such that novel techniques are needed to use them effectively in a particle
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collider. Research on this type of accelerator is also ongoing at the MPP. Both the plasma
wakefield accelerator and muon collider are depicted in Figure 1.
All of the experimental activities described in the previous paragraphs have a strong
international component. The teams work in collaborations varying in size from
approximately 100 researchers (CRESST, XENON, Double CHOOZ, MAGIC, HESS,
GERDA) with typically ten institutes from five or more countries, to teams numbering over
1000 researchers with institutes from dozens of countries (ATLAS). The theoretical activities
are also very international, with groups belonging to EU networks and strong partnerships
with leading institutes worldwide. This international component has long been a key element
of particle physics and will remain so in the future.
Particle physics encompasses a broad range of theoretical and experimental research,
extending from the smallest dimensions, masses, and time intervals, to the very largest
scales. MPG scientists are at the leading edge in developments in mathematical physics as
well as in extracting extremely precise quantitative predictions based on existing theories.
Meanwhile, experiments are on the verge of taking data at the energy frontier at the LHC
and at completely new levels of sensitivity in dark matter and neutrino experiments. The
MPG is poised to reap the fruit of many years of efforts in this field of research, and looks
forward with excitement to what the next years will bring in particle physics.
References
[1] http://www.cresst.de/
[2] http://www.mpi-hd.mpg.de/hfm/HESS/
[3] http://magic.mppmu.mpg.de/
[4] Supersymmetric Higgs bosons in weak boson fusion, W. Hollik, T. Plehn, M. Rauch, H. Rzehak, Phys.
Rev. Lett. 102 (2009) 091802
[5] http://wwwatlas.mppmu.mpg.de/
[6] Seeing through the String Landscape
-a String Hunter’s Companion in Particle Physics and
Cosmology, Dieter Lust,
JHEP
0903:149,2009.
32
[7] Phenomenology of 10 Dark Sectors, G. Dvali, M. Redi, Phys. Rev. D80:055001, 2009
[8] Theory of Neutrinos: A White Paper, R.N. Mohapatra et al. , Rept. Prog. Phys. 70:1757-1867, 2007 Rept.
Prog. Phys. 72, 106201 (2009)
[9] http://doublechooz.in2p3.fr/Status_and_News/status_and_news.php
[10] http://www.mpi-hd.mpg.de/gerda/
[11] Proton Driven Plasma Wakefield Acceleration, A. Caldwell, K. Lotov, A. Pukhov, F. Simon, Nat. Phys. 5
(2009) 363.
Glossary
MPIK: Max Planck Institute for Nuclear Physics in Heidelberg
AEI: Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Golm
CRESST: Collaboration for Cryogenic Rare Event Search with Superconducting
Thermometers
LHC: Large Hadron Collider at CERN
GERDA GERmanium Detector Array experimental collaboration
XENON: Experimental Collaboration for dark matter search based on liquid Xenon
technology
Double CHOOZ: Experimental Collaboration on neutrino physics at CHOOZ reactor
LHC-b Experimental Collaboration at CERN studying production and decay of b type
quarks
Belle II Experimental Collaboration at KEK accelerator facility in Japan studying
production and decay of b type quarks
PWA Plasma wakefield acceleration
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