Exotic Phenomena in the early Universe
“The physics of the early Universe: Experimental search for exotic phenomena
at the LHC relevant for Astroparticle physics and Cosmology”
Proposal for a “Forskerprosjekt” within the NFR “FRINAT” program
F. Ould-Saada et al.
Oslo, 3 June 2008
Introduction
High Energy Particle Physics is the study of the basic constituents of matter and the
interactions among them. The current theory which elegantly summarizes the understanding
of the field, the Standard Model (SM), describes interactions between elementary particles
grouped in 3 families, or generations, of quarks and leptons as shown in Figure 1. Forces are
mediated through (gauge) boson exchange: photon for the electromagnetic (EM) force, W and
Z for the weak nuclear force, and gluons for the strong nuclear “colour" force. The fact that
particle masses are very small makes gravity negligible compared to the other three forces.
Leptons are free particles. They can be charged (e−, μ−, τ−), in which case they feel both
electromagnetic and weak forces, or neutral (neutrinos: νe, νμ, ντ), then they interact only
weakly. Quarks are particles that are sensitive to all three interactions. There are no free
quarks in nature. We observe only composite states of quarks called hadrons, of which the
proton and the neutron are the most widely known examples.
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In our present understanding the physical universe
can be described by six fundamental quarks and
leptons and three microscopic forces, or interactions,
governed by force carriers which are bosons. Higher
symmetries than the ones behind the Standard Model
might be needed to relate leptons and quarks (Grand
Unified Theories, GUT) and matter particles and
forces carriers (Supersymmetry, SUSY). The
gravitational interaction, mediated through the
graviton, is not (yet) part of the Standard Model.
Superstring theories, where particles correspond to
vibrations, in 10 space dimensions, of extended
objects, called strings, are the current candidates for
the ultimate unified theory.
Figure 1
The world of the infinitely small obeys some laws or symmetries. Charge conjugation, C,
parity, P, and time reversal T are some examples of such symmetries. The electromagnetic
and strong forces are based on exact (dynamical) symmetries, leading to conservation of the
electric charge, in one case, and of “color" charge, in the other case. The SM describes
(almost) all current particle physics data. One of its great successes is the unification of
electromagnetism (long range, macroscopic) and weak nuclear force (short range,
microscopic). The resulting electroweak symmetry must be (spontaneously) broken at low
energies in order to give the weak bosons (W and Z), as well as all matter particles, masses.
This implies the existence of a scalar particle, the Higgs boson, the only missing building
element of the SM.
The SM answers many of the questions about the structure and stability of matter. However,
it is believed that the model is only an effective, low-energy approximation of some more
fundamental theory. The fact that quarks and gluons behave as free particles at high energies
(asymptotic freedom) is predicted by Quantum Chromo Dynamics (QCD). Their confinement
in hadrons is experimental evidence that is not very well understood.
The fact that neutrinos have mass, as demonstrated by solar and atmospheric neutrino
experiments, is considered as a first sign of physics beyond the SM. Current data also hint at
unification between strong and electroweak forces at much larger energies, of the order of
1016GeV, such that higher symmetries are required. Current observations show that 4% of the
universe is made of normal matter, whereas 21% is dark matter and 75% dark energy. The
candidate theories must answer other questions like: Why are there 3 families of quarks and
leptons? Are the quarks and leptons really fundamental? What is the origin of dark matter and
dark energy in the universe? What is the origin of matter-antimatter asymmetry in the
universe? How can Gravity be unified with the other three fundamental forces?
The Large Hadron Collider LHC at the CERN laboratory in Geneva will be the first particle
accelerator to explore directly a new energy frontier, the TeV scale. By colliding beams of
protons or lead nuclei, the LHC will probe deeper into matter than ever before, reproducing
conditions in the first nanoseconds in the life of the Universe. The LHC should either confirm
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the existence of the SM Higgs particle, or provide experimental hints of the real nature of the
origin of mass.
The fundamental question of why the observed Universe is built entirely of matter, while
matter and anti-matter should have been created in equal amounts in the Big Bang, is still
unanswered. To explain the observed asymmetry (which gave rise to our existence!) the socalled CP asymmetry of elementary interactions is needed. In fact weak interactions described
by the SM predict CP asymmetries, however it turns out that the magnitude of these is far too
small to explain the matter-anti-matter asymmetry of the Universe. The dark matter problem
comes from observational evidence that most of the mass of our universe cannot be due to the
ordinary matter we know. A large fraction of matter must consist of unknown, weakly
interacting particles (dark matter). A supersymmetric extension of the SM provides probably
the best known candidate for such a dark matter particle. It also necessitates the existence of
the light Higgs boson, in agreement with the present bounds from precision electroweak
measurements. Supersymmetric partners of SM particles provide natural cancellations of
interactions which otherwise would make the Higgs boson very heavy, and thus solve one of
the basic theoretical problems of the SM. SUSY unifies matter (fermions) and force carriers
(bosons). It is a kind of “matter-force duality".
Exotics at the LHC
Recent theoretical attempts to explain the huge difference in the strengths of the gravitational
interaction compared to the interactions comprising the Standard Model propose that gravity
can propagate in new space-time dimensions that are inaccessible to the other interactions.
Extra space dimensions were originally suggested by Kaluza and Klein to unify Gravity and
Electromagnetism. More recently, they are required by Superstring theories, where a particle
becomes a string vibrating in 10 space dimensions. Superstring theory is a candidate theory
that may solve one of the most outstanding problems, namely the consistency of quantum
mechanics (particles and waves) and general relativity (space and time), thus allowing
unification of all four fundamental forces at the Planck energy of 1019GeV.
Even more recently, it was suggested that gravity could become strong already at the TeV
scale accessible at LHC. This would allow production of massive gravitons - particles
mediating gravity. The graviton can propagate in the extra dimensions, which in some
scenarios are compactified on circles of radius R. For distances smaller than R, Newton's
formula of gravitation must be modified.
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Other dramatic scenarios allow for microscopic
black holes. As particles approach each other in a
particle accelerator, their gravitational attraction
increases steadily. When they are extremely close,
they may enter space with more dimensions. The
extra dimensions would allow gravity to increase
more rapidly so a black hole can form. Once
produced these black holes rapidly decay to a
spectrum of particles through the Hawking
evaporation process, sending a unique pattern of
radiation. The figure on the right shows an ATLAS
simulated black-hole event.
Starting in 2008, four experiments, among them ATLAS, will collect data that should reveal
new physics phenomena. Among the open questions, LHC may reveal the origin of particle
masses, explain dark matter, investigate extra space dimensions, and help understand the
origin of matter in the Universe. Several aspects of LHC physics touch on the interface
between particle physics and cosmology, such that one already talks about Astro-CosmoParticle physics.
The ATLAS experiment will certainly contribute to a more satisfactory understanding of
matter, energy, space and time. The members of the Norwegian High Energy Particle Physics
community have made significant contributions to the construction of the ATLAS detector,
Grid computing and the preparation of the software necessary to tackle the first physics data.
A Need for a strong PhD programme
The Norwegian HEPP community welcomes the Norwegian Research Council’s initiative
“FRINAT”.
It is a known fact that, within High Energy Physics, the number of PhD students per professor
is much lower in Norway that most of the countries involved in LHC. Currently there is
significantly less than one Ph.D student per professor. Compared to the investments made it is
very important to make sure that more students take advantage of high quality education at the
frontiers of knowledge and technology.
We have been rather successful in attracting master students to join the LHC-related
activities. During the last 3 years, however, we could not offer a PhD grant even to the most
brilliant students. Due to lack of Norwegian Ph.D grants, six of the recently graduated master
students from Oslo and Bergen are preparing PhD’s in Germany, Slovenia and other places,
posts generally won in strong international competition.
There are currently10 master students finishing in 2008-2009. We therefore welcome the
“FRINAT” initiative, and hope that it can allow us to keep some of these talented students
within our projects. This is very important for the HEPP involvement as the finishing master
students “master” all the tools needed to tackle the LHC data. They have been instructed to
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take and monitor data, take part in shifts and involved in detailed preparations for physics
analysis through the Grid.
The project “Exotic phenomena in the early Universe” will concentrate on the following
questions:
• What is the (new) physics in the early Universe?
• Are there new fundamental symmetries, leading to new gauge bosons?
• Why is Gravity so weak? Are there extra space dimensions where gravitons “hide”?
What can we learn from microscopic black-holes?
• Why are neutrino masses so small? Is the “Seesaw mechanism” the answer? Where
are the Right-handed neutrinos? What is the nature of these neutrinos?
• Do we have the necessary statistical tools to differentiate between the various new
physics scenarios and the competing Standard Model processes?
In what follows we describe proposals for 5 PhD projects related to searches for Exotic
phenomena:
• The quest for extra space dimensions.
o Extra spatial dimensions and graviton searches. If, through the existence of
extra spatial dimensions, gravity becomes strong already at LHC energies,
graviton production may be possible. This includes both narrow graviton
resonances (Randall and Sundrum model, RS), as well as real graviton
production (Arkani-Hamed, Dimopoulos, Dvali scenario, ADD).
o Extra spatial dimensions and microscopic Black holes. As particles approach in
a particle accelerator, their gravitational attraction increases steadily. When
they are extremely close, they may enter space with more dimensions. The
extra dimensions would allow gravity to increase more rapidly so a black hole
can form. Once produced these black holes rapidly decay to a spectrum of
particles through the Hawking evaporation process, sending a unique pattern of
radiation. A black hole decays equally to all Standard Model degrees of
freedom.
• The quest for extra symmetries.
o New neutral gauge bosons. Many of the theories beyond the Standard Model
require extra symmetries which often lead to one or more gauge bosons, such
as an extra neutral Z’.
o Majorana neutrinos and the Seesaw mechanism. Models with extra neutral
gauge bosons often contain charged gauge bosons W’ as well. The most
attractive candidate for W’ is the WR gauge boson associated with the left-right
symmetric models. These models seek to provide a spontaneous origin for
parity violation in weak interactions. The models require the introduction of
right-handed neutrinos, which can facilitate the Seesaw mechanism for
explaining the smallness of the ordinary neutrino masses.
• Tools and methods for discoveries at the LHC
o New physics and statistical analysis. Studying such new physics scenarios at
the LHC requires a thorough understanding of statistical analysis, and many
physics results can only be obtained through further development of such
techniques. It is vital to have experts that can identify statistical tools and
methods appropriate for the entire spectrum of exclusion, discovery and
measurement in the LHC environment, as part of any exotics search
programme.
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Several basic physics simulation studies have been performed together with master and
doctoral students. Some of the activities are being followed up further, through more detailed
and precise simulations of the ATLAS detector, more statistics possible through The Nordic
Grid infrastructure, improvement of analysis methods, and inclusion of new theoretical ideas.
The Norwegian HEPP community is heavily involved in preparations for analysis of the data
which ATLAS will soon start recording. This includes Grid middleware, distributed data
management and analysis and contributions to simulation and analysis tools.
There is room for many doctoral and master theses that the project members will supervise,
with the help of additional post-docs recently hired. The work will be done within some of the
ATLAS physics working groups and will involve wide international collaboration. At a more
local level, strong links to the Nordic and Norwegian theoreticians, astrophysicists and
cosmologists will widen the scope.
PhD proposals
We will mostly concentrate on two-lepton final states as a common denominator for various
searches for new physics. The majority of the phenomena described below will lead to final
states with 2 charged leptons (same-sign or opposite sign) without missing energy. This
complements current studies of 2 leptons with missing energy as a sign of supersymmetric
particles. Some technical studies related to particle reconstruction and identification, as well
as studies of concurrent Standard Model backgrounds, are in common, as are the advanced
statistical techniques required to perform such analyses.
Most of the studies described below are based on fast simulation of the ATLAS detector,
often started by local master and PhD students together with senior members of the proposed
project. Some studies are only performed at the generator level. While preparing for the first
LHC data, the PhD candidates will
¾ study the various signals as well as the corresponding Standard Model backgrounds
using a fully simulated detector. Currently large amount of events are being produced
on the Grid. The members of the Norwegian ATLAS community are experienced in
using the various ATLAS software and Grid tools, as well as analysis packages.
¾ make an overview of the existing data at colliders and those obtained by nonaccelerator based experiments or inferred from astrophysics and cosmology. The
contact with astrophysics groups and theoreticians will lead to a better understanding
and interpretation of the results.
¾ The main activity of all 5 PhD candidates will be to
o take part in data taking at CERN, Geneva,
o understand the first signals and reconstruct some basic particles such as Z0
decaying into e+e− and μ+μ−,
o study the background processes relevant to the phenomena they will study,
o analyse a large sample of data,
o present the results, first locally, then within the relevant ATLAS physics
groups and during ATLAS physics weeks,
o combine ATLAS search results with other experiments in particle physics
(CMS, the other general-purpose proton-proton detector at LHC, CDF and D0
at the Tevatron), astroparticle physics and cosmology.
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Extra space dimensions and search for Gravitons at LHC
Extra space dimensions were originally suggested by Kaluza and Klein to unify Gravity and
Electromagnetism. More recently, they are required by Superstring theories, where a particle
becomes a string vibrating in 10 space dimensions. Superstring theory may solve one of the
most outstanding problems, namely the consistency of quantum mechanics (particles and
waves) and general relativity (space and time), thus allowing unification of all four
fundamental forces at the Planck energy of 1016TeV. Even more recently, it was suggested
that gravity could become strong already at the TeV scale accessible at LHC. This would
allow production of gravitons (particles mediating gravity) which would disappear in the extra
dimensions, as well as mini black holes.
Master and PhD students in Oslo and Bergen studied graviton production in pp collisions.
Simulation studies within the RS model [1] of pp -> G -> e+e−, μ+μ−, γγ showed that gravitons
with masses up to mG ~2 TeV may be discovered [3]. Moreover, through a precise
measurement of the decay angular distribution it may be possible to discriminate between spin
2 (graviton) and spin 1 (gauge boson) or 0 (scalar such as Higgs) intermediate bosons.
The ADD model [2] introduces extra dimensions to explain the relative weakness of gravity.
Real gravitons can propagate in the extra dimensions, which are compactified on circles of
radius R. For distances smaller than R, Newton's formula of gravitation must be modified.
The graviton momentum in the extra dimensions appears as a mass term. Production channels
[4] pp -> G + jet or G + photon, where the graviton G escapes the detector, lead to a large
missing transverse energy and momentum. The background includes events with neutrinos.
Discovery in ATLAS can be made up to effective Planck masses of MP = 5-8 TeV (δ= 2) and
MP = 5-6 TeV (δ = 3; 4), where δ is the number of extra space dimensions. Some models with
large extra dimensions, like the ADD model, are already constrained by astrophysical
considerations [6]. For example, a neutron star would shine in 100 MeV gamma rays from the
decays of gravitationally trapped gravitons. The failure of EGRET to see such a signal puts a
restrictive bound on the size of the extra dimensions, excluding δ = 2.
Heavy graviton exchange in di-lepton production leads to additional hard Bremsstrahlung,
due to the graviton-photon coupling [5]. This could be a striking signal of extra dimensions, in
both the ADD and RS models. Also, a new, integrated measure of angular distributions has
been proposed. At low luminosities, this might be preferable over an angular distribution.
An analysis of ATLAS data to search for graviton decaying into di-leptons would be an
excellent topic for a PhD work starting in 2009. Optionally, radiative decays [5] or decays
into di-photons [3] can be studied. If the graviton is discovered, the next step would be to
measure its properties such as the spin.
REFERENCES
[1] L. Randall and R. Sundrum, A large mass hierarchy from a small extra dimension, Phys.
Rev. Lett. 83, 3370 (1999); 83, 4690 (1999).
[2] N. Arkani-Hamed, S. Dimopoulos, G. Dvali, Phys. Lett. B 429, 263 (1998); Phys. Rev. D
59, 086004 (1999).
[3] Mustafa Hussain, Simulation of a search for the narrow graviton resonance at the LHC,
Cand. Scient. awarded by the University of Oslo, May 2006. Supervisor: F. Ould-Saada.
[4] Torkjell Huse, PhD thesis, University of Oslo, to be submitted in June 2008.
Supervisors: S. Stapnes, F. Ould-Saada.
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[5] Erik Dvergsnes, PhD awarded by the University of Bergen, 2004. Supervisor: P. Osland;
E. Dvergsnes, P. Osland, N. Öztürk, Graviton-induced bremsstrahlung, Phys. Rev. D 67,
074003 (2003)
[6] S. Hannestad & G. Raffelt, Phys. Rev. D67 (2003) 125008; erratum-ibid D69 (2004)
029901.
Extra space dimensions and Microscopic Black-holes
Various aspects of black hole production and decay were explored [1] within the context of
the ATLAS experiment through a fast simulation study with the CHARYBDIS black hole
generator. A parton level study was performed to shed light on characteristics of the generator
decay model. Signatures of black hole events at the detector level were investigated.
Detection with ATLAS is rather easy due to the large production cross section and the striking
properties of the events: large multiplicity, high sphericity, high transverse energy, and low
missing transverse momentum. The discovery potential was evaluated for various values of
the effective Planck mass MP and the number of extra dimensions δ, using both a simple
fixed-temperature, blackbody approximation of the black hole decay and a more refined
model with a time-evolving temperature and grey-body spectra. The results indicated that if
MP is below 5 TeV, a discovery would be possible within a month of LHC running at low
luminosity. A strong dependence on MP is observed, whereas the dependence on the number
of extra dimensions is rather weak.
It is a very interesting PhD topic to follow the work of reference [1] using data to be collected
by ATLAS. Strategies for identifying microscopic black hole events at future colliders are
recently reviewed in reference [2].
REFERENCES
[1] Robindra Prabhu, Studies of Higher Dimensional Black Holes with the ATLAS detector
at the LHC. Cand. Scient., awarded by the University of Oslo, June 2005. Supervisors: F.
Ould-Saada, Lars Bugge.
[2] B. Koch, M. Bleicher, H. Stoecker, Black Holes at LHC?, arXiv:hep-ph/0702187v1
Search for new neutral gauge bosons
New massive and electrically neutral gauge bosons are a common feature of physics beyond
the Standard Model. They are present in most extensions of the SM gauge group, including
models in which the SM is embedded into a unifying group. They can also arise in certain
classes of theories with extra dimensions. One of the prime motivations for an additional Z’
has come from string theory, in which certain compactifications lead naturally to an E6 gauge
group containing two U(1) subgroups with 2 corresponding gauge bosons. Models with extra
neutral gauge bosons often contain charged gauge bosons W’ as well.
The most attractive candidate for W’ is the WR gauge boson associated with the left-right
symmetric models. These models seek to provide a spontaneous origin for parity violation in
weak interactions. Note that these models require the introduction of right-handed neutrinos,
which can facilitate the seesaw mechanism for explaining the smallness of the ordinary
neutrino masses.
Searches for an extra neutral Z’ - a heavier version of the Z0, one of the mediators of weak
interactions - have been simulated [1]. A Z’ can be produced at LHC through the process pp > Z0 +X. A discovery could be made, after one year of running at high luminosity, up to
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masses of order 4 TeV by looking at the leptonic decays Z’-> e+e-, μ+μ− . [2] High statistics
would allow the study of Z’ properties and discrimination between various models could be
achieved.
An analysis of ATLAS data to search for Z’ decaying into di-leptons would be an excellent
topic for a PhD work starting in 2009. If a Z’ is discovered, one the challenges would be to
extract the forward-backward charge asymmetry, a quantity very sensitive to the various Z’
models [3].
REFERENCES
[1] Heidi Sandaker, SemiConductor Tracker Development and Physics Simulation , Dr.
Scient. awarded by the Universitety of Oslo, September 2005. Supervisors: Steinar Stapnes,
F. Ould-Saada
[2] I.A.Goultvin et. Al., Phys.Atom.Nucl.70:56-62, 2007
[3] Dittmar, Nicollerat and Djouadi, Z' studies at the LHC: an update, hep-ph/030702
Majorana Neutrinos at LHC and Sea-Saw mechanism
In spite of the remarkable agreement of the Standard Model (SM) with the current
experimental data, too many fundamental questions have not been satisfactorily answered yet.
For instance, parity violation in the weak interactions is introduced by hand and neutrinos are
considered massless. There is now very convincing evidence from atmospheric and solar
neutrino data that neutrinos have a very small but definitely non-zero mass.
The Left-Right Symmetric Model is an alternative theory which restores parity symmetry at
high energy by extending the SU(2)L x U(1)Y gauge group of the SM to SU(2)L x SU(2) R
U(1) B-L and by introducing three new heavy gauge bosons : W+R , W−R and Z’.
The experimental confirmation of standard neutrinos with non-zero mass and the theoretical
possibility of lepton number violation find a natural explanation when new heavy Majorana
neutrinos exist. As opposed to Dirac particles, a Majorana particle and its antiparticle are
identical. Then the Left-Right Symmetric Model, together with the Seesaw mechanism, may
provide an explanation for the lightness of the three left-handed neutrinos (νl) by introducing
three heavy right-handed neutrinos (Nl).
The production and decay of new possible heavy Majorana neutrinos can be studied in
hadronic collisions [1]. New bounds on the mixing of these particles with standard neutrinos
are estimated according to a fundamental representation suggested by grand unified models. It
is shown that Majorana neutrinos can clearly be observed in final states with same-sign
dileptons and a charged weak vector boson. . An ATLAS study of pp -> WR -> e Ne,
followed by Ne -> e W, leading to 2 same-sign di-leptons and a charged W decaying into 2
jets (or a lepton and a neutrino), was reported in reference [2]. The discovery potential for a
new neutral Z' gauge boson decaying into two Majorana neutrinos pp -> Z' -> NeNe signals
has also been studied in [3].
Very recently, a study on the possibility of distinguishing new heavy Majorana neutrino
models at LHC energies is presented in reference [4]. The authors present signatures and
distributions that can indicate the theoretical origin of the new Majorana particles. The single
and pair production of heavy Majorana neutrinos are calculated and the model dependence is
discussed. Same-sign dileptons in the final state provide a clear signal for the Majorana nature
of heavy neutrinos, since there is lepton number violation. Mass bounds on heavy Majorana
neutrinos allowing model discrimination are estimated. Reference [5] discusses how to probe
the Seesaw mechanism at LHC.
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Cosmology can provide upper bounds on neutrino masses. For example, the large-scale
distribution of matter in the universe is sensitive to neutrino masses. On the other hand, we
need to learn more about neutrino masses in the laboratory in order to pin down the correct
cosmological model [6].
An analysis of ATLAS data to search for Majorana neutrinos, produced through of pp -> WR > e Ne, and decaying into e W, following references [1-5] would be an excellent topic for a
PhD work starting in 2009. The final state to reconstruct consists of 2 same sign charged
leptons and jets. Optionally, the process Z' -> NeNe can be looked at. The final state would
consist of an opposite sign di-lepton and 4 jets. The two processes have different
backgrounds. The latter process can also be studied in connection with the Z’ decay into l+l−
described above.
REFERENCES
[1] F. M. L. Almeida, Jr, Y. A. Coutinho, J. A. Martins Simões, M. A. B. do Vale, “Signature
for heavy Majorana neutrinos in hadronic collisions”, Phys. Rev. D 62, 075004 (2000).
[2] J. Collot, A. Ferrari, “Search for new gauge bosons and right-handed Majorana neutrinos
in pp collisions at 14 TeV”, ATL-PHYS-99-018.
[3] A. Ferrari, J. Collot, “Sensitivity study for a Z’ boson decaying into right-handed
Majorana neutrinos at LHC in the ATLAS detector”, ATL-PHYS-2000-034
[4] F.M.L., de Almeida et al., “Discriminating among the theoretical origins of new heavy
Majorana neutrinos at CERN LHC”, 10.1103/PhysRevD.75.075002
[5] B. Bajc, M. Nemevsek, G. Senjanovic, Probing Seesaw at LHC, hep-ph/0703080
[6] Øystein Elgarøy, Upper limits on neutrino masses from cosmology, Nucl. Phys. B (Proc.
Suppl.) 168 (2007) 51
New physics and statistical analysis
The statistical methods used in the analysis of experimental high-energy physics have
improved together with advances in accelerator and particle detector technology [0], the
motivation often being to strive to extract the maximum amount of information out of data
which have been obtained after significant investments in terms of time, money and
intellectual effort. Particle physicists have traditionally performed measurements with great
care to optimize the sensitivity of the measurement, to precisely estimate and propagate
statistical and experimental uncertainties and to present the results with a standard statistical
language, but in the absence of a confirmed signal and especially in searches for new
phenomena there have been numerous more or less rigorous ways of treating the data and
presenting the results. In the generation of experiments before the LHC, e.g. the experiments
at the LEP electron-positron collider, the analysis and presentation of search results was
carefully re-examined and found to be ripe for improvement. Methods to propagate
experimental errors [1], to present measurements and searches in a common framework [2]
and to present robust results for combinations of search results at the experimental sensitivity
bound [3] were introduced and applied to a wide variety of search experiments [e.g. 4].
The B-factory experiments BaBar and Belle [5] have performed a huge number of
measurement and search experiments and in some cases early inconclusive search results have
developed into discoveries and measurements after large increases in data samples were
obtained. The environment in events at the LHC will be quite distinct from LEP where the
signals are clean but small and the background small and well-understood, and from the Bfactories where enormous samples of data with essentially fully reconstructed events have
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been collected and clearly independent background-enriched and signal-enriched samples
exist. At the LHC the signals will tend to be larger than at LEP but the background will be
much larger and overlap significantly the signal. In addition the complexities of the signals
and the huge amount of background in the detector will essentially forbid the complete
reconstruction of events.
The goals of this project are mostly common with the goals of the newly formed Atlas
Statistics Forum: To identify statistical tools and methods appropriate for the entire spectrum
of exclusion, discovery and measurement in the LHC environment, to further develop them if
necessary, to make these tools and methods easily available to the Atlas physics community
(possibly but perhaps not exclusively through a common software interface), to demonstrate
their use in relevant physics channels, to facilitate the combination of Atlas search channels
which have a common physics origin (e.g. the different di-lepton final states described above
and Higgs boson decays), and to facilitate the combination of ATLAS search results with
other experiments (mainly with CMS) .
In addition to many DELPHI and LEP publications this activity has lead to a cand.scient
degree [6] on an object-oriented interface to search statistics code developed for LEP and a
PhD on the Higgs boson search with the DELPHI experiment at LEP with emphasis on the
challenging missing energy final state (e+-e->HZ->Hnunubar) [7].
REFERENCES
[0] The Review of Particle Properties, W.-M. Yao et al., Journal of Physics G 33, 1 (2006)
The PHYSTAT conference series
[1] R.D.~Cousins and V.L.~Highland, Nucl. Instr. and Meth. A320 (1992) 331.
[2] G.~Feldman and R.~Cousins, Phys Rev D 57 (1998) 3873-3889.
[3] A. L. Read, Proceedings of Advanced Statistical Techniques in Particle Physics (Durham,
March 2002) 11-21.
[4] G. Abbiendi et. al, Search for the Standard Model Higgs Boson at LEP, Phys. Lett. B 565
(2003) 61-75
[5] The BaBar experiment; The Belle experiment:
[6] Borg, A.L., Preparation of a computer program for statistical analysis of searches for new
particles at the LHC/ATLAS experiment, Cand. Scient. thesis, University of Oslo, 2002.
Supervisor: A.L. Read.
[7] Haug, S., Polynomial Discrimination and Weighted Counting in a Standard Model Higgs
Boson Search , Dr. Scient thesis, University of Oslo, 2004. Supervisor: A.L. Read.
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