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I. INTRODUCTION:
I(a). Mysterious dark matter in the universe:
There is enough evidence for the existence of the dark matter (DM) in the universe.
Shown here is a collision of two galaxy clusters
with compelling visual of
splitting dark matter components (blue parts)
from nomal matter components (pink parts).(*)
An astromonical precisionmeasurement by
Wilkinson Microwave Anisotropy Probe (WMAP) reveals that 23% of the universe
(or Ω = 0.23) is composed of cold dark matter (CDM).
However, we still don't know what the CDM is.
Particle physics theorists have been trying to construct new models
that are consistent with the Standard Model (SM),
but includes a CDM candidate paricle.
(*)
An evidence for
Dark Matter in the Universe (taken from NASA web site)
I(b). Dark matter particle in supersymmetry:
Supersymmetry (SUSY) uniquely opens the possibility to directly connect
the SM with an ultimate unification
of the fundamental interactions.
When combined with supergravity grand unification
(SUGRA GUT), it resolves a number
of problems inherent in the SM and
predicts grand unification at the GUT scale MGUT ~ 1016
Giga-electron-Volts (GeV),
subsequently verified at LEP experiments in 90's.
A minimal framework of supergravity model
(the minimal supergravity or mSUGRA model) is
consistent with all existing experimenatal data and
provides a leading candidate (lightest neutralino or χ10) for CDM observed in
universe.
Ω = 0.23 constrains mSUGRA model, suggesting four distinct parameter regions:
coannhilation (CA) region,
focus point (FP) region,
A-funnel region, and
bulk region.
"SUSY mass" spectra in each region are unique.
This means we have to employ different experimental techniques
to probe all four.
I(c). Dark matter particles at the Large Hadron Collider (LHC):
Can we search for dark matter in laboratories?
Yes. If SUSY is correct, the LHC at CERN
is powerful enough to produce SUSY paricles including the SUSY dark matter particle
in proton-proton (pp) collisions at 14 Tera-electron-Volts (TeV).
Two detectors (ATLAS and CMS) will be ready to detect the collisions
in fall 2008.
Each experimentist must be a super-detective who
can solve the mysterious dark matter puzzle
from millions of trillions of pp collisions.
I(d). Compact Muon Solenoid (CMS) - dark matter particle detector:
The CMS (21 m x 15 m x 15 m, 12,500 tonnes) is one of two super-fast & super-sensitive detectors,
consisting of 15 heavy elements, collecting derbies from the collision and converting a visual image for us.
Shown is such a visual, called an event display, from one of "cosmic ray" runs in July.
You see a nice straight-line "track" (muon) hitting
various components of the CMS detector.
The Texas A&M University (TAMU) experimental
"Collder Physics" group is a member institution.
I(e). Why FP?:
In 2002,
R. Arnowitt, B. Dutta, and T. Kamon launched a global phenomenology project
to study cosmologically-motivated SUSY signals at 3 colliders:
(a) Tevatron, (b) ILC and (c) LHC.
Within the mSUGRA model, there are four regions that are equally motivated.
Taking into account
the measurements of anomalous muon magnetic moment and Br(b → s γ),
they began with the CA region
at the Tevatron and the ILC in 2002,
followed by studies at the LHC in 2004.
Fruitful collaboration with other colleagues amplified
the publication productivity. Below are papers we published,
where each student's (post-doc's) name is indicated in boldface (italic):
- [#7 of TeV Pheno Project]
R. Arnowitt, B. Dutta, T. Kamon, and M. Tanaka, Phys. Lett. B538 (2002) 121
- [#2 of ILC Pheno Project]
V. Khotilovich, R. Arnowitt, B. Dutta, and T. Kamon, Phys.Lett. B618 (2005) 182
- [#1 of LHC Pheno Project]
R. Arnowitt, B. Dutta, T. Kamon, N. Kolev, and D. Toback, Phys. Lett. B 639 (2006) 46
- [#2 of LHC Pheno Project]
R. Arnowitt, A. Aurisano, B. Dutta, T. Kamon,
N. Kolev, P. Simeon, D. Toback, and P. Wagner,
Phys. Lett. B. 649 (2007) 73
- [#3 of LHC Pheno Project]
R. Arnowitt, B. Dutta, A. Gurrola, T. Kamon, A. Krislock, and D. Toback,
Phys. Rev. Lett. 100 (2008) 231802
Since this CA work is complete,
the gear is shifted to the FP region.
This is #6 of LHC Pheno Project.
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You might wonder what LHC Pheno Projects #4 and #5 are.
In 2007, B. Dutta and T. Kamon expanded the global phenomenology project
by including cosmologically-motivated SUSY signals in non-standard cosmology.
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II. ANALYSIS:
II(a). Reference points:
Our analysis begins with choosing a reference point in FP region.
We first explore three FP points:
FP1:
M(gluino) = 698,
ΔM(χ30, χ10) = 285;
ΔM(χ20, χ10) = 91;
ΔM(χ30, χ20) = 194
FP2:
M(gluino) = 857,
ΔM(χ30, χ10) = 62,
ΔM(χ20, χ10) = 56,
ΔM(χ30, χ20) = 7
FP3:
M(gluino) = 889,
ΔM(χ30, χ10) = 81,
ΔM(χ20, χ10) = 59,
ΔM(χ30, χ20) = 22
Since FP3 is similar
to the ATLAS reference point, possessing an interesting feature
in its neutralino decays,
we choose FP3 (m0 = 3550 GeV,
m1/2 = 314 GeV, A0 = 0, tanβ = 10, μ > 0,
Mtop = 175 GeV) for
our systematic study.
All SUSY masses are listed
here.
Below is a table
of SUSY masses (in GeV) and leptonic decay branching ratios
for χ20 and χ30 that are important
for this study. Our attention goes to the decays shown in the diagram.
| M(g~) = 889.25 |
M(χ30) = 197 |
M(χ20) = 175 |
M(χ10) = 116 |
| ΔM(χ30, χ10) = 81 |
ΔM(χ20, χ10) = 59 |
ΔM(χ30, χ20) = 22 |
Br(χ20 → χ10 l l) = 6.8% (e+μ)
Br(χ30 → χ10 l l) = 6.6% (e+μ) |
II(b). Event characterization:
We study the final states of the SUSY events
using ISAJET (event generation), PGS4 (fast detector simulation with
a CMS configuration), and ROOT (analysis) packages.
It is evident that the final state is characterized by large multiplicity jets
(~6 jets with pT(j) > 50 GeV), two or more leptons,
and large missing transverse energy from gluino decay.
The events are selected with (i) pT(l) > 10 GeV, pT(j) > 50 GeV;
(ii) ΔR(l, l) > 0.4, ΔR(l, j) > 0.4;
(iii) TrackIso(l) < 3 GeV.
The dilepton mass distributions show
its characteristic two end points, refelecting the decays of two heavier neutralinos.
Furthermore, because of "focus point" implying a small value of
μ parameter in mSUGRA, the Higgsino nature in χ20 and χ30
is large.
Thus χ20 and χ30 strongly couple with heavier quark(s) as
illustrated in the figure.(*)
This also means its branching ratios for gluino decaying
to those neutralinos are large.
Below are some numerical values at our FP3 point.
(*) This figure came in my mind when Will asked me about μ. He has another design
and plan to reveal at the Texas APS meeting.
-
Br(g~ → χ20 tt) = 10.2%
vs.
Br(g~ → χ20 uu) = 0.8%
-
Br(g~ → χ30 tt) = 11.1%
vs.
Br(g~ → χ30 uu) = 0.009%
Thus we have to identify (a) the gluino decay by looking at high multiplicy
of jets and b-jets and (b) the neutralino decays by looking at dilepton mass edges.
III. SUMMARY:
The goal of this research is to construct a new method for probing 23% of the universe
at the LHC in mSUGRA FP region.
We have first performed a systematic study to characterize various final states expected
in the FP region. One of the final states is events with
multi-jets (often from top-quark decays),
two or more leptons plus large missing transverse energy.
Our next study (Phase 2) is emphasized on an identification of top quark(s) and
reconstruction of gluino mass.
ACKNOWLEDGMENTS
We thank Sherry Yennello
for her organization of this summer's successful REU program
as the Cyclotron REU Director.
We also thank Vadim Khotilovich and Jonathan Asaadi
for technical help in setting up computer, user account, and ROOT program.
W.F. thanks Jim Pivarski
and his REU colleague, Paul Geffert (advisor, David Toback),
for their stimulated discussions.
This work is supported in part by DOE grant
DE-FG02-95ER40917.
W.F. is supported by NSF REU.
The work of A.G. is supported by DOEd GAANN.
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Miscellaneous:
Grand Unified Theories (GUTs): Particle physics theories that explain the unification
of three of four fundamental forces in nature: strong, weak and electromagnetic forces.
The Standard Model (SM) is known to have its structural defects to prevent from explaining physics
at much higher energy than ~100 GeV.
Supersymmetry (SUSY) rescues us to be able to construct
a theory that can explain physics up to the GUT scale (~1016 GeV).
mSUGRA: The minimal supergravity (or mSUGRA) model is a leading theory of
particle physics which connects the SM with two Higgs doublets and universality.
Since it is a minimal framework, the model can be
specified by 4 parameters plus one sign
(m0,
m1/2,
A0,
tanβ, and sign of μ)
and
has been used as many benchmark studies at the
Tevatron,
at the
ILC, and at the
LHC.
mSUGRA Parameters:
m0 = common scalar mass at GUT scale;
m1/2 = common gaugino mass at GUT scale;
A0 = common trilinear couping at GUT scale;
tanβ = ratio of two Higgs vacuum expectation values;
μ = Higgsio mixing parameter
Mysterious universe: Dark energy (73%), dark matter (23%), and nomal matter (4%).
Standard Model (SM): The SM particles (6 quarks, 6 leptons, 3 gauge particles)
are just accounted for 4% of universe.
Cold Dark Matter:
23% of universe is composed of cold dark matter.
If cold dark matter is an elementary particle, we need a new particle theory
since none of the SM particles possess the dark matter properties.
Such a new theory guides us how to search it at various experiments at colliders and underground labs.
Two of leading theories are SUSY (or SUGRA-GUT or mSUGRA) or Extra Dimension.
"Partcle Physics and Cosmology (PPC)" - T. Kamon designed this "PPC" cube for the 2nd international conference
on PPC (PPC09)(*) to visualize the interconnection between particle physics
and cosmology.
See
public talks
for more details.
If you are interested in more scientific talks,
see here.
(BTW: This cube was inspired by my 12-yrs. old daughter who was watching "Transformers Movie"
with me.)
See also the SM cube (Window Movie).
(*) The PPC conference was founded by TAMU in 2007.
Dark Matter Hunters:
The TAMU High Energy Physics group, including theorists, has extensively been searching for the
(both "warm" and "cold") dark matter particles at the
Tevatron
and at the
LHC,
as well as various underground dark-matter detection experiments
(e.g., ZEPELIN, CDMS, LUX).
For example, Dr. Weinberger, one of our post-docs, is featured by
Science Grid This Week Feature - Computing the unseen: the search for dark matter
(Feb. 6, 2008)
for his search for
dark matter through Bs → μμ events at the Tevatron.
Why ILC?
The International Linear Collider (ILC) is proposed
to do precision measurements. The power of the collider machine is
complementary to the LHC.
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