× 100 for a grid in xdelay and ydelay . . . . . . . . . 219
6.16 Ionization energy vs. calibrated phonon energy. . . . . . . . . 220
Projected sensitivity of CDMS II and WIMP upper limits from
recent experiments. . . . . . . . . . . . . . . . . . . . . . . . . 225
A.1 Raw output of GEANT based Monte Carlo. . . . . . . . . . . 229
The time I have spent working on CDMS has been a very enjoyable and exciting one. In addition to the wide variety of physics problems and challenges
that have come my way, my life has been enriched by the people I have associated with during this time. Of these people, I mention first Dan Akerib,
my thesis advisor. I have benefitted immensely from Dan’s broad knowledge
of physics, perspective on issues, his ability to quickly grasp and explain new
concepts, and his talent for separating the essentials from the details. Before
long, I realized how fortunate I was to have an advisor like Dan. I am proud
to be his first graduate advisee. I hope that many more young physicists will
have the opportunity to experience the leadership, humility, encouragement,
and friendship that he brings as a thesis advisor.
I thank the two research associates I have worked with in the CWRUCDMS group, Alex Bolozdynya and Richard Schnee, for their guidance and
friendship. I thank Alex for giving me a feel for the practical, hardwareoriented side of experimental physics which I now relish. I also treasure the
many conversations we have had on more esoteric topics. I have learned much
from Richard on subjects ranging from poker to data analysis. I admire his
overall knowledge of the experiment, and had a good opportunity to make use
of it during the last few months of thesis writing. Special thanks to him for
helping me out in many ways during the last few months. Don Driscoll was
my first office mate and colleague at CWRU. I have greatly enjoyed talking
physics and solving problems with him. I have happily become addicted to his
humor and optimism as well as the wonderful company of his family, Diana
and Astra. Gensheng Wang, the next graduate student to join the group
has been a pleasure to work with. In addition to the general comradery we
have developed, I have enjoyed and learned much from our interesting and
often lively discussions. I also thank him and his family for treating me on
many occasions to the best Chinese food I have ever tasted. In the last year,
I have had the pleasure of working with Sharmila Kamat. Her hard work,
and questions have been very helpful to me. I hope I have done a decent
job of answering some of those questions. I would also like to thank the
undergrads, Matt, Mo, Tim P, Peter, Aaron, and Tim J for their tireless
work and for making the lab a fun and lively workplace. Thanks also to
Cheshana Marshal, and Mike Stamatikos for their excellent company.
I would like to thank all the secretaries in the physics department for
making my life much easier than it could have been. I also thank my thesis
committee for their interest and patience. I am grateful to Lawrence Krauss
for pointing me in the direction of Dan and CDMS when I started looking
for research work a while back.
I thank Bernard Sadoulet and Blas Cabrera for welcoming me to the
CDMS collaboration. My exposure to the rest of the CDMS collaboration
occurred mainly over the month-long visits I made to Stanford. During these
stays and later, I have had the opportunity of working closely with Rick
Gaitskell whose guidance and encouragement have been valuable to me. I
am also grateful to Tom Shutt for his help and patient explanations during my
first days in CDMS. I also thank Angela Da Silva whose work was the starting
point for many of the studies described here. I was fortunate to have been
paired with Steve Eichblatt on most of these studies. I have greatly enjoyed
his company and friendship. I have enjoyed many interesting and informative
discussions with Dan Bauer, Sae Woo Nam, Andrew Sonnenschein, Roland
Clarke, and Sunil Golwala during my visits to Stanford. I am especially
thankful to Sunil for his perseverance and hard work in making Run 19 the
success that it was. I thank Steve Yellin, Dennis Seitz, Laura Baudis, Maria
Isaac, Paul Brink, Patrizia Meunier, Tarek Saab, and Vuk Mandic for helping
me with much of the work described in this dissertation. Although I haven’t
worked on specific projects with many of the other members of CDMS, I owe
all of them a big thank you because I have benefitted from all of their hard
I thank all my friends and relatives all over the world! Special thanks to
Chaminda, Kathy and the kids, and my friend Bala for their encouragement
and support during the last few months of thesis writing. I am grateful to
Christine for her company, and friendship. I also thank my brother Gehan
for his support and words of encouragement during trying times. Finally but
most importantly, I thank my parents. I would not be who I am or where I
am if not for the love, freedom, and opportunities they have given me. Thank
The Limiting Background in a Dark Matter Search at Shallow Depth
A convincing body of evidence from observational and theoretical astrophysics suggests that matter in the universe is dominated by a non-luminous,
non-baryonic, non-relativistic component. Weakly Interacting Massive Particles (WIMPs) are a proposed particle candidate that satisfy all of the above
criteria. They are a front-runner among dark matter candidates because their
predicted contribution to matter in the universe is cosmologically significant
and because they may arise naturally from supersymmetric (SUSY) models
of particle physics. The Cryogenic Dark Matter Search (CDMS) employs
advanced detectors sensitive to nuclear recoils caused by WIMP scatters and
capable of rejecting ionizing backgrounds.
The first phase of the experiment, conducted at a shallow site, is limited
by a background of neutrons which are indistinguishable from WIMPs in
terms of the acquired data. By accounting for and statistically subtracting
these neutrons, CDMS I provides the best dark matter limits to date over a
wide range of WIMP masses above 10 GeV/c2. These results also exclude
the signal region claimed by the DAMA annual modulation search at a >71%
The second phase of the experiment, located at a deep site, is scheduled
to begin data acquisition in 2002. Due to longer exposures, larger detector
mass, and low background rates at this site, data from CDMS II are expected
to improve on present WIMP sensitivity by about two orders of magnitude.
Emphasized in this work are the research topics in which I have been
directly involved. These include the work described in Chapters 3 and 5
with regard to the development and use of simulation tools, detailed studies
into the limiting neutron background, and the present understanding of this
background in relation to CDMS I and CDMS II. I was also involved in
several detector development projects in preparation for CDMS II. Analysis
of test data from a ZIP detector, planned for use in CDMS II, is presented
in Chapter 6.
WIMP Dark Matter
Much of the theoretical and observational work in present-day astrophysics
revolves around the dark matter problem. Particle physics also plays an
integral role in research into this subject. The Cryogenic Dark Matter
Search (CDMS) is designed for the direct detection of Weakly Interacting
Massive Particles (WIMPs), a strongly motivated dark matter candidate.
In this chapter, I will briefly outline the reasoning and evidence behind the
dark matter problem, the need for non-baryonic cold dark matter, the
motivation for WIMPs, and some specifics regarding their detection.
The dark matter problem refers to the lack of luminous matter for
explaining certain astronomical observations under the framework of
conventional gravity. This discrepancy between theory and observation may
be explained by a “dark” component of matter in the universe. The
discussion in this chapter is phrased in terms of this assumption. Another
solution to the dark matter problem may be the discovery of a new theory
of gravity and inertia that does not require additional matter to reconcile
theory and observation. Several such theories have been proposed.
However, none of them have gained widespread acceptance due mainly to
aesthetic reasons1 . In either case, experimental searches for dark matter,
like CDMS, serve an important purpose. They are useful for detecting or
setting limits on several proposed dark matter candidates.
The development below follows several standard text books on
cosmology [3, 4, 5]. The details may be found in these references.
The Robertson-Walker metric, which is the outcome of assuming
that the universe is homogeneous and isotropic on large scales, is given by
+ r2 dθ2 + r2 sin2 θdφ2
1 − kr2
ds2 = dt2 − R2 (t)
where R(t) is a scale factor with units of length. It is scaled to give k the
values +1, −1, or 0, for positively curved, negatively curved, and flat
universes. The Einstein equations for this metric simplify to the Friedmann
equations given below.
(ρ + p) +
Here, units with c = 1 have been used. The matter density and pressure of
the “fluid” filling the universe are given by ρ and p respectively. Newton’s
constant is represented by G while k is the curvature index as given above.
Terms involving the cosmological constant Λ can be absorbed into the first
term of each r.h.s. above, if it is viewed as a “fluid” component having
energy density ρΛ = Λ/8πG and pressure pΛ = −ρΛ . For matter and
radiation the pressure p is given by 0 and ρ/3, respectively.
Several authors have recently claimed that the predictions of one such theory, MOdified
Newtonian Dynamics (MOND) compare unfavourably with existing data. [1, 2]
The Hubble parameter is given by
According to equation 1.2, a flat universe (k = 0) implies that
ρtotal = ρ + ρΛ = 3H 2 /8πG. This value of ρtotal is referred to as ρc , the
critical density. Using this definition, equation 1.2 may be rewritten as
= Ωm + ΩΛ = Ω
H 2 R2
where the Ω’s are obtained by dividing the respective densities (ρ and ρΛ )
by ρc . Note that both Ω’s are functions of R. Also note that a Ω geater
than, less than, or equal to unity correspond to positively-curved,
negatively-curved, and flat universes, respectively. The Hubble constant H0
is the present value of H. It is usually quoted to be 
H0 = 71 ± 7 km/sec/Mpc = 100h km/sec/Mpc
where the dimensionless parameter h is useful for expressing the uncertainty
in H0 when quoting cosmological parameters. For example, using this
value, the current critical density is estimated at 1.1 × 10−6 h2 GeV/cm3.
Light emitted in the past is redshifted by a factor 1 + z given by
where R0 is the present value of the scale factor while R is the value of the
scale factor at the time that light was emitted. According to equation 1.5,
the r.h.s. of equation 1.7 is a function of the curvature k, Ωm , and ΩΛ . The
redshift (z) in the l.h.s. is a measurable.
When a disk and a point are separated by a large distance, the solid
angle subtended by the disk at the point depends on the curvature of space.
Light rays from the point to the edges of the disk will be convergent,
divergent, and straight for positive-curvature, negative-curvature and flat
universes respectively. Curvature-measurement experiments where the
point is an observer and the disk is the a far-away object of known size are
called standard ruler tests. Standard candle tests, where the redshift of an
object is recorded against its luminosity distance, also yield information on
Constraints on Ωm and ΩΛ
Distance versus redshift measurements on high-redshift supernovae Ia [7, 8]
and measurements of the first-Doppler-peak angular size in the Cosmic
Background Radiation (CBR) [9, 10], are highly successful instances of the
two methods outlined above. Since z is a measurable and the r.h.s of
equation 1.7 depends on curvature and the Ω’s, measuring curvature using
these methods puts constraints on specific functions of Ωm and ΩΛ .
Experimental constraints on Ωm and ΩΛ are obtained using measurements
at several redshifts. While to first order, the angular size of the first CBR
peak is only sensitive to Ω = Ωm + ΩΛ , the shape of the angular power
spectrum of temperature anisotropy and positions of other peaks in that
spectrum can be used to obtain possible ranges in Ωm and ΩΛ [11, 12].
These experiments, together with other observations  have in recent
years provided accurate evidence in favor of a particular cosmological
model. This model is described by
This model is contrary to previous expectations that Ω = 1 and Ωm ≥ 0.9.
In recent years, the above model has lead to a concentrated effort to find a
good particle physics motivation for a non-zero Λ.
A theoretical prejudice for Ω = 1 exists for two reasons.
Equations 1.2 and 1.3 can be used to show that Ω = 1 is the only static
solution for Ω. Furthermore, values of Ω different from unity will lead to
large deviations from unity in a very short time. Therefore, in the early
universe, Ω must have been extremely close to unity in order to be
consistent with the present observation of a nearly flat universe. Therefore,
if Ω is not exactly unity, a mechanism for extreme fine tuning is required to
explain its minute deviation from unity in the early universe. This is the
first theoretical reason for expecting that Ω = 1. The other is due to a
popular class of theories known as inflation. These theories were first
developed to explain the absence of magnetic monopoles. Soon thereafter,
it was realized that they also explain the startling homogeneity of the
cosmic microwave background radiation which is incident on the earth
today from a large number of causally disconnected parts of the universe.
In inflation theories, an exponential expansion of a previously-small
causally-connected patch of space is used to explain this homogeneity. The
exponential expansion drives the scale factor R to a large value, thus
making the second term on the l.h.s. of equation 1.5 negligibly small.
Therefore, even if the curvature k is non-zero, Ω is driven to unity.
Evidence for Non-Baryonic Cold Dark
The galaxy luminosity density in the nearby universe is measured to be 
Lg = 3.3 × 108 hL /Mpc3 .
where L refers to the sun’s luminosity. Given this luminosity density, the
matter density will equal ρc when the mass to luminosity ratio is
where M refers to the solar mass. Thus, the matter density in the universe
cannot be equal to the critical density or a significant fraction of it, if most
of the matter in the universe is in objects like like stars
(M/L < 10Modot /Lodot ).
It addition to the above argument, there are several direct
measurements that support the existence of a dark matter problem. The
most familiar of these are from the rotation curves of spiral galaxies and
observations of galaxy clusters.
Galactic Rotation Curves
The tangential velocity of stars in the plane of a spiral galaxy can be
measured using redshifts of stellar absorption lines. About 83% of the
luminous matter in a typical spiral galaxy is contained within a radius Ropt
of about 10 kpc. The radio emission line of neutral hydrogen can be used to
trace velocities beyond this point to about 2Ropt . A set of velocity
curves obtained in this way for several typical spiral galaxies is shown in
figure 1.1. The crucial feature of these plots is that the velocity curve
flattens beyond r > Ropt. According to Newtonian gravity, which is
applicable in this case, the tangential velocities are governed by
If most matter in the galaxy were luminous, the velocity curve beyond Ropt
is therefore expected to vary as 1/ r. The observed rotation curves clearly
indicate the presence of a non-luminous component. The expected rotation
curve due to the luminous matter is also displayed in figure 1.1. The
dashed curve indicates the rotation curve due to a particular “dark halo”
model used by the authors of . Surveys of spiral-galaxy rotation curves
typically yield mass-to-light ratios greater than 10 hM /L .
Figure 1.1: Rotation curves of spiral galaxies. The radius is given in units
of Ropt. The velocities are normalized to the velocity at Ropt. Data are
shown with error bars. The dashed curve is the velocity contribution due
to an assumed dark halo, which is modeled to have a density distribution
proportional to r3 /(r2 + a2) where a is a constant . The dotted and solid
curves represent the expected rotation curves due to luminous matter and
the combination of dark and luminous components. Figure taken from 
Clusters of Galaxies
Galaxy clusters are gravitationally bound systems of up to several thousand
galaxies. Because of their large size, galaxy clusters are expected be a fair
sampling of the universe. Mass-to-light measurements on galaxy clusters
can therefore be used to estimate Ωm for the universe.
Several methods are used to extract mass-to-light ratios from galaxy
clusters. The first method uses the peculiar velocities of galaxies to
estimate gravitational potential energy in a cluster . The gravitational
potential is obtained from peculiar velocities through the virial theorem
which states that
energy respectively. This method is valid as long as the cluster is in a state
of dynamic equilibrium. The “first” discovery of the dark matter problem is
attributed to Zwicky, who in 1933 used this method on the Coma cluster.
X-ray emission from hot intracluster gas can also be used for
mass-to-light estimates. Hydrostatic equilibrium is assumed for gas in the
central part of the cluster. The observed x-ray maps are then fit to models
of temperature and density distributions of the gas [18, 19]. Gravitational
lensing of background galaxies by clusters has also been used to measure
the dark matter content of clusters . Most mass-to-light estimates
obtained from galaxy clusters lie in the range (250 − 450)hM /L . This
implies that Ωm is in the range 0.18 to 0.32.
Cluster Baryon Fraction
As with mass-to-light ratios, the ratio of baryon density to matter
density in galaxy clusters is expected to be a fair sampling of the baryon
Figure 1.2: Measurements of mass-to-light ratio as a function of dynamical
scale. Figure taken from 
fraction of the universe. Once the baryon fraction of a cluster is measured,
it can be combined with constraints on baryon density provided by Big
Bang nucleosynthesis, discussed below, to obtain Ωm . These estimates are
in good agreement with mass-to-light estimates from galaxy clusters, with
typical quoted Ωm s of about 0.4±0.1 .
Consistency of Evidence
The above methods and other studies (eg. large scale flows, Virgo infall)
yield a coherent picture of matter density in the universe. All estimates of
Ωm from large scale structures, with fair sampling of the universal matter
density, give values consistent with the picture described by equation 1.8.
Figure 1.2 shows the light to mass ratios from a large number of
measurements carried out at different length scales .