Theses
Measurement of Dielectron Invariant Mass Spectra in Au + Au Collisions at psNN = 200GeV
with HBD in PHENIX
Jiayin Sun, 2016
Dileptons are emitted throughout the entire space-time evolution of heavy
ion collisions. Being colorless, these electromagnetic probes do not participate
in the final-state strong interactions during the passage through the hot
medium, and retain the information on the conditions of their creation. This
characteristic renders them valuable tools for studying the properties of the
Quark Gluon Plasma created during ultra-relativistic heavy ion collisions.
The invariant mass spectra of dileptons contain a wealth of information on
every stage of the evolution of heavy ion collisions. At low mass, dilepton
spectra consist mainly of light meson decays. The medium modification of
the light vector mesons gives insight on the chiral symmetry restoration in
heavy ion collisions. At intermediate and high mass, there are significant
contributions from charm and bottom, with a minor contribution from QGP
thermal radiation. The region was utilized to measure cross sections of open
charm and open bottom, as well as quarkonium suppression as demonstrated
by PHENIX.
An earlier PHENIX measurement of dielectron spectra in heavy ion collisions,
using data taken in 2004, shows significant deviations from the hadronic
decay expectations. The measurement, however, su↵ered from an unfavoriii
able signal to background ratio. Random combination of electron-positron
pairs from unrelated sources, mostly Dalitz decay of ⇡0 and external conversion
of decay photon to electrons, is the main contributor to the background.
Mis-identified hadrons are another major background source.
To improve the situation, the Hadron Blind Detector (HBD), a windowless
proximity focusing Cerenkov detector, is designed to reduce this background
by identifying electron tracks from photon conversions and ⇡0 Dalitz
decays. The detector has been installed and operated in PHENIX in 2009 and
2010, where reference p+p and Au+Au data sets were successfully taken. We
will present the dielectron results from the analysis of the Au+Au collisions,
and compare the measured mass spectra to theoretical expectations.
Measuring the anti-quark contribution to the proton spin using parity violating W
production in polarized proton proton collisions
Ciprian Gal, 2014
Since the 1980s the spin puzzle has been at the heart of many experimental measurements.
The initial discovery that only ∼30% of the spin of
the proton comes from quarks and anti-quarks has been refined and cross checked
by several other deep inelastic scattering (DIS) and semi inclusive
DIS (SIDIS) experiments. Through measurements of polarized parton distribution
functions (PDFs) the individual contributions of the u, d, ¯u, ¯d,
quarks have been measured. The flavor separation done in SIDIS experiments requires
knowledge of fragmentation functions (FFs). However, due to the
higher uncertainty of the anti-quark FFs compared to the quark FFs, the quark polarized
PDFs (∆u(x), ∆d(x)) are significantly better constrained than
the anti-quark distributions (∆¯u(x), ∆ ¯d(x)). By accessing the antiquarks directly
through W boson production in polarized proton-proton collisions
(u ¯d → W+ → e+/µ+ and du¯ → W− → e −/µ−), the large FF uncertainties are avoided
and a cleaner measurement can be done. The parity violating single
spin asymmetry of the W decay leptons can be directly related to the polarized
PDFs of the anti-quarks. The W± → e± measurement has been performed with
the PHENIX central arm detectors at √s = 510 GeV at the Relativistic Heavy Ion
Collider (RHIC) and is presented in this thesis.
Approximately 40 pb−1 of data from the 2011 and 2012 was analyzed and a large parity
violating single spin asymmetry for W± has been measured. The
combined data for 2011 and 2012 provide a single spin asymmetry for both charges:
• W+: −0.27 ± 0.10(stat) ± 0.01(syst)
• W−: 0.28 ± 0.16(stat) ± 0.02(syst)
These results are consistent with the different theoretical predictions at the
1σ level. The increased statistical precision enabled and required a more
careful analysis of the background contamination for the this measurement. A method
based on Gaussian Processes for Regression has been employed to
determine this background contribution. This thesis contains a detailed description
of the analysis together with the asymmetry results and future
prospects.
Double Longitudinal Helicity Asymmetries in Pion Production from Proton Collisions,
Studies of Relative
Luminosity Determination, and the Impact on Determination of the Gluon Spin in
the Proton
Andrew Manion, 2014
Polarized proton-proton collisions at RHIC are being used to study
the origin of proton spin, which arises from the spin and orbital
angular momentum of its constituent quarks and gluons. Measurements
at the PHENIX experiment at √s = 200 GeV of Aπ0LL,
the double longitudinal helicity asymmetry in neutral pion production,
are used in global analyses of world polarized scattering data,
where they are particularly important in constraining the sector
of gluon polarization. These measurements have ruled out maximal
gluonic spin contributions and are consistent with a small
or zero contribution. In the latest measurements, the statistical
precision of the data has reached the systematic limit, prompting
investigation into the largest of the systematic uncertainties, the
determination of relative luminosity. Details of the 2009 measurement
at PHENIX of Aπ0LL and its inclusion in the global analysis will
be presented along with recent studies on systematic uncertainties,
including a 2012 study that varied the angles of the beams in the
PHENIX interaction region.
Systematic studies of soft direct photon production in Au+Au collisions at psNN =
200GeV
Benjamin Bannier, 2014
Direct photons are produced during all stages of a heavy-ion collision.
Due to their very small interaction cross section with the dense hadronic
medium, they can escape the collision almost undisturbed and transport
information about their production environment to a detector making them
an excellent probe in heavy-ion physics.
The observation of both a large yield and strong elliptical flow v2 of
soft direct photons in heavy ion collisions at RHIC has sparked a lot of
interest. While a large yield seems to point towards abundant production
from the early, hot stages of the interaction, large elliptical flow can be better
understood in a picture of predominately late production when the overall
flow of the medium has built up. Telling different production scenarios for
soft direct photons apart has been diffcult.
We map out the centrality-dependence of direct photon observables and
present results for dependence of the soft direct photon yield and flow as functions
of centrality in the momentum range 0.4GeV/c < pT < 5.0GeV/c
from a sample of externally converted photons. Here we exploit the good
momentum resolution of our detector for charged particles at low momenta
and reconstruct photons in electron-positron pairs from conversions in specific
locations in the detector material. We find that the yield of soft direct
photons has approximately a power-law dependence on the number of participants
in the collision, and that their flow is en par with the flow of photons
from hadron decays, indicative of relatively late production.
Low Momentum Direct Photons as a Probe of Heavy Ion Collisions
Richard Petti, 2013
Relativistic heavy ion collisions have been a major research interest
in the field of nuclear physics for the past few decades. Large
collider facilities have been constructed to study the exotic matter
produced in relativistic heavy ion collisions, one of which is
the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National
Laboratory in Upton, NY. Essential to the study of heavy
ion collisions are probes that are produced in the collision itself.
Photons are a very useful probe of the collisions, since they escape
the fireball virtually unmodified and carry with them information
about the environment in which it was produced. Recent interest
in low momentum direct photons has increased, due to the onset of
the “thermal photon puzzle” and the apparent inability for typical
models to explain both a large direct photon yield excess and large
azimuthal production asymmetry (v2) at low momentum measured
by PHENIX. The focus of this thesis will be the measurement of
direct photons at low momentum with the PHENIX detector in
√sNN = 200GeV Au+Au collisions.
Low momentum direct photons (direct is any photon not from
a hadron decay) are notoriously difficult to measure in a heavy
ion environment, due to large decay photon backgrounds, neutral
hadron contamination, and worsening calorimeter resolution. A
novel technique for measuring direct photons via their external
conversion to di-electron pairs has been developed. The method
virtually eliminates the neutral hadron contamination due to the
very clean photon identification based on di-electron pair invariant
mass cuts. The direct photon fraction, Rγ, defined as the ratio of
the yield of inclusive photons to hadron decay photons, is measured
through a double ratio further reducing systematic uncertainties to
manageable levels at low momentum. The direct photon fraction
is converted to a direct photon invariant yield and a detailed look
at the centrality dependence of the excess yield is presented. This
dependence is confronted with recent theoretical calculations predicting
novel production mechanisms of direct photons and possible
solutions to the “thermal photon puzzle”.
Measurements of Cross sections and Double Longitudinal Asymmetries of π± production
in p + p collisions to constrain the Gluon Spin contribution to the Proton Spin
Sook Hyun Lee, 2013
The spin of the proton is known to be 1
2!. Although its angular
momentum sum rule in terms of constituent quark and gluon components
has been established, its detailed decomposition is poorly
known. What fraction is attributed to the spin (polarization) and
orbital angular momentum component is completely unknown, and
how much of the spin component is from the quarks and gluons
is only partially known. Dedicated experiments in the past few
decades have measured the sum of quark and anti-quark spin contribution
to account for only ∼25% of the proton spin, whereas
separating the sea-quark polarizations or constraining the contribution
of gluon polarization is still a subject of active experimental
research.
The Relativistic Heavy Ion Collider (RHIC) is a unique facility
that provides collisions between polarized protons and thereby excellent
tools to study the role of gluons in the proton intrinsic
angular momentum. The double longitudinal asymmetry ALL of
single inclusive production allows access to the polarized gluon
distribution ∆g. It does so when the asymmetry measurements
are incorporated into the so-called global analysis where polarized
parton distribution functions and fragmentation functions are simultaneously
fitted to best describe various measurements from
different experiments. While π0 at PHENIX and jets at STAR
have mainly been putting constrains on ∆G, the first moment of
∆g, other channels that provide complementary information on
∆G are critical.
The high pT charged pion production is expected to be sensitive
to the sign of ∆G. The isospin symmetry with other pion species
will enables us to visually see the sign via the ordering of ALL
of the three pion species even without performing global analysis.
The interpretation can also be cross checked with the one drawn
from global analysis, where the dominance of q−g scattering in π±
production enhances the sensitivity. For this dissertation, high pT
charged pion production at mid-rapidity in polarized p+p collisions
at √s = 200 GeV has been analyzed. In this work, I developed
a new analysis including the Hadron Blind Detector, a gas-based
Cerenkov detector, to overcome the major challenge, a large fraction
of electrons misidentified with π±, and achieved >98% purity
in π± sample. Along with ALL, invariant differential cross section
has been measured for different charges separately to validate
the current perturbative Quantum Chromo-dynamics framework.
Through these first successful measurements, we demonstrated π±
is a promising channel to extract crucial information on ∆G in
that complete discussions will be available with further constrained
charge-separated fragmentation functions and improved statistics.
Single Electrons from Decays of Heavy Quarks Produced in Cu+Cu Collisions at the Relativistic
Heavy Ion Collider
Nicole Apadula, 2013
The PHENIX experiment at the Relativistic Heavy Ion Collider
(RHIC) has measured charm and bottom quark production at midrapidity
in p+p, d+Au, and Au+Au collisions at √
s = 200 GeV
through their semi-leptonic decay into electrons. The large mass
of the charm and bottom quarks means they are formed predominately
by gluon-gluon fusion in the initial hard scatterings at RHIC
and thus experience the full evolution of the medium, making them
a good probe of medium effects. The yield in central Au+Au collisions
is suppressed relative to p+p collisions, suggesting that the
heavy quarks lose a significant portion of their initial energy in
the medium. The d+Au results are enhanced relative to the p+p,
pointing to cold nuclear matter effects that are masked by the hot
medium in the Au+Au collisions. Studies of the intermediately
sized Cu+Cu system provide a way to explore these competing effects
as a function of system size and number of participating nucleons.
In this dissertation, measurements of electrons from the decays
of heavy quarks produced in Cu+Cu collisions are presented.
We examine the interplay between hot and cold nuclear matter
effects on open heavy flavor by comparing the results to those already
measured in Au+Au and d+Au collisions. It has already
been shown in the central Au+Au that partonic energy loss models
are insufficient to describe the level of suppression. New models
that include cold nuclear matter effects and the addition of meson
dissociation are shown and compared to the Cu+Cu results.
Direct Photon Tagged Jets in 200 GeV
Au+Au Collisions at PHENIX
Megan Connors, 2011
A hot dense medium called the quark gluon plasma (QGP) has
been created at the Relativistic Heavy Ion Collider (RHIC). Quarks
and gluons are deconfined in the QGP state, but many of its properties
are still under investigation. One interesting observation is
that high momentum partons (quarks and gluons), which result
from hard scatterings in the initial collision, lose energy as they
travel through the medium. These partons fragment into the particles
observed in the detector. Since fully reconstructing all the
“jet” particles associated with the initial parton is complicated by
the high multiplicity background produced in heavy-ion collisions,
two particle correlations which trigger on a high momentum, pT ,
particle and measure the yield of associated particles in the event
as a function of the azimuthal angle, ∆φ, are used instead.
Di-hadron correlations are useful for observing suppression of the
away-side (∆φ > π/2) jet yield and some features potentially due
to the medium’s response to the lost energy. However, the hadron
triggers, since they are fragments of a modified jet themselves, are
biased to be near the surface of the medium and the jet energy is
unknown. Since photons do not interact via the strong force, they
are unmodified by the medium and provide an unbiased trigger.
Direct photons result directly from the hard scattering. They balance
the energy of the opposing parton and provide knowledge of
the opposing jet momentum. Therefore, by measuring the hadron
yield on the away-side, opposite the direct photon trigger, the jet
fragmentation function, which describes how partons fragment into
hadrons, can be measured as a function of zT = phT/pT γ. By comparing
the spectra in Au+Au collisions to that in p+p collisions,
the effective modifications of the fragmentation function can be
quantified.
Using the data collected by PHENIX during the 2007 RHIC Run,
suppression of the away-side yield and the modified fragmentation
function is measured via direct photon-hadron correlations. By including
lower pT hadrons in the measurement, the altered shape of
the modified fragmentation function is studied. Possible enhancement
of the lowest zT particles suggests that the energy lost at high
pT is redistributed to low pT particle production.
A Measurement of Electrons From Heavy Quarks in p+p Collisions at √s = 200 GeV
Harry Themann, 2011
The Relativistic Heavy Ion Collider (RHIC) at BNL offers a unique
opportunity in that it is capable of colliding protons and nuclei,
including asymmetric collisions of different species. Open heavy
quarks, that is charm or bottom not forming bound cc¯ or b ¯b pairs
are important probes of the Quark Gluon Plasma at the Relativistic
Heavy Ion Collider at BNL. They are formed at the initial collision
of the nuclei and thus any effect to their transverse momentum
spectra or azimuthal distribution can only come from their interaction
with the matter created in the collision. One of the most
powerful techniques of measuring these effects is to divide AuAu
data by appropriately scaled pp data. This work focuses on providing
the best possible pp reference both in scope and precision.
Transverse momentum (pT ) spectra of electrons from semileptonic
weak decays of heavy flavor mesons in the range of 0.3 < pT <
15.0GeV/c have been measured at mid-rapidity (|y| < 0.35) beyond
the previous published range of pT < 9.0GeV/c. This is done
using a new technique exploiting the observed characteristics of energy
deposition in the PHENIX electromagnetic calorimeters. We
present this technique as well as the final measurement compared
to FONLL theory predictions of open charm and bottom cross section.
Dielectron Mass Spectra in √sNN = 200 GeV Cu+Cu Collisions at PHENIX
Sarah Campbell, 2011
The dielectron mass spectrum consists of light vector meson decays, correlated
heavy quark contributions and decays from other hadronic and photonic sources.
Thermal radiation and modifications to the light vector mesons may provide
additional signals at low masses above known hadronic sources. The PHENIX
√sNN = 200 GeV Au+Au and Cu+Cu analyses have measured a centrality dependent
excess in the the low mass region between 0.15 GeV/c2 and 0.75 GeV/c2.
Between the φ and the J/ψ, in the intermediate mass range, the correlated heavy
quark decays are the primary dielectron source; direct photons may augment this
region as well.
The Cu+Cu system is sensitive to the onset of the dielectron excess. By studying
the Cu+Cu mass spectra and yields as a function of pair pT and collision centrality
we obtain further understanding of its behavior. Comparisons to the PHENIX
Au+Au and p+p measurements an extrapolations from theory are presented.
Search for jet interactions with quark-gluon plasma
John Chen, 2011
A hot, dense QCD medium is created in heavy ion collisions at
the Relativistic Heavy Ion Collider at Brookhaven National Laboratory.
This new type of matter is opaque to energetic partons,
which suffer a strong energy loss in the medium. Two particle
correlations are a powerful tool to study the jet properties in the
medium and provide information about the energy loss mechanism
and jet-medium interactions. When triggering on high pT particles,
the away-side shape depends strongly on the pT of the associated
particles.
In this analysis, we present the inclusive photon-hadron two
particle azimuthal correlations measured in Au+Au collisions at
√sNN = 200 GeV by PHENIX experiment. In order to study
jet-medium interactions, we focus on intermediate pT , and subtract
particle pairs from the underlying event. Jet-like correlations
appear modified in central Au+Au compared to p+p, in both the
trigger and opposing jet. The trigger jet is elongated in pseudorapidity
(the “ridge”), while the opposing jet shows a double peak
structure (”head” and “shoulder”). We decompose the structures
in ∆η and ∆φ to disentangle contributions from the medium and
the punch-through and trigger jets. Upon correcting the underlying
event for elliptic flow, the ridge is observed for associated
particle pT below 3 GeV/c; it is broad in rapidity and narrow in
∆φ. The away side correlated particle yield is enhanced in central
collisions. The yield of particles in the shoulder grows with centrality
while the away side punch-through jet is suppressed. Remarkably,
the ridge closely resembles the shoulder in the centrality
dependence of particle yield and spectra.
There has been great debate about the origin of the ridge and
shoulder. A favored explanation is that the structure is due to
features of the collective flow of particles in the underlying event,
particularly the fluctuation-driven triangular flow, quantified by
the third Fourier component, v3. We measure higher order Fourier
harmonics in two ways, and use the results to give the shape of
particle correlations in the underlying event. We decompose the
power spectrum for the medium and for jets measured in p+p
collisions.
When including the higher harmonics of the collective flow (v3,
v4) in the shape of the underlying events in two particle correlations,
the ridge and shoulder no longer exist after subtraction. The
jet function in Au+Au looks like p+p in which the away side jet
is suppressed and broadened. There is also a pedestal-like structure
in the jet function. Since the higher harmonics only change
the shape of the underlying background, the pedestal is simply the
redistribution of the ridge and shoulder particle yields.
In conclusion, when jets pass through the medium, the away
side jet is suppressed and the shape is broadened. This also brings
out extra particles with spectra slightly harder than the medium,
but softer than jet fragments. These are probably from the jetmedium
interaction.
Cold Nuclear Matter Effects on Heavy Quark Production in Relativistic Heavy Ion Collisions
John Durham, 2011
The experimental collaborations at the Relativistic Heavy Ion Collider
(RHIC) have established that dense nuclear matter with partonic
degrees of freedom is formed in collisions of heavy nuclei at
√sNN = 200 GeV. Information from heavy quarks has given significant
insight into the dynamics of this matter. Charm and bottom
quarks are dominantly produced by gluon fusion in the early stages
of the collision, and thus experience the complete evolution of the
medium. The production baseline measured in p + p collisions
can be described by fixed order plus next to leading log perturbative
QCD calculations within uncertainties. In central Au+Au
collisions, suppression has been measured relative to the yield in
p + p scaled by the number of nucleon-nucleon collisions, indicating
a significant energy loss by heavy quarks in the medium. The
large elliptic flow amplitude v2 provides evidence that the heavy
quarks flow along with the lighter partons. The suppression and
elliptic flow of these quarks are in qualitative agreement with calculations
based on Langevin transport models that imply a viscosity
to entropy density ratio close to the conjectured quantum lower
bound of 1/4π. However, a full understanding of these phenomena
requires measurements of cold nuclear matter (CNM) effects,
which should be present in Au+Au collisions but are difficult to
distinguish experimentally from effects due to interactions with the
medium.
This thesis presents measurements of electrons at midrapidity from
the decays of heavy quarks produced in d+Au collisions at RHIC.
A significant enhancement of these electrons is seen at a transverse
momentum below 5 GeV/c, indicating strong CNM effects
on charm quarks that are not present for lighter quarks. A simple
model of CNM effects in Au+Au collisions suggests that the
level of suppression in the hot nuclear medium is comparable for
all quark flavors.
Probing the Nucleus with d+Au Collisions at RHIC
Zvi Citron, 2011
The Relativistic Heavy Ion Collider (RHIC) was built to produce
and study Quark Gluon Plasma (QGP), the phase of matter
thought to exist under conditions sufficiently hot and dense to
create a medium in which the degrees of freedom are quarks and
gluons rather than color neutral hadrons. Already in its early years
of running, the data from RHIC provided tantalizing evidence of
QGP signatures in Au+Au collisions at √sNN = 200 GeV. A crucial
part of understanding the putative QGP in Au+Au collisions
is to have both a well understood reference as well as a robust
control experiment. Proton-proton collisions at the same √sNN
serve as the baseline for heavy ion collisions at RHIC, and play
an invaluable role in setting our frame of reference in interactions
that do not create any nuclear medium. For the control experiment,
RHIC’s ability to collide asymmetric beams is utilized and
d+Au collisions are used. Unlike p+p collisions, in the d+Au system
there is a nuclear medium present - the heavy Au nucleus -
and so we may study this system to distinguish initial state cold
nuclear matter effects from final state effects that occur in the hot
dense medium of Au+Au collisions.
Beyond its use as a control experiment, the d+Au collision system
presents the opportunity for important study of nuclear and
nucleonic structure, it is after all necessary for our colored parton
theory to operate in the nucleus as well as in a QGP. Deuteron -
gold collisions at RHIC are a powerful tool for shedding light on
cold nuclear matter effects.
This thesis describes two analyses of d+Au collisions measured
by the PHENIX experiment at RHIC. The first is a measurement
of the midrapidity yield of unidentified charged hadrons in the
2003 RHIC run. This is used a key baseline for understanding
particle production in Au+Au collisions as well as a detailed look
at the Cronin effect. The second analysis measures rapidity separated
two-particle production where one of the particles is at either
forward or backward rapidity and the other at midrapidity. These
measurements probe different x regions of the Au nucleus and there
investigate shadowing, anti-shadowing and other cold nuclear matter
effects.
Measurement of
Fast Parton Interactions
with Hot Dense Nuclear Matter
via Two-Particle Correlations
at PHENIX
Michael McCumber, 2009
Deconfinement of color charge in nuclear matter at high energy
density is a topic of considerable theoretical interest and experimental
effort. Predicted in QCD, a new phase of deconfined matter,
the quark gluon plasma, is thought to describe a transitional
period of the early universe following the Big Bang. The extremely
high energy density medium created in relativistic collisions of large
nuclei at RHIC afford an opportunity to study the properties of
quark gluon plasma in a laboratory setting.
Fast partons (quarks and gluons) transiting the produced medium
have been observed to experience a large energy loss. Correlations
between pairs of final state particles at high transverse momenta
(pT ! 4 GeV/c) map the hadron jets resulting from these partons
and show that partons crossing the medium are nearly fully absorbed.
The mechanism of energy loss on length scales comparable
to the nucleus is not fully understood, so more differential measurements
are needed to constrain theoretical models. Quenching
as a function of the path length through the medium adds a new
dimension of experimental discrimination on energy loss and initial
state geometry. The resulting away-side suppression patterns indicate
that surviving fast partons cross the nuclear overlap region
with little energy loss.
The transiting partons deposit energy locally in the medium. The
resulting medium excitations may lead to measurable signals related
to the medium properties. Pair correlations at low pT (" 4
GeV/c) can reflect the medium response. Comparison of correlations
in heavy ion collisions with baseline measurements in protonproton
collisions show modifications to the correlation shape and
yields. Two new structures are found, both extended in rapidity,
one centered at small azimuthal opening angle ∆φ (known as
the “ridge”) and the other occurring at ∆φ = π ± 1.1 rad (“shoulder”).
Comparisons between the two raise the possibility that both
phenomena may result from the same mechanism. The medium response
correlations are consistent with collective excitation theory,
but pose challenges to Cherenkov gluon radiation and deflected jet
models.