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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.
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.
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.
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”.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Search
THESES
Measurement of Dielectron Invariant Mass Spectra in Au + Au Collisions at psNN = 200GeV with HBD in PHENIX
Jiayin Sun, 2016
Measuring the anti-quark contribution to the proton spin using parity violating W
production in polarized proton proton collisions
Ciprian Gal, 2014
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
Low Momentum Direct Photons as a Probe of Heavy Ion Collisions
Richard Petti, 2013
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
Single Electrons from Decays of Heavy Quarks Produced in Cu+Cu Collisions at the Relativistic
Heavy Ion Collider
Nicole Apadula, 2013
Direct Photon Tagged Jets in 200 GeV Au+Au Collisions at PHENIX
Megan Connors, 2011