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Nuclear seminar

Accessing Quark Transversity Through Azimuthal Single-Spin Asymmetries of Charged Pions in Jets at STAR

A complete, fundamental understanding of the proton must include knowledge of the underlying
spin structure. The transversity distribution (h_1 (x)), which describes the transverse spin structure
of quarks inside of a transversely polarized proton, is only accessible through channels that couple
h_1 (x) to another chiral odd distribution, such as the Collins fragmentation function (∆D (z, j_T )).
Significant Collins asymmetries of charged pions have been observed in semi-inclusive deep inelastic
scattering (SIDIS) data. These SIDIS asymmetries combined with e^+ e^- process asymmetries from
Belle have allowed for the extraction of h_1 (x) and ∆D (z, j_T ). The current uncertainties on h_1 (x)
are large compared to the corresponding quark momentum and helicity distributions and reflect
the limited statistics and kinematic reach of the available data. In transversely polarized hadronic
collisions, Collins asymmetries may be isolated and extracted by measuring the spin dependent
azimuthal distributions of charged pions in jets. An exploratory STAR analysis with the 2006
s = 200 GeV dataset hinted at a charge dependent Collins asymmetry and motivated a dedicated
transversely polarized proton run in 2012 where an order of magnitude more data was collected (20 pb^{−1})
at an average polarization of 63%. This measurement, coupled with the same measurement
at √s=510 GeV and interference fragmentation function (IFF) measurements at √s = 200 and
500 GeV at midrapidity (|η| < 1) access higher momentum scales than the existing SIDIS data,
and will allow for a comprehensive study of evolution and factorization of the Collins channel.
Preliminary results from the √s = 200 and 500 GeV Collins and IFF analyses will be presented.

Event Series:

Nuclear Seminar: Exploring Gluon and Antiquark Polarization in the Proton with STAR

With the recent discovery of the Higgs boson at the Large Hadron Col-
lider, the mechanism through which fundamental particles acquire mass in
the Standard Model of particle physics is now complete. However, the vast
majority of the visible mass of the universe resides in protons and neutrons
which are not fundamental, but composite particles of the quarks and glu-
ons whose interactions are described by Quantum Chromodynamics (QCD).
These strong interactions are responsible for 99% of the proton and neutron
masses, and therefore these bound states of quarks and gluons provide an
ideal laboratory to study QCD and elucidate our understanding of visible
matter in the universe. To that end, one of the primary goals of the STAR
experiment at the Relativistic Heavy Ion Collider is to use spin as a unique
probe to unravel the internal structure and the QCD dynamics of the nucleon
by studying high-energy polarized proton collisions. In this talk, I will dis-
cuss what we have learned about the origin of the proton's spin, emphasizing
recent developments in gluon and antiquark polarization.

Event Series:

Nuclear Seminar: Measurement of the dμd Quartet-to-Doublet Fusion Ratio (q : d) and the μd Hyperfine Rate (qd) Using the Fusion Neutrons from μ− Stops in D2 Gas.

The MuSun experiment will determine the μd capture rate (μ−+d → n+n+e) from the doublet hyperfine state d, of the muonic deuterium atom in the 1S ground state to a precision of 1.5%. Modern Effective Field Theories (EFT) predict that an accurate measurement of d would determine the two nucleon weak axial current. This will help in understanding weak nuclear interactions such as the stellar thermonuclear proton-proton fusion reactions, neutrino interactions and double beta decay. The experiment took place in the E3 beamline of Paul Scherrer Institute (PSI) using a muon beam. Muons were stopped in a cryogenic time projection chamber (cryo-TPC) filled with D2 gas. This was surrounded by plastic scintillators and multiwire proportional chambers for detecting the decay electrons and an array of eight liquid scintillators for detecting neutrons. The goal of this dissertation is to measure the dμd quartet-to-doublet fusion ratio (q : d) and μd hyperfine rate (qd) using the fusion neutrons from μ− stops in D2 gas. The dμd molecules undergo muon catalyzed fusion (MCF) reactions from the doublet and the quartet state with rates d and q, yield 2.45 MeV monoenergetic fusion neutrons. Encoded in the time dependence of the fusion neutrons are the dμd formation rates d, q and qd. Consequently, the investigation of the fusion neutron time spectrum enables the determination of these kinetics parameters that are important in the extraction of d from the decay electron time spectrum. The final results of this work yield q : d = 82.05 ± 4.01 and qd = 39.67 ± 0.4 μs−1. 1

Event Series:
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