Bulletin of the American Physical Society
APS April Meeting 2016
Volume 61, Number 6
Saturday–Tuesday, April 16–19, 2016; Salt Lake City, Utah
Session X3: To the Heart of the MatterInvited
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Sponsoring Units: DNP DCOMP Chair: Xiaochao Zheng, University of Virginia Room: Ballroom B |
Tuesday, April 19, 2016 10:45AM - 11:12AM |
X3.00001: Direct measurements of astrophysically important $\alpha$-induced reactions Invited Speaker: Melina Avila Understanding stellar evolution is one of the primary objectives of nuclear astrophysics. Reaction rates involving $\alpha$-particles are often key nuclear physics inputs in stellar models. For instance, there are numerous $(\alpha,p)$ reactions fundamental for the understanding of X-ray bursts and the production of $^{44}$Ti in core-collapse supernovae. Furthermore, some $(\alpha,n)$ reactions are considered as one of the main neutron sources in the s-process. However, direct measurements of these reactions at relevant astrophysical energies are experimentally challenging because of their small cross section and intensity limitation of radioactive beams. The active target system MUSIC offers a unique opportunity to study $(\alpha,p)$ and $(\alpha,n)$ reactions because its segmented anode allows the investigation of a large energy range in the excitation function with a single measurement. Recent results on the direct measurement of $(\alpha,n)$ and $(\alpha, p)$ measurements in the MUSIC detector will be discussed. [Preview Abstract] |
Tuesday, April 19, 2016 11:12AM - 11:39AM |
X3.00002: Precise test of the unitarity of the CKM matrix via superallowed nuclear beta decay Invited Speaker: Hyo-In Park Superallowed 0$^{+}$ $\rightarrow$ 0$^{+}$ nuclear beta decay between isospin T = 1 analogue states is a sensitive probe for studying the fundamental properties of the weak interaction. Today, the most precise measurements of the decay strengths (or $ft$ values) of fourteen superallowed transitions, ranging from $^{10}$C to $^{74}$Rb, provide a direct determination of the vector coupling constant $G_{V}$, and lead to the most precise value of $V_{ud}$, the up-down quark-mixing element of the Cabbibo-Kobayashi-Maskawa (CKM) matrix. When $V_{ud}$ is combined with the other top-row elements, $V_{us}$ and $V_{ub}$, the sum of squares of the top-row elements of the CKM matrix satisfies the unitarity condition at the level of $\pm$0.06\%.\footnote{J.C. Hardy and I.S. Towner, \text{Phys. Rev. C} {\bf91}, 025501 (2015).} The impact of this result on searches for new physics beyond the Standard Model motivates further work to improve even further the precision of the CKM-matrix unitarity sum. Our current focus is on measurements to constrain the uncertainty in calculations of the isospin-symmetry-breaking corrections needed to determine $V_{ud}$ from the experimental data. This can be achieved with high-precision comparisons of the $ft$ values from four pairs of accessible mirror superallowed decays with $A \leq 42$. This presentation reports our results for the mass-38 pair, $^{38}$Ca $\rightarrow$ $^{38m}$K and $^{38m}$K $\rightarrow$ $^{38}$Ar, and our progress on measuring $^{42}$Ti decay. The measured ratio of the mirror $ft$ values for $A = 38$ agrees well with the corrections currently used, and points the way to even tighter constraints on the unitarity of the CKM matrix. If the three mirror pairs, with $A = 26, A = 34$ and $A = 42$ confirm and strengthen our present conclusion, it will become possible to shrink the systematic uncertainty on $V_{ud}$, reduce the uncertainty on the CKM-matrix unitarity sum, and further constrain the scope for possible extensions to the Standard Model. [Preview Abstract] |
Tuesday, April 19, 2016 11:39AM - 12:06PM |
X3.00003: Experimental results on jet quenching Invited Speaker: Doga Gulhan It is still an open question whether or not the study of the modification of jets in heavy ion collisions can teach us about the properties of the color charged medium. The information on jet-medium interactions obtained from experimental results at RHIC and LHC, including results from 5.02 TeV PbPb collisions that took place at the end of 2015, are discussed. Which measurements can add more information? How close is phenomenology to implement the theoretical predictions in a way that can allow us to extract information from experimental data? [Preview Abstract] |
Tuesday, April 19, 2016 12:06PM - 12:33PM |
X3.00004: From C to Parton Sea: How Supercomputing Reveals Nucleon Structure Invited Speaker: Huey-Wen Lin Studying the structure of nucleons is not only important to understanding the strong interactions of quarks and gluons, but also to improving the precision of new-physics searches. Since a broad class of experiments, including the LHC and dark-matter detection, require interactions with nucleons, the mission to probe femtoscale physics is also essential for disentangling Standard-Model contributions from potential new physics. These SM backgrounds require parton distribution functions (PDFs) as inputs. However, after decades of experiments and theoretical efforts, there still remain many unknowns, especially in the sea flavor structure and transversely polarized structure. In a discrete spacetime, we can make a direct numerical calculation of the implications of QCD using sufficiently large supercomputing resources. A nonperturbative approach from first principles, lattice QCD, provides hope to expand our understanding of nucleon structure, especially in regions that are difficult to observe in experiments. In this work, we present a first direct calculation of the Bjorken-x dependence of the PDFs using Large-Momentum Effective Theory (LaMET) and comment on the surprising result revealed for the nucleon sea-flavor asymmetry. [Preview Abstract] |
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