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A Handbook devoted to the poet W.B. Yeats (1865-1939) that examines how his work as a poet, playwright, critic, and public figure in the late 19th through the mid-20th century continues to influence writing in English, Irish, and worldwide Anglophone literatures.

The fundamental building blocks of the proton—quarks and gluons—have been known for decades. However, we still have an incomplete theoretical and experimental understanding of how these particles and their dynamics give rise to the quantum bound state of the proton and its physical properties, such as its spin 1 . The two up quarks and the single down quark that comprise the proton in the simplest picture account only for a few per cent of the proton mass, the bulk of which is in the form of quark kinetic and potential energy and gluon energy from the strong force 2 . An essential feature of this force, as described by quantum chromodynamics, is its ability to create matter–antimatter quark pairs inside the proton that exist only for a very short time. Their fleeting existence makes the antimatter quarks within protons difficult to study, but their existence is discernible in reactions in which a matter–antimatter quark pair annihilates. In this picture of quark–antiquark creation by the strong force, the probability distributions as a function of momentum for the presence of up and down antimatter quarks should be nearly identical, given that their masses are very similar and small compared to the mass of the proton 3 . Here we provide evidence from muon pair production measurements that these distributions are considerably different, with more abundant down antimatter quarks than up antimatter quarks over a wide range of momenta. These results are expected to revive interest in several proposed mechanisms for the origin of this antimatter asymmetry in the proton that had been disfavoured by previous results 4 , and point to future measurements that can distinguish between these mechanisms. Quark–antiquark annihilation measurements provide a precise determination of the ratio of down and up antiquarks within protons as a function of momentum, which confirms the asymmetry between the abundance of down and up antiquarks.

The nucleon (proton and neutron) electromagnetic form factors (FFs) are fundamental quantities in nuclear and elementary particle physics as they are essential ingredients needed to parametrize the internal structure of composite particles. The spatial distribution of charge and magnetization within the nucleon is encoded in the electric GE(p,n) and magnetic GM(p,n) FFs. These FFs have been extensively and precisely measured using elastic electron scattering. In addition, the combination of the proton and neutron FFs allows for the separation of the up- and down-quark contributions to the nucleon FFs. In this work, we improve on and extend to low- and high-Q2 values our original analysis and extract the up- and down-quark contributions to the nucleon electromagnetic FFs using worldwide data. In particular, we emphasize new precise data which cover the low-Q2 region which is sensitive to the large-scale structure of the nucleon. We compare our results to previous extractions and several recent theoretical calculations and models.

A high-precision parity-violating electron–quark scattering experiment provides measurements of a combination of electron–quark weak couplings with a precision five times higher than the single previous direct study, confirming the predictions of the electroweak particle-physics theory and providing constraints on parity-violating interactions beyond the standard model. Parity-violating asymmetry revisited Parity symmetry — or mirror-image symmetry — implies that flipping left and right does not change the laws of physics. Violation of parity symmetry in the weak nuclear force was discovered in the mid-1950s and parity violation in electron scattering was important in establishing, and is now used to test, the standard model of particle physics. This study reports a high-precision electron–quark scattering experiment that provides a measurement of the parity-violating asymmetry with a precision of five times higher than the single previous direct study via this scattering process. The results confirm the predictions of electroweak particle-physics theory, while providing constraints on parity-violating interactions beyond the standard model. Symmetry permeates nature and is fundamental to all laws of physics. One example is parity (mirror) symmetry, which implies that flipping left and right does not change the laws of physics. Laws for electromagnetism, gravity and the subatomic strong force respect parity symmetry, but the subatomic weak force does not 1 , 2 . Historically, parity violation in electron scattering has been important in establishing (and now testing) the standard model of particle physics. One particular set of quantities accessible through measurements of parity-violating electron scattering are the effective weak couplings C 2 q , sensitive to the quarks’ chirality preference when participating in the weak force, which have been measured directly 3 , 4 only once in the past 40 years. Here we report a measurement of the parity-violating asymmetry in electron–quark scattering, which yields a determination of 2 C 2 u  −  C 2 d (where u and d denote up and down quarks, respectively) with a precision increased by a factor of five relative to the earlier result. These results provide evidence with greater than 95 per cent confidence that the C 2 q couplings are non-zero, as predicted by the electroweak theory. They lead to constraints on new parity-violating interactions beyond the standard model, particularly those due to quark chirality. Whereas contemporary particle physics research is focused on high-energy colliders such as the Large Hadron Collider, our results provide specific chirality information on electroweak theory that is difficult to obtain at high energies. Our measurement is relatively free of ambiguity in its interpretation, and opens the door to even more precise measurements in the future.

The protons and neutrons in a nucleus can form strongly correlated nucleon pairs. Scattering experiments, in which a proton is knocked out of the nucleus with high-momentum transfer and high missing momentum, show that in carbon-12 the neutron-proton pairs are nearly 20 times as prevalent as proton-proton pairs and, by inference, neutron-neutron pairs. This difference between the types of pairs is due to the nature of the strong force and has implications for understanding cold dense nuclear systems such as neutron stars.

The spatial distribution of charge and magnetization within the nucleon (proton and neutron) is encoded in the elastic electromagnetic form factors G E ( p , n ) and G M ( p , n ) . These form factors have been precisely measured utilizing elastic electron scattering, and the combination of proton and neutron form factors allows for the separation of the up- and down-quark contributions to the nucleon form factors. We expand on our original analyses and extract the up- and down-quark contributions to the nucleon electromagnetic form factors from worldwide data with an emphasis on precise new data covering the low-momentum region, which is sensitive to the large-scale structure of the nucleon. From these, we construct the flavor-separated Dirac and Pauli form factors and their ratios, and compare the results to recent extractions and theoretical calculations and models.

The fundamental building blocks of the proton—quarks and gluons—have been known for decades. However, we still have an incomplete theoretical and experimental understanding of how these particles and their dynamics give rise to the quantum bound state of the proton and its physical properties, such as its spin1. The two up quarks and the single down quark that comprise the proton in the simplest picture account only for a few per cent of the proton mass, the bulk of which is in the form of quark kinetic and potential energy and gluon energy from the strong force2. An essential feature of this force, as described by quantum chromodynamics, is its ability to create matter–antimatter quark pairs inside the proton that exist only for a very short time. Their fleeting existence makes the antimatter quarks within protons difficult to study, but their existence is discernible in reactions in which a matter–antimatter quark pair annihilates. In this picture of quark–antiquark creation by the strong force, the probability distributions as a function of momentum for the presence of up and down antimatter quarks should be nearly identical, given that their masses are very similar and small compared to the mass of the proton3. Here we provide evidence from muon pair production measurements that these distributions are considerably different, with more abundant down antimatter quarks than up antimatter quarks over a wide range of momenta. These results are expected to revive interest in several proposed mechanisms for the origin of this antimatter asymmetry in the proton that had been disfavoured by previous results4, and point to future measurements that can distinguish between these mechanisms.

Alleles of interferon (IFN) regulatory factor 8 (IRF8) are associated with susceptibility to both systemic lupus erythematosus (SLE) and multiple sclerosis (MS). Although high-type I IFN is thought to be causal in SLE, type I IFN is used as a therapy in MS. We investigated whether IRF8 alleles were associated with type I IFN levels or serologic profiles in SLE and MS. Alleles that have been previously associated with SLE or MS were genotyped in SLE and MS patients. The MS-associated rs17445836G allele was associated with anti-double-stranded DNA (dsDNA) autoantibodies in SLE patients (meta-analysis odds ratio=1.92). The same allele was associated with decreased serum IFN activity in SLE patients with anti-dsDNA antibodies, and with decreased type I IFN-induced gene expression in peripheral blood mononuclear cell from anti-dsDNA-negative SLE patients. In secondary progressive MS patients, rs17445836G was associated with decreased serum type I IFN. Rs17445836G was associated with increased IRF8 expression in SLE patient B cells. In summary, IRF8 rs17445836G is associated with human autoimmune disease characterized by low-type I IFN levels, and this may have pharmacogenetic relevance as type I IFN is modulated in SLE and MS. The association with autoantibodies and increased IRF8 expression in B cells supports a role for rs17445836G in humoral tolerance.

Background Alleles of IRF8 have been associated with susceptibility to both systemic lupus erythematosus (SLE) and multiple sclerosis (MS). While type I interferon (IFN) is thought to be causal in SLE, type I IFN is used as a therapy in MS. Objectives We investigated whether the IRF8 alleles were associated with differences in serum IFN in SLE or MS. Methods Single nucleotide polymorphisms (SNPs) in IRF8 (associated with SLE and MS respectively) were genotyped in 627 SLE patients of African-American, European-American, and Cretan origin, 73 MS patients, and matched controls. Serum type I IFN was measured using a functional reporter cell assay. Results The MS-associated rs17445836 G allele was associated with the presence of anti-dsDNA autoantibodies in SLE patients across all ancestral backgrounds (meta-analysis OR=1.92). The same allele was associated with decreased serum IFN activity in SLE patients with anti-dsDNA antibodies, and the subgroup of MS patients with secondary progressive MS. The rs17445836 G allele was associated with decreased type I IFN-induced gene expression in PBMC from SLE patients who lacked anti-dsDNA antibodies. No associations were observed with the rs12444486 allele. Conclusions The rs17445836 G allele was associated with decreased type I IFN responses in both SLE and MS patients. The association of this allele with low IFN SLE and with MS, a condition characterized by low circulating type I IFN levels, suggests that this allele is associated with autoimmunity in the setting of low type I IFN levels. These data also suggest a role for this allele in humoral autoimmune responses. Disclosure of Interest None Declared

The atomic nucleus is one of the densest and most complex quantum-mechanical systems in nature. Nuclei account for nearly all the mass of the visible Universe. The properties of individual nucleons (protons and neutrons) in nuclei can be probed by scattering a high-energy particle from the nucleus and detecting this particle after it scatters, often also detecting an additional knocked-out proton. Analysis of electron- and proton-scattering experiments suggests that some nucleons in nuclei form close-proximity neutron–proton pairs 1 – 12 with high nucleon momentum, greater than the nuclear Fermi momentum. However, how excess neutrons in neutron-rich nuclei form such close-proximity pairs remains unclear. Here we measure protons and, for the first time, neutrons knocked out of medium-to-heavy nuclei by high-energy electrons and show that the fraction of high-momentum protons increases markedly with the neutron excess in the nucleus, whereas the fraction of high-momentum neutrons decreases slightly. This effect is surprising because in the classical nuclear shell model, protons and neutrons obey Fermi statistics, have little correlation and mostly fill independent energy shells. These high-momentum nucleons in neutron-rich nuclei are important for understanding nuclear parton distribution functions (the partial momentum distribution of the constituents of the nucleon) and changes in the quark distributions of nucleons bound in nuclei (the EMC effect) 1 , 13 , 14 . They are also relevant for the interpretation of neutrino-oscillation measurements 15 and understanding of neutron-rich systems such as neutron stars 3 , 16 . Electron-scattering experiments reveal that the fraction of high-momentum protons in medium-to-heavy nuclei increases considerably with neutron excess, whereas that of high-momentum neutrons decreases slightly, in contrast to shell-model predictions.