Modified structure of protons and neutrons in correlated pairs

The atomic nucleus is made of protons and neutrons (nucleons), which are themselves composed of quarks and gluons. Understanding how the quark–gluon structure of a nucleon bound in an atomic nucleus is modified by the surrounding nucleons is an outstanding challenge. Although evidence for such modification—known as the EMC effect—was first observed over 35 years ago, there is still no generally accepted explanation for its cause1,2,3. Recent observations suggest that the EMC effect is related to close-proximity short-range correlated (SRC) nucleon pairs in nuclei4,5. Here we report simultaneous, high-precision measurements of the EMC effect and SRC abundances. We show that EMC data can be explained by a universal modification of the structure of nucleons in neutron–proton SRC pairs and present a data-driven extraction of the corresponding universal modification function. This implies that in heavier nuclei with many more neutrons than protons, each proton is more likely than each neutron to belong to an SRC pair and hence to have distorted quark structure. This universal modification function will be useful for determining the structure of the free neutron and thereby testing quantum chromodynamics symmetry-breaking mechanisms and may help to discriminate between nuclear physics effects and beyond-the-standard-model effects in neutrino experiments

A comparison of the National Center for Health Statistics and new World Health Organization growth references for school-age children and adolescents with the use of data from 11 low-income countries

Background: In 2007 new World Health Organization (WHO) growth references for children aged 5–19 y were introduced to replace the National Center for Health Statistics (NCHS) references. Objective: This study aimed to compare the prevalence of stunting, wasting, and thinness estimated by the NCHS and WHO growth references. Design: NCHS and WHO height-for-age z scores were calculated with the use of cross-sectional data from 20,605 schoolchildren aged 5–17 y in 11 low-income countries. The differences in the percentage of stunted children were estimated for each year of age and sex. The z scores of body mass index–for-age and weight-for-height were calculated with the use of the WHO and NCHS references, respectively, to compare differences in the prevalence of thinness and wasting. Results: No systematic differences in mean z scores of height-for-age were observed between the WHO and NCHS growth references. However, z scores of height-for-age varied by sex and age, particularly during early adolescence. In children for whom weight-for-height could be calculated, the estimated prevalence of thinness (WHO reference) was consistently higher than the prevalence of wasting (NCHS reference) by as much as 9% in girls and 18% in boys. Conclusions: In undernourished populations, the application of the WHO (2007) references may result in differences in the prevalence of stunting for each sex compared with results shown when the NCHS references are used as well as a higher estimated prevalence of thinness than of wasting. An awareness of these differences is important for comparative studies or the evaluation of programs. For school-age children and adolescents across all ranges of anthropometric status, the same growth references should be applied when such studies are undertaken