A Novel Feature of Valence Quark Distributions in Hadrons

Examining the evolution of the maximum of valence quark distribution, qV, weighted by Bjorken x, h(x,t)≡xqV(x,t), it is observed that h(x,t) at the peak becomes a one-parameter function; h(xp,t)=Φ(xp(t)), where xp is the position of the peak, t=logQ2, and Q2 is the resolution scale. This observation is used to derive a new model-independent relation which connects the partial derivative of the valence parton distribution functions (PDFs) in xp to the quantum chromodynamics (QCD) evolution equation through the xp derivative of the logarithm of the function Φ(xp(t)). A numerical analysis of this relation using empirical PDFs results in an observation of the exponential form of the Φ(xp(t))=h(xp,t)=CeDxp(t) for leading to next-to-next leading order approximations of PDFs for the range of Q2, covering four orders in magnitude. The exponent, D, of the observed “height-position” correlation function converges with the increase in the order of approximation. This result holds for all the PDF sets considered. A similar relation is observed also for the pion valence quark distribution, indicating that the obtained relation may be universal for any non-singlet partonic distribution. The observed “height-position” correlation is used also to indicate that no finite number of exchanges can describe the analytic behavior of the valence quark distribution at the position of the peak at fixed Q2.

Funciones de estructura antisimétricas de hadrones holográficos

En esta tesis se estudió a través de la correspondencia AdS/CFT el proceso de dispersión inelástica profunda de leptones cargados sobre hadrones de la teorı́a N = 4 súper Yang-Mills con grupo de gauge SU(N) en el régimen de acoplamiento fuerte. Los hadrones considerados fueron el glueball, de espı́n 0, y el glueballino, de espı́n 1/2. Partiendo de las amplitudes de dispersión de cuerdas cerradas de la teorı́a de supercuerdas IIB en un fondo AdS5×S5 se obtuvieron las funciones de estructura de los hadrones en distintos regı́menes de la variable de Bjorken x. En el primer régimen el cálculo se corresponde a amplitudes de supergravedad del tipo IIB, mientras que para valores de x chico es necesario calcular las amplitudes de dispersión en la teorı́a de supercuerdas. Se estudió el rol central que tiene el término de Chern-Simons en la presencia de las funciones de estructura antisimétricas para ambos tipos de hadrones. Por último, en el régimen de x exponencialmente chico, donde la fı́sica de Regge se vuelve relevante, el análisis dual se hizo a través de la técnica del Pomerón, obteniendo las funciones de estructura en dicho régimen y realizando comparaciones con datos experimentales obtenidos del protón. Como resultado se obtuvieron buenos ajustes y predicciones para la función antisimétrica g1.

Central Exclusive Production at LHCb

The LHCb collaboration has measured central exclusive production of J/ф, ф(2S), and ϒ mesons as well as J/фJ/ф, J/фф(2S), ф(2S)ф(2S), and xcxc meson pairs in proton-proton collisions. The analyses of ϒ and charmonium pairs are performed at the centre-of-mass energies of 7 TeV and 8 TeV, and those of J/ф and ф(2S) are done at 7 TeV and 13 TeV. The analysis at 13 TeV involves the use of new shower counters. These allow a reduction in the background by vetoing events with activity in an extended region in rapidity. The measurements of central exclusive production at LHCb are sensitive to gluon distributions for Bjorken-x values down to 2×10−6 (at 13 TeV). An overview of the LHCb results is presented and compared to existing measurements of other experiments and theoretical calculations.

EMC effect at small Bjorken x values

The Bessel-inspired behavior of parton densities at small Bjorken x values is used along with “frozen” and analytic modifications of the strong coupling constant [1] to study the so-called EMC effect. Among other results, this approach allowed predicting small x behavior of the gluon density in nuclei.

3 parton production at DIS at small x

We use the spinor helicity formalism to calculate the cross section for production of three partons of a given polarization in Deep Inelastic Scattering (DIS) off proton and nucleus targets at small Bjorken x. The target proton or nucleus is treated as a classical color field (shock wave) from which the produced partons scatter multiple times. The resulting expressions are used to study azimuthal angular correlations between produced partons in order to probe the gluon structure of the target hadron or nucleus as well as to study energy loss in DIS reactions.