# 2015 High Energy and Particle Physics Prize of the European Physical Society

## Physicists who predicted the structure of the proton to be awarded the prestigious EPS High Energy Physics prize

The has been awarded jointly to five theoretical physicists: James D. Bjorken (SLAC National Accelerator Laboratory, Stanford, USA) "for his prediction of scaling behaviour in the structure of the proton that led to a new understanding of the strong interaction" and to Guido Altarelli (University of Roma Tre, Rome, Italy and CERN, Geneva, Switzerland), Yuri Dokshitzer (Laboratory of Theoretical and High Energy Physics, Paris, France and St. Petersburg Nuclear Physics Institute, Gatchina, Russia), Lev N. Lipatov (National Research Center "Kurchatov Institute", Petersburg Nuclear Physics Institute, Gatchina, Russia) and Giorgio Parisi (University of Rome, La Sapienza, Rome, Italy) "for developing a probabilistic field theory framework for the dynamics of quarks and gluons, enabling a quantitative understanding of high-energy collisions involving hadrons."

The award ceremony will take place at the EPS-HEP 2015 conference in Vienna (http://epshep2015.eu/) on 27 July.

In the quest for understanding the deep structure of matter, it was clear by the end of 1950s that the nucleus consists of smaller constituents, protons and neutrons, called nucleons. It was also proposed that these particles were in fact composite, and made of smaller particles called quarks.

However, physicists had no idea of how to observe these smaller pieces, nor did they have a theory that could consistently describe their dynamical properties. In 1968, J.D. Bjorken investigated the mathematical properties of the scattering of highly energetic electrons off protons, the so-called deep-inelastic scattering, in the hypothetical limit when the protons have infinite momentum. He found that the structure of the proton should then be independent of the energy transferred from the electron, as the quantity that determines the resolution scale. This property, called scaling behaviour of the proton structure, led him to propose that the scattering of the electron occurs on point-like constituents of the proton, dubbed partons. His findings were soon confirmed experimentally, and the partons coincided with the quarks postulated earlier.

These developments eventually led to the construction of a quantum field theory of the strong interaction: quantum chromo-dynamics (QCD). The resulting parton model introduces probabilistic momentum distributions for partons (duly identified as quarks and the gluons that bind them) inside the proton. Collisions involving energetic protons are described by elementary collision processes with partons in the initial state. A consistent formulation of this parton-model picture in the context of QCD perturbation theory was achieved in 1977 by G. Altarelli and G. Parisi, as well as independently by Y.

Dokshitzer who built on earlier work of V.N. Gribov and L.N. Lipatov. Although physicists are still not able to compute the momentum distributions in the proton, the equations derived in 1977, called DGLAP evolution equations, describe the QCD-induced variation of parton momentum distributions with the resolution scale. Furthermore, they provide a physical explanation of logarithmic deviations from Bjorken scaling in terms of parton radiation prior to their violent interaction. The QCD-improved parton model is a very successful framework that has been validated experimentally to high precision on a multitude of experimental measurements. At present it forms the basis of precise quantitative predictions of cross sections for scattering processes at hadron colliders. As such it is a cornerstone of the interpretation of all measurements at the Large Hadron Collider, including not only processes with already known particles in the final state but also searches for new particles, such as the Higgs boson discovered in 2012.

Professor Yves Sirois (Ecole Polytechnique Paris), the current secretary of the EPS HEPP Board, said, "This prize recognizes essential theoretical contributions that paved the way to the modern understanding of scattering processes involving hadrons, allowing ultimately to interpret the data of many generations of leading experiments at HERA, the Tevatron, and the LHC colliders in terms of the fundamental processes involving quarks and gluons".

#### Source and contact:

Prof. Thomas Lohse, chair of the EPS-HEPP Board, lohse@physik.huberlin.de.#### Original publications:

J.D.Bjorken, Asymptotic Sum Rules at Infinite Momentum, Phys.Rev. 179 (1968) 1547. V.N.Gribov and L.N.Lipatov, Deep Inelastic Electron Proton Scattering in Perturbation Theory, Sov.J.Nucl.Phys. 15 (1972) 438. G.Altarelli and G.Parisi, Asymptotic Freedom in Parton Language, Nucl.Phys. B126 (1977) 298. Y.L.Dokshitzer, Calculation of the Structure Functions for Deep Inelastic Scattering and e+ e- Annihilation by Perturbation Theory in Quantum Chromodynamics, Sov.Phys.JETP 46 (1977) 641.#### For further reading

http://www.scholarpedia.org/article/Bjorken_scalinghttp://www.scholarpedia.org/article/QCD_evolution_equations_for_parton_densitiesPress Release