Bertrand Ducloué

Tests phénoménologiques de la chromodynamique quantique perturbative à haute énergie au LHC: Dans la limite des hautes énergies, la petite valeur de la constante de couplage de l'interaction forte peut être compensée par l'apparition de grands logarithmes de l'énergie dans le centre de masse. Toutes ces contributions peuvent être du même ordre de grandeur et sont resommées par l'équation de Balitsky-Fadin-Kuraev-Lipatov (BFKL). De nombreux processus ont été proposés pour étudier cette dynamique. L'un des plus prometteurs, proposé par Mueller et Navelet, est l'étude de la production de deux jets vers l'avant séparés par un grand intervalle en rapidité dans les collisions de hadrons. Un calcul BFKL ne prenant en compte que les termes dominants (approximation des logarithmes dominants ou LL) prédit une augmentation rapide de la section efficace avec l'augmentation de l'intervalle en rapidité entre les jets ainsi qu'une faible corrélation angulaire. Cependant, des calculs basés sur cette approximation ne purent pas décrire correctement les mesures expérimentales de ces observables au Tevatron. Dans cette thèse, nous étudions ce processus à l'ordre des logarithmes sous-dominants, ou NLL, en prenant en compte les corrections NLL aux facteurs d'impact, qui décrivent la transition d'un hadron initial vers un jet, et à la fonction de Green, qui décrit le couplage entre les facteurs d'impact. Nous étudions l'importance de ces corrections NLL et trouvons qu'elles sont très importantes, ce qui conduit à des résultats très différents de ceux obtenus à l'ordre des logarithmes dominants. De plus, ces résultats dépendent fortement du choix des échelles présentes dans ce processus. Nous comparons nos résultats avec des données récentes de la collaboration CMS sur les corrélations angulaires des jets Mueller-Navelet au LHC et ne trouvons pas un bon accord. Nous montrons que cela peut être corrigé en utilisant la procédure de Brodsky-Lepage-Mackenzie pour fixer le choix de l'échelle de renormalisation. Cela conduit à des résultats plus stables et une très bonne description des données de CMS. Finalement, nous montrons que, à l'ordre des logarithmes sous-dominants, l'absence de conservation stricte de l'énergie-impulsion (qui est un effet négligé dans un calcul BFKL) devrait être un problème beaucoup moins important qu'à l'ordre des logarithmes dominants.

We present version 2.2 of the High Energy Jets (HEJ) Monte Carlo event generator for hadronic scattering processes at high energies. The new version adds support for two further processes of central phenomenological interest, namely the production of a W boson pair with equal charge together with two or more jets and the production of a Higgs boson with at least one jet. Furthermore, a new prediction for charged lepton pair production with high jet multiplicities is provided in the high-energy limit. The accuracy of HEJ 2.2 can be increased further through an enhanced interface to standard predictions based on conventional perturbation theory. We describe all improvements and provide extensive usage examples. HEJ 2.2 can be obtained from https://hej.hepforge.org.

We describe the calculation of the QCD contribution to same-sign W W -pair production, pp\to e^\pm \nu_e \mu^\pm \nu_\mu jj p p → e ± ν e μ ± ν μ j j , resumming all contributions scaling as \alpha_W^4 \alpha_s^{2+k}\log^k(\hat s/p_\perp^2) α W 4 α s 2 + k log k ( s ̂ / p ⊥ 2 ) ~. These leading logarithmic contributions are enhanced by typical cuts used for Vector Boson Scattering (VBS) studies. We show that while the cross sections are little affected by these corrections, other more exclusive observables relevant for experimental studies are affected more significantly.

When computed to next-to-leading order in perturbative QCD, the non-linear Balitsky-Kovchegov (BK) equation for the high-energy evolution of the dipole-hadron scattering appears to be unstable. We show that this instability can be avoided by using the rapidity of the dense hadronic target (instead of that of the dilute dipole projectile) as the evolution time. Using this variable, we construct a collinearly-improved version of the BK equation, where the dominant radiative corrections to the kernel -- those enhanced by double collinear logarithms -- are resummed to all orders.

Reaching next-to-leading order accuracy in perturbative calculations of particle production in QCD at high energy is essential for reliable phenomenological applications. In recent years, the Color Glass Condensate effective theory (the natural framework for such calculations) has indeed been promoted to NLO accuracy. However, the first NLO calculation of single-inclusive hadron production met with an unexpected difficulty: the cross-section suddenly turns negative at transverse momenta of the order of a few GeV, in a range where perturbation theory is expected to be reliable. We summarize recent efforts to understand and solve this issue, as well as to develop a running coupling scheme that can be used to consistently describe various processes in this formalism.

The standard formulation of the high-energy evolution in perturbative QCD, based on the Balitsky-Kovchegov equation, is known to suffer from severe instabilities associated with radiative corrections enhanced by double transverse logarithms, which occur in all orders starting with the next-to-leading one. Over the last years, several methods have been devised to resum such corrections by enforcing the time-ordering of the successive gluon emissions. We observe that the instability problem is not fully cured by these methods: various prescriptions for performing the resummation lead to very different physical results and thus lack of predictive power. We argue that this problem can be avoided by using the rapidity of the dense target (which corresponds to Bjorken x) instead of that of the dilute projectile as an evolution time. This automatically ensures the proper time-ordering and also allows for a direct physical interpretation of the results. We explicitly perform this change of variables at NLO. We observe the emergence of a new class of double logarithmic corrections, potentially leading to instabilities, which are however less severe, since disfavoured by the typical BK evolution for “dilute-dense” scattering. We propose several prescriptions for resumming these new double-logarithms to all orders and find only little scheme dependence: different prescriptions lead to results which are consistent to each other to the accuracy of interest. We restore full NLO accuracy by completing one of the resummed equations (non-local in rapidity) with the remaining NLO corrections.