High Energy Physics Phenomenology

An impressive effort is being placed in order to develop new strategies that allow an efficient computation of multi-loop multi-leg scattering amplitudes, with a particular emphasis in removing spurious singularities and instabilities. In this Letter, we describe an innovative technique to obtain the causal representation of any multi-loop multi-leg amplitude in Quantum Field Theory, taking advantage of the fact that all the information about thresholds is encoded in the cusp structure. By using the Loop-Tree Duality (LTD) theorem, we first obtain a compact causal representation through the nested residue computation for several topological families. From their representation, we extract all the possible combinations of causal propagators,λ±i, and then study their geometrical properties. We found that the causal propagators can be built starting from the cusp matrix, through a suitable definition of connected partitions. The causal representation for a given topological family is obtained by summing over all the possible products ofλ±ithat originate connected and oriented partitions of the underlying topology. These results are compatible with Cutkowsky rules, containing only physical thresholds which noticeably improves the numerical stability. Moreover, we explicitly show that diagrams with the same number of cusps exhibits similar causal structures, regardless of the number of loops.

We present a model for the vector and axial form factors of the transitionsB¯¯¯¯0??D?��?in good agreement with the presently available data and based on the present theoretical knowledge, combining a) the safe lattice QCD predictions atmQ=??andw=1; b) the predictions at generalwof a relativistic, covariant quark model atmQ=??, including the well tested Godfrey and Isgur spectroscopic model and which agrees with lattice QCD atw=1; c) the constraint of Bjorken and Neubert relating Semi-leptonic (SL) and Class I Non-leptonic (NL) decays, which shows thatB¯¯¯¯0??D0(2300)+???strongly constrains?1/2(w)to be much smaller than?3/2(w), in agreement with the theoretical expectation; d) the general HQET expansion which constrains the1/mQcorrections (cf \cite{LLSW}, denoted hereafter asLLSW).An important element in the understanding of data is the large contribution of virtualD(??Vto the broad structures seen in SL decays at lowD(???masses - which makes it difficult to isolate the broad resonances denoted asD1/2in the following.

QCD jets produced in heavy-ion collisions at LHC or RHIC energies partially evolve inside the produced hot and dense quark gluon plasma, offering unique opportunities to study QCD splitting processes in different backgrounds. Induced (modified) splittings are expected to be the most relevant mechanism driving the modifications of in-medium jets compared to vacuum jets for a wide sets of observables. Although color coherence among different emitters has been identified as an essential mechanism in studies of the {\it QCD antenna radiation}, it is usually neglected in the multi-gluon medium-induced cascade. This independent gluon emission approximation can be analytically proved to be valid in the limit of very large media, but corrections or modifications to it have not been computed before in the context of the evolution (or rate) equation describing the gluon cascade. We propose a modified evolution equation that includes corrections due to the interference of subsequent emitters. In order to do so, we first compute a modified splitting kernel following the usual procedure of factorizing it from the subsequent Brownian motion. The calculation is performed in the two-gluon configuration with no overlapping formation times, that is expected to provide the first correction to the completely independent picture.

The novel PQ mechanism replaces the strong CP problem with some challenges in a model building. In particular, the challenges arise regarding i) the origin of an anomalous global symmetry called a PQ symmetry, ii) the scale of the PQ symmetry breaking, and iii) the quality of the PQ symmetry. In this letter, we provide a natural and simple UV completed model that addresses these challenges. Extra quarks and anti-quarks are separated by two branes in the Randall-SundrumR4?S1/Z2spacetime while a hidden SU(NH) gauge field condensates in the bulk. The brane separation is the origin of the PQ symmetry and its breaking scale is given by the dynamical scale of the SU(NH) gauge interaction. The (generalized) Casimir force of SU(NH) condensation stabilizes the 5th dimension, which guarantees the quality of the PQ symmetry.

A new critical endpoint is pinned down in the thermomagnetic-QCD phase structure, which is suggested to be present between the two-flavor and three-flavor massless limits. It is signaled by the electromagnetic scale anomaly in QCD, and is shown to be most eminent in a weak magnetic field regime, which is not well explored on lattice QCD.

A new observable measuring theCPasymmetry in three-body decays, which is called the forward-backward asymmetry inducedCPasymmetry (FBI-CPA),AFBCP, is introduced. This newly defined observable has the dual advantages that 1) it can isolate theCPasymmetry associated with the interference of theS- andP-wave amplitude from that associated with theS- orP-wave amplitude alone; 2) it can effectively double the statistics comparing to the conventionally defined regionalCPasymmetry. We also suggest to perform the measurements of FBI-CPA in some three-body decay channels of charm and beauty hadrons.

In the present paper, we first give a detailed study on the pQCD corrections to the leading-twist part of BSR. Previous pQCD corrections to the leading-twist part derived under conventional scale-setting approach up toO(α4s)-level still show strong renormalization scale dependence. The principle of maximum conformality (PMC) provides a systematic way to eliminate conventional renormalization scale-setting ambiguity by determining the accurateαs-running behavior of the process with the help of renormalization group equation. Our calculation confirms the PMC prediction satisfies the standard renormalization group invariance, e.g. its fixed-order prediction does scheme-and-scale independent. In lowQ2-region, the effective momentum of the process is small and to have a reliable prediction, we adopt four low-energyαsmodels to do the analysis. Our predictions show that even though the high-twist terms are generally power suppressed in highQ2-region, they shall have sizable contributions in low and intermediateQ2domain. By using the more accurate scheme-and-scale independent pQCD prediction, we present a novel fit of the non-perturbative high-twist contributions by comparing with the JLab data.

We propose a low scale renormalizable left-right symmetric theory that successfully explains the observed SM fermion mass hierarchy, the tiny values for the light active neutrino masses, the lepton and baryon asymmetries of the Universe, as well as the muon and electron anomalous magnetic moments. In the proposed model the top and exotic quarks obtain masses at tree level, whereas the masses of the bottom, charm and strange quarks, tau and muon leptons are generated from a tree level Universal Seesaw mechanism, thanks to their mixings with the charged exotic vector like fermions. The masses for the first generation SM charged fermions arise from a radiative seesaw mechanism at one loop level, mediated by charged vector like fermions and electrically neutral scalars. The light active neutrino masses are produced from a one-loop level inverse seesaw mechanism. Our model is also consistent with the experimental constraints arising from the Higgs diphoton decay rate. We also discuss theZ??and heavy scalar production at a proton-proton collider.

Many efforts have been made to reveal the nature of the overabundant resonant structures observed by the worldwide experiments in the last two decades. Hadronic molecules attract special attention because many of these seemingly unconventional resonances are located close to the threshold of a pair of hadrons. To give an overall feature of the spectrum of hadronic molecules composed of a pair of heavy-antiheavy hadrons, namely, which pairs are possible to form molecular states, we take charmed hadrons for example to investigate the interaction between them and search for poles by solving the Bethe-Salpeter equation. We consider all possible combinations of hadron pairs of theS-wave singly-charmed mesons and baryons as well as the narrowP-wave charmed mesons. The interactions, which are assumed to be meson-exchange saturated, are described by constant contact terms which are resummed to generate poles. It turns out that if a system is attractive near threshold by the light meson exchange, there is a pole close to threshold corresponding to a bound state or a virtual state, depending on the strength of interaction and the cutoff. In total, 229 molecular states are predicted. The observed near-threshold structures with hidden-charm, like the famousX(3872)andPcstates, fit into the spectrum we obtain. We also highlight a?c?¯cbound state that has a pole consistent with the cross section of thee+e?????c?¯cprecisely measured by the BESIII Collaboration.

Requiring the effective mass term for a category of fundamental Dirac or Majorana fermions of the same electric charge to be invariant under the transformation?αL(R)???αL(R)+nαz?in the flavor space, wherenαandz?stand respectively for the flavor-dependent complex numbers and the spacetime- and flavor-independent element of the Grassmann algebra, we show thatnαcan be identified as the elementsUαiin thei-th column of the unitary matrixUused to diagonalize the corresponding Hermitian or symmetric fermion mass matrixM?, andmi=0holds accordingly. We find that the reverse is also true. Given the very facts that the charged leptons, up- and down-type quarks all have a strong mass hierarchy and current experimental data allow the lightest neutrino to be (almost) massless, the zero mass limit for the first-family fermions and the translational flavor symmetry behind it are expected to be a natural starting point for model building.