High Energy Physics - Phenomenology (hep-ph)

Abstract: The transverse momentum distributions of J/{\psi}mesons produced in pp collisions at the center-of-mass energy 5 TeV, 7TeV, and 13 TeV are described by the modified Hagedorn function. The fitting results by the modified Hagedorn function are in accord with experimental data measured by the LHCb Collaboration at LHC. The related parameters are obtained and analyzed.

Abstract: Production of electron-positron pairs by a high-energy $\gamma$ photon and a bichromatic laser wave is considered where the latter is composed of a strong low-frequency and a weak high-frequency component, both with circular polarization. An integral expression for the production rate is derived that accounts for the strong laser mode to all orders and for the weak laser mode to first order. The structure of this formula resembles the well-known expression for the nonlinear Breit-Wheeler process in a strong laser field, but includes the dynamical assistance from the weak laser mode. We analyze the dependence of the dynamical rate enhancement on the applied field parameters and show, in particular, that it is substantially higher when the two laser modes have opposite helicity.

Abstract: We study the inclusive $H_b \to X_s \gamma$ decay with $H_b$ a beauty baryon, in particular $\Lambda_b$, employing an expansion in the heavy quark mass at $\mathit{O}(m_b^{-3})$ at leading order in $\alpha_s$, keeping the dependence on the hadron spin. For a polarized baryon we compute the distribution $\displaystyle\frac{d^2\Gamma}{dy \, d \cos \theta_P}$, with $y=2E_\gamma/m_b$, $E_\gamma$ the photon energy and $\theta_P$ the angle between the baryon spin vector and the photon momentum in the $H_b$ rest-frame. We discuss the correlation between the baryon and photon polarization, and show that effects of physics beyond the Standard Model can modify the photon polarization asymmetry. We also discuss a method to treat the singular terms in the photon energy spectrum obtained by the OPE.

Abstract: To avoid the impact from the background events directly from $e^+e^-$ annihilations or $J/\psi$ decays, we propose a novel approach to investigate $\eta$ decays, in particular for its rare or forbidden decays, by using $\eta^\prime\rightarrow\pi\pi\eta$ produced in $J/\psi$ decays at the $\tau-$charm factories. Based on the MC studies of a few typical decays, $\eta\rightarrow \pi\pi$, $\gamma l^+l^- (l= e, \mu)$, $l^+l^-$, as well as $l^+l^-\pi^0$, the sensitivities could be obviously improved by taking advantage of the extra constraint of $\eta^\prime$. Using one trillion $J/\psi$ events accumulated at the Super $\tau$-Charm facility, the precision on the investigation of $\eta$ decays could be improved significantly and the observation of the rare decay $\eta\rightarrow e^+e^-$ is even accessable.

Abstract: Deriving a comprehensive set of reduction rules for Feynman integrals has been a longstanding challenge. In this paper, we present a proposed solution to this problem utilizing generating functions of Feynman integrals. By establishing and solving differential equations of these generating functions, we are able to derive a system of reduction rules that effectively reduce any associated Feynman integrals to their bases. We illustrate this method through various examples and observe its potential value in numerous scenarios.

Abstract: Extending the transverse momentum dependent factorization to thrust dependent observables entails a series of difficulties, ultimately associated to the behavior of soft radiation. As a consequence, the definition of the transverse momentum dependent functions has to be revised, while preserving (and possibly extending) their universality properties. Moreover, the regularization of the rapidity divergences generates non trivial correlations between rapidity and thrust. In this paper, we show how to deal with these correlations in a consistent treatment of the thrust dependence of $e^+ e^- \to h \, X$ cross section, where the hadron transverse momentum is measured with respect to the thrust axis. In this framework all results obtained in the past few years properly fit together, leading to a remarkable phenomenological description of the experimental measurements.

Abstract: In this contribution, we make use of the QCD perturbative fragmentation function formalism to describe the one-particle inclusive fragmentation of a heavy quark produced in $e^+e^-$ annihilation at $\mathcal{O}(\alpha_S^2)$. We perform the computation analytically in Mellin-space. We resum soft-gluons effects in initial conditions and coefficient functions and perform evolution up to NNLL accuracy, obtaining the first NNLO + NNLL prediction for charm quark production. We study the impact of different matching schemes and Landau pole prescriptions in soft-gluon resummation, finding significant differences. We extract simple non-perturbative fragmentation functions for $B$ and $D^*$ mesons by comparing the perturbative prediction with the data from CLEO, BELLE and LEP experiments. We find that for charm mesons the experimental results from CLEO/BELLE and from LEP are not reconcilable with the standard DGLAP evolution.

Abstract: In this paper, we propose a Green's basis and also a new physical basis for dimension-seven (dim-7) operators, which are suitable for the matching of ultraviolet models onto the Standard Model effective field theory (SMEFT) and the deviation of renormalization group equations (RGEs) for dim-7 operators in the SMEFT. The reduction relations to convert operators in the Green's basis to those in the physical basis are achieved as well, where some redundant dim-6 operators in the Green's basis are involved if the dim-5 operator exists. Working in these two bases for dim-7 operators and with the help of the reduction relations, we work out the one-loop RGEs resulting from the mixing among different dimensional operators for the dim-5 and dim-7 operators up to $\mathcal{O} \left( \Lambda^{-3} \right)$ in the SMEFT. These new results complete the previous results for RGEs of the dim-5 and dim-7 operators and hence can be used for a consistent one-loop analysis of the SMEFT at $\mathcal{O} \left( \Lambda^{-3} \right)$.

Abstract: We use parity (P) to set $\theta_{QCD}$ to zero in the minimal left-right symmetric model, add a heavy vectorlike quark family, and obtain in a novel manner the Nelson Barr (NB) form associated so far only with spontaneous CP solution to the strong CP Puzzle. Our solution does not have the `coincidence of scales problem', that typically plagues NB models. P protects $\bar{\theta}$, if it breaks at a scale $v_R$ below the mass $M$ of the heavy quarks, and $\bar{\theta} \sim 10^{-9} (v_R/M)^2$ is radiatively generated, which can be acceptably small. On the other hand, if $M < v_R$, the $\bar{\theta} \sim 10^{-9}$ generated by the NB mechanism is too large, but $\delta_{CKM}$ can be obtained without the NB mechanism, and surprisingly a lower irreducible $\bar{\theta} \sim (10^{-13}~to~10^{-10}) ln( {v_R/M)}$, testable by neutron EDM experiments is generated. No leptonic CP violation is generated (Dirac phase $\delta_{CP} = 0~or~\pi$ in PMNS matrix) which makes the minimal model testable by neutrino experiments. We also discuss some challenges in a non-minimal model that generates leptonic CP violation. Lastly but importantly, we find with $SU(2)_L \times SU(2)_R$ gauge group, that there is an \textit{automatic} NB solution on imposing CP (global symmetry usually imposed to get the NB form is accidental once we have $SU(2)_R$), which does not require mirror or generalized parity and is more economical, as it works also with just 1 mirror generation.