Search results for " phenomenology"
showing 10 items of 3693 documents
Charge asymmetries of top quarks at hadron colliders revisited
2011
A sizeable difference in the differential production cross section of top- compared to antitop-quark production, denoted charge asymmetry, has been observed at the Tevatron. The experimental results seem to exceed the theory predictions based on the Standard Model by a significant amount and have triggered a large number of suggestions for "new physics". In the present paper the Standard Model predictions for Tevatron and LHC experiments are revisited. This includes a reanalysis of electromagnetic as well as weak corrections, leading to a shift of the asymmetry by roughly a factor 1.1 when compared to the results of the first papers on this subject. The impact of cuts on the transverse mome…
Dijets at Tevatron Cannot Constrain SMEFT Four-Quark Operators
2019
We explore the sensitivity of Tevatron data to heavy new physics effects in differential dijet production rates using the SMEFT in light of the fact that consistent and conservative constraints from the LHC cannot cover relatively low cutoff scales in the EFT. In contrast to the results quoted by the experimental collaborations and other groups, we find that, once consistency of the perturbation expansion is enforced and reasonable estimates of theoretical errors induced by the SMEFT series in $\frac{E}{\Lambda}$ are included, there is no potential to constrain four-quark contact interactions using Tevatron data. This shows the general difficulty of constraining physics model-independently …
LHC-scale left-right symmetry and unification
2013
We construct a comprehensive list of nonsupersymmetric standard model extensions with a low-scale left-right (LR)-symmetric intermediate stage that may be obtained as simple low-energy effective theories within a class of renormalizable SO(10) grand unified theories. Unlike the traditional minimal LR models many of our example settings support a perfect gauge coupling unification even if the LR scale is in the LHC domain at a price of only (a few copies of) one or two types of extra fields pulled down to the TeV-scale ballpark. We discuss the main aspects of a potentially realistic model building conforming the basic constraints from the quark and lepton sector flavor structure, proton deca…
The Weak-Magnetic Moment of Heavy Quarks
1997
With initial and final particles on-shell, the anomalous weak-magnetic dipole moments of b and c quarks are electroweak gauge invariant quantities of the effective couplings Zb\bar{b} and Zc\bar{c}, respectively, and good candidates to test the Standard Model and/or new physics. Here we present a complete computation of these quantities within the Standard Model. We show that decoupling properties with respect to heavy particles do take place in the weak magnetic moment. The obtained values, a_b(M_Z^2)=(2.98-1.56i)x10^(-4) and a_c(M_Z^2)=(-2.80+1.09i)x10^(-5) are dominated by one-gluon exchange diagrams. The electroweak corrections are less than 1% of the total magnitude.
Relativistic constituent quark model with infrared confinement
2009
We refine the relativistic constituent quark model developed in our previous papers to include the confinement of quarks. It is done, first, by introducing the scale integration in the space of alpha-parameters, and, second, by cutting this scale integration on the upper limit which corresponds to an infrared cutoff. In this manner one removes all possible thresholds presented in the initial quark diagram. The cutoff parameter is taken to be the same for all physical processes. We adjust other model parameters by fitting the calculated quantities of the basic physical processes to available experimental data. As an application, we calculate the electromagnetic form factors of the pion and t…
Hard Exclusive Reactions and the Structure of Hadrons
2001
We outline in detail the properties of generalized parton distributions (GPDs), which contain new information on the structure of the nucleon and which enter the description of hard exclusive reactions. We highlight the physics content of the GPDs and discuss the quark GPDs in the large $N_c$ limit and within the context of the chiral quark-soliton model. Guided by this physics, we then present a general parametrization for these GPDs. Subsequently we discuss how these GPDs enter in a wide variety of hard electroproduction processes and how they can be accessed from them. We consider in detail deeply virtual Compton scattering and the hard electroproduction of mesons. We identify a list of …
Weak decays of doubly heavy hadrons
1995
We explore the application and usefulness of the heavy quark symmetry to describe the weak decays of hadrons (mesons and baryons) containing two heavy quarks. Firstly, we address the internal dynamics of a heavy-heavy bound system with the help of estimates based on potential models, showing an approximate spin symmetry in the preasymptotic quark mass region including charmonium, bottonium and $B_c$ meson states. However, no asymptotic spin symmetry should hold in the infinite quark mass limit in contrast to singly heavy hadrons. Predictions on semileptonic and two-body nonleptonic decays of $B_c$ mesons are shown. Furthermore, the stemming flavor and spin symmetries from the interaction be…
Heavy Baryon Specroscopy from the Lattice
1996
The results of an exploratory lattice study of heavy baryon spectroscopy are presented. We have computed the full spectrum of the eight baryons containing a single heavy quark, on a $24^3\times 48$ lattice at $\beta=6.2$, using an $O(a)$-improved fermion action. We discuss the lattice baryon operators and give a method for isolating the contributions of the spin doublets $(\Sigma,\Sigma^*)$, $(\Xi',\Xi^*)$ and $(\Omega,\Omega^*)$ to the correlation function of the relevant operator. We compare our results with the available experimental data and find good agreement in both the charm and the beauty sectors, despite the long extrapolation in the heavy quark mass needed in the latter case. We …
First lattice calculation of the B-meson binding and kinetic energies
1995
We present the first lattice calculation of the B-meson binding energy $\labar$ and of the kinetic energy $-\lambda_1/2 m_Q$ of the heavy-quark inside the pseudoscalar B-meson. This calculation has required the non-perturbative subtraction of the power divergences present in matrix elements of the Lagrangian operator $\bar h D_4 h$ and of the kinetic energy operator $\bar h \vec D^2 h$. The non-perturbative renormalisation of the relevant operators has been implemented by imposing suitable renormalisation conditions on quark matrix elements, in the Landau gauge. Our numerical results have been obtained from several independent numerical simulations at $\beta=6.0$ and $6.2$, and using, for t…
Study of semileptonic and nonleptonic decays of the B-c(-) meson
2006
We evaluate semileptonic and two--meson nonleptonic decays of the $B_c^-$ meson in the framework of a nonrelativistic quark model. The former are done in spectator approximation using one--body current operators at the quark level. Our model reproduces the constraints of heavy quark spin symmetry obtained in the limit of infinite heavy quark mass. For the two--meson nonleptonic decays we work in factorization approximation. We compare our results to the ones obtained in different relativistic approaches.