The two-loop helicity amplitudes for $q \bar q' \to V_1 V_2 \to 4~\mathrm{leptons}$
Thomas Gehrmann, Andreas von Manteuffel, Lorenzo Tancredi
备注：
34 pages, 3 figures, results available in computer readable format on HepForge at http://vvamp.hepforge.org ; fixed typos in electroweak coupling definitions, version accepted for publication in JHEP
We compute the two-loop massless QCD corrections to the helicity amplitudes for the production of two massive vector bosons in quark-antiquark annihilation, allowing for an arbitrary virtuality of the vector bosons: $q \bar q' \to V_1V_2$. Combining with the leptonic decay currents, we obtain the full two-loop QCD description of the corresponding electroweak four-lepton production processes. The calculation is performed by projecting the two-loop diagrams onto an appropriate basis of Lorentz structures. All two-loop Feynman integrals are reduced to a basis of master integrals, which are then computed using the differential equations method and optimised for numerical performance. We provide a public C++ code which allows for fast and precise numerical evaluations of the amplitudes.
The lepton helicity distributions in the polarized Drell-Yan process at RHIC energy are investigated. For the events with relatively low invariant mass of lepton pair in which the weak interaction is negligible, only the measurement of lepton helicity can prove the antisymmetric part of the hadronic tensor. Therefore it might be interesting to consider the helicity distributions of leptons to obtain more information on the structure of nucleon from the polarized Drell-Yan process. We estimate the QCD corrections at O(alpha_s) level to the hadronic tensor including both intermediate gamma and Z…
As continuation of our previous paper we further develop our new method for calculating helicity amplitudes of jet-like QED processes described by tree diagrams, applying it to lepton pair production. This method consists in replacing spinor structures for real and weakly virtual intermediate leptons by simple transition vertices. New vertices are introduced for the pair production case, and previous bremsstrahlung vertices are generalized to include virtual photons inside the considered jet. We present a diagrammatic approach that allows to write down in an efficient way the leading helicity …
The lepton helicity distributions in the polarized Drell-Yan process at RHIC energy are investigated. In the absence of the weak interaction, only the measurement of lepton helicity can prove the antisymmetric part of the hadronic tensor. Therefore it might be interesting to consider the helicity distributions of leptons to obtain more information on the structure of nucleon from the polarized Drell-Yan process. We estimate the QCD corrections at ${\cal O} (α_s)$ level to the hadronic tensor including both intermediate $γ$ and $Z$ bosons. We report the numerical analyses on the $Z$ pole and sh…
We compute the two-loop massless QCD corrections to the helicity amplitudes for the production of two electroweak gauge bosons in the gluon fusion channel, $gg \to V_1 V_2$, keeping the virtuality of the vector bosons $V_1$ and $V_2$ arbitrary and taking their decays into leptons into account. The amplitudes are expressed in terms of master integrals, whose representation has been optimised for fast and reliable numerical evaluation. We provide analytical results and a public C++ code for their numerical evaluation on HepForge at http://vvamp.hepforge.org .
The lepton pair production $e^- e^+ \to e^- e^+ l^- l^+ $ or $μ^- μ^+ \to μ^- μ^+ l^- l^+ $ is studied in the dominant cross section region of scattering angles ${m_j}/{E_j} \stackrel{<}{\sim} θ_j \ll 1 $. An analytical expression is found for all 64 helicity amplitudes of these processes. The accuracy of the obtained formulae is given omitting only terms of the order of ${m_j^2}/{E_j^2}$, $θ_j^2$ and $ θ_j {m_j}/{E_j}$. The result has a compact form convenient both for analytical and numerical calculations of various cross sections in this dominating scattering regime.
It is pointed out that the heavy singlet neutrinos characteristic of leptogenesis develop asymmetries in the abundances of the two helicity states as a result of the same mechanism that generates asymmetries in the standard lepton sector. Neutrinos and standard leptons interchange asymmetries in collisions with each other. It is shown that an appropriate quantum number, B-L', combining baryon, lepton and neutrino asymmetries, is not violated as fast as the standard B-L. This suppresses the washout effects relevant for the derivation of the final baryon asymmetry. One presents detailed calculat…
The hypermagnetic helicity density at the electroweak phase transition (EWPT) exceeds many orders of magnitude the galactic magnetic helicity density. Together with previous magnetic helicity evolution calculations after the EWPT and hypermagnetic helicity conversion to the magnetic one at the EWPT, the present calculation completes the description of the evolution of this important topological feature of cosmological magnetic fields. It suggests that if the magnetic field seeding the galactic dynamo has a primordial origin, it should be substantially helical. This should be taken into account…
Knowledge of two-loop QCD amplitudes for processes $q\bar q \to V_1 V_2 \to (l_1\bar l'_1)(l_2 \bar l'_2)$ is important for improving the theoretical description of four-lepton production in hadron collisions. In this paper we compute these helicity amplitudes for all intermediate vector bosons, $V_1 V_2 = γ^*γ^*, WW, ZZ, WZ, W γ^*$, including off-shell effects and decays to leptons.
We present the joint helicity amplitudes for $J/ψ\to Λ\barΛ$, $Λ(\barΛ)$ decays to different final states in the helicity frame. Two observables to search for $CP$ violation in $J/ψ\toΛ\barΛ$ can be expressed with the information of helicity angles of baryon and antibaryon. Four decay parameters of $Λ$ and $\barΛ$, namely, $α_-,α_+,α_0$ and $\barα_0$, can be obtained with the joint helicity amplitude equations by the likelihood fit method. With the data sample of $10^{10}$ $J/ψ$ decays accumulated by BESIII, the precision of the measurements is estimated to be about $10^{-3}$.
Using the helicity method we derive complete formulas for the joint angular decay distributions occurring in semileptonic hyperon decays including lepton mass and polarization effects. Compared to the traditional covariant calculation the helicity method allows one to organize the calculation of the angular decay distributions in a very compact and efficient way. In the helicity method the angular analysis is of cascade type, i.e. each decay in the decay chain is analyzed in the respective rest system of that particle. Such an approach is ideally suited as input for a Monte Carlo event generat…
Three-loop helicity amplitudes of four-lepton scattering in QED
Higgs Centre for Theoretical Physics, School of Physics and Astronomy The University of Edinburgh（爱丁堡大学物理与天文学学院希格斯理论物理中心）；Department of Mathematical Sciences, University of Liverpool（利物浦大学数学科学系）；Dipartimento di Fisica e Astronomia, Universita di Padova（帕多瓦大学物理与天文系）；INFN, Sezione di Padova（意大利国家核物理研究所帕多瓦分部）
The unexpectedly large transverse polarization measured in the B-to-phi K* decay poses the question whether it is accounted for as a strong interaction effect or possibly points to a hidden nonstandard weak interaction. We extend here the perturbative argument to the helicity structure of the two-body baryonic decay and discuss qualitatively on how the baryonic B decay modes might help us in understanding the issue raised by the B-to-VV decays. We find among others that the helicity +1/2 amplitude dominates in the leading order in the B (b-bar q) decay and that the dominant amplitude is sensit…
Decay rate, forward-backward asymmetry and polarization asymmetries of final state leptons in B-> K_{1}tau ^{+}tau ^{-}, where K_{1} is the axial vector meson, are calculated in Standard Model and in the universal extra dimension (UED) model. The sensitivity of the observables on the compactification radius $R$, the only unknown paramter in UED model, is studied. Finally, the helicity fractions of the final state K_{1} are calculated and their dependence on the compactification radius is discussed. This analysis of helicity fraction is briefly extended to B->K^{*}l ^{+}l ^{-}(l =e,mu) and comp…
We study the helicity distributions of light flavor quark-antiquark ($q \bar{q}$) pairs in the nucleon sea. The valence quarks are handled by adopting the light-cone SU(6) quark-spectator-diquark model and the sea $q \bar{q}$ pairs are treated from statistical consideration by introducing the helicity suppression factors $l_q(x)$ and $\bar l_q(x)$ to parametrize the helicity distributions of q-flavor sea quark and antiquark respectively, while $Δl_q(x)=l_q(x)-\bar l_q(x)$ represents a combined effect of helicity contribution due to sea $q \bar{q}$ pairs. From fitting the nucleon polarization a…
We study hypermagnetic helicity and lepton asymmetry evolution in plasma of the early Universe before the electroweak phase transition (EWPT) accounting for chirality flip processes via inverse Higgs decays and sphaleron transitions which violate the left lepton number and wash out the baryon asymmetry of the Universe (BAU). In the scenario where the right electron asymmetry supports the BAU alone through the conservation law $B/3 - L_{eR}=const$ at temperatures $T>T_{RL}\simeq 10 TeV$ the following universe cooling leads to the production of a non-zero left lepton (electrons and neutrinos) as…
We study hypermagnetic helicity and lepton asymmetry evolution in plasma of the early Universe before the electroweak phase transition (EWPT) accounting for chirality flip processes via inverse Higgs decays and sphaleron transitions which violate the left lepton number and wash out the baryon asymmetry of the Universe (BAU). In the scenario where the right electron asymmetry supports the BAU alone through the conservation law $B/3 - L_{eR}=const$ at temperatures $T>T_{RL}\simeq 10~TeV$ the following universe cooling leads to the production of a non-zero left lepton (electrons and neutrinos) as…
$W$ bosons are produced at LHC from a forward-backward symmetric initial state. Their decay to a charged lepton and a neutrino has a strong spin analysing power. The combination of these effects results in characteristic distributions of the pseudorapidity of the leptons decaying from $W^+$ and $W^-$ of different helicity. This observation may open the possibility to measure precisely the $W^+$ and $W^-$ rapidity distributions for the two transverse polarisation states of $W$ bosons produced at small transverse momentum.
Motivated by the recent hints of lepton flavor universality violation observed in semileptonic $B$ decays, we analyze how to test flavor and helicity structures of the corresponding amplitudes in view of future data. We show that the general assumption that such non-standard effects are controlled by a $U(2)_q \times U(2)_\ell$ flavor symmetry, minimally broken as in the Standard Model Yukawa sector, leads to stringent predictions on leptonic and semileptonic $B$ decays. Future measurements of $R_{D^{(*)}}$, $R_{K^{(*)}}$, ${\mathcal B}(\bar B_{c,u}\to \ell \bar ν)$, ${\mathcal B}(\bar B \to π…
In view of the recent measurement of the $F_L^{D^*}$ magnitude in the $\bar{B} \to D^{*} \barν_ττ$ reaction we evaluate this magnitude within the standard model and for a family of models with the $γ^μ-αγ^μγ_5$ current structure for the quarks for different values of $α$. At the same time we evaluate also the transverse contributions, $M=-1$, $M=+1$, and find that the difference between the $M=-1$ and $M=+1$ contributions is far more sensitive to changes in $α$ than the longitudinal component.
Antiproton proton annihilation reactions allow unique access to the moduli and phases of nucleon electromagnetic form factors in the time like region. We present the helicity amplitudes for the unequal mass single photon reaction $p \bar{p}\to l^+ l^-$ in the s channel including the lepton mass. The relative signs of these amplitudes are determined using simple invariance properties. Helicity amplitudes for the annihilation reaction $p \bar{p} \to N \bar{N}$ are also given, where $N$ is any spinor particle with structure. Crossing relations between the $e p \to e p$ scattering and the $p \bar{…
Using a subtraction method, we derive the formulae suitable for use in Monte-Carlo event generators to give the angular distribution for the gluon-quark induced NLO corrections in Drell-Yan lepton pair production. We also give the corresponding helicity density matrix for W and Z boson production.
We give a general parameterization of the Lambda_b --> Lambda(1520) gamma decay amplitude, applicable to any strange isosinglet spin-3/2 baryon, and calculate the branching fraction and helicity amplitudes. Large-energy form factor relations are worked out, and it is shown that the helicity-3/2 amplitudes vanish at lowest order in soft-collinear effective theory (SCET). The suppression can be tested experimentally at the LHC and elsewhere, thus providing a benchmark for SCET. We apply the results to assess the experimental reach for a possible wrong-helicity b --> s gamma dipole coupling in La…
We present a next-to-leading order QCD calculation of the cross section and longitudinal spin asymmetry in single-inclusive charged-lepton production, pp -> l X, at RHIC, where the lepton is produced in the decay of an electroweak gauge boson. Our calculation is presented in terms of a multi-purpose Monte-Carlo integration program that may be readily used to include experimental spin asymmetry data in a global analysis of helicity parton densities. We perform a toy global analysis, studying the impact of anticipated RHIC data on our knowledge about the polarized anti-quark distributions.
We examine the transverse momentum $Q_\perp$ dependence of the helicity structure functions for massive vector bosons of mass $Q$ in hadron reactions. We demonstrate that large logarithmic terms of the form $\ln(Q/Q_\perp)$ in the helicity structure functions have the same origin as the logarithmic terms in the angular-integrated cross section and that they can be resummed to all orders in the strong coupling $α_s$, in the same way as the angular-integrated cross section. As a consequence of current conservation, the resummed part of the helicity structure functions preserves the Lam-Tung rela…
We present the differential rates for exclusive $\Bbar\to X \ell_1 \ell_2$, where $\ell_1$ is a charged massless lepton and $\ell_2$ is a charged or neutral massless lepton, and $X$ is a mesonic system up to spin 2. The cases of interest are semileptonic (SL) $\Bbar\to \xcu\ellm \barnuell$ decays, and $\Bbar \to X_s \ellm \ellp$ where the the di-lepton can be $c \bar{c}$ resonances or non-resonant electroweak penguins (EWP). We consider helicity amplitudes having non-zero relative phases that can be potential new sources for CP-violation. Our motivations for these additional phases include a c…
We derive the two-loop QCD helicity amplitudes for the processes $l q \to l qg$ ($l \bar q \to l \bar q g$) and $l g \to l q\bar q$, which are the partonic reactions yielding $(2+1)$-jet final states in deep inelastic lepton nucleon scattering. The amplitudes are obtained by analytic continuation of the known helicity amplitudes for $e^+e^- \to q\bar q g$. We separate the infrared divergent and finite parts of the amplitudes using Catani's infrared factorization formula. The analytic results for the finite parts of the amplitudes are expressed in terms of one- and two-dimensional harmonic poly…
The Hot Big Bang model predicts the existence of a \emph{cosmic neutrino background}. The number of particles and anti-particles in this primordial bath of neutrinos can be different -- a memory of processes that took place at very early epochs. If neutrinos were massless, this asymmetry would not change once neutrinos froze out. However, in the case of massive particles, the asymmetry is not protected by conservation laws and can get erased via helicity-flipping scatterings off matter inhomogeneities. We evaluate this helicity-flipping rate and demonstrate that if relic lepton asymmetry ever …
In this paper, we show how to calculate analytically the one-loop helicity amplitudes for the process $q\bar{q} rightarrow t\bar{t}$ induced by KK gluon, using the spinor-helicity formalism. A minimal set of Feynman rules which are uniquely fixed by gauge invariance and the color representation of the KK gluon are derived and used in the calculation. Our results can be applied to a variety of models containing a massive color octet vector boson.
We study the small--angle double bremsstrahlung in $e^-e^\pm$ scattering for a jet kinematics where both photons move along the electron direction. This region gives the main contribution to the cross section. We present analytic expressions for all 64 amplitudes with arbitrary helicity states of the initial and final leptons and the produced photons convenient for analytic and numerical studies. The accuracy of the obtained amplitudes is given omitting only terms of the order of m^2/E_j^2, θ_j^2 and θ_j m/E_j. The helicity amplitudes for the cross channel γe^\pm \to γe^+e^- + e^\pm are given.…
We present the one-loop QCD corrections to the helicity amplitudes for the processes $q\qb \to W^+ W^-, Z Z, W^\pm Z, W^\pm γ$, or $Z γ$, including the subsequent decay of each massive vector boson into a pair of leptons. We also give the corresponding tree-level amplitudes with an additional gluon radiated off the quark line. Together, these amplitudes provide all the necessary input for the calculation of the next-to-leading order QCD corrections to the production of any electroweak vector boson pair at hadron colliders, including the full spin and decay angle correlations.
The self-couplings of the electroweak gauge bosons are probed at hadron colliders through the production of a massive gauge boson and a photon. To extend the theoretical description of this type of final states towards next-to-next-to-leading order (NNLO) in QCD, we derive the two-loop QCD corrections to the helicity amplitudes describing the production of a massive gauge boson in association with a real photon. Our results are obtained by applying projectors to the general parton-level tensor structure. The leptonic decay of the vector boson is included, thus allowing for a fully exclusive de…
We compute the part of the two-loop virtual amplitude for the process $gg \to V_1 V_2 \to (l_1 \bar l'_{1}) (l_2 \bar l'_2)$, where $V_{1,2}$ are arbitrary electroweak gauge bosons, that receives contributions from loops of massless quarks. Invariant masses of electroweak bosons are allowed to be different from each other. Our result provides an important ingredient for improving the description of gluon fusion contribution to the production of four-lepton final states at the LHC.
We investigate the long distance contribution via charmed hadron loops in the processes $η_c$, $χ_{c0}$ and $h_c$ decaying to baryon anti-baryon pairs, which are supposed to be highly suppressed by the helicity selection rule as a consequence of the perturbative QCD framework. With an effective Lagrangian method, our estimation result indicates that such hadron loops play an important role in these hadronic decays. It is a further test of the evading mechanism for the helicity selection rule in charmonium baryon-antibaryon decays.
We perform a helicity amplitude analysis for the processes of $e^+e^-$$\rightarrow$$J/ψ,ψ(2S)\rightarrowΞ(1530)\barΞ(1530)\rightarrowππΞ\barΞ\rightarrowΛ\bar\Lambda4π\rightarrow p\bar{p}6π$. The joint angular distribution for these processes are obtained, which allows a proper estimation of the detection efficiency using helicity amplitude information. In addition, the sensitivities of measurement of hyperon asymmetry decay parameters for these processes are discussed.
The two body charmless decays of $B_s$ meson to light vector mesons are analyzed within the framework of QCD factorization. This approach implies that the nonfactorizable corrections to different helicity amplitudes are not the same. The effective parameters $a_i^h$ for helicity $h=0,+,-$ states receive different nonfactorizable contributions and hence are helicity dependent, contrary to naive factorization approach where $a_i^h$ are universal and polarization independent. The branching ratios for $\bar{B}_s\to VV$ decays are calculated and we find that branching ratios of some channels are of…
At future electron-positron colliders, one of the largest irreducible backgrounds to top searches in the channel `4 jets + lepton + missing energy' comes from QCD events of order $α_s^2$. We compute here such processes exactly at the parton level by resorting to 2 -> 6 matrix elements exploiting helicity amplitude techniques. We adopt a typical selection procedure based on the tagging of a high momentum and separated lepton. We finally outline kinematic differences between signal and background events that can be exploited to further reduce such a QCD noise.
We present a general method for calculating inclusive cross sections for the production of heavy quarkonium states with definite polarization within the NRQCD factorization approach. Cross sections for polarized production can involve additional matrix elements that do not contribute to cross sections for unpolarized production. They can also include interference terms between parton processes that produce $Q \bar Q$ pairs with different total angular momentum. The interference terms cancel upon summing over polarizations. Our method can be generalized to $N$ dimensions and is therefore compat…
Final state interactions in quark fragmentation may give origin to non zero values of the off-diagonal element rho_(1,-1) of the helicity density matrix of vector mesons V produced in current jets, with a large energy fraction x_E; the value of rho_(1,-1)(V) is related to the hard constituent dynamics and tests unusual properties of it. Some recent data on phi, K^* and D^* produced in e^+ e^- annihilations at LEP show such effects. Predictions are given here for rho_(1,-1) of heavy mesons produced in nucleon-nucleon, gamma-nucleon and lepton-nucleon interactions.
We compute the single-spin asymmetry $A_{UL}$ in semi-inclusive deep-inelastic scattering of unpolarized leptons and longitudinally polarized protons at large transverse momentum of the produced hadron. Our calculation is performed in collinear factorization at the lowest order of QCD perturbation theory. For photon exchange the asymmetry is T-odd and receives contributions from the interference of the tree level and one-loop absorptive amplitudes. We consider the behavior of the spin asymmetry at low transverse momentum where contact to the formalism based on transverse-momentum dependent dis…
We present the two-loop helicity amplitudes for the scattering of massless quarks in QCD. We use projector techniques to compute the coefficients of the general four-quark amplitude at up to two-loops in conventional dimensional regularisation and use these coefficients to derive the helicity amplitudes in the 't Hooft-Veltman scheme. The structure of the infrared divergences agrees with that predicted by Catani while expressions for the finite remainders for $q \bar q \to Q\bar Q$ and the processes related by crossing symmetry are given in terms of logarithms and polylogarithms that are real …
We generalise the Jacob-Wick helicity formalism, which applies to sequential decays, to effective field theories of rare decays of the type $B \to K_{J_K}(\to K π) \bar{\ell}_1 \ell_2$. This is achieved by reinterpreting local interaction vertices $\bar b Γ'_{μ_1 ..μ_n} s \bar \ell Γ^{μ_1 ..μ_n} \ell$ as a coherent sum of $1 \to 2$ processes mediated by particles whose spin ranges between zero and $n$. We illustrate the framework by deriving the full angular distributions for $B \to K\bar{\ell}_1 \ell_2$ and $B \to K^*(\to K π) \bar{\ell}_1 \ell_2$ for the complete dimension-six effective Hami…
We show that the baryon asymmetry of the universe can be explained in models where the Higgs couples to the Chern-Simons term of the hypercharge group and is away from the late-time minimum of its potential during inflation. The Higgs then relaxes toward this minimum once inflation ends which leads to the production of (hyper)magnetic helicity. We discuss the conditions under which this helicity can be approximately conserved during its joint evolution with the thermal plasma. At the electroweak phase transition the helicity is then converted into a baryon asymmetry by virtue of the chiral ano…
This paper presents the high-precision theoretical predictions for $e^+e^- \to l^-l^+$ scattering. Calculations are performed using the {\tt SANC} system. They take into account complete one-loop electroweak radiative corrections as well as longitudinal polarization of initial beams. Reaction observables are obtained using the helicity amplitude method with taking into account initial and final state fermion masses. Numerical results are given for the center-of-mass energy range $\sqrt{s}=250-1000$ GeV with various degrees of polarization.
We apply the concept of helicity from classical hydrodynamics to elucidate two problematical issues in cosmic string physics. Helicity, the space integral of the scalar product of a velocity-like field with its vorticity field (curl), can be defined for a complex scalar field in analogy with fluids. We dwell on the topological interpretation of helicity as related to the linking of field lines of the vorticity field. Earlier works failed to fully implement this interpretation for cosmic strings by missing a term connected with the linking of these lines inside the strings. As a result paradoxi…
A detailed analysis of $Z^{'0}Z^{'0}$, $K^{+}K^{-}$ and $K^{0}\bar{K}^{0}$ pair production in $e^{+}e^{-}$ collisions is presented by using helicity amplitudes. The trilinear bosons couplings in the $SU(3)_{C}\otimes SU(3)_{L}\otimes U(1)_{X}$ models without exotic electric charges are also calculated. We carry out the mentioned analysis for two models, one of them is a one family model which is an $E_6$ subgroup \cite{b1} and the other one is a three family model with right handed neutrinos\cite{b2,b3}. These models do not contain exotic electric charges. For them, we give explicit formulae a…
We study electron-positron annihilations into six jets at the parton level in perturbative Quantum Chromo-Dynamics (QCD), via the elementary processes $e^+e^-\ar q\bar q gggg$, $e^+e^-\ar q\bar q q'\bar q' gg$ and $e^+e^-\ar q\bar q q'\bar q' q''\bar q''$, for massive quarks $q, q'$ and $q''$ and massless gluons $g$. Several numerical results of phenomenological relevance are given, at three different collider energies and for a representative selection of jet clustering algorithms. We also present helicity amplitudes and colour factors needed for the tree-level calculation
We analyze the azimuthal dependence of the heavy-quark-initiated contributions to the lepton-nucleon deep inelastic scattering (DIS). First we derive the relations between the parton level semi-inclusive structure functions and the helicity $γ^{*}Q$ cross sections in the case of arbitrary values of the heavy quark mass. Then the azimuth-dependent ${\cal O}(α_{s})$ lepton-quark DIS is calculated in the helicity basis. Finally, we investigate numerically the properties of the $\cosϕ$ and $\cos2ϕ$ distributions caused by the photon-quark scattering (QS) contribution. It turns out that, contrary t…
We have derived the tensor amplitudes for partial wave analysis of $ψ\toΔ\barΔ$, $Δ\to p π$ within the helicity frame, as well as the amplitudes for the other decay sequences with same final states. These formulae are practical for the experiments measuring $ψ$ decaying into $p \bar{p}π^+ π^-$ final states, such as BESIII with its recently collected huge $J/ψ$ and $ψ(2S)$ data samples.
Inelastic QED processes, the cross sections of which do not drop with increasing energy, play an important role at high-energy colliders. Such reactions have the form of two-jet processes with the exchange of a virtual photon in the t-channel. We consider them in the region of small scattering angles $m/E \lesssim θ\ll 1$, which yields the dominant contribution to their total cross sections. A new effective method is presented and applied to QED processes with emission of real photons to calculate the helicity amplitudes of these processes. Its basic idea is similar to the well-known equivalen…
In order to observe a signal of possible CP violation in top-quark couplings, we have studied energy correlation of the final leptons in $e^+e^-\to t\bar{t} \to \ell^+\ell^-X / \ell^\pm X$ at future linear colliders. Applying the recently-proposed optimal method, we have compared the statistical significances of CP-violation-parameter determination using double- and single-lepton distributions. We have found that the single-lepton-distribution analysis is more advantageous.
Recently we considered the electromagnetic decays of the orbitally-excited SU(6) 70-plet baryons in large-N_c QCD, fitting to the helicity amplitudes measured in photoproduction experiments. Using the results of this analysis, we predict the helicity amplitudes for the decays N^* \to Δγand Δ^* \to Δγ. These decays can be studied at a number of new experimental facilities, and thus provide another nontrivial test of the lowest order large-N_c predictions.
