Helicity Amplitude Method Research Articles

MUMUG: a fast Monte Carlo generator for radiative muon pair production (a pedagogical tutorial)

A fast leading-order Monte Carlo generator for the process $e^+e^-\to\mu^+\mu^-\gamma$ is described. In fact, using the $e^+e^-\to\mu^+\mu^-\gamma $ process as an example, we provide a pedagogical demonstration of how a Monte Carlo generator can be created from scratch. The $e^+ e^- \to \mu^+ \mu^- \gamma$ process was chosen, since in this case we are not faced with either too trivial or too difficult a task. Matrix elements are calculated using the helicity amplitude method. Monte Carlo algorithm uses the acceptance-rejection method with an appropriately chosen simplified distribution that can be generated using an efficient algorithm. We provide a detailed pedagogical exposition of both the helicity amplitude method and the Monte Carlo technique, which we hope will be useful for high energy physics students.

One-loop electroweak radiative corrections to lepton pair production in polarized electron-positron collisions

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.

William James Stirling CBE. 4 February 1953—9 November 2018

James Stirling's wide-ranging contributions to the development and application of quantum chromodynamics were central in verifying QCD as the correct theory of strong interactions, and in computing precise predictions for all types of collider processes. He published more than 300 papers on a vast range of phenomenological topics, including some of the most highly cited of all time in particle physics. His research, always full of insight, focused on the confrontation of theoretical predictions with experimental results. Amongst many key contributions, he developed the helicity amplitude method and used it to show that the CERN ‘monojet’ events, thought to be a possible signal of new physics, were due to vector boson plus jet production. The method has since facilitated the calculation of many other important processes. At Durham he formed a famous long-standing collaboration that set the standard for determining the quark and gluon distributions in the proton. Besides his intellectual brilliance, his personal qualities of humility, modesty, diligence and fairness made him an outstanding scientific leader and administrator. He played a major role in the foundation of the Institute for Particle Physics Phenomenology in Durham and served as its first Director. In 2005 he was appointed Pro-Vice Chancellor for Research at Durham. He moved to the Cavendish Laboratory in Cambridge in 2008, becoming Head of the Department of Physics in 2011. Then in 2013 he was appointed to the newly created position of Provost, the chief academic officer, at Imperial College, London, from which he retired in August 2018.

Measuring properties of a Heavy Higgs boson in the Hto toverline{t}to b{W}^{+}overline{b}{W}^{-} decay

Suppose a heavy neutral Higgs or scalar boson H is discovered at the LHC, it is important to investigate its couplings to the standard model particles as much as possible. Here in this work we attempt to probe the CP-even and CP-odd couplings of the heavy Higgs boson to a pair of top quarks, through the decay Hto toverline{t}to b{W}^{+}overline{b}{W}^{-} . We use the helicity-amplitude method to write down the most general form for the angular distributions of the final-state b quarks and W bosons. We figure out that there are 6 types of angular observables and, under mathrm{C}mathrm{P}tilde{mathrm{T}} conservation, one-dimensional angular distributions can only reveal two of them. Nevertheless, the H couplings to the toverline{t} pair can be fully determined by exploiting the one-dimensional angular distributions. A Higgs-boson mass of 380 GeV not too far above the toverline{t} threshold is illustrated with full details. With a total of 104 events of Hto toverline{t}to b{W}^{+}overline{b}{W}^{+} , one can determine the couplings up to 10-20% uncertainties.

Spinor Helicity Amplitudes Method

In this paper, we introduce the spinor helicity amplitude method. We provide the approaches to handle with massless fermions, massive spinors and polarization vectors respectively, then applied this method to some examples such as e + e −→μ + μ −, e − γ→e − γ, $\bar {q}q \to gg$ processes. Among the examples, $\bar {q}q\to gg$ process presented in the antecedent literatures, but we provide a more detailed calculation, and others are gave for the first time. We summarize all kinds of the situations, and give the needed formulas in appendix, which will be of use to collect these analysis formulas in one place.

Carlomat: A program for automatic computation of lowest order cross sections

The current version of carlomat, a program for automatic computation of the lowest order cross sections of multiparticle reactions, is described. The program can be used as the Monte Carlo generator of unweighted events as well. Program summary Program title: carlomat Catalogue identifier: AEDQ_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEDQ_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 101 613 No. of bytes in distributed program, including test data, etc.: 1 092 251 Distribution format: tar.gz Programming language: Fortran 90/95 Computer: all Operating system: Linux Classification: 4.4, 11.2 Nature of problem: Description of two particle scattering reactions with possibly up to 10 particles in the final state with a complete set of the Feynman diagrams in the lowest order of the Standard Model. Solution method: The matrix element for a user specified process and phase space parametrizations, which are necessary for the multichannel Monte Carlo integration of the lowest order cross sections and event generation, are generated automatically. Both the electroweak and quantum chromodynamics lowest order contributions are taken into account. Particle masses are not neglected in the program. Matrix elements are calculated numerically with the helicity amplitude method. Constant widths of unstable particles are implemented by modifying mass parameters in corresponding propagators. Restrictions: The number of external particles is limited to 12. Only the Standard Model is implemented at the moment in the program. No higher order effects are taken into account, except for assuming the fine structure constant and the strong coupling at appropriate scale and partial summation of the one particle irreducible loop corrections by introducing fixed widths of unstable particles. Running time: Generation of the Fortran code with carlomat on a PC with the Pentium 4 3.0 GHz processor for reactions with 8 particles in the final state relevant for the associated top quark pair and Higgs boson production and decay takes about 10 minutes CPU time. This relatively long time of the code generation is determined by a lot of write to and read from a disk commands which have to be introduced in order to circumvent limitations of the Fortran compilers concerning possible array sizes. The compilation time of generated routines depends strongly on a compiler used and an optimization option chosen. Typically, for the reactions mentioned, it takes about one hour to compile all the routines generated. Most of the time is used for the compilation of the kinematical routines. The execution time of the Monte Carlo (MC) integration with about 2 million calls to the integrand amounts typically to a few hours and, if the MC summing over polarizations is employed, it is dominated by computation of the phase space normalization.

Possible manifestation of new scalar interaction in P-odd asymmetries in Λb→Λτ+τ- decay

Using the helicity amplitude method and including a new scalar type interaction in the matrix element of the exclusive semileptonic Λb→Λτ+τ- decay, P-odd asymmetries with polarized and unpolarized heavy baryons are investigated. The result is obtained that the study of P-odd asymmetries can be promising for establishing the new scalar sector beyond the SM.

Contribution of superstring Z′ boson on the polarization effects in proton proton collisions

Abstract In this work we investigate the single- and the double-spin asymmetries at the collisions of polarized protons pp → (γ *, Z 0, Z′) + X within the scope of QCD, the electroweak interaction and superstring E 6 theory. The helicity amplitude method is used. Analytical expressions for the single- and the double-spin asymmetries are obtained and their dependence on the transverse momentum of the lepton pair is investigated at the three different values of invariant masses of the lepton pair. The pure contribution coming from the superstring Z′ boson on the single- and double- spin asymmetries has been extracted. The results obtained allow investigation of the spin structure of the proton.

Partial wave analysis of [formula omitted] at BESII

The decay ψ′→π+π−π0 is analyzed using a sample of 14 million ψ′ events taken with the BESII detector at the BEPC, and the branching fraction is measured to be B(ψ′→π+π−π0)=(18.1±1.8±1.9)×10−5. A partial wave analysis is carried out using the helicity amplitude method. ψ′→ρ(770)π is observed, and the branching fraction is measured to be B(ψ′→ρ(770)π)=(5.1±0.7±1.1)×10−5, where the first error is statistical and the second one is systematic. A high mass enhancement with mass around 2.15 GeV/c2 is also observed. Attributing this enhancement to the ρ(2150) resonance, the branching fraction is measured to be B(ψ′→ρ(2150)π→π+π−π0)=(19.4±2.5−3.4+11.5)×10−5. The results will help in the understanding of the longstanding “ρπ puzzle” between J/ψ and ψ′ hadronic decays.

Single top quark production and decay at next-to-leading order in hadron collisions

We present a calculation of the next-to-leading order QCD corrections, with one-scale phase space slicing method, to single top quark production and decay process $p\bar{p},pp\to t\bar{b}+X\to b\ell\nu\bar{b}+X$ at hadron colliders. Using the helicity amplitude method, the angular correlation of the final state partons and the spin correlation of the top quark are preserved. The effect of the top quark width is also examined.

Ee4fγ—A program for e+e−→4f,4f γ with nonzero fermion masses

A computer program ee4f γ for calculating cross-sections of any four fermion final state of e + e −-annihilation at high energy and the corresponding bremsstrahlung reaction that is possible in the framework of the Standard Model is presented. As the fermion masses are arbitrary, the cross-sections for channels that do not contain e + and/or e − in the final state can be computed without any collinear cut, the on-shell top quark production can be studied and the Higgs boson exchange can be incorporated in a consistent way. The program can be used as a Monte Carlo generator of unweighted events as well. Program summary Title of program: ee4f γ Version: 1.0 (February 2004) Catalogue identifier: ADTQ Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADTQ Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Licensing provisions: none Computers: all Operating systems: Unix/Linux Programming language used: FORTRAN 90 CPC Program Library subprograms used: RANLUX, ACPR RANLUX 79 (1994) 111—a random number generator Memory required to execute with typical data: 4.0 Mb No. of bits in a word: 32 No. of bytes in distributed program, including test data, etc.: 364 490 No. of lines in distributed program, including test data, etc.: 45 278 Distribution format: tar gzip file Nature of physical problem: Description of all e + e −→4 fermions and corresponding bremsstrahlung reactions that are possible in the Standard Model (SM) to lowest order and with nonzero fermion masses at center of mass energies typical for next generation linear colliders. Such reactions are relevant, typically, for W-pair or intermediate mass Higgs boson production and decay. Method of solution: Matrix elements are calculated with the helicity amplitude method. The phase space integration is performed numerically utilizing a multi-channel Monte Carlo method. Restrictions on complexity of the problem: No higher order effects are taken into account, except for assuming the fine structure constant and the strong coupling at the appropriate scale and partial summation of those one particle irreducible loop corrections which are inducing the (fixed) finite widths of unstable particles. Typical running time: The running time depends strongly on a selected channel and desired precision of the result. The results of the appended test run have been obtained on a 800 MHz Pentium III processor with the use of Absoft FORTRAN 90 compiler in about 9 minutes. In order to obtain a precision level below one per mile a few million calls to the integrand are required. This results in less than one hour running time for the fastest channel e +e −→ν μ ν ̄ μν τ ν ̄ τ and more than 100 hours running time for the slowest channel e + e −→ e + e − e + e −. However, the typical running time for channels that are relevant for the W-pair or Higgs boson production is several hours.

Diffractive light quark jet and gluon jet production at hadron colliders in the two-gluon exchange model

Massless diquark jet and gluon jet production at large transverse momentum in the coherent diffractive processes at hadron colliders are studied in the two-gluon exchange parametrization of the Pomeron model. We use the helicity amplitude method to calculate the cross section formulas. We find that for the light quark jet and gluon jet production the diffractive process is related to the differential off-diagonal gluon distribution function in the proton. We estimate the production rate for these processes at the Fermilab Tevatron by approximating the off-diagonal gluon distribution function by the usual diagonal gluon distribution in the proton. We also use the helicity amplitude method to calculate the diffractive charm jet production at hadron colliders, by which we reproduce the leading logarithmic approximation result of this process we previously calculated.

S usy23 v2.0: An event generator for supersymmetric processes at e+e− colliders

S usy23 is a Monte Carlo package for generating supersymmetric (SUSY) processes at e + e − colliders. Twenty-three types of SUSY processes with 2 or 3 final state particles at tree level are included in version 2.0. S usy23 addresses event simulation requirements at e + e − colliders such as LEP. Matrix elements are generated by GRACE with the helicity amplitude method for processes involving massive fermions. The phase space integration of the matrix element gives the total and differential cross sections, then unweighted events are generated. Sparticle widths and decay branching ratios are calculated. Each final state particle may then decay according to these probabilities. Spin correlations are taken into account in the decays of sparticles. Corrections of initial state radiation (ISR) are implemented in two ways, one is based on the electron structure function formalism and the second uses the parton shower algorithm called QEDPS. Parton shower and hadronization of the final quarks are performed through an interface to JETSET.

Asymmetry Effects in Polarized Hadron Scattering

We calculate the single-spin and double-spin asymmetry differential cross sections for the polarized hadron scattering PP → l+ l- + jet up to O(αs) by the helicity amplitude method. Numerical results of the differential cross sections, which can be used to probe the spin contents of the proton, are obtained from several sets of polarized parton distribution functions.

WPHACT 1.0 — A program for WW, Higgs and four-ferminon physics at e+e− colliders

WPHACT ( W W and Higgs Physics with PHACT) is a MC program and unweighted event generator which computes all Standard Model processes with four fermions in the final state at e + e − colliders. It is based on an helicity amplitude method which allows precise and fast evaluations of the matrix elements both for massless and massive fermions. Fermion masses for b quarks are exactly taken into account. QED initial state and Coulomb corrections are evaluated, while QCD final state corrections are included in an approximate formulation. Cuts can be easily introduced and distributions for any variable at parton level can be implemented. The contributions to the processes of neutral Standard Model or Susy Higgs can be included. Anomalous couplings effects for the triple coupling can be computed. An interface to hadronization is provided and Jetset can be directly called from the program.

Helicity amplitudes for multiple bremsstrahlung in massless non-abelian gauge theories

We formulate the helicity amplitudes for multiple bremsstrahlung processes in massless gauge theories in terms of spinor inner-products. In the non-abelian case (QCD) we show that contributions from the diagrams with gluon self-coupling can be decomposed into parts to be included together with QED-like diagrams in gauge-invariant subsets, and the amplitudes of the parts are related to the amplitudes of QED-like diagrams. This enables us to make use of the simplifying power of the helicity amplitude method as far as possible.