Yan Process Research Articles

FEWZ 2.0: A code for hadronic Z production at next-to-next-to-leading order

We introduce an improved version of the simulation code FEWZ ( Fully Exclusive W and Z Production) for hadron collider production of lepton pairs through the Drell–Yan process at next-to-next-to-leading order (NNLO) in the strong coupling constant. The program is fully differential in the phase space of leptons and additional hadronic radiation. The new version offers users significantly more options for customization. FEWZ now bins multiple, user-selectable histograms during a single run, and produces parton distribution function (PDF) errors automatically. It also features a significantly improved integration routine, and can take advantage of multiple processor cores locally or on the Condor distributed computing system. We illustrate the new features of FEWZ by presenting numerous phenomenological results for LHC physics. We compare NNLO QCD with initial ATLAS and CMS results, and discuss in detail the effects of detector acceptance on the measurement of angular quantities associated with Z-boson production. We address the issue of technical precision in the presence of severe phase-space cuts. Program summary Program title: FEWZ Catalogue identifier: AEJP_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEJP_v1_0.html Program obtainable from: CPC Program Library, Queenʼs 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.: 6 280 771 No. of bytes in distributed program, including test data, etc.: 173 027 645 Distribution format: tar.gz Programming language: Fortran 77, C++, Python Computer: Mac, PC Operating system: Mac OSX, Unix/Linux Has the code been vectorized or parallelized?: Yes. User-selectable, 1 to 219 RAM: 200 Mbytes for common parton distribution functions Classification: 11.1 External routines: CUBA numerical integration library, numerous parton distribution sets (see text); these are provided with the code. Nature of problem: Determination of the Drell–Yan Z / photon production cross section and decay into leptons, with kinematic distributions of leptons and jets including full spin correlations, at next-to-next-to-leading order in the strong coupling constant. Solution method: Virtual loop integrals are decomposed into master integrals using automated techniques. Singularities are extracted from real radiation terms via sector decomposition, which separates singularities and maps onto suitable phase space variables. Result is convoluted with parton distribution functions. Each piece is numerically integrated over phase space, which allows arbitrary cuts on the observed particles. Each sample point may be binned during numerical integration, providing histograms, and reweighted by parton distribution function error eigenvectors, which provides PDF errors. Restrictions: Output does not correspond to unweighted events, and cannot be interfaced with a shower Monte Carlo. Additional comments: !!!!! The distribution file for this program is over 170 Mbytes and therefore is not delivered directly when download or E-mail is requested. Instead a html file giving details of how the program can be obtained is sent. Running time: One day for total cross sections with 0.1% integration errors assuming typical cuts, up to 1 week for smooth kinematic distributions with sub-percent integration errors for each bin.

Hard- and soft-fermion-pole contributions to single transverse-spin asymmetry for Drell–Yan process

We study the single transverse-spin asymmetry for the Drell–Yan lepton pair production based on the twist-3 mechanism in the collinear factorization. We calculate all the hard-pole (HP) and the soft-fermion-pole (SFP) contributions to the single-spin-dependent cross section originating from the quark–gluon correlation functions in the transversely polarized nucleon in the leading order with respect to the QCD coupling constant. Combined with the soft-gluon-pole (SGP) contribution, this completes the corresponding twist-3 cross section. In the real photon limit, where all the HP contributions are transformed into the SFP contribution, we find that the SFP partonic hard cross section for the two independent quark–gluon correlation functions coincides in each scattering channel, as in the case of the inclusive light-hadron production. Our result enables one to extract the quark–gluon correlation functions from the forthcoming experiments at several facilities such as RHIC and J-PARC.

Drell–Yan process at Large Hadron Collider

Drell–Yan process at LHC, \(q\bar {q}\to Z/\gamma ^\ast \to \ell ^+\ell ^-\), is one of the benchmarks for confirmation of Standard Model at TeV energy scale. Since the theoretical prediction for the rate is precise and the final state is clean as well as relatively easy to measure, the process can be studied at the LHC even at relatively low luminosity. Importantly, the Drell–Yan process is an irreducible background to several searches of beyond Standard Model physics and hence the rates at LHC energies need to be measured accurately. In the present study, the methods for measurement of the Drell–Yan mass spectrum and the estimation of the cross-section have been developed for LHC operation at the centre-of-mass energy of 10 TeV and an integrated luminosity of 100 pb − 1 in the context of CMS experiment.

Single spin asymmetry in πp Drell–Yan process

We study the single spin asymmetries for the πp↑→μ+μ−X process. We consider the asymmetries contributed by the coupling of the Boer–Mulders function with the transversity distribution and the pretzelosity distribution, characterized by the sin(ϕ+ϕS) and sin(3ϕ−ϕS) azimuthal angular dependence, respectively. We estimate the magnitude of these asymmetries at COMPASS by using proper weighting functions. We find that the sin(ϕ+ϕS) asymmetry is of the size of a few percent and can be measured through the experiment. The sin(3ϕ−ϕS) asymmetry is smaller than the sin(ϕ+ϕS) asymmetry. After a cut on qT, we succeed in enhancing the asymmetry.

Five-quark components in baryons

While several experiments based on the Drell–Yan process have revealed the presence of light antiquarks in the proton, the experimental signatures for strange quark components remain consistent with 0. Phenomenological studies of meson photoproduction on nucleons with hadronic models indicate that the underprediction of the NΔ transition strengths by the three quark model may be attributed to the missing “meson cloud" contributions. If qqqqq̄ configurations are included in the baryon wave functions the conclusions that emerge are that (a) a combination of at least three different qqqqq̄ configurations are required for a satisfactory description of the nucleon properties and (b) that the vanishing of the axial form factor of the N(1535) resonance is a natural consequence of the cancellation of the contributions of the qqq and qqqqq̄ configurations.

Measuring the flavor asymmetry in the sea quarks of the proton with the Drell–Yan process

The proton is a composite object made of fundamental, strongly interacting quarks. Many of the features of the proton can be described by a simple picture based on three “valence” quarks bound by the exchange of gluons. However, protons are much more complex objects with the vast majority of their mass dynamically generated by quantum chromodynamics (QCD). This mass manifests itself through a “sea” of gluons and quark–antiquark pairs. By measuring Drell–Yan scattering, the Fermilab E-906/Drell–Yan experiment will study the sea quark distribution in the proton and, in particular, the unusually large asymmetry between anti-up and anti-down quarks measured by earlier Drell–Yan experiments. This asymmetry cannot simply be generated through pair creation, but rather indicates an underlying, fundamental antiquark component in the proton. Using the same technique, the E-906/SeaQuest experiment will also investigate the differences between the antiquark distributions of the free proton and a proton bound in a nucleus. Nuclear binding is expected to modify the quark distributions and it has long been known that the overall quark distributions are different (the EMC effect). Surprisingly, present data suggest, however, that the antiquark distributions are not modified. To accomplish these goals, the experiment will use a 120 GeV proton beam extracted from the Fermilab Main Injector. While the experiment will be taking advantage of equipment from earlier Drell–Yan experiments, the changes in kinematics of the experiment require several, significant upgrades to the spectrometer. Japanese institutions, including Tokyo Institute of Technology, KEK, RIKEN, Kyoto University and Yamagata University, are making substantial contributions to this upgrade. The collaboration expects to begin data collection in Summer, 2010.

Mass-degenerate heavy vector mesons at hadron colliders

We study the LHC phenomenology of two mass-degenerate heavy gauge bosons with the same quantum numbers as Z and γ. We give a leading-order estimate of the number of events expected in Drell–Yan processes in terms of the parameters of the model (mass and coupling) and of the LHC machine specifications (integrated luminosity and energy). We consider the feasibility of measuring a forward–backward asymmetry for various choices of the parameters and estimate the potential reach. We comment on how the results may affect future collider design and apply our results to a specific model of electro-weak symmetry breaking by way of example.

Gauge invariance, causality and gluonic poles

We explore the electromagnetic gauge invariance of the hadron tensor of the Drell–Yan process with one transversely polarized hadron. The special role is played by the contour gauge for gluon fields. The prescription for the gluonic pole in the twist 3 correlator is related to causality property and compared with the prescriptions for exclusive hard processes. As a result we get the extra contributions, which naively do not have an imaginary phase. The single spin asymmetry for the Drell–Yan process is accordingly enhanced by the factor of two.

Composite vectors at the Large Hadron Collider

An unspecified strong dynamics may give rise to composite vectors sufficiently light that their interactions, among themselves or with the electroweak gauge bosons, be approximately described by an effective Lagrangian invariant under $SU(2)_L\times SU(2)_R/ SU(2)_{L+R}$. We study the production at the LHC of two such states by vector boson fusion or by the Drell--Yan process in this general framework and we compare it with the case of gauge vectors from a $SU(2)_L\times SU(2)_R\times SU(2)^N$ gauge model spontaneously broken to the diagonal SU(2) subgroup by a generic $\sigma$-model. Special attention is payed to the asymptotic behaviour of the different amplitudes in both cases. The expected rates of multi-lepton events from the decay of the composite vectors are also given. A thorough phenomenological analysis and the evaluation of the backgrounds to such signals, aiming at assessing the visibility of composite-vector pairs at the LHC, is instead deferred to future work.

Single Transverse Spin Asymmetries at Parton Level

Two factorization approaches have been proposed for single transverse spin asymmetries. One is the collinear factorization, the other is the transverse-momentum-dependent factorization. They have been previously derived in a formal way by using diagram expansion at hadron level. If the two factorizations hold or can be proven, they should also hold when we replace hadrons with parton states. We examine these two factorizations at parton level with massless partons. It is nontrivial to generate these asymmetries at parton level with massless partons because the asymmetries require helicity-flip and nonzero absorptive parts in scattering amplitudes. By constructing suitable parton states with massless partons we derive the two factorizations for the asymmetry in Drell–Yan processes. It is found from our results that the collinear factorization derived at parton level is not the same as that derived at hadron level. Our results with massless partons confirm those derived with single massive parton state in our previous works.

Polarization effects in Drell-Yan processes

Effects of polarization of hadrons and constituent quarks in Drell-Yan processes are considered; they are one of the most efficient tools for investigation of the quark structure of hadrons. Special attention is paid to such important parton distribution functions as the transversity and T-odd Sivers and Boer—Mulders functions whose study is necessary for understanding the effects connected with the nonzero transverse component of the quark momentum. An original method for direct extraction of transversity and Boer—Mulders function in the proton from the data on Drell—Yan processes, in which a maximum of one hadron in the initial state is transversely polarized, is presented. This method possesses a number of important advantages. The method is applied both to Drell—Yan processes with a valence antiquark (antiproton-proton and pion-proton collisions) and with a sea antiquark (proton-proton, proton-deuteron, and deuteron-deuteron collisions). Theoretical estimates of asymmetries and cross sections for setups at RHIC (BNL, US), NICA (JINR, Russia), COMPASS (CERN, Switzerland), PAX (GSI, Germany), and J-PARC (Japan) are presented for evaluation of the measurability of transversity and T-odd distributions. These theoretical estimates are accompanied by calculations of statistical uncertainties for measured asymmetries using the new Monte Carlo generator of Drell—Yan events. The duality of Drell—Yan processes and those of production of J/Φ resonance is studied, and it may allow one to considerably reduce statistical uncertainties of parton distributions. Kinematical conditions, for which this duality can be observed, are evaluated.

HADRON STRUCTURE STUDY IN FORTHCOMING DRELL–YAN EXPERIMENTS: COMPASS PROJECT AT CERN

The study of Drell–Yan (DY) processes involving the collision of an (un)polarised hadron beam on an (un)polarised proton target can result in a fundamental improvement of our knowledge on the transverse momentum dependent (TMDs) parton distribution functions (PDFs) of hadrons. The production mechanism of J/ψ and J/ψ - DY duality can also be addressed. One of the forthcoming polarised DY experiments (COMPASS (SPS, CERN)) is discussed in this context. The most important features of this project are briefly reviewed, as well as its sensitivity to the various transverse momentum dependent spin asymmetries.

Radiative corrections in K e4 decay

The final state interaction of pions in the decay K±→π+π−e±ν allows one to obtain the value of the isospin and angular momentum zero pion–pion scattering length a00. To extract this quantity from experimental data the radiative corrections (RC) have to be taken into account. Based on the lowest order results and the factorization hypothesis, we get the expressions for RC in the leading and next-to leading logarithmical approximation. It is shown that the decay width dependence on the lepton mass me through the parameter \(\sigma=\frac{\alpha}{2\pi}(\ln\frac{M^{2}}{m_{e}^{2}}-1)\) has the standard form of the Drell–Yan process and is proportional to the Sommerfeld–Sakharov factor. The numerical estimations are presented.

APPLIED HIGH ENERGY QCD

This review stresses the theoretical elements that underlie a wide range of phenomenological studies of high-energy QCD, which include both soft and hard processes. After a brief introduction to the basics of QCD, various aspects of QCD-based phenomenology are covered: color transparency, hadronization of color charges, Regge phenomenology, parton model, Bjorken scaling and its violation, DGLAP evolution equation, BFKL formalism, GLR-MQ evolution equation and saturation. In the last part of the review, we employ the light-cone dipole formalism to describe deep inelastic lepton scattering, Drell — Yan processes, direct photon production, diffraction, quark and gluon shadowing in nuclei, the Cronin effect and nuclear broadening.

High energy resummation of Drell–Yan processes

We present a computation of the inclusive Drell–Yan production cross-section in perturbative QCD to all orders in the limit of high partonic centre-of-mass energy. We compare our results to the fixed order NLO and NNLO results in MS ¯ scheme, and provide predictions at N 3LO and beyond. Our expressions may be used to obtain fully resummed results for the inclusive cross-section.

Transversity and T-odd PDFs from Drell–Yan processes with pp, pD and DD collisions

We estimate the single-spin asymmetries (SSA) which provide the access to transversity as well as to Boer–Mulders and Sivers PDFs via investigation of the single-polarized Drell–Yan (DY) processes with pp, pD and DD collisions available to RHIC, NICA, COMPASS, and J-PARC. The feasibility of these SSA is studied with the new generator of polarized DY events. The estimations performed demonstrate that there exist kinematical regions where SSA are presumably measurable. Most useful for PDFs extraction are the limiting kinematical ranges, where one can neglect the sea PDFs contributions which occur at large values of the Bjorken variable x. It is of interest that, contrary to the Sivers PDF, the transversity PDF is presumably accessible only in a particular kinematical region. Contrary to the option with the symmetric collider mode (RHIC, NICA), this is of importance for the COMPASS experiment and the future J-PARC facility, where the fixed-target mode is available.

Enhanced production of TeV right-handed neutrinos through the large magnetic moment

We examine the effects of magnetic moments of right-handed neutrinos, whose masses are set at around TeV scale, then it is plausible to have a large enhancement for the production cross section of TeV scale right-handed neutrinos though the Drell–Yan process, e+e−→γ, Z∗→NiNj(i≠j), which is within the reach of the future linear collider (ILC).

Flavorful [formula omitted] signatures at LHC and ILC

There are lots of new physics models which predict an extra neutral gauge boson, referred as Z′-boson. In a certain class of these new physics models, the Z′-boson has flavor-dependent couplings with the fermions in the Standard Model (SM). Based on a simple model in which couplings of the SM fermions in the third generation with the Z′-boson are different from those of the corresponding fermions in the first two generations, we study the signatures of Z′-boson at the Large Hadron Collider (LHC) and the International Linear Collider (ILC). We show that at the LHC, the Z′-boson with mass around 1 TeV can be produced through the Drell–Yan processes and its dilepton decay modes provide us clean signatures not only for the resonant production of Z′-boson but also for flavor-dependences of the production cross sections. We also study fermion pair productions at the ILC involving the virtual Z′-boson exchange. Even though the center-of-energy of the ILC is much lower than a Z′-boson mass, the angular distributions and the forward–backward asymmetries of fermion pair productions show not only sizable deviations from the SM predictions but also significant flavor-dependences.

Drell–Yan double-spin asymmetry [formula omitted] in polarized [formula omitted] collisions: Wandzura–Wilczek contribution

The longitudinal-transverse spin asymmetry ALT in the polarized Drell–Yan process depends on the twist-3 spin-dependent distributions of nucleon. In addition to the contributions expressed as matrix element of the twist-3 operators, these distributions contain the so-called Wandzura–Wilczek part, which is completely determined by a certain integral of the twist-2 spin-dependent parton distributions. We demonstrate that the recently obtained empirical information on the transversity distribution allows a realistic estimate of the Wandzura–Wilczek contribution to ALT for the case of polarized proton–antiproton collisions. In particular, our results in the Wandzura–Wilczek approximation indicate that rather large ALT can be observed in the proposed spin experiments at GSI, and its behavior as a function of dilepton mass obeys novel pattern, compared with the other double-spin asymmetries ATT and ALL. Our results provide a guide for testing a signal of effects originating from the twist-3 operators associated with quark–gluon correlation.

Inhomogeneous Shadowing Effect in High-Energy p-A Drell–Yan Process

Having studied the initial state energy loss versus nuclear shadowing for the Drell–Yan dimuon pair production in the color string model, the inhomogeneous shadowing effect is considered in this paper. We find that the inhomogeneous shadowing effect does amend the rate of energy loss per unit path length, -dE/dz. Finally, the theoretical results for the Drell–Yan differential cross-section ratios are compared with the E772 and E866 data. It is found that the theoretical results are in good agreement with the experimental data.