Gauge Boson Production Research Articles

Higgs-boson production at small transverse momentum

Using methods from effective field theory, we have recently developed a novel, systematic framework for the calculation of the cross sections for electroweak gauge-boson production at small and very small transverse momentum q_T, in which large logarithms of the scale ratio m_V/q_T are resummed to all orders. This formalism is applied to the production of Higgs bosons in gluon fusion at the LHC. The production cross section receives logarithmically enhanced corrections from two sources: the running of the hard matching coefficient and the collinear factorization anomaly. The anomaly leads to the dynamical generation of a non-perturbative scale q_* ~ m_H e^{-const/\alpha_s(m_H)} ~ 8 GeV, which protects the process from receiving large long-distance hadronic contributions. We present detailed numerical predictions for the transverse-momentum spectrum of the Higgs boson, finding that it is quite insensitive to hadronic effects.

Massive neutral gauge boson production as a probe of nuclear modifications of parton distributions at the LHC

We analyze the role of nuclear modifications of parton distributions, notably, the nuclear shadowing and antishadowing corrections, in production of lepton pairs from decays of neutral electroweak gauge bosons in proton-lead and lead-collisions at the LHC. Using the Collins-Soper-Sterman resummation formalism that we extended to the case of nuclear parton distributions, we observed a direct correlation between the predicted behavior of the transverse momentum and rapidity distributions of the produced vector bosons and the pattern of quark and gluon nuclear modifications. This makes Drell-Yan pair production in $pA$ and $AA$ collisions at the LHC a useful tool for constraining nuclear PDFs in the small-$x$ shadowing and moderate-$x$ antishadowing regions.

Searching for supersymmetry inZ’decays

I investigate production and decay of heavy neutral gauge bosons Z' in GUT-inspired U(1)' groups and in the Sequential Standard Model. In particular, decays into supersymmetric particles, such as slepton, chargino and neutralino pairs, as predicted in the MSSM, are accounted for, with a special interest in final states with leptons and missing energy. For a representative point of the parameter space, it is found that the inclusion of supersymmetric decay modes has an impact of 200-300 GeV on the Z' mass exclusion limits.

Diffractive gauge bosons production beyond QCD factorization

We discuss single diffractive gauge boson $({\ensuremath{\gamma}}^{*},{W}^{\ifmmode\pm\else\textpm\fi{}},Z)$ production in proton-proton collisions at different (Large Hadron Collider and Relativistic Heavy Ion Collider) energies within the color dipole approach. The calculations are performed for gauge bosons produced at forward rapidities. The diffractive cross section is predicted as a function of the fractional momentum and invariant mass of the lepton pair. We found a dramatic breakdown of the diffractive QCD factorization caused by an interplay of hard and soft interactions. Data from the CDF experiment on diffractive production of $W$ and $Z$ are well explained in a parameter-free way.

WWZ/γproduction in the large extra dimensions model at the LHC and ILC

We investigate the effect induced by the Kaluza-Klein (KK) graviton in the ${W}^{+}{W}^{\ensuremath{-}}\ensuremath{\gamma}/Z$ production in the framework of the large extra dimensions (LED) model at both the CERN Large Hadron Collider (LHC) and the International Linear Collider (ILC). The integrated cross sections and various kinematic distributions in the LED model are presented and compared with those in the standard model. The results show that the contributions from KK-graviton exchange remarkably affect the observables of the triple gauge boson (${W}^{+}{W}^{\ensuremath{-}}\ensuremath{\gamma}/Z$) production processes at both the ILC and LHC, particularly either in the high transverse momentum region or in the central rapidity region. We also find that the relative LED discrepancy for the ${W}^{+}{W}^{\ensuremath{-}}\ensuremath{\gamma}/Z$ production at the LHC is generally larger than that at the ILC due to the additional LED contribution via $gg$ fusion subprocess and the KK-graviton exchanging resonant effect induced by the continuous large colliding energy in $pp$ collision. We conclude that the ${W}^{+}{W}^{\ensuremath{-}}\ensuremath{\gamma}$ and ${W}^{+}{W}^{\ensuremath{-}}Z$ productions at the LHC could have the distinct advantage over at the ILC from the aspect of effectively exploring the LED signal in measuring ${W}^{+}{W}^{\ensuremath{-}}\ensuremath{\gamma}/Z$ production.

Parton distributions with LHC data

We present the first determination of parton distributions of the nucleon at NLO and NNLO based on a global data set which includes LHC data: NNPDF2.3. Our data set includes, besides the deep inelastic, Drell–Yan, gauge boson production and jet data already used in previous global PDF determinations, all the relevant LHC data for which experimental systematic uncertainties are currently available: ATLAS and LHCb W and Z rapidity distributions from the 2010 run, CMS W electron asymmetry data from the 2011 run, and ATLAS inclusive jet cross-sections from the 2010 run. We introduce an improved implementation of the FastKernel method which allows us to fit to this extended data set, and also to adopt a more effective minimization methodology. We present the NNPDF2.3 PDF sets, and compare them to the NNPDF2.1 sets to assess the impact of the LHC data. We find that all the LHC data are broadly consistent with each other and with all the older data sets included in the fit. We present predictions for various standard candle cross-sections, and compare them to those obtained previously using NNPDF2.1, and specifically discuss the impact of ATLAS electroweak data on the determination of the strangeness fraction of the proton. We also present collider PDF sets, constructed using only data from HERA, the Tevatron and the LHC, but find that this data set is neither precise nor complete enough for a competitive PDF determination.

Gauge-boson production with multiple jets near threshold

Signatures of new physics beyond the Standard Model are often characterized by large missing transverse energy ($\not E_T$) produced in association with multiple jets. The dominant Standard Model background to such processes comes from gauge-boson production in association with jets. A standard search strategy involves looking for an excess in the $m_{eff}$ distribution, where $m_{eff}= \not E_T +\sum_{J} p^T_J$ and $p^T_J$ denotes the transverse momentum of the $J$-th jet. The region of large $m_{eff}$ is dominated by jet production near threshold, giving rise to large Sudakov logarithms that can change the magnitude and shape of the $m_{eff}$ distribution. We present an effective theory framework for the resummation of such threshold logarithms. We perform an analysis for exclusive jet production using the N-jettiness global event shape, which allows theoretical control to also be maintained over large logarithms induced by vetoing additional jets. As a first step, we give explicit numerical results with next-to-leading-log (NLL) resummation for $pp \to \gamma + 2$ jets in the large $m_{eff}$ region.

Gauge boson production at colliders – Predictions for precision studies

The status of today’s theoretical description of Standard Model (SM) processes involving gauge bosons at the Tevatron and the LHC, and of the tools that are used in their phenomenological studies are reviewed. A few recent ideas to further improve on the way technically challenging calculations can be performed also are discussed.

Neutral triple gauge boson production in the large extra dimensions model at linear colliders

We consider the neutral triple-gauge boson production process in the context of large extra dimensions (LED) models including the Kaluza-Klein (KK) excited gravitons at future linear colliders, say ILC(CLIC). We consider $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\gamma}$, $\ensuremath{\gamma}\ensuremath{\gamma}Z$, $\ensuremath{\gamma}ZZ$, and $ZZZ$ production processes, and analyze their impacts on both the total cross section and some key distributions. These processes are important for new physics searches at linear colliders. Our results show that KK graviton exchange has the most significant effect on ${e}^{\ensuremath{-}}{e}^{+}\ensuremath{\rightarrow}\ensuremath{\gamma}ZZ$ among the four processes with relatively small ${M}_{S}$, while it has the largest effect on ${e}^{\ensuremath{-}}{e}^{+}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\gamma}$ with larger ${M}_{S}$. By using the neutral triple-gauge boson production we could set the discovery limit on the fundamental Plank scale ${M}_{S}$ up to around 6--9 TeV for $\ensuremath{\delta}=4$ at the 3 TeV CLIC.

LHC signals of noncustodial warped 5D models

We study the implications at the LHC for a recent class of noncustodial warped extra-dimensional models where the ${\mathrm{AdS}}_{5}$ metric is modified near the infrared brane. Such models allow for TeV Kaluza-Klein excitations without conflict with electroweak precision tests. We discuss both the production of electroweak and strong Kaluza-Klein gauge bosons. As we will show, only signals involving the third generation of quarks seem to be feasible in order to probe this scenario.

Neutral triple electroweak gauge boson production in the large extra-dimension model at the LHC

We study the prospects of probing large extra-dimension models at the LHC through neutral triple gauge boson production processes. In theories with extra dimensions these processes result from the exchange of a tower of massive graviton modes between the SM particles. We consider $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\gamma}$, $\ensuremath{\gamma}\ensuremath{\gamma}Z$, $\ensuremath{\gamma}ZZ$, and $ZZZ$ production processes, and present our results for various kinematic distributions at the LHC for $\sqrt{S}=14\text{ }\text{ }\mathrm{TeV}$.

Electroweak gauge-boson production at small q T : Infrared safety from the collinear anomaly

Using methods from effective field theory, we develop a novel, systematic framework for the calculation of the cross sections for electroweak gauge-boson production at small and very small transverse momentum q_T, in which large logarithms of the scale ratio M_V/q_T are resummed to all orders. These cross sections receive logarithmically enhanced corrections from two sources: the running of the hard matching coefficient and the collinear factorization anomaly. The anomaly leads to the dynamical generation of a non-perturbative scale q_* ~ M_V e^{-const/\alpha_s(M_V)}, which protects the processes from receiving large long-distance hadronic contributions. Expanding the cross sections in either \alpha_s or q_T generates strongly divergent series, which must be resummed. As a by-product, we obtain an explicit non-perturbative expression for the intercept of the cross sections at q_T=0, including the normalization and first-order \alpha_s(q_*) correction. We perform a detailed numerical comparison of our predictions with the available data on the transverse-momentum distribution in Z-boson production at the Tevatron and LHC.

QCD results from the LHC

A selection of results from the 2010 data taking period of the ATLAS and CMS experiments at the LHC at a proton-proton centre-of-mass energy of s = 7 TeV is presented. These results comprise differential jet cross sections for varying jet multiplicities, the investigation of properties of large rapidity gaps spanned by a dijet system, the production of heavy gauge bosons together with jets, and finally the investigations of properties of top quark pair production.

SUSY QCD corrections to electroweak gauge boson production with an associated jet at the LHC

We study the stability of the neutral- and charged-current Drell-Yan process in association with a jet as a standard candle at the LHC under the inclusion of O(alpha_s) supersymmetric QCD (SQCD) corrections within the MSSM. We include the decay of the electroweak gauge boson into dileptons, i.e. we consider the production of charged lepton--anti-lepton or lepton-neutrino final states with one hard jet. We find that the SQCD corrections are negligible for the integrated cross section. Only at high lepton transverse momentum can they induce effects of the percent level.

Phenomenological aspects of new physics at high energy hadron colliders

A general introduction is given in chapter 1. Chapter 2 outlines the main features of the Standard Model (SM) of particle physics and the theoretical motivations for going beyond it. We subsequently provide brief descriptions of a few popular models that aim to solve the issues that arise within the SM. In chapter 3 we describe the general Monte Carlo method and show how it can be used to construct a class of computational tools called Monte Carlo event generators. We describe the main generic features of event generators and how these are implemented in the HERWIG++ event generator. By applying resummation techniques, we provide, in chapter 4, analytical calculations of two types of hadron collider observables: global inclusive variables and the transverse energy of the QCD initial state radiation (ET), associated with the either Drell-Yan gauge boson production or Higgs boson production. In both cases we provide comparisons to results obtained from Monte Carlo event generators. In chapter 5 we examine two well-motivated models for new physics: one of new heavy charged vector bosons (W prime) and a model motivated by strong dynamics electroweak symmetry breaking that contains new resonances, leptoquarks, that couple primarily to quarks and leptons of the third generation. In the prior model, we improve the current treatment of the W' by considering interference effects with the SM W and construct an event generator accurate to next-to-leading order which we use to conduct a phenomenological analysis. For the leptoquark model, we provide an implementation in the HERWIG++ event generator and use it to form a strategy for mass reconstruction.

NNLO benchmarks for gauge and Higgs boson production at TeV hadron colliders

The inclusive production cross sections for W+,W− and Z0-bosons form important benchmarks for the physics at hadron colliders. We perform a detailed comparison of the predictions for these standard candles based on recent next-to-next-to-leading order (NNLO) parton parameterizations and new analyses including the combined HERA data, compare to all available experimental results, and discuss the predictions for present and upcoming RHIC, SPS, Tevatron and LHC energies. The rates for gauge boson production at the LHC can be rather confidently predicted with an accuracy of better than about 10% at NNLO. We also present detailed NNLO predictions for the Higgs boson production cross sections for Tevatron and LHC energies (1.96, 7, 8, 14 TeV), and propose a possible method to monitor the gluon distribution experimentally in the kinematic region close to the mass range expected for the Higgs boson. The production cross sections of the Higgs boson at the LHC are presently predicted with an accuracy of about 10–17%. The inclusion of the NNLO contributions is mandatory for achieving such accuracies since the total uncertainties are substantially larger at NLO.

Noncommutative standard model and polarization in charged gauge boson production at the LHC

We study the pair production of charged gauge bosons at the LHC in a noncommutative extension of the standard model. We use angular distributions in the decays of the gauge bosons to partially reconstruct polarized cross sections. We use this, together with CP considerations, to construct more sensitive observables that allow to separate space-time from space-space noncommutativities.

LHC signals for coset electroweak gauge bosons in warped/composite pseudo-Goldstone boson Higgs models

The framework of a warped extra dimension with the Standard Model (SM) fields propagating in it is a very well-motivated extension of the SM since it can address both the Planck-weak and flavor hierarchy problems of the SM. Within this framework, solution to the little hierarchy problem motivates extending the SM electroweak (EW) 5D gauge symmetry in such a way that its breakdown to the SM delivers the SM Higgs boson. We study signals at the large hadron collider (LHC) for the extra EW (called coset) gauge bosons, a fundamental ingredient of this framework. The coset gauge bosons, due to their unique EW gauge quantum numbers [doublets of SU(2)_L], do not couple at leading order to two SM particles. We find that, using the associated production of the charged coset gauge bosons via their coupling to SM bottom quark and a (light) KK top quark, the LHC can have a 3\sigma reach of \sim 2 (2.6) TeV for the coset gauge boson masses with \sim 100 (1000) fb^{-1} luminosity. Since current theoretical framework(s) suggest an {\em indirect} lower limit on coset gauge boson masses of >3 TeV, luminosity or energy upgrade of LHC is likely to be crucial in observing these states.

Radiative corrections to longitudinal and transverse gauge boson and Higgs production

Radiative corrections to gauge boson and Higgs production computed recently using soft-collinear effective theory (SCET) require the one-loop high-scale matching coefficients in the standard model. We give explicit expressions for the matching coefficients for the effective field theory (EFT) operators for q qbar -> VV and q qbar -> phi^+ phi for a general gauge theory with an arbitrary number of gauge groups. The group theory factors are given explicitly for the standard model, including both QCD and electroweak corrections.

Soft and collinear functions for the standard model

Radiative corrections to high-energy scattering processes were given previously in terms of universal soft and collinear functions. This paper gives the collinear functions for all standard model particles, the general form of the soft function, and explicit expressions for the soft functions for fermion-fermion scattering, longitudinal and transverse gauge boson production, single $W/Z$ production, and associated Higgs production. An interesting subtlety in the use of the Goldstone boson equivalence theorem for longitudinal ${W}^{+}$ production is discussed.