Electroweak radiative corrections to hadronic precision observables at TeV energies
We illustrate the impact of full one-loop weak corrections onto b-jet-, prompt-photon- and Z-production at Tevatron and Large Hadron Collider (LHC)
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arXiv subjects
E. Maina
Publications and source records attributed to E. Maina.
One-loop weak corrections to the $b\bar b$ cross section at TeV energy hadron colliders
We investigate one-loop weak corrections to the production cross section of two $b$-jets at Tevatron and Large Hadron Collider (LHC). We establish that they are small at inclusive level but dominant in exclusive observables that have a non-trivial dependence on the helicity structure of the hard subprocesses. Such effects can serve as a test of the Standard Model (SM) and, conversely, they should be taken into account in future experimental analyses aiming at extracting possible signals of new physics.
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Weak Corrections to Three-Jet Production in Electron-Positron Annihilations: 1) The Factorisable Contributions
We report on the calculation of the factorisable one-loop weak-interaction corrections to the initial and final states for three-jet observables in electron-positron annihilations. We show that such corrections are of a few percent at $\sqrt s=M_Z$. Hence, while their impact is not dramatic in the context of LEP1 and SLC, where the total error on the measured value of $α_{\mathrm{S}}$ is larger, at a future Linear Collider, running at the $Z$ mass peak (e.g., GigaZ), they ought to be taken into account in the experimental fits, as here the uncertainty on the value of the strong coupling constant is expected to be at the 0.1% level or even smaller. The calculation has been performed using helicity amplitudes so that it can be applied to the case of polarised beams.
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WPHACT 2.0: A fully massive Monte Carlo generator for four fermion physics at e^+ e^- colliders
WPHACT 2.0 is the new fully massive version of a MC program and unweighted event generator which computes all Standard Model processes with four fermions in the final state at e^+ e^- colliders. The program can now generate unweighted events for any subset of all four fermion final states in a single run, by making use of dedicated pre-samples which can cover the entire phase space. Improvements with respect to WPHACT 1.0 include the Imaginary Fermion Loop gauge restoring scheme, new phase space mappings, a new input system, the possibility to compute subsets of Feynman diagrams and options for including ISR via QEDPS, running alpha_{QED}, CKM mixing, resonances in q bar(q) channels.
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Effects of jet algorithms from higher order QCD in $W^\pm$ mass determinations at LEP2
We analyse the impact of systematic effects due to the scale dependence of QCD corrections in combination with the use of different jet clustering algorithms in the measurement of the $W^\pm$ mass in the fully hadronic decay mode of $W^+W^-$ pairs produced at LEP2. We consider higher order contributions induced by both virtual and real gluon radiation onto the electroweak CC03 and CC11 channels through ${\cal O}(α_{s})$ at the parton level. We find that the associated uncertainties can be of order 100 MeV, thus competitive with those possibly arising in the non-perturbative regime and indeed above the current experimental estimates.
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Four-Fermion Production in Electron-Positron Collisions
This report summarises the results of the four-fermion working group of the LEP2-MC workshop, held at CERN from 1999 to 2000. Recent developments in the calculation of four-fermion processes in electron-positron collisions at LEP-2 centre-of-mass energies are presented, concentrating on predictions for four main reactions: W-pair production, visible photons in four-fermion events, single-W production and Z-pair production. Based on a comparison of results derived within different approaches, theoretical uncertainties on these predictions are established.
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QCD corrections to $e^+ e^- \to u~\bar d~s~\bar c$ at Lep 2 and the Next Linear Collider: CC11 at O(α_s)
QCD one-loop corrections to the full gauge invariant set of electroweak diagrams describing the hadronic process $e^+ e^- \to u~\bar d~s~\bar c$ are computed. Four-jet shape variables for $WW$ events are studied at next-to-leading order and the effects of QCD corrections on the determination of the $W$--mass in the hadronic channel at Lep 2 and NLC is discussed. We compare the exact calculation with a ``naive''approach to strong radiative corrections which has been widely used in the literature.
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QCD corrections to charged current processes at the Next Linear Collider: CC10, CC11 and CC20 at O(alpha_s)
One-loop QCD corrections to the full gauge invariant set of electroweak diagrams describing the charged current processes $e^+ e^- \to u \bar d μ\barν_μ$ (CC10), $e^+ e^- \to u \bar d e^- \barν_e$ (CC20) and $e^+ e^- \to u \bar d s \bar c$ (CC11) are computed. We compare the exact calculation with a ``naive''approach to strong radiative corrections which has been widely used in the literature and discuss the phenomenological relevance of QCD corrections for NLC physics.
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Report of the Working Group on `W Mass and QCD' (Phenomenology Workshop on LEP2 Physics, Oxford, April 1997)
The W Mass and QCD Working Group discussed a wide variety of topics relating to present and future measurements of M(W) at LEP2, including QCD backgrounds to W+W- production. Particular attention was focused on experimental issues concerning the direct reconstruction and threshold mass measurements, and on theoretical and experimental issues concerning the four jet final state. This report summarises the main conclusions.
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CC10 at ${\cal O}(α_s )$: QCD corrections to $e^+ e^- \to μ^- \bar ν_μu \bar d$ at LEP2 and the Next Linear Collider
QCD one-loop corrections to the semileptonic process $e^+ e^- \to μ^- \bar ν_μu \bar d$ are computed. We compare the exact calculation with a ``naive'' approach to strong radiative corrections which has been widely used in the literature and discuss the phenomenological relevance of QCD contributions for LEP2 and NLC physics.
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$b \bar b W^+ W^-$ production at hadron colliders. Top signal and irreducible backgrounds
We compute complete tree level matrix elements for $gg , q \bar q \rightarrow b \bar b W^+W^-$. We analyze the irreducible backgrounds to top signal at the Tevatron and at the LHC. Their contribution to the total cross section is about $5 \%$ at the LHC, due to single resonant channels. Several distributions with contributions from signal and backgrounds are presented.
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Perturbative rates and colour rearrangement effects in four-jet events at LEP2
An important issue in the direct reconstruction method of determining the $W$ mass from $q\bar q Q\bar Q$ events at LEP2 concerns the impact of the relatively unknown QCD interconnection effects. It has been suggested that a study of `short string' states, in which colour singlet states are formed from $q \bar Q $ and $Q\bar q$ pairs with small phase--space separation, could shed important light on this issue. We show that such configurations can also be generated by conventional background $e^+e^-\ar 4$~parton processes, in particular QCD $q \bar q g g$ and $q \bar q Q \bar Q$ and non--resonant electroweak $q \bar q Q \bar Q$ production. We study the colour and kinematic structure of these background contributions, and estimate the event rate to be expected at LEP2. We find that the QCD processes are heavily suppressed, but that non--resonant $q \bar q Q \bar Q$ production may be comparable in rate to the expected `short string' signal from $W^+W^-$ production.
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QCD corrections to the production of a heavy quark pair plus a hard photon in $e^+ e^-$ annihilation
We present complete results for the $O(α_s)$ corrections to the production of a heavy quark pair plus a hard photon, in $e^+ e^-$ annihilation, including both photon and $Z$ intermediate states. Virtual corrections are calculated analytically in $4 - 2 ε$ dimensions, together with the soft approximation to the real emission diagrams. After cancellation of infrared divergences, real and virtual contributions are combined numerically to obtain the cross section for the production of two heavy quark jets plus a hard photon in next-to-leading order. The resulting fully differential cross section can be used to construct arbitrary distributions with the desired experimental cuts.
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Determination of the mass of the W boson
Previous studies of the physics potential of LEP2 indicated that with the design luminosity of 500 inverse picobarn one may get a direct measurement of the mass of the W-boson with a precision in the range 30 - 50 MeV. This report presents an updated evaluation of the estimated error on the mass of the W-boson based on recent simulation work and improved theoretical input. The most efficient experimental methods which will be used are also described.
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$e^+e^- \to b\bar b W^+W^-$ events at the Next Linear Collider: colour structure of top signal and irreducible background
We examine the colour structure and charged particle yield for both the $t \bar t$ signal and the irreducible background processes contributing to \eebbww\ production close to the \ttb\ threshold. The charged particle multiplicity for the various components of the cross section is computed as a function of several kinematic variables. Our study may have important implications for recently proposed studies of interconnection phenomena in \ttb\ production at high--energy $e^+e^-$ colliders
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NLO QCD corrections to the production of off--shell WW pairs at $e^+ e^-$ colliders
QCD corrections to $e^+e^- \ra WW \ra q_1 \bar q_1 q_2 \bar q_2$ are computed and presented in a form which allows to impose realistic cuts on the structure of the observed events. QCD radiation substantially modifies the jet--jet invariant mass distributions from which the value of the $W$ mass will be extracted. When the range of allowed jet--jet masses is restricted, as it has been proposed in order to suppress non--$WW$ backgrounds, the total cross section is also deeply affected. If all events are forced to four jets, combining the two partons with smallest invariant mass, and the reconstructed masses are required to satisfy $ \left| M_{Ri} -M_W \right| \le 10$ GeV $i$=1,2 the lowest order cross section can be reduced by more than 40\%.
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Six--jet decay of off--shell WW pairs at $e^+ e^-$ colliders
Six--jet events via WW pairs, $e^+e^- \ar W^+W^- \ar q_1 \overline{q_1} q_2 \overline{q_2} g g$ are studied at tree level using helicity amplitudes. This is the dominant production mechanism for six--jet final states at Lep II energy. ISR effects are taken into account. Total production rates as a function of $y_{cut}$ are given. The relevance of these processes for the issue of colour reconnection is discussed. The cross section for five--jet production via WW pairs at Lep II is also presented.
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Higgs Production and Decay via $e^+e^-\rightarrow Z^0 H^0 \rightarrow b\bar bW^+W^-$ and Irreducible Backgrounds at the Next Linear Collider
The complete matrix element for $e^+e^-\ar b\bar bW^+W^-$ has been computed at tree--level and applied to $Z^0H^0$ production followed by $Z^0\ar b\bar b$ and $H^0\ar W^+W^-$, keeping into account all irreducible backgrounds, which are dominated by \ttb production, at the Next Linear Colliders. We find that, depending on the center of mass energies and on the search strategies, this channel can be useful for the study of the parameters of the Standard Model Higgs boson over the most part of the heavy mass range.
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