arXiv subjects
Publications and source records attributed to J. Ohnemus.
The processes $pp/ pbar p \to Z γ+ X \to \ell^+ \ell^- γ+ X$ ($\ell=e, μ$) and $pp/ pbarp \to Z γ+ X \to \barννγ+ X$ are calculated to ${\cal O}(α_s)$ for general $ZZγ$ and $Zγγ$ couplings. The impact of ${\cal O}(α_s)$ QCD corrections on the observability of $ZZγ$ and $Zγγ$ couplings in $Zγ$ production at the Tevatron and the Large Hadron Collider (LHC) is discussed.
The process $p\,p\hskip-7pt\hbox{$^{^{(\!-\!)}}$} \rightarrow W^{+} W^{-} + X \rightarrow \ell^+_1 ν_1 \ell^-_2 \bar ν_2 + X$ is calculated to ${\cal O}(α_s)$ for general $C$ and $P$ conserving $WWV$ couplings ($V=γ,\, Z$). The prospects for probing the $WWV$ couplings in this reaction are explored. The impact of ${\cal O}(α_s)$ QCD corrections and various background processes on the observability of non-standard $WWV$ couplings in $W^+ W^-$ production at the Tevatron and the Large Hadron Collider (LHC) is discussed in detail. Sensitivity limits for anomalous $WWV$ couplings are derived at next-to-leading order for the Tevatron and LHC center of mass energies, and are compared to the bounds which can be achieved in other processes. Unless a jet veto or a cut on the total transverse momentum of the hadrons in the event is imposed, the ${\cal O}(α_s)$ QCD corrections and the background from top quark production decrease the sensitivity of $p\,p\hskip-7pt\hbox{$^{^{(\!-\!)}}$} \rightarrow W^{+} W^{-} + X \rightarrow \ell^+_1 ν_1 \ell^-_2 \bar ν_2 + X$ to anomalous $WWV$ couplings by a factor two to five.
We discuss the direct measurement of the trilinear vector boson couplings in present and future collider experiments. The major goals of such experiments will be the confirmation of the Standard Model (SM) predictions and the search for signals of new physics. We review our current theoretical understanding of anomalous trilinear gauge boson self-interactions. If the energy scale of the new physics is $\sim 1$ TeV, these low energy anomalous couplings are expected to be no larger than ${\cal O}(10^{-2})$. Constraints from high precision measurements at LEP and low energy charged and neutral current processes are critically reviewed.
The QCD radiative corrections to hadronic diboson production are reviewed. The radiative corrections for $W^{\pm}γ$, $Zγ$, $ZZ$, $W^+W^-$, and $W^{\pm} Z$ are discussed. Similarities and differences in the behavior of the order $α_s$ cross sections for these processes are emphasized.
The cross section for producing $WZ$ pairs at hadron colliders is calculated to order $α_s$ for general $C$ and $P$ conserving $WWZ$ couplings. The effects of the next-to-leading-order corrections on the derived sensitivity limits for anomalous $WWZ$ couplings are discussed. The prospects for observing the approximate amplitude zero, which is present in the standard model $WZ$ helicity amplitudes, are also discussed.
We investigate the angular distribution of the lepton pair in the process $p \bar{p} \rightarrow γ^{\ast} + X \rightarrow \ell^+\ell^- + X$, where the virtual photon is produced at high transverse momentum. The angular distribution of the leptons is very sensitive to possible nonperturbative effects, such as a nontrivial vacuum structure of QCD, and offers a good chance to test such effects. We present complete ${\cal O}(α_s^2)$ calculations of the decay lepton distributions in the lepton pair rest frame. An order ${\cal O}(α_s)$ Monte Carlo study of the lepton angular distributions, with acceptance cuts and energy resolution smearing applied to the leptons, is also presented.
The process $p\,\pbar \rightarrow W^{\pm}Z + X \rightarrow \ell^\pm_1 ν_1 \ell_2^+ \ell_2^- + X$ is calculated to ${\cal O}(α_s)$ for general $C$ and $P$ conserving $WWZ$ couplings. At the Tevatron center of mass energy, the QCD corrections to $WZ$ production are modest. At Large Hadron Collider (LHC) energies, the inclusive QCD corrections are large, but can be reduced significantly if a jet veto is imposed. Sensitivity limits for the anomalous $WWZ$ couplings are derived from the next-to-leading order $Z$ boson transverse momentum distribution for Tevatron and LHC energies. Unless a jet veto is imposed, next-to-leading order QCD corrections decrease the sensitivity to anomalous $WWZ$ couplings considerably at LHC energies, but have little influence at the Tevatron. We also study, at next-to-leading order, rapidity correlations between the $W$ and $Z$ decay products, and the $ZZ/WZ$ and $WZ/Wγ$ cross section ratios. These quantities are found to be useful tools in searching for the approximate zero present in the Standard Model $WZ$ helicity amplitudes. The prospects for observing the approximate amplitude zero at the Tevatron and the LHC are critically assessed.
We demonstrate that supersymmetric decay modes of the neutral Higgs bosons of the MSSM could well make their detection extremely difficult when produced singly in $\gam\gam$ collisions at a back-scattered laser beam facility.
Further kinematical variables are suggested, in which to compare the putative leptonic $W$ plus 4-jet top quark signal with the QCD background. We show that the lepton rapidity asymmetry, the $p_T$-ranking of the tagged $b$-jet, the double-tag probability, the reconstructed $t+\bar t$ invariant mass, and the lepton energy in the parent top quark rest-frame, all display interesting differences between signal and background.
The measurement of the angular distribution of leptons from the decay of a $W$, $Z$, $γ^{\ast}$, or $\Jpsi$ produced at high transverse momentum in hadronic collisions provides a detailed test of the production and decay mechanism of the spin one state. In the absence of cuts on the final state leptons, the lepton angular distribution in the lepton pair rest frame is determined by the polarization of the spin one state. At leading order in perturbative QCD the general structure of the decay lepton distribution arising from a $W,Z [γ^{\ast},\Jpsi]$ is controlled by six [four] invariant structure functions. In the presence of cuts, the lepton angular distributions are dominated by kinematic effects rather than polarization effects. We present Monte Carlo studies for Tevatron energies and discuss how polarization effects can be highlighted in the presence of cuts.
The process $p p \to Z γ+ X \to \ell^- \ell^+ γ+ X$, where $\ell$ denotes a lepton, is calculated to order $α_s$. Total and differential cross sections, with acceptance cuts imposed on the leptons and photon, are given for the Tevatron and LHC center of mass energies. In general, invariant mass and angular distributions are simply scaled up in magnitude by the QCD radiative corrections, whereas in transverse momentum distributions, the QCD radiative corrections increase with the transverse momentum.
A Monte Carlo study of the polar and azimuthal angular distributions of the lepton pair arising from the decay of a $W$ or $Z$ boson produced at high transverse momentum in hadronic collisions is presented. In the absence of cuts on the final state leptons, the lepton angular distribution in the gauge boson rest frame is determined by the gauge boson polarization. Numerical results for the lepton angular distributions in the Collins--Soper frame with acceptance cuts and energy resolution smearing applied to the leptons are presented. In the presence of cuts, the lepton angular distributions are dominated by kinematic effects rather than polarization effects, however, some polarization effects are still observable on top of the kinematic effects. Polarization effects are highlighted when the experimental distributions are divided by the Monte Carlo distributions obtained using isotropic gauge boson decay.
Leptoquarks can be produced in pairs by gluon--gluon fusion and quark--antiquark annihilation at hadron colliders. While HERA is the proper machine for single production of $(eu)$ and $(ed)$ type leptoquarks, the flavor species of ($μu$), ($μd$) and ($τu$), ($τd$) type leptoquarks can be produced at hadron colliders very efficiently. Besides exploiting gluon-quark collisions, leptoquarks can also be produced singly by colliding the quarks in one proton beam with leptons $e,μ,τ$ generated by splitting photons which are radiated off the quarks in the other proton beam. For Yukawa couplings of the size $α$ leptoquark masses up to about 300~GeV can be generated at the Tevatron while the LHC can produce leptoquarks with masses up to about 3~TeV. [Leptoquarks involving heavy quarks can be produced singly at a lower rate, determined by the heavy flavor flux in the proton beam.]
The identification of the intermediate-mass Higgs process $γγ\to H \to b \bar b$ will be one of the most important goals of a future photon linear collider. Potentially important backgrounds from the continuum $γγ\to c \bar c, b \bar b$ leading-order processes can be suppressed by a factor $m_q^2/s$ by using polarized photon beams in the $J_z=0$ initial-state configuration. We show that the same $m_q^2/s$ suppressions do not necessarily apply to the radiative processes $γγ\to c \bar c g, b \bar b g$. These processes can mimic the two-jet topology of the Higgs signal when two of the three partons are collinear, or when one of the partons is soft or directed down the beam pipe. We calculate the contribution of these processes to the two-jet background in the $J_z=0$ channel. We investigate the effect of imposing additional event shape, jet width and secondary vertex cuts on both signal and background, and show that with reasonable detector capabilities it should be possible to reduce the background to a manageable level.
The processes $p p \rightarrow V_1 V_2 + X \rightarrow \ell_1 \bar \ell_1 \ell_2 \bar \ell_2 + X$, where $V_i = W^{\pm}$ or $Z$ and $\ell_i$ denotes a lepton, are calculated to ${\cal O}(α_s)$. Total and differential cross sections, with acceptance cuts imposed on the final state leptons, are given for the Tevatron and LHC center of mass energies. Inclusive and exclusive $0$-jet and $1$-jet cross sections are given. The transverse momenta spectra of the leptons are significantly enhanced at high $p_T^{}$ by the QCD radiative corrections, especially at the LHC energy. Invariant mass and angular distributions are scaled up in magnitude by the QCD radiative corrections, but are little changed in shape.
We demonstrate that the Standard Model amplitude for $f_1 \bar f_2 \rightarrow W^\pm Z $ at the Born-level exhibits an approximate zero located at $\cosθ= (g^{f_1}_{-} + g^{f_2}_{-}) / (g^{f_1}_{-} - g^{f_2}_{-})$ at high energies, where the $g^{f_i}_{-}$ ($i=1,2$) are the left-handed couplings of the $Z$-boson to fermions and $θ$ is the center of mass scattering angle of the $W$-boson. The approximate zero is the combined result of an exact zero in the dominant helicity amplitudes ${\cal M}(\pm,\mp)$ and strong gauge cancelations in the remaining amplitudes. For non-standard $WWZ$ couplings these cancelations no longer occur and the approximate amplitude zero is eliminated.
Non--strongly interacting supersymmetric particles -- sleptons, charginos, neutral\-inos, and charged Higgs bosons -- are difficult to detect at the Large Hadron Collider. We therefore examine the possibility of producing particles of this type in virtual $γγ$ collisions at the LHC. Since photons can be emitted from protons which do not break up in the radiation process, very clean events can be generated, compensating to some extent for the small event numbers. Higher rates are expected, at the expense of stray hadrons, for events in which one or two protons break up.
Events with a leptonic W-decay plus jets should contain top-quark signals, but a QCD W+jets background must be separated. We compare transverse W-momentum, jet multiplicity, and b-tagging separation criteria, and find that the main background after tagging comes from mistagging. We illustrate how to extract the mass m_t via event reconstructions and how to confirm signal purity by lepton angular distributions.