Off-shell Cross Sections Research Articles

Higgs boson properties and tests of the Standard Model at CMS

The measurements of Higgs boson properties, mass, spin/CP, couplings, off-shell cross sections and constraints on invisible width are summarized and compared with standard model expectations. The analysis is based on pp collision data collected at centre-of-mass energies of 7 and 8 TeV by the CMS experiment at the LHC, corresponding to integrated luminosities of 5 fb−1 and 20 fb−1, respectively.

Higgs in bosonic channels (CMS)

The main Higgs boson decays into bosonic channels will be considered, presenting and discussing results from the latest reprocessing of data collected by the CMS experiment at the LHC, using the full dataset recorded at centre-of-mass energies of 7 and 8 TeV. For this purpose, results from the final Run-I papers for the H → ZZ → 4ℓ, H → γγ and H → WW analyses are presented, focusing on the Higgs boson properties, like the mass, the signal strenght, the couplings to fermions and vector bosons, the spin and parity properties. Furthermore, the Higgs boson width measurement exploiting the on-shell versus the off-shell cross section (in the H → ZZ → 4ℓ and H → ZZ → 2ℓ2ν decay channels) will be shown.

Higgs in bosonic channels (CMS)

The main Higgs boson decays into bosonic channels will be considered, presenting and discussing results from the latest reprocessing of data collected by the CMS experiment at the LHC, using the full dataset recorded at centre-of-mass energies of 7 and 8 TeV. For this purpose, results from the final Run-I papers for the H → ZZ → 4ℓ, H → γγ and H → WW analyses are presented, focusing on the Higgs boson properties, like the mass, the signal strenght, the couplings to fermions and vector bosons, the spin and parity properties. Furthermore, the Higgs boson width measurement exploiting the on-shell versus the off-shell cross section (in the H → ZZ → 4ℓ and H → ZZ → 2ℓ2ν decay channels) will be shown.

Coupling extraction from off-shell cross sections

In this note, we present a novel method of extracting the couplings of a new heavy particle to the Standard Model states. Contrary to the usual discovery process which involves studying the on-shell production, we look at regions away from resonance to take advantage of the simple scaling of the cross-section with the couplings. We apply the procedure to the case of a heavy quark as an illustration.

Jet Vetoes interfering with H → WW

Far off-shell Higgs production in $H \rightarrow WW,ZZ$, is a particularly powerful probe of Higgs properties, allowing one to disentangle Higgs width and coupling information unavailable in on-shell rate measurements. These measurements require an understanding of the cross section in the far off-shell region in the presence of realistic experimental cuts. We analytically study the effect of a $p_T$ jet veto on far off-shell cross sections, including signal-background interference, by utilizing hard functions in the soft collinear effective theory that are differential in the decay products of the $W/Z$. Summing large logarithms of $\sqrt{\hat s}/p_T^{veto}$, we find that the jet veto induces a strong dependence on the partonic centre of mass energy, $\sqrt{\hat s}$, and modifies distributions in $\sqrt{\hat s}$ or $M_T$. The example of $gg\rightarrow H \rightarrow WW$ is used to demonstrate these effects at next to leading log order. We also discuss the importance of jet vetoes and jet binning for the recent program to extract Higgs couplings and widths from far off-shell cross sections.

Dilepton Production from Parton Interactions in the Early Stage of Relativistic Heavy-Ion Collisions

We address the dilepton production from the parton interactions in the early stage of relativistic heavy-ion collisions within the parton-hadron-string dynamics (PHSD) off-shell transport approach. The description of partons in PHSD is based on the dynamical quasiparticle model (DQPM) matched to reproduce lattice QCD results in thermodynamic equilibrium. According to the DQPM the constituents of the strongly interacting quark-gluon plasma (sQGP) are massive and off-shell quasi-particles (quarks and gluons) with broad spectral functions. In order to address the electromagnetic radiation of the sQGP, we derive off-shell cross sections of qq̄ → γ*, qq̄ → γ*+ g and qg → γ*q (q̄g → γ*q̄) reactions taking into account the effective propagators for quarks and gluons from the DQPM. Dilepton production in In+In collisions at 158 AGeV and in Au+Au at GeV is calculated by implementing these off-shell processes into the PHSD transport approach. By comparing to the data of the NA60 and PHENIX Collaborations, we study the relative importance of different dilepton production mechanisms and point out the regions in phase space where partonic channels are dominant.

Dilepton production at SPS and RHIC energies within the PHSD off-shell transport approach

The microscopic parton-hadron-string dynamics (PHSD) transport approach describes the full evolution of relativistic heavy-ion collisions: from the initial hard scatterings to the partonic phase in the early hot and dense reaction region (when the local energy density is above ≈1 GeV/fm3) followed by hadronization and off-shell hadron propagation and interactions. The description of quarks and gluons in PHSD is based on the dynamical quasiparticle model (DQPM) matched to reproduce lattice QCD results in thermodynamic equilibrium. The DQPM describes QCD properties in terms of single-particle Green's functions and leads to the notion of effective quasiparticles which are massive and have broad spectral functions (due to large interaction rates). Dressing the quark and gluon lines with effective propagators provided by the DQPM, we obtain cross sections of dilepton production in the reactions q + q̄ → l+l− (Born mechanism), q + q̄ → l+l− + g (quark annihilation with the gluon Bremsstrahlung in the final state), and q/q̄ + g → l+l− + q/q̄ (gluon Compton scattering) by off-shell quarks and gluons in the strongly interacting partonic phase. After implementing the off-shell cross sections for the partonic processes into the PHSD transport approach, we address the dilepton production from the parton interactions in the early stage of relativistic heavy-ion collisions at SPS and RHIC energies. Dilepton production in In + In collisions at 158 AGeV and in Au + Au at √s = 200 GeV is calculated. By comparing to the data of the NA60 and PHENIX Collaborations, we study the relative importance of different dilepton production mechanisms and point out the regions in phase space where partonic channels are dominant.

Dilepton production by dynamical quasi-particles in the strongly interacting quark–gluon plasma

We address the dilepton production by the constituents of the strongly interacting quark–gluon plasma (sQGP). In order to make quantitative predictions for dilepton rates at experimentally relevant low dilepton mass (O(1GeV)) and strong coupling (αS ∼ 0.5–1), we take into account non-perturbative spectral functions and self-energies of the quarks, antiquarks and gluons. For this purpose, we use parametrizations of the quark and gluon propagators provided by the dynamical quasi-particle model (DQPM) matched to reproduce lattice quantum chromodynamics (QCD) data. The DQPM describes QCD properties in terms of the single-particle Green's functions and leads to the notion of the constituents of the sQGP being effective quasi-particles, which are massive and have broad spectral functions (due to large interaction rates). By ‘dressing’ the quark and gluon lines with the effective propagators, we derive the off-shell cross sections for dilepton production in the reactions (Born mechanism), (quark annihilation with the gluon bremsstrahlung in the final state), (gluon–Compton scattering), and (virtual gluon decay, virtual quark decay). In contrast to previous calculations of these cross sections, we account for virtualities of all the quarks and gluons. We find that finite masses of the effective quasi-particles not only screen the singularities typical of the perturbative cross sections with massless quarks, but also modify the shape of the dilepton production cross sections, especially at low dilepton mass Q and at the edges of the phase space. Finally, we use the calculated mass-dependent cross sections to identify the dependence of the dilepton rates on the spectral function widths of the initial and final quarks and gluons, which has not been estimated so far. The results demonstrate that the multiple partonic scatterings encoded in the broad spectral functions of the dynamical quasi-particles have considerable effect on the dilepton rates.

Dileptons from the nonequilibrium quark-gluon plasma

According to the dynamical quasiparticle model (DQPM) – matched to reproduce lattice QCD results in thermodynamic limit, – the constituents of the strongly interacting quark-gluon plasma (sQGP) are massive and off-shell quasi-particles (quarks and gluons) with broad spectral functions. In order to address the electromagnetic radiation of the sQGP, we derive off-shell cross sections of qq̄ → γ*, qq̄ → γ* + g and qg → γ* q(q̄g → γ*q̄) reactions taking into account the effective propagators for quarks and gluons from the DQPM. Dilepton production in In+In collisions at 158 AGeV is studied by implementing these processes into the parton-hadron-string dynamics (PHSD) transport approach. The microscopic PHSD transport approach describes the full evolution of the heavy-ion collision: from the dynamics of quasi-particles in the sQGP phase (when the local energy density is above ~ 1 GeV/fm) through hadronization and to the following hadron interactions and off-shell propagation after the hadronization. A comparison to the data of the NA60 Collaboration shows that the low mass dilepton spectra are well described by including a collisional broadening of vector mesons, while the spectra in the intermediate mass range are dominated by off-shell quark-antiquark annihilation, quark Bremsstrahlung and gluon-Compton scattering in the nonperturbative QGP. In particular, the observed softening of the mT spectra at intermediate masses (1 GeV ≤ M ≤ 3 GeV) is approximately reproduced.

Erratum: Quark initial state interaction in deep inelastic scattering and the Drell-Yan process [Phys. Rev. D71, 034009 (2005)

We pursue a phenomenological study of higher twist effects in high energy processes by taking into account the off-shellness (virtuality) of partons bound in the nucleon. The effect of parton off-shellness in deep inelastic ep->eX scattering and the Drell-Yan process (pp->l+ l- X) is examined. Assuming factorization and a single-parameter Breit-Wigner form for the parton spectral function, we develop a model to calculate the corresponding off-shell cross sections. Allowing for a finite parton width of the order of 100 MeV, we reproduce the data of both DIS and the triple differential Drell-Yan cross section without an additional K-factor. The results are compared to those from perturbative QCD and the intrinsic-kT approach.

Quark initial state interaction in deep inelastic scattering and the Drell-Yan process

We pursue a phenomenological study of higher-twist effects in high-energy processes by taking into account the off-shellness (virtuality) of partons bound in the nucleon. The effect of parton off-shellness in deep inelastic $ep\ensuremath{\rightarrow}eX$ scattering (DIS) and the Drell-Yan process ($pp\ensuremath{\rightarrow}l\overline{l}X$) is examined. Assuming factorization and a single-parameter Breit-Wigner form for the parton spectral function, we develop a model to calculate the corresponding off-shell cross sections. Allowing for a finite parton width $\ensuremath{\approx}100\text{ }\text{ }\mathrm{M}\mathrm{e}\mathrm{V}$, we reproduce the data of both DIS and the triple-differential Drell-Yan cross section without an additional $K$-factor. The results are compared to those from perturbative QCD and the intrinsic-${k}_{T}$ approach.

Inclusive quasifree electrofission cross section for238U

We present results from a joint theoretical and experimental study of inclusive quasifree electrofission of ${}^{238}\mathrm{U}.$ The off-shell cross sections for the quasifree reaction stage have been calculated within the plane wave impulse approximation with distortion corrections included in the effective momentum approximation. Proton and neutron single-particle momentum distributions were calculated in the macroscopic-microscopic approach. The fissility for proton and neutron single hole excited states of the residual nuclei ${}^{237}\mathrm{Pa}$ and ${}^{237,238}\mathrm{U}$ was calculated within the compound nucleus model. Final state interaction corrections to residual nucleus excitation energy were calculated using the imaginary part of the optical potential. The total inclusive electrofission cross section was measured with high absolute precision, and all principal partial contributions are analyzed, in particular, the quasifree one.

Quasi-free 238U [formula omitted] cross section in macroscopic–microscopic approach

We present the result of a theoretical study of inclusive quasi-free electrofission of 238U. The off-shell cross sections for the quasi-free reaction stage have been calculated within the Plane Wave Impulse Approximation (PWIA) using a macroscopic–microscopic description of the proton and neutron single particle momentum distributions. Electron wave function distortion corrections were included using the effective momentum approximation. The fissility for the proton single hole excited states of the residual nucleus 237Pa was calculated both with and without contributions of the pre-equilibrium emission of the particles. The fissility of the 237,238U residual nuclei was calculated within the compound nucleus model. Final state interaction correction to the fissility was calculated using a pure imaginary optical potential. The (e,e′f) cross sections, thus obtained, were compared with available experimental data.

E-p off-shell cross section in quasielastic (e,e'p) reactions.

By carrying out calculations on $^{208}\mathrm{Pb}$(e,e'p) reactions at different electron beam energies for fixed knocked-out proton kinematics, we examined whether, even in the presence of Coulomb distortions of the electron waves, the (e,e'p) cross section can still be separated into a reduced cross section ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{red}}$(${\mathit{p}}_{\mathit{m})}$ and the e-p off-shell cross section. We found that if one uses an effective momentum approximation in calculating the e-p off-shell cross section, one gets a reduced cross section which is independent of the electron kinematics within a few percent.

Double and triple differential cross sections for K-shell ionisation by relativistic electron impact

Expressions for the double and triple differential cross sections for K-shell ionisation by relativistic electrons are derived with the plane wave impulse approximation (PWIA). Assuming the bound atomic electrons to be on-shell, the double differential cross section factorises in essence into the Moller cross section and the Compton profile. The triple differential off-shell cross section is given for Moller and Bhabba scattering. The results are compared with experimental data.

Simultaneous electron-photon excitation of hydrogen and helium

Total and differential cross sections for simultaneous excitation of H(2s), H(3s) and He(1s 2s) by electron impact in the presence of a photon field have been calculated in second-order perturbation theory. The characteristics of the results are interpreted in terms of their physical origin. The off-shell excitation cross section for the He(1s 2s) state is found to be one order of magnitude smaller than that for excitation of the H(2s) state. This is consistent with our expection that atoms with higher dipole-polarisability should favour larger off-shell cross sections for transitions betweens-states.

Off-shell electron-nucleon cross sections: The impulse approximation

With the goal of improving the analysis of quasi-elastic (e, e′) and (e, e′N) experiments, ambiguities in the impulse approximation are investigated by comparing various off-shell extrapolations of the Rosenbluth cross section. It is found that these ambiguities are especially large for low incident electron energies and momentum transfers and that they can be reduced by applying various physical constraints, such as those due to Lorentz invariance and current conservation. In particular the latter is important: most off-shell cross sections that have been used do not treat this correctly and, as a consequence, in the limit of the photon point give infinite photoabsorption cross sections.

Off-energy-shell behavior of theTmatrix for realistic localN−Npotentials with soft or hard cores

Off-energy-shell T matrix elements for any partial wave are calculated using realistic local two-nucleon potentials with soft or hard cores. The off- shell behaviors are discussed in relation to on-shell values. Off-shell cross sections involved in the impulse approximation of nuclear reactions are analyzed. The validity of some usual on-shell approximations is examined.