Recoil Region Research Articles

A dynamical (e,2e) investigation of the structurally related cyclic ethers tetrahydrofuran, tetrahydropyran, and 1,4-dioxane

Triple differential cross section measurements for the electron-impact ionization of the highest occupied molecular orbitals of tetrahydropyran and 1,4-dioxane are presented. For each molecule, experimental measurements were performed using the (e,2e) technique in asymmetric coplanar kinematics with an incident electron energy of 250 eV and an ejected electron energy of 20 eV. With the scattered electrons being detected at -5°, the angular distributions of the ejected electrons in the binary and recoil regions were observed. These measurements are compared with calculations performed within the molecular 3-body distorted wave model. Here, reasonable agreement was observed between the theoretical model and the experimental measurements. These measurements are compared with results from a recent study on tetrahydrofuran [D. B. Jones, J. D. Builth-Williams, S. M. Bellm, L. Chiari, C. G. Ning, H. Chaluvadi, B. Lohmann, O. Ingolfsson, D. Madison, and M. J. Brunger, Chem. Phys. Lett. 572, 32 (2013)] in order to evaluate the influence of structure on the dynamics of the ionization process across this series of cyclic ethers.

Twist-3 light-cone distribution amplitudes of the scalar mesons within the QCD sum rules and their application to the $B \rightarrow S$ transition form factors

We investigate the twist-3 light-cone distribution amplitudes (LCDAs) of the scalar mesons $a_0$, $K^{\ast}_0$ and $f_0$ within the QCD sum rules. The QCD sum rules are improved by a consistent treatment of the sizable $s$-quark mass effects within the framework of the background field approach. Adopting the valence quark component $(\bar{q}_1 q_2)$ as the dominant structure of the scalar mesons, our estimation for their masses are close to the measured $a_0(1450)$, $K^{\ast}_0(1430)$ and $f_0(1710)$. From the sum rules, we obtain the first two non-zero moments of the twist-3 LCDAs $\phi^{s,\sigma}_{a_0}$: $\langle \xi_{s,a_0}^{2(4)} \rangle=0.369 \;(0.245)$ and $\langle \xi_{\sigma,a_0}^{2(4)} \rangle=0.203 \;(0.093)$; those of the twist-3 LCDAs $\phi_{K^*_0}^{s,\sigma}$: $\langle \xi_{s,K^{\ast}_0}^{1(2)} \rangle =0.004\;(0.355)$ and $\langle \xi_{\sigma,K^{\ast}_0}^{1(2)} \rangle =0.018\;(0.207)$; and those of the twist-3 LCDAs $\phi_{f_0}^{s,\sigma}$: $\langle \xi_{s,f_0}^{2(4)} \rangle=0.335 \;(0.212)$ and $\langle \xi_{\sigma,f_0}^{2(4)} \rangle=0.196 \; (0.088)$, respectively. As an application of those twist-3 LCDAs, we study the $B \to S$ transition form factors by introducing proper chiral currents into the correlator, which is constructed such that the twist-3 LCDAs give dominant contribution and the twist-2 LCDAs make negligible contribution. Our results of the $B \to S$ transition form factors at the large recoil region $q^2 \simeq 0$ are consistent with those obtained in the literature, which inversely shows the present twist-3 LCDAs are acceptable.

Bs → f0(980) Transition Form Factors Within the kT Factorization Approach

In the paper, we apply the kT factorization approach to deal with the Bs → f0 (980) transition form factors in the large recoil regions, i.e. the small q2 regions. For the purpose, we adopt the B-meson wave-functions ΦB, and δ that include the three-Fock states contributions to do our discussion. Although the scalar meson f0(980) is widely perceived as the 4-quark bound state (scenario 2), but the distribution amplitudes of 4-quark states are still unknown to us, so we adopt 2-quark model (scenario 1) for scalar meson f0(980) in our discussion. By varying the B-meson wave-function parameters within their reasonable regions, we obtain F0(0) = F+(0) = 0.20 ± 0.02, FT(0) = 0.24 ± 0.02. Our present results for these form factors are consistent with the light-cone sum rule results obtained in the literature.

Resummation of rapidity logarithms in B meson wave functions

We construct an evolution equation for the B meson wave functions in the k T factorization theorem, whose solutions sum the double logarithms associated with the light-cone singularities, namely, the rapidity logarithms. The derivation is subtler than that of the Sudakov resummation for an energetic light hadron, due to the involvement of the effective heavy-quark field. The renormalization-group evolution in the factorization scale needs to be included in order to derive an ultraviolet-finite and scale-invariant kernel for resumming the rapidity logarithms. It is observed that this kernel is similar to that of the joint resummation for QCD processes in extreme kinematic regions, which combines the threshold and k T resummations. We show that the resummation effect maintains the normalization of the B meson wave functions, and strengths their convergent behavior at small spectator momentum. The resummation improved B meson wave functions are then employed in the leading-order analysis of the B → π transition form factors, which lead to approximately 25 % deduction in the large recoil region.

Estimation of semileptonic decays ofBcmeson toS-wave charmonia with nonrelativistic QCD

We study the semileptonic differential decay rates of the ${B}_{c}$ meson to S-wave charmonia, ${\ensuremath{\eta}}_{c}$, and $J/\ensuremath{\Psi}$ at next-to-leading order accuracy in the framework of nonrelativistic QCD. In the heavy quark limit, ${m}_{b}\ensuremath{\rightarrow}\ensuremath{\infty}$, we obtain analytically the asymptotic expression for the ratio of the next-to-leading order form factor to the leading-order form factor. Numerical results show that the convergence of the ratio is perfect. At the maximum recoil region, we analyze the differential decay rates in detail with various input parameters and polarizations of $J/\ensuremath{\psi}$, which can now be checked in the LHCb experiment. Phenomenologically, the form factors are extrapolated to the minimal recoil region, and then the ${B}_{c}$-to-charmonium semileptonic decay rates are estimated.

Cornering new physics in b → s transitions

Abstract We derive constraints on Wilson coefficients of dimension-six effective operators probing the b → s transition, using recent improved measurements of the rare decays B s → μ + μ −, B → Kμ + μ − and B → K * μ + μ − and including all relevant observables in inclusive and exclusive decays. We consider operators present in the SM as well as their chirality-flipped counterparts and scalar operators. We find good agreement with the SM expectations. Compared to the situation before winter 2012, we find significantly more stringent constraints on the chirality-flipped coefficients due to complementary constraints from B → Kμ + μ − and B → K * μ + μ − and due to the LHCb measurement of the angular observable S 3 in the latter decay. We also list the full set of observables sensitive to new physics in the low recoil region of B → K * μ + μ −.

Electron impact single ionization of the water molecule in the second Born approximation

A general formalism is presented to calculate the triple-differential cross section for the ionization of neutral atoms and simple molecules with the chemical form XHn in the frame of the second Born approximation using the closure approximation. The first Born term and a part of the second term are calculated analytically and the remainder is calculated semi-analytically. A new method is given to eliminate analytically the singularities occurring in the second Born term. The post-collision interaction between the scattered and ejected electrons is considered approximately using the Gamow factor. To check the accuracy of the present formalism, the ionization of the atomic hydrogen by electron impact is considered. The present formalism is developed to investigate the single ionization of the water molecule by electron impact. The comparison of our results with the experiments and previous theoretical findings shows good agreement and contrary to the previous theoretical calculations, our results predict two peaks in the recoil region.

Electron-impact excitation of the(2p2)1Dand(2s2p)1Poautoionizing states of helium

An experimental study of the excitation of the ($2{p}^{2}$) ${}^{1}D$ and ($2s2p$) ${}^{1}{P}^{o}$ autoionizing states of helium by 250-eV electron impact is presented. The ejected-electron angular distributions and energy spectra are measured in coincidence with the corresponding scattered electrons for a scattering angle of $\ensuremath{-}{13}^{\ensuremath{\circ}}$ and for a range of ejected-electron angles in both the forward and backward directions. Resonance profiles are analyzed in terms of the Shore-Balashov parametrization to obtain the resonance asymmetry ${A}_{\ensuremath{\mu}}$ and yield ${B}_{\ensuremath{\mu}}$ parameters and the direct ionization cross section $f$. The spectra and their parameters are compared to the previous measurements of Lower and Weigold [J. Phys. B. 23, 2819 (1990)] and McDonald and Crowe [J. Phys. B 26, 2887 (1993)]. Comparison is also made with the recent theoretical triply differential cross-section calculations based on the first and second Born approximations. In general, good qualitative agreement is found between the experimental results. Some differences are found at the forward and backward directions. These differences in the shape and magnitude of the cross sections are attributed to the different incoming electron energies used in the experiments. The second Born approximation with inclusion of the three-body Coulomb interaction in the final state agrees reasonably well with experiments in the binary region. However, the ${}^{1}{P}^{o}$ resonance yield parameter ${B}_{\ensuremath{\mu}}$ is significantly overestimated at the recoil region, giving a relatively large recoil peak, in contradiction to the experiment. There is also a discrepancy between the two theories available for the ${}^{1}D$ resonance yield parameter ${B}_{\ensuremath{\mu}}$ in this region. Remaining discrepancies between theories and experiments are also discussed.

More benefits of semileptonic rare B decays at low recoil: CP violation

We present a systematic analysis of the angular distribution of Bbar -> Kbar^\ast (-> Kbar pi) l^+ l^- decays with l = e, mu in the low recoil region (i.e. at high dilepton invariant masses of the order of the mass of the b-quark) to account model-independently for CP violation beyond the Standard Model, working to next-to-leading order QCD. From the employed heavy quark effective theory framework we identify the key CP observables with reduced hadronic uncertainties. Since some of the CP asymmetries are CP-odd they can be measured without B-flavour tagging. This is particularly beneficial for Bbar_s,B_s -> phi(-> K^+ K^-) l^+ l^- decays, which are not self-tagging, and we work out the corresponding time-integrated CP asymmetries. Presently available experimental constraints allow the proposed CP asymmetries to be sizeable, up to values of the order ~ 0.2, while the corresponding Standard Model values receive a strong parametric suppression at the level of O(10^-4). Furthermore, we work out the allowed ranges of the short-distance (Wilson) coefficients C_9,C_10 in the presence of CP violation beyond the Standard Model but no further Dirac structures. We find the Bbar_s -> mu^+ mu^- branching ratio to be below 9*10^-9 (at 95% CL). Possibilities to check the performance of the theoretical low recoil framework are pointed out.

The benefits of $ \bar{B} \to {\bar{K}^*}{l^{+} }{l^{-} } $ decays at low recoil

Using the heavy quark effective theory framework put forward by Grinstein and Pirjol we work out predictions for \( \bar{B} \to {\bar{K}^*}{l^{+} }{l^{-} } \), l = e, μ, decays for a softly recoiling \( {\bar{K}^*} \), i.e., for large dilepton masses \( \sqrt {{{q^2}}} \) of the order of the b-quark mass m b . We work to lowest order in Λ/Q, where \( Q = \left( {{m_b}\sqrt {{{q^2}}} } \right) \) and include the next-to-leading order corrections from the charm quark mass m c and the strong coupling at \( \mathcal{O}\left( {{{{m_c^2}} \left/ {{{Q^2},{\alpha_s}}} \right.}} \right) \). The leading Λ/m b corrections are parametrically suppressed. The improved Isgur-Wise form factor relations correlate the \( \bar{B} \to {\bar{K}^*}{l^{+} }{l^{-} } \) transversity amplitudes, which simplifies the description of the various decay observables and provides opportunities for the extraction of the electroweak short distance couplings. We propose new angular observables which have very small hadronic uncertainties. We exploit existing data on \( \bar{B} \to {\bar{K}^*}{l^{+} }{l^{-} } \) distributions and show that the low recoil region provides powerful additional information to the large recoil one. We find disjoint best-fit solutions, which include the Standard Model, but also beyond-the-Standard Model ones. This ambiguity can be accessed with future precision measurements.

Radiative and semileptonic B decays involving higher K-resonances in the final states

We study the radiative and semileptonic B decays involving a spin-J resonant \(K_{J}^{(*)}\) with parity (−1)J for \(K_{J}^{*}\) and (−1)J+1 for K J in the final state. Using large energy effective theory (LEET) techniques, we formulate \(B\to K_{J}^{(*)}\) transition form factors in the large recoil region in terms of two independent LEET functions \(\zeta_{\perp}^{K_{J}^{(*)}}\) and \(\zeta_{\parallel}^{K_{J}^{(*)}}\), the values of which at zero momentum transfer are estimated in the BSW model. According to the QCD counting rules, \(\zeta_{\perp,\parallel}^{K_{J}^{(*)}}\) exhibit a dipole dependence in q 2. We predict the decay rates for \(B\to K_{J}^{(*)}\gamma\), \(B\to K_{J}^{(*)}\ell^{+}\ell^{-}\) and \(B\to K_{J}^{(*)}\nu \bar{\nu}\). The branching fractions for these decays with higher K-resonances in the final state are suppressed due to the smaller phase spaces and the smaller values of \(\zeta^{K_{J}^{(*)}}_{\perp,\parallel}\). Furthermore, if the spin of \(K_{J}^{(*)}\) becomes larger, the branching fractions will be further suppressed due to the smaller Clebsch–Gordan coefficients defined by the polarization tensors of the \(K_{J}^{(*)}\). We also calculate the forward–backward asymmetry of the \(B\to K_{J}^{(*)}\ell^{+}\ell^{-}\) decay, for which the zero is highly insensitive to the K-resonances in the LEET parametrization.

Experimental investigation of the triple differential cross section for electron impact ionization of N2 and CO2 molecules at intermediate impact energy and large ion recoil momentum

The (e,2e) triple differential cross sections (TDCS) are measured for the ionization of nitrogen and carbon dioxide molecules in a coplanar asymmetric geometry for a wide range of ejected electron energies and at an incident energy about 500–700 eV. This kinematics corresponds to a large momentum imparted to the ion, and is meant to enhance the recoil scattering. The experimental binary and recoil angular distributions of the TDCS are characterized both by a shift towards larger angles with respect to the momentum transfer direction and by a large intensity in the recoil region, in particular for the ionization of the ‘inner’ N2(2σg) molecular orbital. The data are compared with the results of calculations using the first Born approximation–two centre continuum (FBA–TCC) theoretical model for treating differential electron impact ionization. The experimentally observed shifts and recoil intensity enhancement are not predicted by the model calculations, which rather yield a TDCS symmetrically distributed around the momentum transfer direction, and completely fail in describing the recoil distribution. It is hoped that these new results will stimulate the development of more refined theories for correctly modelling single ionization of molecules.

ExclusiveBsdecays to the charmed mesonsDs+(1968,2317)in the standard model

The transition form factors of ${\overline{B}}_{s}^{0}\ensuremath{\rightarrow}{D}_{s}^{+}(2317)$ and ${\overline{B}}_{s}^{0}\ensuremath{\rightarrow}{D}_{s}^{+}(1968)$ at large recoil region are investigated in the light cone sum rules approach, where the heavy quark effective theory is adopted to describe the form factors at small recoil region. With the form factors obtained, we carry out a detailed analysis on both the semileptonic decays ${\overline{B}}_{s}^{0}\ensuremath{\rightarrow}{D}_{s}^{+}(1968,2317)l{\overline{\ensuremath{\nu}}}_{l}$ and nonleptonic decays ${B}_{s}\ensuremath{\rightarrow}{D}_{s}^{+}(1968,2317)M$ with $M$ being a light meson or a charmed meson under the factorization approach. Our results show that the branching fraction of ${\overline{B}}_{s}^{0}\ensuremath{\rightarrow}{D}_{s}^{+}(2317)\ensuremath{\mu}{\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}$ is around $2.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, which should be detectable with ease at the Tevatron and CERN LHC. It is also found that the branching fractions of ${\overline{B}}_{s}^{0}\ensuremath{\rightarrow}{D}_{s}^{+}(1968)l{\overline{\ensuremath{\nu}}}_{l}$ are almost 1 order larger than those of the corresponding ${B}_{s}^{0}\ensuremath{\rightarrow}{D}_{s}^{+}(2317)l{\overline{\ensuremath{\nu}}}_{l}$ decays. The consistency of predictions for ${B}_{s}\ensuremath{\rightarrow}{D}_{s}^{+}(1968,2317)L$ ($L$ denotes a light meson) in the factorization assumption and ${k}_{T}$ factorization also supports the success of color transparency mechanism in the color allowed decay modes. Most two-charmed meson decays of ${B}_{s}$ mesons possess quite large branching ratios that are accessible in the experiments. These channels are of great importance to explore the hadronic structure of charmed mesons as well as the nonperturbative dynamics of QCD.

Radiative and semileptonicBdecays involving the tensor mesonK2*(1430)in the standard model and beyond

We study semileptonic and radiative $B$ decays involving the strange tensor meson ${K}_{2}^{*}(1430)$ in the final state. Using the large energy effective theory (LEET) techniques, we formulate the $B\ensuremath{\rightarrow}{K}_{2}^{*}$ transition form factors in large recoil region. All the form factors can be parametrized in terms of two independent LEET functions ${\ensuremath{\zeta}}_{\ensuremath{\perp}}$ and ${\ensuremath{\zeta}}_{\ensuremath{\parallel}}$. The magnitude of ${\ensuremath{\zeta}}_{\ensuremath{\perp}}$ is estimated from the data for $\mathcal{B}(B\ensuremath{\rightarrow}{K}_{2}^{*}(1430)\ensuremath{\gamma})$. Assuming a dipole ${q}^{2}$ dependence for the LEET functions and ${\ensuremath{\zeta}}_{\ensuremath{\parallel}}/{\ensuremath{\zeta}}_{\ensuremath{\perp}}=1.0\ifmmode\pm\else\textpm\fi{}0.2$, for which the former consists with the QCD counting rules, and the latter is favored by the $B\ensuremath{\rightarrow}\ensuremath{\phi}{K}_{2}^{*}$ data, we investigate the decays $B\ensuremath{\rightarrow}{K}_{2}^{*}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ and $B\ensuremath{\rightarrow}{K}_{2}^{*}\ensuremath{\nu}\overline{\ensuremath{\nu}}$, where the contributions due to ${\ensuremath{\zeta}}_{\ensuremath{\parallel}}$ are suppressed by ${m}_{{K}_{2}^{*}}/{m}_{B}$. For the $B\ensuremath{\rightarrow}{K}_{2}^{*}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ decay, in the large recoil region where the hadronic uncertainties are considerably reduced, the longitudinal distribution $d{F}_{L}/ds$ is reduced by 20\char21{}30% due to the flipped sign of ${c}_{7}^{\mathrm{eff}}$ compared with the standard model result. Moreover, the forward-backward asymmetry zero is about $3.4\text{ }\text{ }{\mathrm{GeV}}^{2}$ in the standard model, but changing the sign of ${c}_{7}^{\mathrm{eff}}$ yields a positive asymmetry for all values of the invariant mass of the lepton pair. We calculate the branching fraction for $B\ensuremath{\rightarrow}{K}_{2}^{*}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ in the standard model. Our result exhibits the impressed resemblance between $B\ensuremath{\rightarrow}{K}_{2}^{*}(1430){\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$, $\ensuremath{\nu}\overline{\ensuremath{\nu}}$, and $B\ensuremath{\rightarrow}{K}^{*}(892){\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$, $\ensuremath{\nu}\overline{\ensuremath{\nu}}$.

B-Meson Wavefunction by a Comparative Analysis of the B → π, K Transition Form Factors

Properties of the B-meson light-cone wavefunction up to next-to-leading order Fock state expansion are studied by a comparative study of the B → π, K form factors within the kT factorization approach and the light-cone sum rule analysis. The form factors F+, 0,TB → π and F+,0,TB → K are carefully re-calculated up to O(1/mb2) within the kT factorization approach in the large recoil region. The QCD light-cone sum rule is applicable in the large and intermediate energy regions, and the QCD light-cone sum rule results of Ball and Zwicky [Phys. Rev. D 71 (2005) 014015] are adopted for such a comparative study. It is found that when the two phenomenological parameters λ̄ [0.50, 0.55] and δ ∊ [0.25, 0.30], the results of F+,0,TB → π(Q2) and F+,0,TB → K(Q2) from these two approaches are consistent with each other in the large recoil energy region.

CP asymmetries in B̄ → K̄*( → K̄π)l̄land untagged B̄s,Bs→ ϕ( →K+K−)l̄ldecays at NLO

The decay → *( → π)l offers great opportunities to explore the physics at and above the electroweak scale by means of an angular analysis. We investigate the physics potential of the seven CP asymmetries plus the asymmetry in the rate, working at low dilepton mass using QCD factorization at next-to leading order (NLO). The b → s CP asymmetries are doubly Cabibbo-suppressed ≲1% in the Standard Model and its extensions where the CKM matrix is the only source of CP violation. Three CP asymmetries are T-odd, and can be (1) in the presence of non-standard CP violation. The T-even asymmetries can reach (0.1), limited by the small strong phases in the large recoil region. We furthermore point out an easy way to measure CP phases from time-integrated, untagged d, Bd → K*( → K0π0)l and s, Bs → ϕ( → K+K−)l decays. Analyses of these CP asymmetries can rule out, or further support the minimal description of CP violation through the CKM mechanism. Experimental studies are promising for (super) flavor factories and at hadron colliders.

B → K Transition Form Factor with Tensor Current within the kTFactorization Approach

We apply the kT factorization approach to deal with the B → K transition form factor with tensor current in the large recoil regions. Main uncertainties for the estimation are discussed and we obtain FTB→K(0) = 0.25 ± 0.01 ± 0.02, where the first error is caused by the uncertainties from the pionic wavefunctions and the second is from that of the B-meson wavefunctions. This result is consistent with the light-cone sum rule results obtained in the literature.

(e,2e) ionization of helium and the hydrogen molecule: signature of two-centre interference effects

Relative (e,2e) triply differential cross sections (TDCS) are measured for the ionization of the helium atom and the hydrogen molecule in coplanar asymmetric geometry at a scattered electron energy of 500 eV and ejected electron energies of 205, 74 and 37 eV. The He experimental results are found to be in very good agreement with convergent close-coupling calculations (CCC). The H2 experimental results are compared with two state-of-the-art available theoretical models for treating differential electron impact ionization of molecules. Both models yield an overall good agreement with experiments, except for some intensity deviations in the recoil region. Similar (e,2e) works were recently published on H2 with contrasted conclusions to the hypothesis that the two H nuclei could give rise to an interference pattern in the TDCS structure. Murray (2005 J. Phys. B: At. Mol. Opt. Phys. 38 1999) found no evidence for such an effect, whereas Milne-Brownlie et al (2006 Phys. Rev. Lett. 96 233201) reported its indirect observation. In this work, based on a direct comparison between experimental results for He and H2, we observe an oscillatory pattern due to these interference effects, and for the first time the destructive or constructive character of the interference is observed, depending on the de Broglie wavelength of the ejected electron wave. The experimental finding is in good agreement with the theoretical prediction by Stia et al (2003 J. Phys. B: At. Mol. Opt. Phys. 36 L257).

B→K transition form factor up to $\mathcal{O}(1/m^2_b)$ within the kT factorization approach

We apply the k(T) factorization approach to deal with the B-->K transition form factor F-+,0(B-->K) (q(2)) in the large recoil regions. The B-meson wave functions Psi(B) and Psi(B) that include the 3-particle Fock states' contributions are adopted to give a consistent PQCD analysis of the form factor up to O(1/m(b)(2)). It has been found that the two wave functions Psi(B) and Psi(B) can give sizable contributions to the form factor and should be kept for a better understanding of the B-meson decays. Next the contributions from different twist structures of the kaon wave function are discussed, including SUf(3)-breaking effects. A sizable contribution from the twist-3 wave function Psi(p) is found, whose model dependence is discussed by taking two groups of parameters that are determined by different distribution amplitude moments obtained in the literature. It is also shown that F-+,0(B-->K) (0) = 0.30 +/- 0.04 and [F-+,0(B-->K) (0)/F-+,0(B-->pi) (0)] = 1.13 +/- 0.02, which are more reasonable and consistent with the light-cone sum rule results in the large recoil regions.

Triply differential (e,2e) cross sections for ionization of the nitrogen molecule at large energy transfer

Measurements of the (e,2e) triply differential cross sections (TDCS) are presented for the ionization of the nitrogen molecule in coplanar asymmetric geometry at an incident energy of about 600 eV and a large energy transfer to the target. The experimental results are compared with state-of-the-art available theoretical models for treating differential electron impact ionization of molecules. The experimental TDCS are characterized by a shift towards larger angles of the angular distribution with respect to the momentum transfer direction, and by a large intensity in the recoil region, especially for ionization of the ‘inner’ 2σg molecular orbital. Such shifts and intensity enhancement are not predicted by the model calculations which rather yield a TDCS symmetrically distributed around the momentum transfer direction.