Forward-backward Asymmetry Parameter Research Articles

Study of forward–backward correlation and multiplicity fluctuations of pions produced in p–p collisions at recent LHC energies

The two-particle rapidity correlation among the pions produced in p–p collisions at recent LHC energies 2.76, 5.02, 7 and 13 TeV are studied in this paper by using the analysis of events generated by UrQMD-3.4, AMPT-2.26 and PYTHIA-8.3. Our investigation for bin-to-bin fluctuation using the variance of forward–backward asymmetry parameter in the forward–backward pseudo-rapidity windows is done through the simulated data. In addition to that, energy-wise variation of correlation strength (β), the probability distribution of roughness parameter () and number density fluctuation parameter (d ik ) are also incorporated in this investigation. Also, we have tried to study the same for the data of different impact parameters generated using the UrQMD model, and the comparison is made with its minimum biased events data-set. The analysis signifies, that the signature of dynamical fluctuation is not so prominent but increases a bit with the increase in energy. An increase in the correlation strength with the increase in energy signifies the contribution of long-range correlation is more in comparison to the short-range one. However, there is no such accountable effect of various impact parameters seen in the present study.

Multiplicity distribution of backward shower particle in total disintegrated events revisited with Weibull distribution

The well-known statistical distribution function, Weibull distribution function, has been employed to study the multiplicity distribution and multiplicity moments for the backward shower particles produced in case of total disintegrated events of [Formula: see text]O–AgBr, [Formula: see text]Ne–AgBr and [Formula: see text]Si–AgBr interactions at (4.1–4.5) AGeV/c. Multiplicity fluctuation of backward shower particles in case of total disintegrated events by the method of scaled variance and the forward–backward asymmetry parameter had also been investigated.

A geometric approach to decoding molecular structure and dynamics from photoionization of isotropic samples.

We propose a geometric approach to the description and analysis of photoelectron angular distributions resulting from isotropic samples in the case of few-photon ionization by electric fields of arbitrary polarization. This approach formulates the standard photoionization observables - the bl,m expansion coefficients of the photoelectron angular distribution, in terms of geometrical properties of the vector field D⃑(k⃑) ≡ 〈k⃑|d⃑|0〉 describing the electronic transition from a bound state |0〉 into a scattering state |k⃑〉 - the photoionization transition dipole. Besides revealing selection rules for the enantio-sensitivity of bl,m coefficients in multiphoton ionization, our approach yields very compact expressions for both chiral and achiral molecules revealing how the molecular rotational invariants couple to the rotational invariants of the setup defined by the electric field polarization and the arrangement of photoelectron detectors. We apply this approach to one-photon ionization and find that the forward-backward asymmetry parameter b1,0, emerging exclusively in chiral molecules and encoded in the field B⃑(k⃑) ≡ iD⃑*(k⃑) × D⃑(k⃑), is sensitive only to the components of D⃑(k⃑) perpendicular to k⃑, while the regular asymmetry parameter b2,0 emerging in chiral and achiral molecules is sensitive only to the component of D⃑(k⃑) parallel to k⃑. Next, we analyze resonantly enhanced two-photon ionization and show that b0,0 and b1,0 can be written in terms of an effectively stretched D⃑(k⃑), and how b1,0 and b3,0 can be used to probe B⃑(k⃑).

Signature of lepton flavor universality violation in Bs→Dsτν semileptonic decays

Deviation from the standard model prediction is observed in many semileptonic $B$ decays mediated via $b\ensuremath{\rightarrow}c$ charged current interactions. In particular, current experimental measurements of the ratio of branching ratio ${R}_{D}$ and ${R}_{{D}^{*}}$ in $B\ensuremath{\rightarrow}{D}^{(*)}l\ensuremath{\nu}$ decays disagree with standard model expectations at the level of about $4.1\ensuremath{\sigma}$. Moreover, the recent measurement of the ratio of branching ratio ${R}_{J/\mathrm{\ensuremath{\Psi}}}$ by LHCb, where ${R}_{J/\mathrm{\ensuremath{\Psi}}}=\mathcal{B}({B}_{c}\ensuremath{\rightarrow}J/\mathrm{\ensuremath{\Psi}}\ensuremath{\tau}\ensuremath{\nu})/\mathcal{B}({B}_{c}\ensuremath{\rightarrow}J/\mathrm{\ensuremath{\Psi}}\ensuremath{\mu}\ensuremath{\nu})$, is more than $2\ensuremath{\sigma}$ away from the standard model prediction. In this context, we consider an effective Lagrangian in the presence of vector and scalar new physics couplings to study the implications of ${R}_{D}$ and ${R}_{{D}^{*}}$ anomalies in ${B}_{s}\ensuremath{\rightarrow}{D}_{s}\ensuremath{\tau}\ensuremath{\nu}$ decays. We give a prediction of several observables such as the branching ratio, ratio of branching ratio, forward backward asymmetry parameter, $\ensuremath{\tau}$ polarization fraction, and convexity parameter for the ${B}_{s}\ensuremath{\rightarrow}{D}_{s}\ensuremath{\tau}\ensuremath{\nu}$ decays within the standard model and within various new physics scenarios.

Ionization of N2 in collisions with fast electrons: Evidence of an interference effect

Absolute double differential cross sections (DDCS) of electron emission were measured for ionization of ${\mathrm{N}}_{2}$ by fast electrons with energy 7 keV. Measurements were performed for different electron emission angles and energies. Evidence of oscillation due to Young-type interference was observed in the DDCS ratios for all angles. The frequency for large backward angle is found to be larger compared to that for small forward angle. Consequently, the forward-backward asymmetry parameter reveals the oscillatory structure even more clearly. The oscillations observed for both experimental-to-theoretical DDCS ratios and forward-backward asymmetry were well explained by the Cohen-Fano model of interference in a molecular double slit. A periodic deviation of the Cohen-Fano model from the asymmetry parameter data reveals the presence of a higher-frequency component. The first Born model was employed to explain the results of molecular nitrogen for which a complete-neglect-of-differential-overlap approximation was used along with an effective atomic number.

Electron DDCS in ionization of O2 by 4.5 MeV/u bare O-ions: A comparative study of the angular asymmetry between O2 and H2

We have measured double differential cross-sections (DDCS) of low energy electrons emitted from O2 in collisions with 72MeV bare O ions. The derived forward backward asymmetry parameter spectrum shows monotonically increasing behavior. No oscillation due to interference was observed in the forward-backward angular asymmetry unlike that for molecular hydrogen for which oscillation was unambiguous.

Energy and angular distributions of the low-energy electron emission in collisions of 4 MeV/u bare F ions with He atoms

The energy and angular distributions of double-differential cross sections (DDCS) of electron emission from He in collisions with 4 MeV/u F9 + ions are reported. The derived single-differential distributions and the total cross sections are also reported. The measured distributions of the low-energy electrons between 1 and 400 eV over a wide angular range between 20° and 160° are compared with the state-of-the-art quantum mechanical models. The first Born (B1) and the continuum distorted wave-eikonal initial state (CDW-EIS) approximations are used for this purpose. The DDCS for a given angle was found to fall by a few orders of magnitude over the electron energy range studied. The CDW-EIS model provides excellent agreement with the energy distributions and the angular distributions. The electron energy dependence of the forward–backward asymmetry parameter shows monotonically increasing behaviour. This has been explained very well in terms of the CDW-EIS model, which includes the two-centre effect. A large deviation from the B1 is also observed. We have also derived the single-differential distributions in terms of the angle as well as the electron energy. These distributions are also well reproduced by the CDW-EIS model.

The dependence of the forward-backward asymmetry parameter on the molecular orientation in angular distributions of emitted electrons in the H2 ionisation induced by bare ion impact

We have studied the angular distributions of electron emission in collisions of bare ions with H2 oriented target using molecular wave functions calculated with the Schwinger variational iteractive method within the Continuum Distorted Wave-Eikonal Initial State approximation (CDW-EIS-SVIM). By calculating the forward-backward asymmetry parameter for different molecular orientations we show that this parameter depend not only on the perturbation strength, Sp = Zp/vp, where Zp and vp are respectively the projectile charge and velocity (two-centre effect) and on the outgoing electron velocity, but may also depend on how the molecule is aligned with respect to the projectile incident axis. Doubly differential cross sections (DDCS) for fixed in space molecular target were calculated. The asymmetry parameter was compared with both experimental and theoretical orientation averaged results.

Young-type interference effect on angular distribution of secondary electrons emitted fromH2in collisions with fast electrons

The Young-type interference arising due to the spatial coherence has been investigated in the electron emission spectrum from fast electron impact ionization of the inversion symmetric homonuclear diatomic molecule ${\mathrm{H}}_{2}$. The evidence of the interference effect in the angular distribution of the double differential spectrum of the secondary electron is found. The signature of constructive interferences has been identified in the soft-collision regions as well as in binary encounters. The observed oscillation in the forward-backward asymmetry parameter is explained in terms of the Cohen-Fano-type interference coupled with the angular dependence of oscillation frequency. A comparative study indicates a marked difference between the angular asymmetry in the case of fast heavy ion $({\mathrm{F}}^{9+})$ and electron collisions with ${\mathrm{H}}_{2}$ at a similar velocity.

Angular distribution of low-energy electron emission in collisions of 6-MeV/u bare carbon ions with molecular hydrogen: Two-center mechanism and interference effect

We report the energy and angular distribution of electron double differential cross sections (DDCS) in collision of $6\text{\ensuremath{-}}\mathrm{MeV}∕\mathrm{u}\phantom{\rule{0.3em}{0ex}}{\mathrm{C}}^{6+}$ ions with molecular hydrogen. We explain the observed distributions in terms of the two-center effect and the Young-type interference effect. The secondary electrons having energies between 1 and $1000\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ are detected at about 10 different emission angles between $30\ifmmode^\circ\else\textdegree\fi{}$ and $150\ifmmode^\circ\else\textdegree\fi{}$. The measured data are compared with the state-of-the-art continuum distorted wave-eikonal initial state and the first Born model calculations which use molecular wave function. The single differential cross sections are derived and compared with the theoretical predictions. The oscillations due to the interference effect are derived in the DDCS ratios using theoretical cross sections for the atomic H target. The effect of the atomic parameters on the observed oscillations is discussed. An evidence of interference effect has also been shown in the single differential cross section. The electron energy dependence of the forward-backward asymmetry parameter shows a monotonically increasing behavior for an atomic target, such as He, which could be explained in terms of the two-center effect only. In contrast, for the molecular ${\mathrm{H}}_{2}$ the asymmetry parameter reveals an oscillatory behavior due to the Young-type interference effect superimposed with the two-center effect. The asymmetry parameter technique provides a self-normalized method to reveal the interference oscillation which does not require either a theoretical model or complementary measurements on the atomic H target.

Ionization dynamics in fast ion-atom collisions. I. Energy and angular distributions of low-energy electrons emitted in ionization of He by bare carbon ions

We have studied the two-center effect (TCE) in ion-atom ionization by measuring the energy and angular distributions of the double-differential cross sections (DDCSs) ${(d}^{2}\ensuremath{\sigma}/d\ensuremath{\varepsilon}d\ensuremath{\Omega})$ of the low-energy electrons emitted in a collision of He atoms with 2.5 MeV/nucleon ${\mathrm{C}}^{6+}$ ions. The electrons with energies between 0.1 and 300 eV were detected for 13 different emission angles between $15\ifmmode^\circ\else\textdegree\fi{}$ and $160\ifmmode^\circ\else\textdegree\fi{}.$ From the measured DDCSs we have deduced the single differential cross sections such as $d\ensuremath{\sigma}/d\ensuremath{\Omega}$ and $d\ensuremath{\sigma}/d\ensuremath{\varepsilon}$ and the total cross section. The data have been compared with the continuum distorted wave eikonal initial state calculations with H-like and Hartree-Fock-Slater wave functions for the initial and final state of the electron and first Born calculations. The forward-backward asymmetry parameter also has been deduced to study the TCE in detail. In the following paper [Lokesh C. Tribedi et al., Phys. Rev. A 58, 3626 (1998)] we have explored the collision dynamics by deducing the momentum distributions of the electron, the recoil ion, and the projectile.

Integral and Differential Cross Sections for the Electron-Impact Excitation of Ar7+Ions

Electron-impact excitation of Ar 7+ is studied theoretically with the R-matrix method. Excitation cross sections are calculated for the transitions from the ground state (3s) to the 3p, 3d, 4s, 4p and 4d excited states at the impact electron energies from threshold to 110 eV, allowing for coupling between the lowest eleven states. All the cross sections show complicated resonance structure. Differential cross sections (DCSs) are also calculated for the 3s → 3p, 3d, 4s transitions both at the resonance and at the off-resonance energies. The DCS at the off-resonance position shows dominance of backward scattering. This is examined in more detail with the calculation of forward-backward asymmetry parameter. At the resonance, DCS changes its shape. This is shown at several selected resonant peaks. The present theoretical cross sections well reproduce experimental data available for the 3s → 3p transition.

Implications of, constraints on, and signatures of a fourth-generation lepton triplet.

The possibility of a fourth-generation weak SU(2)/sub /ital L// triplet of leptons is further investigated. Constraints on the masses of the charged triplet of leptons are derived from their contribution to the /rho/ parameter. Partial-wave considerations are then applied to the lepton-triplet--Higgs-boson interactions, where it is argued that these interactions are likely to become strongly interacting at high energies. Finally, possible signatures of the lepton triplet are considered. These include the ratio GAMMA/sub /ital Z///GAMMA/sub W/, the partial width GAMMA(/ital Z//r arrow//ital ff/bar //), the cross section sigma(/ital e//sup +/e/sup /minus///r arrow/f/bar f/), and the forward-backward asymmetry parameter.

On the high-energy behaviour of a minimal extension of the Weinberg-Salam model

The electron-positron annihilation cross section near the heavy neutral vector-boson (Z) mass is calculated in the standard Weinberg-Salam (WS) model as well as in an SU(2)*U(1)*U'(1) (primed) model. Simple analytic expressions are presented for different parity-violating and asymmetry parameters in both the WS and primed models. The numerical results for these parameters indicate that the two models might be distinguishable at higher energies if the forward-backward asymmetry parameter and the parity-violating asymmetry parameters are measured.