Coulomb-nuclear Interference Region Research Articles

TOTEM-ATLAS ambiguity: Shouldn't one worry?

The values of the parameters ρpp,σtotpp and Bpp extracted in the region of Coulomb-nuclear interference (CNI) in the ATLAS and TOTEM experiments at energy s=13 TeV are studied based on the general expression for the Coulomb-nuclear amplitude. Due to the significant differences between these data, we undertake both their joint analysis and the retrieval of parameter values separately for each experiment. A significant incompatibility of these parameters published by the ATLAS and TOTEM experiments is shown. This is especially noticeable in the case of σtot and the situation is similar to what once happened at the TEVATRON: the values of σtotATLAS with account of errors do not overlap with σtotTOTEM.

Hadron Polarimetry for the Electron-Ion Collider

The Electron-Ion Collider (EIC) will be the first collider to use both polarized electron beams and polarized proton and light ion beams. It will therefore offer unique opportunities to study the structure of nucleons and to answer fundamental questions in QCD. The uncertainties on the polarization measurement translate directly into the uncertainties of final physics observables. Hence, a precise measurement of the hadron beam polarization and a good control of systematic uncertainties are critical for the success of the spin program at the EIC. Contrary to the case of electron beam polarimetry, which uses physical processes derived from first principles that allow a high precision extraction of the electron beam polarization, for hadron beams no such process is available. The currently best used methods rely on the process of elastic scattering in the Coulomb-Nuclear Interference (CNI) region, for which there are only effective models available. The experience from RHIC, the only existing polarized proton collider, will be detailed and the challenges of the measurements at the EIC will be addressed. In particular, measurements of the present RHIC polarimeters and simulations of the future EIC polarimeters will be presented.

Coulomb phase corrections to the transverse analyzing power AN(t) in high-energy forward proton-proton scattering

Study of polarized proton-proton elastic scattering in the Coulomb-nuclear interference region allows one to measure the forward hadronic single spin-flip amplitude including its phase. However, in a precision experimental data analysis, a phase-shift correction ${\ensuremath{\delta}}_{C}$ due to the long distance Coulomb interaction should be taken into account. For unpolarized scattering, ${\ensuremath{\delta}}_{C}$ is commonly considered as well established. Here, we evaluate the Coulomb phase shifts for the forward elastic proton-proton single spin-flip electromagnetic and hadronic amplitudes. Only a small discrepancy between the spin-flip and nonflip phases was found which can be neglected in the high-energy forward elastic $pp$ studies involving transverse spin. Nonetheless, the effective alteration of the hadronic spin-flip amplitude by the long-distance electromagnetic corrections can be essential for interpretation of the experimental results.

TOTEM, recent experimental review

The TOTEM (TOTal cross section, Elastic scattering and diffraction dissociation Measurement at the LHC) experiment, located at the interaction point 5 of the LHC, has measured the total, elastic and inelastic proton-proton crosssections, using a luminosity independent method, based on the optical theorem, in a center-of-mass energy range from 2.76 to 13 TeV. The elastic scattering was investigated in a wide range of the squared four-momentum transfer |t| allowing study of the Coulomb-nuclear interference region down to |t| ∼ 8 × 10−4 GeV2. This made possible the first measurement of the ρ parameter at √s = 13 TeV, ρ being the ratio between the real and the imaginary part of the nuclear elastic scattering amplitude at t = 0. This measurement, combined with the total crosssection results, led to the exclusion of all the models classified and published by the COMPETE Collaboration. The results obtained by TOTEM are indeed compatible with predictions of a colourless 3-gluon bound state exchange in the t-channel of proton-proton elastic scattering, as postulated by alternative theoretical models both in the Regge-like framework and in the modern QCD framework. This result has been confirmed, with a significance of 5.4σ, also by the comparison with the pp data measured by the D0 collaboration at Fermilab. In this contribution the TOTEM experiment results will be described, along with the actual experiment status, the future physics program for the LHC Run 3.

Probing proton halo effects in the 8B+64Zn collision around the Coulomb barrier

Proton halo effects in the 8B+64Zn reaction at an energy around 1.5 times the Coulomb barrier have been studied at HIE-ISOLDE CERN using, for the first time, the only existing postaccelerated 8B beam. This, together with the use of a high granularity and large solid angle detection system, allowed for a careful mapping of the elastic angular distribution, especially in the Coulomb-nuclear interference region. Contrary to what is observed for the one-neutron halo nucleus 11Be on the same target in a similar energy range, the analysis of the elastic scattering angular distribution shows only a modest suppression of the Coulomb-nuclear interference peak, with no remarkable enhancement of the total reaction cross-section. Inclusive angular and energy distributions of 7Be produced in direct reaction processes have also been measured. The comparison of these data with the results of theoretical calculations for the elastic and non-elastic breakup contributions indicate that both processes are important. Overall, the experimental data suggest a 8B collision dynamics at the barrier very different from the one of neutron halo nuclei, showing only modest effects of coupling to continuum. This behaviour can be interpreted as due to the presence of the additional Coulomb interactions halo-core and halo-target together with the presence of the centrifugal barrier felt by the valence proton of 8B.

Probing the Pomeron spin structure with Coulomb-nuclear interference

Polarized pp elastic scattering at small angles in the Coulomb-nuclear interference (CNI) region offers a unique opportunity to study the spin structure of the Pomeron. Electromagnetic effects in elastic amplitude can be equivalently treated either as Coulomb corrections to the hadronic amplitude (Coulomb phase), or as absorption corrections to the Coulomb scattering amplitude. We perform the first calculation of the Coulomb phase for the spin-flip amplitude and found it significantly exceeding the widely used non-flip Coulomb phase. The alternative description in terms of absorption corrections, though equivalent, turned out to be a more adequate approach for the Coulomb corrected spin-flip amplitude. Inspired by the recent high statistics measurements of single-spin asymmetry with the HJET polarimeter at the BNL, we also performed a Regge analysis of data, aiming at disentangling the Pomeron contribution. However, in spite of an exceptional accuracy of the data, they do not allow to single out the Pomeron term, which strongly correlates with the major sub-leading Reggeons. A stable solution can be accessed only by making additional ad hoc assumptions, e.g. assuming the Pomeron to be a simple Regge pole, or fixing some unknown parameters. Otherwise, in addition to the STAR data at s=200GeV new measurements, say at 100GeV or 500GeV, could become decisive.

Measurement of proton-proton elastic scattering into the Coulomb region at Pbeam = 2.5, 2.8 and 3.2 GeV/c

The proton–proton elastic differential cross section at very small four momentum transfer squared has been measured at three different incident proton momenta in the range of 2.5 to 3.2 GeV/c by detecting the recoil proton at polar angles close to 90∘. The measurement was performed at COSY with the KOALA detector covering the Coulomb–nuclear interference region. The total cross section σtot, which has been determined precisely, is consistent with previous measurements. The values of the slope parameter B and the relative real amplitude ratio ρ determined in this experiment alleviate the lack of data in the relevant energy region. These precise data on ρ might be an important check for a new dispersion analysis.

Analysis of 6Li+16O elastic scattering using different potentials

Available experimental angular distributions for 6Li elastically scattered from 16O nucleus in the energy range 13.0–50.0 MeV are reanalyzed within the framework of optical potential, double folding optical potential as well as cluster folding potential. Special interest was paid to the cluster folding based on the well-known cluster structure of 6Li. Elastic scattering data for 6Li+16O system plotted as a function of momentum transfer showed that the real Coulomb nuclear interference region independent of the bombarding energy. This structural behavior for the data could be used to define the interaction potential with some certainty and to extract reliable values for the renormalization factors.

Two-Pomeron eikonal approximation for the high-energy elastic diffractive scattering of nucleons

It is demonstrated that the elastic diffractive scattering of nucleons at collision energies higher than 540 GeV and transferred momenta lower than 2 GeV, including the Coulomb-nuclear interference region, can be described in the framework of a very simple Regge-eikonal model where the eikonal is just a sum of two supercritical Regge pole terms. The predictive value of the proposed approximation is verified.

Peculiarities of 6Li +12C elastic scattering

Using the phenomenological optical potential and two different semi-microscopic potentials, namely double folding and cluster folding (CF), the available experimental data for 6Li elastically scattered from [Formula: see text]C nucleus at energies 50–600[Formula: see text]MeV are reanalyzed. On the basis of the well-known cluster structure of 6Li as a composite nucleus consisting of a core “alpha” with a valence particle “deuteron” orbiting this core, special attention was paid to the CF potential. Elastic scattering data for 6Li+[Formula: see text]C system plotted as a function of momentum transfer showed that the real Coulomb nuclear interference region is independent of the bombarding energy. The aforementioned structural behavior for the data could be used to define the potential with some certainty. In addition to a Woods–Saxon imaginary potential of fixed radius, the real part of the potential derived from the cluster structure of 6Li was successful in reproducing the experimental data in the whole angular range. Coupled channel (CC) calculation effects are also performed by coupling to 6Li resonant state ([Formula: see text], [Formula: see text][Formula: see text]MeV).

Search for polarized antiproton production

The production of antiprotons is studied in view of possible polarization effects as basis for a polarized antiproton beam. If antiprotons are produced with some polarization, a quite simple procedure for the generation of a polarized antiproton beam could be worked out. The experiments are performed at the CERN PS test beam T11 where secondary particles with momenta around 3.5 GeV/c are selected. The polarization analysis is performed by measuring the asymmetry of the elastic $\bar {p} p$-scattering in the Coulomb-nuclear interference region. The detection system includes Cherenkov and tracking detectors for the particle identification and the 3d track reconstruction. Details on the detection system and the status of the analysis are given.

Analysis and implications of precision near-forward TOTEM data

Very precise data on elastic proton-proton scattering at $\sqrt{s}=7$, $8$ and $13$ TeV have been obtained by the TOTEM group at LHC in the near-forward region (momentum transfers down to $|t| = 6 \times 10^{-4}\ {\rm GeV}^2$ at $\sqrt{s}=8$ TeV and to $|t| = 8 \times 10^{-4}\ {\rm GeV}^2$ at $\sqrt{s}=13$ TeV). The Coulomb-nuclear interference has been measured with sufficient accuracy for TOTEM to establish the falloff of the $\rho$ parameter with increasing energy. The predictions from a previously studied model are shown to be in good agreement with the data and thus allow us to draw rather firm conclusions about the structure of the near-forward nuclear amplitude. We point out that due to a zero in the real part of the nuclear amplitude occurring at a very small momentum transfer--that can migrate into the Coulomb-nuclear interference (CNI) region at higher energies--much care is needed in extracting the numerical value of $\rho$ for such energies. Thus, the true value of $\rho$ would be higher than the TOTEM value for $\rho$ found under the hypothesis that the real part of the elastic nuclear amplitude is devoid of such a zero in the CNI region.

The elastic differential pp cross-section at 13 TeV: an empirical model analysis

We discuss the differential elastic pp cross-section data measured at 13 TeV, through the Coulomb Nuclear Interference region until past the dip. We show that data are consistent with the asymptotic predictions from an empirical model and examine the presence of a zero for the real part of the elastic amplitude near the Coulomb region, well before the dip.

Influence of single-neutron stripping on near-barrier He6+Pb208 and He8+Pb208 elastic scattering

The influence of single-neutron stripping on the near-barrier elastic scattering angular distributions for the $^{6,8}\mathrm{He}+^{208}\mathrm{Pb}$ systems is investigated through coupled reaction channels (CRC) calculations fitting recently published data to explore the differences in the absorptive potential found in the scattering of these two neutron-rich nuclei. The inclusion of the coupling reduces the elastic cross section in the Coulomb-nuclear interference region for $^{8}\mathrm{He}$ scattering, whereas for $^{6}\mathrm{He}$ its major impact is on the large-angle elastic scattering. The real and imaginary dynamic polarization potentials are obtained by inverting the CRC elastic scattering $S$-matrix elements. These show that the main absorptive features occur between 11 and 12 fm for both projectiles, while the attractive features are separated by about 1 fm, with their main structures occurring at 10.5 fm for $^{6}\mathrm{He}$ and 11.5 fm for $^{8}\mathrm{He}$.

Measurement of elastic pp scattering at $$\sqrt{\hbox {s}} = \hbox {8}$$ s = 8 TeV in the Coulomb–nuclear interference region: determination of the $$\mathbf {\rho }$$ ρ -parameter and the total cross-section

The TOTEM experiment at the CERN LHC has measured elastic proton-proton scattering at the centre-of-mass energy $\sqrt{s}$ = 8 TeV and four-momentum transfers squared, |t|, from 6 x $10^{-4}$ GeV$^2$ to 0.2 GeV$^2$. Near the lower end of the |t|-interval the differential cross-section is sensitive to the interference between the hadronic and the electromagnetic scattering amplitudes. This article presents the elastic cross-section measurement and the constraints it imposes on the functional forms of the modulus and phase of the hadronic elastic amplitude. The data exclude the traditional Simplified West and Yennie interference formula that requires a constant phase and a purely exponential modulus of the hadronic amplitude. For parametrisations of the hadronic modulus with second- or third-order polynomials in the exponent, the data are compatible with hadronic phase functions giving either central or peripheral behaviour in the impact parameter picture of elastic scattering. In both cases, the $\rho$-parameter is found to be 0.12 $\pm$ 0.03. The results for the total hadronic cross-section are $\sigma_{tot}$ = (102.9 $\pm$ 2.3) mb and (103.0 $\pm$ 2.3) mb for central and peripheral phase formulations, respectively. Both are consistent with previous TOTEM measurements.

Evidence for Non-Exponential Differential Cross-Section of pp Elastic Scattering at Low |t| and √s= 8 TeV by TOTEM

Recently published and preliminary results of the TOTEM experiment are presented, emphasizing a recent discovery of a non-exponential behaviour of the differential crosssection of elastic proton-proton scattering, that TOTEM measured with an unprecedented precision at the centre-of-mass energy √ s = 8 TeV based on a high-statistics data sample obtained with the β * = 90 m optics of CERN LHC. Both the statistical and systematic uncertainties remained below 1%, except for the t-independent contribution from the overall normalisation. This measurement allowed TOTEM to exclude a purely exponential differential cross-section in the range of four-momentum transfer squared 0.027 t | 2 with a significance greater than 7σ. In this context we also highlight the innovative TOTEM recalibration of LHC optics, that used elastic scattering data measured by the world’s largest and most complex Roman Pot detector system, and discuss recent preliminary TOTEM data on the Coulomb-Nuclear interference region with its physics implications.

Improving results on transverse double spin asymmetries in the CNI region at STAR

Double spin effects in polarized pp-elastic scattering in the Coulomb nuclear interference (CNI) region are sensitive to small contributions to the nuclear amplitude in addition to Pomeron exchange dominating at high energies. Measurements of double spin asymmetries require external luminosity normalization using collision counts for all spin combinations. Several possible sources of such data from various STAR subsystems were thoroughly analyzed to make the best choice. BBC arrays were found to be free of double spin effects to the level of ∼ 2 × 10−4 thus leading to the systematic uncertainty ∼10−3 in the value of (ANN + ASS)/2.

Elastic scattering of the halo nucleus11Be on64Zn

Elastic scattering cross sections for the halo nucleus 11 Be incident on 64 Zn in vicinity of Coulomb barrier are calculated using the microscopic double folding model. The parametrized densities taking into account for the halo part are folded with the effec- tive nucleon-nucleon interaction (M3Y) to yield the microscopic double folded potential. The dynamic polarization potential has been computed using the dipole strength distri- bution from the cluster model and from experiments. The calculated differential cross section with the inclusion of the imaginary part of the dynamic polarization potential to the double folded potential show a suppression in the Coulomb-nuclear interference region.

Evaluation of the pion–nucleon sigma term from CHAOS data

We have reanalyzed the π±p scattering data at low energy in the Coulomb-nuclear interference region as measured by the CHAOS group at TRIUMF with the aim to determine the pion–nucleon σ term. The resulting value σ=(44±12) MeV, while in agreement with lattice QCD calculations and compatible with other recent analyses, is significantly lower than that from the GWU–TRIUMF analysis of 2002.

Model-independent data reductions of elastic proton–proton scattering

New developments in empirical analyses of the proton–proton differential cross section data at high energies are reported. Making use of an unconstrained model-independent parametrization for the scattering amplitude and two different fit procedures, all the experimental data in the center-of-mass energy interval 19.4–62.5 GeV are quite well described (optical point and data above the region of Coulomb-nuclear interference). The contributions from the real and imaginary parts of the amplitude beyond the forward direction are discussed and compared with the results from previous analyses and phenomenological models. Extracted overlap functions (impact parameter space) are outlined and a critical discussion on model-independent analyses and results are also presented.