arXiv subjects

Otto Nachtmann

Publications and source records attributed to Otto Nachtmann.

At least 55 records · Page 3Linked to original sources

arXiv2017

Exclusive diffractive production of hadrons in $pp$ collisions

We discuss central exclusive diffractive dihadron production in proton-proton collisions at high energies. The calculation is based on a tensor pomeron model and the amplitudes for the processes are formulated in an effective field-theoretic approach. We include a purely diffractive dipion continuum, and the scalar and tensor resonances decaying into the $π^{+} π^{-}$ pairs as well as the photoproduction contributions ($ρ^{0}$, Drell-Söding). We discuss how two pomerons couple to tensor meson $f_{2}(1270)$ and the interference effects of the scalar and tensor resonances and the dipion continuum. The theoretical results are compared with existing CDF and CMS experimental data. We discuss also the Drell-Hiida-Deck type mechanism with centrally produced $ρ^{0}$ meson associated with a very forward/backward $πN$ system. For the $pp \to pp π^{+} π^{-} π^{+} π^{-}$ reaction we consider both the $σσ$ and $ρρ$ contributions as well as the triple Regge exchange mechanism. Predictions for planned or being carried out experiments (STAR, ALICE, ATLAS, CMS) are presented. We show the influence of the experimental cuts on the integrated cross section and on various differential distributions for outgoing particles.

hep-ph↗

arXiv2016

Central production of $ρ^{0}$ in $pp$ collisions with single proton diffractive dissociation at the LHC

We consider the $pp \to pp ρ^{0} π^{0}$ and $pp \to pn ρ^{0} π^{+}$ processes at LHC energies. Our description is based on the nonperturbative framework of tensor pomeron and tensor reggeon exchanges. We discuss the Drell-Hiida-Deck type mechanism with centrally produced $ρ^{0}$ meson associated with a very forward/backward $πN$ system. The considered processes constitute an inelastic (non-exclusive) background to the $p p \to p p ρ^0$ reaction in the case when only the centrally produced $ρ^{0}$ meson decaying into $π^{+} π^{-}$ is measured, the final state protons are not observed, and only rapidity-gap conditions are checked experimentally. We compare our results for the $γπ^{+} \to ρ^{0} π^{+}$ reaction with the experimental data obtained by the H1 collaboration at HERA. We present several differential distributions for the $pp \to pn ρ^{0} π^{+}$ reaction and estimate the size of the proton dissociative background to the exclusive $p p \to p p ρ^0$ process. The ratio of integrated cross sections for the inelastic $p p \to p N ρ^0 π$ processes, where $p N ρ^0 π$ stands for $p n ρ^0 π^{+}$ plus $p p ρ^0 π^{0}$, to the reference reaction $pp \to \ pp ρ^0$ is of order of (7--10)\%. We present also the ratios of the $ρ^0$ rapidity and transverse momentum distributions for the inelastic $p p \to p N ρ^0 π$ versus the elastic $pp \to \ pp ρ^0$ reaction. Our results may be used to investigate the $γπ\to ρ^{0} π$ process at LHC energies.

hep-ph↗

arXiv2016

Exclusive production of two and four pions in proton-proton scattering

We consider exclusive $pp \to pp π^{+} π^{-}$ and $pp \to pp π^{+} π^{-}π^{+} π^{-}$ reactions at high energies. The calculation is based on a tensor pomeron model and the amplitudes for the processes are formulated in an effective field-theoretic approach. In the case of $pp \to pp π^{+} π^{-}$ process we consider both diffractive and photoproduction mechanisms and we include the non-resonant $π^{+}π^{-}$ continuum and the resonance $f_{0}(500)$, $f_{0}(980)$, $f_{2}(1270)$, $ρ(770)$ contributions. We discuss how two pomerons couple to tensor meson $f_{2}(1270)$ and the interference effects of resonance and dipion continuum. We find that the relative contribution of resonances $ρ(770)$, $f_2(1270)$ and dipion continuum strongly depends on the cut on proton transverse momenta. In the case of exclusive central $4 π$ production we include the contribution via the intermediate $σσ$ and $ρρ$ states. For both processes the theoretical results have been compared with the experimental data and predictions for planned or being carried out experiments (e.g. STAR, ATLAS-ALFA) are presented.

hep-ph↗

arXiv2016

Exclusive diffractive production of $π^+ π^-$ pairs within tensor pomeron approach

We discuss exclusive central diffractive production of $π^{+} π^{-}$ in proton-(anti)proton collisions at high energies. Based on a tensor pomeron model we present results of the purely diffractive dipion continuum, the scalar $f_{0}(500)$, $f_{0}(980)$ and tensor $f_{2}(1270)$ resonances decaying into the $π^{+} π^{-}$ pairs as well as the photoproduction mechanism ($ρ^{0}$, Drell-Söding). We discuss how two pomerons couple to the tensor meson $f_{2}(1270)$ and the interference effects of resonance and dipion continuum contributions. The theoretical results are compared with existing STAR, CDF, and CMS experimental data. Predictions for planned or being carried out experiments (ALICE, ATLAS) are presented. We find that the relative contribution of resonant $f_2(1270)$ and dipion continuum strongly depend on the cut on proton transverse momenta (or four-momentum transfer squared $t_{1,2}$) which may explain some controversial observations made by different ISR experiments in the past. The cuts may play then the role of a $ππ$ resonance filter.

hep-ph↗

arXiv2016

Helicity in Proton-Proton Elastic Scattering and the Spin Structure of the Pomeron

We discuss different models for the spin structure of the nonperturbative pomeron: scalar, vector, and rank-2 symmetric tensor. The ratio of single-helicity-flip to helicity-conserving amplitudes in polarised high-energy proton-proton elastic scattering, known as the complex r_5 parameter, is calculated for these models. We compare our results to experimental data from the STAR experiment. We show that the spin-0 (scalar) pomeron model is clearly excluded by the data, while the vector pomeron is inconsistent with the rules of quantum field theory. The tensor pomeron is found to be perfectly consistent with the STAR data.

hep-ph↗

arXiv2016

Exclusive diffractive production of $π^{+}π^{-}π^{+}π^{-}$ via the intermediate $σσ$ and $ρρ$ states in proton-proton collisions within tensor pomeron approach

We present first predictions of the cross sections and differential distributions for the central exclusive reaction $pp \to pp π^{+} π^{-} π^{+} π^{-}$ being studied at RHIC and LHC. The amplitudes for the processes are formulated in terms of the tensor pomeron and tensor $f_{2 R}$ reggeon exchanges with the vertices respecting the standard crossing and charge-conjugation relations of Quantum Field Theory. The $σσ$ and $ρρ$ contributions to the $π^{+} π^{-} π^{+} π^{-}$ final state are considered focussing on their specificities. The correct inclusion of the pomeron spin structure seems crucial for the considered sequential mechanisms in particular for the $ρρ$ contribution which is treated here for the first time. The mechanism considered gives a significant contribution to the $pp \to pp π^{+} π^{-} π^{+} π^{-}$ reaction. We adjust parameters of our model to the CERN-ISR experimental data and present several predictions for the STAR, ALICE, ATLAS and CMS experiments. A measurable cross section of order of a few $μb$ is obtained including the experimental cuts relevant for the LHC experiments. We show the influence of the experimental cuts on the integrated cross section and on various differential distributions.

hep-ph↗

arXiv2016

Central exclusive diffractive production of $π^{+}π^{-}$ continuum, scalar and tensor resonances in $pp$ and $p \bar{p}$ scattering within tensor pomeron approach

We consider central exclusive diffractive dipion production in the reactions $pp \to pp π^{+} π^{-}$ and $p\bar{p} \to p\bar{p} π^{+} π^{-}$ at high energies. We include the dipion continuum, the dominant scalar $f_{0}(500)$, $f_{0}(980)$, and tensor $f_{2}(1270)$ resonances decaying into the $π^{+} π^{-}$ pairs. The calculation is based on a tensor pomeron model and the amplitudes for the processes are formulated in terms of vertices respecting the standard crossing and charge-conjugation relations of Quantum Field Theory. The formulae for the dipion continuum and tensor meson production are given here for the first time. The theoretical results are compared with existing STAR, CDF, CMS experimental data and predictions for planned or being carried out experiments (ALICE, ATLAS) are presented. We show the influence of the experimental cuts on the integrated cross section and on various differential distributions for outgoing particles. Distributions in rapidities and transverse momenta of outgoing protons and pions as well as correlations in azimuthal angle between them are presented. We find that the relative contribution of resonant $f_2(1270)$ and dipion continuum strongly depends on the cut on proton transverse momenta or four-momentum transfer squared $t_{1,2}$ which may explain some controversial observations made by different ISR experiments in the past. The cuts may play then the role of a $ππ$ resonance filter. We suggest some experimental analyses to fix model parameters related to the pomeron-pomeron-$f_{2}$ coupling.

hep-ph↗

arXiv2016

Geometric phases causing lifetime modifications of metastable states of hydrogen

Externally applied electromagnetic fields in general have an influence on the width of atomic spectral lines. The decay rates of atomic states can also be affected by the geometry of an applied field configuration giving rise to an imaginary geometric phase. A specific chiral electromagnetic field configuration is presented which geometrically modifies the lifetimes of metastable states of hydrogen. We propose to extract the relevant observables in a realistic longitudinal atomic beam spin-echo apparatus which allows the initial and final fluxes of the metastable atoms to be compared with each other interferometrically. A geometry-induced change in lifetimes at the 5%-level is found, an effect large enough to be observed in an available experiment.

physics.atom-ph↗

arXiv2014

The $ρ^0$ and Drell-Söding contributions to central exclusive production of $π^+ π^-$ pairs in proton-proton collisions at high energies

We present a study of the central exclusive $π^{+} π^{-}$ production via the photoproduction mechanism in nucleon-nucleon collisions. The photon-pomeron/reggeon and pomeron/reggeon-photon exchanges both for the $ρ^{0}$ resonance contribution and the Drell-Söding contribution are considered. The amplitudes for the processes are formulated in terms of vertices respecting the standard crossing and charge-conjugation relations of Quantum Field Theory. The coupling parameters of tensor pomeron and reggeon exchanges are fixed based on the H1 and ZEUS experimental data for the $γp \to ρ^{0} p$ reaction. We present first predictions of this mechanism for the $pp \to pp π^{+} π^{-}$ reaction being studied at COMPASS, RHIC, Tevatron, and LHC. We show the influence of the experimental cuts on the integrated cross section and on various differential distributions for outgoing particles. Distributions in rapidities and transverse momenta of outgoing protons and pions as well as correlations in azimuthal angle between them are presented. We compare the photoproduction contribution to $π^{+} π^{-}$ distributions with double pomeron/reggeon two-pion continuum. We discuss whether the high-energy central production of $ρ^{0}$ mesons could be selected experimentally.

hep-ph↗

arXiv2014

Spin correlations in the Drell-Yan process, parton entanglement, and other unconventional QCD effects

We review ideas on the structure of the QCD vacuum which had served as motivation for the discussion of various non-standard QCD effects in high-energy reactions in articles from 1984 to 1995. These effects include, in particular, transverse-momentum and spin correlations in the Drell-Yan process and soft photon production in hadron-hadron collisions. We discuss the relation of the approach introduced in the above-mentioned articles to the approach, developed later, using transverse-momentum-dependent parton distributions (TDMs). The latter approach is a special case of our more general one which allows for parton entanglement in high-energy reactions. We discuss signatures of parton entanglement in the Drell-Yan reaction. Also for Higgs-boson production in pp collisions via gluon-gluon annihilation effects of entanglement of the two gluons are discussed and are found to be potentially important. These effects can be looked for in the current LHC experiments. In our opinion studying parton-entanglement effects in high-energy reactions is, on the one hand, very worthwhile by itself and, on the other hand, it allows to perform quantitative tests of standard factorisation assumptions. Clearly, the experimental observation of parton-entanglement effects in the Drell-Yan reaction and/or in Higgs-boson production would have a great impact on our understanding how QCD works in high-energy collisions.

hep-ph↗

arXiv2014

Exclusive central diffractive production of scalar, pseudoscalar and vector mesons

We discuss exclusive central diffractive production of scalar ($f_{0}(980)$, $f_{0}(1370)$, $f_{0}(1500)$), pseudoscalar ($η$, $η'(958)$), and vector ($ρ^{0}$) mesons in proton-proton collisions. The amplitudes are formulated in terms of effective vertices required to respect standard rules of Quantum Field Theory and propagators for the exchanged pomeron and reggeons. Different pomeron-pomeron-meson tensorial (vectorial) coupling structures are possible in general. In most cases two lowest orbital angular momentum - spin couplings are necessary to describe experimental differential distributions. For the $f_{0}(980)$ and $η$ production the reggeon-pomeron, pomeron-reggeon, and reggeon-reggeon exchanges are included in addition, which seems to be necessary at relatively low energies. The theoretical results are compared with the WA102 experimental data, in order to determine the model parameters. For the $ρ^{0}$ production the photon-pomeron and pomeron-photon exchanges are considered. The coupling parameters of tensor pomeron and/or reggeon are fixed from the H1 and ZEUS experimental data of the $γp \to ρ^{0} p$ reaction. We present first predictions of this mechanism for $pp \to pp π^{+} π^{-}$ reaction being studied at COMPASS, RHIC, Tevatron, and LHC. Correlation in azimuthal angle between outgoing protons and distribution in pion rapidities at $\sqrt{s} = 7$TeV are presented. We show that high-energy central production of mesons could provide crucial information on the spin structure of the soft pomeron.

hep-ph↗

arXiv2014

Photoproduction of pi+ pi- pairs in a model with tensor-pomeron and vector-odderon exchange

We consider the reaction (gamma p) to (pi+ pi- p) at high energies. Our description includes dipion production via the resonances rho, omega, rho-prime and f2, and via non-resonant mechanisms. The calculation is based on a model of high energy scattering with the exchanges of photon, pomeron, odderon and reggeons. The pomeron and the C=+1 reggeons are described as effective tensor exchanges, the odderon and the C=-1 reggeons as effective vector exchanges. We obtain a gauge-invariant version of the Drell-Soeding mechanism which produces the skewing of the rho-meson shape. Starting from the explicit formulae for the matrix element for dipion production we construct an event generator which comprises all contributions mentioned above and includes all interference terms. We give examples of total and differential cross sections and discuss asymmetries which are due to interference of C=+1 and C=-1 exchange contributions. These asymmetries can be used to search for odderon effects. Our model is intended to provide all necessary theoretical tools for a detailed experimental analysis of elastic dipion production for which data exist from fixed target experiments, from HERA, and are now being collected by LHC experiments.

hep-ph↗

arXiv2014

Applications of the tensor pomeron model to exclusive central diffractive meson production

We discuss exclusive central diffractive production of scalar ($f_{0}(980)$, $f_{0}(1370)$, $f_{0}(1500)$), pseudoscalar ($η$, $η'(958)$), and vector ($ρ^{0}$) mesons in proton-proton collisions. We show that high-energy central production of mesons could provide crucial information on the spin structure of the soft pomeron. The amplitudes are formulated in terms of effective vertices respecting standard rules of Quantum Field Theory and propagators for the exchanged pomeron and reggeons. For the scalar and pseudoscalar meson production, in most cases, two lowest orbital angular momentum - spin couplings are necessary to describe WA102 experimental differential distributions. Different pomeron-pomeron-meson tensorial (vectorial) coupling structures are possible in general. For the $ρ^{0}$ production the photon-tensor pomeron/reggeon exchanges are considered and the coupling parameters are fixed from the H1 and ZEUS experimental data of the $γp \to ρ^{0} p$ reaction. We present first predictions of this mechanism for the $pp \to pp (ρ^{0} \to π^{+} π^{-})$ reaction being studied at COMPASS, RHIC, Tevatron, and LHC. We analyse influence of the experimental cuts on integrated cross section and various differential distributions for produced mesons.

hep-ph↗

arXiv2013

Exclusive central diffractive production of scalar and pseudoscalar mesons; tensorial vs. vectorial pomeron

We discuss consequences of the models of "tensorial pomeron" and "vectorial pomeron" for exclusive diffractive production of scalar and pseudoscalar mesons in proton-proton collisions. Diffractive production of $f_{0}(980)$, $f_{0}(1370)$, $f_{0}(1500)$, $η$, and $η'(958)$ mesons is discussed. Different pomeron-pomeron-meson tensorial coupling structures are possible in general. In most cases two lowest orbital angular momentum - spin couplings are necessary to describe experimental differential distributions. For $f_{0}(980)$ and $η$ production reggeon-pomeron, pomeron-reggeon, and reggeon-reggeon exchanges are included in addition, which seems to be necessary at relatively low energies. The theoretical results are compared with the WA102 experimental data. Correlations in azimuthal angle between outgoing protons, distributions in rapidities and transverse momenta of outgoing protons and mesons, in a special "glueball filter variable", as well as some two-dimensional distributions are presented. We discuss differences between results of the vectorial and tensorial pomeron models. We show that high-energy central production, in particular of pseudoscalar mesons, could provide crucial information on the spin structure of the soft pomeron.

hep-ph↗

arXiv2013

A Model for Soft High-Energy Scattering: Tensor Pomeron and Vector Odderon

A model for soft high-energy scattering is developed. The model is formulated in terms of effective propagators and vertices for the exchange objects: the pomeron, the odderon, and the reggeons. The vertices are required to respect standard rules of QFT. The propagators are constructed taking into account the crossing properties of amplitudes in QFT and the power-law ansaetze from the Regge model. We propose to describe the pomeron as an effective spin 2 exchange. This tensor pomeron gives, at high energies, the same results for the p-p and p-pbar elastic amplitudes as the standard Donnachie-Landshoff pomeron. But with our tensor pomeron it is much more natural to write down effective vertices of all kinds which respect the rules of QFT. This is particularly clear for the coupling of the pomeron to particles carrying spin, for instance vector mesons. We describe the odderon as an effective vector exchange. We emphasise that with a tensor pomeron and a vector odderon the corresponding charge-conjugation relations are automatically fulfilled. We compare the model to some experimental data, in particular to data for the total cross sections, in order to determine the model parameters. The model should provide a starting point for a general framework for describing soft high-energy reactions. It should give to experimentalists an easily manageable tool for calculating amplitudes for such reactions and for obtaining predictions which can be compared in detail with data.

hep-ph↗

arXiv2012

The New F_L Measurement from HERA and the Dipole Model

From the new measurement of F_L at HERA we derive fixed-Q^2 averages . We compare these with bounds which are rigorous in the framework of the standard dipole picture. The bounds are sharpened by including information on the charm structure function F_2^(c). Within the experimental errors the bounds are respected by the data. But for 3.5 GeV^2 <= Q^2 <= 20 GeV^2 the central values of the data are close to and in some cases even above the bounds. Data on F_L/F_2 significantly exceeding the bounds would rule out the standard dipole picture at these kinematic points. We discuss, furthermore, how data respecting the bounds but coming close to them can give information on questions like colour transparency, saturation and the dependencies of the dipole-proton cross section on the energy and the dipole size.

hep-ph↗

arXiv2010

On the Energy Dependence of the Dipole-Proton Cross Section in Deep Inelastic Scattering

We study the dipole picture of high-energy virtual-photon-proton scattering. It is shown that different choices for the energy variable in the dipole cross section used in the literature are not related to each other by simple arguments equating the typical dipole size and the inverse photon virtuality, contrary to what is often stated. We argue that the good quality of fits to structure functions that use Bjorken-x as the energy variable - which is strictly speaking not justified in the dipole picture - can instead be understood as a consequence of the sign of scaling violations that occur for increasing Q^2 at fixed small x. We show that the dipole formula for massless quarks has the structure of a convolution. From this we obtain derivative relations between the structure function F_2 at large and small Q^2 and the dipole-proton cross section at small and large dipole size r, respectively.

hep-ph↗

arXiv2009

Ioffe Times in DIS from a Dipole Model Fit

We present a study of Ioffe times in deep inelastic electron-proton scattering. We deduce 'experimental' Ioffe-time distributions from the small-x HERA data as described by a particular colour-dipole-model fit. We show distributions for three representative gamma*-proton c.m. energies W and various values of the photon virtuality Q^2. These distributions are rather broad for transversely and very narrow for longitudinally polarised virtual photons. The Ioffe times for W=150 GeV, for example, range from around 1000 fm for Q^2=1 GeV^2 to around 10 fm for Q^2=100 GeV^2. Based on our results we discuss consequences for the limitations of applicability of the dipole picture.

hep-ph↗