Search

We study the prompt production of the $\chi_c(1^+)$ and $\chi_b(1^+)$ mesons at high energies. Unlike $\chi(0^+,2^+)$ production, $\chi (1^+)$ mesons cannot be created at LO via the fusion of two on-mass-shell gluons, that is $gg\to\chi_{c,b}(1^+)$ are not allowed. However, the available experimental data show that the cross sections for $\chi_c(1^+)$ and $\chi_c(2^+)$ are comparable. We therefore investigate four other $\chi(1^+)$ production mechanisms: namely, (i) the standard NLO process $gg\to\chi_{c,b}(1^+)+g$, (ii) via gluon virtuality, (iii) via gluon reggeization and, finally, (iv) the possibility to form $\chi_{c,b}(1^+)$ by the fusion of three gluons, where one extra gluon comes from another parton cascade, as in the Double Parton Scattering processes.

We note that the definition of diffractive events is a matter of convention. We discuss two possible `definitions': one based on unitarity and the other on Large Rapidity Gaps (LRG) or Pomeron exchange. LRG can also arise from fluctuations and we quantify this effect and some of the related uncertainties. We find care must be taken in extracting the Pomeron contribution from LRG events. We show that long-range correlations in multiplicities can arise from the same multi-Pomeron diagrams that are responsible for LRG events, and explain how early LHC data can illuminate our understanding of `soft' interactions., 19 pages, 8 figures; v2: minor changes, version accepted by EPJC

We show how parton distributions unintegrated over the parton transverse momentum, k_t, may be generated, at NLO accuracy, from the known integrated (DGLAP-evolved) parton densities determined from global data analyses. A few numerical examples are given, which demonstrate that sufficient accuracy is obtained by keeping only the LO splitting functions together with the NLO integrated parton densities. However, it is important to keep the precise kinematics of the process, by taking the scale to be the virtuality rather than the transverse momentum, in order to be consistent with the calculation of the NLO splitting functions., Comment: 20 pages, 8 figures. v2: version to appear in Eur. Phys. J. C

We justify the practical use of the Shuvaev integral transform approach to calculate the skewed distributions, needed to describe diffractive processes, directly from the conventional diagonal global parton distributions. We address doubts which have been raised about this procedure. We emphasise that the approach, on the one hand, satisfies all theoretical reqirements, and, on the other hand, is consistent with DVCS data at NLO. We construct an easily accessible package for the computation of these skewed distributions., Comment: 21 pages, 10 figures. New title. Extra Fig. 2 and extra Section 5 to compare with alternative treatment of GPDs. Numerical results unchanged. To be published in EPJC

We calculate the probability that the rapidity gaps in diffractive processes survive both eikonal and enhanced rescattering. We present arguments that enhanced rescattering, which violates soft-hard factorization, is not very strong. Accounting for NLO effects, there is no reason to expect that the black disc regime is reached at the LHC. We discuss the predictions for the survival of the rapidity gaps for exclusive Higgs production at the LHC., 15 pages, 6 figures, version to be published in EPJC

We show how the early data runs of the LHC can provide valuable checks of the different components of the formalism used to predict the cross sections of central exclusive processes. The `soft' rapidity gap survival factor can be studied in electroweak processes, such as W+gaps events, where the bare amplitude is well known. The generalized gluon distribution, in the appropriate kinematic region, can be probed by exclusive Upsilon production. The perturbative QCD effects, especially the Sudakov-like factor, can be probed by exclusive two- and three-jet production. We discuss the possible role of enhanced absorptive corrections which would violate the soft-hard factorization implied in the usual formalism, and suggest ways that the LHC may explore their presence., Comment: 28 pages, 12 figures; text clarified, new figure added, version to be published in Eur. Phys. J. C

We investigate the uncertainty in the value of the strong coupling $\alpha_S(M_Z^2)$ when allowing it to be a free parameter in the recent MMHT global analyses of deep-inelastic and related hard scattering data that was undertaken to determine the parton distribution functions (PDFs) of the proton. The analysis uses the standard framework of leading twist fixed--order collinear factorisation in the ${\overline {\rm MS}}$ scheme. We study the constraints on the value of $\alpha_S(M_Z^2)$ coming from the individual data sets by repeating the NNLO and NLO global analyses at various fixed values of $\alpha_S(M_Z^2)$ about its optimum values, spanning the range $\alpha_S(M_Z^2)=0.108$ to $0.128$ in units of $0.001$, and making all PDFs sets available. The inclusion of the measurements of the cross section for inclusive $t\bar{t}$ production in the global fit allows us to explore the correlation between the values taken for the mass $m_t$ of the top quark and $\alpha_S(M_Z^2)$. We find that the best fit values are $\alpha_S(M_Z^2)=0.1201\pm0.0015$ and $0.1172\pm 0.0013$ at NLO and NNLO respectively, with the central values changing to $\alpha_S(M_Z^2)=0.1195$ and $0.1178$ when the world average of $\alpha_S(M_Z^2)$ is used as a data point. We investigate the interplay between the uncertainties on $\alpha_S(M_Z^2)$ and the uncertainties on the PDFs. In particular we calculate the cross sections for key processes at the LHC and show how the uncertainties coming from the standard MMHT2014 PDFs and from $\alpha_S(M_Z^2)$ can be provided independently and can be combined., Comment: 30 pages, 62 figures. Final version - minor corrections to text

We propose a new variable, R_j, in order to identify exclusive double-diffractive high E_T dijet production. The variable R_j is calculated using the transverse energy E_T and pseudorapidity of the jet with the largest E_T. For a purely exclusive event the value of R_j -> 1, if we were to neglect hadronisaton and the detector resolution effects. To illustrate the expected R_j-distribution we also compute exclusive three-jet production; and, moreover, include jet smearing effects. By studying the predictions as a function of the size of the rapidity interval, delta(eta), which allows for additional gluon radiation, one can probe the QCD radiation effects which are responsible for the Sudakov suppression of the exclusive amplitude. In this way we may check, and improve, the formalism used to predict the cross sections of exclusive double-diffractive Higgs boson (and/or other New Physics) production., 18 pages, 6 figures. More details are given for the calculation of exclusive 3-jet production. The R_j distributions of exclusive 3-jet production in Figs. 4 and 5 are corrected. This version is to be published in Eur. Phys. J. C

We review a physical content of the two simplest models of reggeon field theory: namely the eikonal and the Schwimmer models. The AGK cutting rules are used to obtain the inclusive, the inelastic and the diffractive cross sections. The system of nonlinear equations for these cross sections is written down and analytic expressions for its solution are obtained. We derive the rapidity gap dependence of the differential cross sections for diffractive dissociation in the Schwimmer model and in its eikonalized extension. The results are interpreted from the partonic viewpoint of the interaction at high energies., Comment: 18 pages, 8 figures. Refs. [20],[21] added, and the comparison with results of [21] inserted

We consider the prompt hadroproduction of J/psi, psi' and the Upsilon (1S,2S,3S) states caused by the fusion of a symmetric colour-octet two-gluon state and an additional gluon. The cross sections are calculated in leading-order perturbative QCD. We find a considerable enhancement in comparison with previous perturbative QCD predictions. Indeed, the resulting cross sections are found to be consistent with the values measured at the Tevatron and RHIC, without the need to invoke non-perturbative `colour-octet' type of contributions., Comment: 21 pages, 10 figures; several clarifying sentences and an additional reference have been added

The double-diffractive production of $\chi_c$ and $\chi_b$ mesons, with a rapidity gap on either side, is studied, using both the Regge formalism and the perturbative QCD approach. Due to the rather low scale, the exclusive double-diffractive process $pp\to p + \chi + p$ is predicted to dominate, whereas the probability that the incoming protons dissociate is expected to be relatively small. We evaluate the corresponding $\chi$ production cross sections at the Tevatron and LHC energies. For the double-diffractive process with proton dissociation, it is possible to select events with large transverse momenta transferred through the rapidity gaps, by measuring the transverse energy, $E_\perp$ , flows in the proton fragmentation regions. Then the large $E_\perp$ provides a scale to justify the use of perturbative QCD, and to allow a spin-parity analysis of the centrally produced system to be performed, by studying the azimuthal angular correlations between the directions of the forward and backward $E_\perp$ flows. The central production of the new X(3872) charmonium state is considered.

We present the general rules for double-Reggeon production of objects with different spins and parities. The existing experimental information on resonance production in these central exclusive diffractive processes is discussed. The absorptive corrections are calculated and found to depend strongly on the quantum numbers of the produced states. The central exclusive diffractive production of 0^+ and 0^- Higgs bosons is studied as an illustrative topical example of the use of the general rules. The signal for diffractive 0^+ and 0^- Higgs production at the LHC is evaluated using, as an example, the minimal supersymmetric model, with large tan(beta)., Comment: 19 pages, 7 figures. Version to be published in EPJC: Subsection 3.2 and one reference added; Figures 4,5,6,7 modified

We describe how unintegrated parton distributions can be calculated from conventional integrated distributions. We extend and improve the 'last-step' evolution approach, and explain why doubly-unintegrated parton distributions are necessary. We generalise k_t-factorisation to (z,k_t)-factorisation. We apply the formalism to inclusive jet production in deep-inelastic scattering, mainly at leading-order, but we also study the extension to next-to-leading order. We compare the predictions with recent HERA data., Comment: 32 pages, 11 figures. Version to appear in Eur. Phys. J. C

We show that high energy hadronic reactions which contain a rapidity gap and a hard subprocess have a specific dependence on the kinematic variables, which results in a characteristic behaviour of the survival probability of the gap. We incorporate this mechanism in a two-channel eikonal model to make an essentially parameter-free estimate of diffractive dijet production at the Tevatron, given the diffractive structure functions measured at HERA. The estimates are in surprising agreement with the measurements of the CDF collaboration. We briefly discuss the application of the model to other hard processes with rapidity gaps., 18 pages, 5 figures, Latex

We study the theoretical accuracy of various methods that have been proposed to measure the luminosity of the LHC pp collider, as well as for Run II of the Tevatron p barp collider. In particular we consider methods based on (i) the total and forward elastic data, (ii) lepton-pair production and (iii) W and Z production.

We present a formalism for high energy soft processes, mediated by Pomerons, which embodies pion-loop insertions in the Pomeron trajectory, rescattering effects via a two-channel eikonal and high-mass diffractive dissociation. It describes all the main features of the data throughout the ISR to Tevatron energy interval. We give predictions for soft diffractive phenomena at the LHC energy, and we calculate the different survival probabilities of rapidity gaps which occur in various diffractive processes., 32 pages, 12 figures, LaTeX; version to be published in EPJC, minor changes to the text, references and footnotes

We show that the conventional prescription used for DGLAP parton evolution at NLO has an inconsistent treatment of the contribution from the infrared (IR) region. We illustrate the problem by studying the simple example of QED evolution, treating the electron and photon as partons. The deficiency is not present in a physical approach which removes the IR divergency and allows calculation in the normal 4-dimensional space., 15 pages, 2 figures, erratum at the end of the article

We show that the diffractive pp (and p\bar{p}) data (on \sigma_tot, d\sigma_el/dt, proton dissociation into low-mass systems, \sigma^D(low M), and high-mass dissociation, d\sigma/d(\Delta\eta)) in a wide energy range from CERN-ISR to LHC energies, may be described in a two-channel eikonal model with only one `effective' pomeron. By allowing the pomeron coupling to the diffractive eigenstates to depend on the collider energy (as is expected theoretically) we are able to explain the low value of \sigma^D(low M) measured at the LHC. We calculate the survival probability, S^2, of a rapidity gap to survive `soft rescattering'. We emphasize that the values found for S^2 are particularly sensitive to the detailed structure of the diffractive eigenstates., Comment: 20 pages, 6 figures. Version to be published in EPJC. Typos corrected in eqs.(4) and (11)

We investigate the effect of extending the standard MSTW parameterisation of input parton distribution functions (PDFs) using Chebyshev polynomials. We find evidence that four powers in the polynomial are sufficient for extremely high precision. Applying this to valence and sea quarks we find an improvement in the global fit, but a significant change only in the small-$x$ valence up-quark PDF, $u_V$. We investigate the effect of also extending, and making more flexible, the `nuclear' correction to deuteron structure functions. The extended `Chebyshev' parameterisation results in an improved stability in the deuteron corrections that are required for the best fit to the `global' data. The resulting PDFs have a significantly, but not dramatically, altered valence down-quark distribution, $d_V$. For the extended set of MSTW PDFs, their uncertainties can be obtained using 23, rather than the usual 20, orthogonal `uncertainty' eigenvectors. Since the dominant effect is on the valence quarks, we present a detailed study of the dependence of the valence--sea separation on the predictions for the decay lepton charge asymmetry which results from $W^\pm$ production at the LHC, illustrating the PDFs and the $x$ range probed for different experimental scenarios. We show that the modified MSTW PDFs make significantly improved predictions for these data at the LHC, particularly for high values of the $p_T$ cut of the decay lepton. However, this is a special case, since the asymmetry is extremely sensitive to valence--sea details, and in particular to the combination $u_V-d_V$ of valence PDFs for $x \sim M_W/\sqrt{s}$ at low lepton rapidities. We show that the predictions for a wide variety of total cross sections are very similar to those obtained using the MSTW2008 PDFs, with changes being much smaller than the PDF uncertainties., Comment: 49 pages, 50 figures. Final version. The inclusion of some new figures and some extended discussion and presentation of explicit results, in particular a new sub-section 4.5 on variation in the number of parameters used. The main conclusions and extracted PDFS are unchanged

Central exclusive Higgs boson production, pp -> p+H+p, at the LHC can provide important complementary contribution to the comprehensive study of the Higgs sector in a remarkably clean topology. The bb(bar) Higgs decay mode is especially attractive, and for certain BSM scenarios may even become the discovery channel. Obvious requirements for the success of such exclusive measurements are strongly suppressed and controllable backgrounds. One potential source of background comes from additional gluon radiation which leads to a three-jet bb(bar)g final state. We perform an explicit calculation of the subprocesses gg -> q(bar)qg, gg -> ggg in the case of `internal' gluon radiation from the spectator, t-channel screening gluon, when the two incoming active t-channel gluons form a colour octet. We find that the overall contribution of this source of background is orders of magnitude lower than that caused by the main irreducible background resulting from the gg^{PP} -> b(bar)b subprocess. Therefore, this background contribution can be safely neglected., Comment: 19 pages, 5 figures. To be published in Eur. Phys. J. C

We describe the formalism, and present the results, for a triple-Regge analysis of the available pp and p\bar{p} high-energy data which explicitly accounts for absorptive corrections. In particular, we allow for the gap survival probability, S^2, in single proton diffractive dissociation. Since for pp scattering the value of S^2 is rather small, the triple-Pomeron vertex obtained in this analysis is larger than that obtained in the old analyses where the suppression caused by the absorptive corrections was implicitly included in an effective vertex. We show that the bare triple-Pomeron coupling that we extract from the pp and p\bar{p} data is consistent with that obtained in a description of the gamma p -> J/psi + Y HERA data. The analyses of the data prefer a zero slope, corresponding to the small size of the bare vertex, giving the hope of a smooth matching to the perturbative QCD treatment of the triple-Pomeron coupling., 26 pages, 12 figures; two figures added, references added, version to appear in Eur.Phys.J.C

We present a `new generation' model for high energy proton-proton `soft' interactions. It allows for a full set of multi-Pomeron vertices, as well as including multichannel eikonal scattering. It describes the behaviour of the proton-proton total, sigma(total), and elastic dsigma(el)/dt, cross sections together with those for low and high mass proton dissociation. Although the model contains a comprehensive set of multi-Pomeron diagrams, it has a simple partonic interpretation. Including the more complicated multi-Pomeron vertices reduces the absorptive effects as compared to the predictions in which only the triple-Pomeron vertex is considered. Tuning the model to describe the available `soft' data in the CERN ISR - Tevatron energy range, we predict the total, elastic, single- and double-diffractive dissociation cross sections at the LHC energy. An inescapable consequence of including multichannel eikonal and multi-Pomeron effects is that the total cross section is expected to be lower than before: indeed, we find sigma(total) \simeq 90 mb at the LHC energy. We also present differential forms of the cross sections. In addition we calculate soft diffractive central production., 42 pages, 24 figures, 2 tables. Fig.20(d) corrected. Version to be published in EPJC

We present LO, NLO and NNLO sets of parton distribution functions (PDFs) of the proton determined from global analyses of the available hard scattering data. These MMHT2014 PDFs supersede the `MSTW2008' parton sets, but are obtained within the same basic framework. We include a variety of new data sets, from the LHC, updated Tevatron data and the HERA combined H1 and ZEUS data on the total and charm structure functions. We also improve the theoretical framework of the previous analysis. These new PDFs are compared to the `MSTW2008' parton sets. In most cases the PDFs, and the predictions, are within one standard deviation of those of MSTW2008. The major changes are the $u-d$ valence quark difference at small $x$ due to an improved parameterisation and, to a lesser extent, the strange quark PDF due to the effect of certain LHC data and a better treatment of the $D \to \mu$ branching ratio. We compare our MMHT PDF sets with those of other collaborations; in particular with the NNPDF3.0 sets, which are contemporary with the present analysis., Comment: Final version. Some additional comments/clarifications and minor corrections. 83 pages, 116 figures. PDFs can be obtained from http://www.hep.ucl.ac.uk/mmht

The ‘optimal’ factorization scale $$\mu _0$$ is calculated for open heavy quark production. We find that the optimal value is $$\mu _F=\mu _0\simeq 0.85\sqrt{p^2_T+m_Q^2} $$ ; a choice which allows us to resum the double-logarithmic, $$(\alpha _s\ln \mu ^2_F\ln (1/x))^n$$ corrections (enhanced at LHC energies by large values of $$\ln (1/x)$$ ) and to move them into the incoming parton distributions, PDF $$(x,\mu _0^2)$$ . Besides this result for the single inclusive cross section (corresponding to an observed heavy quark of transverse momentum $$p_T$$ ), we also determined the scale for processes where the acoplanarity can be measured; that is, events where the azimuthal angle between the quark and the antiquark may be determined experimentally. Moreover, we discuss the important role played by the $$2\rightarrow 2$$ subprocesses, $$gg\rightarrow Q\bar{Q}$$ at NLO and higher orders. In summary, we achieve a better stability of the QCD calculations, so that the data on $$c{\bar{c}}$$ and $$b{\bar{b}}$$ production can be used to further constrain the gluons in the small x, relatively low scale, domain, where the uncertainties of the global analyses are large at present.