Search

We consider the central exclusive production of high $E_T$ jets $pp\to p+(X+{\rm dijet})+p$. In particular we study the possible contamination of the purely exclusive signal by semi-exclusive production where no other secondaries are emitted in one hemisphere, between the highest $E_T$ dijet and the recoil proton, while in the other hemisphere a third jet, plus possibly additional hadron activity, is allowed, but still separated from the incoming proton by a large rapidity gap. This process arising from the fusion of a hard and a soft Pomeron has not been considered before. It turns out that it gives a negligible contribution. %contamination, which was not known before the calculation. The calculation involves a careful treatment of the QCD colour structure of the amplitudes., 15 pages, 4 figures, version to be published in EPJC

We address a long standing problem concerning the scale behaviour of parton densities in the low $x$, low $Q^2$ domain. We emphasize the important role of absorptive corrections at low $x$ and use knowledge of diffractive deep inelastic scattering to exclude the absorptive effect from conventional deep inelastic data. In this way we obtain a significantly different low $x$ behaviour of the gluon density, which is now much better described by linear DGLAP evolution. Accounting also for a second power correction, which arises from the freezing of $\alpha_s$ at low $Q^2$, leads to an essentially flat behaviour of the low $x$ gluon density., Comment: 16 pages, 7 figures, 3 tables, text and 2 figures improved

We present the new MSHT20 set of parton distribution functions (PDFs) of the proton, determined from global analyses of the available hard scattering data. The PDFs are made available at NNLO, NLO, and LO, and supersede the MMHT14 sets. They are obtained using the same basic framework, but the parameterisation is now adapted and extended, and there are 32 pairs of eigenvector PDFs. We also include a large number of new data sets: from the final HERA combined data on total and heavy flavour structure functions, to final Tevatron data, and in particular a significant number of new LHC 7 and 8 TeV data sets on vector boson production, inclusive jets and top quark distributions. We include up to NNLO QCD corrections for all data sets that play a major role in the fit, and NLO EW corrections where relevant. We find that these updates have an important impact on the PDFs, and for the first time the NNLO fit is strongly favoured over the NLO, reflecting the wider range and in particular increased precision of data included in the fit. There are some changes to central values and a significant reduction in the uncertainties of the PDFs in many, though not all, cases. Nonetheless, the PDFs and the resulting predictions are generally within one standard deviation of the MMHT14 results. The major changes are the $u-d$ valence quark difference at small $x$, due to the improved parameterisation and new precise data, the $\bar d, \bar u$ difference at small $x$, due to a much improved parameterisation, and the strange quark PDF due to the effect of LHC $W,Z$ data and inclusion of new NNLO corrections for dimuon production in neutrino DIS. We discuss the phenomenological impact of our results, and in general find reduced uncertainties in predictions for processes such as Higgs, top quark pair and $W,Z$ production at post LHC Run-II energies., Comment: 140 pages, 96 figures, 16 tables. Published version plus very minor alterations - version 4

We discuss the exclusive $J/\psi$ photoproduction process, as measured recently at LHCb, as a means of constraining and ultimately determining the low $x$ and low $Q$ gluon PDF. The scale dependence of the theoretical prediction for this process is shown to be systematically improved via a taming of the known $\bar{\text{MS}}$ result, this amounts to resumming logarithmically enhanced small-$x$ terms and implementing a small-$Q$ power correction. The cross section level predictions allow the behaviour of the gluon PDF in the low $(x, Q)$ domain to be determined., Comment: 4 pages, 3 figures. Proceedings of Light Cone 19 - QCD on the light cone: from hadrons to heavy ions (LC2019), submitted to PoS

We evaluate the contribution of inelastic intermediate states (such as $p\to N^*$ excitations) to the phase between the one-photon-exchange and the `nuclear' high energy $pp$ scattering amplitudes as $t\to 0$, caused by multiphoton diagrams. It turns out to be rather small - much smaller than to have any influence on the experimental accuracy of the measurements of $\rho$, defined to be the ratio of the real to imaginary parts of the forward `nuclear' amplitude., Comment: 11 pages, 5 figures, version published in Phys.Rev. D

We study the rapidity dependence of the cross section on the rapidity of the central exclusive production cross sections of C-even mesons in $pA$ and $AA$ collisions, where $A$ is a heavy ion. We observe qualitatively different behaviour of the contributions arising from $\gamma$-Odderon and Pomeron-Pomeron fusion mechanisms. This can be used to extract the Odderon signal from the events of $f_2$ mesons exclusively produced in the forward region. Estimates, obtained using expected values of the Odderon cross section, indicate that the $\gamma$-Odderon contribution may exceed by a few times the Pomeron-induced background in Pb-Pb collisions. Moreover, the Odderon effect can be clearly seen in terms of the asymmetry in $pA$ and $AA$ collisions with the beam and target reversed. It is particularly interesting to note that the asymmetry for $\gamma$-Odderon fusion reaches its maximum value close to 1 in the forward direction, whereas the asymmetry for the Pomeron-Pomeron fusion contribution is small. The role of additional interactions of the $f_2$ meson with nucleons in the heavy ion, and also the contributions from secondary Reggeons, are estimated. The photon-Odderon contribution has a large normalisation uncertainty but the enhances cross-section in the forward region combined with a large asymmetry increases the chance of experimentally detecting the Odderon., Comment: 19 pages, 4 figures. Version published in EPJC containing extra references

COMPASS is a fixed target experiment at CERN SPS aimed to study hadron structure and spectroscopy. Hadron identification in the momentum range between $3$ and $55 GeV/c$ is provided by a large gaseous Ring Imaging Cherenkov Counter, RICH-1. To cope with the challenges imposed by the new physics program of COMPASS, RICH-1 has been upgraded by replacing four MWPC-based photon detectors with newly developed MPGD-based photon detectors. The architecture of the novel detectors is a hybrid combination of two layers of THGEMs and a Micromegas. The top of the first THGEM is coated with CsI acting as a reflective photo-cathode. The anode is segmented in pads capacitively coupled to the APV-25 based readout. The new hybrid detectors have been commissioned during the 2016 COMPASS data taking and stably operated during the 2017 run. In this paper design, construction, operation and performance aspects of the novel photon detectors for COMPASS RICH-1 are discussed., Comment: 11 Pages; 9 Figures; 8 Sections including References; 18 References; Proceedings for INSTR20 conference accepted in JINST for publication

We describe a QCD based model which incorporates the main properties of the inclusive particle distributions expected for diffractive processes, including the diffractive dissociation at high energies. We study, in turn, the total cross section, $\sigma_{tot}$, the differential elastic, $d\sigma_{el}/dt$, cross section, the dependence of the single proton dissociation cross section, $\xi d^{SD}/d\xi $, on the momentum fraction, $\xi = 1-x_L$, lost by the leading proton, the multiplicity distributions in inelastic (non-diffractive) collisions and in the processes of dissociation. Besides this we calculate the mean transverse momenta of the `wee partons' (secondaries) produced in the case of dissociation (that is in the processes with a large rapidity gap) and compare it with that in inelastic interactions., Comment: 27 pages, 13 figures

We argue that the so-called maximal Odderon contribution breaks the `black disk' behaviour of the asymptotic amplitude, since the cross section of the events with Large Rapidity Gaps grows faster than the total cross section. That is the `maximal Odderon' is not consistent with the unitarity., Comment: 11 pages, 4 figures, Section 2 extended, version to be published in Phys.Lett.B

In recent years information on the transversity distribution $h_1$ has been obtained combining the Collins asymmetry results from semi-inclusive deep inelastic scattering (SIDIS) data on transversely polarized nucleon targets with the information on the fragmentation function of a transversely polarized quark from the asymmetries measured in $e^+e^-$ annihilation into hadrons. An alternative method was proposed long time ago, which does not require the $e^+e^-$ data, but allows one to get ratios of the $u$ and $d$ quark transversity distributions from the SIDIS data alone. The method utilizes the ratio of the difference of the Collins asymmetries of positively and negatively charged hadrons produced on transversely polarized proton and deuteron targets. We have applied this method to the COMPASS proton and deuteron data, and extracted the ratio $h_1^d/h_1^u$. The results are compared to those obtained in a previous point--by--point extraction based both on SIDIS and $e^+e^-$ data., Comment: 7 pages, 4 figures

We recall the main properties of inclusive particle distributions expected for Pomeron-proton and Pomeron-Pomeron interactions. Due to the small size of the Pomeron we expect larger transverse momenta of secondaries and a smaller probability of Multiple Interactions, that is a narrower multiplicity distribution. We propose to compare the spectra of secondaries produced in the Pomeron and the proton interactions in terms of the Feynman $x_F$ variable. The main difference should be observed for a relatively large $x_F$, that is near the edge of rapidity gaps. Such data offer the opportunity to illuminate the properties of the `soft' or `Regge' Pomeron, which drives the minimum-bias type of events in high energy $pp$ interactions. Besides this, there should be a good opportunity to observe a glueball in the Pomeron fragmentation region., 7 pages, 1 figure, text improved, to be published in J.Phys.G-letters

We explore the possibility of observing Odderon exchange in proton--ion collisions at the LHC, via the ultraperipheral photoproduction of C-even mesons. As well as the signal, we consider in detail the principle backgrounds, due to QCD--initiated production (i.e. double Pomeron exchange) and $\gamma\gamma$ fusion. We find that while the photon--initiated background is dominant at very small momentum transfer, this can be effectively removed by placing a reasonable cut on the transverse momentum of the produced meson. In the case of QCD--initiated production, we show this is in general strongly suppressed by the small probability of no additional particle production in the central detector, namely the survival factor. In some scenarios, this suppression is sufficient to permit the observation of Odderon exchange in Pb$-p$ collisions in a clean environment, or else to place bounds on this. We in addition identify the cases of $\pi^0$ and $\eta(548)$ production as particularly promising channels. Here, the QCD--initiated background is absent for $\pi^0$ due to isospin conservation and very small for $\eta(548)$ due to its dominantly flavour octet nature and odd parity., Comment: 26 pages, 4 figures

We present the MMHT2015qed PDF set, resulting from the inclusion of QED corrections to the existing set of MMHT Parton Distribution Functions (PDFs), and which contain the photon PDF of the proton. Adopting an input distribution from the LUXqed formulation, we discuss our methods of including QED effects for the full, coupled DGLAP evolution of all partons with QED at $\mathcal{O}(\alpha)$, $\mathcal{O}(\alpha\alpha_{S})$, $\mathcal{O}(\alpha^2)$. While we find consistency for the photon PDF of the proton with other recent sets, building on this we also present a set of QED corrected neutron PDFs and provide the photon PDF separated into its elastic and inelastic contributions. The effect of QED corrections on the other partons and the fit quality is investigated, and the sources of uncertainty for the photon are outlined. Finally we explore the phenomenological implications of this set, giving the partonic luminosities for both the elastic and inelastic contributions to the photon and the effect of our photon PDF on fits to high mass Drell-Yan production, including the photon--initiated channel., Comment: 54 pages, 34 figures. Typo corrected in Eq.(7)

We discuss the causes which can limit the accuracy of the predictions based on the conventional PDFs when including in global parton analyses the data at moderate scales $\mu$. The first is the existence of power corrections ${\cal O} (Q_0^2/\mu^2)$ due to the double counting of contributions arising from the region below the input scale $Q_0$. The second concerns the possible inclusion of the BFKL re-summation of the $(\alpha_s\ln (1/x))^n$ terms. The third is the treatment of the heavy-quark thresholds. We show how to include the heavy-quark masses ($m_h$ with $h=c,b,t$) in DGLAP evolution which provides the correct {\it smooth} behaviour through the threshold regions and how to subtract the low parton virtuality $|k^2, Comment: 15 pages, 3 figures. arXiv admin note: text overlap with arXiv:1307.3508

We argue that if the elastic proton-proton cross section increases with energy, the Froissart-like high energy behaviour of the elastic amplitude (which corresponds to a `black disk' of radius $R(s)=c\ln s-\beta\ln(\ln s)$) is the only possibility to satisfy the unitarity equation at each value of the impact parameter, $b$. Otherwise the cross section of events with Large Rapidity Gaps grows faster than the total cross section at the same $b$. That is, these `gap' events require {\it maximal} growth of the high-energy (asymptotic) cross section and of the interaction radius $R(s)$ in order to be consistent with unitarity., Comment: 9 pages, 3 figures. text added, arXiv admin note: substantial text overlap with arXiv:1801.07065

Inspired by the new TOTEM data on elastic $pp$ scattering at 13 TeV, we study the possibility to describe all the diffractive collider data ($\sigma_{\rm tot}, d\sigma_{\rm el}/dt$, $\rho\equiv$Re$A$/Im$A$, \sd) in a wide interval of energy (0.0625 to 13 TeV) in the framework of a two-channel eikonal model. We show that a satisfactory description can be achieved without an odd-signature (Odderon) exchange contribution. We consider the possible role of the QCD Odderon which may improve the description of $\rho$ and discuss the importance of the odd-signature term if the amplitude were to exceed the black disc limit., Comment: 17 pages, 4 figures, version published in Phys. Lett. B

We compare the rapidity, $y$, and the beam energy, $\sqrt{s}$, behaviours of the cross section of the data for $D$ meson production in the forward direction that were measured by the LHCb collaboration. We describe the observed cross sections using NLO perturbative QCD, and choose the optimal factorization scale for the LO contribution which provides the resummation of the large double logarithms. We emphasize the inconsistency observed in the $y$ and $\sqrt{s}$ behaviours of the $D$ meson cross sections. The $y$ behaviour indicates a very {\it flat} $x$ dependence of the gluon PDF in the unexplored low $x$ region around $x\sim 10^{-5}$. However, to describe the $\sqrt{s}$ dependence of the data we need a steeper gluon PDF with decreasing $x$. Moreover, an even steeper behaviour is needed to provide an extrapolation which matches on to the well known gluons found in the global PDF analyses for $x\sim 10^{-3}$. The possible role of non-perturbative effects is briefly discussed., 12 pages, 4 figures, this article supersedes arXiv:1705.08845, version to be published in PRD

We investigate the impact of the high precision ATLAS and CMS 7 TeV measurements of inclusive jet production on the MMHT global PDF analysis at next-to-next-to-leading order (NNLO). This is made possible by the recent completion of the long-term project to calculate the NNLO corrections to the hard cross section. We find that a good description of the ATLAS data is not possible with the default treatment of experimental systematic errors, and develop a simplified solution that retains the dominant physical information of the data. We then investigate the fit quality and the impact on the gluon PDF central value and uncertainty when the ATLAS and CMS data are included in a MMHT fit. We consider both common choices for the factorization and renormalization scale, namely the inclusive jet transverse momentum, $p_\perp$, and the leading jet $p_\perp$, as well as the different jet radii for which the ATLAS and CMS data are made available. We find that the impact of these data on the gluon is relatively insensitive to these inputs, in particular the scale choice, while the inclusion of NNLO corrections tends to improve the data description somewhat, and gives a qualitatively similar though not identical impact on the gluon in comparison to NLO., 21 pages, 15 figures, 4 tables. Version accepted for publication in EPJC

To obtain more precise parton distribution functions (PDFs) it is important to include data on inclusive high transverse energy jet production in the global parton analyses. These data have high statistics and the NNLO terms in the perturbative QCD (pQCD) description are now available. Our aim is to reduce the uncertainty in the comparison of the jet data with pQCD. To ensure the best convergence of the pQCD series it is important to choose the appropriate factorization scales, $\mu_F$. We show that it is possible to absorb and resum in the incoming PDFs and fragmentation function ($D$) an essential part of the higher $\alpha_s$ order corrections by determining the `optimal' values of $\mu_F$. We emphasize that it is necessary to optimize different factorization scales for the various factors in the cross section: indeed, both of the PDFs, and also the fragmentation function, have their own optimal scale. We show how the values of these scales can be calculated for the LO (NLO) part of the pQCD prediction of the cross section based on the theoretically known NLO (NNLO) corrections. After these scales are fixed at their optimal values, the residual factorization scale dependence is much reduced., Comment: 10 pages, 1 figure, version to be published in EPJC

We discuss the possibility of identifying invisible objects (e.g. dark matter) via missing mass in Central Exclusive Processes; that is, in events where only forward protons are detected at the LHC. We show that the signal must have a cross section greater than 0.25 fb in order that it can be detected. We estimate the huge background caused by soft proton dissociation and evaluate the requirements of the detector `veto' system needed to sufficiently suppress these events. In addition, we discuss the related process leading to the possible identification of the particles from the so-called `compressed-mass Beyond the Standard Model (BSM)' scenarios. In this case the background is not so severe., 15 pages, 3 figures

Proton transverse-spin azimuthal asymmetries are extracted from the COMPASS 2010 semi-inclusive hadron measurements in deep inelastic muon-nucleon scattering in those four regions of the photon virtuality $Q^2$, which correspond to the four regions of the di-muon mass $\sqrt{Q^2}$ used in the ongoing analysis of the COMPASS Drell-Yan measurements. This allows for a future direct comparison of the nucleon transverse-momentum-dependent parton distribution functions extracted from these two alternative measurements. Various two-dimensional kinematic dependences are presented for the azimuthal asymmetries induced by the Sivers transverse-momentum-dependent parton distribution function. The integrated Sivers asymmetries are found to be positive with an accuracy that appears to be sufficient to test the sign change of the Sivers function predicted by Quantum Chromodynamics., 13 pages, 6 figures

We present a point-by-point determination of the Sivers distributions from hadron leptoproduction data. The method, which relies on some simple assumptions, is based on the combined analysis of proton and deuteron observables. We make use of the single-spin asymmetries measured by COMPASS in semi-inclusive deep inelastic scattering of 160 GeV muons on transversely polarized proton and deuteron targets., 5 figures

COMPASS is a fixed target experiment at CERN SPS aimed to study Hadron Structure and Spectroscopy. Hadron Identification in the momentum range between 3 and 55 GeV/c is provided by a large gaseous Ring Imaging Cherenkov Counter (RICH-1). To cope with the challenges imposed by the new physics program of COMPASS, RICH-1 have been upgraded by replacing four MWPCs based photon detectors with newly developed MPGD based photon detectors. The architecture of the novel detectors is a hybrid combination of two layers of THGEMs and a MicroMegas. The top of the first THGEM is coated with CsI acting as a reflective photo-cathode. The anode is segmented in pads capacitively coupled to the APV-25 based readout. The new hybrid detectors have been commissioned during 2016 COMPASS data taking and stably operated during 2017 run. In this paper all aspects of the novel photon detectors for COMPASS RICH-1 are discussed., 4 Pages, 8 Figures, Proceedings for conference NDIP2017

We discuss the salient features of exclusive vector meson production in heavy ion collisions at LHC energies. Special attention is paid to the space-time picture of the process. We account for both coherent and incoherent contributions. The explicit quantitative predictions are given for the $\rho$-meson differential cross section in lead-lead collisions in different kinematical configurations relevant for the LHCb and ALICE experiments., Comment: 16 pages, 7 figures, text, references and figures 5 and 6 improved

We study exclusive vector meson photoproduction, $\gamma p \to V + p$ with $V=J/\psi$ or $\Upsilon$, at NLO in collinear factorisation, in order to examine what may be learnt about the gluon distribution at very low $x$. We examine the factorisation scale dependence of the predictions. We argue that, using knowledge of the NLO corrections, terms enhanced by a large $\ln(1/\xi)$ can be reabsorbed in the LO part by a choice of the factorisation scale. (In these exclusive processes $\xi$ takes the role of Bjorken-$x$.) Then, the scale dependence coming from the remaining NLO contributions has no $\ln(1/\xi)$ enhancements. As a result, we find that predictions for the amplitude of $\Upsilon$ production are stable to within about $\pm 15\%$. This will allow data for the exclusive process $p p \to p\Upsilon p$ at the LHC, particularly from LHCb, to be included in global parton analyses to constrain the gluon PDF down to $x\sim 10^{-5}$. Moreover, the study of exclusive $J/\psi$ photoproduction indicates that the gluon density found in the recent global PDF analyses is too small at low $x$ and low scales., Comment: 21 pages, 6 figures. Text significantly improved, references added, version to be published in J.Phys.G

We investigate the effect of including the HERA run I + II combined cross section data on the MMHT2014 PDFs. We present the fit quality within the context of the global fit and when only the HERA data are included. We examine the changes in both the central values and uncertainties in the PDFs. We find that the prediction for the data is good, and only relatively small improvements in $\chi^2$ and changes in the PDFs are obtained with a refit at both NLO and NNLO. PDF uncertainties are slightly reduced. There is a small dependence of the fit quality on the value of $Q^2_{\min}$. This can be improved by phenomenologically motived corrections to $F_L(x,Q^2)$ which parametrically are largely in the form of higher-twist type contributions., Comment: 16 pages, 8 figures

We investigate the variation in the MMHT2014 PDFs when we allow the heavy quark masses $m_c$ and $m_b$ to vary away from their default values. We make PDF sets available in steps of $\Delta m_c =0.05~{\rm GeV}$ and $\Delta m_b =0.25~{\rm GeV}$, and present the variation in the PDFs and in the predictions. We examine the comparison to the HERA data on charm and beauty structure functions and note that in each case the heavy quark data, and the inclusive data, have a slight preference for lower masses than our default values. We provide PDF sets with 3 and 4 active quark flavours, as well as the standard value of 5 flavours. We use the pole mass definition of the quark masses, as in the default MMHT2014 analysis, but briefly comment on the $\overline{\rm MS}$ definition., Comment: 20 pages, 12 figures. Version accepted for publication in EPJC. Please note certain LHAPDF files available prior to 4 December 2015 were not formatted correctly

We briefly discuss some of the developments since the publication of the MMHT14 parton distributions. In particular we explore the impact of recent LHC data for $W^\pm,Z$ and $t\bar{t}$ production, and perform a preliminary new analysis including these data. In this re-fit (which we tentatively call `MMHT16') there are few changes of significance in the central values of the PDFs, but some data reduce the uncertainties, mainly in the strange and valence quark distributions. We find that an extended $\bar{d}-\bar{u}$ parametrization only leads to minor changes, with the difference going to zero as $x \to 0$. We comment on the determination of the photon PDF., Comment: 7 pages, 4 figures, contribution to the proceedings of Diffraction2016 which was held in Sicily, Sept. 2-8

We argue that the secondaries produced in high energy hadron collisions are emitted by small size sources distributed over a much larger area in impact parameter space occupied by the interaction amplitude. That is, Bose-Einstein correlation of two emitted identical particles should be described by a `two-radii' parametrization ansatz. We discuss the expected energy, charged multiplicity and transverse momentum of the pair (that is, $\sqrt{s},~N_{\rm ch}, k_t$) behaviour of both the small and large size components., Comment: 7 pages, 1 figure

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

Observations of rare processes containing large rapidity gaps at high energy colliders may be exceptionally informative. However the cross sections of these events are small in comparison with that for the inclusive processes since there is a large probability that the gaps may be filled by secondary particles arising from additional soft interactions or from gluon radiation. Here we review the calculations of the probability that the gaps survive population by particles from these effects for a wide range of different processes., 47 pages, 17 figures, short section and references added, Topical Review to be published in J.Phys.G

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 quantify the central inclusive background contributions to exclusive Higgs production at the LHC arising from double-Pomeron-exchange processes. We consider the H --> bb(bar) signal. We study processes mediated by the fusion of two `hard' Pomerons, and also by the fusion of `soft' Pomerons. The latter background is found to be very small, and the former is found to be less than the exclusive signal., 10 pages, 2 figures, 1 table

The study of transverse spin and transverse momentum effects is an important part of the scientific program of COMPASS, a fixed target experiment at the CERN SPS taking data since 2002. The studies are carried on by measuring the hadrons produced in deep inelastic scattering (DIS) of 160 GeV/c muons off different targets. Among the possible asymmetries in the hadron azimuthal distributions, particularly interesting are the Collins and Sivers asymmetries which the COMPASS Collaboration has measured using transversely polarised deuteron and proton targets. The new results for charged pions and kaons obtained from the 2010 run with a transversely polarised proton target are presented., presented at: SPIN2012, 20th International Spin Physics Symposium, Dubna, Russia, 17-23 September, 2012

The main characteristics of the COMPASS experimental setup for physics with hadron beams are described. This setup was designed to perform exclusive measurements of processes with several charged and/or neutral particles in the final state. Making use of a large part of the apparatus that was previously built for spin structure studies with a muon beam, it also features a new target system as well as new or upgraded detectors. The hadron setup is able to operate at the high incident hadron flux available at CERN. It is characterised by large angular and momentum coverages, large and nearly flat acceptances, and good two and three-particle mass resolutions. In 2008 and 2009 it was successfully used with positive and negative hadron beams and with liquid hydrogen and solid nuclear targets. This article describes the new and upgraded detectors and auxiliary equipment, outlines the reconstruction procedures used, and summarises the general performance of the setup., Comment: 91 pages, 101 figures and 7 tables

We present a point-by-point determination of the valence transversity distributions from two different types of processes: single-hadron production and dihadron production, both in semi-inclusive deep inelastic scattering and e+e- annihilation. The extraction is based on some simple assumptions and does not require any parametrization. The transversity distributions obtained from Collins effect in single-hadron production and from interference effects in dihadron production are found to be compatible with each other., Comment: 16 pages, 7 figures; added references

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

The partonic transverse momentum, k_t distribution plays a crucial role in driving high-energy hadron interactions. If k_t is limited we have old fashioned Regge Theory. If k_t increases with energy the interaction may be described by perturbative QCD. We use BFKL diffusion in ln k_t, supplemented by a stronger absorption of low k_t partons, to estimate the growth of the mean transverse momenta with energy. This growth reveals itself in the distribution of secondaries produced at the collider energies. We present a simple, BFKL-based, model to demonstrate the possible size of the effect. Moreover, we propose a way to evaluate experimentally the shape of the parton transverse momenta distribution by studying the spectra of the (D or B) mesons which contain one heavy quark., Comment: 10 pages, 3 figures

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 study the effect of absorptive corrections due to parton recombination on the parton distributions of the proton. A more precise version of the GLRMQ equations, which account for non-linear corrections to DGLAP evolution, is derived. An analysis of HERA F_2 data shows that the small-x gluon distribution is enhanced at low scales when the absorptive effects are included, such that a negative gluon distribution at 1 GeV is no longer required., 10 pages, 3 figures

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

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 study diffractive hard dijet production, with one or two rapidity gaps, at high energies. We emphasize that both hard and Regge factorization are broken in these processes. We show that a multi-Pomeron-exchange model for screening effects gives a specific pattern for the breakdown of factorization, which is in good agreement with diffractive dijet data collected at the Tevatron., 6 pages, 1 figure

We present the latest results of studies within the MMHT PDF framework. We discuss the impact of the most recent ATLAS 7 TeV jet data, demonstrating that while a good fit can be achieved for individual jet rapidity bins, it is not possible to achieve a good description of the data when all bins are fitted. We examine the role that the experimental correlated systematic uncertainties play in this, and demonstrate that by simply decorrelating no more than two sources of error between rapidity bins a remarkably improved description of the data can be achieved. We then study the impact of NNLO corrections, showing that a mild decrease in the fit quality is produced. We also present the results of including new LHC $W$, $Z$, $W+c$ and $t\overline{t}$ data on the MMHT14 PDF set, showing that a marked decrease in the $s+\overline{s}$ uncertainty is in particular achieved. Finally, some discussion of the latest work towards the inclusion of the photon PDF within the MMHT framework is presented., 13 pages, 8 figures. To appear in the proceedings of the XXIII Cracow Epiphany Conference

I present results on continuing updates in PDFs within the framework now called MMHT14 due to both theory improvements and the inclusion of new data sets, including most of the up-to-date LHC data. A new set of PDFs is essentially finalised, with no changes expected to the PDFs presented here., 6 pages, 5 figures. Proceedings of the 37th International Conference of High-Energy Physics (ICHEP2014), 2-9 July 2014, Valencia, Spain

We consider the main factors which cause the variation of the value of the local slope of the elastic pp cross section B(t)=d[\ln(d\sigma_el(pp)/dt]/dt with t. Namely, we discuss the role of the pion-loop insertion in the pomeron trajectory, the t-dependence of the pomeron-nucleon coupling and the role of the eikonalization of the proton-proton amplitude in both the one- and two-channel eikonal models., Comment: 16 pages, 11 figures; IPPP/14/86, DCPT/14/172, version to be published in J.Phys.G