Search

We provide an overview of the status of Monte-Carlo event generators for high-energy particle physics. Guided by the experimental needs and requirements, we highlight areas of active development, and opportunities for future improvements. Particular emphasis is given to physics models and algorithms that are employed across a variety of experiments. These common themes in event generator development lead to a more comprehensive understanding of physics at the highest energies and intensities, and allow models to be tested against a wealth of data that have been accumulated over the past decades. A cohesive approach to event generator development will allow these models to be further improved and systematic uncertainties to be reduced, directly contributing to future experimental success. Event generators are part of a much larger ecosystem of computational tools. They typically involve a number of unknown model parameters that must be tuned to experimental data, while maintaining the integrity of the underlying physics models. Making both these data, and the analyses with which they have been obtained accessible to future users is an essential aspect of open science and data preservation. It ensures the consistency of physics models across a variety of experiments., Comment: 164 pages, 10 figures, contribution to Snowmass 2021

We report on the results of our recent research published in [1] that shows that AdS/QCD generates predictions for the rate of diffractive \rho-meson electroproduction that are in agreement with data collected at the HERA electron-proton collider [2,3]. Preliminary results of this research were presented in [4]., Comment: 4 pages, 1 figure. Talk given by Ruben Sandapen at the seventh biennial International Workshop on Diffraction in High-Energy Physics (Diffraction 2012) September 10-15, 2012, Lanzarote, Canary Islands (Spain). To appear in the Workshop Proceedings

We use an AdS/QCD holographic wavefunction to generate predictions for the rate of diffractive {\rho}-meson electroproduction that are in reasonable agreement with data collected at the HERA electron-proton collider., Comment: 4 pages, 3 figures, to appear in the Proceedings of 20th International Workshop on Deep-Inelastic Scattering and Related Subjects (DIS 2012), Bonn, Germany, 26-30 March 2012

We show that AdS/QCD generates predictions for the rate of diffractive rho meson electroproduction that are in agreement with data collected at the HERA electron-proton collider., Comment: 9 pages, 4 figures. Figure 3 added to illustrate that the AdS/QCD prediction is impressive. Accepted for publication in Physical Review Letters

We report on a successful extraction of the twist-2 and twist-3 Distribution Amplitudes (DAs) of the rho meson using the HERA data on diffractive rho photoproduction. We extract these DAs using several Colour Glass Condensate (CGC) inspired and a Regge inspired dipole models. All our extracted DAs are consistent with Sum Rules and lattice predictions., Comment: 4 pages, 2 figures, presented at the 19th International Workshop on Deep Inelastic Scattering and Related Subjects (DIS 2011), Jefferson Lab, Newport News, April 2011. To appear in the Proceedings

We explore how the colour of any new TeV-scale resonances that decay into top quark pairs can be identified by studying the dependence of the observed cross-section on a central jet veto. To facilitate this study, colour octet resonance production was implemented in Pythia8 and colour singlet resonance production is simulated after minor modifications. We find that the colour of a 2 TeV resonance can be identified with 10/fb of data at a centre-of-mass energy of 14 TeV for a wide range of couplings, but only if the uncertainty in the theoretical prediction is dramatically reduced from its current level., Comment: 18 pages, 10 figures

This report is based on a ten-week program on "Gluons and the quark sea at high-energies", which took place at the Institute for Nuclear Theory in Seattle in Fall 2010. The principal aim of the program was to develop and sharpen the science case for an Electron-Ion Collider (EIC), a facility that will be able to collide electrons and positrons with polarized protons and with light to heavy nuclei at high energies, offering unprecedented possibilities for in-depth studies of quantum chromodynamics. This report is organized around four major themes: i) the spin and flavor structure of the proton, ii) three-dimensional structure of nucleons and nuclei in momentum and configuration space, iii) QCD matter in nuclei, and iv) Electroweak physics and the search for physics beyond the Standard Model. Beginning with an executive summary, the report contains tables of key measurements, chapter overviews for each of the major scientific themes, and detailed individual contributions on various aspects of the scientific opportunities presented by an EIC., Comment: 547 pages, A report on the joint BNL/INT/Jlab program on the science case for an Electron-Ion Collider, September 13 to November 19, 2010, Institute for Nuclear Theory, Seattle; v2 with minor changes, matches printed version

We extract the leading twist-2 and subleading twist-3 Distribution Amplitudes (DAs) of the rho meson using the HERA data on diffractive rho photoproduction. We do so using several Colour Glass Condensate (CGC) inspired and a Regge inspired dipole models. We find that our extracted twist-2 DA is not much model dependent and is consistent with QCD Sum Rules and lattice predictions. The extracted twist-3 DA is more model dependent but is still consistent with the Sum Rules prediction., Comment: 21 pages, 10 figures, 3 tables. Section 6 revised, figures 8 and 9 and table 3 updated. Conclusions essentially unchanged

Diffractive rho meson production has been identified as one of the important processes where saturation can be probed at a future Electron-Ion Collider (EIC). A source of uncertainty in making predictions for this process lies within the assumed form of the meson light-cone wavefunction. We report here the results of reference [1] where a Regge-inspired dipole model was used to extract this wavefunction as well as the corresponding leading twist-2 Distribution Amplitude from the current accurate HERA data. In addition, we shall check the robustness of the conclusions in reference [1] by using alternative Colour Glass Condensate dipole models., Comment: 9 pages, 5 figures. Proceedings Contribution for the workshop "Gluons and the quark sea at high energies: distributions, polarization, tomography", Institute for Nuclear Theory, Seattle, U.S. September 13 to November 19, 2010

We extract the light-cone wavefunctions of the rho meson using the HERA data on diffractive rho photoproduction. We find good agreement with predictions for the distribution amplitude based on QCD sum rules and from the lattice. We also find that the data prefer a transverse wavefunction with enhanced end-point contributions., Comment: 13 pages, 7 figures, significant improvements over the original version with a new section on distribution amplitudes added

We review the subject of central exclusive particle production at high energy hadron colliders. In particular we consider reactions of the type A + B -> A + X + B, where X is a fully specified system of particles that is well separated in rapidity from the outgoing beam particles. We focus on the case where the colliding particles are strongly interacting and mainly they will be protons (or antiprotons) as at the ISR, SppS, Tevatron and LHC. The data are surveyed and placed within the context of theoretical developments., Comment: 64 pages

We investigate the theoretical description of the central exclusive production process, h1+h2 -> h1+X+h2. Taking Higgs production as an example, we sum logarithmically enhanced corrections appearing in the perturbation series to all orders in the strong coupling. Our results agree with those originally presented by Khoze, Martin and Ryskin except that the scale appearing in the Sudakov factor, mu=0.62 \sqrt{\hat{s}}, should be replaced with mu=\sqrt{\hat{s}}, where \sqrt{\hat{s}} is the invariant mass of the centrally produced system. We confirm this result using a fixed-order calculation and show that the replacement leads to approximately a factor 2 suppression in the cross-section for central system masses in the range 100-500 GeV., Comment: 41 pages, 19 figures; minor typos fixed; version published in JHEP

We compute the rate for diffractive upsilon meson production at the Tevatron and the LHC. The upsilon is produced diffractively via the subprocess gamma + p -> upsilon + p where the initial photon is radiated off an incoming proton (or antiproton). We consider the possibility to use low angle proton detectors to make a measurement of the gamma p cross-section and conclude that a measurement of the cross-section at a centre of mass energy in excess of 1 TeV is possible at the LHC. This is in the region where saturation effects are likely to reveal themselves., Comment: 14 pages

After a brief resume of the theory underpinning the central exclusive process (CEP) pp \to p+H+p, attention is focussed upon Higgs bosons produced in the Standard Model, the MSSM and the NMSSM. In all cases, CEP adds significantly to the physics potential of the LHC and in some scenarios it may be crucial., Comment: Talk presented at the workshop "New Trends in HERA Physics", Ringberg Castle, Tegernsee, 5-10 October 2008

After a brief introduction to the physics of soft gluons in QCD we present a surprising prediction. Dijet production in hadron-hadron collisions provides the paradigm, i.e. h_1 +h_2 \to jj+X. In particular, we look at the case where there is a restriction placed on the emission of any further jets in the region in between the primary (highest p_T) dijets. Logarithms in the ratio of the jet scale to the veto scale can be summed to all orders in the strong coupling. Surprisingly, factorization of collinear emissions fails at scales above the veto scale and triggers the appearance of double logarithms in the hard sub-process. The effect appears first at fourth order relative to the leading order prediction and is subleading in the number of colours., Comment: Talk presented at the workshop "New Trends in HERA Physics", Ringberg Castle, Tegernsee, 5-10 October 2008

In a previous paper we reported the discovery of super-leading logarithmic terms in a non-global QCD observable. In this short update we recalculate the first super-leading logarithmic contribution to the 'gaps between jets' cross-section using a colour basis independent notation. This sheds light on the structure and origin of the super-leading terms and allows them to be calculated for gluon scattering processes for the first time., Comment: 21 pages

We present the FP420 R&D project, which has been studying the key aspects of the development and installation of a silicon tracker and fast-timing detectors in the LHC tunnel at 420 m from the interaction points of the ATLAS and CMS experiments. These detectors would measure precisely very forward protons in conjunction with the corresponding central detectors as a means to study Standard Model (SM) physics, and to search for and characterise New Physics signals. This report includes a detailed description of the physics case for the detector and, in particular, for the measurement of Central Exclusive Production, pp --> p + phi + p, in which the outgoing protons remain intact and the central system phi may be a single particle such as a SM or MSSM Higgs boson. Other physics topics discussed are gamma-gamma and gamma-p interactions, and diffractive processes. The report includes a detailed study of the trigger strategy, acceptance, reconstruction efficiencies, and expected yields for a particular p p --> p H p measurement with Higgs boson decay in the b-bbar mode. The document also describes the detector acceptance as given by the LHC beam optics between the interaction points and the FP420 location, the machine backgrounds, the new proposed connection cryostat and the moving ("Hamburg") beam-pipe at 420 m, and the radio-frequency impact of the design on the LHC. The last part of the document is devoted to a description of the 3D silicon sensors and associated tracking performances, the design of two fast-timing detectors capable of accurate vertex reconstruction for background rejection at high-luminosities, and the detector alignment and calibration strategy., Comment: 178 pages, 128 figures. Updated Timing section. Figures compressed to marginal resolution due to arxiv.org file size requirements. A higher quality document accessible at http://www.fp420.com/papers.html

The simplest supersymmetric model that solves the mu problem and in which the GUT-scale parameters need not be finely tuned in order to predict the correct value of the Z boson mass at low scales is the Next-to-Minimal Supersymmetric Standard Model (NMSSM). However, in order that fine tuning be absent, the lightest CP-even Higgs boson h should have mass ~100 GeV and SM couplings to gauge bosons and fermions. The only way that this can be consistent with LEP limits is if h decays primarily via h->aa->4 tau or 4j but not 4b, where a is the lighter of the two pseudo-scalar Higgses that are present in the NMSSM. Interestingly, m_a < 2 m_b is natural in the NMSSM with m_a > 2 m_tau somewhat preferred. Thus, h -> 4 tau becomes a key mode of interest. Meanwhile, all other Higgs bosons of the NMSSM are typically quite heavy. Detection of any of the NMSSM Higgs bosons at the LHC in this preferred scenario will be very challenging using conventional channels. In this paper, we demonstrate that the h -> aa -> 4 tau decay chain should be visible if the Higgs is produced in the process pp -> p+h+p with the final state protons being measured using suitably installed forward detectors. Moreover, we show that the mass of both the h and the a can be determined on an event-by-event basis., Comment: 23 pages

We reconsider the calculation of a non-global QCD observable and find the possible breakdown of QCD coherence. This breakdown arises as a result of wide angle soft gluon emission developing a sensitivity to emission at small angles and it leads to the appearance of super-leading logarithms. We use the `gaps between jets' cross-section as a concrete example and illustrate that the new logarithms are intimately connected with the presence of Coulomb gluon contributions. Numerical estimates of their potential phenomenological significance are presented., Comment: 7 pages, 3 figures; proceedings of Workshop on Diffraction in High-Energy Physics - DIFFRACTION 2006, Sep. 5-10 2006, Adamantas, Milos island, Greece

We confront a very wide body of HERA diffractive electroproduction data with the predictions of the colour dipole model. We focus upon three different parameterisations of the dipole scattering cross-section and find good agreement for all observables. There can be no doubting the success of the dipole scattering approach and more precise observations are needed in order to expose its limitations., Comment: 28 pages, 26 figures. Several corrections in response to referee report

We examine the possibility of producing gluino pairs at the LHC via the exclusive reaction pp -> p+gluino+gluino+p in the case where the gluinos are long lived. Such long lived gluinos are possible if the scalar super-partners have large enough masses. We show that it may be possible to observe the gluinos via their conversion to R-hadron jets and measure their mass to better than 1% accuracy for masses below 350 GeV with 300/fb of data., Comment: 13 pages, 9 figures. Minor corrections to version 1

We reconsider the calculation of a non-global QCD observable and find the possible breakdown of QCD coherence. This breakdown arises as a result of wide angle soft gluon emission developing a sensitivity to emission at small angles and it leads to the appearance of super-leading logarithms. We use the `gaps between jets' cross-section as a concrete example and illustrate that the new logarithms are intimately connected with the presence of Coulomb gluon contributions. We present some rough estimates of their potential phenomenological significance., Comment: 21 pages, 7 figures

We review the calculation for Higgs production via the exclusive reaction pp -> p+H+p. In the first part we review in some detail the calculation of the Durham group and emphasise the main areas of uncertainty. Afterwards, we comment upon other calculations., Comment: 19 pages. 11 figures. Contribution to the workshop "HERA and the LHC"

We use perturbative QCD to calculate the parton level cross section for the production of two jets that are far apart in rapidity, subject to a limitation on the total transverse momentum Q0 in the interjet region. We specifically address the question of how to combine the approach which sums all leading logarithms in Q/Q0 (where Q is the jet transverse momentum) with the BFKL approach, in which leading logarithms of the scattering energy are summed. This paper constitutes progress towards the simultaneous summation of all important logarithms. Using an "all orders" matching, we are able to obtain results for the cross section which correctly reproduce the two approaches in the appropriate limits., Comment: 29 pages, 9 figures, minor corrections to text and improved figures

We use data on the deep inelastic structure function F_2 in order to constrain the cross-section for scattering a colour dipole off a proton. The data seem to prefer parameterisations which include saturation effects. That is they indicate that the strong rise with energy of the dipole cross-section, which holds for small dipoles, pertains only for r < r_s(x) where r_s(x) decreases monotonically as x decreases. Subsequent predicitions for the diffractive structure function F_2^{D(3)} also hint at saturation, although the data are not really sufficiently accurate., Comment: 15 pages, 4 figures

We compare the predictions of two distinct dipole models for inclusive and exclusive diffractive processes. While only one of these dipole models contains perturbative saturation dynamics, we show that the predictions of both models are fully consistent with the available HERA data, indicating no compelling evidence for saturation at present HERA energies., Comment: 8 pages, 4 figures, presented at the 26th Montreal-Rochester-Syracuse-Toronto (MRST) conference held at Concordia University, Montreal, Canada, 12th-14th May 2004. To appear in the proceedings

Theoretical progress in Higgs boson production and background processes is discussed with particular emphasis on QCD corrections at and beyond next-to-leading order as well as next-to-leading order electroweak corrections. The residual theoretical uncertainties of the investigated processes are estimated in detail. Moreover, recent investigations of the MSSM Higgs sector and other extensions of the SM Higgs sector are presented. The potential of the LHC and a high-energy linear e+e- collider for the measurement of Higgs couplings is analyzed., Comment: 169 pages, Proceedings 3rd Les Houches Workshop: Physics at TeV Colliders, Les Houches, France, 26 May - 6 Jun 2003

We confront the colour glass condensate motivated dipole model parameterization of Iancu, Itakura and Munier with the available HERA data on the diffractive structure function F2D3 and with existing dipole model parameterizations. Reasonably good agreement is found with only two adjustable parameters. We caution against interpreting the success of the model as compelling evidence for low-x perturbative saturation dynamics., Comment: 8 pages, 6 figures

We present a detailed comparison of a variety of predictions for diffractive light vector meson production with the data collected at the HERA collider. All our calculations are performed within a dipole model framework and make use of different models for the meson light-cone wavefunction. There are no free parameters in any of the scenarios we consider. Generally we find good agreement with the data using rather simple Gaussian motivated wavefunctions in conjunction with dipole cross-sections which have been fitted to other data., Comment: 28 pages, 17 figures. Predictions for the Colour Glass Condensate model of Iancu et al. have now been included, in addition to the dipole models previously considered

Diffractive vector meson photoproduction accompanied by proton dissociation is studied for large momentum transfer. The process is described by the non-forward BFKL equation which we use to compare to data collected at the HERA collider., Comment: 39 pages, 29 figures

Diffractive vector meson photoproduction accompanied by proton dissociation is studied for large momentum transfer. The process is described by the non-forward BFKL equation, for which a complete analytical solution is found. The scattering amplitudes for all combinations of helicity are presented., Comment: 16 pages. Minor typographical errors corrected

We discuss two novel possibilities in Higgs physics. The first is that, by adding a real Higgs triplet to the Standard Model, it is possible for the lightest Higgs boson to be as heavy as 500 GeV without any fine tuning. The second, somewhat orthogonal, possibility concerns the MSSM with explicit CP violation. This model is known to permit a light Higgs boson with mass below 50 GeV which may have avoided detection at LEP and may also avoid detection at the Tevatron and LHC. We suggest that diffraction may provide the key to excluding or observing such a Higgs., Comment: 4 pages. Talk presented at "QCD and High Energy Hadronic Interactions", Rencontres de Moriond

Light CP-violating Higgs bosons with mass lower than 70 GeV might have escaped detection in direct searches at the LEP collider. They may remain undetected in conventional search channels at the Tevatron and LHC. In this Letter we show that exclusive diffractive reactions may be able to probe for the existence of these otherwise elusive Higgs particles. As a prototype example, we calculate diffractive production cross-sections of the lightest Higgs boson within the framework of the Minimal Supersymmetric Standard Model with explicit CP violation. Our analysis shows that the challenging regions of parameter space corresponding to a light CP-violating Higgs boson might be accessible at the LHC provided suitable proton tagging detectors are installed., Comment: 9 pages, LaTeX, 2 figures, version as to appear in Phys. Rev. D

We perform an analysis of the Renormalization Group evolution of the couplings in an extension to the Standard Model which contains a real triplet in the Higgs sector. Insisting that the model remain valid up to 1 TeV allow us to map out the region of allowed mass for the Higgs bosons. We conclude that it is possible for there to be no light Higgs bosons without any otherwise dramatic deviation from the physics of the Standard Model., Comment: A study of the decoupling limit added. Published in JHEP06(2003)059

This talk will review selected topics in rapidity gap physics. In particular I will discuss diffractive jet production and the possibility of searching for the higgs boson using diffraction at the LHC; the dipole picture of diffraction and saturation; and those processes where a large momentum is transferred across the rapidity gap, for which there has been recent progress both experimentally and theoretically., Comment: 14 pages. Talk presented at HCP2002, Karlsruhe

A detailed study is presented of elastic WW scattering in the scenario that there are no new particles discovered prior to the commissioning of the LHC. We work within the framework of the electroweak chiral lagrangian and two different unitarisation protocols are investigated. Signals and backgrounds are simulated to the final-state-particle level. A new technique for identifying the hadronically decaying W is developed, which is more generally applicable to massive particles which decay to jets where the separation of the jets is small. The effect of different assumptions about the underlying event is also studied. We conclude that the channel WW -> jj+l+nu may contain scalar and/or vector resonances which could be measurable after 100 fb^(-1) of LHC data., Comment: 29 pages

We explore the diffractive interaction of a proton with an anti-proton which results in centrally produced dijets. This process has been recently studied at the Tevatron. We make predictions within an Ingelman-Schlein approach and compare them to the recent data presented by the CDF collaboration. Earlier calculations resulted in theoretical cross-sections which are much larger than those observed by CDF. We find that, after consideration of hadronisation effects and the parton shower, and using parton density functions extracted from diffractive deep inelastic scattering at HERA, it is possible to explain the CDF data. We need to assume a gap survival probability of around 10% and this is in good agreement with the value predicted by theory. We also find that the non-diffractive contribution to the process is probably significant in the kinematical region probed by the Tevatron., Comment: 12 pages, 6 figures, 3 tables

We perform a global fit to high energy precision electroweak data in a Higgs model containing the usual isospin doublet plus a real isospin triplet. The analysis is performed in terms of the oblique parameters S, T and U and we show that the mass of the lightest Higgs boson can be as large as 2 TeV.

We explore QCD calculations for the process gamma p -> V X where V is a vector meson, in the region s >> -t and -t >> Lambda_QCD^2. We compare our calculations for the J/psi, phi and rho mesons with data from the ZEUS Collaboration at HERA and demonstrate that the BFKL approach is consistent with the data even for light mesons, whereas the two-gluon exchange approach is inadequate. We also predict the differential cross-sections for the Upsilon and omega for which no data are currently available., Comment: 9 pages including 2 postscript figures

The diffractive production of vector bosons at the Tevatron is studied. We take a hard pomeron flux and use the H1 parton density functions extracted from their measurement of F_2^{D(3)}. To this we add a reggeon exchange contribution. We find that the ratio of diffractive to non-diffractive W boson production is in good agreement with the CDF data. We note that the poorly understood reggeon contribution might be as large as the pomeron contribution in the kinematic range of the data. We also note that our pomeron exchange contribution is much smaller than earlier predictions due to our use of a hard pomeron flux. All our results have been obtained using the lowest order matrix elements and we have compared to the results obtained using a modified version of Herwig which is capable of generating a wide variety of diffractive scattering processes. Gap survival is estimated using Pythia and is found to be around 60%. We make predictions which could be compared to future measurements of diffractive Drell-Yan production., Comment: 11 pages, 4 figures

We describe a formalism for solving the BFKL equation with a coupling that runs for momenta above a certain infrared cutoff. By suitably choosing matching conditions proper account is taken of the fact that the BFKL diffusion implies that the solution in the infrared (fixed coupling) regime depends upon the solution in the ultraviolet (running coupling) regime and vice versa. Expanding the BFKL kernel to a given order in the ratio of the transverse momenta allows arbitrary accuracy to be achieved., Comment: 11 pages, 2 figs

We have modified the HERWIG event generator to incorporate diffractive interactions. All standard HERWIG hard subprocesses are available., Comment: The update contains a revised, expanded version of the original POMWIG manual; 9 pages, 2 figures

We have performed a detailed phenomenological investigation of the hard colour singlet exchange process observed at the Tevatron in events that have a large rapidity gap between outgoing jets. We include the effects of multiple interactions to obtain a prediction for the gap survival factor. Comparing the data on the fraction of gap events with the prediction from BFKL pomeron exchange we find agreement provided that a constant value of alpha_s is used in the BFKL calculation. Moreover, the value of alpha_s is in line with that extracted from measurements made at HERA., Comment: 5 pages, 5 figures. Talk given at the Tevatron Run 2 QCD and Weak Boson Physics workshop, Nov 4-6 1999

We examine the prospects for studying the diffractive production of high pt photons in the process gamma+gamma -> gamma+X at a future linear collider operating in both ee and gamma-gamma modes. The high luminosity associated with a linear collider make it the ideal place to measure this process., Comment: 5 pages. Contribution to the 2nd ECFA/DESY study on the physics and detectors for a linear electron-positron collider

We study the diffractive production of high pt photons at HERA. We have implemented the process as a new hard sub-process in the HERWIG event generator in order to prepare the ground for a future measurement., Comment: 4 pages, 4 figures. Contribution to the 1999 UK Phenomenology Workshop on Collider Physics, Durham, UK

We employ a parameterisation of the proton dipole cross section previously extracted from electroproduction and photoproduction data to predict the diffractive structure function F2D(3)(Q^2, beta, xpom). Comparison with HERA H1 data yields good agreement., Comment: 5 pages,4 figures, Latex2e, uses espcrc1.sty. Presented at "Hadron 99" Beijing, August 1999. Reference added, erroneous citation corrected

We calculate the subjet rates for jets produced in hadron collisions. The kt algorithm is used to define the jets and allows the theoretical calculation to sum both the leading and next-to-leading logarithms in the resolution variable, ycut. We also ensure that our calculation matches exactly the leading order in alpha_s result and has sensible behaviour near thresholds., Comment: 23 pages, 22 figures

The study of those rapidity gap processes where a large momentum is transferred across the rapidity gap provides an ideal opportunity to understand the gap producing mechanism wholly within the framework of QCD perturbation theory. The current theoretical and experimental status of this `high-t diffraction' is reviewed., Comment: 3 pages, 1 figure. Talk presented at 7th International Workshop on Deep Inelastic Scattering, Zeuthen, Germany, April 1999. Revised version: references altered