DSpace Collection:

Predictions for neutrino structure functions.

Mon, 01 Jan 2001 00:00:00 GMT

Title: Predictions for neutrino structure functions. Authors: Kretzer, S; Olness, Frederick I; Scalise, R J; Thorne, Robert S; Yang, U K Abstract: The first measurements of xF3 are higher than current theoretical predictions. We investigate the sensitivity of these theoretical predictions upon a variety of factors including: renormalization scheme and scale, quark mass effects, higher twist, isospin violation, and PDF uncertainties.

Measuring supersymmetric particle masses at the LHC in scenarios with baryon number R-parity violating couplings.

Thu, 01 Feb 2001 00:00:00 GMT

Title: Measuring supersymmetric particle masses at the LHC in scenarios with baryon number R-parity violating couplings. Authors: Allanach, B C; Barr, Alan; Drage, L; Lester, Christopher G; Morgan, D; Parker, Michael A; Webber, Bryan R; Richardson, P Abstract: The measurement of sparticle masses in the Minimal Supersymmetric Standard Model at the LHC is analysed, in the scenario where the lightest neutralino, the ˜χ01 , decays into three quarks. Such decays, occurring through the baryon-number violating coupling λ′′ ijk, pose a severe challenge to the capability of the LHC detectors since the final state has no missing energy signature and a high jet multiplicity. We focus on the case λ′′ 212 6= 0 which is the most difficult experimentally. The proposed method is valid over a wide range of SUGRA parameter space with λ′′ 212 ∼ 10−5−0.1. Simulations are performed of the ATLAS detector at the Large Hadron Collider. Using the ˜χ01 from the decay chain ˜qL → ˜χ02 q → ˜lRℓq → ˜χ01 ℓℓq, we show that the ˜χ01 and ˜χ02 masses can be measured by 3-jet and 3-jet + lepton pair invariant mass combinations. At the SUGRA point m0 = 100 GeV, m1/2 = 300 GeV, A0 = 300 GeV, tan β = 10, μ > 0 and with λ′′ 212 = 0.005, we achieve statistical (systematic) errors of 3 (3), 3 (3), 0.3 (4) and 5 (12) GeV respectively for the masses of the ˜χ01 , ˜χ02 , ˜lR and ˜qL, with an integrated luminosity of 30 fb−1.

The Running coupling BFKL anomalous dimensions and splitting functions.

Thu, 01 Mar 2001 00:00:00 GMT

Title: The Running coupling BFKL anomalous dimensions and splitting functions. Authors: Thorne, Robert S Abstract: I explicitly calculate the anomalous dimensions and splitting functions governing the Q2 evolution of the parton densities and structure functions which result from the running coupling BFKL equation at LO, i.e. I perform a resummation in powers of ln(1/x) and in powers of β0 simultaneously. This is extended as far as possible to NLO. These are expressed in an exact, perturbatively calculable analytic form, up to small power-suppressed contributions which may also be modelled to very good accuracy by analytic expressions. Infrared renormalons, while in principle present in a solution in terms of powers in αs(Q2), are ultimately avoided. The few higher twist contributions which are directly calculable are extremely small. The splitting functions are very different from those obtained from the fixed coupling equation, with weaker power-like growth ∼ x−0.25, which does not set in until extremely small x indeed. The NLO BFKL corrections to the splitting functions are moderate, both for the form of the asymptotic power-like behaviour and more importantly for the range of x relevant for collider physics. Hence, a stable perturbative expansion and predictive power at small x are obtained. March 2001 1 Royal

MRST global fit update.

Thu, 31 May 2001 23:00:00 GMT

Title: MRST global fit update. Authors: Thorne, Robert S; Martin, A D; Stirling, W James; Roberts, R G Abstract: We discuss the impact of the most recent data on the MRST global analysis - in particular the new high-ET jet data and their implications for the gluon and the new small x structure function data. In the light of these new data we also consider the uncertainty in predictions for physical quantities depending on parton distributions, concentrating on the W cross-section at hadron colliders.

Attempts at explaining the NuTeV observation of dimuon events.

Thu, 31 May 2001 23:00:00 GMT

Title: Attempts at explaining the NuTeV observation of dimuon events. Authors: Dedes, A; Dreiner, Herbert K; Richardson, P Abstract: The NuTeV Collaboration has observed an excess in their di-muon channel, possibly corresponding to a long-lived neutral particle with only weak interactions and which decays to muon pairs. We show that this can not be explained by pair production of neutralinos in the target followed by their decay far downstream in the detector via a LLE R-parity violating (RPV) operator, as suggested in the literature. In the parameter region allowed by LEP the event rate is far too small. We propose instead a new neutralino production method via B-mesons, which can fully explain the observation. This is analogous to neutrino production via -mesons. This model can be completely tested and thus also possibly excluded with NOMAD data. If it is excluded, the NuTeV observation is most likely not due to physics beyond the Standard Model. Our model can also be tested at the current and future B-factories. This opens up a new way of testing a long-lived neutralino LSP at fixed-target experiments and thus a possibility to close the gap between collider and cosmological tests of R-parity violation. We also discuss a possible explanation in terms of a neutral heavy lepton mixing with the Standard Model neutrinos. The flavour structure of the observation can be accounted for but the production rate is far too low.

Supersymmetric models and collider signatures.

Thu, 31 May 2001 23:00:00 GMT

Title: Supersymmetric models and collider signatures. Authors: Hinchliffe, I; Richardson, P Abstract: We briefly review the SUGRA, GMSB and AMSB supersymmetry breaking models. We then discuss the phenomenological differences between them and consequent characteristic experimental signatures. This is followed by a review of the discovery potential for supersymmetry at the Tevatron, LHC and a future e+e− linear collider.

Extracting the flavor structure of a baryon number R-parity violating coupling at the LHC.

Thu, 31 May 2001 23:00:00 GMT

Title: Extracting the flavor structure of a baryon number R-parity violating coupling at the LHC. Authors: Allanach, B C; Barr, Alan; Parker, Michael A; Richardson, P; Webber, Bryan R Abstract: The detection of the flavour content of jets produced from R-parity violating neutralino decays is investigated in the case where one baryon-number violating coupling dominates. Simulations are performed of the ATLAS experiment at the LHC for all couplings, other than λ′′ tjk since neutralino decays through these couplings are very suppressed. Secondary vertex distributions and muons produced by heavy-quark (b- and c-) jets allow discrimination between LSP decay modes. The dominant coupling can be identified at better than 3.5 σ in almost all cases, with the only remaining ambiguity caused by the inability to distinguish strange from down quarks.

HERWIG 6.3 release note.

Sat, 30 Jun 2001 23:00:00 GMT

Title: HERWIG 6.3 release note. Authors: Corcella, G; Knowles, I G; Marchesini, G; Moretti, Stefano; Odagiri, Kosuke; Richardson, P; Seymour, Michael H; Webber, Bryan R Abstract: A new release of the Monte Carlo program HERWIG (version 6.3) is now available. The main new features are new (MRST) built-in parton distribution functions, more SM gauge boson production processes, 2 ! 3 MSSMHiggs production processes, an option to handle negative event weights, and an interface to the CIRCE beamstrahlung program.

Detecting exotic heavy leptons at the large hadron collider.

Tue, 31 Jul 2001 23:00:00 GMT

Title: Detecting exotic heavy leptons at the large hadron collider. Authors: Allanach, B C; Harris, Chris M; Parker, Michael A; Richardson, P; Webber, Bryan R Abstract: New almost-degenerate charged and neutral heavy leptons are a feature of a number of theories of physics beyond the Standard Model. The prospects for detecting these at the Large Hadron Collider using a time-of-flight technique are considered, along with any cosmological or experimental constraints on their masses. Based on a discovery criterion of 10 detected exotic leptons we conclude that, with an integrated luminosity of 100 fb−1, it should be possible to detect such leptons provided their masses are less than 950 GeV. It should also be possible to use the angular distribution of the produced particles to distinguish these exotic leptons from supersymmetric scalar leptons, at a better than 90% confidence level, for masses up to 580 GeV.

The Spectrum of the MSSM with nonstandard supersymmetry breaking.

Tue, 31 Jul 2001 23:00:00 GMT

Title: The Spectrum of the MSSM with nonstandard supersymmetry breaking. Authors: Hetherington, J P J Abstract: The Minimal Supersymmetric Standard Model (MSSM) can include two soft breaking terms which are often neglected: a non-analytic scalar trilinear coupling and a Higgsino bilinear term. A set of high-scale boundary conditions consistent with the reparameterisation invariance which the model possesses is obtained. The three-family renormalisation group equations for the MSSM with these terms are presented. The ranges of the universal high-scale values of these couplings which lead to an acceptable TeV-scale theory are obtained, as is the supersymmetric particle spectrum at this scale. The effect of the new terms on fine-tuning is presented. SOFTSUSY, an existing program for calculating SUSY particle spectra, has been used, with as few modifications as possible.

AMEGIC++ 1.0: A Matrix element generator in C++.

Fri, 31 Aug 2001 23:00:00 GMT

Title: AMEGIC++ 1.0: A Matrix element generator in C++. Authors: Krauss, F; Kuhn, R; Soff, G Abstract: The new matrix element generator AMEGIC++ is introduced, dedicated to describe multi–particle production in high energy particle collisions. It automatically generates helicity amplitudes for the processes under consideration and constructs suitable, efficient integration channels for the multi–channel phase space integration. The corresponding expressions for the amplitudes and the integrators are stored in library files to be linked to the main program.

QCD matrix elements + parton showers.

Fri, 31 Aug 2001 23:00:00 GMT

Title: QCD matrix elements + parton showers. Authors: Catani, S; Krauss, F; Kuhn, R; Webber, Bryan R Abstract: We propose a method for combining QCD matrix elements and parton showers in Monte Carlo simulations of hadronic final states in e+e− annihilation. The matrix element and parton shower domains are separated at some value yini of the jet resolution, defined according to the kT -clustering algorithm. The matrix elements are modified by Sudakov form factors and the parton showers are subjected to a veto procedure to cancel dependence on yini to next-to-leading logarithmic accuracy. The method provides a leading-order description of hard multi-jet configurations together with jet fragmentation, while avoiding the most serious problems of double counting. We present first results of an approximate implementation using the event generator APACIC++.

Jet fragmentation in e+e- annihilation.

Fri, 31 Aug 2001 23:00:00 GMT

Title: Jet fragmentation in e+e- annihilation. Authors: Biebel, O; Nason, Paolo; Webber, Bryan R Abstract: A short review of theoretical and experimental results on fragmentation in e+e− annihilation is presented. Starting with an introduction of the concept of fragmentation functions in e+e− annihilation, aspects of scaling violation, multiplicities, small and large x, longitudinal, gluon, light and heavy quark fragmentation are summarized.

Spin correlations in Monte Carlo simulations.

Fri, 31 Aug 2001 23:00:00 GMT

Title: Spin correlations in Monte Carlo simulations. Authors: Richardson, P Abstract: We show that the algorithm originally proposed by Collins and Knowles for spin correlations in the QCD parton shower can be used in order to include spin correlations between the production and decay of heavy particles in Monte Carlo event generators. This allows correlations to be included while maintaining the step-by-step approach of the Monte Carlo event generation process. We present examples of this approach for both the Standard and Minimal Supersymmetric Standard Models. A merger of this algorithm and that used in the parton shower is discussed in order to include all correlations in the perturbative phase of event generation. Finally we present all the results needed to implement this algorithm for the Standard and Minimal Supersymmetric Standard Models.

MRST2001: Partons and alpha(s) from precise deep inelastic scattering and Tevatron jet data.

Sun, 30 Sep 2001 23:00:00 GMT

Title: MRST2001: Partons and alpha(s) from precise deep inelastic scattering and Tevatron jet data. Authors: Martin, A D; Roberts, R G; Stirling, W James; Thorne, Robert S Abstract: We use all the available new precise data for deep inelastic and related hard scattering processes to perform NLO global parton analyses. These new data allow an improved determination of partons and, in particular, the inclusion of the recent measurements of the structure functions at HERA and of the inclusive jets at the Tevatron help to determine the gluon distribution and S better than ever before. We find a somewhat smaller gluon at low x than previous determinations and that S(M2Z ) = 0.119 ± 0.002 (expt.) ± 0.003 (theory).

NNLO global parton analysis.

Fri, 01 Mar 2002 00:00:00 GMT

Title: NNLO global parton analysis. Authors: Martin, A D; Roberts, R G; Stirling, W James; Thorne, Robert S Abstract: We perform a NNLO (and a LO) global parton analysis in which we include the new precise data for deep inelastic scattering from HERA and for inclusive jet production at the Tevatron, together with the improved knowledge of the three-loop splitting functions. The results are compared with our recent NLO analyses. The LO fit produces significantly worse results in general, but gives a surprisingly good fit to the Tevatron high-ET jet data. For the approximate NNLO analysis we notice a slight improvement in the quality of the global fit, and find that the partons are changed by up to 10% at Q2 = 10 GeV2, in particular in the x < 0.01 regime. †Royal

HERWIG 6.4 release note.

Sat, 01 Dec 2001 00:00:00 GMT

Title: HERWIG 6.4 release note. Authors: Corcella, G; Knowles, I G; Marchesini, G; Moretti, Stefano; Odagiri, Kosuke; Richardson, P; Seymour, Michael H; Webber, Bryan R Abstract: A new release of the Monte Carlo program HERWIG (version 6.4) is now available. The main new features are: spin correlations between the production and decay of heavy fermions, i.e. top quarks, leptons and SUSY particles; polarization effects in SUSY production processes in lepton-lepton collisions; an interface to TAUOLA for decays; MSSM Higgs processes in lepton-lepton collisions.

Implementing initial state radiation for lepton induced processes in AMEGIC++.

Fri, 01 Mar 2002 00:00:00 GMT

Title: Implementing initial state radiation for lepton induced processes in AMEGIC++. Authors: Schalicke, A; Krauss, F; Kuhn, R; Soff, G Abstract: We have implemented the method of Yennie, Frautschi, and Suura up to first order in α for the simulation of QED Initial State Radiation in lepton induced processes in AMEGIC++. We consider s–channel processes via the exchange of scalar or vector resonances at electron and muon colliders.

Implementation of supersymmetric processes in the HERWIG event generator.

Sun, 31 Mar 2002 23:00:00 GMT

Title: Implementation of supersymmetric processes in the HERWIG event generator. Authors: Moretti, Stefano; Odagiri, Kosuke; Richardson, P; Seymour, Michael H; Webber, Bryan R Abstract: We describe the implementation of supersymmetric processes in the HERWIG Monte Carlo event generator. We define relevant parameter and mixing conventions and list the hard scattering matrix elements. Our implementation is based on the Minimum Supersymmetric Standard Model, with the option of R-parity violation. The sparticle spectrum is completely general. Both hadron–hadron and lepton–lepton collisions are covered. This article supplements a separate publication in which the general features of HERWIG 6.2 are described, and updates the treatment of supersymmetry to version 6.4.

Matching NLO QCD computations and parton shower simulations.

Sun, 31 Mar 2002 23:00:00 GMT

Title: Matching NLO QCD computations and parton shower simulations. Authors: Frixione, Stefano; Webber, Bryan R Abstract: We propose a method for matching the next-to-leading order (NLO) calculation of a given QCD process with a parton shower Monte Carlo (MC) simulation. The method has the following features: fully exclusive events are generated, with hadronization according to the MC model; total exclusive rates are accurate to NLO; NLO results for distributions are recovered upon expansion in αs; hard emissions are treated as in NLO computations while soft/collinear emissions are handled by the MC simulation, with the same logarithmic accuracy as the MC; and matching between the hard- and soft/collinear-emission regions is smooth. A fraction of events with negative weight is generated, but unweighting remains possible with reasonable efficiency. The method is clarified using a simple toy model, and illustrated by application to the hadroproduction of W+W− pairs.