Showing papers in "European Physical Journal C in 2008"

Herwig++ Physics and Manual

Abstract: In this paper we describe Herwig++ version 2.3, a general-purpose Monte Carlo event generator for the simulation of hard lepton-lepton, lepton-hadron and hadron-hadron collisions. A number of important hard scattering processes are available, together with an interface via the Les Houches Accord to specialized matrix element generators for additional processes. The simulation of Beyond the Standard Model (BSM) physics includes a range of models and allows new models to be added by encoding the Feynman rules of the model. The parton-shower approach is used to simulate initial- and final-state QCD radiation, including colour coherence effects, with special emphasis on the correct description of radiation from heavy particles. The underlying event is simulated using an eikonal multiple parton-parton scattering model. The formation of hadrons from the quarks and gluons produced in the parton shower is described using the cluster hadronization model. Hadron decays are simulated using matrix elements, where possible including spin correlations and off-shell effects. Comment: 153 pages, program and additional information available from http://projects.hepforge.org/herwig . Updated description to Herwig++ version 2.3 and added one author

First results from DAMA/LIBRA and the combined results with DAMA/NaI

Abstract: The highly radiopure ≃ 250 kg NaI(Tl) DAMA/LIBRA set-up is running at the Gran Sasso National Laboratory of the INFN. In this paper the first result obtained by exploiting the model independent annual modulation signature for Dark Matter (DM) particles is presented. It refers to an exposure of 0.53 ton×yr. The collected DAMA/LIBRA data satisfy all the many peculiarities of the DM annual modulation signature. Neither systematic effects nor side reactions able to account for the observed modulation amplitude and to contemporaneously satisfy all the several requirements of this DM signature are available. Considering the former DAMA/NaI and the present DAMA/LIBRA data all together (total exposure 0.82 ton×yr), the presence of Dark Matter particles in the galactic halo is supported, on the basis of the DM annual modulation signature, at 8.2 σ C.L.; in particular, in the energy interval (2–6) keV, the modulation amplitude is (0.0131±0.0016) cpd/kg/keV and the phase and the period are well compatible with June 2nd and one year, respectively.

Comparative study of various algorithms for the merging of parton showers and matrix elements in hadronic collisions

Abstract: We compare different procedures for combining fixed-order tree-level matrix-element generators with parton showers. We use the case of W-production at the Tevatron and the LHC to compare different implementations of the so-called CKKW and MLM schemes using different matrix-element generators and different parton cascades. We find that although similar results are obtained in all cases, there are important differences.

Parton Distributions for LO Generators.

Abstract: We present a study of the results obtained by combining LO partonic matrix elements with either LO or NLO parton distributions. These are compared to the best prediction using NLO for both matrix elements and parton distributions. The aim is to determine which parton distributions are most appropriate to use in those cases where only LO matrix elements are available, e.g. as in many Monte Carlo generators. Both LO and NLO parton distributions have flaws, sometimes serious, for some processes, so a modified optimal LO set is suggested. We investigate a wide variety of processes, and the new modified LO* PDF works at least as well as, and often better than, both LO and NLO PDFs in nearly all cases.

Flavor physics of leptons and dipole moments

Abstract: This chapter of the report of the “Flavor in the era of the LHC” Workshop discusses the theoretical, phenomenological and experimental issues related to flavor phenomena in the charged lepton sector and in flavor conserving CP-violating processes. We review the current experimental limits and the main theoretical models for the flavor structure of fundamental particles. We analyze the phenomenological consequences of the available data, setting constraints on explicit models beyond the standard model, presenting benchmarks for the discovery potential of forthcoming measurements both at the LHC and at low energy, and exploring options for possible future experiments.

Automating dipole subtraction for QCD NLO calculations

Abstract: In this publication the construction of an automatic algorithm to subtract infrared divergences in real QCD corrections through the Catani–Seymour dipole subtraction method [1,2] is reported. The resulting computer code has been implemented in the matrix element generator AMEGIC++ [3]. This will allow for the automatic generation of dipole subtraction terms and their integrals over the one-parton emission phase space for any given process. If the virtual matrix element is provided as well, this then directly leads to an NLO QCD parton level event generator.

The role of the quark and gluon GPDs in hard vector-meson electroproduction

Abstract: Electroproduction of light vector mesons is analyzed on the basis of handbag factorization. The required generalized parton distributions are constructed from the CTEQ6 parton distributions with the help of double distributions. The partonic subprocesses are calculated within the modified perturbative approach. The present work extends our previous analysis of the longitudinal cross section to the transverse one and other observables related to both the corresponding amplitudes. Our results are compared to recent experimental findings in detail.

κ-Minkowski spacetime and the star product realizations

Abstract: We investigate a Lie algebra-type κ-deformed Minkowski spacetime with undeformed Lorentz algebra and mutually commutative vector-like Dirac derivatives. There are infinitely many realizations of κ-Minkowski space. The coproduct and the star product corresponding to each of them are found. An explicit connection between realizations and orderings is established and the relation between the coproduct and the star product, provided through an exponential map, is proved. Utilizing the properties of the natural realization, we construct a scalar field theory on κ-deformed Minkowski space and show that it is equivalent to the scalar, nonlocal, relativistically invariant field theory on the ordinary Minkowski space. This result is universal and does not depend on the realizations, i.e. the orderings, used.

Possible implications of the channeling effect in NaI(Tl) crystals

Abstract: The channeling effect of low energy ions along the crystallographic axes and planes of NaI(Tl) crystals is discussed in the framework of corollary investigations on WIMP dark matter candidates. In fact, the modeling of this existing effect implies a more complex evaluation of the luminosity yield for low energy recoiling Na and I ions. In the present paper related phenomenological arguments are developed and possible implications are discussed at some extent.

Dynamical parton distributions of the nucleon and very small-x physics

Abstract: Utilizing recent DIS measurements (F2,L) and data on dilepton and high-ET jet production we determine the dynamical parton distributions of the nucleon generated radiatively from valence-like positive input distributions at optimally chosen low resolution scales. These are compared with ‘standard’ distributions generated from positive input distributions at some fixed and higher resolution scale. It is shown that up to the next-to-leading order NLO( $\overline{\text{MS}}$ , DIS) of perturbative QCD considered in this paper, the uncertainties of the dynamical distributions are, as expected, smaller than those of their standard counterparts. This holds true in particular in the presently unexplored extremely small-x region relevant for evaluating ultrahigh energy cross sections in astrophysical applications. It is noted that our new dynamical distributions are compatible, within the presently determined uncertainties, with previously determined dynamical parton distributions.

The Determination of $\alpha_s$ from Tau Decays Revisited

Abstract: We revisit the determination of α S (m 2 ) using a fit to inclusive τ hadronic spectral moments in light of (1) the recent calculation of the fourth-order perturbative coefficient K 4 in the expansion of the Adler function, (2) new precision measurements from BABAR of e+e− annihilation cross sections, which decrease the uncertainty in the separation of vector and axial-vector spectral functions, and (3) improved results from BABAR and Belle on τ branching fractions involving kaons. We estimate that the fourth-order perturbative prediction reduces the theoretical uncertainty, introduced by the truncation of the series, by 20% with respect to earlier determinations. We discuss to some detail the perturbative prediction of two different methods: fixed-order perturbation theory (FOPT) and contour-improved perturbative theory (CIPT). The corresponding theoretical uncertainties are studied at the τ and Z mass scales. The CIPT method is found to be more stable with respect to the missing higher order contributions and to renormalization scale variations. It is also shown that FOPT suffers from convergence problems along the complex integration contour. Nonperturbative contributions extracted from the most inclusive fit are small, in agreement with earlier determinations. Systematic effects from quark-hadron duality violation are estimated with simple models and found to be within the quoted systematic errors. The fit based on CIPT gives α S (m 2 )=0.344±0.005±0.007, where the first error is experimental and the second theoretical. After evolution to M Z we obtain α S (M 2 )=0.1212±0.0005±0.0008±0.0005, where the errors are respectively experimental, theoretical and due to the evolution. The result is in agreement with the corresponding N3LO value derived from essentially the Z width in the global electroweak fit. The α S (M 2 ) determination from τ decays is the most precise one to date.

B, D and K decays

Abstract: With the advent of the LHC, we will be able to probe New Physics (NP) up to energy scales almost one order of magnitude larger than it has been possible with present accelerator facilities. While direct detection of new particles will be the main avenue to establish the presence of NP at the LHC, indirect searches will provide precious complementary information, since most probably it will not be possible to measure the full spectrum of new particles and their couplings through direct production. In particular, precision measurements and computations in the realm of flavour physics are expected to play a key role in constraining the unknown parameters of the Lagrangian of any NP model emerging from direct searches at the LHC. The aim of Working Group 2 was twofold: on one hand, to provide a coherent, up-to-date picture of the status of flavour physics before the start of the LHC; on the other hand, to initiate activities on the path towards integrating information on NP from high-pT and flavour data.

Exclusive processes in position space and the pion distribution amplitude

Abstract: We suggest to carry out lattice calculations of current correlators in position space, sandwiched between the vacuum and a hadron state (e.g. pion), in order to access hadronic light-cone distribution amplitudes (DAs). In this way the renormalization problem for composite lattice operators is avoided altogether, and the connection to the DA is done using perturbation theory in the continuum. As an example, the correlation function of two electromagnetic currents is calculated to the next-to-next-to-leading order accuracy in perturbation theory and including the twist-four corrections. We argue that this strategy is fully competitive with direct lattice measurements of the moments of the DA, defined as matrix elements of local operators, and offers new insight in the space-time picture of hard exclusive reactions.

Excited heavy tetraquarks with hidden charm

Abstract: The masses of the excited heavy tetraquarks with hidden charm are calculated within the relativistic diquark-antidiquark picture. The dynamics of the light quark in a heavy-light diquark is treated completely relativistically. The diquark structure is taken into account by calculating the diquark-gluon form factor. New experimental data on charmonium-like states above open charm threshold are discussed. The obtained results indicate that X(3872), Y(4260), Y(4360), Z(4248), Z(4433) and Y(4660) could be tetraquark states with hidden charm.

Agegraphic dark energy as a quintessence

Abstract: Recently, a dark energy model characterized by the age of the universe, dubbed "agegraphic dark energy", was proposed by Cai. In this paper, a connection between the quintessence scalar-field and the agegraphic dark energy is established, and accordingly, the potential of the agegraphic quintessence field is constructed.

Is X(3872) really a molecular state

Abstract: After taking into account both the pion and sigma meson exchange potential, we have performed a dynamical calculation of the D-0(D) over bar*(0) system. The s meson exchange potential is repulsive from heavy quark symmetry and numerically important for a loosely bound system. Our analysis disfavors the interpretation of X(3872) as a loosely bound molecular state if we use the experimental D*D pi coupling constant g = 0.59 and a reasonable cutoff around 1 GeV, which is the typical hadronic scale. Bound state solutions with negative eigenvalues for the D-0(D) over bar* system exist only with either a very large coupling constant (twice the experimental value) or a large cutoff (Lambda similar to 6GeV or beta similar to 6GeV(2)). In contrast, there probably exists a loosely bound S-wave B (B) over bar* molecular state. Once produced, such a molecular state would be rather stable, since its dominant decay mode is the radiative decay through B* -> B gamma. Experimental search of these states will be very interesting.

Vector and axial nucleon form factors: A duality constrained parameterization

Abstract: We present new parameterizations of vector and axial nucleon form factors. We maintain an excellent descriptions of the form factors at low momentum transfers, where the spatial structure of the nucleon is important, and use the Nachtman scaling variable ξ to relate elastic and inelastic form factors and impose quark–hadron duality constraints at high momentum transfers where the quark structure dominates. We use the new vector form factors to re-extract updated values of the axial form factor from neutrino experiments on deuterium. We obtain an updated world average value from νμd and pion electroproduction experiments of MA=1.014± 0.014 GeV/c 2. Our parameterizations are useful in modeling neutrino interactions at low energies (e.g. for neutrino oscillations experiments). The predictions for high momentum transfers can be tested in the next generation electron and neutrino scattering experiments.

Wilson line correlator in the MV model: relating the glasma to deep inelastic scattering

Abstract: In the color glass condensate framework the saturation scale measured in deep inelastic scattering of high energy hadrons and nuclei can be determined from the correlator of Wilson lines in the hadron wavefunction. These same Wilson lines give the initial condition of the classical field computation of the initial gluon multiplicity and energy density in a heavy ion collision. In this paper the Wilson line correlator in both adjoint and fundamental representations is computed using exactly the same numerical procedure as has been used to calculate gluon production in a heavy ion collision. In particular the discretization of the longitudinal coordinate has a large numerical effect on the relation between the color charge density parameter g2μ and the saturation scale Qs. Our result for this relation is Qs≈0.6g2μ, which results in the classical Yang–Mills value for the “gluon liberation coefficient” c≈1.1.

Charged black holes in phantom cosmology

Abstract: In the classical relativistic regime, the accretion of phantom-like dark energy onto a stationary black hole reduces the mass of the black hole. We have investigated the accretion of phantom energy onto a stationary charged black hole and have determined the condition under which this accretion is possible. This condition restricts the mass-to-charge ratio in a narrow range. This condition also challenges the validity of the cosmic-censorship conjecture since a naked singularity is eventually produced due to accretion of phantom energy onto black hole.

Interacting modified variable Chaplygin gas in a non-flat universe

Abstract: A unified model of dark energy and matter is presented using the modified variable Chaplygin gas for interacting dark energy in a non-flat universe. The two entities interact with each other non-gravitationally, which involves a coupling constant. Due to dynamic interaction, a variation in this constant arises that henceforth changes the equations of state of these quantities. We have derived the effective equations of state corresponding to matter and dark energy in this interacting model. Moreover, the case of phantom energy is deduced by putting constraints on the parameters involved.

One-loop corrections to the Drell-Yan process in SANC (II) : The neutral current case

Abstract: Radiative corrections to the neutral current Drell–Yan-like processes are considered. Complete one-loop electroweak corrections are calculated within the SANC system. Theoretical uncertainties are discussed. Numerical results are presented for typical conditions of LHC experiments.

Perturbative photon fluxes generated by high-frequency gravitational waves and their physical effects

Abstract: We consider the electromagnetic (EM) perturbative effects produced by high-frequency gravitational waves (HFGWs) in the GHz band in a special EM resonance system, which consists of fractal membranes, a Gaussian beam (GB) passing through a static magnetic field. Under the synchro-resonance condition, coherence modulation of the HFGWs to the preexisting transverse components of the GB is predicted to produce the transverse perturbative photon flux (PPF), which has three novel and important properties. (1) The PPF has a maximum at a longitudinal symmetrical surface of the GB where the transverse background photon flux (BPF) vanishes; (2) the resonant effect will be high sensitive to the propagating directions of the HFGWs; (3) the PPF reflected or transmitted by the fractal membrane exhibits a very small decay to be compared with a very large decay of the much stronger BPF. Such properties might provide a new way to distinguish and display the perturbative effects produced by the HFGWs. We also discuss the high-frequency asymptotic behavior of the relic GWs in the microwave band and the positive definite issues of their energy-momentum pseudo-tensor.

Interacting dark energy with inhomogeneous equation of state

Abstract: We have investigated the model of dark energy interacting with dark matter by choosing inhomogeneous equations of state for the dark energy and a nonlinear interaction term for the underlying interaction. The equations of state have dependencies either on the energy densities, the redshift, the Hubble parameter or the bulk viscosity. We have considered these possibilities and have derived the effective equations of state for the dark energy in each case.

Collider aspects of flavor physics at high Q

Abstract: This chapter of the “Flavor in the era of LHC” workshop report discusses flavor-related issues in the production and decays of heavy states at the LHC at high momentum transfer Q, both from the experimental and the theoretical perspective. We review top quark physics, and discuss the flavor aspects of several extensions of the standard model, such as supersymmetry, little Higgs models or models with extra dimensions. This includes discovery aspects, as well as the measurement of several properties of these heavy states. We also present publicly available computational tools related to this topic.

CP violation in the general two-Higgs-doublet model: a geometric view

Abstract: We discuss the CP properties of the potential in the general two-Higgs-doublet model (THDM) This is done in a concise way using real gauge invariant functions built from the scalar products of the doublet fields The space of these invariant functions, parametrising the gauge orbits of the Higgs fields, is isomorphic to the forward light cone and its interior CP transformations are shown to correspond to reflections in the space of the gauge invariant functions We consider CP transformations where no mixing of the Higgs doublets is taken into account as well as the general case where the Higgs basis is not fixed We present basis independent conditions for explicit CP violation which may be checked easily for any THDM potential Conditions for spontaneous CP violation, that is CP violation through the vacuum expectation values of the Higgs fields, are also derived in a basis independent way

New high statistics measurement of Ke4 decay form factors and ππ scattering phase shifts

Abstract: We report results from a new measurement of the Ke4 decay K±→π+π-e±ν by the NA48/2 collaboration at the CERN SPS, based on a partial sample of more than 670 000 Ke4 decays in both charged modes collected in 2003. The form factors of the hadronic current (F,G,H) and ππ phase difference (δ=δs-δp) have been measured in ten independent bins of the ππ mass spectrum to investigate their variation. A sizeable acceptance at large ππ mass, a low background and a very good resolution contribute to an improved experimental accuracy, a factor two better than in the previous measurement, when extracting the ππ scattering lengths a0 0 and a0 2. Under the assumption of isospin symmetry and using numerical solutions of the Roy equations, the following values are obtained in the plane (a0 0,a0 2): a0 0=0.233±0.016stat±0.007syst,a0 2=-0.0471±0.011stat±0.004syst. The presence of potentially large isospin effects is also considered and will allow comparison with precise predictions from Chiral Perturbation Theory.

The thermal production of strange and non-strange hadrons in e + e − collisions

Abstract: The thermal multihadron production observed in different high energy collisions poses two basic problems. (1) Why do even elementary collisions with comparatively few secondaries (e + e − annihilation) show thermal behavior? (2) Why is there in such interactions a suppression of strange particle production? We show that the recently proposed mechanism of thermal hadron production through Hawking–Unruh radiation can naturally account for both. The event horizon of color confinement leads to thermal behavior, but the resulting temperature depends on the strange quark content of the produced hadrons, causing a deviation from full equilibrium and hence a suppression of strange particle production. We apply the resulting formalism to multihadron production in e + e − annihilation over a wide energy range and make a comprehensive analysis of the data in the conventional statistical hadronization model and the modified Hawking–Unruh formulation. We show that this formulation provides a very good description of the measured hadronic abundances, fully determined in terms of the string tension and the bare strange quark mass; it contains no adjustable parameters.

Design, performance, and calibration of CMS forward calorimeter wedges

Abstract: We report on the test beam results and calibration methods using high energy electrons, pions and muons with the CMS forward calorimeter (HF). The HF calorimeter covers a large pseudorapidity region (\(3\leq|\eta|\leq5\)), and is essential for a large number of physics channels with missing transverse energy. It is also expected to play a prominent role in the measurement of forward tagging jets in weak boson fusion channels in Higgs production. The HF calorimeter is based on steel absorber with embedded fused-silica-core optical fibers where Cherenkov radiation forms the basis of signal generation. Thus, the detector is essentially sensitive only to the electromagnetic shower core and is highly non-compensating (e/h≈5). This feature is also manifest in narrow and relatively short showers compared to similar calorimeters based on ionization. The choice of fused-silica optical fibers as active material is dictated by its exceptional radiation hardness. The electromagnetic energy resolution is dominated by photoelectron statistics and can be expressed in the customary form as \(\frac{a}{\sqrt{E}}\oplus{b}\). The stochastic term a is 198% and the constant term b is 9%. The hadronic energy resolution is largely determined by the fluctuations in the neutral pion production in showers, and when it is expressed as in the electromagnetic case, a = 280% and b = 11%.

Studying the MSSM Higgs sector by forward proton tagging at the LHC

Abstract: We show that the use of forward proton detectors at the LHC installed at 220 m and 420 m distance around ATLAS and/or CMS can provide important information on the Higgs sector of the MSSM. We analyse central exclusive production of the neutral \(\mathcal{CP}\)-even Higgs bosons h and H and their decays into bottom quarks, τ leptons and W bosons in various MSSM benchmark scenarios. Using plausible estimates for the achievable experimental efficiencies and the relevant background processes, we find that the prospective sensitivity of the diffractive Higgs production will allow one to probe interesting regions of the MA–tanβ parameter plane of the MSSM. Central exclusive production of the \(\mathcal{CP}\)-even Higgs bosons of the MSSM may provide a unique opportunity to access the bottom Yukawa couplings of the Higgs bosons up to masses of MH≲ 250 GeV. We also discuss the prospects for identifying the \(\mathcal{CP}\)-odd Higgs boson, A, in diffractive processes at the LHC.

Viscous Dark Cosmology with Account of Quantum Effects

Abstract: The analytic properties of the energy density ρ(t) of the cosmic fluid and the Hubble parameter H(t) are investigated close to the future singularity t=t s assuming different forms for the equation of state. First, it is shown that the inclusion of quantum effects coming from the conformal anomaly modifies the singularity. Thereafter, we consider the effect coming from the bulk viscosity in the fluid. The viscosity tends to reduce the magnitude of t s, but does not alter the singularity itself (the exponent). The main emphasis is laid on the simple case when the equation of state is p=w ρ, with w a constant.