Experimental determination of the b quark mass in DELPHI

The running mass of the b quark as defined in the MS-bar renormalization scheme, m_b, was measured at the M_Z scale using 2.8 million hadronic Z^0 decays collected by the DELPHI experiment at LEP. The result is m_b(M_Z) = 2.67 +- 0.25 (stat.) +- 0.34 (frag.) +- 0.27(theo.) GeV/c^2 which differs from that obtained at the Upsilon scale, by m_b(M_\Upsilon/2)-m_b(M_Z) = 1.49 +- 0.52 GeV/c^2. This measurement, performed far from the $b\bar{b}$ production threshold, provides the first experimental observation of the running of the quark masses.Comment: Talk given at the QCD 97 conference held in Montpellier, July 1997. Also available here http://hep.ph.liv.ac.uk/~martis

Exploring alternative symmetry breaking mechanisms at the LHC with 7, 8 and 10 TeV total energy

In view of the annnouncement that in 2012 the LHC will run at 8 TeV, we study the possibility of detecting signals of alternative mechanisms of ElectroWeak Symmetry Breaking, described phenomenologically by unitarized models, at energies lower than 14 TeV. A complete calculation with six fermions in the final state is performed using the PHANTOM event generator. Our results indicate that at 8 TeV some of the scenarios with TeV scale resonances are likely to be identified while models with no resonances or with very heavy ones will be inaccessible, unless the available luminosity will be much higher than expected

Experimental studies of QCD using flavour tagged jets with DELPHI

Identified $b\overline{b}g$ and $q\overline{q}\gamma$ events from DELPHI are used to measure the ratio of the mean charged particle multiplicity distribution between gluon and quark jets. The dependence of this ratio with the jet energy is established using about three million Z$^0$ decays. Results from all other detectors are discussed and compared. A nice agreement is found among all them. The ratio between the normalized total three-jet cross sections of $b\overline{b}g$ and $q\overline{q}g, q \equiv u,d,s$ events is also determined. The preliminary value obtained indicates that $b$ quarks are experimentaly seen to radiate less than light quarks due to their higher mass. The suggested experimental error is $\sim$300 MeV for the $b$ mass determination at the M$_Z$ scale.Comment: Latex, 5 pages, 3 figures,to appear in the Proceedings of the High Energy Physics International Euroconference on Quantum Chromodynamics (QCD '96), Montpellier, France, 4-12th July 1996. Ed. S. Narison, Nucl Phys. B (Proc. Suppl.

e+e−→bbˉW+W−e^+e^- \to b\bar b W^+W^- events at the Next Linear Collider: colour structure of top signal and irreducible background

We examine the colour structure and charged particle yield for both the $t \bar t$ signal and the irreducible background processes contributing to \eebbww\ production close to the \ttb\ threshold. The charged particle multiplicity for the various components of the cross section is computed as a function of several kinematic variables. Our study may have important implications for recently proposed studies of interconnection phenomena in \ttb\ production at high--energy $e^+e^-$ collidersComment: 14 pages, Latex, 8 figures uuencoded, complete ps file available via anonymous ftp at: ftp://axpc.hep.phy.cam.ac.uk/disk$alpha1:[public.hep9514]CAVENDISH_9514.ps http://www.hep.phy.cam.ac.uk/theory/papers/index.htm

Boson-boson scattering and Higgs production at the LHC from a six fermion point of view: four jets + lν\nu processes at \O(\alpha_{em}^6)

Boson-boson scattering and Higgs production in boson-boson fusion hold the key to electroweak symmetry breaking. In order to analyze these essential features of the Standard Model we have performed a partonic level study of all processes $q_1 q_2 \to q_3 q_4 q_5 q_6 l \nu$ at the LHC using the exact matrix elements at \O(\alpha_{em}^6) provided by \Phase, a new MC generator. These processes include also three boson production and the purely electroweak contribution to \toptop production as well as all irreducible backgrounds. Kinematical cuts have been studied in order to enhance the VV scattering signal over background. \Phase has been compared with different Monte Carlo's showing that a complete calculation is necessary for a correct description of the process.Comment: 26 pages, 19 figure

Light, long-lived and secluded: can gluinos be driven out from LEP1 data ?

We briefly report about a possible settlement of the still ongoing dispute concerning the existence of SUSY signals in 4jet events at LEP1. We base our arguments on a simple selection strategy exploiting secondary vertex tagging and kinematical constraints, which could allow one to access or exclude gluino events for a broad range of masses and lifetimes.Comment: 10 pages, LaTeX, 3 PostScript figures embedded with epsfig. Complete ps paper and figures available also at ftp://ftae3.ugr.es/pub/rmt/ugrft70.p

How well can the LHC distinguish between the SM light Higgs scenario, a composite Higgs and the Higgsless case using VV scattering channels?

A complete parton level analysis of ll + four jets l = e,mu and 3lv + two jets production at the LHC is presented, including all processes at order \ordEW, \ordQCD and \ordQCDsq when appropriate. The infinite Higgs mass scenario, which is considered as a benchmark for strong scattering theories and is the limiting case for composite Higgs models, and one example of a model incorporating a Strongly Interacting Light Higgs are confronted with the Standard Model light Higgs predictions. This analysis is combined with the results in the lv + four jets channel presented in a previous paper, in order to determine whether a composite Higgs signal can be detected as an excess of events in boson--boson scattering.Comment: Introduced some representative Feynman diagrams. Rearranged section 4. Typos fixed. Published in JHE

Spinor techniques for massive fermions with arbitrary polarization

We present a new variant of the spinor techniques for calculating the amplitudes of processes involving massive fermions with arbitrary polarization. It is relatively simple and leads to basic spinor products. Our procedure is not more complex than CALCUL spinor techniques for massless fermions. We obtain spinor Chisholm identities for massive fermions. As an illustration, expressions are given for the amplitudes of electron-positron annihilation into fermions-pairs for several polarizations.Comment: 14 pages, 2 figure

Perturbative rates and colour rearrangement effects in four-jet events at LEP2

An important issue in the direct reconstruction method of determining the W mass from q\bar q Q\bar Q events at LEP2 concerns the impact of the relatively unknown QCD interconnection effects. It has been suggested that a study of `short string' states, in which colour singlet states are formed from q \bar Q and Q\bar q pairs with small phase--space separation, could shed important light on this issue. We show that such configurations can also be generated by conventional background e^+e^-\ar 4~parton processes, in particular QCD q \bar q g g and q \bar q Q \bar Q and non--resonant electroweak q \bar q Q \bar Q production. We study the colour and kinematic structure of these background contributions, and estimate the event rate to be expected at LEP2. We find that the QCD processes are heavily suppressed, but that non--resonant q \bar q Q \bar Q production may be comparable in rate to the expected `short string' signal from W^+W^- production