Search results for "Quarks"
showing 10 items of 68 documents
Searches for heavy neutrinos from Z decays
1992
We have searched for possible fourth family heavy neutrinos, pair produced in Z0 decays, in a sample of about 112 000 hadronic Z0 final states collected with the DELPHI detector. For all mixing matrix elements we exclude a new Dirac neutrino lighter than 44.5 GeV at a 95% confidence level, if the neutrino couples to the electron or muon family, and lighter than 44.0 GeV, if the neutrino couples to the tau family. Depending on the values of the mixing element and to which lepton family the neutrino couples, we obtain mass limits up to 46.2 GeV. For all mixing matrix elements we exclude a new Majorana neutrino lighter than 39.0 GeV, if it couples to the electron or the muon family, and lighte…
High precision measurement of Gamma(Z->bbbar)/Gamma(Z->hadrons) with the DELPHI detector at LEP collider
1997
Entre las medidas disponibles a la energía en centro de masas correspondiente al polo del bosón Z, la fracción de la anchura parcial a pares de quarks bbbar y su anchura parcial hadrónica, Rb0 = Gamma(Z -> bbbar) / Gamma(Z -> hadrones),tiene actualmente un especial interés. Prácticamente todas las correcciones radiativas electrodébiles y de QCD cancelan al realizar el cociente, de forma que Rb0 es esencialmente sensible sólo a las correcciones al vértice Z -> bbbar, como el fuerte acoplamiento CKM al quark top. Dado el excelente acuerdo entre el Modelo Estándar y la mayor parte de las observaciones de precisión, junto con el reciente descubrimiento del quark top y la determinación directa d…
eta - eta' - glueball mixing
2010
We have revisited glueball mixing with the pseudoscalar mesons in the MIT bag model scheme. The calculation has been performed in the spherical cavity approximation to the bag using two different fermion propagators, the cavity and the free propagators. We obtain a very small probability of mixing for the eta at the level of $0.04-0.1% and a bigger for the eta' at the level of 4-12%. Our results differ from previous calculations in the same scheme but seem to agree with the experimental analysis. We discuss the origin of our difference which stems from the treatment of our time integrations.
Triquark correlations and pentaquarks in a QCD sum rule approach
2005
The role of quark correlations in the description of hadron dynamics in many domains of physics, from low energy dynamics to very hot(dense) systems, is being appreciated. Strong correlations of two quarks (diquark) have been widely investigated in this respect. Recently, we have proposed a dynamical scheme to describe the $\Theta^+$ pentaquark in which also three quark correlations (triquark) were instrumental in producing a low mass exotic state. We perform a study, within the QCD sum rule approach including OPE and direct instanton contributions, of triquark correlations and obtain two quasi-bound light $ud\bar{s}$ color quark clusters of 800 MeV and 930 MeV respectively.
Searches for scalar top and scalar bottom quarks at LEP2
1997
Searches for scalar top and bottom quarks have been performed with data collected by the ALEPH detector at LEP. The data sample consists of 21.7 pb^-1 taken at sqrt{s} = 161, 170, and 172~GeV and 5.7 pb^-1 taken at sqrt{s} = 130 and 136~GeV. No evidence for scalar top quarks or scalar bottom quarks was found in the channels stop --> c chi, stop --> b l snu, and sbottom --> b chi. For the channel stop --> c chi a limit of 67 GeV/c^2 has been set on the scalar top quark mass, independent of the mixing angle between the supersymmetric partners of the left and right-handed states of the top quark. This limit assumes a mass difference between the stop and the chi of at least 10 GeV/c…
Multi-channel search for squarks and gluinos in root s=7 TeV pp collisions with the ATLAS detector at the LHC
2013
A search for supersymmetric particles in final states with zero, one, and two leptons, with and without jets identified as originating from b-quarks, in 4.7 fb[superscript −1] of √s = 7 TeV pp collisions produced by the Large Hadron Collider and recorded by the ATLAS detector is presented. The search uses a set of variables carrying information on the event kinematics transverse and parallel to the beam line that are sensitive to several topologies expected in supersymmetry. Mutually exclusive final states are defined, allowing a combination of all channels to increase the search sensitivity. No deviation from the Standard Model expectation is observed. Upper limits at 95 % confidence level…
Search for quarks in proton-proton interactions at √s = 52.5 GeV
1978
The results of an experiment to search for quarks in pp collisions, at a total centre-of-mass energy √s = 52.5 GeV, are reported. The experiment was sensitive to fractionally charged particles with 0.035e ⩽ |Q| ⩽ 0.67e,β =v/c ⩾ 0.1, and masses up to 21 GeV/c2. The 90% confidence level on the ratio «quark flux/charged-particle flux» is 5.11 · 10−11 for |Q| = 1/3e. This value holds true for particles produced with a mean transverse momentum of 0.4 GeV/c.
Search for Down-Type Fourth Generation Quarks with the ATLAS Detector in Events with One Lepton and Hadronically Decaying W Bosons
2012
This Letter presents a search for pair production of heavy down-type quarks decaying via b′→Wt in the lepton+jets channel, as b′¯b′→W−tW+¯t→b¯bW+W−W+W−→l±νb¯bq¯qq¯qq¯q. In addition to requiring exactly one lepton, large missing transverse momentum, and at least six jets, the invariant mass of nearby jet pairs is used to identify high transverse momentum W bosons. In data corresponding to an integrated luminosity of 1.04 fb−1 from pp collisions at √s=7 TeV recorded with the ATLAS detector, a heavy down-type quark with mass less than 480 GeV can be excluded at the 95% confidence level.
Instanton induced quark dynamics and the pentaquark
2004
We analyze the existence of the exotic $\Theta^+$ from the perspective of instanton induced quark dynamics. The 't Hooft interaction gives strong attraction in specific channels of the triquark $ud\bar s$ and diquark $ud$ configurations. In particular it leads to a light $ud\bar s$ triquark cluster, with the mass around $750 {\rm MeV}$, in the I=0, $S=1/2$ and color 3 configuration, and a light $ud$-diquark configuration, with mass $440 {\rm MeV}$, in the I=0, S=0 and color {$\bar{3}$} configuration. If we consider the pentaquark as a bound state of such triquark and diquark configurations in a relative L=1 state we obtain good agreement with the data. The small width of $\Theta^+$ has a na…
The Higgs Era
2016
Después del descubrimiento de un bosón escalar con masa alrededor de 125, el Modelo Estándar parece funcionar perfectamente, pero todavía hay mucho espacio (y necesidad) para nueva física. A pesar de su enorme éxito, el Modelo Estándar deja muchas preguntas sin responder y muchas cuestiones abiertas. A falta del conocimiento de una simetría fundamental de la Naturaleza que nos indique el camino, hemos de analizar fenomenologicamente cualquier extensión del Modelo que pueda responder al menos a alguna de estas preguntas. En este trabajo estudiamos dos tipos de extensiones del Modelo Estándar, las dos relacionadas con el sector escalar. En la primera parte de la tesis estudiamos la posible ex…