Search results for "Collider"
showing 10 items of 1690 documents
The Scale Dependence of the Hadron Multiplicity in Quark and Gluon Jets and a Precise Determination of $C_{A}/C_{F}$
1999
\frac{C_A}{C_F} = 2.246 \pm 0.062~(stat.) \pm 0.080~(syst.) \pm 0.095~(theo.) Data collected at the Z resonance using the DELPHI detector at LEP are used to determine the charged hadron multiplicity in gluon and quark jets as a function of a transverse momentum-like scale. The colour factor ratio, \cacf, is directly observed in the increase of multiplicities with that scale. The smaller than expected multiplicity ratio in gluon to quark jets is understood by differences in the hadronization of the leading quark or gluon. From the dependence of the charged hadron multiplicity on the opening angle in symmetric three-jet events the colour factor ratio is measured to be: C_A/C_F = 2.246 \pm 0.0…
A measurement of the branching fractions of the b quark into charged and neutral b hadrons
2003
The production fractions of charged and neutral b-hadrons in b-quark events from Z0 decays have been measured with the DELPHI detector at LEP. An algorithm has been developed, based on a neural network, to estimate the charge of the weakly-decaying b-hadron by distinguishing its decay products from particles produced at the primary vertex. From the data taken in the years 1994 and 1995, the fraction of bbar-quarks fragmenting into positively charged weakly-decaying b-hadrons has been measured to be: f^+ = (42.09 +/- 0.82 (stat.) +/- 0.89 (syst.))%. Subtracting the rates for charged Xibar_b^+ and Omegabar_b^+ baryons gives the production fraction of B^+ mesons: f_Bu = (40.99 +/- 0.82 (stat.)…
Nuclear PDF constraints from p+Pb collisions at the LHC
2015
As the current nuclear PDF analyses are mainly constrained by fixed-target Drell-Yan and deeply inelastic scattering data only the quark nuclear modifications at fairly large $x$ values are in a good control. Inclusive pion production in d+Au collisions at RHIC provides some constraints for gluons but due to the limited kinematic reach of the data the gluon modifications remain uncertain especially at small values of $x$. In this talk, we discuss how the existing data from p+Pb collisions at the LHC can improve the nuclear PDF fits and which measurements would be sensitive to the small-$x$ gluons. In particular, we consider inclusive hadron production, compare this to direct photons, and sh…
Search for neutral and charged Higgs bosons in $e^+ e^-$ collisions at $\sqrt{s}$= 161 GeV and 172 GeV
1998
A search for neutral and charged Higgs bosons has been performed in the data collected by the {\sc DELPHI} detector at centre-of-mass energies of 161~GeV and 172~GeV. The analysis assumes either the pair-production of charged Higgs bosons, ${\mathrm H}^{\pm}$, or the production of the lightest neutral Higgs boson, h, with either a Z or a neutral pseudoscalar Higgs boson, A. All final state topologies expected from the decay of h and A %neutral Higgs particles into hadrons or a pair of $\tau$ leptons, and from the decay of ${\mathrm H}^{\pm}$ %charged Higgs bosons into a pair of quarks or a $\tau \nu_{\tau}$ pair have been considered. %In the case of the associated production with a Z boson,…
MEASUREMENT OF THE FORWARD-BACKWARD ASYMMETRY OF CHARM AND BOTTOM QUARKS AT THE Z-POLE USING D-ASTERISK(+/-)-MESONS
1995
The forward-backward asymmetries for the processes $$e^ + e^ - \to c\bar c$$ and $$e^ + e^ - \to b\bar b$$ at theZ resonance are measured using identifiedD *± mesons. In 905,000 selected hadronic events, taken in 1991 and 1992 with the DEL-PHI detector at LEP, 4757D *+→D 0π+ decays are reconstructed. Thec andb quark forward-backward asymmetries are determined to be: $$\begin{gathered} A_{FB}^{c\bar c} = 0.077 \pm 0.029(stat) \pm 0.012(sys), \hfill \\ A_{FB}^{b\bar b} = 0.059 \pm 0.062(stat) \pm 0.024(sys). \hfill \\ \end{gathered} $$ Constraining theb asymmetry to the value measured by DELPHI using independent analyses, the charm asymmetry is determined to be: $$A_{FB}^{c,const} = 0.068 \pm…
EPPS16: Nuclear parton distributions with LHC data
2017
We introduce a global analysis of collinearly factorized nuclear parton distribution functions (PDFs) including, for the first time, data constraints from LHC proton-lead collisions. In comparison to our previous analysis, EPS09, where data only from charged-lepton-nucleus deep inelastic scattering (DIS), Drell-Yan (DY) dilepton production in proton-nucleus collisions and inclusive pion production in deuteron-nucleus collisions were the input, we now increase the variety of data constraints to cover also neutrino-nucleus DIS and low-mass DY production in pion-nucleus collisions. The new LHC data significantly extend the kinematic reach of the data constraints. We now allow much more freedom…
Consistent measurements of alpha(s) from precise oriented event shape distributions
2000
An updated analysis using about 1.5 million events recorded at $\sqrt{s} = M_Z$ with the DELPHI detector in 1994 is presented. Eighteen infrared and collinear safe event shape observables are measured as a function of the polar angle of the thrust axis. The data are compared to theoretical calculations in ${\cal O} (\alpha_s^2)$ including the event orientation. A combined fit of $\alpha_s$ and of the renormalization scale $x_{\mu}$ in $\cal O(\alpha_s^2$) yields an excellent description of the high statistics data. The weighted average from 18 observables including quark mass effects and correlations is $\alpha_s(M_Z^2) = 0.1174 \pm 0.0026$. The final result, derived from the jet cone energ…
First global next-to-leading order determination of diffractive parton distribution functions and their uncertainties within the {\tt xFitter} framew…
2018
We present {\tt GKG18-DPDFs}, a next-to-leading order (NLO) QCD analysis of diffractive parton distribution functions (diffractive PDFs) and their uncertainties. This is the first global set of diffractive PDFs determined within the {\tt xFitter} framework. This analysis is motivated by all available and most up-to-date data on inclusive diffractive deep inelastic scattering (diffractive DIS). Heavy quark contributions are considered within the framework of the Thorne-Roberts (TR) general mass variable flavor number scheme (GM-VFNS). We form a mutually consistent set of diffractive PDFs due to the inclusion of high-precision data from H1/ZEUS combined inclusive diffractive cross sections me…
Direct search for charged Higgs bosons in decays of top quarks
2001
We present a search for charged Higgs bosons in decays of pair-produced top quarks in pbar p collisions at sqrt(s) = 1.8 TeV using 62.2 pb^-1 of data recorded by the D0 detector at the Fermilab Tevatron collider. No evidence is found for signal, and we exclude at 95% confidence most regions of the (M higgs, tan beta) parameter space where the decay t->H b has a branching fraction greater than 0.36 and B(H -> tau nu) is large.
Evidence for production of single top quarks and first direct measurement of |Vtb|.
2007
The D0 Collaboration presents first evidence for the production of single top quarks at the Fermilab Tevatron ppbar collider. Using a 0.9 fb^-1 dataset, we apply a multivariate analysis to separate signal from background and measure sigma(ppbar->tb+X,tqb+X) = 4.9 +- 1.4 pb. The probability to measure a cross section at this value or higher in the absence of signal is 0.035%, corresponding to a 3.4 standard deviation significance. We use the cross section measurement to directly determine the CKM matrix element that describes the Wtb coupling and find 0.68 < |Vtb| <= 1 at 95% C.L. within the standard model.