The standard model of particle physics contains parameters-such as particle masses-whose origins are still unknown and which cannot be predicted, but whose values are constrained through their ...interactions. In particular, the masses of the top quark (Mt) and W boson (MW) constrain the mass of the long-hypothesized, but thus far not observed, Higgs boson. A precise measurement of Mt can therefore indicate where to look for the Higgs, and indeed whether the hypothesis of a standard model Higgs is consistent with experimental data. As top quarks are produced in pairs and decay in only about 10-24 s into various final states, reconstructing their masses from their decay products is very challenging. Here we report a technique that extracts more information from each top-quark event and yields a greatly improved precision (of ± 5.3 GeV/c2) when compared to previous measurements. When our new result is combined with our published measurement in a complementary decay mode and with the only other measurements available, the new world average for Mt becomes 178.0 ± 4.3 GeV/c2. As a result, the most likely Higgs mass increases from the experimentally excluded value of 96 to 117 GeV/c2, which is beyond current experimental sensitivity. The upper limit on the Higgs mass at the 95% confidence level is raised from 219 to 251 GeV/c2.
We present a search for electroweak production of single top quarks in the s-channel and t-channel using neural networks for signal-background separation. We have analyzed 230 pb(-1) of data ...collected with the DO detector at the Fermilab Tevatron Collider at a center-of-mass energy of 1.96 TeV and find no evidence for a single top quark signal. The resulting 95% confidence level upper limits on the single top quark production cross sections are 6.4 pb in the s-channel and 5.0 ph in the t-channel.
We present a measurement of the top quark pair (t (t) over bar) production cross section (sigma(t (t) over bar)) in pp collisions at a center-of-mass energy of 1.96 TeV using 230 pb(-1) of data ...collected by the DO detector at the Fermilab Tevatron Collider. We select events with one charged lepton (electron or muon), large missing transverse energy, and at least four jets, and extract the t (t) over bar content of the sample based on the kinematic characteristics of the events. For a top quark mass of 175 GeV, we measure sigma(t (t) over bar) 6.7(-1.3)(+1.4)(stat)(-1.1)(+1.6)(syst) +/- 0.4(lumi) pb, in good agreement with the standard model prediction.
We present a measurement of the top quark pair (a) production cross section in pp collisions at root s = 1.96 TeV using events with two charged leptons in the final state. This analysis utilizes an ...integrated luminosity of 224-243 pb(-1) collected with the DO detector at the Fermilab Tevatron Collider. We observe 13 events in the e(+)e(-), e mu and mu(+)mu(-) channels with an expected background of 3.2 +/- 0.7 events. For a top quark mass of 175 GeV, we measure a t (t) over bar production cross section of sigma(t (t) over bar) = 8.6(-2.7)(+3.2) (stat) +/- 1.1(syst) +/- 0.6(lumi) pb, consistent with the standard model prediction.
The ATLAS Simulation Infrastructure Abdesselam, A.; Alviggi, M. G.; Banfi, D. ...
The European physical journal. C, Particles and fields,
12/2010, Letnik:
70, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The simulation software for the ATLAS Experiment at the Large Hadron Collider is being used for large-scale production of events on the LHC Computing Grid. This simulation requires many components, ...from the generators that simulate particle collisions, through packages simulating the response of the various detectors and triggers. All of these components come together under the ATLAS simulation infrastructure. In this paper, that infrastructure is discussed, including that supporting the detector description, interfacing the event generation, and combining the GEANT4 simulation of the response of the individual detectors. Also described are the tools allowing the software validation, performance testing, and the validation of the simulated output against known physics processes.
We present a search for electroweak production of single top quarks in ≈90 pb−1 of data collected with the DØ detector at the Fermilab Tevatron collider. Using arrays of neural networks to separate ...signals from backgrounds, we set upper limits on the cross sections of 17 pb for the s-channel process pp̄→tb+X, and 22 pb for the t-channel process pp̄→tqb+X, both at the 95% confidence level.
The flavour-tagging algorithms developed by the ATLAS Collaboration and used to analyse its dataset of
s
=
13
TeV
pp
collisions from Run 2 of the Large Hadron Collider are presented. These new ...tagging algorithms are based on recurrent and deep neural networks, and their performance is evaluated in simulated collision events. These developments yield considerable improvements over previous jet-flavour identification strategies. At the 77%
b
-jet identification efficiency operating point, light-jet (charm-jet) rejection factors of 170 (5) are achieved in a sample of simulated Standard Model
t
t
¯
events; similarly, at a
c
-jet identification efficiency of 30%, a light-jet (
b
-jet) rejection factor of 70 (9) is obtained.
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