4,211 research outputs found
Can Doubly Strange Dibaryon Resonances be Discovered at RHIC?
The baryon-baryon continuum invariant mass spectrum generated from
relativistic nucleus + nucleus collision data may reveal the existence of
doubly-strange dibaryons not stable against strong decay if they lie within a
few MeV of threshold. Furthermore, since the dominant component of these states
is a superposition of two color-octet clusters which can be produced
intermediately in a color-deconfined quark-gluon plasma (QGP), an enhanced
production of dibaryon resonances could be a signal of QGP formation. A total
of eight, doubly-strange dibaryon states are considered for experimental search
using the STAR detector (Solenoidal Tracker at RHIC) at the new Relativistic
Heavy Ion Collider (RHIC). These states may decay to Lambda-Lambda and/or
proton-Cascade-minus, depending on the resonance energy. STAR's large
acceptance, precision tracking and vertex reconstruction capabilities, and
large data volume capacity, make it an ideal instrument to use for such a
search. Detector performance and analysis sensitivity are studied as a function
of resonance production rate and width for one particular dibaryon which can
directly strong decay to proton-Cascade-minus but not Lambda-Lambda. Results
indicate that such resonances may be discovered using STAR if the resonance
production rates are comparable to coalescence model predictions for dibaryon
bound states.Comment: 28 pages, 5 figures, revised versio
Measurement of the Luminosity in the ZEUS Experiment at HERA II
The luminosity in the ZEUS detector was measured using photons from electron
bremsstrahlung. In 2001 the HERA collider was upgraded for operation at higher
luminosity. At the same time the luminosity-measuring system of the ZEUS
experiment was modified to tackle the expected higher photon rate and
synchrotron radiation. The existing lead-scintillator calorimeter was equipped
with radiation hard scintillator tiles and shielded against synchrotron
radiation. In addition, a magnetic spectrometer was installed to measure the
luminosity independently using photons converted in the beam-pipe exit window.
The redundancy provided a reliable and robust luminosity determination with a
systematic uncertainty of 1.7%. The experimental setup, the techniques used for
luminosity determination and the estimate of the systematic uncertainty are
reported.Comment: 25 pages, 11 figure
Jet production in charged current deep inelastic e⁺p scatteringat HERA
The production rates and substructure of jets have been studied in charged current deep inelastic e⁺p scattering for Q² > 200 GeV² with the ZEUS detector at HERA using an integrated luminosity of 110.5 pb⁻¹. Inclusive jet cross sections are presented for jets with transverse energies E_{T}^{jet} > 5 GeV. Measurements of the mean subjet multiplicity, 〈n_{sbj}〉, of the inclusive jet sample are presented. Predictions based on parton-shower Monte Carlo models and next-to-leading-order QCD calculations are compared to the measurements. The value of α_{s} (M_{z}), determined from 〈n_{sbj}〉 at y_{cut} = 10⁻² for jets with 25 < E_{T}^{jet} < 119 GeV, is α_{s} (M_{z}) = 0.1202 ± 0.0052 (stat.)_{-0.0019}^{+0.0060} (syst.)_{-0.0053}^{+0.0065} (th.). The mean subjet multiplicity as a function of Q² is found to be consistent with that measured in NC DIS
Measurement of event shapes in deep inelastic scattering at HERA
Inclusive event-shape variables have been measured in the current region of
the Breit frame for neutral current deep inelastic ep scattering using an
integrated luminosity of 45.0 pb^-1 collected with the ZEUS detector at HERA.
The variables studied included thrust, jet broadening and invariant jet mass.
The kinematic range covered was 10 < Q^2 < 20,480 GeV^2 and 6.10^-4 < x < 0.6,
where Q^2 is the virtuality of the exchanged boson and x is the Bjorken
variable. The Q dependence of the shape variables has been used in conjunction
with NLO perturbative calculations and the Dokshitzer-Webber non-perturbative
corrections (`power corrections') to investigate the validity of this approach.Comment: 7+25 pages, 6 figure
Constraints on the χ_(c1) versus χ_(c2) polarizations in proton-proton collisions at √s = 8 TeV
The polarizations of promptly produced χ_(c1) and χ_(c2) mesons are studied using data collected by the CMS experiment at the LHC, in proton-proton collisions at √s=8 TeV. The χ_c states are reconstructed via their radiative decays χ_c → J/ψγ, with the photons being measured through conversions to e⁺e⁻, which allows the two states to be well resolved. The polarizations are measured in the helicity frame, through the analysis of the χ_(c2) to χ_(c1) yield ratio as a function of the polar or azimuthal angle of the positive muon emitted in the J/ψ → μ⁺μ⁻ decay, in three bins of J/ψ transverse momentum. While no differences are seen between the two states in terms of azimuthal decay angle distributions, they are observed to have significantly different polar anisotropies. The measurement favors a scenario where at least one of the two states is strongly polarized along the helicity quantization axis, in agreement with nonrelativistic quantum chromodynamics predictions. This is the first measurement of significantly polarized quarkonia produced at high transverse momentum
Beauty photoproduction measured using decays into muons in dijet events in ep collisions at =318 GeV
The photoproduction of beauty quarks in events with two jets and a muon has
been measured with the ZEUS detector at HERA using an integrated luminosity of
110 pb. The fraction of jets containing b quarks was extracted from the
transverse momentum distribution of the muon relative to the closest jet.
Differential cross sections for beauty production as a function of the
transverse momentum and pseudorapidity of the muon, of the associated jet and
of , the fraction of the photon's momentum participating in
the hard process, are compared with MC models and QCD predictions made at
next-to-leading order. The latter give a good description of the data.Comment: 32 pages, 6 tables, 7 figures Table 6 and Figure 7 revised September
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