20,506,648 research outputs found
High pT leading hadron suppression in nuclear collisions at sqrt(s_NN) = 20 -- 200 GeV: data versus parton energy loss models
Experimental results on high transverse momentum (leading) hadron spectra in
nucleus-nucleus collisions in the range sqrt(s_NN) = 20 -- 200 GeV are reviewed
with an emphasis on the observed suppression compared to free space production
in proton-proton collisions at the corresponding center-of-mass energies. The
transverse-momentum and collision-energy (but seemingly not the in-medium path
length) dependence of the experimental suppression factors measured in central
collisions is consistent with the expectations of final-state non-Abelian
parton energy loss in a dense QCD medium.Comment: Two typos correcte
The 4-particle hydrogen-antihydrogen system revisited: twofold Hamiltonian symmetry and natural atom antihydrogen
Modern ab initio treatments of H-Hbar systems are inconsistent with the logic
behind algebraic Hamiltonians H(+-)=H(0)+/-deltaH for charge-symmetrical and
charge-asymmetrical 4 unit charge systems like H(2) and HHbar. Since these 2
Hamiltonians are mutually exclusive, only the attractive one can apply for
stable natural molecular H(2). A wrong choice leads to problems with antiatom
Hbar. In line with earlier results on band and line spectra, we now prove that
HL chose the wrong Hamiltonian for H(2). Their theory explains the stability of
attractive system H(2) with a repulsive Hamiltonian instead of with the
attractive one, representative for charge-asymmetrical system HHbar. A new
second order symmetry effect is detected. Repulsive HL Hamiltonian H(+) applies
at long range but at the critical distance, attractive charge-inverted
Hamiltonian H(-)takes over and leads to bond H(2) but in reality, HHbar, for
which we give an analytical proof. Another wrong asymptote choice in the past
also applies for atomic antihydrogen Hbar, which has hidden the Mexican hat
potential for natural hydrogen. This generic solution removes most problems,
physicists and chemists experience with atomic Hbar and molecular HHbar,
including the problem with antimatter in the Universe.Comment: at the instituional UGent archive, 37 pag, 10 fig, tabb, version as
submitted, abstract shortene
Parton recombination at all
Hadron production at all in heavy-ion collisions in the framework of
parton recombination is reviewed. It is shown that the recombination of thermal
and shower partons dominates the hadron spectra in the intermediate
region. In collisions, the physics of particle production at any
is basically the same as at . The Cronin effect is described as a
result of the final-state instead of the initial-state interaction. The
suppression of at high is due to the reduction of the soft
parton density on the deuteron side, thus resulting in less pions produced by
recombination, an explanation that requires no new physics. In
collisions large ratio is obtained because the thermal partons can
contribute to the formation of proton more than they do to the pion.Comment: 12 pages + 5 figures. Invited talk at Hard Probes 200
Two-pion Bose-Einstein correlations in central Pb-Pb collisions at = 2.76 TeV
The first measurement of two-pion Bose-Einstein correlations in central Pb-Pb
collisions at TeV at the Large Hadron Collider is
presented. We observe a growing trend with energy now not only for the
longitudinal and the outward but also for the sideward pion source radius. The
pion homogeneity volume and the decoupling time are significantly larger than
those measured at RHIC.Comment: 17 pages, 5 captioned figures, 1 table, authors from page 12,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/388
Suppression of charged particle production at large transverse momentum in central Pb-Pb collisions at TeV
Inclusive transverse momentum spectra of primary charged particles in Pb-Pb
collisions at = 2.76 TeV have been measured by the ALICE
Collaboration at the LHC. The data are presented for central and peripheral
collisions, corresponding to 0-5% and 70-80% of the hadronic Pb-Pb cross
section. The measured charged particle spectra in and GeV/ are compared to the expectation in pp collisions at the same
, scaled by the number of underlying nucleon-nucleon
collisions. The comparison is expressed in terms of the nuclear modification
factor . The result indicates only weak medium effects ( 0.7) in peripheral collisions. In central collisions,
reaches a minimum of about 0.14 at -7GeV/ and increases
significantly at larger . The measured suppression of high- particles is stronger than that observed at lower collision energies,
indicating that a very dense medium is formed in central Pb-Pb collisions at
the LHC.Comment: 15 pages, 5 captioned figures, 3 tables, authors from page 10,
published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/98
Observation of the TeV gamma-ray source MGRO J1908+06 with ARGO-YBJ
The extended gamma ray source MGRO J1908+06, discovered by the Milagro air
shower detector in 2007, has been observed for about 4 years by the ARGO-YBJ
experiment at TeV energies, with a statistical significance of 6.2 standard
deviations. The peak of the signal is found at a position consistent with the
pulsar PSR J1907+0602. Parametrizing the source shape with a two-dimensional
Gauss function we estimate an extension \sigma = 0.49 \pm 0.22 degrees,
consistent with a previous measurement by the Cherenkov Array H.E.S.S.. The
observed energy spectrum is dN/dE = 6.1 \pm 1.4 \times 10^-13 (E/4 TeV)^{-2.54
\pm 0.36} photons cm^-2 s^-1 TeV^-1, in the energy range 1-20 TeV. The measured
gamma ray flux is consistent with the results of the Milagro detector, but is
2-3 times larger than the flux previously derived by H.E.S.S. at energies of a
few TeV. The continuity of the Milagro and ARGO-YBJ observations and the stable
excess rate observed by ARGO-YBJ along 4 years of data taking support the
identification of MGRO J1908+06 as the steady powerful TeV pulsar wind nebula
of PSR J1907+0602, with an integrated luminosity above 1 TeV about 1.8 times
the Crab Nebula luminosity.Comment: 6 pages, accepted for pubblication by ApJ. Replaced to correct the
author lis
Probing the QCD equation of state with thermal photons in nucleus-nucleus collisions at RHIC
Thermal photon production at mid-rapidity in Au+Au reactions at
= 200 GeV is studied in the framework of a 2D+1 hydrodynamical
model that describes efficiently the bulk identified hadron spectra at RHIC.
The combined thermal plus NLO pQCD photon spectrum is in good agreement with
the yields measured by the PHENIX experiment for all Au+Au centralities. Within
our model, we demonstrate that the correlation of the thermal photon slopes
with the charged hadron multiplicity in each centrality bin provides direct
empirical information on the underlying degrees of freedom and on the equation
of state, , of the strongly interacting matter.Comment: Version to appear in EPJ-C (extended discussion and refs. and a few
corrections
Nucleon form factors, B-meson factories and the radiative return
The feasibility of a measurement of the electric and magnetic nucleon form
factors at -meson factories through the radiative return is studied. Angular
distributions allow a separation of the contributions from the two form
factors. The distributions are presented for the laboratory and the hadronic
rest frame, and the advantages of different coordinate systems are
investigated. It is demonstrated that values up to 8 or even 9 GeV
are within reach. The Monte Carlo event generator PHOKHARA is extended to
nucleon final states, and results are presented which include Next-to-Leading
Order radiative corrections from initial-state radiation. The impact of angular
cuts on rates and distributions is investigated and the relative importance of
radiative corrections is analysed.Comment: 9 pages, 11 figures. Final version to appear in Eur. Phys. J.
A description of the ratio between electric and magnetic proton form factors by using space-like, time-like data and dispersion relations
We use the available information on the ratio between the electric and
magnetic proton form factors coming from recently published space-like data and
from the few available time-like data. We apply a dispersive procedure on these
data to evaluate the behaviour of this ratio, as a complex function, for all
values of q^2.Comment: 12 pages, 7 Encapsulated Postscript figures, uses epsfig, rotating,
exscale, amsmath, cite, latexsym, graphics, color packages, added reference
Recommended from our members
ReSCon '11, Research Student Conference: Book of Abstracts
The fourth SED Research Student Conference (ReSCon2011) was hosted over three days, 20-22 June 2011, in the Hamilton Centre at Brunel University. The conference consisted of technical presentations, a poster session and social events. which focused on current research being conducted within the School of Engineering and Design by postgraduate research students from the School. The conference is held annually, and ReSCon plays a key role in contributing to research and innovations within the School
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