537 research outputs found
Charmonium propagation through a dense medium
Attenuation of a colourless cĚc dipole propagating with a large momentum through a hot medium originates from two sources, Debye screening (melting), and inelastic collisions with surrounding scattering centres (absorption). The former never terminates completely production of a bound charmonium in heavy ion collisions, even at very high temperatures. The latter, is controlled my the magnitude of the dipole cross section, related to the transport coefficient, which is the rate of transverse momentum broadening in the medium. A novel procedure of Lorentz boosting of the SchrĂśdinger equation is developed, which allows to calculate the charmonium survival probability employing the path-integral technique, incorporating both melting and absorption. A novel mechanism of charmonium regeneration in a dense medium is proposed
Jet lag effect and leading hadron production
We propose a solution for the long standing puzzle of a too steeply falling
fragmentation function for a quark fragmenting into a pion, calculated by
Berger [1] in the Born approximation. Contrary to the simple anticipation that
gluon resummation worsens the problem, we find good agreement with data. Higher
quark Fock states slow down the quark, an effect which we call jet lag. It can
be also expressed in terms of vacuum energy loss. As a result, the space-time
development of the jet shrinks and the -dependence becomes flatter than in
the Born approximation. The space-time pattern is also of great importance for
in-medium hadronization.Comment: 11 pages, 5 figure
Synthesizing greenhouse gas fluxes across nine European peatlands and shrublands: responses to climatic and environmental changes
In this study, we compare annual fluxes of methane
(CH4), nitrous oxide (N2O) and soil respiratory carbon dioxide
(CO2) measured at nine European peatlands (n = 4) and
shrublands (n = 5). The sites range from northern Sweden
to Spain, covering a span in mean annual air temperature
from 0 to 16 �C, and in annual precipitation from 300 to
1300mmyrâ1. The effects of climate change, including temperature
increase and prolonged drought, were tested at five
shrubland sites. At one peatland site, the long-term (>30 yr)
effect of drainage was assessed, while increased nitrogen deposition
was investigated at three peatland sites.
The shrublands were generally sinks for atmospheric CH4,
whereas the peatlands were CH4 sources, with fluxes ranging
from â519 to +6890 mgCH4-Cmâ2 yrâ1 across the studied
ecosystems. At the peatland sites, annual CH4 emission
increased with mean annual air temperature, while
a negative relationship was found between net CH4 uptake
and the soil carbon stock at the shrubland sites. Annual
N2O fluxes were generally small ranging from â14
to 42 mgN2O-Nmâ2 yrâ1. Highest N2O emission occurred
at the sites that had highest nitrate (NOâ
3 ) concentration
in the soil water. Furthermore, experimentally increased
NOâ
3 deposition led to increased N2O efflux, whereas prolonged
drought and long-term drainage reduced the N2O efflux.
Soil CO2 emissions in control plots ranged from 310
to 732 gCO2-Cmâ2 yrâ1. Drought and long-term drainage generally reduced the soil CO2 efflux, except at a hydric
shrubland where drought tended to increase soil respiration.
In terms of fractional importance of each greenhouse gas
to the total numerical global warming response, the change in
CO2 efflux dominated the response in all treatments (ranging
71â96 %), except for NOâ
3 addition where 89% was due to
change in CH4 emissions. Thus, in European peatlands and
shrublands the effect on global warming induced by the investigated
anthropogenic disturbances will be dominated by
variations in soil CO2 fluxes
Particle-yield modification in jet-like azimuthal di-hadron correlations in Pb-Pb collisions at = 2.76 TeV
The yield of charged particles associated with high- trigger
particles ( GeV/) is measured with the ALICE detector in
Pb-Pb collisions at = 2.76 TeV relative to proton-proton
collisions at the same energy. The conditional per-trigger yields are extracted
from the narrow jet-like correlation peaks in azimuthal di-hadron correlations.
In the 5% most central collisions, we observe that the yield of associated
charged particles with transverse momenta GeV/ on the
away-side drops to about 60% of that observed in pp collisions, while on the
near-side a moderate enhancement of 20-30% is found.Comment: 15 pages, 2 captioned figures, 1 table, authors from page 10,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/350
Search for a W' boson decaying to a bottom quark and a top quark in pp collisions at sqrt(s) = 7 TeV
Results are presented from a search for a W' boson using a dataset
corresponding to 5.0 inverse femtobarns of integrated luminosity collected
during 2011 by the CMS experiment at the LHC in pp collisions at sqrt(s)=7 TeV.
The W' boson is modeled as a heavy W boson, but different scenarios for the
couplings to fermions are considered, involving both left-handed and
right-handed chiral projections of the fermions, as well as an arbitrary
mixture of the two. The search is performed in the decay channel W' to t b,
leading to a final state signature with a single lepton (e, mu), missing
transverse energy, and jets, at least one of which is tagged as a b-jet. A W'
boson that couples to fermions with the same coupling constant as the W, but to
the right-handed rather than left-handed chiral projections, is excluded for
masses below 1.85 TeV at the 95% confidence level. For the first time using LHC
data, constraints on the W' gauge coupling for a set of left- and right-handed
coupling combinations have been placed. These results represent a significant
improvement over previously published limits.Comment: Submitted to Physics Letters B. Replaced with version publishe
Search for the standard model Higgs boson decaying into two photons in pp collisions at sqrt(s)=7 TeV
A search for a Higgs boson decaying into two photons is described. The
analysis is performed using a dataset recorded by the CMS experiment at the LHC
from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an
integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross
section of the standard model Higgs boson decaying to two photons. The expected
exclusion limit at 95% confidence level is between 1.4 and 2.4 times the
standard model cross section in the mass range between 110 and 150 GeV. The
analysis of the data excludes, at 95% confidence level, the standard model
Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The
largest excess of events above the expected standard model background is
observed for a Higgs boson mass hypothesis of 124 GeV with a local significance
of 3.1 sigma. The global significance of observing an excess with a local
significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is
estimated to be 1.8 sigma. More data are required to ascertain the origin of
this excess.Comment: Submitted to Physics Letters
Measurement of the Lambda(b) cross section and the anti-Lambda(b) to Lambda(b) ratio with Lambda(b) to J/Psi Lambda decays in pp collisions at sqrt(s) = 7 TeV
The Lambda(b) differential production cross section and the cross section
ratio anti-Lambda(b)/Lambda(b) are measured as functions of transverse momentum
pt(Lambda(b)) and rapidity abs(y(Lambda(b))) in pp collisions at sqrt(s) = 7
TeV using data collected by the CMS experiment at the LHC. The measurements are
based on Lambda(b) decays reconstructed in the exclusive final state J/Psi
Lambda, with the subsequent decays J/Psi to an opposite-sign muon pair and
Lambda to proton pion, using a data sample corresponding to an integrated
luminosity of 1.9 inverse femtobarns. The product of the cross section times
the branching ratio for Lambda(b) to J/Psi Lambda versus pt(Lambda(b)) falls
faster than that of b mesons. The measured value of the cross section times the
branching ratio for pt(Lambda(b)) > 10 GeV and abs(y(Lambda(b))) < 2.0 is 1.06
+/- 0.06 +/- 0.12 nb, and the integrated cross section ratio for
anti-Lambda(b)/Lambda(b) is 1.02 +/- 0.07 +/- 0.09, where the uncertainties are
statistical and systematic, respectively.Comment: Submitted to Physics Letters
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