1,527 research outputs found
Nonperturbative QCD Vacuum Effects in Nonlocal Quark Dynamics
A straightforward calculation reveals the essentially nonlocal character of
the leading heavy interaction arising from nonperturbative gluon
field correlations in the model of a fluctuating QCD vacuum. In light of this
quarkonium spin splitting ratio predictions which have supported the scalar
confinement ansatz are reconsidered as a specific example of possible
consequences for spectroscopy.Comment: Latex, 9 page
Cosmological implications of the Higgs mass measurement
We assume the validity of the Standard Model up to an arbitrary high-energy
scale and discuss what information on the early stages of the Universe can be
extracted from a measurement of the Higgs mass. For Mh < 130 GeV, the Higgs
potential can develop an instability at large field values. From the absence of
excessive thermal Higgs field fluctuations we derive a bound on the reheat
temperature after inflation as a function of the Higgs and top masses. Then we
discuss the interplay between the quantum Higgs fluctuations generated during
the primordial stage of inflation and the cosmological perturbations, in the
context of landscape scenarios in which the inflationary parameters scan. We
show that, within the large-field models of inflation, it is highly improbable
to obtain the observed cosmological perturbations in a Universe with a light
Higgs. Moreover, independently of the inflationary model, the detection of
primordial tensor perturbations through the B-mode of CMB polarization and the
discovery of a light Higgs can simultaneously occur only with exponentially
small probability, unless there is new physics beyond the Standard Model.Comment: 28 LaTeX pages, 6 figure
Precise Charm- and Bottom-Quark Masses: Theoretical and Experimental Uncertainties
Recent theoretical and experimental improvements in the determination of
charm and bottom quark masses are discussed. A new and improved evaluation of
the contribution from the gluon condensate to the
charm mass determination and a detailed study of potential uncertainties in the
continuum cross section for production is presented, together with a
study of the parametric uncertainty from the -dependence of our
results. The final results, MeV and
MeV, represent, together with a closely related lattice
determination MeV, the presently most precise
determinations of these two fundamental Standard Model parameters. A critical
analysis of the theoretical and experimental uncertainties is presented.Comment: 12 pages, presented at Quarks~2010, 16th International Seminar of
High Energy Physics, Kolomna, Russia, June 6-12, 2010; v2: references adde
Multiplicity dependence of jet-like two-particle correlations in p-Pb collisions at = 5.02 TeV
Two-particle angular correlations between unidentified charged trigger and
associated particles are measured by the ALICE detector in p-Pb collisions at a
nucleon-nucleon centre-of-mass energy of 5.02 TeV. The transverse-momentum
range 0.7 5.0 GeV/ is examined,
to include correlations induced by jets originating from low
momen\-tum-transfer scatterings (minijets). The correlations expressed as
associated yield per trigger particle are obtained in the pseudorapidity range
. The near-side long-range pseudorapidity correlations observed in
high-multiplicity p-Pb collisions are subtracted from both near-side
short-range and away-side correlations in order to remove the non-jet-like
components. The yields in the jet-like peaks are found to be invariant with
event multiplicity with the exception of events with low multiplicity. This
invariance is consistent with the particles being produced via the incoherent
fragmentation of multiple parton--parton scatterings, while the yield related
to the previously observed ridge structures is not jet-related. The number of
uncorrelated sources of particle production is found to increase linearly with
multiplicity, suggesting no saturation of the number of multi-parton
interactions even in the highest multiplicity p-Pb collisions. Further, the
number scales in the intermediate multiplicity region with the number of binary
nucleon-nucleon collisions estimated with a Glauber Monte-Carlo simulation.Comment: 23 pages, 6 captioned figures, 1 table, authors from page 17,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/161
Multi-particle azimuthal correlations in p-Pb and Pb-Pb collisions at the CERN Large Hadron Collider
Measurements of multi-particle azimuthal correlations (cumulants) for charged
particles in p-Pb and Pb-Pb collisions are presented. They help address the
question of whether there is evidence for global, flow-like, azimuthal
correlations in the p-Pb system. Comparisons are made to measurements from the
larger Pb-Pb system, where such evidence is established. In particular, the
second harmonic two-particle cumulants are found to decrease with multiplicity,
characteristic of a dominance of few-particle correlations in p-Pb collisions.
However, when a gap is placed to suppress such correlations,
the two-particle cumulants begin to rise at high-multiplicity, indicating the
presence of global azimuthal correlations. The Pb-Pb values are higher than the
p-Pb values at similar multiplicities. In both systems, the second harmonic
four-particle cumulants exhibit a transition from positive to negative values
when the multiplicity increases. The negative values allow for a measurement of
to be made, which is found to be higher in Pb-Pb collisions at
similar multiplicities. The second harmonic six-particle cumulants are also
found to be higher in Pb-Pb collisions. In Pb-Pb collisions, we generally find
which is indicative of a Bessel-Gaussian
function for the distribution. For very high-multiplicity Pb-Pb
collisions, we observe that the four- and six-particle cumulants become
consistent with 0. Finally, third harmonic two-particle cumulants in p-Pb and
Pb-Pb are measured. These are found to be similar for overlapping
multiplicities, when a gap is placed.Comment: 25 pages, 11 captioned figures, 3 tables, authors from page 20,
published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/87
Grassland Resistance and Resilience after Drought Depends on Management Intensity and Species Richness
The degree to which biodiversity may promote the stability of grasslands in the light of climatic variability, such as prolonged summer drought, has attracted considerable interest. Studies so far yielded inconsistent results and in addition, the effect of different grassland management practices on their response to drought remains an open question. We experimentally combined the manipulation of prolonged summer drought (sheltered vs. unsheltered sites), plant species loss (6 levels of 60 down to 1 species) and management intensity (4 levels varying in mowing frequency and amount of fertilizer application). Stability was measured as resistance and resilience of aboveground biomass production in grasslands against decreased summer precipitation, where resistance is the difference between drought treatments directly after drought induction and resilience is the difference between drought treatments in spring of the following year. We hypothesized that (i) management intensification amplifies biomass decrease under drought, (ii) resistance decreases with increasing species richness and with management intensification and (iii) resilience increases with increasing species richness and with management intensification
Heavy quarkonium: progress, puzzles, and opportunities
A golden age for heavy quarkonium physics dawned a decade ago, initiated by
the confluence of exciting advances in quantum chromodynamics (QCD) and an
explosion of related experimental activity. The early years of this period were
chronicled in the Quarkonium Working Group (QWG) CERN Yellow Report (YR) in
2004, which presented a comprehensive review of the status of the field at that
time and provided specific recommendations for further progress. However, the
broad spectrum of subsequent breakthroughs, surprises, and continuing puzzles
could only be partially anticipated. Since the release of the YR, the BESII
program concluded only to give birth to BESIII; the -factories and CLEO-c
flourished; quarkonium production and polarization measurements at HERA and the
Tevatron matured; and heavy-ion collisions at RHIC have opened a window on the
deconfinement regime. All these experiments leave legacies of quality,
precision, and unsolved mysteries for quarkonium physics, and therefore beg for
continuing investigations. The plethora of newly-found quarkonium-like states
unleashed a flood of theoretical investigations into new forms of matter such
as quark-gluon hybrids, mesonic molecules, and tetraquarks. Measurements of the
spectroscopy, decays, production, and in-medium behavior of c\bar{c}, b\bar{b},
and b\bar{c} bound states have been shown to validate some theoretical
approaches to QCD and highlight lack of quantitative success for others. The
intriguing details of quarkonium suppression in heavy-ion collisions that have
emerged from RHIC have elevated the importance of separating hot- and
cold-nuclear-matter effects in quark-gluon plasma studies. This review
systematically addresses all these matters and concludes by prioritizing
directions for ongoing and future efforts.Comment: 182 pages, 112 figures. Editors: N. Brambilla, S. Eidelman, B. K.
Heltsley, R. Vogt. Section Coordinators: G. T. Bodwin, E. Eichten, A. D.
Frawley, A. B. Meyer, R. E. Mitchell, V. Papadimitriou, P. Petreczky, A. A.
Petrov, P. Robbe, A. Vair
Threshold production of unstable top
We develop a systematic approach to describe the finite lifetime effects in
the threshold production of top quark-antiquark pairs. It is based on the
nonrelativistic effective field theory with an additional scale rho^(1/2) m_t
characterizing the dynamics of the top-quark decay, which involves a new
expansion parameter rho=1-m_W/m_t. Our method naturally resolves the problem of
spurious divergences in the analysis of the unstable top production. Within
this framework we compute the next-to-leading nonresonant contribution to the
total cross section of the top quark-antiquark threshold production in
electron-positron annihilation through high-order expansion in rho and confirm
the recently obtained result. We extend the analysis to the
next-to-next-to-leading O(alpha_s) nonresonant contribution which is derived in
the leading order in rho. The dominant nonresonant contribution to the
top-antitop threshold production in hadronic collisions is also obtained.Comment: 20 pages, 7 figures; v2: added a section on invariant mass cuts and
one reference, minor changes in Introduction, results unchanged, matches
published versio
Standard Model Theory for the FCC-ee Tera-Z stage
The future 100-km circular collider FCC at CERN is planned to operate in one
of its modes as an electron-positron FCC-ee machine. We give an overview
comparing the theoretical status to the experimental demands of one of four
foreseen FCC-ee operating stages, Z-boson resonance energy physics, called the
FCC-ee Tera-Z stage for short. The FCC-ee Tera-Z will deliver the highest
integrated luminosities as well as very small systematic errors for a study of
the Standard Model (SM) with unprecedented precision. In fact, the FCC-ee
Tera-Z will allow the study of at least one more perturbative order in quantum
field theory compared to the LEP/SLC precision. The real problem is that the
present precision of theoretical calculations of the various SM observables
does not match that of the anticipated experimental measurements. The
bottle-necks to overcoming this situation are identified. In particular, the
issues of precise QED unfolding and the correct calculation of SM
pseudo-observables are critically reviewed. In an Executive Summary, we specify
which basic theoretical calculations are needed to meet the strong experimental
expectations at the FCC-ee Tera-Z. Several methods, techniques and tools needed
for higher-order multi-loop calculations are presented. By inspection of the
Z-boson partial and total decay width analyses, it is argued that at the
beginning of operation of the FCC-ee Tera-Z, the theory predictions may be
tuned to be precise enough not to limit the physics interpretation of the
measurements. This statement is based on anticipated progress in analytical and
numerical calculations of multi-loop and multi-scale Feynman integrals and on
the completion of two-loop electroweak radiative corrections to the SM
pseudo-observables this year. However, the above statement is conditional as
the theoretical issues demand a very dedicated and focused investment by the
community.Comment: Published versio
Metabolomics Unravel Contrasting Effects of Biodiversity on the Performance of Individual Plant Species
In spite of evidence for positive diversity-productivity relationships increasing plant diversity has highly variable effects on the performance of individual plant species, but the mechanisms behind these differential responses are far from being understood. To gain deeper insights into the physiological responses of individual plant species to increasing plant diversity we performed systematic untargeted metabolite profiling on a number of herbs derived from a grassland biodiversity experiment (Jena Experiment). The Jena Experiment comprises plots of varying species number (1, 2, 4, 8, 16 and 60) and number and composition of functional groups (1 to 4; grasses, legumes, tall herbs, small herbs). In this study the metabolomes of two tall-growing herbs (legume: Medicago x varia; non-legume: Knautia arvensis) and three small-growing herbs (legume: Lotus corniculatus; non-legumes: Bellis perennis, Leontodon autumnalis) in plant communities of increasing diversity were analyzed. For metabolite profiling we combined gas chromatography coupled to time-of-flight mass spectrometry (GC-TOF-MS) and UPLC coupled to FT-ICR-MS (LC-FT-MS) analyses from the same sample. This resulted in several thousands of detected m/z-features. ANOVA and multivariate statistical analysis revealed 139 significantly changed metabolites (30 by GC-TOF-MS and 109 by LC-FT-MS). The small-statured plants L. autumnalis, B. perennis and L. corniculatus showed metabolic response signatures to increasing plant diversity and species richness in contrast to tall-statured plants. Key-metabolites indicated C- and N-limitation for the non-leguminous small-statured species B. perennis and L. autumnalis, while the metabolic signature of the small-statured legume L. corniculatus indicated facilitation by other legumes. Thus, metabolomic analysis provided evidence for negative effects of resource competition on the investigated small-statured herbs that might mechanistically explain their decreasing performance with increasing plant diversity. In contrast, taller species often becoming dominant in mixed plant communities did not show modified metabolite profiles in response to altered resource availability with increasing plant diversity. Taken together, our study demonstrates that metabolite profiling is a strong diagnostic tool to assess individual metabolic phenotypes in response to plant diversity and ecophysiological adjustment
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