3,561 research outputs found
Coherent Radiative Parton Energy Loss beyond the BDMPS-Z Limit
It is widely accepted that a phenomenologically viable theory of jet
quenching for heavy ion collisions requires the understanding of medium-induced
parton energy loss beyond the limit of eikonal kinematics formulated by
Baier-Dokshitzer-Mueller-Peigne-Schiff and Zakharov (BDMPS-Z). Here, we
supplement a recently developed exact Monte Carlo implementation of the BDMPS-Z
formalism with elementary physical requirements including exact energy-momentum
conservation, a refined formulation of jet-medium interactions and a treatment
of all parton branchings on the same footing. We document the changes induced
by these physical requirements and we describe their kinematic origin.Comment: 8 pages, 4 figure
Efficient simulation of relativistic fermions via vertex models
We have developed an efficient simulation algorithm for strongly interacting
relativistic fermions in two-dimensional field theories based on a formulation
as a loop gas. The loop models describing the dynamics of the fermions can be
mapped to statistical vertex models and our proposal is in fact an efficient
simulation algorithm for generic vertex models in arbitrary dimensions. The
algorithm essentially eliminates critical slowing down by sampling two-point
correlation functions and it allows simulations directly in the massless limit.
Moreover, it generates loop configurations with fluctuating topological
boundary conditions enabling to simulate fermions with arbitrary periodic or
anti-periodic boundary conditions. As illustrative examples, the algorithm is
applied to the Gross-Neveu model and to the Schwinger model in the strong
coupling limit.Comment: 5 pages, 4 figure
From Loop Groups to 2-Groups
We describe an interesting relation between Lie 2-algebras, the Kac-Moody
central extensions of loop groups, and the group String(n). A Lie 2-algebra is
a categorified version of a Lie algebra where the Jacobi identity holds up to a
natural isomorphism called the "Jacobiator". Similarly, a Lie 2-group is a
categorified version of a Lie group. If G is a simply-connected compact simple
Lie group, there is a 1-parameter family of Lie 2-algebras g_k each having
Lie(G) as its Lie algebra of objects, but with a Jacobiator built from the
canonical 3-form on G. There appears to be no Lie 2-group having g_k as its Lie
2-algebra, except when k = 0. Here, however, we construct for integral k an
infinite-dimensional Lie 2-group whose Lie 2-algebra is equivalent to g_k. The
objects of this 2-group are based paths in G, while the automorphisms of any
object form the level-k Kac-Moody central extension of the loop group of G.
This 2-group is closely related to the kth power of the canonical gerbe over G.
Its nerve gives a topological group that is an extension of G by K(Z,2). When k
= +-1, this topological group can also be obtained by killing the third
homotopy group of G. Thus, when G = Spin(n), it is none other than String(n).Comment: 40 page
Twisted Mass Finite Volume Effects
We calculate finite volume effects on the pion masses and decay constant in
twisted mass lattice QCD (tmLQCD) at finite lattice spacing. We show that the
lighter neutral pion in tmLQCD gives rise to finite volume effects that are
exponentially enhanced when compared to those arising from the heavier charged
pions. We demonstrate that the recent two flavour twisted mass lattice data can
be better fitted when twisted mass effects in finite volume corrections are
taken into account.Comment: 17 pages, revte
LPM-Effect in Monte Carlo Models of Radiative Energy Loss
Extending the use of Monte Carlo (MC) event generators to jets in nuclear
collisions requires a probabilistic implementation of the non-abelian LPM
effect. We demonstrate that a local, probabilistic MC implementation based on
the concept of formation times can account fully for the LPM-effect. The main
features of the analytically known eikonal and collinear approximation can be
reproduced, but we show how going beyond this approximation can lead to
qualitatively different results.Comment: 4 pages, 3 figures - To appear in the conference proceedings for
Quark Matter 2009, March 30 - April 4, Knoxville, Tennessee; v2: removed line
number
Testing the Color Charge and Mass Dependence of Parton Energy Loss with Heavy-to-light Ratios at RHIC and LHC
The ratio of nuclear modification factors of high-pT heavy-flavored mesons to
light-flavored hadrons (``heavy-to-light ratio'') in nucleus-nucleus collisions
tests the partonic mechanism expected to underlie jet quenching. Heavy-to-light
ratios are mainly sensitive to the mass and color-charge dependences of
medium-induced parton energy loss. Here, we assess the potential for
identifying these two effects in D and B meson production at RHIC and at the
LHC. To this end, we supplement the perturbative QCD factorized formalism for
leading hadron production with radiative parton energy loss. For D meson
spectra at high but experimentally accessible transverse momentum (10 < pT < 20
GeV) in Pb-Pb collisions at the LHC, we find that charm quarks behave
essentially like light quarks. However, since light-flavored hadron yields are
dominated by gluon parents, the heavy-to-light ratio of D mesons is a sensitive
probe of the color charge dependence of parton energy loss. In contrast, due to
the larger b quark mass, the medium modification of B mesons in the same
kinematical regime provides a sensitive test of the mass dependence of parton
energy loss. At RHIC energies, the strategies for identifying and disentangling
the color charge and mass dependence of parton energy loss are more involved
because of the smaller kinematical range accessible. We argue that at RHIC, the
kinematical regime best suited for such an analysis of D mesons is 7 < pT < 12
GeV, whereas the study of lower transverse momenta is further complicated due
to the known dominant contribution of additional, particle species dependent,
non-perturbative effects.Comment: 21 pages RevTex, 9 Figure
Chemical ionization tandem mass spectrometer for the in situ measurement of methyl hydrogen peroxide
A new approach for measuring gas-phase methyl hydrogen peroxide [(MHP) CH_3OOH] utilizing chemical ionization mass spectrometry is presented. Tandem mass spectrometry is used to avoid mass interferences that hindered previous attempts to measure atmospheric CH_3OOH with CF_3Oâ clustering chemistry. CH_3OOH has been successfully measured in situ using this technique during both airborne and ground-based campaigns. The accuracy and precision for the MHP measurement are a function of water vapor mixing ratio. Typical precision at 500 pptv MHP and 100 ppmv H_2O is ±80 pptv (2 sigma) for a 1 s integration period. The accuracy at 100 ppmv H_2O is estimated to be better than ±40%. Chemical ionization tandem mass spectrometry shows considerable promise for the determination of in situ atmospheric trace gas mixing ratios where isobaric compounds or mass interferences impede accurate measurements
Light hadrons in 2+1 flavor lattice QCD
This talk will focus on recent results by the MILC collaboration from
simulations of light hadrons in 2+1 flavor lattice QCD. We have achieved high
precision results in the pseudoscalar sector, including masses and decay
constants, plus quark masses and Gasser-Leutwyler parameters from well
controlled chiral perturbation theory fits to our data. We also show
spectroscopy results for vector mesons and baryons.Comment: To appear in the proceedings of the First Meeting of the APS Topical
Group on Hadronic Physics, Fermilab, Batavia, Illinois, Oct. 24-26, 200
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