共查询到20条相似文献,搜索用时 0 毫秒
1.
B. I. Ermolaev M. Greco S. I. Troyan 《The European Physical Journal C - Particles and Fields》2008,58(1):29-35
We examine the recent COMPASS data on the spin structure function g
1 singlet. We show that it is rather difficult to use the data in the present form in order to draw conclusions on the initial
parton densities. However, our tentative estimate is that the data better agree with positive than with negative initial gluon
densities. 相似文献
2.
3.
We discuss the scale dependence of
s
in connection with jet multiplicities on theZ pole in the framework of perturbative QCD. Several scale defining procedures are applied to jet fractions and compared to recent measurements at LEP.Dedicated to Prof. Dr. H.J. Mang on the occasion of his 60th birthday 相似文献
4.
P.?Stok?osa W.?P?aczek M.?Skrzypek 《The European Physical Journal C - Particles and Fields》2009,60(2):213-232
We present two Monte Carlo algorithms of the Markovian type which solve the modified QCD evolution equations at NLO level.
The modifications with respect to the standard DGLAP evolution concern the argument of the strong coupling constant, α
S. We first analyze the z-dependent argument and then the k
T
-dependent one. The evolution time variable is identified with the rapidity. The two algorithms are tested to 0.05% precision
level. We find that the NLO corrections in the evolution of the parton momentum distributions with k
T
-dependent coupling constant are of the order of 10 to 20%, and in the small-x region even up to 30%, with respect to the LO contributions.
The project is partly supported by the EU grant MTKD-CT-2004-510126, realized in a partnership with the CERN Physics Department
and by the Polish Ministry of Science and Information Society Technologies grant No. 620/E-77/6.PR UE/DIE 188/2005-2008. 相似文献
5.
In this note we discuss possible separations of exact, massive, tree-level spin amplitudes into gauge-invariant parts. We
concentrate our attention on processes involving two quarks entering a color-neutral current and, thanks to the QCD interactions,
two extra external gluons. We will search for forms compatible with parton-shower languages, without applying approximations
or restrictions on phase space regions. Special emphasis will be put on the isolation of parts necessary for the construction
of evolution kernels for individual splittings and to some degree for the running coupling constant as well. Our aim is to
better understand the environment necessary to optimally match hard matrix elements with parton-shower algorithms. To avoid
complications and ambiguities related to regularization schemes, we ignore, at this point, virtual corrections. Our representation
is quite universal: any color-neutral current can be used; in particular, our approach is not restricted to vector currents
only.
This work is partially supported by RTN European Programme, MRTN-CT-2006-035505 (HEPTOOLS, Tools and Precision Calculations
for Physics Discoveries at Colliders). 相似文献
6.
Sevil Salur 《The European Physical Journal C - Particles and Fields》2009,62(1):119-125
Jets can be used to probe the physical properties of the high energy density matter created in collisions at the Relativistic
Heavy Ion Collider (RHIC). Measurements of strong suppression of inclusive hadron distributions and di-hadron correlations
at high p
T
have already provided evidence for partonic energy loss. However, these measurements suffer from well-known geometric biases
due to the competition of energy loss and fragmentation. These biases can be avoided if the jets are reconstructed independently
of their fragmentation details—quenched or unquenched. In this paper, we discuss modern jet reconstruction algorithms (cone
and sequential recombination) and their corresponding background subtraction techniques required by the high multiplicities
of heavy ion collisions. We review recent results from the STAR experiment at RHIC on direct jet reconstruction in central
Au+Au collisions at
GeV. 相似文献
7.
We re-evaluate the non-perturbative contribution to the thrust distribution in e
+
e
−→ hadrons, in the light of the latest experimental data and the recent NNLO perturbative calculation of this quantity. By
extending the calculation to NNLO+NLL accuracy, we perform the most detailed study to date of the effects of non-perturbative
physics on this observable. In particular, we investigate how well a model based on a low-scale QCD effective coupling can
account for such effects. We find that the difference between the improved perturbative distribution and the experimental
data is consistent with a 1/Q-dependent non-perturbative shift in the distribution, as predicted by the effective coupling model. Best fit values of α
s
(91.2 GeV)=0.1164−0.0026+0.0028 and α
0(2 GeV)=0.59±0.03 are obtained with χ
2/d.o.f.=1.09. This is consistent with NLO+NLL results but the quality of fit is improved. The agreement in α
0 is non-trivial because a part of the 1/Q-dependent contribution (the infrared renormalon) is included in the NNLO perturbative correction.
Address after 1 October 2008: Rudolf Peierls Centre for Theoretical Physics, 1 Keble Road, Oxford OX1 3NP, UK. 相似文献
8.
Y. Kiyo J. H. Kühn S. Moch M. Steinhauser P. Uwer 《The European Physical Journal C - Particles and Fields》2009,60(3):375-386
The next-to-leading order analysis for the cross section for hadroproduction of top-quark pairs close to threshold is presented.
Within the framework of non-relativistic QCD a significant enhancement compared to fixed-order perturbation theory is observed
which originates from the characteristic remnant of the 1S peak below production threshold of top-quark pairs. The analysis includes all color-singlet and color-octet configurations
of top-quark pairs in S-wave states and, for the dominant configurations, it employs all-order soft-gluon resummation for the hard parton cross section.
Numerical results for the Large Hadron Collider at
TeV and
TeV and also for the Tevatron are presented. The possibility of a top-quark mass measurement from the invariant-mass distribution
of top-quark pairs is discussed. 相似文献
9.
Camelia Mironov Ramona Vogt Gerd J. Kunde 《The European Physical Journal C - Particles and Fields》2009,61(4):893-898
We propose a new method for identifying and isolating
events through semileptonic decays of the
pair. Employing these decay dileptons to tag the jet in a specific kinematic region provides a clean signature of jets associated
with heavy-quark production. The measurement, in both pp and heavy-ion collisions, is essential for addressing heavy-quark fragmentation in vacuum and in a dense medium. We present
next-to-leading order calculations of
production (leading order in
production) in
TeV pp collisions at the LHC and discuss the feasibility of the measurement in heavy-ion collisions at
TeV. 相似文献
10.
Ivan Vitev Ben-Wei Zhang Simon Wicks 《The European Physical Journal C - Particles and Fields》2009,62(1):139-144
We report selected results from a recent in-depth study of jet shapes and jet cross sections in ultra-relativistic reactions
with heavy nuclei at the LHC. We demonstrate that at the highest collider energies these observables become feasible as a
new, differential and accurate test of the underlying QCD theory. Our approach allows for detailed simulations of the experimental
acceptance/cuts that help isolate jets emerging from a dense QGP. We show for the first time that the pattern of stimulated
gluon emission can be correlated with a variable quenching of the jet rates and provide an approximately model-independent
approach to determining the characteristics of the medium-induced bremsstrahlung spectrum. The connection between such cross
section attenuation and the in-medium jet shapes is elucidated. 相似文献
11.
Redamy Perez-Ramos 《The European Physical Journal C - Particles and Fields》2009,62(3):541-545
The energy evolution of average multiplicities and multiplicity fluctuations in jets produced in heavy-ion collisions is investigated
from a toy QCD-inspired model. In this model, we use modified splitting functions accounting for medium-enhanced radiation
of gluons by a fast parton which propagates through the quark–gluon plasma. The leading contribution of the standard production
of soft hadrons is enhanced by a factor
while next-to-leading order (NLO) corrections are suppressed by
, where the parameter N
s
>1 accounts for the induced soft gluons in the medium. Our results for such global observables are cross-checked and compared
with their limits in the vacuum. 相似文献
12.
13.
Using the QCD sum rules we calculate the neutron-proton mass difference at zero density as a function of the difference in bare quark massm
d—m
u. We confirm results of Hatsuda, Høgaasen and Prakash that the largest term results from the difference in up and down quark condensates, the explicitC(m
d—m
u) entering with the opposite sign. The quark condensates are then extended to finite density to estimate the Nolen-Schiffer effect. The neutron-proton mass difference is extremely density dependent, going to zero at roughly nuclear matter density.The Ioffe formula for the nucleon mass is interpreted as a derivation, within the QCD sum rule approach, of the Nambu-Jona-Lasinio formula. This clarifies theN
c counting and furthermore provides an alternative interpretation of the Borel mass. 相似文献
14.
15.
I discuss the temperature dependence of quarkonium correlators calculated in lattice QCD. I show that the dominant source
of the temperature dependence comes from the zero-mode contribution, while the temperature dependence associated with the
melting of bound states is quite small. I study the zero-mode contribution quantitatively for various quark masses and show
that it is well described by a quasi-particle model with temperature-dependent heavy quark mass. As a byproduct, an estimate
of the medium dependence of the heavy-quark mass is obtained. 相似文献
16.
D. V. Shirkov 《Physics of Particles and Nuclei Letters》2008,5(6):489-493
In quantum theory, physical amplitudes are usually presented in the form of a Feynman perturbation series in powers of coupling
constant α. However, it is known that these amplitudes are not regular functions at α = 0.
For QCD, we propose new sets of expansion parameters w
k
(α
s
) that reflect singularity at α
s
= 0 and should be used instead of powers α
s
k
. Their explicit form is motivated by the so-called Analytic Perturbation Theory. These parameters reveal saturation in a
strong coupling case at the level α
s
eff (α
s
1) = w
1(α
s
1) ∼ 0.5. They can be used for the quantitative analysis of divers low-energy amplitudes.
We argue that this new picture with non-power sets of perturbation expansion parameters, as well as the saturation feature,
is of a rather general nature.
The text was submitted by the author in English.
A preliminary version with the main results was published in [1]. 相似文献
17.
Gouranga C. Nayak 《The European Physical Journal C - Particles and Fields》2009,59(4):891-898
In this paper we implement the Schwinger–Keldysh closed-time path integral formalism in non-equilibrium QCD in accordance
to the definition of the Collins–Soper fragmentation function. We consider a high-p
T parton in QCD medium at initial time τ
0 with an arbitrary non-equilibrium (non-isotropic) distribution function
fragmenting to a hadron. We formulate the parton-to-hadron fragmentation function in non-equilibrium QCD in the light-cone
quantization formalism. It may be possible to include final-state interactions with the medium via a modification of the Wilson
lines in this definition of the non-equilibrium fragmentation function. This may be relevant to the study of hadron production
from a quark–gluon plasma at RHIC and LHC. 相似文献
18.
19.
Alejandro?Ayala Eleazar?Cuautle Isabel?Domínguez Antonio?Ortiz Guy?Pai? 《The European Physical Journal C - Particles and Fields》2009,62(3):535-540
The experimental results on transverse momentum azimuthal hadron correlations at RHIC have opened a rich field for parton
energy loss analysis in heavy-ion collisions. Recently, a considerable amount of work has been devoted to study the shapes
of the “away-side” jet which exhibit an interesting and unexpected “double hump” structure not observed in the analogous treatment
of the pp data. Driven by the possibility that the latter result might just mean that such a structure exists already in the case of
pp collisions, but that its relative intensity could be small, here we use the Event Shape Analysis to show that it is possible
to identify and select well defined event topologies in pp collisions, among which a double hump structure for the away-side jet emerges. Using two shape parameters, the sphericity
in the transverse plane and the recoil to analyze a sample of PYTHIA generated pp collisions at
GeV, we show that this structure corresponds to two jets emitted in the backward hemisphere. Finally, we show that Q-PYTHIA
qualitatively reproduces the decrease in the yield of dijet events and the increase of the double hump structure in the away
side observed in heavy-ion collisions. The implications for the treatment of parton energy loss in heavy-ion collisions are
discussed. 相似文献
20.
B. Hippolyte 《The European Physical Journal C - Particles and Fields》2009,62(1):237-242
Measurements at low transverse momentum will be performed at the LHC for studying particle production mechanisms in pp and heavy-ion collisions. Some of the experimental capabilities for bulk matter physics are presented, focusing on tracking
elements and particle identification. In order to anticipate the study of baryon production for both colliding systems at
multi-TeV energies, measurements for identified species and recent model extrapolations are discussed. Several mechanisms
are expected to compete for hadro-production in the low momentum region. For this reason, experimental observables that could
be used for investigating multi-parton interactions and help understanding the “underlying event” content in the first pp collisions at the LHC are also mentioned. 相似文献