2032681
All tree-level amplitudes in massless QCD
We derive compact analytical formulae for all tree-level color-ordered gauge theory amplitudes involving any number of external gluons and up to four massless quarkanti-quark pairs. A general formula is presented based on the combinatorics of paths along a rooted tree and associated determinants. Explicit expressions are displayed for the next-to-maximally helicity violating (NMHV) and next-to-next-to-maximally helicity violating (NNMHV) gauge theory amplitudes. Our results are obtained by projecting the previously-found expressions for the super-amplitudes of the maximally supersymmetric super Yang-Mills theory ($$ \mathcal{N} = 4 $$ SYM) onto the relevant components yielding all gluon-gluino tree amplitudes in $$ \mathcal{N} = 4 $$ SYM. We show how these results carry over to the corresponding QCD amplitudes, including massless quarks of different avors as well as a single electroweak vector boson. The public Mathematica package GGT is described, which encodes the results of this work and yields analytical formulae for all $$ \mathcal{N} = 4 $$ SYM gluon-gluino trees. These in turn yield all QCD trees with up to four external arbitrary-flavored massless quark-anti-quark pairs.
119223768
No Lee-Wick Fields out of Gravity
We investigate the gravitational one-loop divergences of the standard model in large extra dimensions, with gravitons propagating in the ($4+\ensuremath{\delta}$)-dimensional bulk and gauge fields as well as scalar and fermionic multiplets confined to a three-brane. To determine the divergences we establish a cutoff regularization which allows us to extract gauge-invariant counterterms. In contrast to the claim of a recent paper [F. Wu and M. Zhong, Phys. Rev. D 78, 085010 (2008).], we show that the fermionic and scalar higher derivative counterterms do not coincide with the higher derivative terms in the Lee-Wick standard model. We argue that even if the exact Lee-Wick higher derivative terms were found, as in the case of the pure gauge sector, this would not allow to conclude the existence of the massive ghost fields corresponding to these higher derivative terms in the Lee-Wick standard model.
33472339
Gravitational corrections to Yukawa and phi{4} interactions.
We consider the lowest order quantum gravitational corrections to Yukawa and phi{4} interactions. Our results show that quantum gravity leads to contributions to the running coupling constants if the particles are massive and therefore alters the scaling behavior of the standard model. Furthermore, we find that the gravitational contributions to the running of the masses vanish.
119250225
Comparing efficient computation methods for massless QCD tree amplitudes: Closed analytic formulas versus Berends-Giele recursion
Recent advances in our understanding of tree-level QCD amplitudes in the massless limit exploiting an effective (maximal) supersymmetry have led to the complete analytic construction of tree-amplitudes with up to four external quark-anti-quark pairs. In this work we compare the numerical efficiency of evaluating these closed analytic formulae to a numerically efficient implementation of the Berends-Giele recursion. We compare calculation times for tree-amplitudes with parton numbers ranging from 4 to 25 with no, one, two and three external quark lines. We find that the exact results are generally faster in the case of MHV and NMHV amplitudes. Starting with the NNMHV amplitudes the Berends-Giele recursion becomes more efficient. In addition to the runtime we also compared the numerical accuracy. The analytic formulae are on average more accurate than the off-shell recursion relations though both are well suited for complicated phenomenological applications. In both cases we observe a reduction in the average accuracy when phase space configurations close to singular regions are evaluated. We believe that the above findings provide valuable information to select the right method for phenomenological applications.
120290404
Gravitational Corrections to Non-Gauge Interactions
We compute the lowest order quantum gravitational corrections to Yukawa and ϕ 4 interactions.
208866883
New results on event-by-event ratio fluctuations in Pb+Pb collisions at CERN SPS energies
Event-by-event fluctuations of produced particle multiplicities are believed to be sensitive to a deconfinement phase transition and the critical point of strongly interacting matter. The NA49 collaboration has conducted a systematic study of various fluctuation observables. In this contribution, recent results on hadron ratio fluctuations in central Pb+Pb collisions at –17.3 GeV are reported. These results are complemented by the centrality dependence of hadron ratio fluctuations at . A universal scaling was found, describing the energy and centrality dependence of the (K+ + K−)/(π+ + π−) and ratio fluctuations purely by a change in the observed average hadron multiplicities. This scaling is broken for the fluctuations of the and K+/p ratios, possibly hinting at a change in the baryon number–strangeness correlation.
16599979
Global Properties of Nucleus-Nucleus Collisions
QCD as a theory of extended, strongly interacting matter is familiar from big bang evolution which, within the time interval from electro-weak decoupling (10−12 s) to hadron formation (5 × 10−6 s), is dominated by the expansion of quark–gluon matter, a color conducting plasma that is deconfined. In the 1970s deconfinement was predicted [1–5] to arise from the newly discovered “asymptotic freedom” property of QCD; i.e., the plasma was expected to be a solution of perturbative QCD at asymptotically high square momentum transfer Q 2, or temperature T
119462879
High pT spectra of identified particles produced in Pb+Pb collisions at 158 A GeV beam energy
Results of the NA49 collaboration on the production of hadrons with large transverse momentum in Pb+Pb collisions at 158 A GeV beam energy are presented. A range of up to pT = 4 GeV/c is covered. The nuclear modification factor RCP is extracted for π±, K± and p, and the baryon to meson ratios and Λ/K0s are studied. All results are compared to other measurements at SPS and RHIC and to theoretical calculations.
118004237
Hadron Formation and the Phase Diagram of QCD Matter
We investigate hadronic species freeze-out systematics in A+A collisions at low SPS energies, corresponding to a baryochemical potential above 300 MeV, analyzing NA49 hadron production data in the framework of the statistical hadronization model, and in the UrQMD hadronic transport model. Observing no deviation from universal grand canonical hadro-chemical equilibrium freeze-out, we argue that the observed hadronic freeze-out points should universally signal the boundary line of the hadronic phase in the QCD matter phase diagram.
120318226
Hadron ratio fluctuations in heavy-ion collisions