Scattering amplitudes with open loops Cascioli, F; Maierhöfer, P; Pozzorini, S
Physical review letters,
2012-Mar-16, Letnik:
108, Številka:
11
Journal Article
Recenzirano
Odprti dostop
We introduce a new technique to generate scattering amplitudes at one loop. Traditional tree algorithms, which handle diagrams with fixed momenta, are promoted to generators of loop-momentum ...polynomials that we call open loops. Combining open loops with tensor-integral and Ossola-Papadopoulos-Pittau reduction results in a fully flexible, very fast, and numerically stable one-loop generator. As demonstrated with nontrivial applications, the open-loop approach will permit us to obtain precise predictions for a very wide range of collider processes.
A
bstract
We present a fully automated implementation of next-to-leading order electroweak (NLO EW) corrections in the O
pen
L
oops
matrix-element generator combined with the
Sherpa
and M
unich
Monte ...Carlo frameworks. The process-independent character of the implemented algorithms opens the door to NLO QCD + EW simulations for a vast range of Standard Model processes, up to high particle multiplicity, at current and future colliders. As a first application, we present NLO QCD + EW predictions for the production of positively charged on-shell
W
bosons in association with up to three jets at the Large Hadron Collider. At the TeV energy scale, due to the presence of large Sudakov logarithms, EW corrections reach the 20-40% level and play an important role for searches of physics beyond the Standard Model. The dependence of NLO EW effects on the jet multiplicity is investigated in detail, and we find that
W
+ multijet final states feature genuinely different EW effects as compared to the case of
W
+ 1 jet.
We report on the first calculation of next-to-next-to-leading order (NNLO) QCD corrections to the inclusive production of Z-boson pairs at hadron colliders. Numerical results are presented for pp ...collisions with centre-of-mass energy (s) ranging from 7 to 14 TeV. The NNLO corrections increase the NLO result by an amount varying from 11% to 17% as s goes from 7 to 14 TeV. The loop-induced gluon fusion contribution provides about 60% of the total NNLO effect. When going from NLO to NNLO the scale uncertainties do not decrease and remain at the ±3% level.
We present theoretical predictions for the production of top-quark pairs with up to three jets at the next-to leading order in perturbative QCD. The relevant calculations are performed with
Sherpa
...and
OpenLoops
. To address the issue of scale choices and related uncertainties in the presence of multiple scales, we compare results obtained with the standard scale
H
T
/
2
at fixed order and the M
i
NLO procedure. Analyzing various cross sections and distributions for
t
t
¯
+
0
,
1
,
2
,
3
jets at the 13 TeV LHC we find a remarkable overall agreement between fixed-order and M
i
NLO results. The differences are typically below the respective factor-two scale variations, suggesting that for all considered jet multiplicities missing higher-order effects should not exceed the ten percent level.
Theoretical uncertainties in the simulation of tt¯bb¯ production represent one of the main obstacles that still hamper the observation of Higgs-boson production in association with top-quark pairs in ...the H→bb¯ channel. In this Letter we present a next-to-leading order (NLO) simulation of tt¯bb¯ production with massive b-quarks matched to the Sherpa parton shower. This allows one to extend NLO predictions to arbitrary tt¯bb¯ kinematics, including the case where one or both b-jets arise from collinear g→bb¯ splittings. We find that this splitting mechanism plays an important role for the tt¯H(bb¯) analysis.
A
bstract
We present next-to-leading order (NLO) predictions including QCD and electroweak (EW) corrections for the production and decay of off-shell electroweak vector bosons in association with up ...to two jets at the 13 TeV LHC. All possible dilepton final states with zero, one or two charged leptons that can arise from off-shell W and Z bosons or photons are considered. All predictions are obtained using the automated implementation of NLO QCD+EW corrections in the O
pen
Loops matrix-element generator combined with the Munich and Sherpa Monte Carlo frameworks. Electroweak corrections play an especially important role in the context of BSM searches, due to the presence of large EW Sudakov logarithms at the TeV scale. In this kinematic regime, important observables such as the jet transverse momentum or the total transverse energy are strongly sensitive to multijet emissions. As a result, fixed-order NLO QCD+EW predictions are plagued by huge QCD corrections and poor theoretical precision. To remedy this problem we present an approximate method that allows for a simple and reliable implementation of NLO EW corrections in the MePs@Nlo multijet merging framework. Using this general approach we present an inclusive simulation of vector-boson production in association with jets that guarantees NLO QCD+EW accuracy in all phase-space regions involving up to two resolved jets.
We describe several techniques for the calculation of multi-loop integrals and their application to heavy quark current correlators. As new results, we present the four-loop correction to the second ...and third physical moments in the low-energy expansions of vector, axial-vector and scalar quark current correlators. Using the Ward identity, we obtain the third and fourth moments for the pseudo-scalar correlator. We briefly discuss the impact of these results on the determination of the charm quark mass and the strong coupling constant using lattice simulations for the current correlators and of the charm- and bottom-quark mass from experimental data for
σ
(
e
+
e
−
→
hadrons
)
.
We present an NLO simulation of WWbb production with massive b-quarks at the LHC. Off-shell and non-resonant contributions associated with top-pair and single-top channels and with leptonic W-boson ...decays are consistently taken into account using the complex-mass scheme. Thanks to the finite b-quark mass, WWbb predictions can be extended to the whole b-quark phase space, thereby including Wt-channel single-top contributions that originate from collinear g
→
bb splittings in the four-flavour scheme. This provides a consistent NLO description of tt and Wt production and decay, including quantum interference effects. The simulation is also applicable to exclusive 0- and 1-jet bins, which is of great importance for Higgs-boson studies in the H
→
WW channel and for any other analysis with large top backgrounds and jet vetoes or jet bins.
Kira—A Feynman integral reduction program Maierhöfer, P.; Usovitsch, J.; Uwer, P.
Computer physics communications,
September 2018, 2018-09-00, Letnik:
230
Journal Article
Recenzirano
Odprti dostop
In this article, we present a new implementation of the Laporta algorithm to reduce scalar multi-loop integrals appearing in quantum field theoretic calculations to a set of master integrals. We ...extend existing approaches by using an additional algorithm based on modular arithmetic to remove linearly depen- dent equations from the system of equations arising from integration-by-parts and Lorentz identities. Furthermore, the algebraic manipulations required in the back substitution are optimized. We describe in detail the implementation as well as the usage of the program. In addition, we show benchmarks for concrete examples and compare the performance to Reduze 2 and FIRE 5. In our benchmarks we find that Kira is highly competitive with these existing tools.
Program title: Kira
Program Files doi:http://dx.doi.org/10.17632/v3cmsnfrnn.1
Licensing provisions: GPLv3
Programming language:C++
External routines/libraries used:Fermat 1, gateToFermat 2, GiNaC 3,4, yaml-cpp 5, zlib 6 and SQLite3 7
Nature of problem: The reduction of Feynman integrals to master integrals leads in general to a system of equations which contains redundant, i.e. linearly dependent, equations. In particular, for multi-scale problems, the algebraic manipulation of these redundant equations can lead to a substantial increase in runtime and memory consumption without affecting the results.
Solution method: The program identifies linearly dependent relations based on modular arithmetic with the help of an algorithm presented in Ref. 8. Afterwards the program brings a linearly independent system of equations in a triangular form. Furthermore, the algebraic manipulations required in the back substitution are optimized.
Restrictions: the CPU time and the available RAM
References:
1 R. H. Lewis, Computer Algebra System Fermat, https://home.bway.net/lewis/.
2 M. Tentioukov, gateToFermat, http://science.sander.su/FLink.htm.
3 C. W. Bauer, A. Frink, and R. Kreckel, Introduction to the GiNaC framework for symbolic computationwithin the C++ programming language, J. Symb. Comput. 33 (2000) 1, arXiv:cs/0004015 cs-sc.
4 J. Vollinga, GiNaC: Symbolic computation with C++, Nucl. Instrum. Meth. A559 (2006) 282-284, arXiv:hep-ph/0510057 hep-ph.
5 YAML, YAML Aint Markup Language, http://yaml.org.
6 J.-L. Gailly and M. Adler, ZLIB, http://zlib.net.
7 SQLite, SQLite3, version: 3.14.2, https://www.sqlite.org.
8 P. Kant, Finding Linear Dependencies in Integration-By-Parts Equations: A Monte Carlo Approach, Comput. Phys. Commun. 185 (2014) 1473-1476, arXiv:1309.7287 hep-ph.
Charged gauge boson pair production at the Large Hadron Collider allows detailed probes of the fundamental structure of electroweak interactions. We present precise theoretical predictions for ...on-shell W+ W- production that include, for the first time, QCD effects up to next to next to leading order in perturbation theory. As compared to next to leading order, the inclusive W+ W- cross section is enhanced by 9% at 7 TeV and 12% at 14 TeV. The residual perturbative uncertainty is at the 3% level. The severe contamination of the W+ W- cross section due to top-quark resonances is discussed in detail. Comparing different definitions of top-free W+ W- production in the four and five flavor number schemes, we demonstrate that top-quark resonances can be separated from the inclusive W+ W- cross section without a significant loss of theoretical precision.
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