Nuclear Theory·nucl-th

Wednesday·August 12, 2015

13 papers—8 primary·5 cross-listed

09
[Submitted on 10 Aug 2015] (cross-list from hep-ph)
Employing Helicity Amplitudes for Resummation
Ian Moult🇺🇸 · Iain W. Stewart🇺🇸 · Frank J. Tackmann🇩🇪 · Wouter J. Waalewijn🇳🇱
Many state-of-the-art QCD calculations for multileg processes use helicity amplitudes as their fundamental ingredients. We construct a simple and easy-to-use helicity operator basis in soft-collinear effective theory (SCET), for which the hard Wilson coefficients from matching QCD onto SCET are directly given in terms of color-ordered helicity amplitudes. Using this basis allows one to seamlessly combine fixed-order helicity amplitudes at any order they are known with a resummation of higher-order logarithmic corrections. In particular, the virtual loop amplitudes can be employed in factorization theorems to make predictions for exclusive jet cross sections without the use of numerical subtraction schemes to handle real-virtual infrared cancellations. We also discuss matching onto SCET in renormalization schemes with helicities in $4$ - and $d$ -dimensions. To demonstrate that our helicity operator basis is easy to use, we provide an explicit construction of the operator basis, as well as results for the hard matching coefficients, for $pp \to H + 0, 1, 2$ jets, $pp \to W / Z / γ + 0, 1, 2$ jets, and $pp \to 2, 3$ jets. These operator bases are completely crossing symmetric, so the results can easily be applied to processes with $e^{+} e^{-}$ and $e^{-} p$ collisions.
Comments:
41 pages + 20 pages in Appendices, 1 figure, v2: journal version
Subjects:
High Energy Physics — Phenomenology (hep-ph); Nuclear Theory (nucl-th)
arXiv:
1508.02397 [pdf]
PRD(2016)·50 citations
10
[Submitted on 10 Aug 2015] (cross-list from hep-ph)
Di-photon "Ridge" in p+p and p+A collisions at RHIC and the LHC
Alex Kovner🇺🇸 · Amir H. Rezaeian🇨🇱
We obtain prompt di-photon cross-section in proton-nucleus collisions in Hamiltonian light-cone approach within a hybrid approximation, treating the projectile proton in the parton model and the target nucleus in the Color-Glass-Condensate approach. We study in details the di-photon correlations in quark-nucleus and proton-nucleus collisions at RHIC and the LHC. We show that the single fragmentation di-photon produces the away side correlations peak, and the double fragmentation component of prompt di-photon is responsible for the near-side peak, and the long-range in rapidity near-side azimuthal collimation, the so-called "ridge" structure. We study the transverse momentum, density and energy dependence of the di-photon ridge and show that it strongly depends on the kinematics and saturation dynamics. We show that while di-photon ridge exists at the LHC in quark-nucleus collisions, the effect disappears in proton-nucleus collisions at the LHC. At RHIC the ridge-type structure persists at low transverse momenta of di-photon even in proton-nucleus collisions. We argue that di-photon correlation measurments in p+A collisions can help to discriminate among models and understand the true origin of the observed di-hadron ridge in p+A collisions. We also show that in addition to the ridge structure, prompt di-photon correlation also exhibits some distinct novel features, including the emergence of away side double-peak structure at intermediate transverse momenta.
Comments:
23 pages, 11 figures; v2: results unchanged, a clarifying Appendix added. The version to appear in PRD
Subjects:
High Energy Physics — Phenomenology (hep-ph); High Energy Physics — Experiment (hep-ex); Nuclear Experiment (nucl-ex); Nuclear Theory (nucl-th)
arXiv:
1508.02412 [pdf]
PRD(2015)·22 citations
11
[Submitted on 11 Aug 2015] (cross-list from math-ph)
The Nonclassical Diffusion Approximation to the Nonclassical Linear Boltzmann Equation
Richard Vasques
We show that, by correctly selecting the probability distribution function $p (s)$ for a particle's distance-to-collision, the nonclassical diffusion equation can be represented exactly by the nonclassical linear Boltzmann equation for an infinite homogeneous medium. This choice of $p (s)$ preserves the $t r u e$ mean-squared free path of the system, which sheds new light on the results obtained in previous work.
Comments:
8 pages, 2 figures
Subjects:
Mathematical Physics (math-ph); math.MP (math.MP); Nuclear Theory (nucl-th)
arXiv:
1508.02462 [pdf]
Appl.Math.Lett.(2016)·3 citations
12
[Submitted on 11 Aug 2015] (cross-list from hep-ph)
Effects of kinematic cuts on net-electric charge fluctuations
Frithjof Karsch🇩🇪 · Kenji Morita🇯🇵 · Krzysztof Redlich🇩🇪
The effects of kinematic cuts on electric charge fluctuations in a gas of charged particles are discussed. We consider a very transparent example of an ideal pion gas with quantum statistics, which can be viewed as a multi-component gas of Boltzmann particles with different charges and masses. Cumulants of net-electric charge fluctuations $χ_{n}^{Q}$ are calculated in a static and expanding medium with flow parameters adjusted to the experimental data. We show that the transverse momentum cut, $p_{t_{min}} \leq p_{t} \leq p_{t_{max}}$ , weakens the effects of Bose statistics, i.e. contributions of effectively multi-charged states to higher order moments. Consequently, cuts in $p_{t}$ modify the experimentally measured cumulants and their ratios. We discuss the influence of kinematic cuts on the ratio of mean and variance of electric charge fluctuations in a hadron resonance gas, in the light of recent data of the STAR and PHENIX Collaborations. We find that the different momentum cuts of $p_{t_{min}} = 0.2$ ~GeV (STAR) and $p_{t_{min}} = 0.3$ ~GeV (PHENIX) are responsible for more than 30\% of the difference between these two data sets. We argue that the $p_{t}$ cuts imposed on charged particles will influence the normalized kurtosis { $κ_{Q} σ_{Q}^{2} = χ_{4}^{Q} / χ_{2}^{Q}$ } of the electric charge fluctuations. In particular, the reduction of $κ_{Q} σ_{Q}^{2}$ with increasing $p_{t_{min}}$ will lead to differences between PHENIX and STAR data of $O (6%)$ which currently are buried under large statistical and systematic errors. We furthermore introduce the relation between momentum cut-off and finite volume effects, which is of relevance for the comparison between experimental data and lattice QCD calculations.
Comments:
10 pages, 11figures, Section II is extended. To appear in Phys.Rev.C
Subjects:
High Energy Physics — Phenomenology (hep-ph); High Energy Physics — Lattice (hep-lat); Nuclear Experiment (nucl-ex); Nuclear Theory (nucl-th)
arXiv:
1508.02614 [pdf]
PRC(2016)·49 citations
13
[Submitted on 11 Aug 2015] (cross-list from hep-ph)
Exclusive decays $B \to ℓ^{-} ν ˉ$ and $B \to D^{(*)} ℓ^{-} ν ˉ$ in the covariant quark model
Mikhail A. Ivanov🇷🇺 · Jürgen G. Körner🇩🇪 · Chien-Thang Tran🇷🇺
We study the exclusive leptonic and semileptonic $B$ decays $B \to ℓ^{-} ν ˉ$ and $B \to D^{(*)} ℓ^{-} ν ˉ$ in the framework of the covariant quark model with built-in infrared confinement. We compute the relevant form factors in the full kinematical momentum transfer region. The calculated form factors are used to evaluate branching fractions and polarization observables of the above transitions. We compare our results with experimental data and results from other theoretical studies.
Comments:
48 pages, 26 figures. References and discussion added. Accepted by Phys.Rev.D for publication
Subjects:
High Energy Physics — Phenomenology (hep-ph); High Energy Physics — Experiment (hep-ex); Nuclear Theory (nucl-th)
arXiv:
1508.02678 [pdf]
PRD(2015)·71 citations

Affiliations

first authorsco-authorsvia INSPIRE

Employing Helicity Amplitudes for Resummation

Di-photon "Ridge" in p+p and p+A collisions at RHIC and the LHC

The Nonclassical Diffusion Approximation to the Nonclassical Linear Boltzmann Equation

Effects of kinematic cuts on net-electric charge fluctuations

Exclusive decays B→ℓ−νˉ and B→D(∗)ℓ−νˉ in the covariant quark model

Affiliations

Exclusive decays $B \to ℓ^{-} ν ˉ$ and $B \to D^{(*)} ℓ^{-} ν ˉ$ in the covariant quark model