Hadronic Cross Sections Research Articles

Quantifying comparisons of threshold resummations

We explore similarities and differences between widely-used threshold resummation formalisms, employing electroweak boson production as an instructive example. Resummations based on both full QCD and soft-collinear effective theory (SCET) share common underlying factorizations and resulting evolution equations. The formalisms differ primarily in their choices of boundary conditions for evolution, in moment space for many treatments based on full QCD, and in momentum space for treatments based on soft-collinear effective theory. At the level of factorized hadronic cross sections, these choices lead to quite different expressions. Nevertheless, we can identify a natural expansion for parton luminosity functions, in which SCET and full QCD resummations agree for the first term, and for which subsequent terms provide differences that are small in most cases. We also clarify the roles of the non-leading resummation constants in the two formalisms, and observe a relationship of the QCD resummation function D(α s ) to the web expansion.

TtH anomalous coupling in double Higgs production

We study the effects of top-Higgs anomalous coupling in the production of a pair of Higgs boson via gluon fusion at the Large Hadron Collider (LHC). The introduction of anomalous ttH coupling can alter the hadronic double Higgs boson cross section and can lead to characteristic changes in certain kinematic distributions. We perform a global analysis based on available LHC data on the Higgs to constrain the parameters of ttH anomalous coupling. Possible overlap of the predictions due to anomalous ttH coupling with those due to anomalous trilinear Higgs coupling is also studied. We briefly discuss the effect of the anomalous ttH coupling on the HZ production via gluon fusion which is one of the main backgrounds in the HH → γγb $ \overline{b} $ channel.

Higher-order electroweak corrections to the partial widths and branching ratios of the Z boson

Recently, the calculation of fermionic electroweak two-loop corrections to the total width of the Z boson and hadronic Z-peak cross-section in the Standard Model has been presented, where “fermionic” refers to diagrams with closed fermion loops. Here, these results are complemented by presenting contributions of the same order for the Z-boson partial widths, which are the last missing pieces for a complete description of Z-pole physics at the fermionic two-loop order. The definition of the relevant observables and the calculational techniques are described in detail. Numerical results are presented conveniently in terms of simple parametrization formulae. Finally, the remaining theoretical uncertainties from missing higher-order corrections are analyzed and found to be small compared to the current experimental errors.

Reviewing recent results from Pierre Auger Observatory

We present some results from the last three years of operation of the Pierre Auger Observatory. This is a short version of the review talk presented at the 18th International Symposium on Particles, Strings and Cosmology (PASCOS2012). The main topics are related to ultra-high energy cosmic rays and their interactions: energy spectrum, mass composition, neutrino spectrum and hadronic cross section at energies above 1018 eV.

Hadronic total cross sections at high energy and the QCD spectrum

We show how to obtain the leading energy dependence of hadronic total cross sections, in the framework of the nonperturbative approach to soft high-energy scattering based on Wilson-loop correlation functions, if certain nontrivial analyticity assumptions are satisfied. The total cross sections turn out to be of “Froissart” type, $ \sigma_{\mathrm{tot}}^{{\left( {hh} \right)}} $ (s) ~ B log2 s for s → ∞. We also discuss under which conditions the coefficient B is universal, i.e., independent of the hadrons involved in the scattering process. In the most natural scenarios for universality, B can be related to the stable spectrum of QCD, and is predicted to be B th ≃ 0.22 mb, in fair agreement with experimental results. If we consider, instead, the stable spectrum of the quenched (i.e., pure-gauge) theory, we obtain a quite larger value $ B_{\mathrm{th}}^{(Q) } $ ≥ 0.42 mb, suggesting (quite surprisingly) large unquenching effects due to the sea quarks.

Two-loop fermionic electroweak corrections to the Z-boson width and production rate

Improved predictions for the Z-boson decay width and the hadronic Z-peak cross-section within the Standard Model are presented, based on a complete calculation of electroweak two-loop corrections with closed fermion loops. Compared to previous partial results, the predictions for the Z width and hadronic cross-section shift by about 0.6 MeV and 0.004 nb, respectively. Compact parametrization formulae are provided, which approximate the full results to better than 4 ppm.

E+e−hadronic cross section and the muong− 2

After a brief introduction to the muon anomalous magnetic moment, the current status of the ( g − 2) µ measurement is given. The three standard deviations of discrepancy between a µ calculations and the direct measurement by the E821 experiment at the Brookhaven National Laboratory (USA) calls for more precise measurements of the e + e − hadronic cross section. This article contains a not inclusive review of the main results and the state of the art on the hadronic cross section measurements from BABAR and other experiments.

Hadron production ine+e−collisions atBABARand implications on the muon anomalous magnetic moment

The BABAR Collaboration has an intensive program of studying hadronic cross sections at low-energy e+e− collisions, accessible via initial-state radiation. Our measurements allow significant improvements in the precision of the predicted value of the muon anomalous magnetic moment. These improvements are necessary for shedding light on the current ∼ 3.5σ difference between the predicted and the experimental values. We have published results on a number of processes with two to six hadrons in the final state. We report here the results of recent studies with the final states which constitute the main contribution to the hadronic cross section in the energy region between 1 and 3 GeV, as e+e− → K+K−, e+e− → KS0KL0 and e+e− → 4 hadrons.

Testing the Standard Model at the precision frontier: the anomalous magnetic moment of the muon

The anomalous magnetic moment of the muon (g − 2)μ is one of the most precisely measured quantities in particle physics (0.54 ppm). There is a long-standing discrepancy of 3-4 standard deviations between the direct measurement of (g − 2)μ and its theoretical evaluation. This theoretical prediction is subdivided into three contributions: QED, weak and hadronic. The QED and weak parts can be determined in perturbative approaches with very high precision. Thus, the hadronic uncertainty dominates the total theoretical uncer- tainty. Within the hadronic uncertainty, the largest contribution stems from the vacuum polarization term, which can be evaluated with the measurement of the inclusive hadronic cross section in e + e − annihilation. The second largest contribution to the hadronic uncer- tainty stems from the so-called Light-by-Light amplitudes. They have to be evaluated via theoretical models. These models require transition form factor measurements as input. Existing and future measurements of the relevant hadronic cross sections and transition form factors are presented.

Precise measurements of the hadronic cross sections with the CMD-3 and SND detectors at the VEPP-2000e+e−collider

It’s good known the difference in 3.5 sigma between the muon (g – 2) experimental result and its calculation in Standard Model. To clarify this intriguing discrepancy the new stage of muon (g – 2) measurement is under preparation at Fermi National Laboratory. On other hand, regular data taking for measurements of hadron cross section production in e + e − annihilations is under way since December 2010 at the electron-positron collider VEPP-2000 with the CMD-3 and SND detectors. The collected data sample corresponds to about 60 inverse picobarns of integrated luminosity per detector in the energy range from 0.32 up to 2 GeV. Preliminary analysis of various hadronic cross sections has been performed.

HADRONIC CROSS SECTION MEASUREMENT AT BES-III

Measurements of the R ratio are closely related to two pre quantities, the anomalous magnetic moment of the muon (g - 2), and the value of the electromagnetic fine structure constant [Formula: see text]. Hadronic contributions to both quantities can be derived via dispersion integrals, using experimental R data as input. For the phenomenological evaluations of these contributions, different energy ranges of hadronic cross section data are required. At BES-III, Initial State Radiation (ISR) from an existing Ψ(3770) dataset will be used for measurements of hadronic cross sections below [Formula: see text], while for higher energies a dedicated energy scan program will be performed up to [Formula: see text].

STATUS OF MONTE CARLO GENERATORS FOR ENERGY SCAN AND RADIATIVE RETURN

A short description of generators used in low energy experiments measuring the hadronic cross section with scan and radiative return methods is presented.

MEASUREMENT OF THE HADRONIC CROSS SECTION AT KLOE/KLOE-2

The KLOE experiment at the DAΦNE ϕ-factory in Frascati was the first to use Initial State Radiation to precisely determine the e+e− → π+π−(γ) cross section below 1 GeV. This measurement is particularly important to test the Standard Model calculation for the anomalous magnetic moment of the muon, (g-2), where a long standing 3σ discrepancy between measurement and theory is observed. In 2005 and 2008 KLOE published two measurements of the π+π− cross section with the photon emitted at small angle. In 2011 an independent measurement of the π+π− cross section with the photon emitted at large angle was published using data at a collision energy of 1 GeV, 20 MeV below the ϕ-peak. A new analysis, published in 2013, derives the pion form factor directly from the bin-by-bin π+π−γ/μ+μ−γ ratio significantly reducing the systematic uncertainty, while the previous measurements were normalized to the DAΦNE luminosity using large angle Bhabha scattering. We present the final result of this measurement, as well as a preliminary value for the combined result of all KLOE hadronic cross section measurements, and evaluate the hadronic contribution to the muon anomaly.

HADRONIC VACUUM POLARISATION IN g - 2 AND αQED

The status of the hadronic vacuum polarisation contributions to g - 2 of the muon and to the running QED coupling is briefly reviewed. The role of recent hadronic cross section measurements and remaining puzzles are discussed.

HADRONIC CROSS SECTION MEASUREMENTS AT SND

The preliminary results on multihadron processes obtained with the SND detector at the e+e− collider VEPP-2000 in the energy range from 1 to 2 GeV are presented. The results are interesting in connection with their contribution to the total hadronic cross section, the muon g-2 and a possibility to study excited vector mesons properties. The nucleon anti-nucleon production was also studied.

Nonlinear effects and the behavior of total hadronic and photonic cross sections

In this work we use an eikonalized minijet model where the effects of the first nonlinear corrections to the DGLAP equations are taken into account. The contributions coming from gluon recombination effects are included in the DGLAP+GLRMQ approach for the free proton in the context of saturation models. The parameters of the model are fixed to fit total $pp$ and $\bar p p$ cross sections, including the very recent data from LHC, HiRes and P. Auger Collaborations. Glauber and multiple scattering approximations are then used to describe the inclusive inelastic proton-Air cross section. Photoproduction cross sections, without change of parameters fixed before, are also obtained from the model using Vector Meson Dominance and the Additive Quark Model. We show and discuss our main results as well as the implications of saturation effects in the behavior of total hadronic and photonic cross sections at very high energies.

Strange freezeout

We argue that known systematics of hadron cross sections may cause different particles to freeze out of the fireball produced in heavy-ion collisions at different times. We find that a simple model with two freezeout points is a better description of data than that with a single freezeout, while still remaining predictive. The resulting fits seem to present constraints on the late stage evolution of the fireball, including the tantalizing possibility that the QCD chiral transition influences the yields at S=2700GeV and the QCD critical point those at S=17.3GeV.

Shear viscosity of hadrons withK-matrix cross sections

Shear viscosity \eta and entropy density s of a hadronic resonance gas are calculated using the Chapman-Enskog and virial expansion methods using the K-matrix parametrization of hadronic cross sections which preserves the unitarity of the T -matrix. In the \pi-K-N-\eta- mixture considered, a total of 57 resonances up to 2 GeV were included. Comparisons are also made to results with other hadronic cross sections such as the Breit-Wigner (BW) and, where available, experimental phase shift parametrizations. Hadronic interactions forming resonances are shown to decrease the shear viscosity and increase the entropy density leading to a substantial reduction of \eta/s as the QCD phase transition temperature is approached.

A study on analytic parametrizations for proton–proton cross-sections and asymptotia

A comparative study on some representative parametrizations for the total and elastic cross-sections as a function of energy is presented. The dataset comprises pp and scattering in the c.m. energy interval 5 GeV–8 TeV. The parametrization for the total cross-section at low and intermediate energies follows the usual reggeonic structure (non-degenerate trajectories). For the leading high-energy pomeron contribution, we consider three distinct analytic parametrizations: either a power (P) law, or a log-squared (L2) law or a log-raised-to-γ (Lγ) law, where the exponent γ is treated as a real free fit parameter. The parametrizations are also extended to fit the elastic (integrated) cross-section data in the same energy interval. Our main conclusions are the following: (1) the data reductions with the logarithmic laws show strong dependence on the unknown energy scale involved, which is treated here either as a free parameter or fixed at the energy threshold; (2) the fit results with the P law, the L2 law (free scale) and the Lγ law (fixed scale and exponent γ above 2) are all consistent within their uncertainties and with the experimental data up to 7 TeV, but they partially underestimate the high-precision TOTEM measurement at 8 TeV; (3) once compared with these results, the L2 law with fixed scale is less consistent with the data and, in the case of a free scale, this pomeron contribution decreases as the energy increases below the scale factor (which lies above the energy cutoff); (4) in all cases investigated, the predictions for the asymptotic ratio between the elastic and total cross-sections, within the uncertainties, do not exceed the value 0.430 (therefore, below the black-disc limit) and the results favor rational limits between 1/3 and 2/5. We are led to conclude that the rise of the hadronic cross-sections at the highest energies still constitutes an open problem, demanding further and detailed investigation.

Low-Energy e+e− Hadronic Annihilation Cross Sections

The past decade has seen very significant progress in the measurement of the cross section for the annihilation process e+e−→hadrons. Different experimental approaches have helped achieve a more complete and fairly consistent picture. I provide a short review of the procedures used to obtain a continuous description of cross sections combining all available results from experiments. These data are necessary input to calculations of hadronic vacuum polarization effects that use dispersion integrals in an energy regime in which perturbative QCD cannot be applied. Applications include the hadronic contribution to the muon magnetic anomaly prediction in the Standard Model and the running of the electromagnetic coupling through the scale dependence of α(s), particularly its value at the Z boson mass, which is necessary for electroweak precision tests. Another application is for QCD studies that aim to accurately determine the strong coupling and its running, αS(s), and the heavy quark masses.