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DOE OSTI2026

Gradient Flow for Parton Distribution Functions: First Application to the Pion

Parton distribution functions (PDFs) are central to precision QCD phenomenology. Their Mellin moments can be computed on the lattice, but direct determinations using local operators, besides ⟨𝑥⟩, face severe challenges from reduced hypercubic symmetry, limiting results to the lowest moments. A recently proposed method resolves these issues using gradient flow. We demonstrate the efficacy of this method by computing ratios of flavor nonsinglet pion PDF moments up to ⟨𝑥 5 ⟩ on four lattice spacings at 𝑚 𝜋 ≃ 411 MeV. The moments and reconstructed PDF agree quantitatively with recent phenomenological extractions.

Francis, Anthony [National Yang Ming Chiao Tung Un↗

DOE OSTI2020

Precision measurement of the neutral pion lifetime

The explicit breaking of the axial symmetry by quantum fluctuations gives rise to the so-called axial anomaly. This phenomenon is solely responsible for the decay of the neutral pion π0 into two photons (γγ), leading to its unusually short lifetime. Here, we precisely measured the decay width Γ of the π 0 → γγ process. The differential cross sections for π0 photoproduction at forward angles were measured on two targets, carbon-12 and silicon-28, yielding Γ(π 0 → γγ)=7.798±0.056(stat.)±0.109(syst.) eV, where stat. denotes the statistical uncertainty and syst. the systematic uncertainty. We combined the results of this and an earlier experiment to generate a weighted average of Γ(π 0 → γγ)=7.802±0.052(stat.)±0.105(syst.) eV. Our final result has a total uncertainty of 1.50% and confirms the prediction based on the chiral anomaly in quantum chromodynamics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2016

Search for light neutron-rich isotopes in stopped pion absorption

The results based on the spectroscopy of superheavy hydrogen isotopes ({sup 4−7}H), heavy helium isotopes ({sup 6,7}He), and heavy lithium isotopes ({sup 7−12}Li) produced in stopped pion absorption by light nuclei were analyzed. Search for nuclear states was performed in inclusive and correlation measurements of missing mass spectra. A broad range of excitation energies studied in correlation measurements provided the possibility of search for isobaric analog states and cluster resonances. A comparison with experimental and theoretical results of other authors was conducted.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2015

Eta photoproduction in a combined analysis of pion- and photon-induced reactions

The $\eta N$ final state is isospin-selective and thus provides access to the spectrum of excited nucleons without being affected by excited $\Delta$ states. To this end, the world database on eta photoproduction off the proton up to a center-of-mass energy of $E\sim 2.3$ GeV is analyzed, including data on differential cross sections, and single and double polarization observables. The resonance spectrum and its properties are determined in a combined analysis of eta and pion photoproduction off the proton together with the reactions $\pi N\to \pi N$, $\eta N$, $K\Lambda$ and $K\Sigma$. For the analysis, the so-called J\"ulich coupled-channel framework is used, incorporating unitarity, analyticity, and effective three-body channels. Parameters tied to photoproduction and hadronic interactions are varied simultaneously. Furthermore, the influence of recent MAMI $T$ and $F$ asymmetry data on the eta photoproduction amplitude is discussed in detail.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2019

Spectral functions of nucleon form factors: Three-pion continua at low energies

We study the imaginary parts of the isoscalar electromagnetic and isovector axial form factors of the nucleon close to the 3 pi-threshold in covariant baryon chiral perturbation theory. At the two-loop level, the contributions arising from leading and next-to-leading order chiral pi N-vertices, as well as pion-induced excitations of virtual Delta(1232)-isobars, are calculated. It is found that the heavy baryon treatment overestimates substantially these 3 pi-continua. From a phenomenological analysis, that includes the narrow omega(783)-resonance or the broad a 1 -resonance, one can recognize small windows near threshold, where chiral 3 pi-dynamics prevails. However, in the case of the isoscalar electromagnetic form factors G$s\atop{E,M}$(t), the radiative correction provided by the pi 0 gamma-intermediate state turns out to be of similar size.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2014

Study of the production of charged pions, kaons, and protons in pPb collisions at $$\sqrt{s_{NN}} =\; $$ s N N = 5.02 $$\,\text {TeV}$$ TeV

Spectra of identified charged hadrons are measured in pPb collisions with the CMS detector at the LHC at sqrt(sNN) = 5.02 TeV. Charged pions, kaons, and protons in the transverse-momentum range pt approximately 0.1-1.7 GeV and laboratory rapidity abs(y) < 1 are identified via their energy loss in the silicon tracker. The average pt increases with particle mass and the charged multiplicity of the event. The increase of the average pt with charged multiplicity is greater for heavier hadrons. Comparisons to Monte Carlo event generators reveal that EPOS LHC, which incorporates additional hydrodynamic evolution of the created system, is able to reproduce most of the data features, unlike HIJING and AMPT. The pt spectra and integrated yields are also compared to those measured in pp and PbPb collisions at various energies. The average transverse momentum and particle ratio measurements indicate that particle production at LHC energies is strongly correlated with event particle multiplicity.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2015

Measurement of pion, kaon and proton production in proton–proton collisions at $\sqrt{s} = 7 TeV$

The measurement of primary π ± , K ± , p and $\bar{p}$ production at mid-rapidity (|y|< 0.5) in proton–proton collisions at $\sqrt{s}$ = 7 TeV performed with a large ion collider experiment at the large hadron collider (LHC) is reported. Particle identification is performed using the specific ionisation energy-loss and time-of-flight information, the ring-imaging Cherenkov technique and the kink-topology identification of weak decays of charged kaons. Transverse momentum spectra are measured from 0.1 up to 3 GeV/c for pions, from 0.2 up to 6 GeV/c for kaons and from 0.3 up to 6 GeV/c for protons. The measured spectra and particle ratios are compared with quantum chromodynamics-inspired models, tuned to reproduce also the earlier measurements performed at the LHC. Lastly, the integrated particle yields and ratios as well as the average transverse momenta are compared with results at lower collision energies.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2018

Neutral pion and $$\eta $$η meson production in p–Pb collisions at $$\sqrt{s_{\mathrm{NN}}}$$sNN = 5.02 TeV

Neutral pion and η meson invariant differential yields were measured in non-single diffractive p–Pb collisions at $ \sqrt{s}=5.02 $ TeV with the ALICE experiment at the CERN LHC. The analysis combines results from three complementary photon measurements, utilizing the PHOS and EMCal calorimeters and the Photon Conversion Method. The invariant differential yields of π0 and η meson inclusive production are measured near mid-rapidity in a broad transverse momentum range of 0.3 4 GeV / c at 0.483 ± 0. 015 stat± 0. 015 sys. A deviation from mT scaling is observed for pT< 2 GeV / c. The measured η/ π 0 ratio is consistent with previous measurements from proton-nucleus and pp collisions over the full pT range. The measured η/ π 0 ratio at high pT also agrees within uncertainties with measurements from nucleus–nucleus collisions. The π 0 and η yields in p–Pb relative to the scaled pp interpolated reference, RpPb, are presented for 0.3 < pT< 20 GeV / c and 0.7 < pT< 20 GeV / c, respectively. The results are compared with theoretical model calculations. The values of RpPb are consistent with unity for transverse momenta above 2 GeV / c. These results support the interpretation that the suppressed yield of neutral mesons measured in Pb–Pb collisions at LHC energies is due to parton energy loss in the hot QCD medium.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

Two-pion femtoscopic correlations in Be+Be collisions at $\sqrt{s_{\text {NN}}} = 16.84$ GeV measured by the NA61/SHINE at CERN

This paper reports measurements of two-pion femtoscopic correlations in Be+Be collisions at a beam momentum of 150$A\,\hbox {GeV}\!/\!c$ (energy available in the center-of-mass system for nucleon pair $\sqrt{s_{\text {NN}}} = 16.84$ GeV) by the NA61/SHINE experiment at the CERN SPS accelerator. The obtained momentum space correlation functions can be well described by a Lévy distributed source model. The transverse mass dependence of the Lévy source parameters is presented, and their possible theoretical interpretations are discussed. The results show that the Lévy exponent $\alpha $ is approximately constant as a function of $m_{\text {T}}$ , and far from both the Gaussian case of $\alpha = 2$ or the conjectured value at the critical endpoint, $\alpha = 0.5$. The radius scale parameter R shows a slight decrease in $m_{\text {T}}$ , which can be explained as a signature of transverse flow. Finally, an approximately constant trend of the intercept parameter $\lambda $ as a function of $m_{\text {T}}$ was observed, similar to previous NA44 S + Pb results (obtained with a Gaussian approximation, but unlike RHIC results).

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2021

Entanglement minimization in hadronic scattering with pions

Recent work [S. R. Beane, D. B. Kaplan, N. Klco and M. J. Savage, Phys. Rev. Lett. 122, 102001 (2019), arXiv:1812.03138.] conjectured that entanglement is minimized in low-energy hadronic scattering processes. It was shown that the minimization of the entanglement power (EP) of the low-energy baryon–baryon [Formula: see text]-matrix implies novel spin–flavor symmetries that are distinct from large-[Formula: see text] QCD predictions and are confirmed by high-precision lattice QCD simulations. Here, the conjecture of minimal entanglement is investigated for scattering processes involving pions and nucleons. The EP of the [Formula: see text]-matrix is constructed for the [Formula: see text] and [Formula: see text] systems, and the consequences of minimization of entanglement are discussed and compared with large-[Formula: see text] QCD expectations.

Physics↗

DOE OSTI2013

On the potential role of the Collins effect in A_N in pp --> pion X

Transverse single spin asymmetries in pp --> pion X processes, while on a quite firm ground experimentally, are still a much debated phenomenological issue. We consider them in a transverse momentum dependent factorization scheme. After revisiting a previous result, we give new estimates of the Collins contribution by adopting the latest information on the Collins and transversity functions, as extracted from SIDIS and e+e- data.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2022

Measurement of Transverse Spin Dependent Azimuthal Correlations of Charged Pion(s) in $p^{\uparrow} p$ Collisions at $\sqrt s = 200$ GeV at STAR

At the leading twist, the transversity distribution function, h^{q}_{1}(x) h 1 q ( x ) , where x x is the longitudinal momentum fraction of the proton carried by quark q q , encodes the transverse spin structure of the nucleon. Extraction of it is difficult because of its chiral-odd nature. In transversely polarized proton-proton collisions ( p^\uparrow p p ↑ p ), h_{1}^{q}(x) h 1 q ( x ) can be coupled with another chiral-odd partner, a spin-dependent fragmentation function (FF). The resulting asymmetries in hadron(s) azimuthal correlations directly probe h_{1}^{q}(x) h 1 q ( x ) . We report the measurement of correlation asymmetries for charged pion(s) in p^\uparrow p p ↑ p , through the Collins and the Interference FF channel.

Pokhrel, Babu↗

DOE OSTI2022

Multidimensional, high precision beam spin asymmetry measurements of semi-inclusive pion production off the proton with CLAS12

High precision measurements of the polarized electron beam-spin asymmetry in SIDIS from the proton have been performed using a 10.6 GeV incident electron beam and the CLAS12 spectrometer at Jefferson Lab. The proceeding reports a multidimensional study of the structure function ratio F^{sin\phi}_{LU}/F_{UU} F L U s i n ϕ / F U U extracted from single pion ( \pi^{+} π + , \pi^{-} π − , \pi^{0} π 0 ) SIDIS data over a large kinematic range in z z , x_{B} x B , P_{T} P T and virtualities Q^{2} Q 2 ranging from 1~GeV ^2 2 up to 7~GeV ^2 2 . F^{sin\phi}_{LU} F L U s i n ϕ is a twist-3 quantity that can reveal novel properties of quark-gluon correlations within the nucleon.

Diehl, Stefan↗

DOE OSTI2014

Measurement of Charged Pions from Neutrino-produced Nuclear Resonance

A method for identifying stopped pions in a high-resolution scintillator bar detector is presented. I apply my technique to measure the axial mass M Δ A for production of the Δ(1232) resonance by neutrino, with the result M Δ A = 1.16±0.20 GeV (68% CL) (limited by statistics). The result is produced from the measured spectrum of reconstructed momentum-transfer Q 2 . I proceed by varying the value of M Δ A in a Rein-Sehgal-based Monte Carlo to produce the best agreement, using shape only (not normalization). The consistency of this result with recent reanalyses of previous bubble-chamber experiments is discussed.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2015

Beam Spin Asymmetry Measurements for Two Pion Photoproduction at CLAS

The overarching goal of this analysis, and many like it, is to develop our understanding of the strong force interactions within the nucleon by examining the nature of their excitation spectra. As the resonances of these spectra have very short lifetimes (tau = 1x10 -23 s) and often have very similar masses, it is often impossible to directly observe resonances in the excitation spectra of nucleons. Polarization observables allow us to study the resonances by looking at how they affect the spin state of final state particles. The beam asymmetry is a polarization observable that allows us to detect the sensitivity of these resonances, and other transition mechanisms, to the electric vector orientation of incident photons. Presented in this thesis are first measurements of the beam asymmetries in the resonant region for the reaction channel pgamma p --> p π + π - focusing on the intermediate mesonic states rho^0 and f^0, and the final state pions. The analysis used data from the g8b experiment undertaken at the Thomas Jefferson National Accelerator Facility (JLab), the first experiment at JLab to use a linearly polarized photon beam. Using the coherent Bremsstrahlung facility and the CLAS detector of Hall B at JLab allowed for many multi-channel reactions to be detected and the first measurements of many polarization observables including those presented here. A brief overview of the theoretical framework used to undertake this analysis is given, followed by a description of the experimental details of the facilities used, then a description of the calibration of the Bremsstrahlung tagging facility which the author undertook, and finally the analysis is presented and the resulting measurements.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2016

Electroproduction of the neutral pion off 4He

Deeply virtual exclusive processes offer a unique opportunity to study the internal structure of the nucleon and nuclei. The goal of this work is to extract the beam-spin asymmetry in deeply virtual coherent neutral pion electroproduction, e^4He to e'^4He'pi^0, using the CLAS detector in the experimental Hall B at Thomas Jefferson National Accelerator Facility. The data were collected in 2009 with a 6 GeV longitudinally polarized electron beam impinging on a 30 cm long, 6 atm Helium-4 gaseous target. In order to ensure that the process is coherent, a new Radial Time Projection Chamber was used to detect and identify low energy recoil a-particles. The Beam Spin Asymmetry in the coherent deep exclusive regime was measured at Q^2 = 1.50 GeV^2, xB = 0.18 and -t = 0.14 GeV^2. The measured asymmetry has an amplitude of 10%+/-5% and has the opposite sign compared the asymmetry measured for pi^0 production on the proton.

Torayev, Bayram↗

DOE OSTI2015

Exploring the Structure of the Proton via Semi-inclusive Pion Electroproduction

Measurements of the multiplicity, $cos \phi_h$ moment, and $cos 2\phi_h$ moment of the semi-inclusive deep inelastic scattering (SIDIS) cross-section have been performed. The data set used was the E1-F run from the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson National Lab. The run used a 5.498 GeV longitudinally polarized electron beam and an unpolarized liquid hydrogen target. Both charged pion channels were studied over a broad range of kinematics (0:1 < $x$ < 0:6, 1:0 < $Q^2$ < 4:7 GeV 2 , 0:0 < $z$ < 0:9, 0:0 < $P^2_{h\perp}$ < 1:0 GeV2, and -180 < $\phi_h$ < 180 ). These measurements give access to transverse momentum dependent parton distribution functions (TMDs) which describe the transverse motion of quarks and gluons inside of the proton. Results show evidence of a TMD known as the Boer-Mulders function being non-zero via a flavor dependence on the $cos \phi_h$ and $cos 2\phi_h$ modulations of the data. A non-zero Boer-Mulders function requires a non-zero quark orbital angular momentum ($L_q$) contribution to the proton spin, thus giving insights into the 28 year old puzzle of the missing proton spin.

43 PARTICLE ACCELERATORS↗

DOE OSTI2018

Proton and Neutral Pion Identification at ME in MINERvA-Scintillator $\nu_{\mu} + N \to \mu^- + \pi^0 + X$ (no mesons) [Slides]

Motivation: Give to MINERvA the first semi-exclusive cross-section analysis with neutral pions at medium energy; This result will have much more statistics than what was measured in the low energy beam (O. Altinok et al. Phys.Rev. D96 (2017) no.7, 072003 ); Provide constraints in the cross section in a range of energy as will be seeing for DUNE.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗