<p>We present a study of the transverse size of parton showers and their diminishing interaction with the medium in the high virtuality stage of jet evolution. We consider the process of a hard quark produced in deep inelastic scattering off a large nucleus. Single gluon radiation from this quark, in the absence of scattering, is rederived using wave packets. This allows for a derivation of the quantum uncertainty size of the hard quark, at the point of splitting. This uncertainty size is then incorporated within a Monte Carlo shower routine yielding transverse shower sizes noticeably larger than the classical antenna size of the shower. No clear relation is found between the full uncertainty size of the shower and the virtuality of the originating parton. The single gluon emission from the hard quark is then reanalyzed for the case of single rescattering off the remainder of the nucleus. A relation is derived between the jet transport coefficient <math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent="true"><mi>q</mi><mo>̂</mo></mover></math> and the gluon Transverse momentum-dependent parton distribution function (gTMDPDF). Solving this relation, for a simple case, clearly demonstrates the weakening of <math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent="true"><mi>q</mi><mo>̂</mo></mover></math> with the virtuality of the hard splitting parton.</p>

United States

Canada

Amit Kumar

Abhijit Majumder

Chathuranga Sirimanna

Japan

Yasuki Tachibana

Modified coherence and the transverse extent of jets

<p>Various azimuthal anisotropies (<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi>v</mi><mn>1</mn></msub><mo>,</mo><msub><mi>v</mi><mn>2</mn></msub><mo>,</mo><msub><mi>v</mi><mn>3</mn></msub><mo>,</mo><msub><mi>v</mi><mn>4</mn></msub></mrow></math>), at high transverse momentum (high <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi>T</mi></msub></math>), are shown to arise from the asymmetric scattering of transverse polarized quarks and gluons, arising from unpolarized nucleons (the Boer-Mulders effect) and resulting in unpolarized hadrons (the Collins effect). Combined with the asymmetric scattering of partons from polarization independent but transverse momentum dependent (TMD) distributions, we obtain a possible mechanism to understand the azimuthal anisotropy of hadrons at large transverse momentum observed in <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi><mtext>−</mtext><mi>p</mi></mrow></math> collisions. Constraining the ratio of polarization dependent TMD distributions to polarization independent distributions by comparing with the data from <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi><mtext>−</mtext><mi>p</mi></mrow></math> collisions, we find that scaling the acquired transverse momentum of the initial state partons from the proton, due to prescattering before the hard interaction, with the length of the nucleus (<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msubsup><mi>k</mi><mo>⊥</mo><mn>2</mn></msubsup><mo>∝</mo><msup><mi>A</mi><mrow><mn>1</mn><mo>/</mo><mn>3</mn></mrow></msup></mrow></math>), straightforwardly yields the azimuthal anisotropy at high <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi>T</mi></msub></math> in <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi><mtext>−</mtext><mi>A</mi></mrow></math> collisions.</p>

Finland

Ismail Soudi

<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>T</mi></math>-odd parton distribution functions and azimuthal anisotropy at high transverse momentum in <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi><mtext>−</mtext><mi>p</mi></mrow></math> and <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi><mtext>−</mtext><mi>A</mi></mrow></math> collisions

We explore the possibility that the initial transverse momentum distribution of unpolarized and polarized partons within unpolarized nucleons, both with and without the anisotropy of unpolarized hadrons produced in the fragmentation of outgoing partons, could lead to the observed azimuthal anisotropy of high transverse momentum (high-<math><msub><mrow><mi>p</mi></mrow><mrow><mi>T</mi></mrow></msub></math>) hadrons produced in high energy proton-proton (p-p) or proton-ion (p-A) collisions. Including simple Gaussian forms for transverse momentum dependent parton distribution functions (PDF) and fragmentation functions, and assuming an <math><msup><mrow><mi>A</mi></mrow><mrow><mn>1</mn><mo>/</mo><mn>3</mn></mrow></msup></math> enhancement of a PDF in p-Pb collisions, we show that the observed anisotropy, with no modification to the angle integrated spectra (<math><msub><mrow><mi>R</mi></mrow><mrow><mi>p</mi><mi>A</mi></mrow></msub><mo>=</mo><mn>1</mn></math>) for 5 GeV <math><mo>≲</mo><msub><mrow><mi>p</mi></mrow><mrow><mi>T</mi></mrow></msub><mo>≲</mo><mn>50</mn></math> GeV, can be straightforwardly understood as arising from a few processes dominated by gluon-gluon to gluon-gluon scattering.

Azimuthal anisotropy at high transverse momentum in - And - Collisions

The widespread notion that jets quenched in a quark-gluon-plasma (QGP) are similar in their parton flavor composition to jets in vacuum is critically examined. We demonstrate that while the soft to semihard (low to intermediate transverse momentum <math><msub><mi>p</mi><mi>T</mi></msub></math>) sector of vacuum jets are predominantly bosonic i.e., composed of gluons, sufficiently quenched jets can have an intermediate momentum sector that is predominantly fermionic, dominated by quarks and antiquarks. We demonstrate, using leading-order perturbative QCD processes, that the rate of flavor conversion from a gluon traversing the QGP as part of a jet, to a quark or antiquark, versus the reverse process, grows steadily with falling <math><msub><mi>p</mi><mi>T</mi></msub></math>. Simple diagrammatic estimates are followed by a variety of realistic simulations in static media, which demonstrate qualitatively similar yet quantitatively different fermion enhancements. The relation of this increase in flavor to the observed baryon enhancement at intermediate <math><msub><mi>p</mi><mi>T</mi></msub></math> is studied in a fully realistic simulation.

Gojko Vujanovic

China

Wen-Jing Xing

Shanshan Cao

Quenching jets increases their flavor

We present the first calculation of the jet transport coefficient <math><mover><mi>q</mi><mo>^</mo></mover></math> in quenched and (<math><mrow><mn>2</mn><mo>+</mo><mn>1</mn></mrow></math>)-flavor QCD on a 4D Euclidean lattice. The lightlike propagation of an energetic parton is factorized from the mean square gain in momentum transverse to the direction of propagation, which is expressed in terms of the thermal field-strength field-strength correlator. The leading-twist term in its operator product expansion is calculated on the lattice. Continuum extrapolated quenched results, and full QCD estimates based on unrenormalized lattice data, over multiple lattice sizes, are compared with (non)perturbative calculations and phenomenological extractions of <math><mover><mi>q</mi><mo>^</mo></mover></math>. The lattice data for <math><mover><mi>q</mi><mo>^</mo></mover></math> show a temperature dependence similar to the entropy density. Within uncertainties, these are consistent with phenomenological extractions, contrary to calculations using perturbation theory.

Germany

Johannes Heinrich Weber

Jet transport coefficient <math><mrow><mover><mrow><mi>q</mi></mrow><mrow><mo>^</mo></mrow></mover></mrow></math> in lattice QCD

A detailed reanalysis of the single gluon emission rate at next-to-leading twist is carried out. As was the case in prior efforts, the problem is cast in the framework of deep-inelastic scattering (DIS) of an electron off a large nucleus. The quark produced in the interaction propagates through the remaining nucleus and engenders scattering and gluon radiation, which is calculated in the limit of one rescattering. This medium induced single gluon emission rate forms the basis of several energy loss calculations in both DIS and heavy-ion collisions. Unlike prior efforts, a complete transverse momentum gradient expansion of the hadronic tensor, including <math><msub><mi>N</mi><mi>c</mi></msub></math> suppressed terms, phase terms, and finite gluon momentum fraction terms, ignored previously, is carried out. These terms turn out to be surprisingly large. In contrast to prior efforts, the full next-to-leading twist gluon emission kernel is found to be positive definite and slowly increasing with the exchanged transverse momentum. Phenomenological consequences of these new contributions are discussed.