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Studies of Hadronization Mechanisms Using Pion Electroproduction in Deep Inelastic Scattering from Nuclei

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Studies of Hadronization Mechanisms Using Pion Electroproduction in Deep Inelastic Scattering from Nuclei Studies of Hadronization Mechanisms using Pion Electroproduction in Deep Inelastic Scattering from Nuclei Will Brooks, Hayk Hakobyan, Cristian Peña, Miguel Arratia Universidad Técnica Federico Santa María Valparaíso, Chile for the CLAS collaboration related posters: π0, T. Mineeva; Σ-, G. & I. Niculescu • Goal: study space-time properties of parton propagation and fragmentation in QCD: • Characteristic timescales • Hadronization mechanisms • Partonic energy loss • Use nuclei as spatial filters with known properties: • size, density, interactions • Unique kinematic window at low energies Comparison of Parton Propagation in Three Processes DIS D-Y RHI Collisions Accardi, Arleo, Brooks, d'Enterria, Muccifora Riv.Nuovo Cim.032:439­553,2010 [arXiv:0907.3534] Majumder, van Leuween, Prog. Part. Nucl. Phys. A66:41, 2011, arXiv:1002.2206 [hep- ph] Deep Inelastic Scattering - Vacuum tp h tf production time tp - propagating quark h formation time tf - dipole grows to hadron partonic energy loss - dE/dx via gluon radiation in vacuum Low-Energy DIS in Cold Nuclear Medium Partonic multiple scattering: medium-stimulated gluon emission, broadened pT Low-Energy DIS in Cold Nuclear Medium Partonic multiple scattering: medium-stimulated gluon emission, broadened pT Hadron forms outside the medium; or.... Low-Energy DIS in Cold Nuclear Medium Hadron can form inside the medium; then also have prehadron/hadron interaction Low-Energy DIS in Cold Nuclear Medium Hadron can form inside the medium; then also have prehadron/hadron interaction Amplitudes for hadronization inside and outside the medium can interfere Comparison of pT broadening data - Drell-Yan and DIS ) 0.22 2 + - + 0.2 HERMES ! , ! , K CLAS/JLab preliminary !+ 0.18 E772 0.16 E866 0.14 0.12 0.1 0.08 0.06 0.04 0.02 Transverse Momentum Broadening (GeV 0 0 50 100 150 200 250 Mass number (JLab/HERMES data shifted for better view) • New, high-precision data with identified hadrons! + 2 • CLAS π : 81 four-dimensional bins in Q , ν, z h, and A New: dependence of pT broadening on Feynman x z Pb xF CLAS preliminary • xF and zh are partially correlated π π • Feynman x is the fraction pL/max{ pL} in the γ*-N CM system • Separate current (xF>0) and target (xF<0) fragmentation • First observation that pT broadening originates in both regimes New: dependence of pT broadening on φpq lead iron carbon CLAS preliminary curves shown contain terms in cos(φpq) and cos(2φpq) only statistical uncertainties shown • Expectation within classical picture: any distribution seen in carbon will become more ‘washed out’ in heavier nuclei • Not seen! first observation of quantum effect in pT broadening - persistent effect in all bins; more study needed to quantify uncertainties New: dependence of pT broadening on W h Entries 9 Mean 2.43 3.4< <4.0 0.5<Z <0.6 RMS 0.234 # "+ 2 T p ! CLAS preliminary 0.05 0.04 lead 0.03 iron 0.02 carbon 0.01 0 2 2.2 2.4 2.6 2.8 3 W • Some variations, but no systematic evidence for influence of resonance region Production Time Extraction - Geometrical Effects Lp5 1/3Entries 10 Mean 4.759 Quark Path Length * Nuclear Density vs. A RMS 1.253 Lp=20 fm 0.6 0.5 0.4 Lp=5 fm 0.3 Lp=3 fm 0.2 Lp=2 fm Quark Path Length * Nuclear Density 0.1 Lp=1 fm Lp=0.5 fm 0 0 1 2 3 4 5 6 7 8 9 10 Mass number to the 1/3 power (A1/3) Production Time Extraction - Geometrical Effects Lp5 1/3Entries 10 Mean 4.759 Quark Path Length * Nuclear Density vs. A RMS 1.253 Lp=20 fm 0.6 0.5 0.4 Lp=5 fm 0.3 Lp=3 fm 0.2 Lp=2 fm Quark Path Length * Nuclear Density 0.1 Lp=1 fm Lp=0.5 fm 0 0 1 2 3 4 5 6 7 8 9 10 Mass number to the 1/3 power (A1/3) Production Time Extraction - Geometrical Effects Lp5 1/3Entries 10 Mean 4.759 Quark Path Length * Nuclear Density vs. A RMS 1.253 Lp=20 fm 0.6 0.5 0.4 Lp=5 fm 0.3 Lp=3 fm 0.2 Lp=2 fm Quark Path Length * Nuclear Density 0.1 Lp=1 fm Lp=0.5 fm 0 0 1 2 3 4 5 6 7 8 9 10 Mass number to the 1/3 power (A1/3) Production Time Extraction - Geometrical Effects Lp5 1/3Entries 10 Mean 4.759 Quark Path Length * Nuclear Density vs. A RMS 1.253 Lp=20 fm 0.6 0.5 0.4 Lp=5 fm 0.3 Lp=3 fm 0.2 Lp=2 fm Quark Path Length * Nuclear Density 0.1 Lp=1 fm Lp=0.5 fm 0 0 1 2 3 4 5 6 7 8 9 10 Mass number to the 1/3 power (A1/3) Production Time Extraction - Geometrical Effects Fits of Z-Scaled Broadening vs. A1/3 0.35 Q2=3 GeV2, =3.5 GeV, Z =0.45 i / Q2=3 GeV2, =3.5 GeV, Z =0.55 0.3 i / 2 2 ) 2 Q =3 GeV , =3.5 GeV, Z =0.65 i / 0.25 0.2 0.15 Lp5 1/3Entries 10 Mean 4.759 Quark Path Length * Nuclear Density vs. A RMS 1.253 0.1 -Scaled Broadening (GeV Lp=20 / fm 0.6 Z 0.05 JLab/CLAS preliminary 0.5 0 2 2.5 3 3.5 4 4.5 5 5.5 6 Mass Number to the 1/3 power (A1/3) 0.4 Lp=5 fm 0.3 Lp=3 fm 0.2 Lp=2 fm Quark Path Length * Nuclear Density 0.1 Lp=1 fm Lp=0.5 fm 0 0 1 2 3 4 5 6 7 8 9 10 Mass number to the 1/3 power (A1/3) Geometrical model Geometrical model • Three-parameter model: • scale factor (~proportional to transport coefficient) • production time (distributed exponentially) • effective absorption cross section Geometrical model • Three-parameter model: • scale factor (~proportional to transport coefficient) • production time (distributed exponentially) • effective absorption cross section • Fourth parameter: quark dE/dx, recently explored Geometrical model • Three-parameter model: • scale factor (~proportional to transport coefficient) • production time (distributed exponentially) • effective absorption cross section • Fourth parameter: quark dE/dx, recently explored • Simultaneous fit of pT broadening and multiplicity ratio in the same 3-fold kinematic bin in Q2, ν, and z Geometrical model • Three-parameter model: • scale factor (~proportional to transport coefficient) • production time (distributed exponentially) • effective absorption cross section • Fourth parameter: quark dE/dx, recently explored • Simultaneous fit of pT broadening and multiplicity ratio in the same 3-fold kinematic bin in Q2, ν, and z • Realistic nuclear densities • Path begins at point with probability proportional to density • Part of path is quark, part of path is (pre-) hadron Results from combined fit 3 or 4 parameter geometric model various bins in Q2, ν, and z fits are consistent with τp=1.6 - 2 fm/c (<Rcarbon/c) C Fe Pb 2 1/3 π+ 1/3 ∆kT vs. A RM vs. A 2 1/3 π+ 1/3 ∆kT vs. A RM vs. A 2 1/3 π+ 1/3 ∆kT vs. A RM vs. A New: exploring a direct measurement of quark energy loss using π+ energy spectrum • Order of magnitude can be estimated from theory as ~100 MeV/fm, although factors of up to 10 are disputed • Measurements of energy loss in cold nuclear matter have been attempted with 800 GeV protons • 6 fm * 100 MeV/fm = 600 MeV ...not easy to see with Drell-Yan muon pairs from 800 GeV protons on tungsten; but should be ‘easy’ to see with 2 GeV pions produced in DIS How to directly measure quark energy loss? • Energy loss is predicted on very solid grounds to be independent of energy for a medium that is thin enough. • “Thin enough” depends on energy, see next slide; if medium is thicker than “thin enough” it still loses energy • If the energy loss is independent of energy, it will produce a shift of the energy spectrum, for higher energies. • We can look for a shift of the Pb energy spectrum compared to that of the deuterium energy spectrum shift Pion energy Energy Loss in pQCD (BDMPS version) dE ΔE L L < Critical Lqˆ − dx ∝ dE L L > Critical Eqˆ − dx ∝ L LCritical Eq at L = LCritical, Lˆq Eq ˆq ; L Critical ∝ · ∝ ˆq E ν few GeV, ˆq 0.02 0.1GeV2/fm, q ≈ ≈ ≈ − E q (multiplied by an Rlead Rcarbon −→ ˆq ≈ − unknown prefactor) Full lineshapes, deuterium and Pb (CLAS EG2 data) with and without cut on xFeynman (normalized to area 1.0) Nu*Zh+0. {Nu*Zh>0.&&Nu*Zh<2.5-0.&&TargType==2} pb1pbd1d Entries 318125833457619043321 744673 Mean 0.67490.8025 1.3021.325 RMS 0.52940.47230.50120.5032 0.05 shift 0.04 Pion energy 0.03 with cut on xFeynman 0.02 0.01 0 0 0.5 1 1.5 2 2.5 Epion (GeV) Minimize RMS of shifted fitted spectra CLAS Preliminary Minimum RMS Pb = 0.42 MeV at ΔE~575 MeV Pb D CLAS Preliminary Fe Minimum RMS = 0.52 MeV at ΔE~405 MeV CLAS Preliminary CLAS Preliminary C Minimum RMS Resulting ΔE consistent with independently = 0.98 MeV at determined values of pT broadening! ΔE~120 MeV Conclusions Conclusions • systematic features of π+ pT broadening have been explored, such as xF dependence and W dependence. The xF dependence indicates broadening comes from both current and target fragmentation for z<0.5 Conclusions • systematic features of π+ pT broadening have been explored, such as xF dependence and W dependence.
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