Hadron Formation Lengths: QCD Confinement in Forming Systems

Transcription

1 Hadron Formation Lengths: QCD Confinement in Forming Systems Will Brooks Jefferson Lab Workshop on Intersections of Nuclear Physics with Neutrinos and Electrons May 5, 2006

2 Main Focus Space-time characteristics of hadronization Formation lengths, production lengths for several hadron species: How do hadrons form, and how long does it take? Quark energy loss and multiple scattering How big is the energy loss, and how does it behave? E.g., an answer for normal nuclear density and temperature might be: 100 MeV/fm at 5 fm, proportional to distance squared Numerous other studies: correlations, associated photons

3 Steps: How Does Confinement Assemble Hadrons? Step 1: Isolate correct interaction mechanisms - physical picture Models Lattice studies require technical development, not happening soon Multi-hadron hadron,, multi-variable studies needed Step 2: Extract characteristic parameters and functional dependencies ncies Length scales Flavor dependence, size dependence Energy loss Correlation functions

4 Interaction mechanisms Ubiquitous sketch of hadronization process: string/color flux tube Confinement process isn t t finished yet, these are colored pairs: How do we get to colorless systems? Is the above picture reality, or just a nice story?(!)

5 Interaction mechanisms An alternative mechanism: gluon bremsstrahlung model : RG GY All hadron color explicitly resolved in this picture Experimentally, it is completely unknown what the dominant mechanism is

6 Experimental Studies: Hadron Attenuation hadronization distance scales and reaction mechanisms from nuclear DIS We can learn about hadronization from nuclear DIS Nucleus acts as a spatial analyzer for outgoing hadronization products

7 Hadron formation lengths: order-of of-magnitude expectations Given hadron size R h, can build color field of hadron in its rest frame in time no less than t 0 ~R h /c.. In lab frame this is boosted: E t = m f R h If take, e.g., the pion mass, radius 0.66 fm, E = 4 GeV,, then t f ~ 20 fm. Slightly more sophisticated, if gluon emission time dominates 2Eh(1 z) t f, kt ΛQCD 0. 2 GeV 2 2 k + m T h If take, e.g., pion mass, z = E hadron /ν = 0.5, E h = 4 GeV,, then t f ~ 13 fm Well-matched to nuclear dimensions!

8 Time scales for hadron formation Can identify multiple time scales? E.g., isolate the production length and the formation length? Production length t p Time required to produce colorless pre-hadron or colorless dipole Signaled by medium-stimulated energy loss via gluon emission Formation length t f Time required to produce fully-developed hadron Signaled by interactions with known hadron cross sections No gluon emission It is essentially unknown what these time scales are

9 Experimental Studies: Propagation of Quarks Quarks lose energy by gluon emission as they propagate In vacuum Even more within a medium proportional to the medium s s gluon density propagating quark gluons Medium-stimulated loss Calculation by BDMPS quarks in nuclei L nucleus This can be directly measured on a series of nuclei Signature of production length t p Corresponds to a quark-gluon correlation function

10 Analogous Processes in QED and QCD Analog to multiple scattering and energy loss: quark multiple scattering and quark energy loss: Dominant term in QED: propagating quark gluons ΔE E ~ L quarks in nuclei L nucleus Dominant term in QCD: ΔE E ~ L 2?? QCD analog of LPM effect

11 Experiments Addressing the Same Topics Experimental avenues Semi-inclusive inclusive deep inelastic scattering on nuclei 1970 s s CERN EMC ea e Xh,, energy transfer ~ GeV 2000 s s HERA HERMES e + A e + Xh,, 12 and 26 GeV beam 2000 s s Jefferson Lab CLAS, ea e Xh,, 5 GeV beam Drell-Yan reaction 1980 s s CERN SPS NA spectrometer: πa Xμ + μ -, 140 and 280 GeV beam 1990 s Fermilab pa Xμ + μ -, 800 GeV beam 2000 s Fermilab pa Xμ + μ -, 120 GeV beam, waiting for funding Relativistic heavy ion reactions 2000 s s BNL RHIC AA everything, 200 GeV/u colliding beams International, multi-institutional institutional quest for 30 years, but most progress since 2000

12 Recent Workshop

13 Experimental Data CLAS (5 GeV,, 11 GeV [future]) and Hermes (12 GeV,, 27 GeV)

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15 CLAS EG2 Targets Two targets in the beam simultaneously 2 cm LD2, upstream Solid target downstream Six solid targets: -Carbon -Aluminum (2 thicknesses) -Iron -Tin -Lead Al + empty target

16 Observable Number 1 p T Broadening Definition of Transverse Momentum Broading e π + γ* p T Nucleus A e (All JLab results will be for π + )

17 p T broadening: A dependence, large ν,, mid-z, π + Transverse momentum broadening vs. nuclear radius is ~linear One plot of dozens available Saturation behavior? Can estimate production length? Clearly see the L 2 behavior! Lead nucleus Iron nucleus Carbon nucleus Preliminary JLab/CLAS/EG2 data

18 Medium-Induced Quark Energy Loss assuming perturbative formula (Dozens of plots like this) de/dx (GeV/fm) de/dx (GeV/fm) Preliminary JLab/CLAS/EG2 data Carbon nucleus Lead nucleus Struck quark energy (GeV) de/dx (GeV/fm) de/dx (GeV/fm) Iron nucleus Struck quark energy (GeV) Struck quark energy (GeV) Struck quark energy (GeV)

19 Observable Number 2 Hadronic Multiplicity Ratio Hadronic multiplicity ratio

20 Interpretation of Hadronic Multiplicity Ratio (concrete example in hadronization picture, for 4 nuclei) HERMES parameterization for pion formation length: τ = 1.4 ν 1 ( z) fm 14 N 40 Ar 84 Kr Example: z = 0.5, ν = 9 GeV, τ = 6.3 fm, ~ radius Pb 197 Au z

21 HERMES data Summary of HERMES data to date: Mostly 27 GeV positron beam, some 12 GeV beam Targets include D, He, N, Kr, Xe (Xe results not yet released) Excellent PID (RICH) except for early nitrogen targets identify p + - o, K + -, proton and antiproton Limited luminosity -> typically 1D binning, lower Q 2

22 Hermes 27 Hermes 27 GeV Data

23 CLAS 5 GeV Data: Hadronic Multiplicity Ratio vs. Z Preliminary CLAS/JLab data Hayk Hakobyan C(e,e π + ) Q 2 >1 GeV2, W>2 GeV Fe(e,e π + ) Q 2 >1 GeV2, W>2 GeV Statistical accuracy permits binning into ν, p T, and Q 2 in addition to z and A Pb(e,e π + ) Q 2 >1 GeV2, W>2 GeV

24 Examples of multi-variable slices of CLAS 5 GeV data (dozens of such plots exist for C, Fe, Pb) Preliminary CLAS/JLab data Hayk Hakobyan Z dependence Q 2 dependence Arbitrary units ν dependence p T2 dependence

25 CLAS12 (a few years from now) Preshower EC Forward Cerenkov Forward DC Forward EC Forward TOF Torus Cold Ring Inner Cerenkov Central Detector Coil Calorimeter

26 Accessible Hadrons (12 GeV)

27 12 GeV Anticipated Data Bins in yellow are accessible at 6 GeV

28 Sample of Models Gluon bremsstrahlung model Gluon radiation + hadronization model Twist-4 pqcd model Medium-induced induced gluon radiation only Rescaling models Gluon emission, partial deconfinement,, nuclear absorption PYTHIA-BUU coupled channel model Fundamental interaction + coupled channel nuclear final state interaction

29 Gluon Bremsstrahlung Model Authors B. Kopeliovich,, J. Nemchik,, E. Predazzi,, A. Hayashigaki Time and energy dependent model for energy loss by gluon emission coupled to a hadron formation scheme Gluon emission: Two time constants Q 2 dependence Hadron formation: Color dipole cross section Struck quark and nearest antiquark projected to hadronic wave function

30 Gluon Bremsstrahlung Model and HERMES Data

31 Twist-4 pqcd Model Authors: X.-N. Wang, E. Wang, X. Guo,, J. Osborne No hadronization in this picture: Hadrons form outside nucleus Energy loss from medium-stimulated gluon radiation causes nuclear attenuation Leading twist-4 4 modifications to pqcd fragmentation functions due to induced gluon radiation from multiple scattering Strength of a quark-gluon correlation function is a free parameter Other similar efforts: F. Arleo, U.A. Wiedemann

32 Twist-4 pqcd Model

33 Rescaling Models Authors: A. Accardi,, H. Pirner,, V. Muccifora Rooted in work by Nachtmann, Pirner,, Jaffe, Close, Roberts, Ross, de Deus, from 1980 s Nuclear attenuation comes from: Partial deconfinement of quarks in nucleus in combination with gluon radiation Nuclear reinteraction and absorption

34 Rescaling Model, EMC/HERMES Data

35 Rescaling Model, HERMES Flavor-separated Kr Data

36 PYTHIA-BUU Coupled Channel Model Authors: T. Falter, W. Cassing,, K. Gallmeister,, U. Mosel PYTHIA-BUU PYTHIA for e-p interaction BUU (Boltzmann( Boltzmann-Uehling-Uhlenbeck) ) coupled channel transport model for final state interactions Can describe the data without modification of fragmentation functions, hadron formation time ~0.5 fm in hadron rest frame

37 PYTHIA-BUU Coupled Channel Model and Hermes Kr Data

38 Impact of Models Summary ~4-6 6 modern models currently on the market: Varying degrees of sophistication All can fit the published data using different assumptions Need more data: Hermes heavy target data, flavor-separated hadrons, high energies CLAS lower-energy, energy, high statistics, wide range of targets, wide range of final states Need lattice calculations (~5-10 years away)

39 Conclusions Understanding how confinement acts in forming hadronic systems is a fundamental question in QCD Using the nucleus as analyzer offers a wealth of new information Hadron production/formation lengths, quark energy loss the main focus Other important connections Jet quenching at RHIC Drell-Yan at Fermilab Neutrino-nucleus nucleus interactions Understanding these very large hadron attenuation effects at a detailed level is crucial for the physics of this workshop! Hermes data gave first glimpse Massive dataset from CLAS becoming available 12 GeV: : the definitive program

40 Drell-Yan Yan: : 800 GeV protons on a variety of nuclear targets (McGaughey,, Moss, Peng,, Ann. Rev. Nucl.. Part. Sci.. 49, 271 (1999) Transverse momentum broadening from Fermilab Drell-Yan Experiments Incident quark, x 1 Target anti-quark, x 2 μ + μ -

41 Connection to Relativistic Heavy Ion Physics One proposed signature of the Quark Gluon Plasma is jet quenching: the suppression of high p T jets Jet quenching caused by radiative energy loss would be an indication of high partonic density, e.g., QGP Hadron formation might give an alternative explanation for jet quenchingq Nuclear DIS is closely related to propagation of partons in AA collisionsc p T (A-A) A) ν (DIS). Relevant energies are ~few GeV

42 Relevance to RHIC and LHC Relativistic Heavy-Ion Collisions Deep Inelastic Scattering π π e e Initial quark energy is known Properties of medium are known

43 Recent Model Approaches: Semi-Inclusive DIS on Nuclei The essential reaction mechanism has not been isolated: Hadron forms inside nucleus or outside? Gluon Bremsstrahlung (Kopeliovich) Gluon radiation of colored quark Formation of color singlet pre-hadron Color transparency modulates pre-hadron (color dipole) attenuation Hadron attenuates in medium Twist-4 pqcd Model (Wang) Medium-induced gluon radiation modifies fragmentation function No hadronization Non-abelian LPM effect predicted Can extrapolate to predict jet quenching in RHI collisions

44 Recent Model Approaches: Semi-Inclusive DIS on Nuclei Semi-Classical Transport (Mosel, Falter) Stationary String Model (Akopov) Initial state from Pythia Detailed final state interactions within a coupled-channel BUU transport model Phenomenological FSI Three fastest particles pqcd In-Medium (Arleo) Rescaling Models (Accardia, Pirner) Medium-modified gluon distribution Transport coefficient measurements for nuclear matter to more nucleus variables (λ A >λand 0 ) from Drell-Yan The models vary in sophistication, but all can describe the HERMES data! Differentiate among models: extend observables Partial deconfinement of nucleon in => PDF s and frag. fcns. modified

45 Gluon Bremsstrahlung Model Authors: B. Kopeliovich, J. Nemchik, E. Predazzi

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47 Gluon Bremsstrahlung Model (prediction of 1996) and HERMES Nitrogen Data

48 Gluon Bremsstrahlung Model and Hermes Data

49 Twist-4 pqcd Model Authors: X.-N. Wang, E. Wang, X. Guo,, J. Osborne No hadronization in this picture: Hadrons form outside nucleus Energy loss from medium-stimulated gluon radiation causes nuclear attenuation Leading twist-4 4 modifications to pqcd fragmentation functions due to induced gluon radiation from multiple scattering Strength of a quark-gluon correlation function is a free parameter

50 Twist-4 pqcd Model

51 Rescaling Model, EMC/HERMES Data

52 Rescaling Model, HERMES Flavor-separated Kr Data

53 Impact of Models Summary ~4-6 6 modern models currently on the market: Varying degrees of sophistication All can fit the published data reasonably well using different assumptionsa Need more data: Hermes heavy target data, flavor-separated hadrons, high energies CLAS lower-energy, energy, high statistics, wide range of targets, wide range of final states Need high standards for theory! Need lattice calculations (~5+ years away)

54 Jlab,, CLAS, and CLAS12 CW electron accelerators allow breakthrough measurements High average luminosity from 100% duty factor Excellent beam characteristics Second and third generation measurements of this kind will come from CLAS/CLAS12 Luminosity ~10 34 cm - 2 s - 1 now, later Large acceptance allows study of two-particle correlations CLAS12 will be a superb instrument for these studies (11 GeV, good PID, high luminosity)

55 CLAS the CEBAF Large Acceptance Spectrometer

56 Charged particle angles Neutral particle angles 8-70 Momentum resolution ~0.5% (charged) Angular resolution ~0.5 mr (charged) Identification of p, π + /π -, K + /K -, e - /e +

57 The CLAS12 Central Detector TOF lightguide TOF light guide Flux return Microstrip Central Tracker Superconducting solenoid coil (Bmax = 5T) Central EC Central TOF

58 CLAS12

59 Nuclear Quark/Hadron Propagation Experimental Method in CLAS Use a range of targets, light to heavy DIS kinematics + measure hadrons Single electron trigger Nuclear medium modifies fragmentation functions and p T distributions 5 GeV beam: Q 2 4 GeV 2, ν 4.5 GeV 11 GeV beam: Q 2 9 GeV 2, ν 9 GeV Measure π +,,0,0, η, ω, η, η φ, Κ +,,0,0, p, Λ, Σ +,0, Ξ 0, 0,

60 CLAS EG2 Physics Focus Search for Color Transparency Measure rho absorption vs. Q 2 at fixed coherence length Compare absorption in deuterium, carbon, aluminum, and iron Quark Propagation through Nuclei Measure attenuation and transverse momentum broadening of hadrons (π,( K) in DIS kinematics Compare absorption in deuterium, carbon, iron, tin, and lead E E02-104

61 CLAS EG2 Running Conditions Beam energies: 4 GeV (7 days) and 5 GeV (50 days) Luminosity: x (D+Fe), 1.3 x (D+Pb) Data taking: DC occupancy < 3%, deadtime 7% (D+Pb( D+Pb) ) and 14% (D+Fe( D+Fe) Number of triggers: 0.6 billion (D+Fe( D+Fe,, 4 GeV) 2 billion (D+Fe( D+Fe,, 5 GeV) 1.5 billion (D+Pb( D+Pb,, 5 GeV) 1 billion (D+C, 5 GeV)

62 CLAS Kinematic Coverage and Particle Identification at 6 GeV Directly identified particles

63 CLAS EG2 Online Physics Results K0 short, D+Fe (Simple vertex cut) Q 2 (GeV 2 ) 27 M D+Pb electrons (5% of total) K0 short, D+Pb (Simple vertex cut) W (GeV) deuterium iron lead ρ ρ ρ

64 Hayk Hakobyan, Yerevan State U. CLAS EG2, very preliminary, 5% of total data set DIS kinematics, Q 2 >1, all ν Carbon Iron Lead No acceptance correction (small, two targets in the beam) Not final calibrations (should be nearly irrelevant, bins are huge) No fiducial cuts (probably ok, two targets in beam) No radiative correction (effect primarily cancels in ratios) No correction for pi+ from rho (need full statistics to correct for this)*** Few-percent kaon contamination in region GeV No isospin correction for heavy targets(~5%?) No x F cuts

65 CLAS EG2 Preliminary 5% of data No acceptance correction (small, two targets in the beam) Not final calibrations (should be nearly irrelevant, bins are huge) No fiducial cuts (probably ok, two targets in beam) No radiative correction (effect primarily cancels in ratios) No correction for pi+ from rho (need full statistics to correct for this)*** Few-percent kaon contamination in region GeV No isospin correction for heavy targets(~5%?) No x F cuts Catherine Silvestre (Nantes)

66 CLAS Kinematic Coverage and Particle Identification at 6 GeV Directly identified particles

67 Sample of Anticipated 6 GeV Data 14 N 40 Ar 5 GeV 84 Kr 197 Au

68 12 GeV Anticipated Data

69 Complications Where can pqcd be used? factorization Target/current fragmentation Resonances in final states

70 Benefits of CLAS/Jlab High luminosity and duty factor Can access lower-rate rate reactions, higher p T, higher Q 2 Can study multivariable dependence Large acceptance for multiple hadrons Access to many hadrons through decay channels Correlate final-state particles with leading hadron ( grey tracks ) Can test fragmentation model