Electromagnetic probes from nuclei to flux tubes

Transcription

1 Electromagnetic probes from nuclei to flux tubes Dan Watts University of Edinburgh University of Surrey November 2010

2 Lecture outline Generating intense photon beams Current facilities: Tagged polarised photon beams Future facilities: Quasi-real tagged polarised photon beam at JLAB Two selected physics topics The euation of state for neutron rich matter Direct probe of the nature of light uark confinement

3 Microtron techniue (MAMI, Jefferson Lab) e - beam accelerated by rf cavities High intensity electron beams Tune magnetic field to ensure path through magnets multiple of λ of accelerating field. e - arrive back in phase Continuous beam (high d.f.) Strecher ring techniue (Spring8, Bonn, Frascati, GRAAL) e - beams fed in from linac Accelerated and stored in ring. - Useable beam bled off slowly Many built for synchrotron radiation can exploit!! Tend to have poorer duty factors beam properties and less stable operation than microtrons

4 High intensity electron beams From JLAB at 6 GeV to JLAB at 12 GeV add Hall D (and beam line) Upgrade magnets and power supplies CHL-2 Enhance euipment in existing halls Beam Power: 1MW Beam Current: 90 µa Max Pass energy: 2.2 GeV Max Enery Hall A-C: 10.9 GeV Max Energy Hall D: 12 GeV

5 Real photon beams from electron beams Laser back-scattering (LEGS, FRASCATI, DUKE, Spring8) E γ smaller than beam energy Facilities up to 4 GeV Tagged Bremsstrahlung (MAMI, Bonn, Lund, Jlab) Eγ up to ~beam energy Facilities up to 6 GeV (9GeV ~ Yr 2012) γ- beam Both techniues can produce polarised γ beams Recoil e Transverse polarisation (Electric field vector oscillates in a plane) Circular polarisation (Electric field rotates Clockwise or anticlockwise) e- beam

6 Quasi-real photon tagging Photoproduction in CLAS12 Electron scattering kinematics e θ e Forward tagger -Q 2 = E 2 P 2 (Q 2 = 0 for real photon) Q 2 = 4 E e E e sin 2 (θ ε /2) e γ Q 2 ~0 if electron scatters in very forward direction Virtual photon ~ same properties as real photon Termed Quasi-real N Photon tagged by detecting the scattered electron at low θ e <4 o High energy photons eg. upgraded JLAB: 7 < E γ < 10.5 GeV Quasi-real photons are linearly polarized Analytic function of θ e, Q 2 and ( E e -- E e ) Electron beam=11 GeV Θ e = 2 o -5 o High Luminosity (can even use thin gas target!) Euivalent photon flux N γ ~ γ sec -1

7 Photon beam facilities E γ max (GeV) I γ max (s -1 MeV -1 ) E γ (FWHM) (MeV) Pol γ lin (%) Pol γ circ (%) Legs ~ B C

8 Two selected physics topics The euation of state for neutron rich matter Probing confinement in the light uark sector Coherent pion photoproduction to measure matter form factor The hunt for hybrids Transition matter FF s

9 Matter form factor and the neutron skin Our knowledge of the shape of stable nuclei is presently incomplete Neutron skin r n -r p (fm) Relativistic mean field Skyrme HF 208 Pb e.g. 208 Pb RMS charge radius accuracy < fm RMS neutron radius accuracy ~0.2 fm!! Horowitz et al. PRC (2001) Piekarewicz et al. NPA 778 (2006) Mass number of nucleus (A)

10 Neutron skin and the euation of state negligible n = density (fm -3 ) α = (N-Z)/A n 0 = nuclear density

11 Neutron stars

12 208 Pb Neutron skin and and Neutron neutron stars stars Thick neutron skin Low transition density in neutron star New data from X-Ray telescopes mass, radii, temp of neutron stars! Solid Liuid Proton fraction as a function of density in neutron star Rutel et al, PRL (2005) Horowitz, PRL (2001) Horowitz, PRC (2001) Carriere, Astrophysical Journal 593 (2003) Tsuruta, Astrophysical Journal Lett. 571 (2002) Direct URCA Cooling n p + e - + ν e - + p n + ν Constrains gravitational wave emission from neutron stars Freuency and damping modes!! arxiv: (2009)

13 Coherent pion photoproduction Photon probe Interaction well understood π 0 meson produced with ~eual probability on protons AND neutrons. Select reactions which leave nucleus in ground state Reconstruct π 0 from π 0 2γ decay Angular distribution of π 0 PWIA contains the matter form factor dσ/dω(pwia) = (s/m N2 ) A 2 ( π */2k γ ) F 2(E γ,θ π ) 2 F m () 2 sin 2 θ π π 0 final state interactions - use latest complex optical potentials tuned to π-a scattering data. Corrections modest at low pion momenta

14 Crystal Ball at MAMI γ Eγ ~ 2 MeV 10 8 γ sec -1 Upstream view into the ball γ PID and MWPC subsystem TAPS 528 BaF 2 crystals Crystal Ball 672 NaI crystals

15 208 Pb Coherent pion photoproduction total σ Theoretical calculations: Unitary isobar model with complex π 0 -A interaction PWIA DWIA FULL (+ self energy) Crystal Ball TAPS99 (unpublished) Eγ from threshold to ~190 MeV is the window to get best access to FF

16 208 Pb Coherent pion photoproduction analysis for neutron skin E γ = MeV E γ = MeV E γ = MeV E γ = MeV Momentum transfer (fm-1) E γ = MeV E γ = MeV E γ = MeV E γ = MeV Zoom in on 1 st minima Red line - coh pi model passed through detector acceptance Momentum transfer (fm-1) Red line Coh pi model plus detector resolution (Geant 4) Skin extraction - Linear interpolation between predicted distributions for different skin thicknesses χ2 minimising fit to the data 2 parameter Fermi Fn. Charge radii = e - scattering; ntn radii interval fm

17 (p,d) pickup Neutron skins extracted from fit Further work in progress interpolated fit between Skyrme model predictions (Alex Brown) Proton scattering ic artoms (CERN) Antiprotoni 208 Pb Neutron ski in (fm) ~ Direct URCA threshold Preliminary results rule out direct URCA cooling in neutron stars! Photon Energy (MeV)

18 Transition matter form factors γ π 0 E x, Detect nuclear decay photon in the same detector as the π 0 decay photons Counts [arb. Units] 3 10 Entries Mean 3 In coincidence sin 2 (2α) 7 RMS 1 with 12 cos C(γ,π 2 Underflow 0 (2α) ) 6 Height Centroid 4 Sigma 0. p3 9 p4-0. Transition matter form factor with an EM probe! 12 C(γ,π 0 ) 12 C(4.4 MeV) E γ = MeV Takaki -- Takaki : no N -- Tryasuchev E γ [MeV] Alpha (deg) θ π0 CM Tarbert, DP Watts et. al. Phys. Rev. Lett (2008)

19 Eγ dependence of spin dependent / independent in medium 12 C(γ,π 0 ) 12 C(4.4MeV) E γ =175± 5 MeV 12 C(γ,π 0 ) 12 C(4.4MeV) E γ =310± 10 MeV 12 C(γ,π 0 ) 12 C(4.4MeV) E γ =420± 20 MeV

20 Confinement the process creating the nucleus One of the outstanding issues in modern physics - full understanding of Quantum Chromodynamics QCD underpins understanding of mass and binding of the atomic nucleus, neutron stars, QGPs... Ability to calculate phenomena at the nucleonic and nuclear distance does not yet exist. Need to test and understand the confinement process Major advances in uality of experimental data and theory In the coming decade (lattice, holographic dual)

21 Beyond the uark model:hybrids and exotica Quarks are confined inside colourless hadrons Combine to 'neutralize' the colour force mesons baryons Other uark-gluon configuration can give colorless objects molecules pentauarks glueball mesons hybrid mesons QCD does not prohibit such states but not yet unambiguously observed

22 Normal mesons No excitation of the gluonic field (or flux tube) Total angular momentum C-parity symmetry Under operation of changing uarks to antiuarks,> = > C=+1,> = - > C=-1 S=S 1 +S 2 J= L+S P = (-1) L+1 C= (-1) L+S If the uark spins align then it is known as a vector meson If anti aligned known as a scalar meson Can define an intrinsic parity for the meson (extra +1 factor as parity opposite to ) Normal meson: flux tube in ground state m=0 (no ang momentum in flux tube) CP=(-1) S+1

23 Normal Meson uantum numbers Meson(Mass) S J P C CP π 0 (135) η (547) ω(782) Φ(1020) Normal meson: flux tube in ground state CP=(-1) S+1

24 What if we excite the flux tube?? Extra degree of freedom in the meson. Extra possibilities in relation between the uantum numbers Normal meson: flux tube in ground state m=0 CP=(-1) S+1 Hybrid meson: flux tube in excited state m=1 CP=(-1) S m=1 flux tube alone has J PC 1 -+ or 1 +- Combine excited flux tube uantum number with those of the uarks Exotic Quantum Nos Unambiguous experimental signature for the presence of gluonic degrees of freedom in the meson!!

25 Lattice QCD indicates they should be there! Dudek et. al. PRL (2009)

26 Why photoproduction? Only get exotic uantum numbers if excite a vector meson (S=1) Most attempts to date used pion beams. S=0 exotics produced with low rate Cannot get a beam of vector mesons. However high energy photons interact dominantly from fluctuations into a pair (Vector meson dominance) Photoproduction rate for exotics is expected to be comparable to regular mesons!!

27 Quasi-real photon tagger for CLAS at JLAB Calorimeter + tracking device + veto Electron energy/momentum Photon energy (ν=e-e') Polarization ε 1 = 1 + ν 2 /2EE' Electron angles Q 2 = 4 E E' sin 2 ϑ/2 ϕ polarization plane Veto for photons δν /ν ~0.1% Rates in the forward tagger Inelastic electro-production Elastic radiative tail Moeller scattering e e N Tagger g v Signal CLAS12 e e' e' ' N Tagger g v L e ~10 35 cm -2 s -1 N Background CLAS12 (N γ ~ γ/s) e'' Tagger e'' CLAS12 e' Atomic electron R ~10kHz R ~100kHz R ~10MHz

28 Forward tagger GEANT4 in CLAS Simulations detector at JLab

29 The forward tagger in CLAS Forward tagger Tungsten Moller shield CLAS12 Solenoid Veto

30 Veto design Will be placed in front of forward tagger crystals Reuirements - high rate capability, readout in high magnetic field (1-5 T) Location of readout electronics Location of photo-detectors Newly developed, high gain SiPMs will be tested with fibre readout EU (FP7) seed funding for Edinburgh to lead development of the device + JLAB funding Rohacell back plate

31 Hermetic charged/neutral particle detector Forward Detector CLAS12 in more detail TORUS Magnet Forward SVT tracker HT Cerenkov Counter LT Cerenkov Counter Forward TOF System Preshower calorimeter E.M. Calorimeter Hall-B - CLAS12 Detector Central Detector SOLENOID magnet Barrel silicon tracker Central TOF Proposed updates Micromegas (CD) Neutron detector (CD) Forward Tagger

32 Disentangling the uantum numbers - PWA 1) the isobar model γ e.g. 3π system Exotic state J PC Statistical errors in the data used for the PWA extraction will become negligible with the new generation of experiments p p Key to establishing the feasibility of (and analysing) the next generation experimental data is assessing and controlling systematic errors Edinburgh has key role: Shown that PWA with necessary accuracy to unambiguously isolate exotic partial waves is achievable using CLAS(12) and forward tagger Simulation studies (D. Glazier) form basis of first forward tagger proposal to PAC

33 Partial wave analysis Edinburgh-IU- INFN-JLab Benchmark channel: γ p n π + π + π - The process is described as sum of 8 isobar channels: a2 ρ π (D-wave) a1 ρ π (S-wave) a2 ρ π (D-wave) pi2 ρ π (P-wave) pi2 ρ π (F-wave) pi2 f2 π(s-wave) pi2 f2 π (D-wave) pi1 ρ π (P-wave) (exotic) Exotic wave recovered with negligible leakage!! Amplitudes calculated by Szczepaniak (indiana) CLAS12 acceptance projected and fitted Can isolate exotic with 1% contribution with a few weeks of beamtime!!

34 Detailed systematic studies are crucial... ρπ decay channel J PC =1 -+ Exotic signal?? Reanalysis of higher statistic sample Possible evidence of exotic meson π 1 (1600) in π p p π π π + (E852- Brookhaven) Not confirmed in a re-analysis of a higher statistic sample

35 Using the nucleus to improve systematics Our group leads new initiative to exploit production from nuclei in hybrid searches Momentum exchange in t-channel well suited to the nuclear wavefunction Nuclear production kills nucleon resonance production which complicates PWA Wide range of spin/isospin filters available!! Tag with low energy nuclear gamma. 12 C 12 C(2 + ;T=0; 4.4MeV) 66 Li Li 1+;T=0 6 Li 0+;T=1; 3.6 MeV 6 Li 3+;T=0; 2.2 MeV Measure isoscalar and isovector in same measurement?

36 Incoherent count rates Rate ~15k day -1 Count rate estimates assume modest decay gamma array at backward angle ( deg) With 50% efficiency Rate ~1.2k day -1 First calculations collaboration between Sandy Donnachie and Helmy Sherif

37 Possible crystals to detect the nuclear decay photons Phoswich design - good timing properties of LaBr to be combined with high stopping power, lower cost scintillators Planned for Spiral set up collaboration to share costs of crystals/readout for use at JLAB

38 Other aspects of forward tagger programme Edinburgh co-spokesperson on Ω photoproduction proposal using forward tagger Cascades: Doubly strange nucleons expect correspondence with excited states of nucleon (but.. much narrower states than in light uark sector, different dependences on dynamical processes)

39 Summary New generation of experiments with electromagnetic probes give clean information on strongly interacting matter from the scale of large nuclei down to the interactions between the light uarks of the nucleus There is an exciting decade ahead of us in terms of understanding the strong interaction!