Transcript iop-3-2005
Exotic Hybrid Mesons. J. D. Kellie Department of Physics and Astronomy Glasgow University. Presentation to IoP Manchester 2005. 1 Summary of talk. 1. Introduction. 2. Exotic Hybrid Mesons a) Why they are of interest. b) How they can be produced. c) How they can be identified and their properties measured. d) Current experimental evidence. 3. Conclusions. 2 Introduction. • This talk presents two experiments designed to investigate QUARK CONFINEMENT by attempting to find evidence of gluonic excitations within mesons in both the light (u, d, s) and charmed (c) quark sectors. • The experiments are GLUEX which forms part of the Jefferson Laboratory upgrade which will increase the electron beam energy from 6 to 12 GeV, and PANDA which is part of the GSI upgrade. • GLUEX will use tagged linearly polarised photons – a source of vector mesons – incident on a hydrogen target. A hermetic spectrometer, based on a superconducting solenoid, will measure the reaction products. Photons of around 9 GeV are required. • The upgrade has obtained U.S. Department of Energy support. 3 PANDA will be located in the high-energy storage ring HESR at the international FAIR facility at GSI, and will use anti-proton proton annihilation to study gluonic excitations, glueballs and hybrids in the charmonium mass range. The antiproton beam – momentum extending to 15 GeV/c – is incident on a fixed hydrogen target inside a superconducting solenoid, which, together with a forward large acceptance dipole magnet, will form the basis of the PANDA spectrometer. 4 Why Exotic Hybrid Mesons are of interest. Quark confinement can be explained in terms of the qq interaction which arises from gluonic exchange between the quarks. Mesons, with their qq structure, are ideal for studying the interaction. qq states have well defined quantum numbers. If gluonic degrees of freedom are added, the resulting states are hybrid mesons. When hybrid meson states have quantum numbers that do not belong to the basic set of qq states they are called exotic hybrid mesons. If states with exotic quantum numbers are discovered, this will clarify the role played by gluons in the confinement of quarks. GLUEX and PANDA are specifically designed to measure the 5 quantum numbers of excited mesons. Lattice QCD → Flux Tube Model Flux tube forms between qq Quark Confinement. neutron (d,u,d) π (d, u ) proton (u,u,d) Confinement arises from flux tubes and their excitation leads to a new spectrum of mesons From G. Bali 6 Understanding Confinement The Ideal Experiment The Real Experiment 7 Normal Mesons – qq color singlet bound states Spin/angular momentum configurations & radial excitations generate our known spectrum of light quark mesons. Starting with u - d - s we expect to find mesons grouped in nonets - each characterized by a given J, P and C. JPC = 0– + 0++ 1– – 1+ – 2++ … JPC = 0– – 0+ – 1– + 2+ – … Not-allowed: exotic Allowed combinations 8 Hybrid Mesons Hybrid mesons 1 GeV mass difference (p/r) Normal mesons 9 Quantum Numbers of Hybrid Mesons Quarks S0 L 0 Excited Flux Tube J PC J PC 0 like Hybrid Meson 1 1 J PC 1 1 p, K Exotic S 1 L 0 J PC J PC 1 like 1 1 J PC 0 1 2 0 1 2 , Flux tube excitation (and parallel quark spins) lead to exotic JPC 10 Mass (GeV) Radial excitations Meson Map – GLUEX mass range Each box corresponds to 4 nonets (2 for L=0) qq Mesons 2.5 Glueballs 2.0 1.5 2 +– 2 –+ 1 –– 1– + 1 +– 1 ++ 0 +– 0 –+ Hybrids 2 –+ 0 –+ 2 ++ exotic nonets 0 ++ 1.0 L=0 1 2 3 4 (L = qq angular momentum) 11 Meson Map – PANDA mass range. 12 add Hall D (and beam line) 12 GeV CEBAF Upgrade magnets and power supplies CHL-2 Enhance equipment in existing halls 13 GlueX / Hall D Detector Barrel Lead Glass Calorimeter Detector Detector Review Oct 20-22, 2004 Solenoid Coherent Brem. Photon Beam Note that tagger is 80 m upstream of detector Target Electron Beam from CEBAF Time of Flight Cerenkov Counter Tracking 14 HESR storage ring at FAIR. PANDA facility. 15 PANDA spectrometer. 16 Participation of the GLASGOW group at GLUEX and PANDA. GLUEX. •Design of the GLUEX tagging spectrometer. •Assessment of suitable diamond radiators. PANDA. •Solenoid and Forward Spectrometer magnetic field design studies. •Development of Grid Computing. •Design of Cerenkov detectors – with Edinburgh. 17 PANDA magnetic field calculations – showing the effects of field clamps. Vertical section: Field component along the axis. Vertical section: Field component transverse to the axis. 18 How Exotic Hybrid Mesons are produced at GLUEX Basic interaction. X e N N ,, The incident photons – vector mesons –are excited into states X which decay into many different channels. A hermetic spectrometer detects the reaction products. Requirements. •JLab energy upgraded from 6 to 12 GeV. •Source of linearly polarised photons, (determine parity). •New detector to measure reaction products. 19 Production of Linearly Polarised Photons. • When the electron beam of energy E interacts with a 0 carefully aligned thin diamond wafer, linearly polarised coherent bremsstrahlung, as well as incoherent bremsstrahlung, is produced. • If the energies E of the residual electrons are measured in a tagging spectrometer, the energy of the bremsstrahlung photon is E 0 E. • The ratio of coherent to incoherent bremsstrahlung is enhanced if the photons pass through a narrow collimator. 20 Coherent Bremsstrahlung This technique provides requisite energy, flux and polarization flux 12 GeV electrons Incoherent & coherent spectrum 40% polarization in peak photons out collimated electrons in spectrometer diamond crystal tagged with 0.1% resolution E (GeV) 21 The GLUEX tagger. Two identical magnets, each ~50 tonnes. Vacuum chamber, designed to withstand vacuum force of ~ 70 tonnes. Focal Plane The focal plane consists of a broad-band low resolution hodoscope covering 0.25E E γ 0.95E used for diamond alignment and 0 0 monitoring, and a high resolution microscope positioned for 8.5GeV E γ 9GeV and operating at ~ 5 106 Hz. per channel. 22 Diamond Assessment using X-rays. X-ray topography Rocking curve measurements. 23 Diamond X-ray Analysis - very good diamond. Topographs. dbg dbr Topographs of 3 slices (100 μm in thickness) cut from a single synthetic diamond with the top slice nearest the seed. Each shows the same pattern, but the relative area of the central region increases with distance from the seed. Rocking Curves for bottom slice. dbb FWHM~10 microradians 24 How exotic hybrid mesons can be identified and their properties measured. Use a large acceptance detector hermetic coverage for charged and neutral particles. typical hadronic final states: f1hKKphKKpppp b1pppppppp pppp high data acquisition rate. Perform partial-wave analysis identify quantum numbers as a function of mass. check consistency of results in different decay modes. 25 Rates High statistics means high rates At 107, the total hadronic rate is » 37kHz Initially tagged /s the tagged hadronic rate is » 1.4kHz 8 Design detector for 10 At 108, the total hadronic rate is » 370kHz the tagged hadronic rate is » 14kHz JLab CLAS runs at 107 already. 107 Running at 107 for 1 year will exceed current photoproduction data by several orders of magnitude and will exceed current p data. 26 Topologies t-channel meson photoproduction p,K, X p photons pions n,p protons 10-60o ~1GeV/c 27 Background Topologies /N* production is a significant background to the simple t-channel production. p,K, X There is interesting physics in this channel, p it is just more complicated to analyze. photons pions n,p p protons forward backwards slow pions 28 The GlueX Detector Tracking Calorimetry Particle ID Magnetic Field 29 Calorimetry Forward Calorimeter LGD Existing lead glass detector ~2500 blocks E/E · 0.036+0.073/E1/2 » 100 MeV · E · 8 GeV Barrel Calorimeter BCAL Expected po and h resolutions Lead-scifiber sandwich 4m long cylinder E/E · 0.020+0.05/E1/2 ~20MeV · E · » 3 GeV 200ps timing resolution z-position of shower time-of-flight Upstream Photon Veto UPV Veto photons ~20MeV · E · 300 MeV 30 Tracking Forward Region FDC 4 packages of planar drift chambers anode + cathode readout six planes per package xy=150m active close to the beam line. Central Region CDC cylindrical straw-tube chamber 23 layers from 14cm to 58cm 6o stereo layers r=150m z» 2mm minimize downstream endplate dE/dx for p<450 MeV/c Necessary for protons 31 Particle Identification Time-of-flight Systems Forward tof ~80ps BCAL ~200ps Start counter Cherenkov Detector DIRC p K p separation dE/dx Information The CDC will do dE/dx p<450 MeV/c 32 Requirements for a good partial wave analysis. • Hermetic Detector for charged particles • and photons. • Uniform, understood acceptance. • Excellent resolution to reduce backgrounds. • Linear polarized photons. • High statistics data sets. • Sensitive to many final states. 33 Current Experimental Evidence for Exotic Hybrid Mesons. BNL E852 Experiment with 18GeV / c π on a hydrogen target. a) π p π1 (1600)p π1 : J PC ρ0 π π ππ f 1π ηπ π π b1π π π π π0 π0 1 - PWA mass peak unstable. b) π p π1 (2001)p Statistics for b) are low, but p (2001) more in line with 1 LQCD in terms of mass and decay modes. GlueX will have much higher statistics and be able to find nonets of states. 34 CONCLUSIONS. •The discovery of exotic hybrid mesons will provide strong evidence that quarks interact by the exchange of gluons, and hence greatly increase our understanding of quark confinement. •The GlueX and PANDA experiments have the required versatility, acceptance, resolution and particle ID to determine the quantum numbers of mesonic states. •In addition to being able to identify exotic hybrid mesons, GlueX and PANDA will measure the spectroscopy of meson states in both the light and charmed quark sectors. •GlueX and PANDA have the potential to search for glueballs, quark molecular states etc. •GlueX and PANDA are complementary experiments with common physics goals, but using entirely different types of facilities. 35