The Hunt for the Hybrid Meson Exploring the dynamics of quark confinement Richard Jones University of Connecticut Physics and Astronomy Colloquium Series Dartmouth College, Feb. 6, 2004

2 Outline Introduction  the strong interaction  confinement in QCD  quark potentials and the quarkonium spectrum Meson Spectroscopy  production and detection  analysis of the final state  quantum numbers and exotic mesons Experimental Searches for Exotics  proton-antiproton annihilation  pion-excitation experiments  photo-excitation experiments

3 Introduction: the strong nuclear force protons: positive electric charge neutrons: no charge like charges repel new force must be present  strong to overcome electrostatic repulsion  short-ranged to prevent collapse What holds the nucleus together?

4 Theoretical foundations Hideki Yukawa proposes theory of the nuclear force (1935)  mediated by spinless exchange particle called the   meson  mass of  meson about 250 times that of the electron  meson later discovered (Lattes, Muirhead, Occhialini, Powell, 1947)

5 Experimental advances experiments soon revealed many more new particles involved in strong interactions  protons and neutrons lightest particles in a large spectrum of strongly-interacting fermions called baryons  pions lightest member of equally numerous sequence of strongly-interacting bosons called mesons many more…

6 Quark model pattern suggests substructure  Murray Gell-Mann  quarks  George Zweig  aces quarks:  fractional electric charge!  spin 1/2  come in flavors (up, down, …) baryons = three quarks mesons = quark-antiquark pair Gell-MannZweig -1/3e +2/3e

7 More experimental advances experiments at Stanford Linear Accelerator Center (Friedman, Kendall and Taylor, 1968)  rendition of Rutherford experiment  scattered electrons off protons  looked at large momentum transfers  found point-like charges inside proton new charges initially called partons, but  fractional charges confirmed  scattering consistent with massless quarks

8 … and more quarks discovery of J/  meson  in November 1974 (BNL, SLAC)  interpreted as bound state of new flavor of quark called charm  predicted as weak partner of strange quarks discovery of  meson in August, 1977 (Fermilab)  interpreted as bound state of new flavor called bottom  new partner predicted at higher mass, to be called top ultra-heavy quark finally observed in 1995 (Fermilab)  weak interaction comparable with strong at 180 GeV/c 2 ! no more quarks expected below mass scale ~1 TeV/c 2

9 no single isolated quark was ever seen in a detector  heavy quarks decay to light quarks via weak interactions  light quarks “dress” themselves in anti-quarks to form mesons  mesons are seen in detectors What kind of theory might explain this? … and yet, confinement

10 Confinement in atomic physics consider the hydrogen atom where  =1/137, weak coupling  no confinement  atom can be ionized with energy E 0  isolated protons exist as physical states V r n=1 n=2

11 Confinement in atomic physics Note the energy scale: What happens if  ~ 1 or greater?  grows to the same size as mass-energy mc 2  is of same order as mc 2  special relativity changes things How might we study these effects?  consider Z > 1  for Z = 140,  = 1.02

12 Confinement in atomic physics Warning!  relativistic corrections to the Hamiltonian shift the g.s. energy E 1 from this simple extrapolation of E 0  the Dirac equation must be solved Qualitative results  something new happens when E 1 > 2mc 2  the bare nucleus spontaneously grows an electron in its g.s.  a positron (anti-electron) simultaneously flies off  process continues until ionization energy of atom < 2mc 2 The Z=180 nucleus is confined to the neighborhood of its electrons – i.e. physical states must have Q < 180 !

13 Confinement in atomic physics Can this effect be observed in experiment?  nuclei with Z >100 are increasingly unstable and radioactive  compound nuclei can be created in A+A collisions with a lifetime of order s  lifetime is too short to do atomic spectroscopy Experiment with heavy ion collider was performed at G.S.I. in Darmstadt, Germany  positron emission rate was monitored vs. Z of beams  some excess yield was seen for Z > 160 Is there some other system for which  ~ 1 for which real spectroscopy is possible?

14 Confinement in nuclear physics this atomic physics analogy is imperfect  only one of the two charges is large  for true  ~ 1 BOTH charges must grow  new things happen when B.E. > 2mc 2  new matter-antimatter pairs spontaneously created  vacuum is unstable!  a new phase is formed to replace the ordinary vacuum  “empty space” becomes full of particles  the Dirac equation is of little use  field theory is the only approach

15 Confinement in nuclear physics other differences from forces in atomic physics The underlying theories are formally almost identical! QEDQCD 1 kind of charge (q)3 kinds of charge (r,g,b) force mediated by photonsforce mediated by gluons photons are neutralgluons are charged (eg. rg, bb, gb)  is nearly constant  s strongly depends on distance

16 LQCD: the static quark potential V(r<<r 0 ) ~ 1/r  1-gluon exchange  asymptotic freedom V(r>>r 0 ) ~ r  like electrodynamics in 1d  confinement

17 Lattice field theory: a new frontier hypercubic space-time lattice quarks reside on sites, gluons reside on links between sites lattice excludes short wavelengths from theory (regulator) regulator removed using standard renormalization systematic errors  discretization  finite volume quarks gluons

18 LQCD: how well does it do? best test is with heavy quarkonium (quenched approx.)  s ~ 0.2 reveals static V qq (r) contains effects of strong coupling at large distances shows confinement! good agreement with experimental spectrum

19 LQCD: what is a hybrid meson? Intuitive picture within Born-Oppenheimer approximation  quarks are massive – slow degrees of freedom  gluons are massless – generate effective potential Glue can be excited ground-state flux-tube m=0 excited flux-tube m=1

20 Meson Spectroscopy production and detection analysis of the final state quantum numbers and exotic mesons

21 Production e + e - annihilation pp annihilation  p collisions  p collisions

22 Detection Forward Calorimeter Cerenkov Counter Time of Flight Solenoid Barrel Calorimeter Tracking Target

23 Analysis reactions tend to produce all sorts of mesons  many flavors (mixtures of up, down, strange …)  many spins and parities only the lightest are “stable”: , k,  pseudoscalar nonet) all other mesons decay to pseudoscalars and photons must be reconstructed by their kinematics  energies of decay products  angles of decay products  respect special relativity, i.e. use rest frame of decaying particle  lab  cm

24 Consider a final state that contains a  +  - pair  what might decay to  +  - ?  consult selection rules parent mesons are identified by resonances in  +  - mass spectrum empirical rule: isobar model of strong interactions  Two-body decay modes are dominant  Multiparticle final states should be described by a cascading sequence of two-body decays from heavier resonances What do we see?

25 at 18 GeV/c suggeststo partial wave analysis Some assembly required… Data from E852, BNL:

26 Classification Ordinary mesons (qq)  defined by the Constituent Quark Model  decay model built on CQM generally successful  spectrum is well understood (experiment, CQM, QCD) Exotic mesons  new states predicted on the basis of confinement in QCD  of special interest are gluonic excitations Glueballs Hybrids  spectrum not well understood  little is known about decays

27 quark-antiquark pairs u d s u d s J=L+S P=(-1) L+1 C=(-1) L+S G=C (-1) I (2S+1) L J 1 S 0 = S 1 = 1 -- ,K*  ’,K L= a 2,f 2,f’ 2,K 2 a 1,f 1,f’ 1,K 1 a 0,f 0,f’ 0,K 0 b 1,h 1,h’ 1,K 1 L=  3,  3,  3,K 3  2,  2,  2,K 2  1,  1,  1,K 1  2,  2,  ’ 2,K 2 L= radial orbital Ordinary mesons

28 m=0 CP=(-1) S+1 m=1 CP=(-1) S Flux-tube Model CP={(-1) L+S }{(-1) L+1 } ={(-1) S+1 } S=0,L=0,m=1 J=1 CP=+ J PC =1 ++,1 -- S=1,L=0,m=1 J=1 CP=- J PC =0 -+, , ,2 +- J PC = 1 -+ or 1 +- Quantum numbers of hybrids J=L+S P=(-1) L+1 C=(-1) L+S G=C (-1) I (2S+1) L J 1 S 0 = S 1 = 1 -- start with CQM rules: add angular momentum of the string

qq Mesons L = Each box corresponds to 4 nonets (2 for L=0) Radial excitations exotic nonets 0 – – – 1 – + 1 – – 2 – – – + 2 – Glueballs Hybrids GeV

30 Searches for Exotic Mesons proton-antiproton annihilation pion-excitation experiments photo-excitation experiments

31 Searches: proton-antiproton annihilation + - Crystal Barrel CERN/LEAR

32  1 (1400) antiproton-neutron annihilation Mass = 1400 ± 20 ± 20 MeV/c 2 Width= 310 ± MeV/c 2 Same strength as the a 2. Produced from states with one unit of angular momentum. Without  1  2 /ndf = 3, with = 1.29 PWA of np     CBAR Exotic

33 Significance of exotic signal.

34 Hybrid mass predictions Flux-tube model: 8 degenerate nonets 1 ++, ,0 +-,1 -+,1 +-,2 -+,2 +- ~1.9 GeV/c 2 Lattice calculations UKQCD (97) 1.87  0.20 MILC (97) 1.97  0.30 MILC (99) 2.11  0.10 Lacock(99) 1.90  0.20 Mei(02) 2.01  0.10 S=0 S=1 MILC, hep-lat/

35 Searches: pion excitation experiments E852 BNL/MPS

36 Partial Wave Analysis Benchmark resonances PWA of   p      +

37   p    p (18 GeV) The a 2 (1320) is the dominant signal. There is a small (few %) exotic wave. Interference effects show a resonant structure in  . (Assumption of flat background phase as shown as 3.)    Mass = MeV/c 2 Width= MeV/c 2 a2a2  PWA: exotic signal

38 A second exotic signal! Leakage From Non-exotic Wave due to imperfectly understood acceptance Exotic Signal  1 (1600) 3  m=  =  ’ m=  =

39 Searches: photo-excitation experiments glueballs hybrid mesons + - +

40 Photoproduction of hybrids A pion or kaon beam, when scattering occurs, can have its flux tube excited  or  beam Quark spins anti-aligned Much data in hand with some evidence for gluonic excitations (tiny part of cross section) q q before q q after q q q q before  beam Almost no data in hand in the mass region where we expect to find exotic hybrids when flux tube is excited Quark spins aligned _ _ _ _

41 Production cross sections Model predictions for regular vs exotic meson prodution with photon and pion probes Szczepaniak & Swat

42 18 GeV Compare statistics and shapes ca Events/50 MeV/c 2 19 GeV SLAC ca Complementary probes

43 GlueX experiment Lead Glass Detector Solenoid Electron Beam from CEBAF Coherent Bremsstrahlung Photon Beam Tracking Target Cerenkov Counter Time of Flight Barrel Calorimeter Note that tagger is 80 m upstream of detector Event rate to processor farm: 10 kHz and later 180 kHz corresponding to data rates of 50 and 900 Mbytes/sec respectively 12 GeV gamma beam MeV energy resolution high intensity (10 8  /s) plane polarization

44 Jefferson Lab site Hall D will be located here

45 Add Arc Add Cryomodules Upgrade plan

46 Summary and Outlook Regularities in the spectrum of light hadrons was a key to the discovery of the building blocks of the nucleus and of the theory of strong interactions. Precise predictions of the properties of light hadrons are still very difficult within QCD, but lattice QCD can overcome these difficulties, provided the systematic errors are controlled, and rapid advances in computing power are leading to unprecedented accuracy in predicting observables. Recent experimental results have fueled renewed interest in hadron spectroscopy to test the theory.