Physics and Astronomy Colloquium Series Dartmouth College Feb

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

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

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

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

Experimental advances n experiments soon revealed many more new particles involved in strong interactions many more… 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 5 ¨

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

More experimental advances n 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 ¨ n new charges initially called partons, but fractional charges confirmed ¨ scattering consistent with massless quarks ¨ 7

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

… and yet, n 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 ¨ confinement n What kind of theory might explain this? 9

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

Confinement in atomic physics n Note the energy scale: n What happens if a ~ 1 or greater? ¨ ¨ ¨ n <T> grows to the same size as mass-energy mc 2 <U> is of same order as mc 2 special relativity changes things How might we study these effects? ¨ ¨ consider Z > 1 for Z = 140, a = 1. 02 11

Confinement in atomic physics n Warning! ¨ ¨ n Qualitative results ¨ ¨ n 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 something new happens when E 1 > 2 mc 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 < 2 mc 2 The Z=180 nucleus is confined to the neighborhood of its electrons – i. e. physical states must have Q < 180 ! 12

Confinement in atomic physics n Can this effect be observed in experiment? ¨ ¨ ¨ n Experiment with heavy ion collider was performed at G. S. I. in Darmstadt, Germany ¨ ¨ n nuclei with Z >100 are increasingly unstable and radioactive compound nuclei can be created in A+A collisions with a lifetime of order 10 -21 s lifetime is too short to do atomic spectroscopy positron emission rate was monitored vs. Z of beams some excess yield was seen for Z > 160 Is there some other system for which a ~ 1 for which real spectroscopy is possible? 13

Confinement inanalogy nuclear physics n this atomic physics is imperfect ¨ ¨ ¨ n only one of the two charges is large for true a ~ 1 BOTH charges must grow new things happen when B. E. > 2 mc 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 14

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

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

Lattice field theory: a new frontier n n n 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 gluons quarks 17

LQCD: how well does it do? n best test is with heavy quarkonium (quenched approx. ) n as ~ 0. 2 reveals static Vqq(r) n n contains effects of strong coupling at large distances n shows confinement! good agreement with experimental spectrum n 18

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

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

Production n e+e- annihilation n pp collisions - + - + + n gp collisions + + 21

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

Analysis n reactions tend to produce all sorts of mesons many flavors (mixtures of up, down, strange …) ¨ many spins and parities ¨ n n n only the lightest are “stable”: p, k, h (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 ¨ qlab qcm 23

What do we see? n Consider a final state that contains a p+p- pair ¨ what might decay to p+p ? ¨ n consult selection rules parent mesons are identified by resonances in p+p- mass spectrum n 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 24

Some assembly required… Data from E 852, BNL: suggests at 18 Ge. V/c to partial wave analysis 25

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

Ordinary mesons quark-antiquark pairs orbital u s d d s u J=L+S P=(-1) L+1 C=(-1) L+S G=C (-1) I (2 S+1) 1 S L J = 0 -+ 3 S = 1 -1 0 r 3, w 3, f 3, K 3 r 2, w 2, f 2, K 2 L=2 r , w , f , K 1 1 p 2, h’ 2, K 2 3 -2 -1 -2 -+ a 2, f’ 2, K 2 a 1, f’ 1, K 1 L=1 a , f’ , K 0 0 b 1, h’ 1, K 1 2++ r, w, f, K* L=0 p, h, h’, K 1 -0 -+ radial 1++ 0++ 1+- 27

Quantum numbers of hybrids n n start with CQM rules: add angular momentum of the string JPC = 1 -+ or 1+CP={(-1)L+S}{(-1)L+1} ={(-1)S+1} Flux-tube Model m=0 CP=(-1) S+1 m=1 CP=(-1) S J=L+S P=(-1) (2 S+1) L+1 C=(-1) G=C (-1) L+S I L J 1 S -+ = 0 0 3 S = 1 -1 S=1, L=0, m=1 S=0, L=0, m=1 J=1 CP=JPC=0 -+, 0+- J=1 CP=+ JPC=1++, 1 -- 1 -+, 1+2 -+, 2+- 28

Each box corresponds to 4 nonets (2 for L=0) qq Mesons Radial excitations 2 –+ 0 –+ 2 ++ Glueballs 2. 0 1. 5 2 +– 2 –+ 1 –– 1– + 1 +– 1 ++ 0 +– 0 –+ Hybrids 2. 5 exotic nonets 0 ++ 1. 0 L=0 1 2 3 4 0++ 1. 6 Ge. V 29

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

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

CBAR Exotic antiproton-neutron annihilation PWA of np ® hp 0 pp 1(1400) Mass = 1400 ± 20 Me. V/c 2 Width= 310 ± 50 +50 -30 Me. V/c 2 Same strength as the a 2. Produced from states with one unit of angular momentum. Without p 1 c 2/ndf = 3, with = 1. 29 32

Significance of exotic signal. 33

Hybrid mass predictions Flux-tube model: 8 degenerate nonets 1++, 1 -- 0 -+, 0+-, 1 -+, 1+-, 2 -+, 2+- ~1. 9 Ge. V/c 2 S=0 S=1 MILC, hep-lat/0301024 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 34

Searches: pion excitation experiments E 852 BNL/MPS - + + 35

Partial Wave Analysis PWA of p- p ® p-p-p+ Benchmark resonances 36

PWA: exotic signal p-p -> hp- p (18 Ge. V) Mass = 1370 +-16+50 -30 Me. V/c 2 p 1(1400) Width= 385 +- 40+65 2 -105 Me. V/c The a 2(1320) is the dominant signal. There is a small (few %) exotic wave. a 2 p 1 Interference effects show a resonant structure in 1 -+. (Assumption of flat background phase as shown as 3. ) 37

A second exotic signal! 1(1600) Exotic Signal Leakage From Non-exotic Wave due to imperfectly understood acceptance 3 p m=1593+-8+28 -47 G=168+-20+150 -12 ph’ m=1597+-10+45 -10 G=340+-50 38

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

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

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

Complementary probes Compare statistics and shapes ca. 1993 @ 18 Ge. V Events/50 Me. V/c 2 ca. 1998 BNL @ 19 Ge. V 28 SLAC 4 1. 0 1. 5 2. 0 2. 5 42

Glue. X experiment n n 12 Ge. V gamma beam Me. V energy resolution high intensity (108 g/s) plane polarization Coherent Bremsstrahlung Photon Beam www. gluex. org Lead Glass Detector Barrel Calorimeter Solenoid Note that tagger is 80 m upstream of detector Tracking Target Electron Beam from CEBAF Time of Flight Cerenkov Counter Event rate to processor farm: 10 k. Hz and later 180 k. Hz corresponding to data rates of 50 and 900 Mbytes/sec respectively 43

Hall D will be located here Jefferson Lab site 44

Upgrade plan Add Cryomodules Add Arc Add Cryomodules 45

Summary and Outlook n n n Regularities in the spectrum of light hadrons was a key to the discovery of the building blocks of the nucleus and of 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 theory. 46