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Hadron Spectroscopy Hadron Spectroscopy Jo Dudek Jefferson Lab / Old Dominion University NNPSS 2007 NNPSS - Hadron Spectroscopy hadrons defined to be those particles which experience the ‘strong nuclear force’ these days we have a strong suspicion that this force is QCD, and that hadrons are made up of dynamically confined quarks and gluons QCD is a gauge field theory = q¯(iγµ∂ m)q + g q¯γµt q Aa 1 F a F µν L µ − a µ − 4 µν a just one parameter of its own - the coupling ‘constant’, g quark masses also appear but they don’t really ‘belong’ to QCD the ta are the generator matrices of the group SU(3) [ta, tb] = ifabctc this doesn’t look too bad - quite like QED which we have few problems with in fact it is an enormously challenging problem to find solutions for now I will just point out that g is not a small “number” so probably the perturbation theory (expansion in g) so useful in QED won’t work here there are small numbers though - the quark masses (mu,d ~ O(1) MeV) NNPSS - Hadron Spectroscopy hadrons fall into two categories based upon spin fermionic baryons 1 3 5 J = 2 , 2 , 2 . most famously the stable proton and the long-lived neutron bosonic mesons J = 0, 1, 2 . none are stable, but the lightest, the pion plays a fundamental role in nuclei NNPSS - Hadron Spectroscopy charmonium - the ‘easy’ case one set of hadrons that are particularly simple are the charmonium mesons each box represents an observed particle ! ψ 3686 ! ηc 3638 χc2 3555 hc 3525 particles fall in groups - ‘gross structure’ χc1 3510 { χc0 splitting within a group - ‘fine structure’ 3415 } reminds us of quantum mechanics of atoms a reasonable description of the spectrum of charmonium comes from solving a J/ψ 3097 ηc 2980 Schrödinger equation assuming a potential between a charm quark and an anti-charm quark mn = 2mc + En 0−+ 1−− 0++ 1++ 1+− 2++ 2−+ 1 2 ψ + V (r)ψ = Enψ − mc ∇ NNPSS - Hadron Spectroscopy charmonium potential model a common ‘guess’ for the potential is α V (r) = + br short distance − r r one gluon exchange long distance confinement solve the Schrödinger equation with this potential L = 2 giving the gross energy level L = 1 structure L = 0 NNPSS - Hadron Spectroscopy charmonium potential model non-relativistically reducing diagrams like this give rise to fine-structure producing terms in the hamiltonian that are suppressed by inverse powers of mc !σ !σ ‘hyperfine interaction’ q · q¯ Σ! L! ‘spin-orbit interaction’ · L = 1 L = 0 splits up the levels just as in the experimental spectrum NNPSS - Hadron Spectroscopy charmonium charmonium (and the heavier bottomonium) seem to be well enough described as quantum mechanical problems we seem to have avoided much of the complexity of field theory (suspicious?) hadron spectroscopy might be an easy subject ? let’s examine the lighter meson spectrum... start with things that appear to be generally true - symmetries NNPSS - Hadron Spectroscopy symmetries of light hadrons experimentally it is found that the strong interaction is invariant under the parity operation (sends !r !r ) provided one assigns→ − an intrinsic parity to hadron states e.g. p = + p P| ! | ! π = π P| ! −| ! NNPSS - Hadron Spectroscopy symmetries of light hadrons certain light hadrons, through their masses (and couplings to other states), appear to sit in definite representations of SU(2) - ‘isospin’ I, I | z! e.g. the proton and the neutron have approximately the same mass, with no ⇒ p 1 1 other baryon having a similar mass form an isospin doublet n = 2 , 2 we observe three different charged pions, all with roughly the same| ! mass ± 0 ! " ⇒ form an isospin triplet π± = 1, 1 , π = 1, 0 | ± ! | ! ! there is a single isolated meson state with mass~550 MeV, which we call the η ! " ! " ! ! ⇒ this is an isospin singlet η = 0, 0 | ! | ! experimentally it is found that the strong interaction is to an excellent approximation isospin invariant, so that for example, an isospin 1 meson cannot decay into a set of mesons having total isospin 0 through the strong interaction (the electromagnetic interaction is not isospin invariant) e.g. the strong interaction cross-section for π+p scattering is the same as that for π-n π+p = 1, +1 1 , + 1 = 3 , + 3 | ! | ! ⊗ | 2 2 ! | 2 2 ! 1 1 3 3 π−n = 1, 1 , = , | ! | − ! ⊗ | 2 − 2 ! | 2 − 2 ! NNPSS - Hadron Spectroscopy symmetries of light hadrons an operation known as charge conjugation exists, which turns particle states into antiparticle states, up to a phase it is possible for neutral bosons to be eigenstates of this operation e.g. γ = γ , and we say the photon has negative ‘charge parity’ C| ! −| ! e.g. π 0 = + π 0 where we can determine the ‘charge parity’ from the exptal obserCvation| ! of π |0 ! γ γ → experimentally we find that the strong interactions are invariant under the charge conjugation operation by merging an isospin transformation and charge conjugation one finds an (invariant) operation on charged & neutral boson states iπ e− Iy G ≡ C without going into details this defines a conserved ‘G-parity’ for meson states e.g. π± = π± G| ! −| ! for a neutral boson G = C (-1)I NNPSS - Hadron Spectroscopy approximate symmetries furthermore, there appears to be an approximate SU(3) symmetry if we look at a broader selection of hadron states extra conserved quantum number: strangeness e.g. K* → K π has strong interaction decay (τ~10-23 s) conserved strangeness process: K*(S=1) → K(S=1) π(S=0) e.g. K → π π has weak interaction decay (τ~10-10 s) strangeness not conserved: K(S=1) → π(S=0) π(S=0) (broken) symmetry clearly seen in baryon masses: representations of SU(3) include singlets, octets, decuplets ... 0 + ++ n p ∆− ∆ ∆ ∆ S=0 ~200 MeV Σ Σ0 Σ+ 0 Σ + S=-1 − Λ Σ∗− Σ∗ ∗ 0 0 Ξ∗− Ξ∗ S=-2 Ξ Ξ− Ω S=-3 NNPSS - Hadron Spectroscopy labelling a meson so then a neutral non-strange meson state can be labelled by the (strong- interaction conserved) quantum numbers IG J P C electrically charged non-strange mesons are not eigenstates of C NNPSS - Hadron Spectroscopy experimental hadron spectrum there are a small number of hadrons that cannot decay through the strong interaction they instead decay electromagnetically or weakly with a relatively long lifetime e.g. π± has cτ ~ 8 m, K± has cτ ~ 4 m, π0→γγ charged particles and photons ionise matter and so are ‘easy’ to detect the other hadrons are short-lived resonances and are detected via their ‘stable’ decay products e.g. ρ±→ π±π0 with cτ ~ O(fm) NNPSS - Hadron Spectroscopy resonances in pp say we’ve got a beam of pions that we fire at a proton target one possible reaction is p p→pp p observe the angular and invariant-mass distributions of the two pions 2 2 2 2 f(θ, m ) = fL(m )PL(cos θ) 2 ! ! m ! L ! θ ! ! !" ! 2 µ ! ! ! ! π m = (p1 + p2) (p1 + p2)µ ! ! e.g. say the pion state} is p+p0 - the reconstructed invariant mass might look like two possible isospins contribute π+π0 = 1, +1 1, 0 | ! | ! ⊗ | ! = a 1, +1 + b 2, +1 | ! | ! NNPSS - Hadron Spectroscopy resonances in pp there isn’t a peak at the same position in p+p+ - since the strong interactions are isospin invariant, we can eliminate the isospin 2 possibility ⇒ we have an isospin 1 resonance, X (X±, X0) the G-parity of this resonance can be inferred immediately GX = GπGπ = (-1)(-1) = +1 hence the neutral member X0 has C = -1 information on the spin of the resonance comes from the angular distribution of pions 2 experimentally this is found to behave like cos θ when mππ ~ 770 MeV 2 2 cos θ = |PL=1(cos θ)| ⇒ J=1 L the parity of two particles in a relative L-wave is P1P2(-1) , so that with Pπ = -1 & using the parity invariance of strong interactions we have PX = -1 this is the rho meson IG JPC=1+ 1- - NNPSS - Hadron Spectroscopy invariant mass dependence of a resonance the rho meson appeared as a bump-like structure in the two-pion invariant mass in many cases resonant bumps can be described by some variant of the Breit- Wigner formula: 1 2 1 2 ∼ (E E0) + Γ − 4 Γ at the quantum mechanical amplitude level 1 A(E) = E E + i 1 Γ − − 0 2 ! " E0 admits a simple non-relativistic interpretation: iEt 1 e− iE t Γt A(t) = dE e− 0 e− 2 E E + i 1 Γ ∼ ! − 0 2 Γt P (t) e− ∼ ‘relativistic’ version corresponds to a simple pole of the S-matrix m Γ T elastic (s) = − 0 BW(rel) s m2 + im Γ − 0 0 NNPSS - Hadron Spectroscopy not always so simple - pp isospin 0 angular independent piece (PL=0(cos θ) ⇒ J=0) scalar, isoscalar channel IG JPC=0+ 0++ not clear what is going on here tempting (and many are tempted) to fit the data as a simple sum of Breit-Wigner fns ¡ this is not allowed ! unitarity is a strong constraint on elastic scattering ImT = T 2 elastic 2| | T BW ( rel ) ( s = m 0 ) = i - already saturates unitarity there are ways around this to deal with this case where multiple resonances overlap method is rarely unique & hence analyses of this type can be rather controversial even with very high quality data NNPSS - Hadron Spectroscopy sometimes very simple - pp isospin 2 for elastic scattering can express the T-matrix via a 400 800 1200 single phase δ T (s) = eiδ(s) sin δ(s) ‘phase shift’ resonance peak when T = i ⇒ δ=π/2 clearly no resonances with isospin 2 and J=(0,2,4)++ NNPSS - Hadron Spectroscopy higher mass resonances pp L = 2 has JPC = 2++ ”tensor meson” f2(1270) also has other decay channels branching fractions NNPSS - Hadron Spectroscopy ppp to access negative G-parity states we’ll need at least three pions in the π+ π- π0 channel the invariant mass shows two resonances below 1GeV in the charged π+ π- π- channel, there is quite a lot going on η ω getting spin information here is a non-trivial task - leads into a model of hadron production isobar model of partial
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