Hadron Spectroscopy at GlueX and Beyond (1) Justin Stevens Preliminaries
Goal:
A self-contained introduction to hadron spectroscopy and an overview of recent excitement in the field
Outline:
Introduction to QCD and hadron spectroscopy
Why study spectroscopy through QCD?
Classification of hadrons
Heavy quark spectroscopy: “The XYZ story”
Light quark spectroscopy (tomorrow and Wednesday)
HUGS 2018 Justin Stevens, 2 Standard Model Coupling 10-6 1/137 1 Strength:
Higgs Boson
Higgs H Spin 1 Spin 1/2
HUGS 2018 Justin Stevens, 3 Standard Model Coupling 10-6 1/137 1 Strength:
Higgs Boson
Higgs H Spin 1 Spin 1/2
HUGS 2018 Justin Stevens, 4 Quarks and hadrons
Proposed to explain proton structure and properties of other states observed at the time
proton = uud u neutron = udd u | i d | i d J =1/2 u J =1/2 d
Color charge analogous to electric charge: atoms are electrically neutral and hadrons are color neutral (or color singlets)
Another flavor of “light” quarks: strange
u d u s
⇡+ = ud¯ K+ = us¯ | i HUGS 2018 ↵ Justin Stevens, 5 Quarks and hadrons
Proposed to explain proton structure and properties of other states observed at the time
proton = uud u ++ = uuu u | i d | i u J =1/2 u J =3/2 u
Color charge analogous to electric charge: atoms are electrically neutral and hadrons are color neutral (or color singlets)
Another flavor of “light” quarks: strange
u d u s
⇡+ = ud¯ K+ = us¯ | i HUGS 2018 ↵ Justin Stevens, 6 Quarks and hadrons
Proposed to explain proton structure and properties of other states observed at the time
proton = uud u ++ = uuu u | i d | i u J =1/2 u J =3/2 u
Color charge analogous to electric charge: atoms are electrically neutral and hadrons are color neutral (or color singlets)
Another flavor of “light” quarks: strange
u d u s
⇡+ = ud¯ K+ = us¯ | i HUGS 2018 ↵ Justin Stevens, 7 Quarks and hadrons
Proposed to explain proton structure and properties of other states observed at the time
proton = uud u ++ = uuu u | i d | i u J =1/2 u J =3/2 u
Color charge analogous to electric charge: atoms are electrically neutral and hadrons are color neutral (or color singlets)
Another flavor of “light” quarks: strange
u d u s
⇡+ = ud¯ K+ = us¯ | i HUGS 2018 ↵ Justin Stevens, 8 Quarks and hadrons
Proposed to explain proton structure and properties of other states observed at the time
proton = uud u ++ = uuu u | i d | i u J =1/2 u J =3/2 u
Color charge analogous to electric charge: atoms are electrically neutral and hadrons are color neutral (or color singlets)
Another flavor of “light” quarks: strange
u d u s
⇡+ = ud¯ K+ = us¯ | i HUGS 2018 ↵ Justin Stevens, 9 Quarks and hadrons
Proposed to explain proton structure and properties of other states observed at the time
proton = uud u ++ = uuu u | i d | i u J =1/2 u J =3/2 u
Color charge analogous to electric charge: atoms are electrically neutral and hadrons are color neutral (or color singlets)
Another flavor of “light” quarks: strange
u d u s
⇡+ = ud¯ K+ = us¯ | i HUGS 2018 ↵ Justin Stevens, 10 Color interactions in QCD
High energy (short distance) limit
Interactions are weak: quarks are “asymptotically free”
QCD is calculable using perturbation theory, e.g. Higgs production at LHC
Low energy (long distance) limit
Interactions are strong and increase with distance, so quarks are confined
QCD is not calculable perturbatively, but recent, dramatic progress in lattice QCD
Opportunity to study QCD in strongly coupled bound states, i.e. hadrons
HUGS 2018 Justin Stevens, 11 Aside: what about nucleon structure?
HUGS 2018 Justin Stevens, 12 Comparing E&M and QCD E&M QCD
Photons (gluons) mediate forces between electric (color) charges q q g ɣ q¯ q¯ Gluon self-interaction produces a potential that grows ~linearly with distance
HUGS 2018 Justin Stevens, 13 Confined states of quarks and gluons Observed mesons and baryons well q st q q q described by 1 principles QCD q But these aren’t the only states mesons baryons permitted by QCD
Lattice QCD: Science (2008)
HUGS 2018 Justin Stevens, 14 Confined states of quarks and gluons Observed mesons and baryons well q st q q q described by 1 principles QCD q But these aren’t the only states mesons baryons permitted by QCD
q q q q q q q q q ...
tetraquark pentaquark ...
Phys. Lett. 8 (1964) 214
HUGS 2018 Justin Stevens, 15 Confined states of quarks and gluons Observed mesons and baryons well q st q q q described by 1 principles QCD q But these aren’t the only states mesons baryons permitted by QCD
q q ⇤b J/ pK q q q ! q q q q tetraquark pentaquark
HUGS 2018 Justin Stevens, 16 Confined states of quarks and gluons Observed mesons and baryons well q st q q q described by 1 principles QCD q But these aren’t the only states mesons baryons permitted by QCD
q q q q q q q q q tetraquark pentaquark
g q Do gluonic degrees of freedom q g g manifest themselves in the bound states we observe in nature? glueball hybrid meson
HUGS 2018 Justin Stevens, 17 Classifying mesons
General properties: mass, electric charge, quark flavor q q Grouped by quantum numbers: J PC Angular momentum: J~ = L~ + S~ Parity: Invert spatial coordinates Spin 0 P =( 1)L+1 Charge conj.: particle ↔ antiparticle C =( 1)L+S
strangeness Allowed JPC for q q ¯ mesons: PC + + ++ ++ electric charge J =0 , 1 , 1 , 0 , 2 ...
HUGS 2018 Justin Stevens, 18 Classifying mesons
General properties: mass, electric charge, quark flavor q q Grouped by quantum numbers: J PC Angular momentum: J~ = L~ + S~ Parity: Invert spatial coordinates Spin 1 P =( 1)L+1 Charge conj.: particle ↔ antiparticle C =( 1)L+S
strangeness Allowed JPC for q q ¯ mesons: PC + + ++ ++ electric charge J =0 , 1 , 1 , 0 , 2 ...
HUGS 2018 Justin Stevens, 19 11
11
Lattice QCD spectrum Dudek et al. PRD 88 (2013) 094505
2000 uu¯ + dd¯ ss¯
1500
Meson Mass (MeV) = ss¯ 1000 | i ! = uu¯ + dd¯ ↵ 500 ⇡0 = uu¯ dd¯ J PC ↵ 3 FIG. 11: Isoscalar (green/black) and isovector (blue) meson spectrum on the m⇡ = 391 MeV, 24 128 lattice. The vertical heightNote: of each box indicates the statistical uncertainty on the mass determination. States outlined in⇥ orange are the lowest-lying states having dominant overlap with operators featuring a chromomagnetic construction – their interpretation as the lightest HUGS 2018 hybrid meson supermultiplet willJustin be Stevens, discussed later. 20
extrapolation might be the complex resonance pole posi- to the spatial volume at m⇡ = 391 MeV, and we note that 3 tion, but we do not obtain this in our simple calculations since only a 16 volume was used at m⇡ = 524 MeV, the using only “single-hadron” operators. mass shown there may be an underestimate. + We discuss the specific case of the 0 and 1 sys- Figure 19 shows the octet-singlet basis mixing angle, 3 FIG. 11: Isoscalar (green/black) and isovector (blue) meson spectrum on the m⇡ = 391tems MeV, in the 24 next128 subsections. lattice. The vertical ✓ = ↵ 54.74 , which by definition must be zero at the ⇥ 4 height of each box indicates the statistical uncertainty on the mass determination. States outlined in orange are the lowest-lying SU(3)F point . While we have no particularly well mo- states having dominant overlap with operators featuring a chromomagnetic construction – their interpretation as the lightest tivated form to describe the quark mass dependence, it hybrid meson supermultiplet will be discussed later. E. The low-lying pseudoscalars: ⇡, ⌘, ⌘0 is notable that the trend is for the data to approach a phenomenologically reasonable value 10 [1, 45–47]. In lattice calculations of the type performed in this ⇠ paper, where isospin is exact and electromagnetism does extrapolation might be the complex resonance pole posi- to the spatial volumenot at m feature,⇡ = 391 the MeV,⇡ and ⌘ andmesons we note are exactly that stable and ⌘ 3 0 tion, but we do not obtain this in our simple calculations since only a 16 volumeis rendered was used stable at sincem⇡ = its 524 isospin MeV, conserving the ⌘⇡⇡ decay F. The low-lying vector mesons: ⇢, !, using only “single-hadron” operators. mass shown there maymode be is an kinematically underestimate. closed. Because of this, many of + We discuss the specific case of the 0 and 1 sys- the caveats presented in Section III B do not apply. Fig- Figure 20 shows the e↵ective masses of !, and ⇢ prin- Figure 19 shows the octet-singlet basis mixing angle, 3 ure 17 shows the quality of the principal correlators from cipal correlators on the m⇡ = 391 MeV, 24 128 lattice. tems in the next subsections. ✓ = ↵ 54.74 , which by definition must be zero at the ⇥ 4 which we extract the meson masses, in the form of an The splitting between the ⇢ and ! is small but statisti- SU(3)F point . Whilee↵ective we have mass, no particularly well mo- cally significant, reflecting the small disconnected contri- tivated form to describe the quark mass dependence, it bution at large times in this channel. At the pion masses E. The low-lying pseudoscalars: ⇡, ⌘, ⌘0 1 (t) is notable that the trend is for them data= tolog approach, a (16) presented in this paper, the ! and mesons are kine- e↵ t (t + t) phenomenologically reasonable value 10 [1, 45–47]. matically stable against decay into their lowest thresh- In lattice calculations of the type performed in this ⇠ old channels, ⇡⇡⇡ and KK. In Figure 21 we show the for the lightest quark mass and largest volume consid- quark mass and volume dependence of the low lying vec- paper, where isospin is exact and electromagnetism does ered. The e↵ective masses clearly plateau and can be tor mesons along with the relevant threshold energies. not feature, the ⇡ and ⌘ mesons are exactly stable and ⌘0 described at later times by a constant fit which gives a is rendered stable since its isospin conserving ⌘⇡⇡ decay F. The low-lyingmass in vector agreement mesons: with the⇢, two!, exponential fits to the principal correlator that we typically use. mode is kinematically closed. Because of this, many of Figure 18 indicates the detailed quark mass and vol- the caveats presented in Section III B do not apply. Fig- Figure 20 shows the e↵ective masses of !, and ⇢ prin- 4 Here we are using a convention where ⌘ =cos✓ 8 sin ✓ 1 , ume dependence of the ⌘ and ⌘0 mesons. We have already ⌘ =sin✓ 8 +cos✓ 1 with 8, 1 having| thei sign conventions| i | ini 3 | 0i | i | i ure 17 shows the quality of the principal correlators from cipal correlators on thecommentedm⇡ = 391 on the MeV, unexplained 24 128 sensitivity lattice. of the ⌘0 mass Eqn 5. which we extract the meson masses, in the form of an The splitting between the ⇢ and ! is small⇥ but statisti- e↵ective mass, cally significant, reflecting the small disconnected contri- bution at large times in this channel. At the pion masses 1 (t) presented in this paper, the ! and mesons are kine- me↵ = log , (16) t (t + t) matically stable against decay into their lowest thresh- old channels, ⇡⇡⇡ and KK. In Figure 21 we show the for the lightest quark mass and largest volume consid- quark mass and volume dependence of the low lying vec- ered. The e↵ective masses clearly plateau and can be tor mesons along with the relevant threshold energies. described at later times by a constant fit which gives a mass in agreement with the two exponential fits to the principal correlator that we typically use. Figure 18 indicates the detailed quark mass and vol- 4 Here we are using a convention where ⌘ =cos✓ 8 sin ✓ 1 , ume dependence of the ⌘ and ⌘0 mesons. We have already ⌘ =sin✓ 8 +cos✓ 1 with 8, 1 having| thei sign conventions| i | ini | 0i | i | i commented on the unexplained sensitivity of the ⌘0 mass Eqn 5. “Conventional” charmonium
“Simple” c c¯ spectrum well described by quark model expectation with expected electromagnetic transitions
HUGS 2018 Justin Stevens, 21 Experimental strategy B KX Search for new particles !+ X ⇡ ⇡ J/ Bumps in mass spectra !
Unique decay distributions
Next measure:
mass and width
decay modes
quantum numbers: J PC
Identify patterns and compare with QCD and models
HUGS 2018 Justin Stevens, 22 Detection strategy
Charged particles (π±, K±, p)
Momentum of “track” through ionization
Neutral particles (ɣ, n, KL)
Calorimeter mesures energy deposit
Particle identification measure velocity: timing or Cherenkov radiation
Combine to obtain 4-vector (p, E)
HUGS 2018 Justin Stevens, 23 Detection strategy 2 c GlueX Data 104 0 ⇡ ω background (MC) p p 103 ⌘ γp → pγγ ! 102 Counts / 10 MeV/
10
1 0 0.2 0.4 0.6 0.8 1 γγ Invariant Mass (GeV/c2)
HUGS 2018 Justin Stevens, 24 Decays: Width and Branching Ratios
Decay width:
Breit-Wigner resonance width, however difficult to separate from experimental resolution 2 c 0 GlueX Data 104 ⇡ ω background (MC) 103 ⌘ γp → pγγ 0 102 Counts / 10 MeV/
1 0 0.2 0.4 0.6 0.8 1 γγ Invariant Mass (GeV/c2)
HUGS 2018 Justin Stevens, 25 Decays: Width and Branching Ratios
Decay width:
Breit-Wigner resonance width
Branching ratios: B = /
HUGS 2018 Justin Stevens, 26 Spectroscopy: a global endeavor
Heavy quarks Light quarks
+ e e p probes Electromagnetic pp¯ pp pp¯ ⇡p
probes Hadronic
HUGS 2018 Justin Stevens, 27 Light quark experiments
Heavy quarks Light quarks
+ e e p probes Electromagnetic pp¯ pp pp¯ ⇡p
probes Hadronic
HUGS 2018 Justin Stevens, 28 Compass: Diffractive πp scattering
Long history of hadron spectroscopy in pion production experiments
Rich structure in π-π+π- spectrum, modeled as intermediate resonances X- and π+π- Isobars
Compass: PRD 95, 032004 (2017) HUGS 2018 Justin Stevens, 29 Jefferson Lab 12 GeV Upgrade
Upgrade maximum electron beam energy from 6 to 12 GeV
Add new experimental Hall D with a dedicated photon beam
HUGS 2018 Justin Stevens, 30 in Hall D
Linearly polarized photon beam from CEBAF 12 GeV
Large acceptance detector for both charged and neutral particles
HUGS 2018 Justin Stevens, 31 in Hall B
CEBAF delivers 11 GeV electron beam to Hall B
Linearly polarized photons through quasi-real photoproduction
Electron scattering provides access to hybrid baryons
Forward Tagger (FT)
HUGS 2018 Justin Stevens, 32 Heavy quark experiments
Heavy quarks Light quarks
+ e e p probes Electromagnetic pp¯ pp pp¯ ⇡p
probes Hadronic
HUGS 2018 Justin Stevens, 33 Heavy quark production and detection + Inclusive: pp BX or ⇤ X Exclusive: e e cc¯ ! B !
ps =2 4.6 GeV
ps = 13 TeV Pro: high rate Con: messy Pro: controlled Con: statistics HUGS 2018 Justin Stevens, 34 Experimental strategy B KX Search for new particles !+ X ⇡ ⇡ J/ Bumps in mass spectra !
Unique decay distributions
Next measure:
mass and width
decay modes
quantum numbers: J PC
Identify patterns and compare with QCD and models
HUGS 2018 Justin Stevens, 35