Meson spectroscopy - glueballs
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Transcript of Meson spectroscopy - glueballs
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Meson spectroscopy - glueballs
• Making a glueball• Decay products• Kaon detection• Kinematic fitting• Partial wave analysis• Glueball candidates & spectra• Further Research
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Meike Door
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Making a glueball
J/ψ: meson, mass: 3097 MeV/c2 , lifetime ~10-20s, SpinParity (JP): 1-
Glueball: particle consisting solely of gluons , JP=0+
Short lifetime: decay products are among others K+K- ,K0K0, π+π- , …qq
Quantum Numbers are conserved
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Measure the decay products of a glueball
• Let‘s focus on K+K-, because this is the favored decay process
(shown by M.S. Chanowitz, „chiral suppression of scalar-Glueball decay“, Physical review letters, 2005)
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Kaon detection (16.09.09)
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Charged particles: Use drift chamber & TOF
B-field in the drift chamber →
Kaons follow a curved path → calculate the momentum of the Kaon
Drift chamber xy view
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Energy deposition dE/dx in the chamber → particle identification
Kaons are well distinguishablefrom other particles
TOF: mass & velocity of particle
Kaon detection (16.09.09)dE
/dx
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Intermezzo: Kaons• Spin parity 0-, lifetime ~10-8s
• K+ : K- : K0: :
• due to weak interaction there is a coupling between K0K0 . → the physical Kaon states are a mixure!
→ Discovery of two kinds of neutral Kaons: Kl0 (long-lived, ~10-8s)
and Ks0(short-lived, ~10-10s). Subject of another Semimar!
• But: Kaons are decay products of other reactions, too… how can you prove that a measured Kaon came from a glueball?
usus
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ds K0 ds_
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Conservation of energy & momentum
K
K
PE
K
K
PE
&
Remember last week:
²)²()²( 2121 gKKKK MPPEE Invariant mass of the glueball
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Requirement for being a glueball !
Glueball decays in e.g. Kaons: energy & momentum has to be conserved!
²²² PEm
Decay into two Kaons:
(in natural units, c=1)
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Addtionally, must hold.
Advantage: The mass of a kaon is very well known:mK = 493,677±0,013 MeV
E and are measured with an uncertainty.→ modify E and so that mK is obtained
This is called kinematic fitting!
²²² 111 KKK mPE
P
P
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Kinematic fitting §3.4
Idea: use known properties of a given process to improve the measurements and resolution.
Mathematical method…
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Until now we know…
• what are the decay products of a glueball (Kaons favored)
• how Kaons are detected
• under which conditions Kaons are a decay product of glueball
• How to improve measurements & resolution Kinematic fitting
²)²()²( 2121 gKKKK MPPEE
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Invariant mass spectrum
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We had a glueball…What was its mass, witdh, spin, parity?
Apply Partial Wave Analysis (PWA) (widely used in high energy physics).Use the fact that
Cross section is measureable.
How does the amplitude look like? Write it as Legendre Polynomial * weighting factor
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²),( f Cross section ~ Amplitude²
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PWA
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lmlm AYfml,
),(),( Legendre Polynomial x weighting factor
→Spherical harmonic
→
Partial wave amplitude
Get Alm using the orthogonality relation:
'''' ),(),( mmlllmml YYd *
→
Put in measured amplitude
→ Calculate partial wave amplitude Alm
(This is for spin=0. Spin ≠0 is more complicated)
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Glueball spin
Spin 0: no favored direction after decay
→ uniform angular distribution, flat spectrum
Spin ≠ 0 : favored direction after decay
→ no uniform angular distribution, no flat spectrum
Learn about the spin of your glueball
Diagrams: „Physics at BES-III“, Kuang-Ta Chao and Yifang Wang, 2008, §3.5.513
Θ is the decayangle w.r.t. thereconstructedmomentum ofthe glueball
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What about parity?• Glueballs can have all
states indicated in the mass spectrum
• Kaons: 0-
• There are JPC states, which supress decay into KK
→ Can you imagine why?
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Conservation of charge/parity violated
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Scalar Glueball candidates
• f0(1370)
• f0(1500)
• f0(1710)
Note:There are also vector- and tensor- glueballs, and pseudoscalar glueballs. Too much for today!
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Glueball f0(1370)
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3 particles measured: φ, π+ , π-
f0(980) candidate for multiquark or molecule states(Chapter 11, Physics of soft Pions)
f0(1790) discussed later
f0(980)
f0(1370)f0(1790)
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Glueball f0(1370)
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f0(1370)
Figure: broad enhancement around f0(1370)
BES II: strong f0(1370) signal for J/ψ → π+ π-
No significant f0(1370) signal for J/ψ → K+ K-
Mass M=1350±50MeVWidth Γ = 265±40MeV
f0(980)
f0(1790)
PWA: peak around 1370MeV comes from a dominant f0(1370) term that interferes with an f2(1270) and a smaller f0(1500)
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Glueball f0(1500)
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backgroundf2(1270)
f0(1500) observed in many experiments,Interpreted as non- state
Strong f2(1270) signal
Shoulder at ~1.45 GeV
Enhancement at ~1.7GeV
Peak at ~2.1GeV
Shaded area: background
~1.45GeV~1.7GeV
~2.1GeV
J/ψ→γ π+ π-
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Glueball f0(1500)
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• PWA: background flat, peaks are left over Two 0+ states in the 1.45GeV & 1.75GeV mass regions
In contrast to f2(1270), the f0(1500) is not directly observed in certain production channels of J/ψ hadronic decays (f0(1500) has a larger branching ratio to ππ than KK)
→ but existance of a 0++ scalar at 1.5GeV is suggested for other production channels with a larger branching ratio to KK than to ππ
f0(1500):M = 1466±6±20 MeV
→ future: search for f0(1500) in more decay modes & studying its spin-parity
J/ψ→γ π+ π-
f2(1270),
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Glueball f0(1710)
• f0(1710) main competitor of f0(1500) for the lightest 0+ glueball (large production rate, also in J/ψ decay)
• Different experiments before BES gave different masses, widths and spin-parities
• Most experiments give JP= 0+ • BES-II: M=1738±30MeV; Γ=125±20MeV, JP= 0+
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Glueball f0(1710)
Where does the background come from?
Spectrum: „Physics at BES-III“, Kuang-Ta Chao and Yifang Wang, 2008, §9.3.2 21
f2‘(1525)f0(1710) Pseuodoscalar glueball (not treated
in this lecture)
J/ψ→γKK_
M(K+K-) (GeV/c²)
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Glueball f0(1710)
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PWA fit projection
Peak at 1765±13MeV
This state may be the f0(1710) ,
the f0(1790) or a superposition!
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f0(1790) & f0(1710): 1 or 2 states?
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f0(1790)
left: f0(1790) peak, right: f0(1710) peak. Masses lie close together!
PWA shows that these states are distinct
f0(1710)
J/ψ→γKK_
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Current & Further Research
• 1~2GeV: at least 3 scalar mesons (glueball candidates) (f0(1370), f0(1500) & f0(1710)) which are explored
• Behaviour has unexpected features• Questions arise about → structure→ decay mechanism→ glueball (yes or no?)→ If yes: is the glueball pure or mixed?→ nature of the candidates remains a mystery
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BES-III
• Large data sample & excellent detector performance will increase the sensitivity
• Search for other hadron states is possible with BES-III
• Recently, two additional scalar meson candidates (f0(1790)& f0(1810)) were reported by BES-II → confirmation required by BES-III
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References
• „Physics at BES-III“, Kuang-Ta Chao and Yifang Wang, 2008
• M.S. Chanowitz, „chiral suppression of scalar-Glueball decay“, Physical review letters, 2005
• BES-III detector properties, G.J. Tambave, lecture 16th sept. 2009
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