glueballs from gluon jets at the LHC
Wolfgang Ochs Max-Planck-Institut für Physik, München
status of glueballs: theory, experimental scenarios leading systems in gluon jets, LEP results proposals for LHC
with Peter Minkowski (Univ. Bern)
hadron2011, Munich, June 13, 2011
W. Ochs, glueballs at LHC – p.1 QCD expectations for glueballs early prediction: bound states of self-interacting gluons scenarios for glueball phenomenology Fritzsch-Minkowski ’75 Lattice QCD quenched approximation (only gluons) lightest state J P C = 0++: mass ∼ 1600 ± 200 MeV
unquenched results (including qq¯) lightest gluonic flavour singlet: mass ∼ 1000 MeV UKQCD ’06: Hart et al. mass ∼ 1500 MeV UKQCD ’10: Richards et al. some problems:
extrapolation to small lattice spacing, small mq; decay to ππ
W. Ochs, glueballs at LHC – p.2 QCD sum rules 2 gluonic resonances to satisfy sum rules for 0++ Mgb1 ≃ 1 GeV, Mgb2 ≃ 1.5 GeV either 2 gb states (NV) or a mixed gb-qq¯ system (HKMS)
Narison-Veneziano ’89 (broad Mgb1) Harnett-Kleiv-Moats-Steele ’08-’11
Experimental searches extra state in spectrum besides flavour nonets enhanced production in “gluon rich” processes suppression in γγ processes
W. Ochs, glueballs at LHC – p.3 glueball in scalar meson spectrum possible solution: f0(1710) f0(1500) 3 isoscalars: 2 nonet qq¯ states f0(1370) one extra state:→ glueball M ∼ 1.5 GeV Amsler, Close ’96 ... f0(980) f0(600)/σ could be from light nonet: qq,¯ 4q, KK¯ problem: f0(1370) not seen in energy-independent analyses (ππ) alternative possibility: f0(1500) f0(980) qq¯ nonet (no f0(1370)) Minkowski, W.O. ’98 f0(600)/σ glueball MBW ∼ 1 GeV Narison W. Ochs, glueballs at LHC – p.4 gluon rich processes produce gb = (gg) . . .
1. central production in pp collisions:
double Pomeron exchange: pp → pf gb pf 2. J/ψ → γ gb 3. pp¯ → π gb 4. b → sg: B → K gb 5. gluon jet at high energy: e+e− → qqg¯ , pp → g + X: g → gb + X reactions 1-4 proceed at low energies, role of gluon not obvious example:
ALICE @ LHC: (double Pomeron): excess of f0(980) and f2(1270) (qq¯)! Pomeron structure at HERA: large qq¯ singlet component at z=1.
⇒ only in reaction 5 a gluon can be identified
W. Ochs, glueballs at LHC – p.5 leading systems in gluon jets
u → π+(ud¯)+ X: leading meson at large x carries initial quark in analogy: g → gb(gg)+ X: leading meson is a glueball, carries initial gluon (?)
nonperturbative jet model for flavour singlet object (η, η′,ω,gb) (analogy to Field Feynman model) C.Peterson, T.F.Walsh, ’80 fragmentation functions g → gb at large x P. Roy, K. Sridhar ’97 H. Spiesberger, P.M. Zerwas ’00 rapidity gap analysis, study charge and mass of leading cluster W. O., P. Minkowski ’00
W. Ochs, glueballs at LHC – p.6 different colour neutralization processes
colour charges separated beyond confinement radius r & Rc: ⇒ colour neutralization by pair production a) initial qq¯: b) initial gg
colour triplet neutralization (P3) colour triplet neutralization Q = 0, ±1 electric charge Q = 0, ±1 (P8) colour octet neutralization Q = 0 colour octet mechanism is precondition for leading glueballs
W. Ochs, glueballs at LHC – p.7 rapidity gap analysis
rapidity gap isolates leading cluster (charge Qlead, mass Mlead)
||| ||| 1 E+pk −−−−−−−−−−−−−−−− > y rapidity: y = ln − 2 E pk ∆y for large rapidity gaps ∆y :
limiting distribution of charge Qlead
Qlead = 0, ±1 for (qq¯), probabilities from fragmentation models
Qlead = 0 for (gg)
charges |Qlead| > 1 are suppressed (multiquark exchanges)
⇒ Results from LEP on Qlead and Mlead from DELPHI, OPAL, ALEPH
W. Ochs, glueballs at LHC – p.8 rapidity gap analysis: leading charge Qlead
gluonjet quarkjet ∆y = 1.5 DELPHI
excess Qlead = 0 in gluon jet dependence on ∆y vs. MC (JETSET), excess 5-10% W. Ochs, glueballs at LHC – p.9 leading charge Qlead in gluon jets
identified b¯bg events ALEPH gluonjet,nogap gluonjet,withgap ALEPH ALEPH 0.035
0.3 g-jet data dN/dQ JETSET dN/dQ 0.03 g-jet data 3jets JETSET+GAL 3jets 0.25 JETSET 1/N 1/N 0.025 JETSET+GAL
0.2 AR0 0.02 AR1
0.15 0.015
0.1 0.01
0.05 0.005
0 0 -4 -2 0 2 4 6 -4 -2 0 2 4 6
Qg Qg
JETSET ok Qlead = 0 excess of ∼ 40% (JETSET)
(GAL, AR refer to color reconnection models) W. Ochs, glueballs at LHC – p.10 rapidity gap analysis: cluster mass for Qlead = 0
DELPHI OPAL gluonjet gluonjet quarkjet gluonjet
0.4 (a) OPAL Jetset 7.4 Ariadne 4.11
leading Herwig 6.2 0.2 Quark jet dN dM background 1 N
0 0 1 2 3 4 5 6 7 8 2 Mleading (GeV/c )
2 (b) OPAL Jetset 7.4 Ariadne 4.11 +- leading Herwig 6.2 Quark jet dN dM background 1 N
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 M+- (GeV/c2) leading 1 (c) OPAL Jetset 7.4 Ariadne 4.11 +-+- leading Herwig 6.2 Quark jet dN dM background 1 N
0 0.5 1 1.5 2 2.5 3 3.5 4 M+-+- (GeV/c2) leading charged + neutrals excess at mass < 2.5 GeV (2σ)
+ − gluon jets: excess of low mass Mlead < 3 GeV no ρ in π π , f0(1500) in 4π?
W. Ochs, glueballs at LHC – p.11 Advantages at LHC
higher energy of gluon jets → larger rapidity gaps quark and gluon jets at comparable energies in the same experiment higher statistics
W. Ochs, glueballs at LHC – p.12 separation of gluon and quark jets at LHC
1. leading order processes quark jets in γ + jet events (qg → γq) gluon jets in di-jet events (at small xT ) rates from pdf’s and parton parton cross sections
pT xT g in di-jet q in γ+ jet Tevatron (CDF) 1.8TeV 50 0.056 60% 75% LHC (G& S) 7TeV 200 0.057 60% 80% 50 0.014 75% 90% 800 0.229 25% 75% J. Gallicchio and M.D. Schwartz, 4/2011 quark jets: an 80% purity is ok for the study of leading systems (quarks fragment harder than gluons)
2. gluon bremsstrahlung gluon jets: from 3 jet events with high purity (> 90 %)
W. Ochs, glueballs at LHC – p.13 selection of gluon jets
⇒ trigger on total transverse energy select 3 jet events: soft gluon jet from bremsstrahlung: qqg or ggg production of low energy jet:
dσ αs αs 2 = σq 2 Pgq(xg)+ σg 2 Pgg(xg) dxg dpT 2πpT 2πpT
fraction of gluon jets: − 2 F (x )= σqPgq(xg )+σg Pgg (xg ) (P (x )= 4 1+(1 xg) ,...) g g σq(Pgq (xg )+Pqq(xg ))+σgPgg (xg ) gq g 3 xg for xg → 0: F (x )= 1 ; R = σg g g 1+4xg/(8+18Rg) g σq examples: xg = 0.2; Rg = 1 ⇒ Fg ≈ 95%
xg = 0.5; Rg = 1 ⇒ Fg ≈ 85% W. Ochs, glueballs at LHC – p.14 studies at LHC
1. Repeat rapidity gap studies at LEP in new environment:
⇒ larger rapidity gaps (∆y ∼ 4) (factor 10 in energy, ln 10 = 2.3); Q = 0, ±1 closer to asymptotics;
learn more about colour neutralization of gluon P3,P8
⇒ mass peaks in Q = 0 system? problem: limited angular acceptance due to rapidity gap
2. alternative approach: resonance production directly
⇒ mass spectra M(ππ), M(KK¯ ), M(4π) ... in jets study their x-dependence in quark and gluon jets
⇒ define reference x-distributions: "leading" (like u → π+) and "suppressed" (like u → π−, g → π)
W. Ochs, glueballs at LHC – p.15 large x fragmentation
meson quarkjet gluonjet triplet neutr. octet neutr. qq¯ : {ref : ρ, f2}, f0 leading suppressed suppressed gb : f0 suppressed suppressed leading qq¯ : f0, strongly mixed leading suppressed leading (?)
4q : σ, f0(980) (?) suppressed suppressed suppressed
W. Ochs, glueballs at LHC – p.16 x− dependent mass spectrum
cluster mass spectrum for xcluster small (many combinations)
glueballs among isoscalars
cluster scalar meson 0 (ππ) f0(600)/σ, f0(980), f0(1500) 0 (4π) f0(1370)(?), f0(1500) 0 (KK¯ ) f0(980), f0(1500) f0(1710)
xcluster large (one or few combinations)
W. Ochs, glueballs at LHC – p.17 Summary glueballs predicted in QCD since the very beginning no clear evidence yet new chance finding glueballs in gluon jets at LHC
large rapidity gaps - increased Qlead = 0 excess x-dependence of mass spectra in q and g jets important hints from LEP ⇒ new fragmentation component beyond JETSET clear excess of Qlead = 0 jets (up to 40%) not enough ρ? gluon jets may not be built from quark strings only
W. Ochs, glueballs at LHC – p.18