glueballs from gluon jets at the LHC Wolfgang Ochs - - PowerPoint PPT Presentation

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 JPC = 0++: mass ∼ 1600 ± 200 MeV unquenched results (including q¯ q) 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-q¯ q 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 q¯

q states f0(1370)

• ne extra state:→ glueball M ∼ 1.5 GeV

Amsler, Close ’96 . . .

f0(980) f0(600)/σ

could be from light nonet:

q¯ q, 4q, K ¯ K

problem:

f0(1370) not seen in energy-independent analyses (ππ)

alternative possibility:

f0(1500) f0(980) q¯ q 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. p¯

p → π gb

• 4. b → sg:

B → K gb

• 5. gluon jet at high energy: e+e− → q¯

qg, 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) (q¯ q)! Pomeron structure at HERA: large q¯ q 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+(u ¯ d) + 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 q¯ q: 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) || | | || − − − − − − − − − − − − − − −− > y rapidity: y = 1

2 ln E+p E−p

∆y for large rapidity gaps ∆y : limiting distribution of charge Qlead Qlead = 0, ±1 for (q¯ q), 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

gluon jet quark jet

∆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 gluon jet, no gap gluon jet, with gap

ALEPH g-jet

data JETSET JETSET+GAL

Qg 1/N3jets dN/dQ

0.05 0.1 0.15 0.2 0.25 0.3

• 4
• 2

2 4 6

ALEPH g-jet

data JETSET JETSET+GAL AR0 AR1

Qg 1/N3jets dN/dQ

0.005 0.01 0.015 0.02 0.025 0.03 0.035

• 4
• 2

2 4 6

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 gluon jet gluon jet quark jet gluon jet

0.2 0.4 1 2 3 4 5 6 7 8

Mleading (GeV/c2) 1 N dN dMleading

OPAL Jetset 7.4 Ariadne 4.11 Herwig 6.2 Quark jet background

(a)

2 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

M

leading +-

(GeV/c2) 1 N dN dM

leading +-

OPAL Jetset 7.4 Ariadne 4.11 Herwig 6.2 Quark jet background

(b)

1 0.5 1 1.5 2 2.5 3 3.5 4

M

leading +-+-

(GeV/c2) 1 N dN dM

leading +-+-

OPAL Jetset 7.4 Ariadne 4.11 Herwig 6.2 Quark jet background

(c)

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) 1.8 TeV 50 0.056 60% 75 % LHC (G& S) 7 TeV 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 dxgdp2

T = σq

αs 2πp2

T Pgq(xg) + σg

αs 2πp2

T Pgg(xg)

fraction of gluon jets: Fg(xg) =

σqPgq(xg)+σgPgg(xg) σq(Pgq(xg)+Pqq(xg))+σgPgg(xg)

(Pgq(xg) = 4

3 1+(1−xg)2 xg

,. . . ) for xg → 0: Fg(xg) =

1 1+4xg/(8+18Rg);

Rg = σg

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(K ¯ K), 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 quark jet gluon jet triplet neutr.

• ctet neutr.

q¯ q : {ref : ρ, f2}, f0 leading suppressed suppressed gb : f0 suppressed suppressed leading q¯ q : 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 (ππ)0 f0(600)/σ, f0(980), f0(1500) (4π)0 f0(1370)(?), f0(1500) (K ¯ K)0 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