glueballs from gluon jets at the LHC

Transcription

1 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

2 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 PC = 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 m q ; decay to ππ W. Ochs, glueballs at LHC p.2

3 QCD sum rules 2 gluonic resonances to satisfy sum rules for 0 ++ M gb1 1 GeV, M gb2 1.5 GeV either 2 gb states (NV) or a mixed gb-q q system (HKMS) Experimental searches Narison-Veneziano 89 (broad M gb1 ) Harnett-Kleiv-Moats-Steele extra state in spectrum besides flavour nonets enhanced production in gluon rich processes suppression in γγ processes W. Ochs, glueballs at LHC p.3

4 glueball in scalar meson spectrum ossible solution: 0(1710) 0(1500) 3 isoscalars: 2 nonet q q states 0(1370) one extra state: glueball M 1.5 GeV Amsler, Close (980) 0(600)/σ could be from light nonet: q q, 4q, K K problem: f 0 (1370) not seen in energy-independent analyses (ππ) lternative possibility: 0(1500) 0(980) q q nonet (no f 0 (1370)) 0(600)/σ glueball M BW 1 GeV Minkowski, W.O. 98 Narison W. Ochs, glueballs at LHC p.4

5 gluon rich processes produce gb = (gg) central production in pp collisions: double Pomeron exchange: pp p f gb p f 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 eactions 1-4 proceed at low energies, role of gluon not obvious xample: LHC: (double Pomeron): excess of f 0 (980) and f 2 (1270) (q q)! omeron 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

6 leading systems in gluon jets π + (u d)+x: leading meson at large x carries initial quark n analogy: 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

7 different colour neutralization processes olour charges separated beyond confinement radius r R c : colour neutralization by pair production a) initial q q: b) initial gg olour triplet neutralization (P 3 ) colour triplet neutralization Q = 0,±1 lectric charge Q = 0,±1 (P 8 ) colour octet neutralization Q = 0 olour octet mechanism is precondition for leading glueballs W. Ochs, glueballs at LHC p.7

8 rapidity gap analysis apidity gap isolates leading cluster (charge Q lead, mass M lead ) > y rapidity: y = 1 2 ln E+p E p y or large rapidity gaps y : limiting distribution of charge Q lead Q lead = 0,±1 for (q q), probabilities from fragmentation models Q lead = 0 for (gg) charges Q lead > 1 are suppressed (multiquark exchanges) Results from LEP on Q lead and M lead from DELPHI, OPAL, ALEPH W. Ochs, glueballs at LHC p.8

9 rapidity gap analysis: leading chargeq lead gluon jet quark jet y = 1.5 DELPHI xcess Q lead = 0 in gluon jet s. MC (JETSET), excess 5-10% dependence on y W. Ochs, glueballs at LHC p.9

10 leading chargeq lead in gluon jets 1/N 3jets dn/dq identified b bg events gluon jet, no gap ALEPH g-jet data JETSET JETSET+GAL 1/N 3jets dn/dq ALEPH gluon jet, with gap ALEPH g-jet data JETSET JETSET+GAL AR0 AR Q g Q g JETSET ok GAL, AR refer to color reconnection models) Q lead = 0 excess of 40% (JETSET) W. Ochs, glueballs at LHC p.10

11 rapidity gap analysis: cluster mass forq lead = 0 DELPHI OPAL gluon jet gluon jet quark jet gluon jet dn dm leading 1 N (a) OPAL Jetset 7.4 Ariadne 4.11 Herwig 6.2 Quark jet background M leading (GeV/c 2 ) dn +- dm leading 1 N 2 (b) OPAL Jetset 7.4 Ariadne 4.11 Herwig 6.2 Quark jet background dn +-+dm leading 1 N (c) M leading (GeV/c 2 ) OPAL Jetset 7.4 Ariadne 4.11 Herwig 6.2 Quark jet background charged + neutrals M leading (GeV/c 2 ) excess at mass < 2.5 GeV (2σ) luon jets: excess of low mass M lead < 3 GeV no ρ in π + π, f 0 (1500) in 4π? W. Ochs, glueballs at LHC p.11

12 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

13 separation of gluon and quark jets at LHC. leading order processes uark jets in γ + jet events (qg γq) luon jets in di-jet events (at small x T ) ates from pdf s and parton parton cross sections p T x T g in di-jet q in γ+ jet Tevatron (CDF) 1.8 TeV % 75 % LHC (G& S) 7 TeV % 80 % % 90 % % 75% J. Gallicchio and M.D. Schwartz, 4/2011 uark jets: an 80% purity is ok for the study of leading systems (quarks fragment harder than gluons). gluon bremsstrahlung luon jets: from 3 jet events with high purity (> 90 %) W. Ochs, glueballs at LHC p.13

14 selection of gluon jets trigger on total transverse energy elect 3 jet events: soft gluon jet from bremsstrahlung: qqg or ggg roduction of low energy jet: dσ x g dp 2 T = σ q α s 2πp 2 T P gq (x g )+σ g α s 2πp 2 T P gg (x g ) raction of gluon jets: σ g(x g ) = q P gq (x g )+σ g P gg (x g ) σ q (P gq (x g )+P qq (x g ))+σ g P gg (x g ) (P gq (x g ) = 4 3 or x g 0: 1 g(x g ) = 1+4x g /(8+18R g ) ; R g = σ g σ q xamples: x g = 0.2; R g = 1 F g 95% x g = 0.5; R g = 1 F g 85% 1+(1 x g ) 2 x g,... ) W. Ochs, glueballs at LHC p.14

15 studies at LHC 1. Repeat rapidity gap studies at LEP in new environment: larger rapidity gaps ( y 4) (factor 10 in energy, ln10 = 2.3); Q = 0,±1 closer to asymptotics; learn more about colour neutralization of gluon P 3, P 8 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

16 largexfragmentation meson quark jet gluon jet triplet neutr. octet neutr. q q : {ref : ρ,f 2 },f 0 leading suppressed suppressed gb : f 0 suppressed suppressed leading q q : f 0, strongly mixed leading suppressed leading (?) 4q : σ,f 0 (980) (?) suppressed suppressed suppressed W. Ochs, glueballs at LHC p.16

17 x dependent mass spectrum luster mass spectrum for cluster small (many combinations) glueballs among isoscalars cluster scalar meson (ππ) 0 f 0 (600)/σ, f 0 (980), f 0 (1500) (4π) 0 f 0 (1370)(?), f 0 (1500) (K K) 0 f 0 (980), f 0 (1500) f 0 (1710) cluster large (one or few combinations) W. Ochs, glueballs at LHC p.17

18 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 Q lead = 0 excess x-dependence of mass spectra in q and g jets important hints from LEP new fragmentation component beyond JETSET clear excess of Q lead = 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