The COMPASS experiment at CERN: hadron spectroscopy and open charm results

Transcription

1 COMPASS The COMPASS experiment at CERN: hadron spectroscopy and open charm results Oleg Kouznetsov JINR, Dubna On behalf of the COMPASS Collaboration BEACH2012 Wichita

2 COmmon Muon and Proton Apparatus for Structure and Spectroscopy ~210 physicists 11 counties 29 Institutes Hadron beam COMPASS I Muon beam 27/07/2012 BEACH2012 2

3 COmmon Muon and Proton Apparatus for Structure and Spectroscopy ~210 physicists 11 counties 29 Institutes Hadron beam COMPASS I Muon beam Unpolarised physics Polarised physics 27/07/2012 BEACH2012 3

4 COmmon Muon and Proton Apparatus for Structure and Spectroscopy ~210 physicists 11 counties 29 Institutes Hadron beam COMPASS I Muon beam Unpolarised physics Polarised physics Exc. V meson production Trans polar.. Long. polar. Λ hyperon polarization Quark densities q Pentaquarks Quark densities T q Gluon densities g 27/07/2012 BEACH2012 4

5 COmmon Muon and Proton Apparatus for Structure and Spectroscopy ~210 physicists 11 counties 29 Institutes Hadron beam COMPASS I Muon beam Coulomb production Diffractive scattering Central production Exc. V meson production Unpolarised physics Trans polar.. Polarised physics Long. polar. Λ hyperon polarization Quark densities q Pentaquarks Quark densities T q Gluon densities g 27/07/2012 BEACH2012 5

6 COmmon Muon and Proton Apparatus for Structure and Spectroscopy ~210 physicists 11 counties 29 Institutes Hadron beam COMPASS I Muon beam Coulomb production Polarizabilities Glueballs Hybrids Diffractive scattering Spectroscopy Central production Exc. V meson production Λ hyperon polarization Unpolarised physics Trans polar.. Quark densities q Polarised physics Long. polar. Multiquarks Pentaquarks Quark densities T q Gluon densities g 27/07/2012 BEACH2012 6

7 COMPASS LHC Lake LEMAN N CERN (France) SPS CERN (Suisse)

8 COMPASS in µ run NIM A 577(2007) 455 ECALs & HCALs Trigger Hodoscopes Drift Chambers RICH SM1 MWPCs SM2 µ Filters/Walls Straws GEMs Si Micromegas SciFi veto 27/07/2012 BEACH2012 8

9 COMPASS in µ run NIM A 577(2007) LiD target µ run Trigger Hodoscopes ECALs & HCALs Drift Chambers RICH SM1 MWPCs SM2 µ Filters/Walls Straws GEMs Si Micromegas SciFi veto 27/07/2012 BEACH2012 9

10 COMPASS in µ run NIM A 577(2007) LiD target µ run Trigger Hodoscopes ECALs & HCALs Drift Chambers RICH SM1 MWPCs SM2 µ Filters/Walls LH 2 target π run Straws GEMs Si Micromegas SciFi veto 27/07/2012 BEACH

11 Data taken :160 GeV µ on 6 LiD L,T 2006 :160 GeV µ on 6 LiD L 2007 :160 GeV µ on NH 3 T 2010 :160 GeV µ on NH 3 T 2011 :200 GeV µ on NH 3 L Outline Part I: nucleon spin structure Charmed D mesons & a gluon contribution to the nucleon spin 27/07/2012 BEACH

12 :160 GeV µ on 6 LiD L,T 2006 :160 GeV µ on 6 LiD L 2007 :160 GeV µ on NH 3 T 2010 :160 GeV µ on NH 3 T 2011 :200 GeV µ on NH 3 L GeV π, µ on Pb (2 weeks ) Data taken GeV π on LH GeV p,π + on LH 2, Pb, Ni, W Outline Part I: nucleon spin structure Charmed D mesons & a gluon contribution to the nucleon spin Part II: hadron reactions Search for exotic states in diffractive dissociation and central production 27/07/2012 BEACH

13 :160 GeV µ on 6 LiD L,T 2006 :160 GeV µ on 6 LiD L 2007 :160 GeV µ on NH 3 T 2010 :160 GeV µ on NH 3 T 2011 :200 GeV µ on NH 3 L GeV π, µ on Pb (2 weeks ) Data taken GeV π on LH GeV p,π + on LH 2, Pb, Ni, W 2112, SPS/LHC shutdown, 2014, 2015, 2016 Outline Part I: nucleon spin structure Charmed D mesons & a gluon contribution to the nucleon spin Part II: hadron reactions Search for exotic states in diffractive dissociation and central production Conclusion & COMPASS-II proposal 27/07/2012 BEACH

14 Part I: nucleon spin structure SLAC CERN DESY JLab RHIC E80 E130 E142/3 E154/5 EMC SMC COMPASS I HERMES CLAS/HALL-A Phenix/Star A worldwide effort since decades 27/07/2012 BEACH

15 Key apparatus: polarised target 3 He 4 He dilution refrigerator (T~50mK) solenoid 2.5T dipole magnet 0.6T acceptance ± 180 mrad Deuterated lithium ammonia 6 LiD/NH 3 (d/p) 50/90% pol. 40/16% dil. factor µ biggest /fabricated for SMC coll. Special shift during the data taking 27/07/2012 BEACH

16 Measurement of asymmetry 27/07/2012 BEACH

17 Nucleon spin puzzle since 1988 ½= ½ Σ + G + <L z > Measurement of G is important for two reasons: quarks gluons orb. mom. past present future - as an element of nucleon spin puzzle - possible role of axial anomaly in the a 0 interpretation (a 0 Σ) a 0 (= Σ) is measured to be experiments ~ instead of expected 0.6 a 0 = 27/07/2012 BEACH

18 π, p, K X t p Gluon polarisation g/g from µ N scattering Photon Gluon Fusion (PGF) Open Charm γ * g cc D X clean channel but experimentally difficult σ 100 nb limited statistics 0 N A ]] = R PGF <a LL > < g/g> + A bkg Spin asymmetry of cross sections for longitudinal polarizations of beam and target, parallel and antiparallel High-pT Hadron Pairs γ * g qq hh easy to get a statistics but physical background 2 cases Q 2 <1 GeV 2 (90% stat) & Q 2 > 1 GeV 2 (10% stat) 27/07/2012 BEACH

19 D* tag D 0 K π π 0 no D* tag D 0 K π N D =8650 D 0 K π N D =37750 Open charm (example) D D π * soft BR π 68% + D 0 K BR 4% Also were used: D 0 Kπππ D 0 Kππ 0 (no π 0 rec) D 0 K sub π (no K ID) Total number: D 0 mesons 27/07/2012 BEACH

20 high p T hadron pairs 10% of statistics - considerably higher statistics but physical background, resolved photons processes (last 4) are important only for low Q 2 QCD- Compton LO DIS qg + + gq + qq + gg 27/07/2012 BEACH

21 World direct measurements on g/g in LO 5 points from COMPASS LSS 10 DSSV 08 preliminary All measurements compatible with 0 QCD fits G as direct measurements point to a small value 27/07/2012 of G.. axial anomaly BEACH2012 contribution is small a 0 Σ21

22 Accessing G: QCD fits to world data (DIS, SIDIS) From Leader, Spin-2008 From DSSV, PRL 101, 2008 LSS-06 : Phys. Rev. D73, 2006 AAC 06 : Phys. Rev. D74, 2006 DSSV-08: Phys. Rev. Lett. 101, 2008 G(x) may be: positive, negative, or sign-changing! QCD analysis shows small first moment of g G G~ 2.5 is needed to restore Σ~ /07/2012 BEACH

23 Summary for Part I The worlds result for direct measurements of the gluon polarisation g/g major contribution of COMPASS indicates a small value of G (the first moment of g) confirmed by polarised pp at RHIC global QCD analysis of g 1 data confirms a small value of G: G = "spin crisis is unsolved yet Next step: a contribution of the angular orbital momentum of quarks and gluons in nucleon spin decomposition 27/07/2012 BEACH

24 Part II: Hadron reactions GeV π, µ on Pb (short run) GeV π on LH GeV p,π +,π on LH 2, Pb, Ni, W Beam intensity: had/s Negative: 96% π, 3.5% K Positive: 75% p, 25% π A large amount of data were collected with hadron beam in 2008/2009 ( times world statistics). π - p π + π - π - and π 0 π 0 π - (2008/2009 data) π - p 5π-final state at low four-momentum transfer (2004 data) π - p ηπ and η π (2008/2009 data) π + π - in central production pp p fast π + π p slow (2009 data) Spectroscopy using initial/final states with strangeness 27/07/2012 BEACH

25 Mesons in the Constituent Quark Model Spin-parity rules for bound q q system Quark spins couple to total intrinsic spin S = 0 (singlet) or 1 (triplet) Relative orbital angular Momentum L r and total spin S r r r couple r to meson spin J = S + L Parity P = ( _ 1) L+1 Charge conjugation C = ( _ 1) L+S Forbidden J PC = 0 --, 0 +-, 1 -+, 2 +-, 3 -+,... QCD allows for states beyond the CQM qqg Hybrids, glueballs, gg ggg, multi-quark states qqqq Exotic mesons have quantum numbers forbidden for Particularly interesting: J PC -exotic states q q 27/07/2012 BEACH

26 From old experiments: hybrids with J PC = 1 + Light meson sector exotics J PC =1 + : π 1 (1400) π N ηπ N 0 pn π π η pp 2π 0 η (E852, VES) (Crystal Barrel) (Crystal Barrel) π 1 (1600) π N ρπ N (E852, VES) η π N (Crystal Barrel) π 1 (2000) f 1 1 (1285) π N b (1235) π N pp b (1235) 1 ππ π N f (1285) π N (Crystal Barrel) 1 b (1235) π N 1 resonant nature controversial... The identification of exotics demands complete information on all neighbouring states and thus requires in particular: Reconstruction of final states containing both neutral and charged particles Observation of the same meson resonance in several different channels Production of resonances in different reactions 27/07/2012 BEACH

27 Difractive dissociation: π - p π - π - π + p Soft scattering of beam hadron off nuclear target (remains intact); Beam particle is excited into some intermediate state X X decays into n-body final state Partial-Wave Analysis (PWA) Isobar model: X - decay is chain of successive two-body decays Wave : unique combination of isobar and quantum numbers Fit of spin-density matrix for major waves with Breit- Wigner 27/07/2012 BEACH

28 Difractive dissociation: π - p π - π - π + p 96M exclusive events (2008) 27/07/2012 BEACH

29 Difractive dissociation: π - p π - π - π + p Left: Dalitz plot for a 2 (1320), events selected by ± Γ 0 around a 2 mass. Right: Dalitz plot for π 2 (1670) with ± 0.5Γ 0 27/07/2012 BEACH

30 Intensities of Major Waves a 1 (1260) π 2 (1670) a 2 (1320)

31 Diffractive dissociation: 3π final states π Pb π π π Pb + π p π π π p + π π π π 0 0 p p 420k events 96M events > 2.4M events Target: 3 mm Pb Trigger: Multiplicity No RPD Target: 40 cm LH 2 Trigger: Recoil proton RPD Cross-check: tracking vs ECAL Search for exotic J PC =1 + in all 3 samples 27/07/2012 BEACH

32 Diffractive pion dissociation (2004 data, Pb target) COMPASS, Phys. Rev. Lett. 104 (2010) PWA analysis of events mom transfer 0.1 < t < 1 (GeV/c) 2 quasi-free nucleons in Pb a 1 (1260), a 2 (1320) and π 2 (1670) are clearly visible 27/07/2012 BEACH

33 2004 data Pb target vs 2008 proton target 27/07/2012 BEACH

34 2008 proton target: π + π - π - vs π 0 π 0 π - Peak at 1.67 GeV/c 2 Phase motion indicates resonant behavior Structure at 1.2 GeV/c 2 unstable w.r.t. fit model 27/07/2012 BEACH

35 2008 proton target: π + π - π - vs π 0 π 0 π - Peak at 1.67 GeV/c 2 Phase motion indicates resonant behavior Structure at 1.2 GeV/c 2 unstable w.r.t. fit model Non-resonant background to be understood (Deck effect) 27/07/2012 BEACH

36 π Pb π π + π π + π Pb Light-meson frontier: access to mesonic states in 2 GeV/c 2 region Little information from previous experiments Known states: π 2 (1670); π(1800) Possible π 2 (2200) signal

37 Glueballs Quenched L-QCD prediction Lightest glueballs: M M ~ 1.7GeV/ c ~ 2.4GeV/ c ( J ( J = 0 = 2 2 PC PC + + ) ) Experimental candidates: f 0 (1500) (Crystal Barrel, WA102) J PC =0 ++ mixing with isoscalar mesons! Higher masses: exotic: J PC = 2 +-, 0 +- M ~ 4.3 GeV/c 2 [Y. Chen et al., Phys. Rev. D 73, (2006)]

38 Central production pp p fast π + π p slow Selection of central events: cut m(p fast π ± ),m(p slow π ± ) > 1.5 GeV/c 2 Invariant mass p fast π - Invariant mass p fast π + 27/07/2012 BEACH

39 pp p fast π + π p slow x F distribution π + π invariant mass Analysis similar to previous experiments (WA102) Comparable results 27/07/2012 BEACH

40 Final states with strangeness: π & K beams Glueball candidates decaying into KK f 0 (1380),f 0 (1500): qq mixing with gg? Hybrid candidates decaying into KKπ π(1800): usual 3 1 S 0 or Hybrid? K from beam identified by CEDAR K + K - identified by RICH: 10 GeV/c< P K < 30 GeV/c π p K + K π p, π p K s 0 K s 0 π p, π p K s 0 K L 0 π p, Κ p Κ π π + p,. 27/07/2012 BEACH

41 Summary for Part II COMPASS has excellent potential to contribute for hybrids, glueballs, multi-quark states searches in diffractive dissociation & central production Evidence for these QCD allowed states still not beyond doubt A large amount of data were collected with hadron beam in 2008/2009 ( times world statistics) Already observed the spin exotic wave π 1 (1600) in data from 2004 run The same analysis of 2008 data (π + π - π - and π 0 π 0 π - ) is still in progress Search for glueballs in central production has been started Measures charged and neutral channels: Independent consistency check. Measures kaonic final states 27/07/2012 BEACH

42 COMPASS II COMPASS-II was approved by the CERN Research Board: Dec. 1, COMPASS II 2012, SPS/LHC shutdown, 2014, 2015, 2016 Primakoff DVSC & DVMP Study Generalised Parton distributions Unpolarized SIDIS Fragmentation Functions, s-pdfs, TMDs Polarizabilities of π and K Drell-Yan Universality of TMDs 27/07/2012 BEACH

43 Recoil proton detector for 2.5 m long LH 2 target (Saclay/Freiburg) 4 m long scintillator slabs ~ 300 ps timing resolution 30º prototype tested successfully Gandalf Project: 1 GHz digitalisation of the PMT signal to cope with high rate 27/07/

44 New large-angle electromagnetic calorimeter ECAL0 (Dubna) ECAL1&2 DVCS+BH Requirements - Photon energy range GeV - Size: 360cm x 360cm ; - Granularity 4x4 6x6 cm 2 - Energy resolution < 10.0%/ E (GeV) - Thickness < 50 cm, - Insensitive to the magnetic field. new shashlyk modules for tests in plates made of Sc 0.8 mm /Pb 1.5 mm 251mm or 15 radiation length Prototype under studies Shaschlyk module with AMPD readout Avalanche Micropixel Photo Diodes 3 x 3 mm 2, densityof pixels /mm 2 Total length470mm with electronics 27/07/ mm

45 First Measurement of Chiral Dynamics in PRL 108, (2012) 27/07/2012 BEACH

46 Testing ChPT π Z π Zγ 2-loop chiral prediction measurments 27/07/2012 BEACH

47 Pion polarisability measurements COMPASS features Energy: 190 GeV High-I pion beam Muon beam = ref. For d anlgles: (α π +β π ) Back d angles (α π β π ) Unique at Compass : kaon component in hadron beam: kaon polarisability accessible availability of a muon beam (point like) for comparison/systematics switching between pion and muon beam within few hours possible 27/07/2012 BEACH

48 Projections for polarisabilities Two Primakoff test runs were performed in 2004 & /07/2012 BEACH

49 Isobar model: X decays via sequence of 2-body decays Intermediate resonances: isobars Partial wave: χ = J PC M ε [isobar R]L Decay amplitudes A χ (m,τ) calculable 3 variables for each 2-body vertex 3π decay: contain angular distributions and isobar parameterizations Partial Wave Analysis ( θ ϕ ) in mother r.f., {,,,, } m,, mother m θ φ θ φ τ GJ GJ m R H H Reflectivity basis: linear combinations p j m ( m) j m P ( 1) j ε = θ p ε m p j m θ ( m ) 1 2, m > 0 = 1 2, m = 0 0, m < 0

50 PWA Technique Illinois / Protvino / Munich Program BNL / Munich Program 1. PWA of angular distributions in 40 MeV mass bins ε ε I ( τ, m T A ( τ, m) indep N r ) = ε =± 1 r= 1 T ε ir i Production amplitudes extended maximum likelihood fit A ε ( τ, m) ε Decay amplitudes (Zemach tensors, D functions) 41 partial waves i=j PC M ε [...]L i [...] = (ππ) S, ρ(770), f 0 (980), f 2 (1270), ρ 3 (1690) Background wave added incoherently No assumption on resonant behavior is made at this point! 2. Mass-dependent χ 2 fit to results of step 1 6 waves Parameterized by Breit-Wigner Coherent background for some waves ir i 2