COMPASS - a facility to study QCD Eva-Maria Kabu, Institut f ur - PowerPoint PPT Presentation

COMPASS - a facility to study QCD Eva-Maria Kabuß, Institut f¨ ur Kernphysik, Mainz University for the Compass collaboration Hadron 2011 M¨ unchen, 13.-17.11.2011 – Compass experiment – What we have done – What we want to do COMPASS

What have we done COMPASS Compass is data taking since 2002 studying (E, p ) ’ ’ Nucleon spin puzzle: S N = 1 2 = 1 2 ∆Σ + ∆ G + L (E, p) e q • muon scattering on polarised p(NH 3 ) and d( 6 LiD) * γ h u with long. and transv. target polarisation N d h u π • addendum in 2010 (transv. p) and 2011 (long. p) • all three leading twist PDFs investigated ( f 1 , g 1 , h 1 ) π + Results: quark spin responsible for 30% of nucleon spin gluon contribution small in x range covered hardly any information on orbital angular momentum Hadron spectroscopy • 190 GeV/ c hadron beams ( π , p, K) on unpol. targets (liquid H 2 , Pb, Ni, Cu ,W) • searches for exotics, hybrids and glueballs • pion polarisabilities E. Kabuß, Munich, 14.6.2011 1

What will we do COMPASS Improve the 1-dimensional picture of the nucleon Generalized parton distribution (GPD) longitudinal momentum structure plus transverse spatial structure accessible in exclusive reaction like DVCS or DVMP Flavour separation and fragmentation in semi-inclusive deep inelastic scattering (SIDIS) improvement of strange quark distribution and fragmentation Transverse momentum dependent distributions (TMD) dynamic picture using intrinsic transverse momenta of partons accessible in SIDIS and Drell-Yan processes QCD at very low momentum transfers using Primakoff reactions to access inverse Compton scattering pion/kaon polarisabilities, testing chiral perturbation theory Compass II proposal: submitted in May 2010 for 5 years of data taking in the first phase approved in December 2010 for initially 3 years of data taking E. Kabuß, Munich, 14.6.2011 2

SPS proton beam: 1.4 10 13 /spill of 4.8s, 400 GeV/c COMPASS � Secondary hadron beams ( � ������������� 8 /spill, 150-270 GeV/c � Tertiary muon beam (80% pol): 2.10 8 /spill, 100-200 GeV/c -> Luminosity ~ 5 × 10 32 cm -2 s -1 with polarised targets COMPASS LHC CNGS Gran Sasso 732 kms SPS 3

SPS proton beam: 1.4 10 13 /spill of 4.8s, 400 GeV/c COMPASS � Secondary hadron beams ( � ������������� 8 /spill, 150-270 GeV/c � Tertiary muon beam (80% pol): 2.10 8 /spill, 100-200 GeV/c -> Luminosity ~ 5 × 10 32 cm -2 s -1 with polarised targets COMPASS LHC CNGS Gran Sasso 732 kms SPS high energy beam(s), broad kinematic range, large angular acceptance 4

COMPASS spectrometer Polarised target COMPASS 1m 3 He−Precooler Superconducting Solenoid Muon Wall 2 Acceptance (70mrad) E/ CAL2 H Dilution refrigerator Targets Trigger Hodoscopes SM2 ) Muon Wall 1 COMPASS 2002-03 data 2 6 (GeV 10 E/ CAL1 H 10 RICH µ SM1 2 Q 1 5 10 Target -1 10 4 10 -2 10 3 10 -3 10 target material: 6 LiD, NH 3 polarisation: 50%, 90% -4 2 10 10 -6 -5 -3 -4 -2 -1 10 10 10 10 10 10 1 x E. Kabuß, Munich, 14.6.2011 5

Primakoff experiments with π, K π − Z → π − Zγ π θ π π chiral perturbation theory predicts low energy behaviour Q 2 θ E γ γ � d σ πγ � + C · s − m 2 d σ πγ π = P ( α π , β π ) (A,Z) (A,Z) dΩ cm dΩ cm s 2 point Primakoff P ( α π , β π ) = (1 − cos θ cm ) 2 ( α π − β π )+(1+cos θ cm ) 2 ( α π + β π ) s 2 m 4 π +(1 − cos θ cm ) 3 ( α 2 − β 2 )( s − m 2 π ) 2 24 s γ • deviation from pointlike due to pion polarisabilities θ • measurements: α π − β π (at backward angles), α π + β π γ π 2-loop chiral prediction π α π − β π = (5 . 7 ± 1 . 0)10 − 4 fm 3 Inverse Compton α π − β π from 4 to 14 · 10 − 4 fm 3 experiments: E. Kabuß, Munich, 14.6.2011 6

Pion polarisability measurement 0.4 b] µ 0.3 [ CM Ω /d 0.2 2 s=3m σ π d E < 20 GeV γ 2 s=5m π 0.1 2 s=8m • effect increases with s 2 π 2 • effects due to α π − β π much s=15m π larger than for α π + β π - - - = 3.00, = -2.86 α β π π 0.02 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 cos θ CM unique at Compass : • kaon component in hadron beam: kaon polarisability accessible • availability of a muon beam (point like) for comparison and systematics • switching between pion and muon beam within few hours possible E. Kabuß, Munich, 14.6.2011 7

Projections for polarisabilities COMPASS • already two (test)measurements performed, clear signal from Primakoff events Number of Events COMPASS 2004 Hadron − Pb 3.0 mm π − Pb 2.0+1.0 mm π − Pb 1.6 mm π 4 10 − Cu 3.55 mm π − C 23.5 mm π − Pb 2.0+1.0 mm µ 3 10 2 10 10 1 0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01 2 2 Q [(GeV/c) ] • expected precision of the new measurement: in 120 d α π − β π α π + β π α 2 − β 2 ( 10 − 4 fm 3 ) ( 10 − 4 fm 3 ) ( 10 − 4 fm 5 ) 90 d with π , 30 d of µ beam 2-loop ChPT prediction 5 . 70 ± 1 . 0 . 016 ± 0 . 10 16 Compass sensitivity ± 0 . 66 ± 0 . 25 ± 1 . 94 E. Kabuß, Munich, 14.6.2011 8

Generalised parton distributions γ γ * q q’ Factorisation for x − ξ x + ξ hard Q 2 large, t < 1 GeV 2 soft GPDs p p’ t = − ∆ 2 H f , E f , � H f , � E f ( x, ξ, t ) • generalised parton distributions for quarks • limits: q ( x ) = H ( x, 0 , 0) normal PDF � F ( t ) = d x H ( x, ξ, t ) elastic form factor • Ji’s sumrule � 1 � � J f = 1 H f ( x, ξ, t ) + E f ( x, ξ, t ) lim d x x 2 t → 0 − 1 J f : total angular momentum contribution of quark f E. Kabuß, Munich, 14.6.2011 9

Nucleon tomography • GPDs allow simultaneous measurement of longitudinal momentum and transverse spatial structure t = − ∆ 2 • for ξ → 0 : ⊥ purely transverse and � d 2 ∆ ⊥ (2 π ) 2 e − i ∆ ⊥ · b ⊥ H f ( x, 0 , − ∆ 2 q f ( x, b ⊥ ) = ⊥ ) • b ⊥ distance to center of momentum ( b in figure is b ⊥ ) E. Kabuß, Munich, 14.6.2011 10

Why GPDs at Compass ? COMPASS • CERN high energy muon beam: • unique kinematic range – 100–160 GeV, 80% polarisation between HERA and – µ + and µ − with opposite polarisation HERMES/JLab – intermediate x : Q 2 (GeV 2 ) 20 COMPASS 160 GeV HERMES 27 GeV = ⇒ sea and valence quarks JLab 11 GeV ZEUS +H1 – high x limit from acceptance 10 9 8 – Q 2 up to 8 GeV 2 7 6 = ⇒ limit from cross section 5 with L = 10 32 cm − 2 s − 1 4 3 • planned measurements: 2 – deeply virtual Compton scattering COMPASS y=0.9 – deeply virtual meson production 1 y=0.05 -2 -1 10 10 1 x Phase 1 : 2.5 m long unpolarised liquid H 2 target = ⇒ GPD H Phase 2 : transversely polarised liquid NH 3 target = ⇒ GPD E E. Kabuß, Munich, 14.6.2011 11

Experimental requirements COMPASS • two competing processes: DVCS and BH • Bethe-Heitler dominates at low x , µp − → µγp used a reference yield • measurement with µ + and µ − with opposite polarisation S CS,U ≡ d σ + ↓ + d σ −↓ D CS,U ≡ d σ + ↓ − d σ −↑ • yield Re(H) and Im(H) • additionally deeply virtual meson production Experimental set-up • 2.5 m long liquid hydrogen target • 4 m long recoil proton detector (2 layers) • ’hermetic’ coverage with electro- magnetic. calorimetry E. Kabuß, Munich, 14.6.2011 12

Test measurement 2009 COMPASS • data taking with µ + (8 times more stat.) and µ − at about nominal intensity • 40 cm liquid H 2 target and small recoil proton detector • measure BH events plus relative DVCS and DVMP contributions • comparison of µ + and µ − data: µ − flux is factor of 3 lower at 160 GeV = ⇒ limitation on overall luminosity 0.005 < x < 0.01 0.01 < x < 0.03 x > 0.03 Bj Bj Bj nb of events nb of events nb of events 278 events |BH+DVCS| 2 134 events |BH+DVCS| 2 54 events |BH+DVCS| 2 35 8 80 2 2 2 |BH| |BH| |BH| 7 70 30 6 60 25 5 50 20 PRELIMINARY PRELIMINARY PRELIMINARY 4 40 15 3 30 10 2 20 5 10 1 0 0 0 -150 -100 -50 0 50 100 150 -150 -100 -50 0 50 100 150 -150 -100 -50 0 50 100 150 (deg) (deg) (deg) φ φ φ ⇒ clear DVCS signal observed at Q 2 > 1 GeV 2 , x > 0 . 03 = E. Kabuß, Munich, 14.6.2011 13

Projected results COMPASS projections with • Transverse imaging : 2 years of data B ( x ) ∼ 1 / 2 � r 2 ⊥ ( x ) � ε global = 10 % no model dependence L = 1222 pb − 1 • Azimuthal dependence : comparison to different models ⇒ c I = 1 ∝ Re ( F 1 H ) Beam Charge and Spin Asymmetry E µ =160 GeV 1 Q 2 4 GeV 2 0.03 x B 0.07 ≤ ≤ ≤ ≤ 0.3 8 B (GeV -2 ) VGG Reggeized (x,t)-correlation 0.2 VGG Factorized (x,t)-dependence 6 α ' = 0.125 Mueller fit on world data 0.1 (with JLab Hall A) 4 α ' = 0.26 < Q 2 > = 3.2 GeV 2 (without JLab Hall A) ZEUS < Q 2 > = 4 GeV 2 H1-HERA I 0 < Q 2 > = 8 GeV 2 2 H1-HERA II < Q 2 > = 2 GeV 2 -0.1 COMPASS 0 280 days at 160 GeV -0.2 -2 0 20 40 60 80 100 120 140 160 180 -4 -3 -2 -1 φ 10 10 10 10 E. Kabuß, Munich, 14.6.2011 14