compass : a facility to study qcd co common m muon and p proton a apparatus for s structure and s...
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COMPASS : a Facility to study QCD
COMMON MUON and PROTON APPARATUS for STRUCTURE and SPECTROSCOPY
1
N. d’Hose (CEA Saclay)
Studies until now 2011: Nucleon Spin with high energy polarized beams + polarized targets:
longitudinal spin: gluon and quark helicity distribution
transverse spin and transverse momentum dependent distribution Spectroscopy with hadron beams + LH2 (or solid) targets:
Search of hybrids and glueballs to better understand quark and gluon confinement
Primakoff with π, K beam Test of Chiral Perturb. Theory
DVCS & DVMP with μ beams Transv. Spatial Distrib. with GPDs
SIDIS (with GPD prog.) Strange PDF and Transv. Mom. Dep. PDFs
Drell-Yan with beams Transverse Momentum Dependent PDFs
New Program (SPSC-P-340) until ~2016 recommended by SPSC and approved by Research Board
COMPASS-II: a Facility to study QCD
COMMON MUON and PROTON APPARATUS for STRUCTURE and SPECTROSCOPY
1
SPS proton beam: 2.6 1013/spill of 9.6s each 48s, 400 GeV/c Secondary hadron beams (, K, …): 6.108 /spill, 50-200 GeV/c Tertiary muon beam (80% pol): 4.6108 /spill, 100-200 GeV/c -> Luminosity ~ 1032 cm-2 s-1 GPD with + and a 2.5m long LH target ~1.2 1032 cm-2 s-1 DY with - and a 1.1m long NH3 target
LHC
high energy beams, broad kinematic range, large angular acceptance
SPS
CNGSGran Sasso 732 kms
COMPASS
60m
Meson Spectroscopy3
Hadron Reactions at COMPASS
COMPASS: • production mechanisms studied in parallel using proton, pion and kaon beams• H2, Ni and Pb targets• identification of charged and neutral particles in a large and flat acceptance • 10 world statistics
PWA analysis (international task force for COMPASS, JLab, PANDA…)
4
Search of the controversial hybrids 1-(1600) with JPC=1-+ in the reaction -p 1-(1600) p -p +--p
+ Pb (2009 data) 2.3M events+ other targets W, Ni
Search of the controversial hybrids 1-(1600) with JPC=1-+ in the reaction -p 1-(1600) p -p +--p
a1(1260) 2(1670)
a2(1320) 1(1600)
JPCM(isobar,)L
Pb 2004 data
PRL104 (2010) 241803
Search of the controversial hybrids 1-(1600) with JPC=1-+ in the reaction -p 1-(1600) p -p +--p
a1(1260) 2(1670)
a2(1320) 1(1600)
Phase between 1(1600) and a1(1260)
JPCM(isobar,)L
Clear 1-(1600) signal in
+-- for Pb target
PRL104 (2010) 241803
Pb 2004 data
Nuclear Medium effect
Nuclear effect: Production of Spin Projection M=1 states enhanced for larger A
Pbfrom Pb (2004) and H2 (2008) targets
Pb
Pb
Pb
Pb
H2
H2
H2
H2
7
QCD at low energy:
Primakoff experiments with π, K or inverse Compton Scattering on π, K
8
the point-like cross section is measured with the muon beam
Deviation due to polarisabilities
The chiral perturbation theory (ChPT) predicts the low-energy behavior of the cross section with s varying from threshold (mπ
2) to a few mπ2
π (or Κ)
π (or Κ)
γ Q2 0 γ s = (p+p)2 = ( CM energy)2
t = (cos cm -1) (s - mπ
2)2s
γ
)θcos,,( )(
. cm2
2
likepointcmcm
P
s
msC
d
d
d
d
Pion Polarisabilities and Chiral predictions 9
The pion: fundamental role for QCD at low-energy Goldstone boson (spontaneous breaking of chiral symmetry) lightest quark-gluon bound state system understanding its internal structure is a fundamental challenge
The polarisabilities give the deformation of the pion shape by an EM field
> 0 S=0 diamagnetic contr. <0
2-loop ChPT prediction: + = (0.2 0.1) 10-4 fm3
- = (5.7 1.0) 10-4 fm3 Previous experiments: - varies from 4 to 14 .10-4 fm3
accuracy 0.025 0.66
Goal ofCOMPASSin 120 days
Px
p ’X Deep Inelastic Scattering
Q²xB
x
p
γ*
Parton Distribution q ( x )
x boost
x P
z
y
QCD at high energy: Deep Inelastic Scattering
’
10
Quark with momentum x in a nucleon
Quark with spin parallel to the nucleon spinin a longitudinally polarized nucleon
Quark with spin parallel to the nucleon spinIn a transversely polarized nucleonChiral odd, accessible in SIDIS and DY
3 twist-2 PDFs
or f1q(x)
or g1q(x)
Unpolarized PDF
Helicity PDF
Transversity PDF
or h1q(x)
Helicity quark distributions
From polarized semi-inclusive DIS analysis,Using charged pions and kaons which tag quark flavour on both proton and deuteron polarized targets Full quark flavour decomposition (at LO)
Longitudinal spin asymmetry
We need unpolarized PDF (MRST04)and parameterization of FF from DSS (DeFlorian, Sassot, Stratmann, PRD75 (2007) 114010)
quark Fragmentation Function (FF)
11
• COMPASS PLB693 (2010) 227 full flavour separation at LO down to x=0.004
o HERMES PRD71 (2005)
-DSSV predictions at NLO (DeFlorian, Sassot, Stratmann, Vogelsang PRD80 (2009) 034030)
LO Helicity quark distributions
Good agreement with global fits to g1 inclusive data
The flavor asymmetry of the sea helicity distributions is compatible with 0(while there is a considerable wide asymmetry in the unpolarized case)
s ~ 0 but the DSSV fit suggests negative values at smaller x
12
Transverse spin or momentum PDF 13
q
hq
2q
qhqTT
2q
Dqe
DΔqΔeCollA
Sivers Collins
qhq
2q
qhq1Tq
2q
Dqe
De
SivA
fCollins fragmentation function, depends on spin
Usual quark fragmentation function
at LO:
• SIDIS with transversely polarized target
• Measure simultaneously two azimuthal asymmetries (h, S)
Sivers: Nucleon spin & quark transverse momentum
Collins: Outgoing hadron direction & quark transverse spin
m p m p h+/-
Transversity PDF
or h1q(x)
= f1T(x)
One Transverse Momentum PDF
beyond the collinear approximation, quark transverse momentum (kT) effects
Transversity from Collins asymmetry
Fit to AdCOLL(x) from COMPASS and Ap
COLL(x) from HERMES
and Collins fragmentation function from BELLE (e+e- scatt.)
in good agreement with new proton data
14
COMPASS proton 2007
Sivers TMD from Sivers asymmetry
Positive hadrons
Negative hadrons
Comparison with predictions from Anselmino et al., based on fit of Hermes-p and Compass–d data
Extraction of Sivers fct(HERMES p and COMPASS d)
x
Present proton data not in fit-COMPASS signal < HERMES signal-Possible W dependence-proton 2010 data to be analysed
= f1T(x)
15
COMPASS proton 2007
After SIDIS, Drell-Yan to study TMDs
Drell –Yan π- p +-X with intense pion beam (up to 109 /spill) with the transversely polarised NH3target with the COMPASS spectrometer equipped with an absorber
Cross sections:
In SIDIS: convolution of a TMD with a fragmentation function
In DY: convolution of 2 TMDs ‘’’’ complementary information and universality test
16
-
‘
Why DY is very favourable at COMPASS? 17
dominated by the annihilation of a valence anti-quark from the pion and a valence quark from the polarised protonThis will be the only such experimental measurement for at least 5-10 years
1rst moment of Sivers function for u quark
large acceptance of COMPASSin the valence quark region for p and π where SSA are expected to be larger
Access to TMDs for incoming pion target nucleon (TMDs as Transversity, Sivers, Boer-Mulders, pretzelosity)
Experimental check of the change of sign of TMDs confronting Drell-Yan and SIDIS results
The T-odd character of the Boer-Mulders and Sivers function implies that these functions are process dependent
)()( 11 DYfSIDISf TT
)()( 1 1 DYhSIDISh Boer-Mulders
Sivers
In order not to be forced to vanish by time-reversal invariancethe SSA requires an interaction phase generated by a rescattering of the struck parton in the field of the hadron remnant
18
FSI ISI
Need experim
ental v
erifica
tion
Test
of c
onsiste
ncy
o
f the a
pproach
COMPASS + HERMES have already measured non zero Sivers SSA in SIDIS
Results from test measurements in 200919
2009 with an absorber up to 1.5 108 /spill (without radioprotection pb)
Expected: 3600±600 J/ and 110±22 DYMeasured: 3170±70 J/ and 84±10 DY
4 < M+- < 9 GeV 2 < M+- < 2.5 GeV
M+- GeV
1- safe domain for Drell-Yan
Large Combinatorial Background S/B ~ 1
Open charm decays (from D0) smaller than 15% of signal
2.9 < M+- < 3.2 GeV
4 < M+- < 9 GeV
2-reasonable domain for Drell-Yan
2 < M+- < 2.5 GeV
3 domains of study
3- domain for J/ mechanism
2.9 < M+- < 3.2 GeV
if q-qbar dominates over g-g fusionDrell-Yan can be considered
20Predictions for Drell-Yan at COMPASS
Sivers function in the safe dimuon mass region 4 < M+- < 9 GeV
2 years of data190 GeV pion beam6 .108 π-/spill (of 9.6s)1.1 m transv. pol. NH3 targetLumi=1.2 1032 cm-2s-1
Blue line with grey band: Anselmino et al., PRD79 (2009)Black solid and dashed: Efremov et al., PLB612 (2005)Black dot-dashed: Collins et al., PRD73 (2006) Squares: Bianconi et al., PRD73 (2006)Green short-dashed: Bacchetta et al., PRD78 (2008)The change of sign can already be seen in 1 year
Deeply Virtual Compton Scattering
Generalized Partons Distrib. H( x,,t )
p ’p’
P’GPDs
* Q²
x+ x-
p
t
x P
y
z
b
x boost
( Px, b )
from inclusive reactions to exclusive reactions
Observation of the Nucleon Structure in 1 dimension in 1+2 dimensions
21
Px
p ’X Deep Inelastic Scattering
Q²xB
x
p
γ*
Distrib. de Partons q ( x )
x boost
x P
z
y
’
22
From Wigner phase-space-distributions (Ji, PRL 2003, Belitsky, Ji, Yuan PRD 2004)We can build « mother-distributions » (Meissner, Metz, Schlegel, JHEP 0908:056 2009)
and derive
k
Exploring the 3-dimensional phase-space structure of the nucleon
GPD: Generalised Parton Distribution (position in the transverse plane) TMD:Transverse Momentum Distribution (momentum in the transv. plane)
PDF GPD TMD
New research fields
Allow for a unified description of form factors and parton distributions Allow for transverse imaging (nucleon tomography) and give access to total angular momentum (through E)
Generalized Partons Distributions (H,E,…)
Tomographic parton images of the nucleon Impact parameter b
Longitudinal momentum fraction x
23
x 0.003 x 0.03 x 0.3
CERN High energy muon beam
100 - 190 GeV μ+ and μ- available 80% Polarisation
with opposite polarization Will explore the intermediate xBj region
Uncovered region between ZEUS+H1 and HERMES+Jlab
before new colliders may be available presently only 2 actors: Jlab and COMPASS
Transverse structure at x~10-2 essential input for phenemenology of high- energy pp collision (LHC)B
What makes COMPASS unique for GPDs? 24
Experimental requirements for DVCS
μ’
p’μ
Phase 1 ~ 2.5 m Liquid Hydrogen Target ~ 4 m Recoil Proton Detector (RPD)
ECALs upgraded
+ ECAL0 before SM1
ECAL2
ECAL1
μ p μ’ p
SM1
SM2
25
Contributions of DVCS and BH at E=160 GeV
θμ’μ
*
pDeep VCS Bethe-Heitler
BH dominates study of Interference DVCS dominatesexcellent Re TDVCS study of dDVCS/dt
reference yield or Im TDVCS Transverse Imaging
26
d |TDVCS|2 + |TBH|2 + Interference TermMonte-CarloSimulationfor COMPASSset-up with only ECAL1+2
MissingDVCS acceptancewithout ECAL0
θμ’μ
*
p
Deeply Virtual Compton Scattering
dDVCS /dt ~ exp(-B|t|)
Phase 1: DVCS experiment to study the transverse imaging
with +, - beam + unpolarized 2.5m long LH2 (proton) target SCS,U d( +) + d( -) sin. 1
IntDVCSunpol
BH sKdd
Using SCS,U and integration over and BH subtraction
dσ(μpμp) = dσBH + dσDVCSunpol
+ Pμ dσDVCSpol
+ eμ aBH Re ADVCS + eμ Pμ aBH Im ADVCS
27
DVCS: Transverse imaging at COMPASS dDVCS/dt ~ exp(-B|t|)
2 years of data160 GeV muon beam2.5m LH2 target global = 10%
28
without any model we can extract B(xB)
B(xB) = ½ < r2 (xB) >
r is the transverse size of the nucleon
DVCS: Transverse imaging at COMPASS dDVCS/dt ~ exp(-B|t|)
14Transverse size of the nucleon
0.65 0.02 fm
H1 PLB659(2008)
1.
0.5
2r
?xB
COMPASS
without any model we can extract B(xB)
B(xB) = ½ < r2 (xB) >
r is the transverse size of the nucleon
DVCS: Transverse imaging at COMPASS dDVCS/dt ~ exp(-B|t|)
B(xB) = b0 + 2 α’ ln(x0/xB)
ansatz at small xB
inspired by Regge Phenomenology:
with the projected uncertainties we can determine :
α’ slope of Regge traject
B with an accuracy of 0.1 GeV-2
α’ with an accuracy 2.5 if α’ 0.26 with ECAL1+2 if α’ 0.125 with ECAL0+1+2
Phase 1: DVCS experiment to constrain GPD H
with +, - beam + unpolarized 2.5m long LH2 (proton) target
and Im(F1 H) and Re(F1 H)cos10
IntInt cc Intc 1,0
SCS,U d( +) + d( -) sin. 10IntDVCSBH sKcd Ints1
DCS,U d( +) - d( -)
Deeply Virtual Compton Scattering
Re H (,t)= P dx H(x,,t) /(x-)
Im H (,t)= H(x= ,,t)
xB / (2-xB)
29
dσ(μpμp) = dσBH + dσDVCSunpol
+ Pμ dσDVCSpol
+ eμ aBH Re ADVCS + eμ Pμ aBH Im ADVCS
Beam Charge and Spin Difference (using DCS ,U)
Comparison to different models
2 years of data160 GeV muon beam2.5m LH2 target global = 10%
Systematic error bands assuming a 3% charge-dependent effect between + and - (control with inclusive evts, BH…)
θμ’μ
*
p
30
’=0.8’=0.05
2008-9 tests: observation of BH and DVCS events
31
10 BH events expected ~30 « pure » DVCS eventswith ~10 0 contamination
During the hadron program with 1m long recoil proton detector (RPD) and 40cm long LH2 target and the 2 existing ECAL1 and ECAL2
2008: observation of exclusive single photon production, εglobal = 0.13 +/- 0.05 confirmed εglobal = 0.1 as assumed for simulations
2009: observation of BH and DVCS events
GPDs investigated with Hard Exclusive Photon and Meson Production
the t-slope of the DVCS cross section ……………… LH2 target +
RPD……phase 1 transverse distribution of partons
the Beam Charge and Spin Sum and Difference and Asymm…………….phase 1 Re TDVCS and Im TDVCS for GPD H determination
the Transverse Target Spin Asymm………polarised NH3 target +
RPD……phase 2 GPD E and angular momentum of partons (future addendum)
Summary for GPD @ COMPASS32
Only selected topics among the large harvest of (present and incoming) results on Spin and Spectroscopy
The main topics of COMPASS-II are: GPD with DVCS and DVMP TMD with DY and SIDIS precise unpolarised PDF measurements Chiral perturbation theory - soft QCD
+ updated programs on spectroscopy, DY, GPD…
Jlab 12 GeV, huge luminosity, very promising facility
and for the next 10 years, before ENC, EIC, eRHIC, LHeC, COMPASS@CERN can be a major player in QCD physics using its unique 200 GeV hadron and polarised muon beams
Conclusions33
4
22
cm
2
cm)()( (
)(. )θcos1()θcos1(
cmcm2
2
pt
m
s
s
msC
d
d
d
d
))(24
2)
2(3
)cmθcos1( 22s
ms
Pion Polarisabilities measurement5
Polarisability effect with increasing s at backward or forward angle
The 3 components
( - ), ( + ), (2 - 2)can be measured
point-like case
for ChPT prediction
DYd
))2),(,( ) sin( ) 2221
2111212
21
2STTTTTT kxkxk(kkdkd hh, Z
)2),(,( ) sin( ) 2221
2111212
21
2STTTTTTT kxkxk(kkdkd hh, Y
STTTTTTT kxkxk(kkdkdT ff, sin )S ),(,( ) 2221
2111212
21
2( X
)( 2,(,( ) cos ) )1 2221
2111212
21
2 TTTTTT kxkxk(kkdkd hh, W
The Drell-Yan process in π- p15
Lepton plane
Collins-Soper frame (of virtual photon)
, lepton plane wrt hadron plane
target rest frame S target transverse spin vector /virtual photon
Hadron plane
Access to TMDs for incoming pion target nucleon TMD as Transversity, Sivers, Boer-Mulders, pretzelosity
π- 190 GeV
DY and COMPASS set-up17
Key elements for a small cross section investigation at high luminosity 1. Absorber (lesson from 2007-8 tests) to reduce secondary particle flux2. COMPASS Polarised Target3. Tracking system and beam telescope4. Muon trigger (LAS of particular importance - 60% of the DY
acceptance)5. RICH1, Calorimetry – also important to reduce the background
xF=x-xp
Results from test measurements in 200919
<Pt> = 1 GeV/c COMPASS sensitive to TMDs
21Predictions for Drell-Yan at COMPASS
xF=x-xpxF=x-xp xF=x-xp xF=x-xp
Statistical accuracy from 0.01 to 0.02 in two years
-0.2 < xF < 0.85
21Predictions for Drell-Yan at COMPASS
xF=x-xp xF=x-xp xF=x-xp
Statistical accuracy 12% in two years
xF=x-xp
22 Summary for Drell-Yan @ COMPASS
First ever polarised Drell-Yan experiment sensitive to TMDs
Pure valence u dominance because of the - beam
Key measurements: TMDs universality SIDIS DY change of sign check from SIDIS to DY for Sivers and Boer-Mulders study of J/ production mechanism
After a series of beam tests, the feasibility is proven
In a phase 2 (future addendum), measurement with anti-proton beams will be envisaged
Px
p ’X Deep Inelastic Scattering
Q²xB
x
p
γ*
Parton Distribution q ( x )
x boost
x P
z
y
QCD at high energy: Deep Inelastic Scattering
’
11
While unpolarised light quark PDF well constrained, strange quark distributions are not so well known
COMPASSwith SIDIS
Semi-Inclusive Deep Inelastic Scattering8
Semi-Inclusive DIS measurements during GPD program with a pure proton target with RICH detector and Calorimeters
Charge separation and identification K+, K-, K0, +, -, 0, …
Major progress as compared to previous experiments to improve Unpolarised PDF
Hadron multiplicities at LO PDF quark Fragmentation Functiondepend on x depend on z (fraction of energy of the outgoing hadron)
Final goal: extensive measurements (x,z,…) to provide input to NLO global analysis for PDF and FF
Projection for 1 week with 2.5m LH2 target high statistics
Projection of errors for s(x) 9
Short term goal: LO analysis from COMPASS data alone integrated over z
Semi-Inclusive Deep Inelastic Scattering11
Semi-Inclusive DIS measurements during GPD program with a pure proton target with RICH detector and Calorimeters
Cahn effect info on <kT>
Boer-Mulders TMD Collins FF + Cahn effect
measurement of Acos h and Acos 2h
+ knowledge of Collins FF Boer-Mulders TMD
determination
Unpol. cross section:
14 Projection of errors for Acos h and Acos 2h
12
• Projection proton (LH2) 1 week
• COMPASS 2004 deuteron 4 weeks
+ flavor separation using RICH particle identification
Input for global analysis
h
Semi-Inclusive Deep Inelastic Scattering10
Asymmetries in the azimuthal angle h
of the outgoing hadron around the virtual photoncan reveal quark transverse spin and quark transverse momentum (kT) effectsbeyond the collinear approximation
At leading twist, not only f1q(x, kT), g1L
q(x, kT), h1q(x, kT)
but also 5 otherTransverse Momentum Dependent PDF (TMD (x, kT))which do not survive after integration on kT
2 examples of TMDs
The Boer-Mulders function
correlates the quark kT andthe quark transverse spin (unpol N)
The Sivers function
correlates the quark kT andthe nucleon spin (transv. Pol. N)
gluon polarization
Direct measurement (at LO untill now ):
global QCD analysis :
Use Q2 evolution of spin Dependent gluon and singletQuark distribution g1(x,Q2)
COMPASS NLO fit of g1 data:2 solutions with |G|=0.2 – 0.3
14
gluon polarization
Longitudinal spin asymmetry
13