RISULTATI RECENTI DI
COMPASS
Andrea Bressan
INFN e Università di Trieste
Two classes of
phenomena
 Longitudinal Spin Case
Transverse Spin Case
12/22/2015
Andrea Bressan
2
ΔG Measurements in
COMPASS
Open charm – D0 and D*
events
All deuterium data analyzed (2002-2006)
D0 selection: Kp invariant mass + cuts on kinematics + RICH PID
D0  Kπ
D*  D0πs  Kππ
 G 
 1
 G 

12/22/2015
S
S
SB
Andrea Bressan
4
ΔG/G da open-charm
NEW: PRESENTED @ DIS2008
12/22/2015
Andrea Bressan
5
ΔG/G da adroni ad alto pT
Q2 > 1 (GeV/c)2
Photon Gluon
Fusion ~ 30%
Leading Order
QCD Compton
PGF


PGF
LL
ALL  R
12/22/2015
a
Andrea Bressan
G  A
BKGR
G
6
adroni ad alto pT: data/MC
Data /MC
12/22/2015
Andrea Bressan
7
Sommario ΔG/G da
COMPASS
COMPASS
QCD fits to g1
New high pT
|G| ~ 0.2-0.3
Q2=3 (GeV/c)2
New open charm
12/22/2015
Andrea Bressan
8
Transverse Spin effects in
COMPASS
What we are after? (Trans)
three quark distribution functions (DF) are necessary to describe
the structure of the nucleon at LO
q(x)
f1q (x)
unpolarised DF
quark with momentum xP in a nucleon
well known – unpolarised DIS
 vector charge
q(x)
g1q(x)
 axial charge
quark with spin parallel to the nucleon
spin in a longitudinally polarised nucleon
known – polarised DIS
Tq(x) = q↑↑(x) - q↑↓(x)
h1q(x),
transversity DF
 tensor charge
12/22/2015
helicity DF
quark with spin parallel to the
nucleon spin in a transversely
polarised nucleon
largely unknown
ALL 3 OF EQUAL IMPORTANCE
Andrea Bressan
10
Single hadron asymms.
Collins and Sivers terms in SIDIS cross sections depend
on different combination of angles:
C = h - s’
Collins angle
 S = h - s
Sivers angle
h azimuthal angle of the hadron
s azimuthal angle of the transverse spin of the initial quark
s’ azimuthal angle of the transverse spin of the fragmenting quark
12/22/2015
Andrea Bressan
s’ = p - s (spin flip)
11
Proton 2007 data…
450 TB collected (by november)!
~20% of the data processed quasi on-line!
First data crunching! All 459 TB processed
end by January-Febrary
Stability cheks done!
Systematic studies done!
First results by May!
12/22/2015
Andrea Bressan
12
2007 Transverse data
statistics
2007 run: May to November
equally shared between transverse and longitudinal
Period
“Weeks”
Target Polarization
Transverse polarization
1
25 / 26
-+- / +-+
data taking:
2
27 / 28
-+- / +-+
3
30 / 31
+-+/ -+-
4
39 / 40
+-+/ -+-
5
41 / 42a
-+- / +-+
6
42b / 43
+-+/ -+-
Data used for these results
several stability tests have been performed
• detectors and triggers performances
• event reconstruction
• K0 reconstruction
• distributions of kinematical variables:
( zvtx, Em’, m’, xBj Q2,y, W, Ehad, hadLab , qhadLab , hadGNS , qhadGNS , pt)
12/22/2015
Andrea Bressan
13
Collins asymmetry – proton
NEW: PRESENTED @ TRANSVERSITY2008
data
statistical errors only; systematic errors ~ 0.3 sstat
at small x, the asymmetries are compatible with zero
in the valence region the asymmetries are different from zero,
of opposite sign for positive and negative hadrons,
and have the same strength and sign as HERMES
12/22/2015
ECT*, June 12, 2008
Andrea Bressan
Anna Martin
14
Comparison with predictions
12/22/2015
Andrea Bressan
15
SIVERS Mechanism
 The Sivers DF
Δ0T q is probably the most famous between TMDs…
 gives a measure of the correlation between the transverse momentum and
the transverse spin
 Requires final/initial state interactions of the struck quark with the spectator
system and the interference between different helicity Fock states to survive
time-reversal invariance
 Time-reversal invariance implies:
Δ0T q (x,k 2T )SIDIS   Δ0T q (x,k 2T )DY
…to be checked – Drell-Yan program in COMPASS
 In SIDIS:
Nh  S   Nh0   1  AhS  sin S
12/22/2015

ASiv 
Andrea Bressan
h
S
A

f  PT
 e  Δ qD
 e  qD
q
2
q
q
T
0
2
q
h
q
h
q
16
Sivers asymmetry – proton
NEW: PRESENTED @TRANSVERSITY2008
data
statistical errors only; systematic errors ~ 0.5 sstat
the measured symmetries are small, compatible with
zero
12/22/2015
ECT*, June 12, 2008
Andrea Bressan
Anna Martin
17
Sivers asymmetry– proton
data
comparison with the most recent predictions from M. Anselmino et al.
arXiv:0805.2677
12/22/2015
ECT*, June 12, 2008
Andrea Bressan
Anna Martin
18
Results: Sivers asymmetry
comparison with predictions from
S.Arnold, A.V.Efremov, K.Goeke, M.Schlegel and P.Schweitzer, arXiv:0805.2137
12/22/2015
ECT*, June 12, 2008
Andrea Bressan
Anna Martin
19
UNPOLARIZED
ASYMMETRIES
unpolarised target
SIDIS cross-section
3 independent azimuthal modulations
in h , the hadron azimuthal angle in GNS
12/22/2015
ECT*, June 12, 2008
Andrea Bressan
Anna Martin
21
unpolarised target SIDIS
cross-section
acceptance
to extract the asymmetries
the azimuthal distributions have to be
corrected by the apparatus acceptance
 dedicated MC simulations
for L and T target polarisation data
initial azimuthal distribution
12/22/2015
Andrea Bressan
final azimuthal distribution
22
results: sin modulation
A sin /  s
s 
2y 1  y
1  (1  y) 2
error bars:
statistical
errors
bands:
systematical
errors
12/22/2015
ECT*, June 12, 2008
Andrea Bressan
Anna Martin
23
results: cosΦ modulation
A cos /  c
c 
12/22/2015
ECT*, June 12, 2008
Andrea Bressan
Anna Martin
2(2  y) 1  y
1  (1  y) 2
24
results: cos 2Φ modulation
A cos 2 /  c 2
c2 
12/22/2015
2(2  y)
1  (1  y) 2
ECT*, June 12, 2008
Andrea Bressan
Anna Martin
25
COMPASS FUTURE
PROGRAM
 Proposal to CERN in preparation
(deadline December 2008)
Transverse SPIN effects
Drell-Yan with pSIDIS on protons
 GPD via DVCS
12/22/2015
Andrea Bressan
26
FOTORIVELATORI basati su TH(ick)
GEM


necessita’ future di COMPASS
tradizione ed interesse per rivelatori di frontiera
R&D per lo sviluppo di fotorivelatori
a gas di terza generazione basati su MPGD
(Micro Pattern Gaseous Detector)
W2
W2:
D=0.3 mm
Pitch=0.7 mm
Rim=0.1 mm
Thick=0.4mm
P1:
D=0.8 mm
Pitch=2 mm
Rim=0.04 mm
Thick=1mm
P1
R3
R3 :
D=0.2 mm
Pitch=0.5 mm
Rim=0.01 mm
Thick=0.2mm
27
Over !
12/22/2015
Andrea Bressan
28