Transcript MC studies of a new Trigger - Istituto Nazionale di Fisica

COMPASS
Measurements of
Transverse Spin Effects
in SIDIS
Andrea Bressan
Torino
01/04/08
Outlook
COMPASS
 Introduction
 Asymmetries

Transversity DF





from Single Hadron
from Two Hadrons
from Hyperons
Sivers DF
Other TMD
 Near/Far Future
 Conclusions
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What you will w/not find
COMPASS
 Review of all Deuteron Results
 w/not 2007 proton results
They are on their way…
 ½ PB of data processed
 Data quality checks ongoing
 Late Spring

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Spectrometer
COMPASS
• longitudinally polarised muon beam
• longitudinally or transversely polarised
target
• momentum and calorimetry measurements
• particle identification
SM2
E/HCAL
E/HCAL
SM1
Polarised
Target
m beam
7/6/2015
MuonWall
Muon Wall
RICH
Beam:
• Luminosity
• intensity
• momentum
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SciFi
Silicon
Micromegas
GEMs
Straws
SDC
MWPC
W45
5 . 1032 cm-2 s-1
2 108 µ+/spill (4.8s/16.8s)
160 GeV/c
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Target
COMPASS
solid state target operated in frozen spin mode
– 4He Dilution
refrigerator (T~50mK)
3He
superconductive
Solenoid (2.5 T) Dipole (0.5 T)
2002-2004: 6LiD
dilution factor f = 0.38
polarization PT = 50%
~20% of the time
transversely polarised
two 60 cm long cells
with opposite polarisation (systematics)
dN/dz
4000
2000
0
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-1000
0
zvtx (mm)
1000
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during data taking with
transverse polarization
• dipole field always 
• polarization reversal in the
2 cells after ~ 5 days
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Event Selection
DIS cuts:
COMPASS
Statistics 2002 - 2004:
8.5 * 106 positive hadrons
7.0 * 106 negative hadrons
 Q2 > 1 (GeV/c)2
 0.1 < y < 0.9
 W > 5 GeV/c2
All hadron selection:
 z > 0.20
 pt > 0.1 GeV/c
Plus for leading hadron:
 zl > 0.25
 No signals in the CALOs
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from neutral particles with
z > zl
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Hadron Identification
COMPASS
 Hadron identification is based on RICH response: several studies
performed on the stability in time of the detector.
 Cherenkov thresholds: π ~ 2 GeV/c
K ~ 9 GeV/c
p ~ 17 GeV/c
 2 σ π/K separation at 43 GeV/c

In the leading hadron sample:
~76% pions
~12% kaons
positive
negative

3.4M
2.8M
leading K
0.7M
0.4M
leading
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Kinematics – inclusive
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COMPASS
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Kinematics - SI
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COMPASS
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K0 Reconstruction
COMPASS
Z(V0) – Z(prim. vtx.) > 10 cm
● angle of target pointing < 0.01 rad
● |M– MK0| < 20 MeV/c²
● pt, arm > 0.025 GeV/c
● pt, K0 > 0.1 GeV/c
K0 target pointing
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Armenteros
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S/B ~ 15
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COMPASS
TRANSVERSITY
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Transversity DF
DTq(x) = q↑↑(x) - q↑↓(x)
h1q(x),
dq(x),
dTq(x)
COMPASS
q=uv, dv, qsea
quark with spin parallel to the
nucleon spin in a transversely
polarised nucleon
Properties:
• probes the relativistic nature of quark dynamics
• no contribution from the gluons  simple Q2 evolution
• Positivity: Soffer bound…………….. 2 | ΔTq |  q + Δq Soffer, PRL 74 (1995)
• first moments: tensor charge……….Δ q   dx Δ q(x)
T
T
• sum rule for transverse spin
1 1
in Parton Model framework…………   DT q  Lq  Lg
2 2
Bakker, Leader, Trueman, PRD 70 (04)
• it is related to GPD’s
• is chiral-odd: decouples from inclusive DIS
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How To
COMPASS
the Transversity DF is chiral-odd:
survives only by the product with another chiral-odd function
can be measured in SIDIS on a transversely polarised target via “quark
polarimetry”
L Nl’ h X
Collins Asymmetry (Collins FF)
L Nl’ L X
L polarization (FF of q L)
L Nl’ hh X Two hadrons asymmetry (Interference FF)
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Single hadron asymms.
COMPASS
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
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s’ =  - s (spin flip)
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Collins Effect
COMPASS
Collins effect:
a quark with an upward (downward) polarization, perpendicular to the
motion, prefers to emit the leading meson to the left (right) side with
respect to the quark direction
i.e.
the fragmentation function of a transversely polarized quark has a spin
dependent part
Dhq (z,phT )=Dhq (z,phT )+Δ0TDhq (z,phT )×sin(φh -φs' )
And the resulting measured asymmetry
Nh   C )  Nh0 
 1  AhC  sinC 
AColl 
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h
C
A

f  PT  Dnn
 C  h  s  
 e  Δ q Δ D
 e  qD
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q
2
q
q
T
2
q
0
T
h
q
h
q
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Unidentified Hadrons
COMPASS
• only statistical errors shown (~1%), systematic errors
considerably smaller
• small asymmetries compatible with 0 for both + and – hadrons
[NP B765 (2007) 31-70]
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Identified Hadrons
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COMPASS
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Naïve Interpretation
COMPASS
(parton model, valence region)
• proton data
 unfavored Collins FF ~ – favored Collins FF
at variance with unpol case
ΔT0 D2   ΔT0 D1
u quark dominance (d quark DF ~ unconstrained)
• deuteron data
some (small) effect expected even if
ΔT D2   ΔT
 cancellation between DTu (x) and DTd (x)
access to DTd (x)
Genova, 27 febbraio 2008
0
F. Bradamante
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0
D1
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Two Hadrons
z-axis
= virtual photon direction
x-z plane = lepton scattering plane
COMPASS
R= angle between lepton
scattering plane and
two-hadron plane
S= azimuthal angle of initial
quark versus lepton
scattering plane
S´= π - S
(fragmenting quark)

R


z1 P2  z 2 P1
z1  z 2
R
(A. Bacchetta, M. Radici, hep-ph/0407345)
7/6/2015(X. Artru, hep-ph/0207309)
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RS= R - S´
= R + S - π
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Azimuthal Asymmetries
z=z1+z2
Target single spin asymmetry ARS(x,z,Mh2):
N (ΦRS ) = N0 { 1 ± A
±
sinRS
UT
sinΦRS }
N±(RS): Number of events for
target spin up (+) and
down (-)
A RS
2
h
q
q
T
2
q
∠h
q
h
q
2
h
2
h
A RS
Dqh , H q∠h
Under measurement
in e+e- (BELLE)
expected to depend on the hadron
pair invariant mass
(X. Artru, hep-ph/0207309)
7/6/2015
and
1
sinRS
=
A UT
f PT D
f: Dilution factor ≈ 0.38
D: Depolarisation factor
D=(1-y)/(1-y+y2/2)
PT: Target polarisation ≈ 0.5
∑e Δ q(x) H (z, M )
( x, z,M ) =
∑e q(x) D (z, M )
2
q
COMPASS
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Selection
7/6/2015
COMPASS
DIS cuts:
Hadron selection:
• Q2 > 1 GeV2/c2
• 0.1 < y < 0.9
• W > 5 GeV/c2
• z1,2 > 0.1 (current fragmentation)
• xF1,2 > 0.1
• z1+z2 < 0.9 (exclusive rho)
• RICH identification of π, K
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Two hadron asymm.s
COMPASS
Expected Small
7/6/2015
(Radici/Bacchetta, PRD74(2006)114007)
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Identified 2 Hadrons
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COMPASS
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z-order two identified
hadrons
COMPASS
 Motivations:
 Hadrons with higher relative energy carry more
information about the fragmenting quark polarization
 For leading hadron pairs signal enhancement is
predicted
PID with 2003-04 rich
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z-order two identified
hadrons
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COMPASS
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L Polarimetry
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COMPASS
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Results
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COMPASS
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COMPASS
SIVERS DF
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SIVERS Mechanism
 The Sivers DF
COMPASS
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 T2 )SIDIS   Δ0Tq (x,k T2 )DY
…to be checked
 In SIDIS:
Nh  S )  Nh0   1  AhS  sin S
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
ASiv 
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h
S
A

f  PT
 e  Δ qD
 e  qD
q
2
q
q
T
0
2
q
h
q
h
q
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Unidentified hadrons
COMPASS
• only statistical errors shown (~1%), systematic errors considerably smaller
• small asymmetries compatible with 0 for both + and – hadrons
[NP B765 (2007) 31-70]
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Identified Hadrons
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Andrea Bressan
COMPASS
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Naïve Interpretation
COMPASS
• proton data
asymmetry for + > 0, asymmetry for - ≈ 0
 Sivers DF for d-quark ≈ - 2 Sivers DF for u-quark
2
• deuteron data
preliminary 2002-2004
the measured asymmetries
compatible with zero suggest
F. Bradamante
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Small SIVERS on D: hint
for small Lg
COMPASS
the measured asymmetry on deuteron compatible
with zero has been interpreted as
Evidence for the Absence of
Gluon Orbital Angular Momentum in the Nucleon
S.J. Brodsky and S. Gardner, PLB643 (2006) 22
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COMPASS
Other TMD’s
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Sidis Xsection
COMPASS
From A. Bacchetta et al.,
JHEP 0702:093,2007.
e-Print: hep-ph/0611265
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Results - LO
COMPASS
g1T is the only parton DF which is
chiral-even, T-even, leading twist function
in addition to the unpolarised DF and to the helicity DF
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Results - LO
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COMPASS
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COMPASS
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COMPASS
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Transverse Target SSA for
exclusive r Q2>1)
COMPASS
 Motivations:
 Hard exclusive meson production (HEMP) is a way,
complementary to DVCS, to access GPDs
 Vector mesons Transverse Target Single Spin
Asymmetry AUT(h,S) connected to GPD E
Ji Sum Rule
 E allows flip of proton helicity, while quark helicity is
not flipped → overall helicity is not conserved
 angular momentum conservation implies transfer of
orbital angular momentum
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Selection
COMPASS
Besides standard DIS cuts:
• Q2 > 1 GeV2/c2
• 0.1 < y < 0.9
• W > 5 GeV/c2
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Exclusive r selection:
•only 3 outgoing particles m, , 
•0.01 < pT2 < 0.5 [(GeV/c)2]
• missing energy -2.5 GeV< Emiss<2.5 GeV
• Inv. Mass -0.3 MeV/c2 < M – Mr < 0.3 MeV/c2
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Results
7/6/2015
COMPASS
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COMPASS
2007 RUN
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Target in 2007
COMPASS
solid state target operated in frozen spin mode
2007: NH3
dilution factor f = 0.14
polarization PT = 90%
23
cells
Coupling
hole
Microwave
power
Beam
line
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Thermal
screen
Cavity
1
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Cavity 2
Cavity
3
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Projected errors for full
statistics (45x1012 m)
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COMPASS
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Summary
COMPASS
precise deuteron data from COMPASS are now available
Collins and Sivers asymmetries h±, ±, K±
+ other 6 TMDs [sin(3h-S), sin(S), sin(2h-S), cos(h-S), cos(2h-S),
cos(h)]
Two hadron asymmetries , K, KK
Transverse Lamba polarization
all the measured deuteron asymmetries are very small, and compatible with
zero
COMPASS data on deuteron allows to determine the d-quark contribution
present phenomenological studies can describe at the same time the
BELLE (FF), the HERMES (proton) and COMPASS (deuteron)…and have
allowed a first extraction of the transversity distribution DTq
At the same way COMPASS and HERMES data have allowed a first
extraction of the Sivers DF
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Coming Soon
COMPASS
 Deuteron
 Cahn and Boer-Mulders asymmetries  unpol
 g2
 Proton
 Collins + Sivers…
 Little further in time… all the rest
Future programs
 Drell-Yan [see … M. Chiosso talk]
 Precision measurement … for better flavour separation and
measurements of first moments…
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COMPASS
THAT’S ALL…
(FOR NOW!)
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