Transcript Diapositiva 1 - Istituto Nazionale di Fisica Nucleare

Spin physics in Drell-Yan processes:
past and future experiments
Michela Chiosso
University of Torino – Dip. Fisica Generale
I.N.F.N - Torino
IWHSS 2008
Torino – April 2nd 2008
Drell-Yan dilepton production

beam
beam p
dd u
u
u
d
*

u


u
2pL
x f  x1  x 2 
S
M2

 x1  x 2
S
Target p
This process is electromagnetic and exactly calculable
Propagator of the virtual photon in the amplitude  factor M-2 in the cross section


d2σ
4α2 π 1
2 q
q
q
q
=
e
f
(x
)f
(x
)
+
f
(
x
)f
(x2 )

q
1
2
1
2
2
dM dxF 9M s x1 + x 2
Past Drell-Yan experiments
This process was first described by Sydney Drell and Tung-Mow Yan in 1970.
First quantitative Drell-Yan experiments: late 1970s  waiting for spectrometers able
to measure pbarn cross-sections in the presence of much larger background.
- Omega, CERN (Corden at al, 1978, 1980)
- CIP, FERMILAB (Anderson et al 1976, Hogan et al
1979)
- NA3, CERN (Badier et al 1979, 1989)
- CFS, FERMILAB (Lederman et al, 1982)
- CHFMNP, CERN (Antreasyan et al)
- E605, FERMILAB (Moreno et al, 1989)
-…
1980s – 1990s
1970s - 1980s
- E615, FERMILAB (Conway et al, 1989)
- NA10, CERN (Anderson et al, 1984)
- E772, FERMILAB (McGaughey et al, 1994)
- NA51, CERN (Abreu et al, 1998)
- E866 (E.A. Hawker et al, 1998)
-…
Present and future Drell-Yan facilities
pp
RHIC
J-PARC
FAIR
collider

pd
pp
COMPASS
(CERN-SPS)
NICA
(JINR)
E906
(FMI)
CMS
(CERN-LHC)
S=(200)2 GeV2
fixed target
collider
fixed target
S= 60-100 GeV2
S=200 GeV2
S=30-80 GeV2
p d  fixed target




p (D )p (D )
pp,pd
pp
S=100-400 GeV2
S=400 GeV2
fixed target
S=230 GeV2
collider
S=196 TeV2
Les Bland
Yuji Goto
Klaus Peters
This talk
Alexander Sorin
Investigating PDFs with Drell-Yan processes
Since many years Drell-Yan process has been playing a key role in the
study of parton distribution functions (PDFs)
Flavor asymmetry of nucleon parton distribution functions
E772, E866, NA51, E906
, k mesons parton distribution functions
NA10, E615, NA3
Boer-Mulders transverse momentum dependent parton distribution
Function (TMD PDFs)
E615, NA10, E866
Recently it is drawing back attention as a unique tool to directly access
spin dependent parton distribution functions
Transversity
Sivers function
Flavor asymmetry in the nucleon sea
E772, E866, NA51, E906
Light sea-quarks flavor asymmetry
Pion quarks distribution function
NA10, -----E615
P.J. Sutton, A.D. Martin, Phys.Rev.D, 45 (7) 1992
No
available experimental
data at small x (x≤0.2) to
gluon distribution:
determine sea-quark distr.
Prompt photon production unambiguously
Performed
various fits with
valence distribution: sea carrying an increasing
Fit to Drell-Yan data fraction of pion momentum

sea-quark distribution:
Fit to Drell-Yan data
valence distr. are the
same, but different contr. to
DY through quark charge
squared
Light mesons parton distribution functions
, k mesons parton distribution functions
NA10, E615, NA3
pion structure function
NA3 :
uk ( x) / u ( x)
Angular distribution & Boer-Mulders PDF
Unpolarized angular distribution in Collin-Soper frame
 2
 1  d   3  

2

1


cos



sin
2

cos


sin

cos
2

 
  

d

2
 
  4  

λ=1,μ=ν=0 at LO, in collinear approximation  transversely polarized ,
no transverse momenta:
Lam-Tung relation: 1--=0
NLO prediction: small deviation from
 ≠ 0:
Boer-Mulders effect if PT2«M2
NLO corrections, at large qt
1  cos2  for Pt<3GeV/c
Angular distribution & Boer-Mulders PDF
1.There is a sizable cos2 asymmetry ( up to 0.3) in the unpolarized
pion-induced Drell-Yan: the Lam-Tung sum rule is violated beyond the
QCD-improved parton model.
NA10 , E615
2.  = -1 at large xf
E615
3. No azimuthal asymmetry in proton-induced Drell-Yan
E866
Angular distribution & Boer-Mulders PDF
Violation of Lam-Tung sum
rule
Boer-Mulders function
azimuthal dependence 1
h


h1 h1

f1 f1

can lead to
Angular distribution & Boer-Mulders PDF
E866 at Fermilab
800 GeV/c p+d
No noticeable flavour asymmetry between
800 GeV/c p+p


h1 (u ) and h1 (d )
Sea-quark Boer-Mulders function is relatively small
Spin dependent parton distribution functions
Polarized Drell-Yan
Transversity  direct access to h1( x ) without convolution with

fragmentation function H1 ( x), like in SIDIS
Sivers function

1 T
f
2
( x, kT )
Boer-Mulders function

2
h1 ( x, kT )
Spin dependent parton distribution functions
Double polarization (
p p ,pp
)
d  h1( x1)  h1( x2 )  cos(2  (  S1  S2 ))
transversity
Single polarization (
pp ,   p

)
d  f1(x1,kT )  f1 T (x2,kT )  sin(  S ) 
2
2
Sivers

d  h1 (x1,kT )  h1(x2,kT )  sin(  S )
2
2
Boer-Mulders transversity

No polarization ( p p,pp, 

1

1
p
)
d  h (x1,kT )  h (x2,k T )  sin(  S )
2
Boer-Mulders
2

1 T

1 T
f (x1,kT )DY  f (x2,kT )SIDIS
2
2
Spin dependent PDFs with Drell-Yan processes
What do we need to access spin dependent PDFs through DY?
polarized beam + polarized target (beam)
or
unpolarized beam + polarized target
Valence
qbeam qt arg et
p pp p p
High luminosity:
very small DY cross section
Larger asymmetries (SSA and DSA)
in region with large valence quark
content.
Spin physics with Drell-Yan processes
in COMPASS
Measures of single-spin DY asymmetries:
Beam:
- 
Polarised DY  p
 S=100÷400 GeV2
SSA ~ sin(s) + sin(s)

2
f1 T ( x 2,k T )
Polarized target:

h1 ( x1,k T )  h1( x2,k T )
2
2
NH3 / 6LiD
Polarization: >80% / >40%
Unpolarised DY

-p
Dilution factor: 0.15 / 0.35
p
d ~ cos(2)  h1 ( x1·,k T )  h1 ( x2,k T )
2
2
Luminosity
Using - beam it is necessary to make an assumption
connecting pion and proton PDFs
~1031 cm-2 s-1
COMPASS spectrometer layout
ECAL1
Ibeam ~ 1108 p/s
PT
Beam
25% of current PT
Polarized target
180 mrad acceptance
2 cells, 15 cm each
HCAL1
MW1
Kinematic range
M<J/y  dominated by semileptonic
decay of charmed hadrons
M>J/y Drell-Yan dilepton production
major contribution
M must be large enough to apply pQCD
But production rate falls off rapidly with M
Safe region:
M 2 J /y

s
Out from resonances regions dominated by
strong production mechanism
Kinematic range
x1, x2
M2
  x1x 2 
S
Q2  M2
Sizeable single and double spin asymmetries
in valence quark region
COMPASS
s=100-400 GeV2
M=4-9 GeV/c2
=0.16-0.8 @ s=100 GeV2
=0.08-0.4 @ s=200 GeV2
=0.05-0.3 @ s=300 GeV2
=0.04-0.2 @ s=400 GeV2
Kinematic range
x2
COMPASS
S=100GeV2
S=200GeV2
x2
S=300GeV2
x2
x1
x1
S=100GeV2
x1
S=300GeV2
Q2
Q2
Kinematic range: COMPASS acceptance
 in “valence” region: 0.1  x1/2  0.5
x1vs x2

Sensitive to Sivers effect at low PT:
PT << Q
PT
Kinematic range: COMPASS vs other experiments
x2 vs x1
E615
E866
COMPASS
Spin physics in Drell-Yan processes at COMPASS
SUMMARY
What…
Spin dependent PDFs
Boer-Mulders function
transversity, Sivers function,
How…
1·108 - beam on NH3/6LiD polarized target
expected luminosity: 1031 cm-2 sec-1
expected events rate: ~ 34000 in 150 days of run
S=300 GeV2 , M(+-): 4-9 GeV/c
When
Beyond 2010
November 11-12, 2007
Test beam at CERN SPS:
verified radiation conditions, PT and spectrometer peformances
with high intensity hadron beam
EOI
in preparation