Experiments Sensitive to Low-x
Gluon Density
L.C. Bland, Brookhaven National Laboratory
OUTLINE
•
Introduction: physics goals
•
Inclusive particle production
•
Forward di-pion correlations: probing low x at RHIC
•
Conclusions
•
Outlook
Quantifying the Properties of Hot
QCD Matter (INT-10-2a)
Seattle, 1 June 2010
Relativisitic Heavy Ion Collider
2
RHIC as a Polarized Proton Collider
RHIC pC Polarimeters
Absolute Polarimeter (H jet)
BRAHMS
PHOBOS
Completed in run 5
Completed in run 6
Siberian Snakes
polarized p+p collisions
Siberian Snakes
62  s  500 GeV
PHENIX
STAR
Spin Rotators
(longitudinal polarization)
Spin Rotators
(longitudinal polarization)
Pol. H Source
LINAC
BOOSTER
Helical Partial Siberian Snake
200 MeV Polarimeter
AGS
AGS pC Polarimeter
Strong AGS Snake
3
STAR
•
Large acceptance near midrapidity
•
Windows to large rapidity
4
Goals I
What is the structure of the proton?
Proton structure is
understood via parton
distribution functions f(x).
These functions are
global fits to world data
and give the probability
to find a parton (quark,
gluon or antiquark)
carrying a fraction x of
the proton’s momentum.
5
World Data for
(unpolarized)
DIS
Combination of fixed-target experiments and
results from the HERA collider (DESY)
provides a precise determination of the x
and Q2 dependence of the F2 structure
function and is the primary data for global
fits to parton distribution functions.
Summary plot from arXiv:hep-ex/0507024, and references therein.
Rapid rise of the gluon density at lowx evident from F2(x)/lnQ2 at fixed x
(Prytz relation)
6
Fixed Target Experiments
Deep Inelastic Scattering from Nuclear Targets
Kinematic Coverage Restricted to Fixed Target Experiments (no EIC, yet)
From Hirai, Kumono, Nagai PRC
70 (2004) 044905, and
references therein
• Growth of gluon distribution at low-x within the proton cannot continue forever
• Gluon density in nucleus only known to x~0.02 since g(2x)~F2(x,Q2)/ln(Q2)
7
Gluon Saturation and the Color Glass Condensate
t = ln(1/x)
Iancu, Venugopalan
hep-ph/0303204
•
Does the low-x gluon density saturate, and is this a highenergy phase of matter?
•
Would a Color Glass Condensate be universal for both
nuclear DIS and hadronic probes of nuclei at high energy?
8
Why is Gluon Saturation Important?
“The Color Glass Condensate is important to search for at RHIC because …it
provides a rigorous QCD theoretical description of the initial state in A+A from
which the QGP must evolve”
New Forms of QCD Matter Discovered at RHIC, M. Gyulassy and L. McLerran
[nucl-th/0405013]
“Some crucial open questions that need to be addressed are… what is the gluon
momentum distribution in the atomic nucleus?”
Study of the Fundamental Structure of Matter with an Electron-Ion Collider,
A. Deshpande, R. Milner, R. Venugopalan, W. Vogelsang,
Ann.
Rev. Nucl. Part. Sci. 55 (2005) 165.
9
RHIC Hard-Scattering Probes
 Polarized proton collisions / hard scattering probes of DG
 d+Au collisions / hard scattering probes of nuclear gluon density
quark
pion or jet
quark
gluon
c
d     dxa  dxb  dzc f a ( xa ) f b ( xb ) Dc ( zc )dˆ ab
a ,b , c
Describe p+p particle production at RHIC energies (s  62 GeV)
using perturbative QCD at Next to Leading Order,
relying on universal parton distribution functions and fragmentation functions
10
RHIC Spin Probes - II
Unpolarized cross sections as benchmarks and heavy-ion references
0 +cross
Large
,K,p
sections
for p+p,
s=200 GeV
p + rapidity
pp

s
=GeV
200
GeV
+ p, sX,
= 200
PRD 76 (2007) 051106
PRL 97 (2006) 252001
jets
PRL 98 (2007) 252001
PRL 92 (2004) 171801
direct g
PRL 98 (2007) 012002
Good agreement between experiment and theory
11
 calibrated hard scattering probes of proton spin and low-x gluons
Why do forward 0 production in a hadron collider?
0
E
p
E
d N
qq
xqp
q
xgp
qg
EN
p
Au
2E 
s
s  2E N
E
z

q
Eq
h  ln(tan( ))
2
p h
xg  T e g
xq  xF / z
s
(collinear approx.)
Q 2 ~ pT2
xF 
• Large rapidity  production (h~4) probes asymmetric partonic collisions

• Mostly high-x valence quark + low-x gluon

p  p   ,h  3.8, s  200GeV
<z>
0
• 0.3 < xq< 0.7
• 0.001< xg < 0.1
<xq> NLO pQCD
Jaeger,Stratmann,Vogelsang,Kretzer
• <z> nearly constant and high 0.7 ~ 0.8
<xg>
• Large-x quark polarization is known to be large from DIS
• Directly couple to gluons  probe of low x gluons
12
Expectations from Color Glass Condensate
()
t related to rapidity of
tln1xproduced hadrons.
1 
dAu
R

dAu
2
*
197

pp
D. Kharzeev
hep-ph/0307037
As y grows
Iancu and Venugopalan, hep-ph/0303204
CGC expects suppression of forward hadron production
13
STAR
STAR d+Au forward π0
PRL 97, 152302
η = 4.0
(NPA 765, 464)
Sizable suppression
1 
dAu
R

dAu
2
*
197

pp
pQCD+Shadowing expects suppression, but not enough
CGC gives best description on pT dependence
14
RdAu rapidity dependence
η = 04
BRAHMS PRL 93, 242303
STAR
PRL 97, 152302
η=4
0
1 
dAu
R

dAu
2
*
197

pp
Observe significant rapidity dependence
similar to expectations from the CGC framework
15
An initial glimpse: correlations in d+Au
PRL 97, 152302 (2006)
• are suppressed at
small <xF> and <pT,π>
consistent with
CGC picture
25<E<35GeV
<pT,π> ~
1.0 GeV/c
Fixed h,as
E & pT grows
<pT,π> ~
1.3 GeV/c
π0: |<η>| = 4.0
h±: |η| < 0.75; pT > 0.5 GeV/c
• are similar in d+Au
and p+p at larger
<pT,π> (<xF>)
Asexpected
expected by
as
by
HIJING
HIJING
16
Guzey, Strikman and Vogelsang
Phys. Lett. B603 (2004) 173
• constrain x value of gluon probed by high-x quark
by detection of second hadron serving as jet
surrogate.
• span broad pseudorapidity range (-1<h<+4) for
second hadron  span broad range of xgluon
• provide sensitivity to higher pT for forward 0 
reduce 23 (inelastic) parton process
contributions thereby reducing uncorrelated
background in Df correlation.
PYTHIA Simulation
17
Run-8 Results from
STAR Forward Meson Spectrometer
(FMS)
Full azimuth spanned with nearly contiguous
electromagnetic calorimetry from -1<h<4
 approaching full acceptance detector
18
STAR Detector
Forward Meson Spectrometer
commissioned/operated in
RHIC run 8.
Forward direction can be viewed at STAR, but present instrumentation is limited
and not completely compatible with high luminosity polarized p+p collisions
19
STAR Forward Meson Spectrometer
• 50 larger acceptance
than the run-3 forward pion
detector (FPD).
• 2 azimuth for 2.5<h<4.0
• Discriminate single g from
0gg up to ~60 GeV
North half of FMS
before closing
Runs
3-6
FPD
Phys. Rev. Lett. 101:222001 (2008)
Run
8
FMS
20
Two forward pions can probe very low xg
For plot:
1) Pythia simulation 3) Plot rapidity of all other
2) Trigger on one 0 pions between 1.5 GeV/c and
forward (3<h<4) with pT of the trigger pion as
function of x
pT>2.5 GeV/c
Forward-Forward
FMS-FMS
This region can be
probed with FMSendcap calorimeter
Forward-Central
FMS-Barrel
calorimeter
See Ermes Braidot, Quark
Matter 2009 proceedings,
arXiv:0907.3473
Forward-Forward
probes lowest x
21
Forward-Central Correlations
A brief summary of findings from Quark Matter 2009
E. Braidot, arXiv:0907.3473
22
Azimuthal Correlations (Dh3)
“GSV” Selection
“GSV” Selection
2.5 GeV/c<pT(<h>=3)
1.5 GeV/c<pT(|h|<0.9)<pT(<h>=3)
dAu – pp = 0.09±0.04
“GSV” selection leads to clear back-to-back peak
with similar pp/dAu widths as expected by pQCD
E. Braidot, arXiv:0907.3473
23
Azimuthal Correlations (Dh3)
“lower-pT” Selection
“GSV” Selection
2.5 GeV/c<pT(<h>=3)
1.5 GeV/c<pT(|h|<0.9)<pT(<h>=3)
dAu – pp = 0.09±0.04
“lower-pT” Selection
2.0 GeV/c<pT(<h>=3)
1.0 GeV/c<pT(|h|<0.9)<pT(<h>=3)
dAu – pp = 0.19±0.03
Evidence of pT dependent azimuthal broadening of signal
E. Braidot, arXiv:0907.3473
24
STAR
Forward+central correlations
Const (BG)
“gsv”
pp
Peak Area
Peak Width
dAu
Df
pT1 1.5
pT2 0.5
2.0
1.0
2.5
1.5
3.0 GeV/c
2.0 GeV/c
• dAu width larger than pp (consistent with FMS-BEMC results)
• dAu back-to-back peak area larger than pp at lower pT
25
• Work in progress towards even lower p A. Ogawa, CATHIE/TECHQM
Summary
Forward-Central Correlations
•
Run-8 FMS results reproduce run-3 FPD Gaussian widths and other features.
•
Comparison of Df0(FMS)+0(EMC) for pp and dAu indicates azimuthal broadening in
dAu.
•
Data are qualitatively consistent with a pT dependent picture of gluon saturation of
the gold nucleus.
26
Forward-Forward Correlations
Probing Au nucleus at lowest x
A brief summary of new results since QM09
arXiv:1005.2378
Forward-Forward
Forward-Central
27
Forward Di-pion Selection
Selection motivated by Guzey, Strikman, Vogelsang, Phys. Lett. B603 (2004) 173.
1) Find all clusters in the FMS
2) Find leading PT(gg) pair and require that it be above UpperThreshold=2.5 GeV/c
3) Find all other possible sub-leading cluster pairings with
0.05<M(gg)<0.25 GeV/c2
PT(gg)>LowerThreshold=1.5 GeV/c
Sub-leading PT 0
Leading PT 0
Forward-Forward pair masses:
Leading
All subleading
pp
Leading
All subleading
dAu
M(gg)
28
M(gg)
leading
Forward-Forward Df(00)
Beam view
Plots are normalized to number of events with
leading PT pion > 2.5 GeV.
subleading
Plots do not yet have any efficiency corrections.
Df0
(near side)
Df
Df0
(near side)
Df
(away side)
pp data
Observation of expected away-side correlations in pp
Near side correlations similar for both pp and dAu
Df
(away side)
dAu data
(dAu)σ(pp)=0.11±0.06
29
Azimuthal broadening from pp to dAu
Forward-Forward Df(00): PT Dependence
Leading PT pion > 2.5 GeV 2.0 GeV
Plots do not have any efficiency corrections.
Df0
(near side)
Df
(away side)
pp data
Azimuthal broadening is PT dependent.
PT dependence is stronger for ForwardForward than Forward-Central
Df
(away side)
Df0
(near side)
dAu data
(dAu)σ(pp)=0.52±0.05
30
Comparing peripheral / central dAu collisions
Use Au-side BBC as measure of collision centrality.
EAST BBC faces the Au beam.
Sum all charge in small tiles of East BBC.
d
This is roughly proportional to the
multiplicity of particles between
3.5<h<5.0 (18 ADC counts per MIP)
Au
East BBC
375 cm
Define “peripheral” events to look like pp.
Central
dAu data
Peripheral
pp data
31
Centrality dependence of forward di-pion decorrelation
Leading PT pion > 2.0 GeV
dAu
peripheral
dAu all
data
pp data
Away-side peaks evident in peripheral
dAu and pp.
dAu data
dAu
central
Away-side peaks in peripheral dAu are
roughly 50% wider than in pp.
Significant dependence on centrality is
evident in azimuthal decorrelation.
32
Centrality from Multiplicity - I
For p+p collisions, BBC response can be quantitatively described by
PYTHIA+GSTAR+slow-simulator. The slow-simulator converts GEANT energy
deposition to ADC count via measured photostatistics and measured phototube
properties.
33
Centrality from Multiplicity - II
Extend p+p strategy to d+Au via HIJING+GSTAR+slow-simulator, and correlate
experimental measure of multiplicity (SQ) with impact parameter from HIJING.
34
Centrality from Multiplicity - III
For d+Au collisions, BBC SQ response can be semi-quantitatively described by
HIJING+GSTAR+slow-simulator. Data shows higher probability than simulation for
largest SQ
35
Centrality from Multiplicity - IV
Correlate simulated BBC SQ response with impact parameter from HIJING.
SQ range
<b> (fm)
b (fm)
SQ<500
6.8
1.7
SQ>2000
2.7
1.3
36
Efficiencies / p+p - I
Strategy: apply same reconstructions used for data to PYTHIA+GSTAR simulations.
If model description is valid, then compare reconstructions to azimuthal correlations
from PYTHIA alone.
37
Conclusion: PYTHIA+GSTAR simulations describe the p+p data
Efficiencies / p+p - II
Strategy: given similarity of data to PYTHIA+GSTAR simulations, associate PYTHIA
primaries with reconstructed PYTHIA+GSTAR clusters to understand signal and
background.
Conclusion: background is primarily combinatoric
g(0
1
)g(0
2)
38
Efficiencies / d+Au
Strategy: basic notion is to repeat the strategy that worked for p+p, replacing
PYTHIA+GSTAR by HIJING+GSTAR. Begin by looking at HIJING 1.383 forward dipion correlations.
39
Conclusion: HIJING 1.383 looks nothing like p+p or d+Au(peripheral)
Efficiencies / d+Au
Why does HIJING 1.383 not work for p+p or d+Au(peripheral)?
Strategy: investigate what HIJING does
Conclusion: HIJING 1.383 uses Duke-Owen 1 parton distribution functions that
predate HERA e+p collider; consequently, HIJING does not know about the rapid
40
rise of the gluon density at low x.
Efficiencies / d+Au
Why does HIJING 1.383 not work for p+p or d+Au(peripheral)?
Strategy: use PYTHIA to investigate impact of parton distribution functions on
forward di-pion azimuthal correlations
Conclusion: rapid rise of gluon density at low-x is required for consistency with p+p
forward di-pion azimuthal correlations (and forward 00 Df in peripheral d+Au)41
Efficiencies / d+Au
(over)estimate impact of additional d+Au multiplicity by embedding study
Strategy: quantify impact of additional multiplicity in d+Au relative to p+p by
embedding forward di-pion events from p+p PYTHIA/GSTAR into d+Au minimum
bias
Conclusion: overestimated impact of multiplicity does not cause disappearance42of
away-side correlations in d+Au(central) collisions
Forward Dipion Correlations
pT dependence
Phase space available at large rapidity limits ability to probe the scale dependence
of the low-x gluon density. Model studies for p+p indicate less discriminatory
power between different parton distribution functions, as pT increases.
43
Back-to-back Angular Correlations
pQCD 22 process =back-to-back di-jet (Works well for p+p)
Forward jet
p
p
d+Au in HIJING
Kharzeev, Levin, McLerran (NPA748, 627)
Mid-rapidity jet
With high gluon density
21 (or 2many) process = Mono-jet ?
Mono-jet
Dilute parton
system (deuteron)
Dense gluonfield (Au)
PT is balanced by
many gluons
CGC predicts suppression of back-to-back correlation
Conventional shadowing changes yield, but not angular correlation
44
Comparison w/ Marquette
CGC with Q02=1.5 GeV2
CM doing b strips
to make better
comparison
Cyrille Marquet: arXiv:0708.0231
Nucl.Phys.A796:41-60,2007
arXiv:1005.4065
b=0
CGC calculation “predicts” away-side peak disappearance for central d+Au
45
Comparison with Tuchin
Kirill Tuchin: arXiv:0912.5479v1
normalized to peak height
CGC with Q02 ~ ?
Quantitative differences between CGC calculations of Marquet and Tuchin
Tuchin calculation confirms basic disappearance of away-side peak in central d+Au
46
What is the Common Factor?
Transverse Single Spin Asymmetries and Low-x Physics
PRL 101 (2008) 222001.
Collinear parton distribution functions are an incomplete description of the structure of
hadrons. kT dependence (i.e., gluons) is an essential step beyond the simple parton
model. Many issues then arise in theoretical descriptions of data.
The most robust measurements to confront theory are Drell-Yan production 47
Mapping Saturation Scales
need to get to lower x
Parton Gas
Qs,g2
CGC
x in 2  2 process
x in 2  1 process
A.Ogawa
48
Conclusions
• Jet-like correlations are observed in forward dipions produced in p+p and
peripheral d+Au collisions
• Jet-like correlations of forward dipions produced in p+p collisions
require a rapidly rising gluon density at low x, consistent with HERA
data.
• Away-side forward di-pion correlations are strongly suppressed in
central d+Au collisions
• Gluon saturation occurs at momentum fractions and scales that are relevant to
RHIC collisions, based on qualitative consistency between measurement and
color-glass condensate expectations of away-side peak disappearance for
forward dipion correlations
49
Outlook
Large Rapidity Drell Yan Production
Large rapidity (xF) means asymmetric partonic collisions, xF=x1-x2
Importance of large x1 for spin physics is natural if spin is dominantly from
valence quarks.
RHIC can access x2~10-4 by large-xF Drell-Yan production. DY has the most
robust theoretical understanding of any hadroproduction experiments. Future
measurements include
• transverse single spin asymmetries in pp  like/unlike color charge forces
• accessing lowest x possible at RHIC, with robust theory understanding
• angular distribution of di-leptons  Lam-Tung violations in pp/p(d)Au
Major forward upgrades at STAR or PHENIX required
Letter of Intent submitted to 2010 program committee for DY feasibility50
Backup
51
Off-peak Analysis
• Off-mass-peak azimuthal correlations for d+Au(peripheral) similar to on-peak results
• Evidence for jet-like correlations
• Azimuthal broadening essentially identical on-peak and off-peak
52
Transverse Single Spin Asymmetries
Definition and Examples
pi, momentum vector,
(i can be initial or final state)
Spin vector, S
•
Transverse single spin asymmetries (SSA) refer to either final state
polarization (P) or analyzing power, where S is determined in the initial state.
•
In general, parity-allowed transverse SSA ~ S (p1  p2) / |P|; i.e., projection of
the transverse spin vector onto a plane defined by two momenta.
Classic Examples of Transverse SSA in Hadro-production
•
p + p  L + X  hyperons from unpolarized proton collisions are polarized
•
h(,p,…) + p  g* + X  l+l¯ + X  violation of Lam-Tung relation in Drell Yan
•
p + p   + X  analyzing power for pion (meson) production
53
(Unpolarized) Drell-Yan Production
Virtual photon angular dependence…
1 d 2
3
 2


2

1


cos



sin
2

cos
f

sin

cos
2
f

 d 4 (  3) 
2
Lam-Tung relation interrelates ,, given that quarks have spin ½ …
1    2
Lam-Tung relation (analog of Callan-Gross relation for DIS)
NA10 ¯+W, p=194 GeV/c
Z. Phys. C31 (1986) 513.
 1-2 Lam-Tung violations
E615 ¯+W, p=252 GeV/c
PRD 39 (1989) 92
E866 p+p/d, pp=800 GeV/c
PRL 39 (2007) 082301
54
Lam-Tung Violations
from E615
 + W, p = 294 GeV/c
PRD 39 (1989) 92
Present understanding: LamTung violations (at low pT) are due
to the Boer-Mulders function.
[D. Boer, PRD 60 (1999) 014012]
55
Hyperon
Polarization
p+BeLX, pp=400 GeV/c
PRD 40 (1989) 3557
Hyperons produced in unpolarized
collisions are observed (via weak
decay asymmetry) to be polarized.
Weak decay asymmetry…
1 dN
*

1


P
cos

L L
*
N d cos
* : polar angle of decay in rest frame
L=0.642±0.013 (weak decay asymmetry)
PL, polarization with respect to axis
P is found to be a function of xF, pT
56
Transverse Single-Spin Asymmetries (AN)
Analyzing power is a tool to measure polarization and is one example of
transverse single spin asymmetries (SSA) with origins yet to be fully understood
Probing for
(1) orbital motion within transversely polarized protons;
(2) Evidence of transversely polarized quarks in polarized protons.
57
A Brief History…
p  p    X
s=20 GeV, pT=0.5-2.0 GeV/c
• QCD theory expects very small
(AN~10-3) transverse SSA for particles
produced by hard scattering.
• The FermiLab E-704 experiment
found strikingly large transverse singlespin effects in p+p fixed-target
collisions with 200 GeV polarized
proton beam (s = 20 GeV).
• Similar AN(xF) observed at lower s
0 – E704, PLB261 (1991) 201.
+/- - E704, PLB264 (1991) 462.
•
•
58
Expectations from Theory
What would we see from this gedanken experiment?
F0 as mq0 in vector gauge theories, so AN ~ mq/pT
or,AN ~ 0.001 for pT ~ 2 GeV/c
Kane, Pumplin and Repko PRL 41 (1978) 1689
59
Two of the Explanations for Large Transverse SSA
Collins mechanism requires
transverse quark polarization and spindependent fragmentation
Sivers mechanism
requires spin-correlated transverse
momentum in the proton (orbital motion).
SSA is present for jet or g
60
Require experimental separation of Collins and Sivers contributions
xF Dependence of Inclusive 0 AN
RHIC Runs 3,5,6 with FPD
STAR
PRL 101, 222001 (2008)
arXiv:0801.2990v1 [hep-ex]
Fits to SIDIS
(HERMES) is
consistent with
data
AN at positive xF
grows with
increasing xF
U. D’Alesio, F. Murgia
Phys. Rev. D 70, 074009 (2004)
arXiv:hep-ph/0712.4240
C. Kouvaris, J. Qiu, W. Vogelsang, F. Yuan,
Phys. Rev. D 74, 114013 (2006).
61
pT Dependence of Inclusive 0 AN
RHIC Runs 3,5,6 with FPD
STAR
B.I. Abelev et al. (STAR) PRL 101 (2008) 222001
• xF dependence is
consistent with Sivers
model
• Rising pT dependence is
not explained
STAR, PRL 101 (2008) 222001
62
xF and pT dependence of AN for
p+p±+X, s=62 GeV
I. Arsene, et al. PRL101 (2008) 042001
• AN(+) ~ -AN(-), consistent with results at lower s and u,d valence differences
• At fixed xF, evidence that AN grows with pT
63
Conclusions and Summary
•
Transverse spin asymmetries are present at RHIC
energies
•
Transverse spin asymmetries are present at large
rapidity (h)
•
Particle production cross sections and correlations are
consistent with pQCD expectations at large h where
transverse spin effects are observed
•
Essential to go beyond inclusive meson production to
disentangle dynamical origins
64
Drell Yan Feasibility
Study at IP2
Letter of Intent submitted to 21-22 June 2010 BNL
Program Advisory Committee
xF Dependence of Inclusive 0 AN
RHIC Run 6 with FPD/FPD++
STAR
PRL 101, 222001 (2008)
arXiv:0801.2990 [hep-ex]
Fits to SIDIS
(HERMES) is
consistent with
data
AN at positive xF
grows with
increasing xF
U. D’Alesio, F. Murgia
Phys. Rev. D 70, 074009 (2004)
arXiv:hep-ph/0712.4240
C. Kouvaris, J. Qiu, W. Vogelsang, F. Yuan,
Phys. Rev. D 74, 114013 (2006).
66
Attractive vs Repulsive Sivers Effects
Unique Prediction of Gauge Theory !
Simple QED
example:
DIS: attractive
Drell-Yan: repulsive
Same in QCD:
As a result:
Transverse Spin Drell-Yan Physics at RHIC (2007)
http://spin.riken.bnl.gov/rsc/write-up/dy_final.pdf
67
Transverse spin direct g
Theory expects repulsive color charge
interactions to result in an opposite sign
to spin-correlated momentum imbalance
for g+jet.
Bacchetta et al., PRL 99, 212002
also Kouvaris,Qiu,Vogelsang and Yuan
and, Teryaev and Ratcliffe
•
Estimate that sign change is accessible with Lint = 30 pb-1 and Pbeam=0.65
•
Best done at s = 200 GeV for 0/g separation
•
Best done before removal for STAR Forward TPC
As part of the 2008 update to Plans for the RHIC Spin Physics Program
68
Future Opportunities
Transverse spin for forward g+jet
Test of predictive power of theory (A. Bacchetta et al. PRL 99 (2007) 212002)
Restricting the measurement of the forward photon to Eg>35 GeV at <hg>=3.2
produces a signal:background ratio of 2.1.
69
As part of the 2008 update to Plans for the RHIC Spin Physics Program
Future Opportunities
Transverse spin for forward g+jet
Test of predictive power of theory
104 useable forward photon + jet coincidences are expected in a 30 pb-1 data
sample with 60% beam polarization
70
As part of the 2008 update to Plans for the RHIC Spin Physics Program
RHIC is a Unique Collider…
Source:
•
•
•
http://www.agsrhichome.bnl.gov/RHIC/Runs/
…capable of colliding essentially all positive ions over a broad range of s
…with large L/s, where L is free space at interaction region  large xF possible
…with a broad and diverse physics program aimed at important questions
o What is quark-gluon plasma?  heavy-ion collisions
o How does the proton get its spin?  polarized proton collisions
71
o Does the gluon density saturate in a heavy nucleus?  d+Au/p+Au collisions
Status
• Luminosity gains projected for s=200 GeV polarized proton collisons were not
realized, so Lint=30 pb-1 and Pbeam=65% for transverse spin direct photon would
be challenging.
• Theory community has revisited whether color-charge interactions are robustly
calculable [arXiv:1001.2977] for transverse single-spin asymmetries for
processes other than Drell Yan production
• Low-x/saturation physics looks to be very interesting at RHIC collision energies.
Non-universality of kT dependent distribution functions for di-jets may impact
small-x as well as transverse spin [arXiv:1003.0482]. This should not be the
case for low-x probed by Drell-Yan production
 establishing the requirements for a large-xF Drell Yan
production experiment will provide the most robust test of
theory for transverse spin, and lead to future avenues that
provide the most robust interconnections between low-x
probed at RHIC and low-x probed at eRHIC.
72
Collision Energy Dependence of Drell Yan Production
Comments…
•
qq  γ* has σ̂  1/ŝ
• partonic luminosities increase with s
• net result is that DY grows with s
• in any case, largest s probes lowest x
 Consider large-xF DY at s=500 GeV
Transverse Spin Drell-Yan Physics at RHIC (2007)
http://spin.riken.bnl.gov/rsc/write-up/dy_final.pdf
73
Pair mass from bare EMcal
arXiv:0906.2332
arXiv:0907.4396
• pair mass backgrounds well modeled
• J/ye+e- observation at <xF>~0.67 emboldens DY consideration
74
Motivations for DY Feasibility at IP2
Report to NSAC from the Subcommittee on Performance Measures (August, 2008)
http://www.sc.doe.gov/np/nsac/docs/PerfMeasEvalFinal.pdf
• Timeliness – HP13 milestone completion by 2015. This could be
accomplished during W program if 3IR impact is acceptable.
• Acceptance/background rejection – severe space constraints at STAR and
PHENIX require major changes in the forward direction. Space constraints are
not present at IP2.
• Is charge sign a requirement?
Objective of DY feasibility test is to establish the requirements for future major
forward upgrades at STAR and PHENIX that would be used in a future p+Au or
d+Au run that would emphasize Drell Yan production to probe low-x through 75
scaling violations or virtual photon pT dependence.
Requirements for DY
Background Reduction
•
electron/hadron discrimination / Q. What hadronic suppression required?
•
Charged/Neutral discrimination and photon conversion background
•
Is charge sign discrimination required for like-sign pair subtraction?
76
Schematic of detector considered
Run-12 configuration
(PHOBOS split-dipole expected to be in place, but not used)
• Hcal is existing 9x12 modules
from E864 (NIM406,227)
• EMcal is modeled as only
(3.8cm)2x(45cm) lead glass
• Preshower would require
construction
http://www.star.bnl.gov/~akio/ip2/topview2.jpeg
77
Schematic of detector considered
Run-13 configuration
(Uses PHOBOS Split Dipole for charge sign)
• Hcal is existing 9x12 modules
from E864 (NIM406,227)
• EMcal is modeled as only
(3.8cm)2x(45cm) lead glass
• Preshower would require
construction
• PHOBOS split-dipole magnetic
field in GEANT model
• Fiber tracker stations and MWPC
require construction
http://www.star.bnl.gov/~akio/ip2/topview_run13.jpeg
78
DY Expectations
• Non-zero AN expected at
moderate to large xF
• Measurement with accuracy
dAN<0.02 should be of great
interest
• With Pbeam=50%, require 10K
events for dAN=0.02
• Uses Sivers function from EPJ
A39 (2009) 89, that fits preliminary
HERMES results and COMPASS
deuteron results
• s=500 GeV predictions very
similar, since xF=x1-x2 is the
relevant parameter (private
communication)
Anselmino, et al PRD 79 (2009) 054010 [arXiv:0901.3078]
79
Previous Work
p+p DY at ISR, s=53,63 GeV
Phys. Lett. B91 (1980) 475
Comments (note: large xF at collider breaks new ground)…
• e+e- low-mass DY done at ISR and by UA2 [see review J.Phys. G19 (1993) D1]
• UA2 [PLB275 (1992) 202] did not use magnet / CCOR did [PLB79 (1979) 398]
• most fixed target experiments do +- DY
80
e+e- DY expectations at large xF at s=500 GeV
Model 1 = EMcal (2m)2 / (0.2m)2 beam hole at 10m / no magnetic field
Model 2 = L/R modular EMcal (0.9mx1.2m) at 5m / no magnetic field
Comments…
• reasonable efficiency can be obtained for large-xF DY with existing equipment
• final estimates of DY yield must follow estimates of background rejection
• critical question for decadal planning: is charge sign discrimination required?
81
Lepton daughters from g*
Most important contributions for g* xF>0.1 at s=500 GeV …
• high energy electrons and positrons (E>10 GeV)
• require detection at very forward angles
• e+(e-) from g* little affected by “modest” isolation (20mr half-angle cone)
82
• best solution for charge sign would be a dipole magnet (difficult for any collider)
Azimuthal angle for g*e+e• e+ and e- in separate modules
except when g* has large pT
• Azimuthal angle required for
analyzing power measurement
• Resolution is primarily from
measuring energies of e+ and e• Model 2 covers full azimuth
despite modular coverage
83
Dileptons from open beauty at large xF
Model 1 = EMcal (2m)2 / (0.2m)2 beam hole at 10m / no magnetic field
Model 2 = L/R modular EMcal (0.9mx1.2m) at 5m / no magnetic field
Comments…
• open beauty dileptons are a background 2x larger than DY for PHENIX 
• direct production of open beauty results in ~15% background at large xF
84
• large forward acceptance for the future would require discrimination (isolation)
Backgrounds
• h±/e± discrimination – requires estimates of p+p collisions and EMcal response
• charged/neutral discrimination
• photon conversion background – requires estimates of p+p collisions and materials
• PYTHIA 5.7 compared well to s=200
GeV data [PRL 97 (2006) 152302]
• Little change until “underlying event”
tunings for LHC created forward havoc
 Stick to PYTHIA 6.222 for estimates
hep-ex/0403012
85
Strategy for estimates
• ~1012 p+p interactions in 50 / pb at s=500
GeV  full PYTHIA/GEANT not practical
• Parameterize GEANT response of EMcal and
use parameterized response in fast simulator
applied to full PYTHIA events
• Estimate rejection factors from GEANT for
hadron calorimeter and preshower detector
(both critical to h±/e± discrimination)
GEANT simulation of EMcal
response to E>15 GeV ± from
PYTHIA 6.222 incident on
(3.8cm)2x45cm lead glass
calorimeter. GEANT response
not so different from 57-GeV
pion test beam data from CDF
[hep-ex/0608081]
• Explicit treatment in fast simulator to estimate
pathlengths through key elements (beam pipe
and preshower), to simulate photon conversion
to e+e- pair
• Estimate effects from cluster merging in EMcal
(d < edcell / use e=1 for estimates)
• Estimate/simulate EMcal cluster energy and
position resolutions. E=15%/E and
x(y)=0.1dcell, used to date for 0gg rejection.
86
Background Estimate
Comments:
• Conversion photons significantly reduced by 0gg veto
• Preshower thickness tuned, although perhaps is not so critical given photon veto
• Linearly decreasing dN/df estimates smaller hadronic background  increased
sophistication needed for reliable estimates, although other model uncertainties could
87
easily dominate.
Magnetic Field Used for Charge
Sign Simulations
• The plan is to reuse the split-dipole
magnet at IP2 designed, built and
operated by the PHOBOS
collaboration.
• PHOBOS provided their field map
and geometry files for GEANT for
simulation studies.
• Compared to use at IP10, splitdipole is rotated by 180o around
vertical axis, to move aperture
restriction from coils close to IP.
Interaction
Point
Vertical component of B versus x,z at y=0 from
PHOBOS split-dipole magnet
88
Raytracing DY di-electrons through apparatus
• Assumes vertex distribution
with z=20 cm  relies on
9MHz RF system to reduce the
diamond size. The z location of
beam-pipe crossings would be
broadened otherwise
• 2-mm square scintillating fibers
are assumed for tracking
stations at z=80 and 200 cm.
• MWPC assumed for tracking
station at z=470 cm
x-z , y-z and r-z views of trajectories through
apparatus planned for run 13, including splitdipole field, used for charge-sign determination
89
Deflections from split-dipole field
•
dr is distance in x-y plane at tracking
station between zero-field track
intercept and full-field track intercept
•
difference between positive and
negative charged particles produced
in collisions is twice larger
•
Strategy to determine charge sign is
to measure impulse delivered by
magnet by measuring deviation of
point at z=470 cm from line fitted to
vertex and z=80,200 cm space
points
90
Staging
Assumptions:
1) ~4 week polarized proton test run at s=500 GeV in RHIC run 11
2) 12 week polarized proton W production run at s=500 GeV in RHIC run 12
3) 12 week polarized proton W production run at s=500 GeV in RHIC run 13
Planned Staging:
1) Hcal + newly constructed BBC at IP2 for RHIC run 11 with goals of
establishing impact of 3IR operation and demonstrate calibration of Hcal to
get first data constraints on charged hadron backgrounds
2) Hcal + EMcal + neutral/charged veto + BBC for RHIC run 12 with goals of
zero-field data sample with Lint>50 / pb and Pbeam=50% to observe
dileptons from J/y, Uand intervening continuum. Split-dipole tests
envisioned.
3) Hcal + EMcal + neutral/charged veto + BBC + split-dipole for RHIC run 13
with goals data sample with Lint>50 / pb and Pbeam=50% to observe
dileptons from J/y, U and intervening continuum to address whether
91
charge sign discrimination is required
Conclusions
•
Acceptance with existing modular apparatus looks adequate for Drell Yan
(DY) feasibility experiment
•
First estimates show that DY can dominate over hadronic, conversion
photon and open beauty backgrounds
•
Requirements for charge sign determination in run-13 stage of DY feasibility
experiment have been established, and will require construction of fiber
tracking stations and MWPC.

Proceed with development of Letter of Intent for DY feasibility test aimed at
running in parallel with W measurement, pending demonstration that impact
of third IR is acceptable
92
Transverse Spin Asymmetries at Midrapidity
p+p  0/h± + X, s = 200 GeV
PRL 95 (2005) 202001
Transverse single spin asymmetries are consistent with zero at midrapidity
93
STAR Results vs. Di-Jet Pseudorapidity Sum
Run-6 Result
VY 1, VY 2 are calculations by
Vogelsang & Yuan, PRD 72 (2005) 054028
AN pbeam
 (kT(50%+
 S)T)
Emphasizes
jet
quark Sivers
Boer & Vogelsang, PRD 69
(2004) 094025
pbeam
into page
jet
Idea: directly measure kT by observing momentum imbalance
of a pair of jets produced in p+p collision and attempt to
measure if kT is correlated with incoming proton spin
AN consistent with zero
~order of magnitude smaller in pp  di-jets than in semi-inclusive DIS
quark Sivers asymmetry!
PRL 99 (2007) 142003
STAR
94
Can L be reconstructed via 0n?
Jet-like n0
(collision vertex)
L0n
Combinatoric
ngg
(collision vertex)
Reconstructed versus simulated vertex
for triggered events
With the vertex, Mggn can be reconstructed.
Backgrounds mostly from Lg final states.
•
Forward L0n reconstruction appears feasible with FHC + FMS
•
Yields are model dependent, and may require elimination of hadronic showering in FMS
95