Putting a New Spin on Things: Exploring QCD at RHIC

1
Putting a New Spin on Things Exploring QCD at
RHIC

• Topics
• Studying QCD at RHIC
• A (Brief) Spin Physics Primer
• Gluon Spin Contribution (Upgrades)
• Parton Angular Momentum and Spin
• Antiquark Spin Contribution with W/- bosons

2
Quantum Chromodynamics
With a collider you can
use QCD in this regime
to probe QCD in this regime.
3
The Proton
as viewed with a high energy (short wavelength)
probe
as viewed with a low energy (long wavelength)
probe
3 valence quarks ? charge
momentum mass spin
u
?
u
d
10-15 m
Using spin we can probe the structure of the
proton!
4
Experimental Access to Proton Structure
Deep Inelastic Electron-Proton Scattering
electron momentum momentum transfer energy
transfer relative quark momentum ?
Bjorken x
5
Quark and Gluon Distribution Functions
(subscript for parent hadron, superscript for
parton)
6
How Much Do We Know?
Unpolarized
Polarized
7
DIS Experimental knowledge of spin dependent
quark and gluon distributions
AAC, M. Hirai, S. Kumano and N. Saito,
Phys.Rev.D69054021,2004
EMC,SMC at CERN E130 ? E155 at SLAC HERMES,
DESY
Q21GeV2
Spin Crisis
8
Exploring Proton Structure with Quark and Gluon
Probes
At ultra-relativistic energies the proton
represents a jet of quarks and gluons
For example, direct photon production (probes
gluon content with quark probes)
9
Polarized p-p at RHICA New Experimental Method
for the Study of Proton Structure
RHIC pC Polarimeters
Absolute Polarimeter (H jet)
Siberian Snakes
BRAHMS PP2PP
PHOBOS
Siberian Snakes
Spin Flipper
PHENIX
STAR
Spin Rotators
Partial Snake
Helical Partial Snake
Strong Snake
Polarized Source
PHENIX Lumi. monitoring GL1-1P scaler board.
LINAC
AGS
BOOSTER
200 MeV Polarimeter
Rf Dipole
AGS Internal Polarimeter
AGS pC Polarimeter
10
PHENIX spin physics program ?g, ?q/?q, dq

• 2 central arms
• electrons, photons, hadrons
• charmonium J/?, ? -gt ee-
• vector meson r, w, ? -gt ee-
• high pT po, p, p-
• direct photons
• open charm
• hadron physics
• 2 muon arms
• onium J/?, ?, ? -gt mm-
• vector meson ? -gt mm-
• open charm

Excellent trigger and DAQ capabilities multiple
trigger signature important for spin physics can
be taken in parallel with high bandwidth!
11
Measuring Asymmetries
Assuming both incoming protons are polarized
The difference Ds is proportional to the Df we
want to measure.
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The RHIC Spin Program
Transverse single/double spin physics
Flavor Decompsition
Gluon Polarization
Transversity Sivers vs. Collins effects
physics of higher twists Pion interf.
Fragmentation
p0,,- Production
W physics Longitudinal single spin physics
Heavy Flavors
Transverse single spin physics Phenix-Local
Polarimetry
Prompt Photon
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G(x,Q2) and q(x,Q2) pQCD work beautifully
Tevatron HERA!
J. Pumplin et.al JEHP 0207012 (2002)
D0 Jet Cross Section
ZEUS F2
14
at RHIC? q(x,Q2), g(x,Q2) and D(z,Q2) pQCD
Good agreement with NLO pQCD and experiment! ?
We can use a NLO pQCD analysis to extract
spin dependent quark and gluon distributions
from RHIC data!
15
Gluon Spin Contribution
16
First Results ALLp0
17
First Results ALLcluster
18
How Significant Is This?
This could be the discovery of the century.
Depending, of course, on how far down it goes.
19
Digging Deeper
First Moment ?G ?01?G(x)dx
Gehrmann-Stirling Models
Much of the first moment ?G ??G(x)dx might
emerge from low x! Some theoretical
guidance ?G(x) x G(x) but G(X)
diverges faster than x-1 ! NEED TO
EXTEND MEASUREMENTS TO LOW x !!
20
Baseline Acceptance
Muon Arm
Muon Arm
EMCAL
0 f coverage 2p
EMCAL
-3 -2 -1 0
1 2 3
rapidity
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Adding Silicon The PHENIX VTX
Provides displaced vertex jet measurement over
Df 2p

• Specifications
• Large acceptance (Df 2 p and h lt 1.2)
• Displaced vertex measurement sDCA lt 40 mm
• Charged particle tracking sp/p 5 p at high pT
• Detector must work for both of heavy ion and pp
  collisions.
• Technology Choice
• Hybrid pixel detectors developed at CERN for
  ALICE
• Strip detectors, sensors developed at BNL with
  FNALs SVX4 readout chip

VTX
Construction Underway
22
Forward Silicon The FVTX

• Detector
• 4 layers
• Vertical planes
• 75 ?m radial pitch, 7.5 phi segmentation (2 13
  mm)
• Maximize z and r extent to give good resolution
  and 3 hits/track as much as possible
• 2600K channels
• Status
• Recently favorably reviewed for FY08 start
• Bootstrapped by LANL LDRD funds to construct one
  octant prototype
• Submitted proposal to DOE for FY08 funding start

Note Prompt J/ys scaled down by 100
23
Expanded Acceptance I
Muon Arm
Muon Arm
EMCAL
0 f coverage 2p
EMCAL
VTX FVTX
-3 -2 -1 0
1 2 3
rapidity
VTX and FVTX add charged particle tracking close
to the event vertex over a wide kinematic range.
24
The PHENIX NoseCone Calorimeter (NCC)

• ge identification
• g p0 separation
• e/m isolation
• jet identification
• e/g/jet triggers

25
Expanded Acceptance II
Muon Arm
Muon Arm
EMCAL
0 f coverage 2p
EMCAL
VTX FVTX
-3 -2 -1 0
1 2 3
rapidity
NCC and MPC add EM Calorimetry in the forward
direction.
26
xgluon With Improved Acceptance
27
Gluon polarization DG(x) - Direct Photons

• ?-jet is
• Very clean way to measure the gluon
• Measuring the angle of the jet gives you access
  to xgluon

x
Todd Kempel Chaoyun Bao
28
Gluon polarization DG Heavy Flavor
Polarized pp collisions
Gluon polarization can be measured by double-spin
asymmetry ALL.

• charm and bottom identification by displaced
  vertex
• Jet identification with larger acceptance

29
Parton Angular Momentum
30
The Sivers Effect
The Sivers Effect is a correlation between the
spin of the proton and the transverse momentum of
the partons.
31
Di-Jet Sivers Predictions
Original prediction asymmetry as large as 4-5.
Should be easy to see in short PHENIX transverse
run.
Sivers is not universal the manifestation of
the effect depends on interference with initial
and final state amplitudes.
32
PHENIX Analysis
Yellow Beam Polarized
Blue Beam Polarized
Red trigger along spin Blue trigger opposite
spin
Analysis by Feng Wei, Iowa State University
33
Flavor Separated Antiquark Contributions
34
W Production in p p Collisions
Weak interaction violates parity
quark/antiquark helicities fixed!
(left-handed quarks)
(right-handed antiquarks)
35
Flavor separation of the spindependent quark and
anti-quark distributions
Parity violation of the weak interaction in
combination with control over the proton spin
orientation gives access to the flavor spin
structure in the proton!
For W- interchange u and d.
Experimental Requirements ? tracking at
high pT ? event selection for muons
difficult due to background muons from
hadron decays and beam backgrounds
(timing resolution!). ? good rejection of
backgrounds in the offline
analysis.
W
Z
36
PHENIX Muon Trigger Upgrade
R2
R3
R1(ab)
r3.40m
MuTr
Rejection 12,000, beam background immune.
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Existing MuID LL1 System
Another quality product from Iowa State
University!
accepted event data
16 bits GL1 data
blue logic triggers
(4 bits per arm shallow/deep)
Global Level-1
MuID LL1
backplane
backplane
GL1-1P
GL1-3
GL1-2
GL1-1
GenLL1
GenLL1
20 horizontal fibers
20 vertical fibers
40 Gbit/s per arm!
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PHENIX ALW/- Sensitivity

• Machine and detector requirements
• PHENIX ?Ldt800pb-1, P0.7
• Muon trigger upgrade!

2009 to 2012 running at vs500 GeV is projected
to deliver ?Ldt 980pb-1
x
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Upgrade Schedule
2002 2003 2004 2005 2006 2007 2008 2009 2010 2011
MPC
VTX (barrel)
FVTX
NCC
MuTrigger
RD Phase
Construction Phase
Ready for Data
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Physics Timeline
see Spin report to DOE http//spin.riken.bnl.gov/
rsc/
L 1x1031cm-2s-1
6x1031cm-2s-1
1.6x1032cm-2s-1
P 0.5 0.6 0.7

vs .. 200 GeV ......... 500
GeV
2005 2006 2007
2008 2009 . 2012
(RHIC II)
10 pb-1 ?275pb-1
.. 980pb-1
_at_ 200GeV
_at_ 500GeV
Inclusive hadrons Jets 25 Transverse
Physics Charm Physics

direct photons
bottom physics

W-physics
ALL(hadrons, Jets)
ALL(charm)
AL(W)
ALL(?)
41
The Electron-Ion Collider (EIC)
eLIC
eRHIC
Jefferson Laboratory
Brookhaven National Laboratory
Proposal to build a high luminosity polarized
electron-Ion collider part of Nuclear Physics
Long Range Plan, EICC Collaboration formed.
42
Summary

• We are pursuing a rich program of spin physics at
  RHIC!
• The polarized gluon distribution function DG will
  be measured through a variety of processes
• Inclusive hadrons, photon-tagged jets, heavy
  flavor
• Transverse spin may allow access to parton
  angular momentum
• The polarized proton program at RHIC will address
  the flavor dependent quark and antiquark spin
  distribution functions.
• The PHENIX upgrade program will provide the event
  detection and selection necessary to access this
  physics
• Silicon Tracking
• VTX and FVTX
• Electromagnetic Calorimetry
• NCC
• New Level-1 Trigger Capabilities
• Muon Trigger Upgrade

43
BACKUP
44
Collins Fragmentation
A simple model to illustrate that spin-orbital
angular momentum coupling can lead to left right
asymmetries in spin-dependent fragmentation
String breaks and a dd-pair with spin 1 is
inserted.
Proton spin is pointing up!
L -1
u
p picks up L-1 to compensate for the pair S1
and is emitted to the right.
u-quark absorbs photon/gluon and flips its Spin.
45
Local Polarimetry in STAR and PHENIX
STAR
PHENIX Exploit AN in forward
neutron production in ZDC (zero
degree hadronic calorimeters)
A(F)
A(F)
rotator off
rotator on
?
PL/P gt 0.99 blue yellow
46
Flavor separation of the spindependent quark and
anti-quark distributions in pp collisions _at_500GeV
Parity violation of the weak interaction in
combination with control over the proton spin
orientation gives access to the flavor spin
structure in the proton!
For W- interchange u and d.
Experimental Requirements ? tracking at
high pT ? good rejection of
backgrounds in analysis. ? event selection
for muons difficult due to background
muons from hadron decays and beam
backgrounds
W
Z
47
Trigger Rate and Rejection (W Physics)
REAL DATA
Design Luminosity vs 500 GeV s60mb L
2x1032/cm2/s
m momentum distribution
PTgt10GeV/c
HQ signal
PTgt20GeV/c
Total X-sec rate12MHz
W signal
DAQ LIMIT 1-2kHz (forµarm)
50
25
Required RF 10,000
Momentum GeV/c
Need Momentum Selectivity in the LVL-1 Trigger!
48
Upgrades Combine to Improve W/- Measurement
Hadron contamination?

• W/- measurement in polarized pp collisions to
  get at sea quark
• contributions to proton spin
• Isolation cut with FVTX/NCC detectors could be
  used to suppress hadron background to W/- signal.

49
Open Heavy Flavor at Forward Rapidity

• Open Heavy Flavor Physics Interests
• What is the total cross section for charm/beauty
  in all collision systems?
• Does asymmetry measurement in polarized pp
  collisions indicate contribution of gluons to
  proton spin?
• Is there modification of charm production in
  cold nuclear matter gluon saturation,
• Is there modification of charm production in
  heavy-ion collisions energy loss in medium
• Do heavy quarks flow?

50
Trigger Algorithm (RPC)
?(?)angle I angle II momentum cut
?(?) deg lt0.7 lt1.0 lt2.0
rejection 36000 19980 10090
Uncorrelated noise rate should be lt 10Hz/cm2.
RPC1 resolution 0.4cm
51
Trigger Algorithm (MuTR)
INPUT single m 15GeV/c vertex z0
52
Siberian Snakes
Gg number of spin precessions in one orbit
d ? 0? ? nsp Gg ? n No imperfection
resonances Partial Siberian snake (AGS) d
180? ? nsp Gg ½ n No imperfection
resonances and No Intrinsic resonances Full
Siberian Snake Two Siberian Snakes in RHIC
snake
snakes
53
Spin Structure from DIS
Photon probes couple to partons with electric
charge
Photon probes couple to partons with electric
charge
Parton Distribution functions (PDF) Helicity
average distribution quarks q(x) well
known gluons g(x) moderately well known
Helicity difference distribution quarks
?q(x) moderately well known gluons ?g(x)
unknown Helicity flip (transversity)
distribution quarks dq(x) unknown
Current Experiments
Need gluon and W probes to explore gluon spin
structure and sea quark spin structure!
Field started with polarized source
and targets about 1975 Yale/SLAC
collaboration
Field started with polarized source
and targets about 1975 Yale/SLAC
collaboration
54
Dg From a Global NLO pQCD Analysis With
Projected Future Direct Photon Data at 200 GeV
from RHIC
M. Hirai, H.Kobayashi, M. Miyama et al.
AAC Preliminary
AAC Preliminary
55
Dg From Direct Photons

• Prompt photon
• clear interpretation
• gluon Compton process dominant
• experimentally challenging
• good background reduction with high performance
  EMCal at PHENIX
• xg 0.04 0.3

statistics in full design luminosity and
polarization
56
Dg From Heavy Flavor (I)

• Heavy flavor
• xg ? 0.01 and smaller
• J/? production
• open heavy flavor production
• uncertainties
• J/? production mechanism
• prompt J/? or decay
• background
• charm and bottom
• ...

57
Dg From Heavy Flavor (II)

• Heavy flavor
• e-µ coincidence
• clean bottom-only at high mass
• background evaluation with like-sign pairs

H. Sato / W. Xie
58
Existing MuID LL1 Algorithm
Logical tubes formed by OR of physical tubes
across panels in each gap.
Rejection Factor 250-500
The most probable trajectory for a vertex muon
striking a gap-1 logical tube is to continue on a
path of equal dx/dz (vertical tubes) or dy/dz
(horizontal tubes). Tubes w/ the same dx/dz (or
dy/dz) get the same index.
59
Muon Piston Calorimeter

• Array of PbWO4 crystals mounted inside a recess
  in the Muon magnet pistons.
• 412 crystals covering 3.1 lt h lt 3.7
• Physics motivation
• Spin asymmetries in forward region

Installed and taking data!
60
Key RHIC Spin Measurements

• DG through a variety of physical processes
  (double-spin asymmetries)
• Inclusive pions
• Heavy Flavor
• Direct Photon Jet
• Transverse Spin Physics
• Sivers (jets and heavy flavor)
• Collins fragmentation function
• Interference fragmentation
• Flavor separation of quark/antiquark asymmetries
• W/- production in polarized pp collisions at 500
  GeV

61
at RHIC? q(x,Q2), g(x,Q2) and D(z,Q2) pQCD
PHENIX p0 cross section a ?lt0.35
Phys.Rev.Lett.91241803,2003
STAR p0 cross section a 3.4lt?lt4.0
Phys.Rev.Lett.92171801,2004
gluon fragmentation !?
Good agreement between NLO pQCD
calculations and experiment ? can use a NLO pQCD
analysis to extract spin dependent quark
and gluon distributions from RHIC data!
62
RHIC Physics (Heavy Ions)
nucl-ex/0611007 (submitted to Phys. Rev. C)
hep-ex/0609010 (accepted by Phys. Rev. Lett.)
nucl-ex/0611018 (submitted to Phys. Rev. Lett.)
The matter is strongly coupled
nucl-ex/0611019 (submitted to Phys. Rev. Lett.)
The matter is dense
The matter may melt and/or regenerate J/ys
nucl-ex/0611020 (submitted to Phys. Rev.
Lett.) hep-ex/0611020 (submitted to Phys. Rev.
Lett.)
The matter modifies jets
The matter is hot
63
How We Do It