eRHIC: Science and Perspective

Richard G. Milner RHIC Planning Meeting October 29-20, 2005

eRHIC: Science and Perspective

• Introduction• Science of eRHIC• Realization• Summary

Study of the Fundamental Structure of Matter with an Electron-Ion ColliderA. Deshpande, R. Milner, R. Venugopalan, W. Vogelsanghep-ph/0506148, to appear in Ann. Revs. Nucl. Part. Sci.


The study of the fundamental structure of matter

• QCD tells us that the nucleon and atomic nuclei are made of pointlike constituents bound by powerful gluon fields

• The valence quark region is well explored experimentally and reasonably well understood theoretically

• Frontier research in QCD demands a concerted experimental effort directed at the role of the gluons and sea quarks

• A new accelerator which directly probes the quarks and gluons is required

Lepton probeHigh center of mass energyHigh luminosity precisionPolarized lepton, nucleonOptimized detectors

• This accelerator is urgently needed to make progress in this field of research and has substantial discovery potential


Deep inelastic scattering – the experimental cornerstone of

QCD


Running of the strong coupling αs


Quark and gluon momentum in proton


Lattice QCD making significant progress

quenchedapproximation

full QCD

agreement with experiment at level of few percent

HPQCDhep-lat/0304004


Nucleon axial charge in full lattice QCD

LHPC Collaborationhep-lat/0510062


Why a Collider ?

• High Ecm (20 to 100 GeV)

large range of x, Q2

x range (10-4 to 1): valence, sea quarks,

glue

Q2 range ( 1 to 1000 GeV2): utilize

evolution equations of QCD

• High luminosity ~ 1033 cm-2 s-1

• High polarization ( ~ 70%) of lepton, nucleon achievable

• Complete mass range of nuclear targets

• Collider geometry allows complete reconstruction of final state

Qmax2 = ECM

2 · x


Why eRHIC?

• RHIC is the world’s first and only polarized proton collider

• In addition, RHIC accelerates heavy ion beams to 100 GeV/nucleon

• eRHIC would capitalize on ~ $ 1 billion investment in RHIC

• RHIC scientific program strongly motivates eRHIC • Leadership from BNL in evolution of lepton-ion

collider since 1999• eRHIC is viewed at BNL as a necessary and

natural upgrade of RHIC after 2010 as the heavy ion and spin programs mature


The Electron-Ion Collider (EIC)• A high luminosity (~1033cm-2s-1) polarized lepton-ion collider is

the QCD machine hadronic physicists worldwide require for future research

• Workshops Seeheim, Germany 1997 MIT 2000

IUCF 1999 BNL 2002 BNL 1999 JLab 2003 Yale 2000

• Electron Ion collider (EIC) received very favorable review of science case in US 2001 Nuclear Physics Long Range Plan, with strong endorsement for R&D

• NSAC in March 2003, declared EIC science `absolutely central’ to Nuclear Physics

• DOE Office of Science 20 Year Strategic Plan in 2004: eRHIC

identified as a long term (realized ~ 2015) priority


EIC Steering Committee

• A. Caldwell (MPI Munich)• A. Deshpande (StonyBrook)• R. Ent (JLab)• G. Garvey (LANL)• R. Holt (ANL)• E. Hughes (Caltech)• K.-C. Imai (Kyoto Univ.)• R. Milner (MIT)• P. Paul (BNL)• J.-C. Peng (Illinois)• S. Vigdor (Indiana Univ.)


eRHIC will be a unique accelerator


Scientific Highlights• nucleon structure

sea quarks and gluespin and flavor structurenew parton distributions

• Meson structure, K are Goldstone Bosons of QCDessential to nuclear binding

• hadronizationevolution of parton into hadronprocess in nuclei of fundamental interest

• nucleirole of partonsinitial conditions for relativistic heavy ion

collisions• matter under extreme conditions

saturation of parton distributionsnew phenomena, e.g. colored glass condensate


The Spin Structure of the Nucleon

•Bjorken Sum Rule

• From NLO-QCD analysis of DIS measurements …ΔG = 1.0±1.2

•quark polarization Δq(x)first 5-flavor separation from

HERMES

• transversity δq(x)a new window on quark spinazimuthal asymmetries from

HERMES and JLab

•gluon polarization ΔG(x)RHIC-spin and COMPASS will

provide some answers!

•orbital angular momentum Lhow to determine? GPD’s ?

½ = ½ + G + Lq + Lg

ΔΣ ≈ 0.2


Inclusive spin-dependent DIS


QCD Test: Bjorken Sum Rule

• Γ1p - Γ1

n = 1/6 gA [1 + Ο(αs)]

• Sum rule verified to ±10%

• eRHIC can approach ±1% G. Igo

Precision QCD test


Polarized parton densities of the proton

Blümlein and Böttcher

SU(3) flavorsymmetryassumed


HERMES Flavor Decomposition of Quark Spin

Semi-inclusiveDIS


Gluon polarization data from COMPASS

2002+3 data analysis; as much again in 2004 on disk


Gluon polarization from STAR

STAR preliminary 2003+4 results for double longitudinal spin asymmetry ALL versus jet pT in p + p → jet + X, compared with NLO pQCD

New more precise data from PHENIX


Spin structure function gp1 at low

xx = 10-4 0.7Q2 = 0 104 GeVeRHIC 250 x 10 GeV

Lumi=85 inv. pb/day

x = 10-3 0.7Q2 = 0 103 GeVFixed target experiments1989 – 1999 Data

10 days of eRHIC runAssume: 70% Machine Eff. 70% Detector Eff.


eRHIC determination of polarized quarks and anti-

quarks

Kinney and Stösslein


Gluon polarization from di-jets in LO for eRHIC

Deshpande et al.


Parity violating lepton scattering

• requires a positron beam• determines new combinations of Δu, Δd, Δs etc.• analog sum rule to Bjorken

A. Deshpande


Saturation: the Color Glass Condensate


Using Nuclei to Increase the Gluon Density

• Parton density at low x rises as• Unitarity saturation at some• In a nucleus, there is a large enhancement of the parton

densities / unit area compared to a nucleon

•

ExampleQ2=4 (GeV/c)2

< 0.3 A = 200

Xep=10-7 for XeA = 10-4

1x

2sQ

2 1 13 3

2

/

/

6 for 200

A A A

N N N

G R GA A

G r AG

A

2

21

134

3

eA s

ep s

X Qx Q

A


Gluon Momentum Distribution from DIS


RHIC Data consistent with Gluon Saturation


Longitudinal structure function FL

• Extracted from scaling violations of F2 • Experimentally can be determined directly• Highly sensitive to effects of gluon• With precise enough F2 and FL one can extract the coefficient λ of the saturation scale• Logarithmic derivatives of F2 and FL with Q will be sensitive to CGC


Quarks in a Nucleus“EMC Effect”

Space-Time Structure of Photon

F2A/F2

D

10-4 10-3 10-2 10-1 1 x

Can pick apart the spin-flavor structure of EMC effect by technique of flavor tagging, in the region where effects of the space-time structure of hadrons do not interfere (large !)

Nuclear attenuation negligiblefor > 50 GeV hadrons escapenuclear medium undisturbed


eRHIC projections for nuclear quark distributions

T. Sloan1 pb-1


Sea-Quarks and Gluons in a NucleusDrell-Yan No Nuclear Modifications to

Sea-Quarks found at x ~ 0.1 Where is the Nuclear Binding?

Flavor tagging can also (in principle)disentangle sea-quark contributions

Constraints on possible nuclearmodifications of glue come from1) Q2 evolution of nuclear ratio of F2 in Sn/C (NMC)2) Direct measurement of J/Psi production in nuclear targets

Compatible with EMC effect?Precise measurements possible of Nuclear ratio of Sn/C (25 GeV) J/Psi production (ELIC)

F2

G


Nuclear Binding Natural Energy Scale

of QCD: O(100 MeV) Nuclear Binding Scale

O(10 MeV) Does it result from a

complicated detail of near cancellation of strongly attractive and repulsive terms in N-N force, or is there another explanation?

How can one understand nuclear binding in termsof quarks and gluons?

Complete spin-flavor structureof modifications to quarks andgluons in nuclear system may bebest clue.


eRHIC - machine design aspects– First detailed document (252

pages) reporting on the eRHIC accelerator and interaction region (IR) design studies

– Collaborative effort between BNL, MIT-Bates, BINP and DESY

– Goal:• Develop an initial design for

eRHIC• Investigate accelerator physics

issues most important to its design

• Evaluate luminosities that could be achieved with minimal R&D effort including IR design

• Identify specific R&D extensive accelerator aspects which could lead to significantly higher luminosities

– Review planned in June 2005http://www.bnl.gov/eic/http://www.bnl.gov/eic/

See talk by V. Ptitsyn


eRHIC: ring-ring design•Collisions at 12 o’clock interaction region•10 GeV, 0.5 A e-ring with 1/3 of RHIC circumference (similar to PEP II

HER)•Inject at full energy 2 – 10 GeV•Existing RHIC interaction region allows for typical asymmetric

detector (similar to HERA or PEP II detectors)

AGS

BOOSTER

TANDEMS

RHIC

2 – 10 GeV e-ring

e-cooling

2 -10GeV Injector

LINAC

Required modifications of RHIC:• Electron cooling• Increase of total current 120 → 360 bunches•Additional spin rotators


eRHICeRHIC

e-/e+ Ring e-/e+ Ring ParametersParameters

10 GeV electrons – 250 GeV 10 GeV electrons – 250 GeV protonsprotons

Ion beam Energy p 250 GeV

Circumference(m) 1278

Electron Energy (GeV) 10

Bending radius(m) 81

Bunch spacing(m) 10.6

Number of bunches 120

Bunch population 1.00E+11

Beam current(A) 0.45

Energy loss/turn (MeV) 11.7

Accelarting voltage(MV) 25

Total rad. Power(MW) 5.27

Syn. Rad. Power/m (KW) in Arc 9.63

Self-pola. Time at 10GeV(minutes) 22.03

Emittance-x, no coupling (n m.rad) 56.6

Beta function at IP (cm) y*/x

*19.2/26.6

Emittance Ratio (y/x) 0.18

Beam size at IP(um) σx 104

Beam size at IP(um) σy 52

Momentum spread σE 9.61E-04

Bunch length (cm) σz 1.17

S.R. damping time(x) (mS) 7.3

Beta tune x 26.105

Beta tune y 22.145

Natural chromaticity ξx/ξy -35.63/-33.84

Luminosity (10^33/cm^2/s) 0.44

• Luminosity limited by beam-beam tune shifts:ξi = 0.0065 ξe = 0.08 •Luminosity assumes collisionsat two other IPs• Dedicated operations yields Luminosity ~ 1033 cm-2 s-1


eRHIC: linac-ring design – Two possible designs are

presented in the ZDR– Electron beam is

transported to collision point(s) directly from superconducting energy recovery linac (ERL)

– Features:• Higher luminosity (~ X 5)

possible• Rapid reversal of

electron polarization• Machine elements free

region approx. 5m• Simpler IR region design:

Round beams possible• Multiple interaction

regions• No positrons


eRHIC: interaction region design - ZDR assumes IR region with zero

crossing angle

– Crab-crossing scheme considered (rotate ion bunch into direction of electron beam): However, required

deflecting RF voltage: V=14.4MV

factor 10 larger then for KEKB crab cavities so excluded for now.

– Initial design: dipole winding in the superconducting electron low- quadrupoles

– Limitation: 1m machine element free region


Review of eRHIC ZDR• Detailed technical review of both ring-ring and linac-ring

designs by Machine Advisory Committee in June 2005• Excellent technical criticism• Status of the present design studies is not yet advanced

enough to make a decision• Ring-ring: challenging project, can be extrapolated from to-

day’s experience and technology. Suggests pursuing ring-ring design as first priority with goal to develop a CDR

• Linac-ring: much more attractive regarding luminosity. Requires successful extrapolations on a number of fronts that are beyond what can be achieved to-day. Requires intensive R&D.

• Concentrate first on R&D which is useful to ring-ring and ring-linac options, e.g. ERL prototype, cooling of ion beams, and beam dynamics.


eRHIC lepton ring vs. PEP-II rings

• Ring type: all are collider lepton rings• Energy range: 5~10 GeV vs. 3-9 GeV• Beam currents: 0.5 ~1A vs. 1.5 ~ 3 A

similar bunch current, PEP-II has more bunches• Beam dimensions: comparable

emit.: ~50 nm rad, beta_ave: ~15 m (arc, rf)• Collective effects:similar

In general, technical approaches are very similar.


• RF : 90% from PEP-II with minimum modifications

• Magnet/power supply: ~ 50 % from PEP-II• Feedback: 100%• Beam diagnostics / control: ~ 50%• Vacuum: now we count it as 20% ~ 30% • Injector: design not clear at this point.

• A large amount of PEP-II components could be used for the eRHIC lepton ring.

• A preliminary estimate shows that we may save 20% to 30% of cost by using PEP-II components. More detailed studies underway.

D. Wang/BatesPreliminary conclusions


eRHIC - Detector design • General purpose

unpolarized/polarized ELECTRon-A- compact central detector with

specialized forward/rear tagging detectors- required free region ± 3m

Ae

eA

• Forward detector unpolarized eA- specialized detector system to detect complete final state in eA collisions- Require dfree region ± 5 m• Common detector subsystems• R&D coherent with heavy ion detector upgrades

See talk by B. Surrow


eRHIC realization• Science case must be strengthened and sharpened• Accelerator case must be developed so that the

science can be made to happen in a timely fashion • eRHIC must be presented coherently in the context of

future RHIC plans • It is essential to energize and organize the hadronic

community worldwide in support of eRHIC• We expect a new Long Range planning exercise

within the next several years• We have to be ready to argue for eRHIC in the

context of other communities arguing strongly for other priorities


Summary• A high luminosity lepton-ion collider offers

a very exciting future for the study of the fundamental structure of matter

• eRHIC appears to be a very attractive means to realize it in a cost-effective and timely way

• It is essential to develop the most powerful eRHIC case with a sense of urgency

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eRHIC: Science and Perspective

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