A High Luminosity Electron-Ion Collider to Study the Structure of Matter

Transcription

1 A High Luminosity Electron-Ion Collider to Study the Structure of Matter Introduction Scientific motivation Realization Summary Study of the Fundamental Structure of Matter with an Electron-Ion Collider A. Deshpande, R. Milner, R. Venugopalan, W. Vogelsang hep-ph/ , Ann. Rev. Nucl. Part. Sci. 55, 165 (2005). 1

2 Why study QCD? It is the only fully consistent theory that we are certain that describes the real world in the limit m q 0, no free parameters All the interactions are a consequence of deep symmetry principles like gauge invariance and chiral symmetry Most of the visible phenomena are emergent quarks and gluons are not seen This makes QCD the only (wonderful) laboratory for exploring the dynamics of a non-trivial, consistent relativistic theory QCD is the central thrust of Nuclear Physics in the U.S. R.L. Jaffe 2

3 Scientific Motivation for EIC The origin of spin How does the spin structure of the nucleon arise from the quark and gluon constituents? The glue that binds us all What is the role of glue in hadron structure, in atomic nuclei, and in the spin structure of the nucleon? 3

4 Why a high luminosity lepton-ion collider? Lepton probe provides precision but requires high luminosity to be effective High E cm large range of x, Q 2 x range: valence, sea quarks, glue Q max2 = E CM2 x Q 2 range: utilize evolution equations of QCD High polarization of lepton, nucleon achievable Complete range of nuclear targets Collider geometry allows complete reconstruction of the final state 4

5 EIC will be a unique accelerator Quarks discovered EIC Gluon momentum distribution measured Nucleon spin structure studied 5

6 Q 2 and x Range of EIC Q^ ep -> ex Kinematic Range 10 x 250 GeV 12 GeV Fixed Target E e =5-10 GeV E p = GeV s ½ = GeV x Bj =10-4 to 0.7 Q 2 =0 to 10 4 (GeV/c) 2 polarization of e ±, p, 3 He ~ 70% heavy ion beams of all elements high luminosity > cm -2 s x 6

7 Why now? Understanding the fundamental structure of matter of central importance in physics, e.g. - Spin structure of the nucleon - Partonic understanding of nuclei Over last 40 years increasing sophistication both in experimental techniques and theoretical understanding of DIS Lepton-nucleon capability disappearing at high energy lepton facilities (SLAC, Fermilab, CERN, and DESY) To push past the present frontiers and carry out a comprehensive study of the role of glue, the planning of the next generation facility is a matter of urgency 7

8 EIC evolution Substantial international interest in high luminosity (~10 33 cm -2 s -1 ) polarized lepton-ion collider over decade Workshops Seeheim, Germany 1997 MIT, USA 2000 IUCF, USA 1999 BNL, USA 2002 BNL, USA 1999 JLab, USA 2004 Yale, USA 2000 BNL,USA 2006 EIC received favorable review of science case in US 2001 Nuclear Physics Long Range Plan, with strong endorsement for R&D At BNL Workshop in March 2002, a plan was formulated to produce a conceptual design for EIC within three years NSAC in March 2003, declared EIC science `absolutely central to future of Nuclear Physics EIC identified in November 2003 as future priority in DOE Office of Science 20 year planning EIC viewed as part of the future at both BNL and JLab. 8

9 Spin intrinsic angular momentum Spin arose from quantum mechanics and relativity and is an essential element of our understanding of the structure of matter Atoms and nuclei The shell structure of atoms and nuclei can be well understood in terms of their spin-½ constituents in a mean potential and the Schrödinger equation Constituents can be directly observed in scattering experiments and the spin structure verified We understand in a fundamental way the stability of matter and why chemical elements and nuclear isotopes have their rich structure observed in our physical world. 9

10 Spin structure of the proton Most of the mass in the world around us arises from QCD, predominantly from the gluons. QCD tells us that the proton consists of spin-½ quarks interacting via exchange of spin-1 gluons. This is a highly relativistic system described by a non- Abelian gauge theory, completely unlike an atom or nucleus. The quark model has had great success in predicting the spins of baryons: consequence of symmetry. We have learned that the quark model breaks down in understanding proton structure from scattering experiments. ½ = ½ Σ + G + Lq + Lg It is essential to understand how the proton spin-½ arises from its quark and gluon constituents and their orbital angular momentum contributions. 10

11 Lepton Scattering Uses the extremely well understood electroweak probe to scatter from the constituents. At low energies (< 1 GeV) the scattering process is well understood in terms of scattering from hadrons. At high energies (> 20 GeV) the scattering is directly interpretable in terms of scattering from the pointlike quarks deep inelastic scattering. The spin asymmetry when detecting the scattered lepton and the coincident constituent is directly sensitive to the polarization of the constituent. This works well for atoms and nuclei, at the nucleon level. It is drastically different in lepton scattering from the proton at high energies. 11

12 J=1 Example: The Spin Structure of the Deuteron L=0 S=1 1 = L + S L=2 S=1 p n p n S-state 96% of g.s. wave function D-state 4% of g.s. wave function 12

13 850 MeV (e,e p) Scattering from Polarized Deuterium BLAST unpublished, preliminary Scattering asymmetry shown vs. initial momentum p m of detected proton The solid line is the theoretical prediction using Monte-Carlo (Arenhövel) At low p m, the scattering is dominated by the S-state in the deuteron At high p m, the asymmetry changes sign and is predominantly due to the D- state of the deuteron. 13

14 Deep Inelastic Scattering = E 2 CM Directly interpretable in terms of QCD Struck quark hadronizes, typically to a meson Quark momentum fraction is 0<x<1 Valence quarks: 0.1 x 1 Sea quarks and gluons: x 0.1 Q 2 =E 2 CM x => low x reached with high E CM Forty years of data has provided a precise determination of the momentum distribution of the quarks and gluons in the proton. 14

15 Determining quark distributions cross sec tion F2 ( x) g 1 spin asymmetry F F x e f x f x ( ) = i[ i ( ) + i ( )] i = u, d, s... g x e f x f x 2 ( ) = i[ i ( ) i ( )] i = u, d, s,... f i (x) = probability to find a quark with momentum x and flavor i polarized along the nucleon spin 2 15

16 The Proton at High Energies Parton model QCD-logarithmic corrections 16

17 The proton is a highly relativistic system of quarks and gluons sea quarks valence quarks # of valence quarks # quarks 17

18 High energy structure of the proton 18

19 Spin-dependent DIS Spin polarize longitudinally both the proton and lepton Measure the spin scattering asymmetry in DIS Inclusive asymmetry directly interpretable in terms of QCD => polarizations of quarks (both valence and sea) under some assumptions Pioneering experiments initiated at SLAC and CERN in the 1980 s. Surprising result that the quark model breaks down motivated a second generation of experiments at SLAC, CERN, and DESY. Scientific interest has motivated a tremendous technical development: polarized electron beams, polarized proton and neutron targets, world s first polarized proton collider as well as intensive theoretical developments. 19

20 The Spin Structure of the Nucleon ½ = ½ ΔΣ + ΔG + L q + L g We know from lepton scattering experiments over the last three decades that: quark contribution Σ 0.2 gluon contribution G 1 ± 1 valence quark polarizations as expected measured anti-quark polarizations are consistent with zero 20

21 EIC will allow access to all aspects of nucleon spin g 1p (x) measured at lower x G accurately determined through several independent channels simultaneously quark polarizations accurately determined Transverse spin fully explored Access to processes over a large range of x and Q 2 => possible determination of the contribution of orbital angular momentum L q,g 21

22 Spin-dependent DIS detecting only the scattered lepton Measurements on neutron and proton with assumptions yield quark polarizations vs. x Q 2 dependence yields gluon spin Neutron and proton data can be combined to test QCD Bjorken Sum Rule: Γ 1 p - Γ 1 n = 1/6 g A [1 + Ο(α s )] Sum rule verified at present to ±10% 1% with EIC Kinematic range accessible with existing accelerators exhausted. 22

23 g p 1 at low x related to G(x) x = Q 2 = GeV Fixed target experiments Data x = Q 2 = GeV EIC 250 x 10 GeV Lumi=85 pb -1 /day 10 days of erhic run Assume: 70% Machine Eff. 70% Detector Eff. 23

24 Gluon polarization from RHIC-spin New more precise data ay SPIN 2006 STAR preliminary results for double longitudinal spin asymmetry A LL versus jet p T in p + p jet + X, compared with NLO pqcd 24

25 G(x,Q 2 ) at EIC Best determination from scaling violations of g 1 (x,q 2 ) EIC will extend range in x ( down to 1 x10-4 ) and Q 2 improve existing measurements by a factor of 3 in 1 week! Direct measure via photon-gluon fusion (down to 3 X 10-3 ) di-jets, high P T hadrons Successfully used at HERA NLO calculations exist Constrains shape in mid x region 1 fb -1 in ~2 weeks at EIC Scaling violation data plus di-jet analysis will yield total uncertainty ~ 5% after 1 year 25

26 HERMES Flavor Decomposition of Semi-inclusive DIS Quark Spin 26

27 EIC determination of polarized quarks and anti-quarks 27

28 Orbital Angular Momentum At this time, it is not known how to determine the contribution of orbital angular momentum. One promising avenue being pursued is to measure a photon or meson in coincidence with the scattered lepton. New types of quark distribution functions arise. JLab@12 GeV will explore this avenue in the valence quark region at low Q 2 EIC will carry out measurements complementary to those at JLab at higher Q 2. Study of this important issue by both theorists and experimentalists continues 28

29 Parity violating lepton scattering W + and W - exchange probed via parity violating scattering This measurement requires a polarized positron beam. It will provide new combinations of Δu, Δd, Δs etc. Δs and Δsbar can be isolated. There is an analog sum rule to the Bjorken Sum Rule. 29

30 Quarks and gluons in Nuclei EMC effect: quark momenta are modified compared to the free nucleon F 2A /F 2 D Gluons in nuclei: almost no experimental information -shadowing - saturation - initial conditions for perfect liquid Fast partons traversing nuclei - hadronization x 30

31 EIC projections for nuclear quark distributions 1 pb -1 T. Sloan 31

32 RHIC Data consistent with Gluon Saturation 32

33 Ratio of Gluon densities in Lead to Proton at Q 2 = 5 (GeV/c) 2 Factor 3 uncertainty in glue => factor 9 uncertainty in semi-hard HI-parton cross-sections at LHC! R. Venugopalan 33

34 Initial conditions for the perfect liquid/qgp McLerran, Ludlam; Physics Today 34

35 Using Nuclei to Increase the Gluon Density x Parton density at low x rises asx δ 2 Unitarity saturation at someq s In a nucleus, there is a large enhancement of the parton densities / unit area compared to a nucleon G / π R G G r A G 6 fo r A = ep A A 3 A 3 A A 2 N / π N N 2 ( Q s ) = X 4 3 ea A 2 ( Q s ) δ 1 Example: Q 2 =4 (GeV/c) 2 δ< 0.3 ea at erhic same parton A = 200 density as ep at LHC energies! X ep =10-6 for X ea =

36 Longitudinal structure function F L Extracted from scaling violations of F 2 Experimentally can be determined directly Highly sensitive to effects of gluon With precise enough F 2 and F L one can extract the coefficient λ of the saturation scale Logarithmic derivatives of F 2 and F L with Q will be sensitive to CGC 36

37 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 terms of quarks and gluons? Complete spin-flavor structure of modifications to quarks and gluons in nuclear system may be best clue. 37

38 erhic ring-ring ZDR design Collisions at 12 o clock interaction region. 10 GeV, 0.5 A e-ring with 1/3 of RHIC circumference Positron and electron beams. Inject at full energy 5 10 GeV. Collision luminosity ~ cm -2 s -1. Cost scale understood. Favorably reviewed by peers GeV e-ring 2-10GeV Injector RHIC LINAC BOOSTER AGS e-cooling TANDEMS 38

39 erhic: linac-ring concept 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 39

40 ERL-based ELIC concept Ion Linac and pre-booster Electron Cooling IR IR IR Solenoid Snake 3-7 GeV electrons GeV light ions Electron Injector CEBAF with Energy Recovery Beam Dump 40

41 Ring-Ring ELIC concept Use present CEBAF as injector to electron storage ring Add light-ion complex 41

42 EIC Detector J. Pasukonis, B.Surrow Design 2: General purpose (unpolarized/polarized ELECTRon-A) (4) Simulated eca event (VNI) Top view Detailed design in progress. 42

43 First Demonstration of Optical Stochastic Cooling Low emittance hadron beams at colliders are essential for attaining high luminosity operation: RHIC erhic, LHC, etc. Cooling of the beam is essential for high luminosity. Optical stochastic cooling (OSC) is a promising technique which has never been demonstrated. A MeV electron storage ring is an ideal machine on which to demonstrate OSC An experiment is in preparation by a collaboration involving MIT, Indiana Univ., BNL and LBL to mount an experiment to provide a first demonstration of OSC at the Bates SHR. Funding proposal submitted to both DOE and NSF. Design of the experiment getting underway. 43

44 Present (Technically Driven) Schedule Estimate for BNL realization Q Q Q Q Q4 NSAC approval CD0 R&D funding CD1 CD2 CD3 (begin construction) CD4 (commissioning begins) 44

45 Summary A high luminosity lepton-ion collider will open up a dramatic new window into the role of gluons in the structure of matter. It will enable precision measurements in present terra incognita on fundamental scientific questions - spin structure of the nucleon - role of gluons and quarks in nuclei Polarized e ± probes in collider geometry will offer unprecedented access to the sea quarks and gluons of the nucleon and atomic nuclei. There is a realistic machine design at a luminosity of ~ cm -2 s -1 with a well understood cost. There are more ambitious machine concepts under development. It is essential that the broad community of physicists work together to make EIC a reality in about a decade. 45