1e.c. aschenauerstony brook student seminar, february 2010
TRANSCRIPT
Stony Brook Student Seminar, February 2010 1
The new kid on the blockThe Electron Ion Collider
E.C. Aschenauer
Stony Brook Student Seminar, February 2010 2
The Standard Model
E.C. Aschenauer
QED: Theory of the Electromagnetic InteractionQCD: Theory of the Strong Interaction
Stony Brook Student Seminar, February 2010 3
A reminder of Quantum Electro-Dynamics (QED)
E.C. Aschenauer
Theory of electromagnetic interactions Exchange particles (photons) do not carry electric
charge Flux is not confined: V(r) ~ 1/r, F(r) ~ 1/r2
Coupling constant (α): Interaction StrengthIn QED: αem = 1/137
√α √α
Feynman Diagram: e+e- annihilation
Vem (r) =−q1q2
4πε0r=αem
r
Stony Brook Student Seminar, February 2010 4
Vs (r) =43α s
r+ kr
Quantum Chromo-Dynamics (QCD) - 1973
E.C. Aschenauer
Theory of strong (nuclear) interactions Three colour charges: red, green and blue Exchange particles (gluons) carry colour charge and
can self-interact Flux is confined: V(r) ~ r, F(r) ~ constant
q
qg
√α√α
αs = strong coupling constant ≈ 0.3
long range force ~ r
gluons can self-interact - more vertices are allowed
~1/r at short range
Stony Brook Student Seminar, February 2010 5
Features of Quantum Chromo-Dynamics
E.C. Aschenauer
Confinement
At large distances, the effective coupling between quarks is large, resulting in confinement (V(r) ~ r)
Free quarks are not observed in nature
Asymptotic freedom
At short distances, the effective coupling between quarks decreases logarithmically
Under such conditions, partons appear to be quasi-free
0.2 fm 0.02 fm 0.002 fm
Stony Brook Student Seminar, February 2010 6
QED vs QCD
E.C. Aschenauer
Potentials:
long range aspect of Vs(r) leads to quark confinement and the existence of nucelons
QED QCD
Charges electric (2) colour (3)
gauge bosons γ g (8)
charged? no yes
coupling strength
αem = e2/4π ≈ 1/137 αs ≈ 0.3
Vs (r) =43α s
r+ krVem (r) =−
q1q2
4πε0r=αem
r
Stony Brook Student Seminar, February 2010
Deep Inelastic Scattering
E.C. Aschenaue
r
7
( , )E k( ', ')E k
q 2 2 2( ')Q q k k 'E E 2
2
Qx
Mv
hEz
Important kinematic variables:
cross section:
DF FF
( ) )( () ~ hf f
hf
f
d zdx Dz q
2tp
Photon:Photon:
Hadron:Hadron:
Quark:Quark:
The structure of the proton – simple view
E.C. Aschenuer
Stony Brook Student Seminar, February 20108
Quark splitsinto gluonsplitsinto quarks …
Increasing resolution (large angle scattering = large Q2)
Increasing resolution: we see “small” partons How do the quarks behave at low-x
E.C. Aschenaue
r
Stony Brook Student Seminar, February 201010
Deep Inelastic Scattering
( , )E k( ', ')E k
q
Important kinematic variables:
cross section:
DF FF2
~'
Ld
d dEW
1 2 1 22( )
p p i ig q s q p qsF gs pF gW q
ε ε
1 2 3 4
1 1 1( ) ( ) ( )
6 2 2r s t u s u s tb b b b
Spin 1
'E E
hEz
Photon:
Hadron:
Quark:
2tp
Collider:
Q2 =−q2 =−(k−k')2 =2EeEe'(1+cosΘe)
x =Q2
sy
Stony Brook Student Seminar, February 2010
“Structure Function” & quark densities
E.C. Aschenaue
r
11
Strong increase of sea quarks towards low x
Density increases with Q2
more partons by magnified view
F2(x)=e2x(q(x)+q(x))quark density
Dynamic creation of quarks at low x
Stony Brook Student Seminar, February 2010 11
d2empNC
dxdQ2 =2παem2 Y+
xQ4 (F2 −y2Y+
FL ±Y−Y+
xF3)
Measure Glue through DIS
E.C. Aschenauer
small x
large x
q(x, Q2 )+ q(x,Q2 ) g(x,Q2 ) q(x, Q2 )−q(x,Q2 )
Observation of large scaling violations
Gluon density dominates
Stony Brook Student Seminar, February 2010 12
What do we know till know
E.C. Aschenauer
F2=C(Q2)x-lQ2 = 1GeV 0.2fmEM proton radius: 0.9fm
gp
Transition, Why, How?
x<0.01 HERA: =−ln ∂F2 / ∂ ln x
Does the rise of F2 set in at the same Q2 for nuclei?
eRHIC: =−ln∂F2 / ∂ lnx
Stony Brook Student Seminar, February 2010 13
d2empNC
dxdQ2 =2παem2 Y+
xQ4 (F2 −y2Y+
FL ±Y−Y+
xF3)
FL: measures glue directly
FL ~ αs G(x,Q2) requires √s scan Q2/xs = y
⇒ G(x,Q2) with great precision
E.C. Aschenauer
Stony Brook Student Seminar, February 2010 14
Parton Saturation
E.C. Aschenauer
at small x linear evolution gives strongly rising g(x) BK/JIMWLK non-linear evolution
includes recombination effects saturation
Dynamically generated scale Saturation Scale: Q2
s(x) Increases with energy or decreasing x
Scale with Q2/Q2s(x) instead of x and Q2
separately as~1 as << 1 Question:What is the relation between saturationand the soft regime? Confinement?
HERA (ep):
Despite high energy range: F2, Gp(x, Q2) only outside the
saturation regimeRegime where non-linear QCD
(saturation phenomena) matter
(Q < Qs) not reached, but close
Only way in ep is to increase √s
EIC (eA): Instead extending x, Q reach
increase Qs
Pocket formula for nuclei
Remember: Q2 > Qs2 ⇒ α s =α s(Q
2 )
Q2 < Qs2 ⇒ α s =α s(Qs
2 )
Qs2 (x, A) ~ Q0
2 A
X⎛⎝⎜
⎞⎠⎟
1/3
Stony Brook Student Seminar, February 2010 15
The Nuclear Enhancement Factor
E.C. Aschenauer
Enhancing Saturation effects: Probes interact over distances L ~ (2mnx)-1
For probes where L > 2RA (~ A1/3), cannot distinguish between nucleons in the front or back of of of the nucleus.
Probe interacts coherently with all nucleons. Probes with transverse resolution 1/Q2 (<< Λ2
QCD) ~ 1 fm2 will see large colour charge fluctuations.
This kick experienced in a random walk is the resolution scale.
Simple geometric considerations lead to:
Nuclear Enhancement Factor:
Enhancement of QS with A: ⇒ non-linear QCD regime reached at significantly lower
energy in e+A than in e+p
Stony Brook Student Seminar, February 2010 16
Probing Saturation regime
HERA (ep) energy range higher G(x,Q2) very limited reach of the saturation regime
eRHIC (eA) will probe deeply into the saturation region
E.C. Aschenauer
Stony Brook Student Seminar, February 2010 17
d2empNC
dxdQ2 =2παem2 Y+
xQ4 (F2 −y2Y+
FL ±Y−Y+
xF3)
F2: for Nuclei
E.C. Aschenauer
Assumptions: 10GeV x 100GeV/n
√s=63GeV Ldt = 4/A fb-1
equiv to 3.8 1033 cm-2s-1
T=4weeks; DC:50% Detector: 100% efficient
Q2 up to kin. limit sx Statistical errors only
Note: L~1/A
Stony Brook Student Seminar, February 2010 18
Diffractive physics: ep vs eA
E.C. Aschenauer
xIP = mom. fraction of pomeron w.r.t. hadron
HERA/ep: ~20% of all events are hard diffractive Diffractive cross-section σdiff/σtot in e+A ? Predictions: ~25-40%? Diffractive structure functions
FLD for nuclei and p extremely sensitive
Exclusive Diffractive vector meson production: dσ/dt ~ [xG(x,Q2)]2 !!
Distinguish between linear evolution and saturation models
?
Curves: Kugeratski, Goncalves, Navarra, EPJ C46, 413
ddt t=0
(γ * A→ MxA) ~α 2 GA(x,Q2 )⎡⎣ ⎤⎦
2
DIS: Diffraction:
Stony Brook Student Seminar, February 2010 19
How to measure coherent diffraction in e+A ?
Beam angular divergence limits smallest outgoing Qmin for p/A that can be measured
Can measure the nucleus if it is separated from the beam in Si (Roman Pot) “beamline” detectors pTmin ~ pAθmin
For beam energies = 100 GeV/n and θmin = 0.08 mrad:
These are large momentum kicks, much greater than the binding energy (~8MeV) Therefore, for large A, coherently
diffractive nucleus cannot be separated from beamline without breaking up
E.C. Aschenauer
species (A)
pTmin (GeV/c)
d (2) 0.02Si (28) 0.22Cu (64) 0.51In (115) 0.92Au (197) 1.58U (238) 1.90
Need to rely on rapidity gap
method simulations look good high eff. high purity
possible with gap alone ~1% contamination ~80% efficiency
depends critical on detector hermeticity
improve further by veto on breakup of nuclei (DIS) Very critical
mandatory to detect nuclear fragments from breakup
n: Zero-Degree calorimeter p, Afrag: Forward
Spectrometer New idea: Use U instead of
AU
Stony Brook Student Seminar, February 2010 20
Measure the Gluon Form Factor
E.C. Aschenauer
RA = 1.2A1/3fmElastic scattering on full nucleus long wavelength gluons (small t)
Expectation for 1M J/y
Requirement:Momentum resolution < 10MeVgreat t resolutionNeed to detect nuclei break-upproductsdetect e’ in FED
Basic Idea:Studying diffractive exclusive J/y productionat Q2~0 Ideal Probe:large photo-production cross sectiont can be derived from e,e’ and J/ y 4-momentum
Stony Brook Student Seminar, February 2010 21
Important to understand hadron structure: Spin
E.C. Aschenauer
SqDq
DG
Lg
SqLq
dq1Tf
SqDq
DG
Lg
SqLq dq1Tf
Is the proton spinning like this?
“Helicity sum rule”
12h= P, 12 |J QCD
z |P, 12 = 12q
∑ Sqz+Sg
z+ Lqz
q∑ +Lg
z
total u+d+squark spin
angular momentum
gluonspin Where do we go with
solving the “spin puzzle” ?
N. BohrW. Pauli
Stony Brook Student Seminar, February 2010
How to measure Quark Polarizations
E.C. Aschenaue
r
27
3/ 2 (~ )q x
rSγ* +
rSN =
32
N qSS
1/ 2 (~ )q x
rSγ* +
rSN =
12
N qSS
Virtual photon g* can only couple to quarks of opposite helicity
Select q+(x) or q-(x) by changing the orientation of target nucleon spin or helicity of incident lepton beam
', ','||
', '
1 e h e he h
B Te h e h
N L N LA
P P N L N L
', ',', 1 / 2 3 / 2
|| ', ',1 / 2 3 / 2
~e h e h
e he h e hA
Asymmetry definition:qq q
inclusive DIS: only e’ info used semi-inclusive DIS: e’+h info used
Stony Brook Student Seminar, February 2010 23
How to measure DS and DG
E.C. Aschenauer
DG: Indirect from scaling violation
g1@eRHIC
Integrated Lumi: 5fb-1
Stony Brook Student Seminar, February 2010
Polarised quark distributions
E.C. Aschenaue
r
29
Correlation between detected hadron and struck qf
“Flavor – Separation”Inclusive DIS:
Semi-inclusive DIS:
( )u d u d s s
( , , , , , )u d u d s s
A PQdddddddddddddddddddddddddddd
Extract Dq by solving:
1, 1, 1, 1, 1,( ( ), ( ), ( ), ( ), ( ))Kp p d d dA A x A x A x A x A xπ π
dddddddddddddd( , , , , , 0)
u d u d s sQ
u d su d s
dddddddddddddd
In LO-QCD:
A1h(x,Q2 , z) =
1/2h −3/2
h
1/2h + 3/2
h~C
eq2q(x,Q2 )Dq
h(z,Q2 )
eq'2 q'(x,Q2 )Dq'
h (z,Q2 )q'∑q∑ q(x,Q2 )
q(x,Q2 )2( , , )h
qP x Q z MC
DF
FF
( ) )( () ~h
f
f
f
h
fq xd z d D z
Stony Brook Student Seminar, February 2010
Measured Asymmetries
E.C. Aschenaue
r
30
Proton
Deuterium
1 ~ 0 ( )KA K us
statistics sufficient for 5 parameter fit
( , , , , , 0)u d u d s s
Qu d su d s
dddddddddddddd
Stony Brook Student Seminar, February 2010
Polarized Quark Densities
E.C. Aschenaue
r
31
good agreement with NLO-QCDPolarised opposite to proton spin
Du(x) > 0
First complete separation of pol. PDFs without assumption on sea polarization
Polarised parallel to proton spin
Dd(x) < 0
Du(x), Dd(x) ~ 0
No indication for Ds(x) < 0
In measured range (0.023 – 0.6)
0.002 0.043u 0.054 0.035d 0.028 0.034s
Stony Brook Student Seminar, February 2010 27
Polarized Quark Distributions
E.C. Aschenauer
DSSV: arXiv:0904.3821
eRHIC: 10GeV@250GeV at 9 fb-1
X
0.2
-0.8
0.
0.8DSSV: Global Analysis of world data
Stony Brook Student Seminar, February 2010
The Gluon Polarization
E.C. Aschenauer
28
x
RHICrange
0.05· x · 0.2can be extended a bit by changing
√s to 64GeV & 500GeV
small-x0.001< x < 0.05
large-xx > 0.2
Dg(x) very small at medium x best fit has a node at x ~ 0.1 huge uncertainties at small x
Dg(x) small !?g(x) dx
0.
1
∫ = −0.084@10GeV 2Need to enlarge x-range
*g p D0 + X
Stony Brook Student Seminar, February 2010 29
Beyond form factors and quark distributions
E.C. Aschenauer
Generalized Parton Distributions
Proton form factors, transverse charge & current densities
Structure functions,quark longitudinalmomentum & helicity distributions
X. Ji, D. Mueller, A. Radyushkin (1994-1997)
Correlated quark momentum and helicity distributions in transverse space - GPDs
Stony Brook Student Seminar, February 2010
How to access GPDs?
E.C. Aschenauer
30
quantum number of final state selects different GPDs: theoretically very clean DVCS (g): H, E, H, E VM ( , , ): r w f H E info on quark flavors PS mesons ( , )p h : H E ~
~ ~
~
ρ0 2u+d, 9g/4
ω 2u-d, 3g/4f s, g
ρ+ u-d
J/ψ g
p0 2Du+Ddh 2Du-Dd
J
qz =
12
xdx H q + E q( )−1
1
⎛⎝
⎞⎠t→ 0
J q
z =12
qq∑ + Lq
z
q∑
12=J q
z + J gz =
12
qq∑ + Lq
z
q∑ + J g
z
Stony Brook Student Seminar, February 2010 31
Proton Tomography
[M. Burkardt, M. Diehl 2002]
FT (GPD) : momentum space impact parameter space:
probing partons with specified long. momentum @transverse position b
T
polarized nucleon:
[x=0]
from lattice
d-quarku-quark
E.C. Aschenauer
Stony Brook Student Seminar, February 2010 32
DVCS @ eRHIC
E.C. Aschenauer
dominated by gluon contributions Need wide x and Q2 range to extract GPDs Need sufficient luminosity to bin in multi-dimensions
Stony Brook Student Seminar, February 2010 33
EIC Scope - QCD Factory
e-
e+
p
Unpolarized andpolarized leptons4-20 (30) GeV
Polarized light ions (He3) 215 GeV/u
Light ions (d,Si,Cu)Heavy ions (Au,U)50-100 (130) GeV/u
Polarized protons50-250 (325) GeV
Electron accelerator RHIC
70% e- beam polarization goalpolarized positrons?
Center mass energy range: √s=28-200 GeV; L~100xHera
e-
Mission: Studying the Physics of Strong Color Fields
E.C. Aschenauer
Stony Brook Student Seminar, February 2010 34
The Relativistic Heavy-Ion Collider @ BNL
RHICBRAHMSPHOBOS
PHENIXSTAR
AGS
TANDEMS
v = 0.99995⋅c = 186,000 miles/sec
E.C. Aschenauer
Stony Brook Student Seminar, February 2010 35
EIC: one solution eRHIC @ BNL
E.C. Aschenauer
STAR
PH
EN
IX
2 x 200 m SRF linac4 (5) GeV per pass5 (4) passes
Polarized e-gun
Beamdump
4 to 5 vertically separatedrecirculating passes
Cohere
nt
e-c
oole
r
5 mm
5 mm
5 mm
5 mm
20 GeV e-beam
16 GeV e-beam
12 GeV e-beam
8 GeV e-beam
Com
mon
vacu
um
ch
am
ber
Gap 5 mm total0.3 T for 30 GeV
eRHICdetector
MeRH
IC
dete
ctor
10-20 GeV e x 325 GeV p 130 GeV/u Au
possibility of 30 GeV @low current operation
MeRHICeRHIC with
CeC
p (A) e p (A) e
Energy, GeV250 (100
)4
325 (125)
20 <30
>
Number of bunches
111 166
Bunch intensity (u) , 1011 2.0
0.31
2.0 (3)
0.24
Bunch charge, nC
32 5 32 4
Beam current, mA
320 50 42050 <10
>
Normalized emittance, 1e-6 m, 95% for p / rms for e
15 73 1.2 25
Polarization, % 70 80 70 80
rms bunch length, cm
20 0.2 4.9 0.2
β*, cm 50 50 25 25
Luminosity, x 1033, cm-2s-1
0.1 -> 1 with CeC
2.8
Stony Brook Student Seminar, February 2010 36
A typical High Energy Detector
E.C. Aschenauer
Particle types: neutrinos (missing energy) muons m hadrons , , p K p
quarks, gluons jets electrons, photons, p0
charged particles
beam pipeRough Classification track detectors for charged particles
“massless” detectors gas detectors solid state detectors
magnet coil(solenoid, field || beam axis)
Calorimeter for energy measurement electromagnetic
high Z material (Pb-glas) absorber (mostly Fe)
flux return yoke + active material
hadronic heavy medium (Fe, Cu, U)
+ active material
Stony Brook Student Seminar, February 2010 37
Detector Requirements from Physics
E.C. Aschenauer
ep-physics the same detector needs to cover inclusive (ep -> e’X), semi-
inclusive (ep -> e’hadron(s)X) and exclusive (ep -> e’p ) preactions
large acceptance absolutely crucial (both mid and forward-rapidity) particle identification is crucial
e, p, K, p, n over wide momentum range and scattering angleexcellent secondary vertex resolution (charm)
particle detection to very low scattering angle around 1o in e and p/A direction
in contradiction to strong focusing quads close to IP small systematic uncertainty (~1%/~3%) for e/p polarization
measurements very small systematic uncertainty (~1%) for luminosity
measurement eA-physics
requirements very similar to epchallenge to tag the struck nucleus in exclusive and diffractive reactions.difference in occupancy must be taken into account
Stony Brook Student Seminar, February 2010 38
First ideas for a detector concept
E.C. Aschenauer
Dipol3Tm
Dipol3Tm
Solenoid (4T)
ZDC
FPD
FED// //
Dipoles needed to have good forward momentum resolution Solenoid no magnetic field @ r ~ 0
DIRC, RICH hadron identification p, K, p high-threshold Cerenkov fast trigger for scattered lepton radiation length very critical low lepton energies
Stony Brook Student Seminar, February 2010 39
MeRHIC Detector in Geant-3
E.C. Aschenauer
Stony Brook Student Seminar, February 2010 40E.C. Aschenauer
and Summary
EIC many avenues for
further important
measurements and
theoretical developments
we have just explored the tip of the iceberg
you are here
Lq,g
DsDg
Dutot , Dd
tot
spin sum rule
Knowledge about
Gluons in p /A
Come and join us
Stony Brook Student Seminar, February 2010 41E.C. Aschenauer
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