Nuclear Physics at Jefferson LabPart II
R. D. McKeownJefferson LabCollege of William and Mary
Taiwan Summer SchoolJune 29, 2011
3
Strange Quarks in the Nucleon
3
• Strange quarks-antiquarks virtual pairs produced by gluons
• Contribution to proton’s magnetism - (Stern’s discovery)?- QCD analog of Lamb shift in atoms
• Study using small (few parts per million) left-right difference in electron-proton force
challenging experiments!
4
Strange Quarks in the Nucleon
Mass:
u u
dp
uud
u
su
s
valence
sea
proton
4.0)(ˆ
2
NdduumN
NssmN sp-N scattering
Spin:
1.01.0
10.020.0
s
sdu
gq JLsdu 2
1
2
1Polarized deep-inelastic scattering
HERMES semi-inclusive
n-p elastic scattering
09.015.0 s
7
Neutral weak form factors
•Electromagnetic interaction
•Neutral weak interaction
g
p
Z0
p
GEg,p, GM
g,p GEZ,p, GM
Z,p
GAZ,p
8
Use Isospin Symmetry
pZM
nM
pMW
sM
pZM
nM
pMW
dM
pZM
pMW
uM
GGGG
GGGG
GGG
,,,2
,,,2
,,2
)sin41(
)sin42(
)sin43(
(p n) = (u d)
For vector form factors theoretical CSB estimates indicate < 1% violations (unobservable with currently anticipated uncertainties)
(Miller PRC 57, 1492 (1998) Lewis and Mobed, PRD 59, 073002(1999)
9
Parity-violating electron scatteringPolarized electrons on unpolarized target
For a proton: (Cahn & Gilman 1978)
LR
LRA
g Z0
g 2
Forward angles Backward angles
12
SAMPLE ExperimentPolarizedInjector
WienFilter
AcceleratorE = 125 MeV600 pulses/sIpk = 3 mAIave = 44 APB = 36%
Energy
BeamCurrent
Fast phase shift(energy) feedback
K11
Beam currentfeedback
SAMPLEDetector
Lumi
Position,Angle,Charge
Halo
MollerPolarimeter
Beam positionfeedback
13
Experimental procedure
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Positive Helicity Negative Helicity 60 Hz
Random (New)Sequence
ComplementSequence
Pulse Pair
AY Y
Y YMeasured
Positive Negative
Positive Negative
• Asymmetry between pulses separated by 1/60 sremove effects due to 60 Hz
• Rapid helicity reversal reduce effects of long-term drifts
• Slow helicity reversal remove helicity-correlated electronics effects
19
• Nucleon models continue to struggle, with some indication that higher mass poles are important
• Precise lattice QCD - motivated prediction:
(Leinweber, et al., PRL 97, 022001 (2006)
• New unquenched lattice QCD result:
Doi, et al., arXiv:0903.3232
Theory Update
20
HAPPEX-III Results
A(Gs=0) = -24.158 ppm ± 0.663 ppm Gs
E + 0.52 GsM = 0.005 ± 0.010(stat) ± 0.004(syst) ± 0.008(FF)
APV = -23.742 ± 0.776 (stat) ± 0.353 (syst) ppm
preliminary
preliminary
21
Measuring the Neutron “Skin” in the Pb Nucleus
21
crust
Neutron Star Lead Nucleus
skin
10 km
10 fm
• Parity violating electron scattering• Sensitive to neutron distribution• First clean measurement• Relevant to neutron star physics• Currently running in Hall A
22Page 22
Lead (208Pb) Radius Experiment : PREXElastic Scattering Parity-Violating Asymmetry
Z0 : Clean Probe Couples Mainly to Neutrons
Applications : Nuclear Physics, Neutron Stars, Atomic Parity, Heavy Ion Collisions
• The Lead (208Pb) Radius Experiment (PREX) determines the neutron radius to be larger than the proton radius by +0.35 fm (+0.15, -0.17).
• This result represents model-independent confirmation of the existence of a neutron skin, with relevance for neutron star calculations.
• Plans for follow-up experiment to reduce uncertainties by factor of 3. This can quantitatively pin down the symmetry energy, an important contribution to the nuclear equation of state.
A neutron skin of 0.2 fm or more has implications for our understanding of neutron stars and their ultimate fate
Rel. mean field
Nonrel. skyrme
PREXPREX data
26
Qweak
Luminosity monitors
Luminosity monitors
scanner
Precise determination of the weak charge of the proton
Qw= -2(2C1u+C1d) =(1 – 4 sin2 qW)
28
Target cell
• 35 cm target cell, designed with CFD • Target tested and stable up to 160 A. Sufficient reserve cooling power to easily reach 180 A. Highest power LH2 target.
• When using 960Hz spin flip rate, the target density fluctuations (an unknown before commissioning) appear to be small compared to expected counting statistical uncertainty (per quartet) of ~220 ppm.
Qweak LH2 Cryotarget
33
New combination of: Vector quark couplings C1q Also axial quark couplings C2q
PV Deep Inelastic Scattering
iii fff
For an isoscalar target like 2H, structure functions largely cancel in the ratio at high x
b(x)
3
10(2C2u C2d )
uv dv
u d
a(x) =C1i Qi fi
+(x)i
Qi
2 fi+(x)
i
e-
N X
e-
Z* *
y 1 E / E b(x) C2i Qi fi
(x)i
Qi
2 fi(x)
i
x xBjorken
At high x, APV becomes independent of x, W, with well-defined SM prediction for Q2 and y
Sensitive to new physics at the TeV scale
a(x) 3
10(2C1u C1d ) 1
2s
u d
6.0
APV GFQ2
2a(x) Y (y) b(x)
0
1
at high x
a(x) and b(x) contain quark distribution
functions fi(x)
PVDIS: Only way to measure C2q
34
SoLID Spectrometer
Baffles
GEM’s
Gas Cerenkov ShashlykCalorimeter
ANL design
JLab/UVA prototype
Babar Solenoid
International Collaborators:China (Gem’s)Italy (Gem’s)Germany (Moller pol.)
35
Statistical Errors (%)
4 months at 11 GeV
2 months at 6.6 GeV
Error bar σA/A (%)shown at center of binsin Q2, x
Strategy: sub-1% precision over broad kinematic range for sensitive Standard Model test and detailed study of hadronic structure contributions
37
SoLID: Comprehensive PVDIS Study
• Measure AD in NARROW bins of x, Q2 with 0.5% precision• Cover broad Q2 range for x in [0.3,0.6] to constrain HT• Search for CSV with x dependence of AD at high x• Use x>0.4, high Q2, and to measure a combination of the Ciq’s
Strategy: requires precise kinematics and broad range
x y Q2
New Physics no yes no
CSV yes no no
Higher Twist yes no yes
2
23)1(
11 x
QxAA CSVHT Fit data to:
C(x)=βHT/(1-x)3
40
New JLab Experiment
Polarized Beam• Unprecedented polarized luminosity
• unprecedented beam stability
Liquid Hydrogen Target• 5 kW dissipated power (2 X Qweak)
• computational fluid dynamics
Toroidal Spectrometer• Novel 7 “hybrid coil” design
• warm magnets, aggressive cooling
Integrating Detectors• build on Qweak and PREX
• intricate support & shielding
• radiation hardness and low noise
44
Future PV Program at Jlab
PV Moller Scattering:
• Custom Toroidal Spectrometer• 5kw LH Target
SOLID (PVDIS):• High Luminosity on LD2 and LH2 • Better than 1% errors for small bins• Large Q2 coverage• x-range 0.25-0.75• W2> 4 GeV2
44INT EIC Workshop, Nov. 2010
48
On the horizon: A New Muon g-2 Experiment at Fermilab
Update: Oct 2010: Dam(Expt – Thy) = 297 ± 81 x 10-11
3.6 s
BNL E821
2010 e+e- Thy
3.6 s
x10-11
Future Goals
Goal: 0.14 ppm
Expected Improvement
D. Hertzog
48INT EIC Workshop, Nov. 2010
49
Cosmology and Dark Matter
R. D. McKeown June 15, 2010
49
• Dark sector is new physics, beyond the standard model• Many direct searches for dark matter interacting with
sensitive detectors (hints, no established signal yet…)• Controversial evidence for
excess astrophysical positrons…
→ many predictions for new physics
50
PAMELA Data on Cosmic Radiation
Va. Tech. Physics Colloquium, Dec. 3, 2010
50
Surprising rise in e+ fraction
But not p
• Could indicate low mass A’ (MA’ < 1 GeV )
• Or local astrophysical origin??
53
New Opportunity: Search for A’ at JLab
Search for new forces mediated by ~100 MeV vector boson A’
with weak coupling to electrons:
Irrespective of astrophysical anomalies: • New ~GeV–scale force carriers are important category of physics beyond the SM• Fixed-target experiments @JLab (FEL + CEBAF) have unique capability to explore this!
53Va. Tech. Physics Colloquium, Dec. 3, 2010
g – 2 preferred!
55
HPS in Hall B• Forward, compact spectrometer/vertex detector identifies heavy photon candidates with invariant mass and decay length.
• EM Calorimeter provides fast trigger and electron ID.
• Small cross sections and high backgrounds demand large luminosities. HPS survives beam backgrounds by spreading them out maximally in time, capitalizing on 100% CEBAF duty cycle and employing high rate DAQ.
• All detectors are split above and below the beam to avoid the “wall of flame” from multiple Coulomb scattered primaries, bremsstrahlung, & degraded electrons.
56
DarkLight at JLab FEL
100 MeV10 mA
Reconstruct all final stateparticles and achieve aninvariant mass resolutionof 1 MeV/c2 or betterover the range 10 to 100MeV/c2.
Toroidal magneticspectrometer with abending power of 0.05 to 0.16 T-m with a wirechamber tracker for theleptons, a radial TPC for proton detection and a scintillator for triggering.
57
Jlab Future
• Clearly we have a exciting and growing program to search for new physicsbeyond the standard model.
• But we have a substantial program ofimportant experiments exploring QCD
- confinement mechanism- nucleon tomography
• And there are prospects for a future newfacility: Electron Ion Collider (EIC)
One more lecture…
Top Related