Upload
others
View
5
Download
0
Embed Size (px)
Citation preview
Parity-‐Viola+ng Electron Sca3ering at JLab
David S. Armstrong College of William & Mary
MENU 2013 Rome, Italy Oct 2 2013
(replacing JulieBe Mammei, who kindly provided most of the slides)
Parity-‐Viola+ng Electron Sca3ering at JLab
Electroweak interac+on (neutral current)
-‐ not one of the canonical probes for Hadron Physics (i.e. strong or electromagne+c)
-‐ nevertheless, much of the program should be of interest to this community
-‐ Focus today: -‐ new results since MENU2010 -‐ plans for 12 GeV era at Jefferson Lab
Outline
1) Intro to Parity-‐Viola+ng Electron Sca3ering (PVES)
2) The Vector Strange Form Factors a ≈ completed program
3) Qweak: first results on the proton’s weak charge
4) Neutron radius in Heavy Nuclei
5) Standard Model Tests with PVES: plans at JLab-‐12 GeV
1) Search for physics Beyond the Standard Model – Low energy (Q2 <<M2) precision tests complementary
to high energy measurements
10/2/2013 MENU 2013 4
• Neutrino mass and their role in the early universe 0νββ decay, θ13 , β decay,… • Ma3er-‐an5ma3er asymmetry in the present universe EDM, DM, LFV, 0νββ, θ13 • Unseen Forces of the Early Universe Weak decays, PVES, gμ-‐2,…
LHC new physics signals likely will need addi5onal indirect evidence to pin down its nature
• Neutrons: LifeSme, P-‐ & T-‐ViolaSng Asymmetries (LANSCE, NIST, SNS...) • Muons: LifeSme, Michel parameters, g-‐2, Mu2e (PSI, TRIUMF, FNAL, J-‐PARC...) • PVES: Low-‐energy weak neutral current couplings, precision weak mixing angle
(SLAC, Jefferson Lab, Mainz) new since MENU2010: first result from QWeak
2) Study nucleon and nuclear proper+es – Strange quark content of nucleon new since MENU2010: HAPPEX-‐III – Neutron radius of heavy nuclei new since MENU2010 – first PREx results
A brief history of parity viola+on
R
R
L
L
1930s – weak interac+on needed to explain nuclear β decay
1950s – parity viola+on in weak interac+on; V-‐A theory to describe 60Co decay
1970s – neutral weak current events at Gargamelle
late 1970s – parity viola+on observed in electron sca3ering -‐ SLAC E122
10/2/2013 MENU 2013 5
Parity-‐viola+ng electron sca3ering
10/2/2013 MENU 2013 6
APV ∝
G FQ2
4πα 2gA
e gVT + βgV
e gAT( ) ~ 10−4Q2 GeV2⎡⎣ ⎤⎦
2
Electroweak interference
SLAC Experiments
10/2/2013 MENU 2013 7
SLAC E122 – crucial confirma+on of WSG electroweak model
• Electron-‐deuteron deep inelas+c sca3ering • High luminosity: photoemission from NEA GaAs cathode • Rapid helicity-‐flip (sign of e-‐ polariza+on) • Polarimetry to determine beam polariza+on • Magne+c spectrometer: backgrounds and kinema+c
separa+on
APV ~ 100 ± 10 ppm
SLAC E158 – 1999 • electron-‐electron sca3ering -‐ purely leptonic interac+on • electron-‐electron weak a3rac+ve force had never been
measured! APV ~ -‐131 ± 14 ± 10 ppb
sin2θW=0.20±0.03
sin2θW=0.2403±0.0013
Genesis of a Strange Idea
Puzzle: Ini+al DIS measurements of spin-‐structure of nucleon (EMC): valence quarks contribute unexpectedly low frac+on to total spin -‐ “Spin Crisis”
Possible reconcilia+on: large frac+on of spin from ? eg. D. B. Kaplan and A. Manohar, Nucl. Phys. B310, 527 (1988).
Theore+cal realiza+on: not only did many available nucleon model calcula+ons allow this, but they also allowed (and in some cases favored) large strange quark contribu+ons to other proper+es of nucleon.
Consterna+on and excitement: at the +me, data gave no constraint on strange contribu+ons to charge distribu+on and magne+c moment!
Challenge: how to isolate strange vector form factors?
Answer: exploit the weak neutral current as a probe
ss
strange quark contribu+on
Define the nucleon form factors associated with a given quark currentq as:
N | qγ µq | N = ψN F1qγ µ + F2
qiσµνq
ν
2MN
⎛
⎝⎜
⎞
⎠⎟ ψN
Assume isospin symmetry, and we have
this
and this
are well known
what about this?
(Assume neutral weak charges are known)
MENU 2013 10/2/2013
Nucleon Form Factors
NC and EM probe same hadronic flavor structure, with different couplings:
Assume Charge Symmetry:
Qweak: Proton’s weak charge
12
EM Charge Weak Charge
u 2/3
d -‐1/3
P (uud) +1
N (udd) 0 -‐1
“Accidental suppression”
sensi+vity to new physics
-‐ Neutral current analog of electric charge:
10/1/2013 MENU 2013
Qweak: Proton’s weak charge
Use four-‐fermion contact interac+on to parameterize the effec+ve PV electron-‐quark couplings (mass scale and coupling)
Large θ Small θ
Erler, Kurylov, and Ramsey-‐Musolf, PRD 68, 016006 2003
Planned 4% measurement of proton’s weak charge -‐ probes TeV scale new physics
New physics:
new Z', leptoquarks, SUSY ...
For electron-‐quark sca3ering:
MENU 2013 10/1/2013
SM
Qweak kinema+cs Hadronic term extracted from fit
Extrac+ng the weak charge
14
Hadron structure enters here: electromagne+c and electroweak form factors…
The previous strange form factor program (experiments at MIT/Bates, JLab and MAMI) allow us to subtract our hadronic contribu+on
Reduced asymmetry more convenient:
10/1/2013 MENU 2013
The Qweak Apparatus
15
Main detectors 8-‐fold symmetry
VerJcal driK chambers (VDC)
(rotate)
Trigger scinJllator
Shield wall
Lintels
QTOR
Lead collar
Cleanup collimators
Target
Tungsten plug Horizontal driK
chambers (HDC) (rotate)
e- beam e- beam
Acceptance defining collimator
QTOR
MENU 2013 10/2/2013
Reduced Asymmetry
Hadronic part extracted through global fit of PVES data.
16
in the forward-‐angle limit (θ=0)
MENU 2013 10/1/2013
V
V
V
0.0 0.1 0.2 0.3 0.4 0.5 0.60.0
0.1
0.2
0.3
0.4
Q
Q
Q
�
Q
X�����This ExperimentQ����HAPPEX����SAMPLEV���PVA4O���G0�������SM (prediction)
X
Data Rotated to the Forward-Angle Limit
[GeV/c]Q 22
ç
ç
W�A
/A��α�Q���Ϊ������B
(�����,e
= 0
)p
0Q
2Q
2
ep
MENU 2013
The C1q & the neutron’s weak charge
17 MENU 2013 10/1/2013
0.22
0.24
-0.70 -0.65 -0.60 -0.55 -0.50 -0.45 -0.40
0.18 0.17
0.16
0.15
0.14 0.13
0.12
C1u < C1d
C1u
+ C
1d
sin e |2
-
-
-
-
-
-
-
-
-
0.20
0.26
133Cs APV PVES
Inner Ellipses - 68% CLOuter Ellipses - 95% CL
ZW
The C1q & the neutron’s weak charge
18 MENU 2013 10/1/2013
0.22
0.24
-0.70 -0.65 -0.60 -0.55 -0.50 -0.45 -0.40
0.18 0.17
0.16
0.15
0.14 0.13
0.12
C1u < C1d
C1u
+ C
1d
sin e |2
-
-
-
-
-
-
-
-
-
0.20
0.26
133Cs APV PVES
Inner Ellipses - 68% CLOuter Ellipses - 95% CL
ZW
Combining this result with the most precise atomic parity viola+on experiment we can also extract, for the first +me, the neutron’s weak charge:
Qweak – first result
19 10/1/2013 MENU 2013
First result (4% of data set):
Lots of work to push down systema+c errors, but no show-‐stoppers found….
Expect final result in 12-‐18 months +me.
More details: Fundamental Symmetries 3, this aaernoon
A of Auxiliary Measurements
20 10/1/2013 MENU 2013
Qweak has data (under analysis) on a variety of observables of poten+al interest for Hadron physics:
• Beam normal single-‐spin asymmetry* for elas+c sca3ering on proton • Beam normal single-‐spin asymmetry for elas+c sca3ering on 27Al • PV asymmetry in the region. • Beam normal single-‐spin asymmetry in the region. • Beam normal single-‐spin asymmetry near W= 2.5 GeV • Beam normal single-‐spin asymmetry in pion photoproduc+on • PV asymmetry in inelas+c region near W=2.5 GeV (related to box diagrams) • PV asymmetry for elas+c/quasielas+c from 27Al • PV asymmetry in pion photoproduc+on
*: aka vector analyzing power aka transverse asymmetry; generated by imaginary part of two-‐photon exchange amplitude (pace Wim van Oers)
N → Δ
N → Δ
γZ
208Pb 208Pb
208Pb
2
Neutron skin – PREx
MENU 2013 10/2/2013
Exploit the large weak charge of neutron to extract radius of neutron distribu+on in heavy nucleus; theore+cally clean probe.
First result of PREx experiment at JLab: PRL 108(2012)112502
APV = 0.656 ± 0.060 (stat) ± 0.014 (syst) ppm
PREX, PREX II and CREX
Theory from P. Ring et al. Nucl. Phys. A 624 (1997) 349
208Pb more closely approximates infinite nuclear ma3er The 48Ca nucleus is smaller, so APV can be measured at a Q2 where the figure of merit is higher
and are expected to be correlated, but the correla+on depends on the correctness of the models
The structure of 48Ca can be addressed in detailed microscopic models
Measure both and -‐ test nuclear structure models over a large range of A
MENU 2013
More info: yesterday’s talks by G. Urcioli and M. Thiel in Fundamental Symmetries 2
Future: PVES at JLab in 12 GeV era
10/2/2013 MENU 2013 23
MOLLER -‐ precision Standard Model test by measuring weak charge of electron in PV electron-‐electron sca3ering
(revisit SLAC E158)
SOLID -‐ precision Standard Model test by measuring PV DIS on deuteron: access the quark weak axial couplings C2q
(also – a similar measurement was made at 6 GeV in Hall A at Jefferson Lab, X. Zheng et al., being readied for publica+on, but I’m not authorized to show the results; stay tuned…)
Large kinema+c coverage: disentangle CSV and higher-‐twist effects
2.3% MOLLER uncertainty
Coupling constants
Mass scale
Lepton compositeness – strong coupling – 47 TeV
LEP2 (gLR and sum) mass scale sensi+vity ~5.2 and 4.4 TeV
e- e-
e- e-
MOLLER at 12 GeV
MENU 2013 10/2/2013
Doubly-‐charged scalar, heavy Z’, SUSY, dark Z…
MOLLER and weak mixing angle
10/2/2013 MENU 2013 25
MS)Z
(MWθ2sin
MS)Z
(MWθ2sin
0.23 0.231 0.232
Moller ± 0.00029
0,lfbA 0.23071 ± 0.00053
)τ
(PlA 0.23131 ± 0.00041
(SLD)lA 0.23070 ± 0.00026
0,bfbA 0.23193 ± 0.00029
1 10 100 1000MH [GeV]
0.227 0.227
0.228 0.228
0.229 0.229
0.23 0.23
0.231 0.231
0.232 0.232
0.233 0.233
0.234 0.234
0.235 0.235
sin2 θ
eff(e
)
ALR(had)
AFB(b)
MOLLER
E158
LHC events
APV
Reminder: at tree-‐level QW
e = (1 – 4 sin2θW)
Higgs discovery at LHC allows firm predic+on of MOLLER asymmetry in Standard Model
SOLID – accessing the C2q’s
10/2/2013 MENU 2013 28
X N
e-‐
Z* γ*
e-‐
Cahn and Gilman, PRD 17 1313 (1978) polarized electrons on deuterium
10/2/2013 MENU 2013 29
Large θ Small θ
Red ellipses are PDG fits
Blue bands represent expected data: Qweak (leK) and PVDIS-‐-‐6GeV (right)
Green bands are proposed SOLID PVDIS
SOLID – Parity-‐Conserving Physics
10/2/2013 MENU 2013 31
-‐ SIDIS with Transversely Polarized 3He approved 90 days
-‐ SIDIS with Longitudinally Polarized 3He approved 35 days
-‐ SIDIS with Transversely Polarized Proton approved 120 days
-‐ Near Threshold Electroproduc+on of J/Psi approved 60 days
PVDIS approved for 169 days (half of full request)
Summary
10/2/2013 MENU 2013 33
Grazie to the MENU 2013 organizers for invi+ng Julie3e Mammei to give this talk, and for accep+ng me as a poor subs+tute….
• Strange vector form factors of proton – small, consistent with zero.
• Qweak: First measurement of proton’s weak charge, consistent with Standard Model, 25x more data on tape
• PREx: two-‐sigma evidence for neutron “skin” of 208Pb; will improve a�er JLab comes online a�er 12 GeV upgrade, and will extend to 48Ca
• MOLLER and SOLID: major programs a�er JLab upgrade two complementary Standard Model tests.