Electroweak Physics at Low Energy

August 18, 2006 Electroweak Physics at Low Energy

Electroweak Physics atLow EnergyKrishna S. Kumar

University of Massachusetts, Amherst


Outline

• Scope of the topic• Overview of experimental efforts

• Parity-violating electron scattering

• E158 at SLAC• Possible future measurements


What is Electroweak Physics?• SU(3)c X SU(2)L X U(1)Y structure of Standard

Model• Probe most general, unified structure of

electromagnetic and weak interaction currents• Centered around W and Z boson interactions

• Central parameters: αQED, MZ, GF• Derived parameters: MW and sin2θW• QCD and hadron/nuclear structure generally

viewed as an “annoying” background; must be studied to make progress sometimes

Low EnergyEnergy scale of the interactions studied is much smaller than the scale of electroweak symmetry breaking and the gauge boson masses


Context

• The electroweak theory has been tested to extraordinary precision

• Typical scale is 0.1%• No significant deviations, but a couple of possible anomalies

• The Higgs boson is “expected” to be light

• We have a self-consistent effective quantum field theory of electroweak interactions up to a scale of 100GeV

• There has been much progress, but many questions remain


Some Questions in EW Physics

• Why 3 generations of particles?• Why is the weak boson mass ~ 100 GeV?• What is the origin of mass?• How did matter dominate over anti-matter?

• Is there a single unifying force?• Why are neutrinos so light?• Are neutrinos Majorana particles?• Why is the Top so heavy?• …….


The High Energy Frontier

• Directly access the TeV scale• Explore the origin of SU(2) X U(1) symmetry breaking

• Measure the arbitrary parameters as precisely as possible

• Confirm or rule out Supersymmetry


Low Energy EW Physics

• Symmetries & Conservation Laws– Atomic and Nuclear systems offer rare, unique access

to EW interactions– Search for violations of “unwanted” symmetries– Indirect access to very high energy scales– Clues to Grand Unified Theories– High statistics → Rare processes

• Evolution of Electroweak parameters– Test theory at quantum loop level– Indirect access to high energy particles


Broad Thrusts• Neutrino Physics

– Oscillations and the MSN matrix– Tritium Beta Decay and Double Beta Decay

• Muon Physics– g-2 anomaly– Precision muon decay parameters– Charged lepton number violation

• Semi-leptonic Weak Decays– Standard Model CP Violation– Tests of CKM unitarity– Anomalous charged current interactions– Proton decay

• Electric Dipole Moment Searches– “Table Top” Searches for T Violation: SUSY nemesis!

• Neutral Weak Interaction Studies


Weak Neutral Current Interactions(WNC)

electrons, quarks, nucleons, nuclei

electrons, neutrinos

• Many contrasts to W interactions:– Flavor conserving– Mixing with electromagnetism: central to unification– Better control in the laboratory

• Most new theories have new neutral bosons

• Precision knowledge of Z couplings to all fermions to interpret collider data

• unique electroweak quantum loop effects


Model Independent AnalysisNeutral Current Interactions are Flavor Diagonal

“GIM Suppressed” Physics is usually orthogonal to weak decays

e.g. Z’ limits are typically 1-2 orders of magnitude weaker than WR limits

Relying on specific models to sell a measurement is not good strategy:

Models will go in and out of fashion over a 5-10 year span

or f1 f2 → f1 f2f1 f 1 → f2 f 2Consider

Lf1 f2=

g2

Λ ij2 ηij

i, j= L,R∑ f 1iγµ f1i f 2 jγ

µ f2 j

Any new physics model can be characterized in this way

One goal of EW measurements at all energy scales is to measure Λ’s


WNC Interactions at Low Q2

Q2 << scale of EW symmetry breaking

LEPII, Tevatron, LHC access scales greater than Λ ~ 10 TeV

Logical to push to higher energies, away from the Z resonance

Parity-violatingHigh energy: LL+RR=VV Low energy: LL-RR


Atomic Parity Violation


Neutrino DIS


A Classic Paper


Parity Violation in Electron Scattering?

Neutron β Decay Electron-protonWeak Scattering

Parity-violating

θE

E’

Q2 = 4E ′ E sin2 θ2

4-momentum transfer


Accessing Parity Violation

•One of the incident beams longitudinally polarized•Change sign of longitudinal polarization•Measure fractional rate difference


Weak Electromagnetic Interference

10−4 ⋅ Q2APV ~ (GeV2)

Neutral weak force first measured in the early ‘70s: sin2θW~1/4

Do the weak and electromagnetic amplitudes interfere?A critical test for the correct gauge structure of the t

Many new theories (other than SU(2) X U(1)) predicted weak neutral currents, but many cases had no interference predicted.

h


Observation of Weak-Electromagnetic Interference

It was realized independently in the mid 70s at SLAC:

APV in Deep Inelastic Scattering off liquid Deuterium: Q2 ~ 1 (GeV)2

•Established the basic experimental technique•Cleanly observed weak-electromagnetic interference•Parity Violation in Weak Neutral Current Interactions•sin2θW = 0.224 ± 0.020: same as in neutrino scattering


An Improved SLAC E122? PV Deep Inelastic Scattering?

Kinematic coverage could not remove QCD uncertainty at 1% level

A New Idea

δ(sin2 ϑW )sin2 ϑW

≅ 0.05δ(APV )

APV

e- e-e

e

e

ee

-

-

-

-- e-

γZγZ 00

If Parity Violation in atoms is sensitive to New Physics,

Parity Violating Electron Scattering can do better

Purely leptonic reaction

Need to measure APV that is proportional to 1-4sin2θW:Elastic electron-electron or electron-proton scattering

APV ∝ me Elab (1− 4sin2 ϑW ) Tiny!

electron-electron(Møller) scattering

σ ∝1

Elab

Figure of Merit rises linearly with Elab


E158 Collaboration & Chronologyty-Violating Left-Right Asymmetry In Fixed Target Møller Scat

At the Stanford Linear Accelerator Center

Goal: error small enough to probe TeV scale physics

E158 Collaboration E158 ChronologyFeb 96: Workshop at PrincetonSep 97: SLAC EPAC approvalMar 98: First Laboratory Review1999: Design and Beam tests2000: Funding and construction2001: Engineering run2002-2003: Physics2004: First PRL2005-2006: Final publications

•Berkeley•Caltech•Jefferson Lab•Princeton•Saclay

•SLAC•Smith •Syracuse•UMass•Virginia

8 Ph.D. Students60 physicists


E158 New Physics Reach


Tiny Asymmetry

Highest electron beam energy with longitudinal beam polarization:50 GeV at the Stanford Linear Accelerator Center

Imagine measuring the length of Central Park in NYC with 2 different meter scales: answer is expected to differ by 1 mm!

Raw asymmetry ~ 1 x10-7 (100 ppb) δ(APV) ~ 10-8 (10 ppb)

Need 1016 events Count at ~ 1 GHz

Tiny signal (weak force) buried in known background (QED)

apparatusmodulator lockin input

Lockin Amplifier output

injector accelerator target spectrometer detector


Optical PumpingModulate longitudinal polarization of the electron beam

Beam helicity is chosen pseudo-randomly at 120 Hz• sequence of pulse quadruplets•Data analyzed as “pulse-pairs”


Experimental Technique

E158

(X,Y,ϑ X ,ϑ Y , E)

12 • Nwindow


Systematic Control


Systematic Control

CID Gun

CID Gun

VaultVault

Source Laser

Source Laser

RoomRoom

IA Feedback LoopIA Feedback LoopIA cell applies a helicity-correlated phase shift to the beam.

The cleanup polarizer transforms this into intensity asymmetry.

POS Feedback LoopPOS Feedback LoopPiezomirror can deflect laser beam on a pulse-to-pulse basis.Can induce helicity-correlated position differences.

““Double” Feedback LoopDouble” Feedback LoopAdjusts ∆CP, ∆PS to keep IA & Piezo corrections small (~ ppm & ~100 nm).

Very slow feedback (n = 24k pairs).


Stanford Linear Accelerator Center


Experimental Apparatus

•Polarized Beam•Precision Beam Monitoring•Liquid Hydrogen Target•Spectrometer•Detectors


σtoroid ≤ 30 ppmσBPM ≤ 2 micronsσenergy ≤ 1 MeV

Beam Monitoring

Agreement (MeV)

Resolution 1.05 MeV

Pulse by pulse monitoring at 1 GeV and 45 GeV


End Station A


Liquid Hydrogen Target

Simplest source of target electrons is liquid hydrogen

Z/A = 1No nuclei or neutrons

Refrigeration Capacity 1 kOperating Temperature 20Length 1.5Flow Rate 5 mVertical Motion 6 inche


Kinematics


E158 Plan View in ESA

Spectrometer magnets

Concrete shieldingtarget Detector

cart


E158 Spectrometer


Downstream Configuration


Detector Concept


ProfileDetectorwheel

LuminosityMonitorregion

PMT LeadHolder/shield

Detector Cart•Cu-fused silica sandwich•20 million 17 GeV electrons at 120 Hz•100 MRad radiation dose



Run 1: Spring 2002Run 2: Fall 2002Run 3: Summer 2003

1020 Electrons on Target

Run 1: Apr 23 12:00 – May 28 00:00, 2002Run 2: Oct 10 08:00 – Nov 13 16:00, 2002Run 3: July 10 08:00 - Sep 10 08:00, 2003

48 GeV: 14.5 revs

APV Sign Flips

Physics Runs

g-2 spin precession45 GeV: 14.0 revs

Data divided into 75 “slugs”: - Wave plate flipped ~ few hours - Beam energy changed ~ few days


Analysis of Calorimeter Response

electron flux

Basic Idea:

: quartz: copper

light guide

PMTshielding

air

Radial and azimuthal segmentation

•Corrections for beam fluctuations•Average over runs•Statistical tests•Beam polarization and other normalization


Beam Asymmetries

Charge asymmetry at 1 GeV

Charge asymmetry agreement at 45 GeV

Energy difference in A line

Energy difference agreement in A line

Position agreement ~ 1 nmPosition differences < 20 nm


Raw Asymmetry Statistics

Ai − A σ i

Ai − A σ i

σi ≈ 200 ppm σi ≈ 600 ppbN = 818N = 85 Million


Final Analysis of All 3 Runs

45 GeV: 14.0 revsg-2 spin precession

48 GeV: 14.5 revs

APV = (-131 ± 14 ± 10) x 10-9

APV ≈ −1×10−7 × Ebeam × Pbeam × (1− 4sin2 ϑ W )≈ 250ppbPhys. Rev. Lett. 95 081601 (2005)


Electroweak Physics

sin2θW = e2/g2

→ test gauge structure of SU(2)×U(1)

3%

•Czarnecki and Marciano•Erler and Ramsey-Musolf•Sirlin et. al.•Zykonov


Status in 1997

sin2θw

Q (GeV)


NatureVol 435

26 May 2005

NEWS AND VIEWS

Status since 2005

* Limit on ΛLL ~ 7 or 16 TeV

* Limit on SO(10) Z’ ~ 1.0 TeV

* Limit on lepton flavor violating coupling ~ 0.01GF

(95% confidence level)

sin2θeff = 0.2397 ± 0.0010 ± 0.0008

6σ

PRL 95 081601 (2005)


Contact Interaction LimitsLEP and LEPII: 18 TeV for ΛVV

8 TeV for ΛLL

leptonic

E158: 12 TeV for ΛLL

A 500 GeV ILC will reach 40 TeV for ΛVV

20 TeV for ΛLL

Tevatron:Semi-leptonic

~ 15 TeV for ΛVV

LHC: ~ 50 TeV for ΛVVMuch weaker limits on ΛLL: this is the window for WNC at low energy

δ(sin2 ϑW ) ≤ 0.0003520 TeV for ΛLLBenchmark:

In PV Møller Scattering


Weak Mixing Angle at High EnergyThe Average: sin2θw = 0.23122(17)

⇒ mH = 89 +38-28 GeV⇒ S = -0.13 ±0.10

ALR

sin2θw = 0.2310(3)↓

mH = 35 +26-17 GeVS= -0.11 ± 17

Rules out the SM!

AFB (Z→ bb)sin2θw = 0.2322(3)

↓mH = 480 +350-230

GeVS= +0.55 ± 17 Rules out SUSY!

Favors Technicolor!

Rules out Technicolor!Favors SUSY!

3σ apart

Average of “ruling out SM” and “discovering Technicolor” = Consistency with SUSY!

W. Marciano, CIPANP06


Weak Mixing Angle: FutureWeak mixing angle and W Mass measurements play a special role:Most precisely measured “derived” EW parameters

• The two best measurements have “issues”– B forward-backward asymmetry– SLD left-right asymmetry

• MW measurements getting close, but cannot completely replace the weak mixing angle

• Improving both measurements at LHC will be very challenging– Mw limited by energy scale issues– Weak mixing angle limited by QCD systematics

• Leptonic Sector– Giga-Z option of ILC– Neutrino Factory


Møller Scattering with JLab Upgrade

Z pole asymmetries

•Comparable to single Z pole measurement: shed light on disagreement•Best low energy measurement until ILC or ν-Factory•Could be done ~ 2012-13

Jefferson Lab

Address longstanding discrepancy between hadronic and leptonic Z asymmetries


Far Future: Fixed Target at ILC

(world average ~0.00016)

Measure contribution from scalars to oblique correctionst

Z

H

b

new

physicsδmH

mH

≈10% for δ sin2 θW ≈ 0.00004Critical crosscheck

ALR and MW at future colliders:

0.000080.001δsin2(θW)

0.0080.015δALR (ppm)

1-20.15ALR (ppm)

2 × 1074 × 106Time (s)

90%85%Pe

120120Pulse Rate (Hz)

14 × 10114.5 × 1011Intensity/pulse

250-50048Energy (GeV)

LCE158K.K, Snowmass 96Systematics extremely challenging!Energy scale to 10-4, polarimetry to 0.15%

Møller scattering at the ILC

σ ∝1

Elab

Figure of Merit riseslinearly with Elab

• Fixed target has advantages for systematics• Could work with ILC “exhaust”beam


Summary• Progress beyond our current understanding of electroweak theory requires:– Exploring the 1 TeV frontier– Exploring rare processes at lower energy– Precision EW measurements over a range of energy scales

• WNC experiments play a central role in testing the electroweak theory

• SLAC E158 has produced the most precise low energy weak mixing angle measurement

• Parity-violating Moller scattering will likely play an important role in bringing EW measurements to closure

Documents

Electroweak Physics at Low Energy