The Q weak Experiment at JLab

Transcription

1 The Q weak Experiment at JLab A search for parity violating new physics at the TeV scale! by measurement of the Protonʼs weak charge.! Roger D. Carlini Jefferson Laboratory Scatter longitudinally polarized electrons from liquid hydrogen Flip the electron spin and see how much the scattered fraction changes The difference is proportional to the weak charge of the proton Hadronic structure effects determined from global PVES measurements. Thomas Jefferson National Accelerator Facility Page 1

2 The Qweak Collaboration A search for parity violating new physics at the TeV scale! by measurement of the Protonʼs weak charge.! (Funded by DOE, NSF, NSERC and the State of Va) 1Spokespersons 2Project Manager D. Androic, D. Armstrong, A. Asaturyan, T. Averett, R. Beminiwattha, J. Benesch, J. Birchall, P. Bosted, C. Capuano, R. D. Carlini1 (PI), G. Cates, S. Covrig, M Dalton, C. A. Davis, W. Deconinck, K. Dow, J. Dunne, D. Dutta, R. Ent, J. Erler, W. Falk, H. Fenker, J.M. Finn, T. A. Forest, W. Franklin, M. Furic, D. Gaskell, M. Gericke, J. Grames, K. Grimm, D. Higinbotham, M. Holtrop, J.R. Hoskins, K. Johnston, E. Ihloff, M. Jones, R. Jones, K. Joo, J. Kelsey, C. Keppel, M. Khol, P. King, E. Korkmaz, S. Kowalski1, J. Leacock, J.P. Leckey, J. H. Lee, L. Lee, A. Lung, D. Mack, J. Mammei, J. Martin, D. Meekins, A. Micherdzinska, A. Mkrtchyan, H. Mkrtchyan, N. Morgan, K. E. Myers, A. Narayan, Nuruzzaman, A. K. Opper, S. A. Page1, J. Pan, K. Paschke, S. Phillips, M. Pitt, B. (Matt) Poelker, Y. Prok, W. D. Ramsay, M. Ramsey-Musolf, J. Roche, N. Simicevic, G. Smith2, T. Smith, P. Souder, D. Spayde, R. Suleiman, B. Sawatzky, E. Tsentalovich, W.T.H. van Oers, B. Waidyawansa, W. Vulcan, P. Wang, S. Wells, S. A. Wood, S. Yang, R. Young, H. Zhu, C. Zorn College of William and Mary, University of Connecticut, Instituto de Fisica, Universidad Nacional Autonoma de Mexico, University of Wisconsin, Hendrex College, Louisiana Tech University, University of Manitoba, Massachusetts Institute of Technology, Thomas Jefferson National Accelerator Facility, Virginia Polytechnic Institute & State University, TRIUMF, University of New Hampshire, Yerevan Physics Institute, Mississippi State University, University of Northern British Columbia, Ohio University, Hampton University, University of Winnipeg, University of Virginia, George Washington University, Syracuse University, Idaho State University, University of Connecticut, Christopher Newport University, University of Zagreb

3 Precision Tests of the Standard Model Standard Model is known to be the effective low-energy theory of a more fundamental underlying structure. Finding new physics: Two complementary approaches: Energy Frontier (direct) : eg. Tevatron, LHC Precision Frontier (indirect) : (aka Intensity Frontier) µ(g-2), EDM, ββ decay, µ e γ, µα eα, K+ π + νν, etc. ν - oscillations Atomic Parity violation Parity-violating electron scattering Often at modest or low energy Hallmark of the Precision Frontier: Choose observables that are precisely predicted or suppressed in Standard Model. If new physics is found in direct measurements (LHC), precision measurements useful to determine e.g. couplings

4 Parity Violating Electron Scattering: Weak Neutral Current Amplitudes Interference: σ ~ M EM 2 + M NC 2 + 2Re(M EM* )M NC scatter electrons of opposite helicities from unpolarized target Interference with EM amplitude makes Neutral Current (NC) amplitude accessible Small (~10-6 ) cross section asymmetry isolates weak interaction First discussed: Ya. B Zel dovich JETP 36 (1959)

5 Weak Charges Govern strength of neutral current interaction with fermion Charge Particle u d Proton uud Neutron udd Electric +2/3-1/ Weak (vector) -2 C 1u = + 1 8/3 sin 2 θ W -2 C 1d = /3 sin 2 θ W Q p w = 1-4 sin 2 θ W 0.07 Q n w = -1 Note accidental suppression of Q w p sensitivity to new physics Q p weak is a well-defined experimental observable. Q p weak has a definite prediction in the electroweak Standard Model. Q e weak : electron s weak charge was measured in PV Møller scattering (E158).

6 Running of sin 2 θ w Plot! Qweak decreasing Qweak increasing far Solid Curve by: J. Erler, M. Ramsey-Musolf and P. Langacker close ν-dis is NuTeV after theoretical corrections by Bentz, Cloet, Londergan and Thomas arxiv: v3 [nucl-th] 23 Aug 2010

7 PVES and Hadronic Structure Effects Neutral-weak form factors Axial form factor assume charge symmetry: Proton weak charge (tree level) Strangeness (Now measured to be relatively small!) Note: Parity-violating asymmetry is sensitive to both weak charges and to hadron structure.

8 Two Decades of Strange Form Factor Searches Provides the ideal data base to extract the hadronic corrections for Qweak! time SAMPLE (MIT/Bates) Q 2 = 0.1 HAPPEx-I (JLab/Hall A) Q 2 = 0.48 PV-A4 (MAMI) Q 2 = 0.23, 0.11 G 0 (JLab/Hall C) Q 2 = HAPPEx-II/helium (JLab/Hall A) Q PV-A4 (backward) PRL 102, (2009) Q 2 = 0.22 G 0 (backward)* Q 2 = 0.23, 0.6 HAPPEx-III (forward Aug-Oct, 2009) Q 2 = 0.63

9 Q p Weak : Extract from Parity-Violating Electron Scattering As Q 2 0 M EM measures Q p proton s electric charge M NC measures Q p Weak proton s weak charge (at tree level) Correction involving hadronic form factors. Exp determined using global analysis of recently completed PVES experiments. The lower the momentum transfer, Q, the more the proton looks like a point and the less important are the form factor corrections.

10 Parity-Violating Asymmetry Extrapolated to Q 2 = 0 (Young, Carlini, Thomas & Roche, PRL 99, (2007) ) 1σ bound from global fit to all PVES data (as/of 2007) PDG PDG SM Q p weak Qweak

11 All Data & Fits Plotted at 1 σ Isoscalar weak charge Standard Model Prediction (anticipated uncertainty assuming SM) HAPPEx: H, He G 0 (forward): H, PVA4: H SAMPLE: H, D Young, Carlini, Thomas & Roche, PRL Isovector weak charge

12 Energy Scale of an Indirect Search Estimate sensitivity to new physics Mass/Coupling ratio add new contact term to the electron-quark Lagrangian: Erler et al. PRD 68, (2003) Λ = mass g = coupling TeV scale can be reached with a 4% Qweak experiment. If the weak charge didn t happen to be suppressed, would have to do a 0.4% measurement to reach the TeV-scale.

13 New Physics: Examples Extra neutral gauge bosons: Z eg. E6 SO(10) U(1)ψ GUT, SUSY, left/right symmetric models, technicolor, string theories Composite fermions Leptoquarks (scalar LQs can arise in R-parity violating SUSY) M.J. Ramsey-Musolf PRC 60(1999)015501; PRD62(2000) J. Erler, A. Kurylov, M.J. Ramsey-Musolf PRD 68(2003)016006

14 The Qweak Measurement (ep parity violation) Longitudinally polarized (rapid helicity flip) electron beam (85%) 35 cm spectrometer 8.4 o beam dump LH2 target! GeV, 180 µa Slightly more likely to hit a proton if the electron is spinning to the left (parity violation) Expect A z = (σ + - σ - )/(σ + + σ - ) -0.3 ppm ±5 x 10-9 statistics in ~2200 hours Use eight detectors at 900 MHz each and in current integrating mode

15 Schematic of the Qweak Experiment Region I, II and III detectors are for Q 2 measurements at low beam current Elastically Scattered Electron Luminosity Monitors Region III Drift Chambers ~3.2 m Region II Drift Chambers Toroidal Magnet Region I Detectors Eight Fused Silica (quartz) Čerenkov Detectors - Integrating Mode Primary Collimator with 8 openings 35 cm Liquid Hydrogen Target Polarized Electron Beam, GeV, 150 µa, P ~ 85%

16 Qweak s Relative difficulty factor Statistical Errors: Higher beam currents Higher polarizations High power targets Normalization/systematic errors: Polarimetry Q 2 measurements Figure by K. Paschke Additive systematic errors: improved control of helicity correlated beam properties

17 Error Budget 2% on A PV 4% on Q w 0.3% on sin 2 θ W Uncertainty ΔA PV /A PV ΔQ w /Q w Statistical (~2,5k hours at 150 µa) 2.1% 3.2% Systematic: 2.6% Hadronic structure uncertainties % Beam polarimetry 1.0% 1.5% Absolute Q 2 determination 0.5% 1.0% Backgrounds 0.5% 0.7% Helicity correlated beam properties 0.5% 0.7% Total: 2.5% 4.1% Final error on Δsin 2 θ W / sin 2 θ W includes QCD uncertainties (1-loop) in calculation of the running 0.2% 0.3%.

18 Electroweak Radiative Corrections Q p Weak Standard Model (Q 2 = 0) ± Q p Weak Global Fit Value (Young, et.al.) ± Q p Weak Experiment anticipated uncertainty 0.0XX ± Source Q p Weak Uncertainty Δ sin θ W (M Z ) ± Zγ box ± Δ sin θ W (Q) hadronic ± WW, ZZ box - pqcd ± Charge symmetry 0 Total ± Erler et al., PRD 68(2003) First Modern Estimates of 2 Boson Exchange effects on Q p Weak at our Kinematics TBE (Gorchtein & Horowitz) (PRL 102, ) ~6% to 7% Correction +1.5% TBE (Sibirtsev, Melnitchouk, Thomas) (PRD 82, ) ~6.6% - 0.6% This correction is still ~4% at a Q 2 of 0.1 GeV 2 so we already remove part of it with the B term determination from other measurements!

19 Recent History of the γz-box Calculation! Gorchtein and Horowitz used a dispersion relation to analyze the box (PRL 102, ).! Sibirtsev et al. found an analytic result that was greater by a factor of 2 (PRD 82, ).! Carlson et al. agreed well with Sibirtsev et al. (PRD 83, ) - both in magnitude and uncertainty.! Ramsey-Musolf joined Gorchtein and Horowitz - and also confirmed the factor of 2 (ArXiv: ) but get a bigger uncertainty, but using an extremely conservative approach to estimating uncertainties probably too conservative.! More work in underway to reduce theoretical uncertainty.!

20 Rapid Spin Flips at 960/sec to Suppress Target Boiling Noise Our Spare Gun Inverted Gun #2 at Test Cave Large grain niobium electrode Problematic field emission at 140kV Repeated BCP treatment, no measurable field emission at 225kV Have since demonstrated many months of beam delivery at 200kV Primary Gun Inverted Gun #1 at CEBAF Operational since July, 2009 Stainless steel electrode Operated at 100kV for HAPPEx and PRex 150uA) Operated at 130kV for Qweak 200 ua), improved transmission

21 Slow Helicity Reversals New Two-Wien spin Flipper in addition to Half Wave Plate flip spin each week to cancel out HC laser spot variation

22 The Qweak Exp! Qweak measurement of proton weak charge through parity violating electron scattering ongoing in Hall C Luminosity monitors Luminosity monitors scanner

23 Qweak (partial shielding shown)

24 Main Detector System Toroidal Spectrometer Produces 8 Beam Spots Each focus is ~2 meters long Elastic focus blue Inelastics - red 8 fused silica radiators 200 cm x 18 cm x 1.25 cm Spectrosil 2000 Rad-hard, low luminescence (expensive) 900 MHz e - per bar Light collection by TIR 5 Angstroms rms polish (even more expensive) 5 PMTs with gain = 2000 S20 photocathodes (I k = 3 na) Current mode readout (I a = 6 µa)

25 Light Output from Quartz Bar Custom 18 bit ADC

26 Tracking System to Precisely Determine Q 2 Acceptance Why a Tracking System? easy answer Goal: 0.5% on Q 2 Subtleties: Moments of Q 2 acceptance Light-weighted response of Cerenkov Check on collimators, toroid field Diagnostics on backgrounds Radiative corrections

27 Tracking System to Determine Q 2 Acceptance Tracks reconstraucted to quartz detector bar by W&M Chambers Scanner map at front of a quartz detector bar

28 Qweak ~2.5 kw LH 2 Cryo-target System! First use of a Fluid dynamics simulation used in the design to minimize boiling noise risk - in the liquid or at the windows. Additional safeguards: large raster size ~(3mm x 3mm), faster pump speed, and more cooling directed onto windows... Faster helicity reversal up to1 ms flip rate. Common mode rejection of boiling noise, line noise and undesirable beam properties increases significantly as Helicity reversal/readout rate is raised.

29 Qweak ~2.5 kw LH 2 Cryo-target System! The highest power cryotarget ever built! 35 cm long liquid hydrogen (LH 2 ) Designed using CFD Target density fluctuations are small compared to statistical uncertainty (per quartet)! Target cell beam direction LH 2 flow Flow Space only

30 New Compton Polarimeter System! 1% precision with the Compton at is anticipated via cross-calibration against existing saturated Fe Møller polarimeter.

31 Meeting the Critical Metric: ΔA Observed width very close to expected width! Using more rf cavity beam monitors (aka BCMʼs) during the next run will drive this down further.! Background & shielding improvements have also dramatically reduced excess width.!

32 ΔA/A Projections (assumes 235 ppm MD, 87% pol) summer down Statistical goal = 2.1%

33 Run I and Run II Anticipated Uncertainties Q P W = - 2 (2C 1u + C 1d )! Isoscalar weak charge! Anticipated constraints if Q P W is consistent with the Standard Model.! Young, Carlini,! Thomas & Roche, PRL! Isovector weak charge!

34 Summary! Interpretable precision measurement of the proton s weak charge in the simplest system. Most hadronic structure effects determined from global PVES measurements. Other theoretical uncertainties calculated to be small. Because Q p Weak happens to almost cancel, it is very sensitive to the value of sin 2 θ W, making possible a ~10 σ test of the running. Sensitive search for parity violating new physics with CL of 95% at the ~2.3 TeV scale. If the LHC observes a new neutral boson with mass Λ, our results could help identify it by constraining the magnitude and sign of the coupling-to-mass ratio g e-p /Λ. Measurement unique to Jlab that is unlikely to be repeated in the foreseeable future so we need to push the precision envelope. This experiment builds the scientific and technical foundation for a next generation Møller measurement at a 12 GeV JLab.

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