Muon Capture on the Proton: First Physics Results from the MuCap Experiment Steven Clayton University of Illinois Urbana Champaign Contents: 1.Physics Context 2.Expt. Diagram 3.Expt. Design 4.1 st Physics Results IUCF Nuclear Physics Seminar MuCap Collaboration V.A. Andreev, T.I. Banks, B. Besymjannykh, L. Bonnet, R.M. Carey, T.A. Case, D. Chitwood, S.M. Clayton, K.M. Crowe, P. Debevec, J. Deutsch, P.U. Dick, A. Dijksman, J. Egger, D. Fahrni, O. Fedorchenko, A.A. Fetisov, S.J. Freedman, V.A. Ganzha, T. Gorringe, J. Govaerts, F.E. Gray, F.J. Hartmann, D.W. Hertzog, M. Hildebrandt, A. Hofer, V.I. Jatsoura, P. Kammel, B. Kiburg, S. Knaak, P. Kravtsov, A.G. Krivshich, B. Lauss, M. Levchenko, E.M. Maev, O.E. Maev, R. McNabb, L. Meier, D. Michotte, F. Mulhauser, C.J.G. Onderwater, C.S. Özben, C. Petitjean, G.E. Petrov, R. Prieels, S. Sadetsky, G.N. Schapkin, R. Schmidt, G.G. Semenchuk, M. Soroka, V. Tichenko, V. Trofimov, A. Vasilyev, A.A. Vorobyov, M. Vznuzdaev, D. Webber, P. Winter, P. Zolnierzcuk Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia Paul Scherrer Institute (PSI), Villigen, Switzerland University of California, Berkeley (UCB and LBNL), USA University of Illinois at Urbana-Champaign (UIUC), USA Université Catholique de Louvain, Belgium TU München, Garching, Germany University of Kentucky, Lexington, USA Boston University, USA

 - Stopping in Hydrogen Quickly forms a  p atom, transitions to ground state, transitions to singlet hyperfine state. Bohr radius a ≈ a 0 m e /m  ≈ a 0 /200 –Most of the time, the  decays: –Occasionally, it nuclear captures on the proton:  - + p   + n rate  S BR~10 -3  - + p   + n +  BR~10 -8, E>60 MeV  -   + e - + e rate 0 = 1/   + BR≈0.999 Complications: molecular formation/transitions, transfer to impurity atoms, … The goal of  Cap is  s to 1% precision H2H2  p

EW current key probe for nucleon structure Understanding hadrons from fundamental QCD lattice QCD chiral effective field theory (ChPT)  Goldstone boson of spontaneously broken symmetry, sys. expansion in q/  model independent predictions fundamental properties and symmetry tests interesting processes (astrophysics) interface to lattice calculations … Nuclear Physics Context nucleon level quark level   (1-  5 ) u d W  W  relevant degrees of freedom ? q q

Muon Capture and Axial Nucleon Structure  - + p   + n rate  S Lorentz, T invariance + second class currents suppressed by isospin symm. Vector form factors q 2 = m  2 g V = (5) g M = (25) Vector form factors q 2 = m  2 g V = (5) g M = (25) Conserved Vector Current CVC strong program JLab, Mainz,... Main motivation for  Cap studies Axial form factors q 2 = m  2 g A = 1.245(3) g P = 8.3 ± 50% Axial form factors q 2 = m  2 g A = 1.245(3) g P = 8.3 ± 50%

Axialvector Form Factor g A Exp. History Axial radius Lattice QCD  +N scattering consistent with  electroproduction (with ChPT correction) introduces 0.4% uncertainty to  S (theory) PDG 2006 Edwards et al. LHPC Coll (2006) Bernard et al. (2002)

g P determined by chiral symmetry of QCD: n  p --  g  NN FF g P = (8.74  0.23) – (0.48  0.02) = 8.26  0.23 PCAC pole term Adler, Dothan, Wolfenstein ChPT leading order one loop two-loop <1% N. Kaiser Phys. Rev. C67 (2003) g P basic and experimentally least known weak nucleon form factor solid QCD prediction via ChPT (2-3% level) basic test of QCD symmetries g P basic and experimentally least known weak nucleon form factor solid QCD prediction via ChPT (2-3% level) basic test of QCD symmetries Recent reviews: T. Gorringe, H. Fearing, Rev. Mod. Physics 76 (2004) 31 V. Bernard et al., Nucl. Part. Phys. 28 (2002), R1 Pseudoscalar Form Factor g P

 S Calculations 1%

Processes to Determine g P  - + p   + n OMC rate  S BR~ experiments, typical precision 10-15%, Saclay 4%  - + p   + n +  RMC BR~10 -8, E>60 MeV  He   + 3 H pion electroproduction … 279±25 events BR  (k>60MeV)=(2.10±0.21)x10 -8 Wright et al. (1998) authors  stat (s -1 ) comment theory 1993Congleton & Fearing13041B theory 1996Congleton & Truhlik1502±321B + 2B exp 1998Ackerbauer et al1496.0±4.0 3 He TPC theory 2002Marcucci et al.1494±91B+2B, T beta constraint rad. corrections?

Muon Atomic/Molecular State in Experiment must be known to connect with theory.  p(F=0) H 2 density  pp  OP  s ~ 664 s -1 p  p(J=1) Ortho  om ≈ ¾  s p  p(J=0) Para  = 1 (Liquid) Rel. Population Time after  p Formation  = 0.01 (~10 bar gas) Time after  p Formation Rel. Population  pm ≈ ¼  s

no overlap theory & OMC & RMC large uncertainty in OP  g P  50% ? no overlap theory & OMC & RMC large uncertainty in OP  g P  50% ? Precise Theory vs. Controversial Experiments ChPT OP (ms -1 ) gPgP  - + p   + n + TRIUMF  Cap precision goal exp theory TRIUMF 2005  - + p   + Saclay

Relevant Kinetics in 10 bar H 2  pp  OP  p(F=0) ss p  p(J=1) Ortho  om p  p(J=0) Para  pm   p(F=1) TT c d pd c d dd dd c Z pZ c Z ZZ  Z ~ Z 4 n +  + (Z-1)* H 2 must be pure isotopically and chemically: c d < 1 ppm, c Z < 10 ppb

Unambiguous interpretation capture mostly from F=0  p state at 1% LH 2 density Lifetime method   →e decays measure   to 10ppm,     S = 1/   - 1/    to 1%  Clean  stop definition in active target (TPC) to avoid:  Z capture, 10 ppm level Ultra-pure gas system and purity monitoring to avoid:  p + Z   Z + p, ~10 ppb impurities Isotopically pure “protium”  to avoid:  p + d   d + p, ~1 ppm deuterium diffusion range ~cm  Cap Experimental Strategy fulfill all requirements simultaneously unique  Cap capabilities

Lifetime Method  Cap measures the lifetime of  - in 10 bar hydrogen gas. -- e-e- T zero T electron Data Acquisition TT Repeat times for a 10 ppm precision lifetime measurement. H2H2 TT Log N events slope =  ≡ 1/   s = – 1/   +

 Cap Detailed Diagram  Tracking of Muon to Stop Position in Ultrapure H 2 Gas  Tracking of Decay Electron

Commissioning and First Physics Data in 2004 (Target Pressure Vessel, Pulled Back)

Time Spectra  -e impact parameter cut huge background suppression diffusion (deuterium) monitoring --  +  SR in 50G  + as reference identical detector systematics different physics blinded master clock frequency N e - t + B Impact Parameter Cut

3D tracking w/o material in fiducial volume Clean  Stop Definition in Active Target 10 bar ultra-pure hydrogen, 1% LH kV/cm drift field ~5.4 kV on 3.5 mm anode half gap bakable glass/ceramic materials Time Projection Chamber (TPC) Beam ViewSide View  Beam  Stop y (Drift Time) z (Anode Number)x (Strip Number)

Ultrapure Gas System: Continuous H 2 UltraPurification System Commissioned 2004 c N2, c O2 < 0.01 ppm

In situ Impurity Monitoring: TPC “Sees” High-Z Captures  Beam  Stop Z>1 Capture (recoil nucleus) Capture Time Capture Time [ns] Z>1 Capture Time

Isotopically Pure “Protium” Target 2) On-site isotopic purifier 2006 (PNPI, CRDF) 1) Generate H 2 from deuterium-depleted water (c d ~1 ppm) World Record c d < 0.1 ppm

c D Monitoring: Data Analysis Approach  Stop Position  Decay Position  d Diffusion Path  -e Vertex Cut  d can diffuse out of acceptance region:  signal proportional to number of  d, and therefore to c D. Impact Parameter Cut [mm] Fitted Rate + Offset [s -1 ] Fits to Exponential Decay + Constant No Impact Parameter Cut c D (Production) c D (Natural H2) = ± Data

c D Monitoring: External Measurement Measurements with New ETH Zürich Tandem Accelerator: 2004 Production Gas, c D = 1.44 ± 0.13 ppm D 2005 Production Gas, c D = 1.45 ± 0.14 ppm D 2006 Production Gas (isotope separation column), c D < 0.06 ppm D The “Data Analysis Approach” gives a consistent result: 2004 Production Gas, c D = ( ± ) × (122 ppm D) = 1.49 ± 0.12 ppm

Correction for Deuterium: Zero Extrapolation D Concentration (c D )0 Production Data (D-depleted Hydrogen) Calibration Data (Natural Hydrogen) Extrapolated Result from fits to data (y = N e - t + B)

Muon-On-Demand concept Muon-On-Demand Beamline Single muon requirement (to prevent systematics from pile-up) limits accepted  rate to ~ 7 kHz, while PSI beam can provide ~ 70 kHz - kV kV Kicker Plates 50 ns switching time  detector TPC Fig will be improved ~3 times higher rate dc kicked 2-Dec-2005  Lan kicker TRIUMF rf design

TPC fiducial cuts Start time fit Consistency Studies Event selection cuts 6 mm Inside TPC 2004 Data

Consistency Studies (continued) 2004 Data divided into 96 chronological groups

Corrections & preliminary results data 2004 MuCapPDG events =1/  (s -1 )  S (s -1 )1/  (s -1 ) - x   18 + x   8.3 internal correction (s -1 ) uncertainty (s -1 ) statistics 12 Z>1 impurities-146  stop definition 2  d diffusion -121  p diffusion -31  +p scatter -52 pileup11 det combination5 clock3 total systematics-339 total sys & stat-3315 external correction (s -1 ) uncertainty (s -1 ) pp  formation 194 OP transition /  in  p system 12 1/   PDG 8.3 total external369.5

 Cap and  S calculations rad. corrections Goldman (1972) Czarnecki Marciano Sirlin (2006) private comm. preliminary MuCap agrees within ~1  with  S theory Thorough theory studies needed for next MuCap 1% stage ! preliminary

within one sigma of chiral prediction, no dramatic discrepancy nearly independent of molecular physics ( OP ) has overlap with old OMC, barely with recent RMC result final result (’06 and ’07 data) will reduce error to 7% within one sigma of chiral prediction, no dramatic discrepancy nearly independent of molecular physics ( OP ) has overlap with old OMC, barely with recent RMC result final result (’06 and ’07 data) will reduce error to 7%  Cap and g P preliminary

 2006 data and 2007 plans  events  - achieved in 2006  events  + and suppl. measurements in 2007  S with 1% uncertainty   +d proposal planned for 2007 Summary and Plans  Cap theory*  S 730  g P 6.95   0.23  Preliminary results 2004 data  Ideas for ultrapure H 2 TPC welcome *including Czarnecki et al. rad. corrections preliminary

Extra Slides

Main Corrections to Lifetime Result 2004 Data (No Cut)

Old Muon Stop Definition 1 Strong “Gondola Effect”* Gondola Observed Decay Rate  - Production Data *The “gondola effect” is a large (anomalous) variation with gondola number of the fitted muon decay rate.

Old Muon Stop Definition 3 Rejected TPC stops (“Bad Tracks”) have the opposite trend. “All Tracks” in the old stop definition means a good linear fit to the track shape is not required, and the EH pixels do not have to be anywhere in particular in the track. The only cut is a fiducial one. “All Tracks” do not show a significant gondola effect!

Tom’s Version of the “Gondola Effect” Largely reduced compared to mine, but still present.

New MIAS Muon Stop Definition 1 2) Within the ROI, find all groups of next- nearest-neighbor connected EL pixels, in pixel space, with at least one EH pixel.* 1) Define a Region of Interest (ROI) of TPC data based on the muon entrance counters [T entrance, T entrance + 25  s] Goal of new stop definition: to be more robust against noise and distorted tracks, and to identify tracks that need more attention. * “Pixel Space” is the 2-dimensional space of: (drift time divided by TDC400 clock period, anode number); it corresponds to spatial (y, z) coordinates. “Raw data” pixels “Island” of pixels Pixels that are part of this island are indicated by a square around the dot. In this case, all pixels shown are part of the same island. The long line and large blue and green squares (1 each) are explained on the next slide.

Define some island characteristics: maximum anode number of EL pixels (AnodeMaxEL) maximum anode number of EH pixels (AnodeMaxEH) minimum anode number of EL pixels (AnodeMinEL) minimum anode number of EH pixels (AnodeMinEH) These maxima/minima anode numbers (and similarly for the drift time dimension) are indicated by the large squares (green for EL pixels, blue for EH pixels). An island is a track if: AnodeMaxEL – AnodeMaxEH <= 2 AnodeMinEH – AnodeMinEL >= 8 In other words, “IsTrack” is a minimal cut and means: the track cannot extend too far beyond the apparent stop point (EH pixels), and it also must have a sufficient tail leading up to the stop point. New MIAS Muon Stop Definition 2 3) Identify if the island is a good stop (“IsTrack”). If an island is found to be a track (IsTrack), then a line is fit to the pixels for possible cuts or beam (y, y’) studies. The (y, z) stop location is defined as that of the most downstream EH pixel, the one with the minimum drift time.

New MIAS Muon Stop Definition 3 4) If the island is not a track (“!IsTrack”), check if it is a spot (“IsSpot”) An island is a spot if: AnodeMaxEL – AnodeMaxEH <= 2 AnodeMinEH – AnodeMinEL <= 2 DriftTimeMinEH – DriftTimeMinEL <= 3 (?) DriftTimeMaxEL – DriftTimeMaxEH <= 3 (?) The (y, z) location of the spot is defined as the average of the EVH pixels if present, otherwise of the EH pixels. (This may be changed to be the average of the earliest EVH or EH pixels.)

New MIAS Muon Stop Definition 4 Location in x (strips dimension) Y (Drift Time) found from anodes Stop x is taken to be the average of the strips that have a hit at the stop time (T stop ) determined from the anodes. The maximum and minimum x of the muon stop are based on strips hit at T stop, T stop +1 and T stop -1.

TPC Event Categories 2 Y (or Drift Time) Z (anode number) Extra EL pixel in Region of Interest (ROI) Occasionally (~2% of good stops), there are one or more EL pixels, unassociated with any EH-containing island, and downstream of the beginning of the muon track.* Y (or Drift Time) Z (anode number) * Most of the “extra EL pixels” upstream of the beginning of the track are merely from when the muon had not yet sufficiently slowed to leave a connected track.

TPC Event Categories 3 Y (or Drift Time) Z (anode number) Many Extra EL pixels in Region of Interest (ROI) Very rarely (<10 -4 per muon stop), there are 8 or more extra EL pixels. Most (~90%) point to the stop position. Y (or Drift Time) Z (anode number) A gap (circled) of 2 or more pixels separates these from the EH-containing island. These are likely muon-proton scattering events, in which the recoiling proton has enough energy to make an EH pixel, and which can be mistaken for a good muon stop.

TPC Event Categories 4 Y (or Drift Time) Z (anode number) Many Extra EL pixels in Region of Interest (ROI) Very rarely (<10 -4 per muon stop), there are 8 or more extra EL pixels. A few (10%) do not point to the stop position. Y (or Drift Time) Z (anode number) This is probably pileup undetected by the entrance counters. ?

TPC Event Categories This histogram shows the number of each of the indicated TPC events.

Fitted Rate vs. Category This region will be zoomed in the next slide. A higher rate is consistent with these events being muon-proton scatters out of the fiducial volume, some of which capture on high-Z material.

Fitted Rate vs. Category (zoom1) Selecting tracks with exactly 1 extra EL pixel in the ROI shows a very strong gondola effect. This region will be zoomed in the next slide.

Fitted Rate vs. Category (zoom2) Rejecting tracks with any extra EL pixels naturally shows the opposite effect than that highlighted in the previous slide. Clearly we must be very careful about rejecting, even implicitly, muon stops based on extra EL pixels in the Region of Interest. These show no gondola effect.

Residuals of Lifetime Fits IsTrackIsTrack with 1 extra EL pixel in ROI The decay rate ( ) is fixed to that found in the IsTrack fit (left), and A and B are allowed to vary. Fit Function: N e (t) = A e - t + B

Residuals of Lifetime Fits Fit Function: N e (t) = A e - t + B Gondola 1Gondola 5Gondola 8 These are IsTrack with 1 extra EL pixel in ROI. is fixed from the fit to all IsTrack events.

Spatial Distribution of Extra EL Pixels Y [mm] Z [mm]

Spatial Distribution of Extra EL Pixels wrt Muon Stop Position Z extra EL pixel – Z muon stop Y extra EL pixel – Y muon stop 0 < T Decay < 1  s The distributions are indexed by the observed decay time of the muon (electron appearance time wrt the muon entrance time). 3 < T Decay < 4  s5 < T Decay < 6  s Y extra EL pixel – Y muon stop Z extra EL pixel – Z muon stop 7 < T Decay < 8  s9 < T Decay < 10  s The center of the EL distibution either moves higher in the TPC with time, or the extra EL pixels are associated with the electron.

Spatial Distribution of Extra EL Pixels wrt Muon Stop Position The distributions are indexed by the observed decay time of the muon (electron appearance time wrt the muon entrance time.

Spatial Distribution of Extra EL Pixels wrt Muon Stop Position Z extra EL pixel – Z muon stop Y extra EL pixel – Y muon stop Gondola 1 The distributions are indexed by the gondola number of an electron (e-Detector track), 0 < t Decay < 1  s. Gondola 3Gondola 5 Y extra EL pixel – Y muon stop Z extra EL pixel – Z muon stop Gondola 7Gondola 9 These appear to be consistent with the hypothesis that the electrons occasionally cause an EL pixel somewhere along their respective paths.

Lifetime Fits with New and Old Stop Definitions Prod50  - data. Fit range: 0.1 to 19.5  s Old stop definition

Fall 2005 Run 2005  Cap Upgrades Waveform Digitizers on electron hodoscope Higher TPC voltage  detection of Alvarez events? In situ Humidity Sensor  understand Z>1 capture yield? Neutron Detectors installed  constrain op ?  Lan kicker: “Muon-on-request”  3x the rate of DC beam

p 3D tracking w/o material in fiducial volume Problem 2:  stops in wall. Solution: tracking to stop position. -- 10 bar ultra-pure hydrogen, 1% LH kV/cm drift field >5 kV on 3.5 mm anode half gap bakable glass/ceramic materials Observed muon stopping distribution E Time Projection Chamber (TPC) e-e-

Muon Tracking in the  Cap TPC Beam ViewSide View  Beam  Stop  3D tracking of muon stop position: Only stops well within fiducial volume are accepted.  Also, monitoring of High-Z impurities via recoil nucleus  Stop Z>1 Capture Capture Time

Z>1 impurities Yield Y for  Z  (Z-1)+n+ Y  100 c Z  ~ 1.6 Y Gas chromatography

 s is Well-Predicted Theoretically g V, g M and g A are well known (SM symmetries and data). g P from data is controversial (next slide), but theory prediction is solid (PCAC, ChPT). Measurement of  s to 1% determines g p to 7%. nucleon current nucleon level quark level   (1-  5 ) u d p n W L W = G F V ud 1 √2

Muon Capture probes Axial Structure of Nucleon Form factors parameterize microscopic QCD structure nucleon current + second class currents nucleon current + second class currents Charged Current Interaction nucleon level quark level   (1-  5 ) u d p n W  - + p   + n rate  S BR~10 -3  - + p   + n +  BR~10 -8, E>60 MeV

Nucleon Weak Form Factors nucleon weak CC formfactors q 2 = -0.88m  2 g V = (5) g A = 1.245(3) g M = (25) g P = ? nucleon weak CC formfactors q 2 = -0.88m  2 g V = (5) g A = 1.245(3) g M = (25) g P = ? g V, g M, g A determined by SM symmetries and data, contribute <0.3% uncertainty to  S g P determined by chiral symmetry of QCD: n  p --  g  NN FF g P = (8.74  0.23) – (0.48  0.02) = 8.26  0.23 PCAC pole term Wolfenstein ChPT leading order one loop two-loop <1% N. Kaiser Phys. Rev. C67 (2003) Recent reviews: T. Gorringe, H. Fearing, Rev. Mod. Physics 76 (2004) 31 V. Bernard et al., Nucl. Part. Phys. 28 (2002), R1 Courtesy P.Kammel nucleon current + second class currents nucleon current + second class currents

Experimental facility Muon Source PSI accelerator (ring cyclotron) generates 590 MeV proton beam protons hit graphite target and produce pions pions decay to muons Muon Beam Properties Particles: μ + or μ – Momentum ~ MeV/c rate ~ 50 kHz Location Paul Scherrer Institute (PSI), Switzerland