A determination of the formation rate of muonic hydrogen molecules from the MuCap experiment Sara Anita Knaack Final Exam June 25 th, 2012

Outline The MuCap experiment Motivation for the measurement of Λ S Muon kinetics Molecular (ppμ) state A measurement of the molecular formation rate, λ ppμ The experiment Event selection Analysis of the decay electron time spectrum Systematic effects Results Fit stability Analysis of the capture neutron time spectrum Consistency check Conclusions

Muon capture in the Standard Model Weak force interaction Sensitive to the hadronic environment spectator quarks Described in field theories χPT p n

The hadronic form factors Theory  g S (q 2 ), g T (q 2 )  0 Experiment  g V (q 2 ), g M (q 2 ), g A (q 2 ) G F and V ud also known Leaves g P (q 2 =-0.88m   ) less well determined experimentally Muon capture on the proton g P (q 2 =-0.88m   )=8.36(23) from χPT to NLO

A measurement of the singlet μp state capture rate, Λ S, determines the pseudoscalar form factor g P First MuCap result

The lifetime method – the MuCap experiment Muon decay μ -  e - ν μ ν e BR=O(1) Capture μ - +p  n+ν BR=O(1.5x10 -3 ) Molecular state effects μp+H 2  [(ppμ)pe] + +e - λ - = λ + + Λ S - Δ ppμ λ +  1 ppm relative precision – MuLan - Phys. Rev. Lett. 106, (2011) Λ S  1% relative precision λ -  10 ppm relative precision

A muon stopping in hydrogen  p Singlet  = (5) s -1  p Triplet  3/4 1/4  S =711.5 s -1  T =12 s -1 Enters the n≈14 quantum state Prompt (≈10 ns) cascade into singlet state Radiative transitions, Coulomb de-excitation, and Auger interactions Irreversibly de-excites to the singlet state under thermal conditions.

The ppμ molecular state Collisional process -  pp  =φλ pp  Dominant mode of formation Electric dipole transition J=1 “ortho” state Normalized rate of λ ppμ =1.8 (9) x 10 6 s -1 J=0 “para” state formation is suppressed λ pf =7.5 x 10 3 s -1 J=1 J=0

Molecular state kinetics  p Singlet pp  Ortho  = (5) s -1 op = 6.6(3.4) x10 4 s -1  p Triplet   pp  =φλ pp  pp  Para  S =711.5 s -1    P =213.6 s -1  O =541.5 s -1 3/4 1/4  pf =φλ pf

Ambiguity of Λ S Interpretation due to the molecular state kinetics Measurements of g P liquid hydrogen target Sensitive to op The MuCap result is less sensitive to this knowledge Gaseous hydrogen at 10 bar

 = 1 (Liquid) Rel. Population Time after  p Formation  = 0.01 (~10 bar gas) Time after  p Formation Rel. Population Relative population of  p, pp  -o and pp  -p states vs. time ( ns) Placing the hydrogen gas under 10 bar at room temperature minimizes the formation of molecular states and their effect on the  S extraction Systematic error for the MuCap measurement Source of uncertaintyFirst MuCap resultFinal - Statistical error 12.5 s s -1 - Systematic error 8.4 s s -1 Correction for Δ pp  ( =2.3(5) x10 6 s -1 ) (λ op ) 23.5  4.3  3.9 s -1 < 2 s -1 The improved precision of the final MuCap result requires an improved determination of λ pp 

The current uncertainty of λ pp  Historical variation of results and conditions Systematic < 2 s -1  < 10 % relative precision =2.3(5) x 10 6 s -1 Bystritskii et al. Soviet Physics 43(4), 606 (1976) Hydrogen gas doped to 30 ppm (atomic) with Xe Impurity elements introduce competing processes involving the muon Liquid Solid Gas

Muon kinetics in an argon-doped hydrogen gas  p Singlet pp    Ar  pp  =φλ ppμ ≈2.2 x10 4 s -1  pAr =φc Ar λ pAr ≈4.5 x10 4 s -1 ≈20 ppm (atomic)  Ar ≈1.3 x10 6 s -1   = x10 6 s -1 n μA r(t), n μp (t) and n ppμ (t)

Muon kinetics with Z>1 elements Muon Transfer – collisional process Scales approximately Z 2 Muon Capture Remnant (Z-1) nucleus remains. The rate Λ Z scales with Z eff 4 # of protons Wavefunction overlap

The decay electron time spectrum analysis Four kinetic rates dominate λ μ, Λ ppμ, Λ pAr and Λ Ar r pp    r  p   +  pp  +  pAr r  Ar   +  Ar C pp  =  pp  /(  pp  +  pAr ) C  p =  pAr  Ar /((  pp  +  pAr )(  pp  +  pAr -  Ar )) C  Ar =  pAr /(  pp  +  pAr -  Ar ) O(10 8 ) events Known muon decay rate λ μ Determine Λ ppμ, Λ pAr and Λ Ar to %-level precision.

Capture neutron time spectrum Dominated by capture form μAr state 7% of all muon stop events Can obtain O(10 6 ) events Extract r μp and r μAr Internal consistency check

The MuCap experimental setup Muon beam Entrance detectors, μSC Muon timing, t μ Time projection chamber (TPC) Decay electron detectors ePC chambers (tracking) eSC hodoscope (t e - timing) Eight liquid scintillator neutron detectors Neutron timing, t n

Muon beam Kicker Separator Quadrupoles MuCap detector TPC Slit  E3 beamline Delivers 7x10 4 muon/s p≈32.6 MeV/c ≈5 MeV Electrostatic kicker Reduces beam rate by 100 With μSC implements a single muon event structure Pileup protection

The time projection chamber target 80 anode wires, z axis 35 strips cathode strips, x axis Time to digital converter readout 2-D unit  pixel EL, EH, EVH thresholds 2.0 kV/cm drift field V drift of 5.5 mm/μs The hydrogen gas is both a target and an active detection volume

Selecting a muon decay event Stop Location EH and EL pixels Fiducial volume criteria Particle track criteria Straight-line fits χ 2, total length l Time coincident electron information Decay time: t decay =t e -t μ Same criteria used for the main analysis Documented extensively Time relative to muon entrance, t μ y z y x

Neutron detectors Liquid organic scintillator detectors FADC (analog) readout of pulses Sensitive to fast neutrons (MeV-scale energy) Gamma rays and electrons Different pulse shapes from neutron and gamma ray hits pulse shape discrimination, PSD. Determines time of the neutron t n

Selection of capture neutron events Requires a good muon stop a coincident neutron hit (±35000 ns) t capture =t n -t μ Electron veto: coincident electron hits within ns of muon entrance Charge deposition in the capture process Typical capture event in the TPC Another observable for muon capture y z y x

Summary of data taken with the argon-doped hydrogen gas Decay electron time spectrum 4.9x10 8 analysis-selected events. 40 ns binning Capture neutron time spectrum 1x10 6 analysis-selected events 60 ns binning Capture recoil time spectrum 200 ns binning Not presented further

Atomic systematic effects  p Singlet pp  h    Ar    pp   pAr f 1-f  Ar Prompt formation of the μAr state Direct stops Excited-state transfer - μp cascade f=4.95(99)x10 -4 Bound μAr state decay rate effect Relativistic orbit - time dilation Phase-space suppressed h=0.985(3) Relative efficiency Nuclear charge e Ar =0.9956(25)  Ar decay electrons detected with (relative) efficiency e Ar μAr, μp, free

Full kinetics model  p Singlet pp  Ortho h  op  Ar   pp   pAr efficiency e Ar f 1-f  Ar pp  Para SS   OO PP  Λ pf

Description of the time spectrum Differential equations, initial conditions, full time spectrum. Atomic physics parameters f, h, and e Ar relative contribution of μAr state decays The hydrogen kinetic rates, λ μ, Λ S, λ op, Λ pf, Λ O, and Λ P Directly affect the time distribution of events The fit function is A n e (t)+B

Fit to the decay electron time spectrum Basic fit results Λ ppμ =2.208(65) x 10 4 s -1 Λ pAr =4.529(15) x 10 4 s -1 Λ Ar =1.302(14) x 10 6 s -1 χ2/Ndf=0.983(64) One external systematic correction

Summary of results Disappearance rate results r μp = (80) x 10 6 s -1 where r μp =λ μ +Λ ppμ +Λ pAr +Λ S +Λ pf r μAr =1.750(16) x 10 6 s -1 where r μAr =hλ μ +Λ Ar Statistics limited results Scrutiny of fit procedure Correlated and non- linear features

χ 2 Map of the Λ ppμ, Λ pAr, and Λ Ar parameter space Variation of the χ 2 relative to the minimum value The Δχ 2 =1 contour is consistent with the ±1 σ of the fit results Reflects the correlation of these parameters The χ 2 variation is controlled and smoothly varying

Fit reproducibility 10 4 pseudo data histograms Fit function result Same statistics as data Reliability of the central values As well as the reported fit errors

Capture Neutron Kinetics  p Singlet pp  Ortho h  op  Ar   pp   pAr Relative efficiency, e H, of 5.2 MeV neutrons e H =1.833(80) ≈ 1-3 MeV neutrons f 1-f  Ar SS  OO  n n (t)=e H (  S n  p (t)+  O n Ortho (t))+  Ar n  Ar (t) r  p =  +  pp  +  pAr +  S +  pf r  Ar = h  +  Ar

Timing calibration 5.2 MeV neutron -> time-of-flight of ≈18 ns The time-of-arrival, t H, and the spread s H Extracted from the pure hydrogen data t H to ±2 ns precision s H =15.5(5.9) ns 1-3 MeV neutrons from capture onto argon Arrive ≈11 ns (2 MeV) later than t H Time window of ±8 ns No sharp transition feature Physical energy distribution not well understood t Ar =t H +11±8 ns s Ar =15.5(10.9) ns 566 mm t n -t μ [ns] Neutron Time Spectrum

Fit start-time scan and sensitivity to systematics Change in fit results: t Ar is varied by ±8 and ±4 ns r μAr dominates in the first 1500 ns Early time window data determines rate r μAr and r μp are correlated variation in both rates with start-time The start-time of 600 ns minimizes sensitivity Allowed 1 σ limit of variation Stable result

Neutron time spectrum results Timing calibration systematic uncertainties The r μAr result is “systematics limited” Well-understood corrections Prompt μAr formation Capture from hydrogen states Relative efficiency of 5.2 MeV neutrons Comparable precision for r μp

Comparison of results 1 and 2 σ contours of the electron and neutron time spectrum analysis results 1 σ agreement for r μp 0.5 σ agreement for r μAr Consistent with statistics The neutron time spectrum - large systematic effects Timing calibration Background Others The more precise results of the electron time spectrum are the main results of this work.

Normalized molecular formation rate result, λ ppμ Known gas density φ=0.0115(1) ≈3% relative precision Agrees well with theory Faifman: 1.8(9) x 10 6 s -1 Differs from Bystritskii et al. at 2.3 σ Only comparable gaseous target Solid Liquid Gas

Final MuCap result Identical conditions as the λ - measurement Exceeds 10% benchmark < 2 s -1 uncertainty to Δ ppμ Clear interpretation of correction

A precision determination of  s also determines the pseudoscalar coupling constant g p.  - + p  n + -- pn q 2 = -0.88m  2 Most precise experimental determination of g p Test of chiral symmetries and low energy QCD The electron time spectrum is described with a single lifetime; reduced due to capture. MuCap measures  S by comparing the  - decay rate in hydrogen to the muon lifetime. Recently measured to 1 ppm precision Phys. Rev. Lett. 99, (2007) and Phys. Rev. Lett. 106, (2011) In leading order:

Muons and the Weak Force Fundamental particle m μ =105.7 MeV/c 2 Decays through the weak interaction lifetime of τ μ ≈2.2 μs decay rate λ μ. Measured to 1 ppm relative precision by the MuLan collaboration λ μ = (5) s -1 Also a significant update to the knowledge of the Fermi Constant G F

The Hadronic Form Factors G-Parity  g S (q 2 ), g T (q 2 )  0. CVC + Electron scattering  g V (q 2 =-0.88m m ) = ± g M (q 2 =-0.88m m ) = ± Neutron beta decay  g A (q 2 =-0.88m m ) = ± Leaves g P (q 2 =-0.88m  ) ill determined experimentally. Muon Capture on the proton g P (q 2 =-0.88m  )=8.36(23) from χPT to NLO

Decay Electron Event Selection 3-D tracking from ePC1 and ePC2 Timing t e, from eSC hodoscope 16 segments two 0.5 mm thick layers Four photo multiplier tubes (PMTs “Paired” - fourfold coincidence (±25 ns) t decay =t e -t μ Impact Parameter b b ≤ 120 mm Electron and muon stop data processed separately at the raw stage of the analysis. Bulk data processing at NCSA

TDC Electronics: threshold discrimination EL “low” threshold Muon track MeV EH “high” threshold MeV Increased dE/dX as the μ - slows Signature of a stop EVH “very high” threshold MeV Recoiled pulse in muon capture onto a Z>1 nucleus Digitization applied at the clock boundary of 200 ns sampling bins. 2-D coordinate for the TPC, anode wire : time of a sample bin pixel The cathode strips are read out in a similar way.

Selecting a muon decay event Stop Location Down-stream most anode at EH: z The time of first EH pixel, t Stop, y via y=(t Stop -t μ )/v drift Coincident cathode strip pixels, x Fiducial volume criteria Stop location and all EH – pixels “MuStop” fiducial volume. All EL (or higher) pixels “Track” fiducial volume. Particle track criteria Straight-line fit to EL pixels χ 2, total length l Decay time: t decay =t e -t μ MuCap criteria Documented extensively Time relative to muon entrance, t μ y z y x

Oxygen Impurity Correction Induce muon transfer and capture. C O =0.23 ppm and C N =0.23 ppm Two sets of simulated time spectra varying C O and C N Known N and O transfer and capture rates x10 5 more statistics than in the data Linear coefficient of variation, k Correction Δ=k C O C O =0.12(11) ppm Applied to Λ ppμ and Λ pAr

Fit start-time scan A sub-set of the data is chosen with start-time moved back from 120 ns Fit stop-time fixed at ns Amplitude and background fixed From “free” 120 – ns result Allowed 1 σ limit of variation

Pulse Waveform Analysis Resampling from 5.9 ns to 0.3 ns Interpolation fit to rate data Peak time determines t n Slow, i S, and total, i Σ, integrals Discriminant r PSD =i S /i Σ Upper and lower limits Vary with the total Integral l = c + k/i Σ Specific to each detector A final 400 ADC bit ≤ i Σ ≤ 5000 ADC bit 0.5 MeV MeV energy cut

Neutron Background Paired eSC hit and neutron coincident to same muon stop Prompt -150<t e -t n <50 ns “misidentified” electron hits Separation of beam- correlated background

CPE Interference Fiducial volume conditions unstable Early times, before 1000 ns Rejected from the analysis p C =60.8(7.0)% Disconnected events, predominant after 2000 p D =15.1(3.7)% CPE -> particle track in the analysis p T =35.7(6.2)% Correction applied using the time spectrum of the disconnected “track” events f CPE =(p D -p C )/p T =1.28(31) Systematic errors 3.1 x 10 2 s -1 for r μp 1.42 x 10 4 s -1 for r μAr Connected Disconnected

Fit start-time scan and sensitivity to systematics No systematic correction Prompt μAr formation μp state capture events Relative efficiency of μp state capture events Full result – correction for capture from the ortho-molecular sate Including the relative efficiency A start-time of 600 ns minimizes sensitivity to these systematics as well.

The capture neutron time spectrum n n (t)=e H (  S n  p (t)+  O n Ortho (t))+  Ar n  Ar (t) n  p ’(t) = -r  p n  p (t) n  Ar ’(t) =  pAr n  p (t) -r  Ar n Ar ’(t) n Ortho ’(t)=  pp  n  p (t)-(  +  O + op )N Ortho (t) Where n  p (0)=1-f and n  Ar (0)= f, r  p =  +  pp  +  pAr +  S +  pf r  Ar = h  +  Ar Results for r  p and r  Ar respectively. Capture from the para-molecular state is neglected. Fit function: A n n (t) +B

Oxygen Impurity Correction Induce muon transfer and capture. C O =0.23 ppm and C N =0.23 ppm Two sets of simulated time spectra varying C O and C N λ pN =0.34 x s -1 and Λ N =0.069 x 10 6 s -1 λ pO = 1.7 x s -1 and Λ O =0.102 x 10 6 s -1 x10 5 more statistics than in the data Linear coefficient of variation, k each of the kinetic rates Correction for each rate, Δ Δ=k C O C O =0.12(11) ppm Δ=191(175) s -1 is applied to Λ ppμ Δ=27(24) s -1 is applied to Λ pAr