Λ b Baryon Studies Dongliang Zhang (University of Michigan) on behalf of Collaboration Hadron215, Jefferson Lab September 13-18, 215
Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 1/18 Λ b baryon Dongliang Zhang [UM] Hadron215 14/9/215 Λ b is the lightest b-baryon m 562 MeV Beyond the reach of B-factories Was less studied Lifetime Polarization Decay properties Included in this talk: Mass and lifetime measurement [Phys. Rev. D 87 (213) 322] Λ b J/ψΛ decay asymmetry parameter and helicity amplitude measurement [Phys. Rev. D 89 (214) 929] Observation of Λ b ψ(2s)λ decay and Γ(Λ b ψ(2s)λ )/Γ(Λ b J/ψΛ ) measurement [arxiv:157.822 [hep-ex], submitted to Phys. Lett. B.]
Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 2/18 detector Dongliang Zhang [UM] Hadron215 14/9/215
Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 3/18 J/ψ and Λ candidates Dongliang Zhang [UM] Hadron215 14/9/215 Candidates / 15 MeV J/ψ candidates Two combined muons, η < 2.5 Sufficient Inner Detector hits In J/ψ or ψ(2s) mass window 5 4 3 2 1 L = 77 pb s = 7 TeV Λ candidates Two good ID tracks, p T > 4 MeV pπ and pπ + combination, in Λ mass window Candidates / 3.25 MeV 1 16 14 L = 77 pb 12 s = 7 TeV 1 8 6 4 2 3 28 29 3 31 32 33 34 (MeV) m µµ 18 19 11 111 112 113 114 115 (MeV) m pπ
Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 4/18 Λ b reconstruction Λ b fit m µ + µ and m pπ (m pπ +) fixed to J/ψ and Λ mass values Dihadron vertex point to dimuon vertex χ 2 /N dof < 3 Also fitted to Bd J/ψ(µ+ µ )KS (π+ π ) hypothesis for Bd background rejection cuts In lifetime measurement 45 Refitted Λ Λ b 4 p T > 3.5 GeV L xy > 1 mm P Λ b P B d >.5 54 55 56 57 58 59 M (MeV) J/ψ Λ Dongliang Zhang [UM] Hadron215 14/9/215 Candidates / 17 MeV 35 3 25 2 15 1 5 L = 4.9 fb s = 7 TeV candidates Λ b candidates 474 Λ b 481 Λ b
Dongliang Zhang [UM] Hadron215 14/9/215 Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 5/18 Λ b lifetime measurement Proper decay time Mass-lifetime unbinned likelihood fit L = τ = L xym PDG p T N [f sig M s(m i δ mi )T s(τ i δ τi )w s(δ mi, δ τi ) + (1 f sig )M b (m i δ mi )T b (τ i δ τi )w b (δ mi, δ τi )] i=1 Mass M s(m i δ mi ): Gaussian M b (m i δ mi ): 1st order polynomial Proper decay time T s(τ i δ τi ): exponential with efficiency correction T b (τ i δ τi ): prompt: Dirac delta + symmetric exponential non-prompt: two exponential w s,b (δ mi, δ τi ) is the PDF for δ mi and δ τi, from data and same for signal and background
Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 6/18 Λ b mass and lifetime Candidates / 17 MeV 8 7 6 5 4 3 2 1 s = 7 TeV L = 4.9 fb Signal Background m Λb = 5619.7 ±.7 MeV = 1.18 ±.3 54 55 56 57 58 59 S M σ M = 31.1 ±.8 MeV N sig = 2184 ± 57 N bkg = 597 ± 16 χ 2 /N dof = 1.9 m J/ψ Λ (MeV) Candidates /.46 ps 3 1 2 1 1 s = 7 TeV L = 4.9 fb Phys. Rev. D 87 (213) 322 = 1.449 ±.36 ps 2 2 4 6 8 1 12 τ Λb S τ σ τ = 1.5 ±.2 =.117 ±.3 ps χ 2 /N dof = 1.9 Signal Background τ (ps) m Λ b = 5619.7 ±.7(stat) ± 1.1(syst) MeV τ Λ b = 1.449 ±.36(stat) ±.17(syst) ps Consistent with results from other experiments and theoretical predictions Main systematic uncertainty from muon trigger efficiency and Λ reconstruction bias Dongliang Zhang [UM] Hadron215 14/9/215
Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 7/18 Λ b J/ψ(µ+ µ )Λ (pπ ) decay Dongliang Zhang [UM] Hadron215 14/9/215 Asymmetric decay In subsequent decay: 4 helicity combination dn d cos θ = 1 2 (1 + P α b cos θ) Amplitude λ Λ λ J/ψ a + = a + e iω+ 1/2 a = a e iω -1/2 b + = b + e iρ+ -1/2-1 b = b e iρ 1/2 1 Full PDF of Ω = (cos θ, φ, cos θ 1, φ 1, cos θ 2, φ 2 ): w(ω) = 1 (4π) 3 19 i= f 1i ( A)f 2i (P, α Λ )F i (Ω) 7 free parameters: P, 6 helicity parameters in A
Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 8/18 Parametrization Dongliang Zhang [UM] Hadron215 14/9/215 For α b measurement, using P =, reduced to 6 terms (i =, 2, 4, 6, 18, and 19) Parameters A α b = a + 2 a 2 + b + 2 b 2 a k + = + k = a+ 2 + b + 2 b a 2 + b 2 + = ω + ρ + = ω ρ i f 1i F i 1 1 2 (k+ 2 + k2 1) + α b(k+ 2 k2 ) cos θ 1 1 4 4 [(3k2 3k2 + 1) + 3α b(1 k 2 k2 + )].5(3 cos2 θ 2 1) 6 1 4 [(k2 + + k2 1) + α b(3 + k+ 2 k2 )].5(3 cos2 θ 2 1) cos θ 1 3 18 [ 1 α b 2 2 k 2 (1 k2 ) cos( ) 1+α b 2 k+ 2 (1 k2 + ) cos( +)] sin θ 1 sin θ 2 cos θ 2 cos(φ 1 + φ 2 ) 19 3 [ 1 α b 2 2 k 2 (1 k2 ) sin( ) 1+α b 2 k+ 2 (1 k2 + ) sin( +)] sin θ 1 sin θ 2 cos θ 2 sin(φ 1 + φ 2 )
Dongliang Zhang [UM] Hadron215 14/9/215 Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 9/18 Method of moments Least square fit χ 2 = i ( F i expected F i )V 1 ij ( F j expected F j ) F j measured from data V ij is the covariance matrix F i expected depends on the parameters A F i expected ( A) = = j j F i (Ω ) 1 (4π) 3 T (Ω, Ω)w( A, Ω) dωdω f 1j ( A)f 2j (α Λ )F i (Ω )T (Ω, Ω)F j (Ω) dω dω = j f 1j ( A)f 2j (α Λ )C ij The effects of detector are in C ij T (Ω, Ω) is the resolution and efficiency function
Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 1/18 Efficiency correction from MC Dongliang Zhang [UM] Hadron215 14/9/215 C ij = 1 (4π) 3 ɛ N mc T N mc F i (Ω n)f j (Ω n ) n=1 F i (Ω )F j (Ω)T (Ω, Ω) dω dω Fraction /.1.1.8 Simulation Fraction /.1.1.8 Simulation.6.6.4.4 Flat PDF.2 α b =1 α b =.3 α b =.5.5 1 cosθ 1 Flat PDF.2 α b =1 α b =.3 α b =.5.5 1 cosθ 2
Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 11/18 Event selection & Background 211 7 TeV data Λ b selection Exclude tracks from primary vertex τ >.35 ps Loose Bd veto: P Λ > P b Bd 556 < m < 568 MeV Combinatorial background Estimated from sidebands (SB) Peaking background Bd J/ψ(µ+ µ )KS (π+ π ) Yield from the mass fit F i from Mont Carlo Parameter [534, 59] MeV [556, 568] MeV N sig 14 ± 5 124 ± 4 N Comb 19 ± 8 234 ± 16 N B d 21 ± 9 73 ± 3 54 55 56 57 58 59 Dongliang Zhang [UM] Hadron215 14/9/215 Events / 1 MeV Events /.1 3 25 2 15 1 5 2 15 1 5 s = 7 TeV, Λ b +Λ b N sig = 14 ± 5 N B = 21 ± 9 d N Comb. = 19 ± 8 Prob(χ 2 ) =.89 Left SB L dt = 4.6 fb s = 7 TeV, L dt = 4.6 fb 211 Signal Bd bkg Comb. bkg Right SB m J/ψΛ (Λ ) [MeV] [556, 568] MeV [54, 552] MeV [572, 584] MeV.5.5 1 cos θ 1 F 2
Dongliang Zhang [UM] Hadron215 14/9/215 Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 12/18 Phys. Rev. D 89 (214) 929 Results Correlation Fit results α b =.3 ±.16(stat) ±.6(syst) k + =.21 +.14.21 (stat) ±.13(syst) k =.13 +.2.13 (stat) ±.15(syst) corresponding to A( 1/2, ) =.17 +.12.17 ±.9 A( 1/2, 1) =.59 +.6.7 ±.3 A( 1/2, ) =.79 +.4.5 ±.2 A( 1/2, 1) =.8 +.13.8 ±.6 Main systematic uncertainties MC statistics Background shape modeling value min χ 2 6 5 4 3 2 1 Parameter α b k + k α b 1.41.19 k + 1.2 k 1 3σ 2σ s = 7 TeV, L dt = 4.6 fb pqcd: (.14.17) Λ b + Λ b best α b =.3 best χ 2 (α )=3.15 min b HQET:.78.5.5 1 LHCb.5 ±.17 ±.7 Between two theory predictions Consistent with LHCb results α b Ref: LHCb [Phys. Lett. B 724 (213) 27]; pqcd [Phys. Rev. D 65 (22) 743]; HEQT [Phys. Lett. B 614 (25) 165173];
Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 13/18 Λ b ψ(2s)λ selection Fiducial range p T (Λ b ) > 1 GeV, η(λ b ) < 2.1 (Kinematic range) p T (µ ± ) > 4 GeV, η(µ ± ) < 2.3 p T (Λ ) > 2.5 GeV Other selectionc cuts p T (π ) >.45 GeV τ >.35 ps, P(Λ b ) > P(B d ) Events / 1 MeV 14 s = 8 TeV, 2.6 fb 12 Λ b + Λ b N = 694 ± 13 1 sig 8 Λ b J/ψΛ Λ b signal B reflection Events / 1 MeV 5 s = 8 TeV, 2.6 fb 4 N sig = 854 ± 84 3 B d J/ψK S B signal Λ b reflection 6 4 2 2 1 54 55 56 57 58 59 m(j/ψ Λ (Λ )) [MeV] 51 52 53 54 55 S m(j/ψ K ) [MeV] Dongliang Zhang [UM] Hadron215 14/9/215
Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 14/18 Λ b ψ(2s)λ selection Dongliang Zhang [UM] Hadron215 14/9/215 Events / 15 MeV 3 25 2 15 s = 8 TeV, 2.6 fb Λ b + Λ b = 63 ± 38 N sig Λ b ψ(2s)λ Bd ψ(2s)k S Λ b signal B reflection Events / 15 MeV 1 8 6 s = 8 TeV, 2.6 fb = 124 ± 28 N sig B signal Λ b reflection 1 4 5 2 54 55 56 57 58 m(ψ(2s) Λ (Λ )) [MeV] 51 52 53 54 55 S m(ψ(2s) K ) [MeV] Simultaneous Λ b and B d mass fit Signal: modified Gaussian p(x) = exp[.5 x 1+1/(1+.5 x) ], x = (m m )/σ Bd and Λ b reflection using MC templates Non-resonant background: exponential functions
Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 15/18 Events / 2 MeV 25 2 15 1 5 s = 8 TeV, 2.6 fb Λ b + Λ b = 977 ± 12 N sig J/ψ signal Background 27 28 29 3 31 32 33 34 m(µ + µ ) [MeV] Events / 2 MeV 18 16 14 12 1 8 6 4 2 s = 8 TeV, 2.6 fb Λ b + Λ b = 724 ± 45 N sig ψ(2s) signal Background 34 35 36 37 38 39 4 41 42 m(µ + µ ) [MeV] Events / 2 MeV 25 s = 8 TeV, 2.6 fb Λ J/ψ Λ 2 b N sig = 771 ± 12 15 Λ signal Background Events / 2 MeV 25 2 15 s = 8 TeV, 2.6 fb Λ b ψ(2s) Λ = 72 ± 38 N sig Λ signal Background 1 1 5 5 16 18 11 112 114 116 118 m(pπ ) [MeV] 16 18 11 112 114 116 118 m(pπ ) [MeV] Dongliang Zhang [UM] Hadron215 14/9/215
Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 16/18 Branching ratio Dongliang Zhang [UM] Hadron215 14/9/215 Yields from fits Λ b J/ψΛ B J/ψKS Λ b ψ(2s)λ B ψ(2s)ks N sig 694 ± 13 854 ± 84 63 ± 38 124 ± 28 m sig [MeV] 562.4 ±.4 5274.7 ± 2.3 5618.2 ± 1.2 5272.4 ± 4.9 σ sig [MeV] 19.7 ±.5 19.2 ± 2.2 14.3 ± 1.1 16.7 ± 4.1 Acceptance correction N cor = N sig /A Efficiency (A eff ) and acceptance (A kin ) in fiducial range p T (Λ b ) > 1 GeV, η(λ b ) < 2.1 p T (µ ± ) > 4 GeV, η(µ ± ) < 2.3 p T (Λ ) > 2.5 GeV Channel A eff [%] A kin [%] Λ b J/ψΛ 4.16 ±.2 7.57 ±.6 Λ b ψ(2s)λ 4.3 ±.3 9.61 ±.7
Dongliang Zhang [UM] Hadron215 14/9/215 Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 17/18 arxiv:157.822 [hep-ex] Branching ratio In kinematic range p T (Λ b ) > 1 GeV, η(λ b ) < 2.1: Γ(Λ b ψ(2s)λ ) Γ(Λ b J/ψΛ ) = N cor(λ b ψ(µ+ µ )Λ ) N cor (Λ b J/ψ(µ+ µ )Λ ) B(J/ψ l + l ) B(ψ(2S) l + l ) =.51 ±.33(stat) ±.16(syst) ±.11(B) B(J/ψ µ + µ ) =.5961 ±.33 [PDG] B(ψ(2S) µ + µ ) = B(ψ(2S) e + e ) =.789 ±.17 [PDG] Systematic uncertainties Signal extraction fits (2.8%) MC statistics (1.3%) Λ b polarization model (1.1%) In the range.5.8 for B meson decays; smaller than calculation.8 ±.1 [Phys. Rev. D 88 (213) 11418]
Introduction Λ b reconstruction Lifetime measurement Helicity study Λ b ψ(2s)λ Summary 18/18 Summary Dongliang Zhang [UM] Hadron215 14/9/215 Studying Λ b production and decay using data Lifetime measurement α b measurement m Λ b τ Λ b = 5619.7 ±.7(stat) ± 1.1(syst) MeV = 1.449 ±.36(stat) ±.17(syst) ps α b =.3 ±.16(stat) ±.6(syst) a + =.17 +.12.17 (stat) ±.9(syst) a =.59 +.6.7 (stat) ±.3(syst) b + =.79 +.4.5 (stat) ±.2(syst) b =.8 +.13.8 (stat) ±.6(syst) Observation of Λ b ψ(2s)λ and the measured branching ratio Γ(Λ b ψ(2s)λ ) Γ(Λ b J/ψΛ ) =.51 ±.33(stat) ±.16(syst) ±.11(B)
Backup 19/18 Backup: Efficiency correction of lifetime measurement Dongliang Zhang [UM] Hadron215 14/9/215 Efficiency.28.26 Efficiency.5.45.24.22 τ.2 c Λb ε(τ ) ~ e Simulation s = 7 TeV c Λb = 113 ± 56 ps.18 1 2 3 4 5 6 τ (ps).4.35.3 Simulation s = 7 TeV τ c Bd ε(τ ) ~ e c Bd = 94 ± 43 ps 1 2 3 4 5 6 τ true (ps)
Dongliang Zhang [UM] Hadron215 14/9/215 Backup: B d lifetime measurement Backup 2/18 Candidates / 13 MeV 4 = 5279.6 ±.2 MeV 35 3 25 2 15 s = 7 TeV L = 4.9 fb Signal Background M Bd S M σ M = 1.11 ±.1 = 27.8 ±.2 MeV N sig = 1753 ± 11 N bkg = 169 ± 65 χ 2 /N dof = 1.3 Candidates /.46 ps 4 1 3 1 2 1 s = 7 TeV L = 4.9 fb τ Bd S τ σ τ = 1.59 ±.12 ps = 1.6 ±.2 =.17 ±.3 ps χ 2 /N dof = 1.3 Signal Background 1 5 51 515 52 525 53 535 54 545 55 (MeV) M J/ψ K S 1 2 2 4 6 8 1 12 14 τ (ps)
Dongliang Zhang [UM] Hadron215 14/9/215 Backup: full PDF Backup 21/18 i f 1i f 2i F i a + a+ + a a + b + b + + b b 1 1 1 a + a+ a a + b + b + b b P cos θ 2 a + a+ a a b + b + + b b α Λ cos θ 1 3 a + a+ + a a b + b + b b P α Λ cos θ cos θ 1 4 a + a+ a a + 1 2 b + b + + 1 2 b b 1 1 2 (3 cos2 θ 2 1) 1 5 a + a+ + a a + 1 2 b + b + 1 2 b b P 2 (3 cos2 θ 2 1) cos θ 6 a + a+ + a a 1 2 b + b + + 1 2 b b α Λ 1 2 (3 cos2 θ 2 1) cos θ 1 7 a + a+ a a 1 2 b + b + 1 2 b b 1 P α Λ 2 (3 cos2 θ 2 1) cos θ cos θ 1 8 3Re(a + a ) P α Λ sin θ sin θ 1 sin 2 θ 2 cos φ 1 9 3Im(a + a ) P α Λ sin θ sin θ 1 sin 2 θ 2 sin φ 1 1 3 2 Re(b b + ) P α Λ sin θ sin θ 1 sin 2 θ 2 cos(φ 1 + 2 φ 2 ) 3 11 2 Im(b b + ) P α Λ sin θ sin θ 1 sin 2 θ 2 sin(φ 1 + 2 φ 2 ) 12 3 Re(b 2 a+ + a b + ) P α Λ sin θ cos θ 1 sin θ 2 cos θ 2 cos φ 2 3 13 Im(b 2 a+ + a b + ) P α Λ sin θ cos θ 1 sin θ 2 cos θ 2 sin φ 2 Re(b 2 a + a + b + ) P α Λ cos θ sin θ 1 sin θ 2 cos θ 2 cos(φ 1 + φ 2 ) 14 3 15 16 3 2 Im(b a + a + b + ) P α Λ cos θ sin θ 1 sin θ 2 cos θ 2 sin(φ 1 + φ 2 ) 3 2 Re(a b + b a + ) P sin θ sin θ 2 cos θ 2 cos φ 2 17 3 2 Im(a b + b a + ) P sin θ sin θ 2 cos θ 2 sin φ 2 18 3 2 Re(b a a + b + ) α Λ sin θ 1 sin θ 2 cos θ 2 cos(φ 1 + φ 2 ) 19 3 2 Im(b a a + b + ) α Λ sin θ 1 sin θ 2 cos θ 2 sin(φ 1 + φ 2 )
Backup 22/18 MC weighted using the fit results Events /.1 2 18 16 14 12 1 8 6 4 2 s = 7 TeV, L dt = 4.6 fb χ 2 test prob:.98 Λ b +Λ b data Reweighted Λ b Pythia Λ b Background cos θ 1 1 1.5.5 1 Events /.5 35 3 25 2 15 5 s = 7 TeV, L dt = 4.6 fb χ 2 test prob:.57 2 (3 cos2 θ 2 1) Λ b +Λ b data Reweighted Λ b Pythia Λ b Background.5.5 1 F 2 F 4 Events /.4 25 2 15 s = 7 TeV, L dt = 4.6 fb χ 2 test prob:.8 Λ b +Λ b data Reweighted Λ b Pythia Λ b Background Events /.2 14 12 1 8 s = 7 TeV, L dt = 4.6 fb χ 2 test prob:.63 Λ b +Λ b data Reweighted Λ b Pythia Λ b Background 1 6 5 4 2.5.5 1.4.2.2.4 1 F 6 2 sin θ 1 1 sin θ 2 cos θ 2 cos(φ 1 + φ 2 ) 2 sin θ 1 sin θ 2 cos θ 2 sin(φ 1 + φ 2 ) Dongliang Zhang [UM] Hadron215 14/9/215 F 18
Sidebands comparison Events /.1 4 3 2 s = 7 TeV, L dt = 4.6 fb [556, 568] MeV [54, 552] MeV [572, 584] MeV Backup 23/18 Events /.4 3 2 s = 7 TeV, L dt = 4.6 fb [556, 568] MeV [54, 552] MeV [572, 584] MeV 1 1.5.5 1.5.5 1 1 2 (3 cos2 1 θ 2 1) F 4 2 (3 cos2 θ 2 1) cos θ 1 F 6 Events /.5 3 2 s = 7 TeV, L dt = 4.6 fb [556, 568] MeV [54, 552] MeV [572, 584] MeV Events /.5 3 2 s = 7 TeV, L dt = 4.6 fb [556, 568] MeV [54, 552] MeV [572, 584] MeV 1 1.4.2.2.4 1 F 18 2 (sin θ 1 1 sin θ 2 cos θ 2 cos(φ 1 + φ 2 ).4.2.2.4 2 (sin θ 1 sin θ 2 cos θ 2 sin(φ 1 + φ 2 ) Dongliang Zhang [UM] Hadron215 14/9/215 F 19
Backup 24/18 Other angular variables Fraction /.1.1.8.6 Simulation Fraction / (π/16) rad.6.5.4.3 Simulation.4 Flat PDF.2 α b =1 α b =.3 α b =.5.5 1 cosθ.2 Flat PDF.1 α b =1 α b =.3 α b = 3 2 1 2 3 φ [rad] 1 Fraction / (π/16) rad.6.5.4.3 Simulation.2 Flat PDF.1 α b =1 α b =.3 α b = 3 2 1 2 3 Flat PDF.1 α b =1 α b =.3 α b = 3 2 1 2 3 φ [rad] φ +φ [rad] Dongliang Zhang [UM] 2 Hadron215 1 2 14/9/215 Fraction / (π/16) rad.5.4.3.2 Simulation
Backup 25/18 Backup: Λ b ψ(2s)λ selection Dongliang Zhang [UM] Hadron215 14/9/215 Λ b and Λ b Events / 1 MeV 8 7 6 5 4 3 2 1 s = 8 TeV, 2.6 fb Λ b N sig = 3523 ± 89 Λ b signal B reflection Events / 1 MeV 7 6 5 4 3 2 1 = 8 TeV, 2.6 fb s Λ b N sig = 3414 ± 92 Λ b signal B reflection 54 55 56 57 58 59 m(j/ψ Λ ) [MeV] 54 55 56 57 58 59 m(j/ψ Λ ) [MeV] Threshold function A (m m p m π ) B exp[c (m m p m π ) + D (m m p m π ) 2 ]