1. Wolfgang Menges, Queen Mary Measuring |V ub | from Semileptonic B Decays Wolfgang Menges Queen Mary, University of London, UK Institute of Physics: Particle Physics 2006 11 th April 2006

2. Wolfgang Menges, Queen Mary V ub Introduction CP violation is described by the CKM matrix in the SM the consistency of the Unitarity Triangle is tested by measuring its angles and sides one side is related to |V ub | |V ub | can be measured in semileptonic, charmless B decays Experimental challenge: suppress continuum background suppress charm decay Theoretical challenge: extrapolate from partial to total BF

3. Wolfgang Menges, Queen Mary Unitarity Triangle Fits B d mixing:  m d B s mixing:  m s /  m d J/  K 0 : sin2  b  ul : |V ub | D*l : |V cb | kaon decays: ε K http://ckmfitter.in2p3.fr charmless two-body:  interference in B  DK: 

4. Wolfgang Menges, Queen Mary PEP2 and BaBar Detector Čerenkov Detector (DIRC) 144 quartz bars 11000 PMTs 1.5 T solenoid ElectroMagnetic Calorimeter 6580 CsI(Tl) crystals Drift CHamber 40 stereo layers Instrumented Flux Return iron/RPCs/LSTs (muon/neutral hadrons) Silicon Vertex Tracker 5 layers, double sided strips e + (3.1 GeV) e - (9 GeV) Collected luminosity: ~ 330 fb -1 Analysed Luminosity: 80/210 fb -1

5. Wolfgang Menges, Queen Mary Inclusive b → ul  Strategies B → X u l decays are described by 3 variables u quark turns into one or more hadrons q 2 = lepton-neutrino mass squared m X = hadron system mass E l = lepton energy Not to scale! Signal events have smaller mX  larger El and q2

6. Wolfgang Menges, Queen Mary Lepton Endpoint Select electrons with 2.0 < E l < 2.6 GeV –Push below the charm threshold  Larger signal acceptance  Smaller theoretical error –Accurate subtraction of background is crucial! off-resonance data events with p e > 2.8 GeV fit b->clv composition in bkg subtraction –Measure the partial BF –S/B =1/15 for the endpoint E l > 2.0 GeV B A B AR total BF is ~2  10  3 B A B AR PRD73:012006,2006 MC signal b  ulv Data – bkgd. Data MC bkgd. b  clv  B (10 -4 ) = 5.72 ± 0.41 stat ± 0.65 sys Luminosity: 80 fb -1

7. Wolfgang Menges, Queen Mary E l and q 2 Try to improve signal-to-background Use p v = p miss in addition to p e  calculate q 2 –Define s h max (E l, q 2 ) = the maximum m X squared Cutting at s h max < m D 2 removes b  clv while keeping most of the signal –S/B = 1/2 achieved for E l > 2.0 GeV and s h max < 3.5 GeV 2 b  clvb  clv E l (GeV) q 2 (GeV 2 ) b  ulvb  ulv Smaller systematic errors B A B AR Extract signal normalize bkg Measured partial BF: B A B AR PRL 95:111801  B (10 -4 ) 3.54 ± 0.33stat ± 0.34 sys Luminosity: 80 fb -1

8. Wolfgang Menges, Queen Mary m X and q 2 Must reconstruct all decay products to measure m X or q 2 –Use (fully-reconstructed) hadronic B tag Suppress b → c l v by vetoing against D ( * ) decays –Reject events with K –Reject events with B 0 → D * + ( → D 0  + ) l − v Measure the partial BF in regions of (m X, q 2 ) – m X 8 GeV 2 :  B (10 -4 ) = 8.7 ± 0.9 stat ± 0.9 sys Luminosity: 211fb -1 B A B AR hep-ex/0507017 lepton v X

9. Wolfgang Menges, Queen Mary |V ub | from Branching Fraction Results of different calculations/ HQE parameters using the same partial branching fraction: For converting a branching fraction into |V ub |the phase space acceptance is needed: 1) BLNP:BaBar B  X c l moments m b (SF) = 4.61 ± 0.08 GeV  π 2 (SF) = 0.15 ± 0.07 GeV 2 3) BLL: BaBar B  X c l moments m b (1S) = 4.74 ± 0.06 GeV 2) BLNP:Belle B  X s  spectrum m b (SF) = 4.52 ± 0.07 GeV  π 2 (SF) = 0.27 ± 0.23 GeV 2 4) DGE: HFAG average m b (MS) = 4.20 ± 0.04 GeV 3) |V ub |=(4.82±0.36 exp ±0.46 SF+theo )  10 -3 2) |V ub |=(4.65±0.34 exp ±0.23 theo )  10 -3 +0.46 –0.38 SF 1) |V ub |=(5.00±0.37 exp ±0.46 SF ±0.28 theo )  10 -3 4) |V ub |=(4.86±0.36 exp ±0.22 theo )  10 -3 Example for m x /q 2

10. Wolfgang Menges, Queen Mary Status of Inclusive |V ub | |V ub | determined to  7.4% –The SF parameters can be improved with b → s , b  clv measurements –What ’ s in the theory error? Subleading SF and Weak Annihilation effects. |V ub | world average summer 2006 Statistical  2.2% Expt. syst.  2.7% b  clv model  1.9% b  ulv model  2.1% SF params. (m b,   2 )  4.1% Theory  4.2% BLNP + SF from b->clv, b->s 

11. Wolfgang Menges, Queen Mary Exclusive b  ul Measure specific final states, e.g., B →  lv –Can achieve good signal-to-background ratio –Branching fractions are O(10 -4 )  statistics limited Need form factors to extract |V ub | –Theo. Uncertainties complementary to inclusive approach ! f + (q 2 ) calculations exist based on: –Lattice QCD (q 2 > 15 GeV 2 ) “ quenched ” calculations  15% uncertainty new unquenched calculations (hep-lat/0409116,0408019) –Light Cone Sum Rules (q 2 < 14 GeV 2 )  10% uncertainty –Quark models (ISGW2) … and other approaches One FF for B →  lv (massless lepton)

12. Wolfgang Menges, Queen Mary Status of Exclusive |V ub | use BF and FF predictions to calculate |V ub | needs also assumption of shape of full q 2 range All calculations consistent! incl. |V ub |[10 -3 ] = 4.45  0.20  0.26 (BLNP)

13. Wolfgang Menges, Queen Mary Precise determination of |V ub | complements sin2  to test the validity of the Standard Model – 7.4% accuracy achieved so far  5% possible? Close collaboration between theory and experiment is important Inclusive |V ub | : – Much exp. and theo. progress in the last 2 years Exclusive |V ub | : –Significant exp. progress in the last year, but further improvement in B →  FF calculations needed - Accuracy of  for exclusive |V ub | in the next few years (?) Important to cross-check inclusive vs. exclusive results The Unitarity Triangle: 2004  2006