Federica Legger Polarized radiative  b decays at LHCb

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  • Slide 1
  • Federica Legger Polarized radiative b decays at LHCb
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  • 2 Outline Theoretical motivations Angular distributions Observables b production at LHC Selection of b (x) events at LHCb Status and perspectives
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  • 3 Theoretical motivations b p s d u b d u bb b (X) p k s d u b d u bb u u p k Electromagnetic penguin b s In the SM the photon is predicted to be left-handed, but could have a right-handed component in LR symmetric models; Effective Hamiltonian at LO in s : leftright
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  • 4 Photon polarization measurements Melikov, Nikitin, Simula, PLB 442, 381 (1998) Grossman, Pirjol, JHEP06, 029 (2000) Atwood, Gronau, Soni, PRL 79, 185 (1997) Mannel, Recksiegel, JPG: NPP 24, 979 (1998) LHCb B factories Knecht, Schietinger, PLB 634, 403 (2006) Gronau, Pirjol, PRD 66, 054008 (2002) B-B interference First measurements of K* polarization in B->K*l+l- by Belle/Babar e + e - conversion Exp. statusTheor. Refs. Latest world average sin2 = 0.0 0.3 Higher K* resonances Difficult to disentangle resonance structure (BaBar, hep/0507031) Gronau, Grossman, Pirjol, PRL 88, 051802 (2002) Charmonium res. interference No results so far... b-baryons Hiller, Kagan, PRD 65, 074038 (2002) Exploit ang. correlations between polarized initial state and final state. Under study at LHCb
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  • 5 Polarized b (1115) decays Angular distributions for b ( (1115) p ) depend on photon polarization and constrain Hiller, Kagan, Phys Rev D65, 074038 (2002) P B = b polarization p = weak decay parameter Evtgen distribution
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  • 6 2 indipendent measurements for r, or b polarization measurement: a discrepancy with the value measured in semileptonic b c l X decays would indicate the presence of non-standard right-handed b c currents Direct CP violation at NLO: O(1%) in SM but 10% if NP! r probes the ratio of CP even contributions to NLO Hamiltonian Observables for b (1115)
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  • 7 (X) resonances (X) parameters (PDG 2004) + BR( b (X) ) (slide 16) 1690 1520 1670 1600 spin = 1/2 spin = 3/2 From the experimental point of view the decay b (1115) is quite hard to observe (c = 7.89 cm) Can we increase the statistics by using heavier resonances?
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  • 8 Need angular distributions helicity formalism Helicity formalism for b (px) b polarization Polarization density matrix : Helicity amplitudes b (X) (X) px + - - Jacob, Wick, Ann Phys 7, 404 (1959)
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  • 9 Results : J = 1/2 Photon angular distribution depends on photon helicity parameter which is related to |r| Proton angular distribution flat because of P conservation Helicity formalism HQET
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  • 10 Results : J = 3/2 helicity can now assume the values: 1/2, 3/2 4 helicity amplitudes
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  • 11 Helicity formalism : J = 3/2 Decay probability:
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  • 12 Similar dependence to spin 1/2 resonances but now depends on the asymmetry of b baryons produced with different helicities Photon angular distribution
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  • 13 Photon helicity is independent of the helicity of the final state formed in the hadronization process Because of parity conservation in strong interactions, the ratio of baryons produced with helicity 3/2 and 1/2 = ratio of baryons produced with helicity -3/2 and -1/2 Assumptions
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  • 14 Photon angular distribution dependence can be factorized with the photon helicity parameter and strong parameter no theoretical predictions for but we can extract it from the proton angular distribution Results : J = 3/2
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  • 15 (X) with helicity 3/2 dominates 1 same amount of 1/2 and 3/2 helicity 1 (X) with helicity 1/2 dominates 1 Possible scenarios for 3/2 - 1/2 p 3/2 1 3/2 1/2 p 3/2 -3 3/2 p 3/2 0 |r| can be probed by measuring 3/2 and SM prediction
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  • 16 b production at LHC: bb cross section in pp collision = 500 b 10% of produced bb hadronize in B hadrons b dominates ( 90%) b produced with transversal polarization Predictions are P B ~ 20% ATLAS plans to measure it with a statistical precision better than 1% BR ( b (1115) = 4.15 10 -5 BR ( b (1520) = 1.30 10 -5 BR ( b (1670) = 0.70 10 -5 BR ( b (1690) = 0.70 10 -5 p1p1 p2p2 bb n Ajaltouni, Conte, Leitner, b into -vector decays , Phys Lett B, 614 (2005) Feasibility of Beauty Baryon Polarization Measurement in b J decay channel by ATLAS Atlas note Calculations based on Hiller (2002)+PDG2004
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  • 17 Data sample & Tools DaVinci v12r16 No particular method to optimize cuts values: PT, IPS (with respect to all primaries) cuts on final states Mass window = 4 for resonances with intrinsic width Cut values chosen to kill bb events while maintaining higher possible efficiencies b (1115) pol = long, full300k evts b (1670) pol = long, full 300k evts b (1670) pol = transv, full 300k evts b (1670) phsp, full 300k evts bb inclusive (DC04-v2) 39M evts
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  • 18 (1115) p c = 7.89 cm About 14% of interact before decay or decay after LHCb spectrometer lost 305000 (generated) 262464 (DoI) T1 T2T3 VELO TT T track Upstream track Long track Downstream track Velo track B Y (T) z (m) - 0.2 - 0.4 - 0.6 - 0.8 - 1.0 - 1.2 0 0 2 46 8 candidates (associated to MC truth)
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  • 19 Vertex fit for b (1115) (1115) vertex: refit the vertex explain apply cut on mass and unconstrained chi square b vertex = (1115) + photon fit: PV + the direction Choose the PV with minimum chi square p PV
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  • 20 UP DLL p- > 6 DLL p-k > 0 PT > 500 MeV sIPS > 4 Charged tracks selection ( (1115)) p: DOWN DLL p- > 10 DLL p-k > 8 PT > 2500 MeV sIPS > 3 LONG DLL p- > 6 DLL p-k > 4 PT > 1600 MeV sIPS > 4 UP PT > 250 MeV sIPS > 4 :: DOWN PT > 350 MeV sIPS > 3 LONG PT > 350 MeV sIPS > 4 Hard DLL and PT cuts on protons to suppress background Slow momentum pions
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  • 21 UL 2 < 2 m < 27 MeV PT > 500 MeV sIPS > 4 FS > 5 (1115) selection LL 2 < 6 m < 6 MeV PT > 500 MeV sIPS > 4 FS > 4 LD 2 < 2 m < 6 MeV PT > 1500 MeV DD 2 < 2 m < 11 MeV PT > 2000 MeV sIPS > 3 FD > 300 mm = 1.2 MeV = 2.8 MeV
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  • 22 b ( (1115) p ) LL PT > 2500 MeV 2 < 2 m < 300 MeV ( b ) < 0.15 UL PT > 500 MeV 2 < 2 m < 300 MeV ( b ) < 0.15 LD PT > 1000 MeV 2 < 1 m < 300 MeV ( b ) < 0.15 PT > 3200 MeV/c (LL) PT > 3400, 3800 MeV/c for UL, DD, LD PT (in b direction) [2250, 3000] MeV/c selection b selection DD PT > 2000 MeV 2 < 1 m < 300 MeV ( b ) < 0.15
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  • 23 Efficiencies for b (1115) tot = 0.011 % no events selected in 39M bb incl Yield = 747 / year B/S < 42 @ 90 % CL = 78.1 MeV 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 x 10 -4 BR measurement
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  • 24 b (1670) selection Protons: Only Long tracks DLL p- > 5 DLL p-K > 0 Exclusive DLL selection PT > 600 MeV sIPS > 3 Kaons: Only Long tracks DLL K- > 5 DLL K-p > 0 Exclusive DLL selection PT > 600 MeV sIPS > 3 PT > 2600 MeV 1600 MeV < PT (in b direction) < 2800 MeV (1670) : 2 < 6, m < 100 MeV PT > 1500 MeV sIPS > 4 b : m < 200 MeV FS > 2; PT > 2000 MeV ( b ) < 0.01 PV bb p
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  • 25 Efficiencies for b (1670) Phase space tot = 0.225 % Annual yield: 2515 long. pol tot = 0.224 % Annual yield: 2507 trans. pol tot = 0.228 % Annual yield: 2553 no events selected in 39M bb incl. B/S = 18.2 @ 90% CL TDR, after L0 x L1 B s , tot = 0.220 % B d K* , tot = 0.156 % After HLT Generic B s,d /K* , = 64 MeV = 69.4 MeV
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  • 26 Photon polarization b (1670) selected evts. transversally polarized) efficiency corrected (from unpolarized decays)
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  • 27 Statistical sensitivity on |r| Still far from the SM expected value, but interesting if NP is present! LHCb could be the first to measure the photon polarization in b-> s transitions b (1115) b (1670)
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  • 28 Conclusions Selection for b (1115) and b (1670) ready BR studies feasible Angular asymmetries studies ongoing: promising photon polarization measurement Can we separate the (1670) and the (1690)? Still observing these resonances could give indications on their production mechanism... LHCb note(s) in preparation
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  • Federica Legger Backup slides
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  • 30 Photon polarization (eff. correction) b (1670) transversally polarized) b (1670) phase space) efficiency
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  • 31 b ( (1670) p K) Charged tracks: false p true p Protons: Only Long tracks DLL p- > 5 DLL p-k > 0 Exclusive DLL Kaons Only Long tracks DLL K- > 5 DLL K-p > 0 Exclusive DLL false K true K bb incl signal bb incl signal
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  • 32 b ( (1670) p K) Charged tracks: bb incl signal bb incl signal Protons: PT > 600 MeV sIPS > 3 Kaons PT > 600 MeV sIPS > 3 bb incl signal bb incl signa