Anomalies and deviations in heavy-flavour physics @GreigCowan (Edinburgh) Birmingham, Dec 2nd 2015
• Introduction to the LHCb experiment • b → sl + l − FCNC decays • Lepton (non-)universality • CP violation in the beauty + charm systems 2 / 60
The LHC https://ideas.lego.com/projects/94885 3 / 60
The LHCb detector 2008 JINST 3 S08005 Covers 4% of solid angle, but accepts 40% of heavy quark production cross section. 4 / 60
A typical LHCb event [2008 JINST 3 S08005] � nPV s � ∼ 2 . 0 σ ( pp → bbX ) ∼ 80 µb � nTracks � ∼ 200 σ ( cc ) ∼ 1500 µb ~ 1 cm b p p HLT2 DiMuon trigger b 5 / 60
Run-1 data sample 2012 (2 fb − 1 @8TeV) ∼ 900 physicists from 64 Efficiency > 93% universities/laboratories in 16 2011 countries. (1 fb − 1 @7TeV) O (100k) bb pairs produced/sec. 2010 LHCb designed to run at lower luminosity than ATLAS/CMS. LHCb tracking/PID is sensitive to pile-up. LHC pp beams are displaced to reduce instantaneous luminosity - stable running conditions. �L� 2011 ∼ 2 . 7 × 10 32 cm − 2 s − 1 �L� 2012 ∼ 4 . 0 × 10 32 cm − 2 s − 1 6 / 60
Searching for New Physics ON-SHELL OFF-SHELL Cannot produce particles Higher energy particles can with mc 2 > E appear virtually in quantum loops → flavour physics NP? History: top quark mass predicted by quark mixing 7 / 60
Rare (FCNC) b -hadron decays 8 / 60
b → s transitions b → s “penguin” decays are loop/CKM suppressed. FCNC can be crucial to finding out where to look for NP. Model independent effective Hamiltonian, where heavy degrees of freedom have been integrated out in short-distance H eff = − 4 G F α e V tb V ∗ � C i O i + C ′ i O ′ � � √ Wilson coefficients, ( C i ). ts i 2 4 π i B 0 → K ∗ (892) 0 µ + µ − O 9 ( ′ ) = [ sγ µ P L ( R ) b ][ lγ µ l ] q 2 ≡ m ( l + l − ) 2 [Blake, Gershon, Hiller, Annu. Rev. Nucl. Part. Sci. 2015] 9 / 60
B 0 → K ∗ (892) 0 µ + µ − [LHCb-PAPER-2015-051] 20 ] 4 2398 ± 57 events, excluding the charmonia. c LHCb 18 / 4 10 2 [GeV 16 � Ω ≡ (cos θ l , cos θ K , φ ) 14 2 3 10 q 12 10 10 2 8 6 10 4 2 0 1 5.2 5.3 5.4 5.5 5.6 5.7 � m ( K + - + - ) [GeV/ c ] 2 µ µ Di-muon final state is experimentally clean signature, but BR ∼ 10 − 7 . P → V V ′ decay, fully described by q 2 ≡ m ( µ + µ − ) 2 and 3 helicity angles. B 0 → K ∗ µ + µ − has rich system of observables (rates, angles, asymmetries) that are sensitive to NP. d 4 Γ[ B 0 → K ∗ 0 µ + µ − ] 11 = 9 � I j ( q 2 ) f j ( � I j → I j for B 0 Ω) , d q 2 d � 32 π Ω j =1 � � � dΓ � � � dΓ d q 2 + d¯ d q 2 + d¯ � � Γ Γ I j + ¯ I j − ¯ � � S j = I j , A j = I j d q 2 d q 2 10 / 60
B 0 → K ∗ (892) 0 µ + µ − [LHCb-PAPER-2015-051] 2 2 c LHCb c LHCb Events / 5.3 MeV/ Events / 10 MeV/ 2 2 1.10 < q 2 < 6.00 GeV / c 4 1.10 < q 2 < 6.00 GeV / c 4 100 100 Describe m ( Kπ ) with Breit-Wigner for P- 50 50 wave and LASS for S- wave K + π − 0 0 5200 5400 5600 0.8 0.85 0.9 0.95 π − µ µ − π − + + + m ( K ) [MeV/ c 2 ] m ( K ) [GeV/ c 2 ] π Events / 0.1 Events / 0.1 LHCb LHCb LHCb Events / 0.1 2 2 2 1.10 < q 2 < 6.00 GeV / c 4 1.10 < q 2 < 6.00 GeV / c 4 1.10 < q 2 < 6.00 GeV / c 4 100 100 100 50 50 50 0 0 0 -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 -2 0 2 θ θ φ cos cos [rad] l K S i , A i ’s extracted using a max likelihood fit. /c 2 around K ∗ (892) 0 . Example fits in ± 50 MeV For the first time the Kπ S-wave is accounted for. 11 / 60
B 0 → K ∗ (892) 0 µ + µ − : some observables [LHCb-PAPER-2015-051] 1 L 5 F S 0.5 LHCb LHCb 0.8 SM from ABSZ SM from ABSZ 0.6 0 0.4 0.2 S 6 s ≡ 4 S 1 c ≡ F L 3 A FB -0.5 0 0 5 10 15 0 5 10 15 2 2 2 [GeV / 4 ] 2 [GeV / 4 ] q c q c 3 4 A A 0.5 0.5 LHCb LHCb 0 0 + many other observables not shown -0.5 -0.5 0 5 10 15 0 5 10 15 2 2 2 [GeV / 4 ] 2 [GeV / 4 ] q c q c Some observables have physical boundaries ⇒ use Feldman-Cousins for uncertainties. CP -asymmetries consistent with zero, as expected, but some deviations in CP -averaged observables (the S j ’s). 12 / 60
B 0 → K ∗ (892) 0 µ + µ − : the anomaly [LHCb-PAPER-2015-051] “Theoretically clean” observables less dependent on hadronic form factors [Descotes-Genon et al JHEP 05 (2013) 137] . These divide out the hadronic uncertainties to leading order. S j =4 , 5 , 7 , 8 P ′ i =4 , 5 , 6 , 8 = Tension from the 1 fb − 1 LHCb result remains. � F L (1 − F L ) 1 5 ' P LHCb 0.5 SM from DHMV A χ 2 fit to all CP -averaged 0 observables shows a 3 . 4 σ 2 . 8 σ, 3 . 0 σ from SM shift from SM prediction -0.5 -1 0 5 10 15 2 2 [GeV / 4 ] q c 13 / 60
b → sµ + µ − branching fractions lower than predictions B 0 → K ∗ (892) 0 µ + µ − [JHEP 08 (2013) 131] [JHEP 06 (2014) 133 ] [JHEP 06 (2015) 115] [JHEP 09 (2015) 179] Λ b → Λ µ + µ − (Bham) B 0 s → φµ + µ − 14 / 60
Observation of B 0 s → µ + µ − CKM suppressed and helicity suppressed (( m µ /m B ) 2 ). Dominant uncertainty will be B ( B 0 s → µµ ) SM = (3 . 66 ± 0 . 23) × 10 − 9 improved via refined Lattice B ( B 0 → µµ ) SM = (1 . 06 ± 0 . 09) × 10 − 10 QCD calcs. [PRL 112, 101801 (2014)] Sensitive to scalar and pseudoscalar NP couplings, e.g., in MSSM B ∝ (tan β ) 6 d B 0 s → µ + µ − µ + b ✁ W + Z 0 B 0 t s W − s µ − 30 years of effort! f B 0 s → µ + µ − µ + b ✁ X + X 0 B 0 t s W − s µ − 15 / 60
Observation of B 0 s → µ + µ − [CMS + LHCb, Nature 522, 68-72 (2015)] CMS and LHCb (LHC run I) ) 16 2 c Candidates / (40 MeV/ Data Use multi-variate techniques 14 Signal and background to suppress background. → µ µ − 0 + B 12 s − 0 → µ + µ B Results consistent with SM at B 0 3 . 0 σ Combinatorial bkg. 10 Semileptonic bkg. ∼ 2 σ . 8 Peaking bkg. B 0 Constrains S and P s 6 . 2 σ 6 contributions. 4 One to watch during LHC 2 Run-2. 0 5000 5200 5400 5600 5800 2 m [MeV/ c ] − µ + µ 16 / 60
Global fits for Wilson coeffs [Descotes-G et al, arXiv:1510.04239] Other global fits exist! 2D fit with ( C NP , C NP 9 ′ ) floating 9 → 4 . 5 σ deviation from SM Inputs from branching fractions and angular observables from b → sll decays, BR( B → X s γ ), BR( B 0 s → µ + µ − ),. . . . Many fits performed with different subsets of the observables and different theoretical inputs (form factors, power corrections, charm loops). C NP < 0 plays central role explaining many deviations seen in b → sll transitions. 9 Possible Z ′ ? Leptoquarks? [many authors] How well do we understand QCD-effects? [Lyon, Zwicky] 17 / 60
Lepton universality R K ≡ B ( B + → K + µ + µ − ) B ( B + → K + e + e − ) ,. . . 18 / 60
Lepton universality ( B + → K + l + l − ) [PRL 113,151601 (2014)] In the SM only the Higgs boson has non-universal lepton couplings. This results in SM predictions of ∼ unity for various decay-rate ratios. R K ≡ B ( B + → K + µ + µ − ) SM = 1 ± O (10 − 2 ) B ( B + → K + e + e − ) 2 . 6 σ deviation Can be described assuming NP only in b → sµµ . Very interesting given indications of non-SM physics in other b → sµµ FCNC decays and 2 . 4 σ excess in H → τµ at CMS [PLB 749 (2015) 337] . Future: Make similar measurements using other decays - R ( φ ) , R ( K ∗ ) , R (Λ) (Bham). 19 / 60
Lepton universality ( B 0 → D ∗ + lν ) CKM mechanism well tested, but room for NP if coupling more to 3rd generation (e.g., charged Higgs). B-factories already reporting deviation from theoretically clean SM prediction. Form-factors cancel in the ratio. Tree-level int., unlike b → sll FCNC R ( D ∗ ) ≡ B ( B 0 → D ∗ + τν τ ) B ( B 0 → D ∗ + µν µ ) Interesting given hints of non-universality in B + → K + l + l − decays ( R K ) and excl/incl measurements of V ub , V cb . 20 / 60
Lepton universality ( B 0 → D ∗ + lν ) [PRL 115, 111803 (2015)] Very challenging measurement at hadron collider (no beam constraints and large m (GeV /c ) miss backgrounds). 4000 Candidates / (75 MeV) ) 4 9.35 < q 2 < 12.60 GeV 2 /c 4 LHCb /c 2 Candidates / (0.3 GeV B ( τ → µν µ ν τ ) = (17 . 41 ± 0 . 04)% 3000 Signal and normalisation have same final 2000 state particles. Large samples of events, triggering on charm. 1000 Require significant B, D, τ flight distances. Use Pulls 2 -2 0 2 4 6 8 10 isolation MVA. -2 -2 0 2 4 6 8 10 Template fit to kinematic variables → 2 4 m 2 (GeV /c ) miss E * (MeV) µ Candidates / (75 MeV) 4000 2 2 4 9.35 < q < 12.60 GeV /c LHCb 3000 2000 1000 Pulls 10 2 500 1000 1500 2000 2500 -2 10 500 1000 1500 2000 2500 E * (MeV) µ R ( D ∗ ) ≡ B ( B 0 → D ∗ + τν τ ) B ( B 0 → D ∗ + µν µ ) 21 / 60
Recommend
More recommend
