Selected results on heavy flavour physics at LHCb Matthew CHARLES - PowerPoint PPT Presentation

! Selected results on heavy 
 flavour physics at LHCb Matthew CHARLES (UPMC/LPNHE) 1

Plan • Quick intro to heavy flavour physics & LHCb • Searches for CP violation in charm decays • Baryon spectroscopy • Onward and upward 2

Introduction 3

Particle Physics today • Our strawman theory, the Standard Model, works embarrassingly well. • We know that it's incomplete, that there is New Physics to discover. • Matter-antimatter asymmetry; dark matter; dark energy; neutrino masses; gravity; etc • Our collective goal today: discover that NP . • Our goal for tomorrow: understand it. 4

The Standard Model The fundamental particles... u c t quarks γ W ± d s b Z g μ τ e leptons H bosons ν e ν μ ν τ [... plus their antiparticles] fermions ... and how they interact 5

Chirality • In the Standard Model, chirality is a big deal. • The SU(2) -- i.e. weak -- interaction only talks to left-handed fermions. • So in the SM, each quark generation is represented as • a doublet of left-handed particles with SU(2) interactions • two singlet right-handed particles • These weak flavour eigenstates are different from the mass eigenstates, but can be expressed as superpositions of them. • Phase convention: up-type quarks are aligned. Then: � � � � � � u c t weak isospin + 1 / 2 , , d ′ s ′ b ′ weak isospin − 1 / 2 L L L • { u, c, t } are also mass eigenstates • {d', s', b' } are linear combinations of mass eigenstates... 6

The CKM Matrix • Write linear relation between mass eigenstates (d,s,b) and flavour eigenstates (d',s',b') as a matrix V CKM :     d 0 d s 0  = V CKM s    b 0 b • Notation: ⎛ ⎞ V ud V us V ub V cd V cs V cb V CKM = ⎜ ⎟ ⎝ ⎠ V td V ts V tb • The complex elements of this matrix are free parameters in the SM and have to be determined experimentally. • Unitarity constraints & removal of unphysical phases => 4 actual free params 7

The CKM Matrix ⎛ ⎞ V ud V us V ub V cd V cs V cb V CKM = http://www.quantumdiaries.org/2012/05/10/needle-in-a-haystack/ ⎜ ⎟ ⎝ ⎠ V td V ts V tb Image by Anna Phan, Quantum Diaries. 
 These two elements have non-tiny complex phases ± Current best-fit magnitudes, from PDG 2014: ⎛ ⎞ 0 . 97427 ± 0 . 00014 0 . 22536 ± 0 . 00061 0 . 00355 ± 0 . 00015 V CKM = 0 . 22522 ± 0 . 00061 0 . 97343 ± 0 . 00015 0 . 0414 ± 0 . 0012 ⎝ ⎠ 0 . 00886 +0 . 00033 0 . 0405 +0 . 0011 0 . 99914 ± 0 . 00005 8 − 0 . 00032 − 0 . 0012

CP violation • C = charge conjugation • P = parity • Weak interaction violates C and P . • It can also violate CP . This occurs when a process and its CP conjugate have different rates, for example: 
 Γ (D → f) ≠ Γ ( D → f ) • How can this happen? • It's always, always an interference effect. For example... 9

CPV u ¯ s W + c u c s s ¯ CPV requires strong and s weak phase differences. u u ¯ ¯ ¯ ¯ u u SM tree SM penguin | | P = | P | e i θ p e i φ p ≡ r | T | e i ( θ t + ∆ θ ) e i ( φ t + ∆ φ ) T = | T | e i θ t e i φ t Total amplitudes: a D = T + P = | T | e i θ t e i φ t � 1 + re i ∆ θ e i ∆ φ � a D = T + P = | T | e i θ t e − i φ t � 1 + re i ∆ θ e − i ∆ φ � Difference in the rates prop. to: | a D | 2 − | a D | 2 = − 4 r sin ∆ θ sin ∆ φ A = | a D | 2 � | a D | 2 | a D | 2 + | a D | 2 = � 4 r sin ∆ θ sin ∆ φ Asymmetry: ' � 2 r sin ∆ θ sin ∆ φ if r ⌧ 1 2 + O ( r ) 10

CPV u ¯ s ˜ W + e c u c s c u s ¯ ¯ s s s u u ¯ ¯ ¯ ¯ ¯ ¯ u u u u SM tree SM penguin NP penguin ... and NP could contribute, changing the amount of CPV. Thus, test for NP: Does CP asymmetry match SM expectation? A = | a D | 2 � | a D | 2 | a D | 2 + | a D | 2 = � 4 r sin ∆ θ sin ∆ φ Asymmetry: ' � 2 r sin ∆ θ sin ∆ φ if r ⌧ 1 2 + O ( r ) 11

CPV in charm • There's a lot more to say about CPV... • ... but for today we'll focus on one specific type: 
 direct CP violation in SCS decays of charm mesons. • Of interest because expected CP asymmetries in the SM are small, but can often be enhanced by NP . • How small? Generically up to O(10 − 3 ) in direct CPV. 
 Much theory progress on this in recent years. Grossman, Kagan & Nir, PRD 75, 036008 (2007) • Thus: a sensitive probe of NP . • Very well suited to LHCb. Bianco, Fabbri, Benson & Bigi, Riv. Nuovo. Cim 26N7 (2003) Brod, Kagan & Zupan, Phys.Rev. D86 014023 (2012) Bigi, arXiv:0907.2950 Gedalia, Kamenik, Ligeti & Perez, PLB 714 55 (2012) Bobrowski, Lenz, Riedl & Rorhwild, JHEP 03 009 (2010) Giudice, Isidori & Paradisi, JHEP 1204 060 (2012) Bigi, Blanke, Buras & Recksiegel, JHEP 0907 097 (2009) Hiller, Hochberg & Nir, Phys.Rev. D85 116008 (2012) 12 etc etc etc

LHCb 13

LHC 14

LHCb y HCAL M5 ECAL M4 SPD/PS 5m M3 M2 Magnet RICH2 M1 T3 T2 T1 TT Vertex Locator z 5m 10m 15m 20m 15

LHCb data-taking in Run 1... 8 TeV: 2 fb − 1 7 TeV: 1 fb − 1 7 TeV: 0.035 pb − 1 16

... and Run 2 13 TeV As of June 8 Photo (c) Zefram 13 TeV 17

Why LHCb? • Very nice detector (for charged tracks) • Precision vertexing & tracking • Hadron & muon ID across wide momentum range • Huge statistics! Cross-sections in acceptance: • σ b b (7 TeV) = 49 ± 7 μ b => 20kHz of b b • σ c c (7 TeV) = 1419 ± 134 μ b => 600 kHz of c c • σ b b (13 TeV) = 101 ± 10 μ b => 40kHz of b b • σ c c (13 TeV) = 2940 ± 240 μ b => 1.2 MHz of c c Eur.Phys.J. C71 (2011) 1645 ; Nucl.Phys.B 871, 1 ; JHEP 1510 (2015) 172 ; JHEP 1603 (2016) 159 18

Overflowing with charm & beauty • In Run 1 we got 600 kHz of charm, and in Run 2 this rises to 1.2 MHz! • We can't keep all of this (or even read it all out) • Use trigger system to select the events that are cleanest and most interesting for physics. • Hardware (L0): fast but crude decision, 1 MHz output • Software (HLT): inclusive, then exclusive reconstruction • Big challenge & goal for the upgrade: making the trigger smarter (and the output leaner) so that we can save keep the physics efficiency up. 19

Search for CPV in D + → K − K + π + Phys. Rev. D 84, 112008 (2011) -- 35 pb − 1 20

D + → K − K + π + : Why? How? • SCS charm decay, CP asymmetry sensitive to NP • Could measure asymmetry integrated across PHSP ... A = B ( D + → K − K + π + ) − B ( D − → K + K − π − ) B ( D + → K − K + π + ) + B ( D − → K + K − π − ) . • ... but we can be smarter. • These are 3-body decays, so there are many interfering contributions • e.g. D + → 𝜚π + , K *0 K + , a 0 (1430) 0 π +, ... • NP might show up in some but not others. • Strong phase varies across the phase space. 21

D + → K − K + π + : Dalitz plot PHSP density is uniform, so all structure * is due to |A| 2 . ) 3 4 /c 2 (GeV LHCb 3 10 2.5 + K - K 2 m 2 10 2 1.5 10 1 1 0.5 1 1.5 2 2 4 2 m (GeV /c ) - * Neglecting efficiency variation, background, etc. + K π 22


 
 
 D + → K − K + π + : Asymmetries • Idea: divide D + and D − Dalitz plots into bins, count yield in each, check for differences in distribution. • Any significant difference => CP violation! • Work with normalised yields so that overall effects, e.g. σ (D + ) ≠ σ (D − ), cancel out. • To test significance, calculate figure of merit: 
 N bins ◆ 2 ✓ di ff erence in normalised yields in bin i χ 2 = X uncertainty on the di ff erence i with NDF = N bins − 1. The per-bin significance, S CP ,i • Careful choice of binning necessary for sensitivity. 
 Studied with toy MC. 23

D + → K − K + π + : Binnings One binning, and (jumping ahead) the per-bin significances: 24

D + → K − K + π + : Validation with MC • Generated toy MC (isobar model from CLEO-c) • Baseline: no CP asymmetries generated • Verified that we don't produce false positives • Effect of K + /K − detector efficiency asymmetry included • Then tested sensitivity by introducing CPV: CPV Adaptive I Adaptive II p (3 σ ) h S i p (3 σ ) h S i no CPV 0% 0.84 σ 1% 0.84 σ 2 � in φ (1020) phase 5% 1.6 σ 2% 1.2 σ 3 � in φ (1020) phase 38% 2.8 σ 12% 1.9 σ 4 � in φ (1020) phase 76% 3.8 σ 41% 2.7 σ 5 � in φ (1020) phase 97% 5.5 σ 79% 3.8 σ 6 � in φ (1020) phase 99% 7.0 σ 98% 5.2 σ 6 . 3% in κ (800) magnitude 16% 1.9 σ 24% 2.2 σ 11% in κ (800) magnitude 83% 4.2 σ 95% 5.6 σ 25

D + → K − K + π + : The data 2010 data only (0.035 fb − 1 at 7 TeV) 15000 ) ) 2 2 Events / ( 0.28 MeV/c Events / ( 0.48 MeV/c (a) (b) LHCb LHCb 40000 lower upper lower middle upper 10000 20000 5000 + D + + D D s 0 0 1800 1850 1900 1800 1850 1900 1950 2000 2 2 m (MeV/c ) m (MeV/c ) - - + + + + K π π K K π CF control mode 
 Signal mode 
 CF control mode 
 3760k D + → K − π + π + 370k D + → K − π + π + 515k D s+ → K − π + π + • Key control sample: D s+ → K − π + π + • Also used D + → K − π + π + ; sidebands of both modes 26