Flavour physics in the LHC era

Transcription

1 Maria Laach school, september 2012 An introduction to Flavour physics in the LHC era and quest for New Physics (an experimentalist s point of view) Clara Matteuzzi INFN and Universita Milano-Bicocca 1

2 Outline Lesson I Introduction The flavor structure of the Standard Model Lesson II Heavy flavor physics at e+e- colliders and hadron colliders Tests of the Standard Model: where do we stand Lesson III Flavour Physics in 2012 and beyond: where to go? Heavy flavor measurements perspectives and New Physics in flavor physics 2

3 Where is Flavour Physics now The Standard Model seems to describe well all the phenomena created and observed in the laboratory. 3

4 However.. Big Bang (~ 14 billion years ago) matter and antimatter equally produced followed by annihilation n baryon /n γ ~ Why didn t all the matter annihilate? No evidence found for an antimatter world elsewhere in the Universe One of the requirements to produce an asymmetric final state (our world) from a symmetric matter/antimatter initial state (the Big Bang) is that CP symmetry must violated [Sakharov, 1967] CP is violated in the Standard Model, through the weak mixing of quarks For CP violation to occur there must be at least 3 generations of quarks So problem of baryogenesis may be connected to why three generations exist, even though all normal matter is made up from the first (u, d, e, ν e ) The way to probe CP violation is through the study of quark mixing In particular, hadrons containing the b-quark show large CP asymmetries However, the CP violation in the SM is not sufficient for baryogenesis Other sources of CP violation expected good field to search for new physics 4

5 How to go further? 1. Search for rare kaon decays ex. search for K + π + νν (BR SM ~ 10-10!) theoretically clean measurement of V td 2. LHC-generation b physics experiments Hadron colliders give enormous b production rate (~10 12 bb pairs/year at LHCb) All b-hadron species produced many CP measurements possible, measure very rare decays 3. Super-B factories (Japan, Italy?) could reach 50 ab-1 5

6 Strengths of indirect approach Can in principle access higher scales and therefore see effect earlier: Third quark family inferred by Kobayashi and Maskawa (1973) to explain small CP violation measured in kaon mixing (1964), but only directly observed in 1977 (b) and1995 (t) Neutral currents (ν+n ν+n) discovered in 1973, but real Z discovered in 1983 Can in principle also access the phases of the new couplings: NP at TeV scale needs to have a flavour structure to provide the suppression mechanism for already observed FCNC processes once NP is discovered, it is important to measure this structure, including new phases Complementary to the direct approach After all CHARM and TOP quarks were first seen not because produced directly, but via their effects in FCNC processes in K and B physics respectively 6

7 Why b-physics? In many New Physics scenarios, large effects are seen in third family: Looking at new physics through radiative corrections many times imply factors Δm 2 Moreover, the long lifetime of the b-quark helps experimentalists. Why we do not see CP violation in the strong interactions and we do see a very small effect in the EW sector? The B-hadrons provides a laboratory where theoretical predictions can be precisely compared with experimental results 7

8 New physics through.. Δb = 1: Decays through penguin b new particles d, s Δb = 2 : Oscillations through box b d,s new particles d, s b or l l through tree b d,s new particles d, s b or l l The Standard Model tree process not affected. 8

9 How to produce heavy quarks? Large statistics of heavy flavours: e+e- cc, bb Z (LEP,SLC) B-factories pp (pp) cc, bb, tt Tevatron (Fermilab) LHC (CERN) 9

10 Machines and experiments Machine Experiment beam Energy (GeV) σ bb b Fraction LEP ADLO e+e nb 1/5 PEP II BABAR e+e nb 1/4 KEK-B BELLE e+e nb 1/4 LHC LHCb pp µb 1/240 Tevatron CDF, D0 pp µb 1/500 10

11 The LEP and SLC legacy Very efficient experimental techniques: displaced vertex,lepton ID, Tag with: leptons, jet charge, fragmentation products, D hadrons Measurements of: B spectroscopy Λ b 0, B s 0,Ξ b, B *,.. lifetimes at 1-4 % precision V cb = ( ) 10-3 V ub = ( ) 10-4 B o -B o oscillations Δm d =( ) ps -1 Δm s > 14.9 ps -1 11

12 What is a B-factory Asymmetric e + e colliders. B-meson factory e + e Υ(4S) BB 12

13 What is a B-factory The ϒ(4S) decays to B 0 B 0, B + B - in a coherent L=1 state 13

14 An event at the B-factories The anatomy of a CP event 14

15 An event at the B-factories B meson reconstruction through the observables: 15

16 CP violation from the B-factories The measurement of sin β and its impact on the unitarity triangle 16

17 The measurement of angle β To measure a phase needs interference: two amplitudes (e.g. 2 decay amplitudes), or decay and mixing. 0 (if one decay mechanism) sin2β 17

18 CP violation from the B-factories Using the decay B 0 J/ψ K S 0 B 0 J/ψ K S Purity 97 % W To measure the ratio: A(t) = Γ(B 0 J/ψ K S ) - Γ(B 0 J/ψ K S ) Γ(B 0 J/ψ K S ) + Γ(B 0 J/ψ K S ) needs to know the flavor at production 18

19 Tagging the flavour at production How do we know what is the flavor at the production? Looking at the opposite B produced: The flavour tagging at the B-factory: figure of merit ε D 2 30% 19

20 CP violation from the B-factories Δt (ps) 20

21 CP violation from the B-factories 21

22 CP violation from the B-factories 22

23 B-physics at hadron colliders 23

24 From e+e- to hadron colliders 24

25 B Physics at Hadron Colliders Advantages Large cross section: 250 µb (at LHC - 7TeV) much larger than at B-factories Produce all B-hadron species: B 0, B s0, Λ b, B c, (with large boost) Disadvantages difficult to trigger, bandwidth restrictions Messy environment 25

26 Typical LEP (ALEPH) event e + e - Z bb A typical CDF bb event A typical BB event at the Υ(4S) Fish-eye views in the plane transverse to the beams 26

27 The proton composition 27

28 The proton composition parton distribution functions p p underlying event X = W, Z, top, jets, SUSY, H, higher-order pqcd corrections; accompanying radiation, jets We don t know Which partons hit each other What their momentum is What the other partons do We know roughly (2-30%) The parton content of the proton The cross sections of processes 28

29 The kinematic variables 29

30 The b-physics at LHC 30

31 ATLAS/CMS Central detectors, η <2.5 Will do B-physics using high Pt muon triggers, mostly with modes involving di-muons. Purely hadronic modes triggered by the tagging muon. LHCb Designed to maximize B-acceptance (within cost and space constraints) Forward spectrometer, 1.9<η<4.9 More b-hadrons produced at low angles. Single arm is OK as b-quarks are correlated. Rely on much softer, lower Pt triggers, efficient also for purely hadronic decays. 31

32 B hadrons have a mass of ~ 5 GeV and therefore tend to be produced with asymmetric x values of the partons boosted along the beam direction: θ (B), rad 32

33 CP violating oscillation amplitudes are damped by: B s D s π proper time resolution wrong flavour tag good decay vertex resolution good momentum resolution --- ideal resolution and tag --- realist. tag --- realist. tag+resolution --- realist. tag+res+bg+acc background good particle identification Detector performance 33

34 δp/p(lhcb) 0.35% 0.55% p distribution for B tracks Mass Resolution in MeV/c 2 Resolution dominated by multiple scattering (over detector resolution) up to 80 GeV Typical B track in LHCb (p>12 GeV): hits: 98.7% correctly assigned Efficiency >95% Ghost rate <7% 34

35 ATLAS Si Tracker CMS Si Tracker LHCb VELO Surface 65 m m m 2 N channels 6 M 10 M 170 k Size 50x400 µm (pixel) 150x150 µm (pixel) 40 µm (strip) Distance to beam 5 cm 4 cm 0.8 cm B lifetime: LHCb: 36 fs, ATLAS: 83 fs, CMS: 77 fs (2π Δm -1 s ~350 fs) - CDF ~ 87 fs fully reco decays PRL (2006) B s J/ψ φ 35

36 Performance of the RICH (detector based on Cerenkov effect) Search for a signal from the decay B s π+π- (turquoise line) Must separate channels of identical topology: B Kπ (red dashed), B KK (yellow) B s Kπ (brown), B π+π- (orange dashed) Λ b pk (purple), Λ b pπ (green) Clara Matteuzzi 36

37 For Mixing & CP measurements it is crucial to know the b-flavor at t=0 opposite side same side Opposite side tags: Only works for bb production mechanism: Lepton (muon or electron) or jet charge Same side tags: Identify Kaon from B s fragmentation 37

38 Figure that matters: εd 2 Efficiency ε of tagging (right or wrong) Dilution D is fraction of correct tags hadron colliders: εd 2 = % compared to 30% at B-factories with the additional problem that the other B can oscillate before the tagging 38

39 A dedicated trigger is necessary because b fraction is only <1% of inelastic cross-section Relevant decays branching fractions: BR ~

40 Conclusions of lecture II LHC is at present a unique opportunity to measure loop processes in b decays and find out if there is new mechanism to understand the origin of CP violation beyond the Standard Model 40