The Λ b 0 baryon at LHCb

Transcription

1 The Λ b 0 baryon at LHCb UNIVERSITY OF BIRMINGHAM SCHOOL OF PHYSICS AND ASTRONOMY SEMINAR 21/10/15 Peter Griffith University of Birmingham, UK 1

2 Contents B-physics at the LHC Heavy baryons in B-physics The LHCb detector Understanding the Λ b 0 baryon at the LHC Key measurements FCNC decays with Λ b 0 Λ b 0 pk μ + μ in detail Summary University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 2

3 Flavour physics, SM and BSM Hierarchy Problem The very successful Standard Model?? Dark Matter Gravity??? Matter/antimatter asymmetry Dark Energy University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 3

4 B-physics at LHCb: LHCb V ub measurement B-physics presents many ways to test and constrain the SM Excellent probes for New Physics & precise measurements of SM CP measurements FCNC observables (bs->ll etc.) New intermediate states/particles reconstructed decay B X 3872 K University of Birmingham Seminar Lb baryon at LHCb Peter Griffith 4

5 B-physics at LHCb: Abundant b-quark production at the LHC σ p p b bx ~ 80μb ~ 100,000 b b pairs per second 40% of heavy quark production within the acceptance of LHCb Production fraction, f Λ b 0 f d ~0.4 plenty of Λ b 0 s at the LHC! (20% of b hadrons) Λ b 0 has half integer spin opens the door for unique measurements University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 5

6 The LHCb detector VELO - high precision tracking and tagging ~4mm from beam able to reconstruct secondary vertices (B meson flight distance ~10mm) Magnet 4Tm Polarity regularly switched to cancel systematic effects RICH large background rejection from PID Extensive muon detection system with clean muon triggering O(1mm) Trackers Calorimeters SPD PD ECAL HCAL University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 6

7 Data taking at LHCb Atlas/CMS Total amount of data after Run1: 3fb 1 ~1fb 1 in 2011 < κ > ~2.7e32 cm 2 s 1 ~2fb 1 in 2012 < κ > ~4.0e32 cm 2 s 1 Comparatively low LHCb employs lumi levelling constant rather than high instantaneous luminosity preferred. Some precision measurements require very well known luminosity PID system becomes saturated at higher luminosities LHCb ~5 khz read out rate to disk (1 khz originally planned ) High operational efficiency (~2% deadtime) University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 7

8 Λ b 0 at LHCb Interesting measurements and discoveries University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 8

9 Measuring V ub with Λ b 0 pνμ Previous inclusive measurements by Babar and Belle Large disagreement between inclusive and exclusive measurements new particle with right-handed coupling? V 2 ub V 2 = B Λ0 b pμ ν μ cb B Λ 0 b Λ + c μ ν μ R FF Where R FF is the ratio of relevant form factors New LHCb measurement removes the need for a new particle. But why the initial disagreement? ε R ~ 0.2 new particle would have ~20% coupling strength of the W boson Λ b 0 pμ ν μ candidates are reconstructed using m corr = m 2 hμ + p 2 + p University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 9 Visible mass Candidates with 100MeV/c 2 uncertainty are selected Transverse momentum of hμ pair

10 Resonances in Λ b 0 J/ψpK Two resonances observed in Λ b 0 J/ψpK Consistent with pentaquark state with content quark cuud Six dimensional amplitude fit. Using just Λ states is not adequate. Two additional states required P c (4450) P c (4380) Mass (MeV/c 2 ) ± ± J P 5 2 Significance, σ arxiv: University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 10

11 long downstream Measurement of the Λ b 0 polarisation Decay amplitudes First of its kind at a hadron collider Uses Λ b 0 J/ψΛ decays Decay of a spin 1 2 particle into spin 1 and 1 2 particles Angular analysis performed on all three angles to Transverse polarisation parameter Angular distributions Transverse production polarisation: 0.06±0.07±0.02 Appears to be small (O(10%) or less) Not so favourable for studying photon helicity in Λ b 0 Λγ and Λ b 0 Λ γ decays if small arxiv: University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 11

12 Λ b 0 lifetime measurement Lifetime measured with Λ b 0 J/ψpK decays Relative to B 0 J/ψπ + K lifetime 1fb 1 of data Acceptance ratio of Λ b 0 and B 0 Decay time distributions for Λ b 0 and B 0 Unprecedented precision dominates world average arxiv: τ LHCb 0 Λb = ± ± ps University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 12

13 Rare Decays at LHCb FCNC s can occur through loops Highly suppressed Sensitive to new physics e.g. additional diagrams from new BSM particles in loops Numerous observables many very sensitive to NP b μ + μ b s s μ μ + An effective field theory is employed b s b s μ + μ μ + μ LHCb ideal for studying rare FCNC decays of mesons and baryons, e.g. b s High resolution tracking High performance PID Muon signals clean at LHCb University of Birmingham 21/10/15 All Wilson coefficients calculable predictive Wilson coefficients NP can be seen in deviations of Wilson coefficients Operators (local interaction terms) Lb baryon at LHCb Peter Griffith 13

14 Λ b 0 Λ μ + μ Branching ratio measurement Previously measured at CDF LHCb-PAPER No signal observed at low q 2 at either CDF or LHCb but results consistent with SM Now updated to 3fb 1, with angular analysis University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 14

15 Λ b 0 Λ μ + μ 3fb 1 update BR as a function of q 2 First evidence of the signal at low q 2! (3σ) Slight deviation from SM predictions similar to other b sll measurements Forward backward asymmetries measured Leptonic A FB Hadronic A FB arxiv: University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 15

16 Λ b 0 pk μ + μ Branching fraction measurement University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 16

17 Λ b 0 pk μ + μ Non-resonant Λ b 0 pk μ + μ Rare FCNC decay through excited states Likely dominated by Λ b 0 Λ (1520)μ + μ m(pk ) structure known Unobserved All variables blinded in mass region of the Λ b 0 Very limited theoretical knowledge Branching fraction predictions (in units of 10e6) for SCA (SM1) and MCN (SM2) models. (a and b without and with LD charmonium contributions respectively) arxiv: m(pk ) in Λ b 0 J/ψ μ + μ pk SUSY Wilson coefficients from M. J. Aslam, Y.-M. Wang and C.-D. Lu, University of Birmingham 21/10/15 Phys. Rev. D 78, Lb (2008) baryon at LHCb Peter Griffith 17

18 Λ b 0 pk μ + μ branching fraction measurement Measured relative to Λ b 0 J/ψpK Simpler calculation Cancellation of systematic effects Lack of theoretical and experimental knowledge Lots of correcting to be done DATA No clean separation of Λ (1520) MC m(pk ) in Λ b 0 J/ψpK MC is produced with only phasespace kinematics Dimuon mass squared Λ b 0 pk μ + μ MC Typical differential decay rate (B 0 K 0 μ + μ ) as function of dimuon mass squared Experimentally motivated model for decay structure would need full amplitude analysis University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 18

19 Λ b 0 pk μ + μ branching fraction measurement Measured relative to Λ b 0 J/ψpK Simpler calculation Cancellation of systematic effects Could see large signal where MC statistics are relatively low Lack of theoretical and experimental knowledge Lots of correcting to be done DATA No clean separation of Λ (1520) MC Dimuon mass squared Λ b 0 pk μ + μ MC Typical differential decay rate (B 0 K 0 μ + μ ) as function of dimuon mass squared m(pk ) in Λ b 0 J/ψpK MC is produced with only phasespace kinematics Experimentally motivated model for decay structure would need full amplitude analysis University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 19

20 Analysis strategy: The LHCb dataflow Trigger Stripping Selection University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 20

21 Analysis strategy: The LHCb dataflow Trigger Stripping Selection Muon triggers only at L0 Offline Loose cuts to minimise kinematic biases Large mass windows accept full kinematic phase space DecayTreeFitter Cuts Particle identification Neural Network University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 21

22 Analysis strategy: The LHCb dataflow Trigger Stripping Selection Muon triggers only at L0 Offline Loose cuts to minimise kinematic biases Large mass windows accept full kinematic phase space Training Samples: Background: Λ 0 b J/ψpK sideband (> 6 GeV) Signal: Λ 0 b pk μ + μ MC Most powerful variables: DecayTreeFitter χ 2 Proton p T Kaon IP χ 2 s Optimised with punzi figure of merit σ chosen to be 5 B+ σ 2 DecayTreeFitter Cuts Particle identification Neural Network Imposes kinematic constraints based off decay chain to final state Proton p T background vs signal from neural net output 500 MeV Proton p T > 500MeV Λ b 0 vtx χ 2 /DoF < 5.0 q 2 < 17.6 GeV 2 Cuts on the `probnn` variables Kaon: Probability of being kaon > 0.2 Probability of being proton < 0.8 Proton: Probability of being proton > 0.2 Probability of being kaon< 0.8 Probability of being pion < 0.7 University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 22

23 Analysis strategy: The LHCb dataflow Trigger Stripping Selection Muon triggers only at L0 Offline Loose cuts to minimise kinematic biases Large mass windows accept full kinematic phase space Training Samples: Background: Λ 0 b J/ψpK sideband (> 6 GeV) Signal: Λ 0 b pk μ + μ MC Most powerful variables: DecayTreeFitter χ 2 Proton p T Kaon IP χ 2 s Optimised with punzi figure of merit σ chosen to be 5 B+ σ 2 DecayTreeFitter Cuts Particle identification Neural Network Imposes kinematic constraints based off decay chain to final state Proton p T background vs signal from neural net output 500 MeV Proton p T > 500MeV Λ b 0 vtx χ 2 /DoF < 5.0 q 2 < 17.6 GeV 2 Cuts on the `probnn` variables Kaon: Probability of being kaon > 0.2 Probability of being proton < 0.8 Proton: Probability of being proton > 0.2 Probability of being kaon< 0.8 Probability of being pion < 0.7 University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 23

24 Analysis strategy: acceptance and efficiency Geometric acceptance Between 10 and 400 mrad Stripping/ selection Strong dependence on kinematics for most selections and acceptances issue with data/mc mismatch Trigger PID Neural Net PID behaviour difficult to replicate in MC. Highly dependent on kinematics Evaluated using calibration samples - Data containing high rate, well understood decays. - Calibration samples binned in kinematic variables - Essentially a look up table for MC event efficiency ε = N pass N PID efficiency is not stable over time and changes with magnet polarity needs to be treated separately University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 24

25 Dealing with MC discrepancies Correcting Λ b 0 production kinematics Use another Λ b 0 mode? Only Λ b 0 J/ψpK has similar phasespace coverage Need strong cuts to achieve pure sample Λ b 0 pk μ + μ decay structure Correlation between all angles and the two masses Need model of at least 5D to account for all correlations* ε cosθ b, cosθ l, Δφ, m pk, q 2 Λ b 0 Needs to perform well with high no. of dimensions and finite MC stats * 7D if we do not assume negligible production polarisation University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 25

26 Dealing with MC discrepancies Correcting Λ b 0 production kinematics Use LHCb s f Λ /f d measurement Measured as function of P T and η Can use well known B d decay to extract Λ 0 b kinematic correction factor B 0 J/ψK s obtain clean sample with loose cuts Fit B 0 P T and η at generator level Fit Λ b 0 P T and η at generator level *background subtracted using fit model to m(b 0 ) (s-weighting) Fit B 0 P T and η In data* and MC Small corrections on B 0 MC LHCb LHCb w = f 0 Λ b P f T, η pdf B 0 P T, η d pdf 0 Λb P T, η 1 w B 0 P T, η Corrected MC doi: /jhep08(2014)143 Perfect agreement not expected. Corrects for production kinematics only Background subtracted data University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 26 MC Physics motivated Independent of Λ b 0 decay Works!

27 Dealing with MC discrepancies Finding a goldilocks model (or an adventure in failed techniques!) Always a trade off between accuracy and speed Speed important when it comes to systematics (e.g Toy MC s) Need to find a trade-off that s just right Λ b 0 pk μ + μ decay structure Correlation between all angles and the two masses Need model of at least 5D to account for all correlations* ε cosθ b, cosθ l, Δφ, m pk, q 2 Needs to perform well with high no. of dimensions and finite MC stats * 7D if we do not assume negligible production polarisation University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 27

28 Dealing with MC discrepancies Kernel density estimation? Phase space populated with kernels to estimate the pdf. A form of data-smoothing Good for low statistics/many dimensions Estimate density in 1D projections Then perform correlated multidimensional KDE on full phase-space Perform boundary correction Good: Incredibly accurate! Slices through each plain of the multi-dimensional phasespace, unfolded into 1 dimension, showing ratio between MC after full reconstruction/selection and generator level MC weighted with the KDE pdf. Bad: Incredibly CPU intensive! University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 28

29 Dealing with MC discrepancies Neural Network? Good: Bad: Very fast! Comparison of weighted generator level MC and full selection MC Not so accurate University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 29

30 Dealing with MC discrepancies - efficiency Legendre polynomials Reweighting function generated for each event. Models acceptance effects assuming flat at generator level True for angles (spherically isotropic in phase-space MC) Not true for mass Model both MC samples and perform ratio of efficiency functions ε event cosθ b, cosθ l, Δφ, m pk 2, q 2 = i,j,k,l,m c ijklm = c 0 M ijklm (2i + 1)(2j + 1)(2k + 1)(2l + 1)(2m + 1) M ijklm = 1 N events 0 c ijklm P i cos θl P j cos θb P k Δφ P l m pk 2 P m (q 2 ) c 0 is chosen to be ½ and controls overall integral. Mostly arbitrary as whole model is normalised to the integrated phase-space efficiency N events P i cos θl P j cos θb P k Δφ P l m pk 2 P m (q 2 ) University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 30

31 Dealing with MC discrepancies - efficiency Fast enough and accurate enough! cosθ l cosθ b Δφ q 2 m pk 2 5D scan Efficiency correction now largely independent of MC model Systematic uncertainty evaluated with `bootstrapping` University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 31

32 Calculating B Λ b 0 pk μ + μ B(Λ b 0 J/ψpK ) Corrected yield Event weight from background subtraction N = i w i B Λ 0 b pk μ + μ ε i B(Λ 0 b J/ψpK ) = N Λ b 0 pk μ + μ N(Λ b 0 J/ψpK ) = i w μμ μμ i /εi j w J/ψ J/ψ j /εj Event efficiency correction Systematics propagation: σ N = i w i ε i 2 σ tot N = σ N 2 + N N obs σ shape (N) + Includes sources of systematic uncertainty on acceptance/efficiency Other sources, e.g yield extraction model uncertainties University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 32

33 Sources of systematic uncertainty Λ b 0 lifetime Yield extraction fit LHCb unofficial Efficiency model + MC statistics Absolute uncertainty on Λ b 0 pk μ + μ Relative uncertainty near negligible thanks to LHCb and CMS lifetime measurements Particle Identification PID systematic is combination of two uncertainties: Statistical uncertainty from bin populations of correction table Uncertainty from bin widths ( can hide fine structure in PID efficiency space) Λ b 0 kinematics correction Corrected MC Background subtracted data MC University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 33

34 Yield extraction LHCb unofficial Λ b 0 J/ψpK m b m c B s 0 K + K μ + μ yield constrained by fitting mass reflection in upper sideband Λ b 0 pk μ + μ to be unblinded in the coming weeks followed by publication -stay tuned! m a Take side band clean of Λ b 0 Swap proton kaon Signal: Double crystal ball B s 0 and B 0 backgrounds: DCB (opposite sign tails) Combinatorial: exponential B 0 s pdf parameters constrained from MC. Yield constrained from data using: m cf m bf m bf I = x dx Bs x dx + Bs x dx Bs m a m a m c University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 34

35 Summary - Λ b 0 pk μ + μ First official observation and branching fraction measurement Blind analysis Blind in another sense! No experimental knowledge Almost no theoretical knowledge b sll studies currently a hot topic Opens the door for further measurements with Run2 data CP-asymmetry Forward-backward asymmetry Amplitude analysis Still in progress but almost there Scheduled for review this winter University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 35

36 Summary - Λ b 0 physics at LHCb Some of the most precise Λ b 0 measurements yet Still a lot less well known than the mesonic b-sector Anomalies seen in b sll decays at LHCb prompt further study of these hadronic counterparts Recent LHCb results from Λ b 0 rare decays hint at similar pattern of SM discrepancy! To take full advantage of the Λ b 0 sector requires a lot of groundwork f Λb 0 Experimentally: τ Λb 0,, polarisation, branching fraction measurements of ideal control channels, e.g f d Λ 0 b J/ψpK, Λ 0 b J/ψpπ Theoretically: Form factors, predictions (BR, A FB ) etc. Some surprises, e.g discovery of states consistent with pentaquark in Λ b 0 J/ψpK! University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 36

37 Fig1: penguin decay Thanks for listening! University of Birmingham 21/10/15 Lb baryon at LHCb Peter Griffith 37