Rare B decays at LHCb
Institute of High Energy Physics (ι«θ½ζ)
Yiming Li (ζδΈιΈ£)
On behalf of the LHCb collaboration
HFCPV-2018, Zhengzhou, 26 Oct 2018
Content
Why rare b decays?
Rare b decays at LHCb:
Leptonic decays
Radiative decays
π β π ππ
LFU tests
Summary and prospects
2018.10.26 Yiming Li, HFCPV-2018, Zhengzhou 2
Why rare b decays?
Flavour-Changing Neutral Currents (FCNC) highly suppressed in the SM
π β π (π) decays: only via penguin or box diagrams
=> sensitive to new physics contributions
2018.10.26 Yiming Li, HFCPV-2018, Zhengzhou 3
SM NP?
Wilson coefficients of interest: C7
(β) β couplings to photons C9
(β), C10(β) β couplings to leptons
LHCb detector
A forward spectrometer for heavy flavour physics
β A general purpose detector at forward region
1250 members, 79 institutes, 18 countries
2018.10.26 Yiming Li, HFCPV-2018, Zhengzhou 4
2 < Ξ· < 5
Excellent lifetime and mass resolution
Efficient particle identification with low mis-ID rate
JINST 3 (2008) S08005 IJMPA 30 (2015) 153022
LHC
> 450 papers published /submitted
Operation
Run I: 3 fb-1
End of Run II: > 9 fb-1
~ 5 times more wrt Run I
accounting for ππ π increase
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7 TeV
8 TeV
13 TeV
9 fb-1
A wealth of physics results produced with Run I already; new results emerging including Run II data
More to expect at HFCPV: β βCPV in B decays at LHCbβ, Yuehong Xie β βCharm CPV and rare decays at LHCbβ, Liang Sun β βSpectroscopy studies at LHCbβ, Jibo He
Leptonic π β ππ decays
Highly suppressed in SM, theoretically clean final state
Very sensitive to new physics contribution
π΅(π )0 β π+πβ
First observation of π΅π 0 β π+πβ by a single
experiment using 4.4 fb-1 (2011~2016)
7.6 π significane
No evidence yet for π΅0 β π+πβ
First measurement of π΅π 0 β π+πβ lifetime
π(π΅π 0 β π+πβ) = 2.04 Β± 0.44 Β± 0.05 ps
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PRL 119 (2017) 191801
1 in SM; [-1, 1] in NP
π΅(π )0 β π+πβ
π decay very challenging due to missing neutrinos
First direct limit on π΅π 0 β π+πβ and world best limit on π΅0 β π+πβ
Exploiting excellent vertex resolution in hadronic final state ππππ
Search with 3 fb-1 (Run I)
2018.10.26 Yiming Li, HFCPV-2018, Zhengzhou 8
PRL 118 (2017) 251802
SMοΌ
β¬ π΅π 0 β ππ = 7.73 Β± 0.49 Γ 10β7
β¬ π΅0 β ππ = 2.22 Β± 0.19 Γ 10β8
Radiative π β π πΎ decays
Studied in detail in B factories
Most sensitive to C7(β)
Photon polarization dominantly left-handed in SM;
up to 50% right-handed in SM extension!
L L/R
π»β, πβ, π , β¦
π΅+ β πΎ+π+πβπΎ
Photon polarization in π΅+ β πΎ+π+πβπΎ
First direct observation (5.1π) of photon polarization using 3 fb-1 Run I data
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PRL 112 (2014) 161801
π΅π 0 β π πΎ
Photon polarization in Bs studied for the first time
Time-dependent decay rate sensitive to right- over left-handed πΎ polarization
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PRL 118 (2017) 021801
SM prediction:
PLB 664 (2008) 174
π β π πΎ in baryonic decays
Opportunity in baryonic decays
π¬π0 β π¬0πΎ
SM predicting BR ~ (0.06 β 1) Γ 10-5.
Polarization of πΎ accessible in angular analysis
π―πβ β π―βπΎ, πΊπ
β β πΊβπΎ
Difficulty: no secondary vertex, many
downstream tracks
Results expected soon
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Semileptonic decays π β π ππ
Different regions of dimuon mass squared sensitive to different Wilson coefficients
Observables:
- Branching ratios
- Angular variables
- Lepton universality
Branching ratios of π β π π+πβ decays
Data consistently below SM predictions; tension at 1βΌ3π level
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Angular analyses in π΅0 β πΎβ0π+πβ
Rich angular structure in π β π ππ, where P5β is less dependent on form-factor
LHCb only global analysis: 3.4π deviation from SM
Update with Run II ongoing
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Local deviation:
2.8π 3.0π
New physics vs. Charm loops
Significance depending on hadronic charm-loop uncertainties
arXiv: 1406.0566
More angular analyses
Many analyses of π΅(π ) β πππ have been
produced with Run I data only, and limited by statistics
Updates including Run II ongoing
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Phase difference in π΅+ β πΎ+π+πβ
Tension in P5β:
NP in short distance? Or sizable long-distance effects away from resonances?
Phase difference between the short- and long-distance amplitudes measured by fitting the full dimuon mass spectrum
Including resonances: π, π, π, π½/π, π 2π , π 3770 , π 4040 , π 4160 , π(4415)
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EPJC (2017) 77:161
3π tension
Exploring new modes: π β πππ
First evidence found in π΅π 0 β πΎ β0π+πβ
3.4π using 4.6 fb-1 data (up to 2016)
Extremely challenging due to contamination from π΅ 0 β πΎ β0π+πβ
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JHEP 07 (2018) 020
Exploring new modes: baryonic decay
First observation of Ξπ0 β ππβπ+πβ.
First b to d transition in baryons
JHEP 04 (2017) 029
New modes (cont.): π¬π0 β π¬ π+πβ
Angular moments measured for π¬π0 β π¬ π+πβ
Focus on high q2 region (15, 20) (GeV/c2)2
Full bases of observables measured with limited statistics, compatible with SM predictions
2018.10.26 Yiming Li, HFCPV-2018, Zhengzhou 19
JHEP 09 (2018) 146
Forward-backward symmetry in lepton-/hadron-side /combined:
Lepton Flavour Universality tests
π π =β¬ π΅βπππ
β¬(π΅βπππ) accurately predicted in SM βΌ πͺ(1%)
Different interaction of e/π in matter
β experimentally challenging!
Significant Bremsstrahlung from e
Large threshold in ECAL due to pileup
π πΎ , π (πΎβ)
Double ratios measured for systematic cancellation and cross-checks
π π =β¬ π΅βπππ
β¬(π΅βπππ)=
β¬ π΅βπππ
β¬(π΅βππ½/π(ππ))Γ
β¬ π΅βππ½/π(ππ)
β¬(π΅βπππ)
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PRL 113 (2014) 151601 JHEP 08 (2017) 055
2.1 ~ 2.5 π 2.6 π
LFU tests in other modes: π΅π 0 β π π+πβ, π΅+ β πΎ+π+πβπ+πβ, π¬π
0 β ππΎβπ+πβ, β¦
Global fits of all π β π ππ decays
LFU observables consistent with Branching Ratio and angular measurements
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PRD 96 (2017) 055008
Proposed models to explain the anomalies: - Lepto-quarks - Pati-Salam models - Massive vector bosons - β¦
Lepton Flavour Violation in π΅ π 0 β πΒ±πβ
LFV forbidden in the SM, can be enhanced by NP
Search using Run I data compatible with background-only hypothesis
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JHEP 09 (2018) 146
Looking into the future
Upgrade I:
Removal of L0 β full software trigger enhancing efficiencies (esp. hadronic FS)
Readout at inelastic 30 MHz rate
Sub-detector replacement for higher occupancy & radiation hardness
Upgrade II (+Ib):
Fully exploiting the potential at HL-LHC era
Coping with pileup ~50
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More details: β βLHCb upgrade and prospectsβ, Zhenwei Yang
50 fb-1. 300 fb-1. Now
Prospects of rare decays
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arXiv: 1808.08865 Physics case for an LHCb Upgrade II With 300 fb-1 in HL-LHC:
π΅π β ππ
β’ Ξβ¬~ 0.16 Γ 10β9
β’ Ξβ¬ π΅π βππ
β¬ π΅βππ~10%
β’ Ξππππππ
~2%, time-dep. CP
π β π πΎ
β’ Systematics dominated in πΎπππΎ, ππΎ
β’ ΞπΌπΎ~ 4%(10%) in Ξπ0 β
ΞπΎ (Ξπβ β ΞβπΎ)
π β π (π)ππ & LFU
Summary
LHCb searched for rare b decays in many final states with high precision
Quite a few 2/3-sigma level tension with SM:
β’ b β sll BR, P5β, R(K), R(K*), β¦
Most results fully exploiting Run I data, intensive work with Run II ongoing (4 times more b yield expected!)
End of Run II pp collision but not end of the storyβ¦
A busy schedule for upgrade will provide ever higher luminosity
Hence higher precision and more possibilities
New ideas / interpretations welcome!
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