Rencontres de Moriond 2019- Electroweak interaction session CP violation in charm decays in LHCb Giulia Tuci, on behalf of the LHCb collaboration giulia.tuci@cern.ch La Thuile,21/03/2019 Young Scientists Forum
CPV in charm Charm transitions are a unique portal for obtaining a novel access ❖ to flavor dynamics there might exist some New Physics coupling ➢ only to up-type quarks expected CPV in charm ≾ 10 -3 → difficult to ➢ observe it experimentally Finally CPV in charm has been observed! (see Federico’s talk on this morning session) ❖ Now it’s the moment to start a systematic exploration of all the charm ❖ hadrons decay channels to do a quantitative study of CPV D 0 → K S 0 K S 0 and D 0 → K S 0 K *0 decay channels are perfect candidates due to ❖ the significant size of the expected effect PRL 119 (2017) 251801 I will focus on the first one ❖ 1 Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb
A CP in D 0 → K S 0 K S 0 In D 0 → K S 0 K S 0 decay channel amplitudes are suppressed ❖ → A CP could be enhanced at a level of ~1% PRD 92 (2015) 054036 Provides independent information on CPV: sensitive to a different mix of ❖ CP-violating amplitudes w.r.t. D 0 → K + K - and D 0 → π + π - PRD 85 (2012) 034036 Exchange B.R. (D 0 → K S 0 K S 0 ) = (1.8 ± 0.4) x 10 -4 Previous measurements Penguin annihilation CLEO PRD 63 (2001) 071101 LHCb (Run1) JHEP 10 (2015) 055 Belle PRL 119 (2017) 171801 2 Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb
Measurement methodology π - Time-integrated measurement. Quantity that we want to measure: ❖ π + K 0 π - S π tag π + K 0 S D 0 p p Quantity measured in LHCb → D * ❖ Production asymmetry: initial state pp is not CP symmetric B π - π + Asymmetric detector acceptance + material interaction different for particles/antiparticles D *+ → D 0 π + decay used to tag D 0 ❖ To remove production and detection asymmetries D 0 → K + K - is used as a ❖ calibration channel A CP (K S 0 K S 0 )= ΔA raw +A CP (K + K - ) I ndependently measured by LHCb on Run1 dataset with a precision of ~0.1% PLB767(2017)177 3 Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb
D 0 → K S 0 K S 0 @ LHCb 0 are difficult to select at trigger level K S ❖ π - 𝜐 (K 0 S ) = 0.9 x 10 -10 s , < 𝛾𝛿 >~80 → 𝛾𝛿 c 𝜐 ~ 216 cm π + ➢ K 0 π - S K S 0 decays often outside vertex detector acceptance ➢ π + π tag K 0 No dedicated trigger in LHCb Run1 ➢ S D 0 p p Great step forward made in Run2! D * ➢ Data samples collected in 2015-2016 (~2fb -1 ) ❖ 0 reconstructed from LL sample: both K S ➢ Long tracks LD sample: one K S 0 is Long and the other ➢ one is Downstream 4 Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb
LHCb results 2015+2016 dataset [ JHEP11(2018)048 ] A raw extracted with a fit to Δm=m(D*)-m(D 0 ) distribution. Total yields: 1067 ± 41 ❖ “Magnet up” polarity “Magnet up” polarity A CP (LL)=(+6.7 ± 3.8 ± 0.9 )% A CP (LD)=(-5.3 ± 7.4 ± 1.3 )% A CP (K S 0 K S 0 ) =(4.3 ± 3.4 ± 1.0)% Combining with Run1 LHCb analysis 0 ) =(2.3 ± 2.8 ± 0.9)% A CP (K S 0 K S Consistent with no-CPV hypothesis and previous results. Statistically limited. Belle result: A CP = (-0.02 ± 1.53 ± 0.17)% 5 Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb
Prospects with current data Additional 3.9 fb -1 of data collected in 2017-2018 ❖ Some improvements have been applied to the trigger ❖ Now sensitive also to events where both K S 0 are downstream! ➢ Investigating the possibility to re-optimise the selection w.r.t the ❖ presented analysis to gain more statistics Statistical uncertainty for the full Run2 analysis could be better ❖ than √L improvement 6 Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb
Prospects with future runs New data will arrive in 2021 with an almost completely new detector and ❖ trigger system Instantaneous luminosity will increase by a factor of 5 (2 x 10 33 cm -2 s -1 ) ➢ Effort is being made to keep high efficiency for downstream tracks ❖ reconstruction despite the higher luminosity As an example, R&D work to realize a dedicated downstream ❖ tracking unit that can be integrated in the DAQ architecture and act as an “embedded track-detector” CERN-LHCC-2017-003 LHCb has the potential to further improve this measurement in the ❖ years to come → Stay tuned! 7 Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb
Backup slides Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb
The LHCb experiment Calorimeters: particle identification Cherenkov detector: Muon particle identification chambers Vertex detector(VELO) Magnet Tracking stations: trajectory of charged particles → momentum Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb
A CP in D 0 → K S 0 K S 0 o t s t u p n i inputs to ~6 x 10 -4 A sd =0 in the SU(3) flavour symmetry - ❖ A b small ma ≠0 in the SU(3) flavour ❖ symmetry CPV could be as large as 1% ❖ Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb
A CP in D 0 → K S 0 K S 0 A raw extracted with a fit to Δm=m(D*)-m(D 0 ) distribution ❖ Peaking background reduced with cut based selection, e.g. ❖ D 0 → K S 0 π + π - , reduced performing selections on m(K S 0 ) and flight distance ➢ D 0 → K S 0 π + π - D 0 → K S 0 K S 0 D 0 → π + π - π + π - Combinatorial background reduced using kNN classifier ❖ Results on LL and LD sample and on the two separate magnet polarities ❖ compatible within 2σ Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb
D 0 → K S 0 K S 0 vs D 0 → K + K - and D 0 → π + π - Tree diagram Penguin diagram Penguin exchange diagram D 0 → K + K - , D 0 → π + π - D 0 → K S 0 K S 0 Exchange diagram Penguin annihilation diagram Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb
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