CP violation in charm decays in LHCb Giulia Tuci, on behalf of the - - PowerPoint PPT Presentation

Giulia Tuci,

• n behalf of the LHCb collaboration

giulia.tuci@cern.ch

La Thuile,21/03/2019

Young Scientists Forum

Rencontres de Moriond 2019- Electroweak interaction session

CP violation in charm decays in LHCb

Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb

CPV in charm

❖ Charm transitions are a unique portal for obtaining a novel access to flavor dynamics

➢ there might exist some New Physics coupling

• nly to up-type quarks

➢ expected CPV in charm ≾10-3 → difficult to

• bserve it experimentally

1

❖ 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 ❖ D0→ KS

0KS 0 and D0→ KS 0K*0 decay channels are perfect candidates due to

the significant size of the expected effect ❖ I will focus on the first one

PRL 119 (2017) 251801

Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb

ACP in D0→ KS

0 KS

❖

In D0→ KS

0 KS 0 decay channel amplitudes are suppressed

→ACP could be enhanced at a level of ~1%

❖

Provides independent information on CPV: sensitive to a different mix of CP-violating amplitudes w.r.t. D0→ K+K- and D0→π+π-

2

Previous measurements

Exchange Penguin annihilation

CLEO PRD 63 (2001) 071101 LHCb (Run1) JHEP 10 (2015) 055 Belle PRL 119 (2017) 171801 PRD 92 (2015) 054036

B.R. (D0 → KS

0KS 0) = (1.8 ± 0.4) x 10-4

PRD 85 (2012) 034036

Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb 3

Measurement methodology

❖

Time-integrated measurement. Quantity that we want to measure: ❖ Quantity measured in LHCb → ❖ D*+ → D0π+ decay used to tag D0

❖

To remove production and detection asymmetries D0→ K+K- is used as a calibration channel ACP(KS

0KS 0)= ΔAraw+ACP(K+K-)

Production asymmetry: initial state pp is not CP symmetric Asymmetric detector acceptance + material interaction different for particles/antiparticles

B

Independently measured by LHCb on Run1 dataset with a precision of ~0.1% PLB767(2017)177

π+ π-

p p πtag D0 D* π+ π+ π- π- K0

S

K0

S

Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb

D0→ KS

0 KS 0 @ LHCb

❖ KS

0 are difficult to select at trigger level

➢

𝜐(K0

S) = 0.9 x 10-10 s , <𝛾𝛿>~80→ 𝛾𝛿c𝜐 ~ 216 cm

➢ KS

0 decays often outside vertex detector acceptance

➢ No dedicated trigger in LHCb Run1 ➢ Great step forward made in Run2!

4 p p πtag D0 D* π+ π+ π- π- K0

S

K0

S

❖

Data samples collected in 2015-2016 (~2fb-1)

➢ LL sample: both KS

0 reconstructed from

Long tracks ➢ LD sample: one KS

0 is Long and the other

• ne is Downstream

Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb 5

Consistent with no-CPV hypothesis and previous results. Statistically limited. Belle result: ACP= (-0.02 ± 1.53 ± 0.17)%

LHCb results 2015+2016 dataset [JHEP11(2018)048]

ACP(LL)=(+6.7 ± 3.8 ± 0.9)% ACP(LD)=(-5.3 ± 7.4 ± 1.3)% ACP(KS

0KS 0) =(4.3±3.4±1.0)%

Combining with Run1 LHCb analysis

ACP(KS

0KS 0) =(2.3±2.8±0.9)%

“Magnet up” polarity “Magnet up” polarity

❖ Araw extracted with a fit to Δm=m(D*)-m(D0 ) distribution. Total yields: 1067 ± 41

Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb 6

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 KS

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

Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb 7

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 1033 cm-2s-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” ❖ LHCb has the potential to further improve this measurement in the years to come → Stay tuned!

CERN-LHCC-2017-003

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

Vertex detector(VELO)

Cherenkov detector: particle identification Calorimeters: particle identification Muon chambers Magnet Tracking stations: trajectory of charged particles → momentum

Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb

ACP in D0→ KS

0 KS

~6 x 10-4 i n p u t s t

• inputs to
• ❖

Asd=0 in the SU(3) flavour symmetry ❖ Ab 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

❖ Araw extracted with a fit to Δm=m(D*)-m(D0 ) distribution ❖ Peaking background reduced with cut based selection, e.g.

➢ D0 → KS

0 π+ π-, reduced performing selections on m(KS 0) and flight distance

❖ Combinatorial background reduced using kNN classifier ❖ Results on LL and LD sample and on the two separate magnet polarities compatible within 2σ

ACP in D0→ KS

0 KS

D0 → KS

0 KS

D0 → KS

0 π+ π-

D0 →π+ π- π+ π-

Giulia Tuci, 21/03/2019 CPV in charm decays in LHCb

D0→ KS

0 KS 0 vs D0→ K+K- and D0→ π+π-

Tree diagram Penguin annihilation diagram Exchange diagram Penguin diagram Penguin exchange diagram

D0→ K+K- , D0→ π+π- D0→ KS

0KS