Measurements with doubly-charmed hadrons Daniel Vieira University - - PowerPoint PPT Presentation

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Measurements with doubly-charmed hadrons

Daniel Vieira University of Chinese Academy of Sciences On behalf of LHCb collaboration 9th International Workshop on Charm Physics 22/05/2018

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Outline

Introduction LHCb Experimental status Ξcc++ observation Lifetime measurement Prospects and conclusion

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Doubly Heavy Baryons

Two SU(4) baryon 20-plets 
 (JP = 1/2+ and JP = 3/2+) Each has a doubly charmed SU(3) triplet (Ξcc++(ccu), Ξcc+(ccd) and Ωcc+(ccs))

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JP = 1 2

+

JP = 3 2

+

JP = 3/2+ baryons decay via strong interactions to JP = 1/2+ JP = 1/2+ baryons decay weakly to other flavours

Ξcc++ - Mass

Theoretical predictions of Ξcc++ mass are 3.5-3.7 GeV/c2 and expected to be close to Ξcc+[1-6]

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https://indico.cern.ch/event/632400/

Ξcc++ - Lifetime

Large ambiguity in theoretical predictions [7-12]

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Reference Ξcc++ (ps) Ξcc+ (ps) Ωcc+ (ps) Karliner, Rosner, 2014

0.185 0.053

Kiselev, Likhoded, Onishchenko, 1998

0.430 +/- 0.100 0.110 +/- 0.010

Kiselev, Likhoded, 2002

0.460 +/- 0.050 0.160 +/- 0.050 0.270 +/- 0.060

Guberina, Melic, 
 Stefancic, 1998

1.050 0.200 0.300

Chang, Li, Li, Wang, 2007

0.670 0.250 0.210 W-exchange

SELEX experiment Ξcc+ measurement

Observed Ξcc+→ Λc+K-π+ and Ξcc+→pD+K- decays Short lifetime: 𝞄Ξcc+ < 33 fs @90% CL Large production: exceeding theoretical predictions by orders of magnitude Mass (combined): 3518.7 ± 1.7 MeV

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15.9 events 5.62 events

PRL 97, 162001 (2006)

Other Ξcc+ searches

Results not confirmed by

• ther experiments (Focus,

BaBar, Belle and LHCb)

JHEP 12 (2013) 090

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Belle

SELEX Ξcc+

PRL 97, 162001 (2006)

LHCb

Good momentum resolution: p/δp ≈ 0.5% - 1% (5-200 GeV) Good vertex resolution: IP resolution ≈ 20 µm Particle identification: 𝜗(𝐿 → 𝐿) ~95%, Mis-ID 𝜗 (𝜌 → 𝐿) ~5%

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LHCb

L0 hardware trigger: looks for transverse energy signatures Software high level trigger: near real time alignment and calibration in trigger

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LHCb

Run I: 1.0 fb-1 at 7 TeV (2011) + 2.0 fb-1 at 8 TeV (2012) Run II: 0.3 fb-1 (2015) + 1.7 fb-1 (2016) + 1.7 fb-1 (2017) at 13 TeV

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Ξcc++→Λc+K-π+π+

Most promising channel for observation [13] See talk in parallel session tomorrow by

• Prof. Yu 


https://indico.inp.nsk.su/event/10/session/7/ contribution/29

Lifetime measurement
 important to confirm weakly decaying nature

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Dataset, reconstruction and selection

2016 data (1.7fb-1) Signal kinematical distributions taken from MC Background distributions taken from wrong-sign 
 Ξcc++→ Λc+ K- π+π- samples First reconstruct Λc+→pK-π+, then combine with K-π+π+

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Dataset, reconstruction and selection

Selection is performed in three steps: Trigger (dedicated trigger line): Kinematical constraints, PID, track and vertex quality Preselection: Ghost probability, kinematical constraints, vertex and PV refit quality MVA: Kinematical constraints and Vertex refit quality

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Mass fit

Signal yield: 313 ± 33

mΞ++

cc = 3621.40 ± 0.72(stat) ± 0.27(syst) ± 0.14(Λc)MeV

PRL 119 (2017) 112001

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𝑛(Ξcc++) = 𝑛(Λc+𝐿-𝜌+) − 𝑛(Λc+) +

Local significance > 12𝜏

Mass value consistent with theoretical range of predictions! Not consistent with Ξcc+ SELEX measurement [MLHCb(Ξcc++) - MSELEX(Ξcc+) = 103 ± 2 MeV ]

Mass fit - Run I data

Also significant in 2012 and consistent with 2016 data Signal yield: 113 ± 21

PRL 119 (2017) 112001

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Local significance > 7𝜏

]

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c ) [MeV/

++ cc

Ξ (

cand

m

3500 3600 3700

2

c Candidates per 5 MeV/ 10 20 30 40 50 60 70 Data Total Signal Background

LHCb 8 TeV

δM(2012, 2016) = 0.8 ± 1.4MeV

Signal properties

Measuring its lifetime is crucial to establish the weak nature

• f its decay and for comparison with theoretical predictions

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PRL 119 (2017) 112001

Lifetime measurement

We perform a lifetime measurement, using the Ξcc++→ Λc+K-π+π+ decay, relative to the control channel of same topology, Λb0→Λc+π−π+π− Unbinned maximum likelihood fit of the background subtracted Ξcc++ decay time distribution (sFit)

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fΞ++

cc (t) = hΛ0 b(t) ×

✏Ξ++

cc (t)

✏Λ0

b(t) × e

−

1 τ Ξ++ cc

−

1 τΛ0 b

! t

Lifetime measurement

We perform a lifetime measurement, using the Ξcc++→ Λc+K-π+π+ decay, relative to the control channel of same topology, Λb0→Λc+π−π+π− Unbinned maximum likelihood fit of the background subtracted Ξcc++ decay time distribution (sFit) Λb0 data and acceptances incorporated to PDF as histograms

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fΞ++

cc (t) = hΛ0 b(t) ×

✏Ξ++

cc (t)

✏Λ0

b(t) × e

−

1 τ Ξ++ cc

−

1 τΛ0 b

! t

Lifetime measurement

We perform a lifetime measurement, using the Ξcc++→ Λc+K-π+π+ decay, relative to the control channel of same topology, Λb0→Λc+π−π+π− Unbinned maximum likelihood fit of the background subtracted Ξcc++ decay time distribution (sFit) Λb0 data and acceptances incorporated to PDF as histograms PDG Λb0 lifetime (1.470 +/- 0.010 ps) used as input

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fΞ++

cc (t) = hΛ0 b(t) ×

✏Ξ++

cc (t)

✏Λ0

b(t) × e

−

1 τ Ξ++ cc

−

1 τΛ0 b

! t

Lifetime measurement

We perform a lifetime measurement, using the Ξcc++→ Λc+K-π+π+ decay, relative to the control channel of same topology, Λb0→Λc+π−π+π− Unbinned maximum likelihood fit of the background subtracted Ξcc++ decay time distribution (sFit) Λb0 data and acceptances incorporated to PDF as histograms PDG Λb0 lifetime (1.470 +/- 0.010 ps) used as input Measurement performed with Λb0 and result consistent with PDG value

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fΞ++

cc (t) = hΛ0 b(t) ×

✏Ξ++

cc (t)

✏Λ0

b(t) × e

−

1 τ Ξ++ cc

−

1 τΛ0 b

! t

Data samples and selection

LHCb 2016 (~1.7 fb-1 - 13TeV) Mostly same selection criteria of

• bservation

Specific hardware trigger: Large hadronic ET deposit from decay products; Or large ET deposit in calorimeters or muon stations from tracks other than the decay products Decay time range: 0.10-2.00 ps Mass fits used to obtain sWeights

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NΞ++

cc = 304 ± 35

NΛ0

b = 3397 ± 119

]

2

c ) [MeV/

+

π

+

π

−

π

+ c

Λ ( m

5500 5600 5700

)

2

c Candidates / (5 MeV/

100 200 300 400 500 600 700 800

Data Total fit Signal Background

LHCb Preliminary (b)

]

2

c ) [MeV/

+

π

+

π

−

K

+ c

Λ ( m

3500 3600 3700

)

2

c Candidates / (5 MeV/

20 40 60 80 100 120 140 160

Data Total fit Signal Background

LHCb Preliminary (a)

LHCB-PAPER-2018-19 LHCB-PAPER-2018-19

Decay time

Normalized, background subtracted decay time distributions

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0.5 1 1.5 2

Decay time [ps]

0.00 0.05 0.10 0.15 0.20

Normalized yield / (0.095 ps)

++ cc

Ξ

b

Λ

LHCb Preliminary

LHCB-PAPER-2018-19

Decay time and acceptance

MC samples reweighted to match PT distributions in data

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0.5 1 1.5 2

Decay time [ps]

0.5 1 1.5 2 2.5 3

Acceptance (arbitrary scale)

++ cc

Ξ

b

Λ

LHCb simulation

✏(t) = Nrec(t) Ngen(t)

LHCB-PAPER-2018-19

Decay time [ps]

0.5 1 1.5 2

Yield / (0.095 ps)

10 20 30 40 50 60

LHCb Preliminary

Data Fit

Results

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τΞ++

cc = 0.256+0.024

−0.022 ps (stat. only)

Statistical errors from MC and Λb0 samples

• btained from pseudoexperiments

LHCB-PAPER-2018-19

σMC = 0.007 ps

σΛ0

b = 0.006 ps

Systematic uncertainties

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Source Uncertainty (ps) Signal and background mass models 0.005 Correlation of mass and decay-time 0.004 Binning 0.001 Data-simulation differences 0.004 Resonant structure of decays 0.011 Hardware trigger threshold 0.002 Simulated Ξcc++ lifetime 0.002 Λ0b lifetime uncertainty 0.001 Sum in quadrature 0.014

Total systematic uncertainties significantly 
 lower than statistical uncertainties

Additional Checks

Sample splitting Charge Polarity Number of primary vertices L0 requirements Binned approach

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Future prospects

Ξcc++: Other decay modes: Ξcc++→Ξc+π+ Ξcc++→Λc+π+ Ξcc++→pD+K-π+ Production cross-section Other states: Ξcc+ Ωcc+

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Conclusions

A significant structure consistent with Ξcc++ was observed in LHCb 2016 data 


PRL 119, 112001 (2017)

Measured mass is consistent with theoretical range of predictions* Lifetime is measured to be:


LHCB-PAPER-2018-19


Many studies more to come: Other Ξcc++ decay modes and properties Other doubly-charmed baryon states

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Ξ++

cc

= 0.256 +0.024

−0.022 (stat.) ± 0.014 (syst.) ps

mΞ++

cc = 3621.40 ± 0.72(stat) ± 0.27(syst) ± 0.14(Λc)MeV

*not compatible with SELEX Ξcc+ mass measurement

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Backup

LHCb

JINST 3 (2008) S08005 IJMPA 30 (2015) 1530022

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LHCb

Interaction Region

JINST 3 (2008) S08005 IJMPA 30 (2015) 1530022

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LHCb

• Vertex Locator (vertex reconstruction)

• Impact parameter resolution: 20μm

• Decay time resolution: 45 fs (𝜐}~1.5 p

JINST 3 (2008) S08005 IJMPA 30 (2015) 1530022

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LHCb

• Tracking system (particle reconstruction)
• 𝜗 Tracking ~ 96%
• 𝜀𝑞⁄𝑞 ~ 0.5%-1% (5-200 GeV)
• 𝜏(mB→cc) ≈ 22MeV

JINST 3 (2008) S08005 IJMPA 30 (2015) 1530022

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LHCb

• Magnet
• Bending power: 4 Tm
• Reversable polarity

JINST 3 (2008) S08005 IJMPA 30 (2015) 1530022

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LHCb

• RICH detectors (𝐿⁄𝜌/𝑞 separation)
• 𝜗 (𝐿 → 𝐿) ~95%
• Mis-ID 𝜗 (𝜌 → 𝐿) ~5%

JINST 3 (2008) S08005 IJMPA 30 (2015) 1530022

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LHCb

• Calorimeters
• L0
• Scintilating detectors
• ID and energy

JINST 3 (2008) S08005 IJMPA 30 (2015) 1530022

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LHCb

• Muon chambers
• Multi-Wire Proportional Chamber (MWPC)
• MisID rates between 6.2% and 0.33 %

JINST 3 (2008) S08005 IJMPA 30 (2015) 1530022

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Additional tests

Multiple candidates: not creating fake narrow structure Checking combinations of tracks from Λc+ and Ξcc++ : not peaking MVA efficiency as a function of mass: very smooth Varying threshold value of MVA selector and using cut based selection: structure stays significant Varying particle ID selections: no peaking structure emerging in WS combinations, structure stays in RS sample Run I sample used as crosscheck: same structure seen

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Additional tests

Structure still significant with 𝛖Ξcc > 5σ Consistent with weak decay

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Systematic uncertainties

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References

[1]D.-H. He et al., Evaluation of spectra of baryons containing two heavy quarks in bag model, Phys. Rev. D70 (2004) 094004, arXiv:hep-ph/ 0403301. [2]C.-H. Chang, C.-F. Qiao, J.-X. Wang, and X.-G. Wu, Estimate of the hadronic production of the doubly charmed baryon Ξcc under GM-VFN scheme, Phys. Rev. D73 (2006) 094022, arXiv:hep-ph/0601032. [3]W. Roberts and M. Pervin, Heavy baryons in a quark model, Int. J. Mod. Phys. A23 (2008) 2817, arXiv:0711.2492. [4]J.-R. Zhang and M.-Q. Huang, Doubly heavy baryons in QCD sum rules, Phys. Rev. D78 (2008) 094007, arXiv:0810.5396. [5]A. Valcarce, H. Garcilazo, and J. Vijande, Towards an understanding of heavy baryon spectroscopy, Eur. Phys. J. A37 (2008) 217, arXiv: 0807.2973. [6]Z.-G. Wang, Analysis of the 1 + doubly heavy baryon states with QCD sum rules, Eur. 2 Phys. J. A45 (2010) 267, arXiv:1001.4693. [7]V. V. Kiselev, A. K. Likhoded, and A. I. Onishchenko, Lifetimes of dou- bly charmed baryons: Xi(cc)+ and Xi(cc)++, Phys. Rev. D60 (1999) 014007, arXiv:hep-ph/9807354. [8]B. Guberina, B. Melic, and H. Stefancic, Inclusive decays and lifetimes of doubly charmed baryons, Eur. Phys. J. C9 (1999) 213, arXiv:hep-ph/ 9901323. [9]D. Ebert, R. N. Faustov, V. O. Galkin, and A. P. Martynenko, Mass spectra of doubly heavy baryons in the relativistic quark model, Phys. Rev. D66 (2002) 014008, arXiv:hep-ph/0201217. [10] V. V. Kiselev and A. K. Likhoded, Baryons with two heavy quarks, Phys. Usp. 45 (2002) 455, arXiv:hep-ph/0103169, [Usp. Fiz. Nauk172,497(2002)]. [11]C.-H. Chang, T. Li, X.-Q. Li, and Y.-M. Wang, Lifetime of doubly charmed baryons, Commun. Theor. Phys. 49 (2008) 993, arXiv:0704.0016. [12]M. Karliner and J. L. Rosner, Baryons with two heavy quarks: Masses, production, decays, and detection, Phys. Rev. D90 (2014), no. 9 094007, arXiv:1408.5877. [13]F.-S. Yu, H.-Y. Jiang, R.-H. Li, C.-D. L, W. Wang, and Z.-X. Zhao, arXiv:1703.09086.

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