Measurements of strange and non strange beauty production in PbPb collisions … · 2018. 11. 22. · Ta-Wei Wang (MIT), Quark Matter 2018 (Venezia, Italy 14-19, May, 2018) Flavor

Measurements of strange and non strange beauty production in PbPb collisions at

5.02 TeV with the CMS detector

Ta-Wei Wang (MIT)on behalf of the CMS collaboration

Quark Matter, Venezia, Italy15, May, 2018

Ta-Wei Wang (MIT), Quark Matter 2018 (Venezia, Italy 14-19, May, 2018)

Flavor dependence parton energy loss

�2

• Heavy flavor are predominantly produced in the early hard scattering, timescale < QGP formation ➜ full information of QGP evolution history

• Heavy ➜ negligible thermal production and annihilation rate

• Parton mass dependence of energy loss

• Medium induced energy loss (Eloss): 1. Collisional 2. Radiative

• Kinematics: “Dead cone effect” [1]: gluon radiation is suppressed at angles < quark mass/energy

• Eloss in light quarks > Eloss in heavy quarks

• Suppression of induced radiation at low pT and the disappearance of this effect at high pT

• A measure to quantify this: nuclear modification factor (RAA)

[1] Y.L. Dokshitzer, D. E. Kharzeev, Phys. Lett. B 519 (2001) 199.

Collisional Radiative


Strangeness enhancement & recombination mechanism

�3

• Proposed an enhancement of strangeness in the QGP state comparing to hadron gas [1]

• Verified by RHIC in strange baryon production measurements [2]

• Suggested that heavy quarks could hadronise via a recombination with other quarks in the medium in addition to fragmentation

[1] J. Rafelski et. al, Phys. Rev. Lett. 48, 1066[2] STAR Collaboration, Phys. Rev. C 77, 044908

[3] ALICE Collaboration, arXiv:1804.09083

- Beauty quark: even cleaner probe

- First time probing combination of beauty and strange

• ALICE measurement of Ds production show indications of higher Ds meson RAA w.r.t other D meson species [3]

Ta-Wei Wang (MIT), Quark Matter 2018 (Venezia, Italy 14-19, May, 2018) �4

• B quark decay kinematic:

• Comparable long decay length (exploit this)

• Precise vertexing & tracking

• High statistics dataset

• No-hadronic PID utilized

b quark ➞ B+, B0, Bs

Primary vertex

Secondary vertex

Hadronize

Decay

Charged tracks

cτ O(500) μm

QGP

Measuring b hadron


Measuring b hadron

�5

• Strategy 1, exclusive: full reconstruction of the B hadron decay chain• Exclusive decay channels:

• B+ ➞ J/ψ(μμ) + K (branch ratio ~ 0.06%)

• Bs ➞ J/ψ(μμ) + φ(KK) (branch ratio ~ 0.03%)


Primary vertex

Secondary vertex

Hadronize

Decay

cτ O(500) μm

QGP

K

µ

µ

Non-prompt J/ψ

B+


• Strategy 1, exclusive: full reconstruction of the B hadron decay chain

• Pros: Access to the original B hadron kinematics Distinguish B hadrons, e.g. B+ v.s. Bs

• Cons: Low Statistic Combinatorial background


Primary vertex

Secondary vertex

Hadronize

cτ O(500) μm

QGP

K

µ

µ

K

Non-prompt J/ψ

Φ

Bs

Decay

Measuring b hadron


• Strategy 2, inclusive: reconstruction some of the daughter resonance of b hadrons

• Inclusive decay channels:

• b hadrons ➞ J/ψ(μμ)

• b hadrons ➞ D0(πK)


Primary vertex

Secondary vertex

Hadronize

cτ O(500) μm

QGP µ

µ

Non-prompt J/ψ

Decay

Measuring b hadron


• Strategy 2, inclusive: reconstruction some of the daughter resonance of b hadrons

• Pros: High statistics

High reconstruction efficiency

• Cons:

Convolution of b mesons and b baryons decay


Primary vertex

Secondary vertex

Hadronize

cτ O(500) μm

QGP Decay

D0

π

K

Measuring b hadron


Full reconstruction: B+

�9

• Signal channel: B+ ➜ J/ψ K+

• Charged tracks are assigned a kaon mass

• Muon pair + track ➜ common vertex fitting ➜ fitting for yield extraction ➜ efficiency correction ➜ RAA

• Cut optimization is crucial to beat down the massive combinatorial background

Tracks

J/ψ + track common vertex fit

B candidates

B

µ+

Primary vertex

Secondary vertex

µ-

B decay

B meson

J/ψ candidates

MVA cut optimizationBoosted Decision Tree (BDT):

• Track kinematics (pT, rapidity…)• Vertex fitting probability• Opening angle• Decay length

Non-prompt J/ψ Muon pairs

K+


Full reconstruction: Bs

�10

B

µ+

Primary vertex

Secondary vertex

µ-

B decay

B mesonK+

Φ

Non-prompt J/ψ

K-

• Signal channel: Bs ➜ J/ψ Φ

• Charged tracks are assigned a kaon mass

• Muon pair + track pairs ➜ common vertex fitting ➜ fitting for yield extraction ➜ efficiency correction ➜ RAA

• Cut optimization is crucial to beat down the massive combinatorial background

Track pairs

J/ψ + Φ common vertex fit

B candidates

J/ψ candidates

MVA cut optimizationBoosted Decision Tree (BDT):

• Track kinematics (pT, rapidity…)• Vertex fitting probability• Opening angle• Decay length

Muon pairs

Φ candidates


Signal extraction: B+

�11

Phys. Rev. Lett. 119, 152301

Signal: Double Gaussian with same mean

Peaking BG: error function + two sided Gaussian

• Signals extracted: fit on invariant mass spectra (maximum likelihood)

• Peaking background: B hadrons other than the signal decay channel

- Examples: B+ → J/ψ + K*+ or B0 → J/ψ + K*0, K0…etc

pp PbPb


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Signal extraction: Bs

�12

Signal: Double Gaussian with same mean

• Narrow natural width of Φ meson ➜ no peaking background components

• First fully reconstructed Bs meson measurement in heavy-ion collision by CMS

HIN-17-008

pp PbPb


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ψPrompt J/ from b hadronsψJ/

Background

(5.02 TeV)-1bµPbPb 368

CMS

�13

Partial reconstruction: non-prompt J/ψ

1. Muon pair fit to a common vertex ➞ J/ψ candidates

2. 2D Fit on invariant mass and decay length spectra

3. Extracted yields corrected using a data-driven approach (tag & probe)

All J/ψprompt J/ψ:

direct production + feed down (ex ψ’)

non-prompt J/ψ: from B decays (ex B

➜ J/ψ X)

• Distinguish non-prompt component from prompt component

• Utilize the fact that b hadrons has a long decay length

b

μ+

μ-

Non-prompt J/ψ

(mm)ψJ/l3− 2− 1− 0 1 2 3 4

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ts /

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Background

(5.02 TeV)-1bµPbPb 368

CMS

arXiv:1712.08959 Submitted to Eur. Phys. J. C


Partial reconstruction: non-prompt D

b

πK

Non-prompt D

• Distinguish non-prompt component from prompt component

• Distance of closest approach (DCA)

1. Track pair fit to a common vertex ➞ D candidates

2. Signal extraction in DCA interval ➞ DCA distribution

3. Template fit (from simulation) on DCA distribution to extract non-prompt DHIN-16-016

W. Xie’s poster


Non-prompt J/ψ & D @ 5.02 TeV


Non-prompt J/ψ RAA v.s. Npart and rapidity

�16

• Strong suppression of non-prompt J/ψ production

• Increased suppression with event activity in both collision energy

• No significant difference in RAA between the two collision energies

• Interplay between: 1). initial momentum spectrum. 2). increase energy loss ??

arXiv:1712.08959 (5.02 TeV), EPJC 77 (2017) 252 (2.76 TeV)

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CMS


• At 2.76 TeV, some mild indication of rapidity dependance was seem

• No y-dependence at 5 TeV, and the results are compatible with the 2.76 TeV results

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CMS

Non-prompt J/ψ RAA v.s. Npart and rapidity


Non-prompt J/ψ RAA v.s. pT

• Measurement down to pT 3 GeV at forward rapidity

• RAA dependence for low pT J/ψ

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from b hadrons0D|y|<1

globaluncertainty

CUJETEPOS2+MC@sHQTAMUPHSD

�19

• Suppression of non-prompt D production in PbPb collisions

X. Jiechen et al., Journal of High Energy Physics 2 (2016) 169 P. B. Gossiaux et al, Nucl. Phys., A931 (2014) 581M. He et al., Physics Letters B 735 (2014) 445 – 450 T. Song et al., hys. Rev. C 92 (2015)

HIN-16-016

Non-prompt D0 RAA v.s. pT

• Compatible with theory prediction that includes both collisional and radiative energy loss (e.g., CUJET)

• The model including only collisional energy loss (PHSD), seems to predict a different behavior compared with other models and data at high-pT

W. Xie’s poster


B+ & Bs @ 5.02 TeV


• Suppression of B+ meson production in PbPb collisions

• B+ meson RAA ~ 0.3 to 0.6 with no obvious trend within uncertainty

• Compatible with theory prediction within uncertainty for pT 10-50 GeV/c

• Necessity for high pT measurement : distinguishing pQCD vs AdS/CFT base models

M. He et al., Physics Letters B 735 (2014) 445 – 450 M. Djordjevic, Phys. Rev. C 94 (Oct, 2016) 044908 X. Jiechen et al., Journal of High Energy Physics 2 (2016) 169 W. A. Horowitz, Phys. Rev. D 91 (2015) 085019P. B. Gossiaux et al, Nucl. Phys., A931 (2014) 581

Phys. Rev. Lett. 119, 152301

B+ nuclear modification factor


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from b hadrons |y|<10D

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1.8 < |y| < 2.4|y| < 2.4global

uncertainty

�22

HIN-16-016

Phys. Rev. Lett. 119, 152301

• Compatible results from three beauty RAA measurements (non-prompt D, B+, and non-prompt J/ψ)

• Beauty seems to separate from charm and light flavor at ~ 20 GeV

• RAA between prompt D, charged particle, B+, non-prompt J/ψ and non-prompt D merging above ~ 20 GeV

• Note:• Rapidity range• Inclusive vs exclusive• B to J/ψ or D kinematic

RAA zoo: B v.s. D v.s. light

arXiv:1708.04962

JHEP 04 (2017) 039


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0BAA R+B

TAMUCUJET

�23

Bs nuclear modification factor

HIN-17-008

• Results derived in two Bs pT bins (7~15 and 15~50 GeV)

• Indication of less suppression of Bs

comparing to B+

• Substantial statistical and systematic uncertainty

⁉

M. He et al., Physics Letters B 735 (2014) 445 – 450 X. Jiechen et al., Journal of High Energy Physics 2 (2016) 169


Bs nuclear modification factor

HIN-17-008

• Bs / B+ RAA ratio

• Correlated systematic uncertainties sources (e.g., muon acceptance) are cancelled

• CMS’s capability to perform fully reconstructed Bs measurement

• Incoming LHC run ➜ more precise measurement

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0BTAMUCUJET

M. He et al., Physics Letters B 735 (2014) 445 – 450 X. Jiechen et al., Journal of High Energy Physics 2 (2016) 169


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CMSPreliminary ratioAA R+/Bs

0BTAMUCUJET

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from b hadrons:ψJ/

from b hadrons |y|<10D

|y|<10prompt D|<1ηcharged hadrons |

|y| < 2.4±B

1.8 < |y| < 2.4|y| < 2.4global

uncertainty

�25

• CMS beauty quark measurements (exclusive and inclusive), results are consistent

• Suppression for B+, non-prompt J/ψ, and non-prompt D

• Beauty seems to separate from charm and light flavor at ~ 20 GeV

• First Bs measurement in heavy ion collision (albeit with a substantial uncertainty)

• Comparison between B+ and Bs at low pT unveils information on flavor recombination.

• Future HL-LHC data with more precise measurements

Summary & outlook

- The MIT group's work was supported by US DOE-NP


Backups


Signal extraction: non-prompt D

Signal: Double Gaussian

with same mean

Combinatorial BG: 3rd order polynomial

Swapped BG: Wrong track mass

assumption

• Signal region DCA

— Sideband region DCA (X 0.5)

HIN-16-016


HIN-16-016

pp PbPb

• Two components template fit (from simulation) on data DCA distribution

• Corrected for potential difference in resolution between data and simulation

Signal extraction: non-prompt D


Non-prompt cross sections

�29

• Cross section measured between pT 6.5~50

GeV, |y|<2.4 (J/ψ) and 2~100 GeV, |y|<1.0(D0)

arXiv:1712.08959 HIN-16-016

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Non-prompt J/ψ fraction

• Proportion of measured J/ψ mesons coming from b-hadron decays

A. PbPb > pp ➜ indication of RAA non-prompt > prompt ??

B. Strong pT dependence, non-prompt fraction increases with pT (20% to 60%)

C. No significant dependence on rapidity is observed

arXiv:1712.08959. Submitted to Eur. Phys. J. C

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http://arxiv.org/abs/1712.08959


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Differential RAA v.s. Npart and pT

arXiv:1712.08959

• Similar information, more suppressed for higher pT and in central collision


MVA optimization

�32

To reach high signal to background significance (low production rate of b) → Multivariate analysis (MVA) for cut value optimization

Variables employed in MVA• track kinematic: track pT and pseudorapidity• Probability of the vertex fit (χ2): B secondary vertex fitting probability• Normalized d0: normalized distance between primary vertex and B secondary vertex• Opening angle(θ): angle between B meson displacement vector and B meson momentum

Primaryvertex

Secondaryvertex

B+

pB θMaximize Figure of merit: S∕√(S+B)• S: signals from simulation (normalized to FONLL prediction)• B: background from real data (sidebands of the B mass

spectrum)• Optimization conducted independently for pp and PbPb


Signal extraction: Bs

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HIN-17-008

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10|<1.45)BData (13 TeV, |y|<2.1)BData (13 TeV, |y

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|<1.45)BData (7 TeV, scaled to |y|<2.1)BData (7 TeV, scaled to |y

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Phys. Lett. B 771 (2017) 435 Phys. Rev. Lett. 119, 152301

Cross section in pp


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�35

Phys. Rev. Lett. 119, 152301 HIN-17-008

- Result derived in 5(3) B+(Bs) pT bins, from 7 to 50 GeV in |y| < 2.4- Consistent with FONLL predictions [1]

[1] M. Cacciari, M. Greco, P. Nason, “The pT Spectrum in Heavy-Flavour Hadroproduction”, JHEP 007, 9805 (1998)


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8.5%−8.3, +PbPb:

Cross section in PbPb

�36

Phys. Rev. Lett. 119, 152301 HIN-17-008


Cold nuclear matter effect?

(GeV/c)T

p0 10 20 30 40 50 60

pPb

R

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

| < 1.93CM

: |yψNonprompt J/ < 1.93

CM: -2.86 < y+B

Open beauty

(5.02 TeV)-1, pp 28.0 pb-1pPb 34.6 nb

CMS

• Suppression observed in both non-prompt J/ψ and B meson in PbPb collisions• Consequence of CNM effect? e.g.

Modification of nPDF and potential

nuclear absorption

• CMS measurements of both non-prompt J/ψ[1] and B meson[2] in pPb collisions• No evidence of suppression observed

in pPb collisions• The suppression seen in PbPb is likely

a final state effect.

[1] EPJC 77 (2017) 269

[2] Phys. Rev. Lett. 116, 032301


CMy

3− 2− 1− 0 1 2

pPb

R

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

< 30 GeV/cT

: 10 < pψNonprompt J/ < 60 GeV/c

T: 10 < p+B

Open beauty

(5.02 TeV)-1, pp 28.0 pb-1pPb 34.6 nb

CMS

(GeV/c)T

p0 10 20 30 40 50 60

pPb

R

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

| < 1.93CM

: |yψNonprompt J/ < 1.93

CM: -2.86 < y+B

Open beauty

(5.02 TeV)-1, pp 28.0 pb-1pPb 34.6 nb

CMS

[1] EPJC 77 (2017) 269

[2] Phys. Rev. Lett. 116, 032301

Cold nuclear matter effect?


B to J/ψ kinematics difference

�39

• The difference in pT between parent B meson and J/ψ

[GeV/c]T

p0 5 10 15 20 25 30

Bin

num

ber

Tp

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

PYTHIA

T pψJ/

Tparent B p


Future

�40

(GeV)T

p1 10 210

AAR

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6 CMSProjection

= 5.02 TeVNNs pp + PbPb

Centrality 0-100%

Charged hadrons-1 < 50 GeV), 0.2 nbT(p

-1 > 50 GeV), 10 nbT(p-1 < 20 GeV), 0.2 nbT (p0D

-1 > 20 GeV), 10 nbT (p0D-1, 10 nb+B

-1, 10 nbψNon-prompt J/

B meson projection central value based on Djordjevic (PRC 94 (2016) 044908)

• Future prospect: HL-LHC• 25 times more statistic expected

(factor of 5 reduction in uncertainty)

• Comparison @ low pT ➜ more definite conclusion on flavor

dependence energy loss

Documents

Measurements of strange and non strange beauty production in PbPb collisions … · 2018. 11. 22. · Ta-Wei Wang (MIT), Quark Matter 2018 (Venezia, Italy 14-19, May, 2018) Flavor