Recent results from ATLAS in beauty and charm physics · [] introduction to b às lltransitions [] overview of ATLAS B (s)à!+!-measurement [] overview of ATLAS B d 0àK*!+!-angular

Recent results from ATLAS in beauty and

charm physicsHeather Russell, McGill UniversityOn behalf of the ATLAS CollaborationLake Louise Winter Institute, 10 – 16 February 2019

[] introduction to b à s ll transitions

[] overview of ATLAS B(s) à !+!- measurement

[] overview of ATLAS Bd0 à K*!+!- angular analysis

[] prospects for future B-physics measurements on ATLAS

2

Overview

Lake Louise Winter Institute 2019Heather Russell, McGill University

https://twiki.cern.ch/twiki/bin/view/AtlasPublic/BPhysPublicResultsAll public ATLAS B-physics results:

https://twiki.cern.ch/twiki/bin/view/AtlasPublic/BPhysPublicResults

3

Some background: b à sℓ+ℓ- transitions


b s

µ+

µ−

t

γ, Z0

W−b s

µ+

µ−ν

W− W+

tNO flavour-changing neutral currents at tree level in the SM – but can be created with box diagrams/loop diagrams

b s

µ+

µ−ν

H− H+

t

3

Some background: b à sℓ+ℓ- transitions


b s

µ+

µ−

d̃i

γ, Z0

χ̃0b s

µ+

µ−

d̃i

H0

g̃

b s

µ+

µ−

t

γ, Z0

W−

b s

µ+

µ−Z ′

b s

µ+

µ−ν

W− W+

tNO flavour-changing neutral currents at tree level in the SM – but can be created with box diagrams/loop diagrams

New physics can enter at tree, loop, or box-level:

4

B(s) à !+!- andBd0 à K*!+!-


t

tZ0

b

s

W+

µ+

µ�

"#$ b s

µ+

µ−

t

γ, Z0

W−"%$d d_ _ &∗$

Both Bs à !+!- andBd0 à K*!+!- decays are suppressed in the Standard Model:

ℬ(Bs → !+!-) = (3.65 ± 0.23) ×10−9

ℬ(Bd → !+!-) = (1.06 ± 0.09) ×10−10 ℬ(Bd➝K*!+!-) = (1.02 ± 0.09) × 10−6

Loop suppressedCKM-suppressedHelicity suppressed

Loop suppressedCKM-suppressed

4

B(s) à !+!- andBd0 à K*!+!-


t

tZ0

b

s

W+

µ+

µ�

"#$ b s

µ+

µ−

t

γ, Z0

W−"%$d d_ _ &∗$

Both Bs à !+!- andBd0 à K*!+!- decays are suppressed in the Standard Model:

Low-rate processes can be significantly modified by small BSM effects

Indirectly sensitive to BSM particles at mass scales up to 10 TeV – far beyond the reach of direct searches

Simultaneous branching ratiomeasurement of processesB0s à !+!- and B0d à !+!-

5

Measurement of B(s) à !+!-


Submitted to JHEP, arXiv:1812.03017

4800 5000 5200 5400 5600 5800Dimuon invariant mass [MeV]

1

10

210

310

Even

ts /

40 M

eV ATLAS Simulation-1 = 13 TeV, 26.3 fbs X-µ +µ ®b

decayscB

Semi-leptonic decays

-µ +µ ® s0B

-µ +µ ® 0B

Measured relative to well-known decay mode

https://arxiv.org/abs/1812.03017


5





1

10

210

310

Even

ts /

40 M


decayscB


-µ +µ ® s0B

-µ +µ ® 0B


Signal yield in Bd or Bs channel



5





1

10

210

310

Even

ts /

40 M


decayscB


-µ +µ ® s0B

-µ +µ ® 0B



Fiducial acceptance*efficiency for B0 dimuon system



5





1

10

210

310

Even

ts /

40 M


decayscB


-µ +µ ® s0B

-µ +µ ® 0B




Fiducial acceptance*efficiency and signal yield for J/"K+ system



5





1

10

210

310

Even

ts /

40 M


decayscB


-µ +µ ® s0B

-µ +µ ® 0B




Fiducial acceptance*efficiency and signal yield for J/"K+ system

ratio of hadronisation probabilities of a b-quark into B+ vs. B0 (B0

s).

(from LHCb)



1

10

210

310

Even

ts /

40 M


decayscB


-µ +µ ® s0B

-µ +µ ® 0B

backgrounds from partially reconstructed Bà!!X decays

6

B(s) à !+!-: backgrounds I



Missing energy from ”X” tracks not included in reconstructed vertex means mB appears lower



1

10

210

310

Even

ts /

40 M


decayscB


-µ +µ ® s0B

-µ +µ ® 0B

backgrounds from partially reconstructed Bà!!X decays

6

B(s) à !+!-: backgrounds I



4800 5000 5200 5400 5600 5800Mass of two misidentified muons [MeV]

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Even

ts /

40 M

eV

h h'®Total B

±

p± K® 0B-K+ K® s

0B

-p+p ® 0B

±

K±p ® s0B

ATLAS Simulation-1 = 13 TeV, 26.3 fbs

Blinded region

Missing energy from ”X” tracks not included in reconstructed vertex means mB appears lower

fully hadronic semileptonicpeaking B à hh’

e.g. B à "+ "-

"± à !±#e.g. B à !+ # "-

"± à !±#

true muons, but not produced promptly in the decay of a B meson.

hadron misidentification

[also backgrounds from muons from fake MS tracks combined with real hadronic ID tracks]



02

4

6

81012

14

16

18

Even

ts /

40 M

eV

ATLAS-1 = 13 TeV, 26.3 fbs

0.4163 < BDT <= 1

2015-2016 data

Total fit

Continuum background

X background-µ +µ ®b

Peaking background-µ +µ ® 0 + B-µ +µ ® s

0B

7

B(s) à !+!-: backgrounds II



0

10

20

30

40

50

60

70

Even

ts /

40 M

eV


0.3312 < BDT <= 0.4163

2015-2016 data

Total fit




0B


0

20

40

60

80

100

120

140

Even

ts /

40 M

eV


0.2455 < BDT <= 0.3312

2015-2016 data

Total fit




0B


0

100

200

300

400

500

600

700

Even

ts /

40 M

eV


0.1439 < BDT <= 0.2455

2015-2016 data

Total fit




0B

1- 0.8- 0.6- 0.4- 0.2- 0 0.2 0.4 0.6BDT output

00.020.040.060.08

0.10.120.140.160.180.2

Frac

tion

of e

vent

s / 0

.05

ATLAS Simulation MC-µ +µ ® s0B

Continuum bkg MC MCcB

X MC-µ +µ ®b MC

(s)0Semi-leptonic B

Increasing BDT response

Combinatorial backgrounddisentangled using a BDT with three categories of variables relating to:


(b) the muons

(c) the rest of the event

(a) the B vertex


8

B(s) à !+!-: results


[limits are diagonal ovals because the mass resolution is insufficient to resolve the two signals]

Submitted to JHEP, arXiv:1812.03017Unbinned maximum likelihood fit to m(!!) in each of the 4 BDT bins

Signal yield result:Ns = 80 ± 22, Nd = −12 ± 20

Most stringent limit to date!


0 1 2 3 4 5 6 7

]9- [10)- µ +µ ® s0BB(

0.2-

0

0.2

0.4

0.6

0.8

1

1.2

]9- [1

0)-

µ +µ

® 0B

B(

SM

ATLAS 2015-2016 data

Run 1 + 2015-2016 data

Likelihood contours for ) = 2.3, 6.2, 11.8Lln(D-2

8








0 1 2 3 4 5 6 7

]9- [10)- µ +µ ® s0BB(

0.2-

0

0.2

0.4

0.6

0.8

1

1.2

]9- [1

0)-

µ +µ

® 0B

B(

SM

ATLAS Run 1 data

2015-2016 data

Run 1 + 2015-2016 data


8








0 1 2 3 4 5 6 7

]9- [10)- µ +µ ® s0BB(

0.2-

0

0.2

0.4

0.6

0.8

1

1.2

]9- [1

0)-

µ +µ

® 0B

B(

SM

ATLAS 2015-2016 data

Run 1 + 2015-2016 data

LHCb Run 1 + partial Run 2 data


8








9

Angular analysis of Bd0 à K*!+!-


JHEP 10 (2018) 047, arXiv:1805.04000

Full differential angular distribution:

Angular coefficients Sihave large uncertainties

è study ratios to minimize theory uncertainties

q2 = (dilepton invariant mass)2fraction of longitudinally polarised K* mesons

0 2 4 6 8 10 12 14

]2 [GeV2q

[MeV]µµpKm5000 5200 5400 5600

Even

ts /

40 M

eV

0

1000

2000

3000

4000

5000ATLAS

-1 = 8 TeV, 20.3 fbsDataTotal Fit ModelSignalCombinatorial

L(2S) y ® bL +(2S) Ky ® +B(2S) K*y ® sB

10

Bd0 à K*!+!-: event selection


[MeV]µµpKm5000 5200 5400 5600

Even

ts /

40 M

eV

0

10000

20000

30000

40000

50000ATLAS

-1 = 8 TeV, 20.3 fbsDataTotal Fit ModelSignalCombinatorial

L y J/® bL + Ky J/® +B K*y J/® sB

" (resonance removed from analysis)

Signal regionq2 # [0.04, 6]\[0.98, 1.1] GeV2

Analysis performed in q2 bins:[0.04, 2.0], [2.0, 4.0], [4.0, 6.0]

[0.04, 4.0], [1.1, 6.0][0.04, 6.0]

Four reconstructed objects:K $ ! !

All must be in the fiducial tracking volume of ATLAS: |%| < 2.5

m(K $) # [846, 946] MeV (K* mass)m(K$!!) # [5.150, 5.7] GeV (B0 mass)

Important - tracks come from thesame location: B0 vertex χ2/n.d.f. < 2

&(2s)J/&

Control regions:extract mass and width of B0

J/& &(2s)

[MeV]µµpKm5200 5400 5600

Even

ts /

25 M

eV

0

10

20

30

40-1 = 8 TeV, 20.3 fbsATLAS

2 [4.0, 6.0] GeVÎ 2S5 fold, q

DataTotal Fit ModelSignalBackground

11

Bd0 à K*!+!-: angular fit results

Lake Louise Winter Institute 2019Heather Russell, McGill UniversityLqcos

0 0.2 0.4 0.6 0.8 1

Even

ts /

0.04

0

10

20


2 [4.0, 6.0] GeVÎ 2S5 fold, qDataTotal Fit ModelSignalBackground

Kqcos 1- 0.5- 0 0.5 1

Even

ts /

0.08

0

10

20

30

40



[rad]f0 1 2 3

pEv

ents

/ 0.

04

0

10

20



", #K, #L fit simultaneously in each q2 bin:[shown: 4 < q2 < 6 GeV2]

0 5 10 15]4c/2 [GeV2q

1-

0.5-

0

0.5

15'P

(1S)

y/J

(2S)

y

LHCb dataBelle data

ATLAS dataCMS data

SM from DHMVSM from ASZB

12

Bd0 à K*!+!-: results


0 2 4 6 8 10]2 [GeV2q

1-

0.5-

0

0.5

1

1.5

2

5P'

DataCFFMPSV fittheory DHMVtheory JC

ATLAS -1 = 8 TeV, 20.3 fbs

0 2 4 6 8 10]2 [GeV2q

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8LF

DataCFFMPSV fittheory DHMVtheory JC

ATLAS -1 = 8 TeV, 20.3 fbs

Tension in higher q2 bins amongst most measurements:

NB: uses LHCb data!

13

Looking forward – HL-LHC


0 2 4 6 8 10]2 [GeV2q

1-

0.5-

0

0.5

1

1.5

2

5P' ATLAS JHEP 10 (2018) 047

CFFMPSV fittheory DHMVtheory JC

6µ10µATLAS HL 6 (stat. only)µ10µATLAS HL

ATLAS Preliminary-1 = 14 TeV, 3000 fbs

Projections from Run 1 dataµµ K*®B

Upgraded inner tracker, higher statistics allow much higher precision on BR measurement

ATL-PHYS-PUB-2019-003

2 3 4 5 6]-9 ) [10-µ +µ ® s

0B( B

0.1-

0

0.1

0.2

0.3

0.4

0.5

0.6]-9

) [1

0- µ + µ

® 0B(

B

ATLAS Simulation Preliminary-µ+µ ® (s)

0Bworking point x60 Run1 statistics stat + syst

stat onlySM prediction

ATL-PHYS-PUB-2018-005

Expect dramatically reduced uncertainties on Bd0 à K*!+!-parameters, even after accounting for harsher muon pT thresholds

[] B(s) à !+!- measurement presents most stringent B à !+!- limit to date

[] Bd0 à K*!+!- angular analysis, compatible

with results from other experiments

[] run 4 HL-LHC prospects show promising improvement in sensitivity

[] look forward to results with the full 140 fb-1

run 2 dataset14

Conclusions


0 1 2 3 4 5 6 7

]9- [10)- µ +µ ® s0BB(

0.2-

0

0.2

0.4

0.6

0.8

1

1.2

]9- [1

0)-

µ +µ

® 0B

B(

SM

ATLAS Run 1 data2015-2016 dataRun 1 + 2015-2016 dataLHCb Run 1 + partial Run 2 data


0 5 10 15]4c/2 [GeV2q

1-

0.5-

0

0.5

15'P

(1S)

y/J

(2S)

y

LHCb dataBelle data

ATLAS dataCMS data

SM from DHMVSM from ASZB

2 3 4 5 6]-9 ) [10-µ +µ ® s

0B( B

0.1-

0

0.1

0.2

0.3

0.4

0.5

0.6]-9

) [1

0- µ + µ

® 0B(

B

ATLAS Simulation Preliminary-µ+µ ® (s)

0Bworking point x60 Run1 statistics stat + syst

stat onlySM prediction

Backup

Variable Description

pBT

Magnitude of the B candidate transverse momentum�!pT

B.

�2

PV,DV xy Compatibility of the separation�!�x between production (i.e. associated PV) and decay (DV)

vertices in the transverse projection:�!�xT ·⌃ �1

��!�xT

·�!�xT, where ⌃��!�xT

is the covariance matrix.

�Rflight Three-dimensional angular distance between�!p B

and�!�x:

p↵2D

2 + (�⌘)2

|↵2D | Absolute value of the angle in the transverse plane between�!pT

Band�!�xT.

Lxy Projection of�!�xT along the direction of

�!p BT

: (�!�xT ·�!pT

B)/|�!pTB |.

IP3D

B Three-dimensional impact parameter of the B candidate to the associated PV.

DOCAµµ Distance of closest approach (DOCA) of the two tracks forming the B candidate (three-dimen-

sional).

��µµ Azimuthal angle between the momenta of the two tracks forming the B candidate.

|d0 |max-sig. Significance of the larger absolute value of the impact parameters to the PV of the tracks

forming the B candidate, in the transverse plane.

|d0 |min-sig. Significance of the smaller absolute value of the impact parameters to the PV of the tracks

forming the B candidate, in the transverse plane.

Pmin

LThe smaller of the projected values of the muon momenta along

�!pTB

.

I0.7 Isolation variable defined as ratio of |�!pTB | to the sum of |�!pT

B | and the transverse momenta of

all additional tracks contained within a cone of size �R =p

(��)2 + (�⌘)2 = 0.7 around the

B direction. Only tracks matched to the same PV as the B candidate are included in the sum.

DOCAxtrk DOCA of the closest additional track to the decay vertex of the B candidate. Only tracks

matched to the same PV as the B candidate are considered.

Nclose

xtrkNumber of additional tracks compatible with the decay vertex (DV) of the B candidate with

ln( �2

xtrk,DV)<1. Only tracks matched to the same PV as the B candidate are considered.

�2

µ,xPVMinimum �2

for the compatibility of a muon in the B candidate with any PV reconstructed in

the event.

15

B(s) à !+!-: BDT variables


Number of signal events insufficient to fit the full angular distribution è trigonometric folding used to reduce the number of fit parameters

Perform multiple fits, extracting FL, S3, and one Si per fit[Use FL, S3 results with lowest systematic uncertainty]

Example: for S5, transform:

Note: cannot extract S6 (AFB) or S9 in this manner

16

Bd0 à K*!+!-: parameter extraction


Asymmetric, so cancel in folding:

sin(ϕ)sin(2θL)cos(θL)

Recent results from ATLAS in beauty and charm physics · [] introduction to b às lltransitions [] overview of ATLAS B (s)à!+!-measurement [] overview of ATLAS B d 0àK*!+!-angular

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