matter-antimatter differences using muons

Matter-Antimatter differencesusing muons

G.Borissov, Lancaster University, UK

representing DØ collaboration

CERN seminarGeneva, 29 October 2013

DØ Result on anomalous Dimuon Charge Asymmetry using Full Tevatron Data Set

29 October 2013 G.Borissov, Matter-Antimatter differences using muons 2

Matter-Antimatter imbalance

• Mystery of imbalance between matter and antimatter in the universe – one of the big questions which need to be addressed by particle physics

• Understanding CP violation and the mechanisms which produce it may be the key to resolving it– CP violation reflects the differences in properties of particles and

antiparticles

– Essential ingredient to explain the imbalance between matter and antimatter


CP violation

• CP violation is naturally included in the Standard Model (SM) through the complex phase of the CKM mass mixing matrix– The single apex of the

Unitarity triangle is the best confirmation of the SM mechanism of CP violation

• BUT: CP violation accounted for by the SM is not sufficient to explain the observed imbalance between matter and antimatter

• Searching for new sources of CP violation is important to understand the evolution of our universe and test the SM


Null Test

• "Null test" is the strategy in this search– Measure CP violation in processes where the SM prediction and its

theoretical uncertainty are expected to be small compared to the experimental sensitivity

• In this case, statistically significant deviation of the experimental result from zero would indicate unambiguously the contribution of new physics

• Like-sign dimuon charge asymmetry is a special kind of null test– Subject of this seminar


Like-sign dimuon charge asymmetry

– N++, N −− − number of events with two muons of the same charge

• Excluding the detector-related effects, this asymmetry can be produced only by CP-violating processes

• Theoretically well predicted quantity– SM expectation and its uncertainty are smaller than the experimental

sensitivity – sensitive null tests

• Inclusive measurement – Large statistics is available

– As yet unknown sources of CP violation could contribute in this asymmetry

NN

NNA


• Mixing of neutral B0 or Bs0 mesons:

– Produces CP violation in mixing

Known CP-violating processes

BX

X

0)( sB

0)( sB

mix

)()( 0)(

0)(

0)(

0)( XBBXBB ssss


• Interference of the B0 (Bs0) decays with and without mixing to the

final state accessible to both

– Produces CP violation in interference of decays with and without mixing

• Example: – D(*)+D(*)− produce both μ+ and μ− , but only μ− contributes to the like-sign

dimuon asymmetry

Known CP-violating processes

0)(

0)( ss BB and

XDDDBB (*)(*)(*)00 )( with

mix

CPf

BX 0)( sB

0)( sB 0

)( sB

))(())(( 0)(

0)(

0)(

0)( CPssCPss fBBfBB

interference


Physics observables

• Asymmetry generated by CP violation in mixing depends on "wrong sign" semileptonic charge asymmetry aq

sl of Bq0 meson (q=d,s)

• Asymmetry generated by CP violation in interference depends on width difference of the Bq0

meson system ΔΓq / Γq

– ΓL , ΓH are the width of light (L) and heavy (H) mass eigenstates of Bq0 system

– Contribution of Bs0 meson is strongly suppressed

– See: GB, B. Hoeneisen, PRD 87, 074020 (2013)

• We extract these quantities from the measured dimuon asymmetry

sdqXBBXBB

XBBXBBa

qqqq

qqqqq ,;)()(

)()(0000

0000

sl

sdqHqLqq ,;,,


Measurement overview• Measure raw asymmetry A by simple counting of the like-

charge dimuon events

• Identify all background contributions and measure the background asymmetry Abkg directly in data– Background is, by definition, any process producing the dimuon

charge asymmetry and not related to CP violation

– Difference in the interaction of particles and antiparticles with the detector material is the main source of Abkg

• Determine a model-independent asymmetry ACP as:

bkgCP AAA


Inclusive muon charge asymmetry

• We also measure the inclusive muon charge asymmetry

– n+ (n−) is the number of detected positive (negative) muons

• Measure the background charge asymmetry abkg of inclusive muons directly in data– Background is, by definition, any process producing the inclusive muon

charge asymmetry and not related to CP violation

• Determine a model-independent asymmetry aCP of inclusive muons as:

nn

nna

bkgCP aaa


Closure test

• The SM expectation of aCP(SM) is much less than ACP(SM) :

• Consistency of measured aCP = a – abkg with zero provides a stringent closure test of our procedure to measure abkg

• It also validates the procedure to measure Abkg

– Sources of the background asymmetries abkg and Abkg are the same

– Their measurement procedure is also the same

)()( SMASMa CPCP

Large collected statistics of events containing muons (~2×109) allows us to test the agreement between a and abkg

with statistical precision of ~3×10-4


DØ DetectorKey elements for

this measurement:• Muon system• Muon trigger• Solenoid + Toroid• Tracking with precise

vertex detector• interaction• Polarities of magnets

were regularly reversed• Low punch-through• Wide acceptance up to |η|~2

pp

G.Borissov, Matter-Antimatter differences using muons 13

• Polarities of DØ solenoid and toroid were regularly reversed

• Data samples with different magnet polarities are of about thesame size and are added togetherwith proper weights

• This allows to cancel the first order detector effects in the charge asymmetries – Trajectory of the negative particle becomes

exactly the same as the trajectory of the positive particle with the reversed magnet polarity

Reversal of Magnet Polarities

Changing polarities is an important feature of DØ detector, which reduces significantly systematics

in charge asymmetry measurements


Previous results• DØ published 3 results on this subject

• Last result published in 2011 with 9 fb-1 (PRD 84, 052007,2011):

– aCP is consistent with zero

– ACP significantly deviates from zero

• Interpreting this result as CP violationin B0

(s) mixing we obtain:

– 3.9 σ deviation from the SM– Contribution from CP violation in interference was not included

in this result

)% (syst) (stat) 093.0172.0787.0( ssls

dsld aCaC

)%063.0067.0276.0(

)%079.0042.0034.0(

CP

CP

A

a

(2011) 052007 84, PRD

1; sdssls

dsld CCaCaC


New measurement

• Increased luminosity from 9.0 fb-1 to 10.4 fb-1

• Detailed study of the dependence of asymmetry on muon impact parameter (IP)

• Detailed study of the dependence of asymmetry on the muon kinematic parameters (pT ,|η|)

• Alternative method to measure the background contribution– Important cross-check of the previously used method

• Additional CP-violating process is included when interpreting the obtained results– CP violation in interference of decays with and without mixing


IP dependence• Asymmetry aCP is measured in 3 exclusive IP samples

IP sample

muon IP (μm)

IP=1 0 – 50

IP=2 50 – 120

IP=3 120 – 3000

Inclusive muon sample

IP distributions of one muon in the like-sign dimuon sample when the other muon has IP in the IP=1 (full line), IP=2 (dashed line), and IP=3 (dotted line) range.


IP dependence

• Asymmetry ACP is measured in 6 non-overlapping samples according to IP1 and IP2 of two muons

IP sample 1st muon IP (μm) 2nd muon IP (μm)

IP1,IP2=11 0 – 50 0 – 50

IP1,IP2=12 0 – 50 50 – 120

IP1,IP2=13 0 – 50 120 – 3000

IP1,IP2=22 50 – 120 50 – 120

IP1,IP2=23 50 – 120 120 – 3000

IP1,IP2=33 120 – 3000 120 – 3000

Like-sign dimuon sample


(pT , |η|) dependence• Additionally, each IP sample is divided into 9 bins depending on

the muon transverse momentum (pT) and pseudorapidity (η)– General kinematic selection of muons:

• 1.5 < pT < 25 GeV; |η| < 2.2

• pT > 4.2 GeV OR |pZ| > 5.4 GeV

Bin pT (GeV) |η|

1 <5.6 <0.7

2 5.6 – 7.0 <0.7

3 >7.0 <0.7

4 <5.6 0.7 – 1.2

5 >5.6 0.7 – 1.2

6 <3.5 1.2 – 2.2

7 3.5 – 4.2 1.2 – 2.2

8 4.2 – 5.6 1.2 – 2.2

9 > 5.6 1.2 – 2.2

cen

tral

inte

rmed

iate

forw

ard


Our measurement

• Measure the asymmetry aCP in 3×9 non-overlapping samples of inclusive muon events with different IP and (pT ,|η|)

• Measure the asymmetry ACP in 6×9 samples of like-sign dimuon events with different IP1, IP2 and (pT ,|η|)– Division into the IP samples helps to distinguish between different

CP-violating processes contributing to aCP and ACP

– Background varies considerably in different IP samples

– Division into (pT ,|η|) bins allows a better measurement of background contribution, which can vary significantly in different kinematic regions

– Division into (pT ,|η|) bins also provides a rigorous test of our measurement procedure


Our measurement

• aCP and ACP are obtained in each in each (pT ,|η|) bin i and in each IP sample :

– Asymmetries ai(IP) and Ai(IP1,IP2) are determined by simple counting of events

– Asymmetries aibkg(IP) and Ai

bkg(IP1,IP2) are measured in data with minimal input from simulation

)IP,(IP)IP,(IP)IP,(IP

(IP)(IP)(IP)

21bkg2121CP

bkgCP

iii

iii

AAA

aaa


Origin of abkg and Abkg

• The dominant background contribution comes from the charge asymmetry of kaon identified as a muon (K→μ)

• Origin of this asymmetry:– because the reaction K−N→Yπ has no K+N analogue

• K+ meson travels further than K− in the material, and has more chance to – decay K→μν

– punch-through the detector material and produce a signal in muon detector

• Asymmetry of K→μ should be positive

• Asymmetry of π→μ should be significantly smaller– because of the small difference in the (πN) cross section between

positive and negative pions

)()( NKNK N − nucleon

29 October 2013 22

Background asymmetries

• Measure charge asymmetry for each type of particle: aK , aπ , ap

– e.g., aK is measured using kaons from K*0 →K− π+ decay

• Also measure charge asymmetry of muon detection (δ)– using "tag and probe" method and

J/ψ→μ+μ− decay

• Measured directly in data

• Measured in each (pT ,|η|) bin

• Average values:

)%02.013.0(

)%42.343.1(

)%08.006.0(

)%10.010.5(

p

K

a

a

a

central

inte

rmed

iate

forward


)IP,(IP)IP,(IP)IP,(IP

(IP)(IP)(IP)

21bkg2121CP

bkgCP

iii

iii

AAA

aaa

Background asymmetries

• Background asymmetries in each (pT ,|η|) bin i and each IP sample

– fKi , fπ

i , and fpi are fractions of K→μ, π→μ, p→μ in inclusive muon sample

– FKi , Fπ

i , and Fpi are fractions of K→μ, π→μ, p→μ in dimuon sample

– aμi , Aμ

i are the charge asymmetries of muon detection

iip

ip

iiik

ik

ibkg

iip

ip

iiik

ik

ibkg

AaFaFaFA

aafafafa


Background fractions

• K→μ, π→μ, p→μ are produced mainly in light quark and gluon jets

• Therefore, their tracks originate from the primary interaction point

• Muons from b-quark decay often have large IP

• Background fractions should decrease significantly in the sample with large IP (IP=3)

),,(

)()()(

sduq

XbbppggXppXqqpp


Charge asymmetry of inclusive muons aCP


Background fractions

• Measured directly in data

• Measure in each (pT ,|η|) bin and in each IP sample

• Alternative independent method is used to cross check the measurement

• Background vary by a factor >7 between IP=1 (low IP) and IP=3 (high IP)

– Only statistical uncertainties are shown

IP=1 IP=2 IP=3

fK, (%) 20.30±0.34 7.71±0.24 2.69±0.14

fπ, (%) 39.13±2.09 15.39±0.83 5.32±0.30

fp, (%) 0.73±0.21 0.28±0.09 0.08±0.09


Contributions to Background asymmetry

– Only statistical uncertainties are given

• Contribution from K→μ dominates in the samples with low IP

• Background is considerably reduced for IP=3

• For IP=3 the kaon and muon detection asymmetries have approximately the same magnitude

IP=1 IP=2 IP=3

K→μ (%) +1.037±0.029 +0.385±0.017 +0.134±0.010

π→μ (%) −0.022±0.040 −0.019±0.016 −0.007±0.006

p→μ (%) −0.010±0.012 −0.004±0.005 −0.001±0.001

aμ (%) −0.050±0.009 −0.102±0.017 −0.128±0.021

abkg (%) +0.954±0.053 +0.250±0.027 −0.001±0.023

a (%) +0.930±0.003 +0.277±0.006 −0.049±0.005

29 October 2013 28

Asymmetry aCP

• Raw asymmetry is large for IP=1 sample

• Background asymmetryexplains it

• aCP is consistent with zero for all IP samples

(IP)(IP)(IP) bkgCP aaa

Sample a (%) abkg (%) aCP (%)

All IP +0.670±0.002 +0.702±0.042 −0.032±0.042±0.061

IP=1 +0.930±0.003 +0.954±0.053 −0.024±0.053±0.075

IP=2 +0.277±0.006 +0.259±0.027 +0.018±0.028±0.024

IP=3 −0.049±0.005 −0.001±0.023 −0.048±0.024±0.011


SM expectation

• SM expectation for the inclusive muon asymmetry aCP is ~10-5

• It is strongly suppressed by the large fraction of non-CP violating processes contributing to the inclusive muon sample– e.g. c→μX or b→μX without oscillation

• Consistency of aCP with zero is an important "closure test" to verify our procedure of background measurement

We observe this consistency in all IP samples, while the raw asymmetry varies considerably and even changes the sign


Closure test: IP=1

• Test consistency with zero of ai

CP for all (pT ,|η|) bins

• Raw asymmetry varies by more than 1.5%

• Background asymmetry abkg explains these variations

• Asymmetry aCP is consistent with zero within 0.05%

χ2=7.54/8 d.o.f


Closure test: IP=2



• Raw asymmetry varies by more than 1%



χ2=3.48/8 d.o.f


Closure test: IP=3



• Raw asymmetry varies by more than 0.5%



χ2=10.8/8 d.o.f


Conclusions on aCP

• Measurement of aCP confirms a validity of our procedure of background measurement– Background varies considerably in different (pT ,|η|) bins

– Background changes by a factor >7 between different IP samples

– Still we get a consistent result: a and abkg agree within 0.03%

• This closure test validates our procedure of the background measurement

• Provides confidence in the measurement of like-sign dimuon asymmetry ACP where the same method of background measurement is used


Charge asymmetry of like-sign dimuons ACP


Dependence on (pT ,|η|)

• There is an overall deviationfrom zero of ACP

– This value is shown in the last bin

– Only statistical uncertainties are shown in the plot

• Raw asymmetry A varies considerably in (pT ,|η|) bins

• Asymmetry Abkg explains thesevariations

2 entries per each dimuon event

)%055.0064.0235.0()IP all( CPA

χ2=7.6/8 d.o.f


Measurements in IP samples

χ2=6.8/8 d.o.f

χ2=5.0/8 d.o.f

χ2=5.0/8 d.o.f

χ2=5.8/8 d.o.f

χ2=7.5/8 d.o.f

χ2=3.5/8 d.o.f


Measurements in IP samples• Same pattern is observed for the

measurements in different IP1, IP2 samples

– good stability of ACP in (pT ,|η|) bins

– Overall shift of ACP

• Deviation from zero of the combination of all our asymmetrymeasurements (3 aCP and 6 ACP) is

9/5.36d.o.f./2

Strong evidence (4.1 σ) of non-zero charge asymmetry


Stability in time

• We published several papers on the dimuon asymmetry

• The result on the residual dimuon asymmetry is very stable – Change of luminosity 1.0 → 10.4 fb-1

– Change of the analysis method

arXiv:1310.0447[hep-ex]

PRD74, 092001 (2006)

PRD82, 032001 (2010)

PRD84, 052007 (2011)


Comparison with the SM• Two SM contributions to the dimuon asymmetry are expected:

– CP violation in mixing– CP violation in interference of decays with and without mixing

• Second contribution is dominant– For all IP:

– The second contribution was established just recently [ GB, B. Hoeneisen, PRD 87, 074020 (2013) ]

– It was not considered in our previous publications

4int4mix

int5mix

10)8.05.3()(;10)1.08.0()(

0)(;10)2.08.0()(

SMASMA

SMaSMa

CPCP

CPCP

)()()(

)()(intmix

mix

SMASMASMA

SMaSMa

CPCPCP

CPCP


Comparison with the SM

• Comparing 9 measurements (3 measurements of aCP and 6 measurements of ACP) with the SM prediction we get:

3.6 standard deviations from the SM prediction

9/0.31d.o.f./2


Interpretation• If we assume that the measured charge asymmetries are produced

by CP violation in mixing and CP violation in interference, we can extract ad

sl, assl, and ΔΓd/Γd

– Asymmetries depend linearly on them

– Coefficients Kd, Ks, and KΓ are different in different IP sub-samples

– These coefficients are determined using the MC input and measured quantities

d

dss

dd

ss

dd

KaKaKA

akaka

)IP,IP()IP,IP()IP,IP()IP,IP(

)IP()IP()IP(

21sl21sl2121CP

slslCP


Results

• We get:

– deviates from the SM prediction by 3.0 σ

• Result is consistent with independent DØ measurements of– ad

sl : PRD 86, 072009 (2012)

– assl : PRL 110, 011801 (2013)

)%38.150.0(

)%99.082.0(

)%43.062.0(

d

d

ssl

dsl

a

a

muon asymmetry 66.0

03.0

61.0

,

,

,

s

d

sd


Impact of ΔΓd / Γd

• The contribution of CP violation in interference depends on the value of ΔΓd / Γd

• This quantity is measured experimentally with poor precision

• SM expectation:

• Contrarily to other quantities, NP contribution to ΔΓd / Γd is not experimentally constrained yet

210)8.15.1(( WA),experimentd

d

210)08.042.0(SM)( d

d


Example of impact of ΔΓd / Γd

• Deviation from SM is sensitive to the value of ΔΓd / Γd

• Using the WA value of ΔΓd / Γd instead of the SM, we get:

– In this example the deviation from the SM prediction for ad

sl , assl is 1.9 σ

(fixed) 0150.0

79.0

)%74.035.0(

)%42.062.0(

,

d

d

sd

ssl

dsl

a

a

Independent measurements of ΔΓd / Γd are required


Combination with other DØ results

• Combining our 3 results (see figure) we get more precise values:

– Deviates from the SM prediction by 3.1 σ

• The measurements are consistent– χ2/d.o.f. = 4.4/2

)%15.179.0(

)%58.033.1(

)%29.009.0(

d

d

ssl

dsl

a

a

The most precise measurement of these quantities so far in a single experiment

55.0

24.0

34.0

,

,

,

s

d

sd

G.Borissov, Matter-Antimatter differences using muons 46

Comparison with other experiments

• Other measurementsBaBar, Belle: ad

sl PRL 111, 101802 (2013), HFAG, arXiv:1207.1158 [hep-ex]

LHCb: assl arXiv:1308.1048 [hep-ex]

Belle: ΔΓd / Γd PRD 85, 071105 (2012)

• Our combination of all world results gives:

– Deviates by 2.9 σ from the SM prediction

• Good consistency of all measurements– χ2/d.o.f. = 4.3/3

)%91.051.1(

)%43.083.0(

)%21.009.0(

d

d

ssl

dsl

a

a

47.0

23.0

25.0

,

,

,

s

d

sd


Conclusions• Measurements of aCP and ACP in

different IP samples are obtained

• Evidence with confidence 4.1 σ of deviation of muon asymmetry from zero

• Our results deviate from the SM prediction by 3.6 σ

• Available in arXiv: 1310.0447[hep-ex]


Backup slides


Measurement of kaon asymmetry

• Define sources of kaons:

• Require that the kaon is identified as a muon

• Build the mass distribution separately for positive and negative kaons

• Compute asymmetry in the number of observed events

KK

KK

)1020(

0*

→ K+ K− decay

N(K+→μ+) + N(K−→μ−)

N(K+→μ+) − N(K−→μ−)


Alternative method to measure background

• Up to now we used the fraction of K*0 to measure the fraction of K→μ events in our sample (K*0 method)

• Recently we developed an independent method to measure background using local muon variables (LV method)

• Background muons come from K→μν and π→μν decays

• When we reconstruct a muon, the charged track in the central tracker is associated with the local track in the muon detector

• If a muon comes from kaon or pion decay, the track parameters of the central track and local track are different


Local variables• Distribution of local variables is significantly different for

signal (S muons) and background (L muons)

• It is very similar for K→μ and π→μ because of similar origin

centrallocal PPX / |)()(| centrallocalY


Measuring the fraction of background

• Use the templates for L muons and S muons to measure the total background fraction

• Templates are obtained in data

• Background fraction is measured independently for each (pT, |η|) bin

• Here the example of the fit for bin 2 is presented

• Ratio π/K is fixed to the expected value


Comparison of two methods• Use local variables (LV) of muon detector to measure background

• Compare it with the "K*0" method used so far in the dimuon papers

• Very good agreement between two methods, including the IP dependence

• Uncertainties for both methods do not include the systematics

• Systematics for two methods is different and independent

Method All IP IP=1 IP=2 IP=3

Local variables (%) 42.56±0.87 55.10±0.94 22.73±1.15 8.51±0.14

K*0 method (%) 46.73±1.76 60.19±2.21 23.38±1.01 8.09±0.47

relative difference (%)

−8.92±3.90 −8.46±3.71 −2.78±6.47 5.19±6.35

Background fraction in the inclusive muon sample

LV method is used as an important cross check of the K*0 method, providing the estimate of its systematic uncertainty


World Average Inputs• Direct measurements of as

sl= (-0.50 ± 0.52)%

– D0: Phys. Rev. Lett., 110, 011801 (2013) assl= (-1.12 ± 0.76)%

– LHCb: arXiv:1308.1048 assl= (-0.06 ± 0.63)%

• Direct measurements of adsl= (+0.23 ± 0.26)%

– Previous B-Factory Results (HFAG) adsl= (-0.05± 0.56)%

– D0: Phys. Rev. D 86, 072009 (2012) adsl= (+0.68± 0.47)%

– New BaBar: Phys. Rev. Lett. 111, 101802 (2013) adsl= (+0.06 ± 0.38)%

• Direct Measurement of ΔΓ/Γ = (+1.5± 1.8)%– Previous B-Factory Results (HFAG)

• Combined with the muon asymmetries using 3-dimensional χ2 minimisation and the correlations.


WA Combination Plots

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