Evidence for an anomalous like-sign dimuon charge ... - Fermilab

Fermilab Wine & Cheese Seminar, 14 May 2010

Evidence for an anomalous like-sign dimuon charge asymmetry G.Borissov G Borissov Lancaster University, UK

Matter – Antimatter Asymmetry • One of major challenges of particle physics – explain the dominance of matter in our Universe • Number of particles and antiparticles produced in the Big Bang is expected to be equal t0 • For some reason matter becomes more antimatter abundant in the early stages of Universe • Antimatter completely annihilated • Hence we're left only with matter today One of conditions (A. Sakharov) required to explain this process – properties of particles and antiparticles must be different (CP violation)

matter

t  t1 antimatter

matter

t  today antimatter

matter 2

CP violation in SM • CP violation is naturally included in the standard model through the quark mixing (CKM) matrix • Many different measurements of CP violation phenomena are in excellent agreement g with the SM: – All measurements are consistent with a single apex of this unitarity triangle plot

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Dimuon charge asymmetry - Fermilab Wine & Cheese seminar

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CP violation in SM • The SM disagrees with one experimental fact – our existence – The SM source of CP violation is not sufficient to explain p the imbalance between matter and antimatter – See e.g. P. Suet, E. Sather, Phys.Rev.D51, 379-394 (1995) – Some S theoretical h i l studies di claim l i up to 10 orders d off magnitude i d deficit d fi i off the CP violation provided by the SM

• New sources of CP violation are required to explain the matter dominance Search for new sources of CP violation – an important task of current and future experiments

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CP violation in mixing • Main goal of our measurement is to study CP violation in mixing in the Bd and Bs systems • Magnitude of this CP violation predicted by SM is negligible comparing p g to ppresent experimental p sensitivity y • Contribution of new physics can result in a significant modification of the SM pprediction, which can be tested experimentally – Small SM value simplifies detection of possible deviation

A measurement of CP violation significantly different from zero would be unambiguous evidence of new physics

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Dimuon charge asymmetry X

B

B

0

B0

X

• We measure CP violation in mixingg usingg the dimuon charge asymmetry of semileptonic B decays:

N A  N b sl

 b  b

N N

 b  b

– Nb++, Nb−− − number of events with two b hadrons decaying semileptonically and producing two muons of the same charge – One O muon comes ffrom di directt semileptonic il t i decay d b → μ−X – Second muon comes from direct semileptonic decay after neutral B meson mixing: g B0  B 0    X 2010/05/14


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Dimuon and semileptonic charge asymmetry • Absl is equal to the charge asymmetry of "wrong sign" semileptonic il i B decays: d    ( B   X )   ( B   X) b a slb   A sl  (B    X )   (B    X )

– S See Y. G Grossman, Y. Nir, i G G. Raz, PRL 997, 1151801 1801 (2006) – "Right sign" decay is B→μ+X – "Wrong Wrong sign sign" decays can happen only due to flavour oscillation in Bd and Bs – Semileptonic charge asymmetry can also be defined separately f Bd andd Bs : for

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0  0   ( B   X )   ( B   X) q q q a sl  ; q  d, s 0  0   ( B   X )   ( Bq  Wine  X&)Cheese seminar Dimuonq charge asymmetry - Fermilab

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Absl at the Tevatron • Since both Bd and Bs are produced at the Tevatron, Absl is a linear combination of adsll and assll : – Need to know production fractions of Bd and Bs mesons at the Tevatron – Measured by the CDF experiment

Aslb  (0.506  0.043)a sld  (0.494  0.043)a sls Unlike the experiments at B factories, the Tevatron gives a unique possibility to measure the charge asymmetry of both Bd and Bs

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Absl and CP violation • Non-zero value of Absl means that the semileptonic decays of Bq0 and Bq0 are different • It implies CP violation in mixing – it occurs only due to the mixing in the Bd and Bs systems

• It is quantitatively described by the CP violating phase q of the B0q (q = d,s)) mass matrix:  q a  tan(q ) M q q sl

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Absl and the standard model • Standard model predicts a very small value of Absl: Aslb  ( 2.3 00..56 )  10 4 – using prediction of adsl and assl from A. Lenz, U. Nierste, hep-ph/0612167 – New physics contribution can significantly change this value by changing the CP violating phases d and s

Our goal is to measure Absl and compare it with the standard model prediction

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Measurement Method

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Experimental observables • Experimentally, we measure two quantities: • Like-sign Like sign dimuon charge asymmetry:

N   N  A   N  N  • Inclusive muon charge g asymmetry: y y n  n a  n  n – N++, N−− – the number of events with two like-sign dimuons – n+, n− – the number of muons with given charge

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Experimental observables and Absl • Semileptonic B decays contribute to both A and a; • Both A and a linearly depend on the charge asymmetry Absl a  k Aslb  abkg A  K Aslb  Abkg – recall that Absl = absl

• In addition, there are detector related background contributions t ib ti Abkg andd abkg Our task is: • Determine the background contributions Abkg and abkg • Find the coefficients K and k b • Extract the asymmetry A sl - Fermilab Wine & Cheese seminar 2010/05/14 Dimuon charge asymmetry

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Background contribution a  k Aslb  abkg A  K Aslb  Abkg

• Sources of background muons: – – – –

Kaon and K d pion i decays d K+→μ+ν, π+→μ+ν or punch-through h th h proton punch-through False track associated with muon track Asymmetry of muon reconstruction

We measure all background contributions directly in data, with a reduced input from simulation Wi h this With hi approachh we expect to controll andd decrease d the systematic uncertainties 2010/05/14


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Correlated background uncertainties t i ti • The same background processes contribute to both Abkg and abkg • Therefore, the uncertainties of Abkg bk and abkg bk are correlated • We take advantage of the correlated background contributions, and obtain Absl from the linear combination:

A  A   a – Coefficient α is selected such that the total uncertainty of Absl is minimized

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Measurement Details

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Data in this analysis: 6.1 fb-1

This measurement

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Reversal of Magnet Polarities • Polarities of DØ solenoid and toroid are reversed regularly • Trajectory j y of the negative g particle becomes exactly the same as the trajectory of the positive particle with the reversed magnet polarity • by analyzing 4 samples with different polarities (++, −−, +−, −+) • the difference in the reconstruction efficiency between positive and negative particles is minimized Changing polarities is an important feature of DØ detector, which reduces significantly systematics in charge asymmetry measurements 18

Event selection • Inclusive muon sample: – – – – –

Charged particle identified as a muon 1.5 < pT < 25 GeV muon with pT < 4.2 GeV must have |pZ| > 6.4 GeV |η| < 2.2 Distance to primary vertex: 2.8 GeV to suppress events with two muons from the same B decay Dimuon charge asymmetry - Fermilab Wine & Cheese seminar

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Blinded analysis

The central value of Absll was extracted from the full data set only after the analysis method and all statistical and systematic uncertainties had been finalized

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Raw asymmetries a  k Aslb  abkg A  K Aslb  Abkg

• We select: – 1.495×109 muon in the inclusive muon sample – 3.731×106 events in the like-sign dimuon sample

• Raw R asymmetries: ti a  ( 0.955  0.003))% A  ( 0.564  0.053)%

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n  n a  n  n

N   N  A   N  N 


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Detailed description of background a  k Aslb  abkg A  K Aslb  Abkg

• Background contribution abkgg to inclusive muon sample: abkg  f k a k  f a  f p a p  (1  f bkg ) – fK , fπ , and fp are the fractions of kaons, pions and protons identified as a muon in the inclusive muon sample – aK , aπ , and ap are the charge as asymmetries mmetries of kaon, kaon pion, pion and proton tracks – δ is the charge asymmetry of muon reconstruction – fbkg = fK + fπ + fp

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Detailed description of background a  k Aslb  abkg A  K Aslb  Abkg

• Background g contribution Abkg to like-sign g dimuon sample: p Abkg  Fk Ak  F A  F p A p  ( 2  Fbkg ) – FK , Fπ , and Fp are the fractions of kaons, pions and protons identified as a muon in the like-sign dimuon sample – AK , Aπ , and Ap are the charge asymmetries of kaon, pion, and proton tracks – Δ is the charge asymmetry of muon reconstruction – Fbkg = FK + Fπ + Fp ;

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Kaon detection asymmetry abkg  f k a k  f a  f p a p  (1  f bkg ) Abkgg  Fk Ak  F A  F p Ap  ( 2  Fbkgg )

• The largest background asymmetry, and the largest background contribution comes from the charge asymmetry of kaon track identified as a muon (aK, AK) • Interaction cross section of K+ and K− with the detector material is different, especially for kaons with low momentum – e.g., g , for p( p(K)) = 1 GeV:

 ( K  d )  80 mb  ( K  d )  33 mb b

• It happens pp because the reaction K−N→Yπ has no K+N analogue g 2010/05/14


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Kaon detection asymmetry abkg  f k a k  f a  f p a p  (1  f bkg ) Abkgg  Fk Ak  F A  F p Ap  ( 2  Fbkgg )

• K+ meson travels further than K− in the material, and has more chance h off decaying d i to a muon • It also has more chance to punch-through and produce a muon signal i l • Therefore, the asymmetries aK, AK should be positive • All other th background b k d asymmetries t i are found f d to t be b about b t ten t times less This asymmetry is difficult to model, and measuring it directly in data significantly reduces the related systematic uncertainties 2010/05/14


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Measurement of kaon asymmetry

abkg  f k a k  f a  f p a p  (1  f bkg )

Abkg  Fk Ak  F A  F p A p  ( 2  Fbkg )

 → K+ K− decay

• Define sources of kaons: K *0  K  

 (1020)  K  K 

• 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

N(K+→μ+) + N(K−→μ−)

N(K+→μ+) − N(K−→μ−) 2010/05/14


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Measurement of kaon asymmetry abkg  f k a k  f a  f p a p  (1  f bkg ) Abkgg  Fk Ak  F A  F p Ap  ( 2  Fbkgg )

• Results from K*0→K+π− and (1020)→K+K− agree well – For F th the diff difference between b t two t channels: h l χ2/dof /d f =5.4 54/5

• We combine the two channels together:

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Measurement of aπ, ap abkg  f k a k  f a  f p a p  (1  f bkg ) Abkg  Fk Ak  F A  F p Ap  ( 2  Fbkg )

• The asymmetries aπ , ap are measured using the decays KS →π+ π− and Λ→p π− respectively • Similar measurement technique q is used

Data

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aK

aπ

ap

(+5.51 ± 0.11)%

+(0.25 ± 0.10)%

(+2.3 ± 2.8)%


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Measurement of fK , FK abkg  f k a k  f a  f p a p  (1  f bkg ) Abkgg  Fk Ak  F A  F p Ap  ( 2  Fbkgg )

• Fractions fK , FK are measured using the decays K*0 →K+π− • We measure fK*0 , FK*0 • We find fK*0/fK using the similar decay K*+ →K KSπ− – In this decay we measure fK*+/fKs and convert it into fK*0/fK

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Measurement of fπ , fp , Fπ , Fp abkg  f k a k  f a  f p a p  (1  f bkg ) Abkg  Fk Ak  F A  F p Ap  ( 2  Fbkg )

• Fractions fπ , fp , Fπ , Fp are obtained using fK , FK with an additional input from simulation on the ratio of multiplicities nπ / nK and np / nK

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Summary of background composition f bkg  f k  f  f p

• We get the following background fractions in the inclusive muon events: (1−fbkg)

fK

fπ

fp

MC

(59 0±0 3)% (59.0±0.3)%

(14 5±0 2)% (25.7±0.3)% (14.5±0.2)% (25 7±0 3)%

(0 8±0 1)% (0.8±0.1)%

Data

(58.1±1.4)%

(15.5±0.2)% (25.9±1.4)%

(0.7±0.2)%

– Uncertainties for both data and simulation are statistical – Simulation fractions are given as a cross-check only, and are not used in the analysis – Good agreement between data and simulation within the systematic y uncertainties assigned g 2010/05/14


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Muon reconstruction asymmetry abkg  f k a k  f a  f p a p  (1  f bkg ) Abkg  Fk Ak  F A  F p A p  ( 2  Fbkg )

• Reversal of toroid and solenoid polarities cancel the first-order detector effects • Quadratic terms in detector asymmetries still can contribute into tthee muon uo reconstruction eco st uct o asymmetry asy et y • Detector asymmetries for a given magnet polarity adet ≈ O(1%) • We can expect the residual reconstruction asymmetry :

    O(0.01%)

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Muon reconstruction asymmetry abkg  f k a k  f a  f p a p  (1  f bkg ) Abkg  Fk Ak  F A  F p A p  ( 2  Fbkg )

• We measure the muon reconstruction asymmetry y y using J/ψ→μμ events • Average asymmetries δ and Δ are:   ( 0.076  0.028)%   ( 0.068  0.023)%

• To be compared with:

a  ( 0.955  0.003)% A  ( 0.564  0.053)%

Such small values of reconstruction asymmetries are a direct consequence of the regular reversal of magnet 2010/05/14 Dimuon charge asymmetry - Fermilab Wine & Cheese seminar polarities during data taking

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Summary of background contribution abkg  f k a k  f a  f p a p  (1  f bkg ) Abkgg  Fk Ak  F A  F p A p  ( 2  Fbkgg )

• We obtain: fKaK (%) or FKAK (%)

fπaπ (%) or FπAπ (%)

fpap (%) or FpAp (%)

(1-fbkg)δ (%) or (2-Fbkg)Δ (%)

abkg or Abkg

Inclusive

0 854±0 018 0.854±0.018

0 095±0 027 0.095±0.027

0 012±0 022 0.012±0.022

−0 044±0 016 −0.044±0.016

0 917±0 045 0.917±0.045

Dimuon

0.828±0.035

0.095±0.025

0.000±0.021

−0.108±0.037

0.815±0.070

• All uncertainties are statistical • Notice N i that h background b k d contribution ib i is i similar i il for f inclusive i l i muon and dimuon sample: Abkg ≈ abkg 2010/05/14


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Signal contribution k Aslb  a  abkg K Aslb  A  Abkg

n  n a  n  n

N   N  A   N  N 

• After subtracting the background contribution from the "raw" asymmetries a and A, A the remaining residual asymmetries are proportional to Absl • In add addition t o to the t e oscillation osc at o process p ocess Bq0  Bq0    X , several other decays of b- and c-quark contribute to inclusive muon and like-sign dimuon sample  0 0 • All processes except Bq  Bq   X don't produce any charge asymmetry, but rather dilute the values of a and A by contributing ib i in i the h denominator d i off these h asymmetries i

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Coefficients k and K k Aslb  a  abkg K Aslb  A  Abkg

• Coefficients k and K take into account this dilution of " "raw" " asymmetries i a andd A • They are determined using the simulation of b- and c-quark decays – These decays are currently measured with a good precision, and this input p from simulation pproduces a small systematic y uncertainty y

• Coefficient k is found to be much smaller than K, because many more non-oscillating b- and c-quark decays contribute to the asymmetry a: k  0.041  0.003 K  0.342  0.023 2010/05/14

k  0.12  0.01 K 36

Bringing everything together • Using all results on background and signal contribution we get two separate measurements of Absl from inclusive and like-sign dimuon samples: Aslb  ( 0.94  1.12 ((stat))  2.14 ((syst) y ) ))% ((from inclusive ) Aslb  ( 0.736  0.266 (stat)  0.305 (syst) )% (from dimuon ) – Uncertainties of the first result are much larger, because of a ssmall a coe coefficient c e t k = 0.0 0.041±0.003 0.003 – Dominant contribution into the systematic uncertainty comes f from the th measurementt of fK and FK fractions

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Closure test • Th The contribution t ib ti off Absl in i the inclusive muon asymmetry a is suppressed by k = 0.041±0.003 • The value of a is mainly determined by the background asymmetry abkg • We measure abkg in data, data and we can verify how well does it describe the observed asymmetry a • We compare a and abkg as a function of muon pT • We g get χ2/dof = 2.4/5 for the difference between these two distributions

Excellent agreement between the expected and observed values of a , including a pT dependence 2010/05/14


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Combination of measurements • Many uncertainties in these two measurements are correlated • Obtain the final result using the linear combination: A  A   a  ( K   k ) Aslb  ( Abkg   abkg )

– The parameter α is selected such that the total uncertainty of Absll is minimized

• Since Abkgg ≈ abkgg and the uncertainties of these quantities are correlated, we can expect the cancellation of background uncertainties in A' for α≈1 • The signal asymmetry Absl does not cancel in A' for α ≈ 1 because: k  K 2010/05/14


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Combination of measurements • Optimal value of α is obtained by the scan of the total uncertainty of Absll obtained from A A' • • The value α=0.959 is selected:

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Final result

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Final result • From A' = A −α a we obtain a value of Absl : Aslb  ( 0.957  0.251 (stat)  0.146 (syst) )%

• To be compared with ith the SM prediction: Aslb ( SM )  ( 0.023 00..005 006 )%

• This result differs from the SM prediction by ~3.2 σ

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Statistical and systematic uncertainties Absl dimuon

Absl combined

Dominan nt uncertaainties

Absl inclusive muon

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Consistency tests • We modify selection criteria, or use a part of sample to test the stability of result • 16 tests in i totall are performed f d • Very big variation of raw asymmetry t A (up ( to t 140%) due d to variation of background, but Absll remains stable

Developed method is stable and gives consistent result after ft modifying dif i selection l ti criteria it i in i a wide id range 2010/05/14


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Dependence on dimuon mass • We compare the expected and observed dimuon charge asymmetry for different masses of μμ pair • The expected and observed asymmetries agree well for Absl = −0.00957 • No singularity in the M(μμ) shape h supports t B physics h i as the th source of anomalous asymmetry Dependence on the dimuon mass is well described by the analysis method 2010/05/14


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Comparison with other measurements • In this analysis we measure a linear combination of adsl and assl:

Aslb  0.506 a sld  0.494 a sls • Obtained result agrees g well with other measurements of adsl and assl

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preliminary combination • Our (preliminary) combination of all measurements of semileptonic charge asymmetry shows a similar deviation f from th SM. the SM

SM

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Value of assl • Obtained Absl value can be translated to the semileptonic charge asymmetry of Bs meson • We need additional input of adsl = −0.0047±0.0046 measured at B factories • We obtain: a sls  ( 1.46  0.75)%

• To be compared with the SM prediction: a slb ( SM )  ( 0.0021  0.0006)%

• Disagreement with the SM is reduced because of additional experimental input of adsl 2010/05/14


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Comparison with other measurements • Obtained value of assl can be translated into the measurement of the CP violating phase s and ΔΓs • This constraint is in excellent agreement with an independent measurement of s and ΔΓs in Bs→J/ψ decay • This result is also consistent with the CDF measurement in this channel

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Combination of results • This measurement and the result of DØ analysis in Bs→J/ψ can be combined together • This combination excludes the SM value of s at more than % C.L. 95%

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This result at a glance • Evidence of an anomalous charge asymmetry in the number of muons produced in the initially CP symmetric pp interaction • This asymmetry is not consistent with the SM prediction at a 3.2σ level • This new result is consistent with other measurements • We observe that the number of produced particles of matter (negative muons) is larger than the number of produced particles of antimatter • Therefore, Th f the h sign i off observed b d asymmetry is i consistent i with ih the sign of CP violation required to explain the abundance of matter in our Universe This result may provide an important input for explaining the matter dominance in our Universe 2010/05/14


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Conclusions • New measurement of Absl is performed Aslb  ( 0.957  0.251 (stat)  0.146 (syst) )%

• Almost all relevant rele ant quantities q antities are obtained from data with ith minimal input from simulation • Closure test shows good agreement between expected and observed asymmetries y in the inclusive muon sample; p ; • Result differs from the SM prediction by 3.2σ • Result is consistent with other measurements of CP violation in mixing • Dominant uncertainty is statistical – precision can be improved with more luminosity 2010/05/14


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Backup slides

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Absl and CP violation • Non-zero value of Absl means that the semileptonic decays of Bq0 and Bq0 are different; • It implies CP violation in mixing; – it occurs only due to the mixing in Bd and Bs ;

• Quantity describing CP violation in mixing is the complex phase q of the B0q (q = d,s)) mass matrix: p  Mq M q   12 * ( M q )

M q12  i   q    12 * M q  2 ( q )

 q12    q 

M q  M H  M L  2 M q12  q   L   H  2  q12 cos q  M q12  q  arg  12   q 

• aqsl is related with the CP violating phase q as:  q a  tan(q ) M q q sl

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Measurement of fK , FK • Fractions fK , FK are measured using the decays K*0 →K+π− selected in the inclusive muon and like-sign dimuon samples respectively; i l • Kaon is required to be identified as a muon; • We measure fractions fK*0 , FK*0; Inclusive muon sample

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like-sign dimuon sample


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Peaking background contribution • Decay ρ0→π+π− produces a peaking background in the (Kπ) mass • The mass distribution from ρ0→π+π− is taken from simulation

ρ0→π+π−

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Measurement of fK , FK • To convert these fractions to fK , FK we need to know the fraction R(K*0) of charged kaons from K*0 →K+π− and the efficiency to reconstruct an additional pion ε0 :

f K *0  f K R( K ) 0 ; FK *0  FK R( K ) 0 *0

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*0


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Measurement of fK , FK • We also select decay K*+ →KSπ+; • We have:

N K *  N K S R( K * ) c – R(K*+) is the fraction of KS mesons from K*+ →KSπ+ decay; – εc is the efficiency to reconstruct an additional pion;

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Measurement of fK, FK • R(K*+) = R(K*0) due to isospin invariance: – verified with the available data on production of K*+ and K*0 in jets at different energies (PDG); – Also confirmed by simulation; – Related systematic uncertainty 7.5%

• ε0 = εc because the same criteria are used to select the pion in K*+→KSπ+ and K*0 →K+π− – Verified in simulation; – Related systematic uncertainty 3%; 2010/05/14


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Measurement of fK, FK • With these conditions applied, we obtain fK , FK as: N ((K KS ) fK  f K *0 * N (K ) N (KS ) FK  FK *0 * N (K )

– The same values N(KS ), ) N(K*++) are used to measure fK , FK ;

• • We assume that the fraction R(K*0) of charged kaons coming from K*0 →K+π− decay is the same in the inclusive muon and like-sign dimuon sample; – We verified this assumption in simulation;

• We assign the systematic uncertainty 3% due to this assumption; 2010/05/14


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Measurement of fπ, Fπ • We use as an input: – Measured fractions fK , FK; – Ratio of multiplicities of pion and kaon nπ /nK in QCD events taken from simulation; – Ratio of multiplicities of pion and kaon Nπ /NK in QCD events with one additional muon taken from simulation; – Ratio of probabilities for charged pion and kaon to be identified as a muon: P(π→μ)/ P(K→μ) ; – Systematic uncertainty due to multiplicities: 4%

• • We obtain fπ , Fπ as:

2010/05/14

f  f K

P (   ) n P ( K   ) nK

F  FK

P (   ) N  P( K   ) N K


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Measurement of P(π→μ)/ P(K→μ) • The ratio of these probabilities is measured using decays KS →π+ π− and (1020)→K+K− ; • We obtain: P (   ) / P ( K   )  0.540  0.029

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Measurement of fp, Fp • Similar method is used to measure the fractions fp , Fp ; • The decay Λ→pπ− is used to identify a proton and measure P(p→μ)/ P(K→μ); • We obtain: P ( p   ) / P ( K   )  0.076  0.021

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Muon reconstruction asymmetry • We measure the asymmetry of muon reconstruction usingg decays J/ψ→μ+μ−; – Select events with only one id ifi d muon andd one identified additional track; – Build J/ψ ψ meson in these events;; – Extract muon reconstruction asymmetry from the asymmetry in the number of events with positive and negative muon;

2010/05/14

n(μ+) + n(μ−)

n(μ (μ+) − n(μ (μ−)


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Track reconstruction asymmetry • We measure track reconstruction asymmetry using events with one muon and 1 additional track; • We compute the expected track asymmetry using the same method as in the main analysis, and we compare it with the observed asymmetry; • The difference δ = atrk t k− aexp corresponds to a possible residual track reconstruction asymmetry; • We find the residual track reconstruction asymmetry consistent with zero:   ( 0.011  0.035)% 2010/05/14


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Processes contributing to a and A c Aslb  a  abkg C Aslb  A  Abkg





n n a  n  n Process

N   N  A    N N a

A

Bq0  Bq0    X

Yes

Yes

b  c   X

Yes

Yes

Yes

No

Yes

No

B    X (without oscillation) c   X

• All processes except Bq0  Bq0    X don't produce any charge asymmetry, y y but rather dilute the values of a and A by y contributing in the denominator of these asymmetries; 66

Processes contributing to a and A

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Consistency tests

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Consistency tests (cont.)

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72