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Matter-Antimatter differences using muons

Matter-Antimatter differences using muons. D Ø Result on anomalous Dimuon Charge Asymmetry using Full Tevatron Data Set. G.Borissov, Lancaster University, UK representing D Ø collaboration CERN seminar Geneva, 29 October 2013. Matter-Antimatter imbalance.

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Matter-Antimatter differences using muons

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  1. Matter-Antimatter differencesusing muons DØ Result on anomalous Dimuon Charge Asymmetry using Full Tevatron Data Set G.Borissov, Lancaster University, UK representing DØ collaboration CERN seminarGeneva, 29 October 2013

  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 G.Borissov, Matter-Antimatter differences using muons

  3. 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 G.Borissov, Matter-Antimatter differences using muons

  4. 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 G.Borissov, Matter-Antimatter differences using muons

  5. 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 G.Borissov, Matter-Antimatter differences using muons

  6. Known CP-violating processes • Mixing of neutral B0 or Bs0 mesons: • Produces CP violation in mixing G.Borissov, Matter-Antimatter differences using muons

  7. Known CP-violating processes • 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 interference G.Borissov, Matter-Antimatter differences using muons

  8. Physics observables • Asymmetry generated by CP violation in mixing depends on "wrong sign" semileptonic charge asymmetry aqsl 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 , ΓHare 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 G.Borissov, Matter-Antimatter differences using muons

  9. 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: G.Borissov, Matter-Antimatter differences using muons

  10. 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: G.Borissov, Matter-Antimatter differences using muons

  11. 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 Large collected statistics of events containing muons (~2×109) allows us to test the agreement between a and abkgwith statistical precision of ~3×10-4 G.Borissov, Matter-Antimatter differences using muons

  12. DØ Detector Key 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 G.Borissov, Matter-Antimatter differences using muons

  13. Reversal of Magnet Polarities • 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 Changing polarities is an important feature of DØ detector, which reduces significantly systematics in charge asymmetry measurements G.Borissov, Matter-Antimatter differences using muons

  14. 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 includedin this result G.Borissov, Matter-Antimatter differences using muons

  15. 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 G.Borissov, Matter-Antimatter differences using muons

  16. IP dependence • Asymmetry aCP is measured in 3 exclusive IP samples 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. G.Borissov, Matter-Antimatter differences using muons

  17. IP dependence • Asymmetry ACP is measured in 6 non-overlapping samples according to IP1 and IP2 of two muons Like-sign dimuon sample G.Borissov, Matter-Antimatter differences using muons

  18. (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 central intermediate forward G.Borissov, Matter-Antimatter differences using muons

  19. 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 G.Borissov, Matter-Antimatter differences using muons

  20. 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 Aibkg(IP1,IP2) are measured in data with minimal input from simulation G.Borissov, Matter-Antimatter differences using muons

  21. 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 N − nucleon G.Borissov, Matter-Antimatter differences using muons

  22. Background asymmetries intermediate central forward • 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 andJ/ψ→μ+μ−decay • Measured directly in data • Measured in each (pT ,|η|) bin • Average values:

  23. Background asymmetries • Background asymmetries in each (pT ,|η|) bin i and each IP sample • fKi , fπi , andfpiare fractions of K→μ, π→μ, p→μ in inclusive muon sample • FKi , Fπi , andFpiare fractions of K→μ, π→μ, p→μ in dimuon sample • aμi , Aμi are the charge asymmetries of muon detection G.Borissov, Matter-Antimatter differences using muons

  24. 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) G.Borissov, Matter-Antimatter differences using muons

  25. Charge asymmetry of inclusive muons aCP G.Borissov, Matter-Antimatter differences using muons

  26. 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 G.Borissov, Matter-Antimatter differences using muons

  27. 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 G.Borissov, Matter-Antimatter differences using muons

  28. Asymmetry aCP • Raw asymmetry is large for IP=1 sample • Background asymmetryexplains it • aCP is consistent with zero for all IP samples

  29. 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 G.Borissov, Matter-Antimatter differences using muons

  30. Closure test: IP=1 • Test consistency with zero of aiCP for all (pT ,|η|) bins • Raw asymmetry varies by more than 1.5% • Background asymmetry abkgexplains these variations • Asymmetry aCP is consistent with zero within 0.05% χ2=7.54/8 d.o.f G.Borissov, Matter-Antimatter differences using muons

  31. Closure test: IP=2 • Test consistency with zero of aiCP for all (pT ,|η|) bins • Raw asymmetry varies by more than 1% • Background asymmetry abkgexplains these variations • Asymmetry aCP is consistent with zero within 0.03% χ2=3.48/8 d.o.f G.Borissov, Matter-Antimatter differences using muons

  32. Closure test: IP=3 • Test consistency with zero of aiCP for all (pT ,|η|) bins • Raw asymmetry varies by more than 0.5% • Background asymmetry abkgexplains these variations • Asymmetry aCP is consistent with zero within 0.05% χ2=10.8/8 d.o.f G.Borissov, Matter-Antimatter differences using muons

  33. 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 G.Borissov, Matter-Antimatter differences using muons

  34. Charge asymmetry of like-sign dimuons ACP G.Borissov, Matter-Antimatter differences using muons

  35. Dependence on (pT ,|η|) 2 entries per each dimuon event • 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 χ2=7.6/8 d.o.f G.Borissov, Matter-Antimatter differences using muons

  36. Measurements in IP samples χ2=5.8/8 d.o.f χ2=6.8/8 d.o.f χ2=5.0/8 d.o.f χ2=7.5/8 d.o.f χ2=5.0/8 d.o.f χ2=3.5/8 d.o.f G.Borissov, Matter-Antimatter differences using muons

  37. 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 Strong evidence (4.1 σ) of non-zero charge asymmetry G.Borissov, Matter-Antimatter differences using muons

  38. 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 PRD74, 092001 (2006) PRD82, 032001 (2010) PRD84, 052007 (2011) arXiv:1310.0447[hep-ex] G.Borissov, Matter-Antimatter differences using muons

  39. 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 G.Borissov, Matter-Antimatter differences using muons

  40. 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 G.Borissov, Matter-Antimatter differences using muons

  41. Interpretation • If we assume that the measured charge asymmetries are produced by CP violation in mixing and CP violation in interference, we can extract adsl, 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 G.Borissov, Matter-Antimatter differences using muons

  42. Results • We get: • deviates from the SM prediction by 3.0 σ • Result is consistent with independent DØ measurements of • adsl : PRD 86, 072009 (2012) • assl : PRL 110, 011801 (2013) muon asymmetry G.Borissov, Matter-Antimatter differences using muons

  43. 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 G.Borissov, Matter-Antimatter differences using muons

  44. Example of impact of ΔΓd / Γd • Deviation from SM is sensitive to the value of ΔΓd / Γd • Using the WA value of ΔΓd / Γdinstead of the SM, we get: • In this example the deviation from the SM prediction for adsl , asslis 1.9 σ Independent measurements of ΔΓd / Γdare required G.Borissov, Matter-Antimatter differences using muons

  45. 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 The most precise measurement of these quantities so far in a single experiment G.Borissov, Matter-Antimatter differences using muons

  46. Comparison with other experiments • Other measurements BaBar, Belle: adsl 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 G.Borissov, Matter-Antimatter differences using muons

  47. Conclusions • Measurements of aCP and ACP in different IP samples are obtained • Evidence with confidence 4.1 σof deviation of muon asymmetry from zero • Our results deviate from the SM prediction by 3.6 σ • Available in arXiv: 1310.0447[hep-ex] G.Borissov, Matter-Antimatter differences using muons

  48. Backup slides G.Borissov, Matter-Antimatter differences using muons

  49. Measurement of kaon asymmetry → K+ K− decay • 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 N(K+→μ+) + N(K−→μ−) N(K+→μ+) − N(K−→μ−) G.Borissov, Matter-Antimatter differences using muons

  50. 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 G.Borissov, Matter-Antimatter differences using muons

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