We report a few recent results from the CDF and D0 Collaborations in the sector of Heavy Flavors. They include measurements of CP-violating asymmetries in B hadron charmless decays, studies of the kinematical properties of flavor changing neutral current decays characterized by the transition of a quark b to a quark s and a muon pair, a search for Bs decay into a muon pair, and measurements of mixing asymmetries in semi-leptonic decays of Bs mesons. All these

measurements are sensitive to the possible presence of new physics

Ristori, L.

English

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DOI 10.1393/ncc/i2014-11636-3Colloquia: LaThuile13IL NUOVO CIMENTO Vol. 36 C, N. 6 Novembre-Dicembre 2013Searches for new physics in the charm and beauty sectorat the TevatronL. Ristori on behalf of the CDF and D0 CollaborationsINFN, Sezione di Pisa - Pisa, Italy andFermi National Accelerator Laboratory - Batavia, Il, USAricevuto il 20 Giugno 2013; approvato l’1 Luglio 2013Summary. — We report a few recent results from the CDF and D0 Collabora-tions in the sector of Heavy Flavors. They include measurements of CP -violatingasymmetries in B hadron charmless decays, studies of the kinematical properties offlavor changing neutral current decays characterized by the transition of a quark bto a quark s and a muon pair, a search for Bs decay into a muon pair, and mea-surements of mixing asymmetries in semi-leptonic decays of Bs mesons. All thesemeasurements are sensitive to the possible presence of new physics.PACS 13.20.He – Decays of bottom mesons.PACS 13.30.Eg – Hadronic decays of baryons.PACS 14.40.Nd – Bottom mesons.PACS 11.30.Er – Charge conjugation, parity, time reversal, and other discretesymmetries.1. – IntroductionThe Tevatron was shut down for good on September 30, 2011 and both CDF and D0have now approximately 10 fb−1 of recorded data to be analyzed. This contribution willreport on a few new results obtained during the last year, since 2012 winter conferences,by the CDF and D0 Collaborations about processes that are particularly sensitive topossible deviations from the Standard Model.The heavy flavor production rates at the Tevatron were very high, actually higher thanat B-factories, with the handicap that interesting events were just a small fraction of thetotal production rate. But some very important detector features helped overcome thehandicap, like excellent mass resolution, precision vertex reconstruction, and a powerfultrigger on secondary vertices. Another very important feature was the forward-backwardsymmetry of the detectors and the CP -symmetric pp¯ initial state. These result in an im-portant advantage over the LHC in measuring small CP asymmetries because you areguaranteed to get the same number of particles and antiparticles in the acceptance.c© Societa` Italiana di Fisica 208SEARCHES FOR NEW PHYSICS IN THE CHARM AND BEAUTY SECTOR AT THE TEVATRON 209]2c [GeV/-+m5.0 5.2 5.4 5.6 5.82 cCandidates per 10 MeV/110210310410-+ K0B-K+ K0sB-+ 0B-K+ K0B+- K0sB- pK0b- p0b-+ 0sBMultibody B decaysCombinatorial bkgdatatotal2 cCandidates per 10 MeV/-1 = 9.30 fbL dtCDF Run II Preliminary Fig. 1. – Invariant mass distribution of pairs of tracks coming from a secondary vertex. Pionrest mass is assumed for both tracks. Many different processes overlap to form a single broadpeak. Individual contributions, as resulting from the fit, are shown with different colors.2. – ACP in charmless B-hadron decaysCharmless B-hadron decays are particularly interesting because they are more directlyconnected to the elements of the CKM mixing matrix and their study is important totest its unitarity and detect the possible effects of new physics. Particularly important isthe measurement of charge asymmetry (Acp) in this class of decays [1,2]. Acp is definedas the difference of opposite flavor branching ratios divided by the sum:(1) Acp =B(b→ f)− B(b¯→ f¯)B(b→ f) + B(b¯→ f¯) .Acp can be measured counting the number of opposite flavor decays and correctingfor detector efficiencies.The data sample for this analysis is collected by the trigger on displaced tracks [3,4].We search for two tracks coming from a secondary vertex and build the invariant massassuming the π meson mass for both tracks. Figure 1 shows the mass spectrum on a logscale. Many individual modes overlap in a single broad peak. We use particle ID andkinematics in a multi-dimensional un-binned fit to separate the contributions of differentdecays (also shown in the figure). The shapes of the individual contributions are obtainedfrom Monte Carlo.To distinguish different particle species we use both kinematics and dE/dx. Whilelooking for small effects, we need to correct for small charge asymmetries in detectoracceptance and efficiency. The CDF detector is not charge symmetric due to the par-ticular structure of the wiring of the central chamber [5] which results in an asymmetryin reconstruction efficiency between positive and negative charged tracks of order of apercent. Additionally, the efficiency for detecting K+ and K− is slightly different due tothe intrinsically different properties of the interactions of opposite sign K mesons withmatter. To correct for these detector induced asymmetries CDF has studied a sample210 L. RISTORI on behalf of the CDF and D0 COLLABORATIONS]2-mass [GeV/c+π-Invariant K1.8 2 2.2 2.42Candidates per 5 MeV/c00.511.52610× 3445±) = 7257239 +π- K→0N(D/ndf = 250/2322χ)-1Data (5.9 fbFit RSπ K→D  WSπ K→D ππ →D Multibody D decaysCombinatorial backg.σ/Δ-202]2-mass [GeV/c-π+Invariant K1.8 2 2.2 2.42Candidates per 5 MeV/c00.511.52610× 3460±) = 7378944 -π+ K→0DN(/ndf = 250/2322χ)-1Data (5.9 fbFit RSπ K→D  WSπ K→D ππ →D Multibody D decaysCombinatorial backg.σ/Δ-202]2-mass [GeV/cπInvariant K1.8 2 2.2 2.4Asymmetry-0.02-0.0100.010.02 0.033)%±) = (-0.832 +π- K→0A(D/ndf = 250/2322χ)-1Data (5.9 fbFitσ/Δ-202Fig. 2. – Results of the combined fit of the tagged D0 → K−π+ samples. Distribution of D0πsmass for charm (left), and anticharm (center) decays, and asymmetry as a function of the mass(right). Fit results are overlaid.of about 14 million D0 and D¯0 meson decays into K−π+ and K+π−. Given the CP -symmetric initial state (pp¯) and the forward-backward symmetric detector acceptance,we know that an equal number of D0 and D¯0 mesons are produced in the CDF detectorand, therefore, any measured asymmetry of K−π+ versus K+π−, must be artificiallyproduced by detector asymmetries.Figure 2 shows the raw charge asymmetry as a function of the K−π+ invariant mass.We clearly see the narrow D0 peak (negative) over a broader D¯0 peak (positive) generatedby the wrong mass assignment to the decay products. The number we need, to correctthe raw asymmetries, is the height of the narrow negative peak. It represents the chargeasymmetry of the detection efficiency for K−π+ versus K+π−. It is of the order of 1%or less and is momentum dependent. A sample of Λ → pπ is used, in a similar way, tocorrect the measurement of the Λ0b asymmetries.The CP asymmetry is obtained by taking the difference of the rates of oppositeflavor decays divided by their sum. The correction for residual detector asymmetriesobtained from the study of D0 to Kπ decays is applied to the raw rates. This precisionmeasurement is possible because we know that b and b¯ are produced equally in thedetector acceptance given the CP symmetry of the initial state as are the D0 and D¯0mesons used for computing corrections.Table I shows the latest CDF results on B0, Bs and Λb charmless decays. The resultfor ACP (B0 → K−π+) has a significance larger than 5.0σ. The updated result forACP (B0s → K−π+) has a significance of 3.0σ and confirms the LHCb evidence for thesame decay [6], with the same level of precision. The averaged value of this result andthe LHCb measurement is +0.24±0.05 which differs from zero with a significance of 4.8σand results in a strong evidence of CP violation in the B0s meson system. The observedasymmetries for Λ0b → pK− and Λ0b → pπ− are consistent with zero.Note that measurements of B-baryon asymmetries are still unique to CDF. They areall based on the assumption of symmetric production which is very good at the Tevatrongiven the pp¯ initial state.SEARCHES FOR NEW PHYSICS IN THE CHARM AND BEAUTY SECTOR AT THE TEVATRON 211Table I. – Direct CP -violating asymmetries results. The first quoted uncertainty is statistical,the second is systematic. N is the number of events determined by the fit for each decay mode.Mode Nb→f Nb¯→f¯ ACP (b→ f)(%)B0 → K+π− 5313± 109 6348± 117 −8.3± 1.3± 0.4B0s → K−π+ 560± 51 354± 46 +22± 7± 2Λ0b → pπ− 242± 24 206± 23 +6± 7± 3Λ0b → pK− 271± 30 324± 31 −10± 8± 43. – b→ sμ+μ−decaysThe transition of a b quark to an s quark is another golden probe for new physics.This is because it’s a flavor changing neutral current process (FCNC), very much sup-pressed in the Standard Model and sensitive to the presence of new objects in the loops.The corresponding decay modes are three-body or more and they provide a number ofkinematical variables whose distributions can be studied in search for deviations frompredictions based on the Standard Model.Figure 3 shows the mass spectra of the processes that were studied at CDF. All sixprocesses are characterized by a transition b to s plus the production of a μ+μ− pair.They are the following:B+ → K+ μ+μ−,B0 → K∗0(892) μ+μ−,B0 → K0s μ+μ−,B+ → K∗+(892) μ+μ−,B0s → φ μ+μ−,Λ0b → Λ μ+μ−.By binning signal events in invariant mass squared of the muon pair (q2), we obtaindifferential branching fraction distributions. They are shown in fig. 4 for four of the sixprocesses, with the whole statistics available to CDF and compared to a prediction basedon the Standard Model. No significant deviation from the predictions is observed.Figure 5 shows isospin asymmetries calculated as the difference divided by the sumof the rates for B0 to K0μ+μ− and B+ to K+μ+μ−:(2) A(∗)i =B(B0 → K(∗)0μ+μ−)− B(B+ → K(∗)+μ+μ−)B(B0 → K(∗)0μ+μ−) + B(B+ → K(∗)+μ+μ−) .This is shown both for Ai (in blue) and A∗i (in red). The plot is again binned in q2.The SM prediction for this quantity is zero for all q2. LHCb observed a three sigmaeffect in the highest q2 bin [7] which is not confirmed, nor can be excluded, by thismeasurement.212 L. RISTORI on behalf of the CDF and D0 COLLABORATIONS)2K) (GeV/cμμM(5 5.1 5.2 5.3 5.4 5.5 5.6 5.72Candidates per 20 MeV/c020406080100120140160180200220  24±Yield: 323 -1CDF Run II Preliminary L=9.6fb-μ+μ+ K→+BDataTotal FitSignalBackground)2) (GeV/cSKμμM(5 5.1 5.2 5.3 5.4 5.5 5.6 5.72Candidates per 20 MeV/c051015202530 8±Yield: 32 -1CDF Run II Preliminary L=9.6fb-μ+μS K→0BDataTotal FitSignalBackground)2) (GeV/c*0KμμM(5 5.1 5.2 5.3 5.4 5.5 5.6 5.72Candidates per 20 MeV/c020406080100120140160 20±Yield: 288 -1CDF Run II Preliminary L=9.6fb-μ+μ*0 K→0BDataTotal FitSignalBackground)2) (GeV/c*+KμμM(5 5.1 5.2 5.3 5.4 5.5 5.6 5.72Candidates per 20 MeV/c024681012141618 6±Yield: 24 -1CDF Run II Preliminary L=9.6fb-μ+μ*+ K→+BDataTotal FitSignalBackground)2) (GeV/cΛμμM(5.3 5.4 5.5 5.6 5.7 5.8 5.9 62Candidates per 20 MeV/c05101520253035  8±Yield: 51 -1CDF Run II Preliminary L=9.6fb-μ+μΛ →b0ΛDataTotal FitSignalBackground)2) (GeV/cφμμM(5 5.1 5.2 5.3 5.4 5.5 5.6 5.72Candidates per 20 MeV/c0510152025303540 9±Yield: 62 -1CDF Run II Preliminary L=9.6fb-μ+μφ →0sBDataTotal FitSignalBackgroundFig. 3. – Invariant mass distributions for the six decay modes analyzed.)2/c2 (GeV2q0 2 4 6 8 10121416182022]-1 )2/c2(GeV-7 [102dB/dq00.10.20.30.40.50.6-1CDF Run II Preliminary L=9.6fb-μ+μ K→BDataSM)2/c2 (GeV2q0 2 4 6 8 10 12 14 16 18]-1 )2/c2(GeV-7 [102dB/dq00.20.40.60.811.21.41.61.82-1CDF Run II Preliminary L=9.6fb-μ+μφ →0SBDataSM)2/c2 (GeV2q0 2 4 6 8 10 12 14 16 18]-1 )2/c2(GeV-7 [102dB/dq00.20.40.60.811.2-1CDF Run II Preliminary L=9.6fb-+* KBDataSMμμ)2/c2 (GeV2q0 2 4 6 8 10 12 14 16 18 20]-1 )2/c2(GeV-7 [102dB/dq-1-0.500.511.522.533.54-1CDF Run II Preliminary L=9.6fb-μ+μΛ →b0ΛDataSMSM)-610×(B=4.0)-610×(B=1.95Fig. 4. – Differential branching ratio distributions for four of the decay modes.SEARCHES FOR NEW PHYSICS IN THE CHARM AND BEAUTY SECTOR AT THE TEVATRON 213)2/c2 (GeV2q0 2 4 6 8 10 1214 16 18 20 22IA-2-1.5-1-0.500.511.52-1CDF Run II Preliminary L=9.6fb-μ+μ* K→B-μ+μ K→BFig. 5. – Isospin asymmetries as defined in eq. (2) binned in the invariant mass of the μ+μ−pair (q2), shown both for Ai (blue) and A∗i (red).Figure 6 shows the definition of some kinematical variables in the B to K∗μ+μ−decay. Particularly interesting are the muon forward backward asymmetry, based on theangle θμ between the positive muon and the B meson in the di-muon rest frame and K∗polarization, based on the angle θK between the kaon and the B meson in the Kπ restframe. Both quantities are shown as a function of q2 in fig. 7. Contributions from B0and B+ are averaged. No significant deviation from the Standard model is observed.4. – Bs to μ+μ−The decay of Bs to μ+μ− is another very important channel to search for possibleeffects of new physics. The branching ratio is predicted to be very small by the StandardModel (≈ 3 × 10−9) [8, 9] because it is a flavor changing neutral current process and isFig. 6. – Definition of geometrical variables θμ and θK used in the definition of the muonforward-backward asymmetry and K∗ polarization, respectively.214 L. RISTORI on behalf of the CDF and D0 COLLABORATIONS)2/c2 (GeV2q0 2 4 6 8 10 12 14 16 18FBA-0.6-0.4-0.200.20.40.60.81DataSM-μ+μ* K→B -1CDF Run II Preliminary L=9.6fb)2/c2 (GeV2q0 2 4 6 8 10 12 14 16 18LF00.20.40.60.81DataSM-μ+μ* K→B -1CDF Run II Preliminary L=9.6fbFig. 7. – Forward-backward muon asymmetry (left) and K∗ polarization (right) in B → K∗ →μ+μ− decays as a function of q2. Charged and neutral B’s are averaged.helicity suppressed. The decay rate can therefore be significantly enhanced even by smallcontributions due to the presence of new particles in the diagrams.The measurement of the Bs to μ+μ− branching ratio has been recently updatedby the D0 Collaboration [10]. The analysis uses a multivariate technique based on aboosted decision tree to suppress backgrounds. The branching ratio is normalized to B+to J/ψ K. A blind analysis is performed in the di-muon mass range between 4.0 and7.0GeV/c2 with a blinded region from 4.9 to 5.8GeV/c2 (fig. 8).The expected number of background events in the signal window is 4.3 ± 1.6 with aStandard Model signal yield of 1.23 ± 0.13. Opening the box revealed three events inthe signal window as shown in fig. 8. This translates to an upper limit for the branchingratio of 15×10−9 at 95% CL, a significant improvement from the previous D0 result [11].5. – CP violation in semi-leptonic decaysNeutral B mesons mix, so they oscillate into their antiparticles. The rate of a Boscillating into a B¯ may be different from the opposite process where a B¯ oscillates intoa B. We can define a semi-leptonic mixing asymmetry as asl equal to the differencedivided by the sum of the rates of a B and a B¯ oscillating into their antiparticles anddecaying into a final state containing a lepton:(3) aqsl =Γ(B¯0q → B0q → +X)− Γ(B0q → B¯0q → −X)Γ(B¯0q → B0q → +X) + Γ(B0q → B¯0q → −X),where q can be d or s according to the flavor of the B mesons.These mixing asymmetries are predicted by the Standard Model to be of the orderof 10−4 to 10−5 and, therefore, too small to be measured by current experiments [12].Any significant deviation from zero would be unequivocal proof of the existence of newphysics.A surprising result was obtained by the D0 Collaboration in 2011 in a study of thecharge asymmetry of the production rate of pairs of muons of the same sign [13] whichshowed a significant deviation from zero and therefore a disagreement with the Stan-SEARCHES FOR NEW PHYSICS IN THE CHARM AND BEAUTY SECTOR AT THE TEVATRON 215) (GeV)μμM(4 4.5 5 5.5 6 6.5 7Events / 25 MeV02468101214161820Expected background 1.5±4.0 -1DØ, 10.4 fb) (GeV)μμM(4.9 5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8Events / 25 MeV00.511.522.533.54 Signal region eventsControl region events 5× SM μμ → 0sBBackground estimate-1DØ, 10.4 fbFig. 8. – Blinded region (arrows) and signal window (shaded) in the invariant mass distributionfor the Bs → μ+μ− search (top). Three events fall in the signal window (bottom).dard Model prediction. The significance of the discrepancy was more than 3σ and wasinterpreted as arising from CP violation in B mixing.]2) [GeV/c±πφM (1.8 2 2.22)/6 MeV/c± sD± μN(102030310× (weighted) ν ±s D±μ →) s0B (s0B 1337±) =  203513 ±sN(D 1173±) =  47965 ±N(D-1D0 Run II, 10.4 fbSignal FitBackground Fit)2)   (GeV/cπ) - M(KD*ππM = M(KΔ0.14 0.15 0.16 0.17)2D*) / (0.5 MeV/cμN(024681012310×-1D0 Run II, 10.4 fbFig. 9. – φπ invariant mass distribution showing D+’s and D+s ’s produced in association witha muon and due respectively to B0 and B0s decays (left). The D∗+ peak (right) is due to B0decay.216 L. RISTORI on behalf of the CDF and D0 COLLABORATIONSslda-0.04 -0.02 0 0.02sls a-0.04-0.0200.02sls aslsD0 asldNew WA a) 68% C.L.>120(IPslbA) 68% C.L.<120(IPslbACombinationStandard ModelFig. 10. – Current status of mixing asymmetry measurements shown as bands in the (adsl, assl)-plane. The different slopes of the bands are due to a different estimated content of Bd and Bsdecays in the corresponding samples.The dimuon asymmetry is sensitive to asymmetries from both B0d and B0s mesonmixing, with approximately equal contributions from each. Given the significance of thisresult, it is important to try to separate the possible contributions of B0d and B0s tothe mixing asymmetry. This was done studying the three decay modes shown in fig. 9where a B meson decays into a D meson and a muon plus anything [14,15]. Raw yieldsare obtained by fitting the D+, D∗+ and Ds mass peaks separately for positive andnegative charge. A D+(D−) or a D∗+(D∗−) corresponds to the decay of a Bd(B¯d) whilea D+s (D−s ) corresponds to the decay of a Bs(B¯s). The raw asymmetry is corrected fordetector effects and divided by the fraction of decays from oscillated B mesons obtainedfrom Monte Carlo simulation. Other signal sources are assumed to be charge symmetric.The results areadsl = [+0.68± 0.45(stat.)± 0.14(syst.)]%,assl = [−1.12± 0.74(stat.)± 0.17(syst.)]%.These numbers are both consistent with zero and with the previous result [13]. Theoverall combination is pictured in fig. 10 as the superposition of bands in the (adsl, assl)plane and is still about three sigma away from the Standard Model.6. – ConclusionRecent results from the analysis of CDF and D0 data show that some importantprecision measurements in flavor physics can still be done at the Tevatron and can stillSEARCHES FOR NEW PHYSICS IN THE CHARM AND BEAUTY SECTOR AT THE TEVATRON 217be competitive and complementary to similar studies at dedicated facilities (B-factories)or at the LHC. Data taking has ended but analysis continues and more interesting resultsin flavor physics will be coming in the near future from the Tevatron.REFERENCES[1] Gronau M. and Rosner J. L., Phys. Lett. B, 482 (2005) 71.[2] Lipkin H. J., Phys. Lett. B, 621 (2000) 126.[3] Ashmanskas W. et al., Nucl. Instrum. Methods A, 518 (2004) 532.[4] Ristori L. and Punzi G., Annu. Rev. Nucl. Part. Sci., 60 (2010) 595.[5] Affolder T. et al., Nucl. Instrum. Methods A, 526 (2004) 249.[6] Aaij R. et al. (LHCb Collaboration), Phys. Rev. Lett., 108 (2012) 201601.[7] LHCb Collaboration, arXiv:1205.3422 [hep-ex], (2012).[8] De Bruyn K. et al., Phys. Rev. Lett., 109 (2012) 041801.[9] Buras A. J. et al., Eur. Phys. J. C, 72 (2012) 2172.[10] Abazov V. M. et al. (D0 Collaboration), Phys. Rev. D, 87 (2013) 072006.[11] Abazov V. M. et al. (D0 Collaboration), Phys. Rev. Lett., 94 (2005) 071802.[12] Lenz A. and Nierste U., JHEP, 06 (2007) 072.[13] Abazov V. M. et al. (D0 Collaboration), Phys. Rev. D, 84 (2011) 052007.[14] Abazov V. M. et al. (D0 Collaboration), Phys. Rev. D, 86 (2012) 072009.[15] Abazov V. M. et al. (D0 Collaboration), Phys. Rev. Lett., 110 (2013) 011801.

Searches for new physics in the charm and beauty sector at the Tevatron

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Searches for new physics in the charm and beauty sector at the Tevatron

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