Rare lepton decays of the B0(s) mesons are sensitive probes of New Physics. The search for the decays B0(s)→ μ+μ− may provide information on the presence of new (pseudo-) scalar particles. Also the study of the electroweak penguin

processes are powerful probes for new physics, as physics beyond the Standard Model (SM) can enter via virtual particles at the same level as SM physics. The Tevatron collider opened the way to high precision Heavy Flavor physics at hadron collider experiments, while the LHCb experiment definitively proved the physics performances are competitive with the B-factories. This is a short review of the searches of rare decays performed at the hadron colliders

Archilli, F.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2014-11672-yColloquia: IFAE 2013IL NUOVO CIMENTO Vol. 37 C, N. 1 Gennaio-Febbraio 2014Rare decays at hadron collidersF. ArchilliINFN, Laboratori Nazionali di Frascati - Frascati (RM), Italyricevuto l’1 Ottobre 2013Summary. — Rare lepton decays of the B0(s) mesons are sensitive probes of NewPhysics. The search for the decays B0(s) → μ+μ− may provide information on thepresence of new (pseudo-) scalar particles. Also the study of the electroweak penguinprocesses are powerful probes for new physics, as physics beyond the StandardModel (SM) can enter via virtual particles at the same level as SM physics. TheTevatron collider opened the way to high precision Heavy Flavor physics at hadroncollider experiments, while the LHCb experiment definitively proved the physicsperformances are competitive with the B-factories. This is a short review of thesearches of rare decays performed at the hadron colliders.PACS 13.20.-v – Leptonic, semileptonic, and radiative decays of mesons.PACS 13.20.He – Decays of bottom mesons.PACS 14.40.Nd – Bottom mesons (|B| > 0).1. – IntroductionThe study of rare decays in the Heavy Flavour sector is a complementary approach todetect New Physics (NP) with respect to the direct searches. While the latter approachaims to discover NP through the production of real new particles, the precise measure-ments of rare process can probe virtual new particles in loop diagrams. The effects ofthese particles can be observed as a deviation in the Standard Model (SM) expectationfor observables such as branching fraction, angular distribution, either in CP -violatingprocesses or in very rare decays involving Flavour-Changing Neutral Currents (FCNC).Since the effect of heavy new particles does not decouple in weak and Yukawa interac-tions, the precise measurement of FCNC can reveal NP that may be well above the TeVscale or can provide key information on the couplings and phases of these new particlesif they are visible at the TeV scale.2. – B0(s) → μ+μ−The search for B0(s) → μ+μ− is one of the most promising ways to constrain theparameters of any extended Higgs sector. These decays are highly suppressed in the SMc© CERN on behalf of the LHCb CollaborationCreative Commons Attribution 3.0 License (http://creativecommons.org/licenses/by/3.0) 100RARE DECAYS AT HADRON COLLIDERS 101because they are both FCNC and helicity suppressed. The SM model predictions forthese decays are [1, 2]B(B0s → μ+μ−)SM = (3.2± 0.2)× 10−9,B(B0 → μ+μ−)SM = (1.0± 0.1)× 10−10.NP enhancement of the B(B0(s) → μ+μ−) are already constrained by the recentsearches done at the hadron collider experiments to be smaller or at the same levelof the SM prediction. However, there still remains room for contributions from NP, forexample a destructive interference between SM and NP process could lower the observedbranching fractions.In this review, I present the last results of the searches of B0(s) → μ+μ− decaysperformed by different experiments at the hadron colliders.At the Tevatron, the D0 experiment has recently published [3] an update of the B0s →μ+μ− searches with 10.4 fb−1. In order to decrease the huge combinatorial background,two different BDT are used: one effective on the couple of muons coming from sequentialdecays such as b → μ−vc with c → μ+vX, and a second for the double semileptonicdecays. To reduce the systematic uncertainties the result is normalized to B± → J/ψK±decays. The CLs method is used to evaluate the branching fraction upper limit. Theexpected limit B(B0(s) → μ+μ−) < 23 × 10−9 at 95% CL and the observed B(B0(s) →μ+μ−) < 15× 10−9 at 95% CL.Also ATLAS has published a blind analysis based on 2.4 fb−1 data, collected in2011 [4]. The sample is divided into three regions of pseudo-rapidity (η) where theATLAS Collaboration observes different resolution of the mass. Therefore the mass win-dows width ranges from 116 to 171MeV/c2. The branching fraction is measured withrespect to the decay B± → J/ψK± in order to minimize systematic uncertainties in theevaluation of the efficiencies and acceptances. The observed upper limit evaluated withthe CLs method is B0s → μ+μ− < 22× 10−9 at 95% CL.The CMS experiment has published a blind analysis using 5 fb−1 data collected in2011 [5]. Also in this case the data sample is divided into two regions corresponding totwo different background conditions. The first subsample containing only tracks with|η| < 1.4 and a mass resolution ∼ 36MeV/c2 and a second subsample containing atleast one track with |η| < 1.4 and a resolution of ∼ 86MeV/c2. CMS performs a cut-based analysis optimized on MC signal and data-sidebands before the unblinding. Thebranching fraction is evaluated by measuring the ratio to a more abundant B-hadrondecay B± → J/ψK±. The upper limits on the branching fractions are determined usingthe CLs method. The combined upper limits for the barrel and endcap channels areB(B0(s) → μ+μ−) < 7.7×10−9 at 95% CL. and B(B0 → μ+μ−) < 1.8×10−9 at 95% CL.The LHCb experiment, already in summer 2012, set the most restrictive upper lim-its [6] on the branching fractions, B(B0s → μ+μ−) < 4.5 × 10−9 and B(B0 → μ+μ−) <1.0 × 10−9 at 95% CL by using only the data sample at 7TeV center-of-mass energycollected during 2011.Recently LHCb experiment updated this measurement including 1.1 fb−1 recordedduring 2012 at√s = 8TeV. Assuming a SM rate for these two decays at the end of theselection we expect ∼ 24 B0s → μ+μ− events and ∼ 2.8 B0 → μ+μ− events in the signalregion.102 F. ARCHILLI]9−) [10−μ+μ→s0BB(2 4 6 8sCL00.20.40.60.81LHCb(8TeV)1−(7TeV) +1.1 fb1−1.0 fb]9−) [10−μ+μ→0BB(0.2 0.4 0.6 0.8 1 1.2 1.4sCL00.20.40.60.81LHCb(8TeV)1−(7TeV) +1.1 fb1−1.0 fbFig. 1. – Expected CLs (dashed black line) under the hypothesis of observing background-only(left) for the B0 → μ+μ− and CLs under background-plus-signal events according to the SMrate (right) for B0s → μ+μ−, with the yellow area covering the region of ±1σ of compatibleobservations; the observed CLs is given by the blue dotted line; the expected (observed) upperlimits at 90% and 95% CL are also shown as dashed and solid grey (red) lines.The events are classified in bins of the dimuon invariant mass mμμ and bins of amultivariate classifier, BDT, based on kinematic and geometrical variables. In each binthe expected numbers of signal and background events are derived. The signal massshape is assumed to be a Crystal Ball shape, where the mean and the resolution areobtained on data while the transition point is derived from simulations. The BDT shapeis obtained on data by extracting with a fit to the mass distribution, in each BDT bin,the yields of B0(s) → h+h−. For the combinatorial background, the mass and the BDTshape are obtained simultaneously by interpolating, in each BDT bin, the mass side-bands into the signal regions with an exponential function. The number of expectedsignal events, for a given branching fraction hypothesis, is evaluated by normalizing tothe decays B0 → Kπ, B+ → J/ψK+ and B0s → J/ψφ. In order to avoid any bias,the mass region mμμ = [m(B0) − 60MeV/c2,m(B0s ) + 60MeV/c2] is blinded until theanalysis is finalized.The comparison of the distributions of observed events and expected backgroundevents, using the full (2011+2012) data sample, results in a p-value (1 − CLb) of 11%for the B0 → μ+μ− decay. From our data we constrain the B0 → μ+μ− branchingfraction to be less than 9.4 × 10−10 at 95% CL [7], which is the world-best limit froma single experiment. The expected and observed CLs values are shown in fig. 1 for theB0 → μ+μ− and B0s → μ+μ− channels, each as a function of the assumed BF.The probability that background processes can produce the observed number of B0s →μ+μ− candidates or more is 5×10−4 and corresponds to a statistical significance of 3.5 σ.The value of the B0s → μ+μ− branching fraction is obtained from an unbinned likelihoodfit to the mass spectrum, performed simultaneously in different BDT bins. The invariantmass distribution of selected B0(s) → μ+μ− candidates with BDT > 0.7 is shown in fig. 2with the fit results overlaid.From the fit we obtain [7] B(B0s → μ+μ−) = (3.2+1.4−1.2(stat)+0.5−0.3(syst)) × 10−9, whichis in agreement with the SM expectation. This is the first evidence for the decay B0(s) →μ+μ−. In order to compare the upper limit on B(B0s → μ+μ−) with the theoreticalprediction, this value has to be multiplied by 0.911± 0.014, which takes into account theeffective lifetime of the Bs meson [8].RARE DECAYS AT HADRON COLLIDERS 103]2c [MeV/−μ+μm5000 5500 6000)2 cCandidates / (50 MeV/02468101214 LHCb(8TeV)1−(7TeV) +1.1 fb1−1.0 fbBDT > 0.7Fig. 2. – Invariant-mass distribution of selected B0(s) → μ+μ− candidates (black points) forcombined 2011 and 2012 dataset and for BDT > 0.7. The result of the fit is overlaid (blue solidline) and the different components detailed: B0s → μ+μ− (red long dashed), B0 → μ+μ− (greenlong dashed), B0(s) → h+h− (pink dotted), B0 → π−μ+vμ (black dashed), B0(+) → π0(+)μ+μ−(light blue dot dashed), and combinatorial (blue long dashed).3. – CP-asymmetry in B0 → K0∗μ+μ−The direct CP-asymmetry ACP is defined as ACP = Γ(B¯0→K¯∗0μ+μ−)−Γ(B0→K∗0μ+μ−)Γ(B¯0→K¯∗0μ+μ−)+Γ(B0→K∗0μ+μ−)and it is predicted to be O(10−3) in the SM [9,10]. The theoretical prediction has a smalluncertainty due to suppression of form factor uncertainties. This could be significantlyenhanced in NP models up to 15% [11].The analysis of B0 → K∗0μ+μ− is performed with 1 fb−1 of data recorded by LHCbduring 2011. The resonant regions of the J/ψ and the ψ(2S) are excluded. The decayis selected using a boosted decision tree and an event-by-event correction is appliedto correct for experimental biases. The two ACP asymmetries, one for each polarityof the LHCb dipole magnet, are mediated with an equal weight. The production andinteraction asymmetries are corrected for using the B0 → J/ψK∗0 decay mode as acontrol channel. The ACP is extracted from simultaneous fit to B0 mass in six binsof dimuon invariant mass, q2, and is shown in fig. 3. The result integrated over q2 isACP(B0 → K∗0μ+μ−) = −0.072± 0.040(stat)± 0.005(syst) [12].]4/c2 [GeV2q0 5 10 15 20CPA-0.3-0.2-0.100.10.2LHCbFig. 3. – CP-asymmetry of B0 → K∗0μ+μ− as a function of dimuon invariant mass squared, q2.104 F. ARCHILLI]4c/2 [GeV2q0 5 10 15 20FBA-0.2-0.100.10.2LHCbLHCb]4c/2 [GeV2q0 5 10 15 20HF00.20.4Theory Binned theoryLHCbLHCbFig. 4. – The forward-backward asymmetry AFB and the flat parameter FH in B+ → K+μ+μ−as a function of the dimuon invariant mass squared [13]. The theory predictions are taken fromref. [14] and [15]. AFB is expected to be negligible in the SM for the full q2 range.4. – B+ → K+μ+μ−The analysis of B+ → K+μ+μ− is performed using 1 fb−1 data recorded during 2011by the LHCb experiment. The differential branching fraction is evaluated using B+ →J/ψK+ as a normalization channel. This measurement is performed in 7 bins of q2. Theintegrated branching fraction, taking the region of the excluded charmonium resonancesinto account, is B(B+ → K+μ+μ−) = (4.36± 0.15(stat)± 0.18(syst))× 10−7 [13].The differential decay rate of B+ → K+μ+μ− can be written as 1Γ dΓ(B+→K+μ+μ−)d cos θ=34 (1−FH)(1−cos2 θ)+ 12FH+AFB cos θ, where AFB is the forward-backward asymmetryand FH is a flat parameter. AFB is predicted to be zero in the SM while FH ∼ 0. Anevent-by-event correction is applied to account for experimental effects. Also in that caseAFB and FH are measured in 7 bins of q2 by a likelihood fit in the invariant mass of Kππsystem and cos θ. The distributions of AFB and FH are shown in fig. 4. The results areconsistent with the SM expectations.5. – B± → K±μ+μ−In the SM the b → d+− transition is even more suppressed than b → s+−,by a factor |Vtd|/|Vts|. Furthermore, this process has never been observed so far. Thepredicted SM branching fraction for B+ → π+μ+μ− is (1.96±0.21)×10−8 [16]. However,many new physics models predict enhanced branching fractions for this decay [17]. Thebest limit has been published by Belle and it is B(B+ → π+μ+μ−) < 6.9× 10−8 at 90%of CL [18].The analysis performed by LHCb is based on a sample of 1.0 fb−1 of data collectedin 2011. The selection is based on a multivariate discriminant (BDT), trained using thekinematic properties of the daughters and the vertex quality of the B candidate. TheJ/ψ and ψ(2S) resonances are excluded using a veto on the dimuon mass.The signal yields extracted from the fit is 25.3+6.7−6.4. This result corresponds to anexcess of 5.2 σ with respect to the null hypothesis and consequently it represents the firstobservation of a b → d+− decay. Normalizing the observed signal to the B+ → J/ψK+decay, LHCb obtained [19]B(B+ → π+μ+μ−) = (2.4± 0.6(stat)± 0.2(syst))× 10−8,which is in agreement with the SM expectation.RARE DECAYS AT HADRON COLLIDERS 1056. – ConclusionsThe Tevatron collider opened the way to high-precision Heavy Flavor physics athadron collider experiments, both through detector and trigger strategies and throughadvanced analysis techniques. It is well demonstrated that Heavy flavour physics at thehadron collider can be fully competitive especially for hadronic modes and very rare de-cays. We have presented the search of B0(s) → μ+μ− decays at Tevatron and LHC. TheLHCb experiment has set the first observation of the B0s → μ+μ− decay: B(B0s → μ+μ−)is measured to be equal to (3.2+1.4−1.2(stat)+0.5−0.3(syst))×10−9 with a 3.5 σ significance. Alsothe study of rare electroweak penguin decays are a very active research area. Up tonow all measurements are compatible with the SM and set strong constraints on a broadrange of new physics models.REFERENCES[1] Buchalla G. and Buras A. J., Nucl. Phys. B, 548 (1999) 309.[2] Buras A. J., arXiv:1012.1447; Gamiz E. et al., Phys. Rev. D, 80 (2009) 014503; BurasA. J., Phys. Lett. B, 566 (2003) 115; Buras A. J. et al., Eur. Phys. J. C, 72 (2012) 2172.[3] Abazov V. M. et al. (D0 Collaboration), Phys. Rev. D, 87 (2013) 072006arXiv:1301.4507 [hep-ex].[4] Aad G. et al. (ATLAS Collaboration), Phys. Lett. B, 713 (2012) 387 arXiv:1204.0735[hep-ex].[5] Chatrchyan S. et al. (CMS Collaboration), JHEP, 04 (2012) 033 arXiv:1203.3976[hep-ex].[6] Aaij R. et al. (LHCb Collaboration), Phys. Rev. Lett., 108 (2012) 231801 arXiv:1203.4493v2 [hep-ex].[7] Aaij R. et al. (LHCb Collaboration), Phys. Rev. Lett., 110 (2013) 021801[8] de Bruyn K. et al., arXiv:1204.1735 [hep-ex].[9] Bobeth C., Hiller G. and Piranishvili G., JHEP, 07 (2008) 106 arXiv:0805.2525[hep-ph].[10] Altmannshofer W., Ball P., Bharucha A., Buras A. J., Straub D. M. and WickM., JHEP, 01 (2009) 019 arXiv:0811.1214 [hep-ph].[11] Alok A. K., Datta A., Dighe A., Duraisamy M., Ghosh D. and London D., JHEP,11 (2011) 122 arXiv:1103.5344 [hep-ph].[12] Aaij R. et al. (LHCb Collaboration), arXiv:1210.4492 [hep-ex].[13] Aaij R. et al. (LHCb Collaboration), arXiv:1209.4284 [hep-ex].[14] Bobeth C., Hiller G., van Dyk D. and Wacker C., JHEP, 01 (2012) 107arXiv:1111.2558 [hep-ph].[15] Bobeth C., Hiller G. and van Dyk D., JHEP, 07 (2011) 067 arXiv:1105.0376 [hep-ph].[16] Song H.-Z., Lu¨ L.-X. and Lu G.-R., Commun. Theor. Phys., 50 (2008) 696.[17] Wang J.-J., Wang R.-M., Xu Y.-G. and Yang Y.-D., Phys. Rev. D, 77 (2008) 014017arXiv:0711.0321 [hep-ph].[18] Wei J.-T. et al. (Belle Collaboration), Phys. Rev. D, 78 (2008) 011101 arXiv:0804.3656 [hep-ex].[19] RAaij et al. (LHCb Collaboration), JHEP, 12 (2012) 125 arXiv:1210.2645 [hep-ex].

Rare decays at hadron colliders

http://eprints.bice.rm.cnr.it/18473/1/ncc10684.pdf

Rare decays at hadron colliders

Abstract

Similar works

Full text

Available Versions

Scientific Open-access Literature Archive and Repository