The LHC collider will soon produce the largest sample of B mesons, opening several new paths to the discovery of New Physics processes through the study of their rare and very rare decays. The strategies for the data analysis and the

expected performances for several key decay channels are reviewed and compared for the three LHC experiments that are ready to reconstruct and select the B mesons: LHCb, ATLAS and CMS

Sarti, A.

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DOI 10.1393/ncc/i2009-10431-7Colloquia: IFAE 2009IL NUOVO CIMENTO Vol. 32 C, N. 3-4 Maggio-Agosto 2009Rare B decays at LHCA. SartiLaboratori Nazionali di Frascati - Frascati, Italy(ricevuto il 19 Settembre 2009; pubblicato online il 16 Ottobre 2009)Summary. — The LHC collider will soon produce the largest sample of B mesons,opening several new paths to the discovery of New Physics processes through thestudy of their rare and very rare decays. The strategies for the data analysis and theexpected performances for several key decay channels are reviewed and compared forthe three LHC experiments that are ready to reconstruct and select the B mesons:LHCb, ATLAS and CMS.PACS 13.20.He – Leptonic, semileptonic, and radiative decays of bottom mesons.PACS 13.25.Hw – Hadronic decays of bottom mesons.1. – IntroductionThe Large Hadron Collider (LHC) experiments at CERN will soon start collectingdata from proton-proton interactions with a center-of-mass energy greater than 5TeV.A large fraction of the particles produced in the pp interactions will contain a b quark.In this perspective, given the very high design luminosity of the collider and the largebb production cross-section, the LHC experiments are going to analyze the largest Bmeson sample ever produced and collected. Even if the experimental environment isnot as clean as in e+e− collider experiments the LHC experiments will contribute to asubstantial improvement in the experimental knowledge of the rare B meson decays withrespect to present B factories (BaBar, Belle) and hadron colliders (CDF, D0) results.2. – Experimental set-upThree detectors, LHCb [1], ATLAS [2] and CMS [3], are almost ready to acquirethe data produced by the particles originating from the proton-proton collisions in theLHC interaction points every 25 ns. While LHCb is a forward spectrometer specificallydesigned and optimized for doing B meson physics studies, ATLAS and CMS are generalpurpose experiments covering nearly the 4π solid angle around the interaction point.A very large number of B mesons and hadrons will be produced at the LHC. Theexpected cross-section for bb production at 14TeV is ∼ 500μb, yielding 5 · 104 bb pairsproduced every second at a luminosity (L) of 1032 cm−2 s−1. However the total ppcross-section is two orders of magnitude higher, ∼ 100mb in the LHCb simulations,demanding an efficient trigger with a strong background rejection.c© Societa` Italiana di Fisica 171172 A. SARTISince most of the interesting B meson decays have a large transverse momentum(pT ) and some of the key rare decays involve leptons (e, μ) as final states, the trackreconstruction and particle identification (PID) performances play a central role in theirselection. ATLAS and CMS achieve a pT resolution less than 2%, a transverse impactparameter resolution of 10μm and a μ reconstruction efficiency greater than 98% butcannot resolve kaons and pions in B meson decays. LHCb achieves a relative momen-tum resolution ∼ 0.4, an impact parameter resolution of 30μm and a reconstructionefficiency for tracks traversing the full detector larger than 95%. LHCb has also ex-cellent PID capabilities achieving a μ identification efficiency greater than 95% witha misidentification smaller than 1% for tracks with p > 3GeV/c and a high kaon-pion discrimination power and identification efficiency in the 1–100GeV/c momentumrange.The trigger of all the three experiments follows almost the same logic: after a hardwaretrigger, named L0 in LHCb and L1 in ATLAS and CMS, that uses the input of the fastestdetectors such as the calorimeters and muon detectors, the affordable rate is obtainedthrough a software trigger (HLT) that uses information from all the sub-detectors. Inaddition, in LHCb, a pile up veto is used in order to clean up the environment frommultiple interactions, since the number of expected pp interactions for each beam crossinggoes from ∼ 1 to ∼ 33 when increasing the luminosity from 2 · 1032 cm−2 s−1 (LHCb)to 1033 cm−2 s−1 (ATLAS, CMS). The L0 and L1 rates are, respectively, ∼ 1MHz and< 100 kHz. The LHCb HLT rate is 2 kHz, out of which 200Hz is the bandwidth allocatedfor exclusive B selections and the remaining 1.8 kHz are used to acquire control samplesand inclusive b samples for data mining. The ATLAS and CMS HLT rates are ∼ 200Hz,and ∼ 10% of this rate can be used for doing B physics studies.3. – Rare B meson decays overviewRare B meson decays can be used as sensitive probes of Standard Model (SM) orSM extension (new physics or NP) predictions. Their properties are currently the lessexperimentally constrained: while some of the Bd rare decays have already been studiedby the B factories experiments, the whole Bs sector is poorly constrained and thus openedfor SM validation or NP discovery searches. Among the very large number of interestingdecay modes that can be used to constrain NP models, in this paper we only considertwo different approaches.The first one is related to the study of b mesons and hadrons decaying into hh′ finalstates (where h, h′ stand for kaon and/or pion) that can be used [4] to make the firstobservation of the Bs → KK transition, validate U-spin relations between Bd and Bsdecays and extract the CKM angles γ and φs.The second approach is related to the B semi-leptonic decays that can be theoreticallydescribed in terms of the Operator Product Expansion (OPE) with a parametrizationof NP processes through modified Wilson coefficients and operators [5]. Different tran-sitions, such as b → sll and b → sγ, are sensitive to different operators and can beused to constrain NP models contributions. While the inclusive approaches represent agreat challenge for hadron collider experiments, the exclusive decays can be addressedmore easily, and observables can be found in which the theoretical hadronic uncertaintiesare well under control. For example, the b → sll forward-backward asymmetry (AFB),its zero value (s0) and the ratio of branching ratios of μμ and ee modes have SM [6]predictions that can be largely affected by NP contributions.RARE B DECAYS AT LHC 173Table I. – LHCb sensitivities with 2 fb−1 to the most relevant physics measurements understudy.Quantity σ(stat.) Quantity σ(stat.)ACPK+π− 0.0025 ACPπ+K− 0.016ACPpπ− 0.016 ACPpK− 0.009Adirπ+π− 0.04 Amixπ+π− 0.04Corr(Adirπ+π− , Amixπ+π−) −0.03 AdirK+K− 0.047AmixK+K− 0.035 Corr(AdirK+K− , AmixK+K−) 0.024. – Non-leptonic two body decaysThe analysis of non-leptonic decays is feasible only in the LHCb experiment, wherethe kaons and pions from the B mesons can be discriminated with high efficiency. Twelvedifferent decay modes have been studied: Bd, Bs and Λb decays into ππ, πK, Kπ andKK pairs. The selection has been designed in order not to use the PID information.Only kinematics and geometrical constraints have been imposed achieving nearly thesame efficiencies for all the various modes. The cuts have been optimized maximizingthe signal (S) over background (B) S√S+Bratio. The total selection efficiency is ∼ 1.3%with an expected yield, assuming an integrated luminosity of 2 fb−1, of nearly 380kevents in total. LHCb will match the expected final CDF statistics (Lint ∼ 3 fb−1),with Lint ∼ 100 pb−1. The annual yield for signal, combinatorial and physical (mis-reconstructed three body decay) backgrounds at 95% confidence level are, respectively,[187,508]k, [44,255]k and [59,306]k events.The CP parameters can be extracted from an Unbinned Maximum Likelihood Fitto all the Probability Density Functions (PDFs) of the various decay modes and back-grounds on the collected data sample. The signal decay modes invariant mass (IM) PDFsare modeled in terms of the reconstructed tracks momentum asymmetry(β = p+−p−p++p−)taking into account the radiative tails modeled using QED calculations [7]. The combi-natorial background IM PDF is going to be extracted from the IM side-bands.The PID PDFs will be extracted from the control samples. The PDFs describingthe time-dependent rates are known from theoretical calculations and do depend on thedecay channel. Key ingredients common to all the decay modes are the proper timeresolution, that is modeled as a sum of three Gaussians with sigmas 30, 55 and 120 fsrespectively, and the effective tagging efficiencies (eff = tag · (1− 2ω)2) that depend onthe decay mode and are in the 4.5–6.5% range.The CP sensitivities have been obtained from a large number of unbinned likelihoodfits to MC samples generated with input from the full MC simulation and a number ofevents corresponding to different values of Lint. Results for Lint = 2 fb−1 are reportedin table I. By combining the CP observables it is possible to extract the sensitivities onthe γ and φs CKM angles: a resolution of 7 degrees on γ and a 10% relative resolutionon φs, for a large NP φs value: ∼ 0.7, are obtained with Lint = 2 fb−1.5. – Semi-leptonic and radiative decaysThe LHCb and ATLAS experiments have studied the reconstruction and selection ofseveral b → sll modes like Bd → K0∗μ+μ−, Bs → φμ+μ−, B+ → K+μ+μ−, B+ →174 A. SARTIK+∗μ+μ− and Λb → Λ0μ+μ−. The expected yields are of the order of few thousandevents in one (LHCb) or three (ATLAS) nominal years, to be compared with the onethousand events collected by Tevatron and B factory experiments together. The mainsources of background are the combinations of tracks from two semi-leptonic b decays andfrom a semi-leptonic b decay and a muon from the b → c → μ decay chain. By measuringthe AFB as a function of the μμ invariant mass LHCb will be able to measure s0 with anabsolute uncertainty of 0.5GeV2 with the data collected in one nominal year of runningwhile ATLAS is going to achieve a 4% statistical error on s0 in three nominal years.Slightly larger statistical uncertainties are expected from the analysis of Λb → Λ0μ+μ−(6%). LHCb also expects to reconstruct 10k ee and 20k μμ events, achieving a relativeresolution on Rk ∼ 4% with 10 fb−1 Lint.The LHC experiments can also reconstruct and select the radiative Bs → φγ andΛb → Λ0γ decays, performing the time-dependent analysis of the decay rates. Fewthousand events are expected in a nominal year of LHCb running (3 years for ATLAS)with S/B ∼ O(1). The expected resolution on CP parameters is of the order of 10% forthe tagged analysis and 20% for the untagged one (current results from B factories onBd mesons have a 40% uncertainty).6. – ConclusionsThree LHC experiments are getting ready to reconstruct and analyze the largest Bmeson sample ever collected. The pp interactions will open several new windows on theb meson physics allowing the first accurate study of Bs meson and Λb hadrons to becross-checked with known results from the already deeply studied Bd mesons system.Exciting new results from the study of B to hh′ decays (LHCb) and semi-leptonic andradiative decays (all the three experiments) will be available, with performances that arecompetitive, since the first years, with results already obtained from the B factories orTevatron experiments.Using the unified offline selection of all the b→ hh′ decays, LHCb is going to collectnearly 100k signal events with an integrated luminosity of 0.5 fb−1 (first year of datataking), achieving a sensitivity on CP observables of the order of 3–5%. An uncertaintyon γ of ∼ 7◦ can be achieved taking into account an U-spin breaking at the level of∼ 20%. An uncertainty σ(s0) of ∼ 0.5GeV2 is expected after a nominal year of LHCbdata taking (2 fb−1) studying the AFB spectrum as a function of the μμ invariant massin the Bd → K∗μμ decay mode. After three nominal years of data taking the ATLASexperiment is going to be able to see deviations from SM predictions and set strong limitson NP models. CMS studies on this channels are ongoing and the expected performanceswill be published soon. The radiative b → sγ decays have also been studied. Theexpected resolutions on the CP parameters in a nominal year of data taking are at the10–20% level.REFERENCES[1] Alves A. A. et al. (LHCb Collaboration), JINST, 3 (2008) S08005.[2] Aad G. et al. (ATLAS Collaboration), JINST, 3 (2008) S08003.[3] Chatrchyan S. et al. (CMS Collaboration), JINST, 3 (2008) S08004.[4] Carbone A. et al., Charmless charged two-body B decays at LHCb, CERN-LHCB-2008-052.[5] Hiller G., Phenomenology of New Physics [hep-ph/0308180].[6] Beneke M. et al., Eur. Phys. J. C, 41 (2005) 173.[7] Baracchini E. and Isidori G., Phys. Lett. B, 633 (2006) 309.

Rare B decays at LHC

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Rare B decays at LHC

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