The LHCb experiment will play soon an important role in the Sector of B-Physics by performing new key measurements looking for New Physics. In this paper we will discuss some relevant measurements feasible with early data

Carbone, A.

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DOI 10.1393/ncc/i2009-10442-4Colloquia: IFAE 2009IL NUOVO CIMENTO Vol. 32 C, N. 3-4 Maggio-Agosto 2009LHCb perspectives with early dataA. CarboneINFN, Sezione di Bologna - viale B. Pichat 6/2, 40127 Bologna, Italy(ricevuto il 19 Settembre 2009; pubblicato online il 28 Ottobre 2009)Summary. — The LHCb experiment will play soon an important role in the sectorof B-Physics by performing new key measurements looking for New Physics. In thispaper we will discuss some relevant measurements feasible with early data.PACS 13.20.He – Decays of bottom mesons.1. – IntroductionThe full physics programme of LHCb will be certainly realized by collecting severalfb−1 of integrated luminosity. However, LHCb can achieve important results alreadywith a few hundreds of pb−1. Due to the complexity of the LHC machine and to themany unknowns connected to its start-up, it is very difficult to foresee how much timewill be needed to collect a given amount of integrated luminosity. However, consideringthe nominal operation of LHC (i.e. an instantaneous luminosity of 2× 1032 cm−2 s−1 atthe LHCb interaction point and 107 s of data taking per year), LHCb should be able tocollect about 0.5 fb−1 of data in a couple of months of running.One of the unresolved questions of particle physics nowadays is the presence of physicsbeyond the Standard Model (SM) expected at the scale of the TeV and not yet observed.Recently, some new measurements seem to indicate an evidence of New Physics (NP) inthe flavour sector. Such an evidence was first claimed by the UTfit Collaboration, whichemployed in their analysis (in particular) the measurements of the Bs mixing phaseperformed by the CDF and DØ experiments at the Tevatron. They found that the Bsmixing phase deviates about 3 standard deviations from the SM expectation [1]. Anothercrucial measurement for the quest of NP in the flavour sector is that of the branchingratio of the very rare decay Bs → μ+μ−. This FCNC mode is expected to have withinSM a branching ratio of (3.35 ± 0.32) × 10−9, but it can be significantly enhanced bythe presence of NP. The current upper limit measured by CDF and DØ and averaged byHFAG is 4.7× 10−8 [2, 3].Another relevant puzzle which still survives from the beginning of the B-factory livesand can be solved by LHCb in its early data taking phase, involves the direct CP asym-metry term measured by Belle [4] and BaBar [5] with the Bd → π+π− decay, where adiscrepancy between the two experiments at the level of 2 standard deviations exists.c© Societa` Italiana di Fisica 127128 A. CARBONEFig. 1. – Bs → μ+μ− branching ratio exclusion limit (assuming no signal is present) at 90% CL.The band between the dashed lines corresponds to a statistical uncertainty at 90% CL due tothe limited Monte Carlo statistics used to evaluate the expected background.In the following, the LHCb perspectives with the very first data will be brieflydiscussed.2. – Exploiting 0.2 fb−1 of integrated luminosityIn this section we will discuss the LHCb potential for some key measurements thatwill be performed by collecting an integrated luminosity of about 0.2 fb−1.2.1. The search for decay Bs → μ+μ− decay . – According to the most recent analysis,LHCb will reach the same sensitivity as CDF at the end of its data taking (assumingthat CDF will take data until the end of 2010 collecting 8 fb−1 of integrated luminosity).This corresponds to excluding a region up to a value of the branching ratio of about2× 10−8 as shown in fig. 1.2.2. Measurements of the Bs mixing phase through the Bs → J/ψφ decay . – LHCb willtrigger and reconstruct 12k Bs → J/ψφ decays [6]. The expected sensitivity on the Bsmixing phase is 0.1 rad. The HFAG avarage [3] on the Bs mixing phase is 2βs = 0.77+0.29−0.37.This means that LHCb will confirm or disprove the forementioned evidence of NP veryearly, if the current central value is real.2.3. B → h+h−′ decays. – LHCb will trigger and reconstruct 40k B → h+h−′ de-cays [7]. The family of the B hadron decays into pairs of charmless charged mesonsor baryons comprise a rich set of channels, each one characterized by charged and time-dependent CP asymmetries. The LHCb Collaboration realized a quite advanced analysiswhich allows to fit several decay modes simultaneously. Figure 2 shows, as an example,the distribution of the ππ invariant mass(1) of 12 different final states, with the fit resultsuperimposed.(1) The invariant mass spectrum is built assuming that all the decay products are pions. Theparticle identification is used as an observable in the final fit in order to disentangle the variouscomponents.LHCb PERSPECTIVES WITH EARLY DATA 129Fig. 2. – Distribution of the B → h+h−′ invariant mass from a Monte Carlo sample corre-sponding to an integrated luminosity of 0.2 fb−1, with the result of the maximum likelihood fitsuperimposed. The various signal and background components contributing to the whole lineshape are also shown.The LHCb sensitivity for the CP asymmetry terms will reach the precision of theB-factories for what concerns the Bd sector (direct and mixing-induced CP asymmetryterms Cπ+π−CP , Sπ+π−CP and the charge asymmetry AK+π−CP ), while in the Bs sector LHCbwill improve the already measured charge asymmetry Aπ+K−CP by CDF [8] and will give,most probably, the first measurement for the time-dependent asymmetry terms CK+K−CPand SK+K−CP . Table I reports the sensitivities expected for the CP asymmetry terms withan integrated luminosity of 0.2 fb−1.3. – Exploiting 0.5 fb−1 of integrated luminosityBy collecting some more statistics LHCb will explore other measurements, amongstwhich one of the most interesting involves the Bd → K∗0μ+μ− decay. The numberof expected reconstructed events is about 2k. Considering that the B-factories havecollected few hundreds of events and CDF less than one hundred, LHCb will surpassthe present statistics already within a few weeks of data taking. This FCNC decayproceeds via a b → s transition through a loop diagram. NP processes can thereforeenter at the same level as the SM processes, making the decay a sensitive probe of NPcontributions. The branching ratio as a function of the di-muon invariant mass squaredq2 and the forward-backward asymmetry (AFB)(2) can both be affected in many NPscenarios [9]. The zero crossing-point of this asymmetry as a function of q2 has receivedparticular theoretical attention. The SM predicts a value q2 = 4.360.33−0.31 GeV2/c4 [10].The sensitivity of LHCb corresponding to L = 0.5 fb−1 is about 1GeV2/c4.(2) This asymmetry is constructed from the number of forward- and backward-emitted positivemuons in the di-muon rest frame.130 A. CARBONETable I. – LHCb sensitivities for the B → h+h−′ CP asymmetry terms, expected with 0.2 fb−1of integrated luminosity.Quantity Current experimental LHCb Quantity Current experimental LHCbknowledge 0.2 fb−1 knowledge 0.2 fb−1Sπ+π−CP −0.65± 0.07 0.13 SK+K−CP 0.77± 0.18(∗) 0.11Cπ+π−CP −0.38± 0.06 0.13 CK+K−CP 0.09± 0.04(∗) 0.15AK+π−CP −0.098+0.012−0.011 0.008 Aπ+K−CP 0.39± 0.08± 0.05 0.05(∗) These quantities are still unmeasured, we show a prediction of their values obtained using U-spin symmetryarguments. For more details see [11].4. – ConclusionsAlthough LHCb will realize its full programme with several fb−1 of integrated lumi-nosity, it will be extremely competitive with some measurements already exploiting veryearly data. By collecting about 0.2 fb−1 of data, LHCb will confirm or disprove the evi-dence of NP in the Bs mixing phase claimed by the Tevatron experiments. In addition,it will look for NP through other outstanding measurements, like the branching fractionof the rare Bs → μ+μ− decay. Other important measurements achievable with 0.2 fb−1of integrated luminosities will involve the CP asymmetry terms of the Bd → π+π− andBs → K+K− decays. Increasing the collected data up to 0.5 fb−1, it will possible toexplore NP effects also employing the Bd → K∗0μ+μ− decay, by measuring its forward-backward asymmetry.REFERENCES[1] Bona M. et al. (UTfit Collaboration), Nuovo Cimento B, 123 (2008) 854.[2] Aaltonen T. et al. (CDF Collaboration), Phys. Rev. Lett., 100 (2008) 101802.[3] See http://www.slac.stanford.edu/xorg/hfag.[4] Ishino H. et al. (Belle Collaboration), Phys. Rev. Lett., 98 (2007) 211801.[5] Aubert B. et al. (BABAR Collaboration), arXiv:0807.4226 [hep-ex].[6] Calvi M. et al., LHCb-2009-025; CERN-LHCb-2009-025.[7] Carbone A. et al., LHCb-2007-059; CERN-LHCb-2007-059.[8] Abulencia A. et al. (CDF Collaboration), Phys. Rev. Lett., 97 (2006) 211802.[9] Ali A., Mannel T. and Morozumi T., Phys. Lett. B, 273 (1991) 505.[10] Beneke M. et al., Eur. Phys. J. C, 41 (2005) 173.[11] Bates A. et al., LHCB-ROADMAP6-001.

LHCb perspectives with early data

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LHCb perspectives with early data

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