K0S decays are a new area of interest for LHCb. Latest results on the K0S → μ+μ− search are reported using data collected by the LHCb experiment at a center-of-mass energy of √s = 7TeV, corresponding to an integrated luminosity of 1 fb−1. The upper limit at 90% CL on the branching ratio of this decay is found to be 9 × 10−9. This limit is 30 times lower than the previous world best. LHCb prospects for other K0S decays and other strange mesons decays are also reported

Marin Benito, C.

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DOI 10.1393/ncc/i2015-11853-2Colloquia: LaThuile14IL NUOVO CIMENTO Vol. 37 C, N. 6 Novembre-Dicembre 2014K0S decays at LHCbC. Marin Benito(∗) on behalf of the LHCb CollaborationUniversitat de Barcelona - Barcelona, Spainricevuto il 31 Luglio 2014Summary. — K0S decays are a new area of interest for LHCb. Latest results on theK0S → μ+μ− search are reported using data collected by the LHCb experiment ata center-of-mass energy of√s = 7TeV, corresponding to an integrated luminosityof 1 fb−1. The upper limit at 90% CL on the branching ratio of this decay is foundto be 9 × 10−9. This limit is 30 times lower than the previous world best. LHCbprospects for other K0S decays and other strange mesons decays are also reported.PACS 13.20.Eb – Decays of K mesons.1. – IntroductionStrange mesons have played a major role in the history of particle physics. When firstobserved, back in the late 1940s, they triggered large interest. Experiments showed thatkaons were produced through the strong force but they were not decaying through thisinteraction, thus being long-lived particles. This fact helped to understand in a betterway the weak interaction, which was pretty unknown at that moment, and motivatedthe GIM mechanism and the prediction of the c quark [1].Another key point in the history of kaon physics was the discovery of charge-parity(CP ) violation. It was first observed in 1964 in the decay of K0L → π+π− at theBrookhaven National Laboratory [2]. Charge-parity violation was completely unexpectedat that moment and its observation triggered many advances in the understanding of par-ticle physics.Strange meson decays are very interesting both from the theoretical and experimen-tal point of view. On one side, they are theoretically clean and involve flavour changingneutral currents (FCNC) with the strongest Cabibbo-Kobayashi-Masakawa (CKM) sup-pression (VtsVtd ∼ 10−4), making them very sensitive to New Physics (NP) effects. Fromthe experimental side, there is a copious production of strange particles at the LHC andthe number of possible final states is limited, allowing for clean studies of these decays.(∗) E-mail: carla.marin.benito@cern.chc© CERN on behalf of the LHCb CollaborationCreative Commons Attribution 3.0 License (http://creativecommons.org/licenses/by/3.0) 154K0S DECAYS AT LHCb 1551.1. LHCb detector for strange decays. – The LHCb detector is a single-arm forwardspectrometer described in detail in ref. [3]. The detector is optimized for the study of band c hadrons. Although strange mesons have in average lower masses and larger decaytimes, they are produced in copious amounts at the LHC. During 2011, at a center-of-mass energy of√s = 7TeV, the LHC produced ∼ 1013 K0S per fb−1 inside the LHCbacceptance [4].Due to the larger decay times of strange mesons, many of them decay outside thevertex locator (VELO). This fact implies that the decay vertex is not directly observedand the daughter tracks are only detected in the subsequent subdetectors. These tracksare referred to as downstream tracks and have in average worse momentum resolution.Tracks that are detected also inside the VELO are referred to as long tracks.When using downstream tracks to reconstruct the daughters of a decay, there is noinformation on the decay vertex. However, when the mother is a charged particle, it leavesalso hits in the VELO that can be matched to the downstream tracks to extrapolate theposition of the decay vertex. This technique helps to improve a lot the selection of thesedecays as shown in sect. 3.3.1.2. LHCb trigger for strange decays. – The LHCb trigger consists of a hardware stage,that selects high pT and ET signatures based on the information from the calorimetersand muon chambers, followed by a software stage that after selecting displaced tracksusing tracking and vertexing information, performs a full event reconstruction.The LHCb trigger is not designed to select strange decays, since it takes advantageof the distinct signatures of b and c hadron decays. Nevertheless, in 2011, around 1/3 ofthe events that passed the trigger contained a reconstructible K0S → π+π−.During 2011, the di-μ invariant mass requirement in the software trigger was incom-patible with the K0S mass. Thus, only triggers selecting single muons were useful to selectdecays like K0S → μ+μ−. In 2012, a low mass di-μ line compatible with the K0S masswas included. This resulted in an increase in the trigger efficiency for K0S → μ+μ− of afactor 3. Additional studies are ongoing to improve this efficiency for LHC Run 2.2. – K0S → μ+μ−The K0S → μ+μ− decay is a FCNC with no tree-level contribution in the StandardModel (SM), with a predicted branching fraction (BR) of (5.1± 0.2)× 10−12 [6]. Withinthe SM, there are two contributions to the amplitude of this process [5]: the long-distancecontribution, which is mediated by virtual photons, and the short-distance, which ismediated by electroweak bosons. The last one is dominated by the CP -violating part ofthe quark level transition s→ dll, making this decay very sensitive to NP contributions.From the experimental side, the previous best measurement dates from 1973 whenthe upper limit on the BR was set to 3.1 × 10−7 at 90% confidence level (CL) [7], farfrom the SM prediction.2.1. LHCb analysis. – The LHCb collaboration reported in 2012 an improved mea-surement of the BR limit for the decay K0S → μ+μ− [4]. The analyses used 1 fb−1 ofdata collected at a center-of-mass energy of√s = 7TeV.The K0S candidates were reconstructed using μ pairs from long tracks and selectedby a multivariate algorithm, namely a Boosted Decision Tree (BDT). The BDT rejectedcandidates coming from random combination of two muons and muons resulting from156 C. MARIN BENITO on behalf of the LHCb COLLABORATION]2cInvariant mass [MeV/460 480 500 520 540)2 cCandidates / (1 MeV/01020304050 LHCb3 10Fig. 1. – Mass spectrum for selected K0S → π+π− candidates. The points correspond to the massreconstructed under the π+π− mass hypothesis for the daughters, while the triangles correspondto the mass reconstructed under the μ+μ− mass hypothesis.the interaction of particles produced in the pp collision vertex with the material in theregion of the VELO.The K0S → π+π− channel was used as normalization channel. The separation of thisdecay from the signal channel was studied using K0S → π+π− decays reconstructed andselected in the same way as the signal channel, both in the π+π− and μ+μ− mass hy-potheses. As shown in fig. 1, the two mass peaks are separated by 40MeV/c2. Thisseparation, combined with the LHCb mass resolution of about 4MeV/c2 for such com-bination of tracks, was used to discriminate between the two channels.The search region in the di-μ invariant mass was defined between [492, 504]MeV/c2.The background level was calibrated by interpolating the observed yield in the massside-bands to the signal region. A model with two components was used: a power lawto describe the tail of K0S → π+π− decays where both pions are misidentified as muonsand an exponential function describing the combinatorial background. Other sourcesof background were found to be negligible. The invariant mass distribution of selectedcandidates is shown in fig. 2.To translate the number of K0S → μ+μ− signal decays into a branching fractionmeasurement a normalisation was performed in BDT bins. Using the value of the BR ofK0S → π+π− from ref. [8], around 2× 10−4 SM candidates were expected per BDT bin.The modified frequentist approach (or CLs method) [9] was used to asses the com-patibility of the observation with expectations as a function of the BR of K0S → μ+μ−.The observed distribution of events was compatible with the background expectations,as shown in fig. 3. The measured upper limit is 11(9)×10−9 at 95(90)% confidence level,a factor of thirty below the previous world best limit.2.2. K0S → μ+μ− prospects at LHCb. – LHCb has improved by a factor of 30 theprevious world best upper limit measurement but there is still need for improvement toget to the SM prediction of (5.1± 0.2)× 10−12. The most interesting region is below the10−10 limit where most new physics models arise.K0S DECAYS AT LHCb 157]2c [MeV/m470 480 490 500 510 520 )2 cCandidates / ( 1 MeV/051015202530354045TOSLHCbFig. 2. – Background model fitted to data events which are triggered on the signal object (TOS),independently of the rest of the event. The vertical lines delimit the search region.The current limit measurement makes use of only one third of the total amount ofdata recorded by the LHCb detector in Run 1. In addition, as mentioned in sect. 1.2 athreefold improvement in the trigger efficiency was already obtained from the adoption ofnew trigger strategies in 2012. Some extra improvement may be obtained by making useof downstream muon tracks. With these improvements, the LHCb collaboration expectsto reach the 10−10 limit in its upgrade phase, with an accumulated luminosity of 40 fb−1.Fig. 3. – CLs curves. The solid line corresponds to the observed CLs. The dashed line corre-sponds to the median of the CLs for an ensemble of background-alone experiments. The darkband covers 65% (1σ) of the CLs curves obtained in the background only pseudo-experiments,while the light band covers 95% (2σ).158 C. MARIN BENITO on behalf of the LHCb COLLABORATION3. – Other rare strange prospects at LHCbWith the publication of the K0S → μ+μ− analysis, strange decays have become a newarea of interest at LHCb with many ongoing studies on this field. A selection is reportedhere.3.1. K0S → π0μ+μ−. – The K0S → π0μ+μ− decay measures the indirect CP violatingcomponent of the related KL → π0μ+μ− decay, allowing to extract the direct CP violat-ing contribution of the last one. This can provide input to determine the imaginary partof the element Vtd of the mixing matrix. Moreover, the K0S → π0μ+μ− decay can be usedto study the structure of the K → πγ∗ form factor, as it is expected to be dominated bythis contribution.The NA48 collaboration reported in 2012 the first observation of this decay [10] anda BR measurement of (2.9+1.5−1.2 ± 0.2)× 10−9 with an uncertainty of ∼ 50%.The most challenging issue of this analysis at LHCb is the π0 reconstruction. Differentpossibilities have been studied using simulated data and the most feasible option is thereconstruction of π0 → γγ. Although few K0S → π0μ+μ− events are expected in the3 fb−1 currently accumulated by the LHCb detector, this decay may be observed in theLHC Run 2 and surely after the LHCb upgrade.3.2. K0S → 4. – The predicted branching fraction of K0S → 4 in the SM are expectedto be [11]K0S → eeee ∼ 10−10,K0S → μμee ∼ 10−11,K0S → μμμμ ∼ 10−14.Any deviation from these values may hint to contributions from NP. No experimentalresults have been reported so far.For those decays containing electrons in the final state, the reconstruction of theseparticles is the most challenging issue at LHCb. Studies on simulated data allow toextract an expected mass resolution of ∼ 20 (∼ 10)MeV/c2 for K0S → eeee (K0S → μμee).These studies also show a clear displacement of the mass peak due to the energy loss of theelectrons. However, the expected separation to the normalization channel K0S → ππeewith the two pions misidentified as electrons or muons, respectively, combined with thequoted mass resolution, should allow to discriminate between the different channels.Using the well measured K0S → ππee decay as normalization channel, the expectedsingle event sensitivity, assuming no background in the search region, with the 3 fb−1 ofdata accumulated by the LHCb detector is found to be ∼ 10−6 (∼ 10−7) for the K0S →eeee (K0S → μμee) channel. Similar studies are being performed with the K0S → μμμμchannel.3.3. K+ mass prospects. – Currently experimental results show a disagreement be-tween the most precise measurements of the K+ mass [12]. LHCb could give a competi-tive result on this puzzle by means of the K+ → πππ decay.The K+ candidates are reconstructed combining 3 π from downstream tracks, whichhave no information from the decay vertex. Random combinations of 3 π not coming froma decay of a K+ are easily selected if no requirement on the decay vertex is made. Studiesusing simulated data at LHCb show a great selection improvement when the tracks areK0S DECAYS AT LHCb 159]2 Mass [MeV/c+K400 450 500 550 600Entries0100200300400500600LHCb Simulation]2 Mass [MeV/c+K400 450 500 550 600Entries020406080100120140160LHCb SimulationFig. 4. – K+ invariant mass distribution for simulated events. Left: no matching requirementapplied. Right: after matching requirement.required to be matched to the hits left by the K+ in the VELO. Figure 4 shows theinvariant mass distribution of the K+ before and after the matching requirement. Thistechnique rejects most of the background with high efficiency on the signal events. Moreinformation of this technique can be found in [13].A rough estimate of systematic uncertainties shows that the precision on the massmeasurement is limited at the moment by statistics. LHC Run2 may provide enoughdata to the LHCb detector to allow for a competitive measurement of the K+ mass.3.4. Σ+ → pμ+μ− prospects. – The HyperCP collaboration at Tevatron reported theobservation of 3 Σ+ → pμ+μ− events with 0 background and a branching fraction for thisdecay of (8.6+6.6−5.4 ± 5.5)× 10−8 [14]. This evidence had wide relevance since all the threeobserved events lie within the expected di-μ mass resolution of ∼ 0.5MeV/c2, possiblypointing towards a Σ+ → pX0(→ μ+μ−) decay. The new intermediate state wouldhave a mass of 214.3 ± 0.5MeV/c2. The existence of a light neutral particle decayingto a muon pair would have many consequences and several experiments have performedspecific tests of this hypothesis [15-18].The LHCb collaboration aims for a direct search for the Σ+ → pμ+μ− decay, takingadvantage of the huge amount of Σ+ produced at LHC pp collisions — approximately 40%of the events contain one. To increase the statistics, both long and downstream tracksare used for this analysis. In the case of downstreeam tracks, the matching techniqueexplained in sect. 3.3 is used to purify the selection. Studies on simulated data show avery good mass resolution for this decay, σ ∼ 2MeV/c2. Using Σ+ → pπ0(→ e+e−γ)with undetected photon as normalization channel, a single event sensitivity of ∼ 5×10−9can be reached with the current statistics (3 fb−1).4. – ConclusionsAlthough the LHCb detector is not designed for strange physics, it has the potentialto significantly contribute in the study of this type of decays. A world best upper limitmeasurement was set for the branching fraction of K0S → μ+μ− to 9.0 × 10−9 at 90%CL, improving in a factor 30 the previous world best result. After this analysis, strangephysics have become a new area of interest for LHCb and several studies are being carriedout to asses the sensitivity of some of the most relevant measurements in the field.160 C. MARIN BENITO on behalf of the LHCb COLLABORATIONREFERENCES[1] Glashow S. L., Iliopoulos J. and Maiani L., Phys. Rev. D, 2 (1970) 1285.[2] Christenson J. H., Cronin J. W., Fitch V. L. and Turlay R., Phys. Rev. Lett., 13(1964) 138[3] LHCb Collaboration, JINST, 3 (2008) S08005.[4] LHCb Collaboration, JHEP, 01 (2013) 090.[5] Isidori G. and Unterdorfer R., JHEP, 01 (2004) 009.[6] Ecker G. and Pich A., Nucl. Phys. B, 366 (1991) 189.[7] Gjesdal S., Presser G., Steffen P., Steinberger J., Vannucci F. et al., Phys. Lett.B, 44 (1973) 217.[8] Particle Data Group, J. Phys. G, 37 (2010) 075021.[9] Read A. L., J. Phys. G, 28 (2002) 2693.[10] NA48 Collaboration, Phys. Lett. B, 599 (2004) 197.[11] D’Ambrosio G., Greynat D. and Vulvert G., Eur. Phys. J. C, 73 (2013) 2678.[12] Particle Data Group, Phys. Rev. D, 86 (2012) 010001.[13] Contu A., CERN-LHCb-PUB-2014-032.[14] HyperCP Collaboration, Phys. Rev. Lett., 94 (2005) 021801.[15] CLEO Collaboration, Phys. Rev. Lett., 101 (2008) 151802.[16] KTeV Collaboration, Phys. Rev. Lett., 107 (2011) 201803.[17] E391a Collaboration, Phys. Rev. Lett., 102 (2009) 051802.[18] LHCb Collaboration, Phys. Rev. Lett., 110 (2013) 211801.

K0S decays at LHCb

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K0S decays at LHCb

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