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At the Lhcb Experiment Summer student project report: Search for the decay ∗ ± ∗± 퐷푠(2317) → 퐷푠 훾 at the LHCb experiment Lukas Calefice1,2,* and Supervisor: Ricardo Vázquez Gómez2,** 1Fakultät Physik, Technische Universität Dortmund, 2LHCb collaboration, CERN, *[email protected], **[email protected] September 2018 Abstract ∗ ± ∗± The summer student project treated the search for the radiative decay 퐷푠(2317) → 퐷푠 훾 with the 2016 and 2017 data sets collected by the LHCb experiment corresponding to 3.3 fb−1 at a centre- ∗ ± ∗± ∗ ± of-mass energy of 13 TeV. For the search the ratio BR(퐷푠(2317) → 퐷푠 훾)/BR(퐷푠(2317) → ± 0 ∗ ± ∗± 퐷푠 휋 ) was investigated, but no hint for the existence of 퐷푠(2317) → 퐷푠 훾 was found. 1 Introduction The spectroscopy of heavy-light mesons can mostly be described successfully with non-relativistic quark potential models in the limit of Heavy Quark Effective Theory (HQET), in which the mesons can be considered as a hydrogen atom. The limit requires that 푚Q → ∞, which is the mass of the ⃗ meson’s heavy quark. In that case the spin of the heavy quark 푆Q is decoupled from the orbital angular momentum 푙 ⃗ between the quarks and states in the spectrum are identified by the quantum numbers 푗 푗 = 퐿 + 푠q, 퐿 and 퐽. Therefore the 푃-wave (퐿 = 1) states are expected to be doubly degenerated in the spin of the heavy quark. Moreover new symmetries called Heavy Flavour Symmetry and Heavy Spin Symmetry come along with the HQET that make it possible to estimate several mass states and branching fractions. [1][2] The HQET was applied successfully to the spectroscopy of the strange-charmed mesons until the ∗ + ∗ + surprising discoveries of the 퐷푠(2317) and the 퐷푠(2460) states by the BABAR and CLEO collabo- ∗ ± ∗ ± rations in 2003. [3][4]. The 퐷푠1(2536) and 퐷푠2(2573) 푃-wave states with 푗 = 3/2 are known since their discoveries by the ARGUS [5] and the CLEO [6] collaborations and match the mass predictions ∗ ± ∗ ± by HQET very well. Therefore the 퐷푠(2317) and 퐷푠(2460) states are supposed to be the missing ± 푗 = 1/2 푃-wave states in 퐷푠 mass spectrum. Most (but not all, e.g. [7]) of the theory papers predict these missing 푗 = 1/2 푃-wave states to have higher masses than those measured, even above the 퐷퐾 and 퐷∗퐾 mass thresholds [8][9][10] and the widths of these states to be broad (hundreds of MeV) [11]. As they are observed below these thresholds they can decay through the strong interaction only via isospin violating modes. Therefore the radiative processes are expected to have sizeable branching fractions, but they have not been seen so far. All these differences between the measurements and the theory predictions raise the questions of how applicable quark potential models and HQET are in ± describing these states and whether they are part of the 퐷푠 spectrum or not. Thus the searches for the ∗ ± ∗± radiative decays such as 퐷푠(2317) → 퐷푠 훾 are important tools for testing HQET in this application. Several other theoretical papers suggest different approaches to solve these problems by treating the states as hadronic molecules [12][13] or multiquarks compounds such as tetraquarks [14]. 1 ∗ ± 2 Former searches for decay channels of the 퐷푠(2317) ∗ ± When the BABAR collaboration found the 퐷푠(2317) resonance, it was seen in the decay channel ± 0 퐷푠 휋 , which is so far the only decay channel found. The existence of the decay channel was confirmed by the CLEO [4] and the BELLE collaborations [15] in the same year. These experiments also searched, ∗ ± ∗± among others, for the radiative decay 퐷푠(2317) → 퐷푠 훾, but did not find a hint for its existence, so they set upper limits on the ratio ∗ ± ∗± BR(퐷푠(2317) → 퐷푠 훾) 푅 = (1) ∗ ± ± 0 BR(퐷푠(2317) → 퐷푠 휋 ) to be ≤ 5.9 % [4] and ≤ 18 % [15] respectively. The BABAR collaboration performed another search ∗ ± ∗ ± for decay channels of the 퐷푠(2317) and 퐷푠(2460) meson states [16] and set the limit to R to be ≤ 16 %. 3 Strategy for the search The analysis is based on the full data sets of 2016 and 2017 which refer to an integrated luminosity −1 ∗ ± ∗± of 3.3 fb . The search of the decay 퐷푠(2317) → 퐷푠 훾 is performed by measuring the ratio R of ∗± ± 0 ± equation (1). The 퐷푠 is reconstructed in the 퐷푠 훾 decay channel, while the 휋 goes to 훾훾 and 퐷푠 + − ± ∗ ± ∗± ∗ ± ± 0 to 퐾 퐾 휋 . Thus the particles in the final states of 퐷푠(2317) → 퐷푠 훾 and 퐷푠(2317) → 퐷푠 휋 are the same, so that a similar selection for both channels is implemented. The advantage of similar selections yields in the cancellation of many systematic uncertainties in the calculation of the branching ratios which gives ∗ ± ∗± ∗ ± ∗± 휋0 푁(퐷 (2317) → 퐷 훾) BR(퐷 (2317) → 퐷 훾) 휀 휀 훾0휀 훾1 푠 푠 = 푠 푠 sel rec rec ∗ ± 0 ∗ ± ± 0 2훾 훾 2 푁(퐷푠(2317) → 퐷푠 휋 ) BR(퐷푠(2317) → 퐷푠 휋 ) 휀sel ⏟⏟⏟⏟⏟(휀rec ) ≈1 ¨¨ 퐾퐾휋 ¨퐾퐾휋 휀trig 휀rec ¨휀sec ℒ휎(푝푝 → 푐 ̄푐)푓퐷푠(2317) × ¨ , 퐾퐾휋¨ 퐾퐾휋 (2) 휀 ¨휀 휀 ℒ휎(푝푝 → 푐 ̄푐)푓퐷푠(2317) ¨trig¨ rec sec ± where furthermore the cancellation of some efficiencies is assumed. The 퐷푠 meson is reconstructed in the full-hadronic 휙(1020)(→ 퐾+퐾−)휋± channel and the selection is tuned by requirements to the kinematics and PID of the charged tracks as well as by avoiding backgrounds. The results are presented in the next section. For the reconstruction of the neutrals the kinematics as well as the angular ± separation between the neutrals and the 퐷푠 flight directions are studied based on a RapidSim [17] simulation. The small available 푄-value in these decays is going to be used to create a cone around ± the 퐷푠 flight direction that covers the neutrals and excludes background from random photons. For ∗ ± ± 0 0 ∗± the reconstruction of the 퐷푠(2317) → 퐷푠 휋 channel only resolved 휋 are used. For the 퐷푠 yield an ∗ ± ∗± SPlot [18] is performed to extract signal weights. With these weights the 퐷푠(2317) → 퐷푠 훾 yield can be built. ± 4 Reconstruction of the 퐷푠 mesons ± + − ± As mentioned before the 퐷푠 are reconstructed in the full-hadronic 휙(1020)(→ 퐾 퐾 )휋 channel. The data was taken in the stripping line StrippingD2hhhFTCalib_KKPLine in stripping S28r1 for 2016 and S29r2 for 2017. There was a change in the stripping line regarding the invariant mass of the two kaons 푚(퐾+퐾−) to be in the 휙(1020) mass range in S29r2 but not in S28r1. Therefore the data sets have been aligned to each other. This is performed by requiring the invariant mass of the two kaons + − 2 2 푚(퐾 퐾 ) to be within 7 MeV/푐 of 푚휙(1020) = (1019.461 ± 0.016) MeV/푐 [19]. Reconstruction of ± 퐷푠 mesons via the 휙(1020)-resonance is a common selection that is motivated by the Dalitz plot and ± + − ± aim to get a very pure signal. In figure 1a a Dalitz plot for 퐷푠 → 퐾 퐾 휋 of simulated events made 2 ∗ ± ∗± with RapidSim for the 퐷푠(2317) → 퐷푠 훾 is shown. The red vertical lines qualitatively highlight the requirement to 푚(퐾+퐾−). ×103 ] 240 2 1.6 lambda Entries 4.658683e+07 1.5 220 Mean 1.16 )[GeV/c 4 + 1.4 10 Std Dev 32.2 π - 200 1.3 m(K 3 180 1.2 10 1.1 160 1 102 140 0.9 120 0.8 10 100 0.7 −50 0 50 0.6 1 1 1.2 1.4 1.6 1.8 2 → + - π± 2 m(K+ K-)[GeV/c2] m((p K ) K )-2286.46 [MeV/c ] (a) Dalitz plot + misid. + − + (b) 훬푐 → (푝 → 퐾 )퐾 휋 background ×103 ×103 500 400 alpha = 1.9400 ± 0.0025 alpha = 1.7157 ± 0.0021 bkg_yield = 178906 ± 504 bkg_yield = 193130 ± 452 350 mean = 1968.8753 ± 0.0032 400 mean = 1968.8388 ± 0.0027 300 n = 4.269 ± 0.049 n = 9.06 ± 0.13 ± ± sig_yield = 2680035 1218 300 sig_yield = 2957775 1195 250 sigma = 6.6816 ± 0.0026 sigma = 6.3974 ± 0.0023 Events / ( 2.5 ) Events / ( 2.5 ) 200 200 150 100 100 50 0 0 1940 1960 1980 2000 2020 1940 1960 1980 2000 2020 m(K+ K- π±) m(K+ K- π±) ± ± (c) 퐷푠 yield for 2016 (d) 퐷푠 yield for 2017 ± + − ± Figure 1: (a) Dalitz plot of 퐷푠 → 퐾 퐾 휋 from simulated events made with RapidSim for the ∗ ± ∗± ± − + 퐷푠(2317) → 퐷푠 훾 decay channel. (b) contribution of 훬푐 → 푝퐾 휋 for the 2016 data ± set before applying any cuts. 퐷푠 yield after all selection cuts for (c) 2016 and (d) 2017. Additionally cuts to the kinematics of the charged tracks and to the particle identification (PID) of ± the kaons were applied to increase the purity of the 퐷푠 . These require for the transverse momenta + − ± ± + − 푝T(퐾 , 퐾 , 휋 ) > 1.5 GeV/푐, 푝T(퐷푠 ) > 5 GeV/푐 and for the PID ProbNNk(퐾 , 퐾 ) > 0.8. There is + misid. + − + also cut applied to avoid contaminations in the signal window from 훬푐 → (푝 → 퐾 )퐾 휋 where the proton is misidentified as a 퐾+. This is made by assigning the proton mass to the positively charged kaons and recalculating their energy and the invariant mass 푚(퐾+퐾−휋+). The cut requires that misid. + − ± ± |푚((푝 → 퐾 )퐾 휋 ) − 푚(훬푐 )| > 33 MeV. In figure 1b the resulting background before applying ± any cuts is shown for the 2016 data set. The final resulting yields for the 퐷푠 after all these cuts are shown in the figures 1c and 1d.
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