The BESIII spectrometer is hosted at the BEPCII e+e−

collider of the IHEP, Beijing. Since July 2008, it has collected the largest data sample available in the world at the energies of the J/ψ, ψ(2S), ψ(3770) and ψ(4040) resonances; data taking at high luminosities will go on for years. In this work I will describe the experiment peculiarities, showing in particular some of the most recent results involving light hadron spectroscopy, charmonium spectra and transitions, and charm physics. I will also describe in details those contributions coming from the Italian component of the Collaboration, focusing on those hardware projects (Zero Degree Detector, ZDD, and cylindrical GEMs, CGEM) the Italian BESIII researchers have

devoted most of their efforts to

Destefanis, M.

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DOI 10.1393/ncc/i2014-11690-9Colloquia: IFAE 2013IL NUOVO CIMENTO Vol. 37 C, N. 1 Gennaio-Febbraio 2014Latest results and hardware activities from BESIIIM. Destefanis for the BESIII CollaborationUniversita` di Torino and INFN, Sezione di Torino - Torino, Italyricevuto l’1 Ottobre 2013Summary. — The BESIII spectrometer is hosted at the BEPCII e+e− collider ofthe IHEP, Beijing. Since July 2008, it has collected the largest data sample availablein the world at the energies of the J/ψ, ψ(2S), ψ(3770) and ψ(4040) resonances;data taking at high luminosities will go on for years. In this work I will describethe experiment peculiarities, showing in particular some of the most recent resultsinvolving light hadron spectroscopy, charmonium spectra and transitions, and charmphysics. I will also describe in details those contributions coming from the Italiancomponent of the Collaboration, focusing on those hardware projects (Zero DegreeDetector, ZDD, and cylindrical GEMs, CGEM) the Italian BESIII researchers havedevoted most of their efforts to.PACS 13.66.Bc – Hadron production in e−e+ interactions.PACS 14.40.Lb – Charmed mesons.PACS 29.40.Gx – Tracking and position-sensitive detectors.PACS 29.40.Vj – Calorimeters.1. – The BESIII experimentThe Beijing Spectrometer III (BESIII) [1] is hosted on the BEPCII e+e− collider atthe Institute of High Energy Physics (IHEP) in Beijing. With its excellent performancesit offers the appropriate scenario to investigate light hadrons and charmonium physics.The BEPCII (Beijing Electron-Positron Collider II) is designed to provide an instantluminosity up to 1033 cm−2 s−1 with beam currents up to 0.93A. The beam energy (upto 2.3GeV) can be tuned according to the required center of mass energy, for exampleto produce different charmonium resonances as J/ψ, ψ(2S) or ψ(3770). The BESIIIspectrometer, shown in fig. 1, is characterized by a shell-like structure, which provides awide geometrical acceptance, 93% of 4π. Departing from the interaction point, it hostsa 43-layer small-celled, helium-based main drift chamber (MDC), a time-of-flight system(TOF) for particle identification, and an electromagnetic calorimeter (EMC) composedof 6240 CsI (Tl) crystals arranged in a cylindrical shape (barrel) plus two end-caps. Theabove mentioned detectors operate inside a 1T solenoidal magnetic field. The magnetc© Societa` Italiana di Fisica 196LATEST RESULTS AND HARDWARE ACTIVITIES FROM BESIII 197Fig. 1. – 3D view of the BESIII spectrometer.iron yoke is segmented to host a muon chamber system composed of resistive plate cham-bers arranged in 9 layers in the barrel and 8 layers in the end-caps. An electromagneticcalorimeter which covers the zero degree region, ZDD (Zero Degree Detector), has beeninstalled for Initial State Radiations (ISR) studies, and γγ physics.The excellent spectrometer performances allowed to collect the world’s largest datasample at the J/ψ, ψ(2S), ψ(3770) and ψ(4040) resonance energies. Moreover, data werealso collected for XY Z physics, and for the strong and EM relative phase measurementbelow the J/ψ resonance. In order to characterize the spectrometer performances, amongall the published results some of the most significant were chosen.2. – Spectrometer performancesThe investigation of the cc¯ radiative decays allowed to explore a new feature of theJ/ψ → γpp¯ final state to study its production at threshold [2]. Moving closer to thepp¯ production threshold, an increase of the pp¯ pairs was observed. The experimen-tal data were described by means of the Partial Wave Analysis (PWA), and the newstructure was fitted with a Breit-Wigner distribution. The fit components introducedwere: X(pp¯), f2(1920), f0(2100), and 0++ wave, which follows the phase space (PHSP)distribution. The final state interactions (FSI) do not explain in details this behav-ior. Figure 2 shows the different contributions to the obtained γpp¯ final state distri-butions. This analysis allowed to define the characteristics of the new found structure:M = 1832+19−5 (stat)+18−17(syst)± 19(model) MeV, Γ < 76MeV at 90% C.L., Br(J/ψ →γX)Br(X → pp¯) = (9.0+0.4−1.1(stat)+1.5−5.0(syst)± 2.3(model)) · 10−5, and JPC = 0+−.The most precise measurement of the ηc resonance parameters was obtained [3]. Inthis study, the following ηc decays were investigated: KSKπ, KKπ0, ππη, KSK3π,2K2ππ0, and 6π. The invariant mass distributions are shown in fig. 3. A simultaneousfit of the presented ηc decay modes was performed. The resonance shape suggests thatthis resonance interferes with non-ηc decays; this feature was taken into account whenextracting the resonance parameters. The obtained results are the following: M(ηc) =2984±0.6±0.6MeV, and Γ(ηc) = 32.0±1.2±1.0MeV, where the errors are the statisticaland the systematic ones, respectively.For the first time the M1 transition ψ(2S) → γηc(2S) was observed [4]. The ψ(2S)decays into K0SK±π∓ and K+K−π0 were investigated. In this analysis, all the possible198 M. DESTEFANIS for the BESIII COLLABORATIONFig. 2. – γpp¯ distributions. The different lines correspond to the different PWA contributions.backgrounds, including also other charmonium states, were taken into account. Theinvariant mass distributions, presented in fig. 4, were simultaneously fitted with thefunction (E3γ ·BW (m)·fd(Eγ)·ε(m))⊗G(δm, σ), where E3γ is the part connected to the M1transition and fd(Eγ) is the dumping function. The Br(ψ(2S) → γηc(2S)) ·Br(ηc(2S) →KKπ) was found to be (1.30± 0.20± 0.30) · 10−5.The 515 pb−1 data collected at the 4260MeV energy allowed to find a newcharmonium like charged structure, the Zc(3900)± which decays into π±J/ψ [5].The J/ψ was reconstructed via the leptonic decay channels e+e− and μ+μ−. Theinvariant mass distribution obtained considering both the pion charged states,shown in fig. 5, was fitted with a Breit-Wigner function in the peak region.This discovery was confirmed one week later by the Belle Collaboration [6], al-though with a lower statistic. The Z±c resonance parameters are the following:M(Z±c ) = (3899.0± 3.6± 4.9) MeV, and Γ(Z±c ) = (46± 10± 20) MeV. The productionratio is found to be R = σ(e+e−→π±Zc(3900)∓→π+π−J/ψ)σ(e+e−→π+π−J/ψ) = (21.5 ± 3.3 ± 7.5)%. Thenature of the Z±c is still not completely understood, and for this reason the BESIIICollaboration decided to increase the statistics to study its properties and its decays.Fig. 3. – Invariant mass distributions of ηc → KSKπ, KKπ0, ππη, KSK3π, 2K2ππ0, and 6π,from left to right, and from up to down, respectively.LATEST RESULTS AND HARDWARE ACTIVITIES FROM BESIII 199Fig. 4. – Invariant mass distributions of the γηc(2S) → K0SK±π∓ and K+K−π0 final states,respectively.Fig. 5. – Invariant mass distribution of the πJ/ψ final states. The clear structure around3900MeV is the new charged Zc state.During the 2012 data taking run, BESIII collected the integrated luminosity requiredto study the relative phase between the strong and the EM decay amplitudes belowthe J/ψ resonance peak. With this measurement, proposed by the Italian part of theBESIII Collaboration, we want to investigate the phase difference between the resonantamplitude and the non-resonant one by searching for interference in the Q2 behavior.According to the perturbative QCD, the EM amplitudes should be real, and the strongone is expected to be real as well. If this is correct, we should observe and interferencepattern (0◦/180◦), but from the experimental data we have indications of 90◦ relativephases, hence no-interference pattern. The aim of the work was to obtain an inclusive setof exclusive measurements in order to deeply investigate in a model independent way therelative phase. Detailed information about this measurement can be found in [7]. Thehigh level analysis is ongoing for many final states, and the results are expected soon.3. – Hardware developmentBeyond the promising physics results obtained, there is also an important hardwareR&D activity mainly conducted by the Italian part of the Collaboration.The central portion of the drift chambers is reaching the end of its operative activity,and for this reason the BESIII Collaboration is evaluating the possibility to substitute itwith a cylindrical GEM (CGEM) detector. The GEMs are well known detectors [8], char-acterized by a good radiation hardness. The project implies the use of GEMs arranged200 M. DESTEFANIS for the BESIII COLLABORATIONFig. 6. – The ZDD detector installed at BESIII. The arrow indicates the position of the beamline. On both sides, the photomultipliers used to collect the signal from the different zones ofthe ZDD are shown.Fig. 7. – ZDD as online luminometer. The beam bunches can be identified in the upper row,while the collected integrated luminosity per data taking run is shown in the lower row.in a cylindrical geometry, following the example of the only existing CGEM constructedfor the KLOE2 experiment [9]. The peculiarities of the BESIII CGEM detector are notforeseen to be too much different from those of the KLOE2 CGEM. In this scenario, theexpected radiation length is in the order of 1%.The initial state radiation (ISR) technique, heavily used by the Babar Collaboration,is an important experimental technique which allows the simultaneous study of physicsprocesses at different center of mass energies. This technique takes advantage of thoseevents in which a photon emission from one of the beam particles allows a boost ofthe center of mass of the investigated process. The main advantages of the ISR arethe following: the study at the same time of more center of mass energies for the sameprocesses without changing the beam energies, and the possibility to investigate theseprocesses even at threshold exploiting the excellent experimental efficiency and takingadvantage of the boost which gives to the final state particles higher momenta in thelaboratory system. The distribution of the ISR photons is expected to be extremelyforward peaked. This implies that a zero degree detector (ZDD), placed around thebeam line (see fig. 6), should be able to collect alone around 40% of the ISR statistics,while in the whole spectrometer only 20% of the events are foreseen. With this preamble,the Italian component of the Collaboration has build and installed an electromagneticcalorimeter (ZDD). The ZDD is already working as online luminometer, as shown infig. 7, and soon it will be completely interfaced with the DAQ system.LATEST RESULTS AND HARDWARE ACTIVITIES FROM BESIII 201∗ ∗ ∗This work was supported in part by INFN and IHEP Beijing.REFERENCES[1] Asner D. M. et al., Physics at BES-III, arXiv:0809.1869v1[hep-ex] (2008).[2] Ablikim M. et al., Phys. Rev. Lett., 108 (2012) 112003.[3] Ablikim M. et al., Phys. Rev. Lett., 108 (2012) 222002.[4] Ablikim M. et al., Phys. Rev. Lett., 109 (2012) 042003.[5] Ablikim M. et al., Phys. Rev. Lett., 110 (2013) 252001.[6] Liu Z. Q. et al., Phys. Rev. Lett., 110 (2013) 252002.[7] Destefanis M., Measuring the Phase between Strong and EM J/ψ Decay Amplitudes, PoS(Baldin ISHEPP XXI) 031 (2013).[8] Sauli F., Nucl. Instrum. Methods A, 386 (1997) 531.[9] De Robertis G. et al., Technical Design Report of the Inner Tracker for the KLOE-2Experiment, arXiv:1002.2572v1[physics.ins-det] (2010).

Latest results and hardware activities from BESIII

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