A representative sample of recent CMS measurements in Heavy-

Flavor physics, based on proton-proton collision data collected in 2011 at a center-ofmass energy of 7TeV, are reviewed. Particular emphasis is given to exotic quarkonia,

properties of b-hadrons and observation of new hadrons

Pozzobon, N.

English

A representative sample of recent CMS measurements in Heavy-Flavor physics, based on proton-proton collision data collected in 2011 at a center-of-mass energy of 7 TeV, are reviewed. Particular emphasis is given to exotic quarkonia, properties of b-hadrons and observation of new hadrons and structures

Pozzobon, Nicola

CERN Document Server

Heavy flavor production and spectroscopy at  CMS

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DOI 10.1393/ncc/i2014-11625-6Colloquia: LaThuile13IL NUOVO CIMENTO Vol. 36 C, N. 6 Novembre-Dicembre 2013Heavy-Flavor production and spectroscopy at CMSN. Pozzobon on behalf of the CMS CollaborationUniversita` di Padova and INFN, Sezione di Padovavia F. Marzolo, 8, 35131 Padova, Italyricevuto 20 Giugno 2013; approvato l’1 Luglio 2013Summary. — A representative sample of recent CMS measurements in Heavy-Flavor physics, based on proton-proton collision data collected in 2011 at a center-of-mass energy of 7TeV, are reviewed. Particular emphasis is given to exotic quarkonia,properties of b-hadrons and observation of new hadrons.PACS 14.20.Mr – Bottom baryons (|B| > 0).PACS 14.40.Nd – Bottom mesons (|B| > 0).PACS 14.40.Pq – Heavy quarkonia.1. – IntroductionThe mission of the Compact Muon Solenoid experiment at the CERN Large HadronCollider (LHC) includes a wide program of Heavy-Flavor (HF) physics [1]. The im-portance of precision measurements of production and properties of b-hadrons is due tomultiple reasons. Measurements of HF production provide a testing ground for QuantumChromodynamics (QCD) calculations: while next-to-leading order (NLO) contributionsdominate at the LHC energies, large theoretical uncertainties remain due to factorizationand renormalization scales. Moreover, b-jet identification is crucial in searches for NewPhysics, as many SM processes are known to have b-hadrons in their final state, and sev-eral scenarios beyond the SM foresee predominant couplings to the heaviest known quarksand leptons. Therefore, measurements of b-hadron properties provide important tests ofthe SM and any deviation from the predicted values would be an indirect indication ofNew Physics.A representative sample of recent results in HF physics, obtained with the CMS ex-periment housed at the CERN LHC, and based on proton-proton collision data collectedin 2011 at a center-of-mass energy of 7TeV, and made public since the previous editionof the “Re´ncontres de Physique de la Valle´e d’Aoste”, is reviewed herein.The CMS apparatus is described in detail in ref. [2]. It is designed around a super-conducting solenoid, of 6m internal diameter, providing a field of 3.8T. At its core,a silicon pixel detector and a silicon microstrip tracker are immersed in the magneticfield. Together, they allow the measurement of charged particle momenta over the wholec© CERN on behalf of the CMS Collaboration under the Creative Commons Attribution License 3.0 126HEAVY-FLAVOR PRODUCTION AND SPECTROSCOPY AT CMS 127pseudorapidity range |η| < 2.5, where η = − ln tan(θ/2) and θ is the polar angle of thetrack relative to the counterclockwise beam direction. Electromagnetic and hadroniccalorimeters are housed inside the superconducting solenoid too. Muon detectors, basedon drift tubes and resistive plate chambers, are embedded in the steel return yoke of themagnet, up to |η| < 2.4. The resolution in transverse momentum (pT ) measurement ofthe combined tracking making use of both the silicon tracker and the muon detectors isbetter than 1% [3,4].All the events used in the analyses presented herein were collected using the two-leveltrigger system of CMS. The first level is based on custom hardware processors and relieson information from muon detectors [5]. The “high-level trigger” is a processor farmwhich further reduces the event rate before data storage [6]. All the online selectioncriteria in the analyses presented herein require the presence of two muons with aninvariant mass compatible with the J/ψ mass. Occasionally, the additional request of adisplaced dimuon vertex is made.2. – Measurement of the X(3872) production cross section via decays to J/ψππin pp collisions at√s = 7TeVThe X(3872) state was first observed in 2003 at Belle [7], it is an unexpected char-monium state and, until present day, its nature is unclear. The X(3872) was observedin several decay channels also at BaBar [8], at the Tevatron [9,10] and at the LHC [11].The X(3872) is produced both promptly and in B-decays, and angular analysis favorsJPC = 1++. Recent updates were presented at this edition of the “Re´ncontres dePhysique de la Valle´e d’Aoste”. Quantitative prediction of its production cross sectionare calculated in the framework of non-relativistic QCD (NRQCD), in the hypothesisthat X(3872) is standard charmonium.The measurement [12] is based on 4.8 fb−1 of pp collision data at√s = 7TeV collectedin 2011 with prompt J/ψ dimuon triggers. The candidate X(3872) is selected via itsdecay to J/ψπ+π−. An opposite-sign muon-pair with central rapidity, consistent withJ/ψ mass, is selected applying stringent criteria on track selection, and it is combined totwo additional opposite-sign tracks, assumed to be pions. The candidate is selected in apT (J/ψπ+π−) range from 10 to 50GeV and with a pseudorapidity |η(J/ψπ+π−)| < 1.2.The vertex of the four-tracks system is refit at the end of the selection procedure.The production cross section is measured relative to the ψ(2S) one, in the same finalstate. The signal yield is extracted with an unbinned maximum-likelihood fit to thefour-track invariant mass. The fit is performed in five different bins of pT (J/ψπ+π−).The ratio between the two cross sections is calculated asR =NX(3872) ×Aψ(2S) × ψ(2S)Nψ(2S) ×AX(3872) × X(3872) ,where A is the acceptance and  the selection efficiency. Corrections and efficiencyfactors are calculated from simulations assuming the X(3872) is unpolarized and quantumnumbers JPC = 1++. The main background is due to the random combination oftracks and it is reduced with a requirement on the distance in φ × η space between thecandidate J/ψ and additional pions. The result is not sensitive to the number of multipleinteractions per bunch crossing thanks to the good selection of primary vertices. Themajor sources of systematic uncertainties are due to the pion-pair efficiency and to thepT spectrum of candidate X(3872) and ψ(2S).128 N. POZZOBON on behalf of the CMS COLLABORATIONFig. 1. – Left: measured value of the production cross section ratio between X(3872) andψ(2S), differential in pT (J/ψπ+π−). Middle: measured fraction of non-prompt X(3872). Right:measured differential production cross section of promptly produced X(3872).Figure 1 shows, from left to right, the cross section ratio R, the non-prompt fractionof produced X(3872), and the production cross section of prompt X(3872), computedfor each bin of pT (J/ψπ+π−). The non-prompt fraction is measured selecting onlycandidate X(3872) with a pseudo-proper decay length larger than 0.1mm. This selectionretains about the 80% of non-prompt X(3872)’s while the contamination from promptlyproduced X(3872) candidates is less than 0.1%. The prompt X(3872) production crosssection is obtained from the measured values of the non-prompt fraction, R, and theψ(2S) production cross section of 3.53± 0.26 (stat.)± 0.32 (syst.)± 0.14 (lumi.) nb [13].The measured differential cross section is remarkably lower than the one predicted byNRQCD, not consistently with the standard charmonium hypothesis. The integratedprompt X(3872) production cross section has been measured in the same final state wherethe kinematic range is restricted to 10 < pT < 30GeV and |y| < 1.2. In such a region,the average value of R is 0.0682± 0.0032 (stat.)± 0.0065 (syst.) while the average non-prompt fraction of produced X(3872) is 0.260±0.024 (stat.)±0.016 (syst.). The resultingprompt cross section times branching ratio for the production of X(3872) → J/ψπ+π−is 1.06± 0.11 (stat.)± 0.15 (syst.) nb.3. – Observation of the decays B+c → J/ψπ+ and B+c → J/ψπ+π−π+ in ppcollisions at√s = 7TeVThe B+c meson is a unique probe for heavy-quark dynamics which is not accessibleto bb¯ or cc¯ bound states. It is the ground state of two different bound heavy quarks withcompeting decay modes. The measurement of its properties would help in understandingsuch dynamics. The B+c → J/ψπ+π−π+ decay was first observed at LHCb [14] and thismeasurement by CMS is the only experimental confirmation so far.The measurement [15] is based on 4.7 fb−1 of pp collision data at√s = 7TeV col-lected in 2011 with displaced J/ψ dimuon triggers. Both the decays are identified froman opposite-sign muon-pair, consistent with the J/ψ mass, selected applying stringentquality criteria on the selected muon tracks. One or three additional tracks are selectedand assumed to be pions (conjugate states are implicit). The largest-pT B+c candidateis retained and required to have pT (B+c ) > 10GeV and |η(B+c )| < 1.6. Additionalrequirements on 3D secondary-vertex significance are made specifically for each decaychannel.HEAVY-FLAVOR PRODUCTION AND SPECTROSCOPY AT CMS 129Fig. 2. – Left: signal yield from fit to invariant mass for the B+c → J/ψπ+ decay channel. Right:signal yield from fit to invariant mass for the B+c → J/ψπ+π−π+ decay channel.Figure 2 shows the signal yield for each decay channel, obtained from fit to the inva-riant mass distribution of the candidate B+c meson. The number of observed events is330 ± 36 (stat.) ± 23 (syst.) in the B+c → J/ψπ+ decay channel, with a significanceS/√S + B = 10.5. The number of observed events is 108± 19 (stat.)± 14 (syst.) in theB+c → J/ψπ+π−π+ decay channel, with a significance S/√S + B = 6.1.4. – Observation of a new Ξb baryonThere are several predicted baryons with beauty and strange valence quarks accordingto the SM. These include the Ξ0b ground state, with JP = 12+, the Ξ0∗b excited state, withwith JP = 32+, and two states with negative parity. Ξ0b candidates were observed at theTevatron, but their quantum numbers still have to be probed.The measurement [16] is based on 5.3 fb−1 of pp collision data at√s = 7TeV collectedin 2011 with displaced J/ψ dimuon triggers. The search for the Ξ0∗b excited state requiresthe full reconstruction of a complex decay chain with three secondary vertices (conjugatestates are implicit): Ξ0∗b → Ξ−b π+, Ξ−b → J/ψΞ−, with J/ψ → μ+μ−, Ξ− → Λπ−,and Λ→ pπ−. Contamination from K0S is removed with invariant mass constraints. Thereconstruction of each track and of each intermediate step is analogous to those describedin other measurements reported in this contribution.Fig. 3. – Left: reconstruction of the Ξ−b candidate. Middle: parametrization of Q for combina-torial background using uncorrelated momenta of like-charge candidates. Right: fit to data ofsignal plus background for opposite-charge candidates.130 N. POZZOBON on behalf of the CMS COLLABORATIONThe signal yield is extracted from an unbinned maximum-likelihood fit to the differ-ence Q between the mass of the candidate Ξ0∗b and the masses of its decay products:Q = m(J/ψΞ−π+) − m(J/ψΞ−) − m(π+). The combinatorial background is modeledwith Ξ−b candidates combined with like-charge pions. Measured distributions are used togenerate uncorrelated momenta of like-charge candidates to calculate Q.Figure 3 shows, from left to right, the invariant mass distribution for J/ψΞ− pairs,which is used to calculate Q, the parameterization of Q for the combinatorial backgroundin an extended range, and the distribution of Q for candidate Ξ0∗b ’s together with theresult of the fit procedure. This is the first observation, with a 6.9 σ significance, ofa state fitting the Ξ0∗b hypothesis. The measured mass is m = 5945.0 ± 0.7 (stat.) ±0.3 (syst.)± 2.7 (PDG Ξ−b mass) MeV.5. – Concluding remarksThe excellent performance of the LHC and CMS allowed our collaboration to per-form high-quality studies in HF physics. In particular, the excellent performance of thetracker and of muon detectors was crucial in studying fully-reconstructed decays of b-hadrons. We reviewed some of the most recent results in exotic quarkonia, measurementsof properties of b-hadrons, and observation of new hadrons.REFERENCES[1] The CMS Collaboration, J. Phys. G: Nucl. Part. Phys., 34 (2007) 995,doi:10.1088/0954-3899/34/6/S01.[2] The CMS Collaboration, JINST, 3 (2008) S08004,doi:10.1088/1748-0221/3/08/S08004.[3] The CMS Collaboration, CMS Public Analysis Summary TRK-10-005,https://twiki.cern.ch/twiki/bin/view/CMSPublic/DPGResultsTRK.[4] The CMS Collaboration, JINST, 7 (2012) P10002.[5] The CMS Collaboration, report CERN-LHCC-2000-038.[6] The CMS Collaboration, Eur. Phys. J. C, 46 (2006) 605,doi:10.1140/epjc/s2006-02495-8.[7] The Belle Collaboration, Phys. Rev. Lett., 91 (2003) 262001,doi:10.1103/PhysRevLett.91.262001.[8] The BaBar Collaboration, Phys. Rev. D, 71 (2005) 071103,doi:10.1103/PhysRevD.71.071103.[9] The CDF Collaboration, Phys. Rev. Lett., 93 (2004) 072001,doi:10.1103/PhysRevLett.93.072001.[10] The D0 Collaboration, Phys. Rev. Lett., 93 (2004) 162002,doi:10.1103/PhysRevLett.93.162002.[11] The LHCb Collaboration, Eur. Phys. J. C, 72 (2012) 1972,doi:10.1140/epjc/s10052-012-1972-7.[12] The CMS Collaboration, JHEP, 04 (2013) 154.[13] The CMS Collaboration, JHEP, 02 (2012) 011,doi:1007/JHEP02(2012)011.[14] The LHCb Collaboration, Phys. Rev. Lett., 108 (2012) 251802,doi:10.1103/PhysRevLett.108.251802.[15] The CMS Collaboration, CMS Public Analysis Summary BPH-11-003.[16] The CMS Collaboration, Phys. Rev. Lett., 108 (2012) 252002,doi:10.1103/PhysRevLett.108.252002.

Heavy-Flavor production and spectroscopy at CMS

POZZOBON, NICOLA

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Heavy-Flavor production and spectroscopy at CMS

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