Recent ATLAS results in the field of meson physics

Recent results on flavor physics are presented, using data from the ATLAS detector. The Υ(nS ) and charmonium inclusive production cross sections are discussed. J/ψ production associated with a W vector boson is presented. An inclusive analysis of B meson production is shown. Results on the production of heavy quarkonia are complemented by the measurement of inclusive φ meson production. The latest information on heavy flavor spectroscopy and searches for new physics using B-meson rare decays is also covered. These results are based on data samples collected during the 2011 and 2012 LHC running periods. 1 The φ(1020) production cross section measurement As a theory of the physics at high momentum transfer, perturbative quantum chromodynamics (pQCD) is very successful. However, for interactions at low momentum transfers, there is no single best model. The φ(1020) meson is produced in hard scatters during pp interactions as well as in hadronisation. Therefore, accurate measurements of φ(1020) meson production can be used to tune phenomenological fragmentation models. The φ(1020) meson production is measured using 383 μb−1 of 7 TeV data collected in 2010 [1] using the ATLAS detector [2]. The φ(1020) is reconstructed using kaons identified by their energy loss in the pixel detector. The kaons are required to have transverse momentum pT > 230 MeV and total momentum p < 800 MeV. The analysis is restricted to the fiducial region 500< pT(φ) < 1200 MeVand |y(φ)| < 0.8, where y is rapidity. The integrated production cross section for φ(1020) → K+K− is found to be 570±8(stat.)±66(sys.)±20(lumi.) μb. The differential cross sections are shown in Fig. 1 along with comparisons to various Monte Carlo (MC) based predictions. The EPOS-LHC and PYTHIA6 DW tunes give the best agreement. 2 W plus J/ψ associated production The measurement of W plus J/ψ associated production can be used to test the relative contributions of the color singlet (CS) and color octet (CO) processes to QQ̄ production. ae-mail: Rui.Wang@cern.ch DOI: 10.1051/ C © Owned by the authors, published by EDP Sciences, 2014 / 01024 (2014) 201 epjconf EPJ Web of Conferences 4 01024 81 81 , This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20148101024 Figure 1: The φ(1020) → K+K− cross section as a function of pT(φ) (left) and |y(φ)| (right), in the fiducial region of 500 < pT(φ) < 1200 MeV and |y(φ)| < 0.8. The measurement is based on 4.5 fb−1 of 7 TeV pp collision data collected by ATLAS in 2011 [3]. In total, 29.2+7.5 −6.5 prompt J/ψ plus W boson events are observed, corresponding to a signal 5.1 σ significance. As shown in Fig. 2, left, these W boson plus prompt J/ψ events include both single parton scattering (SPS) events and double parton scattering (DPS) events. The DPS-subtracted rate appears to EPJ Web of C onferences


The φ(1020) production cross section measurement
As a theory of the physics at high momentum transfer, perturbative quantum chromodynamics (pQCD) is very successful. However, for interactions at low momentum transfers, there is no single best model. The φ(1020) meson is produced in hard scatters during pp interactions as well as in hadronisation. Therefore, accurate measurements of φ(1020) meson production can be used to tune phenomenological fragmentation models.
The measurement is based on 4.5 fb −1 of 7 TeV pp collision data collected by ATLAS in 2011 [3]. In total, 29.2 +7.5 −6.5 prompt J/ψ plus W boson events are observed, corresponding to a signal 5.1 σ significance.
As shown in Fig. 2, left, these W boson plus prompt J/ψ events include both single parton scattering (SPS) events and double parton scattering (DPS) events. The DPS-subtracted rate appears to EPJ Web of C onferences be dominated by CS (Fig. 2, right). However, both of the leading order (LO) CS and next-to-leading order (NLO) CO predictions are compatible with the measured result at the 2σ level.
3 Measurement of the χ c1,2 and ψ(2S ) production cross sections Together with the J/ψ production cross section, χ c and ψ(2S ) production cross section measurements can give a precise picture of the production of both prompt and non-prompt charmonium below the  DD threshold. The prompt production normally includes both direct production from pp collisions and production from decays of heavier charmonium states (the feed-down contribution). However, since ψ(2S ) is the highest state below the DD threshold, it has no contribution from higher states. The non-prompt production is contributed by the b hadron decays.
The χ c1,2 → J/ψγ production cross section is measured using 4.5 fb −1 of 7 TeV 2011 data in the ranges 10 < p T (J/ψ) < 30 GeV and |y(J/ψ)| < 0.75 [4]. The ψ(2S ) → J/ψππ production cross section is measured using 2.1 fb −1 of 7 TeV 2011 data in the ranges 10 < p T < 100 GeV and |y| < 2.0 [5]. The prompt production data are compared to different models ( Fig. 3 and Fig. 5, left). NLO non-relativistic QCD (NRQCD) is compatible in both cases. The LO Color Singlet Model (CSM) underestimates the χ c1,2 production cross section and the ψ(2S ) production cross section. The k T factorisation approach overestimates the χ c1,2 production cross section and underestimates the ψ(2S ) production cross section. These suggest that higher-order corrections or CO contributions to the cross sections not included in either prediction may be important. Between 20% and 30% of prompt J/ψ mesons are produced in χ c1 feed-down (Fig. 4, left). As is shown in Fig. 4, right and Fig. 5, right, the non-prompt production data are compatible with the FONLL predictions.

Measurement of the Υ(nP) production cross section
As a complement to charmonium system studies, the Υ family allows more dependable theoretical calculations due to its members' larger masses. Additionally, the impact of spin-alignment uncertainties is mitigated. Like charmonium, Υ mesons are produced directly in pp collisions or from decay of excited states. The Υ(1S ) differential cross sections multiplied by the di-muon branching fraction, d 2 /dp T dy×Br(Υ → μ + μ − ).
The Υ(nS ) → μ + μ − production cross sections has been measured using 1.  feed-down naturally. In Fig. 7, the increase in the p T range of 5 GeV to 30 GeV may be related to the feed-down contribution.

Observation of a new χ b state
χ b (nP) states are sought using 4.9 fb −1 of ATLAS 2011 7 TeV data [7], through decay modes of χ b (nP) → Υ(1S , 2S )γ. In this measurement, a di-muon sample with requirments of p T > 4 GeV and pseudorapidity |η| < 2.3 for each muon has been used for Υ(1S , 2S ) candidate selection. A photon is combined with each Υ candidate. Converted photons reconstructed from e + e − track pairs in the Inner Detector (ID) and unconverted photons reconstructed from electromagnetic calorimeter energy deposits are used.

Angular analysis of
The angular distributions of the B d → K * 0 (K + π − )μ + μ − four particle final state are sensitive to physics beyond the Standard Model (SM). Two important observables are the forward-backward asymmetry of the muons, A FB , and the longitudinal polarization fraction of the K * 0 , F L . They can be extracted from the decay width which depends on the angle between those two muon momenta, the angle between the kaon and pion momenta, and the momentum transfer q 2 = m 2 (μ + μ − ). The measurement uses 4.9 fb −1 of 7 TeV 2011 data recorded by ATLAS [8]. As shown in Fig. 9, both A FB and F L as measured by ATLAS are compatible with SM predictions and with other measurements.

Angular analysis of B s → J/ψφ enents
The B s → J/ψφ process is also sensitive to new physics beyond the SM. CP violation in this decay occurs due to interference between direct decays and decays occurring through B 0 s −B 0 s mixing. The CP states are separated statistically through the time dependence of the decay and angular correlations among the final state particles. With 4.9 fb −1 of 7 TeV 2011 data, the ATLAS measurement of the CP violating weak phase is φ s = 0.12 ± 0.25(stat.) ± 0.05(syst.) rad. The ATLAS measurement of the difference of widths of the heavy and light mass eigenstates is ΔΓ s = 0.053±0.021(stat.)±0.010(syst.) ps −1 [9]. Both are compatible with the SM predictions (Fig. 10). 01024-p.7 s → μ + μ − ). The 95% CL limit is indicated by the horizontal (red) line. The green and yellow bands correspond to ±1σ and ±2σ fluctuations on the expectation (dashed line). using 4.9 fb −1 of 7 TeV 2011 data [10]. Six events in the signal region have been observed with 6.75 background events expected. Thus ATLAS sets a limit on the branching ratio at 1.5 × 10 −8 @95% CL (Fig. 11).
EPJ Web of C onferences 01024-p.8 Precise measurements of B-hadron production cross sections in pp collisions at LHC can provide tests of QCD calculations for heavy-quark production at high center-of-mass energies and in wide p T and y ranges. The B + → J/ψK + production cross section is measured using 2.4 fb −1 of early 2011 7 Figure 12: The doubly-differential cross-section for B + production as a function of p T and y (left). The differential cross-section for B + production versus p T , integrated over rapidity, compared with predictions using the FONLL calculation with a hadronization fraction f b→B + of (40.1 ± 0.8)% (right).
TeV data [11]. The differential cross section in the kinematic range 9 GeV < p T < 120 GeV and |y| < 2.25 is shown in Fig. 12. Different model predictions have been compared to the measured data, and they are compatible within theoretical uncertainties.

B c meson observation
The B ± c meson is a bound state of the two heaviest quarks able to form a stable state. Weak decays of the B ± c meson provide a unique probe of heavy quark dynamics that is inaccessible to bb or cc bound states. The B ± c meson is reconstructed using 2011 7 TeV data in the decay mode B c → Jψπ [12]. With 4.3 fb −1 data, 82 ± 17 B c ground state mesons have been extracted using an unbinned maximum likelihood fit (Fig. 13). The B c mass returned by the fit is 6282 ± 7 MeV, in agreement with the world average value.