DOI 10.1393/ncc/i2015-15129-7 Colloquia: LaThuile15
Heavy quark physics in CMS
G. Fedion behalf of the CMS Collaboration
INFN, Sezione di Pisa - Pisa, Italy
received 2 October 2015
Summary. — The most recent results which concern the heavy quark hadrons done in the CMS experiment are reported. The searching area spans over the heavy quark spectroscopy, production cross sections, beauty meson decay properties, rare decays, and CP violation.
PACS 14.40.Pq – Heavy quarkonia.
1. – Introduction
Recent results regarding the heavy quark physics in CMS [1] can be divided in two main categories: the indirect search for new physics beyond the Standard Model and the study of charm and beauty hadrons production and decay. In the former analysis category a measurement which lead to a different results from the Standard Model expectation, gives the hint that new physics might be present in the measured processes, see sect. 2 and 3. While in the latter category, the analyses aim is to do precise measurements within the Standard Model framework, or aim to test the theoretical approximations of the studied process, see sect. 4, 5, 6. 7, 8 and 9.
2. – Search for B(s,d)→ μμ
The rare decays Bs→ μμ and Bd→ μμ were studied combining the CMS data col-lected at the center-of-mass energy of√s = 7 and 8 TeV for a total integrated luminosity
of 5 and 20 fb−1, respectively [2].
The aim of the studies was to search for events of those two rare decays. In case of no evidence of signal were found new production upper limits would have been set. In the Standard Model of particle physics the B(s,d) → μμ decays are highly suppressed since they cannot decay through the tree-level diagrams. The flavour changing neutral-current FCNC transitions are only possible via box or penguin diagrams, see fig. 1. Furthermore the decays are Cabibbo and helicity suppressed. The Standard Model predictions for the branching fraction are B(Bs → μμ) = (3.66 ± 0.23) · 10−9 and B(Bd → μμ) = (1.06± 0.10) · 10−10 [3]. Any measured deviation of the branching fractions with respect
B0 s W + W− Z0 t b s μ+ μ− B0 s W+ ν W− t b s μ− μ+ B0 s X + W− X0 t b s μ+ μ− B0 s X+ ν W− t b s μ+ μ−
Fig. 1. – Bs→ μμ decay Feynman diagrams. From left to right: SM penguin diagram, SM box
diagram, new physics penguin diagram, and new physics box diagram.
to the Standard Model prediction might be a hint of New Physics particles present in box or penguin diagrams, see fig. 1.
With an unbinned maximum-likelihood fit to the dimuon invariant mass distribution the branching fractionB(Bs→ μμ) = (3.0+0.9−0.8(stat.)+0.6−0.4(syst.))· 10−9 is measured. An excess of Bs→ μμ events with respect to background is observed with a significance of 4.3 standard deviations. In case of the decay branching ratioB(Bd→ μμ) < 1.1 · 10−9, an upper limit is determined at 95% of confidence level. Both results are in agreement with the expectations from the standard model. The two-dimensional likelihood scan of the fit is shown in fig. 2.
3. – CP-violating phase φs using Bs →J/ψφ(1020) channel
The CP-violating weak phase φsand the decay width difference ΔΓsof Bsmeson are measured by the CMS experiment at the LHC using a data sample of Bs →J/ψφ(1020) decays [4]. The analysed data sample corresponds to an integrated luminosity of 20 fb−1 collected in pp collisions at√s = 8 TeV.
The weak phase φsof Bsarise from the interference between direct Bs meson decays into a CP eigenstate and decays through mixing to the same final state. It is predicted by the mean of a global fit of the SM parameters to be φs = 0.0363+0.0016−0.0015 rad [5].
) -μ + μ → s 0 BF(B 0 1 2 3 4 5 6 7 8 9 10 -9 10 × ) -μ + μ → 0 BF(B 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 -9 10 × σ 1 σ 2 3σ σ 4 CMS SM CMS s=7 TeV, L=5 fb-1 s=8 TeV, L=20 fb-1 ) -μ + μ → s 0 BF(B 0 1 2 3 4 5 -9 10 × lnLΔ 2 0 2 4 6 8 10 12 14 16 18 20 σ 1 σ 2 σ 3 σ 4 -9 10 × ) -0.9 +1.0 (3.0 )= -μ + μ → s 0 BF(B σ 4.3 SM ) -μ + μ → 0 BF(B 0 0.2 0.4 0.6 0.8 1 -9 10 × lnLΔ 2 0 2 4 6 8 10 12 14 16 18 20 σ 1 σ 2 σ 3 σ 4 -10 10 × ) -1.8 +2.1 (3.5 )= -μ + μ → 0 BF(B σ 2.0 SM
Fig. 2. – Two-dimensional likelihood scan for B(Bs → μμ) and B(Bd → μμ). The red point
[rad] s φ -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 ] -1 [pss ΓΔ 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 CMS CMS Preliminary -1 =8 TeV L=20 fb s 68% C.L. 90% C.L. 95% C.L. Standard Model
Fig. 3. – Two-dimensional scan of the likelihood function for φs and ΔΓs, the best fit values
and the C.L. contours are shown in comparison with the SM prediction.
Being the predicted φs with a good accuracy and close to zero, a significant deviation
of the measured weak phase with respect to the prediction, might be the sign of NP particles in the mixing decay of Bs.
An angular analysis of the Bsdecay based on three angles (θT, ψT, and φT) is needed
in order to disentangle the two CP states (odd and even). The differential decay rate of Bsis studied in terms of proper decay time and the angular variables. In order to increase the sensitivity on φs, the flavour of Bsat production time (flavour tagging) is used as an observable in the fit of data. The total tagging power of tagging is (0.97± 0.03)%.
Assuming a polarisation-independent CP violation model and no direct CP violation, the values for the weak phase and the decay width difference between the Bs mass eigenstates are measured to be:
φs=−0.03 ± 0.11 (stat) ± 0.03 (syst) rad,
ΔΓs= 0.096± 0.014 (stat) ± 0.007 (syst) ps−1.
In fig. 3 the two-dimensional scan of the likelihood function for φsand ΔΓsis shown.
The value of φs is in statistical agreement with the SM fit prediction. The ΔΓs is
confirmed to be non-zero.
4. – B+→ ψ(2S)φK+ rare decay
The B+ → ψ(2S)φK+ invariant mass peak was detected by the CMS experiment using 19.6 fb−1 integrated luminosity at √s = 8 TeV of center-of-mass energy. The unbinned maximum likelihood fit was performed on the invariant mass distribution to extract the peak characteristics. The likelihood function is composed by the signal PDF, which is parametrized as a double Gaussian. While the background PDF is parametrized as a first-order Chebyshev polynomial. In fig. 4 the mass distribution and the best fit projection is shown. The peak centroid is compatible with the B+mass and the measured resolution is about 3 MeV. The obtained signal events might be used to measured the B+→ ψ(2S)φK+ decay branching fraction.
(GeV) ± K φ (2S) ψ m 5.22 5.24 5.26 5.28 5.3 5.32 Candidates / 0.002 GeV 0 20 40
60 Mass resolution σ = ~ 3 MeV
=8 TeV) s ( -1 L=19.6 fb CMSPreliminary
Fig. 4. – B+→ ψ(2S)φK+ invariant mass distribution and relative best fit. The lines represent the signal-plus-background fit (solid) and the background-only component (dashed).
5. – Bc decays
At a center-of-mass energy √s = 7 TeV in proton-proton collisions, the ratio of the
production cross sections times branching fractions σ(B±c )× B(Bc±→ J/ψπ±)/σ(B±)×
B(B±→ J/ψK±) is studied [6]. The analysed data sample corresponds to an integrated
luminosity of 5.1 fb−1. The ratio is determined to be (0.48± 0.05(stat) ± 0.03(syst) ± 0.05(τBc))× 10−2 in the kinematic region that requires Bc± and B±x mesons with
trans-verse momentum pT> 15 GeV and rapidity |y| < 1.6. A previous measurement of the same ratio done by LHCb is in agreement with the present result taking into account for the different kinematic regions used in the analyses.
The ratio of the branching fractions R =B(Bc± → J/ψπ±π±π∓)/B(Bc± → J/ψπ±) is also measured. The efficiencies were assessed using a model-independent method. The measured ratio is R = 2.55± 0.80(stat) ± 0.33(syst)+0.04−0.01(τBc), consistent with the
previous LHCb result.
In figs. 5 the invariant mass distributions for Bc± → J/ψπ±π±π∓ (left) and Bc± →
J/ψπ± (right) are shown.
invariant mass (GeV)
-π + π + π ψ J/ 6 6.1 6.2 6.3 6.4 6.5 6.6 Events / 0.015 GeV 0 20 40 60 80 100 ) > 15 GeV + c (B T p )| < 1.6 + c |y(B (7 TeV) -1 5.1 fb CMS
invariant mass (GeV) + π ψ J/ 5.8 5.9 6 6.1 6.2 6.3 6.4 6.5 6.6 6.7 Events / 0.015 GeV 10 20 30 40 50 60 ) > 15 GeV + c (B T p )| < 1.6 + c |y(B (7 TeV) -1 5.1 fb CMS
Fig. 5. – Invariant mass distributions for Bc±→ J/ψπ±π±π∓(left) and Bc±→ J/ψπ±) (right).
Invariant mass [GeV] π π ψ J/ 5.15 5.2 5.25 5.3 5.35 5.4 5.45 5.5 5.55 Events / 9 MeV 0 50 100 150 200 250 300 350 Data Fit Signal Combinatorial background Peaking background CMS (7 TeV) -1 5.3 fb
invariant mass [GeV]
-π + π ψ J/ 5.15 5.2 5.25 5.3 5.35 5.4 5.45 5.5 5.55 Pull -2 0 2 5.25 5.3 5.35 5.4 5.45 Events / 4.5 MeV 0 200 400 600 800 1000 Data Fit Signal Combinatorial background CMS (7 TeV) -1 5.3 fb
invariant mass [GeV]
-K + K ψ J/ 5.25 5.3 5.35 5.4 5.45 Pull -4 -2 0 2
Fig. 6. – Invariant mass distributions for Bs→ J/ψf0(980) (left) and Bs →J/ψφ(1020) (right).
The fit function projections is also shown.
6. –B(Bs→ J/ψf0(980))/B(Bs →J/ψφ(1020)) ratio
At a centre-of-mass energy of √s = 7 TeV in proton-proton collisions, the ratio of
branching fractions Rf0/φ =B(Bs→ J/ψf0(980))/B(Bs → J/ψφ(1020)) is measured [7].
The analysed data sample corresponded to an integrated luminosity of 5.3 fb−1. The measured ratio is Rf0/φ = 0.140± 0.013(stat) ± 0.018(syst), where the first uncertainty
is statistical and the second is systematic. This result is consistent with theoretical predictions and previous measurements.
In figs. 6 the invariant mass distributions for Bs → J/ψf0(980) (left) and Bs → J/ψφ(1020) (right) are shown.
7. – J/ψ and ψ(2S) production
At a centre-of-mass energy of √s = 7 TeV in proton-proton collisions, the
double-differential cross sections of promptly produced J/ψ and ψ(2S) mesons are measured as a function of transverse momentum pT and absolute rapidity |y| [8]. The analysed J/ψ and ψ(2S) data samples correspond to integrated luminosities of 4.55 and 4.90 fb−1, respectively. The differential cross section as a function of pT only, where the rapidity is integrated for values lower than 1.2, is also measured. With the present analysis the highest-pTvalues in the context of the measured cross sections has been reached.
In figs. 7 we show the differential cross sections as a function of pTtimes the dimuon branching fractions for four rapidity bins and integrated over the range|y| < 1.2 (left) and the J/ψ and ψ(2S) differential cross sections integrated over rapidity (right). 8. – Υ(nS) production
At a centre-of-mass energy of√s = 7 TeV in proton-proton collisions, the differential
cross sections as a function of transverse momentum pTfor the production of Υ(nS)→
μμ (n = 1, 2, 3) are presented [9]. The analysed data samples correspond to integrated
luminosities of 4.9 fb−1. The Υ(nS) candidates are selected from those with dimuon rapidity|y| < 1.2 and transverse momentum 10 < pT< 100 GeV.
This measure extends the maximum pT range from 70 to 100 GeV with respect the previous results. The measurements show a transition from an exponential to a power-law
[GeV] T p 0 20 40 60 80 100 120 [nb / GeV] T p d y / d 2 σ d -5 10 -4 10 -3 10 -2 10 -1 10 1 10 2 10 ψ J/ (2S) ψ = 7 TeV s pp -1 | < 1.2, 4.55 fb (2.4%) y CMS, | -1 | < 0.75, 2.3 pb (3.5%) y ATLAS, | -1 | < 1.2, 4.90 fb (10.6%) y CMS, | -1 | < 0.75, 2.1 fb (2.4%) y ATLAS, | | < 1.2 y FKLSW, | Power-law fits [GeV] T p 0 20 40 60 80 100 120 [nb / GeV] y d T p / d σ d× B -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 1 CMS -1 2) × | < 1.2 ( y | | < 0.3 y | | < 0.6 y 0.3 < | | < 0.9 y 0.6 < | | < 1.2 y 0.9 < | of 2.2% not included Luminosity uncertainty ψ J/ (2S) ψ ψ : L = 4.55 fb J/ (2S) : L = 4.90 fb ψ -1 = 7 TeV s pp 2
Fig. 7. – Left: J/ψ and ψ(2S) the differential cross sections as function of pTtimes the dimuon
branching fractions for four rapidity bins and integrated over the range|y| < 1.2. Right: J/ψ and ψ(2S) differential cross sections integrated over rapidity.
behavior at pt 20 GeV for the three Υ states. In figs. 8 we show: the μμ invariant mass distribution and the well definite Υ(nS) peaks (left) and the relative differential cross sections as a function of pT (right).
9. – χbn production
At a centre-of-mass energy of√s = 8 TeV in proton-proton collisions, the production
cross section ratio σ(χb2)/σ(χb1) is presented [10]. The analysed data samples correspond
to integrated luminosities of 20.7 fb−1.
The prompt χb1(1P) and χb2(1P) are detected using the radiative decays χb1,2(1P)→
Υ(1S)+γ. The emitted photons are reconstructed using the decay to electron pairs while interacting with the tracker material (converted photons). That permitted the
χb1,2(1P) peaks to be resolved. The selected χb1,2(1P) kinematic region is defined by the
photon pseudorapidity|η| < 1.0, the Υ(1S) rapidity |y| < 1.5, and the Υ(1S) transverse momentum 7 < pT< 40 GeV. Events / 10 MeV 0 1000 2000 3000 4000 |y| < 0.6 < 12 GeV T 10 < p (a) (7 TeV) -1 4.9 fb CMS [GeV] μ μ M 9 9.5 10 10.5 11 Pull -4 -2 0 2 4 [GeV] T p 0 20 40 60 80 100 ) [pb/GeV] − μ + μ Br(× T /dpσ d -5 10 -4 10 -3 10 -2 10 -1 10 1 10 2 10 3 10 4 10 (7 TeV) -1 4.9 fb CMS (1S) Υ 0.1 × (2S) Υ 0.01 × (3S) Υ (a) CMS 2011 |y| < 1.2 0.5 × CMS 2010 |y| < 2.4 NLO theory
Fig. 8. – Left: μμ invariant mass distribution and best fit projections. Right: the Υ(nS) differential cross sections as a function of pT.
[GeV] γ μ μ m 9.7 9.75 9.8 9.85 9.9 9.95 10 10.05 Events/5 MeV 20 40 60 80 100 120 140 160 < 11 GeV Υ T 7 < p (8 TeV) -1 20.7 fb CMS [GeV] Y T p 5 10 15 20 25 30 35 40 ) b1 χ( σ ) / b2 χ( σ 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 (8 TeV) -1 20.7 fb CMS
BF uncertainty (9%) not included
| < 1 γ η | < 1.5, | Y CMS, |y | < 4.5 χ LHCb, 2.0 < |y arXiv:1409.0693 CMS average [GeV] Y T p 5 10 15 20 25 30 35 40 ) b1 χ( σ ) / b2 χ( σ 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 [GeV] Y T p 5 10 15 20 25 30 35 40 ) b1 χ( σ ) / b2 χ( σ 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Fig. 9. – Left: μμγ invariant mass distribution of the χb1,2(1P) peaks and fit function projections
for the first Υ rapidity bin. Right: cross sections ratio σ(χb2)/σ(χb1) as a function of the
Υ(1S) pT.
The cross section ratio is measured as function of Υ(1S) transverse momentum. The measured average value of the cross section is 0.85± 0.07(stat + syst) ± 0.08 (BF), where the first uncertainty is the combination of the statistical and systematic uncertainties and the second is due to the uncertainty of the branching fractions.
In figs. 9 we show: the μμγ invariant mass distribution of the χb1,2(1P) peaks for the
first Υ rapidity bin (left) and the relative cross sections ratio as a function of the Υ(1S)
pT (right). The cross section ratio does not shows any significant dependence on the Υ(1S) transverse momentum.
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