Krätschmer, Ilse

Raics, Péter

Trócsányi, Zoltán

Ujvári, Balázs

Journal of Physics Conference Series

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University of Debrecen Electronic Archive

Journal of Physics: Conference Series
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Quarkonium production measurements and
searches for exotic quarkonia at CMS
To cite this article: Ilse Krätschmer and the CMS Collaboration 2014 J. Phys.: Conf. Ser. 556 012012
 
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Quarkonium production measurements and searches
for exotic quarkonia at CMS
Ilse Kra¨tschmer
On behalf of the CMS Collaboration
Institute of High Energy Physics Vienna, Austria
E-mail: ilse.kratschmer@cern.ch
Abstract. The CMS experiment at the LHC studies conventional and exotic quarkonia. CMS
measured the quarkonium polarization of prompt Ψ(nS) and Υ(nS) states, using 2011 data taken
in pp collisions at
√
s = 7 TeV. No strong polarization was observed in any of the states. Using
the Ψ(nS) polarization results and other polarization hypotheses, the prompt Ψ(nS) differential
production cross section was determined up to and even beyond 100 GeV. The production
of X(3872) in its decay to J/ψpi+pi− was also measured, using data taken in pp collisions at√
s = 7 TeV. The cross section times the branching fraction of the X(3872) relative to the one
of ψ(2S) and the fraction of X(3872) originating from B decays were determined. With the
help of these results, the prompt X(3872) differential cross section times branching fraction as
a function of pT was extracted. Furthermore, a search for the exotic quarkonium, Xb, decaying
to Υ(1S) pi+pi− in the mass range of 10 to 11 GeV was conducted, using data collected in pp
collisions at
√
s = 8 TeV. No evidence of a Xb signal was observed. An upper limit on the
relative inclusive production cross section times branching ratio of the Xb and Υ(2S) states at
95% CL was set.
1. Introduction
Quarkonia are an ideal system to study how the strong interaction binds quarks into hadrons
since the formation of the initial quark-antiquark pair and the transformation into a bound
state happens at very different timescales [1]. Quarkonium production and polarization are
strongly intertwined. This paper starts with a short discussion of CMS quarkonium polarization
measurements followed by recent results on the quarkonium production by CMS. CMS also
conducted searches and studies of exotic quarkonia which are presented in Section 4.
2. Quarkonium polarization
Quarkonium polarization measurements are one of the most challenging ones in the description
of quarkonium production. The polarization of JPC = 1−− quarkonium states can be studied
through the angular decay distribution of the leptons (here muons). The most general observable
distribution of a parity-conserving dilepton decay of vector particles can be written as
W (cosϑ, ϕ|~λ) = 3/(4pi)
(3 + λϑ)
(1 + λϑ cos
2 ϑ + λϕ sin
2 ϑ cos 2ϕ+ λϑϕ sin 2ϑ cosϕ) , (1)
where ~λ = (λϑ, λϕ, λϑϕ) represents the frame-dependent polarization parameters and ϑ and
ϕ are the polar and azimuthal angles of the positive muon with respect to the z-axis of the
XI International Conference on Hyperons, Charm and Beauty Hadrons (BEACH 2014) IOP Publishing
Journal of Physics: Conference Series 556 (2014) 012012 doi:10.1088/1742-6596/556/1/012012
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution
of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Published under licence by IOP Publishing Ltd 1
 [GeV]
T
p
0 20 40 60 80 100 120
 [ n
b  
/  G
e V
]
y d Tp
 /  
d
σ d
-510
-410
-310
-210
-110
1
10
210
ψJ/
 = 7 TeVspp
-1CMS Preliminary, |y| < 1.2, 4.9 fb
-1CMS, |y| < 0.9, 37 pb
-1ATLAS, |y| < 0.75, 2.3 pb
 [GeV]
T
p
0 20 40 60 80 100 120
 [ n
b  
/  G
e V
]
y d Tp
 /  
d
σ d
-510
-410
-310
-210
-110
1
10
210
(2S)ψ
 = 7 TeVspp
-1CMS Preliminary, |y| < 1.2, 4.9 fb
-1CMS, |y| < 1.2, 37 pb
-1ATLAS, |y| < 0.75, 2.1 fb
Figure 1. Differential production cross section for the prompt J/ψ (left) and ψ(2S) state
(right), for |y| <1.2, compared to previous CMS [10] and ATLAS measurements [11, 12]. In case
of the ψ(2S), preliminary ATLAS results are used. The global uncertainties from the integrated
luminosity and from the branching fractions are not shown.
chosen reference frame [2, 3]. CMS used three different reference frames: the center-of-mass
helicity frame (HX) coinciding with the direction of the quarkonium momentum, the Collins-
Soper frame [4] and the perpendicular helicity frame [5].
CMS determined the three frame-dependent polarization parameters and the frame-invariant
quantity λ˜ = (λϑ + 3λϕ)/(1 − λϕ) in the three reference frames for the prompt ψ(nS) (n=1,2)
and Υ(nS) (n=1,2,3) mesons. A dimuon data sample taken in pp collisions at
√
s = 7 TeV
corresponding to a total integrated luminosity of 4.9 fb−1 was used. Quarkonia are seen to
be produced essentially unpolarized, with observed polarization levels much smaller than those
predicted by most theoretical calculations [6, 7].
3. Quarkonium production
The experimental measurement of quarkonium production strongly depends on the polarization.
The most recent CMS measurement of the prompt ψ(nS) production cross section [8] uses
four different polarization scenarios in the acceptance calculation: no polarization, two extreme
polarizations (λHXϑ = ±1) and the polarization as measured by CMS averaged over all dimuon pT
and |y| ranges. The same data sample as in the ψ(nS) polarization measurements was used. The
efficiency and acceptance corrected yields of the prompt ψ(nS) mesons were extracted using an
unbinned maximum likelihood fit to the dimuon invariant mass and decay length distribution [9].
Figure 1 shows the unpolarized double differential cross section of the prompt ψ(nS) mesons
in comparison to CMS measurements using 2010 data [10] and results from ATLAS [11, 12].
The most recent CMS measurement considerably extends the reach in transverse momentum,
pT, with respect to the previous one. In case of the ψ(2S), the findings of CMS disagree with
the preliminary result of ATLAS at high pT. However, the final result of ATLAS [13] shows
better agreement with the CMS measurements.
XI International Conference on Hyperons, Charm and Beauty Hadrons (BEACH 2014) IOP Publishing
Journal of Physics: Conference Series 556 (2014) 012012 doi:10.1088/1742-6596/556/1/012012
2
4. Exotic quarkonia
In the last decade, a plethora of exotic states decaying into heavy quarkonia have been observed.
Amongst the first ones to be discovered was the X(3872) state [14].
CMS studied the production of X(3872) decaying to J/ψpi+pi− in pp collisions at
√
s =
7 TeV [15]. The data used corresponds to a total integrated luminosity of 4.8 fb−1. The
inclusive cross section times branching fraction of X(3872) relative to the one of ψ(2S) was
determined as a function of pT. Additionally, the fraction of X(3872) originating from B
decays was measured to be 6.56 ± 0.29 (stat.) ± 0.65 (syst.)%. Using these results and
the prompt ψ(2S) cross section measurment [10], the differential production cross section of
X(3872) was calculated. Figure 2 shows a comparison of the measured rate with Non Relativistic
Quantum ChromoDynamics (NRQCD) calculations [16]. While the prediction is larger than the
measurement, the dependence on pT is reasonably well modelled.
CMS also looked for the bottomonium counterpart, Xb, of the X(3872) state [17]. The
analysis was based on a data sample collected in pp collisions at
√
s = 8 TeV, corresponding to
an integrated luminosity of 20.7 fb−1. The mass of the Xb has been predicted to lie between 10
and 11 GeV [18, 19, 20]. Therefore, a search for the Xb decaying to Υ(1S)pi
+pi− was conducted in
the mass range of 10 and 11 GeV, excluding the Υ(2S) and Υ(3S) regions. No excess signal was
observed. Upper limits on the inclusive production cross section times the branching fraction of
the Xb relative to the one of Υ(2S) were set at the 95% confidence level (CL). The upper limits
range between 0.9 - 5.4%, as shown in Fig. 3. These are the first upper limits on the production
of a possible Xb from a hadron collider.
5. Summary
CMS performed measurements of the polarizations of the prompt ψ(nS) and Υ(nS) states, using
data collected in pp collisions at
√
s = 7 TeV. No strong polarization was observed for any
of the quarkonium states studied. Using the ψ(nS) polarization results and other polarization
hypotheses, the double differential cross sections of the prompt ψ(nS) states were determined.
These measurements considerably extend the reach in pT up to and even beyond 100 GeV.
CMS also conducted studies with exotic quarkonia like the production of the X(3872) using
data taken in pp collisions at
√
s = 7 TeV. The fraction of X(3872) originating from B decays
was measured to be (6.56 ± 0.29 (stat.) ± 0.65 (syst.))%. The inclusive cross section times
) [GeV]-pi +pi ψ(J/
T
p
10 15 20 25 30
 
[ n b
/ G
e V
]
Β
 
⋅
 T
 
/ d
p
X (
3 8
7 2
)
p r
o m
p t
σ
 
d
-210
-110
1
|y| < 1.2
 = 7 TeVsCMS   
-1L = 4.8 fb
LO NRQCD 
LO NRQCD uncertainty Figure 2. Measured differen-
tial cross section for the prompt
X(3872) production times branch-
ing fraction, B, of the decay to
J/ψpi+pi− as a function of pT. The
inner error bars represent the sta-
tistical uncertainty, while the outer
error bars indicate the total un-
certainty. The NRQCD calcula-
tions [16] are shown by the solid red
line with the dotted lines represent-
ing the uncertainty.
XI International Conference on Hyperons, Charm and Beauty Hadrons (BEACH 2014) IOP Publishing
Journal of Physics: Conference Series 556 (2014) 012012 doi:10.1088/1742-6596/556/1/012012
3
 [GeV]
bX
M
10 10.2 10.4 10.6 10.8 11
)
pi
pi
( 1 S
)
Υ
→
( 2 S
)
Υ (
B
 
×
 
( 2 S
)
Υ
σ
)
pi
pi
( 1 S
)
Υ
→ b
( X
B
 
×
 bX
σ
R
 =
 
-210
-110
Median Expected
Observed
 Expectedσ 1±
 Expectedσ 2±
CMS Preliminary
s = 8 TeV
L -1= 20.7 fb Figure 3. Exclusion limits on the
production cross section times the
branching fraction of the Xb →
Υ(1S)pi+pi− relative to the one of
Υ(2S)→ Υ(1S)pi+pi− as a function
of the Xb mass, at 95% CL. The
solid curve shows the observed
limits, while the dashed curve
represents the limits expected for a
pure background hypothesis.
branching fraction of X(3872) relative to the one of ψ(2S) and the cross section times branching
fraction of the prompt X(3872) were determined as a function of pT.
A search for the counter part of the X(3872) in the bottomonium spectrum, the Xb, was
performed with
√
s = 8 TeV data collected in pp collisions. No evidence of a Xb signal was
found, but upper limits on the relative inclusive production cross section times branching ratio
of the Xb and Υ(2S) at 95% CL were set. These are the first upper limits for Xb with hadron
collider data.
6. Acknowledgments
The author is supported by the Austrian Science Fund (FWF): P24167.
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XI International Conference on Hyperons, Charm and Beauty Hadrons (BEACH 2014) IOP Publishing
Journal of Physics: Conference Series 556 (2014) 012012 doi:10.1088/1742-6596/556/1/012012
4


Quarkonium production measurements and searches for exotic quarkonia at CMS

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Quarkonium production measurements and searches for exotic quarkonia at CMS

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