DOI 10.1393/ncc/i2015-15020-7 Colloquia: IFAE 2014 IL NUOVO CIMENTO 38 C (2015) 20
Measurements of Λ
+ cproduction in pp collisions at
√
s = 7 TeV
with ALICE
E. MeninnoINFN, Sezione di Napoli and Universit`a degli Studi di Salerno, Salerno, Italy
received 7 January 2015
Summary. — A Large Ion Collider Experiment (ALICE) has been designed to
study the strongly interacting medium created in heavy-ion collisions at LHC ener-gies, the Quark Gluon Plasma (QGP). Heavy quarks (charm and beauty), produced in the early stages of the collisions, are among the most powerful probes to study this state of matter. We report about the charmed baryon Λ+c measurement in pp
collisions at√s = 7 TeV with the ALICE experiment, through the reconstruction
of the decay channels Λ+
c → pK−π+ and Λ+c → p ¯K0 (and charge conjugates).
PACS 14.20.Lq – Charmed baryons (|C| > 0, B = 0).
1. – Introduction
Charm and beauty quarks belong to the most powerful probes to study the Quark Gluon Plasma in heavy-ion collisions: produced in hard parton scattering processes occurring in the early stages of the collision, they traverse the QCD medium, interact with its constituents and experience the whole evolution of the medium. In particular,
the Λ+
c measurement in Pb-Pb collisions should answer the question if the enhancement
of the baryon-to-meson ratio in nucleus-nucleus collisions relative to pp collisions,
ob-served at intermediate pT at RHIC and LHC in the light-flavour sector, also holds in
the heavy-flavour sector. Such an enhancement could point to hadron production by coalescence at intermediate momentum but also radial flow could play a role [1, 2]. The measurement in pp collisions provides the necessary baseline to understand the results in Pb-Pb collisions and it is needed, together with results for D mesons, to measure the total charm cross section.
The ALICE detector consists of a central barrel at mid-rapidity and a muon spec-trometer at forward rapidity. A subset of the central barrel detectors is essential for the
Λ+
c analysis: the Inner Tracking System (ITS) for vertex reconstruction and tracking,
the Time Projection Chamber (TPC) for tracking and Particle Identification (PID) and the Time of Flight detector (TOF) for PID. Two hadronic decay channels were studied:
c
CERN on behalf of the ALICE Collaboration
2 E. MENINNO ] 2 ) [GeV/c -π + π Invariant Mass ( 0.45 0.46 0.47 0.48 0.49 0.5 0.51 0.52 0.53 0.54 2 Entries / 0.5 MeV/c 2 4 6 8 10 12 14 6 10 × = 7 TeV s pp events 8 10 × 2.75 -π + π → S 0 K p analysis S 0 K → c Λ selection for 25/05/2012
Fig. 1. – Invariant mass of K0
S candidates.
Λ+c → pK−π+ (and charge conjugate) with a branching ratio (BR) of (5.0± 1.3)%(1)
and Λ+
c → pKS0 with an overall(2) branching ratio of (0.8± 0.2)%.
This analysis was performed on a data sample of about 3× 108events collected with
a minimum-bias trigger(3) during the 2010 LHC run with pp collisions at√s = 7 TeV.
2. – Λ+c analysis strategy
Λ+
c is the lightest baryon carrying open charm, with a rest mass of 2.286 GeV/c2
and a decay length (cτ ) of 60 μm [3]. The measurement is based on the reconstruction
of displaced secondary decay vertices. For the decay channel Λ+c → pK−π+ single
tracks are selected at first. Pairs of tracks with opposite charge are constructed and
topological selections are applied on the vertices defined for each pair of tracks. A
third track is finally attached to a pair, forming a Λ+
c candidate and the vertex of the
triplet is calculated. The identification of pions, kaons and protons reduces the large combinatorial background: a Bayesian approach is used to combine the response of the TPC and TOF detectors. Particle species are assigned to the tracks according to the maximum probability criterion: a track is identified as species i if the Bayesian probability W (i) to be of species i is the highest.
For the decay channel Λ+c → pKS0, pairs of tracks with opposite charge are selected in
the invariant mass window of the K0
S, applying topological selections and excluding pairs
that, with the (p, π) or (e+, e−) mass assignments, are compatible with Λ decays or
pho-ton conversions. After this selection, the purity of the sample of KS0 is very high (fig. 1).
These K0
S candidates are subsequently combined with single tracks originating from the
interaction point, forming Λ+
c candidates. Protons are identified using a “number of
sigma cut” approach, where the measured energy deposit (TPC) or time-of-flight (TOF) are compared with the values expected for protons.
(1) This BR includes resonant as well as non-resonant decays with a proton, a kaon and a pion in the final state.
(2) Λ+
c → p ¯K0 with BR (2.3± 0.6)%; ¯K0 → KS0 with a probability of 50% and finally
K0
S→ π+π−with BR (69.20± 0.05)%.
(3) The minimum bias trigger selects all the events with at least one hit in the innermost layers
of the ITS or in one of the scintillator arrays, positioned in the forward and backward regions of the experiment, in coincidence with signals from the beam pickup detectors.
MEASUREMENTS OF Λ+
c PRODUCTION IN pp COLLISIONS AT
√
s = 7 TeV WITH ALICE 3
) 2 p (GeV/c S 0 Invariant Mass K 2.22 2.24 2.26 2.28 2.3 2.32 2.34 2.36 2 Entries / 5 MeV/c 13 14 15 16 17 18 19 3 10 × 0.002 ± Mean = 2.283 0.003 ± Sigma = 0.006 1.2 ± ) 3.5 σ Significance (3 436 ± ) 1267 σ S (3 ) 0.0096 σ S/B (3 <2.0 GeV/c T p ≤ 1.0
p
S 0K
→
cΛ
) 2 p (GeV/c S 0 Invariant Mass K 2.22 2.24 2.26 2.28 2.3 2.32 2.34 2.36 2 Entries / 5 MeV/c 6 7 8 9 10 11 3 10 × 0.002 ± Mean = 2.284 0.002 ± Sigma = 0.010 1.2 ± ) 4.7 σ Significance (3 385 ± ) 1529 σ S (3 ) 0.0145 σ S/B (3 <3.0 GeV/c T p ≤ 2.0 events 8 10 × = 7 TeV, 3.16 s pp ) 2 p (GeV/c S 0 Invariant Mass K 2.22 2.24 2.26 2.28 2.3 2.32 2.34 2.36 2 Entries / 5 MeV/c 2 2.5 3 3.5 4 3 10 × 0.002 ± Mean = 2.288 0.002 ± Sigma = 0.004 1.1 ± ) 3.4 σ Significance (3 150 ± ) 446 σ S (3 ) 0.0269 σ S/B (3 <4.0 GeV/c T p ≤ 3.0 ) 2 p (GeV/c S 0 Invariant Mass K 2.22 2.24 2.26 2.28 2.3 2.32 2.34 2.36 2 Entries / 5 MeV/c 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 3 10 × 0.004 ± Mean = 2.285 0.004 ± Sigma = 0.013 1.1 ± ) 3.5 σ Significance (3 152 ± ) 496 σ S (3 ) 0.0255 σ S/B (3 <5.0 GeV/c T p ≤ 4.0 11/10/2012Fig. 2. – Invariant mass distribution of Λ+c → pK−π+candidates. The signal is observed in four pT bins in the range 2 < pT < 6 GeV/c.
3. – Results
A clear Λ+
c signal can be observed in both decay channels, as shown in figs. 2 and 3,
in the pT range [2, 6[ GeV/c. We are not able to observe a Λ+c signal for pT < 1 GeV/c,
because the combinatorial background is huge and the resolution on the track position
is not sufficient to separate the Λ+
c decay vertex from the interaction point.
2.2 2.22 2.24 2.26 2.28 2.3 2.32 2.34 2.36 2 Entries / 5 MeV/c 3500 3600 3700 3800 3900 4000 4100 4200 4300 0.001 ± Mean = 2.286 0.001 ± Sigma = 0.004 1.0 ± ) 3.8 σ Significance (3 221 ± ) 801 σ S (3 ) 0.0181 σ S/B (3 ) 2 (GeV/c π Invariant Mass pK ) < 3 GeV/c c Λ ( t 2 < p 2.22 2.24 2.26 2.28 2.3 2.32 2.34 2.36 2 Entries / 10 MeV/c 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 0.003 ± Mean = 2.290 0.002 ± Sigma = 0.011 1.1 ± ) 4.8 σ Significance (3 226 ± ) 986 σ S (3 ) 0.0236 σ S/B (3 15/05/2012 + π pK → c Λ events 8 = 7 TeV, 3.x10 s pp ) 2 (GeV/c π Invariant Mass pK )< 4 GeV/c c Λ ( t 3 < p 2.22 2.24 2.26 2.28 2.3 2.32 2.34 2.36 2 Entries / 10 MeV/c 450 500 550 600 650 700 750 0.002 ± Mean = 2.289 0.002 ± Sigma = 0.007 1.1 ± ) 3.9 σ Significance (3 90 ± ) 332 σ S (3 ) 0.0474 σ S/B (3 ) 2 (GeV/c π Invariant Mass pK ) < 5 GeV/c c Λ ( t 4 < p 2.22 2.24 2.26 2.28 2.3 2.32 2.34 2.36 2 Entries / 10 MeV/c 40 60 80 100 120 140 160 180 200 0.003 ± Mean = 2.290 0.003 ± Sigma = 0.010 1.5 ± ) 4.1 σ Significance (3 77 ± ) 209 σ S (3 ) 0.0877 σ S/B (3 ) 2 (GeV/c π Invariant Mass pK ) < 6 GeV/c c Λ ( t 5 < p
Fig. 3. – Invariant mass distribution of Λ+c → pKS0 candidates. The signal is observed in four
4 E. MENINNO
4. – Perspectives for future measurements
For the measurement of the Λ+
c production cross section, corrections for reconstruction
and selection efficiency and detector acceptance need to be applied. These are the next steps of this analysis.
As was demonstrated, the Λ+
c analysis in pp collisions is strongly limited by the large
combinatorial background. The larger statistics, which will be available with the increase in luminosity expected during Run II, will improve the precision of our results.
The observation of Λ+c in Pb-Pb collisions is very challenging with the present
detec-tor. However, the ITS upgrade (planned for the second long LHC shutdown in the years
2018-2019) [4] will improve the track impact parameter resolution (by a factor of∼ 3),
the tracking efficiency and pT resolution at low pT. These new features will enable us to
measure Λ+
c for the first time in Pb-Pb collisions.
REFERENCES
[1] Abelev B. et al. (ALICE Collaboration), Phys. Rev. Lett., 111 (2013) 223301. [2] Fries R. and Muller B., Eur. Phys. J. C, 34 (2004) S279.
[3] Beringer J. et al. (Particle Data Group), Phys. Rev. D, 86 (2012) 010001.
[4] Abelev B. et al. (The ALICE Collaboration), J. Phys. G: Nucl. Part. Phys., 41 (2014) 087002.
