Measurementof mesonsviatheirthreepiondecay p with ALICE ......Oak Ridge National Laboratory, TN,...
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Measurement of ω mesons via their three pion decay with ALICE in pp collisions at √ s = 7 TeV Florian Jonas for the ALICE collaboration Westf¨ alische Wilhelms-Universit¨ at M ¨ unster, Germany Oak Ridge National Laboratory, TN, United States 137th LHCC Meeting 2019 Abstract In this contribution, the differential invariant cross section of ω meson production at mid-rapidity in proton- proton collisions at √ s = 7 TeV is presented. The meson is reconstructed using its π + π - π 0 decay channel, requiring the identification and measurement of charged pions using ALICE’s tracking capabilities as well as the reconstruction of neutral pions via their decay to two photons. All methods available in ALICE to mea- sure photons at mid-rapidity are used: Its two calorimeters, the EMCal and the PHOS, as well as the so-called Photon Conversion Method (PCM), which allows to measure photons via pair conversions. In addition, two hy- brid approaches are used for the neutral pion reconstruction which combine calorimeter photon measurements with the PCM. Introduction ALICE [1] is designed as a heavy-ion experiment and its research mainly fo- cuses on the properties of the quark- gluon plasma (QGP) – a phase of strongly interacting matter in which quarks and gluons exist as deconfined particles. Due to the high energy den- sities reached in Pb-Pb collisions at the LHC, the creation of the QGP is expected and supported by measure- ments up to this point. Studying the particles created in a collision (in this case the ω meson) offers important insights into the different stages of the collision and the obtained meson production cross sections have a variety of applications: • Theoretical Motivations: – By comparing the measured cross sections to theory predictions, the underlying models and assumptions can be tested and constrained (e.g. the used Fragmentation Function or Parton Distribution Function). • Experimental Motivations: – The obtained cross sections are needed as input for other analyses such as direct photon [2] and di-electron measurements to determine the background of decay photons and leptons, respectively. – The cross sections are probes for the QGP: The suppression of hadron production in Pb-Pb collisions at high transverse momentum (p T ) with respect to pp collisions is expected due to the presence of the QGP. Furthermore, a decrease of the ω mass and increase in its width is expected due to partial restoration of chiral symmetry in the QGP [3]. Event Selection • The data used in this analysis consists of about 440 million minimum bias pp events at √ s = 7 TeV (integrated luminosity L int ≈ 7 nb -1 ), recorded at the LHC in 2010. The minimum bias trigger requires a hit in either of the two V0 detectors or a hit in the Silicon Pixel Detector (SPD), the innermost layer of the Inner Tracking System (ITS). • The same amount of events is generated using the Monte Carlo event generator P YTHIA 6 and the generated particles are further propagated through the ALICE detectors using G EANT 3. Photon Reconstruction Three different techniques are available in ALICE to measure photons at mid-rapidity: Photon Conversion Method (PCM) • Coverage: |η | < 0.9, 0 ◦ <ϕ< 360 ◦ • Photons can undergo pair creation in de- tector material and the created e + e - pairs are visible as two charged tracks with op- posite curvature. • Reconstruction of secondary-vertex ⇒ conversion point of original photon • Allows photon measurements down to very low momentum (p T ∼ 0.2 GeV/c). Photon Spectrometer (PHOS): • Coverage: |η | < 0.13, 260 ◦ <ϕ< 320 ◦ [4] • Consists of 10 752 lead-tungstate crystals. • Smaller coverage than EMCal but higher granularity. • Optimised for low momenta photons (down to p T ∼ 0.3 GeV/c). Electromagnetic Calorimeter (EMCal): • Coverage: |η | < 0.7, Δϕ = 40 ◦ [5] • Layered Pb-scintillator sampling calorimeter • Designed for high-p T measurements Charged Pion Measurement • Reconstruction of tracks in the Time Projection Chamber (TPC) and the ITS • Identification of charged pion tracks via dE /dx measurement in the TPC Invariant Mass Analysis 0.66 0.68 0.7 0.72 0.74 0.76 0.78 0.8 0.82 0.84 0.86 0.88 ) 2 c (GeV/ 0 π - π + π M 0 100 200 300 400 500 Counts ALICE work in progress = 7 TeV s pp, INT1 triggered rec w/ PCM 0 π c < 5.0 GeV/ T p : 4.0 < ω Raw real events Mixed event BG Remain. BG BG subtracted Fit 0.66 0.68 0.7 0.72 0.74 0.76 0.78 0.8 0.82 0.84 0.86 0.88 ) 2 c (GeV/ 0 π - π + π M 0 100 200 300 400 500 600 700 Counts ALICE work in progress = 7 TeV s pp, INT1 triggered rec w/ PCM-EMC 0 π c < 5.0 GeV/ T p : 4.0 < ω Raw real events Mixed event BG Remain. BG BG subtracted scaled by 2.0 Fit 0.66 0.68 0.7 0.72 0.74 0.76 0.78 0.8 0.82 0.84 0.86 0.88 ) 2 c (GeV/ 0 π - π + π M 100 - 0 100 200 300 400 500 Counts ALICE work in progress = 7 TeV s pp, INT1 triggered rec w/ PCM, PHOS 0 π c < 5.0 GeV/ T p : 4.0 < ω Raw real events Mixed event BG Remain. BG BG subtracted scaled by 2.0 Fit 0.66 0.68 0.7 0.72 0.74 0.76 0.78 0.8 0.82 0.84 0.86 0.88 ) 2 c (GeV/ 0 π - π + π M 0.2 - 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 3 10 × Counts ALICE work in progress = 7 TeV s pp, INT1 triggered rec w/ EMC 0 π c < 6.0 GeV/ T p : 5.0 < ω Raw real events Mixed event BG Remain. BG BG subtracted scaled by 2.0 Fit • The ω is reconstructed via its decay to π + π - π 0 by calculating the in- variant mass of all three-pion combinations in a given event [6]. • Neutral pions are reconstructed via their decay to two photons [7]. Five different methods are used for the π 0 reconstruction that differ in how each of its decay photons is measured: PCM, PCM-EMCal, PCM-PHOS, EMCal and PHOS • The underlying background is described using event mixing, π 0 sideband mixing or like-sign mixing. • Any residual background is described using a linear fit and the signal is then fitted using a Gaussian. ω (782) [8] m = 782.65(12) MeV/c 2 Γ Tot =8.49(8) MeV/c 2 Decays: Γ π + π - π 0 /Γ Tot = 89.20(7) % ··· π 0 [8] m = 134.9970(5) MeV/c 2 c τ = 25.5 nm Decays: Γ γγ /Γ Tot = 98.823(34) % ··· Results • The five individual measurements are combined using the Best Lin- ear Unbiased Estimate (BLUE) method [9] which weights the mea- surements according to their stat. and sys. uncertainties. • The cross section of ω meson production in pp collisions at √ s = 7 TeV is successfully measured for 1.8 <p T < 16 GeV/c. • Parameterization of the cross section possible using Tsallis function. Comparison to other measurement: • Data in agreement with stand-alone PHOS measurement [10]. • Improved momentum resolution at low p T . Comparison to theory: • P YTHIA 8.2 with Monash 2013 tune in agreement with data over whole mo- mentum range. 2 3 4 5 6 7 8 9 10 20 ) 3 c -2 (pb GeV 3 p d σ 3 d E 0 π - π + π → ω data: norm. unc. 3.5% data: PHOS PUB-787 PYTHIA 8.2 Monash 2013 Tsallis fit ALICE work in progress = 7 TeV s pp, 2 3 4 5 6 7 8 9 10 20 Tsallis data 2 3 4 5 6 7 8 9 10 20 ) c (GeV/ T p data PYTHIA 0 2 4 6 8 10 12 14 16 ) c (GeV/ T p 0 π / ω = 7000 GeV, work in progress s ,pp 0 π - π + π → ω = 200 GeV, PHENIX s ,pp 0 π - π + π → ω = 200 GeV, PHENIX s ,pp γ 0 π → ω = 200 GeV, PHENIX s ,pp - e + e → ω = 62 GeV, ISR s ,pp γ 0 π → ω Particle ratios: • ω/π 0 ratio in agreement with mea- surements at lower collision energies ⇒ supporting observation that ratio is independent of collision energy at high p T • Constant cross section ratio at high p T can be used for m T -scaling [11] of other mesons. Summary • The cross section of ω meson production is measured for the first time using all available methods in ALICE to measure photons at mid-rapidity, allowing to access a wide momentum range. • Results consistent with stand-alone PHOS measurement & P YTHIA theory predictions. • ω/π 0 ratio in agreement with measurements at lower collision energies. References [1] K. Aamodt et al. The ALICE experiment at the CERN LHC. JINST, 3:S08002, 2008. [2] Jaroslav Adam et al. Direct photon production in Pb-Pb collisions at √ s NN = 2.76 TeV. Phys. Lett., B754:235–248, 2016. [3] Yuji Tsuchimoto. In-Medium Modifications of Low-Mass Vector Mesons in PHENIX at RHIC. Nucl. Phys., A830:487C–490C, 2009. [4] V Man’ko et al. ALICE Photon Spectrometer (PHOS): Technical Design Report. Technical Report CERN-LHCC-99-004, 1999. [5] P Cortese et al. ALICE Electromagnetic Calorimeter Technical Design Report. Technical Report CERN-LHCC-2008-014. ALICE-TDR-14, Aug 2008. [6] F. Jonas. Measurement of ω and η mesons via their three pion decay with ALICE in pp collisions at √ s =7 TeV, Sep 2018. CERN-THESIS-2018-313. [7] B. Abelev et al. Neutral pion and η meson production in proton-proton collisions at √ s =0.9 TeV and √ s =7 TeV. Phys.Lett., B717:162–172, 2012. [8] M. Tanabashi et al. Review of particle physics. Phys. Rev. D, 98:030001, Aug 2018. [9] A. Valassi et al. Information and treatment of unknown correlations in the combination of measurements using the BLUE method. Eur. Phys. J., C74:2717, 2014. [10] S. Acharya et al. Production of ω(782) in pp collisions at √ s = 7 TeV. May 2018. ALICE-PUBLIC-2018-004. [11] L. Altenk¨ amper et al. Applicability of transverse mass scaling in hadronic collisions at energies available at the CERN Large Hadron Collider. Phys. Rev., C96(6):064907, 2017.
Transcript of Measurementof mesonsviatheirthreepiondecay p with ALICE ......Oak Ridge National Laboratory, TN,...
Measurement of ω mesons via their three pion decaywith ALICE in pp collisions at
√s = 7TeV
Florian Jonas for the ALICE collaborationWestfalische Wilhelms-Universitat Munster, GermanyOak Ridge National Laboratory, TN, United States137th LHCC Meeting 2019
Abstract
In this contribution, the differential invariant cross section of ω meson production at mid-rapidity in proton-proton collisions at
√s = 7 TeV is presented. The meson is reconstructed using its π+π−π0 decay channel,
requiring the identification and measurement of charged pions using ALICE’s tracking capabilities as well asthe reconstruction of neutral pions via their decay to two photons. All methods available in ALICE to mea-sure photons at mid-rapidity are used: Its two calorimeters, the EMCal and the PHOS, as well as the so-calledPhoton Conversion Method (PCM), which allows to measure photons via pair conversions. In addition, two hy-brid approaches are used for the neutral pion reconstruction which combine calorimeter photon measurementswith the PCM.
Introduction
ALICE [1] is designed as a heavy-ionexperiment and its research mainly fo-cuses on the properties of the quark-gluon plasma (QGP) – a phase ofstrongly interacting matter in whichquarks and gluons exist as deconfinedparticles. Due to the high energy den-sities reached in Pb-Pb collisions atthe LHC, the creation of the QGP isexpected and supported by measure-ments up to this point.
Studying the particles created in a collision (in this case the ω meson) offers important insights intothe different stages of the collision and the obtained meson production cross sections have a varietyof applications:
• Theoretical Motivations:
– By comparing the measured cross sections to theory predictions, the underlying models andassumptions can be tested and constrained (e.g. the used Fragmentation Function or PartonDistribution Function).
• Experimental Motivations:
– The obtained cross sections are needed as input for other analyses such as direct photon [2]and di-electron measurements to determine the background of decay photons and leptons,respectively.
– The cross sections are probes for the QGP: The suppression of hadron production in Pb-Pbcollisions at high transverse momentum (pT) with respect to pp collisions is expected due tothe presence of the QGP. Furthermore, a decrease of the ω mass and increase in its width isexpected due to partial restoration of chiral symmetry in the QGP [3].
Event Selection
• The data used in this analysis consists of about 440 million minimum bias pp events at√s = 7 TeV
(integrated luminosity Lint ≈ 7 nb−1), recorded at the LHC in 2010. The minimum bias triggerrequires a hit in either of the two V0 detectors or a hit in the Silicon Pixel Detector (SPD), theinnermost layer of the Inner Tracking System (ITS).
• The same amount of events is generated using the Monte Carlo event generator PYTHIA6 andthe generated particles are further propagated through the ALICE detectors using GEANT3.
Photon Reconstruction
Three different techniques are available in ALICE to measure photons at mid-rapidity:
Photon Conversion Method (PCM)•Coverage: |η| < 0.9, 0◦ < ϕ < 360◦
• Photons can undergo pair creation in de-tector material and the created e+e− pairsare visible as two charged tracks with op-posite curvature.• Reconstruction of secondary-vertex⇒ conversion point of original photon•Allows photon measurements down to
very low momentum (pT ∼ 0.2 GeV/c).
Photon Spectrometer (PHOS):•Coverage: |η| < 0.13, 260◦ < ϕ < 320◦ [4]•Consists of 10 752 lead-tungstate crystals.• Smaller coverage than EMCal but higher
granularity.•Optimised for low momenta photons
(down to pT ∼ 0.3 GeV/c).
Electromagnetic Calorimeter (EMCal):•Coverage: |η| < 0.7, ∆ϕ = 40◦ [5]• Layered Pb-scintillator sampling calorimeter•Designed for high-pT measurements
Charged Pion Measurement
• Reconstruction of tracks in the Time Projection Chamber (TPC) and the ITS
• Identification of charged pion tracks via dE/dx measurement in the TPC
Invariant Mass Analysis
0.66 0.68 0.7 0.72 0.74 0.76 0.78 0.8 0.82 0.84 0.86 0.88
)2c (GeV/0π π +
πM
0
100
200
300
400
500
Co
un
ts
ALICE work in progress = 7 TeVspp,
INT1 triggered rec w/ PCM0
π
c < 5.0 GeV/T
p: 4.0 < ω
Raw real events
Mixed event BG
Remain. BG
BG subtracted
Fit
0.66 0.68 0.7 0.72 0.74 0.76 0.78 0.8 0.82 0.84 0.86 0.88
)2c (GeV/0π π +
πM
0
100
200
300
400
500
600
700
Co
un
ts
ALICE work in progress = 7 TeVspp,
INT1 triggered rec w/ PCMEMC0
π
c < 5.0 GeV/T
p: 4.0 < ω
Raw real events
Mixed event BG
Remain. BG
BG subtracted
scaled by 2.0
Fit
0.66 0.68 0.7 0.72 0.74 0.76 0.78 0.8 0.82 0.84 0.86 0.88
)2c (GeV/0π π +
πM
100−
0
100
200
300
400
500Co
un
ts
ALICE work in progress = 7 TeVspp,
INT1 triggered rec w/ PCM, PHOS0
π
c < 5.0 GeV/T
p: 4.0 < ω
Raw real events
Mixed event BG
Remain. BG
BG subtracted
scaled by 2.0
Fit
0.66 0.68 0.7 0.72 0.74 0.76 0.78 0.8 0.82 0.84 0.86 0.88
)2c (GeV/0π π +
πM
0.2−
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
310×
Co
un
ts
ALICE work in progress = 7 TeVspp,
INT1 triggered rec w/ EMC0
π
c < 6.0 GeV/T
p: 5.0 < ω
Raw real events
Mixed event BG
Remain. BG
BG subtracted
scaled by 2.0
Fit
• The ω is reconstructed via its decay to π+π−π0 by calculating the in-variant mass of all three-pion combinations in a given event [6].•Neutral pions are reconstructed via their decay to two photons [7].
Five different methods are used for the π0 reconstruction that differin how each of its decay photons is measured:PCM, PCM-EMCal, PCM-PHOS, EMCal and PHOS• The underlying background is described using event mixing,π0 sideband mixing or like-sign mixing.•Any residual background is described using a linear fit and the signal
is then fitted using a Gaussian.
ω(782) [8]
m = 782.65(12) MeV/c2
ΓTot = 8.49(8) MeV/c2
Decays:Γπ+π−π0/ΓTot = 89.20(7) %
· · ·
π0 [8]
m = 134.9970(5) MeV/c2
cτ = 25.5 nm
Decays:Γγγ/ΓTot = 98.823(34) %
· · ·
Results
• The five individual measurementsare combined using the Best Lin-ear Unbiased Estimate (BLUE)method [9] which weights the mea-surements according to their stat.and sys. uncertainties.• The cross section of ω meson
production in pp collisions at√s = 7 TeV is successfully measured
for 1.8 < pT < 16 GeV/c.• Parameterization of the cross section
possible using Tsallis function.
Comparison to other measurement:•Data in agreement with stand-alone
PHOS measurement [10].• Improved momentum resolution at
low pT.
Comparison to theory:• PYTHIA8.2 with Monash 2013 tune in
agreement with data over whole mo-mentum range.
2 3 4 5 6 7 8 9 10 20
)3
c2
(p
b G
eV
3p
dσ
3d
E
0π
π
+π → ωdata:
norm. unc. 3.5%
data: PHOS PUB787
PYTHIA 8.2 Monash 2013
Tsallis fit
ALICE work in progress = 7 TeVspp,
2 3 4 5 6 7 8 9 10 20
Tsa
llis
da
ta
2 3 4 5 6 7 8 9 10 20)c (GeV/
Tp
da
taP
YT
HIA
0 2 4 6 8 10 12 14 16)c (GeV/
Tp
0π
/
ω = 7000 GeV, work in progresss ,pp 0π
π+
π →ω
= 200 GeV, PHENIXs ,pp 0π
π+
π →ω
= 200 GeV, PHENIXs ,pp γ0
π →ω
= 200 GeV, PHENIXs ,pp
e+ e→ω
= 62 GeV, ISRs ,pp γ0
π →ω
Particle ratios:• ω/π0 ratio in agreement with mea-
surements at lower collision energies⇒ supporting observation that ratio is
independent of collision energy athigh pT
•Constant cross section ratio at highpT can be used for mT-scaling [11] ofother mesons.
Summary• The cross section of ω meson production is measured for the first time using all available methods
in ALICE to measure photons at mid-rapidity, allowing to access a wide momentum range.• Results consistent with stand-alone PHOS measurement & PYTHIA theory predictions.• ω/π0 ratio in agreement with measurements at lower collision energies.
References[1] K. Aamodt et al. The ALICE experiment at the CERN LHC. JINST, 3:S08002, 2008.
[2] Jaroslav Adam et al. Direct photon production in Pb-Pb collisions at√sNN = 2.76 TeV. Phys. Lett., B754:235–248, 2016.
[3] Yuji Tsuchimoto. In-Medium Modifications of Low-Mass Vector Mesons in PHENIX at RHIC. Nucl. Phys., A830:487C–490C, 2009.
[4] V Man’ko et al. ALICE Photon Spectrometer (PHOS): Technical Design Report. Technical Report CERN-LHCC-99-004, 1999.
[5] P Cortese et al. ALICE Electromagnetic Calorimeter Technical Design Report. Technical Report CERN-LHCC-2008-014. ALICE-TDR-14, Aug 2008.
[6] F. Jonas. Measurement of ω and η mesons via their three pion decay with ALICE in pp collisions at√s = 7 TeV, Sep 2018. CERN-THESIS-2018-313.
[7] B. Abelev et al. Neutral pion and η meson production in proton-proton collisions at√s = 0.9 TeV and
√s = 7 TeV. Phys.Lett., B717:162–172, 2012.
[8] M. Tanabashi et al. Review of particle physics. Phys. Rev. D, 98:030001, Aug 2018.
[9] A. Valassi et al. Information and treatment of unknown correlations in the combination of measurements using the BLUE method. Eur. Phys. J., C74:2717, 2014.
[10] S. Acharya et al. Production of ω(782) in pp collisions at√s = 7 TeV. May 2018. ALICE-PUBLIC-2018-004.
[11] L. Altenkamper et al. Applicability of transverse mass scaling in hadronic collisions at energies available at the CERN Large Hadron Collider. Phys. Rev., C96(6):064907, 2017.
