measurements of heavy quarkonium production and polarization at cms

Measurements of Heavy Quarkonium Production and Polarization at CMS

Linlin ZHANG （张琳琳） and Sijin QIAN （钱思进）Peking University

中国物理学会 高能物理分会 2014 学术年会 武汉，湖北， 4 月 21 日

Introduction

Υ(nS) differential cross section at CMS

Quarkonium (Υ(nS) and prompt Ψ(nS) ) polarization at CMS

Summary

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Outline

CMS Collaboration, “Prompt and non-prompt J/ψ production in pp collisions at sqrt(s) = 7 TeV”, Eur. Phys. J. C71 (2011) 1575.

CMS Collaboration, “Measurement of the Y(1S), Y(2S), and Y(3S) cross sections in pp collisions at sqrt(s) = 7 TeV", Phys. Lett. B727 (2013) 101.

CMS Collaboration, “Measurement of the Y(1S), Y(2S), and Y(3S) Polarizations in pp Collisions at sqrt(s) = 7 TeV”, Phys. Rev. Lett. 110 (2012) 081802.

CMS Collaboration, “Measurement of the prompt J/ψ and (2S) polarizations in pp Collisions at sqrt(s) = 7 TeV”, Phys. Lett. B727 (2013) 381.

Linlin ZHANG on behalf of the CMS Collaboration, “Quarkonium production and polarization in CMS”, Proceedings of Charm 2013 conference in Manchester, UK, 9/2013.

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Introduction (1/2)

L. Zhang and S. Qian (PKU)

Quarkonium production has the potential to clarify (non-perturbative) hadron formation and other QCD features

However, after decades of theoretical and experimental researches, quarkonium production remains a mystery

Best theory candidate is NRQCD: effective field theory, treats quarkonia as non-relativistic systems (heavy quark masses)

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Introduction (2/2)

Production X-section PolarizationL. Zhang and S. Qian (PKU)

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The CMS detector

CMS provides excellent dimuon mass resolution (σM/M ~0.6%) and large pT

reach

Use decay channel Υ(nS)→μ+μ-, covering kinematic phase space: 10 < pT

< 100 GeV, |y|< 0.6

Acceptance from CMS measured Υ(nS) polarization Muon efficiencies are evaluated from data-driven (Tag-&-Probe) method

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Υ(nS) differential cross section

L. Zhang and S. Qian (PKU)

The Υ(nS) differential cross section time dimuon branching ratio integrated ove |y| < 0.6 in a given pT bin of width △pT

The acceptance is the fraction of Υ decays satisfying all dimuon requirements in which each muon also meets the single-muon kinematic selection

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Υ(nS) differential cross section

L. Zhang and S. Qian (PKU)

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Υ(nS) differential cross section

Cross sections measured up to very large pT (pT >> M)→ stringent QCD test

There is a transition from nearly exponential cross section decrease with the increasing pT to power-law behavior for all three Υ(nS) states

This suggests a change in the nature of the production process

L. Zhang and S. Qian (PKU)

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Comparison to ATLAS and previous CMS measurements

Good agreement with ATLAS results and previous CMS results using 2010 data

L. Zhang and S. Qian (PKU)

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Quarkonium polarization: variables and frames

λθ = +1 : “transverse” polarizationJz = ± 1

Jz = 0

λθ = +1λφ = λθφ = 0

λθ = –1λφ = λθφ = 0

λθ = 1 : “longitudinal” polarization

L. Zhang and S. Qian (PKU)

Helicity axis (HX): direction of quarkonium momentum in Lab frame

Collins-Soper axis (CS): direction of relative velocity of colliding particles (p1- p2)

Perpendicular helicity axis (PX): perpendicular to CS (p1+ p2)

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Quarkonium polarization: variables and frames

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Need to measure the full angular distribution

L. Zhang and S. Qian (PKU)

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Quarkonium polarization measurements

L. Zhang and S. Qian (PKU)

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Quarkonium polarization measurements

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The analysis framework

2-D projection

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Systematic uncertainties

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Υ(nS) polarization in the HX frame, |y| < 0.6

L. Zhang and S. Qian (PKU)

Color octet matrix elements are fit to CMS hadro-production data only

Theoretical calculations account for feed-down contributions to Υ(1S) and Υ(2S) states

Calculations for Υ(3S) may change once including feed-down from χb(3P)

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Υ(nS): Comparison to NLO NRQCD

L. Zhang and S. Qian (PKU)

No signs of strong polarizations

The Ψ(2S) is not affected by feed-down from heavier quarkonia

→ easier comparison to theory

Error bars show total uncertainties at 68.3% CL

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Prompt Ψ(nS) polarization in the HX frame

L. Zhang and S. Qian (PKU)

CMS results disagree with existing NLO NRQCD calculations Calculations use a global fit of color octet matrix elements to photo- as

well as hadroproduction data Theoretical predictions are for the polarization of the directly produced J/Ψ 's, not accounting for the feed-down from decays of P-wave states

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Ψ(nS): Comparison to NLO NRQCD

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Possible solution?

The transverse components can be largely canceled between the 3S1

[8] and 3PJ

[8] channels, leaving the remaining terms to be dominated by the unpolarized J/ψ

K.-T. Chao et al, PRL108 (2012) 242004

Υ(1S), Υ(2S) and Υ(3S) differential cross sections for 10 < pT < 100 GeV were measured by CMS using data collected in 2011 A transition from exponential (low pT) to power-law (high pT) in the

differential cross sections has been observed

Three anisotropy parameters and the frame invariant parameter are measured in three polarization frames (HX, CS and PX) for the Υ(1S), Υ(2S), Υ(3S) and prompt J/Ψ and Ψ(2S)

No evidence of strong polarizations, transverse or longitudinal

Polarization measurements for most cases disagree with existing theoretical calculations

Further experimental and theoretical efforts are necessary to fully understand quarkonium production; the measurement of the polarizationsof the P-wave states (χc and χb) is very important 22

Summary

L. Zhang and S. Qian (PKU)

Backup

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Quarkonium Production

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Dimuon mass fits at different pT bins

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Υ Cross sections and ratiosY(2S) Y(3S)

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Quarkonium Polarization

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Definition of the PPD

Single-muon efficiencies carefully measured with data-driven Tag&Probe method

Carefully studied to avoid artificial spurious polarizations Efficiencies are accounted for on an event-by-event basis

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Efficiencies

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Υ(nS) polarization in the HX frame, 0.6 < |y| < 1.2

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Υ(nS) polarization in the PX frame, |y| < 0.6

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Υ(nS) polarization in the PX frame, 0.6 < |y| < 1.2

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Υ(nS) polarization in the CS frame, |y| < 0.6

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Υ(nS) polarization in the CS frame, 0.6 < |y| < 1.2

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Prompt Ψ(nS) polarization in the PX frame

Error bars show total uncertainties at 68.3% CL

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Prompt Ψ(nS) polarization in the CS frame

Error bars show total uncertainties at 68.3% CL

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J/Ψ: Comparison to NLO NRQCD

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Ψ(2S): Comparison to NLO NRQCD