Two-Particle Azimuthal Correlations As a Probe of Collective Behaviour in Deep Inelastic Ep Scattering at HERA
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Two-particle azimuthal correlations as a probe of collective behaviour in deep inelastic ep scattering at HERA Dhevan Gangadharan On behalf of the ZEUS Collaboration Universit¨atHeidelberg DIS 2021, 14 April 2021 1/29 Dhevan Gangadharan On behalf of the ZEUS Collaboration (Uni Heidelberg) DIS 2021, 14 April 2021 1 / 29 High-energy heavy-ion collisions: initial state target Initial state: elliptical in X-Y plane Most spatial configurations in the initial state correspond to partially overlapping nuclei. Z axis Consequently, the initial scattering leaves behind an elliptically eccentric zone in the transverse plane. X axis projectile b 2/29 Dhevan Gangadharan On behalf of the ZEUS Collaboration (Uni Heidelberg) DIS 2021, 14 April 2021 2 / 29 High-energy heavy-ion collisions: final state Y Subsequently, a prominent stage of rescattering is expected that rapidly leads to a local thermal equilibrium. X Hydrodynamics of a QCD fluid is used Rescattering (hydrodynamic stage) to describe the evolution of this matter. P Y This is a non-perturbative process that converts the initial-state spatial eccentricity into a final-state momentum anisotropy. PX 3/29 Dhevan Gangadharan On behalf of the ZEUS Collaboration (Uni Heidelberg) DIS 2021, 14 April 2021 3 / 29 Observations in heavy-ion collisions Pb+Pb centrality Two particle correlations show a clear double ridge, which is interpretated as a sign of fluid-like behaviour. The fluid of QCD matter is referred to as a Quark{gluon plasma (QGP). CMS EPJC (2012) 72:2012 4/29 Dhevan Gangadharan On behalf of the ZEUS Collaboration (Uni Heidelberg) DIS 2021, 14 April 2021 4 / 29 Observations in high-multiplicity p + p collisions p+p The LHC revealed a similar double-ridge in high multiplicity p + p collisions. The finding came as a surprise since a p + p collision was thought to be too small to produce a thermally equilibrated QGP. This motivated the search for similar effects in even smaller ep DIS at HERA. ATLAS PRL 116 172301 5/29 Dhevan Gangadharan On behalf of the ZEUS Collaboration (Uni Heidelberg) DIS 2021, 14 April 2021 5 / 29 Deep inelastic scattering (DIS) DIS Scattered electron DIS is defined by large virtualities: Incoming 2 2 electron Q ΛQCD. Transverse radius (Rt ) and longitudinal length (L) of the probed region are given by: 1 Rt ∼ Q 1 L ∼ PRD 95 114008 mproton x Neutral current (NC) DIS involves exchange of photon or Z boson. 6/29 Dhevan Gangadharan On behalf of the ZEUS Collaboration (Uni Heidelberg) DIS 2021, 14 April 2021 6 / 29 Photoproduction (PhP) Photoproduction PhP is defined by small virtualities: 2 2 Q ΛQCD. Exchange photon may fluctuate into quarks and gluons. Larger interaction regions are probed. Scattering is hadron-like. PhP is not presented here. A complementary ZEUS analysis in photoproduction on this subject will be published soon. 7/29 Dhevan Gangadharan On behalf of the ZEUS Collaboration (Uni Heidelberg) DIS 2021, 14 April 2021 7 / 29 Two-particle correlation function cnf2g Two-particle azimuthal correlations are Transverse Plane measured: Directed Elliptic asymmetry asymmetry cnf2g = hhcos n(φi − φj )ii. 'i is the azimuthal angle of particle i. n is the harmonic. The inner and outer brackets denote an Triangular Quadrangular average within an event and over all events, asymmetry asymmetry respectively. Detector acceptance corrections described in the backup. 8/29 Dhevan Gangadharan On behalf of the ZEUS Collaboration (Uni Heidelberg) DIS 2021, 14 April 2021 8 / 29 The HERA collider and experiments Location: DESY, Hamburg, Germany Data taking: 1992 - 2007 27.6 GeV electrons/positrons 920 GeV protons p ! s = 318 GeV HERA I+II: 500 pb−1 per experiment We present recently published measurements of two-particle correlations in NC DIS with ZEUS: JHEP 04 (2020) 070. 9/29 Dhevan Gangadharan On behalf of the ZEUS Collaboration (Uni Heidelberg) DIS 2021, 14 April 2021 9 / 29 DIS event and track selection Charged particles are tracked in the central tracking detector (CTD) and micro vertex detector (MVD) in a 1.43 T magnetic field. Depleted uranium calorimeters. The barrel and rear ones are used to help identify the scattered electron. A fully contained event is characterized by P i (Ei − Pz;i ) = 55 GeV due to energy and momentum conservation. 10/29 Dhevan Gangadharan On behalf of the ZEUS Collaboration (Uni Heidelberg) DIS 2021, 14 April 2021 10 / 29 DIS event and track selection High Q2 ZEUS Event selection (46 M) FCAL BCAL RCAL DIS triggers Q2 = −(k − k0)2 > 5 GeV2 HAC HAC 0 k0 > 10 GeV r > 15 cm EMC L C o M w θ > 1 rad E e Solenoid magnet Q P 2 47 < (Ei − Pz;i ) < 69 GeV CTD scattered electron jVz j < 30 cm r MVD Track selection for correlation analysis electron proton Reject scattered electron −1:5 < η < 2:0 0:1 < pT < 5:0 GeV ≥ 1 MVD hit DCAXY ;Z < 2 cm ∆R > 0:4 (cone around scattered electron) 11/29 Dhevan Gangadharan On behalf of the ZEUS Collaboration (Uni Heidelberg) DIS 2021, 14 April 2021 11 / 29 ZEUS ZEUS NC DIS 366 pb1 s = 318 GeV 2 2 {2} {2} Q > 5 GeV 1 LEPTO gen 2 c c p < 5.0 GeV ARIADNE gen T 1.5 < η < 2.0 ARIADNE nondiff gen 0.2 p > 0.1 GeV 0.1 p > 0.1 GeV c1f2g &T c2f2g versus chargedT particle multiplicity Nch no |∆η| cut no |∆η| cut 0 0 0 10p > 0.5 GeV 20 30 0 10p > 0.5 GeV 20 30 0.1 T T {2} {2} 1 |∆η| > 2 2 |∆η| > 2 c1f2g is better described by the ARIADNE c c 0.05 generator. 0 c2f2g is better described by the LEPTO generator. −0.1 0 Neither model works well for both harmonics. 0 10p > 0.5 GeV 20 30 0 ARIADNE 10 nondiff gen 20 30 T 0.2 The diffractive component in ARIADNE only {2} {2} 1 ARIADNE nondiff gen |∆η| > 2 2 c c slightly influences c f2g . 0 p > 0.5 GeV 2 T |∆η| > 2 0.1 Massless jets were reconstructed from the generated hadrons with the kT algorithm and −0.2 ≤ 1 jet: LEPTO gen ≤ 1 jet: ARIADNE gen Et > 2 GeV, ∆R = 1. = 2 jets: LEPTO gen 0 = 2 jets: ARIADNE gen Jets can explain the observed correlations. 0 10 20 30 0 10 20 30 Nch Nch 12/29 Dhevan Gangadharan On behalf of the ZEUS Collaboration (Uni Heidelberg) DIS 2021, 14 April 2021 12 / 29 ZEUS ZEUS NC DIS 366 pb1 s = 318 GeV 2 2 {2} {2} Q > 5 GeV 1 LEPTO gen 2 c c p < 5.0 GeV ARIADNE gen T 1.5 < η < 2.0 ARIADNE nondiff gen 0.2 p > 0.1 GeV 0.1 p > 0.1 GeV c1f2g &T c2f2g versus chargedT particle multiplicity Nch no |∆η| cut no |∆η| cut 0 0 Short-range (j∆ηj ∼ 0) correlations are strongest 0 10p > 0.5 GeV 20 30 0 10p > 0.5 GeV 20 30 0.1 T T {2} {2} at low Nch 1 |∆η| > 2 2 |∆η| > 2 c c 0.05 Longe-range correlations 0 (j∆ηj > 2, orange-black pairs) of the first harmonic are negative 0 −0.1 and the largest in magnitude 0 10p > 0.5 GeV 20 30 0 ARIADNE 10 nondiff gen 20 30 T 0.2 {2} {2} 1 ARIADNE nondiff gen |∆η| > 2 2 c c 0 p > 0.5 GeV T |∆η| > 2 0.1 −0.2 ≤ 1 jet: LEPTO gen ≤ 1 jet: ARIADNE gen = 2 jets: LEPTO gen 0 = 2 jets: ARIADNE gen 0 10 20 30 0 10 20 30 Nch Nch 13/29 Dhevan Gangadharan On behalf of the ZEUS Collaboration (Uni Heidelberg) DIS 2021, 14 April 2021 13 / 29 Ridge figures in DIS 0.5 < p < 5.0 GeV ZEUS T s = 318 GeV 2 ≤ Nch < 10 15 ≤ Nch < 30 Q2 > 5 GeV2 ) ) ϕ ϕ 1.2 ∆ ∆ , 1 , η η ∆ ∆ C( C( 1 0.8 0.8 0.6 4 4 ∆ 3 ∆ 3 ϕ 3 2 ϕ 3 2 2 1 η 2 1 η 1 0 ∆ 1 0 ∆ 0 −1 0 −1 −1 −3−2 −1 −3−2 Jet peak centered at ∆' ∼ ∆η ∼ 0. Away-side ridge at high Nch is expected from tilted dijets. No double ridge visible at high Nch 14/29 Dhevan Gangadharan On behalf of the ZEUS Collaboration (Uni Heidelberg) DIS 2021, 14 April 2021 14 / 29 Ridge comparisons between the LHC and HERA DIS Pb+Pb p+Pb centrality CMS CMS EPJC (2012) 72:2012 PLB 718 (2013) 795 p+p e+p DIS 0.5 < p < 5.0 GeV ZEUS T s = 318 GeV 2 ≤ Nch < 10 15 ≤ Nch < 30 Q2 > 5 GeV2 ) ) ϕ ϕ 1.2 ∆ ∆ , 1 , η η ∆ ∆ C( C( 1 0.8 0.8 0.6 4 4 ATLAS ∆ 3 ∆ 3 ϕ 3 2 ϕ 3 2 PRL 116 172301 2 1 η 2 1 η 1 0 ∆ 1 0 ∆ −1 −1 0 −2 0 −2 −1 −3 ZEUS −1 −3 JHEP 04 (2020) 070 15/29 Dhevan Gangadharan On behalf of the ZEUS Collaboration (Uni Heidelberg) DIS 2021, 14 April 2021 15 / 29 Summary and outlook Two-particle azimuthal correlations have been measured in ep neutral current deep inelastic scattering with ZEUS. Comparisons of the observed correlations with available models of DIS suggest that the measured correlations are dominated by contributions from jets. The correlations do not indicate the kind of collective behaviour recently observed in high-multiplicity hadronic collisions at the highest RHIC and LHC energies.