Long-range azimuthal correlations in 2.76 & 13 TeV pp with ATLAS - PowerPoint PPT Presentation

Long-range azimuthal correlations in 2.76 & 13 TeV pp with ATLAS Andy Buckley University of Glasgow MPI@LHC 2015, Trieste, 2015-11-26 1/9

Introduction to the ridge The “near-side ridge” phenomenon has been one of the most prominent and enduring physics puzzles at the LHC. Expected/hoped-for effect in pA and AA collisions, from collective flow theory paradigm. Discovery in high-multiplicity pp was a surprise! Ridge in p + Pb due to global sinusoidal modulation of particle production. . . same in pp ? Today show ATLAS’ latest ridge Phys. Rev. Lett. 110, 182302 (2013), measurements, cf. arXiv:1509.04776 arXiv:1212.5198 2/9

Datasets pp data taken with ATLAS: 4 pb − 1 at 2 . 76 TeV and 14 nb − 1 at 13 TeV. Pile-up low in both cases: � µ � ∼ 0 . 5 at 2 . 76 TeV and ∼ 0 . 04 at 13 TeV. Charged tracks with p T > 300 MeV and | η | < 2 . 5 used Events / 3 7 7 10 10 6 6 as input to correlation measures. 5 5 10 10 4 4 Only tracks with p T > 400 MeV 3 3 10 ATLAS ATLAS 10 2 2 s =2.76 TeV s =13 TeV counted in N rec ch . 10 10 1 1 0 50 100 150 0 50 100 150 200 rec rec N N ch ch 3/9

Correlation observables 1: S , B , and C in ∆ η , ∆ φ Raw observable is 2p correlation C (∆ η, ∆ φ ) = S (∆ η, ∆ φ ) / B (∆ η, ∆ φ ) . S and B are distributions of particle a , b separations for a & b in same event and mixed events respectively. Division cancels acceptance effects and systematics. Not explicitly unfolded, but tracking efficiencies used as weights 1 /ǫ ( p a T , η a ) ǫ ( p b T , η b ) . a,b a,b ATLAS 0.5<p <5.0 GeV ATLAS 0.5<p <5.0 GeV T T s =13 TeV ≤ rec s =13 TeV rec ≥ 10 N <30 N 120 ch ch ) 1.1 ) φ φ 1.02 ∆ ∆ 1.05 , , η η ∆ ∆ 1 1 C( C( 0.95 0.98 4 4 -4 -4 -2 -2 2 2 ∆ ∆ φ φ 0 0 η η ∆ ∆ 0 0 2 2 4 4 Dominant structure is the dijet system with “this” jet around ( 0 , 0 ) and the “other” jet’s far ridge in ∆ η at ∆ φ ∼ π . Near-side ridge at ∆ φ ∼ 0. 4/9

Correlation observables 2: per-particle yields To focus on long-range ridge Results: effects, integrate over large ) ) φ | ∆ η | to define S , B , and C ATLAS (a) φ ∆ φ (b) Y( ) ∆ ∆ Y( Y( periph 0.7 ∆ η ∆ φ 2.0<| |<5.0 FY ( )+G 2.45 s =13 TeV a,b ridge periph 0.5<p <5.0 GeV Y + FY (0) functions in ∆ φ only, e.g. T s =2.76 TeV templ ∆ φ Y ( ) 2.4 0.6 s =13 TeV � 5 ≤ 0 N rec <20 ch ≤ rec 40 N <50 ch 2.35 S (∆ φ ) = d | ∆ η | S (∆ η, ∆ φ ) 0.5 2.3 2 ) ) φ 0 2 4 φ 0 2 4 ∆ 3.1 (c) ∆ (d) Y( Y( 3.6 3.05 Useful to convert to a 3.55 3 s =2.76 TeV s =13 TeV 3.5 per-particle correlation yield: ≤ rec ≤ rec 50 N <60 60 N <70 ch ch 2.95 3.45 � B (∆ φ ) d ∆ φ 2.9 3.4 Y (∆ φ ) = C (∆ φ ) · ) ) φ 0 2 4 φ 0 2 4 ∆ ∆ 5.7 (e) (f) π N a Y( Y( 4.2 = S (∆ φ ) � 5.6 s =2.76 TeV s =13 TeV π N a ≤ rec rec ≥ 4.1 70 N <80 N 90 ˆ ch ch B (∆ φ ) 5.5 4 0 2 4 0 2 4 Y measures the average number ∆ φ ∆ φ of long-range correlation Increasing modulation with N ch fills in the partners per “trigger” particle a near-side minimum ⇒ produces the ridge, at a given ∆ φ . and narrows + heightens the far-side peak. 5/9

Interpretation: yield fits Results: In p + Pb collisions, the ridge results from sinusoidal global ) ) φ φ ∆ φ ATLAS (a) Y( ) (b) ∆ ∆ Y( 0.7 ∆ η Y( periph ∆ φ 2.0<| |<5.0 FY ( )+G modulation of single-particle 2.45 s =13 TeV a,b ridge periph 0.5<p <5.0 GeV Y + FY (0) T s =2.76 TeV templ ∆ φ Y ( ) azimuthal angle distributions. 2.4 0.6 s =13 TeV ≤ rec 0 N <20 ch ≤ rec 40 N <50 ch 2.35 This new study uses template 0.5 2.3 ) ) 0 2 4 0 2 4 φ φ fitting of Y to investigate whether 3.1 (c) (d) ∆ ∆ Y( Y( 3.6 3.05 the pp ridge has the same origin: 3.55 3 s =2.76 TeV s =13 TeV 3.5 ≤ rec ≤ rec Y templ (∆ φ ) = FY periph (∆ φ ) + Y ridge (∆ φ ) 50 N <60 60 N <70 ch ch 2.95 3.45 2.9 where 3.4 ) ) 0 2 4 0 2 4 φ φ ∆ (e) ∆ 5.7 (f) Y( Y( Y ridge (∆ φ ) = G [ 1 + 2 v 2 , 2 cos ( 2 ∆ φ )] . 4.2 5.6 s =2.76 TeV s =13 TeV ≤ rec rec ≥ 4.1 70 N <80 N 90 ch ch 5.5 G is fixed by template 4 normalisation = data; 0 2 4 0 2 4 ∆ φ ∆ φ Y periph taken from Y periph and Y ridge as open points and blue lowest-multiplicity data bin; line respectively; Y templ in red fits several F and v 2 , 2 free parameters for χ 2 fit. data features with 2 params on one sinusoid. 6/9

Interpretation: testing single particle modulation If the ridge is formed by sinusoidal 2,2 modulation of individual particle ATLAS s =2.76 TeV s =13 TeV v 0.006 0.006 production, then v 2 , 2 should 0.004 0.004 factorise: 0.002 0.002 v 2 , 2 ( p a T , p b T ) = v 2 ( p a T ) v 2 ( p b T ) . 20 40 60 80 100 20 40 60 80 100 120 2 0.1 s =2.76 TeV 0.1 s =13 TeV v Tested for 3 p b T bins vs track 0.05 0.05 b 0.5<p <5.0 GeV multiplicity N rec T ch . Extract v 2 from ∆ η 2.0<| |<5.0 b 0.5<p <1.0 GeV T a 0.5<p <5.0 GeV b 2.0<p <3.0 GeV T T combinations of p a , b T bins in v 2 , 2 : 20 40 60 80 100 20 40 60 80 100 120 rec rec N N ch ch � 2 ≤ rec 0.1 ATLAS 50 N <60 0.1 s =13 TeV v ch v 2 ( p T 1 ) = v 2 , 2 ( p T 1 , p T 2 ) / v 2 , 2 ( p T 2 , p T 2 ) . 0.05 0.05 ≤ rec s =2.76 TeV 40 N <50 Results: top row shows fitted v 2 , 2 , ch ∆ η ≤ rec 2.0<| |<5.0 s =13 TeV 70 N <80 ch b 0.5<p <5.0 GeV rec ≥ N 100 middle shows v 2 . Latter shows T ch 0 1 2 3 4 1 2 3 4 p a [GeV] p a [GeV] clear agreement between p b T and T T Extraction of v 2 ( p a T ) showing stability substantial independence of multiplicity at both √ s . Bottom vs p b T bin and N rec ch . row shows p T dependence of v 2 . 7/9

Interpretation: modulation strength vs. N rec ch Relative size of ridge modulation ∼ Gv 2 , 2 / FY periph ( 0 ) ⇒ study G and F vs N rec ch : ch dependence of Y periph extraction by Different datasets study N rec subdividing the N periph ∈ [ 0 , 20 ] range. ch v 2 , 2 fairly stable with N rec ch + linear growth of G + flattening of F ⇒ increase in ridge visibility with N rec ch 8/9

Interpretation: modulation strength vs. N rec ch Relative size of ridge modulation ∼ Gv 2 , 2 / FY periph ( 0 ) ⇒ study G and F vs N rec ch : ch dependence of Y periph extraction by Different datasets study N rec subdividing the N periph ∈ [ 0 , 20 ] range. ch v 2 , 2 fairly stable with N rec ch + linear growth of G + flattening of F ⇒ increase in ridge visibility with N rec ch 8/9

Summary ◮ The near-side ridge is still a major puzzle in LHC physics. Still there in high-multiplicity 13 & 2.76 TeV pp events! ◮ A new fit of the ∆ φ modulation in these pp events shows excellent consistency with a single 2 ∆ φ Fourier mode, which not only produces the near side ridge but also beneficially modifies the away side peak with increasing N rec ch . ◮ Comparison of the v 2 , 2 Fourier coefficient in ∆ φ yields between p T bins of trigger and partner tracks reveals single-particle modulation coefficients, v 2 , independent of N rec ch and p b T : consistent with azimuthal modulation of individual particle production. ◮ It hence appears that the pp and p + Pb ridge phenomena have the same source.. . but exactly what that is remains to be seen! 9/9