ALICE Results on Jets and Photons

Transcription

1 ALICE Results on Jets and Photons Yue Shi Lai Miami2018 December 15, 2018 (Yue Shi Lai) December 15, / 17

2 Introduction Jets: Remnants from surviving quark and gluons, after interaction with the plasma, produced plentifully Asymptotic freedom causes flux tube to rip and hadronizes into a spray of particles I will talk about results with Substructure (color coherence when forming jets) Identified deuterons (do hadrons coalesce into composite particles?) (Yue Shi Lai) December 15, / 17

3 Inclusive Jets/RAA RAA = rate relative to superposition (of independent nucleons colliding) If the medium is linear: RAA measures the opacity (ignoring whether there is a wavelength shift or not) Result marginally consistent with several models (models tend to be tuned not too far off RAA ) plead,ch behaves like a quark T discriminator, and selects a subset of jets (otherwise combinatorial fake jets below 100 GeV) (Yue Shi Lai) December 15, / 17

4 N-subjetiness Thaler, Van Tilburg (JHEP 1103:015, 2011) P k N candidate subjets: p T,k minf R 1,k, R 2,k,..., R N,k g P k p T,kR 0 R 0 the characteristic radius (e.g. the dashed circle below) τ N 0: everything lines up, N or fewer subjets (open circle symbols) τ N 0: energy away from N subjets, at least N + 1 subjets J. Thaler, K. Van Tilburg, JHEP 1103:015 (2011) (Yue Shi Lai) December 15, / 17

5 N-subjetiness pp p s = 13 TeV Pb-Pb p s = 2.76 TeV Recoil jet against a high-pt TT = trigger track only Cambridge-Aachen deconstruction Interpretation of the observed difference is difficult. Note that the difference can be in the trigger track (jet fragmentation) itself. (Yue Shi Lai) December 15, / 17

6 N-subjetiness pp p s = 13 TeV Pb-Pb p s = 2.76 TeV Modest amount of difference also seen in kt deconstruction p Comparison at the same snn is difficult, because ALICE cannot take low energy pp (at heavy ion energies) with peak luminosity (slow TPC readout) (Yue Shi Lai) December 15, / 17

7 SoftDrop Shared Momentum Fraction Larkoski, Marzani, Thaler (Phys. Rev. D 91, , 2015) β Rij SoftDrop condition min.p T,i, p T,j / > z cut p T,i + p T,j R 0 Default z cut = 0.1, β = 0 p T,2 z g = if the SoftDrop condition is satisfied p T,1 + p T,2 Found to behave like the splitting function in pp (and dangerous to assume for Pb-Pb) A. Larkoski et al. Phys. Rev. Lett. 119, (2017) (Yue Shi Lai) December 15, / 17

8 SoftDrop Shared Momentum Fraction In Pb-Pb track only, a small ( 30%) modification data/pythia at the highest z g was observed Notice unlike CMS, log scale and not self normalized No net enhancement (without self-normalization and inclusive/no minimum opening angle) (Yue Shi Lai) December 15, / 17

9 SoftDrop Shared Momentum Fraction In pp (but at p s = 13 TeV) the data/pythia is increasing with zg Systematics is different due to this being track + EM clusters, and fully corrected to particle level (Yue Shi Lai) December 15, / 17

10 Deuterons in Jets Deuteron production in alignment with the dijet geometry Hadrons do coalesce into composite particles (Yue Shi Lai) December 15, / 17

11 Prompt Photon Prompt photons: Rare (suppressed by α EM ) kinematic tags for strong interacting with the plasma (Non-isolated) low-p T prompt photon spectra in pp and p-pb Prompt photon azimuthal anisotropy/elliptic flow in Pb-Pb Isolated prompt photon + hadron correlation P. Stankus, Ann. Rev. Nucl. Part. Sci. 55, 517 (2005) (Yue Shi Lai) December 15, / 17

12 Low p T Prompt Photon arxiv: (submitted to PRC) Inclusive prompt photons are measured by first measuring all possible hadron yield, and determine decay contribution (Yue Shi Lai) December 15, / 17

13 Low p T Prompt Photon R γ = γ incl,meas πsim 0 πmeas 0 γ incl,meas γ decay,sim γ decay,meas Only p T > 8 GeV really accessible (Yue Shi Lai) December 15, / 17

14 Prompt Photon v 2 arxiv: (submitted to PLB) Similar technology as before v 2 is the relative strength of the 2nd order Fourier component (over the 1st) how a particle is produced (v 2 = 0 if isotopic) At low p T, prompt photons are thermal emissions, so finite v 2 expected due to finite impact parameter (Yue Shi Lai) December 15, / 17

15 Isolated Photon + Hadron Photon acts like the back-scattered electron in deep inelastic scattering Measurement without jets, determine yield by integrating over correlated hadrons (a leading order approximation) Photon 20 GeV interesting to study partons interacting with the plasma, but difficult for ATLAS/CMS due to radiation length in front of the ECAL (Yue Shi Lai) December 15, / 17

16 Isolated Photon + Hadron Measurement is baseline for future Pb-Pb measurement Innovative usage of ALICE Si tracking only + calorimeter ( CALOFAST ), and calorimetric triggers p This is also why it is the only pp/p-pb measurement at snn = 5 TeV I am showing (Yue Shi Lai) December 15, / 17

17 Conclusion Many measurement, particularly with jets/photons in the low energy range of 20 GeV, where ATLAS/CMS material budget (and field strength) makes measurement difficult Plasma modification of jets is only a modest effect, sitting above the large variability of (no plasma) QCD processes Result from new ALICE CALOFAST mode for rare, high p T processes (Yue Shi Lai) December 15, / 17