BBeauty Production with ALICE at the LHC
Erin F. Gauger for the ALICE Collaboration
The University of Texas at Austin, Austin TX 78712, USA, [email protected]
Abstract.
In this manuscript, various beauty production measurementsusing the ALICE detector will be presented. We will show new measure-ments of non-prompt D mesons in pp collisions at √ s = 5 .
02 TeV andbeauty-tagged jet production in p–Pb collisions at √ s NN = 5 .
02 TeV.The R AA of beauty-hadron decay electrons in central Pb–Pb collisionsand the v of beauty-hadron decay electrons in semi-central Pb–Pb col-lisions at √ s NN = 5 .
02 TeV will also be discussed.
Keywords: heavy quarks, beauty quarks, nuclear modification factor
In hadronic collisions, beauty quarks are produced early via hard-scattering pro-cesses with large momentum transfer. Because of this early production, thebeauty quark is an excellent probe of the Quark-Gluon Plasma (QGP) [1,2]formed in heavy-ion collisions. Traveling through the QGP, beauty quarks in-teract with other partons via collisional and radiative processes and lose energy.The beauty quark ( m b (cid:39) .
18 GeV/ c [4]) is expected to lose less energy thanlighter quarks, since collisional processes depend on the mass of the particleand the dead cone effect [3] would hamper radiative energy loss. Since charm( m c (cid:39) .
27 GeV /c [4]) and beauty quarks are both produced early in the col-lision but have different masses, it is useful to compare beauty and charm mea-surements to test our understanding of mass-dependent energy loss in heavy-ioncollisions.Measurements of beauty production in pp collisions are important to testperturbative QCD (pQCD) calculations, as well as to provide the baseline forPb–Pb measurements. In p–Pb collisions, beauty-production measurements arecrucial to isolate initial-state and cold nuclear matter effects, both of whichwould be present in Pb–Pb measurements. In ALICE, beauty production is measured via beauty-hadron decay electrons,non-prompt D mesons, and beauty-tagged jets. All three measurements relyon the excellent vertex reconstruction and impact parameter resolution of theALICE detector to isolate the decay particles and jets from beauty decay. a r X i v : . [ nu c l - e x ] A p r E. Gauger for the ALICE Collaboration
Roughly 10% of beauty hadrons decay directly into electronic final states (e.g.B − → e+X), and another 10% decay to charm hadrons which further decay toelectrons (e.g. B − → D → e+X) [4]. This high branching ratio coupled withthe excellent electron identification of the ALICE detector makes it convenientto study beauty quarks by measuring the production of beauty-hadron decayelectrons. Beauty-hadron decay electrons have been measured in pp, p–Pb, andPb–Pb collisions [5,6,7].Electrons from beauty-hadron decays must be separated from other sourcesof electrons, such as photon conversion, Dalitz decays, and charm-hadron de-cays. This separation is achieved by exploiting the relatively long decay length ofbeauty hadrons ( τ B ≈ µ m /c ) versus non-beauty hadrons (e.g. τ D = 60–300 µ m /c ) [4]. The long decay length gives beauty-hadron decay elec-trons a longer distance of closest approach to the primary vertex ( d ), thus mak-ing the beauty-electron d distribution much wider than that from other sources(Fig. 1, bottom left). The difference in d shape allows us to fit the electron d distribution with templates from Monte Carlo simulations in order to extract thebeauty-hadron decay electron yield. Once the yields are obtained in both pp andPb–Pb collisions, the nuclear modification factor R AA = d N AA / d p T T AA ∗ d σ pp / d p T can becalculated. The pp reference spectrum for the beauty-hadron decay electron R AA measurement in Pb-Pb at √ s NN = 5 .
02 TeV was obtained by a pQCD-drivenscaling of the cross section measured at √ s = 7 TeV [5]. In order to obtain a v measurement, the yield is measured for electrons that lie both in and out of theevent plane, and the v is calculated according to v = R π N in − plane − N out − of − plane N in − plane + N out − of − plane ,where N refers to the number of electrons measured, and R is the resolution cor-rection for the event plane.Two beauty-electron measurements in ALICE are R AA in 0–10% and v in 20–40% central Pb–Pb collisions at √ s NN = 5 .
02 TeV. In Fig. 1, top left,the beauty-electron R AA is shown along with the R AA of heavy-flavor decayelectrons (from both charm and beauty decays). Though the systematic errorbars are large, a hint of an increased R AA of beauty-hadron decay electrons whencompared with heavy-flavor electrons at low- p T is observed. This is consistentwith our expectations of the mass dependence of energy loss in the QGP medium.At high p T , the two distributions overlap, in part because at higher momentum,the heavy-flavor electron sample becomes dominated by beauty-hadron decayelectrons. Fig. 1 (top right) shows a comparison of the R AA of beauty-hadrondecay electrons with models that include both collisional and radiative energyloss. We see that the theoretical models are in good agreement with data.Finally, the v of beauty-hadron decay electrons is shown in Fig. 1 (bottomright). The v is non-zero; in fact, between 1 . < p T < c , the significanceof the measurement for a positive v is 3.49 σ . This hints that the beauty quarkmay participate in the collective behavior of the medium. The v measurementof heavy-flavor decay electrons [8], also shown in Fig. 1, is similar to that ofelectrons from beauty decays. eauty production with ALICE 3 ) c (GeV/ T p AA R e → c, b | < 0.8 η | | < 0.6 η | e → c) → b ( | < 0.8 η | | < 0.6 η | ALICE Preliminary = 5.02 TeV NN s ALI−PREL−308490 ) c (GeV/ T p AA R MC@sHQ+EPOS2PHSDDjordjevic e → c) → b ( | < 0.8 η | | < 0.6 η | ALICE Preliminary = 5.02 TeV NN s ALI−PREL−308498ALI-PREL-319387 ALI-PREL-319441
Fig. 1.
Various results of beauty-hadron decay electrons in ALICE. The top two panelsshow the R AA in 0–10% Pb–Pb collisions. The bottom-left panel shows an example ofa MC template fit of the d of electrons to extract those from beauty-decay. Thebottom-right panel shows the v of beauty-hadron decay electrons in 20–40% Pb–Pbcollsions. mesons With the ALICE detector, beauty production is also studied by measuring non-prompt D mesons from beauty-hadron decays. The measurement is performedin pp collisions at √ s = 5.02 TeV. The non-prompt D mesons (along with theircharge conjugates) are reconstructed via the decay channel to K − π + (branchingratio ∼ . mesons ( f non-prompt ) in oursample. At low- p T , f non-prompt almost reaches 95%, an unprecedented purity forthis measurement (see Fig. 2). The cross-section of non-prompt D mesons iscompared with FONLL [10] predictions (pQCD) in Fig. 2. As in previous mea-surements [9], the two are in agreement, though the measurement lies on theupper edge of the FONLL uncertainty band. E. Gauger for the ALICE Collaboration ) c (GeV/ T p non - p r o m p t f = 5.02 TeV s pp, ALICE Preliminary ALI−PREL−319757 ) c - ) ( pb G e V y d T p / ( d σ d ALICE Preliminary = 5.02 TeV s pp, ± ± |<0.5 y from b hadrons, | D DataFONLL ) c (GeV/ T p F O N LL D a t a ALI−PREL−319648
Fig. 2.
Left: the fraction of non-prompt to inclusive D mesons in the sample. Right:The cross section of non-prompt D mesons in pp collisions at √ s = 5.02 TeV comparedto FONLL [10]. A more direct access to the initial parton kinematics is obtained by measuringbeauty-tagged jets. This has been done for the first time in ALICE in p–Pbcollisions at √ s NN = 5 .
02 TeV. Jets are selected using the anti- k T algorithm[11], and a resolution parameter of R = 0.4. To achieve a high purity of b-jetsin the sample, the long lifetime of beauty hadrons is exploited once more. Jetsthat contain a three-pronged secondary vertex are selected, and a number oftopological requirements are applied to increase the b-jet purity. In particular,a cut is applied on the displacement significance ( SL xy >
7) of the secondaryvertex. The SL xy is defined as the distance between primary and secondaryvertex in the xy -plane divided by the resolution of that distance. The resultsof this measurement are shown in Fig. 3, where the estimated purity ( ∼ In this manuscript the ALICE results on beauty production were discussed withparticular focus on beauty-hadron decay electrons in Pb–Pb collisions, non-prompt D mesons in pp collisions, and beauty-tagged jets in p–Pb collisions. Allthree analyses took advantage of the long lifetime of beauty hadrons to separatethe beauty signal from background sources. The non-prompt D meson measure-ment agrees with FONLL predictions, and the beauty-tagged jet measurements eauty production with ALICE 5
10 20 30 40 50 60 70 80 90 100 / G e V ) c ) ( m b η d c h T , j e t p / ( d σ d − − − − − ALICE Preliminary = 5.02 TeV NN s Pb − p = 0.4 R , T k charged b-jets, anti- ) c (GeV/ chT,jet p
10 20 30 40 50 60 70 80 90 100 R a t i o t o da t a Datasystematic uncertaintyPOWHEG HVQPOWHEG systematic uncertainty
ALI−PREL−323633
10 20 30 40 50 60 70 80 90 100 / G e V ) c ) ( m b η d c h T , j e t p / ( d σ d − − − − − ALICE Preliminary = 5.02 TeV NN s Pb − p = 0.4 R , T k charged b-jets, anti- ) c (GeV/ chT,jet p
10 20 30 40 50 60 70 80 90 100 R a t i o t o da t a Datasystematic uncertaintyPOWHEG dijet EPPS16POWHEG systematic uncertainty
ALI−PREL−323637 ) c (GeV/ ch, recoT, jet p
10 20 30 40 50 60 70 80 90 100 b - j e t pu r i t y b-tagged charged jets| < 0.5 labjet η = 0.4, | R , T k anti- > 7 xy L σ / xy L < 0.03 cm SV σ Data-driven evaluationPOWHEG-based evaluationSystematics of the POWHEG-based evaluation
ALICE Preliminary = 5.02 TeV NN s Pb − p ALI−PREL−323641
Fig. 3.
Top left and right: the cross-section of beauty-tagged jets in p–Pb collisionscompared with POWHEG. Bottom: the purity of beauty jets in the sample. agree with POWHEG models. In Pb–Pb collisions, we see a hint of the mass-dependent energy loss in the QGP, as well as a non-zero v for beauty-hadrondecay electrons. Acknowledgements:
This work was supported by U.S. Department of EnergyOffice of Science under contract number de–sc0013391.
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