Analysis of b-jet production in p–Pband pp collisions at √sNN= 5 TeV
with ALICE
Artem Isakov, NPI CAS
for the ALICE Collaboration

12th Lomonosov conferenceNuclear modification factor
Quark-gluon plasma (QGP) is created in heavy-ion (HI) collisions:

Signatures of QGP:
● Collective flow → QGP acts like nearly-perfect liquid● Jet quenching → QGP slows penetrating patrons

Nuclear modification factor compares particle yields in HI and in pp collisions scaled by
the number of binary NN collisions (Ncoll):d N AAd pT
R AA =d N pp
⟨ N coll ⟩d pT

Different collision systems:

RpA ≠ 1 → presence of cold nuclear matter (CNM) effects● R< 1 → indication of final state effects (medium energy loss)
AA●12th Lomonosov conference

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b jets
Inclusive b jets in p-Pb at√sNN = 5.02 TeV, full jets[*]

Properties of b-quark:● large mass (4.62 GeV/c2) → it can be created

only in initial hard scatterings. Its production
rate can be calculated from pQCD
● long lifetime → it survives through the wholeevolution of QGP“Dead cone”
θdead cone effect – gluon radiation from massive
quarks is suppressed at angles θ < m/E
ALICE has excellent capabilities at low-pT[*] CMS, CERN-PH-EP/2013-037

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ALICE experiment

Time Projection Chamber● Track reconstruction● Particle identification via
specific energy loss

Inner Tracking System● Track reconstruction● Primary and secondary
vertex reconstruction|ηtrack| < 0.9● Full azimuth● 0.5 T solenoid● p> 0.15 GeV/cT, track●V0 - scintillator arrays located at forward
and backward pseudorapidity fortriggering and centrality determination
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b-jet tagging via impact parameter
Two independent methods were used for b-jet tagging:1) Impact parameter (IP) - distance of closest approach of jet constituents to primary vertex.
2) Secondary vertex (SV) - properties of most displaced 3-prong secondary vertex.

b-jet candidate selection with IP method:● Impact parameter significance, sdxy = δ·dxy / σdxy, iscalculated for each track in the jet:
δ – impact parameter signdxy – 2D impact parameterσdxy – uncertainty of dXY measurement● sdxy of the tracks are sorted in descending order● b jets are tagged by imposing the criteria:
2th most displaced track,tresholdd xy >d xy , threshold parameter varied:

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Isakov Artem5b-jet tagging via secondary vertex
Two independent methods were used for b-jet tagging
1) Impact parameter - distance of closest approach of jet constituents to primary vertex.
2) Secondary vertex (SV) - properties of most displaced 3-prong secondary vertex.

b-jet candidate selection with SV method:Jet● 3 prong SV is made out of jet constituents● In each event we consider the most displaced SV➔

Secondaryvertex (SV)Minimal significance of the SV displacement: SLxy = LXY/σLxy
LXY – distance between primary and secondary vertices
σLxy – uncertainty of LXY measurement√∑ d 2ii=1

di – distance of closest approach (DCA) of i-th prong to the SVxy

L ±σLx y● Upper limit on the SV resolution: σSV =3Primaryvertex

Default SV cut: σSV < 0.03 cm, LXY/σLxy > 7

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Corrections of b-jet spectra
The obtained spectrum of b-jet candidates needs to be corrected to account for purity andefficiency of b-jet tagging:dN bprimarydN raw
P- jet= b - jet candidates × ε bb
dpT , jet ch
dpT , jet chεb − the probability that true b jet will pass SV tagging selections (efficiency)
Pb − the fraction of true b jets among all tagged b-jet candidates (purity)b-jet tagging efficiency is estimated from
EPOS+PYTHIA detector-level simulation:εb =N selectedb - jets
N ball- jets

allN b - jets − the number of b-jets without any

constraint on parameters of the SV
selectedN b - jets − the number of b-jets that werereconstructed after the SV cuts

b-jet tagging efficiency in p-Pb for the default SV cuts: εb ≈ 35 %, εc ≈ 11 %, εLF ≈ 1 %12th Lomonosov conference

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Data driven method for b-jet purity
The purity of the b-jet candidate was estimated by hybrid method that utilizes data-driven
template fitting and POWHEG simulations:
The data driven method describes themeasured distribution of invariant mass ofSV as a linear combination of MCtemplates:n SV =P b⋅T b +P c⋅T c + P LF⋅T LF
1=P b +P c + P LF
nSV − measured SV invariant mass
Tb , Tc , TLF − MC template for each jet flavor
Pb , Pc , PLF − purity for each jet flavor

The template fitting method fails for jets with momenta larger than 30–40 GeV/c

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POWHEG based method for b-jet purity
POWHEG method - based on b and c-jets spectra calculated by Next-to-Leading Order (NLO)
POWHEG generator:

P b=

N b εbN b εb + N c εc +N LF ε LF

Nb , Nc − POWHEG pT spectrum of b and c-jets
folded with the response matrix
NLF− pT spectrum of light flavour-jets (LF):

N LF =N raw −N b −N c
Nraw − raw pT spectrum of inclusive jetsεb, εc, εLF − efficiency of SV tagging for b, c and
LF jets

Different POWHEG settings were tested against the template fit results to find plausible
POWHEG settings (regularization and renormalization scale)12th Lomonosov conference

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9The differential production cross section

ppp-Pb

The measurement of b-jet the production cross section agrees with POWHEG calculations

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The measurement of b-jet fraction in pp collisions

The measured b-jet fraction is consistent with the POWHEG predictions within theuncertainties.

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Measurements of charged b-jet RpPb

No strong CNM effects present in p–Pb.
CMS measured full anti-kT R=0.3 b-jets within −2.5 < ηjet< 1.5.

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Summary●●●Results of differential production cross-section and b-jet fraction in pp collisions
are compatible with POWHEG simulations
Within the current precision the ALICE measurement of charged b-jet RpPb does
not show to be affected by cold nuclear matter effects.
The ALICE measurement of charged b-jet RpPb is compatible with the analogous
CMS measurements for full-jets.

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Backup12th Lomonosov conference

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Bjet Analysis settings
Event selection● Minimum bias trigger (V0 scintillator arrays)● |zvtx| < 10 cm● Pileup rejection
After event selection we have 6 · 108 minimum bias events (pPb)
9.2 · 108 minimum bias events (pp)Track selection● |ηtrack| < 0.9
Background density correction:
Jet selection:● Charged-particle anti-kT, R=0.4 Two leading kT jets are excluded● pT recombination scheme
correctedRAW
p=pT , charged jet
T , charged jet −ρ⋅A jet● pT, constituent > 0.15 GeV/c● |ηjet| < 0.5
A physical jets
pTch,,jetraw

ρ=

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A all jets

×median k

T physical jet

Isakov Artem{A jet15}