The ATLAS b-jet Trigger

Not

revi

ewed

,for

inte

rnal

circ

ulat

ion

only

For approval

(Offline JetProb)10

-Log0 1 2 3 4 5 6 7 8 9 10

Entri

es

-110

1

10

210

310

410

510ATLAS Preliminary

=7TeVs QCD MC

Data

Data with Loose cut at trigger level

Data with Medium cut at trigger level

Data with Tight cut at trigger level

Offline JetProb distribution in data and simulation and the same

distribution in data when a b-jet requirement is added at the

trigger level (both Level 2 and Event Filter). The three working

points correspond to 90%, 70% and 50% b-tagging efficiency on

a b-tagged jet sample using top MC simulation.

Andrea Coccaro for the b-jet trigger signature group 5

July, 2011

Not

revi

ewed

,for

inte

rnal

circ

ulat

ion

only

For approval

[GeV/c]T

Track p0 5 10 15 20 25 30

Entri

es

10

210

310

410


=7TeVsb-jetc-jetlight jetData 2011

Track transverse momentum at the Event Filterlevel. Tracks are reconstructed starting from alow pT jet identified by the Level 1 and are re-quested to fulfill online b-tagging criteria.


July, 2011

Not

revi

ewed

,for

inte

rnal

circ

ulat

ion

only

For approval

Track multiplicity0 5 10 15 20 25 30

Entri

es

1

10

210

310

410

510



Track multiplicity at the Event Filter level.Tracks are reconstructed starting from a low pTjet identified by the Level 1 and are requested tofulfill online b-tagging criteria.


July, 2011

Not

revi

ewed

,for

inte

rnal

circ

ulat

ion

only

For approval

Signed transverse IP significance-10 -5 0 5 10

Entri

es

1

10

210

310



Signed transverse impact parameter significanceof reconstructed tracks at the Event Filter level.Tracks are reconstructed starting from a low pTjet identified by the Level 1 and are requested tofulfill online b-tagging criteria.


July, 2011

The HLT uses the reconstructed tracks information to discriminate b-jets from light jets. b-tagging relies on the relatively long lifetime of B hadrons formed in the hadronisation of b-quarks which allow the B hadron to travel 0.45 mm in its rest frame before it decays.The algorithms exploit this by identifying tracks from the B hadron decay which are displaced from the primary interaction vertex and thus indicate that a long-lived particle was present. Two algorithms based on reconstructing the distance of closest approach, or Impact Parameter (IP), of tracks in both the longitudinal (z0) and transverse (d0) direction with respect to the beam line are used. The d0and its error depend on the beam spot estimate, so the HLT b-tagging relies on a good estimate of the beam spot width and position.

I. Likelihood taggerA likelihood ratio W is used.

W =tracks�

i=1

signal(θi)

background(θi) S(z0) =

z0 − zpvσ(z0)

S(d0) =d0

σ(d0) II. JetProb tagger

A resolution function R(x) is fitted to S(d0) in order to compute the probability that each track comes from the primary vertex:

Ptrack =

� −|S(d0)|

−∞R(x)dx

A corresponding jet probability is computed from the individual track probabilities giving an estimate of the probability of the jet coming from the primary vertex. For b-jets, this probability is expected to be close to zero. Note the dependence of this tagger on the beam spot estimate at the plot above.

• From 2011, the ATLAS b-jet trigger is active and selecting events enriched in heavy flavor.• For the first months of data taking the JetProb b-tagging algorithm is used with other more advanced taggers under commissioning.• One of the main developments currently pursued are data-driven measurements of the online and offline b-tagging efficiencies.

LVL2 Jet probability to originate from PV0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Nor

mal

ised

eve

nts

-210

-110

ATLAS = 7 TeVs

Preliminary

Run 160472 / 160530before HLT beam spot updateafter HLT beam spot update

Due to the high event rate and low cross section of the processes of interest, ATLAS developed a three-tiered trigger system to select events to be recorded.• Level 1 is hardware-based and uses the calorimeters and the muon spectrometer with coarse granularity. It selects regions of interest, which are further analysed at Level 2 and Event Filter.• Level 2 and Event Filter are implemented in software and have a longer time to select events and have access to more detailed information.• The Event Filter has ~ 4 s to take the decision, while Level 2 has ~ 40 ms, allowing a more detailed analysis at the Event Filter.The Level 2 and Event Filter are referred to as the High Level Trigger (HLT).

The Large Hadron C o l l i d e r ( L H C ) collides protons at 7 Te V c e n t e r - o f -momentum energy. T h e A T L A S e x p e r i m e n t i s a g e n e r a l p u r p o s e experiment at the LHC, that measures physics observables of the final state

particles arising from the LHC collisions.

The ATLAS b-jet TriggerDanilo Enoque Ferreira de Lima (University of Glasgow)

On behalf of the ATLAS Collaboration

ATLAS Trigger

b-jet Trigger Selection

The ATLAS experiment

Data/MC Comparison

Outlook

The plots show the signed transverse impact parameter, the selected track multiplicity and transverse momenta at the Event Filter, for simulated events and data collected in 2011.

The selected tracks are used to ca lcu late the s igned impact parameters, which are, in turn, used in the JetProb variable.

Trigger MenuThe b-jet triggers enhances the ATLAS physics potential selecting events with final states containing several b-jets.Various analyses are currently based on b-jet triggers:

• tt̄ analyses with fully hadronic top quark decays• SUSY with 0 leptons• Searches for Higgs with Higgs decaying into bb̄

In order to study its performance and measure efficiencies, a set of unbiased triggers exist.

Effect of HLT on Offline b-taggingThe offline JetProb variable is used during analysis on acquired data to tag b-jets, applying a cut to select jets with -log(JetProb) greater than a value.The effect of the b-jet trigger selection at the Event Filter is shown on data.Three working points of the HLT b-tagging are shown in the plot: “loose”, “medium” and “tight”.

1

2

3

4

5

6

7

Documents

The ATLAS b-jet Trigger