Particle Flow

The Particle Flow algorithm applies selection criteria to tracks and calorimeters clusters before they enter as inputs the jet clustering algorithm. The Particle Flow steps are the following:

1. requirements are applied to tracks, which are selected as “charged particles”; 2. requirements are applied to calorimeter clusters not associated to tracks. The clusters

selected in this way are called “isolated neutral particles”;

3. requirements are applied to calorimeter clusters associated to at least one track. The estimated energy released by the tracks in the calorimeters is subtracted from the associated cluster energy. The resulting object is called “not-isolated neutral particle”.

In the following the treatment of charged particles, isolated neutral particles and not- isolated neutral particles is explained.

Charged particles

Different selection requirements are applied to tracks depending on the category they belong (long, downstream, upstream, VELO tracks, see Section 2.3). This is done to ensure the quality of those entering the jet clustering step. The tracks selection is performed applying requirements to the following observables:

• the track transverse momentum (pT);

• the χ2 _{obtained from the track Kalman fit;}

• Pghost, defined as the probability for a track to be an artifact of the pattern recognition,

not associated to a real particle. It is computed as an output of a neural network [63] that has in inputs the result of the track fit and the track kinematic;

• The momentum resolution, σ(q/p)/(q/p), where q is the particle charge and p the particle momentum.

The list of tracks requirements is displayed in table 3.2 for the four track categories.

Table 3.1: List of requirements applied to tracks by the Particle Flow algorithm. The requirements depend on the track category.

long downstream upstream VELO

pT [MeV/c] - - > 100 -

χ2 _{< 5 < 1.5 < 1.5 < 10}

Pghost < 0.5 - - -

σ(q/p)(q/p) > 10 > 10 > 2 -

VELO tracks do not contain information on the particle momentum, but they provide information on the the Primary Vertex associated to the jet, therefore they are included in the jet clustering. A particle type and mass is also assigned to each track depending on the particle identification decision. The particle type categories are p/¯p, π±_{, µ}±_{, e}± _and

K±_.

Isolated neutral particles

In this step calorimeters clusters not associated to tracks are stored as inputs for the jets clustering.

Neutral particles detected by the ECAL are mainly photons and π0 _{decayed into two}

photons. Their identification relies on the shape of the calorimeter clusters, therefore a likelihood for the photon or π0 _{hypotheses is computed. The π}0 _{are divided into two}

categories depending on how they are detected: merged π0_{, when the two photons are}

almost collinear and produce a single cluster, and resolved π0_{, where the two photons}

are detected as two separate clusters. Requirements are applied to select ECAL clusters isolated from the tracks and to ensure their quality before entering the jet clustering step. The following observables are considered:

• the likelihood for the photon hypothesis (PhotonID); • the cluster transverse energy ET;

• a χ2 _{for each track-cluster combination (χ}2

track−cluster): it evaluates how much the

cluster is likely to be originated by the particle associated to the track.

The list of the requirements applied in the ECAL clusters selection is reported in table 3.2 It can be noticed that different conditions are applied to photon clusters associated to T tracks, which are defined as tracks that have hits only in T1-T2-T3 stations.

Table 3.2: List of requirements applied to ECAL clusters identified as photons or π0.

merged π0 _{resolved π}0 _photons

ET [MeV/c] - - > 200

PhotonID - _{> −4 > −1 (> −2 with T track)}

PhotonID for 1 γ - _{> −2} -

χ2

track−cluster > 25 > 25 > 25 (> 16 with T track)

The selection of isolated HCAL clusters is performed by applying different χ2

track−cluster

requirements for different cluster energy thresholds: a χ2

track−cluster greater than 25 is

required for HCAL clusters with energies below 10 GeV while a χ2

track−cluster greater than

15 is required for energies above 10 GeV. No particle identification requirements are applied on the HCAL clusters selection.

Non-isolated neutral particles

The selection of calorimeter clusters not isolated from tracks works as follow: 1. ECAL clusters with χ2

track−cluster below 25 and HCAL clusters with χ2track−cluster

below 25(16) for energies below(above) 10 GeV are selected;

2. ECAL and HCAL clusters are grouped in the way that clusters in different groups do not share the same associated tracks;

3. the expected energy released in the calorimeters by charged particles pointing to a clusters group is evaluated using an empirical parametrization of E/p as a function of p, where E is the cluster energy and p the track momentum;

4. if the total expected energy of the cluster group (Eexp_{) is larger than 1.8 times the}

measured energy (Em_{) then the clusters group is discarded;}

5. if Em _{> 1.8 E}exp _{then E}exp _{is subtracted from E}m_;

6. the remaining energy is selected as non-isolated neutral particle if its ET is greater

The parametrization of E/p as a function of p is obtained through a procedure called E/p calibration. The calibration has been performed on a data sample of pp collisions at 7 TeV, collected using a minimum bias trigger configuration. From this sample isolated tracks matched to a calorimeter cluster have been selected. Requirements are applied to remove the background from minimum ionizing particles, like muons, that release a small amount of their energy in calorimeters:

• only one PV in the event; • long or downstream track;

• No other tracks within ∆R < 0.5 from the selected track, with ∆R = p∆η2_{+ ∆φ}2_,

where η is the pseudorapidity and φ the azimuthal angle; • track pT greater than 50 MeV/c;

• χ2

track−cluster < 1 for one ECAL or HCAL cluster;

• no other track with χ2

track−cluster < 100 associated to the same cluster;

• cluster transverse energy greater than 200 MeV.

Track-cluster objects are divided in different categories with the following features (track- cluster objects can belong to one or more of these categories):

• the track is associated to a cluster in ECAL but not in HCAL; • the track is associated to a cluster in HCAL but not in ECAL;

• the track is associated to a cluster in ECAL, independently of the presence of a HCAL cluster;

• the track is associated to a cluster in HCAL, independently of the presence of a ECAL cluster;

• the track is associated to a cluster in HCAL;

• both a ECAL and a HCAL cluster are associated to the tracks.

A separate E/p calibration is performed for each of these categories. Moreover the calibration is done separately for tracks identified as hadrons or electrons. In the calibration procedure the mean value of E/p is computed in different intervals of p as the mode of the E/p distribution. Then the E/p dependence from p is fitted with empirical functions. For the first five categories listed above, where only one cluster from ECAL or HCAL is considered, the function used is:

E

p(p) = a1e

−a2p

For the last category E is computed as the sum of ECAL and HCAL energies and the function used is:

E

p(p) = (a1+ a2p + a3p

2_)e−a4p_{+ a}

5.

Few examples of this parametrization are reported in figure 3.1, compared with the E/p vs p obtained using a Monte Carlo sample of pp collision at 7 TeV with a minimum bias trigger configuration.

Figure 3.1: E/p calibration for hadrons. The plots show the calibration for different categories that are, from left to right and top to bottom: the category where a ECAL cluster is associated to a track but bot a HCAL cluster; the category where a HCAL cluster is associated to a track, independently from ECAL; the category where both a ECAL and a HCAL clusters are associated to a track.