Draft: Development of the new Hlt1TauToPhiMuLine

General Description

MR related to the proposal on the implementation of a new Hlt1 trigger line designed to select \phi(1020)\to K^+K^- + \mu decays.

This exclusive line selects \tau\to\phi(KK)\mu events by combining a neutral vertex (a dihadron vertex) with a third track (a muon).

The general idea of the algorithm is (for the complete selections list, see the table below):

• Exploit \phi(1020) mass to select \phi\to K^{+}K^{-} candidates + other kinematic selections.
• Exploit is_muon requirement to select the third track + other kinematic selections.
• Final tau candidate found requiring the \tau mass + DOCA(\phi,\mu) to select the closest \mu to \phi + PT and c\tau of the \phi\mu system.

See my last presentation at the WP3 meeting for further details.

• Name of the new line implemented: Hlt1TauToPhiMu
• Exclusive Hlt1TauToPhiMu rate = 3.5\pm1.0\,\rm{kHz} (first version, v1, now replaced by v1.1).

Update

• A new version of the Hlt1TauToPhiMu (v1.1) line has been implemented, operating at a lower rate compared to the first version.
• The rate was computed using MinBias samples located at: /eos/lhcb/wg/rta/WP3/bandwidth_division/Beam6800GeV-expected-2024-MagDown-Nu7.6/30000000/. The table below summarizes the test results for the new Hlt1TauToPhiMu line, including performance comparisons with Hlt1TwoTrackMVA and Hlt1TrackMuonMVA (the other two important line to select our signal):

Line name	Total events	Inclusive rate [kHz]
Hlt1TwoTrackMVA	11266/564308	598.93 +/- 5.59
Hlt1TauToPhiMu	56/564308	2.98 +/- 0.40
Hlt1TrackMuonMVA	1016/564308	54.01 +/- 1.69
Total Hlt1 rate with Hlt1TauToPhiMu	23503/564308	1249.48 +/- 7.98
Total Hlt1 rate w/o Hlt1TauToPhiMu	23472/564308	1247.83 +/- 7.97

• Based on these results, the exclusive rate of the Hlt1TauToPhiMu line in its new configuration is (1.65 ± 0.30) kHz (new version v1.1).

Table of the implemented selection in version v1.1:

• The new selections implemented in v1.1 are \chi^2_{FD}(\phi) > 10 and a tighter cut on \chi^2_{IP}(\mu) > 9 (previously > 4).