Flavour-violating decays of mixed
top-charm squarks at the LHC

Amit Chakraborty
Theory Center, KEK

Beyond the BSM workshop
October 2, 2018

Based on:
AC, Endo, Fuks, Herrmann, Nojiri, Pani and Polesello; hep-ph 1808.07488

GluinoCMS Twiki page

CMS Twiki page

Post-Higgs era: SUSY searches

Stop

Similar exclusions from ATLAS– Data consistent with SM predictions; Strong constraints
with run-1 and run-2 data, relatively heavy (> 1 TeV)
– Many assumptions & Limitations ...
– Compressed scenario/generation mixing:– light particles (say 500-600 GeV) still possible!
– new search strategies

CompressedStopscenarioFlavor in SM
Yukawa interaction: only source of FV in the SM

Highly suppressed
Off-diagonalterms

Flavor in MSSM●

●Minimal Flavor Violation: Same flavor structure as in SM, Super-CKM
basis (squarks undergo similar rotation as quarks), FV effects areproportional to CKM elements
Beyond MFV: Generation mixing, additional sources of FV
Direct search: Production and Decay changes significantly, relaxed limits!
Strong Limits: low energy flavor data

[Higgs data puts limits on LR-type mixing]●RR-sector (up-type) is almost unconstrained!

[Recent study on non minimal MSSM, global fit, Fuks et. al. JHEP (2015)] [More][Gabbini, Masiero (1989); Gabbiani, Gabrieli, Masiero, Silvestrini (1996); Ciuchino, Degrassi, Gambino, Giudice (1998), Lari, Pape,Porod et al. (2008), Fuks et al (2012), Backovic et al (2015), Crivellin et al. (2016) ... ]The Proposal
Mixing in 2nd & 3rd generation RR up-sector:– Probe lightest mixed state (stop) at the LHC– Sensitivity of “tc+missing energy” channel at current & HL LHC.
(Current Stop searchs: tt+ MET, cc+MET)– Combine search strategies, flavor tagging, probe squark flavorstructure, departures from MFV paradigm

Simplified Model
Model:

SM+ right-handed stop + right-handed scharm+ Gluino + Neutralino (bino) [Production & Decay]

Signatures:

Monte-Carlo set-up●

Signal: LO using MG5, passed to PY8 and thenDelphes; normalized using NLO+NLL xsec●

Bkg: ttbar, single top, VV @NLO, ttbarZ, ttbarW,W+jets, Z+jets @LO; PY8 + Delphes; normalized with NNLO/NLO xsecs●●Squark pair-production: m(u1) = [600, 1500] GeV, m(chi) = 50 GeV,gluino mass = 3 TeV
Jets: Fastjet with R=0.4, anti-kT, ATLAS card.

Recast of LHC 13 TeV data
toptt

charm

cc

Scharm search: ~ 850 GeV @ 13 TeV● Stop search: 1-lepton, jets + MET search at 13 TeV● Translate to 3-parameter plane: m(u1), m(u2) and θ (tc).
●●

Recast: compare signal yields with Model independent limits on
non-SM contributions from the observed data.
Define: R = Nsig / Nnon-SM(obs) ; R > 1 => Excluded!

Event selection - IFinal State : top + charm + MET●Exactly one lepton with pT>25 GeV, |η|<2.5.

●At least one b-tagged jet with pT>50 GeV.

●

|Δφmin(MET, jets)| > 0.6, mT(lep,MET) > 160 GeV, m(bjet,lep) < 160 GeV(Reduces dominantttbar bkg)●

Two search strategies:
Case-A: Veto additional b-jets (εb = 77%), a light jet with pT>100
GeV failing b-tagging.
Case-B: Presence of a c-tagged jet (εc = 30%) with pT>100 GeVand mass > 160 GeV, others with mass > 160 GeV fail b-tagging[S/B improves, statistics low]

Event selection - II●

Asym MT2: aMT2 > 200 GeV (reduce di-lep ttbar)(V1 = lepton, b-jet; V2 = remaining leading jet; MET system = (0,80 GeV)●

Presence of a hard c-jet, construct MT2(lep,b-jet,light/c-jet)(V1 = lepton, b-jet; V2 = hardest non b-tagged jet (case-A)/c-tagged jet (case-B); METsystem = (0,0 GeV)- Squark mass dependent end-point●Optimize:– depends on squark-neutralino mass
splitting– vary between [300,600] in 50 GeV

Sensitivity at 14 TeV
Case-A

The “tc+MET” strategy:
Probing regions of phase space not accessible with current strategy.

Sensitivity at 14 TeV– @ High Lumi run, both are comparable, S/B got improved after C-tagging!
Q: After discovery, “mixed stop” originating from t-c or t-u mixing?
- Estimate by changing the c/b-tagging working point, control

light jet rejection ratese.g., for Higgs coupling: Perez et. al. 2015

Summary– Flavor mixing in right-handed stop and scharm sector, lightest stop is mixed,
Interesting collider signatures– Current strategy excluded stop mass up to 600 GeV, light stop is still viable!
– We study: t + c + MET topology, two search strategies, core difference
inclusion of charm tagging– Mixed Stop mass up to 1.3 TeV can be probed at High Lumi run of LHC– Main Advantage:
Combine search strategies, make use of flavor tagging working points,
Opens the door to probe squark flavor structure, observation of departuresfrom MFV paradigmBack ups

Flavor in MSSM●●New sources of FV appears
Mostly from Soft-SUSY breaking terms (e.g.: gravity mediation, gauge mediation withmessenger mixing, …) [Porod et. al.,●No direct relation with CKM●Generation mixing at EW scale●Independent parameters
Non-Minimal Flavor violation

6x6
Basis:
Today[Gabbini, Masiero (1989); Gabbiani, Gabrieli, Masiero, Silvestrini (1996); Ciuchino, Degrassi, Gambino, Giudice (1998), Lari, Pape,
Porod et al. (2008), Fuks et al (2012), ...]

Consequences of Generation mixing[Fuks et. al. JHEP (2015)][Back]