Fourth Generation Leptons

Linda Carpenter

UC Irvine Dec 2010

Work with Arvind Rajaraman, and Daniel Whiteson

arXiv:1001.1229v1 [hep-ph]

arXiv:1005.4407 [hep-ph] arXiv:1005.0628 [hep-ph]

arXiv:1010.1011 [hep-ph] arXiv:1010.5502 [hep-ph]

• Review 4th Generation Mass Formalism

• Review PDG bounds on 4th Generation Leptons

• Recalculated Bounds From LEP on 4th Generation Neutrinos for stable and unstable scenarios

• Look at Proposed LHC searches for 4th Generation Leptons

Most General Fourth Generation Lepton Sector consists of a charged lepton with a

Dirac Mass, and a neutrino with a Majorana and a Dirac Mass

The neutrino mass matrix is

Diagonalizing one gets

Define two mass eigen states

With mixing angle

And masses

Leptons couple to the charged and neutral currents

where

Mass bounds are different in the case that the lightest leptons is stable or unstable.

Under the assumption that final state leptons are all the same flavor these are LEP’s exclusions.

arXiv:hep-ex/0107015v1

In the case of unstable lightest neutrino, mass bounds are given by LEP 2

From the process

For mixed mass case the neutrino production cross section for the lightest state neutrinos is

Suppressed by the fourth power of the mixing angle. Heavy state pair production and heavy light production are suppressed by phase space. In this way we can lower the mass bound.

LEP search

Relied on looking for 2 well isolated leptons of the same flavor.

Required isolation cone of 30 degrees around the hard leptons

Looked for 60 GeV of hadronic activity, mostly sensitive to hadronic decay of the Ws.

Assuming all 4th gen neutrinos decay to a single final lepton flavor, generate events with MADGRAPH, decay using BRIDGE, and shower events through PYTHIA to get estimated efficiencies for mixed mass search

The only bound on stable neutrinos is from the Z pole measurement of the Z invisible width. The Z invisible width may be corrected by 21 MeV*

PDG quotes 45 GeV for Dirac type and 39.9 for Majorana type stable neutrinos, (The Lower Majorana bound is from a phase space factor)

*(Particle Data Group), J. Phys. G 37, 075021 (2010)P. Abreu et al. [ DELPHI Collaboration ], Phys. Lett. B274, 230-238

(1992)

If there are 2 Majorana neutrino states the stable neutrino mass bound may be

lowered even further

Recall that the Neutrino couplings to the Z boson is suppressed by powers of the

mixing angle

Consider production processes for neutrinos from the Z.

The first process above contributes to the invisible width and the second to the total width.

To maximize the invisible width one wants to compress the neutrinos, however if the N2

states are too light one compromises the total

width.

Existing SUSY searches may already constrain this scenario at LEP, as squark searches look for the jets plus missing energy signal.

One considers N2 pair production. The relevant process is

Consider a search along the lines of the acoplanar jets search pre-selection for LEP’s SUSY squark search. The cuts are as follows

The background is dominated by diphoton events.

Calculated acceptance for cuts following those of squark SUSY search

pre-selection

Hadron Collider Searches for 4th Generation Leptons

a) The case of unstable lightest neutrino

Looking for unstable leptons, One has events with many final state particles particles.

One must have something good to trigger on. Luckily, the signal for Majorana neutrinos is like sign dileptons, which is a very distinctive signal.

Possible Neutrino Event

Half of the event will have same sign dileptons and

Many event will have the final state

For a dilepton search with proposed cuts

One may produce estimated efficiencies

With possible exclusion up to 300 GeV or with 5 inverse fb of data, 3 sigma discovery potential for 225GeV neutrinos.

Searches for a charged lepton neutrino pair are easier than neutrino pair production b.c. one may produce a lepton and neutrino from a W rather than a neutrino pair from a Z.

The resulting cross section is almost two order of magnitude higher than neutrino pair production from a Z.

LHC event producing L N1 pair

We use the following cuts

Benchmark point mN=100 GeV mL=200GeV

Events vs. Number of jets

The production cross section is high, around .1pb

Backgrounds are low for like sign di-lepton events and are mostly from WZ or W photon or W+jets and ttbar with mistagged leptons.

Production cross section

Plot of

Plot of Acceptance

Note that acceptance drops as N1 decreases as its decay products get

soft

Exclusion with 1 inverse fb at 7TeV

Possible exclusion at 95% c.l. for entire mass plane of charged lepton masses up to 250 GeV with 1 inverse fb at 7TeV.

Maximum reach into the mass plane of 350 GeV for charged lepton masses.

Jet distribution can give a hint that the signal is indeed a charged lepton plus neutrino

b) The Case of stable lightest neutrinos

In the case of stable neutrinos most signals of a fourth generation lepton sector will be jets plus missing energy. Take an example,

However with only electroweak cross section, these are quite hard to find at LHC as they may be lost in the QCD background.

As an example 100 GeV neutrino has the same production cross section as a 1.5 TeV gluino(200 fb) without any hard missing energy cuts.

In addition, one if faced with a large W background.

Channels which in the unstable neutrino case would have worked well are now buried, for example

p p LN1 WN1N1

Even for an offshell W, the reconstructed transverse mass of the W has a long tail

One strategy is to consider signal with multiple leptons, where the backgrounds are very small.

One such signal is 4 leptons plus missing energy. Consider the following process,

This search will rely on the following cuts

Exclusion with 20 inverse fb at 14 TeV

More stuff to do:

Consider searches were unstable charged lepton is the lightest particle

possible other lepton signals of stable neutrinos case e.g. 2l+2j+missing energy

Higgs decays

A possible Higgs decay

In the region of intermediate Higgs, 100-160 GeV gg and bbar are the dominant signals.

For the case of light-heavy neutrinos neutrino pair production is proportional to the square of the Dirac mass component of the neutrino. This will easily beat bbar production by a factor of 50-100 for the case of neutrinos with masses from 35-80 GeV.