SEARCH of LIFETIME

Introduction

"Search of Lifetime" is the generic term for searches of new long-lived particles or particles which decay into well known long-lived particles like charm or bottom quarks or tau leptons. Therefore lifetime information can be used to search directly or indirectly for new particles.
These new particles are predicted by Higgs, Technicolor and Compositeness models and R_p violating symmetry. Also known particles like the top quark can be looked for which might be produced at HERA with an enhanced cross section due to anomalous couplings. Anomalous couplings of the top quark which could be generated by new physics beyond the SM have not been tested with high precision by experiments. Top quarks dominantly decay into a long-lived bottom quark: t->bW).
Particles with a detectable decay length of about L>100mu can be identified in the H1 experiment in the tracking detectors (CJC) and in the vertex dector (CST) . Several analysis may be performed at HERA covering new particle production from the low mass (light gluino) up to the kinematic limit of 300 GeV (e.g. scalar quarks) in various different final state topologies.

Search Channels

Higgs:

Whilst the production of the single SM Higgs at HERA is negligible the production of light Higgs particles in the two Higgs-Doublet Model (2HDM) is not. Even more HERA is believed to have the best sensititvity on the world (even better than LHC!) if the light Higgs h is between 5-15 GeV. Current limits from direct searches (LEP) and indirect limits from g-2 experiments cannot exclude a light Higgs in the mass window m_b < m_h < m_upsilon.
Main feature of the 2HDM model is a cancelation of the symmetry breaking phases sin(beta-alpha)~0 resulting in a vanishing
ZZh coupling. h is expected to be dominantly produced in gluon-gluon and photon-gluon fusion. The dominant decay is h -> tau tau if m_h is below twice the bottom mass and h -> bb_bar if above.
Background is expected from gamma gamma -> tau tau production and from bottom quark production by boson-gluon fusion. For this search a lifetime tag is needed for the identification of tau and bottom quarks.

Technicolor:

Although searches for technicolor particles possesing standard SU(3) color are mainly constrained by searches performed at Tevatron there might be a search window for color singlet technipions which would produce via t-channel exchange a long-lived tau and bottom final state: e q -> b tau .
This process violates lepton flavour conservation which however was discovered in neutrino mixing. This process is characterised by very exclusive final states. Background can be strongly reduced when exploiting lifetime information in search.

R_p broken SUSY:

In SUSY models each particle has a supersymmetric partner which differs by spin=1/2. Supersymmetry is broken such that the known SM particles are light and the supersymmetric partners are heavy. SM fermions have scalar particle partners (l_scalar, q_scalar) and SM gauge bosons (gamma, W, Z, gluon) have fermionic partners (photino, Wino, Zino, gluino).
The symmetry can by characterised by a new quantum number called R-parity which is defined as R_p=(-1)^(L+3B+2S) . This quantum number is +1 for SM particles and -1 for SUSY particles. The mechanism of symmetry breaking is an open question. Spontaneous breaking of SUSY has become very attractive in the past years because it predicts neutrino mixing and R-parity violation. The latter means that SUSY particles can be singly produced or decay into SM particles.
R-parity violating SUSY is very interesting to look for at HERA because scalar quarks could be resonantly produced in the reaction: eq -> q_scalar which is directly tested at highest energies.

R_p violating SUSY allows many different scenarios for the decay of the scalar quark:

Neutralino is LSP (lightest supersymmetric particle)

The scalar quark (preferably a scalar top in most scenarios) can decay into a neutralino (Chi_0) which is usually a mixed state of the supersymmetric partners of the higgs and gauge bosons: q_scalar -> Chi_0 q . In scenarios where R-parity is spontaneously broken via bilinear terms the Chi_0 preferably decays into heavy fermions: Chi_0 -> b b_bar nu or Chi_0 -> tau tau_bar nu. Note that in some models the neutralino itself might have detectable lifetime [Porod et al. hep-ph/0011248].

Scalar top is LSP

In that case the scalar stop can only decay into SM particles. For bilinar R-parity violating models highest branching is expected for the decay into heavy fermions: t_scalar -> b tau. This final state is identical to the Technicolor model with technipion exchange (above) and can be tagged using lifetime information.

Compositeness or new fourth generation quarks:

With compositeness models it is tried to explain the existence of the three lepton and quark generations and the mass ordering of the SM fermions by substructure (Rishons, Haplons, Preons, etc.). Fermions are no longer elementary particles. These models generally predict higher excited states of the known fermions or even predict ground states of new particles. For instance in the "Trinity Preon" model [ref.] the existing of a new particle X is predicted which can decay into long-lived particles via: X -> b mu nu_tau or: X -> b tau nu_mu . Also fourth generation particles which have not been discovered yet are expected to decay preferably into the known third generation fermions which are all heavy and have significant lifetime.