Search for New Physics in High Mass Dielectrons
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Introduction
We present an analysis of 200 pb^-1 of run 2 data. We measure the invariant
mass spectrum of di-electrons at high invariant mass values. For the selection
we require one electron in our central calorimeter (|eta|<1) and
the 2nd candidate can either be in the central calorimeter
(central-central "CC") in the plug calorimeter (1.1<|eta|<2.8)
(central-plug "CP").
We find good agreement with the Standard Model expectation and thus set an upper
limit on the production cross section (times branching ratio) for the production
of particles leading to this signature. We do this separately
for spin-0, spin-1 and spin-2 particles since the experimental acceptance
depends on the spin.
We then interpret this cross section limit in terms of physics beyond the
Standard Model: Z', RS Gravitons, sneutrino, technicolor, Little Higgs.
Comparison of Data with Standard Model
The invariant mass distribution of teh CC and CP candidates is shown here:
Central-Central dielectron invariant mass distribution
Central-Plug dielectron invariant mass distribution
The following plots show the mass spectrum of the data compared to the
Standard Model expectation. The main source of background is Drell-Yan.
Jets being misidentified as electrons (labelled "QCD") are another important
source of background.
Central-Central
Central-Plug
Combined result: C-C and C-P
The following plots show the cos(theta) distribution of the data compared with
the Standard Model expectation:
Central-Central
Central-Plug
Combined result: C-C and C-P
The following plots show the efficiency times acceptance of detecting a particle
decaying into a electron-positron pair as function of the mass. Since the spin
of the particle changes the angular distribution of the decay particles we consider
the possible spin of any new particle separately: spin-0 , spin-1 and spin-2.
spin-0 particle acceptance (e.g. Higgs)
spin-1 particle acceptance (e.g. Z')
spin-2 particle acceptance (e.g. RS Graviton)
Limits on New Physics
Since we find good agreement with the SM expectation we deterimed an upper limit on the
cross section of any new particle resulting in this final state at 95% CL. We again do this
for each spin value separately. The following plots show this upper limit. Also shown is the
expected limit. The green band means a 1sigma error on the expected and the yellow a 2sigma error
on the expected limit. The observed limit agrees with the expected limit within 2 sigma at all
mass values.
spin-0 cross section limit
spin-1 cross section limit
spin-2 cross section limit
Interpretation in Specific Models
RPV sneutrino's:
Sneutrino's may decay into e+e- pairs depending on the couling (lambda') of the sneutrino to
the incoming quarks and the branching ratio (BR) of the sneutrino into an electron pair. We exclude
sneutrino's in the plane lamnbda'**2*BR versus sneutrino mass.
Cross section limits on sneu->ee
Excluded region on lambda Br vs. sneutrino mass plane
Z' bosons
These exist in many extensions of physics beyond the Standard Model which attempt
to form GUT's, e.g. E6 models. We present the limit for several Z' models. For illustatration
we also show the Z' mass limit in the case of SM coupling:
Cross section limits on Z'->ee (SM & E6)
Technicolor
techni-rho's and techni-pion's may decay into e+ e- pairs. We show the result
as function of the techni-rho mass. With the present dataset they can not yet
be excluded but with a factor 2-4 more data first constraints in this channel are
possible.
Cross section compared to Technicolor predictions
Little Higgs
Cross section limits on Little Higgs Z'
Excluded region on cot(theta) vs. little Higgs Z' mass plane
Randall-Sundrum graviton
Randall -Sundrum gravitons are excluded by these data in the plane of coupling (k/MPl)
versus effective graviton mass. E.g. for k/M_Pl=0.05 masses less than 500 GeV are ruled out
at 95% C.L..
Cross section limits on G->ee
Excluded region on k/Mpl vs. graviton mass plane
Large Extra Dimensions
These data can also be used to set a limit on the effective Planck scale in ADD type models.
the likelihood distributions:
Likelihood distribution for lambda < 0
Likelihood distribution for lambda > 0
Effective Planck scale limits for 3 lambda conventions
Typical High Mass Event in Central-Central (371 GeV)
COT plot
Lego plot
Koji Ikado