| 1 |
\section{Introduction}
|
| 2 |
\label{sec:intro}
|
| 3 |
|
| 4 |
In this paper we describe a search for physics beyond the standard model (BSM)
|
| 5 |
in a sample of proton-proton collisions at a centre-of-mass energy of 7~TeV.
|
| 6 |
The data sample was collected with the Compact Muon Solenoid (CMS) detector~\cite{JINST} at
|
| 7 |
the Large Hadron Collider (LHC) in 2011
|
| 8 |
and corresponds to an integrated luminosity of \lumifinal.
|
| 9 |
This is an update of a previous analysis performed with a data sample of 34~pb$^{-1}$
|
| 10 |
collected in 2010~\cite{ref:ospaper}.
|
| 11 |
|
| 12 |
The BSM signature in this search is motivated by three general considerations.
|
| 13 |
First, new particles predicted by BSM
|
| 14 |
physics scenarios are expected to be heavy, since they have so far eluded detection.
|
| 15 |
Second, BSM physics signals with high
|
| 16 |
enough cross sections to be observed in our current dataset are expected to be
|
| 17 |
produced strongly, resulting in significant hadronic activity.
|
| 18 |
Third, astrophysical evidence for
|
| 19 |
dark matter suggests~\cite{ref:DM,ref:DM2} that the mass of weakly-interacting
|
| 20 |
massive particles is of the order of the electroweak symmetry breaking
|
| 21 |
scale. Such particles, if produced in pp collisions, could escape detection and give rise to
|
| 22 |
an apparent imbalance in the event transverse energy. We therefore focus on the
|
| 23 |
region of high missing transverse energy (\MET). An example of a specific BSM scenario is
|
| 24 |
provided by R-parity conserving supersymmetric (SUSY) models in which
|
| 25 |
new, heavy particles are pair-produced and subsequently undergo
|
| 26 |
cascade decays, producing hadronic jets and
|
| 27 |
leptons~\cite{Martin:fk,susy1,susy2,susy3,susy4,susy5,susy6}.
|
| 28 |
These cascade decays may terminate in the
|
| 29 |
production of weakly-interacting massive particles, resulting in large \MET.
|
| 30 |
|
| 31 |
The results reported in this paper are part of a broad program of BSM searches
|
| 32 |
in events with jets and \MET, characterized by the number and
|
| 33 |
type of leptons in the final state.
|
| 34 |
Here we describe a search for events containing opposite-sign isolated
|
| 35 |
lepton pairs ($e^+e^-$, $e^{\pm}\mu^{\mp}$, $\mu^+\mu^-$) in addition to the jets
|
| 36 |
and \MET. Results from complementary searches with different final states have
|
| 37 |
already been reported in Ref.~\cite{ref:RA1} {\bf ADD MORE REFS HERE}.
|
| 38 |
|
| 39 |
Our analysis strategy is as follows. In order to select dilepton events, we
|
| 40 |
use high-\pt\ dilepton triggers and a preselection based
|
| 41 |
on that of the $t\bar{t}$ cross section measurement in the dilepton channel~\cite{ref:top}.
|
| 42 |
%, which used a data sample corresponding to an integrated luminosity of 3.1\pbinv.
|
| 43 |
Good agreement is found between this
|
| 44 |
data sample and predictions from SM Monte Carlo (MC) simulations in terms of the event yields
|
| 45 |
and shapes of various kinematic distributions.
|
| 46 |
We search for a kinematic edge in the dilepton mass distribution, which is a characteristic
|
| 47 |
feature of SUSY models in which the opposite-sign leptons are produced via the decay
|
| 48 |
$\chi_2^0 \to \chi_1^0 \ell^+\ell^-$.
|
| 49 |
Because BSM physics is expected to have large hadronic activity and \MET\ as discussed
|
| 50 |
above, we proceed to define 2 signal regions
|
| 51 |
with requirements on these quantities to select about 0.1\%
|
| 52 |
of dilepton $t\bar{t}$ events, as predicted by MC.
|
| 53 |
The observed event yields in the signal regions are compared with the predictions from two
|
| 54 |
independent background estimation techniques based on data control samples,
|
| 55 |
as well as with SM and BSM MC expectations.
|
| 56 |
|
| 57 |
%The search is not optimized in the context of any particular BSM physics, e.g. specific SUSY model.
|
| 58 |
No specific BSM physics scenario, e.g.\ a particular SUSY model, has been used to optimize the search.
|
| 59 |
In order to illustrate the sensitivity of the search, a simplified and practical model of
|
| 60 |
SUSY breaking, the constrained minimal supersymmetric
|
| 61 |
extension of the standard model (CMSSM)~\cite{CMSSM,CMSSM2}, is used. The CMSSM is described by
|
| 62 |
five parameters: the universal scalar and gaugino mass parameters ($m_0$ and $m_{1/2}$, respectively),
|
| 63 |
the universal trilinear soft SUSY breaking parameter $A_0$, the
|
| 64 |
ratio of the vacuum expectation values of the two Higgs doublets ($\tan\beta$), and the sign of the
|
| 65 |
Higgs mixing parameter $\mu$.
|
| 66 |
Throughout the paper, two CMSSM parameter sets, referred
|
| 67 |
to as LM1 and LM3~\cite{TDR}, are used to illustrate possible CMSSM yields. The parameter values
|
| 68 |
defining LM1 (LM3) are $m_0 = 60~(330) \GeVcc$, $m_{1/2} = 250~(240) \GeVcc$, $\tan\beta = 10~(20)\GeV$;
|
| 69 |
both LM1 and LM3 have $A_0 = 0$ and $\mu > 0$. These two scenarios are beyond the exclusion reach
|
| 70 |
of previous searches performed at the Tevatron and LEP. The LM1 scenario was recently excluded
|
| 71 |
by a search performed at CMS in events with jets and \MET~\cite{ref:RA1}.
|
| 72 |
In this analysis, the LM1 and LM3 scenarios serve as benchmarks which
|
| 73 |
may be used to allow comparison of the sensitivity with other analyses.
|
| 74 |
|
| 75 |
|
| 76 |
%In the Standard Model (SM), the main sources of isolated dileptons are Drell-Yan production
|
| 77 |
%($q\bar q$ annihilation into a virtual photon or a $Z$) and $t\bar{t}$ production.
|
| 78 |
%In this search we exclude lepton pairs with invariant mass consistent with a $Z$ boson,
|
| 79 |
%leaving $t\bar{t}$ as the dominant SM background. A separate search for BSM physics in the
|
| 80 |
%$Z+\textrm{jets}$ sample will be described in a forthcoming paper.
|
