First searches for physics BSM at CMS
Carmen Diez Pardos
First searches for physics beyond the Standard Model at CMS
Carmen Diez Pardos, CIEMAT (Madrid)
On behalf of the CMS Collaboration
Email: carmen.diez@ciemat.es
An overview of the searches for new physics beyond the Standard
Model at the CMS experiment [1] with early data delivered by the LHC at
a centre of mass energy of 7 TeV is presented. In particular the results for
searches of new resonances in dijet events and long-lived heavy particles
are discussed.
Dijet searches
Searches with dijets in the final state provide both a test of QCD and
sensitivity to physics beyond the Standard Model (SM). The inclusive dijet
final state is studied using two observables, the dijet invariant mass
spectrum [2,3] and the dijet centrality ratio [4], defined as the number of
events with both leading jets in the inner η region divided over the number
in the outer η region, R=N(|η |<0.7)/N(0.7<| η |<1.3).
The dijet system is defined by the two leading jets reconstructed
using the anti-kt algorithm with | η |<2.5 and |Δ η |<1.3. Events with
Mjj>220 GeV and without imposing pT requirements are selected. Figure 1,
left, shows the dijet mass spectrum up to 2.1TeV using a sample of
integrated luminosity of 2.9 pb-1, compared to the QCD background
simulation. It follows a smoothly falling function with the mass, as
predicted by the Standard Model, without any indication of the presence of
narrow resonances in the data. Thus 95% CL upper limits on the production
of new particles can be established as a function of the dijet mass. They can
be compared with the prediction of different theoretical models, see Figure
1 right, excluding string resonances with masses up to 2.5 TeV, excited
quarks up to 1.58 TeV, axigluons up to 1.42 TeV and E6 diquarks up to
1.60 TeV.
The dijet centrality ratio analysis is more sensitive to the existence of
contact interactions. New physics predicts higher dijet production at lower
values of |η|, while in the SM the dominant t-channel scattering QCD
production is more isotropic. The dijet centrality ratio, defined above, is
roughly flat (R≈0.5-0.6) in the absence of new physics. Two physics
models are considered, motivated by the possibility that quarks are
composite particles: contact interactions and dijet resonances coming from
excited quarks, q*. Figure 2, left, shows the measured ratio compared to
the predictions from NLO QCD and the mentioned new models. Given the
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First searches for physics BSM at CMS
Carmen Diez Pardos
consistency between data (Lint=0.12 pb-1) and the QCD hypothesis, contact
interactions with a scale Λ <1.9 TeV can be excluded at 95% CL, see
Figure 2, right.
Figure 1: Left: Dijet mass spectrum (points) compared to a smooth fit (solid) and to predictions:
QCD (short-dashed), excited quark signals (dot-dashed), and string resonance signals (longdashed). Right: 95% CL upper limits on σ x BR x A for dijet resonances of type gluon-gluon
(open circles), quark-gluon (solid circles), and quark-quark (open boxes), compared to
theoretical predictions for string resonances, excited quarks, axigluons, colorons, E6 diquarks,
new gauge bosons and Randall-Sundrum gravitons.
Figure 2: Left: Measured dijet centrality ratio with 120 nb-1 of integrated luminosity. The ratio is
compared to predictions from NLO QCD plus non-perturbative corrections, contact interactions
with Λ= 0.5, 1.0, and 1.5 TeV, and excited quark resonances with masses of 0.5 and 1.2 TeV.
Right: Limit established for the contact interaction scale Λ. RLL versus Λ for the data (solid
black line), the 95% CLs (solid blue), and the SM expected limit (dashed black line) with 1 and
2σ bands (green/yellow).
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First searches for physics BSM at CMS
Carmen Diez Pardos
Heavy Long Lived Particles
Heavy long lived particles are predicted in many new physics
scenarios, like some flavours of SUSY, GUTs or Split SUSY. In CMS two
methods have been studied for the search for Heavy Stable Charged
Particles (HSCP) [5] and stopped gluinos [6].
The search for HSCP is a signature based search, as they will be
directly observable through the distinctive signature of a slowly moving,
high momentum particle. The analysis isolates candidate events using
momentum and ionization energy loss measurements, by selecting tracks
reconstructed in the inner track detector with high dE/dx and p T.
Additionally, tracks passing muon identification requirements are also
considered. For both selections, the candidate mass is then calculated from
the measured pT and dE/dx. No events survive the selection criteria for a
luminosity about 0.2 pb-1, allowing to establish 95% CL upper limits on
the production cross-section for stau, stop and gluino, see Figure 3. From
the intersection of the limits with the theoretical prediction curves, gluinos
with a mass value less than 271 GeV are excluded with the tracker-only
analysis and with less than 284 GeV with additional muon identification
requirements.
Figure 3: 95% C.L. upper limits on the cross section for production of the different particles
considered in the models and predicted theoretical ones. Analysis of tracker only candidates,
left, and muon identification plus tracker candidates, right.
The search for stopped gluinos complements HSCP searches because
it is sensitive to slower moving particles, with β<0.3. If long-lived gluinos
are produced at CMS they will hadronise into gluino plus quarks or gluon
bound states, “R-hadrons”. These stopped R-hadrons may decay during
time intervals when there are no collisions, which are the periods that CMS
has looked for them. In the absence of a signal, a limit at 95% CL on gluino
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First searches for physics BSM at CMS
Carmen Diez Pardos
pair production over 14 orders of magnitude of gluino lifetime is set, see
Figure 4, left. For a mass difference of 𝑚𝑔̃ −Mχ > 100 GeV with 𝑚𝑔̃ = 200
GeV, gluino lifetimes are excluded in the range (75 ns, 6 μs). Furthermore
with a counting experiment, gluino masses 𝑚𝑔̃ < 200 GeV for a lifetime of
2.6 μs are excluded, and with a time profile analysis, 𝑚𝑔̃ < 200 GeV for a
lifetime of 200 ns. The lifetimes chosen are those for which the counting
experiment and time profile analysis are most sensitive.
Figure 4: Left, expected and observed 95% CL limits on gluino pair production cross-section
using the “cloud model'' of R-hadron interactions as a function of gluino lifetime (from both
counting experiment and the time profile analysis) and, right, as a function of gluino mass.
Commissioning of SUSY searches
A data sample of Lint = 100 pb−1 of pp collisions at √𝑠 = 7 TeV
should be already sensitive to SUSY parameter space well beyond current
Tevatron limits [7], see Figure 5, and in several analyses it should be
enough with the data collected during the 2010 run (about 35 pb-1).
Searches of SUSY particles involve a broad range of signatures with jets,
leptons, and missing transverse energy (MET) and they require a careful
control of background. So currently the efforts aim to test strategies to
suppress and estimate SM backgrounds and validate data-based methods.
The suppression of QCD contributions to MET tails in the event and the
prediction of QCD contribution to lepton samples [8] are examples of these
tasks.
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First searches for physics BSM at CMS
Carmen Diez Pardos
Figure 5: Estimated 95% C.L. exclusion limits for the like-sign dilepton SUSY search, left, and
for the all-hadronic SUSY search, right, expressed in mSUGRA parameter space.
Conclusion
The CMS experiment has searched for evidence of new physics in
several channels using the early data collected. No signals have been
observed with the first 0.1-2.9 pb-1 recorded, allowing to set limits on the
production of new physics predicted by several models. Some results, like
the search for Stopped Gluinos or the existence of contact interactions,
already extend the existing limits from Tevatron. Although higher statistics
are needed to prospect the kinematics phase-space where SUSY signal is
expected, CMS is already testing methods for effectively suppressing and
controlling the SM background, using data-based techniques.
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https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsEXO.
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