Talk given at the
XXIIIth International Symposium on
Lepton and Photon Interactions at High Energy
August 13-18 2007, Daegu, Korea
SEARCHES FOR EXOTIC PHENOMENA
AT COLLIDERS
Claude Vallée
Centre de Physique des Particules de Marseille
At least a factor two increase of the integrated
luminosity is expected until the end of the
program, scheduled for 2009.
The LHC pp collider will open a new area in
the exploration of the high energy frontier. Its
√s value of 14 TeV will gain an order of
magnitude in resolution power for searches of
new physics. Again the integrated luminosity
expected to be accumulated by the experiments
in the first year of running is ~1 fb -1.
NEW PHYSICS MODELS
Abstract
This report summarizes the present status of
searches for new physics performed at the high
energy frontier, excluding the Higgs boson and
supersymmetry which were covered in other
talks at this Conference. A few insights into
the discovery reach of the first year of LHC
running are also given1.
THE HIGH-ENERGY FRONTIER
In the past 15 years, exploration of the high
energy frontier has been dominated by the LEP,
HERA and Tevatron colliders, which are
complementary in terms of initial states (e+e-,
ep and pp respectively). Their different energy
ranges (√s of 0.21, 0.32 and 1.96 TeV
respectively) are partially compensated by the
composite nature of the proton, leading in many
cases to similar sensitivities to new physics,
depending on the models. Most present results
are based on ~1 fb-1 of integrated luminosity.
The LEP experiments have accumulated ~0.9
fb-1 per experiment. At HERA each experiment
could record ~0.5 fb-1 of useful data until the
collider was shutdown on June 30th 2007. H1
and ZEUS have now started to combine their
data sets to maximize the statistical precision of
HERA searches. The Tevatron Collaborations
have presently ~2.5 fb-1 of useful data on tape
but most results are still based on ~1 – 1.3 fb-1.
1
Most of the results being preliminary, no references are
provided. More details are available on the public web sites
of the Tevatron, HERA and LHC experiments.
In order to be as independent as possible of
particular assumptions, new physics is
generally described with effective lagrangians
involving generic couplings to fermions and
bosons.
Lepton-quark resonances (“leptoquarks”) can
be produced either in pair at Tevatron via QCD
gauge coupling, or individually at HERA via
lepton-quark couplings. Flavour Changing
Neutral Currents (FCNC), which are strongly
suppressed in the Standard Model (SM), are
usually described through anomalous couplings
of the fermions to the photon and Z boson.
Contact Interactions, which could result from a
substructure of the fermions, are modeled via
effective 4-fermion “Fermi-like” contact terms
acting at a scale Λ. Similarly production of
excited fermions can be modeled either through
contact interactions or through excitation of
standard fermions by gauge bosons. New
physics with extra dimensions can manifest
itself either through contact-like interactions (in
case of several large extra dimensions), or as a
sequence of
graviton narrow resonances
coupled to fermion and boson pairs at a scale
Mpl (Randall-Sundrum model involving one
small warped extra dimension).
Searches results are generally quantified in
terms of limits on the couplings of the new
particles to the standard ones, as function of
their masses. Extra assumptions on unknown
parameters like decay branching ratios are often
made. When comparing results from different
colliders it is essential to use the same model
and the same assumptions in the limits
derivations.
In the following results are presented as
function of the type of experimental signatures
studied to investigate new physics.
INCLUSIVE SIGNATURES
The main inclusive signature at Tevatron
consists in the production of hadronic high-PT
jets and dijets. The inclusive dijet mass
spectrum is well described on many orders of
magnitude (Fig. 1). The deviation to SM
initially observed at RUN I corresponded to
Contact Interaction scales of 1.6 TeV, but it
was later recognized that this observable is very
sensitive to the gluon distribution at high x
(Fig. 2), which is little constrained by deep
inelastic scattering. Indeed Tevatron jet
measurements are now used as inputs to global
parton distribution fits to constraint the gluon,
and no recent result on contact interactions
have recently been derived from these
measurements.
Figure 2
Systematic, theory and PDF uncertainties on
the data/SM ratio of the inclusive dijet mass
spectrum measured at Tevatron
At Tevatron inclusive missing transverse
momentum is also sensitive to contact
interactions and more particularly to production
of a graviton continuum in models with large
extra dimensions. The measured distribution is
in good agreement with the SM (Fig. 3),
dominated by the production of Z bosons
decaying to neutrinos.
Figure 1
Inclusive dijet mass spectrum measured by
CDF, compared to expectations
Strategies developed to bypass this difficulty
consist in exploiting the different angular
distributions of jets in SM and BSM processes,
jets being produced more centrally in the latter
case. Using such techniques LHC experiments
expect to reach a sensitivity of 8 TeV on the
contact interaction scale with 1 fb-1 of data.
Figure 3
Inclusive missing transverse energy spectrum
measured by CDF, compared to expectations
Effective Planck scales MD below order 1 TeV
are excluded depending on the number nD of
extra dimensions (Fig 4). For nD = 2, this
2
corresponds to an upper limit of 0.36 mm on
the size of the extra dimensions.
interaction terms are excluded by ZEUS up to
scales Λ of order 5 TeV, depending on the
chirality of the couplings (Fig. 7). Comparable
limits were derived at Tevatron with the DrellYan process using integrated luminosities of
~0.2 to 0.4 fb-1.
Figure 4
Lower limits on the effective Planck scale MD
as function of the number of extra dimensions
Figure 6
Ratio data/SM of the Q2 spectra measured by
H1, compared to the SM and a model with a
finite quark size of 0.74 fm
Figure 5
Neutral current 4-momentum transfer squared
Q2 spectrum measured by H1
At HERA the main inclusive signature consists
in the neutral current production of an electronjet system, which can be studied as function of
the 4-momentum transferred Q2 (Fig. 5). No
deviation to the SM is observed, the difference
between the e+p and e-p cross-sections at high
Q2 being due to the different sign of the γ-Z
interference term. H1 sets an upper limit of
0.74 fm on a finite quark radius Rq which
would modify the cross-section by a factor 1Rq2Q2/6 (Fig. 6). Similarly eeqq contact
Figure 7
Limits on Contact Interaction scales derived by
ZEUS as function of the couplings chirality
3
The same process can be studied as a function
of the electron-jet mass to search for leptoquark
resonances (Fig. 8).
Figure 10
Figure 8
Neutral Current inclusive mass spectrum
measured by H1
Charged Currents involving production of a
neutrino-jet system are also investigated (Fig.
9). A good agreement to the SM predictions is
observed and limits on first generation
leptoquarks are derived. As an example, for
leptoquarks coupled to an electron and a uquark, assuming the same decay branching ratio
to electron and neutrino, masses below
Excluded domain for scalar e-u leptoquarks as
function of the leptoquark mass MLQ and the
leptoquark coupling λ
LEPTONIC SIGNATURES
Drell-Yan pair production is the golden
leptonic signature for searches at Tevatron (Fig.
11). Quantification of deviations to SM
Figure 11
Figure 9
Charged Current inclusive mass spectrum
measured by H1
~300 GeV are excluded for couplings λ of the
electromagnetic strength 0.3 (Fig. 10). For λ =
0.1 H1 extends the exclusion domain to higher
masses than Tevatron.
Drell-Yan spectrum measured by CDF in the
electron channel, compared to expectations
allows to set limits on the production of new
resonances such as heavy spin-1 Z’ bosons or
Randall-Sundrum spin-2 gravitons. Z’ bosons
with SM couplings are excluded for masses up
to 920 GeV (Fig. 12).
4
combined their observations of multi-electrons
using the full HERA I+II data sample (Fig. 13).
A good overall agreement to the SM is
observed. The small excess at total scalar PT
ΣPT > 100 GeV is due to the e+p data, where 5
events are observed for 1.8 expected.
H1 has also investigated the muon signatures.
Combining multi-lepton events with electrons
and muons, the number of events observed by
H1 at ΣPT > 100 GeV in the e+p data is 4 for 1.2
expected (Fig. 14).
Figure 12
Upper limit on the production cross-section of
a new spin-1 ee resonance Z’ as function of the
Z’ mass, compared to model predictions
At LHC 1 fb-1 of data will allow to exclude a
SM-like Z’ up to masses of ~3 TeV. In case of
a resonance discovery, its nature (Z’ or
graviton) will be discriminated from the
angular distributions of the decay leptons.
Figure 13
Combined H1-ZEUS multi-electron events
distribution as function of the multi-electron
total scalar momentum ΣPT
At HERA multi leptons are rare signatures
mainly produced by γγ interactions, and could
also reveal new physics. H1 and ZEUS have
Figure 14
Multi-lepton (e+µ) events distribution
measured by H1 as function of the multi-lepton
total scalar momentum ΣPT
Another rare signature sensitive to new physics
consists in events with isolated leptons and
missing transverse momentum. At HERA they
are mainly due to W production and are
expected to show dominantly a low-PT recoil
hadronic system X. At HERA I, a 3-σ excess of
events with a high-PT recoil system was seen by
H1 in the e+p data. The H1 excess stays at the
same statistical level when adding the full
HERA II e+p data, corresponding to a factor 3
increase in the data sample size.
When
combining with ZEUS in a common phase
space, the excess significance decreases to 1.8
σ, with 23 events observed for 14.6 expected at
PTX > 25 GeV (Fig 15).
No such excess is seen in the e-p data. When
combining H1 and ZEUS measurements with
the whole HERA I and II data, an overall good
agreement to SM is found (Fig. 16). The bulk
5
of the measured events provide a clean W
sample for SM studies.
through an anomalous FCNC coupling to the
photon and the u-quark. The b-jet from the top
quark decay would be responsible for the high
PTX values observed. Investigating the
distribution of the reconstructed l--X mass,
only a few events fit the top hypothesis.
Assuming the observed excess to be a statistical
fluctuation, H1 sets improved limits on the
anomalous top coupling to the photon and the u
quark (Fig. 17). Note also that CDF, looking
for possible top decays into a u- or c-quark and
a Z boson, has recently improved the L3 limit
on the anomalous top coupling to the Z boson
shown in Fig. 17.
Figure 15
Combined H1-ZEUS distribution of events with
an isolated lepton and missing PT as function of
the recoil transverse momentum PTX, for the full
HERA I+II e+p data
Figure 17
Excluded domain of anomalous top couplings
to the photon and u quarks (horizontal axis)
and to the Z and u-quarks (vertical axis)
Figure 16
Combined H1-ZEUS distribution of events with
an isolated lepton and missing PT as function of
the recoil transverse momentum PTX, for the full
HERA I+II data
A possible explanation of an excess of events at
high PTX could be the production of top quarks
Leptons and missing transverse momentum are
also an interesting signature at Tevatron, where
this topology is also dominantly due to W
production (Fig. 18). Comparison of the
transverse mass spectrum to predictions allows
to set limits on the production of new heavy W’
bosons. Masses of SM-like W’ bosons are
excluded for masses up to 965 GeV (Fig. 19).
PHOTON SIGNATURES
Photon signatures have gained considerable
attention in the recent years in the context of
Gauge Mediated Supersymmetry Breaking,
where neutralinos can decay into a photon and
6
Figure 18
Transverse mass spectrum measured by D0 for
events with an isolated electron and missing PT
Figure 20
Inclusive di-photon mass spectrum measured by
CDF
The di-photon inclusive mass spectrum
measured at Tevatron is in good agreement
with the SM (Fig. 20), allowing to set limits on
the production of Randall-Sundrum graviton
resonances. When combining with the dielectron channel, graviton masses up to 900
GeV are excluded for a coupling of 0.1 (Fig.
21).
Figure 19
Upper limit on the production cross-section of
a new spin-1 e resonance W’ as function of
the W’ mass, compared to model predictions
a gravitino, and extra-dimensions models,
where gravitons can couple to a pair of photons.
Figure 21
Excluded domain for Randall-Sundrum
gravitons as function of the graviton mass MG
and its coupling k/Mpl
7
CDF has performed a systematic investigation
of all topologies involving isolated photons. No
outstanding eeγγETmiss event similar to that
found in run I was observed in run II up to now
(Fig.
22).
The
run
I
excess
of
lepton+photon+ETmiss events (16 observed for
7.6 expected) is also not confirmed at run II.
MODEL TUNED SEARCHES
Some searches are tailored to test particular
models of new physics. They may involve more
complex event topologies as in signature based
searches, with an event selection tuned to
optimize the sensitivity to a potential signal and
the SM background rejection.
A first example is provided by the search for
excited fermions, namely electrons, at
Tevatron. Here excited electrons can be
produced in association with a standard
electron and decay into a photon-electron
system. The eγ mass spectrum measured in eeγ
topologies shows no deviation to the SM
prediction, dominated by radiative Drell-Yan
pairs (Fig. 24).
Figure 22
Distribution of events with 2 photons and
missing ET measured by CDF as function of the
total scalar momentum of the event HT
All topologies such as the γγ channel recently
analysed (Fig. 23) are in good agreement with
the SM.
Figure 24
e-γ mass spectrum measured by D0 for events
with 2 electrons and 1 photon
Limits on the excited electron mass are derived
within a Contact Interaction model as function
of the Contact Interaction scale Λ (Fig. 25).
Excited electron masses of up to 760 GeV can
be excluded for Λ = 1 TeV.
Figure 23
Distribution of events with 2 photons and 1
measured by CDF as function of the total
scalar momentum of the event HT
H1 has also performed a systematic search for
excited electrons at HERA, investigating the
three potential decay channels into a photon, a
Z or a W boson. No signal is observed and
limits on the excited electron mass are derived
in the context of gauge excitation models as
function of the excitation coupling (Fig. 26).
They extend the region excluded by LEP as
well as by Tevatron, after re-interpretation of
the previous D0 result within the gauge
mediation model. This is an example of the
8
extend those from the LEP experiments (Fig.
27).
Figure 25
Excluded domain on excited electron
production as function of the excited electron
mass me* and the contact interaction scale Λ
importance to use the same model and
assumptions when comparing results from
different colliders.
Figure 27
Excluded domain for excited neutrinos as
function of the excited neutrino mass and its
coupling to gauge bosons f/Λ
The Tevatron experiments have also searched
for new particles produced in pairs and
decaying into a hadronic jet and a Z boson. One
Z is observed through its charged leptonic
decay and the second through its hadronic
decay. Results are interpreted in term of
production of a new 4th generation b’ quark.
Both short-lived and long-lived b’ quarks were
looked for. CDF excludes short-lived b’ quark
masses below 270 GeV for a decay branching
ratio to the Z equal to 1 (Fig. 28).
Figure 26
Excluded domain for excited electrons as
function of the excited electron mass me* and its
coupling to gauge bosons f/Λ
H1 has also searched for excited neutrinos
through their three possible electroweak gauge
decays, restricting the search to the e-p data
which are most sensitive to this process. Again
no signal is observed and the limits derived
Figure 28
Upper limit on short-lived b’ quark production
cross-section and model expectation, as
function of the b’ mass
9
Long-lived b’ quarks are investigated by D0,
selecting pairs of electromagnetic particles
within the Z mass range and non-pointing to the
interaction vertex. Limits on the b’ mass are
derived as function of the b’ life time (Fig. 29).
The excluded domain extends to higher life
times than covered by CDF with muonic Z
decays.
This does not favour the BPT model from
Bjorken et al. which introduces a new righthanded down quark of ~300 GeV to explain the
structure of the CKM matrix.
The Tevatron experiments have performed
searches for 2nd and 3rd generation leptoquarks,
for which they have a better sensitivity than
HERA experiments in general.
The 2nd generation leptoquarks are searched for
by D0 assuming the same decay branching ratio
to a muon and a neutrino, and selecting
candidate events for one leptoquark decaying to
a muon, and the other one to a neutrino. The
resulting µ-jet mass spectrum shows no
deviation to the SM prediction, which is
dominated by W boson production (Fig. 31).
Figure 29
Excluded domain for long-lived b’ quarks as
function of the b’ mass and its life time
Recently CDF has performed a systematic
search for new quarks produced in pairs and
decaying into a standard quark and any gauge
or Higgs boson. All three generations are
investigated, using b-tagging for the 3rd
generation. For a Higgs mass of 120 GeV,
production cross sections of ~1 pb are excluded
for new quark masses up to 350 GeV (Fig. 30).
Figure 31
Mass distribution of D0 2nd generation
leptoquark candidates compared to SM
expectations
2nd generation leptoquark masses below 214
GeV are excluded (Fig. 32).
Figure 30
Upper limit on the production cross-section of
a new heavy quark Q as function of its mass
MQ, compared to the BPT model prediction
The 3rd generation leptoquarks are searched for
through their decays into  leptons. D0 selects
event candidates where the  from one
leptoquark decays muonically, and the  from
the other leptoquark decays hadronically.
Limits are derived assuming either a decay
coupling  to ’s equal to 1 (leptoquarks of
charge -4/3) or in the range 1 - 0.5 (leptoquarks
of charge +2/3). For  = 1, 3rd generation
leptoquark masses below 190 GeV are
10
excluded (Fig. 33). The limit depends only
slightly on  because of the phase space
Figure 32
Upper limit on the production cross-section of
a 2nd generation leptoquark as function of its
mass, compared to model predictions
suppression of the neutrino decay mode which
involves a top quark.
intend to ensure that no particular high-PT
topology has been overlooked in the
investigations for new physics because of
models a-priori’s. Second, in case a deviation to
the SM is observed in some channel, generic
searches allow to quantify the probability
within the SM that a similar deviation be
observed in any channel, if the experiment
would be redone. The difficulties of such
analyses consist in defining particles identifiers
suitable to all possible event topologies in term
of efficiency and background rejection, and to
fully account for all SM processes which can
contribute to the observations, including highorder term corrections, in the predictions.
In this spirit H1 has investigated in its whole
data set all topologies involving high-PT
isolated electrons, muons, photons, jets and
neutrinos. A good agreement of the observed
event rates with the SM is observed for all
topologies. Fig. 34 shows the results for the e +p
HERA II sample, where the largest deviation is
seen in the isolated lepton+PTmiss channel
already discussed.
Figure 33
Figure 34
Upper limit on the production cross-section of
a 3rd generation leptoquark as function of its
mass, compared to model predictions
Comparison of the observed event yields to the
SM predictions for each topology studied in the
H1 generic search (HERA II e+p data)
GENERIC SEARCHES
Generic searches investigate all possible event
topologies in a comprehensive and unified way,
and quantify the overall agreement of the whole
data set to the SM. Such searches are not
optimal to test specific models of new physics,
but allow to address two questions. First they
The agreement to the SM is studied in more
details looking at the total invariant mass or
total scalar PT, ΣPT, distributions of the events.
For each topology, the agreement of the mass
or ΣPT distribution to the SM is quantified by a
probability number. The distribution of these
probabilities for all topologies are compared to
the expectations obtained by simulating many
identical experiments. Fig. 35 shows the result
11
for the e+p sample. Good agreement is
observed, with again the isolated lepton + P Tmiss
channel showing the lowest probability.
Figure 36
Figure 35
Distribution of the probabilities quantifying the
deviations to SM of the ΣPT distributions of all
H1 generic search topologies, compared to the
expectation from SM MC experiments
The CDF experiment has performed a similar
search at Tevatron. Here the analysis includes a
global procedure, VISTA, which adjusts
experimental errors and higher-order theory
uncertainties on the data, using ~16500
distributions of ~350 event classes. After
adjustment the deviation of each distribution to
the expectation is quantified by a probability.
As seen in Fig 36, some distributions remain
poorly described. This is thought to be due to
non-understood underlying event and radiative
effects.
Distribution of the standard deviations to SM of
the shapes of all distributions studied in the
CDF generic search, after VISTA adjustment
SUMMARY
The previous hints of new physics observed at
HERA and Tevatron have not been confirmed
with increased integrated luminosities of ~1fb-1.
New particles are currently excluded for masses
ranging from ~200 GeV to ~1 TeV, depending
on the models and assumptions. The first fb-1 of
LHC data will open a new discovery window
up to ~2 to 3 TeV. Whatever this data reveal, a
good understanding of the SM, in particular
QCD and radiative effects, will be vital to
establish discoveries and interpret the
observations.
Similarly to H1, an algorithm, SLEUTH,
investigates the high-PT tails of some of the
distributions and quantifies their deviations to
the SM. No significant deviation is found.
Using this procedure CDF estimates to 46% the
probability to observe in any final state of a
redone experiment a deviation higher than
observed in the present data.
At LHC inclusive variables like total scalar PT
also provide promising methods to detect new
physics with complex topologies in the early
data.
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