Transcript Looking For New Physics With

Looking For New Physics With
Can Kılıç (Johns Hopkins University)
Work done with David E. Kaplan and Matthew McEvoy
A Long Expected Party
• We have all been
waiting eagerly for
the LHC to turn on in
order to see new
physics.
• The LHC will operate
with a CM energy of
14 TeV and a design
luminosity of
100 fb-1/year.
• The two multipurpose detectors at
the LHC are ATLAS
and CMS.
The LHC: The Lord of the Rings?
“One ring to find them all.”
Three’s Company
• But let us not forget that
there is more than ATLAS
and CMS to the LHC story.
• While I have little to say
about ALICE, LHCB is
designed to study bphysics, and is optimized
for seeing displaced
vertices.
• Furthermore because of its
reduced luminosity, LHCB
is more sensitive to soft
new physics signals with
heavy flavor tags and can
have an edge over the allpurpose detectors.
Looking Forward
• Unlike ATLAS and CMS, the LHCB does not have full coverage, it is limited
to a forward cone of 1.9<<4.9.
• The beam is detuned to 2 fb-1/year in order to reduce pile-up. (LHCB has 1
event/crossing, ATLAS/CMS have 5 at low luminosity and 23 at high
luminosity)
• The main components of the detector are the vertex locator, inner/outer
trackers, Cherenkov detectors, pile-up detector, ECAL, HCAL and muon
chambers.
Less Is More
• The LHCB vertex detectors have 21 stations of 300μ-strip detector
module pairs which are retractable and can move as close as 8mm
to the beam (4cm for CMS, 5cm for ATLAS).
• The data processing is different in LHCB compared to ATLAS/CMS
in that displaced vertices are not required to be part of a jet.
• Sophisticated Cherenkov detectors give excellent particle-id
capabilities and thus improved b-tagging efficiency.
• 2 kHz bandwidth for b-physics (compared to ~15 Hz for
ATLAS/CMS)
SUSY in Distress
• For all its successes (GCU, DM, Hierarchy
Problem), SUSY has a hard time coping
with the LEP bound of mh>114 GeV.
• Pushing the Higgs mass above this bound
in minimal SUSY scenarios requires heavy
stops and leads to severe fine tuning.
• One way out of this conundrum is to
extend the MSSM. We win if we can make
the Higgs heavier or if we can make it
decay in a channel that softens the LEP
bounds.
• In SUSY models with a singlet both these
things are not only possible, but generic. F
term contributions to the Higgs potential
from the singlet can help make the Higgs
boson heavier without severe fine tuning
while the additional degrees of freedom
modify its decay modes.
How I Learned to Stop Worrying and
Love the Singlet
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The μ problem in the MSSM has to do with
explaining the smallness of the μHuHd term in
the superpotential required for viable EWSB.
This problem is avoided in extended SUSY
models where μ=<S>, S being a singlet
superfield.
For a light Higgs boson the largest coupling to
the SM relevant for decays is yb~1/40 which
means any allowed decay mode of the Higgs to
BSM particles is generically dominant. In
particular, the dominant Higgs decay mode in the
NMSSM can be h->aa.
a, being light and part of a singlet can only decay
through mixing with the Higgs, so it adopts
couplings to the SM proportional to the Yukawas.
Consequently, a will generically decay to the
heaviest kinematically allowed fermion pair. The
coupling to down type quarks is further
enhanced by tan β.
For generic regions of parameter space, this
makes the dominant Higgs decay mode 4 bquarks. This is the channel we would like to
discover at LHCB.
A Needle in a Haystack
• The production mechanism for the Higgs is
still through glue-fusion and the cross
section is ~25pb (LO) for mh=115 GeV, this
is expected to increase significantly at
higher orders, but the background (mainly
4b production in QCD) is only done to LO
and is >~100nb.
• We generate the signal using PYHTIA and
the background using ALPGEN (4b / 4b+j)
showered through PYTHIA. For our
analysis sample we require 4 displaced
vertices within the LHCB acceptance
region. This leaves about 0.04 of the signal
and 0.02 of the background.
+ more
Details of Analysis, Estimates of Significance
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Then we construct 4 cones of ΔR=0.6
using the displaced vertices as seeds
and the best pairing is found by
minimizing ΔRp12+ ΔRp22.
The four-momentum of each acandidate (cone pairs) is reconstructed
using calorimeter towers with >1GeV.
The invariant masses ma and mh are
calculated.
We define Q1= mh - 3ma and Q2= mh +
1/3ma and plot the data in a Q1 – Q2
double histogram (bin size 5GeV x
10GeV).
We look for the greatest excess in a 3 x
5 region to find the best fit.
Our current estimate for the
significance in one year’s data is 2-3σ,
depending on the b-tagging efficiencies.
Other Interesting SUSY scenarios for LHCB
(David E. Kaplan, Keith Rehermann arXiv: 0705.3426)
• A different extension of SUSY is through violation of R-parity. Since
there are very strong bounds from proton decay and from lepton
number conservation, the easiest way to implement RPV is through
the superpotential term UcDcDc.
• The collider phenomenology in such models is quite different from
the MSSM, the bounds on superpartner masses is reduced and the
Higgs mass can be significantly below the LEP bound if mh > 2mLSP.
• The ‘LSP’ itself is unstable and decays to three jets through an ofshell squark. This means that SUSY events are devoid of both
leptons and MET, a very bleak scenario for triggering considerations.
The same can be true for Higgs physics as well.
• One distinguishing feature of such scenarios is a macroscopic decay
length for the ‘LSP’, which makes it possible for LHCB to trigger on
such events.
Other Interesting SUSY
scenarios for LHCB
(David E. Kaplan, Keith Rehermann
arXiv: 0705.3426)
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The RPV coupling of the neutralino favors decays
to heavier quarks. This makes the most likely
decay product (cbs), except for a heavy LSP that
can go to (tbs).
Considering squarks as the primary production
process, the analysis requires one displaced
vertex with 5+ tracks in the acceptance and large
invariant mass (>2mb). The main background is
multi b (c) production, in particular events where
two displaced vertices cannot be distinguished.
Depending on RPV parameters, squark masses
up to 700 GeV can be within discovery reach.
Considering Higgs decay to neutralinos as the
primary production process, the analysis requires
both displaced vertices to be within acceptance.
For generic regions of parameter space this leads
to ~103 events/year with negligible background.
More Exotic Possibilities: Hidden Valley Models
Strassler, Zurek (hep-ph:0604261,0605193,0607160)
• In the class of hidden valley models there is
a new physics sector which is neutral under
SM, and the two sectors only talk through
heavy degrees of freedom.
• Because of the energy barrier, it is plausible
for the new physics to have completely
evaded LEP and TeVatron searches.
• In one such model, the new physics is a
copy of QCD, and the primary production
mechanism is that of v-quark pairs which vhadronize.
• Most v-hadrons will be unseen but there
can be states which carry the right quantum
numbers to decay to a SM current, for
scalars the helicity flip required makes Γ~m,
so a likely final state is b-quarks.
More Exotic Possibilities: Hidden Valley Models
Strassler, Zurek (hep-ph:0604261,0605193,0607160)
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The signatures of such models can be very unusual, there can be a large
number of jets in the final state with possible heavy flavor tags, and the vhadrons themselves may have macroscopic decay lengths.
If only few states decay to SM within the detector, the visible part of the
event can be soft and our ability to see the new physics may be limited to
identifying displaced vertices and heavy flavors. In fact the jets can be on
top of each other which is bad for triggering on reconstructed objects.
A different class of models has the Higgs boson as the bridge between the
SM and the new physics, providing a non-SUSY scenario with non-standard
Higgs decays which has very similar collider signatures to the NMSSM, in
such a case covering all bases detector-wise may again be crucial.
Finally, SUSY-HV models can fake RPV, if the LSvP is lighter than the LSsP,
so in SUSY events most of the ‘MET’ is transformed into v-hadrons with
their characteristic phenomenology.
While HV models may appear theoretically unmotivated, many benchmark
BSM frameworks with small Z2 breaking can produce similar signatures.
Conclusions
• Most experimental search strategies are based on a few
benchmark new physics frameworks whose specific
collider signatures may be less generic than we have
come to believe.
• While for most people LHC is synonymous to
ATLAS/CMS, there are BSM physics scenarios that give
rise to softer final state particles without large MET,
many such scenarios can involve heavy flavors which
could give LHCB an edge for early discovery.
• We have shown that LHCB is quite relevant for the
search of a Higgs boson dominantly decaying to 4 b-jets.
There is potential for increasing the significance using
better detector simulation.
• RPV-SUSY and hidden valley scenarios also fall into the
above class of models.
• What is your favorite nightmare LHC scenario? Maybe
LHCB can help..