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 multi-purpose 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 b-physics, 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 all-purpose 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 • 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 b-quarks. 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 • 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 a-candidate (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 of-shell 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) • 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 ModelsStrassler, 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 v-hadronize. • 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 ModelsStrassler, Zurek (hep-ph:0604261,0605193,0607160) • 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 v-hadrons 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..