Higgs Signals and Implications for MSSM

Transcription

1 Higgs Signals and Implications for MSSM Shaaban Khalil Center for Theoretical Physics Zewail City of Science and Technology

2 SM Higgs at the LHC In the SM there is a single neutral Higgs boson, a weak isospin doublet. Three of the components of the two complex fields are absorbed by the W and Z 0 leaving one field H = 0 v + H(x) 2 where v is the vacuum expectation value of the field. The masses are M W = gv 2 M Z = gv 2cosθ W M H = 2 v The mass of the Higgs is not predicted in the SM. The perturbative limit implies that M H 700 GeV.

3 Higgs Boson Feynman Rules Higgs couples to heavy particles No tree level coupling to photons ( ) or gluons (g) M H = 2 v large M H is strong coupling regime

4 SM Higgs Decay at the LHC The Higgs coupling to a pair of fermions is proportional to the fermion mass m f, therefore the decay to heavy quarks and leptons is favored. A light Higgs (M H <130 GeV), decays mainly to bb, a heavy Higgs mainly to four leptons via WW or Z 0 Z 0. For a heavy Higgs the most sensitive channels at CMS are therefore WW l + l + missing energy or Z 0 Z 0 2(l + l ).

5 For a light Higgs the gg bb background dominates the H bb signal. For CMS the most sensitive discovery channel is H γγ (which has a much smaller decay branching ratio) thanks to its crystal -calorimeter. Unless the Higgs is heavy, a significant (5 ) signal will be observed in the invariant mass distribution with about 20fb 1 which corresponds to 2 years of operation at a reduced initial luminosity of cm 2 s 1. Event expect to be produce with L = 1fb 1 M H WW lvlv ZZ 4l γγ

6 SM Higgs Production at the LHC

7 Gluon fusion is the dominant mechanism for Higgs production at the LHC

8 ATLAS and CMS Combine summer '11 Search Limits on the SM Higgs CMS & ATLAS combined search excludes SM Higgs in the mass range GeV

9 Higgs Decay into Two Photons Background model fit to the m γγ distribution, together with a simulated signal (M H =120 GeV). The magnitude of the signal is what would be expected if its cross section were 5 times the SM expectation. The expected exclusion limit at 95% CL is between 1.5 and 2.0 times the SM cross section in the mass range between 110 and 140 GeV. The observed limit disfavours at 95% CL a SM Higgs boson decaying into two photons in the mass range 127 to 131 GeV

10 CDF on Higgs Decays to Diphotons The CDF collaboration has recently released new results from a search for what is probably the clearest signature of Higgs boson decay: pairs of highmass photon candidates Signal is shown in red. The background is in grey, and the CDF data are shown with black points with error bars. signal shown corresponds to the one that CDF was able to exclude at 95% confidence level. For a Higgs mass of 120 GeV: signal is about 20 times what the standard model predicts.

11 CMS and ATLAS searched for a Higgs boson in the four-lepton decay channel, H ZZ 4l with each Z boson decaying to an electron or muon pair. CMS observed seventy-two events with four lepton invariant mass m 4l > 100 GeV, while events are expected from background. The four-lepton mass distribution is consistent with the expectation of standard model background production of ZZ pairs. Upper limits at 95% CL exclude the SM Higgs boson in the ranges GeV, GeV, and GeV. H ZZ 4l

12 Higgs Mass Hierarchy Problem If the cutoff scale is very high, fine tuning of the Higgs boson mass is serious problem. We need to find a reason to keep the Higgs boson mass light.

13 Hierarchy problem and SUSY String and GUT unification A cutoff scale ~ Planck scale (10 19 GeV). SUSY is a symmetry to avoid the fine tuning in the renormalization of the Higgs boson mass at the level of O(10 34 ). In SUSY, the loop diagrams that are quadratically divergent cancel, term by term against the equivalent diagrams involving superpartners. If M H ~ O(100) GeV, the masses of superpartners should be O(1) TeV. Thus, some of the superpartners will be detected at the LHC.

14 Minimal Supersymmetric SM The MSSM is a straightforward supersymmetrization of the SM with minimal number of new parameters. The particle content of MSSM = Two Higgs doublet SM + scalar SUSY partners and fermionic SUSY partners Two Higgs doublets are necessary for fermion Yukawa couplings: H1: down-type-quark and lepton Yukawa couplings H2: up-type-quark Yukawa couplings Different assumptions about SUSY breaking are often made. This leads to quite different phenomenological predictions. A new symmetry, called R-symmetry is introduced to rule out the terms violate baryon and lepton number explicitly and lead to proton decay at unacceptable rates.

15 SUSY Particle Spectrum

16 Higgs Mass in SUSY Models SUSY models include at least two Higgs doublets. This means: 8 degrees of freedom, 3 eaten up by the W± and Z 5 Higgs fields: h 0,A 0,H 0,H ± Connection between Higgs masses and gauge boson masses: Connection between Higgs masses and gauge boson masses: m 2 H,h = 1 2 m2 A + m2 Z ± m 2 A + m 2 Z 2 4m 2 Am 2 Zcos 2 2β m 2 H ± = m 2 W + m 2 A cos 2 β α = m2 h(m 2 Z m 2 h) m 2 A(m 2 A m 2 h) α is the mixing angle between the two neutral Higgs bosons

17 At the tree level: Light higgs is always lighter than Z-boson. Charged Higgs is always heavier than the W-boson 2 parameters determine the model (m A, m h ) or (tan β, m A ) or (tan β, m h ) Decoupling limit: m A >> m Z m h ~ m max h m A ~ m H ~ m H ± cos β α =0 The relation between the SM and the neutral SUSY Higgs couplings is give by

18 Top-stop Radiative Corrections For top-stop, which is a good approximation M 2 t = M 2 S + m 2 t + cos2β S2 w M 2 Z m t X t m t X t M 2 S + m 2 t cos2βs2 wm 2 Z

19 Therefore m 2 h m 2 Z + 3g 2 m 4 t 8π 2 m 2 W ln M2 S m 2 t + X2 t (1 M 2 S X2 t 12M 2 S ) X t =0 No mixing & X t = 6 M S Maximal mixing MSSM Higgs Mass Spectrum Probing neutral Higgs with mass <130 GeV is salient prediction and stringent test for MSSM

20 MSSM Lightest Higgs & SM Higgs What is the difference between a SUSY Higgs and SM Higgs? In MSSM, there are 2 scalar Higgs bosons. The light one is a SM-like one, bur differs in its coupling to all particles and it has an upper bound of 130 GeV. Can one implement the search for the SM Higgs boson in the framework of the MSSM Higgs search? Yes, but corrections are needed due to the different couplings. The MSSM Higgs has decay channels which are either irrelevant or do not exist in the SM. Once a scalar Higgs boson is observed, can one tell if it is a SUSY Higgs boson or a SM one? No definite answer at the LHC.

21 As an example, we consider MSSM benchmark SPS 1a which is consistent will all observations from collider data and cosmology. In this case, one finds Search for MSSM Higgs The branching ratio of this chain is 21%. The signature will be 2 b-jets, missing energy (> 200 GeV) and multiple jets from the quark q and the associated decay of the antigluino/antisquark. The main background stems from tt production, W and Z 0 + jets and b decays to b-quarks. SUSY and Higgs masses in MSSM for SPS 1a

22 A preliminary simulation has been performed, which requires at least 4 jets, 2 tagged b-jets and missing energy above 200 GeV. Here we shows the bb invariant mass distribution for an integrated luminosity of 10 fb 1 Also we show the mass region covered in the m 0 m 1 plot (5σ limit). Clearly the signal in left fig could be observed rather early at LHC.

23 Gluophobic Scenario Cancellation between top & stop quark loops can strongly suppressed the Higgs production cross section at the LHC. This cancellation is more effective for low stop mass, therefore large mixing parameter X t 23

24 The couplings Y hbb, Y hττ ~ sinα Small α eff Scenario cosα the coupling to bb and ττ might be suppressed. This scenario is important for light Higgs boson. including radiative corrections, one define α eff and Suppression occurs for large tanβand relatively low m A 24

25 Conclusions The importance of the LHC for the future of high energy physics cannot be overemphasized. One of the main motivations for the experiments at LHC is to search for Higgs and SUSY particles. Little room left for the SM Higgs! 114 < M H < % CL If SUSY exists at the electroweak scale, then these particles could be easily observed at the LHC. As an example, we assumed the msugra scenario where the LSP is stable and which is the most interesting candidate for dark matter. Discovery of SUSY, if it occurs, would be a revolution of physics in 21st century. If the LHC finds SUSY, this would be one of the greatest achievements in history of theoretical physics.

26 Thank you