New Physics at the TeV Scale ? (What do we expect at LHC ?)

1. New Physics at the TeV Scale ? (What do we expect at LHC ?) Kiwoon Choi (KAIST) (CTP Colloquium @ SNU)

2. LHC is a proton + proton collider with c.m energy 14 TeV and the luminosity 1034 cm-2 s-1 in the 26.6 km tunnel at CERN, Geneva. LHC (Large Hadron Collider) is coming.

3. Swiss Alps Lac Leman Geneva Airport CERN

4. LHC will probe for the first timethe TeV energy scale (~ 10-19 m) which is expected to be the threshold scale of revolutionary new physics. # Why do we expect new physics at TeV ? # What do we expect (hope?) to see at LHC ? # What would be the implications of LHC discoveries for more fundamental physics ?

5. Threshold scales of new physics (c = h/2π = 1) # Scale of atomic spectroscopy: α2em me ~ 10 eV ⇒ Atomic structure and Quantum Mechanics manifesting themselves at scales ≥ αem me ~ 103 eV (uncertainty principle, wavefunction description of physical state, force as an exchange of particle, …) # Scale of nuclear force: mπ ~ 100 MeV ⇒ Quarks, gluons and Quantum Chromodynamics (QCD) manifesting at scales ≥ ΛQCD ~ 1 GeV (asymtotic freedom, confinement, chiral symmetry breaking, …)

6. # Scale of weak force due to electroweak symmetry breaking (EWSB): MW,Z ~ 100 GeV We don’t know yet what is the underlying physics of EWSB, just expect it can be as rich as the new physics encountered at lower threshold scales. There are many reasons to believe that there exists the so-called Higgs boson providing the simplest description of EWSB. However this minimal Higgs description of EWSB suffers from a fine tuning problem, implying that more nontrivial structures should exist , and this is why we expect there exist rich new physics at the TeV scale.

7. yt top yt Higgs Unnatural Higgs boson mass Self energy due to the cloud of virtual top-antitop pairs surrounding Higgs δm2H = - 3y2tΛ2/2π2 (yt ~ 1) (Λ = maximal energyof the top-antitop fluctuations) m2H = (m2H)bare+ δm2H ~ - M2W,Z We always like to have a description valid over a large scale, e.g. valid up to Λ≫MW , however it requires an unnatural fine tuning at the level of M2W/Λ2≪ 1 .

8. Similar problem has been encountered in # Electron mass in non-relativistic QED (or classical electrodynamics) # Cosmological constant of our Universe Electron mass: Electromagnetic self energy due to Coulomb-type virtual photons surrounding an electron of radius re δme = e2/(4π2re) me = (m e)bare + δme = (m e)bare + e2/(4π2re) Unnatural fine tuning is required also if this theory of electron and photon does work up to Λ = 1/re≫ me .

9. Hypothetical physicist knowing only the low energy world at scales ≤ me can imagine three different possibilities. • Composite electron Electron is a fat guy with radius re ~ 1/me ~ 4x10-13m ⇒ δme ~ e2/(4π2re) < me Without directly probing the physics at the composite length scale, one might be able to exclude this possibility by precise low energy multipole expansion data. Anomalous magnetic moment of the electronδμ : Composite structure at re ⇒ δμ ~ ere ~ e/me

10. B)More Universes and Anthropic Selection re ≪ 1/me and δme ~ e2/(4π2re) ≫ me However there exist huge number of different Universes having all possible values of (me)bareand thus of me, and we are living in a special Universe realizing a fine cancellation between (me)bare and δme , yielding me = (m e)bare + δme = 0.51 MeV, since any sizable deviation of me from this value does not allow the DNA helix to replicate itself, thus not allow us to exist now. (T. Regge, 1971)

11. C)More Symmetry There appears a new symmetry at scales ~ me , eliminating the linearly divergent part ( ~ 1/re) of δme, thereby reducing the self energy as δme ≤ me even for Λ = 1/re ≫ me . Indeed relativistic QED has a new symmetry γ5 |e, E, up> = |e, -E, down> = |-e, E, up> (electron) (positron) δme = = [ e2Λ/(4π2) - e2Λ/(4π2 ) ] + 3e2meln(Λ/me)/(16π2) ( γ5 cancellation ) (V. F. Weisskopf, 1934) ≤ meas long as Λ ≤ mee500 ~ 10210 GeV

12. Cosmological Constant (Vacuum Energy Density) Vacuum energy density of generic quantum field: δΩvac = ±Σ ( zero point energy ) ~ Λ4 (Λ = maximum energy of quantum fluctuations) Ωvac = (Ωvac)bare +δΩvac = (3x10-3 eV)4 The success of quantum field theory up to scales ~ MW suggests Λ ≥ MW ~ 1011 eV , then an extreme fine tuning (better than 10-50) is required to get the observed vacuum energy density of our Universe.

13. Again we can imagine three possibilities. A)Composite graviton at sub-millimeter Graviton is a composite object at scale ~ 10-3 eV, so is blind to quantum fluctuations at higher energy scales. (R. Sundrum, 2003) This might be tested by looking for a modification of gravity at sub-millimeter scale. (Principle of general covariance appears as an emergent feature of Nature at sub-millimeter.)

14. B)More Universe and Anthropic Selection Ωvac = (Ωvac)bare + Λ4 = (3x10-3 eV)4 There exist huge number of different Universes having all possible values of (Ωvac)bareand thus of Ωvac , and we are living in a special Universe realizing the extreme fine tuning (better than 10-50) for Ωvac = (3x10-3 eV)4, since only a Universe with - (2x10-3 eV)4 ≤ Ωvac ≤ (10-2 eV)4 can form a galaxy and a life in it. (S. Weinberg,1987)

15. C)More Symmetry At present, we don’t know any symmetry reducing the vacuum energy density Ωvacdown to the observed value ~ (3x10-3 eV)4 . (cf. Kaplan and Sundrum (2005)?) ################################################# Now, we know definitely that more symmetry(γ5 symmetry) is the correct answer to the electron mass puzzle, while at the moment more Universe is considered as the most plausible answer to the cosmological constant puzzle because we don’t know any theoretical scheme to realize the other possibilities (fat graviton or symmetry) .

16. S. Weinberg ...A physicist talking about the anthropic principle runs the same risk as a cleric talking about pornography : no matter how much you say you are against it, some people will think you are a little too much interested. Remarks on multiverse and anthropic principle The multiverse and anthropic principle assume that there exist huge number of different universes including any kind of Universe under the consideration, and our universe has certain particular feature since only a universe with such feature can accommodate life. On the other hand, we are looking for a fundamental principle selecting our universe as a unique possibility. So, talking about the anthropic principle, some of your friends might say“you are abandoning to be a physicist ”.

17. Until very recently, we didn’t take the multiverse and anthropic principle as a serious explanation for any fine tuning problem in physics. But the recent discovery of dark energy which is most likely to be a small but nonzero cosmological constant dramatically changed our attitude (but not the attitude of religious people who believes in the God’s design for unique Universe). The realization that string theory admits huge number of different vacua giving different universes (string landscape) also boosted this change of our attitude.

18. Martin Rees : I bet my dog’s life! Andrei Linde : I bet my own life! How much are some prominent physicists now confident about the multiverse explanation of the cosmological constant ?

19. Steven Weinberg : I bet the lives of both A.Linde & M.Rees’s dog!

20. Christoph Schoenborn (Cardinal Archbishop of Vienna) “Now, at the beginning of the 21st century, faced with scientific claims like neo-Darwinism and the multiverse hypothesis in cosmology invented to avoid the overwhel ming evidence for purpose and design found in modern science, the Catholic Church will again defend human nature by proclaiming that the immanent design evident in nature is real. Scientific theories that try to explain away the appearance of design as the result of`chance and necessity are not scientific at all, but, as John Paul put it, an abdication of human intelligence.”

21. What will be the correct answer to the Higgs mass puzzle: mH2 = (mH2)bare + Λ2 ~ - (300 GeV)2 ? • Composite Higgs at Λ ~ 1 TeV ? Highly disfavored by the measured multipole properties of the W and Z bosons • Multiverse and Anthropic selection with Λ ≫ 1 TeV ? Might be the answer as a Universe with mH2 > 0 or mH2 < - (1 TeV)2 cannot form complex elements for life. (Agrawal et al, 1997) (No exciting new physics at TeV other than the boring Higgs, so a disaster for us !)

22. C)More Symmetry ? Yes, we have a beautiful symmetry (SUPERSYMMETRY) naturally reducing the Higgs self energy as δmH2 ~ MW2. SUSY | fermion > = | boson >( SUSY |quark>=|squark> ) SUSY | boson > = | fermion >( SUSY |photon>=|photino> ) δmH2 = -3yt2[Λ2 – Λ2 + mst2 ln (Λ/mst) ] / (2π2) (SUSY cancellation) SUSY at TeV ( mst = stop mass ~ 1 TeV) gives the desired mH ~ MW ~ 100 GeV without fine tuning. yt top yt Higgs stop + Higgs

23. SUSY is the unique possible extension of the Lorentz symmetry, so no doubt about its existence. The real question is at which scale it does appear, and it is very likely that TeV is the right scale for SUSY to appear as it gives a natural EWSB. In TeV scale SUSY scenario, superpartners of known particles have a mass ~ 1 TeV: Fermion SUSY Boson quark ⇔ squark lepton ⇔ slepton photino ⇔ photon gluino ⇔ gluon

24. In addition to providing natural EWSB , TeV scale SUSY has more attractive features !

25. # Lightest superparticle (typically photino) is stable anda good Dark Matter candidate. Direct Evidence of Dark Matter (Galaxy cluster : 1E 0657-56)

26. # Unification of the strong, weak and electromagnetic gauge couplings

27. If SUSY exists at TeV, LHC will discover it! p p ⇒ gluino or squark pairs ⇒ many jets + leptons + missing ET Glunio Mass ≤ 1 TeV: few months of running Gluino Mass ≤ 3 TeV: several years of running (5σ discovery)

28. LHC can measure sparticle masses accurately. (with an accuracy of order few to 10%) # Cascade decays of sparticle Hinchliffe et al

29. Dilepton invariant mass distribution ~ 77 GeV

30. : transverse mass and momentum of qq system : trialmass and transverse momentum of χ # Transverse mass of sparticle pair (MT2) Lester and Summers Gluino MT2for the process: gluino + gluino⇒ q q χ + q q χ(q=quark, χ=photino) Cho, KC, Kim, Park (2007) (minimization over all possible splitting of the observed missing ET) = Max of mT2(gluino) over all events = a function of the trial photino mass

31. The gluino MT2max(mχ) has a CUSP when trial photino mass = true photino mass, with which one can determine precisely both the gluino mass and the photino mass. Cho, KC, Kim, Park (2007)

32. LHC measurement of gluino, photino, squark, and slepton masses:Gluino MT2 can make the accuracy much better!

33. Mass Ma m2 GUT? String? (GeV) 103 1016 Sparticle masses are generated at very high scale and logarithmically run down to low energy scale. So, accurately measured sparticle spectra at TeV provide an window to more fundamental physics such as grand unification or superstring structure which might exist at extremely high energy scale.

34. KKLT-type string compactification at scale 10-33 m Kachru, Kallosh, Linde, Trivedi (2003) Our world Anti- brane Quark, lepton, gauge boson, superpartners 6D CY space with radius 10-33m * The structure of string flux vacuum at 10-33m can be read from the pattern of superparticle masses at 10-19m.

35. Mechanism to mediate SUSY breaking in KKLT-type string compactifications Choi, Falkowski, Nilles, Olechowski (2005) Messengers of supersymmetry breaking graviton CY volume modulus SUSY breaking brane Our world

36. 질량 질량 Ma Ma m2 m2 거리 거리 10-19m 10-19m 10-25m 10-33m 10-33m Superparticle masses in such mediation scheme show a highly distinctive feature ! Choi, Jeong, Okumura (2005) Mirage Mediation Conventional scenarios Mirage mediation

37. Transmission of SUSY breaking to our world should be an important ingredient of the fundamental theory such as superstring or supergravity theory, which can be tested by the sparticle spectra measured by LHC. Four mediation schemes predicting distinctive patterns of sparticle spectra at TeV: # Gravity mediation ⇒ M1 : M2: M3 = 1 : 2 : 6 # Gauge mediation ⇒ M1 : M2: M3 = 1 : 2 : 6 (but with light gravitino) # Anomaly mediation ⇒ M1 : M2: M3 = 3.3 : 1 : 9 # Mirage mediation ⇒ M1 : M2: M3 = 1 : 1.3 : 2.5

38. Sparticle spectra from these mediation schemes

39. LHC will be able to test the predictions of these mediation schemes, and exclude some or all of them.

40. Conclusion and Summary # We are confronting a critical moment of particle physics. # From July, 2008, LHC will be probing the TeV scale where exciting new physics is expected to be waiting for us. # TeV SCALE SUPERSYMMETRY appears to be the most plausible candidate for the new physics at TeV.

41. Conclusion and Summary # LHC will be able to determine sparticle spectra, thereby providing an window to more fundamental physics such as grand unification, supergravity and superstring structure which might exist at extremely high energy scales.