1. 14 April, 2009C. Dallapiccola, MIT Seminar Mini Black Holes at the LHC as a Signature of Extra Dimensions Carlo Dallapiccola University of Massachusetts, Amherst

2. 14 April, 2009C. Dallapiccola, MIT Seminar Outline Introduction - TeV scale gravity and black holes  Motivation  Theoretical background Black Hole signature Analysis at LHC - ATLAS  Event selection  Observation and Limits

3. 14 April, 2009C. Dallapiccola, MIT Seminar Why the interest in gravitational interactions in high energy physics?

4. 14 April, 2009C. Dallapiccola, MIT Seminar Motivation I: Hierarchy Problem Conventional paradigm: two very disparate fundamental scales in physics  Electroweak Scale (E EW ) ≈ 1000 GeV  Gravitational Scale ( ) = 1.2  10 19 GeV 16 orders of magnitude difference! Striking hierarchy problem that must be some day be addressed -- what is stabilizing this large difference in fundamental scales?

5. 14 April, 2009C. Dallapiccola, MIT Seminar Motivation II: Empirical Electroweak interactions have already been probed at length scales 1/E EW  we know it’s truly a fundamental scale. Gravity has not remotely been probed at length scales 1/M Pl = 10 -33 cm  31 orders of magnitude smaller than scales at which gravity has been tested (0.01 cm). Presumptuous to assume extrapolation of Newton’s Law over these 31 orders of magnitude?

6. 14 April, 2009C. Dallapiccola, MIT Seminar A Proposal: TeV Scale Gravity Perhaps E EW is the only fundamental scale in physics  Fundamental scale even for gravity: M Pl = E EW = 1 TeV  At this energy scale, gravitational interactions comparable to weak interactions  strong gravity  Radiative stability of electroweak scale is resolved without SUSY, etc.  ultraviolet cut-off for the theory is at 1 TeV, where quantum gravity is the new physics But then how do we explain where observed Planck Scale comes from (ie. Why is gravity so weak at large distance scales = low energies)? = effective scale, not in fundamental laws

7. 14 April, 2009C. Dallapiccola, MIT Seminar Theoretical Framework Geometry of extra spatial dimensions is responsible for this apparent hierarchy Observed 3-space = 3-brane on which SM charges and fields are confined/localized. Embedded in a D-dimensional bulk = 3+n+1 spacetime dimensions Only graviton propagates in the extra dimensions String theory  branes on which some fields (open strings) are confined and others (closed strings) are not  prefers n = 7 Observed 3-space = 3-brane on which SM charges and fields are confined/localized. Embedded in a D-dimensional bulk = 3+n+1 spacetime dimensions Only graviton propagates in the extra dimensions String theory  branes on which some fields (open strings) are confined and others (closed strings) are not  prefers n = 7

8. 14 April, 2009C. Dallapiccola, MIT Seminar Theoretical Framework Two popular scenarios: Arkani-Hamed, Dimopoulos, Dvali (ADD) *  Large volume of compact (flat) extra dimensions generates the hierarchy  gravitational field lines spread through bulk. Randall, Sundrum (RS) †  Strong curvature (warping) of small AdS single extra dimension generates the hierarchy  gravity localized on a second brane bounding the extra dimension. * Phys. Lett. B 429, 263 (1998) † Phys. Rev. Lett. 83, 3370 (1999) Focus on this at ATLAS

9. 14 April, 2009C. Dallapiccola, MIT Seminar Compact Extra Dimensions (ADD)  Matter (SM fields) are localized on a 4-d submanifold (SM brane) of a higher dimensional spacetime (bulk)  Gravitational field not localized  propagates in the bulk  The n extra spatial dimensions are compactified at submillimeter length scales R  explains why not observed yet  Newton’s Law ( ) becomes:  Looks just like usual (tested) Newton’s Law, with an effective Planck Scale:

10. 14 April, 2009C. Dallapiccola, MIT Seminar Compact Extra Dimensions: Signatures Gravity “strong” at TeV (M D ) scale Deviations from Newton’s Law at short distance (torsion-balance “Cavendish” expts.) Direct or virtual emission of gravitons by SM particles in accelerator experiments Enhanced production of gravitons in early universe and in certain astrophysical processes Large cross section for black hole production at TeV collision energies

11. 14 April, 2009C. Dallapiccola, MIT Seminar Deviations from Newton’s Law Direct tests of deviations from Newton’s Law (torsion- balance “Cavendish” expts.)  n = 1 already ruled out (R = solar system scale!)  n = 2 still viable (R ≈ 10  m - 1mm)  R 4 TeV  n > 2 unconstrained  Ex.: M D > 4 GeV for n = 3

12. 14 April, 2009C. Dallapiccola, MIT Seminar Astrophysical/Cosmological Signatures Gravitons compete with other processes in carrying away energy in astrophysical phenomena Gravitons decay slowly (~10 9 yrs. or more)  preferentially 2-photon state  Gravisstrahlung accelerates supernovae cooling  Photons from decays of gravitons produced from supernovae contributes to diffuse cosmic gamma ray background  “Halo” of trapped gravitons around neutron stars  source of gamma rays long after supernova  Contribution of gravitons produced early in universe to critical density Stringent constraints (many assumptions): n > 3, M D > ~5 TeV

13. 14 April, 2009C. Dallapiccola, MIT Seminar Accelerator Signatures: Gravitons Graviton momentum in the bulk = Kaluza-Klein (KK) tower of graviton states  ~continuum of states due to large size of extra dimensions Direct graviton production:  Photon + missing E at LEP  Photon + missing E t at Tevatron (LHC)  Jet + missing E t at Tevatron (LHC) Virtual graviton exchange enhancing SM processes  Ex.:  Sensitive to unknown coupling and ultra-violet cutoff Reliable constraints (few assumptions): n > 1, M D > ~1-2 TeV

14. 14 April, 2009C. Dallapiccola, MIT Seminar Accelerator Signatures: Black Holes At CM energies above Planck scale M D black holes can be produced in particle collisions  particles passing within distance smaller than event horizon Naively, cross section for partons a and b to form a black hole is “geometric”:  R S is the horizon size, or Schwarzschild radius  Depends on which fraction of available parton energy goes into forming the black hole (trapped behind horizon).  Convolute with parton distribution functions to get Range of BH masses  depends on eff. impact param.

15. 14 April, 2009C. Dallapiccola, MIT Seminar Black Holes at the LHC At LHC (E CM = 14 TeV), cross section may be quite large Assume some min. BH mass, below which unknown quantum gravity effects are important and classical BH production is lost Use M D = 1 TeV as reference point Perspective: Z  l + l - + jets = 26 pb nMin. M BH (TeV)  (pb) 2540.7 280.34 4524.3 7522.3

16. 14 April, 2009C. Dallapiccola, MIT Seminar Black Hole Search at ATLAS LHC and the ATLAS experiment ATLAS Black Hole event simulation Search strategy and predicted discovery thresholds

17. 14 April, 2009C. Dallapiccola, MIT Seminar The Large Hadron Collider Lake Geneva  14 TeV CMS ATLAS CERN Main Site  Proton-proton collider circumference = 27 km Energy = 7 TeV / beam √s = 14 TeV Stored energy / beam = 350 MJ (!) Bunch spacing = 25 ns  40 MHz crossing rate Design luminosity = 10 34 cm -2 s -1 100 fb -1 / year Number of interactions per crossing ~23

18. 14 April, 2009C. Dallapiccola, MIT Seminar The ATLAS Detector Inner Tracker EM Calorimeter Hadronic Calorimeter Muon Detectors Diameter25 m Barrel toroid length26 m End-cap end-wall chamber span46 m Overall weight 7000 Tons

19. 14 April, 2009C. Dallapiccola, MIT Seminar Black Hole Production Collision: gravitational shock waves of ultrarelativistic particles collide  complex horizon forms Balding: collapse to a more regular “Kerr-Newman” stationary solution  asymmetries and moments (hair) shed by emitting bulk gravitons (energy lost) Spin down: angular momentum lost via emission of high- spin state particles Hawking evaporation: thermal grey-body radiation  High temperature: many high p T particles  democratic: rate of SM particle emission according to degrees of freedom  no couplings  Isotropic: no preferred direction  n dependence: higher T for higher n Mini black hole

20. 14 April, 2009C. Dallapiccola, MIT Seminar BH Evaporation Properties Particle multiplicities and missing E T ( and G) for BH events Particle p T and  LARGE n = 7

21. 14 April, 2009C. Dallapiccola, MIT Seminar Black Hole Backgrounds Primary bkgds. are states with high multiplicity and high p T jets, such as ttbar Requiring a very high p T charged lepton can greatly reduce bkgd.

22. 14 April, 2009C. Dallapiccola, MIT Seminar BH - Bkgd Characteristics BHs Bkgd

23. 14 April, 2009C. Dallapiccola, MIT Seminar Black Hole Event Selection Single jet trigger with 400 GeV threshold: > 99% eff. Uniquely identify objects in the event as muon, electron, photon or hadronic jet Select events with large scalar sum p T  Further require at least one lepton with p T > 50 GeV (QCD dijet reduced by additional 10 3 )

24. 14 April, 2009C. Dallapiccola, MIT Seminar Black Hole Selection Missing E T also characteristic (larger than, say, SUSY)

25. 14 April, 2009C. Dallapiccola, MIT Seminar BH Signal Determination Reconstruct BH Mass: Discovery: fb -1

26. 14 April, 2009C. Dallapiccola, MIT Seminar Classical BHs: Conclusion ATLAS  capable of discovering BHs up to kinematic limit of LHC 5  discovery: few pb -1 data if M thresh = 5 TeV few fb -1 data if M thresh = 8-10 TeV Could be accompanied by bulk graviton signals of jet/photon + missing energy Exciting prospect of resolving difficult hierarchy problem and perhaps even probing quantum gravity! Determining fundamental params. (M D and n) difficult But…Relies on: Large predicted cross-section (many caveats) Extrapolations of QCD dijet backgrounds at high p T from TeV scale to 14 TeV scale (could be off by orders of magnitude)

27. 14 April, 2009C. Dallapiccola, MIT Seminar Classical BHs: Recent Studies Better simulation of mass lost during balding phase: as much as 30% of mass could be lost  lowers cross section by factor of 5- 10. Better simulation of effects of BH with spin: effectively higher temp. BH  fewer, but higher p T emissions (more jet-like). Also, vector emission enhanced by factor 2-3, at expense of fermions  fewer leptons produced. Will increase amount of integrated luminosity needed for discovery and degrade S/B, but will not significantly diminish ability to observe classical BHs at the LHC

28. 14 April, 2009C. Dallapiccola, MIT Seminar Non-Classical Regime Recently argued * that classical BHs at the LHC are unlikely: only valid for M BH >> M D (M min introduced)  Quantum gravity effects important (and largely unknown) for M BH near the Planck mass  Reasonable criteria is that Compton wavelength of colliding partons are within their Schwarzschild radius or that entropy is sufficiently large: M min = 3-4 * M D Steeply falling parton distribution functions make it exceedingly difficult to satisify this relation at LHC energies Instead, we may see mostly phenomena at quantum gravity regime  eg. string balls * P. Meade and L. Randall, J. High Energy Physics 5, 003 (2008)

29. 14 April, 2009C. Dallapiccola, MIT Seminar String Balls String theory is one candidate for partial description of quantum gravity  Highly-excited string states (string balls) could be produced at the LHC  decay thermally (but more jet-like than BHs)  New mass scale introduced  string scale (M S < M D )  Thus, string ball cross-section higher than that of BHs  Select using cuts on  |p T | and jet p T ratios (at least 4 jets)

30. 14 April, 2009C. Dallapiccola, MIT Seminar String Balls: Cross Section Limits Studies: set limits on string-ball cross section for given mass threshold and 100 pb -1 int. luminosity. At 95% C.L. M S > 4.8 TeV M S > 1.6 TeV M D > 2.4 TeV

31. 14 April, 2009C. Dallapiccola, MIT Seminar Conclusion The “big” hierarchy problem, addressing the gigantic disparity of the electroweak and gravitational scales, is one of the biggest in fundamental physics Extra dimensional theories provide a framework in which the hierarchy problem is replaced by the more tractable problem of how to naturally stabilize the large sizes of the extra dimensions The LHC is well-positioned to observe or set stringent limits on the most striking phenomena: mini black hole production, string balls, etc.