Published on November 28, 2007
Slide1: Lisa Randall, Harvard University Phenomenology?: Phenomenology? Bulk gauge bosons Means KK modes of Weak bosons Gluons Fermions As well as gravitons Precise signatures depend on fermion wavefunction profiles: Precise signatures depend on fermion wavefunction profiles Nontrivial profiles help solve flavor problem Masses depend on overlap with Higgs Expect light fermions localized near Planck/Gravity brane Top near Weakbrane since it’s heavy IR KK d s tR uL,dL Definitions: Definitions Fermion Model: Fermion Model Richer Spectrum…But Lower Production Cross Sectionfor the Graviton: Richer Spectrum… But Lower Production Cross Section for the Graviton Light quarks are localized away from Higgs Hence away from TeV brane No Drell-Yan production from quarks Gluons are spread throughout the bulk Hence coupling to graviton down Graviton Interactions: Graviton Interactions Volume suppressed Features of interactions: Features of interactions The suppression by M4L is a factor of 1/N from the dual gauge theory perspective 1/mTeV is local cutoff p k rc because gluon has flat wave function: volume factor Fermion behavior KK Graviton Production: KK Graviton Production w/Liam Fitzpatrick,Jared Kaplan, Liantao Wang Final State? Dominant Decay to right-handed tops: Final State? Dominant Decay to right-handed tops Determining top jets: delta R: Angle between decay products: Determining top jets: delta R: Angle between decay products Angular dependence: spin determination : Angular dependence: spin determination Graviton: some reachOther Bulk Modes?: Graviton: some reach Other Bulk Modes? Gluon KK Mode: Gluon KK Mode Gluon KK mode coupling to light quarks is less suppressed than graviton Gluon KK mode wave function relatively flat in bulk Benefit from light quark coupling: not as for gluon No 1/ML, Gluon KK mode lighter by factor 1.5 Larger reach for gluon KK mode Slide16: Understand from dual point of view in terms of gluon KK mixing-vector meson dominance Note only gluon production from quarks. At tree level, gluon coupling vanishes. Slide17: Gluon wave function Gluon fermion interaction: Gluon fermion interaction Dominates over top jet background: Dominates over top jet background w/Ben Lillie, Liantao Wang However, signal doesn’t dominate over jet background: However, signal doesn’t dominate over jet background Clearly…: Clearly… Efficient top jet identification required, especially for heavier KK gluons Usual method: relies on separated decay products Won’t be true for energetic tops How to identify energetic tops?: Top jet mass measurement Detailed substructure of jets: eg hard lepton Summary So Far: Summary So Far If RS1 solves the hierarchy problem, we should be able to tell Clean KK graviton signal if SM on brane Best signature: spin-2 resonance and mass gap In bulk, gluon KK mode will be important Decays into tops critical Challenge is to maximize energy reach Critical for many possibilities for electroweak sector Models give insights into what to look for End of Part II Part III: Quantum Gravity at the LHC Other Exotics? Black Holes?: Part III: Quantum Gravity at the LHC Other Exotics? Black Holes? Estimate black hole production cross section –claim: just need 2 energetic beams within RS M~TeV=>~100 pb cross section Not suppressed by gauge couplings or phase space factors Original claims: Prolific Production! Spectacular fireball final states! Recent Work: Recent Work How much could we really hope to learn from black holes? Do we even produce them? We will see: LHC unlikely to make classical black holes states that decay with high multiplicity via Hawking radiation However…all is not lost Potentially much more prolifically produced 2 body final states Uncalculable, but we will see distinctive experimental signatures that will distinguish among modes Might teach us about quantum gravity Why Change in Expectations?: Why Change in Expectations? Estimate was always optimistic Understanding uncertainties and making refinements essential PDFs drop rapidly and We are necessarily near black hole production threshold Every term in original estimate must be considered carefully M: quantum gravity scale MBH: black hole mass relative to center of mass energy Criteria for a Black Hole?: Criteria for a Black Hole? MBH>M As advertised, not even convention independent 2p/(M/2)<RS More stringent version of above ADD (n=6) MBH>4M—almost at experimental limit RS MBH>16M—if taken seriously, bhs already out of reach Additional thermality/entropy constraints support these high mass threshold claims What is true threshold energy?: Inelasticity as function of impact parameter: What is true threshold energy?: Inelasticity as function of impact parameter What fraction of com energy goes into black hole Important since PDFs fall rapidly—effectively increases threshold Penrose, D’eath and Payne, Eardley and Giddings, Yoshino and Rychkov Parameterize two Aichelberg-Sexl shock waves (two highly boosted particles) intersecting What fraction of energy gets trapped behind horizon? Of course applies in classical regime but we use to estimate w/ and w/o inelasticity; Impact parameter weighted: w/ and w/o inelasticity; Impact parameter weighted Upshot: Upshot Black hole production threshold (MBH) higher than originally thought Means Lower production cross section Lower reach in black hole mass Translates into lower entropy reach as well Don’t produce classical thermal black holes What do we produce? 2 body final states! Compositeness Searches for Quantum Gravity : Compositeness Searches for Quantum Gravity Measure differential cross section Measure angular dependence through Rh (much less systematic error) Indicator of strong dynamics Clarification: Clarification We don’t really think we can make precise predictions We use models for quantum gravity To see what to look for Take advantage of potentially rich data Ask: what are distinguishing features that Experimentally probe quantum gravity Also note we forbid global quantum number violating transitions so we focus on B-conserving jets and lepton-number conserving processes Eg: Result Model I: Dijet “Black Holes”: Eg: Result Model I: Dijet “Black Holes” Lepton cross section might be key: Lepton cross section might be key Four-fermion operators: large lepton suppression Pdf, alpha, u/s TeVADD:MD=1 From Black Holes: From Black Holes Much higher cross section since large fraction with larger pdfs Even just losing u/s, alpha Summary: Summary Black holes not as “spectacular” as advertised BUT Lots of information about quantum gravity buried in 2->2! Initial increase in rate for more central processes always occurs Could be related to fundamental partons in black holes? R behavior: bh, string resonances, different forms for string, Z’ all distinctive Threshold behavior where interference matters Hadron vs. Lepton cross section Conclusion: Conclusion Physics at a few TeV could be spectacular Might be relatively low end But need to modify strategies to have best capacity for high energy Tops, compositeness searches can be key We could be lucky-low energy clean signals But in any case there should be something there Hopefully we’ll know in a few years!