Download Black Holes

Mini Black Holes at the LHC as a
Signature of Extra Dimensions
Carlo Dallapiccola
University of Massachusetts, Amherst
14 April, 2009
C. 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
14 April, 2009
C. Dallapiccola, MIT Seminar
Why the interest in gravitational
interactions in high energy physics?
14 April, 2009
C. Dallapiccola, MIT Seminar
Motivation I: Hierarchy Problem
• Conventional paradigm: two very disparate
fundamental scales in physics
 Electroweak Scale (EEW) ≈ 1000 GeV
 Gravitational Scale ( M Pl  c GN ) = 1.21019 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?
14 April, 2009
C. Dallapiccola, MIT Seminar
Motivation II: Empirical
• Electroweak interactions have already been probed at
length scales 1/EEW  we know it’s truly a fundamental
scale.
• Gravity has not remotely been probed at length scales
1/MPl = 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?
14 April, 2009
C. Dallapiccola, MIT Seminar
A Proposal: TeV Scale Gravity
• Perhaps EEW is the only fundamental scale in physics
 Fundamental scale even for gravity: MPl = EEW = 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
14 April, 2009
C. 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
14 April, 2009
C. Dallapiccola, MIT Seminar
Theoretical Framework
Two popular scenarios:
Focus on this at ATLAS
• 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)
14 April, 2009
C. 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 ( V   m1m2 M Pl2 r ) becomes:
m1m2 1
 n 1 r  R
n 2
MD
r

m1m2 1
V r  n 2

r  R
n
MD R r
V r 
MD 1TeV
 Looks just like usual (tested) Newton’s Law, with an effective
Planck Scale:
2
n 2 n

Pl
D 
M M
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R
C. Dallapiccola, MIT Seminar
Compact Extra Dimensions: Signatures
M Pl2  M Dn2 R n
Gravity “strong” at TeV (MD) 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
14 April, 2009
C. Dallapiccola, MIT Seminar
Deviations from Newton’s Law
• Direct tests of deviations from Newton’s Law (torsionbalance “Cavendish” expts.)
 n = 1 already ruled out (R = solar system scale!)
 n = 2 still viable (R ≈ 10m - 1mm)
 R < 30m, MD > 4 TeV
 n > 2 unconstrained
 Ex.: MD > 4 GeV for n = 3
14 April, 2009
C. Dallapiccola, MIT Seminar
Astrophysical/Cosmological Signatures
• Gravitons compete with other processes in carrying
away energy in astrophysical phenomena
• Gravitons decay slowly (~109 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 , MD > ~5 TeV
14 April, 2009
C. 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: e e qq  G qg  qG
 Photon + missing E at LEP
 Photon + missing Et at Tevatron (LHC)
 Jet + missing Et at Tevatron (LHC)


• Virtual graviton exchange enhancing SM processes
 
 
 
 Ex.: e e  e e qq  
 Sensitive to unknown coupling and ultra-violet cutoff


Reliable constraints (few assumptions):
n > 1 , MD > ~1-2 TeV
14 April, 2009
C. Dallapiccola, MIT Seminar
Accelerator Signatures: Black Holes
• At CM energies above Planck scale MD 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
2
black hole is “geometric”:  ab BH  RS
 RS is the horizon size, or Schwarzschild radius
 Depends on which fraction of available parton energy sˆ goes
 hole (trapped behind horizon).
into forming the black
 Convolute with parton distribution functions to get  pp BH

• Range of BH masses  depends on eff. impact param.
14 April, 2009
C. Dallapiccola, MIT Seminar

Black Holes at the LHC
• At LHC (ECM = 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 MD = 1 TeV as reference point
n
Min. MBH (TeV)
 (pb)
2
5
40.7
2
8
0.34
4
5
24.3
7
5
22.3
• Perspective: Zl+l- + jets = 26 pb
14 April, 2009
C. Dallapiccola, MIT Seminar
Black Hole Search at ATLAS
• LHC and the ATLAS experiment
• ATLAS Black Hole event simulation
• Search strategy and predicted discovery thresholds
14 April, 2009
C. Dallapiccola, MIT Seminar
The Large Hadron Collider
• Proton-proton collider
circumference = 27 km
Lake Geneva

• Energy = 7 TeV / beam
√s = 14 TeV
• Stored energy / beam
CERN Main Site
= 350 MJ (!)

ATLAS
CMS
• Bunch spacing = 25 ns
 40 MHz crossing rate
• Design luminosity
= 1034 cm-2 s-1
• 100 fb-1 / year
• Number of interactions
per crossing ~23
14 April, 2009
C. Dallapiccola, MIT Seminar
The ATLAS Detector
Muon Detectors
EM Calorimeter
Inner Tracker
Hadronic Calorimeter
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Diameter
Barrel toroid length
End-cap end-wall chamber span
Overall weight
C. Dallapiccola, MIT Seminar
25 m
26 m
46 m
7000 Tons
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 highspin state particles
• Hawking evaporation: thermal grey-body radiation
Mini black hole
 High temperature: many high pT 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
14 April, 2009
C. Dallapiccola, MIT Seminar
BH Evaporation Properties
Particle multiplicities and missing ET ( and G) for BH events
LARGE
n=7
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Particle pT and 
C. Dallapiccola, MIT Seminar
Black Hole Backgrounds
• Primary bkgds. are states with high multiplicity and high pT jets,
such as ttbar
• Requiring a very high pT charged lepton can greatly reduce bkgd.
14 April, 2009
C. Dallapiccola, MIT Seminar
BH - Bkgd Characteristics
Bkgd
BHs
14 April, 2009
C. 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 pT   pT  2.5 TeV
• Further require at least one lepton with pT > 50 GeV
(QCD dijet reduced by additional 103)

14 April, 2009
C. Dallapiccola, MIT Seminar
Black Hole Selection
p
T
 2.5 TeV
Missing ET also
characteristic
(larger than, say,
SUSY)
14 April, 2009
C. Dallapiccola, MIT Seminar
BH Signal Determination
Reconstruct BH Mass:
Discovery:
S
B 5
S  10


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C. Dallapiccola, MIT Seminar
Classical BHs: Conclusion
• ATLAS  capable of discovering BHs up to kinematic limit of LHC
• 5 discovery: few pb-1 data if Mthresh = 5 TeV
few fb-1 data if Mthresh = 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. (MD and n) difficult
But…Relies on:
• Large predicted cross-section (many caveats)
• Extrapolations of QCD dijet backgrounds at high pT from TeV scale
to 14 TeV scale (could be off by orders of magnitude)
14 April, 2009
C. 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 510.
• Better simulation of effects of BH with spin: effectively higher
temp. BH  fewer, but higher pT 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
14 April, 2009
C. Dallapiccola, MIT Seminar
Non-Classical Regime
• Recently argued* that classical BHs at the LHC are
unlikely: only valid for MBH >> MD (Mmin introduced)
 Quantum gravity effects important (and largely unknown) for
MBH near the Planck mass
 Reasonable criteria is that Compton wavelength of colliding
partons are within their Schwarzschild radius or that entropy is
sufficiently large: Mmin = 3-4 * MD
• 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)
14 April, 2009
C. 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 (MS < MD)
 Thus, string ball cross-section higher than that of BHs
 Select using cuts on |pT| and jet pT ratios (at least 4 jets)
14 April, 2009
C. 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. MS > 4.8 TeV
MS > 1.6 TeV
MD > 2.4 TeV
14 April, 2009
C. 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.
14 April, 2009
C. Dallapiccola, MIT Seminar