New Physics at a TeV and the LHC - I Accelerator Basics and the LHC Sreerup Raychaudhuri Tata Institute of Fundamental Research, Mumbai, India IPM String School (ISS 2009), Tehran, Iran April 16, 2009

We learn about nuclear and sub-nuclear physics in two ways: 1.Indirect way – from energy levels of nuclear/particle states (spectroscopy) 2.Direct way – from scattering experiments Typical scattering experiment

Two possible designs for a scattering experiment: 1. ‘Fixed’ target experiment As more and more energy is pumped into the beam, more and more energy is lost in the recoil of the target

2. Collider experiment Full beam energy is available for the process

SLAC 1969 LEP 1991 Tevatron 1994 LHC 2009

How are these high beam energies attained? Cannot make plates too close, for then there will be spark discharges even with high vacuum; instead we make voltage high ⇒ use AC voltage instead of DC voltage

Cannot use a continuous beam any more; must send bunches of particles at a time… …pulsed operation : timing between bunches must match RF…

Interaction rate will now depend on bunch crossings… Luminosity cm -2 s -1 Event rate s -1

As data are gathered over time… No of events Event rate : Integrated luminosity If  Is measured in pb, fb, etc., L is measured in pb -1, fb -1, etc. 1 pb = 1000 fb ⇒ 1 fb -1 = 1000 pb -1

nb -1 /s 10 nb -1 /s From the BNL home page

How are these high luminosities attained? Packing charged particles like electrons and protons into very small volumes is difficult because of the strong electrostatic repulsion; requires very strong focussing magnetic fields from superconducting electromagnets etc.

Working Principle of a Storage Ring Re-use the same bunches many many times…

8.6 Km The LHC is just a giant storage ring

8.6 Km

Buried  100 m  below ground to shield radiation

Section of LHC tunnel showing beam pipe

Some LHC parameters Beam energy : 5 TeV  7 TeV Collision energy : 10 TeV  14 TeV Luminosity : 10 nb -1 s -1 (design) ‘Integrated’ luminosity: 100 fb -1 per year (design) Bunch crossing rate :4  10 7 s -1 Bunch distance: ~ 7 m Bunch size : few cm  1 mm, 16  m (collision pt) No of protons/bunch :1.1  No of magnets: 9593 Magnet temperature: 1.9 K Current: A Magnetic field: 8.3 T

Some amusing LHC facts LHC will consume as much power as domestic sector in Geneva canton LHC budget is comparable to GDP of a small country, e.g. Fiji or Mongolia Vacuum is 10 times better than the surface of the Moon Magnetic fields of 8.3 Tesla are 100,000 times the Earth’s magnetic field Magnets will use 700,000 lit of liquid He and 12,000,000 lit of liquid N Total length of cable could stretch from Earth to Sun 5 times LHC protons will travel at c LHC protons will have energies comparable to that of a flying mosquito Protons used in 10 years would be equivalent to only 7.5  g of hydrogen LHC beams will together have enough energy to melt 1 tonne of copper Data could fill a stack of HD-DVDs 11 Km high (Mt. Everest: 8.8 Km)

barrel radiation sensitive; high efficiency End-cap radiation hardened; low efficiency 2009,

VXD EMC HCAL  Ch

CMS Detector VXD ECAL HCAL  Ch

Particle detection at the LHC No signals at all ; only missing energy Track in VXD ; energy deposit in ECAL Track in VXD ; tracks/deposits in  CH No track in VXD; only deposit in ECAL Hadronic jets ; signals in all devices Decay at the interaction vertex itself Displaced vertices in VXD; deposit in HCAL Decay at the interaction vertex itself Everything must be reconstructed only from these effects

Protons not point particles, but conglomerates of valence quarks (uud) gluons sea quarks (u,d,s,c,b,t) More like two cars crashing and spewing out parts than like the collision of hard billiard balls…

Choice of Variables Partonic system has an (unknown) longitudinal boost Each collision event will have a different   we must choose variables which are independent of longitudinal boosts

1.Transverse momentum : 2.Rapidity : 3.Pseudo rapidity : 4.Angular separation : 5.Invariant mass : Commonest Variables

Signal and Background If a certain final state (including phase space characteristics) is predicted by a theory, the cross-section for producing that final state is called the signal If it is possible to produce the same final state (including phase space characteristics) in an older, well-established theory (e.g. Standard Model), that cross-section is called a background Experimental results will have errors:

What constitutes a discovery? Excess/depletion over background : Assuming random (Gaussian) fluctuations, the probability that this deviation is just a statistical effect is about : Consensus: 3  deviation is exciting; 5  deviation constitutes a discovery; 8  deviation leaves no room for doubt

Limiting the parameter space Once there is a well-established deviation from the background, we compare it with the signal: If the numbers match, we can start claiming a discovery… Usually this matching can always be achieved by tuning the free parameters in the (new) theory… Comparison essentially serves to constrain the parameter space of the (new) theory If we must have very small ≈

Typical new physics bounds arising when experimental cross- sections match with backgrounds : Large N S excluded Small N S allowed If experimental data are there, this is called an exclusion plot If the data are projected, this is called a search limit LHC

If both signal and background are present, the prediction is that experiment will see the sum of both predictions. Typical case: background is large; signal is small In this case and Will be very difficult to observe any signal over the experimental error… Require to reduce the background (without reducing the signal) « »

Kinematic Cuts Fermi’s Golden Rule : Phase space integral has to be over all accessible final states Experimental cross-section may not be able to (wish to) access all the possible momentum final states  phase space integral must be done with appropriate kinematic cuts acceptance cuts : forced on us by the detector properties. selection cuts : chosen to prefer one process over another.

Examples of acceptance cuts: minimum p T for the final states : – very soft particles will not cause showering in ECAL/HCAL – different cuts for barrel and endcap maximum  for the final states : – no detector coverage in/near beam pipe isolation cut on  R for leptonic final states : – no hadronic deposit within a cone of  R = 0.4 – to be sure that the lepton is coming from the interaction point and not from a hadron semileptonic decay inside a jet Will be somewhat different for ATLAS and CMS

Selection cuts can be of different kinds depending on the process and the purpose for which it is made… Example of a selection cut: Suppose we want to select more electrons from the process (1) instead of electrons from the process (2) From simple energy-sharing arguments, the electron in (1) will have more p T than the electron in (2) Impose the selection cut : Ensures that the accessible phase space for (2) shrinks without seriously affecting that of (1) → reflected in the cross-section

A variety of selection cuts can be used to reduce the background without affecting the signal (much). Much of the collider physicist’s ingenuity lies in devising a suitable set of selection cuts to get rid of the background(s). Often the background can be reduced really dramatically – to maybe 1 in 10000… Nevertheless, often this reduction of backgrounds to negligible values may also reduce the already weak signal to less than one event in the whole running life of LHC! High luminosity is essential !!

The proton luminosity is not the end of the story…  what actually matters are : parton distributions  luminosity … actual collisions will happen between partons…

x f(x) Parton density functions (PDFs) from the CTEQ-6 Collaboration (C.P. Yuan et al)

Trade-off between energy and luminosity… If x < 0.4, then maximum available energy at parton level is only about 5 TeV… But to observe most new physics, high luminosity demand restricts us to x < 0.1, i.e. 1 – 2 TeV. LHC probes the TeV scale – but only just…

Physics Goals of the LHC to test known physics, i.e. SM = QCD + GSW model (H boson) to discover new physics, e.g. dark matter, SUSY, extra dim, new symmetries, compositeness, … Q. Why should new physics appear at the TeV scale? Is this just wishful thinking? …or do we have solid reasons?

Significance of the TeV energy scale: top-down approach : GUT or stringy unification must have low energy consequences; high scale SUSY will have low energy manifestations, extra dimensions will become accessible at high enough energies bottom-up approach : hierarchy problem, neutrino masses, GUT evolution aesthetic approach : 18 free parameters in the SM no QCD-EW unification desert scenarios

Capabilities of the LHC Cannot do a blind search… All important final states require a trigger

Huge QCD backgrounds… especially if looking for hadronic final states Cannot see very soft p T jets/leptons/photons LHC has severe limitations….

Can find the Higgs boson of the SM (if it exists) Can find a SUSY signal if kinematics permits Can find a resonant new state Sure shots : Less sure : Can measure Higgs boson couplings Can determine t quark properties to precision Can measure SUSY parameters Can discover exotics, e.g. gravitons, monopoles…

How are we so sure? … next two lectures….