The LHC Technically extremely challenging Peak luminosity Center of mass energy pp collider 26.67km circumference Accelerator Detector Radiation damage High occupancy Interaction rate 25ns

Pixels/blade 24 Blades/disk The CMS Detector Barrel + Forward Pixel Disks FPIX-Disk

Radiation Environment Barrel layers will be 53 cm long. Layers will be placed at 4.3 cm, 7.2 cm, while a third layer at 11.0 c.m will be added later. End disks are located at a distance of 32.5 and 46.5 cm from the interaction point. In LHC radiation environment is harsh. At full luminosity 1st, 2nd and 3rd layer will be exposed to a fluence of about 3  n eq /cm 2, 1.2  n eq /cm 2 and 0.6  n eq /cm 2 per year respectively. Readout chip is expected to survive a particle fluence of 6  n eq /cm 2, so sensors parts have to perform well at least up to this fluence. Total fluence at LHC

P N Hole and electron free region Particle Newly generated electron and hole pair A detector has to be fully or partially depleted! What is depletion? Giving enough reverse bias voltage so that no free carriers are available. In depleted condition, if a particle goes through the detector it will make electron and hole pair. They are attracted to the opposite terminals and get collected by charge sensitive amplifiers- we get a signal! ROC Silicon Sensor

Radiation Damage in Silicon Increase in depletion voltage - most problematic. Increase of leakage current. Decrease of charge collection efficiency.

Sensor Design N N+N+ P+P+ SiO 2 Bulk material is n and the pixels are n +. Towards the end of their lifetime pixel sensors will have depletion voltage more than 300 V. During that period sensors will be operated in partial depletion. To operate in partial depletion it is necessary that the depletion region starts from the pixel side. As the bulk will be inverted to p-type, the pixels are chosen to be n +.

N To protect from outer environment detector is covered with SiO 2. During fabrication process positive charge get trapped in the interface between Si and SiO 2. This oxide charge attracts the electrons in the silicon bulk and makes an electron rich layer at the surface which is called “accumulation layer”. Accumulation layer shortens the gap between the n + ring and the p + implant. Any voltage drop between n+ ring and the p+ implant appears in a tiny area between the accumulation layer and the p+ implant, causing a high electric field and breakdown of the silicon. To disconnect the accumulation layer at several places and maintain a uniform voltage drop p + guard rings are used! V V Breakdown prone area SiO 2 P+P+ N+N+ Trapped electrons Guard Ring

P-stop On the pixel side the induced electron channel will short circuit the pixels. To isolate pixels n + implants are surrounded by p + implants, which are called p-stop. A high inter-pixel resistance is desirable to isolate the pixels. However in the case where all pixels are not grounded, a too high value of inter-pixel resistance will give rise to a high potential difference between neighboring pixel. Hence a opening in the p-stop ring is left open to control the inter-pixel resistance. Also during quality control, the opening in the p-stop will help to bias the pixel array from the outer n + ring.

Engineering Run In engineering run with SINTEF and CSEM we received 20 wafers from SINTEF and 3 wafers from CSEM. Three different type of guard ring designs - 6, 11 and 16 guard rings and 8 different type of p-stop designs were implemented. Most of the sensors had 22  30 pixels. There were couple of full size sensors in the wafer consisting 50  52 pixels. The baseline design was 11 guard ring and double open ring p-stop. Sensors are 275  m thick and have 1-2 K  cm resistivity.

Wafer A SINTEF Wafer P-stop design A P-stop design G P-stop design F

Quality Control Wafers are received from vendors Characterization of sensors leakage current breakdown voltage depletion voltage inter-pixel resistance inter-pixel capacitance Identifying good sensors and dicing them out of the wafer Sending them for irradiation Characterization of sensors after irradiation leakage current breakdown voltage depletion voltage inter-pixel resistance inter-pixel capacitance Identifying the problem and making suitable change in design for next engineering run

Depletion Voltage and Leakage Current Depletion voltage is the voltage when the sensor is fully depleted. It is obtained from the 1/C 2 vs. V reverse_bias graph. Capacitance is proportional to the depleted area, hence after reaching the full depletion capacitance of a sensor remains constant. All the sensors deplete between 150 and 175 V. Leakage current is the current due to the minority career and is measured by applying reverse bias. All the sensors have leakage current less than 100nA/cm 2 at 1.5  V depletion - which was the specified criteria.

Guard Ring Performance When the leakage current increases exponentially and reach a predetermined compliance value we call it a breakdown. Compliance value is 1  A for non irradiated and 1mA for irradiated sensors. All the diode with 6 guard ring breaks down at 600 V. Some of the diodes with 16 guard ring do not break down up to 1000V while some break down earlier. None of the diode with 11 guard ring break down up to 1000 V!

Breakdown Voltage Distribution Most of the sensors breakdown between V. Sensors with p-stop design A, F and G has higher breakdown voltage.

Inter-pixel Isolation Inter-pixel resistance is about 0.1M  before depletion and 100G  after depletion. That corresponds to a negligible amount of voltage difference between neighboring pixels before depletion and about -15V at a bias voltage of -250V.

Inter-pixel Capacitance Inter-pixel capacitance is 1pF before depletion and 100fF after depletion.

Irradiation and Guard ring Performance Twenty six sensors and five diodes were irradiated at Indiana University Cyclotron Facility and UC Davis. IUCF MeV proton beam. UC Davis - 58 MeV proton beam. Total dose equivalent to 1  10 14, 6  and 1  MeV n eq /cm 2. After irradiation, measurement were done at 21 0 C, C. Depletion voltage after irradiation:  = 1  MeV n eq /cm 2 V dep = 50 V  = 6  MeV n eq /cm 2 V dep = 300 V  = 6  n eq /cm 2 As 11 guard rings had the best performance before irradiation, only diodes with 11 guard rings were irradiated. After irradiation diodes with 11 guard ring do not show any breakdown up to 800 V!

Leakage current and Soft Breakdown All sensors (except design F) show a soft breakdown between V. Leakage current deviates from sqrt(V) behaviour. Soft breakdown absent in diode - breakdown occurs at p-stop. Design F doesn’t show any soft breakdown at  = 1  n eq /cm 2 ! At  = 6  n eq /cm 2 two out of three F show soft breakdown.

  Unbonded pixel Hot pixel Bumpbonded pixel Soft Breakdown Measurement done at PSI shows that the extra current is drawn by limited number of pixels breaking down early (design A). Crosses & diamonds - not correlated Possible reasons of breakdown - fabrication defect, handling defect. Design F is simplest design, least prone to fabrication defect.

Inter-pixel Isolation and Capacitance  = 1  n eq /cm 2  = 6  n eq /cm 2 After irradiation the inter-pixel resistance is about 0.1G  for a sensor that received a fluence of 1  n eq /cm 2. It corresponds to a voltage difference of -32 V with the neighboring pixels when the bias voltage is -200 V. The inter-pixel capacitance is about 40fF for a sensor that received a fluence of 6  n eq /cm 2.

Ongoing and Future Steps A study is being carried out to determine the effect of irradiation on depletion voltage and whether oxygen rich silicon provides a better alternative to regular silicon. Inter-pixel resistance and inter-pixel capacitance is being measured again in different way to have a more consistent results. A new design has been submitted to SINTEF for next engineering run. Single open ring p-stop and eleven guard ring design has been chosen as baseline design. In the new engineering run emphasize has been given to optimize the size of the pixels.

The Future of New Physics 2006First collisions Interaction Energy Max. integrated Luminosity LHC starts now Tevatron Data taking Interaction Energy Integrated Luminosity CDF RUN II is a great opportunity to do new physics

New Physics at the Tevatron Important for new physics Produce it Find it Identification of objects that make up the signature Understanding of the calibration and resolution of the detector Understanding background With a center of mass energy of 2TeV and an integrated luminosity of in RunII D0 and CDF will be able to extend searches to new parameter space

Extra Dimension Two fundamental energy scale in nature The electroweak scale ~ 10 3 GeV The Planck scale ~ GeV Electroweak scale, m ew, is an experimental certainty. Planck scale is the scale when gravitational interactions become strong. However no one has explored the validity of Newton’s law at that level! What happens if there is only one fundamental scale and it is m ew ! The usual strength of the gravitational force can arise from this picture if we assume that there are n extra compact spatial dimensions of radius ~R. The Planck scale M Pl(4+n) of this (4+n) dimensional theory is taken to be ~m ew.

Extra Dimensions Two test masses m1, m2 placed within a distance r<<R will feel gravitational potential directed by Gauss’s law in (4+n) dimensions V( r) ~ [m 1 m 2 /M Pl(4+n) n+2 ][1/ r n+1 ] (r<<R) For r>>R the usual 1/r potential is obtained V( r) ~ [m 1 m 2 /(M Pl(4+n) n+2 R n )][1/r] (r>>R) So our effective 4-d M Pl is M Pl ~ M Pl(4+n) n+2 R n Putting M Pl(4+n) ~ m ew we get R = 8  m for n=1, 0.7 mm for n=2, 3 nm for n=3 and 6  m for n=4. And so far no one has tested Newton’s law to distances less than 0.2 mm. Therefore compactified extra dimensions at sub millimeter scale are, in principle, allowed!

Kaluza-Klein mode In the presence of extra dimension there will be a excited modes of wave function which are known as Kaluza-Klein modes. Graviton propagating in the compactified extra dimensions are called Kaluza-Klien gravitons G kk. From the point of view of a 3+1-dimensional space time, the Kaluza-Klein graviton modes are massive, with the mass per excitation mode ~1/R. Since the mass per excitation mode is so small (e.g. 400 ev for n=3 or 0.2 Mev for n=4), a very large number of modes can be excited at high energies. Phase space available to G kk increases by many folds. For a large numbers of modes, accessible at high energies, gravitational coupling is therefore enhanced drastically.

Signatures of Real Graviton Effects Monojets at the collider Single vector boson at the collider Since graviton can propagate in the bulk energy momentum are not conserved in the G kk emission from the point of view of our 3+1 space-time. Collider signature for the G kk leaving our world will be single photons/Z/jets with missing E T.

Signatures of Virtual Graviton Effects Fermion or vector boson pairs at the collider. For a virtual G kk emission the signature will be fermion and vector boson pair production. In the case of pair production via virtual graviton, gravity effects interfere with the standard model. Therefore, production cross section has three terms: SM, interference and the direct gravity effects. By calculating the cross section and subtracting the SM background from it effect of virtual gravity can be found.

Prospect of Run II If large extra dimension exists Run II or LHC is likely to discover them.

Conclusions The work on the CMS Forward Pixel system was a great experience and will be an enormous help for my future work with vertex detectors. CDF has the potential to discover new physics. If LED is not discovered in Run II, a new limit can be put in it’s validity. Looking forward for the data from CDF.